Biolaminated deposits

Lecture Notes in Earth Sciences Edited by Somdev Bhattacharji, Gerald M. Friedman, Horst J. Neugebauer and Adolf Seilacher

9 I

III

III

I

I

Gisela Gerdes

Wolfgang E. Krumbein

Biolaminated Deposits I

I

I

Springer-Verlag Berlin Heidelberg NewYork London Paris Tokyo

Authors Dr. Gisela G e r d e s Prof. Dr. W o l f g a n g E. Krumbem GeomlcrobJology Division, University of Oldenburg Carl-von-Ossietzkystr. 9-11 D - 2 9 0 0 OIdenburg, West G e r m a n y

ISBN 3 - 5 4 0 - 1 7 9 3 7 - 2 Springer-Verlag Berlin Heidelberg N e w York ISBN 0 - 3 8 7 - 1 7 9 3 7 - 2 Spdnger-Verlag N e w York Berhn Heidelberg

This work is subject to copynght All rights are reserved, whether the whole or part of the material ts concerned, specifically the rights of translation, reprinting, re-use of tllustrattons, recitation, broadcasting, reproduction on microfilms or in other ways, and storage in data banks Duplication of this pubtlcatlon or parts thereof Is only permitted under the provisions of the German Copynght Law of September 9, 1965, in its version of June 24, 1985, and a copynght fee must always be paid, Vtolations fall under the prosecution act of the German Copyright Law, © Springer-Vedag Berlin Heidetberg 1987 Printed in Germany Pnnttng and bmding Druckhaus Beltz, Hemsbach/E,ergstr 2132/3140-543210

Petrificata parentes libus

montium calcariorum non filii

sed

sunt, cum omnis calx oriatur ab anima-

(Linnaeus,

Systema Naturae,

Ed. XII, T.

III, p. 154, 1760-1761)

PREFACE

The geological only

significance of life has long attracted mankind.

have single groups of organisms been considered,

building animals,

diatoms or "monera"

(radiolarian,

such as

Not

frame-

globigerins,

fo-

rams),

but unitarian pictures were also drawn concerned with the regu-

lation

and

pathways.

feedback The

of geochemical cycles by

enzyme-controlled

back-coupling system of

inanimate matter fascinated Vernadsky crystallographer,

interacting

metabolic living

and

(1863 - 1945), a mineralogist and

and is again stressed in Lovelock's Gaia

and Krumbein's Bioplanet or Bioid approach.

hypothesis

The role of microorganisms

in this respect is well documented in terms of disintegration of rocks, production and mineralization of organic compounds, oxidation

and reduction of metals,

ore formation. of

biomineral

catalyzation of the

formation and

biogenic

Records of stromatolites arising from the vital activity

microorganisms date back to the earliest known sedimentary environ-

ments of the Precambrian era.

The tified

aim of the work presented here is to document the in-situ straaccretion

microbes. duced the

Part

of sediments attributable to the vital I comments on terms

sedimentary structures and products. occurrence of microbial mats

activity

which relate to microbially Part II is concerned

(potential stromatolites)

in

marginal marine environments of arid and temperate coastlines. modes

of

facies

evolution in subenvironments are shown

integration of sedimentological,

microbiological

of prowith

modern Varying

through

the

and faunistic data. In

Part III structures attributed to the activity of Precambrian, and Lower Jurassic microbial communities are analyzed,

Permian

and some comple-

mentary aspects concerned with the geological potential of microbes are summarized.

Acknowledgements

(Gisela Gerdes)

P r e s e n t e d h e r e is a m o d i f i e d v e r s i o n of m y thesis w h i c h encompasses a number of individual publications. I am indebted to m a n y p e o p l e w h o a c c o m p a n i e d my way over the past years. My b e n e f a c t o r in this w o r k was W.E. Krumbein. He first introduced me to the fascinating system of microbial mats. From Gavish Sabkha and Solar Lake we went on to include the "Farbstreifen-Sandwatt" as parts of the expanding biosed i m e n t a r y system. We then turned our a t t e n t i o n to counterparts of all this in fossil records, spanning the gap b e t w e e n b i o l o g y and geology. My first encounter w i t h a c t u o p a l e o n t o l o g y was during my c o o p e r a t i o n w i t h Wo Sch~fer. His b o o k " A k t u o p a l ~ o n t o l o g i e nach Studien in der Nordsee" was the first scientific w o r k w h i c h I was able to follow through from its conception. His "Schule des Sehens", w h i c h was transformed into reality through the r e o r g a n i z a t i o n of exhibits at the Senckenberg Museum, Frankfurt, remains one of the most m e m o r a b l e imp r e s s i o n s of m y stay in that city. H.-E. Reineck p r o v i d e d support and advice in the fields of actuogeology and actuopaleontology. Our c o l l a b o r a t i o n b e g a n in "Senckenberg am Meer", Wilhelmshaven. I w o u l d like to thank h i m for the interest he shared in m y w o r k and for all his h e l p and advice. During our trips to ancient and m o d e r n d e p o s i t i o n a l environments and through our w o r k in the l a b o r a t o r y he taught me to r e c o g n i z e and understand sedimentary structures. My thanks are further extended to my other benefactor, H. K. Schminke. I am grateful also to colleagues from the G e o m i c r o b i o l o g y team and to K. Wonneberger, my former p a r t n e r at O l d e n b u r g U n i v e r s i t y marine b i o l o g y unit, Wilhelmshaven, for their d i s c u s s i o n and advice. Memories o f our w o r k together on Mellum, in the G a v i s h Sabkha, by Solar Lake and in Elat unite me w i t h Eo Holtkamp. Our stay, laboratory w o r k and accommodation on M e l l u m w e r e made p o s s i b l e b y the M e l l u m Council and in Israel by the H. Steinitz Marine B i o l o g y Laboratory, Elat and its staff. I am p a r t i c u l a r l y g r a t e f u l to F. D. Por for his advice during our stay in Israel. I would also like to thank all for a s s i s t a n c e and care in the p r e p a r a t i o n of drawings, reproductions, photographs, thin sections and checking of the manuscripts: R. Fl~gel, G., K. Oetken and H. Gerdes, W. Golletz, A. Gr~nert, E. Johnston, M. and H. M~ller, I. Raether, V. Schostak, L. Tr~nkle. I e s p e c i a l l y want to thank J. Gifford for her p a t i e n t h e l p in t r a n s f o r m i n g this m a n u s c r i p t into readable English. Finally, I am indebted to Dr. Engel and S p r i n g e r V e r l a g for p u b l i c a tion in the Lecture Notes series. I w o u l d like to thank e v e r y b o d y who made this possible.

S U M M A R Y B i o l a m i n a t e d deposits, p r o d u c e d by m i c r o b i a l communities, were studied in m o d e r n p e r i t i d a l e n v i r o n m e n t s and in the rock record. The term microbial, mat refers to modern, the t e r m s t r o m a t o l i t e to ancient analogs. The t e r m b i o l a m i n a t e d d e p o s i t s was used to e n c o m p a s s b o t h microbial m a t s and stromatolites. M i c r o b i a l mat e n v i r o n m e n t s studied are the Gavish Sabkha, the Solar Lake, b o t h h y p e r s a l i n e b a c k - b a r r i e r systems at the Gulf of Aqaba, Sinai Peninsula, and the " F a r b s t r e i f e n - S a n d w a t t " (versicolored sandy tidal flats) on Mellum, an island in the e s t u a r y e m b a y m e n t of the southern N o r t h Sea coast. Three f a c i e s - r e l e v a n t categories were distinguished: (i) the m a t - f o r m i n g microbiota, (2) e n v i r o n m e n t a l conditions controlling mat types and lithology, (3) b i o t u r b a t i o n and grazing. Cyanobacteria a c c o u n t for b i o g e n i c sediment a c c r e t i o n in all cases studied. T h r e e m a j o r groups occur: filamentous cyanobacteria, coccoid unicells w i t h b i n a r y fission and those w i t h m u l t i p l e fission. In the p r e s e n c e of these groups the following mat types evolve: (i) continuously flat (stratiform) L ~ - l a m i n a e (occur i n all environments studied); (2) translucent, v e r t i c a l l y extended L v - l a m i n a e (only Gavish Sabkha and Solar Lake); (3) n o d u l a r granules (only Gavish Sabkha). Basically, the d e v e l o p m e n t of mats is c o n t r o l l e d by moisture. Thus h i g h - l y i n g parts w h e r e the g r o u n d w a t e r table runs m o r e than 40 cm b e l o w surface are b a r e of mats. These are: The circular slope and e l e v a t e d c e n t e r of the G a v i s h Sabkha, the shorelines of the Solar Lake and the e p i s o d i c a l l y flooded upper supratidal zone of M e l l u m Island. The following situations of w a t e r supply w e r e found to stimulate mat growth: (i) Capillary m o v e m e n t of g r o u n d w a t e r to exposed surfaces, (2) shallowest calm water, b o t h r e a l i z e d in the G a v i s h Sabkha and the Solar Lake. On M e l l u m Island, mats form in the lower supratidal zone, w h i c h is flooded in the spring tide cycle and w e t t e d during low tide by capillary groundwater. S a l i n i t y is almost that of normal seawater, w h e r e a s in the Solar Lake, it ranges from 45 °/oo to 180 °/oo and in the G a v i s h Sabkha, it reaches more than 300 °/oo. S a l i n i t y increase is c o r r e l a t e d w i t h rising c o n c e n t r a t i o n s of m a g n e s i u m and sulfate ions. In the Gavish Sabkha, episodic sheetfloods cause h i g h - r a t e sedimentation w h i c h is a c c i d e n t a l to the living mats. Episodic low-rate s e d i m e n t a t i o n stimulates the mats to grow through the freshly d e p o s i t e d sediment layer. This occurs p r e d o m i n a n t l y on M e l l u m Island due to eolian transport. W i t h i n the G a v i s h Sabkha, m i n e r a l o g y of sediments, c o m m u n i t y structures, standing crops, redox p o t e n t i a l s and p H are h i g h l y c o r r e l a t i v e to the i n c r e a s i n g evenness in m o i s t u r e supply w h i c h is r e a l i z e d b y the i n c l i n a t i o n of the s y s t e m b e l o w mean sea level. These conditions bring about a lateral sequence of facies types w h i c h include (I) siliciclastic b i o l a m i n i t e s at the coastal bar base, (2) nodular to b i o l a m i noid c a r b o n a t e s at saline mud flats, (3) r e g u l a r l y stratified stromatolitic c a r b o n a t e s w i t h ooids and oncoids w i t h i n the h y p e r s a l i n e lagoon, (4) b i o l a m i n a t e d sulfate t o w a r d t h e elevated center. High-magnesium calcite in facies type 3 p r e c i p i t a t e s around d e c a y i n g organic matter and forms also the ooids and oncoids. These occur p r e d o m i n a n t l y w i t h i n h y d r o p l a s t i c L v - l a m i n a e w h i c h p r o v i d e n u m e r o u s n u c l e a t i o n centers. W i t h i n the Solar Lake, facies type 3 (stromatolitic carbonates w i t h ooids and oncoids) is m o s t important, and grows to e x t r a o r d i n a r y thickness at the lake's shelf. The regular a l t e r n a t i o n of dark and light

