Modern and Ancient Lake Sediments (IAS Special Publication 2)

Modern and Ancient Lake Sediments

Modern and Ancient Lake Sediments Edited by Albert Matter and Maurice E. Tucker © 1978 The International Association of Sedimentologists. ISBN: 978-0-632-00234-4

Modern and Ancient Lake Sediments EDITED BY ALBERT MATTER AND MAURICE E. TUCKER

Proceedings of a symposium held at the H.C. 0rsted Institute, University of Copenhagen, 12-13 August 1977. Sponsored by the International Association of Sedimentologists and the Societas Internationalis Limnologiae

S PE C I A L P U B L I C AT I O N N U M B E R 2 O F T H E I NTERNAT I O NA L A S S O C I AT I O N O F S E D IMENTO LO G I ST S . P U B L I S H E D B Y B LA C KWELL S C IE N T I F I C P UB L I C A T I O N S O X F O RD L O N D O N E D I N B U R G H M E L B OU RN E

© 1978 The I nternational Association of Sedimentologists Published by Blackwell Scientific Publications Osney Mead, Oxfoord 8 John Street, London WC I 9 Forrest Road, Edinburgh P.O. Box 9, N orth Balwyn. Victoria. Australia All rights reserved. No part of this publication may be reproduced, stored in a retrieval system. or transmitted, in any form or by any means. electronic, mechanical, photocopying. recording or otherwise without the prior permission of the copyright owner.

First published 1 978 British Library Cataloguing in Publication Data Modern and ancient lake sediments.---{International Association of Sedimentologists. Special publications; No. 2) I . Sediments (Geology)-Congresses 2. Lakes-Congresses I. Matter, Albert I I. Tucker, Maurice E. Ill. Series 551.4'82 QE47 1 .2 ISBN 0-632-00234-4 Distributed in the U.S.A. by Halsted Press, a division of John Wiley & Sons, Inc., New York Printed and bound in Great Britain by Burgess & Son Ltd Abingdon, Oxfordshire

Contents

Introduction A lbert Matter and Maurice E. Tucker

7

Saline lakes and their deposits: a sedimentological approach Lawrence A . Hardie, Joseph P. Smoot and Hans P. Eugster

43

Late Pleistocene-Holocene evolution of the Kivu-Tanganyika Basin Peter Stoffers and Robert E. Hecky

55

Holocene carbonate evolution in Lake Balaton (Hungary): a response to climate and impact of man German Muller and Frank Wagner

81

Permian Saar-Nahe Basin and Recent Lake Constance (Germany): two environments of lacustrine algal carbonates Andreas Schafer and Karl R. G. Stapf

107 Origin of the carbonate sediments in the Wilkins Peak Member of the lacustrine Green River Formation (Eocene), Wyoming, USA Joseph P. Smoot

127 Late Neogene sedimentation in the Black Sea Kenneth J. Hsii and Kerry Kelts

145 Turbidites and varves in Lake Brienz (Switzerland): deposition of clastic detritus by density currents Michael Sturm and Albert Matter

167

Lacustrine facies in the Pliocene Ridge Basin Group: Ridge Basin, California Martin

187

H.

Link and Robert

H.

Osborne

Lacustrine sedimentation in an evaporitic environment: the Ludian (Palaeogene) of the Mormoiron Basin, Southeastern France Georges True

203 Triassic lacustrine sediments from South Wales: shore-zone clastics, evaporites and carbonates Maurice E. Tucker

223

Permo-Triassic lacustrine deposits in the Eastern Karoo Basin, Natal, South Africa D. E. van Dijk, D.

K.

Hobday and A . v

J.

Tankard

vi

Contents

239 Subaqueous clastic fissure eruptions and other examples of sedimentary transposition in the lacustrine Horton Bluff Formation (Mississippian), Nova Scotia, Canada Reinhard Hesse and Harold G. Reading

257 A Proterozoic lacustrine interlude from the Zambian Copperbelt Harry Clemmey

277 Economic significance of playa lake deposits C. C.

Reeves Jr.

Spec. Pubis int. Ass. Sediment. ( 1 978) 2, 1-6

Modern and ancient lake sediments: an introduction

A L B E R T M A T T E R andM A U R I C E E . T U C K E R Geol. Jnst. Universitiit Bern, Sahlistrasse 6, CH-3000 Bern, Switzerland and Department of Geology, The University, Newcastle upon Tyne NEJ 7R U,

