Eolian sediments and processes, Volume 38

DEVELOPMENTS IN SEDIMENTOLOGY 38

EOLIAN SEDIMENTS AND PROCESSES

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 I 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 TO 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 CH’EMISTRY OF 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 11 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 , Editors 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 , Editor INFLUENCE O F ABYSSAL CIRCULATION ON SEDIMENTARY ACCUMULATIONS IN SPACE AND TIME 24 R.E. GRIM and G U V E N BENTONITES 25A G. L A R S E N and G. V. C H I L I N G A R , Editors DIAGENESIS I N SEDIMENTS AND SEDIMENTARY ROCKS, I 26 T. SUDO and S. S H I M O D A , Editors CLAYS AND CLAY MINERALS O F JAPAN 21 M.M. M O R T L A N D and V.C. F A R M E R , Editors INTERNATIONAL CLAY CONFERENCE 1918 28 A. N I S S E N B A U M , Editor HYPERSALINE BRINES AND EVAPORITIC ENVIRONMENTS 29 P. T U R N E R CONTINENTAL R E D BEDS 30 J.R.L. A L L E N SEDIMENTARY STRUCTURES 31 T . SUDO, S . S H I M O D A , H. Y O T S U M O T O and S. A I T A ELECTRON MICROGRAPHS OF CLAY MINERALS 32 C.A. N I T T R O U E R , Editor SEDIMENTARY DYNAMICS O F CONTINENTAL SHELVES 33 G.N. B A T U R I N PHOSPHORITES ON THE SEA FLOOR 34 J.J. F R I P I A T , Editor ADVANCED TECHNIQUES FOR CLAY MINERAL ANALYSIS 35 H. V A N OLPHEN and F. V E N I A L E , Editors INTERNATIONAL CLAY CONFERENCE 1981 36 A. IIJIMA, J.R. HEIN and R . S I E V E R , Editors SILICEOUS DEPOSITS IN THE PACIFIC REGION 31 A. S I N G E R and E. G A L A N , Editors PALYGORSKITE AND SEPIOLITE: OCCURRENCES, GENESIS AND USES

DEVELOPMENTS IN SEDIMENTOLOGY 38

EOLIAN SEDIMENTS AND PROCESSES Edited by

M.E. BROOKFIELD and T.S. AHLBRANDT Department of Land Resource Science, Ontario Agricultural College, University of Guelph, Ont. N1G 2W1 (Canada) 1376 S. Perry Pk. Road, Sedalia, CO 80135 (U.S.A.)

ELSEVIER Amsterdam

-

Oxford

- New

York

- Tokyo

1983

ELSEVIER SCIENCE PUBLISHERS B.V. 1 Molenwerf P.O. Box 211, 1000 AE Amsterdam, The Netherlands Distributors for the United States and Canada: ELSEVIER SCIENCE PUBLISHING COMPANY INC. 52, Vanderbilt Avenue New York, N Y 10017

I . i h i . ~ r \ 01

< snyrr\r

C.11dlogilig In P u h l i i a l i o n 1)dt.I

Main entry under title: Eolian sediments and processes. (Developments in sedimento5oa ; 38) Bibliography: p. 1. Wind erosion--Congresses. 2. Sediments (Geoloa) --Congresses. I. Rrookfield, M. F. (Michael E.) 11. Ahlbrandt, Thomas S. 111. Series.

W597.E55 1983 551.3'7 ISBN 0-444-42233-1

83-14081

ISBN 0-444-42233-1(Vol. 38) ISBN 0-444-41238-7 (Series) 0 Elsevier Science Publishers B.V., 1983

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 Science Publishers B.V., P.O. Box 330, 1000 AH Amsterdam, The Netherlands Printed in The Netherlands

V

PREFACE

This volume i s the r e s u l t of a three day symposium, consisting of f i f t y two papers, on "Eolian sediments and processes" presented a t the 11th International Association of Sedimentologists Congress held in Hamilton, Ontario, in August 1982. Many of the papers included in t h i s volume were given o r a l l y a t t h a t sumposium; however, we have included a few o t h e r s t o broaden or f i l l gaps in our coverage. The symposium and volume were organized t o include the e n t i r e spectrum of e o l i a n i n v e s t i g a t i o n s , ranqing from the niicroscopic level to regional svntheses and f i n a l l y t o ancient eolian deposits and t h e i r i n t e r p r e t a t i o n . Thus, we have included a very diverse g r o u p o f papers in one volume, with the following aims. F i r s t l y , t o bring to your a t t e n t i o n aspects of eolian sediments a n d processes which you may n o t normally consider, b u t which could be illuminating in your own i n v e s t i g a t i o n s : secondly, t o summarize the present s t a t e of eolian research.

No one author can now hope t o keep a b r e a s t of a l l c u r r e n t developiiients in t h i s rapidly expanding f i e l d , y e t summaries a r e p e r i o d i c a l l y needed. Ile r e g r e t n o t being a b l e t o include papers o n important areas of economic research, such as the d e f i n i t i o n of i s o l a t e d eolian r e s e r v o i r s using liigli resolution seismic d a t a . Such material i s generally confidential and will n o t be iiiade a v a i l a b l e until i t s proprietary value i s diminished. We wish t o dedicate t h i s volume t o E.D. McKee, in recognition of h i s pioneering and continuing research on eolian sediments a n d processes. Many of t h e , s t u d i e s reported here a r e an outgrowth of his e f f o r t s and were helped by his encouragement. We would l i k e to thank most of the authors f o r t h e i r careful preparation and modification of t h e i r manuscripts. \le have not standardized s p e l l i n g , so b o t h North American and European was accepted. Lastly, we hope you enjoy reading t h i s voluiiie.

Hichael E. Brookfield Thomas S. Ahlbrandt

This Page Intentionally Left Blank

VII

CONTENTS

Preface

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Introduction 1. McKee, E . D .

Eolian sand bodies o f the world ......................

. . v 1

The sediment

2. Binda, P.L. 3. 4.

5. 6.

On t h e skewness o f some eolian sands from Saudi Arabia ......................................... Schenk, C . J . Textural and s t r u c t u r a l c h a r a c t e r i s t i c s of some experimentally formed eolian s t r a t a . . ................ Smalley, I . J . and Smalley, V . Loess material and loess deposits: formation, d i s t r i b u t i o n and consequences ............. Derbyshire, E. Oriqin and c h a r a c t e r i s t i c s o f some Chinese l o e s s :...... a t two l o c a t i o n s in China ..................... Ruhe, R . V . Clay minerals in t h i n l o e s s , Ohio r i v e r basin,

27 41

51 69

................................................

91

S a l t a t i o n threshold and deposition r a t e modellinq ............................................

103

U.S.A Processes

7 . Iversen, J.D. 8. Gerety, K.M.

and Slingerland, R . Nature of the s a l t a t i n g population in wind tunnel experiments with heterogeneous size-density sands ........................... 9. Greeley, R . , Williams, S.H. and Marshall, J.R. Velocities of windblown p a r t i c l e s in s a l t a t i o n : preliminary

115

laboratory and f i e l d measurements ....................

133

10. Stapor, F.W. J r . , Play, J.P. and Barwis, J . Eolian shape-sorting and aerodynamic t r a c t i o n equivalence in t h e coastal dunes o f Hout Bay, Republic o f South Africa .......... 11. Wasson, R.J.

149

Dune sediment type, sand colour, sediment provenance and hydrology i n t h e Strzelecki-Simpson dunefield, Australia ............................................ Early post-depositional modification of aeolian dune sands ...........................................

197

13. Whitney, PI.

Eolian f e a t u r e s shaped by aerodynamic and v o r t i c i t y processes ............................................

223

14. Tsoar, H.

Wind tunnel modelling o f echo and climbing dunes .....

24 7

12. Pye, K.

165

Recent eolian

VIII

15. Lancaster, N.

C o n t r o l s of dune morphology i n t h e Namib sand sea...

16. Brown, R.A.

The f l o w i n the P l a n e t a r y Boundary Layer

17. Hyde, R . and Iiasson, R.J.

............

261 291

R a d i a t i v e and m e t e o r o l o g i c a l c o n t r o l

on t h e movement o f sand a t Lake Munqo, New South

....................................

Wales, A u s t r a l i a 18. Hesp, P.

311

Morphodynamics o f i n c i p i e n t foredunes i n New South Wales, A u s t r a l i a ....................................

19. Warren, A. and Knott, P.

325

Desert dunes: a s h o r t r e v i e w o f needs

i n d e s e r t dune research and a r e c e n t study o f m i c r o m e t e o r o l o q i c a l d u n e - i n i t i a t i o n mechanisms ........... 20. Flainguet, P I . and Chemin, f1.-C.

343

Sand seas o f t h e Sahara and Sahel:

an e x p l a n a t i o n o f t h e i r t h i c k n e s s and sand dune t y p e by t h e sand budget p r i n c i p l e 21. Anton, D.

........................

353

Flodern e o l i a n d e p o s i t s o f t h e Eastern Province o f Saudi Arabia ........................................

22. Ahlbrandt, T.S.,

Swinehart, J.B.

and flaroney, D.G.

365

The dynamic

Holocene dune f i e l d s o f t h e Great P l a i n s and Rocky Mountain Basins, U.S.A

..............................

379

Ancient e o l i a n 23. Rubin, D.M.

and Hunter, R.E.

Reconstructing bedform assemblaqes

from compound cross-bedding 24. Hunter, R.E.

and Rubin, D.M.

.........................

407

I n t e r p r e t i n g c y c l i c crossbedding,

w i t h an example from t h e Navajo Sandstone ...........

25. Ruegg, G.H.J.

P e r i q l a c i a l e o l i a n e v e n l y laminated sandy d e p o s i t s i n t h e l a t e P l e i s t o c e n e o f N.W.

Europe, a f a c i e s

unrecorded i n modern sedimentoloqical handbooks.. 26. Ross, G.N.

429

...

455

Bigbear Erg: a P r o t e r o z o i c intermontane e o l i a n sand sea i n t h e Hornby Bay Group, Northwest T e r r i t o r i e s , Canada ..............................................

27. Glennie, K.W.

N o r t h Sea area.............................. 28. Steele, R.P.

........

521

L o n g i t u d i n a l draa i n t h e Permian Yellow Sands o f n o r t h - e a s t England

29. Blakey, R.C.

433

Lower Permian Rotliegend d e s e r t sedimentation i n t h e

and Middleton, L.T.

..................................

543

Permian shore1 i n e e o l i a n complex

i n c e n t r a l Arizona: dune changes i n response t o c y c l i c sea-level changes

30. Mader, D.

...................................

551

A e o l i a n sands t e r m i n a t i n g an e v o l u t i o n o f f l u v i a l d e p o s i t i o n a l environment i n M i d d l e Buntsandstein (Lower T r i a s s i c ) o f t h e E i f e l , Federal Republic Germany

.............................................

583

IX

31. Middleton, L.T. and Blakey, R . C .

Processes and c o n t r o l s on the

intertonguing o f t h e Kayenta and Navajo Formations, 32. Marzolf, J.E.

northern Arizona: eolian-fluvial i n t e r a c t i o n s ....... Changing wind and hydrologic regimes during deposition

613

of t h e Navajo and Aztec Sandstones, J u r a s s i c ( ? ) , southwestern United S t a t e s ..........................

635

This Page Intentionally Left Blank

EOLIAN

SAND

BODIES OF THE WORLD

EDWIN

D. MCKEF:

U.S. G e o l o g i c a l Survey, P.O. Box 2 5 0 4 6 , D e n v e r F e d e r a l C e n t e r , Denver, C0 80225

I PITRODIICTI Ow M i n d causes t h e a c c u m u l a t i o n o f sand and t h e m i g r a t i o n o f sand b o d i e s i n many parts o f t h e world,

where i t f o r m s dunes,

A limited

sand s h e e t s and t o n g u e s .

niimber of b a s i c o r s i m p l e dune t y p e s i s r e c o g n i z e d ( F i g . l), b u t a g r e a t number o f varieties,

q e n e r a l l y r e f e r r e d t o as compound o r complex f o r m s , e x i s t .

The f a c t o r s

t h a t , c r e a t e and c o n t r o l t h e s e sand b o d i e s a r e p r i m a r i l y t h e s t r e n g t h and d i r e c t i o n of

wind,

t h e a v a i l a b l e supply o f

f a c t o r s as t o p o g r a p h y ( F i q . vpqptation (Figs.

sand,

and,

t o a lesser extent,

such p h y s i c a l

2 ) and w a t e r b o d i e s ( F i g s . 3, 4 ) and t h e p r e s e n c e o f

5, 6 ) .

F i q . 1. S i m p l e dune t y p e s r e s u l t i n g f r o m u n i d i r e c t i o n a l w i n d s , a r r a n g e d i n normal sequence downwind w i t h d e c r e a s i n g sand s u p p l y o r w i n d v e l o c i t y , and s t a b i l i z a t i o n . A l t h o u g h advances i n t h e s t u d y o f dune sands and o f dune s y s t e m s have been made i n recent years, remain t o images extreme,

many a s p e c t s a r e s t i l l p o o r l y u n d e r s t o o d and numerous problems

be c l e a r l y delineated.

from s a t e l l i t e s

hundreds

Recent of

miles

investigations away

and

have i n v o l v e d L a n d s a t

included,

at

the

opposite

t h e p e t r o g r a p h i c e x a m i n a t i o n o f i n d i v i d u a l sand g r a i n s and t h e a n a l y s i s

o f s t r a t i f i c a t i o n and f a b r i c t h r o u g h l a t e x p e e l s .

The p r i m a r y o b j e c t i v e o f t h e

p r e s e n t d i s c u s s i o n i s t o s u g g e s t p r o b l e m s i n v a r i o u s c a t e g o r i e s t h a t need f u r t h e r investigation. composition,

These c a t e g o r i e s i n v o l v e a l l o f t h e p r i n c i p a l a t t r i b u t e s o f dunes:

texture, structure,

and f a b r i c ; a l s o c o n s i d e r e d a r e t h e t w o p r i n c i p a l

components o f m o s t dune systems, t h a t i s , dune and i n t e r d u n e d e p o s i t s .

2

Sangre d~ C r i s t o “ a n w f o r r s trail Dunes, C o l o r a d c , U.S.A.

F i g . 2.

s a n d ( f o r e g r o u n d ) a t Great Sand

F i g . 3. S a l t polygons i n sebkha i n t e r d u n e , s o u t h o f Dhahran, Saudi A r a b i a ; photo by T. S. A h l b r a n d t .

Fig. 4.

Fog o v e r dunes in Namib Desert, Soutrivier, South West Africa.

Fig. 5. Stabilization of sand by vegetation, dune coppice north o f Sandwich Harbor, South West Africa.

a

Fiq. 6. Veqetation a n d rock b a r r i e r s t h a t r e t a r d dune migration along Kuiseb River, Namib Desert, South West Af r i ca; photo by E. Tad Nichols.

Fig. 7. Large s c a l e , high angle f o r e s e t s i n Ple istoc e ne e o l i a n i t e s , Mallorca, Balearic I s l a n d s; t a b u l a r planar s e t ; photo by M. Esteban.

5 COMPOSITION

The dominant m i n e r a l however,

r e p r e s e n t e d i n dune sands,

i n some r e g i o n s c a l c i u m c a r b o n a t e ( F i g .

forms most o f t h e sand ( F i g . 8 ) . and

other

impurities

occur

in

is,

7 ) and,

o f course, i n others,

quartz; gypsum

P a r t i c l e s o f volcanic rock, f e l d s p a r grains various

proportions

in

some

dune

fields.

L o c a l l y , as w i t h v o l c a n i c r o c k fragments a t G r e a t Sand Dunes, Colorado, such e x o t i c g r a i n s may c o n s t i t u t e more t h a n h a l f o f t h e sand.

Aerial F i g . 8. Mexico, U.S.A.

view o f gypsum dunes,

White Sands N a t i o n a l Monument,

New

A r e c e n t r e v i e w o f c a r b o n a t e e o l i a n i t e s (McKee and Ward, i n p r e s s ) has shown 26 p r i n c i p a l l o c a l i t i e s around t h e w o r l d , m a i n l y i n t h e e q u a t o r i a l b e l t , where 9).

Sand t h a t forms e o l i a n

1 imestones c o n s i s t s l a r g e l y o f o o l i t e s , f o r a m i n i f e r s ,

dunes o f c a r b o n a t e sand a r e w e l l developed ( F i g .

o r s h e l l fragments, and

commonly i s i n d i s t i n g u i s h a b l e i n c o m p o s i t i o n f r o m beach o r b a r sands nearby. This s i m i l a r i t y

i n composition could explain,

i n part,

why r e l a t i v e l y few

c a r b o n a t e e o l i a n i t e s have been r e c o g n i z e d i n t h e e a r l y g e o l o g i c r e c o r d .

6

F i g . 9. Distribution hemispheres.

of

carbonate

eolianites

in

eastern

and

western

TEXTURE D e s p i t e t h e f a c t t h a t many analyses o f sand g r a i n s have been made i n dune f i e l d s throughout t h e world,

v a r i o u s a s p e c t s of t h e d i s t r i b u t i o n p a t t e r n s o f

sandsize and s o r t i n g do n o t seem t o be f u l l y e s t a b l i s h e d o r u n d e r s t o o d except on a l o c a l b a s i s .

Evidence t h a t a m a j o r p a r t o f a l l dune sands a r e o f f i n e

g r a i n s i z e (1/8-1/4

mm) and a r e m o d e r a t e l y w e l l s o r t e d seems t o be e s t a b l i s h e d

( A h l b r a n d t , 1979, p. 24).

L o c a l l y , however, sand on dune f l a n k s i s f o u n d t o be

c o a r s e r on t h e average and more p o o r l y s o r t e d t h a n t h a t on t h e c r e s t s , based on a comparison o f analyses f r o m dunes o f A u s t r a l i a , South West A f r i c a , Sand Dunes, Colorado (E. D. McKee unpub. d a t a ) .

and Great

These d a t a suggest t h e need

7

f o r g r e a t e r uniformity i n sampling, e s p e c i a l l y as regards s p e c i f i c locations on a dune and d i f f e r e n t types and s i z e s of dunes in order t o make possible worldwide comparisons. Analyses regarding grain s i z e and s o r t i n g d i s t r i b u t i o n a t Great Sand Dunes (Fig. 10) i n d i c a t e t h a t t h e higher dunes (>30 m ) contain much more fine-grained sand and much l e s s medium-grained sand than do t h e lower dunes, and, i n a d d i t i o n , they a r e d e f i n i t e l y b e t t e r sorted. In t h i s p a r t i c u l a r dune f i e l d , t h e coarsest and most poorly sorted sand i s on t h e windward slope, b u t how t h i s f e a t u r e compares w i t h t h a t of o t h e r dune f i e l d s i s n o t known. I n any event, much more information i n t h e form of analyses from selected regions t o show d i f f e r e n c e s r e s u l t i n g from o r responsible f o r dune type and dune s i z e a r e needed f o r a b e t t e r understanding of processes and forms. Information on t h e r e l a t i o n s between local wind movements, on and around individual dunes, and t h e r e s u l t i n g t e x t u r a l p a t t e r n s of t h e s e dunes, a l s o i s rare.

COARSE COARSE

MEDIUM

FINE

MEDIUM

FINE

VERY FINE

VERY FINE

COARSE

MOD. WELL SORTED

WELL SORTED

MEDIUM

FINE

VERY FINE

MOD, MODERATELY SORTED WELL SORTED

Fig. 10. Low barchanoid ridge dune, Great Sand Dunes, Colorado, showing d i s t r i b u t i o n of grain size and s o r t i n g along p r o f i l e .

U.S.A.,

8

STRUCTURE Processes o f sand t r a n s p o r t by w i n d C r o s s - s t r a t i f i c a t i o n i n e o l i a n deposits i s t h e product o f several d i s t i n c t processes o f t r a n s p o r t (Hunter, 1981; Kocurek and D o t t , 1981).

These processes

11) t h a t i n c l u d e s v a r i o u s amounts

are (1) s a l t a t i o n o r r i p p l e m i g r a t i o n (Fig.

o f s u r f a c e creep, ( 2 ) g r a i n f a l l o r a d r o p p i n g o f g r a i n s f r o m sand i n suspension (Fig.

12),

and

(3)

avalanching

13,

(Figs.

14)

either

b y mass

movement

(slumping) o r by g r a i n f l o w a g e which i n v o l v e s t h e d o w n h i l l r o l l i n g o r s l i d i n g o f discrete particles.

Experiments made i n a w i n d t u n n e l b y C h r i s t o p h e r J .

Schenk show t h a t t h e s e t h r e e processes f o r m t y p e s o f s t r a t i f i c a t i o n g e n e r a l l y s i m i l a r i n c h a r a c t e r and, i n many examples,

d i f f i c u l t t o d i s t i n g u i s h one f r o m

A few f e a t u r e s , however, a r e d i s t i n c t i v e and, where t h e s e o c c u r i n a

another.

deposit, are diagnostic o f a p a r t i c u l a r o r i g i n . C r i t e r i a f o r recognizing eolian f a b r i c Useful i n d i c a t o r s o f d e p o s i t i o n a l process a r e t h e v a r i o u s forms o f deformational

s t r u c t u r e such as t h r u s t s and flames,

c h a r a c t e r i s t i c o f avalanching;

t h e y commonly f u r n i s h evidence o f t h e degree o f c o h e s i o n i n sand a t t h e t i m e o f deposition.

The b e v e l l e d bottoms o f f o r e s e t s a r e p r e s e r v e d i n some c l i m b i n g

r i p p l e s t r u c t u r e s ; t h e y n o t o n l y i n d i c a t e t h a t sand movement was by s a l t a t i o n , b u t a l s o h e l p t o d e t e r m i n e o r i e n t a t i o n o f t h e r i p p l e d s u r f a c e on t h e dune.

A

t h i r d f e a t u r e p r e s e r v e d i n some e o l i a n c r o s s - s t r a t a c o n s i s t s o f sand f l o w t o e s ( F i g . 1 2 ) , composed o f r e l a t i v e l y c o a r s e sand tongues a t t h e base o f f o r e s e t s i n a m a t r i x o f f i n e r sand d e p o s i t e d as g r a i n f a l l . Many more s t u d i e s need t o b e made on methods o f d i f f e r e n t i a t i n g between f a b r i c s o f t h e t h r e e main t y p e s o f e o l i a n d e p o s i t s , and on comparisons w i t h s t r a t a f r o m o t h e r environments. Basic s t r u c t u r a l types By f a r

t h e commonest t y p e s

tabular-planar

o r wedge-planar

of

s t r u c t u r e i n most

cross-strata.

sand seas a r e e i t h e r

These t y p e s

are t h e

normal

p r o d u c t o f l e e - s i d e a v a l a n c h i n g on dunes; t h e y a r e m o s t l y medium t o l a r g e s c a l e and t h e y d i p a t r e l a t i v e l y h i g h angles.

The f a c t o r s r e s p o n s i b l e f o r each of

t h e s e t y p e s a r e n o t y e t understood, n o r i s i t c l e a r why some dune forms e x h i b i t a g r e a t e r percentage o f t a b u l a r - p l a n a r t h a n wedge-planar s t r a t a .

A

scarcity

of

trough

structures

i n most

e o l i a n deposits

is

generally

recognized; however, some e x c e p t i o n s occur, e s p e c i a l l y i n c o a s t a l dunes.

Large

t r o u g h s t r u c t u r e s f o r m i n g f e s t o o n s ( F i g . 1 5 ) , have been r e c o r d e d f r o m c a r b o n a t e eolianites

i n t h e saddles between " s p i l l - o v e r

1967; MacKenzie, 1964).

lobes"

o f dune r i d g e s

(Ball,

9

F i g . 11. C o n t a c t between a v a l a n c h e s a n d ( b o t t o m ) and r i p p l e - p r o d u c e d s t r a t a ( t o p ) ; l e e b a s e of l a r g e dune; G r e a t Sand Dunes, C o l o r a d o , U.S.A.; p h o t o by n. C . S c h n a b e l .

Fig. 12. Sand-flow t o e s a t b a s e o f a v a l a n c h e d e p o s i t s ( d a r k ) , w i t h n e a r l y s t r u c t u r e l e s s s a n d ( l i g h t ) , p r o b a b l y f r o m g r a i n f a l l o u t , between wedges; G r e a t Sand Dunes, C o l o r a d o , U.S.A.; p h o t o by D. C. S c h n a b e l .

10

F i q . 13. L.enses i n s t r a t a f o r m e d by slump sand masses; s e c t i o n normal t o w i n d p h o t o by d i r e c t i o n on l e e s l o p e o f dune; G r e a t Sand Dunes, C o l o r a d o , U.S.A.; n. C. Schnabel.

F i g . 14. Laminae f r o m a v a l a n c h i n g ( g r a i n f l o w ) o f d r y sand i n w h i c h e a c h s i z e g r a d e was dyed a p a r t i c u l a r c o l o r t o d e t e r m i n e sorting fabric; laboratory experiment.

11

F i q . 15. Bahamas.

Trough cross-bedding Photo by R. Redfern.

i n e o l i a n carbonates;

New P r o v i d e n c e I s l a n d ,

A q u a n t i t a t i v e s t u d y o f t h e p r o p o r t i o n s o f each b a s i c dune s t r u c t u r e t h a t A comparison

occurs i n various d e p o s i t i o n a l environments should be rewarding.

o f t h e c r o s s - s t r a t a t y p e s i n dunes o f d i f f e r e n t c o m p o s i t i o n , d i f f e r e n t f o r m and different

wind

regime

probably

d i s t i n g u i s h i n q between v a r i e t i e s c o n t r o l l i n g genesis. little

or

establish

dune

and

for

additional

criteria

for

recognizing the features

Because t h e t r o u g h s t r u c t u r e i s t h e p r o d u c t o f r e l a t i v e l y

understood

absence

would of

processes

general

and

proportion

is

comparatively

probably

is

an

uncommon,

its

important

indicator

presence, of

d e p o s i t i o n a l environment i n general. D e f o r m a t io n a l S t r u c t u r e s Penecontemporaneous faults

and f o l d s

(Fig.

r o t a t e d blocks (Fig. serve

as

useful

indicators therefore

deformational

structures

including

17),

16), f l a m e s ( F i g .

tools

i n determining genesis.

p r o v i d e i n f o r m a t i o n on t h e

important,

they

(Fig.

types

of

18),

and

1 9 ) a r e s u f f i c i e n t l y numerous i n m o s t e o l i a n d e p o s i t s t o These s t r u c t u r e s a r e good

o f t h e nature o f t h e stress--whether

deformation.

various

break aparts

record the

degree

part

of

of

cohesion

compression o r tension--and

a sand body i n v o l v e d . represented a t

More

the time o f

Thus, t h e y p e r m i t f a i r l y r e l i a b l e d i s t i n c t i o n s t o h e made between

sand bodies t h a t ,

when d e p o s i t e d ,

were

(1) s a t u r a t e d ,

(2) dry,

s u r f a c e ( c r u s t ) , o r ( 4 ) w e t t e d t h r o u g h o u t ( w a t e r drawn o f f ) .

(3) wetted

12

16. S t a i r - s t e p f o l d i n middle p a r t of a v a l a n c h e s l o p e , w e t t e d sand i n dune ; Denver Sediment a r y S t r u c t u r e s Labor a t o r y

Fig.

.

Fig. 17. Deformational s t r u c t u r e s i n a v a l a n c h e s t r a t a , Flame f o l d s i n d r y sand n e a r b a s e o f s l o p e ; Denver Sedimentary S t r u c t u r e s Laboratory.

13

F i g . 18. B r e a k - a p a r t l a m i n a e i n l o w e r p a r t o f s l i p f a c e o f dune w i t h w e t c r u s t ; Denver S e d i m e n t a r y S t r u c t u r e s L a b o r a t o r y .

F i g . 19. R o t a t e d b l o c k s i n dry sand, Sedimentary S t r u c t u r e s Laboratory.

near top o f avalanche slope;

Denver

14 Nine

or

more

principal

types

of

deformational

structure

have

been

recognized, b o t h i n experiments (McKee, Douglass, and R i t t e n h o u s e , 1971) and i n t h e f i e l d (McKee and B i g a r e l l a , 1972).

Some o f t h e s e forms a r e t y p i c a l o f more

t h a n one c o n d i t i o n o f cohesion, b u t o t h e r s seem t o be u n i q u e t o a p a r t i c u l a r c a t e g o r y o f m o i s t u r e c o n t e n t , and t h e r e f o r e a r e e s p e c i a l l y i m p o r t a n t i n d i c a t o r s o f genesis.

L i k e assemblages o f f o s s i l s , a s s o c i a t e d groups o f s t r u c t u r e s a r e

more i m p o r t a n t t h a n i s an i n d i v i d u a l sediment i n which t h e y were formed.

f o r demonstrating t h e nature o f t h e

Further investigations i n t h i s f i e l d w i t h

a d d i t i o n a l c o n f i r m a t i o n o f s t r u c t u r a l i n t e r p r e t a t i o n s o f f e r good o p p o r t u n i t i e s f o r advancing o u r knowledge o f dune sands. D u n e - l i k e s t r u c t u r e s i n o t h e r environments Among t h e

various

features t h a t

commonly a r e a t t r i b u t e d t o t h e e o l i a n

d e p o s i t i o n o f sand a r e t h e c o n s i d e r a b l e l e n g t h and h i g h - a n g l e d i p o f many cross-strata.

S u f f i c i e n t o b s e r v a t i o n s have been r e c o r d e d t o e s t a b l i s h beyond

reasonable doubt t h a t t h e l e e s i d e o r s l i p f a c e o f most modern dunes stands a t 30 degrees o r s l i g h t l y more and t h a t t h e l e n g t h o f t h e f o r e s e t s developed on i t

may be 8 o r 10 meters and o f t e n much more.

Furthermore,

the resulting

s t r u c t u r e s o f these deposits are dominantly o f planar type--either

wedge o r

tabu1 a r . The s i z e and a t t i t u d e o f dune s t r u c t u r e s ,

however, a r e n o t b y themselves

s u f f i c i e n t t o p o s i t i v e l y i d e n t i f y t h e e o l i a n o r i g i n o f a sand d e p o s i t .

There

a r e many e x c e p t i o n s t o t h e normal, r e s u l t i n g f r o m v a r i a b i l i t y i n t h e process o f development. and

G r a i n s i z e , cohesion o f g r a i n s , s t r e n g t h and v a r i a b i l i t y o f w i n d

numerous

other

factors

d e p o s i t i o n a l product.

are

responsible

for

great

differences

i n the

Subsequent processes such as compaction and d i a g e n e s i s

a l s o must b e c o n s i d e r e d (Walker and Harms, 1972). I n t h e r e c o g n i t i o n o f e o l i a n d e p o s i t s and i n t h e i n t e r p r e t a t i o n o f v a r i o u s dune t y p e s ,

i t i s n o t enough t o be f a m i l i a r w i t h wind-formed c h a r a c t e r i s t i c s

alone, such as s t r u c t u r e s , t e x t u r e s and f a b r i c .

A l s o i m p o r t a n t i s t o know how

t h e s e f e a t u r e s compare w i t h s i m i l a r ones i n f l u v i a l and m a r i n e environments. F o r example, e x t e n s i v e , r a t h e r p y r e sand d e p o s i t s a r e known t o o c c u r i n p a r t s o f t h e N o r t h Sea ( H o u b o l t , 1968; McCave, 1971) and o f f t h e New England c o a s t (Jordan,

1962;

Swift,

1975) where t h e c r e s t s o f v e r y l a r g e sand waves w i t h

asymmetrical

form

are

indicated

similarities,

t h e marine l i n e a r

on

sand

bottom ridges

profiles. probably d i f f e r

s u b a e r i a l dunes as d i s c u s s e d b y Walker and M i d d l e t o n (1977).

Despite greatly

these from

They show t h a t

t h e exaggerated v e r t i c a l s c a l e o f t h e N o r t h Sea p r o f i l e s g i v e s a d i s t o r t e d p i c t u r e o f t h e steep-side,

s l o p e a n g l e which

erroneous i d e a o f t h e f o r e s e t l e n g t h s .

i s v e r y l o w and l e a d s t o an

Because l i t t l e i n f o r m a t i o n i s a v a i l a b l e

15 on t h e n a t u r e o f t h e i r characteristics,

stratification,

on t h e i r m i n o r s t r u c t u r e s o r g r a i n

o r on t h e i r r e l a t i o n s t o a s s o c i a t e d sediments o f o t h e r t y p e s

o r t o faunas, v a l i d comparisons can n o t be made y e t betweeen t h e s e marine sands and t h o s e o f e o l i a n o r i g i n .

A

second environment,

comparison,

g r e a t l y i n need o f i n v e s t i g a t i o n f o r purposes o f

i s t h e l a r g e r i v e r system.

Recent o b s e r v a t i o n s on t h e Orinoco

R i v e r i n Venezuela b y C a r l N o r d i n o f t h e U.S. that

sand

deposits

kilometers,

reach

across

the width

G e o l o g i c a l Survey, have shown of

the

and e x t e n d f o r many k i l o m e t e r s downstream.

under w a t e r d u r i n g f l o o d stage,

river,

more t h a n t w o

T h i s sand, which i s

i s exposed as a s e r i e s o f t r a n s v e r s e b a r s

Trenches made b y N o r d i n show l a r g e - s c a l e t a b u l a r -

d u r i n g l o w w a t e r (Fig. 20).

p l a n a r s t r u c t u r e s a t moderate t o h i g h angles t h a t d i p d o w n r i v e r (Fig. Details

f r o m measurements on l e n g t h s ,

angles,

21).

and t h i c k n e s s e s a r e n o t y e t

a v a i l a b l e , b u t t h e importance o f a c a r e f u l analyses o f t h e s e d e p o s i t s s h o u l d be apparent. Shingle structures Lack o f

i n t e r d u n e s r e s u l t s f r o m one dune c l i m b i n g o r m i g r a t i n g up t h e

windward s l o p e o f t h e one ahead.

I n most cases,

t h i s s i t u a t i o n i s brought

about b y t h e presence o f a b a r r i e r such as a r o c k r i d g e o r mountainous t e r r a n e t h a t slows down t h e f o r w a r d movement o f t h e sand masses and causes one dune t o p i l e up on another. Colorado,

U.S.A.,

Such a s i t u a t i o n i s i l l u s t r a t e d b y Great Sand Dunes i n

where t h e Sangre de C r i s t o Range ( F i g . 2) e f f e c t i v e l y b l o c k s

a g e n e r a l e a s t e r l y movement o f t h e dunes. strike

can

be

seen

to

have

Sand r i d g e s w i t h l a r g e l y n o r t h - s o u t h

relatively

short

distances

between c r e s t s .

I n d i v i d u a l dune r i d g e s a r e n o t s e p a r a t e d b y t h e n e a r - h o r i z o n t a l interdunes,

surfaces o f

b u t a r e advancing up g e n t l e windward s l o p e s o f t h e n e x t dune

downwind l i k e o v e r l a p p i n g s h i n g l e s (Fig. 22). Not a l l s h i n g l e s t r u c t u r e s i n t r a n s v e r s e dune f i e l d s a r e caused by mountain barriers. ridges

I n t h e Namib D e s e r t o f South West A f r i c a , f o r i n s t a n c e , l a r g e dune

bordering

the Atlantic

coast

s o u t h o f t h e Kuiseb R i v e r

(Fig.

23)

i l l u s t r a t e dunes s t a c k e d c l o s e t o g e t h e r w i t h o u t f l a t - l y i n g i n t e r d u n e s between; one dune c l i m b s o n t o t h e next.

Here, however, no p h y s i c a l b a r r i e r can be c i t e d

t o e x p l a i n a s l o w i n g down o f t h e n o r t h e a s t w a r d advance o f sand r i d g e s .

One

p o s s i b l e e x p l a n a t i o n i s t h e n o r t h e a s t e r l y w i n t e r w i n d (Berg Wind) t h a t opposes t h e normal w i n d o f f t h e South A t l a n t i c .

Thus,

i n t h i s desert c o n f l i c t i n g

seasonal winds may cause a s h i n g l e development n o t f a r i n l a n d f r o m t h e coast.

16

D e p o s i t s of sand waves a c r o s s t h e O r i n o c o R i v e r i n V e n e z u e l a , f o r m e d F i g . 20. d u r i n g f l o o d s t a g e , p h o t o by C a r l N o r d i n .

T r e n c h showing l a r g e s c a l e f o r e s e t s - i n sand wave d e p o s i t s o f O r i n o c o , F i g . 21. Venezuela; p h o t o by C a r l Mordin.

17

Fig. 22. Great S a n d Dunes, Colorado, U.S.A., view from soutn showing shingle s t r u c t u r e (overlapping d u n es ) ; photo by D. C. Schnabel.

Fig. 23. Overlapping t r a n s v e r s e dunes ( n o n interdune) near Sandwich Bay, South West A f r i c a ; photo by E. Tad Nichols. S t r u c t u r a l evidence of s h i n g l e s t r u c t u r e i s recorded in s t r a t i f i c a t i o n as seen i n trenches located a t t h e base of any t r an sve rse dune t h a t i s climbing t h e g e n tly sloping windward f ace of a preceding dune (Fig. 2 4 ) .

This s t r u c t u r e nor-

mally c o n s i s t s of st e ep l y dipping f o r e s e t s of an e a r l i e r dune, bevelled t o a lowangle s u r f a c e d i p p i n g windward and commonly covered with a veneer of s a n d marking t h e base of t h e new dune.

This i n turn may be p a r t l y covered by high-angle

s t r a t a , dipping downwind and representing avalanching de posits o f t h e new dune.

18

Fig. 24. Low-angle windward-side s t r a t a o v e r l y i n g b e v e l l e d s u r f a c e o f e a r l i e r high a n g l e f o r e s e t s i n s h i n g l e s t r u c t u r e , Great Sand Dunes, Colorado, U.S.A.; photo by D. C. Schnabel.

F i g . 25. I n t e r d u n e a t b a s e o f l a r g e l i n e a r dune, n o r t h e a s t p a r t , Rub a1 K h a l i ; photo by T. S . Ahlbrandt.

19 MORPHOLOGY

Interdunes The s u b j e c t o f i n t e r d u n e s and t h e i r importance as i n t e g r a l p a r t s o f dune systems has been l a r g e l y n e g l e c t e d u n t i l r e c e n t y e a r s

(Kocurek,

1981).

In

modern dune f i e l d s , i n t e r d u n e s o c c u r as e s s e n t i a l l y f l a t s u r f a c e s between dune r i d g e s ( F i g . 25) o r dune mounds; i n a n c i e n t r o c k s t h e y f o r m e i t h e r t h i n , p l a n a r s t r a t a o f mud and s i l t o r a r e r e p r e s e n t e d b y bedding p l a n e s , s e p a r a t i n g s e t s o f cross-strata deposition,

(Hunter,

1981).

Because t h e y a r e formed n o t p r i m a r i l y by wind

b u t r a t h e r as a r e s u l t o f ponding,

o t h e r m i s c e l l a n e o u s processes,

deflation,

p l a n t growth and

o p e r a t i n g between t i m e s o f dune b u i l d u p , t h e y

a r e d i s t i n c t i v e i n s e v e r a l ways (as d i s c u s s e d below) and commonly a r e i n marked c o n t r a s t w i t h t h e e o l i a n sands t h a t e n c l o s e them. C h a r a c t e r i s t i c f e a t u r e s o f i n t e r d u n e d e p o s i t s i n c l u d e adhesion s t r u c t u r e s (Fig.

26),

evaporite

crusts

i r r e g u l a r bedding s u r f a c e s .

(Fig.

3),

carbon

residues

from

plants,

and

T h e i r r e l a t i o n s h i p t o l a g o o n a l sediments, a l l u v i a l

f a n s , and o t h e r d i s t i n c t i v e n o n - e o l i a n d e p o s i t s suggests t h a t t h e y should have numerous

criteria for

recognition,

but

apparently they are s t i l l

commonly

unrecognized among s t r a t a o f p r o b a b l e e o l i a n genesis i n a n c i e n t f o r m a t i o n s . I n v e s t i g a t i o n s o f t h e c h a r a c t e r i s t i c s and s t r a t i g r a p h i c r e l a t i o n s o f i n t e r d u n e deposits

s h o u l d t h e r e f o r e be h i g h l y rewarding,

and,

although t h e interdunes

s e p a r a t i n g dunes o f u n i d i r e c t i o n a l winds a r e f a i r l y w e l l understood (McKee and Moiola,

1975),

virtually

no good d e s c r i p t i o n s

are a v a i l a b l e f o r interdunes

between c l u s t e r s o f s t a r dunes ( F i g . 27) o r rows o f l i n e a r dunes (Fig. 28). O r i g i n o f 1 in e a r dunes The o r i g i n o f l i n e a r dunes, a l s o r e f e r r e d t o as s e i f o r l o n g i t u d i n a l dunes, has l o n g been a m a t t e r o f much c o n t r o v e r s y . p a r a l l e l sand r i d g e s t h a t have two s l i p f a c e s , ridge, d i p p i n g i n opposite directions. l i n e a r dunes o f v a r i o u s sand seas. A f r i c a c h a r a c t e r i s t i c a l l y form zig-zag,

These dunes a r e d e f i n e d as l o n g

i.e.,

one on each s i d e o f t h e

Some m a j o r d i f f e r e n c e s i n f o r m o c c u r i n Those o f t h e L i b y a n D e s e r t o f n o r t h e r n r a t h e r than s t r a i g h t l i n e ridges (Fig.

28); t h o s e o f A u s t r a l i a ' s Simpson D e s e r t commonly develop branches known as Yjunctions

(Fig.

29).

I n t h e Namib Desert o f South West A f r i c a , many l i n e a r

dunes a r e complex w i t h s t a r dunes o r o t h e r t y p e s a l o n g t h e i r c r e s t s (Fig. 30).

P o s s i b l y because o f t h e s e v a r i a t i o n s i n form,

genesis i s r e p r e s e n t e d b y dunes o f t h e l i n e a r c l a s s .

more t h a n one t y p e o f

20

26. A d h e s i o n s t r u c t u r e s i n i n t e r d u n e p l a y a between b a r c h a n s , l d a l v i s Ray, South West A f r i c a .

Fig.

south of

S t a r dune c l u s t e r w i t h i r r e g u l a r l y spaced i n t e r d u n e s ; Sahara, N o r t h F i g . 27. A f r i c a ; a i r p h o t o b y H. T. U . Smith.

21

F i g . 28. Z i g - z a g t r e n d s o f l i n e a r ( s i e f ) dunes n o r t h o f Sebha O a s i s , L i b y a ; a e r i a l view.

Fig. 29. Y - j u n c t u r e i n l i n e a r dune, p h o t o b y J . F. S c h r e i b e r , Jr.

G r e a t Sandy D e s e r t , A u s t r a l i a ;

aerial

22

F i g . 30. Complex l i n e a r and s t a r dunes, southwest o f Gobabeb, Namib D e s e r t , South West A f r i c a ; p h o t o by E . Tad Nichols. P r i n c i p a l hypotheses o f genesis t h a t have been proposed f o r l i n e a r dunes a r e r e f e r r e d t o as

(a) d e f l a t i o n ,

(b)

helical

roll,

( c ) dune m o d i f i c a t i o n and

e v o l u t i o n , ( d ) l e e - s i d e accumulation, and ( e ) b i d i r e c t i o n a l w i n d (McKee, 1982, p.

25).

Arguments f o r and a g a i n s t t h e s e v a r i o u s hypotheses a r e numerous and

combinations o f two o r more e x p l a n a t i o n s seem l i k e l y t o a p p l y i n some sand seas.

Many a d d i t i o n a l f a c t s need t o be g a t h e r e d t h r o u g h d e t a i l e d s t u d i e s ,

especially o f

structures,

and wind d a t a ,

b e f o r e more p o s i t i v e c o n c l u s i o n s

r e g a r d i n g t h e o r i g i n o f dunes o f t h i s c l a s s can be reached. Selective preservation The m i g r a t i o n o f sand b o d i e s i n any dune f i e l d n o r m a l l y i s a process o f essentially

simultaneous

erosion

d e p o s i t i o n on t h e o t h e r ( l e e ) .

on

one

S e t s of

side

of

a

dune

(windward)

d i p p i n g l a m i n a e may, t h e r e f o r e ,

and be

p a r t l y , o r even e n t i r e l y , removed d u r i n g a subsequent b e v e l l i n g o f t h e windward s u r f a c e as a dune advances.

T h i s process u l t i m a t e l y r e s u l t s i n a s e l e c t i v e

p r e s e r v a t i o n w i t h b u r i a l o f o n l y t h e l o w e r o r basal p a r t o f an o r i g i n a l dune d e p o s i t (Kocurek and D o t t , 1981; Rubin and Hunter, 1982). Through t r u n c a t i o n o f t h e upper p a r t s o f f o r e s e t s t h a t were o r i g i n a l l y d e p o s i t e d on a dune s l i p f a c e as seen a t White Sands, New Mexico ( F i g . 3 1 ) , t h e amount o f sand removed f r o m t h e s u r f a c e and t r a n s p o r t e d downwind may v a r y

23 considerably.

Probably

the

sand

supply

available

and

the

strength

and

A t one extreme, these

c o n s i s t e n c y o f t h e wind moving i t a r e major c o n t r o l s .

f a c t o r s may be so weak t h a t a c t u a l t h i n windward-slope d e p o s i t s , d i p p i n g upwind

a t low a n g l e s , may be p r e s e r v e d a s i n t r e n c h e s of t r a n s v e r s e dunes a t Great ( F i q . 11) and i n t h e Permian Coconino Sandstone of Grand Canyon, Arizona. A t t h e o t h e r e x t r e m e , t h e wind may be s o s t r o n g t h a t i t

Sand Dunes, Colorado, U.S.A.

removes a l l sand i n a given p l a c e down t o t h e u n d e r l y i n g i n t e r d u n e . dunes

commonly

removed.

have

a

considerable

percentage

of

the

original

Normal deposit

C l e a r l y , t h i s s u b j e c t d e s e r v e s much more c a r e f u l s t u d y t h a n has been

g i v e n , f o r o n l y t h e s t r u c t u r e s i n t h e lower p a r t of a s l i p f a c e a r e p r e s e r v e d i n t h e q e o l o q i c r e c o r d by t h i s s e l e c t i v e p r o c e s s .

Windward s u r f a c e of barchanoid r i d g e showing b e v e l l e d t o p s o f Fiq. 31. f o r e s e t s as dune m i q r a t e s , White Sands, New Mexico, I1.S.A. RevPrsing dunes R e v e r s i n q dunes ( F i g . 32) probably a r e f a r more common t h a n i s g e n e r a l l y recoqnized,

although

relatively

s t r u c t u r e have been made.

few

detailed

studies

of

their

form

and

E a r l i e s t r e c o r d s of such dunes d a t e back almost a

c e n t u r y t o t h e o b s e r v a t i o n s of Hedin (1896) and Cornish (1897) and t h e y have been examined s i n c e t h a t t i m e i n a number o f d i f f e r e n t sand s e a s .

They a r e

formed under c l i m a t i c regimes i n both humid and a r i d a r e a s , and i n c o l d lowa l t i t u d e and c o l d h i q h - a l t i t u d e

a r e a s , a p p a r e n t l y l a r g e l y a s a f u n c t i o n of

c o n t r a s t i n q wind d i r e c t i o n s (McKee, 1979, p. 106).

24

F i g . 32. R e v e r s i n g dunes o f t r a n s v e r s e t y p e , Santa C a t a r i n a , B r a z i l .

f r o m s o u t h n e a r c o a s t a t Loga,

A l t h o u g h t h e b a s i c f e a t u r e s o f r e v e r s i n g dune development a r e r e c o r d e d , many d e t a i l s c o n c e r n i n g s t r u c t u r e p a t t e r n , r a t e o f movement, r e l a t i o n s t o i n t e r d u n e s and o t h e r a s p e c t s have n o t been c l e a r l y d e f i n e d .

R e v e r s i n g dunes a r e f o r m e d on

t r a n s v e r s e dune r i d g e s i n G r e a t Sand Dunes, C o l o r a d o , U.S.A., l i n e a r dune r i d g e s i n t h e Namib D e s e r t , caused by winds,

S o u t h West A f r i c a .

b u t t h e y o c c u r on They seem t o be

c o n t r o l l e d by m o u n t a i n b a r r i e r s i n some r e g i o n s ,

open d e s e r t t e r r a i n elsewhere.

but are i n

C h a r a c t e r i s t i c p a t t e r n s o f c r o s s - b e d d i n g and

r e l a t i o n s t o i n t e r d u n e development a r e o t h e r f e a t u r e s n e e d i n g i n v e s t i g a t i o n . REFERENCES A h l b r a n d t , T.S., 1979. T e x t u r a l p a r a m e t e r s o f e o l i a n d e p o s i t s , C h a p t e r B, I n Edwin D. McKee ( E d i t o r ) , A S t u d y o f G l o b a l Sand Seas. U.S. Geol. S u r v e y P r o f . Paper 1052, pp. 21-51. B a l l , M.M., 1967. C a r b o n a t e sand b o d i e s o f F l o r i d a and t h e Bahamas. Jour. Sed. P e t r o l o g y , 37: 556-591. C o r n i s h , Vaughan, 1879. On t h e f o r m a t i o n o f sand dunes. Geog. Jour., 9: 278309. H e d i n , Sven, 1896. A j o u r n e y t h r o u g h t h e Takla-Makan D e s e r t Chinese Turkistan. Geog. Jour., 8: 264-278. 1968. Recent s e d i m e n t s i n t h e s o u t h e r n b i g h t o f t h e N o r t h H o u b o l t , J.J.H.C., Sea. G e o l o g i e e n Mijnbouw, 47: 245-273. 1981. S t r a t i f i c a t i o n s t y l e s i n e o l i a n sandstones: Some H u n t e r , R.E., P e n n s y l v a n i a n t o J u r a s s i c examples f r o m t h e w e s t e r n i n t e r i o r U.S.A. SOC. Econ. P a l e o n t o l o g i s t s and M i n e r a l o g i s t s Spec. Pub. 31: pp. 315-329. J o r d a n , G.F., 1962. L a r g e s u b m a r i n e sand waves. S c i e n c e , 136: 839-848.

25 Kocurek, G., 1981. S i g n i f i c a n c e o f i n t e r d u n e d e p o s i t s and bounding s u r f a c e s i n e o l i a n dune sands. Sedimentology, 28: 753-780. Jr., 1981. D i s t i n c t i o n and uses o f s t r a t i f i c a t i o n Kocurek, G. and D o t t , R.H., types i n t h e i n t e r p r e t a t i o n o f e o l i a n deposits. Jour. Sed. P e t r o l o g y , 51: 579-595. MacKenzie, F.T., 1964. Bermuda P l e i s t o c e n e e o l i a n i t e s and paleowinds. Sedimentology, 3: 52-64. McCave, I.N., 1971. Sand waves i n t h e N o r t h Sea o f f t h e coast o f Holland. Marine Geology, 10: 199-225. 1979. A s t u d y o f g l o b a l sand seas. U.S. Geol. Survey Prof. Paper McKee, E.D., 1052, 429 pp. McKee, E.D., 1982. Sedimentary s t r u c t u r e s i n dunes o f t h e Namib Desert, South West A f r i c a . Geol. SOC. America, Spec. Paper 188, 64 pp. McKee, E.D. and B i g a r e l l a , J.J., 1972. Deformational structures i n B r a z i l i a n c o a s t a l dunes. Jour. Sed. P e t r o l o g y , 42: 670-681. Douglass, J.R. and Rittenhouse, Suzanne, 1971. Deformation o f McKee, E.D., l e e - s i d e laminae i n e o l i a n dunes. Geol. SOC. America B u l l . , 82: 359-378. McKee, E.D. and Moiola, R.J., 1975. Geometry and growth o f t h e White Sands U.S. Geol. Survey Jour. Research, 3: 59-66. dune f i e l d , New Mexico. and Ward, W.C., ( i n press). Carbonate dunes and e o l i a n McKee, E.D. limestones. Am. Assoc. Petroleum G e o l o g i s t s Mem. Rubin, D.M. and Hunter, Ralph, 1982. Bedform c l i m b i n g i n t h e o r y and nature. Sedimentology, 29: 121-138. S w i f t , D.J.P., 1975. T i d a l sand r i d g e s and s h o a l - r e t r e a t massifs. Marine Geology, 18: 105-134. and M i d d l e t o n , G.V., 1977. F a c i e s Models 9, E o l i a n Sands. Walker, R.G. Geoscience Canada, 4: 182-190. 1972. E o l i a n o r i g i n of f l a g s t o n e beds, Lyons Walker, T.R. and Harms, J.C., Sandstone (Permian), t y p e area, Boulder County, Colorado, In Environments o f sandstone, carbonate, and e v a p o r i t e d e p o s i t i o n . Mtn. G e o l o g i s t , 9: 279-288.

This Page Intentionally Left Blank

27

ON THE SKEWNESS OF SOPIE EOLIAN SANDS FROM SAUDI A R A B I A P I E R L. BINDA:

Geology Department, U n i v e r s i t y o f Regina, Regina, Saskatchewan, Canada S4S OA2

INTRODUCTION

P o s i t i v e skewness o f g r a i n - s i z e d i s t r i b u t i o n s ,

s. curve departing from

n o r m a l i t y i n h a v i n g a t a i l i n t h e f i n e f r a c t i o n , i s o f t e n quoted as a t y p i c a l characteristic o f eolian transport.

T h i s seems t o h o l d t r u e whether one uses

the s t a t i s t i c a l l y more p r e c i s e measure o f t h e t h i r d moment o r a g r a p h i c a l l y derived approximation o f i t . I n t h e l a t e f i f t i e s and e a r l y s i x t i e s , i n s p i t e o f some c o n t r o v e r s i a l r e s u l t s , the b a l a n c e o f e v i d e n c e showed t h a t skewness can be used i n d i s c r i m i n a t i n g dune from beach sand (Mason and F o l k , 1958; Shepard and Young, 1961; Friedman, 1961; Folk, 1962).

Data on i n l a n d , d e s e r t , dunes seemed t o c o n f i r m , a l b e i t w i t h some

r e s e r v a t i o n s , t h a t p o s i t i v e skewness i s c h a r a c t e r i s t i c o f wind-blown sand. Sieve a n a l y s i s o f e o l i a n sand f r o m areas around Jeddah, p a r t l y c a r r i e d o u t as sedimentology e x e r c i s e s w i t h s t u d e n t s of K i n g A b d u l a z i z U n i v e r s i t y , r e v e a l e d t h a t , a l t h o u g h most g r a i n - s i z e d i s t r i b u t i o n s a r e i n f a c t f i n e skewed, a few coarse ( n e g a t i v e ) skewed sands do o c c u r ( s e e a l s o Binda, i n p r e s s ) . I n o r d e r t o e x p l a i n t h e anomalous d i s t r i b u t i o n s , p a i r s o f c r e s t - a n d - t r o u g h samples o f e o l i a n r i p p l e s were analyzed, as i t was suspected t h a t m i x i n g o f sand from t h e two " m i c r o - e n v i ronments" under a regime o f changing wind d i r e c t i o n was the cause. BACKGROUND

ON STATISTICAL PARAMETERS OF EOLIAN

SANDS

A comprehensive r e v i e w by F o l k (1971) shows t h a t i n i n l a n d dunes t h e mean g r a i n s i z e i s g e n e r a l l y i n t h e 2 - 3 o range and s t a n d a r d d e v i a t i o n between about . 2 5 and .70

0.

A l t h o u g h i n many cases d i f f e r e n c e s a r e n o t s t a t i s t i c a l l y s i g n i -

f i c a n t , c r e s t s a r e g e n e r a l l y b e t t e r s o r t e d than f l a n k s , and l e e w a r d f l a n k s may be b e t t e r s o r t e d t h a n windward ones.

Skewness i s g e n e r a l l y p o s i t i v e w i t h o n l y

a few sands s l i g h t l y n e g a t i v e l y skewed. L o n g i t u d i n a l dunes o f t h e Simpson D e s e r t o f A u s t r a l i a ( F o l k , 1971) have c r e s t s coarser t h a n f l a n k s .

Standard d e v i a t i o n may be as h i g h as 1.0

b e t t e r s o r t e d than f l a n k s .

0,

w i t h crests

Over 92% o f t h e l o n g i t u d i n a l dunes samples analyzed

by F o l k a r e p o s i t i v e l y skewed, w i t h c r e s t s more p o s i t i v e l y skewed t h a n f l a n k s , and windward s l i g h t l y more skewed t h a n l e e w a r d f l a n k .

28 I n t e r p a r a m e t r i c r e l a t i o n s h i p s o f sands from t h e Simpson D e s e r t do n o t r e v e a l any p a r t i c u l a r p a t t e r n , e x c e p t f o r p l o t s o f mean s i z e versus skewness: c o a r s e r t h e mean, t h e more p o s i t i v e t h e skewness, t a i l o f f i n e material.

G. t h e

the

more pronounced t h e

T h i s r e l a t i o n s h i p i s e x p l a i n e d by F o l k n o t as a m i x i n g

of p o p u l a t i o n s ; t h e p o s i t i v e t a i l i s always p r e s e n t , b u t i t becomes more e v i d e n t as t h e p r i m a r y mode s h i f t s t o c o a r s e r s i z e s . F o l k (1968 and 1971) shows t h a t t h e b e s t method o f s t u d y i n g e o l i a n sand s i z e s

i s t h e c o n s t r u c t i o n o f f r e q u e n c y c u r v e s which r e v e a l c h a r a c t e r i s t i c modes. B r i e f l y , wind has a tendency t o p i c k up from t h e d e s e r t f l o o r g r a i n s h a v i n g d i a meters around 2.5

@

(0.177 mm).

Thus, dune sands w i l l be unimodal w i t h a

pronounced mode around t h i s value, whereas d e f l a t i o n areas ( r e g ) , from which t h e 2.5

0

m a t e r i a l i s removed, a r e c h a r a c t e r i z e d by bimodal o r polymodal f r e q u e n c y

d i s t r i b u t i o n s w i t h an obvious s a d d l e i n correspondence w i t h t h i s v a l u e .

The

a n a l y s i s o f modes i n f r e q u e n c y d i s t r i b u t i o n curves has been s u c c e s s f u l l y employed by Binda (1972) and B i n d a and H i l d r e d (1973) f o r t h e r e c o g n i t i o n o f e o l i a n o r i g i n o f some sub-Recent " K a l a h a r i Sands" o f Zambia.

However, SkoEek and S a a d a l l a h

(1972) r e p o r t e d t h a t 12 o u t o f 32 dune sands from t h e Southern D e s e r t o f I r a q have bimodal g r a i n - s i z e frequency d i s t r i b u t i o n s .

They e x p l a i n t h e b i m o d a l i t y as

due t o m i x i n g o f a suspension, d u s t , p o p u l a t i o n w i t h a s a l t a t i o n , sand, p o p u l a t i o n . A h l b r a n d t (1979), i n h i s e x t e n s i v e l i s t i n g o f s t a t i s t i c a l parameters o f i n l a n d dunes f r o m v a r i o u s l o c a l i t i e s , has a few e n t r i e s c l e a r l y l a b e l e d : ward, c r e s t , s l i p f a c e .

barchan, wind-

From these few d a t a i t appears t h a t i n barchans t h e mean

g r a i n s i z e i s n o t dependent upon t h e p o s i t i o n i n t h e dune, b u t perhaps upon t h e a v a i l a b l e m a t e r i a l and wind v e l o c i t y

--

i t varies w i t h l o c a l i t y .

Crests tend t o

be b e t t e r s o r t e d t h a n windward and l e e (s1i p f a c e ) , and a1 1 ( 1 3 ) samples have p o s i t i v e skewness.

A h l b r a n d t comments on skewness b e i n g dependent on mean g r a i n

s i z e , a l t h o u g h t h i s i s more e v i d e n t l y d i s p l a y e d i n h i s f i g u r e 21 ( p l o t o f mean versus skewness f o r i n t e r d u n e m a t e r i a l ) than i n f i g u r e 20 (mean versus skewness f o r i n l a n d dunes), t h e l a t t e r showing a r a t h e r " f l a t " p a t t e r n . R e l e v a n t t o t h e p r e s e n t s t u d y i s a l i s t i n g o f parameters o f r i p p l e t r o u g h s ( 6 samples) and c r e s t s ( 3 samples) f r o m t h e G r e a t Sand Dunes N a t i o n a l Monument of Colorado ( A h l b r a n d t , 1979, p . 4 6 ) . than t h e t r o u g h s .

A l l t h e c r e s t s have c o a r s e r mean d i a m e t e r

Two c r e s t s have p o s i t i v e skewness, one n e g a t i v e , whereas a l l

b u t one t r o u g h have n e g a t i v e skewness.

A c a u t i o n a r y comment on t h e use o f skewness as an i n d i c a t o r o f e o l i a n o r i g i n o f sand i s expressed by B i g a r e l l a (1972, p. 1 4 ) :

" p o s i t i v e skewness, c o n s i d e r e d

by some an i n d i c a t o r o f dune environment, i s open t o q u e s t i o n because n e g a t i v e skewness has a l s o been r e c o r d e d i n dune sand."

29

I

I

I

I

I

I

26'

I

I /1 \

EDGE OF PRECAMBR/AN SHIELD

'r

-

\ 24'

\

c

1

AR

I

MADINAH

-

I

I

I

-\

NAFUD RUMHAT

\

*

\

\

WAD/ KHUL E/S

JEDDAH

I 36O

Figure 1.

I

I 38'

I

I 40'

I 42O

I

I 440

I

I

I

46O

L o c a t i o n map.

LOCALITIES ( F i g . 1) The samples analyzed f o r t h i s s t u d y a r e from: (NR),

Wadi K h u l e i s ( K ) , Nafud Rumhat

and Jeddah ( E a s t and S o u t h ) .

The area o f Wadi K h u l e i s l i e s i n t h e Red Sea c o a s t a l p l a i n a p p r o x i m a t e l y 80 km t o t h e n o r t h e a s t o f Jeddah, about 5 km w e s t o f K h u l e i s v i l l a g e , a t a p p r o x i m a t e l y 22O10' N and 39O20'

E,

and i s bordered t o t h e e a s t and s o u t h e a s t by Precambrian

metavolcanics and T e r t i a r y b a s a l t r e s p e c t i v e l y .

I n some p a r t s o f t h e b a s i n dunes

a r e continuous, i n o t h e r p a r t s t h e y a r e s c a t t e r e d on a d e f l a t e d g r a v e l l y s u r f a c e . Dunes a r e m a i n l y o f t h e barchan type, up t o 3 . 5 m h i g h , 45 m l o n g and 30 m wide, but g e n e r a l l y s m a l l e r ( F i g . 2 ) .

They have a g e n t l e n o r t h w e s t e r l y windward s l o p e

( 3 O t o 8O) and a s t e e p s o u t h e a s t e r l y avalanche face (30° t o 3 2 O ) .

Owing t o

crossing o f p r e v a i l i n g n o r t h w e s t e r l y winds w i t h seasonal n o r t h - n o r t h e a s t e r l i e s , some dunes a t t h e s o u t h e r n edge o f t h e b a s i n a c q u i r e a l o n g i t u d i n a l shape (Zaidi, i n press).

30

Figure 2 .

Siiia11 barchan dunes and d e f l a t e d paveillent a t Wadi Khuleis.

Sairiples NR were c o l l e c t e d a t t h e s o u t h e r n edge of Nafud R u i n h a t , a t approximately

23'30'

N and 43030' E .

Nafud R u i n h a t i s a n o r t h w e s t - t r e n d i n g barchan f i e l d a few

t e n s of k m long and a few kin wide.

The wind i s g e n e r a l l y from the n o r t h , t h e r e -

f o r e t h e s t e e p avalanche s l o p e s f a c e t o t h e s o u t h . 11

iii

long and 2

The dune which was sampled i s

high and a p p e a r s t o be t y p i c a l o f t h e dunes i n t h e a r e a .

rii

At

v a r i a n c e with t h e barchans of Wadi K h u l e i s , t h e ones i n Nafud R u m h a t have an almost smooth s u r f a c e , devoid of n o t i c e a b l e r i p p l e s . E a s t Jeddah samples were c o l l e c t e d i n Jeddah froiii e o l i a n s h e e t s forming a t h i n veneer on t o p of Precambrian o u t c r c p s , e a s t of t h e r i n g - r o a d t h a t c o n n e c t s Plakkah and Madinah r o a d s .

U n t i l a few y e a r s a g o , barchan dunes were a l s o p r e s e n t i n the

a r e a , b u t they have s i n c e been bulldozed t o ):lake p l a c e f o r urban development.

In

t h i s a r e a , p a i r s of samples were c o l l e c t e d from c r e s t s and t r o u g h s of u n i d i r e c t i o n a l e o l i a n r i p p l e s 2 t o 5 cm i n h e i g h t and 15 t o 30 cni i n wavelength ( F i g . 3 ) . The f o u r South Jeddah samples were c o l l e c t e d a p p r o x i m a t e l y 20 kni s o u t h o f J e d d a h , only 200

in1

from the s h o r e l i n e .

They r e p r e s e n t c r e s t s and t r o u g h s o f

u n i d i r e c t i o n a l e o l i a n r i p p l e s formed by wind reworking o f r a i s e d marine t e r r a c e s .

A t t h i s l o c a l i t y , wind r i p p l e s a r e o f c o n s i d e r a b l e magnitude, r e a c h i n g 15 cm i n h e i g h t and 1 m i n wavelength.

31

Figure 3.

Eolian ripples froni East Jeddah.

Figure 4.

E o l i a n ripples a t r i g h t a n g l e s on a dune s u r f a c e a t Wadi Khuleis

32 METHODS Samples o f d u n e s f r o m Wadi K h u l e i s were c o l l e c t e d w i t h a g l a s s j a r , f r o m an a r e a a p p r o x i m a t e l y 20 x 20 cm and t o a d e p t h o f 5 - 10 cm.

As i s apparent f r o m

f i g u r e 4, s h i f t i n g winds produce r i p p l e s a t r i g h t angles t o each o t h e r , t h e r e f o r e t h e samples c o n t a i n b o t h c r e s t and t r o u g h sand.

The samples were q u a r t e r e d and

a p p r o x i m a t e l y 100 grams o f m a t e r i a l was s i e v e d . For t h e d e t a i l e d d i s c r i m i n a t i o n o f r i p p l e c r e s t s and t r o u g h , a c c u r a t e sampling was achieved i n E a s t and South Jeddah and i n Wadi K h u l e i s by t a k i n g s m a l l e r amounts (60

-

70 g) o f sand f r o m t h e s u r f a c e o f c r e s t s and t r o u g h s .

For t h e

t r o u g h s a m e t a l t a b l e spoon proved t o be an adequate t o o l , whereas f o r t h e c r e s t s a f l a t p l a s t i c t o y scoop was used. Samples were s i e v e d a t '5

intervals.

Cumulative percentages were p l o t t e d on

a r i t h m e t i c graph paper and percentages were o b t a i n e d g r a p h i c a l l y .

Statistical

parameters were c a l c u l a t e d a c c o r d i n g t o t h e formulae o f F o l k and Ward (1957). Frequency d i s t r i b u t i o n curves were c o n s t r u c t e d f r o m t h e c u m u l a t i v e curves w i t h t h e t a n g e n t method d e s c r i b e d by F o l k ( 1 9 7 4 ) . RESULTS AND D I S C U S S I O N Ranges and averages o f s t a t i s t i c a l parameters a r e shown i n Table 1.

A full

l i s t i n g o f t h e parameters i s g i v e n i n Binda ( i n p r e s s ) . Dunes _ _ Sands f r o m Nafud Rumhat a r e g e n e r a l l y c o a r s e r g r a i n e d t h a n t h e ones f r o m Wadi Khuleis.

The a r i t h m e t i c mean o f t h e mean d i a m e t e r i s 1.88 0 i n Nafud Rumhat and

2.52 i n Wadi K h u l e i s .

These mean d i a m e t e r s conform w i t h p u b l i s h e d r e s u l t s on

wind-blown sands f r o m o t h e r p a r t s o f t h e w o r l d (Friedman, 1961; F o l k , 1971; A h l b r a n d t , 1979).

I t i s n o t p o s s i b l e t o d i s c r i m i n a t e among v a r i o u s p a r t s o f t h e

dunes on t h e b a s i s o f mean d i a m e t e r .

A l l t h e frequency d i s t r i b u t i o n s , e x c e p t one

( c r e s t ) f r o m Wadi K h u l e i s , a r e unimodal. Nafud RumhZt, and between 2 and 3

0

Modes a r e g e n e r a l l y l e s s t h a n 2

@

in

i n Wadi K h u l e i s , t h e l a t t e r c o n f o r m i n g more

c l o s e l y t o t h e "quanta" model o f F o l k (1971). Sands f r o m Nafud Rumhat a r e g e n e r a l l y b e t t e r s o r t e d t h a n t h e ones f r o m Wadi Khuleis.

I n t h e l a t t e r l o c a l i t y , ranges o f s t a n d a r d d e v i a t i o n o f v a r i o u s p a r t s

of t h e dunes show a c o n s i d e r a b l e o v e r l a p .

I n Nafud Rumhat t h e two c r e s t samples

show b e t t e r s o r t i n g t h a n t h e o t h e r s , b u t t h e s m a l l number o f samples does n o t warrant generalizations.

Skewness i s p o s i t i v e i n a l l Nafud Rumhat samples,

v a r i a b l e a t Wadi K h u l e i s , where, however, leeward samples t a k e n f r o m t h e avalanche f a c e t e n d t o be more symmetrical t h a n o t h e r s ( F i g . 5 ) . The h i g h e r v a l u e s o f k u r t o s i s (more l e p t o k u r t i c c u r v e s ) o c c u r i n b o t h l o c a l i t i e s on t h e c r e s t s ; windward sands t e n d t o have h i g h e r k u r t o s i s t h a n leeward ones, i n agreement w i t h r e s u l t s r e p o r t e d by F o l k (1971).

Table I Range a n d A r i t h m e t i c Mean o f S t a t i s t i c a l P a r a m e t e r s of E o l i a n Sands from Nafud

@ Mean (Mz)

0 S t a n d a r d Deviation(GI)

Sample (No)

Mean

Range

Mean

1.88

0.43 - 0 . 7 1

0.57

2.82

2.52

0.52

-

0.85

0.67

2.67

2.47

0.58 - 0.85

0.72

Range

RumhBt, Wadi K h u l e i s , and Jeddah.

Skewness (SkI ) Range

K u r t o s i s (IQ Nean

Range

M e an

1.1.4 - 2 . 1 0

1.53

Dunes Nafud Rumhat ( 7 )

1.53

-

2.12

-

M.40

M.32

-0.10

-

M.21

M .05

0.69

-0.10

- +0.19

iil.08

0.79 - 1.00

0.87

H.09

-

1.18 0.90

Wadi Khuleis ( 1 5 )

2.21

Wadi Khuleis windward ( 7 )

2.21

Wadi Khuleis crest ( 4 )

2.26 - 2.82

2.45

0.52 - 0.83

0.64

-0.17

- M.21

N .07

0.87 - 1 . 1 8

0.99

Wadi K h u l e i s Lee ( 4 )

2.57

-

2.77

2.69

0.55

0.78

0.62

-0.05

-

-0.01

0.69

-

0.92

0,84

1 . 7 1 - 2.68

2.29

0.43 - 0.69

0.55

-0.21 - M . 3 0

a.08

0.75

-

1.19

1.01

H.13

0.75 - 1.19

1.02

-

H.01

Ripples

East Jeddah ( 2 2 ) E a s t Jeddah crests (11)

1 . 7 1 - 2.68

2.12

0.43 - 0 . 6 4

0.54

-0.05

- a.30

E a s t Jeddah Troughs (11)

2.27

-

2.47

0.40

-

0.56

-0.08

-

2.68

0.69

-

M.18 w . 0 4 M . 4 5 H 10

.

0.87

-

1.18

1.01

0.70

-

2.68

1.27

Wadi K h u l e i s ( 6 )

1.38

2.71

2.24

0.45

0.64

0.60

-0.30

Wadi K h u l e i s crests ( 3 )

1.38 - 2.41

1.62

0.45 - 0.62

0.55

N.12 - M.45

iil.25

0.96 - 2.68

1.68

Wadi K h u l e i s t r o u g h s ( 3 )

2.26

2.71

2.56

0.57

0.64

0.60

-0.30

-

-0.06

0.70

-

0.88

0.86

T o t a l R i p p l e Crests ( 1 4 )

1.38

-

2.68

2.08

0.43 - 0.67

0.54

-0.05

- +0.45

+O. 15

0.75 - 2.68

1.16

T o t a l R i p p l e Troughs ( 1 4 )

2.26 - 2 . 7 1

2,49

0.40 - 0.69

0.57

-0.30

- M.22

w.02

0.70 - 1,18 0 . 9 8

S . Jeddah* r i p p l e crests ( 2 ) S . Jeddah* r i p p l e t r o u g h s ( 2 )

-0.19

1.84

-

-

M.22

- 0.14

0.89 - 1.25

+0.51 - M . 6 2

-

0.85

-0.21

2.14

-

1.31

-

-0.14

1 . 4 1 - 2.31 0.97

-

1.05

*Not computed w i t h e o l i a n r i p p l e s s i n c e t h e y are " i n s i t u " wind-reworked m a r i n e s a n d s . W W

WADI KHULEIS DUNES

WINDWARD

CREST

Figure 5 .

Variations i n frequency d i s t r i b u t i o n curves of dune samples from Wadi Khuleis.

35

The most obvious d i f f e r e n c e between t h e g r ai n s i z e d i s t r i b u t i o n s in the two l o c a l i t i e s where dunes have been sampled i s t h a t the curves of Wadi Khuleis a r e heterogeneous, whereas the ones from Nafud Rumhat a r e f a i r l y s i m i l a r t o each o th e r . As i t was suspected t h a t the heterogenity of the Wadi Khuleis samples was a consequence of the s h i f t i n g i n wind d i r e c t i o n in the near-coastal area of Wadi Khuleis, and of various admixtures of r i p p l e c r e s t and trough sand, i t was decided t o do a d e t a i l e d a n a l y s i s of sand from c r e s t s and troughs of e olia n ripples. Ripples Three types o f r i p p l e s have been sampled f o r t h i s study.

The East Jeddah

samples r e p r e s e n t r i p p l e s t h a t form on s h eet s of wind-blown sand climbing on h i l l s of Precambrian bedrock.

The Wadi Khuleis samples re pre se nt r i p p l e s t h a t

form on dunes i n a shifting-wind realm, b u t a few samples were c a r e f u l l y c olle c te d from places where i t was p o s s i b l e t o do so without mixing c r e s t sand with trough sands.

South Jeddah samples a r e from a marine t e r r a c e o r ra ise d beach on which

wind has produced r i p p l e s .

Therefore, t h e l a t t e r a r e n o t , s t r i c t l y speaking,

e o lia n sands; probably not much s a l t a t i o n t r a n s p o r t i s involved here because of the coarse nature of t h e sand, r o l l i n g being the most probable mechanism. I n a l l si x t e e n p a i r s of r i p p l ed sands analysed, c r e s t samples a r e c oa rse r grained than troughs.

This observation had already been made by Bagnold (1941). I n ten p a i r s , c r e s t s a r e b e t t e r s o r t e d , t h e contrary being t r u e f o r the othe r

six.

Kurtosis i s higher i n troughs i n nine cases and lower in seven.

The d e t a i l e d study of wind-generated r i p p l e s sheds some l i g h t on skewness of eolian sands, t h e parameter which some authors deem c r i t i c a l t o the i d e n t i f i c a tion of e o l i a n t r a n s p o r t .

Not a l l e o l i a n sands have positive skewness; negative

skewness i s common in the troughs of e o l i a n r i p p l e s . Excluding t h e f o ur South Jeddah samples, eo l i a n r i p p l e c r e s t s range from - 0 . 0 5 t o + 0.45 i n skewness, averaging + 0.15; twelve samples have p o s i t i v e skewness, two negative.

Ripple troughs range from - 0.30 t o + 0.22, averaging + 0.01; e i g h t

samples have p o s i t i v e skewness, s i x negative.

I n f i g u r e 6 a r e shown ranges and

mean skewness of dunes and of r i p p l e s . Including the f o u r South Jeddah samples, i n eleven of the sixte e n p a i r s , c r e s t s have more p o s i t i v e ( o r l e s s n eg at i v e) skewness than troughs, in four p a i r s the opposite i s t r u e , and i n one case skewness i s i d e n t i c a l i n c r e s t and trough I n f i g u r e 7 a r e shown t y p i cal frequency d i s t r i b u t i o n curves of c r e s t and trough sands.

In F o l k ' s (1974) terminology, c r e s t samples a r e thus d i s t r i b u t e d :

3

strongly f i n e skewed, 8 f i n e skewed, 5 near symmetrical, whereas trough samples are:

4 f i n e skewed, 9 near symmetrical, 3 coarse skewed.

These r e s u l t s agree

with d a t a from the Great Sand Dunes National Monument l i s t e d i n Ahlbrandt (1979

36

N.

RUMHZT DUNE

( 7 ) - -I

W.KHULElS DUNES (15) 1-1

1

RIPPLE CRESTS . (14)

RIPPLE TROUGHS (14) 1 -

1

1

I

I

I

-.2

-.4

I

0

+.2

I +.4

SKEWNESS (Sk, )

1-1

Figure 6.

RANGE

MEAN

( 7 1 SAMPLES

Ranges and mean skewness o f 50 e o l i a n sands f r o m Saudi A r a b i a

I t i s perhaps p o s s i b l e t o i n f e r from t h i s s t u d y t h a t , p o s i t i v e skewness b e i n g

normal f o r wind-blown sand, t h e n e g a t i v e skewnesses encountered i n r i p p l e t r o u g h s i s b r o u g h t a b o u t e i t h e r by r o l l i n g o f a few coarse g r a i n s down f r o m t h e c r e s t s , o r by c o a r s e g r a i n s t h a t t h e w i n d i s n o t capable o f p u s h i n g up t o t h e c r e s t s and thus l a g behind i n the troughs.

The f o r m e r e x p l a n a t i o n i s more l i k e l y , a t l e a s t

f o r t h e s t r i c t l y e o l i a n samples where c r e s t s d i s p l a y c o a r s e r low p e r c e n t i l e s .

CONCLUSIONS G r a i n - s i z e f r e q u e n c y - d i s t r i b u t i o n curves o f dune sands f r o m Nafud Rumhat and Wadi K h u l e i s a r e a l m o s t a l l unimodal ( 2 1 o u t o f 22) w i t h modes between 1.5 and 2 0 , and 2 and 3 0 , r e s p e c t i v e l y . There i s a c e r t a i n amount o f o v e r l a p i n t h e v a l u e s o f s t a t i s t i c a l parameters f r o m windward, c r e s t , and l e e o f t h e dunes; t h u s i t would be d i f f i c u l t t o a s s i g n an unknown sample t o a s p e c i f i c dune sub-environment.

However, i n t h r e e dunes

37

/ I 2

I

I

I': I \ I

I

I

2

3

C.

\

3

A.

W . KHULEIS

B.

JEDDAH EAST

c. JEDDAH

---

I -I Figure 7 .

I 0

I I

I

I

2

3

4

SOUTH

RIPPLE CREST RIPPLE TROUGH

Frequency d i s t r i b u t i o n curves of selected p a i r s of r i p p l e samples showing t h a t c r e s t s are coarser grained and more p o s i t i v e l y skewed than troughs.

o u t of f o u r , sand from the c r e s t i s b e t t e r sorted and has higher kurtosis than e i t h e r windward o r leeward sand. An i n t e r e s t i n g conclusion can be reached with regard t o skewness, a parameter which i s often c i t e d as being c r i t i c a l t o the i d e n t i f i c a t i o n o f the eolian o r i g i n of a sand. I n Nafud R u m h a t , where the dune surface i s almost smooth, a l l samples are p o s i t i v e l y skewed. I n Wadi Khuleis, where the dunes' surface i s r i p p l e d , and where the wind d i r e c t i o n i s not constant, some samples a r e p o s i t i v e l y skewed, others a r e negatively skewed, regardless o f t h e i r position on the dune. I n general, frequency d i s t r i b u t i o n s of dune sands from Wadi Khuleis a r e more heterogeneous than the ones from Nafud R u m h a t .

38

+.e 0

X

x

X

+2

X

x

X

0

A

0

H

Y

cn Y

+.I

+

0.

0

0 0

+

X

0 - t

+

0

0

0

02%

.+ .+

v,

t n o

0

W

z 3

+

tr

0

+

0

-.I

0

+

+

+

O

+.

.

W Y

+

tn

-. 3

0

I

I

I

1.5

DUNES

Figure 8.

I

1

I

I

I

I

I

I

I

2.o MEAN ( M t ) x N.RUMH~~T

+ W . KHULEIS

I

I

I

2.5

R I PPLES

o CREST 0

TROUGH

Pl o t of mean diameter versus skewness of 50 e olia n sands from Saudi Arabia.

The a n a l y s i s of s i x t e e n p a i r s of r i p p l e c r e s t and t r o u g h samples from Wadi Khuleis and from two Jeddah l o c a l i t i e s shows t h a t most c r e s t sands a r e p o s i t i v e l y skewed, whereas n e g at i v el y skewed o r symmetrical d i s t r i b u t i o n s a r e common i n troughs.

Also, f o r a l l e o l i a n samples analyzed i n t h i s study the re seems t o be

a c o r r e l a t i o n between mean diameter and skewness:

the c oa rse r t h e mean, the

more pronounced the f i n e t a i l ( F i g . 8 ) . I t would be beyond t h e scope of t h i s paper t o embark on a discussion of the

dynamics of r i p p l e formation. However, from the data presented here i t stands t o reason t h a t a s o r t i n g process takes place i n the formation of e o l i a n r i p p l e s which se p a r a t e s more p o s i t i v e l y skewed sand on t he c r e s t from symmetrical t o n e g a ti v e l y skewed sand i n t h e troughs.

T h u s t h e v a r i a t i o n i n skewness among t h e Wadi Khuleis samples may be explained by the mixing of r i p p l e c r e s t and r i p p l e trough sands, brought about by changing

39 It may be i n f e r r e d t h a t , i f a sample has a predominance o f c r e s t m a t e r i a l , i t w i l l wind d i r e c t i o n , and hence the d i f f i c u l t y o f c o l l e c t i n g homogeneous samples. be p o s i t i v e l y skewed.

I f , on t h e o t h e r hand, t r o u g h m a t e r i a l i s predominant,

t h e r e s u l t i n g g r a i n - s i z e d i s t r i b u t i o n w i l l t e n d t o be e i t h e r symmetrical o r n e g a t i v e l y skewed. ACKNOWLEDGMENTS

I wish t o thank t h e s t u d e n t s o f t h e sedimentology c l a s s a t t h e F a c u l t y of Earth S c i e n c e s , King Abdulaziz U n i v e r s i t y , Jeddah, who c o l l e c t e d and s i e v e d some of t h e samples; S a i d Zaidi who f i r s t i n d i c a t e d t o me t h e Wadi Khuleis dune f i e l d ;

Drs. S a i d Oinara and Mohammed Askari who accompanied me t o Wadi Khuleis. D. M. J . Kent and R . Rahmani c r i t i c a l l y r e a d the m a n u s c r i p t .

Drs.

REFERENCES A h l b r a n d t , T . S . , 1979. T e x t u r a l p a r a m e t e r s o f e o l i a n d e p o s i t s . I n : E . O . McKee ( E d i t o r ) , A Study o f Global Sand S e a s . U.S.G.S. P r o f . Paper 1052, p p . 21-51. Bagnold, R . A . , 1941. The P h y s i c s of Blown Sands and Sand Dunes. Methuen & Co. L t d . , London, 265 p p . B i g a r e l l a , J . J . , 1972. E o l i a n environments: t h e i r c h a r a c t e r i s t i c s , r e c o g n i t i o n , and i m p o r t a n c e . I n : J . K . Rigby and W . K . Hamblin ( E d i t o r s ) , Recognition of Ancient Sedimentary Environments. SOC. Econ. P a l e o n t . Mineral. S p e c i a l P u b l i c a t i o n 1 6 , p p . 12-62. Binda. P . L . . 1972. On the sedimentoloqv o f some cover sands from Zambia. R.C.M. Report GR 44, K a l u l u s h i , Zambia. Binda, P . L . ( i n p r e s s ) . G r a i n - s i z e s t u d y o f some e o l i a n sands from Saudi Arabia. B u l l . Fac. E a r t h S c i . , K . A . U . , Jeddah. Binda, P . L . and H i l d r e d , P . R . , 1973. Bimodal g r a i n - s i z e d i s t r i b u t i o n s of some K a l a h a r i - t y p e s a n d s from Zambia. Sedimentary Geology, 10: 233-237. Folk, R . L . , 1962. Of skewnesses and s a n d s . J o u r . Sed. P e t r o l o g y , 32: 145-146. Folk, R . L . , 1968. Bimodal supermature s a n d s t o n e s : p r o d u c t o f the d e s e r t f l o o r . I n t . Geol. C o n g r . , 2 3 r d , Prague, P r o c . S e c t . 8, p p . 9-32. Folk, R . L . , 1971. L o n g i t u d i n a l dunes of the n o r t h w e s t e r n edge of t h e Simpson D e s e r t , Northern T e r r i t o r y , A u s t r a l i a , 1. Geomorphology and g r a i n s i z e r e l a t i o n s h i p s . Sedimentology, 16: 5-54. F o l k , R . L . , 1974. P e t r o l o g y o f Sedimentary Rocks. Hemphill, A u s t i n , Texas, 182 p p . Folk, R . L . and Ward, W . C . , 1957. Brazos r i v e r b a r - a s t u d y i n the s i g n i f i c a n c e of g r a i n - s i z e p a r a m e t e r s . J o u r . Sed. P e t r o l o g y , 27: 3-27. Friedman, G . M . , 1961. D i s t i n c t i o n between dune, beach, and r i v e r sands from t h e i r t e x t u r a l c h a r a c t e r i s t i c s . J o u r . Sed. P e t r o l o g y , 31: 514-529. Mason, C . C . and Folk, R . L . , 1958. D i f f e r e n t i a t i o n o f beach, dune, and e o l i a n f l a t environments by s i z e a n a l y s i s , Mustang I s l a n d , Texas. J o u r . Sed. P e t r o l o g y , 28: 211-226. S h e p a r d , F . P . and Young, R . , 1961. D i s t i n g u i s h i n g between beach and dune s a n d s . J o u r . Sed. P e t r o l o g y , 31: 196-214. SkoEek, V. and S a a d a l l a h , A . A . , 1972. G r a i n - s i z e d i s t r i b u t i o n , c a r b o n a t e c o n t e n t and heavy m i n e r a l s i n e o l i a n s a n d s , Southern D e s e r t , I r a q . Sedimentary Geology, 8 : 29-46. Z a i d i , S.M.S. ( i n press). Geomorphology o f Wadi Khulays a r e a . F a c u l t y E a r t h S c i . , K . A . U . , Research S e r i e s , J e d d a h , Saudi A r a b i a . _I

This Page Intentionally Left Blank

41

.mn

TEXTIJRAL

STRUCTURAL CHARACTERISTICS

FORMED EOLIAN

OF SOME EXPERIMENTALLY

STRATA CHRISTOPHER J . SCHEFIK, U.S. G e o l o g i c a l Survey, P.O. Denver F e d e r a l Center, Denver, CO 80225

Box 25046,

INTRODUCTION

The purpose o f t h i s paper i s t o compare t h e t e x t u r a l and s t r u c t u r a l f e a t u r e s of t h e t h r e e main t y p e s o f e o l i a n s t r a t a based on a s e r i e s o f d e p o s i t s formed i n a wind tunnel.

The b e l i e f h e r e i s t h a t t h e d e p o s i t i o n a l processes c o n t r o l

t e x t u r a l and s t r u c t u r a l c h a r a c t e r i s t i c s o f e o l i a n sediments and, consequently, 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 u s e f u l i n d i s t i n g u i s h i n g between t h e t h r e e main t y p e s of s t r a t i f i c a t i o n i n s m a l l samples, such as core. The main t y p e s o f e o l i a n s t r a t i f i c a t i o n ,

e l u c i d a t e d by H u n t e r (1977 a, b )

f r o m r e c e n t d e p o s i t s a l o n g t h e Texas c o a s t i n c l u d e avalanche, s e v e r a l forms o f e o l i a n r i p p l e s t r a t i f i c a t i o n . Hunter

(1981)

i l l u s t r a t e d how t h e s e t y p e s

grainfall,

and

Kocurek and D o t t (1981) and

o f s t r a t a can be i d e n t i f i e d on

outcrop. The d i f f e r e n t t y p e s o f s t r a t a , as i n h e r e n t i n t h e g e n e t i c c l a s s i f i c a t i o n o f Hunter

(1977a),

reflect

the

processes

of

development o f each t y p e o f s t r a t i f i c a t i o n .

deposition

responsible

for

the

Sand i s d e p o s i t e d by w i n d t h r o u g h

a t l e a s t t w o p r i m a r y processes--by g r a i n f a l l and by a c c u m u l a t i o n d u r i n g r i p p l e migration--leading

t o t h e p r o d u c t i o n o f g r a i n f a l l and r i p p l e s t r a t i f i c a t i o n .

G r a i n f a l l d e p o s i t i o n w i l l o c c u r on a s l o p i n g s u r f a c e u n t i l t h e a n g l e o f i n i t i a l y i e l d i s reached.

A t t h i s p o i n t an avalanche t a k e s p l a c e ,

f o r m a t i o n o f avalanche s t r a t i f i c a t i o n .

resulting i n the

Each o f t h e s e processes r e s u l t s i n

g r a i n arrangements t h a t can be u s e f u l i n i d e n t i f y i n g t h e t y p e o f s t r a t a . METHODS

OF STUDY

Each t y p e o f e o l i a n s t r a t i f i c a t i o n was formed i n a w i n d t u n n e l .

I n t h e view

of t h e t u n n e l shown i n F i g u r e 1, t h e w i n d t r a v e l s f r o m r i g h t t o l e f t a l o n g t h e h o r i z o n t a l f l u m e s e c t i o n (A), r e s u l t i n g i n t h e f o r m a t i o n o f w i n d r i p p l e s t r a t a , t h e n proceeds o v e r t h e s i m u l a t e d f o r e s e t s l o p e (B), where t h e sand i s d e p o s i t e d as g r a i n f a l l i n t h e zone o f f l o w s e p a r a t i o n . qrainfall,

t h e slope eventually

avalanche downslope.

As more sand i s d e p o s i t e d as

becomes u n s t a b l e ,

and t h e g r a i n f a l l s t r a t a

42

F i q . 1.

Photoqraph o f t h e wind t u n n e l .

A f t e r each e x p e r i m e n t , l a t e x p e e l s were p r e p a r e d f r o m a l l o f t h e s t r a t i f i c a t i o n types

i n a direction

parallel

samples c u t f r o m t h e p e e l s . point-countinq technique.

t o t h e wind.

T h i n s e c t i o n s were made f r o m

P o r o s i t y was d e t e r m i n e d i n t h i n s e c t i o n u s i n g a

Care was t a k e n t o a v o i d t h e v a r i o u s e r r o r s t h a t a r e

p o s s i b l e w i t h t h i s method ( H a l l e y , 1 9 7 8 ) . The sand used i n a l l o f t h e e x p e r i m e n t s was a n a t u r a l l y q r a d e d e o l i a n dune sand;

i t was f i n e q r a i n e d and w e l l s o r t e d and h a d p o s i t i v e skewness. R ESVLT? Ripple s t r a t i f i c a t i o n

Commonly, d u r i n q e o l i a n r i p p l e r n i q r a t i o n , s a l t a t i o n bombardment, r i p p l e upwind

and t h e

( F i q 2).

remainder

Deposits

subcritically climbinq translatent each r i p p l e p r e s e r v e d rates

of

deposition,

depends and r a t e s

i s buried by the

f o r m e d by t h i s

process

next

factors,

succeeding

h a v e been t e r m e d

s t r a t a b y H u n t e r (1977a).

on s e v e r a l of

p a r t o f each r i p p l e i s removed by

The amount of

including wind velocity,

r i p p l e migration.

This

process

is

not

r e s t r i c t e d t o h o r i z o n t a l surfaces b u t can occur across slopes approaching t h e a n q l e o f i n i t i a l y i e l d ( F r y b e r q e r and Schenk, 1981).

43

F i g . 2. R e l i e f l a t e x peel i l l u s t r a t i n g a coset o f e x p e r i m e n t a l l y formed e o l i a n ripple stratification. Incomplete preservation o f t h e e n t i r e r i p p l e leads t o Finer grained layers exhibit s t r a t a t h a t a p p e a r p a r a l l e l o r n e a r l y so. positive relief. Arrows b r a c k e t a r i p p l e s t r a t u m . Low a n g l e i l l u m i n a t i o n causes shadows i n c o a r s e r g r a i n e d , r e c e s s e d a r e a s . S c a l e i n cm. R i p p l e s t r a t i f i c a t i o n o f t h i s t y p e i s c h a r a c t e r i z e d by t h i n , parallel

strata

s e p a r a t e d by p l a n a r b o u n d i n q s u r f a c e s

(Fig.

essentially

2).

Peels o f

r i p p l e s t r a t a i l l u s t r a t e w h a t a p p e a r s t o b e a n i n t e r c a l a t i o n o f c o a r s e r and f i n e r qrained layers. peels.

a r e r a r e l y observed. sortinq

The f i n e r q r a i n e d l a y e r s e x h i b i t p o s i t i v e r e l i e f i n t h e

Althouqh t h e s e d e p o s i t s a r e formed d u r i n g r i p p l e m i g r a t i o n ,

foresets

The l a c k o f f o r e s e t s i s e x p l a i n e d p a r t l y by t h e d e g r e e of

P x h i b i t e d b y t h e sand d e p o s i t e d o v e r t h e b r i n k

o f each r i p p l e and

p a r t l y by t h e f a c t t h a t sand does n o t a v a l a n c h e down t h e l e e s l o p e s o f e o l i a n ripples. Ourinq

ripple

miqration,

motion by t h e s a l t a t i o n

the

surface-creep

bombardment

(Bagnold,

population

1941).

is

maintained

in

The g r a i n s m o v i n q as

s u r f a c e c r e e p a r e d r i v e n o v e r t h e r i p p l e b r i n k and c o l l e c t on t h e l e e s l o p e .

A

c e r t a i n amount o f s a l t a t i n q sand i s d e p o s i t e d i n t h e t r o u g h o f each r i p p l e , and t h i s sand i s q e n e r a l l y f i n e r q r a i n e d t h a n t h e s u r f a c e - c r e e p m a t e r i a l d e p o s i t e d on t h e l e e s l o p e .

As t h e r i p p l e migrates, t h e c o a r s e r l e e s l o p e d e p o s i t s cover

t h e f i n e q r a i n s i n t h e t r o u q h , r e s u l t i n q i n a s t r a t u m t h a t i s i n v e r s e l y graded.

44

The inverse grading of r i p p l e s t r a t i f i c a t i o n i s best viewed i n t h i n section (Fig. 3 ) . The contacts between r i p p l e s t r a t a a r e generally s h a r p ; f i n e r grains a t t h e base of one stratum r e s t on t h e coarser grains of t h e stratum below. A t r a i n of miqratinq r i p p l e s can produce a coset of inversely graded s t r a t a . No other eolian depositional process appears t o produce t h i s inverse grading so repeti t i vely.

Fiq. 3. Photomicrograph of r i p p l e s t r a t a . Note t h e inverse grading a n d s h a r p boundaries between s t r a t a . Primary porosity in t h e coarser-grained p a r t of each stratum comnonly i s higher than t h e finer-grained p a r t . Black i s pore space. Photographed under plane polarized l i g h t , usinq a red f i l t e r . Bar s c a l e i s 3 mm long. Primary porosity in experimentally formed r i p p l e s t r a t i f i c a t i o n averaged 3 9 X , t h e lowest of t h e t h r e e main types of s t r a t a . Measurements have shown t h a t t h e porosity can he higher in t h e coarser grained p a r t of each inversely qraded stratum. Ripple s t r a t a a r e commonly preserved as crosswind-formed deposits a t t h e base of t h e high-angle s t r a t a in a dune, a n d some a r e

I n t h i s position, r i p p l e s t r a t a commonly form t h e tangential bedding frequently observed in e o l i a n deposits. i n t e r c a l a t e d with avalanche s t r a t a .

Grainfall s t r a t i f i c a t i o n The t e x t u r e of g r a i n f a l l s t r a t i f i c a t i o n depends d i r e c t l y on t h e velocity h i s t o r y of t h e wind because i t i s formed i n zones of flow separation. As t h e velocity increases, t h e s a l t a t i o n population becomes coarser grained, a n d t h e sand subsequently deposited i n a zone of flow separation a l s o becomes coarser. In addition t h e thickness o f a g r a i n f a l l stratum i s dependent on the duration of t h e wind event. Because of t h e s e dependencies, g r a i n f a l l

45 stratificdtion i s d i f f i c u l t t o characterize texturally.

S e t s of y r a i n f a l l

s t r a t a t h i n and becoiiie f i n e r yrained downslope (Fig. 4 ) .

F i g . 4.. G r a i n f a l l s t r a t a produced by a s e r i e s of wind gusts. Finer yrained 1 ayers e x h i b i t p o s i t i v e r e l i e f i n t h i s l a t e x peel. Contacts between s t r a t a a r e Thickness of adjacent y r a i n f a l l s t r a t a genera l l y n o t s h a r p , b u t gradational. Bar s c a l e i s 2 cm l o n y . can be extremely v a r i ab l e, because of wind gusting. A t h i n se c t i o n view of g r a i n f a l l s t r a t i f i c a t i o n i l l u s t r a t e s t h e yra da tiona l,

r a t h e r than sharp c on t act s ex h i b i t ed by a coset of experimentally formed s t r a t a (Fig. 5 ) . each

of

lowered.

The s t r a t a i l l u s t r a t e d i n Figure 5 were formed during wind gusts, in which,

t h e wind was

gradually

r ai s e d

t o a high p o i n t and then

The arrows point t o co ar s er grained la ye rs deposited during t h e

higher winds. The t e x t u r a l c o n t r a s t can be weak o r strong, depending on t h e d if f e r e n c e between t h e wind v el o ci t y maxima and t h e threshold ve loc ity necessary t o i n i t i a t e sand movement.

46

Fig. 5. Photomicrograph of e x p e r i m e n t a l l y formed g r a i n f a l l s t r a t a . Note t h e q r a d a t i o n a l a s p e c t of t h e s t r a t i f i c a t i o n and t h e t e x t u r a l c o n t r a s t s produced by a s e r i e s of wind qrlsts. Arrows p o i n t t o c o a r s e r - g r a i n e d l a y e r s . Black i s pore space. Photoqraphed under p l a n e p o l a r i z e d l i g h t , u s i n q a red f i l t e r . Rar s c a l e i s .5 cm lonq. Primary p o r o s i t y of t h e g r a i n f a l l

s t r a t a was v a r i a b l e b u t g e n e r a l l y was

i n t e r m e d i a t e between t h e r i p p l e and a v a l a n c h e p o r o s i t i e s , with an a v e r a q e of 4 3%.

Avalanche s t r a t i f i c a t i o n Avalanche s t r a t i f i c a t i o n

i s produced

by t h e r e d e p o s i t i o n

w a i n f a l l d e p o s i t s by slumpinq or sandflow. q e n e r a l l y heqan a s a slump h l o c k , which sandflows a s i t proceeded downslope.

of p r e - e x i s t i n q

Durinq t h e e x p e r i m e n t s , a v a l a n c h e s d i s p e r s e d wholly o r i n p a r t

into

C e r t a i n minor d e f o r m a t i o n a l s t r u c t u r e s ,

such as t h r u s t s , f l a m e s , and f o l d s , a r e c o n s i d e r e d c h a r a c t e r i s t i c of d r y sand a v a l a n c h i n q (McKee e t a l . , 1971). T h r u s t - t y p e s t r u c t u r e s formed d u r i n q a n a v a l a n c h e can be observed i n p e e l s hetween

the

structurps deposits.

boundinq are

formed

surfaces as

a

of slump

a n avalanche s t r a t u m ( F i q . 6 ) . block

overrides

pre-existing

These avalanche

47

F i r r . 6. T h r u s t s t r u c t u r e s f o r m e d d u r i n q s l r r m p i n q on t h e s i m u l a t e d f o r e s e t slope. Thpse s t r u c t u r e s a r e commonly f o r m e d a b o u t m i d - s l o p e above t h e a r e a where slumps d e q e n e r a t e t o s a n d f l o w s . Arrows p o i n t t o t h r u s t s t r u c t u r e s . Dormslope i s t o t h e r i q h t . S c a l e i n cm.

F i q . 7. R e l i e f p e e l i l l u s t r a t i n g t h e t e x t u r a l c o n t r a s t between e x p e r i m e n t a l l y former! s a n d f l o w s t r a t a ( n e q a t i v e r e l i e f i n p e e l ) and f i n e r q r a i n e d g r a i n f a l l strata. N o t e t h a t t h e a v a l a n c h e s t r a t a do n o t f o r m a t a n g e n t i a l c o n t a c t a t t h e base of t h e f o r e s e t s l o n e . S c a l e i n crn.

48

Fiq. 8. Photomicroqraph of a n a v a l a n c h e s t r a t u m formed by sandflowage. F i n e r q r a i n s a r e c o n c e n t r a t e d a l o n q the base of each f l o w ( a r r o w ) and g i v e t h e s t r a t um a weak i n v e r s e grading. S t r a t a d i p t o the r i g h t . Black i s pore Photoqraphed under p l a n e p o l a r i z e d l i q h t , u s i n g a r e d f i l t e r . Bar space. s c a l e i s 1 mm lonq. O n t h e lower h a l f of t h e s i m u l a t e d f o r e s e t s l o p e , sandflow r a t h e r t h a n slump

p r o c e s s e s predominate. During s a n d f l o w s , a l l of t h e g r a i n s are i n motion r e l a t i v e t o one a n o t h e r and d e f o r m a t i o n a l s t r u c t u r e s a r e not produced. In a d d i t i o n , d u r i n g sandflow, t h e f i n e g r a i n s end up e i t h e r a t o r n e a r t h e b a s e of t h e flow (Bagnold, 1954; Middleton, 1970) and a r e e i t h e r l e f t behind on t h e p l a n e o f s h e a r i n g o r a r e i n c o r p o r a t e d i n t o t h e b a s e of t h e a v a l a n c h e s t r a t u m when t h e f l o w movement c e a s e s .

49 T h i s process l e a d s t o s t r a t a t h a t a r e g e n e r a l l y w e l l s o r t e d and c o a r s e r downslope.

Some avalanche s t r a t a e x h i b i t

e f f e c t d i m i n i s h e s downslope.

a weak i n v e r s e g r a d i n g ,

but t h i s

Because t h e c o a r s e r g r a i n s t r a v e l a l o n g t h e edges

and t o p o f t h e flow,

t h e y r e a c h t h e base o f t h e s l o p e f i r s t ,

c o a r s e n i n g downslope.

A t t h e base, t h e avalanche s t r a t a wedge o u t i n t o e i t h e r

grainfall

or

ripple stratification.

There

leading t o a

i s a marked t e x t u r a l c o n t r a s t

between t h e c o a r s e r g r a i n e d s a n d f l o w " t o e s " and t h e f i n e r g r a i n e d g r a i n f a l l o r r i p p l e s t r a t i f i c a t i o n ( F i g . 7). I n t h i n s e c t i o n , t h e i n v e r s e g r a d i n g can be observed i n an avalanche s t r a t u m upslope f r o m t h e s a n d f l o w t o e s , r i p p l e s t r a t a (Fig.

8).

b u t i t i s n o t as pronounced t h e r e as i n t h e

I n g e n e r a l , avalanche s t r a t a a r e w e l l s o r t e d because

o f t h e removal o f t h e f i n e r g r a i n s . thick

Each s t r a t u m commonly i s a few c e n t i m e t e r s

and i s s e p a r a t e d f r o m a d j a c e n t

s t r a t a by sharp c o n t a c t s .

Avalanche

d e p o s i t s were c h a r a c t e r i z e d by t h e l o o s e s t p a c k i n g and t h e h i g h e s t p o r o s i t y (avg. 474,) o f t h e t h r e e t y p e s o f s t r a t a . CONCLUSIONS The processes o f d e p o s i t i o n i m p a r t c h a r a c t e r i s t i c t e x t u r a l and s t r u c t u r a l f e a t u r e s t o each o f t h e main t y p e s o f e o l i a n s t r a t i f i c a t i o n .

These f e a t u r e s ,

such as n a t u r e o f c o n t a c t s ,

and presence o r

grading,

thicknesses o f s t r a t a ,

absence o f s m a l l - s c a l e d e f o r m a t i o n a l s t r u c t u r e s ,

are useful i n discriminating

between r i p p l e and avalanche s t r a t i f i c a t i o n i n o u t c r o p and c o r e samples. REFERENCES Bagnold, R.A., 1941. The p h y s i c s o f blown sand and d e s e r t dunes. Plethuen, London, 265 pp. Bagnold, R.A., 1954. Experiments on a g r a v i t y - f r e e d i s p e r s i o n o f l a r g e s o l i d spheres i n a Newtonian f l u i d under shear. Proc. R. SOC. London, 225A: 49-63. 1981. Wind s e d i m e n t a t i o n t u n n e l experiments on F r y b e r g e r , S.G. and Schenk, C.J., t h e o r i g i n s o f e o l i a n s t r a t a . Sedimentology, 28: 805-821. H a l l e y , R.B., 1978. E s t i m a t i n g p o r e cement volume i n t h i n s e c t i o n . Jour. Sediment. P e t r o l . , 48: 642-650. Hunter, R.E., 1977a. B a s i c t y p e s o f s t r a t i f i c a t i o n i n small e o l i a n dunes. Sedirnentology, 24: 361-387. Hunter, R.E., 1977b. Terminology o f c r o s s - s t r a t i f i e d sedimentary l a y e r s and c l i m b i n g r i p p l e s t r u c t u r e s . J o u r . Sediment. P e t r o l . , 47: 697-706. Hunter, R.E., 1981. S t r a t i f i c a t i o n s t y l e s i n e o l i a n sandstones: some Pennsylvanian t o J u r a s s i c examples f r o m t h e Western I n t e r i o r , U.S.A. SOC. Econ. P a l e o n t . M i n e r a l . Spec. Publ . , 31: 315-329. IKocurek, G. and D o t t , R.H., 1981. D i s t i n c t i o n and uses o f s t r a t i f i c a t i o n t y p e s i n t h e i n t e r p r e t a t i o n o f e o l i a n sand. J o u r . Sediment. P e t r o l . , 51: 579-595. McKee, E.D., Douglass, J.R. and R i t t e n h o u s e , S., 1971. D e f o r m a t i o n o f l e e s i d e l a m i n a e i n e o l i a n dunes. Geol. SOC. Amer. B u l l . , 82: 359-378. 1970. Experimental s t u d i e s r e l a t e d t o t h e problems o f f l y s c h M i d d l e t o n , G.V., s e d i m e n t a t i o n . Geol. Assoc. Canada Spec. Publ., 7: 253-272.

This Page Intentionally Left Blank

51

LOESS MATERIAL ARD LOESS DEPOSITS:

FORMATION, DISTRIBUTION AND CONSEQUENCES

I A N J . SMALLEY and VALERIE SMALLEY:

Dept. o f E a r t h Sciences, U n i v e r s i t y o f

W a t e r l o o , W a t e r l o o , O n t a r i o , Canada N2L 3G1

INTRODUCTION:

THE FOUR STAGES

Loess i s d e p o s i t e d b y w i n d :

t o t h i s i t owes i t s s p e c i a l c h a r a c t e r i s t i c s ,

i . e . t h e n a r r o w s i z e r a n g e i n t h e c o a r s e s i l t r e g i o n , t h e open s t r u c t u r e , t h e t e n d e n c y t o c o l l a p s e when l o a d e d and w e t t e d , and t h e m a n t l i n g o f t h e landscape.

Loess r e f l e c t s t h e a e o l i a n i n f l u e n c e m o s t s t r o n g l y and i t s

l a r g e s c a l e d e p o s i t s a r e among t h e m o s t s p e c t a c u l a r f e a t u r e s o f a e o l i a n geology.

T h e r e i s a p r a c t i c a l a s p e c t t o l o e s s as w e l l , i t f o r m s t h e p a r e n t

m a t e r i a l f o r some o f t h e w o r l d ' s m o s t p r o d u c t i v e a g r i c u l t u r a l s o i l s (Chesworth 1982). The s t u d y o f l o e s s as a d e p o s i t has p r o v i d e d one h u n d r e d and f i f t y y e a r s of c o n t r o v e r s y and e x c i t e m e n t b u t t h e t i m e has come when we s h o u l d f o c u s our a t t e n t i o n on t o l o e s s m a t e r i a l i n a l l i t s forms, r a t h e r than j u s t t h e most s p e c i a l o c c u r r e n c e s .

For a simple b a s i c l o e s s d e p o s i t t o be formed

four major events u s u a l l y occur: has t o b e formed; posited;

(%

20-60 w i )

3 ) , and de-

4 ) , t h e n , u s u a l l y , a f e w p o s t - d e p o s i t i o n a l changes o c c u r and we

have w h a t we c a l l l o e s s .

as:

l ) , the loess-size material

2 ) , t h i s m a t e r i a l has t o be t r a n s p o r t e d ;

l ) ,make;

2), move;

I n s h o r t h a n d t e r m s t h e sequence can be d e s c r i b e d

3), place;

and 4 ) , change.

T h i s f o u r e v e n t sequence h a s been d i s c u s s e d e l s e w h e r e ( S m a l l e y 1966, 1978a) and i t has been p o i n t e d o u t t h a t t h e w i d e r a n g e o f ' l o e s s t h e o r i e s ' o c c u r r e d because i n v e s t i g a t o r s t e n d e d t o c o n c e n t r a t e t h e i r a t t e n t i o n o n one e v e n t , and t o n e g l e c t t h e r e s t .

O b v i o u s l y many l o e s s d e p o s i t s a r e f o r m e d a f t e r

a c o n s i d e r a b l y more complex h i s t o r y , b u t t h e r e a r e a s i g n i f i c a n t number o f m a j o r d e p o s i t s w h i c h f o r m e d a f t e r a v e r y modest e v e n t sequence.

llanecki

and D o m i n i k ( 1 9 7 2 ) have d e s c r i b e d a sequence o f e v e n t s f o r t h e Z w i e r z y n i e c loess i n Poland;

t h i s i s a f a i r l y simple d e p o s i t , p o s s i b l y what Smalley I,

and K r i n s l e y ( 1 9 8 1 ) w i s h e d t o c a l l ' U r l o s s ' .

C u t t s ( 1 9 7 3 ) has d e s c r i b e d

a sequence f o r M a r t i a n ' l o e s s ' , w h i c h has been d i s c u s s e d and d e v e l o p e d b y Smalley and K r i n s l e y ( 1 9 7 9 ) i n an a t t e m p t t o compare a e o l i a n d e p o s i t s on E a r t h and Mars.

The l o n g e s t e v e n t sequence has been p r o p o s e d f o r t h e T a s h k e n t

l o e s s ( S m a l l e y 1980) and as t h i s n i n e e v e n t sequence has a c r i t i c a l b e a r i n g on t h e d e s e r t l o e s s p r o b l e m i t w i l l be d i s c u s s e d l a t e r , i n t h e d e s e r t l o e s s

52 section.

An e l e v e n e v e n t sequence w i l l be p r o p o s e d f o r t h e l o e s s compr s i n g

t h e N o r t h China p l a i n . The q u e s t i o n t h a t a r i s e s , and f o r m s t h e m a j o r w h a t s t a g e do we i d e n t i f y l o e s s ? after the f i r s t

PO

n t o f t h i s paper i s

at

I s i t reasonable t o i d e n t i f y loess material

'make' e v e n t has o c c u r r e d r a t h e r t h a n w a i t i n g t o a p p l y t h e

d e s i g n a t i o n u n t i l a f t e r t h e d e p o s i t has f o r m e d .

T h i s a p p r o a c h may be neces-

s a r y ifwe a r e t o g i v e p r o p e r a t t e n t i o n t o t h e l o e s s a t t h e p e r i p h e r y o f t h e main event r e g i o n s .

I t i s no s u r p r i s e t h a t some o f t h e most s i g n i f i c a n t

p a p e r s i n w h i c h t h i s new a p p r o a c h becomes a p p a r e n t c o n c e r n l o e s s i n B r i t a i n a t t h e end o f t h e n o r t h European l o e s s band.

-

F o r example C a t t e t a1 ( 1 9 7 1 )

s t u d i e d t h e c o n t r i b u t i o n o f l o e s s m a t e r i a l t o t h e s o i l s o f N o r f o l k and were a b l e t o show t h a t i t was a m a j o r c o n s t i t u e n t , even t h o u g h t h e d e p o s i t i o n a l c h a r a c t e r i s t i c s were l a c k i n g .

T h e r e were enough m a t e r i a l c h a r a c t e r i s t i c s

and s u p p o r t i n g e n v i r o n m e n t a l knowledge t o a l l o w them t o s t a t e c o n f i d e n t l y t h a t t h e l o e s s , w h i c h had l o s t some c h a r a c t e r i s t i c s b y w e a t h e r i n g and o t h e r s o i l - f o r m i n g p r o c e s s e s , was t h e m a i n c o n s t i t u e n t o f t h e c o v e r l o a m . Thus we have l o e s s as a c o n s t i t u e n t as w e l l as l o e s s as a d e p o s i t , and i n p r a c t i c a l a n d economic t e r m s l o e s s as a c o n s t i t u e n t may be as i m p o r t a n t as l o e s s as a d e p o s i t . identified.

Two a s p e c t s o f l o e s s as a c o n s t i t u e n t need t o be

What C a t t e t a1 ( 1 9 7 1 ) d e s c r i b e d f o r N o r f o l k was a t h i n ( 7 0 cm)

l o e s s d e p o s i t w h i c h had been i n c o r p o r a t e d i n t o t h e l o c a l c o v e r l o a m

-

an

a c t u a l a e o l i a n e x i s t e d , w h i c h had s u b s e q u e n t l y l o s t i t s d e p o s i t i o n a l characteristics.

It seems r e a s o n a b l e t o i d e n t i f y a n o t h e r t y p e o f a d m i x t u r e

o r c o n s t i t u e n t system, one i n w h i c h occurred.

only

t h e i n i t i a l 'make' e v e n t has

Here we a r e s e e k i n g t o i d e n t i f y l o e s s m a t e r i a l w h i c h has n o t

reached ' d e p o s i t ' s t a t u s b u t which e x i s t s i n s u f f i c i e n t c o n c e n t r a t i o n s i n s o i l systems t o c o n t r i b u t e good s t r u c t u r e and n u t r i e n t s t a t u s .

There w i l l

e x i s t s o i l s w h i c h owe t h e i r h i g h a g r i c u l t u r a l v a l u e t o s l o e s s c o n s t i t u e n t w h i c h has o n l y e x i s t e d as l o e s s ' m a t e r i a l ' .

T h i s phenomenon w i l l be r e -

s t r i c t e d t o l o e s s produced by g l a c i a l a c t i o n . I n t h e e a r l y p a p e r s i n w h i c h e v e n t sequences f o r l o e s s d e p o s i t s were i d e n t i f i e d ( S m a l l e y 1966, S m a l l e y and V i t a - F i n z i , f o r t h e d e f i n a b l e e v e n t s was p r o p o s e d .

1968) a s i m p l e n o m e n c l a t u r e

The 'make' e v e n t s , t h o s e a s s o c i a t e d

w i t h t h e p r o d u c t i o n o f t h e b a s i c l o e s s m a t e r i a l were d e s i g n a t e d P e v e n t s

(P f o r p r o v e n a n c e ) ;

t h o s e a s s o c i a t e d w i t h t r a n s p o r t a t i o n were l a b e l l e d T

e v e n t s and t h o s e w i t h d e p o s i t i o n were e v e n t s b o t h had a

D

designation.

D events.

The ' p l a c e ' and ' c h a n g e '

I t i s j u s t p o s s i b l e t h a t we w i l l be a b l e

t o r e c o g n i z e n a t u r a l s y s t e m s i n w h i c h t h e number o f s i g n i f i c a n t l o e s s f o r m i n g e v e n t s may have any t o t a l r a n g i n g f r o m one t o t e n o r e l e v e n o r more; shows some s u g g e s t e d l i m i t s .

table 1

53 TABLE 1 Some o u t l i n e e v e n t sequences;

f o u r examples chosen t o i l l u s t r a t e

the possible variations i n complexity o f loess-deposit-forming s y s tems Loess

E v e n t Sequence

Chinese l o e s s : N o r t h C h i n a P l a i n Loess n e a r T a s h k e n t Loess i n N o r f o l k , E n g l a n d Loess m a t e r i a l . N. Canada

P1, T1, 01, T2, T3, D2, T4, D3, 04, T5, 05 P1, T1, D1, T2, T3, D2, T4, D3, D4 P1, T1, D1, D2 P1

THE FORMATION OF LOESS MATERIAL Smalley (1966) proposed t h a t g l a c i a l g r i n d i n g p r o v i d e d l o e s s m a t e r i a l f o r the bulk o f the world's loess deposits; p r o v i d e d by d i r e c t g l a c i a l a c t i o n . and V i t a - F i n z i

i.e.

t h e 'make' e v e n t was u s u a l l y

T h i s a s s e r t i o n was r e p e a t e d by S m a l l e y

( 1 9 6 8 ) and s u p p o r t e d b y B o u l t o n ( 1 9 7 8 ) who w r o t e :

" V i t a - F i n z i and S m a l l e y ( 1 9 7 0 ) have a r g u e d t h a t a m a j o r p r o p o r t i o n o f t h e

I w o u l d s u p p o r t them i n t h i s , and

w o r l d ' s s i l t i s produced by g l a c i e r s .

go f u r t h e r t o s u g g e s t t h a t m o s t o f t h i s i s p r o d u c e d i n t h e b a s a l zone o f t r a c t i o n which I b e l i e v e t o be a u n i q u e l y g l a c i a l environment i n which l a r g e forces a t non-inertial

s h e a r c o n t a c t s p r o d u c e f i n e - g r a i n e d wear p r o d u c t s . "

( B o u l t o n 1978, p. 796) R e c e n t l y a l t e r n a t i v e s o u r c e s have been e x p l o r e d and some s u g g e s t e d ' m a k e ' mechanisms s h o u l d be d i s c u s s e d h e r e .