VJ

laminae results from seasonally o s c i l l a t i n g w a t e r depths. These conditions couple b a c k over changing light and salinity intensities t o changing dominance structures of m a t - b u i l d i n g communities. Increasing salinity correlates w i t h d e c r e a s i n g w a t e r depth and a c c o u n t s for the relative a b u n d a n c e of coccoid unicells and diatoms, b o t h active p r o d u cers of e x t r a c e l l u l a r slimes (Lv-laminae). W a t e r depths locally or temporarily i n c r e a s e d favor surface c o l o n i z a t i o n by Mic~ocoleu8 chthonoplastes (Lh-laminae). The b i o l a m i n a t e d deposits of the v e r s i c o l o r e d tidal flats on M e l l u m Island are similar to facies type 1 of the Gavish Sabkha (siliciclastic biolaminites). D i f f e r e n c e s exist in the lithology: Sediments upon or through w h i c h the mats on M e l l u m Island grow are made up of clean sand. The grains originate p r e d o m i n a n t l y from re-worked glacial sediments and are rounded to well rounded. By contrast, the strong a n g u l a r i t y of s i l i c i c l a s t i c grains in the Gavish Sabkha clearly shows their status as p r i m a r y w e a t h e r i n g products. In all environments studied, insects p l a y a s i g n i f i c a n t role. M a i n l y salt b e e t l e s c o n t r i b u t e to the l e b e n s s p u r e n spectrum. There is no indication that b u r r o w i n g and grazing beetles and dipterans are detrimental to the growing mat systems. A c c o r d i n g to the m a r i n e fauna, two distributional barriers exist: (i) p h y s i c a l and (2) b i o g e o c h e m i c a l factors. Physical b a r r i e r s are (a) h y p e r s a l i n i t y and barrier-closing, w h i c h r e s t r i c t the m a r i n e fauna in the G a v i s h Sabkha and the Solar Lake to a few species, m a i n l y m e i o f a u n a l elements such as o s t r a c o d s and copepods. Only in the Gavish Sabkha, one m a r i n e gastropod species occurs w h i c h colonizes mud flats of lower salinity. A salinity barrier of about 70 °/oo separates the g a s t r o p o d h a b i t a t s from the zones of growing mats. Under reduced salinity, the snails are able to destroy the m i c r o b i a l mats completely. (b) D e c r e a s i n g r e g u l a r i t y of flooding in the m i c r o b i a l mat e n v i r o n m e n t of M e l l u m Island excludes intertidal d e f o r m a t i v e b u r r o w e r s such as cockles and lugworms. However, locally the mats are p i e r c e d by numerous d w e l l i n g traces. These stem from small polychaetes and amphipod crustaceans w h i c h are able to spread over the i n t e r t i d a l - s u p r a t i d a l b o u n d a r y and settle up to the MHWS-Ievel. Biogeochemical b a r r i e r s are oxygen d e p l e t i o n w i t h i n the sediments, high ammonia and sulfide contents, which generate through b a c t e r i a l break-down of organic matter. W i t h i n the h i g h l y p r o d u c t i v e mats of Mic~ocoleu8 chthonoplastes on M e l l u m Island, dwelling traces of marine p o l y c h a e t e s and a m p h i p o d crustaceans d i s a p p e a r due to these conditions. Microcoleus chthonoplastes, indiThe name of the m a t - f o r m i n g species, cates its c a p a c i t y to form "soils" (Greek chthonos). While lithology is not altered, the p r e s e n c e of Mic~ocoleu8 mats leads to a h a b i t a t change which excludes t r a c e - m a k i n g "arenophile" i n v e r t e b r a t e species and favors "chthonophile" species w h i c h do not leave traces. S t r o m a t o l i t i c m i c r o s t r u c t u r e s studied in rock specimens were interpreted using m o d e r n analogs: Microcolumnar buildups in P r e c a m b r i a n stromatolites, ooids and oncoids were compared w i t h those of modern microbial mats. The nodular to b i o l a m i n o i d facies type found in the Gavish Sabkha was s u g g e s t e d to be an analog to the P l a t t e n d o l o m i t e facies of Permian Zechstein, North Poland. Studies of the Lower Jurassic ironstone of L o r r a i n e clearly indicate that fungi h a v e b e e n involved in the formation of stromatolites, ooids and oncoids. In conclusion, the comparative study of m i c r o s t r u c t u r e s in m i c r o b i a l mats and stromatolites reveals a b e t t e r u n d e r s t a n d i n g in both fields. In m a n y cases, it was g e o l o g y w h i c h first revealed the s i m i l a r i t y of recent forms to those ancient ones and c o n s e q u e n t l y e n c o u r a g e d r e s e a r c h into them.

CONTENTS

PREFACE ............................................................ ACKNOWLEDGEMENTS ................................................... SUMMARY .% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I.

LAYERED SEDIMENT ACCRETION BY MICROBES INTRODUCTORY REMARKS .......................................... i.

2. II.

III IV V

TERMS i.i. 1.2. 1.3. THE

IN USE ............................................... Stromatolites and subsequent terms ................... Specific fabrics without direct evidence of microbes Biolaminated particles ~ ..............................

PROBLEM

OF

VERSATILITY

.

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

STROMATOLITE ENVIRONMENTS IN T H E P E R I T I D A L ZONE MODERN EXAMPLES ............................................... i.

THE GAVISH SABKHA A HYPERSALINE (GULF OF AQABA, SINAI PENINSULA) -

BACK-BARRIER SYSTEM ............................

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

3 3 5 6 9

13

15

1.1.

Introduction

1.2.

Methods

1.3.

Locality

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

18

1.4.

The physical environment ............................. 1.4.1. Geomorphic relief ............................. 1.4.2. Hydrology ..................................... 1.4.3. Temperatures ..................................

18 18 22 24

1.5.

Lithological framework ........................ ....... 1.5.1. Evaporites .................................... 1.5.2. Carbonates .................................... 1.5.3. Detrital clastics ............................. 1.5.4. Internal fabrics of sheetflood deposits .......

25 25 27 27 29

1.6.

Stromatolitic facies types ........................... 1.6.1. The microbiota ................................ 1.6.2. Major mat-s%ructuring organisms ............... 1.6.3. Character and distribution of stromatolitic facies types .................................. 1.6.4. Facies type-related biogeochemistry ........... 1.6.5. Products of early diagenetic processes ........ 1.6.6. In-situ formation of ooids and oncoids ........ 1.6.7. Microbially modified surface structures .......

30 30 31 34 42 45 49 53

1.7.

Faunal 1.7.1. 1.7.2. 1.7.3. 1.7.4. 1.7.5. 1.7.6.

55 56 56 58 65 66 68

1.8.

Modes

1.9.

Summary

.............................................. and

previous

work

influence on the biolaminated deposits ........ Species composition and distribution .......... Trophic relations ............................. Systematic ichnology .......................... Environmental zonation of trace categories .... Skeletal hard parts ........................... Grazing stress (experimental approach) ........ of

stratification and

conclusions

15 16

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

70

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

72

VIII

2. T H E S O L A R L A K E - I M P O R T A N C E O F S M A L L T E C T O N I C E V E N T S (GULF OF AQABA, SINAI PENINSULA) ..........................

.

75

2.1.

Introduction

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

75

2.2.

Locality

previous

76

2.3.

Bathymetric

2.4.

Sub-environments 2.4.1. The shelf 2.4.2. The slope

and facies types .................... ..................................... and bottom ..........................

78 78 84

2.5.

Lithologic and ichnologic framework .................. 2.5.1. C l a s t i c c o m p o u n d s ............................. 2.5.2. Evaporites .................................... 2.5.3. Ichnologic patterns ...........................

85 85 85 86

2.6.

Summary and conclusions .............................. 2.6.1. Occurrence of facies types compared to the Gavish Sabkha ................................. 2 . 6 . 2 . T i m e i n t e r v a l s r e c o r d e d in s t r o m a t o l i t e s ...... 2.6.3. Importance of small tectonic events ...........

87

and

zones

and

work

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

limnologic

cycle

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

3. V E R S I C O L O R E D TIDAL FLATS (MELLUM ISLAND, S O U T H E R N N O R T H SEA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .........................................

77

87 89 90

93

3.1.

Introduction

3.2.

Methods

3.3.

Locality and previous work ........................... 3.3.1. Recent sedimentological history ............... 3.3.2. General setting of Mellum Island a n d s t u d y a r e a .............. . . . . . . . . . . . . . . . . . . . 3.3.3. P r e v i o u s w o r k . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

95 97

3.4.

The physical environment of mat formation ............ 3.4.1. C l i m a t e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.2. F l o o d i n g f r e q u e n c y . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.3. Salinity ...................................... 3.4.4. Moisture ...................................... 3.4.5. Morphological unconformities ..................

97 97 98 98 98 99

3.5.

Sub-environments and facies .......................... 3.5.1. L o c a l d o m i n a n c e o f m a t - p r o d u c i n g s p e c i e s ...... 3.5.2. S t r a t i f i c a t i o n of living top mats ............. 3.5.3. Internal sedimentary structures ............... 3.5.4. S t a n d i n g c r o p s a n d b i o g e o c h e m i s t r y ............

i01 i01 102 105 106

3.6.

Fauna and ichnofabrics ............................... 3.6.1. Mixed marine-terrestrial composition .......... 3.6.2. Trophic types ................................. 3.6.3. Regional distribution of trophic types ........ 3.6.4. Life habits and ichnofabrics ..................

109 109 iii 114 114

3.7.

D o m i n a n c e c h a n g e a n d its i m p o r t a n c e f o r b i o t u r b a t i o n grades and patterns .................................. 118 3.7.1. Effects of increasing elevation ............... 118 3.7.2. E f f e c t s o f i n c r e a s i n g m i c r o b i a l p r o d u c t i v i t y .. 1 2 4 3.7.3 Promoting and limiting distributlonal factors 125

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

93 94 94 94

IX

3.8.

Intertidal-supratidal sequence ....................... 3.8.1. Change of sedimentary internal structures ..... 3.8.2. Change of sedimentary surface forms ...........

127 128 129

3.9.

Subaerial rise of biolaminated quartz-sand (experimental approach) ..............................

132

Summary

134

3.10.

4.

III.

WHAT

THE

4.

ENVIRONMENTS

HAVE

IN C O M M O N

.......

137

4.3.

Peritidal settings ................................... 4.3.1. The "sabkha cycle" ............................ 4.3.2. Temperate humid coastlines ....................

139 139 140

THE

141

"Purpose"

BETWEEN

organisms

REMARKS

Saltbeetles:

INTRODUCTION

pioneer

- FINAL

4.2.

GAP

as

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

Cyanobacteria

2. M E T H O D S 3.

conclusions

4.1.

SPANNING i.

and

of

dwelling

MICROBIOLOGY

AND

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

137

burrows

138

GEOLOGY

...........

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

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

143

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

DESCRIPTION

AND

INTERPRETATION

OF

FOSSIL

MICROSTRUCTURES

144 ...

145

3.1.

Precambrian Gunflint iron formation, Ontario ......... 145 3.1.1. Provenance of rock samples and previous work .. 1 4 5 3.1.2. Microstructures ............................... 146

3.2.

Permian Zechstein Plattendolomite of North Poland (PZ3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1. Provenance of rock samples and previous work 3.2.2. Microstructures ...............................

148 .. 1 4 8 148

3.3.

Lower Jurassic ironstone, Lorraine ................... 151 3.3.1. Provenance of rock samples and previous w o r k .. 1 5 1 3.3.2. Microstructures ............................... 152

3.4.

Summary

and

conclusions

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

PATHWAYS INVOLVED IN M I C R O B I A L SEDIMENT ACCRETION: A COMPLEMENTARY SUMMARY ....................................

REFERENCES

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

154

157

165

PART

LAYERED

-

INTRODUCTORY

SEDIMENT

REMARKS

I

ACCRETION

ON

TERMS

BY

MICROBES

AND

PROBLEMS

-

"The

name

tions

stromatolite

with

a fine,

structure,

in c o n t r a s t

of

...

oolites

transition tion

between

the

laminated

structures

products

comment

attributable

on terms

tic b a c k g r o u n d . also

to

these

generated

The general

sedimentary

I. i. S t r o m a t o l i t e s

KALKOWSKY patterns

(Table

and

i) with

same

gene-

in use led us

of

microbially

terms

the term s t r o m a t o l i t e

only the structure

of their

aggregates

KRUMBEIN,

Subsequently,

namely

thin

structures.

used

by r e f i n e d

paleomicrobiology

the

structures

of terms

on the t e r m i n o l o g y

by o r g a n i s m s

evidenced

stromatoid

of

cover one and the

lack of d e f i n i t i o n s

produced

more

to s e d i m e n t a r y

of m i c r o o r g a n i s m s

in rocks,

1983).

to

IN USE

terms

remarks

and s u b s e q u e n t

(1908)

is also a

The transi-

from a center point."

relating

that these

introductory

formation

1908)

to the a c t i v i t y

the aim of d e m o n s t r a t i n g

ooid-bag

independance

forma-

laminated

sense there

ooid and stromatoid.

polyooid,

I. TERMS

Here we will

flat

to the c o n c e n t r i c

increasing

(E. KALKOWSKY,

to c a r b o n a t e

or less

In a c e r t a i n

from ooid,

denotes

relates

more

refer

..."

1965;

the main

to

that

(translated

suggestion

KNOLL

layered

so small

of m i c r o p a l e o n t o l o g y

& TYLER,

were

to

"have been

is p r e s e r v e d

this v i s i o n a r y

methods

(BARGHOORN

that m i c r o o r g a n i s m s

which

by

was more and merging

& AWRAMIK,

framework builders

of

into 1983),

stroma-

~olites.