U. K.

Research on lake sediments really began towards the end of the last century. In Europe, lakes in Switzerland received the attention of scientists from the mid­ nineteenth century, with early work concerned with water circulation and chemistry, and then later with the lake sediments themselves. Research by Forel ( 1 886- 1 892) demonstrated the presence of channels and levees on the Rhone delta in Lake Geneva. These Forel attributed to the underflow of sediment-laden river-water. In the early part of this century Nipkow ( 1920, 1 928) sampled the bottom sediments of Lake Zurich and, although primarily interested in the flora, described rhythmically laminated sediments ('non-glacial varves') consisting of a summer calcareous layer and a winter organic-rich layer. He also noted thick detrital layers (turbidites) which could be correlated from core to core. Work on true varves forming in proglacial lakes also goes back to the last century, although interest has largely been focused on their chronologie and stratigraphic use (e.g. De Geer, 19 1 2). In North America, classic work was carried out by Russell ( 1 885) and Gilbert ( 1 890) on the Pleistocene lakes Lahontan and Bonneville. The shore features of Lake Lahontan with their evidence for an ancient deep lake were first recognized in 1 858/9 by Henry Englemann, geologist to an expedition crossing the Carson Desert region. Russell ( 1 885) described the whole Lake Lahontan basin, with its beach terraces, bars, spits and tufas, and erected a stratigraphy of the lake deposits. He was able to demonstrate two deep-lake periods with an intervening period of complete lake desiccation. Russell's interpretation of the lake history has generally been proved correct by later work (Morrison, 1 964). It was to be many years, however, before a detailed description was published of the Great Salt Lake, one ofthe remnants of Lake Bonneville. Eardley's ( 1 938) work on the Great Salt Lake sediments is still one of the few detailed accounts of modern lacustrine carbonates with which ancient lacustrine limestones can be compared.

Modern and Ancient Lake Sediments Edited by Albert Matter and Maurice E. Tucker © 1978 The International Association of Sedimentologists. ISBN: 978-0-632-00234-4

2

A lbert Matter and Maurice E. Tucker

Studies of lake sediments, like those of Fore! mentioned above, and then later studies of sedimentation in reservoirs, such as Lake Mead, Colorado (Grover & Howard, 1 938; Gould, 195 1), have had an important bearing on the evolution of the concept of turbidity currents. The latter of course are an important sediment transport mechanism in continental-margin situations and in marine basins and troughs. Of all ancient lake sequences, the one which has received most attention, has the most extensive literature and is invariably quoted in comparisons, is the Eocene Green River Formation of Wyoming, Utah and Colorado, U.S.A, representing the deposits of lakes Uinta and Gosiute. Classic work was published by Wilmot Bradley from the 1920's onwards (e.g. 1929, 193 1 , 1 964) following reconnaissance mapping of the area by U.S. Geological Survey officers in the late 1 800s. Bradley envisaged a relatively deep, permanently stratified lake, particularly during deposition of rhythmically laminated oil shales in the basin centre. Recent investigations, however, have shown that a playa-lake model is more applicable (Eugster & Surdam, 1 973; Eugster & Hardie, 1 975 and also Smoot, 1978). A notable feature of the Green River Formation is the great range of evaporite and authigenic minerals present (Milton & Eugster, 1 959; Bradley & Eugster, 1969) many of which are unique to this formation. One of the first rock sequences to be interpreted as lacustrine is the Middle Old Red Sandstone (Devonian) of Caithness, N.E. Scotland. The Caithness Flagstones, deposited in Lake Orcadie, have long been famous for the fossil fish they yield, particularly in the Achanarras Limestone. These rocks were initially described in 1829 by Sedgwick & Murchison, and then later by Geikie ( 1 878) and Crampton & Carruthers ( 1 9 1 4). Apart from the fauna, the lacustrine interpretation was also based on the dark grey, carbonaceous and finely-laminated nature of the flagstones, which contrasts markedly with the red conglomerates, sandstones and siltstones that characterize the continental Devonian elsewhere. It is interesting to note that in the later studies of the Orcadian lake sediments (Rayner, 1963; Donovan, 1 975) comparisons were made with the findings of Nipkow ( 1 920, 1 928) from Lake Zurich. Significant slopes around Lake Orcadie are indicated by slumped and brecciated horizons in the flagstones. Crampton & Carruthers ( 1 9 1 4) noted the absence of beach deposits and used this as an argument in favour of a lacustrine interpretation. Recent work, however, has recognized lacustrine shore-line clastics (Donovan, 1 975), as well as stromatolites (Fannin, 1969; Donovan, 1 973). The latter were fairly accurately described by Crampton & Carruthers but interpreted as subaerial tufas. There has been an increasing interest in lake sediments in the past decades with many papers dealing with modern examples in various parts of the world, but relatively few concerned with ancient lakes (see for example references in Picard & High, 1 972). At the International Sedimentological Conference in Nice ( 1 972), it was realized that a number of people are actively working on lake sediments. It was therefore proposed to hold a special symposium on this subject, under the auspices of the International Association of Sedimentologists. The proceedings of that sympo­ sium, held in Copenhagen August 1 2- 1 3, 1 977, coinciding with the International Limnological Congress (S.I.L.), constitute this Special Publication. In inviting contributions to this book, the editors have attempted to cover most sedimentological aspects of lakes, from clastic to chemical and biochemical, and from modern to ancient. Aspects of lake sediments not covered in this book include those of an engineering geological nature, such as the silting up of dammed lakes, and those of environmental importance, such as the poisoning of lake sediments and waters through industrial pollution.

Introduction

3

The book begins with a review paper on arid-zone lakes by Hardie, Smoot & Eugster. In Saline lakes and their deposits: a sedimentological approach the physical and chemical conditions of sedimentation are discussed. Various subenvironments are defined and the characteristic features of their sediments presented. It is then shown how the sequence of facies can be used to interpret the history of the lake basin. Low latitude lakes in Africa are described in Stoffers and Reeky's paper on the Late Pleistocene-Holocene evolution of the Kivu- Tanganyika Basin. The variations in diatom assemblages and mineralogy (Mg-content in carbonates, pyrite-siderite) are described from cores collected in Lake Kivu. The variations are related to the stratification history of the lake, which was controlled by climatic changes of the last 14,000 years. Contemporaneous volcanic activity in the form of sublacustrine springs is also considered important. The control of climate (and later man) on sedimentary mineralogy is also demonstrated by Mtiller and Wagner in Holocene carbonate evolution in Lake Balaton (Hungary) : a response to climate and the impact to man.