The a r c h e t y p a l l o e s s p a r t i c l e can

be e n v i s a g e d as a 30 um q u a r t z p a r t i c l e and t h e make e v e n t has t o p r o d u c e l a r g e q u a n t i t i e s o f these;

t h i s i s t h e m a j o r make q u e s t i o n - how a r e t h e

predominant q u a r t z o a r t i c l e s produced?

Since t h e Smalley (1966) g l a c i a l

g r i n d i n g p r o p o s a l was made i t has been shown, l a r q e l y by Moss and cow o r k e r s (Moss and Green 1 9 7 5 ) b u t a l s o b y R i e z e b o s and Van d e r I i a a l s ( 1 9 7 4 ) , t h a t q u a r t z i n i g n e o u s r o c k s c o n t a i n s many d e f e c t s and f r a c t u r e s - w h i c h make i t r a t h e r weaker t h a n was o r i g i n a l l y e n v i s a g e d .

I n t h e Smalley (1966)

s c e n a r i o t h e o n l y n a t u r a l e n e r g y s o u r c e o f s u f f i c i e n t m a g n i t u d e t o Droduce l a r g e s c a l e f r a c t u r i n g o f s t r o n g q u a r t z was g l a c i a l a c t i o n .

We see now

t h a t t h e q u a r t z i s n o t s o s t r o n q as was o r i g i n a l l y t h o u g h t and t h u s , a l t h o u g h g l a c i a l g r i n d i n g s t a y s as e f f i c i e n t as i t e v e r was, o t h e r n a t u r a l f o r c e s may c r o s s t h e c r i t i c a l e n e r g y t h r e s h o l d and be a b l e t o p r o d u c e s i g n i f i c a n t q u a r t z breakage.

A c l e a r d i s t i n c t i o n s h o u l d be made between ' s i g n i f i c a n t ' b r e a k a g e w h i c h can o r o d u c e l a r g e amounts o f l o e s s m a t e r i a l , and 'some' b r e a k a g e w h i c h m i g h t F r o d u c e p a r t i c l e s on a s m a l l s c a l e .

G l a c i e r s produce s i g n i f i c a n t

b r e a k a g e and c a n a c c o u n t s a t i s f a c t o r i l y f o r t h e p a r t i c l e s u p p l y t o t h e N o r t h American and n o r t h European d e p o s i t s .

A l t e r n a t i v e p a r t i c l e producing

54 mechanisms t e n d t o a p D l y t o o t h e r o f t h e w o r l d ' s d e o o s i t s . t i c u l a r approaches s h o u l d be d i s c u s s e d ;

Three par-

t h o s e o f W h a l l e y e t a1 ( 1 9 8 2 ) ,

Nahon and T r o m p n t t e ( 1 9 8 2 ) and Goudie e t a1 ( 1 9 7 9 ) .

A l l look towards

l o e s s p r o d u c t i o n away f r o m g l a c i a l r e g i o n s , and p r o p o s e p a r t i c l e p r o duction alternatives t o glacial grinding. W h a l l e y e t a1 ( 1 9 8 2 ) f a v o u r sand g r a i n i m p a c t as t h e f o r m a t i o n iiiechanisrii. They c o l l e c t e d t h e p r o d u c t s o f sand a b r a s i o n e x p e r i m e n t s and o b s e r v e d t h e presence o f loess-sized p a r t i c l e s .

They c o n c l u d e d t h a t an a e o l i a n

a b r a s i o n a l o r i g i n f o r some l o e s s i c m a t e r i a l seemed p o s s i b l e , and i n p a r t i c u l a r t h a t t h e Gobi d e s e r t c o u l d s u p p l y m a t e r i a l f o r t h e Chinese loess.

T h e i r abrasion experinients produced s i g n i f i c a n t l y d i f f e r e n t re-

s u l t s f r o m t l i o s e o f Kuenen ( 1 9 6 0 ) who f o u n d t h a t l o e s s - s i z e d m a t e r i a l was n o t produced.

Nahon and T r o m p e t t e ( 1 9 8 2 ) s u g g e s t e d t h a t c h e m i c a l w e a t h e r i n g ,

p a r t i c u l a r l y i n t r o p i c a l a r e a s , i s an a c t i v e a g e n t o f s i l t f o r m a t i o n . Ideak i i i i n e r a l s d i s a p p e a r b y i n c o n g r u e n t d i s s o l u t i o n and q u a r t z i s f r a g m e n t e d by p a r t i a l c h e m i c a l d i s s o l u t i o n .

The s o r t i n g o f t h e d e t r i t a l f r a c t i o n ,

e s s e n t i a l l y s i l t , may be i n i t i a t e d i n s o i l s by t h e p e d o g e n e t i c s e p a r a t i o n o f c l a y and t h e p r o g r e s s i v e d i s s o l u t i o n o f t h e cement.

I n t h i s way i m -

p o r t a n t c o n c e n t r a t i o n s o f s i l t on t h e s u r f a c e a r e o b t a i n e d by s e l e c t i v e r e 1 a t i ve accumul a t io n .

The Goudie e t a1 ( 1 9 7 9 ) mechanism i s a l s o t r o p i c a l

and c h e m i c a l - s a l t we t h e r i n g by s o d i u m s u l p h a t e

-

and t h e i r e x p e r i m e n t s

d i d p r o d u c e sand g r a i n b r e a k a g e and i n d i c a t e a p o s s i b l e s o u r c e o f some loess-sized particles G o u d i e e t a1 ( 1 9 7 9 )

Pye and S p e r l i n g , 1 9 8 3 ) . i n t h e i n t r o d u c t i o n t o t h e i r p a p e r , c i t e d as

f r e q u e n t l y f a v o u r e d p a r t i c l e p r o d u c t i o n mechanisms g l a c i a l g r i n d i n g ( S m a l l e y 1 9 6 6 ) , i n s o l a t i o n w e a t h e r i n g , f r o s t a c t i o n ( Z e u n e r 1949) and s p a l l i n g d u r i n g w i n d t r a n s p o r t ( S m a l l e y and V i t a - F i n z i 1 9 6 8 ) . second o f t h e s e , f r o s t w e a t h e r i n g , f o r glacial grinding.

The

now l o o k s l i k e an e f f i c i e n t complement

The o r i g i n a l o b s e r v a t i o n s b y Zeuner ( 1 9 4 9 ) have

been s u p p o r t e d b y B r o c k i e ' s ( 1 9 7 3 ) i n v e s t i g a t i o n s i n F!ew Z e a l a n d and i t appears t h a t c o l d w e a t h e r i n g p r o c e s s e s m u s t now be c o n s i d e r e d as a m a j o r method f o r p r o d u c i n g l o e s s m a t e r i a l .

I n f a c t c o l d m o u n t a i n s o u r c e s can

be seen as s u p p l y i n g m a t e r i a l f o r w h a t were t h o u g h t o f as ' d e s e r t ' l o e s s d e p o s i t s , and a s i m p l e d i s t i n c t i o n o f l o e s s i n t o e i t h e r ' i c e - s h e e t ' o r ' m o u n t a i n ' may c o v e r t h e i i i a j o r o c c u r r e n c e s ( S m a l l e y 1 9 7 8 b ) . The p a r t i c l e p r o d u c t i o n p r o b l e m needs more i n v e s t i g a t i o n .

The e a r l i e r

g e n e r a l i z a t i o n s a r e becoming more r e f i n e d and a r e b e i n g i n c o r p o r a t e d i n t o comprehensive e v e n t sequence models f o r t h e f o r m a t i o n o f s p e c i f i c d e p o s i t s , b u t more knowledge o f t h e a c t u a l p a r t i c l e f o r m i n g mechanism i s r e q u i r e d , and t h e r e a r e o t h e r h i g h e n e r g y e n v i r o n m e n t s t h a t s h o u l d be i n v e s t i g a t e d .

55 Palmer (1982) has shown t h a t high energy r i v e r s a s s o c i a t e d with deformable rocks can produce l o e s s - s i z e m a t e r i a l and i t s e e m l i k e l y t h a t in New Zealand, a country o f s t e e p r i v e r s , a r e a s o n a b l e p r o p o r t i o n o f l o e s s m a t e r i a l cotlies from t h i s s o u r c e . B u t f r o s t w e a t h e r i n g must c e r t a i n l y be seen a s a iiiajor rnechanisni f o r s u p p l y i n g l o e s s m a t e r i a l . Experimental i n v e s t i g a t i o n s c a r r i e d o u t o v e r t h e l a s t f i f t e e n y e a r s a t t h e CPlRS Centre de Georiiorphologie in Caen have r e c e n t l y been reviewed and suiiimarized ( L a u t r i d o u and Ozouf 1982) a n d i t i s c l e a r t h a t m a t e r i a l o f t h e right s i z e and iiiineralogy can be pro-

duced, i n t h e r e q u i r e d amounts, by f r o s t a c t i o n . SIZE LIMITS AN0 DEFINITIONS S i z e has always been a d e f i n i n g c h a r a c t e r i s t i c of l o e s s , but i f we a r e going t o eiiiphasise t h e m a t e r i a l c h a r a c t e r i s t i c s , t h e l i m i t s need t o be more c a r e f u l l y d e f i n e d .

A major problem has a r i s e n with t h e use o f t h e

term ' s i l t ' , and i n p a r t i c u l a r w i t h t h e way i n which s i l t i s simply used as a s i z e terrn t o f i l l t h e s p a c e between c l a y and sand.

Both c l a y a n d sand

do have c e r t a i n g e o l o g i c a l p r o c e s s e s r e f l e c t e d i n t h e i r c h a r a c t e r i s t i c s i z e s b u t t h e s i l t term l a c k s t h i s j u s t i f i c a t i o n and i t has a very v a r i a b l e range i n t h e v a r i o u s e x t a n t s i z e s c a l e s .

For example i n t h e ISSS ( I n t e r -

n a t i o n a l S o c i e t y o f S o i l S c i e n c e ) system s i l t i s between 2 um and 20 Wni and l o e s s a t 20-60 uin would be c l a s s i f i e d a s a f i n e s a n d .

B u t a 4 uri

p a r t i c l e which would be s i l t on the ISSS system i s c l a y on t h e U.S. of S o i l s s y s t e m .

Bureau

The c h o i c e o f a s i z e range f o r l o e s s m a t e r i a l has t o be a somewhat a r b i t r a r y p r o c e s s - b u t n o t i n the same way t h a t naming a l l p a r t i c l e s i z e classes i s ;

a f t e r a l l we do have a l o t o f accumulated s i z e a n a l y s i s d a t a .

We propose t h a t t h e 20-60

pm

range could be c a l l e d l o e s s - s i z e .

Browzin

(1981) has r e c e n t l y argued f o r a 10-50 urn ' l o e s s - s i z e ' range and t h e r e a r e c e r t a i n h i s t o r i c a l a s p e c t s which can be c i t e d i n s u p p o r t o f t h i s p a r t i c u l a r r a n g e . Russell ( 1 9 4 4 ) , who made c o n s i d e r a b l e e f f o r t s t o produce

a r i g o r o u s d e f i n i t i o n o f l o e s s , p l a c e d h i s s i z e l i m i t s a t 10 and 50 urn; and i f i t ( l o e s s ) i s t o be t h o u g h t o f a s a s i l t y d e p o s i t i t i s c o n v e n i e n t

t h a t t h e s i l t r a n g e s o f t h e U.S. Department o f A g r i c u l t u r e and t h e U.S. Bureau o f S o i l s have upper bounds a t 50 urn. The b e s t reason f o r p u t t i n g t h e lower bound a t 20 um ( r a t h e r than a t 10 um) i s t o o b t a i n t h e maximum d i s t a n c e from t h e s i z e range i n which f i n e a e r o s o l i c d u s t i s found, s a y < l o um. A range o f 20-60 urn p r o v i d e s a b e t t e r s e p a r a t i o n from o t i i e r , m a t e r i a l s than a 10-50 pm range. Manecki e t a1 (198C)) favoured t h e 20-60 urn. r a n g e , which t h e y c a l l e d t h e ' l o e s s i a l ' f r a c t i o n and t h i s seems a range d e s e r v i n g o f some s u p p o r t , a l t h o u g h i t i s f o r ' t a c t i c a l ' r a t h e r than s c i e n t i f i c r e a s o n s t h a t i t might be favoured o v e r t h e 10-50 urn

56

range. The Smalley a n d Vita-Finzi (1968) d e f i n i t i o n of loe ss was: 'Loess i s a c l a s t i c deposit which c o n s i s t s predominantly of quartz part i c l e s 20-50 urn i n diameter a n d which occurs as wind l a i d s h e e t s . '

his

has l a s t e d f a i r l y well and i s s t i l l an acceptable b r i e f d e f i n i t i o n .

Siiia 1 1 ey

and Vita-Finzi (1968, p . 766) quoted a d e f i n i t i o n by Pe ttijohn (1957 p . 377-8) a n d i t i s timely here t o quote a more recent d e f i n i t i o n from a l a t e r e dition o f t h e same work;

t h i s i s l o es s defined within a s t r i c t l y sediniento ogical

framework (PettL john 1975, p . 290-1 ) : "Loess i s an unconsolidated porous s i l t , commonly buff in colour ( l o c a l l y gray, yellow, brown o r r e d ) , ch ar act er i zed by i t s lack of s t r a t i f i c a t i o n and remarkable a b i l i t y t o stand in a v e r t i c a l s lope .

I t conimonly shows a crude columnar s t r u c t u r e . I t i s generally highly calcareous a n d e f f e r v e sc e s in weak aci d . Loess i s e s s e n t i a l l y a s i l t . " THE DESERT LOESS PROBLEM The d e s e r t l o e ss problem begins witJi Obruchev (1911). He f i r s t proposed, on th e b a s i s of f i e l d observations made in the Central Asian d e s e r t s , t h a t t h e r e e x i s t e d a s e p a r a t e , d ef i n ab l e ' h o t ' l o es s which was produced in h o t (sandy) d e s e r t s . The l o es s regions of Central Asia, and those of China, do e x i s t next t o d e s e r t s a n d t h i s appeared t o give support t o the d e s e r t lo e s s hypothesis. Butler (1956) however, a f t e r a careful consideration of the A u s t r a l i a n s i t u a t i o n suggested t h a t "considering the vast areas o f d e s e r t s i n t h e world, and o u r r e l a t i v e ignorance of ' h o t ' l o e s s , the l a t t e r may be more hypothetical t h a n real . " The Butler suggestion provoked Sniallcy and Vita-Finzi (1968) i n t o a survey o f d es er t l oe ss a n d they concluded t h a t th e r e does not e x i s t a s p e c i f i c and e f f i c i e n t mechanism which can produce lo e s s p a r t i c l e s i n h o t d e s e r t environments. Fine p a r t i c l e s , usually l e s s t h a n 10 i n diameter, a r e produced i n d e s e r t s b u t these a r e c a rrie d long d is t a n c e s i n suspension and do n o t form l o es s d eposits. The g r e a t bulk of S a h a r a n d u s t i s of t h i s type a n d i s car r i ed o u t over the A t l a n t i c - the Sahara i s not a producer of l o es s p a r t i c l e s in s i g n i f i c a n t amounts - as Penck (1930 observed long ago. Recently Gouc'ie e t a1 (1979) have proposed p a r t i c l e formation by s a l t weathering a n d lihalley e t a1 (1392) have produced some loe ss-siz e d material by sand grain impacts. The most convincing s t u d i e s of d e s e r t loess f o r mation have been those by Yaalon and coworkers in the '!egev ( e . g . Yaalon and Dan 1974). They have been ab l e t o show t h a t some loe ss p a r t i c l e s a r e produced i n t h e Sinai d e s e r t and c a r r i e d up t o form de posits and contribut i o n s t o s o i l s in t h e Negev a n d o t h e r p a r t s o f I s r a e l . I t i s i n t e r e s t i n g t o note t h a t the p a r t i c l e s a r e formed i n t h e rocky p a r t s of the Sinai de se rt

57

by d i r e c t w e a t h e r i n g o f exposed rocks and t h a t t h e o t h e r p a r t s of t h e d e s e r t do n o t produce l o e s s p a r t i c l e s .

w i l l produce ments;

I t thus seeins l i k e l y t h a t v a r i o u s p r o c e s s e s

modest amounts o f l o e s s s i z e d p a r t i c l e s i n hot d e s e r t e n v i r o n -

t h e q u e s t i o n remains - can t h e s e p r o c e s s e s produce enouqh m a t e r i a l

t o form a

III~J>~ deposit?

Wlialley e t a1 (1982) s u g g e s t e d t h a t t h e impact

mechanism n i g h t c o n t r i b u t e s u b s t a n t i a l amounts o f iiiaterial to t h e Chinese loess d e p o s i t s ;

t h e p a r t i c l e formation p r o c e s s e s o c c u r r i n g i n t h e Gobi

d e s e r t and being e s s e n t i a l l y t h e same a s t h o s e d e s c r i b e d by Sinalley a n d

The most t e l l i n g argument a g a i n s t t h e impact p r o c e s s being a major producer o f l o e s s m a t e r i a l i s t h e l a c k of t h e r e l e v a n t 20-60 Ilm p a r t i c l - s i n t h e p r o d u c t s of t h e P!orth A f r i c a n d e s e r t s . Major Vita-Finzi ( 1 9 6 8 ) .

s t u d i e s on Saharan d u s t have been c a r r i e d o u t r e c e n t l y (Morale8 1979) and t h e r e i s no i n d i c a t i o n t h a t t h e major d u s t p r o d u c t has a mode s i z e o t h e r

t h a n in t h e expected 200,000km2 o r 77%) i s nongla c ia l,

92 so t h a t s e d i m e n t d e r i v e d f r o m b e d r o c k o r s o i l s f o r m e d i n b e d r o c k p r o v i d e d a l l u viurn t o t h e O h i o R i v e r v a l l e y .

C o n s e q u e n t l y , t h e s o u r c e m a t e r i a l f o r l o e s s was

composed o f n o n g l a c i a l a l l u v i u n i as w e l l a s g l a c i a l o u t w a s h .

The s o i l s i n t h e

n o n g l d c i a l r e g i o n a r e I n c e p t i s o l s , A l f i s o l s , and U l t i s o l s ( S o i l S u r v e y S t a f f ,

1975).

I n c e p t i s o l s a r e weakly weathered s o i l s w i t h few d i a g n o s t i c pedologic

features.

A l f i s o l s a r e moderately weathered s o i l s w i t h high-base s t a t u s t h a t

formed under f o r e s t .

U l t i s o l s a r e s t r o n g l y weathered s o i l s w i t h low-base s t a t u s

t h a t f o r m e d u n d e r f o r e s t on g e n e r a l l y g e o l o g i c a l l y o l d e r l a n d s c a p e s .

These

s o i l s o c c u p y a l l t h e t r i b u t a r y d r a i n a g e b a s i n s t h a t d r a i n t o t h e O h i o R i v e r from t h e Green R i v e r i n w e s t e r n K e n t u c k y , e a s t w a r d t h r o u g h West V i r g i n i a , and n o r t h ward i n c l u d i n g t h e A l l e g h e n y R i v e r i n w e s t e r n P e n n s y l v a n i a .

These s o i l s a l s o

o c c u r i n s o u t h e r n p a r t s o f w a t e r s h e d s a c r o s s O h i o and s o u t h e r n I n d i a n a . Wisconsin l o e s s covers a l a r g e p a r t o f t h e uplands i n t h e Ohio drainage b a s i n and i s >5 in t h i c k j u s t e a s t o f t h e Wabash R i v e r v a l l e y i n s o u t h w e s t e r n I n d i a n a and a l s o w e s t e r n K e n t u c k y .

E a s t w a r d i n I n d i a n a and K e n t u c k y and i n

Ohio, t h e l o e s s g e n e r a l l y i s 1 t o 2

m thick.

I n t h e Wisconsin d r i f t r e g i o n the

l o e s s b u r i e s g l a c i a l t i l l o r s t r a t i f i e d d r i f t w i t h o u t an i n t e r v e n i n g p a l e o s o l . On u p l a n d summits i n t h e I l l i n o i a n d r i f t and n o n g l a c i a l r e g i o n s , t h e l o e s s b u r i e s p a l e o s o l s f o r m e d i n t i l l o r s t r a t i f i e d d r i f t o r m a t e r i a l s d e r i v e d froin them o r p a l e o s o l s f o r m e d i n b e d r o c k o r i n i a t e r i a l s d e r i v e d from i t .

A p r o b l e m a r i s e s i n t h e t h i n - l o e s s r e g i o n when c l a y m i n e r a l s a r e used as i n d i c a t o r s o f s o u r c e s o f l o e s s o r as i n d i c a t o r s o f t h e d i s t r i b u t i o n of t h e s e d i ment w i t h i n t h e l o e s s - d i s p e r s i o n system.

Where l o e s s i s t h i n , s o i l s have formed

t h r o u g h o u t t h e l o e s s and merge downward w i t h t h e b u r i e d s o i l s f o r m e d i n t h e substrata.

T h i s phenomenon i s t e r m e d s o i l weldiizc; (Ruhe and O l s o n , 1 9 8 0 a ) .

To

f u r t h e r c o m p l i c a t e t h e d u a l s o i l - s t r a t i g r a p h i c system, a f r a g i p a n (Grossman and C a r l i s l e , 1969) i s formed i n t h e l o w e r p a r t o f t h e l o e s s j u s t above t h e b u r i e d s o i l (Ruhe and O l s o n , 1 9 8 0 a ) .

The l o e s s - d e r i v e d s o i l m o r p h o l o g i c a l l y a p p e a r s t o

be s t r o n g l y d e v e l o p e d and a l s o a p p e a r s t o be s t r o n g l y w e a t h e r e d . a r e v e r y s t r o n g l y a c i d i c w i t h pH v a l u e s r a n g i n g f r o m 3 t o 5.3. then,

The s u b s o i l s The p r o b l e m ,

i s w h a t c l a y m i n e r a l s r e p r e s e n t o r i g i n a l components o f t h e l o e s s s e d i m e n t ,

o r what c l a y m i n e r a l s a r e a l t e r e d products o f s o i l weathering. attacked i n t h i s study.

T h i s problem i s

I n t h i c k l o e s s t h e p r o b l e m c a n be a v o i d e d by s t u d y i n g

t h e c l a y m i n e r a l s i n t h e l o e s s - d e r i v e d s o i l s and t h e u n d e r l y i n g l o e s s p a r e n t m a t e r i a l i n c l u d i n g r e l a t i v e l y unweathered calcareous l o e s s .

METHODS S o i l c o r e s were e x t r a c t e d i n t h e f i e l d w i t h a t r u c k - m o u n t e d h y d r a u l i c s o i l c o r i n g machine.

A l l s i t e s were l o c a t e d o n u p l a n d summits t h a t were e s s e n t i a l l y

f l a t o r w i t h l o w - s l o p e g r a d i e n t so t h a t p o s t - l o e s s e r o s i o n w o u l d b e m i n i m a l . c o r e s c o m p l e t e l y p e n e t r a t e d t h e l o e s s and l o e s s - d e r i v e d s o i l and e x t e n d e d

All

93 downward i n t o the s u b s t r a t u m p a l e o s o l s and where p o s s i b l e i n t o t h e p a r e n t m a t e r i a l s of t h e b u r i e d s o i l s .

The p e d o l o g i c , l i t h o l o g i c , and s t r a t i g r a p h i c niorphologies

of each c o r e were d e s c r i b e d and measured and s e r v e d a s t h e b a s i s f o r sampling for laboratory s t u d i e s .

F i f t y - s i x s i t e s were c o r e d :

Kentucky, and 8 i n Ohio ( F i g . 1 ) .

31 i n I n d i a n a , 17 i n

The number of samples p e r c o r e v a r i e d from

1 5 t o 33 and averaged a b o u t 20. Each sample of each c o r e was a n a l y z e d i n the l a b o r a t o r y f o r p h y s i c a l , chemic a l , and m i n e r a l o g i c p r o p e r t i e s u s i n g methods of t h e U. S . National S o i l Survey Laboratory ( S o i l Survey S t a f f , 1 9 7 2 ) . P a r t i c l e s i z e was determined by t h e pipet method, and s i z e s e p a r a t e s were sand (>62pm), c o a r s e s i l t (62-16pm), f i n e s i l t (16-2pm), and c l a y ( 0

m

EARTH CASE

10

3 5 0 ptn Q U A R T Z P A R T I C L E S

4

v

I-

r

?!

urnr 1 0 . 5 r n / s

5

W

r I

I

I

I

1

I

I

MEAN PARTICLE VELOCITY ( m / s ) 1

I

I

0 20 40 60 80 100 M E A N P A R T I C L E V E L O C I T Y (%urn)

Fig. 7. P a r t i c l e v e l o c i t i e s f o r 350 um q u a r t z g r a i n s measured a t f o u r h e i g h t s above t h e s u r f a c e i n a wind t u n n e l a t 1 b a r p r e s s u r e and a f r e e s t r e a m wind speed o f 10.5 m/s (u, = 0.5 m/s). Figure

10 shows

typical

results for

speeds i n c r e a s e w i t h h e i g h t above with the

p a r t i c l e velocimeter, although

f i l m i s limited.

E a r t h cases;

t h e surface

in

general, p a r t i c l e

similar t o the

t h e range

r e s u l t s obtained

i n the heights

analyzed by

A t t h i s t i m e , i d e n t i c a l experiments have n o t been r u n t o com-

pare r e s u l t s f r o m t h e v e l o c i m e t e r w i t h t h o s e f o r t h e m o t i o n p i c t u r e s . shows

r e s u l t s o b t a i n e d i n t h e Venus

tures.

Wind Tunnel

f r o m analyses o f

Although t h e a b s o l u t e v e l o c i t i e s a r e v e r y low

(a r e f l e c t i o n of t h e low

wind speeds on Venus), n o t e t h a t t h e p a r t i c l e s a c h i e v e n e a r l y t h e as f r e e s t r e a m

wind speed,

in

marked c o n t r a s t

t o Mars

F i g u r e 11 motion p i c -

where

same v e l o c i t y

p a r t i c l e s seldom

reach f r e e s t r e a m v e l o c i t i e s . I n a d d i t i o n , p a r t i c l e v e l o c i t i e s on r i s i n g , f l a t , and were assessed (Fig. Earth increase

12).

throughout

falling trajectories

As p r e d i c t e d by White (1979), p a r t i c l e v e l o c i t i e s the saltation

b e f o r e impact w i t h t h e s u r f a c e ,

t r a j e c t o r y , reaching

a

on

maximum j u s t

142

25 A

E 0

20

W

0

U

u. U 3

15

ln W

> 0

m U

MARS CASE

10

3 5 0 p m SHELL PARTICLES

-

c

u

I

a -

6 5 m/ s

5

W

I I

(

I

1

I

1

0

I

1

I

1 35

I

30 20 25 15 10 MEAN PARTICLE VELOCITY (m/s)

5

I 50

I

I

10 20 30 40 M E A N P A R T I C L E V E L O C I T Y (%Urn)

Fia. 8. P a r t i c l e v e l o c i t i e s f o r f o u r d i f f e r e n t h e i a h t s above t h e s u r f a c e for 356 w a l n u t s h e l l p a r t i c l e s s u b j e c t e d t o a f r e e s t r e a m wind speed o f 65 m/s (u, = 3.4 m/s) i n a low atmospheric p r e s s u r e (6.6 mb) t o s i m u l a t e t h e martian e n v i ronment. 3.0

FIELD STUDIES Although l a b o r a t o r y s i m u l a t i o n s e n a b l e a e o l i a n processes t o be i n v e s t i g a t e d

under c o n t r o l l e d c o n d i t i o n s , q u e s t i o n s the results

as a p p l i e d t o n a t u r a l c o n d i t i o n s .

t i o n s , a f i e l d experiment was particles.

inevitably arise

conducted t o

I n order

v a l i d i t y of

as t o t h e

t o address such ques-

obtain v e l o c i t y data

f o r saltating

The experiment was conducted 3 November 1981 a t Waddell Creek State

Beach i n C a l i f o r n i a , about 90

km s o u t h o f San Francisco.

The beach

i s one o f

t h e w i n d i e s t on t h e c o a s t and i s f a i r l y wide, e n a b l i n g a good s a l t a t i o n c l o u d t o develop across

t h e dry

p a r t o f the

beach b e f o r e

r e a c h i n g t h e area

where the

f i l m i n g t o o k place. Using t h e same i n s t r u m e n t s as ( a t a h e i g h t o f 1.0 m above Greeley e t al., taneously.

t h e surface), p a r t i c l e f l u x (using

1982), and h i g h

The p a r t i c l e

wind v e l o c i t y

employed i n t h e w i n d t u n n e l s ,

speed m o t i o n p i c t u r e s were a l l

c o l l e c t o r s , see o b t a i n e d simul-

v e l o c i m e t e r m a l f u n c t i o n e d and d a t a were

w i t h i t i n t h e f i e l d experiment.

n o t obtained

143

92vm

350um 130

120

Siia

\

E

10(

k

0 0 A

9c

W

> n

z

3 z a

W

8C

7C

a

+ W W

6(

U LL

5c 1

I

5

10

1

1

I

1

20

25

30

35

I

15

PARTICLE VELOCITY ( Y O D E , m / a )

Fig. 9. S t a t i s t i c a l modes o f v e l o c i t i e s as a f u n c t i o n o f f r e e s t r e a m wind speed a t a h e i g h t o f 7.1 cm above t h e s u r f a c e f o r 350 urn and 92 urn s h e l l p a r t i c l e s i n a m a r t i a n s i m u l a t i o n ; p a r t i c l e v e l o c i t y remains c o n s t a n t o v e r a wide range of freestream wind speeds. Four s u c c e s s f u l r u n s were completed, hold.

i n t h e wind t u n n e l runs. is

a l l a t wind speeds j u s t

The m o t i o n p i c t u r e s were analyzed f o l l o w i n g t h e

somewhat d i f f e r e n t

above t h r e s -

same procedures as used

F i g u r e 13 shows t h e r e s u l t s ; a l t h o u g h t h e f r o m t h o s e used

i n the

wind t u n n e l runs,

grain size the particle

v e l o c i t y d i s t r i b u t i o n s a r e w i t h i n t h e range expected.

4.0

SUMMARY AND CONCLUSIONS

Velocities o f speed,

windblown

h e i g h t above t h e

p a r t i c l e s were

ground, and

d e t e r m i n e d as

p a r t i c l e diameter f o r

functions

of wind

various conditions

s i m u l a t i n g Earth, Mars, and Venus i n e n v i r o n m e n t a l wind t u n n e l s .

S i m i l a r data,

although o f l i m i t e d range, were o b t a i n e d f r o m a f i e l d experiment

f o r comparison

w i t h t h e wind t u n n e l r e s u l t s s i m u l a t i n g t h e t e r r e s t r i a l environment.

144

-

-

EARTH CASE

E

-

ASU WIND TUNNEL

0

W

0

a Y

U 3 UJ W

> 0

m

a I-

I

-

Q W

I

I

'0

I

I

I

I

I

J

1

5 6 7 3 4 2 MEAN PARTICLE VELOCITY (m/s)

1 I

I

I

I

I

1

I

70

80

I

20 30 40 50 60 MEAN PARTICLE VELOClTY

10

8

(%Urn)

F i g . 10. P a r t i c l e v e l o c i t i e s o b t a i n e d f r o m a n a l y s e s of h i g h speed m o t i o n p i c t u r e s f o r 400 q u a r t z p a r t i c l e s s u b j e c t e d t o a f r e e s t r e a m w i n d speed of 10.5 m/s (u, = 0.55 m / s ) i n t h e 1.0 b a r a t m o s p h e r i c w i n d t u n n e l a t A r i z o n a S t a t e Un iv e r s i t y

.

I n g e n e r a l , t h e r e s u l t s show

that particles

travel a t higher

speeds w i t h

i n c r e a s e d h e i g h t above t h e ground, and t h a t s m a l l e r p a r t i c l e s t r a v e l f a s t e r t h a n l a r g e r ones. not increase

However, f o r a g i v e n h e i g h t above t h e ground, p a r t i c l e speed does with higher

t h i s i s with reference t o

f r e e s t r e a m w i n d speeds.

modal v a l u e s

and

I t must

be remembered t h a t

n o t t o maximum v a l u e s ; a

few p a r t i -

c l e s do i n c r e a s e i n v e l o c i t y w i t h f r e e s t r e a m w i n d v e l o c i t i e s . Comparisons o f r e s u l t s f o r

E a r t h , Mars,

and Venus r e v e a l

some r e m a r k a b l e

differences.

As shown i n F i g u r e 14, most p a r t i c l e s a c h i e v e speeds n e a r l y equal

t o freestream

w i n d speed o n Venus, b u t

seldom a c h i e v e

Mars; E a r t h c a s e s a r e o f i n t e r m e d i a t e v a l u e s . ences

i n a t m o s p h e r i c d e n s i t y and t o

p l a n e t a r y e n v i r o n m e n t s ( T a b l e 1). V e n u s i a n atmosphere t h a n on and f o r t h e (just

t h e t h r e s h o l d w i n d speeds among

speeds), t h e

speed on the three

P a r t i c l e s a r e more e a s i l y moved i n t h e dense

Mars; c o n s e q u e n t l y , t h r e s h o l d speeds a r e

r a n g e o f w i n d speeds i n

above t h r e s h o l d

h a l f t h e wind

This i s attributed t o the d i f f e r -

w h i c h most grains

movement i s presumed

need n o t

be m o v i n g

very

v e r y low, t o occur fast to

145

-

3

-

VENUS CASE

E

VENUS WIND TUNNEL

0 Y

5 0 0 - 6 0 0 p m QUARTZ PARTICLES

W

0

__t_

ucO = 3.621111s

U

u . 2 K 3 v)

W

> 0

m

-

u 1

-

I-

-

r

-

0

-

W

I I

0

I

1

,

-

I

I

1

1

I

I

I

I

I

I

I

1

60

l

1

1

I

4.0 I

I

I

l

3.5

2.5 3.0 MEAN PARTICLE VELOCITY (m/s)

0

80 90 70 MEAN PARTICLE VELOCITY (%urn)

100

Fig. 11. P a r t i c l e v e l o c i t i e s o b t a i n e d f r o m a n a l y s i s o f h i q h speed m o t i o n p i c t u r e s f o r 500 t o 600 pm q u a r t z p a r t i c l e s s u b j e c t e d t o a f r e e s t r e a m w i n d speed o f 3.62 m/s (u, = 0.2 m/s) a t 30 b a r s a t m o s p h e r i c p r e s s u r e i n t h e Venus Wind Tunnel. achieve

100% o f t h e w i n d speed.

C o n v e r s e l y , p a r t i c l e s on Mars must a c c e l e r a t e

very r a p i d l y t o a c h i e v e t h e speed o f t h e h i g h w i n d s r e q u i r e d f o r despite

the f a c t t h a t s a l t a t i o n path

lengths are

l o n g on Mars

most g r a i n s f a l l t o t h e s u r f a c e b e f o r e a c h i e v i n g even 50-60% o f

t h r e s h o l d , and ( W h i t e , 1979), freestream wind

speed. F u t u r e e x p e r i m e n t s w i l l i n v o l v e e x p a n s i o n o f t h e d a t a base t o a w i d e r r a n g e o f wind-tunnel

and f i e l d c o n d i t i o n s and f o r a w i d e r r a n g e o f p a r t i c l e d i a m e t e r s ,

wind speeds, and measurements f o r

v a r i o u s h e i g h t s above t h e s u r f a c e .

tion, a v e l o c i m e t e r i s under development t h a t w i l l be f i e l d - p o r t a b l e obtain

a l a r g e r number

of velocity

measurements t h a n a r e

I n addii n order t o

presently available

from h i g h - s p e e d m o t i o n p i c t u r e s . ACKNOWLEDGEMENTS A l l a s p e c t s o f t h i s work

were s u p p o r t e d

by t h e P l a n e t a r y

Geology O f f i c e ,

N a t i o n a l A e r o n a u t i c s and Space A d m i n i s t r a t i o n . We t h a n k t h e f o l l o w i n g i n d i v i d u a l s f o r t h e i r c o n t r i b u t i o n t o D.

B a l l f o r p h o t o g r a p h i c s u p p o r t , G.

Kuhl f o r d a t a r e d u c t i o n , and

Beardmore,

P.

Parkes, K.

t h i s study:

Malone,

and D.

R. Leach f o r a s s i s t a n c e i n o b t a i n i n g t h e w i n d t u n -

146 n e l d a t a and development o f t h e p a r t i c l e v e l o c i m e t e r . i n the i n i t i a l

arrangements f o r

Alan P e t e r f r e u n d a s s i s t e a

t h e f i e l d experiment.

R. Leach, M.

We thank

M a l i n , and K. Gerety f o r h e l p f u l comments on t h e manuscript. 60

f0

501

DESCENDING

0

6

4

2

8

10

PARTICLE VELOCITY (m/s) I

0

I

10

, 20

I

30

I

I

40

50

I

60

PARTICLE VELOCITY

70

80

I

I

90

100

(%Urn)

Fig. 12. P a r t i c l e v e l o c i t i e s o b t a i n e d f r o m a n a l y s i s o f high-speed m o t i o n p i c t u r e s f o r 500 t o 600 vm q u a r t z p a r t i c l e s s u b j e c t e d t o a f r e e s t r e a m wind speed of 10.5 m/s (u, = 0.5 m / s ) a t one b a r atmospheric pressure, comparing v e l o c i t i e s f o r g r a i n s on t h e r i s i n g , h o r i z o n t a l , and descending p a r t s o f t h e i r t r a j e c We p r e s e n t l y have no e x p l a n a t i o n f o r t h e bimodal v e l o c i t y d i s t r i b u t i o n tories. o f g r a i n s on t h e h o r i z o n t a l p a r t o f t h e t r a j e c t o r y .

147

-Eo

-

12

4

-

EARTH CASE FIELD EXPERIMENT WADDELL BEACH, CA

W

0

-

10

8

LL

-

Dp

__t_

= 300pm

a a m

6

W

> 0

m 4

W

I

4

2 i 1

0

3

2

5

4

MEAN PARTICLE VELOCITY (m/S) I

I

1

1

I

I

I

I

I

0

10

20

30

40

50

60

70

80

M E AN P A R T I C L E V E L 0 C I T Y ( % uoo)

Fig. 13. P a r t i c l e v e l o c i t i e s o b t a i n e d f r o m a n a l y s i s o f h i g h speed m o t i o n p i c t u r e s f o r n a t u r a l beach sands o f -300 um d i a m e t e r s u b j e c t e d t o a wind speed o f -6 m/s measured a t a h e i g h t o f 1 m above t h e s u r f a c e a t Waddell Creek S t a t e Beach, C a l i f o r n i a .

148

-E 0

30

25

w

0 U

;2 0 3 v)

w 15

> 0

10 I-

I Q

i i 5 r

0

I

I

I

I

I

I

I

I

I

I

10

20

30

40

50

60

70

80

90

100

P A R T I C L E V E L O C I T Y (MODE,%u,)

F i g . 14. Comparison o f v e l o c i t i e s on E a r t h , Mars, and Venus. Two s i z e s o f p a r t i c l e s , each s u b j e c t e d t o r e l a t i v e l y l o w and h i g h w i n d speeds, were t e s t e d under E a r t h and Mars c o n d i t i o n s ; one s i z e p a r t i c l e was t e s t e d a t a l o w v e l o c i t y under Venus c o n d i t i o n s . I n g e n e r a l , g r a i n s a c h i e v e a much h i g h e r v e l o c i t y i n r e l a t i o n t o t h e w i n d speed u n d e r V e n u s i a n c o n d i t i o n s t h a n u n d e r m a r t i a n c o n d i t i o n s , and g r a i n s u n d e r t e r r e s t r i a l c o n d i t i o n s have i n t e r m e d i a t e v e l o c i t i e s . REFERENCES Bagnold, R.A., 1941. The P h y s i c s o f Blown Sand and D e s e r t Dunes: Methuen, London, 265 pp. G r e e l e y , R., R.N. Leach, S.H. W i l l i a m s , B.R. White, J.B. P o l l a c k , D.H. K r i n s l e y , a n d J.R. M a r s h a l l , 1982. R a t e o f Wind A b r a s i o n on Mars: J. _ Geophy. _ _ _ - Res., 87, pp. 10,009-10,024. G r e e l F , R., B.R. White, J.B. P o l l a c k , J.D. I v e r s e n , and R.N. Leach, 1977. Dust NASA T e c h n i c a l Memo. TM s t o r m s on Mars: C o n s i d e r a t i o n s and S i m u l a t i o n s : 78423, pp. 29. G r e e l e y , R., B.R. W h i t e , J.B. P o l l a c k , J.D. I v e r s e n , and R.N. Leach, 1981. Dust s t o r m s on Mars: C o n s i d e r a t i o n s and S i m u l a t i o n s : I n Desert Dust: Origin, C h a r a c t e r i s t i c s , and E f f e c t on Man. Geol. SOC. her.--1 T r o y Pewe, pp. l O T i 2 i 7 ' - - I v e r s e n , J.E., R. G r e e l e y , and J.B. P o l l a c k , 1976. Windblown d u s t on Earth, Mars, and Venus: J. Atmos. S c i . , 2, 2425-2429. I v e r s e n , J.D. and B . R T W W 1 9 8 2 . S a l t a t i o n t h r e s h o l d on E a r t h , Mars, and Venus: S e d i m e n t o l o g y , 29, pp. 111-119. S t u d y of sand movement by w i n d : Phy. S c i . Res. I n s t . , Kawamura, R., 1951. Tokyo Univ., 5, 95-112, ( o r i g i n a l i n Japanese; NASA t r a n s l a t i o n ) . S c h l i c h t i n g , H., 1368. Boundary-Layer Theory: M c G r a w - H i l l Book Company, New York, pp. 748. Schmidt, R.A. 1977. A System T h a t Measures B l o w i n g Snow: U.S. Dept. Agric. Res. Paper, RM -194, 80 pp. WhiteXR-79. S o i l T r a n s p o r t by Winds on Mars: J. Geophy. Res., 84, pp. 4643-4651. Williams, S.H. and R. G r e e l e y , 1983. F l u x o f w i n d b l o w n p a r t i c l e s on Venus: Preliminary laboratory results: NASA Rept. P l a n e t a r y Geology, ( a b s t r a c t , i n press).

149

EOLIAN SHAPE-SORTING AND AERODYNAMIC TRACTION EQUIVALENCE I N THE COASTAL DUNES OF HOUT BAY, REPUBLIC OF SOUTH AFRICA FRANK W. STAPOR, JR.l,

0. Box 12559, Charleson, S. C. 29412

MRR1,P.

JAMES P. MAY, Dept. of Chemistry and Geology, The C i t a d e l , C h a r l e s t o n , S. C. 29409 JOHN BARWIS2, Geology Department, U n i v e r s i t y o f South C a r o l i n a , Columbia, S.C.

INTRODUCTION This study i n v e s t i g a t e s e o l i a n shape-sorting,

aerodynamic t r a c t i o n equiva-

lence, and t h e i n t e r a c t i o n between s u r f a c e c r e e p and s a l t a t i o n i n a c t i v e c o a s t a l dunes near Hout Bay, R e p u b l i c o f South A f r i c a . experience a

These dunes have a p o i n t source,

m a r k e d l y u n i d i r e c t i o n a l wind regime, and a r e composed o f a 50/50

mixture o f q u a r t z and b i o c l a s t i c c a l c i t e sand. observed i n these c a l c i t e g r a i n s .

No d i a g e n e t i c m o d i f i c a t i o n s were

These c a l c i t e g r a i n s were d e r i v e d f r o m

gastropod s h e l l s and a r e more p r i s m a t i c t h a n plate-shaped.

No m i n e r a l o g i c

laminations were observed, u n l i k e t h e more common s i t u a t i o n i n c o a s t a l dunes i n which t h e c a l c i t e g r a i n s a r e plate-shaped and congregated i n t o d i s t i n c t laminae. This sand c o m p o s i t i o n i s i d e a l f o r examining e o l i a n shape-sorting,

given the

s i m i l a r d e n s i t i e s o f q u a r t z and c a l c i t e and t h e 50/50 m i x t u r e t h a t p r o b a b l y precludes g r a i n a v a i l a b i l i t y problems.

O n l y s t o s s s u r f a c e s were sampled i n o r d e r

t o maximize d e p o s i t i o n f r o m e o l i a n t r a c t i o n , s u r f a c e creep and s a l t a t i o n . faces,

s i t e s o f g r a i n f l o w and g r a i n f a l l d e p o s i t i o n ,

Slip

were avoided as were

interdune r e g i o n s . The r e s u l t s o f t h i s s t u d y i n d i c a t e t h a t e o l i a n t r a c t i o n t r a n s p o r t p r e f e r e n t i a l l y s e l e c t s l e s s s p h e r i c a l g r a i n s and t h i s s h a p e - s o r t i n g determines aerodynamic t r a c t i o n e q u i v a l e n c e .

There appears t o be m i n i m a l r a t h e r t h a n c o n t i n u a l

exchange between t h e c r e e p and s a l t a t i o n components d u r i n g s t o s s deposition;

surface

t h e creep component a t any g i v e n t i m e was p r e v i o u s l y p a r t o f t h e

s a l t a t i o n cloud.

-lExxon P r o d u c t i o n Research Co., % h e l l O i l Co.,

P.O.

P.O.

Box 2189, Houston, Texas 77001

Box 527, Houston, Texas 77001

29208

150

0

YLTERS

-

SAMPLE LOCATION

+"4 MAJOR DUNE SLIPFACE

-_- SADDLE CREST ---

F i g . 1: Hout Bay dune f i e l d , with sample l o c a t i o n s . Note n o r t h w e s t e r l y o r i e n t a t i o n o f t r a n s p o r t a x i s , as i n d i c a t e d by f i e l d and s l i p f a c e o r i e n t a t i o n s . Dune s l i p f a c e o r i e n t a t i o n s and t h e o v e r a l l p a r a b o l i c geometry o f t h e f i e l d make i t c l e a r t h a t d e f l a t i o n o f t h e p o c k e t beach p r o v i d e s t h e o n l y source o f t h i s e o l i a n sand.

As a s m a l l c l o s e d system, t h e dune f i e l d t h u s p r o v i d e s an unusual

and i d e a l n a t u r a l l a b o r a t o r y t o s t u d y t h e t e x t u r a l and m i n e r a l o g i c a l e f f e c t s of e o l ian t r a n s p o r t .

151 GEOLOGIC SETTING Hout Bay i s a s o u t h - f a c i n g c r e s c e n t i c embayment a l o n g t h e west f l a n k o f t h e Cape o f Good Hope, South A f r i c a , about 18 km southwest o f Cape Town ( F i g u r e 1). The c o a s t l i n e o f t h e e n t i r e Cape, i n c l u d i n g Hout Bay, c o n s i s t s o f s t e e p - c l i f f e d exposures o f t h e Precambrian Cape G r a n i t e .

High peaks above t h e s e c o a s t a l

c l i f f s , which i n p l a c e s r i s e t o o v e r 1,100 m, a r e h e l d up b y Lower P a l e o z o i c sediments o f t h e Table M o u n t a i n Group.

These r o c k s a r e made up almost e n t i r e l y

o f quartz a r e n i t e s , w i t h m i n o r amounts o f r e d shale. The n o r t h e r n s h o r e l i n e o f Hout Bay i s a t w o - k i l o m e t e r - l o n g pocket beach. sediment comprises

fine-

t o medium-grai ned q u a r t z sand mixed w i t h v a r y i ng

amounts o f b i o c l a s t i c carbonate, l a r g e l y modern g a s t r o p o d d e t r i t u s . component has o n l y two p o s s i b l e sources. beach t o t h e e a s t and west.

Beach

The q u a r t z

One i s t h e e r o d i n g g r a n i t e f l a n k i n g t h e

The o t h e r i s i n t e r m i t t e n t streams d r a i n i n g t h e q u a r t z

arenites t h a t cap t h e peaks s u r r o u n d i n g t h e bay. Dune F i e l d Geometry and Source Extending n o r t h - n o r t h w e s t f r o m t h e p o c k e t beach i s a narrow, 3.5 fairway o f t r a n s v e r s e t o b a r c h a n - l i k e dunes ( F i g u r e 1).

km-long

These dunes c l i m b a

t h r e e - k i l o m e t e r - l o n g ramp t o t h e saddle between L i o n s Head and Suther Peaks, a point a p p r o x i m a t e l y 125 m above mean sea l e v e l ( m s l ) .

Northwest f r o m t h i s saddle

the f i e l d s l o p e s down t o a p a r a b o l i c t e r m i n u s j u s t above Sandy Bay ( F i g u r e 2). The geometry o f t h e f i e l d resembles t h e p a r a b o l i c dunes d e s c r i b e d b y McKee (1979), and r e s u l t s f r o m t h r e e f a c t o r s .

F i r s t , dune f i e l d l o c a t i o n and w i d t h a r e

constrained by s u r r o u n d i n g bedrock topography.

Second, t h e beach i s r e l a t i v e l y

small and p r o v i d e s what amounts t o a p o i n t source o f sand.

T h i r d , dominant winds

are s o u t h e a s t e r l y ( F i g u r e 3 ) and a r e f u n n e l e d northward through t h e e n t r a n c e t o

Hout Bay, c r e a t i n g a w i n d - t u n n e l e f f e c t t h a t r e s u l t s i n a "blowout" o f beach sand. Although t h e wind r o s e i n F i g u r e 3 shows two m a j o r modes, n o r t h w e s t e r l i e s may have l i t t l e e f f e c t o n t h e Hout Bay dune f i e l d .

Most o f t h e f i e l d , because i t l i e s

south o f t h e saddle p o i n t , may be t o p o g r a p h i c a l l y s h i e l d e d f r o m a l l winds b u t those w i t h a s i g n i f i c a n t s o u t h e r l y component.

I n addition,

b d i t e r r a n e a n - t y p e c l i m a t e w i t h a w i n t e r r a i n y season.

t h e Cape h a s a

Strong northwesterl i e s

we u s u a l l y accompanied by r a i n , which should d r a m a t i c a l l y decrease e o l i a n transport.

Consequently,

even i n t h e v i c i n i t y o f t h e saddle,

m a i n NW-oriented y e a r round.

dune s l i p f a c e s

N e v e r t h e l e s s , some b r i n k p o i n t s do show s m a l l

k v e r s a l s l i k e t h o s e d e s c r i b e d by H u n t e r e t a l . ( i n p r e s s ) , as shown i n F i g u r e 4.

152

F i g . 2: O b l i q u e a e r i a l p h o t o g r a p h o f t h e H o u t Bay dune f i e l d . View l o o k i n g s o u t h a c r o s s Sandy Bay, w i t h H o u t Bay i n d i s t a n c e . N o t e s a d d l e p o i n t and t e r m i n u s o f a c t i v e dune i n f o r e g r o u n d .

JANUARY (SUMMER. DRY)

JULY

(WINTER. WET)

CAPE TOWN, R.S.A

F i g . 3: Wind d a t a f o r Cape Town (D. F. M a l a n A i r p o r t ) f o r p e r i o d 1956-1970. Note dominance o f s o u t h e r l y mode ( d r y summer w i n d s ) . S t r o n g e s t n o r t h w e s t e r l i e s o c c u r d u r i n g w i n t e r r a i n s ( J u l y through October). The number i n t h e c e n t e r i s t h e p e r c e n t a g e o f c a l m c o n d i t i o n s ; each a r c r e p r e s e n t s 5 p e r c e n t . Dune F i e l d O r i g i n U n d e r l y i n g t h e m o b i l e dunes j u s t above t h e modern beach i s a s e m i - c o n s o l i d a t e d sequence of P l e i s t o c e n e s h o r e l i n e d e p o s i t s .

T h i s sequence c o n s i s t s o f a r a i s e d

c o b b l e beach, t h e t o p o f w h i c h l i e s a t + 4 m above m s l and r a n g e s up t o 9 m above m s l e l s e w h e r e i n H o u t Bay (Buchanan, 1 9 7 7 ) .

T h i s p e r c h e d beach i s o v e r l a i n b y 4

153

Fig. 4: V e r t i c a l a e r i a l p h o t o o f t h e H o u t Bay dune f i e l d j u s t sout,h o f t h e s a d d l e point. N o t e r e v e r s e d b r i n k p o i n t ( a r r o w ) o n l a r g e dune a t c e n t e r . B a r i s 100 m long and i s o r i e n t e d N-S.

m o f massive, s h e l l - and a r t i f a c t - b e a r i n g sand which i s i n t u r n capped b y 6+ m o f c r o s s - s t r a t i f i e d sand ( I n s k e e p ,

1975). A w h o l e s h e l l I4C d a t e o f 47,100 y e a r s

was r e p o r t e d b y I n s k e e p (1975) f o r a bed 1 m b e l o w t h e t o p o f t h e m a s s i v e u n i t .

I f one c a n presume t h i s d a t e t o be i n f i n i t e o r "dead" w i t h r e s p e c t t o I4C ( s e e Fairbridge,

1971), t h e r a i s e d beach was most 1 i k e l y d e p o s i t e d d u r i n g t h e Sangarnon

(Eemian) 120,000-year

h i g h s t a n d , as p r o p o s e d b y B a r w i s and T a n k a r d ( i n p r e s s ) f o r

a very s i m i l a r s u c c e s s i o n 27 km t o t h e e a s t ,

i n F a l s e Bay.

The dunes t h e r e f o r e p o s t d a t e t h e l a s t i n t e r g l a c i a l .

L i k e those b o r d e r i n g False

Bay, t h e y p r o b a b l y were i n i t i a t e d d u r i n g t h e W i s c o n s i n a n l o w s t a n d ( B a r w i s and Tankard,

i n p r e s s ) and l i k e l y c o u l d h a v e r e m a i n e d a c t i v e e v e r s i n c e . SAMPLING PROCEDURE

Sand samples were c o l l e c t e d a t t h e s i t e s shown i n F i g u r e

1

u s i n g a 15-cm-long

(2.5 cm d i a . ) p l e x i g l a s s c y l i n d e r w h i c h was pushed v e r t i c a l l y i n t o t h e bed.

The

e n t i r e c o n t e n t s o f t h e c y l i n d e r were t h e n r e t a i n e d , i n c l u d i n g p e b b l e s and l a r g e molluscan f r a g m e n t s ,

w h i c h were l a t e r removed i n t h e l a b o r a t o r y .

A l l samples

were t a k e n f r o m t h e s t o s s s u r f a c e s o f a c t i v e dunes t h a t r a n g e d i n c r e s t s p a c i n g

154 from a few t e n s o f meters t o more t h a n 200 m.

No s l i p f a c e samples were taken, nor

were any samples t a k e n f r o m d e e p l y e r o d i n g areas as evidenced b y l o c a l topography and g r a v e l d e f l a t i o n lags. water,

and d r i e d .

The b u l k samples were s p l i t , washed i n d e - i o n i z e d

Two s u b - s p l i t s were t h e n t a k e n o f each sample:

(1)

untreated f r a c t i o n

(2)

q u a r t z f r a c t i o n ( t r e a t e d w i t h HC1 t o remove c a l c i u m carbonate).

Weight p e r c e n t c a l c i u m carbonate was d e t e r m i n e d f r o m t h i s t r e a t m e n t f o r each sample.

Organics, h e a v y m i n e r a l s ,

and o t h e r contaminants were 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 w a r r a n t f u r t h e r t r e a t m e n t . G R A I N CHARACTERISTICS The b i o c l a s t i c c a l c i t e g r a i n s showed no e v i d e n c e o f d i a g e n e t i c m o d i f i c a t i o n s when viewed under a b i n o c u l a r microscope. bladed.

T h e i r shape i s p r i s m a t i c r a t h e r t h a n

T h e i r m i n e r a l o g y - d e n s i t y was determined b y f l o t a t i o n i n a bromoform

s o l u t i o n c a l i b r a t e d t o f l o a t c a l c i t e and q u a r t z b u t t o a l l o w a r a g o n i t e t o s i n k . DETERMINATION OF SETTLING SPEED DISTRIBUTIONS The u n t r e a t e d f r a c t i o n and t h e q u a r t z f r a c t i o n were analyzed f o r s e t t l i n g speed d i s t r i b u t i o n i n The C i t a d e l Sediment A n a l y z e r (CITSA). t y p e a n a l y z e r p a t t e r n e d a f t e r t h a t o f Gibbs (1974). and t h e f a l l d i s t a n c e i s 120 cm.

CITSA i s a s e t t l i n g tube-

The t u b e i s 16 cm i n diameter

The p a r t i c l e s s e t t l e o n t o a pan suspended from

a Cahn E l e c t r o b a l a n c e (Model RTL),

t h e o u t p u t f r o m which c o n s i s t s o f a t i m e

d i s t r i b u t i o n o f accumulated weight,

which i s c o n v e r t e d t o a s e t t l i n g speed

distribution.

The ouput i s f e d t o a s t r i p - c h a r t r e c o r d e r f o r analog d i s p l a y and

through a n a n a l o g - t o - d i g i t a l c o n v e r t e r t o a TRS-80 microcomputer f o r s t a t i s t i c a l t r e a t m e n t o f t h e data. A l l s e t t l i n g speeds were c o r r e c t e d f o r t e m p e r a t u r e v a r i a t i o n t o a standard

v a l u e o f 2OoC.

The t e m p e r a t u r e - c o r r e c t e d s e t t l i n g speed was t h e n c o n v e r t e d t o a

d i m e n s i o n l e s s parameter ( C h i ) b y t h e f o l l o w i n g t r a n s f o r m : x (Chi) = -log2(s/so)

where s i s t h e t e m p e r a t u r e - c o r r e c t e d s e t t l i n g speed i n m/s and so i s standard s e t t l i n g speed o f 1 m/s. (1981).

The use o f t h e Chi parameter i s t r e a t e d i n d e t a i l by May

S e t t l i n g speeds o f t h e q u a r t z f r a c t i o n were d i r e c t l y measured.

Calcite

s e t t l i n g speeds were c a l c u l a t e d b y s u b t r a c t i n g t h e q u a r t z f r a c t i o n v a l u e s from those o f t h e u n t r e a t e d f r a c t i o n . The use o f a w a t e r - f i l l e d s e t t l i n g t u b e t o analyze q u a r t z and c a l c i t e e o l i a n sands was e v a l u a t e d b y t h e f o l l o w i n g t e s t .

Assuming t h a t two p a r t i c l e s , one o f

q u a r t z and one o f c a l c i t e , a r e d e p o s i t e d i n aerodynamic s e t t l i n g e q u i l i b r i u m ,

155 then t h e i r t e r m i n a l s e t t l i n g v e l o c i t i e s a r e i d e n t i c a l .

The d i a m e t e r s o f t h e i r

corresponding nominal spheres can be computed u s i n g t h e f o r m u l a s o f Rubey (1933), Gibbs

g d. (1971),

and Reed

g

z. (1975).

Table 1 shows t h e computed nominal

diameters f o r s e v e r a l t e r m i n a l v e l o c i t i e s i n a i r .

-e t_a l .

The Gibbs _ et _ a l . and t h e Reed

r e s u l t s a r e s i m i l a r , as t h e y were d e r i v e d f r o m t h e same e x p e r i m e n t a l data.

The Rubey v a l u e s a r e s i m i l a r , though s l i g h t l y l e s s f o r slow v e l o c i t i e s ( s m a l l p a r t i c l e s i z e ) and d i v e r g e t o h i g h e r v a l u e s f o r t h e h i g h e r v e l o c i t i e s .

A l l three

formulas c o n s i s t e n t l y p r e d i c t t h a t t h e q u a r t z d i a m e t e r s h o u l d be 1-2% g r e a t e r than t h e a e r o d y n a m i c a l l y e q u i v a l e n t c a l c i t e .

Next, t h e s e q u a r t z and c a l c i t e

computed nominal d i a m e t e r s were c o n v e r t e d back t o a s e t t l i n g v e l o c i t y i n w a t e r using t h e same t h r e e formulas.

Again, t h e Gibbs

ec. and t h e Reed gal. values

are i n c l o s e agreement ( w i t h i n 1-2%), whereas t h e Rubey v a l u e s p r e d i c t slower values on t h e slow end and f a s t e r v a l u e s on t h e f a s t end.

A l l three formulas

p r e d i c t t h a t q u a r t z and c a l c i t e p a r t i c l e s i n aerodynamic s e t t l i n g e q u i v a l e n c e should s e t t l e with d i f f e r e n t v e l o c i t i e s i n water, w i t h t h e q u a r t z v e l o c i t y b e i n g about 1%slower.

T h i s v a l u e i s a p p r o x i m a t e l y t h e same magnitude as t h e e r r o r i n

the s e t t l i n g t u b e apparatus.

I t i s concluded t h a t a w a t e r - f i l l e d s e t t l i n g t u b e

i s s u i t a b l e f o r e v a l u a t i n g q u a r t z and c a l c i t e aerodynamic s e t t l i n g equivalence. And t h i s e q u i v a l e n c e s h o u l d be i n d i c a t e d b y and c a l c i t e s e t t l i n g speeds.

"0 s i g n i f i c a n t d i f f e r e n c e s i n q u a r t z

Significant

d i f f e r e n c e s would suggest g r a i n

a v a i l a b i l i t y problems and/or t h e o v e r p r i n t o f t h e t r a n s p o r t i n g process. RESULTS AND DISCUSSION The q u a r t z and c a l c i t e f r a c t i o n s o f t h e s e Hout Bay e o l i a n sands sampled f r o m stoss s u r f a c e s a r e n o t i n aerodynamic s e t t l i n g e q u i l i b r i u m . speeds average 8% slower.

D i f f e r e n c e s i n mean

2% (95% Confidence I n t e r v a l ) w i t h t h e c a l c i t e f r a c t i o n s b e i n g

D i f f e r e n c e s i n s o r t i n g average 22%

fractions being the b e t t e r sorted.

6% (95% C.I.)

w it h t h e c a l c i t e

Table 2 p r e s e n t s t h e mean, s o r t i n g , skewness,

and k u r t o s i s moment measures which d e s c r i b e i n d i v i d u a l s e t t l i n g speed d i s t r i b u tions.

Mean s e t t l i n g speed v e r s u s s o r t i n g v a l u e s f o r q u a r t z and c a l c i t e

f r a c t i o n s of each sample a r e p l o t t e d i n F i g u r e 5.

As these samples a r e v e r y n e a r l y 50/50 m i x t u r e s of q u a r t z and c a l c i t e , mean weight p e r c e n t c a l c i t e i s 47% & 3% (95% C.I.), most l i k e l y m i n i m a l .

a nest o f q u a r t e r p h i screens.

D i f f e r e n c e s i n q u a r t z and c a l c i t e s i e v e -

determined mean g r a i n s i z e average 13% sands, see Table 3.

problems o f g r a i n a v a i l a b i l i t y a r e

To f u r t h e r examine t h i s problem t h e samples were s i e v e d i n

:6% (95% C.I.)

f o r t h e Hout Bay e o l i a n

However, Ludwick and Henderson (1968) show t h a t s i e v i n g c a n

underestimate t h e modal i n t e r m e d i a t e d i a m e t e r o f low s p h e r i c i t y g r a i n s b y 10% t o

20%. Hence t h e r e appears t o be no s h o r t a g e o f s u i t a b l e c a l c i t e g r a i n s .

156 Table 1: Gibbs,

S e t t l i n g speeds o f q u a r t z and c a l c i t e g r a i n s c a l c u l a t e d by medns o f the

z. (1971),

Reed,

g

a. (1975),

and Rubey (1933) formulae.

d e n s i t y of 0.0012 and a v i s c o s i t y of 0.00018. v i s c o s i t y of 0.01. Vai r ( m/ sec 1

dqtz. (mm)

A i r has a

Water has a d e n s i t y o f 1 and a

Q u a r t z has a d e n s i t y o f 2.65 and c a l c i t e 2.72 dcal. (mm)

% diff.

% diff

Vqtz/water ( m/set)

Vcal/water

1.;4 1.2 1.4 2.0 1.2 1.2 1.3 1.5 1.4 X=1.5

0.00075 0.00158 0.0350 0.00816 0.0192 0.0318 0.0449 0.0585 0.0997

0.00076 0.00161 0.00355 0.00818 0.0195 0.0322 0.0455 0.0590 0.101

1.3 1.9 1.4 0.2 1.6 1.3 1.3 0.9 1.3 x=1.2

1.4 1.2 1.3 2.0 1.8 2.5 1.6 1.6 1.5 X=1.6

0.000745 0.00157 0.00347 0.00807 0.00191 0.0315 0.0446 0.0581 0.0992

0.000755 0.0016 0.00353 0.00808 0.00192 0.0313 0.0449 0.0585 0.100

1.3 1.9 1.7 0.1 0.7 0.6 0.7 0.7 0.8 x.0.9

1.4 1.3 1.4 1.4 2.0 2.2 2.3 2.3 2.7 x=1.9

0.000708 0.00143 0.00299 0.00665 0.0171 0.0311 0.0465 0.0620 0.106

0.000717 0.00145 0.00303 0.00674 0.0171 0.0312 0.0468 0.0625 0.107

1.3 1.4 1.3 1.4 0.0 0.3 0.6 0.8

(m/sec)

A c c o r d i n g t o Gibbs e_t _ a l . (1971)

0.0625 0.125 0.25 0.50 1.0 1.5 2.0 2.5 4.0

0.0291 0.0426 0.0646 0.103 0.174 0.246 0.320 0.397 0.643

0.0287 0.0421 0.0637 0.101 0.172 0.243 0.316 0.391 0.634

A c c o r d i n g t o Reed et a l . (1975)

0.0625 0.125 0.25 0.50 1.0 1.5 2.0 2.5 4.0

0.029 0.0425 0.0642 0.102 0.172 0.242 0.314 0.388 0.625

0.0286 0.042 0.0634 0.1 0.169 0.236 0.309 0.382 0.616

A c c o r d i n g t o Rubey (1933)

0.0625 0.125 0.25 0.50 1.0 1.5 2.0 2.5 4.0

0.0281 0.0400 0.0581 0.0881 0.152 0.236 0.349 0.496 1.15

0.0277 0.0395 0.0573 0.0869 0.149 0.231 0.341 0.485 1.12

0.9 -

X.o.9'

157 Table 2:

S t a t i s t i c a l moment measures which d e s c r i b e t h e s e t t l i n g speed d i s t r i -

butions o f t h e q u a r t z and c a l c i t e f r a c t i o n s .

on s t o s s s u r f a c e s o f t h e Hout Bay dunes. 0.00 i s t h e skewness (Skew.) Sample

Weight %

These e o l i a n sands were sampled

Mean and s o r t i n g a r e i n Chi u n i t s .

and k u r t o s i s ( K u r ) v a l u e o f a log-normal d i s t r i b u t i o n Quartz Fraction

Calcite

Mean

Sorting

Skew.

2 3

45 46

4.74 4.68

.58 .66

-0.18 -0.32

4

45

4.74

.54

0.03

6

52

4.42

.51

0.26

7

53

4.47

.56

8

50

4.62

.52

Calcite Fraction Kur.

Mean

Sorting

Skew.

Kur.

0.42

.41

0.45

5.40 5.27

.34

0.07 0.15

-0.21 -0.10

-0.35

4.78

.39

-0.10

-0.71

0.62

4.80

.47

0.32

0.50

0.10

-0.07

4.76

.46

0.14

-0.21

0.03

0.68

5.13

.39

0.34

-0.31 0.60

9

56

4.44

.53

-0.04

0.72

5.04

.37

0.38

11

54

4.86

.43

-0.04

0.92

5.27

.34

0.33

0.60

12

55

5.04

.44

-0.20

1.92

5.38

.36

0.31

0.06

13 15

46

4.61

.69

-0.13

-0.12

4.88

.48

0.20

0.33

36

4.01

.87

0.23

-1.07

4.88

.72

-0.02

-1.17

16

36

4.94

.49

-0.02

-0.04

5.12

.76

-0.52

0.44

17

45

4.34

.62

0.05

0.08

5.00

.59

-0.33

0.43

18

54

4.72

.46

0.09

0.47

5.13

.39

0.31

1.01

21

46

5.26

.37

-0.05

0.57

5.55

.31

0.31

0.24

24

43

4.94

.35

0.10

0.93

5.26

.37

0.25

0.41

0.20

0.44

5.19

.33

0.36

-0.31

25

46

4.94

.44

26

49

4.93

.37

0.31

1.23

5.33

.36

0.38

0.37

27

51

5.05

.38

0.29

0.70

5.40

.33

0.38

1.29

2a

39

5.16

.40

0.16

0.17

5.32

.32

0.22

0.91

Thus, t h e r e appears t o be a s u b t l e , b u t s i g n i f i c a n t , s h a p e - r e l a t e d r e d u c t i o n i n s e t t l i n g - s p e e d between t h e s e q u a r t z and c a l c i t e f r a c t i o n s . The n a t u r e o f any p r e f e r e n t i a l s p h e r i c i t y s e l e c t i o n d u r i n g e o l i a n t r a c t i o n t r a n s p o r t has been a r a t t e r o f some c o n t r o v e r s y :

s t u d i e s have i n d i c a t e d 1) low s p h e r i c i t y g r a i n s

Selected (Free, 1911; Mattox, 1955; W i nkelmolen, 1971; Stapor, 1973; Veenstra,

1982), 2) h i g h s p h e r i c i t y g r a i n s s e l e c t e d (MacCarthy,

1935;

MacCarthy and

Ihrddle, 1938; Shepard and Young, 1961) and 3) a wind v e l o c i t y dependance i n which

low s p h e r i c i t y p a r t i c l e s a r e e n t r a i n e d b y l o w e r winds and h i g h e r s p h e r i c i t y p a r t i c l e s b y h i g h e r winds ( W i l l i a m s , 1964).

The r e s u l t s o f t h i s s t u d y i n d i c a t e

158

[IUARTZ rn CALCITE

4.0 (FASTER)

4.5

5.5

5.0

(SLOWER)

MEAN SETTLING SPEED (CHI1

F i g . 5: Mean s e t t l i n g speed versus s o r t i n g f o r q u a r t z and c a l c i t e f r a c t i o n s o f e o l i a n sands. These samples a r e from s t o s s surfaces o f t h e Hout Bay dune f i e l d . Mean s e t t l i n g speeds and s o r t i n g values are i n Chi u n i t s .

that

grains o f

lower s p h e r i c i t y

t r a c t i o n transport.

are p r e f e r e n t i a l l y

I n a d d i t i o n , these d i f f e r e n c e s

selected d u r i n g eolian

i n mean s e t t l i n g - s p e e d and

s o r t i n g d e f i n e t h e aerodynamic t r a c t i o n e q u i v a l e n c e between q u a r t z and c a l c i t e d e p o s i t e d on s t o s s s u r f a c e s . The mean s e t t l i n g speed of b o t h q u a r t z and c a l c i t e f r a c t i o n s decreases downtransport.

These g r a d i e n t s ,

a l t h o u g h s i g n i f i c a n t a t t h e 10% l e v e l , are r a t h e r

g e n t l e and account f o r o n l y 21% and 18% of t h e t o t a l v a r i a n c e i n q u a r t z and c a l c i t e mean s e t t l i n g speed, r e s p e c t i v e l y .

These percentages are unexpectedly

l o w g i v e n t h e r e l a t i v e l y s h o r t and narrow t r a n s p o r t path, t h e p o i n t source, and t h e u n i f o r m i t y o f t h e dominant s o u t h e r l y summer winds.

I t may be t h a t the

n o r t h w e s t e r l y w i n t e r winds, a1 though accompanied by r a i n , a r e indeed e f f e c t i v e i n sand t r a n s p o r t .

P l o t s of q u a r t z and c a l c i t e mean s e t t l i n g speed versus

d i s t a n c e a r e shown i n F i g u r e s 6 and 7.

159 Table 3:

R e s u l t s o f t h e s i e v e analyses.

Mean and s o r t i n g a r e i n p h i

units.

0.00 i s t h e skewness (Skew.) and k u r t o s i s ( K u r . ) v a l u e o f a l o g normal d i s t r i b u t i o n . These samples were s i e v e d i n a nest o f q u a r t e r

p h i screens f o r t h i r t y minutes.

Q u a r t z and c a l c i t e means have an

average d i f f e r e n c e of 13% + 6% (95% C. I . ) w i t h t h e c a l c i t e b e i n g smaller Quartz Fraction Sample

Calcite Fraction

Mean

Sorting

Skew.

Kur.

Mean

Skew.

Kur.

02

1.80

.56

0.17

1.13

1.37

1.89 1.80 1.61

* 57 -48

-.44 +. 12

-.34 -.lo

2.72 0.73

.45

+. 24

1.91 0.69 1.92

.56 .47 .44

-.25

03 04 06

2.06 2.15 2.05

.51

+. 10 -.13 -.05 +.03 -.15 -.09 +.42 -.44

0.87 1.25

.49 .55

0.97

1.63 1.73 1.71

1.83 1.82

+.14

07 08 09 11

1.94 1.89 2.10

.54 .47 .40

-.05 -.lo

2.17 1.91

.35 .58

0.53

1.63 2.05

.80 .47

0.72 1.78 2.07 1.57 0.01 1.46 -.70

+.01

0.42

1.83

.57

+.01

+. 18

1.91 1.43

1.98 2.42

.43 .34

+.02

-.02

+.08

0.47

4.12

2.09

.34

+.13

0.47

2.06 2.69 4.73 0.99

2.18

.42

2.16

.39

-.09 -.27

0.15 1.77

2.26 2.28

.39 .32

-.21 -.14

1.28 0.07

12 13 15

16 17 18 21 24 25 26 27 28

1.97 2.06 1.70 1.01 1.76 1.52 1.87 2.30 2.00 1.99 2.00 2.12 2.21

.54 .49 .40 .42 .65 .76 .76 .61 .41 .34 .33

+.03 +. 29

.37 .32

+.40 -.43

.36 .36

+. 13 +. 21

1.69 2.69 3.46 0.15 0.03

Sorting

-.02

+.13 +.lo -.33 -.09 -.30

1.01 0.50

I n d i v i d u a l q u a r t z and c a l c i t e s e t t l i n g - s p e e d d i s t r i b u t i o n s were d i s s e c t e d i n t o log-normal components through t h e use o f t h e ROKE program o f C l a r k (1977).

The

basic s t a t i s t i c a l d i s t r i b u t i o n o f monomineralic sand g r a i n s e x p e r i e n c i n g a p a r t i c u l a r mode o f t r a n s p o r t / d e p o s i t i o n d i s t r i b u t i o n s have been advocated,

i s assumed t o be log-normal.

f o r example,

the hyperbolic

Other

(Barndorff-

N i e l s e n etc.,1982), however, i t i s beyond t h e scope o f t h i s s t u d y t o c r i t i c a l l y discuss t h e m e r i t s o f v a r i o u s d i s t r i b u t i o n s .

The ROKE program employs a

n o n l i n e a r l e a s t - s q u a r e s a l g o r i t h m t o determine t h e mean, s o r t i n g , and p r o p o r t i o n o f two o r more components whose c o m b i n a t i o n b e s t e s t i m a t e s t h e g i v e n d i s t r i b u tion.

160

0

0

5.0 -

Y

RUARTI

0

E] 4.0

0

F i g . 6: Q u a r t z f r a c t i o n mean s e t t l i n g speed p l o t t e d a g a i n s t d i s t a n c e n o r t h from t h e southernmost sample. The s t r a i g h t l i n e i s a r e g r e s s i o n l i n e , s l o p e i s s i g n i f i c a n t a t t h e 10% l e v e l , which accounts f o r 21% o f t h e t o t a l v a r i a n c e inmean s e t t l i n g speed: mean s e t t l i n g speed = 4.52 + O.O0015y, y = d i s t a n c e i n meters n o r t h o f s o u t h e r r m o s t sample.

P r i o r t o d i s s e c t i o n , a one component ROKE a n a l y s i s was made o f each q u a r t z and c a l c i t e f r a c t i o n t o d e t e r m i n e j u s t how w e l l a log-normal d i s t r i b u t i o n d e s c r i b e d each measured s e t t l ing-speed d i s t r i b u t i o n .

Root-mean-square

i n d i c a t e t h e goodness o f t h e log-normal d e s c r i p t i o n .

(RMS) d e v i a t i o n s

Next a two-component ROKE

a n a l y s i s was made and t h e e x t r a c t e d log-normal components accepted i f 1) t h e RMS d e v i a t i o n s decrease t o a t l e a s t one t h i r d o f t h e one component v a l u e and 2) t h e l e s s e r component makes up a t l e a s t 10% o f t h e m i x t u r e . one-component and t h e accepted two-component Tables 4 and 5.

The r e s u l t s o f a l l t h e

ROKE analyses a r e p r e s e n t e d i n

A m i x t u r e o f two log-normal components b e t t e r d e s c r i b e s t h e

measured d i s t r i b u t i o n i n 50% o f t h e q u a r t z and 70% of t h e c a l c i t e f r a c t i o n s .

A

s i n g l e log-normal component b e t t e r d e s c r i b e s t h e o t h e r q u a r t z f r a c t i o n s and 15% of t h e c a l c i t e .

A m i x t u r e o f t h r e e log-normal components b e s t d e s c r i b e s t h e

remaining c a l c i t e f r a c t i o n s . W i t h i n t h e two-component m i x t u r e s , d i f f e r e n c e s i n mean s e t t l i n g speed and s o r t i n g average 8% 2 1.5% (95% C.I.) calcite. 5%

2

and 70% 2 30% (95% C.I.)

respectively for

Q u a r t z f r a c t i o n d i f f e r e n c e s i n mean s e t t l i n g speed and s o r t i n g averge

3% (95% C.I.)

and 90%

2

30% (95% C.I.)

respectively.

Thus, each m i n e r a l

161 Table 4:

R e s u l t s o f t h e one- and two-component ROKE analyses o f t h e q u a r t z

fractions.

Mean and s o r t i n g a r e i n Chi u n i t s .

RMS stands f o r root-mean-square

deviations o f t h e ROKE-predicted m i x t u r e f r o m t h e observed d i s t r i b u t i o n Quartz Sample

2 3 4 6 7 8 9 11 12 13 15 16 17 18 21 24 25 26 27 28

ONE-COMPONENT Mean Sorting

4.77 4.72 4.75 4.40 4.46 4.63 4.45 4.87 5.05 4.64 3.97 4.96 4.35 4.72 5.27 4.94 4.93 4.92 5.03 5.16

.57 .64 .56 .49 .57 .50 .50 .40 .42 .71 .96 .50 .62 .45 .37 .34 .43 .35 .36 .41

RMS

.00774 .0155 .00411 .0134 .00827 .0106 .00909 .00706 .0113 .00752 .0467 .00744 .00719 .00554 .00953 .00607 .0148 .0114 .0118 .0120

Mean1

TWO-COMPONENT Sorting1 %I

Mean2

RMS

Sorting2

4.30

.38

71

4.77

.7 1

.00303

4.71 4.44 4.88

.69 .67 .54

52 58 55

4.56 4.45 4.86

.31 .29 .26

.00284 .00207 .00227

3.38

.33

60

4.99

.45

.00331

4.69

.40

87

4.95

.83

.00166

5.00 4.79

.39 .29

67 58

4.84 5.19

.21 .56

.00204 .00386

4.93 5.05

.28 .31

76 70

5.43 5.50

.43 .52

.0018 .00379

Table 5: R e s u l t s o f t h e one- and two-component ROKE analyses o f t h e c a l c i t e f r a c t i o n f r a c t i o n s . Mean and s o r t i n g a r e i n Chi u n i t s . RMS stands f o r root-mean-square d e v i a t i o n s o f t h e ROKE-predicted m i x t u r e f r o m t h e observed m i x t u r e Calcite Sample

ONE-COMPONENT Mean Sorting

RMS

Mean1

TWO-COMPONENT Sorting1 %I

Mean2

Sorting2

RMS ~~

2 3 4 6 7 8 9

11 12

13 16 17 18 21 24 25 26 27 28

5.40 5.26 4.79 4.77 4.75 5.11 5.02 5.25 5.36 4.86 4.89 5.12 5.05 5.10 5.53 5.25 5.17 5.31 5.37 5.31

.39 .34 .40 .44 .47 .38 .36 .32 .36 -46 .80 .61 .50 .37 .30 .35 .32 .35 .31 .29

.0107 .00512 .00642 .01420 .00719 .0132 .0150 .0188 .0142 .00685 .0288 .0536 -0337 .0127 .0155 .0129 .0140 .0155 .0172 .00805

5.43

.49

69

5.34

.19

.00392

4.65 5.01 4.91 4.83 5.15 5.20

.31 .42 .24 .22 .24 .24

60 54 50 53 67 60

5.03 4.46 5.34 5.27 5.51 5.65

.57 .33 .38 .36 .38 .36

.00275 .00231 .00482 .00485 .00182 .00383

5.00 5.40 5.33 5.03 5.17 5.26 5.26

.28 .17 .37 .21 .24 .21 .22

71 52 76 61 66 72 63

5.44 5.72 5.04 5.44 5.64 5.76 5.42

.43 .34 .13 .34 .37 .33 .41

.00184 .00230 .00360 .00262 .00353 .00200 .00143

162

.

m

.

5.5

..

=.

CALCITE

F i g . 7: C a l c i t e f r a c t i o n mean s e t t l i n g speed p l o t t e d a g a i n s t d i s t a n c e north from t h e southerrmost sample. The s t r a i g h t l i n e i s a r e g r e s s i o n l i n e , slope i s s i g n f i c a n t a t t h e 10% l e v e l , which accounts f o r 18% o f t h e t o t a l variance inmean s e t t l i n g speed: mean s e t t l i n g speed = 4.98 + O.OOOly, y = d i s t a n c e i n meters n o r t h o f southerrmost sample. f r a c t i o n i s composed of a f a s t e r and b e t t e r s o r t e d component and a g e n e r a l l y slower and more p o o r l y s o r t e d one. deposited

from

creep and

These two components may r e p r e s e n t grains

s a l t a t i o n e o l i a n transport,

respectively.

The

interchange of g r a i n s between s a l t a t i o n and surface creep may not be continuous as i n d i c a t e d b y Bagnold (1941). Although these hypothesized creep and s a l t a t i o n components p l o t i n d i f f e r e n t p o r t i o n s o f a s o r t i n g versus mean s e t t l i n g - s p e e d

diagram (Fig.

8),

average

d i f f e r e n c e s i n q u a r t z and c a l c i t e mean s e t t l i n g speed and s o r t i n g are i n d i s tinguishable.

W i t h i n t h e creep components q u a r t z and c a l c i t e f r a c t i o n s have

average d i f f e r e n c e s of 6%

T 1%(95% C.I.)

speed and s o r t i n g r e s p e c t i v e l y .

and 25%

f e r e n c e s i n mean s e t t l i n g - s p e e d and s o r t i n g o f 9% (95% C.I.)

respectively.

b e t t e r sorted.

7% (95% C.I.)

i n mean s e t t l i n g

The s a l t a t i o n components have average d i f -

T 4% (95% C.I.)

and 31%

5%

I n both cases t h e c a l c i t e f r a c t i o n i s t h e slower and

T h i s suggests t h a t t h e processes which produced these two

components are s i m i l a r , a t l e a s t i n regards t o q u a r t z / c a l c i t e d e n s i t y and shape contrasts:

an unexpected r e s u l t g i v e n t h a t g r a i n s i n s u r f a c e creep g e t t h e i r

momentum from t h e impact o f s a l t a t i n g g r a i n s w h i l e t h e l a t t e r g e t t h e i r momentum

163 SALTATION 9 QUARTZ - CALCITE

(FASTER)

\

4.5

CREEP QUARTZ =CALCITE

5.0

5.5

MEAN SETTLING SPEED (CHI)

(SLOWER)

Fig. 8: A p l o t o f mean s e t t l i n g speed versus s o r t i n g f o r q u a r t z / c a l c i t e f r a c t i o n s i n t e r p r e t e d t o be creep and s a l t a t i o n components. Only those samples f o r which the two-component ROKE a n a l y s i s was accepted f o r both mineral f r a c t i o n s are shown i n t h i s diagram. Note t h e two d i s t i n c t , non-overlapping c l u s t e r s . d i r e c t l y from the wind (Bagnold, 1941). However, we are d e a l i n g w i t h d e p o s i t s produced by a sequential r e d u c t i o n i n wind v e l o c i t y . Thus, creep components previously may w e l l have been p a r t o f the s a l t a t i o n cloud. CONCLUSIONS The r e s u l t s o f t h i s p r e l i m i n a r y study i n d i c a t e t h a t e o l i a n t r a c t i o n t r a n s p o r t p r e f e r e n t i a l l y s e l e c t s g r a i n s o f lower s p h e r i c i t y and t h i s s e l e c t i o n d e f i n e s aerodynamic t r a c t i o n e q u i l i b r i u m .

I n a d d i t i o n , the s t a t i s t i c a l d i s s e c t i o n of

settling-speed d i s t r i b u t i o n s suggests t h a t both creep and s a l t a t i o n components

are present deposition,

i n these

stoss

surface deposits.

This

o r c o n d i t i o n s o f f a l l i n g wind v e l o c i t y ,

implies t h a t during

t h e creep and s a l t a t i o n

components are not c o n t i n u o u s l y interchanged. ACKNOWLEDGEMENTS The h e l p o f Mrs. Mary Joe C l i s e , MRRI, i n s i e v i n g these samples i s g r a t e f u l l y acknowledged.

Dr. Graham Gash, MRRI, a s s i s t e d i n the computer p r o g r a m i n g and

data management. Dr. Peter McCabe and Dr. Ken Stanley, both o f Exxon P r o d u c t i o n Research Co., reviewed t h i s paper and made c r i t i c a l comnents which s i g n i f i c a n t l y

164 improved t h e o r i g i n a l manuscript. REFERENCES Bagnold, R. A., 1941. The Physics of Blown Sand and Desert Dunes. Methuen and Co., 265 pp. Barndorff-Nielsen, 0.; Dalsgaard, K.; Halgreen, C.; Kuhlman, H.; M o l l e r , J. T.; and G. Schou, 1982. V a r i a t i o n i n p a r t i c l e s i z e d i s t r i b u t i o n over a small dune. Sedimentology, 29: 53-66. Barwis, J. H., and A. J. Tankard, i n press, P l e i s t o c e n e s h o r e l i n e d e p o s i t i o n and sea-level h i s t o r y a t d w a r t k l i p , South A f r i c a . Jour. Sed. Pet. Buchanan, W. F., 1977, Rescue d i g a t a Late Stone Age Cave, Hout Bay, Cape Province. Unpub. Archaeol. A d d i t i o n a l Proj., Univ. Cape Town, 13 pp. Clark, I . , 1977. ROKE, a computer program f o r n o n l i n e a r least-squares decompcs i t i o n o f mixtures o f distributions. Computers and Geosciences, 3,(2). F a i r b r i d g e , R. W., 1971. Quaternary s h o r e l i n e problems a t INQUA, 1969. Q u a t e r n a r i a , 15: 1-17. Free, E. E., 1911. The movement o f s o i l m a t e r i a l by t h e wind. B u l l . 68, Bureau o f S o i l s , U.S. Dept. o f A g r i c u l t u r e , 37 pp. Gibbs, R. J., 1974. A s e t t l i n g tube system f o r sand-sized analysis. Jour. Sed. Pet. 44: 583-588. Gibbs, R. J., Matthews, M. D., and D. A. Link, 1971. The r e l a t i o n s h i p between sphere s i z e and s e t t l i n g v e l o c i t y . Jour. Sed. Pet., 41: 7-18. Hunter, R. E., Richmond, B. M., and T. R. Alpha, i n press. Storm-controlled o b l i q u e dunes o f the Oregon coast. B u l l . Geol. SOC. Am. Inskeep, R. R., 1976. A note on t h e Melkbos and Hout Bay r a i s e d beaches and t h e M i d d l e Stone Age. S. A f r . Archaeol. B u l l . , 31: 26-28. Ludwick, J. C. and P. L. Henderson, 1968. P a r t i c l e shape and inference o f s i z e from s i e v i n g . Sedimentology, 11: 197-235. MacCarthy, G. R., 1935. E o l i a n sands: a comparison. Am. Jour. Sci., 30: 81-95. MacCarthy, G. R. and 3. W. Huddle, 1938. Shape-sorting o f sand g r a i n s by wind a c t i o n . Am. Jour. Sci., 35: 64-73. Mattox, R. B., 1955. E o l i a n shape s o r t i n g . Jour. Sed. Pet., 25: 111114. a proposed standard parameter f o r s e t t l i n g tube May, J. P., 1981. CHI ( X ) : a n a l y s i s o f sediments. Jour. Sed. Pet., 51: 607-610. McKee, E. D., ed., 1979. A Study o f Global Sand Seas. U. S. Geol. Surv., P r o f . Paper 1052, 429 pp. Reed, W. E . , LeFever, R., and G. J. Moir, 1975, D e p o s i t i o n a l environment i n t e r p r e t a t i o n from s e t t l i n g v e l o c i t y ( p s i ) d i s t r i b u t i o n s . B u l l . Geol. SOC. Am., 86: 1321-1328. Rubey, W. W., 1933. S e t t l i n g v e l o c i t i e s o f gravel, sand, and s i l t p a r t i c l e s . Am. Jour. Science, 25: 325-338. Shepard, F. P. and R. Young, 1961. D i s t i n g u i s h i n g between beach and dune sands. Jour. Sed. Pet., 31: 196-214. Stapor, F. W., 1973. Heavy m i n e r a l c o n c e n t r a t i n g processes and density/shape s i z e e q u i l i b r i a i n t h e marine and c o a s t a l dune sands o f t h e Apalachicola, F l o r i d a , region. Jour. Sed. Pet., 43: 396-407. Veenstra, H. J., 1982. Size, shape and o r i g i n o f sands o f t h e East F r i s i a n I s l a n d s ( N o r t h Sea, Germany). Geol. Mijnbouw, 61: 141-146. Williams, G., 1964. Some aspects o f the e o l i a n s a l t a t i o n load. Sedimentology, 3: 257-287. Winkelmolen, A. M., 1971. R o l l a b i l i t y , a f u n c t i o n a l shape p r o p e r t y o f grains. Jour. Sed. Pet., 41: 703-714.

165

DUNE SEDIMENT TYPES, SAND COLOUR, SEDIMENT PROVENANCE

AND HYDROLOGY I N THE

STRZELECKI-SIMPSON DUNEFIELD, AUSTRALIA R.J. WASSON*:

D e p t . o f B i o g e o g r a p h y and Geomorphology, A u s t r a l i a n Y a t i o n a l

U n i v e r s i t y , C a n b e r r a , A.C.T.,

Australia

INTRODUCTION The l a r g e S t r z e l e c k i - S i m p s o n d u n e f i e l d o f A u s t r a l i a ( F i g . 1) c o n s i s t s p r i n c i p a l l y o f l o n g i t u d i n a l dunes o f v a r i o u s k i n d s .

One o f t h e iiiost s t r i k i n g

r e g i o n a l c h a r a c t e r i s t i c s o f t h e s e dunes i s t h e i r c o l o u r ( F i g . 2 ) .

The b r i g h t

red-brown dunes o f t h e n o r t h e r n Simpson d u n e f i e l d a r e o f t e n c i t e d as b e i n g t y p i c a l o f t h i s d u n e f i e l d , b u t t h e r e a r e a l s o e x t e n s i v e a r e a s o f p a l e brown, orange brown, and w h i t e dunes.

The z o n a t i o n o f t h e s e v a r i o u s l y c o l o u r e d sands

has been i n t e r p r e t e d as t h e r e s u l t o f p r o g r e s s i v e r e d d e n i n g and a g e i n g o f sand g r a i n c o a t i n g s downwind f r o m s o u r c e s o f sand ( W o p f n e r and T w i d a l e , 1 9 6 7 ) . There a r e v a r i o u s l i n e s o f e v i d e n c e w h i c h s u g g e s t t h a t t h i s h y p o t h e s i s i s inadequate.

I n t h i s p a p e r s e d i m e n t s and m o r p h o l o g y w i t h i n each of t h e m a j o r

c o l o u r zones a r e examined, and t h e p r o c e s s e s o f s e d i m e n t m o b i l i z a t i o n f o r dune c o n s t r u c t i o n examined.

I n t h e l i g h t o f t h i s examination t h e hypothesis o f

downwind a g e i n g o f dune sand i s c r i t i c a l l y t e s t e d and f o u n d t o b e i n a d e q u a t e . SPATIAL DISTRIBUTION OF DUNE SEDIMENT TYPES Colour, i r o n c o n t e n t and m i n e r a l o g y ( f o r methods see Append'ix) Two d o m i n a n t g r o u p s o f c o l o u r s o c c u r i n t h e d u n e f i e l d ( F i g . 2 ) : brown (10YR7/4 t o 5YR6/8; (5YR6/8 t o 2.5YR4/8).

Japanese S t a n d a r d S o i l C o l o u r C h a r t ) ;

1, p a l e 2, r e d - b r o w n

N e a r t h e b i g l a k e s , w h i c h f o r m an a r c f r o m Lake Frome t o

Lake E y r e ( F i g s . 1 and 2), t h e dunes a r e p a l e c o l o u r e d .

These p a l e dunes

extend o n l y a s h o r t d i s t a n c e f r o m L a k e s Frome and C a l l a b o n n a , e x t e n d some 300 km f r o m Lake B l a n c h e , and w e l l n o r t h o f Lake Eyre.

Downwind o f t h e a r e a o f

pale dunes e a s t o f Lake Frome, a t r a n s i t i o n zone a b o u t 10-20 km o c c u r s between pale and red-brown dunes.

F u r t h e r n o r t h S t r z e l e c k i Creek f o r m s t h e b o u n d a r y

between p a l e and r e d - b r o w n dunes, a n d t h e b o u n d a r y i s as w i d e as t h e c r e e k ' s immediate f l o o d p l a i n i n some p l a c e s ;

t h a t i s , 1-2 km.

I n t h e Simpson dune-

f i e l d , t h e b o u n d a r y i s g r a d a t i o n a l , once a g a i n o v e r 10-20 km.

Apart from these

obvious b o u n d a r i e s , t h e r e a r e c o l o u r g r a d i e n t s w i t h i n t h e a r e a s o f p a l e dunes.

*

P r e s e n t a d d r e s s : C S I R O , D i v i s i o n o f W a t e r and Land Resources, P.O. Canberra City, A.C.T. 2601, A u s t r a l i a .

Box 1666,

166

Fig. 1. Simpson a n d Strzelecki dunefields, and catchments of streams entering the dunefields.

For example, sands close t o Kallakoopah Creek a r e very pale (10YR7/4) and rapidly darken t o 7.5YR7/4 in a distance of about 25 km downwind. Representative sample points a r e shown on Figure 2. Systematic sampling has been c a r r i e d out on t h r e e t r a v e r s e s within t h e dunefield (Fig. 2 ) . Traverse 1 from Moolawatana Bore t o Hawker Gate i s 140 km long and i s p a r a l l e l t o the dune trend. Each sample was taken from t h e c r e s t s of longitudinal dunes, and cons i s t s of the upper 10 cm a t each point. On Traverse 1 the sands a r e pale near Lake Frome a n d t h e i r hues redden from about 65 km downwind of Moolawatana Bore (Fig. 3 ) . Colour values show no systematic trend. Total f e l d s p a r was determined by s t a i n i n g polished sections with sodium c o b a l t i n i t r i t e a f t e r etching with hydrofluoric acid. Feldspar q u a n t i t i e s show a systematic decline from Lake Frome, b u t maintain low values from 65 km onwards. Heavy mineral content i s higher in the yellower sands near Lake Frome than in the redder sands f u r t h e r e a s t . The change in colour, t o t a l f e l d s p a r content, and heavy mineral content of c r e s t a l sands occurs a t about the same place (Fig. 3 ) .

167

Fig. 2. Dune sand c o l o u r s i n t h e Simpson and S t r z e l e c k i d u n e f i e l d s . C o l o u r s recorded a t p o i n t l o c a t i o n s a r 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 o f l a r g e a r e a s a r o u n d each. The c o l o u r b o u n d a r i e s a r e b a s e d on g r o u n d t r a v e r s e s , c o l o u r a e r i a l photographs, and s a t e l l i t e images. T r a v e r s e 2 ( F i g . 4 ) i s 174 km l o n g and c u t s a c r o s s t h e t r e n d o f t h e l o n g i t u d i n a l dunes ( F i g . 2).

S t a r t i n g a t t h e e a s t e r n end, t h e dunes a r e

red-brown ( d o m i n a n t hue i s 2 . 5 Y R ) Creek i s r e a c h e d a t 130 km. creek.

becoming d o m i n a n t l y 5 Y R u n t i l t h e S t r z e l e c k i

P a l e dunes ( d o m i n a n t l y 7.5YR/6)

o c c u r w e s t of t h e

A l t h o u g h t h e sample number i s s m a l l , t o t a l f e l d s p a r c o n t e n t seems t o be

168

lower in the red sands ( t h e eastern p a r t o f Traverse 2 , known as T1) t h a n in the pale sands ( t h e T2 p a r t o f Traverse 2 ) . Furthermore, the percentage of heavy minerals i s a l s o lower in the red sands t h a n in the pale sands.

H u e '5

(YR) 2.5

I

5-

Heavy minerals 0.2-

-

I

I 1

I

.

. .

*

I

.I

I

t

I

4

I

I

I

I

. . . . . . .

I.

I.

Traverse 1, Strzelecki dunefield.

Fig. 3.

Colour Value

I

. . . . .

Value 6 -

%

I

I

I

.

*

. . .

0

.

I

Location shown on Fig. 2 .

.. ... .... . ..... ... .... .... ., I. I I . , I

0..

0.0

4

0.

0.

0

0.

L

I

. O %

L

"

'

Feldspar 2

O

(East)

Fig. 4.

"

" 20

. 40

60

80

,

I

.

.

0 minerals02

O

I

*

.

4

a

.I

. . . 100

I

.I. 120

%

*. 140

Kilometres

Traverse 2 , Strzelecki dunefield.

Location shown on Fig. 2.

.

..

#

160 (West)

169 Traverse 3 (Fig. 5 ) i s 122 km l o n g , runs p a r a l l e l t o t h e longitudinal dunes, and s t a r t s a t the ' b i g bend' of Kallakoopah Creek in the Simpson Desert. The dune sands a r e pale near the creek, as noted e a r l i e r , becoming red-brown about 80 km downwind.

Colour values decline from south t o north. Heavy mineral content i s generally lower in the pale sands t h a n in the red sands, the reverse t o the pattern found on Traverse 2 .

Hut

I

Fig. 5.

Traverse 3 , Simpson dunefield.

Location shown on Fig. 2.

Traverse 2 was selected t o examine t h e petrographic a n d chemical differences between pale ( T 2 ) a n d red-brown ( T l ) dune sands, in a n attempt t o explain the colour differences. The following r e s u l t s were obtained: 1. The f r a c t i o n of 250 pm t o 1000 pm in diameter was separated and the grain coatings rubbed off by rubber p e s t l e in a mortar. Standard X-ray d i f fraction ( X R D ) analyses of 19 samples (13 from T1 and 6 from T 2 ) of the :oatings revealed mixtures of k a o l i n i t e , i l l i t e , and mixed-layer i l l i t e iontrnori 11 oni t e , with kaol i n i t e dominating. The only d i s c e r n i b l e difference ay in the absence of montmorillonite in some T2 samples. 2 . Chemically scrubbed and ground separations of heavy minerals from 6 iamples on T 1 and 8 samples on T2 were examined by XRD. The dominant minerals ire ilmenite, magnetite, zircon, and r u t i l e . Half t h e samples have s l i g h t l y

170 more z i r c o n t h a n m a g n e t i t e , w h i l e t h e o t h e r h a l f d i s p l a y a d e c l i n e f r o m ilmenite t o r u t i l e .

There i s no d i f f e r e n c e between T1 and T2 i n terms o f

abundant heavy m i n e r a l composition.

Four samples f r o m T1 and 3 f r o m T2 were

examined by XRD more e x t e n s i v e l y , and a l l samples c o n t a i n anatase, s p i n e l , pyroxene, hornblende, t o u r m a l i n e , topaz, g a r n e t , and a p a t i t e , w h i l e a l l samples f r o m b o t h p a r t s o f T r a v e r s e 2 c o n t a i n monazite, sphene, and a u g i t e .

3.

E n e r g y - d i s p e r s i o n X-ray (EDAX) mapping o f s e c t i o n s c u t t h r o u g h g r a i n

c o a t i n g s on two samples each f r o m T I and T2 showed random d i s t r i b u t i o n s o f t h e elements Fe, A l , S i , T i , and K.

The sample a n a l y s i s was c a r r i e d o u t on a l l

samples f r o m T r a v e r s e 2, and t h e same r e s u l t found.

From these analyses i t i s

n o t p o s s i b l e t o conclude whether t h e Fe occurs as c r y s t a l l i t e s between t h e c l a y s , o r occurs p r i n c i p a l l y w i t h i n t h e l a t t i c e s o f t h e c l a y m i n e r a l s .

4. Coatings were rubbed o f f sub-samples o f 250-1000 um g r a i n s f r o m 8 samples f r o m T1 and 6 samples f r o m T2, and t o t a l carbon determined. mean C = 81

?

40 ppm, and f o r T2 mean C = 87 i 48 ppm.

F o r T1

There i s no d i f f e r e n c e

between t h e samples, a l t h o u g h i t must be s a i d t h a t r u b b i n g abrades t h e g r a i n s u r f a c e s s o t h a t d i f f e r e n c e s between c o a t i n g s c o u l d be masked by t h e removal technique.

5.

I r o n as Fe203 was determined f o r b u l k samples ( w i t h heavy m i n e r a l s g r a i n s f o r 13 samples f r o m T1 and 6 samples f r o m T2

removed) o f 250-1000 ( T a b l e 1).

The ranges o f values c a l c u l a t e d by t a k i n g 2a about t h e means do n o t

o v e r l a p , s o t h e sample means a r e d i f f e r e n t . T1 samples t h a n i n t h e T2 samples.

That i s , t h e r e i s more i r o n i n t h e

Examination o f t h e q u a r t z g r a i n s f r o m these

samples i n t h i n s e c t i o n d i d n o t r e v e a l obvious d i f f e r e n c e s i n i m p u r i t i e s ,

it

SO

i s concluded t h a t t h e d i f f e r e n c e s d e t e c t e d i n Fe% r e f l e c t d i f f e r e n c e s i n t h e i r o n content o f t h e g r a i n coatings.

6.

Haematite was r e c o r d e d by XRD analyses i n s e v e r a l samples f r o m b o t h T1

and T2, b u t g e n e r a l l y c r y s t a l l i n e i r o n m i n e r a l s were n o t d e t e c t e d . l i k e l y t o be i n t h e Fe+3 f o r m (Walker, 1979).

A l l iron is

T h i s p r o p o s i t i o n was t e s t e d f o r NO

t h r e e samples from each o f T1 and T2 by d e t e r m i n i n g t o t a l Fe and t h e n Fet3. Fe”

was d e t e c t e d by d i f f e r e n c e . 7.

Examination o f t h i n s e c t i o n s f r o m T1 and T2 shows t h a t g r a i n c o a t i n g s

a r e g e n e r a l l y t h i c k e r i n T1 samples.

Maximum t h i c k n e s s i n T1 g r a i n s i s about

35 urn, w h i l e on T2 t h e y a r e about 10-15 urn. c o a t i n g s i n T1 i s g r e a t e r t h a n f o r T2.

T h i s suggests t h a t t h e volume of

To t e s t t h i s p r o p o s i t i o n c o a t i n g s were

rubbed o f f and t h e i r w e i g h t determined as a p r o p o r t i o n o f t h e b u l k 250-1000 sub-sample f r o m T1 samples and T2 samples. 0.75% and f o r T2 0.38

k

0.17%.

F o r T1 t h e mean v a l u e i s 0.88

*

The range o f v a l u e s f o r 2a about t h e means

o v e r l a p s , so t h e samples a r e n o t d i f f e r e n t .

T h i s r e s u l t may have been caused

by t h e i n a c c u r a c y o f t h e r u b b i n g method, o r i t may be r e a l .

Sonic removal o f

c o a t i n g s ( c f . Walker, 1979) i s o b v i o u s l y a more e f f i c i e n t method.

171 The s i g n i f i c a n t r e s u l t t o emerge f r o m t h e s e analyses i s t h a t t h e redness o f the dune sands i s a f u n c t i o n o f t h e i r o n c o n t e n t o f t h e g r a i n c o a t i n g s .

This

r e s u l t i s d e r i v e d o n l y f r o m T r a v e r s e 1, and so i t was t h o u g h t d e s i r a b l e t o t e s t i t elsewhere.

The same a n a l y s i s was c a r r i e d o u t on T r a v e r s e 3 ( T a b l e l ) , and

i t i s c l e a r t h a t t h e r e d sands have more i r o n t h a n t h e p a l e sands.

TABLE 1 Loca 1 i t y

%Fe

(x

?

s)l

Traverse 1 : T1

0.26 t 0.06

T2 : pale red Curdlawidny : p a l e red

0.18 t 0.03 0.14 ? 0.06 0.41 ? 0.04 0.34 t 0.04 0.56 i 0.04

Traverse 3

2

Coat m i n e r a l ogy2

K a o l i n i t e , i l l i t e : mont., haemati t e K a o l i n i t e , i l l i t e : mont., Haematite, k a o l i n i t e , i l l i t e Haematite, k a o l i n i t e , i l l l i t e Haematite, k a o l i n i t e , i l l i t e Haematite, k a o l i n i t e , i l l i t e

Sample no. (n 1 13 6 3 3 8 8

Mean t s t a n d a r d d e v i a t i o n , as w e i g h t % b a s i s on 250-1000 fraction. Determined by X-ray d i f f r a c t i o n on c o a t i n g s rubbed o f f 250-1000 fraction. Another t e s t was c a r r i e d o u t i n t h e P a r a k y l i a d u n e f i e l d o f South A u s t r a l i a , j u s t west o f Lake Torrens, i n t h e v i c i n i t y o f Curdlawidny Lagoon ( F i g . 6 ) . lake i s bounded by t h r e e r i d g e s which p a r a l l e l t h e l a k e ' s e a s t e r n shore.

The These

ridges a r e l u n e t t e - l i k e and a t l e a s t t h e upper 1-2 m c o n s i s t s o f a e o l i a n sand.

-dune

crest

Fig. 6. Map of dunes downwind o f t h e s o u t h e r n p a r t o f Curdlawidny Lagoon. Sample p o i n t 1 i s i n pale-brown dunes, and 2 i s i n red-brown dunes.

172 L o n g i t u d i n a l dunes e x t e n d downwind f r o m t h e s e t r a n v e r s e r i d g e s .

The sands o f

t h e t r a n s v e r s e r i d g e s and a d j a c e n t l o n g i t u d i n a l dunes a r e p a l e (2,5YR6/6), w h i l e t h e l o n g i t u d i n a l dunes f u r t h e r downwind, and n o t a t t a c h e d t o t h e t r a n s v e r s e r i d g e s , a r e r e d - b r o w n (10YR5/5). F o u r samples were t a k e n f r o m t h e c r e s t s o f one p a l e and one red-brown l o n g i t u d i n a l dune a t C u r d l a w i d n y ( F i g . 6 ) , and each sample was s p l i t .

The Fe

c o n t e n t o f t h e p a l e sand and r e d sand i s d i f f e r e n t u s i n g t h e 2 0 c r i t e r i o n ( T a b l e l ) , t h a t i s , r e d sands c o n t a i n more Fe t h a n p a l e sands.

As n o t e d e a r l i e r , d i f f e r e n c e s i n c o l o u r o c c u r w i t h i n a r e a s d e s i g n a t e d as p a l e o r red-brown.

On T r a v e r s e 3, sand hue r e d d e n s t o w a r d s t h e n o r t h , a t r e n d

which i s p a r a l l e l e d by decreasing c o l o u r value (Fig. 5).

Total i r o n content i s

n o t o n l y h i g h e r i n t h e r e d d e r sands b u t t h e r e i s a t r e n d o f i n c r e a s i n g i r o n content towards t h e north.

That i s , i r o n i s p o s i t i v e l y c o r r e l a t e d w i t h b o t h

c o l o u r and d i s t a n c e f r o m K a l l a k o o p a h Creek.

On T r a v e r s e 1, hue y e l l o w s w i t h i n

T 1 a b o u t 40 km f r o m t h e e a s t e r n end, and t h e n y e l l o w s a g a i n a t S t r z e l e c k i Creek i n t o T2 ( F i g . 4 ) .

These s t e p - l i k e changes i n hue a r e p a r a l l e l e d b y s t e p - l i k e

increases i n c o l o u r value.

T o t a l i r o n c o n t e n t a l s o p a r a l l e l s t h e s e changes i n

c o l o u r , so t h a t i r o n p e r c e n t a g e i s h i g h e s t where hue i s m o s t r e d , t h e n a t 40 km i r o n c o n t e n t f a l l s as hue y e l l o w s , and i r o n c o n t e n t i s l o w e s t where hue i s l e a s t red.

N o t o n l y a r e c o l o u r and i r o n c o n t e n t c o r r e l a t e d between m a j o r zones

o f d i f f e r e n t c o l o u r , b u t s u b t l e changes i n c o l o u r and i r o n c o n t e n t p a r a l l e l each o t h e r w i t h i n each zone. The r e s u l t s p r e s e n t e d i n F i g u r e s 3, 4, and 5 i n d i c a t e t h a t t h e dune sands a r e f a i r l y homogeneous i n t h e i r h e a v y m i n e r a l c o n t e n t , b u t a s i g n i f i c a n t d i f f e r e n c e o c c u r s between T r a v e r s e 1 and 2 i n f e l d s p a r c o n t e n t .

This order o f

m a g n i t u d e d i f f e r e n c e i n f e l d s p a r c o n t e n t between t h e p a l e sands o f t h e t w o traverses i s hard t o i n t e r p r e t i n isolation, but i s l i k e l y t o r e f l e c t a difference

i n t h e s e d i m e n t s f r o m w h i c h t h e dune sands were d e r i v e d .

Grain-size Samples were t a k e n f r o m f l a t p o r t i o n s o f dune c r e s t s o n T r a v e r s e 1 and s i e v e d a t 0.25 fl i n t e r v a l s w i t h o u t t r e a t m e n t o t h e r t h a n oven d r y i n g . t h e g r a p h i c mean

Values o f

(MZ) and i n c l u s i v e g r a p h i c s t a n d a r d d e v i a t i o n (Q,) were c a l -

c u l a t e d u s i n g t h e methods o f F o l k ( 1 9 6 8 ) .

A t t h e 90% c o n f i d e n c e l e v e l , v a l u e s

o f MZ f o r T2 c a n be a p p r o x i m a t e d b y t h e normal d i s t r i b u t i o n , b u t T 1 i s b i m o d a l .

As a consequence, MZ v a l u e s f o r T 1 and T2 were compared u s i n g t h e nonp a r a m e t r i c Mann-Whitney U t e s t .

T h i s t e s t showed t h a t a t p 3-

->

_-- +

FIELD

F i g . 2.

A

MAIN STREAMLINES NEGATIVE FLOW SECONDARY FLOW

\-

-

\DITCH

P l a n v i e w o f a wedge-shaped snow d r i f t i n a d i t c h , showing t h e

dispersal o f flow patterns.

B

-

P r o f i l e v i e w o f a wedge-shaped snow d r i f t i n a d i t c h , showing

dispersal patterns.

227 suspensates, t r a n s p o r t e d by s m a l l - s c a l e v o r t i c e s . Repeated d e l i c a t e b r u s h i n g o f surfaces i n l o w p r e s s u r e v o r t i c i t y and t h e p u l s a t i n g wind p a t t e r n account f o r f i n e shaping, b u r n i s h e d s u r f a c e s and s c u l p t u r a l d e t a i l s such as f l u t e s , v o r t e x p i t s and k n i f e - e d g e d k e e l s , l i k e t h o s e developed on v e n t i f a c t s i n A n t a r c t i c a by s i l t - s i z e d i c e c r y s t a l s (Whitney and S p l e t t s t o e s s e r ,

1982). Also, t h e snow d r i f t

demonstrates t h a t f i n e k e e l s f o r m by simultaneous offsweep f r o m b o t h s i d e s . I f t h e e n t i r e c r e s t l i n e o f t h e snow d r i f t i s i n c l i n e d , and t h e r e i s no appr-

e c i a b l e e l e v a t i o n a t i t s head zone t o c r e a t e s t r o n g n e g a t i v e f l o w back t o windward, t h e n c l o s u r e o f t h e s t r e a m l i n e s may o c c u r a l l a l o n g t h e c r e s t , f o r m i n g leeward t r e n d i n g f l u t e s down t h e c r e s t l i n e . I f , however, t h e d r i f t i s e l e v a t e d i n t h e head zone, some wind impact on t h i s head w i l l d e f l e c t upwards and escape, lowering i n p r e s s u r e and a t t r a c t i n g a s t r o n g n e g a t i v e f l o w up t h e c r e s t . The p r i n c i p a l s t r e a m l i n e s w i l l d i v i d e around t h e head zone and move leeward; b u t negative f l o w , r i s i n g o v e r t h e t o p a t t h e i n t e r f a c e , dominates t h e s c u l p t u r a l p a t t e r n , d e f l e c t i n g t h e f l u t i n g upwind a l o n g t h e c r e s t l i n e , as on sand stream-

ers i n t h e l e e o f v e g e t a t i o n . Flow l i n e s i n t h e a n g l e around t h e basal m a r g i n o f a snow d r i f t a r e i n t h e nature o f v h t u r i s , t h a t i s , a c c e l e r a t e d , c a r r y i n g l o w p r e s s u r e t h a t p u l l s secondary f l o w down l a t e r a l s u r f a c e s o f t h e d r i f t , and t h u s p r o d u c i n g a second

s e t o f secondary f l u t e s ( f i g . 2B). T h i s s e t may remain s h o r t and w i d e l y s e p a r a t ed from t h e s e t r e l a t e d t o t h e t o p f l o w l i n e s , o r i t may e l o n g a t e upwards and either j o i n t h e t o p s e t o r i n t e r d i g i t a t e w i t h it. T h i s i s a l s o a p p l i c a b l e t o dunes and f l u t e d v e n t i f a c t s . Where stream1 i n e s dominate t h e e r o s i v e process, secondary f l u t e s may be absent, o r t h e y may be p r e s e n t as segments t h a t t r a n s e c t l a r q e r f l u t e s eroded by t h e s t r e a m l i n e s . Hence, i n a r e a s o f snow and b r i s k winds, wedge-shaped d r i f t s can a f f o r d visual evidence o f processes and e r o s i o n a l r e s u l t s o f t h e complex d i s p e r s a l o f an aerodynamic system. Thus we i n f e r t h a t what i s produced i n an i n s t a n t a n e o u s event on a snow d r i f t may a l s o be produced on c o n s o l i d a t e d m a t e r i a l s , i n a l i k e manner o v e r an extended p e r i o d o f t i m e , by m i l l i o n s o f r e p e t i t i o n s o f p a t t e r n s

o f f l o w a l o n g t h e same t r e n d s . THE SAND DRIFT OR STREAMER The sand d r i f t has much i n common w i t h wedge-shaped snow d r i f t s i n form, patterns o f w i n d d i s p e r s a l and consequent s c u l p t u r a l d e t a i l . Large streamers, developed i n t h e l e e o f v e g e t a t i o n and l o g s , on Padre I s l a n d Texas, e x h i b i t : separated s e t s o f f l u t i n g , r e l a t e d t o t o p and b o t t o m f l o w l i n e s ; i n t e r d i g i t a t i o n

as i n t h e midground; and l e e w a r d d e f l e c t i o n , as a t t h e r i g h t a l o n g t h e t o p margin. Most small streamers t o t h e l e e o f v e g e t a t i o n have s t r o n g l y r e f l e x e d f l u t i n g along t h e k e e l l i n e ( f i g . 3 B ) . T h i s i s due t o t h e f a c t t h a t t h e p l a n t s t o

228

F i g . 3.

A

- P r o f i l e view o f a wedge-shaped sand d r i f t , showing a s e t o f l a t e r a l f l u t i n g r e l a t e d t o t h e t o p f l o w l i n e , a second s e t r e l a t e d t o a basal f l o w l i n e and i n t e r d i g i t a t i o n o f t h e two s e t s . ldind d i r e c t i o n from t h e l e f t . Padre I s l a n d , Texas.

