DEVELOPMENTS IN SEDIMENTOLOGY 29
CONTINENTAL RED BEDS
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DEVELOPMENTS IN SEDIMENTOLOGY 29
CONTINENTAL RED BEDS
FURTHER TITLES IN THIS SERIES VOLUMES 1. 2.3, 5, 8 and 9 are out of print
4 F.G. T I C K E L L THE TECHNIQUES O F SEDIMENTARY MINERALOGY 6 L. V A N D E R P L A S THE IDENTIFICATION O F DETRITAL FELDSPARS 7 S. D Z U L Y N S K I and E.K. W A L T O N SEDIMENTARY FEATURES O F FLYSCH AND GREYWACKES 10 P,McL.D. DUFF, A. H A L L A M and E.K. W A L T O N CYCLIC SEDIMENTATION 11 C.C. R E E V E S Jr. INTRODUCTION T O PALEOLIMNOLOGY 12 R.G.C. B A T H U R S T CARBONATE SEDIMENTS AND THEIR DIAGENESIS 13 A.A. M A N T E N SILURIAN REEFS O F GOTLAND 14 K.W. G L E N N I E DESERT SEDIMENTARY ENVIRONMENTS 15 C.E. W E A V E R and L.D. P O L L A R D THE CHEMISTRY O F CLAY MINERALS 16 H.H. R I E K E III and G.V. C H I L I N G A R I A N COMPACTION O F ARGILLACEOUS SEDIMENTS 17 M.D. P I C A R D and L.R. HIGH Jr. SEDIMENTARY STRUCTURES O F EPHEMERAL STREAMS 18 G.V. C H I L I N G A R I A N and K.H. W O L F COMPACTION O F COARSE-GRAINED SEDIMENTS 19 W. S C H W A R Z A C H E R SEDIMENTATION MODELS AND QUANTITATIVE STRATIGRAPHY 20 M.R. W A L T E R , Editor STROMATOLITES 21 B. V E L D E CLAYS AND CLAY MINERALS IN NATURAL AND SYNTHETIC SYSTEMS 22 C.E. W E A V E R and K.C. BECK MIOCENE O F THE SOUTHEASTERN UNITED STATES 23 B.C. H E E Z E N , E d f t o r INFLUENCE O F ABYSSAL CIRCULATION ON SEDIMENTARY ACCUMULATIONS IN SPACE AND TIME 24 R.E. G R I M and N. G U V E N BENTONITES 25A 0. L A R S E N and G.V. C H I L I N G A R I A N , E d f t o m DIAGENESIS IN SEDIMENTS AND SEDIMENTARY ROCKS 26 T . SUDO and S. SHIMODA, Edftors CLAYS AND CLAY MINERALS O F JAPAN 27 M.M. M O R T L A N D and V.C. F A R M E R INTERNATIONAL CLAY CONFERENCE 1978 28 A. N I S S E N B A U M , Editor HYPERSALINE BRINES AND EVAPORITIC ENVIRONMENTS
DEVELOPMENTS IN SEDIMENTOLOGY 29
CONTINENTAL RED BEDS P. TURNER Department of Geological Sciences, The University of Aston in Birmingham, Gosta Green, Birmingham, England
ELSEVIER SCIENTIFIC PUBLISHING COMPANY 1980 Amsterdam - Oxford - New York
ELSEVIER SCIENTIFIC PUBLISHING COMPANY 336 Jan van Galenstraat P.O. Box 211, 1000 AE Amsterdam, The Netherlands Distributors for the United States and Canada: ELSEVIER/NORTH-HOLLAND INC. 52,Vanderbilt Avenue New York, N.Y. 10017
Library el Congress Cataloging 10 Pnbllcation Data
Turner, P 1948 Continental red beds. (Ikvebpuents i n sedimentology j 29) Bibliography: p. Includes index. 1. Red beds. I. T i t l e . 11. Series.
&E471*l'87
552l.5 ISBn 0-4W+-blg08-X
80-1903
ISBN 0-444-41908-x (Vol. 29) ISBN 0-444-41238-7 (Series)
Elsevier Scientific Publishing Company, 1980 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Scientific Publishing Company, P.O.Box 330, 1000 AH Amsterdam, The Netherlands 0
Printed in The Netherlands
V
PREFACE AND ACKNOWLEDGEMENTS C o n t i n e n t a l r e d beds a r e a d i s t i n c t i v e s e d i m e n t a r y f a c i e s which have e x c i t e d t h e i n t e r e s t o f g e o l o g i s t s f o r many y e a r s .
T h e i r f e a t u r e s i n c l u d e a v a r i e t y of
a l l u v i a l , d e l t a i c and d e s e r t d e p o s i t s , a h i g h l y d i s t i n c t i v e s u i t e o f d i a g e n e t i c f e a t u r e s and d i s t i n c t i v e m a g n e t i c p r o p e r t i e s .
Red beds a l s o h o s t a d i s t i n c t i v e
t y p e of m i n e r a l i z a t i o n c o n t a i n i n g c o p p e r , uranium a n d vanadium minera Is.
Perhaps
t h e most d i s t i n c t i v e , and c o n t r o v e r s i a l , f e a t u r e of r e d beds is t h e i r c o l o u r .
The
o r i g i n o f t h i s c o l o u r (and t h e o r i g i n o f r e d b e d s ) has been h o t l y d e b a t e d f o r o v e r 100 y e a r s .
E a r l y w o r k e r s , i m p r e s s e d by t h e r e d c o l o u r of some modern d e s e r t
s e d i m e n t s , t h o u g h t t h a t a n c i e n t r e d beds were d e p o s i t e d under d e s e r t , c o n d i t i o n s . L a t e r , t h e American s e d i m e n t o l o g i s t P.D. Krynine championed t h e i d e a t h a t r e d beds were formed by t h e e r o s i o n and r e d e p o s i t i o n of r e d l a t e r i t i c s o i l s i n m o i s t tropical climates.
The a s s o c i a t i o n o f t h e s e two h y p o t h e s e s r e s u l t e d i n t h e wide-
s p r e a d b e l i e f t h a t r e d beds a r e o f some p a l a e o c l i m a t i c s i g n i f i c a n c e .
Later studies
by T.R. Walker showed t h a t d i a g e n e t i c p r o c e s s e s a r e most i m p o r t a n t i n t h e f o r m a t i o n of r e d b e d s , t h a t r e d c o l o u r can form i n s i t u , and t h a t c o n t i n e n t a l r e d beds a r e n o t n e c e s s a r i l y of p a l a e o c l i m a t i c s i g n i f i c a n c e . Much of t h e e a r l y palaeomagnetic work on r e d b e d s was done p r i o r t o o u r c u r r e n t u n d e r s t a n d i n g o f r e d bed d i a g e n e s i s and i t was g e n e r a l l y assumed t h a t r e d beds were magnetized d u r i n g d e p o s i t i o n and e a r l y d i a g e n e s i s by t h e a l i g n m e n t o f d e t r i t a l magn e t i c o x i d e s ( a view c o n s i s t e n t w i t h K r y n i n e ' s h y p o t h e s i s which i s s t i l l w i d e l y h e l d to-day).
Many l a t e r palaeomagnetic s t u d i e s r e v e a l e d complex m a g n e t i z a t i o n s more
c o n s i s t e n t w i t h a d i a g e n e t i c o r i g i n f o r r e d beds.
There i s , however, much v a r i a t i o n
i n t h e d i a g e n e t i c and p a l a e o m a g n e t i c p r o p e r t i e s o f r e d beds.
This v a r i a t i o n , along
w i t h t h e b e l i e f that a review combining t h e d e p o s i t i o n a l , d i a g e n e t i c and palaeomagn e t i c h i s t o r y o f r e d beds would b e o f some v a l u e , i s t h e i n s p i r a t i o n f o r t h i s book. The book would never have been w r i t t e n w i t h o u t t h e h e l p o f numerous f r i e n d s and c o l l e a g u e s who have c o n t r i b u t e d t o d i s c u s s i o n s w i t h me o v e r t h e p a s t decade a l t h o u g h I must stress t h a t I am s o l e l y r e s p o n s i b l e f o r a l l t h e shortcomings which t h e book c o n t a i n s .
I would p a r t i c u l a r l y l i k e t o thank t h e f o l l o w i n g who
p r o v i d e d m e w i t h m a t e r i a l a s s i s t a n c e a t v a r i o u s s t a g e s of t h e work:
E.L.
Boardman, D.W.
C o l l i n s o n , R.A. I x e r , R.G.
J a c k s o n 11, R . J .
P.J.C. N a g e t g a a l , S h e l l U . K . E x p l o r a t i o n and P r o d u c t i o n , R . J . D.J.
R. A r c h e r ,
Moiola,
S t e e l , D.H.
Tarling,
Vaughan, T.R. Walker, B. Waugh and P.A. Z i e g l e r .
The p r o d u c t i o n of t h e m a n u s c r i p t would n e v e r have been p o s s i b l e w i t h o u t t h e a s s i s t a n c e of t h e s t a f f o f G e o l o g i c a l S c i e n c e s a t t h e U n i v e r s i t y o f Aston p a r t i c u l a r l y Cathy Kennedy who typed t h e m a n u s c r i p t and B e v e r l e y P a r k e r who d r a f t e d many o f t h e diagrams.
My w i f e Angela r e a d t h e m a n u s c r i p t and p r e p a r e d
VI the s u b j e c t index. Many a u t h o r s and p u b l i s h e r s have g r a n t e d me p e r m i s s i o n t o use i l l u s t r a t i o n s from t h e i r o r i g i n a l p u b l i c a t i o n s a s f o l l o w s : American A s s o c i a t i o n o f Petroleum G e o l o g i s t s ( F i g s . 2 . 1 1 ,
2.26, 3.13, 3.14, 3 . 2 9 ,
4.35, 4 . 3 7 ) American J o u r n a l of S c i e n c e ( F i g s . 1 . 2 5 , 6.12) American M i n e r a l o g i s t ( F i g . 5 . 1 7 ) Annals de Geophysique ( F i g s . 7 . 1 2 , 7.13) Annals de Physique ( F i g . 7 . 9 ) B.T.
B a t s f o r d Ltd. ( F i g s . 2 . 2 ,
2.5)
B u l l e t i n of t h e G e o l o g i c a l S o c i e t y o f America ( F i g s . 2 . 1 6 ,
2.22, 4 . 2 , 4.23, 4 . 2 9 ,
4 . 3 0 , 4 . 3 1 , 4.32, 4.38, 4.39, 4 . 4 0 , 5 . 1 4 , 5.18, 6 . 6 , 6 . 8 , 6 . 9 , 8.22, 8.23, 8 . 2 4 , 8 . 2 7 , 8.28, 8.29, 8 . 3 0 ) B u l l e t i n of t h e G e o l o g i c a l Survey of G r e a t B r i t a i n ( F i g s . 3 . 3 0 ,
3.25, 3.26, 6.28)
Canadian J o u r n a l o f E a r t h S c i e n c e s ( F i g s . 4 . 5 0 , 8 . 4 , 8 . 5 , 8 . 7 , 8 . 8 , 8.9) E a r t h and P l a n e t a r y S c i e n c e Letters ( F i g s . l . 9 ,
7.19)
Geochimica Cosmochimica Acta ( F i g . 6 . 1 1 ) Geografiska Annaler (Fig.4.11) Geologie en Mijnbouw ( F i g s . 2 . 2 4 , Geological Journal (Figs.1.22,
3.15)
4.19, 4.34, 4 . 4 4 , 4 . 5 8 , 4.59)
Geologisches Rundschau ( F i g s . l . 1 6 ,
1 . 1 7 , 1.31)
Geophysical J o u r n a l of t h e Royal A s t r o n o m i c a l S o c i e t y ( F i g s . 6 . 4 , 7.29, 7.30, 7 . 3 1 , 7.32, 7.33, 7 . 3 4 , 7.38, 7.39,
7.26,
7.27, 7.28,
7.40, 7.42, 7.43, 7.44, 8.10,
8.12, 8 . 1 3 , 8 . 1 4 , 8.15, 8.16) Geosciences Canada ( F i g . 3.19) Ground Water (Fig.6.10) John Wiley and Sons I n c . ( F i g . 2 . 1 7 ) J o u r n a l of t h e G e o l o g i c a l S o c i e t y o f London ( F i g s . l . 1 0 ,
1 . 2 1 , 2.30, 4.52, 4.53)
J o u r n a l of Geomagnetism and G e o e l e c t r i c i t y (Fig.7.5) J o u r n a l of G e o p h y s i c a l R e s e a r c h ( F i g s . 8 . 1 8 , J o u r n a l o f Sedimentary P e t r o l o g y ( F i g s . 1 . 3 6 ,
8 . 1 9 , 8 . 2 5 , 8.26, 8.31) 2 . 7 , 2.20, 2.21, 2.27, 3.11, 3.21,
3.22, 3.23, 4 . 5 , 4 . 6 , 4 . 2 3 , 4.49) Limnology and Oceanography ( F i g . 6 . 2 1 ) Marine Geology ( F i g . 3.6) Mountain G e o l o g i s t (Fig.2.23) Nature (Fig.8.6) Norsk Geologisk T i d d s k r i f t ( F i g . l . 2 0 ) P h i l o s o p h i c a l T r a n s a c t i o n s of t h e Royal S o c i e t y o f London ( F i g . 8 . 1 ) P h y s i c s of t h e E a r t h and P l a n e t a r y I n t e r i o r s ( F i g . 7 . 2 3 , P h y s i c a l Review ( F i g . 7 . 1 0 ,
7.11)
7.24,
7.25)
VII
Proceedings of the Royal Society of London (Fig.7.17) Proceedings of the Yorkshire Geological Society (Fig.4.45) Reports of Progress in Physics (Fig.6.2) Reviews of Modern Physics (Fig.7.7) Science (Figs.7.20, 1.28, 7.14, 7.15) Scottish Journal of Geology (Figs.4.41, 4.36) Sedimentary Geology (Figs.2.3, 4.9, 4.26, 4.47, 4.48, 4.56, 4.61, 4.62, 4.63) Sedimentology (Figs.2.4, 2.6, 2.9, 2.10, 2.12, 2.13, 3.10, 3.17, 4.7, 4.12, 4.13, 4.14, 4.21, 4.22, 4.46, 7.18) Society of Economic Paleontologists and Mineralogists (Figs.l.4, 1.7, 1.11, 1.12, 1.13, 1.14, 1.16, 1.18, 1.19, 1.32, 1.33, 1.34, 3.3) Springer-Verlag (Fig.4.25, 5.1) Tectonophysics (Figs.1.23, 1.24) Transactions of the Gulf Coast Association of Geological Societies (Figs.3.18, 3.1, 3.4, 3.6, 3.7) W.H. Freeman and Co. (Fig.4.28)
This Page Intentionally Left Blank
IX
CONTENTS
CHAPTER 1 THE GEOLOGICAL FRAMEWORK OF CONTINENTAL RED BEDS
i
INTRODUCTION
1
GEOSYNCLINES Pre-orogenic phase Pre-f l y s c h Flysch Mo l a s s e
2 4
4 6 6
PLATE TECTONICS AND SEDIMENTATION D i v e r g e n t p l a t e margins I n t r a c o n t i n e n t a l r i f t s and a u l a c o g e n s Intercontinenta 1 r i f t i n g Convergent p l a t e margins Forearc areas The magmatic a r c Back-arc a r e a s Transform f a u l t s Continental c o l l i s i o n
6 7 7 9 13
THE TECTONIC SETTING OF ANCIENT CONTINENTAL RED BEDS Introduction P r o t e r o z o i c r e d b e d s of w e s t e r n Canada The Caledonian orogen E a r l y Mesozoic r e d beds formed i n graben a s s o c i a t e d w i t h t h e opening of t h e A t l a n t i c Ocean Cenozoic molasse The A l p i n e molasse The molasse of t h e Canadian c o r d i l l e r a
26 26 26 29 35
13 14 15 16 21
39 40 44
THE CLIMATIC SETTING OF ANCIENT CONTINENTAL RED BEDS The s i g n i f i c a n c e of f i r s t - c y c l e a r k o s i c r e d beds The s i g n i f i c a n c e of s i z e - c o m p o s i t i o n t r e n d s
50
THE CLASSIFICATION OF ANCIENT CONTINENTAL RED BEDS I n t r o d u c t i o n and p r e v i o u s c l a s s i f i c a t i o n s Reddening and d e p o s i t i o n a l environment Palaeomagnetic c l a s s i f i c a t i o n of c o n t i n e n t a l r e d b e d s A p r a c t i c a l c l a s s i f i c a t i o n of c o n t i n e n t a l r e d beds
60 60 63 64 66
CHAPTER 2
69
DESERT RED BEDS
51 55
INTRODUCTION
69
MODERN DESERT SEDIMENTS
69
DESERT FLWIAL SEDIMENTS
71
AEOLIAN SAND DEPOSITS Ripples Dunes and d r a a s Barchans Seifs T r a n s v e r s e forms P a r a b o l i c dunes Rhourds The i n t e r n a l s t r u c t u r e of modern and a n c i e n t a e o l i a n d e p o s i t s
72 74 77 78 80 a2 a4 84 85
X The t e x t u r a l c h a r a c t e r i s t i c s of a e o l i a n sands Grain s i z e Roundness and f r o s t i n g
88 89 91
DESERT LAKES AND INLAND SABKHAS
95
THE REDDENING OF DESERT SEDIMENTS Introduction The d e t r i t a l model - Simpson D e s e r t , A u s t r a l i a The d i a g e n e t i c model Sonoran D e s e r t Early diagenetic reddening Late diagenetic reddening
96 96 100 102 104 104
ANCIENT AEOLIAN RED BEDS Introduction D i s t i n g u i s h i n g a e o l i a n from s h a l l o w m a r i n e s a n d s t o n e s The Lower Permian of NW Europe Marginal f a n g l o m e r a t e s and wadi d e p o s i t s Aeolian sands D e s e r t l a k e and sabkha d e p o s i t s D i a g e n e s i s and r e d d e n i n g of t h e R o t l i e g e n d e s Mechanical i n f i l t r a t i o n of c l a y A u t h i g e n i c q u a r t z , f e l d s p a r and c l a y m i n e r a l s Carbonate and s u l p h a t e cements
105 105 107 108 111 113 114 116 117 120 122
CONCLUSIONS AND FURTHER READING
125
CHAPTER 3
126
-
DELTA PLAIN RED BEDS
INTRODUCTION
126
DELTA MODELS
127
DELTAIC FACIES ASSOCIATIONS The d e l t a p l a i n River-dominated d e l t a p l a i n s F l u v i a l d i s t r i b u t a r y channels I n t e r d i s t r i b u t a r y environments Swamps L a c u s t r i n e and l a c u s t r i n e d e l t a - f i l l d e p o s i t s Tide-dominated d e l t a p l a i n s Tidal d i s t r i b u t a r y channels Interdistributary areas The d e l t a f r o n t
131 132 132 132 134 137 138 138 139 139 139 146
DELTA GROWTH AND ABANDONMENT ANCIENT DELTAIC SUCCESSIONS
147
RED BEDS I N ANCIENT DELTA PLAINS Introduction The D i f u n t a Group of n o r t h e a s t e r n Mexico The d e l t a p l a i n f a c i e s Colour v a r i a t i o n s and m i n e r a l o g y The o r i g i n of t h e d e l t a p l a i n r e d beds C a r b o n i f e r o u s r e d beds i n Europe and North America Introduction Secondary p o s t - d i a g e n e t i c r e d d e n i n g b e n e a t h t h e sub-Permian unconformity Red beds o f t h e Upper Coal Measures of t h e UK A. South Wales B . The Blackband and E t r u r i a Marl Groups of North S t a f f o r d s h i r e Upper c a r b o n i f e r o u s r e d beds a t J o g g i n s , Nova S c o t i a
150 150 15 3 155 156 15 8 15 9 15 9 160 163 164 168 175
CONCLUSIUWS AND FURTHER READING
178
XI CHAPTER 4
ALLUVIAL RED BEDS
179
INTRODUCTION
179
RIVER CHANNELS
180
ALLUVIAL FANS Alluvial fan deposits Debris f l o w d e p o s i t s Sheet flood deposits Stream c h a n n e l d e p o s i t s Sieve deposits
182 183 184 185 185 185
PEBBLY BRAIDED RIVERS Longi t u d i n a 1 b a r s Bars i n c u r v e d c h a n n e l r e a c h e s Sedimentary o r g a n i z a t i o n of p e b b l y a l l u v i u m on a l a r g e s c a l e
185 186 188 189
SANDY LOW-SINUOSITY RIVERS
190
EPHEMERAL STREAM CHANNELS
194
MEANDERING RIVERS Introduction Point bars
198 198 200
RIVER BANKS AND FLOODPLAINS Bank d e p o s i t s Floodplains Interfluvial areas
204 204 20 7 208
THE TRANSPORT OF IRON I N RECENT ALLUVIUM
213
ANCIENT ALLUVIAL RED BEDS The Old Red Sandstone reddened p e b b l y a l l u v i u m Internal facies The r e d d e n i n g of p e b b l y a l l u v i u m reddened sandy and muddy a l l u v i u m External facies A. Coarse members B. F i n e members The r e d d e n i n g o f sandy and muddy a l l u v i u m A . F i n e member r e d d e n i n g B. Coarse member r e d d e n i n g The Lower T r i a s s i c r e d beds o f NW Europe Upper C r e t a c e o u s r e d beds o f t h e s o u t h e r n Bohemian Basins
2 20 221 222 2 30 233 233 237 244 246 250 25 3 25 9
CONCLUSIONS AND FURTHER READING
264
CHAPTER 5
265
-
THE DIAGENESIS OF CONTINENTAL RED BEDS
INTRODUCTION
265
SANDSTONE DIAGENESIS The c o m p o s i t i o n o f n a t u r a l w a t e r s and e a r l y d i a g e n e t i c r e a c t i o n s Cementa t i o n M i n e r a l t r a n s f o r m a t i o n and replacement T e x t u r a l changes d u r i n g d i a g e n e s i s Red bed d i a g e n e s i s
265 267 269 272 273 2 74
THE MECHANICAL INFILTRATION OF DETRITAL CLAY Cenozoic a l l u v i u m o f s o u t h w e s t e r n USA and n o r t h w e s t e r n Mexico Mechanically i n f i l t r a t e d c l a y i n a n c i e n t r e d beds
275 275 2 80
DISSOLUTION OF FRAMEWORK SILICATES Feldspars Micas
2 80 282 284
XI1 Ferromagnesian m i n e r a l s
291
CLAY REPLACEMENT
293
AUTHIGENIC MLNERALS Potassium f e l d s p a r Zeolites Clay m i n e r a l s Illite-montmorillonite Illite Kaolinite-dicki te Chlorite Calcite Haematite and p r e c u r s o r o x i d e s Quartz
299 299 302 308 308 30 9 30 9 313 314 316 319
STAGES I N THE DIAGENESIS OF CONTINENTAL RED BEDS
319
CHAPTER 6
323
THE MINERALOGY AND GEOCHEMISTRY OF IRON OXIDES I N RED BEDS
IRON OXIDE MINERALS The t i t a n o m a g n e t i t e s - titanomaghemi t e s The h a e m a t i t e - i l m e n i t e s e r i e s F e r r i c oxyhydroxides
323 32 3 326 3 29
OPAQUE OXIDES I N RED BEDS Titanomagnetite Haematite-ilmenite
3 30 3 30 333
PRECIPITATION AND STABILITY OF FERRIC OXYHYDROXIDES The d e h y d r a t i o n o f f e r r i c oxyhydroxides
338 340
ENVIRONMENTAL CONTROLS ON THE PRECIPITATION AND DIAGENETIC HISTORY OF FERRIC OXYHYDROXIDES Marine c o n d i t i o n s An a n c i e n t example: The C a t s k i l l c l a s t i c wedge Lacustrine conditions A n a n c i e n t example: The . O r c a d i a n Basin
344 345 348 35 1 35 9 365
POST-DEPOSITIONAL INFLUENCE OF GROUNDWATER ON THE MINERALOGY AND GEOCHEMISTRY OF RED BEDS Reddening by groundwater Secondary r e d u c t i o n zones
365 36 7
I R O N CONTENT OF ANCIENT RED BEDS COKCLUSIONS AND FURTHER READING
376 379
CHAPTER 7
38 0
THE MAGNETIZATION OF CONTINENTAL RED BEDS
INTRODUCTION Palaeomagnetism and rock magnetism The p h y s i c a l b a s i s of magnetism Hysteresis A n i s o t r o p y , magnetic domains and t h e t i m e dependence of m a g n e t i z a t i o n
380 380 382 304 3 84
MINERAL MAGNETISM The t i t a n o m a g n e t i t e s and ti tanomaghemi t e s Haema t i t e Grain s i z e e f f e c t s The r e l a t i v e s t a b i l i t y of s p i n c a n t e d and d e f e c t moments
388 389 39 0 394 398
MAGNETIZATION PROCESSES I N SEDIMENTS D e t r i t a l remanent m a g n e t i z a t i o n The o c c u r r e n c e of DRM i n n a t u r e
40 3 40 3 408
XI11 Chemical remanent m a g n e t i z a t i o n CRM of t h e g o e t h i t e - h a e m a t i t e t r a n s i t i o n
411 413
THE MAGNETIC PROPERTIES OF CONTINENTAL RED BEDS Induced m a g n e t i z a t i o n ( J i - H ) a n d i s o t h e m a l remanence (IRM) c u r v e s Ji-T a n a l y s i s The NRM of r e d b e d s Chemical d e m a g n e t i z a t i o n Comparative t h e r m a l d e m a g n e t i z a t i o n o f p i g m e n t , s p e c u l a r i t e a n d r o c k
417 418 424 42 7 429 434
CHAPTER 8
THE PALAEOMAGNETISM OF CONTINENTAL RED BEDS
440
INTRODUCTION
440
PROTEROZOIC BASINS OF WESTERN CANADA The M a r t i n F o r m a t i o n The C h r i s t i e Bay Group
44 1 443 446
THE LATE PRECAMBRIAN OF NORTH WEST SCOTLAND
45 3
THE OLD RED SANDSTONE The Anglo-Welsh B a s i n The O r c a d i a n B a s i n
460 460 46 3
TRIASSIC RED BEDS OF THE WESTERN USA The Moenkopi F o r m a t i o n
46 8 469
LATE CENOZOIC RED BEDS OF BAJA CALIFORNIA
4 75
THE RELATIONSHIP BETWEEN DIAGENESIS AND PALAEOMAGNETISM I N CONTINENTAL RED BEDS Models of remanence a c q u i s i t i o n The p a l a e o m a g n e t i c e v o l u t i o n o f r e d b e d s Type A r e d b e d s Type B r e d b e d s Type C r e d beds
481 481 484 485 487 488
REFERENCES
493
SUBJECT INDEX
5 37
This Page Intentionally Left Blank
1
CHAPTER 1 THE GEOLOGICAL FRAMEWORK OF CONTINENTAL RED BEDS INTRODUCTION C o n t i n e n t a l r e d beds comprise a wide range o f s e d i m e n t a r y f a c i e s r e p r e s e n t i n g t h e whole spectrum of non-marine d e p o s i t i o n a l environments from a l l u v i a l f a n s , r i v e r f l o o d p l a i n s , d e s e r t s , l a k e s and d e l t a s .
They a r e n o t e x c l u s i v e l y r e d a n d o f t e n
c o n t a i n v a r i a b l e p r o p o r t i o n s of i n t e r d i g i t a t e d d r a b s t r a t a which shows a v a r i e t y of other colours.
Most commonly such c o l o u r v a r i a t i o n a r e t h e r e s u l t o f minor v a r i a t -
i o n s i n d e p o s i t i o n a l environments. Red beds a r e c o l o u r e d by f i n e l y d i s s e m i n a t e d f e r r i c o x i d e s , u s u a l l y i n t h e form of h a e m a t i t e (&-Fe 0 ) , and g e n e r a l l y have a c o l o u r between 5R and 10R a c c o r d i n g 2 3 t o t h e Rock Color C h a r t (Goddard, 1951). The o c c u r r e n c e of f e r r i c o x i d e s i n r e d beds i n d i c a t e s t h a t t h e y formed under o x i d i z i n g c o n d i t i o n s b u t i s o f no palaeoc l i m a t i c s i g n i f i c a n c e b e c a u s e r e d beds a r e known t o form i n b o t h a r i d and m o i s t tropical climates.
The a c t u a l mechanisms of r e d bed f o r m a t i o n a r e s t i l l o n l y p o o r l y
understood and i t i s now c l e a r t h a t s e v e r a l d i f f e r e n t p r o c e s s e s a r e i n v o l v e d i n t h e g e n e s i s of a p a r t i c u l a r r e d bed f o r m a t i o n .
G e n e r a l i z a t i o n s r e g a r d i n g t h e o r i g i n of
r e d beds a r e n o t t h e r e f o r e p o s s i b l e . One of t h e most i m p o r t a n t v a r i a b l e s c o n t r o l l i n g t h e f o r m a t i o n of r e d beds i s t h e n a t u r e of t h e b a s i n i n which t h e sediments a r e o r i g i n a l l y d e p o s i t e d .
The n a t u r e of
t h e sedimentary b a s i n i s l a r g e l y determined by t h e t e c t o n i c s e t t i n g which c o n t r o l s t h e r a t e of u p l i f t and e r o s i o n i n t h e s o u r c e a r e a , t h e r a t e of sediment s u p p l y t o t h e b a s i n , and a l s o t h e r a t e o f s u b s i d e n c e a n d d e p o s i t i o n w i t h i n t h e b a s i n i t s e l f . T r a d i t i o n a l l y , c o n t i n e n t a l r e d beds have been r e g a r d e d a s c l a s t i c wedges of sediment formed a l o n g t h e f l a n k s of a c t i v e l y r i s i n g mountain r e g i o n s .
The f o r m a t i o n o f t h e s e
c l a s t i c wedges c l e a r l y i m p l i e s a s t r o n g t e c t o n i c i n f l u e n c e i n t h e form of m a r g i n a l f a u l t i n g and u p l i f t of t h e s o u r c e r e g i o n s .
C o n t i n e n t a l s e d i m e n t s of t h i s t y p e a r e
a d i s t i n c t i v e f e a t u r e of t h e l a t e r s t a g e s o f mountain b u i l d i n g and r e p r e s e n t a d i s t i n c t i v e s e d i m e n t a r y f a c i e s known a s molasse.
They a l s o o c c u r i n a number of
o t h e r t e c t o n i c s e t t i n g s such a s i n t r a c o n t i n e n t a l r i f t systems and t r a n s f o r m ( s t r i k e s l i p ) s e d i m e n t a r y b a s i n s , b o t h o f which may c o n t a i n v a s t t h i c k n e s s e s o f c o n t i n e n t a l sediments w i t h a s s o c i a t e d r e d beds.
There a r e c l o s e p a r a l l e l s between t h e t e c t o n i c
s e t t i n g of Recent c o n t i n e n t a l sediments and a n c i e n t r e d beds. The f i r s t p a r t of t h i s c h a p t e r d e s c r i b e s t h e t e c t o n i c framework o f Recent s e d i m e n t a r y b a s i n s i n r e l a t i o n t o t h e g e o s y n c l i n a l and p l a t e t e c t o n i c c o n c e p t s . T h i s i s followed by a d e s c r i p t i o n of a number of a n c i e n t r e d bed b a s i n s which i l l u s t r a t e t h e c h a r a c t e r i s t i c s e t t i n g s i n which a n c i e n t c o n t i n e n t a l r e d beds a r e
found.
The l a s t two p a r t s a r e concerned w i t h t h e p a l a e o c l i m a t i c s i g n i f i c a n c e and
c l a s s i f i c a t i o n of a n c i e n t c o n t i n e n t a l r e d beds. GEOSYNCLINES Our p r e s e n t u n d e r s t a n d i n g of s e d i m e n t a r y b a s i n s owes much t o t h e development of the geosynclinal concept.
T h i s embodies t h e common a s s o c i a t i o n of t h i c k s e d i m e n t a r y
b a s i n , f o l d i n g , i n t r u s i v e and e x t r u s i v e igneous a c t i v i t y , and mountain b u i l d i n g . Most f e a t u r e s o f g e o s y n c l i n e s can b e i n t e r p r e t e d i n terms of p l a t e t e c t o n i c s , b u t a number c a n n o t , and f o r t h i s r e a s o n t h e r e t e n t i o n o f t h e c o n c e p t , w i t h i t s emphasis on t h e d e s c r i p t i o n of t e c t o n i c s e t t i n g s of s e d i m e n t a r y sequences a t c o n t i n e n t a l margins has proved most u s e f u l (HsU, 1972). The term g e o s y n c l i n e , s t r i c t l y s p e a k i n g , means a l a r g e down-warped s u r f a c e , b u t h a s always been c o n s t r u e d a s a t h e o r e t i c a l c o n c e p t o r i g i n of mountains.
-
a working h y p o t h e s i s f o r t h e
The o r i g i n a l i d e a of H a l l (1859) was t h a t s e d i m e n t a r y l o a d
l e d t o s u b s i d e n c e , which i n t u r n r e s u l t e d i n f o l d i n g , i g n e o u s a c t i v i t y a n d mountain building.
C r i t i c s p o i n t e d o u t t h a t g e o s y n c l i n e s d i d n o t s u b s i d e under s e d i m e n t a r y
l o a d b u t s y n c l i n a l o r g e o s y n c l i n a l d e p r e s s i o n s made t h e a c c u m u l a t i o n s of t h i c k s e d i m e n t a r y sequences p o s s i b l e (Dana, 1873).
Thus, i f s e d i m e n t a r y s u p p l y was slow
o r modest and c o u l d n o t keep pace w i t h s u b s i d e n c e , " g e o s y n c l i n a l " s e d i m e n t s would be b a t h y a l and p e l a g i c ( S u e s s , 1875; Haug, 1900). I t was r e a l i z e d a t a n e a r l y s t a g e t h a t orogeny c o u l d r e s u l t from l a t e r a l comp-
r e s s i o n caused by t h e movement o f ocean f l o o r towards a c o n t i n e n k a l mass and t h a t g e o s y n c l i n e s developed e i t h e r b e s i d e , o r o n , a c o n t i n e n t a l margin.
T h i s view was
s t r o n g l y i n f l u e n c e d by t h e p r e s e n t p o s i t i o n o f t h e Appalachians and t h e w e s t e r n C o r d i l l e r r a , w h e r e a s t h e e a r l y view o f Europeans was t h a t g e o s y n c l i n e s developed between and on t h e a d j a c e n t margins o f two c o n t i n e n t s .
I n t h i s context sedimentation
was c o n s i d e r e d t o t a k e p l a c e p r o g r e s s i v e l y c l o s e r t o one o f t h e c o n t i n e n t s , termed a ' f o r e l a n d ' , d u r i n g g e o s y n c l i n a l development. G e o s y n c l i n a l c l a s s i f i c a t i o n was begun by S c h u c h e r t (1923) and e x t e n d e d by S t i l l e (1936, 1940) who d i s t i n g u i s h e d o r t h o g e o s y n c l i n e s , c h a r a c t e r i z e d by a l p i n e - t y p e deformation and orogeny, and p a r a g e o s y n c l i n e s which a r e c h a r a c t e r i z e d by b l o c k f a u l t i n g and do n o t form mountain c h a i n s .
O r t h o g e o s y n c l i n e s were d i v i d e d i n t o e u g e o s y n c l i n e s
w i t h a v a r i e d igneous s u i t e i n c l u d i n g p r e - o r o g e n i c o p h i o l i t e s , s y n - o r o g e n i c a n d e s i t e s and p o s t - o r o g e n i c g r a n i t e s , and miogeosynclines which l a c k i g n e o u s r o c k s . More d e t a i l e d c l a s s i f i c a t i o n s were developed by Kay (1947, 1951) who sub-divided t h e o r t h o g e o s y n c l i n e and p a r a g e o s y n c l i n e o f S t i l l e (1936).
He retained t h e division
i n t o miogeosyncline and e n g e o s y n c l i n e and a p p l i e d i t t o t h e Appalachian mountains (Fig.l.1).
The p a r a g e o s y n c l i n e s were, however, d i v i d e d i n t o t h r e e t y p e s : exogeo-
s y n c l i n e s , s i t u a t e d on a c o n t i n e n t a l margin and s u p p l i e d by d e t r i t u s from t h e deformed o r t h o g e o s y n c l i n e ; a u t o g e o s y n c l i n e s , c o n s i s t i n g m o s t l y of i n t r a c o n t i n e n t a l
3 c a r b o n a t e b a s i n s i n d e p e n d e n t of t h e o r t h o g e o s y n c l i n e and z e u g o g e o s y n c l i n e s which Eastern New York
20 000
Vermont
New Hampshire
Maine
50
50
0 Distance (miles)
RESTORED SECTION - CAMBRIAN THROUGH MIDDLE ORDOVICIAN
100 fathom line Continental shelf
0
-
1
5000 10 000
/
15 000
20 000 Shelf __*
2 5 000
S1ope+Continental
rise
SEDIMENT THICKNESS OF ATLANTIC CONTINENTAL MARGIN
F i g . 1.1. The Appalachian g e o s y n c l i n e ( a f t e r Kay,1951) compared w i t h t h e p r e s e n t day A t l a n t i c margin ( a f t e r Drake e t a l . , 1959).
a r e i n t r a c r a t o n i c b a s i n s f i l l e d by d e t r i t u s from i n t r a c r a t o n i c mountain c h a i n s . Kay a l s o r e c o g n i z e d a number of s e d i m e n t a r y b a s i n s which h e c o n s i d e r e d t o b e c h a r a c t e r i s t i c of t h e l a t e r s t a g e s of g e o s y n c l i n a l development and termed them " l a t e cycle geosynclinesii.
These i n c l u d e d :
e p i e n g e o s y n c l i n e s d e r i v e d from
mountains which were formed from e u g e o s y n c l i n e s ; t a p h r o g e o s y n c l i n e s which a r e i n t r a c r a t o n i c b a s i n s c h a r a c t e r i s e d by fault-bounded m a r g i n s , and p a r a l i a g e o s y n c l i n e s s i t u a t e d a l o n g c o n t i n e n t a l margins and merging i n t o t h e c o a s t a l p l a i n .
This category
was c r e a t e d t o accommodate t h e Gulf of Mexico. The most w i d e l y r e c o g n i z e d o f t h e s e L a t e Cycle G e o s y n c l i n e s i s t h e Taphrogeos y n c l i n e which i s s i g n i f i c a n t b e c a u s e t h e s e r e p r e s e n t a m a j o r l o c u s o f c o n t i n e n t a l e l a s t i c d e p o s i t i o n and r e d bed f o r m a t i o n and have o f t e n been wrongly compared i n t e c t o n i c s e t t i n g w i t h p o s t - o r o g e n i c molasse.
Taphrogeosynclines a r e elongated
s e d i m e n t a r y b a s i n s which a r e i n t r a c r a t o n i c and c h a r a c t e r i z e d by t i l t e d (= t a p h r o ) f a u l t blocks.
The t y p e example i s t h e T r i a s s i c f a u l t t r o u g h o f t h e e a s t e r n U.S.A.
4
An i m p o r t a n t landmark i n t h e development of t h e g e o s y n c l i n a l c o n c e p t was t h e r e c o g n i t i o n o f t h e importance o f t u r b i d i t y c u r r e n t s (Kuenen, 1957a, 195713, 1958, 1967).
These p r o v i d e d a mechanism whereby l a r g e sand and conglomerate b o d i e s c o u l d
b e emplaced i n deep w a t e r and p r o v i d e d a n e x p l a n a t i o n of how a n i n i t i a l l y deep s e d i m e n t a r y b a s i n c o u l d be f i l l e d .
Drake e t a l . (1959) showed how t u r b i d i t y c u r r e n t s
r e l a t e d t o a n A t l a n t i c - t y p e c o n t i n e n t a l margin
-
a c o n t i n e n t a l s h e l f prism passing
oceanwards i n t o a c o n t i n e n t a l r i s e p r i s m w i t h a t h i c k s u c c e s s i o n of t u r b i d i t e s . D i e t z (1963) and D i e t z and Holden (1966) e x t e n d e d t h e A t l a n t i c margin a n a l o g y a n d s u g g e s t e d t h a t t h e l a t e Mesozoic and Cenozoic m i o g e o s y n c l i n a l s u c c e s s i o n of t h e e a s t e r n USA t h i c k e n e d seawards towards t h e c o n t i n e n t a l e d g e , forming a m i o g e o c l i n e . The A t l a n t i c m i o g e o c l i n e was t h e n compared w i t h t h e Lower P a l a e o z o i c Appalachian geosyncline (Fig.l.2). The g e o s y n c l i n a l c o n c e p t r e a c h e d a s o p h i s t i c a t e d l e v e l immediately p r l o r t o t h e r e v e l a t i o n s o f p l a t e t e c t o n i c s and many o f t h e views which p r e v a i l e d a t t h e time a r e summarized by Aubouin (1965).
He d e f i n e d a n e l e m e n t a r y g e o s y n c l i n e model w i t h
miogeosyncline and e u g e o s y n c l i n e which were f u r t h e r d i v i d e d i n t o a r i d g e and furrow. The Western A l p s , H e l l e n i d e s , Apennines and C a r p a t h i a n s e a c h formed examples and h e s t r e s s e d t h e importance o f d i s t i n c t s t a g e s o f development w i t h t h e m i g r a t i o n of s e d i m e n t a t i o n and d e f o r m a t i o n towards t h e f o r e l a n d d u r i n g orogeny. I t has been known f o r some t i m e now t h a t g e o s y n c l i n a l s e d i m e n t a t i o n i s b r o a d l y c y c l i c and f o u r d i s t i n c t i v e f a c i e s can b e r e c o g n i z e d : flysch
-
pre-orogenic
-
pre-flysch
-
molasse (HsU, 1973) a l t h o u g h t h e r e has been c o n s i d e r e b l e v a r i a t i o n i n t h e
usage o f t h e v a r i o u s terms i n v o l v e d . T h i s sequence of f a c i e s i s a p p l i c a b l e t o many o r o g e n i c b e l t s of d i f f e r e n t a g e s and i n d i f f e r e n t p a r t s o f t h e world. These exposed o r o g e n i c b e l t s o f t e n show t h e p a s s a g e from a basement ( p r e - o r o g e n i c ) p h a s e , t h r o u g h a s t a r v e d ( p r e - f l y s c h ) phase formed a s t h e b a s i n deepened, t o a deep-marine c l a s t i c ( f l y s c h ) p h a s e and f i n a l l y a c o n t i n e n t a l c l a s t i c ( m o l a s s e ) phase formed d u r i n g and a f t e r mountain u p l i f t .
Pre-orogenic phase The f a c i e s of t h e p r e - o r o g e n i c phase a r e v e r y v a r i a b l e and i n c l u d e i g n e o u s a n d / o r metamorphic c r y s t a l l i n e basement, s h a l l o w marine c l a s t i c s ( q u a r t z a r e n i t e s ) and c a r b o n a t e s and o t h e r f a c i e s c o n s i s t e n t w i t h a m i o g e o s y n c l i n a l s e t t i n g . I t i n d i c a t e s t h a t g e o s y n c l i n a l development u s u a l l y b e g i n s w i t h e s t a b l i s h m e n t of s h a l l o w marine s e d i m e n t a t i o n on c o n t i n e n t a l c r u s t . Pre-flvsch T h i s i s a v a r i a b l e f a c i e s which p r e c e d e s t h e f l y s c h s t r a t i g r a p h i c a l l y .
It
c o n s i s t s mainly of f i n e g r a i n e d sediments i n c l u d i n g b l a c k s h a l e s , s i l t s t o n e s , c h e r t s and d a r k l i m e s t o n e s sometimes r e f e r r e d t o a s t h e ' b a t h y a l l u l l ' ( G o l d r i n g , 1962).
5 Late Pracambrsan M Ordov8cian
L Y
I
.r
in 0 0 0 0
n
z
W
W
c
n
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m
.C
v)
.c W
L ln L m
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W
c Y 'c
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m
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U
Fig.l.2.
D e p o s i t i o n and c o l l a p s e o f c o n t i n e n t a l r i s e ( a f t e r D i e t z and Holden, 1966).
I n f a c t two d i s t i n c t i v e s u b f a c i e s c a n be recognized i n t h e p r e - f l y s c h and t h e s e a r e termed t h e Becken ( b a s i n ) and Schwellen ( s w e l l ) s u b f a c i e s i n t h e Hercynian geosyncline. The b a s i n f a c i e s c o n s i s t s o f r e l a t i v e l y t h i c k sequences o f f i n e g r a i n e d sediments d e p o s i t e d as h e m i p e l a g i t e s o r l o w d e n s i t y t u r b i d i t e s whereas t h e swell f a c i e s c o n s i s t s o f condensed l i m e s t o n e s r i c h i n b i o c l a s t i c m a t e r i a l and d e p o s i t e d on a submarine rise. An i m p o r t a n t f e a t u r e i n some p r e - f l y s c h sequences i s t h e o c c u r r e n c e of o p h i o l i t e complexes.
These show t h e upward sequence:
ultramafics
p i l l o w l a v a s and a r e o f t e n a s s o c i a t e d w i t h r a d i o l a r i t e s .
gabbros
+ dykes +
The b a s i c rocks a r e
f r e q u e n t l y s e r p e n t i n i z e d and B a i l e y and McCallien (1960) gave t h e name "Steinmann T r i n i t y " t o t h e t h r e e f o l d a s s o c i a t i o n of r a d i o l a r i t e , s e r p e n t i n i t e , and g r e e n s t o n e s r e c o g n i z e d by Steinmann (1905, 1927).
6
Flysch F l y s c h i s a w i d e l y used term (HsU, 1970) which i s commonly c o n s i d e r e d t o b e a synonym f o r t u r b i d i t e .
Some a u t h o r s p r e f e r t o r e s t r i c t t h e term f l y s c h t o syn-
o r o g e n i c mass-flow d e p o s i t s found i n o r t h o g e o s y n c l i n e s ( e . g . d e R a a f , 1968; S t a n l e y , 1970) and u s e t h e term f l y s c h - l i k e o r f l y s c h o i d f o r s i m i l a r d e p o s i t s found i n l a t e g e o s y n c l i n a l s t a g e s when t e c t o n i c a c t i v i t y i s n o t known. F l y s c h c o n s i s t s m o s t l y of a l t e r n a t i n g s a n d s t o n e s and s h a l e s which a r e o f t e n r e p e a t e d through g r e a t s t r a t i g r a p h i c t h i c k n e s s e s .
A number of f a c i e s a s s o c i a t i o n s
a r e now r e c o g n i z e d ( M u t t i and Ricci L u c c h i , 1972) i n b o t h Recent and a n c i e n t t u r b i d i t e basins including:
b a s i n p l a i n , o u t e r f a n and i n n e r f a n f a c i e s
associations.
Molasse i s d e f i n e d by Van Houten (1969) a s , " C l a s t i c wedges l a i d down i n s h a l l o w marine and nonmarine environments a d j a c e n t t o r i s i n g mountain c h a i n s " .
A wide
v a r i e t y o f f a c i e s a r e r e p r e s e n t e d i n c l u d i n g a l l u v i a l f a n s , f l u v i a l , d e l t a i c , and s h a l l o w marine d e p o s i t s encompassing a wide range of l i t h o l o g i e s r a n g i n g from c o n g l o m e r a t e s , s a n d s t o n e s , mudstones, and t h i n l i g n i t e s and c o a l s . I n t h e g e o s y n c l i n a l c y c l e m o l a s s e g e n e r a l l y f o l l o w s , b u t may b e p a r t l y contemporaneous w i t h , f l y s c h s e d i m e n t a t i o n .
I t i s u s e f u l t o d i s t i n g u i s h between a l a t e -
o r o g e n i c f a c i e s now deformed and a p o s t - o r o g e n i c f a c i e s which i s undeformed. Molasse i s t h e r e f o r e one of t h e main d i a g n o s t i c f e a t u r e s of orogeny and may b e a s e n s i t i v e m o n i t o r o f d e f o r m a t i o n and u p l i f t i n mountain b e l t s . PLATE TECTONICS AND SEDIMENTATION
Most g e o l o g i s t s now a c c e p t t h e main t e n e t s of p l a t e t e c t o n i c s and i t s c o r o l l a r y c o n c e p t s o f c o n t i n e n t a l d r i f t , s e a f l o o r s p r e a d i n g , t r a n s f o r m f a u l t s and s u b d u c t i o n zones a l t h o u g h some (Beloussov, 1970; M e y e r h o f f , l 9 7 0 ) have c h a l l e n g e d t h e s e c o n c e p t s . The term p l a t e t e c t o n i c s (McKenzie, 1972; Dewey, 1972) a r o s e b e c a u s e t h e d i s c r e t e s p h e r i c a l c a p s of e s s e n t i a l l y r i g i d l i t h o s p h e r e which a r e i n r e l a t i v e motion w i t h r e s p e c t t o one a n o t h e r , and w i t h t h e less r i g i d a s t h e n o s p h e r e b e n e a t h , a r e u s u a l l y r e f e r r e d t o a s p l a t e s (McKenzie and P a r k e r , 1967). P l a t e t e c t o n i c t h e o r y l a y s emphasis on t h e h o r i z o n t a l movement of t h e l i t h o s p h e r e and t h e p l a t e t e c t o n i c s e t t i n g ( D i c k i n s o n , 1974a) of s e d i m e n t a r y b a s i n s can b e d e s c r i b e d i n terms of t h e p r o x i m i t y of t h e b a s i n t o a p l a t e margin.
Thus i n t r a p l a t e
s e t t i n g s o f c o n t i n e n t a l o r o c e a n i c c h a r a c t e r c o n t r a s t w i t h p l a t e margin s e t t i n g s . The l a t t e r i n c l u d e t h r e e d i s t i n c t t y p e s of p l a t e j u n c t i o n :
1) d i v e r g e n t where two
l i t h o s p h e r i c p l a t e s a r e moving a p a r t a s ocean f l o o r s p r e a d i n g t a k e s p l a c e ;
2)
c o n v e r g e n t where l i t h o s p h e r e i s consumed a s one p l a t e descends b e n e a t h a n o t h e r ; and
7 3) s t r i k e - s l i p o r t r a n s f o r m where p l a t e s move l a t e r a l l y p a s t one a n o t h e r . The a c c u m u l a t i o n of a t h i c k s e d i m e n t a r y b a s i n r e q u i r e s e i t h e r a n i n i t i a l depressi o n o r p r o g r e s s i v e s u b s i d e n c e , b o t h o f which imply l a r g e v e r t i c a l movements.
i s not i n c o n s i s t e n t with t h e p l a t e t e c t o n i c theory.
This
V e r t i c a l movements t a k e p l a c e
i n r e s p o n s e t o changes i n t h e t h i c k n e s s o f t h e c r u s t , i q t h e t h e r m a l c o n d i t i o n of t h e l i t h o s p h e r e , and i n t h e i s o s t a t i c b a l a n c e between l i t h o s p h e r e and a s t h e n o s p h e r e .
I t i s t h e s e v e r t i c a l movements which l a r g e l y c o n t r o l t h e e v o l u t i o n of s e d i m e n t a r y basins. D i v e r g e n t p l a t e margins D i v e r g e n t p l a t e j u n c t i o n s may o r may n o t b e a s s o c i a t e d w i t h t h e emplacement of o c e a n i c c r u s t and r e l a t e d s e a f l o o r s p r e a d i n g ,
U s u a l l y d i v e r g e n t p l a t e margins a r e
c h a r a c t e r i z e d by a domal u p l i f t w i t h a c e n t r a l r i d g e and median v a l l e y a l t h o u g h r a p i d l y s p r e a d i n g r i d g e s s u c h a s t h e E a s t P a c i f i c Rise l a c k t h e median r i f t v a l l e y . U p l i f t i s due t o thermal e x p a n s i o n , h i g h h e a t f l o w and magmatic a c t i v i t y and i s o f t e n a s s o c i a t e d w i t h t h e emplacement o f o c e a n i c c r u s t and s e a f l o o r s p r e a d i n g . Continuous s p r e a d i n g o v e r a l o n g t i m e r e s u l t s i n t h e w i d e s p r e a d s e p a r a t i o n of t h e c o n t i n e n t a l margins a s i n t h e p r e s e n t - d a y A t l a n t i c and I n d i a n Oceans.
I n other
c a s e s s p r e a d i n g may b e i n t e r r u p t e d and o n l y a c e r t a i n amount o f o c e a n i c c r u s t emplaced a s i n t h e Red Sea.
I f no o c e a n i c c r u s t i s emplaced a n i n t r a c o n t i n e n t a l
r i f t forms a n d t h i s may b e t h e s i t e of a major s e d i m e n t a r y a c c u m u l a t i o n . I n t r a c o n t i n e n t a l r i f t s and a u l a c o g e n s I n t r a c o n t i n e n t a l r i f t s a r e major g l o b a l s t r u c t u r e s w i t h a long h i s t o r y of d e f o r m a t i o n , m a g m a t i s m , and s e d i m e n t a t i o n ,
They a r e c h a r a c t e r i z e d by domal u p l i f t s ,
usually about lkm h i g h , lOOkm wide and 200-3OOkm l o n g , n e g a t i v e g r a v i t y anomalies and by a l k a l i n e v u l c a n i c i t y .
The t y p e example i s t h e E a s t A f r i c a n r i f t system which i s
o v e r 3,OOOkm long and h a s a h i s t o r y e x t e n d i n g back i n t o t h e Precambrian w i t h o u t any a p p a r e n t major l a t e r a l movement ( F i g . l . 3 ) .
The E a s t A f r i c a n r i f t i s c h a r a c t e r i z e d by
a g r a d u a l t o p o g r a p h i c r i s e towards t h e c e n t r a l r i f t which i s between 40 and 50km wide and i s a b o u t 2km below t h e s u r r o u n d i n g p l a t e a u .
W i t h i n t h e r i f t a r e a number o f
Mesozoic and Cenozoic a l k a l i n e v o l c a n i c c e n t r e s and non-volcanic s u c h a s Ruwenzori.
horst structures
T o p o g r a p h i c a l l y t h e r i f t i s complex and i s made more s o by t h e
e n e c h e l o n p a t t e r n o f t h e boundary f a u l t s .
A number of s e d i m e n t a r y b a s i n s t h e r e f o r e
e x i s t and t h e major ones a r e t h e s i t e s of l a r g e p r e s e n t - d a y l a k e s and c o n t a i n t h i c k sequences of l a c u s t r i n e s e d i m e n t s .
Because o f i t s domal s t r u c t u r e t h e maior r i v e r s
flow away from t h e E a s t A f r i c a n r i f t b u t t h e r e i s a l s o a n i n t e r n a l d r a i n a g e .
The
s e d i m e n t a r y b a s i n s a r e t h u s f i l l e d by a l l u v i a l f a n s , d e r i v e d from l o c a l t o p o g r a p h i c h i g h s , and l a c u s t r i n e s e d i m e n t s .
8
F i g . 1.3. The East A f r i c a n R i f t System ( a f t e r King,1970). I n s e t shows a r e a s of l a t e Mesozoic and Cenozoic domal u p l i f t and t r i p l e j u n c t i o n s ( a f t e r Burke and Whiteman, 1973). The B a i k a l r i f t i s a n o t h e r major r i f t system s t r i k i n g l y s i m i l a r t o t h e E a s t African r i f t .
I t s t r e t c h e s f o r a d i s t a n c e of 3,500km from n o r t h w e s t e r n Mongolia,
through t h e mountains o f s o u t h e a s t e r n S i b e r i a i n t o Yakutia and probably a s f a r a s
9
t h e s e a o f Okhotsk (King, 1976).
The r i f t s t r u c t u r e s c o n s i s t o f l i n e a r systems of
fault-bounded i n t e r m o n t a n e d e p r e s s i o n s a l o n g t h e c r e s t o f a r c h e d u p l i f t s .
There
a r e twelve l a r g e d e p r e s s i o n s , 100-7OOkm l o n g and 15-18km i n w i d t h , and numerous smaller depressions.
The b a s i n s a r e f i l l e d by up t o 5,500m o f O l i g o c e n e - P l e i s t o c e n e
c o n t i n e n t a l sediments w i t h i n t e r c a l a t e d v o l c a n i c s . nizable:
Two t e c t o n i c u n i t s a r e recog-
a lower group c o n s i s t i n g of l a c u s t r i n e , swamp and a l l u v i a l d e p o s i t s of
O l i g o c e n e , Miocene and p o s s i b l y lower P l i o c e n e a g e and a n upper group o f l a t e Pleistocene-Holocene age r e s t r i c t e d t o i n t e r m o n t a n e t r o u g h s and d e r i v e d from a h i g h e r mountain r e l i e f e v i d e n t l y under c o l d c l i m a t i c c o n d i t i o n s . O t h e r good examples o f r i f t systems i n c l u d e t h e Oslo g r a b e n w i t h i t s Permian a l k a l i c v o l c a n i c s and t h e t r o u g h s of t h e North Sea (Whiteman e t a l . , Rhine Graben i s one of t h e b e s t known.
1975).
The
I t i s o f T e r t i a r y a g e and i s l o c a t e d over
a dome forming t h e s o u t h e r n arm of a t r i p l e j u n c t i o n c e n t r e d i n F r a n k f u r t (Burke and Dewey, 1973).
I t h a s t h e u s u a l a l k a l i n e v o l c a n i c s and i s a s s o c i a t e d w i t h h i g h
h e a t flow, e a r t h q u a k e s and r e c e n t f a u l t a c t i v i t y ( M u e l l e r , 1970). A d i s t i n c t i v e type of r i f t system i s t h e abandoned r i f t o r a u l a c o g e n . were d i s c o v e r e d by S h a t s k i (1961) and developed i n t o a d o m e - t r i p l e a u l a c o g e n - r i f t c o n c e p t by Burke and Dewey (1973).
These
junction-
Aulacogens a r e deep l i n e a r
t r o u g h s which form t h e t h i r d arms of t r i p l e j u n c t i o n s and e x t e n d from a c o n t i n e n t a l margin o r g e o s y n c l i n e i n t o a f o r e l a n d p l a t f o r m o r c o n t i n e n t .
The developmental
sequence of a n a u l a c o g e n i n v o l v e s e a r l y doming and t h e f o r m a t i o n o f a plume-induced t r i p l e j u n c t i o n r i f t network. of t h e system.
S p r e a d i n g may t a k e p l a c e on any o f t h e t h r e e arms
I f i t t a k e s p l a c e on a l l t h r e e t h e n t h r e e p l a t e s a r e formed, b u t i f
o n l y on two, t h e n t h e f o r m a t i o n of two p l a t e s l e a v e s a n abandoned r i f t o r a u l a c o g e n . One of t h e b e s t known a u l a c o g e n s i s t h e Benue Trough.
T h i s formed from a
Cretaceous t r i p l e j u n c t i o n j o i n i n g S o u t h America and A f r i c a ( F i g . l . 4 ) .
Two arms
s e p a r a t e d t o form t h e S o u t h A t l a n t i c w h i l s t t h e t h i r d was abandoned and remains a s t h e Benue Trough (Burke e t a l . , 1971; Olade, 1975). s u c c e s s i o n ( o v e r 1Okm) of T e r t i a r y s e d i m e n t s .
I t has been f i l l e d by a t h i c k
These p a s s from submarine f a n d e p o s i t s
a t t h e b a s e up through d e l t a i c s e d i m e n t s i n t o c o n t i n e n t a l a l l u v i a l s a n d s t o n e s ( F i g . 1.5). Intercontinental r i f t i n g The emplacement of o c e a n i c c r u s t i n a x i a l r i f t zones i s a s s o c i a t e d w i t h s e a f l o o r s p r e a d i n g a n d s e p a r a t i o n of e a c h h a l f o f t h e r i f t .
These develop i n t o p a s s i v e
' A t l a n t i c ' t y p e c o n t i n e n t a l m a r g i n s , comprising a s h e l f , s l o p e and r i s e , and sometimes termed t r a i l i n g edges. I n t h e e a r l y s t a g e s of i n t e r c o n t i n e n t a l r i f t i n g s e d i m e n t a t i o n i s c l o s e l y a s s o c i a t e d w i t h t h e e r u p t i o n of v a s t amounts of b a s a l t i c l a v a s .
I n t h e Red S e a , which
has a complex s p r e a d i n g h i s t o r y (Richardson and H a r r i s o n , 1 9 7 6 ) , t h e r e i s a n a x i a l
10
o-(3q--+-+ of Guinea
0'
The o r i g i n o f t h e Benue Trough a s a n a u l a c o g e n abandoned d u r i n g c o n t i n e n t a l Fig.l.4. r i f t s e p a r a t i o n o f A f r i c a and S o u t h America d u r i n g t h e C r e t a c e o u s ( a f t e r Hoffman e t a l . , 1974). t r o u g h 4-3Okm wide and o v e r 1,000m deep,
I n the deeper p a r t s of t h e trough a r e h o t
b r i n e p o o l s which form where t r a n s f o m f a u l t s c u t t h e median v a l l e y ( B i g n e l l , 1975). These p o o l s d e p o s i t m e t a l l i f e r o u s s e d i m e n t s r i c h i n o x i d e s of z i n c , c o p p e r , l e a d i r o n and manganese which i n t h e A t l a n t i s I1 deep can b e d i v i d e d i n t o seven l a t e r a l l y
11
-z
I ro
I
I
\
Continental sandstones
Cretaceous rise deposits
--
I
2 '
I
km
~
~ _ _ _ _ _ _ _ ~ ~
~
~
Fig.l.5. Schematic s t r u c t u r a l c r o s s s e c t i o n showing t h e s e d i m e n t a r y f i l l of t h e Benue Trough ( a f t e r Burke, 1972). c o r r e l a t i v e f a c i e s ( B i s c h o f f , 1969). major mineralogy:
The f a c i e s nomenclature i n d i c a t e s t h e i r
d e t r i t a l , i r o n - m o n t m o r i l l o n i t e , goethite-amorphous,
m a n g a n o s i d e r i t e , a n h y d r i t e , and manganite.
sulphide,
An a d d i t i o n a l m a g n e t i t e f a c i e s was
s u b s e q u e n t l y a'dded ( H a c k e t t and B i s c h o f f , 1973).
The o r i g i n s of t h e b r i n e s and
m c t a l l i f e r o u s sediments i n t h e median v a l l e y o f t h e Red Sea has been d i s c u s s e d by
Schoell (1975).
P o s s i b i l i t i e s i n c l u d e i n t e n s e e v a p o r a t i o n of a n i s o l a t e d Red Sea
t o produce b r i n e c o n c e n t r a t i o n s , c o n n a t e b r i n e s e x p e l l e d by geothermal h e a t i n g , a s
well as e more d i r e c t v o l c a n i c o r i g i n . by s o l u t i o n of Miocene e v a p o r i t e s .
Another p o s s i b i l i t y i s t h a t t h e y o r i g i n a t e d
There i s a t h i c k (2-5km) e v a p o r i t e - c l a s t i c
sequence i n t h e Red Sea which was formed f o l l o w i n g a marine i n c u r s i o n a f t e r d e p r e s s i o n o f t h e Red Sea d u r i n g t h e l a t e Eocene and e a r l y O l i g o c e n e (Hutchinson and E n g e l s , 1970).
F u r t h e r s e a f l o o r s p r e a d i n g a n d d e p o s i t i o n of deep s e a oozes
took p l a c e d u r i n g t h e P l i o c e n e and Q u a t e r n a r y .
A s s e a f l o o r s p r e a d i n g p r o g r e s s e s two l a t e r a l l y e q u i v a l e n t s u c c e s s i o n s b u i l d up on t h e now w i d e l y s e p a r a t e d c o n t i n e n t a l m a r g i n s .
I d e a l l y t h e s e c o n t i n e n t a l margins
c o n s i s t of t h i c k sequences of c o n t i n e n t a l s h e l f s e d i m e n t s and a r e u n d e r l a i n by c o n t i n e n t a l s e d i m e n t s , i n c l u d i n g r e d b e d s , which formed i n f a u l t bounded t r o u g h s d u r i n g t h e e a r l y s t a g e s of r i f t i n g .
Oceanwards t h e r e i s a t r a n s i t i o n from c o n t i n e n t a l
r i s e , through a b y s s a l p l a i n t o t h e mid-oceanic r i d g e .
I n v e r t i c a l sequence o c e a n i c
c r u s t p a s s e s upwards through p e l a g i c s e d i m e n t s i n t o t u r b i d i t e s ( F i g . l . 6 ) .
12
CONTINENTAL SHELF
CONTINENTAL RISE
OCEANIC RIDGE
ABYSSAL PLAIN
Pillowed tholelite
C r o s s - s e c t i o n of t h e w e s t e r n A t l a n t i c showing r e l a t i o n s h i p s of c o n t i n e n t a l Fig. 1 . 6 . c r u s t , o c e a n i c c r u s t and s e d i m e n t s ( a f t e r Dewey and B i r d , 1970). Oceanic r i d g e s themselves a r e s t r u c t u r a l l y complex and may be b l o c k f a u l t e d ( e . g . M i d - A t l a n t i c r i d g e ) w i t h normal f a u l t i n g p a r a l l e l t o t h e r i d g e t r e n d and t r a n s f o r m f a u l t i n g perpendicular t o i t .
T h i s produces a complex s y s t e m of r i d g e s and b a s i n s
which a r e t r a n s v e r s e and p a r a l l e l t o t h e r i d g e t r e n d .
Some o f t h e s e a r e up t o
5,000111 deep (Van Andel and Komar, 1969; Van Andel e t a l . ,
1971) and c o n t a i n t h i c k
sequences (500-1,200m) of l a r g e l y u n d i s t u r b e d f i n e g r a i n e d t u r b i d i t e s w i t h p i l l o w l a v a s , i n t r u s i o n s and s e d i m e n t a r y b r e c c i a s a l o n g t h e boundary f a u l t
1975).
s c a r p s (Arcyana,
Away from t h e ocean r i d g e s t h e newly formed l i t h o s p h e r e c o o l s and s i n k s t o
deeper o c e a n i c d e p t h s and t h e e a r l i e r formed r i d g e s e d i m e n t s a r e o v e r l a i n by p e l a g i c sediments.
The p r e c i s e n a t u r e o f t h e s e depends upon a numberof f a c t o r s s u c h a s
o c e a n i c c i r c u l a t i o n , t h e c a r b o n a t e compensation d e p t h and t h e p r o x i m i t y of c l a s t i c sources such a s i s l a n d a r c s .
The p e l a g i c s e d i m e n t s of ocean r i d g e s and a b y s s a l
p l a i n s o f t e n c o n t a i n l a r g e amounts o f hydrogenous manganese n o d u l e s which c o n s i s t o f i r o n and manganese oxyhydroxides w i t h s i g n i f i c a n t c o n c e n t r a t i o n s o f Co, N i and Cu.
These form on t h e f l a n k s of ocean r i d g e s , seamounts and a b y s s a l p l a i n s , m o s t l y
a t d e p t h s g r e a t e r t h a n 4,000m.
13 convergent D l a t e margins Convergent p l a t e margins a r e c h a r a c t e r i z e d by t h e s u b d u c t i o n of o c e a n i c l i t h o s p h e r e b e n e a t h a v o l c a n i c a l l y a c t i v e o v e r r i d i n g p l a t e and t h e development o f a r c t r e n c h systems (Dickinson, 1974a).
Arc-trench systems c a n b e b r o a d l y equated w i t h
t h e e u g e o s y n c l i n e s o f e a r l i e r workers (Dickinson, 1974b) and have t h e f o l l o w i n g f i v e morphotectonic e l e m e n t s :
1) t h e t r e n c h , a b a t h y m e t r i c deep f l o o r e d by ocean c r u s t ;
2 ) t h e s u b d u c t i o n zone b e n e a t h t h e i n n e r w a l l of t h e t r e n c h and t h e t r e n c h s l o p e
break;
3 ) t h e a r c - t r e n c h gap, a b e l t w i t h i n which a f o r e a r c b a s i n may o c c u r between
t h e t r e n c h s l o p e b r e a k and t h e magmatic a r c ; i n t r a - a r c b a s i n s may o c c u r ;
4 ) t h e magmatic a r c , w i t h i n which
and 5 ) t h e back a r c a r e a , w i t h i n which may l i e e i t h e r
a n i n t e r a r c b a s i n f l o o r e d by o c e a n i c c r u s t and s e p a r a t e d from t h e r e a r o f t h e a r c by normal f a u l t s , on a r e t r o a r c b a s i n f l o o r e d by c o n t i n e n t a l basement and s e p a r a t e d from t h e r e a r of t h e a r c by a t h r u s t f a u l t system
(Fig.l.7).
Forearc a r e a s The f o r e a r c a r e a comprises t r e n c h , s u b d u c t i o n zone and a r c - t r e n c h gap.
The
ocean f l o o r seaward of t h e submarine t r e n c h i s made up o f p e l a g i c sediments and t u f f s o v e r l y i n g b a s a l t i c ocean c r u s t .
Within t h e submarine t r e n c h t h e r e i s o n l y a r e l a t i v e -
l y t h i n sedimentary sequence ( n o r m a l l y a b o u t 1,OOOm). Thickness v a r i a t i o n s r e s u l t
from f l u c t u a t i o n s i n t h e s u p p l y o f c l a s t i c d e t r i t u s .
When t h i s i s low t h i n sequences
(50Om) o f p e l a g i c and hemipelagic sediment a r e d e p o s i t e d and g r e a t e r s u p p l y r e s u l t s i n t h e d e p o s i t i o n of t h i c k t u r b i d i t e sequences ( 2 , 5 0 0 1 ~ ) . Seismic p r o f i l e s show t h a t t h e s e a r e l a r g e l y undeformed b u t t h i s i s t o be e x p e c t e d i f t h e r a t e o f sediment s u p p l y i s much h i g h e r t h a n t h e r e l a t i v e r a t e of p l a t e consumption and s u b d u c t i o n zones a r e masked by submarine f a n s ( S i l v e r , 1969).
I t seems u n l i k e l y t h e r e f o r e t h a t
submarine t r e n c h e s a r e t h e s i t e of t h i c k g e o s y n c l i n a l accumulations (Von Huene, 1974). The subduction zone i t s e l f i s d i r e c t l y u n d e r l a i n by deformed and u p l i f t e d oceanics t r a t a most probably c o n s i s t i n g of melanges, crumpled beds and o p h i o l i t e s , and s l i c e d by t h r u s t s .
The s u b d u c t i o n complex i s t h u s i n f e r r e d t o grow by t h e s u c c e s s i v e
increment o f o c e a n i c m a t e r i a l s which a r e jammed a g a i n s t t h e seaward f l a n k a t t h e t r e n c h a x i s o r s c r a p e d from t h e upper s u r f a c e o f t h e descending p l a t e .
The g r e a t
t h i c k n e s s e s of s u b d u c t i o n complexes would seem t h e r e f o r e t o be of t e c t o n i c r a t h e r than s t r a t i g r a p h i c o r i g i n , The a r c - t r e n c h gap ( a c c r e t i o n a r y p r i s m o f K a r i g and Sharman, 1 9 7 5 ) c o m p r i s e s a n o u t e r a r c r i d g e formed from s t a c k s o f t h r u s t s h e e t s which i n d i v i d u a l l y young towards t h e magmatic a r c b u t whose o v e r a l l a g e d e c r e a s e s towards t h e t r e n c h ( K a r i g and Sharman, 1975; Moore, 1975).
Within t h i s zone i s a f o r e a r c b a s i n which may b e
50-lOOkm wide and o v e r 5 , 0 0 0 km long.
These f o r e a r c b a s i n s c o n t a i n t h i c k sequences
of predominantly c l a s t i c sediments which may l a t e r a l l y i n t e r f i n g e r w i t h v o l c a n i c
14
TRENCH,
RA
ARC TSB FAB,+
+
TS B
C
IAB
* ARC
A IB
FTB /
I
C
500 Km I d e a l i z e d diagram i l l u s t r a t i n g t e c t o n i c s e t t i n g s o f sedimentary b a s i n s Fig.l.7. a s s o c i a t e d w i t h a r c - t r e n c h systems. V e r t i c a l e x a g g e r a t i o n i s X10; t r u e d i p a n g l e s o f p l a t e d e s c e n t a r e 600 i n A and 3 0 ' i n B ( a f t e r Dickinson, 1974a). rocks o f t h e magmatic a r c .
Sediments a r e d e r i v e d from t h e o u t e r a r c , t h e magmatic
a r c and by l o n g i t u d i n a l t r a n s p o r t from t h e a d j a c e n t c o n t i n e n t .
A v a r i e t y of f a c i e s
a r e r e p r e s e n t e d i n f o r e a r c b a s i n s and a l t h o u g h t u r b i d i t e s and s h e l f d e p o s i t s a r e abundant c o n t i n e n t a l sediments a r e a l s o r e p r e s e n t e d . f o r e a r c b a s i n s i s t h e 'interdeep'
One of t h e b e s t developed
of t h e Sunda a r c (Van B e m e l e n , 1949) ( F i g . l . 8 ) .
The b a s i n l i e s t o t h e west o f t h e Burma-Sumatra-Java
magmatic a r c and e a s t
Of
a b e l t of f l y s c h and u l t r a b a s i c r o c k s which form t h e o u t e r a r c of m e Arakan and t h e Andaman-Nicobar-Mentawai l i n e o f i s l a n d s .
I n t h e n o r t h t h e f o r e a r c b a s i n forms
t h e w e s t e r n t r o u g h o f Burma where o v e r 8,OOOm of l a t e Cretaceous t o P l i o c e n e m a r i n e , d e l t a i c and f l u v i a l sediments have accumulated.
The a r e a i s now occupied by t h e
r i v e r c o u r s e s of t h e Chindwin and lower Irrawaddy. The mamatic a r c Magmatic a r c s can be d i v i d e d i n t o i n t r a - o c e a n i c a r c s s i t u a t e d between a r e a s of o c e a n i c c r u s t and c o n t i n e n t a l margin a r c s s i t u a t e d on t h e margin o f a c o n t i n e n t
15 n e a r t h e c o n t i n e n t - o c e a n c r u s t a l boundary ( D i c k i n s o n , 1974b) ( F i g . l . 7 ) .
They
i n c l u d e i s l a n d a r c s backed by s h a l l o w e p i c o n t i n e n t a l s e a s a s w e l l a s t h o s e s t a n d i n g a l o n g t h e edges o f c o n t i n e n t a l landmasses.
20'
lrrawsddy Oslra
10'
10' INDIAN OCEAN
Fig.l.8.
The Sunda and Banda a r c s ( a f t e r Reading, 1978).
Magmatic a r c s form t o p o g r a p h i c r i d g e s i n t e r r u p t e d by fault-bounded b a s i n s .
These
i n t r a - a r c b a s i n s a r e m o s t l y e x t e n s i o n a l and may b e r e l a t e d t o v o l c a n o - t e c t o n i c s u b s i d e n c e , a r c h i n g t h a t accompanies u p l i f t o f p a r a c o n t i n e n t a l c r u s t , o r t o t h e development of a n i n c i p i e n t i n t e r a r c b a s i n .
The s e d i m e n t a r y f i l l i s t y p i c a l l y
v o l c a n i c l a s t i c and i n c l u d e s e p i c l a s t i c mass flow d e p o s i t s , c o a r s e p y r o c l a s t i c s , t u f f s and v o l c a n o g e n i c t u r b i d i t e s w i t h t h e c o a r s e r s e d i m e n t s b e i n g d e p o s i t e d around t h e volcanic centres.
C o n t i n e n t a l margin a r c s show a wide v a r i e t y of s e d i m e n t a r y f a c i e s
i n c l u d i n g c o n t i n e n t a l s e d i m e n t s and r e d beds ( D i c k i n s o n , 1974a). Back-arc a r e a s Back-arc a r e a e a r e t e c t o n i c a l l y complex a r e a s c o n s i s t i n g of r i d g e s and b a s i n s
16 b e h i n d t h e magmatic a r c .
Behind i n t r a - o c e a n i c a r c s t h e r e i s f r e q u e n t l y a remnant-
a r c which i s s e p a r a t e d from t h e magmatic a r c by a n i n t e r a r c b a s i n ( F i g . l . 7 ) . Behind t h e remnant a r c may b e a second i n t e r a r c b a s i n , u s u a l l y r e f e r r e d t o a s a m a r g i n a l b a s i n , which s e p a r a t e s t h e remnant a r c from t h e mainland.
I n continental
margin a r c s t h e r e i s no remnant a r c and t h e back-arc m a r g i n a l b a s i n e x t e n d s from t h e c o n t i n e n t a l margins t o t h e magmatic a r c .
These back-arc e n s i a l i c b a s i n s were
c a l l e d r e t r o a r c b a s i n s by D i c k i n s o n (1974a) ( F i g . l . 7 ) . Back-arc a r e a s a r e o f t e n c h a r a c t e r i z e d by h i g h h e a t flow and t h e narrow b e l t o f l i t h o s p h e r e b e h i n d t h e a r c - t r e n c h gap can b e c o n s i d e r e d a s a s e p a r a t e narrow p l a t e . P l a t e motion can t h u s r e s u l t i n s p l i t t i n g and t h e emplacement o f new o c e a n i c c r u s t c a u s i n g t h e f o r m a t i o n of a n i n t e r a r c b a s i n by back-arc s p r e a d i n g ( K a r i g , 1970, 1971). The sediments o f i n t e r a r c b a s i n s i n c l u d e d i s t i n c t i v e t u r b i d i t e a p r o n s
Of
v o l c a n i c l a s t i c d e t r i t u s d e r i v e d from t h e m i g r a t i n g f r o n t a l a r c and r e s t i n g d i r e c t l y on o c e a n i c c r u s t .
I n some i n t e r a r c b a s i n s t h e s i d e away from t h e a r c - t r e n c h system
may b e bounded by a submerged remnant a r c , no e f f e c t i v e c l a s t i c s o u r c e i s p r e s e n t , and p e l a g i c s e d i m e n t a t i o n dominates.
There i s t h u s no t e r r i g e n o u s i n p u t t o i n t e r a r c
b a s i n s and K a r i g and Moore (1975) r e c o g n i z e f o u r dominant t y p e s o f s e d i m e n t ( F i g . l . 9 ) including:
1) v o l c a n i c l a s t i c d e t r i t u s d e r i v e d from t h e magmatic a r c , v o l u m e t r i c a l l y
t h e most i m p o r t a n t and d e p o s i t e d p r i n c i p a l l y a s t u r b i d i t e s d e r i v e d from t h e v o l c a n i c c h a i n
2 ) montmorillonite clays
3 ) b i o g e n i c c a r b o n a t e ooze and
4 ) wind blown,
continentally derived dust. R e t r o a r c b a s i n s a r e d i s t i n c t from a c t i v e i n t e r a r c b a s i n s b e c a u s e t h e y leave a complex s e d i m e n t a r y r e c o r d w i t h a wide v a r i e t y o f f a c i e s .
Marine d e l t a i c and f l u v i a l
environments a r e r e p r e s e n t e d i n sequences up t o 5km t h i c k and d e p o s i t e d i n t e r r e s t r i a l lowlands and e p i c o n t i n e n t a l s e a s between c o n t i n e n t a l a r c s and landmasses.
These
s e d i m e n t s a r e m o s t l y d e r i v e d from t h e magmatic a r c a n d t r a n s p o r t e d i n a t r a n s v e r s e d i r e c t i o n towards t h e c r a t o n a l t h o u g h t h e r e a r e minor c o n t r i b u t i o n s from t h e c r a t o n itself. The C r e t a c e o u s r e t r o a r c b a s i n s o f t h e i n t e r i o r and Rocky Mountain r e g i o n of North America c o n t a i n a v a r i e t y o f marine and c o n t i n e n t a l s e d i m e n t s d e r i v e d from u p l i f t e d s t r a t a i n t h e f o l d - t h r u s t b e l t which was formed by p a r t i a l s u b d u c t i o n b e h i n d t h e a r c (Weimer, 1970).
The main h i g h l a n d s o u r c e s were f o l d e d and f a u l t e d p r e -
Mesozoic s t r a t a l y i n g j u s t west of t h e r e t r o a r c b a s i n b u t e a s t of t h e b a t h o l i t h b e l t which marks t h e main magmatic a r c (Hamilton, 1969).
The sub-Andean zone of t h e
Andes l i e s e a s t of t h e magmatic a r c and c o n s i s t s o f deep i n t e r m o n t a n e t r o u g h s .
Here
a v a r i e t y o f c o a r s e c o n t i n e n t a l c l a s t i c s e d i m e n t s a r e d e p o s i t e d i n a l l u v i a l f a n and f l u v i a l environments. Transform f a u l t s Transform o r s t r i k e - s l i p f a u l t s bound p l a t e s between which movement i s p r i m a r i l y
17 l a t e r a l and l i t h o s p h e r e i s conserved b e i n g n e i t h e r c r e a t e d o r d e s t r o y e d .
Primary
t r a n s f o r m f a u l t s ( G i l l i l a n d and Meyer, 1976) o c c u r a l o n g fundamental b r e a k s in t h e l i t h o s p h e r e and may have had a l o n g h i s t o r y o f t e c t o n i c a c t i v i t y l i k e t h e San Andreas f a u l t system which is o v e r 1,500km l o n g , 500km wide, and h a s been a c t i v e s i n c e t h e Oligocene.
Secondary t r a n s f o r m f a u l t s a r e much s m a l l e r s t r u c t u r e s which
r e s u l t d i r e c t l y from s e a f l o o r s p r e a d i n g . Volcaniclastic debris
Montmorillonite-rich brown clays
Volcaniclastic debris
- 4
I
41-
56-
1
Brown clay or siliceous
6 7. 8
Fig.l.9.
S e d i m e n t a t i o n model f o r back-arc b a s i n s ( a f t e r K a r i g and Moore, 1975).
The San Andreas F a u l t system today l i e s a t t h e s p l i n t e r e d boundary of t h e P a c i f i c and North American p l a t e s , a t e c t o n i c arrangement t h a t a r o s e i n t h e O l i g o c e n e (Fig.l.10).
Transform movement between t h e North American and P a c i f i c p l a t e s has
v a r i e d i n t h e p a s t b u t is c u r r e n t l y a b o u t 5.0cm/year (Atwater and Molnar, 1973). Southern C a l i f o r n i a and n o r t h w e s t Mexico i s t h u s b e i n g deformed a s p a r t of a broad
18
C
B
A
D
.YU
NA QU
0 QU
QU
80 F
E
:: .YU
I I
I
.YU
,
0 QU
*QU
Schematic diagrams of p l a t e arrangements a l o n g t h e w e s t e r n c o a s t of North Fig.l.10. America. P = P a c i f i c , K = K u l a , F = F a r a l l o n , N = North American p l a t e . SE = S e a t t l e , SA = Sacramento, YU = Yurna, G U = G u a d a l a j a r a . Dashed l i n e e n c l o s e s r e g i o n of p l a t e boundary t e c t o n i c complexity. Age i n Ma g i v e n a t lower r i g h t o f e a c h s k e t c h ( a f t e r C r o w e l l , 1979). t r a n s f o r m zone.
W i t h i n t h i s zone t h e o r i g i n of s e v e r a l modern s e d i m e n t a r y b a s i n s
i n c l u d i n g t h e S a l t o n Trough and t h e Gulf of C a l i f o r n i a c a n b e r e l a t e d t o t h e i r p o s i t i o n a t o r n e a r t h e p l a t e boundary ( F i g . 1 . 1 1 ) . Crowell (1974a) h a s shown how t h e s e modern, and o l d e r , s e d i m e n t a r y b a s i n s originate i n a strike-slip tectonic setting. a r e recognized:
Two main t y p e s of s e d i m e n t a r y b a s i n
p u l l - a p a r t b a s i n s and f a u l t - w e d g e . b a s i n s .
The former o c c u r where a l o n g and s t r a i g h t s t r i k e - s l i p f a u l t h a s a s h a r p d o u b l e bend (anastomosing f a u l t ) t h a t s i d e s t e p s t h e main f a u l t t r e n d .
Such double bends
can b e r e s t r a i n i n g , r e s u l t i n g i n o v e r l a p and e l e v a t i o n a t t h e bends ( F i g . 1 . 1 2 ) , r e l e a s i n g i n which a p u l l - a p a r t and s u b s i d e n c e r e s u l t s . a n i d e a l i z e d p u l l - a p a r t b a s i n a r e shown i n F i g . l . 1 3 .
01:
The c h a r a c t e r i s t i c s of
Examples i n c l u d e t h e p r e s e n t
day Gulf o f C a l i f o r n i a and S a l t o n Trough which a r e widening and l e n g t h e n i n g a s c o n t i n e n t a l t e r r a n e t o t h e west moves o b l i q u e l y away from t h e mainland p a r t o f North America.
The S a l t o n Trough c o n t a i n s o v e r 6,000m of
f l u v i a l , l a c u s t r i n e and
26
24
. i
22
30
18
. .:
:.
...:
: .
10
0
14
100 200
19 12
300
km.
40
38
\\
\
36
34
32
30 /
Major fault
6Edg, of basin floor 28
......
Continental margin
Fig.l.11. Major f a u l t s and p r e s e n t s e d i m e n t a t i o n s i t e s of C a l i f o r n i a and Baja C a l i f o r n i a ( a f t e r C r o w e l l , 1974b). marine s e d i m e n t s .
The b a s i n margins c o n t a i n c o a r s e f a n g l o m e r a t e s and a r e marked
by u n c o n f o r m i t i e s w i t h a b r u p t changes i n f a c i e s and t h i c k n e s s .
Much of t h e sediment
i n t h e S a l t o n Trough h a s been s u p p l i e d by t h e a n c e s t r a l and p r e s e n t day Colorado
20 river,
R ESTRA l NIN G BEND
RELEASING BEND
B
SHARP RESTRAINING BEND
SHARP PULL-APART
Fig.l.12. R i g h t s l i p on f a u l t w i t h marked double bends (A) r e s u l t s i n p u l l - a p a r t a t r e l e a s i n g bends (B) a n d d e f o r m a t i o n ( f o l d and t h r u s t f a u l t s ) a t r e s t r a i n i n g bends (C) ( a f t e r C r o w e l l , 1974a). Fault-wedge b a s i n s develop i n a system of anastamosing s u b - p a r a l l e l f a u l t s , a l l w i t h t h e same d i r e c t i o n o f o f f s e t .
With l a t e r a l movement t h e s e s p l i t and r e j o i n
l e a d i n g t o t h e i r convergence, compression and u p l i f t , o r t o d i v e r g e n c e , e x t e n s i o n and s u b s i d e n c e ( F i g . l . 1 4 ) . formed i n t h i s way.
The Ridge Basin ( F i g . l . 1 5 ) i s a m a j o r s e d i m e n t a r y b a s i n
I t o r i g i n a t e d a s a narrow d e p r e s s i o n bounded on t h e
southwest
by t h e s c a r p of t h e San G a b r i e l F a u l t zone and on t h e n o r t h e a s t by h i g h l a n d s r a i s e d by f o l d i n g and by movements on t h e C l e a r w a t e r , L i e b r e , and o t h e r f a u l t s ( F i g . l . 1 5 A ) . The sediments o f t h e Ridge Basin a r e o v e r 12,000m t h i c k and i n c l u d e a v a r i e d assemblage of a l l u v i a l f a n , l a c u s t r i n e and m a r g i n a l marine d e p o s i t s d e p o s i t e d d u r i n g t h e i n t e r v a l from l a t e Miocene t o e a r l y P l e i s t o c e n e .
The P l i o c e n e d e p o s i t s c o n s i s t
mainly of c o a r s e f l u v i a l conglomerates and s a n d s t o n e s d e p o s i t e d a l o n g t h e b a s i n margins and mudstone, s h a l e and s a n d s t o n e d e p o s i t e d under l a c u s t r i n e c o n d i t i o n s i n the c e n t r a l p a r t of the basin.
Rocks which f l o o r and s u r r o u n d t h e Ridge B a s i n
c o n s i s t of g r a n i t e s , g n e i s s e s , and o t h e r metamorphics a s w e l l as T e r t i a r y c l a s t i c s .
21
I
Stretched and attenuated
W
Complex unconformities and overlaps in subsurface Complex intersection Oldest intact basin f i l l Remnants of marginal -.. rocks within volcanics
1
Talus breccias 2nd rapld ’ facies changes basinward Small thrust plates -’ , , Slide blocks
Braided right -slip zone
Fig.l.13.
I
\
]Oblique
result of convergence between boundary righ! slip faults
S k e t c h map o f a n i d e a l i z e d p u l l - a p a r t b a s i n ( a f t e r Crowell, 1974a).
The V i o l i n B r e c c i a , a c o a r s e sedimentary b r e c c i a a l o n g t h e San G a b r i e l F a u l t , c o n t a i n s c l a s t a i n t h e lower p a r t which were d e r i v e d from a l i m i t e d s o u r c e a r e a . T h i s s o u r c e a r e a i s now o f f s e t by 28km a l o n g t h e San G a b r i e l F a u l t (Crowell, 1974b). Upper Miocene marine beds on t h e southwest s i d e of t h e
San G a b r i e l F a u l t have a l s o
been d i s p l a c e d from t h e i r s o u r c e a r e a by t h e same amount (Crowell, 1952).
These
f e a t u r e s a r e c o n s i s t e n t w i t h t h e o r i g i n o f t h e Ridge Basin a s a fault-wedge “hollow” r e s u l t i n g from s t r i k e - s l i p ( F i g . 1.15A). Continental c o l l i s i o n ~
~~
When c o n t i n e n t a l masses a r e b r o u g h t t o g e t h e r by s u b d u c t i o n o r s t r i k e - s l i p movement c o l l i s i o n o c c u r s r e s u l t i n g i n f o l d i n g , t h r u s t i n g and o r o g e n e s i s .
A
consequence o f c o n t i n e n t a l c o l l i s i o n s of t h i s t y p e i s t h e consumption o f n e a r l y a l l t h e o c e a n i c l i t h o s p h e r e and t h e s u t u r i n g o f t h e two a r e a s of c o n t i n e n t a l c r u s t . The c l o s u r e o f o c e a n i c a r e a s i n t h i s way i s r a r e l y c o m p l e t e l y r e g u l a r o r symmetrical and t h e r e may be r e l i c s o f ocean b a s i n p r e s e r v e d between c o l l i s i o n p o i n t s .
r e l i c t b a s i n s a r e r e f e r r e d t o a s remnant-ocean b a s i n s (Graham e t a l . , 1975).
These
As
c o l l i s i o n proceeds remnant-ocean b a s i n s a r e p a r t l y f i l l e d by sediment d e r i v e d from t h e newly formed mountains, and new b a s i n s a r e c r e a t e d .
These i n c l u d e p e r i p h e r a l
b a s i n s (Dickinson, 1974b) which form on t h e s u b d u c t i n g p l a t e o r upon t h e o v e r r i d i n g
22
p l a t e a s modified r e t r o a r c basins.
II Divergence
S k e t c h map t o show u p l i f t a t convergence and s u b s i d e n c e a t d i v e r g e n c e of Fig.l.14. s t r i k e - s l i p f a u l t s ( a f t e r Crowell, 1974a). I n a n c i e n t o r o g e n i c b e l t s i t i s common p r a c t i c e t o d i s t i n g u i s h between f o r e l a n d b a s i n s which a r e s i t u a t e d on t h e s u b d u c t i n g p l a t e from h i n t e r l a n d b a s i n s which a r e formed on t h e o v e r r i d i n g p l a t e .
F o r e l a n d and h i n t e r l a n d b a s i n s a r e a l s o c o m o n l y
d i s t i n g u i s h e d from s u c c e s s o r b a s i n s (King, 1 9 6 6 ) which a r e , i n f a c t , t h e epieugeos y n c l i n e s of Kay (1951).
S u c c e s s o r b a s i n s , b e c a u s e t h e y l i e on f o l d e d e u g e o s y n c l i n a l
r o c k s , p r o v i d e a n i m p o r t a n t r e c o r d of t h e l a t e r s t a g e s o f o r o g e n e s i s ( E i s b a c h e r , 1974). The s e d i m e n t a r y b a s i n s which r e s u l t from t h e e r o s i o n of u p l i f t e d mountain r a n g e s a r e often c o l l e c t i v e l y referred t o a s late-orogenic basins.
A t the present t i m e
t h e s o u t h e r n f l a n k o f t h e a c t i v e l y r i s i n g Himalayas c o n t a i n a number of b a s i n s which a r e up t o 500 km wide and e x t e n d f o r o v e r 1,000 km ( F i g . l . 1 6 ) . These b a s i n s a r e p r i m a r i l y p a r a l l e l t o t h e main t e c t o n i c t r e n d a s a r e t h e main d r a i n a g e c o u r s e s , a l t h o u g h t h e r e a r e a l s o t r a n s v e r s e elements ( F i g . 1.16);.
A l l u v i a l f a c i e s have been
a c c u m u l a t i n g i n t h e s e b a s i n s s i n c e Miocene times and t h e o l d e r d e p o s i t s a l o n g t h e n o r t h e r n margin of t h e a l l u v i a l a p r o n a r e f o l d e d and t h r u s t e d s o t h a t t h e d e p o s i t i o n a l a x i s of t h e b a s i n has m i g r a t e d p r o g r e s s i v e l y southwards w i t h t i m e .
The
23
Upth~d block
Extension & Subsidence
Fig.l.15.A.Block diagram i l l u s t r a t i n g o r i g i n o f t h e Ridge Basin a t a curve i n San Andreas F a u l t ( a f t e r C r o w e l l , 1974b). B. F a c i e s , r e l a t i o n s h i p s i n Ridge Basin showing v e r t i c a l t r a n s i t i o n of marine t o l a c u s t r i n e s e d i m e n t s and l a t e r a l p a s s a g e of a l l u v i a l f a n g l o m e r a t e s through m a r g i n a l l a c u s t r i n e f a c i e s i n t o o f f s h o r e l a c u s t r i n e f a c i e s ( a f t e r C r o w e l l , 1975; Link e t a l . , 1978). The d e t a i l e d r e l a t i o n s h i p s between t e c t o n i c s and a l l u v i a l s t r a t i g r a p h y i n one of these basins
( t h e Jhelum R e - e n t r a n t , P a k i s t a n ) h a s been i n v e s t i g a t e d by V i s s e r and
Johnson (1978).
T h i s formed p a r t o f a r a p i d l y s u b s i d i n g f o r e d e e p which developed
d u r i n g t h e Neogene and Q u a t e r n a r y a s a r e s u l t o f southward d i r e c t e d t e c t o n i s m and r a p i d morphogenetic u p l i f t .
I t p e r s i s t s today a s t h e Indo-Gangetic b a s i n and
c o n t a i n s a sequence of p r e - t e c t o n i c E a r l y Palaeogene c a r b o n a t e s o v e r l a i n by L a t e Palaeogene f l u v i a l - d e l t a i c molasse (Rawalpindi Group) and t h e younger f l u v i a l S i w a l i k Group,
The environment and r a t e of f l u v i a l d e p o s i t i o n i n t h e Upper p a r t
of t h e S i w a l i k Group i n t h e Jhelum R e - e n t r a n t was c o n t r o l l e d by l a r g e - s c a l e s y n t a x i a l t e c t o n i c s i n t h e n o r t h w e s t e r n Himalaya.
V i s s e r and Johnson (1978)
i d e n t i f i e d a n i s o c h r o n o u s i n t e r v a l of f l u v i a l d e p o s i t i o n between a c o r r e l a t i v e
PZ
25 p a i r of v o l c a n i c a s h h o r i z o n s and s p a n n i n g t h e Gauss/Matuyama p o l a r i t y t r a n s i t i o n
(2.4My) ( F k . 1 . 1 7 ) .
0REVERSED POLARITY NORMAL POLARITY
F R4
FR3
M P8
MP5
RT1
FR1
RT2
CH1
Fig. 1.17. L i t h o s t r a t i g r a p h i c v a r i a t i o n a t e i g h t l o c a l i t i e s i n t h e Jhelum R e - e n t r a n t , P a k i s t a n showing upper S i w a l i k r o c k s bounded by two v o l c a n i c a s h h o r i z o n s and t h e p o s i t i o n of t h e Gauss/Matuyama p o l a r i t y t r a n s i t i o n . L i t h o l o g i c symbols: L a t e r a l a c c r e t i o n s a n d s t o n e f a c i e s , s t i p p l e d showing dominant bedform; v e r t i c a l a c c r e t i o n mudstone f a c i e s w i t h d e p o s i t i o n a l f a b r i c p r e s e r v e d , no p a t t e r n ; mudstone-facies m o d i f i e d by p e d o g e n e s i s , h a t c h e d ( a f t e r V i s s e r and Johnson, 1 9 7 8 ) . These f l u v i a l d e p o s i t s c o n s i s t of markedly fining-upwards c y c l e s w i t h w e l l developed l a t e r a l a c c r e t i o n u n i t s ( p o i n t b a r s a n d s ) and v e r t i c a l a c c r e t i o n u n i t s ( f l o o d p l a i n mudstones).
The d e t a i l e d sequence o f t h e v e r t i c a l a c c r e t i o n d e p o s i t s
i n d i c a t e s t h a t t h e f l o o d p l a i n environment c o n s i s t e d o f a stream-proximal, sedimentation-dominant zone and a s t r e a m - d i s t a l , pedogenesis-dominant zone.
The depegdence o f t h e a l l u v i a l s t r a t i g r a p h y on t e c t o n i c s i s i l l u s t r a t e d b y two variables:
t h e p r o p o r t i o n of l a t e r a l a c c r e t i o n s a n d i n any s e c t i o n and a l s o t h e
t o t a l t h i c k n e s s of s e d i m e n t between t h e two d a t e d v o l c a n i c a s h h o r i z o n s .
The
p r o p o r t i o n of l a t e r a l a c c r e t i o n s a n d b o d i e s i s , of c o u r s e , dependent on a g r e a t v a r i e t y of e n v i r o n m e n t a l v a r i a b l e s b u t i n s p i t e o f t h i s t h e r e a p p e a r s t o b e a r e l a t i o n s h i p between t h e t o t a l t h i c k n e s s of l a t e r a l a c c r e t i o n d e p o s i t s and t h e p r o x i m i t y t o major f l u v i a l c o u r s e s .
The t h i c k n e s s of t h e i n t e r v a l e n c l o s e d by t h e
v o l c a n i c a s h e s embodies a d i r e c t measure of t h e s e d i m e n t a t i o n r a t e which can b e c a l c u l a t e d on t h e b a s i s of t h e palaeomagnetic d a t i n g .
The r a t e v a r i e s from a low
of 0.33m/1000 y e a r s i n t h e s o u t h e a s t (FR1 i n F i g . l . 1 7 ) t o a h i g h of 0.4m/lOOO y e a r s
i n t h e n o r t h w e s t (FR4 i n F i g . l . 1 7 ) .
This defines a regular trend of increasing
sediment r a t e towards t h e apex of t h e Jhelum R e - e n t r a n t i n d i c a t i n g t h a t t h e Ree n t r a n t s t r u c t u r e was a m a j o r f o c u s of molasse s e d i m e n t a t i o n .
The s i g n i f i c a n c e o f
R e - e n t r a n t s a p p e a r s t o b e t h a t t h e y form p r e f e r e n t i a l s i t e s f o r t h e p a s s a g e o f m a j o r l o n g i t u d i n a l i n t e r m o n t a n e r i v e r s i n t o t h e molasse b a s i n ,
The p r e c i s e t e c t o n i c
s i g n i f i c a n c e o f R e - e n t r a n t s i s , however, n o t y e t e s t a b l i s h e d .
The f l e x u r e o f t h e
n o r t h w e s t e r n Himalayan s y n t a x i s c o u l d be due t o a northward p r o j e c t i n g s p u r of t h e Indian lithospheric p l a t e c o l l i d i n g with the Eurasian p l a t e ,
Crawford ( 1 9 7 4 ) , however,
b e l i e v e s t h e s y n t a x i s t o b e a s e c o n d a r y f e a t u r e formed between converging E u r a s i a n c r u s t a l b l o c k s t o t h e e a s t and w e s t of t h e s y n t a x i s . THE TECTONIC SETTING OF ANCIENT CONTINENTAL RED BEDS
Introduction The p r e v i o u s s e c t i o n i l l u s t r a t e s t h e f a c t t h a t c o n t i n e n t a l s e d i m e n t s may accumulate i n a wide range of t e c t o n i c s e t t i n g s from p r e - o r o g e n i c r i f t s t o l a t e - o r o g e n i c b a s i n s . A t t h e p r e s e n t - d a y t h e s e c o n t i n e n t a l s e d i m e n t s do n o t c o n t a i n abundant r e d beds
a l t h o u g h i n some c a s e s t h e p r o g r e s s i v e reddening o f t h e o l d e r s e d i m e n t s can b e demonstrated. I n t h e s t r a t i g r a p h i c r e c o r d s t r u c t u r a l and s t r a t i g r a p h i c a n a l y s i s can b e used t o demonstrate t h a t a n c i e n t c o n t i n e n t a l s e d i m e n t s formed i n s i m i l a r t e c t o n i c s e t t i n g s t o t h e i r p r e s e n t - d a y c o u n t e r p a r t s a s f a r back a s t h e P r o t e r o z o i c .
Not a l l t h e s e a n c i e n t
c o n t i n e n t a l sediments a r e r e d b u t t h e r e i s a marked abundance of r e d beds compared t o t h e p r e s e n t - d a y , a f e a t u r e which i n d i c a t e s t h e importance of d i a g e n e t i c reddening. I n t h e s u c c e e d i n g s e c t i o n s t h e t e c t o n i c s e t t i n g o f a number o f d i f f e r e n t r e d bed basins i s described.
They a r e assembled i n s t r a t i g r a p h i c a l sequence and i l l u s t r a t e
t h e wide range o f t e c t o n i c s e t t i n g s i n which a n c i e n t c o n t i n e n t a l r e d beds a r e formed. P r o t e r o z o i c r e d beds of w e s t e r n Canada The e a r l i e s t major r e d bed sequences t o a p p e a r i n t h e s t r a t i g r a p h i c r e c o r d a r e
27
ATHAPUSCOW AULACOOEN
SLAVE PLATFORM
Sio"?"
-
0
SOUTH
D
-
D
-
x) liu
'
NORTH
F i g . l . 1 8 . Stratigraphic cross-section of the Athapuscow aulacogen and adjacent platform with structural cross-sections of the northeast half (above) and southwest half (below) ( a f t e r Hoffman e t a l . , 1974).
28
GRABEN STAGE
TRANSITIONAL STAGE
DOWNWARPING STAGE
POST-GEOSYNCLINAL STAGE
CHURCHILL PROVINCE-ATHAPUSCOW FANGLOMERATE
0
RED LlTHlC SANDSTONE GREYWACKE TURBIDITES
0
QUARTZITE
..... . PEBBLY SUBARKOSE
AULACOGEN
[lkm
10 km
-SLAVE PROVINCE OLISTOSTROME MUDSTONE CARBONATE
@
QUARTZ DlORlTE VOLCANICS
F i g . l . 1 9 . Schematic transverse c r o s s - s e c t i o n s showing the e v o l u t i o n o f the Athapuscow aulacogen ( a f t e r Hoffman e t a l . , 1974).
29
t h o s e a s s o c i a t e d w i t h t h e C o r o n a t i o n g e o s y n c l i n e and Athapuscow a u l a c o g e n (Hoffman, 1973; Hoffman e t a l . , 1974) o f w e s t e r n Canada. i n Fig.8.1.
A t e c t o n i c map of t h e a r e a i s shown
The sequence i n t h e Athapuscow a u l a c o g e n t h i c k e n s from 2200m i n t h e
S l a v e P l a t f o r m i n t h e n o r t h t o o v e r 7000m i n t h e main p a r t of t h e a u l a c o g e n .
Within
t h i s framework a v a r i e t y o f d e p o s i t i o n a l p h a s e s c a n b e r e c o g n i z e d i n c l u d i n g a major molasse sequence, t h e C h r i s t i e Bay Group ( F i g . l . 1 8 ) . i n t h e development o f t h e Athapuscow a u l a c o g e n :
Four s t a g e s c a n b e r e c o g n i z e d
Graben, t r a n s i t i o n a l , downwarping
and p o s t - g e o s y n c l i n a l which i s v e r y s i m i l a r t o t h e developmental sequence s e e n i n o t h e r a u l a c o g e n s (Ham, 1969).
The molasse p h a s e d e v e l o p s d u r i n g t h e downwarping
s t a g e and b e g i n s w i t h a g i g a n t i c o l i s t o s t r o m e i n which a n g u l a r b l o c k s o f stromatol i t i c d o l o m i t e and l i m e s t o n e a r e c h a o t i c a l l y d i s p e r s e d i n a r e d mudstone m a t r i x , I n d i v i d u a l o l i s t o l i t h s a r e up t o 45m t h i c k and a k i l o m e t r e i n l e n g t h and many a r e recumbently f o l d e d or emplaced u p s i d e down.
The o l i s t o s t r o m e i s o v e r l a i n by a 450m
t h i c k sequence o f r e d mudstone w i t h hopper-shaped h a l i t e c a s t s which p a s s e s up i n t o 850m o f r e d cross-bedded l i t h i c s a n d s t o n e s o f a l l u v i a l o r i g i n which were d e p o s i t e d by r i v e r s which flowed towards t h e n o r t h e a s t a l o n g t h e a x i s o f t h e aulacogen.
The
s a n d s t o n e s a r e o v e r l a i n by 230m of r e d mud-cracked s i l t s t o n e s w i t h h a l i t e and gypsum casts. The downwarp s t a g e was followed by a s t a g e o f m i l d c o m p r e s s i o n a l d e f o r m a t i o n ( F i g . l . 1 9 ) i n which t h e molasse and e a r l i e r s e d i m e n t s were f o l d e d .
A t h i c k sequence
o f f a n g l o m e r a t e s was d e p o s i t e d on t h e u p t u r n e d edges of t h e s e o l d e r r o c k s d u r i n g t h e post-geosynclinal stage.
T h i s sequence c o n s i s t s o f 4,OOOm of f l u v i a t l e r e d and b u f f
b o u l d e r conglomerates a n d p e b b l y s a n d s t o n e c l o s e l y a s s o c i a t e d w i t h a l k a l i c b a s a l t flows.
The conglomerate c l a s t s r e c o r d p r o g r e s s i v e e r o s i o n o f p l a t f o r m c o v e r from t h e
u p l i f t and u n r o o f i n g o f t h e basement.
They t e n d t o o c c u r i n homoclines d i p p i n g
p a r a l l e l t o t h e t r e n d of t h e f a u l t s from which t h e y were d e r i v e d .
T h i s f e a t u r e , and
t h e r e c o g n i t i o n of d e x t r a l t r a n s c u r r e n t movement on some of t h e boundary f a u l t s ( R e i n h a r d t , 19691, s u g g e s t s t h a t t h e f a n g l o m e r a t e s may have been d e p o s i t e d i n s t r i k e s l i p b a s i n s l i k e t h o s e d e s c r i b e d by Crowell (1974a). The Caledonian Orogen The Caledonian o r o g e n i c b e l t s t r e t c h i n g through G r e e n l a n d , NW Europe and North America c a n b e i n t e r p r e t e d i n terms o f t h e opening and c l o s u r e of a
proto-Atlantic
o r I a p e t u s ocean (Wilson, 1966; Dewey, 1969; M i t c h e l l and McKerrow, 1975; P h i l l i p s e t a l . , 1976; McKerrow e t a l . , 1977).
Sediments a s s o c i a t e d w i t h t h e e v o l u t i o n of
t h e Caledonian orogen range i n a g e from L a t e Precambrian t o Devonian.
Continental
r e d beds were formed i n t h e i n i t i a l phase of opening i n t h e L a t e Precambrian and a l s o d u r i n g and a f t e r t h e f i n a l c l o s u r e of the p r o t o - A t l a n t i c i n Devonian times. The Sparagmite s u c c e s s i o n o f s o u t h e r n Norway c o n s i s t s o f a complex sequence of greywackes, s h a l e s , c a r b o n a t e s , a r k o s e s , c o n g l o m e r a t e s , and t i l l i t e s up t o 3000m
30 thick.
Red b e d s , p a r t i c u l a r l y amongst t h e a r k o s i c s e d i m e n t s , a r e abundant.
The
sequence i s b e l i e v e d t o have been d e p o s i t e d i n a r i f t v a l l e y which formed d u r i n g t h e i n i t i a l opening of t h e p r o t o - A t l a n t i c (Bjbrlykke e t a l . ,
1976) ( F i g . l . 2 0 ) .
The v a l l e y i s e s t i m a t e d t o have been 60-70km a c r o s s and 2-4km deep.
I n the central
p a r t of t h e b a s i n s e d i m e n t a t i o n was dominated by s h a l e s and greywackes which p a s s upwards i n t o s h a l l o w marine sediments i n c l u d i n g t i l l i t e s .
The a r k o s e s and cong-
lomerates dominated t h e b a s i n margins and were d e p o s i t e d a s c o a r s e c l a s t i c wedges ( f a n d e l t a s ) which p e r i o d i c a l l y prograded i n t o t h e b a s i n .
The L a t e Pre-Cambrian
T o r r i d o n i a n Sandstone of NW S c o t l a n d a c o n t i n e n t a l r e d bed sequence formed i n a s i m i l a r t e c t o n i c s e t t i n g t o t h e s p a r a g m i t e s of Scandanavia.
I t formed t h e north-
w e s t e r l y margin of t h e p r o t o - A t l a n t i c ocean d u r i n g t h e i n i t i a l s t a g e s o f r i f t i n g and s e a - f l o o r s p r e a d i n g . The T o r r i d o n i a n sequence rests w i t h marked unconformity on t h e Lewisian basement
-
i n p l a c e s a p r e - T o r r i d o n i a n rugged topography w i t h a r e l i e f o f 300-4Oh i s e v i d e n t . The unconformity i s o v e r l a i n by a t l e a s t 2300m of t h e S t o e r Group c o n s i s t i n g of r e d s a n d s t o n e s and s h a l e s ,
T h i s i t s e l f i s unconformably o v e r l a i n by t h e T o r r i d o n Group Most
c o n s i s t i n g of a t l e a s t 7000m o f r e d pebbly s a n d s t o n e s , s i l t s t o n e s and s h a l e s .
o f t h e s e r e d beds a r e of a l l u v i a l o r i g i n and i n c l u d e b r a i d e d s t r e a m d e p o s i t s ( S e l l e y ,
1 9 7 0 ) , a l t h o u g h minor marine h o r i z o n s do o c c u r ( G r a c i e and S t e w a r t , 1967).
Williams
(1969a) h a s shown t h a t t h e p a l a e o s l o p e was g e n e r a l l y t o t h e e a s t and s o u t h e a s t w i t h t h e p a l a e o c u r r e n t s r a d i a t i n g from two p o i n t s o u r c e s west of t h e p r e s e n t o u t c r o p n e a r t h e l i n e of t h e Minch F a u l t ( F i g . l . 1 1 ) .
The p r e s e n c e o f a t l e a s t two a l l u v i a l f a n s
i s i m p l i e d and c o n t i n e n t a l d e p o s i t i o n i n t h i s L a t e Precambrian r i f t may have been
c o n c e n t r a t e d a l o n g a t e c t o n i c a l l y c o n t r o l l e d , n o r t h - w e s t e r l y r e t r e a t i n g Lewisian scarp slope.
T h i s i s c o n s i s t e n t w i t h c l a s t s t u d i e s o f t h e T o r r i d o n i a n Group which
confirm t h e i r d e r i v a t i o n from rocks which span t h e range o f Lewisian e v e n t s (3000-1000
My)but which a r e from a h i g h e r c r u s t a l l e v e l t h a n i s now exposed
( A l l e n e t a l . , 1974) ( F i g . l . 2 1 ) . The subsequent c l o s u r e of t h e p r o t o - A t l a n t i c ocean d u r i n g t h e O r d o v i c i a n and S i l u r i a n r e s u l t e d i n c o n t i n e n t a l c o l l i s i o n and t h e f o r m a t i o n of t h e Caledonian Mountains.
T h i s c a n be i n t e r p r e t e d t o have o c c u r r e d f i r s t i n t h e n o r t h e a s t and
l a t e r towards t h e south-west a s a t r i p l e p o i n t m i g r a t e d i n t h i s d i r e c t i o n ( P h i l l i p s e t a l . , 1976).
The e v o l u t i o n o f t h e S c o t t i s h Caledonides c a n a l s o be compared
c l o s e l y w i t h present-day n o r t h e a s t e r n Burma ( M i t c h e l l and McKerrow, 1975).
The
molasse f a c i e s o f t h e Caledonian orogen i s r e f e r r e d t o a s t h e Old Red Sandstone. I t i s a major c o n t i n e n t a l r e d b e d sequence r a n g i n g i n age from l a t e S i l u r i a n t o
Devonian which was d e p o s i t e d i n a number of d i f f e r e n t l a t e - o r o g e n i c s e d i m e n t a r y b a s i n s , some w i t h a marine c o n n e c t i o n ( e x t e r n a l b a s i n s ) and some w i t h no marine connections and i n t e r n a l d r a i n a g e p a t t e r n s ( i n t e r n a l b a s i n s ) .
31
Lower Cambrian
VanQsAs Formation O e I I a x and Coastal
Feldsoalhlc Sandstone
Moelv Tillite and Ekre Shale
< Oeoosilion 01 Glacial ' Sediments
Ring Formation
Biri Limestone and Shale
Shale
Brettuq Formation d... 1
\L
and PhOSDhate Deposits /15";"Carbonate on Shallow Shell
S i m p l i f i e d diagram i l l u s t r a t i n g t h e d e p o s i t i o n a l h i s t o r y and d i s t r i b u t i o n Fig.l.20. of f a c i e s i n t h e c e n t r a l s p a r a g m i t e b a s i n i n s o u t h e r n Norway ( a f t e r B j d r l y k k e e t a l . , 1976). The Midland V a l l e y of S c o t l a n d was a major Old Red Sandstone b a s i n which c o n t a i n s a b o u t 7000m of s e d i m e n t s and a n d e s i t i c v o l c a n i c s .
The b a s i n i s a fault-bounded
g r a b e n ( F r i e n d , 1967; Bluck, 1978) i n which t h e boundary f a u l t s a r e p a r a l l e l t o t h e main Caledonian t e c t o n i c elements. The Lower Old Red Sandstone i s a c o n g l o m e r a t e - l i t h i c a r e n i t e assemblage i n t e r bedded w i t h a c i d t o b a s i c v o l c a n i c s which was d e p o s i t e d i n a t l e s s t two b a s i n s : t h e S t r a t h m o r e b a s i n t o t h e n o r t h w e s t and t h e Lanark b a s i n t o t h e s o u t h e a s t .
Both
t h e s e b a s i n s were f i l l e d l o n g i t u d i n a l l y from t h e n o r t h e a s t (Bluck, 1978) ( F i g . 1.221, t h e S t r a t b o r e b a s i n b e g i n n i n g i n t h e n o r t h e a s t a n d e x t e n d i n g southwestwards so
32 t h a t t h e b a s i n - f i l l o v e r l a p s and t h i n s i n a s o u t h w e s t e r l y d i r e c t i o n .
-O
200 Km
i" f
4
TORRIDON GROUP alluvial fanpile TORRIDON GROUP
-
-
1.0 b.y. 1 . 8 b.y.
> 2 ' 5 b.y. Fig.l.21. R e c o n s t r u c t i o n of t h e p r i n c i p a l pre-Torridon Group u n i t s i n r e l a t i o n t o t h e a r e a of T o r r i d o n (Applecross Formation) d e p o s i t i o n i n NW S c o t l a n d ( a f t e r A l l e n e t a l . , 1974). The Upper Old Red Sandstone sediments were p a r t l y d e r i v e d from t h e Lower Old Red Sandstone over which t h e y unconfonnably l i e .
The b a s i n h a s a n E-W o r i e n t a t i o n and
was f i l l e d l o n g i t u d i n a l l y from t h e s o u t h w e s t , a major r e v e r s a l o f t h e d r a i n a g e p a t t e r n s e e n i n t h e Lower Old Red Sandstone.
The b a s i n began i n t h e Clyde r e g i o n
and was i n i t i a l l y extended by s u c c e s s i v e normal f a u l t s which formed i n r e s p o n s e t o s i n i s t r a l movement of t h e Highland Boundary F a u l t f 1 . 2 2 ) .
33
i
Upper Old Red Sandstone
Schematic diagram o f t h e d i p o f major l o n g i t u d i n a l b a s i n s i n t h e Midland Fig.1.22. V a l l e y o f S c o t l a n d i n Lower and Upper Old Red Sandstone times ( a f t e r Bluck, 1978). The Devonian b a s i n s o f w e s t e r n Norway a r e a l s o i n t e r n a l b a s i n s b u t show evidence o f h a v i n g formed i n a d i f f e r e n t t e c t o n i c regime t h a n t h e Midland V a l l e y of S c o t l a n d . 2 These b a s i n s a r e n o t a b l e f o r t h e i r s m a l l s i z e ((2,OOOkm ), v a s t s t r a t i g r a p h i c t h i c k n e s s e s (up t o 25km) a n d t h e c y c l i c n a t u r e of t h e i r c o a r s e - g r a i n e d a l l u v i a l i n f i l l i n g ( S t e e l , 1976; S t e e l and Aasheim, 1978) ( F i g . 1 . 2 3 ) .
The l o n g e s t of t h e s e
b a s i n s i s t h e Hornelen b a s i n which i s bounded by t h r u s t ( e a s t ) , h i g h - a n g l e f a u l t ( n o r t h and s o u t h ) and unconformable ( w e s t ) margins.
The s u r r o u n d i n g basement rock
h a s a n E-W Caledonian g r a i n and c o n s i s t s o f Precambrian g n e i s s e s , Cambro-Silurian m e t a b a s a l t s , s c h i s t s , q u a r t z i t e s , rnetagreywackes, g r a n o d i o r i t e s and gabbros. The b a s i n was f i l l e d l a r g e l y w i t h sandy a l l u v i u m d e p o s i t e d on a n a l l u v i a l p l a i n o r sandy f a n d e l t a growing t o t h e w e s t and n o r t h w e s t ( S t e e l and Aasheim, 1978). margins of t h e b a s i n a r e f l a n k e d by f a n g l o m e r a t e s ( S t e e l e t a l . ,
The
1977; Larsen and
S t e e l , 1978) which frequently show a coarsening-upwards theme, usually of the order of 100-20Om, and i r r e s p e c t i v e of the a l l u v i a l f a c i e s .
This i s interpreted by
S t e e l ( 1 9 7 6 ) a s an indication of rapid lowering of the basin f l o o r and the consequent
35 p r o g r a d a t i o n of a l l u v i a l s e d i m e n t a t i o n .
I n t h e Hornelen b a s i n a l l u v i a l t r a n s p o r t
was mainly a l o n g t h e a x i s o f t h e b a s i n ( F i g . 1 . 2 3 ) and t h e r e a r e i m p o r t a n t d i f f e r e n c e s i n f a c i e s , t h i c k n e s s e s , g r a d i e n t s and s i z e o f f r i n g i n g a l l u v i a l f a n s on o p p o s i t e s i d e s of t h e basin.
These f e a t u r e s and t h e dominance o f a s a n d s t o n e r a t h e r t h a n
conglomerate s u c c e s s i o n s u g g e s t s a d e p o c e n t r e s y s t e m a t i c a l l y m i g r a t i n g through time i n a s t r i k e - s l i p t e c t o n i c regime (Crowell, 1974a; S t e e l , 1976). The Solund b a s i n i s n o t so c l e a r l y o r g a n i z e d a s t h e Hornelen b a s i n b u t coarseningupwards sequences a r e p r e s e n t .
The s u c c e s s i o n i s dominated by conglomerates which
were d e p o s i t e d by c u r r e n t s flowing t r a n s v e r s e t o t h e b a s i n a x i s , f e a t u r e s which s u g g e s t t h a t t h e b a s i n was formed i n a d i p - s l i p t e c t o n i c regime. E a r l y Mesozoic r e d beds formed i n graben a s s o c i a t e d w i t h t h e opening of t h e A t l a n t i c
Ocean
The break-up of t h e s u p e r c o n t i n e n t Pangaea and t h e opening of t h e North A t l a n t i c Ocean d u r i n g t h e l a s t 200 My h a s been a s s o c i a t e d w i t h t h e f o r m a t i o n o f numerous graben (Fig.1.24).
I n a l l , over 100 s e p a r a t e graben c a n b e i d e n t i f i e d and t h e s e a r e
l i n k e d , forming a n e l a b o r a t e network of t r i p l e r i f t systems (Burke, 1976).
Some of
t h e s e r i f t s developed t o form new o c e a n i c c r u s t b u t many o f them f a i l e d and were f i l l e d w i t h t h i c k sequences o f c o n t i n e n t a l s e d i m e n t s , many o f which a r e now r e d beds. The formation of t h e s e t e n s i o n a l r i f t v a l l e y s i s commonly accompanied by b a s a l t i c volcanism and t h e r a p i d accumulation of s e v e r a l k i l o m e t r e s of sediment.
The
sedimentary f i l l i n g i s f r e q u e n t l y t r a n s v e r s e a l t h o u g h h o r s t s w i t h i n t h e graben s t r o n g l y i n f l u e n c e t h e e a r l y s t a g e s of sediment d i s p e r s a l .
Rifting generally
c o n t i n u e s u n t i l t h e o n s e t ' o f normal marine s e d i m e n t a t i o n b u t i n some c a s e s sediment d i s t r i b u t i o n may be i n f l u e n c e d by graben which a r e o v e r 100 My o l d . Graben o f T r i a s s i c age are a c h a r a c t e r i s t i c f e a t u r e o f t h e c e n t r a l A t l a n t i c a r e a which was t h e f i r s t p a r t o f t h e A t l a n t i c Ocean t o open.
These a r e b e s t known from
t h e North American s i d e o f t h e A t l a n t i c where t h e y form a l i n e a r system of f a u l t bounded b a s i n s a b o u t 2000km i n l e n g t h ( S a n d e r s , 1963).
These a r e g e n e r a l l y f i l l e d
w i t h s e v e r a l k i l o m e t r e s of r e d f l u v i a l and r e d / d r a b l a c u s t r i n e sediments of Carnian t o Lower J u r a s s i c age (Cornet and T r a v e r s e , 1975) i n t e r c a l a t e d w i t h t h o l e i t i c b a s a l t i c l a v a s , s i l l s , and dykes g i v i n g r a d i o m e t r i c a g e s between 210-170 My.
The
graben a r e normally a few t e n s o f k i l o m e t r e s wide and occur up t o 500km i n t o t h e c o n t i n e n t , t h e s t r i k e u s u a l l y b e i n g c o n t r o l l e d by t h e s t r u c t u r a l t r e n d of t h e underl y i n g Appplachian f o l d e d r o c k s , a l t h o u g h t h e Newark graben c r o s s e s t h e Appalachian strike.
Sedimentation w i t h i n t h e graben i s l a r g e l y c o n t r o l l e d by t h e marginal
boundary f a u l t s .
The t e c t o n i c h i s t o r y of any i n d i v i d u a l graben i s , however,
c o m p l i c a t e d and t h e r e i s evidence t h a t s t r i k e - s l i p components a r e i m p o r t a n t i n some c a s e s ( S a n d e r s , 1963).
36
NORTH SEA
ARGENT1
*.-
S k e t c h map i l l u s t r a t i n g t h e d i s t r i b u t i o n o f major g r a b e n a r o u n d t h e Fig.1.24. A t l a n t i c Ocean which formed i n a s s o c i a t i o n w i t h c o n t i n e n t a l r u p t u r e . The g r a b e n between t h e l i n e s V-W and X-Y developed between 210 and 170 My ago; t h o s e s o u t h of t h e l i n e X-Y formed between a b o u t 145 and 125 My ago; t h o s e n o r t h o f t h e l i n e T-Y formed a b o u t 80 My ago and t h o s e n o r t h o f R-S between 80-60 My ( a f t e r Burke, 1976). The H a r t f o r d b a s i n c o n t a i n s a 4km t h i c k s u c c e s s i o n which can b e c o r r e l a t e d f o r a d i s t a n c e o f o v e r 140km.
I t c o n t a i n s t h e E a s t B e r l i n Formation, a 145-45Om t h i c k
sequence of f l u v i a l and l a c u s t r i n e r o c k s o f Lower J u r a s s i c a g e (Hubert e t a l . , 1976). Along t h e e a s t e r n boundary o f t h e b a s i n was a s t e e p escarpment a l o n g which a l l u v i a l f a n s were developed.
These t r a n s p o r t e d d e t r i t u s e a s t w a r d s r a d i a l l y away from t h e
fault-bounded margin (Fig.1.25)
i n t o t h e c e n t r a l p a r t of t h e graben.
Dispersal
h e r e seems a l s o t o have been e s s e n t i a l l y e a s t w a r d s , j u d g i n g from t h e r a t h e r v a r i a b l e p a l a e o c u r r e n t s , which s u g g e s t t h a t t h e g r a b e n was f i l l e d t r a n s v e r s e l y .
The p a l a e o -
s l o p e of t h e g r a b e n f l o o r was g e n e r a l l y t o t h e s o u t h and t h e r e may have been s i g n i f i c a n t d i s p e r s a l o f sediment a l o n g t h e a x i s o f t h e g r a b e n ,
37
B
A
*OI
km
OJ ~ R O D E DEDGE; OF BASIN ,I
__*
Palasocurrents
a
Stream Channel Sendstone
0
Alluvial Fan conglomerats & tlldstom
A. P a l a e o c u r r e n t and p a l a e o s l o p e f o r s a n d s t o n e and conglomerates d e p o s i t e d Fig.1.25. i n s t r e a m c h a n n e l s and a l l u v i a l f a n s of t h e E a s t B e r l i n Formation B. P a l a e o c u r r e n t s f o r r e d s a n d s t o n e and s i l t s t o n e d e p o s i t e d i n s h a l l o w l a k e s on f l o o d p l a i n s . Palaeos l o p e s f o r f l o o d p l a i n r e d mudstone w i t h t h i n s a n d s t o n e l a y e r s showing t h e r e g i o n a l p a l a e o s l o p e o f t h e v a l l e y f l o o r ( a f t e r Hubert e t a l . , 1976).
38 The North Sea (Fig.1.26)
i s a n o t h e r example of a f a i l e d r i f t system a s s o c i a t e d
w i t h t h e opening of t h e North A t l a n t i c (Whiteman e t a l . , 1975; Burke, 1976) a l t h o u g h
C o n t i n e n t a l r e d beds accumulated i n t h e North Sea b a s i n
i n c l u d e t h e Permian
R o t l i e g e n d e s , a d e s e r t f l u v i a l / a e o l i a n s u c c e s s i o n d e r i v e d l a r g e l y from t h e V a r i s c a n mountains i n t h e s o u t h ( G l e n n i e , 1972) and t h e T r i a s s i c , a n e v a p o r i t e / r e d bed sequence w i t h marine i n t e r c a l a t i o n s (Brennand, 1975).
Around t h e s o u t h e r n margin
of t h e North Sea i s a t h i n a p r o n of c o a r s e c l a s t i c s w i t h conglomerates, d e r i v e d from t h e h i g h l a n d s t o t h e s o u t h .
The major p a r t o f t h e s u c c e s s i o n s c o n s i s t s o f
s h e e t s a n d s t o n e s and s h a l e s w i t h e v a p o r i t e s i n t h e upper p a r t and d i s p l a y s p a r t i c u l a r l y w e l l t h e i n f l u e n c e o f b l o c k f a u l t i n g on t h i c k n e s s v a r i a t i o n s (Fig.1.27).
I n the
m a r g i n a l a r e a s t h e T r i a s s i c i s t h i n and rests unconformably on o l d e r r o c k s whereas
39 i n t h e C e n t r a l Graben t h e s u c c e s s i o n i s much t h i c k e r .
SECTION 1 NORTHERN VIKING GRABEN CormorCnt
Dunlin
kult
L Cwnhn)
SECTION 2 CENTRAL NORTH SEA
T r i a s s i c s e c t i o n s i n t h e n o r t h e r n North Sea showing t h e i n f l u e n c e of Fig.1.27. b l o c k - f a u l t i n g on t h e r a t e of s u b s i d e n c e ( a f t e r Brennand, 1975). AS i n o t h e r f a i l e d r i f t s o f t h e North A t l a n t i c margins t h e r e i s evidence o f s t r i k e -
s l i p t e c t o n i c s i n t h e North Sea.
For example, t h e gas f i e l d s which o c c u r i n
R o t l i e g e n d e s s a n d s t o n e s of t h e s o u t h e r n North Sea a p p e a r t o have been a l i g n e d by Hercynian s t r i k e - s l i p movements ( B l a i r ; 1975). Cenozoic molasse C o n t i n e n t a l sediments formed a s c l a s t i c wedges a l o n g c r a t o n i c margins ( f o r e d e e p s
or e x o g e o s y n c l i n e s ) a r e known as molasse and a fundamental p a r t of t h e l a t e r s t a g e s
40
of orogeny.
Molasse i s w e l l - r e p r e s e n t e d i n t h e Cenozoic f o l d mountains b e l t s of t h e
world (Fig.1.28)
o f t e n forming sequences s e v e r a l k i l o m e t r e s t h i c k and c o n t a i n i n g
mtrrl
nalryas
. .
. .... . ...
. . .. L. .:’ parrlic
J
marina
I
I
m+m
Fig.1.28. A. S t r a t i g r a p h i c s e c t i o n s o f v a r i o u s molasse sequences ( s h a d e d ) and d u r a t i o n s i n My. Dots = t h i c k c o n g l o m e r a t i c u n i t s ; V = v o l c a n i c u n i t s . LM (Lower m a r i n e ) , LFW (Lower f r e s h w a t e r ) , UM (Upper m a r i n e ) , and UFW (Upper f r e s h w a t e r m o l a s s e ) . B. T h i c k n e s s of t h e same sequences ( s h a d e d ) and s u b s i d e n c e rates i n metres p e r m i l l i o n y e a r s ( a f t e r Van Houten, 1969). major r e d bed u n i t s . deposition:
There i s some e v i d e n c e t h a t t h e m a j o r p e r i o d s of molasse
Palaeocene/Eocene and Miocene/Pliocene c o r r e s p o n d t o t h e main p e r i o d s
of l i t h o s p h e r i c p l a t e motion i n t h e i r r e s p e c t i v e a r e a s (Van Houten, 1969). The A l p i n e molasse The p r i n c i p a l t e c t o n i c e l e m e n t s of a l p i n e Western Eruope a r e shown i n Fig.1.29. The Molasse n o r t h of t h e C e n t r a l and E a s t e r n Alps accumulated i n a c r a t o n i c b a s i n a b o u t 700km long and 50-14Okm wide which l a y between l a r g e Hercynian b l o c k s .
Other
i m p o r t a n t m o l a s s e b a s i n s a r e t h e A q u i t a i n e B a s i n l y i n g between t h e Massif C e n t r a l and t h e P y r e n e e s , and t h e Ebro B a s i n i m e d i a t e l y s o u t h of t h e P y r e n e e s ( F i g . 1 . 2 9 ) . The Molasse Basin succeeded e a r l y and m i d d l e O l i g o c e n e F l y s c h which p a s s e d t h r o u g h
41 a s l o p e f a c i e s ( t o n m e r g e l ) i n t o a p a r a l i c sequence (Lower Marine Molasse) a s t h e a x i s of t h e A l p i n e Foredeep was d i s p l a c e d towards t h e c r a t o n (Hagn, 1960). The s u c c e e d i n g Molasse i s c h a r a c t e r i s t i c a l l y c o a r s e g r a i n e d , mainly non-marine, and r a n g e s i n a g e from l a t e Oligocene up late-Miocene - e a r l y P l i o c e n e and i s up t o 6000111 t h i c k .
The d e p o s i t s a r e mainly wedge-shaped; i n e a s t e r n S w i t z e r l a n d , f o r
example, t h e proximal Molasse i s a b o u t 4500111 b u t 60km t o t h e n o r t h t h i n s t o o n l y 1800m (FUchtbauer, 1967a).
Along t h e n o r t h e r n margin t h e Molasse i n c l u d e s d e b r i s
from c r a t o n i c s o u r c e s and i n t e r f i n g e r s w i t h marine d e p o s i t s o f t h e Rhine Graben and the Jura d i s t r i c t .
These n e a r sea level d e p o s i t s r e f l e c t a c t i v e u p l i f t of t h e
orogen, h i g h a n g l e f a u l t i n g o f t h e basement b e n e a t h t h e f o r e d e e p and t h e northward movement o f nappes t h a t began s e v e r a l m i l l i o n s o f y e a r s a f t e r t h e main orogeny from e a r l y Eocene t o e a r l y Oligocene ( C l a r k and J P g e r , 1969; Milne, 1969; HsU and Schlanger,l971). The n o r t h e r n A l p i n e Molasse comprises two major and f i v e minor coarsening-upwards megacycles s e v e r a l t o many hundreds o f metres t h i c k (FUchtbauer, 1967a) which r e f l e c t phases o f A l p i n e deformation ( F i g . 1.30). The Lower F r e s h w a t e r and Upper F r e s h w a t e r Molasse ( F i g . l . 3 0 ) a r e s e p a r a t e d by t h e mid-Miocene p a r a l i c t o s h a l l o w marine Upper Marine Molasse which r e f l e c t s reduced d e t r i t a l i n p u t and e x t e n s i v e marine f l o o d i n g o f t h e foredeep.
The r e g i o n a l palaeo-
d r a i n a g e p a t t e r n o f t h e Molasse i s t y p i c a l o f t h a t s e e n i n Cenozoic o r o g e n i c b e l t s l y i n g p a r a l l e l t o t h e a x i s o f t h e f o r e d e e p and t h e mountains.
The Lower Freshwater
Molasse g r a d e s e a s t w a r d s i n t o p a r a l i c and marine f a c i e s whereas t h e Upper Marine Molasse and s u c c e e d i n g Upper F r e s h w a t e r Molasse were t r a n s p o r t e d westwards ( F i g . l . 3 1 ) . The Lower Marine Molasse, which comprises t h e middle Oligocene tonmergel and p a r a l i c s a n d s t o n e w i t h i t s minor i n f l u x of conglomerate ( T r b p y , 1960; FUchtbauer, 1964), f i l l e d t h e f o r e d e e p e a r l y i n mid-Oligocene t i m e ; s e v e r a l hundred metres were d e p o s i t e d a t t h e narrower w e s t e r n end and o v e r a lOOOm i n t h e wider e a s t e r n end where s u b s i d e n c e was more a c t i v e (FUchtbauer, 1964).
The s u c c e e d i n g p a r a l i c s a n d s t o n e s
show s i m i l a r t h i c k n e s s v a r i a t i o n s r a n g i n g from a few t e n s o f metres i n t h e west t o s e v e r a l hundred metres t h i c k i n t h e e a s t .
The s a n d s t o n e s a r e q u a r t z o s e and c a l c -
l i t h i c a r e n i t e s d e r i v e d from t h e F l y s c h , P r e a l p s , and Northern Calcareous Alps. A s s o c i a t e d minor conglomerates a l s o i n d i c a t e c a l c a r e o u s s o u r c e a r e a s and r e f l e c t i n c i p i e n t u p l i f t and nappe emplacement of t h e l a t e A l p i n e o r o g e n i c phase. During l a t e Oligocene and e a r l y Miocene time t h e C e n t r a l Alps were a c t i v e l y u p l i f t e d and r e s u l t e d i n t h e accumulation o f Molasse i n t h e c e n t r a l and w e s t e r n p a r t s of t h e f o r e d e e p .
This began w i t h i n t e r b e d d e d non-marine s a n d s t o n e and mudstone and
w a s followed by t h e development of a f r i n g e o f e i g h t o r n i n e l a r g e a l l u v i a l f a n s
over lOOOm t h i c k ( F i g . l . 3 1 ) s u c c e s s i v e fining-upwards
(Fiichtbauer, 1964; G a s s e r , 1966, 1968).
These comprise
c y c l e s 10-20m t h i c k which i n c l u d e c o a r s e f a n g l o m e r a t e s ,
u s u a l l y s e v e r a l metres t h i c k .
These f a n g l o m e r a t e s grade l a t e r a l l y and d i s t a l l y i n t o
v a r i e g a t e d f l u v i a l fining-upwards c y c l e s , u s u a l l y no more t h a n a few metres t h i c k ,
42
-
Hercynian blocks Cenozoic mountain belts
0
Fig.1.29.
100
200
300 km.
The main t e c t o n i c e l e m e n t s of p a r t of w e s t e r n Europe.
and i n c l u d i n g t h i n beds o f l i g n i t e and f r e s h w a t e r l i m e s t o n e s ( B e r s i e r , 1945).
The
a l l u v i a l f a n s s u p p l i e d sediment t o t h e a x i a l r e g i o n of t h e b a s i n which wss t h e n t r a n s p o r t e d e a s t w a r d s t o t h e marine a r e a which p e r s i s t e d i n s o u t h e a s t e r n Germany and a d j a c e n t A u s t r i a (Lensch, 1961; S t e p h a n , 1965). U p l i f t and d e t r i t a l i n f l u x had waned by Mid-Miocene t i m e and t h e n o r t h e r n A l p i n e f o r e d e e p was i n u n d a t e d by marine c o n d i t i o n s and o n l y a few minor f l a n k i n g a l l u v i a l f a n s remained.
The r e s u l t i n g Upper Marine Molasse i s less t h a n lOOOm t h i c k b u t
e q u i v a l e n t d e p o s i t s a t t h e e a s t e r n end of t h e f o r e d e e p where s u b s i d e n c e was s t i l l g r e a t e r ( s c h l i e r m e r g e l ) a r e more t h a n 2OOm t h i c k .
I n t h e west t h e Upper Marine
Molasse c o n s i s t s of s h a l l o w m a r i n e t i d a l s a n d b o d i e s ( B r i e l , 1962; Van d e r Linden, 1963). The C e n t r a l and E a s t e r n Alps were u p l i f t e d i n l a t e Miocene-Early P l i o c e n e times and t h e H e l v e t i c nappes were t r a n s p o r t e d northwards o v e r i d i n g t h e f o r e d e e p by a
43 EBRO
-Pontian hrmatian Tortonian Helvetian Burdigalian E Aquitanian
-
-ChattianScampian - Ruplian N
Lnnoirsian-Lartorfian Ludian
Priabonian
Et
Lutetian
Y pratian-Cuitian
Fig.l.30. S t r a t i g r a p h i c d i s t r i b u t i o n o f t h i c k Cenozoic conglomerates i n t h e A l p i n e , A q i i i t a i n e and Ebro Basins. The r a n g e s o f conglomerates and o r o g e n i c p h a s e s depend on t h e a b s o l u t e time span a s s i g n e d t o Cenozoic epochs and on t h e assumption t h a t t h e A q u i t a n i a n and P o n t i a n S t a g e s a r e Miocene i n a g e 1. Main and l a t e A l p i n e orogeny (Milne, 1969). Arrow p o i n t e d l i n e s a r e S t i l l e ' s A l p i n e p h a s e s o f deformation ( R i c h t e r , 1927; B r e y e r , 1960) 2. P o s t - F l y s c h , coarsening-upward megacycles (Ftlchtbauer, 1967a) 3 . N o r t h Pyrenean o r o g e n i c p h a s e s ( S c h o e f f l e r , 1971) 4. South Pyrenean o r o g e n i c p h a s e s ( R i b a , 1955; S o l e r and P u i d e f a b r e g a s , 1970; Henry e t a l . , 1971) 5. Conglomeaates from c r a t o n i c b l o c k s ( F e r r e r e t a l . , 1968; G r o s s , 1968) 6. European Cenozoic S t a g e s ( a f t e r Van Houten, 1974). few t e n s of k i l o m e t r e s .
O l d e r proximal molasse d e p o s i t s were f o l d e d and f a u l t e d
b e n e a t h t h e nappes a s t h e a x i s of t h e f o r e d e e p was s h i f t e d t e n s o f k i l o m e t r e s t o t h e north.
The exposed Molasse was e v e n t u a l l y reduced t o l e s s t h a n h a l f i t s o r i g i n a l
width.
The Upper F r e s h w a t e r Molasse was d e p o s i t e d a l o n g t h e l e n g t h of t h e d i s p l a c e d
f o r e d e e p a s l a r g e a l l u v i a l f a n s d u r i n g T o r t o n i a n and P o n t i a n times,
Some of t h e s e
f a n s , such a s t h o s e a t Napf and H b r n l i a r e o v e r 1500m t h i c k ( M a t t e r , 1964, 1970). With i n c r e a s e d u p l i f t of t h e E a s t e r n Alps c l a s t s d e r i v e d from t h e Northern Calcareous Alps dominate t h e e a s t e r n f a n g l o m e r a t e s ( J a n o s c h e k , 1963) and t h e e a s t w a r d d i s p e r s a l p a t t e r n s e e n i n t h e Lower Molasse was now c o m p l e t e l y r e v e r s e d so t h a t e a s t e r n d e t r i t u s was now s p r e a d a c r o s s t h e J u r a D i s t r i c t ( F i g . 1 . 2 9 ) .
The a c c u m u l a t i o n of t h e Upper
F r e s h w a t e r Molasse p r o b a b l y c o n t i n u e d i n t o P l i o c e n e time f i l l i n g t h e f o r e d e e p t o o v e r lOOOm above s e a l e v e l ,
D i s t a l molasse was f o l d e d i n t h e J u r a s and much of
t h e l a t e r r e c o r d was d e s t r o y e d by deep e r o s i o n .
44
Fig.l.31. D i s t r i b u t i o n o f a l l u v i a l f a n s and d i s p e r s a l p a t t e r n s i n t h e n o r t h e r n A l p i n e f o r e d e e p d u r i n g a c c u m u l a t i o n o f Lower Freshwatel; ( C h a t t i a n - A q u i t a n i a n ) and Upper F r e s h w a t e r ( T o r t o n i a n P o n t i a n ) Molasse ( a f t e r FUchtbauer, 1967a). The molasse of t h e Canadian c o r d i l l e r a Molasse d e p o s i t i o n i n t h e Canadian c o r d i l l e r a shows some p a r a l l e l s w i t h t h e A l p i n e molasse.
I t c o n s i s t s of two o r o g e n i c b e l t s :
The P a c i f i c Orogen on t h e West
and t h e Columbian Orogen on t h e E a s t (Wheeler and G a b r i e l s e , 1972).
These two
b e l t s a r e s e p a r a t e d by t h e I n t e r m o n t a n e B e l t which i s c h a r a c t e r i z e d by v o l c a n i c and g r a n i t i c t e r r a i n u n c o n f o m a b l y o v e r l a i n by non-volcanic s u c c e s s o r b a s i n s of Middle J u r a s s i c t o e a r l y T e r t i a r y age.
The v a s t m a j o r i t y of t h e C o r d i l l e r a n m o l a s s e i s
r e l a t e d t o t h e Columbian Orogen and was l a i d down i n f o r e l a n d b a s i n s , i n t r a m o n t a n e v a l l e y s and e u g e o s y n c l i n a l s u c c e s s o r b a s i n s ( E i s b a c h e r e t a l . ,
1974).
The palaeo-
d r a i n a g e p a t t e r n and d e p o s i t i o n o f t h e m o l a s s e was c l o s e l y c o n t r o l l e d by a c t i v e l i n e a r t e c t o n i c elements.
The d i s t r i b u t i o n of m o l a s s e f a c i e s i n t h e Columbian
Orogen i s shown i n F i g . 1 . 3 2 .
A c e n t r a l zone of u p l i f t is f l a n k e d on t h e west by
F o r e l a n d Basins and on t h e e a s t by S u c c e s s o r B a s i n s ; r e l a t i v e t e c t o n i c t r a n s p o r t w i t h i n t h e s e two a r e a s were t o t h e e a s t a l o n g t h e e a s t e r n edge and t o t h e w e s t a l o n g t h e w e s t e r n edge; t h e c e n t r a l zone o f u p l i f t i s a l s o t h e r e f o r e a zone of s t r u c t u r a l divergence.
E a s t of t h e a x i s of t h i s zone t h e r o c k s of t h e Columbian Orogen c o n s i s t
mainly of deformed m i o g e o c l i n a l c a r b o n a t e - q u a r t z i t e s u c c e s s i o n s , whereas west o f t h e a x i s , high-grade metamorphic complexes a r e dominant.
Other major f e a t u r e s of t h e
Columbian Orogen a r e l o n g i t u d i n a l f a u l t z o n e s , well-known s i n c e McConnells' (1896) d e s c r i p t i o n , and now r e g a r d e d a s l o n g - l i v e d , r e p e a t e d l y r e a c t i v a t e d d e e p - c r u s t a l f r a c t u r e s (Monger e t a l . , ( F i g . 1.32).
1972) and a l s o s t r u c t u r a l r e - e n t r a n t s and s a l i e n t s
45
,
j; ...:;.::. :..... , .: :.. , ’.:.: .... .(. ./
. .
::.:, . . ..,
i. ,
Pml Reentrant
... ’.’ .;.;.. :. ... . ....
Molassa Flcies High grade maamorphicn
rm!Y km.
F i g . 1 . 3 2 . Main t e c t o n i c elements i n the Canadian C o r d i l l e r a and d i s t r i b u t i o n o f molasse f a c i e s i n s u c c e s s o r b a s i n s , intramontane v a l l e y s and f o r e l a n d b a s i n of the Columbian Omgen ( a f t e r Eisbacher e t a l . , 1974).
46
The s o u t h e r n F o r e l a n d b a s i n of t h e Columbian Orogen c o n t a i n s two major molasse sequences e a c h r e p r e s e n t i n g a p r o g r a d a t i o n o f c o a r s e g r a i n e d , non marine c l a s t i c s o v e r marine s h a l e s and s a n d s t o n e s (Fig.1.33).