When Lake Balaton had no outlet in Pre-Roman times, low Mg-calcite was precipitated during periods of high-water level and low rates of evaporation, whereas protodolomite and high Mg-calcite were precipitated during periods of low-water level and high evaporation. Since the construction of an outlet for the lake by the Romans, calcite has been precipitated periodically at times of algal blooms. Continuing with lacustrine carbonates, Schafer and Stapf combine the modern and ancient in their Permian Saar-Nahe Basin and Recent Lake Constance (Germany) : two environments of lacustrine algal carbonates. The oncolites of Lake Constance are of two main types, rough and smooth surfaced forms, which are reflections of water depth and the algal community. Two notable features are the preservation of algal filaments within the precipitated calcite and the various fabrics of the algal balls which can be related to the blue-green algal genera. The Permian oncolites show a greater diversity of shape and size, but are distinguished from the Lake Constance examples by the lack of preserved algal filaments. In Origin of the carbonate sediments in the Wilkins Peak Member of the lacustrine Green River Formation (Eocene), Wyoming, U.S.A. by Smoot the carbonate is chiefly peloidal intraclastic dolomite. Smoot reasons that the carbonate is not a lacustrine precipitate, but was derived from disintegration of surface crusts, tufas and caliches which developed on exposed mudflats, in stream beds and on alluvial fans around the lake. These subaerial dolomites were then transported as clasts into the lake by sheet flows following rainstorms. Deep-sea drilling results are reported by Hsii and Kelts in Late Neogene chemical sedimentation in the Black Sea. The sediments are chiefly calcitic chalks, laminated in the lower part, again like Nipkow's carbonate varves of Lake Zurich. It is thought that the carbonate mineralogy was controlled by salinity, with horizons rich in dolomite being formed in one instance when a shallow salt lake existed, and in another through sabkha diagenesis. Siderite, occurring at two levels, is thought to be a direct precipitate at a time of high dissolved-iron input. Deeper water clastic sediments are described by Sturm and Matter in Turbidites and varves in Lake Brienz (Switzerland) : deposition of clastic detritus by density currents. The turbidity currents are generated through river inflow at high stages and high rates of sediment supply. They deposit graded beds with scoured bases on the central basin plain. During normal river stages when the density of the inflowing river water is lower, interflows and overflows lead to the formation of rhythmically-laminated ('varved') sediments below the thermocline. Homogeneous sediments accumulate on the shore terrace and upper slope above the thermocline.

4

A lbert Matter and Maurice E. Tucker

Link & Osborne describe one of the thickest known lacustrine sequences (over 1 2 k m thick) in Lacustrine facies of the Pliocene Ridge Basin Group: Ridge Basin, California. Practically all possible lacustrine facies are developed, including various shore-zone and deltaic clastic sand bodies, sublacustrine fans and turbidites, evaporites, organic mudstones and stromatolites. The thickness and rapid lateral and vertical facies changes are related to vertical and strike-slip faulting along the margins of the Ridge Basin. Tertiary lacustrine sediments from S.E. France are described by True in the next paper: Lacustrine sedimentation in an evaporitic environment: the Ludian (Palaeogene) of the Mormoiron Basin. As is typical of many lakes, coarse clastics were deposited around the edge of the fault-bounded lake basin, during the Lower Ludian, and organic-rich muds accumulated in the centre. Arid conditions during the Upper Ludian led to evaporites and dolomites being precipitated in the basin centre, along with sepiolite and magnesium smectite-rich muds. The effect of climate on lake sediments is also discussed by Tucker in Triassic lacustrine sediments from South Wales: shore-zone clastics, evaporites and carbonates. The depositional environment of the red homogeneous Keuper Marl has been a subject of some debate but in this paper Tucker describes the lateral, marginal equivalents, which are of lacustrine shore-zone origin. Triassic shore terraces with beach gravels, cliff-lines with screes and wave-cut notches are all preserved, cut into Carboniferous Limestone at the lake margin. During periods of lake shoreline retreat (regression), calcretes developed within the shore-zone sediments and sabkha-type evaporites formed within the exposed lake floor sediments (the Keuper Marl). Van Dijk, Hobday & Tankard describe sedimentology and palaeoecology of Permo- Triassic lacustrine deposits in the Eastern Karoo Basin, Natal, South Africa. The sediments consist of repetitive coarsening-upward cycles representing shore-zone and deltaic sandstones prograding over offshore lacustrine siltstones. Coastal bays were infilled by distributary overbank flooding and crevasse splays. A climatic change resulted in shallow playa lakes with emergence structures and evaporites. Vertebrate remains, fish and plants are common throughout with their prevalence and preservation varying from facies to facies. From the Palaeozoic, Hesse & Reading describe Subaqueous clasticfissure eruptions

and other examples of sedimentary transposition in the lacustrine Horton Bluff Formation (Mississippian), Nova Scotia, Canada. Spectacular sandstone dykes and