The rize

term Spongiostromata

fossil

stromata

crustose

include

origin was

growth

was

structures.

stromatolites

carbonate

introduced

as well

precipitation

by PIA

According

(1927) to PIA,

as oncolites,

by crustose

algae.

to

characte-

the Spongio-

and his

theory of

TABLE 1. Terms relating to m i c r o b i a l l y g e n e r a t e d layered structures and particles

Stromatolites Spongiostromata Algal sediments Cryptalgal fabrics Algal mats B l u e - g r e e n algal bioherms Microbial mats

LAYERED STRUCTURES FOSSIL AND RECENT: SYNONYMOUS TERMS

I.

Growth b e d d i n g FABRICS W I T H O U T DIRECT EVIDENCE OF MICROORGANISMS

II.

III. PARTICLES

Subsequent intertidal

(KALKOWSKY, 1908) (PIA, 1927) (BLACK, 1933) (AITKEN, 1967) (GOLUBIC, 1976) (RICHTER et al., 1979) (BROCK, 1976; KRUMBEIN, 1986) (PETTIJOHN & POTTER, 1964)

Fenestral fabrics Thrombolitic fabrics

(TEBBUTT et al. 1965) (AITKEN, 1967)

Oncoids Ooids

(HEIM, 1916) (KALKOWSKY, 1908)

studies of crustose algae in m o d e r n shallow subtidal environments

of the tropics and subtropics have

PIA's idea that calcareous algae have built stromatolites.

Accordingly,

terms created to designate modern analogs of stromatolites were sediments" or "algal mats".

Further terms used are

and b l u e - ~ r e e n algal b i o h e r m s since

and

supported

"algal

"cryptalgal fabrics"

"blue-green algae" were o b s e r v e d to

be most c o m m o n l y involved in stromatolite formation.

The term "blue-green algae" is the traditional b o t a n i c a l assignment. However,

t a x o n o m i c a l l y they are not algae but g r a m - n e g a t i v e l y reacting,

photosynthetic

bacteria.

Thus

group is now "cyanobacteria" 1979c;

RIPPKA et al.,

the t a x o n o m i c a l l y revised name of

(STANIER & COHEN-BAZIRE,

1977;

the

KRUMBEIN,

1979).

However, we should avoid the term "cyanobacterial mats" to d e s i g n a t e m o d e r n analogs of stromatolites

for two reasons:

i. A l t h o u g h many stromatolites are in fact p r o d u c e d via p h o t o s y n t h e tic a c t i v i t y of cyanobacteria, laminated greens"

rock

structures

are

These

structures

can also originate

organotrophic al.,

it seems important to stress that

1985;

bacteria DANIELLI

1981; KRETZSCHMAR,

not e x c l u s i v e l y

(DAHANAYAKE & KRUMBEIN,

& EDINTON,

1982; KRUMBEIN,

produced

from fungi 1985;

by

"blue-

and

chemo-

DAHANAYAKE

1983; D E X T E R - D Y E R et al., 1983).

wavy

et

1984; GYGI,

2. In

the

light of studies on m o d e r n l a m i n a t e d mats w h i c h

display

very complex b i o c o e n o t i c systems including n u m e r o u s groups of and n u m e r o u s m e t a b o l i c pathways, produced

by

diverse

microbes

it is a s s u m e d that s t r o m a t o l i t e s were

microecosystems

rather than

by

"monocultures".

C y a n o b a c t e r i a l and fungal components are often well p r e s e r v e d in matolites while in

due to their e x t r a c e l l u l a r sheaths,

other a s s o c i a t e d p h o t o t r o p h s and a n a e r o b i c h e t e r o t r o p h s

evidence

however,

(AWRAMIK et al.,

regulate the

1978;

KNOLL &

AWRAMIK,

The

i m p o r t a n t for the trapping and p r e c i p i t a t i o n of

biochemical

c a l c i u m carbonate,

1979a,

which

often

bacteria

magnesium,

copper,

iron,

manganese

1972; FRIED~tAN et al.,

1980; F E R G U S O N & BURNE,

1984; E C C L E S T O N et al.,

occur in a s s o c i a t i o n

with

which

support

the s u c c e s s i o n

salts

1973; KRUMBEIN,

1981; NOVITSKY,

1985; W E S T B R O E K et al., stromatolites.

and

is

minerals

Hence

and fungi are c o n s i d e r e d to be the main p r o d u c e r s of

substrate

are

a c t i v i t y of the a s s o c i a t e d b a c t e r i a

1969; MITTERER,

b; W I L S O N et al.,

LUCAS & PREVOT,

not

These,

"physicochemistry" of a mat system and thus

fundamental.

(KITANO et al.,

are

1983).

particularly e.g.

stro-

envelopes and capsules,

subsequent

1983; 1985), cyano-

organic

biochemical

a c t i v i t y of other bacteria.

A c c o r d i n g l y the term "microbial mat" is p r e f e r e n t i a l l y used today to denote m o d e r n analogs of s t r o m a t o l i t e s 1979;

BAULD,

1984; COHEN et al.,

(BROCK,

1984).

1976;

In their u n c o n s o l i d a t e d state,

m i c r o b i a l mats of varying c o m p o s i t i o n are also termed matolites"

(KRUMBEIN,

1983).

"potential

A s a t i s f a c t o r y d e f i n i t i o n of

mats has been given r e c e n t l y by K R U M B E I N

To

K R U M B E I N et al.,

microbial

(1986a).

finish the list of terms a s s o c i a t e d w i t h s t r o m a t o l i t e s and their

m o d e r n analogs we refer to the atlas of p r i m a r y s e d i m e n t a r y of

stro-

P E T T I J O H N & P O T T E R (1964),

who included stromatolites

structures inasmuch

as

they are "a type of growth bedding".

i. 2. S p e c i f i c fabrics w i t h o u t d i r e c t e v i d e n c e of m i c r o o r g a n i s m s

Upon specific

decay,

sediments can be devoid of m i c r o b i a l cell remains

patterns

but

such as fenestral and t h r o m b o l i t i c fabrics can indi-

cate s e d i m e n t a c c r e t i o n by microbes.

F e n e s t r a l fabrics in laminated m i c r o b i a l mats commonly generate gas bubble formation,

shrinkage and d e s s i c a t i o n

(MONTY,

from

1976). The term

"fenestra"

was

suggested by TEBBUTT et al.

p e n e c o n t e m p o r a n e o u s gap in rock frame work, interstices".

(1965) for a "primary

Fenestrae were found w i t h i n laminae of u n i c e l l u l a r cyano-

b a c t e r i a w h i c h possess usually a great p l a s t i c i t y due to large ties

of gel around cell colonies.

gel-supported

If in stratified mat

quanti-

systems,

the

laminae are sandwiched b e t w e e n laminae b u i l t of filamen-

tous microorganisms,

the voids are elongated,

ding plane and d e s c r i b e a laminoid p a t t e r n & TOSCHEK,

or

larger than g r a i n - s u p p o r t e d

follow the general bed-

(LF-A-type; M U L L E R J U N G B L U T H

1969). On the other hand, more e x t e n s i v e layers d o m i n a t e d by

unicellular irregular

organisms and their e x t r a c e l l u l a r slimes can also show arrangement

sedimentary

of fenestrae

(LF-B-type).

Laminated

an

patterns,

augen structures and lensoids as well as the formation

of

oncoids

and ooids in situ can be derived p h y s i c a l l y a c c o r d i n g

to

law

p a t t e r n formation in laminae of d i f f e r e n t v i s c o s i t i e s

(D'ARCY

of

THOMPSON,

1984).

Thrombolitic ments

the

are

fabrics

due

intergrowing

(AITKEN,

1967)

in m i c r o b i a l l y p r o d u c e d

to irregular d i s t r i b u t i o n of decaying

dead

colonies or internal d i s s o l u t i o n of mineral

around colonies of m i c r o o r g a n i s m s

(MONTY,

sedi-

colonies,

precipitates

1976).

i. 3. B i o l a m i n a t e d p a r t i c l e s

The name oncoid was suggested by HEIM les

(1916) for spheroidal

partic-

w i t h n o n - c o n c e n t r i c succession of more or less concentric

laminae

(FLUGEL,

1982). PIA (1927) regarded them as a subgroup of the Spongio-

stromata, above)

and

is

his

theory

of c a r b o n a t e p r e c i p i t a t i o n by

still in use today,

algae

w h i l e HEIM's suggestion was

(see

that

the

formation of oncoids w o u l d be due to the "aggressive activity of bacterial

colonies"

environments involved; around

we should,

nuclei

empty

spaces

activity"

Whether

Oncoids appear in both the fossil record

together with m i c r o b i a l mats. however,

(bioclasts,

consider whether mineral

m i c r o b i a l clots and lumps,

w o u l d be a b e t t e r i n d i c a t i o n

of

precipitates

lithoclasts)

"aggressive

or not ooids are of b i o g e n i c origin is still a The

name

was

or

bacterial

nucleus.

matter

suggested by K A L K O W S K Y for more

spherical or ellipsoidal grains w i t h uniform, a

modern

(i. e. b a c t e r i a l decay of the organic substrate).

controversy.

ting

and

C y a n o b a c t e r i a are commonly

for

or

less

concentric laminae

coa-

The use of the t e r m ooid is rather c o m p l i c a t e d since

it

is

u n d e r s t o o d to include at least two d i f f e r e n t

(FLUGEL, ooids;

1982).

Generally,

TEICHERT,

ooids

and

oolites

kinds

of

origin

(rocks consisting

of

1970) are studied w i t h the consensus that they origi-

nate in h i g h - e n e r g y environments.

However,

several modern mat environ-

ments show h o w it b e c o m e s p o s s i b l e to o b t a i n laminated p a r t i c l e s by the interaction

of

microbial

communities with a

physical

and

chemical

environment.

LUDWIG

&

THEOBALD

(1852) o b s e r v e d the formation of

concentrically

laminated coated grains in the thermal waters of Bad N a u h e i m w h i c h w e r e called

"Erbsensteine"

and oncoid

(HEIM,

i. e.

pisoids,

the terms ooid

(KALKOWSKY,

1916) being unknown at that time.

1908)

The authors noted

c y a n o b a c t e r i a - and d i a t o m - d o m i n a t e d m i c r o b i a l mats in an o p e n - a i r thermal

w a t e r course and r e c o g n i z e d the formation of coated grains

gas

bubbles,

m e t a b o l i c a l l y derived from the

mats.

authors noted that in fall the mat was degrading, release of the "Erbensteine",

around

Furthermore,

the

which resulted in the

and their d e p o s i t i o n d o w n s t r e a m in sandy

d e p r e s s i o n s as pisolites.

These, intimate

as

well as various other studies,

imply the

existence

genetic r e l a t i o n s h i p s b e t w e e n low energy e n v i r o n m e n t s and the

formation of ooids during p a r t i t i o n of m i c r o b i a l communities 1885,

ROTHPLETZ,

MITTERER,

1892, GIESENHAGEN,

1971). F A B R I C I U S

Bahama

ooids,

slimes

of microorganisms,

the

coatings

grains.

of

noticed

and

1922; SIMONE,

(WALTHER,

1981; FLUGEL,

1982;

(1977), w h e n studying the u l t r a s t r u c t u r e of

epilithic coatings of various

particles

with

immediate p r e c i p i t a t i o n of aragonite within

finally the

genesis

of

concentrically

laminated

He concluded that m i c r o b i a l p a r t i t i o n in the genesis of coated

grains is of p r i m a r y importance, while o v e r s a t u r a t i o n of the water with calcite,

The lites the

a g i t a t i o n and even nuclei supply are of secondary importance.

recent finding of m i c r o b i a l mats b u i l d i n g real domal w i t h i n the Bahama Bank e n v i r o n m e n t strengthens the

initiation

stromato-

argument

of ooid formation w i t h i n m i c r o b i a l mats also

for

of the

Bahama Bank ooid shoals.

In summary, should

we p r o p o s e that the genetic d e f i n i t i o n of the term ooid

imply b i o g e n i c i t y rather than abiogenicity.