The lower sequence i n c l u d e s t h e m a r i n e
I f T
'W l y River- Psrkapoo' Assemblago
'Alborta' Assemblage
'Kootenay- Bkirmorr' Aswrnblam
1
Fig.1.33. The f o u r l i t h o l o g i c a l assemblages o f t h e s o u t h e r n F o r e l a n d B a s i n of t h e Columbian Orogen ( a f t e r E i s b a c h e r e t a l . , 1974). Upper F e r n i e Assemblage and t h e non-marine Kootenay-Blairmore Assemblage.
The upper
sequence c o n t a i n s t h e m a r i n e A l b e r t a Assemblage and t h e non-marine B e l l y RiverPaskapoo Assemblage.
Only one molasse sequence i s p r e s e n t i n t h e n o r t h e r n f o r e l a n d
b a s i n and c o n s i s t s o f a n Upper C r e t a c e o u s t o Lower T e r t i a r y c o n t i n e n t a l c l a s t i c wedge o v e r l y i n g Lower C r e t a c e o u s m a r i n e c l a s t i c s . The Lower Molasse (Kootenay-Blairmore Assemblage) i n t h e Canadian C o r d i l l e r a conformably s u c c e e d s s l o p e d e p o s i t s i n t h e Upper p a r t o f t h e F e r n i e Formation; t h e Lower d e p o s i t s (Kootenay Formation) r e p r e s e n t d e l t a i c coal-swamp c o n d i t i o n s ( J a n s a ,
1972) and a r e followed b y a l l u v i a l s e d i m e n t s o f t h e Blairmore Group which c o v e r e d e x t e n s i v e a r e a s of t h e s o u t h e r n f o r e l a n d b a s i n and l i e w i t h e r o s i o n a l d i s c o n f o r m i t y on t h e o v e r l y i n g s e d i m e n t s , a l t h o u g h t h e s e a r e n o t f o l d e d ( N o r r i s , 1964).
I n the
v i c i n i t y of t h e Crowsnest r e - e n t r a n t t h e Lower Molasse i s 150Om t h i c k and h e r e r e d beds a r e developed i n t h e a l l u v i a l p l a i n sediments
(Mellon, 1967; H o l t e r and Mellon,
1972); t h e molasse t h i n s e a s t w a r d s and northwards g r a d i n g i n t o marine s h a l e s i n t h e north (Fig.1.32).
47
L
KOOTENAY
- BLAIRMORE MOLASSE
I BELLY RIVER-PASKAWO MOLASSE
F i g . 1 . 3 4 . I n f e r r e d palaeodrainage p a t t e r n s during the d e p o s i t i o n of KootenayBlairmore molasse (A) and B e l l y River-Paskapoo molasse (B) ( a f t e r Eisbacher e t a l . , 1974).
48 D e p o s i t i o n o f t h e Lower Molasse a l o n g t h e mountain f r o n t was accompanied by t h e n o r t h w e s t e r l y p r o g r a d a t i o n of f l u v i o - d e l t a i c s a n d b o d i e s from t h e e a s t e r n margin o f t h e b a s i n and t h i s i n d i c a t e s g e n e r a l emergence of t h e f o r e l a n d b a s i n .
These e a s t e r n
c l a s t i c s a r e t h e Athabasca O i l Sands (McMurray Formation) and were d e r i v e d from t h e c r y s t a l l i n e Precambrian s h i e l d ( C a r r i g y , 1966, 1967). s t r u c t i o n (Fig.1.34)
P a l a e o g e o g r a p h i c a l recon-
based on H o l t e r and Mellon (1972) and S t o t t (1972) shows t h a t
sediment t r a n s p o r t was t o t h e n o r t h a l o n g t h e a x i s o f t h e F o r e l a n d b a s i n w i t h l a t e r a l i n p u t of d e t r i t u s from l a r g e a l l u v i a l f a n s which developed i n t h e v i c i n i t y of s t r u c t u r a l r e - e n t r a n t s s u c h a s t h e Peace and Crowsnest. The d e p o s i t i o n o f t h e Lower Molasse c u l m i n a t e d i n a w i d e s p r e a d m a r i n e t r a n s g r e s s i o n d u r i n g A l b i a n time.
T h i s p r o g r e s s e d southwards from t h e n o r t h and r e s u l t e d i n t h e
d e p o s i t i o n o f a b o u t lOOOm o f f i n e g r a i n e d s h a l l o w marine c l a s t i c s which i n c l u d e t h e c o a s t a l sands o f t h e well-known Cardium Formation ( M i c h a e l i s and Dixon, 1969; S i n h a , 1970). The t r a n s i t i o n from t h i s m a r i n e A l b e r t a Assemblage t o t h e Upper Molasse ( B e l l y River-Paskapoo Assemblage) took p l a c e g r a d u a l l y d u r i n g t h e Campanian; a t t h e same t i m e t h e r e was s p o r a d i c m o l a s s e d e p o s i t i o n w i t h i n some o f t h e i n t r a m o n t a n e valleys.
The Upper Molasse i s a b o u t 3000m t h i c k and i s t h i c k e s t n e a r t h e Rocky
Mountain F r o n t n e a r t h e Crowsnest r e - e n t r a n t ; e a s t w a r d s i t t h i n s and i n t e r f i n g e r s w i t h marine d e p o s i t s .
Conglomerates a r e r a r e i n t h e Upper Molasse and o c c u r m a i n l y
i n t h e Eocene and O l i g o c e n e where t h e y c a n b e r e l a t e d t o t h e f i n a l morphogenic u p l i f t of t h e Rocky Mountain Belt.
The s u c c e s s i o n i s dominated by f l u v i a l and d e l t a i c
s a n d s t o n e s and s h a l e s , and i n t h e w e s t e r n margin o f t h e f o r e l a n d b a s i n a r e i n t e r bedded Upper C r e t a c e o u s v o l c a n i c s .
D e t a i l e d p e t r o l o g i c a l i n f o r m a t i o n i s g i v e n by
Lerbekmo (19631, Campbell and Lerbekmo (19631, Nelson (1968) and C a r r i g y (1971). The p a l a e o d r a i n a g e p a t t e r n o f t h e Upper Molasse i n t h e s o u t h e r n F o r e l a n d B a s i n was predominantly a x i a l and from t h e n o r t h ( F i g . 1 . 3 4 ) .
I n the northern Foreland
B a s i n , n e a r t h e P e e l r e - e n t r a n t a r e Upper C r e t a c e o u s t o T e r t i a r y e l a s t i c wedges 200Om t h i c k and e q u i v a l e n t t o t h e B e l l y River-Paskapoo Assemblage (Mountjoy, 1967; Young, 1971, Green, 1972).
These were d e r i v e d from deformed P r o t e r o z o i c and Palaeo-
z o i c s u c c e s s i o n s t o t h e west and p r o b a b l y t r a n s p o r t e d i n a n e a s t e r l y f l o w i n g d r a i n a g e network (Young, 1971). The f i n a l phase of molasse d e p o s i t i o n i n t h e f o r e l a n d b a s i n s o f t h e Columbian Orogen o c c u r r e d d u r i n g Eocene-Oligocene t i m e when f a u l t - c o n t r o l l e d i n t r a m o n t a n e b a s i n s r e s u l t e d i n t h e development o f c o a r s e g r a i n e d a l l u v i a l f a n s ( P r i c e , 1965; J o n e s , 1969).
These c o n g l o m e r a t e s r e s t w i t h unconformity on t h e B e l l y River-Paskapoo
c l a s t i c s ( C a r r i g y , 1971) and r e p r e s e n t t h e t r a n s i t i o n from l o n g i t u d i n a l t o s u p e r imposed t r a n s v e r s e d r a i n a g e , r e f l e c t i n g t h e f i n a l morphogenic u p l i f t of t h e Columbian Orogen ( B a l l y e t a l . , 1966). The most remarkable f e a t u r e o f t h e Upper Molasse i s t h a t t h e s o u t h e r n F o r e l a n d Basin r e c e i v e d much of i t s s e d i m e n t from t h e n o r t h e r n p a r t o f t h e Columbian Orogen. T h i s was p r o b a b l y due t o f a u l t i n g and u p l i f t i n t h e n o r t h e r n Rockies which t e r m i n a t e d
49
d e p o s i t i o n and i n i t i a t e d e r o s i o n i n t h a t a r e a .
This d i r e c t reversal of the a x i a l
d r a i n a g e d i r e c t i o n is a n a l o g o u s t o t h a t s e e n i n t h e A l p s , c o i n c i d e s w i t h e x t e n s i v e i n t r a m o n t a n e f a u l t i n g and a p p e a r s t o b e a c h a r a c t e r i s t i c f e a t u r e o f Cenozoic molasse The s i m i l a r i t y o f m o l a s s e b a s i n development i n two q u i t e
deposition (Fig.1.35).
d i s s i m i l a r o r o g e n i c b e l t s i s remarkable and may r e f l e c t two m a j o r p h a s e s of c r u s t a l s h o r t e n i n g a l o n g o p p o s i t e o r o g e n i c f l a n k s , e a c h p r o d u c i n g a major molasse c y c l e . The same phenomenon i s a l s o s e e n i n o l d e r m o l a s s e b a s i n s s u c h a s t h e Old Red Sandstone o f t h e Midland V a l l e y o f S c o t l a n d (Bluck, 1978).
S I
h
COLUMBIAN OROGEN
The two molasse megacycles of t h e s o u t h e r n Columbian Orogen and t h e Fig.1.35. n o r t h e r n Alps. The o n s e t of i n t r a m o n t a n e d e p o s i t i o n i s i n d i c a t e d by 'XI. The arrows i n d i c a t e t h e d i r e c t i o n o f sediment t r a n s p o r t a l o n g t h e b a s i n a x e s ( b a s e d on Schmidt-Thome, 1963 and S t o t t , 1 9 7 2 ) .
50
THE CLIMATIC SETTING OF ANCIENT CONTINENTAL RED BEDS
The c l i m a t i c s i g n i f i c a n c e o f a n c i e n t c o n t i n e n t a l r e d beds i s a c o n t r o v e r s i a l subject.
The o l d e s t c o n t i n e n t a l r e d beds i n t h e s t r a t i g r a p h i c r e c o r d a r e p r o b a b l y
t h o s e i n t h e Dharwar g r e e n s t o n e b e l t s of I n d i a which a r e a b o u t 2500 My o l d ( S r i n i v a s a n and S r e e n i v a s , 1972).
Red beds do n o t , however, become abundant i n
t h e s t r a t i g r a p h i c r e c o r d u n t i l a b o u t 1800-2000 My ago which has been t a k e n a s a n i n d i c a t i o n t h a t f r e e O2 began t o accumulate i n t h e atmosphere a b o u t t h i s time (Cloud, 1968). Red beds a r e c e r t a i n l y a n i n d i c a t i o n o f a n oxygenated atmosphere b u t t h e q u e s t i o n remains whether t h e y a r e a more s p e c i f i c p a l a e o c l i m a t i c i n d i c a t o r .
Traditionally,
r e d beds have been a s s o c i a t e d w i t h h o t , d r y c l i m a t e s and a r i d o r s e m i - a r i d c o n d i t i o n s . E a r l y a u t h o r s were much i m p r e s s e d w i t h t h e o c c u r r e n c e of r e d d e s e r t s e d i m e n t s (Dawson, 1848; Crosby, 1885; G o o d c h i l d , 1896) and r e d beds were s u b s e q u e n t l y associated with a h o t , dry climate.
I t was o n l y u n t i l r e l a t i v e l y r e c e n t l y t h a t
t h e o c c u r r e n c e o f r e d beds i n b o t h d r y and m o i s t t r o p i c a l c l i m a t e s has been s t r e s s e d . D i r e c t e v i d e n c e of t h e n a t u r e o f t h e c l i m a t e w i t h i n t h e d e p o s i t i o n a l b a s i n may b e p r o v i d e d by f a u n a l o r f l o r a l e v i d e n c e .
There may a l s o b e l i t h o l o g i c a l e v i d e n c e of
c l i m a t e and Walker (1974) has d i s t i n g u i s h e d between: a ) a d e s e r t - e v a p o r i t e r e d bed a s s o c i a t i o n i n which r e d beds a r e a s s o c i a t e d w i t h a e o l i a n s a n d s , d e s e r t f l u v i a l s e d i m e n t s and e v a p o r i t e s formed i n p l a y a l a k e s and i n l a n d sabkhas a n d , b ) a m o i s t c l i m a t e - r e d bed a s s o c i a t i o n i n which r e d beds a r e i n t e r b e d d e d and i n t e r f i n g e r with coal-bearing s t r a t a Although t h e s e two a s s o c i a t i o n s a r e c l i m a t i c a l l y q u i t e d i s t i n c t t h e y b o t h i n d i c a t e d e p o s i t i o n w i t h i n 30'
of t h e e q u a t o r and t h e q u e s t i o n a r i s e s whether r e d beds a r e
g e n e r a l l y i n d i c a t i v e of low p a l a e o l a t i t u d e s .
T h i s i s i m p o r t a n t f o r many r e d bed
f o r m a t i o n s which show no d i r e c t e v i d e n c e of t h e n a t u r e of t h e p r e v a i l i n g c l i m a t e i n the depositional basin. Many a u t h o r s have a t t e m p t e d t o make p a l a e o c l i m a t e a s s e s s m e n t on t h e b a s i s o f m i n e r a l o g i c a l composition and t e x t u r e .
T h i s m i n e r a l o g i c a l method i s aimed a t t h e
i d e n t i f i c a t i o n of s o u r c e a r e a composition and t h e n a t u r e o f t h e s o u r c e a r e a weathering.
The r e s u l t s c a n n o t , t h e r e f o r e , b e compared d i r e c t l y w i t h o t h e r p a l a e o c l i m a t e
i n d i c a t o r s because t h e r e could be important differences i n t h e climates of t h e source a r e a and t h e d e p o s i t i o n a l b a s i n .
There i s a l s o t h e a d d i t i o n a l d i f f i c u l t y of p o s t -
d e p o s i t i o n a l changes i n mineralogy and t e x t u r e which c o u l d modify any d i a g n o s t i c f e a t u r e s i n h e r i t e d from t h e s o u r c e a r e a . N e v e r t h e l e s s , t h e r e i s e v e r y i n d i c a t i o n t h a t a n c i e n t c o n t i n e n t a l r e d beds have b o t h a d i s t i n c t i v e d e p o s i t i o n a l and d i a g e n e t i c mineralogical s i g n a t u r e , although the q u a n t i t a t i v e s i g n i f i c a n c e of each i s n o t y e t fully established.
51 The s i g n i f i c a n c e of f i r s t - c y c l e a r k o s i c r e d b e d s There a r e many a n c i e n t examples o f r e d beds which w e r e d e r i v e d d i r e c t l y from c r y s t a l l i n e r o c k s o f a p p r o x i m a t e l y g r a n i t i c c o m p o s i t i o n and which a r e c o n s e q u e n t l y rich i n feldspar.
These s a n d s t o n e s a r e g e n e r a l l y r e f e r r e d t o a s ' a r k o s i c ' o r
' a r k o s e s ' a l t h o u g h t h e r e i s some c o n f u s i o n a s t o t h e e x a c t meaning of t h e term ( O r i e l , 1949).
I n h i s o r i g i n a l d e f i n i t i o n B r o n g n i a r t (1826) u s e d t h e term a r k o s e
t o d e n o t e a c o a r s e c l a s t i c r o c k w i t h abundant f e l d s p a r b u t d i d n o t s p e c i f y t h e e x a c t amount.
Most workers f o l l o w t h e usage o f P e t t i j o h n (1949) and r e s t r i c t t h e
term a r k o s e t o s a n d s t o n e s w i t h more t h a n 25% f e l d s p a r i n t h e framework c o n s t i t u e n t s a l t h o u g h h e l a t e r r e d e f i n e d t h e term a s a s a n d s t o n e w i t h 25% o r more f e l d s p a r r o c k fragments i n which t h e f e l d s p a r formed a t l e a s t h a l f .
+
R e c e m t usage of t h e
term ' a r k o s i c ' s u g g e s t s t h a t i t i s b e s t r e s t r i c t e d t o c o n t i n e n t a l d e p o s i t s s o a s t o
a v o i d c o n f u s i o n w i t h f e l d s p a t h i c greywackes ( s e e P e t t i j o h n , 1975, p.214) which form i n q u i t e a d i f f e r e n t manner.
The term ' f e l d s p a t h i c ' o r s u b a r k o s i c i f o f t e n used
t o d e n o t e thobe s a n d s t o n e s i n which t h e f e l d s p a r c o n t e n t i s between 10-25%. Although a r k o s e s may form by a v a r i e t y o f c l o s e l y r e l a t e d p r o c e s s e s
(Pettijohn,
1975 p.217) t h e y a r e g e n e r a l l y r e g a r d e d , b e c a u s e o f e a s y h y d r o l y s i s , t o r e q u i r e high r e l i e f , r a p i d e r o s i o n , t r a n s p o r t a t i o n , and b u r i a l b e f o r e d e c o m p o s i t i o n c a n t a k e p l a c e i n lowland s o i l s .
They t y p i c a l l y develop a s a l l u v i a l f a n and f l u v i a l d e p o s i t s
a t t h e margins o f t e c t o n i c a l l y a c t i v e fault-bounded g r a b e n s ,
Examples i n c l u d e t h e
L a t e Precambrian T o r r i d o n i a n Sandstone of NW S c o t l a n d ( S e l l e y , 1 9 6 6 ) , t h e Old Red Sandstone i n p a r t s o f S c o t l a n d (Mackie, 1899) t h e Pennsylvanian Lyons and F o u n t a i n Formations o f Colorado ( H u b e r t , 1 9 6 0 ) , t h e T r i a s s i c Newark S e r i e s of C o n n e c t i c u t and o t h e r e a s t e r n s t a t e s (Krynine, 1950) and p a r t s of t h e T e r t i a r y molasse of s o u t h e r n Germany and S w i t z e r l a n d ( G a s s e r , 1968).
The amount o f f e l d s p a r i n a r k o s i c r e d beds
t h u s a p p e a r s t o b e c o n t r o l l e d by t h r e e m a j o r f a c t o r s : a ) t h e proportion of f e l d s p a t h i c c r y s t a l l i n e rock i n t h e source area b ) t h e c l i m a t e w i t h i n t h e s o u r c e a r e a and d e p o s i t i o n a l b a s i n c ) t h e amount o f p o s t - d e p o s i t i o n a l d e s t r u c t i o n o f f e l d s p a r s by i n t r a s t r a t a l s o l u t i o n V a r i a t i o n i n one o r more of t h e s e f a c t o r s a c c o u n t s f o r t h e observed v a r i a t i o n i n t h e f e l d s p a r c o n t e n t o f a r k o s e s which i s c o n s i d e r a b l e ( s e e P e t t i j o h n , 1975 p.215). T r a d i t i o n a l l y a r k o s e s a r e t h o u g h t t o form when e i t h e r c l i m a t i c c o n d i t i o n s were too d r y o r t o o c o l d t o a l l o w much w e a t h e r i n g w i t h t h e r e s u l t t h a t t h e f e l d s p a r remained l a r g e l y undecomposed; o r t h e s o u r c e a r e a was u p l i f t e d and e r o d e d so r a p i d l y t h a t t h e r e was n o t s u f f i c i e n t t i m e f o r f e l d s p a r w e a t h e r i n g .
These a r e r e f e r r e d by Folk
(1968a) a s t h e C l i m a t i c Arkose and t h e T e c t o n i c Arkose r e s p e c t i v e l y .
I n practice,
c l i m a t i c a r k o s e s form on s t a b l e s h i e l d a r e a s and a r e m o s t l y s h a l l o w m a r i n e , beach o r dune d e p o s i t s and a r e r a r e i n n a t u r e . T e c t o n i c a r k o s e s form p r i m a r i l y i n b l o c k - f a u l t e d mountainous a r e a s a s a l l u v i a l f a n s o r piedmonts.
S i n c e t h e s e t e c t o n i c a l l y a c t i v e a r e a s a r e c h a r a c t e r i z e d by r a p i d
e r o s i o n and d e p o s i t i o n , and i f c r y s t a l l i n e r o c k s a r e exposed i n t h e s o u r c e a r e a , f e l d s p a r w i l l b e a b u n d a n t l y p r e s e r v e d and may comprise up t o 50% o f t h e framework constituents.
R e l a t i v e u p l i f t i n t h e s o u r c e a r e a s may b e o f t h e o r d e r o f 5000-8OOOm
and r e s u l t i n t h e exposure o f basement g r a n i t e s and g n e i s s e s .
Rates of u p l i f t a r e
d i s c u s s e d by Crowell ( 1 9 7 4 a , b ) and S t e e l e t a l . (1976). The combination o f rugged r e l i e f and r a p i d e r o s i o n means t h a t p r e d o m i n a n t l y f r e s h d e t r i t u s , i n c l u d i n g f e l d s p a r s , i s formed.
I n humid c l i m a t e s t h e r e may b e a t h i c k
weathered m a n t l e and s o i l i n t h e i n t e r f l u v i a l a r e a s and Krynine (1949) has shown how t h i s can e x p l a i n t h e m i x t u r e o f weathered and unweathered f e l d s p a r s o f t h e same species.
I n a r i d c l i m a t e s r e d s o i l s a r e l a r g e l y a b s e n t i n t h e s o u r c e a r e a s (Walker,
1967b) because t h e r e i s i n s u f f i c i e n t r a i n f a l l f o r t h e development of l a t o s o l s .
In
such a r e a s t h e d e t r i t u s i s n o t r e d , b u t g r e y , y e l l o w o r t a n i n c o l o u r , and o n l y reddens through s u b s e q u e n t b u r i a l and d i a g e n e s i s . I n a n c i e n t a r k o s i c sequences t h e r e i s o f t e n l i t t l e o r no e v i d e n c e of t h e c l i m a t e i n t h e s o u r c e and d e p o s i t i o n a l a r e a .
Of c o u r s e , t h e b e s t e v i d e n c e l i e s i n t h e
p r e s e n c e of fauna o r f l o r a o r s e d i m e n t a r y p a l a e o c l i m a t i c i n d i c a t o r s such a s a s s o c i a t e d e v a p o r i t e s , b u t f o r a v a r i e t y o f r e a s o n s t h e s e may b e a b s e n t . Sedimentary p e t r o l o g i s t s have l o n g been i n t e r e s t e d i n u s i n g t h e mineralogy o f a r k o s e s t o i n t e r p r e t p a l a e o c l i m a t i c c o n d i t i o n s a t t h e t i m e of d e p o s i t i o n .
Important
c l u e s may b e p r o v i d e d by t h e d e g r e e and n a t u r e of f e l d s p a r a l t e r a t i o n (Krynine, 1947; F o l k , 1 9 6 8 a ) p r o v i d e d t h a t t h e e f f e c t s due t o c l i m a t e and d e p o s i t i o n a l p r o c e s s e s can be i s o l a t e d from p r e w e a t h e r i n g e f f e c t s ( e . g . due t o d e u t e r i c o r hydrothermal a l t e r a t i o n ) and d i a g e n e t i c e f f e c t s ( e . g .
t h e i n t r a s t r a t a l d i s s o l u t i o n and r e p l a c e m e n t o f
f e l d s p a r by c l a y j . Krynine showed t h a t i m p o r t a n t c l u e s may b e p r o v i d e d by g r a i n r o u n d n e s s , which g i v e s some i n d i c a t i o n o f t h e r a t e of e r o s i o n and s o u r c e a r e a r e l i e f , t h e a v e r a g e d e g r e e of f e l d s p a r a l t e r a t i o n and a l s o t h e homogeneity o f f e l d s p a r a l t e r a t i o n , t h e homogeneity b e i n g t h e d e g r e e of w e a t h e r i n g between i n d i v i d u a l f e l d s p a r s p e c i e s . F o l k (1968a) h a s shown how f e l d s p a r a l t e r a t i o n m i g h t be i n t e r p r e t e d i n terms o f s o u r c e a r e a topography and c l i m a t e . I n humid c l i m a t e s w i t h rugged topography abundant a n g u l a r f e l d s p a r i s produced and g e n e r a l l y c o n s i s t s o f a m i x t u r e of f r e s h and weathered g r a i n s b e c a u s e o f t h e e r o s i o n of weathered m a n t l e and f r e s h bedrock.
I n a r i d climates fresh feldspar i s
produced, and well-rounded f r e s h f e l d s p a r s may b e a n i n d i c a t i o n of a n a r i d c l i m a t e and low topography (peneplane) c o n d i t i o n s . There have been a number o f s p e c i f i c a p p l i c a t i o n s of f e l d s p a r w e a t h e r i n g and a r k o s e mineralogy o f p a l a e o c l i m a t e i n t e r p r e t a t i o n .
Todd (1968) h a s shown t h a t under
f a v o u r a b l e c i r c u m s t a n c e s m i n e r a l o g i c a l e v i d e n c e can b e used t o p r o v i d e e v i d e n c e o f ancient climates.
H e i n v e s t i g a t e d t h e k a o l i n i t i c w e a t h e r i n g o f t h e Domengine
s a n d s t o n e , one o f t h e Eocene a r k o s i c sequences i n C a l i f o r n i a .
The method i n v o l v e d
i d e n t i f i c a t i o n of f e l d s p a r g r a i n s i n t h i n s e c t i o n u s i n g t h e u n i v e r s a l s t a g e
53
G r a i n s were i d e n t i f i e d a s o r t h o c l a s e ,
measurement o f a x i a l a n g l e s and o p t i c p l a n e s .
o r t h o c l a , s e m i c r o p e r t h i t e , p l a g i o c l a s e , p e r t h i t e and m i c r o c l i n e and i n a l l a t o t a l of 1050 g r a i n s were examined, o f which 19% were d e t e r m i n e d u s i n g t h e U-stage.
Todd
measured t h e g r a i n s i z e , g r a i n r o u n d n e s s , and t h e t y p e and d e g r e e of w e a t h e r i n g f o r each grain.
The d e g r e e of w e a t h e r i n g was d e t e r m i n e d by measuring t h e p e r c e n t a g e
a r e a of t h e g r a i n which showed v i s i b l e a l t e r a t i o n .
A w e a t h e r i n g i n d e x y = logz A
where A i s t h e p e r c e n t a g e v i s i b l e a r e a showing a l t e r a t i o n was c a l c u l a t e d and used 0-1 f r e s h , 1-2 s l i g h t l y w e a t h e r e d , 2-3 m o d e r a t e l y
with the following verbal scale: w e a t h e r e d , 3-4 s t r o n g l y weathered.
The r e s u l t s a r e s m a r i z e d i n T a b l e 1.1.
TABLE 1.1. T e x t u r a l , c o m p o s i t i o n a l and w e a t h e r i n g c h a r a c t e r i s t i c s of f e l d s p a r g r a i n s of t h e Domengine Sandstone (Eocene) Sacramento V a l l e y , C a l i f o r n i a ( a f t e r Todd, 1 9 6 8 ) . No. Grains
Average Mean Size 0
Average Mean Roundness
Composition Mean
Composition S.D.
Mean
S.D.
Y Y
459
1.88
2.67(SA)
Ab+An 33%
2.24
0.66
Orthoclase M i c r o p e r t h i t e 410
1.75
2.94(SA)
Ab+An 39%
2.44
0.74
Plagioclase
2.11
3.13(SR)
Orthoclase
100
An
37%
An 7.7%
1.50
0.78 ~
The r e s u l t s c l e a r l y show t h a t t h e a v e r a g e roundness of t h e t h r e e f e l d s p a r s p e c i e s
i s s i m i l a r b u t t h e p o t a s h f e l d s p a r i s somewhat c o a r s e r t h a n t h e p l a g i o c l a s e .
Rather
s u r p r i s i n g i y t h e o r t h o c l a s e m i c r o p e r t h i t e i s , on a v e r a g e , t h e most weathered o f t h e t h r e e f e l d s p a r s p e c i e s , c l o s e l y f o l l o w e d by o r t h o c l a s e , w i t h p l a g i o c l a s e t h e f r e s h e s t of a l l .
The w e a t h e r i n g of t h e f e l d s p a r s i s p r e d o m i n a n t l y k a o l i n i t i c ; 10% of t h e
g r a i n s shown s e r i c i t i c a l t e r a t i o n , and some t h e "bubbly" a l t e r a t i o n o f Folk (1955). T h i s sequence o f o r t h o c l a s e more weathered t h a n p l a g i o c l a s e ( a n d e s i n e ) i s o f c o u r s e c o n t r a r y t o t h e g e n e r a l l y a c c e p t e d sequence of m i n e r a l s t a b i l i t y . A p l o t of a v e r a g e s i z e of e a c h s p e c i e s o f f e l d s p a r from a number of t h i n s e c t i o n s , when p l o t t e d a g a i n s t a v e r a g e w e a t h e r i n g , shows a number of i n t e r e s t i n g t r e n d s . Although o r t h o c l a s e m i c r o p e r t h i t e i s o v e r a l l , t h e most w e a t h e r e d s p e c i e s , g r a i n s s m a l l e r t h a n 2.0
0
a r e fresher than the equivalent sized orthoclase.
c o n s i s t e n t w i t h t h e i r more a n g u l a r a p p e a r a n c e ,
This i s
The p l a g i o c l a s e w e a t h e r i n g t r e n d
p a r a l l e l s t h o s e o f t h e two a l k a l i f e l d s p a r s b u t i s c l e a r l y f r e s h e r t h r o u g h o u t , except f o r the very coarsest orthoclase grains.
The e s t i m a t e d t r e n d f o r each s p e c i e s
shows a tendency t o show i n c r e a s e d w e a t h e r i n g w i t h i n c r e a s e i n g r a i n s i z e up t o a p o i n t a t which same d e g r e e o f w e a t h e r i n g i s m a i n t a i n e d .
Again t h i s i s c o n t r a r y t o
t h e g e n e r a l l y a c c e p t e d b e l i e f which h o l d s t h a t t h e f i n e r g r a i n s b e c a u s e of t h e i r g r e a t e r s u r f a c e area/volume r a t i o s h o u l d normally show a h i g h e r d e g r e e of weathering.
54
3.0 3-5 2.5
1
0 ORTHOCLASE 0 ORTHOCLASE MICROPERTHITE
A PLAGIOCLASE
-
-
M%o 1.5
-
1.0
-
0.5
-
. .....
I
-1.0
I
I
I
1
0.0
1.0
2.0
3.0
1
-uo
Md R e l a t i o n s h i p between a v e r a g e d e g r e e o f w e a t h e r i n g (My)and a v e r a g e g r a i n Fig.1.36, s i z e (Md) o f f e l d s p a r s p e c i e s i n t h e Domengine Formation (Eocene) Sacramento V a l l e y , C a l i f o r n i a ( a f t e r Todd, 1968). Todd (1968) showed t h a t t h e s e d a t a c o u l d b e r e a d i l y e x p l a i n e d p r o v i d e d t h a t due c o n s i d e r a t i o n was p a i d t o t h e c r y s t a l l o g r a p h i c s t r u c t u r e of t h e i n d i v i d u a l f e l d s p a r s p e c i e s and a l s o t h e a c t i v i t y of d i s s o l v e d i o n s i n t h e s o i l w a t e r , i n c l u d i n g d i s t i n c t i o n between w e a t h e r i n g a n d l e a c h i n g r a t e .
The e x p e r i m e n t a l d a t a and t h e o r e t i c a l
c a l c u l a t i o n s of Hess (1966) show t h a t d u r i n g t h e w e a t h e r i n g of K-feldspar a t l e a s t f o u r d i f f e r e n t sequences o f m i n e r a l p h a s e s c o u l d a c h i e v e s t a b i l i t y depending upon
+ + concentration.
the log K /H
With d e c r e a s i n g s i l i c a t h e s e a r e :
K-feldspar-
montmorillonite+kaolinitej
K-feldspar+
kaolinite
gibbsite
+gibbsite
+ K-mica + g i b b s i t e + + decreases with decreasing
K-feldspar
and i f l o g K /H K-feldspar+
silica
K-mica j k a o l i n i t e 3 g i b b s i t e
For example, a h i g h w e a t h e r i n g r a t e t o g e t h e r w i t h a h i g h l e a c h i n g r a t e may r e s u l t i n r a p i d loss o f K+ from o r t h o c l a s e w i t h l i t t l e o r no development of secondary mica because o f t h e h i g h H+ c o n c e n t r a t i o n . of K
+ would
I f t h e l e a c h i n g r a t e were r e t a r d e d b u i l d up
r e s u l t i n t h e development of secondary mica on t h e f e l d s p a r s u r f a c e ,
A l b i t e , on t h e o t h e r hand, would be more s t a b l e t h a n K - f e l d s p a r i n a n environment
+ +
i n which t h e l o g Na +/ H + c o n c e n t r a t i o n was r e l a t i v e l y h i g h and t h e l o g K /H c o n c e n t r a t i o n was r e l a t i v e l y low.
The more h i g h l y w e a t h e r e d s t a t e of o r t h o c l a s e
55
m i c r o p e r t h i t e r e l a t i v e t o o r t h o c l a s e c a n b e e x p l a i n e d i n terms o f c r y s t a l l o g r a p h i c structure,
P e r t h i t e l a m e l l a e b o u n d a r i e s c a n b e c o n s i d e r e d a s a r r a y s of d i s l o c a t i o n s
(Weertman and Weertman, 1964) which a r e more s u s c e p t i b l e t o w e a t h e r i n g t h a n a r e a s with perfect crystal lattice.
As e x p e c t e d such a r e a s show e v i d e n c e of more e x t e n s i v e
w e a t h e r i n g (Todd, 1968 p.840). These m i n e r a l o g i c a l d a t a can b e t a k e n t o i n d i c a t e t h a t t h e w e a t h e r i n g of t h e Domengine s o u r c e r o c k s i n v o l v e d s l u g g i s h l e a c h i n g o f a l k a l i - r i c h p a r e n t r o c k s under low-relief subtropical conditions.
Todd (1968) a r g u e d t h a t t h e h i g h sodium-low
p o t a s s i u m a c t i v i t y i n s o i l w a t e r c o u l d b e b r o u g h t a b o u t by abundant non-seasonal r a i n f a l l and s u f f i c i e n t v e g e t a t i o n t o u t i l i z e much o f t h e p o t a s s i u m f o r p l a n t growth. S i n c e P l a n t s do n o t u s e sodium b u t r e q u i r e p o t a s s i u m i n abundance, t h e p r e s e n c e of low run-off and l u s h v e g e t a t i o n c a n a d e q u a t e l y e x p l a i n t h e h i g h sodium r e l a t i v e t o p o t a s s i u m a c t i v i t y i n t h e s o i l and t h e observed w e a t h e r i n g c h a r a c t e r i s t i c s of t h e feldspar species.
T o d d ' s h y p o t h e s i s i s s u p p o r t e d by i n d e p e n d e n t p a l a e o b o t a n i c a l
e v i d e n c e ( A x e l r o d , 1966) which shows t h a t t h e s o u r c e a r e a ( S o u t h e r n S i e r r a Nevada) a t t h i s time w i t h 700m peaks was covered on i t s w e s t e r n s l o p e s by b r o a d - l e a f e d e v e r g r e e n s which p r e f e r r e d a warm-temperate c l i m a t e w i t h f r o s t l e s s w i n t e r s w i t h a n a n n u a l t e m p e r a t u r e r a n g e of 15-22OC.
The c l i m a t e p r o b a b l y d i d n o t show marked
s e a s o n a l i t y and s o i l m o i s t u r e l e v e l s were p r o b a b l y r e a s o n a b l y c o n s t a n t w i t h a n a n n u a l p r e c i p i t a t i o n between 60 and 70 i n c h e s p e r y e a r . Todd (1968) a l s o made a n e x a m i n a t i o n of t h e Upper Miocene Lower Modelo s a n d s t o n e i n C a l i f o r n i a which was d e r i v e d from a v a r i e t y o f p l u t o n i c r o c k groups ( S u l l w o l d , 1960).
The o r d e r of f e l d s p a r s t a b i l i t y i s o r t h o c l a s e - m o r e - s t a b l e - t h a n
plagioclase
(An (50%) w i t h a mean w e a t h e r i n g i n d e x ( M y ) of 1 . 3 4 f o r o r t h o c l a s e and 1.56 f o r plagioclase.
Observed w e a t h e r i n g p r o d u c t s i n c l u d e p o t a s s i u m mica ( i l l i t e ) and l e s s e r
amounts of k a o l i n i t e .
These f e a t u r e s c o u l d s u g g e s t a more t e m p e r a t e c l i m a t e , a c t i v e
l e a c h i n g (needed t o produce t h e k a o l i n i t e ) b u t w i t h r e s t r i c t e d removal of potassium i o n s ( t o produce t h e i l l i t e ) .
Comparison o f t h e h a b i t a t s of t h e modern e q u i v a l e n t s
o f t h e S i e r r a Madrean f l o r a which dominated b o t a n i c a l a s s o c i a t i o n i n s o u t h e r n C a l i f o r n i a a t t h i s time ( l a t e Miocene) ( A x e l r o d , 1957) s u g g e s t s p r e c i p i t a t i o n of 75-87cm p e r y e a r and a mean a n n u a l t e m p e r a t u r e of 2OoC.
Thus p r e c i p i t a t i o n and
t e m p e r a t u r e were b o t h somewhat less i n t h i s s o u r c e a r e a t h a n t h a t of t h e Domengine Sandstone.
A s a whole t h e e v i d e n c e i s c o n s i s t e n t w i t h t h e change i n c l i m a t e i p
C a l i f o r n i a from a composition macroclimate i n E a r l y Eocene t o a n i n c r e a s i n g l y c o n t i n e n t a l one t h r o u g h o u t t h e Miocene a s r e l i e f i n c r e a s e d and c l i m a t i c b a r r i e r s became more i m p o r t a n t , The s i g n i f i c a n c e o f s i z e - c o m p o s i t i o n t r e n d s Another method of u s i n g m i n e r a l o g i c a l d a t a t o i n t e r p r e t p a l a e o c l i m a t e c o n d i t i o n s has been d e s c r i b e d by Young e t a l . (1975) and depends upon comparison w i t h s i z e -
56 composition p l o t s of framework g r a i n s i n f i r s t - c y c l e s a n d s t o n e s .
The main
d i f f e r e n c e s between s e m i - a r i d and humid c l i m a t e s a r e shown i n Fig.1.37)
r 60
t I
\
Fragments
Monocrystolline Ouortz
Feldspar
Polycr ysta I line Ouartz
t
Distinction
Total Pyroxene Amphibole. Micos, Heavier, Ooaqws
r
-
0
M
F
kh
L C
C
M
F
C
H
F
C
M
F
--
Grain Size
High-rank metomorphic Low-rank metomorphic Plutonic
Arid
Humid
V a r i a t i o n i n composition o f f i r s t c y c l e Holocene f l u v i a l s a n d from semiFig.1.37. a r i d and humid climates. Weathering i n s e m i - a r i d c l i m a t e s y i e l d s g r e a t e r amounts of r o c k f r a g m e n t s , f e l d s p a r and a c c e s s o r y m i n e r a l s . Weathering i n humid c l i m a t e s produces more p o l y c r y s t a l l i n e and m o n o c r y s t a l l i n e q u a r t z . The r e l a t i o n s h i p s h o l d r e g a r d l e s s of c r y s t a l l i n e s o u r c e rock-type ( a f t e r Mack and S u t t n e r , 1977). between s e m i - a r i d and humid p a l a e o c l i m a t e s may b e p o s s i b l e u s i n g t h i s method.
Mack
and S u t t n e r (1977) a p p l i e d t h e t e c h n i q u e i n a s t u d y o f t h e P e n n s y l v a n i a n F o u n t a i n Formation i n t h e Colorado F r o n t Range.
The F o u n t a i n Formation was d e p o s i t e d a s
a l l u v i a l f a n s which f l a n k e d t h e e a s t e r n s i d e o f t h e A n c e s t r a l R o c k i e s , a n a r e a o f c r y s t a l l i n e Precambrian Rocks s i t u a t e d n e a r t h e l o c a t i o n o f t h e p r e s e n t Colorado F r o n t Range ( T i e j e , 1923; K n i g h t , 1929; H u b e r t , 1960; Howard, 1966) ( F i g . 1.38). A number o f a u t h o r s i n c l u d i n g Wahlstrom (1948) and Hubert (1960) b e l i e v e d t h a t
t h e F o u n t a i n Formation was d e p o s i t e d under humid c l i m a t e c o n d i t i o n s whereas Raup (1966) and Mallory (1972) a r g u e d t h a t i t was d e p o s i t e d under a r i d t o s e m i - a r i d conditions. Mack and S u t t n e r (1977) have a p p l i e d t h i s t e c h n i q u e i n o r d e r t o compare t h e provenance and palaeogeography of t h e F o u n t a i n s a n d s t o n e s w i t h Holocene s a n d s i n t h e same a r e a .
Having e s t a b l i s h e d t h a t t h e Holocene s a n d s were d e r i v e d from t h e same
EXPLANATION
I I
p?)Alluvial fan
I
I Fig.1.38. Middle Pennsylvanian palaeogeography i n t h e Colorado a r e a b a s e d on Krumbein and S l o s s (1963) and D o t t and B a t t e n (1971). s o u r c e r o c k s ( s e e a l s o H u b e r t , 1960, and Howard, 1966) Mack and S u t t n e r (1977) then compensated f o r d i a g e n e t i c m o d i f i c a t i o n such a s f e l d s p a r and hornblende breakdown (Walker, 1967a, 1973) by s u b t r a c t i n g t h e modal amounts and n o r m a l i z i n g t h e r e s t of t h e framework g r a i n s t o 100%. Thus t h e n o r m a l i z e d p o p u l a t i o n s of q u a r t z g r a i n s and rock fragments i n t h e F o u n t a i n Formation and Holocene s a n d s c o u l d r e f l e c t t h e d i f f e r e n c e s ( i f a n y ) i n t h e e f f e c t s of w e a t h e r i n g a n d c l i m a t e i n t h e s o u r c e a r e a . I n a l l t h e s i z e f r a c t i o n s r o c k fragments a r e more abundant i n t h e Holocene sands than i n the Fountain sandstone (Fig.1.39).
Following t h e i n t e r p r e t a t i o n
s u g g e s t e d by Young e t a l . (1975) t h i s r e l a t i o n s h i p was t a k e n by Mack and S u t t n e r (1977) a s a n i n d i c a t i o n t h a t t h e F o u n t a i n Formation c l i m a t e was more humid t h a n t h e Holocene.
Rigorous chemical w e a t h e r i n g i n humid c l i m a t e s more r a p i d l y d e s t r o y s
l a b i l e m i n e r a l s i n r o c k fragments t h e r e b y b r i n g i n g a b o u t t h e i r r a p i d d e s t r u c t i o n . Humid c l i m a t e s a r e c h a r a c t e r i z e d by lower amounts of rock fragments i n a l l sand size classes. P o l y c r y s t a l l i n e q u a r t z fragments show e x a c t l y t h e o p p o s i t e t r e n d t o t h e rock fragments.
I n a l l t h e s i z e f r a c t i o n s p o l y c r y s t a l l i n e q u a r t z i s more abundant i n
t h e F o u n t a i n Sandstone t h a n i t i s i n t h e Holocene sand.
I n p a r t i c u l a r the coarse
s a n d f r a c t i o n shows a much h i g h e r p r o p o r t i o n o f p o l y c r y s t a l l i n e q u a r t z t h a n t h e Holocene sand which produces a much h i g h e r s l o p e i n t h e s i z e - c o m p o s i t i o n t r e n d , These p o l y c r y s t a l l i n e q u a r t z r e s u l t s a u p p o r t t h e humid-climate h y p o t h e s i s and t h e
58 s l o p e d i f f e r e n c e of t h e s i z e - c o m p o s i t i o n p l o t can be e x p l a i n e d by t h e p r e f e r e n t i a l mechanical d e s t r u c t i o n d u r i n g t r a n s p o r t a t i o n of l a b i l e m i n e r a l s on t h e edges o f p o l y m i n e r a l i c rock fragments i n t h e c o a r s e s a n d f r a c t i o n .
T h i s p r o c e s s would produce
a h i g h e r r a t i o of frequency p e r c e n t p o l y c r y s t a l l i n e q u a r t z i n t h e c o a r s e f r a c t i o n t o frequency p e r c e n t i n t h e f i n e f r a c t i o n i n a humid c l i m a t e .
I
PLUTONIC SOURCE
60 *O
1
Rock Fragments
10
0 5
C
M
F
100
Monocrys1.
EXPLANATION Colorado Springs, Colo. Fountain Fm. A Holocene A Boulder, Colo. Fountain Fm. 0 Holocene
-L C
40
2o O
M
C
M
F
F
.
METAMORPHIC SOURCE 100
Golden, Colo. Fountain Fm. 0 Holocene Loveland, Colo. Fountain Fm. 0 Holocene
40
20
0
C
M
F
C
M
F
C
M
F
Grain Size Fig.1.39. V a r i a t i o n i n c o m p o s i t i o n of Holocene f l u v i a l s a n d and s a n d s t o n e s from t h e F o u n t a i n Formation. Each p o i n t i s based on s e p a r a t e 200 p o i n t modal a n a l y s e s of t h e i n d i v i d u a l s i z e f r a c t i o n s ( a f t e r Mack and S u t t n e r , 1977). There i s l i t t l e marked d i f f e r e n c e i n t h e mean frequency p e r c e n t of m o n o c r y s t a l l i n e q u a r t z between t h e F o u n t a i n and Holocene sands ( F i g . 1 . 3 9 ) .
However, a l l s i z e f r a c t i o n s
of t h e F o u n t a i n s a n d s t o n e c o n t a i n more m o n o c r y s t a l l i n e q u a r t z than t h e Holocene s a n d
from t h e same a r e a .
This f e a t u r e i s t a k e n by Mack and S u t t n e r (1977) a s f u r t h e r
s u p p o r t f o r t h e i r argument t h a t t h e F o u n t a i n c l i m a t e was more humid t h a n t h e Holocene because Young e t a l . (1975) have shown t h a t more i n t e n s e chemical w e a t h e r i n g i n humid
c l i m a t e s r e l e a s e s more m o n o c r y s t a l l i n e q u a r t z t h a n i n a r i d o r s e m i - a r i d r e g i o n s . Mack and S u t t n e r (1977) i n t e r p r e t e d t h i s d a t a t o i n d i c a t e t h a t t h e c l i m a t e on t h e e a s t f l a n k of t h e F r o n t Range d u r i n g d e p o s i t i o n o f t h e F o u n t a i n Formation was more humid t h a n i t i s a t p r e s e n t .
T h i s d i f f e r s from t h e c o n c l u s i o n s o f Raup (1966) who
s u g g e s t e d on t h e b a s i s of c l a y m i n e r a l o g y t h a t t h e c l i m a t e was a r i d t o s e m i - a r i d a t t h e time.
Raup's e v i d e n c e was m a i n l y t h e abundance of i l l i t e and mixed l a y e r
i l l i t e - m o n t m o r i l l o n i t e and t h e o n l y minor amounts of k a o l i n i t e i n t h e F o u n t a i n Formation which s u g g e s t e d t o him i n c o m p l e t e l e a c h i n g i n a n a r i d o r s e m i - a r i d c l i m a t e ( c f . Todd, 1968).
The u s e of c l a y m i n e r a l assemblages i s however s e r i o u s l y i m p a i r e d
by d i a g e n e t i c m o d i f i c a t i o n and Walker (1973) h a s a r g u e d t h a t much o f t h e c l a v i n t h e F o u n t a i n Formation, i n c l u d i n g t h e i l l i t e and mixed l a y e r i l l i t e - m o n t m o r i l l o n i t e ,
i s of d i a g e n e t i c o r i g i n . A l s o Van Houten (1972) h a s shown t h a t c l a y m i n e r a l assemblages l i k e t h o s e d e s c r i b e d by Raup (1966) may a l s o b e found i n Recent a l l u v i u m d e r i v e d from c r y s t a l l i n e s o u r c e r o c k s and d e p o s i t e d i n a t r o p i c a l savanna c l i m a t e w i t h 30-40 i n c h e s of r a i d y e a r .
Raup a l s o n o t e d t h a t t h e c l a y m i n e r a l o g y of t h e
F o u n t a i n Formation was s i m i l a r t o t h a t o f t h e Minturn Formation, a F o u n t a i n e q u i v a l e n t on t h e w e s t f l a n k of t h e F r o n t Range which c o n t a i n s e v a p o r i t e s and s u g g e s t e d t h a t t h i s was f u r t h e r e v i d e n c e o f a n a r i d c l i m a t e d u r i n g t h e d e p o s i t i o n o f t h e F o u n t a i n Formation. T h i s paradox i s e a s i l y r e s o l v e d when t h e d e t a i l e d palaeogeography i s c o n s i d e r e d . C o n t i n e n t a l r e c o n s t r u c t i o n s f o r t h e L a t e C a r b o n i f e r o u s - E a r l y Permian ( D o t t and B a t t e n , 1971; Van d e r Voo and F r e n c h , 1974) show t h a t t h e A n c e s t r a l Rockies were within 1 0'
of l a t i t u d e of t h e contemporary e q u a t o r .
I n t h i s s i t u a t i o n , with the
A n c e s t r a l Rockies s i t u a t e d j u s t s o u t h o f t h e e q u a t o r , t h e p r e v a i l i n g e q u a t o r i a l e a s t e r l y winds on a s c e n d i n g t h e A n c e s t r a l Rockies would c a u s e l a r g e amounts of prec i p i t a t i o n on t h e e a s t e r n f l a n k , whereas p r e c i p i t a t i o n on t h e w e s t e r n f l a n k would b e low because of t h e r a i n shadow e f f e c t , t h e r e b y p r o d u c i n g c o n d i t i o n s c o n s i s t e n t w i t h e v a p o r i t e f o r m a t i o n i n t h e Minturn Formation.
I n d e p e n d e n t e v i d e n c e of t h e o r o g r a p h i c /
r a i n shadow i n t e r p r e t a t i o n i s d i s c u s s e d by Vaughn (1964, p.581-582).
I n north-
c e n t r a l Texas t h e f l o r a o f t h e Wolfcampian Lower W i c h i t a Group p r o v i d e s e v i d e n c e o f a humid c l i m a t e whereas Wolfcampian d e p o s i t s i n t h e Four Corners a r e a ( H a l g a i t o S h a l e ) r e f l e c t s d e p o s i t i o n under more a r i d c o n d i t i o n s (Baars', 1962). There a r e many e x c e l l e n t Recent examples of t h e o r o g r a p h i c / r a i n shadow e f f e c t i n t r o p i c a l l a t i t u d e s (Haunuitz and A u s t i n , 1944; Kendrew, 1 9 5 3 ) . Although t h e m i n e r a l o g i c a l d a t a i n t h e s t u d i e s of Todd (1968) and Mack and S u t t n e r (1977) e n a b l e p a l a e o c l i m a t i c i n t e r p r e t a t i o n s which a r e s u p p o r t e d by f a u n a l and f l o r a l e v i d e n c e l i m i t a t i o n s a r e imposed b e c a u s e of t h e d i a g e n e t i c d e s t r u c t i o n o f p o l y c r y s t a l l i n e q u a r t z and r o c k f r a g m e n t s (Walker, 1974; Walker e t a l . , 1978).
The s e l e c t i v e
d e s t r u c t i o n of l a b i l e m i n e r a l s d u r i n g d i a g e n e s i s and t h e r e p l a c e m e n t of i n t e r i o r g r a i n c o n t a c t s by a u t h i g e n i c c l a y may r e s u l t i n t h e d i s a g g r e g a t i o n o f rock fragments and p o l y c r y s t a l l i n e q u a r t z g r a i n s i n t o t h e i r d a u g h t e r m o n o c r y s t a l l i n e q u a r t z u n i t s .
60
Walker (1978) has p r e s e n t e d p e t r o g r a p h i c e v i d e n c e which i n d i c a t e s t h a t s u c h d i a g e n e t i c m o d i f i c a t i o n h a s been i m p o r t a n t i n t h e F o u n t a i n Formation b u t h e h a s n o t p r e s e n t e d any q u a n t i t a t i v e i n f o r m a t i o n t o s u p p o r t h i s c o n c l u s i o n t h a t t h e t e c h n i q u e d e s c r i b e d by Mack and S u t t n e r (1977) c a n n o t b e a p p l i e d t o t h e F o u n t a i n Formation. Such q u a n t i t a t i v e i n f o r m a t i o n on t h e d e g r e e of d e s t r u c t i o n d u r i n g d i a g e n e s i s may b e d i f f i c u l t o r even i m p o s s i b l e t o c o l l a t e b u t d e t a i l e d s t u d i e s a r e c l e a r l y needed i n t h i s f i e l d . There a r e s e v e r a l p o s s i b i l i t i e s . I n t h e f i r s t p l a c e some i n t e r n a l measure i s p o s s i b l e by comparing t h e p r o p o r t i o n s of framework g r a i n s . For example, i n t h e F o u n t a i n Formation t h e r e i s 1 b 2 h times more p o l y c r y s t a l l i n e q u a r t z t h a n i n Holocene Sands from t h e same s o u r c e a r e a
-
a f e a t u r e which s u g g e s t s t h a t d i a g e n e t i c
d i s a g g r e g a t i o n o f p o l y c r y s t a l l i n e g r a i n s c o u l d h a r d l y have been q u a n t i t a t i v e l y important.
Another method might b e t o compare t h e framework m i n e r a l o g y o f zones
which were r a p i d l y cemented and e s c a p e d e x t e n s i v e i n t r a s t r a t a l s o l u t i o n ( s e e Walker
e t a l . , 1978) w i t h uncemented zones which c o u l d have undergone more e x t e n s i v e i n t r a s t r a t a l solution.
The d i f f e r e n c e s would g i v e some i n d i c a t i o n o f t h e e x t e n t
t o which rock fragments and p o l y c r y s t a l l i n e q u a r t z g r a i n s had become d i s a g g r e g a t e d during diagenesis. THE CLASSIFICATION OF ANCIENT CONTINENTAL RED BEDS I n t r o d u c t i o n and p r e v i o u s c l a s s i f i c a t i o n s The p r e v i o u s d i s c u s s i o n s i n t h i s c h a p t e r i n d i c a t e t h a t c o n t i n e n t a l r e d beds o c c u r i n a wide r a n g e o f d i s t i n c t i v e t e c t o n i c s e t t i n g s .
These i n c l u d e f a i l e d c o n t i n e n t a l
r i f t s ( a u l a c o g e n s ) , s t r i k e - s l i p b a s i n s on t r a n s f o r m f a u l t s and a v a r i e t y of l a t e orogenic sedimentary basins.
W i t h i n t h e s e s e t t i n g s a wide r a n g e of c o n t i n e n t a l
sediments a r e deposited i n d e l t a i c , a l l u v i a l and a e o l i a n environments.
Red beds a r e
r e p r e s e n t e d i n a l l of t h e s e d e p o s i t i o n a l environments and t h e a v a i l a b l e e v i d e n c e s u g g e s t s t h a t t h e y may form i r r e s p e c t i v e o f t h e p r e v a i l i n g c l i m a t i c c o n d i t i o n s . D e s p i t e t h e s e c l e a r l y d e f i n e d d i f f e r e n c e s i n c o n t i n e n t a l r e d beds p r e v i o u s c l a s s i f i c a t i o n s have been based a l m o s t e x c l u s i v e l y on c o l o u r v a r i a t i o n s o r t h e g e n e t i c o r i g i n of pigmentary h a e m a t i t e .
T h i s i s u n s a t i s f a c t o r y b e c a u s e i t may
c o n f u s e r e d beds which a r e o f d i v e r s e t e c t o n i c o r s e d i m e n t a r y a s s o c i a t i o n s i m p l y because t h e y have s i m i l a r c o l o u r c h a r a c t e r i s t i c s .
A l s o , t h e u s e of t h e o r i g i n of
pigment a s a c l a s s i f i c a t i o n c r i t e r i o n i s u n s a t i s f a c t o r y b e c a u s e i t c a n h a r d l y l e a d t o mutually exclusive categories.
I t i s now g e n e r a l l y r e c o g n i z e d t h a t t h e v a s t
m a j o r i t y of pigmentary h a e m a t i t e i s o f d i a g e n e t i c o r i g i n ( t h e e x c e p t i o n s i n c l u d e some a e o l i a n s a n d s which c o u l d b e d e r i v e d from l a t e r i t i z e d a l l u v i a l s o u r c e r o c k s ) . T h i s view d o e s , of c o u r s e , c o n t r a s t markedly w i t h t h o s e of e a r l i e r workers. Krynine (1949) proposed a comprehensive c l a s s i f i c a t i o n of r e d beds which u t i l i z e d h i s own i d e a s on t h e o r i g i n o f i r o n o x i d e pigment i n r e d b e d s .
Krynine e n v i s a g e d
t h r e e p o s s i b l e means by which pigmentary o x i d e s c o u l d c o n t r i b u t e r e d c o l o u r a t i o n :
61 d e t r i t a l p a r t i c l e s , p r e c i p i t a t i o n from s o l u t i o n and t h e f o r m a t i o n of a u t h i g e n i c o x i d e s by i n s i t u o x i d a t i o n .
From t h e s e he developed a f o u r - f o l d c l a s s i f i c a t i o n
( F i g . 1.41) which he a d m i t t e d was "pragmatic" and " n o t e n t i r e l y l o g i c a l " (Krynine
RED SOILS
c. 0x1DATlON
I
Primary DETRITAL Continental
I-
.\ RED BEDS
\ Marine or bog / CHEMICAL PRECIPITATES
Fig.l.40. K r y n i n e ' s (1949) c l a s s i f i c a t i o n of r e d beds b a s e d on g e n e s i s and provenance o f f e r r i c o x i d e pigment. 1949, p . 6 0 ) .
These f o u r d i v i s i o n s may b e summarized a s f o l l o w s . I n t h i s c a t e g o r y r e d pigment was b e l i e v e d t o b e produced by
1) P r i m a r y r e d b e d s .
w e a t h e r i n g i n t h e s o u r c e a r e a and i n c o r p o r a t e d d i r e c t l y i n t o t h e d e p o s i t e d sediment e i t h e r w i t h i n t h e r e g o l i t h (Primary R e s i d u a l Red Beds) o r by l o c a l reworking, ( P r i m a r y Reworked Red Beds) o r a f t e r e r o s i o n and t r a n s p o r t (Primary D e t r i t a l Red Beds) 2) Post-depositional
a n d p o s t - d i a g e n e t i c r e d beds.
Produced by t h e o x i d a t i o n of
non-red d e t r i t u s e i t h e r immediately a f t e r d e p o s i t i o n ( P o s t - D e p o s t t i o n a l Red Beds) o r a f t e r b u r i a l , emergence and deep s u b - s u r f a c e o x i d a t i o n ( P o s t - D i a g e n e t i c Red Beds)
3) Secondary r e d beds.
Produced by t h e e r o s i o n a n d r e d e p o s i t i o n of p r e - e x i s t i n g r e d
beds. 4 ) Chemical r e d beds.
Produced by chemical p r e c i p i t a t i o n w i t h i n t h e d e p o s i t i o n a l
basin under 'marine' o r !freshwater' conditions, o r ' a u t h i g e n i c a l l y and i n t r a s t r a t a l l y
62
by i n f i l t r a t i o n w i t h i n t h e s e d i m e n t ' . U n f o r t u n a t e l y , a s i s t h e c a s e w i t h many c l a s s i f i c a t i o n schemes, K r y n i n e ' s c l a s s i f i c a t i o n of r e d beds h a s s u f f e r e d much a b u s e .
Many workers o n l y d i s t i n g u i s h
between ' p r i m a r y ' and ' s e c o n d a r y ' r e d beds and u s e t h e terms i n a d i f f e r e n t s e n s e from t h a t o r i g i n a l l y i n t e n d e d by Krynine.
Primary r e d beds i s t h e term o f t e n a p p l i e d
t o f a c i e s - c o n t r o l l e d r e d beds whether t h e y b e of d e t r i t a l o r d i a g e n e t i c o r i g i n ( i n t h e s t r i c t s e n s e t h e former c o u l d b e denoted p r i m a r y i f t h e y were d e r i v e d from weathered s o u r c e r o c k s w h i l s t t h e l a t t e r a r e p o s t - d e p o s i t i o n a l r e d beds i n K r y n i n e ' s S i m i l a r l y t h e term s e c o n d a r y r e d beds i s o f t e n used t o d e n o t e r e d
classification).
beds formed by i n s i t u w e a t h e r i n g of s e d i m e n t s a f t e r b u r i a l and emergence.
Such
r e d beds were termed p o s t - d i a g e n e t i c r e d beds by Krynine a n d t h e term s h o u l d b e r e s t r i c t e d t o i t s o r i g i n a l meaning t o d e n o t e t h o s e r e d beds formed by t h e e r o s i o n and r e d e p o s i t i o n of o t h e r r e d beds (e.g. Duchesne R i v e r Formation, Anderson and P i c a r d , 1974). The problem w i t h K r y n i n e ' s g e n e t i c c l a s s i f i c a t i o n i s t h a t i n t h e V a s t m a j o r i t y of To overcome t h i s C l a r k (1962) proposed a f i e l d
cases i t i s d i f f i c u l t t o apply.
c l a s s i f i c a t i o n i n which t h e c o l o u r was r e l a t e d t o s e d i m e n t a r y s t r u c t u r e s , t e x t u r e s and f o s s i l c o n t e n t .
He d i s t i n g u i s h e d t h e f o l l o w i n g t y p e s o f r e d b e d s :
1) Shale-pebble r e d .
B r i c k - r e d t o deep r e d i n c o l o u r t y p i c a l l y w i t h p e b b l e s of r e d
material i n a less-red matrix.
Red c o l o u r becomes d e e p e r towards t h e s o u r c e a r e a
and away from t h e s o u r c e a r e a t h e c o l o u r i s c o n c e n t r a t e d i n s h a l e beds. 2 ) Red c l a y conglomerates.
B r i c k r e d t o deep r e d i n c o l o u r and a s s o c i a t e d w i t h a
c r y s t a l l i n e igneous s o u r c e a r e a .
3) V a r i e g a t e d r e d . and s t r e a k s .
Shales generally purple t o red i n colour with greenish lenses
Sandstones w h i t e , l i g h t - g r e y o r g r e e n i s h when f r e s h , f r e q u e n t l y weath-
e r i n g t o d a r k brown.
C o n c r e t i o n a r y h o r i z o n s and v e r t e b r a t e f o s s i l s a r e o f t e n
abundant.
4 ) Cinnamon r e d .
S h a l e s deep r e d w i t h g r e e n i s h m o t t l i n g a n d abundant r i p p l e marks,
mud c r a c k s and v e r t e b r a t e t r a i l s .
S a n d s t o n e s dark-red t o l i g h t g r e y , u s u a l l y f i n e -
medium g r a i n e d and l a t e r a l l y p e r s i s t e n t .
5 ) Brick red. gypsum.
S i l t s t o n e s b r i c k r e d t o o r a n g e t o ochreous y e l l o w , sometimes w i t h
Sandstones l i g h t r e d t o l i g h t brown and o f t e n l e n s o i d .
6) P a s t e l red.
Laminated l a v e n d e r , y e l l o w , p u r e w h i t e ; l i t t l e r e d c o l o u r .
Mainly
f i n e grained, well-sorted quartzose sandstone. 7) Spattered red.
Red c o l o u r i n c o n c e n t r a t i o n s and i n complex r a m i f y i n g s t a i n s i n
a v a r i e t y of g r e e n i s h c o l o u r e d l i t h o l o g i e s . C l a r k (1962) i n t e r p r e t e d t h e s h a l e - p e b b l e , r e d c l a y conglomerate and v a r i e g a t e d r e d beds a s t y p e s of a l l u v i a l r e d beds formed under d i f f e r e n t climatic c o n d i t i o n s . Cinnamon r e d s and b r i c k r e d s were i n t e r p r e t e d a s e s t u a r i n e , d e l t a i c o r s h a l l o w marine r e d beds formed under d e s e r t c o n d i t i o n s and p a s t e l r e d s were c o n s i d e r e d t o b e o f lacustrine origin.
S p a t t e r e d r e d beds were b e l i e v e d by C l a r k t o b e t h e t y p i c a l
r e s u l t o f p o s t - d i a g e n e t i c w e a t h e r i n g and t h u s o f no e n v i r o n m e n t a l s i g n i f i c a n c e . Although t h i s c l a s s i f i c a t i o n scheme i s p r e f e r a b l e t o t h e p u r e l y g e n e t i c scheme o f Krynine i t h a s t h e major f a i l i n g of a t t a c h i n g t o o much s i g n i f i c a n c e t o t h e c o l o u r o f r e d beds.
We now know t h a t pigmentary o x i d e s may form i n r e d beds i n
a v a r i e t y o f ways and so r e d bed c l a s s i f i c a t i o n s b a s e d on c o l o u r o r t h e o r i g i n of colour a r e l i k e l y t o be misleading. Reddening and d e p o s i t i o n a l environment I n Recent s e d i m e n t s i r o n o x i d e i s most c o m o n l y p r e s e n t a s a tan-brown c o l o u r e d compound p r o b a b l y c o n s i s t i n g of amorphous Fe(0H)
and g o e t h i t e . F i n e g r a i n e d 3 h a e m a t i t e i s a p p a r e n t l y n o t p r e s e n t i n s u f f i c i e n t abundance t o i m p a r t a r e d c o l o u r t o t h e sediment (Walker, 1967b). The importance o f d e t r i t a l i r o n h y d r o x i d e s i n p r o d u c i n g r e d beds h a s been demonstrated i n a series o f p a p e r s by Van Houten (1961, 1964, 1 9 6 8 ) , p a r t i c u l a r l y i n t r o p i c a l savanna a l l u v i u m , which c o n t a i n s s u f f i c i e n t t a n o r brown c o l o u r e d i r o n hydroxide i n t h e s i l t and c l a y f r a c t i o n t o t u r n t h e s e d i m e n t r e d a f t e r i t s c o n v e r s i o n i n t o haematite. (1969a).
The f e a s i b i l i t y of s u c h a n ' a g e i n g ' p r o c e s s i s d e s c r i b e d by Berner
The e n r i c h m e n t o f i r o n h y d r o x i d e s i n t h e c l a y and s i l t f r a c t i o n of
f l u v i a t i l e r e d beds goes some way t o e x p l a i n i n g t h e f a c t t h a t r e d s i l t s t o h e s and mudstones g e n e r a l l y c o n t a i n much more i r o n t h a n a s s o c i a t e d s a n d s t o n e s .
The p o s t -
d e p o s i t i o n a l a g e i n g of i r o n h y d r o x i d e s i s l i k e l y t o b e i m p o r t a n t i n a n c i e n t f i n e g r a i n e d a l l u v i u m p a r t i c u l a r l y i n c o a s t a l a l l u v i a l p l a i n s w i t h well-developed f l o o d plain deposits.
I t i s a l s o o f c o n s i d e r a b l e i m p o r t a n c e i n t h o s e r e d beds which a r e
predominantly mudstones and o c c u r i n t e r d i g i t a t e d w i t h non-red c o a l and c o a l - b e a r i n g s t r a t a i n d e l t a i c a s s o c i a t e d sequences,
Ageing of d e t r i t a l hydroxides i s u n l i k e l y
t o b e a n i m p o r t a n t p r o c e s s i n a e o l i a n s a n d s t o n e s b e c a u s e of t h e winnowing of t h e finer fractions. The o t h e r major s o u r c e o f pigment i n r e d beds i s the i n t r a s t r a t a l breakdown of d e t r i t a l i r o n bearing minerals.
T h i s produces a number of a u t h i g e n i c m i n e r a l s ,
i n c l u d i n g pigmentary h a e m a t i t e , a n d has been most a b l y demonstrated by Walker (1967a, 1976) and Walker e t a l . (1978) i n a series of f i r s t c y c l e a r k o s e s d e p o s i t e d under d e s e r t c o n d i t i o n s d u r i n g t h e Cenozoic of t h e s o u t h w e s t e r n USA and n o r t h w e s t e r n Mexico.
F r e s h l y d e p o s i t e d a l l u v i u m c o n t a i n s no d e t r i t a l c l a y a l t h o u g h t h i s may be
mechanically i n f i l t r a t e d during e a r l y diagenesis.
I n t h e s e sediments reddening
p r o g r e s s e s w i t h time as d e t r i t a l f e r r o m a g n e s i a n s i l i c a t e s a r e a l t e r e d by i n t e r s t i t i a l groundwater.
Most o f t h e pigmentary h a e m a t i t e i s t h u s a u t h i g e n i c and many
a n c i e n t f i r s t - c y c l e a r k o s i c r e d beds were p r o b a b l y produced i n a s i m i l a r manner (e.g.
S c h l u g e r a n d Roberson, 1975; Walker and L a r s o n , 1976; Hubert and Reed, 1978).
64
The r e l a t i v e importance of a u t h i g e n i c h a e m a t i t e depends upon a number of f a c t o r s i n c l u d i n g t h e o v e r a l l t e c t o n i c s e t t i n g , c l i m a t e , s o u r c e a r e a , c o m p o s i t i o n and d e p o s i t i o n a l environment.
I t i s l i k e l y t h a t t h e r e a r e i m p o r t a n t amounts of a u t h -
i g e n i c h a e m a t i t e i n most r e d beds b u t i t i s most s p e c t a c u l a r l y developed i n f i r s t c y c l e d e s e r t a l l u v i u m d e r i v e d from c r y s t a l l i n e s o u r c e r o c k s .
Such a l l u v i u m may
o r i g i n a l l y be composed of t o t a l l y f r e s h , u n o x i d i z e d d e t r i t u s because o f r a p i d e o o s i o n and d e p o s i t i o n .
During s u b s e q u e n t b u r i a l h a e m a t i t e would b e produced by
i n t r a s t r a t a l a l t e r a t i o n and t h e pigment c o u l d be c o m p l e t e l y of a u t h i g e n i c o r i g i n . The d i a g e n e t i c model of r e d d e n i n g a l s o a p p e a r s , i n g e n e r a l , t o b e a p p l i c a b l e t o aeolian sands.
Most p r e s e n t - d a y a e o l i a n sands a r e n o t r e d i n c o l o u r b u t t h e
m a j o r i t y of a n c i e n t ones a r e , a f e a t u r e s u g g e s t i n g t h a t d i a g e n e t i c p r o c e s s e s p l a y a n i m p o r t a n t p a r t i n reddening.
There a r e , however, i m p o r t a n t e x c e p t i o n s i n t h a t
some p r e s e n t - d a y a e o l i a n s a n d s a r e d i s t i n c t l y r e d . F o l k (1976) h a s been a b l e t o d e m o n s t r a t e t h a t t h e r e d a e o l i a n s a n d s t o n e s i n t h e
Simpson Desert o f A u s t r a l i a a r e l a r g e l y d e r i v e d from T e r t i a r y f l u v i a l s e d i m e n t s which were p r e v i o u s l y reddened d u r i n g a p e r i o d of r e g i o n a l l a t e r i t i z a t i o n (Schmidt and Embleton, 1976).
Krynine (1949) had p r e v i o u s l y r e c o g n i z e d t h a t many o f t h e
Recent " r e d s a n d s o f t h e d e s e r t " were reworked r e d f l u v i a l s a n d s e.g. S a h a r a , where t h e y a r e d e r i v e d from r e d O l i g o c e n e f l u v i a l s a n d s .
i n the Northern
There i s e v i d e n c e
t h a t some a n c i e n t a l l u v i a l r e d beds may a l s o have been d e r i v e d from p r e v i o u s l y formed r e d beds ( e . g . Anderson and P i c a r d , 1974) b u t t h e s e u s u a l l y show e v i d e n c e of a d d i t i o n a l a u t h i g e n i c pigment. The pigment i n ' a n y p a r t i c u l a r r e d bed sequence may t h u s b e of d i v e r s e o r i g i n . Colour v a r i a t i o n s , o r t h e o r i g i n o f pigment, a r e t h e r e f o r e c o n s i d e r e d t o b e u n s a t i s f a c t o r y methods of c l a s s i f y i n g c o n t i n e n t a l r e d beds. Palaeomagnetic c l s s s i f i c a t i o n o f c o n t i n e n t a l r e d beds C o n t i n e n t a l r e d beds have been used i n p a l a e o m a g n e t i c s t u d i e s f o r many y e a r s and t h e i r magnetic p r o p e r t i e s have been i n v e s t i g a t e d i n d e t a i l ( C o l l i n s o n , 1965a, 1967, 1968b, 1974).
They g e n e r a l l y p o s s e s s a r e l a t i v e l y s t r o n g ( l o m 4 t o
n a t u r a l remanent m a g n e t i z a t i o n (W) which i s c a r r i e d by h a e m a t i t e .
G cm3 g-')
There h a s been
c o n s i d e r a b l e d e b a t e r e g a r d i n g t h e o r i g i n o f h a e m a t i t e i n r e d beds a s well a s t h e o r i g i n ot t h e m a g n e t i z a t i o n and i t s t i m i n g r e l a t i v e t o t h e d e p o s i t i o n a l a g e o f t h e sediments. Krynine (1949) b e l i e v e d t h a t r e d beds c o u l d form by d i r e c t d e r i v a t i o n o f h a e m a t i t e from r e d t r o p i c a l s o i l s ( l a t o s o l s ) .
I n t h i s c a s e ( i f i t were t r u e ) t h e m a g n e t i z a t i o n
o f t h e r e d beds would b e a d e t r i t a l remanent m a g n e t i z a t i o n (DRM) produced by t h e a l i g n m e n t of magnetic p a r t i c l e s d u r i n g s e d i m e n t a t i o n a n d i t would a c c u r a t e l y r e c o r d t h e ambient geomagnetic f i e l d a t t h e time o f d e p o s i t i o n .
Van Houten (1964) b e l i e v e d
t h a t t h e h a e m a t i t e c o u l d form d i a g e n e t i c a l l y by t h e i n s i t u d e h y d r a t i o n of y e l l o w
65 o r brown i r o n hydroxides d e r i v e d from d e e p l y weathered s o i l s b u t n o t n e c e s s a r i l y latosols.
T h i s c o n v e r s i o n i s thought t o t a k e p l a c e r a p i d l y i n t h e g e o l o g i c a l
s u b - s u r f a c e and i f i t d i d o c c u r would produce a chemical remanent m a g n e t i z a t i o n (CRM) by t h e growth o f h a e m a t i t e through i t s c r i t i c a l g r a i n s i z e .
I t would
thus r e c o r d a s h o r t - t e r m i n t e g r a t i o n of t h e ambient geomagnetic f i e l d , a l t h o u g h i t would have s u f f i c i e n t p r e c i s i o n t o be g e n e r a l l y a c c e p t a b l e f o r r o u t i n e palaeo-
magnetic purposes.
More r e c e n t l y , Walker (1976) has a r g u e d t h a t r e d beds i n both
t h e a r i d and warm m o i s t c l i m a t i c a s s o c i a t i o n s form due t o t h e a u t h i g e n e s i s of h e a m a t i t e which h a s formed d i a g e n e t i c a l l y owing t o t h e i n s i t u a l t e r a t i o n o f d e t r i t a l s i l i c a t e minerals.
S i n c e t h i s p r o c e s s c o n t i n u e s w i t h b u r i a l , any m a g n e t i z a t i o n of
t h e r e d bed would b e a CRM.
Moreover, i t would be a long-term i n t e g r a t i o n of t h e
ambient geomagnetic f i e l d , p o s s i b l y c o n t a i n i n g multicomponent m a g n e t i z a t i o n s , and a s s u c h would have l i t t l e o r no palaeomagnetic v a l u e .
Walker has p r e s e n t e d convincing
evidence i n s u p p o r t o f h i s h y p o t h e s i s b o t h f o r t h e T e r t i a r y a r k o s i c sediments of Baja C s l i f o r n i a and a l s o t h e a l l u v i a l d e p o s i t s o f Holocene and P l e i s t o c e n e age i n P u e r t o Rico and t h e Orinoco Basin, The g e o l o g i c a l evidence i n f a v o u r of t h e d i a g e n e t i c o r i g i n o f h a e m a t i t e i n r e d beds i s convincing; a v a r i e t y o f d i a g e n e t i c p r o c e s s e s i n c l u d i n g pigment p r o d u c t i o n , t h e pseudomorphing o f s i l i c a t e s and t h e o x i d a t i o n o f m a g n e t i t e t a k e p l a c e a t v a r i o u s
times throughout t h e d i a g e n e t i c h i s t o r y o f c o n t i n e n t a l r e d beds.
The paleeomagnetic
e v i d e n c e i n d i c a t i n g t h a t h a e m a t i t e c a r r i e s t h e NRM of c o n t i n e n t a l r e d beds i s e q u a l l y compelling and i t must be concluded t h a t t h e m a g n e t i z a t i o n of r e d beds i s a c q u i r e d during diagenesis.
However, t h e magnetic c h a r a c t e r i s t i c s o f r e d beds a r e extremely
complex and g e n e r a l i z a t i o n s r e g a r d i n g t h e t i m i n g o f m a g n e t i z a t i o n i n a i l r e a Deas s h o u l d n o t be made a s t h e y can o n l y be m i s l e a d i n g .
Three b r o a d t y p e s o f r e d bed
m a g n e t i z a t i o n have been r e c o g n i z e d by Turner (1979) which r e f l e c t p r o g r e s s i v e degrees of d i a g e n e t i c a l t e r a t i o n .
These i n c l u d e :
Type A m a g n e t i z a t i o n s which c o n t a i n
e s s e n t i a l l y a s i n g l e component of m a g n e t i z a t i o n and may show d i s c r e t e zones of normal and r e v e r s e d p o l a r i t y .
E a r l y Mesozoic sandy a l l u v i u m a f f o r d s p a r t i c u l a r l y good
examples o f t h i s t y p e o f m a g n e t i z a t i o n ( H e l s l e y and S t e i n e r , 1974; Turner and I x e r , 1977).
Type B m a g n e t i z a t i o n s a r e t y p i c a l l y composite o r multicomponent and were
a c q u i r e d o v e r a l o n g e r t i m e i n t e r v a l , spanning a t l e a s t one geomagnetic f i e l d reversal.
They t h e r e f o r e r e p r e s e n t a more advanced s t a g e of d i a g e n e s i s t h a n t h a t
r e p r e s e n t e d by Type A m a g n e t i z a t i o n s .
Examples a r e most commonly found i n t h e
Upper P a l a e o z o i c ( e . g . Roy and P a r k , 1974; Turner and A r c h e r , 1975).
Type C
m a g n e t i z a t i o n s a r e t h o s e which can be demonstrated t o be complete replacements o f t h e o r i g i n a l m a g n e t i z a t i o n and t h e r e f o r e b e a r no r e l a t i o n s h i p t o t h e d e p o s i t i o n a l a g e of t h e r o c k s .
They a r e most commonly found i n v e r y a n c i e n t r e d beds which have
suffered extensive diagenetic alterations. Precambrian ( e . g . Lomax and B r i d e n , 1977).
A number of examples a r e known from t h e
These t h r e e t y p e s o f m a g n e t i z a t i o n r e p r e s e n t s t a g e s i n t h e c o n t i n u o u s d i a g e n e t i c m o d i f i c a t i o n of c o n t i n e n t a l r e d b e d s .
The p a l a e o m a g n e t i c c h a r a c t e r i s t i c s o f r e d
beds p r o v i d e s a s e n s i t i v e means of d e t e c t i n g d i a g e n e t i c changes and t h u s e n a b l e s t h e d i s t i n c t i o n between r e d beds which have s u f f e r e d d i f f e r e n t d e g r e e s o f d i a g e n e s i s . Many f a c t o r s i n c l u d i n g a g e , m i n e r a l o g i c a l c o m p o s i t i o n and t e x t u r a l p r o p e r t i e s a f f e c t t h e r a t e and e x t e n t of d i a g e n e s i s .
These p r o p e r t i e s a r e , however, d i f f i c u l t t o
monitor and t h e p a l a e o m a g n e t i c t e c h n i q u e a l l o w s a r e l a t i v e l y s i m p l e means of c l a s s i f y i n g them i n t o meaningful g r o u p s , A p r a c t i c a l c l a s s i f i c a t i o n of c o n t i n e n t a l r e d beds
The more r e a l i s t i c approach t o t h e c l a s s i f i c a t i o n of r e d beds must t a k e i n t o a c c o u n t a l l s i g n i f i c a n t c h a r a c t e r i s t i c s i n c l u d i n g d e p o s i t i o n a l , t e x t u r a l and m i n e r a l o g i c a l p r o p e r t i e s , and d i a g e n e t i c f e a t u r e s .
The i n i t i a l d e p o s i t i o n a l
environment i s i m p o r t a n t i n c o n t r o l l i n g t h e s u b s e q u e n t d i a g e n e t i c h i s t o r y .
The
main d e p o s i t i o n a l environment i n which c o n t i n e n t a l r e d beds a c c u m u l a t e , t o g e t h e r w i t h t h e i r main m i n e r a l o g i c a l , t e x t u r a l and d i a g e n e t i c p r o p e r t i e s a r e s u f f i c i e n t l y d i s t i n c t t o e n a b l e them t o b e c l a s s i f i e d i n t h e manner shown i n 7 a b l e 1.2. major a s s o c i a t i o n s of c o n t i n e n t a l r e d beds a r e r e c o g n i z e d :
Three
a l l u v i a l , d e s e r t and
d e l t a p l a i n r e d beds which a r e r e a d i l y d i s t i n g u i s h e d on t h e b a s i s o f t h e i r characteristic facies. A l l u v i a l r e d beds comprise a wide range of d e p o s i t i o n a l f a c i e s r a n g i n g from marginal a l l u v i a l fans and coarse grained braided r i v e r s t o a l l u v i a l p l a i n s with l o w - s i n u o s i t y o r meandering r i v e r s .
A l l u v i a l f a n s and c o a r s e g r a i n e d b r a i d e d r i v e r s
t y p i c a l l y develop i n t e c t o n i c a l l y a c t i v e s e d i m e n t a r y b a s i n s and a r e a p a r t i c u l a r f e a t u r e of s t r i k e - s l i p b a s i n s developed on t r a n s f o r m f a u l t s .
They a l s o o c c u r a s a
m a r g i n a l f a c i e s w i t h i n f a i l e d c o n t i n e n t a l r i f t s and i n a v a r i e t y of l a t e - o r o g e n i c basins.
C l i m a t i c a l l y , a l l u v i a l r e d beds have no s p e c i a l s i g n i f i c a n c e a l t h o u g h
palaeomagnetic e v i d e n c e does s u g g e s t t h a t t h e y may have been d e p o s i t e d w i t h i n 20'-40'
of t h e p a l a e o - e q u a t o r , a f e a t u r e which s u g g e s t s t h a t t h e y do n o t
r e a d i l y form a t h i g h p a l a e o l a t i t u d e s i n c o l d c l i m a t e s .
There a r e no p r e s e n t -
day a l l u v i a l r e d beds and t h e a n c i e n t o c c u r r e n c e s a r e c o n s i d e r e d t o b e a l m o s t e x c l u s i v e l y of d i a g e n e t i c o r i g i n .
I n many examples t h e i n t r a s t r a t a l a l t e r a t i o n o f
d e t r i t a l i r o n - s i l i c a t e s h a s been c o n v i n c i n g l y demonstrated and t h e r e can be l i t t l e doubt of t h e a u t h i g e n i c n a t u r e of t h e pigmentary i r o n o x i d e s .
I n t r a s t r a t a l solution
i s , however, most i m p o r t a n t i n c o a r s e g r a i n e d a l l u v i u m i n which t h e r e i s l i t t l e
i n t e r s t i t i a l c l a y and s i l t y m a t r i x .
Such a l l u v i u m i s e f f e c t i v e l y a p o r o u s , open
system through which o x i d i z i n g groundwater can p a s s a s i t a l t e r s t h e framework mineralogy.
I n f i n e r g r a i n e d a l l u v i u m such a s t h e f l o o d p l a i n s o f meandering r i v e r s
t h e emphasis is s l i g h t l y d i f f e r e n t .
Here t h e s e d i m e n t s c o n t a i n abundant c l a y and
s i l t which c o n t a i n s d e t r i t a l f e r r i c h y d r o x i d e s .
Dehydration of such m a t e r i a l c o u l d
TABLE 1.2. The c l a s s i f i c a t i o n o f c o n t i n e n t a l r e d beds i n t o t h r e e major a s s o c i a t i o n s based on d e p o s i t i o n a l environment, colour c h a r a c t e r i s t i c s and d i a g e n e t i c f e a t u r e s . Depositional C h a r a c t e r i s t i c s
Redness C h a r a c t e r i s t i c s
Diagenetic C h a r a c t e r i s t i c s
A l l u v i a l Fan-Braided R i v e r s
Pebbly a l l w i u m Mostly m u l t i s t o r e y channel deposits interbedded debrisflow and stream Flood deposits
Uniformly red. F i n e r g r a i n e d horizons more i n t e n s e l y red
I n t r a s t r a t a l s o l u t i o n of s i l i c a t e grains. Increase i n mineralogical maturity. Decrease i n t e x t u r a l m a t u r i t y Authigenic q u a r t z , f e l d s p a r , c a l c i t e , clays
A l l u v i a l P l a i n s w i t h high o r low-sinuosity streams
Sandy and muddy alluvium i n FU c y c l e s . Sand u n i t s a r e channel and bank d e p o s i t s . Mudstones a r e f l o o d p l a i n deposits. Pedogenic modifi c a t i o n of f l o o d p l a i n d e p o s i t s common
Variegated. Sandy u n i t s r e d o r drab. Mudstone u n i t s r e d o r variegated
A s above. Also i n v e r s i o n of f e r r i c hydroxides - haematite i n muddy f l o o d p l a i n d e p o s i t s . C a l c r e t e a common f e a t u r e of floodplain deposits
DESERT RED BEDS
Cross s t r a t i f i e d w e l l - s o r t e d sands w i t h s t e e p f o r e s e t . i n c l i n a t i o n s . Interbedded w i t h i n l a n d sabkha and d e s e r t l a k e (gypsum and a n h y d r i t e ) and poorly-sorted wadi deposits .
Uniformly r e d
I n t r a s t r a t a l s o l u t i o n of f e l d s p a r s and ferromagnesian m i n e r a l s . Authigenic q u a r t z and f e l d s p a r . Gypsuma n h y d r i t e cements
CU and FU c y c l e s . S i l t s t o n e s frequently associated with s e a t e a r t h s and o t h e r pedogenic m o d i f i c a t i o n s . Mainly floodp l a i n , well-drained swamp and lacustrine delta deposits
Variegated. Red beds u s u a l l y confined t o A mottled mudstones. appearance i s common
Early diagenetic pyrite. Formation of k a o l i n i t e by humid a c i d leaching. S i d e r i t e . Pedogenic nodules
ALLUVIAL RED BEDS
Alluvial, aeolian sabkhas, d e s e r t l a k e s
DELTA PLAIN RED BEDS D e l t a p l a i n s of river-dominated deltas
produce s i g n i f i c a n t q u a n t i t i e s o f h a e m a t i t e and t h u s c o n t r i b u t e d i r e c t l y t o t h e r e d d e n i n g of t h e sediment. Desert r e d beds i n c l u d e a wide range of f a c i e s d e p o s i t e d i n a h o t , a r i d c l i m a t e .
They i n c l u d e t h e d e p o s i t s o f l a k e s , i n l a n d s a b k h a s , a e o l i a n dunes, and a v a r i e t y of d e s e r t a l l u v i a l sediments. on s t a b l e c r a t o n i c a r e a s . occur.
Desert b a s i n s may b e t e c t o n i c a l l y a c t i v e o r developed
I t i s t h e l a t t e r on which t h e l a r g e s a n d s e a s o f t h e world
Desert a l l u v i a l s e d i m e n t s a r e e s s e n t i a l l y t h e same a s t h e i r c o a r s e g r a i n e d
counterparts i n the general a l l u v i a l category.
They d i f f e r i n b e i n g l a r g e l y ephemeral,
c o n s i s t i n g o f p o o r l y s o r t e d d e t r i t u s which may b e i n t e r b e d d e d w i t h a e o l i a n s a n d s . The r e d d e n i n g o f c o a r s e g r a i n e d d e s e r t a l l u v i u m t a k e s p l a c e by t h e i n t r a s t r a t a l a l t e r a t i o n of i r o n s i l i c a t e s . A e o l i a n s a n d s a r e w i d e l y r e g a r d e d a s one of t h e most d i s t i n c t i v e f a c i e s of c o n t i n e n t a l r e d beds a l t h o u g h t h e y a r e s t i l l p o o r l y u n d e r s t o o d .
They may be more
d i f f i c u l t t o recognize i n t h e s t r a t i g r a p h i c record than i s generally appreciated and i t i s n o t p o s s i b l e t o make g e n e r a l i z a t i o n s a b o u t t h e o r i g i n of t h e i r r e d colouration.
P r e s e n t - d a y a e o l i a n s a n d s e x i s t i n v a r i o u s c o l o u r s t a t e s from r e d
t o d r a b and t h e r e i s e v i d e n c e t h a t b o t h d e t r i t a l a n d d i a g e n e t i c r e d d e n i n g may be important processes. The l a s t r e d bed f a c i e s a s s o c i a t i o n which i s r e c o g n i z e d a r e d e l t a p l a i n r e d beds. These a r e d e p o s i t e d i n d e l t a i c complexes a t c o n t i n e n t a l margins and i n a v a r i e t y o f tectonic settings.
A s t h e d e p o s i t s of f l u v i a l d i s t r i b u t a r y c h a n n e l s and f l o o d -
p l a i n s t h e y b e a r many c h a r a c t e r i s t i c s i n common w i t h some a l l u v i a l r e d beds.
They
a r e d i s t i n g u i s h e d by t h e i r a s s o c i a t i o n w i t h d e l t a i c s e d i m e n t s and a number of distinctive features.
The l a t t e r i n c l u d e a n abundance o f c o l o u r m o t t l e d mudstones,
r o o t l e t h o r i z o n s , s e a t e a r t h s and o c c a s i o n a l c o a l h o r i z o n s .
These f e a t u r e s
i n d i c a t e a m o i s t t r o p i c a l climate a n d many of t h e d e s c r i b e d a n c i e n t d e l t a p l a i n r e d beds were d e p o s i t e d under t h e s e c o n d i t i o n s .
I t i s , however, l i k e l y t h a t d e l t a
p l a i n r e d beds a l s o formed under o t h e r c l i m a t i c c o n d i t i o n s and have been d e s c r i b e d from a r i d c l i m a t e s and t r o p i c a l c l i m a t e s whth s e a s o n a l a r i d i t y .
They may b e
i n d i c a t i v e o f r e l a t i v e l y low p a l a e o l a t i t u d e s . The r e d d e n i n g p r o c e s s e s i n d e l t a p l a i n r e d beds c o n t r a s t w i t h t h o s e o f t h e a l l u v i a l and d e s e r t a s s o c i a t i o n s .
Pigmentary o x i d e s may form by i n t r a s t r a t a l s o l u t i o n
o f d e t r i t a l i r o n s i l i c a t e s b u t t h e predominance o f mudstone u n i t s means t h a t a number o f o t h e r p r o c e s s e s may b e more i m p o r t a n t .
These i n c l u d e t h e d e h y d r a t i o n o f g o e t h i t e ,
t h e r e l e a s e of o x i d e s d u r i n g c l a y m i n e r a l t r a n s f o r m a t i o n s and t h e o x i d a t i o n of f i n e l y disseminated p y r i t e .
Pedogenic p r o c e s s e s p l a y a n i m p o r t a n t r o l e i n t h e p o s t -
depositional reddening of d e l t a p l a i n sediments.
69
CHAPTER 2 DESERT RED BEDS INTRODUCTION
Desert r e d beds comprise a wide v a r i e t y of s e d i m e n t a r y f a c i e s d e p o s i t e d under a r i d and s e m i - a r i d c o n d i t i o n s i n a range of d e p o s i t i o n a l environments.
They i n c l u d e
a l l u v i a l f a n s , d e s e r t f l u v i a l (wadi) d e p o s i t s , a e o l i a n s a n d s and t h e d e p o s i t s of d e s e r t l a k e s and i n l a n d sabkhas w i t h t h e i r a s s o c i a t e d e v a p o r i t e s .
Red c o l o u r a t i o n
i s a u b i q u i t o u s f e a t u r e o f a n c i e n t d e s e r t s e d i m e n t s and many e a r l y workers b e l i e v e d t h a t a l l r e d beds were formed under d e s e r t c o n d i t i o n s .
T h i s b e l i e f was based l a r g e l y
on t h e m i s c o n c e p t i o n t h a t Recent d e s e r t s e d i m e n t s a r e r e d i n c o l o u r .
I n fact the
majority of d e s e r t sediments a r e not red a t t h e present t i m e (although there a r e a few n o t a b l e e x c e p t i o n s ) and r e c e n t work i n d i c a t e s t h a t many of t h e w o r l d ' s d e s e r t s a r e s l o w l y reddening w i t h time a s a r e s u l t of d i a g e n e t i c a l t e r a t i o n . Those a e o l i a n s a n d s which a r e a c t u a l l y r e d i n c o l o u r , a good example b e i n g t h e Simpson Desert of A u s t r a l i a , c a n n o t b e a c c o u n t e d f o r by d i a g e n e s i s a l o n e and a p p e a r t o have been d e r i v e d from f l u v i a l s e d i m e n t s which were p r e v i o u s l y reddened d u r i n g a Pleistocene pluvial climate.
Recent d e s e r t s e d i m e n t s t h e r e f o r e a p p e a r t o show two
d i s t i n c t r e d d e n i n g p r o c e s s e s and t h i s must b e a n i m p o r t a n t f a c t o r when c o n s i d e r i n g t h e o r i g i n o f a n c i e n t d e s e r t r e d beds. I n t h i s c h a p t e r t h e s e d i m e n t a r y environments and f a c i e s of modern d e s e r t sediments a r e reviewed w i t h s p e c i a l r e f e r e n c e t o a e o l i a n s a n d d e p o s i t s
The r e d d e n i n g of
d e s e r t s e d i m e n t s i s d e s c r i b e d u s i n g t h e d e s e r t a l l u v i u m of t h e s o u t h w e s t e r n USA and t h e a e o l i a n sands of t h e Simpson Desert a s examples.
A s e c t i o n i s devoted t o a n c i e n t
a e o l i a n r e d beds and i n c l u d e s d i s c u s s i o n s o f t h e problems of t h e r e c o g n i t i o n of a e o l i a n sands i n t h e s t r a t i g r a p h i c r e c o r d and t h e o r i g i n of t h e i r r e d c o l o u r a t i o n . An a n c i e n t example, t h e Lower Permian o f n o r t h w e s t Europe, i s d e s c r i b e d i n d e t a i l .
MODERN DESERT SEDIMENTS Modern a r i d d e s e r t s a r e b a r r e n t r a c t s of l a n d o v e r which r a i n f a l l i s t o o l i m i t e d t o support vegetation.
The upper l i m i t of r a i n f a l l i s a b o u t 25cm a y e a r and t h e r a t e
o f e v a p o r a t i o n f a r exceeds t h e r a t e of p r e c i p i t a t i o n .
D e s e r t s c o v e r a b o u t 20% of
t h e w o r l d ' s l a n d s u r f a c e a n d a r e c o n c e n t r a t e d between t h e l a t i t u d e s of about 10'
30'
and
n o r t h and s o u t h o f t h e e q u a t o r i n t h e r e g i o n s o f p r e v a i l i n g t r a d e winds ( F i g . 2 . 1 ) .
D e s e r t s o c c u r between t h e s e l a t i t u d e s b e c a u s e t h e r e l a t i v e humidity of t h e descending h i g h - p r e s s u r e a i r of t h e h o r s e l a t i t u d e s (30'
n o r t h and s o u t h o f t h e e q u a t o r )
d e c r e a s e s a s t h e a i r i s compressed a d i a b a t i c a l l y and g i v e s r i s e t o c l o u d l e s s s k i e s . P a r t o f t h e a i r flows towards t h e e q u a t o r and i s d e f l e c t e d westwards by t h e C o r i o l i s
OL I m p o r t a n t f e a t u r e s of d e s e r t s a r e t h e d i u r n a l extremes of t e m p e r a t u r e and humidity which promote w e a t h e r i n g p r o c e s s e s ( O l l i e r , 1 9 6 9 ) .
During t h e c o o l e r hours
before
dawn t h e r e l a t i v e humidity may be a s h i g h a s 100% and heavy dews f r e q u e n t l y c o v e r rock o u t c r o p s .
These dews promote chemical c o r r o s i o n .
M o i s t u r e i s a l s o drawn t o
t h e s u r f a c e by c a p i l l a r i t y and e v a p o r a t i o n r e s u l t s i n t h e growth o f gypsum and h a l i t e c r y s t a l s which c a u s e e x p a n s i o n and mechanical w e a t h e r i n g of t h e h o s t r o c k . Rapid d i u r n a l changes i n t e m p e r a t u r e c a u s e d i f f e r e n t i a l e x p a n s i o n and c o n t r a c t i o n s t r e s s e s between t h e s u r f a c e and t h e u n d e r l y i n g r o c k , r e s u l t i n g i n s p a l l i n g of t h e rock s u r f a c e and t h e s p l i t t i n g of b o u l d e r s .
Igneous rock s u r f a c e s a r e t y p i c a l l y
rounded by e x f o l i a t i o n . The s i l t and s a n d - s i z e d p r o d u c t s of w e a t h e r i n g a r e s e t i n t o motion by t h e wind i n deserts,
T r a n s p o r t t a k e s p l a c e b y s u s p e n s i o n . s a l t a t i o n and s u r f a c e c r e e p
71 (Bagnold, 1941).
The c o a r s e r , more r e s i s t a n t w e a t h e r i n g p r o d u c t s a r e l e f t on t h e
d e s e r t s u r f a c e a s a l a g d e p o s i t t h a t may b e a b r a d e d by s a n d - l a d e n winds i n t o v e n t i facts.
Lag g r a v e l s on t h e d e s e r t s u r f a c e may e f f e c t i v e l y p r e v e n t most of t h e
u n d e r l y i n g s e d i m e n t s from b e i n g removed by t h e wind. D e s e r t sediments may accumulate i n b o t h a c t i v e t e c t o n i c s e t t i n g s where f a u l t controlled basins a r e rapidly subsiding (e.g. cratonic areas (e.g. Sahara).
s o u t h w e s t e r n USA) o r on s t a b l e
Many of t h e d e s e r t b a s i n s have i n l a n d d r a i n a g e
p a t t e r n s which may b e l a r g e l y i n h e r i t e d from P l e i s t o c e n e p l u v i a l e p i s o d e s when t h e c l i m a t e was more humid and s u r f a c e run-off
l e s s ephemeral.
number of d e p o s i t i o n a l zones can be r e c o g n i z e d .
A t t h e present-day a
Around t h e margins of d e s e r t b a s i n s
f l u v i a l p r o c e s s e s f r e q u e n t l y dominate and i n t e c t o n i c a l l y a c t i v e a r e a s a l l u v i a l f a n s may be abundant.
I n t h e c e n t r a l r e g i o n s a e o l i a n p r o c e s s e s dominate and i n t h e s t a b l e
i n t r a c r a t o n i c r e g i o n s l a r g e s a n d s e a s o r ' e r g s ' (Wilson, 1973) may,form.
Desert
l a k e s and i n l a n d sabkhas may a l s o b e found i n t h e c e n t r a l r e g i o n s because t h e w a t e r t a b l e h e r e may b e n e a r t o t h e s u r f a c e . A e o l i a n s a n d s a l s o form i n c o a s t a l r e g i o n s forming e l o n g a t e d dune f i e l d s p a r a l l e l to the coast. and sabkhas.
Here t h e y a r e a s s o c i a t e d w i t h o t h e r c o a s t a l sediments s u c h a s beaches Although t h e s e c o a s t a l dunes g e n e r a l l y have a low p r e s e r v a t i o n p o t e n t i a l
because o f t h e e f f e c t s of m a r i n e t r a n s g r e s s i o n t h e y might be more abundant i n t h e s t r a t i g r a p h i c record than i s generally r e a l i z e d .
The problem of d i s t i n g u i s h i n g
a n c i e n t a e o l i a n sands which p a s s l a t e r a l l y i n t o s h a l l o w marine s a n d b o d i e s a r e considerable.
DESERT FLWIAL SEDIMENTS Heavy r a i n s t o r m s a r e i n f r e q u e n t i n t h e d e s e r t and b e c a u s e of t h e l a c k of v e g e t a t i o n t o b i n d t h e sediment t h e y have a d r a m a t i c e f f e c t when t h e y do occur. Large amounts o f sediment may b e t r a n s p o r t e d , v i a a l l u v i a l f a n s and w a d i s , from t h e h i g h l a n d t o t h e lowland a r e a s .
I f t h e r a i n s t o r m i s p a r t i c u l a r l y heavy t h e
flowing w a t e r may e x t e n d t h e l e n g t h of t h e wadi and e v e n t u a l l y r e a c h t h e s e a ; o f t e n t h e p a s s a g e of w a t e r i s b l o c k e d by a e o l i a n s a n d and temporary l a k e s form.
During
m i l d e r r a i n s t o r m s t h e w a t e r s o a k s i n t o t h e wadi s e d i m e n t s and t h e downward p e r c o l a t i o n may r e s u l t i n t h e d e p o s i t i o n of c l a y i n what were o r i g i n a l l y m a t r i x - f r e e s e d i m e n t s . Under t h e s e c o n d i t i o n s t h e wadi may l o c a l l y become o v e r l o a d e d w i t h sediment.
Over-
l o a d i n g may a l s o o c c u r a t t h e n i c k p o i n t s between h i l l and p l a i n ; h e r e t h e r a p i d r e d u c t i o n i n w a t e r v e l o c i t y a s t h e flow widens l a t e r a l l y f r e q u e n t l y c a u s e s c h a n n e l b r a i d i n g and t h e f o r m a t i o n of a l l u v i a l f a n s .
D e b r i s flows a r e a common o c c u r r e n c e
on a l l u v i a l f a n s when t h e s e d i m e n t / w a t e r r a t i o i s h i g h .
A s t h e w a t e r s u p p l y i n t h e wadi d i m i n i s h e s much of i t s o a k s i n t o t h e sediment f i l l i n g t h e channel.
T h i s w a t e r i s commonly s a t u r a t e d w i t h r e s p e c t t o calcium
c a r b o n a t e and e v a p o r a t i o n r e s u l t s i n r a p i d c e m e n t a t i o n w i t h i n t h e v a d o s e zone.
The
72 s u r f a c e sands and s i l t s d r y o u t v e r y r a p i d l y and a r e u s u a l l y n o t cemented.
The
l a s t a r e a s t o d r y up a r e s t a g n a n t p o o l s i n which mud accumulates from s u s p e n s i o n . These become d e s i c c a t e d and mud c r a c k s form; t h i n mud l a y e r s r e s u l t i n g i n t h e f o r m a t i o n of f r a g i l e c u r l e d mud f l a k e s , which a r e e a s i l y blown away by t h e wind, w h i l s t t h i c k e r and h e a v i e r mud f l a k e s may b e p r e s e r v e d i n s i t u by a c o v e r o f wind blown sand. Because o f t h e ephemeral n a t u r e of d e s e r t f l u v i a l p r o c e s s e s d r y wadi c h a n n e l s a r e f r e q u e n t l y reworked o r f i l l e d by a e o l i a n sand and i n v e r t i c a l p r o f i l e t h e r e may be r a p i d a l t e r n a t i o n of w a t e r - d e p o s i t e d and wind-deposited s e d i m e n t s .
Although
a e o l i a n sands a r e g e n e r a l l y much b e t t e r s o r t e d t h a n f l u v i a t i l e s a n d s t n e reworking o f a e o l i a n s a n d s d u r i n g f l o o d s and t h e d e r i v a t i o n o f a e o l i a n sand from d r y wadi c h a n n e l s can l e a d t o some i n h e r i t a n c e o f t e x t u r a l c h a r a c t e r i s t i c s .
This often
makes t h e i r d i s t i n c t i o n d i f f i c u l t and G l e n n i e (1970) h a s l i s t e d a number o f c r i t e r i a which e n a b l e t h e r e c o g n i t i o n of w a t e r - l a i d d e p o s i t s i n t h e d e s e r t environment:
1) The bedding may be t h a t o f normal stream-flow s e d i m e n t s 2 ) Unsorted mudflow conglomerates may o c c u r 3) Channels and a c c r e t i o n f o r e s e t s a r e common
4 ) Clay laminae o r c l a y d r a p e s a r e common 5 ) Many sands a r e n o t w e l l - s o r t e d , b e i n g sometimes a r g i l l a c e o u s o r p e b b l y 6 ) They a r e commonly cemented by c a l c i t e
7 ) Sand and s i l t l a y e r s a r e commonly m i s s i n g from g r a d e d conglomerates 8) M a j o r i t y of q u a r t z g r a i n s t h a t a r e n o t cemented by c a l c i t e e x h i b i t f r o s t e d ( p i t t e d ) s u r f a c e s under t h e microscope The f o l l o w i n g f e a t u r e s may a l s o e n a b l e t h e r e c o g n i t i o n of s u b a e r i a l e x p o s u r e of w a t e r - l a i d sediment i n d e s e r t environments 1 ) P r e s e n c e o f c u r l e d ( g e n e r a l l y concave-up) c l a y f l a k e s 2 ) Comon p r e s e n c e o f c l a y p e b b l e s
3 ) P r e s e n c e of mud c r a c k s w i t h a sandy i n f i l l 4 ) P r e s e n c e of s a n d s t o n e dykes 5 ) A e o l i a n sand i n t e r b e d d e d w i t h w a t e r - l a i d d e p o s i t s .
The c o n t a c t s may show
e v i d e n c e of f l u v i a l e r o s i o n o r d e f l a t i o n . For e x c e l l e n t i l l u s t r a t i o n of t h e s e and many o t h e r f e a t u r e s o f d e s e r t f l u v i a t i l e sediments t h e r e a d e r i s r e f e r r e d t o t n e books o f G l e n n i e (1970) and P i c a r d and High (1973). AEOLIAN SAND DEPOSITS
A e o l i a n s a n d d e p o s i t s a r e t h e most c h a r a c t e r i s t i c f e a t u r e of d e s e r t s . o c c u r s n o t i n i s o l a t e d dunes b u t i n l a r g e a r e a s known a s s a n d s e a s ( t h e
The s a n d of t h e
n o r t h w e s t e r n S a h a r a ) which c o v e r between o n e - q u a r t e r and o n e - t h i r d of t h e w o r l d ' s deserts.
Wilson (1970) e s t i m a t e d t h a t 99.8% of a l l ' a c t i v e ' a e o l i a n s a n d was found
73 i n e r g s whose a r e a was g r e a t e r t h a n 125km2 and f u r t h e r l a r g e amounts o f a e o l i a n sand a r e t o b e found i n ' f i x e d ' e r g s on many d e s e r t margins ( F i g . 2 . 2 ) .
The e r g s
LARGE DUNE AREAS
Fig.2.2. 1973).
World map of p r i n c i p a l f i x e d and a c t i v e e r g s ( a f t e r Cooke and Warren,
d e r i v e t h e i r sand by t h e winnowing o f p e r i p h e r a l and i n t e r n a l d e s e r t f l u v i a t i l e sediments and by t h e a t t r i t i o n o f s o u r c e r o c k s .
The low d e n s i t y and v i s c o s i t y o f
wind make i t a n e f f i c i e n t sediment s o r t e r ; o n l y s a n d - s i z e d p a r t i c l e s c a n b e moved a s bed-load.
C o a r s e r g r a i n s a r e g e n e r a l l y l e f t behind a s a d e f l a t i o n l a g and f i n e r
sediment goes i n t o s u s p e n s i o n and may b e t r a n s p o r t e d beyond t h e b a s i n of d e p o s i t i o n thus contributing t o t h e general d e f l a t i o n of d e s e r t basins, The n a t u r e of t h e bedforms i n e r g s a r e c o n t r o l l e d by b o t h g r a i n s i z e and t h e t h i c k n e s s o f t h e s a n d c o v e r (Wilson, 1972, 1973) ( F i g . 2 . 3 ) .
I n t h e t h i c k accum-
u l a t i o n s , a s i n t h e Sahara and A r a b i a , t h r e e s c a l e s o f bedform:
r i p p l e s , dunes,
and d r a a s a r e developed b u t i n d e s e r t s where t h e c o v e r i s t h i n , a s i n t h e A u s t r a l i a n d e s e r t , t h e l a r g e r d r a a s a r e n o t developed and dunes form t h e l a r g e s t o b s e r v e d features.
These d i f f e r e n t bedforms a r e o f t e n i n t i m a t e l y r e l a t e d and show h i g h l y
complex p a t t e r n s .
T h i s c o m p l e x i t y r e s u l t s from t h e e f f e c t s of t o p o g r a p h i c s l o p e ,
v a r i a b l e wind d i r e c t i o n s , and t h e f a c t t h a t l a r g e r dunes and d r a a s r e q u i r e a l o n g time t o b e remoulded i n t o a new s h a p e a f t e r changes i n wind d i r e c t i o n and wind speed.
Another problem a r i s e s b e c a u s e of t h e r e l a t i v e l y h i g h t h r e s h o l d v e l o c i t i e s
needed f o r s a n d e n t r a i n m e n t .
T h i s means t h a t g r a i n movement i s a n i n t e r m i t t e n t
74 p r o c e s s and t h e s t r o n g winds which move s a n d r a p i d l y may have a d i f f e r e n t d i r e c t i o n from t h e weaker winds which o p e r a t e through a g r e a t e r p r o p o r t i o n of t h e time (Sharp, 1966).
The mechanics of s a n d t r a n s p o r t by wind a r e s t i l l p o o r l y under-
s t o o d and we s t i l l r e l y h e a v i l y on B a g n o l d ' s (1941) c l a s s i c work.
I n the aeolian
t r a n s p o r t of s a n d g r a i n b a l l i s t i c s and i n t e r g r a n u l a r c o l l i s i a n s a r e t h e dominant p r o c e s s e s whereas i n aqueous t r a n s p o r t f l u i d t u r b u l e n c e i s more i m p o r t a n t . The h i e r a r c h y of bedforms i n a e o l i a n s a n d g e n e r a l l y m a n i f e s t s i t s e l f a s a series o f superimposed forms w i t h d i s t i n c t s i z e c l a s s e s and w i t h v e r y few forms of i n t e r mediate s i z e .
T h i s i s s i m i l a r t o t h e h i e r a r c h y observed i n aqueous bedforms
( A l l e n , 1968) and t h e e x i s t e n c e of a t h r e e - f o l d h i e r a r c n y of r i p p l e s , dunes and sand waves i n s h a l l o w marine environments (Houbolt, 1968) i s m o s t l y c l o s e l y comThe a e o l i a n bedform h i e r a r c h y i s d i s c u s s e d
p a r a b l e t o t h a t s e e n i n a e o l i a n bedforms. I
i n d e t a i l by Wilson (1972) whose c l a s s i f i c a t i o n i s a d o p t e d h e r e ( T a b l e 2 . 1 ) .
This
h i e r a r c h i c a l arrangement a p p l i e s t o t r a n s v e r s e , l o n g i t u d i n a l and o b l i q u e forms and a p a r t from t h e f o u r t h o r d e r i t a l s o a p p l i e s t o e r o s i o n a l a s w e l l a s d e p o s i t i o n a l bedforms (Cooke and Warren, 1973).
Although t h e h i e r a r c h i c a l groups proposed by
Wilson o v e r l a p i n t h e i r s i z e r a n g e s , when t h e wavelength i s p l o t t e d a g a i n s t g r a i n s i z e a s i n F i g . 2 . 4 t h e y can b e s e e n t o form t h r e e d i s t i n c t groups.
I n e a c h of t h e s e
c l a s s e s t h e g r a i n s i z e o f t h e s a n d i n c r e a s e s w i t h t h e wavelength o f t h e bedform. S i n c e a g r e a t e r windspeed i s needed t o move c o a r s e r s a n d i t f o l l o w s t h a t bedform wavelength w i l l i n c r e a s e w i t h windspeed ( s e e a l s o , S h a r p , 1963). Ripples Although a e o l i a n r i p p l e s o c c u r w i t h s i m i l a r v a r i e t y t o t h e i r subaqueous c o u n t e r p a r t s t h e y a r e most commonly a s i m p l e arrangement o f p a r a l l e l r i d g e s a n d t r o u g h s t r a n s v e r s e t o t h e wind d i r e c t i o n .
i s r a r e l y v e r y pronounced.
They may b e s t r a i g h t o r s i n u o u s b u t t h e s i n u o s i t y
Commonly, t h e y a r e asymmetric w i t h a s t e e p e r l e e s l o p e
o r v e r y s m a l l s l i p f a c e and t h e h e i g h t / w a v e l e n g t h r a t i o i s u s u a l l y between 0.02 and 0.08 which i s r a t h e r s m a l l e r t n a n t h a t o f o t n e r a e o l i a n bedforms (Ellwood e t a l . , 1975).
Ridge c r e s t s a r e a l m o s t i n v a r i a b l y c o a r s e r g r a i n e d t h a n t h e t r o u g h s .
E a r l y workers ( e . g . B o u r c a r t , 1928) b e l i e v e d t h a t r i p p l e s , l i k e o t h e r a e o l i a n bedforms, were t h e r e s u l t of aerodynamic i n s t a b i l i t y and t h e r e i s some e v i d e n c e (Bagnold, 1941; W i l s o n , 1972; Cooke and Warren, 1973) t o i n d i c a t e t h a t a s m a l l p r o p o r t i o n of r i p p l e s may b e of t h i s type. o r fluid-drag interface.
These a r e t h e s o - c a l l e d 'aerodynamic'
r i p p l e s which r e s u l t from aerodynamic i n s t a b i l i t y n e a r t h e s a n d / a i r
They t e n d t o have a s t r o n g l o n g i t u d i n a l component i n t h e s h a p e and a l s o
t o be s e v e r a l times l a r g e r t h a n t h e t r a n s v e r s e impact r i p p l e s .
Most a e o l l a n r i p p l e s
a r e r e c o g n i z e d t o have formed by t h e r e g u l a r l y s p a c e d impact of s a n d g r a i n s a s d e s c r i b e d by Bagnoid (1941, p.146-153).
I n wind t u n n e l e x p e r i m e n t s Bagnold found
t h a t t h e r e was e q u i v a l e n c e between t h e l e n g t h o f t h e c h a r a c t e r i s t i c f l i g h t p a t h o f
75
Oriental NORTH AFRICA
Fig.2.3. The Erg O r i e n t a l , A l g e r i a showing i t s p o s i t i o n i n r e l a t i o n t o r e g i o n a l p a t t e r n s of s a n d t r a n s p o r t and t h e r e l a t i o n s h i p between sand c o v e r and bedforms w i t h i n i t , A . Flow d i r e c t i o n s medium s a n d , B. Flow d i r e c t i o n s f i n e s a n d (arrows i n d i c a t e d i r e c t i o n s t a k e n from aedforms) , C . Prominent d r a a t r e n d s ( d i f f e r e n t t r e n d s a r e r e l a t e d t o g r a i n s i z e d i f f e r e n c e s ) , D . P r o p o r t i o n of sand c o v e r , E . Mean s p r e a d - o u t sand t h i c k n e s s (m), and G . Mean d r a a wavelength (km) ( a f t e r Wilson, 1 9 7 3 ) .
76 s a l t a t i n g s a n d g r a i n s and r i p p l e wavelengths.
I n the f i e l d t h e s u r f a c e of r i p p l e s
i s o f t e n covered by c o a r s e r g r a i n s t r a v e l l i n g a s s u r f a c e c r e e p .
When t h e s e g r a i n s
r e a c h r i p p l e c r e s t s t h e y c a n n o t move on b e c a u s e t h e y e x p e r i e n c e fewer bombardments by s a l t a t i n g g r a i n s .
Sharp (1963) has c l e a r l y demonstrated t h e c o n c e n t r a t i o n of
c o a r s e r g r a i n s i n a e o l i a n r i p p l e c r e s t s by s e c t i o n i n g impregnated r i p p l e s . e n v i s a g e d a sequence of r i p p l e f o r m a t i o n a s f o l l o w s :
He
a s t h e wind moves o v e r a f l a t
s a n d bed, s a l t a t i n g s a n d moves t h e s u r f a c e c r e e p ; t h i s e n c o u n t e r s chance i r r e g u l a r i t i e s and p i l e s up; t h e p i l e s of s a n d grow u n t i l a n e q u i l i b r i u m h e i g h t i s r e a c h e d ; t h i s c h a r a c t e r i s t i c h e i g h t and t h e a n g l e of i n c i d e n c e of t h e g r a i n s c o n t r o l t h e wavelength. TABLE 2.1. H i e r a r c h i c a l c l a s s i f i c a t i o n o f a e o l i a n bedform elements ( a f t e r W i l s o n , 1972). Order
Wavelength
Height
Orientation
Possible Origin
Name
1st
300-55OOm
20-45Om
long. o r transverse
aerodynamic instability
draas
2nd
3-600m
0.1- lOOm
long. o r transverse
aerodynamic instability
dunes
3rd
15-250cm
0.2-0.5cm
long. o r transverse
aerodynamic instability
aerodynamic ripples
4th
0.5-2,000cm 1.0- 3,000cm
0.05-100cm 0.05-100cm
transverse long.
impact secondary Taylor-G'ir t l e r vortices
impact r i p p l e s secondary ripple sinuosity
G r a n u l e - r i p p l e s ( S h a r p , 1963) have been d i s c u s s e d by a number o f a u t h o r s (Bagnold, 1941; G l e n n i e , 1970).
They have much g r e a t e r wavelengths t h a n o r d i n a r y
s a n d r i p p l e s ; Bagnold (1941) observed some i n t h e Libyan Desert w i t h a wavelength of up t o 20m and a n a m p l i t u d e of o v e r 60cms.
Granule-ripples usually contain
l a r g e p r o p o r t i o n s o f g r a i n s o v e r 2mm; which may make up t o 20% of t h e r i p p l e a s a whole and up t o 80% of t h e r i p p l e c r e s t .
They a r e f r e q u e n t l y symmetrical i n c r o s s -
s e c t i o n and t h e c o n c e n t r a t i o n s o f c o a r s e g r a i n s i n t h e r i p p l e c r e s t i s c h a r a c t e r istic.
Bagnold (1941) thought t h a t g r a n u l e - r i p p l e s formed when t h e r e had been
long-continued d e f l a t i o n s o t h a t much of t h e f i n e s a n d was removed from t h e s u r f a c e layers.
I t was t h o u g h t t h a t a s u p p l y of f i n e s a l t a t i n g g r a i n s from upwind
m a i n t a i n e d t h e s u r f a c e c r e e p and s i n c e t h e c o a r s e g r a i n s c o u l d n o t move beyond t h e r i p p l e c r e s t t h e upward growth o f t h e g r a n u l e - r i p p l e would s l o w l y c o n t i n u e ; a c c o r d i n g l y he t h o u g h t t h e y were v e r y o l d .
The p r o b a b l e c o n t r o l l i n g f a c t o r of t h e s i z e of
granule-ripples is t h e i r grain size.
The f l i g h t p a t t e r n s of s a l t a t i n g g r a i n s i n
c o a r s e m a t e r i a l a r e g r e a t e r because of t h e i r b e t t e r bounce o f f t h e c o a r s e g r a i n s .
77
S c a t t e r diagram o f g r a i n s i z e (P20 = c o a r s e twenty p e r c e n t i l e ) a g a i n s t Fig.2.4. ripples, B dunes, wavelength f o r a e o l i a n bedforms showing t h e h i e r a r c h y : A C d r a a s ( a f t e r Wilson, 1972).
-
-
-
Because of t h i s t h e wavelength i s g r e a t e r and c o a r s e g r a i n s accumulate a t granuler i p p l e c r e s t s i n t h e same way a s i n normal wind r i p p l e s (Ellwood e t a l . , 1975). Another k i n d of r i p p l e which c o m o n l y o c c u r s i n t h e d e s e r t environment i s t h e a d h e s i o n r i p p l e ( G l e n n i e , 1970) a l t h o u g h t h e s e were f i r s t d e s c r i b e d from c o a s t a l f l a t s i n n o r t h e r n Germany (Reineck, 1955).
Adhesion r i p p l e s form most commonly
on sabkhas and i n damp i n t e r - d u n e a r e a s where sand g r a i n s a d h e r e t o t h e s u r f a c e because of t h e m o i s t u r e c o n t e n t .
G r a i n s t h e n c o n t i n u e t o b e t r a p p e d on t h e upwind
s i d e s of t h e r i p p l e s a n d . c m o n l y grow up t o 5cm h i g h a l t h o u g h t h e y may be o c c a s i o n a l l y much l a r g e r ( G l e n n i e , 1970). ripples is r a t h e r irregular.
The i n t e r n a l s t r u c t u r e o f a d h e s i o n
I t t e n d s t o b e l e n t i c u l a r , which i n s a b k h a s , produces
a n i r r e g u l a r wavy l a m i n a t i o n . Dunes and d r a a s Dunes and d r a a s show v e r y s i m i l a r morphological t y p e s and t h e e s s e n t i a l d i f f e r e n c e between them i s one of s i z e (Fig.2.4). described together,
For t h i s r e a s o n t h e y c a n b e c o n v e n i e n t l y
Dunes have wavelengths measured i n 10's o r 100's of metres and
h e i g h t s i n metres, w h i l e d r a a s have l e n g t h s o f t h e o r d e r of k i l o m e t r e s and h e i g h t s i n 10's of m e t r e s .
The r a t e of movement of b o t h is r e l a t e d t o t h e i r s i z e w i t h t h e
s m a l l e r forms b e i n g a b l e t o move more q u i c k l y (Long and S h a r p , 1964).
Draas move 6 years).
q u i t e s l o w l y ( a b o u t lcm/year) and p r o b a b l y t a k e a long t i m e t o develop (104-10
They p r o b a b l y o r i g i n a t e by t h e n u c l e a t i o n of a number o f dunes and a r e u s u a l l y covered on b o t h windward and leeward f a c e s by dunes.
Those dunes on t h e l e e s i d e
t e n d t o have l a r g e s l i p f a c e s and c l e a r l y c o n t r i b u t e t o t h e m i g r a t i o n of t h e draa. Dunes o r i g i n a t e i n a e o l i a n sand because o f chance i r r e g u l a r i t i e s (Bagnold, 1941).
78 Such i r r e g u l a r i t i e s may b e a g e n t l e d i p i n t h e d e s e r t s u r f a c e o r some u p s t a n d i n g foreign object,
Once a p a t c h of sand i s n u c l e a t e d i n t h i s way i t b e g i n s t o modify
t h e s u r r o u n d i n g a i r flow p a t t e r n . o f 10-15'
I n t h i s way a b a c k s l o p e , u s u a l l y w i t h a n a n g l e
i s developed and w i t h c o n t i n u e d growth d e p o s i t i o n on t n e l e e s l o p e g e t s
p r o g r e s s i v e l y n e a r e r t h e s m i t u n t i l t h e a n g l e of r e p o s e f o r s a n d i s exceeded and s l i p p i n g r e s u l t s (McKee e t a l . ,
1971).
Depending upon a v a r i e t y of f a c t o r s i n c l u d i n g wind c o n d i t i o n s , s a n d t y p e , and s u p p l y of s a n d , dunes may a c q u i r e a v a r i e t y of s h a p e s and s i z e s .
Many a u t h o r s have
a t t e m p t e d t o c l a s s i f y sand dune t y p e s (Melton, 1940; Bagnold, 1941; Holm, 1960; McKee, 1966; Cooper, 1967).
A commonly used c l a s s i f i c a t i o n i s t h a t of McKee (1966)
b u t t h i s has t h e d i s a d v a n t a g e of n o t d i s t i n g u i s h i n g d r a a - s i z e d f e a t u r e s :
1. Barchan 2. T r a n s v e r s e 3. P a r a b o l i c 4. S e i f 5. S t a r 6. Dome-shaped 7. Reversing A more comprehensive c l a s s i f i c a t i o n which can be a p p l i e d t o dune and d r a a - s i z e d f e a t u r e s i s g i v e n by Cooke and Warren (1973) ( s e e F i g . 2 . 5 ) . Bar c hans Barchans a y e c r e s c e n t shaped s a n d mounds formed i n a n u n d i r e c t i o n a l wind by a v a l a n c h i n g of s a n d on t h e s l i p f a c e .
The e x t r e m i t i e s ( h o r n s ) of t h e dune e x t e n d
downwind b e c a u s e t h e y m i g r a t e more r a p i d l y t h a n t h e main body.
S e v e r a l measurements
have been made of t h e r a t e o f b a r c h a n m i g r a t i o n which i s r e l a t e d t o h e i g n t ( F i n k e l 1959; N o r r i s , 1966; H a s t e n r a t h , 1967) and a l s o t o t h e shape o f t h e dune; narrower dunes a r e l i k e l y t o advance more r a p i d l y t h a n b r o a d e r o n e s ,
(Long a n d S h a r p , 1964)
a l t h o u g h b a r c h a n s do p r e s e r v e t h e i r p l a n shape d u r i n g m i g r a t i o n (McKee, 1966). A c t u a l r a t e s o f advance which have been q u o t e d v a r y from 17-47m/annum ( H a s t e n r a t h , 1967) and 15m/annum ( B e a d n e l l , 1 9 1 0 ) . Barchan dunes u s u a l l y o c c u r i n f a i r l y complex p a t t e r n s .and may form b o t h dune and d r a a - s i z e d f e a t u r e s .
I s o l a t e d b a r c h a n s t e n d t o develop i n a r e a s where t h e s u p p l y
of sand i s d i m i n i s h e d such t h a t t h e u n d e r l y i n g pavement c a n n o t be c o m p l e t e l y covered. The s a n d s u p p l y c o n t r o l s t h e s p a c i n g of t h e d u n e s , t h e less t h e amount of s a n d t h e wider t h e dune s p a c i n g , b u t t h e a n g l e of d i p o f s l i p f a c e s i s c o n s i s t e n t l y n e a r 34' n e a r t h e c e n t r a l p a r t of t h e dune and somewhat l e s s i n t h e h o r n s .
These s l i p f a c e s
r e s u l t i n p l a n a r - t a b u l a r u n i t s of c r o s s - b e d d i n g ( F i g . 2 . 6 ) and d i p w i t h i n a n a r c of l e s s t h a n 150' and u s u a l l y a b o u t 90'.
The unimodal d i s t r i b u t i o n of f o r e s e t d i r e c -
t i o n s when p l o t t e d on a s t e r e o g r a p h i c p r o j e c t i o n e n a b l e s barchans t o be d i s t i n g u i s h e d
79
Fig.2.5. The main t y p e s o f a e o l i a n dune and d r a a . The terms u s e d i n B-H may be a p p l i e d t o dunes a s w e l l a s d r a a s . E f f e c t i v e wind d i r e c t i o n i s i n d i c a t e d where a p p r o p r i a t e . I n C and I no one d i r e c t i o n dominates ( a f t e r Cooke and Warren, 1 9 7 3 ) .
80 from o t h e r t y p e s of dunes i n a n c i e n t d e p o s i t s , p a r t i c u l a r l y s e i f dunes ( s e e Fig.2.8).
Main trench, Northwest wall
sw
NE
feet
20
----I
0
1
10
20
30 40
50
60
70
t Base of
I
I
80
90
dune I
I
100 110 120 130 l40feet
Sido tronch, Northeast wall
,
I
dun. I
150 160 170 180 l90 200 210 220 230 240 250 260 270 280 -Bounding
surface
-Selected cross
- stratum
foot
Fig.2.6. I n t e r n a l s t r u c t u r e of a barchan dune a s observed i n e x c a v a t i o n s i n t o t h e gypsum dunes of t h e White Sands N a t i o n a l Monument, New Mexico ( a f t e r McKee, 1966).
Seifs
S e i f s a r e l o n g i t u d i n a l forms e l o n g a t e d p a r a l l e l t o t h e p r e v a i l i n g wind d i r e c t i o n . They cover e x t r e m e l y l a r g e a r e a s of some d e s e r t s such a s t h e Rub a 1 K h a l i ( G l e n n i e , 1970), t h e w e s t e r n Sahara and Libyan d e s e r t s (Bagnold, 1933) and t h e A u s t r a l i a n
deserts (Folk, 1971a,b).
Many of t h e s e a r e d r a a - s i z e d f e a t u r e s which a r e a s s o c i a t e d
w i t h s i n u o u s , v e r y e l o n g a t e d c r e s t l i n e s which a r e known a s s e i f dunes from t h e Arab usage ( s e i f meaning sword).
The c r e s t l i n e s may b i f u r c a t e i n a n upwind d i r e c t i o n
and f r e q u e n t l y shows t h e development o f a l t e r n a t i n g s l i p f a c e s . The o r i g i n o f s e i f s has been a m a t t e r of some d i s c u s s i o n .
Bagnold (1941) b e l i e v e d
t h a t t h e y a r e produced when s t r o n g winds blow from a q u a r t e r o t h e r t h a n t h a t o f t h e g e n e r a l d r i f t of sand caused by t h e more p e r s i s t e n t g e n t l e winds.
McKee and T i b b i t t s
(1964) b e l i e v e d t h a t s e i f s were produced i n t h e v e c t o r o f two converging wind d i r e c t i o n s blowing from two q u a r t e r s a b o u t 90'
apart.
Other a u t h o r s have p r e s e n t e d
81 e v i d e n c e i n f a v o u r o f t h i s i d e a (Madigan, 1946; Wopfner a n d Twidale, 1967). However, a number o f a u t h o r s ( e . g . G l e n n i e , 19701, have p r e s e n t e d e v i d e n c e t h a t l o n g i t u d i n a l dunes a r e produced p a r a l l e l t o winds o f uniform d i r e c t i o n and most workers now f a v o u r t h i s e x p l a n a t i o n .
The s e i f form s u g g e s t s t h a t p a i r e d s p i r a l
v o r t i c e s w i t h a x e s p a r a l l e l t o t h e flow a r e i m p o r t a n t i n t h e i r f o r m a t i o n .
For
example, i n S a u d i Arabia d r a a - s i z e d l o n g i t u d i n a l dunes have smaller s e i f s a l i g n e d d i a g o n a l l y a c r o s s t h e s i d e s o f t h e m a j o r r i d g e s a t a n a n g l e which would b e e x p e c t e d i f three-dimensional vortex flow operated.
I n t n e Simpson Desert t h e minor long-
i t u d i n a l f e a t u r e s and a l s o t h e Y - j u n c t i o n s e x h i b i t a n g l e s o f 30-50'
which would
b e e x p e c t e d from secondary f l o w ( F o l k , 1971b; W i l s o n , 1972). The s t r u c t u r e of s e i f dunes has been d e a l t w i t h by McKee and T i b b i t t s (1964) (see Fig.2.7)
and by G l e n n i e (1970).
I d e a l l y t h e y c o n s i s t of cross-bedded u n i t s
d i p p i n g n e a r l y normal t o t h e e l o n g a t i o n o f t h e r i d g e t h u s p r o d u c i n g two modes a b o u t 120' a p a r t .
T h i s f e a t u r e e n a b l e s t h e d i s t i n c t i o n of b a r c h a n s a n d s e i f s i n v e r t i c a l
s e c t i o n s (Fig.2.8).
mStructunlarr sand
Near-vertical laminae between
inches
B
Encroachmont Dopotit
Accntion Doporit
Fig.2.7. A . I n t e r n a l s t r u c t u r e of a s e i f dune ( a f t e r McKee and T i b b i t t s , 1964). B. G e n e r a l i z e d s t r u c t u r e of a s e i f dune ( a f t e r Bagnold, 1941).
82
D mretjon mminae
l B-B'
Dips only reach 34O in
N
W
E
S
5
Fig.2.8. I d e a l i z e d p l a n view and s t e r e o g r a p h i c p l o t of s l i p f a c e o r i e n t a t i o n s i n barchans and s e i f s . The s i n g l e s l i p f a c e of t h e b a r c h a n produces a unimodal d i s t r i b u t i o n of f o r e s e t d i p d i r e c t i o n whereas t h e a l t e r n a t i n g s l i p f a c e s of t h e s e i f produces a bimodal d i s t r i b u t i o n o f f o r e s e t d i p d i r e c t i o n s ( a f t e r G l e n n i e , 1970). T r a n s v e r s e forms T r a n s v e r s e dunes a r e a l m o s t s t r a i g h t , e l o n g a t e d r i d g e s of sand o r i e n t e d a t r i g h t a n g l e s t o t h e p r e v a i l i n g wind d i r e c t i o n .
They a r e r e g u l a r l y spaced and have
broad i n t e r d u n e a r e a s which sometimes form i n l a n d sabkhas.
G l e n n i e (1970)
d i s c u s s e d t h e o r i g i n of t r a n s v e r s e dunes and s u g g e s t e d t h a t they o r i g i n a t e d i n i n l a n d sabkha a r e a s where t h e damp sabkha s u r f a c e i n h i b i t e d t h e development of barchan horns.
The i n t e r d u n e sabkhas e v e n t u a l l y d i s a p p e a r and a s a n d s e a w i t h
t r a n s v e r s e dunes i s produced ( s e e G l e n n i e , 1970, F i g . 7 7 ) .
Bagnold (1954) c o n s i d e r e d
t r a n s v e r s e dunes t o b e u n s t a b l e forms which broke up i n t o s e i f s o r b a r c h a n s . However, Cooper ( 1 9 5 8 ) , McKee ( 1 9 6 6 ) , and G l e n n i e (1970) a l l b e l i e v e them t o be s t a b l e forms.
For example, t h e y a r e w e l l - s e e n i n t h e e a s t e r n p a r t of t h e Rub a 1
83 K h a l i ( G l e n n i e , 1970, p . 9 5 ) where t h e dunes a r e s e p a r a t e d by i n l a n d s a b k h a s . I n a r e a s of complete sand c o v e r t r a n s v e r s e forms may develop more complex s h a p e s . These normally have a v a l a n c h e f a c e s on t h e i r l e e s i d e s b u t t h e d r a a - s i z e d forms may have g e n t l y s l o p i n g l e e s i d e s wnicn a r e covered w i t n superimposed dunes.
A common
v a r i a t i o n on t h e s i m p l e s t r a i g h t c r e s t e d form i s t h e a k l e p a t t e r n ( F i g . 2 . 5 ) made up o f a l t e r n a t i n g convex and concave downwind s e c t o r s . The i n t e r n a l s t r u c t u r e of t r a n s v e r s e dunes i s d i s c u s s e d by McKee (1966) who n o t e s t h a t cross-bedding i s of t h e planar t a b u l a r type (Fig.2.9) with s t e e p l y dipping
(30-34')
The f o r e s e t s a r e more
f o r e s e t laminae which a r e t y p i c a l l y l o n g and even.
o r l e s s p a r a l l e l t o t h e a c t i v e s l i p f a c e and a r e c u t by i n t e r n a l e r o s i o n s u r f a c e s which a r e comparable t o t h e r e a c t i v a t i o n s u r f a c e s o f f l u v i a l b a r s .
These d i p a t
low a n g l e s which range from n e a r - h o r i z o n t a l n e a r t h e dune c r e s t t o o v e r 20' lower p a r t o f t h e s e t .
i n the
These s u r f a c e s a r e p r o b a b l y t h e p r o d u c t s of r e v e r s e d o r
t r a n s v e r s e wind e p i s o d e s .
Dome-shaped dunes (McKee, 1966; Thompson, 1969) a p p e a r
t o form by g r a d u a l lowering of t h e s l i p f a c e o f t r a n s v e r s e dunes by s u c c e s s i v e erosional events. Detail of asymmetrical trough, Side trench
SE
feet
NW
:I 5
2 0 1
ft.
sw
I
I
I
I
I
0
5
10
15
20
Main trench, North wall
- Bounding surface
-Selected cross-stratum
Fig.2.9. I n t e r n a l s t r u c t u r e of a t r a n s v e r s e dune a s observed i n e x c a v a t i o n s i n t o t h e gypsum dunes of t h e White Sands N a t i o n a l Monument, New Mexico ( a f t e r McKee, 1966).
84 P a r a b o l i c dunes P a r a b o l i c dunes a r e U o r V shaped s a n d r i d g e s w i t h t h e concave s i d e towards t h e wind.
They a r e n o t so common a s some of t h e o t h e r dune t y p e s .
The m i d d l e p a r t o f
t h e dunes t h u s moves forward w i t h r e s p e c t t o t h e s i d e s and McKee (1966) b e l i e v e d t h a t t h i s was because t h e s i d e s were anchored by v e g e t a t i o n t h u s a l l o w i n g t h e c e n t r a l p a r t of t h e dune t o move forward more r a p i d l y .
Characteristically the s l i p
f a c e i s convex upwards. The i n t e r n a l s t r u c t u r e i s d e s c r i b e d by McKee (1966).
Large s c a l e c r o s s - b e d d i n g
i s a g a i n dominant w i t h e r o s i o n s u r f a c e s b e i n g s e p a r a t e d by b u n d l e s of f o r e s e t laminae.
F o r e s e t laminae i n p a r a b o l i c dunes a r e low-angled compared w i t h o t h e r
dunes and t y p i c a l l y , b e c a u s e of t h e o r i e n t a t i o n of t h e s l i p f a c e , t h e y a r e concave downwards ( F i g . 2 . 1 0 ) . Moin trench, Northwest wail
sw
0
NE
10
M
30
10
M
M)
70
80
.)
901cetio3
Main h m h . Wthwest WON NE
sw
I M
0
I0
20
Y)
LO
M
€0
?a
80
'
I
93IediW
'/
Side trench, Southwest wall SE
F i g . 2 . 1 0 . I n t e r n a l s t r u c t u r e o f a p a r a b o l i c dune a s s e e n i n e x c a v a t i o n s i n t o t h e gypsum dunes of t h e White Sands N a t i o n a l Monument, New Mexico ( a f t e r McKee, 1 9 6 6 ) . Rhourds Rhourds ( F i g . 2 . 5 )
and o t h e r complex i n t e r f e r e n c e p a t t e r n s o c c u r where t h e wind
p a t t e r n i s highly variable.
The i n t e r f e r e n c e p a t t e r n i s sometimes r e c o g n i z a b l e a s
t h e i n t e r s e c t i o n of two o r more l i n e a r t r e n d s b u t t h e s e commonly b r e a k down i n t o a
s e r i e s of i s o l a t e d peaks which when d r a a - s i z e d a r e c a l l e d rhourds.
These may b e
o v e r 200m h i g h and most commonly r e s u l t from two dominant wind d i r e c t i o n s which a r e
85
s e p a r a t e d by a n a c u t e a n g l e ,
They a r e b e s t - d e v e l o p e d i n a r e a s of complete sand
c o v e r and a d e s e r t f l o o r l a g o f c o a r s e s a n d u s u a l l y s e p a r a t e s i n d i v i d u a l rhourds. Rhourds may show a complex p a t t e r n o f s l i p f a c e s and i n many c a s e s c a r r y s u p e r imposed dunes.
O t h e r s may t a k e t n e form o f smooth dunes w i t h no well-developed
s l i p f a c e s (McKee, 1966). The i n t e r n a l s t r u c t u r e o f modern and a n c i e n t a e o l i a n d e p o s i t s I n k e e p i n g w i t h t h e mechanism of m i g r a t i o n o f s a n d dunes two t y p e s of sand beds c a n be r e c o g n i z e d .
These were r e f e r r e d t o by Bagnold (1941) a s :
1. A c c r e t i o n d e p o s i t s 2. Encroachment o r a v a l a n c h e d e p o s i t s A c c r e t i o n d e p o s i t s r e p r e s e n t t h e windward g e n t l e s l o p e of sand dunes and c o n s i s t o f r a t h e r t h i n s u b - h o r i z o n t a l laminae which d i p g e n t l y (3-10')
upwind.
They
g e n e r a l l y c o n s i s t of f i r m l y packed s a n d a r e a r e n o t commonly p r e s e r v e d i n a n c i e n t a e o l i a n sandstones. Encroachment d e p o s i t s a r e t n e f o r e s e t laminae produced by a v a l a n c h i n g of sand on t h e s l i p f a c e .
They form t h e f a m i l i a r c r o s s bedding of a e o l i a n s a n d d e p o s i t s .
Laminae t e n d t o b e r a t h e r t h i c k (2-5cm) and d i p 25 t o 34'
downwind.
They
c o n s t i t u t e t h e major p a r t o f a dune body and a r e most commonly p r e s e r v e d i n a n c i e n t aeolian sandstones.
Tne s t e e p l y d i p p i n g f o r e s e t laminae t e n d t o f l a t t e n o u t n e a r
t h e base of t h e s l i p face producing t h e c h a r a c t e r i s t i c asymptotic f o r e s e t s . The laminae o f dune s a n d s a l s o show d e f o r m a t i o n s t r u c t u r e s .
These a r e thought
t o be due t o t h e movement o f p a r t i a l l y c o h e s i v e s a n d masses on t h e s l i p f a c e a f t e r r a i n has f a l l e n (McKee e t a l . , 1971; B i g a r e l l a , 1972; McKee and B i g a r e l l a , 1972). D e t a i l e d d e s c r i p t i o n s o f t h e i n t e r n a l s t r u c t u r e o f i n d i v i d u a l dune t y p e s have been g i v e n i n t h e p r e v i o u s s e c t i o n s .
These a r e based l a r g e l y on t h e dunes o f gypsum
sand a t White Sands, New Mexico and i t i s n o t c l e a r how t y p i c a l t h e y a r e o f a e o l i a n dunes i n g e n e r a l .
A v a i l a b l e e v i d e n c e ( B i g a r e l l a e t a l . , 1969; G l e n n i e , 1970)
s u g g e s t s t h a t t h e y c l o s e l y compare w i t h q u a r t z i t i c dunes.
These d a t a can be used
t o i d e n t i f y d i f f e r e n t dune t y p e s i n a n c i e n t a e o l i a n sequences and u n d e r i d e a l c i r c u m s t a n c e s e n a b l e t h e d i s t i n c t i o n of t r a n s v e r s e from l o n g i t u d i n a l t y p e s ( S h o t t o n , 1956; G l e n n i e , 1970).
T r a n s v e r s e dunes s u c h a s b a r c h a n s produce a s i n g l e mode
o f d i r e c t i o n s when t h e f o r e s e t a t t i t u d e s a r e p l o t t e d on a s t e r e o g r a p h i c p r o j e c t i o n . L o n g i t u d i n a l d u n e s , however, produce a bimodal p a t t e r n b e c a u s e of t h e a l t e r n a t i n g s l i p f a c e s a l t h o u g h t h i s might be more c o m p l i c a t e d because o f t h e i n f l u e n c e of superimposed dunes.
S t u d y o f t h e Lower Permian dune bedding i n NW Europe by G l e n n i e
(1972) s u g g e s t s t h a t l o n g i t u d i n a l dunes may dominate upland a r e a s whereas t r a n s v e r s e t y p e s a r e more abundant i n t h e c e n t r a l p a r t o f t h e d e s e r t b a s i n ( F i g . 2 . 1 1 ) . Trough cross-bedded u n i t s a r e n o t commonly d e s c r i b e d from a e o l i a n s a n d d e p o s i t s b u t have been d e s c r i b e d i n some a n c i e n t examples ( B i g a r e l l a , 1972).
They a r e
86 c o n c e i v a b l y produced by t h e m i g r a t i o n of a n a k l e dune o r d r a a p a t t e r n i n a manner analogous t o t h e p r o d u c t i o n of aqueous t r o u g h c r o s s - b e d d i n g by t h e m i g r a t i o n of l i n g u o i d o r l u n a t e e l e m e n t s ( A l l e n , 1968; B r o o k f i e l d , 1977).
Fig.2.11. S t e r e o g r a p h i c p r o j e c t i o n s ( u p p e r hemisphere) of p o l e s t o Lower Permian dune bedding. A and B a r e from o u t c r o p s i n n o r t h e r n England. C and D a r e from w e l l s i n t h e s o u t h e r n North Sea. Deduced palaeowind d i r e c t i o n s a r e shown. Dashed l i n e s i n d i c a t e a r e a s o f t y p i c a l t r a n s v e r s e ( a ) and l o n g i t u d i n a l dune bedding a t t i t u d e s ( b ) ( a f t e r G l e n n i e , 1972). Bounding s u r f a c e s i n a e o l i a n s a n d d e p o s i t s have been d i s c u s s e d by a number o f authors.
A c l a s s i f i c a t i o n , b a s e d p r i m a r i l y on a n c i e n t examples h a s beefl proposed
by B r o o k f i e l d (1977) ( F i g . 2 . 1 2 ) .
H e d i s t i n g u i s h e s t h r e e o r d e r s o f a e o l i a n bounding
s u r f a c e s which a r e a r r a n g e d i n a h i e r a r c h y a n a l a g o u s t o t h e bedform h i e r a r c h y o f Wilson (1972). First-order
These a r e termed f i r s t , second, and t h i r d - o r d e r bounding s u r f a c e s . bounding s u r f a c e s a r e f l a t - l y i n g o r convex-up bedding p l a n e s c u t t i n g
a c r o s s c r o s s - b e d d i n g and o t h e r dune s t r u c t u r e s ,
The d i s t a n c e s between t h e s e p l a n e s
r a n g e s from 0.3-17m and t h e y t e n d t o b e r e g u l a r l y s p a c e d i n one p a r t i c u l a r formatior Opdyke and Runcorn (1960) c o n s i d e r e d them t o be due t o t h e i n t e r a c t i o n of a e o l i a n d e f l a t i o n and d e p o s i t i o n i n a d j a c e n t a r e a s .
S t o k e s (1968) thought t h a t t h e y were
= o
50
Fig.2.12. S k e t c h diagrams of q u a r r y f a c e s i n t h e Lower Permian dune s a n d s of t h e Dumfries b a s i n , S c o t l a n d showing a e o l i a n bounding s u r f a c e s : F i r s t - o r d e r surfaces t h i c k l i n e s , second-order surfaces - t h i n l i n e s , third-order surfaces dashed l i n e s ( i n s e t 9 ) . Cross laminae a r e shown by d o t t e d l i n e s ( a f t e r B r o o k f i e l d , 1977).