collapse structures are related to contemporaneous earthquake activity associated with nearby fault movements. A Proterozoic lacustrine interlude from the Zambian Copperbelt is described by Clemmey. The lacustrine sediments are distinguished from underlying and overlying marine strata largely on the basis of preservation of delicate trace fossils (the oldest in the world) and sedimentary structures. The latter include synsedimentary folds and faults, gas-burst structures, syneresis cracks and various types of rain print. Equivalent structures are noted from a modern ephemeral lake, also in Zambia. Cycles of offshore to nearshore and sabkha sediments (with original evaporites still preserved) are recognised in the Copperbelt rocks and attributed to periods of lake-shoreline progradation. The final paper in this volume by Reeves illustrates the Economic significance of playa lake deposits. The various sediments and minerals of playa lake basins are described and their industrial uses noted. It is likely that playa lakes will take on an even greater importance in the future with the increasing demand for natural resources. With ancient sediments, problems can arise in distinguishing true lake environ-

Introduction

5

ments (enclosed water bodies) from lagoons, estuaries, deltaic interdistributary bays and other paralic water bodies with a permanent or semi-permanent marine connection. In many cases the fauna or flora alone is sufficiently diagnostic of a continental setting (e.g. Van Dijk, Hobday & Tankard, 1 978; Hsii & Kelts, 1 978). When the fossils are not diagnostic or are absent, however, then the sediments themselves, and their facies sequences and relationships must be used. Few, if any, sedimentary structures are restricted to the lake environment (Picard & High, 1 972) but taken together an assemblage of sedimentary structures can be sufficient to indicate an enclosed water body (e.g. Clemmey, 1978). Sedimentary features which are common in lake sediments include wave-formed ripples, syneresis and desiccation cracks and 'varves'. The mineralogy of the sediments and geochemistry may help too, since certain clay minerals and evaporites are restricted to non-marine settings. The characteristic lacustrine facies sequence is a coarsening-upwards unit representing progradation of lake shore-zone coarse clastics over offshore basin-centre silts and muds (e.g. Van Dijk et al., 1978; Clemmey, 1 978). Two other characteristic features are the very rapid lateral facies changes from beach gravels to offshore silts, particularly at coincident lake margins, and the very rapid vertical changes, of lacustrine into subaerial facies, which occur at non-coincident lake margins (Donovan, 1975; Tucker, 1 978). Facies associations on a larger scale, such as relationships to fluviatile or aeolian sediments, will also provide valuable information on environmental interpretations, but in addition on basin geometry and depositional controls (e.g. Link & Osborne, 1978; True, 1 978; and Tucker, 1 978). There is still much to be learnt about Recent lake sediments to facilitate the interpretation of their ancient equivalents. It is likely that re-evaluation of ancient floodplain, aeolian, deltaic or paralic formations will reveal lacustrine sequences hitherto unrecognized. It is hoped that this book will make some contribution towards an understanding of ancient and modern lake sediments. R E F ER E N C E S BRADLEY, W.H. ( 1 929) Algae reefs and oolites of the Green River Formation. Prof Pap. U.S. geo/. Surv. 154,

203-233. BRADLEY, W.H. ( 1 93 1 ) Origin and microfossils of the oil shale of the Green River Formation of Colorado

and Utah. Prof Pap. U.S. geo/. Surv. 168. BRADLEY, W.H. ( 1 964) Geology ofthe Green River Formation and associated Eocene rocks in south western

Wyoming and adjacent parts of Colorado and Utah. Prof Pap. U.S. geo/. Surv. 496-A. BRADLEY, W.H. & EusTER, H.P. ( 1 969) Geochemistry and palaeolimnology of the trona deposits and

associated authigeni c minerals of the G reen River Formation of Wyoming. Prof Pap. U.S. geo/. Surv. 496-8. CLEMMEY, H. ( 1 978) A Proterozoic lacustrine interlude from the Zambian Copperbelt. In: Modern and Ancient Lake Sediments (Ed. by A. Matter and M. E. Tucker). Spec. Pubis int. Ass. Sediment. 2, 259-278. CRAMPTON, C.B. & CARRUTHERS, R.G. ( 1 9 1 4) The geology of Caithness. Mem. geo/. Surv. U.K. DE GEER, G. ( 1 9 1 2) A geochronology of the last 1 2,000 years. Int. Geo/. Congr. XI Sess. Stockholm, 241-253. DONOVAN, R.N. ( 1 973) B asin margin deposits of the middle Old Red Sandstone at Dirlot, Caithness. Scott. J. Geol. 9, 203-2 1 1 . DONOVAN, R.N. ( 1 975) Devonian lacustrine limestones at the margin of the Orcadian Basin, Scotland. J. geo/. Soc. 131, 489-5 10. EARDLEY, A.J. ( 1 938) Sediments of Great Salt Lake, Utah. Bull Am. Ass. Petrol. Geo/. 22, 1 305- 1 4 1 1 . EuGSTER, H.P. & HARDIE, L.A. ( 1 975) Sedimentation i n an Ancient Playa-Lake Complex: The Wilkins Peak Member of the Green River Formation of Wyoming. Bull. geo/. Soc. Am. 86, 3 1 9-334.