B - R e f l e x e d f l u t i n g c o n v e r g i n g t o a k e e l l i n e i n t h e l e e o f a sand d r i f t i n t h e Kharga Oasis, Egypt. N e g a t i v e f l o w l i n e s j o i n b e f o r e entering the positive flow. windward a c c e n t u a t e t h e u p d r a f t and a l s o cause t h e h i q h e s t p a r t o f t h e d e p o s i t t o f o r m a t t h e windward margin. The u p d r a f t a t t r a c t s n e g a t i v e f l o w u p t h e keel l i n e w h i c h f o r c e s t h e p o s i t i v e f l o w s t i l l h i g h e r . T h i s causes t h e n e g a t i v e f l o w l i n e s t o a t t e n u a t e and converge upward and windward towards t h e k e e l l i n e , formi n q f l u t e s i n t h e i r wake. As t h e p o s i t i v e f l o w bends down t o l e e w a r d o f t h e head zone, p e r i o d i c a l l y t h e a t t e n u a t e d n e g a t i v e f l o w l i n e s f r o m o p p o s i t e s i d e s escape

229 across t h e k e e l l i n e and j o i n w i t h each o t h e r b e f o r e e n t e r i n q t h e d e c l i n i n q p o s i t i v e flow. F i q u r e 4 shows a v e r y s m a l l e r o s i o n a l sand f e a t u r e and diaqram o f t h e f l o w p a t t e r n around i t . U n l i k e t h e streamer i n t h e l e e o f t h e bush, t h i s f e a t u r e had very l i t t l e e l e v a t i o n o r b u l k a t i t s windward end. Hence, t h e r e was l i t t l e t o c r e a t e an u p d r a f t . The wind impinqed o h l i a u e l y on t h e narrow end and t h e s t r e a n l i n e s d i v i d e d around i t w i t h t h e s t r o n q e r f l o w s w i n g i n g c o u n t e r c l o c k w i s e . Those s t r e a m l i n e s c l o s e s t t o t h e f e a t u r e c l o s e d downwind a l o n q a n e a r l y h o r i z o n t a l keel. H e l i c a l scores and s l i q h t depressions i n t h e f l o w t r e n d s i n d i c a t e v e r t i c a l axis v o r t i c e s i n t h e f l o w l i n e s . A n e i g h b o u r i n g sand f e a t u r e o f b a r c h a n o i d f o r m ( f i q . 5 ) a t t a i n e d enough breadth and b u l k i n t h e windward zone t o spread t h e s t r e a m l i n e s c o n s i d e r a b l y more t h a n was t h e case i n t h e k e e l e d f e a t u r e . Furthermore, t h e b u i l d - u p i n height was g r a d u a l f r o m t h e windward m a r q i n t o t h e mid-zone. Thus, when wind swept up t h i s s l o p e a t a l o w a n g l e , t h e r e was no h i q h u p d r a f t and n e g a t i v e f l o w , therefore,

was n o t drawn h i q h above t h e s u r f a c e and g r e a t l y a t t e n u a t e d , b u t

j o i n e d t h e p o s i t i v e f l o w d i r e c t l y , f o r m i n q t h e sharp t r a n s v e r s e k e e l and t h e concave l e e slope, i n s t e a d o f an a x i a l k e e l as was t h e case w i t h t h e streamer i n t h e l e e o f a bush. These two sand s c u l p t u r e s were about a m e t r e a p a r t on a i l i c h i q a n sand dune. There was n o t h i n q i n t h e environment t o i n d i c a t e why one s h o u l d develop as a streamlined f o r m and t h e o t h e r s h o u l d n o t . Hence, t h e e x p l a n a t i o n must l i e i n t h e d i s t r i b u t i o n o f t h e masses so as t o c r e a t e two d i f f e r e n t aerodynamic systems

as t h e wind d i s p e r s e d around them; t h a t i s , d i f f e r e n t d i s t r i b u t i o n o f t h e wind impact, f l o w p a t t e r n s and offsweep p a t t e r n s . Each o f t h e s e t h r e e aspects o f t h e aerodynamic system e x e r t s m a j o r shaping c o n t r o l . Thus, when t h e system i s understood, a g i v e n f e a t u r e has i n t e r p r e t a t i v e value; t h a t i s , i t i s i n d i c a t i v e o f a p a t t e r n o f f l o w , For i n s t a n c e , t h e o r i e n t a t i o n o f t h e k e e l o f t h e streamer ( F i g .

38) and i t s r e f l e x e d f l u t e s r e p r e s e n t t h e escape o f convergent l i n e s o f n e g a t i v e flow p r i o r t o j u n c t u r e w i t h t h e p o s i t i v e f l o w . B u t t h e t r a n s v e r s e k e e l o f f i g u r e

5A i n d i c a t e s d i r e c t j u n c t u r e o f p o s i t i v e and n e g a t i v e f l o w . I n r e p o r t i n g t h e barchan dune s t u d y (Whitney, 1978, p. 15-17),

I n o t e d t h a t t h e k e e l s have been

formed between o p p o s i t e l y moving p o s i t i v e and n e g a t i v e f l o w , i n a case where t r a n s p o r t e d m a t e r i a l s o u t l i n e d t h e f u l l p a t t e r n o f f l o w on b o t h s i d e s o f t h e

c r e s t and converqed a g a i n s t t h e k e e l i n o f f s w e e p and j u n c t u r e . Mainguet (1978) showed t h a t wind impinqed l a r q e l y o b l i q u e l y t o t h e axes o f the g r e a t s e i f dunes o f t h e Sahara. I suggested t o h e r i n 1979 t h a t , i n t h i s case; 1 ) t h e r e had t o be n e g a t i v e f l o w on t h e s i d e s o p p o s i t e t h e w i n d impact,

2 ) t h e w i d t h s o f l i n e s o f i m p a c t i n g w i n d f l o w a r e p r o b a b l y governed by t h e diameters o f t h e v o r t i c e s t h e y c a r r y , and 3) v e l o c i t y i s p r o b a b l y l o w e r i n t h e

230

MAIN STREAMLINES

-+ NEGATIVE FLOW - _ _ + SECONDARY FLOW 71-

N+

YJ

.d

?$

HELICAL SCORE

-/

F i g . 4.

A - S t r e a m l i n e d sand s c u l p t u r e : t o p o f S i l v e r Lake Dunes, Oceana County, M i c h i g a n . About n a t u r a l s i z e . Wind f r o m t h e l e f t . B - Diagram o f t h e f l o w p a t t e r n s on and around t h e sand s c u l p t u r e shown i n f i g . 4A.

231

I.

’

>R--+

-+

(k%

MAIN STREAMLINE NEGATIVE FLOW SECONDARY FLOW HELICAL SCORE

Fig. 5. Barchanoid sand s c u l p t u r e , t o p o f S i l v e r Lake Dunes, Oceana County, Michigan. A - photograph. B - Flow p a t t e r n s . Both n a t u r a l s i z e . Wind from t h e l e f t .

232 i n t e r z o n e s of t h e v o r t i c e s t h a n a t t h e c e n t r e s o f t h e f l o w t r e n d s , t h u s causing f l u c t u a t i n q f o r c e o f t h e o b l i q u e impact on one f l a n k and a f l u c t u a t i n q deqree of n e q a t i v e f l o w on t h e o p p o s i t e f l a n k . ldhere t h e n e q a t i v e f l o w i s s t r o n q e s t , i t would erode t h e l e e f l a n k and t h u s bend t h e keel toward t h e i m p a c t i n g wind. A s e r i e s o f f l o w l i n e s i m p i n g i n g o b l i q u e l y on one f l a n k o f a l i n e a r dune would t h e n produce a sinuous c r e s t . Mainquet (1982, p e r s . comm.) showed me a e r i a l photographs d e m o n s t r a t i n g t h i s h y p o t h e s i s , i n which n e g a t i v e f l o w n e a r t h e tapered l e e s e c t o r s o f l o n g s e i f dunes had segmented t h e dunes, w i t h e s s e n t i a l l y p a r a l l e l l i n e s c u t t i n g o b l i q u e l y a c r o s s t h e dune c r e s t , f i r s t making a number o f s a l i e n t s and f i n a l l y a t e r m i n a l c h a i n o f s e v e r a l s h o r t dunes. The p a r a l l e l i s m o f t h e l i n e s o f n e q a t i v e f l o w showed c l e a r l y t h a t a s i n g l e wind d i r e c t i o n and i t s opposing n e g a t i v e f l o w must have been r e s p o n s i b l e f o r t h e segmentation. The o t h e r concept f o r p r o d u c t i o n o f s i n u o s i t y o f t h e s e dunes i n v o l v e s a l t e r n a t i n g wind d i r e c t i o n s . I n t h i s case, t h e l i n e s o f n e q a t i v e f l o w would n o t be p a r a l l e l u n l e s s t h e two winds were b l o w i n g i n o p p o s i t e d i r e c t i o n s . There i s s t i l l another p o s s i b i l i t y t h a t i s r e l a t e d t o t h e i n t e r d i g i t a t i o n along t h e crest l i n e . I f t h e r i s i n g columns o f secondary f l o w a r e n o t i n phase o r t h e offsweep o f t h e main s t r e a m l i n e s i s n o t symmetrical t o t h e k e e l , t h m i n t e r d i q i t a t i o n and u l t i m a t e l y s i n u o s i t y would r e s u l t . The o r i g i n o f s i n u o s i t y i n s e i f dunes has l o n q been argued f r o m t h e s t a n d p o i n t o f t h e w i n d reqime, b u t p r o b a b l y n o t from t h e p a t t e r n s o f c r e s t a l offsweeps. Since offsweep i s so v e r y i m p o r t a n t i n t h e shaping o f snow d r i f t s and r o c k f e a t u r e s ,

i t mus: a l s o be i m p o r t a n t i n dune

development. J u s t beyond t h e end o f t h e runway a t C a i r o A i r p o r t , Eqypt, I noted a number o f segmented s e i f dunes. I n t h i s area o f Egypt, where winds a r e v a r i a b l e , a s t u d y m i q h t s u p p o r t an a l t e r n a t i n q wind e x p l a n a t i o n . S i n q l e causes may n o t be a p p l i c a b l e , f o r t h e a p p l i c a t i o n o f aerodynamics t o a e o l i a n problems shows t h a t f a c t o r s n o t p r e v i o u s l y c o n s i d e r e d a r e o f q r e a t importance. Shape d i f f e r e n c es a l o n e i n t r o d u c e g r e a t v e r s a t i l i t y i n e n t i r e f l o w systems. THE STREAMLINED YARDANG Yardangs commonly o c c u r as s e t s o f p a r a l l e l r i d g e s i n l a r g e and o f t e n v e r y l o n g yardang f i e l d s i n zones o f h y p e r a r i d i t y t h a t a r e c o n t r o l l e d by wind d i r e c t i o n ; some f i e l d s b e i n g many t e n s o f k i l o m e t r e s l o n g . Yardangs a r e u s u a l l y a t l e a s t t h r e e t i m e s as l o n q as wide (McCauley e t a l . ,

1977). I n d i v i d u a l s range

from f e a t u r e s a m e t r e o r so l o n g , t o g r e a t s t r e a m l i n e d h i l l s s e v e r a l k i l o m e t r e s i n length. The sandstone yardang,shown

i n p r o f i l e on f i g u r e 6, i s about 1.7 metres h i g h

and 4.4 metres l o n q . I t i s l o c a t e d i n t h e u n i d i r e c t i o n a l wind regime o f southwestern Egypt, a r e g i o n o f such h y p e r a r i d i t y t h a t r a i n f a l l s o n l y a few t i m e s each c e n t u r y . For i n s t a n c e , i n 1978, t h e r e s e a r c h group t h a t v i s i t e d an abandon-

233 ed caiiip o f R. A. B a g n o l d , d a t i n g f r o m 1939, f o u n d t h a t t i r e t r a c k s l e f t by h i s v e h i c l e s were s t i l l f r e s h .

/ R

MONODIRETIONAL

0

SHALLOW VORTEX P I T S

F i q . 6. A - Small y a r d a n g and v e n t i f a c t s n e a r B l a c k H i l l , h y p e r a r i d s o u t h w e s t e r n

d e s e r t o f E q y p t . S t r e a m l i n e d f o r m due t o u n i d i r e c t i o n a l w i n d f r o n t h e r i q h t ( n o r t h ) . ( P h o t o b y J . F. PlcCauley, w i t h p e r m i s s i o n , c o u r t e s y o f C a r o l S. B r e e d ) . B - I n f e r r e d wind f l o w p a t t e r n s around t h e s t r e a m l i n e d yardang. The n o r t h w a r d f a c i n q c o n v e x w i n d i m p a c t s u r f a c e a t t h e r i g h t end of t h i s yardang i s e s s e n t i a l l y v e r t i c a l , t h o u q h v e r y s l i g h t l y u n d e r c u t i n t h e l o w e r p a r t . Above t h e v e r t i c a l s e c t i o n o n t h e w i n d w a r d s i d e , t h e r e i s a f l a t t e n e d ,

2 34

i n c l i n e d facet,

a f e a t u r e n o t commonly found on yardanqs. Breed (pers.comm.,

1982) r e p o r t e d t h a t t h e whole c r e s t a l zone was f l a t t e n e d , b u t t h a t n e i g h b o u r i n g yardangs were c r e s t a l l y keeled. From t h e apex, t h e c r e s t l i n e drops away leeward i n t h r e e stages, s t e e p e n i n q i n t h e m i d - s e c t i o n , w i t h a s a l i e n t a n g l e above and a r e e n t r a n t a n g l e below i t s s h o r t m i d s e c t i o n . A sand streamer covers t h e termi n a l s e c t i o n o f t h e low, t a p e r e d l e e end. The f i g u r e d s u r f a c e ( l e f t s i d e ) i s concave i n i t s m i d zone and bears s e v e r a l l a r q e , s h a l l o w p i t s a t t h e t o p o f t h e c o n c a v i t y ; and, above t h e p i t s , s e v e r a l l o n g v e r t i c a l l y o r i e n t a t e d f l u t e s whose bounding r i d g e s t e n d t o converqe towards t h e c r e s t l i n e . S h o r t , v e r t i c a l l y o r i e n t a t e d f l u t e s o c c u r a l o n g t h e b o t t o m m a r g i n o f t h e f a c e . Longer v e r t i c a l l y o r i e n t a t e d f l u t e s o c c u r i n t h e l o w e r p a r t o f t h e head zone a l o n q w i t h s e v e r a l wide, s h a l l o w v o r t e x p i t s on t h e l o w e r and m i d s e c t i o n s o f t h e impact s u r f a c e . The head zone i s s l i q h t l y w i d e r t h a n t h e main body, making a d i s t i n c t s u r f a c e o f d i s c o n t i n u i t y between t h e head zone and l e f t s i d e t h a t i s o r i e n t a t e d o b l i q u e l y f r o m about t h e m i d d l e o f t h e basal m a r g i n up t o t h e base o f t h e f l a t f a c e t ,

c a u s i n g t h i s s i d e o f t h e head zone t o appear t r i a n g u l a r i n t h e i l l u s t r a t i o n . One o f t h e s h a r p e r c o l o u r s l i d e s o f t h i s yardang showed s e t s o f f l u t e s on t h e f l a t f a c e t , separated by a p i t t e d d e p r e s s i o n a c r o s s t h e l o w e r p a r t o f t h e f a c e t ( f i g . 68).

Windward o f t h e yardang, t h e r e i s a s h o r t s h a l l o w moat bordered by a d e p o s i t o f sand, on which Breed i s s t a n d i n g i n f i q u r e 6A. P r i o r t o t h i s photograph, t h e d e p o s i t formed a f l u t e d c o r n u c o p i a w i t h a concave f a c e m o d i f i e d by a c e n t r a l r i d q e ( f i g . 6B). Windward o f t h i s e l e v a t e d d e p o s i t i s a l a r q e f l a t a u r e o l e , t h e c e n t r e o f w h i c h i s t h i n l y covered w i t h rebounded sand. The a d j a c e n t d e s e r t f l o o r i s covered w i t h g r a n u l e s , b u t i n t h e d a r k e r b o r d e r o f t h e a u r e o l e , t h e g r a n u l e c o v e r seems t o t h i c k e n . T h i s b o r d e r zone marks t h e b e g i n n i n g o f t h e d i v i d i n g o f t h e s t r e a m l i n e s i n t h e i n i t i a l p a r t o f t h i s p a r t i c u l a r aerodynamic system. F u l l c l o s u r e o f t h i s system i s l i k e l y t o be some d i s t a n c e t o t h e l e f t o f t h e i l l u s t r a t i o n . I n t h e Kharga a r e a o f Egypt, i n t h e l e e o f yardangs o f a b o u t t h i s same s i z e , t h e qround i s swept f r e e o f g r a n u l e s f o r about 20 metres. Hence, t h e aerodynamic system c r e a t e d by a s a l i e n t f e a t u r e i s c o n s i d e r a b l y l o n q e r t h a n t h e feature i t s e l f . G r a i n impact, u p d r a f t , and n e g a t i v e f l o w r e t u r n i n g f r o m leeward, share i n c r e a t i n g t h e steep windward f a c e : b u t g r a i n impact has l i t t l e e f f e c t elsewhere

on t h i s yardang s i n c e t h e head zone s h i e l d s t h e r e s t o f t h e s u r f a c e , and f l o w l i n e s and small s c a l e n o r m a l - a x i s v o r t i c e s b r i n g g r a i n s and f i n e r suspensates t o s u r f a c e c o n t a c t s i n a p e r s i s t e n t b r u s h i n g mode a l o n g s p e c i f i c t r e n d s o r p r e s s u r e g r a d i e n t s o r i e n t a t e d t o l o c i o f l o w p r e s s u r e b o t h on t h e head zone and over t h e r e s t o f t h e feature.

235 The c l o s u r e o f t h e s t r e a m l i n e s o v e r t h e c r e s t l i n e i s r e s p o n s i b l e f o r t h e development of s h o r t u i d e f l u t e s a l o n a t h e m a r q i n o f t h e m i d and l o w e r p a r t o f the c r e s t l i n e and f o r a t t r a c t i n q l o n g secondary f l o w l i n e s near t h e apex, where d e t a i l e d photographs r e v e a l e d l o n q f l u t e s and sharp r i d q e s c o n v e r q i n g upwards a q a i n s t t h e c r e s t l i n e . Much o f t h e e r o s i o n o f t h e l e f t f a c e may be caused by n e g a t i v e flow, moving t o windward and upward; by secondary f l o w moving toward basal and t o p f l o w l i n e s ; and by more o r l e s s s t a t i c c e n t r e s o f v o r t i c i t y . A l s o , s u b s i d i a r y f l o w i s p a r t i a l l y r e s p o n s i b l e f o r e r o d i n q t h e t o p f a c e t , as evidenced by t h e f l u t i n g . R i s i n g n e g a t i v e f l o w and d e c l i n i n g p o s i t i v e f l o w share i n erodi n q t h e l o n q f l a t t e n e d c r e s t a l s u r f a c e . From t h e r e e n t r a n t a n g l e on t h e c r e s t a l l i n e leeward, some o f t h e s t r e a m l i n e s a l s o become i n t e r f a c i a l i n escaping o v e r the d e c l i n i n g l e e s e c t i o n . F o l l o w i n g impact, l o w p r e s s u r e develops on t h e windward s u r f a c e and b r i n q s a g e n e r a l p a t t e r n o f n e g a t i v e f l o w back t o windward f r o m t h e zone i n t h e l e e o f t h e d e c l i n i n q c r e s t l i n e . L i k e w i s e , u p d r a f t a t t h e base

o f t h e f l a t f a c e t creates low pressure,brinqing l i n e s o f negative f l o w both downward on t h i s f a c e t and a l s o o b l i q u e l y upward o v e r t h e broad c e n t r a l zone o f the l e f t face. I n t e r s e c t i o n o f t h e s e m a j o r l i n e s o f n e g a t i v e f l o w c r o s s i n g t h i s f a c e undoubt e d l y p l a y s a r o l e i n c r e a t i n g t h e v o r t i c i t y t h a t causes t h e l e f t f a c e t o become p i t t e d and concave. Another f a c t o r , however, i s most l i k e l y i n v o l v e d . I n t h e n o r t h e r n hemisphere i r i p i n g i n q winds t e n d t o d e f l e c t s l i q h t l y more c o u n t e r c l o c k wise t h a n c l o c k w i s e around o b j e c t s , commonly c a u s i n q s t r o n q e r u n d e r c u t t i n g o f yardangs and v e n t i f a c t s t o t h e r i g h t t h a n t o t h e l e f t o f t h e i r impact zones. When t h e s t r o n q e r d e f l e c t i o n r i s e s o v e r t h e n o r m a l l y convex windward end o f t h e r i g h t face, i t c r o s s e s t h e c r e s t l i n e o b l i q u e l y . T h i s b r i n g s a p a t t e r n o f n e q a t i v e f l o w o b l i q u e l y up t h e l e f t f a c e f r o m t h e l e f t s i d e o f t h e l e e end t o j o i n w i t h t h e o v e r r i d i n g c r e s t a l offsweep. T h i s n e g a t i v e f l o w produces f l u t e s , v o r t e x p i t s and c o n c a v i t i e s on t h e l e f t f a c e s o f v e n t i f a c t s and yardangs. On t h e l i m e stone yardangs o f llaqub Asyot Plateau, e a s t o f Kharga, Eqypt, i t erodes huge basal m a r g i n a l c a v i t a t i o n s on t h e s o u t h e a s t s i d e s o f t h e l e e t e r m i n i . T h i s i s a most s t r i k i n g example o f t h e e x p r e s s i o n o f t h e o b l i q u e l i n e o f concavo-convex-

i t y . Also, most of t h e l a k e bed yardangs t h a t I saw were e i t h e r s t e e p l y convex

or v e r t i c a l on t h e r i g h t and e i t h e r concave o r g e n t l y i n c l i n e d on t h e l e f t . The axes o f t h e yardangs seemed t o a l i g n p r e t t y w e l l w i t h t h e wind d i r e c t i o n . Hence, b o t h t h e b a s i c wind d i r e c t i o n and t h e d e f l e c t e d wind p a t t e r n r e q i s t e r e r o s i o n p a t t e r n s on t h e s e f e a t u r e s . Flainguet (1978) c l a i m s t h a t t h i s d e x t r a l d e f l e c t i o n a l i g n s l i n e a r dunes o b l i q u e t o t h e wind, b u t a p p a r e n t l y on yardangs and v e n t i f a c t s , i t i s s u b o r d i n a t e t o t h e b a s i c wind d i r e c t i o n . The moat on t h e windward end ( f i g . 6A) i s s h o r t and s h a l l o w and may n o t have a c e n t r a l k e e l e d spetum. The septum i s a common f e a t u r e o f moats where n e g a t i v e

236 f l o w l i n e s meet and r i s e a l o n g t h e impact face. Long, deep moats o f t e n have a k e e l e d septum 1 m e t r e h i q h o r more a t t h e windward base, d i v i d i n g t h e moat i n t o r i g h t and l e f t t r o u g h s . Yardangs o f t h e Kharga a r e a a r e composed o f u n c o n s o l i d a t e d s i l t s d e p o s i t e d i n ponds d u r i n g N e o l i t h i c and Roman t i m e s . D e s p i t e t h e weakness of t h e s e m a t e r i a l s , septa a r e n o t o n l y h i g h b u t o c c a s i o n a l l y e x t e n d up t o t h e apex of t h e yardangs as a windward k e e l . When I s a t down i n t h e shaded r i g h t t r o u g h o f one o f t h e s e l a k e bed yardangs, I n o t e d t h a t warm incoming wiqd was s t r i k i n g me on t h e r i g h t , b u t t h a t c o o l n e g a t i v e f l o w was s t r i k i n g my l e f t arm. Hedin (1903) s t a t e d t h a t around t h e Lop Nor yardanqs o f t h e T a k l a Makhan d e s e r t t h e r e was no s h e l t e r f r o m t h e b i t i n g c o l d winds, and t h a t i n f u r r o w s 4 metres o r more deep w i n d v e l o c i t y seemed even h i g h e r t h a n eisewhere around t h e yardangs ( c i t e d i n McCauley e t a l . ,

1977, p. 1 5 ) . Hence, i t i s c l e a r t h a t w i t h i n

f l u t e s and moats, v e n t u r i s develop t h a t must a c c e l e r a t e e r o s i o n a l o n q t h e l i n e of channeled f l o w . Hedin (1903) a l s o mentioned t l i e f i e r c e eddy e f f e c t f r o m b o t h s i d e s o f t h e l e e end o f a yardang. The same q e n e r a l e f f e c t , b u t t o a m i l d e r deqree, r e s u l t s from t h e m e e t i n q o f t h e l i n e s o f n e g a t i v e f l o w r e t u r n i n g t o t h e windward end o f t h e yardang; b u t here, on some shapes, t h e r e nay be a steep face up which t o d e f l e c t . The m e e t i n q o f t h e s e l i n e s o f n e g a t i v e f l o w above a keeled septum a t t h e windward end e s t a b l i s h e s a p a t t e r n o f simultaneous offsweep o f t h e n e g a t i v e f l o w t h a t c o n t i n u e s up t h e m i d s e c t i o n o f t h e windward f a c e t o f i n a l escape a t t h e apex, c r e a t i n g a windward k e e l between t h e c o n v e r g i n g f l o w l i n e s and a t t h e same t i m e a f f o r d i n g some degree o f p r o t e c t i o n o f t h e windward s u r f a c e f r o m s a n d b l a s t . These k e e l s a l s o g i v e us t h e c l u e t o t h e o r i g i n o f windward k e e l s on t h e g r e a t d o u b l y t e r m i n a t e d yardangs o f t h e p l a t e a u e a s t o f Kharga. I t i s i m p o r t a n t t o n o t e t h a t around t h e l a k e bed yardangs o f t h e Kharga Oasis, t h e r e i s no s h o r t a g e o f sand. The yardangs a r e among, and o f t e n o v e r r i d d e n by, barchan dunes

-

b u t s t i l l m a i n t a i n steep windward f a c e s .

When e v e n t u a l l y a d i v i d i n g septum i n t h e moat o f a l a k e bed yardang i s d e s t r oyed, t h e two l i n e s o f n e g a t i v e f l o w c r e a t e s t r o n g v o r t i c i t v i n t h e i r m e e t i n g t h a t u n d e r c u t s t h e windward end o f t h e yardang, commonly somewhat more t o r i g h t t h a n t o l e f t d u r i n g t h e i n i t i a l stages; b u t , i n t i m e , t h e e n t i r e windward end becomes d e e p l y undermined, p r o d u c i n g a p r o f i l e s i m i l a r t o t h a t o f i c e z a s t r u g i (fig.

7 ) . FlcCauley e t a l . (1979, p. 8224) f i g u r e a group o f small q u a r t z i t e

yardangs on which t h e head zones had become so g r e a t l y u n d e r c u t t h a t t h e y had t o p p l e d . Because o f t h e complex p r e s s u r e r e l a t i o n s h i p s w i t h i n a v o r t e x , v o r a c i o u s e r o s i o n can be o c c u r i n g i n t h e h i g h e r p r e s s u r e p e r i m e t e r w h i l e t h e l o w p r e s s u r e i n t h e c e n t r a l zone may a c t u a l l y r e t a r d e r o s i o n by removing t o o l s . Hence, s a l i e n t s may remain a t p i t c e n t r e s , and t a l l columns may s t a n d on d e s e r t f l o o r s . I n t h e u n d e r c u t zones of t h e l a k e bed yardangs o f t h e Kharga area, v o r t i c i t y has sometimes p r e s e r v e d remarkable p r o t r u s i o n s j u t t i n g t o windward, c a l l e d f i n g e r s , and composed o f d e l i c a t e l y c o h e r e n t p a r t i c l e s . These f i n g e r s a r e o f t e n

237

Fiq. 7.

P r o f i l e view o f l a k e bed yardang w i t h t h e f o r m o f an i c e z a s t r u g i , where n e g a t i v e f l o w has u n d e r c u t t h e windward s i d e and v o r t i c i t y has preserved f i n g e r s extendinq t o windmrd. Kharqa, Eqypt.

no more t h a n a few m i l l i m e t r e s wide, b u t many c e n t i m e t r e s lonq,and a r e s t r u c t u r e s t h a t can n o t w i t h s t a n d t h e m e r e s t t o u c h o f a human hand, y e t a r e o r i e n t a t e d i n t o the wind ( f i g . 7 ) . Seeing these f e a t u r e s on a windward s u r f a c e o f h i g h l y f r i a b l e m a t e r i a l , and l i k e w i s e t h e thousands o f d e l i c a t e f i n g e r s p r o t r u d i n q t o windward on l i m e s t o n e v e n t i f a c t s o f t h e r e g i o n around Kharga, causes one t o wonder what the r o l e o f s a n d b l a s t i n g r e a l l y i s . The presence o f f i n g e r s must s u r e l y denote r e t a r d a t i o n o f e r o s i o n a t c e n t r e s o f v o r t i c i t y , p r o b a b l y a l o n g l i n e s where negative f l o w j o i n s p o s i t i v e f l o w . L i t h o l o g y and t i m e a r e two o t h e r i m p o r t a n t f a c t o r s i n shaping yardangs. The f r i a b l e m a t e r i a l s and t h e coarse g r a i n e d r o c k s i n t h e m o n o d i r e c t i o n a l wind r e q ime, whether sandstone o r weak l a k e beds, t e n d t o erode w i t h v e r t i c a l windward

faces. Many o f t h e l a r g e Nubian Sandstone yardangs s o u t h o f t h e Abu T a r t o u r Plateau i n t h e c e n t r a l western d e s e r t o f Egypt, a r e steep a t b o t h ends and may be h i g h e r t o l e e w a r d t h a n t o windward because o f a r e g i o n a l n o r t h w a r d d i p , t h a t

i s t o windward, and a r a t h e r r e s i s t e n t bed t h a t commonly forms a n o r t h w a r d d e c l i n i n g cap. On t h e Naqub Asyot P l a t e a u , e a s t o f Kharga, t h e e x c e e d i n g l y f i n e - g r a i n ed Eocene Limestone has been c a r v e d i n t o thousands o f d o u b l y t e r m i n a t e d , o r b o a t shaped, yardangs. These yardangs have been f o r m i n g f o r many thousands o f y e a r s . Some have steep windward f a c e s , b u t many have more g e n t l y i n c l i n e d k e e l e d and f l u t e d windward f a c e s . One t y p e i s h i q h e r t o t h e l e e t h a n t o windward and i s almost i n v a r i a b l y d e e p l y u n d e r c u t on t h e l e e s i d e , more t o t h e southeast than the southwest l e e s i d e . T h i s f o r m may have v a s t numbers o f v o r t e x p i t s on t h e l e f t s i d e s , b u t none on t h e r i g h t s i d e s . B o t h s i d e s , however, commonly bear

2 38 remarkable coarse, h i q h l y b u r n i s h e d f l u t e s ( f i o . 8 ) .

F i g . 8. Several small l i m e s t o n e yardanqs w i t h l a r g e basal c o n c a v i t i e s i n t h e i r l e e , some b e a r i n g f l u t e s and v o r t e x p i t s . N o n o d i r e c t i o n a l wind from t h e n o r t h ( r i g h t s i d e ) . Naqub Asyot P l a t e a u , e a s t o f Kharqa, Eqypt. Since t h e yardang i s p a r t o f t h e bedrock, i t does n o t have t h e p o t e n t i a l f o r s h i f t i n g around, a s does a v e n t i f a c t . Since most g e o l o g i s t s c o n s i d e r t h a t t h e l o w f a c e s on v e n t i f a c t s r e p r e s e n t t h e windward s i d e s and t h a t i f t h e s e l o w faces a r e on t h e l e e , t h e n t h e v e n t i f a c t has been r e o r i e n t a t e d , t h e small yardang ( f i g . 6A) i s s i g n i f i c a n t t o t h e s t u d y o f v e n t i f a c t development, p a r t i c u l a r l y i n having a l o w l e e and h i g h steep windward f a c e - f o r so do most o f t h e v e n t i f a c t s associated w i t h it. PATTERNS OF DEVELOPMENT OF BRAZILNUT AND TRIQUETROUS VEIITIFACTS Whereas yardangs a r e c o n s i d e r e d t o have l o n g axes a l i g n e d w i t h t h e wind, t h i s i s n o t always t r u e o f s t r e a m l i n e d v e n t i f a c t s . While, i n g e n e r a l , v e n t i f a c t s remain s t a t i o n a r y t h r o u g h o u t much o f t h e i r h i s t o r y , t h e r e a r e cases where t h e y move, e s p e c i a l l y when reduced i n s i z e . I f t h e l o n q a x i s o f a r o c k i s t r a n s v e r s e t o t h e wind, offsweep w i l l be l a t e r a l , and offsweep k e e l s w i l l t h e n be r i g h t and l e f t i n s t e a d o f windward and l e e . H o s t v e n t i f a c t s f o r m i n b i d i r e c t i o n a l and m u l t i d i r e c t i o n a l wind regimes: t h o s e t h a t f o r m i n m o n o d i r e c t i o n a l regimes a r e s i m i l a r , but d r e i kanter are rare. B r a z i l n u t forms a r e s i m i l a r t o t r i q u e t r o u s forms b u t t e n d t o be b u l k i e r a t the windward end and t o have f o u r o r more f a c e s i n s t e a d o f t h e t h r e e t h a t c h a r a c t e r i z e t h e t r i q u e t r o u s forms. The v e n t i f a c t s used i n t h e s t u d y o f t h e s e two general forms range i n age f r o m Precambrian (Gowganda T i l l i t e ) t o P l e i s t o c e n e and modern

239 v e n t i f a c t s . The N i c h i g a n v e n t i f a c t s were l a r g e l y f r o m l a g g r a v e l s i n t h e i n t e r dune c o r r i d o r s a l o n g Lake M i c h i g a n , and f r o m m o r a i n a l g r a v e l s about 150 metres above Lake M i c h i g a n on S l e e p i n g Bear Moraine, Leelanau County, Michigan. T h i s l a t t e r area has a m u l t i d i r e c t i o n a l wind regime, b u t w i t h l o c a l l y dominant n o r t h west and southsouthwest winds, as i n t h e c l i f f c r e s t s e t t i n q i n which t h e B r a z i l nut ( f i g . 9A) developed. From t h e s t a n d p o i n t o f t h e a x i a l l i n e o f t h e v e n t i f a c t , both o f t h e s e winds impinged o b l i q u e l y on t h e v e n t i f a c t . Hence, t h e f i n e keel i s the r e s u l t a n t o f t h e s e two wind d i r e c t i o n s and t h e i r d e f l e c t i o n a l o n g i t ( f i g .

9B). The d e f l e c t i o n o f t h e k e e l a t t h e l e f t i s due t o n e g a t i v e f l o w up t h e smoothe r surface i n response t o t h e n o r t h w e s t wind r i s i n q o v e r t h e apex. The p r o f i l e

view i s t h a t o f t h e z a s t r u q a form, w i t h a v e r y l a r g e smooth u n d e r c u t f a c e t on t h e windward end. T h i s f a c e t i s s e p a r a t e d f r o m t h e f l a t base under t h e t a p e r e d end by a sharp margin, e x t e n d i n g a c r o s s t h e u n d e r s i d e normal t o t h e c r e s t a l k e e l . Three s h a l l o w , smooth-surfaced round v o r t e x p i t s o c c u r on t h e t a p e r e d base i n t h e offsweep zone. Symmetry among v e n t i f a c t s i s e x t r e m e l y r a r e , even i n m o n o d i r e c t i o n a l regimes. One reason f o r t h i s i s t h a t t h e o r i g i n a l forms a r e r a r e l y symmetrical. Also, i n m u l t i d i r e c t i o n a l wind regimes, one wind m o d i f i e s t h e work o f another. I n e i t h e r m o n o d i r e c t i o n a l o r m u l t i d i r e c t i o n a l wind regimes, dominance o f a d e x t r a l o r s i n i s t r a l wind d e f l e c t i o n t o one s i d e o f t h e o t h e r o f t h e impact f a c e c r e a t e s a l i n e o f p o s i t i v e f l o w opposed by n e g a t i v e f l o w , c r o s s i n g t h e c r e s t o b l i q u e l y and causing c o n v e x i t y t o develop t o windward and c o n c a v i t y t o l e e o f t h e c r e s t . Despite t h e s e s e v e r a l i n f l u e n c e s , B r a z i l n u t and t r i q u e t r o u s forms o c c a s i o n a l l y develop w i t h a f a i r degree o f symmetry. The t r i q u e t r o u s f o r m has two s i d e s and a f l a t t e n e d basal s u r f a c e . I n t h e l a g gravels, f o r m e r beach pebbles, i n t h e dune c o r r i d o r s a l o n g Lake Michigan, e v e r y stage i n t h e h i s t o r y o f t r i q u e t r o u s forms i s r e p r e s e n t e d . F i g u r e 10A and 106 represent i n t e r m e d i a t e stages between a beach p e b b l e and a t r i q u e t r o u s f o r m ( f i g . 1OC): however, i n t h e l a t t e r case t h e winds were o p p o s i t e l y o r i e n t a t e d . The t r i q u e t r o u s f o r m develops i n s i t u , a f a r d i f f e r e n t h i s t o r y t h a n e n v i s i o n e d

f o r i t by o t h e r g e o l o g i s t s . The l a g g r a v e l s a f f o r d t h e f u l l spectrum o f v e n t i f a c t i o n f r o m beach pebbles t o t a t t e r e d w i n d - s h a t t e r e d fragments, i n a v a s t v a r i e t y

o f l i t h o l o g i e s , and a l s o t h e g r e a t advantage o f knowinq something o f t h e o r i g i n a l form o f t h e r o c k p r i o r t o v e n t i f a c t i o n . t i e n c e a t r i q u e t r o u s form i s l i k e l y t o s t a r t as a p l a t y beach p e b b l e w i t h convex s u r f a c e s and steep rounded s i d e s t h a t p e r m i t a submarginal f l o w ( a v e n t u r i ) t o develop around i t s base t h a t p u l l s i n secondary f l o w f r o m t h e s i d e s above and f r o m t h e p e r i p h e r a l base, f l u t i n q t h e sides and i n i t i a t i n g t h e f l a t t e n i n g o f t h e p e r i p h e r y o f t h e base. Since t h e s i d e s are steep, w i n d c r e a t e s an u p d r a f t . \{hen t h i s a i r f a l l s on t h e rounded t o p i t c r e a t e s v o r t i c e s t h a t e v e n t u a l l y erode many p i t s

-

u s u a l l y broad and shallow,

though o c c a s i o n a l l y f u n n e l shaped. A t t h e base, i n t h e l e e o f t h e h i g h e r v e l o c i t y

240

MAIN STREAMLINES NEGATIVE FLOW SECONDARY FLOW

F i g . 9. B r a z i l n u t v e n t i f a c t e r o d e d i n a m u l t i d i r e c t i o n a l w i n d r e g i m e , o f w h i c h t h e n o r t h w e s t wind i s t h e dominant o f t h e two p r i n c i p a l wind d i r e c t i o n s . S l e e p i n g B e a r t l o r a i n e , L e e l a n a u C o u n t y , M i c h i g a n . V e n t i f a c t i s 8.4 cm. long. A - surface view. B - diagram o f f l o w patterns. wind, n e q a t i v e f l o w b e g i n s t o work i t s way u n d e r t h e v e n t i f a c t , c r e a t i n q v o r t i c i t y and e r o d i n g a l e e b a s a l c o n c a v i t y . As t h e v e n t i f a c t i o n p r o c e s s c o n t i n u e s ,

t h i s c o n c a v i t y m i g r a t e s t o w i n d u a r d . The v o r t e x p i t s on t o p expand and c r o w d each o t h e r u n t i l r i m s become hexagonal i n shape. S i d e s b e g i n t o f l a t t e n u n d e r t h e combined i n f l u e n c e o f t h e s u b m a r g i n a l v e n t u r i a n d t h e u p d r a f t and l a t e r a l d e f l e c t i o n : t h e s e a c t i o n s b e g i n t o c r e a t e a s h a r p m a r q i n between s i d e s and t o p which p e r m i t a c c e l e r a t e d f l o w i n t h e l e e w a r d d i r e c t i o n and a t t e n u a t i o n o f t h e o f f s w e e p .

“!s t h e l o w p r e s s u r e a i r escapes, i t p u l l s i n t e r f a c i a l f l o w s f r o m b o t h t h e s i d e

241 and t h e t o p a c r o s s t h e s h a r p m a r g i n . T h i s n o t o n l y c o m p l e t e s t h e f l a t t e n i n g o f t h e t o p s u r f a c e b u t c r e a t e s f l u t i n g . F l u t i n g has a b e t t e r chance o f s u r v i v a l on t h e t o p s u r f a c e t h a n on t h e s i d e s , b u t away f r o m l o c i o f s t r o n g g r a i n i m p a c t , l a t e r a l s u r f a c e s a r e o f t e n w e l l f l u t e d . Many o f t h e t o p s u r f a c e p i t s a r e c o n v e r ted t o f l u t e s , b u t those t h a t remain o f t e n share r i m s w i t h f l u t e s . Both rin type and s u r f a c e c h a r a c t e r o f t h e p i t s a n d f l u t e s a r e s i m i l a r . B o t h f e a t u r e s b e a r numerous m i c r o s c o p i c h e l i c a l and r a d i a l s c o r e s . I t was t h e d i s c o v e r y o f t h e s e scores t h a t showed t h e common o r i g i n , b y v o r t i c i t y , o f f l u t e s and p i t s and i n i t iated t h e s t u d i e s t h a t demonstrated t h e aerodynamic e r o s i o n o f v e n t i f a c t s . I n t h e f l a t t e n i n g o f t h e t o p and s i d e s , t h e n a r r o w i n o p r o c e s s b e q i n s , b e v e l i n g t h e f l a t tened s i d e s and c r e a t i n g t h e o f f s w e e p k e e l s . O f f s w e e p f r o m one s i d e o f a k e e l p u l l s a t r a i l i n g offsweep from t h e opposite side, sharpening t h e terminal keel ( f i q . 10A). The v e n t i f a c t i s now a t a n i n t e r m e d i a t e s t a q e between a beach p e b b l e and t h e t r i q u e t r o u s f o r m ( f i q . lOB)., t h a t i s , a t t h e f l a t t o p p e d s t a g e . I n t h e next p e r i o d o f i t s d e v e l o p m e n t , t h e f l a t t e n i n g o f t h e b a s a l s u r f a c e p r o g r e s s e s inward, d e s t r o y i n g a l l o r m o s t o f t h e b a s a l c o n c a v i t y . When t h e base i s f l a t , t h e b a s a l m a r g i n becomes a t h i n s h a r p l i n e , and t h e b a s a l f l o w now o p e r a t e s above t h e m a r g i n , a t t a c k i n g and d e s t r o y i n g t h e r e m a i n i n q f l u t e s and m a k i n g t h e s i d e s concavo-convex.

I n t h e narrowing process, t h e terminal keels a r e lengthening

u n t i l t h e y j o i n a s a s i n g l e c r e s t a l k e e l ( f i g . l O C ) , and e v e r y s t a g e o f t h i s process i s r e p r e s e n t e d i n t h e l a g g r a v e l s . The l a s t v e n t i f a c t , however, a t t a i n e d i t s shape i n a c h a n n e l where o p p o s i t e w i n d s i m p i n g e d o n t h e t e r m i n i .

F i g . 10. A - f l o w a r o u n d a f l a t - t o p p e d v e n t i f a c t , w i t h emphasis on n o r t h w e s t w a r d flow. B - Top v i e w o f f l a t - t o p p e d v e n t i f a c t , p r o t o t y p e o f t r i q u e t r o u s v e n t i f a c t , o r i e n t a t e d w i t h l o n g a x i s as r e s u l t a n t o f two wind d i r e c t i o n s . C - T r i q u e t r o u s v e n t i f a c t , e r o d e d i n a b i d i r e c t i o n a l w i n d system where t h e t w o o p p o s i t e w i n d s i m p i n g e d on t h e t e r m i n i . L e n g t h 3.2 cm.

242

The t r i q u e t r o u s form miqht seem an ideal shape t o gradually reduce in s i z e , b u t i t i s by no neans t h e end s t a q e , as some have considered. Because t h i s form usually lacks symmetry, concavity may increase on one f l a n k , s h i f t i n q t h e keel l a t e r a l l y u n t i l i t disappears, and t h e f l a t t e n i n o process s e t s in aqain. Appare n t l y , however, basal marginal o u t l i n e s of t h e t r i q u e t r o u s form a r e more likely t o be retained than keels. A t some l o c i on the llaqub Asyot Plateau, Eqypt, the qround i s paved with t h i n , f l a t wafers of limestone, of fusiform and other shapes, no more than about one centimetre long. There a r e , of course, millions of v e n t i f a c t forms. The best known a r e the rare named forms, souqht a f t e r by c o l l e c t o r s , and commonly used t o i l l u s t r a t e ventif a c t s . The t r i q u e t r o u s form i s a c t u a l l y q u i t e r a r e , and takes a long time t o develop. Hence, when i t occurs in f l i n t , a s i t does on t h e fiaqub Asyot Plateau, i t qives us a perspective on t h e lonq period o f a r i d i t y in t h i s part of the world. I n l e s s a r i d regions, f l i n t v e n t i f a c t s a r e so r a r e t h a t I had never seen one: however, t h e Eqyptian d e s e r t i s paved with them. I f wind can shape touqh f l i n t t o a streamlined form, and c r e a t e vortex p i t s and f l u t e s o n t h e i r surfaces, then we do not have t o c a l l upon weatherinq, s o l u t i o n , o r even streams, t o explain erosion o f g r a n i t e , q u a r t z i t e and o t h e r hard rocks in hyperarid environments such a s central Eqypt, where modern stream channels a r e absent, and ancient ones a r e progressively o b l i t e r a t e d by t h e wind t h e deeper t h e d e s e r t i s penetrated. Wind i s capable of finding and exploitinq t h e minutest surface irregularity, For instance, in southwest Eqypt, o r t h o q u a r t z i t e with s i l i c e o u s cement has been converted t o v e n t i f a c t s of spongy, l a c e - l i k e appearance; t h a t i s , bearinq thousa n d s of p i t s and c a v i t i e s where v o r t i c i t y has removed cement, worked inwards and eroded from t h e inside out by loosening sand g r a i n s , which then tumble in the voids t o enlarge t h e inner c a v i t i e s (McCauley e t a l . , 1979). THE NON-STREAMLINED FORMS PRODUCED PRIMARILY BY VORTICITY. This category of wind eroded f e a t u r e s includes vortex p i t s a n d numerous types of s a l i e n t s such a s knobs, domes, p i l l a r s , chimneys, odd-shaped hoodoo forms and conical h i l l s where t h e vortex i s s t a t i o n a r y o r recurrent a t the sane locus and often very l a r q e . Conical h i l l s i l l u s t r a t e t h i s method of erosion. They occur by t h e thousands in Eqypt, e s p e c i a l l y in Cretaceous sandstone, and a r e especially numerous in t h e Abu Simel area along t h e Nile v a l l e y , and in the central western d e s e r t south of t h e Abu Tartour Plateau in association with yardanqs. Saddles form in t h e c r e s t s of t h e yardangs, due t o offsweep and v o r t i c i t y and other facto r s . The intervening c r e s t s then become centres of v o r t i c i t y . The upward escaping a i r r e t a r d s erosion a t the top by removing t o o l s of erosion. This often preserves a small steep-sided cap or tapered cone, b u t the higher pressure a i r , engirdling the lower s e c t i o n , rapidly carves the yardang i n t o a s e r i e s of aligned conical h i l l s . The slopes of these h i l l s become covered with enormous t a l u s blocks

243

because t h e cyclinq wind removes t h e weaker beds, undermininq more massive beds above. Mass wasting then hastens t h e er o s i v e process and continues t o work on the f a l l e n blocks a s t h e wind p en et r at es alonq t he more f r i a b l e l a y e r s . I n time, the h i l l disappears. C i r cu l ar a r e a s bearing a few fraqments of sandstone sometime mark t h e l o c a t i o n of a former h i l l . Ile found in such remnants, and a t the bases of conical h i l l s l i k e t h a t in f i q u r e 1 1 A , blocks somwhat comparable t o McCauley

e t a l ' s (1979) q u a r t z i t e v e n t i f a c t s in t h a t erosion had proqresses f a r back into the rocks: b u t in t h i s cas e, i t \vas alonq beddino planes and did not involve the formation of many p i t s and p er f o r at i o n s . The marqins of the remaining beds, were however, incised i n t o wedqe-shaped f i n g e r s o r i e n t a t e d t o windward ( f i q . 1 1 B ) .

Fig. 11. A - Conical h i l l shaped by v o r t i c i t y and mass wasting. Central western d e s e r t of Egypt. B - Wind erosion along bedding p l an es , c ontributic q t o mass wasting. Central western d e s e r t of Eqypt.

244

COllCLlJS 1Ol.I.S I n windv r e q i o n s , a r i d o r o t h e r w i s e , a l l f e a t u r e s a r e s u b j e c t t o aerodynamic and v o r t i c i t y f o r c e s . I n an aerod.vnamic system, t h e main s t r e a m l i n e s d i v i d e t o windward o f a f e a t u r e , f l o w around i t and c l o s e somewhere 1teLiard. T h i s o f t e n permits l i t t l e contact o f the p r i n c i p a l f l o w p a t t e r n s w i t h the surface o f the f e a t u r e u n l e s s i t s l e e s e c t i o n i s t a p e r e d so as t o i n i t i a t e t h e c l o s u r e o f the s t r e a m l i n e s on t h e f e a t u r e i t s e l f and b r i n o t h e p o s i t i v e f l o w i n t o i n t i m a t e conta c t w i t h t h e s u r f a c e o f t h e l e e s e c t o r o f t h e f e a t u r e . It i s f o r t h i s reason t h a t e r o s i o n i s o f t e n a c c e l e r a t e d i n t h e l e e o f a f e a t u r e and t h a t i n a s i m p l e monod i r e c t i o n a l wind regime o r o p p o s i t e l y b l o w i n g b i d i r e c t i o n a l regime, s t r e a m l i n e d forms develop. I n most o t h e r s e c t o r s around t h e f e a t u r e , e r o s i o n o c c u r s a l o n q l i n e s o f subsi d i a r y f l o w where p r e s s u r e q r a d i e n t s , t r a n s p o r t i n q v o r t i c i t y and susDensates form r e g u l a r p a t t e r n s o f i n t e r f a c i a l f l o w t o some p o i n t o f l o w p r e s s u r e wind-escape such as a m a j o r f l o w l i n e , a k e e l , a v o r t e x p i t o r t h e impact s u r f a c e i t s e l f . Hence, much of t h e shaping and s c u l p t u r i n g o f f l u t e s , p i t s , k e e l s and o t h e r s t r u c t u r e s i s accomplished by t h e s u b s i d i a r y f l o w systems. Thus an aerodynamic system i s f a r more complex t h a n many i n v e s t i q a t o r s have r e a l i z e d and v a r i e s w i t h e v e r y a l t e r a t i o n i n shape and d e t a i l . T h e r e f o r e , i n a m u l t i d i r e c t i o n a l wind, a d i f f e r e n t aerodynamic system r e s u l t s on any one f e a t u r e w i t h e v e r y wind s h i f t , because t h e r e l a t i o n s h i p o f wind d i r e c t i o n t o f e a t u r e o r i e n t a t i o n i s a l t e r e d and

so a l t e r s t h e d i v i s i o n o f t h e s t r e a m l i n e s and t h e s u b s i d i a r y f l o w p a t t e r n s . T h i s amounts t o a shape change. I n a m o n o d i r e c t i o n a l wind t h e f u l l c o n c e n t r a t i o n o f f o r c e s i s m a i n t a i n e d i n a s i n g l e system o f f l o w t r e n d s , a l t e r i n g o n l y g r a d u a l l y as e r o s i o n proceeds. One p a t t e r n common i n a m o n o d i r e c t i o n a l system occurs when t h e c r e s t l i n e o f a f e a t u r e erodes i r r e g u l a r l y , t h u s c a u s i n q l o c i o f s t a t i o n a r y v o r t i c i t y t o develop and c r e a t e c e n t r e s o f v o r t i c i t y around s a l i e n t s . T h i s acticn t o g e t h e r w i t h undermining and mass w a s t i n g produces c o n i c a l h i l l s t h a t a r e so o f t e n a s s o c i a t e d w i t h s t r e a m l i n e d forms. I f aerodynamics and v o r t i c i t y can produce a yardang, t h e s e processes must

a l s o have some f u n c t i o n i n shaping dunes.

AC KNOWLEDGEMEIITS I w i s h t o t h a n k J . F . McCauley, Carol S. Breed, M.J.

G r o l i e r and Bahay I s s i w i

f o r a r r a n g i n g and a s s i s t i n g my s t u d y o f w i n d e r o s i o n i n Egypt, C . Breed f o r f u r n i s h i n g yardang i l l u s t r a t i o n s and R . V . r e a d i n g and c r i t i c i z i n g t h e m a n u s c r i p t .

D i e t r i c h and (1.F. S p l e t t s t o e s s e r f o r

245 REFERENCES

Hedin, Sven, 1903. C e n t r a l A s i a and T i b e t , v o l . 1 81 2. C h a r l e s S c r i b n e r & Sons, Mew York, 608 pp. McCauley, J.F., e t a l . , 1977. Yardangs o f Peru and o t h e r d e s e r t r e q i o n s . U.S. Geol. Survey, I n t e r a q e n c y r e p o r t , A s t r o g e o l o q y , no. 81. McCauley, J.F., e t a l . , 1979. P i t t e d and f l u t e d r o c k s i n t h e western d e s e r t o f Egypt: V i k i n q conparisons. J o u r . Geophys. Res., 84: 8222-8232. Mainguet, M.M., 1978. L ' E r q de F a c h i - B i l m a . C . R . R . S . , Mem. e t Doc., Nnuv. Ser., 18: 1-184. Whitney, M . I . , 1978. The r o l e o f v o r t i c i t y i n d e v e l o p i n g l i n e a t i o n by wind e r o s i o n . Geol. SOC. Amer. B u l l . , 89: 1-18. Whitney, M.I. and S p l e t t s t o e s s e r , J.F., 1982. V e n t i f a c t s and t h e i r f o r m a t i o n : Darwin H o u n t a i n s , A n t a r c t i c a . I n : D.H. Yaalon ( E d i t o r ) , A r i d i c s o i l s and geomorphic processes. Proc. I n t e r n a t . Conf. I n t e r n a t . SOC. S o i l Sci., Jerusalem, I s r a e l . Z e i t s c h r . Geomorph. Catena Suppl., 1 : 175-194.

This Page Intentionally Left Blank

247

WIND TUNNEL MODELING OF ECHO AND CLIMBING DUliES H A I H TSOHR, Department o f Geography, Ben G u r i o n U n i v e r s i t y o f t h e liegev, Beer

Sheva 84 120, P.O.Box 653, I s r a e l . L i s t o f symbols Az A m p l i f i c a t i o n f a c t o r = "2/U1 d

Distance f r o m t h e c l i f f ( l e n g t h )

g

A c c e l e r a t i o n due t o g r a v i t y ( l e n g t h / t i m e 2 )

h

Height o f t h e c l i f f ( l e n g t h )

H

Height o f t h e dune ( l e n g t h )

L

C h a r a c t e r i s t i c h o r i z o n t a l dimension ( l e n g t h )

Re* Roughness Reynolds number

S

Point o f stagnation

Sh Height o f p o i n t o f s t a g n a t i o n ( l e n g t h )

U

Wind v e l o c i t y ( l e n q t h / t i m e )

U1 U n d i s t u r b e d w i n d v e l o c i t y ( l e n g t h / t i m e )

U2 D i s t u r b e d w i n d v e l o c i t y ( l e n g t h / t i m e ) U*

F r i c t i o n v e l o c i t y (length/time)

z0 1

Mean s i z e ( P h i )

+

Roughness l e n g t h o f t h e ground ( l e n g t h )

v+ Standard d e v i a t i o n ( P h i )

Sk$ Skewness ( P h i ) K

$

Kurtosis ( P h i )

v

2 Kinematic v i s c o c i t y ( l e n g t h / t i m e )

a

I n c l i n a t i o n o f t h e c l i f f (degrees) INTRODUCTION E o l i a n sand dunes o r i g i n a t e as a c c r e t i o n s o f sand on e x i s t i n g sand patches,

and may develop i n d e p e n d e n t l y o f f i x e d s u r f a c e f e a t u r e s (Bagnold, 1941). A p a r t

from these s e l f - a c c u m u l a t e d dunes, sand accumulations a r e f o u n d i n f r o n t o f , o r behind, t o p o g r a p h i c o b s t a c l e s such as c l i f f s , shrubs, b o u l d e r s , e t c . (Bagnold,

1941; Cooke and Warren, 1973; Elabbutt, 1977). The f o r m o f such dunes ensues f r o m t h e a c c e l e r a t i o n o f t h e w i n d around t h e o b s t r u c t i o n , t h e d e c e l e r a t i o n i n f r o n t

of, o r behind, i t , and t h e d e f o r m a t i o n o f t h e w i n d d i r e c t i o n around i t . According t o t h e i r l o c a t i o n r e l a t i v e t o t h e o b s t r u c t i o n s , t h e s e dunes can be divided i n t o l e e w a r d and windward a c c u m u l a t i o n s (Cooke and Warren, 1973; Mabbutt,

248 1977

.

I n t h e l e e of the o b s t acl e, sand accumulates t o form l e e dunes ( e . g . S m i t h ,

1963 f i g . 6 ; Smith, 1978, f i g s . 5-2a and 5 - 2 b ) . The c h a r a c t e r i s t i c flow-field t h a t causes l e e f e a t u r e s has been revealed by Greeley e t a l .

1 9 7 4 a ) in wind t u n n e l

s i m u la t i o n s . Wind tunnel and f i e l d work t o determine the a i r f

ON

Dast shrubs shows

s i m i l z r l e e f e a t u r e s (Hesp, 1981). Windward accumulations depend upon t h e slope of the obstac e . They can be subdivided i n t o echo dunes - s i n g l e sand ridges formed p a r a l l e l t o v e r t i c a l cl ffs (Clos-Arceduc, 1 9 6 9 ) , and f e a t u r e l e s s dunes t h a t climb ge ntle slopes (Evans 1962) k n o w n as climbing dunes ( f i g s . 1 and 2 ) .

-

Fig. 1. Echo dunes i n f r o n t of a cucsta c l i f f a t Paiute Point, northern Arizona. Note t h a t t h e s i d e s of t h e echo dune develop i n t o climbing dunes on t h e g e n t l e s l o p e s of t h e g u l l i e s .

Fig. 2 . Climbing dune on a g e n t l e slope in t h e Ploenkopi Pla te a u, northern Arizona.

249

Fig. 3. An echo dune i n f r o n t o f an abandoned r a i l w a y s t a t i o n , n o r t h e r n S i n a i . T h i s paper p r e s e n t s d a t a c o n c e r n i n g t h e f o r m a t i o n o f echo and c l i m b i n g dunes gathered d u r i n g w i n d t u n n e l t e s t s made on s e v e r a l s i m u l a t e d c l i f f s w i t h d i f f e r e n t i n c l i n a t i o n s . Three t y p e s o f measurement were t a k e n : f i r s t , modeling o f echo and c l i m b i n g dunes i n f r o n t o f s i m u l a t e d c l i f f s ; second, t r a c i n g o f eddies and wind turbulence i n f r o n t o f t h e s i m u l a t e d c l i f f s and o v e r t h e echo dune models, through a bubble g e n e r a t o r ; t h i r d , measurements o f w i n d v e l o c i t y i n f r o n t o f t h e s i m u l a t e d c l i f f s and o v e r t h e echo dune models, w i t h a h o t - w i r e anemometer. Echo and c l i m b i n g dunes a r e f o u n d i n f r o n t o f d e s e r t c l i f f s ( f i g s . 1 and 2 ) , coastal c l i f f s (Bowman, 1981, f i g . 2e) and a r t i f i c i a l c o n s t r u c t i o n s i n areas o f s h i f t i n g sands ( f i g . 3 ) . P r e v i o u s s t u d i e s o f echo and c l i m b i n g dunes Deposits caused d i r e c t l y by f i x e d o b s t r u c t i o n s a r e d e s c r i b e d i n Bagnold (1941) as sand shadows. A c c o r d i n g t o Bagnold (1941), sand g r a i n s t h a t s t r i k e a v e r t i c a l

obstacle rebound o f f i t and come t o r e s t i n t h e s t a g n a n t a i r b e f o r e t h e o b s t a c l e , thus f o r m i n g a heap. More sand s l i d e s down t h e s l o p e s and j o i n s sand streams passing a l o n g t h e s i d e s o f t h e o b s t a c l e . A c c o r d i n g t o Clos-Arceduc (1969) and Embleton e t a1

.

(1979), echo dunes a r e generated by a s t a n d i n g v o r t e x o f a i r

created beneath t h e f l o w up and o v e r an escarpment. D e s c r i p t i o n s o f c l i m b i n g dunes are g i v e n by Smith (1954), Evans (1962) and G o l d s m i t h (1978). M o d e l i n g o f dunes and c l i f f s i n t h e w i n d t u n n e l The p r e s e n t s t u d y was conducted a t an open c i r c u i t w i n d t u n n e l w i t h a w i d t h

o f 1 metre, a h e i g h t o f 0.82 metres, and a l e n g t h o f 12.5 metres. Rigorous r e s t r i c t i o n s govern t h e i n t r o d u c t i o n o f sand i n t o t h e w i n d t u n n e l . Studies u s i n g sand have h i t h e r t o o n l y examined sand t r a n s p o r t and r i p p l e s (Bagnold,

250 1941; Ford, 1957; Seppala and Linde, 19781, w i n d s t r e a k s ( G r e e l e y e t a l . ,

1974a,b)

and e r o s i o n of s o i l and dune sands ( C h e p i l and Woodruff, 1963; G i l l e t t e , 1978; L o g i e , 1981). Model s t u d y i n a w i n d t u n n e l r e q u i r e s two t y p e s o f s i m i l a r i t y : 1 ) t h e model and t h e p r o t o t y p e s h o u l d be g e o m e t r i c a l l y s i m i l a r ; 2 ) t h e model and t h e prototype flows s h o u l d be k i n e m a t i c a l l y s i m i l a r (dynamic s i m i l a r i t y ) . A l l l i n e a r dimensions of wooden models a r e r e l a t e d t o t h e c o r r e s p o n d i n g dimensions o f t h e p r o t o t y p e by a c o n s t a n t s c a l e f a c t o r o f about 50. To e s t a b l i s h t h e c o n d i t i o n s f o r dynamic s i m i l a r i t y , a l l t h e f o r c e s r e l e v a n t t o t h e f l o w must be c o n s i d e r e d . T h i s can be achieved w i t h t h e Buckingham P i Theorem ( M i r o n e r , 1979), i n which a l l t h e i m p o r t a n t v a r i a b l e parameters a r e matching factor between t h e model and t h e p r o t o t y p e f l o w s .

It i s i m p o s s i b l e t o r e a c h a s a t i s f a c t o r y dynamic s i m i l a r i t y o f a l l parameters 2 ( I v e r s e n , 1979). Froude number (U / g L ) i s t h e most p o p u l a r parameter as appropriate f o r s i m i l i t u d e (Iversen,

1980). The f u l l scale-model w i n d speed r a t i o would be

j u s t t h e square r o o t o f t h e l e n g t h r a t i o , which i s n o t a r e a l i s t i c v a l u e . Therefore t h e r a t e o f a c c u m u l a t i o n o f sand i n t h e dune i s n o t c o r r e c t l y modeled and t h e f i n a l shape o f t h e dune i s p r o b a b l y somewhat d i f f e r e n t f r o m f u l l s c a l e ( I v e r s e n , 1982, p e r s o n a l communication). The two main l i m i t a t i o n s t o accuracy a r e g r a i n s i z e and wind v e l o c i t y . Dynamic s i m i l a r i t y would r e q u i r e m i n u t e sand g r a i n s t h a t would n o t s a l t a t e and v e l o c i t i e s t h a t would be below t h e t h r e s h o l d o f a l l g r a i n s i z e s . The c l o s e s t p o s s i b l e approxi m a t i o n can be a c h i e v e d by u s i n g t h e s m a l l e s t a v a i l a b l e g r a i n s i z e t h a t s a l t a t e s , and v e l o c i t i e s t h a t approach t h e t h r e s h o l d . The f o u r moment s t a t i s t i c s (McBride, 1971) o f t h e sand used h e r e were:

x4

= 3 . 2 ; 0 $ = 0.58;

Sk

= 0.4;

K

,+

T h i s sand i s f i n e r b y l . O , + t h a n n a t u r a l d e s e r t dune sand.

$

= 4.07.

S t r i c t m a t c h i n g o f Reynolds number i s n o t r e a l l y necessary f o r dynamic s i m i l a r i t , (Howard e t a l . ,

1977). When t h e t u r b u l e n t f l o w i s c o m p l e t e l y developed, t h e specifii

numerical v a l u e o f t h e Reynolds number i s u n i m p o r t a n t as l o n g as we have: Re* = U*z0/v

less than 3

(Sundaram e t a1

. , 1972)

(1)

The o n l y way t o g e t a f u l l y t u r b u l e n t f l o w i s by i n c r e a s i n g zo. T h i s was done by i n t r o d u c i n g a rough s u r f a c e b e f o r e t h e t e s t s e c t i o n i n t h e t u n n e l . RESULTS AND DISCUSSION The a c c u m u l a t i o n o f sand i n f r o n t o f a c l i f f The f o r m a t i o n o f echo dunes was m o n i t o r e d i n f r o n t o f a v e r t i c a l l o w board ( s i m u l a t e d c l i f f ) l o c a t e d a c r o s s t h e t u n n e l and spanning t h e f u l l w i d t h o f t h e t u n n e l ( f i g . 41. The source sand was i n s e r t e d 1.6 metres upwind f r o m t h e simulated

251 c l i f f . The wind v e l o c i t y as measured t h r o u g h a p i t o t t u b e i n t h e m i d d l e o f t h e tunnel was between 6 and 9 metres/second ( j u s t above t h e t h r e s h o l d speed o f t h e sand.).In a l l cases, sand tended t o accumulate a t a c e r t a i n d i s t a n c e f r o m t h e

c l i f f ( f i g . 4).

Fig. 4. A s i m u l a t e d v e r t i c a l c l i f f across t h e wind t u n n e l . N o t i c e t h a t t h e sand accumulates up t o a c e r t a i n d i s t a n c e i n f r o n t o f t h e c l i f f . Measurements o f t h i s d i s t a n c e ( d ) a t d i f f e r e n t c l i f f h e i g h t s ( h ) show t h a t d u r i n g t h e f i r s t s t a g e o f sand a c c u m u l a t i o n i n f r o n t o f t h e s i m u l a t e d c l i f f we do n o t

g e t any sand a t d / h l e s s t h a n 0 . 3 ( t a b l e 1 , f i g . 5 , I

-

111).

TABLE 1 Results o f measurements o f t h e w i d t h ( d ) o f t h e a r e a f r e e o f sand i n f r o n t o f simulated c l i f f s h a v i n g d i f f e r e n t lieights.(h)

h

d

(cm. 1 1.90

(cm. 1 0.60

3.90

1.30

0.32

5.70

1.90

0.33 0.33

7.60

2.50

0.33

9.55

3.70

12.00

4.50

0.39 0.38

252

F i g . 5. Cross s e c t i o n s t h a t r e p r e s e n t s i x stages i n t h e f o r m a t i o n and development o f an echo dune i n t h e w i n d t u n n e l . h = s i m u l a t e d c l i f f h e i g h t ; d = d i s t a n c e f r o m t h e c l i f f ; t = t i m e i n hours f r o m t h e b e g i n n i n g o f the test, When more sand i s added, an echo dune i s g r a d u a l l y c r e a t e d w i t h a c r e s t between 0.5 d / h t o 0.6 d/h ( f i g . 5, I 1 1

-

V I ) . As t h e dune becomes h i g h e r , sand covers

t h e a r e a where d / h i s l e s s t h a n 0.3 ( f i g . 5, I V

-

V I ) , b u t t h e c r e s t l i n e remains

a t 0.5 d/h. When t h e s i m u l a t e d c l i f f i s i n c l i n e d , t h e r a t e s d/h o f t h e accumulation i n f r o n t o f t h e c l i f f i s changed. T a b l e 2 g i v e s t h e v a l u e s o f accumulation, d/h, i n various slope i n c l i n a t i o n s , a

.

The c o r r e l a t i o n between t h e d/h values and those

o f o( i s v e r y good ( r = 0 . 9 8 8 ) . It seems t h a t s l o p e s o f 50 degrees and l e s s bring a b o u t c l i m b i n g dunes, w h i l e s t e e p e r i n c l i n a t i o n s e f f e c t accumulations a t some value o f d/h i n f r o n t o f t h e c l i f f . TABLE 2 The r a t e s o f d/h o f sand a c c u m u l a t i o n i n f r o n t o f i n c l i n e d s l o p e s i n a degrees

d/ h

(degraees)

0

50

0.06

55

0.11

60

0.29

75

0.37

90

253 Long t e r m m o n i t o r i n g o f t h e a c c u m u l a t i o n o f sand i n f r o n t o f 60 degree c l i f f showed t h a t a f t e r 28 hours o f wind f l o w , j u s t above t h e t h r e s h o l d , sand accumulated

up t o d / h v a l u e s equal t o 0.04. A f t e r 52 hours t h e sand s t a r t e d t o c l i m b t h e c l i f f and turned i n t o a c l i m b i n g dune. r l o n i t o r i n g o f a 75 degree c l i f f showed t h a t accumulation a f t e r 24 hours o f wind f l o w was up t o d/h = 0.1,

and t h e c r e s t l i n e

was a t 0.375 d / h . A t t h i s stage t h e sand accumulation had a morphology o f an echo dune ( f i g . 6 ) .

I

I

t=24hr

1

1.5 1:5

.O .o

Fig. 6. A p r o f i l e o f an echo dune, formed i n f r o n t o f a s i m u l a t e d 75 degree slope a f t e r 24 hours o f wind r u n . h = s i m u l a t e d c l i f f h e i g h t , d = d i s t a n c e f r o m the c l i f f .

Measurements o f eddies and w i n d t u r b u l e n c e i n f r o n t o f t h e s i m u l a t e d c l i f f and o v e r t h e echo dune models A bubble g e n e r a t o r was used t o t r a c e t h e eddies and t h e wind t u r b u l e n c e i n f r o n t of t h e s i m u l a t e d c l i f f and on t h e l e e s i d e o f t h e echo dune models. The generator e m i t t e d bubbles made o f helium, a i r and d e t e r g e n t f r o m a p i p e p l a c e d 10 c e n t i m e t r e s above t h e t u n n e l f l o o r , a t a d i s t a n c e o f 1.5 metres f r o m t h e model s. F i g u r e 7 shows t h e t r a c k s o f t h e bubbles w i t h v a r i o u s models. F i g u r e 7A,B shows t h e p a t t e r n o f f l o w i n f r o n t o f t h e s i m u l a t e d c l i f f . When t h e a i r f l o w approaches t h e c l i f f p e r p e n d i c u l a r l y , i t slows down w i t h a consequent b u i l d - u p o f pressure a g a i n s t t h a t f a c e (Eaton, 1981). As a r e s u l t t h e s t r e a m l i n e s s t a r t t o separate f r o m t h e f l o o r a t d/h = 1.0 t o 2.0.

Some o f t h e s t r e a m l i n e s r i s e and

flow o v e r t h e c l i f f , w h i l e o t h e r s make a l o o p and c r e a t e a r e v e r s e - f l o w eddy.

The p o i n t on t h e c l i f f a t which t h e s t r e a m l i n e s d i v i d e i n t o f l o w t h a t t r a v e r s e s the c l i f f and f l o w t h a t c r e a t e s t h e r e v e r s e - f l o w eddy, i s c a l l e d t h e p o i n t o f stagnation

(s).

The h e i g h t o f t h e p o i n t o f s t a g n a t i o n (Sh) i n one case was 0.72h and i n t h e other 0.63h ( f i g . 7A,B). S i m i l a r r e s u l t s were a c h i e v e d by Pande e t a1.(1980) i n f r o n t o f a fence, where t h e upstream s e p a r a t i o n l e n g t h was 1.22h and t h e p o i n t

o f s t a g n a t i o n was a t a h e i g h t o f 0.72h.

A

2

C

r-

2

I

1.5

1 d/h .5

F i g . 7. Tracks o f bubbles i n *Front o f and o v e r d i f f e r e n t models: A,B cliff;

C

-

75 degree slope; D

-

60 degree slope; E

-

0

-

vertical

45 degree slope;

F

-

echo dune model, H/h = 0.18 [ H = dune h e i g h t , h = c l i f f h e i g h t ) ;

G

-

echo dune model, H/h = 0.324;

H

-

echo dune model, H/h = 0.6.

S = s t a g n a t i o n p o i n t , Sh = h e i g h t o f S.

TABLE 3 The r e l a t i v e h e i g h t o f t h e s t a g n a t i o n p o i n t (Sh) i n d i f f e r e n t i n c l i n a t i o n s ( a ) o f the c l i f f a (degrees)

Sh

90

0.63

75

0.44

60

0.27

255 The d e p o s i t i o n o f sand a t d / h g r e a t e r t h a n 0.3 ( t a b l e 1) was a r e s u l t o f t h e reverse f l o w . A t t h a t d i s t a n c e t h e r e was an encounter between two o p p o s i t e d i r e c t ions, which r e s u l t e d i n a l o w e r i n g o f t h e wind v e l o c i t y and caused d e p o s i t i o n . When i n c l i n e d c l i f f s were modeled ( f i g . 7C,O & E ) t h e r e v e r s e f l o w eddy became smaller. Slopes o f 60 and 45 degrees c r e a t e d a small o r i n d i s c e r n i b l e l o o p and the p o i n t o f s t a g n a t i o n was lowered a c c o r d i n g l y . There i s a v e r y good c o r r e l a t i o n ( r = 0.99) between t h e h e i g h t o f t h e p o i n t

o f stagnation

(37)

and t h e i n c l i n a t i o n ( a ) o f t h e c l i f f ( t a b l e 3 ) . The c o r r e l a t -

ion e q u a t i o n i s :

- 0.453

Sh = 12 x

(2)

When Sh = 0, t h e n a i s 38 degrees a c c o r d i n g t o e q u a t i o n 2. T h i s means t h a t t h e r e

w i l l be no s t a g n a t i o n p o i n t and no s e p a r a t i o n i n f r o n t o f slopes whose i n c l i n a t i o n s are l e s s than 38 degrees. T a b l e 2 shows t h a t sand tends t o c l i m b a 50 degree slope, and t h a t when i t g a t h e r s i n some q u a n t i t y i n f r o n t o f a 60 degree s l o p e i t a l s o s t a r t s t o c l i m b . When t h e r e v e r s e - f l o w eddy i s v e r y small case of a 60 degree s l o p e ( f i g . 7D)

-

-

as i n t h e

sand a c c u m u l a t i n g a t t h e base o f t h e c l i f f

causes a r e d u c t i o n o f t h e v i r t u a l s l o p e which i t e v e n t u a l l y begins t o ascend. When echo dune models b u i l t o f wood were p l a c e d i n f r o n t o f t h e s i m u l a t e d c l i f f w i t h shape and p o s i t i o n s i m i l a r t o t h o s e m o d e l l e d w i t h sand ( f i g .

5), t h e bubbles showed c o n s t a n t and p r o m i n e n t r e v e r s e - f l o w eddies between t h e dune and t h e c l i f f ( f i g . 7F,G,H).

I t seems t h a t t h e p r e s s u r e f i e l d b u i l d - u p on t h e

dune i n c r e a s e d t h e r e v e r s e f l o w i n f r o n t o f t h e c l i f f . The s t a g n a t i o n p o i n t moved up t h e c l i f f , as t h e dune model h e i g h t r e l a t i v e t o t h e c l i f f increased. Measurements o f wind v e l o c i t y i n f r o n t o f c l i f f s and o v e r echo dune models Wind v e l o c i t y

measurements were c a r r i e d o u t by a h o t w i r e anemometer, whose

probe was s e n s i t i v e t o t h e l o n g i t u d i n a l v e l o c i t y component o f t h e f l o w . The anemometer probe was p l a c e d on t h e wind t u n n e l f l o o r o r t h e model s u r f a c e , so t h a t the h o t w i r e was 6 m i l l i m e t r e s above t h e s u r f a c e . I n o r d e r t o compare t h e r e s u l t s

o f t h e measurements t a k e n on d i f f e r e n t s i z e s o f model, t h e d i s t u r b e d wind v e l o c i t y o v e r and n e a r t h e models ( U 2 ) , was r e l a t e d t o t h e u n d i s t u r b e d wind v e l o c i t y ( U l ) . The r a t i o between t h e two, U2/U1,

i s c a l l e d t h e speed-up r a t i o o r t h e a m p l i f i c a t i o n

f a c t o r , Az (Bowen and L i n d l e y , 1977) Figure

8A shows t h e a m p l i f i c a t i o n f a c t o r , Az, i n f r o n t o f a s i m u l a t e d c l i f f .

Az s t a r t s t o d r o p a t a d i s t a n c e o f d / h = 3.3 as a r e s u l t o f t h e b u i l d - u p of pressure a g a i n s t t h e c l i f f , and reaches a minimum a t d / h = 0.74, opposite f l o w d i r e c t i o n s meet ( s e e f i g . imum a t d/h

=

where t h e two

7A,B). From t h e r e Az i n c r e a s e s t o a max-

0.275 and t h e n decreases again. T h i s i s t h e area where t h e r e v e r s e

256

B

A

1 .5 0 L

F i g . 8. V a r i a t i o n o f the a m p l i f i c a t i o n f a c t o r (Az) i n f r o n t o f simulated c l i f f s and slopes: A - v e r t i c a l c l i f f ; B - 75 degree slope; C - 60 degree slope; 0 - 50 degree slope; E - 45 degree slope. h = simulated c l i f f height, d = d i s t a n c e from t h e c l i f f o r slope base. flow eddy e x i s t s ( f i g . 7A,B).

A slope i n c l i n e d a t 75 degrees ( f i g . 88) shows t h e

same p a t t e r n , but t h e minimum and maximum p o i n t s s h i f t toward t h e c l i f f ( d/h = 0.5 and 0.125,

r e s p e c t i v e l y ). T h i s i s a r e s u l t o f t h e l o w e r i n g o f t h e height o f

t h e stagnation p o i n t and t h e r e d u c t i o n i n diameter o f t h e reverse-flow eddy (fig.

7 C ) . An i n c l i n e d c l i f f o f 60 degrees does n o t show a prominent r i s e i n Az

c l o s e t o t h e c l i f f ( f i g . 8C), as i s a l s o t r u e f o r a c l i f f i n c l i n e d a t 50 degrees ( f i g . 8D).

A slope i n c l i n e d a t 45 degrees has no p o i n t o f maximum Az i n f r o n t o f

t h e c l i f f ( f i g . 8E). The p o i n t o f minimal Az i s on t h e base o f t h e c l i f f . Measurements o f Az over echo dune models t h a t a r e i n t h e same p o s i t i o n as the models o f f i g u r e 7 (F,G,H)

show a decrease i n Az toward t h e windward base of the

dune model, and then an increase toward t h e c r e s t ( f i g . 9A,B,C).

I n t h e lee,

between t h e dune model and t h e simulated c l i f f t h e r e i s a sharp abatement i n AZ, a subsequent increase, and f i n a l l y another drop toward t h e c l i f f . This pattern can be explained by reference t o f i g u r e 7 (F,G,H).

The r a t e o f increase i n

AZ

on t h e windward slope of t h e modelled dune depends on t h e r e l a t i v e s i z e o f the dune. The increase i n Az on t h e l e e s i d e i s a r e s u l t o f t h e prominent reversef l o w eddy. When t h e echo dune i s small r e l a t i v e t o t h e h e i g h t o f t h e c l i f f , t h e magnitude o f t h e reverse-flow i s h i g h e r than t h e increased wind on t h e dune crest ( f i g . 9A). A t t h i s stage t h e dune develops ( f i g . 5 ) .

257

Az

20

15

3.5

3 2.5 2

gh

1.5

1

.5

0

Fig. 9. V a r i a t i o n o f t h e a m p l i f i c a t i o n f a c t o r (Az) o v e r echo dune models. A : H/h = 0.18; B: H/h = 0.324; C : H/h = 0.6. (H = dune h e i g h t , h = c l i f f height). Where t h e dune has accumulated t o t h e p o i n t t h a t t h e magnitude o f t h e wind on the c r e s t has become h i g h e r t h a n t h e r e v e r s e - f l o w ( f i g . 9B), i t ceases t o grow. It seems t h a t t h e echo dune comes t o a s t e a d y s t a t e when t h e h e i g h t o f t h e dune

(H) i s a b o u t 0.3 t o 0.4h. A t t h i s h e i g h t t h e a i r f l o w on t h e windward s i d e o f t h e dune equals t h e r e v e r s e - f l o w on t h e l e e s i d e and t h e s t a g n a t i o n p o i n t ( S ) i s c l o s e t o t h e r i m o f t h e c l i f f ( f i g . 7G,H), w h i c h means a l o w e r magnitude r e v e r s e - f l o w eddy. The s i t u a t i o n p r e s e n t e d on f i g u r e 9C does n o t e x i s t i n r e a l i t y . Sand t h a t was added t o an echo dune i n a s t e a d y s t a t e c o n d i t i o n would move t o t h e t r o u g h between t h e dune and t h e c l i f f and be c a r r i e d a l o n g i t by a l o n g r o l l e r v o r t e x . T h i s s i t u a t i o n was observed a t P a i u t e P o i n t i n n o r t h e r n A r i z o n a ( f i g . 1). Here t h e s i d e s o f t h e echo dunes d e v e l o p i n t o c l i m b i n g dunes on t h e gentle s l o p e s o f t h e small g u l l i e s . Sand t h a t moves l a t e r a l l y a l o n g t h e t r o u g h between t h e echo dune and t h e c l i f f e n t e r s t h e c l i i i i b i n g dune and i s pushed up slope t o t h e p l a t e a u ( f i g . 1). SUMMARY AND CONCLUSIONS According t o t h e r e s u l t s o b t a i n e d i n t h e w i n d t u n n e l , t h e f o l l o w i n g s t a g e s f o r t h e development o f echo dunes i n f r o n t o f a c l i f f were i d e n t i f i e d .

1) When wind e n c o u n t e r s a v e r t i c a l c l i f f i t s v e l o c i t y b e g i n s t o d r o p a t a d i s t a n c e o f about d / h = 3, and a t t a i n s minimum v e l o c i t y a t d / h = 0.75.

Closer t o t h e c l i f f

2 58 t h e v e l o c i t y i n c r e a s e s and t h e d i r e c t i o n r e v e r s e s as a r e s u l t o f a reverse-eddy flow. 2 ) Sand d e p o s i t s s t a r t a t a d i s t a n c e o f d / h = 2.0 t o 0.4.

I n no case was sand

f o u n d between t h e c l i f f ( d / h = 0) and d / h = 0.3,

a t the s t a r t o f deposition.

3 ) When sand p i l e s up i t c r e a t e s an "echo-dune".

I n t h e l e e o f t h i s dune a separ-

a t i o n eddy develops which has t h e same phase as t h e r e v e r s e - f l o w eddy. The reverse. f l o w i s s t r e n g t h e n e d and a t t a i n s h i g h e r v e l o c i t i e s t h a n t h e wind above t h e dune c r e s t . T h i s b r i n g s a b o u t a r a p i d i n c r e a s e i n t h e h e i g h t o f t h e dune. 4 ) As t h e dune g a t h e r s h e i g h t , t h e magnitude o f t h e wind f l o w above t h e dune c r e s t i n c r e a s e s . A t a h e i g h t o f about 0.3 t o 0.4h t h e v e l o c i t y o v e r t h e c r e s t e q u a l s t h e r e v e r s e - f l o w v e l o c i t y . The dune a t t h i s stage i s i n a s t e a d y s t a t e . The same amount o f sand t h a t j o i n s t h e dune l e a v e s i t , t h r o u g h a r o l l e r v o r t e x movement a l o n g t h e t r o u g h between t h e dune and t h e c l i f f .

5 ) When t h e c l i f f i s i n c l i n e d , t h e s i z e o f t h e r e v e r s e - f l o w eddy decreases. Slopes of g r e a t e r t h a n 55 degrees have small r e v e r s e - f l o w eddies which have no e f f e c t on t h e sand, which t h e n t e n d s t o accumulate a t t h e base o f t h e s l o p e and t o climb i t . T h i s i s t h e f o r m a t i o n o f a " c l i m b i n g dune".

ACKNOWLEDGEMENTS T h i s work was conducted a t t h e P l a n e t a r y Geology L a b o r a t o r y o f t h e Department of Geology, A r i z o n a S t a t e U n i v e r s i t y . The a u t h o r i s d e e p l y i n d e b t e d t o Professor Ronald G r e e l e y f o r h i s h e l p , a d v i c e and s u g g e s t i o n s d u r i n g t h e s i m u l a t i o n proceedu r e . Comments and c r i t i c a l r e v i e w by P r o f e s s o r James D. I v e r s e n and D r . Andrew Warren a r e g r a t e f u l l y acknowledged. The work was s u p p o r t e d by t h e P l a n e t a r y Geology O f f i c e , N.A.S.A.,

t h r o u g h t h e Mars Data A n a l y s i s Program.

REFERENCES Bagnold, R.A., 1941. The P h y s i c s o f Blown Sand and D e s e r t Dunes. Chapman and Hall, London, 265 pp. Bowen, A.J. and L i n d l e y , D., 1977. A w i n d - t u n n e l i n v e s t i g a t i o n o f t h e wind speed and t u r b u l e n c e c h a r a c t e r i s t i c s c l o s e t o t h e ground o v e r v a r i o u s escarpment shapes. Boundary-Layer Met., 12: 259-271. Bownan, D., 1981. E f f i c i e n c y o f e i g e n f u n c t i o n f o r d i s c r i m i n a n t a n a l y s i s o f suba e r i a l n o n - t i d a l beach p r o f i l e s . Mar. Geol., 39: 243-258. C h e p i l , W.S. and Woodruff, N.P., 1963. The p h y s i c s o f w i n d e r o s i o n and i t s control, Adv. Agronomy, 15: 211-302. Clos-Arceduc, A., 1969. Essai d ' e x p l i c a t i o n des formes d u n a i r e s sahariennes. Etud des P h o t o - I n t e r p r e t a t i o n , P a r i s , I.G.N., 63 pp. Cooke, R.U. and Warren, A., 1973. Geomorphology i n D e s e r t s . B.T. B a t s f o r d , London, 374 pp. Eaton, K.J., 1981. B u i l d i n g and t r o p i c a l windstorms. Overseas B u i l d i n g Notes, No. 188, 17 pp. Embleton, C., Thornes, J.B. and Warren, A., 1979. The n a t u r e o f f l u i d m o t i 0 n . h : C. Embleton and J . Thornes ( E d i t o r s ) , Process i n Geomorphology. Edward Arnold, London, pp. 39-72.

259 Evans, J.R., 1962. F a l l i n g and c l i m b i n g sand dunes i n t h e Cronese ( " C a t " ) mountt a i n s , San Bernadino County, C a l i f o r n i a . J. Geol., 70: 107-113. Ford, E.F., 1957. The t r a n s p o r t o f sand by wind. Trans. Am. Geophys. Union, 38: 171-175. G i l l e t t e , D . , 1976. T e s t s w i t h a p o r t a b l e wind t u n n e l f o r d e t e r m i n i n g wind e r o s i o n t h r e s h o l d v e l o c i t i e s . Atmos. Env., 12: 2309-2313. Goldsmith, V . , 1978. Coastal dunes. I n : R.A. D a v i s ( E d i t o r ) , Coastal Sedimentary Environments. S p r i n g e r - V e r l a q , New York, pp. 171-235. Greeley, R., I v e r s e n , J.D., P o l l a c k , J.B., Udovich, Id. and White, B., 1974a. Wind t u n n e l s i m u l a t i o n o f l i g h t and d a r k s t r e a k s on Mars. Science, 183: 847-849. Greeley, R., I v e r s e n , J.D., P o l l a c k , J.B., Udovich, N. and White, B., 1974b. Wind t u n n e l s t u d i e s o f m a r t i a n a e o l i a n processes. Proc. R. SOC. London, 341k: 331-360. Hesp, P.A., 1981. The f o r m a t i o n o f shadow dunes. J o u r . Sed. P e t r o l . , 51: 101-112. Howard, A.D., Morton, J.B., Gad-el-Hak, M. and P i e r c e , D.B., 1977. S i m u l a t i o n model o f e r o s i o n and d e p o s i t i o n on a barchan dune. N.A.S.A., CR-2838, 77 pp. Iversen, J.D., 1979. D r i f t i n g snow s i m i l i t u d e . J o u r . H y d r a u l i c s D i v . , A.S.C.E., 105, HY6: 737-753. Iversen, J.D., 1980. D r i f t i n g snow s i m i l i t u d e - t r a n s p o r t r a t e and roughness modeling. Jour. G l a c i o l . , 26: 393-403. Logie, M., 1981. Wind t u n n e l e x p e r i m e n t s on dune sands. E a r t h S u r f a c e Process. Landf., 6: 365-374. Mabbutt, J.A., 1977. D e s e r t Landforms. M.I.T. Press, Flassachusetts, 340 pp. McBride, E.F., 1971. Mathematical t r e a t m e n t o f s i z e d i s t r i b u t i o n d a t a . I n : R.E. Carver ( E d i t o r ) , Proceedures i n Sedimentary P e t r o l o g y . J. Wiley, New York, pp. 109-127. Mironer, A . , 1979. E n g i n e e r i n g F l u i d Mechanics. McGraw-Hill, New York, 592 pp. Pande, P . K . , Parkash, R. and Agarwal, M.L., 1980. Flow p a s t f e n c e i n t u r b u l e n t 106, H Y 1 : 191-207. boundary l a y e r . J o u r . H y d r a u l i c s D i v . , A.S.C.E., Seppala, 1.1. and L i n d e , K., 1978. Wind t u n n e l s t u d i e s o f r i p p l e f o r m a t i o n . Geogr. Annal., Ser. A, 60: 29-42. Smith, H.T.U., 1954. E o l i a n sand on d e s e r t mountains. Geol. SOC. Arner. B u l l . , 65: 1306-1307. Smith, H.T.U., 1963. E o l i a n geomorphology, wind d i r e c t i o n , and c l i m a t i c change i n n o r t h A f r i c a ( f i n a l r e p o r t ) . U.S. A i r Force Cambridge Res. Labs., AFCRL-63-443, 48 PP. 1978. F i e l d t r i p t o t h e dunes a t S u p e r s t i t i o n f l o u n t a i n . I n : R. Smith, R.S.U., Greeley, M.B. Womer, R.P. Papson, and P.D. Spudis ( E d i t o r s ) , A e o l i a n f e a t u r e s o f s o u t h e r n C a l i f o r n i a : A comparative P l a n e t a r y geology guidebook. A r i z o n a S t a t e Univ., C o l l e g e o f t h e D e s e r t and NASA-Ames Res. C e n t e r . pp. 66-71. Sundaram, T.R., Ludwiq, G.R. and S k i n n e r , G.T., 1972. Modeling o f t h e t u r b u l e n c e s t r u c t u r e o f t h e atmospheric s u r f a c e l a y e r . Amer. I n s t . Aeron. Jour., 10: 743-750.

This Page Intentionally Left Blank

261

CONTROLS OF DUNE HORi'HOLOGY 11.1 THE NAI.113 SAND SEA

N . LANCASTER, Desert E c o l o g i c a l Research U n i t , I l a l v i s Bay, S.W.A./Namibia.

IN'ROOUCTION

The advent o f s a t e l l i t e and o t h e r remote sensing imagery o f d e s e r t areas has shown t h e g r e a t v a r i e t y o f dune morphology i n sand seas, b u t a t t h e same t i m e et _ a1 (1979) has f a c i l i t a t e d t h e i r mapping, as IlcKee and Breed (1976) and 3reed _ have demonstrated. Host i n v e s t i g a t i o n s o f dune morphology have been l a r g e l y concerned w i t h t h e typology o r gross morphology o f dunes and t h e i r s p a t i a l d i s t r i b u t i o n (e.g. 1958, Holm 1960, Mabbutt 1968, Mainguet 1976, 1978, Besler 1980).

Plonod

A potentially

valuable area i s t h e study o f dune morphometry, p a r t i c u l a r l y t h e l e n g t h , h e i g h t and spacing o f dunes, b u t t h i s has r e c e i v e d l i t t l e a t t e n t i o n (Wilson 1972, Breed and Grow 1979, Lancaster 1981a).

However, t h e r e have been few s t u d i e s o f t h e

factors which c o n t r o l t h e morphology and morphometry of dunes w i t h i n a sand seaSuch s t u d i e s may make an important c o n t r i b u t i o n t o our understanding o f t h e factors which c o n t r o l t h e shape and s i z e o f dunes and t h e ways i n which e o l i a n sand bodies accumulate. The a i m o f t h i s paper

is t o d e s c r i b e t h e morphology and morphometry o f dunes

i n t h e Namib sand sea and t o r e l a t e t h e s p a t i a l v a r i a t i o n i n these p r o p e r t i e s t o changes i n t h e g r a i n s i z e and s o r t i n g c h a r a c t e r i s t i c s o f t h e sands and i n wind regimes.

I n t h i s paper, data obtained from remote sensing imagery i s

combined w i t h f i e l d observations and measurements o f dune morphometry and sediments a t 26 s i t e s throughout t h e sand sea (Fig. 1). C h a r a c t e r i s t i c s o f dunes i n t h e Namib sand sea

2 . It s t r e t c h e s

The Namib sand sea occupies an "rea o f approximately 34 000 km

f o r over 300 km between L u d e r i t z (26's)

and t h e Kuiseb r i v e r (23°-23030'S) on

t h e A t l a n t i c c o a s t o f southern A f r i c a .

Dunes a r e found over i t s 100

-

120 km

width from sea l e v e l t o t h e v i c i n i t y o f t h e 1 000 m c o n t o u r a t t h e base o f t h e Great Escarpment, which borders t h e e a s t e r n s i d e o f t h e sand sea.

The v a r i e t y

o f dune forms i n such a small area has a t t r a c t e d t h e a t t e n t i o n o f previous

262

I

\

0

50 kin

Fig. 1. Location of sites at which dunes were sampled and morphometric measurements made.

investigators, particularly Barnard (1973) who divided the northern part o f the sand sea i n t o three zones: a coastal belt o i transverse dunes, an interior zone of "longitudinal" dunes, which he believed were formed by deflation and an eastern zone of complex "multicyclic" dunes. Besler (1980)produced a complex map of dune types in the sand sea as part of a study of the origins of the sand sea. VORPHOLOGY Three major dune types occur in the Namib sand sea. Their distribution is

DUNE

263

F i g . 2. The Namib Sand Sea: mosiac of LANDSAT images. 22232-08083; 22232-08090;22232-08092;22287-08131;22237-08133.

shown by the satellite image in Fig. 2. ard the map based upon LANDSAT imagery, supplemented by air photo interpretation, in Fig. 3. The proportion of dunes of different types i s given in Table 1 . Terminology employed follows McKee (1579).

2 64

’So

239

2LC

25 Sylvia Hill

26

-

0

20 km

T

Fig. 3. Dune types in the Namib Sand Sea. Map compiled from LANDSAT imagery, supplemented by air photo interpretation. 1 Complex linear dunes 2 Compound linear dunes a Straight dunes b Anastomosing and reticulate patterns 3 Simple linear dunes 4 a Barchans b Simple transverse and barchanoid ridges c Compound transverse dunes 5 Star dunes and chains of star dunes 6 Rolling dunes without slip faces 7 Sand sheets.

265 TAELE 1 Relative importance o f dune types i n t h e Namib sand sea Dune t .v. w

Area covered ( % o f sand sea) 74.04

Linear ComDlex Compound S t r a ig h t Anastomosing and r e t i c u l a t e patterns Simple Transverse and barchanoid Barchans Simple Compound S t a r dunes and chains o f s t a r dunes Low r o l l i n g dunes w i t h o u t s l i p faces Sand sheets

36 -89 35.02 19.56 15.46 2.13 13.32 0.02 3.48 9 -82 9.31 1.54 3.33

Transverse and barchanoid dunes o f simple and compound form a l i g n e d normal t o They a l s o

SSUto SW winds occur i n a s t r i p up t o 20 km wide along t h e coast.

occur l o c a l l y f u r t h e r i n l a n d where dune p a t t e r n s a r e d i s t u r b e d , f o r example west o f t h e Tsondab F l a t s and a t Sossus V l e i . areas o f t r a n s v e r s e and c r e s c e n t i c dunes. extends from E l i z a b e t h 8ay t o S y l v i a H i l l .

There a r e two major c o a s t a l

The southern area o f these dunes It s t a r t s as a t r a i n o f

i r r e g u l a r l y spaced barchans and barchanoid r i d g e s extending from t h e beaches of E l i z a b e t h and Chamais Bays.

North o f L u d e r i t z , an e x t e n s i v e area o f simple

transverse and barchanoid r i d g e s fans o u t i n t o t h e main sand sea, w h i l s t along the coast t h e r e i s a zone, up t o 10 km wide, o f compound t r a n s v e r s e dunes which extends t o S y l v i a H i l l .

The n o r t h e r n group o f t r a n s v e r s e dunes begins on t h e

coast southeast o f Fleob Bay and continues t o t h e n o r t h e r n boundary o f t h e sand sea a t t h e Kuiseb r i v e r .

Most dunes i n t h i s area a r e l a r g e compound forms

c o n s i s t i n g o f a main t r a n s v e r s e r i d g e w i t h small barchanoid r i d g e s on t h e i r crests and upper southwest slopes.

Between Meob and Conception Bays t h e r e

are areas o f barchans and small t r a n s v e r s e and barchanoid r i d g e s .

Some o f

these j o i n t h e main area o f compound t r a n s v e r s e dunes from t h e west. Linear dunes on N-S t o NW-SE alignments a r e t h e dominant dune form i n t h e Nanib sand sea.

Complex l i n e a r dunes w i t h a sinuous sharp c r e s t l i n e , s t a r

form peaks a t i n t e r v a l s and barchanoid r i d g e s on t h e i r e a s t e r n f l a n k s a r e widely developed i n n o r t h e r n and c e n t r a l p a r t s o f t h e sand sea.

I n western

areas small simple l i n e a r dunes commonly cross t h e c o r r i d o r s between t h e major dunes.

I n southern p a r t s o f t h e sand sea most l i n e a r dunes a r e o f compound

form and c o n s i s t of 2

a broad rounded s w e l l .

-

5 p a r a l l e l o r converging small r i d g e s running along Along t h e e a s t e r n margins o f t h e sand sea compound

266

and simple linear dunes, largely fixed by vegetation, occur in anastomosing and reticulate patterns. Transitional to these dunes are broad linear ridges with a retjculate pattern of small dune crests on their summits. Dunes of star form are found in three areas along the eastern margins of the sand sea: north of Tsondab Vlei, around Sossus Vlei and in three groups along the eastern edge of the southern part o f the sand sea. Few of these dunes have a true stellate form. Many consist of a characteristic narrow, relatively short, steep sided ridge with a preferred crestal orientation, usually NW-SE and curving arms on alignments roughly perpendicular to this. Frequently dunes of star type are found in chains (c.f. the compound forms of Breed and Grow 1979) and are transitional to complex linear dunes. Within the sand sea, sand covered areas without significant dune development are rare. On the southern margins o f the sand sea an extensive gently rolling sand plain extends for 20 km north of the Koichab river. South and southwest of the Uri Hauchab mountains there is an area of rolling dunes without slip faces up to 30 m high, comparable with the Mrey6 of Monod (1958). Throughout the sand sea but particularly in southern areas low rolling dunes without slip faces (c.f. the "zibar" of Holm, 1960) occur in the interdune corridors between complex and compound linear dunes, frequently on a trend normal to that of the 1 inear dunes. DUNE MORPHOMETRY Transverse dunes Spacing o f transverse dunes in the Namib sand sea is variable (Fig. 4) and ranges from 100 to 1 400 m with a mean o f 610 m. Most transverse dunes have spacings between 200 and 1 000 m. Those with spacings of 200 - 300 m or less are generally simple forms and tend to be low (5 - 10 m high) transverse or barchanoid ridges. More widely spaced dunes are invariably of compound form with the main dunes having a spacing of 600 - 1 000 m, rather less than those described as compound crescentic by Breed and Grow (1979),but comparable with simple transverse dunes in the United Arab Emirates, A1 Jiwa and Takla Makan. On the upper parts Of their stoss slopes small (2 - 5 m high) barchanoid and crescentic dunes with spacings of 50 - 80 m are frequently developed.

Rolling dunes without slip faces in the area south of the Uri Hauchab mountains (site XVI) have a similar morphometry to that of compound transverse dunes.

267

These dunes have spacings of 1 000 - 1 100 rn with a height o f 20 - 30 m. On their stoss slopes and rounded crests are small rounded undulations similar to the "zjbsr" o f Holm (1960) with a spacing of 170 - 180 rn.

:."I r-!Tm P 1993 rn

4,

Linear

!

0 ' 1200

1800 O

ZLOO

W

1800

ZLOO

rn

rn

X 1L32 rn

10 600

1200

::m!h

." 301

0

0

P610 rn

.1

600

Transverse

1200

rn

Dune spacing

Fig. 4. Spacings of transverse, linear and star dunes in the Namib sand sea. There is a systematic variation in dune spacing within the area of compound transverse dunes. This is particularly striking in the northern group o f dunes. Dune spacing increases northwards from 700 to 1 000 - 1 100 m in the area northeast o f Conception Bay, then decreases progressively to 300 - 400 m north of Sandwich Harbour. Using data from sample sites only, Fig. 5a shows that there is a close relationship between transverse dune height and spacing (r = 0.75, significant at the 0.05 level). Similarly close relationships have been established by Lancaster (1982b) for simple and compound transverse and barchanoid dunes in the Skeleton Coast dunefield on the northern Namib coast. Linear dunes Aspects of the rnorphometry of linear dunes in the Namib sand sea have been discussed by Lancaster (1981 ). Their morphometric characteristics are shown

2 68

0

0

so0 Dune spoclng r m l

1000

x

01

.

800

1000

3001

I

’

complex

..

y~OllX.1L3L r =

2500

1500 2000 Dune spacing Iml

x

0-72

2800

c

n;11

X

300

!5@0

1000

2000

2500

Dune SSOL8ng Irnl

Fig. 5. R e l a t i o n s h i p s between dune h e i g h t and spacing: A Transverse dunes B Linear dunes C S t a r dunes. i n Fig. 6.

Dune spacings range from 1 200 t o 2 800 m, w i t h a mean o f 1 993 m

and most l i n e a r dunes a r e 1 800

-

2 200 m a p a r t .

I n a d d i t i o n , barchanoid

r i d g e s w i t h a mean spacing o f 87 m, commonly j o i n i n g t h e main dune c r e s t i n an en echelon manner, occur on t h e e a s t e r n f l a n k s o f many complex l i n e a r dunes, p a r t i c u l a r l y i n western and c e n t r a l areas o f t h e n o r t h e r n p a r t o f t h e sand sea. Spacing o f t h e m u l t i p l e r i d g e s o f compound l i n e a r dunes averages 160 m, o r approximately one t e n t h t h a t o f t h e average spacing o f t h e main dunes

( 1 680 m).

of 838 m.

Widths o f t h e l i n e a r dunes a r e commonly 600 Most complex l i n e a r dunes a r e 80

-

-

1 000 m w i t h a mean

150 m h i g h , w h i l s t compound

269

-

.)

7

25.

0,

Dune

.

height

0

600

1200 m

Dune w d l h

? 1332

i 1993

L 50!

Dune i p o c l n g

Dune FpocNng

Fig. 6. Morphometric characteristics o f linear and star dunes in the Namib sand sea. linear dunes are between 30 - 50 m in height. In terms o f their size and spacing complex and compound linear dunes in the Namib sand sea are comparable with similar dunes in the Rub a1 Khali and southern Sahara. The height and spacing o f linear dunes in the Namib sand sea varies systematically, as described by Lancaster (1982~). Such dunes are highest at 150 - 170 m in the central parts o f the sand sea and become progressively smaller towards its margins. Throughout the southern parts o f the sand sea linear dunes are less than 50 m high. The spacing o f this type o f dune varies in a similar way, with the most widely spaced dunes occuring between the Tsondab and Tsauchab valleys (Fig. 8a and 8b). This apparent relationship between linear duie height and spacing is born out by Fig. 5b, which shows that, as with transverse dunes, the height and spacing

270

of linear dunes is closely correlated (r = 0.77, significant at the 0.05 level) Similar, but less clear, relationships (Fig. 7a) occur between the width and spaciq of linear dunes (r = 0.54) confirming the data of Breed and Grow (1979)

I : 0 28X r.051

.

.

251 75

A

X complex Compound

.

0 1 800

1000

1500 O""P

I

:0 - 2 7 X

t

2000 Irnl

2506

2800

spac8ng

252.10

B

7.068 n

i

11

Fig. 7. Relationships between dune width and spacing: Linear dunes B Star dunes. A

Star dunes -~ Spacings o f star and related chains of star dunes widely, from 600 to 2 600 m (Fig. 6). Most dunes of spacings between 1 000 and 1 800 m , with a mean of 1 dunes are between 400 and 1 000 m and average 651 m.

in the Namib sand sea vary this type however have 332 m. Widths of star Star dunes of the type

271

Found in the Namib are thus narrower and more closely spaced than the comlex 1 inear dunes.

I

A

I

15'

Fig. 8. Spatial variation in dune height ( A ) and dune spacing ( 6 ) in the Nanib sand sea. The largest star dunes occur in the vicinity of Sossus Vlei, where they may reach heights o f 200 - 300 m. Star dunes in the southern parts of the sand sea are generally 80 - 100 m high, and relatively closely spaced ( + 1 000 rn). Mean star dune height for the sand sea is 145 m, or rather greater than that of linear dunes. In many areas there are small barchanoid or reversing dunes, with spacings of 90 - 120 m between the star dunes. As with other dune types described above, star dune height and spacing are closely correlated (r = 0.72, significant at the 0.05 level) Fig. 5c.

272

Although Breed and Grow (1979) could find no statistically significant relationship between star dune width and spacing for their world-wide sample of these dunes, such a relationship can be established for the Namib star dunes (Fig. 7b) (r = 0.68, significant at the 0.05 level). The pattern of dune alignments I n many areas of the Namib sand sea the pattern of dune alignments consists of a major or dominant trend, together with one or two subsidiary elements. From place to place, the relative importance of these trends differs. Thus a major trend in one area may be continued as a subsidiary trend elsewhere, or a minor trend i s accentuated to dominate in another area, as Fig. 9 illustrates. The trend of the crests of most compound and simple transverse dunes i s generally 300 - 320°, with NE facing slip faces, tending to swing round slightly inland (Fig. 9e). In the Conception - Meob area simple transverse and barchan dunes have slip faces to the N or NNE and redge trends of 240 280'. Especially along the inland margin of their distribution, maiy compound transverse dunes exhibit prominent N-S oriented linear elements which cross from one dune to another (Fig. 9f). These may be considered incipient linear dunes in many cases. The major dune trend in the sand sea is that of the linear dunes, which have a HNhJ-SSE to NNE-SSU alignment in its central and northern areas (Fiq. 9a). I n the southern parts of the sand sea compound linear dunes occur on NW-SE alignments The trend of fixed anastomosing and reticulate linear dunes along the eastern margins of the sand sea is strongly affected by topography in many places, but elsewhere is generally WNW-ESE to NW-SE. Nithin the complex linear dune landscape two subsidiary trends occur. Crest trends of east flank barchanoid dunes are 320 - 330' (Fig. 9c), with NNE to NE oriented slip faces in western areas, swinging round eaztwards to 340 - 355O, with slip faces facing east. This appears to continue the trend of the transverse dunes along the coast. Corridor crossing dunes follow a consistent alignment of 060' or WSW-ENE in western parts of the sand sea from Sossus Vlei northwards, and locally in the southern part of the sand sea (Fig. 9b). The trend of the major ridges of most star dunes follows the pattern of the linear dunes as Fig. 9a illustrates. Subsidiary trends of star dune arms are LISU-ENE following that of the crossing dunes (Fig. 9b), whilst barchanoid and reversing dunes amongst the star dunes continue the trend of east flank barchanoid dunes elsewhere.

273

Fio. 9. Dune alignments in the Namib sand sea: A

B

C 0 E F

Linear dunes and major ridges of star dunes Corridor crossing dunes and secondary ridges of star dunes East flank and related barchanoid dunes in linear and star dune landscapes: bar indicates ridge trend; tick, dominant slip face orientation Low rolling dunes without slip faces in interdune corridors Transverse dunes: bar indicates crest trend; tick, major slip face orientation Oblique linear elements in transverse dune areas.

274

The pattern of dune morphology in the sand sea Considering all dune types, the pattern of dune morphology and morphometry, particu’3rly dune height and spacing, varies in a systematic way throughout the s?nd sea. The largest and thus the most widely spaced dunes are found in the central and some northern parts of the sand sea, with progressively smsller dunes towards the margins. With the exception of small areas of star dunes, most dunes in the southern parts of the sand sea are less than 50 m high and are generally less than 1 700 m apart. A line drawn from the coast south of Meob Bay through the Uri Hauchab mountains effectively divides the Namib sand sea into two parts. The southern section is characterised by low, mostly compound, linear dunes, with wide areas of rolling dunes without slip faces and a narrow zone of compound transverse dunes along the coast. The northern and central section is dominated by large, complex linear dunes, locally grading into chains of star dunes and changing eastwards to lower reticulate and anastomosing compound linear dunes, largely fixed by vegetation. Along the coast is a belt of moderately sized compound transverse dunes. CONTROLS OF DUNE MORPHOLOGY

The morphology of desert dunes is principally a product of the interaction between the sand surface and the wind. This interaction is modified by the growth of the dunes which project into the airflow and deflect it in their vicinity . Thus two major influences on dune morphology may be recognised: the character of the dune sands, particularly their grain size and sorting characteristics, and the character of the wind regime. Further, as sand seas are the product of an ongoing process of sediment accumulation, the effects of this are also important. An additional factor in the eastern part of the Namib sand sea is the presence of a partial vegetation cover on the dunes. Many features of the reticulate and anastomosing simple and compound linear dunes in these areas can be related to development of blowouts and parabolic dunes during years o f low rainfall, which are revegetated after occasional heavy rains (e.g. 1976-1978) The influence of grain size and sorting The evidence for relationships between dunes of different types arid sand with different grain sizes and sorting characteristics is confusing. In the Algerian Sahara, Bellair (1953) found that barchan and transverse dunes were composed o f well sorted unimodal sands but complex linear and star dunes were formed of bi

275

or trimodal sands. However, Alimen et_a1 (1958) and Capot-Rey and Gremion (1964) could find no consistent relationship and pointed to the complex patterns of grain size and sorting on larger dunes. McKee (1966) found that there was a progressive decrease in grain size and increase i n sorting from dunes of dome type, through barchans and transverse dunes to parabolic dunes at Llhite Sands, New Mexico. Similarly, i n the Sudan, Warren (1970) put forward evidence to indicate that sands from undulating sand sheets were coarser and less well sorted than those from transverse dunes, which were in turn coarser and less well sorted than those from linear dunes. A number 0-f studies have noted the relationship between coarse bi or multi modal sands and low rolling dunes without slip faces or "zibar" (Capot-Rey 1947, McKee and Tibbitts 1964, Warren 1972, Tsoar 1978, Lancaster 198213). Sands from adjacent linear or transverse dunes are also consistently finer and better sorted than those of low rolling dunes. Further, Warren (1972) noted that, whilst the low rolling crests were transverse to the resultant sand flow direction, the linear dunes were on oblique alignments.

There is also some evidence to suggest that the grain size characteristics of the sand may control the spacing of dunes, especially those of transverse form. Wilson (1972) suggested that, for each class of his bedform hierarchy dune spacing was proportional to the size of the coarse fraction o f the sand of which it was composed. Such a hypothesis was confirmed by Lancaster (1982b) who -Found a close relationship between the grain size of the coarse 5th percentile and the spacing of simple and compound transverse dunes i n the northern Namib.

In the Namib sand sea, it is possible to demonstrate that dunes of different types are associated with sands of different characters. But how far this is a Genetic relationship i s uncertain. The closest relationship to emerge is that between areas of low rolling dunes without slip faces and coarse, poorly sorted, multimodal or bimodal sands with mean grain sizes between 1.71 and 2.07 phi and a phi standard deviation between 0.87 - 1.45. Such dunes occur widely in the southern parts of the Namib satid sea, and i n western coastal areas northeast o f Conception Bay. They also occur i n the interdune corridors between linear dunes in northern and central parts of the sand sea. Although most of these dunes are spaced between 80 and 150 m apart, with an amplitude o f 1 - 4 IV they may be up to 30 m high and 1 100 m apart in some areas (e.g. site XVI). The absence of slip faces, even

276

in large forms, may be attributed to the long grain paths of coarse sands when in saltation, an observation suggested also by the widespread association o f mega ripples with these dunes. In all areas sand from low rolling dunes is consistently coarser and less well sorted than that of adjacent linear dunes. The alignments of low rolling dunes without slip faces (Fig. 9d) show them to be transverse to south, and locally south southwest, winds in northern and central parts of the sand sea and to southeasterly winds in southern areas. Although winds from these directions are less frequent than south southwest or southwest winds, they are usually the strongest winds from this sector, and thus the only ones capable of moving coarse sands to any extent.

A 0 0

0 Low rolling dunes

0 0

0

04 1.5

20

x

2-5

Transverse dunes Linear dunes Star dunes

3.0

Mean

Lo

01

b ul Y

Mean

Fig. 10. Grain size and sorting character of sands from different dune types (phi units). Mean values from crest sands at sample sites in Fig. 1.

277

Apart from the above there are few consistent relationships between sand grain size and sorting characteristics and dune types, in the Namib sand sea, as Fi;. 10 indicates. Crestal sands from dunes of star type tend to be the best sorted of all dune sands. Sands from some areas of transverse dunes, at sites IX, XXI and XXII are clearly coarser and less well sorted than sands from linear dunes, but those from site XXIII are very similar to those of compound linear dunes in the southern parts of the sand sea. Sands from compound transverse dunes at site XI are intermediate in composition, but much less well sorted than nearby linear dunes at site 111. There thus seems to be some evidence for a progressive increase i n sorting and a fining of sands from transverse through linear to star dunes. This may be the result of the pattern of sand movements on different dune types. On transverse dunes this i s unidirectional, with sands being buried on avalanche slopes, to be recycled later as these deposits are re-exposed by the advance of the dune. Crestal areas of linear dunes are reversed seasonally and undergo more frequent resorting. On star dunes the sands are exposed to multi directional winds and constant resorting as Folk (1971) predicted. However, the apparent change i n the grain size and sorting character of different dune types in the Namib may equally well be explained by their position relative to sand transport paths, with transverse dunes located i n near source upwind areas and linear and star dunes i n downwind areas. It might be expected that different elements o f a complex pattern of dunes would be composed of sands with different grain sizes. Grain size, by controlling the threshold velocity for sand movement, controls the effective wind regime and thus may strongly influence dune alignments. However, i n most areas o f the Namib sand sea this i s not the case. Figure 1 1 indicates that there i s a progressive change i n grain size and sorting from the interdune and plinths to the crestal areas of the linear dunes. Sands from east flank barchanoid and crossing dunes are very similar i n composition, being slightly less well sorted and coarser than adjacent crest sands, but the differences are n o t significant i n terms of threshold velocities and the movement of sand by the wind. Locally, greater differences between elements of the dune landscape do exist. At Sossus Vlei small barchanoid and reversing dunes on the vlei surface are composed of grey brown sands which are coarser and less well sorted (average mean grain size = 2.06, average standard deviation = 0.49) than adjacent large complex star dunes (F= 2.38, oI = 0 . 2 7 ) .

ID EPWP

X

1.5

EF S C

20

2.5

Mean Fig. 1 1 . Grain size and sorting variations in the linear dune landscape ( p h i u n i t s ) . Mean values from s i t e s in Fig. 1 . ID Interdunes EP,WP East and west plinths X Corridor crossing dunes EF East flank barchanoid dunes S Slip faces C Crests.

IDP

RD

T

T

N X

Mean

Grain size ana sorting differences between dune types a t s i t e Ix (phi u n i t s ) . Mean values from each dune type. ID Interdunes Low rolling dunes without s l i p faces RD T Transverse and barchanoid c r e s t s P Plinths o f linear dunes X WSW - ENE linear dune c r e s t s N N - S linear dune c r e s t s .

F i g . 12.

279

Northwest of the Tsondab Flats (site IX), the N-S linear dunes are disrupted as they cross former river courses and a complex pattern occurs, with transverse and barchanoid dunes aligned transverse to southwest winds, and linear dunes on WSW-ENE and N-S alignments. Between them are areas of low rolling dunes. Differences between the grain size and sorting character of these elements of tt:e dune landscape lend support to the hypothesis of a progressive improvement in sorting from low rolling dunes through transverse to linear and star dunes. Figure 12 shows that clear differences in grain size and sorting exist between adjacent dune types. Thus low rolling dunes without slip faces are significantly coarser and less well sorted (Ks = 2.04 2.27, zI = 0.87 - 0.96) than transverse and barchanoid ridges, which have average mean grain sizes between 2.31 and 2.36 phi and an average st.andard deviation of 0.55 - 0.60. Low,simple linear dunes on WSW-ENE alignments (similar to corridor crossing dunes elsewhere) are finer still ( 6 = 2.46) and much better sorted (oI = 0.35) than transverse dunes but similar in composition to crestal sands of the N-S linear dunes. Grain size may have an important influence on dune spacing, as suggested by Wilson (1972) who argued that dune spacing was a function of the windspeed required to move the coarse sand on the dune surface, which had the effect of "protecting" finer sands from movement. In the Namib sand sea, the size of this fraction is approximated by the grain size of the 5th percentile of surface sands from dune crests. In the northern group of compound transverse dunes, spacing changes from south to north, increasing from 700 - 800 m in the south to a maximum of 1 100 - 1 200 m in central parts of this strip of dunes and then declining again steadily to 300 - 400 m at the Kuiseb river. Although grain size was sampled at widely spaced intervals, it appears to follow a similar pattern, with coarse sands being associated with the most widely spaced dunes. For the sand sea as a whole, a clear relationship between the grain size of the 5th percentile of sand from dune crests and the spacing o f transverse and related dunes can be demonstrated, as F i g . 13 shows (r = -0.88, significant at the 0.05 level). This is perhaps the most important effect of grain size on dune morphology. However, as Lancaster (1981 ) has shown, no such relationship can be established for linear or star dunes, and their spacing shows no relationship to any aspect of their grain size character. If anything, large, widely spaced dunes of these types tend to be composed of finer sand than smaller dunes. The above suggests that, although the spacing of all dunes may be aerodynamically determined, different factors affect the spacing of linear and

280

transverse dunes, because of their different relationship to the wind. Thus in areas of transverse dunes, winds are primarily from one narrow directional sector (Fryberger and Dean, 1979) and the spacing between these dunes can be reasonab!y compared to the width of the zone in which the wind i s disturbed as it passes over the dune. The size of this zone will differ with dune height and also with the effective wind velocity, which is in turn controlled by grain size. Howard _ et _ a1 (1978)have further suggested that the equilibrium size o f transverse and barchanoid dunes may result from the merging of dunes o f different sizes and rates of movement. Dunes of coarser sand will probably move more slowly than those of fine sand, because of the higher effective wind velocities required. They will normally trap more rapidly moving dunes and hence tend to grow in size. Thus dunes of coarse sand will be larger, and more widely spaced than those of finer sands.

i5001

y

= 2 07- 0 O O l X

r - -088 n = 1L

\

- 1000-E m

:: 2 500.