-
-
c o n t r o l l e d by groundwater l e v e l s i n t h e f o l l o w i n g manner:
a s t h e s u r f a c e of
s e d i m e n t a t i o n r i s e s s p a c e i s c r e a t e d f o r groundwater s a t u r a t i o n and w i t h n e a r s u r f a c e w a t e r t a b l e and s t r o n g e r t h a n u s u a l wind a c t i o n t h e d r y s a n d w i l l b e s t r i p p e d o f f and t h e zone of s a t u r a t i o n exposed.
A change i n wind s t r e n g t h would
c a u s e dunes t o m i g r a t e o v e r t h e s u r f a c e t h u s c r e a t i n g a bedding p l a n e . seems u n l i k e l y
-
This process
t h e major o b j e c t i o n b e i n g t h a t t h e w a t e r t a b l e i n s a n d s e a s i s n o t
h o r i z o n t a l b u t forms a subdued r e p l i c a o f t h e bedform geometry (McKee and Moiola, 1975).
B r o o k f i e l d ( 1 9 7 7 ) a t t r i b u t e s f i r s t - o r d e r bounding s u r f a c e s t o d r a a m i g r a t i o n .
A s dunes m i g r a t e and climb o v e r e a c h o t h e r a n d a c r o s s i n t e r d u n e a r e a s t h e b a s e s of
e a r l i e r bedforms a r e n o t c o m p l e t e l y removed and a p p e a r a s t r u n c a t e d f o r e s e t s i n t h e
88 a r e a s of d e f l a t i o n (McKee, 1966; McKee and Moiola, 1975).
Examples of f i r s t - o r d e r
bounding s u r f a c e s i n t h e Permian s a n d s t o n e s o f t h e D u m f r i e s h i r e b a s i n i n s o u t h e r n S c o t l a n d and t h e T r i a s s i c s a n d s t o n e s a t Frodsham, C h e s h i r e a r e g i v e n by B r o o k f i e l d (1977).
B r o o k f i e l d (1977) a l s o g i v e s d a t a on t h e r a t e of m i g r a t i o n of d r a a s and
d i s c u s s e s t h e e f f e c t s of c l i m b i n g bedforms and s u b s i d e n c e r a t e s on t h e b u i l d - u p of f i r s t - o r d e r bounding s u r f a c e s . Second-order bounding s u r f a c e s a r e c u t by f i r s t - o r d e r bounding s u r f a c e s a n d c o n t a i n w i t h i n them cross-bedded u n i t s ( F i g . 2 . 1 2 ) .
Although o n l y s p a r s e d a t a i s
a v a i l a b l e B r o o k f i e l d s u g g e s t s a c h a r a c t e r i s t i c s p a c i n g of between 1 and 1.5m w i t h i n c l i n a t i o n a n g l e s from 0-15'.
They a r e c o n s i d e r e d by B r o o k f i e l d (1977) t o mark
t h e p a s s a g e of dunes a c r o s s d r a a s , o r t o l o n g i t u d i n a l dunes m i g r a t i n g a c r o s s t h e lower l e e s l o p e s of d r a a s o r a l t e r n a t i v e l y t o t h e c l i m b i n g of dunes down a d r a a which does n o t have a well-developed s l i p f a c e .
This draa i n t e r p r e t a t i o n f o r
second-order bounding s u r f a c e s a c c o u n t s f o r t h e t y p i c a l sweeping f o r e s e t laminae c h a r a c t e r i s t i c of many a n c i e n t a e o l i a n s a n d s t o n e s w i t h o u t having t o i n v o k e prolonged reworking by cross-winds ( S h a r p , 1966). T h i r d - o r d e r bounding s u r f a c e s a r e c o n s i d e r e d by B r o o k f i e l d (1977) t o b e r e a c t i v ation surfaces.
These a r e d i s c o n t i n u i t i e s i n t h e l a t e r a l sequence of c r o s s - b e d s
which t a k e t h e form o f e i t h e r a p l a n a r o r convex-upwards e r o s i o n s u r f a c e .
The
c r o s s - b e d s u n d e r l y i n g t h e r e a c t i v a t i o n s u r f a c e a r e t r u n c a t e d , whereas t h o s e above a r e conformable ( A l l e n , 1 9 7 4 ~ ) . R e a c t i v a t i o n s u r f a c e s i n a e o l i a n sands may i n d i c a t e change i n wind d i r e c t i o n o r v e l o c i t y , o r a l t e r n a t i v e l y , t h e a c t i o n of v o r t i c e s g e n e r a t e d by bedforms upstream.
There i s l i t t l e e v i d e n c e t o s u p p o r t t h e f i r s t o f
t h e s e hypotheses and i t seems more l i k e l y t h a t t h e r e a c t i v a t i o n s u r f a c e s a r e t h e r e s u l t of l o c a l a i r flow changes caused by c o n f i g u r a t i o n a l changes i n dune p a t t e r n s . For example, a dune coming under t h e i n f l u e n c e of e d d i e s t o t h e l e e o f upwind dunes c o u l d have i t s c r e s t rounded o f f and t h e s l o p e o f t h e upper lee s i d e c o n s i d e r a b l y reduced, thus producing a r e a c t i v a t i o n surface. The t e x t u r a l c h a r a c t e r i s t i c s o f a e o l i a n sands The s i m i l a r i t y of a e o l i a n and aqueous c r o s s - b e d d i n g makes i t v e r y d i f f i c u l t on occasions t o recognize a n c i e n t a e o l i a n sands,
The d i f f i c u l t y may be p a r t l y overcome
by c a r e f u l s t u d y o f t h e t e x t u r a l p r o p e r t i e s o f a e o l i a n sand which may b e v e r y distinctive.
D i s c r i m i n a t i o n of a e o l i a n sand from o t h e r t y p e s of s a n d u s i n g s c a t t e r
diagrams of g r a i n s i z e p a r a m e t e r s ( e . g . Friedman, 1961) has now l a r g e l y been abandoned b e c a u s e of t h e o v e r l a p s between d i f f e r e n t e n v i r o n m e n t s , p a r t i c u l a r l y beaches (Moss, 1962, 1963).
A more r e a l i s t i c approach i s t o c o n s i d e r t h e whole
g r a i n s i z e spectrum i n c l u d i n g t h e p r o p o r t i o n s of d i f f e r e n t s u b p o p u l a t i o n s .
Folk
(1968b), f o r example, h a s shown t h a t d e s e r t f l o o r s a n d s a r e t y p i c a l l y bimodal and supermature and t h a t t h e s e c h a r a c t e r i s t i c s a r e connnon i n a n c i e n t a e o l i a n s a n d s t o n e s .
89 O t h e r t e x t u r a l p r o p e r t i e s which a r e commonly o b s e r v e d i n a e o l i a n s a n d s i n c l u d e t h e h i g h d e g r e e of roundness and t h e s u r f a c e p i t t i n g ( f r o s t i n g ) o f q u a r t z g r a i n s .
The
roundness o f a e o l i a n sand g r a i n s r e s u l t s from t h e h i g h l e v e l o f p h y s i c a l a t t r i t i o n which t h e y e x p e r i e n c e .
The f r o s t i n g c a n n o t b e a t t r i b u t e d t o sample impact marking
and i s more l i k e l y t h e r e s u l t o f s m a l l s c a l e s o l u t i o n and p r e c i p i t a t i o n of s i l i c a during t h e d i u r n a l temperature cycle.
Some of t h e s e problems a r e now c o n s i d e r e d
i n more d e t a i l . Grain s i z e Wind i s a p a r t i c u l a r l y good s o r t i n g medium and i t s e f f e c t s can b e b e s t i l l u s t r a t e d by c o n s i d e r i n g t h e t h r e e r e l a t e d s i z e - f a m i l i e s of s e d i m e n t a r y p a r t i c l e s a s s o c i a t e d with deserts.
These a r e :
1) c o a r s e s a n d and g r a v e l which a r e e a s i l y r o l l e d by t h e wind b u t a r e t o o l a r g e t o s a l t a t e ; t h e s e accumulate i n w e l l - s o r t e d groups (e.g.
ripple crests)
2 ) medium and f i n e sand which t r a v e l s l a r g e l y by s a l t a t i o n and which accumulates a s w e l l - s o r t e d s a n d dunes 3) v e r y f i n e s a n d and c o a r s e s i l t which, i f u n d i s t u r b e d and on a f l a t s u r f a c e , i s t o o s m a l l t o b e e a s i l y moved by t h e wind; b u t when d i s t u r b e d may t r a v e l g r e a t distances (e.g. dust storms). The s a l t a t i o n p o p u l a t i o n , 2) a b o v e , i s t h e p r i n c i p a l c o n s t i t u e n t o f a e o l i a n sand dunes.
Udden(1898) showed t h a t dune c r e s t s were p r e d o m i n a n t l y i n t h e 0.25-0.125mm
s i z e (2-30) r a n g e r e g a r d l e s s o f l o c a l i t y and was w e l l aware of t h e good s o r t i n g of a e o l i a n sands.
Many o t h e r a u t h o r s have commented on t h e f i n e g r a i n s i z e and
good s o r t i n g of a e o l i a n dunes b u t t h e r e a r e s u r p r i s i n g l y few s y s t e m a t i c a n a l y s e s f o r i n l a n d d e s e r t s and r a t h e r more f o r c o a s t a l dunes.
Alimen (1957a,b) and
C h a v a i l l i o n (1964: have s t u d i e d many samples from t h e S a h a r a , Moiola and Weiser (1968) s t u d i e d 30 samples from s e v e r a l d e s e r t s and i n v e s t i g a t e d t h e r e l a t i o n s h i p s between mean g r a i n s i z e , s o r t i n g , and t h e h i g h e r moment measures.
W a i t t (1969)
c o n s i d e r e d s t u d i e d g r a i n s i z e p a r a m e t e r s i n r e l a t i o n t o dune p o s i t i o n i n a West Texas dune f i e l d .
Folk (1968b, 1971b) has s t u d i e d t h e a e o l i a n s a n d s of t h e Simpson
Desert i n A u s t r a l i a i n some d e t a i l .
F u r t h e r d a t a i s p r o v i d e d by McKee and T i b b i t t s
( 1 9 6 4 ) , Sharp ( 1 9 6 6 ) , Warren (1971, 1972) and Skozek a n d S a a d a l l a h (1972).
The
g r a i n s i z e p a r a m e t e r s of a e o l i a n d u n e s , a s r e p o r t e d by t h e s e a u t h o r s , can b e c o n s i d e r e d a s a continuum between two d i s t i n c t i v e t y p e s .
The b e s t known o f t h e s e
i s t h e f i n e g r a i n e d v e r y w e l l - s o r t e d t y p e d e s c r i b e d by Udden ( 1 9 9 8 ) .
This i s
t y p i c a l l y p o s i t i v e l y skewed w i t h a s h a r p c u t - o f f towards t h e c o a r s e r g r a i n s and has a t a i l of f i n e m a t e r i a l (Fig.2.13E,F).
T h i s f i n e m a t e r i a l i n a e o l i a n dunes i s
p r o b a b l y due t o t h e s e t t l e m e n t o f d u s t a f t e r t h e c e s s a t i o n o f winds (Sokolow, 1894). Another v e r y common sand found i n dune a r e a s i s a bimodal sand w i t h a c o a r s e mode a t a b o u t 0 . 6 m (0.750) and a f i n e mode a t a b o u t 0.12mm ( 3 . 0 0 ) (Bagnold, 1941;
90
Fig.2.13. R e p r e s e n t a t i v e g r a i n s i z e d i s t r i b u t i o n s ( s o l i d l i n e s ) from a e o l i a n sands o f t h e Simpson Desert, A u s t r a l i a , The dashed l i n e s show how i n d i v i d u a l d i s t r i b u t i o n s can b e broken down i n t o c o a r s e (C), medium (M), and f i n e ( F ) G a u s s i a n s u b p o p u l a t i o n s ( a f t e r F o l k , 1971b).
91 F o l k , 1968b, 1971b; Warren, 1971, 1972).
Typically t h e g r a i n diameter r a t i o i n
t h e s e bimodal s a n d s i s between 4 : l and 8 : l and t h e y a r e b e s t developed i n f l a t i n t e r d u n e a r e a s , p a r t i c u l a r l y downwind o f s e i f s ( F o l k , 1971b; Warren, 1972) (Fig.2.13B,C).
Many z i b a r a r e a l s o bimodal (Cooke and Warren, 1973) b e c a u s e t h e
c o a r s e g r a i n s e f f e c t i v e l y s e a l o f f t h e s u r f a c e so t h a t o n l y s t r o n g winds can b u i l d dunes. The o r i g i n o f bimodal s a n d s on t h e d e s e r t f l o o r h a s been a m a t t e r o f some discussion.
F o l k (1968b) a r g u e d t h a t t h e b i m o d a l i t y r e s u l t e d from t h e s e l e c t i v e
removal of t h e s a l t a t i o n p o p u l a t i o n from i n i t i a l l y u n s o r t e d d e t r i t u s .
H e thus
c o n s i d e r e d t h e c o a r s e mode t o r e p r e s e n t t h e s u r f a c e c r e e p and t h e f i n e mode t o r e p r e s e n t c o h e s i v e m a t e r i a l w i t h a h i g h c r i t i c a l t h r e s h o l d f o r movement ( F i g . 2 . 1 4 ) . F i n e r s e d i m e n t s do i n d e e d have h i g h c r i t i c a l t h r e s h o l d f o r movement (Bagnold, 1941, p.88).
However t h i s i s p a r t l y due t o i n t e r p a r t i c l e c o h e s i o n (Smalley, 1964, 1970)
and o n l y a p p l i e s t o v e r y w e l l s o r t e d f i n e s a n d s ; when t h e f i n e sand e x i s t s i n a p o o r l y s o r t e d m i x t u r e i t w i l l b e c o n s t a n t l y bombarded by s a l t a t i n g c o a r s e r g r a i n s . An
The d i s t u r b a n c e c r e a t e d i s t h u s l i k e l y t o c a u s e e n t r a i n m e n t of t h e f i n e sands.
a l t e r n a t i v e , and more s a t i s f a c t o r y , e x p l a n a t i o n of b i n o d a l i s m has been p u t forward by Warren (1972).
T h i s h a s been c a l l e d t h e " p r o t e c t i o n i s t theory".
I n t h i s theory
t h e s a n d s i n s a l t a t i o n i n c l u d e some p a r t i c l e s t h a t a r e f i n e enough t o f i t i n t o t h e i n t e r p a r t i c l e v o i d s of t h e c r e e p l o a d , so t h a t when t h e y s t r i k e t h e bed t h e y p e n e t r a t e t h e v o i d s and a r e p r o t e c t e d from f u r t h e r bombardment.
The s l i g h t l y c o a r s e r f r a c t i o n
o f t h e s a l t a t i o n l o a d i s t o o l a r g e f o r t h e v o i d s so t h a t i t remains on o r n e a r t h e s u r f a c e of t h e bed where i t i s bombarded by o t h e r p a r t i c l e s and moved onward. s a n d i s t h u s f i l t e r e d o u t and a c c u m u l a t e s on dunes.
This
This p r o t e c t i o n i s t theory
e x p l a i n s t h e e x i s t e n c e o f bimodal s a n d s on dunes a s w e l l a s on a r e a s where t h e r e h a s been n e t d e f l a t i o n .
I t a l s o e x p l a i n s t h e o c c u r r e n c e o f c o r e s o f f i n e sand w i t h i n
r i p p l e s ( S h a r p , 1963) and bimodal s a n d s on t h e h o r n s of b a r c h a n s ( V e r l a q u e , 1958). These g r a i n s i z e c h a r a c t e r i s t i c s a r e w i d e l y o b s e r v e d i n a n c i e n t a e o l i a n s a n d s t o n e s . Some of them, from t h e Lower Permian d e s e r t of NW Europe a r e i l l u s t r a t e d i n Fig.2.15. Roundness and f r o s t i n g The most c h a r a c t e r i s t i c f e a t u r e s o f a e o l i a n s a n d g r a i n s a r e t h e h i g h d e g r e e of roundness and t h e i r f r o s t e d s u r f a c e a p p e a r a n c e .
These f e a t u r e s a l o n e have o f t e n been
used t o i d e n t i f y a e o l i a n e n v i r o n m e n t s , a p r a c t i c e which i s n o t t o b e recommended b e c a u s e a e o l i a n sand g r a i n s can be d e r i v e d and reworked i n t o s h a l l o w marine environments and a l s o b e c a u s e s i m i l a r f e a t u r e s may r e s u l t from subaqueous t r a n s p o r t (Margolis and K r i n s l e y , 1971).
However well-rounded g r a i n s w i t h well-developed
s u r f a c e f r o s t i n g do p r o v i d e a n i m p o r t a n t c l u e t o a n a e o l i a n o r i g i n ( G l e n n i e , 1972). There have been a number of s c a n n i n g e l e c t r o n microscope (SEM) s t u d i e s of t h e s u r f a c e f e a t u r e s of a e o l i a n s a n d g r a i n s ( M a r g o l i s and K r i n s l e y , 1971; K r i n s l e y and
92
UNSORTED FLUVIAL DETRITUS SOURCE MATERIAL
.
.
Md Sd.+ Fn. Sd MOVED EASIEST
DEFLATION PROCESS DUNE
BIMODAL PRODUCT Bimodal Cr. Sd.+VFn.Sd.
Unimodal Fn. -Md. Sd.
Fig.2.14. I d e a l i z e d diagram d e p i c t i n g t h e o r i g i n of bimodal a e o l i a n s a n d s by d e f l a t i o n . F i n e sand i s removed from u n s o r t e d f l u v i a l d e t r i t u s b e c a u s e t h i s i s most e a s i l y s a l t a t e d . F i n e r s e d i m e n t remains b e h i n d b e c a u s e i t i s t o o c o h e s i v e w h i l s t c o a r s e r g r a i n s c a n n o t b e moved by s a l t a t i o n ( a f t e r F o l k , 1968b). Doornkamp, 1973) and i t i s now r e c o g n i z e d t h a t b o t h g r a i n impacts and s o l u t i o n / r e p r e c i p i t a t i o n a r e important i n t h e i r formation.
K r i n s l e y e t a l . (1976) r e c o g n i z e d
f o u r t y p e s of t e x t u r a l f e a t u r e s t h a t seem t o b e c h a r a c t e r i s t i c of a e o l i a n q u a r t z g r a i n s from modern h o t d e s e r t s f F i g . 2 . 1 6 ) .
1. "Upturned p l a t e s " .
These a r e p a r a l l e l r i d g e s r a n g i n g i n l e n g t h from a b o u t 0.5Ym
t o a b o u t 1Ofm and c o v e r i n g t h e s u r f a c e s of most g r a i n s g r e a t e r t h a n 4 0 0 - 5 0 0 ~i n diameter.
These u p t u r n e d p l a t e s a r e i n t e r p r e t e d a s c l e a v a g e s c a r p s r e s u l t i n g from
" a b r a s i o n f a t i g u e " ( P a s c o e , 1961) due t o s a l t a t i o n i m p a c t s .
2 . Equidimensional o r e l o n g a t e d e p r e s s i o n s 20 t o 250 m i n l e n g t h .
These d e p r e s s i o n s
tend t o be on s m a l l e r g r a i n s and a r e i n t e r p r e t e d by K r i n s l e y e t a l . (1976) a s p o s s i b l y b e i n g due t o d i r e c t r a t h e r t h a n g l a n c i n g i m p a c t s between s a l t a t i n g o r creeping grains.
3. Smooth s u r f a c e s on s m a l l e r g r a i n s , 90-400ym i n s i z e .
These a r e due t o p r e -
c i p i t a t i o n and s o l u t i o n o f s i l i c a and a r e u n a f f e c t e d by a b r a s i o n t o any g r e a t e x t e n t because such g r a i n s a r e t r a n s p o r t e d mainly i n s u s p e n s i o n .
4 . A r c u a t e , c i r c u l a r o r p o l y g o n a l c r a c k s (Lucci and Casa, 1968) commonly found on
93
Fig.2.15. Thin s e c t i o n photomicrographs showing some c h a r a c t e r i s t i c t e x t u r e s o f a e o l i a n sands. All from t h e R o t l i e g e n d e s (Lower Permian) of t h e S o l e P i t a r e a , North Sea. A. F i n e g r a i n e d , w e l l - s o r t e d s a n d from a dune t o p . B . Medium g r a i n e d s a n d from t h e i n t e r m e d i a t e p a r t of a dune. C. Bimodal sand from a dune base. Note t h a t t h e f i n e r mode i s a n g u l a r and t h e c o a r s e r mode rounded.
94
Fig.2.16. A. A q u a r t z s a n d g r a i n from t h e Algodones dunes ( s o u t h e r n C a l i f o r n i a ) . The g r a i n i s smooth and rounded w i t h a s u g g e s t i o n o f u p t u r n e d p l a t e s on t h e lower r i g h t s i d e B. Closeup of A. Note u p t u r n e d p l a t e s (UP) s l i g h t l y rounded by s o l u t i o n C. Quartz sand g r a i n from T r i a s s i c o f Devonshire showing e l o n g a t e o r rounded d e p r e s s i o n s and upturned p l a t e s on t h e u p p e r l e f t - h a n d c o r n e r D. Closeup o f C. Note s i m i l a r i t y t o B e x c e p t t h a t t h e u p t u r n e d p l a t e s (W) a r e l a r g e r ( a f t e r K r i n s l e y e t a l . , 1976).
95 g r a i n s 90 t o 150um i n s i z e .
These f e a t u r e s might b e t h e r e s u l t of p h y s i c a l o r
chemical w e a t h e r i n g and a r e n o t p r e s e n t on l a r g e r g r a i n s b e c a u s e i t would p r o b a b l y b e removed by a b r a s i o n d u r i n g s a l t a t i o n o r c r e e p . These f e a t u r e s have been r e c o g n i z e d i n a v a r i e t y o f a e o l i a n s a n d s from t h e d e s e r t s of Libya, A r a b i a , A u s t r a l i a , New Mexico and N e w Mexico and were a l s o d e s c r i b e d from E a r l y T r i a s s i c r e d beds from South Devonshire:
c o n s i d e r e d by t h e a u t h o r s t o b e t h e
o l d e s t known example of a e o l i a n s u r f a c e t e x t u r e s unmodified by d i a g e n e s i s . DESERT LAKES AND INLAND SABKHAS The t o p o g r a p h i c a l l y l o w e s t a r e a s o f many d e s e r t s o f t e n c o n t a i n ephemeral s a l i n e lakes.
More permanent l a k e s r e s u l t b e c a u s e o f t h e i n l a n d d r a i n a g e p a t t e r n and t h e
f a c t t h a t t h e w a t e r t a b l e i s v e r y c l o s e t o t h e s u r f a c e i n t h e c e n t r a l p a r t of t h e
INFLUENT SEEPAGE
EFFLUENT SEECAOE
INFLUENT SEEPAGE
/
ALLUVIAL FANS PLAYA EVAPORITES
LACUSTRINE
A
A//
BEDROCK
Fig.2.17. Schematic diagram of a d e s e r t b a s i n showing t h e d i s t r i b u t i o n of s e d i m e n t a r y f a c i e s and t h e w a t e r c i r c u l a t i o n ( a f t e r Davis and De Wiest, 1 9 6 6 ) . d e s e r t basin (Fig.2.17).
The d i s t r i b u t i o n o f s e d i m e n t a r y f a c i e s and groundwater
c i r c u l a t i o n a r e closely interdependent.
I n t e c t o n i c a l l y a c t i v e basins a l l u v i a l fans
a r e developed around t h e margins and t h e s e have i n f l u e n t d r a i n a g e .
These a r e
f o l l o w e d , towards t h e c e n t r e of t h e b a s i n , by a b e l t o f a e o l i a n s a n d and i n t h e v e r y c e n t r e of t h e b a s i n where t h e d r a i n a g e becomes e f f l u e n t , d e s e r t l a k e s may develop. They a l s o m i g h t form where dune s a n d s b l o c k a wadi c h a n n e l f o r example ( s e e G l e n n i e , 1970, F i g . 4 9 ) .
These d e s e r t l a k e s a r e r e f e r r e d t o a s p l a y a s i n North America and
i n l a n d sabkhas i n t h e Middle E a s t . They range i n s i z e from j u s t a few s q u a r e metres 2 up t o thousands o f km , Lake E y r e , A u s t r a l i a , f o r example, had a maximum a r e a l e x t e n t
of a b o u t 8,000km2 (Twidale, 1972).
The more r e c e n t h i s t o r y o f p r e s e n t - d a y p l a y a s i s
96 h i g h l y complicated and many of them were p e r e n n i a l l a k e s i n t h e p a s t ( S t o e r t z and E r i k s o n . 1974) e s p e c i a l l y d u r i n g P l e i s t o c e n e times. When t h e d e s e r t l a k e d r i e s up, d u r i n g t h e h o t s e a s o n , t h e s i l t a n d c l a y d e p o s i t e d when t h e l a k e was f u l l becomes d i s r u p t e d .
T h i s may b e by t e r r e s t r i a l a n i m a l s o r o t h e r
a g e n c i e s which l e a v e abundant t r a c k s on t h e sediment s u r f a c e .
Wind blown s a n d g r a i n s
may s t i c k t o t h e damp sabkha s u r f a c e and a s m o i s t u r e c o n t i n u e s t o r i s e by c a p i l l a r i t y a d d i t i o n a l g r a i n s w i l l a d h e r e and a d h e s i o n r i p p l e s w i l l form.
These a r e one o f t h e
most c h a r a c t e r i s t i c sedimentary s t r u c t u r e s o f i n l a n d sabkhas.
D e s i c c a t i o n polygons
develop and may c u r l up a t t h e edges l e a v i n g c r a c k s which a r e f i l l e d from above by wind blown sand o r from below by t h e i n j e c t i o n of a s l u r r y o f wet sand (Oomkens, 1966; G l e n n i e , 1970).
E v a p o r a t i o n of t h e s a l i n e l a k e w a t e r l e a d s t o t h e c o n c e n t r a t i o n
of s a l t s and a c r u s t of h a l i t e may form on t h e former l a k e bottom; gypsum c r y s t a l s ma) grow w i t h i n t h e sediment c a u s i n g f u r t h e r d i s r u p t i o n o f bedding s t r u c t u r e s .
Other
m i n e r a l s p e c i e s may b e i m p o r t a n t i n d e s e r t l a k e s and t h e s e may show a z o n a l a r r a n g e ment w i t h t h e more s o l u b l e s a l t s b e i n g d e p o s i t e d towards t h e c e n t r e of t h e b a s i n (Hunt e t e l . , 1966; Amiel and Friedman, 1971). I n Death V a l l e y , C a l i f o r n i a t h r e e main m i n e r a l f a c i e s c a n be r e c o g n i z e d e a c h w i t h s u b - f a c i e s determined on t h e b a s i s o f s u r f a c e morphology o r t h e amount o f admixture (Hunt e t a l . , 1966).
clastic
C a r b o n a t e s , s u l p h a t e s , and c h l o r i d e s show a r o u g h l y
c o n c e n t r i c z o n a t i o n w i t h t h e c h l o r i d e f a c i e s occupying by f a r t h e l a r g e s t c e n t r a l a r e a (Fig.2.18).
H a l i t e i s v o l u m e t r i c a l l y t h e dominant m i n e r a l i n t h e b a s i n and t h e
c a r b o n a t e and s u l p h a t e f a c i e s a r e d e f i n e d on t h e b a s i s of r e l a t i v e l y minor amounts of c a l c i t e and gypsum.
The c a r b o n a t e f a c i e s e r c u r s a t t h e t r a n s i t i o n from d i s t a l
f a n t o s a l t p a n (Fig.2.19), fine i n t o the basin.
t h e c a l c i t e o c c u r r i n g w i t h i n t h e c l a s t i c s e d i m e n t s which
These c a l c i t e - b e a r i n g s i l t s a r e e n c r u s t e d on t h e s u r f a c e by
h a l i t e b e n e a t h which i s a l a y e r o f gypsum nodules.
I n t h e s u l p h a t e f a c i e s gypsum
i s p r e c i p i t a t e d a t t h e s u r f a c e a s e i t h e r a massive l a y e r o r a s a c r y s t a l mush w i t h
a n i r r e g u l a r m i c r o r e l i e f (Fig.2.18).
The c h l o r i d e s c o n t a i n a number o f s u b - f a c i e s ,
t h e most e x t e n s i v e o f which c o n s i s t s of a smooth s i l t l a y e r up t o 15cm t h i c k which
i s u n d e r l a i n by up t o 30cm o f h a l i t e and rests on c l a s t i c sediment.
The s i l t l a y e r
c o n t a i n s s m a l l nodules of s u l p h a t e s and b o r a t e s sometimes cemented by c a r b o h a t e and h a l i t e and forming a hardpan.
The h a l i t e probably formed a t t h e t o p o f t h e c a p i l l a r y
f r i n g e of t h e w a t e r t a b l e . THE REDDENING OF DESERT SEDIMENTS
Introduction The reddening o f d e s e r t dune sands i s c e n t r a l t o t h e problem o f r e d bed formation. E a r l y workers impressed by r e d dune sands i n d e s e r t s were o f t h e o p i n i o n t h a t a n c i e n t r e d beds were a l s o of d e s e r t o r i g i n (Dawson, 1848; Crosby, 1885, 1891; Goodchild, 1896; B a r r e l l , 1908).
I t was g e n e r a l l y b e l i e v e d t h a t t h e r e d n e s s o f d e s e r t s a n d s was
L6 Fig.2.18. M i n e r a l z o n a t i o n i n t h e Death V a l l e y p l a y a of C a l i f o r n i a . I n t h e s u l p h a t e zone massive gypsum has a t h i n s k i n o f a n h y d r i t e . T h i s i s u n d e r l a i n by c a l c i t i c sandy s i l t . I n t h e c a r b o n a t e zone (expanded s e c t i o n ) s u r f a c e h a l i t e o c c u r s a s a b l i s t e r - l i k e c r u s t above t h e gypsum o f t h e s u l p h a t e l a y e r . The c a l c i t e of t h e c a r b o n a t e l a y e r o c c u r s a s s m a l l e u h e d r a l c r y s t a l s i n a s i l t y and sandy m a t r i x ( a f t e r Hunt e t a l . , 1966). caused by t h e low w a t e r t a b l e and l a c k of o r g a n i c m a t t e r which r e s u l t e d i n a n o x y g e n a t i n g environment (Dawson, 1848).
I n t h i s environment t h e d e s e r t sun was
presumed t o c a u s e t h e d e h y d r a t i o n of yellow f e r r i c h y d r o x i d e s t o r e d f e r r i c o x i d e (Crosby, 1885, 1891; B a r r e l l , 1908). Although t h e r e are many r e c o r d s o f r e d s u r f a c e m a t e r i a l s i n d e s e r t s t h e y a r e by no means e x c l u s i v e l y so ( G l e n n i e , 1970).
I n t h e a r i d Sudan Buursink (1971) found
v e r y few r e d o r r e d d i s h s o i l s and i n A u s t r a l i a , L i t c h f i e l d (1962, 1963) found drab soils.
V a r i e d c o l o u r s a r e much more t y p i c a l ; Warren (1970) r e p o r t e d 5YR 516 and
even r e d d e r c o l o u r s i n t h e f i x e d a e o l i a n s a n d s of t h e s e m i - a r i d Sudan and White (1971) n o t e d c o l o u r s of 5YR 516-8 and 2.5yR 416-8 i n s i m i l a r f i x e d a e o l i a n s a n d s i n t h e Republic of Niger.
Worral (1969) summarized much o f t h e work on t h e sands of
t h e s o u t h e r n S a h a r a and p o i n t e d o u t t h e wide r a n g e i n c o l o u r f r o n r e d , r e d d i s h - y e l l o w a n d reddish-brown.
G l e n n i e (1970) r e p o r t s r e d a e o l i a n s a n d s from t h e T r u c i a l Coast.
The r e d a e o l i a n sands o f t h e A u s t r a l i a n d e s e r t s a r e well-known ( L i t c h f i e l d , 1962, 1963; F o l k , 1976, 1978).
86 M 3E)NW INlWVNVd
3
Fig.2.19. R e l a t i o n s h i p between f r i n g i n g a l l u v i a l f a n s , s a l t p a n s and t h e d i s t r i b u t i o n of p l a n t s i n Death V a l l e y ( a f t e r Hunt e t a l . , 1966). Enlargement shows a h y p o t h e t i c a l c r o s s - s e c t i o n of l a r g e s c a l e d e s i c c a t i o n f i s s u r e s found i n some p l a y a s ( a f t e r N e a l , 1965). and e l s e w h e r e i n Mexico c o l o u r s of 10R 614 a r e known (Walker, 1967a).
The b r i g h t e s t
c o l o u r s i n r e d d e s e r t s o i l s a r e o f t e n found i n t h e B h o r i z o n a f e a t u r e which can b e a t t r i b u t e d t o more advanced w e a t h e r i n g , i r o n i l l u v i a t i o n , and a l s o t o t h e f a c t t h a t t h e r e i s no c o l o u r masking by o r g a n i c m a t t e r i n t h i s h o r i z o n . Many a u t h o r s have n o t e d t h a t a e o l i a n sand dunes become r e d d e r w i t h t i m e .
A
number o f examples a r e g i v e n by N o r r i s and N o r r i s (1961) and t h e s u b j e c t is d i s c u s s e d by N o r r i s ( 1 9 6 9 ) , G a u t i e r (1935, p . 4 4 ) .
T r i c a r t and Brochu (1955) have observed
t h a t smaller, presumably more a c t i v e and younger, Saharan d u n e s , a r e y e l l o w , whereas t h e l a r g e r e v i d e n t l y f i x e d dunes t e n d t o b e r e d i n c o l o u r . confirmed G a n t i e r ' s o b s e r v a t i o n s on a q u a n t i t a t i v e b a s i s .
Alimen (1957a,b) Logan (19601, i n t h e
d e s e r t s of s o u t h w e s t A f r i c a , showed t h a t younger dunes on t h e c o a s t were p a l e y e l l o w w h i l s t t h e o l d e r , i n l a n d dunes were r e d i n c o l o u r .
Many o t h e r workers have made
s i m i l a r o b s e r v a t i o n s i n c l u d i n g T e r r y (1957) f o r w e s t e r n A u s t r a l i a , Stephens (19611, J a c k s o n (1962) and Wopfner and Twidale (1967) f o r c e n t r a l A u s t r a l i a ; P r i c e (1962) f o r s o u t h e r n Texas, Dolan (1970) i n n o r t h C a r o l i n a , and G l e n n i e (1970) f o r t h e T r u c i a l Coast.
I t i s n o t c l e a r i n a number of t h e s e c a s e s whether t h i s i s due t o
t h e l a c k o f t i m e needed f o r t h e development of pigment o r t o t h e removal o f pigment by a b r a s i o n from t h e more a c t i v e , younger dunes.
For a l o n g t i m e t h e r e d c o l o u r of d e s e r t dune s a n d s has been a t t r i b u t e d t o haematite grain coatings.
T h o u l e t (1881) made chemical a n a l y s e s of h a e m a t i t e - c o a t e d
Saharan sands and P h i l l i p s (1882) r e p o r t e d t h a t r e d s a n d s from t h e Arabian d e s e r t c o n t a i n e d 0.21% Fe203.
P h i l l i p s (1882) a l s o made t h e i m p o r t a n t o b s e r v a t i o n t h a t
t h e r e d pigment must have been a c q u i r e d a f t e r t h e rounding of t h e s a n d o r i t would have o t h e r w i s e been worn away.
However t h e r e have been few d e t a i l e d s t u d i e s of t h e
o x i d e m i n e r a l o g y of r e d d e s e r t s o i l s and dunes.
Walker (1967a) c o u l d n o t d e t e c t
c r y s t a l l i n e h a e m a t i t e i n many of t h e r e d s o i l s of t h e Sonoran d e s e r t and i t i s l i k e l y t h a t more p o o r l y c r y s t a l l i n e o x i d e s o r h y d r o x i d e s , p r o b a b l y i n i n t i m a t e a s s o c i a t i o n with clay minerals, a r e important colouring agents.
These may e x i s t a s g r a i n c o a t i n g s
o r a s i n t e r s t i t i a l i r o n stained c l a y matrix. There i s c o n s i d e r a b l e e v i d e n c e t h a t modern d e s e r t s e d i m e n t s become r e d d e r w i t h age.
F i n e g r a i n e d h a e m a t i t e g r a i n c o a t i n g s and h a e m a t i t e - s t a i n e d c l a y m a t r i x a r e
responsible for the colouration.
These f e a t u r e s a r e c o m p a t i b l e w i t h a r e d d e n i n g
p r o c e s s which i n v o l v e s t h e i n s i t u p r o g r e s s i v e h y d r o l y z a t i o n of ferromagnesian m i n e r a l s ( R e i f e n b e r g , 1947, 1950; R i m , 1951; S h o t t o n , 1956; P r i c e , 1962; Walker, 1967a; N o r r i s , 1969).
Three i m p o r t a n t f a c t o r s , h e a t and m o i s t u r e , time a r e
i m p o r t a n t i n t h e i n t r a s t r a t a l a l t e r a t i o n of f e r r o m a g n e s i a n m i n e r a l s and subsequent d e g r e e o f r e d d e n i n g which might r e s u l t .
Heat i s i m p o r t a n t i n s p e e d i n g up chemical
r e a c t i o n s ; i n d e s e r t s u r f a c e s e d i m e n t t e m p e r a t u r e s between 70-85OC a r e commonly r e c o r d e d ( W a l t h e r , 1900; Mohr and Van Baren, 1954; Cloudsley-Thompson, 1968) and some workers have commented t h a t r e d n e s s was most i n t e n s e n e a r t h e s u r f a c e because o f s o l a r h e a t i n g ( P a s s a r g e , 1904). Moisture i s c l e a r l y important i n t h e formation of hydrated weathering products and a c e r t a i n m i m i m u m amount i s needed i f i n t r a s t r a t a l s o l u t i o n i s t o proceed. There i s c o n s i d e r a b l e e v i d e n c e o f t h e a b i l i t y o f d e s e r t s a n d s t o r e t a i n m o i s t u r e f o r l o n g p e r i o d s under t h e s e c o n d i t i o n s ( C v i j a n o v i c h , 1953).
The c o n d e n s a t i o n of m o i s t u r e
t o form i n t e r n a l dew is a l s o well-known ( C o r n i s h , 1914; R i m , 1951; V e r l a q u e , 1958; Engel and S h a r p , 1958); t h e consequence i s t h a t even d u r i n g t h e d r y s e a s o n dunes may be damp j u s t a metre o r so below t h e s u r f a c e and u n s t a b l e s i l i c a t e m i n e r a l s may b e i n prolonged c o n t a c t w i t h m o i s t u r e e s p e c i a l l y i n f i x e d dunes.
A l t e r a t i o n of f e r r o -
magnesian m i n e r a l s and t h e f o r m a t i o n o f h a e m a t i t e c o u l d t a k e p l a c e under t h e s e conditions, The e v i d e n c e does s u g g e s t however t h a t t h i s a l t e r a t i o n p r o c e s s i s a r e l a t i v e l y slow one and o n l y s e e n t o good e f f e c t i n P l e i s t o c e n e and P l i o c e n e sediments which have been i n p r o l o n g e d c o n t a c t w i t h oxygenated ground w a t e r .
Walker (1967a) i n
h i s c l a s t i c s t u d y o f t h e Sonoran D e s e r t a l l u v i u m showed t h a t t h e f o r m a t i o n of c r y s t a l l i n e h a e m a t i t e and r e d c o l o u r a t i o n c o u l d t a k e up t o 106 y e a r s .
The impl i c a t i o n h e r e i s t h a t t h e r e l a t i v e s c a r c i t y o f modern r e d dune sands i s due t o t h e
f a c t t h a t s u f f i c i e n t t i m e has n o t been a v a i l a b l e f o r t h e i n t r a s t r a t a l a l t e r a t i o n t o develop. The l o n g p e r i o d o f time which i s needed f o r i n s i t u r e d d e n i n g makes i t
100 d i f f i c u l t t o a c c o u n t f o r t h o s e r e d a e o l i a n s a n d s which do o c c u r a t t h e p r e s e n t time; f o r example, some of t h e l a r g e r dunes i n t h e Saharan e r g s c a n be no more t h a n a few thousand y e a r s o l d (Wilson, 1970) and i n t h e Sudanese f i x e d dunes s t a b i l i t y i s c o n s i d e r e d t o have l a s t e d f o r o n l y some 9,000 y e a r s (Warren, 1970).
The p r e s e n t l y
a c t i v e r e d sand dunes of t h e A u s t r a l i a n d e s e r t s a r e a l s o d i f f i c u l t t o e x p l a i n by t h i s d i a g e n e t i c model. A p o s s i b l e c l u e t o t h i s problem l i e s i n t h e once w i d e l y h e l d b e l i e f t h a t r e d n e s s
i n deserts is associated with wetter ciimates i n the past.
This i s p a r t i c u l a r l y
a p p l i c a b l e t o A f r i c a (Grabham, 1926; B o u r c a r t , 1928; S a n d f o r d , 1935; Kubiena, 1955; Meckelein, 1957; Cooke, 1961) and A u s t r a l i a ( C r o c k e r , 1946; Madigan, 1946; Mabbutt, 1965).
T h i s d e t r i t a l model f o r t h e o r i g i n of r e d d e s e r t s a n d s h a s been r e c e n t l y
r e v i v e d by F o l k (1976). The d e t r i t a l model
- Simpson Desert, A u s t r a l i a
The Simpson Desert is a r e d e r g i n c e n t r a l A u s t r a l i a , a b o u t 300 miles i n d i a m e t e r , which c o n s i s t s o f NEJW t r e n d i n g s e i f dunes developed on P l e i s t o c e n e f l u v i a l s e d i m e n t s . The d e s e r t i s t h e r e s u l t of t h e d e s i c c a t i o n o f o l d e r a l l u v i a l p l a i n s which were formed a t t i m e s o f much h e a v i e r r a i n f a l l and l e s s e n e d e v a p o r a t i o n , e s p e c i a l l y d u r i n g t h e P l e i s t o c e n e (Madigan, 1946).
F o l k (1976) s t u d i e d t h e n o r t h w e s t e r n edge of t h e
e r g and sampled t h e c r e s t s and f l a n k s of a c t i v e dunes and a l s o t h e i n t e r v e n i n g deflationary f l a t s o r reg,
The dunes, composed o f w e l l - s o r t e d f i n e s a n d , a r e b r i g h t
r e d i n c o l o u r whereas t h e c o a r s e r r e g sediments a r e d u l l e r and d a r k e r .
This i s
confirmed by q u a n t i t a t i v e measurement of hue, v a l u e and s a t u r a t i o n u s i n g t h e Munsell S o i l Colour C h a r t on s i e v e d
40
grades.
The r e s u l t s a r e summarized i n Fig.2.20.
In
t h e dune s a n d s t h e f i n e and v e r y f i n e s a n d s (between 2.00 and 4.50) a r e a l m o s t a l l of hue 2.5YR ( r e d d i s h - o r a n g e ) , v a l u e 4/ ( d a r k ) and s a t u r a t i o n / 7 ( v e r y s t r o n g ) . The c o a r s e r s e d i m e n t s i n t h e dunes (up t o 1.00) a r e more o r a n g e w i t h hue 3.5YR, v a l u e
5 / (medium) and s a t u r a t i o n / 6 .
Similar r e s u l t s a r e seen i n t h e reg sediments but
t h e hue i s s l i g h t l y more o r a n g e , t h e v a l u e s l i g h t l y d a r k e r and t h e s a t u r a t i o n a l m o s t a f u l l shade weaker (Fig.2.20).
F o l k (1976) n o t e d on i n s p e c t i o n of s i n g l e s a n d g r a i n s
w i t h t h e b i n o c u l a r microscope t h a t t h e c o l o u r d i s t r i b u t i o n i s bimodal; c o a t s on t h e g r a i n s a r e e i t h e r s t r o n g " r e d d i s h orange" (10R t o 2.5YR) o r p a l e r " y e l l o w i s h orange" (7.5YR) w i t h o n l y a few g r a i n s of i n t e r m e d i a t e hue.
The d a r k e r , r e d d e r l i n e s o c c u r
i n p i t s i n t h e g r a i n s ( s e e F o l k , 1976, F i g . 1 ) and a r e c o v e r e d by a t h i n c o a t i n g of chemically-precipitated "turtle-skin"
silica.
The more y e l l o w hues a r e caused by
a s o f t " f l u f f " of l i m o n i t i c m a t e r i a l (Fig.2.21). Folk (1976) showed t h a t t h e r e d d i s h m a t e r i a l i n t h e p i t s i s a c u t a n o r c l a y i r o n - o x i d e s k i n (Brewer, 1964) and a r g u e d t h a t t h e r e d c u t a n and y e l l o w " f l u f f " a r e of d i f f e r e n t a g e s .
The r e d c u t a n s a r e c l e a r l y o l d e r and a r e t h o u g h t t o have o r i g -
i n a t e d d u r i n g a p e r i o d o f P l e i s t o c e n e ( o r e a r l i e r ) l a t e r i t i z a t i o n whereas t h e y e l l o w
101
I
I
DUNES
I
I
REG
1
5
1
6
I
7
I
0
SATURATION
Fig.2.20. Raw c o l o u r d a t a f o r samples from dune and r e g environments i n t h e Simpson Desert. G r a i n s i z e of t h e sample f r a c t i o n shown on t h e v e r t i c a l s c a l e . Each r e a d i n g of hue, v a l u e ( l i g h t n e s s ) and s a t u r a t i o n (chroma) shown by a d o t . Mean c o l o u r r e a d i n g s a s a f u n c t i o n o f g r a i n s i z e shown by curved l i n e s . Both dune and r e g show r e d d e s t , d a r k e s t , and most i n t e n s e c o l o u r s f o r t h e f r a c t i o n between a b o u t 28 and 40. Bottom diagram superimposes t h e mean t r e n d l i n e s f o r b o t h environments; g r e a t e s t d i f f e r e n c e between dune and r e g i s t h e weaker s a t u r a t i o n o f t h e r e g sediments ( a f t e r F o l k , 1976). " f l u f f " i s Holocene and s t i l l accumulating today.
The c o n t r a s t i n c o l o u r between
f i n e r and c o a r s e r sand f r a c t i o n s i s a t t r i b u t e d t o t h e f a c t t h a t t h e f i n e s a n d , because i t r e p r e s e n t s t h e s a l t a t i o n p o p u l a t i o n , i s c o n s i s t e n t l y mobile and does n o t a l l o w t h e accumulation of t h e l i m o n i t i c f l u f f .
There i s c o n s i d e r a b l e evidence
i n f a v o u r o f t h e widespread f o r m a t i o n o f l a t e r i t e d u r i n g p r e - P l e i s t o c e n e o r P l e i s t o c e n e time i n c e n t r a l A u s t r a l i a and l a t e r i t e s and i r o n p i s o l i t e s a r e r e p o r t e d by T e r r y (19341, C a r r o l l ( 1 9 3 9 ) , Whitehouse ( 1 9 4 0 ) , Crocker ( 1 9 4 6 ) , Madigan (1946), and Mabbutt (19h5). The s i g n i f i c a n c e of p r i o r l a t e r i t i z a t i o n and c l i m a t i c change i n t h e formation
of o t h e r r e d a e o l i a n s a n d s i s n o t c l e a r because s u f f i c i e n t s t u d i e s have n o t y e t been carried out.
Many d e s e r t b a s i n s a r e s i t e d on a n c i e n t a l l u v i a l p l a i n s , a f e a t u r e
102
10R- 25YR red orange
-
S k e t c h showing t h e sequence of g r a i n c o a t i n g s on reddened a e o l i a n sand Fig.2.21. from t h e Simpson Desert 1) H a e m a t i t i c c l a y c u t a n , formed i n P l e i s t o c e n e l a t e r i t i c s o i l and p r e s e r v e d o n l y i n p i t s ; 2 ) t h i n s i l i c a c o a t w i t h bumpy s u r f a c e , g i v i n g a g r e a s y l u s t r e o r i g i n a t i n g i n t h e p r e s e n t d e s e r t regime; 3 ) l i m o n i t i c s u r f a c e " f l u f f " of Holocene o r i g i n ( a f t e r F o l k , 1976). which s u g g e s t s t h a t t h e p r o c e s s c o u l d b e i m p o r t a n t e l s e w h e r e , p a r t i c u l a r l y i n p l a c e s l i k e t h e Sahara where dune s a n d s a r e known t o show downwind c o l o u r v a r i a t i o n s . The d e t r i t a l model has n o t y e t been f u l l y i n v e s t i g a t e d i n a n c i e n t a e o l i a n s a n d s t o n e s , a l t h o u g h many e a r l i e r workers c o n s i d e r e d i t i m p o r t a n t ( e . g . Dunham, 1953). The d i a g e n e t i c model
-
Sonoran D e s e r t
The Sonoran d e s e r t of n o r t h w e s t e r n Mexico and t h e s o u t h e a s t e r n USA c o n t a i n s a sequence of l a t e T e r t i a r y and Q u a t e r n a r y d e s e r t a l l u v i u m a s s o c i a t e d w i t h m a r g i n a l marine d e p o s i t s and a e o l i a n s a n d ( F i g . 2 . 2 2 ) .
The d e s e r t a l l u v i u m h a s been s t u d i e d
i n d e t a i l by Walker (1967a, 1976) and Walker e t e l . (1978).
I t c o n s i s t s of f i r s t -
c y c l e a r k o s i c d e t r i t u s d e r i v e d from f r e s h c r y s t a l l i n e basement which l a c k s any s i g n i f i c a n t s o i l development.
The a l l u v i u m i s non-red a t t h e t i m e of d e p o s i t i o n
and t h e e a r l i e s t d e t e c t a b l e r e d d e n i n g o c c u r s i n d e p o s i t s of l a t e P l e i s t o c e n e a g e . I n t h i s e a r l y s t a g e of r e d d e n i n g , which a l s o i n c l u d e s l a t e T e r t i a r y d e p o s i t s , t h e pigment c o n s i s t s o f amorphous f e r r i c o x i d e which c a n n o t b e i d e n t i f i e d by X-ray d i f f r a c t i o n a n a l y s i s (Walker, 1976, p.281). reddish-yellow
( e . g . 5yR 6 / 6 ) .
A t t h i s stage the deposits a r e typically
B r i g h t r e d c o l o u r a t i o n ( e . g . 8R 4 / 6 t o 9R 4 / 6 1 and
f i n e l y c r y s t a l l i n e h a e m a t i t e o n l y d e v e l o p i n o l d e r Miocene d e p o s , i t s .
103
A
GYNITIC ROCK$
LOW-TIDE TERRACE SAND
I
Fig.2.22. A. L o c a t i o n map B. Diagrammatic c r o s s s e c t i o n showing f a c i e s r e l a t i o n s h i p s o f R e c e n t , P l e i s t o c e n e , and P l i o c e n e s e d i m e n t s i n n o r t h e a s t e r n Baja C a l i f o r n i a , Mexico ( a f t e r Walker, 1967a). Walker (1967a, 1976) p r e s e n t e d c o n v i n c i n g e v i d e n c e t h a t t h e s e r e d beds formed i n s i t u by t h e p r o g r e s s i v e a l t e r a t i o n of d e t r i t a l ferromagnesian s i l i c a t e m i n e r a l s . S i n c e t h e r e has been no a p p r e c i a b l e c l i m a t i c change t h r o u g h o u t t h e d e p o s i t i o n a l h i s t o r y of t h e a r e a t h e s t u d y p r o v i d e s c o n v i n c i n g e v i d e n c e t h a t r e d beds can form i n a h o t , a r i d c l i m a t e and t h a t s e a s o n a l l y h i g h r a i n f a l l o r p r i o r l a t e r i t i z a t i o n a r e n o t p r e r e q u i s i t e s f o r t h e i r formation. The d i a g e n e t i c p r o c e s s e s l e a d i n g t o t h e f o r m a t i o n o f r e d beds i n t h i s a r e a a r e d e s c r i b e d by Walker e t a l . (1978).
They depend v e r y h e a v i l y on t h e t y p i c a l ground-
water c i r c u l a t i o n p a t t e r n seen i n d e s e r t b a s i n s (Fig.2.17).
Owing t o t h e h i g h
p e r m e a b i l i t y o f m a r g i n a l a l l u v i a l f a n s t h e w a t e r t a b l e g r a d i e n t s a t t h e b a s i n margin a r e low and t h e s u r f a c e d r a i n a g e p a t t e r n i s i n f l u e n t . o c c u r s w a t e r flows i n t o t h e a l l u v i u m ,
Whenever s u r f a c e r u n o f f
E f f l u e n t seepage only occurs near the c e n t r e
o f t h e b a s i n where t h e s a t u r a t e d zone l i e s n e a r e r t o t h e s u r f a c e and groundwater i s l o s t by e v a p o t r a n s p i r a t i o n . These c o n d i t i o n s e n s u r e t h a t groundwater m i g r a t e s towards the centre of t h e basin.
Two t y p e s of d i a g e n e t i c p r o c e s s e s o c c u r which l e a d t o t h e r e d d e n i n g o f t h e s e
104 d e s e r t sediments.
These a r e ' e a r l y ' d i a g e n e t i c and ' l a t e ' d i a g e n e t i c reddening.
E a r l y d i a g e n e t i c reddening The i n f l u e n t seepage i n m a r g i n a l a l l u v i u m r e s u l t s i n t h e mechanica of d e t r i t a l c l a y i n t o p r e v i o u s l y c l a y - f r e e sediment.
infiltration
This clay coats d e t r i t a l
g r a i n s and f r e q u e n t l y c o n t a i n s small amounts o f i r o n a s hydroxide c o a t i n g s and i n the c l a y mineral l a t t i c e .
A f t e r d e p o s i t i o n t h i s c l a y i s more o r less c o n t i n u o u s l y
bathed i n oxygenated groundwater and w i t h t i m e t h e c l a y b e g i n s t o redden due t o t h e formation of i r o n o x i d e .
The i r o n o x i d e may form by ' a g e i n g ' o f t h e d e t r i t a l i r o n
hydroxide o r by o x i d a t i o n of t h e c l a y m i n e r a l l a t t i c e .
I n e i t h e r case i t i s t h i s
type of e a r l y d i a g e n e t i c reddening which i s mainly r e s p o n s i b l e f o r t h e r e d d i s h yellow (5YR 6 / 6 ) pigment t h a t c h a r a c t e r i z e d much o f t h e younger ( P l i o - P l e i s t o c e n e ) d e s e r t a l l u v i u m o f t h e s o u t h e a s t e r n USA. Late d i a e e n e t i c reddeninn When u n s t a b l e framework s i l i c a t e s a r e i n prolonged c o n t a c t w i t h oxygenated groundwater t h e y a r e s u s c e p t i b l e t o a l t e r a t i o n by h y d r o l y s i s .
The a l t e r a t i o n
i n v o l v e s t h e f o r m a t i o n o f d i s s o l u t i o n v o i d s and t h e replacement of framework g r a i n s by mixed l a y e r i l l i t e - m o n t m o r i l l o n i t e .
I r o n - b e a r i n g s i l i c a t e s such a s h o r n b l e n d e ,
pyroxene, and b i o t i t e c a n b e observed i n v a r i o u s s t a g e s of a l t e r a t i o n and e l e c t r o n microprobe a n a l y s e s show t h a t t h e replacement c l a y c o n t a i n s i r o n o x i d e which i s d i f f u s i n g outwards, away from t h e decomposing g r a i n .
This process causes the
r e l e a s e o f i o n s ( i n c l u d i n g sodium, potassium, c a l c i u m , magnesium, aluminium, s i l i c o n , and i r o n ) i n t o t h e c i r c u l a t i n g groundwater.
As t h e groundwater moves t h r o u g h t h e
sediment i t s composition i s modified and whenever t h e l o c a l c o n c e n t r a t i o n of d i s s o l v e d i o n s i s s u f f i c i e n t l y g r e a t a u t h i g e n i c m i n e r a l phases a r e p r e c i p i t a t e d i n t h e p o r e spaces.
The d e s e r t a l l u v i u m o f n o r t h w e s t e r n Mexico and t h e s o u t h w e s t e r n USA c o n t a i n s
a d i s t i n c t i v e s u i t e of e u h e d r a l a u t h i g e n i c m i n e r a l s i n c l u d i n g : mixed l a y e r i l l i t e - m o n t m o r i l l o n i t e ,
potassium f e l d s p a r ,
q u a r t z , c a l c i t e and h a e m a t i t e .
It i s the
c r y s t a l l i n e , a u t h i g e n i c h a e m a t i t e formed by t h i s l a t e d i a g e n e t i c p r o c e s s which g i v e s t h e o l d e r (Miocene) d e p o s i t s o f t h e r e g i o n t h e i r b r i g h t r e d (9R 4 / 6 1 c o l o u r a t i o n . The c o n d i t i o n s needed f o r t h e f o r m a t i o n of r e d beds by i n t r a s t r a t a l a l t e r a t i o n of framework s i l i c a t e s have been summarized by Walker (1967a) a s f o l l o w s :
1) occurrence o f i r o n - b e a r i n g d e t r i t a l g r a i n s 2) post-depositional conditions favouring i n t r a s t r a t a l a l t e r a t i o n of the i r o n bearing
grains
3 ) i n t e r s t i t i a l Eh-pH c o n d i t i o n s t h a t a r e s u i t a b l e f o r t h e f o r m a t i o n o f i r o n o x i d e s
4 ) absence of subsequent r e d u c t i o n o f t h e f e r r i c i r o n 5 ) enough t i m e f o r i n t r a s t r a t a l a l t e r a t i o n s of t h e i r o n - b e a r i n g g r a i n s and t h e f e r r i c hydroxide formed by t h e a l t e r a t i o n t o be c o n v e r t e d i n t o h a e m a t i t e , and p o s s i b l y
105 6 ) r e l a t i v e l y warm (above 65-70°F)
temperature.
Red beds can form wherever t h e s e c o n d i t i o n s a r e m e t .
The i m p o r t a n t f a c t o r i s t h a t
c l i m a t e i s n o t a n i m p o r t a n t r e q u i s i t e f o r r e d bed f o r m a t i o n .
These r e s u l t s t h u s
c o n t r a d i c t t h e c l a i m t h a t r e d s o i l s i n d e s e r t s a r e t r a n s p o r t e d from, o r r e l i c t s o f , l a t e r i t i c s o i l s which formed i n p r e v i o u s h o t , m o i s t climates. The m a j o r i t y o f a n c i e n t r e d beds a r e now c o n s i d e r e d t o b e of d i a g e n e t i c o r i g i n and formed i n a manner s i m i l a r t o t h a t d e s c r i b e d by Walker (1967a, 1 9 7 6 ) .
The model
a p p l i e s e q u a l l y w e l l t o b o t h a n c i e n t a e o l i a n s a n d s ( e . g . Kessler, 1978) and a n c i e n t d e s e r t a l l u v i u m ( e . g . G l e n n i e e t a l . , 1978). ANCIENT AEOLIAN RED BEDS Introduction The m a j o r i t y o f t h e ' c l a s s i c ' examples o f a n c i e n t a e o l i a n s a n d s t o n e s come from t h e l a t e P a l a e o z o i c and E a r l y Mesozoic of t h e w e s t e r n USA ( T a b l e 2 . 2 ) .
O t h e r examples
i n c l u d e t h e T r i a s s i c Botucatu Sandstones of B r a z i l (Almeida, 1953; B i g a r e l l a , 1972). and t h e Lower Permian ( R o t l i e g e n d e s ) s a n d s t o n e s o f B r i t a i n and t h e North Sea b a s i n (Laming, 1966; G l e n n i e , 1970, 1972; Waugh, 1970b) which o c c u r i n a s s o c i a t i o n w i t h d e s e r t a l l u v i a l and i n l a n d sabkha d e p o s i t s . The r e c o g n i t i o n o f a n c i e n t a e o l i a n s a n d s t o n e s i s by no means a n e a s y m a t t e r and t h e c r i t e r i a d e v i s e d by G l e n n i e (1970) may b e u s e f u l l y a p p l i e d :
1. Bedding h o r i z o n t a l o r l a r g e and small s c a l e c r o s s - b e d d i n g , showing c o n s t a n t o r multiple orientation 2. I n d i v i d u a l laminae w e l l - s o r t e d , e s p e c i a l l y i n f i n e r g r a i n s i z e s ; s h a r p g r a i n s i z e d i f f e r e n c e s between laminae a r e common
3 . G r a i n s i z e commonly r a n g e s from s i l t (60 m) t o c o a r s e sand.
Maximum s i z e f o r
wind t r a n s p o r t i s c . l.Ocm
4. L a r g e r s a n d g r a i n s t e n d t o b e w e l l - s o r t e d 5 . Clay d r a p e s a r e v e r y r a r e 6. Sands a r e f r e e of c l a y 7. Uncemented q u a r t z g r a i n s a r e f r o s t e d
8. Mica i s g e n e r a l l y a b s e n t Perhaps t h e most c h a r a c t e r i s t i c ( b u t n o t n e c e s s a r i l y d i a g n o s t i c ) f e a t u r e of a n c i e n t a e o l i a n s a n d s t o n e s i s t h e l a r g e s c a l e of t h e cross-bedding.
These a r e most
commonly p l a n a r t a b u l a r u n i t s a l t h o u g h t r o u g h s ( e . g . K n i g h t , 1929) a r e a l s o described.
I n d i v i d u a l s e t s may b e up t o 35m t h i c k and m u l t i p l e s e t s may occupy
g r e a t s t r a t i g r a p h i c thicknesses (Table 2.2).
One o f t h e b e s t d e s c r i p t i o n s of a n c i e n t
a e o l i a n c r o s s - b e d d i n g has been made by Walker and Harms (1972) from t h e Permian Lyons Sandstone of Colorado ( F i g . 2 . 2 3 ) .
I n v e r t i c a l cross s e c t i o n t h e cross-bedding appears
t o be p l a n a r t a b u l a r b u t on t h e exhumed l e e f a c e t h e r e a r e v e r y low a m p l i t u d e sand
106 TABLE 2.2.
C h a r a c t e r i s t i c s of t h e c l a s s i c a e o l i a n s a n d s t o n e s o f t h e w e s t e r n USA. Formation
CASPER
Age
Penn-Perm
Approx.Max. Thickness
Thick
Dip
Type
Ref e r e nce
240m
min 15m,
15-25'
troughs
S t e i d tmann
,
1974 COCONINO
Perm
3 30m
CEDAR MESA
Perm
450m
DE CHELLY
Perm
WHITE R I M
huge"
25- 30'
randomly B a a r s , 1962 oriented wedge-shaped sets
up t o 30m
up t o 25
planar tabular
B a a r s , 1962
3 30m
up t o 35m
15-35'
planar tabular
B a a r s , 1962
Perm
200m
up t o 20m
19-27'
planar tabular
Baars and S e a g e r , 1970
LYONS
Perm
40m
up t o 13m
25-28'
planar tabular
Walker and Harms, 1972
WINGATE
U.Trias
130m
up t o 15m
up t o 30'
NAVAJO
L.Jurassic
700m
up t o 30m
20-30'
U.J u r a s s i c ENTRADA ( S l i c k Rock Member)
280m
up t o 8 m
Dane, 1935 planar S a n d e r s o n , 1974 tabular Freeman and i n lower V i s h e r , 1975 p a r t (3-10m) trough i n upper p a r t (up t o 6m ) "wedging set" "sweeping aeolian c r o s s beds"
r i p p l e s t h a t s u g g e s t t h a t s a n d was blown a c r o s s t h e dune f a c e .
C r a i g and Shawe, 1975
The p l a n view shows
t h a t t h e e n t i r e dune advanced down d i p n o t by a v a l a n c h i n g down t h e s l i p f a c e b u t by t h e a d d i t i o n t o t h e l e e f a c e of wedges of s a n d blown i n t o p l a c e a c r o s s t h e dune. According t o Walker and Harms (1972) t h i s a l s o e x p l a i n s t h e low d i p a n g l e (25') t h e c r o s s beds which would b e n e a r e r t h e a n g l e of r e p o s e f o r d r y s a n d (34')
of
i f dune
m i g r a t i o n had been by a v a l a n c h i n g . I n f a c t t h e a n g l e of 34'
i s r a r e l y a t t a i n e d by a n c i e n t a e o l i a n s a n d s t o n e s a l t h o u g h
t h i s c o u l d be due p a r t l y t o p o s t - d e p o s i t i o n a l r e d u c t i o n by compaction ( G l e n n i e , 1972). The most abundant t y p e of a n c i e n t a e o l i a n c r o s s - b e d d i n g i s t h e p l a n a r t a b u l a r t y p e a p p a r e n t l y produced by t r a n s v e r s e dune t y p e s i n c l u d i n g barchans ( e . g . S h o t t o n , 1956).
107
Fig.2.23. Geometry of a e o l i a n c r o s s - b e d d i n g a s s e e n i n t h e Lyons S a n d s t o n e , Lyons Quarry (Permian, Colorado) ( a f t e r Walker and Harms, 1972). L o n g i t u d i n a l dune t y p e s a r e s u r p r i s i n g l y r a r e i n t h e s t r a t i g r a p h i c r e c o r d a l t h o u g h t h e Permian Yellow Sands o f NE England p r o v i d e a p o s s i b l e example ( G l e n n i e , 1972; Magraw, 1975). D i s t i n g u i s h i n g a e o l i a n from s h a l l o w marine s a n d s t o n e s A number of a n c i e n t s a n d s t o n e u n i t s p r e v i o u s l y r e g a r d e d a s a e o l i a n i n o r i g i n
( i n c l u d i n g some i n T a b l e 2 . 2 ) have r e c e n t l y been r e i n t e r p r e t e d a s s h a l l o w marine deposits.
Baars (19621, f o r example, c r i t i c i z e d t h e a e o l i a n i n t e r p r e t a t i o n of
t h e Cedar Mesa s a n d s t o n e s a s d i d Baars and S e a g e r (1970) f o r t h e White R i m sandstones ( s e e Table 2.2). cross-bedding.
I n b o t h c a s e s t h e i r argument was b a s e d on t h e s t y l e of
The Cedar Mesa i s c h a r a c t e r i z e d by .."moderate t o low a n g l e c r o s s -
s t r a t i f i c a t i o n , v e r y low a n g l e t o h o r i z o n t a l t h i n s a n d b e d s , long sweeping c u r v e s from m o d e r a t e l y d i p p i n g c r o s s - s t r a t a i n t o h o r i z o n t a l f o r e s e t b e d s , t h i n s i m p l e s e t s of low a n g l e c r o s s - s t r a t a "
( B a a r s , 1962, p.178).
The d e p o s i t s were r e i n t e r p r e t e d
a s l i t t o r a l o r beach sands ( B a a r s , 1962) b u t t h i s i n t e r p r e t a t i o n c a n n o t a c c o u n t f o r t h e s e t s up t o 30m t h i c k which a r e unknown i n any p r e s e n t - d a y beach environment. The c r i t i c i s m of t h e a e o l i a n i n t e r p r e t a t i o n o f t h e White R i m Sandstone was based on t h e i r r e g u l a r b a r - l i k e topography on t h e t o p of t h e White R i m .
The b a r topography
i s mantled by a v e n e e r o f wave-rippled sand which was a l m o s t c e r t a i n l y d e p o s i t e d i n
a marine environment.
A p p a r e n t l y i g n o r i n g t h e 20m t h i c k c r o s s beds i n t h e White R i m
Sandstone Baars and S e a g e r (1970) r e i n t e r p r e t e d t h e whole f o r m a t i o n a s s h a l l o w marine on t h e b a s i s of t h e b a r - l i k e topography and v e n e e r o f wave r i p p l e d sand.
A more
s a t i s f a c t o r y i n t e r p r e t a t i o n would b e t h a t t h e White R i m Sandstone i s a n a e o l i a n sand
108 which has s u f f e r e d marine reworking i n i t s upper p a r t . The s h a l l o w marine d e p o s i t s which most c l o s e l y resemble a e o l i a n s a n d s and which t h u s c r e a t e t h e most c o n f u s i o n i n i n t e r p r e t a t i o n a r e submarine s a n d waves and t i d a l current ridges.
These a r e common on many s h e l f a r e a s (Houbolt, 1968; McCave, 1971;
S w i f t , 1975) and f r e q u e n t l y show c r o s s - b e d d i n g which i t i s c l a i m e d c l o s e l y r e s e m b l e s a e o l i a n dune bedding ( S t a n l e y e t a l . , 1971, p . 1 3 ) ( F i g . 2 . 2 4 ) .
Thus, w i t h i n c r e a s i n g
knowledge of cross-bedded submarine s a n d s a number of s a n d s t o n e s c l a s s i c a l l y r e g a r d e d a s a e o l i a n have been r e i n t e r p r e t e d a s s h a l l o w marine s a n d waves o r t i d a l c u r r e n t ridges.
The most n o t a b l e amongst t h e s e i s t h e J u r a s s i c Navajo S a n d s t o n e , b u t t h e
r e i n t e r p r e t a t i o n o f Freeman a n d V i s h e r (1975) h a s been s t r o n g l y c r i t i c i z e d by many a u t h o r s ( F o l k , 1977; P i c a r d , 1977; R u z y l a , 1977; Steidmann, 1977).
The r e i n t e r p -
r e t a t i o n of t h e Permian Yellow Sands a s a t i d a l c u r r e n t r i d g e by P r y o r (1971b) i s a l s o open t o c r i t i c i s m .
P r y o r (1971a) l i k e n e d t h e W e i s s l i e g e n d e s of Germany t o t h e
t i d a l c u r r e n t r i d g e s d e s c r i b e d by Houbolt (1968) c l a i m i n g t h a t Houbolt (1968) showed .."cross-stratification i n thickness..".
i n t h e submarine sand r i d g e s o f t h e North Sea t o exceed 40m
I n f a c t H o u b o l t ' s (1968) c r o s s s e c t i o n s show c o n s i d e r a b l e v e r t i c a l
e x a g g e r a t i o n and t h e t r u e d i p on t h e " s t e e p " f a c e s i s o n l y 5 o r 6'
In
(Fig.2.24).
g e n e r a l , t h e t r u e deep o f " s t e e p " f a c e s on submarine sand waves and r i d g e s i s r a t h e r s h a l l o w (up t o 1 0 ' )
and does n o t b e a r c l o s e comparison w i t h t h e v e r y t h i c k , s t e e p l y
d i p p i n g cross-beds which t y p i f y a n c i e n t a e o l i a n s a n d s t o n e s . many a e o l i a n u n i t s has c e r t a i n l y been o u t of p l a c e .
The r e i n t e r p r e t a t i o n of
I t may b e t h a t we r a t h e r under-
e s t i m a t e t h e amount of a e o l i a n s a n d i n t h e s t r a t i g r a p h i c r e c o r d . A l l t h e a e o l i a n s a n d s which have been s u b j e c t t o s e r i o u s r e i n t e r p r e t a t i o n l a t e r a l l y i n t e r f i n g e r w i t h s h a l l o w marine d e p o s i t s and do n o t c o n t a i n abundant d e s e r t s e d i m e n t s of o t h e r f a c i e s .
A e o l i a n s a n d s t o n e s can b e c o n f i d e n t l y i d e n t i f i e d when t h e y o c c u r i n
i n t e r n a l b a s i n s a s s o c i a t e d w i t h d e s e r t f l u v i a l and i n l a n d sabkha d e p o s i t s a n d , of c o u r s e , show t h e t y p i c a l t e x t u r a l p r o p e r t i e s and c r o s s - b e d d i n g ,
An e x c e l l e n t example
i s a f f o r d e d by t h e Lower Permian of NW Europe. The Lower Permian of NW Europe The Lower Permian o f Northwestern Europe i n c l u d i n g t h e North Sea a r e a i s one of t h e best-known a n c i e n t exantples of t h e d e s e r t - e v a p o r i t e r e d bed a s s o c i a t i o n ,
It i s
g e n e r a l l y known a s t h e " R o t l i e g e n d e s " ( t h e " r e d beds" which u n d e r l i e t h e Upper Permian Z e c h s t e i n s e q u e n c e ) and r e p r e s e n t s a c o n t i n e n t a l c l a s t i c sequence d e p o s i t e d under semi-desert conditions.
R o t l i e g e n d e s f a c i e s have been r e c o g n i z e d i n England,
S c o t l a n d ( S h e r l o c k , 1926, 1928, 1 9 4 7 ) , s o u t h e r n Norway ( H o l t e d a h l , 1934; Dons and G y W , 19671, t h e Rhine Graben i n F r a n c e and w e s t e r n Germany ( L a p p a r e n t , 1906; H a r r a s s o w i t z , 1 9 2 6 ) , T h u r i n g i a and Harz i n c e n t r a l and e a s t e r n Germany (Gagel, 1926; Born, 1926) and i n Poland and o t h e r p a r t s of F r a n c e , Germany, t h e P y r e n e e s , and R u s s i a ( S h e r l o c k , 1947).
North Sea e x p l o r a t i o n h a s shown t h a t t h e s e s c a t t e r e d
109
r25 I
B
215' sea level
35"
Well Bank
340°160~280"realevel
100"
Well Bank
4/ \ \ \
0
0
'2 km
1
235" sea level
55'
Smith Knoll
0
0
1
2km
Fig.2.24. A . P r o f i l e s o f s a n d waves i n t h e s o u t h e r n s i g h t o f t h e North Sea. V e r t i c a l s c a l e p o s i t i o n e d w i t h r e s p e c t t o s e a l e v e l = Om. Lengths o f s e c t i o n s a b o u t 3800m ( A ) , 2800m (B), 900m (C) and 1220m (D). C a l c u l a t e d d i p s o f " s t e e p f a c e s " a v e r a g e s 5 O ( a f t e r McCave, 1971). B. S p a r k e r p r o f i l e s o f t i d a l c u r r e n t r i d g e s i n t h e e a s t c o a s t o f England. Although t h e y look l i k e a s y m m e t r i c a l c r o s s bedded f e a t u r e s , compare t h e h o r i z o n t a l and v e r t i c a l s c a l e s t o s e e c o n s i d e r a b l e v e r t i c a l e x a g g e r a t i o n . C a l c u l a t e d s t e e p s l o p e a v e r a g e 5-60 ( a f t e r H o u b o l t , 1968).
110 o u t c r o p s form p a r t of a t l e a s t two f a u l t bounded s e d i m e n t a r y b a s i n s which l a y a l o n g t h e n o r t h e r n margin o f t h e V a r i s c a n mountain c h a i n . WNW-ESE a r e s e p a r a t e d by t h e Mid-North Sea
-
These b a s i n s
which t r e n d
RingkBbing-Fyn High.
The p a l e e o -
geography and f a c i e s d i s t r i b u t i o n o f t h e R o t l i e g e n d e s , b a s e d on Z i e g l e r (1975, 1978)
i s shown i n Fig.2.25.
Fig.2.25.
R o t l i e g e n d e s palaeogeography of NW Europe ( , a f t e r P . A . Z i e g l e r , 1978).
The Lower R o t l i e g e n d e s were d e p o s i t e d on a d e s e r t s u r f a c e c a r v e d o u t of a blockf a u l t e d s t r u c t u r a l c o n f i g u r a t i o n s i m i l a r t o t h a t d e s c r i b e d by Kent (1974, 1975) f o r e a s t e r n England and t h e North Sea.
A t t h i s t i m e (Late Carboniferous
-
E a r l y Permian)
t h e r e was widespread v o l c a n i c a c t i v i t y , p a r t i c u l a r l y i n t h e Oslo Graben and t h e a r e a t o t h e e a s t o f t h e N e t h e r l a n d s , and a l s o i n t h e s m a l l e r i s o l a t e d b a s i n s s u c h a s s o u t h west England (Laming, 1965, 1966) and t h o s e i n t h e s o u t h of S c o t l a n d (Mykura, 1965; Wagner, 1966; B r o o k f i e l d , 1 9 7 8 ) .
I n the e a s t e r n Netherlands t h i s Stephanian t o Early
Permian v o l c a n i c i t y r e s u l t e d i n t h e a c c u m u l a t i o n of o v e r 1,000m of q u a r t z p o r p h y r i e s , and s p i l i t i c l a v a s and t u f f s and t h e v o l c a n i c s a r e i n t e r b e d d e d w i t h conglomerates and c o n g l o m e r a t i c s a n d s t o n e s which r e s u l t e d from penecontemporaneous e r o s i o n .
In
some a r e a s c o n t i n e n t a l d e p o s i t i o n was c o n t i n u o u s through l a t e Westphalian-StephanianE a r l y Permian b u t o t h e r a r e a s were s u b j e c t e d t o e r o s i o n and deep w e a t h e r i n g f a r i n t o t h e Permian.
I n some of t h e s e a r e a s , such a s t h e Midlands of B r i t a i n and t h e Midland
V a l l e y of S c o t l a n d t h e deep d e s e r t w e a t h e r i n g was r e s p o n s i b l e f o r t h e secondary r e d d e n i n g of g r e y W e s t p h a l i a n Coal Measures ( B a i l e y , 1926; Mykura, 1960a). I t was
111 on t h i s d e s e r t s u r f a c e t h e n , c o n s i s t i n g m o s t l y o f W e s t p h a l i a n and S t e p h a n i a n s e d i m e n t s , i n p l a c e s w i t h contemporary v o l c a n i c s , t h a t t h e R o t l i e g e n d e s were d e p o s i t e d (Fig.2.26).
ES WLCANICS
Fluvial (Wadi) Conglomomtor, hndrtonor L Shalor
Fig.2.26. Conceptual c r o s s s e c t i o n s through t h e R o t l i e g e n d e s b a s i n i n s o u t h e r n North Sea and e a s t e r n N e t h e r l a n d s j u s t p r i o r t o t h e Z e c h s t e i n t r a n s g r e s s i o n . Cross s e c t i o n s a r e s o u t h of t h e Mid-North Sea High and Fyn G r i n s t e d High r e s p e c t i v e l y ( a f t e r G l e n n i e , 1972). The R o t l i e g e n d e s sediments a r e d i v e r s e and show many c h a r a c t e r i s t i c s analogous t o t h o s e of Recent d e s e r t s e d i m e n t s .
The f o l l o w i n g a c c o u n t of R o t l i e g e n d e s f a c i e s i s
based on S h o t t o n ( 1 9 5 6 ) , Laming ( 1 9 6 6 ) , G l e n n i e (1970, 19721, P i p e r ( 1 9 7 0 ) , Waugh ( 1 9 7 0 a , b ) , G l e n n i e e t a l . (1978), Marie ( 1 9 7 5 ) , and B r o o k f i e l d (1977, 1978). Marginal f a n g l o m e r a t e s and wadi d e p o s i t s Coarse g r a i n e d f l u v i a l c l a s t i c s i n c l u d i n g wadi-fan b r e c c i a s and f a n g l o m e r a t e s o c c u r t h r o u g h o u t , and a l o n g t h e s o u t h e r n margin of t h e V a r i s c a n mountains, and a l s o a s f l a n k d e p o s i t s around o t h e r h i g h l a n d a r e a s i n t h e Midlands o f England, t h e Lake D i s t r i c t , S o u t h Devon and t h e S c o t t i s h Basins ( F i g . 2 . 2 5 ) . d e s c r i b e d wadi d e p o s i t s from t h e s o u t h e r n North Sea.
G l e n n i e (1972) has
These c o n s i s t of a l t e r n a t i o n s
of subrounded-rounded q u a r t z i t e c o n g l o m e r a t e , brown-red s a n d s t o n e s , and dark r e d clay.
The r e d c l a y s have been c r a c k e d and c u r l e d by d e s i c c a t i o n and a l s o i n t r u d e d
112 by s a n d s t o n e dykes i n t h e same manner a s Recent wadi s e d i m e n t s . The d e s e r t f l u v i a t i l e s e d i m e n t s a r e s e e n t o b e s t e f f e c t i n some o f t h e o n s h o r e o u t c r o p s s u c h a s t h o s e i n SW England d e s c r i b e d by Laming ( 1 9 6 6 ) .
Here p o o r l y s o r t e d
w a t e r - l a i d b r e c c i a s were t r a n s p o r t e d t o t h e e a s t and n o r t h - e a s t a s i n d i c a t e d by c l a s t i m b r i c a t i o n and t h e i n c r e a s e i n roundness of l i m e s t o n e c l a s t s w h i l e i n t e r b e d d e d a e o l i a n sands show a palaeowind d i r e c t i o n t o t h e n o r t h - w e s t ( F i g . 2 . 2 7 ) .
The w a t e r -
l a i d d e p o s i t s show many o f t h e f e a t u r e s t y p i c a l of ephemeral d e s e r t a l l u v i u m They were p r o b a b l y d e p o s i t e d a s a l l u v i a l f a n s on which s h e e t f l o o d i n g
Fig.2.27. The Permian o u t c r o p of s o u t h Devon showing t h e t r a n s p o r t d i r e c t i o n s o f wadi d e p o s i t s and t h e palaeowind d i r e c t i o n ( a f t e r Laming, 1966):.
113 was of common o c c u r r e n c e .
The bedding s t y l e of t h e b r e c c i a s i s m a i n l y f l a t w i t h
trough c r o s s beds i n some down-fan a r e a s .
Other s i g n i f i c a n t sedimentary s t r u c t u r e s
i n c l u d e d e s i c c a t i o n c r a c k s , r a i n p r i n t s , and s a n d s t o n e dykes.
The w a t e r - l a i d
d e p o s i t s a r e f i r m l y cemented by c a l c i t e which was p r e c i p i t a t e d s h o r t l y a f t e r deposition a s i n other desert deposits.
In p l a c e s t h e w a t e r - l a i d a l l u v i u m i s i n t e r b e d d e d w i t h a e o l i a n s a n d ,
These a r e
g e n e r a l l y w e l l - s o r t e d , l a m i n a t e d s a n d s which a r e r e l a t i v e l y p o o r l y cemented. The s u c c e e d i n g wadi s e d i m e n t s have s h a r p e r o s i o n a l b a s e s i n d i c a t i n g t h e e r o s i o n of wind blown sands which p r e v i o u s l y b l o c k e d wadi c h a n n e l s . A e o l i a n sands Wadi s e d i m e n t s may b e o v e r l a i n o r i n t e r b e d d e d w i t h a e o l i a n dune sands ( G l e n n i e , 1970, 1972).
I n onshore a r e a s t h e r e a r e a number of Permian s a n d s t o n e s p a r t s of
which can b e s a f e l y r e g a r d e d a s d e s e r t dune d e p o s i t s .
These i n c l u d e t h e B r i d g n o r t h
Sandstone ( S h o t t o n , 1937, 1954) and i t s e q u i v a l e n t s i n t h e Vale of Clwyd, t h e C o l l y h u r s t Sandstone ( C o l t e r and B a r r , 1975) some o f t h e s a n d s t o n e s i n t h e s o u t h Devonshire b a s i n (Laming, 1 9 6 6 ) , and t h e s a n d s t o n e s on t h e Permian b a s i n s of t h e South of S c o t l a n d i n c l u d i n g t h e Mauchline, T h o r n h i l l , Lochmaben and Dumfries and t h e C o r r i e Sandstone of Arran ( P i p e r , 1970; B r o o k f i e l d , 1977, 1978).
The Yellow Sands
o f NE England, f o r a long t i m e r e g a r d e d a s a e o l i a n (Hodge, 1 9 3 2 ) , were r e i n t e r p r e t e d a s s h a l l o w m a r i n e dunes by P r y o r (1971b) b u t a r e a l s o l i k e l y t o b e of a e o l i a n o r i g i n . These s a n d s t o n e s f u l f i l l t h e c r i t e r i a e s t a b l i s h e d by G l e n n i e (1970) f o r t h e r e c o g n i t i o n of a e o l i a n d e p o s i t i o n . The a e o l i a n dune s a n d s comprise i n d i v i d u a l u n i t s o f r e d s a n d s t o n e s e p a r a t e d by bounding s u r f a c e s ,
Each u n i t u s u a l l y s t a r t s a t t h e b a s e w i t h s u b h o r i z o n t a l , f i n e l y
l a m i n a t e d s a n d s t o n e s and p a s s e s upwards i n t o more s t e e p l y d i p p i n g f o r e s e t laminae ( s l i p f a c e s ) which may a p p r o a c h 30'
i n d i p , b u t n e v e r 34'
T h i s d i f f e r e n c e can b e a c c o u n t e d f o r by compaction.
a s s e e n i n modern dunes;
These major bounding s u r f a c e s
a r e i n t e r p r e t e d a s erosion surfaces associated with the migration of draas.
I n the
North Sea a e o l i a n dunes i n t h e R o t l i e g e n d e s a r e r e a d i l y r e c o g n i z a b l e by t h e i r welll a m i n a t e d c r o s s - b e d d i n g and p e t r o l o g i c a l c h a r a c t e r i s t i c s .
A s i n t h e onshore o u t c r o p s
t h e g r a i n s i z e of t h e s a n d s i s m o s t l y fine-medium and o c c a s i o n a l l y c o a r s e g r a i n e d n e a r t h e b a s e of i n d i v i d u a l u n i t s .
The f i n e r g r a i n s a r e g e n e r a l l y s u b a n g u l a r and t h e
c o a r s e r ones subround o r round w i t h a f r o s t e d a p p e a r a n c e .
Clay m a t r i x , a p a r t from a u t h i g e n i c c l a y , i s a b s e n t and mica g e n e r a l l y
uncommon. scarce.
Bimodal sands a r e n o t
Some of t h e t e x t u r a l c h a r a c t e r i s t i c s o f R o t l i e g e n d e s a e o l i a n s a n d s a r e shown
i n Fig.2.15. Another f e a t u r e worthy of n o t e a r e t h e f u l g u r i t e s which o c c u r i n t h e C o r r i e Sandstone of t h e I s l e o f Arran.
These a r e r o d s o f f u s e d s a n d w i t h a s t a r - s h a p e d
c r o s s s e c t i o n which a r e i n t e t p r e t e d a s p a l a e o - l i g h t e n i n g
f l a s h e s (Harland and Hacker,
114 1966; P i p e r , 1970). They p r o v i d e good e v i d e n c e o f t h e former e x i s t e n c e o f d e s e r t s torms
.
Continuous d i p m e t e r measurements o v e r a wide a r e a of t h e North Sea shows t h a t dune s l i p f a c e s d i p c o n s i s t e n t l y westwards ( G l e n n i e , 1972).
It i s not possible to
t e l l from c o r e s a l o n e , however, what dune t y p e s p r e v a i l e d i n t h e s e dune f i e l d s . T h i s a l s o a p p l i e s t o many of t h e o n s h o r e a r e a s , w h e r e two-dimensional o u t c r o p p r e c l u d e s t h e r e c o g n i t i o n o f dune t y p e s .
However, S h o t t o n (19561, b e l i e v e d t h a t t h e
v a s t m a j o r i t y of Permian dunes were of t h e b a r c h a n t y p e on t h e b a s i s of unimodal d i s t r i b u t i o n s of s l i p f a c e laminae and Waugh (1970b) c o n s i d e r e d t h e P e n r i t h Sandstone t o have a l s o been d e p o s i t e d a s b a r c h a n dunes.
G l e n n i e (1972) on t h e o t h e r
hand was of t h e o p i n i o n t h a t s e i f d u n e s , on t h e b a s i s o f bedding a t t i t u d e s i n b o t h o u t c r o p s and w e l l c o r e s , were p r o b a b l y more numerous t h a n b a r c h a n s on t h e h i g h e r ground n e a r e r t h e V a r i s c a n Mountains, whereas b a r c h a n s o r t r a n s v e r s e dunes were more abundant i n t h e i n t e r i o r . Permian p a l a e o m a g n e t i c d a t a ( B r i d e n e t a l . , 1970; Van d e r Voo and French, 1974) e n a b l e a r e a l i s t i c c o n t i n e n t a l r e c o n s t r u c t i o n t o be made d e s p i t e t h e f a c t t h a t when a n a l y s e d i n d e t a i l t h e r e a r e some d i s c r e p a n c i e s i n t h e r e s u l t s (Briden e t a l . , 1970; Turner and Vaughan, 1977).
The r e l i a b l e d a t a ( s e e Van d e r Voo and F r e n c h , 1974,
p.109) p l a c e s B r i t a i n a b o u t 10°N of t h e Permian p a l a e o e q u a t o r ( F i g . 2 . 2 8 ) and t h u s provides important corroborative evidence f o r the postulated o r i g i n s of t h e Rotliegendes.
Moreover, i f t h e palaeowind d i r e c t i o n s a r e superimposed on t h e p a l a e o -
l a t i t u d e r e c o n s t r u c t i o n t h e n i t can be s e e n t h a t t h e palaeowinds were p r o b a b l y t h e
NE t r a d e winds and t h e R o t l i e g e n d e s d e s e r t can b e c o n s i d e r e d a s a N o r t h e r n hemisphere " t r a d e wind d e s e r t " ( G l e n n i e , 1972). D e s e r t l a k e and sabkha d e o o s i t s I n t h e SE p a r t o f t h e s o u t h e r n R o t l i e g e n d e s b a s i n t h e a e o l i a n s a n d s a r e succeeded by t h e Ten Boer Member a s u c c e s s i o n of r e d mudstones w i t h minor amounts o f s i l t s t o n e and s a n d s t o n e w i t h n o d u l a r a n h y d r i t e .
The s a n d s t o n e s commonly show a f f i n i t i e s w i t h
b o t h aqueous and a e o l i a n t r a n s p o r t ; mudcracks and c u r l e d mudflakes a r e common i n d i cating subaerial desiccation.
Adhesion r i p p l e s a r e commonly a s s o c i a t e d w i t h n o d u l a r
a n h y d r i t e and t h e i n t e r p r e t a t i o n i s t h a t t h e s e d e p o s i t s r e p r e s e n t t h o s e of a n i n l a n d sabkha which was s u b j e c t e d t o a l t e r n a t e f l o o d i n g and d e s i c c a t i o n . The i n l a n d sabkha sequence g r a d e s l a t e r a l l y i n t o a c l a y - r i c h sequence c h a r a c t e r i z e d by t h e p r e s e n c e of h a l i t e .
A d e t a i l e d p a l a e o g e o g r a p h i c a l map showing t h e l a t e r a l
f a c i e s arrangement i s g i v e n i n F i g . 2 . 2 9 .
T h i s sequence i s well-known a s t h e
H a s e l g e b i r g e f a c i e s of t h e R o t l i e g e n d e s ( R i c h t e r - B e r n b u r g , 1955; Kent and Walmsley, 1970) and is t h e dominant component i n t h e c e n t r a l p a r t of t h e s o u t h e r n b a s i n where t h e R o t l i e g e n d e s may b e up t o 1500m t h i c k a s opposed t o 250m a t t h e b a s i n margin.
115
.
Fig.2.28. Permian p a l a e o l a t i t u d e s f o r NW Europe ( b a s e d on Van d e r Voo and French, 1974) w i t h superimposed palaeowind d i r e c t i o n s . The r e c o n s t r u c t i o n p e r m i t s i n t e r p r e t a t i o n t h a t R o t l i e g e n d e s dune s a n d s were d e p o s i t e d i n a n o r t h e r n hemisphere " t r a d e wind d e s e r t " . H a l i t e comprises a b o u t 30% o f t h e H a s e l g e b i r g e f a c i e s and a n h y d r i t e i s s c a r c e , o n l y o c c u r r i n g a s cement o r a n h y d r i t i c c l a y ,
By a n a l o g y w i t h Recent h o t d e s e r t s t h e
H a s e l g e b i r g e f a c i e s i s i n t e r p r e t e d a s t h e d e p o s i t s of a s a l i n e d e s e r t l a k e a l t h o u g h l a k e s of t h i s magnitude a r e n o t known a t t h e p r e s e n t b u t o c c u r r e d a t o t h e r times i n t h e p a s t ( G l e n n i e , 1972).
In o r d e r t o accumulate t h e t h i c k sequences o f Ten Boer sabkha f a c i e s and H a s e l g e b i r g e l a k e f a c i e s r e l a t i v e s u b s i d e n c e must have o c c u r r e d and k e p t pace w i t h sedimentation.
Water was p r o b a b l y s u p p l i e d t o t h e c e n t r e of t h e R o t l i e g e n d e s b a s i n
from t h e V a r i s c a n mountains i n t h e s o u t h v i a t h e m a r g i n a l a l l u v i a l f a n s and wadis. T h i s R o t l i e g e n d e s d e s e r t b a s i n was t e r m i n a t e d by t h e Z e c h s t e i n marine t r a n s g r e s s i o n which i s r e p r e s e n t e d by t h e K u p f e r s c h i e f e r and t h e l a t e r a l l y e q u i v a l e n t Marl S l a t e of NE England.
T h i s t r a n s g r e s s i o n may have been t h e c a t a s t r o p h i c f l o o d i n g
of a sub-sea l e v e l b a s i n ( T u r n e r e t a l . , 1978; Smith, 1979) b u t t h e r e a r e i n d i c a t i m s t h a t i t took p l a c e i n a number of s t a g e s .
The f i r s t e v i d e n c e of marine c o n d i t i o n s
i s found i n t h e B l n d e r s c h i e f e r (Plumhoff, 1966) which c o n s i s t s o f a t h i n band w i t h a r i c h m a r i n e fauna a b o u t 2.5m below t h e l p w e s t Z e c h s t e i n l i m e s t o n e .
Also, Glennie
(1972) r e p o r t s s a n d s t o n e s which a r e c o n s i d e r e d t o have been slumped and homogenized
116 by flowing w a t e r .
I n a number o f p l a c e s , two s u c h h o r i z o n s o c c u r , s e p a r a t e d by a
temporary development of dune sand c l e a r l y i n d i c a t i n g t h e e p i s o d i c n a t u r e of t h e transgression.
=Mudstones
and Halite
=Wadi
Sediments
B A d h e s i o n Ripples
Fig.2.29. Schematic R o t l i e g e n d e s f a c i e s d i s t r i b u t i o n o f f t h e e a s t c o a s t o f England. South of t h e lake-margin sabkha dune s a n d s form t h e dominant sediment; t h e y a r e i n t e r bedded w i t h a v a r i a b l e p r o p o r t i o n o f wadi s e d i m e n t s a l o n g a n a x i s t r e n d i n g N from t h e p r e s e n t N o r f o l k c o a s t , and w i t h sabkha s e d i m e n t s , i n c l u d i n g a d h e s i o n r i p p l e s , a l o n g a n E-W a x i s through t h e n o r t h e r n S o l e P i t a r e a . Superimposed on t h i s map a r e c o n t o u r s of t h e e s t i m a t e d d i f f e r e n c e between t h e p r e s e n t and maximum d e p t h s of b u r i a l of t h e o v e r l y i n g B u n t e r S h a l e ( s o l i d l i n e s ) and t h e amount o f p o r o s i t y d e s t r u c t i o n a t t r i b u t e d t o compaction (dashed l i n e s ) . The Dowsing f a u l t zone was p r o b a b l y a c t i v e from R o t l i e g e n d e s t i m e ( a f t e r G l e n n i e e t a l . , 1978). D i a n e n e s i s and r e d d e n i n n of t h e R o t l i e n e n d e s F o r t h e most p a r t t h e R o t l i e g e n d e s a r e u n i f o r m l y r e d i n c o l o u r .
They p a s s upwards
117 i n t o marine s a n d s t o n e s ( W e i s s l i e g e n d e s ) which were never reddened (Nemec and Porebski, 1977a,b).
The r e d c o l o u r a t i o n o f t h e R o t l i e g e n d e s s a n d s t o n e s has been
debated f o r many y e a r s .
Dunham (1953) was w e l l aware t h a t t h e c o l o u r was due t o
g r a i n c o a t i n g s and m a i n t a i n e d t h a t i t developed p r i o r t o cementation.
He r e f e r r e d
t o t h e pigmentary m a t e r i a l a s t u r g i t e and b e l i e v e d t h a t i t o r i g i n a t e d i n d e s e r t m a r g i n a l a r e a s by l a t e r i t i c w e a t h e r i n g under a s u b t r o p i c a l , humid c l i m a t e .
Dunham
s u g g e s t e d t h a t t h e t u r g i t e was t r a n s p o r t e d t o s h a l l o w lagoons where i t c o a t e d sand g r a i n s d u r i n g s e d i m e n t a t i o n ; a mechanism of reddening i n e f f e c t i d e n t i c a l t o t h e Primary r e d bed model of Krynine (1949).
Shotton (1956), w h i l s t accepting t h a t
s u c h a mechanism might a c c o u n t f o r t h e r e d c o l o u r o f some o f t h e f l u v i a l r e d beds i n t h e New Red S a n d s t o n e , c l e a r l y thought i t a n u n s u i t a b l e e x p l a n a t i o n f o r t h e a e o l i a n s a n d s t o n e s s u c h a s t h e P e n r i t h Sandstone and t h e B r i d g n o r t h Sandstone because o f t h e absence o f w a t e r i n t h e a e o l i a n environment. S h o t t o n (1956, p.454) on t h e o t h e r hand q u i t e c l e a r l y s u g g e s t e d t h a t i n t r a s t r a t a l s o l u t i o n o f i r o n - b e a r i n g m i n e r a l s was r e s p o n s i b l e f o r t h e r e d c o l o u r a t i o n of t h e a e o l i a n sandstones..."The
o n l y method I c a n s u g g e s t i s t h e c a p i l l a r y movement of
dew w a t e r and o c c a s i o n a l r a i n f a l l i n t h e top l a y e r s o f s a n d , c r e e p i n g a s f i l m s over t h e s u r f a c e s and c o a t i n g them w i t h t u r g i t e from i r o n - b e a r i n g c o n s t i t u e n t s w i t h i n t h e sand i t s e l f . ,
,'I
Subsequent r e s e a r c h , n o t a b l y by Waugh ( 1 9 7 0 a , b ) , G l e n n i e e t a l . (1978), K e s s l e r (1978) and Hancock (1978) h a s s u p p o r t e d t h e c o n t e n t i o n s of S h o t t o n a l t h o u g h t h e i r c o n c l u s i o n s a r e based mainly on t h e o v e r a l l d i a g e n e t i c p i c t u r e and p a r t i c u l a r l y t h e p r e s e n c e o f a u t h i g e n i c phases.
Diagenetic f e a t u r e s i n Rotliegendes sandstones
include:
1) g r a i n d i s s o l u t i o n and t h e replacement o f framework s i l i c a t e s by c l a y 2 ) mechanical i n f i l t r a t i o n of c l a y
3 ) f o r m a t i o n o f a u t h i g e n i c q u a r t z , f e l d s p a r , and c l a y 4 ) f o r m a t i o n o f c a l c i t e and a n h y d r i t e cements Although t h e e f f e c t s o f b u r i a l d i a g e n e s i s a r e w e l l s e e n i n t h e R o t l i e g e n d e s t h e e a r l y d i a g e n e t i c f e a t u r e s a r e o f t e n extremely w e l l preserved.
There a r e many
e x c e l l e n t i l l u s t r a t i o n s o f t h e s e f e a t u r e s (e.g. Waugh, 1970a,b, 1978; G l e n n i e , e t a l . , 1978; Hancock, 1978; Kessler, 1978). Mechanical i n f i l t r a t i o n o f c l a v The r e d c o l o u r e d g r a i n c o a t i n g s i n some of t h e R o t l i e g e n d e s a e o l i a n s a n d s t o n e s can o f t e n be demonstrated t o have a g e o p e t a l f a b r i c ( F i g . 2 . 3 0 ) .
This f a b r i c u s u a l l y takes
t h e form o f a c o a t i n g on t h e p o s t - d e p o s i t i o n a l upper s u r f a c e of t h e d e t r i t a l g r a i n s and may t h i c k e n a t g r a i n c o n t a c t s o r even show meniscus b r i d g e s due t o s u r f a c e tension e f f e c t s .
When examined i n d e t a i l , u s i n g SEM, g r a i n c o a t i n g s o f t h i s type
a r e s e e n t o c o n s i s t o f l a y e r s o f c l a y f l a k e s o r i e n t a t e d p a r a l l e l t o t h e s u r f a c e of
118
Fig.2.30. Photomicrographs of r e d c l a y - i r o n o x i d e g r a i n c o a t i n g s i n R o t l i e g e n d e s a e o l i a n s a n d s t o n e s . A . Well-rounded s a n d g r a i n s showing c l a y - o x i d e g r a i n c o a t i n g s . Note t h e i r even t h i c k n e s s and a b s e n c e a t g r a i n c o n t a c t s ( C o r r i e S a n d s t o n e , A r r a n , S c o t l a n d ) . B. Clay-oxide c o a t i n g s of uneven t h i c k n e s s s u g g e s t i n g a geopedal o r i g i n ( S o l e P i t a r e a , North S e a ) . C . Clay-oxide i n f i l l i n g p i t s on a q u a r t z g r a i n s u g g e s t i n g o r i g i n a s a c l a y c u t a n ( P e n r i t h S a n d s t o n e , Cumbria). I). Deformed g r a i n of c l a y o x i d e , p r o b a b l y a c o m p l e t e l y pseudomorphed f e r r o m a g n e s i a n s i l i c a t e ( S o l e P i t a r e a , North S e a ) .
119
Fig.2.31. Photomicrographs of a u t h i g e n i c q u a r t z and f e l d s p a r i n R o t l i e g e n d e s a e o l i a n sandstones. A . Monocrystalline q u a r t z grains with o p t i c a l l y continuous overgrowths i n d i f f e r e n t e x t i n c t i o n p o s i t i o n s ( P e n r i t h S a n d s t o n e ) . B. Polyc r y s t a l l i n e q u a r t z g r a i n showing o p t i c a l c o n t i n u i t y between e a c h s u b c r y s t a l and i t s overgrowth ( P e n r i t h S a n d s t o n e ) . C. P l a g i o c l a s e w i t h o p t i c a l l y c o n t i n u o u s overgrowth ( S o l e P i t a r e a , N o r t h S e a ) .
120 t h e d e t r i t a l g r a i n ( s e e Kessler, 1978, P l a t e 2 ) . t h a t of a u t h i g e n i c c l a y .
T h i s f a b r i c i s q u i t e d i s t i n c t from
More commonly no g e o p e t a l f a b r i c i s r e c o g n i z a b l e , t h e r e d
c l a y o x i d e c o a t i n g i s o f uniform t h i c k n e s s a r o u n d t h e d e t r i t a l g r a i n s b u t a b s e n t a t g r a i n c o n t a c t s ( F i g . 2.30). Crone ( 1 9 7 5 ) , Walker (1976) and Walker e t a l . (1978) have d e m o n s t r a t e d t h a t g e o p e t a l c l a y f a b r i c s of t h i s t y p e form from t h e mechanical i n f i l t r a t i o n of c l a y by i n f l u e n t s e e p a g e of s u r f a c e r a i n o r f l o o d w a t e r s ,
I n t h e Cenozoic d e p o s i t s o f t h e
a r i d p a r t s o f N o r t h America t h i s i n f i l t r a t e d c l a y i s n o t r e d when d e p o s i t e d b u t reddens w i t h t i m e .
T h i s happens b e c a u s e o f t h e f o r m a t i o n of h a e m a t i t e by t h e
r e a c t i o n of oxygenated groundwater w i t h i r o n a d s o r b e d on c l a y s u r f a c e s and i n c l a y mineral l a t t i c e s .
There seems t o b e no r e a s o n why t h i s p r o c e s s i s n o t d i r e c t l y
a c c o u n t a b l e f o r t h e g e o p e t a l f a b r i c s observed i n t h e R o t l i e g e n d e s a e o l i a n r e d beds; i t is a l s o p r o b a b l e t h a t t h e non-geopetal
a s i m i l a r manner.
c l a y - o x i d e g r a i n c o a t i n g s were formed i n
There a r e no i n d i c a t i o n s t h a t t h e c l a y - o x i d e c o a t i n g s were formed
i n a more p l u v i a l c l i m a t e and l a t e r r e d e p o s i t e d i n a e o l i a n dunes ( c f . F o l k , 1976). Indeed many of t h e c o a r s e , well-rounded a e o l i a n g r a i n have s i n g l e c l a y o x i d e c o a t i n g s which c o u l d o n l y have formed p o s t - d e p o s i t i o n a l l y . A u t h i g e n i c q u a r t z , f e l d s p a r and c l a y m i n e r a l s A u t h i g e n i c o v e r g r o w t h s , p a r t i c u l a r l y of q u a r t z o r f e l d s p a r , a r e a common f e a t u r e of t h e Permian R o t l i e g e n d e s .
Q u a r t z overgrowths a r e v o l u m e t r i c a l l y more i m p o r t a n t
t h a n f e l d s p a r and have been d e s c r i b e d i n d e t a i l from t h e P e n r i t h Sandstone by Waugh (1970a,b).
The P e n r i t h Sandstone i s s i l i c i f i e d by q u a r t z overgrowths i n t h e
n o r t h e r n p a r t o f i t s o u t c r o p i n t h e Vale o f Eden b u t remains u n s i l i c i f i e d i n t h e s o u t h e r n p a r t where i t i s i n t e r b e d d e d w i t h c a l c i t e cemented f a n g l o m e r a t e s (brockram). (Waugh, 1970b, F i g . 4 ) . The overgrowths which t a k e t h e form of b i p y r a m i d a l q u a r t z c r y s t a l s o c c u r on a v a r i e t y o f q u a r t z t y p e s , i n c l u d i n g u n s t r a i n e d , s t r a i n e d , and polycrystalline grains.
The q u a r t z type i s i n s t r u m e n t a l i n c o n t r o l l i n g t h e t y p e of
overgrowth which i s produced.
Thus, s i n g l e c r y s t a l s t r a i n e d , and u n s t r a i n e d and
p o l y c r y s t a l l i n e overgrowths were r e c o g n i z e d by Waugh (1970a,b) and t h e s e a r e shown i n Fig.2.31. silcrete.
The s i l i c i f i e d P e n r i t h Sandstone can r e a d i l y b e i n t e r p r e t e d a s a d e s e r t
Other s i l i c i f i e d deposits e.g.
t h e Lower Permian p e t r i f i e d f o r e s t s o f
Germany have been i n t e r p r e t e d a s s i l c r e t e s (Schwarzbach, 1963, p . 6 5 ) .
Silcretes are
a common f e a t u r e o f modern d e s e r t s and have been d e s c r i b e d by Woolnough (19301, Sandford (1935) and A l l e y (1977).
The s i l i c e o u s cement i n s i l c r e t e s is u s u a l l y
r e p o r t e d a s o p a l o r chalcedony b u t Williamson (1957) h a s r e c o r d e d rounded a e o l i a n grains with o p t i c a l l y continuous quartz r i m s . Waugh (1970b) a t t r i b u t e d t h e s o u r c e o f t h e s e c o n d a r y s i l i c a i n t h e P e n r i t h Sandstone t o s i l i c e o u s d u s t produced by wind a b r a s i o n .
This i s consistent with the
g r e a t i m p o r t a n c e of a i r b o r n e d u s t i n t h e f o r m a t i o n of d e s e r t s o i l s (Yaalon, 1973).
121 Waugh went on t o s u g g e s t t h a t t h i s d u s t w i t h a h i g h l y s o l u b l e s u r f a c e l a y e r was d i s s o l v e d by a l k a l i n e groundwater and d e s e r t dews which i n t u r n were drawn t o t h e s u r f a c e of dunes by e v a p o r a t i o n .
On e v a p o r a t i o n o p t i c a l l y c o n t i n u o u s q u a r t z
overgrowths were p r e c i p i t a t e d a r o u n d i n d i v i d u a l g r a i n s .
The f o r m a t i o n o f q u a r t z
overgrowths i n t h i s way a c c o u n t s f o r t h e f a c t t h a t t h e c o r e s of P e n r i t h Sandstone barchans a r e f r e q u e n t l y l e s s s i l i c i f i e d and a l s o ( b e c a u s e o f d i f f e r e n c e s i n groundw a t e r c o m p o s i t i o n ) f o r t h e o c c u r r e n c e of u n s i l i c i f i e d P e n r i t h Sandstone a s s o c i a t e d with t h e marginal fanglomerates.
Here where c a r b o n a t e r o c k s were common i n t h e s o u r c e
a r e a t h e groundwaters were l i k e l y t o have been more a c i d . The d i s t r i b u t i o n o f a u t h i g e n i c q u a r t z i n t h e a e o l i a n s a n d s t o n e s of t h e s o u t h e r n N o r t h Sea i s a l s o i r r e g u l a r .
G l e n n i e e t a l . (1978) n o t e s t h a t a u t h i g e n i c q u a r t z i s
more abundant i n dune t o p s e t s than b o t t o m s e t s b u t a t t r i b u t e d i t t o b u r i a l d i a g e n e s i s . Although t h e amount of a u t h i g e n i c q u a r t z was t o o g r e a t t o be a c c o u n t e d f o r by p r e s s u r e s o l u t i o n a l o n e ( G l e n n i e e t a l . , 1978, p . 3 2 ) s u g g e s t e d t h a t ' f e l d s p a r l e a c h i n g and a l t e r a t i o n o f u n d e r l y i n g m o n t m o r i l l o n i t e and i l l i t e / m o n t m o r i l l o n i t e , r a t h e r t h a n a b r a s i o n d u s t , p r o v i d e d the s o u r c e o f e x t r a s i l i c a .
T h i s i s l i k e l y t o be the c a a e
f o r t h e l a t e d i a g e n e t i c q u a r t z which forms d u r i n g b u r i a l d i a g e n e s i s b u t i t c a n n o t a c c o u n t f o r t h e l a r g e volumes o f a u t h i g e n i c q u a r t z which undoubtedly form d u r i n g e a r l y d i a g e n e s i s , a s i n t h e c a s e of t h e P e n r i t h Sandstone. Authigenic f e l d s p a r (Fig.2.31) i s a minor, b u t c h a r a c t e r i s t i c c o n s t i t u e n t of Lower Permian s a n d s t o n e s .
Most commonly t h e a u t h i g e n i c f e l d s p a r o c c u r s a s over-
growths on d e t r i t a l g r a i n s b u t a l s o a s a p o r e - f i l l i n g cement.
K - f e l d s p a r overgrowths
a r e commonly s t o i c h i o m e t r i c K A l S i 0 w i t h -2V of 20-25' s u g g e s t i n g p o t a s s i u m i n t e r 3 8 m e d i a t e s a n i d i n e (Waugh, 1978). A u t h i g e n i c p l a g i o c l a s e s a l s o o c c u r b u t a r e much more scarce than the K-feldspar.
Hancock (1978) d e s c r i b e d a u t h i g e n i c overgrowths of
a l b i t e a n d a l b i t e cement c r y s t a l s from t h e R o t l i e g e n d e s o f West Germany.
The
r e q u i s i t e i o n s f o r t h e development o f a u t h i g e n i c f e l d s p a r a r e a l m o s t c e r t a i n l y p r o v i d e d by i n t r a s t r a t a l d i s s o l u t i o n of d e t r i t a l s i l i c a t e s ( i n c l u d i n g f e l d s p a r s ) and v o l c a n i c r o c k fragments.
The d i a g e n e t i c h i s t o r y o f f e l d s p a r i n t h e Lower
Permian s a n d s t o n e s i s p r o b a b l y , l i k e t h a t of q u a r t z , l o n g and complex, and a u t h i g e n i c f e l d s p a r was p r o b a b l y produced d u r i n g b u r i a l d i a g e n e s i s a s w e l l a s d u r i n g e a r l i e r diagenesis. The r e c o g n i t i o n o f d i s s o l u t i o n and a u t h i g e n e s i s i n f e l d s p a r g r a i n s i s o f c r u c i a l importance t o t h e r e d bed q u e s t i o n .
I t d e m o n s t r a t e s t h a t i n t r a s t r a t a l s o l u t i o n was
a n i m p o r t a n t p r o c e s s , and t h a t d e t r i t a l f e r r o m a g n e s i a n s i l i c a t e s must a l s o have suffered its effects. Permian s a n d s t o n e s .
There a r e v e r y few r e l i c t f e r r o m a g n e s i a n g r a i n s i n tk Lower Kessler (1978, P l a t e 2 , F i g . 6 ) i l l u s t r a t e s a p a r t i a l l y d i s s o l v e d
hornblende g r a i n b u t t h i s i s r a r e .
Following Walker (1967a) one m i g h t assume t h a t
t h e p a u c i t y of u n s t a b l e f e r r o m a g n e s i a n s i l i c a t e s i s due t o t h e i r s u b s e q u e n t d i s solution.
I n s u p p o r t of t h i s i s t h e f a i r l y common o c c u r r e n c e o f g r a i n s o f c l a y w i t h
a t h i c k e r o u t e r s h e l l of haematite (Fig.2.30)
which c o u l d b e c o m p l e t e l y r e p l a c e d by
122 ferromagnesian m i n e r a l s .
However, i n c o n t r a s t t o t h e f i r s t - c y c l e a r k o s e s d e s c r i b e d
by Walker they a r e u n l i k e l y t o have been v e r y abundant i n t h e Lower Permian which was d e r i v e d mainly from W e s t p h a l i a n / S t e p h a n i a n s e d i m e n t a r y r o c k s .
I n some a r e a s
e . g . Northern Germany v o l c a n i c d e t r i t u s i s i m p o r t a n t and t h e i n t r a s t r a t a l s o l u t i o n of such g r a i n s was p r o b a b l y a n i m p o r t a n t s o u r c e o f i r o n and o t h e r e l e m e n t s . The r e l a t i v e importance o f m e c h a n i c a l l y i n f i l t r a t e d c l a y and i n t r a s t r a t a l d i s s o l u t i o n of ferromagnesian g r a i n s o r v o l c a n i c c l a s t s i n t h e r e d d e n i n g of t h e Lower Permian s a n d s t o n e s i s n o t c l e a r .
Both p r o c e s s e s c e r t a i n l y o p e r a t e d and i t i s
l i k e l y t h a t i n combination t h e y c o u l d a c c o u n t f o r t h e o b s e r v e d c o l o u r a t i o n .
In
e i t h e r c a s e r e d n e s s was produced a t a n e a r l y s t a g e of d i a g e n e s i s . The most common a u t h i g e n i c c l a y m i n e r a l s i n t h e Lower Permian s a n d s t o n e s a r e i l l i t e and c h l o r i t e and i n some a r e a s t h e r e i s minor k a o l i n i t e .
The i l l i t e o c c u r s
i n a v a r i e t y o f forms which r e a d i l y e n a b l e i t s d i s t i n c t i o n from m e c h a n i c a l l y i n f i l trated illite.
Whereas t h e l a t t e r h a s a c l a s t i c t e x t u r e p a r a l l e l t o g r a i n s u r f a c e s
( s e e Kessler, 1978, P l a t e 2 , F i g . 3 ) t h e a u t h i g e n i c i l l i t e o c c u r s a s randomly o r r a d i a l l y o r i e n t a t e d f l a k e s which may s e p a r a t e outwards i n t o f i b r e s ( s e e G l e n n i e , e t a l . , 1978, P l a t e 2 , Fig.B).
Combined i l l i t e and c h l o r i t e i s u b i q u i t o u s i n a n c i e n t
r e d beds and i t i s r e a s o n a b l e t o assume ( c f . Walker, 1976; G l e n n i e e t a l . , 1978) t h a t much of i t was o r i g i n a l l y mixed l a y e r i l l i t e - m o n t m o r i l l o n i t e produced by silicate alteration.
The t r a n s f o r m a t i o n o f i l l i t e - m o n t m o r i l l o n i t e t o i l l i t e and
t h e i n c r e a s e i n i l l i t e c r y s t a l l i n i t y w i t h i n c r e a s e i n t e m p e r a t u r e and d e p t h of b u r i a l g i v e s some i n d i c a t i o n o f t h e d e g r e e o f b u r i a l d i a g e n e s i s (Ktlbler, 1968).
The e f f e c t
o f a u t h i g e n i c c l a y m i n e r a l s on s a n d s t o n e p e r m e a b i l i t y i s d i s c u s s e d by S t a l d e r (1973)
-
A u t h i g e n i c i l l i t e i n Lower Permian s a n d s t o n e s i s f r e q u e n t l y i r o n - r i c h and a l t h o u g h t h e r e have been no d e t a i l e d s ' t u d i e s o f i t s t h i n s e c t i o n p e t r o g r a p h y i t i s l i k e l y t o have reddened d u r i n g d i a g e n e s i s by r e a c t i o n w i t h oxygenated g r o u n d w a t e r , C h l o r i t e , o c c u r r i n g a s t h i n hexagonal p l a t e s , formed a t t h e same t i m e a s i l l i t e . There i s no d i r e c t e v i d e n c e y e t r e p o r t e d which i n d i c a t e s t h a t t h e a l t e r a t i o n o f a u t h i g e n i c c h l o r i t e c o n t r i b u t e s d i r e c t l y t o r e d bed c o l o u r a t i o n .
Few o t h e r c l a y
m i n e r a l s sre r e p o r t e d from t h e R o t l i e g e n d e s , k a o l i n i t e b e i n g noteworthy.
This often
o c c u r s a s books o f pseudohexagonal c r y s t a l s which choke p o r e s p a c e s and i t i s normally i n t e r p r e t e d a s a n i n d i c a t i o n of a n a c i d phase of d i a g e n e s i s ( p e r h a p s r e s u l t i n g from m e t e o r i c w a t e r ) r e s u l t i n g from u p l i f t a f t e r deep b u r i a l ( M i l l o t , 1970; Hancock, 1978).
A u t h i g e n i c i l l i t e and k a o l i n i t e a r e i l l u s t r a t e d i n F i g . 2.32.
Carbonate and s u l p h a t e cements Two cementing a g e n t s a r e normally s e e n i n t h e R o t l i e g e n d e s s a n d s t o n e s : and a n h y d r i t e
(Fig.2.33).
calcite
C a l c i t e t y p i c a l l y o c c u r s a s a v e r y e a r l y cement i n wadi
s a n d s t o n e s and f a n g l o m e r a t e s .
This i s c o n s i s t e n t with the f e a t u r e s observed i n
modern d e p o s i t s formed under s i m i l a r c i r c u m s t a n c e s ( B l i s s e n b a c h , 1954; G l e n n i e , 1970;
123
Fig.2.32. SEM photographs of a u t h i g e n i c i l l i t e and k a o l i n i t e i n R o t l i e g e n d e s s a n d s t o n e s from t h e North Sea a r e a . A . I l l i t e , showing t y p i c a l boxwork t e x t u r e . B. A u t h i g e n i c q u a r t z c r y s t a l s forming p o r e s p a c e s plugged w i t h pseudohexagonal f l a k e s of k a o l i n i t e and some f i b r o u s i l l i t e .
124
Fig.2.33. C a l c i t e and a n h y d r i t e cement i n Rot'liegendes s a n d s t o n e s . A . C a l c i t e cement i n a wadi b r e c c i a . Growth zones a r e d e f i n e d by i r o n o x i d e i n c l u s i o n s (South Devon). B. A n h y d r i t e cement i n a dune s a n d s t o n e ( S o l e P i t a r e a , N o r t h S e a ) .
125 S t a l d e r , 1975). I n a e o l i a n s a n d s t o n e s c e m e n t a t i o n i s n e v e r p e r v a s i v e and many o f t h e dune sandstones a r e only loosely consolidated.
There a r e i m p o r t a n t d i f f e r e n c e s between t h e
Lower Permian s a n d s t o n e s o f B r i t a i n and t h e R o t l i e g e n d e s of t h e s o u t h e r n North Sea. The former show v e r y l i t t l e i n t h e way of c a r b o n a t e o r a n h y d r i t e cements and i t must b e presumed t h a t t h e cements were removed a t some s t a g e .
I n the Rotliegendes
gypsum i s s e e n a s a n e a r l y cement i n some dunes b u t i t i s u s u a l l y c o n v e r t e d t o a n h y d r i t e on b u r i a l ( G l e n n i e e t a l . ,
1978).
The a n h y d r i t e shows a p o i k i l i t i c o r
v o i d f i l l i n g h a b i t ( F i g . 2 . 3 3 ) and i s o f t e n a s s o c i a t e d w i t h d o l o m i t e which o f t e n shows similar textures.
Dolomite a l s o o c c u r s a s b l o c k y a n h e d r a l cement f i l l i n g p o r e s p a c e s
o r a s a g g r e g a t e s of f i n e e u h e d r a l c r y s t a l s .
As w i t h a n h y d r i t e t h e r e a r e i n d i c a t i o n s
t h a t d o l o m i t e may have been a n e a r l y p h a s e t o c r y s t a l l i z e i n t h e dune s a n d s t o n e s , b u t many a u t h o r s (e.g. G l e n n i e e t a l . , 1978) r e g a r d i t t o b e a p r o d u c t of b u r i a l diagen-
esis.
The f o r m a t i o n of d o l o m i t e by r e p l a c e m e n t i n t h i s manner i f of i n t e r e s t because
of t h e well-known a s s o c i a t i o n o f r e p l a c e m e n t d o l o m i t e s w i t h i r o n hydroxide o r h a e m a t i t e cements and zones ( K a t z , 1971; S t a l d e r , 1975).
Under c e r t a i n c i r c u m s t a n c e s
t h e l e a c h i n g o f i r o n from c a r b o n a t e s d u r i n g d o l o m i t i z a t i o n c o u l d p r o v i d e a n i m p o r t a n t s o u r c e of pigmentary h a e m a t i t e , CONCLUSIONS AND FURTHER READING
Desert r e d beds i n c l u d e two d i s t i n c t i v e t y p e s of r e d b e d s , d e s e r t a l l u v i u m and a e o l i a n sands.
The former p r o v i d e c l a s s i c examples o f r e d bed f o r m a t i o n by long-
c o n t i n u e d i n t r a s t r a t a l a l t e r a t i o n i n b o t h Cenozoic a l l u v i u m and a number o f a n c i e n t r e d beds.
The r e d d e n i n g h i s t o r y of a e o l i a n s a n d s is more c o m p l i c a t e d .
Red a e o l i a n
s a n d may b e d e r i v e d from l a t e r i t i z e d a l l u v i a l s e d i m e n t s and t h e n b e s u b j e c t e d t o i n t r a s t r a t a l a l t e r a t i o n and f u r t h e r . r e d d e n i n g ,
The g e o l o g i c a l s e t t i n g s of some
a n c i e n t r e d b e d s , s u c h a s t h e R o t l i e g e n d e s , a r e c o n s i s t e n t w i t h t h i s view and more work i s needed t o e l u c i d a t e t h e r e l a t i v e i m p o r t a n c e of p r e - d e p o s i t i o n a l a n d p o s t d e p o s i t i o n a l reddening. Comprehensive d e s c r i p t i o n s of modern d e s e r t s e d i m e n t s a r e g i v e n by t h e f o l l o w i n g . They a l s o i n c l u d e some d i s c u s s i o n of t h e problems o f r e d c o l o u r a t i o n i n d e s e r t s o i l s and s a n d s . Cooke, R.U.
and Warren, A . , 1973. Geomorphology i n Deserts. B.T.
Batsford Ltd.,
London, 374 pp. G l e n n i e , K.W.,
1970. Desert Sedimentary Environments. E l s e v i e r , Amsterdam, 222 pp.
126
CHAPTER 3 DELTA PLAIN RED BEDS
INTRODUCTION D e l t a s a r e b o d i e s of c l a s t i c s e d i m e n t formed i n s u b a e r i a l and s h a l l o w w a t e r environments by t h e i n f l o w of r i v e r s and i n which a g r a d a t i o n i n t o o f f s h o r e , g e n e r a l l y f i n e r g r a i n e d f a c i e s can be t r a c e d ,
I n many d e l t a s t h e d e p o s i t i o n a l
i n f l u e n c e of r i v e r s may b e masked by waves and t i d e s and t h e d e p o s i t s of t h e s e may b e v e r y d i f f i c u l t t o r e c o g n i z e i n t h e s t r a t i g r a p h i c r e c o r d .
Other d e l t a s a r e
dominated by t h e d e p o s i t i o n a l i n f l u e n c e of t h e s u p p l y i n g r i v e r s and a r e c h a r a c t e r i z e d by a s u b a e r i a l a l l u v i a l p l a i n known a s t h e d e l t a p l a i n .
Most o f t h e a n c i e n t
d e l t a i c s u c c e s s i o n s which have been d e s c r i b e d a r e o f t h i s r i v e r - d o m i n a t e d t y p e and c o n t a i n s e d i m e n t a r y sequences which can be a t t r i b u t e d t o d e p o s i t i o n i n t h e d e l t a p l a i n environment. The n a t u r e of t h e d e l t a p l a i n environment i s dependent upon s e v e r a l i n t e r - r e l a t e d v a r i a b l e s of which c l i m a t e , w a t e r d i s c h a r g e ( r a t e and v a r i a b i l i t y ) , and sediment l o a d ( q u a n t i t y and g r a i n s i z e ) a r e p a r t i c u l a r l y i m p o r t a n t .
I n A r c t i c and a r i d
c l i m a t e s where p r e c i p i t a t i o n i s e r r a t i c and v e g e t a t i o n i s s p a r s e , b r a i d e d c h a n n e l p a t t e r n s w i t h c o a r s e - g r a i n e d bed l o a d s t e n d t o develop on t h e d e l t a p l a i n and sandy o r pebbly sequences a r e d e p o s i t e d .
I n humid, t r o p i c a l c l i m a t e s p r e c i p i t a t i o n i s
h i g h and v e g e t a t i o n abundant f a v o u r i n g t h e f o r m a t i o n of meandering c h a n n e l s w i t h a h i g h suspended sediment l o a d and mud-dominated sequences predominate. Red beds may form i n any of t h e s e c l i m a t i c s e t t i n g s b u t a r e p a r t i c u l a r l y charact e r i s t i c and best-known from d e l t a p l a i n d e p o s i t s of m o i s t t r o p i c a l c l i m a t e s ( w i t h or without seasonal a r i d i t y ) .
T y p i c a l l y t h e s e r e d beds a r e developed i n m o t t l e d
o r polychrome mudstones d e p o s i t e d o v e r wide a r e a s of t h e d e l t a p l a i n . A s i n d e s e r t and a l l u v i a l r e d beds t h e r e a r e no good modern a n a l o g u e s of d e l t a p l a i n r e d b e d s , a f e a t u r e which s u g g e s t s t h a t t h e y a r e o f d i a g e n e t i c o r i g i n . The p r o c e s s e s l e a d i n g t o r e d d e n i n g o f muddy d e l t a p l a i n s e d i m e n t s a r e c l o s e l y dependent on ground w a t e r c o n d i t i o n s a n d , i n many c a s e s , a r e a s s o c i a t e d w i t h pedogenesis.
These p r o c e s s e s c o n t r a s t markedly w i t h r e d d e n i n g p r o c e s s e s i n f i r s t - c y c l e
arkosic alluvium.
They i n c l u d e t h e r e l e a s e o f i r o n o x i d e s d u r i n g c l a y m i n e r a l
t r a n s f o r m a t i o n s and t h e o x i d a t i o n o f f i n e l y d i s s e m i n a t e d p y r i t e a s w e l l a s t h e i n t r a s t r a t a l a 1 t e r a t i o n of i r o n - s i l i c a t e s . I n t h i s c h a p t e r t h e s e d i m e n t s o f modern d e l t a s and t h e i r a n c i e n t a n a l o g u e s a r e described i n d e t a i l .
T h i s i s f o l l o w e d by a d e s c r i p t i o n of t h e o c c u r r e n c e of r e d
127 beds i n a n c i e n t d e l t a p l a i n s e d i m e n t s and a d i s c u s s i o n o f t h e i r o r i g i n .
Special
r e f e r e n c e i s made t o t h o s e r e d beds which formed i n m o i s t t r o p i c a l c l i m a t e s and a r e i n t e r - d i g i t a t e d with coal-bearing s t r a t a i n t h e Carboniferous
System o f n o r t h e r n ,
Europe. DELTA MODELS Moore and A s q u i t h (1971) d e f i n e a d e l t a a s :
"The s u b a e r i a l and submerged c o n t i -
guous sediment mass d e p o s i t e d i n a body of w a t e r (ocean o r l a k e ) p r i m a r i l y by t h e a c t i o n of a r i v e r " .
D e l t a s formed a t ocean margins a r e c o n s i d e r a b l y more i m p o r t a n t
t h a n t h o s e formed i n l a k e s and a r e t h e s o l e s u b j e c t o f t h i s c h a p t e r .
Present-day
d e l t a s a r e r e l a t i v e l y young b e c a u s e t h e y have o n l y formed s i n c e t h e p o s t - P l e i s t o c e n e r i s e i n s e a l e v e l ; f o r t h e most p a r t , however, t h e y a r e v e r y l a r g e and t h e i r s u b a e r i a l e x t e n t may b e i n e x c e s s o f 100,000km
2
(Smith, 1966).
The d i s t r i b u t i o n , o r i e n t a t i o n and i n t e r n a l geometry of d e l t a i c d e p o s i t s i s c o n t r o l l e d by a v a r i e t y of f a c t o r s i n c l u d i n g c l i m a t e , w a t e r d i s c h a r g e , sediment l o a d , river-mouth p r o c e s s e s , waves, t i d e s , c u r r e n t s , w i n d s , s h e l f s l o p e and t h e t e c t o n i c s and geometry o f t h e r e c e i v i n g b a s i n (Wright e t a l . , 1973).
The r e s u l t i s t h a t d e l t a s
show c o n s i d e r a b l e v a r i a t i o n and a n u n d e r s t a n d i n g o f t h e c o n t r o l l i n g p r o c e s s e s i s n e c e s s a r y b e f o r e any m e a n i n g f u l c l a s s i f i c a t i o n can b e made. C l i m a t e , w a t e r d i s c h a r g e , and sediment l o a d a r e c l o s e l y i n t e r - r e l a t e d .
I n moist
t r o p i c a l r e g i o n s p r e c i p i t a t i o n i s g e n e r a l l y h i g h r e l a t i v e t o e v a p o t r a n s p i r a t i o n and r u n o f f t e n d s t o b e h i g h and s t e a d y .
The i m p o r t a n c e of chemical w e a t h e r i n g i n such
regions leads t o high dissolved-load sediment concentrations. r i v e l y s t a b l e meandering c h a n n e l p a t t e r n s predominate.
Consequently, r e l a -
I n Arctic o r a r i d conditions
where p r e c i p i t a t i o n i s spasmodic and v e g e t a t i o n s p a r s e , b r a i d e d c h a n n e l p a t t e r n s w i t h l a r g e bed l o a d s t e n d t o predominate (Coleman a n d W r i g h t , 1975; M i a l l , 1976). When a sediment-laden r i v e r e n t e r s a body o f s t a n d i n g w a t e r , depending on t h e d e n s i t y d i f f e r e n c e s between t h e r i v e r and t h e l a k e o r s e a i n t o which i t f l o w s , one of t h r e e t y p e s of flow d i s p e r s a l may o c c u r : A ) I n f l o w more dense and forms a p l a n a r j e t a l o n g t h e bottom.
commonly r e s u l t s .
A turbidity current
The d e p o s i t s which form from t h e s e bottom c u r r e n t s a r e u s u a l l y
c l a s s i f i e d a s submarine fans.
B) I n f l o w e q u a l l y dense o f t e n o c c u r s when rivers e n t e r l a k e s .
Under t h e s e c o n d i t i o n s
t h e sediment i s d i s p e r s e d r a d i a l l y and t h e competency i s r a p i d l y l o s t . a r c u a t e zone o f a c t i v e d e p o s i t i o n t h u s r e s u l t s
-
A narrow
the c l a s s i c a l Gilbertian delta.
C) I n f l o w less dense i s t h e c a s e which u s u a l l y p r e v a i l s i n marine d e l t a s because f r e s h w a t e r i s less dense t h a n s e a w a t e r ,
The t y p e of sediment d i s p e r s a l which t h e n
t a k e s p l a c e depends on t h e s t r e n g t h o f waves, t i d e s and l o n g s h o r e c u r r e n t s . The energy c o n d i t i o n s t h a t e x i s t i n t h e s e a a t t h e r i v e r mouth e x e r c i s e t h e main c o n t r o l on t h e d e p o s i t i o n a l environment and t h e morphology of t h e r e s u l t i n g d e l t a .
128 The most u s e f u l c l a s s i f i c a t i o n of d e l t a t y p e s i s one based on t h e r e l a t i v e s t r e n g t h s of f l u v i a l and marine p r o c e s s e s ( c f . F i s h e r e t a l . , Galloway, 1975).
1969;. Coleman and W r i g h t , 1975;
F i s h e r e t a l . (1969) d i s t i n g u i s h e d between h i g h - c o n s t r u c t i v e
d e l t a s dominated by f l u v i a l p r o c e s s e s , and h i g h - d e s t r u c t i v e d e l t a s dominated by b a s i n a l processes (Fig.3.1).
H i g h - c o n s t r u c t i v e d e l t a s i n c l u d e t h e l o b a t e and b i r d -
f o o t types w h i l s t t h e h i g h - d e s t r u c t i v e d e l t a s i n c l u d e t h e wave-dominated and t i d e dominated t y p e s .
Although t h i s c l a s s i f i c a t i o n i s u s e f u l b e c a u s e i t stresses t h e
importance o f v e r t i c a l f a c i e s sequence and t h e a r e a l d i s t r i b u t i o n o f f a c i e s i t has a number of d i s a d v a n t a g e s and h a s n o t been w i d e l y a d o p t e d .
I n the f i r s t place the
scheme p l a c e s t o o much emphasis on t h e two end-members o f what i s r e a l l y a c o n t i n uous spectrum o f d e l t a - t y p e s .
Secondly, t h e u s e of t h e term h i g h - d e s t r u c t i v e i s
m i s l e a d i n g because a l l d e l t a s , by d e f i n i t i o n a r e c o n s t r u c t i v e w h i l s t a c t i v e and t h e
term i s a l s o confused w i t h t h e s o - c a l l e d " d e s t r u c t i o n a l phase" of d e l t a s ( S c r u t o n , 1960) which t a k e s p l a c e a f t e r d e l t a abandonment. A more w i d e l y used c l a s s i f i c a t i o n i s t h a t proposed by Galloway (1975) which employs a t e r n a r y diagram t o d e p i c t t h e f i e l d s o f f l u v i a l - , wave-, and t i d e dominated d e l t a s (Fig.3.2). Fluvial-dominated
Fig.3.1. (1969).
High-constructive
d e l t a s occur when waves, t i d a l c u r r e n t s , and
and h i g h - d e s t r u c t i v e d e l t a s a s d e f i n e d by F i s h e r e t a l .
129 longshore c u r r e n t s a r e weak.
Rapid seaward p r o d u c t i o n t a k e s p l a c e and a v a r i e t y of
c h a r a c t e r i s t i c f l u v i a l l y - d o m i n a t e d d e p o s i t i o n a l environments d e v e l o p s .
A t t h e mouth
o f each d i s t r i b u t o r y c h a n n e l subaqueous l e v e e s may form a s t h e j e t of w a t e r e n t e r s the sea (Fig.3.3).
The main s e d i m e n t l o a d i s t h e n d e p o s i t e d i n a d i s t r i b u t o r y mouth
b a r which becomes f i n e r g r a i n e d seawards.
With c o n t i n u e d p r o g r a d a t i o n t h e r i v e r
g r a d i e n t becomes s h a l l o w e r and t h e flow becomes less competent.
During p e r i o d s of
h i g h d i s c h a r g e t h e s u b a e r i a l l e v e e s may b e b r e a c h e d upstream forming a c r e v a s s e through which t h e f l o w may b e d i v e r t e d towards t h e s e a .
This i s often a shorter
r o u t e t h a n that p r o v i d e d by t h e former d i s t r i b u t a r y c h a n n e l and i t may r e s u l t i n t h e f o r m a t i o n of c r e v a s s e - s p l a y d e p o s i t s i n i n t e r d i s t r i b u t a r y bays.
RIVER
bY
DOMINATED
DOMINATED
Rhono
WAVE ENERGY
TIDE ENERGY
Fig.3.2. A c l a s s i f i c a t i o n o f d e l t a t y p e s b a s e d on v a r i a t i o n s i n t r a n s p o r t a t i o n p a t t e r n s on t h e d e l t a ( a f t e r Calloway, 1975). River-dominated d e l t a s t e n d t o b e of two main s u b t y p e s ,
The f i r s t i s t h e b i r d -
f o o t d e l t a which h a s r e l a t i v e l y few d i s t r i b u t a r i e s , s h o e s t r i n g s a n d s , and d i s c r e t e mouth b a r d e p o s i t s .
T h i s t y p e of d e l t a t e n d s t o form when t h e r i v e r d i s c h a r g e i s
s t e a d y and has a h i g h s u s p e n s i o n l o a d ( s e e Fig.3.1D).
The second t y p e i s normally
l o b a t e i n o u t l i n e and t h e r e a r e a g r e a t e r number of d i s t r i b u t a r i e s e a c h of which t e n d s t o be more ephemeral.
I n t h i s t y p e t h e s e d i m e n t s tend t o b e c o a r s e r g r a i n e d
w i t h a g r e a t e r p r o p o r t i o n of bed l o a d and t h e mouth b a r d e p o s i t s merge l a t e r a l l y
130 i n t o s h e e t sands ( s e e Fig.3.1C)
Fig.3.3. Subenvironments a t a d i s t r i b u t a r y mouth i n a r i v e r - d o m i n a t e d d e l t a , South P a s s , M i s s i s s i p p i d e l t a ( a f t e r Coleman and G a g l i a n o , 1965).
I n wave-dominated d e l t a s mouth b a r d e p o s i t s a r e c o n t i n u a l l y reworked i n t o a s e r i e s of superimposed c o a s t a l b a r r i e r sands (Fig.3.4A).
These w i l l t e n d t o dominate t h e
v e r t i c a l s u c c e s s i o n of f a c i e s w i t h s a n d b o d i e s a l i g n e d g e n e r a l l y p a r a l l e l t o t h e c o a s t i n c o n t r a s t t o t h e sand b o d i e s o f r i v e r - d o m i n a t e d d e l t a s which w i l l be g e n e r a l l y perpendicular t o the coast.
In tide-dominated d e l t a s t h e t i d a l r a n g e i s so h i g h t h a t t i d a l c u r r e n t s i n d i s t r i b u t a r y c h a n n e l become t h e p r i n c i p a l s o u r c e of sediment d i s p e r s a l e n e r g y .
Around,
and seaward o f t h e d i s t r i b u t a r y mouth t h e sediment may b e reworked i n t o a series of p a r a l l e l , l i n e a r o r d i g i t a t e r i d g e s which a r e a l i g n e d p a r a l l e l t o t h e t i d a l c u r r e n t s .
131 DELTAIC FACIES ASSOCIATIONS C l a s s i c a l l y t h r e e s t r u c t u r a l elements can be recognized i n d e l t a s : - t o p s e t , fores e t and b o t t o m s e t .
The t o p s e t d e p o s i t s o f a d e l t a a r e t h e most i m p o r t a n t and
comprise a v a r i e t y o f s e d i m e n t s i n c l u d i n g marsh d e p o s i t s and d e l t a - f r o n t s i l t s and sands.
These a r e c u t by r i v e r and d i s t r i b u t a r y c h a n n e l s w i t h n a t u r a l l e v e e s and
crevasse splays.
Muddy s e d i m e n t s w i t h s h e l l l a y e r s a r e o f t e n d e p o s i t e d i n
i n t e r d i s t r i b u t a r y bays ( S c r u t o n , 1960).
T o p s e t d e p o s i t s can b e d i v i d e d i n t o
t h o s e d e p o s i t e d s u b a e r i a l l y ( d e l t a p l a i n f a c i e s ) and t h o s e d e p o s i t e d subaqueously (delta front faciesl. h
A
B
-
WAVE DOMINATED
c
b
TIM -DOMINATED
Fig.3.4. A . A wave-dominated d e l t a , t h e modern Rh8ne d e l t a d e l t a , t h e modern Gulf of Papua ( a f t e r F i s h e r , 1969).
B. A tide-dominated
The f o r e s e t d e p o s i t s a r e t h o s e o f t h e p r o d e l t a o r d e l t a f r o n t s l o p e .
These a r e
p r e d o m i n a n t l y s i l t y c l a y s , sometimes w i t h s a n d , which a r e formed o f f t h e main d e l t a distributaries.
F r e q u e n t l y i n c o r p o r a t e d i n t o t h e p r o d e l t a a r e g u l l i e s 2-8m deep
132 and up t o a s much a s lkm from e a c h o t h e r i n t h e M i s s i s s i p p i b u t r a t h e r s m a l l e r on t h e R h h e d e l t a (Van S t r a a t e n , 1959). Bottomset d e p o s i t s a r e o f f - s h o r e c l a y s which a r e under t h e i n f l u e n c e of a c t i v e deltas.
These b o t t o m s e t d e p o s i t s a r e up t o 1 0 m t h i c k i n t h e M i s s i s s i p p i d e l t a and
a r e u n d e r l a i n by m a r g i n a l d e p o s i t s of P l e i s t o c e n e a g e . I n f a c t t h i s t h r e e - f o l d d i v i s i o n i s somewhat a r b i t r a r y and o n l y r e a l l y a p p l i e s t o the classical "Gilbertian delta".
The two most a c t i v e d e p o s i t i o n a l zones of a d e l t a
a r e b o t h w i t h i n t h e t o p s e t a r e a and i t i s , t h e r e f o r e , more p r a c t i c a b l e t o make a two-fold d i v i s i o n i n t o t h e d e l t a f r o n t which i n c l u d e s t h e s h o r e l i n e and seawardd i p p i n g p r o f i l e e x t e n d i n g o f f s h o r e ; and t h e d e l t a p l a i n which i s t h e low-lying a r e a developed behind t h e d e l t a f r o n t . The d e l t a p l a i n D e l t a p l a i n s a r e low-lying a r e a s which a r e c u t by a c t i v e and abandoned d i s t r i b a t a r y channels.
These c h a n n e l s t r a n s f e r t h e t o t a l d i s c h a r g e o f t h e a l l u v i a l s y s t e m
s u p p l y i n g t h e d e l t a t o t h e d e l t a f r o n t environment.
The i n t e r - c h a n n e l a r e a s c o n t a i n
a v a r i e t y of b a y s , f l o o d p l a i n s , l a k e s , t i d a l f l a t s , m a r s h e s , swamps and s a l i n a s . I n m o i s t t r o p i c a l and s u b t r o p i c a l c l i m a t e s d e l t a p l a i n s a r e t h i c k l y v e g e t a t e d and mangrove swamps, f r e s h w a t e r l a k e s , swamps and marshes may b e common ( e . g . t h e N i g e r d e l t a ( A l l e n , 1 9 7 0 ~ )and t h e M i s s i s s i p p i d e l t a s (Coleman, 1966).
I n a r i d regions,
however, v e g e t a t i o n i s s p a r s e and d e l t a p l a i n s may b e c h a r a c t e r i z e d by c a l c r e t e s e . g . t h e Ebro d e l t a (Maldonado, 1975) o r s a l i n a s w i t h gypsum and h a l i t e e.g. N i l e d e l t a ( E i n s e l e and Werner, 1968).
the
Sometimes d e l t a s i n a r i d c l i m a t e s a r e
covered by e x t e n s i v e dune f i e l d s e s p e c i a l l y i n d e l t a s w i t h b a r r i e r - b e a c h s h o r e l i n e s where sand i s eroded from t h e beach r i d g e s e . g . SZo F r a n c i s c o d e l t a .
D e l t a s formed i n
tundra r e g i o n s a r e c h a r a c t e r i z e d by p i n g o s , p a t t e r n e d ground and o t h e r c r y o g e n i c features.
Tundra v e g e t a t i o n may accomulate i n thaw ponds e.g.
t h e Mackenzie and
C o l v i l l e d e l t a s (Mackay, 1963; Naidu and Mowatt, 1975). Most d e l t a p l a i n s a r e a f f e c t e d by e i t h e r f l u v i a l o r t i d a l p r o c e s s e s and i n some c a s e s t h e r e i s r a t h e r a d e l i c a t e b a l a n c e between t h e two.
Wave-influenced d e l t a
p l a i n s a r e r a r e b e c a u s e t h e s e d e v e l o p b a r r i e r - b e a c h s h o r e l i n e s which p r o t e c t t h e d e l t a p l a i n from wave a c t i v i t y .
D e l t a p l a i n s which a r e dominated by f l u v i a l
p r o c e s s e s may p a s s downstream i n t o a t i d e dominated d e l t a p l a i n ( e . g . delta).
t h e Niger
A l t e r n a t i v e l y t h e y may b e e n c l o s e d by beach r i d g e s a t t h e seaward end (e.g.
t h e Rhone d e l t a ) o r p a s s d i r e c t l y i n t o t h e d e l t a f r o n t ( e . g .
the Mississippi deltas).
River-dominated d e l t a p l a i n s Fluvial d i s t r i b u t a r y channels The dominant f e a t u r e s a r e t h e f l u v i a l d i s t r i b u t a r y c h a n n e l s which a r e c l o s e l y
133 s i m i l a r t o channels i n conventional a l l u v i a l s e t t i n g s . I n low l a t i t u d e s where suspended-load c h a n n e l s a r e common d i s t r i b u t a r y channel p a t t e r n s a r e c l o s e l y dependent on g r a d i e n t .
I n upstream r e a c h e s t h e channel p a t t e r n
tends t o be meandering b u t a s t h e g r a d i e n t d e c r e a s e s downstream t h e channel p a t t e r n o f t e n changes from meandering t o r e l a t i v e l y s t r a i g h t ( F i g . 3 . 5 ) .
I n high l a t i t u d e s
WIDTH /DEPTH
100
40
20
10
SINUOSITY
Fig.3.5. 60186
G e n e r a l i z e d diagram o f downstream changes i n channel c h a r a c t e r i s t i c s of suspended l o a d f l w i a l systems ( a f t e r Morton and Donaldson, 1978).
where ths p r o p o r t i o n o f bed l o a d i s g r e a t e r and t h e d i s c h a r g e more s p o r a d i c b r a i d e d channels t e n d t o develop.