We p r o p o s e further

that the term ooid should be p l a c e d into the g e n e t i c a l l y linked sequence

of

laminated s e d i m e n t a r y bodies and strata w h i c h

participation

of m i c r o o r g a n i s m s

(see KALKOWSKY,

form

1908).

under

the

The term ooid

(modi-

TABLE 2. C l a s s i f i c a t i o n of b i o l a m i n a t e d deposits and p a r t i c l e s fied after D A H A N A Y A K E & KRUMBEIN, 1986)

Laminated particles

L a m i n a t e d structures Single structure

Criteria

Assemblage

Single particle

Assemblage

Single Assemp a r t i c l e blage

Stromatolite

Ooid

Oolite

Oncoid

Stromatoloid rock

Ooloid

Ooloid rock

Oncoloid Oncoloid rock

GENESIS Biogenic

Stromatoid

A b i o g e n i c Stromatoloid

Irregular rounded

Concentric discontinous

Concentric continuous

Planar to conical

LAMINAT I ON

(**)

Regular rounded

Tabular, domed or columnar

MORPHOLOGY

(*)

(*)

Oncolite

For o o i d s / o o l o i d s larger than 2 m m in diameter the terms p i s o l o i d (pisolite/pisoloid rock) may be used

(**) For o n c o i d s / o n c o l o i d s oncoid/microoncoloid used

less than 2 mm in d i a m e t e r the terms micro(microoncolite/microoncoloid rock) m a y be

w o u l d then define a regularly concentric, ly

c o n c e n t r i c coated grain.

more

the term oncoid an irregular-

Finally the term stromatoid w o u l d include

or less s t r a t i f o r m lamina types

(Lh- and L v - l a m i n a e as

in this volume), h e m i s p h e r o i d structures

described

(like the d o m a l LLH- and sepa-

rate v e r t i c a l l y stacked SH-types c l a s s i f i e d by LOGAN et al., also BATHURST,

1971).

Logically,

t h o c h t h o n o u s or allochthonous)

rocks composed of ooids

are then oolites,

lites and of stromatoids are s t r o m a t o l i t e s

to

(e. g.

gical o r i g i n is not unequivocal. ted like"

the

suffix

(ooid-like,

"-oloid"

onco-

(1981) and BUICK

geyserites)

(1984) we

deposits

or

or w h e r e the biolo-

In this case the said authors sugges-

(OEHLER,

oncoid-like)

(either au-

of oncoids are

avoid the a b o v e - m e n t i o n e d terms if l a m i n a t e d

p a r t i c l e s are clearly a b i o g e n i c

1964; see

(Table 2).

F o l l o w i n g the suggestions of BUICK et al. propose

pisoid/

1972) w h i c h means a

appearance.

advice w i t h respect to the whole sequence

"stromatolite-

We p r o p o s e to follow

(Table 2).

this

2. THE P R O B L E M OF V E R S A T I L I T Y

Fig. in

1 has b e e n c o n s t r u c t e d to illustrate the p r o b l e m of v e r s a t i l i t y

m i c r o b i a l c o m m u n i t i e s w h i c h may leave a lasting record

sediments.

Laminated

types d o m i n a t e d by d i f f e r e n t major taxa m o o r g a n o t r o p h i c bacteria;

Fig.

(cyanobacteria,

zontally,

che-

There are filamen-

for example, w h i c h arrange themselves either h o r i -

(concordant to b e d d i n g planes)

Furthermore,

so

fungi or

p l a n a r l y or r a d i a l l y and leave b e h i n d lamina o r i e n t e d either

stratiformally

allows

ancient

IA). The w e a l t h of d i f f e r e n t taxa solely

in the group of c y a n o b a c t e r i a has to be considered. tous cyanobacteria,

in

or laminoid structures can derive from c o m m u n i t y

experience

or u p w a r d l y convex.

of p r e s e n t - d a y m i c r o b i a l mat

environments

us to state that life strategies of the m i c r o b e s c o n c e r n e d

varied

that

consistency.

they are able to develop over

substrate

of

are

varying

P r e s u m a b l y w a t e r is a v a i l a b l e p e r m a n e n t l y or periodically.

It

is t h e r e f o r e i n a d v i s a b l e to restrict the formation of stromatolites

to

any one p a r t i c u l a r e n v i r o n m e n t w h i c h w o u l d imply in

f a c i e s - i n d i c a t i v e role

(Fig. IB).

general

their

In the light of the great v a r i a b i l i t y

of l a m i n a - f o r m i n g microbes,

it is e s s e n t i a l to look c a r e f u l l y at other

available

and p a l e o n t o l o g i c a l

sedimentological

facies are concerned. nor

always d e p t h - d e p e n d e n t

5tASSARI,

1980).

environmentally taxa

Dominance induced.

(JANNASCH

&

WIRSEN,

sequence

is

organisms ment

also

1981;

The b i o l o g i c a l growth h a b i t of

is also important.

MONTY,

1977;

each

sediment particles.

made p o s s i b l e w i t h o u t

single

The process of burial

S e d i m e n t a t i o n is often

and m i c r o b i a l mats act as sticky fly papers and bind a l l o c h t h o n o u s

as

structures in m i c r o b i a l mats may be m a i n l y

in turn influences the m o r p h o g e n e s i s .

re-establishment

i n f o r m a t i o n as far

S t r o m a t o l i t e s are n e i t h e r n e c e s s a r i l y intertidal

(SHINN,

and

involved,

1983) w h i c h capture

Growth of the m i c r o b i o g e n i c sedimentation

as

migrating

o v e r r i d e others in order to find the most favorable environ-

(e. g. by phototaxis).

This sort of self-burial is typical and can

be seen in b o t h lacustrine and m a r i n e l o w - e n e r g y environments.

Burial gives rise to various kinds of p e n e c o n t e m p o r a n e o u s within

the

degrees

organic s u b s t r a t e

(Fig. IC):

of b a c t e r i a l d e c a y occur;

tion

and

to

diffusion,

shrinkage,

with

varying

some p o l y s a e c h a r i d e chains and com-

plexes are more r e c a l c i t r a n t than others;

place.

Micromilieus

processes

degassing,

d i s s o l u t i o n and

gas b u b b l e forma-

precipitation

The d i f f e r e n t p l a s t i c i t y of substrates and their b e h a v i o u r

c o m p a c t i o n has to be considered.

Faunal influence c o m p l i c a t e s

take due the

10

overall

situation in so far as feeding and excretion

gration,

and the spreading of microbial

tion of thrombolitic

lead to

disinte-

colonies may support the forma-

fabrics and intraclasts.

MARINE

NON-SEDM I ENTATO IN SUBAERA I LLYEXPOSED LOWSEDM I ENTATO IN

LACUSTRN IE SUBMERSED

PERO I DC I ALLYRAPID SEDM I ENTATO IN

TERRESTRA IL

I•ERIA

FUNGI

/

/

RADA I L,HOR¢ ZONTAL / VERTC I AL / /

~

J

/

OFDF IFERENTMAJORTAXA? ~

C. PENECONT~PORANEOUS

~

~k

~

b ~

C

~

~

EARLY DA I GENESS I (DEGRADATO I N,MN I ERALPRECIPITATION) BIOTURBATIONG , RAZN IG

COMPACTO I NOFSUBSTRATESOFDF IFERENTPLASTICITY Fig. i. Schematic representation relating stromatolitic structures and parameters possibly involved in their formation and early diagenesis. The concern of this scheme is to demonstrate that the appearance of a stromatolitic structure is indicative neither of one single phylum nor of one single environment.

11

The

unifying

principle within the complexity of stromatolitic

brics is that they are products of microbes which by their physiology and

and arrangement in time and space interact with a

chemical

environment

to produce a laminated

fa-

morphology,

pattern

physical (KRUMBEIN,

1983). This basic definition is irrespective of the existence of specific growth patterns (biostromate or biohermal buildups, ticles,

and

laminoid

fenestral

laminated par-

fabrics) which may be

explained

by

biotopic and microbiocoenotic as well as by physical modifications.

A key to the recognition of biotopic and biocoenotic characteristics encoded within sedimentary structures is the study of microbial mats in modern environments. matolites

may

conditions

A limitation of the actualistic approach to stro-

be that many present-day

1980;

KNOLL,

the

strial, mats

ecological

1985a). This consequently

explain why well developed thick stromatolitic sequences are

developed in the present than in the past. to

and

do not function at the same rate and level of efficiency as

in the past (REINECK & SINGH, may

depositional

However,

present-day extension of shelf flats and some lacustrine

and deep sea environments,

less

though restricted special

present-day

witness clearly the constancy of the biolaminite

terre-

microbial

tradition.

The

following chapters deal with stromatolite environments in the peritidal zone

which

apparently include types of potential

always had world-wide distribution.

stromatolites

that

Two of our main study areas are in

the semi-arid tropics and are part of the desert coast adjacent to

the

Gulf

others

are

North

Sea

of

Aqaba

supratidal coastal dies

graben system (Sinai Peninsula) while the

flats

of offshore embankments in the

southern

region which is located in the temperate-humid zone.

describe microfacies types,

stratification microbial structures

bio- and ichnofabrics and

The stumodes

which display depositional dynamics that interfere with

activity.

Comparing

the formative

environments

of

the important role played by climate and geomorphic

becomes evident.

of

these relief

PART

STROMATOLITE

ENVIRONMENTS

-

MODERN

II

IN

THE

EXAMPLES

PERITIDAL

-

ZONE

"Alle jene Gebiete,

w e l c h e ich auf der geolo-

gischen

'Salzthon'

babe,

Karte sind

als

nichts

weiter

ausgeschieden

als

eingedampfte

L a g u n e n und m e e r e n t b l ~ s s t e r Strand." WALTHER,

i. THE G A V I S H S A B K H A

-

(JOHANNES

1888)

A H Y P E R S A L I N E B A C K - B A R R I E R SYSTEM

(GULF OF AQABA,

SINAI PENINSULA)

i. i. I n t r o d u c t i o n

Facies

is the p r o d u c t of specific d e p o s i t i o n a l and b i o t o p i c

tions acting w i t h i n a certain e n v i r o n m e n t biotopic arid

c o n d i t i o n in the Gavish Sabkha,

tropics,

remarkably even are

stable in the annual cycle.

The a v a i l a b i l i t y of

flourish in the Gavish Sabkha

The

Gavish

strong the

in subsurface contact w i t h the

sea.

e v a p o r a t i o n is c o n s t a n t l y r e c h a r g e d by seepage

moistened

is mats

Water

of

sea

by

loss

by

seawater.

Thus

is p r o v i d e d w i t h p e r m a n e n t s h a l l o w - w a t e r environments mud flats.

is

salt swamp).

Sabkha is a t o p o g r a p h i c low separated from the

but

system

water

e n v i r o n m e n t if m i c r o b i a l

(sabkha is a t r a n s l i t e r a t i o n

the arabic term sabkhat or sebkat meaning

bar-closing

1958). A specific

a coastal e n v i r o n m e n t in the

is that the h o r i z o n t a l g r a d i e n t of surface m o i s t u r e

more critical than the h y p e r s a l i n e to

(TEICHERT,

condi-

These p r e v a i l i n g conditions can

be

and

interrupted

a l t h o u g h not always p e r m a n e n t l y changed by w i n t e r flashfloods.

The purpose of this chapter is threefold: of

topographic

sedimentary tolitic

on the d e v e l o p m e n t of

microbially

produced

structures w h i c h r e p r e s e n t analogs of conspicuous

structures

1985a), provide

moisture

(i) to document the effect

(2)

in the g e o l o g i c a l record

(see for

stromaKNOLL,

framework

which

further i n f o r m a t i o n about the e n v i r o n m e n t of deposition,

(3) to

interpret

the

tO d o c u m e n t the lithological and faunal

example

mode

of s t r a t i f i c a t i o n of the sabkha deposits

as

the

16

result

of

changes b e t w e e n long-lasting

conditions

fair-weather

and

short but catastrophic sheetflooding.

i. 2. M e t h o d s

Field

work was carried out from July to October 1981 and

to March 1982. bed

It focussed on the coring and d o c u m e n t a t i o n of undistur-

sediments,

fauna,

on

February

on the sampling of m i c r o b i a l mat m a t e r i a l and

benthic

m e a s u r e m e n t s of p h y s i c o c h e m i c a l p a r a m e t e r s and on the docu-

m e n t a t i o n of surface structures.