A lbert Matter and Maurice E. Tucker

6

EuGSTER, H.P. & SURDAM, R.C. ( 1 973) Depositional environment of the Green River Formation of

Wyoming: A preliminary report. Bull. geo/. Soc. Am. 84, 1 1 1 5 - 1 1 20.

FANNIN, N . G.T. ( 1 969) Stromatolites from the middle Old Red Sandstone of Western Orkney. Geo/. Mag.

106, 77-8 8 . FOREL, F.A. ( 1 885) Les ravins sous-lacustre des fleuves glaciaires. C. r. hebd. Seanc. A cad. Sci., Paris, 101, 725-728. FOREL, F.A. ( 1 892) Le Leman, 1. GEIKIE, A. ( 1 87 8) On the Old Red Sandstones of Western Europe. Trans. Roy. Soc. .£din. 28, 345-452. GILBERT, G.K. ( 1 890) Lake Bonneville. Mongrr. U.S. geol. Surv. 1. GROVER, N.C. & HowARD, C.S. ( 1 938) The passage ofturbid water through Lake Mead. Proc. Am. Soc. Civ. Engr,. 103, 720-7 32. · GouLD, H.R. ( 1 95 1 ) Some quantitative aspects of Lake Mead turbidity currents. in: Turbidity Currents and the Transportation of Coarse Sediments to Deep Water: a Symposium. Spec. Pubis Soc. econ. Paleont. Miner. , Tulsa, 2, 34-52. HsD, K.J. & KELTS, K. ( 1 978) Late Neogene chemical sedimentation in the Black Sea. In: Modern and Ancient Lake Sediments (Ed. by A. Matter and M. E. Tucker). Spec. Pubis int. Ass. Sediment. 2, 1 29- 1 45 . LINK, M.H. & OSBORNE, R.H. ( 1 978) Lacustrine facies i n the Pliocene Ridge B asin Group: Ridge B asin, California. In: Modern and Ancient Lake Sediments (Ed. by A. Matter and M. E. Tucker). Spec. Pubis int. Ass. Sediment. 2, 1 67- 1 89. MILTON, C. & EuGSTER, H.P. ( 1 959) Mineral assemblages of the Green River Formation. In: Researches in Geochemistry (Ed. by P. H. Abelson), pp. 1 1 8- 1 50. John Wiley & Sons, New York. MORRISON, . B . ( 1 964) Lake Lahontan: geology of southern Carson Desert, Nevada. Prof Pap. U.S. geol. Surv. 401. NIP�ow, F. ( 1 920) Vorlaufige Mitteilungen iiber Untersuchungen des Schlammabsatzes im Ziirichsee. Z. Hydro/. l, 1 00- 1 22. NIPKOW, F. ( 1 928) Ober das Verhalten der Skelette planktischer Kieselalgen in geschichtetem Tiefenschlamm des Ziirich- und Baldeggersees. Z. Hydro/. 4, 7 1 - 1 20. PICARD, M.D. & H IG H L.R. ( 1 972) Criteria for recognizing lacustrine rocks. In: Recognition of Ancient Sedimentary Environments (Ed. by J. K. Rigby and W. K. Hamblin). Spec. Pubis Soc. econ. Paleont. Miner., Tulsa, 16, 108-145. RAYNER, D.H. ( 1 963) The Achanarras Limestone of the middle Old Red Sandstone, Caithness, Scotland. Proc. Yorks. geo/. Soc. 34, 1 1 7- 1 3 8 . RuSSELL, I . C. ( 1 885) Geological history of Lake Lahontan, a Quaternary lake o f northwestern Nevada. Monogr. U.S. geol. Surv. 2. SEDGWICK, A. & MuRCHISON, R.I. ( 1 829) On the Old Conglomerates and other Secondary Deposits of the north coast of Scotland. Proc. geo/. Soc. Lond. I, 1 -77. SMOOT, J.P. ( 1 97 8) Origin of the carbonate sediments in the Wilkins Peak Member of the lacustrine Green River Formation (Eocene), Wyoming, U.S.A. In: Modern and Ancient Lake Sediments (Ed. by A. Matter and M. E. Tucker). Spec. Pubis int. Ass. Sediment. 2, 109- 1 27 . TRUC, G. ( 1 978) Lacustrine sedimentation i n an evaporitic environment: the Ludian (Palaeogene) o f the Mormoiron Basin, S.E. France. In: Modern and Ancient Lake Sediments (Ed. by A. Matter and M. E. Tucker). Spec. Pubis int. Ass. Sediment. 2, 1 89-203. TUCKER, M.E. ( 1 978) Triassic lacustrine sediments from South Wales in shore-zone clastics, evaporites and carbonates. In: Modern and Ancient Lake Sediments (Ed. by A. Matter and M. E. Tucker). Spec. Pubis int. Ass. Sediment. 2, 205-224. VAN DuK, D .E., HOBDAY, O.K. & TANKARD, A.J. ( 1 978) Permo-Triassic lacustrine deposits in the Eastern Karoo Basin, Natal, South Africa. In: Modern andAncient Lake Sediments (Ed. by A. Matter and M. E. Tucker). Spec. Pubis int. Ass. Sediment. 2, 225-239. ,

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7-4 1

Saline lakes and their deposits: a sedimentological approach

L A W R E N C E A . H A R D I E , J O S E P H P . S M O O T and HANS P. EUGSTER Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, Maryland, U.S.A.