X

0

0

10

05 Groin

20

15

size of 51h percentile

25

lphi un115I

Fig. 13. Relationship between grain size of coarse 5th percentile o f crest sands and spacing of transverse and barchanoid dunes.

The overall tendency for linear dunes is to extend at a small angle to the most persistent sand moving winds (Tsoar, 1978). Because they extend parallel to each other, the rate of movement of one dune will not affect the adjacent dunes and their grain size character will be immaterial in controlling spacing and aerodynamic effects may be paramount. Although Tsoar (1978) suggested that the spacing of linear dunes was 1 1 - 15 times their height, or the distance at which the disturbance of the wind by the dune should have ceased, formative winds blow obliquely to the dune and thus the separation

281

between dunes possible that winds than do normslly been

along the line of the wind i s much greater. However it is wide obstructions, like dunes, may have a greater effect upon narrow ones, such as hedges and lines of trees, which have considered by micrometeorologists.

The influence of wind regimes Compared to many desert areas, a relatively large amount of information on winds i s available for the Namib. Data for the central Namib are summarised et _ a1 (in press). Additional data for stations A,B and E i n fig. 14 by Lancaster _ was obtained from information in Ward (1983). Major spatial variations in available wind energy and directional variability occur in the Namib. In northern areas, and apparently throughout the sand sea, the frequency and strength o f winds from south to southwest directions decreases from the coast to inland areas. Additionally the most frequently occurring wind in this sector changes direction from south or south southwest on the coast to southwest in central areas and west southwest to west on the eastern edge of the sand sea. During the winter months, the eastern Namib in particular i s affected by light to moderate easterly to northeasterly winds corresponding to a katabatic flow from the escarpment zone bordering the desert. From time to time, when regional pressbre gradients are normal to the coast strong to very strong northeast to east "berg winds" blow. These, too, most frequently affect eastern areas. The nature of wind regime changes from north to south are less well known. Descriptions in Royal Navy and South African Air Force (1944) and from unpublished data on file at Gobabeb indicate that the Luderitz area experiences a very high energy wind regime, dominated by south and south southeast winds. Data from Koichab Pan and Aus (Royal Navy and South African Air Force, 1944) indicate a decrease in wind energy inland, but a much windier climate than in northern areas. Winds at Aus are from three main directions, SW, SE and NW, whilst at Koichab Pan, SW and SE sand movements apparently dominate. Resultant sand movement i s thus towards the north. Coastal areas of the Namib are characterised by a relatively high energy, narrow unimodal wind regime. Near Walvis Bay 80% of sandflow is from the SSW (Fig. 14a). Similar, but higher energy, wind regimes occur at Kolmanskop near Luderitz, where 92% of annual sandflow is from SSW and SSE. Thus, as in

282

many other sand seas (Fryberger and Dean, 1 9 7 9 ) , such a wind regime gives rise to transverse and barchanoid dunes. Barchans occur in the southern areas o f higher wind energy.

) wind roses

Fig. 14. Sand movements i n the northern parts of the Namib sand sea. Resultant sandflows in tonnes m-’yr-’Inland, available wind energy drops sharply and the areas of linear dunes in the central and northern parts o f the sand sea are characterised by a low to moderate energy wind regime. Towards the coast northeast to east winds are infrequent and the wind regime i s wide unimodal (Fig. 14b), with 80% of the sand movement from SE to SW directions. In the centre of the area of linear dunes (Fig. 14c and 14d), the annual wind regime is bimodal, with a major

283

sobtherly mode ( S t o SW) accounting f o r 60 - 80% of t h e annual sand movement and a minor N N E t o E mode giving r i s e t o 15 - 20% of annual sandflow.

Locally,

as F i a . l k shows, a t h i r d mode, N N W t o N, appears and i s responsible f o r 6 - 8% of annual sandflow. Seasonally t h e importance of t h e s e modes changes considerably (Fig. 15). The southerly mode p e r s i s t s a l l y e a r , b u t accounts for only 12 - 15% of sandflow i n May t o August and peaks a t 70 - 80% in September t o November. Sand movements from N N E - E dominate in winter months when they average 50 - 60% of monthly sandflows, b u t a r e r a r e in other months. The northerly mode i s most important in November t o February when i t may account f o r 12 - 15% of sandflow.

Jan

April

July

Oct

\

1/25 L

Fiq. 15. Seasonal changes i n sand movements in t h e n o r t h e x p a r t s o f t h e Namib sand sea. Data f o r s t a t i o n s B , C and F of Fig. 14. Resultant sandflows in tonnes m-lyr-’.

284

In the central and northern parts of the sand sea alignments of linear dunes are N-S to NNE-SSW, yet the major sand moving winds are from the south to south southwest and resultant sandflow directions are at angles of 20 - 40' to the dunes. Similar observations led Besler (1980)to conclude that the linear dunes were a product of late Glacial wind regimes when southerly winds may have dominated, and that they were being modified slightly under present conditions. Coi?troversy has surrounded the relationship of linear dunes to wind directions. Process studies by Tsoar (1978) and a review of the available evidence by Lancaster (1982a) have facilitated the development of a general model for linear dune formation. This proposes that linear dunes form in a bimodal wind regime where the modes are less than 180' apart. Linear dunes are not necessarily aligned with the resultant direction of sand movement, but frequently form at a small angle (20 - 40') to the most persistent sand moving winds. These winds are diverted to move sand parallel to the dune where they cross the crest and are instrumental in extending the dune. The contribution of winds from different directions to the overall morphology of the dune is thus related to the angle at which they cross its crestline and to their persistence. In the central and northern parts of the Namib sand sea, persistent SII - SSlf winds cross linear dune crests at optimal angles for dune extension and are thus responsible for the overall trend of the dunes. The effect of NE - E winds i s mainly to reverse the slip face position and to create what are effectively reversing dunes on those parts of the sinuous crestline which are transverse to SSW and NNE winds. Because sand stays i n those parts of the dune, such areas tend to grow upward, and with the contribution of northerly winds in summer, form star peaks. This model i s supported by studies of internal structures by McKee (1982). Such a model can also explain the coexistence of different dune alignments in a single area. Elements of the pattern at angles of 20 - 40' to the persistent winds will be selected and progressively extended. These may be equivalent to the left and right hand oblique elements of Cooke and Warren (1973). In central and northern parts of the Namib sand sea two linear elements occur on N-S and WSW-ENE alignments. The major wind is oblique to them, but transverse to east flank barchanoid elements (Fig. 16). Of the linear oblique elements the N-S is dominant, partly because it lies upwind and thus traps most sand. The corridor crossing dunes are at an angle of 50' to dominant sand movements and thus consequently experience less extension and more deposition. Their form is also emphasised by north and north northwest

285

winds, giving a small reversing ridge.

Linear dunes

East flank

/

Most pers Ist en t sand moving winds

Fie. 16. Relationships between wind direction and linear dune alignment. Towards the east of the sand sea, sand flows from the southerly sector swing round to SW and I.ISI.l (Fig. 14e and 14f). The alignments of linear dunes and their east flank barchanoid dunes i n the central parts of the area follow the pattern, as illustrated by Fig. 9c. The frequency and sand moving effect of east and northeast winds increases eastwards and may even become the dominant sand movement sector. Thus 10 km east of Gobabeb ( F i a . 14e) 64% of sandflow i s from SU and SSW and 15% from NNE to ESE. Close to the eastern margin of the sand sea (Fig. 14f), 53% of sandflow is from NE to E and 35% from l*ISW to WW. Major sandflows are therefore directly opposed in direction and amount, and Under these conditions dune extension winds cross the dunes at close to 90'. will be minimal and the dunes will become effectively reversing types. In such a situation sand stays on the dune, which does not extend, but grows upwards by deposition. Many of the star form dunes along the eastern edge of the Namib are of this type. Yet not all dunes in the eastern Namib are of star form and most are simple and compound linear dunes. Many of the easterly winds are relatively light and the effects of topography are required to concentrate the winds and increase their velocity, as suggested by McKee (1982). Hence the

286

association o f the groups of star and reversing dunes with prominent valleys extending from the escarpment zone. The largest of these valleys i s that o f the Tsauchsb river leading to Sossus Vlei, which also has the largest concentration and maximum size of star dunes. In the southern Namib, winter winds from the northwest also play a part, and in a trimodal wind regime (Fryberger and Dean, 1979) take on a more truly stellate form. Effects of the sand accumulation process Regional changes in wind regimes over the Namib result in a decrease in wind energy and a parallel increase in its directional variability from south to north and west to east. Studies of sand seas elsewhere (Wilson 1971, Mainguet 1978, Fryberger and Ahlbrandt 1979) indicate that such changes are a major influence on the locus of sand accumulations. Thus many sand seas are located in areas of low total or resultant wind energy. This appears to be the case in the Namib, where sand i s moved from higher energy wind regimes with little directional variability in southern and western coastal areas, to accumulate in central and northern areas of the sand sea which have lower energy wind regimes and opposed wind directions. In these areas, the largest dunes occur, representing the zone o f maximum sand accumulation, whilst most dunes in the southern sand sea are relatively small. As has been suggested elsewhere (Mainguet and Callot 1974, Lancaster 1982c) the pattern of dune size and spacing in a sand sea i s a reflection of the ratio between wind energy and sand availability. Areas of low dunes are found where wind energy is great relative to sand supply, and in the case of linear dunes also when most winds blow at a small angle to the dune. Large linear and star dunes will occur where sand i s abundant, but total or resultant wind energy is low, and winds at large angles to the dunes frequent. It is also probable that dunes of simple and compound varieties will predominate where net sand transport dominates and complex and star dunes in zones of net sand accumulation. A similar pattern was observed by blainguet (1978)for the Erg Fachi-Bilma in Niger. One effect of the movement o f sand into zones of net sand accumulation will be to concentrate coarse sands in upwind areas. Coarse sands will be moved by the wind less frequently and then mostly as surface creep or traction load. They will thus move more slowly than fine sand which will be transported more rapidly in saltation by a greater frequency of winds. Consequently, dune types such as low rolling dunes without slip faces will be most common in upwind areas. Such a situation appears to apply in the Namib sand sea.

287

COIiCLUS IONS Spatial variability i n wind regimes can account for most of the observed pattern of dune types in the Namib sand sea. Narrow unimodal wind regimes in coast31 areas produce transverse and barchanoid dunes. Wide unimodal to bimodal regimes are common and are responsible for the formation and maintenance of the linear dunes which dominate the sand sea. Such dunes extend at a small angle (20 - 40') to the most persistent sand moving winds (SSW -SW). Where winds at high angles to the dunes are frequent, deposition rather than dune extension occurs and dunes grow in size. On the eastern margins of the sand sea, topographically induced funnelling of easterly katabatic winds creates complex reversing and star dunes. In such areas, sand, once on the dune, rarely leaves it and dunes attain large sizes. Regional changes i n wind regimes result in the accumulation of sand in central and northern areas of the sand sea, which are characterised by large, widely spaced dunes of complex morphology. Most southern areas are zones o f active sand transport and possess relatively low simple or compound dune forms. The major effect of sand grain size and sorting on dune morphology is its control o f the spacing of transverse and barchanoid dunes. Throughout the sand sea, but particularly in southern areas, coarse bimodal or multimodal sands are associated with subdued or low rolling dunes, generally without slip faces. Elsewhere, although sands with different grain size and sorting characters are associated with different dune types, this is frequently a product of different source areas and of the nature of the sorting process on dunes of differing morphologies. In general, therefore, the character o f the wind regime plays a major role in determining the morphology of dunes in the Namib sand sea. This suggests that further study of winds in sand seas, at varying scales, is of paramount importance in seeking to explain the form and movement of desert dunes. ACKNOWLEDGEMENTS I thank the C.S.I.R. and Transvaal Museum for assistance and the Directorate of Nature Conservation, S.W.A.A. for facilities and permission to work i n the Namib Desert Park. Consolidated Diamond Mines (S.W.A.) are also thanked for permission to investigate dunes in southern parts of the sand sea. E.D. McKee, Andrew Goudie and John Rogers reviewed this paper. I much appreciate their comments and suggestions for its improvement. I thank J.D. Ward o f t h e Kuiseb Environmental P r o j e c t f o r access t o h i s u n p u b l i s h e d wind d a t a .

288

REFERENCES

Alimen, N.H., Buron, M. and Chavaillon, J . , 1958. CaractPres granulometriques s d ' e r g s d u Sahara nord-occidental. Academie des Sciences, de ~ u ~ l o u edunes P a r i s , C.R. 247: 1753-1761. Barna-d, N.S., 1973. Duinformasies in d i e Sentrale Namib. Tegnikon, Des 1973. 5 : 2-13.

B e l l a i r , P.,1953. Sables desertiques e t morphologie eolienne. I n : Proceedings 1 9 t h International Geological Congress, Algiers, 1952. Vol 7 : 113-118. Besler, H., 1980. Die Dunen-Namib: Entstehung u n d Dynamik eines Ergs. S t u t t g a r t e r Geographische Studien. 96: 241 pp. Breed, C.S., Fryberger, S.G., Andrews, S.C., Mc Cauley, C., Lennartz, F. Gebel, D. and Horstman, K., 1979. Regional s t u d i e s of sand seas using Landsat (ERTS) imagery. In: E.D. McKee ( E d i t o r ) , A S t u d y of Global Sand Seas. United S t a t e s Geological Survey, Professional Paper 1052: 305-398. Breed, C.S. and Grow, T., 1979. Morphology and d i s t r i b u t i o n of dunes in sand seas observed by remote sensing. I n : E.D. McKee ( E d i t o r ) , A Study of Global Sand Seas. United S t a t e s Geological Survey Professional Paper 1052: 253-304. Capot-Rey, R., 1947. L'Edeyen de Mourzouk. Travaux I n s t i t u t e de Recherches Sahariennes. 4: 67-109. Capot-Rey, R. and Gremion, M., 1964. Remarques sur quelques sables Sahariennes. Travaux I n s t i t u t e de Recherches Sahariennes. 23: 153-163. Cooke, R . U . and Ilarren, A., 1973. Geomorphology in Deserts. Batsford. Folk, R.L., 1971. Longitudinal dunes of t h e northwestern edge of t h e Simpson Desert, Northern T e r r i t o r y , Australia. 1 : Geomorphology and grain s i z e r e l a t i o n s h i p s . Sedimentology. 16: 5-54. Fryberger, S.G. and Ahlbrandt, T.S., 1979. Mechanisms f o r t h e formation of eolian sand seas. Z e i t s c h r i f t f u r Geomorphologie, N . F . 23: 440-460. Fryberger, S.G. and Dean, G., 1979. Dune forms and w i n d regime. In: E.D. McKee ( E d i t o r ) , A S t u d y of Global Sand Seas. United S t a t e s Geological Survey Professional Paper 1052: 137-170. Holm, D.A., 1960. Desert geomorphology in t h e Arabian Peninsula. Science, 132: 1369-1379.

Howard, A . D . , Morton, J.B., Gad-el Hak, M. a n d Pierce, D., 1978. Sand t r a n s p o r t model of barchan dune equilibrium. Sedimentology, 25: 307-338. Lancaster, J . , Lancaster, N . and Seely, M.K., Climate of t h e c e n t r a l Namib Desert. Madoqua, ( i n p r e s s ) . Lancaster, N., 1981. Aspects of t h e morphometry of l i n e a r dunes of t h e Namib Desert. South African Journal o f Science, 77: 366-368. Lancaster, N . , 1982a. Linear Dunes. Progress in Physical Geography, 6 : 475-504. Lancaster, N., 1982b. Dunes on t h e Skeleton Coast, S.W.A./Namibia: Geomorphology a n d grain s i z e r e l a t i o n s h i p s . E a r t h Surface Processes and Landforms, 7: 575-537. Lancaster, N . , 1982c. Spatial v a r i a t i o n s in l i n e a r dune morphology and sediments in t h e Namib Sand Sea. In: Proceedings 5th SASQUA Conference, Paleoecology of Africa, 15 ( i n p r e s s ) . blcKee, E.D., 1966. Structure of dunes a t Ilhite Sands National llonument, New Mexico ( a n d a comparison w i t h s t r u c t u r e s of dunes from o t h e r selected a r e a s ) . Sedimentology, 1 : 1-69. McKee, E.D., 1979. Introduction t o a s t u d y of global sand s e a s . I n : E.D. McKee ( E d i t o r ) , A S t u d y of Global Sand Seas. United S t a t e s Geological Survey Professional Paper 1052: 3-19. HcKee, E.D., 1982. Sedimentary s t r u c t u r e s in dunes of t h e Namib Desert, South West Africa. Geological Society of America, Special Paper 188: 64 pp. McKee, E . D . and Breed, C.S., 1976. Sand Seas of t h e Norld. United S t a t e s Geological Survey Professional Paper 929: 81-88. McKee, E . D . and T i b b i t t s , G.C., 1964. Primary s t r u c t u r e s of a s e i f dune and assbciated deposits in Libya. Journal of Sedimentary Petrology, 34: 5-17.

289

Mabbutt, J.A., 1968. Aeolian landforms in central Australia. Australian Geographical Studies, 6 : 139-150. blainguet, rl., 1976. Propositions pour une nouvelle classification des edifices s ? b l e v x boliens, d'apre's les imape des satellites Landsat 1 , Gemini, Noaa 3. Zeitschrift fur Geomorphologie, N.F. 20: 3 , 275-296. Mainguet, M., 1978. L'erg de Fachi-Bilma (Tchad - Niger). Memoires et Documents C.N.R.S. 18: 184 pp. Mainguet, M. and Callot, Y., 1974. Air photo study of typology and interrelations between the texture and structure o f dune patterns i n the Fachi-Bilma erg, Sahara. Zeitschrift fur Geomorphologie, Supplement Bd 20: 62-63. Monod, Th., 1953. Majabat Al-koubra. Memoires de 1 ' Instuit Francais D'AfriqueNoire. 52: 406 pp. Royal Navy and South African Air Force, 1944. Weather on the coasts o f southern Africa: Vol 2, Pt 1: The west coast of Africa from the River Congo to Olifants River. 1-61. Tsoar, H., 1978. The dynamics of longitudinal dunes. Final Technical Report, European R2search Office, United States Army. 171 ppUard, J.D., 1983. Sand dynamics along the Kuiseb River. In: Huntley, B.J.(Editor) Kuiseb Environmental Project: the development of a monitoring base1 ine. South African National Scientific Programs, Report 68. Varren, A., 1970. Dune trends and their implications in central Sudan. Zeitschrift fur Geomorphologie, Supplement Bd 10: 154-180. llarren, A., 1972. Observations on dunes and bimodal sands in the Tenere Desert. Sedimentology, 19: 37-44. Wilson, I.G., 1971. Desert sandflow basins and a model for the development of ergs. Geographical Journal, 137: 180-197. Nilson, I.G., 1972. Aeolian Bedforms - their development and origins. Sedimentology, 19: 173-210.

This Page Intentionally Left Blank

291

THE FLOW IN THE PLANETARY BOUNDARY LAYER R.A.

BRDUN: P o l a r S c i e n c e C e n t e r , U n i v e r s i t y o f Washington, 4057 R o o s e v e l t Way N.E.,

S e a t t l e , l l l a s h i n g t o n 98105, U . S . A .

OVERVIEW The p u r p o s e o f t h i s p a p e r i s t o g i v e some b a s i c i n f o r m a t i o n a b o u t t h e f l o w i n t h e P l a n e t a r y Boundary L a y e r (PBL). I t i s d e s i g n e d t o i n t r o d u c e t h e f l o w f e a t u r e s which a r e l i k e l y t o b e s i g n i f i c a n t t o t h e s e d i m e n t o l o q i s t s . c o n c e n t r a t e d o n t h e a v e r a g e , o r mean f l o w . flow analysis i s presented.

Therefore i t i s

However, a n o v e r a l l p i c t u r e o f PBL

T h i s i s t o p r o v i d e an h i s t o r i c a l p e r s p e c t i v e , t h e

problems f a c e d , t h e a s s u m p t i o n s made and t h e t r i u m p h s needed i n o r d e r t o f u r n i s ' l

a f i n a l , r a t h e r sirriple, p i c t u r e o f t h e PBL f l o w .

T h i s knowledae s h o u l d h e l p

geophysical f l u i d d y n a m i c i s t s t o i n f e r c h a r a c t e r i s t i c f l o w c o n d i t i o n s from sedimentary d e p o s i t c h a r a c t e r i s t i c s .

I t should also serve t o e x p l a i n t h e nature

of d e p o s i t s based o n t h e PBL f l o w c h a r a c t e r i s t i c s .

A t t h e minimum, i t s h o u l d

a l e r t anyone c o n c e r n e d w i t h phenomena a t a f l u i d b o u n d a r y w h i c h has a s c a l e w h i c h i s s i g n i f i c a n t w i t h r e s p e c t t o t h e boundary l a y e r depth t o t h e f a c t t h a t secondary flows may b e i m p o r t a n t . L i k e t h e w e l l - k n o w n e x p r e s s i o n o f t h e f o r e s t n o t seen f o r t h e t r e e s , i n v e s t i g a t i o n s o f t h e f l o w i n t h e PBL f r e q u e n t l y a r e p r e o c c u p i e d w i t h t h e t r e e s , i n t h i s case r e p r e s e n t e d b y t u r b u l e n t e d d i e s .

E l a b o r a t e i n s t r u m e n t a t i o n and t e c h n i q u e s

have been d e v e l o p e d f o r m e a s u r i n g , c o u n t i n g , c a t e g o r i z i n g and a v e r a q i n g t h e t u r b u l e n c e i n v a r i o u s ways.

T h i s i s a h i g h l y i n s t r u c t i v e and a v e n e r a b l e p r o c e s s

f o r b u i l d i n g a l a r g e s c a l e s o l u t i o n from fundamental b u i l d i n g b l o c k s .

However,

t h e r e a r e two p r o b l e m s w i t h t h i s p r o c e s s . F i r s t , t h e b u i l d i n g blocks, t h e t u r b u l e n t eddies, a r e s t i l l d i f f i c u l t t o p i n down w i t h s p e c i f i c measurements, o r w i t h a a e n e r a l d e f i n i t i o n . mixing motion o f molecules,

U n l i k e t h e random,

t h e t u r b u l e n t e d d i e s a r e d e p e n d e n t o n t h e mean f l o w ,

and t e n d t o m i n g l e b e f o r e m i x i n g w i t h v a r i o u s d e g r e e s o f e f f i c i e n c y , a g a i n depending o n t h e mean f l o w c h a r a c t e r i s t i c s .

I n i t i a l l y , small scale eddies i n t e r -

a c t t o f o r m a mean f l o w , w h i c h i s t h e n u n s t a b l e t o i n f i n i t e s i m a l p e r t u r b a t i o n s , and l a r g e s c a l e e d d i e s d e v e l o p .

I n some c a s e s t h e s e e d d i e s r e a c h a n e q u i l i b r i u m

and r e s u l t i n a s e c o n d a r y f l o w .

These o r g a n i z e d e d d i e s become p a r t o f ( a n d

a l t e r ) t h e mean f l o w s t a t e .

I t i s d i f f i c u l t t o measure t h e l a r g e e d d i e s ( w h i c h

have d i m e n s i o n s o f k i l o m e t e r s i n t h e a t m o s p h e r e ) .

The s e c o n d a r y f l o w i s a f i n i t e

p e r t u r b a t i o n , w i t h m a g n i t u d e s a r o u n d 10% o f t h e mean f l o w . d i f f e r e n t f r o m t h e random t u r b u l e n t e d d y v e l o c i t i e s .

This i s n o t very

Careful averaging i s

292 necessary t o d i s t i n g u i s h v a r i o u s s c a l e t u r b u l e n c e eddies from t h e organized secondary f l o w .

P u r s u i n g t h e f o r e s t a n a l o q y , one t y p e o f t r e e b u i l d s a f o r e s t ,

i n which another type o f f r e e f l o u r i s h e s , q i v i n g r i s e t o another type o f forest. T h e r e e x i s t s a good i l l u s t r a t i o n o f t h e p o s i t i o n o f t h e s c i e n t i s t t r y i n q t o tie i s l i k e a b l i n d e x p e r i m e n t e r a t t e m p t i n o t o

measure g e o p h y s i c a l t u r b u l e n c e .

determine t h e v e h i c u l a r t r a f f i c c h a r a c t e r i s t i c s on a r o a d which c a r r i e s bicycles H i s i n s t r u m e n t s c o u n t o n l y w h e e l s , and he knows t h e t o t a l number o f

and c a r s .

v e h i c l e s and p e o p l e t r a n s p o r t e d . 2.9 w h e e l s / v e h i c l e ,

He i s l i k e l y t o come up w i t h a n a v e r a q e of

and w i l l p r o b a b l y d e v e l o p a model w i t h a v e h i c l e w i t h t h r e e

w h e e l s , one o f w h i c h d o e s n ' t t o u c h g r o u n d p a r t o f t h e t i m e ( a m o t o r c y c l e w i t h sidecar).

An a v e r a g e number ( s a y , 1 . 7 ) o f p e o p l e c o u l d b e p l a c e d i n t h e v e h i c l e ,

and t h e a v e r a g e f l o w m i g h t be s u c c e s s f u l l y p a r a m e t e r i z e d - - f o r t h e s e s p e c i f i c conditions.

When t h e c o n d i t i o n s change, e.g.,

r a i n i n h i b i t i n g c y c l i n n , o r an

OPEC gas embargo r e s t r i c t i n g sedan d r i v i n g , t h e p a r a m e t e r i z a t i o n m u s t change. F u r t h e r m o r e , d r a s t i c changes i n t h e model a r e n e c e s s a r y when t h e s t a t i s t i c s change, such as changed d r i v i n g h a b i t s f o r a 2.3 w h e e l / v e h i c l e a v e r a n e , o r a new v e h i c l e ( " o f t h e second k i n d " ) c o n c e p t f o r a 3 . 2 w h e e l / v e h i c l e a v e r a g e .

The

s c i e n t i s t i s u s u a l l y up t o p r o d u c i n g i m a g i n a t i v e r e v i s i o n s , b u t t h e model g e n e r a l l y becomes cumbersome.

The p r o b l e m , o f c o u r s e , i s t h a t t h e p a r a m e t e r i z a -

t i o n i s based o n a f a u l t y u n d e r s t a n d i n g o f t h e b a s i c e l e m e n t s o f t r a n s p o r t a t i o n . The o t h e r p r o b l e m i s a l a c k o f a good p i c t u r e o f t h e f o r e s t

=

t h e mean f l o w .

I n g e o p h y s i c a l f l u i d dynamics, t h e s c a l e s a r e l a r q e , a n d i n s t r u m e n t s and sample

volumes a r e s m a l l .

F o r i n s t a n c e , t h e l a r g e , two-km r o l l s w h i c h I w i l l d i s c u s s ,

may t a k e 20 m i n u t e s t o h o u r s t o p a s s o v e r a g i v e n p o i n t . a l o n g r e c o r d and a s t e a d y s t a t e f i e l d i s r e q u i r e d .

To d e t e c t t h e s e eddies,

C o n s e q u e n t l y , t h e y were not

g e n e r a l l y r e c o g n i z e d a s a n i n t e g r a l p a r t o f t h e mean PBL f l o w u n t i l r e c e n t l y . E a r l i e s t observations were o f seagulls, which soared i n c i r c u l a r p a t t e r n s f o r l o w w i n d s ( c o n v e c t i o n a l o n e ) and l i n e a r p a t t e r n s p a r a l l e l t o t h e mean w i n d f o r moderate winds.

G l i d e r p i l o t s learned t o recognize t h e l i n e a r u p d r a f t regions D i s t a n c e r e c o r d s w e r e s e t by

b y f o l l o w i n g t h e c l o u d s w h i c h accompanied them.

f l y i n g a l o n g t h e mean w i n d u n d e r t h e c l o u d " s t r e e t s " .

They a l s o u n d o u b t e d l y

became aware o f t h e d o w n d r a f t s e x i s t i n g h a l f w a y between a d j a c e n t s t r e e t s . The w e a l t h o f s a t e l l i t e p i c t u r e s s h o w i n g u b i q u i t o u s c l o u d s t r e e t s and l i n e a r c h a r a c t e r i s t i c s o f many l a r g e s c a l e f l o w s e s t a b l i s h e d t h e i m p o r t a n c e o f these flow patterns.

However, measurements o f t h e windspeeds and d i r e c t i o n s o f both

t h e r o l l f l o w a n d t h e l a r g e s c a l e mean f l o w has been a t t e m p t e d o n l y i n t h e past decade.

C u r r e n t l y , t h e r e e x i s t many q u a l i t a t i v e i n d i c a t i o n s o f t h e s e steady-state

s e c o n d a r y f l o w e d d i e s embedded i n t h e PBL---so

many t h a t we m u s t assume t h a t they

a r e a n i n t r i n s i c p a r t o f t h e mean f l o w s o l u t i o n . ments have been v e r y l i m i t e d .

However, q u a n t i t a t i v e measure-

293 There a r e d i v e r s e methods a v a i l a b l e f o r d e s c r i b i n g t h e f l o w i n t h e PBL.

They

range i n c o m p l e x i t y from f i n i t e d i f f e r e n c i n g t h e complete e q u a t i o n s t o t h e s i m p l e a n a l y t i c s o l u t i o n s o f f e r e d by f i r s t o r d e r c l o s u r e ( K - t h e o r y ) .

The model w h i c h

I

am advocating s t a r t s w i t h t h e Navier-Stokes e q u a t i o n s , and accents t h e Boussinesq eddy v i s c o s i t y assumption f o r t h e s m a l l s c a l e eddies o n l y .

A f i n i t e perturbation

i s c a l c u l a t e d u s i n g energy c r i t e r i a f o r e q u i l i b r i u m , and t h i s produces l a r g e s c a l e coherent s t r u c t u r e s embedded w i t h i n t h e boundary l a y e r as p a r t o f t h e mean f l o w .

I b e l i e v e t h a t we a r e now a t t h e p o i n t where we can g i v e a f a i r l y good p i c t u r e Of

the mean PBL f l o w t o g e t h e r w i t h i t s secondary f l o w , and t h e v a r i a b i l i t y o f

both w i t h changing c o n d i t i o n s .

I n t h i s p i c t u r e , I b e l i e v e there are explanations

f o r v a r i o u s s e d i m e n t a t i o n phenomena. I t i s a p p a r e n t t h a t t h e wave forms o c c u r r i n g i n g e o p h y s i c a l f l u i d dynamics a r e s i m i l a r t o those f o u n d i n sedimentary d e p o s i t s .

The s e i f sand dunes 100

meters h i g h , hundreds o f km l o n q , a r e b u t one v e r y s t r i k i n g example o f t h i s f a c t . The i n i t i a l s a t e l l i t e p i c t u r e w h i c h i n s p i r e d me (and o t h e r s ) t o s t u d y t h e two dimensional waves w i t h c r e s t s and t r o u g h s n e a r l y p a r a l l e l t o t h e mean f l o w i n the atmosphere was t h a t o f c l o u d s t r e e t s shown i n f i g u r e 1 .

However, a l s o a v a i l -

able a t t h a t t i m e were Gemini photos of t h e s e i f sand dunes as seen i n f i g u r e 2. The t a n t a l i z i n g s i m i l a r i t y i n s c a l e s and wavelenqths s t r o n g l y suqqested atmospheric f o r c i n g .

However, when I f i r s t p r e s e n t e d t h i s idea, some a t m o s p h e r i c

p h y s i c i s t s were i n c r e d u l o u s t h a t a p e r t u r b a t i o n f l o w i n t h e PBL c o u l d move such mountains o f sand. be s t r o n g evidence.

However, t h e remarkable c o i n c i d e n c e of s c a l e s seems t o me t o A paper by Hanna (1969) documented these s i m i l a r i t i e s .

As

y e t , I have n o t seen a q u a n t i t a t i v e a n a l y s i s f r o m t h e s e d i m e n t o l o q i s t ' s p o i n t o f view y i e l d i n g t i m e s c a l e s . I n t h e remainder o f t h e paper

I w i l l t r y t o p r o v i d e a summary o f o u r under-

standing o f t h e PBL f l o w i n c l u d i n g t h e b a s i s f o r development o f secondary f l o w s , which a r e s i g n i f i c a n t f o r sedimentary d e p o s i t s on c e r t a i n s c a l e s . d i s c u s s t h e c o n d i t i o n s i n w h i c h v a r i o u s f l o w s develop.

Then I w i l l

For f u r t h e r d i s c u s s i o n ,

see Brown (1980). HISTORY

The g e n e r a l e q u a t i o n s o f f l u i d f l o w i n a continuum, t h e Navier-Stokes equat i o n s , were a v a i l a b l e around 1880.

Immediately, Boussinesq o f f e r e d t h e h y p o t h e s i s

t h a t c o m p l e t e l y t u r b u l e n t f l o w , as i n a c o n v e c t i v e PBL, c o u l d be t r e a t e d assuming the momentum and h e a t t r a n s p o r t were accomplished by t h e s m a l l t u r b u l e n t eddies i n a s i m i l a r f a s h i o n t o t h e m o l e c u l a r m o t i o n which u n d e r l i e s d i f f u s i o n .

I n 1894,

i n h i s two-year d r i f t a c r o s s t h e A r c t i c Ocean, Nansen observed t h a t t h e pack i c e m o t i o n was a b o u t 45" t o t h e r i g h t o f t h e s u r f a c e wind v e c t o r . assigned r e s p o n s i b i l i t y t o t h e r o t a t i n g frame o f r e f e r e n c e .

He c o r r e c t l y I n 1904, Ekman

i n c l u d e d t h e v i r t u a l C o r i o l i s f o r c e i n t h e Navier-Stokes e q u a t i o n s and found an

2 94

F i g u r e 1 . A p o l l o photoqraph o f t h e Georqia c o a s t , 4 A o r i l 1968, frori 125 miles. The c l o u d s t r e e t s a r e over t h e l a n d , a l i g n e d approxiiiiately with the mean wind >which i s due n o r t h . The a r e a p i c t u r e d i s about 100 km s q u a r e , t h e cloud rows a r e 2-3 km a p a r t a t about 1 km h e i q h t . North i s a t t h e t o D .

295

Figure 2 . Gemini ohotoqraph o f s e i f sand dunes. The a r e a o i c t u r e d i s about 100 km s q u a r e and t h e dune rows a r e s e p a r a t e d by a b o u t 2 k i l o m e t e r s . Mean wind f l o w i n t h e a r e a i s p a r a l l e l t o t h e dunes.

296

exact solution f o r the balance between C o r i o l i s , viscous and pressure gradient f o r c e s . This i s the Ekman PBL s o l u t i o n , which p r e d i c t s a velocity p r o f i l e in the' form of a logarithmic s p i r a l (Ekman, 1905). A sketch i s shown i n f i q u r e 3. Looking down, the surface flow i s 45" counterclockwise from the f r e e stream flow. A p l o t of the locus of the velocity vectors i s called a hodoqraph, shown in the

x-y plane i n f i g u r e 3. Each level of flow represents the flow down the pressure gradient, retarded by f r i c t i o n and viewed from a ( d i f f e r e n t ) non-inertial frame of reference. This i s a unique solution of the flavier-Stokes equations f o r three f o r c e s , which a r e l e f t in the boundary layer l i n i t equations. I t i s so eleqant t h a t i t i s taught i n every oceanography and atmospheric science beqinninq dynamics c l a s s . This i s d e s p i t e the f a c t t h a t a qenuine Ekman s p i r a l has nossibly never been observed. I say possibly because one may have been found i n the high Arctic Ocean under the pack i c e i n a 1960's expedition.

Projection of Wind Vectors on S u r f a c e is Hodograph

of E k m a n Spiral (height in meters)

Figure 3. A sketch of PEL wind v e l o c i t i e s above the e a r t h . the vectors on t h e surface forms a hodograph.

The projection of

297

A SKETCH O F THE ANALYTIC UNDERSTANDING OF MEAN PBL FLOW no /

observational t

I

2-1 ayer oa t c hed so 1 u t i on

S t u a r t --+ Fa1 1e r Lilly's Instability of the Ekman s o l u tion

3 o u s s i n e sq ' s vi scosi t y

c

I

1880

Benard ' s convection cell s 1900

1920

1940

Modified Ekman solution with secondary flow s %abl i za t i on

1960

Ekman la ve r -obse rva tion in A rc tic Ocean

1980

2000

YEAR

Figure 4. A sketch of progress i n the a n a l y t i c understanding of the average PBL flow s i n c e c i r c a 1880.

I can move quickly t h r o u q h t h e h i s t o r y by usinq the schematic of f i q u r e 4. Now, I would never show t h i s hiqhly personalized view of the history of PBL u n d w standing t o a group of boundary l ay er s c i e n t i s t s . This i s because I have l e f t o f f many small v e r t i c a l jumps i n understandinq.

This re pre se nts how well I , a s a

qeophysical f l u i d dynamicist, would have understood the nean flow in the PBL havinq read the l i t e r a t u r e u p t o any qiven d a t e . Takinq o f f from t h e base of the HavierStokes equations, the f i r s t j u m p was the eddy v i s c o s i t y hyoothesis.

Benard's

experiments showed r eq u l ar convection cel I s , and Rayleiqh provided the mathematical solution. This was a good beginning, and s a t e l l i t e photos reveal such c e l l s on geophysical s c a l e s ( f i g u r e 5 ) . Continuing o n the schematic, note t h a t I have made 100% understanding a t somewhat more than present knowledge with due modesty, and an eye t o f u r t h e r fundinq. This i s a l s o done with t h e recognition t h a t PBL s c i e n t i s t s of Ekman a n d Ta ylor' s day probably f e l t t h a t a near 100% understanding had been achieved with the Ekman/ Taylor s o l u t i o n s .

Figure 6 shows linked Ekman s p i r a l s and the correspondinq hodo-

graphs i n t h e ocean-atmosphere i n t e r f a c e .

Another Ekman s p i r a l would occur a t the

ocean bottom.

However, t h er e was steady erosion i n the confidence in Ekman's solution a s experiments continued t o f a i l t o f i n d an Ekman s p i r a l . I n the 193Os, Rossby and ilontqomery added t h e r ecen t l y developed Prandtl loqarithmic la ye r t o the bottom of t h e Ekman l ay er f o r a two-layer s o l u t i o n .

This resolved the problerr

298

F i g u r e 5 . NOAA 4 p h o t o q r a p h o f c l o u d s i n t h e B e r i n g Sea. The a r e a i s a p p r o x i m a t e l y 1600 km s q u a r e ; c l o u d s t r e e t s a r e s e F a r a t e d b y 2 - 5 km and t h e downstream c e l l s a r e a b o u t 20-50 km i n d i a m e t e r . The f l o w i s f r o m t h e u p p e r r i q h t (NE) t o t h e l o w e r l e f t . I t changes f r o m a s t r o n g f l o w i n t h e c l o u d s t r e e t r e g i o n t o l o w sFeeds i n t h e c e l l u l a r r e g i o n . b e t w e e n t h e m a t h e m a t i c a l Ekman s o l u t i o n and t h e o b s e r v a t i o n a l l y e s t a b l i s h e d o a r a l l e l flow loqarithniic v e l o c i t y p r o f i l e i n the l a y e r near the surface.

However,

l a c k o f o b s e r v a t i o n a l a q r e e m e n t c a s t i n c r e a s i n g d o u b t on t h e s o l u t i o n s f o r t h e PBL flow.

I n t h e 19605, t h e Ekman m a t h e m a t i c a l s o l u t i o n was f o u n d t o be u n s t a b l e t o

infinitesimal perturbations.

S i n c e t h e PBL i s r i f e w i t h i n f i n i t e s i m a l perturbations,

and t h e y qrow e x n o n e n t i a l l y , t h i s s i g n a l l e d t h e end of a b e l i e f t h a t Ekrnan's solut i o n c o u l d be exoected t o s a t i s f a c t o r i l y r e p r e s e n t boundary f l o w .

299

e of reference

velocity-30cm/sec

Figure 6 . Sketch of linked Ekman s p i r a l s in t h e ocean and atmosphere. Note the s c a l e change from atmosphere t o ocean. Many meteorologists assumed t h a t t h e f a u l t l a y in the eddy-viscosity assumption, Progress was made in understanding the turbulence. However understandinq, or p r e d i c t a b i l i t y , and the attempts t o d i r e c t l y account f o r the turbulence came t o the f r o n t .

of the mean flow reached a new low. I n c i d e n t a l l y , the Arctic observation of t h e Ekman s p i r a l was compatible with the mathematics, since the Reynolds number of the flow was very low, below the c r i t i c a l f o r i n s t a b i l i t i e s . Usual flows in the ocean or atmosphere a r e a t Reynolds numbers q r e a t l y in excess of the c r i t i c a l .

300

rileanwhile, t h e r e were many i n d i c a t i o n s i n t h e ocean and t h e atmosphere t h a t p e r s i s t e n t , s t e a d y - s t a t e wave s o l u t i o n s e x i s t e d i n t h e mean f l o w .

Based on obser-

v a t i o n s o f g l i d e r p i l o t s and s e a g u l l s , K u e t t n e r d i s c u s s e d t h e presence o f r o l l v o r t i c e s i n t h e atmosphere, and Langmuir, i n v e s t i g a t i n q windrows i n t h e ocean, proposed and measured r o l l v o r t i c e s i n t h e w a t e r boundary l a y e r . I n 1970, a s o l u t i o n was found wherein t h e i n s t a b i l i t y qrew o n l y t o a f i n i t e p e r t u r b a t i o n v a l u e and t h e n came t o e q u i l i b r i u m .

There i s now a s t a b l e , steady-

s t a t e s o l u t i o n t o t h e N a v i e r - S t o k e s e q u a t i o n s f o r t h e PBL which i n c l u d e s an embedded secondary f l o w o f h o r i z o n t a l v o r t i c e s w i t h a l t e r n a t i n q c i r c u l a t i o n and o r i e n t e d a l o n g t h e mean f l o w d i r e c t i o n o f f l o w .

Furthermore, t h e presence o f t h e s e vortices

e x p l a i n s why o b s e r v a t i o n s o f t h e f l o w which q e n e r a l l y were o f t o o s h o r t d u r a t i o n t o i n c l u d e more t h a n a p o r t i o n o f a r o l l , had i n c o n s i s t e n t hodoqraphs. shown i n f i q u r e s 7 and 8.

This f a c t i s

F i q u r e 7 shows a schematic o f t h e r o l l c i r c u l a t i o n i n

t h e Ekman l a y e r and a hodoqraph o f t h e mean f l o w .

F i g u r e 8 shows t h e hodograph

which would r e s u l t i f a q u i c k ( s e v e r a l minutes i n t h e atmosphere) a s c e n t b a l l o o n were used t o measure t h e v e r t i c a l v e l o c i t y p r o f i l e a t v a r i o u s s t a t i o n s i n t h e r o l l . The dashed l i n e s i n f i g u r e 4 r e p r e s e n t t h e assumed l e v e l o f u n d e r s t a n d i n g o f many members of t h e meteorology community, who e i t h e r d i d n ' t know o f , o r questioned t h e v a l i d i t y o f t h e o b s e r v a t i o n s and t h e i n s t a b i l i t y t h e o r y . i s s o n i c e t h a t i t was h a r d t o q i v e up.

A l s o , Ekman's solution

T h i s p e r s i s t e n c e was j u s t i f i e d somewhat

when t h e q u a s i - n o n l i n e a r s o l u t i o n i n d i c a t e d t h a t t h e Ekman s o l u t i o n was s t i l l valid if one added a f i n i t e p e r t u r b a t i o n secondary f l o w t o t h e eddy-laminar f l o w solution. Much o f PBL modeling r e q u i r e s o n l y an average v a l u e f o r t h e momentum, h e a t o r moisture f l u x f o r a l a r g e scale region.

For i n s t a n c e , t h e g r i d D o i n t s on an atmos-

p h e r i c g e n e r a l c i r c u l a t i o n model a r e 400 km a p a r t .

I t was p o s s i b l e t o determine an

eddy v i s c o s i t y c o e f f i c i e n t t o p a r a m e t e r i z e t h e a d v e c t i v e e f f e c t o f t h e l a r q e eddies. However, t h e b a s i c d i f f u s i o n e q u a t i o n i s Q = K dS/dz, where Q i s t h e d i f f u s e d quant i t y , and S t h e p o t e n t i a l f u n c t i o n , e.q.,

or

9=

heat, S = temperature.

when Q i s momentum f l u x , S i s v e l o c i t y ;

The l i m i t s r e q u i r e d by t h e fundamental theorem o f

CalCUlUS t o d e f i n e t h e d e r i v a t i v e must n o t a f f e c t t h e random c h a r a c t e r o f t h e eddies which a r e modeled i n t h e eddy c o e f f i c i e n t K.

T h i s means t h e r e must be a l o t o f them

l e f t i n t h e t h i n l a y e r where t h e average d e r i v a t i v e i s d e f i n e d ( t h e continuum hypothesis).

T h i s r e q u i r e s an eddy d e f i n i t i o n l i k e t h e b l i n d t r a n s p o r t m o d e l e r ' s

v e h i c l e of t h e second k i n d .

S t i l l , f o r areas l a r g e enouqh t o c o n t a i n many r o l l s ,

a h y p o t h e t i c a l v a r i a b l e K - p r o f i l e can work r e a s o n a b l y w e l l i n m o d e l i n q s p e c i f i c average v a l u e s . necessary.

However, i n f i n i t e , and d i s c o n t i n u o u s

K d i s t r i b u t i o n s were sometimes

301

Typical Secondary Flow in the Planetary Boundary Layer (Modified Ekman Layer) Mean Wind Hodograuh

F i g u r e 7. S k e t c h o f s e c o n d a r y r o l l f l o w w i t h a mean w i n d hodograph. The a l t e r n a t i n g r o t a t i o n o f t h e v o r t i c e s p r o d u c e s c l o u d bands i n enhanced v e r t i c a l v e l o c i t y r e g i o n s . The t u r n i n g o f t h e w i n d f r o m f r e e s t r e a m ( G e o s t r o p h i c ) t o t h e s u r f a c e t h r o u q h a n g l e E i s shown.

Z/8=1

STAT1ON A 1

-o.o#oo

-0.OkGG5k-

-0.050 -0.025

v/u, SHORT TERM VELOCITY

HODOGRAPHS IN THE PEL

F i g u r e 8. Hodographs s h o w i n g t h e i n s t a n t a n e o u s v e l o c i t y p r o f i l e s a t s t a t i o n s A, B, and C i n f i g u r e 7. 6 i s t h e Ekman s c a l e h e i q h t ; PBL h e i q h t i s a n p r o x i m a t e l y ~ 6 .

302 P o l l u t i o n n o d e l e r s u s i n g t h - l a r g e s c a l e a v e r a q e d Ks, may have f o u n d t h e i r averane p o l l u t a n t v a l u e s w e r e adequa-t?, b i i t a n o t h e r c r i t e r i a ,

t h e maximun c o n c e n t r a t i o n s ,

were o f t e n o b s e r v e d t o b e muc:i g r e a t e r t h a n t h i s a v e r a q e . c o n p a r a b l e t o t h e dent11 o f t h e l a y e r . t i o n o f t h e secondary f l o i r s .

T h i s o c c u r r e d o n a scale

These phenomena a r e e x p l a i n e d b v a considera-

The r o l l s cause a l t e r n a t i n q l i n e s o f a s c e n d i n n and

d e s c e n d i n g n o t i o n , a n d c o r r e s p o n d i n g l i n e s o f c o n v e r q e n c e a n d d i v e r q e n c e a t t h e to? and b o t t o m as seen i n f i g u r e 9 .

These l i n e s o f c o n c e n t r a t i o n have been o b s e r v e d

i n moisture, insects, e l e c t r i c f i e l d s ,

t u r b u l e n c e and d u s t .

SCJW i n v e s t i o a t i o n s o f t h e t r a n s F o r t o f s e d i m e n t have been c o n c e r n e d w i t h l a w

s c a l e a v e r a q e s w h i l e o t h e r s have been c o n c e r n e d w i t h sria11 s c a l e v a r i a t i o n s , eacli w i t h an a p p r o p r i a t e PBL f l o i i s o l u t i o n . t h i n q about the other.

O f c o u r s e , each s c a l e f l o u c a n t e l l son’?-

For i n s t a n c e , i n t h e atmos?here, the s i m l l s c a l e c l o u d

s t r e e t s a r e r o u t i n e l y u s e d t o d e t e r m i n e t h e d i r e c t i o n o f t h e mean f l o w ( n a r a l l e l t o the s t r e e t s ) .

L i k e w i s e c e r t a i n p a t t e r n s o f s e d i n e n t a t i o n t e l l s o m e t 9 i n g about

t h e p r e v a i l i n q mean f l o w d i r e c t i o n and n i a q n i t u d e .

Soiw u n d e r s t a n d i n ? o f t h e theory

f o r secondary flow w i l l a i d i n t h i s e v a l u a t i o n .

F i g u r e 9. S k e t c h o f S e c o n d a r y f l o w v o r t i c e s s h o w i n q c o n v e r g e n c e / d i v e i - g e n c e p a t t e r n s , d e n s i t y o f a n y o a s s i v e m a t e r i a l and mean f l o w a v e r a g i n g volume.

303 Secondary Flow and Naves Eknan's solution was f o r the three force balance, P /p - f V - K U z z = 0 X

P /p

Y

f U

f

-

K Uzz= 0

where P i s pressure, p i s d e n s i t y , f i s the Coriolis parameter equal t o twice the local r o t a t i o n of the e a r t h , U a n d V a r e the x and y components of the velocity, K i s eddy v i s c o s i t y , a n d s u b s c r i p t s x , y a n d z denote p a r t i a l d i f f e r e n t i a t i o n .

1.2

-

I

E = 20"

,'

1.0 -

I

F .---.

Q'OO

L

8.2

8.2

0.8 -

is=i I

0

s= I

I

3

20.6 -

I

I I

8

I

\ \ \ \\

\

0

-0.4

-0.2

v/

0.0

ug

0.2

-0.4

-0.2

0.0

02

v / ug

Figure 10. Hodographs of mean flow solutions from ( 1 ) (dashed) and ( 2 ) ( s o l i d ) . E i s t h e angle between the freestream flow (Ug) end the r o l l a x i s ; 6 i s the Ekman s c a l e height = (ZK/f)k. The l e f t p r o f i l e i s f o r s t a b l e layer s t r a t i f i c a t i o n , the r i g h t i s f o r unstable s t r a t i f i c a t i o n . The solution i s a s p i r a l o r a hodoqraph, shown in f i g u r e 10 as the dashed l i n e s . llhen the Reynolds number f o r the flow (based o n the heiqht, H), 'ie = U H / K exceeds a c e r t a i n , small value, the i n e r t i a l force i s l a r q e with respect t o the viscous f o r c e .

The f l u i d cannot s u s t a i n the shearino and develops a more e f f i c i e n t momentum f l u x ( s t r e s s ) mode; i n s t a b i l i t y waves a r i s e . These qrow u n t i l the\( destroy o r rearranqe the flow s o l u t i o n . This can be shown by considerino the t o t a l velocity

304 as c o n s i s t i n g o f a n a v e r a g e f l o w w h i c h i s t h e h o r i z o n t a l a v e r a g e o v e r t h e e n t i r e domain; p l u s a f i n i t e p e r t u r b a t i o n v e l o c i t y w h i c h has t h e f o r m o f t h e i n s t a b i l i t y waves and a z e r o mean o n l a r g e h o r i z o n t a l s c a l e s ; p l u s t h e p e r t u r b a t i o n component w h i c h r e D r e s e n t s t h e random, s m a l l s c a l e t u r b u l e n c e ,

-

U = U,(mean)

-

+ u p ( z e r o mean waves) + u- ' ( t u r b u 1 e n c e ) . x

The s m a l l s c a l e p e r t u r b E t i o n s , c o s i t y t e r m , K dU/dz.

(u',

v',

w'),

a r e r e p r e s e n t e d i n t h e eddy v i s -

The ( u p , v 2 , w 2 ) t e r m s a r e a l l o w e d t o grow t o a f i n i t e

p e r t u r b a t i o n and f o r m t h e c o h e r e n t e d d i e s o f t h e s e c o n d a r y f l o w .

The shape i s

determined from t h e i n s t a b i l i t j / a n a l y s i s o f t h e f l o w p r o f i l e determined from the solution t o (1).

The m a g n i t u d e i s d e t e r m i n e d f r o m t h e e n e r g y e q u a t i o n f o r t h e

t r a n s f e r o f e n e r q y between t h e mean f l o w and t h e p e r t u r b a t i o n .

When t h e mean pllis

f i n i t e p e r t u r b a t i o n t e r m s a r e s u b s t i t u t e d i n t o ( 1 ) and a h o r i z o n t a l a v e r a g e i s __ t a k e n , one a d d i t i o n a l t e r m , w 2 v 2 i s n o n z e r o , Px/p

- f V - K Uzz = 0

P/p

+ f U - K V Z z = G

Y

Re

The s o l u t i o n t o t h i s inhomogeneous e q u a t i o n f o r t h e mean f l o w c a n be w r i t t e n as s i m p l y t h e sum o f t h e Ekman s o l u t i o n p l u s a mean component due t o t h e secondary flow,

_Urn

-

= UE

f i 7 u r e 10.

+ -U p .

The m o d i f i e d hodographs a r e shown b y t h e s o l i d l i n e s i n

These a r e t h e new s o l u t i o n s f o r t h e a v e r a q e o v e r t h e e n t i r e domain.

If a n a v e r a g e i s t a k e n w h i c h i s s h o r t w i t h r e s p e c t t o t h e s e c o n d a r y f l o w wavelength, o r of i n s u f f i c i e n t time f o r several wavelenqths t o advect p a s t t h e s i t e , then the

a v e r a g e w o u l d n ' t b e z e r o , and hodographs l i k e t h o s e o f f i g u r e 8 m i g h t

be o b s e r v e d , d e p e n d i n g o n t h e s t r a t i f i c a t i o n o f t h e l a y e r and where i n t h e a l t e r n a t i ng c o n v e r g e n c e / d i v e r q e n c e zones t h e a v e r a q e was t a k e n . I n Ekman's s i m p l e s t s o l u t i o n , a c o n s t a n t K was assumed.

T h i s i s c r i t i c i z e d on

t h e a r g u m e n t t h a t K m u s t go t o z e r o a t t h e b o t t o m , where t h e e d d i e s m u s t v a n i s h ( n o room), and t h e t o p , where t h e z e r o eddy v i s c o s i t y s o l u t i o n w o r k s f i n e .

However,

t h e t w o - l a y e r s o l u t i o n adds t h e l o g a r i t h m i c v e l o c i t y w h i c h c o r r e s p o n d s t o a l i n e a r l y increasing K proportional t o

z a t t h e b o t t o m ; and t h e s t r e s s t e r m , K dU/dz, qoes

z e r o w i t h dU/dz a t t h e t o p , r e n d e r i n g t h e v a l u e o f K m o o t t h e r e .

to

Another c r i t i c i s m

o f K-theory i s t h a t a d i f f u s i o n c o e f f i c i e n t cannot represent t h e advection o f e d d i e s whose s c a l e i s t h a t o f t h e PBL d e p t h .

The s e c o n d a r y f l o w s o l u t i o n e x p l i c i t l y

d e s c r i b e s t h e s e e d d i e s as p a r t o f t h e mean f l o w and p a r a m e t e r i z e s o n l y t h e s m a l l scale eddies w i t h the d i f f u s i o n c o e f f i c i e n t . One o f t h e e f f e c t s o f t h e s e c o n d a r y f l o w i n t h e mean i s t h a t t h e s u r f a c e f l o w direction i s closer t o the freestream direction. from t h e s u r f a c e t o t h e f r e e s t r e a m .

That i s , t h e f l o w t u r n s l e s s

The Eknan l a y e r t e n d e n c y t o have f l o w speeds

305 h i g h e r t h a n t h e f r e e s t r e a m i s enhanced i n v a r y i n q d e o r e e , d e p e n d i n q uoon l a y e r H h i l e t h e magnitude o f t h e secondary f l o w i t s e l f i s simply a

stratification.

f i n i t e p e r t u r b a t i o n o n t h e mean f l o w , t h e m o d i f i c a t i o n t o t h e mean f l o w a t any l a y e r c a n be l a r q e , d e p e n d i n q o n l a y e r s t r a t i f i c a t i o n .

T h i s r e s u l t s , amonq o t h e r

t h i n g s , i n a mean v e l o c i t y i n t h e u p p e r p a r t o f t h e PBL w h i c h may be s u b s t a n t i a l l y greater than t h e f r e e stream v e l o c i t y .

A l s o , m i x i n g i s q r e a t e r and more momentum,

heat o r o t h e r passive m a t e r i a l s can be t r a n s p o r t e d v e r t i c a l l y .

The s o l u t i o n a t a

p o i n t depends upon t h e s t e a d y - s t a t e f r e e - s t r e a m f l o w , s u r f a c e rouqhness and l a y e r stratification.

Y . km

r

1

I

I

I

1

'0

'

I

Y . km

F i g u r e 11. ( w h e r e $,

L a t e r a l c r o s s - s e c t i o n of s e c o n d a r y f l o w . =

VP,

-$y = w p ) ,

The s t r e a m l i n e f u n c t i o n , @

and t h e l a t e r a l s e c o n d a r y f l o w v e l o c i t y , v2/Ug,

a r e shown.

A c r o s s - s e c t i o n o f t h e s e c o n d a r y f l o w i s shown i n f i g u r e 11.

Under t h e assump-

t i o n s o f t h e s o l u t i o n , t h e r e i s no change i n t h e d i r e c t i o n normal t o t h e s e s e c t i o n s , o r n e a r l y i n t h e d i r e c t i o n o f t h e mean f l o w . t h e l a t e r a l component of t h e mean f l o w .

The t i l t t o t h e v o r t i c e s i s caused b y

T h i s p i c t u r e changes f o r d i f f e r e n t l a y e r

306 stratificstions.

As t h e b a s i c f l o w b o u n d a r y c o n d i t i o n s change, a p o i n t - b y - p o i n t

change i n t h e e q u i l i b r i u m s o l u t i o n w i t h s e c o n d a r y f l o w c o u l d b e f o u n d .

Because o f

t h e enhanced m i x i n g t h e a d j u s t m e n t t i m e i s f a i r l y s h o r t , a b o u t o n e - h a l f

h o u r i n the

atmosphere, compared t o 12-24 h o u r s i f i t depended o n d i f f u s i v e m i x i n g a l o n e . an a v e r a g e w i n d , t h i s t r a n s l a t e s t o a h o r i z o n t a l d i s t a n c e o f a b o u t 20 km.

For

One

s h o u l d a l w a y s remember t h a t i n p r a c t i c e , t h e e n t i r e p r o c e s s i s u s u a l l y s t r o n g l y i n f l u e n c e d , if n o t d r i v e n , by c o n v e c t i v e e n e r g y .

However, f o r r e a s o n s based on

t h e b a s i c n a t u r e o f t h e i n s t a b i l i t i e s , I have a r g u e d t h a t t h e c h a r a c t e r i s t i c wavel e n g t h o f t h e secondary f l o w i s q e n e r a l l y o f t h e o r d e r o f t w i c e t h e l a y e r depth. C o n v e c t i v e e n e r g y a l w a y s pushes t h e f l o w t o w a r d more p a r a l l e l f l o w ( w i t h l e s s t u r n i n g ) , and e n e r g i z e s t h e s e c o n d a r y f l o w . T h e r e i s a n o t h e r p o s s i b l e f a c t o r i n f l u e n c i n g t h e shape o f t h e r o l l s .

This

o c c u r s when t h e s e c o n d a r y f l o w i s c o n f i n e d t o a n imposed b o u n d a r y l a y e r d e p t h . The m o s t common c a s e i n t h e atmosphere o c c u r s when a c a p p i n g i n v e r s i o n e x i s t s dlle

to s y n o p t i c s c a l e i n f l u e n c e s .

A s t a t i o n a r y h i g h p r e s s u r e r e g i o n o r a downslope

( k a t a b a t i c ) w i n d m i g h t cause t h i s c o n d i t i o n .

A c o m p l e t e a n a l y s i s h a s n ' t been done,

however, p r e l i m i n a r y r e s u l t s i n d i c a t e t h a t a l i d f o r c e s l o n g e r w a v e l e n q t h c i r c u l a tions.

SOME APPLICATIONS The s e i f sand dunes have a l r e a d y been m e n t i o n e d , b u t m e r i t more a t t e n t i o n .

These

a r e l o n g i t u d i n a l dunes, 20 t o 60 m e t e r s h i g h , e x t e n d i n g u p t o hundreds o f km i n l e n g t h h a v i n g w a v e l e n g t h s f r o m 1 t o 8 km, b u t t y p i c a l l y a b o u t 2 km.

A qood summary

o f t h e i r o c c u r e n c e i n t h e w o r l d ' s d e s e r t s was done b y Hanna ( 1 9 6 9 ) .

He f o u n d t h a t

t h e i r o r i e n t a t i o n was i n v a r i a b l y i n t h e d i r e c t i o n o f t h e p r e v a i l i n g w i n d s .

Cases

e x i s t where dune a l i g n m e n t t u r n s i n a c c o r d a n c e w i t h t h e l o c a l p r e v a i l i n g w i n d s . O t h e r cases show one s e t o f l o n g i t u d i n a l dunes o v e r l y i n g a n o t h e r , s u g g e s t i n q d i f f e r e n t w i n d and c l i m a t e c o n d i t i o n s d u r i n g d i f f e r e n t b u i l d i ng p e r i o d s .

The sand

t r i n s p o r t depends o n t h e cube o f t h e w i n d v e l o c i t y , and s t o r m e p i s o d e s may o c c u r i n d i r e c t i o n s o t h e r than p r e v a i l i n g winds. Evidence o f t h i s can a l s o be found i n superimposed l o n g i t u d i n a l dunes.

B a r c h a n dunes c a n o c c u r i n r e g i o n s where t h e

p r e v a i l i n g w i n d d i r e c t i o n changes i n d i f f e r e n t p e r i o d s .

They c o u l d r e s u l t f r o m

t h e s u p e r - p o s i t i o n o f d i f f e r e n t v o r t e x d i r e c t i o n s , o r f r o m some o t h e r dynamic, c o n v e c t i v e o r t o p o g r a p h i c feedback mechanism.

The Yardangs, s m a l l s c a l e l o n g i -

t u d i n a l dunes w i t h k n i f e - e d g e r i d g e s and U-shaped v a l l e y s , a r e c o m p a t i b l e w i t h vortex flow distributions. I f l o n g i t u d i n a l a t m o s p h e r i c v o r t i c e s do c a u s e dunes, t h e n t h e d i f f e r e n t wave-

l e n g t h s would c o r r e l a t e t o d i f f e r e n t c h a r a c t e r i s t i c boundary l a y e r s (and c l i m a t i c conditions).

There i s a p a u c i t y o f d a t a i n t h e d e s e r t r e g i o n s t o i n v e s t i g a t e this

correlation.

N e v e r t h e l e s s , t h e o r e t i c a l w a v e l e n g t h s f o r e x p e c t e d a t m o s p h e r i c vortices

f a l l p r e c i s e l y i n t h e r a n g e o f t h e l o n g i t u d i n a l dune w a v e l e n g t h s .

There i s a problem in t h a t theory p r ed i ct s l a t e r a l advection of the r o l l s . I f they c o n s t a n t l y moved sideways, t h e r o l l s would n o t be a s l i k e l y t o build lonqit u d i n a l dunes, or even s t a t i o n a r y cloud s t r e e t s .

However, there a r e qood q u a n t i t a t i v e arguments which make p l au s i b l e a fixed r o l l loc a tion. One i s the tendency of convective energy t o favor zero phase speeds ( n o l a t e r a l movement). Another i s the p o s i t i v e feedback furnished by t h e d i f f e r e n c e i n heatinq c h a r a c t e r i s t i c s between the dunes a n d the troughs, which enhances convection alonq the dune c r e s t s . The study of p e r i g l a c i a l winds i n conjunction with loe ss and g l a c i a l debris deposits involves the i n t e r a c t i o n between PBL flow c h a r a c t e r i s t i c s , climate a n d sedimentation p a t t e r n s . The p e r s i s t e n t , k at ab at ic winds a ssoc ia te d with the g la c ie r s and the d e p o s i t ch ar act er can give evidence of g l a c i a l presence, e xte nt a n d d u r a t i o n . The high d en s i t y of the cold a i r can move gra ins more e f f e c t i v e l y .

I n f a c t , s a l t a t i o n and d r i f t forming processes i n t h e snow can be s i m i l a r t o t h a t i n d u s t a n d sand, providing information on a s h o r t e r time s c a l e . Longitudinal d r i f t s orie nte d with the wind form quickly i n a snow storm.

This i s e v i d e n t i n c u r r e n t An t ar ct i c s t u d i e s .

F in a l l y , t h e r e has been much i n t e r e s t i n the d i f f e r e n t aspects of aeolian processes on Mars and Venus.

O n Mars, longitudinal clouds frequently occur as seen in f i g u r e 1 2 , and barchan a n d t r an s v er s e dune systems have been observed. SUMMARY I have presented the flow s o l u t i o n s f o r t h e geophysical PBL.

I t a pplie s to any

atmosphere, o r l a r g e body of water. Although a r o t a t i n q frame of reference produces the turning i n t h e PBL, turning from any cause w ill produce i n s t a b i l i t y waves above a c r i t i c a l Reynolds number.

T h e o r i e n t a t i o n of these waves with

respect t o the mean flow c h a r a c t e r i s t i c s i s shown in f i g u r e 13.

These waves

apparently o f t e n come t o a n equilibrium a t a f i n i t e value, producing a secondary flow. A knowledge of the nature of these flows w ill ?resumably aid the sediment o l o g i s t ' s study of d ep o s i t i o n . I n reading f o r t h i s paper, I have noted t h a t this i s only a s t e p i n an enormously complex problem, so I ' l l t r y t o capsulize the r e s u l t s . The most e a s i l y observed parameters a r e the wavelength and the angle between the wave a x i s and the mean flow.

Roughly, i n PBL flow, the pe rturba tion wave-

lengtli equals twice the depth of the l a y e r of the flow influenced by the bottom (and thereby influencing s ed i men t at i o n ) . Longitudinal pa tte rns a r e frequently associated with t h e i n f l e c t i o n p o i n t i n s t a b i l i t y , which occurs due to the turninq of the mean flow.

Flow v e l o c i t i e s must be above a moderate minimum value.

Con-

vection i n the presence of a moderate mean flow a l s o produces longitudinal waves of t h i s wavelength.

These waves a r e r e l a t i v e l y long, corresponding t o the deep

layers containing the i n s t a b i l i t y .

I f conditions such a s l a r g e s c a l e atmosnheric

subsidence o r k a t a b a t i c winds produce a capping inversion on the PBL flow, then

308

F i g u r e 12. Cumulus c l o u d s o n Mars, t a k e n f r o m t h e V a r s O r b i t e r . The area i s at 1 2 " S , 90"W. The c e l l s a r e a p p r o x i m a t e l y 6 km i n d i a m e t e r , e l o n q a t e d i n t h e w i n d d i r e c t i o n . The i n d i c a t e d a l i g n m e n t i s o f r o l l s 8 - 1 0 kni a p a r t w i t h clouds a t a b o u t 2 . 5 km h e i g h t . T h i s s e p a r a t i o n / h e i g h t r a t i o o f 3 . 2 - 4 i s i n aqreement w i t h t h e r o l l theory.

309

(a)

j_______ Y

convergence/ divergence /44

, wave direction / ' '

Figure 13. Sketch o f v e l o c i t y shear and c o r r e s p o n d i n g i n s t a b i l i t y waves. ( a ) i l l u s t r a t e s t h e i n f l e c t i o n p o i n t i n e v i t a b l y a s s o c i a t e d w i t h a speed chanse (Vt-Vo) between two l a y e r s o f d e p t h , h. The r e s u l t i n g wave c r e s t s ( i n t h e lower p a r t o f t h e f i g u r e ) a r e normal t o t h e f l o w d i r e c t i o n ( $ = 0 ) . These ( c ) shows t h e would be a s s o c i a t e d w i t h r i p p l e s , b i l l o w s o r t r a n s v e r s e dunes. I n t h i s case t h e case o f p u r e t u r n i n g between l a y e r s ( V t - V o = 0, E = 3 0 " ) . i n e v i t a b l e i n f l e c t i o n p o i n t i n s t a b i l i t y produces waves w i t h c r e s t s p a r a l l e l t o t h e mean f l o w i n t h e l a y e r . I n ( b ) , a mixed case i s shown. wavelengths can be l o n g e r t h a n t w i c e t h e d e p t h o f t h e l a y e r .

There a r e a l s o s p e c i a l

flow c o n d i t i o n s ( s h a r p t u r n i n q i n t h e l a y e r near t h e s u r f a c e f o r i n s t a n c e ) which lead t o l o n g i t u d i n a l p a t t e r n s on t h e s u r f a c e w i t h s c a l e s o f 10s o f m e t e r s . When t h e f l o w i s p e r p e n d i c u l a r t o t h e wave c r e s t s , t h e dynamic i n f l e c t i o n p o i n t i n s t a b i l i t y i s t h e l i k e l y source o f enerqy. called Kelvin-Helmholz i n s t a b i l i t y . c o r r e s p o n d i n g l y s h o r t e r wavelengths.

T h i s i n s t a b i l i t y can o c c u r i n t h i n l a y e r s w i t h B i l l o w s i n c l o u d s , r i p p l e s i n sand and t r a -

verse sand dunes may be d i r e c t r e s u l t s .

L a t e r a l wave p a t t e r n s a r e f r e q u e n t l y

observed superimposed on l o n g i t u d i n a l ones. l a t e r a l waves.

T h i s case o f p u r e speed shear i s o f t e n

No e q u i l i b r i u m t h e o r y e x i s t s f o r these

S t r o n g i n t e r a c t i o n w i t h t h e s u r f a c e topoqraphy i s p o s s i b l e when

the roughness h e i g h t s a r e s i q n i f i c a n t compared t o t h e t h i n l a y e r depth.

These flow

s i t u a t i o n s go beyond t h e s i m p l e a n a l y t i c s o l u t i o n s which I have discussed. J u s t as m e t e o r o l o g i s t s use t h e l o w - l e v e l c l o u d s t r e e t s t o i n f e r t h e wind d i r e c tions f o r t h e i r a n a l y s i s , t h e s e d i m e n t a t i o n c h a r a c t e r i s t i c s can be used t o p r o v i d e useful c l i m a t i c i n f o r m a t i o n i n r e g i o n s and t i m e s where o t h e r d a t a a r e scarce.

This

i s t h e case f o r sparse measurement r e g i o n s such as t h e d e s e r t s and t h e ocean f l o o r s , ancient f l o w s and f l o w on o t h e r p l a n e t s f o r i n s t a n c e .

A knowledge o f t h e i n s t a -

b i l i t y waves i s most h e l p f u l i n u n d e r s t a n d i n q t h e complex b u t o b v i o u s l y o r o a n i z e d patterns which o c c u r i n g e o p h y s i c a l f l o w s and t h e i r boundaries.

310 References Brown, R . A . , 1980. L o n g i t u d i n a l i n s t a b i l i t i e s and secondary flows i n t h e planetar 653-697. boundary l a y e r : a review. Rev. o f Geophys. and Space P h y s i c s , z(3), Ekman, \!. W . , 1905. O n the i n f l u e n c e o f the e a r t h ' s r o t a t i o n on ocean c u r r e n t s Arkiv. Math. A s t r o . F y s i k . , z(11). 1-53. Hanna, S . R . , 1969. The f o r m a t i o n of l o n q i t u d i n a l sand dunes by l a r q e h e l i c a l e d d i e s i n the atmosphere. J . Appl. M e t e o r o l . , 8 ( 6 ) , 874-880. Rossby, C . G . and R . B . Hontqomery, 1935. The l a y e r s o f f r i c t i o n a l i n f l u e n c e in wind and ocean c u r r e n t s . HIT P a p e r s , 3 ( 3 ) , 101 P O .

311

RADIATIVE AND METEOROLOGICAL CONTROL ON THE MOVEMENT OF SAND AT LAKE MUNGO, N.S.W., AUSTRALIA R. HYDE, School o f Earth Sciences, Macquarie U n i v e r s i t y , Sydney, N.S.W., Australia

R.J. WASSON*, Department o f Biogeography and Geomorphology, The A u s t r a l i a n National U n i v e r s i t y , Canberra, A.C.T., A u s t r a l i a

INTRODUCTION Most studies o f a e o l i a n t r a n s p o r t have been p r i m a r i l y concerned w i t h the movement o f d r y sand and t h e o r i e n t a t i o n o f dunes t o the p r e v a i l i n g sand-moving winds (Bagnold, 1941; Fryberger, 1979; Warren, 1979; Wilson, 1972). This preoccupation w i t h d r y sand may be an acceptable p r o p o s i t i o n f o r dunes i n a completely a r i d environment where t h e subsurface l a y e r s are dry, b u t i n other s i t u a t i o n s t h i s approach does n o t take i n t o account t h e presence o f a wet core o f sand below the surface and t h e p o t e n t i a l c o n t r o l t h i s could e x e r t on the

movement o f sand.

I n wet-core dunes i t i s u s u a l l y o n l y the f i r s t few tens o f

centimetres, a t most, t h a t a r e d r y and r e a d i l y mobile.

Therefore, i n strong

winds, t h e surface d r y l a y e r may be r a p i d l y removed, and t h e p o t e n t i a l decrease i n sand transpor.t, once t h e wet core i s exposed, must be considered i n any complete a n a l y s i s o f sand t r a n s p o r t . Wet-core dunes are n o t unique t o A u s t r a l i a , and t h e i r existence has been noted elsewhere by several workers (e.g. Sharp, 1966;

Tsoar, 1978).

Nor do we

be1 ieve t h a t wet cores are a t r a n s i e n t phenomenon o c c u r r i n g o n l y a f t e r periods of rain.

Rather, they probably represent the normal subsurface c o n d i t i o n o f

dunes i n areas outside hyper-arid environments, and t h e i r dynamic i m p l i c a t i o n s need t o be investigated. The increase i n the threshold v e l o c i t y r e q u i r e d t o move wet sand has long been recognized (e.g.

Chepil , 1956;

Woodruff and Siddoway, 1965).

However,

what has n o t been considered i s the r o l e o f s o l a r r a d i a t i o n i n the d r y i n g o f subsurface layers, and t h e p o t e n t i a l c o n t r o l t h a t t h i s could have on the amount

o f dry sand a v a i l a b l e f o r subsequent t r a n s p o r t by t h e wind. I t i s n o t t h e i n t e n t i o n o f t h i s paper t o examine i n d e t a i l the complex r a d i a t i v e and energy exchanges i n v o l v e d i n t h e d r y i n g o f near-surface sand. Rather i s i t an attempt t o discuss t h e i n t e r n a l s t r a t i f i c a t i o n and moisture content o f a l i n e a r dune i n a semi-arid environment i n terms o f the meteorol o g i c a l and shortwave r a d i a t i v e conditions during the previous twelve months. *present address: CSIRO D i v i s i o n o f Water and Land Resources, Canberra, A.C.T.

312

In this way we hope to draw attention to the need to take into account factors other than just the wind velocity when considering the dynamics of sand transport and dune mobilization. The dunefield to be discussed in this paper is situated at Lake Mungo in the far west of New South Wales, Australia (see Fig. 1). Lake Mungo is one of the

Fig. 1. Map of southeastern Australia showing mean annual rainfall isohyets and location of Mildura and Lake Mungo. Willandra Lakes, and the structure of the lunettes surrounding the lakes is well documented (Bowler, 1971; 1978). The present-day environment is one where the clay and quartz sand lunettes around Lake Mungo are being eroded and old lunette deposits (which are >14 000 yrs B.P.: Bowler, 1976) are now capped by a sheet of highly mobile, unvegetated, aeolian sand currently moving eastwards onto the surrounding plain. The quartz sand-sheet is surmounted by linear dunes forming in the lee of residuals of lunettes being eroded (see Fig. 2). The axes o f the linear dunes are oriented approximately west-east and, in the area being investigated, this is at right-angles to the edge of the lake.

31 3

I+

I

Large s l i p face S _ Small slip face

__

_ - . - Dune

crest - line

Margins of d u n e

of lunette material @ Residual A old

- O - O

- O

5km

I km

70 m

B

c

Fig. 2. a ) Lake Mungo. b ) Sand-sheet overriding lunette on eastern side of Lake Mungo. c ) Linear dune forming in lee of remnant knob on eroding lunette. Our i n t e r e s t in Lake Mungo s t a r t e d in July 1978 when a two-metre trench was dug across the t o p of one linear dune on the sand-sheet (see Fig. 2 ) . Details of the s t r a t i f i c a t i o n and internal moisture content of t h i s dune are given in Figure 3. Of major i n t e r e s t t o us were the evidence of a recent reversal in orientation of the internal s t r a t i f i c a t i o n of the dune near i t s c r e s t , and the existence of an extensive layer of dry sand below the surface of the northern flank of the dune. Although extensive digging was carried out on the southern

314

I

A

14 Distance From cenlle Llne (metres)

Fig. 3. North-south transect across linear dune a t Lake Mungo, showing g r a v i metric moisture content, internal s t r a t i f i c a t i o n and position of buried dry layer i n July 1978. side, no equivalent subsurface dry layer could be found. The only dry sand that could be seen on the southern flank was a shallow layer a few centimetres deep just below the surface, which significantly was not present on the opposite side of t h e dune. I t i s t h i s reversal in dune-crest orientation and the disposition of dry and moist layers t h a t we will now attempt t o explain. THE METEOROLOGICAL AND CLIMATIC ENVIRONMENT AT LAKE MUNGO In common with many semi-arid regions of the world, the area around Lake Mungo i s not well endowed with reliable long-term meteorological records. The only measurement routinely made in the area i s r a i n f a l l , which i s of prime i n t e r e s t t o pastoral i n t e r e s t s i n the region, b u t these data are often irregular. Reliable, long-term records are available from Mildura, approximately 100 km southwest of Lake Mungo (see Fig. 11, where the Australian Bureau of Meteorology maintains a f u l l weather-station w i t h both surface- and upper-air measurements. The station a l s o records half-hourly integrated values of global shortwave radiation. Rai nfal 1 The annual average r a i n f a l l f o r Mildura is 250 m and i s s e l a t i v e l y evenly distributed throughout the year (see Fig. 4a). However, i n the 12 months preceding July 1978 the pattern of r a i n f a l l was e r r a t i c (see Fig. 4a), with

315

A

S

O

N

D

J

F

M

A

4 0 r rnrn M i I dura

i

0,

L

aug

,

I

M

J

J

(b)

I

I

j an

,

”

J

. L , . . ,b

L jul

Fig. 4. a ) Monthly average rainfall (30-yr record) and actual rainfall (hatched) , August 1977-July 1978; Mildura. b ) Daily rainfall August 1977July 1978; Mildura.

nwnthly t o t a l s generally l e s s than the average, except f o r June and July 1978 when approximately double the average rainfall was recorded. Rainfall amounts on individual days during the 12-month period are shown in Figure 4b. Wind speed and direction Three-hourly wind-speed and -direction s t a t i s t i c s from Mildura were used t o obtain a measurement of potential sand-moving winds a t Lake Mungo. These were calculated u s i n g the method described by Fryberger (1979) and a value of 6 m s - l a t 10 metres f o r the threshold wind speed. The annual d r i f t potentials from each direction, along w i t h the resultant d r i f t potential ( R D P ) , are shown i n Figure 5 , and monthly d r i f t potentials a r e given i n Figure 6. Using Fryberger’s (1979) c l a s s i f i c a t i o n , the wind environment i s wide unimodal and low energy, since the total d r i f t potential is only 162. The data i n Figure 5 show t h a t

316

the RDP vector i s within 10" of the axes of the linear dunes a t Lake Mungo. Monthly d r i f t potentials i n Figure 6 show marked seasonal changes in both potentials and the direction o f the resultant d r i f t potential f o r each month. Between January and June d r i f t potentials were small, b u t increased rapidly in July and maintained t h e i r strength and direction into December. Since the axes of the dunes are approximately west-east, the monthly resultant d r i f t potentials show a seasonal change i n the across-dune component of the RDP, being towards the south between April and November, and towards the north during the remainder of the year.

F i g . 5. Annual d r i f t potential a t Mildura and axial orientation of linear dune a t Lake Mungo.

ja n

0

xc)i

feb

mar

X L ju n

oct

Fig. 6.

-

nov

0

5

0

10

dec

10 DRIFT POTENTIAL

20 RESULTANT DRIFT POTENTIAL

Monthly potentials and resultant d r i f t potentials a t Mildura.

317 Wind speeds and d i r e c t i o n s f o r the period A u g u s t 1977 t o July 1978 inclusive were a l s o examined t o see whether wind conditions d u r i n g t h i s period were similar t o those given by t h e long-term wind p o t e n t i a l s plotted i n Figure 6. Instead of constructing monthly sand r o s e s , t h e 1977/78 wind-drift p o t e n t i a l s in Figure 7 have been resolved i n t o across- and along-dune components. For these c a l c u l a t i o n s a s l i g h t l y simpler approach was adopted and the l i n e s in Figure 7 a r e values of Vz(V-Vt), where V i s the measured wind speed and V t the threshold speed of 6 ms-1. The data i n Figure 7 show overall consistency with the longer-term s t a t i s t i c s i n Figure 6 with a seasonally changing across-dune component superimposed upon a dominant along-dune p o t e n t i a l ; between August 1977 and t h e beginning of October potential across-dune t r a n s p o r t was predominantly towards the south, and towards t h e north between October and the end of March. Subsequent along- and across-dune p o t e n t i a l s were extremely low, but by July t h e across-dune potential towards the south had reappeared.

r

4000 t o w a r d s n o r t h I

4000 L t o w a r d s s o u t h 1

I

I

aug

I

I

I

I

j an

I

I

I

I

I

1

jul

4000 t o w a r d s e a s t

F

I

t

4808 t o w a r d s w e s t

Fig. 7. Daily across- and along-dune wind p o t e n t i a l s Y 2 (V-V,), A u g u s t 1977July 1978; Mildura.

318 Radiation The final parameter t o be discussed i s shortwave radiation from the Sun. We realize t h a t any detailed discussion of the radiative and energy balance over a dune should also deal w i t h the components of incoming and outgoing longwave radiation, net radiation (available radiation) , the flux of heat into the sand, and the t r a n s f e r of sensible and l a t e n t heat from the dune into the atmosphere. None of these fluxes were measured during the period being studied and they are almost impossible t o estimate. However, we believe t h a t the amount of solar radiation potentially available f o r absorption by the dune will be an indication of the amount of energy available f o r e i t h e r drying the surface of the wet core i f i t became exposed d u r i n g periods of strong winds, or t o increase the depth of the surface dry layer when wind speeds were low. Shortwave radiation incident a t the outside of the atmosphere can be calculated quite e a s i l y using established trigonometric expressions (e.g. Sellars, 1965; Kondrat'ev, 1969; Paltridge and P l a t t , 1976), which take into account seasonal and diurnal changes i n the Sun's zenith distance and the eccentricity of the Earth's o r b i t around the Sun. The amount of radiation received a t the surface i s , however, l e s s than t h a t received a t the outside of the atmosphere, because of absorption, scattering and reflection w i t h i n the atmosphere. In addition, a proportion of the shortwave radiation incident a t the surface i s reflected back t o space. Finally, radiation incident upon sloping surfaces has t o be dealt w i t h separately, taking into account the slope of the surface and i t s orientation r e l a t i v e t o the Sun. Since we were primarily interested i n calculating the amount of s o l a r radiation available f o r absorption by the dune, i t was necessary t o calculate the transmissivity of the atmosphere, using data from Mildura, and t o determine the albedo ( r e f l e c t i v i t y ) of the sand a t Lake Mungo. An average daily value of atmospheric transmissivi t y was determined by comparing measured values of global shortwave radiation ( d i r e c t plus diffuse) a t Mildura with daily t o t a l s calculated for the outside of the atmosphere a t the same l a t i t u d e (34O14'S). Although some s l i g h t seasonality was apparent, an average value of 0.71 has been used in subsequent calculations. To determine the albedo of the sand a t Lake Mungo, incoming and reflected shortwave radiation were measured above a f l a t t i s h section of the sand-sheet d u r i n g September and December 1981, using Kipp solarimeters and a Middleton pyrano-albedometer. These r e s u l t s are shown i n Figure 8. Paltridge and Platt (1976) showed t h a t most surfaces displayed a Fresnel type r e f l e c t i v i t y curve which could be expressed as a ( e ) = al + ( l - a l ) exp C-k(90-e)l

where e i s the solar elevation, al i s the albedo of the surface a t elevations

319

Sun's

01

1

I

1

30

Fig. 8.

I

I

elevation I

I

60

I

90

Change of albedo w i t h Sun's elevation over sand a t Lake Mungo.

above 50" and k i s a constant which, l i k e a, will vary between surfaces. In Figure 8 the curved l i n e was plotted using the expression given above with a1 0.46 and k = 0.1, and i t is c l e a r t h a t t h i s equation adequately describes the relationship between solar elevation and albedo a t Lake Mungo. To calculate the radiation incident upon each flank of the dune i t isnecessary t o know the angle of the slope ( i ) and the azimuth of the normal of the slope w i t h respect t o south ( a l ) (see Fig. 9 ) . W i t h this information, plus data on the change i n the Sun's azimuth and zenith distance ( 0 ) throughout the day, the equations given by Sellars (1965) can be used to calculate the zenith distance (01) of the Sun r e l a t i v e t o each flank throughout the day. Daily t o t a l s of radiation can then be calculated, taking into account the average transmissivity of the atmosphere. For convenience, a value of 214" was used f o r the slope of

320

Dune

4

South

Fig. 9. Schematic diagram showing orientation o f the linear dune at Lake Mungo to the Sun. (Symbols explained in the text).

4000

r

10's o f kilojoules/sq.m

NORTH SLOPE

0 j an

jul

Fig. 10. Daily totals o f global radiation August 1977-July 1978; Mildura. The curves show daily totals o f solar radiation potentially absorbed on each side o f the dune.

321

both flanks of the dune a t Lake Mungo (shown by the dashed l i n e i n Figure 3). Since the dunes a r e oriented approximately west-east, the azimuth of the normal o f l h e slopes with respect t o south was taken as zero (a1 = 0 ) . These calculations were carried out f o r each day between August 1977 and July 1978 and the results are given in Figure 10. Also shown are the daily totals of global radiation received a t Mildura d u r i n g the same period. The envelope containing these data is the maximum global solar radiation that could have been received a t Mildura during the year. These measurements take into account the transmissivity of the atmosphere and the depletion of solar radiation on cloudy days, b u t are not values of absorbed radiation, since they do not take into account solar radiation reflected away from the surface. The Mildura data show the marked seasonal change i n incoming solar radiation that occurs a t t h i s l a t i t u d e , w i t h l e s s than a third of the sumner daily t o t a l s being received by a horizontal surface in winter. The two solid lines in Figure 10 are the daily t o t a l s of shortwave radiation potentially absorbed by each flank of the dune throughout the 12-month period. Whereas the radiation potentially available f o r absorption by the north flank does not change much throughout the year, radiation available on the south flank is affected by both the adverse slope of the dune and h i g h values of albedo d u r i n g the winter months. This results in very small daily t o t a l s being absorbed i n May, June and July. A t the height of sumner, solar radiation on the southern flank exceeds that on the northern side o f the dune, an apparent anomaly, b u t actually caused by the Sun rising and s e t t i n g south of the axis of the dune during the summer halfyear.

DISCUSSION The exact sequence of events a t Lake Mungo prior t o the trench being dug during July 1978 cannot be determined since no observations of any kind had previously been made over the dune. Despite t h i s , i t i s possible t o r e l a t e the features revealed by the trench w i t h meteorological and radiative conditions d u r i n g the preceding few months. The existence of a thick, subsurface, dry layer on the northern flank of the dune, and i t s absence on the opposite side, suggest that northward sand transport across the dune had been occurring i n the preceding months. The only period when t h i s was possible was between October 1977 and the end of March 1978, when both the along- and across-dune wind potentials were high (see Fig. 7 ) . This coincided w i t h a period when rainfall was low and generally below the average (see F i g s . 4a, 4b), and was also the time of the year when large amounts o f solar radiation would have been absorbed by each side of the dune (see Fig. 10). These factors are thought t o have combined t o produce a continuing supply o f dry sand f o r transport across the dune t o i t s northern flank. In the

322 following months both along- and across-dune wind potentials were low and there would have been l i t t l e , i f any, transport of sand on the dune. During May, June and July there were a number of days with small amounts of r a i n , dampening the surface sand (see Fig. 4b). Figure 7 shows t h a t , d u r i n g July, an acrossdune component of wind potential towards the south has reappeared, and evidence for the actual transport of sand back across the dune can be seen in the s t r a t i fication near the c r e s t of the dune (see Fig. 3 ) . This occurred a t a time when the flanks of the dune were probably damp, t h u s impeding the transport of sand along and across the dune. Figure 3 also shows the existence of a shallow layer of dry sand very close t o the surface on the southern flank of the dune, although there was no sign of a corresponding near-surface layer on the northern flank. Since a t this time of the year the amount of solar radiation available for absorption by the south-facing side of the dune was low (see Fig. 6 1 , sand must have been transported from the opposite side of the dune, following efficient u t i l i z a t i o n of solar radiation i n drying surface sand on the northern flank. Figure 9 shows t h a t i n June and July the northern side of the dune can potentially absorb almost as much radiation as i t d i d d u r i n g the summer months because of i t s favourable orientation t o the Sun. Therefore, on sunny days d u r i n g l a t e June and July, near-surface sand on the northern flank would have been dried and subsequently transported across the dune when wind conditions were favourable. Since the moisture content of the sand i n the reverse s t r a t i fied c r e s t of the dune was moist (see Fig. 3 ) , there must have been a continuing sequence of radiative drying, transport and rewetting of sand d u r i n g t h i s winter period. The shallow, dry layer of sand on the southern flank of the dune can be regarded only as a transient feature and the damp sand above i t is evidence t h a t this layer would probably disappear d u r i n g the next rain or drizzle. CONCLUSIONS In this paper we have attempted t o explain the internal s t r a t i f i c a t i o n and moisture content of a l i n e a r dune a t Lake Mungo i n terms of wind speed and direction, r a i n f a l l and solar radiation d u r i n g the preceding 12 months. The sequence of meteorological events i n the year preceding our observations correl a t e s w i t h the features observed i n the dune i n July 1978 and i l l u s t r a t e s the need f o r factors other than just wind velocity t o be considered when discussing the dynamics of sand dunes. Of special importance is the role of shortwave radiation from the Sun i n the drying of wet sand exposed a t the surface of dunes and the degree t o which an exposed wet core limits potential sand transport along o r across a dune.

323 REFERENCES Bagnold, R.A., 1941. The Physics of Blown Sand and Desert Dunes. Methuen, London, 265 pp. Bowler, J.M., 1971. Pleistocene salinities and climatic change: evidence from lakes and lunettes in S.E. Australia. In: D.J. Mulvaney and J. Golsen (Editors), Aboriginal Man Environments in Australia. Australian National University Press, Canberra, pp. 46-63. Bowler, J.M., 1976. Recent developments in reconstructing late quaternary environments in Australia. In: R.L. Kirk and A.G. Morne (Editors), The Origin of the Australians. Human Biology Series No. 6. Australian Institute of Aboriginal Studies, Canberra, pp.55-57. Bowler, J.M. and Magee, J.W., 1978. Geomorphology of the Mallee Region in semi-arid northern Victoria and western New South Wales. Proc. Roy. SOC. Vic., 90, 1: 5-26. Chepil, W.S., 1956. Influence of moisture on erodibility of soil by wind. Soil Sci. SOC. Am. Proc., 20: 288-292. Fryberger, S.G., 1979. Dune forms and wind regime. In: E.D. McKee (Editor), A Study of Global Sand Seas. U.S. Geological Survey Professional Paper 1052. Washington, pp. 137-170. Kondrat'ev, K.I., 1969. Radiation in the Atmosphere. Academic Press, New York, 912 pp. Paltridge, G.W. and Platt, C.M.R., 1976. Radiative Processes in Meteorology and Climatology. Elsevier Scientific Publishing Co., New York, 318 pp. Sellars, W.D., 1965. Physical Climatology. The University of Chicago Press, Chicago, 272 pp. Sharp, R.P., 1966. Kelso dunes, Mohave Desert, California. Geol. SOC. Am. Bull. , 77: 1045-1074. Tsoar, H. , 1978. The dynamics of longitudinal dunes. Final Technical Report, European Research Office, U.S. Army, London, 171 pp. Warren, A., 1979. Aeolian processes. In: C. Embleton and J. Thornes (Editors), Processes in Geomorphology. Edward Arnold, p p . 3 2 5 - 3 5 1 . Wilson, I.G., 1972. Aeolian bedforms, their development and or.igins. Sedimentology, 19: 173-210. Woodruff, N.P. and Siddoway, F.H., 1965. A wind erosion equation. Proc. Soil Sci. SOC. Am., 29: 602-608.

This Page Intentionally Left Blank

325

MORPHODYNAI.1ICS OF INCIPIENT 1 OHEDUNES IN NL W SOUTH \\ALES, AUSTRALIA PATRICK HESP D i v i s i o n o f R e s o u r c e E1driagemc.n Depdr t m e n t o f Aqr i c u l t i r e 3 a r r a h R o a d , S o u t h P e r t h , W.A. 6 1 5 1

INTRODUCTION F o r e d u n e s are t h e f o r e m o s t v e g e t a t e d s a n d d u n e s o c c u r r i n y on t h e b d c k s h o r e zone of sandy beaches.

They a r e g e n e r a l l y f o r m e d p a r a l l e l t o t h e c o a s t .

a u t h o r d i s t i n g u i s h e s two b a s i c t y p e s o f f o r e d u n e s : established foredunes.

The

i n c i p i e n t f o r e d u n e s and

I n c i p i e n t f o r e d u n e s dre t h e i n i t i a l f o r e d u n e f o r m e d by

t h e t r a p p i n g o f sand w i t h i n pioneer v e g e t a t i o n s p e c i e s .

Where t h e s e d u n e s a r e

c o l o n i s e d by a g r o u p o f ' w o o d y ' v e g e t a t i o n s p e c i e s ( e . q . mat a n d t u f t e d p l a n t s ) , they are termed e s t d b l i s h r d f o r e d u n e s . I n r e c e n t y e a r s w h i l s t t h e r e h a s b e e n a s i g n i f i c a n t i n c r e a s e i n r e s e a r c h on t h e d y n a m i c s o f u n v e g e t a t e d c o a s t a l a n d d e s e r t d u n e s a n d r e l a t e d phenomena ( e g . Walker a n d M a t s u k u r a , 1 9 7 9 ; Howard e t a l . , 1 9 7 8 ; G r e e l e y e t a l . , 1 9 8 0 ) , t h e r r has n o t b e e n a s i g n i f i c a n t p a r a l l e l i n c r e a s e i n r e s e a r c h on v e g e t a t e d coastal dunes, e s p e c i a l l y f o r e d u n e s .

I n p a r t i c u l a r , i f one is interested i n derivinq

e x p l a n a t i o n s o f f o r e d u n e m o r p h o l o g y , t h e s e m i n a l w o r k of O l s o n ( 1 9 5 8 a , b ) s t i l l provides t h e most d e t a i l e d d i s c u s s i o n a v a i l a b l e on dynamic p r o c e s s e s o p e r a t i n g on f o r e d u n e s .

T h i s p a p e r t h e n , i s c o n c e r n e d w i t h d e t a i l i n g a s p e c t s o f t h e morpho-

d y n a m i c s of i n c i p i e n t f o r e d u n e s , a s t h e s e r e p r e s e n t t h e i n i t i a l s t a r t i n g p o i n t i n foredune formation.

I n d e t a i l , t h e f o l l o w i n g is an attempt t o d e r i v e l i n k -

ages between e c o l o g i c , aerodynamic and t r a n s p o r t p r o c e s s e s , and observed dune morphologies.

It c o n c e n t r a t e s o n i n c i p i e n t f o r e d u n e s formed by s a n d d e p o s i t i o n

w i t h i n l a t e r a l l y e x t e n s i v e ( a l o n g s h o r e ) c o l o n i e s of e i t h e r p l a n t s e e d l i n q s o r rhiromeslstolons.

I t i s n o t c o n c e r n e d w i t h i n c i p i e n t f o r e d u n e s i n i t i a t e d by

s a n d d e p o s i t i o n w i t h i n d i s c r e t e p l a n t s o r g r o u p s of p l a n t s

ds

d e s c r i b e d by

S a l i s b u r y ( 1 9 5 2 ) , Land ( 1 9 6 4 ) , G o d f r e y d n d G o d f r e y ( 1 9 7 6 ) , Woodhouse ( 1 9 7 8 ) a n d Hesp ( 1 9 8 1 ) . EXPERIMENTAL PROCEDURES Much o f t h e t o p o g r a p h i c a n d v e g e t a t i o n s u r v e y d a t a p r e s e n t e d b e l o w were o b t a i n e d from t h r e e - d i m e n s i o n a l ,

permanent survey p l o t s l o c a t e d a t v a r i o u s foredune-beach

s i t e s i n F e n s e m b a y m e n t , M y a l l L a k e s r e g i o n , N e w S o u t h Wales (see Thorn e t a l . , 1981, a n d S h o r t a n d H e s p , 1 9 8 2 , f o r a w i d e r d i s c u s s i o n of t h i s l o c a t i o n ) .

In thc

p l o t s , t h e c h a n g e i n s u r f a c e e l e v a t i o n o n e a c h c o r n e r of e v e r y o n e metre s q u , i r c

326 was surveyed w i t h a l e v e l , and numbers o r percent cover o f each p l a n t species, i n each square metre were counted o r estimated.

Monthly, and occasional d a i l y

surveys were conducted over a t h r e e year p e r i o d .

P l o t dimensions were e i t h e r 10

o r 20 m alongshore, and 30 m wide. Aerodynamic s t u d i e s were undertaken u t i l i s i n g a maximum o f twelve, low i n e r t a , Rimco m i n i a t u r e cup anemometers (cup diameter 32 mm, s t a l l i n g speed 0 . 1 ms-l; see Bradley, 1969).

The anemometers were mounted on 12 mm diameter, aluminium

masts, and on t h e surface.

Instantaneous sand t r a n s p o r t observations were made

v i s u a l l y , and by use o f slow motion f i l m .

Mean, r e l a t i v e sand t r a n s p o r t r a t e s

were a l s o determined using Leatherman (1978) sand t r a p s . Observations o f i n t e r n a l s t r u c t u r e were made i n hand-dug trenches u t i l i s i n g techniques described by B i g a r e l l a ( 1 9 7 2 ) , i n c l u d i n g n a i l and s t r i n g g r i d s and photography.

BIOLOGICAL PROCESSES I n New South Wales, i n c i p i e n t foredunes are predominantly i n i t i a t e d by the growth of S p i n i f e x h i r s u t u s .

Spinifex hirsutus L a b i l l (Hairy Spinifex; family:

Gramineae) i s a p e r e n n i a l n a t i v e grass commonly about 25 t o 30 cm high, w i t h s t o u t rhizomes and s t o l o n s which can extend f o r s e v e r a l metres.

Male p l a n t s

have two-flowered s p i k e l e t s , arranged i n spikes which are c l u s t e r e d i n groups of 4, 5 o r 6. g l o b u l a r head.

Female p l a n t s have numerous s p i k e l e t s arranged i n a l a r g e , dense, I t f l o w e r s d u r i n g spring-summer

occurs by t h r e e mechanisms:

(Beadle e t a l . ,

1972).

Growth

germination o f seeds, shoot p r o d u c t i o n , and l a t e r a l

spread o f rhizomes and s t o l o n s . Seeds ( u s u a l l y caryopses i n s p i k e l e t s ) are e i t h e r wind blown from landward source areas, or wave t r a n s p o r t e d t o t h e beach and backshore.

I n New South Wales,

w e s t e r l y o f f s h o r e winds, which occur throughout autumn and w i n t e r , appear primarily r e s p o n s i b l e f o r t h e t r a n s p o r t o f seeds onto t h e beach.

Here they a r e incorporated

i n t o the beach sediments by t h e a c t i o n o f swash and a e o l i a n sand t r a n s p o r t . Maximum germination occurs i n s p r i n g , p a r t i c u l a r l y e a r l y October through November, on t h e upper p o r t i o n o f t h e backshore zone a t t h e l i m i t o f s p r i n g t i d e swash. Swash deposited seeds may germinate as a d i s c r e t e group from, o r as an attached group t o a p r i o r S p i n i f e x vegetated foredune.

The s e e d l i n g zone may be o f vary-

i n g width, d e n s i t y and d i s t r i b u t i o n . The beach may a l s o be vegetated by t h e seaward growth o f S p i n i f e x rhizomes and stolons.

A t i n t e r v a l s o f 10 t o 15 cm nodes a r e formed from which r o o t s and semi-

d i s c r e t e p l a n t s emerge. (Hesp, 1982).

S t o l o n growth r a t e s range from 0.4 t o 1.5 m per month

Shoot growth responds t o b o t h a temperature regime, a t t a i n i n g a

maximum growth p e r i o d i n summer, and t o a c c r e t i o n r a t e .

327 AERODYNAMICS OF- SPINIFEX SURFACES A s soon a s S p i n i f e x p l a n t s c o l o n i s e t h e b a c k s h o r e , t h e y i n c r e a s e t h e aerodynamic roughness of t h e s u r f a c e and i n t e r a c t w i t h t h e a i r f l o w i n g o v e r t h e s u r f a c e .

In

an a t t e m p t t o d e t e r m i n e t h e n a t u r e o f i n t e r a c t i o n s between S p i n i f e x d e n s i t y and d i s t r i b u t i o n , f l o w dynamics and s a n d t r a n s p o r t , two q u e s t i o n s a p p e a r e d t o be of i n i t i a l importance:

1) what was t h e s t r u c t u r e o f wind f l o w a c r o s s a r e l a t i v e l y

homogeneous v e g e t a t e d s u r f a c e , and 2 )

what e f f e c t d i d l a t e r a l v a r i a t i o n s i n

p l a n t d e n s i t y and d i s t r i b u t i o n have on t h e f l o w s t r u c t u r e ? F i g u r e 1 i l l u s t r a t e s v e l o c i t y p r o f i l e s measured a c r o s s a r e l a t i v e l y homogeneous, The p r o f i l e s a r e e x p r e s s e d a s a

Spinifex dominated, s m a l l i n c i p i e n t foredune.

r a t i o o f a mean v e l o c i t y measured a t 4 m h e i g h t , upwind on t h e u n v e g e t a t e d beach. Table 1 l i s t s p r o f i l e s t a t i s t i c s , d e r i v e d from a l e a s t s q u a r e s r e g r e s s i o n a n a l y s i s (e.g. Reitsma, 1978), f o r p r o f i l e s one t o f o u r .

Correlations for profile f i t

are a l s o given. F i g u r e 1 i l l u s t r a t e s t h a t t h e lower p o r t i o n o f t h e p r o f i l e s d i s p l a y a r e l a t i v e l y uniform t e n d e n c y t o d e c r e a s e i n n e a r - s u r f a c e v e l o c i t y w i t h i n c r e a s i n g f e t c h downwind o f t h e change i n r o u g h n e s s ( i . e .

beach t o v e g e t a t i o n ) .

T h i s downwind

d e c r e a s e i n n e a r - s u r f a c e v e l o c i t y i s accompanied by a downwind i n c r e a s e i n relative shear velocity.

I n a d d i t i o n , r e l a t i v e roughness lengths i n c r e a s e t o

p r o f i l e 3 , f i v e metres downwind o f t h e l e a d i n g e d g e ( T a b l e 1; c f . Kutzbach, 1961; Bradley, 1968).

Note t h a t v e g e t a t i o n h e i g h t r e m a i n s c o n s t a n t a p p r o x i m a t e l y

one h a l f of a metre downwind of t h e l e a d i n g e d g e . Mean v e l o c i t y i n c r e a s e s i n t h e u p p e r p a r t of t h e p r o f i l e s one t o f o u r d u e t o There is a l s o a

lower p r o f i l e d r a g and f l o w c o m p r e s s i o n o v e r t h e f o r e d u n e .

d r a m a t i c r e d u c t i o n i n v e l o c i t y , and f o r m a t i o n of a s e p a r a t i o n e n v e l o p e i n t h e l e e of t h e dune c r e s t . types of r i d g e s (e.g.

This t y p e o f flow s t r u c t u r e has been observed f o r v a r i o u s Taylor and Gent, 1974).

The e f f e c t o f l a t e r a l v a r i a t i o n s i n p l a n t d e n s i t y o n f l o w s t r u c t u r e a r e now b r i e f l y examined.

I m p l i c a t i o n s of t h e aerodynamic d a t a f o r sand t r a n s p o r t i n

Spinifex a r e discussed i n t h e following section. W h i l s t n a t u r a l p l a n t p o p u l a t i o n s may be o c c a s i o n a l l y r e l a t i v e l y homogeneous,

t h e y more commonly d i s p l a y s p a t i a l v a r i a b i l i t y i n d e n s i t y and d i s t r i b u t i o n .

In

o r d e r t o a s s e s s t h e n a t u r e of f l o w s t r u c t u r e i n c a n o p i e s o f v a r y i n g d e n s i t y , mean wind v e l o c i t y p r o f i l e s were measured s i m u l t a n e o u s l y w i t h i n a d j a c e n t low and high d e n s i t y S p i n i f e x h i r s u t u s p o p u l a t i o n s . both.

Figure 2 depicts t h e r e s u l t s .

F e t c h morphology was i d e n t i c a l f o r

The lower d e n s i t y p r o f i l e i l l u s t r a t e s t h e

g r e a t e r f l o w p e n e t r a t i o n of t h e v e g e t a t i o n , and o c c u r r e n c e o f h i g h e r v e l o c i t i e s n e a r t h e s u r f a c e , compared t o t h e h i g h e r d e n s i t y p r o f i l e .

Above t h e v e g e t a t i o n

canopy, t h e p r o f i l e s r e v e r s e a s f l o w a c c e l e r a t e s o v e r t h e h i g h e r d e n s i t y vegetation.

R e l a t i v e roughness l e n g t h s , c a l c u l a t e d using a l l s i x v e l o c i t i e s on

each p r o f i l e ,

were 3 . 6 cm and 8 . 4 cm f o r t h e

low and h i g h

density

328

/-. 3 05 \ I0

I

0

2

3

4

5 6 Disiance fin)

7

8

9

I0

!I

F i g . 1. Mean w i n d v e l o c i t y p r o f i l e s e x p r e s s e d a s a p e r c e n t a g e o f t h e u n d i s t u r b e d , upwind f l o w a t 4 m h e i g h t , m e a s u r e d o v e r a s m a l l , a s y m m e t r i c i n c i p i e n t f o r e d u n e , Hawks Nest. The d a s h e d l i n e i n d i c a t e s t h e t o p o f t h e 25 cm h i g h S p i n i f e x h i r s u t u s c o v e r . P r o f i l e l o c a t i o n s a r e i n d i c a t e d by d o t s ( s u r f a c e ) a n d n u m b e r s (lower) on t h e dune. profiles respectively.

The d a t a show t h a t a s p l a n t c o v e r o r d e n s i t y v a r i e s

l a t e r a l l y , t h e w i t h i n - c a n o p y flow s t r u c t u r e a l s o v a r i e s l a t e r a l l y . S i m i l a r l y , mean v e l o c i t y p r o f i l e s m e a s u r e d p r o g r e s s i v e l y downwind a c r o s s s u r f a c e s on which p l a n t d e n s i t y v a r i e s d i s p l a y a l i k e flow s t r u c t u r e t o t h a t described above(Hesp, 1982).

T h a t i s , Lhe

p l a n t d e n s i t y and d i s t r i b u t i o n .

f l o w a d j u s t s r a p i d l y t o local changes i n

T h e s e o b s e r v a t i o n s c o n c u r w i t h t h e work of

M u l h e a r n a n d F i n n i g a r i ( 1 9 7 8 ) a n d R a u p a c h e t a l . (1980) who d e m o n s t r a t e t h a t s p a t i a l v a r i a t i o n i n f l o w v e l o c i t y i s a common f e a t u r e of f l o w s o v e r s u r f a c e s where roughness is s p a t i a l l y v a r i a b l e . TABLE 1.

Profile

A i r - p l a n t i n t e r a c t i o n d a t a f o r p r o f i l e s 1 t o 4 of F i g u r e 1.

Shear Velocity u,(m/s)

Roughness l e n g t h Ln (cm)

.10

0.11

.16 .38 .50

0.43 8.17 8.73

Correlation co-efficient .96 .99 .93 .98

329

90. 70.

50.

30.

15.

4-

Fig. 2 . Mean wind v e l o c i t y p r o f i l e s measured s i m u l t a n e o u s l y a c r o s s a h i g h d e n s i t y ( b r o k e n l i n e ) and moderate-low d e n s i t y ( c o n t i n u o u s l i n e ) S p i n i f e x vegetated f e t c h . SAND TRANSPORT

Once t h r e s h o l d v e l o c i t i e s a r e r e a c h e d , s a n d g r a i n s a r e i n c o r p o r a t e d i n t o t h e wind f l o w , a n d i n t e r a c t w i t h t h e v e g e t a t i o n .

I n o r d e r t o e l u c i d a t e t h e sand

t r a n s p o r t p r o c e s s e s which t h e a u t h o r b e l i e v e s o p e r a t e w i t h i n v e g e t a t e d s u r f a c e s , two t y p e s i t u a t i o n s dre d i s c u s s e d below.

The f i r s t c o n s i d e r s a n i d e a l s i t u a t i o n

where t h e s u r f a c e is i n i t i a l l y f l a t and c o v e r e d w i t h a c o n t i n u o u s , homogeneous, high d e n s i t y c o v e r of S p i n i f e x ; t h e s e c o n d d e a l s w i t h a s i t u a t i o n where t h e vegetation density is s p a t i a l l y variable. S i t u a t i o n 1:

High d e n s i t y S p i n i f e x

I t is assumed t h a t a e o l i a n s a n d t r a n s p o r t i s o c c u r r i n g on t h e upwind, unvegetated beach.

As t h e s a n d f l o w meets t h e l e a d i n g e d g e of t h e S p i n i f e x

vegetation, observations i n d i c a t e t h a t s e v e r a l events occur i n combination.

For

a g i v e n m o d e r a t e wind v e l o c i t y ( s a y 5-7 m s e c - l a t z = 1 0 cm) r i p p l e f o r m a t i o n c e a s e s , a n d t h e r e i s a r a p i d d e c e l e r a t i o n o f f l o w v e l o c i t i e s n e a r t h e bed i n t h e l e a d i n g e d g e r e g i o n ( f i g u r e 1; see a l s o Townsend, 1966; Sadeh e t a l . , 1971; Kotoda, 1 9 7 9 ) .

L i m i t e d f i e l d e x p e r i m e n t s c o n d u c t e d u t i l i s i n g Leatherman ( 1 9 7 8 )

330 s a n d t r a p s s i t e d a c r o s s t h e l e a d i n g e d g e r e g i o n , show t h a t t h e r a t e o f s a n d t r a n s p o r t d e c r e a s e s downwind a s n e a r - s u r f a c e f l o w v e l o c i t y d e c r e a s e s (Hesp, 1982). A n a l y s i s of slow-motion f i l m s a n d t o p o g r a p h i c m i c r o - s u r v e y s i n d i c a t e t h a t sand t r a n s p o r t a p p r o a c h e s a minimum r a t e , a t some p o i n t downwind o f t h e l e a d i n g edge of t h e vegetation.

I n a d e n s e c a n o p y , o b s e r v a t i o n s show t h a t when s a n d g r a i n s

s a l t a t e i n t o t h e v e g e t a t i o n , t h e y may d i s l o d g e o t h e r g r a i n s , b u t d i s l o d g e d g r a i n s c a n n o t move f a r b e c a u s e o f t h e p r e s e n c e o f v e g e t a t i o n .

In addition, the

downwind r e d u c t i o n i n t h r e s h o l d v e l o c i t y n e g a t e s t h e o p p o r t u n i t y f o r f u r t h e r s a l t a t i o n t o take place ( f i g . 1 ) .

Thus, f o r a g i v e n windspeed t h e r e a p p e a r s t o

b e a l i m i t t o t h e w i d t h , o r f e t c h o v e r which upwind i n d u c e d s a l t a t i o n w i l l occur i n dense vegetation. The c r e e p l o a d moves a c r o s s t h e l e a d i n g e d g e u n d e r t h e canopy.

A t , or v e r y

n e a r t h e s u r f a c e , Sadeh e t a l . ( 1 9 7 1 ) h a v e shown t h a t t h e p r e s e n c e o f stems e x e r t a d r a g o n t h e flow.

I i n f e r t h a t t h i s a d d i t i o n a l d r a g , p l u s t h e downwind

r e d u c t i o n i n s a l t a t i o n bombardment, would l e a d t o a p r o g r e s s i v e downwind d e crease i n t r a c t i o n load within t h e vegetation. S i t u a t i o n 2:

Variable density Spinifex

O b s e r v a t i o n s i n d i c a t e t h a t lower d e n s i t y v e g e t a t e d s u r f a c e s d i f f e r i n s e v e r a l r e s p e c t s from t h e h i g h d e n s i t y s u r f a c e ( S i t u a t i o n 1 ) d i s c u s s e d a b o v e .

Firstly,

s a n d t r a n s p o r t i n d u c e d upwind o f t h e v e g e t a t i o n , may t r a v e l f u r t h e r i n t o t h e vegetation a s near-surface v e l o c i t i e s a r e higher than i n t h e higher density case (Fig. 2).

S e c o n d l y , i n c o m i n g g r a i n s may i n d u c e f u r t h e r t r a n s p o r t t o t a k e

p l a c e w i t h i n t h e v e g e t a t i o n by s a l t a t i o n c o l l i s i o n s .

T h i r d l y , my o b s e r v a t i o n s

i n d i c a t e t h a t c r e e p a n d s a l t a t i o n a p p e a r t o b e i n d u c e d by l i f t and d r a g f o r c e s o p e r a t i n g w i t h i n t h e more open r e g i o n s i n v e g e t a t i o n , i n d e p e n d e n t l y o f upwind induced s a l t a t i o n e j e c t i o n s (Fig. 2 ) .

These o b s e r v a t i o n s a c c o r d w i t h Kuhlrnan's

(1957, 1959) o b s e r v a t i o n s of sand t r a n s p o r t on s u r f a c e s e x h i b i t i n g a v a r i a b l e vegetation cover. DUNE MORPHOLOGY The d i s c u s s i o n above s u g g e s t s t h a t a t a g i v e n p o i n t downwind o f t h e l e a d i n g edge i n d e n s e v e g e t a t i o n , s a n d t r a n s p o r t r e a c h e s a minimum.

Upwind i n d u c e d

s a l t a t i n g g r a i n s have a l l been t r a p p e d , a n d t h e r e i s l i t t l e , i f a n y , w i t h i n canopy i n d u c e d s a n d t r a n s p o r t .

The e f f e c t o f a downwind d e c r e a s e i n s a n d

t r a n s p o r t i s t o produce a t h i n lamina o f sand of a given width, t h e dimensions of which are a p p a r e n t l y d e p e n d e n t o n p l a n t d e n s i t y and wind v e l o c i t y .

If the

wind v e l o c i t y r e m a i n s r e l a t i v e l y c o n s t a n t f o r a p e r i o d of time, c o n t i n u o u s d e p o s i t i o n of s a n d o v e r a r e l a t i v e l y f i n i t e d i s t a n c e p r o d u c e s a series of l a m i n a e

of s i m i l a r w i d t h , a n d r e s u l t s i n t h e f o r m a t i o n o f a wedge- or t r i a n g u l a r - s h a p e d dune.

The windward s l o p e o f t h e dune i s l o n g and low, t h e l e e w a r d s l o p e s h o r t

331 and s t e e p .

The d u n e i s t r i a n g u l a r i n s h a p e b e c a u s e t h e r e i s a downwind i n c r e a s e

i n t h e r a t e of s a n d d e p o s i t i o n .

Although v e l o c i t i e s may a b r u p t l y d e c r e a s e j u s t

w i t h i n t h e l e a d i n g e d g e of v e g e t a t i o n , sand g r a i n s do n o t f a l l v e r t i c a l l y t o t h e bed, b u t f o l l o w t r a j e c t o r i e s s u s t a i n e d by upwind i n d u c e d momentum.