One of t h e major d i f f e r e n c e s between f l u v i a l d i s t r i b u t a r y
c h a n n e l s and a l l u v i a l channels is i n t h e abundance o f channel s w i t c h i n g o r a v u l s i o n . F l w i a l d i s t r i b u t a r y channels are f r e q u e n t l y abandoned because s h o r t e r and s t e e p e r r o u t e s are produced a s t h e d e l t a p r o g r a d e s seawards.
The abandonment o f d i s t r i b u t a r y
c h a n n e l s e n a b l e s b a s i n a l p r o c e s s e s t o t a k e o v e r i n t h e 16wer r e a c h e s o f t h e former channels and i n t h e Rh6ne d e l t a beach sands a r e d e p o s i t e d i n abandoned channel mouths ( K r u i t , 1955; Van S t r a a t e n , 1960). The f a c i e s sequences o f d i s t r i b u t a r y c h a n n e l s a r e g e n e r a l l y s i m i l a r t o those of a l l u v i a l channels and t h e s e a r e d e s c r i b e d i n d e t a i l i n Chapter 4.
Fining-upwards
sequences b e g i n n i n g w i t h a channel l a g followed by t r o u g h c r o s s bedded sands and c r o s s l a m i n a t e d s i l t s and c u l m i n a t i n g w i t h s o i l h o r i z o n s a r e t y p i c a l (Oomkens, 1967, 1974).
The fining-upwards may r e s u l t from l a t e r a l m i g r a t i o n of t h e channel o r from
channel abandonment i n which c a s e i t might b e f i l l e d by suspended l o a d sediment from overbank f l o o d i n g o f a d j a c e n t c h a n n e l s ,
One of t h e major consequences of t h e r a p i d
134 s w i t c h i n g o f d i s t r i b u t a r y c h a n n e l s i s t h a t t h e y t e n d t o be s h o r t - l i v e d compared w i t h t h e i r upstream e q u i v a l e n t s . low w i d t h t o d e p t h r a t i o .
This tends t o . r e s u l t i n sand bodies with a r e l a t i v e l y I n t h e d e l t a s o f t h e Rhine and Rh6ne t h i s r a t i o d e c r e a s e s
from a b o u t 1000 f o r a l l u v i a l c h a n n e l s t o a b o u t 50 f o r d i s t r i b u t a r y c h a n n e l s n e a r t h e s h o r e l i n e (Oomkens, 1974). I n t e r d i s t r i b u t a r y environments The i n t e r d i s t r i b u t a r y a r e a s i n r i v e r - d o m i n a t e d d e l t a p l a i n s c o n t a i n a wide v a r i e t y of environments and s e d i m e n t a r y f a c i e s . and f r e s h w a t e r l a k e s . throughcrevasses.
These i n c l u d e i n t e r d i s t r i b u t a r y b a y s , swamps
Sediment i s s u p p l i e d t o t h e s e a r e a s by overbank f l o o d i n g o r
Overbank f l o o d s r e s u l t i n t h e s h e e t - f l o w of sediment-laden w a t e r s
o v e r l a r g e a r e a s w i t h t h e d e p o s i t i o n of f i n e g r a i n e d s u s p e n s i o n l o a d d e p o s i t s . These may o r i g i n a l l y b e f i n e l y - l a m i n a t e d b u t i n t e r n a l s t r u c t u r e s a r e soon d e s t r o y e d by r o o t p e n e t r a t i o n and burrowing.
Overbank f l o o d i n g a l s o r e s u l t s i n t h e build-up
of l e v e e s b e c a u s e t h e c o a r s e r sediment i s r a p i d l y d e p o s i t e d n e a r t h e c h a n n e l banks during flood.
Repeated f l o o d i n g r e s u l t s i n t h e a l t e r n a t i o n o f t h i n e r o s i v e l y - b a s e d
sands and muds.
The sands may b e p a r a l l e l o r c r o s s l a m i n a t e d and f i n e away from t h e
c h a n n e l margin.
The muds show e v i d e n c e o f emergence i n t h e form of r o o t l e t h o r i z o n s .
C r e v a s s i n g i s a n o t h e r i m p o r t a n t f l o o d - g e n e r a t e d p r o c e s s i n which f l o o d w a t e r s e s c a p e from t h e c h a n n e l t h r o u g h d i s t i n c t c r e v a s s e c h a n n e l s i n t h e l e v e e s .
T h i s may
r e s u l t i n t h e f o r m a t i o n of c r e v a s s e - s p l a y s which a r e a r e a l l y r e s t r i c t e d wedges of sediment which form a p r o n s a l o n g t h e lower f l a n k s o f t h e l e v e e s .
Crevasse-splay
sand b o d i e s c o m o n l y show s h a r p , e r o s i o n a l b a s e s and may b e l e n t i c u l a r i n shape. u s u a l l y show c r o s s - b e d d i n g which i n d i c a t e s u n i d i r e c t i o n a l flow c o n d i t i o n s .
They
The over-
l y i n g muddy s e d i m e n t s may show e v i d e n c e of emergence. I n t h e M i s s i s s i p p i d e l t a s h a l l o w c r e v a s s e c h a n n e l s w i t h a s s o c i a t e d minor mouth b a r s a r e a n i m p o r t a n t f e a t u r e of i n t e r d i s t r i b u t a r y bays (Coleman e t a l . , 1964; Coleman and G a g l i a n o , 1964) ( F i g . 3.6).
As t h e s e c r e v a s s e / c h a n n e l mouth b a r c o u p l e t s p r o g r a d e
i n t o t h e i n t e r d i s t r i b u t a r y bay a v e r t i c a l sequence i s produced i n which b i o t u r b a t e d bay muds p a s s upwards i n t o c r o s s - l a m i n a t e d s i l t s and s a n d s .
When t h e c r e v a s s e chan-
n e l s a r e c l o s e l y s p a c e d a l a t e r a l l y e x t e n s i v e f r o n t e x t e n d s i n t o t h e bay p r o d u c i n g t h e s u b d e l t a s d e s c r i b e d by G a g l i a n o e t a l . (1971) and G a g l i a n o and v a n Beek (1975) (Fig. 3.6). I n t h e open bays of t h e M i s s i s s i p p i d e l t a t h e upper p a r t s o f c r e v a s s e sands may b e reworked i n t o s a n d s p i t s ( F i s k e t a l . , 1954).
These produce coarsening-upwards u n i t s
of w e l l - s o r t e d s a n d s w i t h f l a t l a m i n a t i o n s and w a v e - r i p p l e s . Large i n t e r d i s t r i b u t a r y a r e a s o f f l u v i a l dominated d e l t a p l a i n s a r e made up of o r g a n i c c l a y s and p e a t d e p o s i t s ( F i s k , 1960).
These s e d i m e n t s g e n e r a l l y l a c k any
w e l l d e f i n e d bedding b e c a u s e t h e y a r e e x t e n s i v e l y burrowed by p l a n t r o o t s and by
s e d i m e n t d w e l l i n g organisms.
T y p i c a l d e p o s i t s a r e t h u s homogeneous m i x t u r e s o f c l a y
135
4B
'
A Dry Cypress Bayou complex B Grand Liard complex C West Bay complex
A. A minor mouth b a r - c r e v a s s e channel c o u p l e t i n a n i n t e r d i s t r i b u t a r y bay Fig.3.6. of the modern M i s s i s s i p p i d e l t a ( a f t e r Coleman e t a l . , 1964) B. S u b d e l t a s of t h e modern M i s s i s s i p p i d e l t a ( a f t e r Coleman and G a g l i a n o , 1964). and s i l t w i t h p l a n t remains.
I n a r i d climates t h e p l a n t remains may b e much reduced
and mud-cracks and a l g a l mats may be p r e s e n t .
Detailed s t u d i e s of d e l t a p l a i n
swamp d e p o s i t s has been made by Coleman e t a l . ( 1 9 6 4 ) , Coleman (1966), Kolb and Van Lopik (1966) and Ho and Coleman (1969).
The d e l t a i c p l a i n merges i n l a n d
w i t h t h e a l l u v i a l p l a i n o f t h e lower M i s s i s s i p p i , t h e main s t r e a m s h e r e a r e t h e M i s s i s s i p p i and t h e A t c h a f a l a y a which p a r t n o r t h o f Baton Rouge.
The Atchafalaya
flows i n t o A t c h a f a l a y a Bay n e a r t h e w e s t e r n margin of t h e d e l t a i c p l a i n . A d e t a i l e d s t u d y of t h e swamp d e p o s i t s i n t h e A t c h a f a l a y a Basin of t h e M i s s i s s i p p i D e l t a has been made by Coleman (1966) and Ho and Coleman (1969). recognized f o u r t y p e s of i n t e r d i s t r i b u t a r y swamp environments:
Coleman (1966) poorly drained
swamp, well d r a i n e d swamp, f r e s h w a t e r l a c u s t r i n e , l a c u s t r i n e d e l t a f i l l and channel fill.
136
I
I
,
I
Em l= ..#.I
I
I 1
I
F i g . 3 . 7 . Physical and b i o l o g i c a l p r o p e r t i e s of a sediment core (No.199) through swamp d e p o s i t s of the Atchafalaya Basin, Louisiana ( a f t e r Coleman, 1966).
137 Swamps The swamps a r e low, f l a t a r e a s which a r e p e r i o d i c a l l y c o v e r e d o r s a t u r a t e d w i t h w a t e r and s u p p o r t a c o v e r o f woody v e g e t a t i o n :
swamps c o v e r e x t e n s i v e a r e a s , over
90% of t h e t o t a l a r e a of t h e A t c h a f a l a y a Basin.
The a r e a s n e a r t o r i v e r channels
a r e g e n e r a l l y w e l l d r a i n e d b u t i n i s o l a t e d a r e a s away from t h e r i v e r c h a n n e l d r a i n a g e i s poor.
T h i s d i f f e r e n c e i n d r a i n a g e c h a r a c t e r i s t i c s means t h a t i n w e l l d r a i n e d
swamps sediments a r e d e p o s i t e d under a l t e r n a t i n g o x i d i z i n g and r e d u c i n g c o n d i t i o n s , whereas t h e s e d i m e n t s of p o o r l y d r a i n e d swamps a r e d e p o s i t e d w h o l l y under r e d u c i n g conditions.
The r e s u l t a n t m i n e r a l o g i c a l c h a r a c t e r i s t i c s e n a b l e t h e d i s t i n c t i o n of
t h e two environments i n sediment c o r e s (Coleman, 1966). P o o r l y d r a i n e d swamp d e p o s i t s a r e made up o f b l a c k , o r g a n i c - r i c h muds w i t h o c c a s i o n a l t h i n l y - l a m i n a t e d s i l t s i n t r o d u c e d by f l o o d w a t e r s .
The sediments a r e
e x t e n s i v e l y b i o t u r b a t e d and b e c a u s e of t h e r e d u c i n g c o n d i t i o n s which p r e v a i l p l a n t m a t e r i a l i s w e l l p r e s e r v e d w i t h t h i n woody p e a t beds randomly i n t e r c a l a t e d throughout t h e sequence.
There i s abundant p y r i t e ( 0 . 5 - 3 . 5 % ) which o c c u r s a s small c u b e s ,
i s o l a t e d g l o b u l a r masses and a s a replacement of s m a l l r o o t s .
V i v i a n i t e (Fe P 0 3 2 8
8.H 0) a l s o o c c u r s commonly (up t o 2%) a s s m a l l n o d u l e s i n c l o s e a s s o c i a t i o n w i t h 2
pyrite. Well d r a i n e d swamp d e p o s i t s a r e s i g n i f i c a n t l y d i f f e r e n t from t h o s e o f t h e p o o r l y d r a i n e d swamps:
t h e c o n t e n t of o r g a n i c m a t t e r i s lower and t h e d e p o s i t s c o n s i s t of
clays with isolated s i l t lenses.
The most s i g n i f i c a n t d i f f e r e n c e , however, l i e s i n
t h e abundance and t y p e of a u t h i g e n i c m i n e r a l s .
There i s much less p y r i t e and v i v i a -
n i t e i n t h e well d r a i n e d swamp d e p o s i t s and a g r e a t e r abundance of c a l c i t e and i r o n o x i d e nodules.
The c a l c i t e n o d u l e s a r e e x t r e m e l y abundant and o c c u r b o t h a s l a r g e
(2cm) p o o r l y cemented n o d u l e s , and s m a l l ( l e s s t h a n 5mm) w e l l cemented nodules. C o n c e n t r a t i o n s of c a l c i t e a l s o o c c u r a l o n g bedding p l a n e s and o t h e r laminae. , o x i d e nodules a r e a l s o abundant i n t h e w e l l d r a i n e d swamp d e p o s i t s . u s u a l l y 0.5-6m
Iron
These a r e
i n s i z e and p r o b a b l y c o n s i s t o f well cemented i r o n hydroxide.
They
a r e s c a t t e r e d t h r o u g h o u t t h e sequence w i t h t h e l a r g e r n o d u l e s c o n c e n t r a t e d a t p a r t i c u l a r horizons.
I r o n o x i d e s a l s o o c c u r a s r i m s around p l a n t r o o t l e t s .
The f o r m a t i o n o f nodules d u r i n g t h e b u r i a l o f swamp d e p o s i t s h a s been d i s c u s s e d by Ho and Coleman (1969).
I n a c o r e o f a p p r o x i m a t e l y 35m t h e y n o t e d t h a t nodules
o c c u r r e d throughout b u t t h o s e n e a r t h e bottom of t h e c o r e were l a r g e r and b e t t e r c o n s o l i d a t e d t h a n t h o s e n e a r t h e t o p . They a l s o n o t e d a g r e a t e r number of nodules when o r g a n i c m a t t e r o r s h e l l s were p r e s e n t . P y r i t e was s i m i l a r l y c o n c e n t r a t e d i n o r g a n i c - r i c h a r e a s , a l t h o u g h r o o t l e t s and wood p a r t i c l e s had s u f f e r e d less from p y r i t e r e p l a c e m e n t i n t h e upper p a r t of t h e c o r e t h a n i n t h e d e e p e r p a r t s o f t h e core. A l s o , w e l l - c r y s t a l l i z e d p y r i t e cubes o c c u r r e d i n t h e lower p a r t s o f t h e c o r e .
138 X-ray r a d i o g r a p h s d i s p l a y v e r y w e l l t h e d i f f e r e n c e between w e l l - c r y s t a l l i z e d n o d u l e s and i n c i p i e n t n o d u l e s .
The former have w e l l - d e f i n e d b o u n d a r i e s whereas t h e
l a t t e r have d i f f u s e b o u n d a r i e s which i n d i c a t e s t h e d i f f u s i o n of t h e a u t h i g e n i c m i n e r a l s i n t o t h e s u r r o u n d i n g sediment.
D i f f e r e n t i a l t h e r m a l and X-ray d i f f r a c t i o n
s t u d i e s by Ho and Coleman (1969) showed t h a t t h e n o d u l e s were composed of s i d e r i t e perhaps w i t h some i r o n o x i d e where t h e i n t e r s t i t i a l cementing m a t e r i a l s c o n s i s t mainly of c a l c i t e and i r o n o x i d e s o f unknown s p e c i f i c t y p e . Ho and Coleman (1969) a t t r i b u t e d t h e f o r m a t i o n of t h e n o d u l e s and t h e cementing m a t e r i a l s t o i n c r e a s e d s o l u b i l i z a t i o n o f m e t a l c a t i o n s under r e d u c i n g c o n d i t i o n s a n d / o r low pH.
The main s u p p l y of m e t a l s was from d e s o r p t i o n from d e t r i t a l c l a y s
a l t h o u g h some m e t a l s may a l s o have been i n c r e a s e d from o r g a n i c m a t e r i a l s .
The
p r e s e n c e of n i t r o g e n i n t h e nodules s u g g e s t e d t o Ho and Coleman (1969) t h a t n o d u l e growth was r e l a t e d t o a d i f f u s i o n mechanism which i n v o l v e d n i t r o g e n e o u s compounds. The u t i l i z a t i o n of o r g a n i c compounds by micro-organisms a s a n e n e r g y s o u r c e f o r o x i d a t i o n - r e d u c t i o n r e a c t i o n s i s v e r y w e l l known and nodule growth p r o b a b l y began when mi'crobiological p r o d u c t i o n of d i s s o l v e d c a r b o n a t e exceeded t h e d i f f u s i o n r a t e and l o c a l s u p e r s a t u r a t i o n r e s u l t e d . L a c u s t r i n e and l a c u s t r i n e d e l t a - f i l l d e p o s i t s F r e s h w a t e r l a c u s t r i n e d e p o s i t s form i n v e r y s h a l l o w (3-4m) b u t e x t e n s i v e l a k e s which have o n l y v e r y weak wave o r c u r r e n t a c t i v i t y . dark grey o r black clays with s i l t lenses.
The d e p o s i t s a r e p r i m a r i l y
Coleman (1966) showed by means o f X-ray
r a d i o g r a p h s t h a t t h e c l a y s were f i n e l y l a m i n a t e d and s u g g e s t e d t h a t t h i s was due t o t h e a l t e r n a t i o n of f l o c c u l a t e d and n o n - f l o c c u l a t e d c l a y l a y e r s which might have r e s u l t e d from r a p i d changes i n pH, c a t i o n , and sediment c o n c e n t r a t i o n .
The f l o c c -
u l a t e d l a y e r s show s y n e r e s i s c r a c k s ; b i o t u r b a t i o n i s a cornon f e a t u r e and l a m e l l i branchs i n growth p o s i t i o n a l s o o c c u r .
I n c o n t r a s t t o t h e w e l l - d r a i n e d swamp
environment p y r i t e and v i v i a n i t e a r e abundant and i r o n o x i d e n o d u l e s a r e a b s e n t . L a c u s t r i n e d e l t a f i l l sequences a r e produced when a s t r e a m e n t e r s t h e f r e s h w a t e r lake.
T h i s b r i n g s i n c o a r s e r s e d i m e n t s and a coarsening-upwards g r a d a t i o n a l sequence
of l a c u s t r i n e c l a y , s i l t y c l a y and s a n d s t o n e i s produced.
L e n t i c u l a r bedding and
r i p p l e - b e d d i n g a r e common i n t h e c o a r s e r b e d s , a l o n g w i t h l o a d s t r u c t u r e s and s o f t sediment deformation.
Fauna and b i o t u r b a t i o n i s n o t a s common a s i n some o f t h e
other facies. Tide-dominated d e l t a p l a i n s D e l t a p l a i n s became dominated by t i d a l c u r r e n t s i n a r e a s of moderate t o h i g h t i d a l range.
During t h e f l o o d s t a g e t i d a l w a t e r flows up t h e d i s t r i b u t a r y c h a n n e l s
and b r e a k s o v e r t h e c h a n n e l banks t o f l o o d t h e i n t e r d i s t r i b u t a r y a r e a .
The d e l t a
p l a i n may t h u s b e t e m p o r a r i l y i n u n d a t e d w i t h marine w a t e r s which a r e d r a i n e d o f f
139 during t h e ebbing t i d e , Tidal d i s t r i b u t a r y channels These d i f f e r from f l u v i a l d i s t r i b u t a r y c h a n n e l s i n having a funnel-shaped form w i t h a h i g h width-depth r a t i o .
I n t h e N i g e r d e l t a ( A l l e n , 1965b, 1 9 7 0 ~ )t h e r e a r e
more t h a n 20 t i d a l i n l e t s 9-15m i n d e p t h and c u t t i n g t h e b e a c h - b a r r i e r s h o r e l i n e . The c h a n n e l flows a r e dominated by dune bed forms b u t a t t h e c o n f l u e n c e o f d i s t r i b u t a r y c h a n n e l s a r e l a r g e a r e a s of m u d f l a t s and s a n d b a r s known a s " i n n e r d e l t a s " (Fig.3.8).
I n some o t h e r tide-dominated d e l t a s s u c h a s t h e Mekong and t h e Ganges-
Brahmaputra t h e dominant bedforms of t h e t i d a l c h a n n e l s a r e l i n e a r s a n d r i d g e s aligned p a r a l l e l with the channel trend (Fisher e t a l . , 1975) ( F i g . 3 . 9 ) .
1969; Coleman and W r i g h t ,
These r i d g e s may by up t o s e v e r a l k i l o m e t r e s l o n g , a few hundred
metres wide and between 10-20m h i g h , The i n t e r n a l s t r u c t u r e of t h e s e t i d a l c h a n n e l s u s u a l l y b e g i n s w i t h an i n t r a f o r m a t i o n a l l a g c o n t a i n i n g a f r a g m e n t a l marine fauna.
These p a s s upwards i n t o s m a l l
s c a l e t r o u g h c r o s s beds and t h e n i n t o f i n e r g r a i n e d c r o s s l a m i n a t e d beds sometimes w i t h f l a s e r o r l e n t i c u l a r bedding ( T e r w i n d t , 1971).
The g r a i n s i z e g r a d u a l l y
d e c r e a s e s upwards and t h e amount of b i o t u r b a t i o n a l s o i n c r e a s e s .
The channel f a c i e -
may p a s s upwards i n t o swamp d e p o s i t s o r c o a s t a l b a r r i e r sands (Oomkens, 1974) ( F i g . 3.10). Interdistributary areas The a r e a s between t i d a l d i s t r i b u t a r y c h a n n e l s i n c l u d e i n t e r t i d a l f l a t s and lagoons which a r e c r o s s e d by minor t i d a l c r e e k s .
form mangrove swamps a s i n t h e Niger d e l t a .
The i n t e r t i d a l f l a t s may be v e g e t a t e d and Here t h e swamps a r e d i s s e c t e d by numerous
small meandering c r e e k s s e r v e d by a d e n d r i t i c d r a i n a g e p a t t e r n ( A l l e n , 1965b).
In
more a r i d r e g i o n s such a s t h e Colorado R i v e r d e l t a (Thompson, 1968; Meckel, 1975) t h e i n t e r d i s t r i b u t a r y a r e a s t e n d t o b e d e s i c c a t e d m u d f l a t s and s a n d f l a t s on which l o c a l s a l t pans develop. The d e l t a f r o n t The d e l t a f r o n t i s t h a t a r e a i n which c h a n n e l i z e d f l u v i a l c u r r e n t s e n t e r t h e b a s i n w a t e r s and undergo flow e x p a n s i o n , d e c e l e r a t i o n and d i s p e r s i o n .
The sediment
c a r r i e d by t h e r i v e r w a t e r s come9 under t h e i n f l u e n c e o f a v a r i e t y of b a s i n a l proc e s s e s i n c l u d i n g waves, t i d e s , l o n g s h o r e d r i f t and o t h e r c u r r e n t s (Wright and Coleman, 1974).
The r e s u l t i s t h a t t h e d e l t a f r o n t i s t h e most a c t i v e d e p o s i t i o n a l
zone of t h e d e l t a (Coleman and G a g l i a n o , 1965).
The r e l a t i v e importance of f l u v i a l ,
wave and t i d a l p r o c e s s e s v a r i e s c o n s i d e r a b l y i n t h e d e l t a f r o n t w i t h t h e r e s u l t t h a t a number of d i f f e r e n t d e l t a t y p e s c a n b e r e c o g n i z e d ( F i g . 3 . 1 ) .
140
1
2
0 , 1 km 3
Fig.3.8. The Forcados t i d a l d i s t r i b u t a r y c h a n n e l o f t h e Niger d e l t a showing t h e p o s i t i o n i n g of i n n e r d e l t a s a t t h e c o n f l u e n c e of two c h a n n e l s ( a f t e r NEDECO, 1961).
Accretionary bar
Fig.3.9. The Mekong d e l t a showing l i n e a r s a n d r i d g e s i n t h e lower r e a c h e s o f t i d a l d i s t r i b u t a r y c h a n n e l s ( a f t e r F i s h e r e t a l . , 1969).
141 The M i s s i s s i p p i d e l t a f r o n t i s dominated by f l u v i a l o u t f l o w c h a r a c t e r i s t i c s . The d e p o s i t i o n of s e d i m e n t a t d i s t r i b u t a r y mouths forms a series of i s o l a t e d l u n a t e mouth b a r s which p r o t r u d e i n t o t h e Gulf o f Mexico (Fig.3.11).
These comprise a
bar-back a r e a which i n c l u d e s minor c h a n n e l s , subaqueous l e v e e s and 'mid-channel b a r s ' , a b a r - c r e s t l o c a t e d o f f s h o r e from t h e d i s t r i b u t a r y mouth, and a b a r - f r o n t (sometimes c a l l e d t h e ' d i s t a l b a r ' ) which s l o p e s o f f s h o r e t o t h e p r o d e l t a (Fig.3.11).
A Metres
Mangroveswamp
B
Tidalchannel
Metres
hannel base
marine
16
So11horizon
Fig.3.10. T i d a l c h a n n e l sequences from t h e N i g e r d e l t a A. A n i n s h o r e sequence t e r m i n a t i n g i n mangrove swamp d e p o s i t s B. A s h o r e l i n e sequence t e r m i n a t i n g i n c o a s t a l b a r r i e r s a n d s ( a f t e r Oomkens, 1974).
142 Mouth b a r s advance r a t h e r l i k e d u n e s , w i t h t h e bar-back and b a r - c r e s t b e i n g e r o d e d w h i l s t t h e bar-front progrades (Coleman e t a l . ,
1974).
During t h e extreme
river f l o o d of 1973 t h e b a r - c r e s t aggraded r a p i d l y w i t h up t o 3m of s e d i m e n t b e i n g
deposited.
As t h e f l o o d s u b s i d e d t h i s s e d i m e n t was reworked by r i v e r c u r r e n t s and
r e d e p o s i t e d on t h e b a r f r o n t .
Mouth b a r p r o g r a d a t i o n was t h e r e f o r e most r a p i d
immediately a f t e r peak f l o o d (Coleman e t a l . , 1974).
0
- 30 - 60 - 90
D i s t r i b u t a r y c h a n n e l s and d i s t r i b u t a r y mouth b a r s i n t h e modern Fig.3.11. M i s s i s s i p p i d e l t a ( a f t e r F i s k e t a l . , 1954). The sequences produced by mouth b a r p r o g r a d a t i o n a r e l a r g e s c a l e c o a r s e n i n g upwards u n i t s .
A t t h e b a s e p r o d e l t a muds p a s s upwards i n t o i n t e r b e d d e d muds, s i l t
and sand r e f l e c t i n g t h e i n t e r a c t i o n between sediment-laden w a t e r s from t h e d i s t r i b u t a r y o u t l e t s and wave p r o c e s s e s .
These p a s s upwards i n t o s a n d s w i t h c u r r e n t -
produced s t r u c t u r e s ( F i s k e t a l . , 1954; F i s k , 1961; Coleman and Wright, 1975) (Fig.3.12). I n t h e M i s s i s s i p p i d e l t a t h e p r o g r a d a t i o n o f i n d i v i d u a l d i s t r i b u t a r y mouth b a r s
is v i r t u a l l y u n c o n t e s t e d by b a s i n a l p r o c e s s e s .
This has r e s u l t e d i n t h e formation
of r a d i a t i n g ' b a r f i n g e r s a n d s ' which g i v e t h e M i s s i s s i p p i d e l t a i t s c h a r a c t e r i s t i c b i r d f o o t shape ( F i s k , 1961).
The b a r f i n g e r s a n d s a r e biconvex e l o n g a t e s a n d b o d i e s
up t o 30km l o n g , 5-8km wide, and w i t h a r e l a t i v e l y uniform t h i c k n e s s of 70m ( F i g . 3,13).
143
c - Thin soils and Peat beds
li c - .
-_,-
sequences represenling rhe repeated infilltng of tnrerdtsrriburarv
tlnrerbedded muds,
silts and sands
tSlump
sheet
tHomo
enous muds. laminayed or bioturbated
Composite i d e a l i z e d sequence produced by mouth b a r p r o g r a d a t i o n i n t h e Fig.3.12. M i s s i s s i p p i d e l t a ( a f t e r Coleman and W r i g h t , 1 9 7 5 ) . I n most o t h e r p r e s e n t - d a y d e l t a s f l u v i a l p r o c e s s e s a r e n o t so dominant a t t h e d e l t a f r o n t and i n t e r a c t i n g wave and t i d a l p r o c e s s e s i n p a r t i c u l a r a r e a dominant f e a t u r e of t h e Rhone d e l t a .
The d e l t a f r o n t c o n s i s t s of l a t e r a l l y e x t e n s i v e beach
r i d g e s i n f r o n t of which i s a r e l a t i v e l y s t e e p o f f s h o r e s l o p e o f up t o 2' 1955; van S t r a a t e n , 1960) (Fig.3.54).
(Kruit,
P r o g r a d a t i o n of t h e d e l t a f r o n t t a k e s p l a c e
by t h e a c c r e t i o n of beach r i d g e s and p r o g r a d a t i o n of t h e mouth b a r producing c o a r s e n i n g upwards sequences ( L a g a a i j and K o p s t e i n , 1964; Oomkens, 1967) ( F i g . 3.15). The sequences b e g i n w i t h b i o t u r b a t e d o f f s h o r e c l a y s and p a s s upwards i n t o f i n e l y l a m i n a t e d s i l t s and t h e n i n t o r i p p l e l a m i n a t e d s i l t s and sands and t h e n f i n a l l y i n t o w e l l - s o r t e d , h o r i z o n t a l l y bedded s a n d s of b e a c h - b a r r i e r a s p e c t .
The s c a r c i t y of
d i s t r i b u t a r y c h a n n e l sands and t h e wave-dominated n a t u r e o f t h e s h e e t sands e n a b l e t h e d i s t i n c t i o n from f l u v i a l dominated d e l t a f r o n t s . The N i g e r d e l t a i s a n example i n which b o t h wave and t i d a l p r o c e s s e s a r e i m p o r t a n t a t the delta front.
The e f f e c t s o f t i d a l p r o c e s s e s a r e c o n f i n e d t o d i s t r i b u t a r y
mouth b a r s w h i l s t waves o p e r a t e o v e r a l a r g e a r e a o f t h e d e l t a f r o n t .
The s h o r e l i n e
144
Cross section Natural Levee
Fig.3.13.
Sparse to abundant faun Marsh
I
I
Bar f i n g e r sands o f the M i s s i s s i p p i d e l t a ( a f t e r F i s k , 1961).
I
F i g . 3.14.
'..
Distributary channel
,
I
The morphology of the Rh8ne d e l t a ( a f t e r van Andel and Curray, 1960).
145 t h u s c o n s i s t s predominantly o f wave g e n e r a t e d beaches o r c h e n i e r s s e p a r a t e d by t i d e dominated d i s t r i b u t a r y c h a n n e l s .
D e l t a s o f t h i s t y p e do n o t form obvious p r o t r u s i o n s
i n t o t h e s e a and have f a c i e s b e l t s p a r a l l e l t o t h e c o a s t ( A l l e n , 1965b, 1970c; Oomkens, 1974) (Fig.3.16). A
6
41-
0 50100% Sand. mainly horizontally 0 .......... bedded. that shows an upward increase in grain size
0 Upward increase in clay content
..... . . . . . .*. . . .......
-
cross bedded 10
Base of
50100%
j -
= Q
$11
Distinct upward increase 20 in number and thickness of intercalated silt and sand beds between clays
I
+i .. .. .. .. ................
silt and sand
6 + 9
i
Clay with thin silt and sand beds
m
= Horizontally bedded
- Crossbadded
6 sumpsdbeds U Burrows 6 Shells, shell debris
0 Plants, plant debris
Fig.3.15. D e l t a f r o n t c o a r s e n i n g upward sequences from t h e Rhone d e l t a A . P a s s i n g g r a d u a l l y upwards i n t o a c o a s t a l b a r r i e r s a n d B. A t r u n c a t e d sequence a t 13m o v e r l a i n by a n e r o s i v e based d i s t r i b u t a r y c h a n n e l sequence ( a f t e r Oomkens, 1967). The d i s t r i b u t a r y c h a n n e l s of t h e Niger d e l t a a r e t i d a l c h a n n e l s and have a shallow, sandy mouth b a r a t t h e e n t r a n c e which p r o b a b l y forms a s a r e s u l t of t h e expansion of tidal currents. platform’.
The mouth b a r s and beach f a c e s l o p e g e n t l y seaward t o a ‘ d e l t a f r o n t
T h i s i s a t e r r a c e between 5-10m deep and up t o 20km wide.
Beyond i t t h e
d e l t a f r o n t s l o p e s down i n t o t h e p r o d e l t a which i s a low-energy environment o n l y m i l d l y a f f e c t e d by waves, t i d a l c u r r e n t s a n d t h e Guinea C u r r e n t . F a c i e s sequences from t h e N i g e r d e l t a f r o n t have been d e t e r m i n e d from numerous c o r e d b o r e h o l e s and have been d e s c r i b e d by Weber (1971) and Oomkens (1974).
146
Fig.3.16.
D e p o s i t i o n a l environments of t h e modern Niger d e l t a ( a f t e r A l l e n , 1 9 7 0 ~ ) .
The sequences ( F i g . 3 . 1 7 ) a r e between 10-30m t h i c k and b e g i n w i t h b i o t u r b a t e d muds which p a s s upwards i n t o i n t e r b e d d e d muds, s i l t s and s a n d s ,
These p a s s i n t o e i t h e r
w e l l - s o r t e d , p a r a l l e l - l a m i n a t e d beach f a c e s a n d s o r t i d a l c h a n n e l d e p o s i t s . DELTA GROWTH AND ABANDONMENT D e l t a s o f t e n have a complex h i s t o r y i n which two p h a s e s of development can b e recognized ( S c r u t o n , 1960).
The f i r s t o f t h e s e is t h e c o n s t r u c t i o n a l phase i n which
t h e d e l t a p r o g r a d e s and b u i l d s - u p a v e r t i c a l sequence of p r o d e l t a d e p o s i t s o v e r l a i n by d e l t a f r o n t s i l t s and s a n d s and f i n a l l y d e l t a marsh d e p o s i t s and p e a t beds.
The
second i s t h e d e s t r u c t i o n a l phase which t a k e s p l a c e a f t e r t h e abandonment of d e l t a growth because o f t h e a v u l s i o n of a l l u v i a l o r d i s t r i b u t a r y c h a n n e l s .
The topmost
beds of t h e d e l t a a r e t h e n a t t a c k e d by waves and c u r r e n t s and may b e c o m p l e t e l y reworked, Compaction may r e s u l t i n l o c a l marine t r a n s g r e s s i o n s .
147
Fig.3.17. D e l t a f r o n t sequences o f t h e N i g e r d e l t a . A. T e r m i n a t i n g i n a c o a s t a l b a r r i e r s a n d B. Terminated by a t i d a l c h a n n e l i n l e t ( a f t e r Oomkens, 1974). The c o n c e p t of two-phase development of d e l t a s i s somewhat i d e a l i z e d and o n l y r e a l l y applicable t o river-dominated d e l t a s l i k e the Mississippi.
Here seven
p a r t i a l l y o v e r l a p p i n g d e l t a l o b e s c a n b e r e c o g n i z e d (Kolb and van Lopik, 1 9 6 6 ) (Fig.3.18).
These l o b e s have been s u c c e s s i v e l y abandoned d u r i n g t h e l a s t 6,000
y e a r s o r so and have undergone v a r i o u s s t a g e s of d e s t r u c t i o n .
The e a r l y s t a g e s
i n c l u d e smoothing of t h e s h o r e l i n e w i t h t h e f o r m a t i o n of b a r r i e r i s l a n d s w h i l s t more abvanced s t a g e s a r e r e p r e s e n t e d by t h e d e p o s i t i o n o f f o s s i l i f e r o u s marine c l a y s on top of t h e former d e l t a p l a i n . ANCIENT DELTAIC SUCCESSIONS I n g e n e r a l a n c i e n t d e l t a i c s u c c e s s i o n s t e n d t o show t h e f o l l o w i n g c h a r a c t e r i s t i c s :
1) t h e y a r e f r e q u e n t l y t h i c k ( s e v e r a l hundreds t o thousands of m e t r e s ) 2 ) they c o n t a i n a c o n s i d e r a b l e volume of sand o r s i l t
3 ) c o a l beds a r e commonly p r e s e n t
4 ) t h e f a u n a l c o n t e n t of i n t e r b e d d e d u n i t s may i n d i c a t e m a r i n e , b r a c k i s h and f r e s h w a t e r d e p o s i t i o n a l environments.
148
Plaqueminer
S t . Bernard
Lafourche
Teche
Maringouin
Km
The Recent M i s s i s s i p p i d e l t a system showing t h e d i s t r i b u t i o n o f d e l t a Fig.3.18. complexes and l o b e s ( a f t e r F r a z i e r , 1967).
5 ) s e d i m e n t a r y s t r u c t u r e s i n d i c a t e s h a l l o w w a t e r d e p o s i t i o n by t r a c t i o n c u r r e n t s 6 ) they o f t e n g r a d e i n an o f f s h o r e d i r e c t i o n i n t o f i n e r g r a i n e d c l a s t i c d e p o s i t s . S p e c i f i c r e c o g n i t i o n of a n c i e n t r i v e r - , wave-, o r tide-dominated d e l t a s i s p o s s i b l e by a n a l o g y w i t h Recent d e l t a s and depends h e a v i l y on v e r t i c a l and l a t e r a l f a c i e s d i s t r i b u t i o n (Fig.3.19).
I n f a c t the v a s t majority of ancient d e l t a i c
sequences which have been d e s c r i b e d a r e o f t h e r i v e r - d o m i n a t e d t y p e ; t h i s may be p a r t l y due t o t h e f a c t t h a t a n c i e n t wave- and t i d e dominated d e l t a s a r e l i k e l y t o be d i f f i c u l t t o d i s t i n g u i s h from o t h e r s h a l l o w marine s e q u e n c e s . Ancient r i v e r - d o m i n a t e d d e l t a sequences a r e r e p r e s e n t e d by r e l a t i v e l y l a r g e - s c a l e coarsening-upwards c y c l e s which r e c o r d t h e p a s s a g e from o f f s h o r e muds i n t o s h o r e l i n e sands a s t h e d e l t a f r o n t p r o g r a d e s seawards.
When complete t h e sequences a r e
t r u n c a t e d by a f l u v i a l d i s t r i b u t a r y c h a n n e l sequence w i t h a s s o c i a t e d d e l t a p l a i n marsh d e p o s i t s ( p e a t o r c o a l ) . Ancient d e l t a i c s u c c e s s i o n s g e n e r a l l y commence w i t h a t h i c k , u n i f o r m i n t e r v a l of mudstones o r s i l t s t o n e s d e p o s i t e d a t t h e b a s e of t h e d e l t a f r o n t and f u r t h e r o f f s h o r e .
149
.
.
interdistributary shell bank distributary channel fill
marsh 1 delta distributarychannel dolta marsh stacked beach ridges
proximal distributary mouth bar
shoreface
d.pdrits distal distributary mouth bar deposits
-
pro delta shale
- WAVE DOMINATED DELTA
'
pro-deltashale
delta marsh tidal channel
d.por*
Ecoal with roots
tidal flat tidal channel
lowangle crossbedding herringbone crossbedding trough, festoon crossbedding
offrhore tidal sand ridges
TIDE DOMINATED DELTA
ripple marks shells, shell fragments bioturbation,trace fossils limestone cyclic units,showing direction of coarsening
F i g . 3 . 1 9 . Idealized v e r t i c a l sequences of r i v e r - , t i d e - , and wave-dominated ancient d e l t a s . The thickness and r e l a t i v e abundance of the f a c i e s may very considerably but t h e i r stratigraphic sequence i s more or l e s s constant ( a f t e r Miall, 1976).
150 V a r i a t i o n s i n t h e s u p p l y of suspended s e d i m e n t a r e r e f l e c t e d i n s l i g h t g r a i n s i z e v a r i a t i o n s which on o c c a s s i o n s g i v e t h e f a c i e s a s t r i p e d o r banded a p p e a r a n c e ( F i g . 3.20).
P l a n t d e b r i s i s f r e q u e n t l y abundant b u t t h e r e i s o n l y a s p a r s e m a r i n e
fauna presumably b e c a u s e of t h e h i g h s e d i m e n t a t i o n r a t e .
Thin e r o s i v e - b a s e d beds
o f f i n e g r a i n e d s a n d s t o n e sometimes o c c u r i n t h i s mudstone f a c i e s ( e . g . de Raaf
e t a l . , 1965) and p r o b a b l y r e p r e s e n t t u r b i d i t y c u r r e n t s g e n e r a t e d n e a r t h e d i s t r i b u t a r y mouth d u r i n g f l o o d p e r i o d s . Above t h e s e mudstones t h e amount o f c o a r s e s i l t s t o n e and f i n e s a n d s t o n e beds become more numerous.
These show c r o s s - l a m i n a t i o n and r i p p l e s i n d i c a t i n g depos-
i t i o n i n a g i t a t e d water above t h e wave b a s e .
I n some c a s e s t h e r e a r e c o a r s e
e r o s i v e - b a s e d s a n d s t o n e s i n which p l a n e l a m i n a t i o n s g i v e way t o c r o s s - l a m i n a t i o n . These may i n d i c a t e t h e d e p o s i t i o n of f l o o d - g e n e r a t e d s e d i m e n t s a t t h e b a s e of t h e d e l t a f r o n t under waning c u r r e n t c o n d i t i o n s .
F u r t h e r upwards t h e s a n d s t o n e beds
become t h i c k e r and more numerous and dominate t h e s u c c e s s i o n .
These s a n d s r e p r e s e n t
t h e d e l t a f r o n t and a r e t h e d e p o s i t s o f d i s t r i b u t a r y mouth b a r s .
The geometry o f
t h e s e sand b o d i e s i s most commonly t h a t of a s a n d - s h e e t b u t a n c i e n t b a r - f i n g e r s a n d s a r e d e s c r i b e d ( C o l l i n s o n and Banks, 1975). The d e l t a i c c y c l e i s completed by f a c i e s r e p r e s e n t i n g t h e d e l t a p l a i n .
These
i n c l u d e s m a l l - s c a l e (4-10m) coarsening-upwards sequences which r e p r e s e n t t h e i n f i l l i n g of i n t e r d i s t r i b u t a r y bays by c r e v a s s e - s p l a y s , c r e v a s s e channel c o u p l e t s ( E l l i o t , 1974).
l e v e e c o n s t r u c t i o n o r mouth b a r -
Such c y c l e s may b e t e r m i n a t e d by p a l a e o -
s o l s o r c o a l seams.
Large s c a l e c h a n n e l f i l l sequences w i t h b a s a l l a g d e p o s i t s and cross-bedded s a n d s t o n e s which show a n o v e r a l l f i n i n g upwards t r e n d o f t e n t e r m i n a t e r i v e r - d o m i n a t e d d e l t a i c cycles.
They r e p r e s e n t d i s t r i b u t a r y c h a n n e l sequences and a s s u c h show
u n i d i r e c t i o n a l flow c h a r a c t e r i s t i c s o f t e n w i t h abundant e v i d e n c e of s t a g e f l u c t u a t ions.
Both h i g h and l o w - s i n u o s i t y c h a n n e l s a r e p r o b a b l y r e p r e s e n t e d i n a n c i e n t
d e l t a i c sequences.
A s y e t t h e r e a r e no d e t a i l e d d e s c r i p t i o n s o f wave o r tide-dominated d e l t a s i n t h e s t r a t i g r a p h i c r e c o r d a l t h o u g h Van de Graaf (1972) and Hubert e t a l . (1972) have d e s c r i b e d bimodal ( t i d a l ) c u r r e n t p a t t e r n s from C r e t a c e o u s d e l t a p l a i n d e p o s i t s i n t h e Western I n t e r i o r , USA.
I n t h e same a r e a t h e Cody Shale-Parkman Sandstone d e l t a
shows d e l t a f r o n t f a c i e s which a r e b e s t i n t e r p r e t e d a s wave-produced b e a c h - b a r r i e r s a n d s t o n e (Hubert e t a l . , 1972; A s q u i t h , 1974). A number of examples of a n c i e n t d e l t a i c d e p o s i t s a r e g i v e n i n T a b l e 3.1.
RED BEDS I N ANCIENT DELTA PLAINS Introduction Red beds i n d e l t a p l a i n environments a r e t h e least-known and most p o o r l y unders t o o d o f t h e t h r e e major a s s o c i a t i o n s o f c o n t i n e n t a l r e d b e d s ,
I n general they
151
30
Flnr grained micaceous r~pplelaminaled sandsloner and sdtstones with impoverished ripples
Erosive.based unit of trough cross bedded sandstones with interbedded SlltStOneS
25
Wavy and cross-laminated siltstones. with frequent loading
20
Planar cross qlratified sandstones wilh unidireri ional Dalaeocurrenlr
B
5
E
15
Banded or striped silty mudstones with Planolites
/....... . . ...........,.. . . .. .....;.. ..... . . ........ .-... ......... ...... . .. . . . . . ... ,
ia
,
Massive sandstones locally cross sirallfed wlth lntemal erowon surfaces and OCCasicmal soft sediment deformation
E Fissile black mudstones with abundant Plandites and thin bands and nodules (
Basal lag deposit above an erosional Surlace mth a rehef 01
6 5 m Abundant ironstone pebbles and mudstone flakes
Underlying delta front progradatlon saquenc
Fig.3.20. R e p r e s e n t a t i v e s e c t i o n s of a n c i e n t r i v e r - d o m i n a t e d d e l t a s . A . A d e l t a f r o n t sequence from t h e Coal Measures o f S o u t h Wales B. A f l u v i a l - d i s t r i b u t a r y c h a n n e l sequence l y i n g on eroded d e l t a f r o n t d e p o s i t s from t h e Namurian o f South Wales ( b o t h a f t e r K e l l i n g and George, 1971).
form t h i n , l a t e r a l l y d i s c o n t i n u o u s u n i t s i n o t h e r w i s e d r a b s t r a t a .
Their association
w i t h d e l t a i c s e d i m e n t a t i o n may n o t b e obvious b u t can be deduced from t h e f a c t t h a t t h e y a r e i n t e r b e d d e d w i t h t y p i c a l d e l t a i c s e d i m e n t s and commonly cap c o a r s e n i n g upwards ( p r o g r a d a t i o n a l ) d e l t a i c p r o f i l e s .
S i n c e t h e d e l t a p l a i n i s dominated by
TABLE 3.1. Examples of a n c i e n t d e l t a i c complexes Forma t i o n
Age
Middle Vicksburg
Lower O l i g i o c e n e
Delta type
Reference
U.S.A.
Wave-dominated
Gregory (1966)
Upper Wilcox Gp.
Eocene
U.S.A.
Wave-domina t e d
F i s h e r (1969)
Lower Wilcox
Eocene
U.S.A.
River-domina t e d
F i s h e r and McGowen (1967)
Sentinel Butte
Palaeocene
U.S.A.
River-dominated
Cherven (1978)
D i f u n t a Gp.
Upper Cretaceous - Palaeocene
Mexico
River-domina t e d
McBride e t a l . (1975)
Cody S h a l e Parkman Sands tone
Upper Cretaceous
U.S.A.
Wave-domina t e d
Hubert e t a l . (1972)
C i s c o Group
Carboniferous
U.S.A.
River-domina t e d
Galloway and Brown (1973)
-
Canyon Group
Carboniferous
U.S.A.
River-domina t e d
Erxleben (1975)
Haslingden F l a g s
Carboniferous
U.K.
River-domina t e d
C o l l i n s o n and Banks (1975)
Grindslow S h a l e s - Kinderscont G r i t
Carboniferous
U.K.
River-dominated
Walker (1966) C o l l i n s o n (1969)
Yoredales
Carboniferous
U.K.
River-domina t e d
E l l i o t (1975)
f l u v i a l d e p o s i t i o n a l environments t h e r e s u l t a n t r e d beds may c l o s e l y resemble a l l u v i a l r e d beds.
They a r e s i m i l a r i n t h a t t h e y o c c u r i n fining-upwards u n i t s and
a r e b e s t - d e v e l o p e d i n t h e f i n e member ( f l o o d p l a i n ) p a r t of t h e u n i t .
Usually there
i s c o n s i d e r a b l e e v i d e n c e of emergence i n t h e form o f mudcracks, r o o t l e t h o r i z o n s
and pedogenic f e a t u r e s .
I n C a r b o n i f e r o u s r e d beds o f t h i s t y p e a c o n n e c t i o n w i t h
d e l t a i c s e d i m e n t a t i o n i s i m p l i e d by t h e o c c u r r e n c e o f abundant s e a t e a r t h s and occasional t h i n coals within t h e red beds,
D e l t a p l a i n r e d beds c o n t r a s t w i t h
a l l u v i a l r e d beds i n t h a t t h e y a r e f r e q u e n t l y c o n f i n e d t o mudstones which may show a wide range of c o l o u r m o t t l i n g .
E x t e n s i v e pedogenic m o d i f i c a t i o n s ( i n c l u d i n g
c a r b o n a t e n o d u l e s , r o o t l e t s a n d s e a t e a r t h s ) and t h e i n s i t u o x i d a t i o n of p l a n t d e b r i s and p r e v i o u s l y formed p y r i t e a r e a l s o commonly observed. Although d e l t a s may a c c u m u l a t e o v e r a wide range of c l i m a t i c s e t t i n g s , a n c i e n t d e l t a p l a i n r e d beds a p p e a r t o b e p a r t i c u l a r l y c h a r a c t e r i s t i c of m o i s t t r o p i c a l climates.
T h i s i s i n d i c a t e d by t h e o c c u r r e n c e of i n t e r d i g i t a t e d c o a l s and o t h e r
e v i d e n c e of abundant v e g e t a t i o n c o v e r .
T h i s c o n t r a s t s markedly w i t h many a n c i e n t
a l l u v i a l r e d beds which a p p a r e n t l y ' formed under more a r i d c o n d i t i o n s and show many s i m i l a r i t i e s t o t h e d e s e r t a l l u v i u m d e s c r i b e d by Walker e t a l . (1978).
Therefore,
a l t h o u g h i t has been a r g u e d t h a t t h e o r i g i n of r e d beds i n d e s e r t and m o i s t t r o p i c a l c l i m a t e s may be b r o a d l y s i m i l a r (Walker, 1 9 7 4 ) , i t seems l i k e l y t h a t t h e r e may be i n t r i n s i c d i f f e r e n c e s i n t h e r e d d e n i n g of m o i s t t r o p i c a l d e l t a p l a i n s and a r i d , d e s e r t alluvium.
These a r e c o n s i d e r e d h e r e w i t h s p e c i a l r e f e r e n c e t o t h e Coal
Measures o f hW Europe. The D i f u n t a Group of n o r t h e a s t e r n Mexico The D i f u n t a Group o f n o r t h e a s t e r n Mexico comprises a t h i c k (4km) sequence o f s e d i m e n t s d e r i v e d from v o l c a n i c r o c k s and d e p o s i t e d i n f l u v i a l , d e l t a i c , and s h e l f environments.
The f l u v i a l systems s u p p l i e d d e l t a s t h a t prograded e a s t w a r d i n t o t h e
a n c e s t r a l Gulf o f Mexico (Fig.3.21A).
I t p r o v i d e s a n e x c e l l e n t example of t h e
f o r m a t i o n of r e d beds i n d e l t a p l a i n sediments which were d e p o s s t e d i n a sub-humid t o s e m i - a r i d c l i m a t e having s t r o n g s e a s o n a l a r i d i t y . V a r i a t i o n s i n sediment i n p u t , s u b s i d e n c e r a t e s , and sediment compaction i n t h e D i f u n t a Group r e s u l t e d i n a complex i n t e r t o n g u i n g of d e l t a i c - p a r a l i c f a c i e s ( F i g . 3.21B).
The d e l t a i c f a c i e s of t h e D i f u n t a Group a r e d e s c r i b e d by McBride e t a l .
(1975) and t h e o r i g i n o f t h e r e d beds which a r e a c h a r a c t e r i s t i c f e a t u r e of t h e d e l t a p l a i n f a c i e s , i s d i s c u s s e d by McBride (1974). The C e r r o H u e r t a , Las Ima'genes, and E n c i n a s Formations a r e mainly r e d beds w i t h minor amounts of g r e e n , p u r p l e , brown, o l i v e and y e l l o w beds t h a t a r e i n t e r p r e t e d a s d e l t a p l a i n f a c i e s (Fig.3.21C). i n c o n t r a s t t o the grey formations,
These beds a r e of uniform l i t h o l o g i c a l c h a r a c t e r These i n c l u d e t h e Ca8on de T u l e , C e r r o Grande
and Rancho Neuvo Formations which v a r y w i d e l y i n l i t h o l o g i c a l c h a r a c t e r r e f l e c t i n g
154
.Sari Antonio
#Delta
and coastal plain
k&ENES C A h
DEL TULE
CERRO HUERTA CERRO DEL PUEBLO
PAMAS
I I
I
Fig.3.21. The D i f u n t a Group of n o r t h e a s t e r n Mexico A . L o c a t i o n of P a r r a s and La Popa d e p o s i t i o n a l b a s i n s B. L i t h o s t r a t i g r a p h y . The c r o s s - h a t c h e d u n i t s a r e p r e dominantly r e d f o r m a t i o n s which t h i n o u t t o t h e e a s t . Cretaceous-Palaeocene boundary i s a b o u t t h e middle of t h e Rancho Nuevo Formation C , Environmental i n t e r p r e t a t i o n . Note t h e c l o s e a s s o c i a t i o n between r e d beds and d e l t a p l a i n / c o a s t a l p l a i n f a c i e s ( a f t e r McBride, 1974).
155 d e p o s i t i o n i n d e l t a f r o n t , d e l t a p l a t f o r m , p r o d e l t a , and s h e l f d e p o s i t s (McBride, 1974; McBride e t a l . , 1975). The d e l t a p l a i n f a c i e s The d e l t a p l a i n d e p o s i t s of t h e D i f u n t a Group a r e c h a r a c t e r i z e d by a n abundance o f r e d beds (80%) i n c y c l i c sequences of c l a y s t o n e (lo%), mudstone ( 2 0 % ) , s i l t s t o n e (20%) and v e r y f i n e s a n d s t o n e s (30%) which a l t e r n a t e w i t h c h a n n e l - f i l l s a n d s t o n e bodies.
Between 50-60% o f t h e d e l t a - p l a i n d e p o s i t s a r e a r r a n g e d i n 2-5m t h i c k
coarsening-upwards c y c l e s which b e g i n w i t h c l a y s t o n e r e s t i n g on a n e r o s i o n s u r f a c e of s a n d s t o n e and g r a d e upwards through mudstone and s i l t s t o n e t o v e r y f i n e s a n d s t o n e (Fig.3.22).
These c y c l e s a r e i n t e r p r e t e d a s f r e s h w a t e r and b r a c k i s h w a t e r l a k e and
marsh d e p o s i t s .
The g r a d u a l f i l l i n g of l a k e s and marshes by l a c u s t r i n e d e l t a s and
p r o g r a d i n g s h o r e l i n e s i s known t o produce coarsening-upwards sequences of t h i s type (Coleman, 1966; W i l l i a m s , 1968).
SILTSTONE. 80 % BURROW MOTTLED 20% CUR~ENT-RIPPLE BEDDED SILT 70%1 CALCITE 20Ki CLAY 10% COMMON CALCITE NODULES
------MUDSTONEi BEDDING SIMILAR TO CLAYSTONE CLAY 50K1SILT 30x1 CALCITE 20% 1 SPARSE CALCITE NODULESi LOCALLY ABUNCYINT OOGONIA
Fig.3.22. Schematic diagram of a t y p i c a l coarsening-upwards d e l t a p l a i n c y c l e of t h e D i f u n t a Group ( a f t e r McBride, 1974).
156 The upper p a r t of t h e s e c y c l e s f r e q u e n t l y c o n t a i n c a l c i t e n o d u l e s (mean s i z e 2cm) which become p r o g r e s s i v e l y more abundant t o comprise a b o u t 20% a t t h e t o p of some s a n d s t o n e members.
They a r e i n t e r p r e t e d a s c a l c a r e o u s p a l a e o s o l s .
Also a t t h e t o p
of t h e c y c l e s t h e r e o c c u r s i n u o u s , v e r t i c a l t u b e s which a r e i n t e r p r e t e d a s r o o t scars.
P l a n t s may have grown submerged ( m a r s h e s ) , on d r i e d l a k e b e d s , o r on over-
bank a r e a s n e a r d i s t r i b u t a r y c h a n n e l s .
The s c a r c i t y o f p l a n t d e b r i s i n t h e d e p o s i t s
c o u l d thus b e a t t r i b u t e d t o r a p i d decomposition and o x i d a t i o n p r i o r t o b u r i a l . would be favoured by w e l l - d r a i n e d swamps o r s e a s o n a l d e s i c c a t i o n .
This
The a b s e n c e o f
d e s i c c a t i o n even i n a r e a s where s u b a e r i a l e x p o s u r e i s i n d i c a t e d s u g g e s t s t h a t good d r a i n a g e i s t h e more l i k e l y c a u s e ( t h e w e l l - d r a i n e d swamp d e p o s i t s of t h e M i s s i s s i p p i d e l t a c o n t a i n v i r t u a l l y no o r g a n i c m a t t e r ) . The coarsening-upwards c y c l e s a r e p e r i o d i c a l l y i n t e r u p t e d by reddish-brown o r brown c h a n n e l - f i l l s a n d s t o n e u n i t s up t o 1Om t h i c k .
These c o n t a i n c r o s s - b e d d i n g i n
t h e lower p a r t w i t h o c c a s i o n a l i n t r a f o r m a t i o n a l l a g s and g i v e way upwards t o horizontal lamination.
These a r e i n t e r p r e t e d a s t h e d e p o s i t s of low s i n u o s i t y
d i s t r i b u t a r i e s which d r a i n e d t h e lower p a r t s of t h e d e l t a p l a i n . Colour v a r i a t i o n s and m i n e r a l o g y I n k e e p i n g w i t h o t h e r reddened d e l t a p l a i n d e p o s i t s t h e D i f u n t a Group shows a wide range of c o l o u r v a r i a t i o n s i n c l u d i n g r e d , p u r p l e , g r e e n , g r e y , o l i v e , y e l l o w and brown.
Most o f t h e d e l t a p l a i n f a c i e s i s made up of r e d beds which r a n g e from
hues of r e d (5R, 7.5R, 10R) t o r e d d i s h brown (2.5YR). of c o l o u r on g r a i n s i z e .
There i s a s t r o n g dependence
C l a y s t o n e s have s t r o n g e r s a t u r a t i o n (chroma numbers of
3 t o 5 v e r s u s 2 t o 3 f o r c o a r s e r - g r a i n e d r o c k s ) and g r e a t e r l i g h t n e s s v a l u e s .
Most
o f t h e r e d s i l t s t o n e s and c l a y s t o n e s a r e u n i f o r m l y c o l o u r e d t h r o u g h o u t a l t h o u g h a few beds have brown m o t t l e s i n burrows and r o o t s c a r s .
A l l the f i n e grained rocks
a r e c a l c a r e o u s (between 11-19’77 CaCO ) and t h e o n l y c l a y m i n e r a l s p r e s e n t a r e i l l i t e
3
and c h l o r i t e .
The r e d c o l o u r i s due t o h a e m a t i t e which i s p r e s e n t a s g r a i n c o a t i n g s
and i n c l o s e a s s o c i a t i o n w i t h c l a y m i n e r a l s .
I r o n d i s t r i b u t i o n s a r e shown i n
Fig.3.23. Green ( 5 G Y ) beds are l i t h o l o g i c a l l y s i m i l a r t o t h e r e d beds c o n s i s t i n g predomi n a n t l y of c l a y s t o n e and mudstone.
The c o l o u r i s a t t r i b u t e d t o f e r r o u s i r o n i n
c h l o r i t e and i l l i t e ( K e l l e r , 1953) i n t h e absence of h a e m a t i t e and o r g a n i c m a t t e r . The m a j o r i t y of t h e s e g r e e n beds o c c u r b e n e a t h c h a n n e l s a n d s t o n e s whose c o l o u r r a n g e s from l i g h t brown t o g r e y and t h e c o n t a c t w i t h a d j a c e n t r e d beds i s s h a r p b u t c l e a r l y t r a n s e c t s bedding p l a n e s .
T h i s f e a t u r e i s c l e a r l y of p o s t - d e p o s i t i o n a l
o r i g i n and s u g g e s t s t h a t t h e g r e e n beds have formed by t h e r e d u c t i o n o f r e d beds o r p r o t o - r e d beds by r e d u c i n g groundwaters which seeped from t h e o v e r l y i n g c h a n n e l sands.
A s i m i l a r e x p l a n a t i o n h a s been s u g g e s t e d f o r g r e e n zones u n d e r l y i n g f l u v i a l
channel s a n d s t o n e s ( F r i e n d , 1 9 6 6 ) .
The t o t a l i r o n c o n t e n t of t h e g r e e n beds i s
157
4
RED PURPLE A GREEN 0 GREY m
a& A.
OLIVE 1 YELLOW 0 ME AN VALUES
+
m n
>,,mo A
-2 2
O.0
A
o +-o
1
a+- m-t 0
Cl
,
m 2
1
moo 3
I
4
I
96 TOTAL FE
Fig.3.23. P l o t s o f t o t a l Fe v s . Fe3+ and F e 2 + v s . Fe3+ i n D i f u n t a c l a y s t o n e samples of d i f f e r e n t c o l o u r ( a f t e r McBride, 1974). v i r t u a l l y i d e n t i c a l w i t h t h a t of t h e r e d beds b u t t h e Fe2+/Fe3+ r a t i o i s v e r y d i f f e r e n t ( 2 . 8 1 a s opposed t o 0.45 i n t h e r e d b e d s ) ( F i g . 3 . 2 3 ) .
This contrasts
markedly w i t h many o t h e r g r e e n beds formed by s e c o n d a r y r.eduction which u s u a l l y show comparable Fe2+ c o n t e n t s b u t much less Fe3+ t h a n a s s o c i a t e d r e d beds.
The
d e p l e t i o n of Fe3+ i n s e c o n d a r i l y reduced g r e e n beds i s a t t r i b u t e d t o r e d u c t i o n and removal of Fe2+ i n s o l u t i o n ( s e e C h a p t e r 6 ) and t h e r e l a t i v e i n c r e a s e i n Fe
2+
i n t h e D i f u n t a g r e e n beds might i n d i c a t e t h a t i r o n has been accomodated i n a u t h i g e n i c c l a y m i n e r a l s r a t h e r than b e i n g removed (McBride, 1974).
An a l t e r n a t i v e view i s
t h a t t h e g r e e n beds were n e v e r f u l l y o x i d i z e d and t h a t t h e pigmentary h a e m a t i t e of t h e r e d beds formed by t h e complete o x i d a t i o n o f f e r r o u s i r o n - b e a r i n g i l l i t e and chlorite, P u r p l e (5RP 5 / 2 -
5P 5 / 2 ) c l a y s t o n e and s i l t y c l a y s t o n e s show s i m i l a r t e x t u r e s
and composition t o r e d b e d s o f t h e same g r a i n s i z e . 0.5-1.5m
P u r p l e beds a r e t y p i c a l l y
t h i c k and may b e t r a n s i t i o n a l between r e d and g r e e n zones.
They a r e
u s u a l l y r o o t - m o t t l e d zones w i t h a s h a r p upper c o n t a c t and a n uneven lower c o n t a c t which always t r a n s e c t s bedding p l a n e s .
Purple colouration i s a c h a r a c t e r i s t i c
f e a t u r e of r e d beds which have c o n t a i n e d s i g n i f i c a n t q u a n t i t i e s of o r g a n i c m a t t e r , s u c h a s r o o t p e n e t r a t e d h o r i z o n s , b u t i t s p r e c i s e o r i g i n i s n o t w e l l understood. The s i m i l a r i t y of t h e r e d and p u r p l e beds i n c o m p o s i t i o n and l i t h o l o g y s u g g e s t s t h a t d i f f e r e n c e s i n i r o n o r manganese c o n t e n t ( S t r a k h o v , 1958) o r t h e mixing of g r e e n and r e d m i n e r a l s (Ortlam, 1967) a r e n o t a p p l i c a b l e .
The d i s p e r s i o n of
168 pigmentary h a e m a t i t e i n mudrocks i s a n i m p o r t a n t f a c t o r i n c o l o u r a t i o n and s o i s t h e p a r t i c u l a r g r a i n s i z e of h a e m a t i t e p r e s e n t .
The b r i g h t e s t r e d c o l o u r a t i o n
r e s u l t s from f i n e l y d i s p e r s e d h a e m a t i t e of e x t r e m e l y f i n e g r a i n s i z e ( 1 - 2 ~ ) whereas p u r p l e c o l o u r a t i o n a p p e a r s t o r e s u l t from c o a r s e r g r a i n e d (3-5rm) h a e m a t i t e which i s less e v e n l y d i s p e r s e d .
These c o n d i t i o n s c o u l d r e s u l t i n r o o t d i s t u r b e d
mudrocks where t h e decomposition and o x i d a t i o n o f o r g a n i c m a t t e r i n l o c a l i z e d zones c r e a t e s v a r i a b l e Eh-pH c o n d i t i o n s w i t h p r e f e r e n t i a l growth o f h a e m a t i t e i n t h e more oxygenated a r e a s . The importance of o r g a n i c m a t t e r i s a l s o s e e n i n o l i v e , y e l l o w , and g r e y beds where i t s p r e s e n c e a c t u a l l y c o n t r i b u t e s t o rock c o l o u r a t i o n .
O l i v e beds (5GY 5 / 1 )
a r e t h e r e s u l t o f t h e c o l o u r mixing o f g r e e n c l a y m i n e r a l s and b l a c k o r g a n i c m a t t e r . The f o r m a t i o n of h a e m a t i t e i n t h e s e b e d s , a n d i n yellow b e d s , may have been i n h i b i t e d i n t h e p r e s e n c e of o r g a n i c m a t t e r and much of t h e Fe3+ c o n t e n t ( F i g . 3 . 2 3 ) may b e i n t h e form of i r o n hydroxide ( g o e t h i t e ) .
Grey beds (N4-N6) a r e n o t abundant i n t h e
d e l t a p l a i n f a c i e s b e i n g more c h a r a c t e r i s t i c of t h e marine d e p o s i t s i n t h e D i f u n t a Group.
The c o l o u r o f t h e s e g r e y beds i s p a r t l y due t o o r g a n i c m a t t e r i n t h e form
of f i n e l y comminuted p l a n t d e b r i s and a l s o t o f i n e g r a i n e d p y r i t e .
The l a t t e r o c c u r s
a s i n f i l l i n g s of f o r a m i n i f e r a n chambers and a l s o a s s c a t t e r e d framboids.
It is
l i k e l y t h a t t h e g r e y beds r e p r e s e n t p o o r l y - d r a i n e d ground i n which r e d u c i n g c o n d i t i o n s p r e v a i l e d below t h e s e d i m e n t s u r f a c e ( c f . Coleman, 1966). Brown c o l o u r e d beds a r e r e s t r i c t e d t o s i l t s t o n e and v e r y f i n e s a n d s t o n e and a p p e a r t o have formed by t h e w e a t h e r i n g of p r e v i o u s l y g r e y beds.
The c h i e f c o l o u r i n g a g e n t
a p p e a r s t o b e g o e t h i t e which h a s formed by t h e o x i d a t i o n o f p y r i t e and t h e h y d r a t i o n of h a e m a t i t e .
Such a p r o c e s s may be a n i m p o r t a n t i n t e r m e d i a t e s t a g e i n t h e f o r m a t i o n
of r e d beds i n c o a s t a l d e p o s i t i o n a l environments ( C z y s c i n s k i e t a l . , 1978). The o r i g i n of t h e d e l t a p l a i n r e d beds I n view o f t h e f a c t t h a t no modern a l l u v i u m i s a s r e d a s a n c i e n t r e d beds (Van Houten, 1968; Walker, 1974) i t must b e presumed t h a t t h e sediment which accumu l a t e d on t h e d e l t a p l a i n was d r a b .
S i n c e t h i s sediment p r o b a b l y c o n t a i n e d
s i g n i f i c a n t amounts of i r o n h y d r o x i d e s and o t h e r i r o n - b e a r i n g m i n e r a l s i n a s s o c i a t i o n w i t h t h e abundant c l a y f r a c t i o n (Walker and Honea, 1969; Van Houten, 1972) r e d c o l o u r a t i o n most l i k e l y developed i n oxygenated c o n d i t i o n s above t h e w a t e r t a b l e . A number of f e a t u r e s s u g g e s t t h a t p e d o g e n e s i s and r e d d e n i n g were c l o s e l y r e l a t e d
w i t h i n t h i s zone.
These i n c l u d e r o o t s c a r s which a r e abundant i n p u r p l e a n d r e d
b e d s , t h e absence of b e d d i n g , and t h e abundance of c a l c i t e nodules of i n f e r r e d pedogenic o r i g i n .
The s c a r c i t y o f d i a g n o s t i c s o i l m i c r o f a b r i c s s u g g e s t s t h a t t h e
D i f u n t a s o i l s were n o t mature.
These f a c t o r s , a s w e l l a s t h e abundance of a s s o c i a t e d
r e d and g r e e n i n t r a c l a s t s i n r e d and brown c h a n n e l s a n d s t o n e s , s u g g e s t s t h a t t h e i n i t i a l reddening may have been r e l a t i v e l y r a p i d , p e r h a p s t a k i n g o n l y s e v e r a l
thousand y e a r s a s opposed t o t h e hundreds o f thousands o f y e a r s needed t o redden sandy d e s e r t a l l u v i u m (Walker, 1967a).
The r e a s o n s f o r t h i s r a p i d p o s t - d e p o s i t i o n a l
r e d d e n i n g l i e s i n t h e r e l a t i v e importance o f t h e i n s i t u a g e i n g o f f e r r i c hdyroxides and i r o n - b e a r i n g c l a y m i n e r a l s .
The c l a y - r i c h n a t u r e of t h e o r i g i n a l non-red
a l l u v i u m means t h a t i t l i k e l y c o n t a i n e d s i g n i f i c a n t amounts of f e r r i c hydroxides which c o u l d ' a g e ' i n t o h a e m a t i t e g i v e n s u i t a b l e c o n d i t i o n s ( B e r n e r , 1969a; Walker and Honea, 1969; Van Houten, 1972).
O t h e r d i a g e n e t i c p r o c e s s e s may a l s o have
c o n t r i b u t e d t o r e d c o l o u r a t i o n ; t h e s e i n c l u d e t h e i n t r a s t r a t a l a l t e r a t i o n of i r o n b e a r i n g s i l i c a t e s such a s b i o t i t e and t h e m a r t i t i z a t i o n of d e t r i t a l m a g n e t i t e g r a i n s (McBride, 1974, p.764).
Another p o s s i b l e r e d d e n i n g mechanism i n v o l v e s t h e formation
of h a e m a t i t e by t h e o x i d a t i o n of f i n e l y d i s s e m i n a t e d p y r i t e ( C h a l l i s , 1975; C z y s c i n s k i e t a l . , 1978).
P y r i t e o x i d a t i o n h a s been e x t e n s i v e l y s t u d i e d ( S i n g e r
and M o r r i s , 1969; Smith and Schumate, 1970) and i s p a r t i c u l a r l y s i g n i f i c a n t i n t h e development of a c i d s u l p h a t e s o i l s (Van Breeman, 1973).
Pyrite w i l l rapidly
o x i d i z e when i n c o n t a c t w i t h oxygenated groundwater, t h e r a t e o f o x i d a t i o n b e i n g l a r g e l y d e t e r m i n e d by t h e o x i d a t i o n of f e r r o u s i r o n .
The mechanism i s probably
a two-stage one i n which f e r r o u s i r o n i s r e l e a s e d by p a r t i a l o x i d a t i o n :
+ 02 + Fez+ + SO:- and t h e n 2 completely oxidized: Fez++ Fe3+ and s u b s e q u e n t l y p r e c i p i t a t e d a s f e r r i c oxyhy-
FeS
droxide.
During p r o g r e s s i v e b u r i a l t h i s f e r r i c oxyhdyroxide would ' a g e ' i n t o haem-
a t i t e p r o v i d i n g t h a t o x i d i z i n g c o n d i t i o n s c o n t i n u e d t o p r e v a i l ( B e r n e r , 1969a). C a r b o n i f e r o u s r e d beds i n Europe and North America Introduction During t h e Upper C a r b o n i f e r o u s i n Europe and North America l a r g e amounts of c l a s t i c sediment accumulated i n r i v e r - d o m i n a t e d d e l t a s .
These d e l t a s s t r a d d l e d
e q u a t o r i a l l a t i t u d e s and were o f t e n covered i n l u s h v e g e t a t i o n which accumulated t o f o m p e a t and s u b s e q u e n t l y c o a l seams (Coal Measures).
Within these coal-bearing
d e p o s i t s a r e abundant i n t e r d i g i t a t e d r e d beds which were formed under m o i s t t r o p i c a l climatic conditions.
T h i s a s s o c i a t i o n o f r e d beds w i t h c o a l - b e a r i n g s t r a t a i s q u i t e
d i s t i n c t i v e from t h e red b e d - e v a p o r i t e a s s o c i a t i o n which forms under a r i d c l i m a t i c c o n d i t i o n s (Walker, 1974). The Coal Measures i n t h e UK and USA p r o v i d e e x c e l l e n t examples of r e d beds which formed w i t h i n r i v e r - d o m i n a t e d d e l t a i c s u c c e s i o n s a l t h o u g h t h e y have n o t been intensively studied.
I n NW Europe t h e Coal Measures were d e p o s i t e d i n a major p a r a l i c
b a s i n which i s now r e p r e s e n t e d by a number o f i n d i v i d u a l c o a l f i e l d s which a r e r e l a t e d t o important depositional c e n t r e s (Fig.3.24).
I n t h e Midland V a l l e y o f S c o t l a n d
1060m of Coal Measures a r e p r e s e r v e d and i s o p a c h y t e s t u d i e s i n d i c a t e a d e p o s i t i o n a l t r o u g h w i t h l o c a l a t t e n u a t i o n a t t h e north-west and s o u t h - e a s t margins.
I n the
Pennine P r o v i n c e 3050m a r e p r e s e r v e d i n t h e c e n t r e of t h e b a s i n b u t a g a i n marked
160 a t t e n u a t i o n o c c u r s i n m a r g i n a l a r e a s and i n t h e s o u t h e r n m o s t p a r t t h e Coal Measures r e s t on Pre-Carboniferous r o c k s .
S i m i l a r l y i n South Wales 244Om accumulated i n t h e
c e n t r e of t h e b a s i n n e a r Swansea and t h e r e i s r a p i d t h i n n i n g towards t h e e a s t .
Fig.3.24. Palaeogeography of NW Europe d u r i n g t h e Upper C a r b o n i f e r o u s . The t h i c k n e s s v a l u e s a r e f a r t h e t o t a l Upper C a r b o n i f e r o u s ( a f t e r P.A. Z i e g l e r , 1978). The Coal Measures accumulated mainly i n river-dominated d e l t a s and d e l t a p l a i n s and comprise b o t h coarsening-upwards and fining-upwards s e q u e n c e s .
I n fact the
complexity of l o c a l s u c c e s s i o n i s s u c h t h a t many e a r l y s t u d i e s c o n c e n t r a t e d on s t a t i s t i c a l methods i n t h e s e a r c h f o r a a e n e r a l i z e d s e d i m e n t a t i o n u n i t (Duff a n d Walton, 1962; Duff e t a l . , 1967; Doveton, 1971).
However many l o c a l s u c c e s s i o n s
have now been d e s c r i b e d i n d e t a i l and comparisons w i t h modern d e l t a s a r e now p o s s i b l e (Reading, 1978). Secondary ( p o s t - d i a g e n e t i c ) r e d d e n i n g b e n e a t h t h e sub-Permian unconformity S t u d i e s of t h e o r i g i n of r e d beds i n t h e Coal Measures of t h e UK have been confused by t h e p r e s e n c e of s e c o n d a r i l y reddened ( w e a t h e r e d ) s t r a t a b e n e a t h t h e sub-Permian unconformity.
There i s good e v i d e n c e t h a t deep p e n e t r a t i o n o f o x i d i z i n g c o n d i t i o n s
b e n e a t h che a r i d Permian l a n d s u r f a c e was r e s p o n s i b l e f o r t h e r e d d e n i n g of what b o u l d o t h e r w i s e be ' n o r m a l ' g r e y Coal Measures i n a number of a r e a s ( B a i l e y , 1926; T r o t t e r , 1951; Mykura , 1960s). Goodchild (1896, p.71) o r i g i n a l l y s u g g e s t e d t h a t t h e r e d c o l o u r a t i o n of Coal
161 Measures i n S c o t l a n d was due t o d e p o s i t i o n of f e r r i c o x i d e from p e r c o l a t i n g s o l u t i o n s of f e r r o u s b i c a r b o n a t e which h e b e l i e v e d were d e r i v e d from Permo-Triassic l a k e s . The r e d d e n i n g o f t h e C a r b o n i f e r o u s beds of A r r a n was d e s c r i b e d by B a i l e y (1926) who c o n s i d e r e d t h a t t h e r e d c o l o u r i n t h e s e r o c k s had formed i n s i t u by t h e o x i d a t i o n of f e r r o u s i r o n - b e a r i n g m i n e r a l s .
The p r e s e n c e of p a r t i a l l y and c o m p l e t e l y o x i d i z e d
i r o n s t o n e n o d u l e s and reddened p l a n t d e b r i s s u p p o r t s t h i s view.
Simpson and Richey
(1936) d e s c r i b e d t h e r e d d e n i n g which a f f e c t s b o t h t h e Barren Red and P r o d u c t i v e Coal Measures i n t h e Sanquhar and T h o r n h i l l d i s t r i c t s o f S c o t l a n d .
They c o n s i d e r e d
t h e r e d c o l o u r of t h e B a r r e n Red Measures t o b e a n o r i g i n a l f e a t u r e .
Near Sanquhar,
however, Permian l a v a s r e s t d i r e c t l y on P r o d u c t i v e Coal Measures which a r e c o l o u r e d r e d d i s h - p u r p l e and l a c k a c o a l h o r i z o n ( C r e e p i e Coal) which i s p r e s e n t e l s e w h e r e a t t h e same h o r i z o n .
I n t h i s c a s e Simpson and Richey (1936, p.57-58) c o n s i d e r e d t h e
r e d d e n i n g t o be t h e r e s u l t o f d e e p l y p e n e t r a t i n g o x i d a t i o n of i r o n - b e a r i n g sediments d u r i n g t h e Permian t i m e s . The r e d d e n i n g of C a r b o n i f e r o u s s t r a t a i n NW England h a s been d e s c r i b e d by T r o t t e r (1939, 1953, 1954).
T r o t t e r demonstrated t h a t b o t h t h e C a r b o n i f e r o u s Limestone and
t h e Coal Measures a r e reddened where t h e y a r e unconformably o v e r l a i n by PermoT r i a s s i c r o c k s , t h e maximum d e p t h of r e d d e n i n g b e i n g a b o u t 600m.
The reddening
a p p a r e n t l y r e s u l t e d from t h e o x i d a t i o n of s i d e r i t e (Clayband i r o n s t o n e s and n o d u l e s , and p y r i t e ) t o h a e m a t i t e b e f o r e t h e Permo-Triassic r o c k s were d e p o s i t e d . T r o t t e r (1954) r e c o g n i z e d a zone o f complete o x i d a t i o n immediately below t h e prePermo-Triassic l a n d s u r f a c e , u n d e r l a i n e i t h e r by a zone of p a r t i a l o x i d a t i o n o r by a l t e r n a t i n g zones o f p a r t i a l o x i d a t i o n and non-oxidation.
Trotter attributed this
d e e p - s e a t e d o x i d a t i o n t o p e r i o d s of p r o l o n g e d d r o u g h t , d u r i n g t h i c h t h e s u b s t r a t u m i n t h e vadose zone became a e r a t e d , a l t e r n a t i n g w i t h s h o r t p e r i o d s o f r a i n f a l l i n a n a r e a of h i g h r e l i e f .
The maximum d e p t h o f r e d d e n i n g i s t h u s determined by t h e l e v e l
below which t h e pre-Permo-Triassic
w a t e r t a b l e n e v e r sank.
The a l t e r n a t i o n of
p a r t i a l l y o x i d i z e d and u n o x i d i z e d zones might be e x p l a i n e d i f t h e lower l e v e l of t h e w a t e r t a b l e were o v e r l a i n by s e v e r a l perched water t a b l e s . I n NE England t h e C a r b o n i f e r o u s s t r a t a a r e a l s o reddened b e n e a t h t h e Permian unconformity (Anderson and Dunham, 1953).
The d e p t h o f r e d d e n i n g h e r e , however,
does n o t exceed a b o u t 15m and T r o t t e r (1953) s u g g e s t e d t h a t t h i s was due t o t h e f a c t t h a t t h e pre-Permian s u r f a c e i n t h i s a r e a was a low c o a s t a l p l a i n b o r d e r i n g t h e t r a n s g r e s s i v e Z e c h s t e i n s e a and t h e upper l i m i t of permanent s a t u r a t i o n of t h e s t r a t a would b e s i m i l a r t o t h e l e v e l i n t h e a d j a c e n t s e a . The most d e t a i l e d d e s c r i p t i o n o f t h e f e a t u r e s a s s o c i a t e d w i t h s e c o n d a r y reddening
is t h a t o f Mykura (1960a) f o r t h e A y r s h i r e c o a l f i e l d . Here reddened s t r a t a o c c u r at a b r o a d l y uniform d e p t h b e n e a t h t h e Permian unconformity though t h e s t r a t i g r a p h i c a l h o r i z o n v a r i e s from t h e Upper Coal Measures down t o
t h e Upper Limestone Group.
The lower s u r f a c e o f r e d d e n i n g shows c o n s i d e r a b l e i r r e g u l a r i t y i n d i c a t i n g t h a t t h e p e r c o l a t i o n o f w a t e r and a i r was o f t e n g r e a t e r a l o n g f a u l t p l a n e s and p o r o u s , sandy
162 beds ( F i g . 3 . 2 5 ) .
The r e d d e n i n g t o o k p l a c e p r i m a r i l y by i n s i t u o x i d a t i o n b u t t h e r e
i s a l s o e v i d e n c e t h a t h a e m a t i t e p r e c i p i t a t e d from groundwater s o l u t i o n s .
This i s i n
t h e form of t h i n f i l m s of h a e m a t i t e which a r e common i n c o a l s r e p l a c e d by l i m e s t o n e and a l s o h a e m a t i t e cements which o c c u r i n b r e c c i a t e d p o r c e l l a n o u s mudstones.
COAL LIMESTONE SEATEARTH SANDSTONE REDDENED STRATA ARGILLACEOUS STRATA
Diagram i l l u s t r a t i n g t h e p o s s i b l e r e l a t i o n s h i p between f a u l t i n g and t h e Fig.3.25. lower l i m i t o f r e d d e n i n g i n t h e Coal Measures of A y r s h i r e ( a f t e r Mykura, 1960a). Reddening i n t h e A y r s h i r e Coal Measures i s u s u a l l y a s s o c i a t e d w i t h t h e r e p l a c e ment of c o a l by l i m e s t o n e .
The r e p l a c e m e n t p r o c e s s r e s u l t s i n a marked r e d u c t i o n
of t h e seam w i t h t h e o c c a s i o n a l development of 'cone i n c o n e ' s t r u c t u r e . u s u a l l y r e p l a c e d by c a l c i t e f i r s t which may i n t u r n b e d o l o m i t i z e d .
Coal i s
I n some o t h e r
a r e a s such a s t h e S o u t h D e r b y s h i r e c o a l f i e l d s ( P o t t e r y Clay Group) o x i d a t i o n of c o a l seams may r e s u l t i n c a v i t i e s c a u s i n g c o l l a p s e of t h e o v e r l y i n g s t r a t a ( M i t c h e l l , 1944).
L a t e r s t a g e s of t h e replacement i n v o l v e t h e f o r m a t i o n of h a e m a t i t e v e i n l e t s
and f i b r o u s gypsum and c a r b o n a t e s . Mykura (1960a) c o n s i d e r e d t h a t t h e g r e a t d e p t h of o x i d a t i o n below t h e Permian unconformity ( a b o u t 50Om) c a n b e p a r t l y e x p l a i n e d by t h e l o n g p e r i o d o f t i m e between t h e end of Coal Measure d e p o s i t i o n and t h e d e p o s i t i o n o f t h e e a r l i e s t Permian r o c k s w h i l s t t h e a r e a was a l a n d s u r f a c e .
The main r e d d e n i n g and t h e a l t e r a t i o n o f t h e
h i g h e r c o a l s a p p e a r s t o have t a k e n p l a c e b e f o r e d e p o s i t i o n of t h e Permian r o c k s and
163 w h i l s t t h e groundwater was r e l a t i v e l y f r e e o f c a l c i u m and magnesium.
After the
d e p o s i t i o n o f t h e Permian l a v a s w e a t h e r i n g and decomposition may have s u p p l i e d much c a l c i u m and magnesium t o t h e groundwater s o l u t i o n s which were r e s p o n s i b l e f o r t h e a l t e r a t i o n of c o a l s t o c a r b o n a t e s . Not a l l t h e r e d beds i n t h e A y r s h i r e c o a l f i e l d a r e of s e c o n d a r y o r i g i n and a number a p p e a r t o b e a s s o c i a t e d w i t h contemporary e r o s i o n s u r f a c e s .
Mykura (1960a,
p.91) d e s c r i b e s m o t t l e d r e d c l a y s t o n e s o v e r l a i n e r o s i v e l y by c o a r s e p e b b l y sands t o n e s c o n t a i n i n g fragments of reddened and non-reddened C a r b o n i f e r o u s s t r a t a . S i m i l a r l y t h e r e d beds i n t h e Barren Red Measures w i t h t h e i r a s s o c i a t e d ' S p i r o r b i s ' l i m e s t o n e s were p r o b a b l y reddened d u r i n g d i a g e n e s i s and a r e u n l i k e l y t o b e due t o sub-Permian w e a t h e r i n g .
The Coal Measures may t h e r e f o r e c o n t a i n r e d beds produced
d u r i n g two q u i t e d i s t i n c t p h a s e s of r e d d e n i n g a n d i t may b e e x t r e m e l y d i f f i c u l t t o d i s t i n g u i s h t h e two.
Secondary r e d beds produced by t h e w e a t h e r i n g of"norma1'
g r e y s e d i m e n t s a p p e a r t o b e c h a r a c t e r i z e d by t h e f o l l o w i n g f e a t u r e s :
1) a l t e r a t i o n of clayband i r o n s t o n e ( s i d e r i t e s h e e t s and n o d u l e s ) t o h a e m a t i t e and f e r r i c h y d r o x i d e s 2 ) o c c u r r e n c e o f reddened p l a n t d e b r i s and reddened non-marine l a m e l l i b r a n c h s 3 ) r e p l a c e m e n t of c o a l seams by c a r b o n a t e s , gypsum and h a e m a t i t e
4 ) a s p a t i a l r e l a t i o n s h i p o f t h e reddened s t r a t a w i t h a n unconformity o r major fault-planes. Red beds of t h e Umer Coal Measures of t h e UK
Red beds a r e a c h a r a c t e r i s t i c f e a t u r e o f t h e Upper Coal Measures i n t h e UK and a r e t y p i c a l l y i n t e r d i g i t a t e d w i t h non-red s t r a t a ,
They p o s s e s s a number o f d i s t -
i n c t i v e s e d i m e n t o l o g i c a l f e a t u r e s which i n d i c a t e t h a t they must have formed a t a r e l a t i v e l y e a r l y post-depositional
s t a g e ( d u r i n g d i a g e n e s i s ) and n o t by t h e sub-
s e q u e n t w e a t h e r i n g of p r e v i o u s l y l i t h i f i e d d r a b s e d i m e n t s .
Some g e n e r a l i z e d
sequences of Upper Coal Measures showing t h e s t r a t i g r a p h i c a l d i s t r i b u t i o n of r e d beds a r e i n d i c a t e d i n Fig.3.26. The widespread d i s t r i b u t i o n o f r e d beds i n t h e Upper Coal Measures and t h e i r a s s o c i a t i o n w i t h ' S p i r o r b i s ' l i m e s t o n e s i s o f t e n t a k e n t o i n d i c a t e a change t o more a r i d conditions.
T h i s may b e so, b u t s e d i m e n t o l o g i c a l s t u d i e s i n d i c a t e t h a t whereas
d e l t a f r o n t sequences a r e abundant i n t h e Lower and Middle Coal Measures t h e Upper Coal Measures a r e dominated by f l u v i a l sequences of t h e d e l t a p l a i n and i t s a l l u v i a l f r i n g e ( e . g . K e l l i n g , 1968, 1969).
The marine i n c u r s i o n marked by t h e topmost
marine band ( A n t h r a c o c e r a s c a m b r i e n s e ) is o f r e l a t i v e l y l i m i t e d a r e a l e x t e n t ( F i g . 3.27) and by t h i s t i m e r e d bed f a c i e s a s s o c i a t e d w i t h m a r g i n a l f l u v i a l sediments were a l r e a d y well-developed.
The changes which o c c u r r e d d u r i n g t h e Upper Coal
Measures f o l l o w e d a p e r i o d of f o l d i n g and f a u l t i n g i n t h e Coalbrookdale c o a l f i e l d . P r i o r t o t h i s t h e Pennine and South-west P r o v i n c e s had m a i n t a i n e d s e p a r a t e b a s i n s
164
Fig.3.26. Upper Coal Measures c o r r e l a t i o n from Somerset t o L a n c a s h i r e showing t h e s t r a t i g r a p h i c a l d i s t r i b u t i o n of r e d beds ( a f t e r P o o l e , 1977). of d e p o s i t i o n b u t t h e t e c t o n i c a c t i v i t y r e s u l t e d i n w i d e s p r e a d u p l i f t and e r o s i o n which was f o l l o w e d by d e p o s i t i o n o v e r t h e former Wales-Brabant Massif ( F i g s . 3.26, 3.27). A.
South Wales
Red beds o c c u r a t a number o f h o r i z o n s i n t h e Upper Coal Measures o f t h e S o u t h Wales c o a l f i e l d ( F i g . 3 . 2 7 ) and have been d e s c r i b e d from t h e L l y n f i and Rhondda Beds (Downing and S q u i r r e l , 1965; K e l l i n g , 1968, 1969) and a l s o t h e Swansea Beds ( A r c h e r , 1965). The Rhondda Beds were d e p o s i t e d i n a n E-W t r e n d i n g b a s i n which was e n c l o s e d on t h r e e s i d e s b u t open t o t h e w e s t .
The d e p o s i t s c o n s i s t o f sand-dominated a l l u v i u m
a r r a n g e d i n fining-upwards c y c l e s a n d , less commonly, coarsening-upwards c y c l e s
165
FACIES OF ANTHRACOCERAS CAMBRIENSE M.B. AT ACME OF INCURSION
Faunal f a c i e s a t t h e acme of t h e Top ( A n t h r a c o c e r a s cambriense) Marine Fig.3.27. Band i n B r i t a i n showing t h e p r o b a b l e l i m i t s o f t h e i n c u r s i o n and t h e s p r e a d of r e d bed f a c i e s o v e r t h e Wales-Brabant M a s s i f . The c o a l f i e l d s a r e shown i n Black (modified from C a l v e r , 1969). Fig.3.29).
The p r i n c i p a l s t r e a m s were a p p a r e n t l y meandering and r e s u l t e d i n t h e
dominance of l a t e r a l a c c r e t i o n u n i t s i n t h e c e n t r a l p a r t o f t h e b a s i n were subs i d e n c e was moderate t o r a p i d .
I n t h e same a r e a coarsening-upwards sequences which
166
SCALE FEET MERES 0 7 0 A
SOUTH WEST south walr ( W o n W o o d l o d , E m & S h p h 1957) OROVESEND SEAMS
1OOo- -Jod NSEA KJUR
North Staffordrhin(b0don EarpWl)
R e p r e s e n t a t i v e s e c t i o n s o f t h e upper p a r t o f W e s t p h a l i a n C and Fig.3.28. Westphalian D showing t h e c o r r e l a t i o n between t h e South Wales, Pennines and S c o t t i s h p r o v i n c e s ( a f t e r C a l v e r , 1969). b e g i n w i t h d a r k mudstones c o n t a i n i n g f r e s h w a t e r b i v a l v e s and o s t r a c o d e s , resemble v e r y c l o s e l y t h e l a c u s t r i n e d e l t a - f i l l sequences d e s c r i b e d by Coleman (1966). The r e d beds i n t h e Rhondda Beds a r e r e s t r i c t e d t o t h e e a s t e r n p a r t o f t h e b a s i n where t h e sequence i s a t t e n u a t e d b e c a u s e of i t s p r o x i m i t y t o t h e p o s i t i v e m a r g i n a l a r e a ( S q u i r r e l and Downing, 1964).
Here t h e s a n d s t o n e s a r e c o a r s e -
g r a i n e d o r t h o q u a r t z i t e s and j u d g i n g from t h e l o w - v a r i a b i l i t y of c u r r e n t v e c t o r s
167 and t h e n a t u r e of t h e fining-upwards sequences t h e y were d e p o s i t e d i n r e l a t i v e l y s m a l l , p r o b a b l y b r a i d e d c h a n n e l s o f s l i g h t s i n u o s i t y ( K e l l i n g , 1968).
Fig.3.29. Schematic model o f p o s t u l a t e d environments of d e p o s i t i o n i n t h e lower Rhondda Beds of South Wales. The n o r t h - s o u t h dimension i s somewhat f o r e s h o r t e r e d . Diagonal s h a d i n g aver t h e e a s t e r n p a r t o f t h e a r e a i n d i c a t e s t h e approximate d i s t r i b u t i o n o f r e d beds ( a f t e r K e l l i n g , 1968). The c o a r s e c l a s t i c s a r e mainly p a l e g r e y t o w h i t e i n c o l o u r whereas t h e i n t e r v e n i n g mudstones may b e g r e y , g r e e n o r r e d , a f e a t u r e i n common w i t h many o t h e r Coal Measure r e d b e d s .
The r e d mudstones may b e u n i f o r m l y r e d b u t more u s u a l l y
a r e m o t t l e d w i t h p a t c h e s , s t r e a k s and s p o t s of g r e e n .
The c o l o u r b o u n d a r i e s may
b e p a r a l l e l t o bedding p l a n e s b u t o c c a s i o n a l l y t r a n s e c t them.
A very c h a r a c t e r i s t i c
f e a t u r e o f t h e m o t t l e d r e d h o r i z o n s i s t h e i r a s s o c i a t i o n w i t h s e a t e a r t h s and t h e presence of s e a t e a r t h - l i k e textures.
These t e x t u r e s , u s u a l l y r e f e r r e d t o a s
" b a s t a r d s e a t e a r t h s " , c o n t a i n abundant r o o t s c a r s b u t r a r e l y show t h e p r e s e n c e of actual rootlets.
They may c o n t a i n s m a l l s i d e r i t e n o d u l e s which have a c r u d e
168 spherulitic or radial texture. The r e d beds a r e t h u s c l o s e l y a s s o c i a t e d w i t h p a l a e o s o l s , a c h a r a c t e r i s t i c f e a t u r e of r e d beds formed i n d e l t a p l a i n environments ,
The m i n e r a l o g y of t h e d i f f e r e n t
c o l o u r e d mudstones i s v e r y s i m i l a r , b e i n g composed mainly of i l l i t e and k a o l i n i t e w i t h minor amounts o f c h l o r i t e .
There a r e some i m p o r t a n t chemical d i f f e r e n c e s ,
however, w i t h t h e r e d mudstones b e i n g markedly e n r i c h e d i n f e r r i c i r o n (7% Fe 0 2 3 a s opposed t o 2.5% Fe203 i n t h e g r e e n mudstones). T h i s c l o s e a s s o c i a t i o n of r e d beds w i t h p a l a e o s o l s i s even more e v i d e n t h i g h e r up i n t h e sequence i n t h e Swansea Beds and Grovesend Beds ( A r c h e r , 1965) ( F i g . 3 . 2 8 ) . Here t h e r e d beds o c c u r a t t h r e e main s t r a t i g r a p h i c a l h o r i z o n s e a c h forming p a r t of a s i m i l a r c y c l i c u n i t (Fig.3.30).
The r e d beds a r e r e s t r i c t e d t o mudstone u n i t s and
a r e t y p i c a l l y v a r i e g a t e d w i t h reddish-brown and r e d d i s h - p u r p l e m o t t l i n g b e i n g prominent.
In p l a c e s b r i c k r e d s p o t s o c c u r and c a n b e s e e n t o s u r r o u n d ' p y r i t e
crystals.
The r e d beds show abundant r o o t s c a r s and a r e c l o s e l y a s s o c i a t e d w i t h
bastard seatearths.
These t y p i c a l l y have a soapy a p p e a r a n c e w i t h numerous l i s t r i c
s u r f a c e s and o n l y r a r e l y show p r e s e r v e d r o o t l e t s .
Spherulitic s i d e r i t e is again
common i n t h e s e s e a t e a r t h s and a s s o c i a t e d s i d e r i t e i r o n s t o n e s have a brown o r s t r a w c o l o u r e d a p p e a r a n c e whereas t h e y a r e normally g r e y i n t h e s u r r o u n d i n g g r e y s e d i m e n t s . Like o t h e r C o a l Measure examples t h e s e r e d beds c a n n o t have been d e p o s i t e d a s r e d d e t r i t u s b e c a u s e t h i s does n o t e x p l a i n t h e i r m o t t l e d a p p e a r a n c e , t h e r e d h a l o e s around p y r i t e c r y s t a l s , o r t h e o c c u r r e n c e of o c c a s i o n a l reddened p l a n t fragments. S i m i l a r l y , t h e i n t e r c a l a t i o n o f t h e r e d beds w i t h normal g r e y s t r a t a i n d i c a t e s t h a t sub-Permian w e a t h e r i n g i s a l s o a n u n l i k e l y e x p l a n a t i o n .
Archer (1965) p r e f e r r e d
a s i m i l a r mechanism t o t h a t s u g g e s t e d by McBride (1974) f o r t h e D i f u n t a Group and Downing and S q u i r r e l (1965) f o r t h e L l y n f i and Rhondda Beds.
T h i s mechanism i n v o l v e s
t h e lowering of t h e w a t e r t a b l e and t h e development of w e l l - d r a i n e d , conditions.
oxygenated
T h i s would e n a b l e t h e d e s t r u c t i o n of p l a n t m a t e r i a l t h u s p r e v e n t i n g
t h e p r e s e r v a t i o n o f c o a l seams.
Reddening c o u l d t a k e p l a c e by t h e o x i d a t i o n of
p y r i t e and a l s o by t h e p r e c i p i t a t i o n o f g o e t h i t e and h a e m a t i t e formed from i r o n r e l e a s e d from c l a y m i n e r a l s .
B.
The Blackband and E t r u r i a Marl Groups o f North S t a f f o r d s h i r e
In t h e North S t a f f o r d s h i r e c o a l f i e l d (Fig.3.27) t h e Upper Coal Measurea a r e a b o u t 1500m t h i c k and i n c l u d e two m a j o r r e d bed f o r m a t i o n s , t h e E t r u r i a Marl Group and t h e K e e l e Group,
These a r e s e p a r a t e d by o v e r 120m of g r e y mudstones and sand-
s t o n e s w i t h t h i n c o a l s (Newcastle Group) and i t i s c l e a r t h a t t h e r e d beds a r e a n i n t r i n s i c f a c i e s and n o t due t o s e c o n d a r y w e a t h e r i n g b e n e a t h t h e P e m o - T r i a s s i c unconformity. Red beds f i r s t a p p e a r i n t h e Upper Coal Measures o f N o r t h S t a f f o r d s h i r e in t h e Blackband Group.
T h i s i s a sequence o f p r e d o m i n a n t l y g r e y non-marine s t r a t a
169
Fig.3.30. Comparative s e c t i o n s of t h e L l y n f i and Rhondda Beds i n t h e n o r t h e a s t p a r t o f t h e South Wales c o a l f i e l d showing t h e d i s t r i b u t i o n o f r e d beds ( a f t e r Downing and S q u i r r e l , 1965). c o n s i s t i n g of mudstones, s i l t s t o n e s and t h i n s a n d s t o n e s sometimes a r r a n g e d i n coarsening-upwards u n i t s .
Near t h e middle p a r t o f t h e Group i s a sequence of t h i c k
c o a l s ( F i g . 3 . 3 1 ) and above t h i s a r e a number of t h i n n e r c o a l s which a r e a s s o c i a t e d with s i d e r i t i c ironstones.
These i r o n s t o n e s , known a s ' b l a c k b a n d i r o n s t o n e s '
g e n e r a l l y o c c u r immediately above t h e c o a l seams (Fig.3.31) p e l o i d a l s i d e r i t e and carbonaceous m a t t e r .
and c o n s i s t of laminated
S i d e r i t i z e d freshwater bivalves a r e
170 p r e s e n t a t some h o r i z o n s and t h e i r o n s t o n e may p a s s l a t e r a l l y i n t o l i m e s t o n e s w i t h mussels of t h e Anthraconauta p h i l l i p s i i g r o u p , o s t r a c o d e s and f i s h .
The blackband
i r o n s t o n e s c l e a r l y r e p r e s e n t widespread l a c u s t r i n e c o n d i t i o n s and t h e s u c c e e d i n g coarsening-upward u n i t s most l i k e l y i n d i c a t e p r o g r a d a t i o n a l f i l l i n g of t h e l a k e s , p o s s i b l y by l a c u s t r i n e d e l t a s .
N f W C b S I ' i GIOUP
UPPER COAL
I
L ~ '
1
/
MEASURES ILbCIWNO GROUP
MIDDLE
f
COAL MEASURES
*
7 I 1 BbNIURY M1)INl UND
LOWER
COAL MEASURES
Fig.3.31. G e n e r a l i z e d sequence o f t h e Coal Measures o f t h e North S t a f f o r d s h i r e c o a l f i e l d w i t h a d e t a i l e d s u c c e s s i o n o f t h e Blackband Group.
!
171 The v e r t i c a l and l a t e r a l d i s t r i b u t i o n of r e d beds i n t h e sequence between t h e Bassey Mine and Blackband c o a l s i s shown i n Fig.3.32.
The r e d beds a r e b e s t
developed i n t h e s o u t h e r n a n d s o u t h - w e s t e r n p a r t o f t h e c o a l b a s i n where t h e succ e s s i o n i s t h i n n e s t and become p r o g r e s s i v e l y t h i n n e r towards t h e c e n t r e of t h e b a s i n so t h a t i n d i v i d u a l r e d bed u n i t s a r e wedge-shaped w i t h a h o r i z o n t a l upper s u r f a c e and a n o b l i q u e lower s u r f a c e ( F i g . 3 . 3 2 ) . These f e a t u r e s s u g g e s t t h a t groundwater d r a i n a g e c o n d i t i o n s a t t h e t i m e o f d e p o s i t i o n e x e r c i s e d a n i m p o r t a n t c o n t r o l on t h e d i s t r i b u t i o n of r e d beds.
Marginal
a r e a s may have been of h i g h e r r e l i e f and t h u s b e t t e r d r a i n e d t h a n a r e a s i n t h e c e n t r e of t h e b a s i n .
Better d r a i n a g e i n t h e m a r g i n a l a r e a s would e n a b l e t h e o x i d a t i o n of
o r g a n i c m a t t e r and t h e development of o x i d i z i n g c o n d i t i o n s w i t h i n t h e d e p o s i t e d sediments so t h a t r e d beds c o u l d e v e n t u a l l y develop.
In t h e c e n t r e of t h e b a s i s ,
on t h e o t h e r hand, poor d r a i n a g e c o u l d r e s u l t i n t h e p r e s e r v a t i o n o f o r g a n i c m a t t e r and t h e development of r e d u c i n g c o n d i t i o n s s o t h a t r e d beds were u n a b l e t o develop. The geometry of t h e r e d beds can b e e x p l a i n e d by v e r t i c a l f l u c t u a t i o n s i n t h e l e v e l of t h e w a t e r t a b l e which may have been a s s o c i a t e d w i t h t e c t o n i c a c t i v i t y .
Relative
u p l i f t o f t h e m a r g i n a l a r e a s r e s u l t e d i n t h e w i d e s p r e a d development o f r e d bed c o n d i t i o n s ( F i g . 3 . 3 3 ) w h i l s t r e l a t i v e s u b s i d e n c e r e s u l t e d i n t h e r e c e s s i o n of r e d bed c o n d i t i o n s and w i d e s p r e a d development o f p e a t swamps and s u b s e q u e n t l y l a c u s t r i n e conditions. The Blackband Group p a s s e s upwards i n t o t h e E t r u r i a Marl Group a major r e d bed f o r m a t i o n a b o u t 30Om t h i c k .
The E t r u r i a Marl i s s t r o n g l y d i a c h r o n o u s , i t s b a s e i s
h a r d t o d e f i n e and t h e r e i s c o n s i d e r a b l e t h i c k n e s s v a r i a t i o n , p a r t l y due t o t h e 'Symon Unconformity' which o c c u r s between t h e E t r u r i a Marl and Halesowen Beds i n t h e South S t a f f o r d s h i r e C o a l f i e l d ( F i g . 3 . 2 6 ) . The E t r u r i a Marl shows many of t h e c h a r a c t e r i s t i c f e a t u r e s o f r e d beds formed I n errmciatfon with coal-bearing s t r a t a .
Much of t h e sequence i s dominated by r e d
mudatone and sandy mudetone which commonly show p a l a e o s o l f e a t u r e s .
These i n c l u d e
s e a t e a r t h s w i t h r o o t s c a r s i n r e d and p u r p l e mudstones, d i s t i n c t c o l o u r m o t t l i n g o f r e d , p u r p l e , y e l l o w , and g r e e n ( c f . McBride, 1974) and a n a b s e n c e o f bedding. t h e r e are occasional t h i n c o a l s with t h e u s u a l grey-coloured s e a t e a r t h s .
Also
Towards
t h e s o u t h , and i n t h e South S t a f f o r d s h i r e c o a l f i e l d , t h e E t r u r i a Marl c o n t a i n s s a n d s t o n e u n i t s ( e s p l e y s ) which are river c h a n n e l d e p o s i t s . based and u s u a l l y p o o r l y s o r t e d and t e x t u r a l l y immature.
These a r e e r o s i v e l y -
I n t h e North S t a f f o r d s h i r e
a r e a t h e s e channel s a n d s t o n e s c o n t a i n l a t e r a l a c c r e t i o n u n i t s and were e v i d e n t l y d e p o s i t e d by meandering s t r e a m s .
F u r t h e r s o u t h ( F i g . 3 . 2 7 ) t h e c h a n n e l sands a r e
t h i c k e r and more a b u n d a n t , and l o c a l l y d e b r i s - f l o w u n i t s o c c u r i n d i c a t i n g t h e p r e s e n c e of m a r g i n a l a l l u v i a l f a n s .
172
KEY L.B.M. 1st. Upper Bassey Mine Limestone L.B.M. Lst. Lower Borrey Mine Limestone Limestone L--L
-
J
b
5
L a
€3’
Coal Red Beds Blackband Ironstone Non-marine Lamellibranchs Ortracoda Spirorbis Ettheria Fish
W
-
0
1
2
3
4
Fig.3.32. Diagrammatic c r o s s - s e c t i o n i l l u s t r a t i n g t h e inter-wedging of r e d beds i n t o t h e North S t a f f o r d s h i r e Blackband Group s u c c e s s i o n .
173 The o r i g i n o f t h e r e d beds i n t h e E t r u r i a Marl i s c l o s e l y a s s o c i a t e d w i t h s o i l forming p r o c e s s e s and a p p e a r s t o have been s i m i l a r t o t h e D i f u n t a Group d e s c r i b e d by McBride (1974).
There i s no e v i d e n c e t o i n d i c a t e t h a t upland s o i l s on t h e Wales-
Brabant Massif were e r o d e d and s u p p l i e d r e d d e t r i t u s t o t h e E t r u r i a Marl a s s u g g e s t e d by Hoare (1960) and Earp ( 1 9 6 1 ) .
The p r i n c i p a l o b j e c t i o n t o t h i s h y p o t h e s i s i s t h a t
s o i l s a r e w e l l developed w i t h i n t h e E t r u r i a Marl and p e d o g e n e s i s and reddening might be expected t o be c l o s e l y r e l a t e d .
L14CWRlNE CONDITIONS RED BEDS RESTRICTED
I
I
SUBSIDENCE
J.
\
@
I I
LACUSTRINE REGRESSION ZONE OF RED BED FORMATION MIGRATES TOWARDS BASIN CENTRE ORMING CONDlTl
@
LACUSTRINE TRANSORESSION
CONTINUED FORMATION OF RED BEDS DURING DIAGENESIS Fig.3.33. Schematic r e p r e s e n t a t i o n of t h e c o n d i t i o n s which r e s u l t e a i n m e inLeLd i g i t a t i o n of r e d and g r e y beds i n t h e Blackband Group of t h e North S t a f f o r d s h i r e coa 1f i e l d .
174 The c h e m i s t r y and m i n e r a l o g y of t h e E t r u r i a Marl s u g g e s t s t h a t p a l a e o s o l s may b e of b o t h s e a t e a r t h and f e r r a l l i t i c (Dury, 1969) t y p e s ( F i g . 3 . 3 4 ) .
Red mudstones
of t h e E t r u r i a Marl a r e f r e q u e n t l y r i c h i n k a o l i n i t e which i s c l o s e l y a s s o c i a t e d with i r o n oxides.
K a o l i n i t e s a r e notable f o r o f t e n containing adsorbed i r o n
(Weaver and P o l l a r d , 1973) and i t i s q u i t e p o s s i b l e t h a t p i g m e n t a t i o n o f t h e E t r u r i a Marl was c l o s e l y a s s o c i a t e d w i t h t h e f o r m a t i o n of i r o n o x i d e s from d i s o r d e r e d k a o l i n i t e s which underwent c r y s t a l l i n i t y changes d u r i n g d i a g e n e s i s ( c f . S l a n s k a , 1976).
The mudstones o f t h e E t r u r i a Marl may t h u s have been reddened a t
a relatively early post-depositional stage.
T h i s i s s u p p o r t e d by p a l a e o m a g n e t i c
e v i d e n c e which i n d i c a t e s t h a t t h e mudstones were magnetized d u r i n g Upper Carboniferous times (Fig.3.35). colour.
The s a n d s t o n e s o f t h e E t r u r i a Marl may b e r e d o r d r a b i n
The d r a b s a n d s t o n e s c o n t a i n a u t h i g e n i c p y r i t e and a p p a r e n t l y were l i t h i f i e d
under r e d u c i n g c o n d i t i o n s i n o r g a n i c - r i c h groundwater.
The r e d s a n d s t o n e s , however,
show e v i d e n c e of prolonged d i a g e n e s i s under o x i d i z i n g c o n d i t i o n s .
They a r e t e x t u r a -
l l y immature a l t h o u g h much c l a y m a t r i x may have been produced by t h e i n s i t u breakdown o f v o l c a n i c rock fragments ( c f . Walker e t a l . ,
1978) and t h e r e i s abundant
a u t h i g e n i c q u a r t z , c a l c i t e , and h a e m a t i t e i n k e e p i n g w i t h many o t h e r a n c i e n t sandy r e d beds.
Palaeomagnetic r e s u l t s from s a n d s t o n e s o f t h e E t r u r i a Marl show s t e e p e r
i n c l i n a t i o n s c o n s i s t e n t w i t h l a t e r d i a g e n e s i s and more p r o l o n g e d reddening.
Si%
0
SANDSTONES MUDSTONES
Al 0 Fe203 Fig.3.34. $ernary diagram showing t h e r e l a t i v e p r o p o r t i o n s of SiOg, A1203, and Fe 0 i n s a n d s t o n e s and mudstones o f t h e E t r u r i a Marl. 2 3
175
0
SANDSTONE
A
MUDSTONE LOCAL FIELD DIRECTION
*
O
*
+
OO O
Fig.3.35. Palaeomagnetic d i r e c t i o n s ( a f t e r t h e r m a l c l e a n i n g ) from s a n d s t o n e s and mudstones of t h e E t r u r i a Marl. The mudstones i n d i c a t e e q u a t o r i a l p a l a e o l a t i t u d e s t y p i c a l of t h e Upper C a r b o n i f e r o u s o f t h e UK which s u g g e s t s r e l a t i v e l y e a r l y postd e p o s i t i o n a l reddening. The s t e e p e r i n c l i n a t i o n s of t h e s a n d s t o n e s s u g g e s t much l a t e r , p o s s i b l y Permian, r e d d e n i n g and a l o n g e r complex d i a g e n e s i s . The p r o j e c t i o n is s t e r e o g r a p h i c , open symbols = upward v e c t o r s , c l o s e d symbols = downward v e c t o r s , circumference = horizontal. Upper C a r b o n i f e r o u s r e d beds a t J o g g i n s , Nova S c o t i a The C a r b o n i f e r o u s sequence a t J o g g i n s , Nova S c o t i a has been d e s c r i b e d by Duff and Walton (1973).
T h i s i s a d e l t a p l a i n sequence which prograded n o r t h - e a s t w a r d s
and was d e r i v e d from igneous and metamorphic r o c k s l i k e t h o s e s e e n t o t h e s o u t h i n t h e Cobequid Arch (Way, 1969).
The s e d i m e n t s s t u d i e d by Duff and Walton (1973)
form t h e b a s e of t h e Middle f i n e f a c i e s of Hacquebard and Donaldson (1964) and a r e considered t o be Westphalian B t o Westphalian C i n age.
A wide v a r i e t y of f a c i e s
a r e r e p r e s e n t e d i n c l u d i n g g r e y mudstone and s h a l e , c h a n n e l and s h e e t sandstone, r e d
176 and red-green m o t t l e d mudstone, non-marine l i m e s t o n e , s e a t e a r t h and c o a l . Walton (1973) r e c o g n i z e d t h r e e d i v i s i o n s :
Duff and
Lower, Middle and Upper beds and n o t e d
d i f f e r e n t f a c i e s arrangements between them ( F i g . 3 . 3 6 ) .
1
4
...... ..:... 2 .,L.
-. I . .
,
Uct1
Fig.3.36. 1973).
Sedimentary sequences a t J o g g i n s , Nova S c o t i a ( a f t e r Duff and Walton,
The s a n d s t o n e s i n t h e J o g g i n s s e c t i o n t a k e t h e form o f l e n t i c u l a r c h a n n e l sands t o n e s (mean t h i c k n e s s 4m) and s h e e t s a n d s t o n e s (mean t h i c k n e s s 2m).
The l a t t e r
a r e most commonly m u l t i s t o r e y u n i t s i n which l e a v e s o f s a n d s t o n e some 0.50m t h i c k a r e s e p a r a t e d by t h i n mudstone p a r t i n g s a l t h o u g h t h e s e c a n b e s e e n t o t h i c k e n towards t h e c h a n n e l s a n d s t o n e s .
An i m p o r t a n t f e a t u r e of t h e s a n d s t o n e s i s t h a t
t h e y a r e p e n e t r a t e d by e r e c t t r e e stumps which c o n t a i n entombed amphibians and r e p t i l e s ( C a r r o l l , 1967). O t h e r p a l a e o n t o l o g i c a l e v i d e n c e r e g a r d i n g t h e c o n d i t i o n s of d e p o s i t i o n comes from t h e o c c u r r e n c e o f t h i n b i v a l v e and o s t r a c o d e l i m e s t o n e s which a r e o f t e n i n t e r bedded w i t h c o a l s .
The most abundant o s t r a c o d e s a r e C a r b o n i t a a l t i l i s ( J o n e s and
K i r b y ) and C. c f . s a l t e r i a n a (Jones and K i r b y ) and t h e b i v a l v e s i n c l u d e ( C u r v i r i m u l a s p . a n d c f . N a i a d i t e s e l o n g a t a (Dawson) and N . P o l l a r d and M.A.
longus (Dawson) a s r e p o r t e d by J.E.
C a l v e r i n Duff and Walton (1973).
There have been many d i s c u s s i o n s
r e g a r d i n g t h e p a l a e o e c o l o g i c a l s i g n i f i c a n c e of o s t r a c o d e s and b i v a l v e s i n c o a l b e a r i n g sequences p a r t i c u l a r l y w i t h r e s p e c t t o s a l i n i t y v a r i a t i o n s and no c l e a r
177 p i c t u r e seem t o have emerged ( C a l v e r , 1968; P o l l a r d , 1969; E a g e r , 1970).
Duff and
Walton (1973) concluded t h a t t h e o c c u r r e n c e o f Curvirimula and N a i a d i t e s i n a s s o c i a t i o n w i t h C a r b o n i t a was a n i n d i c a t i o n o f b r a c k i s h t o f r e s h w a t e r c o n d i t i o n s w i t h t h e p o s s i b i l i t y t h a t t h e b i v a l v e - r i c h l i m e s t o n e s were more near-marine and t h e o s t r a c o d e - r i c h l i m e s t o n e s were more n e a r - f r e s h w a t e r . Duff and Walton (1973) compared t h e Upper and Lower beds o f t h e J o g g i n s s e c t i o n using standard transion matrix techniques. diagrams a r e shown i n Fig.3.37. t h e Lower and Upper beds.
Representative facies relationship
These i l l u s t r a t e some i m p o r t a n t d i f f e r e n c e s between
The r e d beds which show t h e f a m i l i a r a s s o c i a t i o n w i t h
s e a t e a r t h s o c c u r o n l y i n t h e Lower b e d s ; t h e s e a r e r i c h e r i n l i m e s t o n e and f o s s i l i f e r o u s mudstones.
They a r e c o m p l e t e l y a b s e n t from t h e Upper beds where
l i m e s t o n e s a r e less abundant and t h e s a n d s t o n e s much more abundant ( s a n d s t o n e / s h a l e r a t i o ' = 0.63 a s opposed t o 0.23 i n t h e Lower b e d s .
Duff and Walton (1973) a s s e s s e d
t h e o v e r a l l environment o f d e p o s i t i o n a s t h a t o f a d e l t a p l a i n w i t h back-swamps and s h a l l o w l a k e s and major d i s t r i b u t a r y c h a n n e l s .
D e p o s i t i o n a l s i t e s were p r o b a b l y
v a r i e d and i n c l u d e d t h e c h a n n e l and i t s m a r g i n s , c r e v a s s e s p l a y s , l a c u s t r i n e d e l t a s and t h e s u r r o u n d i n g f l o o d b a s i n . The d i f f e r e n c e s between t h e Upper and Lower beds p r o v i d e a n i m p o r t a n t c l u e t o t h e o r i g i n of t h e r e d beds.
The upper beds c o n t a i n i n g t h e t h i c k e s t c o a l s , abundant
s a n d s t o n e s and r e l a t i v e l a c k of non-marine f a u n a s s u g g e s t s a s i t e low on t h e d e l t a p l a i n i n which c o n t i n u o u s swamp c o n d i t i o n s a l l o w e d t h e a c c u m u l a t i o n of t h i c k sequenc e s of p e a t ,
The Lower beds on t h e o t h e r hand were c o n s i d e r e d by Duff and Walton
(1973) t o have accumulated a t a h i g h e r p o s i t i o n on t h e d e l t a p l a i n which e n a b l e d non-marine f a u n a s t o f l o i r i s h and t h e c r e a t i o n of b e t t e r d r a i n a g e c o n d i t i o n s i n which o x i d i z i n g c o n d i t i o n s and r e d bed f o r m a t i o n may have p r e v a i l e d on t h e i n t e r v e n i n g mud f l a t s which would have been above t h e w a t e r t a b l e . T h i s e x p l a n a t i o n i s c o n s i s t e n t w i t h t h a t p u t forward f o r o t h e r d e l t a p l a i n r e d beds.
S p e c i f i c r e d d e n i n g mechanisms were p r o b a b l y complex b u t t h e d e h y d r a t i o n of
d e t r i t a l c l a y - o x i d e complexes and o x i d a t i o n o f e a r l i e r formed p y r i t e a r e c o n s i d e r e d t o have been i m p o r t a n t .
4t
Shalo
t\ -COAL
I ' /
Limoston.
Fig. 3.37. F a c i e s r e l a t i o n s h i p diagrams f o r t h e C a r b o n i f e r o u s sediments a t J o g g i n s , Nova S c o t i a ( a f t e r Duff and Walton, 1973).
178 CONCLUSIONS AND FURTHER M A D I N G Although t h e r e a r e no modern examples o f d e l t a p l a i n r e d beds i t seems l i k e l y t h a t they formed i n a v a r i e t y o f c l i m a t i c s e t t i n g s .
They a r e most c h a r a c t e r i s t i c
o f a n c i e n t river-dominated d e l t a s which formed i n m o i s t t r o p i c a l c l i m a t e s and t h u s c o n t r a s t markedly w i t h d e s e r t r e d beds.
The mechanism of r e d d e n i n g a l s o c o n t r a s t s
w i t h d e s e r t r e d beds and p e d o g e n e s i i r a t h e r t h a n i n t r a s t r a t a l a l t e r a t i o n may be t h e dominant p r o c e s s . The f o l l o w i n g a r e recommended f o r f u r t h e r r e a d i n g : B r o u s s a r d , M.L.
( E d i t o r ) , 1975. D e l t a s
-
Models f o r E x p l o r a t i o n , Houston Geol.
SOC., Houston, 555 pp. Mohr, E . C . J .
and Van Baren, F.A.,
New York, 490 pp.
1954. T r o p i c a l S o i l s . I n t e r s c i e n c e Publ.,
179
CHAPTER 4 ALLWIAL RED BEDS INTRODUCTION A l l u v i a l s e d i m e n t s c o n s i s t of a g r e a t v a r i e t y o f p e b b l y , sandy and muddy d e p o s i t s . They accumulate i n a v a r i e t y of f l u v i a l d e p o s i t i o n a l environments under a l l c l i m a t i c Present-day a l l u v i a l sediments a r e not red, nor i s a l l a n c i e n t alluvium.
conditions.
A n c i e n t a l l u v i a l s e d i m e n t s a r e , however, common i n t h e s t r a t i g r a p h i c r e c o r d and comprise t h e most i m p o r t a n t group of a n c i e n t c o n t i n e n t a l r e d beds.
They a r e t h e most
e x t e n s i v e l y s t u d i e d r e d beds b u t a r e s t i l l o n l y p o o r l y u n d e r s t o o d and we owe much of o u r p r e s e n t knowledge t o T.R. Walker and c o l l e a g u e s who have made a n i n t e n s i v e s t u d y of Cenozoic d e s e r t a l l u v i u m i n t h e s o u t h w e s t e r n USA and n o r t h w e s t e r n Mexico.
Walker
(1967a) showed t h a t t h i s d e s e r t a l l u v i u m was reddened p o s t - d e p o s i t i o n a l l y by t h e i n t r a s t r a t a l a l t e r a t i o n of d e t r i t a l i r o n s i l i c a t e m i n e r a l s .
I t s h o u l d be s t r e s s e d
t h a t t h i s a l l u v i u m i s of f i r s t - c y c l e o r i g i n and i s r i c h i n d e t r i t a l ferromagnesian silicates,
The same c o n d i t i o n s do n o t a p p l y t o a l l a n c i e n t a l l u v i a l r e d beds.
When a p p l i e d t o a n c i e n t a l l u v i a l r e d bed s e q u e n c e s t h e d i a g e n e t i c model of Walker c a n n o t , i n i t s e l f , a d e q u a t e l y a c c o u n t f o r some o f t h e observed v a r i a t i o n s .
For
example, d e p o s i t i o n a l environment i s c l e a r l y a n i m p o r t a n t c o n t r o l l i n g f a c t o r because d i f f e r e n t environments show d i f f e r e n t c o l o u r d i s t r i b u t i o n p a t t e r n s .
A l l u v i a l fans
and c o a r s e g r a i n e d a l l u v i a l sequences f r e q u e n t l y show u n i f o r m l y r e d c o l o u r a t i o n which
i s c o n s i s t e n t w i t h a d i a g e n e t i c o r i g i n l i k e t h a t d e s c r i b e d by Walker.
Fine grained
a l l u v i a l s e q u e n c e s , l i k e t h o s e d e p o s i t e d by meandering r i v e r s , a r e g e n e r a l l y q u i t e different.
Here t h e a l l u v i u m i s d i f f e r e n t i a t e d i n t o c o a r s e ( c h a n n e l ) and f i n e ( f l o o d -
p l a i n ) members and t h e r e a r e i m p o r t a n t c o l o u r d i f f e r e n c e s between t h e two, t h e former o f t e n b e i n g d r a b and t h e l a t t e r r e d .
The s u g g e s t i o n h e r e i s t h a t d e p o s i t i o n a l and
e a r l y p o s t - d e p o s i t i o n a l p r o c e s s e s have been e f f e c t i v e i n producing t h e red- non-red differentiation.
The i m p l i c a t i o n i s t h a t t h e a l l u v i u m o r i g i n a l l y c o n t a i n e d sub-
s t a n t i a l amounts of f i n e g r a i n e d i r o n o x i d e o r i r o n h y d r o x i d e i n a s s o c i a t i o n w i t h t h e c l a y f r a c t i o n of t h e a l l u v i u m .
T h i s was c o n c e n t r a t e d i n t h e f l o o d p l a i n e n v i r -
onment and became s t a b i l i z e d under d r y , oxygenated c o n d i t i o n s , e v e n t u a l l y producing a red colour.
I n t h e s a t u r a t e d channel e n v i r o n m e n t , t h e s e i r o n o x i d e s and hydroxides
become u n s t a b l e , e s p e c i a l l y under r e d u c i n g c o n d i t i o n s , and were e v e n t u a l l y removed i n solution,
Van Houten (1972) h a s shown t h e importance o f d e t r i t a l f e r r i c hyd-
r o x i d e s i n Recent t r o p i c a l a l l u v i u m .
The s o u r c e of t h e s e hydroxides i s n o t p r e c i s e l y
known and t h e i m p o r t a n t q u e s t i o n i s whether o r n o t they have been d e r i v e d from upland r e d s o i l s . I n t h i s c h a p t e r t h e d e p o s i t i o n of p r e s e n t - d a y a l l u v i u m i s reviewed and t h e t r a n s p o r t of i r o n o x i d e s and h y d r o x i d e s i n t h e s e sediments i s d e s c r i b e d .
The
180 various post-depositional processes including the 'ageing' of f e r r i c hydroxides and t h e i n t r a s t r a t a l s o l u t i o n of i r o n s i l i c a t e s a r e a l s o d e s c r i b e d and t h e i r importance i n a n c i e n t r e d bed sequences a s s e s s e d .
Gross g e n e r a l i z a t i o n s about
t h e o r i g i n of a l l u v i a l r e d b e d s ' a r e n o t p o s s i b l e .
I n d i v i d u a l environments i n a
p a r t i c u l a r r e d bed sequence may have been reddened by d i f f e r e n t p r o c e s s e s and d i f f e r e n c e s i n m i n e r a l o g i c a l c o m p o s i t i o n and p o s t - d e p o s i t i o n a l h i s t o r y may e x e r t a s t r o n g c o n t r o l l i n g i n f l u e n c e on d i f f e r e n t r e d bed s e q u e n c e s .
RIVER CHANNELS When s e e n i n p l a n view r i v e r c h a n n e l s may t a k e on a v a r i e t y of forms which depend upon t h e flow s t a g e o f t h e r i v e r ,
The r i v e r c h a n n e l p a t t e r n i s c o n t r o l l e d
by a number of v a r i a b l e s i n c l u d i n g t h e sediment l o a d and i t s c h a r a c t e r i s t i c s , and a l s o t h e amount and n a t u r e of t h e d i s c h a r g e .
A f t e r t h e work o f Leopold and Wolman
(1957) t h r e e b a s i c c h a n n e l p a t t e r n s can be r e c o g n i z e d :
s t r a i g h t , meandering and
b r a i d e d which a r e d i s t i n g u i s h e d on t h e b a s i s of s i n u o s i t y ( r a t i o of Thalweg l e n g t h t o V a l l e y l e n g t h ) and t h e p r e s e n c e of a l l u v i a l i s l a n d s ,
S t r a i g h t c h a n n e l s have a
s i n u o s i t y of a b o u t 1.0 and meandering c h a n n e l s a s i n u o s i t y o f g r e a t e r than 1 . 5 . A b r a i d e d channel i s one which flows i n two o r more anastomosing c h a n n e l s around a l l u v i a l islands (Fig.4.1).
N a t u r a l channel p a t t e r n s i n t e r g r a d e w i t h e a c h o t h e r
and b r a i d e d and meandering c h a n n e l s r e p r e s e n t t h e end p o i n t s o f a c o n t i n u o u s spectrum. Even t h e same channel may show changing p a t t e r n s a l o n g i t s l e n g t h ; t h e same channel a p p e a r i n g t o meander a t b a n k f u l s t a g e s and b r a i d e d a t low f l o w s t a g e s ( R u s s e l l , 1954).
Because of t h i s continuum of c h a n n e l p a r t e r n s S c h u m (1963) proposed a more
d e t a i l e d c l a s s i f i c a t i o n proposing f i v e c l a s s e s :
straight, transitional, regular,
i r r e g u l a r and t o r t u o u s c h a n n e l p a t t e r n s . The f a c t o r s c o n t r o l l i n g c h a n n e l p a t t e r n a r e complex a l t h o u g h s l o p e and d i s c h a r g e a r e two of t h e most i m p o r t a n t .
Leopold and Wolman (1957) showed t h a t i n t h e r i v e r s
they s t u d i e d b r a i d e d and meandering c h a n n e l s c o u l d be d i s t i n g u i s h e d by t h e r e l a t i o n ship: -0.44
S = 0.06Qb
(4.1)
where S = s l o p e and Qb= b a n k f u l d i s c h a r g e .
Thus f o r a g i v e n d i s c h a r g e meanders
would be e x p e c t e d t o form on t h e s m a l l e r s l o p e s , o r a l t e r n a t i v e l y a t t h e same s l o p e a b r a i d e d c h a n n e l c o u l d be e x p e c t e d t o have a h i g h e r d i s c h a r g e t h a n a meandering channel.
T h i s i s w e l l i l l u s t r a t e d on t h e R i v e r Rhine where S c h a f e r (1973) h a s shown
t h a t t h e upper r e a c h e s a r e b r a i d e d ( s l o p e = 0.97%) and t h e lower r e a c h e s meandering (slope = 0.025%).
The r e l a t i o n s h i p , however, does n o t h o l d f o r a l l r i v e r s and o t h e r
f a c t o r s have a n i m p o r t a n t r o l e . discharge.
Amongst t h e s e a r e bank s t a b i l i t y and s e d i m e n t
Some r i v e r s have a meandering c h a n n e l p a t t e r n i n a r e a s where t h e banks
a r e s t a b i l i z e d by v e g e t a t i o n o r f o r e s t a t i o n b u t t a k e on a b r a i d e d p a t t e r n i n a r e a s where t h e banks a r e more r e a d i l y eroded.
Increased sediment load r e s u l t s i n a
181
bank L i n e d
A
B
C
b
b'
75 0 7.5 50 75 100
Fig.4.1.
The c l a s s i f i c a t i o n o f r i v e r c h a n n e l s a c c o r d i n g t o Leopold and Wolman (1957)
tendency towards b r a i d i n g and some l a r g e r i v e r s w i t h s m a l l s l o p e s have changed from a meandering t o a b r a i d e d p a t t e r n because of an i n c r e a s e i n sediment l o a d . example of t h i s i s t h e Brahmaputra R i v e r (Coleman, 1969).
A good
I t follows t h a t the
p r o p o r t i o n of bed l o a d t o a v a i l a b l e d i s c h a r g e may b e a n i m p o r t a n t cause of b r a i d i n g i n r i v e r c h a n n e l s (Morisawa, 1 9 6 8 ) . The c r o s s - s e c t i o n a l form of a r i v e r c h a n n e l depends upon a number of f a c t o r s i n c l u d i n g d i s c h a r g e and t h e n a t u r e and amount of sediment l o a d . d i s t i n g u i s h e d between bed-load,
Schumm (1968)
mixed-load and suspended-load s t r e a m s .
Fig.4.2
i l l u s t r a t e s t h e c h a n n e l morphology of t h e s e d i f f e r e n t t y p e s o f s t r e a m and t h e n a t u r e of t h e i r d e p o s i t s .
Meandering c h a n n e l s t e n d t o develop i n suspended l o a d s t r e a m s
because t h e y produce h i g h l y c o h e s i v e banks which a r e r e s i s t a n t t o e r o s i o n .
Low-
s i n u o s i t y c h a n n e l s a r e more c h a r a c t e r i s t i c of bed l o a d s t r e a m s because t h e s e have bank m a t e r i a l s which a r e g e n e r a l l y l e s s r e s i s t a n t t o e r o s i o n .
Present-day a l l u v i a l
sediments c a n be c o n v e n i e n t l y d i v i d e d i n t o f o u r major groups on t h e b a s i s of channel p a t t e r n and t y p e o f a l l u v i u m :
a l l u v i a l f a n s , pebbly b r a i d e d r i v e r s , sandy b r a i d e d
182 r i v e r s , and meandering s t r e a m s .
Each of t h e s e has a n c i e n t a n a l o g u e s i n t h e
s t r a t i g r a p h i c a l r e c o r d and forms major r e d bed s e q u e n c e s .
F i g . 4 . 2 . Scheme i l l u s t r a t i n g t y p e s of s i n g l e - c h a n n e l and m u l t i p l e - c h a n n e l s y s t e m s , a s s o c i a t i o n s , and t h e i r c h a r a c t e r i s t i c s . A . Channel morphology B. A l l u v i a l d e p o s i t s ( m o d i f i e d a f t e r Schumm, 1968).
ALLUVIAL FANS A l l u v i a l f a n s a r e cone-shaped d e p o s i t s which g e n e r a l l y develop i n a r e a s o f h i g h r e l i e f which a r e undergoing r a p i d e r o s i o n .
They u s u a l l y o c c u r a l o n g t h e f a u l t -
bounded margins of s e d i m e n t a r y b a s i n s and may c o a l e s c e l a t e r a l l y t o form a piedmont slope o r bajada.
The p r o c e s s e s forming a l l u v i a l f a n s have been most e x t e n s i v e l y
s t u d i e d i n a r i d and s e m i - a r i d r e g i o n s where r a i n f a l l i s i n f r e q u e n t and v e g e t a t i o n sparse.
Humid-climate, o r v e g e t a t e d a l l u v i a l f a n s , have r e c e i v e d much l e s s a t t e n t i o n
and a r e more p o o r l y known.
R e l e v a n t d e s c r i p t i o n s i n c l u d e t h o s e of Hoppe and Ekman
(1964) and Winder ( 1 9 6 5 ) . A l l u v i a l f a n s p a s s l a t e r a l l y i n t o a v a r i e t y of s e d i m e n t a r y environments i n c l u d i n g l a k e s , a l l u v i a l p l a i n s and marine s h o r e l i n e s .
Because of t h e i r l o c a t i o n on a c t i v e
f a u l t s c a r p s a l l u v i a l f a n s b u i l d t h i c k sequences of s y n t e c t o n i c sediments i n t e r b e d d e d w i t h t h o s e of t h e a s s o c i a t e d s e d i m e n t a r y environments.
A number of f a c t o r s , i n c l u d i n g t h e s i z e of t h e catchment a r e a , s o u r c e a r e a l i t h o l o g y and c l i m a t e c o n t r o l t h e s i z e and shape of a l l u v i a l f a n s .
I n semi-arid
r e g i o n s t h e f a n a r e a (A ) and t h e a r e a of t h e d r a i n a g e b a s i n a r e r e l a t e d by: F AF = cA: (4.2)
183 where c and n a r e e m p i r i c a l l y d e r i v e d exponents ( B u l l , 1964; Hooke, 1965; Beaumont, 1972); n g e n e r a l l y b e i n g between 0 . 8 and 1.0 and c v a r y i n g between 0 . 1 5 and 2.10 depending on s o u r c e a r e a l i t h o l o g y , t e c t o n i c a c t i v i t y and c l i m a t e . Most f a n s have a concave-up p r o f i l e when s e e n i n r a d i a l s e c t i o n and t h i s i s n o t u s u a l l y a s i m p l e curve b u t a s e r i e s o f e v e n l y s l o p i n g segments which may be due t o i n t e r m i t t e n t s o u r c e a r e a u p l i f t ( e . g . B u l l , 1964)
LOWER FAN
Y ZONE OF COALESCENCE
X MIDFAN
X
CHANNEL PROFILE
F i g . 4 . 3 . P l a n view and r a d i a l p r o f i l e o f a n a l l u v i a l f a n ( b a s e d on B u l l , 1964 and Hooke, 1 9 6 7 ) . Alluvial fan deposits Channels r a d i a t e from t h e f a n apex and d i s s e c t t h e f a n s u r f a c e .
Usually there
i s no b r e a k i n shape between t h e canyon f l o o r and t h e main channel s u g g e s t i n g t h a t d e p o s i t i o n i s i n i t i a t e d by l a t e r a l e x p a n s i o n a t t h e f a n apex r a t h e r t h a n by a decrease i n slope.
The main c h a n n e l i s commonly i n c i s e d on t h e upper p a r t of t h e
f a n and emerges a t a lower l e v e l , c a l l e d t h e ' i n t e r s e c t i o n p o i n t ' by Hooke (1967) (Fig.4.3). The d e p o s i t s of a l l u v i a l f a n s have been c l a s s i f i e d by B u l l (1972) i n t o f o u r main t y p e s :
d e b r i s flow d e p o s i t s , s h e e t f l o o d d e p o s i t s , s t r e a m channel d e p o s i t s
and s i e v e d e p o s i t s .
184 D e b r i s flow d e p o s i t s D e b r i s flows a r e d e n s e , v i s c o u s masses of s e d i m e n t i n which t h e m a t r i x can s u p p o r t and t r a n s p o r t l a r g e c l a s t s up t o b o u l d e r s i z e .
The d e b r i s flows may s o l i d i f y n e a r
t h e margins t h u s forming d e b r i s flow l e v e e s , o r i n a c e n t r a l p l u g where s h e a r i s i n s u f f i c i e n t t o overcome t h e s t r e n g t h of t h e deforming mass (Johnson, 1970; C a r t e r , 1975; Middleton and Hampton, 1976).
When t h e c e n t r a l s o l i d p l u g expands t o t h e
f u l l t h i c k n e s s of t h e flow i t w i l l c e a s e t o move. D e b r i s flow d e p o s i t i o n r e q u i r e s s o u r c e a r e a l i t h o l o g i e s which produce s u b s t a n t i a l amounts of f i n e d e t r i t u s and s t e e p s l o p e s which promote r a p i d e r o s i o n and s u r f a c e run-off.
They most commonly o c c u r t h e r e f o r e , n e a r t h e p r o x i m a l p a r t o f t h e f a n and
may b e i n i t i a t e d d u r i n g i n c r e a s e d run-off
(e.g.
Sharpe and Nobles, 1953).
The
d e p o s i t s a r e g e n e r a l l y p a r a l l e l s i d e d , u n s t r a t i f i e d beds which a r e e l o n g a t e and p a r a l l e l t o the fan-surface (Fig.4.4).
100
N \
\
\
\ \
\
\ \ \
I
I
I \
I
I I 1
/ I
I
I Probable debris flow lobm
m w i sflow ~nrrr
aPossible s h e dqmsits
Present channel DOpotitiOMl lobes probably sheet ftooc
The upper p a r t of t h e T r o l l h e i m F a n , Death V a l l e y showing t h e d i s t r i b u t i o n Fig.4.4. of r e c e n t d e b r i s flow l o b e s and l e v e e s . The unshaded a r e a s a r e mainly fragments of o l d e r f a n s u r f a c e s , There i s a r e l i e f of a b o u t lOOm o v e r t h e a r e a shown ( a f t e r Hooke, 1967).
185 C h a r a c t e r i s t i c a l l y t h e y a r e . p o o r l y - s o r t e d , m a t r i x s u p p o r t e d and show a r e l a t i v e l y r a p i d d e c r e a s e i n maximum c l a s t s i z e down t h e f a n s u r f a c e . Sheet flood deposits S h e e t f l o o d s o c c u r most commonly below t h e i n t e r s e c t i o n p o i n t and a r e r e l a t i v e l y low v i s c o s i t y f l o o d s which expand a t t h e downstream ends o f c h a n n e l s .
Locally they
develop i n t o s h a l l o w s h e e t flows w i t h u p p e r flow regime c o n d i t i o n s and p a s s downf a n i n t o p a t t e r n s of b r a i d e d c h a n n e l s and b a r s which d i s s e c t t h e upper s u r f a c e of t h e sediment.
The d e p o s i t s a r e f a i r l y w e l l - s o r t e d sand and g r a v e l w i t h i n t e r n a l
l e n t i c u l a r i t y , s c o u r i n g , minor c r o s s - b e d d i n g and c r o s s - l a m i n a t i o n . Stream c h a n n e l d e p o s i t s Stream c h a n n e l d e p o s i t s o c c u r most a b u n d a n t l y i n t h e upper p a r t of t h e f a n above t h e i n t e r s e c t i o n p o i n t ( B u l l , 1972).
Below t h i s p o i n t f l o o d s may b e l a r g e l y uncon-
f i n e d and develop i n t o s h e e t f l o o d s .
The d e p o s i t s of s t r e a m c h a n n e l s a r e m o s t l y
l e n t i c u l a r , p o o r l y - s o r t e d s a n d s and g r a v e l s w i t h i m b r i c a t i o n developed i n t h e c o a r s e r g r a v e l s and c r o s s - b e d d i n g i n t h e sandy b e d s .
They a r e t h u s e a s i l y d i s t i n g u i s h e d
from d e b r i s flow d e p o s i t s . Sieve deposits S i e v e d e p o s i t s o c c u r j u s t below t h e i n t e r s e c t i o n p o i n t where h i g h l y permeable o l d e r d e p o s i t s c a u s e t h e flow t o d i m i n i s h r a p i d l y a s i n f l u e n t seepage o c c u r s . r e s u l t i s a clast-supported gravel lobe (Fig.4.4) poorly imbricated gravel.
The
c o n s i s t i n g of w e l l - s o r t e d and
S i e v e d e p o s i t s may have l o b e s w i t h c l e a r l y d e f i n e d down-
s t r e a m margins (Wasson, 1974).
During l a t e r b u r i a l t h e i r w e l l - s o r t e d
t e x t u r e may
be d e s t r o y e d by t h e m e c h a n i c a l i n f i l t r a t i o n o f f i n e r sediment (Walker, 1976). PEBBLY BRAIDED RIVERS Pebbly b r a i d e d r i v e r s a r e well-developed i n g l a c i a l outwash a r e a s , o r c l i m a t e a l l u v i a l f a n s , and a l s o i n t h e wadis of s e m i - a r i d r e g i o n s ,
humid-
The g r o s s
morphology of t h e s e c o a r s e g r a i n e d b r a i d e d r i v e r systems i s u s u a l l y a s h a l l o w f a n o r v a l l e y t r a i n i n which v a l l e y w a l l s c o n f i n e t h e flow.
Much o f o u r knowledge of
t h e s e r i v e r s comes from t h e g l a c i a l outwash p l a i n s o f I c e l a n d (HjulstrMm, 1952; K r i g s t r b m , 1962; B l u c k , 1974) and n o r t h w e s t e r n Canada and Alaska (Williams and R u s t , 1969; Boothroyd, 1972; R u s t , 1972; Smith, 1974). The b r a i d e d p a t t e r n i n r i v e r s o f t h i s t y p e a r e c a u s e d by t h e development of channel bars of various s i z e s .
The p r e c i s e c a u s e of b r a i d i n g i s n o t f u l l y known, b u t
f l u c t u a t i o n s i n d i s c h a r g e ( F a h n e s t o c k , 1963; Church, 1972) which a r e a f e a t u r e of p e r i g l a c i a l a r e a s would a p p e a r t o be a n i m p o r t a n t f a c t o r .
I t i s c o n v e n i e n t t o group
186 the channel bars of pebbly braided streams i n t o t h r e e types:
longitudinal bars,
b a r s i n curved c h a n n e l r e a c h e s and t r a n s v e r s e b a r s a l t h o u g h t h e l a t t e r a r e more common i n d i s t a l r e a c h e s and w i l l b e c o n s i d e r e d i n sandy low s i n u o s i t y r i v e r s . Lonai t u d i n a l b a r s L o n g i t u d i n a l b a r s a r e c h a r a c t e r i s t i c a l l y diamond o r lozenge-shaped i n p l a n view and a r e t h e dominant bedform o f p e b b l y b r a i d e d spreams ( F i g . 4 . 5 ) .
The upper s u r f a c e s
of t h e s e b a r s may b e c o v e r e d w i t h t r a n s v e r s e r i b s (McDonald and B a n e r j e e , 1971) of c o a r s e c l a s t s w i t h well-developed i m b r i c a t i o n and t h e l o n g a x e s o f c l a s t s o r i e n t a t e d t r a n s v e r s e t o t h e flow d i r e c t i o n .
These t r a n s v e r s e r i b s t e n d t o show a r e g u l a r
s p a c i n g which i s a f u n c t i o n of g r a i n s i z e and may r e s u l t from upper flow regime a ntidune transport. The i n t e r n a l s t r u c t u r e o f l o n g i t u d i n a l b a r s c o n s i s t s of g r a n u l e t o c o b b l e g r a d e g r a v e l s which a r e m o s t l y m a s s i v e o r w i t h a c r u d e h o r i z o n t a l s t r a t i f i c a t i o n .
The
g r a v e l s may be m a t r i x - f i l l e d o r openwork (Smith, 1974) and a r e f r e q u e n t l y a r r a n g e d i n fining-upwards u n i t s .
Large s c a l e cross-bedded g r a v e l s a r e n o t f r e q u e n t l y
r e p o r t e d b u t may develop a s a r e s u l t of a v a l a n c h i n g .
These a v a l a n c h e f o r e s e t s may
show r e a c t i v a t i o n s u r f a c e s ( C o l l i n s o n , 1970) which r e s u l t from f a l l i n g s t a g e m o d i f i c a t i o n of t h e b a r f r o n t .