A B S TRAC T Saline lakes (lakes with >5000 ppm dissolved solutes) are common throughout the arid regions of the world. Their distribution is controlled by tectonic setting and climate, and thus their deposits take on an importance beyond their size and abundance in the geological column. To exploit this aspect of saline lakes an understanding is needed of their sedimentary record. Saline lakes occupy the hydrographically lowest areas of closed drainage basins and are surrounded by a complex of interrelated depositional subenvironments that result mainly from the characteristics of the inflow. Our approach is to identify distinctive subenviron­ ments, each subject to a distinctive set of hydrological, biological, chemical and sedimentological processes and hence each with a diagnostic set of sedimentary features in their deposits, as follows: (I) alluvia/fan; coarse gravelly wedges composed of braid channel deposits, incised channel fills, sieve deposits and debris flows; (2) sandflat; flat unchannelled sandy apron at base of fan; planar and wavy laminated coarse sand (upper flow regime bedforms); (3) dry mudflats; exposed plain of mudcracked muddy sediment fringing the saline lake, covered with thin saline efflorescent crusts; sediment laminated but disrupted by mudcracks, sheetcracks, and saline mineral growth; (4) ephemeral saline lake consisting of an inner salt pan (thin beds of crystalline salts with mud partings) and an outer saline mudflat ( massive mud crowded with salt crystals that have destroyed layering); (5) perennial saline · lake; bottom sediment of laminated carbonates, gypsum, etc., or, if very saline, thin-bedded halite, etc.; (6) dunefield (aeolian deposits); (7) perennial stream floodplain (braid or meander deposits); (8) ephemeral stream floodplain (braid deposits); (9) springs; travertine and tufa mounds and sheets; ( I 0) shoreline features (deltas, beach ridges, spits, etc.). By recognizing from the sedimentary record which subenvironments were present and how they were arranged in space and time, we can interpret the history of a saline lake basin, either modern or ancient.

Modern and Ancient Lake Sediments Edited by Albert Matter and Maurice E. Tucker © 1978 The International Association of Sedimentologists. ISBN: 978-0-632-00234-4

8

L. A. Hardie, J.

P.

Smoot and H. P. Eugster

I N TRO D U C T I O N

Closed drainage basins with saline lakes occupying the hydrographically lowest areas are common throughout the arid regions of the world today. By saline lake we mean a lake that normally contains water carrying more than 5000 ppm of dissolved solutes, the upper salinity tolerance of most freshwater aquatic organisms (Beadle, 1 974, p. 264). Such saline lakes may be perennial (like Great Salt Lake, Utah) or ephemeral (like the Etosha Pan, southern Africa), may be hundreds of metres deep (like the Dead Sea, Middle East) or seldom covered by more than a few centimetres of brine (like Saline Valley, California), rnay expand to thousands of square kilometres at high stand (like Lake Eyre, South Australia) or never become larger than a fraction of a square kilometre (like the Basque Lakes, British Columbia), and may occur at elevations below sea level (like the Death Valley salt pan, California) or thousands of metres above sea level (like Lake Uyuni, Bolivia). All these saline lakes have in common an environmental setting in which annual evaporation exceeds annual inflow, and so their distribution and characteristic features are primarily controlled by the climate of the basins. Tectonism, however, can create orographic (rain-shadow) deserts far north or south of the arid sub-tropical global belts, so control of the distribution of many saline lakes is first and foremost a tectonic one. It is these climatic and tectonic controls that make saline lakes and their deposits take on a geologic importance well beyond their size and abundance in the geologic record. The processes that operate in saline lakes are hydrological, chemical, biological and sedimentological and are all intimately interdependent. A great deal is now known about the chemical aspects of saline lake brines (see, for example, Hardie & Eugster, 1 970; Eugster & Hardie, in press), something is known of the hydrology of closed lake systems (Langbein, 1 96 1 ; Jones, 1 965; Reeves, 1 968, pp. 1 34-1 53), a little is known about the unusual biota of saline lakes (Beadle, 1 94, pp. 259-282), but very little work has been done on the sedimentology ofsaline lake basins. While we will discuss each of these aspects, it is the last one, the sedimentology, that we want to emphasize in this paper. This aspect carries the greatest importance in ancient saline lake deposits because it is the sediments, both clastic and chemical, that preserve the most legible record of the hydrological and physical environment, a record without which the chemical processes could not be properly interpreted. Our presentation here of the sedimentary characteristics of saline lake basins represents only a tentative beginning and is far from comprehensive because many of the crucial sedimentological observations are missing. We hope, however, that this will be enough to encourage other workers to look more carefully at the details of the depositional features, particularly the facies patterns, of saline lake basins both modern and ancient.