This r e s u l t s

i n d e p o s i t i o n b e i n g lowest a t t h e l e a d i n g e d g e , and i n c r e a s i n g l y g r e a t e r downwind of t h e l e a d i n g e d g e .

A dune shape such a s t h a t depicted i n f i g u r e 1

results. Where t h e v e g e t a t i o n d e n s i t y i s e i t h e r low, or t h e d u n e s u r f a c e e x h i b i t s l a r g e s p a t i a l v a r i a t i o n s i n d e n s i t y ( a s i n s i t u a t i o n 2 above), t r i a n g u l a r dunes a r e not a s common.

R a t h e r , l o n g e r , a s y m m e t r i c , convex d u n e s a r e formed.

This appears

t o be due t o t h e i m p o r t a n c e o f l o c a l s a n d t r a n s p o r t i n a r e a s o f low d e n s i t y ,

o r v a r i a b l e d i s t r i b u t i o n s , w i t h i n t h e canopy. I t i s s u g g e s t e d a b o v e t h a t f o r a g i v e n wind v e l o c i t y

,

t h e f e t c h (or w i d t h ) o v e r

which most a e o l i a n s a n d d e p o s i t i o n o c c u r s i s d e t e r m i n e d by p l a n t d e n s i t y .

If

t h e change i n e l e v a t i o n o v e r time o f a series o f a d j a c e n t p o i n t s a l o n g an i n c i p i e n t f o r e d u n e s t o s s f a c e i s m o n i t o r e d , and t h e p l a n t d e n s i t y a r o u n d t h o s e p o i n t s is a s s e s s e d , t h e c h a n g e i n e l e v a t i o n s h o u l d t h e n b e a f u n c t i o n of p l a n t d e n s i t y and wind v e l o c i t y .

T h a t i s , t h e e l e v a t i o n a t o n e p o i n t on t h e f o r e d u n e

s t o s s f a c e s h o u l d b e h i g h e r where upwind p l a n t d e n s i t y is h i g h ( s i n c e s a n d i s supposedly t r a p p e d s o o n e r w i t h i n t h e s e l o c a t i o n s ) , t h a n where p l a n t d e n s i t y i s low. F i g u r e 3a i l l u s t r a t e s two p l o t s o f numbers o f S p i n i f e x ( d e n s i t y ) and s a n d a c c r e t i o n , f o r e a c h v e g e t a t e d , s q u a r e metre closest t o t h e beach on e a c h a d j a c e n t l i n e w i t h i n a c o n t i n u o u s l y m o n i t o r e d s i t e ( s e e s e c t i o n 2 f o r d e t a i l s ) . The s o l i d l i n e and crosses i l l u s t r a t e a v e r a g e a c c r e t i o n (m 3 p e r s q u a r e m e t r e ) and a v e r a g e S p i n i f e x numbers f o r t h e p e r i o d November 18, 1978 t o 3 a n u a r y 2 0 , 1979 ( 3 s u r v e y s , 64 day a v e r a g e ) .

The b r o k e n l i n e and c i r c l e s i l l u s t r a t e a v e r a g e a c c r e t i o n f o r

t h e same p e r i o d b u t p l a n t numbers a s a t 3 a n u a r y 2 0 , 1 9 7 9 f o r t h e same l o c a t i o n s . Correlation c o e f f i c i e n t s a r e i n d i c a t e d , and a r e s i g n i f i c a n t a t t h e 95 p e r c e n t l e v e l . Although c o r r e l a t i o n s a r e low, t h e l i n e s i n d i c a t e t h a t t h e r e e x i s t s a r e a s o n a b l e r e l a t i o n s h i p between p l a n t d e n s i t y and s a n d a c c r e t i o n i n t h e s e l e c t e d l o c a t i o n . F i g u r e 3 b i l l u s t r a t e s t h e m o r p h o h g i c r e s u l t where s a n d h a s been t r a n s p o r t e d a c r o s s two a d j a c e n t z o n e s of v a r y i n g v e g e t a t i o n d e n s i t y .

The d a t a is d e r i v e d

from s u r v e y s and p l a n t c o u n t s i n t h e same p l o t s i t e a s t h a t n o t e d above.

The

f i g u r e shows t h e numbers of S p i n i f e x p l a n t s c o u n t e d w i t h i n e a c h s q u a r e metre along two o n e metre wide l i n e s , 5 metres a p a r t o n March

2 4 , 1979.

I t may be

seen t h a t t h e r e are many more S p i n i f e x p l a n t s p e r s q u a r e metre on l i n e 5 compared t o l i n e 10, and t h a t t h e h i g h e s t d e n s i t y z o n e of S p i n i f e x p l a n t s p e r s q u a r e

metre o n l i n e 5 is a l s o l o c a t e d f u r t h e r s e a w a r d s t h a n l i n e 10. of t h e i n c i p i e n t f o r e d u n e i s a l s o shown f o r l i n e 5 and l i n e 10.

The morphology The f o r m o f t h e

dune a t l i n e 5 is h i g h e r , s h o r t e r a n d more a s y m m e t r i c t h a n t h e dune form a t l i n e 10.

332 A l a g e f f e c t on sand d e p o s i t i o n may a l s o be observed on l i n e 5.

F i g u r e 4 i l l u s t r a t e s two p o r t i o n s o f an i n c i p i e n t foredune which was i n i t i a t e d by s e e d l i n g g e r m i n a t i o n d u r i n g October ( s p r i n g ) , 1978.

The area shown i n f i q u r e

4c and d i n i t i a l l y d i s p l a y e d a p p r o x i m a t e l y t w i c e t h e d e n s i t y o f t h e a d j a c e n t (200 m a p a r t ) a r e a i l l u s t r a t e d i n f i g u r e 4a and b.

The h i g h e r d e n s i t y i n c i p i e n t

foredune p o r t i o n has a narrower b a s a l w i d t h , and i s h i q h e r t h a n t h e l o w e r density f oredune zone.

7 /x

x

o x

0 0

25

X 50

75

100

125

I

1

150

175

Distance seawards (m)

Spinfex numbers (density)

F i g . 3a. R e l a t i o n s h i p between S p i n i f e x h i r s u t u s d e n s i t y and sand a c c r e t i o n f o r each 24-25 square metre l o c a t i o n on a l l P l o t l i n e s (1-10) f o r 20.1.79, and f o r t h e p e r i o d 18.11.78 - 20.1.79. F i g . 3b. Morphology of l i n e s 5 and 10, and S p i n i f e x numbers w i t h i n t h e 20-30 sq.m. zone (down t h e dune) a t 24.3.79. A s h o r t e r , asymmetric r i d g e forms i n t h e h i g h e r d e n s i t y ( l i n e 5 ) zone.

Swales Swales a r e concave t r o u g h s w h i c h o f t e n

occur i n t h e l e e o f foredunes.

The

d a t a p r e s e n t e d i n f i g u r e s 3 and 4 show t h a t where v e g e t a t i o n d e n s i t y i s moderate t o h i g h , t h e i n c i p i e n t foredune t r a p s and r e t a i n s t h e l a r g e s t volume o f incoming a e o l i a n sand.

Swales a r e t h u s commonly ' f o r m e d ' as m i n i m u m d e p o s i t i o n zones, as

suggested by S a l i s b u r y ( 1 9 5 2 ) , Ranwell (1972) and Quinn ( 1 9 7 7 ) .

Swale d e p t h

depends on t h e d e n s i t y o f p l a n t s on t h e foredune t o seaward, and wind v e l o c i t y . Swale d e p t h (and w i d t h ) w i l l be l e s s where v e g e t a t i o n d e n s i t y i s l o w o r wind v e l o c i t i e s are

h i g h , as more sand w i l l be t r a n s p o r t e d i n t o t h e swale zone.

Swale w i d t h t h e n depends on one o f t h e f o l l o w i n g :

the i n i t i a l location of the

333 Spinifex seedliny zone with respect t o

d

landward foredune (see s e c t i o n 3) ;

the d e n s i t y o f thc seawdrd v e g e t a t i o n r o n e ( a s a b o v e ) ; t h c width o f scawdrd

rhiLome g r o w t h ; m d w i n d v e l o c i t y (ds a b o v e ) .

F i g . 4. E v o l u t i o n o f t w o p o r t i o n s of d n i n c i p i e n t f o r e d u r i r f o r m e d by S p i n i f e x s e e d l i n g c o l o n i s d t i o n o f t h e b a c k s h o r e d u r i n g O c t o b e r , 1978. ( d ) and ( b ) i l l u s t r d t e t h c l o w - m o d e r d t e d e n s i t y p o r t i o n ( 2 4 . 3 . 7 9 drid 26.4.80), arid ( c ) d n d ( d ) t l i c h i g h d e n s i t y p o r t i o n (samf' d d t e s ) . T t i r f u r t h e r s e d w a r d a new S p i n i f e x s e e d l i n g z o n e , the w i d r r p o t r n t i d l swdle

width.

The l o c d t i o n o f t h e s e e d l i n g z o n e i s d e t e r m i n e d by t h e i r i t r r a c t i o r i o f

spring t i d e heighL and bedch morphology, and/or t h e rate o f beach p r o q r a d d t i o n . The w i d e r t h e b e a c h , o n a v e r d y e , t h e f u r t h e r s e d w a r d w i l l t h e s e e d l i n g Lone b e l o c a t e d , dnd h e n c e t h e w i d e r t h e swdle b e t w e e n dn o l d e r d u n e and t h e s e e d l i n g zorle.

R a p i d b e a c h p r o g r d d a t i o n w i l l medn t h a t t i m e - d i s c r e t e s e r d l i n q Lories mdy

p o t e n t i a l l y be w i d e l y s e p a r a t e d .

S i m i l a r comments a p p l y t o areas experiericiricj

r a p i d s e a w a r d c o l o n i s a t i o n by S p i n i f e x r h i r o m r s .

334 E f f e c t o f v a r i a t i o n s i n wind v e l o c i t y V a r i a t i o n s i n wind v e l o c i t y i n d u c e v a r i a t i o n s i n t h e volume o f s a n d t r a n s p o r t e d , t h e aerodynamic behaviour o f t h e v e g e t a t i o n , and dune morphology.

As w i n d v e l -

o c i t i e s i n c r e a s e o v e r v e g e t a t i o n , t h e c a n o p y i s p e n e t r a t e d more e f f e c t i v e l y ( e s p e c i a l l y d u r i n g g u s t s ) , a n d s h e a r stresses a r e o r d e r s of m a g n i t u d e t h a n when wind s p e e d s a r e low ( F i n n i g a n , 1 9 7 9 ) .

greater

Furthermore, with increasing

w i n d v e l o c i t y , t h e v e g e t a t i o n i s f o r c e d closer t o t h e g r o u n d ( s t r e a m l i n e d ) , a n d roughness l e n g t h s ( z ) are lowered (Sellers, 1965;

Thom, 1 9 7 1 , 1 9 7 2 ) .

Upwind

o f t h e v e g e t a t i o n , t h e s a l t a t i o n ' c o l u m n ' i s h i g h e r , g r a i n t r a j e c t o r i e s are l o n g e r , a n d t h e v o l u m e o f s a n d i n t r a n s p o r t i s much g r e a t e r ( W i l l i a m s , 1 9 6 4 ) . The p o t e n t i a l f o r s a n d t r a n s p o r t ( i n c l u d i n g e r o s i o n ) i s a l s o much g r e a t e r w i t h i n the vegetation. of

I n a d d i t i o n , i f sand d e p o s i t i o n o c c u r s o v e r a l o n q enough period

time, o r i f t h e r a t e o f s d n d t r a n s p o r t i s h i g h , v e g e t a t i o n may b e c o m p l e t e l y

b u r i e d , a n d i n c o m i n g s a n d may t h e n b y p a s s t h e o r i g i n a l d e p o s i t i o n z o n e ( i . e . z0 w i l l a p p r o a c h z e r o ) .

S h o r t term m o n i t o r i n g of s i n g l e storm e v e n t s , a n d l o n g term s u r v e y i n q o f i n c i p i e n t foredune formation (Hesp, 1982) i l l u s t r a t e s t h a t for a given S p i n i f e x d e n s i t y , sand t r a n s p o r t and d e p o s i t i o n w i l l t a k e p l a c e over a l o n g e r d i s t a n c e d u r i n g h i g h w i n d v e l o c i t i e s t h a n d u r i n g low t o m o d e r a t e w i n d v e l o c i t i e s . O b s e r v a t i o n s i n d i c a t e t h a t w h e r e t h e v e g e t a t i o n i s f a i r l y d e n s e , most s a n d i s d e r i v e d f r o m t h e b e a c h , u p w i n d of t h e v e g e t a t e d z o n e .

However, w h e r e t h e

v e g e t a t i o n d e n s i t y i s low t o m o d e r a t e , more s a n d i s t r a n s p o r t e d f r o m w i t h i n t h e v e g e t a t i o n a s wind v e l o c i t i e s i n c r e a s e .

Velocity p r o f i l e s adjust very rapidly

i n response t o local v a r i a t i o n s i n v e g e t a t i o n d e n s i t y (Fig. 2; and Bradley, 1968). By i m p l i c a t i o n , s a n d t r a n s p o r t w o u l d a l s o o c c u r more r e a d i l y a s v e g e t a t i o n d e n s i t y d e c r e a s e s , a n d t h u s w i t h i n - c a n o p y t r a n s p o r t becomes i n c r e a s i n g l y i m p o r t a n t a s velocities increase.

I n a d d i t i o n , it is l i k e l y t h a t a p o r t i o n of t h e flow a t

t h e l e a d i n g e d g e o f t h e v e g e t a t i o n moves u n d e r t h e c a n o p y .

D u r i n g h i g h wind

v e l o c i t i e s S a d e h e t a 1 . ( 1 9 7 1 ) n o t e t h a t j e t t i n g commonly o c c u r s a r o u n d e a c h p l a n t stem.

T h i s i n c r e a s e s t h e a v e r a g e v e l o c i t y n e a r t h e b e d , and would p r e -

sumably i n c r e a s e t h e p o t e n t i a l f o r sand t r a n s p o r t w i t h i n v e g e t a t i o n . t i g u r e 5 i l l u s t r a t e s two p h o t o g r a p h s , t a k e n o n e month a p a r t , o f a p o r t i o n o f

a c o n t i n u o u s l y m o n i t o r e d f o r e d u n e p l o t (same drea a s F i g . 4 a , b ) .

Sand d e -

p o s i t i o n and e r o s i o n h a s o c c u r r e d across a wide zone i n t h e h i g h e r v e l o c i t y e v e n t ( 5 b ) , and dune h e i g h t is lowered w h i l s t dune l e n g t h is i n c r e a s e d . E f f e c t of v a r i a t i o n s i n p l a n t growth p a t t e r n s Where t h e i n i t i a l l o n g s h o r e d i s t r i b u t i o n o f S p i n i f e x s e e d l i n g s i s c o n t i n u o u s ,

lateral v a r i a t i o n s i n density (alongshore) produce foredune morphologic variations. T h e i n c i p i e n t f o r e d u n e w i l l b e h i g h e r a n d o f s h o r t e r b a s a l w i d t h i n areas o f

335 h i g h e r d e n s i t y , a n d lower a n d l o n g e r i n a r e a s o f lower d e n s i t y . produces a l o n g s h o r r 1958b).

This variation

lee s l o p e and c r e s t a l s i n u o s i t y (see f i g . 4 d , and O l s o n ,

When r h i z o m e l s t o l o n c o l o n i s a t i o n i s t h e d o m i n a n t p r o c e s s , s p a t i a l

v d r i a t i o n i n s u r f a c e c o v e r mdy b e p r o n o u n c e d .

F o r e x a m p l e , s i r r q l c o r sevcrdl

r h i z o m e s may a l t e r n a t e w i t h u n v e g e t a t e d z o n e s a l o n g s h o r e , a n d c r e s t a l s i n u o s i t y

i s a common f e a t u r e o f t h e i n c i p i e n t f o r e d u n e .

F i g . 5. T e m p o r a l v a r i a t i o n of a n i n c i p i e n t f o r e d u n e f o l l o w i n g d p e r i o d o f low-moderate wind v e l o c i t i e s ( a ; 2 6 . 8 . 7 9 ) , a n d a p e r i o d o f h i g h wind v e l o c i t i e s ( b ; 24.9.79).

The mode of b e a c h v e g e t a t i v e c o l o n i s a t i o n , a n d s p a t i a l v d r i d t i o r r s i n s h o o t and r h i z o m e growth s t r u c t u r e a n d l o c a t i o n p r o d u c e v a r i a t i o n s i n i n c i p i e n t f o r e dune morphology.

T h r e e major i n c i p i e n t f o r e d u n e m o r p h o l o g i e s may b e r e c o q n i s e d :

ramps, terraces and r i d g e s .

Ramp i n c i p i e n t f o r e d u n e s d e v e l o p p r i m a r i l y o n

i n h e r i t e d s e a w a r d s l o p i n g s u r f a c e s c o l o n i s e d by S p i n i f e x r h i z o m e s o r s e e d l i n g s . The ramp m o r p h o l o g y i s m a i n t a i n e d w h e r e s e a w a r d r h i z o m e g r o w t h k e e p s p d c e

with

sand d e p o s i t i o n , o r w h e r e s e e d l i n g d e n s i t y is low a n d s a n d d e p o s i t i o n o c c u r s throughout t h e vegetated zone allowing in-place build-up o f t h e sloping surface.

Terrace i n c i p i e n t f o r e d u n e s d e v e l o p w h e r e ( a ) S p i n i f e x s e e d l i n y s p r o p a g a t e on f l a t beach ( i n h e r i t e d morphology); or (b) where t h e v e g e t a t i o n

d

d e n s i t y i s low

t o m o d e r d t e , o r d i s t r i b u t i o n s a r e i r r e g u l a r , s a n d i s more a b l y a n d more e v e n l y t r a n s p o r t e d through t h e whole v e g e t a t i o n zone.

Terraces may b e e x t e n s i v e

where b e a c h p r o g r a d a t i o n i s r a p i d , a n d t h e r a t e o f s e a w a r d S p i n i f e x rhizome and s h o o t c o l o n i s a t i o n i s h i g h .

Ridge i n c i p i e n t foredunes i n i t i a l l y develop

b e d c u s e o f p r e f e r e n t i a l d e p o s i t i o n o f s a n d w i t h i n t h e f i r s t f e w metres o f

d

high d e n s i t y S p i n i f e x zone. T e m p o r a l v a r i a t i o n s i n g r o w t h p a t t e r n s i n d u c e f u r t h e r m o r p h o l o g i c charrge. s h o o t s and r h i z o m e s p r o d u c e d w i t h i n e x i s t i n g c o m m u n i t i e s c o l o n i s e t h e seaward u n v e g e t a t e d b e a c h , p r o d u c i n g a s e a w a r d d i s p l a c e m e n t o f t h e major z o n e o f s a n d d e p o s i t i o n and a r e s u l t i n g c h a n g e i n t h e i n c i p i e n t f o r e d u n e morpholoyy (see

New

336 e . g . O l s o n , 1958b, p l a t e 1 D ; McKenzie, 1 9 5 8 ) .

For e x a m p l e , ramp f o r e d u n e mor-

p h o l o g i e s may d e v e l o p t h r o u g h an e v o l u t i o n a r y c o n t i n u u m from t e r r a c e s t o r i d g e s where r h i z o m e s c o l o n i s e seaward s u r f a c e s , t r a p p i n g s a n d t o i n i t i a l l y form a t e r r a c e , and t h e n a r i d g e a s s e a w a r d growth i s i n h i b i t e d by t i d a l i n u n d a t i o n , or a s density increases. SEDIMENTARY STRUCTURES I n t h e f o l l o w i n g , two e x c a v a t i o n s a r e i l l u s t r a t e d t o i n d i c a t e t h e t y p i c a l i n t e r n a l s t r u c t u r e o f i n c i p i e n t f o r e d u n e s formed i n S p i n i f e x h i r s u t u s .

The

t e r m i n o l o g y u s e d i s t h a t of McKee and Wier ( 1 9 5 3 ) . F i g u r e 6 i l l u s t r a t e s a t r e n c h e x c a v a t e d i n a s m a l l i n c i p i e n t f o r e d u n e approxi m a t e l y SO metres west of a c o n t i n u o u s l y m o n i t o r e d p l o t .

I n t h i s a r e a , a low

d e n s i t y zone o f S p i n i f e x s e e l i n g s p r o p a g a t e d on t h e b a c k s h o r e .

Aeolian sand

d e p o s i t i o n o c c u r r e d t h r o u g h o u t , and beyond t h e S p i n i f e x z o n e . The lowermost s e t ( A i n f i g . 6 d ) i s bounded a t t h e t o p by a l i n e o f d r i f t m a t e r i a l , m o s t l y wood and s e e d f r a g m e n t s ( s e e b o t t o m f i g . 60).

T h i s was d e -

p o s i t e d by s p r i n g t i d e s w a s h , a n d t h e S p i n i f e x h i r s u t u s growing above o r i g i n a t e d from s e e d s d e p o s i t e d w i t h i n t h i s l i t t e r .

T h i s lowermost s e t o f

s t r a t a i s a s i m p l e t a b u l a r s e t c o m p r i s i n g e x t r e m e l y l o w - a n g l e , medium-scale ( l i k e l y t o be l a r g e - s c d l e ; i . e . l e n g t h g r e a t e r t h a n 6 m), e v e n , p a r a l l e l , swash deposited laminae.

Laminae i n t h e f a c e a t t h e r e a r o f t h e t r e n c h ( n o r m a l t o

p r e v a i l i n g wind) a r e h o r i z o n t a l . a t a n g l e s o f less t h a n

4O,

The rest ( t r e n c h f r o n t and p a r a l l e l f a c e ) d i p

and most commonly a t 1' t o .'2

Near t h e m i d d l e o f

t h e t r e n c h t h e d i r e c t i o n o f d i p c h a n g e s from a seaward t o a landward o n e . The s e c o n d s e t ( B i n f i g . 6 d ) i s a s m a l l , s i m p l e l e n t i c u l a r set c o m p r i s i n g medium-scale, l o w - a n g l e c r o s s - l a m i n a e which a r e e v e n , and g e n e r a l l y p a r a l l e l . The wavy s t r a t a i n t h e b a c k s e t (windward) z o n e ( f i g . 6 c ) p r o b a b l y r e s u l t from This set (B) r e p r e s e n t s t h e i n i t i a l a c c r e t i o n u n i t w i t h i n

human f o o t p r i n t s .

the Spinifex seedlings. The sets ( C , D , E) l y i n g above s e t 6, a p p e a r t o be s i m p l e ( n o n - e r o s i o n a l l e n t i c u l a r sets. other.

B d c k s e t bounding s u r f a c e s a r e p r e d o m i n a n t l y p a r a l l e l t o e a c h

The sets r e s u l t from minimal b a c k s e t d e p o s i t i o n and maximum t o p s e t and

foreset deposition.

Wind v e l o c i t i e s were p r o b a b l y h i g h e r on a v e r a g e t h a n i n t h e

d e p o s i t i o n a l e v e n t ( s ) f o r m i n g s e t B. d i p s of less t h a n 13',

A l l c r o s s - s t r a t a i n s e t s C , D and E had

and most were i n t h e r a n g e S o t o 9".

t h e uppermost p o r t i o n o f s e t E ( i . e . c r o s s - s t r a t a t o be recognised.

The s a n d c o m p r i s i n g

E i i i was u n f o r t u n a t e l y t o o d r y f o r t h e

The c u r v a t u r e and s h a p e o f t h e E i i u n i t , which

p a r a l l e l s t h e f i r s t v i s i b l e lamina i n t h e backset s t r a t a , i n d i c a t e s t h a t t h i s u n i t is p a r t of t h e E l e n t i c u l a r set.

S e t shape i n d i c a t e s t h a t i n d i v i d u a l

l a m i n a e were t h i c k e s t i n t h e c e n t r e ( d u n e c r e s t ) , and t h i n n e s t a t t h e windward and l e e w a r d e n d s .

Metres seaward

W W U

338 The c r o s s - s t r a t a i n d i c a t e t h e d e v e l o p m e n t from an i n i t i a l low v e l o c i t y micror i d g e f o r m a t i o n , t h r o u g h a p e r i o d o f m o d e r a t e a n d h i g h e r v e l o c i t y t e r r a c e forma t i o n , t o a r i d g e capping.

S t r a t a comprising t h e ' t e r r a c e ' sets continue

landwards ( f i g . 6 a , b) i n t o a n o l d e r vegetated i n c i p i e n t foredune zone.

Aeolian

a c c r e t i o n a l s t r a t a d e p o s i t e d o n t h e upwind b e a c h a r e c o n t i n u o u s w i t h t h e ' t e r r a c e ' strata.

Ridge d e v e l o p m e n t ( u n i t E i i ) p r i n c i p a l l y o c c u r r e d a s t h e d e n s i t y of T h i s p r o c e s s was a i d e d , i n p a r t by t h e o p e r a t i o n

t h e local vegetation increased.

of wind v e l o c i t y e v e n t s which were more c o n s t a n t , and of less v e l o c i t y t h a n p r e v i o u s e v e n t s which formed t h e ' t e r r a c e ' s t r a t a . B a c k s e t and f o r e s e t c r o s s - s t r a t a a r e c o n t i n u o u s a c r o s s t h e whole i n c i p i e n t foredune.

B i g a r e l l a e t a l . (1969) a l s o found t h i s t y p e of continuous s t r a t i f i c a t i o

i n a " s a n d r i d g e dune" ( a f o r e d u n e ) .

McKee ( 1 9 7 9 , p . 9 7 ) s t a t e s t h a t " t h e

abundance o f s a n d s u p p l y and t h e s l o w r a t e of movement ( b e c a u s e o f i n t e r n a l dampness) a r e r e s p o n s i b l e f o r t h i s f e a t u r e " .

I n t h e trench i l l u s t r a t e d i n

f i g u r e 6 , c o n t i n u o u s c r o s s - s t r a t a a r e formed by c o n t e m p o r a n e o u s d e p o s i t i o n a c r o s s t h e whole dune s u r f a c e . Figure 7 i l l u s t r a t e s a trench vegetated i n c i p i e n t foredune.

excavated i n a l a r g e r , o l d e r , moderately

The l o w e s t t a b u l a r s e t (1 on f i g . 7 ) c o n s i s t s

o f v e r y low a n g l e , p a r a l l e l c r o s s - l a m i n a e i n which d i p s d o n o t e x c e e d 5'.

The

lower-most p o r t i o n o f t h i s s e t i s i n t e r p r e t e d on t h e b a s i s of h e i g h t above MSL a s b a c k s h o r e swash l a m i n a e which g r a d e upwards i n t o a e o l i a n l a m i n a e .

s e t h a s b e e n swash e r o d e d on t h e s e a w a r d e d g e ( s e t 2 ) .

This

F i n e heavy-mineral

l a m i n a e were p r e s e n t w i t h i n t h e p l a n a r wedge-shaped s e c o n d s e t . The t h i r d set i s a s i m p l e , l e n t i c u l a r set c o m p r i s i n g c o n v e x , g e n e r a l l y T h i s s e t r e p r e s e n t s i n i t i a l r i d g e formatior

p a r a l l e l , continuous cross-laminae. within Spinifex.

Above t h e t h i r d s e t , t h e r e i s a t l e a s t o n e s e t o f l e n t i c u l a r ,

convex, continuous cross-laminae.

The l o w e r bounding s u r f a c e o f t h e s e t ( s ) i s

p l a n a r , a t l e a s t i n t h e upwind s e c t i o n . downwind o f t h e r i d g e crest o f set 3 . (mid-way t o c r e s t ) , t o 11'

14'

t o 6'

(downwind e n d ) .

Deposition has predominatly occurred The c r o s s - l a m i n a e d i p from 6'

( j u s t upwind o f c r e s t ) , t o 1 0'

upwind t o

( l e e of crest),

Cross-laminae a r e continuous a c r o s s t h e crest, j o i n i n g

t h e l o w e r s e t ( 3 ) t a n g e n t a l l y a t low a n g l e s downwind.

I n s p e c t i o n of t h e i n i t i a l

convex set ( 3 ) and uppermost s e t ( s ) , shows t h a t t h e c r o s s - l a m i n a e t h i c k e n landw a r d s i n t o t h e c e n t e r o f t h e s e t ( t h e most c o n v e x , r i d g e p o r t i o n ) , and t h e n e i t h e r d i m i n i s h i n s i z e downwind, o r t e r m i n a t e .

This depositional process

r e s u l t s i n ridge formation. I n summary, s i m p l e and p l a n a r , t a b u l a r and l e n t i c u l a r s e t s of l o w - a n g l e , continuous cross-laminae dominate t h e s e i n c i p i e n t foredunes. d i p a t a n g l e s o f l e s s t h a n 15'.

Most c r o s s - s t r a t a

Goldsmith (1973, 1978) h a s a l s o s t r e s s e d t h e

abundance o f l o w - a n g l e c r o s s - b e d d i n g i n f o r e d u n e s .

339

F i g . 7 . I n t e r n a l s e d i m e n t a r y s t r u c t u r e s of a n i n c i p i e n t f o r e d u n e . T r e n c h d e p t h i s 0.95 cm ( c r e s t t o b a s e ) , a n d l e n g t h i s 5 m. Lower s e t s a r e i d e n t i f i e d (see t e x t ) , a n d t h e a r r o w i n d i c a t e s t h e c r e s t s u r f a c e . Wind f l o w s f r o m l o w e r right t o upper left.

CONCLUSION I n c i p i e n t f o r e d u n e s formed w i t h i n l a t e r a l l y c o n t i n u o u s S p i n i f e x p o p u l a t i o n s r e s u l t from a complex i n t e r a c t i o n between b i o l o g i c , aerodynamic, and geomorphic processes.

T h i s p a p e r h a s o n l y b r i e f l y d i s c u s s e d some o f t h e major f a c t o r s

i n v o l v e d , a n d many a d d i t i o n a l f a c t o r s ( e . g . wave e r o s i o n ; p r e s e n c e o f o t h e r p l a n t s p e c i e s w i t h i n S p i n i f e x z o n e s ) dct t o i n t r o d u c e V a r i a t i o n s i n f i n a l d u n e morphology a n d i n t e r n a l s t r u c t u r e .

However, s e v e r a l g e n e r a l p o i n t s may b e made

concerning i n c i p i e n t foredune e v o l u t i o n : ( i ) The n e a r - s u r f a c e f l o w s t r u c t u r e w i t h i n S p i n i f e x v e g e t a t i o n p r i n c i p a l l y

varies dccording t o density/distribution variations.

When s a n d i s t r a n s p o r t e d

w i t h i n t h e f l o w , s e d i m e n t a t i o n p a t t e r n s a r e a l s o i n p a r t d e t e r m i n e d by p l a n t cover.

Thus f o r e d u n e h e i g h t i n c r e a s e s and b a s a l w i d t h d e c r e a s e s as p l a n t

density increases.

F o r e d u n e s t e n d t o b e more a s y m m e t r i c , w e d g e - s h a p e d f o r m s

as p l a n t d e n s i t y i n c r e a s e s .

L o n g s h o r e v a r i a t i o n s i n crestal and lee s l o p e

s i n u o s i t y o f dunes is a p r o d u c t of v a r i a t i o n s i n p l a n t d e n s i t y and d i s t r i b u t i o n . ( i i ) As w i n d v e l o c i t y i n c r e a s e s , P o r e d u n e h e i g h t d e c r e a s e s a n d d u n e l e n g t h

increases for a given vegetation d e n s i t y .

34 0 ( i i i ) S w a l e s a r e o f t e n formed

ds

non- t o l i m i t e d - d e p o s i t i o n a l z o n e s .

( i v ) S e d i m e n t a r y s t r u c t u r e s a r e dominated by l o w - a n y l e c r o s s b e d s . and b a c k s e t s a r e c o n t i n u o u s , and s l i p f a c e s a r e g e n e r a l l y a b s e n t .

Topsets

For a g i v e n

wind v e l o c i t y , s h o r t e r , s t e e p e r wedge- o r t r i d n g u l d r - s h d p e d s e t s of c r o s s l a m i n a e a r e formed i n h i g h d e n s i t y S p i n i f e x z o n e s .

Where t h e d e n s i t y o f S p i n i f e x

i s low t o m o d e r a t e , l o n g e r , l o w e r more convex s e t s o c c u r .

Individudl cross-

lamina a r e t h i c k e r i n t h e higher d e n s i t y case than i n t h e lower d e n s i t y c a s e . F o r a g i v e n v e g e t a t i o n d e n s i t y , ttre h i g h e r t h e a v e r a g e wind v e l o c i t y , t h e l o n g e r and l o w e r t h e s e t s , and t h e g r e a t e r t h e d e g r e e of b a c k s e t l o w - a n g l e truncation.

A s a v e r a g e v e l o c i t y i n c r e a s e s , i t i s e x p e c t e d t h e r e would be

t r e n d from s i m p l e t o p l a n a r l e n t i c u l a r s e t s .

d

These c o n c l u s i o n s i n d i c a t e t h a t

a modification i s r e q u i r e d t o Yaalori's (1975) suggestion t h a t a r e l a t i o n s h i p between p l a n t d e n s i t y and l o w - a n g l e c r o s s - b e d d i n g might be e x p e c t e d . angle (less than 2 0')

c r o s s - b e d s do d o m i n a t e w e l l v e g e t a t e d s u r f a c e s .

LowHowever,

a s p l a n t d e n s i t y i n c r e a s e s t h e s c a l e o f c r o s s - b e d d i n g d e c r e a s e s and t h e s l o p e increases. ACKNOWLEDGEMENTS T h i s s t u d y formed p a r t o f a d o c t o r a l r e s e a r c h programme f u n d e d by the Department of Geography, U n i v e r s i t y o f S y d n e y , and t h e S o i l C o n s e r v a t i o n S e r v i c e of N e w S o u t h Wales. Frank Burrows k i n d l y l o a n e d t h e a u t h o r h i s anemometers, R o b e r t Hyde p r o v i d e d g u i d d n c e i n i n i t i a l d a t a c o l l e c t i o n a t t e m p t s , B j o r n Kjerfve a s s i s t e d i n d a t a a n d l y s i s , and Alex C o l v i n , L i n d a Huzzey and C a r o l Luddington s u r v e y e d d u n e s u n d e r a l l c o n d i t i o n s . A d d i t i o n a l d a t a were c o l l e c t e d w i t h ttre s u p p o r t o f ttre W e s t e r n A u s t r a l i a n Department of A g r i c u l t u r e . G l e n n i s Fdrrow kindly typed t h e manuscript. R t t ERENCES

B e a d l e , N . C . W . , O.D. E v a n s , R . C . C d r o l i n , and M . D . T i n d a l e , 1 9 7 2 . F l o r a o f t h e Sydney R e g i o n . A . H . a n d A.W. Heed, 724 pp. B i g a r e l l a , 3.3., 1972. Eolian environments: Their c h a r a c t e r i s t i c s , recognition, and i m p o r t a n c e . I n : R i g b y , 3 . K . and W . K . Hamblin ( e d ' s ) , R e c o g n i t i o n of A n c i e n t S e d i m e n t a r y E n v i r o n m e n t s . S.t .P.M. Spec.Pub. No. 16: 12-62. B i g a r e l l a , 3 . 3 . , R . N . B e c k e r , and G . M . D u a r t e , 1969. C o a s t a l dune s t r u c t u r e s from P a r a n a ( B r a z i l ) . Mar. G e o l . , 7: 5 - 5 5 . B r a d l e y , E.F., 1968. A m i c r o m c t e o r o l o g i c a l s t u d y o f v e l o c i t y p r o f i l e s and s u r f a c e d r a g i n t h e r e g i o n m o d i f i e d by a change i n s u r f a c e r o u g h n e s s . Q . 3 . Roy. Met. S O C . , 9 4 : 361-379. B r a d l e y , E . F . , 1969. A s m a l l , s e n s i t i v e anemometer s y s t e m f o r a g r i c u l t u r a l m e t e o r o l o g y . Agr. Net., 6 : 185-193. F i n n i g a n , 3 . 3 . , 1979. T u r b u l e n c e i n waving wheat IT. S t r u c t u r e of momentum t r a n s f e r . B o u n d a r y - l a y e r Met., 1 6 : 213-236. G o l d s m i t h , V . , 1 9 7 3 . I n t e r n a l geometry and o r i g i n of V e g e t a t e d c o a s t a l sand d u n e s . 3. S e d . P e t . , 4 3 ( 4 ) : 1128-1142. G o l d s m i t h , V . , 1978. C o a s t a l Dunes. I n : R . A . D a v i s J r . ( e d . ) , C o a s t a l S e d i mentary E n v i r o n m e n t s . S p r i n g e r - V e r l a g , C h p t . 4 : 171-230. G o d f r e y , P . 3 . and M.M. G o d f r e y , 1976. B a r r i e r l s l a n d Ecology o f Cape Lookout N a t i o n a l S e a s h o r e and V i c i n i t y , N o r t h C a r o l i n a . N a t . P a r k S e r . S c i . Monograph S e r . No. 9 , 1 6 0 pp.

341 Greeley, R . , R . Leach, B. White, 3. Iversen and J . P o l l a c k , 1980. Threshold windspeeds f o r sand on Mars: Wind t u n n e l s i m u l a t i o n s . Geophys, Res. L e t t e r s , L 7 ( 2 ) : 121-124. hesp, P . A . , 1981. The f o r m d t i o n o f shadow dunes. J. Sed. P e t . , 5 1 ( 1 ) : 101-111. Hesp, P . A . , 1982. Dynamics and Morphology of Foredunes i n South E a s t A u s t r a l i a . Ph.D. D i s s e r t a t i o n , U n i v . Sydney. Howard, A . D . , 3 . 0 . Morton, M . Gad-€1-Hak, and D . B . P i e r c e , 1978. Sand t r a n s p o r t model of barChdn dune e q u i l i b r i u m . Sedimentology, 25: 307-338. Kotoda, K . , 1979. Wind p r o f i l e and aerodynamic p a r a m e t e r s above and w i t h i n a p l a n t canopy. A n n . R e p . I n s t . C e o s c i . , Univ. Tsukuba, No. 5 : 11-14. Kuhlman, H . , 1957. S a n d f l u g t og K l i t d a n n e l s e . Geograf. T i d s s k r i f t , 56: 1-19. Kuhlman, H. 1959. Q u a n t i t a t i v e measurements of a e o l i a n sand t r a n s p o r t . Geograf. T i d s s k r i f t , 57: 51-74. Kutzbach, J . C . , 1961. I n v e s t i g a t i o n s of t h e m o d i f i c a t i o n of wind p r o f i l e s by a r t i f i c i a l l y c o n t r o l l e d s u r f a c e roughness. Ann. Rep. No. Dd-36-039-SC80282: 71-113. D e p t . M e t e o r o l . U n i v . Wisconsin, Madison, Wisconsin. Land, L.S., 1964. E o l i a n c r o s s - b e d d i n g i n t h e beach dune environment, S a p e l o I s l a n d , Georgia. J. Sed. P e t . , 3 4 ( 2 ) : 389-394. Leatherman, S . P . , 1978. A new a e o l i a n sand t r a p d e s i g n . Sedimentology, 25: 303-306. McKee, E . D . , 1979. Sedimentary s t r u c t u r e s i n dunes. I n McKee, E . D . ( e d . ) , A Study of Global Sand S e a s . Geol. Surv. P r o f . Paper 1052: Chpt. E: 83-134. McKee, E . D . and G.W. Weir, 1953. Terminology f o r s t r a t i f i c a t i o n and c r o s s s t r a t i f i c a t i o n i n s e d i m e n t a r y r o c k s . Geol. Soc. A m e r . B u l l . , 64: 381-390. McKenzie, P . , 1958. T h e development of sand beach r i d g e s . Aust. 3 . S c i . , 20: 213-14. Mulhearn, P . 3 . and 3 . 3 . F i n n i a a n , 1978. T u r b u l e n t flow o v e r a very rouqh, random s u r f a c e . Boundary-layer Met,, 15: 109-32. Olson, 3 . S . , 1958a. Lake Michigan dune development. 1. Wind-velocity p r o f i l e s . 3 . G e o l . , 66: 254-63. Olson, 3 . S . , 1958b. Lake Michigan dune development. 2 . P l a n t s a s a g e n t s and t o o l s i n geomorphology. 3. G e o l . , 66: 345-51. Q u i n n , C . M . , 1977. Sand Dunes. Formation, E r o s i o n and Management. An Foras F o r b a r t h a , D u b l i n , 92 pp. Ranwell, D.S., 1972. Ecology of S a l t Marshes and Sand Dunes. Chapman and H a l l , London, 258 pp. Raupach, M . R . , A S . Thorn, and I . Edwards, 1980. A wind-tunnel s t u d y of t u r b u l e n t flow c l o s e t o r e g u l a r l y a r r a y e d rough s u r f a c e s . Boundary-Layer Met., 18: 373-97. Reitsma, T . 1978. W i n d - p r o f i l e Measurements i n d Maize Crop. A g r i c . Res. Rept. 882, ISBN 90 220 0684 0 ; A g r i c . Pub. Doc. C e n t r e , Waqeningen, 103 p p . Sadeti, W . Z . , 3 . E . Cermak, and T . Kawatani, 1971. Flow over high roughness e l e m e n t s . Boundary-Layer Met., 1: 321-44. S a l i s b u r y , E . , 1952. Downs and Dunes. T h e i r P l a n t L i f e and Environment. 6 . B e l l and Sons L t d , London. S e l l e r s , W . D . , 1965. P h y s i c a l C l i m a t o l o g y . Univ. of Chicago P r e s s . S h o r t , A . D . and P.A. Hesp, 1982. Wave, beach and dune i n t e r a c t i o n s i n s o u t h e a s t e r n A u s t r a l i a . Mar. G e o l . , 48: 259-284. Taylor, P.A. and P.R. Gent, 1974. A model of a t m o s p h e r i c boundary-layer flow above an i s o l a t e d two-dimensional ' h i l l ' ; an example of flow above ' g e n t l e t o p o g r a p h y ' . Boundary-Layer Met. 7 : 349-362. Thom, A.S., 1971. Momentum a b s o r p t i o n by v e g e t a t i o n . Q . 3 . Roy. Met. SoC., 97: 414-428. Thom, A.S., 1972. Momentum, mass and h e a t exchange of v e g e t a t i o n . Q . 3 . Roy. Met. S a c . , 98: 124-134. Thom, B.G., G.M. Bowman, and P.S. Roy, 1981. L a t e Q u a t e r n a r y e v o l u t i o n of c o a s t a l sand b a r r i e r s , P o r t Stephens-Myall Lakes a r e a , c e n t r a l NSW, A u s t r a l i a . Q u a t . Res. 15: 345-364.

34 2 Townsend, A.A., 1966. T h e f l o w i n a t u r b u l e n t boundary l a y e r a f t e r a change i n s u r f a c e roughness. 3 . F l u i d Mech., 26: 255-266. Walker, H.3. and Y . Matsukura, 1979. Barchans and h a r c h a n - l i k e dunes a s develope i n two c o n t r a s t i n g a r e a s w i t h r e s t r i c t e d s o u r c e r e g i o n s . Ann. Rep. I n s t . G e o s c i . , Univ. Tsukuba, No. 5 : 43-46. W i l l i a m s , G . , 1964. Some a s p e c t s of t h e e o l i a n s a l t a t i o n l o a d . Sedimentology, 3 : 257-207. Woodhouse, W . W . 3 r . , 1978. Dune B u i l d i n g and S t a b i l i s a t i o n w i t h V e g e t a t i o n . U.S. Army, Corps o f E n g r s . , Spec. Rept. No. 3 , 112 pp. Yadlon, D . H . , 1975. D i s c u s s i o n o f " I n t e r n a l geometry of v e g e t a t e d c o a s t a l sand dunes". 3 . Sed. P e t . , 45: 359.

343

DESERT DUNES: A SHORT R E V I E W OF NEEDS I N DESERT DUNE RESEARCH AND A RECENT STUDY

OF MICROMETEOROLOGICAL DUNE-INITIATION MECHANISMS A. WARREN and P. KNOTT: Department o f Geography, U n i v e r s i t y C o l l e g e London, Gower S t . ,

London W C l E 6BT, Enqland.

I n t h e l a s t decade t h e r e have been v e r y c o n s i d e r a b l e a d d i t i o n s i n o u r underIt i s t r u e t h a t a t the 'steady' scale -

standing o f t h e morohology o f d e s e r t dunes.

the s c a l e o f t h e movement o f sand - t h e r e have been few fundamental advances. Although we do n o t have b o t h more r i g o r o u s and more p r a c t i c a l formulae f o r r e l a t i n g sand movement t h e wind v e l o c i t y ( L e t t a u & L e t t a u , i n press and Hsu, 1973), and we have a l s o had o u r p e r c e p t i o n o f t e r r e s t r i a l processes sharpened by debates about wocesses on o t h e r p l a n e t s ( e . g . Sagan & Bagnold, 1975, Greeley, 1979), we have seen a s l o w e r t h a n usual advance i n fundamental s t u d i e s o f moving sand.

A t the 'graded' scale

-

t h a t o f a dune

-

we a r e now b e g i n n i n g t o have p r e s e n t -

able models o f t h e processes o f dune movement (Howard e t a1

.,

1977) and we a l s o

have c a r e f u l s t u d i e s o f dune s u r f a c e processes by Tsoar (1978) and K n o t t (1979). A t t h e ' c y c l i c ' s c a l e - t h a t o f whole sand seas, and f o l l o w i n g Schumm and

L i c h t y ' s (1964) argument, t h a t o f m i l l e n n i a

-

we now have enormously improved

knowledge of g l o b a l and even p l a n e t a r y e r g s (Mainguet & C a l l o t , 1978, McKee, 1979, Breed e t a l . ,

1979, Tsoar e t a l . ,

1979).

For d e s e r t dunes, as f o r a l l geomorphology, advances come l a r g e l y f r o m r e d u c t i o n s i n t h e s c a l e of e n q u i r y : most o f t h e v a r i a t i o n i n l a r g e s c a l e ' c y c l i c ' p a t t e r n s i s understandable, u l t i m a t e l y , o n l y i f graded s c a l e processes a r e e x p l a i n e d and most of these, i n t u r n , a r e u n d e r s t a n d a b l e o n l y i f t h e r e a r e reasonable models a t t h e 'steady' scale.

O f course, each s c a l e a l s o has elements t h a t can o n l y be under-

stood a t t h a t s c a l e , and, moreover, we must acceDt ' b l a c k boxes' o f D o o r l y understood concepts t o r e a c h any k i n d o f e x p l a n a t i o n o f g r o s s p a t t e r n s .

Desoite t h i s

unavoidable r e d u c t i o n , we want t o argue t h a t t h e s c a l e o f s t u d y t h a t w i l l y i e l d and i s now y i e l d i n g t h e g r e a t e s t advance i s t h e ' g r a d e d ' s c a l e o f t h e s i n g l e dune: the o r o p e r s t u d y o f dunes i s t h e dune. There a r e f o u r reasons.

F i r s t , we b e l i e v e t h a t we have models a t t h e steady

scale t h a t a r e s u f f i c i e n t f o r s t u d y a t t h e graded s c a l e : Howard e t a l . (1977) Tsoar (1978) and K n o t t (unpubl

.

1979) a l l made use o f a range o f stead;(

scale

models b u t found t h a t t h e i r d i f f e r e n c e s were n o t c r i t i c a l .

Second, i t i s a t t h i s scale t h a t s t u d i e s o f subsqueous dunes a r e y i e l d i n g t h e g r e a t e s t advances (e.g.

A l l e n , 1976).

T h i r d , we have a b a t t e r y o f t e c h n i q u e s and now models f o r t h i s s c a l e

344 o f s t u d y ( K n o t t & Idarren, 1980, Howard e t a l ,

1976).

F o u r t h , a number o f c r i t i c a l

o u e s t i o n s a r e s t i l l o u t s t a n d i n g a t t h e c y c l i c s c a l e t h a t can o n l v be r e s o l v e d a t t h e graded s c a l e .

Ide w i l l t a k e o n l y one examnle o f t h e s e , b e f o r e d e s c r i b i n n the

F i q . 1 . S i n a i : b a r c h a n s and s l o u k s w i t h d i f f e r i n q t r e n d s i n t h e l e e o f a n escarn rnent. N o r t h i s t o t h e t o p o f t h e n i c t u r e . The o r i n c i n l e w i n d i s f r o m s l i o h t l y w e s t o f n o r t h . S c a l e 1:50,000 a p p r o x .

345 outcome of some new work. The r e l a t i o n of the trend of l i n e a r dunes t o the ambient wind regiqe i s s t i l l unresolved. I t i s t r u e t h a t Fryberger and Dean's (1979) broad survey of l i n e a r dunes found t h a t t h e i r alignment was o f t e n closely associated with the r e s u l t a n t of broad unimodal p a t t e r n s , b u t Muriel Brookfield (1970) found p e r s i s t e n t deviations of the two in Central A u s t r a l i a , ldarren (1972) and Plainguet & C a l l a t (1978) a l s o noted c o n s i s t e n t discrepancies in the Fachi-Blima Erg, and Warren (1976a) seoorted a notable mismatch i n the Pleistocene Nebraska Sand H i l l s . Yet, another examole, s t r i k i n g l y s i m i l a r t o P l a t e VI i n Mainguet and C a l l o t ' s study comes fr,om Sinai (Fig.1) where the barchans and slouk show d i s t i n c t trends. Are these discrepancies explicable with Wilson's (1972) hypothesis of 'oblique d u n e s ' , with Wipperman's (1969) ideas about i n s t a b i l i t y of vortices i n the Ekman boundary layer, or with Warren's (1976b) theory about the roughness f a c t o r in the Ekman s p i r a l ? ( i t s e l f a t variance with the ' C o r e o l i s ' hypothesis of Maingy~et & C a l l o t , 1978.) A resolution of these arguments i s c r i t i c a l t o i n t e r p r e t a t i o n s of ancient dune patterns ( e . 9 . Warren, 1970, Fryberger, 1980). B u t the resolution can only come from s t u d i e s a t the graded s c a l e of the flow around individual dunes.' An example: Dune I n i t i a t i n g Processes

A fundamental process in the formation of dunes i s t h e i r i n i t i a t i o n . We have moved away, i n general, from the idea of ,chance obstacles, l i k e camels o r nebkha, t o the idea t h a t dunes can be i n i t t a t e d , somehow, by converging o r diverging flow (Wilson, 1972). B u t how does t h i s begin i n t h e atmosphere? One of us (Knott) studied t h i s process in the v i c i n i t y of I n Salah in Algeria ( K n o t t , 1979) where t h e r e i s a strong unimodal wind regime ( F i g . 2 ) associated with barchans in the small Erg Sidi Moussa. I

Knott used t h r e e principal methods of observation. I n the Eulerian mode he tracked f i r s t p i l o t balloons q t 15 second i n t e r v a l s with two t h e o d o l i t e s 500 m a p a r t , and second tetroons released a t a predetermined constant density surface; he a l s o used tetroon t r i a d s t o study convergence a n d divergence. I n the Lagrantian mode he used cup and strain-guage anemoters on booms on a 10 metre mast a t 1 , 2 , 4 a n d 8 metres above the surface. His f i r s t discovery of relevance-here was t h a t on 15 out of h i s 16 days of observation a temoerature inversion developed overnight from about 2200 hrs t o 0800 hrs a f t e r which t h e r e was a raDid change t o a d i a b a t i c and then suoeradiabatic conditions. The inversion tended t o deeoen continuously from about 10 t o about 200 m u n t i l 0700 hrs and i n t e n s i f i e d from 0530 o r 0630 h r s . A t i t s most intense i t was 3.O0C/1O0 m . ( J u l y 1 7 , 1976) ( F i g . 3 ) . His second relevant discovery, not s u r p r i s i n g i n view of o t h e r s t u d i e s of inversions (Offen & Kline, 1975), was t h a t a j e t developed near the inversion where the wind was deroupled from surface f r i c t i o n . I t was d i s t i n c t between 0000 and 0630 h r s . For example, on July ZZnd, 1976 a t 0630 hrs t h e r e was a wind

34 6 o f 18.5 m sec-'

a t 200 m w h i l e a t 10 m t h e wind was o n l y 4.9 m s e c - l .

Wind shear

was commonly 7 m s e c - l / 1 0 0 m . ( F i g . 4 ) . The t h i r d r e l e v a n t d i s c o v e r y ( a l s o n o t s u r p r i s i n g i n view o f t h e e a r l i e r work) was about t h e d i s p e r s i o n o f t h e j e t i n t h e e a r l y morning: i n t e n s e ' b u r s t s ' o f energy were f i r s t r e l e a s e d t o t h e s u r f a c e i n ' w o u n d j e t s ' . from t h e f l i g h t o f t e t r o o n s . t e t r o o n s were grounded.

These were discovered

Some o f t h e downward j e t s were so i n t e n s e t h a t

Others m e r e l y p r o j e c t e d t h e t e t r o o n downward and then

allowed i t t o r i s e again (Fig.5).

Waves i n t h e j e t were f a i r l y s t a b l e b e f o r e s u n r i s e when t h e h e i g h t v a r i a t i o n s o f t e t r o o n s were o f l o w a m o l i t u d e and s h o r t

F i g . 2 . I n Salah and t h e Erg S i d i Moussa. L o c a t i o n o f e x p e r i m e n t a l s i t e s . The l o w e r d i a g r a T shows t h e y e a r l y p a t t e r n o f sand movement and i t s r e s u l t a n t above threshold. t h e 6 m sec

34 7 wavelength, b u t t h e y p r o g r e s s e d t o h i g h a m p l i t u d e and l o n g wavelengths b y about

1200 h r s .

In t h e e a r l y morning v e r t i c a l v e l o c i t i e s seldom exceeded

but by 1000 h r s t h e y were o c c a s i o n a l l y

2 2

-f

2 m sec-l,

m sec-l.

The e x i s t e n c e o f some process such as ground j e t s was c o n f i r m e d by two f u r t h e r observations.

F i r s t , t h e o b s e r v a t i o n s o f t h e t e t r o o n t r i a d s showed t h e r e t o be

divergence a t l o w l e v e l s ( F i g . 6 ) .

Second t h e Lagrangian s t r a i n quage measurements

showed v e r y e r r a t i c t u r b u l e n c e i n t h e morning w i t h d i s t i n c t ' b u r s t s ' o f h i g h e r i n t e n s i t y ( h i g h u"

and h i g h

I s o l a t i n g these by eye r e v e a l e d t h a t t h e

U").'

f r a c t i o n o f t h e t i m e o c c u n i e t by b u r s t s was 0 . 3 and t h a t t h e y D e r s i s t e d f o r 3-4 minutes ( F i g . 7 ) .

In summary, t h e m e t e o r o l o g i c a l measurements showed: ( 1 ) Rapid c o o l i n g of t h e s u r f a c e a f t e r sunset: an i n v e r s i o n aDpeared i n w h i c h buoyant and mechanical m i x i n g a r e suopressed below t h e i n v e r s i o n .

a ) Temperature Profiles

3001

, { ,

200

2

100 50 10

pr

0430 ".. , $400.0400

32

,

36

34

b ) Velocity Profiles

,

,

,

,

3a 40 ("C1

42

Temperature

300-

P?

4 6 8 10 12 14 16 18 20

i t

0500i

/

Wind Speed (m/sec)

PP

200-

150~ ~ 2 5 3 0

100-

5010-

cf

,. Q

200

300

P

/b600

150 100 50 10 4 6 8 10 12 14 16 18 20

150 loo-

0630

22nd July, 1976 I

Fig.3. Temperature and v e l o c i t y p r o f i l e s , 22nd J u l y 1976. Records from p i l o t b a l l o o n a s c e n t s .

Times a r e shown.

348 ( 2 ) A j e t formed above the inversion ( a t c 200 m ) and i n t e n s i f i e d towards

sunrise (Fig.8 ( 3 ) The j e t then became unstable f i r s t in i n t e n s i f y i n g 2-0 waves and then in 3-D v o r t i c e s and f i n a l l y

( 4 ) Ground j e t s appeared a t about 1100 hrs disnersing the inversion by midda:,

Time-height Section of the Wind Field (m.sec:’)

10 Time

Fig.4. The wind f i e l d 22nd July 1976.

Data from tetroon t r i a d s .

We can hypothesise t h a t the t r a n s f e r of horizontal momentum from high t o low a l t i t u d e s could be the mechanism whereby the s t a t i c threshold of sand movement ( V , t ) i s f i r s t surpassed in the e a r l y morning. This would be a l o c a l i s e d process moving s a n d downwind t o accumulate as t h e beginnings of a new dune. Once one transverse dune h a d grown by trapping sand in the c l a s s i c Bagnoldian manner, K n o t t (1979) hypothesised from o t h e r findings (Fig. 9 ) t h a t another dune could not grow in the turbulent zone downwind: i t could only grow a t a c e r t a i n distance. A regularly spaced groun of transverse dunes would form i n t h i s way.

Answers t o some of the f a s c i n a t i n g questions raised by regional dune surveys, a n d t o some of the problems of i n t e r p r e t i n g ancient ergs can only come from s t u d i e s such as these a t the dune s c a l e . Symbols

.___

u

mean wind velocity

u ’ deviation of actual from mean wind v e l o c i t y .

V,t

threshold shear v e l o c i t y .

34 9

350

Vertical Profile of Divergence

0000 0900

1300

1900

3007

-E 200I

E 150-

.-0,

I”

100-

50-

Fig.6.

Divergence p a t t e r n s 1 1 t h August 1976.

Measurements w i t h t e t r o o n t r i a d s .

Velocity Traces at 10m height 22nd July, 1976 -

0930

31 TIME ( G M T )

TIME ( G M T )

F i g . 7 . V e l o c i t y t r a c e s a t 10 m. by eye.

S t r a i n - g u a g e measurements.

Bursts i d e n t i f i e d

351

Time Variation of Turbulence Intensity-22nd July,1976

0.3

-

0.2-

0.1-

0

06 08 10 12 14 16 18 20 Time(GMT) Fig.8.

1

Tiiiie v a r i a t i o n o f t u r b u l e n c e i n t e n s i t v .

F i o . 9 . Hynothesis o f f l o w round a b a r c h a n , u s i n n measurecients a t n o i n t shown.

352 REFERENCES A l l e n , J . R . L . , 1976. Computational models f o r dune t i m e - l a g : g e n e r a l i d e a s , d i f f i c u l t i e s and e a r l y r e s u l t s . Sedimentary G e o l . , 15: 1-53. Breed, C . S . , G r o l i e r , M . J . and McCauley, J . F . , 1979. Morohology and d i s t r i b u t i o n of common ' s a n d ' dunes on Mars: comparison w i t h E a r t h . J . Geoohys. Res., 8 4 : 81 83-9204. B r o o k f i e l d , M u r i e l , 1970. Dune t r e n d s and wind reqime i n c e n t r a l A u s t r a l i a . S e i t z . f b r Geomorph. S u p p l . 10: 121-53. F r y b e r g e r , S . G . , 1980. Dune forms and wind regime, M a u r i t a n i a , '*lestA f r i c a : i m p l i c a t i o n s f o r p a s t c l i m a t e . I n : 11. S a r n t h e i n , S c i b o l d , E . and Rognon, P . ( E d i t o r s ) , Sahara and Surrounding S e a s , Balkema, Rotterdam, 79-96. F r y b e r g e r , S . G . and Dean, G . , 1979. Dune forms and wind regime. I n : E . D . McKee ( E d i t o r ) , A Study of Global Sand S e a s , U.S. Geol. Survey P r o f . Paner 1052, 137-1 70. G r e e l e y , R . , 1979. S i l t - c l a y a g g r e g a t e s on Mars: a model f o r one f o r m a t i o n of ' s a n d ' . J . Geophys. R e s . , 8 4 : 6248-6254. Howard, A . D . , Morton, J . B . and Gad-el-Hak, PI., 1977. S i m u l a t i o n model of e r o s i o n and d e p o s i t i o n on a barchan dune. R e p t . NASA, CR-2838 C o n t r a c t NGR-47-005172, 82 p p . Howard, A . D . , Morton, J . B . , Gad-el-Hak, PI. & P i e r c e , D . B . , 1978. Sand t r a n s p o r t model o f barchan dune e q u i l i b r i u m . Sedimentology, 25: 307-338. Hsu, S.A., 1973. Computing e o l i a n sand t r a n s p o r t from s h e a r v e l o c i t y measurements. J . G e o l . , 81: 739-743. Knott, P . , 1979. The s t r u c t u r e and p a t t e r n of dune-forming winds. Unpubl. P h . D . T h e s i s , Univ. o f London, 2 v o l s . , 403 and 249 p p . Knott, P . and Narren, A . , 1980. Aeolian P r o c e s s e s . I n : A. Goudie, e t a l . ( E d i t o r ) , Geomorphological Techniques, George A l l e n & U n w i n , London, 226-246. L e t t a u , Katharina and Hans, unpub. Experimental and m i c r o m e t e r o l o g i c a l f i e l d s t u d i e s o f dune m i g r a t i o n . Chap.IX i n Univ. o f !Jisconsin Report on t h e C l i m a t e o f Coastal P e r u . Mai?guet, M . and C a l l o t , Y . , 1978. L'Erg de Fachi-Bilma ( T c h a d - N i g e r ) . C o n t r i b u t i o n a l a c o n n a i s s a n c e de l a dynamique des e r g s e t d e s dunes des zones a r i d e s chaudes. Me'm. e t Documents, 18: S e r v . de documentation e t de c a r t o g . geog. Ed. CNRS, P a r i s , 184 PD. McKee, E . D . ( E d i t o r ) , 1979. A s t u d y of g l o b a l sand s e a s . U . S . Geol. Survey Prof. Paper 1052: 429 p p . Dffen, G . R . and K l i n e , S . J . , 1975. A proposed model of t h e b u r s t i n g p r o c e s s i n t u r b u l e n t boundary 1a y e r s . J . F1 u i d Mech . , 70: 209-228. Sagan, C . and Bagnold, R . A . , 1975. F l u i d t r a n s o o r t on e a r t h and a e o l i a n t r a n s p o r t on Mars. I c a r u s , 26: 209-218. Schumm, S.A. and L i c h t y , R . W . , 1964. Time, s p a c e and c a u s a l i t y i n geomorphology. Amer. J . S c i . 263, 2: 110-119. T s o a r , H., 1978. The dynamics o f l o n g i t u d i n a l dunes. Dept. of Geog. Ben Gurion U n i v . B e ' e r Sheva, I s r a e l , U.S. Army Grant no. DA-ERD-76-9-072, 171 p p . T s o a r , H . , G r e e l e y , R . and P e t e r f r e u n d , A . R . , 1979. Mars: the North P o l a r Sand Sea and r e l a t e d wind p a t t e r n s . J . Geophys. Res., 84: 8167-8180. Warren, A . , 1970. Dune t r e n d s and t h e i r i m p l i c a t i o n s i n the c e n t r a l Sudan. Z e i t s c h r . f b r Geomorphologic Suppl. 1 0 , 154-179. I l a r r e n , A . , 1976a. Morphology and Sediments o f the Nebraska Sand H i l l s i n r e l a t i o n t o P l e i s t o c e n e Winds and the development o f a e o l i a n bedforms. J . G e o l . , 84: 685-700. Warren, A . , 1976b. Dune t r e n d and the Ekman s p i r a l . Nature ( L o n d . ) , 259: 653-654. Idilson, I . G . , 1972. Aeolian bedforms - t h e i r development and o r i g i n . Sedimentology, 19: 173-210. Wipperman, F., 1969 The o r i e n t a t i o n o f v o r t i c e s due t o i n s t a b i l i t y o f t h e Ekmanboundary 1a y e r . B e i t r a g e z u r Physik d e r Atmosph2re, 42: 225-244.

353

SAND SEAS OF THE SAHARA AND SAHEL: AN EXPLANATION OF THEIR THICKNESS AND SAND DUNE TYPE BY THE SAND BUDGET PRINCIPLE M. MAINGUET and f1.-C.

CHEPIIN, L a b o r a t o i r e de Geoqraphie Physique Zonale,

U n i v e r s i t e de Reims, Reims, France TNTRWJCTION U n t i l now, t h e d e f i n i t i o n o f sand seas has been d e s c r i p t i v e and has taken account o n l y o f t h e sand d e p o s i t as t h e s t a t i c component (Capot-Key, 1970, PlcKee, 1979) w h i l e n e g l e c t i n g t h e dynamic component r e p r e s e n t e d by t h e sandc a r r y i n q winds which i r a v e r s e t h e sand seas. The two dynamic a e o l i a n parameters, t r a n s p o r t a t i o n and d e p o s i t i o n , combine t o determine t h e sand sea sand budqet. The i n t e r a c t i o n between sand i m p o r t and sand e x p o r t w i t h i n t h e sand sea determines t h e t h i c k n e s s o f t h e sand c o v e r and t h e t y p e o f dunes formed. A f t e r d e f i n i n g t h e i d e a o f sand budqet, and dunes formed by e r o s i o n and depo s i t i o n , t h e s e d e f i n i t i o n s w i l l be used i n a model which i s a p p l i e d t o t h e Sahara system. The r e s u l t s w i l l e x p l a i n and demonstrate t h e reasons f o r t h e r a r i ' o f sand i n t h e h y p e r a r i d Sahara and t h e v a r i a t i o n s i n t h e t h i c k n e s s o f t h e sand covers. THE SAIJD SEA SAND BUDGET Mainguet and C a l l o t (1978) have demonstrated by means o f t h e p r i n c i p l e o f loac s u b s t i t u t i o n f o r l a r g e systems ( B r o o k f i e l d , 1970, Hainguet e t a l . ,

1980, Wilson,

1973), u s i n g t h e "Grand Erg de Fachi B i l m a " as an example, t h a t t h e same winds a r e a b l e t o d e p o s i t and remove t h e sand. When t h e i n p u t o f sand i s g r e a t e r t h a n t h e o u t p u t , t h e sand budget i s p o s i t i v i and t h e sand sea t h i c k n e s ; i t s s u r f a c e i s moulded i n t o dunes o f d e p o s i t i o n c o a l e s c i n g barchans, barchan c h a i n s o r t r a n s v e r s e c h a i n s . T h i s behaviour can be seen i n t h e s a h e l i a n sand sea o f N i g e r where t h e sand c o v e r , moulded i n t o l a r g e and small sand h i l l s , reaches a t h i c k n e s s o f 60 metres. The sand budget i s p o s i t i v e , even though t h e r e a r e l o c a l l y conspicuous d e f l a t i o n s t r e a k s i n d i c a t i n g t h e b e g i n n i n g o f a n e t l o s s o f sand. When t h e o u t p u t o f sand i s g r e a t e r t h a n t h e i n p u t , t h e sand budget i s n e g a t i v e and t h e sand sea t h i n s ; d e f l a t i o n c o r r i d o r s w i t h l e v e l l e d s u r f a c e s appear, which a r e dunes o f e r o s i o n , o r sand seas o f t h e sand r i d g e t y p e . The b e s t examples o f t h i s t y p e o f sand sea, a l m o s t f r e e o f sand, a r e t h e " E r g s I g u i d i and Chech".

3 54 There, t h e c o r r i d o r s a r e t e n t i m e s as wide as t h e dunes w h i c h s e p a r a t e them. A c o n t i n u a l n e g a t i v e sand budget can l e a d t o t h e disappearance o f sand r i d g e s and t h i s o c c u r s when t h e d e f l a t i o n c o r r i d o r s merge. Under t h e s e c o n d i t i o n s , t h e sand sea i s reduced t o a l a y e r o f c o a r s e sand, which i s a winnowed residue,as t h e o l d d e f l a t i o n c o r r i d o r s t e n d t o c u t down i n t o bedrock. A good example o f t h i s i s t h e n o r t h e a s t e r n p a r t o f t h e Tenere. APPLICATION OF THESE I D E A S TO THE SAHARAN SYSTEM U n t i l now, i t was t h o u g h t t h a t t h i c k a e o l i a n sand d e p o s i t s were an i n d i c a t i o n

of a r i d i t y (Capot-Rey,

1970, Monod, 1958, T r i c a r t and C a i l l e u x , 1964). The obser-

v a t i o n o f s c a r c i t y o f sand i n t h e Sahara i s c o v e r e d w i t h sand sheets

-

-

where o n l y 20 p e r c e n t o f t h e surface

i s i n c o n t r a s t t o t h e c o n t i n u o u s GO m e t r e t h i c k

sand c o v e r o f t h e s o u t h e r n s a h e l i a n margins. These f e a t u r e s l e d us t o c a t e g o r i c a l l y r e j e c t t h e commonly accepted concept t h a t t h e sand c o v e r t h i c k n e s s i n d i c a t e s , i n a r i d r e g i o n s , a r i d i t y i n p a s t t i m e s (Hainguet, 1982). An a t t e m p t w i l l be made t o demonstrate t h a t t h e h e t e r o q e n e i t y o f sand accumu l a t i o n i n t h e Sahara and i t s m a r g i n s must be c o n s i d e r e d i n i t s e n t i r e t y f o r t h e best palaeoclimatic i n t e r p r e t a t i o n o f aeolian deposits. The two Saharan margins, t h e n o r t h e r n subhumid and t h e s o u t h e r n s a h e l i a n , do n o t e x h i b i t t h e same b e h a v i o u r i n r e g a r d t o a e o l i a n phenomena. These behavioural d i f f e r e n c e s r e s u l t , i n s p i t e o f a NE-SW u n i t y i n wind c i r c u l a t i o n , f r o m a palaeod i f f e r e n c e : i n t h e n o r t h Sahara Q u a t e r n a r y ; i n t h e s o u t h Sahara

-

more permanent humid c o n d i t i o n s d u r i n g t h e more a r i d c o n d i t i o n s . I n s u p p o r t o f t h i s ,

Dresch (1982) demonstrated t h a t , d u r i n g t h e Q u a t e r n a r y , t h e n o r t h e r n m a r g i n o f t h e Sahara d i d n o t have an a r i d o r h y p e r a r i d c l i m a t e , and t h a t t h e Maghreb and t h e A t l a s Mountains s t a y e d s e m i a r i d w i t h a f u n c t i o n i n g h y d r o g r a p h i c network whose widespread ephemeral streams p r o v i d e d sand and s i l t t o be m o b i l i z e d by t h e wind. The l a r g e Western and E a s t e r n sand seas, which w i l l be c a l l e d "Mediterranean Sand Seas", s t i l l b e n e f i t f r o m t h i s supply, as i s t e s t i f i e d by t h e i r dune types ( f i g . l ) , The bouquets o f l i n e a r dunes and t h e barchan c h a i n s which f o r m t h e n o r t h e r n t w o t h i r d s o f t h e "Great Western Erg", a s w e l l as t h e barchan c h a i n s and s t a r dunes o f t h e n o r t h e r n two t h i r d s o f t h e "Great E a s t e r n t r g " , i n d i c a t e sand i n p u t i s g r e a t e r t h a n sand o u t p u t . I n t h e s o u t h e r n Sahara ( f i g . 2), t h e o p p o s i t e s i t u a t i o n p r e v a i l s . The area s u p p l y i n g sand f r o m upwind i s t h e Sahara i t s e l f , where t h e p e r e n n i a l l y a r i d c l i m a t e reduce t h e e f f e c t i v e n e s s o f t h e source. A l a r q e p a r t o f t h e sand e x p o r t ed f r o m t h e Sahara i s t r a p p e d i n t h e Sahel r e g i o n , w h i l e some i s blown i n t o t h e A t l a n t i c Ocean. Under t h e p r e s e n t c l i m a t i c d e t e r i o r a t i o n , t h e e x p o r t f r o m t h e Sahel r e g i o n i s i n c r e a s i n g because o f a d e c r e a s i n g e f f e c t i v e n e s s o f t h e f r a g i l e

355 v e g e t a t i o n cover.

+ .South A t l a s Mediterranean ergs

3-- ;I.

High group n o r t h o f t h e c e n t r a l Sahara

3. H i g h group : T a s s i l i , Haggar, Tademait, l i m i t t h e passage o f sand between t h e e x p o r t i n g m e d i t . e r g s and t h e h y p e r a r i d Sahara.

--

H y p e r a r i d Sahara

2. Sand accumulations o f t h e mediterranean ergs 2a. N o r t h e r n area : p o s i t i v e sand budget 2b. Southern area : n e g a t i v e sand budget

4. Very d e f i c i e n t h y p e r a r i d s e c t o r .

FIG. 1. SOURCE AND SAND DEPOSITS : NORTH SAHARA MODEL

H y p e r a r i d Sahara

. A r i d Sahara

121

1-2. D e f i c i e n t areas p r o v i d i n g sand

++

.Sahel i a n e r g s

,3. Area o f e f f e c t i v e a c c u m u l a t i o n i n t h e vegetated saharo-sahelian and s a h e l i a n a r e a s

.Sahel i a n - s o u d a n i a n zone

41

4. E x p o r t area below t h e Sahel

.FIG. 2. SOURCE AND SAND DEPOSITS : SOUTH SAHARA MODEL

356 VERIFICATION OF THE SAHARAN MODEL OF SAND DISTRIBUTION A.Sand d i s t r i b u t i o n i n t h e Sahara and on i t s s a h e l i a n m a r g i n s From Meteosat s a t e l l i t e images, i t i s g e n e r a l l y p o s s i b l e t o a n a l y s e t h e d i s t r i b u t i o n of sand a c c o r d i n g t o ground r e f l e c t a n c e ( f i g . 3 ) . One must, however, be c a r e f u l s i n c e t h e d i f f e r e n c e s i n r e f l e c t a n c e can i n d i c a t e : a ) d i f f e r e n c e s i n t h i c k n e s s o f t h e sand c o v e r ; b ) v a r i o u s degrees o f i n t e n s i t y o f a e o l i a n a c t i v i t y ; c ) d i f f e r e n c e s i n t h e d e n s i t y and n a t u r e o f t h e v e g e t a t i o n . F o r example, t h e n o r t h e a s t e r n p a r t o f t h e Tenere, where t h e sand c o v e r i s t h i n o r l o c a l l y none x i s t a n t , i s more r e f l e c t i v e t h a n t h e v e r y t h i c k Haoussa sand sea. The reason i s t h a t t h e n o r t h e a s t e r n Tenere i s b a r r e n and t r a v e r s e d by v e r y a c t i v e sand-laden winds, w h i l e t h e Haoussa sand sea i s v e g e t a t e d and i n f l u e n c e d by winds w i t h very l i t t l e s a n d - c a r r y i n g c a p a c i t y . Thus, s a t e l l i t e imaqes a l o n e a r e i n s u f f i c i e n t f o r p r o p e r i n t e r p r e t a t i o n and a d d i t i o n a l knowledge must be drawn on. ( i ) The l a c k o f sand i n t h e c e n t r a l Sahara Again, based on t h e s a t e l l i t e images, t h e h y p e r a r i d Sahara shows few sand seas w i t h dunes. O n l y 2 p e r c e n t o f i t s a r e a has t h i c k d e p o s i t s which a r e : t h e c e n t r e o f t h e "Mourzouk Erg", t h e " B r u s s e t and B r e a r d Ergs", and a band t h a t s t r e t c h e s f r o m t h e southwestern f o o t o f T i b e s t i , n o r t h o f T e r m i t , t h r o u g h t h e "Bilma Erg" and t h e s o u t h e r n Tenere. These t h i c k d e p o s i t s a l l have some l o c a l e x p l a n a t i o n : p r o t e c t i o n a g a i n s t sand e x p o r t by topography i n t h e case o f t h e "Mourzouk Erg"; t h e e f f e c t o f t h e o b s t a c l e o f t h e A 9 r f o r t h e " B r u s s e t and B r e a r d Ergs"; and t h e c o n v e r g e n t e f f e c t s downwind o f T i b e s t i f o r t h e B i l m a Tenere band. On t h e o t h e r hand, a l m o s t s a n d - f r e e seas, where t h e sand dunes a r e separated by l a r g e i n t e r d u n e s , occupy

15 p e r c e n t o f t h e Sahara. These i n d i c a t e a v e r y

n e g a t i v e sand budget: examples a r e t h e G r e a t Sand Sea, t h e Calansho Sand Sea, t h e Rebiana Sand Sea, t h e "Erg Chech" and t h e "Erg I g u i d i " ( f i g s . 4,5). ( i i ) The s o u t h e r n p a r t o f t h e Sahara (medium sand c o v e r ) T h i s r e g i o n has a t h i c k e r sand c o v e r t h a n t h e c e n t r a l Sahara. The sand sea topography c o n s i s t s o f r e c e n t sand r i d g e s which a r e l a r g e r t h a n t h e i n t e r d u n e c o r r i d o r s between them, i n d i c a t i n g a l e s s n e g a t i v e sand budget t h a n t h e c e n t r a l Saharan sand seas. T h i s s o u t h e r n p a r t o f t h e Sahara i s i n t e r m e d i a t e i n c h a r a c t e r between t h e c e n t r a l Sahara, d e f i c i e n t i n sand, and t h e f i e l d s o f d e n s e l y vegetated dunes o f t h e Sahel: i t forms a b e l t c o n t a i n i n g p o c k e t s whose l e n g t h reaches f r o m 3O0E.to 26OW. These p o c k e t s a r e formed by t h e m a s s i f s o f Ennedi, A f r , Adrar o f I f o g h a s , A d r a r o f C h i n g u e t t i and Tagant. The sand seas o f t h i s b e l t a r e , from e a s t t o west: t h e Sudan ghoz, e a s t o f Ennedi; t h e Chad sand sea, between t h e Mourdi d e p r e s s i o n and Lake Chad; t h e s o u t h e r n p a r t o f t h e B i l m a Erg and t h e Tenere; t h e sand seas o f T a l a k and Azaouak a t M a l i ; t h e M a k t e i r , t h e I j a f e n e ,

E

L aJ '3

2

357

.. .. *. .. .. .. .. .. ..

.r LL

c;,

m

~ ~ m e m l ~ h w r n

m L W

3 Y m

..

3 58

F i g . 4. G r o w t h i n " v e n t i l a t i o n " o f an e r g w i t h e n l a r q e m e n t o f i n t e r d u n e c o r r i d o r s . T h i s L a n d s a t 1 i n a q e o f 1 6 t h tdoveriber, 1972 ( s c a l e a b o u t 1:1,000,000) c o v e r s t h e n o r t h e r n p a r t o f t h e " E r g Chech". T h i s e r q , f o r m e d e n t i r e l y o f sand r i d q e s has a n e g a t i v e sand b u d q e t . On t h e image, t h e r a t i o o f r i d g e w i d t h s (Cd) t o i n t e r d u n e c o r r i d o r w i d t h s ( C l ) decreases from west t o east. I n s e c t i o n A , t h e e m p t y i n g r a t i o ( E = C l / C d ) i s f r o m 1 t o 0.5; i n s e c t i o n B, f r o m 0 . 4 t o 0.2. T h i s d e c r e a s e i n t h e r a t i o c o n f i r m s t h e i n c r e a s e i n sand e x p o r t i n t h e e r g f r o m w e s t t o e a s t , a s we p a s s away f r o m t h e s h e l t e r i n g ,?nd w i n d shadow a r e a o f t h e m a s s i f t o t h e n o r t h w e s t . t h e Ouarane, t h e Amoukrouz, t h e A z e f a l a n d t h e A k c h a r i n M a u r i t a n i a . ( i i i ) The t h i c k l y c o v e r e d s o u t h e r n s a h e l i a n r e g i o n o f t h e Sahara These r e g i o n s c o n s i s t o f b a r c h a n c h a i n s , t r a n s v e r s e r i d g e s and h i l l s o f sand

359

F i g . 5. S a n d r i d g e s and i n t e r d u n e c o r r i d o r s on a n a e r i a l p h o t o q r a p h o f " E r q Chech". T h i s a e r i a l p h o t o q r a p h ( s c a l e 1:50,000) shows t h e s a n d r i d n e s and i n t e r d u n e c o r r i d o r s o f f i g u r e 4 i n more d e t a i l . It s h o m : t h e a s y m m e t r i c f o r m o f t h e r i d g e s , more r e c t i l i n e a r o n t h e e a s t t h a n o n t h e w e s t , and t h e i r r e q u l a r s u r f a c e morphology. The r i d q e s c o n s i s t o f e x t r e m e l y s i n u o u s l i n e a r dunes, t e r m i n a t i n g a r o u n d d e p r e s s i o n s c a l l e d " q h o r r a f a s " . The i n t e r d u n e c o r r i d o r s a r e d e s t i t u t e o f sand, b u t r o u g h n e v e r t h e l e s s . The s t r i k i n g s c a r c i t y o f sand i n t h e sand sea i s shown b e t t e r t h a n on t h e s a t e l l i t e image. The l o n g i t u d i n a l sand r i d g e s have a maximum w i d t h o f 500 m e t r e s , w h i l e t h e i n t e r d u n e a r e a s r e a c h a w i d t h o f 3500 m e t r e s . and t h e i r sand seas have a p o s i t i v e sand b u d g e t . The b e s t examples a r e : t h e llreyye, t h e Aoukar, t h e M a u r i t a n i a n p a r t o f t h e A f t o u t - e s - S a h e l i

sand seas and

t h e Haoussa sand sea (Grove, 1 9 5 8 ) . The l a s t , w h i c h i s v e r y t h i c k , i s p a r t i a l l y i n t e r r u p t e d upwind

o f t h e A d e r D o u t c h i f o r l a c k o f a sand s u p p l y : downwind t h e

edifices

chanoid sand e d i f i c e s

Yardangs

Sandy v a l l e y s

0Exposed bedrock

a

I--]

!'

!

.

2

,, L.

: , I

.. -.-,u.w.

361 B. Reasons f o r t h e s c a r c i t y o f sand i n t h e Sahara

( i ) A d e f i c i e n t supply

1) Lack o f l o c a l supply. The Sahara i s an o l d d e s e r t , p r o b a b l y d a t i n q from t h e Miocene. Sarntheim (1978) demonstrated t h a t t h e f i r s t a e o l i a n sands o f Saharan o r i g i n were d e p o s i t e d i n t h e A t l a n t i c Ocean d u r i n g t h e Oliqocene. F o r t h e Miocene, seasonal a r i d i t y i s documented by Jaeger (1975), and r e a l a r i d i t y a t t h e end o f t h e Miocene by Maley (1980). The p r e v a i l i n g a r i d i t y l i m i t s t h e supply o f sediment, s i n c e t h e p o t e n t i a l sand sources a r e a l l used up. The s o i l cover has been eroded: f l u v i a t i l e d e p o s i t s have had t h e i r s a n d - s i l t - c l a y f r a c t i o n s winnowed away: l i b e r a t i o n o f sand by c o r r a s i o n i s h i n d e r e d by d e s e r t v a r n i s h which s t r e n q t h e n s t h e r o c k . The s c a r c i t y o f sand i n t h e L i b y a n hamadas, between t h e G r e a t E a s t e r n Erq and t h e Calansho Sand Sea, can be e x p l a i n e d by t h e n a t u r e o f t h e s e c a l c a r e o u s hamadas l a c k i n g i n q u a r t z .

2 ) Shortage o f e x t e r n a l supply. The p r e s e n t l y s e m i - a r i d r e g i o n s o f t h e Maghreb have known l e s s a r i d p e r i o d s , w i t h t h e f o r m a t i o n o f ephemeral stream d e p o s i t s , as Dresch (1982) has c l e a r l y shown. However, t h i s sediment source can n o t be t r a n s p o r t e d by t h e w i n d a c r o s s t h e mountainous b a r r i e r s o f t h e n o r t h e r n Sahara: t h e winds a r r i v e a t t h e n o r t h e r n e r g s f r e e o f sand. ( i i ) Dynamic causes f o r t h e absence o f d e p o s i t s

1) The s h e l t e r e f f e c t . The topography o f t h e a r i d and h y p e r a r i d Sahara i s n o t f a v o u r a b l e f o r d e p o s i t i o n because o f b a r r i e r s . The f u s e d p l a t e a u x o f T a d e m a i t - T a s s i l i - H o g g a r i s a s u f f i c i e n t b a r r i e r t o sand movement t o f o r m t h e T a n z e r o u f t and i t s e x t e n s i o n s t o t h e n o r t h and south, where absence o f sand i s t h e r u l e ( f i g . 7 ) . L i k e w i s e , t h e Sahara Saraoui i s sand f r e e downwind o f t h e n o r t h Sahara A t l a s c h a i n . 2 ) Topographic i n c r e a s e i n wind v e l o c i t y . The h i g h e r t h e wind v e l o c i t y , t h e g r e a t e r t h e c a p a c i t y f o r sand t r a n s p o r t a t i o n . Topographic o b s t a c l e s produce wind a c c e l e r a t i o n e f f e c t s which can be f u r t h e r i n c r e a s e d when t h e s e o b s t a c l e s form c o r r i d o r s . Whenever t h e wind v e l o c i t y i n c r e a s e s , t h e wind power increases, as i n d i c a t e d by s m a l l e r d i s t a n c e s between t h e s t r e a m l i n e s . llhere t h e w i n d v e l o c i t i e s a r e h i g h , sand t r a n s p o r t i s t h e main a e o l i a n process. Where t h e w i n d v e l o c i t i e s a r e l o w , t h e sand c a r r y i n g c a p a c i t y decreases and d e p o s i t i o n occurs. Both o f t h e s e e f f e c t s a r e produced by n a t u r a l as w e l l as by man-made o b s t a c l e s , such as v i l l a g e s , oases, e t c . ( i i i ) The i n c r e a s e i n sand e x p o r t under p r e s e n t dynamic c o n d i t i o n s Over t h e l a s t t w e n t y y e a r s , t h e a r i d i t y has i n c r e a s e d t h e e f f i c i e n c y o f sand t r a n s p o r t by t h e winds. The s p e c i a l topography o f t h e r e g i o n has i n c r e a s e d t h e e x p o r t o f sand f r o m t h e Sahara and a c c e l e r a t e d t h e process o f emptying o f sand from t h e c e n t r a l Sahara ergs, w h i l e moving t h e e x p o r t l i m i t i n g l i n e towards t h e south. As a r e s u l t , t h e s a h e l i a n zone i s becoming an e x p o r t s i t e , w i t h t h e l o c a l

362

m , . R o c k massif responsable f o r shelter effect

El

131*Clouds

n

.She1 t e r w i t h sand t r a n s p o r t

Limit of efficient 'shel t e r ='.Perfect

.Imperfect shel t e r

shel t e r

0.Sand v e i l =.Sand

cover

P I G . 7. SHELTER EFFECT OF THE COMBINED TASSILI N'AJJER-HOGGAR-TADEMAIT PLATEAUX FROM 1:7.000.000 METEOSAT SATELLITE IMAGE

363 development of d e f l a t i o n s t r i a t i o n s . A l l q r a i n s i z e s l e s s than 400 microns are removed by t h e wind, as i s observed i n Niger. North of Lake Chad, where t h e dunes a r e t r a n s v e r s e chains, these l o n q i t u d i n a l s t r i a t i o n s a r e superimposed, g i v i n g a chequered p a t t e r n t o t h e sand sea surface. I n t h e t h i c k sand sheets of Niger, p a r t i c u l a r l y i n t h e vegetated Haoussa erq, west o f Lake Chad, t h e sand l o s e s cohesion and d e f l a t i o n s t r i a t i o n s appear as soon as t h e v e q e t a t i o n cover i s degraded. S a t e l l i t e images can be used t o c o n f i r m these s t r i a t i o n s up t o 10' N., v e r i f y i n g t h a t t h i s area i s t e n d i n q towards a negative sand budget. CONCLUSIONS The d i f f e r e n c e i n behaviour between t h e h y p e r a r i d r e q i o n s o f t h e Sahara, dominated by surfaces underqoing s t r o n g d e f l a t i o n , hence areas o f e x p o r t o f sediment, and t h e semi-arid margins, which a r e surfaces o f d e c c e l e r a t i o n , hence

o f d e p o s i t i o n and accumulation, i s a f a c t which can n o t be neglected i n a e o l i a n sedimentology. It e x p l a i n s n o t o n l y t h e p r o f u s i o n o f a e o l i a n deposits i n t h e semi-arid regions, b u t a l s o t h e p r o f u s i o n o f loess, whose a e o l i a n o r i g i n i n deserts i s u s u a l l y accepted. REF E RENC ES

B r o o k f i e l d , M., 1970. Dune t r e n d s and wind regime i n c e n t r a l A u s t r a l i a . Z. Geomorph. Suppl., 10: 121-158. Capot-Rey, R., 1970. Remarques sur l e s ergs du Sahara. Ann. Gebqr. P a r i s , 431: 2-19. Dresch, J . , 1982. Sur l a semi-aridit; du Maghreb au P l i o - Q u a t e r n a i r e . B u l l . Assoc. Ge'ogr France, 483-4 : 42-45. Grove, A.T., 1958. The a n c i e n t e r g o f Hausaland and s i m i l a r formations on t h e south s i d e o f t h e Sahara. Geogr. Jour., 124: 528-533. Jaeger, J.J., 1975. Les rongeurs du Mioc'ene moyen e t s u p g r i e u r du Maqhreb. Th'ese Sci. M o n t p e l l i e r , fasc. 1, 164pp. Maley, J . , 1980. Les changements c l i m a t i q u e s de l a f i n du T e r t i a r e en A f r i q u e : l e u r consgquence s u r l ' a p p a r i t i o n du Sahara e t sa ve'q6tation. I n : A.J. W i l l i a m s and H. Faure ( e d i t o r s ) , The Sahara and t h e N i l e . A.A. Balkema, Rotterdam, pp. 63-86. Mainguet, M . , 1982a. Les dunes d ' 6 r o s i o n : s i g n i f i c a t i o n morphodynamique e t c l i m a t i q u e de l e u r e x i s t e n c e . Wllrzb. geogr. Arb., 56: 79-92. Mainguet, M., 1982b. L ' g p a i s s e u r des d6pats sableux g o l i e n s e s t - e l l e un i n d i c a t e u r d ' a r i d i t e ? L ' a r i d i t 6 sahareinne. B u l l . Assoc. Ge'ogr. France, 483-4: 64-67. Mainguet, M. and C a l l o t , Y., 1978. L ' e r g de Fachi Bilma (Tchad-Niger). C o n t r i b u t i o n 'a l a connaissance de l a dynamique des ergs e t des dunes des zones a r i d e s chaudes. Mem. e t Doc. , C.N.R.S. Paris, 19, 184 pp. Mainguet, M., Cossus, L. and Chapelle, A.M., 1980. U t i l i s a t i o n des imaqes M6te'os a t pour p r k i s e r l e s t r a j e c t o i r e s eoliennes au s o l , au Sahara e t sur l e s marges sah6liennes. SOC. Photogramm. e t T e l g d i c t . , 78: 1-12. McKee, E.D.(editor), 1979. A s t u d y o f Global Sand Seas. U.S. Geol. Surv., Prof. Paper, 1052, 429 pp. Monod, Th., 1958. Majabat-al-Koubra. C o n t r i b u t i o n 'a 1 'Ctude de "1 'empty q u a r t e r " ouest-saharien. M6m. I.F.A.N., Dakar, 406 pp, 81 p l a t e s . Sarntheim, M., 1978. Sand d e s e r t s d u r i n g g l a c i a l maximum and c l i m a t i c minimum. Nature, 272: 868-890. T r i c a r t , J . and C a i l l e u x , A., 1964. Le model6 des re'gions s'eches. Les Cours de l a Sorbonne, C.D.U., Paris, 2 v o l s . , 129 pp. and 179 pp. (paper t r a n s l a t e d by M.E. B r o o k f i e l d )

.

This Page Intentionally Left Blank

365

PIODERH EOLIAN DEPOSITS OF THE EASTERli P R O V I N C E

OF SAUDI A R A B I A

DANILO ANTON: Sand Research Program, U n i v e r s i t y o f Petroleum and M i n e r a l s , Dharan, Saudi A r a b i a . REGIONAL SETTING AND GEOLOGICAL EVOLUTION The E a s t e r n P r o v i n c e o f Saudi A r a b i a o c c u p i e s t h e e a s t e r n t h i r d o f t h e c o u n t r y , e x t e n d i n g a b o u t 1200 Km. f r o m South Yemen and Oman i n t h e south, t o I r a q and Kuwait i n t h e n o r t h , w i t h an a r e a o f n e a r l y 0.5 Km.

2

(fiq. 1).

The s o u t h e r n p a r t o f t h e p r o v i n c e i n c l u d e s t h e empty sand f i e l d s o f t h e e a s t e r n R u b ' a l K h a l i . The c e n t r a l and n o r t h e r n p a r t s a l o n g t h e A r a b i a n G u l f a r e more v a r i e d , i n c l u d i n g t h e J a f u r a h and Dahna sand f i e l d s , a l l u v i a l v a l l e y s and fans and an e x t e n s i v e sedimentary p l a t e a u . I n t h i s a r e a a r e l o c a t e d most o f t h e towns and t h e b u l k o f economic a c t i v i t y .

I

J..\

,

- /

Riyadh'

240

/"

/

Lat N

I.

&o

400

42O

44O

4S0

40O

SC?

Indian

52O

54O

Long E

F i g . 1 . L o c a t i o n o f Recent dune f i e l d s i n t h e A r a b i a n P e n i n s u l a

Ocean

56O

1

J

58O

366 The area i s u n d e r l a i n by s h e l f sedimentary r o c k s which d i p eastwards o f f t h e Arabian s h i e l d . D u r i n q t h e T e r t i a r y , t h e western edge o f t h e s h i e l d r o s e and t h e e a s t e r n p a r t subsided. I n t h e i n t e r i o r ( c e n t r a l ) homocl i n e , a r e a s which underwent g r e a t e r subsidence accumulated t h i c k e r sedimentary sequences f o r m i n g t h e main sedimentary basins; t h e Rub'al K h a l i , S i r h a n - T u r a y f ,

Northern Arabian

G u l f , and Dibba b a s i n s . The f i r s t w e l l - d e f i n e d T e r t i a r y c o n t i n e n t a l d e p o s i t s a r e o f t h e ? e a r l y Pliocene Iiadrukh Formation which u n d e r l i e s , w i t h d e p o s i t i o n a l c o n t i n u i t y , t h e Pliddle Miocene Dam Formation. The Hadrukh F o r m a t i o n i s m a r i n e a l o n g t h e e a s t e r n edge, near t h e G u l f , and c o n t i n e n t a l elsewhere. I t c o n s i s t s m o s t l y o f m a r l y sandstones, sandy c l a y s and sandy l i m e s t o n e s w i t h c l a y and c h e r t and l e s s f r e q u e n t l y gypsum, s u q q e s t i n q a l l u v i a l and l a c u s t r i n e environments o f a s e m i - a r i d c l i m a t e . Some e o l i a n a c t i v i t y may have occured because, thouqh no e o l i a n s t r u c t u r e s a r e found a t o u t c r o p , P.L. V i n c e n t ( p e r s . comm.,

1982) found f r o s t e d , well-rounded e o l i a n

sand g r a i n s i n s e v e r a l l a y e r s i n and above t h e Hadrukh Formation s u q q e s t i n q a r e l a t e d e o l i a n source. Younqer l a c u s t r i n e and a l l u v i a l d e p o s i t s o f t h e H o f u f Formation o v e r l i e t h e Dam Formation. The t y p e s e c t i o n o f t h i s u n i t (Powers e t al.,

1966) s t a r t s w i t h a basal 20 m e t r e

t h i c k conglomerate i n c l u d i n q b o u l d e r s

and pebbles o f l i m e s t o n e i n a q u a r t z m a t r i x , i n d i c a t i n q an a l l u v i a l environment under s e m i - a r i d c o n d i t i o n s . Above t h i s a r e t h i c k a r g i l l a c e o u s sandstones. A second c l e a r l y i d e n t i f i e d a l l u v i a l e p i s o d e i s seen i n t h e upper p a r t o f t h e type s e c t i o n where a l i m e s t o n e p e b b l e conglomerate i s exposed. Only m i n o r e o l i a n sediments o c c u r i n t h e H o f u f Formation, s u g q e s t i n q t h a t t h e r e was l i t t l e e o l i a n a c t i v i t y d u r i n q i t s d e p o s i t i o n i n Plio-Pliocene t i m e s . Larqe a l l u v i a l fans were formed i n Pliocene-Ouaternary t i m e s and p r o v i d e d a source o f m a t e r i a l f o r l a t e r r e w o r k i n q by t h e wind. The two p r i n c i p a l f a n s which reached t h e e a s t e r n p a r t o f t h e p e n i n s u l a a r e Cladi Dawasir and Wadi Sahba. The Dawasir western f a n formed i m m e d i a t e l y downstream o f t h e qorqe near S u l a y y i l . Eastwards, a l l u v i a l s e d i m e n t a t i o n t o o k p l a c e i n a f l a t wide area down t o t h e p r e s e n t sabkha M a t t i i n t h e U n i t e d Arab E m i r a t e s c o a s t l i n e o f t h e G u l f . Wadi Sahba a l l u v i a l fan e x t e n d s e a s t o f t h e Haradh r e g i o n , beyond t h e l a s t h i l l s o f t h e e a s t e r n s e d i m e n t a r y p l a t e a u , d i p p i n g eastwards under t h e e o l i a n d e p o s i t s o f t h e J a f u r a h sand f i e l d s . The e o l i a n d e p o s i t s i n t h e E a s t e r n P r o v i n c e ( J a f u r a h , Dahna and Rub'al K h a l i ) a r e always f o u n d o v e r l y i n q t h e s e a l l u v i a l sediments.

2. 2.1

EOLIAN SAliD FIELDS AND DEPOSITS J a f u r a h sand f i e l d s The J a f u r a h sand f i e l d s a r e l o c a t e d between 27'

49'

and 24'

South, e a s t o f t h e

30' East m e r i d i a n , i n t h e c o a s t a l l o w l a n d s below t h e 200 m e t r e c o n t o u r l i n e

a l o n g t h e A r a b i a n G u l f l i t t o r a l . They a r e a n a r r o w band i n t h e n o r t h t h a t widens southward t o merge w i t h t h e Rub'al K h a l i sand f i e l d s .

367 I n s p i t e o f a r e l a t i v e l y h i g h humidity, associated w i t h the p r o x i m i t y o f the Gulf, and a r a i n f a l l o f n e a r l y 80 mm./year on t h e n o r t h e r n edge, t h e morphodynamics o f t h e J a f u r a h a r e o f a r i d t y p e . Due t o l o w t o p o g r a p h i c p o s i t i o n , s u r f i c i a l a q u i f e r s a r e o f t e n v e r y s h a l l o w . There has been a l o n g h i s t o r y o f m i q r a t i o n o f water, t h r o u q h a r t e s i a n , n o n - a r t e s i a n sources and w e l l s , f r o m deep and s h a l l o w a q u i f e r s , r a i s i n g s t i l l f u r t h e r t h e w a t e r l e v e l , and l o c a l l y improvi n g t h e q u a l i t y o f t h e s u r f i c i a l groundwater. R e c e n t l y t h i s t r e n d has been s u b s t a n t i a l l y i n c r e a s e d by new a g r i c u l t u r a l developments and e x t e n s i v e d r i l l i n g c o n n e c t i n g up o t h e r w i s e i s o l a t e d a q u i f e r s . A l s o , t h e p r o x i m i t y o f t h e G u l f i n c r e a s e s t h e average h u m i d i t y , d e c r e a s i n g e v a p o r o t r a n s p i r a t i o n . As a r e s u l t , t h e v e g e t a t i o n may be q u i t e dense i n some areas, w i t h s h a l l o w

r e l a t i v e l y sweet s u r f i c i a l qroundwater. I n o t h e r areas, t h e presence o f a s h a l l o w s a l i n e s u r f i c i a l a q u i f e r o r t h e e x c e s s i v e d e p t h o f t h e water l e v e l may p r e v e n t p l a n t growth. I n t h e f i r s t case, t h e f i n a l r e s u l t i s a sabkha p l a i n ; i n t h e second, a f i e l d o f a c t i v e dunes. Thus, i n t h e J a f u r a h area, a r i d c o n d i t i o n s produce dune f i e l d s o n l y i n a p p r o p r i a t e a r e a s . Dune f i e l d s a r e l o c a l and t h e s i z e o f e o l i a n f e a t u r e s s m a l l . Due t o a n e a r l y c o n s t a n t n o r t h e r l y wind and f r e q u e n t l y h i g h wind speeds, barchans and barchanoid r i d g e s a r e t h e commonest d e p o s i t i o n a l forms. I s o l a t e d barchans t r a v e r s e t h e f l a t sabkha s u r f a c e s o r merge i n t o barchan dune f i e l d s . On t h e sabkhas themselves, dunes o f t e n have t h e i r basal l a y e r s preserved, a s m o i s t u r e cominq f r o m t h e s h a l l o w w a t e r t a b l e l e a d s t o s a l t c e m e n t a t i o n which p r e v e n t s d e f l a t i o n below a c e r t a i n l e v e l : w h i l e t h e t o p s a r e l o s t due t o i n c r e a s e d wind v e l o c i t y a s s o c i a t -

ed w i t h decreased s u r f a c e rouqhness. I n t h e dune f i e l d s , t h e dunes remain a c t i v e . I n f o r m a t i o n f r o m a n e t w o r k o f a z i m u t h a l sand t r a p s i n n o r t h J a f u r a h has shown t h a t sand i s m a i n l y t r a n s p o r t e d f r o m t h e N.N.W.

Sand samples were t a k e n f r o m

v a r i o u s geomorphological s i t e s i n t h e sand f i e l d s . S o r t i n g v a r i e s between 1.15 and 2.0, w i t h g r a i n s i z e Hd v a l u e s r a n g i n g f r o m 0.12 t o 0.4 mm. T y p i c a l g r a i n s i z e d i s t r i b u t i o n s a r e shown on f i g u r e s 2 and 3. Q u a r t z c o n t e n t v a r i e s between 90 and 99 p e r c e n t , w i t h l o w e r v a l u e s i n t h e s m a l l e r f r a c t i o n s . The c o l o u r o f t h e sand i s n o r m a l l y around 1 0 YR 7/3 on t h e N u n s e l l c h a r t ( v e r y p a l e brown near l i g h t g r a y ) , p r o b a b l y due t o t h e f a c t t h a t few y e l l o w q r a i n s and no i r o n c o a t i n g s a r e found. Rounded g r a i n s f o r m between 20 and 30 p e r c e n t o f t h e t o t a l , b u t a n g u l a r q r a i n s may f o r m between 1 0 and 30 p e r c e n t o f some samples. 2.2

The Dahna sand f i e l d s The Dahna sand f i e l d s a r e formed o f a l o n g and narrow s t r i p o f e o l i a n sand

from 20'

3 0 ' t o 28'

3 0 ' N o r t h . The w i d t h o f t h e s t r i p does n o t exceed 40 Kin. b u t

i t s l e n g t h reaches more t h a n 1100 Km. The Dahna sand f i e l d s a r e m o s t l y composed o f s e v e r a l r e l a t i v e l y p a r a l l e l

3 68

2.0 mm

0.5

1.0

0.3

0.2

F i g . 2. R e p r e s e n t a t i v e g r a i n s i z e s f o r N o r t h J a f u r a . Number 13 Hd = 0.30 mm.,

So = 1.27.

Number 1 6

-

0.06

0.1

-

0.03

barchan:

p a r a b o l i c dune: Pld = 0.285 mm.,

So = 2.52.

r i d g e s i n c l u d i n g complex dune systems. I n Landsat imaqery t h e p a t t e r n i s n o t always w e l l - d e f i n e d .

I n t h e south, N 20 t o Pi o r i e n t a t e d s t r i p s o c c u r : t h e i r

spacinq v a r i e s between 0.8 and 1.2 Km. and t h e i r l e n g t h s ranqe from 1 0 t o 20 h. F u r t h e r n o r t h , t h e s t r i p s a r e n o t c l e a r l y observed, b u t two d i r e c t i o n s can be d e f i n e d ( N 80 and

N

1 7 0 ) . I n t h e n o r t h e r n s e c t o r , d i r e c t i o n s change from N 100

t o N 120: spacing i s much g r e a t e r ( 3 t o 9 K m . ) and l e n g t h s range from 1 0 t o 100 Km. The f a c t t h a t t h e Dahna sand f i e l d s occupy a r e l a t i v e l y l o w t o p o q r a p h i c p o s i t i o n and a p p r o x i m a t e l y f o l l o w t h e c u r v e d q e o l o q i c a l s t r u c t u r e s , sugqests a q e n e s i s r e l a t e d t o nearby sand sources i n t h e c o n t i g u o u s watersheds. I n t h e area west o f Kurays, f i v e s t r i p s w i t h i n t e r d u n e v e g e t a t e d sand sheets a r e observed. E o l i a n a c t i v i t y i s more o r l e s s r e s t r i c t e d t o t h e dune r i d g e s . Beneath t h e r i d q e s , two p a l a e o s o l s developed i n e o l i a n sands a r e i n t e r e s t i n q evidence o f t h e e x i s t e n c e o f a t l e a s t two humid p e r i o d s i n t e r r u p t i n g t h e e o l i a n

369 dynamics in t h e region ( f i g . 3 ) .

TOP OF THE EXPOS RE

3.3

3.0

MODERN

EOLIAN - - SAND

5

2.1

DUNES 1

(ACTIVE 2 .d (WELL DEFINED STEEPLY 2.1

DIPPING " LAMINAE, DUNAR MORPHOUIQY 1

77-77-7

c

SOIL ( S I L T Y

SAND W I T R SOME CLAY)

LW

18

CaC03 tubes 1.5

(colcified roots

-

EOLIAN SAND (WELL DEFINED

I2

WELL

,DIPPING STEEPLY ,DIPPING

LAMINAE, LAMINAE, ROOTS ROOTS CANALS CANALS

PRESERVED)

0.9

SANDY SOIL ( SILTY-CLAYEY

SA

0.E

EOLIAN SAND WITH SOME ALLUVIAL REWORKING 0.3

I

(VARIABLE

m . 0.0

DIP, MINOR OCCURENCES OF COARSE GRAINED LAYERS)

4

BOTTON OF THE P I T

Fiq. 3. Exposure in p i t , 15 Km. west of Khurays in Dahna sand f i e l d s . I n 1 , 3, and 5 , typical e o l i a n s t r u c t u r e s a r e observed: c r ~ s s b e d d i n q ,s l i g h t l y i r r e g u l a r parallel lamination (dikaka type) and s t e e p l y dipping bedding. The Dahna e o l i a n sediments have well-defined c h a r a c t e r i s t i c s . Grain s i z e Md values range from 0.16 t o 0.45 mm. and So from 1.1 t o .8. Typical cumulative curves a r e shown in f i g u r e s 4 and 5 . The sands a r e strongly reddish (Plunsell 7.5 YR 6/8, fal'ling in the reddish-yellow zones) and quartz represents about 99 per cent of the t o t a l samples with mainly subrounded yellow grains and iron

370 c o a t i n g s i n a p p r o x i m a t e l y 75 t o 8 0 p e r c e n t o f t h e cases.

F i g . 4. R e p r e s e n t a t i v e g r a i n s i z e s f o r J a f u r a h ( n o s . 3 and 4 ) and Dahna (no. 2) dune f i e l d s . No. 3 N o r t h Jafurah,Abuhadriyah hiqhway, shadow f i n e sand: Md = 0.14 mm., So = 1.22. No. 4 - N o r t h Jafurah,Abu H a d r i y a h hiqhway, e o l i a n sand, open a r e a b e h e e n mounds: Pld = 0.315, So = 1.89. No. 2 - Dahna sand f i e l d , f i n e sand i n shadow o f small mound 9 Km. west o f Khurays: Md = 0.188, So = 1.17.

-

Along t h e Dahna l o w c o r r i d o r , small and medium s i z e d a l l u v i a l f a n s a r e seen, i n which r e d d i s h s o i l s have developed. These a l l u v i a l fans, o f p r o b a b l e P l e i s t ocene age, seem t o have been t h e source f o r t h e i r o n c o a t e d g r a i n s i n t h e younge r f o r m a t i o n s , s u p p o r t i n g t h e i d e a t h a t t h e sand f i e l d s o f Dahna a r e m a i n l y c o n t r o l l e d by l o c a l and r e g i o n a l g e o l o q i c a l s t r u c t u r e and qeomorpholoqy, and only t o a l e s s e r e x t e n t by c l i m a t e . 2.3

The R u b ' a l K h a l i The R u b ' a l K h a l i i s t h e l a r q e s t s i n g l e r e g i o n o f a r i d i t y . More t h a n 550,000

Km.'

i n t h e s o u t h e r n h a l f of t h e c o u n t r y (about 50 p e r c e n t o f t h e a r e a i n the

E a s t e r n P r o v i n c e s ) a r e covered w i t h more o r l e s s c o n t i n u o u s e o l i a n accumulations, r a r e l y i n t e r r u p t e d by eroded remnants o f o l d e r r e l i e f ( m a i n l y near t h e edges),

371 100

80

SILT

8 .-c>

60

0)

0

z

0

40

20

Fig. 5. R e p r e s e n t a t i v e g r a i n s i z e s f o r Dahna sand f i e l d s . No. 1 0 - o l d e r e o l i a n sand f r o m t r e n c h , d e p t h 2.0 t o 2.5 m e t r e s : Hd = 0.199.mn. So,= 1.39. 1'0. 20 - t o p o f barchanoid r i d g e , 17 Km. west o f Khurays: Md = 0.184 mm., so = 1.12.

o r by uncovered g r a v e l pavements o r sabkha f l a t s i n some i n t e r d u n e areas. P r e s e n t r a i n f a l l i n t h e r e g i o n seems t o be l e s s t h a n 50 mm.,

although accur-

a t e m e t e o r o l o g i c a l i n f o r m a t i o n i s n o t a v a i l a b l e o v e r most o f t h e r e g i o n . When i t r a i n s , most o f t h e w a t e r j u s t humects t h e s u r f a c e g r a i n s , p r o m p t l y e v a p o r a t i n g o r , d u r i n g heavy r a i n s , i n f i l t r a t e s t h r o u g h t h e t h i c k sandy c o v e r t o t h e water t a b l e i n a r e a s w i t h s h a l l o w s u r f i c i a l a q u i f e r s . Very r a r e l y , a small amount r u n s down t h e s l o p e s of t h e dune r i d g e s , and s o r t i n q o f g r a i n s o c c u r s on micro-pedime n t s s l o p i n g toward t h e i n t e r d u n e d e p r e s s i o n s . No c o n c e n t r a t e d r u n o f f o c c u r s anywhere i n t h e r e g i o n and no aqueous qeomorphic f e a t u r e s can be seen. N e v e r t h e l e s s , t h e R u b ' a l K h a l i i s r e a l l y a complex a r e a w i t h a l o n g h i s t o r y o f c l i m a t i c and geomorphological changes. Two main f a c t o r s have i n f l u e n c e d t h e e v o l u t i o n o f t h e landscape i n t h e r e g i o n . I n t h e f i r s t place, t h e amount o f r a i n f a l l and, a s s o c i a t e d w i t h i t t h e d e n s i t y and permanence o f v e g e t a t i o n c o v e r

372 and secondly, t h e changes o f wind d i r e c t i o n . Changes i n r a i n f a l l have c o n t r o l l e d t h e presence o r absence o f f l u v i a l f l o w , t h e r i s e o r d r o p i n l a k e l e v e l s , s o i l f o r m a t i o n o r e r o s i o n , s a l t a c c u m u l a t i o n and e o l i a n r e w o r k i n q o f e x i s t i n g sand. Chanqes i n wind d i r e c t i o n have m a i n l y a f f e c t e d dune morphology: o l d r i d q e s a r e being r e b u i l t i n a new a l i q n m e n t , complex dunes a r e b e i n g formed, and sand sources changing t h e i r l o c a t i o n . Two main dune systems occur. The o l d e s t one o v e r l i e s t h e P l i o - P l e i s t o c e n e a l l u v i a l s u r f a c e s , and l o c a l l y l a t e P l e i s t o c e n e l a c u s t r i n e d e p o s i t s . These o l d dunes developed a p a l e o s o l d u r i n q e a r l y Holocene t i m e s and were covered by new dunes i n a l a t e r stage. The p a l e o s o l s on t h e dunes a r e composed o f r o o t and stem e n c r u s t a t i o n s (McClure, 1978) and seem t o have been contemporary w i t h t h e younger l a k e s i n t h e i n t e r d u n e d e p r e s s i o n s , w h i c h a r e now covered by modern e o l i a n deposits. S e l e c t e d samples were o b t a i n e d f r o m t h e w e s t e r n and south-western areas. G r a i n s i z e v a r i e s c o n s i d e r a b l y depending on geomorphic p o s i t i o n o f t h e samples. T y p i c a l dune sand Pld v a l u e s range f r o m 0.15 t o 0.4 mm. w i t h good s o r t i n g on h i q h dune r i d g e s (So = 1.1 t o 1.4) and w i t h poor s o r t i n g near d e f l a t i o n zones (So up t o 2.6).

R e p r e s e n t a t i v e c u m u l a t i v e c u r v e s a r e shown on f i g u r e 6. Q u a r t z

i s o f t e n more t h a n 97 p e r c e n t o f t h e samples, w i t h f e l s p a r v a r y i n q f r o m 0.5 t o 2 p e r c e n t and m a f i c m i n e r a l s b e i n g l e s s t h a n 3 p e r c e n t . G r a i n s a r e subrounded

and subanqular, w i t h rounded g r a i n s f o r m i n g a b o u t 25 p e r c e n t o f t h e samples. G r a i n c o l o u r ranqes f r o m y e l l o w t o l i g h t y e l l o w , w i t h d u l l s u r f a c e s and remnants o f i r o n c o a t i n g s i n about 10 t o 20 p e r c e n t o f t h e g r a i n s . The qeneral c o l o u r o f t h e sand v a r i e s between 5 YR 7/6 t o 7/8 and 7.5 YR 7 / 6 t o 7/8 o f t h e Munsell colour charts, corresponding t o reddish-yellow colours.

3. 3.1

QUATERNARY EVOLUTION Background I d e n t i f i c a t i o n o f t h e p r e s e n t , qeomorphic systems r e q u i r e s o n l y f i e l d observ-

a t i o n and m e t e o r o l o g i c a l d a t a . A c t i v e wadis and dune f i e l d s , presence o f g u l l i e s o r r a v i n e s , and o b s e r v a t i o n o f t h e v e g e t a t i o n c o v e r , complemented w i t h d a t a on r a i n f a l l , wind d i r e c t i o n and speed, h u m i d i t y and temperature, can p r o v i d e a good u n d e r s t a n d i n g o f t h e c u r r e n t qeomorphic system. I d e n t i f i c a t i o n o f o l d e r systems r e q u i r e s a d i f f e r e n t approach; sedimentary accumulations, l a n d f o r m s and paleopedogenic processes must be d e s c r i b e d , a n a l y s e d and i n t e g r a t e d i n order t o o b t a i n t h e necessary knowledge t o r e c o n s t r u c t t h e paleoenvironment. Taking i n t o account t h e s e elements, s i x main c l i m a t i c phases a r e proposed for t h e l a t e P l i o c e n e t o Q u a t e r n a r y i n t h e p r e s e n t s t u d y . D u r i n q each phase, t h e d i s t r i b u t i o n o f qeomorphic systems i n t h e r e g i o n changed. D u r i n g 'humid' phases, a r i d systems became s e m i - a r i d systems and s e m i - a r i d systems became semi-humid.

373 100

8 C -SAND-

6C (D

.->

c

0 3

5

0 4c

2(

-

1.0

mm

0.5

0.3

0.2

0.1

0.06

0.03

Fig. 6 . Representative qrain s i z e analyses f o r t h e northwest Rub'al Khali. 110. 5 - slope of s t a r dune: Md = 0.332 nm., So = 1.13. No. 6 - eolian sand on top o f l a r g e dune: Md = 0.201 rnm., So = 1.12. During the ' a r i d ' phases, the opposite phenomenon occured; semi-humid systems became semi-arid, and semi-arid became a r i d . Eolian dynamics and c o r r e l a t i v e sediments a r e a r e s u l t of these c l i m a t i c o s c i l l a t i o n s . Arid phases correspond with periods of sand i n s t a b i l i t y - d e f l a t i o n , ,formation of dunes, e t c . and humid phases a r e c o r r e l a t i v e with a l l u v i a l sedimentation

along wadis, s o i l development o n t h e slopes, and dune immobilization. 3.2

The l a t e Pliocene - e a r l y Pleistocene humid phase

No s i g n i f i c a n t eolian a c t i v i t y seems t o have occured durinq l a t e Pliocenee a r l y Pleistocene times. On t h e contrary, a v a i l a b l e evidence shows the existence

of a r e l a t i v e l y long humid period s t a r t i n q sometime in the l a t e Pliocene and ending in the e a r l y Pleistocene. Widespread gravel deposits (associated with present o r old drainaqe systems), red s o i l s in the western mountains o f t h e Arabian Peninsula, weathering on shield

374 rocks and b a s a l t , a l l u v i a l f a n s and t e r r a c e s , seem to confirm the existence of t h i s humid phase during the l a t e Cenozoic (Anton, 1980; Hoetzl e t a l . , 1978). Two b a s a l t s gave K/Ar ages of 1.1 0.3 Ha. and 3.5 f 0.3 Ma. The gravels

*

were therefore accumulated a f t e r t h i s . Due to sedimentoloqical and qeomorphologi c a l s i m i l a r i t i e s i t i s reasonable t o assume t h a t o t h e r qravelly accumulations and deep weathering in the peninsula a l s o occured during the same period. In t h e Eastern Province, i n the Iiadi Sahba basin, the valleys a r e f i l l e d with a basal coarse g r a v e l l y deposit underlying a f i n e r formation which seems to correspond with t h i s ancient humid period. Downstream of A1 Kharj, a single valley i s observed running eastwards through t h e Dahna eolian c o r r i d o r and down t o the Jafurah sand f i e l d s . Along t h i s v a l l e y , t e r r a c e s composed of gravelly d e p o s i t s , and o f t e n covered with well-defined c a l c i c r u s t s , a r e encountered. These t e r r a c e s a r e normally dissected along the a x i s of t h e valley and t o a l e s s e r degree alonq the edges, next to bedrock outcrops. The elevation of the t e r r a c e s reaches a maximum in the high gradient section west of Haradh s t a t i o n , and decreases in height eastwards, with t h e i r f l a t tops merging w i t h t h e surface of an extensive a l l u v i a l plain ( a c t u a l l y a t r u e alluvial f a n ) with probably s i m i l a r age. The fan i s p a r t i a l l y covered with eolian sand and has been dissected by a l a t e r channel; b u t can s t i l l be e a s i l y traced on Landsat images and observed in the f i e l d . Extensive gravel pavements, consisting mainly of quartz pebbles, a r e observed in many places. No eolian deposits have been i d e n t i f i e d in t h i s f a n , which seems t o be one of t h e main sources f o r the younger eolian sand f i e l d s . The Middle Pleistocene a r i d phase Several l i n e s of evidence show t h a t the Middle Pleistocene was not a s humid a s t h e e a r l y Pleistocene, and t h a t t h i s trend continued u n t i l l a t e Pleistocene times. In the west of the country, t h e younger b a s a l t s of t h e tlarrat a r e n o t strongly weathered and no deep valleys have been c u t into them (Hoetzl e t a l . ,

3.3

1978), which can be i n t e r p r e t e d a s due t o t h e absence of long humid periods during the r e s t of t h e Quaternary.