The

Gavish

Sabkha m i c r o b i a l mats were first studied by us

early summer of 1978.

in

the

At that time the p e r m a n e n t l y w a t e r - c o v e r e d parts

of the sabkha were floored w i t h e x t r a o r d i n a r i l y m u l t i l a m i n a t e d communities at

(KRUMBEIN et al., the

end

multilaminated loads

of

of

Our next

fully

about one year after the floods.

developed

At that time

(KRUMBEIN et al.,

Data

from

ion

1985).

mineralogy mats

In summary we present in

of

which

sabkha

lished

type 3),

crops

and pH in sediments, Undisturbed long, knife,

surface

permanently water-covered

(2) data from p o s t f l o o d

studies:

sediment distribution,

is

(GAVISH studies: waters,

microbial topographic

c o m p o s i t i o n and

already

reestab-

2 and 4), vertical profiles of redox p o t e n t i a l s

faunal distribution.

sediments

were

taken w i t h plastic

50/70 mm diameter). V e r t i c a l slices

of

of m i c r o b i a l communities w h i c h were

(facies type I,

not

sediments

(I) data from p r e f l o o d

r e l a t i o n to the salinity

m o i s t u r e and salinity gradients, standing

were

we adopt data from p r e f l o o d stu-

analyses and m i n e r a l o g y

of surface sediments,

(facies

with

1979).

seawater

concentrations

the

to interpret

type

p r e s e n t e d here were also obtained during the p r e f l o o d situation et al.,

one

reestablished

Thus,

stage of the m u l t i l a m i n a t e d mat

repeatedly recorded in core segments, dies

and

studies

the benthic systems of the p r e f l o o d p e r i o d were

a l t h o u g h new initial stages had already developed. the

1980

m i c r o b i a l mats of the l a g o o n a r y b a s i n were b u r i e d

terrigenous sediments and died off.

c o n d u c t e d in 1981, all

1979). Then two strong sheetfloods occurred,

of 1979 and the other at the b e g i n n i n g of

tubes

(200/400 mm

sections were made w i t h an electric

(45 x 45 mm) were separated and frozen under

shock

in

liquid oxygen. They were s u b s e q u e n t l y dried in a d r y - f r e e z i n g apparatus

17

and

later h a r d e n e d

REINECK, pared

1970).

and

were

examined

taken

(1962).

The

from

Chips

electron

critical

gold

Sections

tions

a dissecting

Pore

core m a t e r i a l

were

(SEM-Type fixed

Stereoscan

then d e h y d r a t e d

selected

- 6 %)

dilution

the samples

905,

photographs by

HAMBLIN scanning

Instruments).

at

appropriate

series

were

HY

were pre-

for

Cambridge

(2

in e t h a n o l / H 2 0

Subsequently

X-ray

described

180,

in g l u t a r a l d e h y d e

point dried.

sections

microscope.

of the sliced

of

the sliced

distribution.

water

0.63

was

carefully

organic

recovered

geochemistry

carried

sediments) Gulton),

work

measurements

and final-

sputtered

Optical

Microbial This work,

mat

with

samples were

as well

examined

collected

a 0.5 mm sieve and

sorted,

identified

with

fixed

Temperature

and

potential)

(air,

water,

(Tastotherm,

(Ingold)

on a

refractometrically

under

millimeasured

to b u r r o w

(150 mm high,

effect

of grazing

on m i c r o b i a l

graze on mat

measurements,

(HOLTKAMP,

sections

were

1985).

Samples were

and counted.

capra)

seawater

were

To study

lab-cultured

agar

which

was

To study the

(Pi~enella conica) were

w h i c h were b r o u g h t

salinity.

passed

Specimens

25 mm in diameter). gastropods

carried

structures

and s e m i - q u a n t i t a t i v e -

(Bledius

in jelly

mats,

at 50 O / o o

and SEM microscopes.

Her data on c o m m u n i t y

of salt beetles

tubes

seawater

light

the h e l p of specialists

behavior allowed

to

samples

isotope

thermoelement

in 4 % formaldehyde.

into glass

allowed

sediment

pH and redox

i00 mm high).

filled

treated with

was

the frac-

1985).

qualitatively

(190 mm diameter,

were

and

electrodes

are included

through

burrowing

with

w i t h E. HOLTKAMP.

fauna was

corers

specimens

sediments.

of

< 0.063 mm.

mineralogical,

& KRUMBEIN,

as the p h y s i c o c h e m i c a l

concentrations

The b e n t h i c

analysis

for

Salinity

study

into the

Instruments).

out in c o l l a b o r a t i o n and p i g m e n t

mm and

a chrome/nickel

and redox p o t e n t i a l

for the

were w e t - s i e v e d

(temperature,

cored

with

(Knick Portamess).

(American

with

for water

(FRIEDMAN

out in freshly

pH

selected

0.2 - 0.063

and s u b d i v i d e d

was m e a s u r e d

voltmeter

cores w e r e

The sediments

- 0.2 mm,

taken

Physicochemical

the

thin

as

> 0.63 mm,

were

ly

F with hardener

for SEM studies.

grain-size

were

under

(Araldit

specimens,

core m a t e r i a l

was

osmolarities,

resin

the sliced

microscopy

material

ly

in an epoxy

F r o m the h a r d e n e d

to

the

lab

and

18

i. 3. L o c a l i t y and p r e v i o u s w o r k

The

Gavish

Sabkha is located in the southern coastal area

Sinai Peninsula,

o p p o s i t e the Strait of Tiran,

34020 ' east longitude, Gulf of Aqaba widely

(Figs.

the

at 28 ° north latitude,

and is a p p r o x i m a t e l y in 400 m distance 2A and 2B). Shallow h y p e r s a l i n e

surrounded by a i r - e x p o s e d saline flats

setting

of

from the

surface w a t e r is

(Fig. 2C).

The

general

describes the Gavish Sabkha as a d e p r e s s i o n w i t h i n an alluvial

fan w h i c h spreads over the coastal plain b e t w e e n the Sinai Massive

and

the

the

shoreline

sabkha, of

of the Gulf

(Fig. 2B).

Further north and south of

the fan is crossed by m a j o r wadi conducts w h i c h on the o c c a s i o n

flash floods t r a n s p o r t t e r r e s t r i a l m a t e r i a l into the coastal

area,

the Gavish Sabkha and the Gulf.

Sea-marginal by C. G. The

sabkhas of the Sinai Peninsula were already

EHRENBERG

Gavish

(HEMPRICH & EHRENBERG,

Sabkha was first recognized by

g e o l o g i s t and geochemist.

were

followed

GAVISH,

GAVISH,

investigations

1971; G A V I S H et al.,

an

Israelian

(GAriSH,

1974,

1980;

1985). These p i o n e e r i n g studies

by studies of the m i c r o b i a l systems

1979; GERDES et al., E.

E.

(1888).

After the war in 1967, he began sedimentolo-

gical, geochemical and h y d r o l o g i c a l F R I E D M A N & GARISH,

mentioned

1828) and J. W A L T H E R

(KRUMBEIN

et

al.,

1985a; E H R L I C H & DOR 1985). To h o n o r the m e m o r y of

who died in 1981, the c o m p r e h e n s i v e results of interdisci-

p l i n a r y research on sea-marginal sabkha environments using the of the G a v i s h Sabkha were compiled

(FRIEDMAN & KRUMBEIN,

example

1985).

I. 4. The p h y s i c a l e n v i r o n m e n t

I. 4. i. G e o m o r p h i c relief

GAVISH et al. tal

bar

consists

softbottom platform. uplift

its

of an u p l i f t e d reef complex and

sediments

of

the sabkha rest on

Studies conducted by F R I E D M A N

(1965,

an

that

the

underlying

present backreef

1972) indicate that such

of reefs o c c u r r e d about 2,000 - 4,000 years

G A V I S H et al. and

(1985) p r o p o s e d that the b e d r o c k underlying the coas-

ago.

Accordingly,

(1985) suggest that the unique round shape of the sabkha

location w i t h i n the alluvial p l a i n could be the result

preexisting

t o p o g r a p h i c low in the u n d e r l y i n g reef p l a t f o r m w h i c h

s u b s e q u e n t l y uplifted.

of

a was

19

N ~

' ~ LAND

."1

EVAPORITI[

~...~__

'~

/'~ F °'~51-~i'_-~' i-~°~~~Z..~ . :.~. ~.-~.;

/ /

i

BEoou,,

SEA

i

STROP1ATOLITIIZ

i

[LASTI[

I

C

Fig. 2. Map of study areas and g e n e r a l setting of the G a v i s h Sabkha° A) Sinai P e n i n s u l a w i t h l o c a t i o n s of the G a v i s h Sabkha and the Solar Lake along the Gulf of Aqaba. M o d i f i e d after F R I E D M A N & KRUMBEIN, 1985. B) A v i e w t o w a r d W showing the round d e p r e s s i o n of the G a v i s h Sabkha at the shore of the Gulf of Aqaba. F r i n g i n g reefs are v i s i b l e at the bottom, foot hills of the Sinai m o u n t a i n s at the top. After F R I E D M A N & KRUMBEIN, 1985. C) I l l u s t r a t i o n of major g e o m o r p h i c elements of the Gavish Sabkha: coastal b a r slope, rims and b a s i n of the lagoon, elevated center. B a r - d i r e c t e d s i l i c i c l a s t i c s and c e n t e r - d i r e c t e d e v a p o r i t e s interfinger w i t h m i c r o b i a l mats w h i c h form at the lower part.

20

The

p r e s e n t - d a y e n v i r o n m e n t forms a round d e p r e s s i o n about 500 m in

diameter level).

and

is at its deepest part -1.80 m b e l o w

The

m.s.l.

(mean

sea

central part of the d e p r e s s i o n is gently elevated and

is

s u r r o u n d e d by a c o n c e n t r i c channel w h i c h is p a r t i a l l y w a t e r - f i l l e d . The circular slopes rise g r a d u a l l y from the channel upwards to the alluvial p l a i n and coastal bar facing.

Three major g e o m o r p h i c elements of the G a v i s h Sabkha can be guished: The b a r r i e r slope,

The

the lagoon and the center

b a r r i e r b l o c k i n g the d e p r e s s i o n has b e e n u p h e a v e d by

d i r e c t e d currents and waves.

lying bed rock and porous sediment infilling. covered

coastline

Seawater is r e p l e n i s h e d through subsurface

conduits formed by c a r b o n a t e c e m e n t a t i o n plates,

is

distin-

(Fig. 2C).

by dry evaporite crusts.

fissures in the under-

The surface of the slope

Several gullies cut through

slope face and merge at the lower end into sandlobes

(Fig.

2C).

the Sea-

w a t e r springs rise at the sandlobe junctions. The gullies are g e n e r a l l y 0.I0 to 0.40 m deep and tend to meander. result

of

sheet floods

(GAVISH,

Gullies and sandlobes are the

1980).

A wadi conduct runs

at

the

coastal bar plateau.

The lagoon is part of the concentric channel halfmoon-shaped surrounded impondments

by

(Fig.

2C).

b a s i n w i t h w a t e r depths of up to 0.60 m. shallow and p a r t i a l l y a i r - e x p o s e d flats

occur

It forms a The basin is

where

various

(about 50 mm deep and two to three meters in

diame-

ter) w h i c h are fed from seawater springs.

The elevation of the center is about 0.50 m above the w a t e r table of the lagoon.

GAVISH p r o p o s e d that g y p s u m a c c u m u l a t i n g b e l o w the surface

caused the "swelling" of sediments and

gradually

u p h e a v e d the center.

Aerial views show three s e d i m e n t a r y plains sloping g e n t l y to NNE 2B):

(i.) The topmost part,

(2.)

the slope w i t h its e x t r e m e l y gentle incline

(Fig.

which is covered by dry evaporite crusts, towards NNE and

(3)

the rim w h i c h is s u r r o u n d e d by the c o n c e n t r i c channel. Surface m o i s t u r e g r a d u a l l y increases toward the lagoon. several and

holes,

Along the slope and the rim are

e x c a v a t e d b y fisher b e d o u i n s

in order to collect brine

to h a r v e s t the p r e c i p i t a t i n g potash and NaCI.

months

the

higher

w a t e r table of the lagoon.

sediments

of the rim are submersed due

and leaves w h i t e g y p s u m crusts.

During to

the

winter

the

slightly

In summer the w a t e r level

retreats

21

IiH

p0; 12°°

Ii Ii [L

March t982 I I

~

II

0

r-GF--~

LB

~

SMF

~ SL

~ A ? \ G U ELEVATED CENTER SW

A O0 h G N GF

~

SMF ~

GF = Gypsum Fiats LB = Lagoonery Bo.sin SMF= Satine Mud Fiats SL = Sand{obes

A

BAR h

i \ I~

~

~ ~':~~'~:,~'~'~':''! ............... .:~

m r-2

GAVISH SABKHA SURFACE WATER }ULF

SAUNITY INCREASE+

SMF

u

ION CONCENTRATIONS (ppm)

GAVISH SABKHA SURFACE WATER

I LB I

/

£ ~ B r i n e Reflux

ION RATIOS (MOLAR).

~o GF

t

~ e

LNFE[~I

SL

+-- . . . . .