The g r a i n s i z e of l o n g i t u d i n a l b a r s g e n e r a l l y
d e c r e a s e s downstream and t h e downstream d e p o s i t i o n a l margins may b e s l i p f a c e s o r r i f f l e s ; o t h e r margins c o n s i s t of l a t e r a l and downstream e r o s i o n t e r r a c e s . The p r o c e s s e s of l o n g i t u d i n a l b a r i n i t i a t i o n and m i g r a t i o n a r e n o t w e l l understood.
They p r o b a b l y form by t h e s e g r e g a t i o n of c o a r s e r c l a s t s a s t h i n rhombohedra1
g r a v e l s h e e t s and grow by t h e development o f l o n g i t u d i n a l s l i p f a c e s and v e r t i c a l a c c r e t i o n t o t h e b a r top.
R u s t (1972) observed t h a t b e d d i n g d i p p e d a t less than
L1mIII
a 1-
1M M
_,.,I 0 Y
“tlC” W I Y W
s-
-l.il
-
M
wI0-u
Fig.4.5. Composite model o f a p e b b l y b r a i d e d r i v e r d e p o s i t b a s e d on t h e Donjek R i v e r i n Alaska ( a f t e r Williams and R u s t , 1969).
187 3 O i n b o t h upstream and downstream d i r e c t i o n and s u g g e s t e d t h a t t h e b a r s u r f a c e i s
the preferred depositional site.
The consequence of t h i s i s t h a t l o n g i t u d i n a l b a r s
s h o u l d m i g r a t e upstream a l t h o u g h t h i s h a s n e v e r been c o n c l u s i v e l y demonstrated because o f t h e obvious p r a c t i c a l d i f f i c u l t i e s , S t u d i e s o f t h e Donjek R i v e r i n Alaska by Williams a n d R u s t (1969) and R u s t (1972) show t h a t s a n d s a r e d e p o s i t e d on t h e t o p and f l a n k s of g r a v e l b a r s and a l s o i n adjacent channels.
These show c r o s s - s t r a t i f i c a t i o n of v a r i o u s t y p e s which i s
produced a s s a n d i s s e g r e g a t e d from g r a v e l d u r i n g lower flow s t a g e s .
Falling
w a t e r l e v e l s r e s u l t i n flow s e p a r a t i o n around t h e g r a v e l b a r and t h e e x i s t i n g a c c r e t i o n topography w i t h t h e r e s u l t t h a t c r o s s s t r a t i f i c a t i o n t r a n s p o r t v e c t o r s a r e v e r y v a r i a b l e and show g r e a t e r d i s p e r s i o n t h a n t r a n s p o r t v e c t o r s s u c h a s i m b r i c a t i o n and l i n e a t i o n which a r e produced d u r i n g h i g h flow s t a g e s (Bluck, 1974). The o v e r a l l e f f e c t i s t h a t g r a v e l b a r d e p o s i t s g e n e r a l l y show fining-upwards sequences ( F i g . 4 . 6 ) which a r e i n d i c a t i v e of waning flow s t a g e .
r
P a r a l l e l laminated
c l a y s and s i l t y c l a y s may r e s u l t from s u s p e n s i o n d e p o s i t i o n i n i s o l a t e d p o o l s which o f t e n form i n abandoned c h a n n e l s .
VERY FINE SANDS, SILTS AND CLAYS WITH HORIZONTAL LAMINATIONS
1.511 FINE-MEDIUM SANDS WITH SMALL SCALE CROSS-BEDDING
MASSIVE AND CRUDELY STRATIFIED GRAVELS
Fining-upwards sequence o f a c h a n n e l - f i l l i n a p e b b l y b r a i d e d r i v e r Fig.4.6. ( b a s e d on W i l l i a m s and R u s t , 1969).
188 Bars i n curved channel r e a c h e s Bars which a r e a t t a c h e d t o e i t h e r bank of a c u r v e d r e a c h u s u a l l y develop a s e x t e n s i o n s of t h e f l a n k s o f l a r g e r l o n g i t u d i n a l b a r s .
They a r e c h a r a c t e r i z e d by
margins which a r e s t r o n g l y o b l i q u e t o t h e main c h a n n e l t r e n d ( F i g . 4 . 7 ) and a r e o f t e n referred t o a s diagonal bars.
The upstream end o f t h e b a r may b e a t t a c h e d t o t h e
bank and t h i s c a u s e s t h e flow t o b e c o n c e n t r a t e d i n a c h a n n e l on t h e o u t s i d e which
i s r e f e r r e d t o a s a r i f f l e reach.
The r i f f l e r e a c h ends a t a s t e e p e r r i f f l e f a c e
which e x t e n d s a c r o s s t h e c h a n n e l a t a n a c u t e a n g l e . The i n t e r n a l s t r u c t u r e o f d i a g o n a l b a r s i s v e r y s i m i l a r t o t h a t o f l o n g i t u d i n a l b a r s c o n s i s t i n g of p a r a l l e l s i d e d s h e e t s of g r a v e l which may be i m b r i c a t e d o r Newly emerging bar
Small deltas 2 1
I
Fig.4.7. Morphology and s t r u c t u r e of a t y p i c a l d i a g o n a l b a r ( l ) , ( 2 ) and ( 3 ) a r e i n d i v i d u a l u n i t s of t h e b a r where (1) i s t h e newly emerging b a r which w i l l e v e n t u a l l y a t t a c h i t s e l f t o ( 2 ) ( a f t e r B l u c k , 1974). structureless.
These g r a v e l s t e n d t o p a s s i n t o cross-bedded s a n d s downstream and
may p a s s upwards i n t o r i p p l e bedded s a n d s d e p o s i t e d under waning f l o o d c o n d i t i o n s . F i n e r sediments may a l s o accumulate i n s l o u g h c h a n n e l s which a r e abandoned a t low water stages.
P r o j e c t i o n s of t h e r i f f l e f a c e i n t o t h e s l o u g h c h a n n e l s i n t h e form
of s m a l l sand l o b e s and d e l t a s a l s o r e f l e c t waning flow due t o c h a n n e l m i g r a t i o n o r f a l l i n g discharge. Diagonal b a r s accumulate by t h e l a t e r a l m i g r a t i o n o f t h e r i f f l e r e a c h w i t h t h e a c c r e t i o n of t h e newly formed b a r o n t o a n o l d e r b a r ,
This r e s u l t s i n t h e formation
o f a g r a v e l p l a t f o r m w i t h a n obvious a c c r e t i o n topography ( F i g . 4 . 7 ) .
189 Sedimentary o r g a n i z a t i o n o f p e b b l y a l l u v i u m on a l a r g e s c a l e Pebbly b r a i d e d r i v e r s of g l a c i a l outwash a r e a s f o m l a r g e f a n s which show s y s t e m a t i c downstream c h a n g e s ,
The s l o p e of t h e f a n s u r f a c e and maximum c l a s t
s i z e both d e c r e a s e downstream a n d t h e r e a r e a s s o c i a t e d changes i n c h a n n e l and b a r morphology (Boothroyd, 1 9 7 2 ) ( F i g . 4 . 8 ) .
Diagonal b a r s may become more i m p o r t a n t
downstream and be a s s o c i a t e d w i t h a n i n c r e a s e d p r o p o r t i o n o f sand and hence c r o s s bedding. I n all, t h e d e p o s i t s o f p e b b l y b r a i d e d s t r e a m s w i l l t e n d t o show r a p i d l a t e r a l and v e r t i c a l f a c i e s changes. and g r a v e l s .
The main u n i t s w i l l c o n s i s t o f e r o s i v e l y based sands
The g r a v e l s w i l l b e m a s s i v e , o r w i t h c r u d e h o r i z o n t a l s t r a t i f i c a t i o n These w i l l p a s s
and may show well-developed i m b r i c a t i o n and fining-upwards.
l a t e r a l l y o r v e r t i c a l l y i n t o l e n t i c u l a r s a n d s which may show t a b u l a r c r o s s - b e d d i n g O c c a s i o n a l s i l t s and f i n e r g r a i n e d sediments w i l l
with reactivation surfaces.
i n d i c a t e t h e o c c u r r e n c e of s u s p e n s i o n d e p o s i t i o n i n p o o l s and abandoned c h a n n e l s .
\
Coarse gravel-> l5cm
a
0 Modium gravol-5-15cm
Fine gravel-< 5cm Mane-boddod sand Cross-boddod (largo-
P , iscale)sand
Rippled sand
Fin.-growl 5
South
4
3
II Medium-gradmidfan I
midlan 2
1
Distance
BP
1
km
2
3
4
5
6
Nor
Fig.4.8. Changes i n b a r t y p e s l o p e and s t r u c t u r e a l o n g a b r a i d e d outwash f a n ( a f t e r Boothroyd, 1 9 7 2 ) .
190 SANDY LOW-SINUOSITY RIVERS Sandy l o w - s i n u o s i t y r i v e r s a r e c h a r a c t e r i z e d by t r a n s v e r s e b a r s ( O r e , 1963; Smith, 1971b) and r e p r e s e n t t h e i n t e r m e d i a t e s t a g e between p e b b l y b r a i d e d r i v e r s and meandering r i v e r s .
They may show a b r a i d e d p a t t e r n b e c a u s e o f t h e development
of mid-channel b a r s o r show a non-braided c h a n n e l i n which t h e major thalweg g e n t l y meanders between a l t e r n a t i n g b a n k - a t t a c h e d b a r s .
The c h a n n e l p a t t e r n commonly
changes w i t h flow s t a g e becoming i n c r e a s i n g l y b r a i d e d a t low d i s c h a r g e a s t h e v a r i o u s bedforms became exposed and e r o d e d .
The c a u s e s o f b r a i d i n g i n t h e s e sandy low-
s i n u o s i t y r i v e r s a r e a g a i n n o t p r e c i s e l y known b u t may i n c l u d e a r e l a t i v e l y h i g h bed-load and r e l a t i v e l y low suspended-load.
Rapid l a t e r a l m i g r a t i o n i s t h e r e f o r e a
t y p i c a l f e a t u r e ( e . g . Gole and C h i t a l e , 1966). The bedforms of sandy l o w - s i n u o s i t y r i v e r s show c o n s i d e r a b l e v a r i a t i o n s and t h e r e i s no u n i v e r s a l l y a p p l i c a b l e c l a s s i f i c a t i o n scheme.
Detailed descriptions i n
i n d i v i d u a l r i v e r s i n c l u d e Coleman (1969) (Brahmaputra), Smith (1970) (South P l a t t e ) , C o l l i n s o n (1970) (Tana) and Cant and Walker (1978) (South Saskatchewan). The most d e t a i l e d d e s c r i p t i o n i s t h a t of Coleman (1969) f o r t h e Brahmaputra R i v e r . T h i s i s a b r a i d e d r i v e r w i t h a bed-load predominantly o f f i n e s a n d and w i t h a h i g h sediment d i s c h a r g e .
The p l a n view o f t h e c h a n n e l b a r s i s g e n e r a l l y diamond shaped
w i t h t h e long a x i s p a r a l l e l t o t h e a v e r a g e flow d i r e c t i o n i n t h e c h a n n e l of f o r m a t i o n (Fig.4.9).
They c o n s i s t o f a n amalgamation o f v a r i o u s bedforms and t e n d t o form
when t h e oncoming sediment l o a d i s g r e a t e r t h a n t h e c a r r y i n g c a p a c i t y of t h e r i v e r . Channel beds thus show r a p i d a g g r a d a t i o n w i t h t h e c h a n n e l becoming w i d e r and s h a l l o w e r c a u s i n g t h e main c u r r e n t t o change c o u r s e i n t h e s e a r c h f o r b e t t e r g r a d i e n t s and l e s s resistence. migration.
The r e s u l t i s a b r a i d e d c h a n n e l which i s c o n s t a n t l y undergoing l a t e r a l Sand b a r n u c l e a t i o n o c c u r s d u r i n g f l o o d s i n a r e a s of s l a c k w a t e r which
A s a n d b a r on t h e Brahmaputra R i v e r showing t h e change i n p o s i t i o n b e f o r e Fig.4.9. and a f t e r f l o o d of 1952. The s o l i d l i n e shows t h e l o c a t i o n of t h e s a n d b a r d u r i n g low s t a g e o f 1952 whereas t h e d o t t e d l i n e shows p o s i t i o n o f t h e same b a r a f t e r p a s s a g e of t h e f l o o d , 1952 ( a f t e r Coleman, 1969).
191 form, f o r example, i n t h e a r e a between two a c t i v e s c o u r c h a n n e l s .
M i g r a t i o n of
c h a n n e l b a r s o c c u r s d u r i n g f l o o d i n g a n d i s m a i n l y due t o t h e movement of bedforms which c o v e r t h e downstream s l o p e s o f t h e b a r s .
These bedforms i n c l u d e r i p p l e s
( h e i g h t up t o 0.3m), m e g a r i p p l e s (up t o 1.5m), dunes (up t o 7.6m) and sandwaves (up t o 15.2m). The s e d i m e n t a t i o n u n i t s produced i n t h i s way b e g i n w i t h l a r g e s c a l e cross-bedding ( F i g . 4 . 1 0 ) w i t h i n d i v i d u a l s e t s up t o l m t h i c k and t h e whole u n i t up t o 7m t h i c k . These u n i t s a r e produced by t h e m i g r a t i o n of g i a n t r i p p l e s d u r i n g f l o o d .
There may
b e s c o u r f i l l u n i t s w i t h i r r e g u l a r lower bounding s u r f a c e s a s s o c i a t e d w i t h t h i s u n i t a s w e l l a s s m a l l r i p p l e bedding and muddy s e d i m e n t d e p o s i t e d a l o n g f o r e s e t laminae and bedding p l a n e s . c r o s s bedded u n i t .
Abundant o r g a n i c m a t e r i a l may a l s o be t r a p p e d i n t h i s
8, d
Hi&ly disturbed soil zone Clay and silt with horizontal hmination and convolutr bedding
Ripplr bdding and horizontal lamination
A E’
3
Mainly l a r p scale cross-bedding
Fig.4.10. I d e a l v e r t i c a l sequence o f t h e s e d i m e n t a r y s t r u c t u r e s of a Brahmaputra R i v e r c h a n n e l b a r based on d a t a from Coleman (1969). O v e r l y i n g t h e l a r g e s c a l e c r o s s bedding i s a zone of s i l t y s e d i m e n t s mainly comp r i s i n g l e n t i c u l a r b o d i e s w i t h c l i m b i n g r i p p l e c r o s s - l a m i n a t i o n and s m a l l - s c a l e r i p p l e bedding. a l s o occur.
I n t e r c a l a t e d l e n t i c u l a r b o d i e s w i t h l a r g e - s c a l e cross-bedding may
These u n i t s a r e d e p o s i t e d a t lower f l o w s t a g e when t h e l a r g e r bedforms
have s t o p p e d moving.
Above t h i s u n i t i s a zone o f s i l t y c l a y s and s i l t showing
p a r a l l e l laminations with occasional t h i n u n i t s of climbing r i p p l e cross-lamination r e s u l t i n g mainly from s u s p e n s i o n d e p o s i t i o n .
Post-depositional deformation s t r u c t u r e s
i n c l u d i n g l o a d s t r u c t u r e s and c o n v o l u t e bedding a r e abundant i n t h i s u n i t and i n t e r p r e t e d by Coleman (1969) a s b e i n g due t o i n c r e a s e d s h e a r s t r e s s caused by a n
192 i n c r e a s e i n c u r r e n t v e l o c i t y and a sudden r i s e i n t u r b u l e n c e .
Above i s a h i g h l y
d i s t u r b e d s o i l zone w i t h burrows and r o o t s . The r e s p o n s e o f c h a n n e l b a r s t o changes i n flow s t a g e i n b r a i d e d s t r e a m s i s w e l l i l l u s t r a t e d by C o l l i n s o n ' s ( 1 9 7 0 ) work on t h e Tana R i v e r i n Northern Norway.
The
r i v e r shows a v a r i e t y o f bedforms r a n g i n g from v e g e t a t e d i s l a n d s , s i d e b a r s , l i n g u o i d b a r s , dunes, and r i p p l e s .
The l i n g u o i d b a r s a r e t h e c h a r a c t e r i s t i c c h a n n e l
b a r and may be up t o 300m l o n g and 1 5 h wide.
S l i p f a c e s on t h e b a r s a r e up t o 2m
h i g h and t h e s t o s s s i d e may b e c o v e r e d by s m a l l e r bedforms. now c o n s i d e r e d t o b e sandwaves (Harms e t a l . ,
These l i n g u o i d b a r s a r e
1 9 7 5 ) which form when t h e d e p t h and
v e l o c i t y of flow a r e above t h o s e a p p r o p r i a t e f o r r i p p l e s b u t below t h o s e f o r dunes. M i g r a t i o n of t h e l i n g u o i d b a r s t a k e s p l a c e d u r i n g f l o o d and produces t a b u l a r c r o s s bedded u n i t s .
However, d u r i n g f a l l i n g flow s t a g e c u r r e n t s become c o n f i n e d t o t h e
t o p o g r a p h i c a l l y lower a r e a s and t h e f r o n t o f t h e b a r may become m o d i f i e d by e r o s i o n of f o r e s e t l a m i n a t i o n s and t h e f o r m a t i o n of c r o s s - l a m i n a t i o n by r i p p l e s m i g r a t i n g across the bar face.
Subsequent r i s e i n flow s t a g e r e s u l t s i n t h e renewed m i g r a t i o n
of t h e b a r and r e a c t i v a t i o n of t h e c r o 5 . s - s t r a t i f i c a t i o n ( F i g . 4 . 1 1 ) .
Hlgh water stage
---__-
lntermedlate water stage
Low water stage
B
A FIo* perpendicular
Rising water stage
A
The development of r e a c t i v a t i o n s u r f a c e s by changing w a t e r s t a g e o v e r a Fig.4.11. l i n g u o i d b a r ( a f t e r C o l l i n s o n , 1970).
193 The S o u t h Saskatchewan R i v e r i n Canada i s a sandy b r a i d e d s t r e a m c h a r a c t e r i z e d by l a r g e s a n d f l a t s (5Om t o 2km i n l e n g t h and 3Om t o 450m w i d e ) , s l i p face-bounded b a r s and d u n e - f i l l e d c h a n n e l s (Cant and Walker, 1978).
A t any one l o c a l i t y t h e r e
a r e commonly one o r two m a j o r c h a n n e l s a n d s e v e r a l minor c h a n n e l s .
The beds o f t h e
major c h a n n e l s a r e f i l l e d w i t h s i n u o u s - c r e s t e d dunes up t o 1.5m h i g h and i n p l a c e s sandwaves ( H a m s e t a l . , 19751 up t o 10m l o n g and 0.3m h i g h a r e common. b a r s l i k e t h o s e d e s c r i b e d by C o l l i n s o n (1970) a r e r a r e ,
Linguoid
The major d e p o s i t s of t h e
channels a r e trough cross-beds. Slipface-bounded b a r s a r e common i n t h e S o u t h Saskatchewan R i v e r and u s u a l l y o c c u r i n a r e a s of f l o w e x p a n s i o n , f o r example a t c h a n n e l j u n c t i o n s , o r where c h a n n e l s widen.
The l a r g e r b a r s a r e up t o 2.5m h i g h and may e x t e n d s e v e r a l hundred m e t r e s d i a g o n a l l y a c r o s s t h e m a j o r c h a n n e l s . These ' c r o s s - c h a n n e l b a r s ' a s t h e y a r e c a l l e d by Cant a n d Walker (1978) produce p l a n a r - t a b u l a r s e t s of c r o s s - b e d d i n g s i m i l a r t o t h o s e d e s c r i b e d by C o l l i n s o n (1970) and Smith (1970, 1972). The s a n d f l a t s a r e a r e a s up t o 2000m long a n d 450m wide which a r e a complex of s m a l l e r f e a t u r e s s i m i l a r t o t h e exposed a r e a s of s a n d on t h e Brahmaputra (Coleman, 1969) and t h e Tana ( C o l l i n s o n , 1970).
The sand f l a t s a r e covered d u r i n g f l o o d and
r e c e n t l y emerged s a n d f l a t s r e v e a l a v a r i e t y of b a r s , sandwaves and r i p p l e s .
The
n u c l e a t i o n and s u b s e q u e n t growth of a s a n d f l a t i s a p p a r e n t l y i n i t i a t e d by t h e emergence o f a c r o s s - c h a n n e l b a r .
A s flow c o n t i n u e s s e d i m e n t i s swept around t h i s
n u c l e u s and expanding flow around t h e downstream end r e s u l t s i n i n w a r d l y d i r e c t e d sediment t r a n s p o r t (Fig.4.12). The r e s u l t i s t h e development o f horns w i t h i n w a r d l y - d i r e c t e d s l i p f a c e s . Asymmetrical s a n d f l a t s form i n t h e same way e x c e p t t h a t t h e y o r i g i n a t e on d i a g o n a l c r o s s - c h a n n e l b a r s and a r e a t t a c h e d t o a s t a b l e bank ( F i g . 4 . 1 2 ) .
Subsequent growth
of t h e s m a l l s a n d f l a t i n t o a l a r g e , complex s a n d f l a t i n v o l v e s a g g r a d a t i o n by d r i v i n g s e v e r a l l a t e r g e n e r a t i o n s o f b a r s o n t o t h e s a n d f l a t from upstream or from a d j a c e n t channels.
I t a l s o i n v o l v e s ellosion o f t h e f l a t s and t h e l i n k i n g of s e p a r a t e sand-
f l a t s by d e p o s i t i o n i n i n t e r v e n i n g c h a n n e l s . The s t r a t i f i c a t i o n sequence o f t h e South Saskatchewan R i v e r i s summarized i n F i g s . 4 . 1 3 and 4.14.
These diagrams can be used t o p l a c e i n t o c o n t e x t t h r e e
i m p o r t a n t t y p e s o f s t r a t i f i c a t i o n sequence which a r e o f g e n e r a l a p p l i c a b i l i t y t o sandy-low-sinuosity streams (Fig.4.14). maximum s a n d f l a t development,
The f i r s t sequence (Fig.4.14A)
shows
Trough c r o s s - b e d s ( c h a n n e l dunes) a r e o v e r l a i n
by a l m t h i c k p l a n a r t a b u l a r s e t ( c r o s s - c h a n n e l b a r ) .
T h i s i s o v e r l a i n by s m a l l
p l a n a r t a b u l a r s e t s w i t h minor c h a n n e l s and r e a c t i v a t i o n s u r f a c e s and a l s o r i p p l e cross-lamination (sand f l a t sequence). The second sequence (Fig.4.14B)
i s an intermediate type with deposition both
i n c h a n n e l s and on sand f l a t s and t h e t h i r d sequence (Fig.4.14C)
i s a channel
dominated sequence w i t h c r o s s - c h a n n e l b a r s which d i d n o t develop i n t o s a n d f l a t s .
194 SYMMETRICAL CROSSCHANNEL BAR
CROSS-CHANNEL BAR NlTH EMERGENT NUCLEUS
SMALL SAND FLAT
1
EXAMPLES OF COMPLEX SAND FLATS
Fig.4.12. Development of s a n d f l a t s on t h e South Saskatchewan R i v e r . S t a g e 1, submerged c r o s s - c h a n n e l b a r w i t h s l i p f a c e . S t a g e 2 emergent n u c l e u s f o l l o w i n g a lowering o f r i v e r s t a g e . I f t h e o r i g i n a l c r o s s - c h a n n e l b a r i s d i a g o n a l t h e n u c l e u s w i l l b e a s y m m e t r i c a l ( 2 A ) o r a d j a c e n t t o a s t a b l e bank (2B). S t a g e 3 , expanding flow downstream c a u s e s t h e development o f h o r n s . S t a g e 4 d e p i c t s t h e f o r m a t i o n of complex sand f l a t s ( a f t e r Cant and Walker, 1978). These sequences a c t u a l l y resemble t h e l a t e r a l a c c r e t i o n p o i n t b a r sequences i n a meandering r i v e r which may c o n s i s t of t r o u g h c r o s s - b e d d i n g a n d p l a n a r t a b u l a r s e t s d e p o s i t e d by s c r o l l - b a r s ( J a c k s o n , 1976).
T h i s emphasizes t h e
d i f f i c u l t y o f d i s t i n g u i s h i n g p r e s e n t - d a y l o w - s i n u o s i t y and meandering r i v e r s on t h e b a s i s of s t r a t i g r a p h i c sequence.
The p r e s e n c e o f a s a n d f l a t s e q u e n c e w i t h
a p l a n a r t a b u l a r c r o s s - c h a n n e l b a r d e p o s i t a t t h e b a s e p r o v i d e s an i m p o r t a n t c l u e t o t h e former p r e s e n c e o f b r a i d e d c h a n n e l s . EPHEMERAL STREAM CHANNELS
Ephemeral s t r e a m s a r e p a r t i c u l a r l y c h a r a c t e r i s t i c o f a r i d and s e m i a r i d r e g i o n s where t h e s u r f a c e run-off is spasmodic because o f t h e c l i m a t i c c o n d i t i o n s .
The
196
Block diagram summarizing t h e main m o r p h o l o g i c a l e l e m e n t s and t h e i r Fig.4.13. d e p o s i t s i n t h e South Saskatchewan R i v e r . The i n s e t shows a p l a n view. S t i p p l e d a r e a s a r e emergent. S i n g l e s h a f t e d arrows show d i r e c t i o n o f bedform movement and double s h a f t e d a r r o w s i n d i c a t e flow d i r e c t i o n . A ( s a n d f l a t ) , B (mixed sand f l a t c h a n n e l ) and C ( c h a n n e l ) r e f e r t o t h e sequences i n Fig.4.14 ( a f t e r Cant and Walker, 1978). d e p o s i t s of ephemeral s t r e a m s show many of t h e f a m i l i a r s e d i m e n t a r y s t r u c t u r e s of permanent s t r e a m flow ( P i c a r d and High, 1973) and may b e d i f f i c u l t t o d i s t i n g u i s h from t h e s e d e p o s i t s .
F r e q u e n t l y , however, t h e y show i n t e r b e d d e d a e o l i a n sands
where s t r e a m c h a n n e l s have been choked by wind-blown sand.
Also wind e r o s i o n may
l e a d t o t h e f o r m a t i o n of a pebble-strewn d e f l a t i o n l a g on t h e former s t r e a m bed ( G l e n n i e , 1970) and c u r l e d mudflakes formed d u r i n g p e r i o d s of d e s i c c a t i o n may be i n c o r p o r a t e d i n t o i n t r a f o r m a t i o n a l conglomerates ( G l e n n i e , 1970; Karcz, 1972). When d i s c h a r g e i s a b n o r m a l l y h i g h s h e e t f l o o d i n g may o c c u r and c o v e r t h e whole v a l l e y f l o o r a s a t B i j o u Creek, Colorado (McKee e t a l . , 1967).
In this particular
c a s e a s h e e t d e p o s i t between l m and Lm t h i c k was d e p o s i t e d , t h e dominant s t r u c t u r e (90-95%) b e i n g upper flow regime p a r a l l e l l a m i n a t i o n s .
I n many f l o o d p l a i n s e c t i o n s
196
SAND FLAT
MIXED INFLUENCE
CHANNEL V.A.
V. A.
FINAL CHAN. FILL, MUD IN SLOUGH-
--- - - - -
SAND FLAT ACCRETION a AGORADATI0N
CONTINUED CHANNEL AGGRADATION IN PLACES WITH C-C BAR
-
CROSS CHANNEL
BAR
-
IN CHANNEL DEPOSITION
IN CHANNEC DEPOSITION
Fig.4.14. Summary s t r a t i g r a p h i c sequences c h a r a c t e r i z i n g a r e a s dominated by sand f l a t development, a r e a s o f mixed sand f l a t and channel i n f l u e n c e , and a r e a s o f channel a n a r a d a t i o n . The arrows i n d i c a t e t h e expected p a l a e o f l o w v a r i a b i l i t y ( a f t e r Cant and Walker, 1978). p a r a l l e l l a m i n a t i o n s form a l l o f t h e s h e e t f l o o d d e p o s i t b u t i n o t h e r s cross-bedding occurs i n t h e upper p a r t of t h e p r o f i l e , i n d i c a t i n g waning flow c o n d i t i o n s .
The
l a s t s t a g e s of t h e f l o o d a t B i j o u Creek were c o n f i n e d t o t h e main channel which c u t i n t o t h e f l o o d p l a i n sands and d e p o s i t e d t r o u g h c r o s s - b e d s w i t h some p a r a l l e l l a m i n a t i o n and p l a n a r cross-bedding
(Fig.4.15).
An i m p o r t a n t f e a t u r e of ephemeral s t r e a m d e p o s i t s a r e t h e p o s t - d e p o s i t i o n a l m o d i f i c a t i o n s i n t e x t u r e which occur. well-washed sands.
The i n i t i a l d e p o s i t s a r e commonly c l e a n ,
A t d e p t h , however, t h e amount of i n t e r s t i t i a l c l a y i n c r e a s e s
due t o t h e e f f e c t s of mechanical i n f i l t r a t i o n by i n f l u e n t seepage (Walker, 1976). The o v e r a l l e f f e c t i s t o p o s t - d e p o s i t i o n a l l y d e c r e a s e t h e s o r t i n g and t e x t u r a l m a t u r i t y of t h e d e p o s i t s .
197
CONVOLUTE
BEWING
MUD tAYER CLIMBING RIPPLE LAMINATION
CONVOLUTE
BEDDING
HORIZONTAL BEDDING (LAMINATED SAND )
-
LARGE SCALE CROSS- BEDDIEIIG (DELTA FORESETS)
HORIZONTAL BEDDING MUD LAYER
CLIMBING RIPPLE LAMINATION
TE U
0
1
HORl20 NTA L BE DOING (LAMINATED SAND 1
OLDER SEDtMENTS
OLDER
SEDIMENTS
F i g . 4 . 2 5 . F l o o d p l a i n sequences formed during a s i n g l e f l o o d of Bijou Creek (Colorado) i n June 1965 (based on McKee e t a l . , 1 9 6 7 ) .
198 MEANDERING RIVERS Introduction Meandering t e n d s t o b e f a v o u r e d by r e l a t i v e l y low s l o p e s and a h i g h suspendedload/bed-load r a t i o .
Using t h e c r i t e r i o n o f Leopold and Wolman (1957) meandering
channels a r e t h o s e w i t h a s i n u o s i t y o f more t h a n 1 . 5 .
I n f a c t a number o f o t h e r
measures of s i n u o s i t y have been used; f o r example B r i c e (1964) used t h e r a t i o : c h a n n e l l e n g t h / l e n g t h of meander b e l t a x i s ( F i g . 4 . 1 6 ) and Schumm (1963) used t h e s t r e a m l e n g t h / v a l l e y l e n g t h and t h e r e i s no g e n e r a l agreement on a n o b j e c t i v e
ratio:
d e f i n i t i o n of meandering.
There a r e some fundamental r e l a t i o n s h i p s between t h e
g e o m e t r i c f e a t u r e s of s t r e a m meanders.
The meander wave l e n g t h a n d a r e a o f t h e
d r a i n a g e b a s i n a r e r e l a t e d by: aAb
A=
(4.3)
i s meander wave l e n g t h , A i s b a s i n a r e a and a and b a r e e m p i r i c a l c o n s t a n t s .
where
S i n c e t h e d i s c h a r g e o f a r i v e r i s r e l a t e d t o i t s d r a i n a g e a r e a , d i s c h a r g e and wavelength a r e also related.
x = cwm
Also (4.4)
A,
= dWn
(4.5)
X
= F R ~
(4.6)
where
3
i s wavelength, W i s c h a n n e l w i d t h , Am i s wave a m p l i t u d e , R is t h e r a d i u s of
c u r v a t u r e , and c , d , f , m , n , and t a r e c o n s t a n t s .
The exponents i n t h e s e e q u a t i o n s
approximate t o u n i t y and t h e r e l a t i o n s h i p s a r e a l m o s t l i n e a r .
-. MEI!iNDER BELT AXIS
Fig.4.16.
Geometric f e a t u r e s o f a s t r e a m meander a c c o r d i n g t o B r i c e (1964).
199 Many e x p l a n a t i o n s have been o f f e r e d a s t h e b a s i c c a u s e of meandering and a number o f f a c t o r s i n c l u d i n g t h e n a t u r e o f t h e b e d - l o a d , g r a d i e n t and d i s c h a r g e may b e i m p o r t a n t .
The dominating f a c t o r i s , however, t n e h e l i c a l flow p a t t e r n
s e e n i n meanders (Fig.4.17).
Maximim flow v e l o c i t i e s a r e found on t h e concave
bank n e a r t n e downstream end and c r o s s from one bank t o t h e o t h e r .
T h i s winding
p a t t e r n produces a c e n t r i f u g a l f o r c e p r o d u c i n g a s u p e r e l e v a t i o n o f t h e w a t e r l e v e l a t t h e o u t s i d e o f t h e bend.
The i n c r e a s e d p r e s s u r e c a u s e d by t h e e x c e s s w e i g h t of
w a t e r p i l e d up i n t h i s way i n t e n s i f i e s t h e h e l i c a l flow a t t h e bend g i v i n g a s t r o n g downward movement on t h e o u t s i d e which c a u s e s e r o s i o n .
The v e l c o i t y components
i n c l u d e a s t r o n g downward component and a much weaker l a t e r a l component.
This
l a t e r a l component o f v e l o c i t y shows a movement a l o n g t h e bottom towards t h e i n n e r p a r t of t h e bend where d e p o s i t i o n o c c u r s ,
Flow mechanisms i n meander b e l t s a r e
d i s c u s s e d by Bagnold ( 1 9 6 0 ) , Leopold e t a l . (1964) and J a c k s o n (1975).
5 Fig.4.17.
The h e l i c o i d a l flow p a t t e r n i n meander bends.
200 An i m p o r t a n t f e a t u r e of meander b e l t s i s t h e l a r g e number o f abandoned c h a n n e l s which a r e seen.
These c h a n n e l c u t - o f f s form i n two main ways ( F i s k , 1947) (Fig.4.18).
The f i r s t a r e c h u t e c u t - o f f s which form d u r i n g f l o o d s t a g e s a s t h e r i v e r s t r a i g h t e n s i t s c o u r s e by e r o d i n g c h u t e s o r s w a l e s .
As a c t i v i t y i n t h e former c h a n n e l i s reduced
i t becomes plugged by bed-load s e d i m e n t s and t h e n f i l l e d w i t h s u s p e n s i o n d e p o s i t s
(Fig.4.18A).
Fig.4.18.
The mechanisms of c h a n n e l s h i f t i n g i n a meander b e l t ( a f t e r F i s k , 1947).
Neck c u t - o f f s a r e formed when concave-bank e r o s i o n b r i n g s two bends c l o s e t o g e t h e r w i t h only a narrow neck s e p a r a t i n g them.
I f t h i s neck is b r e a c h e d t h e r i v e r w i l l
r a p i d l y abandon t h e meander and t a k e t h e s h o r t e s t r o u t e downslope.
The r e s u l t i s
an oxbow l a k e (Fig.4.18B) which becomes f i l l e d w i t h s u s p e n s i o n - l o a d sediment. Point bars P o i n t b a r s a r e t h o s e a r e a s on t h e i n s i d e o f a meander bend which r e s u l t from t h e l a t e r a l a c c r e t i o n o f sediment d u r i n g c h a n n e l m i g r a t i o n .
The s t r u c t u r e o f p o i n t b a r
d e p o s i t s has been d e s c r i b e d by many a u t h o r s i n c l u d i n g F i s k (1944, 19471, F o l k and Ward ( 1 9 5 7 ) , F r a z i e r and Osanik ( 1 9 6 1 ) , Bernard and Major (19631, Harms e t a l . (19631, A l l e n (1964a, 1 9 6 5 a , e , 1 9 7 0 a , b ) , McGowen and Garner ( 1 9 7 0 ) , Bluck (1971) and J a c k s o n (1975, 1976).
The c l a s s i c a l c o n c e p t of p o i n t bar d e p o s i t i o n assumes b a n k f u l
d i s c h a r g e and f u l l y developed h e l i c o i d a l flow around t h e bend.
Decrease i n v e l o c i t y
and d e p t h and t h e upflow component of flow o v e r t h e p o i n t b a r s u r f a c e c a u s e bed s h e a r stress t o f a l l and t h e s u r f a c e a c t s a s a n e l u t r i a t o r g i v i n g a n upslope
201 r e d u c t i o n i n g r a i n s i z e (Fig.4.19).
Floodplain Channel-fill
I
@Cross-bedding
HFIat-bedding
Fig.4.19. I d e a l i z e d diagram o f p o i n t b a r d e p o s i t i o n i n a meandering s t r e a m ( a f t e r A l l e n , 1970a). The shape and s i z e o f p o i n t b a r s i s dependent upon t h e s i z e o f t h e r i v e r channel. I n small streams they a r e simple depositional f e a t u r e s b u t i n l a r g e r i v e r s they a r e more complex and may c o n t a i n a number o f s e p a r a t e u n i t s .
The s u r f a c e o f p o i n t b a r s
may show well-developed a c c r e t i o n topography ( F i s k , 1947; Sundborg, 1956) c o n s i s t i n g of s c r o l l - s h a p e d r i d g e s ( s c r o l l b a r s ) a l t e r n a t i n g w i t h d e p r e s s i o n s ( s w a l e s ) . may be f i l l e d w i t h muddy s e d i m e n t and marshes may develop i n them. represents the depositional event of a s i n g l e flood.
Swales
Each s c r o l l b a r
The t h i c k n e s s of p o i n t b a r
d e p o s i t s may c o r r e s p o n d t o t h e maximum d e p t h o f t h e r i v e r c h a n n e l .
On t h e M i s s i s s i p p i
p b i n t b a r s up t o 25m t h i c k a r e known ( F i s k , 1944, 1947); on t h e Brazos R i v e r , 15-2Om (Bernard and Major, 1963) and 10-15m on t h e Niger R i v e r ( A l l e n , 196513).
For s m a l l e r
r i v e r s t h e t h i c k n e s s may b e o n l y 1-3m. The s t r u c t u r e and g r a i n s i z e o f p o i n t b a r d e p o s i t s f r e q u e n t l y shows a d i s t i n c t i v e v e r t i c a l arrangement.
The c o a r s e s t m a t e r i a l may b e c o n g l o m e r a t i c sediment which i s
found a t t h e b a s e o f t h e p o i n t b a r and r e p r e s e n t s a c h a n n e l l a g d e p o s i t (Fig.4.19). I f a s u f f i c i e n t l y wide range o f g r a i n s i z e m a t e r i a l i s a v a i l a b l e t h e p o i n t b a r w i l l show a g r a d u a l upward d e c r e a s e i n g r a i n s i z e .
This i n d i c a t i o n of e f f e c t i v e decrease
i n s t r e a m power i s a l s o r e f l e c t e d i n t h e sequence o f s e d i m e n t a r y s t r u c t u r e s ;
flat-
bedding p a s s e s upwards i n t o c r o s s - b e d d i n g which i n t u r n g i v e s way t o c r o s s - l a m i n a t i o n i n the f i n e r grained sands and silts.
T h i s sequence i n d i c a t e s p r o g r e s s i v e l y
202
d e c r e a s i n g flow regime and t h e t o p of t h e p o i n t b a r sequence may b e capped by a v e n e e r of muddy sediment d e p o s i t e d from s u s p e n s i o n a s a s w a l e - f i l l .
This o v e r a l l
fining-upwards i n t h e p o i n t b a r sequence i s a f u n c t i o n o f t h e l a t e r a l m i g r a t i o n o f
As l a t e r a l d e p o s i t i o n b u i l d s up on t h e i n n e r b a n k , b a l a n c i n g
t h e r i v e r channel.
e r o s i o n on t h e o u t e r bank, t h e r e i s f o r any p o i n t on t h e s u r f a c e o f t h e p o i n t b a r a d e c r e a s e i n bed s h e a r s t r e s s and s t r e a m power a s t h e thalweg m i g r a t e s l a t e r a l l y away and so f i n e r g r a i n e d d e p o s i t s r e s u l t i n t h e v e r t i c a l sequence ( A l l e n , 1970a). D e t a i l e d d e s c r i p t i o n s o f p o i n t b a r sequences s u g g e s t t h a t t h i s i d e a l d e p o s i t i o n a l model i s o n l y approximated i n n a t u r a l systems. Maryland was s t u d i e d by Leopold e t a l .
The Watts Branch p o i n t b a r i n
(1964) o v e r a p e r i o d of s i x y e a r s d u r i n g
which t h e r i v e r c h a n n e l m i g r a t e d a d i s t a n c e o f more t h a n a c h a n n e l w i d t h and t h e n e t volume o f e r o s i o n and d e p o s i t i o n were e q u a l (Fig.4.20).
The p o i n t b a r shows
a n o v e r a l l fining-upwards b u t g r a v e l l y h o r i z o n s a r e d i s t r i b u t e d a t v a r i o u s l e v e l s above t h e c h a n n e l b a s e and i n d i v i d u a l s e d i m e n t a t i o n u n i t s have a d i s c o n t i n u o u s , l e n t i c u l a r nature.
Y
f
-
33,5 -
w 31.1
1
,
,
0 0 5 1 2 DISTANCE IN METRE
5
L
SURFACE PROFILES OF MEANDER
SUCCESSIVE
Y
3
7
6
CHANNEL AND
8 POINT
9
I
10
11
BAR, 1953- 59
I
I 6,5
5,5 6 OlSrANCE IN METRE
LARGE SCALE CROSS-SECTION OF
SEDIMENTS IN
7
RELATION
GRAVEL BED OLIVE-GRAY
COARSE SAND BROWN STAINED WITH PEBBLES AND LENSES OF SILT BROWN SANOY SILT
TO PROFILES
-
1959 1958
----
1956
...-, .,-
19 5 5 19 53
FINE SAND WITH SOME SILT
Fig.4.20.
8,s
8
EXPLANAILON CLAYEY SILT WITH ORGANIC MATTER
ORANGE BROWN MATTLED SANDY SILT WITH SOME CLAY
0 0
7,s
The Watts Branch p o i n t b a r ( a f t e r Leopold e t a l , , 1964).
203 McGowen and G a r n e r (1970) i n v e s t i g a t e d c o a r s e g r a i n e d p o i n t b a r s o f t h e b i t e R i v e r , L o u i s i a n a and t h e Colorado R i v e r , Texas. (1.4 t o 1 . 7 ) bed-load
These r i v e r s a r e l o w - s i n u o s i t y
s t r e a m s w i t h a r a t h e r h i g h g r a d i e n t which t r a n s p o r t mainly
c o a r s e s a n d , and pebble-cobble s i z e d g r a v e l s .
The r i v e r s meander b e c a u s e of t h e
h i g h s t a b i l i t y o f t h e banks which a r e composed of v e g e t a t e d muddy sediment.
These
c o a r s e g r a i n e d p o i n t b a r s can b e d i v i d e d i n t o a lower a n d a n upper p o i n t b a r (Fig.4.21).
The upper p o i n t b a r i s c h a r a c t e r i z e d by c h u t e s and c h u t e b a r s e d i m e n t s .
CONVEX BANK
CONCAVE BANK
Parallel laminated to thin sand beds
-
rn
6
FLOOD STAGE
4
Parallel laminae
LOW WATER - - - - _ STAG
2 0 UPPER POINT BAR
(Chute and chute bar sediments)
0 10 I rn
Fig.4.21. 1970).
C r o s s - p r o f i l e o f a c o a r s e g r a i n e d p o i n t b a r ( a f t e r McGowen and G a r n e r ,
Chutes a r e c h a n n e l s w i t h r e l a t i v e l y s t e e p s i d e s , f l a t b o t t o m s , and s i n u o u s t r e n d s . They a r e d e e p e s t where t h e y meet t h e main c h a n n e l and d e c r e a s e i n d e p t h s t e a d i l y downcurrent where s c o u r s g i v e way t o d e p o s i t i o n i n t h e form of c h u t e b a r s . can b e 2-5m d e e p , 5-7m wide and e x t e n d f o r hundreds of m e t r e s .
These
The sequence of
sediments i n t h e lower p o i n t b a r i s s i m i l a r t o t h a t s e e n i n t h e c r o s s - o v e r ( t h e s t r a i g h t c h a n n e l r e a c h between two meander l o o p s ) ( F i g . 4 . 2 2 ) . The s t r u c t u r e of c o a r s e g r a i n e d p o i n t b a r s do n o t show t h e t y p i c a l fining-upwards
o f t h e i r f i n e r g r a i n e d e q u i v a l e n t s and t h e i r o r i g i n i s c l e a r l y r a t h e r more complicated than t h e s i m p l e l a t e r a l a c c r e t i o n model.
However, t h e t o p of t h e sequence may b e
t e r m i n a t e d by a n o r g a n i c r i c h c l a y (Fig.4.22) Unit 1
-
w i t h t h e f u l l sequence a s f o l l o w s :
trough cross-bedding (scour pool d e p o s i t ) , Unit 2
-
small s c a l e f o r e s e t
and t r o u g h f i l l c r o s s - b e d d i n g (lower p o i n t b a r / c r o s s - o v e r d e p o s i t ) , U n i t 3 s c a l e f o r e s e t cross-bedding (chute bar d e p o s i t s ) , a n d Unit 4
-
-
large
p a r a l l e l laminae,
s m a l l s c a l e f o r e s e t c r o s s - b e d d i n g and s c o u r and f i l l d e p o s i t s ( f l o o d p l a i n d e p o s i t s ) . V a r i a t i o n s from t h e i d e a l i z e d p o i n t b a r model i n n a t u r a l systems a r e p a r t l y e x p l a i n e d by J a c k s o n (1975, 1976) who has shown t h a t t h e h e l i c o i d a l flow p a t t e r n i n meander bends can b e d i v i d e d i n t o t h r e e zones: i n f l u e n c e o f t h e upstream bend p r e v a i l s bend dominates and
1) a t r a n s i t i o n zone i n which t h e
2 ) a f u l l y developed zone i n which t h e l o c a l
3 ) a n i n t e r m e d i a t e zone i n which one changes i n t o t h e o t h e r .
204
These downstream changes i n flow p a t t e r n a r e r e f l e c t e d i n t h e bedforms produced on t h e s u r f a c e of t h e p o i n t b a r and hence i n t h e s t r a t i g r a p h i c sequence of s t r a t i f i c a t i o n types (Fig.4.23).
Floodplain Deposit
t
Unit 3 Chub Bar Dsporit
Unit 2 Lower Point Bar Deposit
-
Unit 1 Scwr Pool Deposit
Fig.4.22. V e r t i c a l sequence i n a c o a r s e g r a i n e d p o i n t b a r d e p o s i t ( a f t e r McGowen and G a r n e r , 1970). The f u l l y developed zone i s c o n s i s t e n t w i t h t h e i d e a l model of p o i n t b a r d e p o s i t i o n b u t a l s o shows p l a n a r t a b u l a r s e t s produced by s c r o l l b a r s .
I n the
t r a n s i t i o n and i n t e r m e d i a t e zones t h e r e i s no g r a d u a l fining-upwards and g r a i n s i z e v a r i a t i o n s t e n d t o b e r a t h e r a b r u p t , i n p l a c e s even coarsening-upwards, RIVER BANKS AND FLOODPLAINS
Bank d e u o s i t s The most i m p o r t a n t r i v e r bank d e p o s i t s a r e n a t u r a l l e v e e s and c r e v a s s e s p l y
.
They may b e developed on e i t h e r b r a i d e d o r meandering s t r e a m s b u t a r e b e s t developed on l a r g e meandering s t r e a m s w i t h a well-developed
floodplain l i k e the Mississippi
R i v e r (Fig.4.24).
N a t u r a l l e v e e s a r e formed when t h e f l o o d w a t e r s o f a s t r e a m o v e r t o p s i t s bank. V e l o c i t y i s reduced and d e p o s i t i o n o f t h e suspended s e d i m e n t r e s u l t s .
The c o a r s e s t
m a t e r i a l i s d e p o s i t e d n e a r e s t t h e c h a n n e l and g r a i n s i z e d e c r e a s e s away from t h e channel.
The s t r u c t u r e of n a t u r a l l e v e e d e p o s i t s h a s been d e s c r i b e d by F i s k (1944,
1 9 6 1 ) , Lattman (19601, A l l e n ( 1 9 6 5 a ) , Coleman (19691, and S i n g h (1972).
I n many
ways levee s e d i m e n t s resemble t h o s e found i n t h e upper p a r t of p o i n t b a r s .
They may
b e f i n e r g r a i n e d t h a n t h e a s s o c i a t e d c h a n n e l s e d i m e n t s a n d c o n s i s t o f s m a l l scale
205 TRANSITIONAL DEPOSITIONAL FACIES .-.-.-‘niw
3
2
+
FLOW .+.+.+‘LOW FLOW XRRENT IISPERSION IOSS’ 50’ 100‘ TRAT. U
DEPOSITIONAL FACIES
-*
-.WIGH” FLOW + ++‘LOW’ FLOW DUNE HEIGHT (CMI 0 25 50 75 I
T
I
I
0
0TRANSITIONAL INTERMEDIATE
HELM-MAIER
FULLY DEVELOPED
INTERMEDIATE DEPOSITIONAL FACIES
g
I
-. -.
-. _.
-
CURRENT
_-_
‘HIGH’ FLOW FLOW
+ + +‘LOW’
CROSS- W N E HElGHT EM1 5s IW. ITRAT. 0 25 XI I5
ISPERSION
II I.I I n@ I I
MI
Y SIZE
wni)
MEAN VEL. (CM*SECI ‘HIGH‘ FLOW -‘LOW’ FLOW
-
FULLY DEVELOPED DEPOSITIONAL FACIES t_l
I:
.: .:..:I
y ,
,
,
- 3 0
3
E
MEAN S I Z E (F
F i g . 4 . 2 3 . The v a r i a t i o n of d e p o s i t i o n a l f a c i e s i n the Lower Wabash River, I l l i n o i s . The i n s e t shows the d i s t r i b u t i o n of f a c i e s w i t h i n c r e a s i n g channel curvature. V e r t i c a l f a c i e s sequences a r e shown from upstream t o downstream on a p o i n t bar Of s u i t a b l e curvature ( a f t e r Jackson, 1975, 1 9 7 6 ) .
206
r i p p l e bedded and c r o s s - l a m i n a t e d s a n d s w i t h i n t e r b e d d e d muddy l a y e r s w i t h p a r a l l e l laminations,
I d e a l l y t h e s e a r e a r r a n g e d i n r e p e t i t i o n s of t h i n fining-upwards
sequences which mark t h e r e p e a t e d submergence and emergence d u r i n g s u c c e s s i v e f l o o d s (Fig.4.25).
The sandy and muddy l a y e r s a r e of v a r i a b l e t h i c k n e s s .
The mud h o r i z o n s
o f t e n show e v i d e n c e of s u b a e r i a l e x p o s u r e i n t h e form of d e s i c c a t i o n c r a c k s , o r raindrop impressions.
Organic m a t t e r i s abundant i n t h e s e h o r i z o n s b e c a u s e l e v e e s
may s u p p o r t much p l a n t growth.
They may a l s o b e e x t e n s i v e l y m o t t l e d due t o t h e
development of s o i l h o r i z o n s . 9ML M 2 O N LAMINATE0 MUD LAYER
SMALL RIPPLE CROSS BEDMND
MUD LAVER IFINELY LAYNATED I SMALL RIPPLE CROSS M O M N O MUD
LAYER
SMALL RIPPLE
CROSS B E D D M
LAMINAILD Y N D BEDDlHo I
IWRIZDNTAL
SMALL R I P R E
CROSS B E D D I M
Fig.4.24. N a t u r a l l e v e e s a n d r e l a t e d f e a t u r e s o f a meander o f t h e M i s s i s s i p p i R i v e r ( a f t e r F i s k , 1947).
Fig.4.25. Repeated c y c l e s of fining-upward sequences i n t h e n a t u r a l l e v e e d e p o s i t s of t h e Gomti R i v e r , I n d i a ( a f t e r Reineck and S i n g h , 1973).
207
The n a t u r a l levees of t h e Brahmaputra a r e r a t h e r broad and form a n i r r e g u l a r extension i n t o t h e flood b a s i n (Fig.4.26).
Most o f t h e f l o o d w a t e r e s c a p e s from
t h e r i v e r through d i s t i n c t c h a n n e l s r a t h e r t h a n a s s h e e t - l i k e o v e r f l o w s .
The bed
l o a d sediment o f t h e f l o o d w a t e r i s t h e n d e p o s i t e d a s a tongue o r f a n of s a n d c a l l e d a crevasse-splay.
The o v e r l a p p i n g o f c r e v a s s e s p l a y s produces t h e l e v e e s which
c o n s i s t of a complex o f i ' n t e r f i n g e r i n g and o v e r l a p p i n g sandy d e p o s i t s capped by muddy s e d i m e n t ,
A. A s i n g l e c r e v a s s e s p l a y on t h e Brahmaputra B. N a t u r a l l e v e e s formed Fig.4.26. by o v e r l a p p i n g c r e v a s s e - s p l a y s on t h e Brahmaputra ( a f t e r Coleman, 1969).
The s a n d l a y e r s may show l a r g e s c a l e c r o s s - b e d d i n g , h o r i z o n t a l - b e d d i n g and c r o s s l a m i n a t i o n (Fig.4.27).
Away from t h e c h a n n e l t h e sandy l a y e r s which o u t and muddy
sediments become more i m p o r t a n t , g r a d i n g l a t e r a l l y i n t o t h e f l o o d p l a i n sediments. Penecontemporaneous d e f o r m a t i o n s t r u c t u r e s may b e common. Floodplains R i v e r f l o o d p l a i n s a r e r a r e l y i n u n d a t e d and t h e most f r e q u e n t f l o o d s a r e g e n e r a l l y one t o two y e a r s a p a r t (Wolman and Leopold, 1957).
The v e l o c i t i e s o f overbank flows
a r e u s u a l l y r a t h e r h i g h and t h e o v e r a l l s e d i m e n t a t i o n r a t e i s low.
The c o a r s e r sandy
sediment i s u s u a l l y d e p o s i t e d n e a r t o t h e c h a n n e l margins w h i l s t f i n e r m a t e r i a l i s c a r r i e d f u r t h e r on t o t h e f l o o d p l a i n a r e a .
These f i n e r s e d i m e n t s a r e d e p o s i t e d from
.suspension and even d u r i n g m a j o r f l o o d s o n l y accumulate t o a t h i c k n e s s of one o r two centimetres.
The i n t e r n a l s t r u c t u r e o f f l o o d p l a i n d e p o s i t s i n c l u d e s h o r i z o n t a l
l a m i n a t i o n and c l i m b i n g r i p p l e c r o s s - l a m i n a t i o n (McKee e t a l . ,
1967) and i n some
s m a l l r i v e r s i t may b e d i f f i c u l t t o d i f f e r e n t i a t e t h e f l o o d p l a i n and c h a n n e l d e p o s i t s . G e n e r a l l y , however, t h e f i n e g r a i n s i z e and preponderence of h o r i z o n t a l l a m i n a t i o n s make f l o o d p l a i n and c h a n n e l d e p o s i t s r e a d i l y d i s t i n g u i s h a b l e .
The d r y i n g o u t of
f l o o d p l a i n s e d i m e n t s produces d e s i c c a t i o n c r a c k s which i n d i c a t e s u b a e r i a l exposure.
208
I n some a r e a s a e o l i a n a c t i v i t y i s i m p o r t a n t and wind-blown s i l t may accumulate i n l a r g e q u a n t i t i e s on r i v e r f l o o d p l a i n s (Lambrick, 1967).
I
LAMINATED SILTY CLAYS WITH BURROWS
HORIZONTAL BEDDING 2m
LARGE SCALE CROSS BEDDING
SAND AND SILT NO INTERNAL STRUCTURE EXCEPT
Fig.4.27. Sequence of sedimentary s t r u c t u r e s i n t h e n a t u r a l l e v e e d e p o s i t s o f t h e Brahmaputra (based on Coleman, 1969). The l o w e s t - l y i n g p a r t o f a f l o o d p l a i n i s g e n e r a l l y r e f e r r e d t o a s t h e f l o o d b a s i n . This i s a f l a t , f e a t u r e l e s s a r e a which i n humid-climate a r e a s i s p o o r l y d r a i n e d and thickly vegetated.
Semi-permanent
l a k e s o r backswamps may develop.
The abundant
v e g e t a t i o n c a u s e s r o o t d i s t u r b a n c e of sediment and s o i l p r o f i l e s may develop.
Under
f a v o u r a b l e c i r c u m s t a n c e s p e a t may accumulate. Interfluvial areas S o i l f o r m a t i o n i n i n t e r f l w i a l a r e a s i s a n i m p o r t a n t p r o c e s s which can be recognized i n a n c i e n t a l l u v i a l sequences.
A t t h e p r e s e n t t i m e one of t h e moat
i m p o r t a n t c o n t r o l s o f s o i l f o r m a t i o n i s c l i m a t e and t h e s i g n i f i c a n c e of a n c i e n t s o i l s is t h e i r p o t e n t i a l value a s palaeoclimatic indicators. S o i l s may develop a s r e c e n t l y d e p o s i t e d sediments o r p o o r l y c o n s o l i d a t e d bedrock and can be regarded a s a n amalgam of t h e l i t h o s p h e r e , t h e h y d r o s p h e r e , t h e b i o s p h e r e and t h e atmosphere.
They g e n e r a l l y c o n s i s t of d i s t i n c t l a y e r s , u s u a l l y known as
h o r i z o n s which t o g e t h e r form a s o i l p r o f i l e .
The n a t u r e o f t h e v a r i o u s h o r i z o n s
and t h e i r p o s i t i o n i n t h e s o i l p r o f i l e depends upon t h e n a t u r e of t h e s u b s t r a t e and a l s o on c l i m a t i c f a c t o r s .
The changes o f s o i l - t y p e which accompany changes i n
J
209 c l i m a t e and v e g e t a t i o n a r e i l l u s t r a t e d i n F i g . 4 . 2 8 .
Some s o i l s may t a k e hundreds
of thousands o r even m i l l i o n s of y e a r s t o develop and may have w i t n e s s e d a p p r e c i a b l e c l i m a t i c change.
C o n s i d e r a b l e c a r e i s t h e r e f o r e needed i n a s s e s s i n g t h e importance
of c l i m a t e i n b o t h p r e s e n t - d a y and a n c i e n t s o i l s .
wet. cold
Dry, hot Desert soils Desert shrubs
Brown SOIIS
Chestnut soils
Charnorem soils
P d d mils
SOllS
Broadleaf forest
Conifer forest
Tundra
SOllS
Tall
Short grass
Grey brown podzolic sals
Prairie
Tundra
Limit of soil water I
I
I
I I
I
I I
I I
Alkaline soils, closed system 8
Carbonates removed
Neutral I I
soils
I I
Acid soils open system
Fig.4.28. Changes i n s o i l p r o f i l e s which accompany changes i n c l i m a t e and v e g e t a t i o n between t h e Canadian Tundra and t h e d e s e r t s of s o u t h w e s t e r n USA. A t t h e 1 0 0 t h m e r i d i a n t h e a n n u a l p r e c i p i t a t i o n a v e r a g e s a b o u t 50cm ( a f t e r Hunt, 1 9 7 2 ) . The m a j o r i t y o f p r e s e n t - d a y s o i l s have a n o r g a n i c r i c h upper l a y e r which g r a d e s downwards i n t o more m i n e r a l dominated l a y e r s .
The p r e s e r v a t i o n p o t e n t i a l of t h i s
o r g a n i c l a y e r i s g e n e r a l l y low e x c e p t i n swamps and p e a t bogs where t h e w a t e r t a b l e
i s g e n e r a l l y h i g h and t h e d r a i n a g e v e r y p o o r .
Organic m a t t e r d o e s , however, have a
s t r o n g i n f l u e n c e on t h e r e s t o f t h e s o i l p r o f i l e by changing t h e composition o f downward p e r c o l a t i n g r a i n w a t e r .
The a c i d i t y of r a i n w a t e r may b e i n c r e a s e d by thee
a d d i t i o n of v a r i o u s o r g a n i c a c i d s and a s i t moves down through t h e s o i l p r o f i l e i t removes a l k a l i and a l k a l i e a r t h i o n s i n s o l u t i o n and a i d s t h e h y d r o l y s i s of s i l i c a t e g r a i n s and t h e f o r m a t i o n of c l a y m i n e r a l s .
Downward p e r c o l a t i o n may b e improved by
t h e r o o t p e n e t r a t i o n and o t h e r a c t i v i t i e s o f growing p l a n t s .
A familiar ancient
example o f s o i l f o r m a t i o n i n t h i s way i s p r o v i d e d by t h e s e a t e a r t h s below c o a l seams s e e n i n t h e Upper P a l a e o z o i c Coal Measures. which a r e o f t e n e n r i c h e d i n k a o l i n i t e .
These a r e r o o t p e n e t r a t e d s o i l s
Comparison w i t h p r e s e n t - d a y p e a t swamps
s u g g e s t s t h a t t h e k a o l i n i t e formed i n s i t u due t o l e a c h i n g by humic a c i d s which p e r c o l a t e d downwards from t h e o v e r l y i n g p e a t ( S t a u b a n d Cohen, 1978).
210 A t t h e p r e s e n t - d a y i n c e r t a i n t r o p i c a l r e g i o n s t h e r e a r e s o i l s forming which have These a r e t h e d u r i c r u s t s (Langford-
been a major i n f l u e n c e i n t h e r e d bed problem.
Smith and Dury, 1965; Goudie, 1973) which i n c l u d e c a l c r e t e , f e r r i c r e t e , and silcrete.
These may form v e r y t h i c k p r o f i l e s , i n some c a s e s up t o lOOm t h i c k , and
a r e c h a r a c t e r i z e d by a t h i n n e r i n d u r a t e d top-zone o r hardpan.
A t the present t i m e
d u r i c r u s t s show a c l o s e r e l a t i o n s h i p w i t h c l i m a t e and form i n a r i d o r s e m i - a r i d l r e g i o n s of s t a b l e geomorphic l e v e l s a l t h o u g h many o t h e r f a c t o r s i n c l u d i n g bedrock lithology a r e a l s o important. I n many p a r t s o f t h e world t h e 50Omm a n n u a l i s o h y e t seems t o be a s i g n i f i c a n t d i v i d i n g l i n e between c a l c r e t e and o t h e r t y p e s of d u r i c r u s t .
The f o r m a t i o n of
c a l c r e t e i s g e n e r a l l y r e g a r d e d t o t a k e p l a c e by a p e r descensum mechanism.(Goudie,
1973, p.136) i n which c a r b o n a t e i s l e a c h e d from t h e upper s o i l h o r i z o n s by downward p e r c o l a t i n g r a i n w a t e r and i s p r e c i p i t a t e d a t d e p t h a s c a l c i t e o r high-magnesian c a l c i t e during dry periods.
I t i s obvious t h a t t h e a c c u m u l a t i o n s o f any s u b s t a n t i a l
t h i c k n e s s o f c a l c r e t e r e q u i r e s t h e a d d i t i o n of c a r b o n a t e t o t h e s o i l s u r f a c e .
Much
of t h i s c a r b o n a t e i s b e l i e v e d t o b e d e r i v e d from a e o l i a n d u s t s which a r e i m p o r t a n t i n s e m i a r i d r e g i o n s b o r d e r i n g d e s e r t a r e a s (Goudie, 1973, T a b l e 4.1; Yaalon, 1973). S i l c r e t e s a r e c h a r a c t e r i s t i c of a r i d r e g i o n s b u t have n o t been so e x t e n s i v e l y s t u d i e d a s c a l c r e t e s , n o r a r e t h e y so abundant i n t h e s t r a t i g r a p h i c a l r e c o r d .
The
t h i c k e r s i l c r e t e p r o f i l e s i n modern d e s e r t s a r e g e n e r a l l y r e g a r d e d a s p a l a e o s o l s ( e . g . Mabbutt, 1967).
S i l c r e t e s show b r o a d l y s i m i l a r f e a t u r e s t o c a l c r e t e s and may
a l s o o r i g i n a t e by a p e r descensum mechanism.
The s i l i c a i s i n t h e form o f q u a r t z ,
c h a l c e d o n y , o r o p a l and may o c c a s i o n a l l y show o p t i c a l c o n t i n u i t y w i t h h o s t q u a r t z grains.
The s o u r c e o f t h e s i l i c a and i t s p r e c i s e r o l e i n s i l c r e t e f o r m a t i o n i s ,
however, v e r y p o o r l y known.
Bedrock seems t o b e a n i m p o r t a n t f e a t o r because i n t h e
w e s t e r n USA v o l c a n i c r o c k s seem t o r a p i d l y develop s o i l s w i t h well-developed s i l i c a cemented h o r i z o n s , whereas a l l u v i a l s o i l s d e r i v e d from g r a n i t i c r o c k s form s i l i c a cemented h o r i z o n s , o n l y v e r y s l o w l y ( F l a c h e t a l . , 1969). of s i l i c a may b e p r o v i d e d by d e s e r t g r a s s e s .
Another i m p o r t a n t s o u r c e
These a r e known t o have e x c e p t i o n a l l y
h i g h s i l i c a c o n t e n t s and s i l i c a i s cormnonly t h e most i m p o r t a n t element i n c i r c u l a t i o n i n t h e d e s e r t ecosystem (Rodin and B a z i l e v i c h , 1965).
A r i d s o i l s i n New S o u t h Wales
c o n t a i n well-rounded s i l t - s i z e d p a r t i c l e s v e r y s i m i l a r t o t h o s e found i n t h e l e a v e s of l o c a l g r a s s e s ( H a l l s w o r t h and Waring, 1964).
Such p a r t i c l e s c o u l d c o n c e i v a b l y
be t r a n s p o r t e d by wind t o b e s u b s e q u e n t l y m o b i l i z e d , presumably under a l k a l i n e c o n d i t i o n s , and d e p o s i t e d d e e p e r i n t h e s o i l p r o f i l e . F e r r i c r e t e o r l a t e r i t e shows many m o r p h o l o g i c a l f e a t u r e s s i m i l a r t o t h o s e o f c a l c r e t e b u t a p r e c i s e d e f i n i t i o n of l a t e r i t e i s n o t e a s y t o make ( s e e McFarlane,
1976 f o r a r e v i e w ) .
A r a t h e r l o o s e usage l i k e t h a t of S i v a r a j a s i n g h a m e t a l . (1962,
p.5) i s p r e f e r r e d and t h u s i n c l u d e s : " h i g h l y weathered m a t e r i a l 2 ) poor i n humus
1) r i c h i n s e c o n d a r y forms of i r o n , aluminium o r b o t h
3) d e p l e t e d of b a s e s and combined s i l i c a
4) w i t h o r w i t h o u t
211 n o n - d i a g n o s t i c s u b s t a n c e s s u c h a s q u a r t z , l i m i t e d amounts o f w e a t h e r a b l e primary m i n e r a l s o r s i l i c a t e c l a y s ; and 5 ) e i t h e r h a r d o r s u b j e c t t o h a r d e n i n g upon exposure t o a l t e r n a t e w e t t i n g and d r y i n g " . L a t e r i t e s do show a number of developmental s t a g e s which have been d e s c r i b e d by P u l l a n (1967) and McFarlane (1976).
The weathered c r y s t a l l i n e r o c k may be s l i g h t l y
f e r r u g i n i z e d and s l i g h t l y hardened and a t t h i s s t a g e i s known a s v e r m i c u l a r i r o n s t o n e . Continued development l e a d s t o i n c r e a s e d f e r r u g i n i z a t i o n and v e s i c u l e s , 5mm i n d i a m e t e r and f i l l e d w i t h p i n k e a r t h y m a t e r i a l , become a b u n d a n t , l a t e r i t i c ironstone.
This i s a v e s i c u l a r
Continued f e r r u g i n i z a t i o n i n c r e a s e s t h e h a r d n e s s and i f t h e
m a t e r i a l i n t h e mermicules and v e s i c u l e s i s washed o u t a c e l l u l a r i r o n s t o n e i s produced.
N o d u l a r , o o l i t i c , and p i s o l i t i c s t r u c t u r e s a r e common i n l a t e r i t e s and,
a s i n c a l c r e t e s , a r e u s u a l l y cemented t o g e t h e r .
McFarlane (1969) d i s t i n g u i s h e s
between s p a c e d p i s o l i t i c l a t e r i t e and packed p i s o l i t i c l a t e r i t e .
The former
comprises d a r k r e d p i s o l i t e s and o o l i t h s s e p a r a t e d by a m a t r i x of k a o l i n i t e and quartz.
T h i s t y p e does n o t harden on exposure and i s r e a d i l y e r o d e d on exposure
t o produce a remanie of l o o s e p i s o l i t h s .
Packed p i s o l i t i c l a t e r i t e on t h e o t h e r
hand has p i s o l i t h s r e s t i n g one a g a i n s t t h e o t h e r , and t h e y a r e u s u a l l y cemented t o form a tough, hardened mass. L a t e r i t e p r o f i l e s a r e more complex t h a n t h o s e o f c a l c r e t e and i n i n s i t u l a t e r i t e s may c o n t a i n up t o s i x main e l e m e n t s . u n d e r l a i n by t h e i r o n s t o n e c r u s t ,
A t t h e s u r f a c e t h e r e i s a s o i l zone which i s
Below t h e c r u s t ( c u i r a s s e o r c a r a p a c e a c c o r d i n g
t o t h e d e g r e e o f i n d u r a t i o n ) i s a m o t t l e d zone, a p a l l i d zone, a s i l i c i f i e d zone, and t h e z e r s a t z zone.
T h i s p r o f i l e o f t e n shows marked c o l o u r v a r i a t i o n s w i t h t h e
i n d u r a t e d zone b e i n g d a r k i s h r e d o r brown (5yR 3 / 4 ) , t h e p a l l i d zone b e i n g r a t h e r p a l e (5Y 7 / 2 ) w i t h low i r o n c o n t e n t , and t h e m o t t l e d zone b e i n g somewhat mixed (10R 5 / 8 ) ( F a n i r a n , 1970).
The ' z e r s a t z ' zone c o n s i s t s of s o f t , h i g h l y a l t e r e d m a t e r i a l which
r e t a i n s t h e s t c u c t u r e of t h e r o c k from which i t formed. The r e d c o l o u r of l a t e r i t e s i s p r i n c i p a l l y due t o h a e m a t i t e ( a - F e 0 ) and 2 3 g o e t h i t e (o( -FeOOH) a l t h o u g h l e p i d o c r o c i t e ('y-FeOOH) ( d ' C o s t a e t a l . , 1966) and maghaematite ( 7 - F e 0 ) ( B o n i f a s , 1959) have a l s o been i d e n t i f i e d . O t h e r i m p o r t a n t 2 3 c o n s t i t u e n t s a r e k a o l i n i t e and h a l l o y s i t e and a l s o g i b b s i t e A1(0HI3 , boehmite y-AlO(OH),
and d i a s p o r e
%-AlO(OH)
.
Amorphous forms of aluminium o x i d e such
a s c l i a c h i t e (Maignien, 1966) a l s o o c c u r . The g e n e s i s o f l a t e r i t e i s s t i l l o n l y p o o r l y u n d e r s t o o d and t h e r e a r e s t i l l a v a r i e t y o f t h e o r i e s i n c i r c u l a t i o n ( s e e McFarlane, 1976, p .9 1 - 1 0 8 ) .
Intense
t r o p i c a l w e a t h e r i n g o r l o n g e x t e n d e d w e a t h e r i n g l e a d s t o t h e r e l a t i v e accumulation of r e s i d u a l o x i d e s o f aluminium, i r o n , manganese, and t i t a n i u m .
The k a o l i n i t e o r
h a l l o y s i t e which may b e produced by t h e w e a t h e r i n g of b a s i c r o c k s i s e v e n t u a l l y s u b j e c t e d t o s i l i c a loss and c o n v e r t e d t o g i b b s i t e .
The s i l i c a may b e l e a c h e d o u t
of t h e system i n r i v e r w a t e r , d e p o s i t e d a t d e p t h i n t h e p r o f i l e , o r t r a n s p o r t e d l a t e r a l l y f o r v a r y i n g d i s t a n c e s t o form s i l c r e t e ( S t e p h e n s , 1971).
212 Two main mechanisms have been p u t forward f o r t h e development of i n s i t u l a t e r i t e s . The f i r s t of t h e s e i s t h a t l a t e r i t e i s a residuum of t h e r e l a t i v e l y immobile endp r o d u c t s of w e a t h e r i n g . constituent oxides.
The mechanism i m p l i e s two p h a s e s of l i m i t e d m o b i l i t y of t h e
F i r s t , a s h o r t - l i v e d phase when t h e m a t e r i a l s t o b e c o n c e n t r a t e d
a r e r e l e a s e d from t h e i r p a r e n t m a t e r i a l by w e a t h e r i n g p r o c e s s e s and re-grouped i n t o r e l a t i v e l y immobile p r e c i p i t a t e s ; s u b s e q u e n t l y , a s e c o n d , m o b i l e p h a s e , d u r i n g t h e r e s o l u t i o n o r a l t e r a t i o n of t h e residuum, p r o b a b l y by groundwater. mechanism e n v i s a g e s l a t e r i t e a s a p r e c i p i t a t e .
The second main
I n t h e s i m p l e s t c a s e o x i d e s of i r o n
and aluminium a r e t h o u g h t t o be d e r i v e d from t h e p a l l i d zone and t r a n s p o r t e d upwards t o be p r e c i p i t a t e d i n a zone of e n r i c h m e n t .
C a p i l l a r i t y and s e a s o n a l f l u c t u a t i o n s
of t h e w a t e r t a b l e have b o t h been s u g g e s t e d a s t h e c a u s e of l a t e r i t e e n r i c h m e n t . There a r e a number o f problems w i t h b o t h t h e s e models of l a t e r i t e f o r m a t i o n (Goudie, 1973, p.141-144;
McFarlane, 1971, 1976, p.93-94) and most workers now
f a v o u r a compromise i n which l a t e r i t e i s c o n s i d e r e d a s a r e s i d u a l p r e c i p i t a t e ( T r e n d a l l , 1962; de Swardt, 1964; McFarlane, 1976).
Although t h e r e a r e v a r i a t i o n s
on t h e theme t h e model developed f o r Ugandan l a t e r i t e s by McFarlane (1971) s e r v e s t o i l l u s t r a t e t h e mechanism.
I n i t i a l l y i r o n is segregated i n t o p i s o l i t h s within
t h e narrow range o f w a t e r t a b l e o s c i l l a t i o n d u r i n g a l a t e s t a g e of l a n d s u r f a c e The zone of p r e c i p i t a t i o n lowers a s t h e l a n d s u r f a c e i s reduced and t h e
reduction.
p r e c i p i t a t e s accumulate a s a s h e e t a t t h e b a s e o f t h e s o i l .
Eventually the land
s u r f a c e r e d u c t i o n c e a s e s and t h e accumulated s h e e t of p i s o l i t h s becomes a l t e r e d and h y d r a t e d t o form a massive v a r i e t y o f l a t e r i t e .
On t h e i n i t i a t i o n of t h e s u c c e e d i n g
c y c l e of e r o s i o n t h e w a t e r t a b l e once a g a i n b e g i n s t o lower and p i s o l i t h f o r m a t i o n resumes.
With c o n t i n u e d l o w e r i n g t h e zone o f p i s o l i t h f o r m a t i o n lowers l e a v i n g above
i t a spread of p i s o l i t h s .
E v e n t u a l l y t h e w a t e r t a b l e i s u l t i m a t e l y lowered beyond
t h e d e p t h a t which p i s o l i t h s can form and l e a c h i n g t a k e s p l a c e t h r o u g h t h e c a r a p a c e d e p l e t i n g t h e s a p r o l i t e and forming a p a l l i d zone.
F i n a l l y deforestation leads t o
i n d u r a t i o n o f t h e l a t e r i t e and i t s loss of p e r m e a b i l i t y . L a t e r i t e a p p e a r s t o form t h e r e f o r e a s a r e s i d u a l p r e c i p i t a t e d u r i n g l a n d s u r f a c e reduction.
An i m p o r t a n t f e a t u r e of t h i s model does n o t l i m i t t h e f o r m a t i o n of
l a t e r i t e t o t h e s t a b l e p r o f i l e of a p l a n a t i o n s u r f a c e , a b e l i e f which i s s t i l l widely held. L a t e r i t e s a r e i m p o r t a n t i n r e d bed s t u d i e s b e c a u s e e a r l i e r workers b e l i e v e d t h a t they s u p p l i e d much d e t r i t a l h a e m a t i t e and g o e t h i t e t o f l u v i a l s e d i m e n t s .
This
o p i n i o n i s no l o n g e r w i d e l y h e l d mainly b e c a u s e Recent f l u v i a l s e d i m e n t s i n t h e t r o p i c s a r e n o t r e d and a l s o b e c a u s e t h e geomorphic environment of l a t e r i t e g e n e s i s does n o t f a v o u r l a r g e s c a l e f l u v i a l e r o s i o n .
There a r e , u n f o r t u n a t e l y , no d i r e c t
s t u d i e s of t h e f l u v i a l e r o s i o n o f l a t e r i t e on which t o b a s e t h i s a s s u m p t i o n .
The
p s e u d o k a r s t i c f e a t u r e s w i d e l y r e p o r t e d i n l a t e r i t e s ( s e e Goudie, 1973, p.46-48) i n d i c a t e i m p o r t a n t s u b t e r r a n e a n e r o s i o n b u t c o u l d s u p p l y o n l y r e l a t i v e l y small amounts of i r o n o x i d e t o a f l u v i a l system.
213 THE TRANSPORT OF IRON I N RECENT ALLUVIUM
Recent a l l u v i u m i s t y p i c a l l y non-red and c o n t a i n s a l a r g e p r o p o r t i o n o f c r y s t a l l i n e m a t e r i a l eroded from bedrock.
Even i n warm, s e a s o n a l l y humid c l i m a t e s
where reddish-brown s o i l s a r e common i n t h e s o u r c e a r e a , t h e a l l u v i u m i s g e n e r a l l y brown o r brownish-yellow i n c o l o u r .
Haematite m a n t l e s do n o t g e n e r a l l y develop
w i t h i n t h e zone o f i n f l u e n c e of r i v e r c h a n n e l s because s u f f i c i e n t time i s n o t available.
Consequently t h e y a r e unimportant as a s o u r c e of r e d d e t r i t u s in. Recent
a 11w i u m . T h i s c o n t r a s t s w i t h t h e view o f Krynine (1950) who r e p o r t e d examples o f Recent r e d sediments on t h e c o a s t a l piedmont of Tabasco i n s o u t h e r n Mexico and s u g g e s t e d t h a t t h e c o l o u r a t i o n was produced by " r e d mud d e r i v e d from t h e r e d s o i l s washed E a r l i e r , Krynine (1949) had argued
down t h e s l o p e s from t h e mountain i n t e r f l u v e s l ' .
t h a t t h i s was t h e p r i n c i p a l mechanism o f r e d bed f o r m a t i o n i n a n c i e n t f l u v i a l d e p o s i t s and c a l l e d such d e p o s i t s primary r e d b e d s ,
T h i s view was w i d e l y h e l d p r i o r t o t h e
p u b l i c a t i o n s o f Walker (1967a,b) who r e v i s i t e d t h e Tabasco a r e a and showed t h a t Krymine's view i s n o t s u p p o r t e d by t h e c o l o u r of t h e r i v e r a l l u v i u m o r of t h e Recent sediments. Samples were c o l l e c t e d by Walker (1967b) from a l o n g t h e e n t i r e l e n g t h o f t h e Mezcalapa R i v e r from t h e Mexico-Guatemala
b o r d e r t o t h e Gulf o f Mexico (Fig.4.29).
The c o l o u r o f t h e sediments ranges from v e r y d a r k g r e y i s h brown (10 R 3 / 2 ) t o g r e y i s h brown (10 R 5 / 2 ) , t h e c o m o n e s t c o l o u r b e i n g d a r k g r e y i s h , b r o w n (10 R 4 / 2 ) .
Walker
d i d n o t see any Recent r e d alluvium. The s c a r c i t y o f r e d a l l u v i u m i n t h i s t r o p i c a l a r e a i s due t o t h e f a c t t h a t r e d s o i l s are n o t abundant i n t h e s o u r c e a r e a .
Most of t h e soils a r e yellow o r brown
i n c o l o u r and r e d pigment o n l y o c c u r s i n h i g h l y a l t e r e d s o i l s which have formed on s u r f a c e s t h a t a r e o f P l e i s t o c e n e age o r o l d e r ( P s u t y , 1965).
Even i n t h e s e c a s e s
o n l y t h e topmost metre o r so o f t h e s o i l i s r e d and t h e m a j o r i t y o f t h e p r o f i l e i s y e l l o w o r brown i n c o l o u r .
Rapid e r o s i o n e n s u r e s t h a t t h e r e d d e t r i t u s i s q u i c k l y
d i l u t e d and masked by t h e more abundant non-red d e t r i t u s . The r e d d e s t modern a l l u v i u m o c c u r s i n some t r o p i c a l r i v e r s such a s t h o s e i n t h e Orinoco Basin (Walker, 1974).
G e n e r a l l y t h e a l l u v i u m i s y e l l o w i s h o r brownish i n
c o l o u r (2.5Y t o 1OYR) b u t d u r i n g p e r i o d s o f heavy run-off a f t e r s t r o n g storms a r e d d i s h c o l o u r e d ( 5 R 6/61 suspended l o a d i s o f t e n s e e n b u t t h i s d r i e s t o a l i g h t brown ( 7 . 5 R 6/41.
Red pigment s u p p l i e d by t h e e r o s i o n o f r e d s o i l s i s q u i c k l y
masked by non-red d e t r i t u s and some may b e l o s t by r e d u c t i o n of i r o n o x i d e d u r i n g t r a n s p o r t a t i o n (Hinze and Meischner, 1968). Examination o f Recent a l l u v i u m , t h e r e f o r e , s u g g e s t s t h a t i t i s n o t c o l o u r e d r e d by d e t r i t u s eroded from r e d s o i l s .
There i s no e v i d e n c e t h a t Recent t r o p i c a l
a l l u v i u m , or any o t h e r f o r t h a t matter, has a predominantly r e d suspended l o a d , S i m i l a r l y t h e r e i s no e v i d e n c e t h a t t h e muds and s i l t s on t h e a c t i v e f l o o d p l a i n s
214
Fig.4.29.
L o c a l i t y map of s o u t h e r n Mexico ( a f t e r Walker, 1967b).
of these r i v e r s a r e red i n colour.
I t must be c o n c l u d e d , t h e r e f o r e , t h a t t h e
mechanism of primary r e d bed f o r m a t i o n a s d e s c r i b e d by Krynine ( 1 9 4 9 ) , i s n o t s u b s t a n t i a t e d by t h e i n v e s t i g a t i o n of Recent a l l u v i u m . The f e r r u g i n o u s s o i l s of t r o p i c a l r e g i o n s do s u p p l y c l a y f r a c t i o n s pigmented w i t h yellow and brown amorphous i r o n hydroxides t o Recent a l l u v i u m (Van Houten, 1964, 1972). hydroxides.
Red pigment may be p r e s e n t and be masked by t h e more abundant brown I n t r o p i c a l a l l u v i u m eroded from igneous and metamorphic t e r r a i n s
t h e r e a r e a l s o abundant f r e s h s i l i c a t e g r a i n s i n c l u d i n g f e r r o m a g n e s i a n m i n e r a l s . I n t h i s r e s p e c t t h e a l l u v i u m o f a l l c l i m a t i c regimes i s s i m i l a r .
This i s i l l u s t r a t e d
by t h e work of Gibbs (1977) who made a d e t a i l e d s t u d y of t h e t r a n s p o r t a t i o n of i r o n and o t h e r t r a n s i t i o n e l e m e n t s ( C r , Mn, Co, N i , and Cu) i n two l a r g e r i v e r s , t h e Amazon and t h e Yukon.
These r i v e r s d r a i n b a s i n s which have a wide v a r i e t y o f rock
t y p e s and environments, and can b e c o n s i d e r e d r e p r e s e n t a t i v e of t h e i r r e s p e c t i v e
215 c l i m a t i c z o n e s , one t r o p i c a l , t h e o t h e r t u n d r a ,
Gibbs ( 1 9 7 7 ) d i s t i n g u i s h e d between:
d i s s o l v e d m a t e r i a l s , a d s o r b e d m a t e r i a l , m e t a l l i c c o a t i n g s (mainly f e r r i c h y d r o x i d e ) , o r g a n i c complexes and c r y s t a l l i n e m a t e r i a l s . I r o n i s t r a n s p o r t e d m o s t l y a s c r y s t a l l i n e p a r t i c l e s and a s f e r r i c hydroxide c o a t i n g s on t h e s e p a r t i c l e s .
A b s o l u t e v a l u e s of i r o n i n v a r i o u s t r a n s p o r t phases
a r e g i v e n i n T a b l e 4.1 and a r e l a t i v e comparison of t h e Amazon and Yukon R i v e r s i s shown i n Fig.4.30.
The Amazon c a r r i e s 47.2% of i t s t o t a l i r o n a s g r a i n c o a t i n g s and
45.5% i n t h e form o f c r y s t a l l i n e p a r t i c l e s .
I n t h e Yukon R i v e r 40.6% o f t h e t o t a l
i r o n i s c a r r i e d i n t h e form of g r a i n c o a t i n g s and 48.2% i n t h e form of c r y s t a l l i n e TABLE 4.1.
C o n c e n t r a t i o n of i r o n i n t r a n s p o r t p h a s e s i n r i v e r w a t e r and suspended sediment f o r t h e Amazon and Yukon R i v e r . R i v e r w a t e r v a l u e s i n micrograms p e r l i t r e ; s o l i d - p h a s e v a l u e s i n ppm ( a f t e r G i b b s , 1977). Fe
S o l u t i o n and o r g a n i c complexed
Amazon River Solid
34.0
Yukon River Solid
50.0
Sorbed Material
Metallic Coatings
Organic Solids
Crystalline Particles
TOTAL
-
1.3
2 ,390
1.4
26,000
333 3 ,720
2,310 25 ,800
5,065.3 55,521.4
10.0
3 ,600 25,720
976 6,970
4,270 30,500
8,906.0 63,197.5
7.5
Fe
AMAZON
YUKON
P r o p o r t i o n s of t o t a l and a v a i l a b l e i r o n t r a n s p o r t e d i n v a r i o u s p l a s e s Fig.4.30. f o r Amazon and Yukon R i v e r s ( a f t e r G i b b s , 1977).
216 particles.
Only t h e o r g a n i c s o l i d p h a s e t r a n s p o r t s a s i g n i f i c a n t minor amount o f
t h e t o t a l i r o n , 6.6% i n t h e Amazon, and 11%f o r t h e Yukon R i v e r .
The i r o n p h a s e s
t r a n s p o r t e d i n s o l u t i o n and s o r b e d on s o l i d s e a c h c o n t r i b u t e l e s s t h a n 1%of t h e t o t a l i r o n t r a n s p o r t e d by t h e two r i v e r s .
The v a s t m a j o r i t y o f t h e a v a i l a b l e i r o n
i n b o t h r i v e r s is t h u s i n t h e form of t h e g r a i n c o a t i n g s ( F i g . 4 . 3 0 ) . The p o s s i b l e s i z e d i s t r i b u t i o n of i r o n t r a n s p o r t p h a s e s f o r t h e Amazon and Yukon a r e shown i n Fig.4.31.
These p h a s e s v a r y w i t h p a r t i c l e s i z e b u t t h e r e i s a remark-
a b l e s i m i l a r i t y between t h e two r i v e r s .
Fe AMAZON
I
5
h
YUKON
4-
iw 3 -
5
/-2?+&
0 0.1 PARTICLE DIAMETER (pn)
1.0 10 100 7.50 PARTICLE DIAMETER ( r m )
Fig.4.31. Concentration of i r o n i n various transporting phases f o r various p a r t i c l e s i z e s i n Amazon and Yukon R i v e r s ( a f t e r G i b b s , 1977). S u r f a c e r e l a t e d phenomena
-
g r a i n c o a t i n g s , o r g a n i c m a t e r i a l and s o r b e d m a t e r i a l
show an 80-9977 d e c r e a s e i n i r o n from f i n e t o c o a r s e r g r a i n e d m a t e r i a l ,
Crystalline
p a r t i c l e s show a d e c r e a s e o f 57% and 45% i r o n w i t h p a r t i c l e s i z e f o r t h e Amazon and Yukon R i v e r s r e s p e c t i v e l y , p r o b a b l y b e c a u s e o f i n h e r e n t m i n e r a l o g i c a l v a r i a t i o n s (Gibbs , 1967b). The importance of m e t a l l i c g r a i n c o a t i n g s a s a t r a n s p o r t o f phase of i r o n and t h e o t h e r t r a n s i t i o n m e t a l s l e d Gibbs (1977) t o a more d e t a i l e d i n v e s t i g a t i o n o f i t s n a t u r e and o r i g i n .
The t h i c k n e s s o f t h e m e t a l l i c g r a i n c o a t i n g s was c a l c u l a t e d
from t h e i r o n c o n c e n t r a t i o n and p a r t i c l e d i a m e t e r i n e a c h s i z e f r a c t i o n w i t h d e n s i t y and shape assumed.
M i c r o s c o p i c e x a m i n a t i o n s s u g g e s t e d t h e b e s t assumptions o f shape
were p l a t e s f o r m a t e r i a l less t h a n 3-5ym and cubes f o r m a t e r i a l g r e a t e r t h a n 3.5ym. From t h i s t h e s u r f a c e a r e a p e r g r a i n o f m a t e r i a l i s c a l c u l a t e d a n d , assuming a d e n s i t y
217 of 2.8 f o r Fe(OH)3, t h e t h i c k n e s s of a uniform c o a t i n g on t h e p a r t i c l e s f o r each s i z e f r a c t i o n can b e deduced ( T a b l e 4.2 ). TABLE 4.2.
C h a r a c t e r i s t i c s of t r a n s p o r t e d s e d i m e n t s i n Amazon and Yukon R i v e r s ( a f t e r Gibbs,
1977). Particle Shape
Particle diameter
Surface area ( m2/g)
(ym)
Plate
Cube
Fe c o n c e n t r a t i o n i n coating
(70)
Fe(OH3) c o a t i n g thickness
P)
Amazon
Yukon
Amazon
Yukon
0.1
5.2885 x 10l3
4.15
5.3
0.00028
0.00036
0.35
1.1916 x
4.89
3.8
0.0015
0.0011
1.25
3.337
x 10l2
4.28
3.0
0.0046
0.0032
3.5
1.1916 x 1OI2
1.75
2.3
0.0052
0.0069
12.5
1.8459 x 10"
0.37
0.5
0.0071
0.0097
47
4.909
x 10"
0.29
0.2
0.021
0.014
100
2.300
x 10"
0.21
0.33
0.033
0.051
250
9.2306 x 10
0.16
0.56
0.061
0.217
The o r i g i n of t h e s e i r o n hydroxide g r a i n c o a t i n g s i s of p a r t i c u l a r i n t e r e s t because c l a y - i r o n o x i d e c o a t i n g s a r e s u c h a c h a r a c t e r i s t i c f e a t u r e o f a n c i e n t r e d beds.
Gibbs (1977, p.838) c o n s i d e r e d two hypotheses.
The f i r s t was t h a t t h e
c o a t i n g s were p r e c i p i t a t e d i n e q u i l i b r i u m w i t h t h e r i v e r w a t e r w h i l s t t h e p a r t i c l e s were f r e e l y suspended.
T h i s i d e a i s n o t s u p p o r t e d by t h e f a c t t h a t t h e g r a i n
c o a t i n g s a r e n o t of uniform t h i c k n e s s a s one might e x p e c t them t o be.
Also physical
a b r a s i o n o r chemical d i s s o l u t i o n i s n o t a l i k e l y mechanism t o e x p l a i n t h e observed r e l a t i o n s h i p o f t h i c k e r g r a i n c o a t i n g s on c o a r s e r p a r t i c l e s .
The second h y p o t h e s i s ,
and t h e ope c o n s i d e r e d t o b e t h e most l i k e l y e x p l a n a t i o n by Gibbs (1967a), i s t h a t t h e g r a i n c o a t i n g s were formed i n a w e a t h e r i n g ( s o i l ) environment.
He a r g u e d t h a t
c o a r s e r g r a i n s developed a t h i c k e r hydroxide c o a t i n g because t h e y had a h i g h e r p e r m e a b i l i t y which a l l o w e d a g r e a t e r s u p p l y o f p r e c i p i t a t i n g i o n s , whereas t h e f i n e r p a r t i c l e s i n t h e s o i l c r e a t e d a lower p e r m e a b i l i t y and reduced t h e s u p p l y of prec i p i t a t i n g ions. Van Houten (1972) s t u d i e d t h e i r o n and c l a y c o n t e n t o f t r o p i c a l Savanna a l l u v i u m a l o n g t h e f l a n k s o f t h e S i e r r a Nevada de S a n t a M a r t a , n o r t h e r n Colombia.
Here upland
s o i l s a r e r e d d i s h t o r e d d i s h brown i n c o l o u r b u t t h e suspended l o a d of t h e l a r g e r r i v e r s d r a i n i n g t h e a r e a i s y e l l o w i s h - g r e y t o p a l e brown (cf. Walker, 1974).
Also
t h e f r e s h a l l u v i u m and v e r y immature a l l u v i a l s o i l s a r e y e l l o w i s h - g r e y t o shades of brown (10 R t o 5 Y ) a l t h o u g h some o f t h e o l d e r a l l u v i u m may b e r e d t o r e d d i s h brown
(10R t o 5YR).
The a l l u v i u m c o n t a i n s r e l a t i v e l y abundant g r a i n s o f f r e s h i r o n - b e a r i n g
218 m i n e r a l s i n c l u d i n g m a g n e t i t e , amphibole, b i o t i t e , c h l o r i t e , h a e m a t i t e , i l m e n i t e and pyroxene.
T h i s i s c o n s i s t e n t w i t h t h e r e c o g n i t i o n of u n a l t e r e d m i n e r a l s u i t e s
i n o t h e r t r o p i c a l r i v e r s l i k e t h e Amazon ( G i b b s , 1967a) and t h e E u p h r a t e s ( P h i l i p , 1968) and i s a n i n d i c a t i o n t h a t a c t i v e l y e r o d e d t r o p i c a l uplands y i e l d f r e s h m i n e r a l g r a i n s d e r i v e d from b e d r o c k ( V a n Andel, 1959; Van Houten, 1968) a s w e l l a s g r a i n s d e r i v e d from s o i l h o r i z o n s (Gibbs, 1977).
Unaltered mineral s u i t e s a r e a l s o t y p i c a l
of r i v e r s i n t e m p e r a t e r e g i o n s (Packham e t a l . , 1961). show some s i g n s of m i n e r a l a l t e r a t i o n .
The Colombian a l l u v i u m does
I n t h e B horizon of red s o i l s black oxides
a r e less abundant and much m a g n e t i t e has a p p a r e n t l y been c o n v e r t e d t o pigment; amphibole and b i o t i t e a r e a l s o reduced i n abundance b u t t h e r e a r e few obvious s i g n s of p a r t i a l l y a l t e r e d amphiboles. Clay m i n e r a l s i n t h e Colombian a l l u v i u m i n c l u d e m o n t m o r i l l o n i t e (M), i l l i t e ( I ) , mixed l a y e r I M , k a o l i n i t e and c h l o r i t e .
The mixed l a y e r c l a y s and t h e k a o l i n i t e
were a l m o s t c e r t a i n l y d e r i v e d from t h e upland s o i l s and f u r t h e r e v i d e n c e o f t h i s i s p r o v i d e d by t h e p r e s e n c e of 10-26% amorphous t o v e r y d i s o r d e r e d a l u m i n o s i l i c a t e . Such m a t e r i a l i s a c h a r a c t e r i s t i c component o f humid c l i m a t e s o i l s and i s f r e q u e n t l y found i n a s s o c i a t i o n w i t h f e r r i c hydroxide ( M i t c h e l l e t a l . , 1964; F o l l e t t e t a l . , 1965). I r o n c o n t e n t i s c o n c e n t r a t e d i n t h e c l a y f r a c t i o n s o f t h e Colombian a l l u v i u m (Fig.4.32).
I n t h e s a n d - s i l t f r a c t i o n t h e amount of t o t a l i r o n c o r r e l a t e s c l o s e l y
w i t h t h e amount o f i r o n - b e a r i n g d e t r i t a l g r a i n s s u c h a s m a g n e t i t e and b i o t i t e .
The
o l d e r and r e d d e r t e r r a c e d e p o s i t s show a d e c r e a s e i n t o t a l i r o n and a l s o e v i d e n c e of t h e removal o f i r o n from amphibole and b i o t i t e g r a i n s ; most of t h e i r o n i n m a g n e t i t e i s r e t a i n e d i n s p e c u l a r h a e m a t i t e p a r t i c l e s produced by p o s t - d e p o s i t i o n a l alteration.
I n t h e c l a y f r a c t i o n s t h e amount of i r o n v a r i e s :
12.8-15.6% o c c u r s
i n r e d s o i l s , 5.5-8.0% i n a l l u v i u m on b a n k s , 4.8-9.0% i n f l o o d p l a i n s e d i m e n t s and 3.0-5.877
i n older terrace deposits.
An a v e r a g e of 75% of t h e t o t a l i r o n i s c o n t a i n e d
i n t h e c l a y f r a c t i o n s o f t h e a l l u v i u m b u t r a t h e r s u r p r i s i n g l y t h e o l d e r and r e d d e r t e r r a c e d e p o s i t s c o n t a i n l e s s i r o n i n b o t h t h e c l a y and t h e s a n d - s i l t f r a c t i o n s . T h i s i m p l i e s t h a t t h e i n c r e a s e i n r e d n e s s h a s n o t been accompanied by a n i n t r a s t r a t a l t r a n s f e r of i r o n from t h e s a n d - s i l t f r a c t i o n t o t h e c l a y f r a c t i o n .
The Colombian
a l l u v i u m c o n t a i n s a n a v e r a g e of 0.75% f r e e ( e x t r a c t a b l e ) i r o n which c o n s i s t s p r i n c i p a l l y of brown l i m o n i t i c m a t e r i a l d e r i v e d from upland s o i l s .
The main
c o n s t i t u e n t s of l i m o n i t e a r e p r o b a b l y amorphous o r p o o r l y c r y s t a l l i n e Fe(0H) g o e t h i t e (FeOOH).
3
and
The m i n e r a l s a r e known t o b e u n s t a b l e r e l a t i v e t o h a e m a t i t e i n
t h e g e o l o g i c a l environment ( B e r n e r , 1969a) and under f a v o u r a b l e c i r c u m s t a n c e s ( o x i d i z i n g c o n d i t i o n s ) w i l l i n v e r t t o r e d pigmentary h a e m a t i t e by d e h y d r a t i o n .
The
p o s t - d e p o s i t i o n a l a g e i n g of f e r r i c hydroxide i n t h i s manner has long been recognized a s a l i k e l y mechanism f o r t h e f o r m a t i o n of a n c i e n t r e d b e d s .
Post-depositional
r e d d e n i n g i s l i k e l y t o o c c u r p r o v i d i n g t h a t t h e r e i s s u f f i c i e n t f e r r i c hydroxide i n t h e sediment which i s f i n e g r a i n e d and d i s p e r s e d t h r o u g h o u t t h e m a t r i x .
Also,
219 s u i t a b l e geochemical c o n d i t i o n s a r e r e q u i r e d , i n c l u d i n g low o r g a n i c c o n t e n t and Eh-pH r a n g e s w i t h i n t h e s t a b i l i t y f i e l d o f h a e m a t i t e .
The f o r m a t i o n o f h a e m a t i t e
i s a l s o f a v o u r e d by i n c r e a s e d a d s o r b e d c a l c i u m and magnesium i n t h e c l a y f r a c t i o n ( T a y l o r and G r a l e y , 1967) and a d s o r p t i o n o f o x a l a t e i o n s by amorphous f e r r i c hydroxide (Schwertmann, 1970).
ON BANK FLOOD PLAIN LOW TERRACE
Fig.4.32. Weight % t o t a l i r o n i n a l l u v i u m from n o r t h e r n Colombia. 1. I n c l a y f r a c t i o n . 2 . I n s a n d - s i l t f r a c t i o n ( a f t e r Van Houten, 1972). P r e c i s e documentation o f f e r r i c hydroxide ' a g e i n g ' i n Recent a l l u v i u m has n o t been made.
D i f f i c u l t y a r i s e s b e c a u s e o f t h e time-dependent n a t u r e of t h e p r o c e s s .
Many o l d e r t e r r a c e d e p o s i t s a r e r e d d e r b u t i t is n o t c l e a r whether t h i s i s due t o h a e m a t i t e formed by i n t r a s t r a t a l a l t e r a t i o n of d e t r i t a l f e r r o m a g n e s i a n s i l i c a t e s o r by t h e d i r e c t a g e i n g of d e t r i t a l f e r r i c h y d r o x i d e s .
The former p r o c e s s has been
documented i n Recent t r o p i c a l a l l u v i u m by Walker (1974) and t h e amount of f e r r i c
hydroxide i n t r o p i c a l a l l u v i u m i s c e r t a i n l y s u f f i c i e n t t o produce r e d pigment p o s t - d e p o s i t i o n a l l y (Walker and Honea, 1969).
The r e l a t i v e importance o f t h e two
p r o c e s s e s i s l i k e l y t o depend on t h e g r a i n s i z e and m i n e r a l o g y o f t h e a l l u v i u m .
In
f i n e g r a i n e d a l l u v i a l f l o o d p l a i n s , w i t h abundant c l a y and s i l t , d e t r i t a l f e r r i c hydroxides and i r o n - r i c h c l a y s r e p r e s e n t a s i g n i f i c a n t p r o p o r t i o n o f t h e t o t a l i r o n b e a r i n g mineralogy.
Given f a v o u r a b l e c o n d i t i o n s much o f t h i s m a t e r i a l w i l l e v e n t u a l l y
form pigmentary h a e m a t i t e .
I n c o a r s e g r a i n e d d e s e r t a l l u v i u m of t h e t y p e d e s c r i b e d
by Walker e t a l . (1978) t h e r e i s l i t t l e i n t e r s t i t i a ' l m a t r i x a t t h e time of d e p o s i t i o n and t h e i r o n - b e a r i n g m i n e r a l o g y i s dominated by f r e s h g r a i n s o f f e r r o m a g n e s i a n minerals.
I n t h i s c a s e r e d d e n i n g p r o g r e s s e s w i t h sediment b u r i a l a s t h e i r o n - b e a r i n g
g r a i n s a r e i n t r a s t r a t a l l y a l t e r e d r e l e a s i n g i o n s i n t o t h e i n t e r s t i t i a l groundwaters which l a t e r p r e c i p i t a t e a s v a r i o u s a u t h i g e n i c p h a s e s , i n c l u d i n g h a e m a t i t e . ANCIENT ALLUVIAL RED BEDS Ancient a l l u v i a l s e d i m e n t s a r e b e s t r e c o g n i z e d by t h e i r s e d i m e n t a r y o r g a n i z a t i o n , absence of marine f o s s i l s , u n i d i r e c t i o n a l p a l a e o c u r r e n t p a t t e r n and f r e q u e n t evidence of emergence s u c h a s p a l a e o s o l s and mudcracks.
The m a j o r i t y o f a n c i e n t a l l u v i a l
sediments a r e p a r t l y r e d i n c o l o u r and t h e s e comprise t h e most i m p o r t a n t group of a n c i e n t r e d beds.
Not any one of t h e above c r i t e r i a , i s i n i t s e l f , d i a g n o s t i c of an
a l l u v i a l environment b u t i n combination t h e y can b e c o n f i d e n t l y used i n t h e i r identi f i c a tion. There a r e l i m i t a t i o n s i n s t u d y i n g a n c i e n t a l l u v i a l s e d i m e n t s by d i r e c t comparison w i t h Recent s e d i m e n t s .
S i n c e P r o t e r o z o i c times (when t h e e a r l i e s t a l l u v i a l sediments
a r e found) t h e r e have been c o n s i d e r a b l e e v o l u t i o n a r y changes i n t h e atmosphere, t h e c l i m a t i c c y c l e , and a l s o i n t h e abundance and t y p e o f p l a n t l i f e . may i n f l u e n c e a l l u v i a l p r o c e s s e s .
A l l these factors
I n pre-Devonian times, b a f o r e t h e a p p e a r a n c e of
p l a n t l i f e , e r o s i o n and s u r f a c e r u n - o f f were p r o b a b l y h i g h a n d Esoods l a r g e compared w i t h t h e p r e s e n t - d a y (Schumm, 1968).
S i m i l a r l y , t h e appearance of grasses i n t h e
L a t e Cenozoic may have p l a y e d a s i g n i f i c a n t p a r t i n s t a b i l i z i n g a l l u v i a l p r o c a g s e s . R u s s e l l (1956) r e c o g n i z e d f o u r p e r i o d s i n which e v o l u t i o n a r y changes i n t h e p l a n t kingdom c o u l d have i n f l u e n c e d a l l u v i a l p r o c e s s e s :
1. p r e v e g e t a t i o n time (pre-Devonian) 2. p r i m i t i v e v e g e t a t i o n (Devonian-Cretaceous)
3 . modern f l o w e r i n g p l a n t s (Cretaceous-Miocene)
4 . t h e appearance of g r a s s e s (Miocene-present). Despite these influences a n c i e n t a l l u v i a l sediments a r e r e a d i l y recognizable and show some remarkable s i m i l a r i t i e s t o Recent a l l u v i u m .
Many of t h e c h a r a c t e r -
i s t i c Recent a l l u v i a l environments c a n b e i d e n t i f i e d i n a n c i e n t r e d beds and t h e r e a r e a number of e x c e l l e n t summaries e.g. Reading (1978).
Here a s m a l l number of
examples a r e c o n s i d e r e d i n d e t a i l s o t h a t t h e f a c t o r s c o n t r o l l i n g a l l u v i a l
221 s e d i m e n t a t i o n and s u b s e q u e n t reddening can be c o n s i d e r e d i n d e t a i l . The Old Red Sandstone Old Red Sandstone i s t h e name g i v e n t o t h e c o n t i n e n t a l f a c i e s of t h e Devonian i n v a r i o u s p l a c e s around t h e p r e s e n t - d a y North A t l a n t i c margins. molasse f a c i e s of t h e Caledonian orogeny.
I t represents the
These p l a c e s which i n c l u d e England, Wales,
S c o t l a n d , Norway, S p i t z b e r g e n , Greenland and p a r t s o f Canada and t h e USA o r i g i n a l l y formed p a r t o f a l a r g e c o n t i n e n t a l l a n d mass which c a n be r e c o n s t r u c t e d on t h e b a s i s of palaeomagnetic e v i d e n c e ( B u l l a r d e t al., 1965; Smith e t a l . , 1973) (Fig.4.33).
30's
60'
Fig.4.33. C o n t i n e n t a l r e c o n s t r u c t i o n f o r t h e Lower Devonian showing t h e p r o b a b l e e x t e n t of t h e Old Red Fandstone c o n t i n e n t (based on Smith e t a l . , 1973 and Woodrow e t a l . , 1973). The Old Red Sandstone c o n t i n e n t l a y between p a l a e o l a t i t u d e s o f a b o u t 0-30's
and
t h e p a l a e o c l i m a t e , j u d g i n g from comparisons w i t h t h e p r e s e n t - d a y c l i m a t i c p a t t e r n s u s i n g l i t h o l o g y and f a u n a l and f l o r a l c h a r a c t e r i s t i c s (Woodrow e t a l . ,
1973),
ranged from h o t and s e m i a r i d i n t h e s o u t h e r n p a r t o f t h e c o n t i n e n t t o a more m o i s t t r o p i c a l c l i m a t e i n t h e n o r t h e r n p a r t o f t h e landmass.
Within t h e Old Red c o n t i n e n t
222
two t y p e s of s e d i m e n t a r y a s s o c i a t i o n can b e r e c o g n i z e d ( A l l e n e t a l . , 1967).
One
i s a n e x t e r n a l f a c i e s i n which c o a s t a l a l l u v i a l s e d i m e n t s p a s s l a t e r a l l y and i n t e r f i n g e r w i t h marine s t r a t a .
The o t h e r i s a n i n t e r n a l f a c i e s which i s c o n f i n e d t o
i n t e r m o n t a n e b a s i n s , has no a s s o c i a t e d marine s t r a t a and c o n s i s t s m o s t l y of a l l u v i a l f a n and l a c u s t r i n e d e p o s i t s .
The e x t e r n a l f a c i e s i s most i m p o r t a n t i n volume and
a r e a l e x t e n t s t r e t c h i n g from t h e n o r t h e r n p a r t o f c o n t i n e n t a l Europe through s o u t h e r n B r i t a i n t o Canada and t h e USA. These two f a c i e s o f t h e Old Red Sandstone p r o v i d e e x c e l l e n t examples o f two c o n t r a s t i n g t y p e s of a n c i e n t a l l u v i a l r e d beds.
The i n t e r n a l f a c i e s compares c l o s e l y
w i t h Recent p e b b l y a l l u v i u m d e p o s i t e d i n a l l u v i a l f a n s and b r a i d e d r i v e r s a r i d climates.
i n semi-
The d e p o s i t s a r e more o r less u n i f o r m l y r e d which c o n t r a s t s w i t h
t h o s e of t h e e x t e r n a l f a c i e s .
These more c l o s e l y resemble t h e d e p o s i t s o f meandering
r i v e r s d e p o s i t e d i n c o a s t a l a l l u v i a l p l a i n s and a r e t y p i c a l l y v a r i e g a t e d showing a l t e r n a t i o n s of r e d and d r a b sediment. Internal facies
-
reddened p e b b l y a l l u v i u m
The i n t e r n a l f a c i e s o f t h e Old Red Sandstone was d e p o s i t e d i n i n t e r m o n t a n e b a s i n s which a r e d e l i n e a t e d by major boundary f a u l t s .
These f a u l t s were a c t i v e a t t h e t i m e
of d e p o s i t i o n ( F r i e n d , 1967, 1969) and c o n t r o l l e d t h e a c c u m u l a t i o n of g r e a t t h i c k n e s s e s o f sediment ( B r y h n i , 1964a; Bluck, 1969; S t e e l e t a l . , 1977).
The r e s u l t
i s t h a t a l l u v i a l f a n d e p o s i t e ( f a n g l o m e r a t e s ) accumulated i n t h e m a r g i n a l a r e a s and
t h e s e i n t e r f i n g e r w i t h l a t e r a l l y e q u i v a l e n t a l l u v i a l p l a i n s e d i m e n t s which a r e u s u a l l y d i s p e r s e d a l o n g t h e a x i s of t h e b a s i n .
I n some i n t e r n a l b a s i n s t h i c k
l a c u s t r i n e sequences a r e d e v e l o p e d , t h e n o t a b l e example b e i n g t h e O r c a d i a n Basin of C a i t h n e s s (Donovan and F o s t e r , 1972; Donovan e t a l . , 1974) i n which o v e r 5km
of d r a b l a c u s t r i n e c l a s t i c s and c a r b o n a t e s were d e p o s i t e d d u r i n g t h e Middle Old Red Sandstone ( E i f e l i a n - G i v e t i a n ) .
The Midland V a l l e y of S c o t l a n d ( F i g . 4 . 3 4 ) r e p r e s e n t s
a l a r g e i n t e r n a l b a s i n bounded by two NE-SW t r e n d i n g f a u l t complexes, t h e Highland Boundary F a u l t s y s t e m t o t h e N o r t h and t h e S o u t h e r n Uplands F a u l t t o t h e South. The Old Red sequence rests on a b a s a l unconformity and began t o accumulate i n Gedinnian time w i t h up t o 6km of s e d i m e n t a c c u m u l a t i n g i n p l a c e s .
There i s a major
b r e a k i n t h e sequence and t h e Upper Old Red, which i s g e n e r a l l y f i n e r g r a i n e d and more e x t e n s i v e a r e a l l y , rests unconformably on t h e Lower Old Red Sandstone.
The
i m p l i c a t i o n i s t h a t t h e r e was a c o n s i d e r a b l e r e d u c t i o n i n s o u r c e a r e a r e l i e f d u r i n g t h e Middle Old Red Sandstone. Along t h e n o r t h e r n margin of t h e Midland V a l l e y b a s i n t h e Lower Old Red Sandstone c o n t a i n s massive conglomerate u n i t s which comprise up t o 70% of t h e s u c c e s s i o n and t h i n southwards ( A l l a n , 1940).
The conglomerates a s d e s c r i b e d by H i c k l i n g (1908,
19121, Campbell (1913) F r i e n d e t a l . ( 1 9 6 3 ) , Bluck (1967, 1969) show l a r g e v a r i a t i o n i n c l a s t s i z e ; i n t h e n o r t h e a s t e r n p a r t of t h e a r e a c l a s t s o v e r 2m i n s i z e o c c u r b u t
223 s o r t i n g i n t h e c o a r s e g r a i n e d p a r t s i s g e n e r a l l y good ( F r i e n d e t a l . , 1963). Roundness v a r i e s l a t e r a l l y and v e r t i c a l l y ( A l l a n , 1940) b u t u s u a l l y q u a r t z i t e c l a s t s a r e well-rounded even when a s s o c i a t e d D a l r a d i a n metamorphic fragments a r e angular.
A l l a n (1940) s u g g e s t e d t h a t t h i s might be due t o r e c y c l i n g o f p r e v i o u s l y
formed q u a r t z i t e b e a r i n g conglomerate.
The c l a s t s o f q u a r t z i t e and D a l r a d i a n
p h y l l i t e , s c h i s t and g n e i s s a r e c o n s i s t e n t w i t h a provenance i n t h e Highland a r e a s t o t h e North; however contemporaneous a n d e s i t e s and b a s a l t s w i t h i n t h e Midland V a l l e y b a s i n a r e a l s o found a s c l a s t s w i t h i n t h e c o n g l o m e r a t e s .
Upwards, D a l r a d i a n type
c l a s t s become more abundant s u g g e s t i n g p r o g r e s s i v e u n r o o f i n g o f t h e n o r t h e r n h i g h l a n d areas.
I n t h e n o r t h e a s t e r n p a r t o f t h e b a s i n Campbell (1929) found t h a t t h e lower
g r a d e metamorphic zones were p r o g r e s s i v e l y s t r i p p e d o f f u n t i l t h e high-grade zones p r e s e n t l y v i s i b l e were exposed.
Fig.4.34. Scotland
Palaeogeography o f t h e Lower O l d Red Sandstone i n t h e Midland V a l l e y o f A . S t r u c t u r a l d e t a i l s B. Suggested palaeogeography ( a f t e r Bluck, 1978).
224
On t h e s o u t h e r n f l a n k o f t h e b a s i n t h e conglomerates a r e n e i t h e r a s t h i c k n o r a s c o a r s e a s t h e i r n o r t h e r n c o u n t e r p a r t s b u t t h e y do o c c u r i n t e r b e d d e d w i t h v o l c a n i c rocks (Mykura, 1960b).
C l a s t s i n c l u d e O r d o v i c i a n and S i l u r i a n c l a s t i c s i d e n t i c a l
w i t h t h o s e of t h e S o u t h e r n Uplands a r e a t o t h e s o u t h ( M i t c h e l l and Mykura, 1962). The evidence i n d i c a t e s t h a t t h e conglomerates i n b o t h t h e n o r t h e r n and s o u t h e r n a r e a s a r e marginal a l l u v i a l f a n s d e r i v e d from t h e n o r t h and s o u t h r e s p e c t i v e l y (Fig.4.34).
The t h i c k n e s s o f t h e conglomerates i n d i c a t e t h a t downwarping went on
contemporaneously w h i l s t changes i n c l a s t composition s u g g e s t t h a t u p l i f t might have been i m p o r t a n t i n s o u r c e a r e a s . The Upper Old Red Sandstone was d e p o s i t e d i n a more e x t e n s i v e , b u t p r o b a b l y less deep Midland V a l l e y b a s i n .
Conglomerate u n i t s a r e fewer and s a n d s t o n e s and pedogenic
c a r b o n a t e s (Burgess, 1960; W a t e r s t o n , 1965) a r e more abundant.
Even so, b o u l d e r
s i z e d conglomerates o c c u r i n t h e v i c i n i t y o f t h e Highland Boundary F a u l t ( F r i e n d e t a l . , 1963).
D e t a i l e d s t u d y of t h e conglomerates by Bluck (1967, 1969) i n t h e
w e s t e r n p a r t of t h e b a s i n shows t h a t t h e y o c c u r i n sequences which b e g i n w i t h a l l u v i a l f a n d e p o s i t s and end w i t h s a n d s t o n e s and conglomerates o f f l o o d p l a i n type. Bluck (1969) recognized t h r e e such s e q u e n c e s , e a c h b e i n g a r e a l l y i s o l a t e d and i n i t i a t e d by t h e r i s i n g of a n a d j a c e n t fault-bounded b l o c k ; t h e s e e a c h have a d i s t i n c t i v e composition and a r e r e f e r r e d t o a s t h e P o r t e n c r o s s , S k e l m o r l i e , and I n v e r k i p d i s p e r s a l shadows ( F i g . 4 . 3 5 ) .
Within t h e s e a r e a s Bluck (1967) was a b l e
Fig.4.35. D i s p e r s a l shadows recognized on t h e e a s t e r n margin of t h e F i r t h o f Clyde ( a f t e r Bluck, 1969).
225 t o d i s t i n g u i s h f o u r t y p e s o f conglomerate on t h e b a s i s of s t r u c t u r e and t e x t u r e : a l l u v i a l f a n mudflows, a l l u v i a l f a n s t r e a m f l o w , b r a i d e d s t r e a m and f l o o d p l a i n ( r i v e r channel) deposits (Fig.4.36).
V e r t i c a l p r o f i l e s t h r o u g h t h e conglomerates
g e n e r a l l y show a n upward r e d u c t i o n i n g r a i n s i z e t o g e t h e r w i t h a change from f a n mudflow t o f a n s t r e a m t o b r a i d e d s t r e a m and f l o o d p l a i n d.eposition ( F i g . 4 . 3 7 ) .
-s I10 Y, -1s
5 10 15. 20 maximum particlo r m (em)
1
-
1
1
1Q 1 5 maximum particlo sir?(cm)
I
S t r u c t u r a l and t e x t u r a l p r o p e r t i e s of Upper Old Red Sandstone conglomFig.4.36. e r a t e s A . D e b r i s f l o w . B. Stream f l o w . C . B r a i d e d s t r e a m . D . F l o o d p l a i n (based on Bluck, 1967).
Alluvial fan
Braided stream
Alluwal fan
Braided stream
Maximum particle size
i
Stream channel
M.P.S. (cm)
5 10 15 20 25
D m Marl
C
Streamflood
W Mudflow
E m
A
C
M.P.S. (cm)
5 10 15 2 0 25
227 These Upper Old Red Sandstone conglomerates f o m e d a t t h e f o o t of fault-bounded mountains.
An i n i t i a l h i g h r a t e of e r o s i o n l e d t o t h e f o r m a t i o n of a l l u v i a l f a n s
a s s h e e t f l o o d d e p o s i t s accumulated. s o u r c e r e g i o n s were c u t back
A s m a r g i n a l f a u l t a c t i v i t y grew l e s s t h e
and a l l u v i a l f a n s e d i m e n t a t i o n was r e p l a c e d by b r a i d e d
s t r e a m and f l o o d p l a i n s e d i m e n t a t i o n forming fining-upwards sequences (Read and Johnson, 1967; Chisholm and Dean, 1974).
S i m i l a r fining-upwards sequences have been
d e s c r i b e d by N i l s e n ( 1 9 6 9 ) , Deegan (19731, S t e e l ( 1 9 7 4 ) , and S t e e l and Wilson (1975). The fundamental r e l a t i o n s h i p between t e c t o n i c s and s e d i m e n t a t i o n i n i n t e r n a l b a s i n s i s w e l l s e e n i n t h e Old Red b a s i n s o f w e s t e r n Norway ( B r y h n i , 1964a,b; S t e e l , 1976; S t e e l e t a l . , 1977).
The Hornelen Basin (Fig.4.38)
c o n t a i n s a t h i c k n e s s of 25km o f sediment.
is r e l a t i v e l y s m a l l b u t
Marginal f a n g l o m e r a t e s occupy t h e n o r t h e r n
and s o u t h e r n a r e a s a d j a c e n t t o t h e bounding f a u l t s b u t most of t h e b a s i n i s f i l l e d by s a n d s t o n e s which accumulated from westward f l o w i n g r i v e r s on a broad a l l u v i a l plain.
Marginal i n t e r f i n g e r i n g of a l l u v i a l p l a i n and f a n g l o m e r a t e sediments s u g g e s t
t h a t a t c e r t a i n times t h e e n t i r e b a s i n was f l o o d e d by t h e a l l u v i a l p l a i n system. Both t h e m a r g i n a l f a n g l o m e r a t e s and t h e a l l u v i a l p l a i n sediments show a v e r y marked c y c l i c i t y (Kolderup, 1927; S t e e l e t a l . , 1 9 7 7 ) . The a l l u v i a l f a n o r i g i n o f t h e m a r g i n a l d e p o s i t s i s i n d i c a t e d by t h e wedge-shaped geometry, down-wedge d e c r e a s e i n maximum c l a s t s i z e , t h e dominance of l o c a l l y d e r i v e d c l a s t s , and t h e f r e q u e n t e v i d e n c e of s u b a e r i a l exposure (mudcracks and r a i n p r i n t s ) . The remarkable f e a t u r e of t h e s e f a n d e p o s i t s i s t h e u b i q u i t y of sequences which can b e i d e n t i f i e d a t a v a r i e t y of l e v e l s .
coarsening-upwards
A t t h e most g e n e r a l l e v e l
a r e sequences c o n s i s t i n g of s t a c k e d a l l u v i a l f a n s predominantly of t h e same t y p e
i . e . predominantly d e b r i s f l o w , s t r e a m flow o r mixed t y p e ( s e e F i g . 4 . 3 9 ) .
Individual
f a n b o d i e s a l o n g t h e s o u t h e r n margin o f Hornelen Basin a r e 50-80m t h i c k (proximal r e a c h ) w i t h a r a d i u s of a b o u t 5km.
Along t h e n o r t h e r n margin t h e f a n s a r e t h i c k e r
(100-2OOm) b u t a r e of s m a l l e r r a d i u s ( a b o u t lkm). a r e i m p o r t a n t i n b o t h t h e s e t y p e s o f f a n body.
Coarsening-upwards sequences
Also i n d i v i d u a l f a n sequences
commonly c o n s i s t of a number of t a b u l a r s u b u n i t s 10-25m t h i c k , each of which shows a c o a r s e n i n g and t h i c k e n i n g upwards t r e n d .
The f o l l o w i n g f e a t u r e s a r e c h a r a c t e r i s t i c :
1) A g e n e r a l coarsening-upwards i n d i c a t e d by a d e c r e a s i n g s a n d s t o n e p e r c e n t a g e 2 ) Coarsening-upwards of conglomerates a s i n d i c a t e d by mean maximum c l a s t s i z e
3 ) A g e n e r a l thickening-upward of conglomerate beds a s i m p l i e d from a p o s t i i v e bed t h i c k n e s s
- maximum p a r t i c l e
s i z e relationship
4 ) A s h a r p , a p p a r e n t l y p l a n a r upper boundary t o t h e f a n body o v e r l a i n by t h e f i n e s t a l l u v i a l p l a i n sediments 5 ) Lower boundary i s g r a d a t i o n a l b e c a u s e t h e r e i s g r a d u a l coarsening-upward from a l l u v i a l p l a i n s t r u c t u r e t o the f i n e s t ( d i s t a l ) fanglomerates.
T h i s has r e s u l t e d
from a l a t e r a l i n t e r f i n g e r i n g o f f a n and a l l u v i a l p l a i n s e d i m e n t s a s t h e f a n radius got l a r g e r through t i m e .
228
._
Grain Sism
F i g . 4 . 3 8 . S i m p l i f i e d map of the Hornelen Basin ( a f t e r K i l d a l , 1970). The s u c c e s s i o n i s 25km t h i c k and both the a x i a l and marginal d e p o s i t s a r e dominated b y coarsening-upwards sequences ( a f t e r S t e e l e t a l . , 1977).
229
0.5 km
I 7
7
i
20 40cm
Fig.4.39. Internal d e t a i l s of a composite debris flow dominated a l l u v i a l fan wedge which is interfingering with floodbasin sediments along the northern margin of the Hornelen basin ( a f t e r S t e e l e t a l . , 1977).
230 These r e l a t i o n s h i p s a r e b e s t a c c o u n t e d f o r by r e l a t i v e s u b s i d e n c e of t h e b a s i n f l o o r c a u s i n g i n c r e a s e d r e l i e f and d r a i n a g e a r e a i n t h e new upland r e g i o n ( B u l l , 1964, 1968; Hooke, 1968).
S t e e l e t a l . (1977, p.1130) s u g g e s t t h a t t h e main
t e c t o n i c e v e n t s c o n t r o l l i n g t h i s s u b s i d e n c e a r e marked by t h e s h a r p p l a n a r t o p of any f a n body.
The coarsening-upwards sequences t h u s r e p r e s e n t t h e i n c r e a s e d
t r a n s p o r t of d e b r i s and t h e g r a d u a l o u t - b u i l d i n g of t h e n e x t f a n i n a n e f f o r t t o e s t a b l i s h a n e q u i l i b r i u m p r o f i l e a c r o s s t h e b a s i n margin.
These a u t h o r s c o r r e l a t e d
t h e 10-25m s u b c y c l e s w i t h d i s c r e t e e p i s o d e s o f f a u l t movement s u g g e s t i n g t h a t t h i s r e f l e c t e d t h e maximurn a m p l i t u d e o f i n d i v i d u a l p h a s e s o f s u b s i d e n c e .
The
lOOm u n i t s
o v e r l a p e a c h o t h e r i n a n e a s t w a r d d i r e c t i o n i n t h e Hornelen Basin ( B r y h n i , 1964a). S t e e l e t a l . (1977) c o n s i d e r e d t h a t t h e amount of o v e r l a p of s u c c e s s i v e cyclotherms i s c o m o n l y less t h a n 0.25km and t h e r e f o r e s u g g e s t e d t h a t t h i s c o u l d be a c c o u n t e d f o r by a p e r i o d i c , r a p i d e a s t w a r d s h i f t i n t h e main l o c u s o f s u b s i d e n c e ; t h i s would a l s o a c c o u n t f o r t h e p l a n a r t o p s and asymmetry o f t h e cyclothems.
T h i s model a l s o
c o n v e n i e n t l y e x p l a i n s how s u c h a v a s t s t r a t i g r a p h i c t h i c k n e s s (25km) c o u l d accumulate w i t h o u t t h e r e b e i n g a d i f f e r e n c e i n t h e d e g r e e of metamorphism between t h e b a s e and t h e t o p ( B r y h n i , 1964b).
The e a s t w a r d o v e r l a p of cyclothems s u g g e s t t h a t t h e base-
ment may n o t have been d e e p e r t h a n 6km below t h e s u r f a c e of t h e b a s i n .
Since t h i s
i s much less t h a n t h e l a t e r a l e x p a n s i o n o f t h e b a s i n i t seems l i k e l y t h a t sedimenta t i o n was c o n t r o l l e d by a wrench f a u l t s y s t e m ( C r o w e l l , 1974a).
S t e e l e t a l . (1977)
proposed a model i n which t h e b a s i n formed on t h e s o u t h e r n f l a n k of a r i g h t - s l i p f a u l t system (Fig.4.40). The r e d d e n i n g of p e b b l y a l l u v i u m The Old Red Sandstone p e b b l y a l l u v i u m i n t h e Midland V a l l e y of S c o t l a n d shows many
of t h e f e a t u r e s which t y p i c a l l y r e s u l t from t h e i n t r a s t r a t a l a l t e r a t i o n of f e r r o magnesian m i n e r a l s .
Many o f t h e s a n d s t o n e s and c o n g l o m e r a t i c s a n d s t o n e s c o n s i s t
o f framework g r a i n s and c l a y m a t r i x w i t h o u t t h e i n t e r m e d i a t e s i l t s i z e s .
The c l a y
m a t r i x may b e i n t e r s t i t i a l o r c o a t i n g t h e framework g r a i n s and c l o s e l y resembles t h e m e c h a n i c a l l y i n f i l t r a t e d c l a y d e s c r i b e d by Walker (1976).
The p o s t - d e p o s i t i o n a l
o x i d a t i o n of i n f i l t r a t e d c l a y c o u l d have been a s i g n i f i c a n t pigmenting p r o c e s s . The heavy m i n e r a l s u i t e s o f r e d s a n d s t o n e s i n t h e Midland V a l l e y b a s i n i n c l u d e h a e m a t i t e , i l m e n i t e , g a r n e t , a p a t i t e , e p i d o t e , r u t i l e , t o u r m a l i n e , z i r c o n and s c a r c e hornblende and pyroxene.
The s c a r c i t y of h o r n b l e n d e (maximum 3.3% of heavy
m i n e r a l s ) and pyroxene (maximum 2.6% b u t c o m p l e t e l y a b s e n t o v e r most o f t h e a r e a ) must b e a c c o u n t e d f o r b e c a u s e t h e r e i s a n abundant s o u r c e a r e a s u p p l y i n Dalradian metasediments and b a s i c v o l c a n i c s ( B r a i t h w a i t e and Jawad A l i , 1978).
Many of t h e
hornblende g r a i n s show cockscomb a l t e r a t i o n f e a t u r e s l i k e t h o s e s e e n i n Cenozoic d e s e r t a l l u v i u m which must b e due t o i n t r a s t r a t a l p r o c e s s e s .
It can be expected
t h a t l a r g e q u a n t i t i e s of a u t h i g e n i c pigmentary h a e m a t i t e and i n t e r s t i t i a l c l a y
231
...w'-.. Fig.4.40. A. S t r u c t u r a l e l e m e n t s i n Hornelen Basin. B. P a l a e o c u r r e n t s y s t e m ( s o l i d arrows a r e m a r g i n a l f a n g l o m e r a t i c d i s p e r s a l ) . C. A s p e c u l a t i v e model o f b a s i n development a l o n g t h e s o u t h e r n f l a n k o f a r i g h t - s l i p wrench f a u l t ( a f t e r S t e e l e t a l . , 1977). m a t r i x would b e produced by t h i s p r o c e s s .
The o t h e r heavy m i n e r a l groups a r e
c o n s i s t e n t w i t h t h e s e d i m e n t o l o g i c a l e v i d e n c e o f a provenance i n t h e metamorphosed D a l r a d i a n t o t h e n o r t h (Fig.4.41).
However, t h e a b s e n c e of k y a n i t e and s i l l i m a n i t e
s u g g e s t s t h a t t h e more d e e p l y r o o t e d metamorphic zones f u r t h e r n o r t h were n o t unroofed a t t h i s t i m e . A number of o t h e r f e a t u r e s f u r t h e r s u g g e s t s t h a t t h e s e r e d beds a r e of d i a g e n e t i c origin.
These i n c l u d e t h e a l t e r a t i o n and c l a y replacement o f f e l d s p a r s and t h e
p r e s e n c e of d i s s o l u t i o n p i t s and v o i d s i n some m e t a s t a b l e heavy m i n e r a l s such a s garnet.
Haematite i n t h e s e r e d beds o c c u r s a s s m a l l e u h e d r a l c r y s t a l s and w i t h i n
i n t e r s t i t i a l matrix.
I t c a n also b e s e e n r e p l a c i n g b i o t i t e and c h l o r i t e a l o n g
c l e a v a g e p l a n e s and complete pseudomorphs o f h a e m a t i t e a f t e r b i o t i t e o c c a s i o n a l l y occur.
The complete a b s e n c e o f m a g n e t i t e s u g g e s t s t h a t i t has s u f f e r e d complete
232 conversion to haematite by martitization and polished sections reveal abundant martitization textures consistent with this view.
I
Arbuthnott Group
I
T
1
Garvock Grouo A
AMPHIBOLE
P
APATITE
E G H
EPIDOTE GARNET HAEMATITE
I
R T
2
ILMENllE RUTILE TOURMALINE ZIRCON
Fig.4.41. Centres of distribution of heavy mineral groups in the Arbuthknott, Garvock, and Strathmore Groups of the Old Red Sandstone in the Midland Valley of Scotland (after Braithwaite and Jawad Ali, 1978).
233 Some o f t h e s e a l t e r a t i o n s a r e i l l u s t r a t e d i n Fig.4.42.
They r e p r e s e n t a more
advanced s t a g e o f e x a c t l y t h e same p r o c e s s e s a s t h o s e d e s c r i b e d i n Cenozoic d e s e r t a l l u v i u m by Walker (1967a; 1976) and Walker e t a l . (1978).
The same mechanism of
reddening a p p e a r s t o be a p p l i c a b l e t o a l l t h e pebbly a l l u v i u m and f a n d e p o s i t s of t h e Old Red Sandstone i n c l u d i n g che Orcadian b a s i n ( T a r l i n g e t a l . , and A r c h e r , 1977) and p r o b a b l y t h e Norwegian b a s i n s t o o .
1976; Turner
Variations i n the r a t e
of reddening and t h e e x t e n t of t h e d i a g e n e t i c p r o c e s s e s i n v o l v e d a r e t o b e expected These w i l l r e s u l t from v a r i a t i o n s i n t e c t o n i c a c t i v i t y , s o u r c e
i n different areas.
a r e a composition and t h e p a t t e r n and composition o f t h e groundwater flow.
Significant
d i f f e r e n c e s between t h e s e a n c i e n t r e d beds and Cenozoic a l l u v i u m i n c l u d e t h e more advanced o r complete a l t e r a t i o n s e e n i n t h e former, and a l s o i t s d a r k e r c o l o u r .
The
l a t t e r i s t y p i c a l of a n c i e n t r e d beds and a p p a r e n t l y r e s u l t s from long-continued growth o f h a e m a t i t e c r y s t a l s which a r e g e n e r a l l y c o a r s e r i n a n c i e n t r e d beds. External facies
-
reddened sandy and muddy a l l u v i u m
The e x t e r n a l f a c i e s o f t h e Old Red Sandstone was d e p o s i t e d on c o a s t a l a l l u v i a l p l a i n s and g e n e r a l l y l i e s conformably on marine c l a s t i c s o r c a r b o n a t e s .
I n the
B r i t i s h a r e a t h e most i m p o r t a n t e x t e r n a l b a s i n i s t h e Anglo-Welsh b a s i n which s t r e t c h e d a c r o s s s o u t h e r n B r i t a i n and a l s o extended i n t o s o u t h e r n I r e l a n d (Fig.4.43). The t r a n s i t i o n from marine t o a l l u v i a l s e d i m e n t a t i o n i n t h i s b a s i n has been d e s c r i b e d
by Allen (1974a).
A l l u v i a l s e d i m e n t a t i o n was i n i t i a t e d i n r e s p o n s e t o a widespread
r e g r e s e i o n which r e s u l t e d from t h e c l o s u r e of t h e p r o t o - A t l a n t i c ocean d u r i n g t h e
Late S i l u r i a n and Lower Devonian. The a l l u v i a l d e p o s i t s of t h e Anglo-Welsh b a s i n a r e b r o a d l y s i m i l a r t o t h o s e i n
a number of other Old Red Sandstone b a s i n s i n c l u d i n g t h e Appalachians ( A l l e n and F r i e n d , 19681, S p i t z b e r g e n ( F r i e n d , 1965; Moody-Stuart, of Norway (Turner, 1974a,b).
1966) and t h e Oslo Graben
These d e p o s i t s f r e q u e n t l y show two major f a c i e s
a s s o c i a t i o n s , a 'coarse member' c o n s i s t i n g o f s a n d s t o n e s and i n t r a f o r m a t i o n a l conglomerate and a ' f i n e number' o f s i l t s t o n e s and mudstones ( A l l e n , 1965e).
These
f a c i e s a r e a r r a n g e d i n fining-upwards c y c l e s i n which t h e c o a r s e members r e p r e s e n t channel d e p o s i t s and t h e f i n e members f l o o d p l a i n and r i v e r bank d e p o s i t s . A.
Coarse members
The c o a r s e members of Old Red Sandstone fining-upwards c y c l e s c o n t a i n a number of d i s t i n c t i v e f a c i e s which have been s t u d i e d i n d e t a i l by A l l e n (1964a, 1965c, 1965e, 1970a, 1970b, 1974b).
Examples a r e shown i n Fig.4.44.
The b a s e of t h e c o a r s e members r e s t s on a scoured e r o s i o n s u r f a c e and u s u a l l y c o n s i s t s o f a t h i n conglomerate.
I n t r a f o r m a t i o n a l c l a s t s a r e common (mudstones and
c a r b o n a t e s from t h e r i v e r banks and f l o o d p l a i n ) and t h e r e may a l s o be e x t r a f o r m a t i o n a l pebbles.
The conglomerate may show i m b r i c a t i o n and i s i n t e r p r e t e d a s a channel l a g
234
Fig.4.42. Thin s e c t i o n photomicrographs showing t h e d i a g e n e t i c a l t e r a t i o n of framework s i l i c a t e s i n t h e Old Red Sandstone of t h e Midland V a l l e y of S c o t l a n d . A . Volcanic r o c k fragment e x t e n s i v e l y a l t e r e d t o h a e m a t i t e . B. D e t r i t a l g a r n e t , d i s s o l v e d and p a r t l y r e p l a c e d by c a l c i t e . C . Q u a r t z i t e fragment showing i n t e r c r y s t a l reddening. D. B i o t i t e p a r t i a l l y r e p l a c e d by h a e m a t i t e .
235
7 Intermontane Basins Coastal Plain
RCADIAN BASIN
Shallow Seas Deep Seas Upland Areas Volcanic bntrs
LORNE BAS1 HEVIOT BASIN
Pahtmgeography of t h e Lower Old Red Sandstone o f t h e B r i t i s h a r e a Fig.4.43. showing t h e Anglo-Welsh b a s i n . deposit.
Some c o n g l o m e r a t e s , c o n s i s t i n g e x c l u s i v e l y of i n t r a f o r m a t i o n a l m a t e r i a l
(mud and c a l c r e t e c l a s t s ) and l a c k i n g a q u a r t z o s e s a n d m a t r i x may r e p r e s e n t t h e d e p o s i t s o f ephemeral d r a i n a g e systems which developed from time t o t i m e on s l i g h t l y e l e v a t e d i n t e r f l u v i a l a r e a s ( A l l e n and W i l l i a m s , 1 9 7 9 ) . Above t h e b a s a l conglomerate a r e m o s t l y cross-bedded s a n d s t o n e s i n t r o u g h o r p l a n a r t a b u l a r s e t s , most commonly up t o l m t h i c k .
The former a r e more abundant
and a t t r i b u t e d t o t h e m i g r a t i o n o f s i n u o u s - c r e s t e d dunes.
T a b u l a r s e t s a r e formed
from t h e m i g r a t i o n of t r a n s v e r s e b a r s , sandwaves a n d , l e s s commonly, s c r o l l b a r s and c h u t e b a r s .
R e a c t i v a t i o n s u r f a c e s and mud d r a p e s may b e p r e s e n t i n d i c a t i n g
water s t a g e f l u c t u a t i o n s . Coarse member s a n d s t o n e s may show a n unusual t y p e of low-angled c r o s s - b e d d i n g r e f e r r e d t o by A l l e n (1963) a s e p s i l o n c r o s s - b e d d i n g .
When p r e s e n t t h i s u s u a l l y
o c c u p i e s t h e whole t h i c k n e s s of t h e c o a r s e member and i s t y p i c a l l y h e t e r o g e n e o u s ,
236 t h e i n d i v i d u a l s e t s c o n t a i n i n g c r o s s - l a m i n a t e d u n i t s and mud d r a p e s s u g g e s t i n g a These e p s i l o n u n i t s a r e g e n e r a l l y l e s s
v a r i a b l e h y d r o l o g i c a l regime ( F i g . 4 . 4 5 ) .
t h a n 5m t h i c k ( A l l e n , 1965c; Moody-Stuart, 1966) and d i p between 5 and 15'.
The
d i r e c t i o n of d i p can o f t e n b e d e m o n s t r a t e d t o be a t r i g h t a n g l e s t o t h e l o c a l palaeos l o p e and t h e f a c i e s i s t h e r e f o r e i n t e r p r e t e d a s a l a t e r a l a c c r e t i o n d e p o s i t l i k e t h o s e s e e n i n t i d a l c r e e k s (Van S t r a a t e n , 1 9 5 1 ) , t h e l a t e r a l a c c r e t i o n s u r f a c e presumably r e p r e s e n t i n g t h e p o i n t b a r s u r f a c e .
The r e l a t i v e s c a r c i t y of e p s i l o n
c r o s s - b e d d i n g , however, i n d i c a t e s t h a t n o t a l l of t h e Old Red S a n d s t o n e p o i n t b a r s iced e p s i l o n cross-bedding. C
D
E
F
Small scale cross-stratification
Large scale cross-stratification
Sandstone
Conglomerate
Flat-bedding
s~~
R e p r e s e n t a t i v e and s t a n d a r d c o a r s e member s a n d s t o n e from t h e Old Red Fig.4.44. Sandstone. (A,B) Welsh B o r d e r s , (C) Tugford Clee H i l l s , (D) M i t c h e l d e a n , (E) F o r e s t of Dean, (F) Clee H i l l s , (G) S p i t s b e r g e n . Note t h e v a r i a t i o n i n s c a l e and sequence. Where a r a n g e o f t h i c k n e s s i s g i v e n , t h e sequences a r e somewhat i d e a l i z e d s t a n d a r d s ( a f t e r A l l e n , 1965e).
237
O t h e r f a c i e s which commonly occur w i t h i n c o a r s e members a r e p a r a l l e l laminated s a n d s t o n e s and c r o s s - l a m i n a t e d
sandstones.
The former f r e q u e n t l y show primary
c u r r e n t l i n e a t i o n ( A l l e n , 196413) i n d i c a t i n g upper flow regime c o n d i t i o n s i n channel a r e a s o f h i g h v e l o c i t y o r s h a l l o w depth.
The c r o s s - l a m i n a t e d s a n d s t o n e s a r e
u s u a l l y f i n e r and more micaceous and may show c l i m b i n g r i p p l e c r o s s - l a m i n a t i o n which a l l o w s e s t i m a t i o n of t h e i n s t a n t a n e o u s r a t e o f v e r t i c a l bed a c c r e t i o n ( A l l e n , 1971).
Cross-laminated s a n d s t o n e s a r e i n t e r p r e t e d a s t h e d e p o s i t s of m i g r a t i n g
small s c a l e r i u p l e s .
LITHOLOGIES
&--,A ntnformrtional WLondomerato B M a j o r Bedding W a r m
Major bedding planrr
E p s i l o n cross-bedding i n t h e Old Red Sandstone. Fig.4.45. v e r t i c a l exaggeration ( a f t e r Allen, 1 9 6 5 ~ ) . B.
There is a l a r g e
F i n e members
There i s commonly no c l e a r d i s t i n c t i o n between c o a r s e member and f i n e member i n r h e Old Red Sandstone fining-upwards
of f a c i e s .
c y c l e s and t h e f i n e members may show a complex
They a l s o show e v i d e n c e of p o s t - d e p o s i t i o n a l m o d i f i c a t i o n i n c l u d i n g
b i o t u r b a t i o n and pedogenesis. Old Red Sandstone f i n e members f r e q u e n t l y show s a n d s t o n e s i n t e r b e d d e d w i t h s i l t s t o n e s and mudstones.
These a r e g e n e r a l l y less t h a n l m t h i c k w i t h convex tops
and s h a r p b a s e s .
L a t e r a l l y t h e y a r e i m p e r s i s t e n t ( L e e d e r , 1974) and may show
cross-bedding p a s s i n g upwards i n t o c r o s s - l a m i n a t i o n .
Since these sandstones
i n d i c a t e e p i s o d i c waning flow c o n d i t i o n s and a r e e n c l o s e d by f l o o d p l a i n s i l t s and muds t h e y a r e r e a d i l y i n t e r p r e t e d a s c r e v a s s e - s p l a y d e p o s i t s ( A l l e n , 1964a; Leeder, 1974). Another c h a r a c t e r i s t i c f a c i e s o f f i n e members i s t h e r a p i d a l t e r n a t i o n o f c r o s s l a m i n a t e d f i n e s a n d s o r s i l t s w i t h homogeneous mudstone t h e ' a l t e r n a t i n g b e d s ' f a c i e s of A l l e n , 1 9 6 5 ~ ) . T y p i c a l l y t h e i n d i v i d u a l u n i t s a r e r a t h e r t h i n ( l e s s than 25cm) and t h e s i l t s t o n e s show e v i d e n c e o f p e r i o d i c s u b a e r i a l exposure.
The sand-
s t o n e s a r e s h a r p b a s e d , sometimes e r o s i o n a l and may c a r r y a n g u l a r s i l t s t o n e c l a s t s . These r a p i d a l t e r n a t i o n s i n d i c a t e p e r i o d s o f submergence and d r y i n g o u t c o n s i s t e n t w i t h d e p o s i t i o n a s l e v e e s o r from overbank f l o o d i n g . The most c h a r a c t e r i s t i c and abundant f a c i e s of t h e Old Red Sandstone f i n e members i s r e d , r a t h e r homogeneous mudstones and s i l t s t o n e s .
Homogenization may
r e s u l t from b i o t u r b a t i o n and t h e r e i s f r e q u e n t e v i d e n c e of s u b a e r i a l exposure i n t h e form of mudcracks.
The s i l t s t o n e s and mudstones c l e a r l y r e p r e s e n t t h e v e r t i c a l
a c c r e t i o n d e p o s i t s o f t h e r i v e r f l o o d p l a i n s which were s u b j e c t t o p e r i o d i c f l o o d i n g and d r y i n g o u t . These f l o o d p l a i n d e p o s i t s i n t h e Old Red Sandstone c o n t a i n f r e q u e n t l y e x t e n s i v e c o n c r e t i o n a r y c a r b o n a t e u n i t s which t e n d t o be c o n c e n t r a t e d towards t h e t o p o f t h e f i n e members.