The d e p o s i t s t h a t may correspond with t h i s Pleistocene sub-epoch a r e f i n e and r i c h in calcium carbonate o r gypsum, suqgesting a semi-arid t o a r i d climate with l i t t l e runoff and limited f l u v i a l flow. Even though some eolian sediments belonq in t h i s period, a r i d i t y does not seem t o have reached the threshold needed f o r widespread eolian a c t i v i t y . A moderately dense xerophytic vegetation cover would have been adequate t o produce t h i s type of environment. Thic period corresponds with the accumulation o f t h e s i l t y deposits which f i l l almost completely t h e valleys in the Arabian s h i e l d area (Anton, 1980). I t i s possible t h a t p a r t of t h i s material could be reworked l o e s s derived from d e f l a t i o n of t h e more a r i d regions of t h e central and eastern p a r t s of the

375 peninsula, r e t a i n e d by v e g e t a t i o n i n t h e more humid western r e g i o n . 3.4

The l a t e P l e i s t o c e n e humid phase

A l a t e P l e i s t o c e n e humid phase was d e f i n e d i n t h e A r a b i a n s h i e l d by Anton (1980) based on a t h i n l a y e r o f c o a r s e sediment c o v e r i n g t h e M i d d l e P l e i s t o c e n e s i l t s . The d i s s e c t i o n o f Wadi Sahba and llladi B a t i n a l l u v i a l f a n s seems t o be also r e l a t e d t o t h i s l a t e Pleistocene increase i n humidity. I n t h e Rub'al Khal i, presumably c o r r e l a t i v e l a c u s t r i n e d e p o s i t s , d a t e d between 36,031)

and 17,000 y e a r s B.P. were n o t e d by PlcClure (1978).No s i g n i f i c -

a n t e o l i a n a c t i v i t y seems t o have o c c u r e d d u r i n g t h i s stage. 3.5

The l a t e P l e i s t o c e n e

-

e a r l y Holocene a r i d phase

I t i s q e n e r a l l y accepted t h a t , d u r i n g l a t e P l e i s t o c e n e times, a marked i n c r e a s e i n a r i d i t y occured i n t h e A r a b i a n and Saharan d e s e r t s ( H o e t z l e t a l . , 1978; Alayne and Grove, 1979; Butzer, 1980; Anton, 1980). Dune systems s t a r t e d t o f o r m i n t h e v a s t expanses o f t h e R u b ' a l K h a l i and Nafud, and t o a l e s s e r e x t e n t i n J a f u r a h and Dahna. F l u v i a l f l o w decreased t o a minimum. No a l l u v i a l accumulations occured, w i t h t h e p r o b a b l e e x c e p t i o n o f e v a p o r i t e s and some f i n e l o c a l d e p o s i t s . Rub'al K h a l i l a c u s t r i n e sediments, s t r a t i g r a p h i c a l l y l o c a t e d above and below t h e s e e o l i a n d e p o s i t s , were d a t e d between 17,000 and 19,000 y e a r s B.P.

(McClure, 1978). No d a t e s a r e a v a i l a b l e f o r t h e E a s t e r n Province, b u t

a number o f exposures o f e o l i a n sands have been i d e n t i f i e d i n t h e J a f u r a h r e g i o n which appear t o c o r r e s p o n d w i t h t h i s phase. I n t h e Dahna area, o l d e r e o l i a n sands u n d e r l y i n ? two p a l e o s o l s have been observed, which seem t o be c o r r e l a t i v e w i t h t h i s phase ( f i g . 3 ) .

A c o r r e l a t i v e a r i d p e r i o d was i d e n t i f i e d i n t h e N i l e v a l l e y

-

dune i n v a s i o n s

o f t h e f l o o d p l a i n s ( B u t z e r , 1980); i n t h e A f a r and E t h i o p i a n r i f t l a k e s from 17,000 t o 12,000 y e a r s B.P.

(Gasse e t a l . ,

-

1980); and i n most o f t h e Sahar-

i a n and S a h e l i a n l a k e s (Alayne and Grove, 1979). 3.6

The e a r l y Holocene humid phase I n l a t e r times, s h a l l o w l a k e s a g a i n formed i n t h e R u b ' a l K h a l i . These a r e

dated between 9,000 and 6,000 y e a r s B.P.

(McClure, 1978). S o i l s developed on

t h e Dahna sand f i e l d s and c o a r s e a l l u v i a l d e p o s i t s accumulated i n Wadi Sahba and Wadi B a t i n . T h i s suggests a marked i n c r e a s e i n h u m i d i t y c o i n c i d i n g w i t h t h e b e g i n n i n g o f t h e Holocene a b o u t 12,000 t o 10,000 y e a r s ago. I n t h e A r a b i a n s h i e l d , t h e g r a v e l t e r r a c e s were d i s s e c t e d and t h i n c o a r s e d e p o s i t s formed i n t h e wadi v a l l e y s (Anton, 1980). I n t h e A r a b i a n p e n i n s u l a , Hoetzl e t a1.(1978)

proposed an e a r l y Holocene humid phase e x t e n d i n g f r o m 9,000

t o 4,500 y e a r s B.P.

Based on carbon 1 4 d a t e s and r e l a t e d geomorphological data,

376 t h e y a l s o proposed a s h o r t a r i d episode from 8,000 t o 7,000 y e a r s B.P.,

corresp-

o n d i n g t o a " N e o l i t h i c P l u v i a l " . Gasse e t a1.(1980) proposed a l o w e r l i m i t o f 12,000 y e a r s B.P.

f o r t h i s humid t h r o u q h o u t E t h i o p i a . However, f o r t h e more a r i d

Lake Abha b a s i n i n C e n t r a l A f a r t h e y c o n s i d e r t h a t t h e main r i s e i n w a t e r l e v e l happened l a t e r , ar'3und 10,000 y e a r s B.P.

I n t h e E q y p t i a n Sahara, ldendorf and

Hassan (1980) proposed a humid phase s t a r t i n q a b o u t 10,000 y e a r s B.P. w i t h a s h o r t a r i d phase f r o m 7,600 t o 7,400 y e a r s B.P.,

and a l a s t humid phase a b o u t

6,500 y e a r s aqo. I n t h e Dahna sand f i e l d , t h e e o l i a n non-weathered h o r i z o n s seem t o correspond t o d r i e r phases and t h e two p a l e o s o l s t o s u c c e s s i v e humid p e r i o d s ( f i q . 3 ) . The upper p a l e o s o l i s covered by a c t i v e e o l i a n d e p o s i t s which appear t o correspond t o t h e l a t e Holocene a r i d phase.

3.7

The l a t e Holocene a r i d phase The s h a l l o w l a k e s o f t h e R u b ' a l K h a l i s t a r t e d t o d r y up about 6,000 y e a r s B.P.

(McClure, 1 9 7 8 ) . Evidence i n t h e J a f u r a h r e g i o n show t h a t t h e e x t e n s i o n o f t h e dune f i e l d s t o o k p l a c e v e r y q u i c k l y . Present-day m i q r a t i n q barchans on sabkhas a r e g e n e r a l l y a few k i l o m e t r e s ( l e s s t h a n 1 0 Km.) downwind o f i d e n t i f i a b l e source a r e a s . Assuming p r e s e n t m i g r a t i o n r a t e s o f about 1 0 m e t r e s / y e a r , t h e o l d e s t dunes a r e l e s s t h a n 1,000 y e a r s o l d . I f m i g r a t i o n r a t e s were f o r m e r l y slower, the a c t u a l aqe m i g h t be i n c r e a s e d t o 2,000 t o 3,000 y e a r s . Some a u t h o r s (e.g N i c h o l s o n e t a1 ., 1980; Gasse e t a l . ,

1980) sugqest t h a t a s l i q h t l y w e t t e r

episode f r o m 2,500 t o 1,000 y e a r s B.P. may have i n t e r r u p t e d t h i s a r i d phase. I f t h i s i s c o n f i r m e d f o r t h e E a s t e r n Provinces r e g i o n , i t seems l i k e l y t h a t

dune m i g r a t i o n had stopped due t o v e q e t a t i o n s t a b i l i z a t i o n d u r i n g some c e n t u r i e s i n t h e second and t h r i d m i l l e n i a b e f o r e t h e p r e s e n t . Even supposing t h a t dunes remained s t a b l e f o r a thousand y e a r s , and a l l o w i n g s e v e r a l c e n t u r i e s f o r dune formation,

i t i s u n l i k e l y t h a t t h e J a f u r a h dunes a r e o l d e r t h a n 4,000 y e a r s B.P.

Thus, t h e p r e s e n t e o l i a n system o f t h e J a f u r a h sand f i e l d s i s o n l y a r e c e n t development r e l a t e d t o g r a z i n g and o t h e r human a c t i v i t i e s d u r i n g t h e l a s t t h r e e t o f o u r m i l l e n i a . G r a z i n q a c t i v i t i e s were expanded around f o u r thousand y e a r s ago when t h e camel was d o m e s t i c a t e d (Doe, 1971) and i n c o r p o r a t e d i n t o herds, t h u s c o n s i d e r a b l y e x t e n d i n g t h e g r a z i n g areas. I n t h e J a f u r a h , t h e o r i g i n o f the main dunes i s r e l a t e d t o d e g r e d a t i o n o f t h e v e g e t a t i o n c o v e r i n upwind a r e a s . 4.

CONCLUSION The c l i m a t e o f t h e E a s t e r n P r o v i n c e o f Saudi A r a b i a has been e v o l v i n g f r o m

humid and s e m i - a r i d d u r i n q t h e e a r l y T e r t i a r y , t o a r i d and s e m i - a r i d i n Quaterna r y times. D u r i n g t h i s e v o l u t i o n , p l a n t communities have more o r l e s s s u c c e s s f u l l y adapted t o t h e i n c r e a s i n g d r o u g h t and have k e p t a r e l a t i v e l y dense c o v e r on e x i s t i n q

377 sandy s o i l s , a l l o w i n q sand movement o n l y d u r i n q t h e most severe a r i d episodes. Amonq t h e i d e n t i f i a b l e a r i d i t y peaks, o n l y t h e t h r e e most r e c e n t ones have produced widespread e o l i a n e f f e c t s on sandy areas. A v a i l a b l e d a t e s f o r t h r e e peaks a r e : f r o m 17,000 t o 10,000 y e a r s B.P.;

-

from 6,000

5,500 y e a r s B.P.

f r o m 9,000 t o 7,000 y e a r s B.P.;

and

t o t h e present.

The l a s t peak i s composed o f two stages: a f i r s t one i n w h i c h g e n e r a l a r i d i t y developed q r a d u a l l y , p r o b a b l y r e s p o n d i n q t o p l a n e t a r y c l i m a t i c chanqes; and a second one i n which t h e t r e n d t o i n c r e a s i n g a r i d i t y a c c e l e r a t e d , a p p a r e n t l y due t o human a c t i v i t y i n v o l v i n q o v e r q r a z i n q and b u r n i n q o f v e g e t a t i o n i n t h e d r i e s t areas and d e p r e d a t o r y a q r i c u l t u r e i n l e s s a r i d reg,ions. The i n f e r r e d h i s t o r y i s summarized i n t a b l e 1 below.

CORRELATION OF CLIMATIC PHASES ' GEOMORPHOLOGICAL EVOLUTION A N 0 GEOLOGICAL DYNAMICS

TABLE 1 _ .

1011

.................

I

IoluiWYERED BY DUNES

.............

S"*LLOW IDOL

SfMIIRID

I ----+-DUNES

LA16 PLISITOEINE SEMIARID

9

EOLIAN

.............................

'2.9.

I

.....................

WILS WYERfO 8" DUNES

......................

WlLSON DUNPS

SOIL EROSION

378

REFEREllCES Alayne, S.F. and Grove, A.T., 1979. Global maps o f l a k e l e v e l f l u c t u a t i o n s since 30,000 B.P. Q u a t e r n a r y Res., 12: 83-118. Al-Sayari, S.S. and ZBti, J.G., 1978. Quaternary Period i n Saudi Arabia. Springer Verlag, Wien, 334 pp. Anton, D., 1980. C l i m a t i c i n f l u e n c e i n t h e Cenozoic e v o l u t i o n o f t h e Arabicn S h i e l d south. Abs. 26th I n t e r n a t . Geol. Conqr., Paris, 1980. Breed, C.S. e t a l . , 1979. Reqional s t u d i e s o f sand seas u s i n g Landsat (E9TS) imagery. I n : McKee, E.D. ( E d i t o r ) , A study o f q l o b a l sand seas. U.S. Geol. Surv. Prof. Paper, 1052: 305-397. Butzer, K.W., 1980. P l e i s t o c e n e h i s t o r y o f t h e N i l e v a l l e y i n Eqypt and Lower Nubia. I n : Williams, H.A.J. and Faure, H. ( E d i t o r s ) , The Sahara and t h e Nile, A.A. Bal kema, Rotterdam, pp.253-280. Doe, B., 1971. Southern Arabia. NcGraw H i l l , N.Y., p. 267. Gasse, F. e t a l . , 1980. Quaternary h i s t o r y o f t h e A f a r and E t h i o p i a n r i f t lakes. I n : I l i l l i a m s , I1.A.J. and Faure, H. ( E d i t o r s ) , The Sahara and t h e N i l e , fi.A. Bal kema, Rotterdam, pp. 361-400. Hoetzl, H. e t a1 ., 1978. Wadi A r Rimah: Wadi Ad Dawasir and i t s h i n t e r l a n d . I n : Al-Sayari, S.S. and Z B t l , J.G. ( E d i t o r s ) , Quaternary Period i n Saudi Arabia. Springer-Verlaq, M e n , pp, 173-193 and 226-251. HcClure, H.A., 1978. A r Rub'al K h a l i . I n : Al-Sayari, S.S. and Faure, J.G. ( E d i t . ) Quaternary Period i n Saudi Arabia. Springer-Verlaq, IJien, pp. 252-263. Nicholson, S.E., 1980. Saharan c l i m a t e s i n h i s t o r i c times. I n : Williams, I1.A.J. and Faure, H. ( E d i t o r s ) , The Sahara and t h e N i l e , A.A. Balkema, Rotterdam, pp. 173-200. Patterson, R.J. and Kinsman, D.J.oI., 1981. H y d r o l o q i c a l framework o f a sabkha along t h e Arabian G u l f . Amer. Ass. P e t r o l . Geol. B u l l . , 65: 1457-1475. 1966. Geology o f t h e Arabian Peninsula: Sedimentary geology Powers, R.W. e t a1 o f Saudi Arabia. U.S. Geol. Surv. P r o f . Paper, 560-D: 1-147. Wendorf, F. and Hassan, F.A., 1980. Holocene ecoloqy and p r e h i s t o r y i n t h e Egyptian Sahara. I n : Williams, M.A.J. and Faure, H. ( E d i t o r s ) , The Sahara and t h e N i l e , A.A. Balkema, Rotterdam, pp. 407-420.

.,

379

THE DYNAMIC HOLOCENE DUNE FIELDS OF THE GREAT PLAINS AND ROCKY MOUNTAIN BASINS, U.S.A.

THOMAS S. AHLBRANDT, P e t r o s t r a t C o n s u l t a n t s , 9600 E . Arapahoe Rd. S u i t e 250, Englewood, Colorado 80112 (USA) JAMES B. SWINEHART, C o n s e r v a t i o n and Survey D i v i s i o n , I A N R , U n i v e r s i t y o f Nebraska, L i n c o l n , Nebraska 68588-0517 (USA) D A V I D G. MARONEY, Route 2, Ladoga, I n d i a n a 47954 (USA) INTRODUCTION The dune f i e l d s i n t h e n o r t h e r n Great P l a i n s and Rocky Mountain b a s i n s ( F i g . 1) t r a d i t i o n a l l y have been t h o u g h t t o have formed d u r i n g t h e l a t e P l e i s t o c e n e . These c o n c l u s i o n s were based upon q u a l i t a t i v e geomorphic a n a l y s e s such as c o r r e l a t i o n s w i t h l o e s s o r t e r r a c e f i l l sequences o r upon i n f e r e n c e s c o n c e r n i n g s t r o n g k a t a b a t i c winds r e l a t e d t o g l a c i a l p e r i o d s .

However, t h e dune f i e l d s we have s t u d i e d

p r o v i d e no d i r e c t e v i d e n c e o f dune f o r m a t i o n d u r i n g t h e P l e i s t o c e n e a l t h o u g h we a n t i c i p a t e t h a t some l a t e P l e i s t o c e n e e o l i a n a c t i v i t y e v e n t u a l l y may be documented. Our evidence f r o m w e l l - d a t e d l o c a l s t r a t i g r a p h i c sequences, combined w i t h an abundance o f i n d i r e c t evidence, s u p p o r t s a m u l t i p h a s e h i s t o r y o f e o l i a n a c t i v i t y i n t h e s e dune f i e l d s d u r i n g t h e Holocene. Several d i s c i p l i n e s must be c a l l e d on t o e x p l a i n a d e q u a t e l y t h e l a t e P l e i s t o c e n e (Wisconsin) and Holocene h i s t o r y p e r t i n e n t t o t h e dune f i e l d s shown i n F i g u r e 1. These dune f i e l d s and t h e i r m a r g i n a l a r e a s have been occupied by p a l e o - I n d i a n o r younger c u l t u r e s u n t i l q u i t e r e c e n t l y .

A r c h e o l o g i s t s have e s t a b l i s h e d a

s t r a t i g r a p h i c s u c c e s s i o n i n t h e Holocene o f t h i s r e g i o n by u s i n g a r t i f a c t s as "index f o s s i l s ' ' (Frison e t a l . , d a t i n g o f t h e dune f i e l d s .

1974) f a c i l i t a t i n g g r e a t l y t h e r e l a t i v e age

P l e i s t o c e n e and Holocene v e r t e b r a t e f a u n a l successions

p r o v i d e a n o t h e r method o f r e l a t i v e d a t i n g i n t h i s area (Anderson, 1974; Wilson, 1974).

As d e s c r i b e d by Wilson (1974), t h e Holocene succession o f b i s o n i s p a r -

t i c u l a r l y important.

Moreover, some i n f o r m a t i o n on r e g i o n a l c l i m a t e can be

o b t a i n e d f r o m t h e s e successions.

P a l y n o l o g i s t s a l s o have p r o v i d e d a r e l a t i v e

s t r a t i g r a p h i c f l o r a l s u c c e s s i o n i n t h e Wisconsin and Holocene, and t h i s i n f o r m a t i o n has been used t o r e c o n s t r u c t t h e e v o l u t i o n of r e g i o n a l c l i m a t i c p a t t e r n s . The r e c o r d o f Holocene g l a c i a l expansion i n t h e Rocky Mountains p r o v i d e s a d d i t i o n a l c l u e s t o r e g i o n a l Holocene c l i m a t e s . l a r g e measure on r a d i o c a r b o n dates.

However, o u r arguments depend i n

We b e l i e v e t h e s e s e v e r a l d i s c i p l i n e s and

techniques r e i n f o r c e o u r h y p o t h e s i s o f s i g n i f i c a n t e o l i a n a c t i v i t y i n t h e Holocene. We conclude t h a t t h e Holocene has been a v e r y dynamic p e r i o d p u n c t u a t e d by p e r i o d s o f r a p i d e o l i a n s e d i m e n t a t i o n and a b r u p t e n v i r o n m e n t a l changes. We w i l l a t t e m p t t o draw t o g e t h e r v a r i o u s d i s c i p l i n e s i n d i s c u s s i n g t h e d i f f e r


ROCKY M 0 UNTA IN BASINS

c

L

3

a

A

I

9

>

c -

I II

> c

0

a W

Z 3

a

B THOUSAND Y E A R S 6 . P t

- - -Holocene

>

F i g u r e 3 . H o l o c e n e e o l i a n a c t i v i t y i n ( A ) Rocky Plountain b a s i n s o f C o l o r a d o and Wyoming and in (B) Nebraska Sand H i l l s .

TABLE 1.

RADIOCARBON DATES FROM THE KILLPECKER DUNE FIELD, WYOMING

Site

Legal l o c a t i o n

Depth below s u r f a c e (cm)

Radiocarbon age (yrs.BP) and l a b . no.

M a t e r i a l dated Wood (sage) emerging f r o m windward s l o p e o f dune

Sage Wood

SEL,SEL,sec. 15, T. 24N, R. 104W.

0- 10

2202 90 ( 1 - 6 4 8 7 ) l

B i s o n Sand

SEL,SWL,sec.30, T.24N,R. 103W.

30.5

7755 90 (1-6320)1

Upper Sand

Center sec. 1 T.23N,R. 105W.

91

28202 95 (I-8319)~

M a r l and r o o t t u b u l e s i n Upper Sand

Finley

SWk,SW%,sec. 19, T.24N,R.l05W.

122

58455115 (1-6486)2

Marl and r o o t t u b u l e s above M i d d l e Sand

B i s o n bone and charcoal i n Upper Sand

lWood and charcoal samples p r e t r e a t e d t o remove carbonates and humic a c i d s - Teledyne Labs, Westwood, N.J. 2 T r e a t e d w i t h a c i d s o l u t i o n u n t i l 62%, by w e i g h t , o f r o o t - t u b e samples removed, i n n e r 38% then d a t e d - Teledyne Labs, Westwood, N.J.

386 o t h e r s k e l e t a l remains recovered a t t h e s i t e a r e o f animals l i k e those p r e s e n t l y l i v i n g i n t h e area.

R a d i o c a r b o n d a t e s o f 9,830+350 y r s . B.P.

(RL-125) o n c h a r -

c o a l and 10,670+170 y r s . B.P. (RL-208) o n bone i n d i c a t e a n age o f a b o u t 10,000 y r s . B.P. a t t h e Casper s i t e ( A l b a n e s e , 1 9 7 4 ) .

Eolian deposits a t t h i s s i t e overlie

an a l l u v i a l fill c o n s i d e r e d t o b e l a t e P l e i s t o c e n e age b y A l b a n e s e and W i l s o n (1974, p. 1 3 ) . The F i n l e y s i t e a l s o c o n t a i n e d numerous b i s o n bones t h a t o r i g i n a l l y were i d e n t i -

Bison

f i e d as B i s o n o c c i d e n t a l i s b u t a c c o r d i n g t o W i l s o n ( 1 9 7 4 ) c l e a r l y r e s e m b l e a n t i q u u s and a r e H o l o c e n e f o r m s .

The e a r l i e s t c u l t u r a l components f o u n d i n p l a c e

i n t h e K i l l p e c k e r Dunes was a t t h e F i n l e y s i t e ( F i g u r e 4 ) and i n c l u d e s Eden and S c o t t s b l u f f p o i n t s o f t h e p a l e o - I n d i a n Cody Complex a s s o c i a t e d w i t h f r a g m e n t s o f d e c a l c i f i e d b i s o n bone.

F r i s o n e t a l . ( 1 9 7 4 ) e s t a b l i s h e d a t y p o l o g i c a l sequence

o f a r t i f a c t s i n t h e H o l o c e n e o f Wyoming.

U t i l i z i n g t h i s concept o f stratigraphic

a r c h e o l o g y , where numerous s i t e s i n Wyoming i n t e g r a t e s p e c i f i c a r t i f a c t s d i r e c t l y w i t h a d j a c e n t r a d i o c a r b o n d a t e s as shown b y F r i s o n e t a l . ( 1 9 7 4 ) , we c a n conclude t h a t t h e Eden a n d S c o t t s b l u f f p o i n t s a t t h e F i n l e y s i t e t y p o l o g i c a l l y d a t e t o a p e r i o d between 7,000 and 10,000 y r s . B.P.

A r a d i o c a r b o n d a t e o f 5,845k115 y r s .

B.P. was o b t a i n e d f r o m m a r l j u s t above a h o r i z o n c o n t a i n i n g t h e s e a r t i f a c t s n o r t h o f t h e F i n l e y s i t e ( F i g . 4 and T a b l e 1 ) .

F r i s o n ( p e r s . comm.,

1982) r e p o r t e d

t h a t paleo-man a r t i f a c t s were f o u n d i n p l a c e i n t h e d o r m a n t dunes a t G r e a t Sand Dunes, C o l o r a d o . I t i s n o t e w o r t h y t h a t f o s s i l b i s o n r e m a i n s a t t h e s e k i l l s i t e s were w h o l l y of

e a r l y H o l o c e n e age a n d y i e l d e d no e x t i n c t P l e i s t o c e n e t a x a .

The p a l e o - I n d i a n

c u l t u r e s r e s p o n s i b l e f o r t h e k i l l s i t e s a l s o w e r e e a r l y H o l o c e n e as c o n f i r m e d by s t r a t i g r a p h i c , a r c h e o l o g i c and r a d i o c a r b o n d a t i n g m e t h o d s . The Lower, M i d d l e , a n d Upper Sands a t K i l l p e c k e r ( d o r m a n t dune s t r a t i g r a p h y , F i g . 4 ) may r e f l e c t l a t e P l e i s t o c e n e a n d H o l o c e n e c l i m a t i c changes.

The Lower

Sand i s s t r u c t u r e l e s s and d a t a f r o m t r e n c h e s a n d b o r e h o l e s i n d i c a t e d i t i s r e s t r i c t e d t o t h e w e s t e r n ( u p w i n d ) m a r g i n o f t h e dune f i e l d . o n g r a v e l s o f t h e Upper F a r s o n a l l u v i a l f i l l .

The Lower Sand r e s t s

The t e r r a c e a s s o c i a t e d w i t h t h i s

f i l l was c o n s i d e r e d t o b e a l a t e P l e i s t o c e n e t e r r a c e ( W i s c o n s i n - P i n e d a l e ) and Moss (1955, p. 6 4 4 ) .

by Holmes

The Lower Sand a p p e a r s t o have been d e p o s i t e d i n a

r e d u c i n g e n v i r o n m e n t as e v i d e n c e d b y i t s p a l e g r e e n c o l o r , w h i c h may r e f l e c t a colder climate i n the l a t e Pleistocene. f o r t h e Lower Sand i s n o t known.

However, t h e d e p o s i t i o n a l e n v i r o n m e n t

The M i d d l e Sand c o n t a i n s t h e o l d e s t d a t e d material

a t K i l l p e c k e r and r e c o r d s Phase I dune a c t i v i t y .

I t i s s e p a r a t e d f r o m t h e Lower

Sand b y a p a l e o s o l and a zone o f c a l c i u m c a r b o n a t e n o d u l e s . Phase I 1 ( M i d d l e H o l o c e n e ) A b r i e f p l u v i a l p e r i o d a p p e a r s t o have o c c u r r e d between Phase I and Phase I 1 activity.

As u s e d h e r e , p l u v i a l i n d i c a t e s a p e r i o d o f g r e a t e r e f f e c t i v e m o i s t u r e

387 and may b e due t o d e c r e a s e d e v a p o r a t i o n , i n c r e a s e d p r e c i p i t a t i o n , o r b o t h .

Phase

I 1 dune a c t i v i t y seems t o c o i n c i d e w i t h t h e commencement o f t h e a r i d A l t i t h e r m a l p e r i o d a b o u t 7,500 y r s . B.P.

As d i s c u s s e d p r e v i o u s l y , t h e A l t i t h e r m a l i s a s s o c i -

a t e d w i t h s e v e r e a r i d i t y s u f f i c i e n t t o c a u s e d w a r f i n g o f b i s o n , m i g r a t i o n o f human c u l t u r e s and b i s o n and t r a n s i t i o n o f f l o r a t o a p r a i r i e v e g e t a t i o n ( g r a s s e s ) a t t h e expense o f e a r l i e r t h e r m o p h i l o u s f o r e s t s o f d e c i d u o u s t r e e s and p i n e w h i c h d o m i n a t e d t h e e n v i r o n m e n t u n t i l a b o u t 8,000 y r s . B.P.

The x e r i c e n v i r o n m e n t d u r i n g

t h e A l t i t h e r m a l p e r m i t t e d m a j o r dune sand movement f u r t h e r i n t e n s i f i e d by i n c r e a s e d number and d u r a t i o n o f summer d r o u g h t s d u r i n g t h e A l t i t h e r m a l as n o t e d by W r i g h t (1968). Phase I 1 a c t i v i t y seems t o have been t i e d more c l o s e l y t o t h e A l t i t h e r m a l i n t h e i n t e r m o n t a n e dune f i e l d s , t h a n o n t h e p l a i n s and i t d o m i n a t e s b o t h t h e K i l l p e c k e r and F e r r i s dunes i n Wyoming.

A m a j o r h i a t u s o f p e r h a p s 2,500 y r s . d u r a t i o n

o c c u r s between t h e M i d d l e Sand and Upper Sand o n t h e u p w i n d end o f t h e K i l l p e c k e r dunes.

As d i s c u s s e d b y A h l b r a n d t ( 1 9 7 4 ) , t h i s m a j o r h i a t u s p r e d a t e s a b a s a l c a l -

C r e t e i n t h e Upper Sand d a t e d t o 5,845+115 y r s . B . P . a n d p o s t d a t e s M i d d l e Sand a r t i f a c t s t y p o l o g i c a l l y c o r r e l a t i v e t o 7,000 t o 10,000 y r s . B.P. a t t h e F i n l e y s i t e and shown i n c o l u m n a r f o r m i n F i g u r e 4.

The m a j o r e r o s i o n a l d i a s t e m on t h e

upwind end o f t h e K i l l p e c k e r Dunes m u s t be r e f l e c t e d b y m a j o r downwind d e p o s i t i o n d u r i n g t h i s phase.

The F e r r i s Dunes a r e a downwind ( e a s t e r l y ) e x t e n s i o n o f t h e

e a s t e r l y t a i l o f t h e K i l l p e c k e r Dunes.

As shown i n F i g u r e 4 , p a s t and p r e s e n t

sand t r a n s p o r t i n t h e K i l l p e c k e r Dunes i s f r o m w e s t t o e a s t a l o n g w h a t Kolm ( 1 9 7 4 ) c a l l s t h e Wyoming w i n d c o r r i d o r . and C14

Gaylord (1979) s t a t e s t h a t " t h e s t r a t i g r a p h y

d a t e s i n d i c a t e t h a t m o s t o f t h e F e r r i s dune d e p o s i t i o n o c c u r r e d u n d e r r e l -

a t i v e l y d r y c l i m a t i c c o n d i t i o n s between 7,660 and 6,460 y r s . B.P."

He s t a t e s

f u r t h e r t h a t e o l i a n d e p o s i t i o n y o u n g e r t h a n 6,460 y r s . B.P. has a g r e a t e r number and f r e q u e n c y o f i n t e r d u n a l pond l a y e r s s u g g e s t i n g t h a t t h e c l i m a t e has i n g e n e r a l ( w i t h m i n o r f l u c t u a t i o n s ) been l e s s a r i d s i n c e t h i s m i d - H o l o c e n e t i m e . Phase I11 ( L a t e H o l o c e n e ) The t h i r d phase o f H o l o c e n e e o l i a n a c t i v i t y i n i n t e r m o n t a n e b a s i n s p o s t d a t e s t h e T r i p l e Lakes N e o g l a c i a l advance (5,000-3,000 (Benedict,

1973).

y r s . B . P . ) i n t h e Rocky Plountains

M o d e r a t i o n o f t h e x e r i c A l t i t h e r m a l p e r i o d i s marked by m a r l s

and r o o t t u b u l e s i n t h e K i l l p e c k e r Dunes Upper Sand w h i c h i s d a t e d a t 2,820595 y r s . B.P.

(Table 1).

However, s u c h m a r l s a r e l a t e r a l l y d i s c o n t i n u o u s w i t h i n a

dune f i e l d and we m u s t r e l y o n o n l y t h o s e dune f i e l d s t h a t came i n t o e x i s t e n c e d u r i n g Phase 111 t o g e t a more a c c u r a t e d e t e r m i n a t i o n o f t h e o n s e t o f Phase 111. The N o r t h P a r k Dunes i n C o l o r a d o o r i g i n a t e d a t t h i s t i m e .

As n o t e d i n F i g u r e 5

and T a b l e 2, a l l u v i a l m a t e r i a l exposed a l o n g N o r t h Sand Creek and E a s t Sand Creek contains radiocarbon d a t a b l e m a t e r i a l i n t h e form o f peats which predate t h e d o r m a n t dunes.

Dune movement a f t e r 2,000 y r s . B.P. b u t b e f o r e 1,000 y r s . B.P.

388 ______ 0'55'

.

106O15

106910'

106'05'

Tllh TlOh

TlOl

Figure 5. Map of North Park dune f i e l d showing a c t i v e and dormant eolian sand a n d l o c a t i o n o f radiocarbon dated samples. I n s e r t shows s t r a t i g r a p h y a t sampled s i t e s . Map modified from Ahlbrandt and Andrews (1978, Fig. 2 ) . i s demonstrated both by radiocarbon dates of material beneath the dunes a n d by material buried within the dormant dunes. The East Sand Creek s e c t i o n contains a t h i c k ( 9 1 cm) peat bog with well preserved twigs dated 2,100+200 y r s . B . P . a t a depth of 1,006 cm below the surface.

Two radiocarbon dates were made on material

from t h e North Sand Creek section (Fig. 5 ) ; a 2,830+200 y r s . B.P. date f o r a 15 cm peat horizon 686 cm below the s u r f a c e and a 1,070+200 y r s . B . P . date f o r a very t h i n ( 2 . 5 cm) peat l a y e r 610 cm below the surface. Possibly the young date f o r the t h i n peat l a y e r may r e f l e c t contamination, s i n c e a radiocarbon date on

389 TABLE 2.

RADIOCARBON DATES FROM NORTH PARK DUNES, COLORADO Depth below s u r f a c e (cm)

Radiocarbon age (yrs.BP) and l a b . no.

1006

2110t200 (W-3656)I

Peat Bog ( 9 1 cm t h i c k ) w i t h s t i c k s i n place (predune a l l u v i u m )

NEk,NEk,sec.l2, T. 10N, R. 79W.

686

2830+200 (W-3655)I

Peat ( 1 5 cm t h i c k ) (predune a1 1 u v i urn)

N o r t h Sand Creek

NEk,NEk,sec.lZ, T.lON,R.79W.

6 10

1070+200 (W-3644)'

T h i n p e a t (2.5 cm t h i c k ) (predune a1 1u v i um)

N o r t h Sand

SWk,NWk,sec.6, T.lON,R.78W.

0- 10

1250+200 (W-3653)'

Charcoal and wood f r o m t r e e stump emerging f r o m barchan dune

Site

Legal l o c a t i o n

East Sand Creek

NW'*,NWk,sec. 12, T.9N,R. 78W.

N o r t h Sand Creek

Hills

'Meyer Rubin,

Material dated

U.S. G e o l o g i c a l Survey Radiocarbon L a b o r a t o r y , Reston, Va.

c h a r c o a l and wood from a t r e e b u r i e d b y d o r m a n t dunes was 1,250i-200 y r s . B.P. (Table 2 ) .

Dune movement between 2,000 and 1,000 y r s . B.P. r e f l e c t s t h e fitkt

p u l s e o f Phase 111.

A second p u l s e o f Phase I 1 1 seems t o have o c c u r r e d between

500 a n d 1 , 0 0 0 y r s . B.P. i n t h e K i l l p e c k e r Dunes, where dune sand o v e r l i e s bone and c h a r c o a l d a t e d a t 7 5 5 i 9 0 y r s . B.P.

( T a b l e 1 ) . T h i s second p u l s e of Phase I 1 1

a c t i v i t y a p p e a r s t o p o s t d a t e B e n e d i c t ' s ( 1 9 7 3 ) Audubon ( N e o g l a c i a l ) advance (1,850-

950 y r s . B . P . ) . Phase I V ( R e c e n t ) As we s t a t e d p r e v i o u s l y , s e v e r a l dunes f i e l d s - i n c l u d i n g t h e K i l l p e c k e r , F e r r i s , N o r t h P a r k , and G r e a t Sand Dunes, c o n t a i n a r e a s o f p r e s e n t l y a c t i v e dunes.

How-

e v e r , t h e a c t i v e dune a r e a s a r e s u r r o u n d e d b y much l a r g e r a r e a s o f d o r m a n t dunes, as i l l u s t r a t e d f o r K i l l p e c k e r Dunes i n F i g u r e 4 and N o r t h P a r k Dunes i n F i g u r e 5 . The o n l y e v i d e n c e f o r t h e o n s e t o f Phase I V i s t h e d a t e o f 22Oi90 y r s . B.P. f o r dead sage ( A r t e m i s i a ) e m e r g i n g f r o m t h e t r a i l i n g edge o f a c t i v e dunes ( F i g . 4 and T h i s d a t e i n d i c a t e s t h a t Phase I V a c t i v i t y e x t e n d s back a t l e a s t two

T a b l e 1).

hundred y e a r s . HOLOCENE EOLIAN ACTIVITY--NEBRASKA SAND H I L L S The Sand H i l l s o f c e n t r a l Nebraska ( F i g . 1 ) have a n a r e a o f a p p r o x i m a t e l y

2

57,000 km a n d c o n s t i t u t e t h e l a r g e s t sand sea i n t h e Western Hemisphere (Smith, 1965, p . 5 5 7 ) . (e.g.

A v a r i e t y o f c h r o n o l o g i e s f o r dune a c t i v i t y have been p r o p o s e d

Lugn, 1935, p . 161; S m i t h , 1965, p . 573; Reed a n d Dreeszen, 1965, p. 4 ) .

These s h a r e a common theme, t h a t t h e f o r m a t i o n o f t h e l a r g e - s c a l e dune f o r m s comp r i s i n g m o s t o f t h e Sand H i l l s o c c u r r e d d u r i n g t h e l a t e P l e i s t o c e n e .

Another e l e -

ment common t o t h e s e c h r o n o l o g i e s i s t h e l a c k o f d i r e c t e v i d e n c e f r o m w e l l - d a t e d

'

390 l o c a l i t i e s documenting the s t r a t i g r a p h i c r e l a t i o n s h i p s between eolian sand and o l d e r o r younger deposits. For example, Lugn (1935, p . 161) c o r r e l a t e d the majori t y of the Nebraska Sand H i l l s with l a t e Wisconsin Peoria loess occurring e a s t and south of the sand sea. This c o r r e l a t i o n was based primarily on a reported decrease in sand content and thickness of the loess with increasing distance from the margin of the Sand H i l l s . Lugn (1968, p . 161) a l s o reported t h a t the loess and dune sand a r e "somewhat interbedded" in the loess-Sand H i l l s t r a n s i t i o n zone. However, our f i e l d work and t h a t of Ahlbrandt a n d Fryberger (1980) could not subs t a n t i a t e any interbedding. Ahlbrandt a n d Fryberger (1980, p . 2 2 ) concluded t h a t the dune sand unconformably o v e r l i e s t h e loess a t the southeast edge of the Sand H i l l s . They i n t e r p r e t e d sands interbedded in the loess sequence t o have been deposited in a f l u v i a l , n o t e o l i a n , environment. I n a study of the e n t i r e Nebraska Sand H i l l s , Smith (1965) discerned two main periods of eolian a c t i v i t y . The f i r s t was provisionally thought t o have occurred in the e a r l y Wisconsin and formed the l a r g e transverse dunes ( S e r i e s I dunes). He suggested t h a t a s i g n i f i c a n t s h i f t in wind regime occurred in the l a t e Wisconsin and t h a t a l e s s intense period of eolian a c t i v i t y produced smaller longitudinal dunes (Series 1 1 ) , which a r e superimposed on the o l d e r and l a r g e r dunes in many places. Smith based h i s conclusion of a change in wind regime on external dune morphology. Ahlbrandt and Fryberger (1980) examined internal s t r u c t u r e s in dunes c l a s s i f i e d by Smith as longitudinal ( S e r i e s 11) dunes a n d found them t o be transverse dunes. They suggested t h a t only one major episode of dune formation, dominated by a s i n g l e wind regime, was necessary t o generate the sand sea. Warren (1976) a l s o argued t h a t b o t h large- a n d small-scale dunes could have been generated under one wind regime, although he accepted a l a t e Pleistocene age f o r the Sand H i l l s . Smith (1965) believed t h a t post-glacial e o l i a n a c t i v i t y was imi ted t o s u p e r f i c i a l modification of the o l d e r dunes. Previous s t u d i e s by Sears (1961) and Watts a n d Wright (1966) have reported radiocarbon dates of 5,040k95 and 12,600?160 y r s . B . P . ( F i g . 6 ) f o r organ c material recovered from i s o l a t e d cores beneath Sand H i l l s lakes and interdune v a l l e y s . A core taken a t Krause Lake (Fig. 6 ) yielded a date of 12,080i380 y r s . B.P. a t 275280 cm and 3,140t-187 y r s . B . P . a t 120-130 cm (Odgen a n d Hay, 1965, p . 168). I n a d d i t i o n , Stuvier (1969, p . 578) reported a date of 8,950k160 y r s . B.P. on material cored between 1,472-1,482 cm below Swan Lake (Fig. 6 ) . However, a t none of these s i t e s was the s t r a t i g r a p h i c r e l a t i o n s h i p of the dated horizon t o nearby dune sand documented. Before such s i t e s can provide unequivocal data f o r e s t a b l i s h i n g an eolian chronology, t h a t r e l a t i o n s h i p must be e s t a b l i s h e d . This will require a s e r i e s of cores or t e s t holes extending from the interdunes well i n t o the dunes themselves. Until such work i s done, radiocarbon-dated sediments cannot be assumed t o postdate the surrounding dunes simply by t h e i r geographic position or by continuity of sediment within the cores. We conclude, as did F l i n t (1976, p . 525),

391

0

20

40 ,

60KlLOMETERS

1

F i g u r e 6. Map s h o w i n g r a d i o c a r b o n ages, i n y r s . B . P . , o f samples f r o m t h e Nebraska Sand H i l l s . I f more t h a n one sample was d a t e d a t a g i v e n l o c a t i o n o n l y t h e y o u n g e s t age i s shown. S i t e s I - V I I a r e new l o c a l i t i e s d e s c r i b e d i n t h i s p a p e r . Dates shown i n b r a c k e t s a r e f r o m c o r e s i n i n t e r d u n e p o s i t i o n s and t h e i r s t r a t i g r a p h i c r e l a t i o n s h i p t o t h e dune sand has n o t been e s t a b l i s h e d . t h a t a r e a s o n a b l y c o r r e c t c h r o n o l o g y o f e o l i a n a c t i v i t y i n t h e Nebraska Sand H i l l s had n o t been e s t a b l i s h e d b y t h i s e a r l i e r w o r k . I n t h i s p a r t o f o u r s t u d y we w i l l a t t e m p t t o document t h e s t r a t i g r a p h i c p o s i t i o n o f r a d i o c a r b o n d a t e d o r g a n i c - r i c h h o r i z o n s w i t h r e s p e c t t o e o l i a n sand.

With

t h e s e w e l l - d a t e d l o c a l s t r a t i g r a p h i c sequences and a r e i n t e r p r e t a t i o n o f p r e v i o u s d a t e s , we hope t o make some p r o g r e s s t o w a r d s a more f i r m l y based c h r o n o l o g y o f e o l i a n a c t i v i t y i n t h e Nebraska Sand H i l l s such as t h a t shown i n F i g u r e 3B. E a r l y Holocene E a r l y H o l o c e n e e o l i a n a c t i v i t y , i f i t o c c u r r e d a t a l l , i n t h e Sand H i l l s i s p o o r l y documented.

S i t e V I ( F i g s . 6 and 7 ) a t t h e head o f W h i t e t a i l Creek i n t h e

s o u t h w e s t e r n p a r t o f t h e Sand H i l l s y i e l d e d a r a d i o c a r b o n d a t e o f 9,930+140 y r s .

B.P.

( T a b l e 3 ) f o r t h e b a s a l 3 cm o f a 2 . 1 m t h i c k o r g a n i c - r i c h sandy s i l t .

This

u n i t c o n t a i n s some p l a n t r e m a i n s a l o n g w i t h s c a t t e r e d g a s t r o p o d s and p e l e c y p o d s . R i c h a r d M a d o l e o f t h e USGS i n Denver, C o l o r a d o u s e d a m o d i f i e d K i h l t r e a t m e n t ( K i h l , 1975) t o e x t r a c t t h e o r g a n i c c a r b o n f r o m t h i s sample w h i c h c o n t a i n e d some modern r o o t l e t s .

Up t o 6 m o f e o l i a n sand d i r e c t l y o v e r l i e s t h e o r g a n i c - r i c h u n i t

and b a r c h a n dunes u p t o 40 m h i g h o c c u r w i t h i n 0 . 4 km o f t h e d a t e d h o r i z o n ( F i g . 7 ) . I t i s p o s s i b l e t h a t e o l i a n a c t i v i t y , c o r r e l a t i n g w i t h Phase I a c t i v i t y d e s c r i b e d

above f o r Wyoming, commenced s h o r t l y a f t e r d e p o s i t i o n o f t h e o r g a n i c - r i c h u n i t . However, w i t h o u t a d d i t i o n a l s t r a t i g r a p h i c i n f o r m a t i o n and r a d i o c a r b o n d a t i n g , l a t e

392

1020

5 150-f400 8410fllO 1000

980

f 980 a cn J""

-049

I Dismal River Ranch

(A)

II: Dismal River Ronch ( 8 )

IUWarner Bridge 1080

v,

n

W b-

1060

w

860

880

840

860

820

840

z W 0

3

1040

e

1020

l-

1

P Natick

l P C o l l i e r Ranch

1

a

1100

41: Whitetail Creek

AGE

Alluvial sond ond sllt containing organic-rich

1080

1060

Pliocene ( 7 )

Alluvial sond, silt and grovel 0 5 KILOMETER

YU Snake River

V e r t i c a l e xo g g e ro t io n x18

F i g u r e 7 . G e n e r a l i z e d g e o l o g i c s e c t i o n s a t Sand H i l l s s i t e s I - V I I . A l l u v i a l sand and s i l t u n i t c o n t a i n i n g o r g a n i c - r i c h zones may, i n p a r t , be o l d e r t h a n Holocene. Refer t o Table 3 f o r d e t a i l s on s p e c i f i c s i t e s . Wisconsin o r e a r l y Holocene a c t i v i t y i n t h i s area must remain s p e c u l a t i v e . W r i g h t , J r . ( p e r s . comm., y r s . B.P.

H. E .

1982) s u g g e s t e d t h a t t h e r a d i o c a r b o n d a t e o f 8,950+150

(Ogden and Hay, 1965, p . 1 6 8 ) f o r o r g a n i c - r i c h sand a t t h e base o f t h e

14 m c o r e f r o m Swan Lake ( F i g . 6 ) p o s t d a t e d e o l i a n a c t i v i t y o n t h e s o u t h w e s t e r n edge o f t h e Sand H i l l s .

The c o r e was composed e n t i r e l y o f homogeneous, f i n e - g r a i n e d

a l g a l s e d i m e n t h a v i n g no i n t e r b e d s o f sand.

I s o l a t e d b a r c h a n dunes o c c u r i n t h e

v i c i n i t y o f Swan Lake and may have f o r m e d p r i o r t o 8,950 y r s . B . P . Holocene phase o f e o l i a n a c t i v i t y .

However,

i n an e a r l y

i t i s a l s o p o s s i b l e t h a t t h e barchan

393 TABLE 3.

NEW RADIOCARBON DATES FROM THE NEBRASKA SAND HILLS

Site

Legal l o c a t i o n

I-Dismal R i v e r Ranch ( A )

SWL,SE%, sec. 35, T. 2IN, R. 33W.

11-Dismal R i v e r Ranch ( B )

SEh,NEL,sec.30, T.22N, R. 32W.

I II-Warner Bridge NWL, NEL, sec. 2 2 ,

Depth below e o l i a n sand (cm) 40- 45 40- 45 45- 55 345-360 10- 17 30- 45

0- 40

Radiocarbon age (yrs.BP) and l a b . no.

3000+400 3560T 70 3810T 80 4900T500

(NWU-83)4 (BETA-5429) (W-4900)5 (NWU-82)4

5150+400 (NWU-84): 8410+llO (W-4926) 3600+400 (NWU-85)4

1

Thickness of overlying eolian sand (m)

44

40

1

T.21N,R.28W. I V - C o l l i e r Ranch

7220+1202 (1-9839)

14

410-420 530-590

3110+ 80 (W-4923)5 5040z 802 (DIC-2075)

13

199-202

9930+140 ( W-4904)

SWk,NWk,sec.23, T.21N,R. 26W.

80-100

SW%,NEL,sec.21, T.23N,R. 27W.

VI-Whitetail Creek

SEL, SE4, sec. 26, T. 16N,R.38W.

VII-Snake R i v e r

SEL,SWk,sec. T.30N,R.38W.

V- Nat ic k

18,

0- 10

8605 553 (DIC-2074)

6 8

' A l l dates made on o r g a n i c - r i c h sands o r s i l t s except DIC-2075, which was on wood %amp1 e r e c e i v e d h o t NaOH treatment 3Average o f two determinations 940+55 (no NaOH treatment) and 780+60 ( t r e a t e d w i t h NaOH) 4George Coleman, Nebraska Wesleyan-Radiocarbon Laboratory IiMeyer Rubin, U.S. Geological Survey Radiocarbon Laboratory, Reston, Va.

dunes could have m i g r a t e d o v e r the Swan Lake s i t e w i t h o u t l e a v i n g any r e c o r d a s i s t h e c a s e i n d e f l a t i o n a r y i n t e r d u n e s ( A h l b r a n d t and F r y b e r g e r , 1981, p . 2 9 8 ) . Middle Holocene The p r e s e n c e o f a mid-Holocene warm a n d / o r a r i d p e r i o d ( A l t i t h e r m a l o r Hypsit h e r m a l ) l a s t i n g some 2,500 t o 3,000 y r s . has been well documented i n t h e Central and Northern P l a i n s ( W r i g h t , 1970 and 1976) and t h e Rocky Mountains ( B e n e d i c t , 1973). I n western Iowa, B e t t i s (1982) p r e s e n t e d e v i d e n c e o f a major e r o s i o n a l unconformity i n small v a l l e y a l l u v i a l f i l l s spanning a p p r o x i m a t e l y 8,000 - 3,400

2

y r s . B . P . Grigal e t a l . (1976) documented a 580 km dune f i e l d i n n o r t h - c e n t r a l Minnesota t h a t was a c t i v e between a b o u t 8 , 0 0 0 and 5,000 y e a r s ago. In s p i t e of

t h i s e v i d e n c e f o r a major warming and probably a r i d p e r i o d a c r o s s t h e p l a i n s and u n l i k e t h e Wyoming dune f i e l d s , we have no d i r e c t l y d a t e d mid-Holocene e o l i a n a c t i v i t y i n the Nebraska Sand H i l l s . A 4 . 8 m c o r e o b t a i n e d by S e a r s (1961) from Hackb e r r y Lake ( F i g . 6 ) c o u l d be i n t e r p r e t e d t o p o s t d a t e e o l i a n a c t i v i t y i n t h e a r e a .

He o b t a i n e d a r a d i o c a r b o n d a t e o f 5 , 0 4 0 i 9 5 y r s . B . P . on brown ooze ( g y t t j a ) a t a d e p t h of 4 . 5 m and p r o b a b l y n e a r t h e beginning of l a c u s t r i n e d e p o s i t i o n . The p o l l e n p r o f i l e from t h i s c o r e i n d i c a t e s a f o r e s t maximum ( h a l f of t h e f o r e s t p o l l e n i s

3 94 p i n e ) between d e p t h s o f 4 and 4 . 8 m e t e r s .

A g a i n , we h a v e n o s a t i s f a c t o r y means

t o e v a l u a t e t h e s t r a t i g r a p h i c r e l a t i o n s h i p o f t h e s u r r o u n d i n g dune sand t o t h e d a t e d sequence i n t h i s c o r e . B r i c e (1964, p . D5) d a t e d a p e a t a t 8,400+250 y r s . B.P. exposed a l o n g t h e North Loup R i v e r ( F i g . 6 ) .

The p e a t bed i s a b o u t 1 . 5 m e t e r s b e l o w t h e t o p o f a t e r r a c e

w h i c h i s o v e r l a i n b y u p t o 7 m e t e r s o f dune s a n d .

B r i c e (1964, p . D9) a l s o n o t e d

t h a t dune sand m a n t l e s P e o r i a Loess a l o n g t h e n o r t h s i d e o f t h e N o r t h Loup R i v e r and t h a t t h e l o e s s i s " r e m a r k a b l y f r e e o f i n t e r b e d d e d dune s a n d . " dence l e a v e s open t h e p o s s i b i l i t y o f m i d - H o l o c e n e e o l i a n a c t i v i t y .

Brice's eviHowever, t h i s

a r e a i s o n l y a b o u t 40 km e a s t o f t h e D i s m a l - M i d d l e Loup r e g i o n where we have good evidence f o r o n l y late-Holocene a c t i v i t y (see below).

T h e r e f o r e , we c o n s i d e r i t

more l i k e l y t h a t l i t t l e , i f a n y , H o l o c e n e e o l i a n d e p o s i t i o n o c c u r r e d i n t h e N o r t h Loup a r e a p r i o r t o 3,000 y r s . B.P. We f e e l c e r t a i n t h a t , s o o n e r o r l a t e r , r a d i o c a r b o n d a t a b l e m a t e r i a l o r archeol o g i c e v i d e n c e w i l l a l l o w d o c u m e n t a t i o n o f e o l i a n a c t i v i t y d u r i n g t h e mid-Holocene. U n t i l t h e n , we m u s t r e l y o n i n d i r e c t e v i d e n c e f r o m s u r r o u n d i n g a r e a s t o p o s t u l a t e a m a j o r p e r i o d o f dune f o r m a t i o n d u r i n g t h e m i d - H o l o c e n e i n t h e Nebraska Sand H i l l s ( F i g . 38).

V e r y l i t t l e l a t e P l e i s t o c e n e - H o l o c e n e s e d i m e n t s u n d e r l i e t h e dune

sand o u t s i d e o f t h e g e n e r a l a r e a o f t h e D i s m a l , M i d d l e and N o r t h Loup r i v e r s .

In

t h e n o r t h e r n and n o r t h w e s t e r n p o r t i o n s o f t h e sand sea t h e dune sand g e n e r a l l y r e s t s d i r e c t l y o n t h e O g a l l a l a Group ( M i o c e n e ) s e d i m e n t s .

Therefore, obtaining

u s e f u l l i m i t s o n e o l i a n a c t i v i t y t h e r e may p r o v e t o b e q u i t e d i f f i c u l t .

The most

p r o d u c t i v e f u t u r e r e s e a r c h may l i e i n d e t a i l e d d r i l l i n g and c o r i n g o f h o l e s on t r a n s e c t s t h a t c r o s s i n t e r d u n e s and dunes. L a t e Holocene The b e s t documented phase o f e o l i a n a c t i v i t y o c c u r s between a p p r o x i m a t e l y 3,000 and 1,500 y r s . B.P.

i n t h e a r e a s o f t h e Dismal a n d M i d d l e Loup r i v e r s ( F i g s . 7

and 8 ) i n t h e e a s t - c e n t r a l p a r t o f t h e Sand H i l l s . (Pliocene?) alluvial-lacustrine v a l l e y - f i l l

A 60-70

m t h i c k l a t e Cenozoic

sequence t r e n d i n g s o u t h w e s t t o n o r t h -

e a s t i s p r e s e n t i n t h i s a r e a ( F i g . 9 ) b e n e a t h t h e dune sand (Maroney, 1 9 7 8 ) .

This

sequence i s o v e r l a i n by H o l o c e n e f l u v i a l c h a n n e l - f o r m sand b o d i e s t h a t a r e as t h i c k as 15 m and t h a t l o c a l l y c o n t a i n beds o f l a t e r a l l y d i s c o n t i n u o u s , o r g a n i c r i c h sand and s i l t up t o 3 m t h i c k .

The means o f t e n r a d i o c a r b o n d a t e s o b t a i n e d

o n m a t e r i a l f r o m t h e s e o r g a n i c - r i c h u n i t s a t f i v e s i t e s r a n g e f r o m 8,410 t o 3,000 y r s . B.P.

(Table 3 ) .

The s i t e s were chosen b y S w i n e h a r t a n d Maroney because

( e x c e p t a t S i t e 111) s i g n i f i c a n t t h i c k n e s s e s ( u p t o 40 m) o f e o l i a n sand o v e r l i e the dated horizons.

No w e a t h e r i n g p r o f i l e o r o t h e r f e a t u r e i n d i c a t i n g a h i a t u s

a t t h e a l l u v i a l - d u n e sand c o n t a c t e x i s t s a t any o f t h e f i v e s i t e s .

Moreover, t h e

dune sand and a l l u v i a l s e d i m e n t s a p p a r e n t l y a r e n o t contemporaneous f a c i e s because no i n t e r b e d d i n g o f t h e t w o o r g r a d a t i o n f r o m one t o t h e o t h e r i s e v i d e n t .

We

Figure 8. Part o f Landsat iiriage ( E - 1 5 2 9 - 1 6 5 3 0 - 7 ) o f t h e i l i d d l e Loup-Dismal River a r e a in t h e ilehraska Sand H i l l s . Image taken o n J a n . 3 , 1 9 7 4 . Recent snow f a l l coiribined with low w i n t e r sun a n g l e a c c e n t u a t e s t o p o g r a p h i c f e a t u r e s . Priiiiary dune t y p e s o c c u r r i n g w i t h i n t h e S a n d H i l l s a r e i n d i c a t e d along with l o c a t i o n o f g e o l o g i c s e c t i o n A - A ' .

w W ul

396

WEST

EASl AGE

SITE A

SITE

I

;I

\ 71 K

I 'olocene

tm

UNITS

A'

Dune sand

r I080

I Alluvial

sond a n d s i l t contalnlnq local o r q o m c - r i c h horlzons

P l i o c e n e ( 7 ) j Alluvial % I t , sand, and g r a v e l Miocene

1

1 1 I

t

Oqollalo Group I990

1

Dismal River p r o f i l e /

I

~

-.-

0-

---a10

20 Km

10 I

F i g u r e 9 . S c h e m a t i c c r o s s - s e c t i o n A - A ' a l o n g t h e Disiiial R i v e r s h o w i n g l o c a t i o n s o f r a d i o c a r b o n sample s i t e s a n d a n a d d i t i o n a l 18 measured s e c t i o n s . T e s t h o l e 33-B-71 p r o j e c t e d f r o i i i a p p r o x i m a t e l y 5 km n o r t h o f t h e D i s m a l R i v e r . c o n s i d e r t h e dune sand t o kfave formed i n a d i f f e r e n t and l a t e r e n v i r o n m e n t , c e r t a i n l y one w i t h c o n s i d e r a b l y l e s s e f f e c t i v e p r e c i p i t a t i o n .

F u r t h e r m o r e , we p o s t u -

l a t e t h a t t h e c l i m a t i c a n d o t h e r changes n e c e s s a r y t o a l l o w e o l i a n s e d i i i i e n t a t i o r l t o r e p l a c e a l l u v i a l s e d i m e n t a t i o n t o o k p l a c e o v e r t h e same p e r i o d o f t i m e a c r o s s much o f t h e c e n t r a l a n d e a s t e r n Sand H i l l s b u t t h e b e g i n n i n g o f e o l i a n Sedimentation was n o t n e c e s s a r i l y s y n c h r o n o u s t h r o u g h o u t t h e e n t i r e a r e a .

S i m i l a r l y , t h e youngest

a l l u v i a l s e d i m e n t s a t a n y s i t e w o u l d n o t n e c e s s a r i l y r e c o r d t h e end o f a l l u v i a l d e p o s i t i o n everywhere.

Based o n t h e s e a r g u m e n t s , we c o n c l u d e t h a t an e n v i r o n m e n t

f a v o r a b l e f o r e o l i a n s e d i m e n t a t i o n began i n t h i s a r e a soiiietiiiie a r o u n d 3,000 y r s .

B.P.

Now we w i l l e x a m i n e t h e i n d i v i d u a l s i t e s i n more d e t a i l .

A t s i t e I ( F i g . 10A) t h e S o u t h F o r k o f t h e D i s m a l R i v e r has u n d e r c u t i t s n o r t h bank a n d p r o d u c e d an e x p o s u r e 70 m h i g h . a b l y o n a 40 cm t h i c k , l i g h t - g r a y ,

A t t h i s s i t e , e o l i a n sand r e s t s conforin-

f i n e - g r a i n e d a l l u v i a l sand which i s u n d e r l a i n

by a 35 cm t h i c k , v e r y d a r k g r a y o r g a n i c - r i c h ,

diatomaceous s i l t y sand.

This or-

g a n i c - r i c h sand has some s m a l l r o d e n t ( ? ) b u r r o w s i n t h e u p p e r 15 cm a n d has an i r r e g u l a r upper c o n t a c t . dated (Table 3 ) .

T h r e e samples o f t h i s o r g a n i c - r i c h sand were r a d i o c a r b o n

A sample c o l l e c t e d b y t w o o f us ( S w i n e h a r t and M a r o n e y ) i n 1977

f r o i n t h e t o p 5 cm o f t h e o r g a n i c - r i c h s a n d y i e l d e d a d a t e o f 3,000~400 y r s . B.P.

(NWU-83). tion.

The sample c o n t a i n e d a s m a l l amount ( ~ 1 2 )o f modern r o o t l e t c o n t a m i n a -

I n 1980, S w i n e h a r t and R i c h a r d M a d o l e o f t h e U . S . G e o l o g i c a l S u r v e y c o l -

l e c t e d a sample f r o m a p p r o x i m a t e l y 5-15 cm b e l o w t h e t o p o f t h e o r g a n i c - r i c h sand. T h i s sample a l s o c o n t a i n e d a s m a l l amount o f modern r o o t l e t s .

Madole used a

[ m o d i f i e d K i h l t r e a t m e n t ( K i h l , 1975) t o c o n c e n t r a t e t h e o r g a n i c s i n t h e sample

397

F i g u r e 10. R a d i o c a r b o n sample s i t e s I , 1 1 , I V , and V s h o w i n g t h e c o n t a c t between dune sand and a l l u v i a l s a n d . R a d i o c a r b o n d a t e s shown i n y r s . B . P . A, S i t e I on t h e s o u t h f o r k o f t h e Disriial R i v e r . A p p r o x i i i i a t e l y 2 5 111 o f e o l i a n sand i s exposed above t h e a l l u v i a l sand. P h o t o g r a p h t a k e n i n A p r i l , 1980. B . S i t e I 1 on t h e s o u t h f o r k o f t h e D i s m a l R i v e r . A p p r o x i i i i a t e l y 20 in o f dune sand exposed above a l l u v i a l s a n d . P h o t o g r a p h t a k e n i n May 1978. C , S i t e I V a l o n g t h e Dismal R i v e r s o u t h o f H a l s e y F o r e s t . Maxiriiurii h e i g h t o f e x p o s u r e i s 25 111. P h o t o g r a p h t a k e n i n Hay 1978. D. S i t e V a l o n g t h e M i d d l e Loup R i v e r . Combined t h i c k n e s s o f a l l u v i a l s s n d , s i l t , and p e b b l y sand a n d o r g a n i c r i c h s i l t s and p e a t l e n s e s above r i v e r l e v e l i s 6 m. Two s o u t h - s o u t h e a s t d i p p i n g t a b u l a r - p l a n a r e o l i a n c r o s s bed sets a r e v i s i b l e . L o w e s t s e t a p p r o x i m a t e l y 6 in t h i c k . P h o t o g r a p h t a k e n i n June 1980.

398 and a d a t e o f 3,810k80 y r s . B.P.

(W-4900) was o b t a i n e d by Fleyer R u b i n o f t h e USGS

R a d i o c a r b o n Lab i n Reston, V i r g i n i a .

F i n a l l y i n 1982, S w i n e h a r t r e s a m p l e d t h e

t o p 5 cm and c a r e f u l l y e l i m i n a t e d as much o f t h e s m a l l r e s i d u a l amount o f modern r o o t l e t s a s p o s s i b l e f r o m t h e sample.

I t was s u b m i t t e d t o a t h i r d l a b ( B e t a Ana-

l y t i c I n c . ) and a d a t e o f 3,560t70 y r s . B.P.

( B e t a - 5 4 2 9 ) was o b t a i n e d .

Given

t h e l a r g e e r r o r a s s o c i a t e d w i t h sample NWU-83 ( T a b l e 3 ) , we c o n c l u d e t h a t a d a t e o f a b o u t 3,400 y r s . B.P.

r e p r e s e n t s a good e s t i m a t e o f t h e t e r m i n a t i o n o f a l l u v i a l

sedimentation a t t h i s s i t e . I n a d d i t i o n t o t h e above t h r e e r a d i o c a r b o n d a t e s , a sample o f o r g a n i c - r i c h sand from 345 t o 360 cm b e l o w t h e base o f t h e dune s a n d y i e l d e d a s t r a t i g r a p h i c a l l y c o n s i s t e n t age o f 4,900+500 y r s . B.P. ( T a b l e 3 ) .

It i s o f i n t e r e s t t o note that

e s s e n t i a l l y n o o r i g i n a l s e d i m e n t a r y s t r u c t u r e s were o b s e r v e d i n t h e 25 m e t e r s o f dune sand e x p o s e d above t h e a l l u v i a l sand ( F i g . 10A).

I n s t e a d , t h e s e c t i o n con-

t a i n s a s e r i e s o f b r o w n i s h g r a y (10YR6/2) t o brown (10YR6/3) d i s c o n t i n u o u s , wavy bands between 1 and 10 cm t h i c k and spaced 10 t o 25 cm a p a r t .

These bands have

a h i g h e r s i l t p l u s c l a y c o n t e n t t h a n t h e l i g h t e r c o l o r e d sand and a r e s l i g h t l y indurated.

S i m i l a r s t r u c t u r e s w e r e d e s c r i b e d b y A h l b r a n d t and F r y b e r g e r (1980,

p. 1 3 ) and t h e y f o u n d them t o o c c u r t h r o u g h o u t t h e S a n d h i l l s .

They c o n c l u d e d

t h a t t h i s t y p e o f d i s s i p a t i o n s t r u c t u r e was p r o d u c e d b y a f r e e z e - t h a w mechanism. R e g a r d l e s s o f how t h e s e s t r u c t u r e s f o r m , i t m u s t b e a v e r y r a p i d p r o c e s s f o r over 100 bands have f o r m e d i n t h e e o l i a n sand a t s i t e I i n l e s s t h a n p r o b a b l y 2,000 y e a r s ( a l l o w i n g 1,500 y r s f o r a c c u m u l a t i o n o f dune s a n d ) .

We b e l i e v e i t was

s t r u c t u r e s s i m i l a r t o t h e s e and d e s c r i b e d by S m i t h (1968, p . 3 2 ) as "a more weathe r e d and somewhat i n d u r a t e d dune sand, w i t h a d i s t i n c t i v e b r o w n i s h s t r e a k i n g o r mottling,"

t h a t he i n t e r p r e t e d t o r e p r e s e n t "an o l d e r s o i l zone . . . p r o v i s i o n a l l y

assigned t o t h e e a r l i e r p a r t o f t h e l a s t g l a c i a l stage."

C l e a r l y , a t s i t e I,

these d i s s i o a t i o n s t r u c t u r e s a r e n o t t h e product o f a s o i l forming process. S i t e I 1 a l s o i s s i t u a t e d a l o n g t h e S o u t h F o r k o f t h e Dismal R i v e r , ( F i g s . 7 and 8 ) a n d has a s i m i l a r s t r a t i g r a p h y t o s i t e I. B o t h r a d i o c a r b o n samples f r o m t h i s s i t e ( T a b l e 3 ) were d r i e d and s i e v e d t h r o u g h a #230 ( 6 2 ~ 1 )s i e v e and a l l v i s i b l e modern r o o t l e t s were p i c k e d f r o m t h e samples p r i o r t o d a t i n g .

Though a l l u v i a l

d e p o s i t i o n may have ended a t p e r h a p s 4,500 y r s . B.P. a t t h i s s i t e , i t does n o t n e c e s s a r i l y f o l l o w t h a t e o l i a n d e p o s i t i o n began i m m e d i a t e l y a f t e r t h i s t i m e .

At

s i t e I, o n l y 4 km n o r t h e a s t , a l l u v i a l d e p o s i t i o n c o n t i n u e d u n t i l a b o u t 3,400 y r s . B.P.

Reed and D r e e s z e n (1965, p . 6 5 ) f i r s t d e s c r i b e d t h e s e o r g a n i c - r i c h sands

f r o m e x p o s u r e s o n t h e Dismal R i v e r 4 km e a s t o f s i t e 1 1 .

It i s interesting t o

n o t e t h a t t h e y c o n s i d e r e d t h i s h o r i z o n c o r r e l a t i v e e i t h e r w i t h t h e G i l m a n Canyon F o r m a t i o n ( a b o u t 30,000 y r s o l d ) o r w i t h t h e Sangamon s o i l ( p r e - W i s c o n s i n ) and r e p o r t e d i t t o b e o v e r l a i n by 43 m o f " m e d i a l W i s c o n s i n P e o r i a Dune Sand." Our p r i m a r y c o n c e r n i n i n t e r p r e t i n g t h e s t r a t i g r a p h y exposed a l o n g t h e Dismal R i v e r was t h e p o s s i b i l i t y t h a t t h e o r g a n i c - r i c h sands, r a t h e r t h a n b e i n g p a r t o f

~.

.

~

399 an a l l u v i a l f i l l c o n f i n e d t o an o l d e r and somewhat broader Dismal R i v e r .

Site

I11 ( F i g s . 7 and 8), where t h e d a t e d h o r i z o n m i g h t be i n t e r p r e t e d t o be p a r t o f a postdune a l l u v i a l f i l l , c o u l d be used t o s u p p o r t such a h y p o t h e s i s .

I f so, then

one c o u l d argue t h a t t h e dune sand o v e r l y i n g s i t e s I and I 1 r e p r e s e n t s a m i n o r , l a t e Holocene r e a c t i v a t i o n ( i . e . a " s k i n e f f e c t " ) .

However, as F i g u r e 8 shows,

t h e South Fork o f t h e Dismal R i v e r between s i t e s I and I 1 c u t s across t h e a x i a l t r e n d o f l a r g e - s c a l e dunes and i n d i c a t e s t h a t t h e r i v e r and any f i l l s a s s o c i a t e d w i t h i t p o s t d a t e t h e dunes.

I n a d d i t i o n , a t e s t h o l e (33-8-71) d r i l l e d i n 1971

by t h e C o n s e r v a t i o n and Survey D i v i s i o n o f t h e U n i v e r s i t y o f Nebraska 5 km n o r t h o f s i t e I 1 ( F i g . 8 ) p e n e t r a t e d a s i m i l a r s t r a t i g r a p h y t o t h a t exposed a l o n g t h e Dismal R i v e r ( F i g . 9 ) .

A t t h e t e s t h o l e s i t e 22 m o f dune sand i s u n d e r l a i n by

a t l e a s t 4 m o f g r a y sand w i t h i n t e r b e d d e d s i l t s and p e a t .

Unfortunately, n o t

enough o r g a n i c m a t t e r c o u l d be c o l l e c t e d t o o b t a i n a r a d i o c a r b o n d a t e .

Thus, we

b e l i e v e t h e a v a i l a b l e e v i d e n c e i n d i c a t e s t h a t t h e o r g a n i c - r i c h sand u n i t predates t h e f o r m a t i o n o f l a r g e - s c a l e dunes and t h e e s t a b l i s h m e n t o f t h e Dismal R i v e r . S i t e s 111, I V , and V ( F i g s . 7, 8, and l O C , D) a r e a t t h e s o u t h e a s t e r n edge o f t h e a r e a o f l a r g e - s c a l e dunes and where p a r a b o l i c dunes, p r o b a b l y t h e S e r i e s I 1 dunes o f Smith (1965), have been superimposed on l a r g e - s c a l e dunes.

We b e l i e v e

t h a t s i t e V p r e s e n t s i n d i s p u t a b l e evidence f o r s i g n i f i c a n t l a t e Holocene e o l i a n activity.

The M i d d l e Loup R i v e r i s a c t i v e l y c u t t i n g a near v e r t i c a l exposure

here and two t a b u l a r p l a n a r c r o s s bed s e t s w i t h s l i p - f a c e d e p o s i t s d i p p i n g up t o

24'

r e s t on an a l l u v i a l sequence ( F i g . 10D).

A l o c a l c o n c e n t r a t i o n o f t w i g s near

t h e base o f t h e exposure y i e l d e d a r a d i o c a r b o n d a t e o f 5,040+80 y r s B.P.

(DIC-

2075) and an 8 cm t h i c k o r g a n i c - r i c h s i l t , w i t h no v i s i b l e modern r o o t l e t contami n a t i o n , gave a d a t e o f 3,110+80 y r s B.P.

(W-4923) ( T a b l e 3 ) .

A 4 m thickness

o f f i n e t o coarse, t a b u l a r cross-bedded sand w i t h some i n t e r b e d d e d s i l t y sand s e p a r a t e t h e d a t e d o r g a n i c - r i c h s i l t f r o m t h e e o l i a n sand. d a t e o f a b o u t 3,000 y r s B.P.

We would suggest a

f o r t h e c e s s a t i o n o f a l l u v i a l d e p o s i t i o n and t h e

beginning o f e o l i a n deposition. Some rough e s t i m a t e s as t o t h e t e r m i n a t i o n o f t h i s l a t e Holocene e o l i a n a c t i v i t y can be made f r o m a r c h e o l o g i c evidence.

The Kelso s i t e ( F i g . 6 ) d e s c r i b e d by

K i v e t t (1970) as a Woodland complex (a r i p a r i a n - b a s e d c u l t u r e ) s i t e was dated a t

1,150+200 y r s B.P.on c h a r c o a l .

K i v e t t ' s (1970) c h a r a c t e r i z a t i o n o f Woodland

peoples i n d i c a t e s t h e y would n o t have occupied a r e g i o n t h a t was a r i d enough f o r l a r g e - s c a l e dunes t o form.

I n a d d i t i o n , Sears (1961) p o l l e n p r o f i l e f o r Hackberry

Lake ( F i g . 6 ) shows an i n c r e a s e i n sedge and a q u a t i c p o l l e n and t r e e p o l l e n somet i m e a f t e r 1,200 y r s B.P.

T h e r e f o r e , t h e l a t e Holocene phase o f a c t i v i t y i n t h e

Nebraska Sand H i l l s p r o b a b l y ended around 1,500 y r s B.P. Muhs and Madole (1980) i n d i c a t e d t h a t dunes a l o n g t h e South P l a t t e R i v e r i n Colorado and a l o n g t h e Arkansas and Cimarron R i v e r s i n Kansas have s i m i l a r s o i l p r o f i l e s t o t h e V a l e n t i n e s o i l s ( E l d e r , 1969, p. 15) developed on dunes i n t h e

400 a r e a o f t h e Cismal and M i d d l e Loup r i v e r s .

However, s i n c e t h e V a l e n t i n e s o i l s

e x i s t i n t h o s e p a r t s o f t h e Nebraska Sand H i l l s where e o l i a n a c t i v i t y p o s s i b l y was l i m i t e d t o t h e m i d - H o l o c e n e , s o i l s d a t a p r o b a b l y do n o t p r o v i d e a b a s i s f o r d i f f e r e n t i a t i o n o f s o i l s f o r m e d w i t h i n t h e l a s t 8,000 y r s . The b e s t e v i d e n c e t h a t l a t e H o l o c e n e e o l i a n a c t i v i t y was accompanied by a region. a l l y warmer and more a r i d c l i m a t e comes f r o m t h e C o l o r a d o F r o n t Range.

Benedict

(1973, p . 592) d e s c r i b e d an i m p o r t a n t n o n g l a c i a l i n t e r v a l between a b o u t 3,000 and 1,800 y r s B.P. and s a i d i t was a t i m e o f s i g n i f i c a n t s o i l f o r m a t i o n . . . "perhaps on a par with the 'Altithermal'

..."

However t h e e v i d e n c e f r o m s m a l l d r a i n a g e s i n

w e s t e r n Iowa ( B e t t i s , 1982) i n d i c a t e s a p e r i o d o f s o i l f o r m a t i o n o n l y f r o m about

2,000 t o 1,800 y r s B.P. Very L a t e H o l o c e n e t o R e c e n t Evidence f o r q u i t e r e c e n t e o l i a n a c t i v i t y i s found a t several s c a t t e r e d loc a l i t i e s i n t h e Nebraska Sand H i l l s .

A t s i t e V I I a l o n g t h e Snake R i v e r ( F i g s .

6 and 7, T a b l e 3 ) , a n o r g a n i c - r i c h sand d a t e d a t 860.55 d i r e c t l y o v e r l a i n b y up t o 8 m o f dune sand.

y r s B.P. (DIC-2074) i s

The K e l s o s i t e ( F i g . 6) d e s c r i b e d

above has 3 rn o f dune sand o v e r l y i n g t h e o c c u p a t i o n s i t e d a t e d a t 1,150+200y r s B.P.

B r a d b u r y (1980, p. 33) r e p o r t e d

a

d a t e o f 500i200 y r s B . P . o n o r g a n i c - r i c h

i n t e r d u n e sediments n o t associated w i t h a c u r r e n t interdune basin.

It i s not

s u g g e s t e d t h a t l a r g e - s c a l e dunes f o r m e d a t a n y o f t h e s e l o c a l i t i e s .

R a t h e r , during

t h e l a s t 500 t o 800 y e a r s d r o u g h t c o n d i t i o n s p e r s i s t e d l o n g enough i n p a r t s o f t h e Sand H i l l s t h a t v e g e t a t i o n was r e d u c e d enough f o r e o l i a n a c t i v i t y t o o c c u r a number of t i m e s .

Weakly (1962) p r e s e n t e d d e n d r o c h r o n o l o g i c e v i d e n c e f r o m Ash

H o l l o w , Nebraska, f o r s i x d r o u g h t s , e a c h l a s t i n g more t h a n 15 y e a r s , between 586 and 258 y r s B.P.

Ash H o l l o w i s l o c a t e d a b o u t 40 km e a s t o f s i t e V I ( r i g . 6) j u s t

o u t s i d e o f t h e s o u t h e r n edge o f t h e Sand H i l l s .

A t present, except f o r m a l l

b l o w o u t s , t h e Nebraska Sand H i l l s a r e s t a b i l i z e d and d o r m a n t . LATE PLEISTOCENE VERSUS HOLOCENE SAND DUNE ACTIVITY Most p r e v i o u s c h r o n o l o g i e s f o r t h e dune f i e l d s shown i n F i g u r e 1 ( e . g . Frye and Leonard, 1954; S m i t h , 1965; Reed a n d Dreeszen, 1965) p l a c e d m a j o r dune format i o n i n e i t h e r t h e e a r l y o r l a t e Wisconsin.

H o l o c e n e dune a c t i v i t y g e n e r a l , l y ,

was c o n s i d e r e d t o r e p r e s e n t o n l y m i n o r r e w o r k i n g o f o l d e r dunes.

However, a l l

t h e e v i d e n c e u s e d t o i n d i c a t e a P l e i s t o c e n e age f o r t h e s e dune f i e l d s i s e i t h e r circumstantial o r indirect. I f , as we p o s t u l a t e , t h e l a s t 11,000 y e a r s o r so has seen s i g n i f i c a n t sand dune f o r m a t i o n ,

t h e n why s h o u l d i t be d i f f i c u l t t o f i n d d i r e c t e v i d e n c e o f

Pleistocene episodes o f e o l i a n s a n d , a c t i u i t y ?

Certainly conditions o f sufficient

w i n d s , sand s u p p l y and l a c k o f v e g e t a t i v e c o v e r w o u l d have combined s e v e r a l times d u r i n g t h e P l e i s t o c e n e t o a l l o w s i g n i f i c a n t a c c u m u l a t i o n s o f e o l i a n sand.

Moreover,

401

well-documented P l e i s t o cen e eo l i an a c t i v i t y i s evidenced by widespread loess accumulations i n many areas of t h e Great P l ai n s .

Yet evidence seems lacking f o r

the e x i s t e n c e of major Pleistocene dune f i e l d s , why? One explanation f o r t h e i r absence or nonrecognition involves the e f f e c t of cold climate processes on dune f i e l d s .

A s,tudy of t h e North Park Dunes of Colorado by

A h l b r a n d t a n d Andrews (1978) demonstrated t h a t the r a t e of dune migration in a n

a c t i v e dune f i e l d su b j ect t o seasonal cold climate processes was roughly ten times slower than i n e q u i v al en t wind regimes i n a warm climate. Furthermore, they pointec o u t t h a t t h e presence of a high water t a b l e , e i t h e r frozen or unfmzen, gre a tly reduces sand t r a n s p o r t i n cold climate dunes a n d t h a t i n t e r c a l a t i o n of i c e and snow combined w i t h f r eeze and t h a w cycles cause extensive modification of inte rna l s t r u c t u r e s a n d r e t a r d dune growth because of a frozen dune c ore .

Thus, a major

dune f i e l d forming under a g l a c i a l climate would re quire a very long time t o build

and would e x h i b i t deformed a n d disrupted i n t er n al s t r u c t u r e s . Such s t r u c t u r e s would form n o t only concurrently with deposition b u t a l s o during subsequent cold climate periods ( e i t h e r seasonally o r p e r e n n i a l l y ) .

Of course, dune f i e l d s

formed i n i t i a l l y during i n t e r g l a c i a l periods and subjected l a t e r t o cold climate processes a l s o could be d r amat i cal l y modified as i s apparent in the North Park Dunes ( A h 1 b r a n d t and Andrews, 1978), t h e Ki 1 1 pecker Dunes ( A h 1 b r a n d t , 1975), and the Nebraska Sand H i l l s (Ahlbrandt and Fryberger, 1980). The l a t e Wisconsin pe rig la c ia l sand wedges documented by Mears (1981) i n the intermontane basins of Wyoming c l e a r l y i n d i c a t e t h e synchroneity of cold climates a n d minor e olia n sand tr a n s p o r t where sand i s incorporated irl wedges within o u r area of study (Fig. 3 ) . Late P l e i s t o c e n e sh eet d ep o s i t s of e o l i a n sand o r cover sands disrupted by cold climate processes were described. by Ruegg (1981, and t h i s volume) and Koster (1982) from a n approximately equivalent l a t i t u d e in Europe t o o u r study a re a . 2 Carter (1981, 1982) described a l a r g e (7,000 km ) l a t e Pleistocene sand se a , composed of l i n e a r dunes, on the Alaskan Ar ct i c Coastal Plain and suggested the dune f i e l d was a c t i v e from perhaps 40,000 y r s . B . P . u n t i l 12,000 y r s . B . P .

Locally,

the e o l i a n sand i s more than 20 m t h i ck and occurs in subpa ra lle l ridges u p t o 20 km long a n d 1 km wide.

A l t h o u g h l o c a l l y some l a r g e - s c a l e , high-angle cross-

beds a r e preserved, C ar t er (1981, p . 381) noted t h a t " the la rge dunes have been extensively modified by thermokarst processes making them d i f f i c u l t t o de te c t on the g r o u n d o r from a i r c r a f t . "

He f u r t h e r s t a t e d ( p . 382) " s l i p f a c e s t h a t would

have f a c i l i t a t e d the c l a s s i f i c a t i o n of d u n e types in the sand sea have been obliterated." We conclude from t h e above d i s cu s s i o n t h a t any dune f i e l d formed p r i o r t o about

12,000 y r s . B . P . i n o u r study area ( F i g . 1). should bear the d i s t i n c t i v e signa ture

of cold climate processes.

I f l a t e P l ei s t o cen e e olia n sands e x i s t , they probably

would be represented by sand s h eet d ep o s i t s a n d i f any la rge -sc a le dunes a r e preserved they should be highly modified and probably n o t e a s i l y recognized from

402 t h e i r morphology a l o n e .

A l a t e Wisconsin e o l i a n sequence s h o u l d c o n t a i n some

r e c o r d o f t h e c o e x i s t i n g b o r e a l fauna and f l o r a .

I n addition, the s o i l p r o f i l e

on these o l d e r dunes ought t o be b e t t e r developed t h a n t h e s o i l p r o f i l e on known Holocene dunes (Muhs and Madole, 1980). The b e s t p o s s i b i l i t y f o r pre-Holocene dunes i n t h e s t u d y a r e a appears t o be i n s m a l l areas o f subdued dune topography i n n o r t h e a s t e r n Colorado (Madole, 1981). These dunes have s o i l p r o f i l e s t h a t a r e more s t r o n g l y developed t h a n t h o s e o f dunes o v e r l y i n g r a d i o c a r b o n d a t e d m i d - t o - l a t e Holocene f l u v i a l u n i t s i n t h e Nebraska Sand H i 11s . SIGNIFICANCE OF MAJOR HOLOCENE ACTIVITY We b e l i e v e t h i s paper documents m u l t i p l e phases o f s i g n i i c a n t e o l i a n a c t i v i t y i n t h e G r e a t P l a i n s and Rocky Mountains d u r i n g t h e Holocene ( F i g . 3 ) . w i s h t o i m p l y t h a t e o l i a n d e p o s i t i o n was simultaneous i n a1 i t was n o t .

We do n o t

areas because c l e a r l y

However, t h e concept o f an e s s e n t i a l l y t h r e e p a r t c l i m a t i c s u b d i v i s i o n

of t h e Holocene i n which t h e r e i s a g r a d u a l s h i f t t o warmer and/or d r i e r c l i m a t e d u r i n g t h e mid-Holocene and a g r a d u a l r e t u r n t o a l e s s severe c l i m a t e ( W r i g h t , 1976) comes i n t o q u e s t i o n .

L i k e many o t h e r g e o l o g i c processes ( e . g . g l a c i a l ad-

vance and r e t r e a t ; a l l u v i a l d e g r a d a t i o n vs a g g r a d a t i o n ) t h e f a c t o r s c o n t r o l l i n g e o l i a n sand-dune a c t i v i t y a r e complex and v e r y d i f f i c u l t t o q u a n t i f y .

Very pos-

s i b l y t h e l a t e Holocene l a r g e - s c a l e dune f o r m a t i o n we e n v i s a g e o c c u r r i n g i n t h e s o u t h e a s t e r n p a r t o f t h e Nebraska Sand H i l l s d i d n o t r e p r e s e n t a s i g n i f i c a n t enough d e p a r t u r e f r o m t h e r e g i o n a l c l i m a t e t o a f f e c t t h e a l l u v i a l and f l o r a l record t o t h e degree t h a t o c c u r r e d d u r i n g t h e mid-Holocene.

Yet i t does appear t o be

g e n e r a l l y c o i n c i d e n t w i t h a n o n g l a c i a l i n t e r v a l i n t h e Colorado F r o n t Range ( B e n e d i c t , 1973).

I n a d d i t i o n , we t h i n k t h a t t h e Holocene e o l i a n r e c o r d i n d i c a t e s

some r a t h e r a b r u p t t r a n s i t i o n s f r o m p l u v i a l t o a r i d c o n d i t i o n s .

We t e n d t o agree

w i t h many o f t h e arguments o f Bryson e t a l . (1970), who emphasized a m u l t i p h a s e Holocene c l i m a t i c h i s t o r y w i t h f a i r l y a b r u p t b o u n d a r i e s .

However, we do n o t see

any advantage i n a d o p t i n g t h e B l y t t - S e r n a n d e r c l i m a t i c c l a s s i f i c a t i o n advocated by Bryson e t a l . (1970) a t t h i s t i m e .

Much more d a t a , p a r t i c u l a r l y i n t h e form

o f w e l l - d a t e d l o c a l s t r a t i g r a p h i c sequences and accompanying f l o r a l and f a u n a l evidence i s needed i n o r d e r t o t e s t t h e r e l i a b i l i t y o f such a c l a s s i f i c a t i o n . T h i s paper r a i s e s q u e s t i o n s on t h e r a t e o f development o f dune f i e l d s .

Using

conceptual models o f an upwind sand s u p p l y and p r o g r e s s i v e downwind development o f t h e f i e l d , g e o l o g i s t s have s p e c u l a t e d t h a t l a r g e e o l i a n dune bedforms must take many thousands o f y e a r s t o develop. d i f f e r e n t scenario.

However, t h e Nebraska Sand H i l l s r e f l e c t a

The sand sea developed on an u n c o n s o l i d a t e d o r p o o r l y c o n s o l i -

d a t e d a l l u v i a l sand s u b s t r a t e , t h e r e b y e l i m i n a t i n g t h e need f o r l o n g d i s t a n c e sand transport.

The a r i d c l i m a t e and abundant sand s u p p l y combined t o g i v e r a p i d r i s e

t o a l a r g e dune f i e l d .

As d i s c u s s e d above, l a r g e - s c a l e dunes o v e r l i e l a t e

4 03 Holocene a l l u v i u m i n p a r t s o f t h e Nebraska Sand H i l l s .

C l e a r l y , sand seas have

formed r a t h e r r a p i d l y , if we a c c e p t t h e s t r a t i g r a p h i c and r a d i o c a r b o n evidence from dune f i e l d s o f t h e Great P l a i n s and Rocky Mountain b a s i n s .

The dune f i e l d s

described h e r e p r o v i d e a l o o k a t e o l i a n a c t i v i t y d u r i n g a v e r y s h o r t p e r i o d of time, perhaps 10,000 y e a r s , and r e c o r d a v e r y dynamic and complex h i s t o r y .

A t h i r d p o i n t o f d i s c u s s i o n r e l a t e s t o t h e t y p e o f dunes i n these f i e l d s compared t o o t h e r r e g i o n s o f t h e w o r l d .

L i n e a r dunes a r e p r o b a b l y t h e most common

f o r m i n l a r g e dune f i e l d s , y e t , t h e y a r e v i r t u a l l y n o n - e x i s t e n t ( w i t h t h e except i o n o f en echelon barchan dunes i n t h e Sand H i l l s ) i n t h e dune f i e l d s o f t h e Great P l a i n s and t h e Rocky Mountain b a s i n s .

Does t h i s r e f l e c t t h e s h o r t - l i v e d

development o f t h e s e western U n i t e d S t a t e s dune f i e l d s , t h e i r m i d - l a t i t u d e s e t t i n g o r abundant sand s u p p l y ? F i n a l l y , t h e dynamic e o l i a n a c t i v i t y i n t h e Holocene r e f l e c t s responses t o both p o s t g l a c i a l and a r i d p e r i o d s .

E o l i a n a c t i v i t y has been e a s i l y and r e p e a t -

e d l y s t i m u l a t e d d u r i n g t h e Holocene and s h o u l d be an i n d i c a t i o n o f t h e r a p i d i t y w i t h which a s i g n i f i c a n t e p i s o d e o f e o l i a n a c t i v i t y c o u l d r e t u r n t o t h e Great P l a i n s and Rocky Mountain b a s i n s . ACKNOWLEDGEMENTS Four r a d i o c a r b o n d a t e s f r o m t h e Nebraska Sand H i l l s were k i n d l y p r o v i d e d by Meyer Rubin o f t h e U.S.

G e o l o g i c a l Survey Radiocarbon Lab, Reston, V i r g i n i a .

We

thank R i c h a r d Madole f o r h i s a s s i s t a n c e i n c o l l e c t i n g and p r e p a r i n g these samples and f o r h i s h e l p f u l d i s c u s s i o n s . s t i m u l a t i n g discussions.

R. George Corner and Daniel Fluhs a l s o p r o v i d e d

T h i s paper was improved by t h e r e v i e w s o f Raymond

B e n t a l l , David Loope, M a r v i n C a r l s o n and Robert D i f f e n d a l , J r .

V i n c e n t Dreeszen,

d i r e c t o r o f t h e C o n s e r v a t i o n and Survey D i v i s i o n , p r o v i d e d guidance and s u p p o r t f o r t h e Nebraska Sand H i l l s p o r t i o n o f t h i s s t u d y .

We thank t h e many landowners

who gave p e r m i s s i o n t o work and c o l l e c t on t h e i r p r o p e r t y . REFERENCES Ahlbrandt, T.S., 1974. Dune s t r a t i g r a p h y , archaeology and t h e c h r o n o l o g y o f t h e K i l l p e c k e r dune f i e l d . I n : M. W i l s o n ( E d i t o r ) , A p p l i e d Geology and Archaeology: The Holocene H i s t o r y o f Wyoming. Wyoming Geol. Survey, Rept. I n v e s t . , 10:51-60. Ahlbrandt, T.S., 1975. Comparison o f t e x t u r e s and s t r u c t u r e s t o d i s t i n g u i s h e o l i a n environments, K i l l p e c k e r dune f i e l d , Wyoming. Mountain G e o l o g i s t ,

12:61-73. Ahlbrandt, T.S. and Andrews, Sarah, 1978. D i s t i n c t i v e sedimentary f e a t u r e s o f c o l d - c l i m a t e e o l i a n d e p o s i t s , N o r t h Park, Colorado. Palaeogeography, Palaeoc l i m a t o l o g y , Pal aeoecology , 25:327-351. A h l b r a n d t , T.S. and F r y b e r g e r , S.G., 1980. E o l i a n d e p o s i t s i n t h e Nebraska Sand H i l l s . I n : G e o l o g i c and P a l e o e c o l o g i c S t u d i e s o f t h e Nebraska Sand H i l l s . U.S. Geol. Survey P r o f . Paper, 112O:l-24. A h l b r a n d t , T.S. and F r y b e r g e r , S.G., 1981. Sedimentary f e a t u r e s and s i g n i f i c a n c e o f i n t e r d u n e d e p o s i t s . I n : F.G. E t h r i d g e and R.M. F l o r e s ( E d i t o r s ) , Recent and A n c i e n t Nonmarine D e p o s i t i o n a l Environments. SOC. Economic P a l e o n t o l o g i s t s and M i n e r a l o g i s t s S p e c i a l Pub., 31:293-314.

4 04

Albanese, J . P . , 1974. Geology o f t h e Casper a r c h a e o l o g i c a l s i t e , Natrona County, Wyoming. I n : M . Wilson ( E d i t o r ) , Applied Geology and Archaeology: The Holocene H i s t o r y of Wyoming. Wyoming Geol. Survey Rept. I n v e s t . , 10:46-50. Albanese, J. P. and Wilson, M . , 1974. P r e l i m i n a r y d e s c r i p t i o n o f the t e r r a c e s of t h e North P l a t t e R i v e r a t Casper, Wyoming. I n : M . Wilson ( E d i t o r ) , Applied Geology and Archaeology: The Holocene H i s t o r y o f Wyoming. Wyoming Geol. Survey, Rept. I n v e s t . , 10:8-18. Anderson, E l a i n e , 1974. A survey o f t h e l a t e P l e i s t o c e n e and Holocene mammal fauna o f Wyoming. I n : M . Wilson ( E d i t o r ) , Applied Geology and Archaeology: The Holocene H i s t o r y of Wyoming. Wyoming Geol. Survey, Rept. I n v e s t . , 10:79-87. Andrews, S . G . , 1981. Sedimentology o f G r e a t Sand Dunes, Colorado. I n : F.G. Ethridge and R.M. F l o r e s ( E d i t o r s ) , Non-marine D e p o s i t i o n a l Environments: Models f o r E x p l o r a t i o n . SOC. Economic P a l e o n t o l o g y and Mineralogy S p e c i a l P u b l . , 31:270291. B e n e d i c t , J . B . , 1973. Chronology o f c i r q u e g l a c i a t i o n , Colorado F r o n t Range. Q u a t e r n a r y Research, 3:584-599. B e n e d i c t , J . B . , 1979. G e t t i n g away from i t a l l : A s t u d y o f man, mountains, and the two-drought A l t i t h e r m a l . Southwestern Lore, 4 5 , no. 3 : l - 1 2 . B e t t i s 111, E . A . , 1982. Geochronology o f l a t e Wisconsinan and Holocene alluvium i n t h e Flissouri d r a i n a g e of wpstern Iowa (USA) [ a b s . ] . I n : A b s t r a c t s Eleventh I n t ' l . Congress on Sedimentology, I n t ' l . Assoc. Sedimentology, p . 136. Bradbury, J . P . , 1980. Late Quaternary v e g e t a t i o n h i s t o r y of the c e n t r a l Great P l a i n s and i t s r e l a t i o n s h i p t o e o l i a n p r o c e s s e s i n t h e Nebraska Sand H i l l s . U.S. Geol. Survey P r o f . P a p e r , 1120C:25-36. B r i c e , J . C . , 1964. Channel p a t t e r n s and t e r r a c e s of the Loup r i v e r s i n Nebraska. U.S. Geol. Survey P r o f . P a p e r , 422D, 41 p p . Bryson, R . A . , B a e r r e i s , D . A . , and Wendland, W . M . , 1970. The c h a r a c t e r o f l a t e g l a c i a l and p o s t - g l a c i a l c l i m a t i c changes. I n : W. D o r t , J r . and J . K . J o n e s , J r . ( E d i t o r s ) , P l e i s t o c e n e and Recent Environments o f the C e n t r a l G r e a t Plains, Univ. o f Kansas, Dept. Geology S p e c i a l Pub., 3:53-74. Bryson, R . A . , Wendland, W . M . , Ives, J.D., and Andrews, J . T . , 1969. Radiocarbon i s o c h r o n e s on t h e d i s i n t e g r a t i o n of t h e L a u r e n t i d e i c e s h e e t . A r c t i c and Alpine Research, 1 : l - 1 4 . C a r t e r , L. David, 1981. A P l e i s t o c e n e sand s e a on t h e Alaskan A r c t i c Coastal P l a i n . S c i e n c e , 211:381-383. C a r t e r , L . David, 1982. Late Wisconsin d e s e r t i f i c a t i o n i n n o r t h e r n Alaska [ a b s . ] . I n : Geol. SOC. America A b s t r a c t s w i t h Programs, 1982 A n n . Mtg , p . 461. Corner, R . G . , 1977. A l a t e P l e i s t o c e n e - H o l o c e n e v e r t e b r a t e fauna from Red Willow County, Nebraska. Nebraska Acad. S c i e n c e T r a n s . , 4:77-93. Corner, R . G . , 1982. A l a t e P l e i s t o c e n e f a u n a from L i t c h f i e l d , Sherman County, Nebraska [ a b s . ] . Nebraska Acad. S c i e n c e , P r o c . , 92nd A n n . Mtg., p . 46. E l d e r , J . A . , 1969. S o i l s o f Nebraska. C o n s e r v a t i o n and Survey Div., Univ. of Nebraska, Resource Report 2 , 60 p p . F l i n t , R . F . , 1971. G l a c i a l and Quaternary Geology. New York, John Wiley and Sans, I n c . , 892 P P . F l i n t , R . F . , 1976. P h y s i c a l e v i d e n c e o f Quaternary c l i m a t i c change. Qua t e m a ry Research. 6 ~ 5 1 9 - 5 2 8 . F r i s o n , G . C . , Wilson, M . , and Wilson, D . J . , 1974. The Holocene s t r a t i g r a p h i c a r c h a e o l o g y of Wyoming: an i n t r o d u c t i o n ; I n : M . Wilson ( E d i t o r ) , Applied Geology an-d Archaeology: The Holocene H i s t o r y of Wyoming, Wyoming Geol. Survey, Rept. I n v e s t . , 10:108-127. Frye, J . C . and Leonard, A . B . , 1952. P l e i s t o c e n e Geology of Kansas. Kansas Geol. Survey, B u l l . 99, 230 p p . Gaylord, R . D . , 1979. Holocene c l i m a t i c changes r e c o r d e d i n t h e F e r r i s Dune Field, s o u t h e r n Wyoming [ a b s . ] . Geol. SOC. America A b s t r a c t s with Programs, 11, n o . 6 , 263. G r i g a l , D . F . , Severson, R . C . , and G o l t z , G . E . , 1976. Evidence of e o l i a n a c t i v i t y i n n o r t h - c e n t r a b M i n n e s o t a 8,000 t o 5,000 y r s ago. Geol. SOC. America R u l l . , 87:1251-1254. . GrGger, J . , 1973. S t u d i e s on the l a t e Q u a t e r n a r y v e g e t a t i o n h i s t o r y o f n o r t h e a s t e r n Kansas: Geol. SOC. America B u l l . , 84:239-250.

405 Haynes, C . V . , J r . , 1965. G e o c h r o n o l o g y o f l a t e Q u a t e r n a r y a l l u v i u m . I n : R. M o r r i son and H.E. W r i g h t , J r . ( E d i t o r s ) , Means o f C o r r e l a t i o n o f Q u a t e r n a r y Succ e s s i o n s . U n i v . U t a h P r e s s , S a l t Lake City, Utah, p p . 591-631. Hoffman, R . S . a n d Jones, J.K., J r . , 1970. I n f l u e n c e o f l a t e - g l a c i a l and p o s t g l a c i a l e v e n t s o n t h e d i s t r i b u t i o n o f r e c e n t mammals on t h e N o r t h e r n G r e a t P l a i n s . I n : W. P o r t , J r . and J . K . Jones, J r . ( E d i t o r s ) , P l e i s t o c e n e and Recent E n v i r o n m e n t s o f t h e C e n t r a l G r e a t P l a i n s , U n i v . Kansas, D e p t . Geology S p e c i a l Pub., 31355-394. Holmes, C.W. and Moss, J.H., 1955. P l e i s t o c e n e g e o l o g y o f t h e s o u t h w e s t e r n Wind R i v e r M o u n t a i n s , Wyoming. G e o l . SOC. A m e r i c a B u l l . , 66:629-654. Howard, E . B . , S a t t e r t h w a i t e , L., J r . , and Bache, C . , 1941. P r e l i m i n a r y r e p o r t on a b u r i e d Yuma s i t e i n Wyoming. America A n t i q u i t y , 7:70-74. Howard, E.B. and Hack, J . T . , 1943. The F i n l e y S i t e . A m e r i c a n A n t i q u i t y , 8:224-241. Jelgersma, S . , 1962. A l a t e - g a l c i a l p o l l e n d i a g r a m f r o m M a d e l i a , s o u t h - c e n t r a l M i n n e s o t a . Am. J o u r . S c i . , 260, no. 7:522-529. K i h l , R., 1975. P h y s i c a l p r e p a r a t i o n o f o r g a n i c m a t t e r samples f o r 14C d a t i n g . I n : J . T . Andrews, R a d i o c a r b o n d a t e l i s t I 1 f r o m Cumberland P e n i n s u l a , B a f f i n I s l a n d , N.W.T. Canada. A r c t i c and A l p i n e Research, 7:90-91. K i n g , J.E. and L i n d s a y , E.H., 1976. L a t e Q u a t e r n a r y b i q t i c r e c o r d s f r o m s p r i n g d e p o s i t s i n w e s t e r n M i s s o u r i . I n : P r e h i s t o r i c Man and H i s E n v i r o n m e n t s , A Case S t u d y i n t h e Ozark H i g h l a n d : Academic P r e s s , New York, p p . 63-78. K i v e t t , M.F., 1970. E a r l y c e r a m i c e n v i r o n m e n t a l a d a p t a t i o n s . I n : W. D o r t , J r . and J.K. Jones, J r . ( E d i t o r s ) , P l e i s t o c e n e and Recent E n v i r o n m e n t s o f t h e C e n t r a l G r e a t P l a i n s , U n i v . o f Kansas, D e p t . Geology S p e c i a l Pub., 3:93-102. Kolm, K.E., 1974, ERTS MSS i m a g e r y a p p l i e d t o mapping o f sand dunes i n Wyoming. I n : M. W i l s o n ( E d i t o r ) , A p p l i e d G e o l o g y and A r c h a e o l o g y : The Holocene H i s t o r y o f Wyoming. Wyoming G e o l . S u r v e y , R e p t . I n v e s t . 10:34-39. K o s t e r , E . A . , 1982. T e r m i n o l o g y and 1 i t h o s t r a t i g r a p h i c d i v i s i o n o f ( s u r f i c i a l ) sandy e o l i a n d e p o s i t s i n t h e N e t h e r l a n d s : An e v a l u a t i o n . G e o l o g i c en M i j n b o u w , 61:121-129. Lugn, A . L . , 1935. The P l e i s t o c e n e g e o l o g y o f Nebraska. Nebraska G e o l . Survey B u l l . 10, 2nd S e r i e s , 223 pp. Lugn, A.L., 1968. The o r i g i n o f l o e s s e s and t h e i r r e l a t i o n t o t h e G r e a t P l a i n s i n N o r t h A m e r i c a . I n : C . S c h u l t z and J.C. F r y e ( E d i t o r s ) , Loess and R e l a t e d E o l i a n D e p o s i t s o f t h e World, U n i v . o f Nebraska P r e s s , L i n c o l n , Neb., pp.

139-182. Madole, R . F . , 1981. G r e a t P l a i n s e o l i a n p r o c e s s e s . I n : G e o l o g i c a l Research 1982, U.S. G e o l o g i c a l S u r v e y P r o f . Paper, 1275:178. Maroney, D . G . , 1978. A s t r a t i g r a p h i c and p a l e o e c o l o g i c s t u d y o f some L a t e Cenoz o i c s e d i m e n t s i n t h e c e n t r a l Sand H i l l s P r o v i n c e o f Nebraska. D i s s e r t a t i o n , U n i v . Nebraska, L i n c o l n , Nebraska, 181 pp. Mears, B.M., J r . 1981. P e r i g l a c i a l wedges and t h e l a t e P l e i s t o c e n e e n v i r o n m e n t o f Wyoming's i n t e r m o n t a n e b a s i n s . Q u a t e r n a r y Research, 15:171-198. Moss, J.H. and o t h e r s , 1951. E a r l y Man i n Eden V a l l e y . U n i v . P e n n s y l v a n i a Museum, Museum Monographs, 6, 124 pp. Muhs, D . R . and Madole, R.F., 1980. S o i l - g e o m o r p h i c e v i d e n c e f o r w i d e s p r e a d movement o f dune sand o n t h e c e n t r a l G r e a t P l a i n s d u r i n g t h e A l t i t h e r m a l [ a b s . ] . T h i r t y - E i g h t h P l a i n s C o n f e r e n c e Program and A b s t r a c t s : 3 3 . Ogden 111, J.G. and Hay, R.J., 1965. O h i o Wesleyan U n i v e r s i t y n a t u r a l r a d i o c a r b o n measurements 11. R a d i o c a r b o n , 7:166-173. Reed, E . C . and Dreeszen, V . H . , 1965. R e v i s i o n o f t h e c l a s s i f i c a t i o n o f t h e P l e i s t o cene d e p o s i t s o f Nebraska. Nebraska G e o l . S u r v e y B u l l . 23, 65 pp. Ruegg, G e r a r d , 1981. S e d i m e n t a r y f e a t u r e s and g r a i n s i z e o f g l a c i o - f l u v i a l and p e r i g l a c i a l P l e i s t o c e n e d e p o s i t s i n t h e N e t h e r l a n d s and a d j a c e n t p a r t s o f w e s t e r n Germany. V e r h . n a t u r w i s s Ver. Hamburq, 24:133-154. Sears, P.B., 1961.. A p o l l e n p r o f i l e f r o m t h e g r a s s l a n d p r o v i n c e . S c i e n c e , 134:

2038-2039. S m i t h , H.T.U., 1940. G e o l o g i c s t u d i e s i n s o u t h w e s t e r n Kansas. Kansas G e o l . Survey, B u l l . , 34, 244 pp. S m i t h , H.T.U., 1965. Dune m o r p h o l o g y and c h r o n o l o g y i n c e n t r a l and w e s t e r n Nebraska J o u r . Geology, 73:557-578.

406 S m i t h , H . T . U . , 1968. Nebraska dunes compared w i t h t h o s e o f North A f r i c a and o t h e r r e g i o n s . In: C . B . S c h u l t z and J.C. Frye ( E d i t o r s ) , Loess and Related Eolian D e p o s i t s of the World. Univ. o f Nebraska Press, L i n c o l n , Nebraska, p p . 29-47. S t e w a r t , J.D., 1978. Mammals o f the T r a p s h o o t l o c a l f a u n a , l a t e P l e i s t o c e n e of Rooks County, Kansas [ a b s . ] , Nebraska Acad. S c i e n c e 88th Ann. Htg, p p . 45-46. S t u v i e r , Minze, 1969. Yale n a t u r a l r a d i o c a r b o n measurements IX. Radiocarbon, 11~545-658. Van Z a n t , Kent, 1976. Late- and p o s t - g l a c i a l v e g e t a t i o n a l h i s t o r y of n o r t h e r n Iowa. P h . D . d i s s e r t a t i o n , Univ. Iowa, Iowa C i t y , 197 pp. Warren, Andrew, 1976. Morphology and sediments o f the Nebraska Sand H i l l s i n r e l a t i o n t o P l e i s t o c e n e winds and the development of a e o l i a n bed forms. J o u r . Geology, 84:685-700. Watts, W.A. and Wright, H . E . , J r . , 1966. Late-Wisconsin p o l l e n and seed a n a l y s i s from the Nebraska Sand H i l l s . Ecology, 47:202-210. Wilson, Michael, 1974. H i s t o r y of t h e b i s o n i n Wyoming, with p a r t i c u l a r r e f e r e n c e t o e a r l y Holocene forms; i n Applied Geology and Archaeology. I n : M . Wilson ( E d i t o r ) , The Holocene H i s t o r y o f Wyoming. Wyoming Geol. Survey Rept. I n v e s t . , 10: 91-99. Weakly, H . E . , 1962. H i s t o r y o f d r o u g h t i n Nebraska. J o u r . of S o i l and Water C o n s e r v . , 17:271-274. Wright, H . E . , J r . , 1968. H i s t o r y o f the P r a i r i e P e n i n s u l a . I n : The Quaternary of I l l i n o i s , Spec. Publ. U n i v . I l l i n o i s C o l l . A g r i c . , 14:78-88. Wright, H . E . , J r . , 1970. V e g e t a t i o n a l h i s t o r y o f the C e n t r a l P l a i n s . I n : W. Dort, J r . and J.K. J o n e s , J r . , ( E d i t o r s ) , P l e i s t o c e n e and Recent Environments of the C e n t r a l G r e a t P l a i n s . Univ. Kansas Dept. Geology Spec. Pub., 3:157-172. Wright, H . E . , J r . , 1976. The dynamic n a t u r e o f Holocene v e g e t a t i o n . Q u a t . Research, 6: 581-586.

407

RECONSTRUCTING BEDFORM ASSEMBLAGES FRCM C@lPOUND CROSSBEDDI NG D M. RUBIN & RALPH E. HUNTER U.S. Geological Survey, Menlo Park, California 94025, USA

INTRODUCTION During t h e l a s t two decades, sedimentologists have observed t h a t small bedforms a r e commonly superimposed on l a r g e bedforms, a n d they have r e a l i z e d

t h a t many compl icated c o s e t s of crossbeds i n eolian and subaqueous sandstones can be r e a d i l y explained by t h e migration of small bedforms on t h e l e e surfaces of o t h e r l a r g e bedforms (Beutner, Flueckinger, and Gard, 1967; Allen, 1968, 1980; Banks, 1973; Brookfield, 1977; Jones, 1979; Kocurek, 1981). The purposes of t h e present

paper a r e :

( 1 ) t o present techniques t h a t can be used t o

reconstruct t h e geometry of bedforms t h a t deposited compound crossbeds, (2) t o i l l u s t r a t e t h e s t r u c t u r e s produced by several common bedform assemblages, and (3) t o show how t h e s e reconstructions can be used t o i d e n t i f y deposits of ob 1 i que and 1 ong i t u d i nal b e d f o n s .

Compound Crossbedding A c r o s s - s t r a t i f i e d bed i s defined t o be simple i f i t does not contain

internal erosion s u r f a c e s (Fig. 1 A ) .

I n c o n t r a s t , where a c r o s s - s t r a t i f i e d bed

does contain erosional surfaces (Fig. l B ) , t h e s t r u c t u r e of t h e c o s e t has been defined a s f u r i o u s or double crossbedding crossbedding (Harms e t a l . , 1975).

(Reiche,

1938)

or

compound

The c r i t i c a l f a c t o r t h a t determines whether t h e crossbedding deposited by a bedform i s simple o r compound i s t h e absence o r c c u r r e n c e of i n t e r m i t t e n t erosion o n t h e l e e slope.

Where no erosion or-.urs, beds deposited on t h e l e e

slope a r e not t r u n c a t e d , bounding surfaces a r e not generated within t h e s e t , crossbedding i s simple. I n c o n t r a s t , where p a r t s of a l e e slope

and

occasionally undergo erosion, l a y e r s on t h e l e e slope a r e truncated, bounding surfaces a r e produced within t h e compound.

s e t , and t h e r e s u l t i n g crossbedding

is

Two kinds of processes can cause erosion on a l e e slope: ( 1 ) flow changes, such a s s h i f t i n g winds o r reversing c u r r e n t s , t h a t change bedform morphology a n d produce r e l a t i v e l y synchronous bounding surfaces t h a t a r e c a l l e d r e a c t i v a t i o n s u r f a c e s , and ( 2 ) local erosion t h a t may occur continuously ( b u t

a t s h i f t i n g locations) in t h e troughs o r on t h e stoss slopes of small bedforms t h a t migrate across t h e l e e slope of a l a r g e r bedform.

4 08

A

I

Crossbed

Cross-stratified b e d or set of crossbeds

Coset of crossbeds, compound set of crossbeds, or doubly cross-stratified bed Crossbed

\

Cross-stratified crossbed or set of crossbeds F i g . 1. ( A ) S e t o f s i i n p l e c r o s s b e d s ; n o b o u n d i n g s u r f a c e s w i t h i n t h e s e t . (B) Set o f compound c r o s s b e d s ; e r o s i o n a l b o u n d i n g s u r f a c e s w i t h i n t h e s e t . The o r i g i r - and s t r u c t u r e o f compound c r o s s b e d d i n g

produced b y s h i f t i n g

b e d f o r i n s and o t h e r t o p o g r a p h i c f e a t u r e s i s d i s c u s s e d i n t h i s paper.

The o r i g i n

a n d s t r u c t u r e o f compound c r o s s b e d d i n g p r o d u c e d b y f l u c t u a t i n g f l o w ,

and t h e

d i f f i c u l t p r o b l e m o f d i s t i n g u i s h i n g t h e t w o t y p e s o f compound c r o s s b e d d i n g , a r e d i s c u s s e d e l s e w h e r e ( H u n t e r and IRubin, 1 9 8 3 ) . Approach The

primary obstacle a

geometry i s t h a t preserved; surfaces

only

depositional

commonly

enviroment.

s e d i m e n t o l o g i s t faces i n r e c o n s t r u c t i n g bedform

usually only constitute

F o r example,

small

surfaces a

o n l y a small

can

be

small

i n a dune f i e l d ,

s m a l l f r a c t i o n o f t h e s u r f a c e area. depositional,

fragments o f t h e o r i g i n a l bedforms a r e preserved,

fraction

of

and a

depositional

"depositional"

depositional ( l e e ) slopes cover a

O f t h i s f r a c t i o n o f the surface t h a t i s

f r a c t i o n t h a t i s t o p o g r a p h i c a l l y l o w w i l l escape

b e i n g r e w o r k e d b y s u b s e q u e n t dune s t o s s s u r f a c e s a n d t r o u g h s ( R u b i n and H u n t e r ,

1982), a l l o w i n g p r e s e r v a t i o n o f p o s s i b l y o n l y a few p e r c e n t o f t h e o r i g i n a l bedform surface.

I n addition,

where a d u n e ' s l e e s u r f a c e i s c o v e r e d w i t h

s m a l l e r dunes o r o t h e r m i g r a t i n g topographic f e a t u r e s , t h e e r o s i o n a l surfaces o f those

features are

not

presewed either,

a n d t h e r e s u l t i n g d e p o s i t may

r e p r e s e n t l e s s t h a n o n e p e r c e n t o f t h e o r i g i n a l dune s u r f a c e . samples,

From t h e s e t i n y

s e d i m e n t o l o g i s t s a t t e m p t t o r e c o n s t r u c t e n t i r e b e d f o r m s and r e g i o n a l

flow fields.

409 D e s p i t e t h e small sample o f p r e s e r v e d s u r f a c e area, compound crossbedding commonly c o n t a i n s enough i n f o r m a t i o n t o a l l o w d e t a i l e d r e c o n s t r u c t i o n s o f t h e migration direction, the

c r e s t plan, and t r o u g h p r o f i l e o f p r i m a r y bedforms, and

migration direction,

bedforms.