SALINITY INCREASE (

GF- L8 I

S.F

i

IS

~{

3ULF ppm 20 000 10 000

4~ @

4

I

.2

t

;000 "E

I 000

3.4

so#L~

I cE! so4=

-~

.0.2 42.4

300

TOTAL SALINITY (%o]

Fig. 3. Hydrology, salinity and seawater chemistry along a horizontal transect crossing several sub-environments of mat-formation: Sand lobes (SL), saline mud flats (SMF), lagoon (LB) and gypsum flats (GF). A) Generalized horizontal transect showing the connection of the Gavish Sabkha with the Gulf. Hydrodynamic mechanisms are indicated by arrows: Seepage, evaporation and brine reflux. Salinity (upper diagram) increases towards the elevated center. B) Ion concentrations (right) and ratios (left) in surface water including Gulf water (salinity values on abcissa right from GAVISH et. al, 1985; abbreviations refer to sub-environments listed in A). Decrease in Ca and change in ion ratios take place on saline mud flats bordering the lagoon where total salinity is about 85 0/oo.

22

1. 4. 2. H y d r o l o g y

Two

m a j o r effects on the h y d r o l o g i c a l

be distinguished: physical include

(1) physical

system of the Sabkha have

to

factors o p e r a t i n g from the land and

(2)

factors o p e r a t i n g from the sea.

Those o p e r a t i n g from the sea

seawater supply and w a t e r level changes w i t h i n the sabkha

to tidal m o v e m e n t s and w e a t h e r effects re-directed against

w i n d drift,

the

coast,

(e. g.

monsoon)

outside.

w h i c h o c c a s i o n a l l y causes strong w a v e

does not affect the b a r - p r o t e c t e d Gavish

due

Onshoattack Sabkha.

Thus the h y d r o l o g i c a l p r o c e s s e s d e s c r i b e d b e l o w remain b a l a n c e d even if high energy conditions occur in the a d j a c e n t Gulf.

Tidal tidal

influence.

Gulf

range of 0.7 m.

tides are semi-diurnal w i t h a

the Gavish Sabkha at a reduced rate w i t h time delay. of

mean

pools and the b a s i n of the lagoon. is seen in

normal

(up

directly

to

The o p e r a t i o n of these t i d e - i n d u c e d

i0 cm) is common in winter.

This

phenomenon

related to the tidal m o v e m e n t but to seasonal

et al.

1985).

is

variations

communities which colonize these margins.

however,

not in

(monsoon;

Both tidal- and c l i m a t e - i n d u c e d fluctuations of

the w a t e r table do not lead to c o n s i d e r a b l e disturbances of the bial

of

A sudden rise in the w a t e r table greater than

the sea level of the Gulf as a response to climate conditions GAVISH

marginal

the shifting of w a t e r l i n e s at the m a r g i n s

the ponds and the basin.

of

Diurnal m o v e m e n t s

w a t e r levels of about 20 mm h a v e b e e n observed w i t h i n the

fluctuations

annual

This tidal m o v e m e n t affects the w a t e r level

Some faunal

micro-

elements,

w h i c h are restricted to a i r - e x p o s e d w e t l a n d habitats, have to

react by m i g r a t i n g according to the shifting water lines.

Seawater ecological of

the

seepage.

A

significance.

constant supply of seawater This is achieved by

Gavish Sabkha and (ii) the process

(sensu HSU & SIEGENTHALER,

produce

surface seawater

to

respects

salt from seawater.

of

pumping"

formerly used by hill

They c o n s i s t e d of

through gently inclined pipes into

system

greatest

"evaporative

people

a pan w i t h a

volume ratio w h i c h was s u c c e s s i v e l y fed by

running

hydrological

of

1969). The c o m b i n a t i o n of seepage and evapo-

ration is analogous w i t h the b o i l i n g - p a n s to

of

is

(i) the b a s i n m o r p h o l o g y

low the

the Gavish Sabkha differs from

rates basin.

this

in

high of The two

(i) the heat needed to evaporate the water generates from sun

irradiation

and

(ii) the natural pan of the

Gavish

Sabkha

consists

w i d e l y of exposed sediments where the w a t e r table is b e l o w the surface. In these areas "evaporative pumping"

operates

(Fig. 3A).

23

The e v a p o r a t i o n rate is about 4.6 m/year. 30

Relative h u m i d i t y averages

- 50 % w i t h a m e a n annual air t e m p e r a t u r e of 26 °C

h i g h solar irradiation. v e r t i c a l l y and sediments by

P e r c o l a £ i n g seepage seawater sinks into the

and elevates the w a t e r table, evaporation.

w h i c h is lowered s u b s e q u e n t l y

This stimulates upward m o v e m e n t

of

water

the p h r e a t i c zone by e v a p o r a t i v e p u m p i n g and results in a

stant

supply of ions n e c c e s s a r y for mineral

capillary (PURSER,

The

movement

formation.

The

in turn leads to the lateral m i g r a t i o n

vertical of

fluids

lateral m o v e m e n t induced by e v a p o r a t i v e p u m p i n g may not operate

Dhabi

sabkha

(PURSER,

G a v i s h Sabkha, however, water

1985).

as seen,

for example,

The d o w n w a r d i n c l i n a t i o n

in the of

p l a t e s e m b e d d e d in the u p h e a v e d bar

conduits sabkha

the

stimulates the lateral m o v e m e n t of i n t e r s t i t i a l

w i t h i n the s e d i m e n t a r y system as well as the lateral

supply

seepage s e a w a t e r through p e r m e a b l e sediments of the m a r i n e bar. nate

con-

1985).

w h e r e sabkha water tables rise landwards, Abu

constantly

The e v a p o r a t i o n p u m p i n g m e c h a n i s m o p e r a t e s b o t h

laterally.

capillary

within

and

(Fig.

of

Carbo-

2C) p r o b a b l y serve

as

for the seeping seawater. T h e i r gentle i n c l i n a t i o n towards the can

be seen at outcrops t e r r a s s i n g some of the

gullies

which

cross the inward slope of the coastal bar.

Besides the p r e c i p i t a t i o n of evaporite m i n e r a l s at the interfaces by evaporative pumping, posed

to

gypsum)

reflux of brines to the sea is pro-

be an important m e c h a n i s m of mineral

in deeper layers

sediment-air

(GARISH et al.,

accumulation

1985).

(mainly

Local d o l o m i t i z a t i o n

has also been suggested as an outcome of brine reflux,

a l t h o u g h several

other models have b e e n proposed.

S a l i n i t y regimes. A h o r i z o n t a l 340 °/oo,

is

established

s a l i n i t y gradient,

ranging from 50 to

along a t r a n s e c t w h i c h runs from the

slope face of the bar towards the center of the sabkha

The data in Fig.

3A were o b t a i n e d from m e a s u r e m e n t s of interstitial

water

in the gully and sandlobe sediments and of standing surface

ters.

Although

and spring 1982), of

salinity

the data r e p r e s e n t only two annual states they may n o n e t h e l e s s

zones is more or less stable t h r o u g h o u t

1980; G A V I S H et al.,

1985,

wa-

(summer 1981

indicate that the e s t a b l i s h m e n t the

year.

a s s u m p t i o n was r e i n f o r c e d by G A V I S H on several earlier visits 1975,

lower

(Fig. 3A).

see also Fig. 3B).

This

(GAVISH,

24

Short-term

oscillations

the lagoon where wind in the overall conspicuous

can flush-over

stability

change

of salinity

occur

concentrated

of the salinity

in

at the immediate

microbially

regime

produced

brine.

rim

of

This d e v i a t i o n

is r e f l e c t e d

structures

by a

(see

very

section

1.6.3).

Ion c o n c e n t r a t i o n s of

GAVISH's

dissolved SO 4

data

taken

by

(Fig.

CaSO 4

enriched

is

while

depleted

cite highly

in bulk

are enriched

(29 wt.%). reduced

with

concentrations

Mg ++

salinity

similar

a recalculation

to

of

: Ca ++ and Ca ++ measurements our

:

ob-

measurements

to the salinity

in-

lies at 80 to 90 °/oo w i t h i n

the

increase

shows

in p a r a g r a p h

so that b a c t e r i a l

sulfate

takes

a nearly

linear

of the inner

lagoonary

in high m a g n e s i u m

As is shown

is a c c o m p a n i e d

in the surface water,

sediments of the

of salinity

since

on the other hand,

the sediments

concerning

correlates

of C a + + - c o n c e n t r a t i o n s

ratio,

dy

which

Further

Ca++/SO 4 = ratio decreases,

M g + + / C a ++

lagoon,

3B).

3B shows

3A).

of C a + + - c o n c e n t r a t i o n s

flats

by the decrease the

1985)

are quite

up to a solution b a r r i e r

saline mud

ly

(Fig.

Fig.

in surface waters,

These data

later

increase

crease

et al.,

The data are c o r r e l a t e d

GAVISH.

four years

The

(GAVISH

c a l c i u m and sulfate

= ratios.

tained

in surface waters.

calcite

sulfate

of

the

Ca ++ is alrea-

(50 wt.%) these

reduction

The

increase.

shoreline

basin where

1.6.4.,

accordingover.

and

cal-

sediments

interferes

are with

CaSO4-precipitation.

Flashflood passing after

impacts.

the system rainfall

transport

for

Sabkha.

A strong of

evaporation months

in

to

sheetflood

pumping regenerate

Sabkha

in this

area

is i0 nun. Rainfall

is more or less n e g l i g i b l e mountains

freshwater.

the h y d r o l o g i c a l

the Gavish

rainfall

the adjacent

sediment-laden

quences

level

Annual

immediately

and e c o l o g i c a l

in O c t o b e r

completely

(GAVISH et al.,

run-off

sheetfloods

which

tremendous

conse-

conditions

1979 caused

of about 60 cm.

s y s t e m was

causes

Such events have

while

of the

a rise

The steady disturbed.

state It

Gavish

in the water of

took

the five

1985).

i. 4. 3 . ~ T e m p e r a t u r e s

A stable influences

zonal t e m p e r a t u r e - g r a d i e n t of wind,

irradiation

was not o b s e r v e d

and e v a p o r a t i o n

due to changing

operating

in the daily

25

cycle.

In summer,

between

day

nightly

drop

low-water tures

air t e m p e r a t u r e s

and night

temperatures

in air t e m p e r a t u r e

environments

and a n i g h t l y

ring c a p a c i t y

reach

however,

saltcrusts.

ranging

of s h a l l o w - w a t e r

range b e t w e e n

is,

and b e l o w

drop

45 °C and more

between

Data

of

GAVISH

vels. The

The

levels

which

and e v a p o r a t i v e

results

are p r e s e n t e d

g e n t l y dip to NNE

VI

graded

describe

strates

(i) the

relief

above

ration

pumping

(Fig.

different

the buffe-

calculate

4 in the course

level

le-

of a transect: of the center

IV indicates

the b o t t o m of

Sabkha.

(i.

The levels

bar

described

slope.

e. the e l e v a t i o n

of seawater

V

The

b e l o w demon-

(2) the e f f e c t i v e n e s s

recharge

the

niveau

plains

of the coastal

of g e o m o r p h o l o g y

seepage

used to

forming m i n e r a l s

table)

The

in shal-

lower daily tempera-

at d i f f e r e n t

sedimentary

elevations

the g r o u n d w a t e r and

were

part of the G a v i s h

of in-situ

influence

1985)

The

30 °C.

framework

in Fig.

4A).

and

remarkable

crusts.

(0 to -5 cm)

the three

is the deepest

distribution

annual

et al.,

sediments

I to III indicate

the lagoon w h i c h and

(GAVISH

of surface

Here,

20

6 and 8 °C indicate

1. 5. L i t h o l o g i c a l

composition

less

and d i f f e r e n c e s

of the

of

evapo-

prevailing

in

the

cycle.

i. 5. i. E v a p o r i t e s

a) H a l i t e

(Fig.

4B).

High

is over 80 %) a c c u m u l a t e ment

surfaces

other m i n e r a l s water at

(levels

indicates

(level

layer w i t h an

average

frequency

relative

height, levels

at III,

level

the

lutitic

upper

4A)

thickness

(Fig.

4C).

Anhydrite

(levels

The a s s o c i a t i o n

of g y p s u m dehydration,

1 m

It covers

(see p a r a g r a p h gradually

with

of h a l i t e while

layer

a gypsum

c

below).

decreasing the

lower

4B).

occurs

I and VI),

at

to

marine

of the h a l i t e

is 25 - 50 % while (Fig.

sedi-

of h a l i t e

of upward m o v i n g

20 cm.

decreases

frequency

unflooded

abundance

The thickness

of about

of h a l i t e

permanently

averages

B the bulk volume

dry crusts

fraction.

conditions

interface.