The s i m i l a r i t y of t h e s e c o n c r e t i o n a r y c a r b o n a t e s (which were formerly
known a s c o r n s t o n e s ) t o P l i o c e n e o r Q u a t e r n a r y c a r b o n a t e s o f known pedogenic o r i g i n h a s been r e a l i z e d f o r some time.and more r e c e n t l y d e t a i l e d comparisons have been made by A l l e n (1973, 1974d) and Leeder (1975, 19761, D e t a i l e d d e s c r i p t i o n s o f t h e c a l c r e t e s i n t h e S h r o p s h i r e a r e a have been made by A l l e n (1974a) who a p p l i e d t h e terminology developed f o r s o i l s by Brewer (1964). The c a l c r e t e s g e n e r a l l y o c c u r i n beds up t o 2m t h i c k and c o n s i s t o f c a l c i t e g l a e b u l e s (Brewer, 1964, p . 2 5 9 ) , u s u a l l y less t h a n 5cm i n d i a m e t e r s c a t t e r e d i n a groundmass o r s - m a t r i x (Brewer, 1964, p.146).
The g l a e b u l e s v a r y c o n s i d e r a b l y i n
form and i n t e r n a l l y c o n t i i n numerous c a l c i t e v e i n s o r c r y s t a l l a r i a (Brewer, 1964, p.284) a l o n g w i t h d i f f u s e p a t c h e s and broken v e i n s of m a t r i x .
Many Old Red Sand-
s t o n e c a l c r e t e s a l s o show c r y s t a l t u b e s (Brewer, 1964, p.287) up t o s e v e r a l m i l l i m e t r e s i n d i a m e t e r , o f t e n b r a n c h i n g , and l i k e t h e e l o n g a t e d g l a e b u l e s l y i n g a t a h i g h a n g l e t o t h e bedding.
The c a l c r e t e s a r e most commonly developed w i t h i n
an s - m a t r i x o f mudstone o r s i l t s t o n e , b u t i n some a r e a s (e.g. Anglesey, A l l e n , 1 9 6 5 ~ )t h e y a r e f r e q u e n t l y developed i n a s a n d s t o n e groundmass.
Usually a
c h a r a c t e r i s t i c p r o f i l e i s developed i n which r a t h e r s m a l l , s c a t t e r e d g l a e b u l e s and c r y s t a l t u b e s p r o g r e s s i v e l y i n c r e a s e i n s i z e and d e n s i t y upwards, o f t e n assuming c y l i n d r i c a l t o p r i s m a t i c forms s t e e p l y i n c l i n e d t o bedding.
The t o p of t h e p r o f i l e
i s o f t e n a massive l i m e s t o n e which h a s a s h a r p , uneven t o p and may b e o v e r l a i n by
mudstone, s a n d s t o n e o r i n t r a f o r m a t i o n a l conglomerate.
Where t h e c a l c r e t e p r o f i l e
239
B
Thick r d . coarse 8 B s t m w h I.nticles and persoatmi bad8 of wry fin. r i w bedded smdston Invertebrate burron several horizons No suncracks Abundant cilcium carbonate concretions
7 coarse stnstm mm iwanebrate burrows. rippla-b.dd.d sendston kmcbr. and convdU1e lamitwtiom No evldare ROCI
6
5
4
8
f
4
3 scoured surface in l o w r part Scattered siltstona clasts
2
1
0
Details o f Old Red Sandstone f i n e members i n t h e Anglo-Welsh b a s i n Fig.4.46. A . From Lydney B. From Abergavenny ( a f t e r A l l e n , 1964a).
i s developed i n s a n d s t o n e a s l i g h t l y d i f f e r e n t a p p e a r a n c e r e s u l t s :
on t h e lower
p a r t of t h e p r o f i l e g l a e b u l e s may b e d i f f i c u l t t o d i f f e r e n t i a t e from t h e c a l c i t e cememted h o s t r o c k and h i g h e r i n t h e p r o f i l e t h e g l a e b u l e s may b e r e s t r i c t e d t o l a r g e n o d u l a r masses ( F i g . 4 . 4 7 ) . A number of v a r i a t i o n s from t h i s s t a n d a r d c a l c r e t e p r o f i l e do e x i s t and one of
t h e s e i s t h e development o f l a r g e - s c a l e p s e u d o - a n t i c l i n e s .
I n t h e s e , curved
c r y s t a l l a r i a w i t h s h a l l o w d i p d e f i n e t h e ' l i m b s ' of t h e a n t i c l i n e and t h e g l a e b u l e s which a r e n e a r l y v e r t i c a l may b e e i t h e r i n c l i n e d s l i g h t l y inwards towards t h e a x i s
240
3
z 2 v)
2
r' I
0
20
Percent acid-solubk
40
60
80
I1
Percent acid-soluble
LEGEND Mudstone
0
Sandstone
Introformotional conglornerote
Gloebules
Parallel laminations
Irregular bonding
a
0
Limestone
Crors- bedding
Fig.4.47. V e r t i c a l c a l c r e t e p r o f i l e s i n t h e Abdon Limestone Formation. A. Nordybank (581848) B. Abdon L i b e r t y (587668) showing g r o s s l i t h o l o g y and t h e v a r i a t i o n i n w t % of a c i d - s o l u b l e c o n s t i t u e n t s (predominantly c a l c i t e ) ( a f t e r A l l e n , 1974a). of t h e p s e u d o a n t i c l i n e o r t h e y f a n outwards and upwards away from t h e a n t i c l i n a l
a x i s (Fig.4.48)
( A l l e n , 1974a, p.113-115).
Pseudoanticlines a r e , l i k e the standard
c a l c r e t e p r o f i l e , e i t h e r s h a r p l y o v e r l a i n by p l a n e l a m i n a t e d mudstone o r e r o s i v e l y o v e r l a i n by i n t r a f o r m a t i o n a l conglomerate and s a n d s t o n e s of t h e s u c c e e d i n g c o a r s e member.
These p s e u d o a n t i c l i n e s p r o b a b l y r e s u l t from t h e d i s p l a c i v e c r y s t a l l i z a t i o n
of c a l c i t e .
The c a l c r e t e s of t h e Old Red Sandstone show many p e t r o g r a p h i c f e a t u r e s v e r y s i m i l a r t o t h o s e s e e n i n Recent c a l c r e t e s .
These i n c l u d e u n d i f f e r e n t i a t e d and
a g g l o m e r a t i c c r y s t i c p l a s m i c f a b r i c s s e e n i n Recent c a l c r e t e s by Blank and Tynes (1965), G i l e e t a l . (1965) and Williams and Polach (1971) and a l s o mural f a b r i c (Brewer, 1964).
There i s abundant e v i d e n c e o f d i s p l a c i v e c r y s t a l l i z a t i o n of c a l c i t e
(Reeves, 1976; W a t t s , 1977, 1978) and e x f o l i a t e d s k e l e t o n g r a i n s o c c u r i n b o t h Recent (e.g.
Swineford e t a l . , 1958) and Old Red Sandstone c a l c r e t e s .
Cross c u t t i n g
s h e e t - l i k e c r y s t a l l a r i a a r e common i n t h e upper p a r t s of modern c a l c r e t e s and a l s o i n some o f t h e Old Red Sandstone p r o f i l e s ( G i l e e t a l . , Reeves, 1970).
Pseudo-oolitic
1966; N a g t e g a a l , 1969;
and p s e u d o - p i s o l i t i c t e x t u r e s conunonly observed i n
Recent c a l c r e t e s (Swineford e t a l . , 1958; S i e s s e r , 1973; Hay and Reeder, 1978) are a l s o s e e n i n t h e Old Red Sandstone.
241
Mudstone
%Tubes
I
' ~ i a t y ' crystallaria
I
Mudstone
IB
Mudstone
Fig.4.48.
Schematic v e r t i c a l s e c t i o n s i l l u s t r a t i n g p s e u d o - a n t i c l i n a l s t r u c t u r e s i n pedogenic c a r b o n a t e u n i t s i n t h e Old Red Sandstone A. Lydney t y p e B. Freshwater E a s t type ( a f t e r A l l e n , 1974a).
The Old Red Sandstone c a r b o n a t e u n i t s compare v e r y c l o s e l y w i t h t h e c a l c a r e o u s d e p o s i t s o f P l i o c e n e and Q u a t e r n a r y age which a r e v a r i o u s l y r e f e r r e d t o a s c a l c r e t e o r c a l i c h e ( A r i s t a r a i n , 1971a).
The mudstones w i t h s c a t t e r e d g l a e b u l e s a r e v e r y l i k e
t h e g l a e b u l a r s o i l s d e s c r i b e d by Bernard and Le Blanc ( 1 9 6 5 ) , Blokhuis e t a l . (1969) and Singh and Singh (1972).
T h i s r e p r e s e n t s S t a g e I and S t a g e I1 i n t h e morpho-
g e n e t i c scheme o f calcrete development proposed by G i l e e t a l . (1966, p.352).
The
more massive, sometimes laminated c a l c r e t e s a r e s i m i l a r t o t h e pedogenic l i m e s t o n e s d e s c r i b e d by Swineford e t a l . ( 1 9 5 8 ) , G i l e (1961, 1 9 7 0 ) , G i l e and Hawley (1966), Reeves ( 1 9 7 0 ) , A r i s t a r a i n (1971a,b) and Williams (1973) and p r o b a b l y r e p r e s e n t S t a g e 111 and S t a g e I V development '(Gile e t a l . , 1966) ( T a b l e 4 . 3 ) . The c l o s e s i m i l a r i t y o f t h e Old Red Sandstone and r e c e n t c a l c r e t e e n a b l e s some p a r a l l e l s t o b e drawn i n t h e i r c o n d i t i o n s o f f o r m a t i o n s
A t t h e p r e s e n t time t h e s e
c o n d i t i o n s a r e a n a r i d t o s e m i - a r i d climate, markedly s e a s o n a l r a i n f a l l , and a r e l a t i v e l y high mean a n n u a l t e m p e r a t u r e (Goudie, 1973, p.96-111).
These P l i o c e n e
and Quaternary c a l c r e t e s a r e most commonly formed i n t h e s h a l l o w s u b - s u r f a c e o f s t a b l e geomorphic l e v e l s (Ruhe, 1969; G i l e , 1970).
They a r e o f pedogenic o r i g i n
i n which downward p e r c o l a t i n g r a i n w a t e r d i s s o l v e s calcium c a r b o n a t e which is
242
p r e c i p i t a t e d lower i n t h e s o i l p r o f i l e a s b o t h c a l c i t e and high-magnesian c a l c i t e ( ' p e r descensum'
model o f Goudie, 1973).
J u d g i n g from t h e p r o t r a c t e d n a t u r e of
r e c e n t c a l c r e t e p e d o g e n e s i s s u b s t a n t i a l l y t h i c k c a l c r e t e p r o f i l e s i n t h e Old Red Sandstone c o u l d have t a k e n o v e r 104 y e a r s t o form. TABLE 4.3.
S t a g e s of t h e morphogenetic sequences i n t h e development o f c a l c r e t e p r o f i l e s and t h e youngest l a n d s u r f a c e s on which t h e s t a g e s o c c u r ( a f t e r G i l e e t a l . , 1966). Stage
Gravelly s o i l s
D i a g n o s t i c Carbonate Morphology Non g r a v e l l y s o i l s
Youngest Geomorphic s u r f a c e on which s t a g e o f h o r i z o n occurs
Thin d i s c o n t i n u o u s pebble coatings
Few f i l a m e n t s o r f a i n t coatings
F i l l m o r e 26005000 y e a r s
I1
Continuous p e b b l e c o a t i n g s some i n t e r p e b b l e fillings
Few t o common n o d u l e s
Leasburg 5000 years l a t e s t Pleistocene
I11
Many i n t e r p e b b l e f i l l i n g s
Many n o d u l e s and internodular f i l l i n g s
Picacho Late Pleistocene
IV
Laminar h o r i z o n o v e r l y i n g plugged h o r i z o n
(increasing carbonate impregnation)
Picacho Late Pleistocene
I
( t h i c k e n e d l a m i n a r and plugged h o r i z o n s )
J o r n a d a Mid- t o Late Pleistocene Laminar h o r i z o n o v e r l y i n g plugged horizon
La Mesa Mid Pleistocene
The fining-upwards c y c l e s i n t h e Old Red Sandstone p r o b a b l y r e c o r d d e p o s i t i o n i n a v a r i e t y of a l l u v i a l c h a n n e l t y p e s and s e t t i n g s .
Where t h e c o a r s e members a r e
i s o l a t e d i n t h e f i n e member a l l u v i u m t h e y always r e s t on a b a s a l e r o s i o n s u r f a c e . This may be f l a t o r g e n t l y concave; sometimes s t e e p , n e a r v e r t i c a l c h a n n e l margins occur.
Concave-based l e n t i c u l a r c o a r s e members a r e g e n e r a l l y r e g a r d e d t o have
formed i n l o w - s i n u o s i t y c h a n n e l s by v e r t i c a l a g g r a d a t i o n .
F l a t - b a s e d c o a r s e members,
on t h e o t h e r hand, a r e more l i k e l y t o have formed i n meandering c h a n n e l s where l a t e r a l migration ensured erosion a t a constant l e v e l .
Good examples of t h e s e
c h a n n e l t y p e s a r e found i n t h e Old Red Sandstone of S p i t s b e r g e n (Moody-Stuart, 1966) (Fig.4.49). The meandering c h a n n e l i n t e r p r e t a t i o n has been used t o e x p l a i n many Old Red Sandstone sequences which have t h i c k f i n e members w i t h pedogenic c a r b o n a t e s .
Good
examples i n c l u d e t h e Old Red Sandstone o f Anglesey ( A l l e n , 1 9 6 5 c ) , t h e Red Marls o f Pembrokeshire ( A l l e n , 1974b) and t h e Wood Bay Formation o f S p i t s b e r g e n ( F r i e n d and Moody-Stuart, 1972).
These sequences a r e , i n a g e n e r a l way, comparable w i t h
243
DEPOSITS Fine channel fill
Lww Channel Floor
Flood Basin Epsilon cross-beddin! Channel directions I '
Fig.4.49. Models i l l u s t r a t i n g t h e r e l a t i o n s h i p between c h a n n e l - t y p e and c o a r s e member s t r u c t u r e i n t h e Old Red Sandstone of S p i t s b e r g e n ( a f t e r Moody-Stuart, 1966). t h e d e p o s i t s of modern meandering c h a n n e l s which wander through t h e i r own a l l u v i u m . R i v e r s o f t h i s t y p e c u t l a t e r a l l y e x t e n s i v e , f l a t e r o s i o n s u r f a c e s which may b e l i n e d w i t h i n t r a f o r m a t i o n a l conglomerate eroded from t h e f l o o d p l a i n ( F i s k , 1944). Sand d e p o s i t i o n o c c u r s p r i n c i p a l l y by l a t e r a l a c c r e t i o n on p o i n t b a r s (Sundborg, 1956, Harms e t a l . ,
1963) and t h e fining-upwards i n t h e c o a r s e member may b e d i r e c t l y
compared w i t h t h e d e c r e a s e i n g r a i n s i z e on t h e s u r f a c e of modern p o i n t b a r s which r e s u l t s from t h e r e d u c t i o n i n bed s h e a r s t r e s s away from t h e c h a n n e l .
The f i n e
members, w i t h t h e i r e v i d e n c e o f s u b a e r i a l e x p o s u r e , b e a r much s i m i l a r i t y t o t h e t h i c k f l o o d p l a i n d e p o s i t s o f a number of r i v e r s which show e v i d e n c e of c a r b o n a t e pedogen-
esis (Bernard and L e Blanc, 1965).
The t h i n s a n d s t o n e upits w i t h i n t h e f i n e member
s i l t s t o n e s i n d i c a t e i n u n d a t i o n s o f t h e f l o o d p l a i n by f l o o d w a t e r s and a r e p r o b a b l y levee o r crevasse-splay deposits. T h i s i n t e r p r e t a t i o n i s n o t , however, w i t h o u t i t s problems.
I n laterally accreted
p o i n t b a r s e p s i l o n c r o s s - b e d d i n g might b e e x p e c t e d t o b e more abundant t h a n i t apparently i s .
A l s o , modern meandering r i v e r s a r e l a t e r a l l y l i m i t e d by e r o s i o n
s u r f a c e s on t h e c h a n n e l banks o r t h e c l a y p l u g s of abandoned c h a n n e l l o o p s .
Steep
channel margins a r e , t h e r e f o r e , l i k e l y t o b e common i n meandering s t r e a m sediments b u t a r e v e r y r a r e i n t h e Old Red Sandstone (one i s d e s c r i b e d by A l l e n (1964a) from
Tugford) and i n a n c i e n t meandering s t r e a m s e d i m e n t s as a whole.
The r e a s o n s f o r
t h e s e d i s c r e p a n c i e s a r e n o t c l e a r b u t i t i s p o s s i b l e t h a t n o t a l l fining-upwards c y c l e s of t h i s type a r e meandering s t r e a m d e p o s i t s . t h i n c o a r s e members ((2m)
Some c y c l e s show r e l a t i v e l y
which a r e l a t e r a l l y e x t e n s i v e , f l a t - b a s e d and w i t h an
abundance of p a r a l l e l l a m i n a t e d s a n d s .
These c o u l d b e t h e d e p o s i t s o f a r e a l l y
e x t e n s i v e , s i n g l e e v e n t f l o o d s which might o c c u r , f o r example, i n t h e d i s t a l r e g i o n s of large ' a l l u v i a l f a n s ' . I n some o t h e r Old Red Sandstone sequences t h e c o a r s e members a r e dominant and t h e r e i s l i t t l e p r e s e r v a t i o n of t h e f i n e member i n t e r - c h a n n e l d e p o s i t s .
These
d e p o s i t s a r e g e n e r a l l y i n t e r p r e t e d a s t h o s e of l o w - s i n u o s i t y sandy r i v e r s which may have been b r a i d e d .
The p a u c i t y o f muddy f l o o d p l a i n d e p o s i t s may b e p a r t l y due
t o t h e f a c t t h a t t h e y were o r i g i n a l l y p o o r l y developed b e c a u s e o f a low suspended l o a d (Schurmn and Kahn, 1972) o r b e c a u s e of t h e tendency o f t h e s e r i v e r s t o comb t h e i r floodplain eroding the floodplain deposits.
The i n t e r p r e t a t i o n of a n c i e n t low-
s i n u o s i t y s t r e a m d e p o s i t s i s d i f f i c u l t b e c a u s e t h e r e a r e few d e t a i l e d s t u d i e s of modern d e p o s i t s .
Cant and Walker (1976) have made a f a c i e s model of t h e B a t t e r y
P o i n t Sandstone of Quebec u s i n g a combination o f f a c i e s sequence and p a l a e o c u r r e n t information (Fig.4.50).
T h i s b e g i n s w i t h a s c o u r e d b a s e and i n t r a f o r m a t i o n a l
conglomerate which p a s s e s upwards i n t o s a n d s t o n e s w i t h t r o u g h cross-bedding.
Within
t h e sequence a r e p l a n a r t a b u l a r cross-bedded u n i t s whose p a l a e o f l o w d i r e c t i o n s d i v e r g e from t h o s e of t h e t r o u g h c r o s s - b e d s by up t o 90'. c o n s i s t s o f s m a l l - s c a l e t a b u l a r s e t s and c r o s s - l a m i n a t i o n .
The t o p of t h e sequence By comparison w i t h t h e
d e p o s i t s of South Saskatchewan R i v e r (Cant and Walker, 1978) t h i s sequence i s i n t e r p r e t e d i n t h e f o l l o w i n g manner ( s e e Fig.4.14).
The t r o u g h c r o s s - b e d s a r e due
t o s i n u o u s - c r e s t e d h i g r a t i n g dunes i n t h e c h a n n e l and t h e s e d e f i n e t h e main palaeoflow d i r e c t i o n .
The p l a n a r t a b u l a r s e t s w i t h d i v e r g e n t flow d i r e c t i o n s a r e t h e
r e s u l t of c r o s s - c h a n n e l b a r s o b l i q u e t o t h e main c h a n n e l a x i s .
The upper p a r t of
t h e sequence may r e p r e s e n t t h e b a r t o p w i t h s a n d f l a t a c c r e t i o n and c h a n n e l aggradation.
A t t h e v e r y t o p o f t h e sequence may be a t h i n zone o f v e r t i c a l
accretion (floodplain) deposits.
The complexity o f c o a r s e member-dominated sequences
i n t h e Old Red Sandstone i s s u c h t h a t many more d e t a i l e d and d i r e c t comparisons w i t h p r e s e n t - d a y l o w - s i n u o s i t y sandy r i v e r s a r e needed i f t h e y a r e t o b e p r o p e r l y understood. The r e d d e n i n g of sandy and muddy a l l u v i u m One of t h e most c h a r a c t e r i s t i c f e a t u r e s o f r e d beds i n t h e Old Red S a n d s t o n e i s t h e d i f f e r e n c e i n c o l o u r between t h e c o a r s e and f i n e members o f fining-upwards c y c l e s (Fig.4.51).
The f i n e members a r e a l m o s t e x c l u s i v e l y r e d (5R, 10R) whereas t h e c o a r s e
members v a r y t h r o u g h s h a d e s of g r e y (N), g r e e n i s h - g r e y (5GY), o l i v e - g r e y (5Y) and brownish-grey (5yR)
( F r i e n d , 1966; Van Houten, 1973; T u r n e r , 1974a).
245
"\ e-
\
Verjical accretion
l
I Channel floor
Fig.4.50. F a c i e s model f o r t h e B a t t e r y P o i n t Sandstone (Old Red Sandstone, Quebec). The changes i n v e r t i c a l sequence and p a l a e o c u r r e n t s a r e i n t e r p r e t e d a s t h e product o f a low-sinuosity r i v e r ( a f t e r Cant and Walker, 1976). a t h i n d r a b zone may be p r e s e n t n e a r t h e top o r t h e b a s e of t h e f i n e member (Fig.4.51) a l t h o u g h t h i s i s g e n e r a l l y a t t r i b u t e d t o secondary b l e a c h i n g .
I n some sequences,
p a r t i c u l a r l y i n Late Devonian times when p l a n t s were more abundant, drab zones w i t h i n t h e f i n e member may be a s s o c i a t e d w i t h o r g a n i c d e b r i s and c a n r e a d i l y b e i n t e r p r e t e d a s marsh o r backswamp d e p o s i t s i n which r e d u c i n g c o n d i t i o n s were developed s h o r t l y a f t e r d e p o s i t i o n (e.g.
Johnson and Friedman, 1969).
The e s s e n t i a l d i f f e r e n c e between t h e r e d f i n e member d e p o s i t s , and d r a b , c o a r s e member d e p o s i t s , i s t h a t t h e former c o n t a i n a g r e a t e r p r o p o r t i o n o f pigmentary haematite.
Much o f t h i s i s c o n c e n t r a t e d w i t h i n t h e c l a y f r a c t i o n (Walker and
Honea, 1969; Van Houten, 1973) s u g g e s t i n g t h a t t h e d e p o s i t i o n a l mechanism must have had a c o n t r o l l i n g i n f l u e n c e on t h e observed red- non-red d i f f e r e n t i a t i o n .
A t the
same t i m e t h e wide v a r i a t i o n i n c o l o u r of c o a r s e members and t h e abundance of haemati t e i n f i n e members (remembering i t s p a u c i t y i n Recent a l l u v i u m ) does s u g g e s t t h a t
p o s t - d e p o s i t i o n a l p r o c e s s e s were i m p o r t a n t and that t h e r e were d i f f e r e n c e s i n t h e
246
p o s t - d e p o s i t i o n a l h i s t o r y o f c o a r s e and f i n e members.
ENVIRONMENT
tTURES MAIN FEA-. ---
COLOUR
CHANNEL BASE
I (BLEACHED) CALCR ETE
SOIL
MASSIVE SI LTSTONE
FLOODPLAIN
CROSS- LAM1NATED
BANK DEPOSITS
fl
SI LTSTONES
I
OSS-BEDDED SANDSTONES
I
OR }NTERMEDIAT
CHANNEL
\\\k
Fig.4.51. T y p i c a l c o l o u r v a r i a t i o n s s e e n i n a n Old Red Sandstone fining-upwards cyclothem. A.
F i n e member r e d d e n i n g
F r i e n d (1966) s t u d i e d t h e c o l o u r d i s t r i b u t i o n o f fining-upwards c y c l e s i n t h e Old Red Sandstone of t h e C a t s k i l l s and S p i t s b e r g e n by a n a l y s i n g changes i n c o l o u r , c l a y m i n e r a l o g y , g r a i n s i z e and i r o n c o n t e n t ,
The c l a y m i n e r a l o g y o f t h e s e r e d
beds i s v e r y s i m i l a r t o many o t h e r a n c i e n t r e d b e d s ; t h e s a n d s t o n e s a r e r e l a t i v e l y r i c h e r i n c h l o r i t e - p l u s - k a o l i n i t e w h i l s t t h e s i l t s t o n e s from t h e f i n e members a r e relatively richer i n i l l i t e .
Although t h e r e i s some v a r i a t i o n from c y c l e t o c y c l e
t h e r e i s l i t t l e doubt t h a t a mature c l a y m i n e r a l assemblage of t h i s t y p e i s t h e r e s u l t of a u t h i g e n e s i s and c l a y m i n e r a l t r a n s f o r m a t i o n and t h e r e i s t e x t u r a l e v i d e n c e i n f a v o u r o f t h i s ( F r i e n d , 1966, p.279). Although t h e C a t s k i l l and S p i t s b e r g e n r e d beds show a c o n c e n t r a t i o n of haematite i n t h e c o a r s e t o medium c l a y f r a c t i o n s (1-4ym) t h e r e a r e no c l e a r r e l a t i o n s h i p s between c o l o u r and c l a y m i n e r a l o g y (Fig.4.52).
T h i s is t h e c a s e w i t h many a n c i e n t
241
lllitr 1 J
0 0 SMdstOM R d NOW
1,11,111
RIfn to
Fig.4.52. Peak h e i g h t r a t i o s f o r i l l i t e , . c h l o r i t e and k a o l i n i t e f o r r e d and drab s a n d s t o n e s and s i l t s t o n e s i n t h e Old Red Sandstone o f t h e C a t s k i l l Mountains, USA ( a f t e r F r i e n d , 1966). r e d beds (Van Houten, 1973; T u r n e r , 1974a) which i s n o t s u r p r i s i n g because t h e h a e m a t i t e i s g e n e r a l l y e u h e d r a l and most l i k e l y a u t h i g e n i c i n o r i g i n .
A number of
a u t h o r s (Van Houten, 1964, 1973; F r i e n d , 1966; T u r n e r , 1974a) have a r g u e d t h a t t h e c o n c e n t r a t i o n o f pigmentary h a e m a t i t e i n f i n e members r e f l e c t s a d e p o s i t i o n a l c o n c e n t r a t i o n o f brown-coloured, environment.
f e r r i c hydroxide p r e c u r s o r i n t h e f l o o d p l a i n
F r i e n d (1966) e x p l a i n e d t h e d i f f e r e n c e i n c o l o u r between t h e c o a r s e
and f i n e members i n t h e f o l l o w i n g manner (Fig.4.53).
I n t h e f l o o d p l a i n environment
abundant i r o n hydroxides were d e p o s i t e d w i t h t h e c l a y - r i c h suspended l o a d .
Because
of p e r i o d i c d r y i n g , and l o w e r i n g o f t h e water t a b l e , o x i d i z i n g c o n d i t i o n s p r e v a i l e d and i r o n hydroxides e v e n t u a l l y aged i n t o h a e m a t i t e .
I n t h e c h a n n e l environment,
however, t h e sediment remained m o s t l y below t h e w a t e r t a b l e a f t e r d e p o s i t i o n .
In
a r e a s where s u b s t a n t i a l o r g a n i c m a t t e r was p r e s e n t b a c t e r i a l consumption of oxygen c r e a t e d a r e d u c i n g environment i n which f e r r i c hydroxides were u n s t a b l e and removed i n s o l u t i o n and t h e c h a n n e l sediments u l t i m a t e l y became d r a b . S e v e r a l f a c t o r s s u g g e s t t h a t t h i s s i m p l e mechanism s u g g e s t e d by F r i e n d i s , i n essence, correct.
These i n c l u d e t h e p a u c i t y of h a e m a t i t e i n Recent a l l u v i u m
(Walker, 19741, t h e f a c t t h a t c l a y - g r a d e f e r r i c h y d r o x i d e s a r e u n s t a b l e r e l a t i v e t o h e m a t i t e ( B e r n e r , 1 9 6 9 a ) , t h e l a r g e amounts of t o t a l i r o n found i n f i n e members, and a l s o t h e f a c t t h a t t h e geochemical c o n d i t i o n s d e s c r i b e d by F r i e n d a r e commonly m e t w i t h i n modern a l l u v i u l s e d i m e n t s .
A c o n t r i b u t i o n from t h e d i a g e n e t i c break-
down o f s i l i c a t e g r a i n s i s a l s o t o be e x p e c t e d b u t t h e r e l a t i v e importance of t h i s
248
p r o c e s s i s d i f f i c u l t t o a s s e s s b e c a u s e o f t h e l a c k of m i n e r a l o g i c a l e v i d e n c e .
ON DEPOSITION
AFTER DEPOSITION EARLY
TAN-BROWN COLOURED FERRIC OXYHYDROXIDES
1
LATE
OXIDIZING
-b
REDUCING
-b
FERROUS IRON I N DETRITAL SILICATES
HAEMATITE FERROUS CCMPOUNDS REMOVED IN SOLUTION AND PYRITE
INTRASTRATAL ALTERATION
,
OXIDIZING
WATER TABLB
REDUCING CONDITIONS DEVELOPED IN WATERLOGGED SANDS ESPECIALLY I F THEY CONTAIN ORGANIC MATERIAL
Fig.4.53. Suggested sequence and environment o f f o r m a t i o n of i r o n compounds i n Old Red Sandstone fining-upwards c y c l e s ( a f t e r F r i e n d , 1966). A q u a n t i t a t i v e e s t i m a t e o f t h e t i m e s c a l e of r e d d e n i n g i n Old Red Sandstone
f i n e members i s p o s s i b l e b e c a u s e o f t h e o c c u r r e n c e of pedogenic c a l c r e t e u n i t s . These o f t e n o c c u r , a l o n g w i t h r e d mudstones, a s i n t r a f o r m a t i o n a l conglomerates i n t h e Old Red Sandstone ( A l l e n , 1960, 1962a) ( F i g . 4 . 5 4 ) .
O f t e n r e d mudstone c l a s t s
o c c u r i n a d r a b , sandy m a t r i x , a l t h o u g h i t i s more u s u a l t o f i n d t h a t t h e m a t r i x shows some e v i d e n c e of reddening.
I n e i t h e r c a s e t h e f a c t t h a t t h e c l a s t s were
s u f f i c i e n t l y c o n s o l i d a t e d t o s u r v i v e f l u v i a l t r a n s p o r t i s a n i n d i c a t i o n t h a t they were a l m o s t c e r t a i n l y r e d a t t h e t i m e ,
The t i m e s c a l e o f c a l c r e t e development
t h u s g i v e s some e s t i m a t e o f t h e t i m e t a k e n f o r t h e development o f r e d c o l o u r . However, t h i s t i m e s c a l e i s d i f f i c u l t enough t o estimate f o r Recent c a l c r e t e s l e t a l o n e t h o s e i n t h e Old Red Sandstone.
Radiocarbon d a t i n g by Ruhe (1967) o f d u s t -
d e r i v e d c a l c r e t e s i n New Mexico s u g g e s t t h a t c a l c i f i c a t i o n i n c r e a s e s a t t h e r a t e of 1%p e r 1000 y e a r s .
Compatible r e s u l t s have been o b t a i n e d by G i l e and Grossman
(1968) and Williams and P o l a c h (1971).
These r e s u l t s , and t h e o r e t i c a l c o n s i d e r a t i o n s ,
s u g g e s t s t h a t t h i c k c a l c r e t e s have developed o v e r some hundreds o f thousands o f years.
249
A . T h i n s e c t i o n photomicrograph of a n Old Red Sandstone c a l c r e t e showing Fig.4.54. l a r g e c r y s t a l l a r i a c u t t i n g a reddened s i l t s t o n e m a t r i x (Red Marls, F r e s h w a t e r West, Wales). B. Conglomeratic c a l c r e t e showing rounded, p r e v i o u s l y r e d d e n e d , c a r b o n a t e c l a s t s i n c a l c i t e cement (Upper Old Red S a n d s t o n e , P e r t h s h i r e , S c o t l a n d ) .
2 50 Goudie (1973, p.88-89)
c a l c u l a t e d a r a t e of l m o f 80%CaC03 p e r 300,000 y e a r s .
Under t h e s e c i r c u m s t a n c e s (assuming t h a t s i m i l a r r a t e s a p p l i e d i n Old Red Sandstone t i m e s ) i t i s n o t s u r p r i s i n g t h a t t h e mudstones were r e d by t h e t i m e t h e s u c c e e d i n g cyclothem was i n i t i a t e d .
Not o n l y i s t h i s s u f f i c i e n t t i m e f o r t h e d e h y d r a t i o n and
i n v e r s i o n of d e t r i t a l f e r r i c hydroxides i n t o h a e m a t i t e i n t h e manner s u g g e s t e d by Van Houten (1972) b u t s u b s t a n t i a l i n s i t u s i l i c a t e a l t e r a t i o n c o u l d a l s o t a k e p l a c e e s p e c i a l l y under t h e c l i m a t i c c o n d i t i o n s which p r e v a i l e d (Walker, 1967a, 1974, 1976). The r e l a t i v e c o n t r i b u t i o n of e a c h of t h e s e p r o c e s s e s i n r e d bed f o r m a t i o n c a n n o t be deduced from t h e a v a i l a b l e e v i d e n c e a l t h o u g h i t seems l i k e l y t h a t b o t h were i n opera t i o n . B.
Coarse member r e d d e n i n n
The mechanism of red-drab d i f f e r e n t i a t i o n proposed by F r i e n d (1966) does n o t s a t i s f a c t o r i l y e x p l a i n t h e f a c t t h a t many c o a r s e members a r e r e d o r brownish-red i n colour.
T h i s i s e s p e c i a l l y s o i n c o a r s e member-dominated s u c c e s s i o n s i n which
f i n e members a r e t h i n o r a b s e n t .
The i m p l i c a t i o n h e r e i s t h a t r e d u c i n g c o n d i t i o n s
were never developed and t h a t t h e groundwaters d u r i n g d i a g e n e s i s were always oxygenated.
The p o s t - d e p o s i t i o n a l h i s t o r y o f c o a r s e member s a n d s t o n e s i s f u r t h e r
c o m p l i c a t e d by e v i d e n c e which s u g g e s t s t h a t t h e r e were r e d u c i n g groundwaters m i g r a t i n g through t h e s a n d s t o n e b o d i e s u n t i l t h e i r f i n a l compaction.
This takes
t h e form of g r e e n , s e c o n d a r i l y b l e a c h e d zones which o c c u r j u s t below a n d , i n some c a s e s , j u s t above t h e c o a r s e members ( F i g . 4 . 5 1 ) .
These s u g g e s t t h a t r e d u c i n g
groundwaters were e x p e l l e d from t h e porous s a n d s t o n e b o d i e s p r o b a b l y d u r i n g compac t i o n . Colour v a r i a t i o n s i n t h e c o a r s e members of sandy a l l u v i u m i n t h e Old Red Sandstone a r e l a r g e l y due t o t h e r e l a t i v e abundance and d i s t r i b u t i o n of pigmentary clay-oxides (Table 4.4).
In some c l a y - r i c h s a n d s t o n e s t h e framework c o n s t i t u e n t s
have t h i c k c l a y - o x i d e p e l l i c l e s (Fig.4.55) Brewer (1964).
which resemble t h e c l a y - c u t a n s o f
O t h e r c l a y - o x i d e c o a t i n g s show t e x t u r e s which c l o s e l y resemble t h e
m e c h a n i c a l l y i n f i l t r a t e d c l a y t e x t u r e s d e s c r i b e d by Walker (1976) from Cenozoic alluvium.
I n b o t h t h e s e c a s e s t h e s u r v i v a l of t h e h a e m a t i t e c l a y - o x i d e m a t r i x i s
a n i n d i c a t i o n t h a t o x i d i z i n g c o n d i t i o n s p r e v a i l e d d u r i n g d e p o s i t i o n and d i a g e n e s i s . S e v e r a l l i n e s of e v i d e n c e a l s o s u g g e s t t h a t pigmentary h a e m a t i t e h a s been produced by i n t r a s t r a t a l s o l u t i o n of i r o n b e a r i n g g r a i n s i n t h e r e d c o a r s e member s a n d s t o n e s . T h i s e v i d e n c e i n c l u d e s t h e composition o f heavy m i n e r a l s u i t e s , c l a y m i n e r a l o g y , and t h e n a t u r e of t h e a u t h i g e n i c m i n e r a l p h a s e s i n t h e s a n d s t o n e s . A n o t a b l e f e a t u r e of heavy m i n e r a l a n a l y s e s i n t h e Old Red Sandstone i s t h e a l m o s t t o t a l absence of u n s t a b l e f e r r o m a g n e s i a n s i l i c a t e s such a s h o r n b l e n d e and pyroxene.
A l l e n (1974a) p u b l i s h e d a d e t a i l e d s u r v e y of heavy m i n e r a l d i s t r i b u t i o n s
i n t h e O l d Red Sandstone of t h e Anglo-Welsh b a s i n and r e c o g n i z e d t h r e e heavy m i n e r a l
251
Fig.4.55. T e x t u r a l f e a t u r e s of pigmentary i r o n o x i d e s and c l a y - i r o n o x i d e s i n Old Red Sandstone sandy a l l u v i u m . A . P a r t i c u l a t e g r a i n c o a t i n g s of h a e m a t i t e formed by p o s t - d e p o s i t i o n a l p r e c i p i t a t i o n ( R i n g e r i k e Group, S i l u r i a n , Norway). B . Thick p e l l i c l e s o f c l a y - i r o n o x i d e (Lower Old Red S a n d s t o n e , Dyfed, Wales). C. I n t e r s t i t i a l m a t r i x o f c l a y - i r o n o x i d e c o n c e n t r a t e d i n laminae (Lower Old Red Sandstone, Gamrie O u t l i e r , S c o t l a n d ) .
252
associations:
metamorphic ( g a r n e t , s t a u r o l i t e and e p i d o t e ) , igneous ( a n a t a s e ,
a p a t i t e , b r o o k i t e , r u t i l e , sphene, s p i n e l and z i r c o n ) , and p e g m a t i t i c ( t o u r m a l i n e The s t r a t i g r a p h i c a l v a r i a t i o n i n t h e s e t h r e e assemblages i s shown i n Fig.4.56.
only),
TABLE 4.4
R e l a t i o n s h i p s between whole rock c o l o u r and t e x t u r a l d i s t r i b u t i o n of c l a y - o x i d e m i s t u r e s i n Devonian r e d s a n d s t o n e s . Colour
Texture
Moderate Pink 5R7f 4
-
Moderate Orange Pink 10R7f 4
Sparse clay-oxide g r a i n coatings L i t t l e o r no i n t e r s t i t i a l m a t r i x
P a l e Red 5R6f2 10R6f2
Clay-oxide g r a i n c o a t i n g s a b s e n t t o p o o r l y developed. Opaque t o t r a n s l u c e n t i n t e r s t i t i a l m a t r i x more abundant
Moderate Reddish Orange 10R6f 6
Prominent t r a n s l u c e n t c l a y - o x i d e g r a i n c o a t i n g s , I n d i v i d u a l c r y s t a l s t o o small t o be r e s o l v e d . S p a r s e i n t e r s t i t i a l m a t r i x . I f more abundant, homogeneously distributed
Moderate Red 5R5f4 5R4f6
P a r t i c u l a t e g r a i n c o a t i n g s mainly of h a e m a t i t e . Abundant opaque i n t e r s t i t i a l m a t r i x may b e inhomogeneously d i s t r i b u t e d
-
-
Individual crystals g r e a t e r than 2 m can b e r e s o l v e d
P a l e Reddish Brown 10R5f 4
-
Dark Reddish Brown 10R3f4
Abundant t h i c k g r a i n c o a t i n g s of c l a y and h a e m a t i t e and opaque i n t e r s t i t i a l matrix
P a l a e o c u r r e n t a n a l y s i s and t h e s t u d y o f e x o t i c p e b b l e s , when combined w i t h t h e heavy m i n e r a l r e s u l t s s u g g e s t a s o u r c e a r e a t o t h e n o r t h w e s t o f t h e a r e a which i n c l u d e d Moine and D a l r a d i a n igneous and metamorphic r o c k s a s w e l l a s Lower P a l a e o c o i c sediments.
The Downton Group i s n o t a b l e f o r t h e l a r g e amounts of g a r n e t which
s u g g e s t a h i g h grade metamorphic provenance l i k e t h e Moine o r D a l r a d i a n .
The
c o m p o s i t i o n a l change a t t h e end of Downton Group times, when t h e igneous assemblage becomes much more i m p o r t a n t (Fig.4.56)
a p p e a r s t o b e due t o u p l i f t and i s o l a t i o n
of t h e d i s t a n t metamorphic s o u r c e a r e a by r i v e r c a p t u r e o r d r a i n a g e r e v e r s a l . The absence of pyroxene and hornblende i n t h e Old Red Sandstone i s q u i t e marked ( s e e a l s o F l e e t , 1925, 1926; Walder, 1941) d e s p i t e t h e f a c t t h a t i t i s l i k e l y t o have been r e l a t i v e l y abundant i n a s o u r c e a r e a which i s known t o have i n c l u d e d igneous and metamorphic r o c k s .
The observed assemblage i s dominated by m i n e r a l s
w i t h a h i g h l e v e l of p e r s i s t e n c e i n a n c i e n t s a n d s t o n e s ( P e t t i j o h n , 1975, p.496). S i m i l a r r e s u l t s a r e found throughout t h e Old Red c o n t i n e n t and i n view o f t h e known s u s c e p t i b i l i t y of hornblende and pyroxene t o i n t r a s t r a t a l a l t e r a t i o n i n o x i d i z i n g groundwaters i t seems l i k e l y t h a t complete d i s s o l u t i o n h a s t a k e n p l a c e .
The i n i t i a l
a l t e r a t i o n p r o d u c t s a r e most l i k e l y t o have been mixed l a y e r i l l i t e - m o n t m o r i l l o n i t e
2 53
and i r o n o x i d e , much of t h e i r o n o x i d e e v e n t u a l l y c o n t r i b u t i n g rowards t h e pigmentary haematite.
D i r e c t evidence o f i n s i t u s i l i c a t e a l t e r a t i o n i s p r o v i d e d by p a r t i a l l y
haematized c h l o r i t e and b i o t i t e f l a k e s ( t h e s e m i n e r a l s a r e o f comparable s t a b i l i t y t o g a r n e t i n t h e d i a g e n e t i c environment).
Advanced i n s i t u a l t e r a t i o n s a r e i n d i c a t e d
by t h e t o t a l pseudomorphing of b i o t i t e f l a k e s by h a e m a t i t e ( T u r n e r , 1974a; Turner and A r c h e r , 1977) ( F i g . 4 . 5 5 ) .
LEGEND Melomorphic Pegmatitic
DITTON AND A I D O N GPS.
Igneous
osseniblaqe assemblage
assernbtage
LEDBURY FM.
I
2
3
6
8 10
20
40
60
100
Percenl
Fig.4.56. S t r a t i g r a p h i c a l v a r i a t i o n i n t h e number p e r c e n t a g e of heavy m i n e r a l groups i n t h e Old Red Sandstone of t h e Anglo-Welsh b a s i n ( a f t e r A l l e n , 1974a). The c l a y m i n e r a l s u i t e s i n t h e Old Red Sandstone a l s o i n d i c a t e e x t e n s i v e diagenesis.
The main c l a y m i n e r a l s a r e i l l i t e , c h l o r i t e and k a o l i n i t e ; montmorill-
o n i t e b e i n g o n l y r a r e l y r e p o r t e d (e.g. A l l e n , 1974a).
I l l i t e t y p i c a l l y shows a
h i g h c r y s t a l l i n i t y index and may have f o r m e d , i n p a r t a t l e a s t , by t h e t r a n s f o r m a t i o n o f mixed l a y e r i l l i t e - m o n t m o r i l l o n i t e which formed a s a n a u t h i g e n i c phase d u r i n g e a r l y diagenesis. The Lower T r i a s s i c r e d beds of NW Europe The lowermost T r i a s s i c ( S c y t h i a n ) of NW Europe saw t h e accumulation of a pebbly and sandy a l l u v i a l sequence on a n o r t h w e s t e r l y i n c l i n e d p a l a e o s l o p e (Audley-Charles, 1970a,b).
Northwards o f t h e London-Brabant h i g h i s a m a r g i n a l conglomeratic f a c i e s
r e f e r r e d t o a s t h e Bunter Pebble Beds (Fig.k.57)
(Brennand, 1975).
Northwestwards
t h e g r a i n s i z e d e c r e a s e s a s t h e pebbly a l l u v i u m p a s s e s downstream i n t o more sandy
254 alluvium.
There a r e a l s o marked v e r t i c a l changes i n g r a i n s i z e and i n t h e Midlands
o f England t h e Bunter P e b b l e Beds p a s s upwards i n t o a sandy a l l u v i a l sequence (Upper M o t t l e d S a n d s t o n e ) .
Above t h i s i s a renewed i n f l u x o f c o a r s e r , p e b b l y
a l l u v i u m (Lower Keuper S a n d s t o n e ) .
A t a b o u t t h e same t i m e t h e d e p o s i t i o n of c o a r s e
g r a i n e d a l l u v i a l f a n s may have been t a k i n g p l a c e i n NW S c o t l a n d b u t t h e s e d e p o s i t s a r e n o t y e t a c c u r a t e l y d a t e d ( S t e e l and W i l s o n , 1975).
n
Maximum distribution Triassic deposits
--
Distribution solid Muschelkalk carbonates Northern limit Muschelkalk carbonates
-I
Edge Rot salt
1 Areas with Keuper salts
5w
Total Triassic thickness
Fig.4.57. Palaeogeography of t h e lower most T r i a s s i c ( S c y t h i a n ) i n NW Europe ( a f t e r Z i e g l e r , 1978). The sedimentology of t h e Bunter P e b b l e Beds h a s been d e s c r i b e d and i n t e r p r e t e d by Thompson (1970).
These t y p i c a l l y form fining-upwards c y c l e s i n which beds of
s t r u c t u r e l e s s o r i m b r i c a t e d , sometimes uncemented, conglomerate p a s s up i n t o l a r g e s c a l e cross-bedded p e b b l y s a n d s t o n e s and t r o u g h cross-bedded s a n d s (Fig.4.58). mudstones may b e p r e s e n t a t t h e t o p of t h e c y c l e .
Thin
The sequence is i n t e r p r e t e d a s
t h e d e p o s i t s o f a l o w - s i n u o s i t y b r a i d e d s t r e a m s i m i l a r t o t h o s e d e p o s i t i n g pebbly a l l u v i u m a t t h e p r e s e n t day.
T h i s view i s s u p p o r t e d by t h e f a c t t h e d e p o s i t s l i e
above concave e r o s i o n s u r f a c e s and g e n e r a l l y f i n e upwards a l o n g w i t h a d e c r e a s e
INTRAFORMATIONAL u- CONGLOMERATE
EXOTIC PEBBLES GRANULES VENTIFACTS
CHANNEL
LARGE SCALE CROSS-BEDDING
mERmIoN
.SCOURED SURFACE
LAMINATED SHALE
SMALL SCALE CROSSBEDDING
MUDCRACKS
01
01
N
mBLocKy
CHANNEL FULL SEDIMENTS DEPOSITEDAS CHANNEL BAR AND LATERAL BARS. POSSIBLE DEVELOPMENT OF POINT BARS WITH LATERAL ACCRETION SURFACES
INTRAFORMATIONAL MUD CLASTS NEAR BASE. ROLLED DREIKANTER. LARGE SCALE CROSS-BEDDING WITH FORESETS POORLY DEFINED AT BASE. SET THICKNESS GENERALLY DECREASES W A R D S ,
RAPID EROSION OF FLOODPLAIN DEPOSITS
RELATIVELY SUDDEN CESSATION OF FLOOD AND BEDLOAD TRACTION
RED? SECONDARILY BUFF OR GREY COARSE TO MEDIUM GRAINED. POORLY SORTED SANDSTONE, PEBBLY SANDSTONE AND CONGLOMERATE.
EXTRAFORMATIONAL PEBBLES. DISC AND SPHEROID SHAPE CONSISTINGOF QUARTZ, QUARTZITE. PORPHYRY. FELSITE.TUFF. CHERT,SANDSTONE, AND MUDSTONE
VERTICAL ACCRETION DEPOSITS OF LOW SINUOSITY RIVER FLOOD PLAIN
INTERPRETATION
RED MICACEOUS,LAMINATED OR BLOCKY, SILTY MUDSTONES WITH MUD CRACKS. SHARP FLAT UPPER SURFACE.AT TIMES IRREGULAR
CHANNELING
MAIN FACTS
256
i n set thickness.
The s t r u c t u r e l e s s and i m b r i c a t e d conglomerates compare c l o s e l y
with the longitudinal gravel bars of present-day coarse grained braided r i v e r s w h i l s t t h e p r e s e n c e of l a r g e s c a l e l a t e r a l a c c r e t i o n u n i t s may r e s u l t from d i a g o n a l o r l a t e r a l b a r s (Bluck, 1974) o r t h e f o r m a t i o n of g r a v e l meander l o b e s l i k e t h o s e d e s c r i b e d by Gustavson (1978). A d i f f e r e n t t y p e of sequence i s s e e n i n t h e upper p a r t o f t h e Lower Keuper Sandstone ( F i g . 4 . 5 9 ) .
Here t h e bank and f l o o d p l a i n d e p o s i t s a r e t h i c k e r and
conglomerate i s l a r g e l y c o n f i n e d t o t h e b a s e o f t h e c y c l e .
The sequence shows t h e
f a m i l i a r fining-upwards and above t h e conglomerate ( c h a n n e l l a g ) a r e c r o s s - s t r a t a which c o u l d b e p o i n t b a r d e p o s i t s o r p o s s i b l y t r a n s v e r s e b a r s o r sandwaves (Harms e t a l . , 1975).
These p a s s upwards i n t o t r o u g h c r o s s - b e d s which i n d i c a t e channel-
f i l l by s i n u o u s c r e s t e d m e g a r i p p l e s .
Above, i n t e r b e d d e d r i p p l e d s a n d s and mudstones
w i t h d e s i c c a t i o n c r a c k s and r e p t i l e f o o t p r i n t s i n d i c a t e l e v e e and f l o o d p l a i n deposition.
T h i s t y p e of c y c l e s u g g e s t s a more meandering s t r e a m c h a n n e l , p a r t l y
e n t r e n c h e d i n i t s own a l l u v i u m and w i t h well-developed banks and r i v e r f l o o d p l a i n s . I n NW England t h e d i s t a l r e g i o n s of t h i s S c y t h i a n a l l u v i a l complex r e v e a l a d i f f e r e n t p a t t e r n of s e d i m e n t a t i o n .
Here a sandy a l l u v i a l sequence ( S t . Bees
Sandstone) l i e s d i r e c t l y on a n Upper Permian e v a p o r i t i c sequence ( S t . Bees S v a p o r i t e s and S h a l e s ) which i s i n t e r b e d d e d w i t h t h i n m a r g i n a l a l l u v i a l f a n d e p o s i t s from t h e a d j a c e n t Lake D i s t r i c t Massif (brockrams) ( A r t h u r t o n and Hemingway, 1972). The S t . Bees Sandstone c o n s i s t s p r e d o m i n a n t l y o f f i n e and medium g r a i n e d sands t o n e s w i t h minor s i l t s t o n e s and mudstones.
These a r e a r r a n g e d i n l a r g e t a b u l a r
u n i t s which wedge-out l a t e r a l l y between prominent e r o s i o n a l bounding s u r f a c e s (Fig.4.60).
Between t h e bounding s u r f a c e s t h e a l l u v i a l c y c l e s b e g i n w i t h i n t r a -
f o r m a t i o n a l l a g cdnglomerate and p a s s upwards i n t o p l a n a r t a b u l a r cross-bedded u n i t s , p l a n e beds o r pseudo-plane beds (Smith, 1971a).
I n t u r n , these u s u a l l y give
way t o t r o u g h c r o s s - b e d s o r c r o s s - l a m i n a t e d s i l t s t o n e s which may g r a d u a l l y f i n e upwards i n t o a t h i n d e s i c c a f e d mudstone o r s h a l e ,
T h i s sequence c l o s e l y resembles
t h a t which i s produced by l o w - s i n u o s i t y , sandy b r a i d e d r i v e r s .
The p l a n a r t a b u l a r
u n i t s of t h e S t . Bees Sandstone f r e q u e n t l y show r e a c t i v a t i o n s u r f a c e s l i k e t h o s e d e s c r i b e d from t h e l i n g u o i d b a r s o f t h e Tana R i v e r by C o l l i n s o n (1970) and t r a n s v e r s e b a r s o r sandwaves of a s i m i l a r t y p e a r e b e l i e v e d t o have been a n i m p o r t a n t f e a t u r e of t h e S t . Bees Sandstone.
The t r o u g h c r o s s - b e d s r e p r e s e n t m i g r a t i n g s i n u o u s -
c r e s t e d dunes which were t h e main c h a n n e l - f i l l i n g bedforms. The c o l o u r of t h e s e T r i a s s i c sandy and p e b b l y a l l u v i u m i s a l m o s t u n i f o r m l y r e d e x c e p t f o r a r e a s which have o b v i o u s l y been s e c o n d a r i l y b l e a c h e d .
Although t h e
o r i g i n of t h e r e d n e s s has been d e b a t e d f o r some t i m e r e c e n t s t u d i e s ( T u r n e r and I x e r , 1977; I x e r e t a l . , 1979) s u g g e s t t h a t t h e T r i a s s i c d e p o s i t s have had a s i m i l a r d i a g e n e t i c h i s t o r y t o t h e d e s e r t sandy a l l u v i u m d e s c r i b e d by Walker (1976) and Walker e t a l . (1978).
The r e d d e n i n g a p p e a r s t h e r e f o r e t o have been produced by
t h e i n t r a s t r a t a l a l t e r a t i o n of ferromagnesian minerals.
F e a t u r e s which s u p p o r t t h i s
Rrt
m m
O I I-. lmi o
c o
Hss s m m
,3
.m,
m r t a o
W FACTS
!WALL WALE CROQS-BEDDING
LARGE SCALE CROIIS-BEDDINCi
WlTW M W C l A n r .
MuwTolyE
LAMINATED W A L E
INTMFOIIMATIONAL CONGLOMERATE
MUDCRACKS
E R W O N CHANNEL
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B~WT#G.OCCAUONAL COARSE SANOSTONES
RIVER FLOOD PLAIN.
FWE SAIIMTONES FLAT BEfDMNG AND RIPPLE
.
M M A M I U A T E D SHALE, M U W O N E AND
MAINLY VERTICAL ACCRETnm D€W6llS OF THE
INTERCRETATION
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F i g . 4 . 6 0 . The l a t e r a l v a r i a t i o n i n sedimentation sequences of the St. Bees Sandstone a t Fleswick Bay, Cumbria. The sequences were d e p o s i t e d by low-sinuosity braided streams with a sandy bed load.
2 59
view a r e :
1) t h e g e n e r a l d i s t r i b u t i o n o f h a e m a t i t e a s g r a i n c o a t i n g s and i n t e r s t i t i a l m a t r i x 2 ) t h e g e n e r a l a b s e n c e of f e r r o m a g n e s i a n m i n e r a l s and heavy m i n e r a l assemblages dominated by z i r c o n , t o u r m a l i n e and r u t i l e
3 ) t h e o c c u r r e n c e of h a e m a t i t e - c l a y pseudomorphs which c o u l d b e t o t a l l y r e p l a c e d ferromagnesian mineral g r a i n s 4) t h e w i d e s p r e a d o x i d a t i o n and r e p l a c e m e n t of p h y l l o s i l i c a t e s
5 ) t h e i n s i t u replacement and d i s s o l u t i o n of f e l d s p a r
6 ) t h e p r e s e n c e of a d i s t i n c t i v e a u t h i g e n i c s u i t e of m i n e r a l s i n c l u d i n g q u a r t z , f e l d s p a r , c l a y m i n e r a l s , h a e m a t i t e , a n a t a s e and r u t i l e , and c a l c i t e . I n a l l t h e s e f e a t u r e s s u p p o r t t h e view t h a t t h i s r e d T r i a s s i c a l l u v i u m r e p r e s e n t s a n advanced s t a g e of t h e d i a g e n e t i c p r o c e s s e s d e s c r i b e d by Walker e t a l . ( 1 9 7 8 ) . The a n a l o g y i s , however, n o t p e r f e c t b e c a u s e t h e T r i a s s i c sediments were d e r i v e d from a mixed s o u r c e a r e a c o n t a i n i n g i g n e o u s , s e d i m e n t a r y and metamorphic r o c k s and u n s t a b l e f e r r o m a g n e s i a n g r a i n s were n o t a s abundant a s i n t h e Cenozoic a l l u v i u m of t h e s o u t h - w e s t e r n USA.
A consequence of t h i s may have been t h a t d e t r i t a l f e r r i c
h y d r o x i d e s were more abundant and c o n t r i b u t e d d i r e c t l y t o r e d d e n i n g i n t h e f i n e r grained sediments.
The e v i d e n c e does s u g g e s t t h a t i n t h e sandy a l l u v i u m o t h e r
p r o c e s s e s were more i m p o r t a n t .
These i n c l u d e t h e m e c h a n i c a l i n f i l t r a t i o n of c l a y
and i t s s u b s e q u e n t r e d d e n i n g , t h e d i s s o l u t i o n and replacement of l a b i l e s i l i c a t e g r a i n s by c l a y l e a d i n g t o a c o n c e n t r a t i o n o f d i s s o l v e d i o n s i n t h e i n t e r s t i t i a l groundwater and t h e u l t i m a t e p r e c i p i t a t i o n of a u t h i g e n i c m i n e r a l s from t h e groundwater solutions. Upper Cretaceous r e d beds of t h e s o u t h e r n Bohemian Basins
The Upper C r e t a c e o u s i n Czechoslovakia i n c l u d e s a sequence of c o n t i n e n t a l r e d and g r e y beds (Klikov Formation) which were d e p o s i t e d i n two NW-SE b a s i n s developed on w e a t h e r e d c r y s t a l l i n e basement.
f a u l t bounded
The sequence comprises a s e r i e s
of fining-upwards cyclothems on a s m a l l and l a r g e - s c a l e and i s c h a r a c t e r i z e d by r a p i d l a t e r a l and v e r t i c a l changes (Fig.4.61)
which c l o s e l y resembles t h e s t y l e of s e d i -
m e n t a t i o n r e f e r r e d t o a s " f l y s c h i n molasse".
The sequence was d e p o s i t e d i n a l l u v i a l
and l a c u s t r i n e environments s t r o n g l y i n f l u e n c e d by t e c t o n i c u n r e s t i n t h e AlpineCarpathian geosyncline. Colour d i s t r i b u t i o n i n t h e K l i k o v Formation shows a marked dependence on g r a i n s i z e and l i t h o l o g y and t h r e e main v a r i a t i o n s can b e r e c o g n i z e d a s f o l l o w s : L i g h t g r e y , g r e e n i s h l i g h t - g r e y o r y e l l o w i s h - g r e y s a n d s t o n e s ( A ) . These a r e medium t o c o a r s e c o n g l o m e r a t i c s a n d s t o n e s which a r e p o o r l y s o r t e d and o f t e n graded. Red beds (B). These a r e reddish-brown, zones w i t h v e r y v a r i a b l e g r a i n s i z e .
sometimes p u r p l e o r g r e e n i s h - g r e y m o t t l e d They i n c l u d e p o o r l y s o r t e d c o n g l o m e r a t i c sandy
mudstones (B ) and sandy c l a y s t o n e s (B2). 1
260
Grey beds (C). These a r e g r e e n i s h - g r e y o r d a r k g r e y s e d i m e n t s w i t h c a r b o n i z e d p l a n t debris.
They i n c l u d e f i n e t o medium l a m i n a t e d s a n d s t o n e s ( C , )
and c l a y s t o n e s w i t h
s p h a e r o s i d e r i t e n o d u l e s (C 1. 2 The d i s t r i b u t i o n of t h e d i f f e r e n t f a c i e s w i t h r e s p e c t t o d e p o s i t i o n a l environment i s shown i n Fig.4.62.
Although i n d i v i d u a l c y c l e s show some s i m i l a r i t y t o t h e
s t a n d a r d fining-upwards model ( A l l e n , 1965e) t h e r e a r e some i m p o r t a n t d i f f e r e n c e s p a r t i c u l a r l y i n t h e abundance of g r a d e d bedding.
T h i s may r e p r e s e n t t h e f l o o d
d e p o s i t s of a l l u v i a l f a n s which were d e p o s i t e d a s t u r b i d i t y flows i n l a k e s ( e . g , R a t t i g a n , 1966; Duff e t a l . ,
1967).
G e n e r a l i z e d p r o f i l e of t h e Upper C r e t a c e o u s K l i k o v Formation i n s o u t h e r n Fig.4.61. Czechoslovakia showing t h e main t y p e s o f cyclothem ( a f t e r Slefnska’, 19761.
261 The r e d b e d s of t h e K l i k o v Formation a r e g r e a t l y e n r i c h e d i n i r o n r e l a t i v e t o 3+ The a d d i t i o n a l i r o n i s l a r g e l y i n t h e form of Fe ;
t h e a s s o c i a t e d d r a b beds.
whereas t h e r e d and d r a b beds have comparable Fe2+ c o n t e n t s , t h e r e d sediments have Fe3+/Fe2+ r a t i o s g e n e r a l l y g r e a t e r t h a n 5 whereas t h e d r a b beds a r e between 0.5 and 2.
Most of t h e Fe3+ i s i n t h e form of f i n e g r a i n e d pigmentary h a e m a t i t e which i n
t h e r e d beds i s c l o s e l y a s s o c i a t e d w i t h abundant w e l l - c r y s t a l l i z e d k a o l i n i t e . S d n s k a (1976) a r g u e d t h a t t h e i r o n - r i c h , k a o l i n i t i c n a t u r e o f t h e r e d beds must have been produced d u r i n g d e p o s i t i o n by s e d i m e n t a r y d i f f e r e n t i a t i o n and s u g g e s t e d t h a t t h e r e d beds formed i n o x i d i z i n g f l o o d p l a i n environments w h i l s t t h e drab beds formed i n r e d u c i n g c h a n n e l and l a c u s t r i n e environments ( F r i e n d , 1966).
Furthermore,
h e i m p l i e d t h a t h a e m a t i t e and g o e t h i t e may have formed from p o o r l y c r y s t a l l i n e i r o n r i c h k a o l i n i t e s ( C a r r o l l , 1958; Weaver and P o l l a r d , 1973) which underwent i n c r e a s e i n c r y s t a l l i n i t y during diagenesis. LITHOLOGICAL NPE
ENVIRONMENT FLUVIAL
TYPE OF
+LACUSTRINE CROSS-BEDDING LAMINATED GRADCD BEODlNG
SANDSTONES GREY BEDS
+ SILTITONES
% OF WHOLE THICKNE!
l6 %
HORIZONTAL DISCRETE BEDDING
CLAYSTONES
CONGLOMERATIC MUDDY SANDSTONIS OR @D CONOLOMERATIC IDS SANDY MUDSTONES OR SANDY CLAYSlDNES
GRADED BEDDING
38 % ! UNDlSTlNCTlVE BEDDING
SANDSTONES
MASSIVE CROSS-BEDDING GRADED BEDDING
MEDIUM to COARSE GRAINED CONGLOMERATIC
46 %
Fig.4.62. F a c i e s d i s t r i b u t i o n of t h e t h r e e main l i t h o l o g i c a l t y p e s i n t h e Klikov Formation ( a f t e r Sldnska', 1976). The i m p l i c a t i o n of t h i s , of c o u r s e , i s t h a t t h e pigmentary o x i d e s must have been d e r i v e d from soils and weathered p r o f i l e s developed i n t h e s o u r c e a r e a , a l t h o u g h t h e s e need n o t n e c e s s a r i l y b e l a t o s o l s .
Sllnska' (1976) examined s e v e r a l weathered
p r o f i l e s of basement r o c k s ( g r a n i t e s and g n e i s s e s ) , which a r e p r e s e n t e d b e n e a t h t h e Klikov Formation, i n o r d e r t o g a i n i n f o r m a t i o n a b o u t t h e c l i m a t i c environment and n a t u r e o f w e a t h e r i n g a t t h e t i m e of r e d bed f o r m a t i o n .
The weathered c r u s t s v a r y
i n t h i c k n e s s between 2 and 2Om and a r e b e s t developed where t h e o v e r l y i n g r e d beds
262 a r e thinnest.
T h i s r e l a t i o n s h i p i s t o be e x p e c t e d i n a t e c t o n i c a l l y c o n t r o l l e d
environment b e c a u s e w e a t h e r e d p r o f i l e s a r e b e s t developed i n t h o s e a r e a s which have undergone t h e l e a s t s u b s i d e n c e . The w e a t h e r e d c r u s t s d e s c r i b e d by Sla'nskg (1976) a r e r e d and m o t t l e d brownishred with greenish grey s t a i n s .
I n t h e g n e i s s e s t h r e e zones can b e d i s t i n g u i s h e d ,
from t h e t o p downwards: 1) Completely w e a t h e r e d g n e i s s a b o u t 4m t h i c k , l i g h t g r e y i n c o l o u r w i t h p a t c h e s o f b r i g h t r e d , brown and p u r p l e .
I t c o n t a i n s r e s i d u a l q u a r t z , k a o l i n i t e and i r o n
oxides. 2) Brown and r e d m o t t l e d g n e i s s a b o u t 15m t h i c k .
The p r i m a r y t e x t u r e i s s t i l l
v i s i b l e b u t t h e f e l d s p a r s a r e c o m p l e t e l y a l t e r e d and b i o t i t e i s o x i d i z e d .
3 ) Dark g r e e n i s h - g r e y b i o t i t e g n e i s s w i t h k a o l i n i z e d f e l d s p a r s and c h l o r i t i z e d biotite.
T h i s p a s s e s downwards i n t o u n a l t e r e d g n e i s s .
Chemical changes f o r two weathered p r o f i l e s a r e r e p r e s e n t e d i n F i g . 4 . 6 3 u s i n g Loughman's (1969) method.
This depicts r e l a t i v e gains o r losses during the different
s t a g e s of w e a t h e r i n g . There a r e some d i f f e r e n c e s between t h e two p r o f i l e s p a r t i c u l a r l y w i t h r e g a r d t o t h e alumina c o n t e n t of t h e w e a t h e r e d rock and t h a t of r e l a t i v e l y u n a l t e r e d rock. Assuming t h a t t h i s r a t i o i s a n i n d i c a t i o n o f t h e d e g r e e of w e a t h e r i n g t h e n t h e top zone o f p r o f i l e I1 r e p r e s e n t s a more advanced s t a g e t h a n t h e same zone i n p r o f i l e I. The s i l i c a c u r v e f o r b o t h p r o f i l e s shows r e l a t i v e enrichment i n t h e i n t e r m e d i a t e zones.
T h i s i s d i f f e r e n t t o t h e t r e n d observed by G o l d i c h (1938) f o r t h e Morton
g r a n i t e g n e i s s which shows a s t e a d y d e c r e a s e w i t h i n c r e a s e i n w e a t h e r i n g i n t e n s i t y . The Fe 0 c u r v e s show a g r a d u a l and t h e n s h a r p i n c r e a s e a s o x i d a t i o n and h y d r a t i o n 2 3 of t h e i r o n p r o c e e d s , The FeO c u r v e shows t h e o p p o s i t e t r e n d . The a l k a l i m e t a l s o r e l e a c h e d o u t d u r i n g t h e e a r l y s t a g e s of w e a t h e r i n g b u t a p p e a r t o b e more o r l e s s s t a b l e a s weathering progresses. These w e a t h e r e d p r o f i l e s t h u s a p p e a r t o b e v e r y s i m i l a r t o t h e r e d p o d z o l i c p r o f i l e s developed on c r y s t a l l i n e r o c k s , l i k e t h a t on t h e Morton g r a n i t e g n e i s s , d e s c r i b e d by G o l d i c h (1938).
The end-product o f t h i s t y p e of w e a t h e r i n g i s a
s o i l h o r i z o n e n r i c h e d i n k a o l i n i t e and i r o n hydroxide.
T h i s i n d i c a t e s a warm and
humid s o u r c e a r e a climate undergoing i n t e n s i v e w e a t h e r i n g ,
T e c t o n i c u n r e s t probably
p r e v e n t e d t h e f o r m a t i o n of t r u e l a t e r i t i c m a n t l e s b e c a u s e of t h e r a p i d e r o s i o n of the s o i l s ,
Sla'nska' (1976) concluded, t h e r e f o r e , t h a t t h e a l l u v i u m of t h e Klikov
Formation was o r i g i n a l l y brown i n c o l o u r w i t h much of t h e i r o n b e i n g i n t r o d u c e d i n t h e form o f k a o l i n i t e - i r o n o x i d e complexes.
The u l t i m a t e c o l o u r and c o l o u r d i f -
f e r e n t i a t i o n was t h e r e s u l t of s u b s e q u e n t e a r l y d i a g e n e t i c changes.
I n oxidizing
zones h a e m a t i t e c r y s t a l l i z e d from i r o n h y d r o x i d e s w h i l s t i n r e d u c i n g zones a u t h i g e n i c s i d e r i t e and b l e a c h e d p a t c h e s developed.
The s t u d y p r o v i d e s a n i m p o r t a n t example
of t h e i m p o r t a n c e of f e r r i a l i t i c s o i l s i n r e d bed f o r m a t i o n .
Such s o i l s p r o v i d e
f i n e g r a i n e d a l l u v i u m which i s r i c h i n i r o n h y d r o x i d e s and c l a y m i n e r a l s a l t h o u g h
263
t h e u l t i m a t e f o r m a t i o n o f r e d beds depends upon d i a g e n e t i c p r o c e s s e s s u c h a s t h e a g e i n g of i r o n h y d r o x i d e s i n t o h a e m a t i t e and t h e r e l e a s e of i r o n o x i d e s d u r i n g c l a y m i n e r a l t r a n s f o r m a t i o n s . O t h e r a n c i e n t t r o p i c a l w e a t h e r i n g p r o f i l e s have been d e s c r i b e d by Blank (1976) a n d S t u r t e t a l . (1979).
"'1
CONCENTRATION RATIO b (PROFILE2)
Fig.4.63. S u c c e s s i v e c h e m i c a l changes i n i t i a t e d d u r i n g a n c i e n t w e a t h e r i n g of g n e i s s p r i o r t o t h e d e p o s i t i o n of t h e Upper C r e t a c e o u s Klikov Formation. R e s u l t s a r e r e c a l c u l a t e d on a n anhydrous b a s i s . C o n c e n t r a t i o n r a t i o i s t h e r a t i o of t h e c o n t e n t o f an o x i d e a t s p e c i f i c l e v e l t o i t s c o n t e n t i n t h e p a r e n t rock a d j u s t e d t o t h e alumina c o n c e n t r a t i o n r a t i o a s a u n i t ( a f t e r Sla'nska', 1976).
264 CONCLUSIONS AND FURTHER READING
A l l u v i a l r e d beds comprise a v a r i e t y of r e d beds formed i n d i f f e r i n g d e p o s i t i o n a l environments and a wide s p e c t r u m of c l i m a t i c c o n d i t i o n s .
Palaeoclimatic
i n d i c a t o r s , i n c l u d i n g palaeomagnetism, s u g g e s t t h a t t h e m a j o r i t y of a n c i e n t a l l u v i a l r e d beds formed i n s e m i - a r i d c l i m a t e s .
Two i m p o r t a n t p r o c e s s e s a r e
i n v o l v e d i n t h e f o r m a t i o n of a l l u v i a l r e d b e d s :
t h e d e p o s i t i o n and a g e i n g of
d e t r i t a l f e r r i c h y d r o x i d e s , and t h e i n t r a s t r a t a l a l t e r a t i o n of f e r r o m a g n e s i a n minerals.
The former p r o c e s s i s more i m p o r t a n t i n muddy a l l u v i u m where t h e r e i s
much f e r r i c hydroxide i n a s s o c i a t i o n w i t h c l a y s and o t h e r f i n e g r a i n e d m a t e r i a l s . I n t r a s t r a t a l a l t e r a t i o n i s more i m p o r t a n t i n sandy a l l u v i u m , e s p e c i a l l y when i t
i s m i n e r a l o g i c a l l y immature.
The r e l a t i v e i m p o r t a n c e of t h e s e two p r o c e s s e s has
n o t been a s s e s s e d i n most a n c i e n t r e d beds and t h e r e a r e i n d i c a t i o n s t h a t s o u r c e a r e a composition may be a n i m p o r t a n t c o n t r o l l i n g f a c t o r . Comprehensive d e s c r i p t i o n s of a n c i e n t f l u v i a l s e d i m e n t s a r e found i n : Miall, A.D.
( E d i t o r ) , 1978. F l u v i a l Sedimentology. Can. SOC. P e t r o l . Geol.
Memoir 5. I n f o r m a t i o n on c o a r s e p a r t i c l e o x i d e s i n a t r o p i c a l r i v e r i s g i v e n by: R i e z e b o s , P.A., 1979. Compositional downstream v a r i a t i o n of opaque and t r a n s l u c e n t heavy r e s i d u e s i n some modern Rio Magdalena s a n d s (Colombia). Sediment. G e o l . , 24: 197-225.
265
CHAPTER 5 THE DIAGENESIS OF CONTINENTAL RED BEDS INTRODUCTION:
SANDSTONE DIAGENESIS
D i a g e n e s i s i s a c o l l e c t i v e term f o r t h o s e p r o c e s s e s which t a k e p l a c e a f t e r t h e d e p o s i t i o n o f a sediment.
They i n c l u d e p h y s i c a l and c h e m i c a l p r o c e s s e s
which a f f e c t t h e d e t r i t a l m i n e r a l g r a i n s and a l s o t h e changes which t a k e p l a c e i n t h e c h e m i s t r y of t h e i n t e r s t i t i a l c o n n a t e f l u i d s .
The i n t e r a c t i o n between
t h e d e t r i t a l m i n e r a l assemblage and t h e i n t e r s t i t i a l w a t e r i s c o n s t a n t l y changing; d u r i n g t h e e a r l y s t a g e s of d i a g e n e s i s when t h e overburden i s n o t g r e a t t h e i n t e r s t i t i a l w a t e r s w i l l t e n d t o d i s s o l v e t h e more l a b i l e d e t r i t a l g r a i n s t h e r e b y i n c r e a s i n g t h e p o r o s i t y of t h e sediment and a l s o t h e c o n c e n t r a t i o n of t h e d i s s o l v e d c o n s t i t u e n t s .
When t h e c o n c e n t r a t i o n of d i s s o l v e d i o n s i s
s u f f i c i e n t l y g r e a t t h e p r e c i p i t a t i o n of m i n e r a l cements w i l l t a k e p l a c e .
Various
cements, i n c l u d i n g c l a y m i n e r a l s , q u a r t z , c a r b o n a t e s , and f e l d s p a r s commonly occur i n sandstones.
These have t h e e f f e c t o f f i l l i n g p o r e s p a c e s and t h u s r e d u c i n g
t h e o v e r a l l p o r o s i t y of t h e sediment.
F u r t h e r p o r o s i t y r e d u c t i o n may r e s u l t
from mechanical changes s u c h a s compaction which i s a n i m p o r t a n t f e a t u r e of more deeply buried sandstone.
An e x c e l l e n t o u t l i n e of t h e m a j o r f e a t u r e s of s a n d s t o n e d i a g e n e s i s i s g i v e n by P e t t i j o h n e t a l . ( 1 9 7 2 ) ;
1)
d i a g e n e s i s based on: chemical evidence.
These a u t h o r s r e c o g n i z e s i x s t a g e s of s a n d s t o n e
textural
2)
mineralogical
3)
physical
and
4)
These s t a g e s a r e summarized i n Fig.5.1.
N e a r l y a l l t h e i m p o r t a n t r e a c t i o n s which t a k e p l a c e d u r i n g d i a g e n e s i s do so i n a n aqueous s e t t i n g , e i t h e r i n w a t e r s which a r e n e a r t o t h e d e p o s i t i o n a l i n t e r f a c e , o r i n groundwaters which c i r c u l a t e i n d e e p l y b u r i e d s e d i m e n t s l o n g a f t e r deposition.
I n t h i s r e s p e c t i t i s c o n v e n i e n t t o d i s t i n g u i s h between
t h o s e p r o c e s s e s which t a k e p l a c e i n r e c e n t l y d e p o s i t e d s e d i m e n t i n s u r f a c e w a t e r s , and t h o s e which o c c u r i n more d e e p l y b u r i e d sediment i n s u b s u r f a c e w a t e r s .
The
former a r e r e f e r r e d t o a s e a r l y d i a g e n e t i c p r o c e s s e s and t h e l a t t e r a s l a t e diagenetic processes.
Because e a r l y d i a g e n e t i c p r o c e s s e s t a k e p l a c e i n s u r f a c e
w a t e r s t h e d e p o s i t i o n a l environment e x e r t s a s t r o n g c o n t r o l on t h e n a t u r e o f t h e d i a g e n e t i c r e a c t i o n s i n c l u d i n g t h e r e s u l t a n t a u t h i g e n i c m i n e r a l assemblages. For t h i s r e a s o n , t h e d i a g e n e t i c f e a t u r e s of m a r i n e and c o n t i n e n t a l sediments a r e q u i t e d i s t i n c t i v e and a r e r e a d i l y d i s t i n g u i s h e d i n t h e s t r a t i g r a p h i c r e c o r d . C h a r a c t e r i s t i c a u t h i g e n i c p h a s e s of marine and c o n t i n e n t a l s e d i m e n t s a r e l i s t e d i n T a b l e 5.1.
266
Immediately after deposition. Exposed to air or water of depositional environment. Original detritus, high porosity
Buried a fmv metres to tens of metres. Exposed to interstitial waters. Some compaction,some early chemical precipitates possible.
Buried to moderate depths of about lOOOm Pore water may be a brine. Chemical cements may reduce porosity,clays may be altered.
Deep burial to thousands of metres, perhaps with folding. Porosity may be very low from chemical cement and pressure solution.
Incipient metamorphism. Growth of chlorine ond other metamorphic minerals with extensive pressun solution and quartzitic texture.
After uplift and erosion,within tens of metres of land surface. Invasion by metooric water, decementation and"weathering"of clays may increase porosity.
The s t a g e s of d i a g e n e s i s i n r e l a t i o n t o d e p t h o f b u r i a l and i n c r e a s e of p r e s s u r e and t e m p e r a t u r e ( a f t e r P e t t i j o h n e t a l . , 1 9 7 2 ) .
Fig.5.1.
TABLE 5 . 1 . The c h a r a c t e r i s t i c a u t h i g e n i c m i n e r a l s u i t e s of marine s a n d s t o n e s and c o n t i n e n t a l r e d beds. Marine Sandstones
C o n t i n e n t a l Red Beds
Quartz Glauconi t e Chamosite Pyrite Calcite
Quartz Potash Feldspar I l l i te-Montmori l l o n i t e Chlorite Ha ema t i t e Calcite
-
267 The composition of n a t u r a l w a t e r s and e a r l y d i a g e n e t i c r e a c t i o n s
I n s a n d s t o n e s d e p o s i t e d under m a r i n e c o n d i t i o n s t h e i n i t i a l p o r e w a t e r s a r e s e a w a t e r whereas t h o s e i n c o n t i n e n t a l environments can be approximated t o t h e mean composition o f v a r i o u s r i v e r w a t e r s ( T a b l e 5 . 2 ) . are striking.
The d i f f e r e n c e s between t h e two
I n marine w a t e r s t h e rnCa2+/mMg2+ r a t i o i s 0.2 b u t i n r i v e r w a t e r s i s
TABLE 5.2.
Comparison of t h e m o l a l i t y o f s e a w a t e r (Mason, 1966) and mean r i v e r w a t e r ( L i v i n g s t o n e , 1963). sea water
r i v e r water
Dissolved species
PPm
molality
PPm
molality
c 1-
18,980
0.535
7- a
0.00022
Na+
K+
H BO 32+3 Sr F-
10,556
0.459
6.3
0.00027
2649
0.028
11.2
0.00012
1272
0.052
4.1
0.00017
400
0.010
15.0
0.00037
380
0.01
2.3
0.00006
140
58.8
0.00096
65
2.3~10a. 1x10- 4
0.02
2.5~10-
26
4.2~10-~
0.1-0.01
5x10-'
8 1.3
9. 1x10-5 6.8~10-
0.09
1x10-6
1
0.09
5~10-~
13.1
0.00014
H4S104 NO
0.5
1.0~10-~ a. 1x10- 6
1.0
0.00002
Fe
0.01
2x10-7
0.67
0.00001
~l (OH)&
0.01
1~10-
0.24
6 2.5~10-
Totals
34,479
1.106
120.8
0.00235
-
d+Fe 3+
2.2.
The d i f f e r e n c e i s due t o t h e removal of CaCo3 from s e a w a t e r by c a l c a r e o u s -
s h e l l e d organisms.
There is a l s o a n i m p o r t a n t d i f f e r e n c e i n t h e s i l i c a c o n t e n t .
In n a t u r a l w a t e r s , v i r t u a l l y a l l t h e s i l i c a i s i n t h e form of u n d i s s o c i a t e d o r t h o s i l i c i c a c i d (H S i 0 ) b u t i s a b o u t t e n times more abundant i n r i v e r w a t e r
4
than i n sea water.
4
The d i f f e r e n c e i s a l m o s t e n t i r e l y due t o t h e removal o f
s i l i c a from t h e s e a by marine organisms s u c h a s diatoms and r a d i o l a r i a . That s t a g e of d i a g e n e s i s d u r i n g which chemical r a a c t i o n s t a k e p l a c e d u r i n g s h a l l o w b u r i a l was r e f e r r e d t o by Dapples (1962) a s t h e "redoxomorphic s t a g e " . The redoxomorphic s t a g e i s dominated i n marine s e d i m e n t s by r e a c t i o n s i n v o l v i n g o r g a n i c m a t t e r and i n marine mudstones a number o f d e p t h r e l a t e d zones can be r e c o g n i z e d ( C u r t i s , 1977, 1978) ( F i g . 5 . 2 ) .
Beneath a t h i n v e n e e r of oxygenated
s e d i m e n t , r e d u c i n g c o n d i t i o n s p r e v a i l because of oxygen consumption by b a c t e r i a l
268
MICROBIAL REDUCTION THERMAL PROCESSES OF F.3* DECOMPOSITION I----- -I------
OCEAN
I
OXIDATION
It
SIJLPHATE REDUCTION
102m.
10m. 111 FERMENTATtON
-25)
1.0 km .
~
-
IV
DECARBO XYLATION
.. ... ... .... .... .... ..... ..... ..... ...... ...... ...... ...... ....... ...... ....... ...... ....... ...... ...... .... ....
2.5 km
b \
40km.
LIQUID HYDROCARBONS GAS
VI GRAPHITE 7METAMORPHISM
..... ... ..... .... .... .
................ ........ ....... ..... ....... ...... .... .... .....
-
-20?
--/ :
:/
::::;;/ .. :.7
RATE OF INTRODUCTION OF ORGANIC MATTER DERIVED CARBON DIOXIDEI BICARBONATE
-
Fig.5.2. The p r o d u c t i o n o f c a r b o n d i o x i d e w i t h i n d i f f e r e n t d i a g e n e t i c zones. Zones I - I V a p p r o x i m a t e l y c o r r e s p o n d t o Dapples' redoxomorphic s t a g e ( a f t e r C u r t i s , 1978). activity. (e.g. F e 3 ' j
I n t h i s zone s u l p h a t e r e d u c t i o n t a k e s p l a c e and t r a n s i t i o n m e t a l s Fe2+ and Mn4++ i 2 + ) a r e a l s o reduced.
With i n c r e a s i n g d e p t h o r g a n i c
m a t t e r , d e r i v e d l a r g e l y from p h y t o p l a n k t o n , undergoes complex t r a c t i o n s i n c l u d i n g f e r m e n t a t i o n , d e c a r b o x y l a t i o n , and t h e f o r m a t i o n of hydrocarbons.
These zones a r e
d e p t h r e l a t e d and c l o s e l y l i n k e d t o t h e s e d i m e n t a t i o n r a t e (Goldhaber and Kaplan, 1975). The redoxomorphic s t a g e i s t h u s l a r g e l y r e s p o n s i b l e f o r t h e c o l o u r o f sediments (Goldhaber and Kaplan, 1975; C u r t i s , 1978) and a c c o u n t s f o r t h e d i f f e r e n c e s s e e n between marine and c o n t i n e n t a l s e d i m e n t s ,
The a b s e n c e o f a r i c h s o u r c e o f o r g a n i c
m a t t e r ( p h y t o p l a n k t o n ) i n non-marine s e d i m e n t s means t h a t d i a g e n e t i c r e a c t i o n s can t a k e p l a c e under oxygenated c o n d i t i o n s .
T h i s means t h a t t h e i r o n o x i d e s , g o e t h i t e
and h a e m a t i t e may form and import a r e d p i g m e n t a t i o n t o t h e sediment.
I n the
p r e s e n c e of s u b s t a n t i a l o r g a n i c m a t t e r b a c t e r i a l consumption o f oxygen r e s u l t s i n r e d u c i n g c o n d i t i o n s w i t h i n t h e s e d i m e n t and most o f t h e i r o n i s p r e c i p i t a t e d i n t h e
269
f e r r o u s s t a t e , mainly a s p y r i t e .
Thus, t h e c o l o u r o f t h e s e d i m e n t s e v e n t u a l l y
becomes d r a b because of t h e p r e s e r v a t i o n o f o r g a n i c m a t t e r and f i n e l y d i s s e m i n a t e d pyrite, S u b s u r f a c e w a t e r shows much more v a r i a t i o n i n comp,osition than r i v e r w a t e r o r sea w a t e r ,
T h i s i s l a r g e l y due t o i t s complex o r i g i n , b e i n g a m i x t u r e of j u v e n i l e
w a t e r s , m e t e o r i c w a t e r s , ocean w a t e r s , w a t e r s produced from d i a g e n e t i c r e a c t i o n s , and magmatic w a t e r s (White e t a l . , 1963; White, 1965). i n s h a l l o w ((300m)
Waters which a r e p r e s e n t
s a n d s t o n e beds a r e g e n e r a l l y o f m e t e o r i c o r i g i n and c o n t a i n l e s s
+
t h a n 100 ppm d i s s o l v e d s o l i d s c o n s i s t i n g mainly of Na , Ca2+, and Mg2+ c a t i o n s and 2HCO;, SO4 , and C 1 - I a n i o n s . With i n c r e a s e d b u r i a l t h e e l e c t r o l y t e c o n t e n t of s u b s u r f a c e w a t e r s i n c r e a s e s from e s s e n t i a l l y t h a t o f f r e s h w a t e r t o more t h a n t e n times h i g h e r t h a n t h e c o n c e n t r a t i o n of s e a w a t e r . I n deeper s u b s u r f a c e w a t e r s C12r e p l a c e s SO4 a s t h e dominant a n i o n and t h e p r o p o r t i o n of C1- g e n e r a l l y i n c r e a s e s
with s a l i n i t y .
The main p h a s e s p r e c i p i t a t e d from deep s u b s u r f a c e w a t e r s a r e q u a r t z
and some c l a y m i n e r a l s such a s c h l o r i t e . Cementa t i o n A v a r i e t y o f m i n e r a l s o c c u r a s cements i n s a n d s t o n e s .
They o c c u r p r i n c i p a l l y
as p o r e f i l l i n g s and p o r e l i n i n g s and i n c l u d e c l a y m i n e r a l s , q u a r t z , c a r b o n a t e s ,
f e l d s p a r , and h a e m a t i t e .
The p a r a g e n e s i s and morphology of cements v a r i e s between
i n d i v i d u a l s a n d s t o n e s ( s e e D a p p l e s , 1967) a l t h o u g h t h e sequence
2 . q u a r t z and f e l d s p a r
3. c a r b o n a t e s i s o f t e n o b s e r v e d .
1. c l a y m i n e r a l s
However, t h e r e may be
more t h a n one phase of c e m e n t a t i o n f o r any p a r t i c u l a r m i n e r a l ( e . g . q u a r t z ) , and l a t e r cements may r e p l a c e e a r l i e r formed cements.
The most abundant cements i n
s a n d s t o n e s a r e q u a r t z and c a l c i t e . Amorphous s i l i c a i n d i s t i l l e d o r s a l i n e w a t e r s has a n e q u i l i b r i u m s o l u b i l i t y of a b o u t 120 ppm a t room t e m p e r a t u r e .
The s o l u b i l i t y r a p i d l y i n c r e a s e s w i t h
t e m p e r a t u r e and d e p t h of b u r i a l and so o p a l i n e cements i n s a n d s t o n e s a r e a good i n d i c a t i o n of s h a l l o w b u r i a l .
I n d e e p l y b u r i e d s a n d s t o n e s s i l i c a always p r e c i p i t a t e s
a s q u a r t z because t h e s o l u b i l i t y o f q u a r t z i s o n l y a b o u t 5% t h a t of amorphous s i l i c a a t room t e m p e r a t u r e and i n c r e a s e s much less r a p i d l y w i t h t e m p e r a t u r e than does t h e s t a b i l i t y of amorphous s i l i c a .
The amount of t o t a l s i l i c a i n s o l u t i o n i s
markedly dependent on pH and r a p i d l y rises a s pH i n c r e a s e s above 9 . 0
-
9.5.
Most
o f t h e s i l i c a i n n a t u r a l w a t e r s i s i n t h e form of o r t h o s i l i c i c a c i d ( H SiO ) and 4 4 i s found i n g r e a t e s t q u a n t i t y i n s u b s u r f a c e w a t e r s where t h e c o n c e n t r a t i o n i s commonly above t h e s o l u b i l i t y o f q u a r t z ( a b o u t 6 ppm). The p r e c i p i t a t i o n of q u a r t z is a n i m p o r t a n t d i a g e n e t i c p r o c e s s i n s a n d s t o n e s d e p o s i t e d under marine and c o n t i n e n t a l c o n d i t i o n s . p r e c i p i t a t e d a s a n o p t i c a l l y c o n t i n u o u s overgrowth.
Most commonly q u a r t z i s The form of t h e overgrowth i s
f r e q u e n t l y e u h e d r a l b u t may be m o d i f i e d . o r even c o m p l e t e l y i n h i b i t e d , i n t h e
270 p r e s e n c e o f g r a i n c o a t i n g s ( P i t t m a n and Lumsden, 1968; Heald and L a r s s e , 1973). Q u a r t z overgrowths a r e so a b u n d a n t , e s p e c i a l l y i n o l d e r q u a r t z - r i c h s a n d s t o n e s , t h a t t h e s o u r c e of t h e a d d i t i o n a l s i l i c a i s n o t immediately o b v i o u s . t h e d i s s o l u t i o n of s i l i c e o u s organisms sponge s p i c u l e s ) may be i m p o r t a n t .
I n m a r i n e sandstones
( d i a t o m , r a d i o l a r i a n t e s t s and s i l i c e o u s
I n continental deposits, especially those
d e p o s i t e d under a e o l i a n c o n d i t i o n s , t h e p r o d u c t i o n o f s i l i c a d u s t by g r a i n a b r a s i o n may be an i m p o r t a n t s o u r c e o f s i l i c a f o r q u a r t z overgrowths (Waugh, 1970 a , b ) .
In
more d e e p l y b u r i e d s a n d s t o n e s t h e r e i s a g e n e r a l tendency f o r t h e abundance of q u a r t z overgrowths t o i n c r e a s e c a u s i n g s i g n i f i c a n t r e d u c t i o n s i n p o r o s i t y and p e r m e a b i l i t y (FUchtbauer, 1967b).
I n t h i s s i t u a t i o n t h e c o r r e l a t i o n between t h e
d e s t r u c t i o n of m e t a s t a b l e heavy m i n e r a l s and i n c r e a s e d q u a r t z c o n c e n t r a t i o n s u g g e s t s t h a t much of t h e e x t r a s i l i c a was s u p p l i e d by t h e i n t r a s t r a t a l s o l u t i o n of s i l i c a t e m i n e r a l s ( P h i l l i p e t a l . , 1963).
A l s o , p r e s s u r e s o l u t i o n i s l i k e l y t o be i m p o r t a n t
i n more d e e p l y b u r i e d s a n d s t o n e s (Thomson, 1959; Weyl, 1 9 5 9 ) , where i t i s a p p a r e n t l y f a c i l i t a t e d by t h e p r e s e n c e of i l l i t i c c l a y laminae. C a l c i t e i s p r o b a b l y t h e most common chemical cement i n a n c i e n t s a n d s t o n e s and may form w i t h o r w i t h o u t d e s t r u c t i o n of t h e g r a i n s u p p o r t e d framework ( D a p p l e s , 1971). N o n - d e s t r u c t i v e c a l c i t e cement c o n s i s t s m o s t l y of a n h e d r a l s u b e q u a n t c r y s t a l s 20 m o r more i n d i a m e t e r which g e n e r a l l y f i l l p o r e s p a c e s .
I n some i n s t a n c e s t h e volume
of c a l c i t e cement exceeds 30%, t h e o r i g i n a l p o r o s i t y of t h e s e d i m e n t , and t h e d e t r i t a l q u a r t z p a r t i c l e s a r e c o r r o d e d , f e a t u r e s which c l e a r l y i n d i c a t e t h e d e s t r u c t i v e n a t u r e of t h e c e m e n t a t i o n . I n o r d e r t o s a t u r a t e a n aqueous s o l u t i o n w i t h r e s p e c t t o c a l c i t e , i t i s n e c e s s a r y t h a t t h e i o n a c t i v i t y p r o d u c t LCa2+][ p r o d u c t f o r t h i s compound.
CO:-J
exceeds t h e thermodynamic s o l u b i l i t y
T h e r e f o r e i n c r e a s e i n t h e a c t i v i t y o f e i t h e r calcium
i o n on c a r b o n a t e i o n may r e s u l t i n t h e p r e c i p i t a t i o n of CaC03.
S i n c e t h e amount of
calcium i n marine and s u b s u r f a c e w a t e r s i s r e l a t i v e l y s t a b l e and o n l y a f f e c t e d by t h e i n t r a s t r a t a l a l t e r a t i o n of c a l c i u m - b e a r i n g s i l i c a t e s c a l c i u m c a r b o n a t e p r e c i p i t a t i o n u s u a l l y r e s u l t s from s u p e r s a t u r a t i o n c a u s e d by a n i n c r e a s e i n t h e a c t i v i t y of t h e c a r b o n a t e i o n . C a l c i t e and q u a r t z cements commonly c o e x i s t i n s a n d s t o n e s and may show evidence of replacement of one m i n e r a l by t h e o t h e r .
This r e l a t i o n s h i p i s r e a d i l y explained
by r e f e r e n c e t o t h e p H - s o l u b i l i t y c u r v e s of q u a r t z , amorphous s i l i c a , and c a l c i t e (Fig.5.3).
The s o l u b i l i t y of s i l i c a i s u n a f f e c t e d by pH v a l u e s below a b o u t 9
whereas c a l c i t e i s v e r y s o l u b l e i n a c i d i c s o l u t i o n s .
T h e r e f o r e , changes i n pH may
cause s o l u t i o n of one phase and p r e c i p i t a t i o n o f a n o t h e r , a r e a c t i o n which commonly o c c u r s because of t h e f a c t t h a t t h e p H - s o l u b i l i t y c u r v e s i n t e r s e c t a t a v a l u e commonly met w i t h i n g r o u n d w a t e r s ; .
271
ppm
m/l
-5 ~ 1 0 ~
- 2.104 -- lo4 5 .lo3 - 2.lo3 - 103
-5~10~ - 2.102 102 -5 ~ 1 0 ~
-210.1lo1
5001
\n
a 4 20 0 Silica dissolver Calcite dissolves
4
5
6
7 PH
8
9 1 0 1 1
Fig.5.3. A . T o t a l s i l i c a and r e l a t i v e amounts of s i l i c a s p e c i e s i n s o l u t i o n i n w a t e r a t 25OC a s a f u n c t i o n of pH. B. R e l a t i o n s h i p between pH and t h e s o l u b i l i t i e s o f c a l c i t e , q u a r t z , and amorphous s i l i c a ( a f t e r B l a t t e t al., 1980).
M i n e r a l t r a n s f o r m a t i o n and r e p l a c e m e n t s The changing i n t e r s t i t i a l environment which r e s u l t s d u r i n g s a n d s t o n e b u r i a l r e s u l t s i n a number of m i n e r a l o g i c a l t r a n s f o r m a t i o n s and r e p l a c e m e n t s .
Different
i n t e r s t i t i a l c o n d i t i o n s may r e s u l t i n t h e r e p l a c e m e n t of framework c o n s t i t u e n t s o r p r e v i o u s l y formed cement by a n o t h e r ( t h e Locomorphic S t a g e of D a p p l e s , 1962). Examples i n c l u d e t h e d o l o m i t i z a t i o n of f e l d s p a r o r c a l c i t e cement and t h e r e p l a c e ment of a u t h i g e n i c q u a r t z by c a l c i t e .
The b e s t known m i n e r a l t r a n s f o r m a t i o n d u r i n g
d i a g e n e s i s a r e t h e s y s t e m a t i c m o d i f i c a t i o n s which t a k e p l a c e i n c l a y m i n e r a l s during sandstone b u r i a l (Table 5 . 3 ) .
The best-known of t h e s e i s t h e mixed l a y e r
illite-montmorillonite-illite t r a n s f o r m a t i o n ( P e r r y and Hower, 1970, 1 9 7 2 ) .
This
i n v o l v e s d e h y d r a t i o n and r e s u l t s i n m o d i f i c a t i o n of t h e p o r e w a t e r composition. TABLE 5 . 3 . Some of t h e c l a y m i n e r a l r e a c t i o n s which o c c u r d u r i n g s a n d s t o n e , d i a g e n e s i s ( a f t e r P e t t i j o h n e t a l . , 1972). Clay m i n e r a l formed
Precursor
Kaolinite
feldspar
Kaolini te
pore space
Illite
kaolinite
Muscovite
kaolinite
Illite
montmorillonite
Chlorite
montmorillonit e
Montmorillonite
volcanic glass
Glauconite
illite
Components added(+) or subtracted(-)
S t a g e of diagenesis
+(Fez+ ,Mg '+) -(Si02,H20,Na+,Ca2+)
During deep b u r i a l c l a y m i n e r a l s a r e t r a n s f o r m e d i n t o micas ( t h e Phyllomorphic S t a g e of D a p p l e s , 1 9 6 2 ) .
The f o r m a t i o n of s e r i c i t e , m u s c o v i t e , b i o t i t e and c h l o r i t e
i n t h i s manner r e p r e s e n t s t h e end p o i n t o f d i a g e n e s i s a n d marks t h e o n s e t o f metamorphism.
The f o r m a t i o n of micas i s a s s o c i a t e d w i t h t e x t u r a l c h a n g e s , i n p a r t i c u l a r
t h e development of q u a r t z i t i c t e x t u r e , a n d o n l y o c c u r s i n d e e p l y b u r i e d s a n d s t o n e s ( S t a g e s 4 , 5 and 6 of P e t t i j o h n e t a l . , 1972) ( s e e F i g . 5 . 1 ) .
213
T e x t u r a l changes d u r i n g d i a g e n e s i s The o v e r a l l f a b r i c o f a s a n d o r s a n d s t o n e i s dependent upon v a r i o u s f a c t o r s including:
1) manner of d e p o s i t i o n
2) g r a i n s i z e
and 5 ) p h y s i c a l and chemical compaction.
3) s o r t i n g
4 ) c l a s t shape,
The f a b r i c o f a s a n d s t o n e can be
s u b s t a n t i q l l y changed by compaction r e s u l t i n g from overburden p r e s s u r e .
The
importance o f compaction i s n o t o n l y t h a t i t changes t h e f a b r i c o f t h e s a n d s t o n e , c a u s i n g r e d u c t i o n i n p o r o s i t y and p e r m e a b i l i t y , b u t a l s o p l a y s a n i m p o r t a n t r o l e i n c o n t r o l l i n g t h e s u b s e q u e n t d i s t r i b u t i o n o f m i n e r a l cements ( e . g . Modarresi and G r i f f i t h s , 1963).
V a r i o u s schemes have been used t o s p e c i f y s a n d s t o n e f a b r i c
(Emery a n d G r i f f i t h s , 1954; Kahn, 1956a,b; A l l e n , 1962b; Mellon, 1964; G r i f f i t h s , 19671, and most r e l y h e a v i l y on t h e n a t u r e o f g r a i n c o n t a c t s ( F i g . 5 . 4 ) .
During depo-
s i t i o n and s h a l l o w b u r i a l t h e f a b r i c i s d e t e r m i n e d l a r g e l y by t h e p h y s i c a l mechanism of d e p o s i t i o n , g r a i n s h a p e , and t h e packing h e t e r o g e n e i t y ( A l l e n , 1969; Morrow, 1971). I n s a n d s t o n e s which have undergone e a r l y c e m e n t a t i o n and s u f f e r e d l i t t l e o r no compaction t h e main t y p e s of g r a i n c o n t a c t s a r e : convex.
t a n g e n t i a l , l o n g , and concavo-
With i n c r e a s i n g d e g r e e o f compaction t h e number of long g r a i n c o n t a c t s
t e n d s t o i n c r e a s e and i n d e e p l y b u r i e d sands (>2000m) l o n g and s u t u r e d c o n t a c t s a r e abundant ( e . g . T a y l o r , 1950; P h i p p s , 1969).
S u t u r e d c o n t a c t s , caused by
Y
Fig.5.4. The d e f i n i t i o n o f g r a i n c o n t a c t s i n s a n d s t o n e s which a r e used f o r f a b r i c a n a l y s i s ( a d a p t e d from G r i f f i t h s , 1967 and P e t t i j o h n e t a l . , 1972).
274 s o l u t i o n and r e c r y s t a l l i z a t i o n , a r e a c h a r a c t e r i s t i c f e a t u r e of d e e p l y b u r i e d s a n d s t o n e s and a good i n d i c a t i o n of compaction and i n c r e a s e i n geothermal g r a d i e n t . E x t e n s i v e p r e s s u r e s o l u t i o n under t h e s e c o n d i t i o n s i s i n d i c a t e d by abundant s t y l o l i t i c boundaries.
Evidence of p r e s s u r e compaction i s demonstrated by s a n d s t o n e s i n which
t h e volume of t h e p o r e s p a c e p l u s t h e volume of chemical cement i s l e s s t h a n t h e o r i g i n a l p o r e volume o f t h e s a n d .
The o c c u r r e n c e o f h e a l e d f r a c t u r e s and b e n t
m i n e r a l g r a i n s a r e f u r t h e r good i n d i c a t i o n s o f t h e e f f e c t s o f compaction. One of t h e major p h y s i c a l e f f e c t s o f s a n d s t o n e compaction i s t h e r e d u c t i o n i n p o r o s i t y and p e r m e a b i l i t y .
The i n c r e a s e i n q u a r t z c e m e n t a t i o n and p r e s s u r e s o l u t i o n
i n d e e p l y b u r i e d s a n d s t o n e s i s c l o s e l y dependent on p h y s i c a l compaction and t h e s e p r o c e s s e s , a c t i n g i n c o n c e r t , can c o n s i d e r a b l y reduce p o r o s i t y and p e r m e a b i l i t y . C o n t r i b u t i n g f a c t o r s i n c l u d e t h e d e f o r m a t i o n o f d u c t i l e framework g r a i n s and a l s o enhanced c l a y m i n e r a l a u t h i g e n e s i s i n deep s u b s u r f a c e b r i n e s .
The e f f e c t of
compaction and p r e s s u r e s o l u t i o n on p o r o s i t y r e d u c t i o n h a s been d e a l t w i t h theor e t i c a l l y by R i t t e n h o u s e ( 1 9 7 1 a ) .
He found t h a t t h e r e l a t i v e amounts of p o r o s i t y
l o s s due t o s o l u t i o n and c e m e n t a t i o n v a r y g r e a t l y depending upon g r a i n s h a p e , a n g u l a r i t y , packing d i r e c t i o n from which t h e p r e s s u r e was a p p l i e d , and t h e amount of s o l u t i o n which had o c c u r r e d .
A good i l l u s t r a t i o n of t h e s e r e l a t i o n s h i p s i s
g i v e n by P h i l l i p e t a l . (1963). R i t t e n h o u s e (1971b) a l s o made a t h e o r e t i c a l s t u d y of p o r o s i t y r e d u c t i o n caused by t h e compactional d e f o r m a t i o n o f d u c t i l e f r a g m e n t s .
Sandstones with a higher
p r o p o r t i o n of s u c h d u c t i l e g r a i n s might b e e x p e c t e d t o l o s e t h e i r p o r o s i t y on b u r i a l more r a p i d l y t h a n q u a r t z o s e s a n d s t o n e s .
Atwater and Miller (1965) i n a
d e t a i l e d s t u d y of'Miocene and younger sands i n s o u t h e r n L o u i s i a n a showed t h a t i n c r e a s i n g d e f o r m a t i o n of less r e s i s t a n t fragments was t h e main r e a s o n f o r t h e l i n e a r decrease i n p o r o s i t y with depth.
Nagtegaal (1978) h a s a l s o shown t h a t
l i t h i c s a n d s t o n e s a r e a l s o more s u s c e p t i b l e t o a l l f o u r p o r o s i t y and p e r m e a b i l i t y reducing processes:
mechanical compaction, p l a s t i c d e f o r m a t i o n , p r e s s u r e s o l u t i o n ,
and m i n e r a l o g i c a l a l t e r a t i o n of t h e framework c o n s t i t u e n t s . Red bed d i a n e n e s i s C o n t i n e n t a l r e d beds show many of t h e d i a g e n e t i c f e a t u r e s e x h i b i t e d by s a n d s t o n e s and e v i d e n t l y have a c o m p l i c a t e d p o s t - d e p o s i t i o n a l h i s t o r y .
Much o f o u r knowledge
of r e d bed d i a g e n e s i s stems from t h e work o f Walker (1967a, 1976) and Walker e t a l . (1978) who have s t u d i e d t h e d i a g e n e s i s i n f i r s t - c y c l e a r k o s i c d e s e r t a l l u v i u m i n t h e s o u t h w e s t e r n USA and n o r t h w e s t e r n New Mexico.
I n t h i s a r e a a number of d i a g e n e t i c
p r o c e s s e s have been d e s c r i b e d , i n c l u d i n g t h e mechanical i n f i l t r a t i o n of d e t r i t a l c l a y , t h e d i s s o l u t i o n of framework s i l i c a t e s by groundwater and t h e i r replacement by c l a y , and t h e a u t h i g e n e s i s of a s u i t e of m i n e r a l s i n c l u d i n g h a e m a t i t e .
The
u n d e r l y i n g importance of W a l k e r ' s work l i e s i n t h e f a c t t h a t i t d e m o n s t r a t e s two
275 important concepts:
1. I n t r a s t r a t a l s o l u t i o n c o n t r o l s t h e c h e m i s t r y of t h e
i n t e r s t i t i a l groundwater and hence t h e n a t u r e of t h e p r e c i p i t a t e d a u t h i g e n i c phases. 2 . The p r o g r e s s i v e d e s t r u c t i o n of l a b i l e p h a s e s means t h a t g r o s s changes i n m i n e r a l composition and t e x t u r e may t a k e p l a c e d u r i n g t h e d i a g e n e s i s of r e d b e d s . However, i t s h o u l d be s t r e s s e d t h a t n o t a l l a n c i e n t r e d beds a r e f i r s t c y c l e a r k o s e s l i k e t h o s e s t u d i e d by Walker, and many can be shown t o have been d e r i v e d from s o u r c e a r e a s less r i c h i n f e l d s p a r s and fefromagnesian m i n e r a l s .
Consequently,
t h e d i a g e n e t i c changes i n t h e s e r e d beds can be e x p e c t e d t o b e t h a t much l e s s extensive than those seen i n f i r s t cycle arkoses.
Similarly, the processes
d e s c r i b e d by Walker, because o f t h e i r time-dependant n a t u r e , c a n n o t be seen i n o p e r a t i o n i n a n c i e n t r e d beds and t h e i r p r e v i o u s e x i s t e n c e can o n l y be i n f e r r e d . N e v e r t h e l e s s , t h e s t u d y o f many a n c i e n t r e d beds p r o v i d e s compelling e v i d e n c e t h a t d i a g e n e t i c p r o c e s s e s , l i k e t h o s d e s c r i b e d by Walker, have been o p e r a t i v e and caused e x t e n s i v e p o s t - d e p o s i t i o n a l m o d i f i c a t i o n s .
This evidence includes the
p r e s e n c e of d i s s o l u t i o n f e a t u r e s , a p a u c i t y of u n s t a b l e s i l i c a t e g r a i n s and a v a r i e d a u t h i g e n i c s u i t e of m i n e r a l s i n c l u d i n g q u a r t z , f e l d s p a r , c a l c i t e and h a e m a t i t e . THE MECHANICAL INFILTRATION OF DETRITAL CLAY
Cenozoic a l l u v i u m of s o u t h w e s t e r n USA and n o r t h w e s t e r n Mexico S t u d i e s of c o a r s e g r a i n e d d e s e r t a l l u v i u m by Walker (1976) and Walker e t a l .
(1978) show t h a t when o r i g i n a l l y d e p o s i t e d i t i s e s s e n t i a l l y f r e e of i n t e r s t i t i a l clay matrix.
T h i s i s because s t r e a m flow i s a h i g h l y e f f i c i e n t p r o c e s s f o r t h e
s e p a r a t i o n o f c l a y from c o a r s e r g r a i n e d d e t r i t u s ; because c l a y has a much lower s e t t l i n g v e l o c i t y i t s t a y s i n s u s p e n s i o n i n t h e s u r f a c e w a t e r and i s c a r r i e d away a s the heavier coarser p a r t i c l e s a r e l e f t behind.
Modern a l l u v i u m from a wide
v a r i e t y of d e s e r t e n v i r o n m e n t s , and even t h a t d e p o s i t e d by ephemeral s t r e a m s , has been shown by Walker e t a l . (1978, p. 19) t o be e s s e n t i a l l y f r e e of i n t e r s t i t i a l c l a y a t t h e time of d e p o s i t i o n .
However, e x a m i n a t i o n o f samples c o l l e c t e d from
p i t s dug i n t o Holocene a l l u v i u m r e v e a l s t h a t w i t h i n a few f e e t below t h e s u r f a c e abundant i n t e r s t i t i a l c l a y o c c u r s w i t h i n t h e c o a r s e g r a i n e d d e t r i t u s .
This clay
i s commonly c o n c e n t r a t e d on t h e upper s u r f a c e s of b u r i e d p e b b l e s and shows a geopedal type f a b r i c (Crone, 1975).
The c l a y r e p r e s e n t s t h e e a r l y s t a g e s of
mechanical i n f i l t r a t i o n which o c c u r s a f t e r a p e r i o d o f s u r f a c e r u n - o f f .
Clay of
a s i m i l a r o r i g i n i s more abundant i n a l l u v i u m of P l e i s t o c e n e and L a t e T e r t i a r y a g e and i n t h i s s i t u a t i o n t h e c l a y forms w e l l - d e v e l o p e d c o a t i n g s on framework grains (Fig.5.5,
5.6).
M e c h a n i c a l l y i n f i l t r a t e d c l a y i s r e a d i l y r e c o g n i z e d by i t s h e t e r o g e n e o u s p e t r o graphic c h a r a c t e r i s t i c s .
I t c o n s i s t s o f v a r i o u s s i z e s of c l a y p l a t e l e t s which a r e
arranged p a r a l l e l t o the g r a i n surfaces.
I n t h e e a r l y s t a g e s of development t h e
c l a y may form s i n u o u s r i d g e s and b r i d g e s which form a t t h e m e n i s c i of p e l l i c u l a r
a.
CONCENTRATION IN VADOSE ZONE ,
-
0
.
o 0
O
CONCENTRATION NEAR THE WATER TABLE ,GROUND
,GROUND SURFACE SURFACE ZONE OF REWORKING
O
D
.
0
.
SURFACE
0
. .
0
- - - 0- - - - 0- - o
b.
o
0
WATER TABLE
. 0
0
.
.
IMPERMEABLE BARRIER
IMPERMEABLE 0ARRIER
c
CONCENTRATION ABOVE IMPERMEABLE BARRIERS
CONCENTRATION IN PROXIMITY TO SOURCE OF INFLUENT SEEPAGE
d d-I
MARGINAL CONGLOMERATES
...
. 7 .. c
d-2
SHIFTING DRAINAGEWAYS ACTIVE
d-3 INCISED DRAINAGEWAYS
ABANDONED
BEDROCK IMPERMEABLE BARRIER
IMPERMEABLE BARRIER
/
w a t e r between a d j a c e n t g r a i n s .
The c o m p o s i t i o n o f t h e c l a y is a l s o v a r i a b l e and
o f t e n resembles the clays i n the sediment source a r e a . Mechanical i n f i l t r a t i o n of c l a y i n t o permeable s e d i m e n t s i s p r o b a b l y v e r y i m p o r t a n t i n a r i d r e g i o n s where t h e d r a i n a g e i s i n f l u e n t b e c a u s e o f t h e low w a t e r table.
Thus whenever i n f l u e n t seepage accompanies a l l u v i a t i o n t h e mechanical
i n f i l t r a t i o n of c l a y w i l l o c c u r .
T y p i c a l l y , c l a y of t h i s t y p e i s i r r e g u l a r l y
d i s t r i b u t e d w i t h i n t h e s e d i m e n t because o f t h e v a r i a b l e n a t u r e o f i n f l u e n t seepage; t h e c l a y a c c u m u l a t i n g i n t h o s e p l a c e s where s u s p e n s i o n s e t t l i n g i s p o s s i b l e .
The
amount of c l a y which a c c u m u l a t e s depends upon a number o f f a c t o r s , i n c l u d i n g t h e amount of c l a y suspended i n t h e w a t e r a n d t h e f r e q u e n c y of e p i s o d e s o f i n f l u e n t seepage.
I n t h e Cenozoic d e p o s i t s of t h e a r i d r e g i o n s i n North America Walker (1976)
r e c o g n i z e d f o u r mechanisms which c o u l d a c c o u n t f o r t h e o b s e r v e d d i s t r i b u t i o n of mechanically i n f i l t r a t e d clay (Fig.5.5).
1) C o n c e n t r a t i o n s i n t h e vadose zone C o n c e n t r a t i o n s i n t h e vadose zone o c c u r when p e l l i c u l a r f i l m s o f w a t e r a r e n o t f u l l y developed on t h e framework g r a i n s .
Under t h e s e c o n d i t i o n s t h e i n f l u e n t
s u r f a c e w a t e r r e p l e n i s h e s t h e s e f i l m s and does n o t r e a c h t h e w a t e r t a b l e .
The
suspended c l a y i s t h u s r e t a i n e d by t h e p e l l i c u l a r w a t e r and a c c u m u l a t e s on t h e c u r f a c e s of t h e framework g r a i n s a s c o a t i n g s composed of a g g r e g a t e s of c l a y p l a t e l e t s orientated p a r a l l e l t o the surface of the grain.
I n p l a c e s c l a y - b r i d g e s may develop
between a d j a c e n t g r a i n s b e c a u s e o f t h e m e n i s c i between a d j a c e n t f i l m s of p e l l i c u l a r
water ( F i g . 5 . 6 ) . I n t h i s mechanism t h e c l a y accumulates from t h e s u r f a c e downwards and t h e t h i c k n e s s o f t h e zone o f c l a y c o n c e n t r a t i o n depends on how deep t h e i n f l u e n t seepage can p e n e t r a t e .
The s e d i m e n t s n e a r t h e s u r f a c e may be f r e e of i n f i l t r a t e d c l a y
b e c a u s e o f reworking i n c h a n n e l s and p e r i o d i c s h e e t flow.
I f t h i s occurs the
a c c u m u l a t i o n of i n f i l t r a t e d c l a y b e g i n s immediately below t h e zone o f a c t i v e reworking. 2) Concentrations near t h e water t a b l e
I n some c a s e s i n f i l t r a t e d c l a y forms nearyy h o r i z o n t a l l a y e r s which c u t primary d e p o s i t i o n a l s t r u c t u r e s s u g g e s t i n g t h a t t h e y formed n e a r former p o s i t i o n s of t h e w a t e r t a b l e (Fig.5.5B).
Walker (1976) s u g g e s t e d t h a t t h e c o n c e n t r a t i o n s could be
e x p l a i n e d by t h e d i f f e r e n t v e l o c i t i e s of w a t e r m i g r a t i o n , above and below t h e water t a b l e .
Above t h e w a t e r t a b l e i n f l u e n t s e e p a g e w a t e r , having r e p l e n i s h e d
p e l l i c u l a r f i l m s , moves r a p i d l y downward under t h e i n f l u e n c e o f g r a v i t y and c a r r i e s w i t h i t i t s s u s p e n s i o n l o a d of c l a y .
W i t h i n t h e s a t u r a t e d zone below t h e w a t e r
t a b l e , t h e m i g r a t i o n v e l o c i t y i s much lower because h e r e t h e groundwater moves under t h e i n f l u e n c e o f a v e r y low w a t e r t a b l e g r a d i e n t .
When t h e downward
218 m i g r a t i n g w a t e r r e a c h e s t h i s zone i t s v e l o c i t y i s much reduced and t h e suspended c l a y b e g i n s t o s e t t l e o u t , t e n d i n g t o become c o n c e n t r a t e d i n t h e upper p a r t o f t h e s a t u r a t e d zone where t h e i n i t i a l d e c r e a s e i n v e l o c i t y f i r s t o c c u r s .
Fluctuations
i n t h e p o s i t i o n of t h e w a t e r t a b l e w i l l t h u s i n c r e a s e t h e v e r t i c a l t h i c k n e s s of t h e zone o f c o n c e n t r a t i o n .
3 ) C o n c e n t r a t i o n s above impermeable b a r r i e r s C o n c e n t r a t i o n s o f m e c h a n i c a l l y i n f i l t r a t e d c l a y may o c c u r whenever t h e downward m i g r a t i o n o f i n f l u e n t s e e p a g e i s a r r e s t e d by a n impermeable b a r r i e r .
These b a r r i e r s
may be beds of f i n e - g r a i n e d s e d i m e n t w i t h i n c o a r s e a l l u v i u m , b u r i e d s o i l s o r even bedrock.
During p e r i o d s of i n t e r m i t t e n t s u r f a c e r u n - o f f p e r c h e d w a t e r t a b l e s may
form above such b a r r i e r s c r e a t i n g c o n d i t i o n s which a l l o w c l a y t o s e t t l e from s u s p e n s i o n i n t h e manner d e s c r i b e d i n 2 ) .
During d r y p e r i o d s when t h e i n f l u e n t
seepage t e m p o r a r i l y c e a s e s t h e p e r c h e d w a t e r t a b l e s may d r a i n away and under t h e s e c i r c u m s t a n c e s t h e remaining c l a y may form c o a t i n g s and b r i d g e s i n t h e manner d e s c r i b e d i n 1 ) . A l t e r n a t i o n of d r o u g h t and i n f l u e n t s e e p a g e c a u s e s t h e accumulation of l a r g e amounts of i n f i l t r a t e d c l a y i n t h e i n t e r m i t t e n t l y s a t u r a t e d zone; i t may u l t i m a t e l y f i l l i n t e r s t i t i a l v o i d s i n t h e a l l u v i u m above impermeable b a r r i e r s .
4 ) C o n c e n t r a t i o n s i n p r o x i m i t y t o s o u r c e s of i n f l u e n t s e e p a g e The c l o s e a s s o c i a t i o n between i n f l u e n t s e e p a g e and i n f i l t r a t i o n of c l a y c l e a r l y s u g g e s t s t h a t t h e amount of i n f i l t r a t e d c l a y s h o u l d g e n e r a l l y i n c r e a s e i n p r o x i m i t y t o p r i n c i p a l recharge a r e a s .
Thus m a r g i n a l f a n g l o m e r a t e s a r e l i k e l y t o c o n t a i n
more i n f i l t r a t e d c l a y t h a n d i s t a l s a n d s which a r e a long way from t h e r e c h a r g e a r e a and o n l y p e r i o d i c a l l y r e a c h e d by major s t o r m s .
S i m i l a r l y more c l a y i s i n -
f i l t r a t e d i n t o t h e a l l u v i u m b e n e a t h ephemeral c h a n n e l s than i t i s i n t h e i n t e r fluvial divides.
A s c h a n n e l s s h i f t p o s i t i o n s o does t h e zone of c l a y c o n c e n t r a t i o n
b u t where c h a n n e l s a r e i n c i s e d t h e y r e c h a r g e t h e same a r e a f o r a much l o n g e r p e r i o d and l a r g e r amounts of i n f i l t r a t e d c l a y accumulate ( F i g . 5 . 5 ) . The i n t r o d u c t i o n of c l a y i n t o p o r o u s , permeable s e d i m e n t s o c c u r s wherever i n f l u e n t seepage accompanies a l l u v i a t i o n .
Large amounts of c l a y c a n be added t o
c o a r s e g r a i n e d a l l u v i u m i n t h i s way and Walker e t a l . (1978, p . 2 1 ) r e p o r t t h a t t h e o l d e r P l e i s t o c e n e and T e r t i a r y d e p o s i t s of t h e s o u t h w e s t e r n USA and n o r t h w e s t e r n Mexico commonly c o n t a i n 10-20% of m a t r i x c l a y which o r i g i n a t e d i n t h i s manner.
T h i s p r o c e s s t h e r e f o r e makes i m p o r t a n t changes i n t h e t e x t u r e , m i n e r a l o g y ,
and chemical composition by a d d i n g m a t r i x t o sediment which was e s s e n t i a l l y m a t r i x f r e e when d e p o s i t e d . This p o s t - d e p o s i t i o n a l
i n f i l t r a t i o n of c l a y t h u s c a u s e s a d i a g e n e t i c d e c r e a s e
i n t h e t e x t u r a l m a t u r i t y ( F o l k , 1951) o f t h e sediment and may l e a d t o t h e f a l s e c o n c l u s i o n t h a t d e p o s i t i o n was by a mudflow o r some o t h e r p r o c e s s c h a r a c t e r i z e d by
219
M e c h a n i c a l l y i n f i l t r a t e d c l a y i n Cenozoic a l l u v i u m and a n c i e n t r e d beds. Fig.5.6. A . SEM photomicrograph of d e t r i t a l g r a i n c o a t e d w i t h c l a y s k i n (CS) which has been m e c h a n i c a l l y i n f i l t r a t e d . Upper G i l a Group ( P l i o c e n e - P l e i s t o c e n e ) , New Mexico. B . Enlargement of a r e a o u t l i n e d i n A showing broken c l a y s k i n . C . Enlargement of a r e a o u t l i n e d i n B showing c l a y p l a t e l e t s o r i e n t e d p a r a l l e l t o t h e g r a i n s u r f a c e and one s m a l l c r y s t a l o f a u t h i g e n i c f e l d s p a r , D . Thin s e c t i o n photomicrograph of m e c h a n i c a l l y i n f i l t r a t e d c l a y - o x i d e i n Lower Permian ( R o t l i e g e n d e s ) s a n d s t o n e . S o l e P i t B a s i n , North Sea. A-C a r e produced by c o u r t e s y of T.R. Walker.
280 weak s o r t i n g a c t i o n .
S i m i l a r l y t h e b u l k chemical c o m p o s i t i o n of t h e s e d i m e n t w i l l
be changed by t h e a d d i t i o n of c l a y m i n e r a l s which a r e n o r m a l l y r i c h e r i n aluminium and lower i n a l k a l i s and a l k a l i n e e a r t h s t h a n t h o s e o f t h e o r i g i n a l sediment. M e c h a n i c a l l v i n f i l t r a t e d c l a v i n a n c i e n t r e d beds G r a i n c o a t i n g s o f c l a y and i r o n o x i d e a r e a c h a r a c t e r i s t i c f e a t u r e o f a n c i e n t
red beds,
The r e l a t i v e proportions of i r o n oxide (haematite) and clay varies
considerably.
I n some c a s e s t h e g r a i n c o a t i n g s c o n s i s t s o f o n l y p a r t i c u l a t e
c r y s t a l s of h a e m a t i t e which have a p p a r e n t l y been p r e c i p i t a t e d i n s i t u and i n o t h e r s t h e r e may be t h i c k c l a y - o x i d e p e l l i c l e s which resemble t h e c l a y c u t a n s of Brewer ( 1 9 6 4 ) .
Such g r a i n s c o u l d c o n c e i v a b l y have been d e r i v e d from upland
s o i l s , o r more l i k e l y , from s o i l s which formed i n a d j a c e n t f l o o d p l a i n a r e a s . More u s u a l l y t h e g r a i n c o a t i n g s c o n s i s t of a c l a y - o x i d e p e l l i c l e of v a r i a b l e t h i c k n e s s and geopedal t e x t u r e s i n c l u d i n g meniscus b r i d g e s a r e cornon., There can be l i t t l e doubt t h a t t h e s e f e a t u r e s were formed by t h e m e c h a n i c a l i n f i l t r a t i o n of c l a y i n t h e manner s u g g e s t e d by Walker ( 1 9 7 6 ) .
An i m p o r t a n t f e a t u r e of m e c h a n i c a l l y - i n f i l t r a t e d c l a y i s t h a t i t i s n o t r e d when d e p o s i t e d b u t o n l y reddens w i t h time a f t e r b e i n g i n c o n t a c t w i t h oxygenated groundwater.
I n Cenozoic a l l u v i u m t h e r e d d e n i n g of i n f i l t r a t e d c l a y i s s i g n i f i c a n t
because i t t h u s r e p r e s e n t s t h e f i r s t s t a g e of p i g m e n t a t i o n i n r e d bed f o r m a t i o n . I t seems l i k e l y t h a t i n a n c i e n t sandy a l l u v i u m t h e i n i t i a l s t a g e s of r e d d e n i n g occur
l a r g e l y a s a r e s u l t of t h e o x i d a t i o n o f m e c h a n i c a l l y - i n f i l t r a t e d and d e t r i t a l c l a y . DISSOLUTION OF FRAMEWORK SILICATES I n g e n e r a l t h e a l t e r a t i o n of s i l i c a t e s f o l l o w s G o l d i c h ' s m i n e r a l s t a b i l i t y s e r i e s which i s t h e r e v e r s e of Bowen's r e a c t i o n s e r i e s .
T h i s sequence o f a l t e r a t i o n
r e s u l t s because of t h e v a r i a t i o n i n bond s t r e n g t h s between oxygen and c a t i o n s i n the s i l i c a t e minerals (Keller, 1957).
M i n e r a l s which a r e low i n Bowen's r e a c t i o n
s e r i e s s u c h a s a u g i t e , h o r n b l e n d e , and p l a g i o c l a s e t e n d t o be a l t e r e d f i r s t w h i l s t q u a r t z , m u s c o v i t e and o r t h o c l a s e a r e t h e most s t a b l e .
The most s t a b l e bond i n
s i l i c a t e s i s t h e S i - 0 bond and d i f f e r e n c e s i n t h e s t r e n g t h o f t h i s bond between t h e d i f f e r e n t s i l i c a t e groups a r e v e r y s m a l l compared t o t h e d i f f e r e n c e s between oxygen and o t h e r c a t i o n s .
T h e r e f o r e , s i l i c a t e s w i t h h i g h S i / O r a t i o s a r e more
s t a b l e r e g a r d l e s s of which m e t a l l i c i o n s b a l a n c e t h e s t r u c t u r e e l e c t r i c a l l y . Within e a c h of t h e m a j o r s i l i c a t e groups t h e s p e c i e s of m e t a l l i c i o n p r e s e n t d e t e r m i n e s t h e p o s i t i o n of t h e m i n e r a l i n t h e w e a t h e r i n g s t a b i l i t y s e r i e s (Table 5.4).
The o r d e r of i n c r e a s i n g s t r e n g t h of t h e o x y g e n - m e t a l l i c c a t i o n bonds i s e x a c t l y t h e r e v e r s e o f t h e o r d e r of r e s i s t a n c e t o w e a t h e r i n g of t h e m i n e r a l s i n which t h e c a t i o n s a r e i m p o r t a n t .
F o r example, o r t h o c l a s e i s one of t h e more
281 r e s i s t a n t p h a s e s i n Bowen's r e a c t i o n s e r i e s y e t potassium-oxygen bonds a r e v e r y weak.
F o r s t e r i t e , on t h e o t h e r hand, w e a t h e r s r a p i d l y y e t t h e magnesium-oxygen
bond i s r e l a t i v e l y s t r o n g .
The r e a s o n f o r t h i s l i e s i n t h e s t r e n g t h of t h e s i l i c o n -
oxygen bond which p r e v e n t s t h e o r t h o c l a s e s t r u c t u r e from d i s i n t e g r a t i o n d e s p i t e t h e r a p i d removal of p o t a s s i u m i o n s .
F o r s t e r i t e h a s few s u c h bonds and w i t h o u t t h e
s t a b i l i z i n g magnesium atoms t h e s t r u c t u r e i s v e r y u n s t a b l e and r a p i d l y c o l l a p s e s . TABLE 5.4.
Bond s t r e n g t h s between oxygen and common c a t i o n s , c a l c u l a t e d from a r e f e r e n c e s t a t e o f gaseous i o n s (from t e l l e r . 1957). I on
Energy of f o r m a t i o n (Kcal/mole)
K+
299
+ Na
322
H+(in OH-)
5 15
Ca
2+
8 39 9 12
Mg2+ Fe
2+
9 19
A 1 3+
1793
*14+
1878
4+ Ti
2882
si4+
(in (in (in (in (in (in
tectosilicates) phyllosilicates) double-chain i n o s i l i c a t e s ) single-chain i n o s i l i c a t e s ) sorosilicates) nesosilicates)
3110 3123 3127 3131 3137 3147
The d i s s o l u t i o n of s i l i c a t e g r a i n s t a k e s p l a c e h y d r o l y s i s .
Sodium, c a l c i u m ,
and magnesium a r e s e p a r a t e d from t h e s i l i c a t e framework r a t h e r e a s i l y and a r e soon c a r r i e d away i n aqueous s o l u t i o n , whereas i r o n , aluminium, and t i t a n i u m t e n d t o remain behind.
T h i s i s c o n s i s t e n t w i t h t h e t h e o r e t i c a l p r e d i c t i o n ( T a b l e 5 . 4 ) and
i s u s u a l l y observed i n n a t u r e ( G o l d i c h , 1938; Wahlstrom, 1948).
The a c t u a l n a t u r e
of t h e h y d r o l y s i s r e a c t i o n i s r a t h e r c o m p l i c a t e d and depends upon t h e composition of t h e m i n e r a l i n v o l v e d . 5KA1Si308
F o r p o t a s h f e l d s p a r i t may be w r i t t e n :
+ 4H+ + 4HC03- +
16H20
+
(Feldspar)
KA15Si7020(OH)4
+ 8H4Si04 + 4K+ + 4HC03-
( I l l ite)
and under extreme c o n d i t i o n s k a o l i n i t e may form from i l l i t e : 2KA15Si7020(OH)4 + 2H+ 2HC0313H20 -p 5A12Si205(0H)4 + 4H4Si04
+
(Illite)
+
(Kaolinite)
(5.1)
+ 2K+ + 2HC03(5.2)
The h y d r o l y s i s of framework s i l i c a t e s i s f u n d a m e n t a l l y i m p o r t a n t i n r e d bed diagenesis.
Not o n l y does i t m a t e r i a l l y change t h e framework m i n e r a l o g y b u t
h y d r o l y s i s a l s o p r o v i d e s t h e major s o u r c e of i o n s from which a u t h i g e n i c m i n e r a l p h a s e s may form. Feldspars Both t h e major groups o f t h e f e l d s p a r m i n e r a l s , t h e a l k a l i f e l d s p a r s and t h e p l a g i o c l a s e s , commonly o c c u r a s d e t r i t a l g r a i n s i n r e d beds and may show e v i d e n c e of e x t e n s i v e i n s i t u d i s s o l u t i o n .
I n g e n e r a l t h e a l k a l i f e l d s p a r s t e n d t o be more
s t a b l e than t h e p l a g i o c l a s e s and t h e r e a r e a l s o v a r i a t i o n s w i t h i n e a c h group; m i c r o c l i n e i s more s t a b l e t h a n o t h e r a l k a l i f e l d s p a r s and s o d i c p l a g i o c l a s e i s more s t a b l e than c a l c i u m r i c h f e l d s p a r s .
Individual grains of feldspar frequently
r e v e a l marked h e t e r o g e n e i t i e s s u c h a s c o m p o s i t i o n a l z o n i n g , p e r t h i t e s of v a r i o u s k i n d s , p o i k i l i t i c t e x t u r e s , and replacement f i l m s and c o a t i n g s .
The i d e a l framework
s t r u c t u r e i s a l s o c o m p l i c a t e d by l o c a l and d i s t a n t o r d e r i n g i n t h e d i s t r i b u t i o n of s i l i c o n and aluminium i o n s .
The i n h e r e n t c o m p l e x i t y of t h e f e l d s p a r s must t h e r e f o r e
have a s i g n i f i c a n t e f f e c t on any d i s s o l u t i o n p r o c e s s e s . There has been a c o n s i d e r a b l e amount of e x p e r i m e n t a l work on t h e i n i t i a l h y d r o l y s i s of f e l d s p a r s (Correns and von E n g e l h a r d t , 1939; W o l l a s t , 1967; Helgeson,
1971).
Correns and von E n g e l h a r d t (1939) found t h a t p o t a s s i u m , s i l i c a , and
aluminium were r e l e a s e d i n t r u e s o l u t i o n d u r i n g t h e breakdown o f a d u l a r i a a l t h o u g h t h e r a t i o s i m p l i e d t h a t a r e s i d u a l c o a t i n g of
hydrous aluminium s i l ; c a t e ,
with a
Si/A1 g r e a t e r than t h a t of k a o l i n i t e , formed around t h e decomposing g r a i n s .
These
a u t h o r s i n f e r r e d t h a t t h e c o a t i n g m a i n t a i n e d a c o n s t a n t t h i c k n e s s and i n f l u e n c e d t h e r a t e of t h e a l t e r a t i o n r e a c t i o n , a view which i s c o n s i s t e n t w i t h l a t e r experiments on o r t h o c l a s e by W o l l a s t (1967).
Helgeson (1971) p o s t u l a t e d t h e
s e q u e n t i a l a p p e a r a n c e of i n t e r m e d i a t e r e a c t i o n p r o d u c t s such a s g i b b s i t e , k a o l i n i t e , and mica s s zoned s u r f a c e l a y e r s around t h e decomposing f e l d s p a r g r a i n s .
Such a
r e s i d u a l l a y e r h a s n o t been d i r e c t l y observed a s y e t and s c a n n i n g e l e c t r o n microscope (SEM) s t u d i e s r e v e a l t h a t t h e i n i t i a l s t a g e s o f h y d r o l y s i s a r e c h a r a c t e r i z e d by well-developed c o r r o s i o n f i g u r e s on c l e a v a g e s u r f a c e s , a f e a t u r e which i m p l i e s continuous d i s s o l u t i o n .
T h i s i s t h e c a s e i n e x p e r i m e n t a l ' c o n d i t i o n s (Tchoubar,
1965), i n s o i l s Wilson (1975), and d u r i n g t h e e a r l y s t a g e s of s a n d s t o n e d i a g e n e s i s (Walker e t a l . , 1978).
I t is a l s o apparent t h a t various feldspar species a r e
a f f e c t e d by c o n t i n u o u s d i s s o l u t i o n . I n t h e f i r s t c y c l e d e s e r t a l l u v i u m s t u d i e d by Walker e t a l . (1978) f e l d s p a r g r a i n s which have been e t c h e d o r hollowed o u t by d i s s o l u t i o n a r e a common f e a t u r e
(Fig.5.7).
Only p l a g i o c l a s e f e l d s p a r s a r e s e r i o u s l y a f f e c t e d by t h e d i s s o l u t i o n
w h i l s t potassium f e l d s p a r has n o t y e t been d i s c e r n i b l y a f f e c t e d .
The d i s s o l u t i o n
of p l a g i o c l a s e t e n d s t o b e i r r e g u l a r w i t h t h e g r a i n s showing i r r e g u l a r l y e t c h e d ,
283
Fig.5.7. P a r t i a l l y d i s s o l v e d f e l d s p a r s i n Cenozoic a l l u v i u m . A , B t h i n s e c t i o n s ; C-D SEM photomicrographs. A . P e r i p h e r a l l y d i s s o l v e d p l a g i o c l a s e g r a i n . D i s s o l u t i o n v o i d s (DV) s u r r o u n d t h e r e l i c t g r a i n a n d c l a y s k i n (CS) marks t h e o r i g i n a l g r a i n boundary. Thin s e c t i o n ? P l i o c e n e f a n g l o m e r a t e , CaZon R o j o , Baja C a l i f o r n i a . B. I n t e r n a l l y d i s s o l v e d p l a g i o c l a s e ( P I . Most of t h e o r i g i n a l g r a i n now occupied by d i s s o l u t i o n v o i d s . Same d e t a i l s a s A . C . P l a g i o c l a s e p a r t i a l l y r e p l a c e d by c l a y (RC) and s u b s e q u e n t l y d i s s o l v e d , forming d i s s o l u t i o n v o i d s (DV). Same d e t a i l s a s A and B. D . P a r t i a l l y d i s s o l v e d p l a g i o c l a s e g r a i n ( P ) showing well-developed d i s s o l u t i o n v o i d s (DV). Tesuque Formation (Miocene-Pliocene), n e a r Dixon, New Mexico. Produced by c o u r t e s y of T.R. Walker.
or fluted, surface textures.
The n a t u r e of t h e r e l i c t g r a i n s i n d i c a t e t h a t d i s -
s o l u t i o n may proceed inward from t h e p e r i p h e r y of t h e g r a i n , o r i t may s e l e c t i v e l y remove t h e i n t e r i o r o f t h e g r a i n s and produce a hollow " s h e l l " .
G r a i n s may be
completely removed by t h i s p r o c e s s l e a v i n g no c l u e of t h e i r o r i g i n a l p r e s e n c e o t h e r than t h e d i s s o l u t i o n v o i d .
Commonly, however, d i s s o l v e d g r a i n s show c o a t i n g s of
i n f i l t r a t e d o r a u t h i g e n i c c l a y which a r e more r e s i s t a n t t o d i s s o l u t i o n and t h e s e may be l e f t behind t o " g h o s t " t h e o r i g i n a l g r a i n a f t e r i t has c o m p l e t e l y d i s a p p e a r e d D i s s o l u t i o n v o i d s o f t h i s t y p e may become f i l l e d w i t h cement and t h u s p r e s e r v e d , b u t o f t e n t h e y c o l l a p s e and t h e c l a y c o a t i n g becomes mixed w i t h t h e i n t e r s t i t i a l matrix.
When t h i s happens t h e r e i s no r e c o r d of t h e p r e v i o u s e x i s t e n c e o f t h e
dissolution void. The d i s s o l u t i o n of f e l d s p a r i n a n c i e n t s a n d s t o n e s i s w i d e l y r e p o r t e d ( M i l l e r ,
1955; Heald and L a r e s e , 1973; F l e s c h and Wilson, 1974; Waugh, 1 9 7 8 ) .
Potassium
f e l d s p a r s show e v i d e n c e o f more e x t e n s i v e d i s s o l u t i o n t h a n t h e Cenozoic a l l u v i u m d e s c r i b e d by Walker e t a l . (1978) and even though p l a g i o c l a s e may s u f f e r p r e f e r e n t i a l d e s t r u c t i o n d u r i n g t r a n s p o r t ( e . g . P i t t m a n , 1 9 6 9 ) , j u d g i n g from t h e r e l a t i v e abundance of potassium f e l d s p a r and p l a g i o c l a s e i n a n c i e n t c o n t i n e n t a l r e d beds i t would a p p e a r t h a t t h e r e has been w h o l e s a l e d i s s o l u t i o n o f t h e more unstable plagioclase feldspars (Table 5 . 5 ) .
The a v e r a g e s e d i m e n t c o n t a i n s a b o u t
5% f e l d s p a r ( g l a t t e t a l . , 1980) b u t t h i s i s c o n s i d e r a b l y less t h a n r e d beds which f r e q u e n t l y c o n t a i n more than 8-10% f e l d s p a r , t h e m a j o r i t y o f which i s p o t a s h feldspar. The p r e f e r e n t i a l d j s s o l u t i o n o f p l a g i o c l a s e i s t h u s c o n s i d e r e d t o b e a n important diagenetic process.
A s w e l l a s changing t h e b u l k m i n e r a l o g i c a l and chemical com-
p o s i t i o n of t h e s e d i m e n t i t a l s o r e d u c e s t h e a p p a r e n t m i n e r a l o g i c a l m a t u r i t y and by i n c r e a s i n g t h e amount of i n t e r s t i t i a l c l a y m a t r i x c a u s e s a n a p p a r e n t d e c r e a s e i n textural maturity. Micas The micas do n o t show obvious d i s s o l u t i o n f e a t u r e s d u r i n g i n t r a s t r a t a l a l t e r a t i o n but i n keeping with t h e complexity of t h e i r behaviour i n s o i l s , probably y i e l d d i v e r s e a l t e r a t i o n p r o d u c t s , which depend upon t h e n a t u r e o f t h e p a r e n t m i n e r a l and a l s o t h e c o n d i t i o n s i n t h e i n t e r s t i t i a l environment.
The i n f l u e n c e of s t r u c t u r e
i s w e l l - i l l u s t r a t e d by t h e f a c t t h a t t r i o c t a h e d r a l micas ( e . g . b i o t i t e ) a r e more s u s c e p t i b l e to weathering than t h e i r dioctahedral counterparts ( e . g . muscovite) (Deer e t a l . , 1966, p . 193-200; Weaver and P o l l a r d , 1 9 7 3 ) .
The t r i o c t a h e d r a l micas
a r e r e a d i l y c o n v e r t e d t o v e r m i c u l a t e d u r i n g w e a t h e r i n g , a p r o c e s s which i s a p p a r e n t l y connected w i t h t h e o r i e n t a t i o n o f t h e hydroxyl d i p o l e i n t h e o c t a h e d r a l s h e e t ( B a s s e t t , 1960).
I n t r i o c t a h e d r a l micas t h e hydroxyl d i p o l e i s normal t o the
s i l i c a t e s h e e t s o t h a t t h e p r o t o n i c c h a r g e i s a d j a c e n t t o t h e p o t a s s i u m i o n , which
285
i s t h e r e f o r e , i n an unstable p o s i t i o n .
I n t h e d i o a c t a h e d r a l micas t h e hydroxyl
group i s i n c l i n e d and moves t h e p r o t o n f u r t h e r from t h e potassium i o n which i s then i n a more s t a b l e p o s i t i o n between t h e s i l i c a t e s h e e t s . TABLE 5 . 5 .
Comparison of t h e a v e r a g e f e l d s p a r c o n t e n t of v a r i o u s p l u t o n i c i g n e o u s r o c k s w i t h t h a t o f a n c i e n t r e d beds. The r e s u l t s i n d i c a t e t h e r e l a t i v e enrichment of p o t a s h f e l d s p a r i n a n c i e n t r e d b e d s , Data a r e from 1. S e l l e y (1966) 2. Simonen and Kuovo (1955) 3. T u r n e r and McD. Whitaker (1976) 4. Krynine (1950) 5. Huckenholtz (1963).
Granite Syeni t e Granodiorit e Quartz d i o r i t e Diorite Gabbro
P o t a s h F e l d s p a r (%)
P l a g i o c l a s e (7")
40 12
26 12 46 56 64 65
15 6 3 0
Precambrian 1 T o r r i d o n i a n Sands tone J o t n i a n Sandstone'
27
3 1
Palaeozoic R i n g e r i k e Sands t o n e 3
8.3
1
Mesozoic 4 Newark Group
24
6
Cenozoic 5 Oligocene Arkose
18.5
21
0.4
There a r e many f i e l d and e x p e r i m e n t a l s t u d i e s which show t h a t t h e breakdown of b i o t i t e i s a complex p r o c e s s .
There i s ample e v i d e n c e t o i n d i c a t e t h a t b i o t i t e
responds i n d i f f e r e n t ways t o v a r y i n g c o n d i t i o n s d u r i n g d i a g e n e s i s ( T u r n e r and A r c h e r , 1977).
T h e o r e t i c a l c o n s i d e r a t i o n s s u g g e s t t h a t b i o t i t e c o u l d weather t o
v e r m i c u l i t e under a c i d c o n d i t i o n s and t o v e r m i c u l i t e p l u s m o n t m o r i l l o n i t e under n e u t r a l and a l k a l i n e c o n d i t i o n s .
T h i s i s c o n s i s t e n t w i t h t h e f a c t t h a t t h e most
abundant mica c l a y i n c a l c a r e o u s o r a l k a l i n e s o i l s i s m o n t m o r i l l o n i t e (McNeal and S a n s o t e r r a , 1964).
The t r a n s f o r m a t i o n of b i o t i t e t o v e r m i c u l i t e a l t h o u g h
known t o o c c u r w i d e l y i n s o i l s i s o n l y r a r e l y r e p o r t e d t o have o c c u r r e d i n r e d beds ( e . g . S c h l u g e r and Roberson, 1 9 7 5 ) .