I n addition,

crest

plan,

and

trough

profile o f

superimposed

some c o s e t s c o n t a i n evidence i n d i c a t i n g bedform s i z e

(Hunter, 1981; Rubin and Hunter,

1982) o r m i g r a t i o n r a t e s (Hunter and Rubin,

1983). The key t o r e c o n s t r u c t i n g bedform geometry f r o m compound crossbedding i s visualizing

the

translated planes.

structures

through

space

generated

and

later

when

hypothetical

lee

exposed

i n variably

oriented

I n t h e s i m p l e case where f l o w i s steady,

slopes

outcrop

two f a c t o r s c o n t r o l t h e

geometry o f t h e i n t e r n a l s t r u c t u r e s : ( 1 ) t h e shape o f t h e l e e s u r f a c e , and the

direction

that

the

surface

is

translated.

i n t e r p r e t e d by v i s u a l i z i n g v a r i o u s l e e - s l o p e until

finding

A

specific

deposit

(2) is

shapes and m i g r a t i o n d i r e c t i o n s

t h e c o m b i n a t i o n t h a t most c l o s e l y d u p l i c a t e s

observed i n t h e f i e l d .

are

To i l l u s t r a t e t h i s approach,

the structures

s t r u c t u r e s produced by

several common bedform assemblages a r e d e s c r i b e d i n t h e f o l l o w i n g s e c t i o n s . SUPER IMPOSED BEDFORMS

L a r g e bedforms commonly s u p p o r t smal l e r superimposed bedforms because o f fluctuating flow conditions (Allen,

1978) and because l a r g e bedforms g e n e r a t e

n e a r - s u r f a c e boundary l a y e r s t h a t i n t u r n produce small bedforms even i n steady flows

(Rubin and McCulloch,

slope

of

a

l a r g e bedform

1980).

produces

r e c o n s t r u c t i n g bedform assemblages. scour

M i g r a t i o n o f small bedforms over t h e l e e

i n t o t h e l a r g e bedform,

two

structures

that

are

useful

for

F i r s t , t h e t r o u g h s o f t h e small bedforms

t h e r e b y g e n e r a t i n g bounding s u r f a c e s (A1 l e n ,

1968; B r o o k f i e l d , 1977; Jones, 1979).

Second, t h e l e e slopes o f t h e m i g r a t i n g

superimposed bedforms d e p o s i t crossbeds. The s c o u r i n g o f bounding s u r f a c e s b y t h e small bedforms i m p l i e s t h a t t h e small bedforms were c l imbing a t small a n g l e s ( t y p i c a l l y l e s s t h a n 15") r e l a t i v e t o t h e s u r f a c e o f t h e l a r g e bedform ( F i g . bedforms a r e two-dimensional,

2).

Consequently, where t h e small

t h e r e l a t i v e l y p l a n a r bounding s u r f a c e s t h a t t h e y

scour c a n b e measured t o e s t i m a t e t h e s t r i k e and d i p o f t h e l e e s l o p e o f t h e T h i s a p p r o x i m a t i o n may n o t always be a c c u r a t e where bounding

l a r g e dune.

s u r f a c e s d i p a t a n g l e s approaching 15" o r l e s s . Having approximated t h e t r e n d o f a l a r g e bedform w i t h t h e mean s t r i k e d i r e c t i o n o f bounding s u r f a c e s scoured by s m a l l superimposed bedforms, t h e n e x t s t e p i n r e c o n s t r u c t i n g a b e d f o r m assemblage i s t o d e t e r m i n e t h e t r e n d s o f t h e s m a l l bedforms. t h e bounding

Two k i n d s o f p l a n a r s t r u c t u r e s g e n e r a t e d b y t h e s e bedforms--

surfaces

t h a t t h e y scour and t h e crossbeds

that they

deposit--

410

A

B

s 'ig. 2. Compound crossbedding deposited by downslope-migrating dunes. ( A ) Vertical s e c t i o n showing dune morphology. ( B ) Block d i a g r a m showing inte rna l s t r u c t u r e . Crossbeds a n d bounding s u r f aces t h a t se pa ra te crossbeds d i p i n same d i rec t i on.

i n t e r s e c t a l o n g t he t r o u g h 1 i n es of t h e superimposed bedforins. the line of stereonet,

intersection

of

those two

planes,

Consequently,

conveniently plotte d on a

i n d i c a t e s t h e trend of t h e Superimposed bedforms.

Thus, cross-

s t r a t i f i e d beds deposited by small bedforms migrating on t h e l e e slopes of l a r g e bedforms bedforms.

can

be

used

to

determine t h e

tre nds

of

both

s c a l e s of

This r e co n s t r u ct i o n technique a n d o t h e r s discussed in t h e t e x t a r e

smmarized in Table 1. Of a l l t h e assemblages of bedforms t h a t generate compound crossbedding, the

two-dimensional

cas e

where

r eg u l ar

straight-crested

bedforms

migrate

d i r e c t l y downslope i s t h e e a s i e s t t o v i s u a l i z e ( F i g . 2A; o r Allen, 1968, fig. 5.16b),

although downslope migration may be no more common t h a n oblique or

alongsl ope migration.

Superimposed bedforms t h a t migrate downslope deposit

crossbeds and scour bounding s u r f aces t h a t d i p in t h e same d i r e c t i o n , which i s t h e d i p d i r e c t i o n of t h e l e e slope of t h e l a r g e bedform (Fig. 2B). Another

bedform

assemblage

that

g en era te s

compound

crossbedding

superimposed bedforms t h a t migrate d i r e c t l y alongslope (Fig. 3A). t h i s assemblage a r e r e l a t i v e l y d i f f i c u l t crossbeds deposited by alongslope-migrating

is

Deposits of

t o v i s u a l i z e a c c u r a t e l y , because bedforms d i p obliquely down the

primary l e e s l o p e , r a t h e r t h a n d i r e c t l y alongslope (Fig. 38). For example, the compound crossbedding i n F i g . 3C was deposited by superimposed dunes migrating almost exactly alongslope (toward 290" on a l e e slope dipping toward Z O O " ) .

Lee slope of l a r g e dune

N

S t o s s s l o p e of l a r g e dune

/

I

/

I

w

scoured b v superimposed

/I

deposited by superimposed

B

C

I n t e r s e c t i o n line and t r o u g h s of s u p e r i m p o s e d d u n e s t r e n d t o w a r d 200°

Fig. 3. Compound c r o s s b e d d i n g g e n e r a t e d b y a l o n g s l ope( A ) Diagrammatic s k e t c h m i g r a t i n g s t r a i g h t - c r e s t e d dunes. showing o b l i q u e v i e w o f dune m o r p h o l o g y and i n t e r n a l structure. ( B ) B l o c k d i a g r a m showing i n t e r n a l s t r u c t u r e . ( C ) Example from E n t r a d a Sandstone n e a r Page, A r i z o n a ; p r i m a r y dune m i g r a t e d s o u t h w e s t f r o m r i g h t t o l e f t ( t h e d i r e c t i o n o f bounding surface d i p ) ; superimposed dunes m i g r a t e d n o r t h w e s t (away f r o m v i e w e r ) . (D) S t e r e o n e t p l o t P r i m a r y dune m i g r a t e d i n d i p d i r e c t i o n o f b e d s shown i n C. o f t h e b o u n d i n g s u r f a c e s s c o u r e d by s u p e r i m p o s e d dunes. T r e n d o f s u p e r i m p o s e d dunes i s d e f i n e d b y t h e l i n e o f i n t e r s e c t i o n o f t h e bounding s u r f a c e s and crossbedding t h a t t h e superimposed dunes d e p o s i t e d , a s e x p l a i n e d i n t e x t .

412 However,

t h e crossbeds

deposited by the

downslope ( t o w a r d 245").

superimposed

dunes

dip

obliquely

W i t h o u t p e r f o r m i n g a s t e r o n e t a n a l y s i s ( F i g . 3D), t h e

m i g r a t i o n d i r e c t i o n o f t h e superimposed dunes m i g h t b e i n f e r r e d t o b e i n t h e d i p d i r e c t i o n o f t h e c r o s s b e d s d e p o s i t e d b y t h e s u p e r i m p o s e d dunes, and a 45degree e r r o r would r e s u l t . Where

the

dimensional, (Fig.

small

bedforms

superimposed

w i l l approximately p a r a l l e l

4A)

on

a

lee

slope

are

three-

t h e a x e s o f t h e t r o u g h - s h a p e d b o u n d i n g s u r f a c e s t h a t t h e y scour t h e l e e s l o p e o f t h e l a r g e bedform.

C o n s e q u e n t l y , where t r o u g h a x e s h a v e d i v e r g i n g a z i m u t h s , t h e l e e s l o p e o f t h e l a r g e dune c a n b e a p p r o x i m a t e d b y t h e p l a n e t h a t c o n t a i n s t h e t r o u g h a x e s ( F i g . 4B).

Where t h e t r o u g h a x e s have s i m i l a r a z i m u t h s , t h e l e e s u r f a c e o f t h e l a r g e (1) t h e average

dune i s a p p r o x i m a t e d b y a p l a n e t h a t i s d e f i n e d b y t w o l i n e s : t r o u g h a x i s and ( 2 )

t h e l i n e r e p r e s e n t i n g t h e average t r a c e o f t h e trough

bounding 'surfaces as observed i n a s e c t i o n o b l i q u e o r transverse t o t h e trough a x e s ( F i g . 4C). CRABBING BASE-OF-SLOPE SCOUR PITS AND SCALLOPED CROSSBEDDING Migrating capable o f

superimposed bedforms a r e

causing

the

compound c r o s s b e d d i n g .

local

not t h e o n l y topographic

erosion that

produces bounding

features

s u r f a c e s and

Scour p i t s a n d i n t e r v e n i n g s p u r s t h a t c r a b , o r m i g r a t e

l a t e r a l l y , a l o n g t h e b a s e o f a l e e s l o p e c a n a l s o p r o d u c e l o c a l e r o s i o n and thereby generate bounding surfaces.

For reasons discussed below,

deposits

produced b y t h e s e f e a t u r e s r e q u i r e d i f f e r e n t i n t e r p r e t i v e techniques from those developed f o r superimposed bedforms. As a s i n g l e s c o u r p i t m i g r a t e s w i t h o u t c h a n g i n g shape o r d i r e c t i o n , l e a d i n g s u r f a c e erodes t h e sediment t h a t t h e scour p i t encounters, trailing

surface i s preserved b y deposition.

s i n g l e trough-shaped s e t o f crossbeds.

The r e s u l t i n g

its

and i t s

structure

is a

The t r o u g h a x i s i n d i c a t e s t h e p a t h o f

m i g r a t i o n , w h i c h i s n e a r l y h o r i z o n t a l b e c a u s e t h e s c o u r p i t o c c u r s a t t h e base o f t h e l e e slope,

a n d t h e shape o f t h e c r o s s b e d s i n d i c a t e s t h e shape o f t h e

scour p i t ' s t r a i l i n g s u r f a c e (Fig. r e w o r k i n g b y subsequent s c o u r p i t s ,

5A).

Where t h e t r o u g h - s h a p e d s e t escapes

a s when m i g r a t i o n c e a s e s o r where s c o u r

p i t s a r e w i d e l y spaced a n d d o n o t m i g r a t e o n o v e r l a p p i n g p a t h s , t h e s e t may be preserved r e l a t i v e l y completely (Fig.

I n many c a s e s ,

however,

5B).

trough-shaped

s e t s a r e r e w o r k e d b y subsequent

s c o u r p i t s , e i t h e r a t random o r i n some o r g a n i z e d p a t t e r n .

F o r example, where

a t r a i n o f s c o u r p i t s m i g r a t e s a l o n g t h e b a s e o f a m i g r a t i n g l e e s l o p e , each scour

p i t may s c o u r

preceding

scour

pit.

i n t o p a r t o f t h e trough-shaped The r e s u l t i n g

structure

i s a

set

deposited b y the

sequence o f l a t e r a l l y

t r u n c a t e d t r o u g h - s h a p e d s e t s t h a t become p r o g r e s s i v e l y y o u n g e r i n a c o n s i s t e n t

413

Fig. 4. Compound c r o s s b e d d i n g g e n e r a t e d by smal 1 t h r e e - d i m e n s i o n a l bedforms superimposed on l a r g e r bedforms. A. Example from t h e Cntrada Sandstone near B. Block diagram i l l u s t r a t i n g s t r u c t u r e produced by Page, Arizona. superimposed t h r e e - d i m e n s i o n a l bedforrns t h a t m i g r a t e d i n d i f f e r e n t d i r e c t i o n s . Lee s u r f a c e o f t h e l a r g e bedform i s approximated by t h e p l a n e t h a t c o n t a i n s t h e axes of t h e t r o u g h s t h a t were scoured by t h e small dunes. C. Block diagram i l l u s t r a t i n g s t r u c t u r e produced by superimposed three-dimensional bedfoniis t h a t migrated i n t h e same d i r e c t i o n . Lee s u r f a c e of t h e l a r g e bedform i s approximated by t h e p l a n e t h a t i s d e f i n e d by t w o l i n e s : ( 1 ) t h e a v e r a g e trough a x i s , and ( 2 ) t h e a v e r a g e b o u n d i n g - s u r f a c e t r a c e ( d e t a i l s i n t e x t ) .

414

A

B

F i g . 5. Trough-shaped s e t s o f c r o s s b e d s d e p o s i t e d b y m i g r a t i n g s c o u r p i t . (A) ( B ) Example f r o m t h e N a v a j o Sandstone a t Snow Canyon, Utah; s e t Block diagram. i s a p p r o x i m a t e l y 10 m t h i c k .

415

d i r e c t i o n ( F i g . GA and B ) . t h i s process will

The lower bounding s u r f a c e o f a s e t d e p o s i t e d by

usually appear scallop-shaped,

and we t h e r e f o r e use t h e

d e s c r i p t i v e terms s c a l l o p e d c r o s s b e d d i n g o r s c a l l o p s f o r t h e s e s t r u c t u r e s . S c a l l o p e d c r o s s b e d d i n g c a n a l s o form when f l u c t u a t i n g cause

trough t o

a bedform

a l t e r n a t e l y aggrade and

flow c o n d i t i o n s

scour d u r i n g m i g r a t i o n

(Hunter and Ruhin, 1 9 8 3 ) , b u t t h e two k i n d s of s c a l l o p s c a n b e d i s t i n g u i s h e d by the three-dimensional bedforins

during

I n s c a l l o p s d e p o s i t e d by

geometry o f t h e s t r u c t u r e s .

fluctuating

flow, crossbeds d i p

in t h e

same d i r e c t i o n a s

I n s c a l l o p s d e p o s i t e d by c r a b b i n g s c o u r p i t s , c r o s s b e d d i p

bounding s u r f a c e s .

d i r e c t i o n s d e v i a t e from b o u n d i n g - s u r f a c e d i p d i r e c t i o n s ; t h e d e v i a t i o n i n d i p direction hounding

is

not

constant,

but

s u r f a c e s and g r e a t e s t

i s least

i n beds t h a t

i n beds t h a t

immediately o v e r l y

immediately u n d e r l i e t h e next

bounding s u r f a c e ( h o r i z o n t a l s e c t i o n in Fig. 6B). We have i d e n t i f i e d s c a l l o p e d c r o s s b e d d i n g d e p o s i t e d by c r a b b i n g s c o u r p i t s

i n e o l i a n s a n d s t o n e s on t h e Colorado P l a t e a u ( F i g . 6C, D , and E ) , i n f l u v i a l beds i n t h e Wingate Sandstone ( T r i a s s i c ) i n Utah, i n Brazos River d e p o s i t s , and S t r u c t u r e s t h a t a p p a r e n t l y o r i g i n a t e d by

i n modern dunes o f t h e Oregon Coast.

a

similar

process

have been

r e p o r t e d by

Brookfield

(1979).

S c a l l o p s of

undetermined o r i g i n a r e i l l u s t r a t e d by l4cKee (1966, f i g . 6 , 1979, f i g . 153B), Gregory (1950, f i g . 45a and f ) , and B i g a r e l l a and Salaniuni ( 1 9 6 7 , f i g . 1 1 - 1 ) . D i s t i n g u i s h i n g Crabbing Scour P i t s from Sumperimposed Bedforns Scour

pits

along

t h e base

of

a

l e e s l o p e and

superimposed

resemble each o t h e r and may even g r a d e from one t o t h e o t h e r . distinguishing

their

deposits

is

i m p o r t a n t because

d i f f e r e n t i n t e r p r e t i v e t e c h n i q u e s (Table 1 ) .

t w kinds of f e a t u r e s i s size. be

superimposed

on

s t b s t a n t i a l l y smal l e r .

a

primary

bedfonns However,

their deposits require

One m a j o r d i f f e r e n c e between t h e

In o r d e r f o r a t r a i n o f secondary bedfonns t o bedform,

the

secondary bedforms

must

be

Consequently, superimposed bedfonns t e n d t o d e p o s i t

c r o s s - s t r a t i f i e d b e d s t h a t a r e t h i n compared t o t h e compound set d e p o s i t e d b y t h e primary bedform ( F i g . 2 A ) . Unlike superimposed bedforms, which c a n b e thought o f a s small s u r f i c i a l f e a t u r e s , s c o u r p i t s and t h e r i d g e s o r spurs s e p a r a t i n g them c a n e i t h e r b e small superimposed f e a t u r e s o r c a n b e p a r t o f t h e b a s i c bedform s t r u c t u r e and may b e comparable i n s i z e t o t h e primary bedform.

Scour p i t s may b e a s deep a s

bedforms a r e h i g h , and a s a r e s u l t , t h e s e t s d e p o s i t e d by c r a b b i n g s c o u r p i t s may b e a s t h i c k a s t h e e n t i r e c o s e t o f beds d e p o s i t e d by a primary bedform.

The o t h e r m a j o r d i f f e r e n c e between t h e s e two morphological end members i s t h a t s c o u r p i t s along t h e b a s e o f a l e e s l o p e a r e t o p o g r a p h i c a l l y very low. Consequently, t h e y c a n s c o u r a d i s t i n c t i v e s c a l l o p e d s u r f a c e t h a t f o n s t h e

B

_A_

Horizontal section

Miaration direction of primary dune

-

Level of horizontal B section _ _ _ - in -

& ----.-

m

-Lee

slope of

--,-----

\EcTI

Scourpit migration direction

’

Leading (erosional) surface of scour pit

Trilling (depositinonal) surface of scour pit

Vertica.I s’ections’

F i g . 6. O r i g i n o f s c a l l o p e d c r o s s b e d d i n g . ( A ) S c h e m a t i c c o n t o u r map o f dune t o p o g r a p h y . (B) D i a g r a m showing s t r u c t u r e s d e p o s i t e d b y t h e dune and s c o u r p i t s i n A. (C) Scalloped crossbedding i n a plane t h a t i s approximately p a r a l l e l t o t h e m i g r a t i o n d i r e c t i o n o f t h e primary (D) Snow Canyon dune; N a v a j o Sandstone a t Snow Canyon, Utah. S e t i s a p p r o x i m a t e l y 10 in t h i c k . ( E ) Two s e t s o f scallops i n a plane t h a t i s oblique t o d i r e c t i o n o f scour-pit migration. s c a l l o p s i n N a v a j o Sandstone, Z i o n N a t i o n a l Park. S e t s a r e 5-10 m t h i c k . ( F ) Modern a n a l o g f r o m Oregon c o a s t - - d u n e s have s p u r s a n d s c o u r p i t s t h a t m i g r a t e t o w a r d v i e w e r .

418 lower bounding surface of a coset. the

Because the topograpically low p o s i t i o n o f

scour p i t s g i v e s them a very high

preservation p o t e n t i a l ,

scalloped

crossbedding i s a useful tool f o r i n t e p r e t i n g bedform behavior. Direction of Scour-Pit Migration The d i r e c t i o n of migration of a scour p i t can be determined e i t h e r by measuring the a x i s of t h e trough-shaped set t h a t i t deposited o r by measuring t h e s t r i k e of t h e steeply dipping s i d e s of t h e trough.

The trough a x i s i s more

precise in theory, because i t i s l e s s a f f e c t e d by changes in s i z e undergone by t h e scour p i t during migration.

In many outcrops, however, the s t r i k e of t h e

trough s i d e s can be measured more accurately. Orientation of Lee Slope of Primary Bedform

I n c o n t r a s t t o t h e bounding surfaces scoured by superimposed bedforms, those scoured by scour p i t s crabbing along the base of a l e e slope may not approximate t h e trend of the primary bedform. This d i f f e r e n c e a r i s e s because scour p i t s may not always be s u p e r f i c i a l features. They may, t h e r e f o r e , extensively rework t h e bedform on which they e x i s t . Another technique i s necessary t o determine t h e trend of the primary bedform. Where scour p i t s migrate nearly p a r a l l e l t o t h e l e e slope of t h e primary bedform, t h e p r e f e r e n t i a l l y preserved s i d e s of t h e troughs t h a t they scour trend nearly p a r a l l e l t o t h e l e e slope of t h e primary bedform.

I n contrast,

where scour p i t s migrate i n nearly t h e same d i r e c t i o n as primary bedforms, t h e mean d i p d i r e c t i o n of a l l crossbeds deposited within scour p i t s , r a t h e r t h a n bounding s u r f a c e s , approximates t h e d i p d i r e c t i o n of t h e generalized l e e slope. A t a l l intermediate angles, t h e lee-slope d i p d i r e c t i o n l i e s between the mean d i p d i r e c t i o n of t h e crossbeds and t h a t of t h e i r bounding surfaces (Fig. 7).

For convenience we can approximate t h e original lee-slope d i p d i r e c t i o n

with t h e c e n t r a l value of t h i s range by taking t h e mean of those two q u a n t i t i e s - -t he bounding- surface d i p d i r e c t i o n and t h e c rossbed d i p d i r e c t i o n (Fig. 7 ) . I t can be shown g r a p h i c a l l y t h a t t h i s c a l c u l a t e d o r i e n t a t i o n i s most accurate i f crossbed d i p d i r e c t i o n s a r e averaged a t t h e top of a scallop. Evaluating t h i s approximation with c i r c u l a r scour p i t s having a diameter D, center-to-center

spacings ranging from D t o 10D, and a l l angles of crab t h a t

generate compound crossbedding, we f i n d t h a t t h e mean e r r o r i s 11" and the maximum e r r o r 45"; 80 percent of t h e approximations have a n e r r o r l e s s t h a n 20". Where t h e bounding-surface d i p d i r e c t i o n d e v i a t e s by l e s s t h a n 45" from t h e mean crossbed d i p d i r e c t i o n , a s was t h e case f o r many of t h e s e t s we observed in t h e f i e l d , e r r o r s a r e l e s s (23" maximum, 5" mean).

419 (1) POSITIONS

OF

(3) BOUNDING SURFACE

LEE SLOPE

(2) MIGRATION DIRECTION

GENERATED BY MIGRATING

OF PRIMARY DUNE AT EARLY

LATER TIME

(6)DIP DIRECTION OF PRIMARY LEE SLOPE Fig. 7. H o r i z o n t a l s e c t i o n t h r o u g h beds d e p o s i t e d by a dune w i t h c r a b b i n g scour pits. D i p d i r e c t i o n o f g e n e r a l i z e d l e e s l o p e ( 6 ) l i e s between d i p d i r e c t i o n o f crossbeds w i t h i n s c a l l o p s ( 5 ) and d i p d i r e c t i o n o f bounding s u r f a c e s o f s c a l 1 ops

.

S c o u r - P i t Shape and O r i g i n s o f Bounding Surfaces w i t h i n Trougho r Scallop-Shaped S e t s Crossbeds

iri a

trough-shaped

set

d e p o s i t e d by a m i g r a t i n g

scour

pit

p r e s e r v e t h e stlape o f t h e scour p i t ' s t r a i l i n g s u r f a c e ( A l l e n , 1965, Chapter

5.6).

The shape o f t h e l e a d i n g s u r f a c e c a n o n l y b e i n f e r r e d , because i t i s

e r o s i o n a l and l e a v e s no d e p o s i t . A l l t h e examples d i s c u s s e d above i l l u s t r a t e t r o u g h - o r scallop-shaped s e t s t h a t were d e p o s i t e d b y n e a r l y c i r c u l a r scour p i t s m i g r a t i n g i n s t r a i g h t l i n e s , and c r o s s b e d d i n g w i t h i n such s e t s i s s i m p l e (no bounding s u r f a c e s c o n t a i n e d within

sets).

However,

crossbedding

s b s e t s b y i n t e r n a l bounding surfaces. form b y a t migration,

least

three

processes:

within

s c a l l o p s may b e d i v i d e d i n t o

These i n t e r n a l bounding s u r f a c e s can

(1) changes i n d i r e c t i o n o f s c o u r - p i t

( 2 ) changes i n s c o u r - p i t morphology,

i n c l u d i n g s i z e , shape, o r t h e

l o c a t i o n o f t o p o g r a p h i c f e a t u r e s such a s bedforms w i t h i n scour p i t s , and ( 3 ) m i g r a t i o n o f s c o u r p i t s w i t h c o n v o l u t e d t r a i l i n g surfaces. o p e r a t e s where t h e t r a i l i n g surfaces

that

surfaces)

are

m i g r a t i o n (i.e.

dip

in

the

surface o f a general

separated b y a

scour

migration

pit

direction

s u r f a c e d i p p i n g away

an erosional surface).

T h i s l a s t process

i s so i r r e g u l a r t h a t (i.e.

depositional

from t h e d i r e c t i o n o f

S c a l l o p s c o n t a i n i n g subsets formed by

t h i s process a r e i l l u s t r a t e d i n t h e a n c i e n t example d i s c u s s e d below.

420

RECOGNITION OF LONGITUDINAL AND OBLIQUE DUNES Small bedforms superimposed on a l a r g e oblique or longitudinal bedform can b e expected t o migrate systematically with a component in a preferred alongc r e s t d i r e c t i o n (Beutner,

Fluekinger, and Card,

1967; Jones, 1979; Hunter,

I n c o n t r a s t , on a transverse bedform, superimposed bedforms would n o t

1981).

be expected t o migrate with a preferred along-crest component. longitudinal

Consequently,

o r oblique bedforms can be i d e n t i f i e d by documenting t h a t the

superimposed bedforms migrated with a preferred along-crest component over t h e l a r g e bedforms. EXAMPLES

Ancient The Navajo Sandstone a t Diana's Throne ( e a s t of Zion National Park a t 37"

11' l a t i t u d e , 112" 38' l o n g i t u d e ) , Utah, contains a s e t of scallop-shaped beds t h a t p a r t i c u l a r l y well i l l u s t r a t e s t h e i n t e r p r e t i v e potential of t h e techniques presented above. The c h a r a c t e r i s t i c s of t h e exposed beds a r e r e l a t i v e l y simple t o describe, b u t mentally v i s u a l i z i n g the process whereby those beds were deposited by a s i n g l e complicated dune can be an extremely d i f f i c u l t a n d f r u s t r a t i n g exercise. However, t h i s kind of exercise i s unavoidable i f reconstructing dune morphology i s d e s i r e d , o r i f t h e d e p o s i t s of longitudinal a n d oblique dunes a r e t o be recognized. The example i s a s e t of scallop-shaped beds with t h e following generalized c h a r a c t e r i s t i c s (Fig. 8 A ) : ( 1 ) Trends of s c a l l o p axes and s t r i k e s of s c a l l o p s i d e s a r e between 135" a n d 170".

(2)

Scallops a r e each f i l l e d with two subsets of crossbeds.

Crossbeds i n the

basal subset a r e r e l a t i v e l y planar and d i p e a s t (80"-95").

Crossbeds i n

t h e overlying subset a r e curved and d i p in d i r e c t i o n s ranging from southeast t o southwest. Crossbed d i p d i r e c t i o n s within scallops range from 85" t o 240", indicating t h a t the scallops were generated by scour p i t s migrating i n t o t h e southeast quadrant (Fig. 88). The trough-axis t r e n d s and bounding-surface s t r i k e s i d e n t i f y t h e mean d i r e c t i o n o f migration a s approximately 160'. Using t h e technique developed above, t h e generalized d i p d i r e c t i o n of t h e primary l e e slope along which t h e scour p i t s migrated l i e s between t h e mean d i p d i r e c t i o n o f crossbeds a t the tops of t h e scallops (approximately 225") an!

d i r e c t i o n of bounding s u r f a c e s (250').

t h e mean d i p

For convenience, t h e c e n t r a l value of

t h i s range (235"-240") can b e used t o c h a r a c t e r i z e t h e generalized lee-slope dip direction.

421

By t r i a l and e r r o r , we discovered t h a t a "0"-shaped scour p i t with an internal spur (Fig. 8C), when t r a n s l a t e d toward 160" and observed in a n e a s t west-trending outcrop, would generate a scallop with internal s t r u c t u r e s identical t o those observed. The s t r a i g h t s i d e of t h e "D" generates a subset

o f eastward-dipping planar crossbeds, and the curved portion of t h e t r a i l i n g surface generates a subset of southwest-dipping concave-downwind crossbeds. The bounding surface between t h e two subsets i s scoured by an erosional trough north of a spur projecting i n t o t h e scour p i t ( l i n e TR in Fig. 8C). That t r o u g h plunges eastward, and, when t r a n s l a t e d toward 160", generates an eastward-dipping bounding surface t h a t separates t h e two subsets deposited by t h e t r a i l i n g surface of t h e scour p i t (Fig. 80). The eastern s c a l l o p in t h e s e t i l l u s t r a t e d

in

Fig.

8A contains a n

additional f e a t u r e not found in t h e others--a t h i r d subset of crossbeds ( t h a t dip t o w a r d 15"). T h a t subset i s inferred t o have been deposited by a bedform-possibly another spur projecting into the scour p i t - - t h a t migrated northward within t h e southeast-migrating scour p i t . The reconstruction presented above was a r r i v e d a t by t r i a l and e r r o r , b u t Figures 8C and D v e r i f y t h a t t h e proposed bedforms would accurately generate t h e bedding observed

in t h e f i e l d .

Moreover,

no other mechanism we can

envision i s capable of producing even a remotely s i m i l a r s t r u c t u r e . The assemblage of bedforms a r i s i n g from t h i s reconstruction (Fig. 8E) i s r e l a t i v e l y complex, b u t i t contains no f e a t u r e s t h a t a r e unusual i n modern dune fields. For example, a f a i r l y exact modern analog of such an assemblage of bedforms occurs in a dune f i e l d on t h e Oregon coast. In t h a t f i e l d , small dunes migrate along t h e troughs and lower l e e slopes of t h e l a r g e dunes (Fig. 8F and Hunter, Richmond, and Alpha, in press). Topographic depressions, o r scour p i t s , a r e bounded by t h e two s e t s of dunes; migration of t h e secondary dunes along t h e base of t h e primary l e e slope causes t h e scour p i t s t o crab. Scour p i t s i n t h e modern example (Fig. 8F) even contain spurs l i k e those we envision f o r t h e Navajo Sandstone example. A d i f f e r e n c e between t h e s e modern dunes and the reconstructed

ancient example i s t h a t t h e modern dunes a r e

destroyed and regeneraed a s a r e s u l t of seasonally changing winds. The examples of compound crossbedding presented above (Figs. 2-6 and 8 ) a r e r e l a t i v e l y straightforward because t h e i r formation can be visualized a s r e l a t i v e l y steady processes. We have a1 so encountered some extremely complicated examples t h a t suggest unsteady processes: zigzagging scour p i t s and intervening spurs (Hunter and Rubin, 1983), scour p i t s t h a t moved in nmerous d i r e c t i o n s in apparently disorganized fashion, and one scour p i t t h a t appeared

a s though i t may have temporarily followed a v e r t i c a l l y corkscrewing p a t h .

rossbed dip direction

ounding surface strikes

rough axis trends

S'

.I

t

r

B

160"

16

424 No d e r n

Cocipound crossbedding produced by superimposed b e d f o r m has been found i n modern a s well a s a n c i e n t e o l i a n sands.

One l o c a l i t y where Lie have seen such

f e a t u r e s i s on wind-scoured s t o s s s l o p e s of barchanoid dunes j u s t north o f the

m o u t h of S u t t o n Creek, 11

krii

north of Florence, Oregon.

Ilere, t h e coiiipound

crossbedding has a component of d i p t o the r i g h t (southwest) of t h e primary crossbedding, which d i p s t o t h e s o u t h e a s t ( F i g . 9 ) . Short-term observations o f dune processes suggest crossbedding

in

these

dunes

is

produced

by

differences

that

in

the

d i r e c t i o n s of t h e dunes and of f e a t u r e s superimposed on the dunes. superimposed f e a t u r e s

include

coi8iporind

t h e iiiigration These

t h e plan-fon:i s i n u o s i t i e s o f t h e barchanoitl

dunes, downwind-pointed s p u r s , a n d scour p i t s between t h e spurs ( F i g s . GF, 8F, a n d Cooper, 195S, p.

44-45).

Because t h e barchanoid dunes have a n averdge

trend t h a t i s n o t p e r f e c t l y t r a n s v e r s e t o t h e n o r t h e a s t e r l y simiier winds t h a t

Fig. 9. Wind-scoured, nearly h o r i z o n t a l , lower s t o s s slope of dune near S u t t o n Creek, Oregon. Crossbeds a n d bounding s u r f a c e s d i p mainly t o w a r d t h e t o p of photo ( i n t h e general d i r e c t i o n of dune i n i g r a t i o n ) , b u t several s u b s e t s t h a t were deposited by alongslope-migrating spurs have components of crossbed d i p t o the right.

425 shape them (Cooper, 1958, p. 31-33) t h e i r l e e s l o p e s a r e a f f e c t e d b y lee-eddy winds t h a t have an a l o n g s l o p e component (Hunter, 1981).

Under t h e i n f l u e n c e o f

these l e e eddies, t h e s a l i e n t s and spurs t e n d t o m i g r a t e c r a b w i s e a l o n g t h e l e e slopes. the

Given t h e u n i f o r m i t y o f t h e wind c l i m a t e a l o n g t h e Oregon c o a s t d u r i n g

summers,

this

process

could

easily

produce

the

observed

compound

crossbeddi ng. CONCLUSIONS Much o f t h e c o m p l e x i t y o f c r o s s - s t r a t i f i e d

beds i s n e i t h e r random nor

i n e x p l i c a b l e , b u t c a n b e demonstrated t o r e s u l t f r o m t h e m i g r a t i o n o f s m a l l bedforms on t h e l e e s l o p e s o f l a r g e bedforms. M i g r a t i n g bedforms w i t h l e e s l o p e s t h a t never e x p e r i e n c e e r o s i o n d e p o s i t simp1 e c r o s s - s t r a t i f i e d beds. One process t h a t causes e r o s i o n on l e e s l o p e s ( t h e r e b y p r o d u c i n g compound c r o s s b e d d i n g ) i s t h e m i g r a t i o n o f small superimposed bedforms across t h e l e e s l o p e o f a l a r g e bedform.

I n general, superimposed bedforms d e p o s i t

c r o s s - s t r a t i f i e d beds t h a t a r e t h i n compared t o t h e compound c r o s s s t r a t i f i e d b e d d e p o s i t e d by t h e l a r g e bedform.

The m i g r a t i o n d i r e c t i o n

o f t h e l a r g e bedform i s i n t h e d i p d i r e c t i o n o f bounding s u r f a c e s scoured b y t h e superimposed bedforms,

not necessarily i n the d i p direction o f

crossbeds t h a t t h e superimposed bedforms d e p o s i t .

The mean t r e n d o f t h e

superimposed bedforms i s para1 l e l t o t h e i n t e r s e c t i o n o f t h e crossbeds and bounding s u r f a c e s generated b y t h e superimposed bedforms.

The mean

m i g r a t i o n d i r e c t i o n o f t h e superimposed bedforms i s normal t o t h e i r mean trend. Scour p i t s t h a t m i g r a t e l a t e r a l l y , o r c r a b , a l o n g a l e e s l o p e generate compound crossbeds w i t h scallop-shaped l o w e r bounding surfaces.

The d i p

d i r e c t i o n o f t h e g e n e r a l i z e d l e e s l o p e l i e s between t h e d i p d i r e c t i o n s o f crossbeds and bounding s u r f a c e s generated b y t h e m i g r a t i n g scour p i t s . The m i g r a t i o n d i r e c t i o n o f a scour p i t i s p a r a l l e l t o t h e a x i s o f t h e trough-

o r scallop-shaped

s e t o f crossbeds d e p o s i t e d b y t h e m i g r a t i n g

scour p i t and i s p a r a l l e l t o t h e s t r i k e o f t h e bounding s u r f a c e scoured by t h e m i g r a t i n g scour p i t . The d e p o s i t s o f l o n g i t u d i n a l and o b l i q u e bedforms c a n b e r e c o g n i z e d where small

bedforms m i g r a t e d

systematically

w i t h a component

i s preferred

along-crest d i r e c t i o n . ACKNOWLEDCMENTS Ed C1 i f t o n (USGS) and Michael B r o o k f i e l d ( U n i v e r s i t y o f Guelph) reviewed a n e a r l y v e r s i o n o f t h i s paper.

Jeanne Blank d r a f t e d t h e f i n a l i l l u s t r a t i o n s .

426

TABLE 1 TECHNIQUES FOR INTERPRETING COMPOUND CROSSBEDDING DEPOSITED BY SUPERIMPOSED BEDFORMS AND C R A B B I I G SCOUR PITS

TYPE OF SECOliDARY FEATURE I n t e r p r e t i v e Goal

T\r o - D imen s io na 1 Superimposed Bedf o rms

Crabbing Scour P i t s

D i s t ing u i s h ing between superimposed bedforms and c r a b b i n g scour p i t s

Sets a r e generally t h i n compared to coset ; bounding s u r f a c e s a r e n o t scalloped; sets are r e l a t i v e l y tabu1 ar.

S e t s can b e as t h i c k a s coset; 1ower bounding surface of coset is scalloped; i n horizontal plane crossbeds are curved.

Determining orientation o f secondary f e a t u r e s

Trend i s p a r a l l e l t o l i n e of intersection of c rossbeds a nd bound ing surfaces generated by superimposed bedform.

Not appl i c a b l e -- scour p i t s may b e r e l a t i v e l y equidimensional.

Determining m i g r a t ion d ir e c t ion o f secondary features

Migration direction is normal to the trend d e t e r m i n e d above.

is Migration direction parallel to: (1) t r e n d o f trough o r scallop axis, and ( 2 ) bounding-surface strike.

Determining d i p direction o f primary bedform's 1 ee s l o p e

Dip o f primary l e e slope is approximated b y d i p direction of bounding surfaces deposited by superimposed bedforms.

Dip direction of the generalized l e e slope l i e s between t h e f o l l o w i n g two d ir ec t ion s : (1) mean crossbed d i p direction, and (2) d i p d i r e c t i o n o f bounding s u r f a c e s scoured b y m i g r a t i n g scour p i t .

D e t e r m i n i n g shape o f secondary features

Shape o f l e e s l o p e superimposed bedforms preserved by shape c rossb eds

of is of

T r a i l i n g s u r f a c e o f scour p i t s i s p r e s e r v e d b y shape of crossbeds within s c a l l ops.

Determining c r e s t p l a n shape o f primary bedform

Crest-plan shape is i n f e r r e d f r o m shape o f b o u n d i n g s u r f a c e s scoured b y superimposed bedforms.

Crest-plan shape is i n f e r r e d f r o m s i z e , shape, and spacing o f scour p i t s .

.

427 REFERENCES A l l e n , J. R. L., 1968. C u r r e n t r i p p l e s . N o r t h - H o l l a n d Pub. Co., Amsterdam, 433 PA l l e n , J. R. L., 1978. Polymodal dune assemblages: a n i n t e r p r e t a t i o n i n terms o f dune c r e a t i o n - d e s t r u c t i o n i n p e r i o d i c flows. Sedimentary Geology, 20:

17-28. J. R.

L., 1980. Sand waves: a model o f o r i g i n and i n t e r n a l s t r u c t u r e . Sedimentary Geology, 26: 281-328. Banks, Id. L., 1973. The o r i g i n and s i g n i f i c a n c e o f some down-current d i p p i n g c r o s s - s t r a t i f i e d s e t s : Jour. Sed. P e t r o l o g y , 43: 423-427. Beutner, F. C., F l u e c k i n g e r , L. A., and Gard, T. M. 1967. Bedding geometry i n a P e n n s y l v a n i a n channel sandstone. B u l l . Geol. SOC. America, 78: 911-916. B i g a r e l l a , J. J., and Salamuni, Riad, 1967. Some palaeogeographic and p a l a e o t e c t o n i c f e a t u r e s o f t h e Parana Basin: I n B i g a r e l l a , J. J., Becker, R. D., and P i n t o , I. D. ( E d i t o r s ) , Problems i n B r a z i l i a n Gondwana Geology, Internat. Symposium on t h e Gondwana S t r a t i g r a p h y and P a l e o n t o l o g y , l s t , Mara Del P l a t a , p. 235-301. B r o o k f i e l d , M. E., 1977. The o r i g i n o f bounding s u r f a c e s i n a n c i e n t e o l i a n sandstones. Sediment01 ogy , 24: 303-332. B r o o k f i e l d , M. E., 1979. Anatomy o f a Lower P e r i m i a n a e o l i a n sandstone complex, S o u t h e r n Scotland. S c o t t . J. Geol., 15: 81-96. Cooper, W. S., 1958. Coastal sand dunes o f Oregon and Washington. Geol. SOC. America Mem. 72, 169 p. Gregory, H. E., 1950. Geology and geography o f t h e Z i o n Park r e g i o n , Utah and U. S. Geol. Survey, P r o f . Paper 220, 200 p. Arizona. Harms, J. C., Southard, J. B, Spearing, D. R., and Walker, R. G., 1975. Depositional environments a s i n t e r p r e t e d from p r i m a r y sedimentary s t r u c t u r e s and s t r a t i f i c a t i o n sequences. SOC. Econ. P a l e o n t o l o g i s t s and M i n e r a l o g i s t s , S h o r t Course No. 2, D a l l a s , Texas, 1 6 1 p. Hunter, R. E., 1981. S t r a t i f i c a t i o n s t y 1 es i n e o l i a n sandstones: some Pennsylvanian t o J u r a s s i c examples f r o m t h e western i n t e r i o r U.S.A.: In F. G. E t h r i d g e , and R. M. F l o r e s ( E d i t o r s ) , Recent and a n c i e n t non-marine depositional envirorments: models for exploration SOC. Econ. P a l e o n t o l o g i s t s and M i n e r a l o g i s t s , S p e c i a l P u b l i c a t i o n no. 31, pp. 315Allen,

329. Hunter, R. E., Richnond, B. M., and Alpha, T. R., i n press, Storm-controlled o b l i q u e dunes o f t h e Oregon coast. Geol. SOC. America B u l l . Hunter, R. E., and Rubin, D. M., 1983. I n t e r p r e t i n g c y c l i c crossbedding, w i t h an example from t h e Navajo Sandstone. I n M. E. B r o o k f i e l d and T. S. Ahlbrandt (Editors), Aeolian sediments and processes, E l sevier, Amsterdam Jones, C. M., 1979. T a b u l a r cross-bedding i n Upper Carboniferous f l u v i a l channel sediments i n t h e s o u t h e r n Pennines, England. Sedimentary Geology,

.

24: 85-104. Kocurek, Gary, 1981. S i g n i f i c a n c e o f i n t e r d u n e d e p o s i t s and bounding s u r f a c e s i n aeol.ian dune sands. Sedimentology, 28: 753-780. McKee, E. D., 1966. S t r u c t u r e o f dunes a t White Sands N a t i o n a l Monunent, New Mexico (and a comparison w i t h s t r u c t u r e s o f dunes f r o m o t h e r s e l e c t e d areas). Sedimentology, 7: 3-69. McKee, E. D., 1979. A n c i e n t sandstones c o n s i d e r e d t o b e e o l i a n . I n E. 0. McKee, ( E d i t o r ) , A s t u d y o f q l o b a l sand seas U. S. Geol. Survey P r o f . Paper i 0 5 2 , p. 187-233.Rubin, D. M., and McCulloch, D. S., 1980. S i n g l e and superimposed bedforms: a s y n t h e s i s o f San F r a n c i s c o B a y and f l u m e observations. Sedimentary G o 1 ogy , 26: 207-231. Rubin, D. M., and Hunter, R. E. 1982. Bedform c l i m b i n g i n t h e o r y and nature. Sedimentology, 29: 121-138.

This Page Intentionally Left Blank

429

INTERPRETING CYCLIC CROSSBEDDING, WITH A N EXAMPLE FROM THE NAVAJO SANDSTONE RALPH E. HUNTER & DAVID M. RUBIN: U.S. M e n l o P a r k , C a l i f o r n i a 94025, U.S.A.

G e o l o g i c a l Survey

INTRODUCTION The e x i s t e n c e o f c y c l i c c r o s s b e d d i n g and J u r a s s i c )

i n t h e N a v a j o Sandstone ( T r i a s s i c ?

and o t h e r 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 U n i t e d S t a t e s was

n o t e d b y S t o k e s ( 1 9 6 4 ) , who i n t e r p r e t e d t h e c y c l e s a s a n n u a l l a y e r s p r o d u c e d b y (The t e r m " c y c l e " w i l l b e used h e r e f o r b o t h a

s e a s o n a l l y f l u c t u a t i n g winds.

d e p o s i t and f o r t h e wind v a r i a t i o n s t h a t produced t h e deposit.)

Cyclic cross-

bedding i s manifested as repeated v a r i a t i o n s i n t h e s t r u c t u r e o r t e x t u r e o f crossbeds w i t h i n a set.

Where t h e l a m i n a t i o n w i t h i n e a c h c y c l e i s p a r a l l e l t o

t h e bounding surfaces o f t h e c y c l e ,

as

i n t h e c y c l e s described b y Stokes

(1964), t h e s t r u c t u r e i s h e r e r e f e r r e d t o a s "concordant c y c l i c crossbedding" (fig.

1A).

R e l a t e d c y c l e s d e s c r i b e d h e r e b u t n o t b y S t o k e s (1964) a r e man-

i f e s t e d a s compound c r o s s b e d d i n g ( t e r m i n o l o g y o f Harms a n d o t h e r s , 1975, p. 51-

i n w h i c h t h e c r o s s b e d s composing a

FA\,

s e t a r e separated b y surfaces o f

, s i o n and a r e i n t e r n a l l y c r o s s b e d d e d ( f i g . 1B). I n t h e i n t e r p r e t a t i o n o f s e d i m e n t a r y c y c l e s , one o f t h e most f u n d a m e n t a l q u e s t i o n s t o b e answered i s w h e t h e r t h e c y c l i c i t y i s a l l o c y c l i c

-

that is,

p r o d u c e d b y changes i n t h e t o t a l e n e r g y o r m a t e r i a l i n p u t t o t h e s e d i m e n t a r y system

-

1964).

o r autocyclic

-

that

is,

i n d e p e n d e n t o f s u c h changes (Beerbower,

S t o k e s (1964) r u l e d o u t one t y p e o f a u t o c y c l i c i t y

r e p r e s e n t s a s i n g l e a v a l a n c h e down a dune s l i p f a c e .

-

sider another t y p e o f a u t o c y c l i c i t y

-

t h a t each c y c l e

However, h e d i d n o t con-

t h a t each c y c l e r e p r e s e n t s t h e d e p o s i t o f

a dune t h a t moved o v e r a l a r g e r dune. The p r o b l e m o f d i s t i n g u i s h i n g a l l o c y c l i c a n d a u t o c y c l i c c r o s s b e d d i n g a l s o arises

i n the

compound

study o f t i d a l

and f l u v i a l

compound c r o s s b e d d i n g ,

where t h e

s t r u c t u r e o f t h e crossbedding a r i s e s from t h e presence o f e r o s i o n

surfaces ( l i k e those surfaces"

( C o l l inson,

i n fig.

1B) t h a t have come t o b e c a l l e d " r e a c t i v a t i o n

1970).

These s u r f a c e s c a n b e p r o d u c e d b y a1 1 o c y c l i c

mechanisms s u c h a s t i d a l f l o w r e v e r s a l s (Boersma, 1969; K l e i n , 1970; D a l r y m p l e and o t h e r s , 1975; C l i f t o n , 1979; A l l e n , 198Da, 1980b; V i s s e r , 1980; K o h s i e k a n d Terwindt,

1981;

(Collinson, However,

Terwindt,

1970; J a c k s o n ,

1981)

o r changes

i n f l o w speed and

flow

depth

1976; J o n e s , 1977, 1979; J o n e s a n d McCabe, 1980).

s e v e r a l w o r k e r s have shown t h a t e r o s i o n s u r f a c e s o f n e a r l y i d e n t i c a l

a p p e a r a n c e c a n b e f o r m e d b y t h e a u t o c y c l i c mechanism o f s m a l l e r b e d f o r m s m o v i n g o v e r a l a r g e r one ( A l l e n ,

1968,

1973;

Banks, 1973;

McCabe a n d Jones, 1977).

430

A.

B.

F i g . 1. Schematic c r o s s s e c t i o n s o f t w o k i n d s o f c y c l i c crossbedding. (A) Concordant c y c l i c crossbeds. The t w o p a t t e r n s r e p r e s e n t d i f f e r e n t s t r a t i f i c a t i o n t y p e s o r g r a i n s i z e s ; t h e e s s e n t i a l f e a t u r e i s t h a t a l l t h e laminae ( 6 ) Crossw i t h i n c y c l e s and t h e bounding s u r f a c e s o f c y c l e s a r e p a r a l l e l . b e d d i n g whose c y c l i c appearance i s due t o t h e compound s t r u c t u r e o f t h e c r o s s b e d d i n g ( t h a t i s , due t o t h e d i v i s i o n o f t h e s e t i n t o subsets b y s u r f a c e s o f e r o s i o n , h e r e r e p r e s e n t e d b y s o l i d 1i n e s ) . Even i f e r o s i o n s u r f a c e s a r e n o t formed on t h e l e e s l o p e o f t h e l a r g e r bedform, t h e movement o f s m a l l e r bedforms o v e r a l a r g e r one c a n c r e a t e c y c l i c v a r i a t i o n s i n t h e t e x t u r e and s t r u c t u r a l c h a r a c t e r i s t i c s o f crossbeds w i t h i n a s e t (Smith, 1972), and such c y c l e s resemble t h o s e d e s c r i b e d b y Stokes (1964).

I n both

e o l i a n and t i d a l crossbedding, t h e a b i l i t y t o i d e n t i f y a c y c l i c i t y c o n t r o l l e d b y astronomic

processes would

represent

a

powerful

tool

for

interpreting

bedform dynamics and f l o w c o n d i t i o n s .

A m a j o r aim o f t h i s paper

is to

define c r i t e r i a

f o r distinguishing

crossbedding produced b y f l u c t u a t i o n s i n f l o w c h a r a c t e r ( c y c l e s o f t h i s t y p e

w i l l be c a l l e d "fluctuating-flow cycles")

f r o m c r o s s b e d d i n g produced by t h e

movement o f s m a l l e r bedforms over l a r g e r ones ("superimposed-bedform c y c l e s " ) . U s i n g t h e s e c r i t e r i a , we e v a l u a t e and s u p p o r t Stokes' (1964) i d e n t i f i c a t i o n o f fluctuating-flow

cycles

i n t h e Navajo Sandstone and i n t e r p r e t

compound c r o s s b e d d i n g as f l u c t u a t i n g - f l o w c y c l e s .

some o f t h e

We t h e n p r e s e n t a n a n a l y s i s

t h a t s u p p o r t s Stokes' i n t e r p r e t a t i o n o f annual p e r i o d i c i t y .

I n a n o t h e r paper

( R u b i n and Hunter, t h i s volume) we d e s c r i b e o t h e r c y c l i c compound crossbedding t h a t we i n t e r p r e t t o b e o f t h e superimposed-bedform type. DISTINGUISHING THE TYPES OF CYCLICITY C h a r a c t e r o f F l u c t u a t i n g - F l o w Cycles Cyclic periodic

crossbedding

fluctuations

aqueous e n v i r o r m e n t s ,

of

concordant structure

fluctuating-flow

i n flow direction,

t y p e c a n b e produced by

flow velocity,

flow i n

The c y c l e s may o r may n o t b e bounded b y e r o s i o n a l

I n t h e absence o f any e r o s i o n a l s u r f a c e s ( f i g . and

depth o f

o r o t h e r parameters a f f e c t i n g sediment t r a n s p o r t r a t e s

and d e p o s i t i o n a l mechanisms. surfaces.

the

a r e made e v i d e n t b y v a r i a t i o n s

( f o r example,

i n texture

by a l t e r n a t i o n s o f g r a i n f a l l

s t r a t a , i n t h e t e r m i n o l o g y o f Hunter, 1977a).

l A ) , t h e c y c l e s are or

sedimentary

and g r a i n f l o w cross-

431

A.

B. EXPLANATION

+--

Flow a t time of sketch

-

Erosional or hiatal surface

_ _ _ _ Representative C.

---

depositional surface Former bedform surface, now destroyed

Fig. 2. Schematic c r o s s sections of bedforms showing c y c l i c erosion surfaces ( r e a c t i v a t i o n s u r f a c e s ) produced by f l u c t u a t i n g flow. ( A ) Erosion surfaces on upper l e e slope and depositional wedges a t base of l e e slope produced by reversed flow. L e f t , a t end of buildout phase; r i g h t , a t end of backcutting phase. ( 8 ) Differing degrees of lee-slope erosion by reversed flow. Left, s l i g h t erosion; Right, extensive erosion. Note t h a t extent of erosion in f i g . 1 A i s intermediate between those shown i n f i g . 1 B . ( C ) Erosional scallops a t base of l e e slope, produced by periods of i n t e n s i f i e d c i r c u l a t i o n of t h e l e e eddy. Fluctuating-flow c y c l e s bounded by surfaces of erosion ( f i g . 1 B ) generally imply major changes in flow d i r e c t i o n o r , in aqueous flows, changes in water depth. Erosion surfaces produced by r e v e r s a l s of t h e normal-to-bedform component of flow tend t o be b e s t developed on t h e upper l e e slope and t o become nonerosional h i a t a l surfaces downward.

Erosion on t h e upper l e e slope i s often

accompanied by t h e formation of depositional wedges on t h e lower l e e slope ( f i g . 2A; Hunter and o t h e r s , i n press). Erosion surfaces t h a t a r e r e s t r i c t e d t o t h e upper l e e slope a r e formed during r e l a t i v e l y short periods of reversed flow. If t h e reversed flow continues long enough, t h e l e e slope i s extensively c u t back, and any i n i t i a l depositional wedge a t i t s base i s destroyed ( f i g .

2 B ) . The extent of backcutting i s , of course, influenced by additional f a c t o r s such as t h e strength of t h e reversed flow and t h e bedform s i z e . An erosion s u r f a c e may be produced on t h e lower l e e s i d e of a bedform s o l e l y by an increase in flow v e l o c i t y i f t h e increase causes s i g n i f i c a n t l y

432 m o r e v i g o r o u s b a c k f l o w i n t h e l e e eddy. suggest t h a t such e r o s i o n surfaces,

O b s e r v a t i o n s i n modern e o l i a n dunes

i n c o n t r a s t t o t h o s e produced b y reversed

f l o w , t e n d t o b e most c o n s p i c u o u s a t t h e b a s e o f t h e l e e s l o p e ( f i g . The

character

of

fluctuating-flow

cyclic

d i f f e r i n g p l a n - f o r m c u r v a t u r e o f t h e bedform. without

superimposed bedforms,

strike parallel bedform.

t o one a n o t h e r

all

crossbedding

2C).

varies

with

On a s t r a i g h t - c r e s t e d b e d f o r m

t h e crossbeds

and e r o s i o n s u r f a c e s must

even i f t h e f l o w i s n o t t r a n s v e r s e t o t h e

C y c l i c i t y on such bedforms c a n m a n i f e s t i t s e l f o n l y b y changes i n d i p

a n g l e , commonly a s s o c i a t e d w i t h e r o s i o n s u r f a c e s , a n d b y changes i n t h e t e x t u r e o r s t r a t i f i c a t i o n type. On c u r v e d b e d f o r m s ,

fluctuating-flow

c y c l e s can manifest themselves by

v a r i o u s f e a t u r e s n o t p o s s i b l e on s t r a i g h t - c r e s t e d bedforms.

R e l a t i v e l y small

f l u c t u a t i o n s i n f l o w d i r e c t i o n can cause s i g n i f i c a n t s h i f t s i n t h e l o c i o f maximum d e p o s i t i o n o r ,

i f t h e b e d f o r m i s s t r o n g l y enough c u r v e d ,

c a n cause

a l t e r n a t i n g e r o s i o n and d e p o s i t i o n t h a t a r e o u t o f phase o n d i f f e r e n t p a r t s o f t h e l e e slope.

O b s e r v a t i o n s o n modern e o l i a n dunes i n d i c a t e t h a t e r o s i o n s u r -

f a c e s p r o d u c e d b y s m a l l changes i n f l o w d i r e c t i o n a r e more common o n t h e downcurrent-convex s a l i e n t s o r downcurrent-pointed spurs o f curved bedforms t h a n on t h e downcurrent-concave r e - e n t r a n t s ( f i g s . alternations

3A, 3B; H u n t e r , 1977a, f i g .

9).

The

o f d e p o s i t i o n and e r o s i o n f r o m one s i d e t o t h e o t h e r o f t h e s e

f e a t u r e s c r e a t e d i s t i n c t i v e s t r u c t u r e s , some o f w h i c h r e s e m b l e B a g n o l d ' s (1941) i n f e r r e d s t r u c t u r e o f l o n g i t u d i n a l dunes. We r e f e r t o t h e d i s t i n c t i v e p a t t e r n s v i s i b l e i n t r a n s v e r s e s e c t i o n s ( s e c t i o n s normal t o t h e m i g r a t i o n d i r e c t i o n o f t h e bedform) o r i n h o r i z o n t a l exposures a s "zigzagging e r o s i o n s u r f a c e s " ( f i g .

3A) and " i n t e r l e a v e d e r o s i o n s u r f a c e s " current re-entrants

(fig.

3B).

I n t h e r a r e c o n c a v e down-

i n f i l l e d f r o m t w o s i d e s t h a t meet a b r u p t l y a t a c o r n e r ,

nonerosional zigzags a r e formed ( f i g .

3C; K o c u r e k a n d D o t t , 1981, f i g .

12).

C h a r a c t e r o f Superimposed-Bedform C y c l e s A s s m i n g t h a t b e d f o r m s o f d i f f e r e n t s i z e s c a n b e s t a b l e i n t h e same s t e a d y f l o w ( f o r evidence s u p p o r t i n g t h i s assumption, cyclic

crossbedding

cyclic.

o f the

see R u b i n and M c C u l l o c h , 1980),

superimposed-bedform t y p e c a n b e e n t i r e l y auto-

Each c y c l e o f t h i s t y p e i s t h e d e p o s i t o f a s i n g l e s u p e r i m p o s e d b e d -

f o r m , a n d t h e c y c l i c a p p e a r a n c e a r i s e s f r o m t h e s i m i l a r i t y o f t h e superimposed bedforms. Superimposed-bedform c y c l e s d i f f e r c o n s i d e r a b l y , d e p e n d i n g o n w h e t h e r t h e s u p e r i m p o s e d b e d f o r m s e x i s t o n b o t h t h e s t o s s a n d l e e s l o p e s o f t h e m a i n bedf o r m o r whether t h e y e x i s t o n l y o n t h e s t o s s slope,

m o v i n g up i t u n t i l t h e y

r e a c h t h e c r e s t , whereupon t h e i r sand a v a l a n c h e s down t h e s l i p f a c e o f t h e m a i n bedform.

Where superimposed b e d f o r m s e x i s t o n l y o n t h e s t o s s s l o p e o f t h e m a i n

433

A

B

ALTERNATING FLOW DIRECTIONS (for all diagrams)

C F i g . 3. Schematic horizontal sections showing l a t e r a l l y discontinuous erosion surfaces a n d c y c l e s formed by moderate f l u c t u a t i o n s i n flow d i r e c t i o n over bedAppearance i n s t r i k e c r o s s section i s forms t h a t a r e curved in plan form. similar. Symbols same a s in f i g u r e 1. ( A ) Zigzagging erosion surfaces formed o n downcurrent-pointed spur; r e - e n t r a n t s a r e a t edges of sketch. ( B ) Interleaved erosion surfaces formed on downcurrent-convex sal i e n t ; re-entrants a r e a t edges of sketch. ( C ) Zigzagging l a y e r s f i l l i n g a concave-downcurrent reentrant of a bedform.

bedform, no erosion surfaces a r e produced on t h e l e e slope except possibly on i t s uppermost p a r t (McCabe a n d Jones, 1977). The r e s u l t i n g cycles a r e made evident in longitudinal c r o s s section ( s e c t i o n parallel t o t h e migration d i r e c t i o n of t h e bedform) only by v a r i a t i o n s in t e x t u r e

(Smith, 1972) or

s t r a t i f i c a t i o n type ( f i g . 1A). Superimposed bedforms can migrate over t h e l e e slope of a l a r g e r bedform i f t h a t slope i s r e l a t i v e l y g e n t l e ( a t l e a s t a few degrees l e s s than t h e angle of repose). Under such conditions, t h e superimposed bedforms produce c l imbing t r a n s l a t e n t s t r a t a (terminology of Hunter, 1977b). Generally these s t r a t a a r e bounded by erosion surfaces a n d a r e i n t e r n a l l y crossbedded a s well a s being crossbeds in a l a r g e r s e t ( t h e s e t formed by t h e main bedform)(fig. 1B).

In

t h i s paper we discuss only those special f e a t u r e s of such compound crossbedding t h a t a r e useful

i n distinguishing i t from compound crossbedding produced by

flow f l u c t u a t i o n s .

In a r e l a t e d paper (Rubin and Hunter, t h i s volume) we

discuss i n g r e a t e r d e t a i l t h e compound crossbedding generated by t h e migration of superimposed bedforms. The s t r u c t u r e of compound crossbedding produced by superimposed bedforms i s controlled by such f a c t o r s a s t h e c r e s t length, plan-form curvature, and migration d i r e c t i o n of t h e superimposed bedforms r e l a t i v e t o t h e main bed-

434

PLAN FORM OF PRIMARY BEDFORM

I CURVED

STRAIGHT

F i g . 4. B l o c k d i a g r a m s showing v a r i e t i e s o f c y c l i c c r o s s b e d d i n g p r o d u c e d b y m i g r a t i o n o f r e l a t i v e l y s t r a i g h t c r e s t e d , l o n g c r e s t e d superimposed bedforms down o r a l o n g l e e s l o p e o f a l a r g e r , p r i m a r y b e d f o r m . Symbols same a s i n f i g u r e 1. Three s u r f a c e s a r e shown i n e a c h d i a g r a m : a h o r i z o n t a l s e c t i o n ( a t upper l e f t ) , a c r o s s s e c t i o n t h a t i s l o n g i t u d i n a l w i t h r e s p e c t t o main crossb e d d i n g ( a t l o w e r l e f t ) and t h e l e e s l o p e o f t h e b e d f o r m ( a t r i g h t ) . form.

Superimposed b e d f o r m s c a n r a n g e f r o m r e l a t i v e l y s h o r t c r e s t e d a n d c u r v e d

t o r e l a t i v e l y l o n g c r e s t e d and s t r a i g h t . short-crested,

Compound c r o s s b e d d i n g p r o d u c e d b y

c u r v e d superimposed b e d f o r m s i s c h a r a c t e r i z e d b y 1 e n t i c u l a r i t y

o f t h e s u b s e t s a s seen i n c r o s s s e c t i o n s t h a t a r e t r a n s v e r s e w i t h r e s p e c t t o t h e m i g r a t i o n d i r e c t i o n o f t h e superimposed b e d f o r m s (McCabe and J o n e s , 1977). Compound c r o s s b e d d i n g

formed b y t h e m i g r a t i o n o f superimposed bedforms

a l o n g t h e l e e s l o p e o f a l a r g e r b e d f o r m d i f f e r s c o n s i d e r a b l y f r o m t h a t formed b y t h e m i g r a t i o n o f s u p e r i m p o s e d b e d f o r m s down t h e l e e s l o p e ( f i g . 4).

Where

t h e s u p e r i m p o s e d b e d f o r m s h a v e a component o f movement a l o n g t h e l e e s l o p e o f t h e main bedform,

t h e y form subsets of crossbeds t h a t a r e v i s i b l e i n cross

s e c t i o n s t r a n s v e r s e w i t h r e s p e c t t o t h e m i g r a t i o n d i r e c t i o n o f t h e l a r g e r bedform.

Where t h e s u p e r i m p o s e d b e d f o r m s have a component o f movement u p o r down

t h e l e e s l o p e o f t h e main bedform,

t h e y form s u b s e t s o f c r o s s b e d s t h a t a r e

435 visible i n cross sections longitudinal with respect t o the migration direction o f t h e main l e e slope. The p l a n - f o r m c u r v a t u r e o f t h e m a i n b e d f o r m a l s o a f f e c t s t h e s t r u c t u r e o f compound c r o s s b e d d i n g ( f i g .

4).

The s i t u a t i o n i n w h i c h t h e s u p e r i m p o s e d bed-

forms a r e s t r a i g h t e r t h a n t h e m a i n b e d f o r m ( l o w e r l e f t , f i g . 4) i s uncommon. D i s t in g u i s h i ng C r i t e r i a Criteria

for

distinguishing

c y c l e s a r e l i s t e d i n t a b l e 1.

superimposed-bedform

and

f l u c t u a t i n g - f l ow

Where c y c l i c c r o s s b e d s a r e c o n c o r d a n t and d i p a t

t h e a n g l e o f repose ( f i g . l A ) , t h o s e produced b y f l o w f l u c t u a t i o n s a r e probably indistinguishable i n longitudinal

s e c t i o n from t h o s e produced b y t h e s p i l l i n g

o f superimposed bedforms o v e r t h e b r i n k o f t h e main s l i p f a c e . section o r transverse cross section, guishable.

however,

In horizontal

t h e t w o t y p e s may b e d i s t i n -

As n o t e d b y McCabe and J o n e s ( 1 9 7 7 ) , a c y c l i c c r o s s b e d p r o d u c e d b y

s p i l l o v e r o f a s u p e r i m p o s e d b e d f o r m c a n e x t e n d l a t e r a l l y no f a r t h e r t h a n t h e l a t e r a l e x t e n t o f t h a t b e d f o r m ( a d i s t a n c e g e n e r a l l y much l e s s t h a n t h e c r e s t l e n g t h o f t h e m a i n b e d f o r m ) , whereas a c y c l i c c r o s s b e d o f f l u c t u a t i n g - f l o w t y p e should extend l a t e r a l l y along t h e e n t i r e l e e slope o f t h e bedform (unless t h e l e e s l o p e i s so s t r o n g l y c u r v e d t h a t p a r t s o f i t a r e n o n d e p o s i t i o n a l ) . over,

More-

i f t h e superimposed bedform i s n o t p e r f e c t l y p a r a l l e l t o t h e b r i n k l i n e

o f t h e m a i n s l i p f a c e , t h e s p i l l o v e r w i l l m i g r a t e a l o n g t h e m a i n s l i p f a c e and t h u s c r e a t e compound c r o s s b e d d i n g

s i m i l a r t o t h a t shown i n t h e u p p e r - r i g h t

d i a g r a m o f f i g u r e 4. Where c y c l i c c r o s s b e d s a r e c o n c o r d a n t b u t d i p l e s s s t e e p l y t h a n t h e a n g l e o f r e p o s e ( a n d t h e r e f o r e must have b e e n f o r m e d b y d e p o s i t i o n a l p r o c e s s e s o t h e r t h a n a v a l a n c h i n g ) , t h e y were p r o b a b l y f o r m e d b y f l u c t u a t i n g f l o w .

Superimposed

b e d f o r m s c a n f o r m c o n c o r d a n t c r o s s b e d s o n l y i f t h e b e d f o r m s a r e d e s t r o y e d when t h e y r e a c h t h e l e e s l o p e o f t h e m a i n b e d f o r m , a n d a v a l a n c h i n g down a s l i p f a c e i s p r o b a b l y t h e o n l y a d e q u a t e mechanism o f s u c h d e s t r u c t i o n .

Where t h e l e e

slope o f t h e main bedform d i p s l e s s s t e e p l y t h a n t h e angle o f repose, t h e superimposed b e d f o r m s w o u l d n o t b e i m m e d i a t e l y d e s t r o y e d and w o u l d f o r m compound r a t h e r t h a n c o n c o r d a n t c r o s s b e d d i n g . Compound c y c l i c c r o s s b e d d i n g o f f l u c t u a t i n g - f l o w and s u p e r i m p o s e d - b e d f o r m types can b e s t b e d i s t i n g u i s h e d by t h e l e n g t h s and widths o f i n d i v i d u a l subsets ( r e l a t i v e t o t h e t h i c k n e s s o f t h e i n d i v i d u a l s u b s e t s o r o f t h e e n t i r e compound set).

The l e n g t h s a n d w i d t h s o f t h e s u b s e t s a r e measured i n s e c t i o n s t h a t a r e

l o n g i t u d i n a l and t r a n s v e r s e ,

respectively,

with respect t o the migration di-

r e c t i o n o f t h e f e a t u r e t h a t formed t h e subset crossbedding. Lenticularity o f the individual s b s e t crossbedding

subsets i n a transverse section o f t h e

s u g g e s t s a superimposed-bedform o r i g i n

(McCabe and J o n e s ,

436 TABLE 1 Features u s e f u l f o r d i s t i n g u i s h i n g c y c l i c c r o s s b e d d i n g o f superimposedbedform and f l u c t u a t i n g - f l o w o r i g i n s

Concordant c y c l i c c rossbedd ing

Cycl i c crossbedding o f super imp0 sed-b ed f orm o r i g in

C y c l i c crossbedding o f f 1 uc t ua t ing- f 1ow o r ig in

Complete concordance ( i .e., i n b o t h t r a n s v e r s e and 1ong i t u d i n a l c r o s s s e c t i o n ) uncommon, p o s s i b l y none x i s t e n t where crossbeds d i p a t l e s s than angle o f repose

Complete concordance commonly produced

C y c l e widtha commonly s m a l l

C y c l e widtha g r e a t unless plan-form curvature i s great

Probably not d i s t i n g u i s h a b l e i n longitudinal cross section

Compound c y c l i c c r o s sb edd ing

Cycles whose crossbedding indicates formation b y f e a t u r e m i g r a t i n g along a l a r g e r l e e s l o p e produced readily

Cycles whose crossbedding i n d i c a t e s formation by f e a t u r e m i g r a t i n g along a l a r g e r l e e s l o p e produced o n l y where p l a n - f o r m curvature o f t h a t slope i s g r e a t

C y c l e 1 engtha commonly great

Cycle lengtha g e n e r a l l y smal 1

Cycl e w i d t ha commonly small

Cycle w i d t h a g r e a t unless p l an-form c u r v a t u r e o f l a r g e r l e e slope i s great

Features produced b y r e verse f l o w compatible w i t h f o r m a t i o n b y l e e eddy simultaneously w i t h forward flow

Features produced b y r e v e r s e f l o w may i n d i c a t e o r i g i n a t t i m e s when forward f l o w d i d not occur

aCycle w i d t h and l e n g t h a r e t h e l a t e r a l e x t e n t o f t h e c y c l e i n t r a n s v e r s e and l o n g i t u d i n a l cross sections, respectively. A l o n g i t u d i n a l cross section i s one p a r a l l e l t o t h e m i g r a t i o n d i r e c t i o n o f t h e f e a t u r e t h a t produced t h e crossbedding. 1977).

Such l e n t i c u l a r i t y a r i s e s

i n superimposed-bedform

c y c l e s from the

t h r e e - d i m e n s i o n a l f o r m ( t h a t i s , t h e f i n i t e c r e s t l e n g t h s and i r r e g u l a r trough e l e v a t i o n s ) o f t h e superimposed bedforms.

F l u c t u a t i n g - f l o w c y c l e s c a n b e len-

t i c u l a r o n l y i f t h e bedform undergoes s u b s t a n t i a l changes i n f o r m d u r i n g t h e f 1uc t ua t io ns.

437 Where a n i n d i v i d u a l s u b s e t i s v e r y l o n g r e l a t i v e t o i t s t h i c k n e s s a s seen i n a longitudinal

s e c t i o n o f t h e s u b s e t c r o s s b e d d i n g , a superimposed-bedform

o r i g i n i s suggested.

The l e n g t h o f a s u b s e t p r o d u c e d b y a superimposed b e d f o r m

equals t h e d i s t a n c e across which t h e bedform migrated d u r i n g d e p o s i t i o n (except f o r a n y s h o r t e n i n g due t o e r o s i o n b y f o l l o w i n g b e d f o r m s ) , a n d t h i s d i s t a n c e c a n e a s i l y b e a l a r g e m u l t i p l e o f t h e s u b s e t t h i c k n e s s ( R u b i n and H u n t e r , The f o r m a t i o n o f a f l u c t u a t i n g - f l o w thickness,

on t h e o t h e r

hand,

r e q u i r e s extensive b u i l d o u t o f t h e l e e slope

followed by almost equally extensive backcutting ( r i g h t , required distance o f

fluctuating-flow

l i k e l y t h a n a superimposed-bedform o r i g i n . creases,

fig.

26).

As

the

n e a r l y b a l a n c e d b u i l d o u t and b a c k c u t t i n g becomes much

greater t h a n t h e c y c l e thickness,a

o r i g i n becomes much l e s s

C o n v e r s e l y , a s s u b s e t l e n g t h de-

f l u c t u a t i n g - f l ow a n d superimposed-bedform

e q u a l l y probable.

1982).

cycle t h a t i s very long r e l a t i v e t o i t s

o r i g i n s become more n e a r l y

However, no q u a n t i t a t i v e l i m i t s c a n b e p u t o n t h i s r u l e .

The above r u l e o f s u b s e t l e n g t h i s most e a s i l y a p p l i e d where t h e s u b s e t c r o s s b e d d i n g has a component o f d i p a l o n g t h e p r i m a r y l e e slope.

Where t h e

m a i n b e d f o r n i was s t r a i g h t a n d t h e s u b s e t s a r e v e r y l o n g ( u p p e r r i g h t , f i g . a s u p e r i m p o s e d - b e d f o r m i n t e r p r e t a t i o n i s t h e o n l y p l a u s i b l e one. b e d f o r m becomes more h i g h l y c u r v e d , fluctuating-flow

cycles

3A,

(figs.

4),

As t h e m a i n

t h e s u b s e t l e n g t h t e n d s t o decrease,

and

3B) become more d i f f i c u l t t o d i s t i n g u i s h

f r o m s u p e r i m p o s e d - b e d f o r m c y c l e s ( u p p e r l e f t , f i g . 4).

I n such c a s e s , e v i d e n c e

o f r e v e r s a l s i n t h e a l o n g s l o p e component o f f l o w ( f i g s .

3A, 38) almost c e r -

t a i n l y i n d i c a t e s f l u c t u a t i n g flow. Compound c y c l i c c r o s s b e d d i n g b y g e n e r a t e d t h e m i g r a t i o n o f l o n g - c r e s t e d , s t r a i g h t - c r e s t e d s u p e r i m p o s e d b e d f o r m s d i r e c t l y down t h e l e e s l o p e o f a l a r g e r , s t r a i g h t - c r e s t e d b e d f o r m c a n 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 f r o m compound c y c l i c

crossbedding produced b y f l u c t u a t i n g f l o w

diagram o f f i g .

4 with figs.

2A, 2B).

(Compare l o w e r - r i g h t

The r u l e o f s u b s e t l e n g t h ( i n l o n -

g i t u d i n a l c r o s s section) i s a suggestive guide:

The l o n g e r t h e s u b s e t r e l a t i v e

t o s e t o r subset t h i c k n e s s , t h e more a superimposed-bedform i n t e r p r e t a t i o n i s preferrable t o a fluctuating-flow

interpretation.

E v i d e n c e o f r e v e r s a l s i n t h e f l o w a c r o s s a b e d f o r m c a n b e used i n some cases cles.

to

distinguish

fluctuating-flow

cycles

from

superimposed-bedform c y -

I f t h e r e v e r s e d f l o w c a n b e shown t o h a v e accompanied f o r w a r d m i g r a t i o n

o f t h e b e d f o r m , t h e r e v e r s e d f l o w must have b e e n m e r e l y a l e e - e d d y e f f e c t , and superimposed-bedfom c y c l e s a r e n o t r u l e d out. o f reversed

f l o w c a n b e shown t o

I f , o n t h e o t h e r hand, p e r i o d s

have a l t e r n a t e d w i t h

periods o f forward

m i g r a t i o n o f t h e p r i m a r y b e d f o r m , t h e c y c l e s must b e o f f l u c t u a t i n g - f l o w ( H u n t e r , 1981).

type

438 Superimposed-bedform and f l u c t u a t i n g - f l o w c y c l e s c a n c o e x i s t w i t h i n the

I f t w o o r more d i f f e r e n t s t y l e s o r s c a l e s o f c y c l i c i t y

same s e t o f crossbeds. can b e i d e n t i f i e d , field,

1979).

t h e o r i g i n o f each must b e c o n s i d e r e d s e p a r a t e l y (Brook-

As one example o f c o m p l e x i t y , superimposed bedforms may e x i s t

d u r i n g one p a r t o f a c y c l e o f f l o w f l u c t u a t i o n s b u t n o t d u r i n g a n o t h e r p a r t . CYCLIC CROSSBEDDING I N THE NAVAJO SANDSTONE C h a r a c t e r o f t h e C y c l i c Crossbeddinp General f e a t u r e s .

C y c l i c crossbedding f o r which a f l u c t u a t i n g - f l o w o r i g i n

i s c o n c e i v a b l e i s f a i r l y comnon i n p a r t s o f t h e Navajo Sandstone, e s p e c i a l l y i n and near Z i o n N a t i o n a l appearance, however,is

Park,

southwestern

U t a h (Stokes,

1964).

A cyclic

w e l l developed i n o n l y a small f r a c t i o n o f t h e sets.

A

s e t t h a t does c o n t a i n w e l l developed c y c l e s commonly e x t e n d s f o r a t l e a s t hundreds o f m e t e r s and comprises hundreds o f c y c l e s ( f i g . commonly d o n o t have w e l l developed c y c l i c i t y .

5 ) , b u t a d j a c e n t sets

The reasons f o r such r e s t r i c -

t i o n o f w e l l developed c y c l i c i t y t o a small f r a c t i o n o f t h e s e t s have not y e t b een d i scov ered. Two t y p e s o f c y c l i c crossbedding occur i n t h e Navajo Sandstone: m u t u a l l y concordant compound,

(fig.

lA),

such a s t h e examples d e s c r i b e d b y Stokes (1964), and

i n which t h e crossbeds a r e separated by e r o s i o n s u r f a c e s and are

i n t e r n a l l y crossbedded ( f i g .

1B).

Both t y p e s o f c y c l i c crossbedding occur i n

r e l a t i v e l y t a b u l a r s e t s t h a t a r e g e n e r a l l y 5 t o 15 m t h i c k and average about 10 m i n thickness,

near t h e average f o r a l l s e t s o f crossbeds i n t h e Navajo Sand-

s t o n e o f t h e Z i o n area.

As seen i n h o r i z o n t a l s e c t i o n o r i n t r a n s v e r s e cross

s e c t i o n , t h e t w o t y p e s o f c y c l e s a r e s i m i l a r i n b e i n g l a t e r a l l y e x t e n s i v e (no l a t e r a l p i n c h o u t s were seen) and i n h a v i n g l a m i n a t i o n p a r a l l e l t o t h e bounding The t w o t y p e s o f c y c l e s d i f f e r , however,

surfaces o f t h e cycles. ways d e s c r i b e d below.

i n several

Cycles t h a t a r e i n one way o r a n o t h e r i n t e r m e d i a t e be-

tween t h e t w o main t y p e s a r e d e s c r i b e d subsequently. Concordant c y c l i c crossbedding. crossbeds

are

concave-upward

(in

Almost

all

longitudinal

t a n g e n t i a l t o t h e base o f t h e s e t ( f i g s . 6, 7). o c c u r near t h e t o p o f t h e s e t , approach 27", t h e Navajo Sandstone when p o s t d e p o s i t i o n a l ( H u n t e r , 1981).

o f t h e concordant cyclic

cross

section)

and

nearly

The maximum d i p angles, which

which i s t h e a n g l e o f repose i n compaction i s t a k e n i n t o account

The t h i c k n e s s o f a c y c l e t e n d s t o v a r y p r o p o r t i o n a l l y w i t h the

s i n e o f t h e d i p angle,

so t h a t t h e dune advance p e r c y c l e remains nearly

c o n s t a n t f r o m t h e t o p t o t h e base o f t h e s e t a s t h e c y c l e t h i c k n e s s decreases f r o m t o p t o base. about 0.3

I n c y c l e s o f t h i s t y p e , t h e dune advance p e r c y c l e averages

m.

A l t h o u g h Stokes (1964)

described g r a i n - s i z e v a r i a t i o n s t h a t g i v e a Cyclic

439 Dune advance 1.0m 0 7 100,

Dune advance 0.5 1.0m

,,

Dune advance

0 I2001

0 5

1.0m

100

F i g . 5. Dune advance p e r c y c l e f o r a l o n g s e r i e s o f m u t u a l l y c o n c o r d a n t c y c l i c c r o s s b e d s i n t h e N a v a j o Sandstone a l o n g S t a t e Highway 9, 2.7 km (1.7 m i l e s ) west o f t h e e a s t - e n t r a n c e g a t e h o u s e , Z i o n N a t i o n a l P a r k , Utah. Data r e p r e s e n t c o n d i t i o n s midway b e t w e e n t o p and b a s e o f a n 1 1 - m - t h i c k s e t o f c r o s s b e d s ; some c y c l e s t h i c k e n a t t h e expense o f a d j a c e n t c y c l e s t o w a r d t o p o r b a s e o f s e t . Measurements were made f r o m p h o t o g r a p h s and p r o b a b l y have some e r r o r s due t o i r r e g u l a r i t i e s o f o u t c r o p surface. L a y e r s l a b e l e d "A" a r e a v a l a n c h e ( s a n d flow) c r o s s - s t r a t a t h a t a r e n o t assigned t o a c y c l e ; L a y e r s l a b e l e d "(A)" a r e c y c l e s t h a t c o n t a i n a v a l a n c h e c r o s s - s t r a t a a l o n g l i n e o f measurement. Other a v a l a n c h e c r o s s - s t r a t a o c c u r h i g h e r i n t h e s e t , and a l l a v a l a n c h e c r o s s - s t r a t a p i n c h o u t toward base o f set. Layers l a b e l e d "P" a r e cycles, n o t avalanche cross-strata, t h a t p i n c h o u t toward base o f set. L a y e r s l a b e l e d " R " mark t h e a r r i v a l of a phase o f r e i n v i g o r a t e d d e p o s i t i o n t h a t m i g r a t e d t o w a r d b a s e o f s e t during t h e course o f several cycles. appearance, t h e c o n c o r d a n t c y c l e s a r e m a n i f e s t e d m a i n l y b y r e p e a t e d v a r i a t i o n s i n the type o f stratification. d e s c r i b e d b y H u n t e r (1977a,

Most o f t h e t y p e s o f d r y - e o l i a n s t r a t i f i c a t i o n

1 9 8 1 ) , K o c u r e k a n d D o t t ( 1 9 8 1 ) , and F r y b e r g e r and

Schenk (1981) a r e r e c o g n i z a b l e .

Most commonly,

the cycles are defined by al-

t e r n a t i o n s o f what a r e i n t e r p r e t e d a s g r a i n f a l l d e p o s i t s a n d d e p o s i t s formed b y climbing wind r i p p l e s ( f i g . irregular levels

8).

C y c l e s o f t h i s t y p e a r e i n t e r r u p t e d a t wide,

i n t e r v a l s b y s o l i t a r y s a n d f l o w l a y e r s t h a t p i n c h o u t downward,

where

the

dip

angle

of

the

cyclic

at

c r o s s b e d s was t o o l o w f o r sand

440

CROSSBEDS

Fig. 6. Schematic rilodel of [:lost coi1ii:ion tylie of Concordant c y c l i c crossbedding in t h e llavajo Sandstone i n t h e v i c i n i t y of Zion IJational P a r k . Dashed l i n e s i n i n s e t b o x e s , g r a i n f a l l 1ai:iinae; s o l i d l i n e s , t r d n s l a t e n t s t r a t a foriried by s t o s s - e r o s i o n a l c l iiiibing wind r i p p l e s .

.

Lower p a r t o f a s e t o f c o n c o r d a n t c y c l i c c r o s s b e d s a t same l o c a l i t y a s Fig 7. i n f i g u r e 5. Note t a n g e n t i a l b a s e s o f c r o s s b e d s . Another photograph of t h i s s e t o f c r o s s b e d s i s shown by P e t t i j o h n and P o t t e r ( 1 9 6 4 , P l a t e 3 6 ) . flowage. Grainfall

deposits

in

t h e c o n c o r d a n t c y c l e s a r e c h a r a c t e r i z e d by the

f a i n t n e s s and n e a r l y p e r f e c t p a r a l l e l i s m o f t h e l a m i n a t i o n and by t h e absence o f any r e g u l a r l y r e p e a t e d v a r i a t i o n s i n g r a i n s i z e o r o t h e r c h a r a c t e r i s t i c s

w i t h i n a b u n d l e o f g r a i n f a l l laminae ( t h a t i s , by t h e absence o f any c y c l i c i t y

a t t h e s c a l e of individual laminae). grainfall action 1977a).

The i n t e r p r e t a t i o n o f t h e s e d e p o s i t s a s

d e p o s i t s i s not i n t e n d e d t o r u l e o u t t h e o c c u r r e n c e o f some wind

and

tractional

transport

C1 imbing-wind-ripple

across

the

depositional

surface

(Hunter,

d e p o s i t s in t h e concordant c y c l e s c o n s i s t o f

t r a n s l a t e n t s t r a t a ( H u n t e r , 1977b) formed by wind r i p p l e s c l i m b i n g a t v e r y low

441

F i g . 8. Transverse exposure o f concordant c y c l i c crossbeds i n lower p a r t o f s e t a t same l o c a l i t y a s i n f i g u r e 5. A r r o w s inark c y c l e c o n t a c t s , w h i c h a r e t h e upper c o n t a c t s o f i n t e r v a l s i n which 1 i g h t - w e a t h e r i n g c l i m b i n g - w i n d - r i p p l e d e p o s i t s a r e doiTiinant a n d l o w e r c o n t a c t s o f i n t e r v a l s i n w h i c h d a r k - w e a t h e r i n g g r a i n f a l l d e p o s i t s a r e dominant. N o t e upward i n c r e a s e i n p r o p o r t i o n o f g r a i n f a l l d e p o s i t s and i n t h i c k n e s s o f c y c l e s . a n g l e s ( s u b c r i t i c a l a n g l e s o f H u n t e r , 1977b, o r s t o s s - e r o s i o n a l a n g l e s o f R u b i n and H u n t e r ,

1982).

These d e p o s i t s d i f f e r f r o m t h e a s s o c i a t e d g r a i n f a l l de-

p o s i t s b y t h e g r e a t e r d i s t i n c t n e s s and l e n t i c u l a r i t y o f t h e l a m i n a t i o n , tendency

for

r i p p l e-foreset

and b y t h e p r e s e n c e o f s o w

t h e l a m i n a t i o n t o appear c y c l i c , crosslainination

and

preserved

by a

r i p p l e cross

sections.

The

r i p p l e s and r i p p l e - f o r e s e t c r o s s l a m i n a e i n d i c a t e r i p p l e m i g r a t i o n i n a l o n g s l o p e d i r e c t i o n s (coimonly i n b o t h alongslope d i r e c t i o n s ) . The c o n t a c t s o f t h e c o n c o r d a n t c y c l e s a r e s h a r p s u r f a c e s ,

some o f w h i c h

a r e s l i g h t l y wavy b e c a u s e o f p r e s e r v e d w i n d r i p p l e s t h a t t r e n d n e a r l y p a r a l l e l

t o t h e d i p o f t h e surface. and c l i r n b i n g - w i n d - r i p p l e

G r a i n f a l l deposits form t h e lower p a r t o f a cycle, deposits form t h e

upper

part

(fig.

8);

conimonly a

t r a n s i t i o n zone o f t h i n l y i n t e r b e d d e d b u n d l e s o f g r a i n f a l l and c l i i n b i n g - w i n d r i p p l e d e p o s i t s occurs i n the middle o f a cycle. climbing-wind-ripple crossbedding.

The r a t i o o f g r a i n f a l l t o

d e p o s i t s w i t h i n a g i v e n c y c l e i n c r e a s e s upward a l o n g t h e

The l a m i n a t i o n w i t h i n a c y c l e i s p a r a l l e l o r n e a r l y p a r a l l e l t o

t h e c y c l e contacts. Compound c y c l i c c r o s s b e d d i n g .

The compound-crossbed c y c l e s a r e d e f i n e d

p r i m a r i l y b y t h e erosional bounding surfaces t h a t t r u n c a t e crossbeds o f t h e

442

F i g . 9. Schematic model o f most common t y p e o f compound c y c l i c crossbedding i n t h e N a v a j o Sandstone i n t h e v i c i n i t y o f Z i o n N a t i o n a l P a r k .

F i g . 10. Compound s e t o f c y c l i c c r o s s b e d s i n t h e Lamb P o i n t Tongue o f the Highway 89, 8.2 km (5.1 m i l e s ) n o r t h o f Kanab, N a v a j o Sandstone a l o n g U.S. Utah. Most o f t h e a p p a r e n t c u r v a t u r e o f t h e c r o s s b e d s i s due t o outcrop c u r v a t ure. and t h e d i p a n g l e s o f t h e e r o s i o n surfaces a r e moderate, g e n e r a l l y longitudinal cross section,

most o f t h e i r l e n g t h b u t c u r v e i n t o concave-upward n o n e r o s i o n a l t h e base o f t h e set.

2Oo-25O.

In

t h e e r o s i o n s u r f a c e s a r e n e a r l y s t r a i g h t through s u r f a c e s near

The dune advance p e r c y c l e a v e r a g e s a b o u t 1.5 m.

Compound-crossbed c y c l e s a r e d e f i n e d b y v a r i a t i o n s i n t h e t y p e o f s t r a t i f i c a t i o n as w e l l a s b y t h e presence o f e r o s i o n surfaces.

S a n d f l o w deposits,

easily recognizable i n d i p cross section b y t h e i r toes (fig.

l l ) , a r e much more

abundant i n t h e s e c y c l e s t h a n i n t h e concordant cycles.

The s a n d f l o w deposits

443

Fig. 11. Lower p a r t o f compound s e t o f c y c l i c c r o s s b e d s shown i n f i g u r e 10. Sandflow c r o s s - s t r a t a ( S ) a r e r e l a t i v e l y d a r k . Basal wedges ( m a r k e d b y b r a c k e t s ) and b o t t o m s e t d e p o s i t s contemporaneous w i t h s a n d f l ows ( 5 ) a r e r e l a t i v e l y 1ight. i n t e r f i n g e r downward w i t h more g e n t l y d i p p i n g d e p o s i t s , f o r m e d b y c l i m b i n g w i n d ripples o r b y grainfall,

w h i c h make up b o t t o m s e t d e p o s i t s t h a t r e s t o n t h e

lower b o u n d i n g s u r f a c e o f a c y c l e .

A

d i s t i n c t i v e wedge composed o f c l i m b i n g - w i n d - r i p p l e d e p o s i t s ,

deposits,

or grainfall

deposits t y p i c a l l y occurs

cycle near t h e base o f t h e

set,

filling

the triangular

youngest s a n d f l o w c r o s s b e d o f a g i v e n c y c l e , natively,

the

gently

dipping

youngest s a n d f l o w c r o s s b e d ) , 11).

bottomset

space b e t w e e n t h e

t h e base o f t h e s e t ( o r , a l t e r -

deposits

contemporaneous

with

the

and t h e u p p e r b o u n d i n g s u r f a c e o f a c y c l e ( f i g .

L a m i n a t i o n w i t h i n t h i s wedge ( h e r e c a l l e d a " b a s a l

Hunter and o t h e r s ,

planebed

i n e a c h compound-crossbed

wedge",

following

i n press) tends t o f a n o u t from p o i n t s near t h e updip and

downdip c o r n e r s o f t h e wedge.

The l a m i n a t i o n i s t r u n c a t e d b y t h e e r o s i o n a l

bounding s u r f a c e n e a r t h e u p d i p c o r n e r o f t h e wedge b u t i s c o n c o r d a n t w i t h t h e more g e n t l y d i p p i n g

downdip c o n t i n u a t i o n o f t h i s

same s u r f a c e .

The u p d i p

p i n c h o u t s o f t h e wedges s u g g e s t d e p o s i t i o n b y w i n d s t h a t b l e w u p s l o p e . C y c l i c crossbedding intermediate

o f i n t e r m e d i a t e types.

i n one way o r

Among t h e s e t y p e s a r e ( 1 )

Several types o f c y c l e s a r e

a n o t h e r b e t w e e n t h e t w o t y p e s d e s c r i b e d above. Cycles t h a t a r e concordant b u t t h a t resemble t h e

compound-crossbed c y c l e s i n b e i n g r e l a t i v e l y t h i c k ( d u n e advance o f a b o u t 1 rn

444 per c y c l e ) , containing

sandflow d e p o s i t s a s a

conspicuous c o n s t i t u e n t , a n d

having concave-up c u r v a t u r e o n l y near t h e b a s e o f t h e s e t ; closely

( 2 ) cycles t h a t

resemble t h e c o n c o r d a n t - c r o s s b e d c y c l e s e x c e p t t h a t t h e i r b o u n d i n g

s u r f a c e s t r u n c a t e t h e l a i n i n a t i o n w i t h i n t h e c y c l e s a t siila11 a n g l e s ( l e s s t h a n 5 " ) in d i p c r o s s s e c t i o n ; and ( 3 ) c y c l e s whose bounding s u r f a c e s a r e concordant

and s t r a i g h t i n t h e upper p d r t of t h e s e t b u t g r a d e downward i n t o concaveupward e r o s i o n a l s u r f a c e s t h a t foriii s c a l l o p s a t t h c b a s e of t h e s e t , a s shown in f i g u r e 2C.

These t h r e e interriiediate t y p e s o f c y c l e s a r e l e s s coiiunon t h a n

t h e tvo main t y p e s . i v i d e n c e f o r Fluctuating-Flovi O r i g i n The p a r a l l e l i s i n between t h e l a m i n a t i o n w i t h i n t h e c y c l e s a n d t h e b o u n d i n g s u r f a c e s of t h e c y c l e s , a s seen in t r a n s v e r s e c r o s s s e c t i o n s or i n horizontal s e c t i o n s o f b o t h t y p e s o f c y c l e s ( f i g s . 1 2 , 1 3 ) , does not s u p p o r t a n o r i g i n o f t h e s e c y c l e s by t h e iiiigr3tion o f superiiiiposcd dunes along t h e l e e s l o p e of the [:lain dune ( s u c h a s i n t h e upper diaqrains o f f i g . 4 ) .

If superimposed dunes

ivrere i n v o l v e d , t h e y must h a v e been s t r a i g h t c r e s t e d arid p a r a l l e l t o t h e main dune.

Floreover, t h e a b s e n c e o f cycl:

pinchouts i n t r a n s v e r s e c r o s s section and

in h o r i z o n t a l s e c t i o n i n d i c a t e s thiit superiii~posedd u n e s , i f they were involved,

~ u s thave been long c r e s t e d .

Fig. 12. Exhuned upper c o n t a c t o f a s e t o f c o n c o r d a n t c y c l i c c r o s s b e d s i n t h e llavajo Sandstone along S t a t e Highway 9, 2.4 km (1.5 m i l e s ) west of easte n t r a n c e g a t e h o u s e , Zion National Park. Note p a r a l l e l i s m of c y c l e c o n t a c t s a n d l a m i n a t i o n w i t h i n c y c l e s and a b s e n c e o f l a t e r a l p i n c h o u t s .

445

Fig. 13. Exposure o f a coinpound s e t o f c y c l i c c r o s s b e d s i n t h e Lamb P o i n t Tongue o f t h e Navajo Sandstone along U.S. llighway 89, 7.9 kill (4.9 m i l e s ) north of Kanab, Utah. Note g e n e r a l p a r a l l e l i s m of c y c l e c o n t a c t s ( t h e iriost conspicuous r i b s ) and l a m i n a t i o n w i t h i n c y c l e s a s seen i n t h i s view, which n e a r l y p a r a l l e l s s t r i k e of c r o s s b e d d i n g .

The c o n c o r d a n t c y c l i c c r o s s b e d s c o u l d not have been formed by superimposed dunes t h a t e x i s t e d on t h e l e e s l o p e of t h e main dune, f o r t h e n coiiqiound c r o s s bedding would have been formed. produced by t h e a v a l a n c h i n g o f

Moreover, t h e s e c y c l e s c o u l d not have been sand,

previously contdined

iri superii:iposed

dunes, down t h e s l i p f a c e o f t h e main dune, because t h e c y c l e s were d e p o s i t e d o n gently d i p p i n g p a r t s o f t h e l e e s l o p e by p r o c e s s e s o t h e r t h a n a v a l a n c h i n g . Superimposed dunes c o u l d have formed t h e c o n c o r d a n t c y c l i c c r o s s b e d s only i f the a r r i v a l o f superimposed dunes a t t h e c r e s t o f the main dune c o u l d have a l t e r e d wind p a t t e r n s f a r down t h e l e e s l o p e o f t h e main dune.

Such a l t e r -

a t i o n s would b e c o n c e i v a b l e o n l y i f t h e s i z e o f a superimposed dune were a l a r g e f r a c t i o n o f t h a t o f t h e main dune.

However, t h e dune advance per c y c l e

i s such a small f r a c t i o n o f t h e s e t t h i c k n e s s (and an even s m a l l e r f r a c t i o n o f the o r i g i n a l dune h e i g h t ) t h a t t h e superimposed dunes, i f p r e s e n t , must have been v e r y small

i n comparison with t h e main dune.

A superimposed-bedform

o r i g i n , t h e r e f o r e , seems much l e s s l i k e l y t h a n a f l u c t u a t i n g - f l o w o r i g i n f o r the concordant c y c l i c c r o s s b e d s . Deciding between a superimposed-bedform and a f l u c t u a t i n g - f l o w o r i g i n f o r the compound c y c l i c c r o s s b e d s i s even more d i f f i c u l t t h a n f o r t h e c o n c o r d a n t cyclic crossbeds.

A1 though the amounts o f e r o s i o n r e p r e s e n t e d by t h e bounding

surfaces of t h e c y c l e s a r e c e r t a i n l y not t o o l a r g e t o b e e x p l a i n e d by a l t e r nating

dune b u i l d o u t and b a c k c u t t i n g caused by f l o w r e v e r s a l s ,

the erosion

could be e q u a l l y well e x p l a i n e d by t h e m i g r a t i o n o f superimposed bedforms down the l e e slope.

446 The d e p o s i t i o n a l wedges a t t h e b a s e o f a t y p i c a l compound s e t o f crossbeds o f f e r some e v i d e n c e f o r d e c i d i n g b e t w e e n a superimposed-bedform and a fluctuating flow origin.

Such wedges a r e e a s i l y e x p l a i n e d b y f l u c t u a t i n g flow; the

wedges w o u l d h a v e b e e n d e p o s i t e d a t t h e b a s e o f t h e dune s l o p e d u r i n g periods o f r e v e r s e d f l o w , c o i n c i d e n t w i t h e r o s i o n o f t h e u p p e r l e e slope.

To explain

t h e s e wedges b y t h e m i g r a t i o n o f s u p e r i m p o s e d dunes down t h e l e e slope of the m a i n dune,

o n e m u s t suppose t h a t t h e l e e - e d d y c i r c u l a t i o n a t t h e base of the

l e e s l o p e o f t h e m a i n dune v a r i e d i n s t r e n g t h a s t h e superimposed dunesmig r a t e d downslope.

Such a n i n t e r p r e t a t i o n w o u l d b e r e a s o n a b l e o n l y i f t h e size

o f a s u p e r i m p o s e d dune were a l a r g e f r a c t i o n o f t h a t o f t h e m a i n dune. P e r h a p s t h e most c o n c l u s i v e e v i d e n c e f o r a f l u c t u a t i n g - f l o w o r i g i n o f both t y p e s o f c y c l i c c r o s s b e d d i n g i s f e a t u r e s i n d i c a t i v e o f r e v e r s a l s i n t h e alongs l o p e component o f t h e w i n d d u r i n g each c y c l e . National

Park,

sharp-cornered the

zigzag

dune r e - e n t r a n t

junction

f r o m two s i d e s .

a r e concordant

and a s

c r o s s b e d s exposed e l s e w h e r e i n t h e s e t . n e a r Kanab,

Utah,

The l a y e r s o n e i t h e r side of t h i c k a s t h e concordant c y c l i c

I n a s e t o f compound c y c l i c crossbeds

i s an exposure o f zigzagging erosional

s i m i l a r t o t h o s e shown i n f i g u r e 3A. bedding

I n one s e t o f crossbeds i n Z i o n

z i g z a g g i n g l a y e r s s i m i l a r t o t h o s e shown i n f i g u r e 3C f i l l a

i n t h i s and n e a r b y o u t c r o p s s u g g e s t t h a t t h e

d o w n w i n d - p o i n t e d s a l i e n t o f t h e dune,

s u r f a c e s ( f i g . 14)

The d i r e c t i o n a l p r o p e r t i e s o f t h e crosszigzags

formed on a

where t h e t w o s i d e s o f t h e s a l i e n t faced

F i g . 14. C r o s s b e d d i n g i n z i g z a g g i n g s e t s f o r m e d o n t w o s i d e s o f a downwindp o i n t e d dune s a l i e n t i n Lamb P o i n t Tongue o f N a v a j o Sandstone a l o n g west side o f Kanab Creek, 8.2 km (5.1 m i l e s ) n o r t h o f Kanab, Utah. F i v e zigzag apexes are visible.

447

OTHER LOCAL CROSSBEDS

CROSSBEDS DEPOSITED ON SALIENT

BEDS DEPOSITED ON EASTWARDDIPPING SURFACE OF SALIENT

SOUTHWARD-DIPPING SURFACE OF SALIENT

Fig. 15. R e l a t i o n o f z i g z a g g i n g crossbeds ( a t same l o c a l i t y a s i n f i g . 14) t o l a r g e - s c a l e c r o s s b e d d i n g elsewhere i n t h e v i c i n i t y . The t w o modes o f zigzagg i n g crossbeds a r e i n t e r p r e t e d as d e f i n i n g f a c i n g d i r e c t i o n s o f t w o s i d e s o f a downwind-pointed s p u r , whereas t h e mean o f o t h e r crossbedding i s i n t e r p r e t e d a s a p p r o x i m a t i n g average f a c i n g d i r e c t i o n o f dunes i n t h e v i c i n i t y . about 45" t o e i t h e r s i d e o f t h e average f a c i n g d i r e c t i o n o f t h e dune ( f i g . 15).

The v a r i a t i o n s i n wind d i r e c t i o n must have been g r e a t enough t o cause

d e p o s i t i o n and e r o s i o n t o a l t e r n a t e f r o m one s i d e o f t h i s s a l i e n t t o t h e o t h e r . Even w i t h o u t t h e e v i d e n c e o f a f l u c t u a t i n g a l o n g s l o p e component o f t h e wind,

we g r e a t l y f a v o r a f l u c t u a t i n g - f l o w o r i g i n f o r t h e Navajo c y c l i c c r o s s -

bedding d e s c r i b e d i n t h i s paper.

Too many i m p r o b a b l e occurrences a r e r e q u i r e d

f o r a superimposed-bedform i n t e r p r e t a t i o n .

The superimposed dunes would have

had t o b e p a r a l l e l o r v e r y n e a r l y p a r a l l e l t o t h e m a i n dune, would have had t o be so l o n g c r e s t e d t h a t l a t e r a l p i n c h o u t s o f t h e i r d e p o s i t s would b e r a r e l y i f e v e r seen, and would have had t o have a l a r g e i n f l u e n c e o n wind c o n d i t i o n s o v e r t h e e n t i r e l e e s l o p e o f t h e m a i n dune, even where t h e superimposed dunes were r e l a t i v e l y s m a l l and d i e d o u t soon a f t e r p a s s i n g t h e c r e s t o f t h e m a i n dune. Evidence f o r Annual P e r i o d i c i t y Methods o f A n a l y s i s .

Wind f l u c t u a t i o n s r a n g i n g i n p e r i o d f r o m a day t o a

y e a r c o u l d c o n c e i v a b l y g e n e r a t e c y c l i c crossbeds o f t h e s c a l e observed i n t h e Navajo Sandstone (Stokes, 1964).

F l u c t u a t i o n s o f even l a r g e r p e r i o d a r e con-

c e i v a b l e , o f course, b u t a r e n o t c o n s i d e r e d h e r e because a y e a r l y p e r i o d i c i t y i s l o n g enough t o reduce t h e r e q u i r e d wind speeds t o q u i t e r e a s o n a b l e l e v e l s . Besides d a i l y and y e a r l y a s t r o n o m i c a l l y c o n t r o l l e d c y c l e s ,

t h e most prominent

448 wind f l u c t u a t i o n s i n t h e r a n g e o f p e r i o d s c o n s i d e r e d h e r e a r e t h e i m p e r f e c t l y p e r i o d i c ones a s s o c i a t e d w i t h t h e passage o f m e t e o r o l o g i c f r o n t s , t y p i c a l l y a t i n t e r v a l s o f a few days t o a few weeks.

A c h o i c e between t h e s e wind c y c l e s

must b e made on t h e b a s i s o f ( 1 ) e v a l u a t i o n s o f t h e p r o b a b i l i t y t h a t t h e wind c y c l e s c o u l d b e d i s t i n c t and r e g u l a r enough t o c r e a t e t h e observed s e r i e s o f c y c l i c crossbeds, and ( 2 ) e v a l u a t i o n s o f t h e reasonableness o f t h e r a t e o f dune movement i m p l i e d b y t h e observed dune advance p e r c y c l e and b y a hypothesized frequency o f wind f l u c t u a t i o n s .

The reasonableness o f an imp1 i e d m i g r a t i o n

r a t e c a n b e e v a l u a t e d b y comparisons w i t h observed m i g r a t i o n r a t e s o f modern dunes and b y c a l c u l a t i o n s o f t h e wind v e l o c i t y r e q u i r e d t o cause a g i v e n m i g r a t i o n rate. D i s t i n c t n e s s and R e g u l a r i t y o f Wind Cycles. monly w e l l developed i n c o a s t a l c l u d i n g d e s e r t s ( D u b i e f , 1952).

D a i l y ’ w i n d c y c l e s a r e com-

r e g i o n s b u t a l s o occur i n o t h e r areas, i n The s t r e n g t h o f d a i l y wind c y c l e s a t nmerous

l o c a l i t i e s i n t h e U n i t e d S t a t e s c a n b e e a s i l y e v a l u a t e d from t h e Local C l i m a t o l o g i c a l Data sheets p u b l i s h e d m o n t h l y b y t h e Environmental Data S e r v i c e o f t h e U.S.

Department o f Commerce.

cycles a r e well

These d a t a sheets show t h a t d a i l y sea-breeze

developed i n many c o a s t a l

areas,

e s p e c i a l l y during

smmer

months, and a r e c h a r a c t e r i z e d b y l a r g e v a r i a t i o n s i n wind v e l o c i t y t h r o u g h t h e day.

On i n l a n d a r i d p l a i n s ( s u c h a s Las Vegas, Nevada, and Yuma, A r i z o n a ) , i n

c o n t r a s t , t e n d e n c i e s t o w a r d d a i l y wind c y c l e s a r e n o t so w e l l developed. i n t h e c o a s t a l dunes o f C a l i f o r n i a ,

Oregon,

and s o u t h e r n Texas,

Even

where d a i l y

wind c y c l e s d u r i n g t h e summer a r e as w e l l developed a s anywhere i n t h e United States,

m e t e o r o l o g i c f l u c t u a t i o n s w i t h a p e r i o d i c i t y o f about a week prevent

t h e development o f l o n g s e r i e s o f u n i f o r m d a i l y l a y e r s .

The e x p e c t a b l e ir-

r e g u l a r i t y o f d a i l y wind c y c l e s d e t r a c t s f r o m t h e i r p l a u s i b i l i t y a s a n exp l a n a t i o n f o r t h e Navajo c y c l i c c r o s s b e d d i n g , even though t h e Navajo d e s e r t may have been a d j a c e n t t o a c o a s t ( S t a n l e y and o t h e r s , 1971). Wind f l u c t u a t i o n s h a v i n g an approximate p e r i o d o f a few days t o a few weeks a r e prominent i n most temperate and s u b t r o p i c a l c l i m a t e s .

Considered as

a p o s s i b l e cause o f c y c l i c crossbedding, however, such f l u c t u a t i o n s s u f f e r frm a g r e a t r a n g e i n t h e d u r a t i o n and s t r e n g t h o f i n d i v i d u a l c y c l e s .

Moreover, i n

most p a r t s o f t h e w o r l d such f l u c t u a t i o n s v a r y g r e a t l y from season t o season. Such wind c y c l e s a r e t h e r e f o r e u n l i k e l y t o g e n e r a t e l o n g s e r i e s o f uniform c y c l i c crossbeds. Y e a r l y wind c y c l e s i n many p a r t s o f t h e w o r l d a r e w e l l developed and e x h i b i t a s m a l l e r range i n r e s u l t a n t s a n d - t r a n s p o r t i n g power t h a n do c y c l e s o f shorter period.

O f t h e c y c l e s c o n s i d e r e d here, y e a r l y ones a r e t h e most l i k e l y

t o generate long s e r i e s o f cycles s i m i l a r i n thickness.

44 9 I m p l i e d Rates o f Dune Movement.

I n e v a l u a t i n g t h e reasonableness o f a n

i n t e r p r e t e d r a t e o f dune movement, one must t a k e i n t o account t h e dune s i z e , form,

and o r i e n t a t i o n r e l a t i v e t o t h e wind, because t h e r a t e o f movement i s

c o n t r o l l e d b y t h e s e f a c t o r s as w e l l 1982). dune.

a s b y wind s t r e s s

(Rubin and Hunter,

Given t h e same wind s t r e s s , a l o w dune m i g r a t e s f a s t e r t h a n a h i g h Even t a k i n g i n t o account t h e i n c r e a s e d wind s t r e s s over h i g h e r dunes

caused by t h e g r e a t e r crowding o f f l o w l i n e s , measurements show t h a t l o w dunes m i g r a t e f a s t e r t h a n h i g h ones, a t l e a s t i f t h e dunes a r e o f barchan t y p e (Long and Sharp,

1964;

Hastenrath,

1967).

Dune form a f f e c t s t h e r a t e o f dune m i -

g r a t i o n b y i t s e f f e c t on s a n d - t r a p p i n g e f f i c i e n c y .

A dune whose l e e s l o p e

t r a p s a l l t h e sand t h a t passes t h e dune c r e s t w i l l m i g r a t e f a s t e r t h a n a dune o f low sand-trapping e f f i c i e n c y .

Dune o r i e n t a t i o n r e l a t i v e t o t h e wind d i -

r e c t i o n has a d i r e c t e f f e c t o n dune m i g r a t i o n as w e l l as a n i n d i r e c t one due t o i t s e f f e c t on s a n d - t r a p p i n g e f f i c i e n c y .

Other f a c t o r s r e m a i n i n g c o n s t a n t , a

t r a n s v e r s e dune w i l l m i g r a t e f a s t e r t h a n one t h a t t r e n d s o b l i q u e l y t o t h e wind, and

i t s r a t e o f migration w i l l

1 ong it ud in a l

.

I n considering

decrease

as t h e dune becomes more n e a r l y

t h e m i g r a t i o n r a t e s o f modern dunes,

we

restrict

a t t e n t i o n t o dunes l a r g e enough t o b e comparable t o t h e Navajo dunes.

our The

average h e i g h t o f t h e Navajo dunes must have been a t l e a s t a s g r e a t a s t h e average t h i c k n e s s o f t h e p r e s e r v e d s e t s o f crossbeds, about 10 m.

More l i k e l y ,

t h e Navajo dunes averaged a t l e a s t 33 m i n h e i g h t ( R u b i n and Hunter, 1982). The m i g r a t i o n r a t e s o f l a r g e modern dunes have r a r e l y been measured. Among t h e measured dunes most comparable i n s i z e t o t h e Navajo dunes a r e t h e Algodones dunes o f s o u t h e r n C a l i f o r n i a . and p r o b a b l y t r a n s v e r s e , 1979).

These dunes, which a r e about 60 m h i g h

m i g r a t e a t a r a t e o f about 0.4

m per y e a r

(Sharp,

The r e l a t i o n s between dune h e i g h t and m i g r a t i o n r a t e f o r t h e barchans

o f P e r u ( H a s t e n r a t h , 1967) and s o u t h e a s t e r n C a l i f o r n i a (Long and Sharp, 1964), i f e x t r a p o l a t e d a s power f u n c t i o n s t o dune h e i g h t s o f 30 m, i n d i c a t e m i g r a t i o n

r a t e s o f 1 2 a n d 10 m/yr, r e s p e c t i v e l y . movements o f 0.5

Although we have m o n i t o r e d d a i l y dune

m on Oregon c o a s t a l t r a n s v e r s e dunes 5 m h i g h under t h e i n -

f l u e n c e o f sunmer sea-breeze winds t h a t reached a maximum speed o f about 15 m/s (34 m i l e s / h o u r )

i n t h e afternoon,

than one day a t a time. interpretation f o r crossbeds

such r a t e s a r e seldom m a i n t a i n e d f o r more

Modern dune m i g r a t i o n r a t e s , t h e r e f o r e , f a v o r a y e a r l y

t h e wind c y c l e s t h a t

( a v e r a g e dune advance,

0.3

formed b o t h t h e c o n c o r d a n t c y c l i c

m per cycle)

and t h e compound c y c l i c

crossbeds (average dune advance, 1.5 m p e r c y c l e ) .

In c a l c u l a t i n g a wind speed f r o m t h e amount o f dune movement r e p r e s e n t e d by a c y c l i c crossbed, we assume a r e l a t i o n between wind speed and sand t r a n s p o r t o f t h e t y p e f o r m u l a t e d by Ehgnold (1941),

and i n i t i a l l y we assume a

450 v a l u e s used b y Hunter and o t h e r s ( i n p r e s s ) .

We emphasize t h a t o t h e r published

e q u a t i o n s and v a l u e s o f c o n s t a n t s c a n r e s u l t i n sand t r a n s p o r t r a t e s d i f f e r i n g b y a f a c t o r o f t w o o r more f r o m t h o s e c a l c u l a t e d here.

From t h e r e l a t i o n

between wind speed and t r a n s p o r t r a t e , graphs showing t h e m i g r a t i o n r a t e s of dunes o f g i v e n h e i g h t , structed (fig. The

orientation,

and s a n d - t r a p p i n g e f f i c i e n c y c a n b e con-

16).

p l o t s r e l a t i n g dune m i g r a t i o n r a t e ,

wind

speed,

and dune height

i n d i c a t e t h a t a 10-m-high t r a n s v e r s e dune o f p e r f e c t s a n d - t r a p p i n g e f f i c i e n c y m i g r a t e s a t a r a t e o f 0.3

m/day when t h e wind speed a t a h e i g h t o f 10 m over

t h e dune c r e s t i s 14 m/s (32 m i / h r ) and a t a r a t e o f 1.5 speed i s 23 m/s (52 m i / h r ) .

m/day when t h e wind

Such wind speeds a r e q u i t e h i g h i n comparison w i t h

l o n g - t e r m mean wind speeds measured a t t h e p r e s e n t t i m e anywhere i n t h e world. F o r example, t a b l e s i n t h e s e r i e s W o r l d Survey o f C l i m a t o l o g y ( G e n t i l l i , 1971; G r i f f i t h s , 1972; Lydolph, 1977; Schwerdtfeger, 1976) i n d i c a t e t h a t y e a r l y mean w i n d speeds i n h o t t o temperate d e s e r t c l i m a t e s r a r e l y exceed 6 m/s, even along desert

coasts.

The w i n d i e s t nonpolar a r i d o r s e m i a r i d r e g i o n r e c o r d e d i n t h i s

mls

A

10

20

30

40

so

60 m m r

Wind speed 10m above dune crest

Fig. 16. R e l a t i o n o f c a l c u l a t e d dune m i g r a t i o n r a t e t o dune h e i g h t and wind Sol i d curves, speed f o r t r a n s v e r s e dunes o f p e r f e c t s a n d - t r a p p i n g e f f i c i e n c y . m i g r a t i o n r a t e i n m e t e r s p e r year; dashed c u r v e s , m i g r a t i o n r a t e i n m e t e r s per day.

451 s e r i e s i s Patagonia, where t h e y e a r l y mean wind speed ( a t Comodoro Rivadavia, A r g e n t i n a ) i s 9.0

m/s.

The inadequacy o f modern d e s e r t winds t o cause enough

d a i l y sand t r a n s p o r t t o account f o r c y c l e s o f Navajo s c a l e i s s i m i l a r l y i n d i c a t e d b y t h e c a l c u l a t e d sand t r a n s p o r t r a t e s o f F r y b e r g e r (1979). A d a i l y p e r i o d i c i t y i s even more s t r o n g l y d i s f a v o r e d when one c o n s i d e r s

t h a t ( 1 ) t h e dunes may have been c o n s i d e r a b l y h i g h e r t h a n 10 m,

(2) t h e dunes

may have been o b l i q u e t o t h e main winds ( R u b i n and Hunter, i n p r e s s ) , ( 3 ) t h e wind c y c l e was undoubtedly c h a r a c t e r i z e d b y d i r e c t i o n a l as w e l l a s speed v a r i a b i l i t y , and (4) t h e l e n g t h o f t h e day was s l i g h t l y s h o r t e r d u r i n g T r i a s s i c and J u r a s s i c

time than a t present

(Pannella,

1972;

Roserberg and Runcorn:

A l l t h e s e c o n s i d e r a t i o n s suggest t h a t g r e a t e r wind speeds t h a n were

1975).

c a l c u l a t e d would b e r e q u i r e d t o g i v e t h e & s e r v e d amount o f sand t r a n s p o r t p e r cycle.