(the relative

relative

evaporation

IV and V it is n e g l i g i b l e

b) A n h y d r i t e in

This

the total

I in Fig.

The

of h a l i t e

at the elevated,

I and VI).

at the s e d i m e n t - a i r

the center

amounts

together

with halite

and there m a i n l y and a n h y d r i t e

the f r e q u e n c y

in

only the

may indicate

of a n h y d r i t e

in

26

HALITE

ANHYDRITE

GYPSUM

CARBONATES

DETRITAL CLASTICS

75 - D o %

~

50- 75 %

25-50 %

~

0-25%

Fig. 4. A b u n d a n c e of evaporites, c a r b o n a t e s and detrital clastics in surface sediments at d i f f e r e n t e l e v a t i o n levels of the G a v i s h Sabkha. A) Aerial view from E towards W to show e l e v a t i o n levels: I -III: Upper, m e d i a t e and lower parts of the center (white area = gypsum), IV: Lagoon, V - VI: Lower and upper parts of the coastal b a r slope. B) H a l i t e is in greatest a b u n d a n c e at the most elevated levels I and VI. C) A n h y d r i t e is g e n e r a l l y a s s o c i a t e d w i t h h a l i t e (levels I and VI). D) G y p s u m a c c u m u l a t e s at lower levels (mainly level III, see also A). E) C a r b o n a t e s make up 50 to 70 wt.% of sediments of the lagoon (mgc a l c i t e dominant, a s s o c i a t e d w i t h calcite, aragonite, dolomite). F) S a n d - s i z e d s i l i c i c l a s t s d o m i n a t e at lower parts of the coastal bar. G) F i n e r - g r a i n e d detrital clastics g e n e r a l l y occur at all levels.

27

the

lutitic

fraction indicates that it may be a p r i m a r y

precipitate.

The hot and dry climate makes both p r o c e s s e s possible.

c) Gypsu m (Fig. 4D). to

the

Here,

biological

tary

The only zone w h e r e g y p s u m rarely

b u l k volume of sediments is the lagoon

plain

contributes

(level IV in

Fig. 4D).

sulfate r e d u c t i o n is m o s t active. W i t h i n the sedimen-

III in Fig. 4D,

close to the lagoon,

w h i c h is already part of the

center

the bulk volume of g y p s u m averages nearly

but

i00 %,

thus indicating that the growth must be faster than the a c c u m u l a t i o n of clastic

sediments d e p o s i t e d by wind.

The r e l a t i v e f r e q u e n c y of g y p s u m

decreases w i t h increasing h e i g h t w h i l e in turn h a l i t e becomes more more

dominant

(compare the upward d i r e c t e d d i s t r i b u t i o n

to level I in Fig. 4B). gypsum over

B e l o w the h a l i t e crust of the e l e v a t e d center,

is the most a u t h i g e n i c mineral. 1 m,

the

(GAVISH et al.,

W i t h an average

thickness

layers reach well b e l o w the g r o u n d w a t e r table

surface

(Fig. 4E)

C a r b o n a t e s are m o s t frequent in the surface sediments of the its

(70 %),

margins. while

abundant

of

1985).

i. 5. 2. Carbonates

and

and

from level III

The

m a j o r components are

dolomite

carbonate

averages about 5 %.

c o m p o n e n t of the

clastic

calcite

and

lagoon

Mg-calcite

Aragonite,

w h i c h is

an

sediments

outside,

is

m o s t l y a minor c o m p o n e n t or c o m p l e t e l y absent w i t h i n the sabkha.

We

will

this

return to the q u e s t i o n of in-situ carbonate

highly

h y p e r s a l i n e m i l i e u w h e n referring to the

accretion

in

development

of

b i o l a m i n a t e d sediments and m i c r o b i a l habitats.

i. 5. 3. D e t r i t a l clastics

Terrigenous

clastic sediments in b u l k volumes of 0 - 25 % are mixed

w i t h the in-situ forming minerals. and

are

type).

Coarser grains are w i t h o u t

s u p p o r t e d by m a t r i c e s of g y p s u m or c a r b o n a t e mud

contact

(wackstone-

They r e p r e s e n t fragments t r a n s p o r t e d by n o r t h e r l y and s o u t h e r l y

winds w h i c h b l o w several times in the year. The p r o g r a d a t i o n a l t e n d e n c y of

the

center sloping gently to NNE

(indicated in

Fig.

4A)

m o s t l y due to successive s e d i m e n t i n f i l l i n g by s o u t h e r l y w i n d s 1985; GAVISH et al.,

1985).

may

be

(PURSER,

28

Compacted bottoms

and

sphericity,

surface

layers of terrigenous

sandlobes

4F).

The grains are

which suggests p r i m a r y provenance

from w e a t h e r i n g

cambrian granitic

(level V in Fig.

clastics occur at the gully

rocks of the adjacent Sinai Massif

(Fig.

of of

low Pre-

5A and 5B).

Fig. 5. Sheetflood deposits (thin sections from sediment cores). A) Layer of grain-flow deposits between two microbial mat generations. Scale is 1 cm. B) Texture of g r a i n - f l o w deposits showing badly sorted and rounded particles. Scale is 500 pm. C) Inverse grading of sheetflood sediments with water escape trace (or p o s s i b l y escape trace of insect larva). Scale is 1 mm. D) Sediments resulting from slow sinking deposition from suspension consist of silt, clay, plant debris and some coarser siliciclastic fragments. Scale is 250 pm. E) Sheetfloods also transport seagrasses with epiphytic forams. Scale is 1 mm.

29 Since

we carried out our field work a p p r o x i m a t e l y two years after

last strong sheetflood, exception

of

evaporites, cause and

the

the

we found all other s e d i m e n t a r y plains w i t h the

g u l l y b o t t o m s and sandlobes already

carbonate mud or m i c r o b i a l mats.

debris to flow t h r o u g h a

re-covered

That strong

by

sheetfloods

wadi system on top Of the coastal

bar

to spread over the w h o l e of the l o w e r - l y i n g region is evidenced by

several layers of c o a r s e - g r a i n e d m a t e r i a l w i t h i n the b a s i n sediments of the lagoon.

Silt/clay

fractions

(Fig. 4G)

derive from

suspension

clouds

in

freshwater w h i c h fill the d e p r e s s i o n d u r i n g sheetfloods.

1. 5. 4. I n t e r n a l fabrics of s h e e t f l o o d d e p o s i t s

a) Deposits

resulting

from debris flow:

These are

poorly

sorted

m e d i u m - to c o a r s e - s i z e d quartz sand c o n c e n t r a t i o n s w h i c h imply a gravit y - i n d u c e d lateral m o v e m e n t of debris loads in w a t e r Inverse

grading

(Fig. 5C).

is

Around

r e c o r d e d w i t h i n several the

(Figs. 5A and 5B).

siliciclastic

c i r c u l a r slope of the

sabkha

t h i c k n e s s of the debris flow deposits exceeds several dm° slope direction, central basin,

their thickness decreases. beds of debris

sequences

depression

the

In the down-

In sediment cores from the

flow range in size from a few mm to some

cm.

b) C o n c e n t r a t i o n s of silt, and

some coarser s i l i c i c l a s t i c

tions

clay, p l a n t debris, fragments

some foram skeletons

(Fig. 5D):

result from slow sinking d e p o s i t i o n

These

concentra-

from suspension.

The inter-

spersed coarser s i l i c i c l a s t i c fragments w i t h o u t contact indicate transport

in the c l a y - w a t e r fluid phase.

Haloph~la sp. of

the

forams

The intermixed plant detritus

stems from the m a n g r o v e swamps and lagoons to the

Gavish Sabkha in the order of tens (determined as So,ires

sp.

by L.

of

kilometers.

HOTTINGER)

are

t r a n s p o r t e d with seagrass leaves into the G a v i s h Sabkha

Beds

of

silt and clay are rarely d i s t r i b u t e d around

slope

but

are

c h a r a c t e r i s t i c of the sediments of the

where

the

c l a y - w a t e r fluid phase comes to

s u s p e n s i o n are a few mm to several cm thick.

rest.

The

of

north

Epiphytic

mechanically

(Fig. 5E).

the

circular

central deposits

basin from

30

i. 6. S t r o ~ t o l i t i c

facies types

i. 6. i. The m i c r o b i o t a

Various cipate

species of p r o c a r y o t i c and eucaryotic m i c r o o r g a n i s m s parti-

in forming b i o g e n i c structures in the Garish Sabkha

Since taxonomic d e t e r m i n a t i o n s are still uncertain, to the genera of the organisms found.

3).

we will refer only

Several of these genera are pre-

Oscillato~ia, echococcus, Thioc~ps~, Nitzschia, Navicula). sent w i t h more than one species

(Table

(e. g.

Spirulina,

Syn-

TABLE 3: M i c r o o r g a n i s m s in the microbial mats of the Gavish Sabkha

I.

Procaryotes A. U n i c e l l u l a r c y a n o b a c t e r i a

Gloeothece, Synechococcus, Johannesbaptistia, Gloeocapsa, Synechocystl8, Myxosarcina, Pleunocapsa, Chroococcodiopsis B. F i l a m e n t o u s C y a n o b a c t e r i a

Spirulina, oscillatoria, LPP-formsl: Microcoleus, Hydrocoleum, Phormidium, Lyngbya, Plectonema, Schizothrix C. A n o x y p h o t o b a c t e r i a

Chromatium, Thiocapsa, Ectothiorhodospira,

Chlonoflexus

D. C h e m o l i t h o a u t o t r o p h i c b a c t e r i a

Thiobacillus,

Bsggiatoa, Desulfovibrio) 2

E. C h e m o o r g a n o t r o p h i c b a c t e r i a

Pseudomonas, Spirillum, Spirochaeta, Proteus, Desulfovibrio, s°-reducing and other taxa II. P h o t o s y n t h e t i c eucaryotes Diatoms: Mastogloia, Navicula,

Amphora, Nitzschia 3

1 "LPP"-grouping refers to Rippka et al. (1979). LPP stands for the genera Lyngbya, Phormidium and Plectonema which are r e p r e s e n t a t i v s for structural and p h y s i o l o g i c a l c h a r a c t e r i s t i c s of the LPP-Group. M i c r o c o l e u s should be incorporated in LPP 2 P h y s i o l o g i c a l attributes m e n t i o n e d are only p a r t i a l l y significant 3 Only frequent forms;

for more details see Ehrlich & Dor (1985).

31

I. 6. 2. Major mat-structuring organisms

Types structure capsulated

of primary producers which give the Gavish Sabkha mats are

(i) heavily ensheathed

filamentous

their

cyanobacteria

unicellular cyanobacteria with multiple fission (3)

(2)

slime-

ensheathed cyanobacteria with binary fission.

(i) The main

ensheathed filamentous cyanobacteria present is Micro-

coleus chthonoplaste8

(Fig.

6A). It is a cosmopolitan species found in

Fig. 6. Main mat-structuring microorganisms (modified after EHRLICH & DOR, 1985). Scale is i0 pm for all presentations. A) Ensheathed filament bundles of Microcoleu8 chthonoplastes. B) Pleurocapsalean cells encased in polysaccharid capsules. C) Syneehocysti8 sp. D) Gloeothece sp. The latter both represent coccoid unicells in colloidal matrix. E) Resulting depositional structures: Colonies of M. chthonoplaste8 produce horizontally oriented laminae (Lh) , Pleurocapsalean colonies form cauliflower-shaped nodules, SynecHocystis and Gloeothece contribute to porous, slime-enriched layers containing bubbles and some diagonally to vertically oriented filamentous organisms (Lv).

32 various environments e. g. RING et al.,

1983),

al.,

Solar Lake,

1980),

streifen-Sandwatt, 1942; 3).

Laguna Mormona,

Multiple

Egypt

Australia

Mexico

(KRUMBEIN,

1985b, c; STAL,

(BAULD,

1984; SKY-

(STOLZ, 1983; M A R G U L I S et 1978;

southern North Sea coast

GERDES et al.,

which

Spencer Gulf,

COHEN,

(OERSTEDT,

1984), Farb-

1841; HOFFMANN,

1985; see also this part chapter

e n s h e a t h e d filament bundles are typical of this

are rarely if ever found in a diagonal or v e r t i c a l

species

arrangement.

M i c r o b i a l mats d o m i n a t e d by this species reveal a smooth and u n i f o r m i l y flat

microtopography.

h o r i z o n t a l l y layered, Lh-lamina

vertical sections the mat appears as

a

b e d d i n g plane c o n c o r d a n t lamina w h i c h we call

Within

a

(Fig. 6E; see facies type 1 and 3 in p a r a g r a p h 1.6.3).