H Y D R O L O G I C A L A S P E C T S O F S AL I N E L A K E S

We are concerned here only with a brief summary of the general aspects of the hydrology of saline lake basins, in particular what determines whether a lake will be saline or not, perennial or ephemeral, and what kinds of inflow systems move water, dissolved solutes and detrital sediment within the basin. The conditions which must be met for a saline lake to form are ( 1 ) evaporation must

Saline lakes and their deposits

9

exceed inflow, and (2) the basin should be hydrologically closed, or at the least, outflow must be very restricted. Arid (desert) or semi-arid (steppe) climates, where annual evaporation rate is greater than annual inflow rate, occur in a number of settings: (a) the dry, high-pressure, 'horse latitude' global belts of the subtropics, (b) orographic (rain-shadow) deserts and steppes, independent of latitude, (c) mid-latitude, mid­ continent deserts and steppes far from sources of ocean moisture (these dry areas, like the Gobi, Turkestan and Tarim deserts and adj acent vast steppes, may at least in part be orographic), and (d) the dry, high-pressure polar 'deserts'. Closed drainage basins and lakes with no outlet have many origins (see Hutchinson, 1957), some of the more common being (a) tectonic basins, particularly block-fault and rift valleys, (b) wind deflation hollows, (c) interdunal depressions in a windblown dune field, (d) volcanic craters, etc., (e) valleys dammed by lava flows or by landslides, (f) cut-off stream meanders, and (g) stream floodbasins isolated by channel levees. Perhaps the most favourable settings for saline lake development are rain-shadow tectonic basins like the block-fault desert basins of the western United States, combining arid climate with hydrological closure. In these basins the enclosing mountains (horsts) act as effective traps for precipitation keeping the basin floors (graben) arid, yet, at the same time, providing enough inflow for significant solute accumulation in the saline lake. Death Valley, California, is an excellent example of this type of setting, where rainfall on the valley floor is < 6 em per year while in the upfaulted Panamint Range more than 2500 m above the valley floor the annual precipitation averages more than 35 em (Hunt et a!. , 1 966, pp. 5-7). The nature of the inflow to a saline lake-whether the inflow is perennial or intermittent, whether it is high or low compared to evaporation rate-will determine if the lake will be perennial or ephemeral. Stable perennial lake conditions will be favoured by perennial inflow and a low evaporation-to-inflow ratio, while ephemeral lake conditions are brought on by intermittent inflow and a high evaporation-to­ inflow ratio. For example, although the Dead Sea is undergoing evaporation at a rate of 160 em/year resulting in a loss of 1· 58 km3 /year of water, it remains a deep perennial lake (400 m) with an annual water level change of no more than 50 em. This balance is maintained mainly by the perennial inflow of the Jordan River which provides an estimated 1 ·25 km3 of water annually (Neev & Emery, 1967, pp. 72-73). Lake Eyre, Australia, on the other hand, with an annual evaporation rate of about 2 1 5 em/year has no perennial stream input and remains dry except when a rare flashflood inundates the basin with freshwater runoff that is channelled to the lake by a massive ephemeral braid stream system (see Bonython & Mason, 1 953). Inflow to saline lakes can take the form of (a) perennial streams that feed directly into the lake, (b) ephemeral streams, which flow only occasionally with storm runoff or perhaps seasonally with meltwaters from the spring thaw in the mountains, (c) unchannelled sheetflow during storms, (d) perennial or ephemeral springs at the lake edge, and (e) perennial or ephemeral groundwater input. Saline Valley, California (Hardie, 1 968; Eugster & Hardie, 1 978), is an example of an ephemeral saline lake that is recharged only by sheet�ow storm-runoff, springs and groundwater. The Inyo Mountains, over 3000 m above the Saline Valley floor, trap enough precipitation to maintain small perennial mountain streams. These mountain streams sink into the porous alluvial fan gravels as soon as they debouch from the mountain canyons, and the water then flows down to the valley floor as a perennial groundwater body. In places this groundwater surfaces at the dry playa edge as isolated perennial springs but

10

L. A . Hardie,

J.

P. Smoot and H. P.

Eugster

the bulk of this subsurface inflow evaporates from the vadose zone to produce a highly concentrated brine that soaks the sediments of the playa throughout the year, and allows salts to continuously accumulate. Like Lake Eyre, only after sporadic flashfloods does the Saline Valley lake contain a surface water body, but this water reaches the lake by direct sheetflow runoff from the fringing alluvial fans rather than by ephemeral stream channels. It is only during these flashfloods that detrital sediment is transported and deposited in the Saline Valley basin. For detailed information on the hydrology of specific saline lakes the reader is referred to works such as those of Jones ( 1 965) and Phillips & Van Denburgh ( 1 97 1), and for a general discussion o f hydrological cycles in closed basin lakes the study of Langbein ( 1 96 1 ) is recommended.