The b i o t i t e - j v e r m i c u l i t e
transformation
i s known t o be complex, a n d e a r l y work (Walker, 1949) s u g g e s t e d t h a t t h e l o s s of p o t a s s i u m was accompanied by o x i d a t i o n , s u b s t i t u t i o n of hydroxyl f o r oxygen, and t h e loss of o c t a h e d r a l i r o n and magnesium.
The l o s s of l a y e r c h a r g e a s s o c i a t e d
w i t h v e r m i c u l i t i z a t i o n can be g e n e r a l l y a c c o u n t e d f o r by t h e o x i d a t i o n of o c t a h e d r a l i r o n ( N o r r i s h , 1972).
The a p p a r e n t s c a r c i t y o f v e r m i c u l i t e i n r e d beds
may be due t o t h e f a c t t h a t b i o t i t e o x i d a t i o n may n o t have been accompanied by vermiculitization.
I t i s p o s s i b l e f o r v i r t u a l l y complete o x i d a t i o n of b i o t i t e w i t h
l i t t l e o r no v e r m i c u l i t i z a t i o n even though a s u b s t a n t i a l p r o p o r t i o n of t o t a l potassium
2 86
Fig.5.8. Thin s e c t i o n photomicrographs (PPL) showing t h e f o r m a t i o n o f h a e m a t i t e a f t e r b i o t i t e A. Randomly s c a t t e r e d pseudohexagonal f l a k e s of h a e m a t i t e B. Thin t r a n s l u c e n t s h e e t s of h a e m a t i t e C . Thin opaque s h e e t s of h a e m a t i t e D. T h i c k e r , d i s c o n t i n u o u s , s h e e t s of opaque h a e m a t i t e E . P r e f e r e n t i a l development of h a e m a t i t e i n zones k i n k e d due t o compaction F. Thick c o n t i n u o u s s h e e t s o f opaque h a e m a t i t e . T h i s i s t h e s t a g e of r e p l a c e m e n t p r i o r t o complete pseudomorphing of b i o t i t e f l a k e s .
may have been l o s t (Walker, 1949; Seddoh and P e d r o , 1974).
B i o t i t e o x i d a t i o n can
thus p r o c e e d w i t h o r w i t h o u t accompanying v e r m i c u l i t i z a t i o n . must be compensated by o t h e r changes i n t h e b i o t i t e .
The v a l e n c y changes
Three p o s s i b l e mechanisms
have been p o s t u l a t e d and a r e f o r m u l a t e d below f o r a n n i t e t h e f e r r o u s end-member of t h e p h l o g o p i t e - b i o t i t e series (Farmer e t a l . ,
1971):
loss of i n t e r l a y e r c a t i o n s : ( S i 3 A 1 0 1 0 ) F e ~ ( O H ) 2K
+
f0,
+ hH20
l o s s o f hydroxyl p r o t o n s : (Si3A1010)Fe32+ (OH)2 K + %02
+
(Si3A1010)Fe22+ Fe 3+ (OH)2
(Si3A1010)Fe 2+ Fe 3+0 2
K
+ KOH
+ H20
(5.3)
(5.4)
2
l o s s of o c t a h e d r a l i r o n : (Si3A1010)Fe32+ (OH)* K
+
+ %H20 +
%02
( S i 3 A 1 0 1 0 ) F e ~ ( O H ) 2K
+ FeO
OH
(5.5)
The l o s s of i n t e r l a y e r c a t i o n s c a n o n l y be of minor importance a s t h e amount of i r o n o x i d i z e d d u r i n g v e r m i c u l i t i z a t i o n i s v e r y much g r e a t e r t h a n t h e l a y e r c h a r g e . The second mechanism which i n v o l v e s t h e c o n v e r s i o n of hydroxyl t o o x i d e i o n s
i s w e l l e s t a b l i s h e d f o r t h e r m a l o x i d a t i o n of b i o t i t e s and i t may a l s o be i m p o r t a n t i n t h e o x i d a t i v e w e a t h e r i n g o f b i o t i t e (Farmer e t a l . , 1971) a l t h o u g h a g e n e r a l a p p r a i s a l i s d i f f i c u l t b e c a u s e of t h e problem of d i s t i n g u i s h i n g e x c e s s w a t e r from c o n s t i t u t i o n a l hydroxyl (Newman and Brown, 1966; R i m s a i t e , 1967, 1970). The l o s s of o c t a h e d r a l i r o n h a s been p o s t u l a t e d f o r some time i n o r d e r t o e x p l a i n t h e lower i r o n c o n t e n t of w e a t h e r e d o x i d i z e d b i o t i t e s i n s o i l s when compared w i t h f r e s h b i o t i t e from t h e p a r e n t r o c k (Walker, 1949; Wilson, 1 9 7 0 ) .
However, o x i d i z e d
b i o t i t e s do n o t n e c e s s a r i l y have lower i r o n c o n t e n t s ( R i m s a i t e , 1967) and t h e p r e s e n c e of i n t e r l a y e r i r o n o x i d e s h a s been a b l y demonstrated by Farmer e t a l . (1971). These a u t h o r s s u g g e s t e d t h a t e l e c t r o n t r a n s f e r from t h e o c t a h e d r a l f e r r o u s i o n s i s i n i t i a l l y mediated through a hydroxyl o r o x i d e i o n .
This i n i t i a l s t e p probably
i n v o l v e s t h e t r a n s f e r o f a hydrogen atom from a hydroxyl group t o t h e o x i d a n t . L a t e r when remaining f e r r o u s i o n s a r e c o o r d i n a t e d o n l y t o o x i d e i o n s e l e c t r o n s may t r a n s f e r d i r e c t l y from o x i d e i o n s t o t h e o x i d a n t , o r i n d i r e c t l y through b r i d g i n g water molecules. (FeFO;-)5+;
I n t h i s way t h e g r o u p i n g :
(Fe2+Fe3+O:-)4+
i s converted t o
t h e r e s u l t a n t l o c a l c o n c e n t r a t i o n of p o s i t i v e c h a r g e s i s u n s t a b l e
and a (Fe3+02-)+ g r o u p i n g i s e j e c t e d through hexagonal h o l e s i n t h e s i l i c a t e s h e e t t o an i n t e r l a y e r space.
The i n i t i a l i n t e r l a y e r o x i d e i s p r o b a b l y amorphous i r o n
hydroxide o r a c r y s t a l l i n e phase o f p - F e O O H . There i s l i t t l e doubt t h a t t h e o x i d a t i v e decomposition of b i o t i t e i s a charact e r i s t i c f e a t u r e of e a r l y d i a g e n e s i s i n c o n t i n e n t a l a l l u v i u m .
The f a t e of t h e
i n t e r l a y e r o x i d e s does however v a r y presumably a c c o r d i n g t o t h e i n t e r s t i t i a l conditions.
Walker (1967a, 1976) and Walker e t a l . (1978) have d e s c r i b e d i r o n -
r i c h h a l o s around decomposing b i o t i t e g r a i n s and have concluded t h a t t h e s e r e p r e s e n t t h e d i s t r i b u t i o n o f i r o n by oxygenated p o r e w a t e r u l t i m a t e l y forming pigmentary
288
Fig.5.9. SEM photomicrographs showing t h e p r o g r e s s i v e r e p l a c e m e n t of b i o t i t e by s h e e t s of h a e m a t i t e c r y s t a l l i t e s p a r a l l e l t o (001). A l l from t h e Old Red Sandstone of t h e Gamrie O u t l i e r , S c o t l a n d , A. Oxidized b i o t i t e showing i n c i p i e n t a l t e r a t i o n on c l e a v a g e s u r f a c e . B . D e t a i l of c l e a v a g e s u r f a c e w i t h i n c i p i e n t a l t e r a t i o n . Note a b s e n c e of c l e a r l y d e f i n e d c r y s t a l forms. C. Cleavage s u r f a c e showing abundant pseudohexagonal c r y s t a l l i t e s of h a e m a t i t e , D . Advanced s t a g e of a l t e r a t i o n showing haematite c r y s t a l l i t e s coalesced i n t o a s i n g l e sheet.
289
T.S.
S.E.M. I
t I
t
I
I
I
r
I
t
f
.
L
2
a
U
I
I
Fig.5.10. Schematic diagram showing t h e model f o r t h e f o r m a t i o n o f h a e m a t i t e pseudomorphs a f t e r b i o t i t e i n Devonian r e d beds a s s e e n i n t h i n s e c t i o n (TS) and s c a n n i n g e l e c t r o n microscopy (SEM). Four s t a g e s a r e r e c o g n i z e d : 1 * S c a t t e r e d pseudohexagonal f l a k e s o f t r a n s l u c e n t h a e m a t i t e i n i n t e r l a y e r p o s i t i o n s . 2 = Thin d i s c o n t i n u o u s s h e e t s p a r a l l e l t o t h e b i o t i t e c l e a v a g e . 3 = Continuous s h e e t s of opaque h a e m a t i t e . 4 = Complete pseudomorph formed by s h e e t s of h a e m a t i t e packed p a r a l l e l t o t h e b i o t i t e cleavage..
I
U
haematite. (1974).
S i m i l a r h a l o e s have been d e s c r i b e d i n C r e t a c e o u s r e d beds by McBride
B i o t i t e s i n b o t h Cenozoic a l l u v i u m and many a n c i e n t r e d beds show
i n t e r l a y e r s h e e t s of h a e m a t i t e .
These have been s t u d i e d i n d e t a i l i n t h e Old Red
Sandstone of S c o t l a n d by T u r n e r a n d Archer (1977) who d e s c r i b e d a complete sequence of b i o t i t e a l t e r a t i o n r a n g i n g from a p p a r e n t l y f r e s h g r a i n s t o t h o s e c o m p l e t e l y r e p l a c e d by h a e m a t i t e (Fig.5.8).
The i n i t i a l s t a g e s of o x i d a t i o n a r e marked by t h e
development of t h i n t r a n s l u c e n t f l a k e s of h a e m a t i t e a l o n g c l e a v a g e p l a n e s when s e e n i n thin section.
I n k e e p i n g w i t h t h e model o f Farmer e t a l . (1971) i t i s l i k e l y
t h a t t h e s e were i n i t i a l l y
FeOOH
and have s u b s e q u e n t l y undergone i n v e r s i o n .
With p r o g r e s s i v e o x i d a t i o n t h e s e f l a k e s b e g i n t o c o a l e s c e l a t e r a l l y and e v e n t u a l l y t h i n d i s c o n t i n u o u s s h e e t s of h a e m a t i t e become v e r y a b u n d a n t .
These a r e t h i c k e r i n
more e x t e n s i v e l y a l t e r e d b i o t i t e g r a i n s s u g g e s t i n g t h a t i n d i v i d u a l s h e e t s o f h a e m a t i t e a r e s t a c k e d one upon top of t h e o t h e r .
Scanning e l e c t r o n microscope
s t u d i e s confirm t h a t t h e s e i n d i v i d u a l s h e e t s of h a e m a t i t e a r e formed from c o a l e s c e d , s m a l l pseudohexagonal f l a k e s of h a e m a t i t e ( F i g . 5 . 9 ) .
P r e c i s e i n s i t u compositional
a n a l y s i s of t h e f l a k e s i s n o t p o s s i b l e u s i n g e n e r g y - d i s p e r s i v e X-ray a n a l y s i s b u t Turner and Archer (1977) were a b l e t o show t h a t i r o n was t h e major e l e m e n t i n t h e f l a k e s and judged t h a t i r o n o x i d e was t h e most l i k e l y composition.
I n a number of
a n c i e n t r e d beds t h i s o x i d a t i o n of b i o t i t e i s s e e n t o have gone f u l l c y c l e w i t h t h e f o r m a t i o n o f complete h a e m a t i t e pseudomorphs a f t e r b i o t i t e ( T u r n e r , 1974a; Turner and A r c h e r , 1 9 7 7 ) .
Evidence from SEM and t h i n s e c t i o n s t u d i e s i n d i c a t e s
t h a t t h e pseudomorphs a r e formed by t h e s t a c k i n g of i n t e r l a y e r s h e e t s of h a e m a t i t e (Fig.5.10).
Complete pseudomorphs of t h i s t y p e a r e common i n t h e R i n g e r i k e Group
of Norway ( T u r n e r , 1974a). The r e a s o n s why b i o t i t e o x i d a t i o n y i e l d s i n t e r l a y e r i r o n o x i d e s i n some c a s e s and n o t i n o t h e r s a r e f a r from c l e a r .
I r o n - d e p l e t e d b i o t i t e s i n s o i l s a r e analogous
t o t h e l e a c h e d b i o t i t e s w i t h i r o n o x i d e h a l o e s r e p o r t e d by Walker ( 1 9 6 7 a ) , Walker
e t a l . (1978) and McBride (1974).
These p r o b a b l y form under c o n d i t i o n s i n which
i r o n s a n be r e d i s t r i b u t e d outwards from t h e decomposing g r a i n s .
I s m a i l (1969) has
shown t h a t t h e p r o d u c t s of e x p e r i m e n t a l w e a t h e r i n g of b i o t i t e depend upon t h e pH conditions.
I n a n e u t r a l environment o x i d a t i o n r e s u l t s i n a d e c r e a s e i n s u r f a c e
c h a r g e y i e l d i n g a s m e c t i t e - l i k e p r o d u c t , whereas i n a c i d c o n d i t i o n s o x i d a t i o n i s balanced by t h e r e l e a s e of o c t a h e d r a l i r o n which m a i n t a i n s t h e s u r f a c e c h a r g e a t a h i g h l e v e l t o form v e r m i c u l i t e .
I t t h u s seems l i k e l y t h a t v a r i a t i o n i n pH i n
t h e i n t e r s t i t i a l environment i s a n i m p o r t a n t f a c t o r i n d e t e r m i n i n g whether i r o n
i s released i n t o the pore waters during diagenesis o r r e t a i n e d within the b i o t i t e s t r u c t u r e t o form i n t e r l a y e r s h e e t s of h a e m a t i t e o r pseudomorphs of h a e m a t i t e a f t e r biotite.
291 Ferromaenesian m i n e r a l s Ferromagnesian m i n e r a l s a r e g e n e r a l l y u n s t a b l e i n t h e aqueous o x i d i z i n g environment and show w i d e s p r e a d e v i d e n c e of i n s i t u d i s s o l u t i o n i n b o t h Recent w e a t h e r i n g p r o f i l e s (Wahlstrom, 1948) and Cenozoic a l l u v i u m (Walker e t a l . , 1978). O l i v i n e a p p e a r s t o be l e s s s t a b l e t h a n pyroxene which i s g e n e r a l l y c o n s i d e r e d t o be l e s s s t a b l e t h a n amphibole ( G o l d i c h , 1938; Wilson, 1975).
Because o l i v i n e i s
t h e most u n s t a b l e f e r r o m a g n e s i a n s i l i c a t e i t i s n o t a commonly o c c u r r i n g heavy mineral.
During d i a g e n e s i s i t can be e x p e c t e d t o be r a p i d l y a l t e r e d , a f e a t u r e
which p a r t l y a c c o u n t s f o r i t s s c a r c i t y , The most abundant f e r r o m a g n e s i a n s i l i c a t e s i n s e d i m e n t s a r e pyroxenes and amphiboles.
SEM and p e t r o g r a p h i c e x a m i n a t i o n s show t h a t pyroxenes c h a r a c t e r i s t i c a l l y
dgvelop "cockscomb" o r "hacksaw" t e r m i n a t i o n s which a r e c o n s i d e r e d t o r e f l e c t p r e f e r e n t i a l d i s s o l u t i o n a l o n g p l a n e s t r a n s v e r s e t o t h e c - a x i s by Walker e t a l . (1978) ( F i g . 5 . 1 1 ) .
The d i s s o l u t i o n o f pyroxene has a l s o been d e s c r i b e d by Edelman
and Douglas (1931) b u t t h e s e a u t h o r s c o n s i d e r e d t h e "hacksaw" t e r m i n a t i o n s t o r e s u l t from a combination of d i s s o l u t i o n a l o n g c l e a v a g e p a r a l l e l t o t h e c - a x i s and c r y s t a l l o g r a p h i c p l a n e s a t 45-60'
t o the a-axis (Fig.5.11).
This explanation i s
more c o m p a t i b l e w i t h t h e s t r u c t u r e of pyroxene t h a n i s t h e e x p l a n a t i o n of Walker e t a l . (1978). The d i s s o l u t i o n b e h a v i o u r o f hornblende i s q u i t e d i f f e r e n t from pyroxene. Hornblende g r a i n s develop d e l i c a t e n e e d l e - l i k e t e r m i n a t i o n s which a r e p a r a l l e l t o the c-axis.
This r e s u l t s
amphibole double c h a i n .
b e c a u s e of p r e f e r e n t i a l d i s s o l u t i o n p a r a l l e l t o t h e
The l i n k s between a d j a c e n t c h a i n s c o n s i s t of a " f r e e
oxygen-cation bonds" and t h e s e r e p r e s e n t t h e p l a n e o f weakness a l o n g which d i s s o l u t i o n can proceed most r a p i d l y . The d i s s o l u t i o n of hornblende and pyroxene p r o b a b l y c o n t i n u e s f o r a l e a s t t e n s of m i l l i o n s o f y e a r s a f t e r d e p o s i t i o n .
I n t h e Cenozoic a l l u v i u m s t u d i e d by Walker
(1976) and Walker e t a l . (1978) r e l i c t s of t h e s e m i n e r a l s a r e common i n r e d beds which, on t h e b a s i s o f a s s o c i a t e d d a t e d b a s a l t s , were d e p o s i t e d between 9 and 26 My ago and a l s o o c c u r i n d e p o s i t s of Miocene a g e .
I n a n c i e n t r e d beds however d e t r i t a l
hornblende and pyroxene g r a i n s ( a l o n g w i t h o t h e r u n s t a b l e m i n e r a l s ) a r e e x t r e m e l y scarce.
T h i s has l o n g been a t t r i b u t e d t o i n t r a s t r a t a l s o l u t i o n ( P e t t i j o h n , 1941)
a c o n c l u s i o n which i s s u p p o r t e d by s t u d i e s of t h e m i n e r a l assemblages w i t h i n c a l c a r e o u s c o n c r e t i o n s ( B r a m l e t t e , 1941) and i n s h a l e s and a s s o c i a t e d s a n d s t o n e s ( B Z a t t and S u t h e r l a n d , 1969). There a r e a number of i m p o r t a n t consequences o f t h e d i s s o l u t i o n of d e t r i t a l s i l i c a t e grains.
I n t h e f i r s t p l a c e i t s u p p l i e s a n abundant s o u r c e of i o n s
i n c l u d i n g K , Na, Ca, A l , S i , Mg, F e , Mn t o t h e i n t e r s t i t i a l groundwater. of t h e s e a r e p r e c i p i t a t e d l a t e r a s a u t h i g e n i c p h a s e s . o f t h e sediment p r o v i d i n g t h a t
Many
I t a l s o increases the porosity
1) d i s s o l u t i o n v o i d s a r e n o t s u b s e q u e n t l y d e s t r o y e d
292
Fig.5.11. P a r t i a l l y d i s s o l v e d f e r r o m a g n e s i a n s i l i c a t e s i n Cenozoic a l l u v i u m . A . P e r i p h e r a l l y d i s s o l v e d h o r n b l e n d e w i t h w e l l - d e v e l o p e d d i s s o l u t i o n v o i d s (DV) and a c l a y s k i n (CS) marking t h e o r i g i n a l g r a i n boundary. A l s o n o t e t h e needlel i k e t e r m i n a t i o n s on t h e r e l i c t g r a i n . Thin s e c t i o n P l i o c e n e ? f a n g l o m e r a t e , Cazon Rojo, Baja C a l i f o r n i a . B. P a r t i a l l y d i s s o l v e d h o r n b l e n d e showing w e l l developed d i s s o l u t i o n n e e d l e s (DN) and d i s s o l u t i o n v o i d s (DV). SEM photomicrograph. Other d e t a i l s a s i n A. C. D e t a i l of hornblende d i s s o l u t i o n needles. Other d e t a i l s a s i n B. D . P a r t i a l l y d i s s o l v e d a u g i t e g r a i n showing well-developed cockscomb t e r m i n a t i o n s on r e l i c t g r a i n . SEM photomicrograph, Hayner Ranch Formation (Miocene) , New Mexico. Produced by c o u r t e s y of T.R. Walker.
293 by compactive c o l l a p s e o r
2 ) t h a t t h e p r e c i p i t a t i o n of s t a b l e a u t h i g e n i c phases
has n o t c o m p l e t e l y compensated f o r t h e newly c r e a t e d p o r o s i t y .
The m i n e r a l o g i c a l
m a t u r i t y o f t h e s e d i m e n t i s i n c r e a s e d because of t h e d i s s o l u t i o n of f e l d s p a r which incresses the quartz/feldspar r a t i o ,
The t e x t u r e i s a l s o changed because of t h e
r e d u c t i o n i n g r a i n s i z e o f t h e a f f e c t e d m i n e r a l s a n d t h i s may l e a d t o a n i n c r e a s e i n s i l t s i z e d p a r t i c l e s thereby decreasing the t e x t u r a l maturity.
The b u l k chemical
composition of t h e s e d i m e n t may a l s o be changed i f t h e r e l e a s e d i o n s a r e n o t prep r e c i p i t a t e d a s a u t h i g e n i c p h a s e s b u t c a r r i e d away by m i g r a t i n g groundwater. CLAY REPLACEMENT
The d i a g e n e t i c d i s s o l u t i o n of s i l i c a t e g r a i n s i s c l o s e l y a s s o c i a t e d w i t h t h e i n s i t u r e p l a c e m e n t by c l a y . a l l u v i u m (Walker e t a l . ,
T h i s i s a f e a t u r e commonly s e e n i n b o t h Cenozoic
1978) and a l s o i n s o i l s (Wilson, 1975).
The form o f t h e
r e p l a c e m e n t v a r i e s a n d may t a k e p l a c e i r r e g u l a r l y , p e r i p h e r a l l y o r p r e f e r e n t i a l l y a l o n g c r y s t a l l o g r a p h i c p l a n e s , i n c i p i e n t f r a c t u r e s , g r a i n c o n t a c t s i n rock fragments and o t h e r zones of weakness.
Replacement c l a y s a r e t h e r e f o r e a u t h i g e n i c i n o r i g i n
(Wilson and P i t t m a n , 1977) and t h e i r m i n e r a l o g i c a l composition depends upon t h e n a t u r e of t h e h o s t m i n e r a l s . F e l d s p a r s i n s a p r o l i t e s and s o i l s y i e l d a v a r i e t y of d a u g h t e r c l a y m i n e r a l s depending upon t h e e x a c t composition of t h e h o s t phase and t h e i n t e r s t i t i a l c o n d i t i o n s .
In s o i l s i t i s g e n e r a l l y a s s e r t e d t h a t f e l d s p a r a l t e r s d i r e c t l y t o i l l i t e b u t t h i s i s o f t e n d i f f i c u l t t o prove because o f t h e p r e s e n c e of a s s o c i a t e d s e r i c i t i c mica. The f o r m a t i o n of mica from f e l d s p a r s a p p e a r s t o be favoured i n c o n f i n e d e n v i r o n ments l i k e t h o s e e x i s t i n g i n s a p r o l i t e s ( K a t o , 1965).
T h i s i s because of t h e
r e t e n t i o n o f potassium i o n s which a r e e s s e n t i a l f o r t h e mica s t r u c t u r e .
In more
open systems s o l u b l e p o t a s s i u m i o n s a r e removed and t h e f o r m a t i o n of o t h e r c l a y m i n e r a l s such a s v e r m i c u l i t e o r m o n t m o r i l l o n i t e may t a k e p l a c e .
These m i n e r a l s
may be formed d i r e c t l y from f e l d s p a r o r from a s s o c i a t e d mica (Meilhac and Tardy, 1970).
The k a o l i n i z a t i o n of f e l d s p a r i s a complex p r o c e s s which i n v o l v e s t h e
f o r m a t i o n of k a o l i n i t e from a s d l u t i o n phase i n t e r m e d i a t e between p a r e n t f e l d s p a r and d a u g h t e r k a o l i n i t e .
A t l e a s t two morphologies a r e i n v o l v e d :
a p l a t y , book-
l i k e k a o l i n i t e forms under " w e a t h e r i n g c r u s t " c o n d i t i o n s i n which t h e geochemical c o n d i t i o n s p e r s i s t e d e s s e n t i a l l y a t e q u i l i b r i u m o v e r a v e r y long t i m e .
Elongate
k a o l i n s , o f t e n r e f e r r e d t o a s h a l l o y s i t e a p p a r e n t l y form under r e l a t i v e l y h i g h e r l a n d c o n d i t i o n s , o r where s h o r t e r t i m e k i n e t i c p r o c e s s e s were i n v o l v e d ( K e l l e r , 1978).
The f a c t t h a t k a o l i n i z a t i o n i s p r e c e d e d by a s o l u t i o n i n r e a c t i o n w i t h t h e
s o l i d p h a s e s i s i n d i c a t e d by t h e random o r i e n t a t i o n of k a o l i n i t e w i t h r e s p e c t t o t h e p a r e n t f e l d s p a r and a l s o by t h e c o r r o s i o n f e a t u r e s r e p o r t e d by Parharn (19691,
Keller (1976) and Berner and Holdren (1977). The a l t e r a t i o n p r o d u c t s o f t h e f e r r o m a g n e s i a n m i n e r a l s a r e n o t a s e x t e n s i v e l y
294
Fig.5.12. Thin s e c t i o n photomicrographs o f s i l i c a t e m i n e r a l s r e p l a c e d by c l a y i n Cenozoic a l l u v i u m . A. P l a g i o c l a s e g r a i n (P) r e p l a c e d i r r e g u l a r l y by c l a y (RC). P l i o c e n e ? f a n g l o m e r a t e , Cat%on Rojo, Baja C a l i f o r n i a . B. Hornblende g r a i n ( H ) showing p e r i p h e r a l r e p l a c e m e n t by c l a y (RC) which h a s been squeezed between o t h e r framework C . P l a g i o c l a s e g r a i n ( P ) showing p r e f e r e n t i a l c l a y g r a i n s . O t h e r d e t a i l s a s i n A. replacement a l o n g c l e a v a g e p l a n e s . Note t h e development of s i l t - s i z e d r e l i c t s of p l a g i o c l a s e . G i l a Group ( P l i o c e n e - P l e i s t o c e n e ) , Tucson, A r i z o n a . D . G r a n i t i c rock fragment now composed m a i n l y of c l a y (RC) which h a s r e p l a c e d a l l o r i g i n a l m i n e r a l s e x c e p t q u a r t z (Q). O r i g i n a l o u t l i n e of t h e r o c k fragment i s marked by a n o r i e n t e d c l a y s k i n . Tesuque Formation ( M i o c e n e - P l i o c e n e ) , New Mexico. Photomicrographs produced by c o u r t e s y of T.R. Walker.
studied a s the feldspars.
I n g e n e r a l , t h e y a l t e r t o magnesium-rich t r i o c t a h e d r a l
expandable m i n e r a l s and c h l o r i t e , o f t e n i n a mixed l a y e r a r r a n g e m e n t .
Basham
(1974) h a s s t u d i e d t h e a l t e r a t i o n of c l i n o - and ortho-pyroxenes i n some g a b b r o i c s a p r o l i t e s i n NE S c o t l a n d and was a b l e t o d e m o n s t r a t e t h e g r a d u a l replacement of pyroxene by v e r m u c u l i t e pseudomorphs.
The v e r m i c u l i t e shows a h i g h d e g r e e of
preferred orientation indicating, as i n dissolution, the close structural control of the host mineral. Although amphiboles a r e g e n e r a l l y c o n s i d e r e d t o be more r e s i s t a n t t o w e a t h e r i n g t h a n p y r o x e n e s , they n e v e r t h e l e s s y i e l d s i m i l a r a l t e r a t i o n p r o d u c t s .
Hornblende
t y p i c a l l y a l t e r s t o c h l o r i t e and s u b s e q u e n t l y t o mixed l a y e r c h l o r i t e - v e r m i c u l i t e i n s o i l s developed on u l t r a b a s i c m a t e r i a l ( S t e p h e n , 1952; K a t o , 1965).
Wilson and
Farmer (1970) demonstrated t h e a l t e r a t i o n of i r o n - r i c h l a m e l l a e i n hornblende t o mixed l a y e r c h l o r i t e - s a p o n i t e .
The d a u g h t e r c l a y showed no o r i e n t a t i o n r e l a t i o n s h i p
w i t h t h e p a r e n t hornblende o t h e r t h a n a tendency towards p a r a l l e l i s m w i t h t h e cleavage planes. I n t h e Cenozoic a l l u v i u m s t u d i e d by Walker e t a l . (1978) m i g r a t i n g ground w a t e r s e n s u r e a more open type o f system t h a n t h a t found i n most s o i l s and s a p r o l i t e s . The r e s u l t i s a s i m p l e c l a y mineralogy c h a r a c t e r i s t i c a l l y c o n s i s t i n g of randomly i n t e r s t r a t i f i e d mixed l a y e r i l l i t e - m o n t m o r i l l o n i t e (Reynolds and Hower, 1970).
The
c o n t a c t between t h e d a u g h t e r c l a y and r e l i c t p a r e n t m i n e r a l a r e normally s h a r p and t h e r e i s no t r a n s i t i o n a l zone of p a r t i a l r e p l a c e m e n t a t t h e margins of t h e r e l i c s . There a p p e a r s t o be no c l o s e s t r u c t u r a l r e l a t i o n s h i p between t h e o r i e n t a t i o n of t h e d a u g h t e r c l a y s and p a r e n t r e l i c and t h i s , t o g e t h e r w i t h t h e o t h e r f e a t u r e s , s u g g e s t s t h a t t h e replacement c l a y i s formed from a s o l u t i o n i n r e a c t i o n w i t h t h e p a r e n t a 1l u v i um ( F i g .5.12 )
.
The f o r m a t i o n o f replacement c l a y i n t h i s way has s e v e r a l i m p o r t a n t e f f e c t s . The amount of i n t e r s t i t i a l m a t r i x i s g r e a t l y i n c r e a s e d and t h i s d e c r e a s e s t h e t e x t u r a l m a t u r i t y of t h e s e d i m e n t s d i a g e n e t i c a l l y .
Even i n Cenozoic a l l u v i u m c l a y
of replacement o r i g i n may o f t e n comprise more t h a n 15% by volume of t h e rock and produce a g r e y wacke-type t e x t u r e i n which p o o r l y s o r t e d a n g u l a r a n d s u b a n g u l a r grains r e s t i n a clay matrix.
T h i s thus p r o v i d e s c o n v i n c i n g s u p p o r t f o r t h e
d i a g e n e t i c o r i g i n o f t h e greywacke t e x t u r e s (Cummins, 1962; Whetten and Hawkins, 1970).
Completely pseudomorphous r e p l a c e m e n t can o c c u r i n t h i s manner b u t t h e s e
a r e seldom p r e s e r v e d because t h e s o f t c l a y y i e l d s t o compactive p r e s s u r e and i s r e a d i l y squeezed between g r a i n t o become i n d i s t i n g u i s h a b l e from i n t e r s t i t i a l m a t r i x . T h i s t y p e of m a t r i x i s r e f e r r e d t o a s pseudomatrix by Dickinson
(1970a).
Any
evidence t h a t t h e d a u g h t e r c l a y formed by t h e replacement of framework s i l i c a t e s becomes o b s c u r e d w i t h time a s t h e d e g r e e of a l t e r a t i o n and d e p t h of b u r i a l i n c r e a s e s . The i d e n t i f i c a t i o n of replacement c l a y i n a n c i e n t r e d beds i s coneequently v e r y d i f f i c u l t t o prove.
296 Another i m p o r t a n t a f f e c t of t h e c l a y r e p l a c e m e n t p r o c e s s i s t h e r e d u c t i o n i n a v e r a g e g r a i n s i z e of t h e framework c o n s t i t u e n t s because o f t h e tendency f o r framework g r a i n s t o become broken down by incomplete r e p l a c e m e n t . p a r t i c u l a r s u f f e r s from t h i s p r o c e s s .
Plagioclase i n
G r a n i t i c r o c k fragments may be e x t e n s i v e l y
d i s a g g r e g a t e d because o f t h e s e l e c t i v e r e p l a c e m e n t of t h e u n s t a b l e m i n e r a l components ( F i g . 5 . 1 2 ) .
Even e x p e c t e d l y s t a b l e rock fragments s u c h a s q u a r t z i t e
fragments may b e d i s a g g r e g a t e d b e c a u s e of t h e r e p l a c e m e n t of c l a y a l o n g g r a i n boundaries.
The replacement p r o c e s s s i g n i f i c a n t l y changes t h e m i n e r a l composition
o f t h e sediments because o f t h e s e l e c t i v e removal u n s t a b l e s i l i c a t e s l i k e p l a g i o c l a s e and t h e ferromagnesian m i n e r a l s .
This r e s u l t s i n an increase i n the proportion of
t h e s t a b l e m i n e r a l s such a s q u a r t z and t h e r e b y i n c r e a s e s t h e m i n e r a l o g i c a l m a t u r i t y d i a g e n e t i c a l l y i n t h e same way a s d i s s o l u t i o n . One of t h e most i m p o r t a n t a s p e c t s of t h e c l a y r e p l a c e m e n t p r o c e s s i s t h e a s s o c i a t e d r e l e a s e of c e r t a i n elements i n t o t h e groundwater c i r c u l a t i o n v i a t h e d a u g h t e r c l a y . There have been numerous s t u d i e s o f t h e geochemical e f f e c t s o f w e a t h e r i n g ( e . g . G o l d i c h , 1938; Brock, 1943; K e l l e r , 1957; H a r r i s and Adams, 1966; Anderson and Wiklander, 1975).
I n s o i l s and s a p r o l i t e s i t i s commonly o b s e r v e d t h a t i n t h e
m o b i l i z a t i o n and r e d i s t r i b u t i o n of c a t i o n s d u r i n g w e a t h e r i n g t h e a l k a l i e s , p a r t i c u l a r l y sodium, and t h e a l k a l i n e e a r t h m e t a l s a r e more mobile t h a n many of t h e o t h e r c a t i o n s commonly found i n s i l i c a t e l a t t i c e s .
A s i m i l a r p a t t e r n emerges from t h e
geochemical s t u d y of t h e breakdown o f s i l i c a t e m i n e r a l s i n Cenozoic a l l u v i u m by Walker (1967a) and Walker e t a l . (1978).
These a u t h o r s have p a i d p a r t i c u l a r a t t e n t i o n
t o t h e geochemical a l t e r a t i o n of hornblende s i n c e t h i s i s a n i m p o r t a n t s o u r c e of i r o n f o r m i g r a t i n g groundwaters.
A combination of e l e c t r o n p r o b e m i c r d a n a l y s i s
SEM, X-ray d i f f r a c t i o n and c o n v e n t i o n a l p e t r o g r a p h y have been used t o d e m o n s t r a t e t h e s e l e c t i v e removal of c a l c i u m , magnesium and i r o n from t h e p a r e n t h o r n b l e n d e w i t h a c o r r e s p o n d i n g i n c r e a s e i n aluminium and s i l i c o n ( F i g . 5 . 1 3 ) . The d a u g h t e r c l a y (mixed l a y e r i l l i t e - m o n t m o r i l l o n i t e ) i s i r o n - r i c h and quant i t a t i v e a n a l y s e s of hornblende g r a i n s i n P l i o c e n e r e d beds by Walker e t a l . (1967) i n d i c a t e t h a t t h e i r o n c o n t e n t of t h e c l a y r a n g e s from 2.5% t o 16.5% a d j a c e n t t o decomposing g r a i n s .
F i g . 5 . 1 4 shows a s c h e m a t i c diagram o f a p a r t i a l l y r e p l a c e d
hornblende g r a i n ( G r a i n 1) w i t h t h e p o s i t i o n s of microprobe p o i n t a n a l y s e s ( T r a v e r s e
1).
The d i s t r i b u t i o n of major o x i d e s w i t h i n c r e a s i n g d i s t a n c e from t h e p a r e n t
hornblende a r e a l s o shown i n t h i s diagram.
These r e v e a l t h a t t h e d a u g h t e r c l a y
c o n t a i n s 10.8% Fe 0 n e a r t h e p a r e n t hornblende b u t t h i s d e c r e a s e s s t e a d i l y away 2 3 from t h e p a r e n t hornblende t o 3.5% a t a d i s t a n c e of 2 2 0 m. Thin s e c t i o n s of t h e s e P l i o c e n e r e d beds r e v e a l a l l s t a g e s of s t a i n i n g o f t h e a u t h i g e n i c i l l i t e - m o n t m o r i l l o n i t e by i r o n o x i d e s .
I n i t i a l l y t h e s t a i n i s c o n c e n t r a t e d aoong f r a c t u r e s i n t h e
c l a y and on t h e p e r i p h e r y o f t h e a l t e r e d g r a i n s
i s d i f f u s e d throughout t h e c l a y .
,
i n more advanced s t a g e s t h e s t a i n
The outward d e p l e t i o n of i r o n h r s been c l e a r l y
demonstrated i n a number of c a s e s and i t seems r e a s o n a b l e t o cnnclude t h a t t h i s i o n
297
I CLAY I
HORNBLENDE
I CLAY
Fig.5.13. F i v e element microprobe a n a l y s i s a c r o s s a hornblende g r a i n p e r i p h e r a l l y r e p l a c e d by c l a y i n Cenozoic a l l u v i u m , G i l a Group, Tuscon Arizona ( a f t e r Walker e t a l . , 1978).
i s a p o t e n t i a l s o u r c e o f pigmentary h a e m a t i t e a t l h o u g h i t i s f a i r t o s a y t h a t t h e e x a c t manner i n which p a r t i c u l a t e h a e m a t i t e g r a i n c o a t i n g s and e u h e d r a l c r y s t a l s of h a e m a t i t e form from i r o n - s t a i n e d c l a y i s n o t y e t e s t a b l i s h e d .
Much of t h e
i r o n o x i d e may be t r a n s p o r t e d a s amorphous Fe(OH)3 i n f i n e l y p a r t i c u l a t e form (Langmuir and Whittemore, 1971) and s u b s e q u e n t l y be p r e c i p i t a t e d a s g o e t h i t e (d(-FeOOH) o r h a e m a t i t e (W-Fe 0 ) depending upon t h e p r e c i s e geochemical c o n d i t i o n s .
2 3
The d a u g h t e r c l a y i s much d e p l e t e d i n CaO and MgO when compared w i t h t h e p a r e n t hornblende ( F i g s . 5 . 1 3 , 5 . 1 4 ) b u t t h r o u g h o u t t h e d a u g h t e r c l a y t h e y remain c o n s t a n t ; i n t h e s e o x i d e s t h e l o s s i s more r a p i d and t a k e s p l a c e a t t h e i n t e r f a c e between t h e hornblende and t h e c l a y .
A s y e t , comparative q u a n t i t a t i v e d a t a a r e n o t a v a i l a b l e f o r t h e o t h e r m i n e r a l s i n Cenozoic a l l u v i u m such a s p l a g i o c l a s e and p o t a s h f e l d s p a r .
However, t h e r e l e a s e
of e l e m e n t s from t h e s e m i n e r a l s i s w e l l documented i n t h e s u r f i c i a l w e a t h e r i n g of igneous r o c k s ( H a r r i s and Adams, 1966) and e x p e r i m e n t a l l y (Andersson and Wiklander, 1975) and t h e r e can be l i t t l e doubt t h a t t h e s e d a t a a r e a p p l i c a b l e i n a g e n e r a l way t o t h e p h r e a t i c and v a d o s e e a r l y d i a g e n e s i s i n Cenozoic a l l u v i u m .
A s a whole, t h e
breakdown and r e p l a c e m e n t of t h e s e m i n e r a l s r e l e a s e s a v a r i e t y of e l e m e n t s i n c l u d i n g sodium, p o t a s s i u m , c a l c i u m , magnesium and i r o n i n t o t h e i n t e r s t i t i a l groundwater. Depending upon t h e chemical c o n d i t i o n s i n t h e groundwater, t h e r e l e a s e d i o n s m y e i t h e r be c a r r i e d away i n s o l u t i o n o r p r e c i p i t a t e d a s a u t h i g e n i c m i n e r a l s .
298
I
/
S
20
I
I
1
I
40
60
80
100
I
I
I
I
I
1
120 140 160 180 200 220
Distance from hornblende in p
m
F i g . 5 . 1 4 . D i s t r i b u t i o n of major o x i d e s i n a hornblende g r a i n p a r t i a l l y r e p l a c e d by a u t h i g e n i c c l a y . D e t e r m i n a t i o n s by e l e c t r o n probe m i c r o a n a l y s i s . The a s t e r i s k i n d i c a t e s w e i g h t p e r c e n t i n t h e p a r e n t h o r n b l e n d e . T r a v e r s e No.1 and G r a i n No.1 of Walker e t a l . ( 1 9 6 7 ) .
S i g n i f i c a n t removal o f i o n s i n groundwater w i l l o b v i o u s l y a l t e r t h e b u l k c h e m i s t r y of t h e s e d i m e n t from i t s o r i g i n a l composition. I n a n c i e n t r e d beds c l a y r e p l a c e m e n t can a l s o be demonstrated t o have been o f considerable importance.
P o t a s h f e l d s p a r s i n p a r t i c u l a r o f t e n show e v i d e n c e of
more e x t e n s i v e c l a y r e p l a c e m e n t t h a n t h a t s e e n i n Cenozoic a l l u v i u m and i t seems r e a s o n a b l e t o assume t h a t t h i s c o u l d be due t o more prolonged exposure t o i n t e r s t i t i a l fluids.
There i s some d i f f i c u l t y , however, i n making r e a l i s t i c e s t i m a t e s
of t h e q u a n t i t a t i v e importance o f c l a y r e p l a c e m e n t d u r i n g d i a g e n e s i s .
This a r i s e s
b e c a u s e o f t h e d i f f i c u l t y of d i s t i n g u i s h i n g c l a y replacement which h a s t a k e n p l a c e d u r i n g w e a t h e r i n g i n t h e s o u r c e a r e a and t h a t which h a s o c c u r r e d p o s t - d e p o s i t i o n a l l y . V a r i a t i o n s i n t h e normal sequence o f f e l d s p a r s t a b i l i t y may p r o v i d e a n i m p o r t a n t c l u e t o s o u r c e a r e a w e a t h e r i n g ( e . g . Todd, 1968) b u t u s u a l l y t h e replacement p r o c e s s f o l l o w s t h e same sequence d u r i n g s o u r c e a r e a w e a t h e r i n g and d i a g e n e s i s .
Despite t h i s
d i f f i c u l t y i t seems l i k e l y t h a t t h e abundance o f h e a v i l y a l t e r e d f e l d s p a r s i n a n c i e n t r e d beds i s a t l e a s t p a r t l y due t o i n t r a s t r a t a l c l a y replacement.
The
dominant c l a y m i n e r a l s i n a n c i e n t r e d beds a r e i l l i t e and c h l o r i t e , a f e a t u r e which s u g g e s t s t h a t t h e mixed l a y e r illite-montmorrilonite-illite t r a n s f o r m a t i o n i s a f e a t u r e o f l a t e r r e d bed d i a g e n e s i s . AUTHIGENIC MINERALS
The d i s s o l u t i o n and r e p l a c e m e n t of d e t r i t a l s i l i c a t e g r a i n s r e s u l t s i n t h e r e l e a s e of chemical c o n s t i t u e n t s i n t o t h e i n t e r s t i t i a l groundwater s o l u t i o n s .
The
f a t e o f t h e s e r e l e a s e d i o n s depends upon t h e i n t e r s t i t i a l physico-chemical c o n d i t i o n s . The i o n s may be s o l u b l e and be c a r r i e d away i n t h e m i g r a t i n g groundwater o r , under f a v o u r a b l e c i r c u m s t a n c e s , t h e y may be p r e c i p i t a t e d a s a n a u t h i g e n i c m i n e r a l phase. I n t h e Cenozoic r e d beds s t u d i e d by Walker e t a l . (1978) a v a r i e t y o f a u t h i g e n i c p h a s e s i n c l u d i n g q u a r t z , c l a y m i n e r a l s , z e o l i t e s , f e l d s p a r s , h a e m a t i t e , and c a l c i t e have been p r e c i p i t a t e d i n t h i s manner.
This authigenic s u i t e is a l s o a character-
i s t i c o f a n c i e n t r e d beds and i s b e l i e v e d t o have formed i n a s i m i l a r manner. Potassium f e l d s u a r A u t h i g e n i c p o t a s h f e l d s p a r i s one of t h e most commonly o c c u r r i n g d i a g e n e t i c phases i n c o n t i n e n t a l r e d beds.
I n t h e Cenozoic a l l u v i u m of SW United and NW Mexico
t h e a u t h i g e n i c p o t a s h f e l d s p a r o c c u r s i n p a r t a s overgrowths o r d e t r i t a l g r a i n s and a l s o a s s m a l l c r y s t a l s w i t h i n t h e i n t e r s t i t i a l m a t r i x i n a s s o c i a t i o n w i t h mechanically i n f i l t r a t e d c l a y and a u t h i g e n i c c l a y and q u a r t z .
I n p l a c e s , a g g r e g a t e s of f e l d s p a r
c r y s t a l s may f i l l p o r e s p a c e s o r d i s s o l u t i o n v o i d s . A u t h i g e n i c p o t a s h f e l d s p a r i s a l s o a common c o n s t i t u e n t i n t h e a l t e r e d t u f f s and t u f f a c e o u s s a n d s t o n e s of f l u v i a l and l a c u s t r i n e o r i g i n i n t h e w e s t e r n United S t a t e s (Hay, 1966; Sheppard and Gude, 1968, 1969, 1973; Moiola, 1970).
Here i t o c c u r s
300
Fig.5.15. SEM photomicrographs of a u t h i g e n i c f e l d s p a r i n Cenozoic a l l u v i u m A . A u t h i g e n i c p o t a s s i u m f e l d s p a r on d e t r i t a l (A-C) and T r i a s s i c r e d beds (D-F). g r a i n , P l i o c e n e f a n g l o m e r a t e , CaEon Rojo, Baja C a l i f o r n i a . B. Red i n t e r s t i t i a l m a t r i x w i t h a u t h i g e n i c f e l d s p a r , same l o c a l i t y . C . Enlargement of o u t l i n e d a r e a i n B. D . O r i e n t e d c r y s t a l s of a u t h i g e n i c f e l d s p a r on t h e s u r f a c e of a d e t r i t a l f e l d s p a r i n a n e a r l y s t a g e o f overgrowth f o r m a t i o n . E . A more advanced s t a g e c o m p l e t e l y c o v e r i n g t h e g r a i n s u r f a c e . F. A complete overgrowth of p u r e potassium f e l d s p a r w i t h l a r g e , smooth c r y s t a l f a c e s . A l l from Lower Keuper S a n d s t o n e ( T r i a s s i c ) , West Midlands, U.K. A-C produced by c o u r t e s y o f T.R. Walker and D-F by c o u r t e s y of A.D. A l i .
301 predominantly a s overgrowths on g r a i n s of o r t h o c l a s e and o c c a s i o n a l l y p l a g i o c l a s e b u t a l s o a s v e r y f i n e g r a i n e d , low b i r e f r i n g e n t a g g r e g a t e s (Hay and Moiola, 1963). The f e l d s p a r i s o f t e n i n a s s o c i a t i o n w i t h a u t h i g e n i c z e o l i t e s ( a n a l c i m e ) which i n p l a c e s form a s t r a t i g r a p h i c a l l y zoned sequence ( s e e F i g . 5 . 1 6 ) .
The f o r m a t i o n of
a u t h i g e n i c K - f e l d s p a r i n v o l c a n i c l a s t i c d e p o s i t s i s s t i m u l a t e d by r e l a t i v e l y high a l k a l i - i o n t o hydrogen-ion a c t i v i t y r a t i o s and by r e l a t i v e l y h i g h s i l i c a a c t i v i t i e s (Hemley, 1959, 1962; Hay and M o i o l a , 1963, 1964; Mioola and Hay, 1 9 6 4 ) . The z o n a t i o n s e e n i n t h e Esmeralda Formation
(Fig.5.16):
analcime
+
feldspar,
c l i n o p t i l o l i t e , and p h i l l i p s i t e i s a p p a r e n t l y n o t t h e r e s u l t o f i n c r e a s e d hydros t a t i c p r e s s u r e o r t e m p e r a t u r e w i t h d e p t h of b u r i a l b u t i s t h o u g h t by Moiola (1970) t o r e f l e c t d i f f e r e n c e s i n composition of t h e p r e c i p i t a t i n g groundwater. A l l the a v a i l a b l e evidence suggests t h a t the authigenic feldspar i s pure potash f e l d s p a r ( s e e f o r example, Hay and M o i o l a , 1963, F i g . 3 p . 324).
This i s consistent
w i t h t h e r e s u l t s o f a n a l y s e s of a u t h i g e n i c K - f e l d s p a r i n a n c i e n t r e d beds which a r e commonly p u r e p o t a s s i u m f e l d s p a r (Waugh, 1978). A u t h i g e n i c K - f e l d s p a r i s known from many a n c i e n t r e d bed sequences i n c l u d i n g t h e T o r r i d o n i a n Sandstone (Waugh, 1 9 7 8 ) , t h e Old Red Sandstone and p a r t i c u l a r l y t h e Permo-Triassic S a n d s t o n e s o f n o r t h e r n Europe (Reynolds, 1929; W i l l i a m s , 1973; G l e n n i e e t a l . , 1978; K e s s l e r , 1978; Hancock, 1 9 7 8 ) .
I t s h o u l d be n o t e d t h a t
a u t h i g e n i c f e l d s p a r a l s o o c c u r s i n a number o f marine f a c i e s , i n p a r t i c u l a r limes t o n e s ( K a s t n e r , 1971; Buyce and Friedman, 1975). Waugh (1978) h a s made a d e t a i l e d s t u d y o f a u t h i g e n i c K - f e l d s p a r i n B r i t i s h Permo-Triassic Sandstones.
I n t h i n s e c t i o n s t h e s e a r e developed a s overgrowths
around d e t r i t a l c o r e s o f o r t h o c l a s e o r m i c r o c l i n e , o r v e r y r a r e l y , p l a g i o c l a s e . Discrete authigenic c r y s t a l s with an a d u l a r i a n h a b i t a l s o occur within the diss o l u t i o n v o i d s of d e t r i t a l K - f e l d s p a r g r a i n s .
These a u t h i g e n i c c r y s t a l s a r e
randomly o r i e n t e d and i n d i c a t e t h a t d i s s o l u t i o n p r e d a t e s t h e i r p r e c i p i t a t i o n and als'o t h a t a d e t r i t a l s e e d c r y s t a l i s n o t n e c e s s a r y f o r t h e f o r m a t i o n of a u t h i g e n i c K - f e l d s p a r ( c f . K a s t n e r , 1971).
The overgrowths c o n s i s t of p r i s m f a c e s , u s u a l l y
developed p a r a l l e l t o l o n g a x e s of g r a i n s , t r u n c a t e d by t h e b a s a l p i n a c o i d .
The
overgrowths o f t e n d i s p l a y "hacksaw" p r o j e c t i o n s w i t h a d i s t i n c t rhombohedral o u t l i n e , p a r t i c u l a r l y a t t h e d i s t a l ends o f e l o n g a t e g r a i n s .
Those evergrowths
p o s s e s s i n g a h i g h d e g r e e o f c r y s t a l l i n e p e r f e c t i o n show a n o v e r a l l s h a p e which i s markedly rhombohedral.
SEM s t u d i e s w i t h e n e r g y - d i s p e r s i v e a n a l y s i s of X-rays
(EDAX) and e l e c t r o n microprobe a n a l y s i s by Waugh (1978) i n d i c a t e t h a t t h e overgrowths a r e v i r t u a l l y p u r e p o t a s s i u m f e l d s p a r t h e a v e r a g e o f 28 overgrowths b e i n g K 0 = 2 16.94%, A1203 = 18.45% and S i 0 2 = 64.41% and t h e composition of any i n d i v i d u a l overgrowth d i d n o t v a r y by more t h a n
_+
0.5% from t h e above a v e r a g e s .
The a n a l y s e s
i n d i c a t e t h a t t h e overgrowths a r e p u r e , s t o i c h i o m e t r i c K A l S i 0 w i t h a n o p t i c a x i a l 3 8 a n g l e (-2V) r a n g i n g from 20-25'. Using t h e nomenclature o f Smith (1974) t h e f e l d s p a r overgrowths can be c l a s s i f i e d a s p o t a s s i a n i n t e r m e d i a t e s a n i d i n e s . Shenhav (1971)
302 h a s a l s o d e s c r i b e d a u t h i g e n i c . o v e r g r o w t h s o f s a n i d i n e from Lower C r e t a c e o u s s a n d stones of I s r a e l . Waugh ( 1 9 7 8 ) r e c o g n i z e d a s e r i e s o f d i s t i n c t g r o w t h s t a g e s i s t h e d e v e l o p m e n t of t h e potash f e l d s p a r overgrowths.
The i n i t i a l g r o w t h s t a g e r e c o g n i z e d i n t h e
d e v e l o p m e n t o f s m a l l , f l a t t e n e d r h o m b o h e d r a l c r y s t a l s v a r y i n g from 3-30/rm i n l e n g t h (Fig.5.15).
The c r y s t a l s a r e c o m b i n a t i o n s o f t h e b a s a l p i n a c o i d ( 0 0 1 ) a n d p r i s m
(110), w i t h ( 0 1 0 ) b e i n g v e r y n a r r o w o r u s u a l l y a b s e n t . so-called
T h i s morphology i s t h e
' a d u l a r i a ' h a b i t ( B a s k i n , 1956) b u t t h e u s e of t h i s term i s u s u a l l y u s e d
a s a m o r p h o l o g i c a l term a n d n o t a s a d i s t i n c t v a r i e t y of K - f e l d s p a r .
A t this stage
o f d e v e l o p m e n t t h e s e p s e u d o - r h o m b o h e d r a 1 c r y s t a l s show a p r o n o u n c e d p r e f e r r e d o r i e n t a t i o n b e c a u s e o f t h e c r y s t a l l o g r a p h i c c o n t r o l e x e r t e d by t h e h o s t f e l d s p a r .
In t h e e a r l y s t a g e s t h e s e a d u l a r i a - l i k e rhombs a r e s p a r s e l y s c a t t e r e d o v e r t h e d e t r i t a l g r a i n s u r f a c e b u t a s growth proceeds t h e i r s u r f a c e d e n s i t y i n c r e a s e s .
The
Permian a e o l i a n sands such a s t h e Bridgnorth and Mauchline sandstones d i s p l a y t h i s g r o w t h s t a g e v e r y w e l l ; i n t h e f o r m e r c a s e t h e a u t h i g e n i c f e l d s p a r h a s grown d i r e c t l y on t h e d e t r i t a l clay-oxide p e l l i c l e and i n the l a t t e r c a s e t h e a u t h i g e n i c f e l d s p a r h a s grown o v e r a u t h i g e n i c i l l i t e c l e a r l y i n d i c a t i n g t h a t f e l d s p a r p r e c i p i t a t i o n postdates the formation of a u t h i g e n i c c l a y .
The s u b s e q u e n t g r o w t h
p h a s e s i n t h e development o f complete overgrowths i n v o l v e a n i n c r e a s e i n t h e n u m e r i c a l d e n s i t y o f t h e f e l d s p a r rhombs a n d t h e i r m e r g i n g t o g e t h e r t o p r o d u c e l a r g e r smooth c r y s t a l f a c e s .
A t t h i s s t a g e t h e r h o m b o h e d r a l m i c r o s t r u c t u r e may
s t i l l b e d i s c e r n i b l e a n d l a r g e a r e a s o f t h e d e t r i t a l g r a i n s u r f a c e may b e s t i l l f r e e from a u t h i g e n i c f e l d s p a r .
Growth o c c u r s o v e r l a r g e r a r e a s by t h e e x t e n s i o n
o f t h e i n d i v i d u a l c r y s t a l f a c e s w h i c h may r e t a i n a d e g r e e o f i r r e g u l a r i t y b e c a u s e o f t h e i n c o m p l e t e o v e r l a p a n d s t a c k i n g of t h e r h o m b o d e d r a l u n i t s .
A t this stage
t h e o v e r a l l rhombohedral morphology o f t h e complete overgrowt h i s s e e n ; t h e a u t h i g e n i c c r y s t a l s r a n g e i n s i z e from 0 . 2 5 t o 1 . O O m m i n l e n g t h .
The c r y s t a l s
f r e q u e n t l y show hacksaw t e r m i n a t i o n s ( F i g . 5 . 1 5 ) . Zeolites A u t h i g e n i c z e o l i t e s a r e well-known i n C e n o z o i c c o n t i n e n t a l d e p o s i t s w h e r e t h e y occur a s euhedral c r y s t a l s i n pore spaces of f l u v i a l d e p o s i t s and i n p a r t i c u l a r a s a replacement of v i t r i c fragments i n l a c u s t r i n e r h y o l i t i c t u f f s .
The z e o l i t e s a r e
h y d r a t e d a l u m i n o s i l i c a t e s of t h e a l k a l i and a l k a l i n e e a r t h m e t a l s w i t h a n i n f i n i t e l y e x t e n d e d t h r e e - d i m e n s i o n a l a n i o n network and t h u s have t h e a t o m i c r a t i o 0 : (Al+Si) =
2.
They i n c l u d e a w i d e v a r i e t y o f m i n e r a l s p e c i e s ( s e e D e e r , Howie a n d Zussman,
1 9 6 6 , p . 393) b u t t h e o n e s o c c u r r i n g more commonly i n c o n t i n e n t a l d e p o s i t s a r e l i s t e d i n Table 5 . 6 .
Hay ( 1 9 6 3 , 1 9 6 4 ) a n d Hemley ( 1 9 5 9 , 1 9 6 2 ) h a v e shown t h a t
z e o l i t e f o r m a t i o n i s f a v o u r e d by r e l a t i v e l y h i g h a l k a l i i o n / h y d r o g e n i o n a c t i v i t y r a t i o s a n d by r e l a t i v e l y h i g h s i l i c a a c t i v i t i e s .
303 TABLE 5 . 6
I
The i d e a l formulae o f z e o l i t e s which commonly o c c u r i n c o n t i n e n t a l d e p o s i t s . Name
Formula
Analcime
Na
A1Si206
Chabazite
Ca
A12Si4012
Clinoptilolite
(Ca ,K2Na2)
Erionite
(Na2,K2,Ca,Mg)4.5
Mordenite
(Na2,K2,Ca)
A12Si10024
Phillipsite
(&a ,Na ,K)3
A12Si5016
H2° 6H20
A12Si20024
5H20
AlgSi27072 7H20 6H20
I n t h e f l u v i a l d e p o s i t s s t u d i e d by Walker e t a l . (1978) a u t h i g e n i c z e o l i t e s i n t h e form of c l i n o p t i l o l i t e o c c u r wherever v o l c a n i c l a s t i c d e t r i t u s i s a b u n d a n t . I t may l o c a l l y be t h e main cementing a g e n t , b u t more commonly o c c u r s a s s m a l l ,
c l e a r e u h e d r a l c r y s t a l s w i t h low b i r e f r i n g e n c e and h i g h r e l i e f which l i n e i n t e r s t i t i a l voids (Fig.5.17).
The f o r m a t i o n o f z e o l i t e s d u r i n g d i a g e n e s i s i s p a r t i c u l a r l y
f a v o u r e d i n t h e p r e s e n c e of v i t r i c d e t r i t u s , a l t h o u g h t h i s i s n o t t h e o r e t i c a l l y an a b s o l u t e n e c e s s i t y (Hay, 1966, p . 8 3 ) . The c l o s e a s s o c i a t i o n of z e o l i t e a u t h i g e n e s i s and v i t r i c d e t r i t u s i s w e l l - s e e n i n numerous examples i n t h e Cenozoic f l u v i a t i l e and l a c u s t r i n e d e p o s i t s of t h e w e s t e r n U n i t e d S t a t e s (Hay, 1963, 1966, Moiola and Hay, 1964; Sheppard, 1971, Sheppafd and Gude, 1968, 1969, 1973; Moiola, 1970; Walton, 1975).
The z e o l i t e s a r e
most abundant i n a l t e r e d v i t r i c t u f f s and t u f f a c e o u s s a n d s t o n e s and a r e g e n e r a l l y a g r e e d t o have formed d u r i n g d i a g e n e s i s by t h e r e a c t i o n of v o l c a n i c g l a s s w i t h i n t e r s t i t i a l w a t e r , which o r i g i n a t e d a s m e t e o r i c w a t e r (Hay, 1963) o r t h e connate w a t e r of a s a l i n e l a k e ( B r a d l e y , 1929; Hay, 1 9 6 4 ) .
The z e o l i t e m i n e r a l s a r e u s u a l l y
a s s o c i a t e d w i t h o t h e r a u t h i g e n i c m i n e r a l s s u c h a s m o n t m o r i l l o n i t e and K - f e l d s p a r and may show a l a t e r a l l y , o r v e r t i c a l l y zoned s u c c e s s i o n .
The Esmeralda Formation,
f o r example, a L a t e Cenozoic (Miocene-Pliocene) sequence of f l u v i a l and l a c u s t r i n e r o c k s shows a v e r t i c a l z o n a t i o n from t h e bottom upwards of analcime c l i n o p t i l o l i t e and p h i l l i p s i t e ( F i g . 5 . 1 6 )
+ K-feldspar,
(Moiola, 1970).
A n a l c i n e o n l y o c c u r s n e a r t h e b a s e of t h e Esmeralda F o r m a t i o n , i n a s s o c i a t i o n with authigenic K-feldspar.
I t o c c u r s a s c l e a r , s u b h e d r a l - e u h e d r a l c r y s t a l s which
show no e v i d e n c e t h a t i t formed d i r e c t l y from v i t r i c m a t e r i a l o r from t h e r e placement of a n a l k a l i - r i c h z e o l i t e p r e c u r s o r s u c h a s c l i n o p t i l o l i t e ( c f . Hay, 1966; Goodwin and Surdam, 1967; Sheppard and Gude, 1 9 6 9 ) .
I t i s more p r o b a b l e
t h a t t h e analcime p r e c i p i t a t e d d i r e c t l y e i t h e r from t r u e s o l u t i o n o r a n i n t e r m e d i a t e a l u m i n o s i l i c a t e g e l (Marimer and Surdam, 1 9 7 0 ) .
304
m
Gray sandstones siltstones and conglomarmr with whim vkric tuffs with basalt flow and tuff-bnocia
PHILLIPSITE
8000
7m
Pale orange to lightgrey sandstones siltstoms 8nd shales with light gray vitric tuffs.
6OOC I-
w
W LL
z
5000
9z
Greyishoranga and greyish-yellow siltsWnes with white-grey sandstones and yellowish-gray shales intartonging with grayish-orange pebble conglomerates limestone brlccia and coarse sandstones
CLINOPTILOLITE
4000 I I-
3MM
Light grey to grayish-orenge volcanic pebble conglomerates undstones and siltstones and grey sandstones with thin tuffs.
2000
1000
0
Whitr to pda orange t u f f u c ~ ~and l s siliaous sandstones with greyish-orange shdr and yellowish-brown SandstOniL Pde ydlowish-orange to greyish-yellowfaldtprthic and lithk sandstones and siltstones Light brown to grwnish-gray pebble c o n g l ~ r a t wand brrcciu with thin interbods of sandstons siltstone and limrstoner
ANALCIME+ K-FELDSPAR
Fig.5.16. S t r a t i g r a p h y and z e o l i t e z o n a t i o n o f t h e Esmeralda Formation (MiocenePliocene) ( a f t e r Moiola, 1970). The main p a r t o f t h e Esmeralda Formation i s c h a r a c t e r i z e d by c l i n o p t i l o l i t e which o c c u r s c h i e f l y a s a pseudomorphic r e p l a c e m e n t o f g l a s s b u t a l s o a s a cement i n sandstones.
The pseudomorphic r e p l a c e m e n t s u g g e s t s t h a t d i s s o l u t i o n o f g l a s s
s h a r d s preceded t h e c r y s t a l l i z a t i o n o f c l i n o p t i l o l i t e .
The r e a s o n s b e i n g t h a t t h e
l a r g e r pseudomorphs a r e g e n e r a l l y hollow and c h a r a c t e r i s t i c a l l y c o n s i s t of a v e r y t h i n m a r g i n a l f i l m of m o n t m o r i l l o n i t e o r t h i n l a y e r of o p a l , succeeded i n w a r d l y by a thin layer of c l i n o p t i l o l i t e c r y s t a l s oriented perpendicular
td
the inner wall
of t h e s h a r d . P h i l l i p s i t e i s r e s t r i c t e d t o b a s a l t i c t u f f - b r e c c i a s n e a r t h e t o p o f t h e Esmeralda Formation where i t o c c u r s p r i m a r i l y a s a r e p l a c e e m n t of s m a l l pumice f r a g m e n t s . Moiola (1970) showed t h a t t h e v e r t i c a l z e o l i t e z o n a t i o n i n t h e Esmeralda Formation was t h e r e s u l t of t h e r e a c t i o n of m i g r a t i n g groundwater w i t h v o l c a n i c d e t r i t u s ,
305 t h e d i f f e r e n t zones r e p r e s e n t i n g d i f f e r e n c e s i n t h e c o m p o s i t i o n o f t h e groundwater i n c o n t a c t w i t h t h e s e d i m e n t s a t t h e time o f a l t e r a t i o n .
The e a r l i e s t formed m i n e r a l
was p r o b a b l y m o n t m o r i l l o n i t e which formed by m a r g i n a l h y d r o l y s i s o f g l a s s and which
i s f a v o u r e d by a r e l a t i v e l y low Na
+K
t o H a c t i v i t y r a t i o (Hemley, 1 9 6 2 ) .
The
f o r m a t i o n o f m o n t m o r i l l o n i t e i n t h i s way would i n c r e a s e t h e pH and t h e c o n c e n t r a t i o n of a l k a l i i o n s .
The groundwater s o l u t i o n became s a t u r a t e d w i t h r e s p e c t t o c l i n o p -
t i l o l i t e which t h e n p r e c i p i t a t e d .
Conditions favouring i t s formation a r e a
r e l a t i v e l y h i g h S i t o A 1 a c t i v i t y r a t i o and a r e l a t i v e l y h i g h Na
+K
to H ratio.
C o - p r e c i p i t a t i o n of o p a l i n c o n j u n c t i o n w i t h c l i n o p t i l o l i t e i n d i c a t e s t h a t t h e s o l u t i o n s were s u p e r s a t u r a t e d w i t h r e s p e c t t o q u a r t z , t r i d y m i t e and c r i s t o b a l i t e . Zonation of z e o l i t e s a r e a l s o s e e n i n t h e Barstow Formation i n t h e Mud H i l l s a r e a of C a l i f o r n i a (Sheppard and Gude, 1969) b u t h e r e t h e z e o l i t i c assemblages show a l a t e r a l zonation.
The z e o l i t e s o c c u r i n a l t e r e d v i t r i c t u f f s of Miocene a g e and
a r e predominantly a n a l c i m e and c l i n o p t i l o l i t e b u t a l s o i n c l u d e c h a b a z i t e , e r i o n i t e , m o r d e n i t e and p h i l l i p s i t e . c r y s t a l s 0.01
-
Analcime most commonly o c c u r s a s s u b h e d r a l t o e u h e d r a l
0.05mm i n s i z e and may be a s s o c i a t e d w i t h t h e o t h e r z e o l i t e s and
potash feldspars but i s never seen i n a s s o c i a t i o n with r e l i c t g l a s s . o c c u r s a s p r i s m a t i c o r p l a t y c r y s t a l s , most commonly 0.01
-
Clinoptilolite
0.02mm long and i s
a s s o c i a t e d w i t h a u t h i g e n i c c l a y s , p o t a s h f e l d s p a r , o p a l and q u a r t z .
Larger c r y s t a l s
o f c l i n o p t i l o l i t e c o n s i s t o f a uniform c o r e and a p r o g r e s s i v e l y zoned r i m , p r o b a b l y r e p r e s e n t i n g a l e s s s i l i c e o u s o u t e r zone.
C l i n o p t i l o l i t e i s often associated with
a c i c u l a r o r p r i s m a t i c c r y s t a l s o f c h a b a z i t e , e r i o n i t e and m o r d e n i t e ; p h i l l i p s i t e occurs a s small s p h e r u l i t e s 0.1
-
0.3mm i n s i z e .
Sheppard and Gude (1969) e n v i s a g e d t h e f o l l o w i n g sequence o f e v e n t s t o e x p l a i n t h e observed l a t e r a l z o n a t i o n of z e o l i t i c assemblages.
I n i t i a l l y , s o l u t i o n of
s i l i c i c g l a s s by m o d e r a t e l y a l k a l i n e and s a l i n e pore-water r e s u l t e d i n t h e d i r e c t f o r m a t i o n of z e o l i t e s ( e x c e p t a n a l c i m e ) from g l a s s by a s o l u t i o n p r e c i p i t a t i o n mechanism ( D e f f e y e s , 1959).
V a r i a t i o n s i n t h e observed z e o l i t e p a r a g e n e s i s a r e
a t t r i b u t e d t o v a r i a t i o n s i n t h e a c t i v i t y of S i 0 2 , t h e a c t i v i t y o f H 2 0 , and t h e proportions of cations i n the i n t e r s t i t i a l water during diagenesis i n a similar manner t o t h a t d e s c r i b e d by Moiola (1970) f o r t h e Esmeralda Formation.
In the
Barstow Formation t h e e v i d e n c e o f a u t h i g e n i c m i n e r a l . a s s e m b l a g e s s t r o n g l y s u g g e s t s t h e f o r m a t i o n o f analcime from a l k a l i c , s i l i c i c z e o l i t e p r e c u r s o r s , s u c h a s c l i n o p t i l o l i t e (Hay, 1966; Walton, 1975, 1977).
The f o r m a t i o n o f a n a l c i m e r a t h e r t h a n
a l k a l i c , s i l i c i c z e o l i t e s i s favoured by a h i g h Na+:H+ r a t i o (Hess, 1966) r e l a t i v e l y low a c t i v i t y of S i 0 2 (Coombs e t a l . , 1959; Senderov, 1963; Campbell and F y f e , 1965) and p e r h a p s , r e l a t i v e l y low a c t i v i t y of H 2 0 .
The c o m p o s i t i o n a l v a r i a t i o n o f
analcime h e r e a p p a r e n t l y r e f l e c t s i n p a r t t h e c o m p o s i t i o n o f t h e z e o l i t e precursor
-
r e l a t i v e l y s i l i c e o u s analcime forming from a z e o l i t e s u c h a s c l i n o p -
t i l o l i t e whereas r e l a t i v e l y aluminous a n a l c i m e may have formed from a more aluminous precursor such a s p h i l l i p s i t e .
S i m i l a r l y , a u t h i g e n i c p o t a s h f e l d s p a r i s nowhere s e e n
306
Fig.5.17. A u t h i g e n i c z e o l i t e . A. Thin s e c t i o n photomicrograph showing a u t h i g e n i c c l i n o p t i l o l i t e (Z) p a r t l y f i l l i n g i n t e r s t i t i a l v o i d s from t h e Tesuque Formation (Miocene-Pliocene), New Mexico. B. SEM photomicrograph o f t h e same sample showing a u t h i g e n i c c l i n o p t i l o l i t e ( 2 ) c r y s t a l s and a s s o c i a t e d c l a y . C . A u t h i g e n i c c l i n o p t i l o l i t e pseudomorphing g l a s s s h a r d from t h e Esmeralda Formation (MioceneP l i o c e n e ) , Nevada. D. E u h e d r a l a u t h i g e n i c a n a l c i m e from t h e Esmeralda Formation. A-B produced by c o u r t e s y of T.R. Walker and C-D by c o u r t e s y of R . J . Moiola.
i n c o n t a c t w i t h v i t r i c m a t e r i a l and a p p a r e n t l y d i d n o t form d i r e c t l y from v i t r i c m a t e r i a l b u t o n l y through a n i n t e r m e d i a t e phase such a s a n a l c i m e o r c l i n o p t i l o l i t e The f a c t o r s a f f e c t i n g t h e f o r m a t i o n of p o t a s h f e l d s p a r from z e o l i t e s a r e d i s c u s s e d by I i j i m a and Hay (1968) and e x p e r i m e n t a l s y n t h e s i s of p o t a s h f e l d s p a r from c l i n o p t i l o l i t e has been made by Nemecz and V a r j u (1962) and by Sheppard and Gude ( 1 9 6 9 ) . The l a t e r a l v a r i a t i o n i n some a u t h i g e n i c m i n e r a l o g y of t h e Barstow Formation t u f f s of:
nonanalcimic z e o l i t e s
-
analcime
-
K - f e l d s p a r i s p r o b a b l y due t h e r e f o r e t o a
v a r i a t i o n i n s a l i n i t y of t h e p o r e w a t e r s t r a p p e d w i t h i n t h e t u f f s d u r i n g d i a g e n e s i s . The o b s e r v e d z o n a t i o n of a u t h i g e n i c z e o l i t e s can i n many i n s t a n c e s be b e s t a c c o u n t e d f o r by a n open system l e a c h i n g model of d i a g e n e s i s (Hay, 1963; Walton, 1975).
According t o t h i s model, w a t e r f l o w i n g through v o l c a n i c s e d i m e n t d i s s o l v e s
g l a s s a n d becomes e n r i c h e d i n d i s s o l v e d s o l i d s t o t h e c o n c e n t r a t i o n a t which m o n t m o r i l l o n i t e and then z e o l i t e s form.
The c o m p o s i t i o n of t h e groundwater s o l u t i o n i
a t any p a r t i c u l a r p o i n t i n t h e d i a g e n e t i c system i s t h u s a f u n c t i o n of t h e l e n g t h of time t h e w a t e r has been i n c o n t a c t w i t h v o l c a n i c g l a s s , t h e i n i t i a l composition of t h e groundwater and a l s o t h e composition of t h e g l a s s .
As deposition continues
t h e d i s t a n c e which groundwater must m i g r a t e from t h e r e c h a r g e a r e a t o r e a c h t h e point gets greater.
The groundwater i s t h e r e f o r e i n c o n t a c t w i t h s e d i m e n t l o n g e r
b e f o r e i t r e a c h e s t h e f i x e d p o i n t and i s c o n s e q u e n t l y changed from i t s o r i g i n a l composition.
A s t h e composition o f t h e groundwater changed d u r i n g d i a g e n e s i s ,
s e v e r a l m i n e r a l s may form s u c c e s s i v e l y a t e a c h p o i n t , e i t h e r a s r e p l a c e m e n t s o r precipitated i n voids.
The c o m p o s i t i o n a l changes which o c c u r a s w a t e r p a s s e s through
t h e d i a g e n e t i c system a r e r e p r e s e n t e d i n F i g . 5 . 1 8 .
The f i r s t formed m i n e r a l s a r e
m o n t m o r i l l o n i t d and o p a l which form from d i s s o l v e d s i l i c a , aluminium and o t h e r cations a s the glass dissolves.
I n i t i a l h y d r o l y s i s of s i l i c a and alumina d i s s o l v e d
from t h e g l a s s r a i s e s t h e pH. I n t h e a r e a s of m o n t m o r i l l o n i t e f o r m a t i o n most of t h e s i l i c a and aluminium i s consumed, a l t h o u g h e x c e s s s i l i c a forms o p a l , and t h e s o l u t i o n becomes r e l a t i v e l y e n r i c h e d i n a l k a l i s (1-2, F i g . 5 . 1 8 ) .
When t h e changing composition of t h e groundwater
r e a c h e s p o i n t 2 t h e r a t i o of a l k a l i i o n t o hydrogen i o n i s such t h a t z e o l i t e formation i s p r e f e r r e d t o montmorillonite formation.
S i l i c a and aluminium a r e t h e n
consumed i n p r o p o r t i o n s comparable t o t h o s e i n t h e o r i g i n a l g l a s s and o p a l c e a s e s t o form.
The s u b s e q u e n t f o r m a t i o n o f a n a l c i m e p o s e s a problem i n t h a t l a r g e amounts
of aluminium need t o be added d u r i n g t h e c o n v e r s i o n .
V a r i o u s s u g g e s t i o n s have been
p u t forward i n c l u d i n g t h e f o r m a t i o n of a n a l c i m e by t h e r e a c t i o n of hydrous aluminium s i l i c a t e s w i t h a l k a l i n e p o r e w a t e r (Ross, 1928; K e l l e r , 1952) and t h e r e p l a c e m e n t of m o n t m o r i l l o n i t e by q u a r t z .
The l a t t e r e x p l a n a t i o n seems more l i k e l y ; t h i s would
lower t h e s i l i c a a c t i v i t y and r a i s e t h e a l u n i n i u m a c t i v i t y t h u s c a u s i n g c o n v e r s i o n of c l i n o p t i l o l i t e t o a n a l c i m e a t composition 4 ( F i g . 5 . 1 8 ) .
308
/'
1
P"
-
I
Fig.5.18. Changes i n w a t e r composition a s i t moves through t h e d i a g e n e t i c system. During f o r m a t i o n o f m o n t m o r i l l o n i t e g l a s s d i s s o l v e s and s o l u t i o n becomes more a l k a l i n e w i t h concomitant i n c r e a s e i n Al(0H); ( 1 - 2 ) . A t same t i m e , s i l i c a a c t i v i t y i s s e t by e q u i l i b r i u m w i t h amorphous s i l i c a phase t h a t c o n t a i n s some aluminium, b u t conc e n t r a t i o n of o t h e r d i s s o l v e d i o n s i n c r e a s e s . S o l u t i o n r e a c h e s s a t u r a t i o n w i t h c l i n o p t i l o l i t e a t 2. Rapid c o n v e r s i o n of g l a s s t o c l i n o p t i l o l i t e remains t h e same. A t 3 replacement of m o n t m o r i l l o n i t e by q u a r t z lowers s i l i c a a c t i v i t y and r a i s e s aluminium a c t i v i t y , c a u s i n g c o n v e r s i o n o f c l i n o p t i l o l i t e t o a n a l c i m e when composition 4 i s reached ( a f t e r Walton, 1975). The p r e c e d i n g d i s c u s s i o n c l e a r l y i n d i c a t e s t h a t t h e b o u n d a r i e s between a u t h i g e n i c z e o l i t e assemblages a r e d e t e r m i n e d more by groundwater hydrology and c h e m i s t r y t h a n by t e m p e r a t u r e , p r e s s u r e o r d e p t h of b u r i a l .
Even s o , t h e e v i d e n c e from d e s e r t
r e g i o n s o f t h e Western S t a t e s does i n d i c a t e t h a t a minimum t h i c k n e s s of s e d i m e n t , p o s s i b i l i t y 200m o r more, must accumulate b e f o r e groundwater i s i n c o n t a c t long enough f o r a u t h i g e n i c z e o l i t e s t o develop (Walton, 1975). Clay M i n e r a l s Illite-montmorillonite The i n t r a s t r a t a l d i s s o l u t i o n o f framework g r a i n s d u r i n g e a r l y d i a g e n e s i s produces a l u m i n a , s i l i c a , and sodium much o f which i s r e p r e c i p i t a t e d a s a u t h i g e n i c c l a y . Throughout t h e Cenozoic a l l u v i u m of SW U n i t e d S t a t e s a n d NW Mexico s t u d i e d by Walker e t a l . (1978) b o t h r e p r e c i p i t a t e d and r e p l a c e m e n t a u t h i g e n i c c l a y s a r e randomly i n t e r s t r a t i f i e d mixed-layer i l l i t e - m o n t m o r i l l o n i t e c o n t a i n i n g 80-95% expandable l a y e r s ( i . e . n e a r l y p u r e m o n t m o r i l l o n i t e ) .
X-ray d i f f r a c t i o n t r a c e s
t h u s e n a b l e t h e r e a d y d i s t i n c t i o n of t h i s a u g h i g e n i c c l a y from t h e more m i n e r a l -
309 o g i c a l l y heterogeneous mechanically i n f i l t r a t e d c l a y .
However, i n t h i n s e c t i o n
a u t h i g e n i c c l a y o f t h i s type may be d i f f i c u l t t o r e c o g n i z e ; i t o c c a s i o n a l l y shows r a d i a t i n g c r y s t a l s which l i n e i n t e r s t i t i a l v o i d s b u t more u s u a l l y t h e p r e c i p i t a t e d c l a y i s a n u n r e c o g n i z a b l e p a r t of t h e i n t e r s t i t i a l m a t r i x .
However, SEM r e v e a l s a
v e r y c h a r a c t e r i s t i c boxwork o r c e l l u l a r t e x t u r e which confirms i t s a u t h i g e n i c T h i s t e x t u r e i s f i g u r e d by B o r s t a n d K e l l e r (1969) and by
nature (Fig.5.19).
Wilson and P i t t m a n ( 1 9 7 7 ) .
I t v a r i e s somewhat depending upon t h e d e h y d r a t i o n s t a t e
of t h e a u t h i g e n i c s m e c t i t e , a h i g h l y c r e n u l a t e d a p p e a r a n c e r e s u l t i n g when t h e m i n e r a l
i s dehydrated.
Although t h i s t e x t u r e s u p e r f i c i a l l y resembles t h e honeycomb t e x t u r e
s e e n i n a u t h i g e n i c c h l o r i t e and i l l i t e i t s h o u l d n o t be confused w i t h t h e s e because i t d i f f e r s i n a n i m p o r t a n t r e s p e c t i . e . i n d i v i d u a l c r y s t a l s o f s m e c t i t e c a n n o t be
r e s o l v e d u s i n g t h e SEM, i n c o n t r a s t t o t h e c h l o r i t e o r i l l i t e honeycomb which i s formed of i n d i v i d u a l c l a y p l a t e l e t s i n t e r l o c k i n g and normal t o t h e d e t r i t a l g r a i n surface.
I n a s i m i l a r v e i n Wilson a n d P i t t m a n (1977) p o i n t o u t t h a t i t can be
m i s l e a d i n g t o u s e o n l y SEM e v i d e n c e t o d i s t i n g u i s h m o n t m o r i l l o n i t e and mixed l a y e r illite-montmorillonite.
I n g e n e r a l t h e SEM a p p e a r a n c e of mixed l a y e r i l l i t e -
m o n t m o r i l l o n i t e depends upon t h e r e l a t i v e p r o p o r t i o n s of t h e p a r t i c i p a t i n g c l a y minerals:
i f i l l i t e - r i c h t h e s t r u c t u r e may be d i f f i c u l t t o d i s t i n g u i s h from pure
i l l i t e and t a k e s t h e form of s h e e t s w i t h s h o r t l a t h - l i k e d i g i t a t e edges.
Illite
A u t h i g e n i c i l l i t e i s u b i q u i t o u s i n a n c i e n t r e d beds and i s l e s s common i n T e r t i a r y and Q u a t e r n a r y a l l u v i u m .
I n t h i n section i t is readily distinguished
from o t h e r c l a y s by i t s h i g h e r b i r e f r i n g e n c e ; i t commonly forms p o r e l i n i n g s (Fig.5.19).
I n SEM s t u d i e s i l l i t e g e n e r a l l y o c c u r s a s i r r e g u l a r f l a k e s w i t h l a t h -
like projections.
The a u t h i g e n i c growths, a t t a c h e d t o t h e s u r f a c e o f t h e sand
g r a i n s , develop a s s h e e t s t h a t c u r l away from t h e p o i n t of a t t a c h m e n t .
The edges
of t h e s e s h e e t s have p r o j e c t i o n s which may b e r e l a t i v e l y s h o r t o r t h e y may develop i n t o c u r l e d , f i b r o u s p r o j e c t i o n s up t o 10ym long and 0 . 5 ~ " i n w i d t h , t h e s o - c a l l e d fibrous o r hairy i l l i t e . Kaolinite-Dickite K a o l i n i t e and d i c k i t e a r e common a u t h i g e n i c c l a y s i n s a n d s t o n e s and t h e r e a r e a number of r e p o r t e d o c c u r r e n c e s i n r e d beds.
K a o l i n i t e o c c u r s i n t h e Permian
R o t l i e g e n d e s s a n d s t o n e s o f t h e North Sea a r e a ( G l e n n i e e t a l . , 1978; Hancock, 1978;
Kessler, 1978).and a l s o t h e Old Red Sandstone ( F r i e n d , 1966).
i n t h e O l d Red Sandstone o f S c o t l a n d .
Dickite a l s o occurs
K a o l i n i t e and d i c k i t e a r e t h e most
e a s i l y r e c o g n i z e d c l a y m i n e r a l s i n t h i n s e c t i o n ( F o t h e r g i l l , 1955, p. 305; Karpova, 1969, p . 7 ) b e c a u s e t h e y a r e f r e q u e n t l y c o a r s e r than o t h e r c l a y m i n e r a l s p e c i e s . K a o l i n i t e i s n o t d i s t i n g u i s h a b l e from d i c k i t e u s i n g o n l y SEM b u t they a r e r e a d i l y
310
A u t h i g e n i c i l l i t e and mixed l a y e r i l l i t e - m o n t m o r i l l o n i t e i n Cenozoic Fig.5.19. a l l u v i u m and a n c i e n t r e d beds. A . Large c r y s t a l s of a u t h i g e n i c c l a y forming a r a d i a l p o r e l i n i n g . Thin s e c t i o n , Cuchara Formation ( E o c e n e ) , Colorado. B. D e t a i l o f a u t h i g e n i c c l a y forming i n t e r s t i t i a l m a t r i x , SEM photomicrograph, G i l a Group, Tucson, Arizona. C . A u t h i g e n i c mixed l a y e r c l a y (A-C) developed on m e c h a n i c a l l y i n f i l t r a t e d c l a y (MIC). SEM photomicrograph, Hayner Ranch Formation (Miocene), New Mexico. D . A u t h i g e n i c i l l i t e showing t y p i c a l boxwork t e x t u r e . SEM photom i c r o g r a p h , R o t l i e g e n d e s ( P e r m i a n ) , North Sea. A-C produced by c o u r t e s y o f T . R . Walker and D by c o u r t e s y of B. Waugh.
TABLE 5.7.
C h a r a c t e r i s t i c s of a u t h i g e n i c c l a y s ( a f t e r Wilson and P i t t m a n , 1977). Morphology
Form o f Aggregate
Relationship with D e t r i t a l Grains
Thickness of Coating o r Long Dimension o f Aggregate
(ym)
pseudohexagonal
stacked p l a t e s (book)
pore f i l l i n g
2-2500 (generally 2-20)
pseudohexagonal
vermicule
pore f i l l i n g
10-2500 (generally 20-200)
pseudohexagonal
sheet
pore f i l l i n g
0.1-1
pseudohexagonal
p l a t e s (2dimensional cardhouse
pore l i n i n g
2- 10
c u r l e d equidimensional w i t h rounded edges
honeycomb
pore l i n i n g
2- 10
equidimensional w i t h a n g u l a r o r l o b a t e edges
r o s e t t e o r fan
pore. l i n i n g and f i l l i n g
4- 150
fan-shaped f i b r o u s bundles
ca bba gehea d
pore-lining and f i l l i n g
8-40
Illite
i r r e g u l a r with elongate spines
sheet
pore l i n i n g
0.1-10
Montmorillonite
n o t recognizable
wrinkled s h e e t o r honeycomb
pore l i n i n g
2-12
Mixed l a y e r montiuorilloni t e /illi t e
subequant w i t h s t u b b y spines
imbricate sheet t o r a a a e d honevcomb
pore l i n i n g
2- 1 2
K a o l i n i t e and Dickite
Chlorite
(generally 4-20)
312
Fig.5.20. A u t h i g e n i c k a o l i n i t e and c h l o r i t e i n a n c i e n t r e d beds. A. Pseudohexagonal c r y s t a l s of k a o l i n i t e forming a p o r e f i l l i n g . Thin s e c t i o n , Lower Old Red S a n d s t o n e , Gamrie O u t l i e r , S c o t l a n d . B. S t a c k e d c r y s t a l s of k a o l i n i t e o f v a r i a b l e g r a i n s i z e forming a p o r e - f i l l i n g . Note t h a t many of t h e c r y s t a l s a r e twinned a s i n d i c a t e d by n o t c h e s . SEM photomicrograph, R o t l i e g e n d e s ( P e r m i a n ) , North Sea. C . A u t h i g e n i c c h l o r i t e forming a p o r e - l i n i n g , Thin s e c t i o n , R o t l i e g e n d e s ( P e r m i a n ) , North Sea. D . A u t h i g e n i c c h l o r i t e p o r e l i n i n g s . Thin s e c t i o n , T o r r i d o n i a n Sandstone ( L a t e P r e c a m b r i a n ) , NW S c o t l a n d . B i s produced by c o u r t e s y of B . Waugh.
313 s e p a r a t e d u s i n g X-ray d i f f r a c t i o n .
Authigenic k a o l i n i t e - d i c k i t e t y p i c a l l y occurs
i n s a n d s t o n e s a s pseudohexagonal p l a t e s , m o s t l y commonly p l u g g i n g p o r e s p a c e s completely, but a l s o a s pore l i n i n g s (Fig.5.20). The most common h a b i t o f a u t h i g e n i c k a o l i n i t e - d i c k i t e i n s a n d s t o n e s i s t h e c - a x i s s t a c k i n g of pseudohexagonal p l a t e s t o form "books".
I n d i v i d u a l p l a t e s may range from
3 - 2 0 ~i n d i a m e t e r ; i n d i v i d u a l p l a t e s and books of k a o l i n i t e may show grooves o r n o t c h e s which may r e p r e s e n t twinning ( M a n s f i e l d and B a i l e y , 1972).
Kaolinite also
d i s p l a y s a v e r m i c u l a r h a b i t which c o n s i s t s o f a sequence of s t a c k e d pseudohexagonal p l a t e s t h a t may e x t e n d t h e f u l l l e n g t h o f p o r e s p a c e s .
More r a r e l y k a o l i n i t e may
o c c u r a s o v e r l a p p i n g pseudohexagonal p l a t e s c o a t i n g s a n d s i z e d d e t r i t a l g r a i n s o r o t h e r a u t h i g e n i c overgrowths. 2-lO/m i n d i a m e t e r .
These p l a t e s a r e g e n e r a l l y l e s s t h a n
pm
t h i c k and
P o r e f i l l i n g and p o r e l i n i n g k a o l i n i t e i n r e d beds i s g e n e r a l l y
c o n s i d e r e d t o be a r e l a t i v e l y l a t e d i a g e n e t i c f e a t u r e .
C h l o r i t e , a l o n g w i t h i l l i t e i s t h e most commonly o c c u r r i n g c l a y m i n e r a l i n a n c i e n t red beds.
I t o c c u r s a s p o r e l i n i n g s which may a p p e a r p a l e g r e e n o r c o l o u r -
l e s s i n t h i n s e c t i o n and a l s o w i t h i n i n t e r s t i t i a l m a t r i x .
The p o r e l i n i n g s a r e
e x t r e m e l y v a r i a b l e i n s a n d s t o n e s g e n e r a l l y and may t a k e t h e form of p l a t e s a t t a c h e d t o d e t r i t a l sand g r a i n s ( P i t t m a n and Lumsden, 1968; Hayes, 1 9 7 0 ) , r o s e t t e s , honeycombs, o r cabbagehead-like growths (Wilson and P i t t m a n , 1977). of t h e s e growth h a b i t s v a r i e s a s f o l l o w s : cabbagehead.
p l a t e s and r o s e t t e s
The i r o n c o n t e n t honeycomb
C h l o r i t e most commonly develops a s i n d i v i d u a l idiomorphic p l a t e s
a t t a c h e d on edge t o t h e s u r f a c e of t h e s a n d g r a i n s u s u a l l y r a n g i n g from 2 - 1 0 ~i n s i z e . Fig.5.20. Honeycomb s t r u c t u r e d e v e l o p s when t h e p l a t e s a r e a r r a n g e d i n a c e l l u l a r p a t t e r n due t o t h e i n t e r s e c t i o n and c u r v i n g of i n d i v i d u a l c r y s t a l s g i v i n g a honeycomb l i k e a p p e a r a n c e .
A u t h i g e n i c c h l o r i t e a l s o forms r o s e t t e s and fan-shaped
c l u s t e r s a s p o r e l i n i n g s and p o r e f i l l i n g s , u s u a l l y 5 - 2 0 ~i n d i a m e t e r , b u t o c c a s i o n a l l y up t o 1 5 0 ~ . I n d i v i d u a l c r y s t a l s t y p i c a l l y range from 4-18p i n diameter.
The r a r e s t growth h a b i t of a u t h i g e n i c c h l o r i t e i s t h e cabbagehead.
At
low m a g n i f i c a t i o n s t h e s e a p p e a r a s s m a l l e q u i d i m e n s i o n a l g r a i n s a t t a c h e d t o sand sized grains.
A t higher magnifications they appear a s concentric ribbon o r
s h e e t - l i k e l a y e r s of c h l o r i t e w i t h complex o r i e n t a t i o n s . r a n g e from a p p r o x i m a t e l y 8 t o 4
I n d i v i d u a l cabbageheads
0 and ~ most commonly form p o r e l i n i n g s .
In a n c i e n t r e d beds c h l o r i t e i s a c h a r a c t e r i s t i c a u t h i g e n i c p h a s e .
It i s
u s u a l l y a n i r o n - r i c h v a r i e t y a s i n d i c a t e d by t h e r e l a t i v e l y s t r o n g (002) and ( 0 0 4 ) r e f l e c t i o n s ( B r i n d l e y , 1961) and o c c u r s a s d i s c r e t e p o r e l i n i n g s and w i t h i n i n t e r s t i t i a l matrix.
314 Calcite A u t h i g e n i c c a l c i t e i s a common cementing a g e n t i n a n c i e n t r e d s and a l s o i n t h e i r L a t e T e r t i a r y and Q u a t e r n a r y p r e c u r s o r s . The e a r l i e s t formed c a l c i t e cements i n r e d bed p r e c u r s o r s o c c u r a s a r e s u l t of b o t h p h r e a t i c and vadose c e m e n t a t i o n i n humid-semi-arid
a l l u v i a l f a n and ephemeral
wadi s t r e a m d e p o s i t s ( B l i s s e n b a c h , 1954; S t a l d e r , 1 9 7 5 ) .
Here zones of c o n s o l i d -
a t i o n a r e closely associated with successive depositional episodes c l e a r l y indicating t h a t cementation occurred very r a p i d l y a f t e r deposition.
More r a r e l y t h e d e g r e e
o f c a l c i t e c e m e n t a t i o n may c o r r e l a t e w i t h s e d i m e n t a r y s t r u c t u r e s
-
a clear indication
t h a t primary p e r m e a b i l i t y and c a p i l l a r y f o r c e s d e t e r m i n e d by g r a i n s i z e were c o n t r o l l i n g f a c t o r s i n l o c a l i z i n g t h e cement f a b r i c .
The f a c t o r s g o v e r n i n g t h e
p r e c i p i t a t i o n of c a l c i t e from f r e s h w a t e r a r e d i s c u s s e d by MUller e t a l . (19721, B l a t t e t a l . (1972) and Folk and Land (1975).
In a l l u v i a l f a n and wadi c h a n n e l d e p o s i t s c a l c i t e cement t e x t u r e s a t t r i b u t a b l e t o b o t h p h r e a t i c and vadose c e m e n t a t i o n o c c u r ; i n p l a c e s t h e r e may be rhythmic a l t e r n a t i o n s of t h e two.
The most t y p i c a l vadose t e x t u r e i s d r i p s t o n e o r s t a l a c t i t e s
composed of l a m i n a t e d m i c r i t i c s e d i m e n t .
Blocky c a l c i t e cements r e s u l t from p h r e a t i c
cementa t i o n . S t a l d e r (1975) has made a d e t a i l e d s t u d y of t h e c a l c i t e c e m e n t a t i o n o f P l i o c e n e Q u a t e r n a r y f l u v i a t i l e c l a s t i c s i n t h e Oman Mountains. several species:
The c a r b o n a t e s h e r e i n c l u d e
c a l c i t e , high-Mg c a l c i t e and d o l o m i t e which a r e c l o s e l y r e l a t e d
t o t h e a r e a of s u r f a c e r u n - o f f . For example, i n a r e a s w i t h c a r b o n a t e o r mixed
c a r b o n a t e - o p h i o l i t e s o u r c e r o c k s t h e cement i s c a l c i t e .
I n areas with peridolite
s o u r c e r o c k s c a l c i t e , h i g h Mg c a l c i t e and d o l o m i t e o c c u r a s cements.
The o r i g i n
o f Ca2+ was t h e r e f o r e p r i n c i p a l l y from c a r b o n a t e s and from p l a g i o c l a s e s and
pyroxenes p r e s e n t i n l a r g e q u a n t i t i e s i n b a s a l t i c and g a b b r o i c s o u r c e a r e a s .
The
h i g h Mg2+ needed f o r t h e f o r m a t i o n of h i g h Mg c a l c i t e (up t o 30% MgCO,) and dolomite was p r o b a b l y d e r i v e d mainly from o l i v i n e s and pyroxenes i n p e r i d o t i t i c source rocks.
F l u c t u a t i o n s i n t h e s u p p l y of Ca2+ and Mg2+ i o n s r e s u l t e d i n t h e
chemical z o n a t i o n o f cement f a b r i c s which e n a b l e d S t a l d e r (1975) t o conclude t h a t c e m e n t a t i o n r e s u l t e d from s u p e r s a t u r a t i o n by e v a p o r a t i o n . I n a n c i e n t r e d beds c a l c i t e i s g e n e r a l l y c o a r s e l y c r y s t a l l i n e , f i l l i n g b o t h i n t e r g r a n u l a r v o i d s and d i s s o l u t i o n v o i d s .
Completely d i s s o l v e d d e t r i t a l g r a i n s
r e v e a l e d o n l y by t h e i r c l a y o x i d e c o a t i n g s ( g h o s t s ) may be f i l l e d by c a l c i t e cement, a p r o c e s s which p r e v e n t s them from compaction and o b l i t e r a t i o n . common i n a n c i e n t r e d beds ( F i g . 5 . 2 1 ) .
Such g h o s t s a r e
C a l c i t e c e m e n t a t i o n may a l s o be i m p o r t a n t
i n s e a l i n g o f f permeable zones and t h u s p r e v e n t i n g p a r t i a l l y a l t e r e d framework g r a i n s from more e x t e n s i v e d i s s o l u t i o n . Changes i n groundwater c h e m i s t r y may d i s s o l v e e x i s t i n g a u t h i g e n i c c a l c i t e and t h e r e may be complex t e x t u r a l r e l a t i o n s h i p s w i t h c a l c i t e , t h e framework g r a i n s and
315
Fig.5.21. A u t h i g e n i c c a l c i t e i n Cenozoic a l l u v i u m and a n c i e n t r e d beds. A . P a r t i a l l y d i s s o l v e d hornblende g r a i n (H) w i t h o r i g i n a l o u t l i n e marked by p r e s e r v e d c l a y s k i n . D i s s o l u t i o n v o i d i s f i l l e d w i t h c a l c i t e . Thin s e c t i o n . P l i o c e n e ? f a n g l o m e r a t e , Cai'ion Rojo, Baja C a l i f o r n i a . B. A u t h i g e n i c c a l c i t e (Ca) w i t h i n c l u s i o n s o f a u t h i g e n i c p o t a s s i u m f e l d s p a r ( A F ) . SEM photomicrograph, o t h e r d e t a i l s a s i n A. C. Probable completely dissolved s i l i c a t e g r a i n with t h e o r i g i n a l o u t l i n e marked by a p r e s e r v e d c l a y s k i n . D i s s o l u t i o n v o i d i s f i l l e d w i t h c a l c i t e which i s i n o p t i c a l c o n t i n u i t y w i t h t h e i n t e r s t i t i a l c a l c i t e . Thin D. Authigenic c a l c i t e causing s e c t i o n , Upper M o t t l e d Sandstone ( T r i a s s i c ) , U.K. p h y s i c a l d i s p l a c e m e n t of f e l d s p a r fragments by d i s p l a c i v e c r y s t a l l i z a t i o n . Thin s e c t i o n , R i n g e r i k e Group ( S i l u r i a n ) , Norway. A-B a r e produced by c o u r t e s y o f T R Wa l k e r
..
.
316 o t h e r authigenic phases.
I n p a r t i c u l a r , c a l c i t e may c o r r o d e o r p h y s i c a l l y d i s p l a c e
framework g r a i n s such a s t h e more e a s i l y d i s s o l v e d c a l c i c p l a g i o c l a s e s and o f t e n shows c o r r o s i v e r e l a t i o n s h i p s w i t h o t h e r a u t h i g e n i c p h a s e s s u c h a s q u a r t z .
The
f r e q u e n t c o e x i s t e n c e of t h e s e two m i n e r a l s i n r e d bed s a n d s t o n e s i s n o t s u r p r i s i n g because t h e s o l u b i l i t y c u r v e s of q u a r t z and c a l c i t e meet a t a pH o f a b o u t 7.5 which i s commonly found i n n a t u r a l groundwaters. Haematite and p r e c u r s o r o x i d e s F i n e l y d i s p e r s e d a u t h i g e n i c h a e m a t i t e i s t h e p r i n c i p a l c o l o u r i n g a g e n t of cnntinental red beds.
I t s a u t h i g e n i c o r i g i n i s confirmed by t h e e u h e d r a l n a t u r e
of t h e h a e m a t i t e c r y s t a l s and t h e f a c t t h a t t h e y o c c u r a s p o r e l i n i n g s and w i t h i n d i s s o l u t i o n v o i d s , p a r t i c u l a r l y of f e r r o m a g n e s i a n m i n e r a l s . o c c u r a s t h i n , pseudohexagonal p l a t e s a b o u t 1~ i n l e n g t h . on g r a i n s u r f a c e s o r a s s m a l l c l u s t e r s of c r y s t a l s Fig.5.22.
The h a e m a t i t e c r y s t a l s They may o c c u r d i s c r e t e l y Much o f t h e i r o n which
forms a u t h i g e n i c h a e m a t i t e i s r e l e a s e d v i a t h e i n t r a s t r a t a l d i s s o l u t i o n o f ferromagn e s i a n m i n e r a l s s u c h a s h o r n b l e n d e and p r o b a b l y pyroxene.
The f a t e of t h e i r o n , once
r e l e a s e d from s i l i c a t e l a t t i c e s , depends upon t h e chemical c o n d i t i o n s i n t h e i n t e r s t i t i a l ground w a t e r .
I n Eh-pH c o n d i t i o n s which c o r r e s p o n d t o t h e s t a b i l i t y f i e l d
o f f e r r o u s i r o n then t h e i r o n may be t r a n s p o r t e d i n s o l u t i o n i n t h e i n t e r s t i t i a l groundwater.
P r e c i p i t a t i o n w i l l o c c u r when Eh and pH changes and e n t e r s t h e
s t a b i l i t y f i e l d of h a e m a t i t e .
The i n i t i a l p r e c i p i t a t e i s l i k e l y t o b e amorphous
i r o n hydroxide o r g o e t h i t e (Langmuir, 1 9 7 1 ) , which s u b s e q u n n t l y a g e s i n t o h a e m a t i t e ( B e r n e r , 1969a).
I n s u p p o r t of t h i s is t h e f a c t t h a t i n Cenozoic a l l u v i u m w i t h t h e
youngest d e t e c t a b l e r e d d e n i n g ( L a t e P l e i s t o c e n e ) h a e m a t i t e c a n n o t be d e t e c t e d by SEM o r X-ray d i f f r a c t i o n a n a l y s i s (Walker, 1 9 7 6 ) .
A u t h i g e n i c e u h e d r a l h a e m a t i t e i n abundant i n a n c i e n t r e d beds and has been i d e n t i f i e d by a number of a u t h o r s i n c l u d i n g , F r i e n d ( 1 9 6 6 ) , T u r n e r (1974a1, Wilson ( 1 9 7 1 ) , Walker (1976) and Walker and L a r s o n (1976)
( s e e F i g s . 5 . 2 2 and 5 . 2 3 ) .
I x e r e t a l . (1979) have c o n v i n c i n g l y d e m o n s t r a t e d t h e a u t h i g e n i c n a t u r e of h a e m a t i t e i n t h e T r i a s s i c S t . Bees Sandstone of n o r t h e r n England.
Here e u h e d r a l h a e m a t i t e
c r y s t a l s o c c u r n o t o n l y a s f i n e g r a i n e d pigment b u t a l s o a s overgrowths which o c c u r a s rhombohedral and p r i s m a t i c p r o j e c t i o n s o r a s s y n t a x i a l r i m cements ( F i g . 5 . 2 3 ) . Complete overgrowths of h a e m a t i t e develop i n a s i m i l a r manner t o t h a t d e s c r i b e d by Waugh (1970b, 1978) f o r s i l i c a t e g r a i n s .
I n t h e e a r l y s t a g e s t h e authigenic haematite
c o n s i s t s of f l a t rhombohedral c r y s t a l s r a n g i n g from 2-20 m i n l e n g t h , t h e c r y s t a l s being arranged perpendicular t o the d e t r i t a l g r a i n surface.
The o r i e n t a t i o n o f t h e
a u t h i g e n i c c r y s t a l s i s c o n t r o l l e d by t h e s t r u c t u r e of t h e h o s t g r a i n such t h a t i n monocrystalline d e t r i t a l g r a i n s , a l l the authigenic c r y s t a l s a r e arranged i n p a r a l l e l fashion.
In p o l y c r y s t a l l i n e d e t r i t a l g r a i n s t h e d i f f e r e n t o r i e n t a t i o n s
of t h e d i f f e r e n t s u b c r y s t a l s i s r e f l e c t e d i n t h e o r i e n t a t i o n of t h e a u t h i g e n i c
317
Fig.5.22. A u t h i g e n i c pigmentary h a e m a t i t e i n Cenozoic a l l u v i u m and a n c i e n t c o n t i n e n t a l r e d beds. A. SEM photomicrograph of h a e m a t i t e c r y s t a l s l i n i n g a d i s s o l u t i o n v o i d . Hayner Ranch Formation (Miocene), New Mexico. B. E n l a r g e d view of o u t l i n e d a r e a i n A. C . E n l a r g e d view o f o u t l i n e d a r e a i n B showing c l u s t e r s of e u h e d r a l h a e m a t i t e c r y s t a l s . D . SEM photomicrograph o f a u t h i g e n i c h a e m a t i t e i n t h e Moenkopi Formation ( T r i a s s i c ) , Arizona. E . C h a r a c t e r i s t i c t e x t u r a l a p p e a r a n c e of a u t h i g e n i c h a e m a t i t e pigment c o a t i n g d e t r i t a l q u a r t z g r a i n . Thin s e c t i o n photomicrograph, R i n g e r i k e Group ( S i l u r i a n ) , Norway. F. D e t a i l of e u h e d r a l a u t h i g e n i c h a e m a t i t e ( R i n g e r i k e Group). A-D a r e produced by c o u r t e s y of T.R. Walker.
318
Fig.5.23. A u t h i g e n i c i r o n a n d t i t a n i u m o x i d e s i n t h e S t . Bees Sandstone ( T r i a s s i c ) , Cumbria, U.K. A . D e t r i t a l g r a i n w i t h a u t h i g e n i c overgrowth c o n s i s t i n g of s y n t a x i a l rhombohedra1 c r y s t a l s o f h a e m a t i t e . R e f l e c t e d l i g h t . B. D e t r i t a l g r a i n w i t h s y n t a x i a l overgrowth c o n s i s t i n g o f a s i n g l e rhombohedra1 c r y s t a l . R e f l e c t e d l i g h t . C. D e t r i t a l g r a i n w i t h p a r a l l e l rhombohedra1 c r y s t a l s o f a u t h i g e n i c h a e m a t i t e and p e r f e c t l y e u h e d r a l a n a t a s e ( t o p r i g h t ) . SEM. D. P o l y c r y s t a l l i n e d e t r i t a l h a e m a t i t e w i t h a u t h i g e n i c rhombohedra1 c r y s t a l s o f h a e m a t i t e showing t h r e e d i f f e r e n t , c r y s t a l l o g r a p h i c a l l y c o n t r o l l e d , o r i e n t a t i o n s . E. D e t a i l of a u t h i g e n i c rhombohedra1 h a e m a t i t e . F. D e t a i l o f a u t h i g e n i c a n a t a s e c r y s t a l showing t h e b a s a l p i n a c o i d a n d pyramidal f a c e s .
319 h a e m a t i t e c r y s t a l s (Fig.5.23).
Complete overgrowths o f h a e m a t i t e p r o b a b l y form
by t h e growth and s t a c k i n g t o g e t h e r o f i n d i v i d u a l rhombohedral c r y s t a l s i n a s i m i l a r manner t o t h a t s e e n i n t h e f e l d s p a r s .
I x e r e t a l . (1979) showed t h e t t h e
h a e m a t i t e overgrowths show some i m p o r t a n t c o m p o s i t i o n a l v a r i a t i o n .
I n particular
t i t a n i f e r o u s overgrowths w i t h a b o u t 5% T i 0 2 a r e a common f e a t u r e o f some T r i a s s i c red beds. Titanium o x i d e s a r e a l s o a n i m p o r t a n t a u t h i g e n i c phase i n r e d beds.
These
commonly o c c u r i n a n c i e n t r e d beds a s s m a l l d i s c r e t e c r y s t a l s of a n a t a s e o r r u t i l e up t o 4
0 i n~ s i z e ( F i g . 5 . 2 3 ) and l i n i n g p o r e s p a c e s .
a n a t a s e and r u t i l e a l s o o c c u r ,
S y n t a x i a l overgrowths of
A u t h i g e n i c t i t a n i u m o x i d e p r o b a b l y forms i n t h e
same manner a s h a e m a t i t e a l t h o u g h a r e a d y s u p p l y of t i t a n i u m i o n s from framework s i l i c a t e s i s n o t immediately o b v i o u s .
I x e r e t a l . (1979) s u g g e s t e d t h a t t i t a n i u m
i o n s might be r e l e a s e d i n t o t h e i n t e r s t i t i a l groundwaters d u r i n g t h e m a r t i t i z a t i o n of d e t r it a l titanomagnetite. Quartz Q u a r t z i s a n abundant a u t h i g e n i c phase i n r e d beds a n d i s commonly a s o o c i a t e d with authigenic feldspar.
The p r e c i p i t a t i o n of a u t h i g e n i c q u a r t z a c c o u n t s f o r
most of t h e f r e e s i l i c a which i s r e l e a s e d by s i l i c a t e d i s s o l u t i o n , t h e remainder b e i n g used i n c l a y m i n e r a l s and f e l d s p a r . I n t h e Cenozoic a l l u v i u m s t u d i e d by Walker e t a l . (1978) a u t h i g e n i c overgrowths on d e t r i t a l q u a r t z g r a i n s a r e s c a r c e and most of t h e a u t h i g e n i c q u a r t z o c c u r s i n two d i s t i n c t c r y s t a l forms.
These a r e :
1) t a b u l a r c r y s t a l s t h a t a r e f l a t t e n e d
p a r a l l e l t o t h e b a s a l pinacoi'd and which have u n e q u a l l y developed p r i s m f a c e s t h a t g i v e s them a pseudo-orthorhombic form and 2 ) p r i s m a t i c c r y s t a l s which a r e t e r m i n a t e d w i t h rhombohedral f a c e s .
Walker e t a l . (1978) concluded t h a t t h e two c r y s t a l forms
r e f l e c t e d d i f f e r e n t concentrations of s i l i c a i n the groundwaters, o r o t h e r d i f f e r e n c e s i n t h e i n t e r s t i t i a l geochemical c o n d i t i o n s . D i s c r e t e a u t h i g e n i c c r y s t a l s a l s o o c c u r i n a n c i e n t r e d beds b u t s y n t a x i a l overgrowths a r e more abundant ( F i g . 5 . 2 4 ) .
I n some a e o l i a n s a n d s t o n e s t h e s e a r e s o
abundant a s t o resemble s i l c r e t e d u r i c r u s t s of modern d e s e r t s .
The f o r m a t i o n of
q u a r t z overgrowths of t h i s t y p e has been d e s c r i b e d by Waugh ( 1 9 7 0 a , b ) . STAGES I N THE DIAGENESIS OF CONTINENTAL RED BEDS The s t u d y o f d i a g e n e s i s i n f i r s t c y c l e d e s e r t a l l u v i u m made by Walker and h i s c o l l e g u e s p r o v i d e s a s t r i k i n g . e x a m p l e o f t h e way i n which m i n e r a l systems may respond t o changes i n t h e i r p h y s i c a l and chemical environment.
Many of t h e m i n e r a l s
a f f e c t e d by t h e s e d i a g e n e t i c p r o c e s s e s were formed under c o n d i t i o n s much d i f f e r e n t from t h o s e i n which they a r e now found,
Thus, many o f t h e m i n e r a l s a r e n o t i n
e q u i l i b r i u m w i t h t h e i r environment a n d a r e undergoing a l t e r a t i o n s .
These a l t e r a t i o n s
320
Fig.5.23. SEM photomicrographs o f a u t h i g e n i c q u a r t z i n Cenozoic a l l u v i u m and a n c i e n t r e d beds. A . T a b u l a r c r y s t a l s of a u t h i g e n i c q u a r t z which a r e f l a t t e n e d p a r a l l e l t o b a s a l p i n a c o i d and have u n e q u a l l y developed p r i s m f a c e s . P l i o c e n e f a n g l o m e r a t e , Ca8on R o j o , Baja C a l i f o r n i a . B. P r i s m a t i c c r y s t a l s of q u a r t z t e r m i n a t e d w i t h rhombohedra1 f a c e s . E l R i t o Formation ( E o c e n e ? ) , New Mexico. C . A u t h i g e n i c q u a r t z ( Q ) e n c r u s t i n g a u t h i g e n i c p o t a s s i u m f e l d s p a r (AF). Same d e t a i l s a s i n A. D . E u h e d r a l overgrowth of q u a r t z . S t . Bees Sandstone ( T r i a s s i c ) , U.K. A-C a r e produced by c o u r t e s y of T.R. Walker.
321
Fig.5.25. Schematic r e p r e s e n t a t i o n of t h e t h r e e s t a g e s o f d i a g e n e t i c a l t e r a t i o n i n f i r s t - c y c l e d e s e r t a l l u v i u m ( a f t e r Walker e t a l . , 1 9 7 8 ) . c a n be e x p e c t e d t o c o n t i n u e u n t i l t h e m i n e r a l assemblages i s i n e q u i l i b r i u m w i t h i t s physico-chemical environment.
The e x i s t e n c e o f p a r t i a l l y a l t e r e d m i n e r a l
g r a i n s i n d e p o s i t s which a r e tend of m i l l i o n s of y e a r s o l d i n d i c a t e s t h e e q u i l i b r i a t i o n p r o c e s s t a k e s p l a c e v e r y slowly; i n d e e d t h e r e may be r e p e a t e d e p i s o d e s o f e q u i l i b r i a t i o n a s t h e p r e c i p i t a t i o n o f a u t h i g e n i c p h a s e s changes t h e c o n d i t i o n s i n t h e i n t e r s t i t i a l w a t e r and i n i t i a t e s a new phase of a l t e r a t i o n .
I t seems l i k e l y
t h a t i n t r a s t r a t a l a l t e r a t i o n s a r e most r a p i d d u r i n g t h e e a r l y s t a g e s a f t e r d e p o s i t i o n because t h i s i s t h e p e r i o d when t h e most u n s t a b l e d e t r i t a l g r a i n s a r e most abundant. The i n t e r s t i t i a l environment may a l s o be a f f e c t e d by e x t e r n a l c a u s e s s u c h a s c l i m a t e and t e c t o n i s m .
C l i m a t e may a f f e c t t h e amount and t e m p e r a t u r e o f c i r c u l a t i n g
ground w a t e r and a l s o t h e amount of o r g a n i c m a t t e r i n t h e d e p o s i t e d s e d i m e n t , and t h e s e w i l l i n t u r n a f f e c t t h e r a t e of i n t r a s t r a t a l a l t e r a t i o n , the concentration of d i s s o l v e d i o n s and t h e Eh-pH c o n d i t i o n s .
Tectonism may a f f e c t t h e r a t e of
groundwater c i r c u l a t i o n and a l s o t h e i n t e r s t i t i a l t e m p e r a t u r e and p r e s s u r e ; u p l i f t o f t h e s o u r c e r e g i o n s t e n d s t o i n c r e a s e t h e c i r c u l a t i o n r a t e and d e c r e a s e t h e t e m p e r a t u r e and p r e s s u r e ; b u r i a l on t h e o t h e r hand t e n d s t o d e c r e a s e t h e c i r c u l a t i o n r a t e and i n c r e a s e t h e t e m p e r a t u r e and p r e s s u r e .
Hence c l i m a t i c o r t e c t o n i c changes
322 a r e l i k e l y t o produce a n immediate r e s p o n s e o f t h e m i n e r a l assemblage t o t h e changes i n t h e i n t e r s t i t i a l environment. Walker e t a l . (1978) have p r e s e n t e d a s c h e m a t i c r e p r e s e n t a t i o n of t h e d i a g e n e s i s changes which have been observed o r i n f e r r e d i n t h e i r s t u d i e s o f Cenozoic s a n d s t o n e s and conglomerates i n t h e s o u t h w e s t USA and n o r t h w e s t e r n Mexico (Fig.5.25). stages a r e recognized:
Stage 1
-
mechanical i n f i l t r a t i o n of c l a y , S t a g e I1
Three
-
s i g n i f i c a n t a l t e r a t i o n of p y r o x e n e s , amphiboles and c a l c i c p l a g i o c l a s e , S t a g e 111 complete l o s s of p y r o x e n e s , amphiboles and most c a l c i c p l a g i o c l a s e .
-
K - f e l d s p a r and
q u a r t z may show e v i d e n c e of a l t e r a t i o n , newly formed c l a y m i n e r a l s . Many a n c i e n t c o n t i n e n t a l r e d beds show e v i d e n c e of S t a g e I11 a l t e r a t i o n s and can be i n f e r r e d t o have r e s u l t e d from a c o n t i n u a t i o n o f t h o s e p r o c e s s e s c o n s i d e r e d t o be o p e r a t i v e i n f i r s t - c y c l e d e s e r t a l l u v i u m .
T h i s e v i d e n c e may be summarized a s
follows :
1) a b s e n c e of f e r r o m a g n e s i a n s i l i c a t e s such a s h o r n b l e n d e and pyroxene i n r e d beds d e r i v e d from s o u r c e r o c k s i n which t h e y were p l e n t i f u l
2 ) s a n d - s i z e d c o n c e n t r a t i o n s of c l a y squeezed between r e s i s t a n t framework g r a i n s . I f t h e c l a y i s h a e m a t i t e - r i c h i t may r e p r e s e n t a c l a y pseudomorph a f t e r a f e r r o -
magnesian s i l i c a t e
3) absence of s i l t grade d e t r i t u s . framework g r a i n s w i t h a c l a y m a t r i x .
Many a n c i e n t r e d beds c o n s i s t of s a n d - s i z e d Such t e x t u r e s a r e n o t produced d e p o s i t i . o n a l l y
and a r e b e s t e x p l a i n e d i f t h e c l a y i s t h e p r o d u c t of i n f i l t r a t i o n a n d / o r a u t h i g e n e s i s
4 ) d i s s o l u t i o n and c l a y r e p l a c e m e n t of f e l d s p a r s i n d i c a t e s t h a t i n t r a s t r a t a l s o l u t i o n must have p r o g r e s s e d t o t h e p o i n t when r e l a t i v e l y s t a b l e g r a i n s a r e b e i n g a t t a c k e d ; t h u s a n i n d i c a t i o n t h a t l e s s s t a b l e g r a i n s might have been c o m p l e t e l y d i s s o l v e d .
5 ) e u h e d r a l a u t h i g e n i c h a e m a t i t e i s common i n a n c i e n t r e d beds.
It i s frequently
c o a r s e enough t o be o b s e r v e d i n t h i n s e c t i o n and w i t h She SEM i s s e e n t o form p l a t y hexagonal c r y s t a l s and r o s e t t e s of hexagonal c r y s t a l s
6 ) a c h a r a c t e r i s t i c a u t h i g e n i c s u i t e o f q u a r t z , K - f e l d s p a r , c a l c i t e and c l a y m i n e r a l s i s p a r t i c u l a r l y common i n a n c i e n t r e d beds and t e s t i f i e s t h e former e x i s t e n c e o f i n t e r s t i t i a l w a t e r r i c h i n d i s s o l v e d i o n s which were r e l e a s e d by i n t r a s t r a t a l dissolution.
323
CHAPTER 6 THE MINERALOGY AND GEOCHEMISTRY OF IRON OXIDES I N RED BEDS I R O N OXIDE. MINERALS
Abundant i r o n - t i t a n i u m o x i d e m i n e r a l s a r e one o f t h e most c h a r a c t e r i s t i c f e a t u r e s of c o n t i n e n t a l r e d beds.
They a r e n o t o n l y r e s p o n s i b l e f o r t h e
c o l o u r a t i o n o f t h e s e r o c k s b u t a l s o r e p r e s e n t t h e end p r o d u c t s of t h e i r d i a g e n e s i s and a r e r e s p o n s i b l e f o r t h e rock magnetism and hence p a l a e o m a g n e t i c p r o p e r t i e s of red beds,
I r o n - t i t a n i u m o x i d e s which o c c u r i n r e d beds show a wide range of
t e x t u r a l and c o m p o s i t i o n a l v a r i a t i o n b u t can c o n v e n i e n t l y b e c o n s i d e r e d i n two g r o u p s : f i n e p a r t i c l e pigmentary o x i d e s , p r i n c i p a l l y c o n s i s t i n g o f h a e m a t i t e b u t a l s o c o n t a i n i n g o t h e r f e r r i c oxyhydroxides, and c o a r s e p a r t i c l e opaque o x i d e s which contain a v a r i e t y of iron- titanium oxide minerals.
The m i n e r a l o g - i c a l p r o p e r t i e s
o f t h o s e i r o n - t i t a n i u m o x i d e s which commonly o c c u r i n r e d beds a r e l i s t e d i n T a b l e 6.1. The i r o n - t i t a n i u m o x i d e s can b e r e p r e s e n t e d by t h e FeO-Ti02-Fe 0 t e r n a r y 2 3 The t h r e e end p o i n t s o f t h e diagram a r e r e p r e s e n t e d by diagram ( F i g . 6 . 1 ) . h a e m a t i t e and maghemite (Fe 0 ) , r u t i l e ( T i 0 2 ) and w U s t i t e (FeO), W i t h i n t h e 2 3 t e r n a r y diagram t h r e e i m p o r t a n t s o l i d s o l u t i o n s e r i e s a r e r e p r e s e n t e d : 1) t h e t i t a n o m a g n e t i t e s v a r y i n g between m a g n e t i t e (Fe 0 ) and ulvklspinel (Fe T i 0 ) which 3 4 2 4 2 ) t h e h a e m a t i t e - i l m e n i t e s e r i e s v a r y i n g between
have t h e s p i n e l s t r u c t u r e
h a e m a t i t e (Fe 0 ) and i l m e n i t e (FeTiO ) which have t h e rhombohedra1 s t r u c t u r e and 3 2 3 3) t h e p s e u d o b r o o k i t e s e r i e s (Fe T i 0 ) w i t h orthorhombic s t r u c t u r e . 2 5 The titanomannetites-titanomazhemites W i t h i n t h e FeO-Ti0 -Fe 0 t e r n a r y diagram t h e most i m p o r t a n t group o f m i n e r a l s 2 2 3 a r e t h o s e w i t h t h e s p i n e l s t r u c t u r e which l i e i n t h e Fe304-Fe2Ti0 Fe2Ti0 -Fe 0 45 2 3 quadrilateral. T h i s s y s t e m c o n t a i n s t h e t i t a n o m a g n e t i t e s and t h e i r o x i d i z e d e q u i v a l e n t s t h e titanomaghemites ( F i g . 6 . 2 ) .
The l e f t - h a n d s i d e of t h e q u a d r i l a t e r a l
r e p r e s e n t s t h e t i t a n o m a g n e t i t e s and t h e b a s e r e p r e s e n t s t h e s o l i d s o l u t i o n between m a g n e t i t e and maghemite (l/-Fe 0 ). The c o o r d i n a t e s ( x , z ) a r e used t o s p e c i f y 2 3 p o i n t s w i t h i n t h e q u a d r i l a t e r a l where p o i n t s on t h e l e f t - h a n d s i d e c o r r e s p o n d t o z = 0
( O < x < l ) and t h o s e on t h e b a s e t o x = 0 ( O L z < l ) .
The p a r a m e t e r z i s
t h e r e f o r e a n o x i d a t i o n p a r a m e t e r and when z = 1 a l l t h e i r o n i s p r e s e n t a s Fe
3+
T h i s i s t h e c a s e w i t h maghemite which may be r e p r e s e n t e d by t h e formula: Fe3+ a13 0-: ( x = 0 , z = I ) , where r e p r e s e n t s a v a c a n t l a t t i c e s i t e : Maghemite may
a
.
D113
t h e r e f o r e b e d e s c r i b e d a s a c a t i o n d e f i c i e n t s p i n e l and i s one of a whole s e r i e s of i r o n - t i t a n i u m c a t i o n d e f i c i e n t s p i n e l s which a r e r e f e r r e d t o a s t h e
324
Ruth
TAN- BROWN
*BLUE
Hagnotito
Haomatito Maghaomit.
Fig.6.1. The FeO-Ti02-Fe203 t e r n a r y system, showing t h e p r i n c i p a l s o l i d s o l u t i o n series. The dashed l i n e s are some l i n e s of c o n s t a n t Fe:Ti r a t i o a l o n g which o x i d a t i o n may proceed. I s o t r o p i c , a n i s o t r o p i c , and t h e c o l o u r v a r i a t i o n s are t h e o p t i c a l p r o p e r t i e s as viewed i n r e f l e c t e d l i g h t oil-immersion. titanomaghemites.
Those s p i n e l s w i t h a c a t i o n f a n i o n r a t i o o f 314, i.e. when
z = 0 a r e referred to a s the titanomagnetites.
I n n a t u r e s u c h compounds a r e
probably r a t h e r s c a r c e and i t i s common p r a c t i c e t o use t h e term t i t a n o m a g n e t i t e f o r b o t h t h e t r u e t i t a n o m a g n e t i t e s , and a l s o f o r t h e i r o x i d i z e d e q u i v a l e n t s , t h e titanmaghemites.
To overcome t h e c o n f u s i o n O ' R e i l l y (1976) s u g g e s t s r e s t r i c t i n g
t h e term t i t a n o m a g n e t i t e t o t h o s e s p i n e l s w i t h a n o x i d a t i o n parameter 240.1. I r o n - t i t a n i u m s p i n e l s w i t h z h 0 . 1 s h o u l d b e r e f e r r e d t o a s titanomaghemite.
A
g e n e r a l formula f o r a l l t h e i r o n - t i t a n i u m s p i n e l s may be w r i t t e n : 3+ 2+ 02Fe 2-2x + z ( I + x ) R Fe ( I + x ) ( l - z ) R 3(1-R) 4 where R = 8 1 8 + z ( l + x ) . The c r y s t a l s t r u c t u r e of t h e s e s p i n e l s c o n s i s t s of a c l o s e packed a n i o n l a t t i c e w i t h c a t i o n s having f o u r f o l d c o o r d i n a t i o n ( t e t r a h e d r a l s i t e s ) and s i x f o l d coordination (octahedral s i t e s ) .
Each a n i o n has t h r e e o c t a h e d r a l s i t e c a t i o n s
and one t e t r a h e d r a l s i t e c a t i o n a s n e a r e s t neighbours. The c a t i o n d i s t r i b u t i o n f o r m a g n e t i t e i s u s u a l l y w r i t t e n Fe 3+LFe3+, Fez+]
O4
where c a t i o n s o u t s i d e t h e b r a c k e t a r e i n t e t r a h e d r a l s i t e s and t h o s e i n s i d e a r e i n octahedral s i t e s .
I n a c t u a l f a c t t h e c a t i o n d i s t r i b u t i o n i s more c o r r e c t l y
325
113 Fo304 magn6titi
haomatito maghomito
The Fe 0 -Fe T i 0 -Fe T i 0 -Fe 0 q u a d r i l a t e r a l r e p r e s e n t i n g t h e Fig.6.2. c o m p o s i t i o n a l zone f o r s i n g l e phase compounds w i t h s p i n e l s t r u c t u r e below a b o u t 400OC. The h a t c h e d r e g i o n r e p r e s e n t s t h e same zone a t a b o u t 1 2 O O O C ( a f t e r O ' R e i l l y , 1976).
I
w r i t t e n Fe3+
04 b e c a u s e above t h e Verwey t r a n s i t i o n (420°K)
e l e c t r o n hopping between c a t i o n s on o c t a h e d r a l s i t e s r e s u l t s i n i n t e r m e d i a t e oxidation state.
Below t h e Verwey t r a n s i t i o n t h e s t r u c t u r e becomes orthorhombic
and t h e r e a r e f o u r o c t a h e d r a l s p e c i e s s i n c e two o c t a h e d r a l s i t e s now o c c u r .
The
c a t i o n d i s t r i b u t i o n i n i n t e r m e d i a t e members of t h e t i t a n o m a g n e t i t e s e r i e s i s o n l y p o o r l y known and v a r i o u s models have been proposed (Akimoto, 1954; Ngel, 1955; B a n e r j e e a n d O ' R e i l l y , 1966).
B a n e r j e e e t a l . (1967) concluded t h a t t e t r a h e d r a l
s i t e s c o n t a i n b o t h Fe2+ and Fe3+ i o n s t h r o u g h o u t most of t h e series.
A useful
r e v i e w of t h e s u b j e c t i s g i v e n by L i n d s l e y (1976). I n t h e m a g n e t i t e - u l v b s p i n e l series t h e r e i s complete s o l i d s o l u t i o n a t t e m p e r a t u r e s above 6OO0C ( F i g . 6 . 4 ) .
During c o o l i n g t h e s o l i d s o l u t i o n i s much
more r e s t r i c t e d and t h e r e i s a tendency f o r e x s o l u t i o n o f t h e two p h a s e s t o occur.
True e x s o l u t i o n t e x t u r e s a r e , however, r e l a t i v e l y s c a r c e i n m a g n e t i t e -
ulvijspinel s o l i d s o l u t i o n s .
The commonly o b s e r v e d i n t e r g r o w t h s of i l m e n i t e i n
t i t a n o m a g n e t i t e a r e now c o n s i d e r e d t o form d i r e c t l y as a r e s u l t of o x i d a t i o n a t low t o moderate p r e s s u r e s and t e m p e r a t u r e s above 60OoC.
These i l m e n i t e i n t e r g r o w t h s
i n t i t a n o m a g n e t i t e may b e d i v i d e d i n t o t h e f o l l o w i n g t e x t u r a l forms a c c o r d i n g t o t h e Buddington and L i n d s l e y (1964) c l a s s i f i c a t i o n :
326 a)
trellis types
b)
composite ( g r a n u l a r ) t y p e s
c)
sandwich t y p e s
The t r e l l i s t y p e s a r e t h e most common and a r e c l e a r l y o x i d a t i o n p r o d u c t s o f magnetite-ulv8spinel.
The composite and sandwich t y p e s a r e l e s s common and
p r o b a b l y c o n t a i n i l m e n i t e s which formed a s p r i m a r y p r e c i p i t a t e s .
A well-illustrated
d e s c r i p t i o n of t h e v a r i o u s t e x t u r e s i s g i v e n by Haggerty ( 1 9 7 6 a ) .
Fig.6.3. P a r t o f t h e s p i n e l u n i t c e l l . Large open c i r c l e s r e p r e s e n t a n i o n s , s m a l l open c i r c l e s , o c t a h e d r a l s i t e c a t i o n s , and s m a l l h a t c h e d c i r c l e s , t e t r a h e d r a l s i t e c a t i o n s . The broken l i n e shows t h e t r i g o n a l a x i s f o r t h e c e n t r a l o c t a h e d r a l site. A d i f f e r e n t o x i d a t i o n mechanism which o p e r a t e s d u r i n g d e u t e r i c c o o l i n g r e s u l t s i n
f o r m a t i o n of c a t i o n d e f i c i e n t s p i n e l s ( t i t a n o m a g h e m i t e s ) . a t t e m p e r a t u r e s i n t h e r a n g e 400-600°C
This takes p l a c e i n i t i a l l y
and i s a common f e a t u r e of submarine b a s a l t s .
This t y p e o f o x i d a t i o n ( m a g h e m i t i z a t i o n ) a l s o c o n t i n u e s d u r i n g submarine w e a t h e r i n g . This i s s u g g e s t e d by p i l l o w l a v a s which show r a d i a l v a r i a t i o n s i n t h e d e g r e e of m a g h e m i t i z a t i o n , t h e s u r f a c e s b e i n g most h i g h l y o x i d i z e d ( R y a l l and Ade-Hall,
1975).
The h a e m a t i t e - i l m e n i t e s e r i e s Haematite (er-Fe203) and i l m e n i t e (FeTiOg) a r e rhombohedra1 o x i d e s which form a s o l i d s o l u t i o n s e r i e s t h a t i s complete a t h i g h t e m p e r a t u r e s ( c e r t a i n l y above 96OoC and p r o b a b l y above 80OoC) b u t i n t e r r u p t e d by a m i s c i b i l i t y gap a t lower t e m p e r a t u r e s ( L i n d s l e y , 1973).
Haematite has t h e corundum s t r u c t u r e which c o n s i s t s o f a
321
t
-v 1200
Fig.6.4. 1974).
Phase diagram f o r t h e b i n a r y t i t a n o m a g n e t i t e system ( a f t e r Hauptman,
hexagonal-close
packed a n i o n l a t t i c e w i t h c a t i o n s f i t t e d i n t o t h e i n t e r s t i c e s ,
t h e rhombohedra1 u n i t c e l l c o n t a i n i n g f o u r c a t i o n s and s i x a n i o n s .
All c a t i o n
s i t e s a r e e q u i v a l e n t w i t h d i s t o r t e d o c t a h e d r a l symmetry which a r i s e s b e c a u s e of e l e c t r o s t a t i c r e p u l s i o n between n e a r e s t - n e i g h b o u r c a t i o n s i n a d j a c e n t b a s a l planes (Fig.6.5). The c r y s t a l s t r u c t u r e of i l m e n i t e i s v e r y s i m i l a r t o t h a t o f h a e m a t i t e b u t t h e morphology of n a t u r a l i l m e n i t e c r y s t a l s has shown t h a t t h e s p a c e group must have a lower symmetry and R? ( a subgroup of R ~ c )was chosen f o r i l m e n i t e o n - t h i s basis,
Compared t o t h e h a e m a t i t e s t r u c t u r e , one h a l f of t h e i r o n i o n s a r e
r e p l a c e d by t i t a n i u m i n s u c h a way t h a t e v e r y Fe-03-Fe u n i t , and t h e sequence of c a t i o n s a l o n g any @ O O g
u n i t becomes a Fe-03-Ti
axis is
.. ., . Fe-Ti-LI) -Ti-Fe-n-Fe
where t h e hyphens r e p r e s e n t i n t e r v e n i n g p l a n e s o f oxygen l a y e r s .
As i n h a e m a t i t e ,
e v e r y t h i r d p o t e n t i a l o c t a h e d r a l s i t e i s empty; t h e n e a r e s t c a t i o n s t o a n empty
s i t e along arranged i n
0001 0001
b e i n g two i r o n o r two t i t a n i u m .
The Fe-0 - T i u n i t s a r e so 3 t h a t oxygen l a y e r s a l t e r n a t e w i t h m e t a l l a y e r s , which themselves
a l t e r n a t e between l a y e r s of i r o n and l a y e r s of t i t a n i u m . Haematite and i l m e n i t e form a complete s o l i d s o l u t i o n s e r i e s a t h i g h t e m p e r a t u r e s . 2+ The c o m p o s i t i o n may be e x p r e s s e d Fe3+ Fe T i 03, where x i s t h e mole f r a c t i o n of 2-2x x i l m e n i t e . I n t e r m e d i a t e s o l i d s o l u t i o n s c o n t a i n Fe2+ and Fe3+ i n amounts which a r e a p p r o x i m a t e l y p r o p o r t i o n a l t o x and (2-2x) r e s p e c t i v e l y , so i t i s p r o b a b l e t h a t 4+ titanium i s present only a s T i Solvus r e l a t i o n s h i p s o f t h e h a e m a t i t e - i l m e n i t e
.
s e r i e s were e s t a b l i s h e d by Carmichael (1961) on n a t u r a l m a t e r i a l , and a r e v i s e d , e x p e r i m e n t a l s o l v u s was p r e s e n t e d by L i n d s l e y (1973).
The c o n s o l u t e t e m p e r a t u r e
328
Fig.6.5. The corundum s t r u c t u r e . The c a t i o n s ( s m a l l h a t c h e d c i r c l e s ) l i e i n t h e i n t e r s t i c e s between t h e close-packed a n i o n l a t t i c e . Broken l i n e s i n d i c a t e t h e t r i g o n a l a x i s . The d i s t o r t i o n o f oxygen t r i a n g l e s and d i s p l a c e m e n t o f c a t i o n s from t h e median p l a n e a r e a l s o i n d i c a t e d . ( c r i t i c a l p o i n t f o r t h e s o l v u s ) may b e lower t h a n 800°C and t h e m i s c i b i l i t y gap, a l t h o u g h n o t y e t d e t e r m i n e d p r e c i s e l y , o c c u p i e s a narrower f i e l d t h a n t h a t s u g g e s t e d by Carmichael (1961) ( F i g . 6 . 6 ) . E x s o l u t i o n of h a e m a t i t e from i l m e n i t e and v i c e v e r s a i s r e s t r i c t e d t o deeps e a t e d i n t r u s i o n s and i s p a r t i c u l a r l y c h a r a c t e r i s t i c of a n o r t h o s i t e s a n d o t h e r b a s i c s u i t e s , but a l s o occurs i n g r a n i t i c s u i t e s .
The p l a n e o f e x s o l u t i o n i s
p a r a l l e l t o t h e (0001) rhombohedra1 d i r e c t i o n , t h e e x s o l u t i o n t a k i n g p l a c e i n a c c o r d w i t h decomposition a s a consequence o f slow c o o l i n g and s o l v i - i n t e r s e c t i o n . S i n g l e - p h a s e compounds o f i n t e r m e d i a t e composition a r e r a t h e r r a r e i n n a t u r e b u t o c c u r i n some quench-cooled e x t r u s i v e i g n e o u s r o c k s (Haggerty, 1976b). The terminology of Buddington e t a l . (1963) i s c o n v e n i e n t t o u s e f o r i n t e r mediate members o f t h e h e a m a t i t e - i l m e n i t e series: F e r r i a n i l m e n i t e i s a one-phase g r a i n o f i l m e n i t e c o n t a i n i n g v a r y i n g amounts of Fe 0 i n s o l i d s o l u t i o n . 2 3 T i t a n h a e r n a t i t e i s a one-phase g r a i n of h a e m a t i t e c o n t a i n i n g v a r y i n g q u a n t i t i e s of FeTiOg i n s o l i d s o l u t i o n . Haemo-ilmenite i s a two-phase g r a i n c o n s i s t i n g o f t i t a n h a e m a t i t e l a m e l l a e e n c l o s e d i n a ferrianilmenite host. Ilmeno-haematite i s a two-phase g r a i n c o n s i s t i n g o f f e r r i a n i l m e n i t e l a m e l l a e enclosed i n a titanhaematite host. T i t a n h a e m a t i t e s a r e o f p a r t i c u l a r i n t e r e s t b e c a u s e o f t h e i r remarkable m a g n e t i c p r o p e r t i e s which i n c l u d e s e l f - r e v e r s a l (Wescott-Lewis and P a r r y , 1971 a , b ) .
329 Hoffman (1975) proposed a mechanism by which s e l f - r e v e r s a l m i g h t o c c u r .
I
I
8001
700
600 00
60
40
Fe Ti O3 Ilmenite
Mole Percent Fe Ti
O3
1
20
Fe203 Hciematite
Fig.6.6. The j o i n Fe203-FeTiO showing t h e h a e m a t i t e - i l m e n i t e m i s c i b i l i t y gap of Carmichael (1961) (hoop-shaped c u r v e ) and L i n d s l e y ’ s (1973) e s t i m a t e of t h e r e g i o n ( s t i p p l e d a r e a ) w i t h i n which t h e m i s c i b i l i t y gap must l i e . P means t h a t p s e u d o b r o o k i t e s o l i d s o l u t i o n s , R t h a t r u t i l e , was p r e s e n t i n t h e r i m p r o d u c t s ( a f t e r L i n d s l e y , 1973). F e r r i c Oxvhvdroxides The n a t u r a l l y o c c u r r i n g forms o f f e r r i c oxyhydroxide a r e l i s t e d i n T a b l e 6.1. T h e i r o c c u r r e n c e , e x c e p t f o r a k a g a n g i t e , have been d e s c r i b e d by P a l a c h e e t a l . (1944). AkaganLite i s a r a r e m i n e r a l , o c c u r r e n c e s h a v i n g been d e s c r i b e d by Van T a s s e l (1959) and Chandy (1962).
I t i s noteworthy b e c a u s e i t o c c u r s i n m e t e o r i t e s and l u n a r rocks
a s a n o x i d a t i o n p r o d u c t o f l a w r e n c i t e (FeC1 ) and n a t i v e n i c k e l - i r o n ( T a y l o r e t a l . , 2 1974). Amorphous f e r r i c oxyhydroxide i s u s u a l l y g i v e n t h e formula Fe(OH)3 f o r convenience.
P o o r l y c r y s t a l l i n e , c o l l o i d a l m a t e r i a l , w i t h a composition n e a r
2.5 FegO3 4.5 H 2 0 , and a p p a r e n t l y w i t h a d i s t o r t e d h a e m a t i t e - t y p e s t r u c t u r e , i s
g i v e n t h e name f e r r i h y d r i t e (Towe and B r a d l e y , 1967).
I t may b e a s s o c i a t e d w i t h
b a c t e r i a l o x i d a t i o n o f f e r r o u s i r o n s o l u t i o n s (Chukrov, 1973).
Amorphous f e r r i c
oxyhydroxide and g o e t h i t e a r e t h e main c o n s t i t u e n t s of l i m o n i t e which i s t h e name
330 TABLE 6.1. N a t u r a l l y o c c u r r i n g forms of f e r r i c oxyhydroxide F e r r i c oxyhydroxide
I d e a l Formula
Amorphous
i n d e f i n i t e Fe (OH)
Goethite
FeOOH
Akaganeite
FeOOH
Lepidocrocite
FeOOH
Haematite
Fe203 Fe 0 2 3
Maghemi t e
g i v e n t o t h e yellow t o brown c o l o u r e d f e r r i c oxyhydroxide complexes which form a s p r e c i p i t a t e s i n n a t u r a l w a t e r s and a s a n o x i d a t i o n p r o d u c t of i r o n - b e a r i n g minerals.
On d e h y d r a t i o n , o r a g e i n g , g o e t h i t e u s u a l l y i n v e r t s t o h a e m a t i t e .
Haematite i s t h e most s t a b l e f e r r i c o x i d e and i s t h e p r i n c i p a l c o l o u r i n g a g e n t o f r e d beds.
Maghemite o c c u r s mainly a s a n o x i d a t i o n p r o d u c t o f m a g n e t i t e , and
more r a r e l y by t h e d e h y d r a t i o n o f l e p i d o c r o c i t e .
T h i s l a t t e r f e r r i c oxyhydroxide
o c c u r s a s a p r e c i p i t a t e i n n a t u r a l w a t e r s , and i s found r a r e l y i n c o n t i n e n t a l r e d beds.
OPAQUE OXIDES IN RED BEDS
U n f o r t u n a t e l y t h e r e have been few d e t a i l e d s t u d i e s of t h e opaque o x i d e s i n e i t h e r Recent s e d i m e n t s o r a n c i e n t r e d beds and we owe much of o u r p r e s e n t knowledge t o t h e c o n t r i b u t i o n by Van Houten ( 1 9 6 8 ) .
O t h e r o b s e r v a t i o n s have
been made by Chamalaun ( 1 9 6 3 ) , P i c a r d ( 1 9 6 5 ) , and Turner and I x e r (1977). A c c u r a t e c h a r a c t e r i z a t i o n and u n d e r s t a n d i n g o f t h e opaque o x i d e s i n r e d beds i s e s s e n t i a l f o r a p r o p e r u n d e r s t a n d i n g of t h e i r d i a g e n e t i c and p a l a e o m a g n e t i c
history.
Opaque o x i d e s can p r o v i d e u s e f u l i n f o r m a t i o n a b o u t t h e provenance of
c o n t i n e n t a l a l l u v i u m and t h e y a r e a l s o r e s p o n s i b l e f o r t h e m a g n e t i c p r o p e r t i e s of most c o n t i n e n t a l r e d b e d s .
Consequently t h e o c c u r r e n c e o f t h e v a r i o u s o x i d e s
i n igneous r o c k s , and t h e i r d e r i v e d e q u i v a l e n t s i n c o n t i n e n t a l r e d b e d s , a r e described systematically.
In g e n e r a l , t h e c o m p o s i t i o n s a n d modal abundances o f
p r i m a r y opaque o x i d e s i n igneous r o c k s depend upon t h e i n i t i a l b u l k c h e m i s t r y o f t h e h o s t r o c k , on t h e d e p t h of emplacement, and on t h e oxygen f u g a c i t y o f t h e c r y s t a l l i z i n g magma.
S i n c e FeO, T i 0 2 and Cr203 d e c r e a s e w i t h S i 0 2 , b a s i c r o c k s
t e n d t o c o n t a i n l a r g e r c o n c e n t r a t i o n s of o x i d e s t h a n t h e i r i n t e r m e d i a t e s u i t e s
o r a c i d end members.