(2) The most common capsulated u n i c e l l u l a r c y a n o b a c t e r i a w i t h multiple fission is Pleurocapsa sp. Pleurocapsalean cased

by

(formerly Entophys~lis sp.). Species of

form cell colonies where each individual cell

thick concentric lamellated sheaths

(Fig.

6B).

is

A

en-

crucial

d i f f e r e n c e from Microcoleus chthonoplastes is that the coccoid colonies do not form flat and b e d d i n g - p l a n e c o n c o r d a n t mats but reveal d i s c o n t i nuous,

more

sediment

or

less c o n c e n t r i c structures.

surfaces

The

microtopography

w h i c h are c o l o n i z e d by P l e u r o c a p s a l e a n

of

populations

exhibits a p u s t u l a r structure. C a u l i f l o w e r - l i k e nodules are also common at

sediment surfaces

(Fig.

6E;

see also facies type 2

in

paragraph

1.6.3). (3) The

m o s t common s l i m e - e n s h e a t h e d u n i c e l l u l a r c y a n o b a c t e r i a w i t h

b i n a r y fission are Gloeothece sp.

and Synechocysti8 sp.

(Figs. 6C and

D). These organisms account for the vast p r o d u c t i o n of p o l y s a c c h a r i d e s . Sediments

composed

of or interwoven with

sluggish and y o g h u r t - l i k e if

liquid

(e. g.

these

polysaccharides

seawater)

is

maintained,

due to the dispersal and p a r t i a l d i s s o l u t i o n of the mucilage. of

are

the organisms are i r r e g u l a r l y a r r a n g e d in the slime mass

The cells (MARTIN

&

WYATT,

1974). The species m e n t i o n e d c o n t r i b u t e p r e d o m i n a n t l y to immense

slime

layers

supported from

found in the Gavish Sabkha sediments.

coccoid

Since the

mats form v e r t i c a l l y e x t e n d e d layers

which

the flat and b e d d i n g - p l a n e c o n c o r d a n t L h - l a m i n a e of the

leus mats,

we call them L v - l a m i n a e

slimediffer

Microco-

(Fig. 6E; see also facies type 3 in

p a r a g r a p h 1.6.3). These order

unicellular

organisms

increase their

slime

production

in

to escape p h o t o t o x i c conditions when they form the surface mats.

Slime p r o d u c t i o n is also stimulated by

increase in s a l i n i t y and tempe-

33

rature

(CASTENHOLZ,

Sabkha.

1984).

Photosynthetically

All these

conditions

active populations

occur in the Gavish

of other species

(e.

g.

Mierocoleu8 ehthonoplastes) under the translucent mat benefit from

the

production

for

of large quantities

the channelling

of gel since it is an ideal m e d i u m

of light.

~

,' "~I

ELE

•

'

EVAPORITIC'

1

SEA I~

,~i,,

ULLY

~5~-~-

STROHATOLITIC ~

-

./~

•../

ELASTIE-'--'~

Cat6onates Evoporiles Detritol clastics

Lh- laminae, horizontal orientation [ ~

Lv-laminae,vertlcally extended

['~

Ooids and oncoids Pleurocopsoleon nodules

Fig. 7. Local d i s t r i b u t i o n of stromatolitic facies types. The sequence A) to D) correlates to increasing salinity. The p o s s i b i l i t y of superficial water increases from A) to C) and decreases again in type D). A) Siliciclastic biolaminites (gully bottoms and sandlobes). B) Nodular to b i o l a m i n o i d carbonates (saline mud flats). C) Stromatolitic carbonates with ooids and oncoids (lagoon). D) Biolaminated sulfate (gypsum flats).

34

i. 6. 3. C h a r a c t e r and d i s t r i b u t i o n of s t r o m a t o l i t i c facies types

A of

supply of m o i s t u r e to surface sediments where the initial microbial

Gavish

In

Sabkha m o i s t u r e at or close to the s e d i m e n t a r y surfaces

function of topography.

accumulation

(Fig. 4),

e. g. salinity

patterns

tures.

environments,

va-

An infor-

Their

are d e s c r i b e d in the following sections in

structures,

Community

a

(Fig. 3) and in-situ

o v e r v i e w of the lateral sequence is given in Fig. 7.

community

is

this g r a d i e n t will be used as the

riable to d e s c r i b e the lateral d i s t r i b u t i o n of facies types.

vidual

the

Since the t o p o g r a p h i c m o i s t u r e g r a d i e n t p a r a l -

lels other f a c i e s - r e l e v a n t factors, mineral

mal

growth

mats occurs is essential for their development.

indi-

terms

s e d i m e n t a r y textures and

of

struc-

structure is defined h e r e as the v i s u a l l y o b s e r v a b l e

p r o d u c t of species s e l e c t i o n and d o m i n a n c e at a certain place.

i__t. S i l i c i c l a s t i c b i o l a m i n i t e s

This

facies

is composed of quartz sand and

interlayered

biolami-

The b i o l a m i n i t e s characterize the Lh-type w h i c h is Microcoleus-

nites.

dominated Where

(Fig. 8 )

(Fig.

laminae

6A).

Thickness of laminae differs from 50 to 500 Nm.

are thin

(about 50 pm),

they form a

monolayered

mat.

Thicker laminae comprise several generations of mat development,

aided

by

(Fig.

sufficient surface m o i s t u r e during periods of n o n - d e p o s i t i o n

8A).

Environments slope

are the gully bottoms and sandlobes along the

(Fig. 7).

surface.

Here

Vertical

depend on

the g r o u n d w a t e r table runs -5 to -i0

barrier cm

m o v e m e n t s of the g r o u n d w a t e r table b e t w e e n 2 - 5 cm

the external tidal m o v e m e n t in the Gulf.

Sediment

surfaces

are p e r m a n e n t l y w e t t e d by c a p i l l a r y m o v e m e n t of the g r o u n d w a t e r rative

pumping).

ment-air water

Thin evaporite crusts

interfaces.

table

surfaces.

(mainly gypsum)

During w i n t e r time,

form at

(evaposedi-

o s c i l l a t i o n s of the ground-

lead o c c a s i o n a l l y to the i n u n d a t i o n

Salinity

below

of

the

of interstitial w a t e r is 45 - 60 °/oo.

sedimentary No extreme

shifts occur during the annual cycle.

Sediments are m a i n l y t e r r i g e n o u s and consist of b a d l y sorted to

g r a v e l - s i z e d s i l i c i c l a s t i c sand.

(Fig. 8B). These sediments o r i g i n a t e

The grains are of low from debris-flow.

medium

sphericity

35

Fig. 8. D o c u m e n t a t i o n of facies type i: S i l i c i c l a s t i c b i o l a m i n i t e s . A) X - r a y r a d i o g r a p h of a s e d i m e n t core showing m i c r o b i a l mats in 3 cm d e p t h and at the surface. Scale is 1 cm. B) C l o s e - u p of s i l i c i c l a s t i c sediments showing m i c r o b i a l coatings of sediment p a r t i c l e s (center). Thin section. Scale is 1 mm. C) C l o s e - u p of b i o l a m i n i t e s showing m e m b e r s of the mat community: sheathed bundles of Mic~ocoleus chthonop~as#es, diatoms and filamentous sulfur bacteria. SEM-photography. Scale is 50 pm.

community

structure

(Fig.

8C):

The b u l k of the laminae is made of

e n s h e a t h e d b u n d l e s of M~c~ocoleu8 chthonoplastes. Few u n i c e l l u l a r genera

of

c y a n o b a c t e r i a and diatoms are a s s o c i a t e d w i t h

Mic~ocoleus

the

mat. The mat d e v e l o p s -2 to -5 mm b e l o w the w a t e r surface or u n d e r n e a t h evaporite

crusts.

Both s i l i c i c l a s t i c sediments and

support l i g h t - c h a n n e l l i n g

2. N o d u l a r to b i o l a m i n o i d c a r b o n a t e s

This

term

(nodules)

granular,

cauliflower-shaped

e m b e d d e d in w i d e - s p a c e d b i o l a m i n o i d s

(a

w h i c h defines a less s i g n i f i c a n t l y laminated b u i l d - u p of b i o g e n i c

sediments). and

crusts

(Fig. 9 )

facies type is c h a r a c t e r i z e d by

microbial aggregates

evaporite

for p h o t o s y n t h e t i c activity.

The embedding m a t e r i a l consists of intermixed calcite

mucilaginous

sheaths

of

polysaccharides.

Various tubular relicts

filamentous c y a n o b a c t e r i a are visible around

the

of

mud empty

nodules

(Fig. 9A). T r a n s i t i o n a l stages towards laminoid a r r a n g e m e n t s of smaller

36

granulae exhibit facies

and

filaments can be seen (Fig. 9B)°

a mammillate mierotopography

Surfaces of

(Fig. 9C).

The p a t t e r n

the

mats

of

this

type is stimulated by constant shifts of salinity and depth

of

water.

Environment.

The n o d u l e - b i o l a m i n o i d

facies type dominates

flats just beyond and in b e t w e e n the sandlobes flats

saline mud

(Fig. 7). The saline mud

border the lagoonary basin and are p a r t i a l l y built over

by

the

37

Fig. 9. D o c u m e n t a t i o n of facies type 2: N o d u l a r to b i o l a m i n o i d carbonates (saline mud flats at the lagoon's outer rim). A) I n t r a s e d i m e n t a r y nodules e m b e d d e d in c a r b o n a t e mud. A filigrane m e s h w o r k of d i a g o n a l l y to v e r t i c a l l y o r i e n t e d filaments is visible. Dark sediments at top: reduced. Scale is 500 pm. B) I n t r a s e d i m e n t a r y t w i n - n o d u l e and b i o l a m i n o i d structures (note w a v y d i a g o n a l dark line in the right upper corner). Scale is 2 mm. C) M a m m i l l a t e surface m i c r o t o p o g r a p h y , c h a r a c t e r i s t i c of the noduleb e a r i n g zone. Scale is 1 cm. D) Larger nodules sampled at the surface of small w a t e r - f i l l e d puddles. Scale is 2 cm. E) S E M - p h o t o g r a p h y of a colony of n o d u l e - f o r m i n g c y a n o b a c t e r i a (Pleurocapsalean). Scale is 3 pm. F) S E M - p h o t o g r a p h y of nodule c o m p a r t m e n t s showing capsules of former cells. Note radial a r r a n g e m e n t of compartments. Scale is 3 pm. G) D i s s e c t e d nodule showing radial a r r a n g e m e n t of compartments, the empty center and i r o n - r i c h p i g m e n t s around the cortex. Scale is 2 mm. Figs. A, B and G thin sections from sediment cores.

elevated

sandlobe

"bars"

gullies into the lagoon.

of coarse sediment w h i c h p r o j e c t

tions and feeds the g e n t l y downwards w a t e r film. Lower s a l i n i t y at

sloping mud flats w i t h a trickling

(Ente~omo~pha

sp.). Micro-

bial mats do not d e v e l o p here. The seepage w a t e r a c c u m u l a t e s

into

and

the

drains from these e m b a y m e n t s through

central basin.

the junc-

s e a w a t e r springs b e t w e e n 50 - 70 °/oo is

m a r k e d by a thick scum of b e n t h i c m a c r o a l g a e

embayments

from

Seepage seawater merges at the sandlobe

S a l i n i t y increases w i t h

from 70 to 180 O/oo.

in shallow

narrow

passages

distance

seawater

springs

Some very shallow

(maximum

w a t e r cover i0 cm) are subject to s h o r t - t e r m

from

the

embayments

(commonly

diur-

nal) changes of w a t e r cover and air exposure due to changing conditions of

e v a p o r a t i o n and wind velocities.

observed extreme °/oo.

Short-term salinity

ranging from 70 to 150 °/oo. amplitudes

Other

HOLTKAMP

shifts

were

(1985) r e c o r d e d

ranging w i t h i n a few minutes b e t w e e n 130

more

and

embayments w i t h w a t e r levels b e t w e e n 20 and 40 cm

240

remain

w a t e r - f i l l e d during diurnal and annual cycles. The salinity ranges from 70 to 120 °/oo. Flats around the embayments are usually a i r - e x p o s e d and covered

by

thin e v a p o r a t i o n crusts.

Salinities here can reach up

to

180 °/oo.

S e d i m e n t s are m a i n l y c o m p o s e d of f i n e - g r a i n e d carbonates. G r a i n size analyses

of surface sediments show 38 wt.%

and 30 wt.% 6.3 - 20 pm. saccharides

and