Such h i g h wind speeds seem e s p e c i a l l y u n l i k e l y when one c o n s i d e r s t h a t

t h e T r i a s s i c and J u r a s s i c were t i m e s o f l o w p o l e - t o - e q u a t o r

temperature gra-

d i e n t and would n o t b e expected t o have had a s t r o n g atmospheric c i r c u l a t i o n (Donn and Shaw, 1977).

A y e a r l y i n t e r p r e t a t i o n f o r t h e Navajo c y c l i c c r o s s -

bedding i s t h e r e f o r e s t r o n g l y p r e f e r r e d .

I n the yearly interpretation,

the

r e q u i r e d wind speeds a r e q u i t e modest, and t h e sand may n o t have moved d u r i n g many days o f a y e a r ( f i g . 16). CONCL US I O N S

A s e t o f crossbeds may b e composed o f c y c l i c b u n d l e s d e f i n e d by repetitions o f

structural,

textural,

o r mineralogic

features.

The bounding

s u r f a c e s o f a n i n d i v i d u a l c y c l e may b e c o n c o r d a n t w i t h t h e crossbedding w i t h i n t h e c y c l e o r may b e e r o s i o n a l and d i s c o r d a n t , i n which case t h e crossbedding i s compound as w e l l as c y c l i c .

B o t h c o n c o r d a n t and compound c y c l i c crossbedding

can b e produced b y e i t h e r o f t w o processes: eter,

especially flow direction,

f l u c t u a t i o n s i n some f l o w param-

o r t h e m i g r a t i o n o f bedforms o v e r t h e l e e

s l o p e o f a l a r g e r bedform on which t h e y a r e superimposed.

Distinguishing these

t w o p o s s i b l e causes o f c y c l i c c r o s s b e d d i n g i s b y no means simple.

Fluctuating-

f l o w c y c l e s c a n b e s t b e r e c o g n i z e d b y s t r u c t u r a l f e a t u r e s a r i s i n g from r e v e r sal s

i n t h e a1 ongsl ope o r a c r o s s - s l ope component o f flow.

Superimposed-

bedforms c y c l e s c a n b e s t b e r e c o g n i z e d b y f e a t u r e s p r o v i n g t h a t p a r t o f one c y c l e formed contemporaneously w i t h p a r t o f a n o t h e r c y c l e ( t h a t i s , b y e v i d e n c e t h a t t h e c y c l e s a r e c l imbing t r a n s l a t e n t s t r a t a ) present.

I n t h e absence o f such f e a t u r e s ,

,but

such f e a t u r e s a r e seldom

superimposed-bedform

cycles can

comnonly b e r e c o g n i z e d b y f e a t u r e s t h a t a r i s e f r o m t h e superimposed bedforms b e i n g s h o r t e r i n c r e s t l e n g t h t h a n t h e l a r g e r bedform, t r e n d i n g a t a n a n g l e t o t h e l a r g e r bedform through a t l e a s t p a r t o f t h e i r extent, o r p e r s i s t i n g w h i l e m i g r a t i n g f o r c o n s i d e r a b l e d i s t a n c e s o v e r t h e l e e s l o p e o f t h e l a r g e r bedform.

452 By using such c r i t e r i a , we found what we i n t e r p r e t t o be c y c l e s of b o t h fluctuating-flow a n d superimposed-bedform o r i g i n in t h e Navajo Sandstone.

The

c y c l e s of fluctuating-flow o r i g i n include b o t h t h e concordant types f i r s t described by Stokes (1964) a n d compound crossbedding. The superimposed-bedform c y c l e s a r e described in a n accompanying paper (Rubin and Hunter, t h i s volume). From considerations of t h e d i s t i n c t n e s s , r e g u l a r i t y in s t r u c t u r e and thickness, a n d s c a l e of t h e cycles, we strongly support Stokes' (1964) interpretation of t h e c y c l i c i t y a s annual. This i n t e r p r e t a t i o n should eventually prove valuable f o r making more sophisticated i n t e r p r e t a t i o n s of t h e dynamics of the Navajo dunes, b u t t h e problems of why c y c l i c i t y i s well developed only in some s e t s o f crossbeds a n d of why t h e c y c l i c i t y t a k e s a t l e a s t two d i s t i n c t forms need t o be solved before t h e f u l l potential of Stokes' (1964) insight i s realized. REFERENCES Allen, J.R.L., 1968. Current r i p p l e s , t h e i r r e l a t i o n t o p a t t e r n s of water and sediment motion. North-Holland Publishing Co., Amsterdam, 433 pp. Allen, J.R.L., 1973. Features of c r o s s - s t r a t i f i e d u n i t s due t o random and o t h e r changes in bed forms. Sedimentology, 20:189-202. Allen, J.R.L., 1980a. Sand waves: a model of o r i g i n a n d internal structure. Sedimentary Geology, 26:281-328. Allen, J.R.L., 1980b. Sand-wave immobility a n d t h e internal master bedding of sand-wave deposits. Geol. Mag., 117:437-446. 1941. The physics of blown sand a n d d e s e r t dunes. Methuen, Bagnold, R.A., London, 265 pp. Banks, N.L., 1973. The o r i g i n and s i g n i f i c a n c e of some downcurrent-dipping c r o s s - s t r a t i f i e d s e t s . Jour. Sed. Petrology, 43:423-427. Beerbower, J.R., 1964. Cyclothems and c y c l i c depositional mechanisms in In: O.F. Merriam, ( E d i t o r ) Symposim on c y c l i c a l l u v i a l plain sediments. sedimentation. Kansas S t a t e Geol. Survey Bull., 169:31-42. Boersma, J.R., 1969. Internal s t r u c t u r e of some t i d a l mega-ripples on a shoal i n t h e Westerschelde e s t u a r y , t h e Netherlands: report of a preliminary investigation. Geol. en Mijnbouw, 48:409-414. Brookfield, M.E., 1979. Anatomy of a Lower Permian aeolian sandstone complex, southern Scotland. Scottish J. Geol., 15:81-96. Clifton, H.E., 1979. Tidal channel deposits of middle Eocene age, Torrey Pines I n : P.L. Abbott, ( E d i t o r ) , Eocene depositional S t a t e Reserve, California. systems, San Diego, California. P a c i f i c Section of SOC. Econ. Paleont o l o g i s t s Mineralogists, Los Angeles, California, pp. 35-42. Collinson, J.O., 1970. Bedforms of t h e Tana River, Norway. Geog. Annaler, 52Az31-56. Dalrymple, R.W., Knight, R.J., a n d Middleton, G.V., 1975. I n t e r t i d a l sand bars in Cobequid Bay (Bay of Fundy). In: L.E. Cronin, ( E d i t o r ) , Estuarine research. Geology and engineering, 2:293-307. Donn, W.L., and Shaw, D.M., 1977. Model of climate evolution based on cont i n e n t a l d r i f t and polar wandering. Geol. SOC. America Bull., 88:390-396 Dubief, J., 1952. Le vent e t l e deplacement d u sable au Sahara. Inst. Rech. Sahariennes Trav., 8:123-162. Fryberger, S.G., 1979. Dune forms a n d wind regime. I n : E.D. McKee ( E d i t o r ) , A study of global sand seas: U.S. Geol. Survey Prof. Paper, 1052:137-169. Fryberger, S. G., and Schenk,. Christopher, 1981. Wind sedimentation tunnel experiments on t h e o r i g i n s of aeolian s t r a t a : Sedimentology, 28:805-821.

453 C l i m a t e s o f A u s t r a l i a and New Zealand. World G e n t i l l i , J., ( E d i t o r ) , 1971. s u r v e y o f c l i m a t o l o g y , 13, 405 pp. G r i f f i t h s , J. F., ( E d i t o r ) , 1972. Climates o f Africa. World survey o f c l i m a t o l o g y , 10, 604 pp. Southard, J. B., Spearing, D. R., and Walker, R. G., 1975, Harms, J. C., Depositional environments a s i n t e r p r e t e d from primary sedimentary SOC. Econ. P a l e o n t o l o g i s t s s t r u c t u r e s and s t r a t i f i c a t i o n sequences: M i n e r a l o g i s t s S h o r t Course No. 2, L e c t u r e Notes, 1 6 1 pp. The barchans o f t h e Arequipa r e g i o n , s o u t h e r n Peru. H a s t e n r a t h , S. L., 1967. Z. Gemorph., 11:300-331. 1977a. B a s i c t y p e s o f s t r a t i f i c a t i o n i n small e o l i a n dunes. Hunter, R. E., Sediment01 ogy, v. 24, 362-387. Terminology o f c r o s s - s t r a t i f i e d sedimentary l a y e r s and Hunter, R. E., 1977b. c l i m b i n g - r i p p l e s t r u c t u r e s : Jour. Sed. P e t r o l o g y , 47:697-706. Hunter, R. E., 1981. S t r a t i f i c a t i o n s t y 1 es i n e o l i a n sandstones: some Pennsylvanian t o J u r a s s i c examples f r o m t h e western i n t e r i o r U.S.A. In: F. G. E t h r i d g e and R. M. F l o r e s , ( E d i t o r s ) , Recent and a n c i e n t n o m a r i n e depositional env ir o m e n t s : models for exploration SOC. Econ. P a l e o n t o l o g i s t s M i n e r a l o g i s t s Spec. Publ. No. 31, pp. 315-329. Hunter, R. E., Richmond, B. M., and Alpha, T. R., i n press. Storm-controlled o b l i q u e dunes o f t h e Oregon coast. Geol. SOC. America B u l l . Jackson, R. G., 11, 1976. L a r g e s c a l e r i p p l e s o f t h e l o w e r Wabash R i v e r . Sediment01 ogy , 23: 593-623. 1977. E f f e c t s o f v a r y i n g d i s c h a r g e regimes on bed form Jones, C. M., sedimentary s t r u c t u r e s i n modern r i v e r s . Geology, 5:567-570. Jones, C. M., 1979. T a b u l a r cross-bedding i n Upper C a r b o n i f e r o u s f l u v i a l channel sediments i n t h e s o u t h e r n Pennines, England. Sediment. Geol., 2 4: 85-1 04. Erosion surfaces w i t h i n g i a n t f l u v i a l Jones, C. M., and McCabe, P. J., 1980. cross-beds o f t h e c a r b o n i f e r o u s i n n o r t h e r n England. Jour. Sed. P e t r o l o g y , 50: 613-620. K l e i n , G. D., 1970. D e p o s i t i o n a l and d i s p e r s a l dynamics o f i n t e r t i d a l sand bars. Jour. Sed. P e t r o l o g y , 40:1095-1127. H., Jr., 1981. D i s t i n c t i o n s and uses o f Kocurek, Gary, and D o t t , R. s t r a t i f i c a t i o n t y p e s i n t h e i n t e r p r e t a t i o n o f e o l i a n sand. Jour. Sed. P e t r o l o g y , 51: 579-595. Kohsiek, L. ti. M., and T e r w i n d t , J. H. J., 1981. Characteristics o f foreset and t o p s e t b e d d i n g i n m e g a r i p p l e s r e l a t e d t o hydrodynamic c o n d i t i o n s on a n i n t e r t i d a l shoal: I n : S.-0. Nio, R. T. E. Schuttenheim, and T. C. E. v a n Weering, ( E d i t o r s ) , Holocene m a r i n e s e d i m e n t a t i o n i n t h e N o r t h Sea b a s i n . I n t e r n a t l . Assoc. S e d i m e n t o l o g i s t s Spec. Publ. 5, pp. 27-37. Barchan-dune movement i n I m p e r i a l V a l l e y , Long, J. T., and Sharp, R. P., 1964. C a l i f o r n i a . Geol. SOC. h e r i c a B u l l . , 75:149-156. Lydolph, P. E., 1977. C l i m a t e s o f t h e S o v i e t Union. World survey o f c l i m a t o l o g y , 7, 443 pp. 1977, F o r m a t i o n o f r e a c t i v a t i o n s u r f a c e s McCabe, P. J., and Jones, C. M., w i t h i n superimposed d e l t a s and bedforms. Jour. Sed. P e t r o l o g y , 47:707-715. Pannella, G i o r g i o , 1972. P a l e o n t o l o g i c a l evidence o n t h e E a r t h ' s r o t a t i o n a l h i s t o r y s i n c e E a r l y Precambrian. Astrophys. Space Sci., 16:212-237. P e t t i j o h n , F. J., and P o t t e r , P. E., 1964. A t l a s and g l o s s a r y o f p r i m a r y sedimentary s t r u c t u r e s . S p r i n g e r - V e r l a g , New York, 370 pp. Rosenberg, G. D., and Runcorn, S. K., ( E d i t o r s ) , 1975. Growth r h y t h s and t h e h i s t o r y o f t h e E a r t h ' s r o t a t i o n . W i l e y - I n t e r s c i e n c e , New York, 559 pp. Bedform c l i m b i n g i n t h e o r y and nature. Rubin, D. M. and Hunter, R. E., 1982. Sedimentology, 29:121-138. Rubin, D. M. and Hunter, R. E., t h i s volme. R e c o n s t r u c t i n g bedform assemblages f r o m compound crossbedding. Rubin, D. M., and Hunter, R. E., i n press. Why d e p o s i t s o f l o n g i t u d i n a l dunes a r e r a r e l y r e c o g n i z e d i n t h e g e o l o g i c record. ,Nature. S i n g l e and superimposed bedforms: a Rubin, D. M. and McCulloch, 0. S., 1980.

4 54 s y n t h e s i s o f San F r a n c i s c o Bay and f l u m e o b s e r v a t i o n s . Sedimentary Geology, 26:207-231. Schwerdtfeger, Werner, 1976. C1 imates o f C e n t r a l and South America. World survey o f c l i m a t o l o g y , 12, 532 pp. Sharp, R. P., 1979. I n t r a d u n e f l a t s o f t h e Algodones c h a i n , I m p e r i a l V a l l e y , California. Geol. SOC. America Bull., pt. I, 90:908-916. Smith, N. D., 1972. Some s e d i m e n t o l o g i c a l a s p e c t s o f p l a n a r crosss t r a t i f i c a t i o n i n a sandy b r a i d e d r i v e r . Jour. Sed. P e t r o l o g y , 42:624-634. S t a n l e y , K. O., Jordan, W. M., and D o t t , R. H., 1971. New h y p o t h e s i s o f Early J u r a s s i c paleogeography and sediment d i s p e r s a l f o r western U n i t e d States. Am. Assoc. P e t r o l e u n G e o l o g i s t s B u l l . , 55:lO-19. Stokes, W. L., 1964. E o l i a n v a r v i n g i n t h e Colorado Plateau. Jour. Sed. P e t r o l ogy , 34: 429-432. T e r w i n d t , J. H. J., 1981. O r i g i n and sequences o f sedimentary s t r u c t u r e s i n i n s h o r e m e s o t i d a l d e p o s i t s o f t h e N o r t h Sea. In: S.-D. Nio, R. T. E. Schuttenheim, and T. C. E. van Weering, ( E d i t o r s ) , Holocene marine s e d i m e n t a t i o n i n t h e N o r t h Sea b a s i n . I n t e r n a t l . Assoc. Sedimentolcgists Spec. Publ. 5, pp. 4-26. V i s s e r , M. J., 1980. Neap-spring c y c l e s r e f l e c t e d i n Holocene s u b t i d a l larges c a l e bedform d e p o s i t s : a p r e l i m i n a r y note: Geology, 8:543-546.

455

PERISLACIAL EOLIAN E V E N L Y LAMINATED SANDY DEPOSITS IN THE LATE PLEISTOCENE OF N'4 E U R O P E , A F A C I E S U N R E C O R D E D IN MODERN SEDIMENTOLOGICAL HANDBOOKS

G E R A R D H.J. R U E G G ( G e o l o g i c a l Survey of The N e t h e r l a n d s , Box 157,

2000 AD Haarlem, The N e t h e r l a n d s )

INTRODUCTION P u b l i c a t i o n s on sandy e o l i a n d e p o s i t s in t h e world mostly concern dune d e p o s i t s , t h e dominant s e d i m e n t a r y s t r u c t u r e of which i s c r o s s bedding ( t o low-angle bedd i n g ) . Regional sand s h e e t s - n o t a s i n t e r d u n e a r e a s - a r e a much l e s s f r e q u e n t f a c i e s and have been s t u d i e d much l e s s o f t e n . The paragraph t h a t Reineck & Singh (1980) devoted t o sand s h e e t s w i t h o u t pebble l a y e r s runs as f o l l o w s : "There a r e v a s t a r e a s of sand s h e e t s known which a r e devoid o f any kind of p e b b l e s . Such sand s h e e t s a r e made u p of w e l l - s o r t e d e o l i a n sand w i t h well developed h o r i z o n t a l l y laminated s a n d . A combination of r a p i d s e d i m e n t a t i o n , high wind v e l o c i t i e s and f a i r l y uniform g r a i n s i z e o f t h e sand cause d e p o s i t i o n of s h e e t sand with an abundantly developed e v e n l y laminated sand bedding ( c f . Bagnold 1954a; Glennie 1970) 'I.

Hunter (1977) mentions t h e p r o d u c t i o n of "planebed l a m i n a t i o n " by winds of 40 mi l e s p e r hour ( 1 8 m / s e c ) . F r y b e r g e r e t a l . ( 1 9 7 9 ) d e s c r i b e r e c e n t low-angle e o l i a n d e p o s i t s of a sand s h e e t , o c c u r r i n g as a t r a n s i t i o n a l f a c i e s between high-angle e o l i a n dunes and non-eolian d e p o s i t s , n e a r Great Sand Dunes National Monument, Colorado, U.S.A. They l i s t e l e v e n s e d i m e n t a r y f e a t u r e s . Most of t h e s e f e a t u r e s a r e a l s o p r e s e n t i n an e o l i a n s u b f a c i e s under c o n s i d e r a t i o n ( s e e below); g e n e r a l l y , however, f e a t u r e s r e l a t e d t o b i o l o g i c a l a c t i v i t y a r e very s c a r c e in t h i s subf a c i e s in t h e European d e p o s i t s .

The Parsonsburg Sand i n t h e C e n t r a l Delmarva P e n i n s u l a , Maryland and Delaware, d e s c r i b e d by Denny e t a l . ( 1 9 7 9 ) , presumably r e p r e s e n t s a f o s s i l analogue in North America. A r e c e n t d e p o s i t from Banks I s l a n d shown in F i g . 8 of a paper by P i s s a r t e t a l . (1977) i s h i g h l y comparable t o a Dutch p e r i g l a c i a l e o l i a n subf a c i e s ( s u b f a c i e s A , see below), although e v i d e n t l y the Banks I s l a n d d e p o s i t c o n t a i n s abundant o r g a n i c laminae ( f r a g m e n t s of willow s h r u b s ) in c o n t r a s t t o t h e bulk of t h e Dutch d e p o s i t s ; t h e d e p o s i t on Banks I s l a n d seems t o be r e l a t i v e l y s m a l l , and connected with r i v e r v a l l e y s . S i m i l a r ( s u b ) r e c e n t d e p o s i t s seem t o

o c c u r on Cumberland P e n i n s u l a , B a f f i n I s l a n d ( c f . Thompson 1954, c i t e d in Andrews e t a l . 1979 and i n Dyke e t a l . 1 9 8 2 ) .

456

legend L Cross-section Montlerlond

1 = France 2

i

Belgium

3

i

The Ncfherionds

L = West Germany IFRGl

5 = E a s t Germany I G D R I 6 = Polmnd 7

T

USSR

F i g . 1. Map o f The N e t h e r l a n d s s h o w i n g l o c a t i o n s r e f e r r e d t o i n Legends t o t h e f i g u r e s ( l e f t ) , and map o f NW E u r o p e ( r i g h t ) . REGIONAL SETTING Deposits o f t h i s k i n d d a t i n g from t h e Weichselian stage (equivalent t o the W i s c o n s i n s t a g e ) o c c u r i n n o r t h e r n F r a n c e (Somm6 e t a l . 198O), B e l g i u m ( e . 9 . M a r 6 c h a l & M a a r l e v e l d 1955; de P l o e y 1961; Zagwi j n & Paepe 1968; Vandenberghe & Gul l e n t o p s 1977; Vandenberghe 1981), West Germany (Dewers 1932, 1934; Ducker & M a a r l e v e l d 1957; Ruegg 1 9 8 1 ) , E a s t Germany ( N e u m e i s t e r 1971; see a l s o Eissmann 1 9 8 1 ) , P o l a n d ( e . g . K o z a r s k i e t a l . 1969; Nowaczyk 1976; K o z a r s k i 1980) and p r e s u m a b l y a l s o i n t h e USSR ( F i g . 1 ) . I n t h e e a s t e r n p a r t o f t h i s s a n d - b e l t ,

the

d e p o s i t s a r e p r e d o m i n a n t l y p r e s e n t as dune f i e l d s r e l a t e d t o r i v e r ( p r a d o l i n a ) v a l l e y s (Nowaczyk 1976; K o s t e r 1 9 8 2 ) .

I n The N e t h e r l a n d s , p e r i g l a c i a l d e p o s i t s a r e known f o r a t l e a s t s i x g l a c i a l stages (Fig. 2 ) . Important c o n t r i b u t i o n s concerning t h e occurrence, stratigraphy,

457 d a t i n g , and g e n e s i s o f p e r i g l a c i a l ! d e i c h s e l i a n d e p o s i t s have been p u b l i s h e d by, e . g . van d e r Hammen

(1951), van d e r Hammen h V a a r l e v e l d ( 1 9 5 2 ) , ) 4 a r @ c h a l &

M a a r l e v e l d (1955), D u c k e r 7, r l a a r l e v e l d ( 1 3 5 7 ) , Z a g w i j n ( 1 9 6 1 ) , van den T o o r n ( 1 9 6 7 ) , van d e r Hammen e t a l . ( 1 9 6 7 ) , Z a g w i j n & Paepe ( 1 9 6 8 ) , van d e r Hammen & Wijmstra (eds.) (1971), Bisschops (1973), Zagwijn (1974), Maarleveld (1976), K o l s t r u p & W i j m s t r a ( 1 9 7 7 ) , K o l s t r u p ( 1 9 8 0 ) , Vandenberghe ( 1 9 8 1 ) , Vandenberghe & Krook ( 1 9 8 1 ) and K o s t e r ( 1 9 8 2 ) . T h e r e i s a g e n e r a l agreement a b o u t t h e e o l i a n dominated o r i g i n o f t h e s e d e p o s i t s ,

a t p r e s e n t known as t h e Twente F o r m a t i o n .

The d e p o s i t s i n q u e s t i o n d r a p e an i r r e g u l a r t o p o g r a p h y ( g e n e r a l f i e l d name: c o v e r s a n d s ) ; on a r e g i o n a l s c a l e t h e y f o r m s a n d s h e e t s . The d e p o s i t s o c c u r o v e r l a r g e a r e a s , n o r m a l l y as a b l a n k e t w i t h a t h i c k n e s s o f some d e c i m e t e r s t o some m e t e r s ; t h e y a r e a b s e n t o n l y on e l e v a t e d a r e a s where t h e y may n o t have been dep o s i t e d , and i n c e r t a i n l o w r e g i o n s due t o s u b s e q u e n t e r o s i o n . The l a t e r a l and v e r t i c a l r e l a t i o n s as w e l l as t h e r e l a t i v e abundances o f t h e d i s t i n g u i s h e d sub-

lnfcrrcd mein rcmpcriturc In July

Chronostratigraphy

7

HOLOCENE

I

Pracborcd

I

Holocene

I

Eemian

I

Holsteinian

W

a 9 r

F i g . 2. L e f t : Q u a t e r n a r y c h r o n o s t r a t i y r a p h y o f The N e t h e r l a n d s s h o w i n g t h e i n t e r v a l s (hatched) i n which p e r i y l a c i a l d e p o s i t s were formed. R i g h t : c h r o n o s t r a t i g r a p h y and c l i m a t i c c u r v e o f t h e W e i c h s e l i a n ( a f t e r van S t a a l d u i n e n e t a l . 1 9 7 9 ) .

458

F i g . 3. C r o s s - s e c t i o n through t h e Y o n t f e r l a n d ice-pushed r i d g e in t h e eastern p a r t of t h e c e n t r a l Netherlands ( a f t e r van de Meene 1 9 7 7 ) . f a c i e s a r e s t r o n g l y dependent on t h e morphology of t h e s u b s o i l , which permits d i v i s i o n of The N e t h e r l a n d s i n t o a n o r t h e r n , a c e n t r a l , and a southern coversand a r e a . Moreover, t h e r e i s a l s o r o u g h l y a general i n c r e a s e of s i l t f r a c t i o n from N N W t o SSE; in t h e southernmost p a r t of t h e c o u n t r y only l o e s s d e p o s i t s occur.

I n a c o n s i d e r a b l e p a r t of t h e n o r t h e r n a r e a a g e n e r a l l y t h i n cover of periglacial Weichselian d e p o s i t s i s s e p a r a t e d from i d e n t i c a l d e p o s i t s of S a a l i a n age ( E i n d h o ven Formation) by a S a a l i a n b a s a l t i l l ; moreover, v a l l e y f i l l s o c c u r , in which s i g n i f i c a n t l o c a l - f l u v i a l d e p o s i t s may be p r e s e n t . I n t h e c e n t r a l p a r t , the subs t r a t u m was p r e v i o u s l y modelled by t h e S a a l i a n i n l a n d i c e and by Eemian interg l a c i a l l e v e l l i n g p r o c e s s e s ( m a i n l y niarine s e d i m e n t a t i o n in t h e former glacial b a s i n s ) ; h e r e , t h e Twente Formation, predominantly r e p r e s e n t e d by e o l i a n deposits, shows s t r o n g v a r i a t i o n of t h e t h i c k n e s s , r a n g i n g from some t e n s of meters in the

459

former g l a c i a l basins t o n i l l o c a l l y on the ice-pushed ridges ( F i g . 3 & Towards the German border, l o c a l - f l u v i a l s h e e t l i k e i n t e r c a l a t i o n s proportionally increase.

I n t h e southern p a r t of the coversand area, the existence of a horst and graben system i s strongly r e f l e c t e d in the extent and thickness of the p e r i g l a c i a l deposits (where the frequency o f the various subfacies i s largely u n k n o w n ) ; in the Central Graben thicknesses up t o 40 meters are known f o r the stacked p e r i g l a c i a l deposits of the l a s t t h r e e g l a c i a l stages ( E l s t e r i a n , Saalian, Weichselian: Nuenen Group) (Fig. 4 ) . More t o the west, f l a t areas with a cover of eolian dep o s i t s are i n t e r s e c t e d by valley f i l l s in which flowing-water sediments are present.

Fig. 4. Variations i n t h e thickness of the Nuenen Group, sheet 51 East ( a f t e r Bisschops 1973) and sheet 51 West ( J . P . Broertjes, Geological Survey, pers. comm.).

I n many areas of t h e country, these deposits are present a t the surface, and have been l i t t l e affected by Holocene processes. Often a so-called "coversand r e l i e f " i s present, with elevations mostly not exceeding 2 t o 3 , and in no case 5, meters in height (Koster 1982) (Fig. 5 ) . SEDIMENTARY (SUE) FACIES The following four subfacies can be distinguished within the p e r i g l a c i a l dep o s i t s of The Netherlands and adjacent areas: (1) eolian subfacies A , ( 2 ) eolian subfacies 6, ( 3 ) l a c u s t r i n e subfacies, and ( 4 ) flowing-water subfacies ( l o c a l

460

F i g . 5 . Ridges of Late Weichselian coversand on t h e s u r f a c e of a former g l a c i a l basin ( G e l d e r s e V a l l e i ) ; 1. b r o o k s , 2 . coversand r i d g e , 3. ice-pushed r i d g e ( a f t e r Maarlevel d 1 9 6 0 ) . d i s c h a r g e ) . The l i t h o l o g i c a l and s t r u c t u r a l c h a r a c t e r i s t i c s w i l l be mentioned b r i e f l y ; t h e photographs supplement t h e d e s c r i p t i o n s . Eolian s u b f a c i e s A ( s e e F i g s . 6-11) Predominantly unimodal w e l l - s o r t e d uniform s a n d s ; e v e n l y l a m i n a t e d , o f t e n with a " f l o w i n g " appearance w i t h t a p e r i n g u n i t s and low-angle c o n t a c t s ; r a r e l y cross-bedded; o c c a s i o n a l l y with laminae composed of g r a n u l e s o r small p e b b l e s ( m o s t l y w i n d - p o l i s h e d , l e s s commonly as v e n t i f a c t s ) ; l o c a l l y c r a c k s and small i n t e r n a l f r o s t wedges, t h i n moss-peat i n t e r c a l a t i o n s and adhesion r i p p l e l a y e r s a r e p r e s e n t ; i n t e r n a l c r y o t u r b a t i c l e v e l s a r e o f t e n l e s s numerous and t h i n n e r than t h o s e a s s o c i a t e d w i t h t h e n e r t s u b f a c i e s ( c f . Maarleveld 1 9 7 6 ) ; b u r i e d accumulations a r e very i n f r e q u e n t ; v a r i o u s low r e l i e f forms (Maarleveld 1960) inay be a s s o c i a t e d . Where c o a r s e sediment was a v a i l a b l e ( e . g . , n e a r ice-pushed r i d g e s ) , laminae o r l e n s e s ( t h e l a t t e r p o s s i b l y r e p r e s e n t i n g degraded g r a n u l e r i p p l e s ; c f . Bagno d 1941, p . 1 5 5 ) c o n t a i n i n g g r a n u l e s o r small p e b b l e s may o c c u r , as well a s l e v e l s

461 w i t h uniformly dispersed gravel-sized p a r t i c l e s ( c f . Bagnold 1941, p . 159). These

phenomena may a l s o be encountered in r e l a t i v e l y coarse eolian subfacies B deposits Note: the deposits depicted by Ahlbrandt & Fryberger (1982) in t h e i r Figs. 18C a n d 18D, show much resemblance t o eolian subfacies A deposits (except f o r the bioturbation t r a c e s , not present in the l a t t e r ) .

Eolian subfacies B ( s e e Figs. 11-20) I n t e r c a l a t e d yellowish a n d greyish evenly laminated sands ( " l a y e r cake" e f f e c t ) ; greyish laminae are more or l e s s s i l t y ; minor grain-size peaks occur the coarse s i I t and/or

in

t h e 75-88

ym

in

f r a c t i o n s ; i n t e r n a l c r y o t u r b a t i c levels

and f r o s t wedges are commonly p r e s e n t . Whaleback-like low-relief forms may consist of t h i s tyue of deposit (de Jong 1975). Locally, the following f e a t u r e s may be found: t h i c k e r s i l t y layers w i t h a loess o r loess-approaching grain s i z e ( m a i n peak

in

the coarse s i l t or

i n

the 75-88ym f r a c t i o n s ) , peat l a y e r s , and adhesion

r i p p l e l a y e r s ; " v e r t i c a l - p l a t y " crack systems ( c f . V i n k & Sevink,

in

van der

Hammen & Wijmstra 1971) may be preserved. Note: the often v i s i b l e c r i n k l y appearance

in

eolian subfacies B deposits ( s e e

Figs. 12-17) i s t h e r e s u l t of contortion by freeze/thaw action on previously e x i s t i n g laminae, and i s not caused by i n f i l t r a t i o n of very finesdeveloping d i s s i p a t i o n s t r u c t u r e s as defined by Ahlbrandt & Fryberger 1980, 1982. Lacustrine subfacies ( s e e Figs. 21-24) Varying composition of d e p o s i t s ; sandy p a r t s are evenly or wave-ripple laminated; s i l t layers composed of material w i t h a loess or l o e s s - l i k e grain s i z e and g y t t j a layers may be i n t e r c a l a t e d ; frost-thaw and f r o s t s t r u c t u r e s occur r a r e l y . Flowing-water subfacies ( s e e Figs. 25-29) Cross-bedded and cross-laminated deposits ( t h e l a t t e r made by among other things climbing r i p p l e s i n d i c a t i n g declining current v e l o c i t i e s ) , represented by p e r s i s t e n t layers as well as by complex c h a n n e l - f i l l s ; cross-bedded cosets locally contain macoscopic p l a n t remains (reworked), e s p e c i a l l y in c h a n n e l - f i l l s . This subfacies i s a t t r i b u t e d t o local discharge systems, as distinguished from the f l u v i a l deposits of t h e trunks of t h e Rhine-Meuse r i v e r system. RELATION OF SUBFACIES IN TIME A N D PLACE

Although the Twente Formation i s eolian-dominated, a t l e a s t in most of the exposures, l a c u s t r i n e a n d flowing-water deposits play an important r o l e l o c a l l y . Grain-size i n v e s t i g a t i o n s have shown t h a t the water-laid deposits are generally reworked eolian sediments. Many authors ( e . g . van der Hammen e t a l . 1967; Vandenberghe 1981) found a c o r r e l a t i o n between f a c i e s a n d subdivision of the g l a c i a l s t a g e . D u r i n g the Early

462

F i g . 6 . E o l i a n s u b f a c i e s A d e p o s i t showing even l a m i n a t i o n , d e f l a t i o n levels and f r o s t c r a c k s ( a g e : Late ! ! e i c h s e l i a n ) ( r o d s c a l e i n cm); Claricurri, Closterineent .

F i g . 7 . Eolian s u b f a c i e s A d e p o s i t s ( a g e : Late I l e i c h s e l i a n ) o v e r l y i n g Saalian t i l l ; e o l i a n accumulation l e v e l l e d by subsequent e o l i a n d e p o s i t i o n ( l e n g t h o f j o i n t e r 3 3 c m ) ; E r i c a , n e a r Crnmen, motorvay c r o s s i n g .

F i g . 8. E o l i a n subfacies A sedivent (I!eicflselian) o v e r l y i n g Saalian sandur deoosits ( r u l e r l e n g t h 2 5 crn); Z e i s t , rnotorway c r o s s i n g .

F i g . 9. D e t a i l o f upuer sediment i n F i g . 8 ( h e i g h t 27 cri).

P

m

W

464

F i g . 11. Eolian s u b f a c i e s A sediment o v e r l y i n g e o l i a n s u h f a c i e s R d e p o s i t s . The former- i s v i r t u a l l y f r e e of s i l t and more s u s c e p t i b l e t o wind a c t i o n ; ldeelde ( B e l g i u m ) , sand p i t .

465

F i g . 1 2 . Eolian s u b f a c i e s R sediments with a f a i n t v e r t i c a l - p l a t y crack system ( a g e : ivliddle W e i c h s e l i a n ) . The h e i g h t o f t h e l a c q u e r p e e l i s 7 7 cm; L o s s e r , Dinkel v a l l e y , o u t c r o p i n concave bank.

466

F i g . 13. E o l i a n s u b f a c i e s 3 sediments ( a g e : ' l i d d l e l i e i c h s e l i a n ) ; t h e lower part i s c r y o t u r h a t i c a l l y deformed a n d tonord by a r e o i o n a l d e f l a t i o n l e v e l (Ceuningen % r a v e l 3ed); t h e unper p a r t c o n t a i n s s o m e i n t e r c a l a t i o n s l a i d down by flowing water- ( h e i y l i t o f l a c q u e r p e e l : 35 c m ) ; L o s s e r , D i n k e l valley, o u t c r o p i n concave bank,

F i a . 1 4 . F o l i a n s u h f a c i e s " sediments ,"iit'i a consrlicLioii5 i r l t r a f o r m a t i o n a l minor c r y o t u r b a t c d a n d s l i c h t l y s l t i m ~ ?l r i v e l ( a n c : ',li;idle L'leichselian). T'ie h e i g h t o f t h e l a c q u e r p e e l i s 35 ci'1 an3 t h e depth of t h e d p n o s i t i s a b o u t 1 2 . 5 ~1 !ielo./ s i i r f a c Q l c w l ; Voorthuizen, sand p i t .

F i ? . 1 5 . Eolian s u b f a c i e s E sediments s h o ' v i n g

i m n y c r y o t u r b a t e d l e v e l s ( a g e : ' l i d d l e I4eiciiseli a n ) . 'Jete s p e c i f i c s u r f a c e p a t t e r n i n f l u e n c e d b y v e r t i c a l - v l a t y crack systems ( r u l e r 33 ciii l o n q ) . ? e D o s i t s i t u a t e d a b o u t 1 5 ni belo:.i s u r f a c e l e v c l ; ' i o o r t l i u i r e n , sand D i t .

e m v

4 68

F i g . 1 7 . Major c r y o t u r b a t i c a l d e f o r m a t i o n s i n mainly e o l i a n s u b f a c i e s E s e d i m e n t s ; Uitwel l i n g e r g a , aquaduct p i t .

c a s t irl c!olian s u b facias r sedimc'nts ( a w o f t l i e d e q o s i t i s S a a l i a n , age of t h e c a s t i s : ! e i c t i s e l i a n ) ( I i e i g i i t o f lacqirer p e e l : ?.$1 m ) ; Sevenuni, S c h a t h e r p s a n d ; l i t . F i g . 19. Ice-:/edqe

470

F i g . 20. V e r t i c a l - n l a t y Lrrick s y i t e n i v i s i b l e iri e o l i a n S u b f a L i P i C ietiiiiients a b o i r t 1 0 . 5 in below s u r f a c e l i ? v i ' l ( a s p : Y i d t l l ~ : ' : I p i i I . i 5 e l i a n ~ ( r i i l e r : 3') c r l o n s ) ; Voorttiuizi'n, s a n d p i t .

471

2 1 . D-noiits o f t h e l a c u s t r i n e ii.i'>facies; s i l t , i and f i n e sands ,:iitii l m t i c u l a r anri w a v y h e d d i n q ( t i e i o l i t : 45 c v ) ; 7irirl1ioven, r a i l v a y - t u n n e l p i t

Fin.

F i g . 2 2 . L a c u s t r i n e d e p o s i t s showing f i n e sands, s i l t s and g y t t j a , i n t e r s e c t e d by ice-wedge c a s t s ( a b o u t R in b e l o w O . D . ; Middle !.!eictiselian); Den t i e l d e r , d o c k yard p i t .

472

F i g . 23. D e p o s i t s of t h e l a c u s t r i n e s u b f a c i e s , with h a l f - s t a t i o n a r y : i ~ v t ? r i p p l e c o s e t s ; a t ttie b a s e t h e r e a r e d e b r i s flow sedirnents )with remnants of t h e o r i g i n a l sediment resembling ttie o v e r l y i n g d e p o i i t s (about, 10 111 b e l o w O.D.; Middle Weictiselian) ( s c a l e on spade s h a f t in drii); Deri l l e l d e r , dockyard lit.

F i g . 2 4 . D e t a i l o f F i g . 2 3 ( j o i n t e r measures 33 crri).

473

F i q . 2 5 . Flowing-'qater deposits intprcalated w i t h eolian subfacies C deposits (aoe: "iddle ' ! e i c h s e l i a n ) ; l a c q u e r o e e l ranoe 4.4Q-5.SQm !)elow s u r f a c e l e v e l ; Voor-ttiuizen, s a n d p i t .

F i g . 2 6 . I n t e r c a l a t i o n of s h e e t l i k e flowino-:vatcr d e 9 o s i t s i n eolian iiibfacies 4 deonsits; Overdinki'l, s a n d o i t..

474

F i q . 2 7 . C h a n n e l - f i l l s i n c i s e d i n a c r y o t u r h a t i c a l l y d e f o r m e d s u p e r n o s i t i o n of p e r i g l a c i a l l a c u s t r i n e d e p o s i t s , Eemian p e a t , and S a a l i a n p e r i g l a c i a l deposits; Cindhoven, r a i l w a y - t u n n e l n i t .

F i g . 28. C h a n n e l - f i l l w i t h a b u n d a n t p l a n t m a t e r i a l l o c a l l y and p e a t lumps a t the base, i n c i s e d i n e o l i a n s u b f a c i e s R d e p o s i t s s i t u a t e d a b o u t 9 m below O.D. (age: M i d d l e W e i c h s e l i a n ) ; Den H e l d e r , d o c k y a r d p i t .

475

F i g . 29. C r o s i - l a m i n a t e d d e p o s i t l a i d down b y f l o w i n g w a t e r \which have f i l l e d up s h a l l o w c h a n n e l - f i l l s ; a b o u t 16 m h e l o w 0.D. ( a g e : E a r l y ' l e i c h s e l i a n ) ( r u l e r i n crn); Den i i e l d e r , d o c k y a r d p i t . and l a t e W e i c h s e l i a n , m a i n l y l o a m - f r e e c o v e r s a n d s were d e p o s i t e d ,

hereas d u r i n g

t h e M i d d l e W e i c h s e l i a n ( P l e n i g l a c i a l ) m a i n l y loamy c o v e r s a n d s were f o r m e d . Van den T o o r n ( 1 9 6 7 ) f o u n d a s i m i l a r s u p e r p o s i t i o n i n t h e graben a r e a ; he d e s c r i b e d an a n a l o g o u s s e t t i n g f o r t h e u n d e r l y i n g S a a l i a n p e r i g l a c i a l d e p o s i s ( E i n d h o v e n F o r m a t i o n ) . M a a r l e v e l d ( 1 9 7 6 ) s u g g e s t e d t h a t t h e " O l d e r Coversands

(Middle

Weichselian; g e n e r a l l y e o l i a n s u b f a c i e s B ) arose under permafrost c o n d i t i o n s and t h e "Younger C o v e r s a n d s " ( L a t e W e i c h s e l i a n ;

generally e o l i a n subfacies A)

were d e p o s i t e d i n t h e absence o f p e r m a f r o s t . W i t h r e s p e c t t o t h e f o u r s u b f a c i e s , i t appears t h a t d e p o s i t s o f s u b f a c i e s A d o m i n a t e d i n t h e E a r l y and L a t e W e i c h s e l -

i a n s e d i m e n t a t i o n , whereas t h e o t h e r t h r e e s u b f a c i e s d o m i n a t e d u n d e r p l e n i g l a c i a l c o n d i t i o n s . The p r e s e n c e o r absence o f p e r m a f r o s t m i g h t a l s o e x p l a i n why s h e e t s o r c h a n n e l s were f o r m e d w i t h i n t h e f l o w i n g - w a t e r s u b f a c i e s . D e p o s i t s o f t h e l a c u s t r i n e and f l o w i n g - w a t e r s u b f a c i e s seem t o be i m p o r t a n t i n t h e southern coversand area, v i z . ,

t h e C e n t r a l Graben a r e a . Here, t h i c k l e n s e s

o f sandy s i l t ( i n c r o s s - s e c t i o n s up t o more t h a n 10 km w i d e and up t o 5 m t h i c k ) o c c u r ( " B r a b a n t s i l t " ) ( v a n den T o o r n 1967; B i s s c h o p s 1 9 7 3 ) , w h i c h a r e c o n s i d e r e d t o be d e p o s i t s f o r m e d by e o l i a n s u p p l i e d m a t e r i a l i n s h a l l o w s h e e t s o f s t a n d i n g water i n areas w i t h blocked r u n - o f f .

476 SEQUENTIONAL TRENDS The f o l l o w i n g gradual s u p e r p o s i t i o n s ( o r p a r t s of them) have been observed:

+

eolian subfacies A eolian subfacies B

eolian subfacies A

t

and

eolian subfacies B

+

.f

(lacustrine subfacies)

1a c u s t r i ne s ubf a c i e s

+

flowing-water subfacies I n v e s t i g a t i o n of a Weichselian v a l l e y f i l l l e d t o 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 an " i d e a l " p e r i g l a c i a l sequence b u t h e r e w i t h o u t l a c u s t r i n e d e p o s i t s - shown now i n F i g . 30 ( F i g . 2 i n Ruegg 1 9 7 5 ) .

v

H

DESERT PAVEMEN1

G

DRY EOLIbN

F

MOlST EOLIAN

E

WET EOLIAN

D

WATER CURRENT V E L O C l T I LOW TO NIL

c

R&TT*ER F I S T R U N N I N G WATER R IPPLE PHASE LOW ENERGY

6

FAST RUNNING WATER DUNE P W S E LOW ENERGY (OFTEN ASSENT)

&

V E R I FbST RUNNING WbTER HIGH ENERGY

a h k A 7 5L

Fig. 30. Schematic r e p r e s e n t a t i o n of a p e r i g l a c i a l sequence as deduced from d e p o s i t s i n a v a l l e y f i l l n e a r P e e l o , The N e t h e r l a n d s . The lower d i v i s i o n s ( A - D ) were formed under d e c l i n i n g c u r r e n t v e l o c i t y c o n d i t i o n s and t h e upper p a r t ( E - H ) was g e n e r a t e d mainly s u b a e r i a l l y and under i n c r e a s i n g e o l i a n i n f l u e n c e ; l a c u s t r i n e d e p o s i t s a r e n o t p r e s e n t and c r y o t u r b a t i c d e f o r m a t i o n s a r e r a t h e r s c a r c e ( a f t e r Ruegg 1 9 7 5 ) .

G R A I N SIZE F i g . 31 shows t y p i c a l g r a i n - s i z e p o p u l a t i o n s found i n samples of e o l i a n subf a c i e s A and 6. The p a t t e r n of the l a t t e r i s c h a r a c t e r i z e d by t h e o c c u r r e n c e of peaks i n the c o a r s e - s i l t and 7 5 - 8 8 ~ mf r a c t i o n s . As an a s p e c t s of t h e i n v e s t i g a t ion performed f o r t h e c l a s s i f i c a t i o n of Dutch Holocene s o i l s , c a r r i e d o u t by the S o i l Survey I n s t i t u t e ( d e Bakker & S c h e l l i n g 1 9 6 6 ) , a g r a i n - s i z e continuum was found between p e r i g l a c i a l e o l i a n sands and s i l t s ( i n c l u d i n g l o e s s ) ( F i g . 3 2 ) .

477 F i g . 31. R e p r e s e n t a t i v e g r a i n s i z e h i s t o g r a m s f o r e o l i a n subf a c i e s A ( u p p e r t h r e e ) and eolian subfacies B (lower three deposits.

BLA RlCUM

z.

2

&

0‘

u-u

“

2

i

-

s

0-2um

F i g . 32. P r o p o r t i o n s of c l a y , s i l t , and sand in samples from t h e coversand and l o e s s a r e a in The N e t h e r l a n d s ( a f t e r de Bakker & Schelling 1966).

478

TENTATIVE EXPLANATION OF THE EOLIAN SUBFACIES The f o l l o w i n g g e n e r a l remarks d e a l w i t h t h e e o l i a n environment of d e p o s i t i o n . The f i r s t three u n d e r l i n e t h e predominantly n o n - f l u v i a l o r i g i n , and t h e l a s t t h r e e c o n s i d e r t h e e o l i a n s e d i m e n t a t i o n i n more d e t a i l .

1. The more o r l e s s uniform h a b i t u s coupled w i t h the o v e r a l l o c c u r r e n c e and v a r i e g a t e d palaeotopography, as w e l l as t h e kind of s u r f a c e r e l i e f , argue a g a i n s t a f l u v i a l mode of o r i g i n . 2 . The absence of c r o s s bedding and c r o s s l a m i n a t i o n and the dominance of even l a m i n a t i o n , i n combination w i t h t h e f i n e - g r a i n e d l i t h o l o g y , cannot be exp l a i n e d i n terms of f l o w i n g - w a t e r p r o c e s s e s . 3. The d i s t u r b a n c e s c o n s i d e r e d t o be c r y o g e n e t i c a r e g e n e r a l l y u n l i k e disturbances seen i n w a t e r - l a i n n o n - p e r i g l a c i a l d e p o s i t s , as Maarleveld (1976) a l r e a d y concluded f o r " d r o p l i k e r e g u l a r d e f o r m a t i o n s " and G u l l e n t o p s & P a u l i s s e n (1978) f o r a kind of "drop s o i l " . 4. Organic l a y e r s o r r o o t d i s t u r b a n c e s a r e g e n e r a l l y s c a r c e o r a b s e n t , c o n t r a r y t o the s i t u a t i o n i n ( s u b ) r e c e n t e v e n l y laminated d e p o s i t s on Banks I s l a n d ( P i s s a r t e t a l . 1977) and B a f f i n I s l a n d ( e . g . , Dyke e t a l . 1 9 8 2 ) . 5 . On t h e b a s i s of the c r y o t u r b a t i c s t r u c t u r e s , p o l a r d e s e r t c o n d i t i o n s a r e inf e r r e d f o r p a r t of t h e Middle Weichselian ( P l e n i g l a c i a l ) . Twice a complete d i s a p p e a r a n c e of p e r m a f r o s t has been deduced from t h e o c c u r r e n c e of two major d i s t u r b e d l e v e l s ( e . g . , Maarleveld 1 9 7 6 ) . 6 . M e l t i n g - o u t of snow l e a d s t o d i s t u r b a n c e s of v a r i o u s s c a l e s , u p t o complete homogenization ( d i a m i c t i z a t i o n ) , b u t t h i s f e a t u r e i s v i r t u a l l y a b s e n t in the i n v e s t i g a t e d d e p o s i t s . On t h e c o n t r a r y , t h e g e n e r a l l y very s h a r p and cons p i c u o u s l a m i n a t i o n , w i t h laminae i n p l a c e s t r a c e a b l e l a t e r a l l y f o r more t h a n 10 m e t e r s , e x c l u d e s such p r o c e s s e s as m e l t i n g - o u t and a l s o e r o s i o n on a micros c a l e due t o r u n - o f f . The f o l l o w i n g t e n t a t i v e h y p o t h e s i s c o n c e r n i n g t h e d e p o s i t i o n of sandy and s i l t y laminae has been proposed ( c f . Ruegg 1975, 1 9 8 1 ) : pure s a n d laminae : d e p o s i t s from t h e t r a c t i o n c a r p e t on a d r y s e d i m e n t a t i o n surf ace; s i l t y s a n d laminae: a s t h e f o r e g o i n g , b u t s u s p e n s i o n load may s t i c k t o a somet i m e s damp s e d i m e n t a t i o n s u r f a c e ; s i l t layers

: wet d e p o s i t i o n a l s u r f a c e ; t r a c t i o n c a r p e t f i x e d upwind; only

the s u s p e n s i o n load reached t h e s p o t . An i n f l u e n c e of t h e humidity of t h e s e d i m e n t a t i o n s u r f a c e on l o e s s deposition has been d e m o n s t r a t e d by Cegla (1969, 1972; c f . Smalley 1 9 7 5 ) . French ( 1 9 7 6 ) d i s c u s s e d the p o s s i b l e d i f f e r e n c e s between t h e P l e i s t o c e n e perig l a c i a l environment of m i d - l a t i t u d e s and t h a t of p r e s e n t - d a y l a t i t u d e s from a

479 g e o m o r p h i c p o i n t o f v i e w . F o r t h e f o r m e r , he a s s i g n e d i m p o r t a n c e t o t h e r e g i m e o f d i u r n a l s o l a r r a d i a t i o n which l e d t o greater m o b i l i t y o f t h e s u r f i c i a l material He a l s o i n f e r r e d s t r o n g e r w i n d s . The g r e a t i m p o r t a n c e o f w i n d s i s c o n c l u d e d f r o m t h e presumed e x i s t e n c e o f g r e a t e r w i n d g r a d i e n t s due t o t h e s p a t i a l c o n c e n t r a t i o n o f t h e v a r i o u s c l i m a t i c zones. I n a d d i t i o n , a n t i c y c l o n i c c o n d i t i o n s p r e v a i l i n g above t h e e x t e n d e d i c e s h e e t s were p r e s u m a b l y much more s e v e r e . F r e n c h m e n t i o n e d t h e abundance o f s u i t a b l e m a t e r i a l a r i s i n g f r o m t h e i c e - m a r g i n a l p o s i t i o n o f t h e P l e i s t o c e n e p e r i g l a c i a l zone. Because o f t h e c o l d e r c l i m a t e and t h e r o u t e o f t h e w e s t e r l i e s , he supposed a r e l a t i v e l y l o w l e v e l o f p r e c i p i t a t i o n . However, t h i s does n o t a c c o u n t f o r t h e d i f f e r e n c e i n t h e d e g r e e o f P l e i s t o c e n e p e r i g l a c i a l dep o s i t i o n i n N o r t h A m e r i c a ( a s f a r as p u b l i s h e d ) v e r s u s NW Europe. S t r o n g subs i d e n c e and t h e a v a i l a b i l i t y o f an abundance o f s u i t a b l e m a t e r i a l i n an e x t e n s i v e d e l t a i c s e t t i n g i n t h e w e s t e r n p a r t o f t h e l a t t e r r e g i o n m u s t have been i m p o r t a n t contributory factors. CONCLUSIONS I n t h e NW p a r t o f c o n t i n e n t a l Europe, s t r e t c h i n g f r o m n o r t h e r n F r a n c e t o t h e B a l t i c s t a t e s o f t h e USSR, t h e r e i s a l a r g e s a n d - b e l t o f w i n d - d o m i n a t e d d e p o s i t s g e n e r a l l y devoid o f h i g h angle cross-beds formed d u r i n g g l a c i a l stages b u t n o t d i r e c t l y r e l a t e d t o i n l a n d i c e . I n The N e t h e r l a n d s t h e s e d e p o s i t s o r i g i n a t e d i n a t l e a s t s i x g l a c i a l s t a g e s ; s t a c k e d sequences a r e l o c a l l y up t o 40 m e t e r s t h i c k . S i g n i f i c a n t d e p o s i t i o n took p l a c e d u r i n g t h e l a s t g l a c i a l stage; a t present, d e p o s i t s o f t h e l a t t e r s t a g e c o v e r a t l e a s t i n The N e t h e r l a n d s an a r e a o f a p p r o x i m a t e l y 30,000 s q km. A f a c i e s w i t h even l a m i n a t i o n shows s t r o n g l a t e r a l e x t e n s i o n and i s r o u g h l y

concordant w i t h t h e p r e - e x i s t i n g topography ("coversands").

Two s u b f a c i e s have

been r e c o g n i z e d , one w i t h a n d t h e o t h e r w i t h o u t more o r l e s s s i l t y l a m i n a e ; t h e f o r w r i s p a r t o f a c o n t i n u u m o f w h i c h t h e l a t t e r and l o e s s a r e t h e endmembers. The t w o s u b f a c i e s a r e t h o u g h t t o be e o l i a n , r e p r e s e n t i n g e n v i r o n m e n t s w i t h a l t e r n a t i n g w e t and d r y d e p o s i t i o n a l s u r f a c e s o r o n l y d r y d e p o s i t i o n a l s u r f a c e s . Moreo v e r , l o c a l - f l u v i a l and l a c u s t r i n e d e p o s i t i o n a l s o t o o k p l a c e i n t h i s p e r i g l a c i a l environment. The f r e q u e n t o c c u r r e n c e o f t h e s e d e p o s i t s i n The N e t h e r l a n d s and a d j a c e n t a r e a s i s c l e a r l y r e l a t e d t o r e g i o n a l s u b s i d e n c e combined w i t h e x t e n s i v e d e l t a - b u i l d i n g , w h i c h l e d t o an abundance o f s u i t a b l e m a t e r i a l . I n E a s t Germany and P o l a n d , t h e p e r i g l a c i a l deposition i s predominantly r e s t r i c t e d t o broad f l u v i a l v a l l e y s , e s p e c i a l l y t h o s e o f W e i c h s e l i a n p r a d o l i n a s ; t h e r e , dune f i e l d s a r e t h e d o m i n a n t phenomenon. C l i m a t o l o g i c a l l y , t h e d i u r n a l s o l a r r a d i a t i o n p a t t e r n as w e l l as t h e e x i s t e n c e o f s t r o n g m i d - l a t i t u d e w e s t e r l i e s m u s t have been i m p o r t a n t f a c t o r s .

I n t h e s c a r c e l i t e r a t u r e c o n c e r n i n g e v e n l y l a m i n a t e d e o l i a n sand s h e e t s , t h i s

480 f a c i e s i s t h o u g h t t o be r e l a t e d t o t h e a v a i l a b i l i t y o f abundant m a t e r i a l , h i g h w i n d v e l o c i t i e s , and u n i f o r m g r a i n s i z e . ACKNOWLEDGEMEN T S I thank: t h e D i r e c t o r o f t h e Netherlands G e o l o g i c a l Survey f o r p e r m i s s i o n t o

c a r r y o u t t h e i n v e s t i g a t i o n s ; J. Vandenberghe and J. Schwan f o r d i s c u s s i o n s i n t h e f i e l d ; W.H.

Zagwijn,

J.G.

Zandstra, E.A.

K o s t e r , 3. Vandenberghe and T.S.

A h l b r a n d t f o r c r i t i c a l r e a d i n g o f t h e m a n u s c r i p t ; Mrs. A.C.H.M.

Niessen f o r h e l p

w i t h l i t e r a t u r e , e s p e c i a l l y f r o m P o l a n d ; F . W i l l e m s e n f o r some p h o t o g r a p h s , A . K o e r s f o r p r e p a r a t i o n o f t h e d r a w i n g s ; M r s . I. Seeger f o r r e a d i n g t h e E n g l i s h

text,

and Mrs. M . E . I .

Jouini f o r typing.

REFERENCES A h l b r a n d t , T.S. and F r y b e r g e r , S.G., 1980. E o l i a n d e p o s i t s i n t h e N e b r a s k a Sand H i l l s . U.S. G e o l . S u r v e y P r o f . P a p e r 1120A: 1-24. A h l b r a n d t , T.S. and F r y b e r g e r , S.G., 1982. E o l i a n d e p o s i t s . I n : P . A . S c h o l l e and D . R . S p e a r i n g ( e d s . ) , Sandstone D e p o s i t i o n a l E n v i r o n m e n t s . AAPG Mem. 31: 11-47. A l l e n , J.R.L., 1970. P h y s i c a l p r o c e s s e s of s e d i m e n t a t i o n . A l l e n & Unwin, London: 248 p p . Andrews, J . T . , Webber, P.J. and N i c h o l s , H . , 1979. A L a t e Holocene p o l l e n d i a g r a m f r o m P a n g n i r t u n g Pass, B a f f i n I s l a n d , N.W.T., Canada. Rev. P a l a e o b o t . P a l y n o l . , 27: 1 - 2 8 . Bagnold, R . A . , 1941. The p h y s i c s o f b l o w n s a n d and d e s e r t dunes. Yethuen, London: 265 p p . B a k k e r , H. de, en S c h e l l i n g , J., 1966. Systeem van b o d e m c l a s s i f i c a t i e v o o r Nederl a n d . P u d i c , Wageningen: 217 pp. B i s s c h o p s , J.H., 1973. T o e l i c h t i n g b i j de G e o l o g i s c h e K a a r t van N e d e r l a n d 1:50.000. B l a d E i n d h o v e n Oost ( 5 1 0 ) . G e o l . S u r v e y o f The N e t h . , Haarlem: 132 pp. B l a t t , H . , M i d d l e t o n , G . V . and M u r r a y , R . C . , 1972: O r i g i n o f s e d i m e n t a r y r o c k s . P r e n t i c e - H a l l , New J e r s e y : 634 pp. Cegla, J., 1969. I n f l u e n c e o f c a p i l l a r y g r o u n d m o i s t u r e on e o l i a n a c c u m u l a t i o n o f l o e s s . B u l l . Acad. P o l . S c i . G e o l . Geog. S e r . , 1 7 ( 1 ) : 25-27. Cegla, J., 1972. Loess s e d i m e n t a t i o n i n P o l a n d ( i n P o l i s h , w i t h e x t e n s i v e E n g l i s h summary). A c t a U n i v . W r a t i s l a v S t u d . Geogr., 1 7 ( 1 6 8 ) : 53-71. C o l l i n s o n , J.D., 1978. D e s e r t s . I n : H.G. R e a d i n g ( e d . ) , S e d i m e n t a r y e n v i r o n m e n t s and f a c i e s . B l a c k w e l l , London: 80-96. Denny, C.S., Owens, J.P., S i r k i n , L.A. and Meyer, R., 1979. The P a r s o n s b u r g Sand i n t h e C e n t r a l D e l m a r v a P e n i n s u l a , M a r y l a n d and D e l a w a r e . U.S. G e o l . S u r v e y P r o f . P a p e r 1067-B: 1-16. Dewers, F . , 1932. F l o t t s a n d g e b i e t e i n N o r d w e s t d e u t s c h l a n d , e i n B e i t r a g zum L o s s p r o b l e m . Abh. n a t u r w . V e r . Bremen, X X V I I I : 131-204. Dewers, F . , 1934. Probleme d e r F l u g s a n d b i l d u n g i n N o r d w e s t d e u t s c h l a n d . Abh. n a t u r w . Ver. Bremen, X X I X : 324-366. Ducker, A . und M a a r l e v e l d , G.C., 1957. Hoch- und s p a t g l a z i a l e a o l i s c h e Sande i n N o r d w e s t d e u t s c h l a n d und i n den N i e d e r l a n d e n . G e o l . Jb., 73: 215-234. Dyke, A.S., Andrews, J.T. a n d , M i l l e r , G.H., 1982. Q u a t e r n a r y g e o l o g y o f Cumberl a n d P e n i n s u l a , B a f f i n I s l a n d , D i s t r i c t o f F r a n k l i n . G e o l . S u r v e y o f Canada, Mem. 403: 32 p p . Eissmann, L . , 1981. P e r i g l a z i a r e P r o z e s s e und P e r m a f o r s t s t r u k t u r e n aus sechs K a l t z e i t e n des Q u a r t a r s . A l t e n b u r g e r n a t u r w i s s . F o r s c h . , 1: 1-171. F r e n c h , H.M., 1976. The P e r i g l a c i a l E n v i r o n m e n t . Longman, London-New Y o r k : 309 pp. Friedman, G.M. and Sanders, J.E., 1978. P r i n c i p l e s o f s e d i m e n t o l o g y . W i l e y , C h i c h e s t e r : 808 pp. F r y b e r g e r , S.G., A h l b r a n d t , T.S. and Andrews, S., 1979. O r i g i n , s e d i m e n t a r y f e a t -

481 u r e s , and s i g n i f i c a n c e o f l o w - a n g l e e o l i a n " s a n d s h e e t " d e p o s i t s , G r e a t Sand Dunes N a t i o n a l Monument and v i c i n i t y , C o l o r a d o . J o u r . Sed. P e t r . , 49: 733-746. G u l l e n t o p s , F. and P a u l i s s e n , E . , 1978. The d r o p s o i l o f t h e E i s d e n t y p e . B i u l . P e r y g l a c j a l n y , 27: 105-115. Hammen, Th. van d e r , 1951. L a t e - g l a c i a l f l o r a and p e r i g l a c i a l phenomena i n The N e t h e r l a n d s . L e i d s e G e o l . Meded., 17: 71-184. Hammen, Th. van d e r and M a a r l e v e l d , G . C . , 1952. Genesis and d a t i n g o f t h e p e r i g l a c i a l d e p o s i t s a t t h e e a s t e r n f r i n g e o f t h e Veluwe. G e o l . en M i j n b . , (NS) 14: 47-54. Hammen, Th. van d e r , M a a r l e v e l d , G . C . , V o g e l , J.C. and Z a g w i j n , W.H., 1967. S t r a t i graphy, c l i m a t i c s u c c e s s i o n and r a d i o c a r b o n d a t i n g o f t h e L a s t G l a c i a l i n The N e t h e r l a n d s . G e o l . en M i j n b . , 46: 79-95. Hammen, Th. van d e r and W i j m s t r a , T.A. ( e d s . ) , 1971. The Upper Q u a t e r n a r y o f t h e D i n k e l V a l l e y . Meded. R i j k s G e o l . O i e n s t , (NS) 22: 55-214. H u n t e r , R . E . , 1977. B a s i c t y p e s o f s t r a t i f i c a t i o n i n s m a l l e o l i a n dunes. SedimentOlogy, 24: 361-387. Jong, H. de, 1975. M e r k w a a r d i g e d e k z a n d k o p j e s i n de omgeving van Y a r k e l o . Boor en Spade, 1 9 : 79-85. K o l s t r u p , E. and W i j m s t r a , T.A., 1977. A p a l y n o l o g i c a l i n v e s t i g a t i o n o f t h e M o e r s h o o f d , H e n g e l o and Denekamp i n t e r s t a d i a l s i n The N e t h e r l a n d s . G e o l . en M i j n b . , 56: 85-102. K o l s t r u p , E . , 1980. C l i m a t e and s t r a t i g r a p h y i n n o r t h w e s t e r n Europe between 30,000 B.P. and 13,000 B . P . , w i t h s p e c i a l r e f e r e n c e t o The N e t h e r l a n d s . Meded. R i j k s G e o l . O i e n s t , 3 2 ( 1 5 ) : 181-253. K o s t e r , E . A . , 1982. T e r m i n o l o g y and l i t h o s t r a t i g r a p h i c d i v i s i o n o f ( s u r f i c i a l ) sandy e o l i a n d e p o s i t s i n The N e t h e r l a n d s : an e v a l u a t i o n . G e o l . en M i j n b . , 61: 121-129. K o z a r s k i , S . , Nowaczyk, B . , R o t n i c k i , K . and T o b o l s k i , K . , 1969. Problems c o n c e r n i n g t h e e o l i a n phenomena i n W e s t - C e n t r a l P o l a n d 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 c h r o n o l o g y o f phases o f e o l i a n a c t i v i t y . G e o g r a p h i a P o l o n i c a , 17: 231-248. K o z a r s k i , S., 1980. An o u t l i n e o f V i s t u l i a n s t r a t i g r a p h y and c h r o n o l o g y o f t h e G r e a t P o l a n d L o w l a n d . Q u a t . S t u d i e s i n P o l a n d , 2: 21-35. M a a r l e v e l d , G . C . , 1960. Wind d i r e c t i o n s and c o v e r sands i n The N e t h e r l a n d s . B i u l . P e r y g l a c j a l n y , 8: 49-58. M a a r l e v e l d , G . C . , 1976. P e r i g l a c i a l phenomena and t h e mean a n n u a l t e m p e r a t u r e d u r i n g t h e l a s t g l a c i a l t i m e i n The N e t h e r l a n d s . B i u l . P e r y g l a c j a l n y , 26: 57-78. Mare'chal, R. e t M a a r l e v e l d , G . C . , 1955. L ' e x t e n s i o n des phe'nomgnes p e ' r i g l a c i a i r e s en B e l g i q u e e t aux Pays-Bas. Meded. G e o l . S t i c h t i n g , (NS) 8: 77-86. Meene, E . A . van de, 1977. T o e l i c h t i n g b i j de G e o l o g i s c h e K a a r t van N e d e r l a n d 1: 50.000. B l a d Arnhem O o s t ( 4 0 0 ) . G e o l . S u r v e y o f The N e t h . , Haarlem: 147 p p . N e u m e i s t e r , H., 1971. J u n g p l e i s t o z a n e Decksedimente und B o d e n e n t w i c k l u n g i n d e r Umgebung von L e i p z i g . Z p r d v y , V I I I : 23-72. Nowaczyk, B., 1976. E o l i a n c o v e r sands i n C e n t r a l - W e s t P o l a n d . Q u a e s t i o n e s Geog r a p h i c a e , 3: 57-77. P i s s a r t , A., V i n c e n t , J.-S. e t E d l u n d , S.A., 1977. Oe'pbts e t phe'nomgnes e ' o l i e n s s u r l ' ? l e de Banks, t e r r i t o i r e s du Nord-Ouest, Canada. Can. J. E a r t h S c i . , 14: 2462-2480. P l o e y , J. de, 1961. M o r f o l o g i e e n k w a r t a i r - s t r a t i g r a f i e van de Antwerpse N o o r d e r kempen. A c t a G e o g r a p h i c a L o v a n i e n s i a , 1: 1-130. Reineck, H.-E. and S i n g h , I . B . , 1980. D e p o s i t i o n a l s e d i m e n t a r y e n v i r o n m e n t s . S p r i n g e r V e r l a g , B e r l i n : 549 pp. ( s e c o n d e d . ) . Ruegg, G.H.J., 1975. S e d i m e n t a r y s t r u c t u r e s and d e p o s i t i o n a l e n v i r o n m e n t s o f M i d d l e - and U p p e r - P l e i s t o c e n e g l a c i a l t i m e d e p o s i t s f r o m an e x c a v a t i o n a t P e e l o , n e a r Assen, The N e t h e r l a n d s . Meded. R i j k s G e o l . D i e n s t , ( N S ) 26: 17-24. Ruegg, G.H.J., 1981. S e d i m e n t a r y f e a t u r e s and g r a i n s i z e o f g l a c i o - f l u v i a l and p e r i g l a c i a l d e p o s i t s i n The N e t h e r l a n d s and a d j a c e n t p a r t s o f Western Germany. Verh. n a t u w i s s . V e r . Hamburg, (NF) 2 4 ( 2 ) : 133-154. S e l l e y , R.C., 1978. A n c i e n t s e d i m e n t a r y e n v i r o n m e n t s . Chapman and H a l l L t d . , London 287 p p . ( s e c o n d e d . ) .

482 S m a l l e y , I . J . , 1975. Loess, l i t h o l o g y and g e n e s i s . Benchmark P a p e r s i n Geology, 26. Dowden, H u t c h i n s o n & Ross, I n c . , S t r o u d s b u r g , Penns.: 430 p p . Somme', J., Paepe, R. e t L a u t r i d o u , J.P., 1980. P r i n c i p e s , methodes e t systgme de l a s t r a t i g r a p h i e du Q u a t e r n a i r e dans l e Nord-Ouest de l a F r a n c e e t l a B e l g i q u e . I n : J. C h a l i n e ( e d . ) , P r o b l h e s de s t r a t i g r a p h i e q u a t e r n a i r e en F r a n c e e t dans l e s p a y s l i m i t r o p h e s . Supple'ment au B u l l . de l ' A F E Q , N.S. 1: 148-162. S t a a l d u i n e n , C.J. van, A d r i c h e m B o o g a e r t , H.A. van, B l e s s , M.J.M., D o p p e r t , J.W. Chr., H a r s v e l d t , H.M., M o n t f r a n s , H.M. van, O e l e , E., Wermuth, R . A . and Zagwijn, W.H., 1979. The g e o l o g y o f The N e t h e r l a n d s . Meded. R i j k s G e o l . D i e n s t , 3 2 ( 2 ) : 9-49. Toorn, J.C. van den, 1967. T o e l i c h t i n g b i j de G e o l o g i s c h e K a a r t van N e d e r l a n d 1: 50.000. B l a d V e n l o West ( 5 2 W). G e o l . S u r v e y o f The N e t h . , Haarlem: 163 pp. Vandenberghe, J. and G u l l e n t o p s , F., 1977. C o n t r i b u t i o n t o t h e s t r a t i g r a p h y o f t h e W e i c h s e l p l e n i g l a c i a l i n t h e B e l g i a n c o v e r s a n d a r e a . G e o l . en M i j n b . , 56: 123-128. Vandenberghe, J., 1981. W e i c h s e l i a n s t r a t i g r a p h y i n t h e S o u t h e r n N e t h e r l a n d s and N o r t h e r n B e l g i u m . Q u a t . S t u d i e s i n P o l a n d , 3: 111-118. Vandenberghe, J. and K r o o k , L . , 1981. S t r a t i g r a p h y and g e n e s i s o f P l e i s t o c e n e d e p o s i t s a t A l p h e n ( s o u t h e r n N e t h e r l a n d s ) . G e o l . en M i j n b . , 60: 417-426. W a l k e r , R.G. and M i d d l e t o n , G . V . , 1979. E o l i a n sands. I n : R.G. W a l k e r ( e d . ) , F a c i e s m o d e l s . G e o s c i e n c e Canada, R e p r i n t s e r i e s 1: 33-41. Z a g w i j n , W.H., 1961. V e g e t a t i o n , c l i m a t e and r a d i o c a r b o n d a t i n g s i n t h e L a t e P l e i s t o c e n e o f t h e N e t h e r l a n d s . P a r t I: Eemian and E a r l y W e i c h s e l i a n . Meded. G e o l . S t i c h t i n g , 1 4 : 15-45. Z a g w i j n , W.H. u n d Paepe, R., 1968. D i e S t r a t i g r a p h i e d e r w e i c h s e l z e i t l i c h e n A b l a gerungen d e r N i e d e r l a n d e und B e l g i e n s . E i s r e i t a l t e r u. Gegenwart, 19: 129-146. Z a g w i j n , W.H., 1974. V e g e t a t i o n , c l i m a t e and r a d i o c a r b o n d a t i n g s i n t h e L a t e P l e i s t o c e n e o f The N e t h e r l a n d s . P a r t 11: M i d d l e W e i c h s e l i a n . Meded. R i j k s Geol. D i e n s t , ( N S ) 25: 101-111.

483

BIGBEAR ERG: A PROTEROZOIC INTERMONTANE EOLIAN SAND SEA I N THE HORNBY BAY GROUP, NORTHWEST TERRITORIES, CANADA. GERALD M. ROSS, D e p a r t m e n t o f Geology, C a r l e t o n U n i v e r s i t y , Ottawa, Canada, K1S 586; P u b l i c a t i o n #02-83 o f t h e O t t a w a - C a r l e t o n C e n t r e f o r Geoscience Studies INTRODUCTION The r e l a t i v e l y r e c e n t d e s c r i p t i o n o f s e d i m e n t a r y s t r u c t u r e s formed by p r o cesses i n t r i n s i c t o t h e m i g r a t i o n o f e o l i a n bedforms, i n p a r t i c u l a r t h e small s c a l e e o l i a n s t r a t i f i c a t i o n d e s c r i b e d f r o m modern dunes by H u n t e r (1977a) and t h e bounding s u r f a c e h i e r a r c h y described by B r o o k f i e l d (1977) p r o v i d e potent i a l l y p o w e r f u l t o o l s f o r r e c o g n i z i n g and i n t e r p r e t i n g e o l i a n d e p o s i t s ( s e e Hunter,

1981;

amples). and/or

Kocurek and Oott,

1981;

K o c u r e k 1981a,b,

These s t u d i e s a r e complemented

by work

which

s a t e l l i t e imagery w i t h s u r f a c e m e t e o r o l o g i c a l

mechanisms Wilson, McKee,

f o r excellent

ex-

combines a i r p h o t o

data t o i n f e r causal

f o r t h e f o r m a t i o n o f e o l i a n sand seas ( e r g s ) and bedforms ( c f .

1971, Breed,

1972a;

F r y b e r g e r and Ahlbrandt,

and F r y b e r g e r ,

sophisticated

facies

1979).

models

1979;

Breed e t al.,

1979a;

It i s now p o s s i b l e t o c o n s t r u c t f a i r l y

for

ancient

eolianites

which

take

into

c o n s i d e r a t i o n b o t h l a r g e and s m a l l - s c a l e d e p o s i t i o n a l p r o c e s s e s . The p u r p o s e o f t h i s p a p e r i s t o document an e o l i a n o r i g i n f o r p a r t of a sequence

of

Northwest

Proterozoic

Territories

(Y.5Ga) of

redbeds o f

Canada.

t h e Hornby Bay Group

Recognition

of

the

i n the

eolianites

and

r e c o n s t r u c t i o n o f b e d f o r m s i s based on c o m p a r i s o n o f o b s e r v e d s t r a t i f i c a t i o n w i t h t h a t d e s c r i b e d f r o m modern, and some a n c i e n t , e o l i a n s e d i m e n t s . L a t e r a l changes

i n stratification

styles

within

the eolianites

are interpreted i n

t e r m s o f c h a n g i n g b e d f o r m s w i t h i n t h e b a s i n . T h i s f a c i e s change r e f l e c t s t h e complex i n t e r p l a y o f w i n d regime, and c l i m a c t i c f a c t o r s . for

40

sand s u p p l y ,

a i r flow-bedform i n t e r a c t i o n ,

The r e s u l t a n t b a s i n a l b e d f o r m p r o f i l e s ,

km p e r p e n d i c u l a r

to

depositional

strike,

are

which extend

similar

to

those

d e s c r i b e d f r o m modern a r i d e n v i r o n m e n t sand seas. The

example

from

the

Hornby

Bay

Group

represents,

to

the

author's

knowledge, o n l y t h e f i f t h r e p o r t e d o c c u r r e n c e o f P r e c a m b r i a n e o l i a n s a n d s t o n e (Donaldson, 1981;

1965;

Clemmy,

modification

M e i n s t e r and T i c k e l l ,

1976), of

although

arenites

1981; P e t t i j o h n , P o t t e r ,

there

by e o l i a n and S i e v e r ,

1976; are

processes 1973;

Hyde,

studies

1980; which

(Chaudhuri,

Folk,

1968).

Goode and H a l l , suggest 1977;

textural Ramaekers,

T h i s i s unusual i n

v i e w o f t h e w i d e l y h e l d a s s u m p t i o n t h a t most t e r r e s t r i a l s e d i m e n t a t i o n i n t h e

484 P r e c a m b r i a n ( a n d p r e - D e v o n i a n ) was by b r a i d e d s t r e a m complexes (Schumm, 1967; R u s t , 1979;

C o t t e r , 1 9 7 8 ) , t h e emergent p a r t s o f w h i c h w o u l d be s u s c e p t i b l e

t o eolian deflation.

The cause o f t h e a p p a r e n t l a c k o f P r e c a m b r i a n e o l i a n i t e s

has been d i s c u s s e d i n an e a r l i e r p a p e r (Ross, i n p r e s s ) . TERMINOLOGY T h r e e c l a s s e s o f e o l i a n b e d f o r m s have been d i s t i n g u i s h e d on t h e b a s i s o f wavelength, draas "draa"

amplitude,

(Wilson,

and g r a i n s i z e ;

1972a,b).

(Kocurek,

dunes,

and

Some c o n f u s i o n e x i s t s as t o t h e u s e o f t h e t e r m

1981b,

e o l i a n bedforms w i l l

these are wind r i p p l e s ,

p.754).

For the

purpose o f

be r e f e r r e d t o as d r a a ,

this

paper,

compound

which i m p l i e s only t h a t these

were t h e l a r g e s t b e d f o r m s i n t h e e r g and t h a t t h e y were compound s t r u c t u r e s (i.e.

b e d f o r m s t h a t have one o r more s u p e r p o s e d b e d f o r m s ) .

Use o f t h e t e r m

d r a a i n t h i s p a p e r does n o t i m p l y a n y t h i n g a b o u t b e d f o r m s p a c i n g and a m p l i t u d e ( c f . W i l s o n , 1972a,b). R e c e n t advances i n t h e u n d e r s t a n d i n g o f m i g r a t i o n o f e o l i a n b e d f o r m s has l e d t o r e c o g n i t i o n o f a h i e r a r c h a l arrangement o f bounding surfaces ( s u r f a c e s t h a t bound s e t s o f s t r a t i f i c a t i o n ) i n e o l i a n a r e n i t e s ( B r o o k f i e l d , 1977). The descriptive

hierarchy

connotations. (first-order t h e draa;

of

these bounding

Laterally surfaces)

extensive,

surfaces

nearly

carries

flat-lying

implicit planar

genetic surfaces

r e s u l t f r o m m i g r a t i o n o f t h e l a r g e s t e o l i a n bedform,

s e c o n d - o r d e r s u r f a c e s , w h i c h a r e t r u n c a t e d by f i r s t - o r d e r s u r f a c e s ,

r e p r e s e n t m i g r a t i o n o f s u p e r p o s e d b e d f o r m s ( d u n e s ) o v e r t h e d r a a s u r f a c e and third-order

s u r f a c e s f o r m by d e f l a t i o n and r e a c t i v a t i o n o f dune l e e s l o p e s

(Brookfield,

1977).

In this

paper,

s u r f a c e s t h a t bound t h e l a r g e s t

first-order

bounding surfaces

crossbed s e t s

(megasets)

s u r f a c e s bound i n t r a s e t s w i t h i n t h e megasets.

are the

and s e c o n d - o r d e r

I n t h e example d e s c r i b e d i n

t h i s p a p e r t h e l a c k o f c o n t r o l on t h e l a t e r a l e x t e n t and v e r t i c a l d i s t r i b u t i o n of first-order that

b o u n d i n g s u r f a c e s made i t d i f f i c u l t t o u n a m b i g u o u s l y c o n c l u d e

these surfaces

nomenclature w i l ' l

formed

by

draa

migration.

Thus

the

bounding

surface

be a p p l i e d i n a p u r e l y d e s c r i p t i v e sense.

REGIONAL STRATIGRAPHIC SETTING The Coppermine H o m o c l i n e , intracratonic

basins

that

Thelon

Basin,

developed

f o l l o w i n g c e s s a t i o n o f "Hudsonian"

in

(1900

-

and A t h a b a s c a the

northwest

1750Ga)

Basin

are three

Canadian

Shield

orogenic a c t i v i t y (Fig.

1). They r e c o r d a p e r i o d o f p r o t r a c t e d a n o r o g e n i c s e d i m e n t a t i o n t h a t c o n t i n u e d u n i n t e r r u p t e d f o r more t h a n

.6Ga.

I n general

these basins c o n t a i n a basal

s u c c e s s i o n o f t e r r e s t r i a l t o m a r i n e s i l i c i c l a s t i c s o v e r l a i n by t r a n s g r e s s i v e marine carbonates

(the

so-called

"orthoquartzite

t o carbonate

suite",

cf.

D o t t , 1981; Krumbein and S l o s s , 1 9 6 3 ) . The s t r a t i g r a p h i c s u c c e s s i o n s p r e s e r v e d

485

F i g . 1. Main t e c t o n i c elements o f t h e Northwest Canadian S h i e l d . S t i p p l e d b a s i n s (Thelon, Athabasca, and E l u Basins and Coppermine Homocline (upper l e f t ) ) a r e a p p r o x i m a t e l y a g e - e q u i v a l e n t c r a t o n i c cover sequences. Dashed l i n e s i n C h u r c h i l l P r o v i n c e and Wopmay Orogen a r e s t r u c t u r a l s u b d i v i s i o n s . AA: Athapuscow Aulacogen; KB: K i l o h i g o k Basin; MF: McDonald F a u l t ; BF: B a t h u r s t F a u l t ; GSL: G r e a t S l a v e Lake; GBL: G r e a t Bear Lake; LA: Lake Athabasca; CG: C o r o n a t i o n G u l f ; HB: Hudson's Bay. ( m o d i f i e d f r o m Lewry and S i b b a l d , 1980 and Hoffman 1980). i n t h e s e b a s i n s d i s p l a y no evidence o f r e g i o n a l metamorphism and/or mation. The Coppermine Homocline i s a -10km

v o l c a n i c rocks t h a t r e s t unconformably on Lower P r o t e r o z o i c (1.84 Van Schmuss and Bowring, 1980) and Archean (2.6 1978) basement

(Fig.

defor-

t h i c k sequence of sedimentary and

-

3.15Ga,

-

1.92Ga,

Krough and G i b b i n s ,

1 ) . The Homocline c o n t a i n s two s u c c e s s i v e t e r r e s t r i a l

s i l i c i c l a s t i c t o marine c a r b o n a t e c y c l e s

( t h e Hornby Bay and Dismal Lakes

groups: Donaldson and Baragar, 1973; Kerans e t al.,

1981), o v e r l a i n by a t h i c k

sequence of c o n t i n e n t a l p l a t e a u b a s a l t s (Coppermine R i v e r Group) and s h a l l o w !water carbonates and s i l i c i c l a s t i c s o f t h e Rae Group (Baragar and Donaldson, 1973). These rocks were d e p o s i t e d i n an e n s i a l i c c r a t o n i c d e p r e s s i o n ( i n t e r i o r [ p l a t f o r m b a s i n o f Sleep e t al., c r u s t a l extension.

1980) t h a t formed as t h e r e s u l t o f anorogenic

486 U n t i l 1978, t h e Coppermine H o m o c l i n e had o n l y been p a r t i a l l y mapped by heli c o p t e r s u r v e y a t 1 : 2 5 0 , 0 0 0 ( B a r a y a r a n d D o n a l d s o n , 1 9 7 3 ) ; e a r l i e r work was of a

reconnaissance

H o f f m a n (1978,

nature

(Fraser

et

1952) mapped p a r t o f

H o m o c l i n e as p a r t o f r e y i o n a

al.,

1960;

t h e southern

Cook

and

Aitken,

margin o f

1971).

t h e Coppermine

p r o j e c t t o c o v e r Wopmay Orogen. D e t a i l e d mapping

and s e d i m e n t o l o y i c a n a l y s i s o f t h e Hornby Bay a n d D i s m a l u n d e r t a k e n i n 1978 and camp e t e d i n 1 9 8 1 ( K e r a n s e t a l . , I n 1980, t h e a u t h o r mapped t h e a r e a d i s c u s s e d

ILakes groups was 1981; Ross, 1982).

i n t h i s p a p e r a t a scale of

1 : 50,000. The Hornby Bay Group, t h e b a s a l g r o u p o f t h e H o m o c l i n e , thick.

i s inore than 2 k m

I t r e s t s u n c o n f o r i n a b l y on Lower P r o t e r o z o i c basement (Wopmay Orogen,

H o f f m a n , 1 9 8 0 ) and i s c o n f o r m a b l y t o u n c o n f o r m a b l y o v e r l a i n by s i l i c i c l a s t i c s of

t h e D i s m a l Lakes Group ( F i g .

2 and 3 ) .

I t i s composed o f t h r e e u n i t s of

f l u v i a l t o rnari ne s i 1 i c i c l a s t i c s o v e r 1 a i n by t r a n s g r e s s i v e m a r i n e dolostones and r e g r e s s i v e d e l t a i c t o m a r i n e s i l i c i c l a s t i c s . been s u b d i v i d e d i n t o t h r e e d e p o s i t i o n a l

systems

The b a s a l s i l i c i c l a s t i c s have (Bigbear,

F a u l t R i v e r , and

F i g . 2. G e o l o g y o f a p o r t i o n o f t h e Coppermine H o m o c l i n e . F a u l t symbols have t e e t h on downthrown s i d e (eg. n o r m a l f a u l t s ) . TBP: T e s h i e r p i - B i g t r e e p l a n a t i o n s u r f a c e . L i m e s t o n e p a t t e r n r e p r e s e n t s m a r i n e d o l o s t o n e and s i l i c i c l a s t i c s from u p p e r Hornby Bay Group.

487 Lady Nye systems) based on l i t h o f a c i e s assemblages, i n f e r r e d d e p o s i t i o n a l h i s t o r i e s (Kerans e t a l . , The B i g b e a r system,

p a l e o c u r r e n t t r e n d s , and

1981, f o r d e t a i l s ) .

which c o n t a i n s t h e e o l i a n i t e s ,

i s “1.6km

thick,

and

r e c o r d s i n f i l l o f an i n t e r m o n t a n e b a s i n t h a t had a s o u t h e a s t - d i p p i n g paleoslope.

Prolonged e r o s i o n o f t h e B i g b e a r system source area r e s u l t e d i n f o r -

mation o f a l o w - r e l i e f t h e word)

p l a n a t i o n s u r f a c e ( p e n e p l a i n i n t h e l i t e r a l sense o f

(Teshierpi-Bigtree planation surface;

Fig.

2 ) w i t h an area o f a t

l e a s t 3000 km2. Synsedimentary normal f a u l t a c t i v i t y and d e p o s i t i o n o f i n t r a formational

conglomerates

d u r i n g l a t e r p e r i o d s o f B i g b e a r system d e p o s i t i o n

marked t h e b e g i n n i n g o f c r u s t a l u p l i f t and change f r o m a s o u t h e a s t - d i p p i n g t o southwest-dipping

paleoslope.

T h i s was

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

a

s h a l l o w graben a l o n g t h e F a u l t R i v e r ( F i g . 2 ) and i n c i s i o n o f t h e p e n e p l a i n by southwest-flowing

rivers.

Coarse

paraconglomerates,

arenites,

and

pebbly

a r e n i t e s o f t h e F a u l t R i v e r system were d e p o s i t e d by a d r a i n a g e system t h a t Wowed west-southwest

subparallel

conglomerates

cobbles

apparently

contain eroded

during

and

t o the

graben

boulders

paleoslope

of

change

axis.

F a u l t R i v e r system

Bigbear

system

arenites,

and

drainage

pattern

r e o r g a n i z a t i o n . Both t h e B i g b e a r and F a u l t R i v e r systems a r e conformably DISMAL LAKES

LADY NY’E FLUVIAL SYSTEM

FAULT RIVER FLUVIAL SYSTEM

BIGBEAR FLUVIAL SYSTEM BUNN CREEK CONGLOMERATE

meters


60% o f t h e u n i t . T h i s l i t h o l o g y decreases t o t K I

- ---.. >L!JWIENTATION IN THE NORTH SEA AREA

K.W. G L E N N I E Shell Internation4lc Pr.troleum Mid., The Hague, The Netherlands INTRODUCTION During the past two decades, o u r knowledge of the d i s t r i b u t i o n and environment of deposition of the Rotliegend has been added t o g r e a t l y by t h e d r i l l i n g of many wells in t h e North Sea area; f i r s t , in t h e southern North Sea where Rotliegend sandstones form the main r e s e r v o i r f o r g a s (e.g. Van Veen, 1975), and l a t e r in the central North Sea where these sandstones ( i n addition t o Zechstein carbonates) a r e o i l reservoirs in two producing f i e l d s , Auk (Brennand and Van Veen, 1975) a n d Argyll (Pennington,

1975). The "Rot1 iegendes" have long been recognised by miners of the Kupferschiefer in Germany and Poland as t h e red beds t h a t underlie the Late Permian Zechstein sequence of N . W . Europe. The Rotliegend sequences o v e r l i e Carboniferous or older s t r a t a . The o r i g i n of t h e Rotliegend sediments, and of t h e i r equivalents in B r i t a i n , as the deposits of an a r i d continental environment, has been advocated f o r many decades ( e . g . Born, 1921). An a n a l y s i s of the l i t h o l o g i e s and inferred depositional environments of the Rotliegend d e s e r t sequence of t h e southern North Sea was given by Glennie (1972) following extensive s t u d i e s of modern d e s e r t sediments (Glennie, 1970), outcrops of Permian s t r a t a , and of cores and wireline logs from wells. Since then, more has become known of t h e Rotliegend sedimentary rocks, especially in the U K p a r t of the central North Sea (Glennie, 1983), as additional well d a t a i s released t o the public each year by t h e U K Department of Energy. Rotliegend deposition t o o k place in t h e environment of a Northern Hemisphere Trade Wind d e s e r t during t h e l a t e r phases of the major Southern Hemisphere g l a c i a t i o n over Gondwana. I intended t o show t h a t there probably was a causal r e l a t i o n s h i p between Rotliegend deposition and polar g l a c i a t i o n , and t h a t , despite probable lowered global a i r temperatures, the marked a r i d i t y of t h e Rotliegend d e s e r t s was the r e s u l t of winds t h a t e i t h e r were stronger o r , more l i k e l y , t h a t blew a t sand-transporting v e l o c i t i e s f o r much longer periods of the year t h a n i s now t h e case. If c o r r e c t , then t h e l a t e Carboniferous-early Permian peak t o the Gondwana g l a c i a t i o n a l s o implies a stronger coeval wind regime outside t h e polar and equatorial areas of the worl d . Rotliegend deposition was b r o u g h t t o an a b r u p t end e a r l y in the Late Permian as the r e s u l t of the rapid flooding of the continental basins by the marine waters of t h e Zechstein Sea ( s e e e.g. Smith, 1970, 1979).

THE ROTLIEGEND BASINS The ' e a r l y ' Permian Rotliegend sedimentary sequences of the North Sea area were

522

EXISTING MAJOR FAULTS

DUNESAND DUNE 8 WADI

\

SABKHA

MORAY FIRTH BASIN

9/20-2 ELGIN UEVELDE

PALAEOWIND DIRECTION

L

FLUVIALTRANSPORT DIRECTION

0

0

PRE-PERMIAN OUTCROP

B

A E

&

DESERT LAKE

-

DUTCH BANK BASIN

MF

/

a Fig. 1

ROTLIEGEND DEPOSITIONAL/EROSIONAL EDGE

DB

D

100 KM

---L--l

Upper Rotliegend Facies and Palaeowind Map o f North Sea Area

49/26-2 DURHAM

523

deposited in t h r e e , e s s e n t i a l l y east-west trending basins (Fig. 1 ) . The l a r g e s t of these b a s i n s , now known as the Southern Permian Basin ( Z i e g l e r , 1978, 1982) extends almost 1500 km from eastern England t o about t h e Polish-Soviet border. The preservedwidth of t h e basin l o c a l l y approaches 400 k m , and sediments accumulated within i t t o a thickness of over 1500 m ( G i l l , 1967). I n the area of the southern North Sea, t h i s sedimentary sequence has been penetrated by many wells and i s now becoming increasingly recognisable a t depth on modern high-resolution seismic l i n e s . The smaller Northern Permian Basin i s separated from i t s southern neighbour by the east-west trending Mid North Sea-Ringkabing-Fyn system of highs and intervening grabens (Central and Horn Grabens). The basin i s now divided i n t o two parts by the NNW-SSE trending northern extension of the Central Graben. I t s eastern l i m i t seems t o coincide with t h e southerly extension of the NNE-SSW trending Oslo Graben, whereas l i t t l e more than 500 km t o the west, the basin character i s l o s t i n the v i c i n i t y of the Forth Approaches. Rotliegend sequences u p t o 600 m thick have been penetrated by the d r i l l . Rotliegend sandstones exceeding 100 m in thickness, have a l s o been penet r a t e d in several wells u p t o 150 km northwards i n t o t h e Viking Graben (Fig. 1 ) . The r e l a t i v e l y small Moray F i r t h Basin i s separated from the Northern Permian Basin by t h e SW-NE trending Grampian S p u r , over which Rotliegend s t r a t a a r e n o t found. Rotliegend sandstones accumulated t o a thickness of 600 m in the centre o f t h e basin. The western margin of t h e basin coincides with the Great G ! w Fault. Although n o t s t r i c t l y Rotliegend, because they a r e not overlain by Zechstein s t r a t a , sedimentary sequences of s i m i l a r age and f a c i e s a l s o occur in a s e r i e s of half grabens t h a t s t r e t c h in an a r c from the south of England, t h r o u g h the west Midlands of England, t h e Cheshire, Manx-Furness and Ulster Basins t o the Minch Basins of N . W . Scotland (Smith e t a l . , 1974). A s h o r t e r b u t p a r a l l e l a r c of small basins curves from the Vale of Eden, t h r o u g h southwest Scotland, t o t h e I s l e o f Arran (Smith e t a l . , 1974; Brookfield, 1978; Lovell, 1983). The development of these half grabens seems t o be matched by s i m i l a r depressions in S.W. Germany and eastern France, and thus r e f l e c t s t h e e a r l y collapse of the Variscan Highlands a n d i t s northern foreland under an E-W tensional regime. Sedimentary sequences of d e s e r t o r i g i n t h a t extend back i n t o the Pennsylvanian have been described from t h e west-central United S t a t e s ( s e e McKee, 1979a). Making allowance f o r t h e l a t e r opening of t h e A t l a n t i c , these American sequences probably formed p a r t of t h e same Northern Hemisphere b e l t of Trade Wind d e s e r t s as the Rotliegend of N . W . Europe ( s e e Fig. 4 ) . ROTLIEGEND FACIES AND DEPOSITIONAL ENVIRONMENTS The Rotliegend i s divided i n t o two d i s t i n c t rock-stratigraphic u n i t s , the Lower and Upper Rotliegend, on t h e presence or absence of volcanic rocks associated with the sedimentary sequence. Although the Lower Rotliegend i s generally overlain by the Upper, t h e l a t t e r may l o c a l l y be coeval with younger p a r t s of t h e Lower Rotliegend in areas where t h e r e was no Early Permian volcanic a c t i v i t y (Fig. 2 ) .

524

................................. ................................ ................................. MUDSTON ES

................................ ...............................

................... ..................

STEPHANIAN WESTPHALIAN

m

a

C B A

3

NAMURIAN HALITE

R = REWORKED

WADI CGLS

F

= FLUlDlSED

& DEFORMED

NODULAR ANHYDRITE

Fig. 2 a)

-

Rotliegend, Rock-stratigraphic Facies Diagram

Lower Rotliegend The range in composition of t h e basic t o intermediate volcanics of the Lower

Rotliegend, and t h e i r d i s t r i b u t i o n adjacent t o known or inferred f a u l t s , suggests t h a t t h e i r o r i g i n was probably r e l a t e d t o the e a r l i e s t tensional movements connected not only with the Permian basins, b u t a l s o with the North Sea graben systems. Katzung (1975) considered t h a t these f a u l t s were i n i t i a t e d during the StephanianAutunian. The Lower Rotliegend i s best developed in northern Germany, where i t forms a continuous sequence u p t o 2000 m thick ( P l e i n , 1978) t h a t i s inferred t o range in from t h e Stephanian i n t o the Autunian, and in t h e area of t h e Oslo Graben-Bamble Trough. Similar rock associations of about t h e same age, b u t covering generally

agl

smaller a r e a s , a r e known in France, S.W. England, S.W. Scotland, a n d in some of the flank areas of the Mid North Sea-RingkBbing-Fyn Highs ( s e e Ziegler, 1982). No volcanic a c t i v i t y o f Saxonian age i s known on continental Europe (Falke, 1976).

525

b)

ii?p?r Rqtl iegend I n t h e Southern Permian B a s i n , t h e Upper R o t l i e g e n d i s made u p of f o u r d i s t i n c t i v e f a c i e s a s s o c i a t i o n s which have been i n t e r p r e t e d a s t h e p r o d u c t s of s e d i m e n t a t i o n i n f l u v i a l ( w a d i ) a e o l i a n , sabkha and l a c u s t r i n e ( d e s e r t l a k e ) environments; the l a t t e r i n c l u d e s d e p o s i t s of bedded h a l i t e . T h e i r main sedimentary c h a r a c t e r s a s recognised i n c o r e s a r e i l l u s t r a t e d by Glennie (1972) and P l e i n ( 1 9 7 8 ) . These f a c i e s a r e d i s t r i b u t e d from s o u t h t o n o r t h a c r o s s the b a s i n i n e s s e n t i a l l y t h e same o r d e r a s

given above, w i t h t h e d e s e r t - l a k e f a c i e s c o i n c i d i n g with t h e main a x i s of subsidence. Only a narrow s t r i p of r a r e l y d r i l l e d R o t l i e g e n d o c c u p i e s t h e a r e a between t h e l a k e sediments and t h e a r e a of t h e h i g h s t o t h e i r n o r t h . On p r e s e n t e v i d e n c e , t h e same s e d i m e n t a r y f a c i e s ( w a d i , a e o l i a n , sabkha, l a c u s t r i n e ) a l s o occur i n t h e Northern Permian and t h e Ploray F i r t h b a s i n s , a l t h o u g h the p r e s e n c e of a bedded l a c u s t r i n e h a l i t e has y e t t o be demonstrdted. I t should be r e a l i s e d , however, t h a t t h e Permian geology of t h e s e two b a s i n s i s s t i l l i n c o m p l e t e l y known; t h e Rotliegend i n t h e deeper p a r t s of t h e b a s i n s i s r a r e l y reached by the d r i l l and, e x c e p t on t h e Moray c o a s t n e a r Elgin ( F i g . l ) , i t i s n o t seen i n o u t c r o p .

Ihe-fluvlal-sesuences

i) of the Southern Permian Basin comprise l o c a l l y conglome r a t i c s a n d s t o n e s t h a t a r e c h a r a c t e r i s e d by t h e o c c u r r e n c e of c u r l e d c l a y f l a k e s , d e n o t i n g s u b a e r i a l d e s i c c a t i o n . Conglomerates form a g r e a t e r p a r t of t h e sequence i n The N e t h e r l a n d s and i n Germany t h a n i n B r i t i s h w a t e r s , and i n the former two l a n d s they a r e generally rich i n volcanic c l a s t s . The f l u v i a l s a n d s t o n e s commonly a l t e r n a t e with well l a m i n a t e d , h o r i z o n t a l l y t o r e l a t i v e l y s t e e p l y i n c l i n e d (20-30") s a n d s t o n e s t h a t a r e i n t e r p r e t e d t o be aeol t a n , and w i t h s t r u c t u r e l e s s s a n d s t o n e s , which a r e t h o u g h t t o have been homogenised by l i q u e f a c t i o n of p r e v i o u s l y d e p o s i t e d sands d u r i n g l o c a l f l u v i a l f l o o d i n g ( c f . S e l l e y , 1 9 6 9 ) . T h u s , t o g e t h e r , t h e s e beds i n d i c a t e t h e ephemeral n a t u r e of the s t r e a m flow w i t h i n t e r v e n i n g p e r i o d s of s u b - a e r i a l e x p o s u r e , d e s i c c a t i o n and wind a c t i v i t y . Some t h i c k e r mud-cracked c l a y s seem t o have had t h e i r c r a c k s i n f i l l e d w i t h sand from above, whereas o t h e r s were r e c o g n i s a b l y i n j e c t e d from below by a s l u r r y of sand and w a t e r t o form s a n d s t o n e dykes ( G l e n n i e , 1970; 1 9 7 2 ) . The f l u v i a l sequences a r e more common a l o n g the s o u t h e r n margin of t h e b a s i n and i n t h e lower p a r t of t h e R o t l i e g e n d ; many s m a l l e r wadi c h a n n e l s probably d i d n o t e x t e n d a s f a r a s t h e d e s e r t l a k e , b u t t e r m i n a t e d i n small i n t e r d u n e sabkhas. I n t e r m i t t e n t l y a l o n g t h e s o u t h e r n margin of t h e b a s i n , major wadi c h a n n e l s c u t through t h e b e l t of dune s a n d s , b u t even h e r e , the f l u v i a l sands commonly a l t e r n a t e with t h o s e of a e o l i a n o r i g i n ( F i g . 1 ) thus emphasising t h e s p o r a d i c n a t u r e of r a i n f a l l i n t h e a r e a . E l l e n b e r g e t a l . (1976) d e s c r i b e l o c a l d e l t a i c s e d i m e n t a t i o n a l o n g t h e s o u t h e r n margin of the d e s e r t l a k e i n E. Germany, and i n Poland, Pokorski (1978) d e s c r i b e d widespread b r a i d e d river c h a n n e l s and ephemeral f l u v i a l a c t i v i t y .

526

The character of wireline l o g s i n d i c a t e s t h a t f l u v i a l sands a l s o occur in sone p a r t s of the Northern Permian and Moray F i r t h basins. The m a i n d i r e c t i o n s of fluvial t r a n s p o r t have n o t y e t been determined, however.

Ihe-aeollan-sesuencer

penetrated in North Sea wells generally comprise s e t s of ii) moderately t o well-laminated sandstones t h a t a r e horizontally bedded a t the base, and upwards become inclined a t angles t h a t increase t o a maximum of 25" or 30". The set i s then generally overlain by horizontally bedded sandstones forming the base of the succeeding s e t ( s e e cores in Glennie, 1972; 1983). Locally, low-angle sandstones nay be overlain by the steeply-dipping laminae of a sequence of prograding sandstones. The well laminated steeply-dipping sandstones a r e i n t e r p r e t e d t o have resulted from deposition by g r a i n f a l l (Hunter, 1977), and the decimetre-thick, poorly t o non-laminated s e t s from sandflow. Both types a r e common in North Sea wells. Ripples a r e r a r e l y well displayed. I n sub-horizontal bedding, however, s l i g h t changes in lamina thickness within the width of a core (10 cm) possibly i n d i c a t e a n o r i g i n by r i p p l e migration in an interdune area as described by Kocurek (1981). I r r e g u l a r adhesion-ripple laminae have been described by Glennie (1972, Fig. 1 4 ) . They a r e possibly more common in areas where t h e r a t e of subsidence matched t h a t of deposit i o n , as l o c a l l y seems t o have been t h e case in t h e Sole P i t Basin (Glennie, Mudd and Nagtegaal , 1978). The s e t s of laminae seen in North Sea cores between d e f l a t i o n surfaces generally range between about one and seven metres in thickness. Locally, however, s e t s have been measured t h a t comprise u p t o 20 m o r so of continuous bedding. Since most of t h e thinner bedding s e t s probably represent only the preserved lowermost portions o f migrating dunes, the very thick s e t s probably represent an unusual event in which most o f t h e lee-slope bedding of a dune i s preserved. This could happen in a sand s e a , f o r instance, where a temporary wadi channel becomes permanently i n f i l l e d by a s i n g l e phase of migrating dune. I t could a l s o be achieved by the i n f i l l of interdune hollows in a growing sand sea such as those i l l u s t r a t e d by Glennie (1970, Fig. 7 2 ) from t h e A1 Liwa, eastern Arabia. For example, t h e Rotliegend sand sea reached a t o t a l thickness of some 300 m in t h e Sole P i t Basin (Glennie and Boegner, 1981). Thus, although we have no idea of how much of the upper p a r t of such l a r g e dunes was removed during l a t e r dune migration, t h e 20 m thick s e t s of bedding can be taken a s a crude approximation t o t h e minimum height of t h e dune on which i t was formed. I n t h i s context, Glennie and Buller (1983) have presented o t h e r evidence suggesting t h a t many Rotliegend transverse dunes of t h e Southern Permian Basin were probably over 50 m high a t the time of the Zechstein transgression. The bedding a t t i t u d e s of t h e dune sands measured on cores and deduced from dipmeter logs of wells in t h e Southern Permian Basin i n d i c a t e t h a t t h e Mid Permian windr blew over t h e area from e s s e n t i a l l y e a s t t o west (Glennie, 1972; 1983; Ellenberg et a l . , 1976; Van Wijhe e t a l . , 1980). A t l e a s t in t h e western half of t h e Northern

527

Permiarl Basin, however, t h e winds blew from the northwest ( F i g . 1 ) . Thus, a centre of barometric high pressure must have been located over the Elid North Sea High (Glennie, in p r e s s ) . A s i m i l a r c e n t r e of high pressure t o the west of Wyoming and North Cclorado has been proposed by Fryberger (1979) t o account f o r southerlyd i r e c t e d f o r e s e t s found in t h e Pennsylvanian-Permian Weber Sandstone. I n t h e Rotliegend sandstones of many North Sea wells, the r e l a t i v e l y narrow spread of bedding a t t i t u d e s indicates t h a t these sands were deposited largely on dunes o f t h e transverse type (Fig. 3 ) . I n N . E . England ( D in Fig. l ) , however, wind strengths seem t o have been t o o strong f o r transverse dunes t o be s t a b l e , and longitudinal ( s e i f ) dunes were formed. Because of t h e i r almost complete preservation beneath the marine Marl S l a t e (Kupferschiefer equivalent) a t the base of the succeeding Zechstein sequence, much o f t h e i r original shapes can be followed in a s e r i e s of l a r g e quarries ( s a n d p i t s ) , mine s h a f t s a n d borings. These dunes a r e aligned along a n ENE-WSW a x i s (N60'E) a n d have been preserved t o a height of u p t o 60 m above t h e Carboniferous surface (Smith a n d Francis, 1967, F i g . 18 a n d PL 15; Glennie a n d Buller, 1983). I n t h e interdune a r e a s , t h e Marl S l a t e generally r e s t s d i r e c t l y on Carboniferous s t r a t a with no Rotliegend sandstone intervening. The thickness of sand v i s i b l y reworked from the dunes by t h e transgressing Zechstein Sea ranges between some 10 cm in w h a t a r e believed t o be former dune-crest l o c a t i o n s , t o a maximum of around 4 m on the dune flanks (Glennie a n d Buller, 1983). Marine faunas with Zechstein a f f i n i t i e s occur in t h e 2-3 cm immediately below t h e Marl S l a t e (Bell e t a l . , 1979). N

N

N

24 DIPS LIEVELDE-I WIND FROM NIOOOE

127 DIPS 49/26-2 WIND FROM Ni05OE

5 4 DIPS COMBINED DIP DATA FROM FIELD HOUSE AND CRIME RIGGS SAND PITS COUNTY DURHAM WIND F'ROM N60°E

Fig. 3 - The poles of dune bedding (derived from dipmeter l o g s ) plotted on Polar Nets (upper hemisphere) f o r three d i f f e r e n t l o c a l i t i e s in the Southern Permian Basin. The d i s t r i b u t i o n of bedding a t t i t u d e s suggests the type of dune of which they form a p a r t , and arrows give deduced wind d i r e c t i o n s : A ) Limited d i s t r i b u t i o n - simple transverse dune C) Most dips concentrated in areas ( b ) on e i t h e r s i d e of t h e dune a x i s - s e i f dune B) Most bedding a t t i t u d e s concentrated in a general down-wind location on the net b u t with some dips almost a t r i g h t angles t o t h e deduced wind d i r e c t i o n , and so suggestive of limited transverse i n s t a b i l i t y and the possible presence of barchan-like horns - barchanoid dune. Note: Bedding a t t i t u d e s in A o r i g i n a l l y published by Van Wijhe e t a l . (1980) as a rose diagram.

528

This limited amount of reworking i s in marked c o n t r a s t t o the much g r e a t e r t h i c k ness of reworked sandstone l o c a l l y present in the T r i a s s i c - J u r a s s i c E n t r a d a Sandstone of New Mexico (Tanner, 1970). There, dunes, with a n original r e l i e f of over 45 r i , were truncated and had t h e i r interdune areas i n f i l l e d with water-laid sandstones some 30 m thick in the aquatic environment of ‘Lake T o d i l t o ’ . A t outcrop, the s e i f dunes of Durham a r e n o t red b u t yellow in colour, from which they derive t h e i r name of ‘Yellow Sands’. I n North Sea w e l l s , in Germany a n d in Poland, however, the uppermost sands of t h e Rotliegend sequence a r e commonly grey o r white f o r u p t o some 50 m beneath the Kupferschiefer, a n d a r e referred t o as the Wei s s l iegend. The o r i g i n of these sands has been the subject of dispute over the decades, with b o t h aeolian and marine derivations being a t t r i b u t e d t o them ( s e e references in Pryor, 1 9 7 1 a , b ; and Nemec a n d Porebski, 1 9 7 7 ) . I n b o t h the southern North Sea and in Poland, the Weissliegend dune bedding grades i n t o large-scale soft-sediment deformation s t r u c t u r e s . Similar s t r u c t u r e s occur in the middle of the Hopeman Sandstones (Peacock, 1966) on t h e southern coast of the Moray F i r t h near Elgin (Fig. 1 ) . Here, a zone of deformation divides the dune sequence of the area i n t o two u n i t s : a lower u n i t of southward-dipping dune sands f o r which correlation with t h e Rotliegend i s suggested, a n d an upper u n i t of southwest-dipping dune sands of probable Late Permian or Early T r i a s s i c age (Walker, 1973). The creation of t h e deformation a t the t o p of the lower dune u n i t , a n d a l s o of s i m i l a r s t r u c t u r e s i n t h e Weissliegend of t h e North Sea and Poland, a r e a t t r i b u t e d t o the e f f e c t s of the Zechstein transgression (Glennie and Buller, 1983). This event will be referred t o again. i i i ) Ihe-lacusfrlne-s‘9”’”ce i n the Southern Permian Basin (Fig. 1 a n d 2 ) consists primarily of red-brown mudstone with minor s i l t s t o n e . Several horizons of h a l i t e , with a cumulative thickness in excess of 100 m ( P l e i n , 1978), a r e developed over much of the axial p a r t of the basin. The h a l i t e i s concentrated i n the middle p a r t of t h e

Upper Rotliegend sequence in the axial p a r t of t h e basin in Germany, b u t wedges o u t towards the basin margins so t h a t in U K waters, f o r example, i t i s limited t o the lower p a r t of the s e c t i o n . In northern Germany, where the l a c u s t r i n e sequence reaches a thickness of around 1500 m , t h e h a l i t e s a t t a i n a s u f f i c i e n t thickness t o r e a c t d i a p i r i c a l l y ; the sequence i s then r e f e r r e d t o as t h e Haselgebirge f a c i e s . Carbonates a n d b o t h bedded and nodular anhydrite a r e absent from t h e l a c u s t r i n e f a c i e s . The l a c u s t r i n e s t r a t a seem t o be devoid of f o s s i l s except in the metre or so beneath the Kupferschiefer (Plumhoff, 1966). As these f o s s i l s have strong Zechstein a f f i n i t i e s , they should, perhaps more c o r r e c t l y , a l s o be a t t r i b u t e d t o t h e Zechstein marine transgression (Fa1 ke, 1976). The l a c u s t r i n e environment of deposition of t h i s sequence was deduced from the lack of f o s s i l s , from t h e absence of carbonates, and from the non-marine composition of the evaporites (Glennie, 1972). This i n t e r p r e t a t i o n i s supported by t h e low

529

bromide c o n t e n t of t h e R o t l i e g e n d h a l i t e and by t h e s u l p h u r - i s o t o p e a n a l y s i s of t h e a s s o c i a t e d anhydri tes (Hol s e r , 1 9 7 9 ) .

Hol s e r (1979) s u g g e s t s t h a t because of t h e i r

r e l a t i v e l y g r e a t volume, t h e s e s a l t s must have been d e r i v e d from o l d e r d e p o s i t s of marine o r i q i n , and s u g g e s t s exposed Devonian and E a r l y Permian ha1 i t e s of European USSR a s p o s s i b l e s o u r c e s . The w r i t e r ' s e x p e r i e n c e i n modern d e s e r t s convinces him,

however, t h a t t h e r e q u i r e d volume of h a l i t e can be c o n c e n t r a t e d and p r e c i p i t a t e d from l a k e w a t e r s u p p l i e d by a normal system of s e a s o n a l l y ( ? ) f l o w i n g r i v e r s , provided t h e r a t e of e v a p o r a t i o n i s g r e a t enough and of long d u r a t i o n . As w i l l be shown l a t e r , both t h e s e f a c t o r s were probably o p e r a t i v e i n t h e e a r l y Permian Rotliegend d e s e r t . There a r e no known s e d i m e n t a r y s t r u c t u r e s w i t h i n t h e l a c u s t r i n e sequence t h a t a r e i n d i c a t i v e of s u b - a e r i a l d e s i c c a t i o n o r e r o s i o n , a l t h o u g h such s t r u c t u r e s a r e known i n t h e a n h y d r i t i c mudstones of t h e sabkha f a c i e s , which e n c i r c l e s t h i s d e s e r t l a k e . The l a k e i s t h e r e f o r e b e l i e v e d t o have been a permanent f e a t u r e of t h e Southern

A t i t s f u l l e s t e x t e n t , t h e l a k e must have covered an a r e a of some 1000 km e a s t - w e s t , by u p t o 200 km n o r t h - s o u t h . Permian Basin t h r o u g h o u t t h e e a r l i e r Permian.

The e v i d e n c e of f l u v i a l t r a n s p o r t d i r e c t i o n s i n t h e R o t l i e g e n d sequences of t h e Southern Permian Basin i n d i c a t e s t h a t most of t h e w a t e r s u p p l i e d t o t h e d e s e r t l a k e was d e r i v e d from t h e s o u t h , from t h e Variscan Mountains, with only minor q u a n t i t i e s from o t h e r s o u r c e s .

F l u v i a l a c t i v i t y seems t o have been more i m p o r t a n t in t h e e a s t

than i n t h e w e s t . The s o u r c e s of w a t e r f o r t h e n o r t h e r n two b a s i n s a r e unknown. i v ) As we have s e e n , a_sesuenre_of_Sabkha_deeoslts occupied a broad a r e a e n c i r c l i n g t h e d e s e r t l a k e i n t h e Southern Permian B a s i n , and d e p o s i t s of t h i s t y p e a r e found i n s m a l l e r a r e a s of l o c a l s u b s i d e n c e i n t h e Northern Permian and Moray F i r t h Basins (Deegan and S c u l l , 1975; Smith, 1976, F i g . 1 ) . Using t e t r a p o d f o o t p r i n t s f o r d a t i n g , Haubold and Katzung (1978) i n d i c a t e t h a t p l a y a and sabkha sediments a l r e a d y e x i s t e d i n Germany d u r i n g t h e L a t e Autunian. These s e d i m e n t a r y r o c k s comprise poorly-bedded c l a y s with minor s i l t s and sands t h a t d i s p l a y many f e a t u r e s i n d i c a t i v e of a sabkha ( e . g . d e s i c c a t i o n c r a c k s , sandstone d y k e s , adhesion r i p p l e s , a n h y d r i t e n o d u l e s ) .

These f e a t u r e s c o l l e c t i v e l y i n d i c a t e an

a q u a t i c d e p o s i t i o n a l a r e a t h a t was s u b j e c t t o e x t e n s i v e s u b a e r i a l d e s i c c a t i o n i n an a r i d environment ( G l e n n i e , 1 9 7 0 ) . T h u s t h e sabkha sediments r e p r e s e n t t h e a r e a t h a t was covered by w a t e r o n l y d u r i n g t h e maximum e x t e n s i o n s of t h e d e s e r t l a k e .

Although

t h e r e probably were annual f l u c t u a t i o n s i n t h e e x t e n t of t h e l a k e and a s s o c i a t e d sabkha, t h i s i s d i f f i c u l t t o i l l u s t r a t e with a l i m i t e d number of well b o r i n g s . F l u c t u a t i o n s on a much l o n g e r time s c a l e a r e c l e a r l y a p p a r e n t , however, and a r e i n d i c a t e d s c h e m a t i c a l l y on t h e r o c k - s t r a t i g r a p h i c f a c i e s diagram ( F i g . 2 ) , and i n t h e w e l l - l o g c o r r e l a t i o n s i l l u s t r a t e d by Adrichem Boogaert (1976) f o r t h e Netherlands. Another i m p o r t a n t p o i n t t h a t t h e s e f i g u r e s i l l u s t r a t e i s t h a t h a l i t e p r e c i p i t a t i o n i n t h e Southern Permian Basin seems t o have been coeval w i t h t h e maximum development of the aeolian facies.

T h i s p o i n t w i l l be r e f e r r e d t o a g a i n .

530

HERCYNIAN MOUNTAINS