The m i n e r a l o g i c a l p r o p e r t i e s of t h e commonly o c c u r r i n g
i r o n - t i t a n i u m o x i d e s a r e l i s t e d i n T a b l e 6.2. Titanomagnetite T i t a n o m a g n e t i t e o c c u r s commonly i n i n t e r m e d i a t e and b a s i c i g n e o u s r o c k s where
331 TABLE 6.2. Mineralogical p r o p e r t i e s of iron-titanium oxides i n polished s e c t i o n . Name, chemical formula and symmetry
Optical properties
Reflectance i n % (wavelength = 589nm)
Microindentat i o n hardness
Magnetite -304 Cubic
Grey, c o m o n l y w i t h brownish t i n t . Much d a r k e r t h a n haematite. Isotropic
20.3 ( A i r ) 8.00 ( O i l )
592 ( l o a d of 100)
Ulvbspinel Fe2TiOq Cubic
Brown-reddish brown I s o t r o p i c . Usually found a s f i n e e x s o l ution bodies i n ti tanomagneti t e
16.35 ( A i r )
No d a t a
Maghaemi t e y-,Fe203 Cubic
B l u i s h - g r e y . Darker than haematite. Isotropic
22.8 ( A i r ) 9.00 ( O i l )
412 ( l o a d of 5 0 )
Ha ema t i te oc-Fe203 Hexagona 1
Grey-white w i t h a b l u i s h t i n t . Anisot r o p i c : greyish-b lue g r e y i s h yellow. Deep red internal reflections
29.15-25.1 ( A i r ) 14.4-11.0 ( O i l )
1038 (mean of 3 r a n g e s 100, 200, 300 )
Ilmenite FeTi03 Trigonal
Brown sometimes w i t h a pink o r v i o l e t t i n t . Anisotropic: l i g h t greenish grey, brownish g r e y
20.2-17.4 ( A i r ) 7.6-5.8 ( O i l
519-533 ( l o a d o f 100) 659-703
Goe t h i t e
Dull grey t o b r i g h t grey. Anisotropic: g r e y - b l u e , greyy e l l o w , brown, greenish- grey. Brownish-yellow internal reflections
16.1-15.0 5.05-4.0
(Air) (Oil)
Greyish-whi te. Anisotropic: shades o f g r e y . Reddish internal reflections
17.4-11.1 5.55-2.0
(Air) (Oil)
u - FeOOH
Orthorhombic
Lepidocrocite Y-FeOOH Orthorhombic
402 (mean of 3 r a n g e s 100, 200, 300)
i t o f t e n comprises up t o f i v e p e r c e n t by volume.
Increased s u b s t i t u t i o n of
t i t a n i u m i s c h a r a c t e r i s t i c of b a s i c igneous r o c k s and h e r e t i t a n o m a g n e t i t e s 3+ 2+ (Fe2-2x Fel+x T i 4 + 02-)w i t h x a p p r o x i m a t e l y i n t h e range 0.54xCO.8 a r e t y p i c a l . x 4 Titanium poor m a g n e t i t e s a r e u s u a l i n a c i d igneous r o c k s . O t h e r c a t i o n s a r e u s u a l l y p r e s e n t i n t i t a n o m a g n e t i t e s , i n c l u d i n g manganese, n i c k e l , z i n c , and chromium ( C r e e r and I b b e t s o n , 1970).
O r i e n t e d i n c l u s i o n s and o x i d a t i o n l a m e l l a e
p a r a l l e l t o (111) a r e common and i n c l u d e h a e m a t i t e , i l m e n i t e and r u t i l e .
The
o c c u r r e n c e o f t i t a n o m a g n e t i t e s i n igneous rocks i s d e s c r i b e d i n g r e a t d e t a i l by Haggerty (1976b).
Other u s e f u l references i n c l u d e
V i n c e n t and P h i l l i p s ( 1 9 5 4 ) ,
Katsura and Kushiro ( 1 9 6 1 ) , Buddington and L i n d s l e y ( 1 9 6 4 ) , and B u t l e r (1973). T i t a n o m a g n e t i t e i s r e a d i l y o x i d i z e d i n t h e s e d i m e n t a r y environment and u n a l t e r e d g r a i n s a r e s c a r c e i n Recent sediments and s o i l s . i n alluvium.
F r e s h g r a i n s most commonly o c c u r
Van Andel (1959, p. 155), Walker (1967a? p. 365) and Van Houten
(1968, Table 1 and 2 ) a r e amongst t h e few a u t h o r s who r e p o r t t h e o c c u r r e n c e o f r e l a t i v e l y u n a l t e r e d t i t a n o m a g n e t i t e g r a i n s ; some of t h e s e are e u h e d r a l and are less a l t e r e d t h a n t i t a n o m a g n e t i t e g r a i n s i n s o i l s and s a p r o p e l s , a f e a t u r e
i n d i c a t i n g d i r e c t c o n t r i b u t i o n from l i t t l e weathered bedrock.
Unfortunately,
d e s p i t e v a s t numbers o f heavy m i n e r a l a n a l y s e s o f Recent a l l u v i u m , t h e r e i s v i r t u a l l y no i n f o r m a t i o n on t h e d e t a i l e d mineralogy o f i r o n o x i d e s ; most a u t h o r s have been c o n t e n t t o group them a l l a s "opaques" d e s p i t e t h e f a c t t h a t t h e y o f t e n form t h e m a j o r i t y o f t h e heavy m i n e r a l f r a c t i o n .
Most of t h e i n f o r m a t i o n r e g a r d i n g
t i t a n o m a g n e t i t e comes from d e t r i t a l c o n c e n t r a t e s i n beach sands.
Wright (1964) h a s
d e s c r i b e d t h e o x i d a t i o n of t i t a n o m a g n e t i t e d e r i v e d from v o l c a n i c rocks i n New Zealand beach sands.
These show p a r t i a l t o complete a l t e r a t i o n t o homogeneous
t i t a n h a e m a t i t e o r t o t i t a n h a e m a t i t e and pseudobrookite.
Some o f t h e h a e m a t i t i z a t i o n
was c e r t a i n l y i n i t i a t e d i n t h e p a r e n t r o c k , b u t p o s t - d e p o s i t i o n a l a l t e r a t i o n a l s o takes p l a c e (Wright, 1964, p. 431).
Rapid o x i d a t i o n o f m a g n e t i t e and t i t a n o -
m a g n e t i t e t a k e s p l a c e i n t h e weathered zone o f igneous r o c k s and, i n r e s i d u a l p l a c e r s o f o x i d e m i n e r a l s o v e r 50 p e r c e n t h a e m a t i t i z a t i o n i s n o t uncomon (e.g. T s u s u e and I s h i h a r a ,
1975).
I n l a t e r i t i c s o i l s t i t a n o m a g n e t i t e s may b e weathered
t o s t r u c t u r e l e s s p e l l e t s o f brown and r e d o x i d e ( B r i g g s , 1960) and a l l t h e g r a i n s u s u a l l y show some e v i d e n c e o f h a e m a t i t i z a t i o n .
T h i s u s u a l l y t a k e s t h e form o f
r e d f i n e g r a i n e d rims o f h a e m a t i t e , o r t h i n h a e m a t i t e l a m e l l a e n e a r t h e g r a i n margin (Van Houten, 1968, p. 401). (Katsura e t a l . ,
S o i l s developed on b a s i c v o l c a n i c r o c k s
1962; C a r r o l l and Hathaway, 1963; Matsusaka e t a l . , 1965) c o n t a i n
much t i t a n o m a g n e t i t e which shows p a r t i a l t o complete o x i d a t i o n t o titanomaghernite.
In a n c i e n t r e d beds u n a l t e r e d g r a i n s o f m a g n e t i t e a r e v i r t u a l l y unknown.
Those
m a g n e t i t e s which do o c c u r form r e l i c t c o r e s w i t h i n h a e m a t i t e s and show e x t e n s i v e signs of oxidation.
There i s c o n s i d e r a b l e e v i d e n c e t h a t r a p i d h a e m a t i t i z a t i o n
( m a r t i t i z a t i o n ) o f m a g n e t i t e t a k e s p l a c e i n t h e d i a g e n e t i c environment.
This i s
333 s e e n i n t h e p r o g r e s s i v e i n c r e a s e o f m a r t i t e r e l a t i v e t o m a g n e t i t e i n more a n c i e n t r e d beds. Haema t i t e - I l m e n i t e The o c c u r r e n c e o f members o f t h e h a e m a t i t e - i l m e n i t e series i n i g n e o u s r o c k s i s d e s c r i b e d by Haggerty (1976b), and i n metamorphic r o c k s by Rumble (1976).
Members
o f t h e h a e m a t i t e - i l m e n i t e series form a minor c o n s t i t u e n t o f most i g n e o u s r o c k s ; a c i d s u i t e s , g r a n i t i c p e g m a t i t e s and c a r b o n a t i t e s t e n d t o b e r i c h e r i n p y r o p h a n i t e (MnTiO ) w h i l s t h a e m a t i t e - r i c h s o l i d s o l u t i o n s a r e r e s t r i c t e d t o a c i d i n t r u s i v e 3 s u i t e s and a n o r t h o s i t e s , where h a e m a t i t e s o l i d s o l u t i o n s a r e e x s o l v e d , and t h e s e coexist with ilmenite-rich solutions.
I n metamorphic r o c k s h a e m a t i t e - i l m e n i t e
s o l i d s o l u t i o n s a r e found i n a l l metamorphic zones i n a wide v a r i e t y of rock t y p e s . E x s o l u t i o n l a m e l l a e of i l m e n i t e i n h a e m a t i t e , o r h a e m a t i t e i n i l m e n i t e i s u s u a l l y observed.
A s m a l l amount o f p y r o p h a n i t e i s u s u a l l y p r e s e n t and i n magnesian-rich
r o c k s a n a p p r e c i a b l e c o n t e n t o f g e i k i e l i t e (MgTiO
i s n o t e d ( S p r i n g e r , 1974). 3 I n t h e s e d i m e n t a r y environment h a e m a t i t e is t h e most i m p o r t a n t i r o n oxide.
Coarse p a r t i c l e s o c c u r i n i n t i m a t e a d m i x t u r e w i t h o t h e r f i n e g r a i n e d o x i d e s i n many k i n d s o f soil, i n c l u d i n g l a t e r i t e s .
The v a s t m a j o r i t y of h a e m a t i t e i n
w e a t h e r e d p r o f i l e s a p p e a r s t o b e formed by t h e o x i d a t i o n of t i t a n o m a g n e t i t e . Under w e a t h e r i n g c o n d i t i o n s members o f t h e h a e m a t i t e - i l m e n i t e s e r i e s a r e more s t a b l e t h a n m a g n e t i t e (Mohr and Van Baren, 1954; Hartman, 1959; Katsura e t a l . , 1962).
A f t e r p r o l o n g e d o x i d a t i o n i l m e n i t e i t s e l f may b e o x i d i z e d ; i l m e n i t e
e x s o l u t i o n l a m e l l a e i n h a e m a t i t e may b e c o n v e r t e d t o h a e m a t i t e p l u s r u t i l e and i l m e n i t e i t s e l f may b e a l t e r e d t o form ' l e u c o x e n e ' .
This a p p e a r s milky w h i t e i n
r e f l e c t e d l i g h t and normally c o n s i s t s o f f i n e l y d i v i d e d r u t i l e , o r more r a r e l y brookite.
B a i l e y e t a l . (1956) r e c o g n i z e d t h r e e s t a g e s o f i l m e n i t e o x i d a t i o n i n
beach s a n d s : p a t c h y i l m e n i t e
amorphous i r o n - t i t a n i u m o x i d e
r u t i l e , a n a t a s e , and sphene ( l e u c o x e n e ) .
finely crystalline
I n a n c i e n t r e d beds i l m e n i t e i s s c a r c e
and u n o x i d i z e d g r a i n s a r e l a r g e l y c o n f i n e d t o t h e d r a b zones i n r e d bed sequences ( M i l l e r and Folk, 1955; Van Houten, 1968). I n r e c e n t and a n c i e n t sediments t h e r e i s l i t t l e e v i d e n c e t h a t p r i m a r y c o a r s e p a r t i c l e haematites a r e volumetrically important.
I n f a c t haematites of t h i s type
may b e d i f f i c u l t t o r e c o g n i z e u n l e s s t h e y show obvious rhombohedra1 e x s o l u t i o n textures.
The h a e m a t i t e s w i t h " o l e a n d e r - l e a f " shaped e x s o l u t i o n l a m e l l a e of
i l m e n i t e d e s c r i b e d by P h i l l i p s ( i n Van Houten, 1964, p.659) from t h e T o r r i d o n i a n Sandstone are p r o b a b l y primary g r a i n s d e r i v e d from i g n e o u s o r metamorphic r o c k s . , I n a n c i e n t r e d beds t h e opaque o x i d e g r a i n s are u s u a l l y r e f e r r e d t o a s specularite.
They show a wide range o f t e x t u r e s , b u t t h e v a s t m a j o r i t y show
e v i d e n c e o f having formed a f t e r t i t a n o m a g n e t i t e ( s e e F i g . 6 . 7 ) .
Such g r a i n s
a r e r e f e r r e d t o a s m a r t i t e s , and t h e f o r m a t i o n o f h a e m a t i t e from m a g n e t i t e i s
334
Fig.6.7. R e f l e c t e d l i g h t photomicrographs of t y p i c a l c o a r s e - p a r t i c l e i r o n o x i d e s i n c o n t i n e n t a l r e d beds. ' A . O c t a h e d r a l pseudomorph o f h a e m a t i t e a f t e r m a g n e t i t e . B . Haematite s h o i i n g m a r t i t i z a t i o n t e x t u r e ( b o t h from S t . Bees S a n d s t o n e , T r i a s s i c , U.K.). C . M a r t i t e ( R i n g e r i k e Group, S i l u r i a n , Norway). D . Haematite w i t h e x s o l u t i o n d i s c s o f i l m e n i t e and s y n t a x i a l a u t h i g e n i c h a e m a t i t e ( S t . Bees S a n d s t o n e ) E . Rhombohedra1 d e t r i t a l h a e m a t i t e . F . Haematite w i t h rhombohedra1 e x s o l u t i o n l a m e l l a e of i l m e n i t e ( b o t h from R i n g e r i k e Group). A l l g r a i n s a r e 1 0 0 - 2 0 0 ~i n diameter.
335 r e f e r r e d t o a s m a r t i t i z a t i o n (Ramdohr, 1969, p. 9 0 6 ) .
In p a r t i a l l y r e p l a c e d
m a g n e t i t e s , s u c h a s o c c u r i n Recent s e d i m e n t s and Cenozoic r e d b e d s , t h i n p l a t e s o f h a e m a t i t e a r e formed a l o n g t h e o c t a h e d r a l (111) p l a n e s of t h e m a g n e t i t e , resulting i n a triangular alteration pattern.
More r a r e l y t h e m a r t i t i z a t i o n may
develop p a r a l l e l t o growth zones w i t h i n t h e m a g n e t i t e c r y s t a l .
In more a n c i e n t
r e d beds i t i s more u s u a l t o f i n d t h a t o r i g i n a l m a g n e t i t e g r a i n s have been c o m p l e t e l y r e p l a c e d by h a e m a t i t e which s t i l l r e v e a l s t h e t r i a n g u l a r m a r t i t i z a t i o n texture i n polycrystalline grains.
F r e q u e n t l y such g r a i n s show e x s o l u t i o n zones Trellis
of i l m e n i t e which may b e a l t e r e d t o h a e m a t i t e w i t h r u t i l e o r a n a t a s e .
t e x t u r e i n p o l y c r y s t a l l i n e h a e m a t i t e g r a i n s a l s o i n d i c a t e s f o r m a t i o n of h a e m a t i t e a f t e r magnetite (Fig.6.7). Although m a r t i t i z a t i o n may o c c u r o v e r a wide t e m p e r a t u r e range i t i s c l e a r t h a t i t must b e mainly o f low-temperature o r i g i n i n a n c i e n t r e d beds.
However
t h e a c t u a l mechanism of m a r t i t i z a t i o n i n a n c i e n t r e d beds has n o t been s t u d i e d i n any d e t a i l and i t i s n o t c l e a r w h e t h e r h a e m a t i t e fX-Fe203)
forms d i r e c t l y
from m a g n e t i t e (Fe 0 ) o r whether a n i n t e r m e d i a t e p h a s e of maghemite (Y-Fe2O3) 3 4 o r a compound i n t e r m e d i a t e i n c o m p o s i t i o n between Fe304 and Fe 0 i s i n v o l v e d .
2 3
Colombo e t a l . (1964) m a i n t a i n e d t h a t below 6OO0C m a g n e t i t e n e v e r o x i d i z e d t o k - F e 2 0 3 u n l e s s c r - F e 2 0 3 n u c l e i were p r e s e n t b u t G a l l a g h e r e t a l . (1968) have c l e a r l y d e m o n s t r a t e d t h e d i r e c t o x i d a t i o n o f Fe3Opw-Fe2O3 in p a r t i c l e s l a r g e r t h a n 300011 and t e m p e r a t u r e s a s low a s 18OoC.
Under l a b o r a t o r y c o n d i t i o n s t h e
t r a n s i t i o n from Fe30C t o Y-Fe203 h a s been r e s t r i c t e d t o f i n e l y d i v i d e d , submicron s i z e d m a t e r i a l (Readman and O ' R e i l l y , 1975).
1972; Johnson and M e r r i l l , 1973; O'Donovan,
P a r t i c l e s below a b o u t 30002 i n d i a m e t e r always y i e l d )'-Fe203
whilst
p a r t i c l e s above t h i s s i z e a r e o x i d i z e d d i r e c t l y t o o+Fe203 v i a compounds i n t e r m e d i a t e i n composition between Fe304 and Fe203 ( G a l l a g h e r e t a l . , 1968). I m p u r i t i e s , e s p e c i a l l y aluminium and a l s o a d s o r b e d w a t e r , may be i m p o r t a n t i n s t a b i l i z i n g t h e s p i n e l s t r u c t u r e d u r i n g t h e Fe304 Y-Fe203 t r a n s i t i o n i n s m a l l p a r t i c l e s ( s e e B a t e , 1975, f o r a r e v i e w ) . The e x p e r i m e n t a l e v i d e n c e i s c o n s i s t e n t w i t h t h e view t h a t m a r t i t i z a t i o n most commonly p r o c e e d s w i t h o u t t h e f o r m a t i o n of a n i n t e r m e d i a t e Y-Fe 0 phase. 2 3 Van Houten (1968) has made t h e o n l y d e t a i l e d s u r v e y of opaque o x i d e s i n Recent a l l u v i u m and a n c i e n t r e d beds ( F i g . 6 . 8 ) .
The s e d i m e n t s s t u d i e d ranged from
P r e c a m b r i a n - P l e i s t o c e n e i n a g e and i n c l u d e d r e d beds and a s s o c i a t e d drab zones. F o r d e t a i l e d r e s u l t s of i n d i v i d u a l f o r m a t i o n s t h e r e a d e r i s r e f e r r e d t o Van Houten (1968, T a b l e s 4 , 5 and 6 ) .
When s t u d i e d a s a whole t h e r e s u l t s
r e v e a l two main t r e n d s ( F i g . 6 . 9 ) which have l a r g e l y been s u b s t a n t i a t e d by l a t e r workers and which may be summarized a s f o l l o w s :
1) Red beds c o n t a i n abundant h a e m a t i t i z e d b l a c k o x i d e g r a i n s and show a p r o g r e s s i v e i n c r e a s e i n h a e m a t i t e / m a g n e t i t e and h a e m a t i t e / i l m e n i t e r a t i o s i n s u c c e s s i v e l y o l d e r r e d beds.
T h i s d e p l e t i o n t r e n d c o u l d p a r t l y be due t o t h e f a c t t h a t many of t h e
336
Sequ*nr.r M
Sands
I
Fig.6.8. R e l a t i v e abundance o f h a e m a t i t e (H), m a g n e t i t e (M) and i l m e n i t e (I). A . Upland s o i l s B. Recent s e d i m e n t s , c o a s t a l Sonora, Mexico C . Cenozoic r e d beds D. Mesozoic r e d beds E . P a l a e o z o i c r e d beds F. Precambrian r e d beds G . Ancient d r a b sequences H. Black sands i n beach p l a c e r s ( a f t e r Van Houten, 1968).
337 more r e c e n t samples came from v o l c a n i c - r i c h f o r m a t i o n s (Van Houten, 1968, p. 403) b u t a number o f o t h e r f a c t o r s s u g g e s t t h a t t h e t r e n d is real.
These i n c l u d e t h e
f a c t t h a t m a g n e t i t e i s a l s o p r e s e n t i n Recent sediments and some Mesozoic r e d beds, w h i l s t i t i s v i r t u a l l y a b s e n t i n Precambrian and P a l a e o z o i c r e d beds.
The u b i q u i t y
o f m a r t i t e s i n a n c i e n t r e d beds a l s o s u g g e s t s t h a t m a g n e t i t e has been c o n v e r t e d t o h a e m a t i t e i n s i t u t h u s c o n t r i b u t i n g t o t h e observed d e p l e t i o n t r e n d . 2 ) Drab mudstone and s a n d s t o n e s i n r e d bed sequences g e n e r a l l y c o n t a i n v e r y few b l a c k o x i d e s and many o f t h e s e a r e i l m e n i t e o r leucoxene.
When compared w i t h t h e i r
a s s o c i a t e d r e d beds t h e s e d r a b zones n o t o n l y show fewer t o t a l g r a i n s b u t a g r e a t e r p r o p o r t i o n o f i l m e n i t e and T i 0 2 , f e a t u r e s which s u g g e s t t h a t p o s t - d e p o s i t i o n a l d i s s o l u t i o n o f b o t h pigmentary and c o a r s e - p a r t i c l e o x i d e s h a s t a k e n p l a c e ( M i l l e r and F o l k , 1955; Van Houten, 1968, p. 404), i n d i c a t i n g t h e g r e a t e r r e s i s t e n c e of i l m e n i t e and t i t a n i u m o x i d e s t o t h e r e d u c i n g s o l u t i o n s which remove i r o n o x i d e s from r e d beds.
MAGNETITE
HEMATITE
lLMENlTE
SILICATES
-*b
IN ALLUVIUM TRANSIT
YOUNQER
OLMR
Fig.6.9. O r i g i n and f a t e o f opaque o x i d e s and pigment i n r e d bed sequences. C o n f i g u r a t i o n s s u g g e s t changes i n abundance o f e a c h component r e l a t i v e to amount i n s o u r c e r o c k s ( a f t e r Van Houten, 1968).
338 These d a t a r e v e a l a n i m p o r t a n t p r o c e s s which i s i n s t r u m e n t a l i n p r o d u c i n g t h e c h a r a c t e r i s t i c o x i d e m i n e r a l o g y o f r e d beds.
This process i s t h e i n s i t u
c o n v e r s i o n of m a g n e t i t e t o h a e m a t i t e ; i t o p e r a t e d under o x i d i z i n g i n t r a s t r a t a l c o n d i t i o n s and c o n t i n u e d f o r a s long a s such c o n d i t i o n s c o u l d b e m a i n t a i n e d , perhaps even f o r m i l l i o n s of y e a r s . PRECIPITATION AND STABILITY OF FERRIC OXYHYDROXIDES The p r e c i p i t a t i o n o f f e r r i c oxyhydroxides i s t h e s i n g l e most i m p o r t a n t p r o c e s s responsible f o r t h e colouration of continental red beds.
T h i s may t a k e p l a c e a t
v a r i o u s s t a g e s i n t h e d e p o s i t i o n and d i a g e n e s i s o f r e d beds b u t two of t h e more i m p o r t a n t ones a r e t h e p r e c i p i t a t i o n o f f e r r i c oxyhydroxides from i n t e r s t i t i a l w a t e r s d u r i n g d i a g e n e s i s and l a t e r p r e c i p i t a t i o n from c i r c u l a t i n g ground w a t e r s . The f i r s t - f o r m e d p h a s e s a r e u s u a l l y m e t a s t a b l e compounds, such a s amorphous Fe(OH)3 o r g o e t h i t e (FeOOH); t h e s e do n o t n o r m a l l y p e r s i s t i n t h e g e o l o g i c a l environment b u t become d e h y d r a t e d t o form t h e s t a b l e i r o n o x i d e , h a e m a t i t e . The commonest oxyhydroxides found a s p r e c i p i t a t e s i n n a t u r a l w a t e r s a r e g o e t h i t e , amorphous Fe(0H)
and more r a r e l y , l e p i d o c r o c i t e . These oxyhydroxide p r e c i p i t a t e s 3 a r e u s u a l l y m i x t u r e s whose a p p a r e n t thermodynamic s t a b i l i t y o r s o l u b i l i t y depends l a r g e l y on t h e s o l u b i l i t y o f t h e l e a s t s t a b l e p h a s e p r e s e n t ( u s u a l l y t h e amorphous material).
N a t u r a l oxyhydroxide p r e c i p i t a t e s c o n t a i n s u b s t a n t i a l amounts o f
c o l l o i d a l m a t e r i a l and p a r t i c l e s i z e s r a n g i n g from m o l e c u l a r Fe(OHl3 t o microns i z e d c r y s t a l s o r c r y s t a l a g g r e g a t e s i s p o s s i b l e (Langmuir, 1969).
Particle size
i s p r o b a b l y t h e most i m p o r t a n t c o n t r o l on t h e thermodynamic s t a b i l i t y of t h e
oxyhydroxides; o t h e r s b e i n g m i n e r a l o g y , c r y s t a l l i n i t y and t h e p r e s e n c e of impurities. S o l u t i o n r e a c t i o n s f o r a l l t h e oxyhydroxides can b e r e p r e s e n t e d b y * Fe(OH)3 = Fe3+
+ 30H-
(6.1)
OtFeOOH + H20 = Fe3+ + 30H112 6X-Fe203)
(6 * 2)
+ 3/2H20 = Fe3+ + 30H-
(6 * 3 )
The n e g a t i v e l o g a r i t h m o f t h e a c t i v i t y p r o d u c t f o r e a c h of t h e s e r e a c t i o n s , assuming p u r e w a t e r and s o l i d s , mav b e w r i t t e n :
1'
PK = - 1 0 g [ ~ e 3 + 1 [ ~ ~ -
when t h e a c t i v i t y of w a t e r i s n e a r u n i t y .
(6.4) T h i s pK i s , i n e f f e c t , a s o l u b i l i t y
p r o d u c t and v a l u e s r a n g e from a b o u t 37.1 f o r f r e s h , r a p i d l y p r e c i p i t a t e d amorphous m a t e r i a l , t o p r o b a b l y 44 o r more f o r c r y s t a l l i n e g o e t h i t e and h a e m a t i t e (Langmuir, 1969, 1971).
Langmuir and Whittemore (1971) have shown t h a t c o - e x i s t i n g oxyhyd-
r o x i d e s can r e v e r s e t h e i r r e l a t i v e s t a b i l i t i e s b e c a u s e of p a r t i c l e s i z e e f f e c t s . For example, f o r e q u a l s i z e d c r y s t a l s of g o e t h i t e and h a e m a t i t e , t h e former i s more s t a b l e above 7601 b u t becomes less s t a b l e below t h i s p a r t i c l e s i z e . The b e h a v i o u r o f p r e c i p i t a t e d f e r r i c oxyhydroxides i n s o l u t i o n can most
339 c o n v e n i e n t l y b e d e s c r i b e d i n terms of Eh and pH (Hem and Cropper, 1959; G a r r e l s , 1960).
F i g . 6 . 1 0 shows a n Eh-pH diagram w i t h t h e s t a b i l i t y f i e l d s o f s o l i d s and
predominant i o n i c s p e c i e s i n t h e s y s t e m F e - H ~ o - 0 ~ -C02.
Ion-ion b o u n d a r i e s a r e drawn
a t e q u a l i o n a c t i v i t i e s and i o n - s o l i d b o u n d a r i e s a r e drawn f o r 10-3M
dissolved
i r o n s p e c i e s which i s a t y p i c a l c o n c e n t r a t i o n found i n i r o n - r i c h groundwaters (Langmuir and Whittemore, 1971; Whittemore and Langmuir, 1975).
The f e r r i c - f e r r o u s -
f e r r i c oxyhydroxide b o u n d a r i e s are drawn f o r pK v a l u e s of 37.1 and 44.2.
The
d i f f e r e n c e means t h a t f o r a g i v e n Eh and pH a w a t e r can c o n t a i n a b o u t seven o r d e r s of magnitude more d i s s o l v e d i r o n i n e q u i l i b r i u m w i t h f r e s h amorphous m a t e r i a l t h a n i t can i n e q u i l i b r i u m w i t h w e l l c r y s t a l l i z e d g o e t h i t e o r h a e m a t i t e .
I n general
t h e Eh and pH of i r o n - r i c h n a t u r a l w a t e r s p l a c e s them on t h e f e r r i c o r f e r r o u s species
-
f e r r i c oxyhydroxide boundary.
The groundwater i n n a t u r a l a q u i f e r s , most
commonly has a pH between 5 and 8 , and Eh between 0 . 3 and -0.1 v o l t s .
Under t h e s e
c o n d i t i o n s i r o n i s p r e s e n t a s Fe2+ i o n s and s o l i d f e r r i c oxyhydroxide p h a s e s . Acid w a t e r s s u c h a s t h o s e from mine d i s c h a r g e s u s u a l l y have a pH between 2 and 5 and Eh from w i t h i n t h e Fe3+ f i e l d down t o 0 v o l t s . Eh v a l u e s measured i n t h e f i e l d a r e o f t e n between t h o s e p r e d i c t e d from t h e s o l u b i l i t y of Fe(OH)3 and t h e s o l u b i l i t y of h a e m a t i t e (Back and Barnes, 1965; Barnes and Back, 1964).
These a u t h o r s i n t e r p r e t e d t h i s as a n i n d i c a t i o n t h a t
f e r r i c oxyhydroxides s p e c i e s e x i s t e d i n v a r y i n g d e g r e e s of d e h y d r a t i o n and t h a t low Eh c o n d i t i o n s l e a d t o s e l e c t i v e l e a c h i n g o f more s o l u b l e f e r r i c oxyhydroxide p h a s e s l e a v i n g h a e m a t i t e a s t h e l a s t f e r r i c phase t o d i s s o l v e (Hem and Cropper, 1959; G a r r e l s and C h r i s t , 1965; Back and B a r n e s , 1965). The d e h y d r a t i o n s t a t e o f f e r r i c oxyhydroxides has a r o u s e d c o n s i d e r a b l e i n t e r e s t i n t h e c o n t e x t of r e d bed f o r m a t i o n .
I n Recent temperate-humid c l i m a t e s o i l s
g o e t h i t e r a t h e r than h a e m a t i t e i s formed (Schwertmann, 1965).
Under c e r t a i n
c o n d i t i o n s yaghemite may a l s o form ( T a y l o r and Schwertmann, 1974; M u l l i n s , 1977); i t may b e t h a t t h e f o r m a t i o n of h a e m a t i t e is i n h i b i t e d by t h e p r e s e n c e of f i n e
g r a i n e d o r g a n i c matter a s s u g g e s t e d by Schwertmann (1965). I n Recent sediments t h e r e i s abundant yellow o r brown f e r r i c hydroxides. a n c i e n t r e d beds t h e s e are s c a r c e and are r e p l a c e d by abundant h a e m a t i t e .
In This
h a s l e d a number of a u t h o r s t o a r g u e t h a t d e t r i t a l f e r r i c hydroxides a g e t o h a e m a t i t e d u r i n g d i a g e n e s i s by some d e h y d r a t i o n r e a c t i o n (Van Houten, 1964; 1972; Schmalz, 1968; T u r n e r , 1974a).
The p r e c i s e m i n e r a l o g y of i r o n h y d r o x i d e s i n
Recent f i n e - g r a i n e d a l l u v i u m i s v e r y p o o r l y known.
Some a u t h o r s have assumed
t h a t much of i t i s amorphous ( B l a t t e t a l . , 1 9 7 2 ) ; o t h e r s assume t h a t i t c o n s i s t s p r i n c i p a l l y of g o e t h i t e ( B e r n e r , 1971).
Undoubtedly much of t h e f e r r i c hydroxide
i n Recent s e d i m e n t s i s c l o s e l y a s s o c i a t e d w i t h c l a y m i n e r a l s ( C a r r o l l , 1958; S o i l e u and McCracken, 1967; Deshpande e t a l . ,
1968; Walker and Honea, 1969) and
t h e e v i d e n c e does s u g g e s t t h a t p o o r l y c r y s t a l l i n e p h a s e s a r e more abundant than
340
Fig.6.10. Eh-pH r e l a t i o n s h i p s between s o l i d s and predominant i o n i c s p e c i e s i n t h e The p o s i t i o n of t h e F e z + - f e r r i c oxyhydroxide boundary i s system Fe-H20-02-C02. shown f o r pK's o f 37.1 and 4 4 . 2 . T o t a l d i s s o l v e d c a r b o n a t e s p e c i e s e q u a l 10-3M. S o l i d s p e c i e s i n p a r e n t h e s e s are dashed l i n e f i e l d s o n l y ( a f t e r Whittemore and Langmuir, 1975). t r u l y amorphous m a t e r i a l ( F o l l e t t , 1965).
Amorphous m a t e r i a l may be a d s o r b e d on
t h e s u r f a c e s o r edges of c l a y m i n e r a l s o r i n o r g a n i c complexes.
E l e c t r o n micro-
scope s t u d i e s (Greenland e t a l . , 1968) a l s o r e v e a l t h a t c l a y p l a t e l e t s m y be covered by minute p a r t i c l e s o f f r e e f e r r i c o x i d e s which may r e p r e s e n t t h e p o o r l y c r y s t a l l i n e phases many o f which a p p e a r t o b e h y d r a t e d f e r r i c o x i d e r e l a t e d t o h a e m a t i t e (Towe and B r a d l e y , 1967).
The most abundant c r y s t a l l i n e phase i n t h e
ferric hydroxide o f f i n e g r a i n e d a l l u v i u m i s p r o b a b l y g o e t h i t e .
The d e h y d r a t i o n o f f e r r i c oxyhydroxides Yellow o r brown i r o n h y d r o x i d e s i n t h e c l a y f r a c t i o n s of a l l u v i u m are b e l i e v e d
341 t o be a b l e t o c o n v e r t i n t o r e d pigmentary h a e m a t i t e by d e h y d r a t i o n d u r i n g d i a g e n e s i s and t h u s produce r e d beds ( F r i e n d , 1966; Van Houten, 1968).
Since t h i s i s a
d e h y d r a t i o n r e a c t i o n many a u t h o r s have m a i n t a i n e d t h a t a n a r i d o r a t l e a s t s e a s o n a l l y dry climate i s necessary f o r i t t o proceed, thus f o s t e r i n g t h e b e l i e f t h a t ancient r e d beds a r e a p a l a e o c l i m a t i c i n d i c a t o r .
F o r example, Schmalz (1968) a r g u e d t h a t
h a e m a t i t e might form from amorphous i r o n hydroxide o r g o e t h i t e by one of t h e following reactions: 2Fe(OH)3 --+Fe203 + 3H20
(6.5)
2Fe (OH) -+2HFe02 + 2H20 +Fez03 2HFe02-+Fe203
+ 3H20
(6.6)
+ H20
(6.7)
Using a v a i l a b l e d a t a Schmalz (1968) q u o t e d a Gibbs f r e e e n e r g y (aGo) of between 4 . 2 and +2.2 k c a l / m o l e f o r r e a c t i o n ( 6 . 7 ) and concluded t h a t h a e m a t i t e could n o t
form s t a b l y i n t h e p r e s e n c e o f w a t e r . E a r l y phase e q u i l i b r i u m s t u d i e s of t h e d e h y d r a t i o n o f g o e t h i t e ( P o s n j a k and Merwin, 1919; Smith and Kidd, 1949; Schmalz, 1959) s u g g e s t e d t h a t t h e d e h y d r a t i o n t e m p e r a t u r e was a b o u t 130OC.
The l o w e s t t e m p e r a t u r e a t which t h e r e a c t i o n has been
o b s e r v e d e x p e r i m e n t a l l y was r e c o r d e d by Tunnel1 and Posnjak (1931) who c o n v e r t e d n a t u r a l g o e t h i t e t o h a e m a t i t e i n a few weeks a t 100°C i n 0. I N H C l .
However, i n one
of t h e s e c a s e s was t h e e q u i l i b r i u m d e l i n e a t e d by r e v e r s i n g t h e r e a c t i o n : h a e m a t i t e could n o t be hydrated.
Moreover t h e method u s e d : t h e a g e i n g o f f r e s h l y p r e c i p i t a t e d
f e r r i c h y d r a t e g e l s a t v a r i o u s t e m p e r a t u r e s and w a t e r p r e s s u r e s does n o t t a k e due a c c o u n t of f a c t o r s s u c h as pH, which could i n f l u e n c e t h e k i n e t i c s o f t h e r e a c t i o n , n o r t h e a b i l i t y of f o r e i g n i o n s t o c a t a l y s e t h e c r y s t a l l i z a t i o n o f h a e m a t i t e o r g o e t h i t e ( T a y l o r and G r a l e y , 1967; Schwertmann, 1970).
I n f a c t Smith and Kidd
(1949) showed t h a t t h e d e h y d r a t i o n t e m p e r a t u r e was markedly dependent upon pH. Berner (1969) Kas shown t h a t t h i s e q u i l i b r i u m d e h y d r a t i o n t e m p e r a t u r e o f 13OoC i s much t o o h i g h and t h a t f i n e g r a i n e d g o e t h i t e i s u n s t a b l e r e l a t i v e t o h a e m a t i t e
under v i r t u a l l y a l l g e o l o g i c a l c o n d i t i o n s .
H i s method was t o d e t e r m i n e AGO by
measuring t h e r e l a t i v e s o l u b i l i t i e s of g o e t h i t e and h a e m a t i t e i n H C 1 s o l u t i o n s . The r e s u l t s c l e a r l y i n d i c a t e d t h a t f i n e g r a i n e d g o e t h i t e i s more s o l u b l e t h a n h a e m a t i t e a t 85OC ( F i g . 6 . 1 1 ) .
Calculations using d i f f e r e n t i a l s o l u b i l i t y enabled
B e r n e r (1969) t o show t h a t t h e AGO f o r r e a c t i o n ( 6 . 7 ) a t 25OC i s -0.66 k c a l / m o l e and a t O°C i s -0.40 k c a l / m o l e .
He f u r t h e r c a l c u l a t e d t h a t t h e maximum t e m p e r a t u r e
a t which f i n e g r a i n e d g o e t h i t e i s s t a b l e r e l a t i v e t o h a e m a t i t e p l u s p o r e w a t e r i s
a b o u t 4OoC.
Assuming t h a t t h e s e d a t a a r e c o r r e c t , i t i s c l e a r t h a t f i n e g r a i n e d
g o e t h i t e i s u n s t a b l e r e l a t i v e t o h a e m a t i t e under v i r t u a l l y a l l g e o l o g i c a l c o n d i t i o n s . The d e h y d r a t i o n o f f i n e g r a i n e d g o e t h i t e i s t h e r e f o r e l i k e l y t o t a k e p l a c e f a i r l y r a p i d l y , e s p e c i a l l y d u r i n g s e d i m e n t b u r i a l and c o u l d a l s o t a k e p l a c e i n t h e p r e s e n c e of water.
I t i s n o t p o s s i b l e t h e r e f o r e t o draw any c o n c l u s i o n s r e g a r d i n g climate
o r p a l a e o c l i m a t e from t h e d e h y d r a t i o n r e a c t i o n .
342
A Goethita 0 Haematite
8.0
3.0 2.0 1.0
0 0
2
4
6
8
10
12
14
16
18
20
Time Days
F i g . 6 . 1 1 . D i s s o l u t i o n and p r e c i p i t a t i o n c u r v e s f o r g o e t h i t e and h a e m a t i t e i n 0 . 1 2 M H C l a t 85 2 l 0 C . Data a r e p l o t t e d o n l y f o r r u n s where d i s s o l u t i o n was n o t e s s e n t i a l l y complete ( a f t e r B e r n e r , 1969a). The e f f e c t of p a r t i c l e s i z e e f f e c t on t h e d e h y d r a t i o n r e a c t i o n has been s t u d i e d by Langmuir (1971) and Langmuir and Whittemore (1971).
Langmuir's (1971) thermo-
chemical r e s u l t s a r e i n g e n e r a l agreement w i t h t h e s o l u b i l i t y r e s u l t s o f B e r n e r (1969).
F i g . 6 . 1 2 shows t h e p a r t i c l e s i z e e f f e c t on t h e Gibbs f r e e e n e r g y of t h e
goethite dehydration r e a c t i o n a t 25OC.
Langmuir (1971) showed t h a t c o a r s e - g r a i n e d
g o e t h i t e ( > p m ) i s s t a b l e r e l a t i v e t o c o a r s e g r a i n e d h a e m a t i t e i n w a t e r up t o 8OoC a t the E a r t h ' s surface, but t h a t f i n e grained goethite i s unstable r e l a t i v e t o coarse grained haematite.
Langmuir (1971) a l s o showed t h a t f o r k i n e t i c r e a s o n s ,
h a e m a t i t e , once formed, w i l l n o t r e h y d r a t e t o form g o e t h i t e .
This i s c o n s i s t e n t
w i t h t h e f a c t t h a t g o e t h i t e pseudomorphs a f t e r h a e m a t i t e a r e n o t known t o o c c u r (Palache e t a l . , 1944).
The measurement of p a r t i c l e s i z e s i n t h e r e g i o n 100-10008
i s e x t r e m e l y d i f f i c u l t and c a r e must b e t a k e n i n e x t r a p o l a t i n g t h e d a t a t o geological conditions.
Langmuir and Whittemore (1971) found i n t h e l a b o r a t o r y
t h a t f r e s h l y p r e c i p i t a t e d f e r r i c oxyhydroxides c o n s i s t e d of amorphous m a t e r i a l , l e p i d o c r o c i t e , and g o e t h i t e .
Based on X-ray d i f f r a c t i o n and e l e c t r o n microscopy
g o e t h i t e occurred a s rod o r needle-like c r y s t a l s elongated p a r a l l e l to the c-axis w i t h t y p i c a l dimensions b e i n g 50-1002 i n d i a m e t e r and 500-20002 i n l e n g t h ( s e e Lamgmuir and Whittemore, 1971, F i g . 5 ) .
Lepidocrocite occurred with i t s t y p i c a l
l a t h - s h a p e d h a b i t r a n g i n g from 60-2002 t h i c k a l o n g t h e b - a x i s , a v e r a g i n g 4002 p a r a l l e l t o t h e c - a x i s and 23001 long i n t h e c - d i r e c t i o n (Langmuir and Whittemore,
1 9 7 1 , Fig.5;
s e e a l s o t h e e x c e l l e n t e l e c t r o n micrographs of Schwertmann, 1 9 7 0 ) .
Langmuir and Whittemore (1971) a l s o made a s t u d y of n a t u r a l l y o c c u r r i n g f e r r i c
343
40001-T---7 3000
1000
-
- 1000 -
m
0.01
hematite > lp goethite < IN
0.1
1
Particle Size of Cubes (microns)
Fig.6.12. P a r t i c l e s i z e e f f e c t on t h e Gibbs f r e e e n e r g y of t h e g o e t h i t e d e h y d r a t i o n r e a c t i o n ( a f t e r Langmuir, 1971). oxyhydroxides and i r o n c o n t e n t i n 24 w e l l w a t e r s from t h e c o a s t a l p l a i n of New J e r s e y and Maryland,
The suspended oxyhydroxides h e r e a r e p r o b a b l y amorphous
m a t e r i a l and g o e t h i t e w i t h a n a v e r a g e g r a i n s i z e , based on f i l t r a t i o n s t u d i e s , of 10,000-20,000~. A number of i m p l i c a t i o n s emerge from t h e work of Berner (1969a) and Langmuir
(1971).
Although h a e m a t i t e may c r y s t a l l i z e d i r e c t l y from w a t e r d i s s o l v i n g g o e t h i t e
a t low t e m p e r a t u r e s ( c 1 0 O o C ) ,
kinetic considerations indicate that the reaction
must b e e x t r e m e l y s l o w , and t h i s p r o b a b l y e x p l a i n s why i t has n o t been observed i n the laboratory.
The t r a n s f o r m a t i o n of l e p i d o c r o c i t e t o g o e t h i t e , on t h e o t h e r hand,
h a s been shown by Schwertmann and T a y l o r (1972) t o t a k e p l a c e by t h e d i s s o l u t i o n of l e p i d o c r o c i t e , and t h e s u b s e q u e n t n u c l e a t i o n and growth o f g o e t h i t e c r y s t a l s .
The
f o r m a t i o n o f h a e m a t i t e by d e h y d r a t i o n of g o e t h i t e i s l i k e l y t o t a k e p l a c e r e a d i l y i n t h e s e d i m e n t a r y environment, e s p e c i a l l y d u r i n g d i a g e n e s i s , and w i l l be enhanced by i n c r e a s e d t e m p e r a t u r e o r lower w a t e r vapour p r e s s u r e , o r a combination of t h e s e . There have been no c o n v i n c i n g d e m o n s t r a t i o n s of t h e i n s i t u a g e i n g of g o e t h i t e t o haematite, nor indeed i s there convincing d i r e c t evidence t h a t the haematite i n a n c i e n t r e d beds has formed from g o e t h i t e d e h y d r a t i o n .
The r e l a t i v e importance of
g o e t h i t e and amorphous f e r r i c hydroxide a s h a e m a t i t e p r e c u r s o r s i s u n c l e a r . Amorphous f e r r i c hydroxide i s u s u a l l y l e s s s t a b l e than f i n e g r a i n e d g o e t h i t e and thermodynamic and k i n e t i c c o n s i d e r a t i o n s i n d i c a t e t h a t h a e m a t i t e f o r m a t i o n from s u c h m a t e r i a l i s more f a v o u r a b l e t h a n h a e m a t i t e f o r m a t i o n from g o e t h i t e .
I n t h o s e r e d beds which c o n t a i n e d s u b s t a n t i a l amounts o f d e t r i t a l c l a y - o x i d e , f o r example, t r o p i c a l savanna a l l u v i u m , t h e d e h y d r a t i o n o f g o e t h i t e i s l i k e l y t o have been an i m p o r t a n t mechanism of h a e m a t i t e - f o r m a t i o n .
D i r e c t formation of
h a e m a t i t e from amorphous f e r r i c hydroxide i s l i k e l y t o have been i m p o r t a n t i n r e d bed a q u i f e r s . ENVIRONMENTAL CONTROLS ON THE PRECIPITATION AND DIAGENETIC HISTORY OF FERRIC OXYHYDROXIDE S
I r o n i s t r a n s p o r t e d i n t h e s e d i m e n t a r y environment i n a v a r i e t y o f forms. amounts may o c c u r i n t r u e s o l u t i o n e s p e c i a l l y i n more a c i d w a t e r s .
Small
The m a j o r i t y ,
however, i s t r a n s p o r t e d a s c r y s t a l l i n e i r o n hydroxide g r a i n c o a t i n g s and w i t h i n s i l i c a t e mineral l a t t i c e s .
O t h e r i m p o r t a n t mechanisms i n c l u d e t h e t r a n s p o r t of
i r o n which i s adsorbed on t h e s u r f a c e and edges of c l a y m i n e r a l s , and a l s o t h a t The p r o p o r t i o n of i r o n t r a n s p o r t e d by
which i s a s s o c i a t e d w i t h o r g a n i c complexes.
t h e s e d i f f e r e n t mechanisms v a r i e s a c c o r d i n g t o t h e d e p o s i t i o n a l environment.
In
m i n e r a l o g i c a l l y immature s a n d s t o n e s d e r i v e d from c r y s t a l l i n e basement, much of t h e i r o n i s bound i n t h e l a t t i c e s of ferromagnesian s i l i c a t e s .
In f i n e g r a i n e d a l l u v i u m ,
a g r e a t e r p r o p o r t i o n o f t h e i r o n i s t r a n s p o r t e d a s hydroxide g r a i n c o a t i n g s and i n association with clay minerals. The u l t i m a t e f a t e o f t h e s e v a r i o u s forms of i r o n depends upon a number of f a c t o r s i n c l u d i n g t h e geochemical environment d u r i n g d e p o s i t i o n and e a r l y d i a g e n e s i s , and a l s o s u b s e q u e n t l y d u r i n g sediment b u r i a l .
The p o t e n t i a l f o r p r o d u c i n g r e d sediment
i s l a r g e l y d e t e r m i n e d d u r i n g e a r l y d i a g e n e s i s and s u b s e q u e n t b u r i a l .
Iron is
sedimented p r i m a r i l y a s i r o n h y d r o x i d e s and c r y s t a l l i n e p a r t i c l e s ; t h e f a t e of t h e s e i r o n h y d r o x i d e s , which c o n s t i t u t e t h e r e a d i l y a v a i l a b l e i r o n , i s d e t e r m i n e d by t h e Eh-pH c o n d i t i o n s below t h e sediment s u r f a c e .
I n continental alluvium oxidizing
c o n d i t i o n s commonly p r e v a i l , above and below t h e sediment s u r f a c e ,
Under t h e s e
c o n d i t i o n s m e t a s t a b l e i r o n h y d r o x i d e s may ' a g e ' i n t o h a e m a t i t e , s i l i c a t e o x i d a t i o n may c o n t i n u e d u r i n g s e d i m e n t b u r i a l , and pigmentary o x i d e s may b e p r e c i p i t a t e d i n p o r e s p a c e s and i n t e r s t i t i a l m a t r i x t h u s p r o d u c i n g r e d sediment.
I n many marine
and l a c u s t r i n e environments t h e s p e c i a l r o l e which i r o n p l a y s w i t h i n t h e s u l p h u r c y c l e means t h a t q u i t e a d i f f e r e n t d i a g e n e t i c h i s t o r y r e s u l t s .
Available i r o n ,
whether i t b e i n c o l l o i d a l form, a s amorphous Fe(OH)3, o r even a s c r y s t a l l i n e hydroxides o r o x i d e s , may s u f f e r r e d u c t i o n , d i s s o l u t i o n o r s u l p h i d a t i o n and e v e n t u a l l y form i r o n s u l p h i d e ( p y r i t e ) .
A m a j o r consequence o f t h i s i s t h a t t h e s e
sediments a r e g e n e r a l l y d r a b i n c o l o u r .
The i n f l u e n c e which d e p o s i t i o n a l e n v i r o n -
ment e x e r t s on t h e f o r m a t i o n o f f e r r i c oxyhydroxides, and u l t i m a t e l y on s e d i m e n t c o l o u r , i s w e l l - s e e n i n t h e c o l o u r d i f f e r e n t i a t j o n which e x i s t s between i n t e r b e d d e d marine and non-marine s e d i m e n t s and a l s o between i n t e r b e d d e d f l u v i a l and l a c u s t r i n e sediments.
346
Marine c o n d i t i o n s The f a t e o f a v a i l a b l e i r o n i n t h e marine environment i s i n t i m a t e l y bound up with the sulphur cycle.
Kaplan e t a l . (1963) showed t h a t b a c t e r i a l s u l p h a t e
r e d u c t i o n i s t h e most i m p o r t a n t p r o c e s s i n t h e s u l p h u r c y c l e w i t h t h e main genera i n v o l v e d b e i n g D e s u l p h o v i b r i o and Desulphotomaculum.
One o f t h e most i m p o r t a n t
s p e c i e s i s D e s u l p h o v i b r i o d e s u l p h u r i c a n s which can t o l e r a t e s a l i n i t y v a r i a t i o n s between 0 and 12% ( L i t t l e w o o d and P o s t g a t e , 1957) and i s p r i m a r i l y r e s p o n s i b l e f o r t h e r e d u c t i o n o f s u l p h a t e i n marine w a t e r s ( T r u d i n g e r , 1976). B a c t e r i a l s u l p h a t e r e d u c t i o n o c c u r s o n l y i n t h e absence o f oxygen.
These
c o n d i t i o n s a r e commonly met w i t h j u s t below t h e s u r f a c e o f marine sediments because o f t h e r a p i d consumption o f oxygen d u r i n g a e r o b i c r e s p i r a t i o n r e l a t i v e t o t h e r a t e of r e p l e n i s h m e n t . The i n i t i a l p r o d u c t s o f s u l p h a t e r e d u c t i o n , H2S and HS- may combine w i t h i r o n , which i s reduced t o Fe2+ and t h u s made more mobile under t h e s e c o n d i t i o n s , t o form a n " i n t e r m e d i a t e q qm e t a s t a b l e i r o n s u l p h i d e such as mackinawite ( t e t r a g o n a l FeS) a n d / o r g r e i g i t e ( c u b i c Fe3S4) ( F i g . 6 . 1 3 . ) . -
,
Subsequent t r a n s f o r m a t i o n of t h e s e i n t e r m e d i a t e s t o t h e more s t a b l e p y r i t e ( c u b i c FeS ) o c c u r s by t h e a d d i t i o n o f e l e m e n t a l s u l p h u r (Goldhaber and Kaplan, 2 1974, 1975). The r a t e o f s u l p h a t e r e d u c t i o n under l a b o r a t o r y c o n d i t i o n s i s c o n t r o l l e d by t h e abundance o f e l e c t r o n donor ( o r g a n i c m a t t e r ) a v a i l a b l e t o t h e b a c t e r i a (Goldhaber and Kaplan, 1975).
I n s e d i m e n t s , t h e p o s i t i v e c o r r e l a t i o n between % s u l p h u r and
% carbon i n a v a r i e t y o f s e d i m e n t a r y environments (Sweeney, 1972) a l s o s u g g e s t s a
c o n t r o l on p y r i t e p r o d u c t i o n by o r g a n i c carbon.
i s d e r i v e d mainly from marine p l a n k t o n .
Organic matter i n marine sediments
During i t s t r a n s p o r t and b u r i a l t h e
o r g a n i c m a t t e r undergoes t r a n s f o r m a t i o n s i n t o a series of f r a c t i o n s : degraded c e l l u l a r m a t e r i a l d w a t e r s o l u b l e complexes c o n t a i n i n g amino a c i d s , f a t t y a c i d s , and s u g a r s d u l v i c acids-whumic acidsAerogen-(Nissenbaum and Kaplan, 1972). T h i s pathway r e p r e s e n t s i n c r e a s i n g l y p o l y m e r i z a t i o n o f t h e o r g a n i c m a t e r i a l .
It
t h u s f o l l o w s t h a t t h i s more complex o r g a n i c m a t e r i a l i s less s u s c e p t i b l e t o b a c t e r i a l a t t a c k because o f t h e l a c k o f s u i t a b l e s o l u b i l i z i n g and enzyme systems (Alexander,, 1971).
The t o t a l sediment accumulation r a t e may t h e r e f o r e be c l o s e l y The main r e a s o n s f o r t h i s being:
r e l a t e d t o t h e rate of b a c t e r i a l metabolism.
1. t o t a l o r g a n i c carbon p r e s e r v e d i n sediments t e n d s t o be p o s i t i v e l y c o r r e l a t e d w i t h high s e d i m e n t a t i o n r a t e ( B e r n e r , 1972)
2. less complexing and rearrangement
3. f o r t h e most p a r t r a p i d l y
of organic m a t e r i a l occurs during r a p i d b u r i a l
accumulating marine sediments tend t o o c c u r a t s h a l l o w d e p t h s which l e a d s t o a lower p r e s s u r e , s h o r t e r t i m e of exposure t o oxygenated water d u r i n g t r a n s p o r t , and a g e n e r a l l y h i g h e r bottom w a t e r temperature.
This c o r r e l a t i o n between b a c t e r i a l
r a t e and s e d i m e n t a t i o n r a t e i s shown i n Fig.6.14.
346 A-
I
I
4
J.
Fe S2
/
(Fmmboidr)
The pathway o f s e d i m e n t a r y p y r i t e f o r m a t i o n ( a f t e r Goldhaber and Fig.6.13. Kaplan, 1974). The above d i s c u s s i o n i n d i c a t e s t h a t r e a c t i o n s o c c u r r i n g d u r i n g s u l p h u r d i a g e n e s i s c o n s i s t o f a number of t r a n s f o r m a t i o n s i n v o l v i n g s u l p h u r and i r o n i n b o t h t h e s o l i d and aqueous p h a s e s .
Both d i s s o l v e d s u l p h i d e and a c i d v o l a t i l e s u l p h i d e (FeS) a r e
i n v o l v e d i n t h e pathway o f p y r i t e f o r m a t i o n which may b e s i m p l i f i e d : 2-
SO4 +H2S
"Fe"
SO
FeS +FeS2
This sequence emphasises t h e f a c t t h a t d i s s o l v e d s u l p h i d e and a c i d v o l a t i l e s u l p h i d e a r e t r a n s i e n t i n t e r m e d i a t e s which a r e c o n t i n u o u s l y b e i n g formed and removed a t any given t i m e .
P o r e w a t e r d a t a show a s y s t e m a t i c d e p l e t i o n o f d i s s o l v e d s u l p h a t e w i t h
d e p t h ( t i m e ) and t h i s i s a s s o c i a t e d w i t h a n e x p e c t e d i n i t i a l i n c r e a s e i n d i s s o l v e d sulphide (Fig.6.15).
Below t h e d e p t h of z e r o s u l p h a t e , s u l p h i d e p r o d u c t i o n from
t h i s s o u r c e must c e a s e , and d i s s o l v e d s u l p h i d e d e c r e a s e s r e f l e c t i n g p r o g r e s s i v e consumption by r e a c t i o n w i t h s o l i d components, and a l s o d i f f u s i o n of d i s s o l v e d sulphide.
S u l p h i d e consumption i s a l s o o c c u r r i n g a t d e p t h s s h a l l o w e r t h a n t h a t a t
which s u l p h a t e d i s a p p e a r s , b u t i n t h e upper 150cm s u l p h i d e p r o d u c t i o n i s g r e a t e r t h a n t h e s u l p h i d e removal r a t e , hence t h e i n c r e a s e i n t h i s zone.
Since sulphide
p r o d u c t i o n ( s u l p h a t e r e d u c t i o n ) c o r r e l a t e s w i t h s e d i m e n t a t i o n r a t e , one m i g h t e x p e c t t h e maximum c o n c e n t r a t i o n of d i s s o l v e d s u l p h i d e t o show a s i m i l a r r e l a t i o n s h i p . A v a i l a b l e e v i d e n c e s u g g e s t s t h a t t h i s i s t h e c a s e , t h e h i g h e s t maximum d i s s o l v e d s u l p h i d e c o n c e n t r a t i o n s b e i n g found i n t h e most r a p i d l y d e p o s i t e d s h a l l o w marine sediments.
The v a r i a t i o n i n maximum s u l p h i d e c o n c e n t r a t i o n is n o t s o l e l y r e l a t e d
347
0
-51 0
I
I
I
I
200
400
600
800
1000
0
0
0-
1200 1400 1600 1800 2000 2200 2400
Sedimentation rate cm. per 1000 yrs.
Fig.6.14. R e l a t i o n s h i p between s u l p h a t e r e d u c t i o n r a t e v e r s u s s e d i m e n t a t i o n r a t e . S e d i m e n t a t i o n r a t e s : Somes Sound, Marine 0 and Long I s l a n d Sound @ ( B e r n e r , 1 9 7 2 ) , (Koide e t a l . , 1972; Emery, 19601, Carmen Basin A and S a n t a Barbara B a s i n P e s c a d e r o Basin (Van Andel, 1 9 6 4 ) , C a r i a c o Trench 0 ( a f t e r Goldhaber and Kaplan, 1975). t o v a r i a t i o n i n production r a t e but a l s o t o the nature of the dissolved sulphide removal mechanism.
T h i s mechanism m a i n l y i n v o l v e s r e a c t i o n w i t h i r o n o x i d e o r
i r o n h y d r o x i d e (see Goldhaber and Kaplan, 1974 f o r a r e v i e w ) .
R i c k a r d (1974) h a s
s t u d i e d t h e s u l p h i d a t i o n of g o e t h i t e under l a b o r a t o r y c o n d i t i o n s and found t h a t t h e r a t e o f t h i s r e a c t i o n i s dependent upon s u l p h i d e c o n c e n t r a t i o n , pH, and t h e surface area of goethite. The abundance of a c i d v o l a t i l e s u l p h i d e s i s s i m i l a r l y d e t e r m i n e d by t h e r e l a t i v e r a t e s of p r o d u c t i o n and consumption.
S i m i l a r arguments t o t h o s e which a p p l y t o
d i s s o l v e d s u l p h i d e a r e a l s o r e l e v a n t add s i m i l a r r e l a t i o n s h i p s e x i s t (Goldhaber and Kaplan, 1975) F i g . 6 . 1 6 ) .
The s i t u a t i o n i s a c t u a l l y a l i t t l e more c o m p l i c a t e d
b e c a u s e t h e c o n v e r s i o n t o p y r i t e i n v o l v e s a d d i t i o n of e l e m e n t a l s u l p h u r (Fig.6.13). T h e r e f o r e t h e low abundance o f a c i d v o l a t i l e s u l p h u r r e l a t i v e t o p y r i t e s u l p h u r a t low r a t e s o f d e p o s i t i o n may b e a f f e c t e d by a n abundant s u p p l y of z e r o v a l e n t s u l p h u r r e s u l t i n g from l o n g e r e x p o s u r e t o o x i d i z i n g c o n d i t i o n s n e a r t h e sediment-
348
Sulfate mM
Sulfide mM +O
300
t
300L
Fig.6.15. V a r i a t i o n i n d i s s o l v e d s u l p h a t e and s u l p h i d e i n p o r e w a t e r s of a c o r e from Carmen B a s i n , Gulf o f C a l i f o r n i a ( a f t e r Goldhaber and Kaplan, 1 9 7 5 ) . w a t e r i n t e r f a c e ( B e r n e r , 1964).
Under c e r t a i n c o n d i t i o n s t h e f o r m a t i o n o f p y r i t e
does n o t i n v o l v e an i n t e r m e d i a t e s u l p h i d e .
These c o n d i t i o n s a r i s e when t h e
s o l u b i l i t y p r o d u c t o f t h e l e a s t s o l u b l e a c i d v o l a t i l e s u l p h i d e i s n o t exceeded ( p y r i t e i n e q u i l i b r i u m w i t h e x c e s s e l e m e n t a l s u l p h u r i s many o r d e r s o f magnitude l e s s soluble than g r e i g i t e o r mackinawite).
Such u n d e r s a t u r a t i o n c o u l d c o r r e l a t e
w i t h a low s e d i m e n t a t i o n r a t e ; s i m i l a r l y low s e d i m e n t a t i o n r a t e s c o u l d r e f l e c t a reduced p r o p o r t i o n of p y r i t e s u l p h u r h a v i n g p a s s e d t h r o u g h t h e a c i d v o l a t i l e s t a g e . An a n c i e n t example:
The C a t s k i l l c l a s t i c wedge
The Upper Devonian C a t s k i l l Formation i n C e n t r a l P e n n s y l v a n i a p r o v i d e s a f i n e example of c o l o u r change i n r e s p o n s e t o a l t e r n a t i n g marine/non m a r i n e c o n d i t i o n s . I n t h i s a r e a t h e C a t s k i l l Formation c o n s i s t s o f two m o t i f s motif c o n t a i n i n g marine- non-marine i n t e r c a l a t i o n s and
2.
1. t h e I r i s h V a l l e y t h e a l l u v i a l motif
c o n s i s t i n g of fining-upwards cyclothems and d e s c r i b e d by A l l e n and F r i e n d (1968). The I r i s h V a l l e y m o t i f i s n o t a b l e b e c a u s e i t d i f f e r s from many modern c o a s t a l d e p o s i t s and shows no e v i d e n c e of the b a r r i e r i s l a n d - t i d a l
f l a t model proposed b y
A l l e n and F r i e n d (1968), n o r any e v i d e n c e of a v i g o r o u s l y p r o g r a d i n g sandy d e l t a ; t h e t i t l e " C a t s k i l l Delta" i s t h e r e f o r e something of a misnomer. The C a t s k i l l complex r e p r e s e n t s a r e g r e s s i v e sequence b u i l t i n t o t h e Upper Devonian
349
I
1
1
1
200 300 400 Sdirnontation rat. ern. per 1000 p a r s 100
.
1
500
Fig.6.16. R e l a t i o n s h i p between maximum p e r c e n t a g e o f t o t a l reduced s u l p h u r i n t h e Data o f Sweeney ( u n p u b l i s h e d ) from a c i d v o l a t i l e form a g a i n s t s e d i m e n t a t i o n r a t e Marine Del Ray Harbor, Los Angeles 0 Data o f Kaplan e t a l . (1963). Sedimentation Goldhaber a n d Kaplan (1975) ( a f t e r Goldhaber and Kaplan, r a t e from Emery (1960) 1975).
A
b a s i n a s a r e s u l t of t h e Acadian orogeny.
In t h e Susquehanna V a l l e y a r e a o f
c e n t r a l P e n n s y l v a n i a a n i n i t i a l marine t r a n s g r e s s i o n submerged o l d e r , s h a l l o w marine o r s h o r e l i n e d e p o s i t s ( T u l l y Limestone and i t s e l a s t i c c o r r e l a t i v e s , Johnson and Friedman, 1966).
The s u c c e e d i n g r e g r e s s i v e sequence can b e d i v i d e d
i n t o f o u r main t y p e s : 1. a t t h e b a s e , g r e y , m a r i n e s i l t y mudstones ( l o w e r p a r t of Trimmers Rock F o r m a t i o n , a b o u t 80m) Formation, a b o u t 45Om)
2 . t u r b i d i t e s ( b u l k o f Trimmers Rock
3 . a l t e r n a t i n g a g i t a t e d - w a t e r marine and non-marine beds
( I r i s h V a l l e y Member of t h e C a t s k i l l Formation, a b o u t 60Om); and 4. a t t h e top a l l u v i a l p l a i n s a n d s t o n e s and mudstones (Buddys Run Member o f t h e C a t s k i l l F o r m a t i o n , a b o u t 1300m).
The s t r a t i g r a p h i c t e r m i n o l o g y i s a f t e r Hoskins e t a l .
(1963) and Dyson (1963). A t y p i c a l u n i t i n t h e I r i s h V a l l e y m o t i f (Fig.6.17)
has an abrupt base overlain
by b i o t u r b a t e d s a n d s t o n e , m a r i n e o l i v e g r e e n s h a l e s and s i l t s t o n e s , r e d s i l t s t o n e s w i t h minor s a n d s t o n e s , and a cap o f r e d mudstones w i t h r o o t t r a c e s d e s i c c a t i o n cracks, and calcareous nodules.
Walker and Harms (1971, p. 387) e n v i s a g e t h e I r i s h
V a l l e y m o t i f forming i n f i v e s t a g e s . c o a s t a l p l a i n o f low r e l i e f . reworking o f winnowed sand.
The p r e s e n c e o f b r a c h i o p o d s a n d c r i n o i d a l d e b r i s
i m p l i e s a normal m a r i n e environment. i n a gradually shoaling sea.
F i r s t , r a p i d marine transgression over a
Second, a s t a g e o f v e r y slow d e p o s i t i o n and b i o l o g i c a l T h i r d , m a r i n e d e p o s i t i o n o f mud took p l a c e
I n i t i a l d e p o s i t s were o l i v e g r e e n f i s s i l e s h a l e s , b u t
a s t h e s e a s h o a l e d , t h i n w e l l - s o r t e d s a n d s t o n e s were d e p o s i t e d .
F o u r t h , t h e r e was
emergence o f mud f l a t s , w i t h many r o o t s p e n e t r a t i n g t h e s e d i m e n t , some s h a l l o w , ephemeral l a k e s and no m a r i n e f a u n a ,
Symmetrical r i p p l e s formed i n ephemeral l a k e s
350
FINING UPWARD ALLUVIAL MOTIF MAY BE PRESENT AT THIS POSITION I N SOME OF THE MAIN MOTIFS.
'RANSGRESSION
I
11-1
bASTAL PLAIN
I
4 "4
6. MASSIVE, BLOCKY RED MUDSTONE WITH ROOT TRACES, FILLED (?DESSICATIONICRACKS, AND TAN CALCAREOUS NODULES. SMDSTONE RARE UNLESS IN FINING-UPWARD MOTIF. 5. DRAB RED SILTSTONES WITH MANY ROOT TRACES AND SOME FILLED (?DESSICATION)CRACKS. BEDDING IS COMMONLY APPARENT! THERE ARE SANDSTONE BEDS 2-IOCM. THICK WITH SMALL SYMMETRICAL RIPPLES. MARINE fOSSILS ABSENT,VERTICAL AND HORIZONTAL BURROWS AND TRAILS O N MANY BEDS 4. GREEN SILTSTONES AND MUDSTONES WITH FINE-GRAINED SANDSTONE BEDS 10-10 CM.THICK, LOW ANGLE CROSSBEDDED,HORIZONTALLY LAMINATED OR RIPPLE CROSS- LAMINATED, SYMMETRICAL RIPPLES WAVELENGTH 10-15 CM. PRESENT, BRACHIOPODS RARE, BURROWS AND TRAILS COMMON. THIS UNIT IS THIN OR ABSENT IN MANY MOTIFS
-
3. OLIVE GREEN FISSILE SHALE, SCATTERED BRACHIOPODS
4
AND CRINOIDS. THIS UNIT MAY REST DIRECTLY ON BASAL SURFACE OF MOTIF. /OIOTURBATE SANDSTONE WITH SCATTERED OUARTZ GRANULES, 2, IACHIOPODS. CRINOIDS, PHOSPHATIC NOOULES. M3NE FRAOMENTS. \BASAL SURFACE.PLANAR, COMMONLYSHUP. IN PLACES BURROWED AND IRREWLAR.
@*
Wfl
METERS
Fig.6.17.
u bG
I d e a l i z e d I r i s h V a l l e y m o t i f ( a f t e r Walker and Harms, 1971).
and d e s i c c a t i o n c r a c k s i n d i c a t e a l t e r n a t e f l o o d i n g and d r y i n g of t h e c o a s t a l f r i n g e . The c o l o u r of t h e s e f i r s t non-marine d e p o s i t s i s r e d , c o n t r a s t i n g w i t h t h e o l i v e g r e e n and d r a b marine s h a l e s and s a n d s t o n e s b e n e a t h .
F i f t h , h i g h e r on t h e c o a s t a l
p l a i n , t h e r e a r e r e d s i l t s t o n e s and mudstones w i t h d e s i c c a t i o n c r a c k s , r o o t s , and t a n c a l c a r e o u s n o d u l e s , s u g g e s t i n g i n c i p i e n t s o i l development.
A few t h i n f i n i n g -
upwards c y c l e s o c c u r and t h e s e d i m e n t s a r e p r e d o m i n a n t l y r e d . The a b s e n c e of s a n d i n t h e s e s h o r e l i n e s e d i m e n t s s u g g e s t a s i m i l a r i t y w i t h t h e p r o g r a d a t i o n a l C h e n i e r P l a i n of Southwestern L o u i s i a n a (Byrne e t a l . ,
1959; Gould
and McFarlan, 1959) and a l s o t h e n o r t h e r n Colorado R i v e r D e l t a (Thompson, 1968). The p r o g r a d a t i o n o f t h i s C a t s k i l l muddy s h o r e l i n e p r o b a b l y took p l a c e by l o n g s h o r e s u p p l y of sediment from more a c t i v e d e l t a i c a r e a s t o t h e n o r s h ( G l a e s s e r , 1970). There a r e numerous ( a b o u t 25) r e p e t i t i o n s o f t h e I r i s h V a l l e y m o t i f i n t h e Susquehanna V a l l e y area a l t h o u g h i t i s n o t c l e a r whether t h e s e w e r e c o n t r o l l e d sedimentologically o r tectonically.
I n t h e former c a s e l o c a l t r a n s g r e s s i o n and
351 r e g r e s s i o n and hence r e p e t i t i o n of t h e m o t i f c o u l d b e e f f e c t e d by t h e r e l a t i v e p r o x i m i t y of major r i v e r c h a n n e l s .
When t h e s e were i n a r e l a t i v e l y d i s t a n t a r e a
o f t h e c o a s t a l p l a i n r e g i o n a l s u b s i d e n c e and compaction would r e s u l t i n l o c a l transgression.
R e g r e s s i o n would o c c u r where t h e r a t e o f sediment s u p p l y by longshore
d r i f t exceeded t h e r a t e of s u b s i d e n c e .
Such a mechanism i s well-documented i n t h e
L o u i s i a n a C h e n i e r P l a i n and M i s s i s s i p p i d e l t a (Gould and McFarlan, 1959). I n t e r m i t t e n t e p i s o d e s of s u b s i d e n c e , p e r h a p s r e l a t e d t o t e c t o n i c a c t i v i t y i n t h e Acadian Mountain Chain may a l s o have been i m p o r t a n t . I n any c a s e , t h e o v e r a l l r e s u l t was t o produce a s e r i e s of i n t e r f i n g e r i n g marine and non-marine s e d i m e n t s i n t h e I r i s h V a l l e y f a c i e s , which a r e r e f l e c t e d i n t h e red- non-red d i f f e r e n t i a t i o n ( F i g . 6 . 1 8 ) .
I t seems l i k e l y t h a t t h e i m p o r t a n t f a c t o r s
c o n t r o l l i n g s e d i m e n t c o l o u r a t i o n under t h e s e c o n d i t i o n s were t h o s e which were l i n k e d t o t h e d e p o s i t i o n a l environment, n o t a b l y redox p o t e n t i a l and s u l p h i d e a c t i v i t y .
In
t h e marine s e d i m e n t s g r e e n o r g r e y c o l o u r s p r e v a i l e d s i n c e pigmentary i r o n o x i d e s and h y d r o x i d e s c o u l d n o t form b e c a u s e of s u l p h i d a t i o n and r e d u c t i o n b e n e a t h t h e sediment s u r f a c e ,
The non-marine sediment e v e n t u a l l y became r e d b e c a u s e of t h e l a c k
of o r g a n i c matter and o x i d i z i n g c o n d i t i o n s which e n a b l e d i r o n o x i d e s and h y d r o x i d e s t o e v e n t u a l l y form s t a b l e pigmentary o x i d e s .
These r e s u l t s t e n d t o s u p p o r t t h e view
t h a t e n v i r o n m e n t a l l y c o n t r o l l e d c o l o u r d i f f e r e n t i a t i o n of t h i s t y p e i n d i c a t e s t h a t s u b s t a n t i a l amounts o f r e a d i l y a v a i l a b l e f i n e g r a i n e d i r o n o x i d e s and hydroxides must have been p r e s e n t a t t h e time o f d e p o s i t i o n .
The r o l e o f t h e s e d e t r i t a l i r o n
o x i d e s and h y d r o x i d e s i n p r o d u c i n g r e d s e d i m e n t i n r e d beds o f t h i s t y p e i s t h e r e f o r e c o n s i d e r e d t o have been more i m p o r t a n t t h a n t h e s u b s e q u e n t d i a g e n e t i c breakdown of i r o n s i l i c a t e s .
T h i s view i s s u p p o r t e d by t h e f a c t t h a t t h e heavy m i n e r a l s u i t e s
o f a number o f r e d beds a r e much t h e same a s t h e i r marine e q u i v a l e n t s . Lacustrine conditions S i n c e i r o n i s a n e s s e n t i a l m i c r o n u t r i e n t , i t s c y c l i n g i n t h e l a c u s t r i n e environment i s i n t i m a t e l y i n v o l v e d w i t h o r g a n i c a c t i v i t y .
Moreover s i n c e t h e o x i d a t i o n of
Fe2+ t o Fe3+ i s e x e r g o n i c and c a p a b l e o f s u p p l y i n g e n e r g y i t i s u s e d by some microorganisms a s a s o u r c e o f energy.
Only s m a l l amounts o f f e r r o u s i r o n a r e p r e s e n t
i n s o l u t i o n i n oxygenated l a k e w a t e r s b i t i n t h e r m a l l y s t r a t i f i e d l a k e s (Fig.6.19) r e d u c i n g c o n d i t i o n s i n t h e hypolimnion may c o r r e s p o n d t o h i g h e r c o n c e n t r a t i o n s o f ferrous iron i n solution.
Much o f t h e i r o n i n normal l a k e w a t e r i s p r e s e n t a s
f e r r i c h y d r o x i d e i n f l o c c u l a n t form and i s removable by f i l t r a t i o n w i t h membranes of a p o r o , s i t y o f 0.5um (Hutchinson, 1957; Hem and Cropper, 1959; Hem and Skougstad, 1960).
C o l l o i d a l Fe(0H)
3
w i t h a p a r t i c l e s i z e 0.001 t o 0.5ym a l s o o c c u r s .
These
p a r t i c l e s commonly have a p o s i t i v e c h a r g e a l t h o u g h a n e g a t i v e l y charged sol can o c c u r a t h i g h pH.
The p o s i t i v e l y charged i o n s i n s o l u t i o n and n e g a t i v e l y charged
c l a y p a r t i c l e s and o r g a n i c c o l l o i d s , c a n n e u t r a l i z e t h e c h a r g e s on t h e hydroxide
352
ACADIAN MOUNTAINS
A
GREEN MUDS-MARINf
I RED SILTS k MUDS-NON MARINf
Fig.6.18. A model of Upper Devonian s e d i m e n t a t i o n i n s o u t h - c e n t r a l P e n n s y l v a n i a . Red- non-red d i f f e r e n t i a t i o n o c c u r s i n t h e I r i s h V a l l e y f a c i e s and r e f l e c t s nonmarine c o n d i t i o n s as t h e C a t s k i l l s h o r e l i n e p r o g r a d e d northwestwards ( a f marine Walker and Harms, 1971).
-
colloid particles.
These uncharged a g g r e g a t e s may form a r a p i d l y s e t t l i n g
p r e c i p i t a t e , and m e t a l s s u c h a s copper i o n s may b e a d s o r b e d and c o p r e c i p i t a t e d with t h e f e r r i c hydroxide p r e c i p i t a t e . The s o l u b i l i t y and a v a i l a b i l i t y o f i r o n i n l a k e w a t e r s i s g r e a t l y enhanced by t h e f o r m a t i o n o f i r o n complexes w i t h c e r t a i n o r g a n i c m o l e c u l e s .
Many o r g a n i c
bases form s t r o n g s o l u b l e i r o n complexes w i t h f e r r o u s ( G j e s s i n g , 1964) and f e r r i c ions.
I r o n enrichment i s commonly found i n s u r f a c e w a t e r s w i t h a h i g h c o n t e n t
of dissolved organic matter.
Such h i g h c o n c e n t r a t i o n s o r complexed s o l u b l e i r o n
a r e a s s o c i a t e d w i t h h i g h l e v e l s of humic a c i d s ( S h a p i r o , 1 9 5 7 ) , t a n n i c a c i d s (Hem, 1960) and o t h e r l i g n i n d e r i v a t i v e s .
The i n t e n s e yellow-brown c o l o u r o f
bog-waters i s p a r t l y a s s o c i a t e d w i t h t h e s e complexes.
S h a p i r o (1964, 1966, 1969)
a53
EPlLlMNlON
25t I % k
Ib
1;
o;
215
3'0
Tempomtun ('C ) Fig.6.19.
A t y p i c a l thermally s t r a t i f i e d lake.
h a s s t u d i e d t h e complexing o f i r o n w i t h humic d e r i v a t i v e s , e s p e c i a l l y w i t h yellow o r g a n i c a c i d s of low m o l e c u l a r w e i g h t , and c o n c l u d e s t h a t t h e p r i m a r y mechanism i s p e p t i z a t i o n i n which i r o n i s d i s p e r s e d i n a s o l u b i l i z e d form o f Fe(0H)
as a 3 r e s u l t of a d s o r p t i o n o f t h e o r g a n i c a c i d s o n t o t h e s u r f a c e s o f t h e p a r t i c l e s .
Some i r o n i s c h e l a t e d w i t h o r g a n i c a c i d s by weak chemical bonding b u t t h i s i s ~ o t t h e p r i m a r y complexing mechanism. The b e h a v i o u r o f i r o n i n l a k e w a t e r s i s c l o s e l y a s s o c i a t e d w i t h t h a t o f manganese and i t i s p e r t i n e n t t o d i s c u s s t h e b e h a v i o u r o f manganese h e r e . T h e o r e t i c a l s t u d i e s by Hem (1963, 1964) and Stunrm and Morgan (1970) a r e c o n s i s t e n t w i t h t h e b e h a v i o u r o f manganese i n l a k e s .
Although manganese o c c u r s i n s e v e r a l
v a l e n c e s t a t e s Mn3+ i s thermodynamically u n s t a b l e i n aqueous s o l u t i o n s under normal c o n d i t i o n s and Mn4'
compounds a r e i n s o l u b l e a t most e n v i r o n m e n t a l pH v a l u e s .
Like i r o n , d i v a l e n t Mn2+ o c c u r s a t low redox p o t e n t i a l s and pH (Fig.6.20).
Some
form of o x i d i z e d manganese w i l l be i n e q u i l i b r i u m w i t h Mn2+ under o x i d i z i n g c o n d i t i o n s o f h i g h pH and Eh, and some form o f Mn2+ may b e i n e q u i l i b r i u m w i t h manganese c a r b o n a t e under r e d u c i n g c o n d i t i o n s o f low pH and Eh. a r e s l o w e r f o r manganese t h a n f o r i r o n .
R a t e s of o x i d a t i o n
Above a pH of 8.5 a n i n t e r m e d i a t e o x i d e
complex forms i n which Mn2+ i s a d s o r b e d o n t o manganese o x i d e s .
The Mn2+ of t h e s e
o x i d e complexes can r e a c t r e l a t i v e l y r a p i d l y w i t h o t h e r a n i o n s and p r e c i p i t a t e a s MnC03, MnS, and Mn(OH)2.
Manganese i s a d s o r b e d on i r o n o x i d e s and c o p r e c i p i t a t e s
w i t h f e r r i c hydroxides when t h e pH i s g r e a t e r than 6 t o 7 .
Manganese forms
s o l u b l e complexes w i t h b i c a r b o n a t e and s u l p h a t e and i n c r e a s e d b i c a r b o n a t e a c t i v i t y d e c r e a s e s manganese s o l u b i l i t y , b u t a t h i g h c o n c e n t r a t i o n s i t has been found t o
3 54
;j$/ 3 10 12 14 P
Fig.6.20. D i s t r i b u t i o n o f manganese s p e c i e s in r e l a t i o n t o pH and redox p o t e n t i a l (modified from Stumm and Morgan, 1970). reduce the oxidation r a t e .
Manganese i n t h e form of Mn2+ forms s t a b l e complexes
w i t h o r g a n i c m a t t e r , i n a s i m i l a r manner t o i r o n , b u t t h e o p e r a t i o n o f t h e s e in a q u a t i c systems a r e n o t a s w e l l u n d e r s t o o d a s t h o s e w i t h i r o n .
Manganese i s
r e l a t i v e l y abundant i n a l k a l i n e s o i l s a s h y d r a t e d o x i d e s and o r g a n i c complexing p l a y s an i m p o r t a n t r o l e i n r e t e n t i o n of manganese i n a d i s s o l v e d form s u i t a b l e for transport.
Drainage from f o r e s t l i t t e r o f t e n has a h i g h manganese c o n t e n t ,
a g a i n p o i n t i n g t o t h e i m p o r t a n c e of o r g a n i c complexes a s a t r a n s p o r t i n g medium of e a r l y o x i d i z e d m e t a l i o n s . The range of t o t a l i r o n found i n most n e u t r a l o r a l k a l i n e l a k e s v a r i e s from a b o u t 5 0 t o 200 g / l i t r e and c o n s i s t s l a r g e l y of Fe(OH)3, o r g a n i c a l l y complexed i r o n , and a d s o r b e d i r o n i n p a r t i c u l a t e form.
Under o x i d i z e d c o n d i t i o n s , a s i n
t h e e p i l i m n i a of l a k e s , v e r y l a r g e amounts o f i r o n a r e found o n l y i n v e r y a c i d i c w a t e r ( p H 4 3 ) s u c h a s i n l a k e s of v o l c a n i c o r i g i n .
The range of manganese
c o n c e n t r a t i o n s ( a b o u t 10-850 g / l ) i s a l s o v a r i a b l e b u t t h e a v e r a g e q u a n t i t y 35 g / l i s somewhat l e s s t h a n t h a t o f i r o n .
The Fe:Mn r a t i o o f l a k e w a t e r i s
g e n e r a l l y much l e s s t h a n t h e a v e r a g e c r u s t a l r a t i o ( 5 0 : l ) i n d i c a t i n g t h e r e l a t i v e enrichment of manganese w i t h r e s p e c t t o i r o n , and i n agreement w i t h t h e r e a c t i o n e q u i l i b r i a ( H u t c h i n s o n , 1957; L i v i n g s t o n e , 1963). The v e r t i c a l d i s t r i b u t i o n o f i r o n and manganese is r e f l e c t e d i n t h e d i s t r i b u t i o n of redox p o t e n t i a l s .
I o n i c i r o n o f oxygenated w a t e r s o f o l i g o t r o p h i c l a k e s ,
e p i l i m n i a of more p r o d u c t i v e l a k e s , and of c i r c u l a t i n g w a t e r s i s v e r y low.
Ferrous
ions d i f f u s e r e a d i l y from s e d i m e n t s when t h e y a r e reduced t o redox p o t e n t i a l s below
355
a b o u t 200m
and m i g r a t i o n o f manganese may o c c u r a t somewhat g r e a t e r redox
potentials,
T h i s i s w e l l s e e n i n e n t r o p h i c l a k e s (Mortimer, 1971) ( F i g . 6 . 2 1 )
where t h e r e l e a s e of manganese p r e c e d e s t h a t o f i r o n . STRATIFICATION
OVERTURN
8 4
2 1
05 2 1
6 4
2
a 4
8
-04
4
-0 2
0.4
.70
0 2
.8.6
-+
30
.80
20 10
.80
S e a s o n a l d i s t r i b u t i o n i n composition mg/l and w a t e r p r o p e r t i e s of w a t e r Fig.6.21. w i t h i n 3Ocm o f t h e s e d i m e n t s a t a d e p t h o f 14m i n E s t h w a i t e W a t e r , England ( a f t e r Mortimer, 1971). I n v e r y p r o d u c t i v e l a k e s , t h e redox p o t e n t i a l of t h e h y p o l i m n e t i c w a t e r s may f a l l , a s t h e decomposition o f t h e hypolimnion f a l l s t h r o u g h o u t t h e p e r i o d of stratification.
Under t h e s e c o n d i t i o n s s u l p h a t e i s reduced t o hydrogen s u l p h i d e
and H S may b e f u r t h e r produced by b a c t e r i a l decomposition of s u l p h u r c o n t a i n i n g 2 o r g a n i c compounds. S i n c e t h e r e i s s i g n i f i c a n t r e l e a s e of f e r r o u s i r o n a t redox p o t e n t i a l s g r e a t e r t h a n t h o s e needed f o r s u l p h a t e r e d u c t i o n t h e r e i s u s u a l l y much Fez+ i n t h e hypolimnion a t t h e time o f s u l p h i d e f o r m a t i o n .
The f o r m a t i o n o f i r o n
s u l p h i d e s and o t h e r m e t a l s u l p h i d e s can r e s u l t i n a s i g n i f i c a n t r e d u c t i o n o f i r o n towards t h e end of summer. The c y c l i n g and d i s t r i b u t i o n o f i r o n and manganese i s c o n t r o l l e d mainly by t h e
3 56 redox p o t e n t i a l .
B a c t e r i a l and p h o t o s y n t h e t i c metabolism g r e a t l y i n f l u e n c e t h e s e
c o n d i t i o n s and t h e r e b y i n d i r e c t l y r e g u l a t e t h e c y c l i n g o f t h e m e t a l s .
The d i r e c t
u t i l i z a t i o n of i r o n and manganese by b a c t e r i a i n e n e r g e t i c t r a n s f o r m a t i o n s i s o n l y of r e l a t i v e l y minor importance.
The c h e m o s y n t h e t i c u t i l i z a t i o n of e n e r g y from
i n o r g a n i c o x i d a t i o n s i n C02 f i x a t i o n i s r e l a t i v e l y i n e f f i c i e n t .
For example t h e
o x i d a t i o n of Fez+ t o Fe3+ by i r o n b a c t e r i a g i v e s a n e n e r g y f i e l d o f o n l y The c y c l i n g o f i r o n and manganese i s i n f l u e n c e d by two p r o c e s s e s
11 k c a l / m o l e .
(Kuznetsov, 1970).
F i r s t , r e d u c t i o n o f t h e o x i d i z e d combined m e t a l o c c u r s under
a p p r o p r i a t e redox c o n d i t i o n s a s f e r r o u s b i c a r b o n a t e or i s p r e i p i t a t e d a s a s u l p h i d e , example r e a c t i o n s b e i n g : Fe203 FeS
+
+
3H2S +2FeS
+
3H20
+S
(6.8)
+
H2S
(6.9)
2H2C03+Fe(HC03)2
Second, s h e a t h e d and s t a l k e d b a c t e r i a , a l g a e , p r o t o z o a n f l a g e l a t e s , and s p e c i f i c t r u e b a c t e r i a p r e c i p i t a t e f e r r i c and manganic o x i d e s on t h e i r c e l l s . i r o n b a c t e r i a o c c u r i n i r o n - r i c h w a t e r s o f n e u t r a l o r a l k a l i n e pH.
The t r u e Characteristic
r e a c t i o n s of chemoautotrophic b a c t e r i a which d e p o s i t h y d r o x i d e s and o x i d e s a r e : 4Fe(HC03)2 4MnC03
+
+ O2 +
02+
+ 4H2C03 + 4C02
6H20+4Fe(OH)3
2Mn203
+
(+ 58 k c a l )
4C02 (+ 76 k c a l )
(6.10) (6.11)
Some s p e c i e s of L e p t o t h r i x can o x i d i z e b o t h f e r r o u s and manganous s a l t s , whereas G a l l i o n e l l a (Spirophyllum) i s r e s t r i c t e d t o i r o n .
S i n c e a t n e u t r a l pH, and i n t h e
p r e s e n c e o f oxygen Fez+ i s s p o n t a n e o u s l y o x i d i z e d ; t h e i r o n o x i d i z i n g b a c t e r i a a r e r e s t r i c t e d t o a r e a s o f s t e e p redox g r a d i e n t , and a r e e f f e c t i v e l y competing w i t h oxygen f o r t h e reduced i r o n .
They a r e t h e r e f o r e , g e n e r a l l y r e s t r i c t e d t o t h e
i n t e r f a c e a r e a s o f i r o n - r i c h r o c k s e e p s , swamps, and bogs where t h e redox p o t e n t i a l
i s s u f f i c i e n t l y low f o r reduced i r o n t o o c c u r , and t o upper h y p o l i m n e t i c a r e a s . O t h e r groups o f b a c t e r i a i n v o l v e d i n t h e p r e c i p i t a t i o n o f i r o n and manganese a r e h e t e r o t r o p h i c forms s u c h a s C l a d o t h r i x and some L e p t o t h r i x which a r e f i l a m e n t o u s and d e p o s i t i r o n and manganese on t h e c e l l i n s h e a t h d u r i n g t h e metabolism o f o r g a n i c compounds.
The c o l o n i a l , c o c c o i d c e l l s o r s h o r t r o d s o f S i d e r o c a p s a a r e a
common form o c c u r r i n g a t t h e oxic-anoxic i n t e r f a c e zone of hypolimnion-metalimnion, e s p e c i a l l y i n i r o n m e r o m i c t i c l a k e s (Dubinina e t el., 1 9 7 3 ) .
Siderocapsa i s a l s o
w i d e l y d i s t r i b u t e d i n t h e oxygenated zones o f s t r e a m s and l a k e s (Hardman and T h i s genus i s n o t a b l e b e c a u s e i t i s c a p a b l e o f m i n e r a l i z i n g
Henrici, 1939).
humates and i n c r e a s e s i n numbers c o i n c i d e n t w i t h i n c r e a s e s i n i r o n humates d u r i n g p e r i o d s of h i g h r a i n f a l l .
Another w i d e l y d i s t r i b u t e d i r o n - o x i d i z i n g b a c t e r i u m i s
T h i o b a c i l l u s t h i o o x i d a n s which o x i d i z e s i r o n s u l p h i d e (FeS) t o f e r r i c s u l p h a t e
(ZoBell, 1973) : FeS2
+ 3%02 + H20+FeS04
2FeS04 +
10, +
H2S04
+ H2S04
Fe2(S04)3
+
(6.12) H20
(6.13)
357 A l s o , F e r r o b a c i l l u s f e r r o o x i d a n s , which i s found i n v e r y a c i d w a t e r s (pH 53 y m Specularite < 53 y m
X Pigment
0.5
\. I
I
I
I
I
I
1
100 200 300 400 500 600 700 TEMPERATURE (
C)
F i g . 7 . 4 6 . Thermal d e m a g n e t i z a t i o n o f t h e NRM of a remagnetized Devonian Sandstone and t h e DRM of i t s e x t r a c t e d pigment, c l a y - o x i d e , and s p e c u l a r i t e f r a c t i o n s . S p e c u l a r i t e c a r r i e s t h e NRM and must have been remagnetized a s i n d i c a t e d by t h e d i r e c t i o n a l a n a l y s i s o f t h e whole rock NRM i n post-Devonian times, p r o b a b l y by t h e i n s i t u o x i d a t i o n of m a g n e t i t e . Devonian d i r e c t i o n s and t h e s a n d s t o n e must have s u f f e r e d post-Devonian remagn e t i z a t i o n , p r o b a b l y i n Permian t i m e s , j u d g i n g from t h e d i r e c t i o n s of a number
o f s i m i l a r specimens ( T u r n e r and A r c h e r , 1975, p. 248). of s e p a r a t e d pigment, c l a y
+
Thermal d e m a g n e t i z a t i o n
i r o n o x i d e and s p e c u l a r i t e f r a c t i o n s shows t h a t t h e
NRM must r e s i d e i n t h e s p e c u l a r i t e f r a c t i o n .
Hence t h e s p e c u l a r i t e f r a c t i o n must
have been remagnetized i n Permian times and m i n e r a l o g i c a l e v i d e n c e s u g g e s t s t h a t t h e o x i d a t i o n of m a g n e t i t e was r e s p o n s i b l e . CONCLUSIONS AND FURTHER READING The m a g n e t i c p r o p e r t i e s of c o n t i n e n t a l r e d beds i n d i c a t e t h a t h a e m a t i t e i s t h e p r i n c i p a l magnetization c a r r i e r .
Although b o t h t e x t u r a l p h a s e s of h a e m a t i t e ,
439 pigment and s p e c u l a r i t e , a c t a s remanence c a r r i e r s i n p a r t i c u l a r c a s e s , t h e s p e c u l a r i t e i s g e n e r a l l y more i m p o r t a n t .
There i s no e v i d e n c e t h a t t h i s s p e c u l a r i t e
c a r r i e s any m a g n e t i z a t i o n (DRM o r PDRM) which r e l a t e s t o d e p o s i t i o n a l o r e a r l y post-depositional
events.
I n d e e d t h e e v i d e n c e of e x t e n s i v e s p e c u l a r i t e d i a g e n e s i s
( m a r t i t i z a t i o n and a u t h i g e n i c overgrowth) s u g g e s t s t h a t t h e m a g n e t i z a t i o n i s i n t i m a t e l y r e l a t e d t o red-bed d i a g e n e s i s .
T h i s i s c o n s i s t e n t w i t h t h e abundance
of multicomponent m a g n e t i z a t i o n s i n r e d beds which s u g g e s t a long-continued p r o c e s s o f d i a g e n e s i s and a s s o c i a t e d m a g n e t i z a t i o n . The t h e o r y of r o c k magnetism and palaeomagnetism i n c l u d i n g m a g n e t i z a t i o n p r o c e s s e s i n s e d i m e n t s a r e d e s c r i b e d by: McElhinny, M.W.,
1973. Palaeomagnetism and P l a t e T e c t o n i c s . Cambridge U n i v e r s i t y
P r e s s , 358 pp. S t a c e y , F.D.
a n d B a n e r j e e , S.K., 1974. The P h y s i c a l P r i n c i p l e s of Rock Magnetism
E l s e v i e r , Amsterdam, 195 pp. Verosub, K . L . ,
1977. D e p o s i t i o n a l and p o s t - d e p o s i t i o n a l p r o c e s s e s i n t h e
m a g n e t i z a t i o n of s e d i m e n t s . Rev. Geophys. Space. P h y s . , 15.: 129-143.
440
CHAPTER 8
THE PALAEOMAGNETISM OF CONTINENTAL RED BEDS INTRODUCTION
C o n t i n e n t a l r e d beds a r e of s p e c i a l s i g n i f i c a n c e i n palaeomagnetism. were t h e f i r s t s e d i m e n t a r y r o c k s t o be s t u d i e d i n any d e t a i l ( e . g .
They
Clegg e t a l . ,
1954; Runcorn, 1955) and many p o l a r wander p a t h s and c o n t i n e n t a l r e c o n s t r u c t i o n s r e l y h e a v i l y on p a l a e o m a g n e t i c d e t e r m i n a t i o n s from r e d beds.
Many of t h e s e r e s u l t s
were made p r i o r t o t h e r e c e n t advances i n o u r knowledge of r e d bed d i a g e n e s i s b u t t h e y have s u b s e q u e n t l y c o n t i n u e d and g e n e r a l l y w i t h l i t t l e r e g a r d t o t h e d i a g e n e t i c p r o c e s s e s which a r e r e s p o n s i b l e f o r t h e m a g n e t i z a t i o n of r e d beds.
The rock magnetic
e v i d e n c e q u i t e c l e a r l y i n d i c a t e s t h a t h a e m a t i t e i s t h e p r i n c i p a l remanence-carrying phase i n r e d beds.
S i m i l a r l y the diagenetic evidence i n d i c a t e s t h a t haematite i s
mainly an a u t h i g e n i c phase and i t must b e concluded t h a t t h e magnetism of c o n t i n e n t a l r e d beds i s a c q u i r e d d u r i n g d i a g e n e s i s . Palaeomagnetic s t u d i e s have been made on r e d beds of a l l g e o l o g i c a l a g e s , throughout t h e P r o t e r o z o i c and t h e P h a n e r o z o i c , i n c l u d i n g t h e Cenozoic d e s e r t a l l u v i u m o f t h e s o u t h w e s t e r n USA (Larson and Walker, 1 9 7 5 ) .
I n many i n d i v i d u a l
r e d bed f o r m a t i o n s which have undergone e x t e n s i v e d i a g e n e t i c c h a n g e s , t h e p a l a e o magnetic p r o p e r t i e s may p r o v i d e t h e most d e t a i l e d r e c o r d of t h e s e changes.
This
may be i n t h e form of d i f f e r e n t components of m a g n e t i z a t i o n c a r r i e d by d i f f e r e n t t e x t u r a l p h a s e s of h a e m a t i t e which were produced a t d i f f e r e n t s t a g e s o f d i a g e n e s i s . Other magnetic p r o p e r t i e s , such a s blocking temperature s p e c t r a , o r s u s c e p t i b i l i t y changes d u r i n g h e a t i n g , may g i v e i m p o r t a n t c l u e s t o t h e d e p t h o f t e m p e r a t u r e s t o which t h e r o c k s have been h e a t e d .
buria1,or the
The p a l a e o m a g n e t i c s i g n i f i c a n c e
of continental red beds, t h e r e f o r e , i s n o t t h a t they represent an accurate record of t h e ambient geomagnetic f i e l d a t t h e time of d e p o s i t i o n b u t t h a t t h e y a r e e x t r e m e l y s e n s i t i v e i n d i c a t o r s of t h e n a t u r e and e x t e n t of r e d bed d i a g e n e s i s The palaeomagnetic p r o p e r t i e s o f r e d beds a r e e x t r e m e l y v a r i a b l e b u t a number of d i s t i n c t i v e t y p e s o f m a g n e t i z a t i o n ( b a s e d on t h e number of d i s c r e t e components p r e s e n t and t h e t i m e of t h e i r a c q u i s i t i o n ) can b e i d e n t i f i e d .
These d i f f e r e n t
t y p e s of r e d bed m a g n e t i z a t i o n show no s y s t e m a t i c v a r i a t i o n through g e o l o g i c a l time.
I n f a c t , t h e p a l a e o m a g n e t i c s t u d y o f r e d beds of v a r i o u s a g e s p r o v i d e s a n
i m p o r t a n t c o n t r i b u t i o n t o t h e o r i g i n o f r e d beds.
I t shows t h a t d i a g e n e s i s must
be a r a t e p r o c e s s i n which t h e r a t e , i n c l u d i n g t h e r a t e of a c q u i s i t i o n o f magn e t i z a t i o n , i s determined by:
1. t h e o r i g i n a l m i n e r a l o g y o f t h e s e d i m e n t ; m i n e r a l o g i c a l l y immature s e d i m e n t s b e i n g s u s c e p t i b l e t o more e x t e n s i v e a l t e r a t i o n and p r o l o n g e d m a g n e t i z a t i o n
441
2 . t h e d e p o s i t i o n a l c o n d i t i o n s ; f i n e r g r a i n e d s e d i m e n t s b e i n g less s u s c e p t i b l e t o e x t e n s i v e a l t e r a t i o n and p r o l o n g e d m a g n e t i z a t i o n
3. t h e p r e v a i l i n g c l i m a t e ; warm a r i d c l i m a t e s b e i n g f a v o u r a b l e f o r t h e longc o n t i n u e d a l t e r a t i o n and m a g n e t i z a t i o n o f r e d beds. These f e a t u r e s c a n b e d e m o n s t r a t e d by r e f e r e n c e t o r e d beds of v a r i o u s g e o l o g i c a l a g e s and d e p o s i t e d i n a v a r i e t y o f d e p o s i t i o n a l environments under d i f f e r e n t climatic conditions. PROTEROZOIC BASINS OF WESTERN CANADA The n o r t h w e s t e r n p a r t o f t h e Canadian s h i e l d i s s u b d i v i d e d i n t o t h e S l a v e , C h u r c h i l l , and Bear P r o v i n c e s on t h e t e c t o n i c map o f Canada ( S t o c k w e l l , 1968). E a r l y P r o t e r o z o i c s u p r a c r u s t a l r o c k s a r e now p r e s e r v e d m a i n l y i n t h r e e s t r u c t u r a l t h e Epworth b a s i n s o u t h w e s t o f C o r o n a t i o n G u l f , t h e Goulburn b a s i n around
basins:
B a t h u r s t I n l e t , and t h e G r e a t S l a v e b a s i n around t h e e a s t arm of G r e a t S l a v e Lake (Fig.8.l).
These s u p r a c r u s t a l r o c k s formed w i t h i n t h e C o r o n a t i o n g e o s y n c l i n e a l o n g
t h e w e s t e r n margin o f t h e S l a v e P r o v i n c e and a r e now b e s t exposed i n t h e Epworth basin.
Thick s u p r a c r u s t a l r o c k s c o r r e l a t i v e w i t h t h o s e i n t h e g e o s y n c l i n e a l s o
accumulated f a r t o t h e e a s t o f t h e Bear P r o v i n c e i n two a u l a c o g e n s , now exposed
in t h e Goulburn and G r e a t S l a v e b a s i n s . These P r o t e r o z o i c r o c k s , d e s c r i b e d by Hoffman (1973), a r e remarkable i n t h e i r resemblance t o P h a n e r o z o i c g e o s y n c l i n a l sequences.
They imply t h e p r e s e n c e o f a
c o n t i n e n t a l margin and g l o b a l p l a t e i n t e r a c t i o n s d u r i n g t h e E a r l y P r o t e r o z o i c . G e o s y n c l i n a l d e p o s i t i o n began w i t h t h e d e p o s i t i o n o f o r t h o q u a r t z i t e s on a westwardf a c i n g c o n t i n e n t a l s h e l f w i t h o v e r l y i n g s t r o m a t o l i t i c dolomite.
West of t h e s h e l f
e d g e , t h e d o l o m i t e p a s s e s a b r u p t l y i n t o a much t h i n n e r mudstone sequence w i t h d o l o m i t e d e b r i s - f l o w s , and t h e o r t h o q u a r t z i t e i n t o a t h i c k l a m i n a t e d s i l t and muds t o n e sequence w i t h q u a r t z i t e t u r b i d i t e s .
Foundering o f t h e c o n t i n e n t a l s h e l f
r e s u l t e d i n t h e d r a p i n g o f a t h i n l a m i n a t e d p y r i t i c b l a c k mudstone sequence overl a i n by a westward-thickening
c l a s t i c wedge which r e s u l t e d from t h e i n t r u s i o n and
e r o s i o n of b a t h o l i t h s t o t h e w e s t .
The c l a s t i c wedge b e g i n s w i t h a t h i c k sequence
o f t u r b i d i t e s t h a t p a s s e s e a s t w a r d s i n t o c o n c r e t i o n a r y mudstone on t h e p l a t f o r m . The mudstone g r a d e s upward i n t o l a m i n a t e d s h a l y l i m e s t o n e , minor t u r b i d i t e s , o v e r l a i n by cross-bedded r e d l i t h i c s a n d s t o n e s . The p l a t f o r m i s marked by two a u l a c o g e n s
-
deeply subsiding f a u l t troughs t h a t
e x t e n d a t h i g h a n g l e s from t h e g e o s y n c l i n e f a r i n t o t h e i n t e r i o r o f t h e p l a t f o r m . These a u l a c o g e n s r e c e i v e d much t h i c k e r s e d i m e n t a r y sequences and were n e v e r s u b j e c t e d t o b a t h o l i t i c i n t r u s i o n s , r e g i o n a l metamorphism, o r low-angle overt h r u s t i n g c h a r a c t e r i s t i c of t h e g e o s y n c l i n e .
The Athapuscow a u l a c o g e n i n t h e
G r e a t S l a v e Lake a r e a i s i n t e r p r e t e d a s h a v i n g been a n i n c i p i e n t r i f t l o c a t e d over a c r u s t a l a r c h during the c o n t i n e n t a l s h e l f s t a g e of t h e geosyncline, b u t
442
...... ,.... ..,... ...... ..... ,..... .. . . , Middle Proterozoic and younger cover rocks. ...*.....*. ..... Comagmatk granitic and vdcenic rocks of the epizoni Great B u r
itholith
Motamorphorrd eerly Proterozoic wpracrustalrocks of the continental rise and clastic mdy of the Corontion 0.ocyncline intruded by the mowzonal Hapburn batholith Thick u r l y Proterozoic wpracrustel rocks of the continental shelf end ciastic wedge of the Coronation coryncline
Thkk eerly Proterozoic suprrruitd rocks deposited in anlacogens
a 3
Thin only Protarozok wprrmstd rocks of tha continentel platform
Mainly exhumed Archaem k r m n t subjected to eerly Proterozoic cetrlestic and retrogressive metamorphism
3
ExhumadAmhwn basement
Fig.8.1. T e c t o n i c s u b d i v i s i o n and s e d i m e n t a r y b a s i n s of e a r l y P r o t e r o z o i c r o c k s i n t h e n o r t h w e s t e r n Canadian S h i e l d ( a f t e r Hoffman, 1 9 7 3 ) .
443 sagged t o become a c r u s t a l downwarp d u r i n g t h e c l a s t i c wedge s t a g e . F i n a l l y , t h e aulacogen became p a r t o f a r e g i o n a l t r a n s c u r r e n t f a u l t s y s t e m , a l o n g which t h i c k f a n g l o m e r a t e s accumulated i n l o c a l t r o u g h s . The molasse o f t h e C o r o n a t i o n g e o s y n c l i n e c o n s i s t s of a l l u v i a l r e d beds d e r i v e d from t h e b a t h o l i t h i c b e l t t o t h e west.
A t t h e b a s e a r e 125m o f r e d mudstone w i t h
t h i n c r o s s - l a m i n a t e d and mud-cracked b u f f s i l t s t o n e s .
These a r e o v e r l a i n by o v e r
500111 o f r e d mudstone w i t h fining-upwards u n i t s of f r i a b l e r e d l i t h i c s a n d s t o n e s many metres t h i c k .
The s a n d s t o n e s , which commonly show t a b u l a r s e t s o f c r o s s -
bedding up t o 2m t h i c k , a r e g e n e r a l l y w e l l - s o r t e d , c a l c i t e - c e m e n t e d and c o n t a i n abundant a n g u l a r t o sub-rounded c l a s t s of s e d i m e n t a r y r o c k s , i n t e r m e d i a t e t o s i l i c i c v o l c a n i c r o c k s , and minor p l u t o n i c and metamorphic r o c k s . I n t h e Authapuscow a u l a c o g e n t h e molasse phase b e g i n s w i t h 650m of r e d mudstone w i t h h a l i t e c a s t s and e x t e n s i v e o l i s t o s t r o m e s w i t h a n g u l a r b l o c k s o f s t r o m a t o l i t i c d o l o m i t e and l i m e s t o n e .
Above a r e 850m of r e d l a m i n a t e d l i t h i c s a n d s t o n e i n
t a b u l a r cross-bedded u n i t s up t o 5m t h i c k w i t h abundant mud-cracks and r i p p l e marks. The s a n d s t o n e i s o v e r l a i n by 230m of r e d mud-cracked mudstone w i t h b u f f s i l t s t o n e beds and abundant h a l i t e and gypsum c a s t s .
The m o l a s s e phase c u l m i n a t e s w i t h up
t o 4000m of r e d and b u f f f a n g l o m e r a t e s and s a n d s t o n e s which a r e n e a r l y f l a t - l y i n g except c l o s e t o major f a u l t s .
L i t h o l o g i c a l l y t h e sandstones a r e indistinguishable
from t h o s e of t h e g e o s y n c l i n e . These P r o t e r o z o i c r e d b e d s , b e c a u s e o f t h e i r unmetamorphosed n a t u r e , o f f e r a u n i q u e o p p o r t u n i t y o f s t u d y i n g t h e palaeomagnetism of o l d e r Precambrian r e d beds. The o c c u r r e n c e of contemporaneous l a v a flows a l s o p r o v i d e s some independent c o n t r o l on t h e s i g n i f i c a n c e o f t h e p a l a e o m a g n e t i c d a t a . The M a r t i n f o r m a t i o n The M a r t i n Formation c o n s i s t s o f a t l e a s t 3960m of f l u v i a l c o n g l o m e r a t e s , a r k o s i c s a n d s t o n e s , s i l t s t o n e s , a n d e s i t i c - b a s a l t i c l a v a f l o w s , and g a b b r o i c s i l l s preserved i n a small fault-bounded b a s i n t o the south-east of the Great Slave b a s i n ( F r a s e r e t a l . , 1970).
The c l a s t i c s e d i m e n t s a r e unmetamorphosed b u t t h e
v o l c a n i c r o c k s a r e h i g h l y a l t e r e d , p r o b a b l y due t o d e u t e r i c a l t e r a t i o n (Tremblay, 1968).
The M a r t i n Formation r e s t s w i t h a n g u l a r unconformity on t h e metamorphosed
T a z i n Group and a n upper a g e l i m i t i s p r o v i d e d by a Rb/Sr whole rock i s o c h r o n from p e g m a t i t e s c u t t i n g t h e T a z i n Group which y i e l d e d a n a g e o f 1975 (Sassano e t a l . , 1972).
F
20 My
A g a b b r o i c dyke c u t t i n g t h e lower p a r t of t h e M a r t i n
Formation h a s been d a t e d (K/Ar whole r o c k ) a t 1490 flows has y i e l d e d a K / A r whole r o c k a g e of 1635
2
t
100 My and one o f t h e b a s a l t
180 My.
F r a s e r e t a l . (1970)
r e g a r d e d t h e M a r t i n Formation a s having formed between 1830 and 1650 My ago. These r o c k s a r e p r o b a b l y t h e o l d e s t unmetamorphosed c o n t i n e n t a l r e d beds anywhere i n t h e world.
444 The palaeomagnetism of t h e M a r t i n Formation i s descrbbed by Evans and Bingham (1973) and t h e magnetic s u s c e p t i b i l i t y a n i s o t r o p y by Aziz-ur-Rahman e t a l . (1975). Although t h e r e d beds were d e p o s i t e d i n f l u v i a l environments and p r o b a b l y formed by t h e d i a g e n e t i c breakdown o f i r o n - b e a r i n g m i n e r a l s , t h e r e a r e , u n f o r t u n a t e l y , no detailed diagenetic studies reported i n the literature. Evans and Bingham (1973) p r e s e n t e d p a l a e o m a g n e t i c r e s u l t s from r e d b e d s , v o l c a n i c s and dykes from a number o f l o c a l i t i e s i n t h e M a r t i n Formation (Fig.8.2).
Fig.8.2.
Sampling l o c a l i t i e s i n t h e M a r t i n Formation ( a f t e r Evans and Bingham, 1973).
S i t e mean d i r e c t i o n s f o r t w e l v e o f t h e t h i r t e e n s e d i m e n t a r y s i t e s sampled a r e shown
i n F i g . 8 . 3 a f t e r thermal c l e a n i n g a t 500-660°C.
The specimen d i r e c t i o n s o f t h e
t h i r t e e n t h s i t e were random u s i n g W a t s o n ' s (1956) c r i t e r i a . r i b u t i o n s i n Fig.8.3.
The s i t e mean d i s t -
show a tendency t o form two groups w i t h NW d e c l i n a t i o n / u p w a r d
d i p and SE declination/downward d i p .
The r e s u l t s a r e , however, r a t h e r ' s t r e a k e d ' o u t
and t h e p o s i t i o n of s i t e M P and MH s u g g e s t a g r e a t c i r c l e s c a t t e r o f d i r e c t i o n s . Evans and Bingham (1973, p. 1488) thought t h a t s i t e s MP and MQ ( t h e one w i t h random d i r e c t i o n s ) had been magnetized d u r i n g a r e v e r s a l w h i l s t t h e geomagnetic f i e l d was i n an intermediate p o l a r i t y stage.
I n s u p p o r t o f t h i s argument they p r e s e n t e d t h e
r e s u l t s o f i n d i v i d u a l p i l o t specimens which show good thermal ' e n d p o i n t s ' s p r e a d a l o n g a g r e a t c i r c l e p a s s i n g c l o s e t o t h e o v e r a l l mean d i r e c t i o n ( F i g . 8 . 3 ) and v e r y
44 5 N
MARTIN FORMATION N
Fig.8.3. Palaeomagnetic r e s u l t s of t h e M a r t i n Formation (Precambrian) of Saskatchowan p l o t t e d on e q u a l - a r e a s t e r e o g r a p h i c p r o j e c t i o n s . Above t h e behaviour of p i l o t specimens f o r s i t e s MQ and MP i s shown d u r i n g s u c c e s s i v e t e m p e r a t u r e i n c r e m e n t s . Below t h e mean d i r e c t i o n s o f sedimentaxy and igneous s i t e s i s shown a f t e r thermal c l e a n i n g . The open symbols i n d i c a t e upward d i p p i n g v e c t o r s and t h e c l o s e d symbols downward d i p p i n g v e c t o r s ( a f t e r Evans and Bingham, 1 9 7 3 ) .
446 s i m i l a r t o t h e g r e a t c i r c l e s s u g g e s t e d by t h e d i s t r i b u t i o n o f s i t e means.
This
however i s o n l y p a r t l y t r u e b e c a u s e some o f t h e p i l o t specimens from t h e MP s i t e do n o t show s t a b l e end p o i n t s , a f e a t u r e which s u g g e s t s t h e p r e s e n c e o f more than
one component of m a g n e t i z a t i o n . The i n t e r p r e t a t i o n t h a t t h e s e i n t e r m e d i a t e d i r e c t i o n s r e p r e s e n t m a g n e t i z a t i o n s a c q u i r e d d u r i n g a f i e l d r e v e r s a l i s o n l y one p o s s i b i l i t y and one t h a t seems e x t r e m e l y u n l i k e l y i n view of t h e g e o l o g i c a l e v i d e n c e .
This hypothesis requires
t h a t t h e r o c k s were magnetized w i t h i n a few thousand y e a r s of d e p o s i t i o n (assuming t h a t Precambrian and Phanerozoic f i e l d r e v e r s a l s proceeded a t t h e same r a t e ) .
The
a v a i l a b l e p e t r o l o g i c a l e v i d e n c e s u g g e s t s t h a t t h e s e r e d beds have s u f f e r e d d i a g e n e t i c m o d i f i c a t i o n c o n s i s t e n t w i t h t h a t s e e n i n many o t h e r f i r s t - c y c l e a r k o s i c r e d beds. A more p l a u s i b l e a l t e r n a t i v e i s t h a t t h e s i t e mean d i r e c t i o n s c o n t a i n superimposed
normal and r e v e r s e d components which y i e l d i n t e r m e d i a t e r e s u l t a n t v e c t o r s a t some s i t e s (MH, M P , MQ).
I n t h e s e c a s e s i t may be t h a t normal and r e v e r s e d components
a r e of comparable magnitude, whereas t h o s e s i t e s which a r e a n t i p o d a l l y opposed might be dominated by e i t h e r t h e normal o r r e v e r s e d component. c o n s i s t e n t with the geological evidence.
This hypothesis i s
I f t h e m a g n e t i z a t i o n was a c q u i r e d d u r i n g
d i a g e n e s i s t h e n i t i s more t h a n l i k e l y t h a t normal and r e v e r s e d f i e l d components would be i n t e g r a t e d ( i f a r e v e r s a l o c c u r r e d ) i n t h e r e d bed sequence b e c a u s e of t h e t i m e s c a l e of d i a g e n e s i s (Walker, 1967a, 1976).
If o n l y a n t i p a r a l l e l components a r e
i n v o l v e d i n t h i s way t h e n t h e g r e a t c i r c l e d e f i n e d by t h e s i t e mean d i r e c t i o n s may be a r e l i a b l e e s t i m a t e of t h e d i p o l a r a x i s of m a g n e t i z a t i o n which e x i s t e d d u r i n g M a r t i n Formation d i a g e n e s i s ( s i n c e a l l t h e r e s u l t a n t v e c t o r s would l i e i n t h e same plane).
R e s u l t s from contemporaneous igneous r o c k s i n t h e M a r t i n Formation s u g g e s t
t h a t t h i s c o u l d be t h e c a s e b e c a u s e , a l t h o u g h somewhat s c a t t e r e d , t h e y show t h e same a x i s of m a g n e t i z a t i o n a s t h e s e d i m e n t a r y s i t e s .
The s i g n i f i c a n c e of p o l e
p o s i t i o n s from t h e M a r t i n Formation i s d i s c u s s e d by Evans and Bingham (1973, p.
1490-1492).
The C h r i s t i e Bav Group The G r e a t S l a v e Supergroup comprises o v e r 12,000m of unmetamorphosed s e d i m e n t a r y r o c k s and v o l c a n i c s p r e s e r v e d i n t h e E a s t A r m F o l d B e l t of t h e G r e a t S l a v e Lake (Fig.8.l).
In many p l a c e s t h e s t r a t a a r e t i g h t l y f o l d e d and u n c o n f o m a b l y o v e r l a i n
by t h e Et-Ehen Group.
The C h r i s t i e Bay Group i s t h e uppermost of t h e four groups
( S o s a n , K a h o c h e l l a , P e t h e i , C h r i s t i e Bay) r e c o g n i z e d by Hoffman (1969) t o make up t h e G r e a t S l a v e Supergroup.
I t c o n s i s t s o f f o u r conformable f o r m a t i o n s , which a r e
i n d e c r e a s i n g a g e , t h e S t a r k , Tochatwi, P o r t a g e I n l e t , and P e a r s o n Formations. T o g e t h e r t h e s e make up t h e m o l a s s e f a c i e s o f t h e Athapuscow a u l a c o g e n (Hoffman, 1973). Palaeomagnetic r e s u l t s have been r e p o r t e d f o r t h e two o l d e s t f o r m a t i o n s of t h e
441
C h r i s t i e Bay Group, t h e S t a r k and t h e Tochatwi Formations by Bingham and Evans (1976) and Evans and Bingham (1976) r e s p e c t i v e l y . i n Fig.8.4.
Sampling l o c a l i t i e s a r e shown
The S t a r k Formation c o n s i s t s of a t l e a s t 60Cm o f f i n e l y bedded r e d
mudstones w i t h a b u n d a n t , d i s c o n t i n u o u s , p a l e g r e e n r e d u c t i o n s l a y e r s and s p o t s . R i p p l e marks, c r o s s - b e d d i n g , and h a l i t e c a s t s a r e common.
Hoffman (1969) c o n s i d e r e d
t h e S t a r k Formation t o have been d e p o s i t e d i n a s h a l l o w s u b l i t t o r a l marine e n v i r onment o f h i g h a r i d i t y .
K / A r d a t e s from a s s o c i a t e d i g n e o u s r o c k s s u g g e s t t h a t t h e
S t a r k Formation i s between 1600 and 1800 My o l d (Hoffman, 1968).
Sampling l o c a l i t i e s used i n t h e palaeomagnetic s t u d y of S t a r k Formation Fig.8.4. and t h e Tochatwi Formation. Compiled from Bingham and Evans (1976) and Evans and Bingham (1976). D e t a i l e d p a l a e o m a g n e t i c r e s u l t s from t h e S t a r k Formation a r e p r e s e n t e d by Bingham and Evans (1976).
A t o t a l of 160 hand samples from 55 s t r a t i g r a p h i c a l l y
d i s t i n c t s i t e s were used i n t h e s t u d y (Fig.8.4).
The p r e l i m i n a r y r e s u l t s showed
t h e p r e s e n c e of two zones o f d i f f e r e n c e magnetic p o l a r i t y w i t h a t r a n s i t i o n a b o u t 280m above t h e b a s e of t h e S t a r k Formation.
P i l o t specimens from b o t h zones
were s u b j e c t e d t o b o t h AF and t h e r m a l d e m a g n e t i z a t i o n . I n b o t h c a s e s t h e r e was l i t t l e d i r e c t i o n a l change d u r i n g t h e d e m a g n e t i z a t i o n t r e a t m e n t b u t t h e i n t e n s i t y p a t t e r n s , a r e d i f f e r e n t f o r r e v e r s e d specimens and f o r normal specimens.
The p i l o t
448 specimens from t h e r e v e r s e d s i t e s g e n e r a l l y show a d e c r e a s e i n i n t e n s i t y whereas t h e normal s i t e s show a s l i g h t i n c r e a s e i n i n t e n s i t y a t 400 Oe (Fig.8.5).
The
t h e r m a l d e m a g n e t i z a t i o n a g a i n r e s u l t e d i n l i t t l e d i r e c t i o n a l change u n t i l temp e r a t u r e s were above 66OoC and t h e same c o n t r a s t i n b e h a v i o u r between normal and r e v e r s e d s i t e s was o b s e r v e d
.
Large d e c r e a s e s i n i n t e n s i t y o n l y o c c u r a t t e m -
p e r a t u r e s o v e r 6OO0C ( F i g . 8 . 5 ) i n d i c a t i n g t h a t h a e m a t i t e must c a r r y t h e m a j o r p a r t o f t h e remanence.
1.0
1.0
J/
Jo
Om5i
\
REVERSES SITES
J/ JO
0.1
REVERSED SITES
\ NORMAL SITES
I 0.01 L. I I I I ' I
0
goo
1200
1800
20
I
Peak firld(00)
200
400
600
Temperature (c)
Fig.8.5. Normalized i n t e n s i t y changes d u r i n g AF and t h e r m a l d e m a g n e t i z a t i o n o f p i l o t specimens from t h e S t a r k Formation ( a f t e r Bingham a n d Evans, 1976). The i n t e n s i t y changes which o c c u r d u r i n g d e m a g n e t i z a t i o n s u g g e s t t h e p r e s e n c e of a s e c o n d a r y component o f m a g n e t i z a t i o n , a p p r o x i m a t e l y p a r a l l e l t o t h e mag-
n e t i z a t i o n of t h e r e v e r s e d s i t e s b u t opposing t h e normal s i t e s .
Bingham and Evans
(1976, p. 572) a p p l i e d v e c t o r s u b t r a c t i o n of t h e components removed from a number of s i t e s between 0 and 400 Oe. form a c l o s e group (347, f 6 7 ,
With t h e s t r a t a i n t h e i r p r e s e n t p o s i t i o n t h e s e R g 5 = 6 ) n e a r t o t h e p r e s e n t f i e l d (030, +63) a t t h e
c o l l e c t i n g l o c a l i t y , a f e a t u r e s t r o n g l y suggesting t h e a c q u i s i t i o n of a secondary component i n r e l a t i v e l y r e c e n t times.
A s i m i l a r a n a l y s i s of t h e v e c t o r s removed
by h e a t i n g t o 5OO0C a l s o y i e l d e d a group (239, +80,
o(
95
= 4')which
showed l i t t l e
1
449
s c a t t e r b e f o r e any c o r r e c t i o n was made f o r g e o l o g i c a l t i l t .
The s c a t t e r o f t h i s
group i s much g r e a t e r when c o r r e c t e d f o r bedding t i l t and a g a i n a p o s t - f o l d i n g secondary component o f m a g n e t i z a t i o n a p p e a r s t o b e p r e s e n t . The r e s u l t s o f s i t e mean d i r e c t i o n s a f t e r thermal c l e a n i n g a t 5OO0C a r e shown i n Fig.8.6.
The most obvious f e a t u r e of t h e s e r e s u l t s i s t h e p o l a r i t y r e v e r s a l
which o c c u r s a b o u t mid-way through t h e s e c t i o n .
S i t e means i n t h e v i c i n i t y o f t h e
r e v e r s a l show i n t e r m e d i a t e d i r e c t i o n s and t h e r e i s one anomalous s i t e mean d i r e c t i o n i n t h e lower, normal i n t e r v a l of t h e s e c t i o n .
The f e a t u r e s a r e s i m i l a r t o f i e l d
r e v e r s a l s documented i n much younger sediments a n d , i n p a r t i c u l a r , show v i r t u a l geomagnetic p o l e s (VGP's) which f o l l o w a g r e a t c i r c l e p a t h d u r i n g t h e p o l a r i t y t r a n s i t i o n ; t h e pre- and p o s t - t r a n s i t i o n a l VGP's a r e e s s e n t i a l l y a n t i p o d a l and t h e remanent i n t e n s i t y i s reduced by a f a c t o r o f 3 w i t h i n t h e t r a n s i t i o n zone. hiination
Inclination ooD-4so oo +UP +so06
Intensity (lO%mU om3,
10
60
[.-.
Fig. 8.6. S t r a t i g r a p h i c p r o f i l e o f t h e r m a l l y c l e a n e d , s i t e mean d i r e c t i o n s f o r t h e S t a r k Formation a f t e r c o r r e c t i o n f o r g e o l o g i c a l d i p . The i n t e n s i t y i s r e p r e s e n t e d by a f i v e p o i n t moving a v e r a g e ( a f t e r Bingham and Evans, 1 9 7 5 ) .
450 These r e s u l t s have been i n t e r p r e t e d a s a f a i t h f u l r e c o r d o f a n e a r l y P r o t e r o z o i c geomagnetic f i e l d r e v e r s a l and t h e anomalous s i t e mean d i r e c t i o n h a s been i n t e r p r e t e d a s a geomagnetic e x c u r s i o n (Bingham and Evans, 1975; 1976).
I f t h i s were
t r u e then m a g n e t i z a t i o n must have been completed d u r i n g d e p o s i t i o n and e a r l y diagenesis.
The o n l y i n d e p e n d e n t e v i d e n c e f o r t h e a g e o f t h e m a g n e t i z a t i o n o f t h e
S t a r k Formation comes from t h e n e a r b y Mackenzie i n t r u s i o n s . Mackenzie p o l e i s 70'
The f a c t t h a t t h e
from t h e S t a r k Formation p o l e s u g g e s t s t h a t r e g i o n a l r e h e a t i n g
d i d n o t c a u s e r e m a g n e t i z a t i o n of t h e S t a r k Formation.
The a g e of t h e Mackenzie
i n t r u s i o n s (1200 My) p u t s a n upper l i m i t on t h e a g e o f t h e m a g n e t i z a t i o n .
There
i s no d i r e c t e v i d e n c e of t h e p r e c i s e a g e of t h e remanence of t h e S t a r k Formation.
Although t h e palaeomagnetic e v i d e n c e i s c o m p a t i b l e w i t h t h e a c q u i s i t i o n o f remanence s h o r t l y a f t e r d e p o s i t i o n , i t does n o t p r e c l u d e t h e p o s s i b i l i t y of m a g n e t i z a t i o n a t a l a t e r s t a g e of d i a g e n e s i s . Formation i.e.
Some o f t h e p a l a e o m a g n e t i c p r o p e r t i e s o f t h e S t a r k
intermediate d i r e c t i o n s during t h e p o l a r i t y t r a n s i t i o n , t h e presence
of u n s t a b l e m a g n e t i z a t i o n s n e a r t h e p o l a r i t y t r a n s i t i o n and t h e d i s s i m i l a r b e h a v i o u r of normal and r e v e r s e l y magnetized specimens i s e q u a l l y c o n s i s t e n t w i t h a much l a t e r
m a g n e t i z a t i o n i n which a r e v e r s e d zone was superimposed on a 'normal zone'. The Tochatwi Formation (Hoffman, 1968) c o n s i s t s o f up t o 80Om o f f i n e g r a i n e d r e d - b u f f s a n d s t o n e s w i t h s u b o r d i n a t e s h a l e s and conglomerate r e s t i n g conformably The a g e o f t h e Tochatwi Formation i s n o t
on t h e u n d e r l y i n g S t a r k Formation.
p r e c i s e l y known b u t may b e a b o u t 1700 My.
Most o f t h e d e p o s i t s a r e c o n s i d e r e d t o
be o f f l u v i a l o r i g i n by Hoffman (1969) e x c e p t f o r t h o s e n e a r t h e b a s e which may r e p r e s e n t a s h a l l o w marine t i d a l o r i n t e r t i d a l environment. Sampling l o c a l i t i e s used i n t h e p a l a e o m a g n e t i c s t u d y by Evans and Bingham (1976) a r e shown i n Fig.8.4. study.
I n a l l , 29 s t r a t i g r a p h i c a l l y d i s t i n c t s i t e s were u s e d i n t h e
During t h e r m a l d e m a g n e t i z a t i o n t h e m a j o r i t y o f specimens i n t h e Tochatwi
Formation ( r e p r e s e n t e d by t h e N. changes.
s e c t i o n i n Fig.8.4)
show s y s t e m a t i c d i r e c t i o n a l
The d i r e c t i o n s move a l o n g g r e a t c i r c l e s by 20-50'
e n d - p o i n t s between 5OO0C and 60OoC.
before reaching s t a b l e
They a l s o r e v e a l t h e r m a l l y d i s t r i b u t e d b l o c k i n g
t e m p e r a t u r e s p e c t r a w i t h o n l y 1-9% of t h e NRM remaining a t 6OO0C (Fig.8.7). On t h e b a s i s o f t h e r e s u l t s from t h e t h e r m a l d e m a g n e t i z a t i o n 46 spceimens (one from e a c h sample) were h e a t e d t o 5OO0C and 60OoC.
The r e s u l t s showed h i g h e r
w i t h i n - s i t e p r e c i s i o n (Watson, 1956) a t 5OO0C and t h e d i r e c t i o n s f o r t h e e l e v e n non-random s i t e s o f t h e N-section
( e x c l u d i n g s i t e NA) a r e shown i n Fig.8.8
and a f t e r , c o r r e c t i o n f o r g e o l o g i c a l t i l t .
before,
Q u i t e c l e a r l y , t h e r e s u l t s a r e much
b e t t e r grouped b e f o r e c o r r e c t i o n f o r g e o l o g i c a l d i p (Mean d i r e c t i o n 165, +78 w i t h
k = 16) than a f t e r c o r r e c t i o n (k decreased t o 4).
The d e c r e a s e i n k i s s i g n i f i c a n t
a t t h e 99% l e v e l (McElhinny, 1964) i n d i c a t i n g a n e g a t i v e f o l d t e s t (Graham, 1949). The r e s u l t s i n d i c a t e t h a t t h i s N-section i n t h e Tochatwi Formation h a s been remagnetized a f t e r f o l d i n g .
A s i m i l a r p o s t - f o l d i n g r e m a g n e t i z a t i o n h a s been
r e c o g n i z e d i n t h e Kahochella Group, some 70km t o t h e e a s t o f t h i s s e c t i o n
451 (McMurray e t a l . , 1973). The d i r e c t i o n o f m a g n e t i z a t i o n i n t h e Kahochella Group
i s v e r y c l o s e t o t h a t s e e n i n t h e N - s e c t i o n of t h e Tochatwi Formation, i n d i c a t i n g t h a t t h e r e m a g n e t i z a t i o n was o f some r e g i o n a l s i g n i f i c a n c e .
The most l i k e l y
p o s s i b i l i t y i s t h a t r e g i o n a l r e h e a t i n g , d u r i n g a m a j o r i g n e o u s e v e n t , was t h e c a u s e of t h e r e m a g n e t i z a t i o n .
L
I
I
I
200
I
I
Temperature
I
600
400
f
( O C)
Fig.8.7. Thermal d e m a g n e t i z a t i o n c u r v e s f o r t h e Tochatwi Formation (N-section) F o u r t e e n specimens a r e r e p r e s e n t e d by t h e shaded a r e a ( a f t e r Evans and Bingham, 1976). Not a l l of t h e Tochatwi Formation h a s s u f f e r e d complete r e m a g n e t i z a t i o n .
This
i s i n d i c a t e d by a number of s i t e s N A , FA-FE, AC and AD which y i e l d d i r e c t i o n s s i m i l a r t o t h o s e of t h e S t a r k Formation,
S i t e s AC and AD have s u f f e r e d some d e g r e e
o f t h e r m a l metamorphism by i n t r u s i v e b o d i e s of a s i m i l a r a g e t o t h e Mackenzie Diabase ( I r v i n g e t a l . ,
1972).
The p r o g r e s s i v e thermal d e m a g n e t i z a t i o n o f some of
t h e s e specimens i s shown i n F i g . 8 . 9 .
I n i t i a l l y t h e y have a s h a l l o w i n c l i n a t i o n
towards t h e WSW ( t h e Mackenzie d i r e c t i o n f o r t h e a r e a ) b u t move p r o g r e s s i v e l y towards t h e o p p o s i t e q u a d r a n t on h e a t i n g t o h i g h e r t e m p e r a t u r e s .
S t a b l e end-points
w i t h a d i r e c t i o n v e r y s i m i l a r t o t h a t s e e n i n t h e S t a r k Formation s u g g e s t s t h a t t h i s i s t h e o r i g i n a l remanence of t h e Tochatwi Formation,
The mean d i r e c t i o n s of
t h e s e s i t e s a r e s c a t t e r e d b e f o r e c o r r e c t i o n f o r d i p , b u t form a c o h e r e n t group a f t e r c o r r e c t i o n , w i t h a mean d i r e c t i o n 030, -11 ( k = 14,Wg5 = 15').
The remanence
o f t h e s e s i t e s t h e r e f o r e p r o b a b l y p r e c e d e s t h e f o l d i n g , and a l s o t h e d e p o s i t i o n of t h e o v e r l y i n g E t - t h e n Group s e d i m e n t s . The a g e of t h e m a g n e t i z a t i o n of t h e S t a r k Formation and Tochatwi Formation cannot be more p r e c i s e l y d a t e d .
The a g e of t h e E t - t h e n s e d i m e n t s i s n o t p r e c i s e l y known.
They a r e younger t h a n t h e d i o r i t e s and s y e n i t e s which i n t r u d e t h e G r e a t S l a v e Supergroup, which a r e d a t e d a t 1630-1845 My, and o l d e r t h a n t h e Mackenzie d i a b a s e s
452
0 0
.
O. 0.
i-0 0
Fig.8.8. Palaeomagnetic r e s u l t s f o r t h e Tochatwi Formation ( N - s e c t i o n ) a f t e r thermal c l e a n i n g . The v e c t o r s a r e p l o t t e d b e f o r e ( a b o v e ) and a f t e r (below) c o r r e c t i o n f o r g e o l o g i c a l t i l t . The c l o s e g r o u p i n g of t h e v e c t o r s p r i o r t o c o r r e c t i o n f o r g e o l o g i c a l t i l t s u g g e s t s t h a t t h e r o c k s were remagnetized a f t e r f o l d i n g ( a f t e r Evans and Bingham, 1 9 7 6 ) .
453
which c u t them and a r e d a t e d a t 1250 My ( F a h r i g and J o n e s , 1969).
Accepting a n
age of 1700-1750 My f o r t h e S t a r k and Tochatwi Formations means t h a t t h e a g e o f t h e m a g n e t i z a t i o n c a n n o t , f o r c e r t a i n , b e d a t e d t o c l o s e r t h a n w i t h i n a b o u t 500 My of d e p o s i t i o n .
The c l o s e s i m i l a r i t y o f t h e r e s u l t s , p a r t i c u l a r l y t h o s e from t h e
S t a r k Formation, w i t h r e s u l t s from P h a n e r o z o i c r e d beds does s u g g e s t t h a t t h e m a g n e t i z a t i o n o c c u r r e d w i t h i n a few m i l l i o n y e a r s o f d e p o s i t i o n .
The r e s u l t s a r e
q u i t e c o m p a t i b l e w i t h t h e a c q u i s i t i o n of remanence d u r i n g r e d bed d i a g e n e s i s .
N
D1 NRM 200
300 400 450 500 550 575 600
0 3 +C3 NRM
NRM
500 500 575 600 630 660
400 450 500 550 575
C4 NRM
600
200 300 400 450 500 575
630 660
600 630
630 660
D i r e c t i o n a l changes d u r i n g t h e r m a l d e m a g n e t i z a t i o n )oE s i t e s AC and AD Fig.8.9. i n t h e Tochatwi Formation. These r e v e a l s t a b l e d i r e c t i o n s s i m i l a r t o t h o s e i n t h e S t a r k Formation ( a f t e r Evans and Bingham, 1976).
THE LATE PRECAMBRIAN OF NORTH WEST SCOTLAND The palaeomagnetism o f t h e Precambrian s e d i m e n t a r y r o c k s of NW S c o t l a n d has been i n v e s t i g a t e d by I r v i n g ( 1 9 5 7 ) , I r v i n g and Runcorn (1957) and S t e w a r t and I r v i n g (1974); t h e s e r o c k s were amongst t h e f i r s t t o b e s t u d i e d i n d e t a i l , t h e r e s e a r c h b e i n g i n i t i a t e d by S . K . Runcorn i n J u n e 1951.
The r o c k s i n v e s t i g a t e d i n c l u d e t h e
S t o e r Group c o n s i s t i n g o f a t l e a s t 2300m o f r e d s a n d s t o n e s and r e s t i n g unconformably on Lewisian basement.
T h i s i s unconformably o v e r l a i n by t h e T o r r i d o n Group which
c o n s i s t s of a t l e a s t 700Om o f r e d s a n d s t o n e s and s h a l e s .
It i s divided i n t o three
4 54 f o r m a t i o n s ( D i a b a i g , A p p l e c r o s s and A u l t b e a ) and i s o v e r l a i n by t h e Lower Cambrian (Peach e t a l . ,
1907) ( F i g . 8 . 1 0 ) .
G e o c h r o n o l o g i c a l s t u d i e s show t h a t t h e Lewisian basement l a s t s u f f e r e d r e g i o n a l metamorphism a t 1500-1800 My.
Mica and whole r o c k K / A r a g e s o f 1120
2
40 My have
been r e p o r t e d from p a r t s of t h e basement and a l s o from d e t r i t u s i n t h e S t o e r and T o r r i d o n Groups s o t h e s e d i m e n t s must b e younger t h a n t h i s .
Rb/Sr whole r o c k
measurements on s h a l e s and s i l t s t o n e s from t h e S t o e r and T o r r i d o n Groups y i e l d e x c e l l e n t i s o c h r o n s and a g e s of 9 9 1 2 24 My and 796
2
24 My r e s p e c t i v e l y (Moorbath
e t a l . , 1967; Moorbath, 1969).
Fig.8.10. G e n e r a l i z e d s e c t i o n through t h e Caledonian f o r e l a n d showing r e l a t i o n s h i p s of t h e S t o e r and T o r r i d o n Groups t o t h e basement ( a f t e r S t e w a r t and I r v i n g , 1 9 7 4 ) . S e d i m e n t o l o g i c a l s t u d i e s o f t h e r e d f a c i e s o f t h e S t o e r and T o r r i d o n Groups show t h a t t h e y were d e p o s i t e d on a pediment s u r f a c e which formed a t t h e f o o t o f a r e t r e a t i n g Lewisian s c a r p ( W i l l i a m s , 1969 a , b ) (Fig.8.11).
I t i s probable t h a t
s h a l l o w marine environments a r e r e p r e s e n t e d i n p l a c e s ( G r a c i e and S t e w a r t , 1967). The s e d i m e n t s c o n t a i n a v a r i e t y of f i n e - c o a r s e g r a i n e d a r k o s i c s a n d s t o n e s .
The
c o a r s e s a n d s t o n e s o c c u r i n l a t e r a l l y p e r s i s t e n t s h e e t s w i t h e r o s i v e b a s e s and showing f l a t b e d d i n g , t a b u l a r and t r o u g h cross-bedding.
A t t h e b a s e of t h e u n i t s
t h e r e may b e c o n c e n t r a t i o n s of i n t r a - and e x t r a - f o r m a t i o n a l conglomerate.
Finer
sandstones tend t o be l e n t i c u l a r i n n a t u r e occurring with interbedded red s h a l e s and c r o s s l a m i n a t e d s i l t s t o n e .
The s a n d s t o n e s o f t e n show penecontemporaneous
d e f o r m a t i o n s t r u c t u r e s ( S e l l e y , 1969). oned c h a n n e l s .
The r e d s h a l e s a r e o f t e n s e e n t o f i l l aband-
The s a n d s a r e i n t e r p r e t e d a s b r a i d e d c h a n n e l d e p o s i t s o r r e l a t i v e l y
455 h i g h s t r e a m Dower ( S e l l e v . 1965. 1970).
Suggested palaeogeography of n o r t h w e s t S c o t l a n d i n T o r r i d o n i a n ( l a t e Fig.8.11. Precambrian) times ( a f t e r W i l l i a m s , 1969a). The S t o e r and T o r r i d o n i a n r e d beds a r e t h u s f l u v i a l r e d beds ( s e e Chapter 4 ) . P e t r o l o g i c a l l y they a r e f i r s t - c y c l e arkosic sandstones being derived p r i n c i p a l l y from t h e Lewisian basement ( S e l l e y , 1966). S t e w a r t and I r v i n g (1974) p r e s e n t e d p a l a e o m a g n e t i c r e s u l t s f o r f o u r l o c a l i t i e s (Fig.8.12)
which c o v e r e d t h e S t o e r Group i n d e t a i l and t h e lower p a r t of t h e
T o r r i d o n Group.
The NRM o f t h e r e d s a n d s t o n e s i n t h e S t o e r and T o r r i d o n Groups
i s g e n e r a l l y v e r y s t a b l e w i t h b l o c k i n g t e m p e r a t u r e s commonly h i g h e r t h a n 6OO0C a l t h o u g h i n some c a s e s t h e t h e r m a l decay o f t h e NRM i s more r a p i d w i t h b l o c k i n g t e m p e r a t u r e s a s low a s 3OO0C. i n Fig.8.13. haematite.
T y p i c a l t h e r m a l d e m a g n e t i z a t i o n r e s u l t s a r e shown
The r e s u l t s q u i t e c l e a r l y i n d i c a t e t h a t t h e NRM i s c a r r i e d by Only i n t h e S t a c Fada Member, a v o l c a n i c mudflow b r e c c i a , is t h e r e
456
5 km
Fig.8.12. Sampling s i t e s and geology of l a t e Precambrian s e d i m e n t s i n Nw S c o t l a n d . ( a f t e r S t e w a r t and I r v i n g , 1974). any e v i d e n c e t o t h e c o n t r a r y ; h e r e lower b l o c k i n g t e m p e r a t u r e s (50O0-6OO0C) and much lower c o e r c i v e f o r c e s (median d e s t r u c t i v e f i e l d 400-500 Oe) s u g g e s t t h a t m a g n e t i t e o r haemoilmenite c o u l d b e a remanence c a r r i e r .
Although m a g n e t i t e has
been r e p o r t e d from r e d s a n d s t o n e s i n t h e T o r r i d o n Group on t h e b a s i s o f thermomagnetic a n a l y s i s ( P a r r y , 1957; Schwarz, 1 9 6 9 ) , more w i d e s p r e a d i n v e s t i g a t i o n s ( I r v i n g , 1957; S t e w a r t and I r v i n g , 1974) show t h a t i t i s l i k e l y t o b e v e r y s c a r c e . S t e w a r t and I r v i n g (1974, p. 56-57) showed t h a t h a e m a t i t e was t h e o n l y c o a r s e g r a i n e d i r o n o x i d e phase and by p l o t t i n g t h e r m a l s t a b i l i t y (M5oo/%)
a g a i n s t volume
p e r c e n t a g e of s p e c u l a r h a e m a t i t e showed t h a t t h e most s t a b l e specimens were t h o s e w i t h most s p e c u l a r h a e m a t i t e (Fig.8.14).
The s p e c u l a r i t e a l m o s t always shows
m a r t i t i z a t i o n t e x t u r e s when viewed i n p o l i s h e d s e c t i o n s i n d i c a t i n g t h e l i k e l i h o o d t h a t t h e h a e m a t i t e formed by t h e o x i d a t i o n of m a g n e t i t e . The d i r e c t i o n s o f m a g n e t i z a t i o n f o r e a c h sampling s i t e which a r e g i v e n i n Fig.8.14
( s e e S t e w a r t and I r v i n g , 1974, T a b l e s 1 and 2 f o r d e t a i l s ) c o n f i r m t h e
e a r l i e r r e s u l t s of I r v i n g and Runcorn (1957).
The S t o e r Group d i r e c t i o n s a r e a l l
457
DIRECTION DI F F ERE NC E FROM NRM
1.0
A