C H E M I C A L E V O L U T I O N O F S A L I N E LA K E B R I N E S

Chemical aspects of saline lake environments have been discussed by Jones ( 1 966), Jones & Van Denburgh ( 1 966) and Hardie & Eugster ( 1 970), among others, and have been summarized at some length by Eugster & Hardie ( 1 978). These latter authors considered the acquisition of solutes by the inflow waters, the modification of water compositions by early precipitation of alkaline earth carbonates and gypsum, the development of concentrated brines from which the most soluble saline minerals crystallize, and finally the diagenetic reactions between sediments and occluded brines. In this report we will give a brief synopsis of these aspects, and leave the reader to consult the above cited works for detailed information. Saline lakes are known to have a wide range of composl.tions, dominated by the solutes Si02, Ca, Mg, Na, K, HC03, C03, S04 and Cl. Na is by far the most abundant cation, while the anion concentrations are quite variable. The major brine types are (a) Na-C03-Cl-S04, (b) Na-Cl-S04, (c) Na-Mg-Cl-S04 and (d) Ca-Mg-Na-Cl. Hardie & Eugster ( 1970) have identified the factors which determine the composition of a particular salt lake. They found that the final composition of the brines is inherited from the very earliest stages of water evolution, the weathering reactions which occur in the watershed. Since these reactions are dominated by the nature and compositions of the minerals involved, it is basically the bedrock lithology which controls water compositions. Bedrock weathering reactions are of several kinds: (a) congruent dissolution of non-silicates like halite, gypsum, calcite, dolomite; (b) congruent dis­ solution and hydrolysis of non-aluminous silicates like olivines; (c) incongruent dissolution and hydrolysis of alumino-silicates like feldspars to produce clays; and (d) oxidation of metal sulphides to produce metal oxides and sulphate ions, e.g. pyrite gives geothite + SO/-. It should be noted that carbonic acid (atmospheric C02 dissolved in rainwater) weathering is a maj or chemical process in arid closed basins, as is demonstrated by the dominance of HC03- in closed basin inflow waters (see, for example, analyses in Jones, 1 965, and Hardie, 1 968) and by the abundance of clay minerals in the fine sediments washed into desert basins (Droste, 1 9 6 1 ). In this regard the weathering of a plagioclase-rich igneous or metamorphic rock can contribute as much Ca2 + and HC03- to the inflow waters as dissolution of limestones. As the inflow waters with their solutes newly derived from the weathering reactions in the highlands move downslope toward the closed lake, they are subject to evaporative concentration. This includes direct evaporation of surface waters,

Saline lakes and their deposits

11

subsurface evaporation of groundwater in the vadose zone, as well as evapotranspira­ tion. C02-degassing may also take place as the C02-charged groundwaters surface and exchange with the atmosphere. These evaporation and degassing processes lead to ever increasing solute concentrations until supersaturation with the least soluble chemical precipitates is reached. All the inflow waters we have tested (Hardie & Eugster, 1 970), whatever their source bedrock type, show saturation first with alkaline earth carbonates, calcite or aragonite or Mg-calcite (Nesbitt, 1 974). The early precipitation of Ca, Mg and COl has a profound effect on the subsequent fate of the brine chemistry. If HCOl >> Ca + Mg at the point of initial alkaline earth carbonate precipitation, then the subsequent evaporative concentration will produce a water rich in COl + HCOl and depleted in Ca + Mg, a sodium carbonate brine. For water initially with Ca + Mg >> HCOl, a brine enriched in alkaline earths and depleted in COl + HCOl will result. The amount of carbonate formed depends on the initial HCOl/Ca + Mg ratio: if that ratio is small or very large, little carbonate can form. Conversely, for molar ratios near unity, carbonate production can be extensive. In that case, low-magnesian calcites are followed by high-magnesian calcites and eventually protodolomite and in some cases even magnesite, because Mg is enriched

II

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Co rich HC0 3 poor

1

HC03?: Co + Mg Mg?: Co

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Bristol

Dry L.

HC03 rich Co poor

>Co > HC03

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4rich Co poor

\l

Co-No- CI

C03 »Co+Mg Co»Mg

M g c ol c i te ppt n.

low

gypsum Co r\ so4 poor

� II I

in f I o w

unde rsaturated

Na-C03-S04-CI

high Mg cal cite pptn.

(±

pr otodolo mite )

aragonite pptn.

or

Co+Mg »>HC03

Soline Volley Death Vo I ley

"t

Co »

gypsum pptn.

\i

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'); �

HC03

Mg » Co

brine

�

L. M ogod i >Co+ Mg

- No- Cl- s o4 -(C03l

a

Deep Springs

No- Mg-(Co)-CI

No- Mg-S04-CI

br ine

brine

Dead

Sea

B o sque

I

L.

L.

Fig. I. A schematic flow sheet for brine evolution as evaporative concentration progresses (direction of arrows).

12

L . A. Hardie, J.

P.

Smoot and H. P. Eugster

preferentially in the residual waters (Fiichtbauer & Hardie, 1 976). * These effects are shown schematically in simplified form in Fig. l (a more complete scheme is given by Eugster & Hardie, in press, fig. 5). Upon further evaporative concentration many waters next become saturated with respect to gypsum. This is still an early precipitation product, and waters at first precipitation of gypsum will normally have an ionic strength of less than l (Hardie & Eugster, 1970, p. 279). Like calcite, gypsum precipitation is an important branching point, with the subsequent path of the water depending upon the Ca/S04 ratio; that is, waters may now become depleted in Ca or in S04, producing either a Na-Cl-S04 or a Ca-Na-Mg-Cl brine (Fig. 1 ). The nature and fate of the precipitation products will be discussed later, but it is clear that a zonal arrangement may result, with the less soluble carbonates on the outside of the evaporating basin, in contact with more dilute waters, and gypsum closer to the centre (Hunt et a!., 1 966; Hardie, 1 968). The next minerals to form upon evaporation are usually quite soluble and their saturation is not reached until the solute load is increased manyfold. During this stage of evaporative concentration an additional process for increasing the solute load becomes important: dissolution of efflorescent crusts by rain and ephemeral runoff. Such crusts are formed at or near the surface and they are the product of complete evaporation to dryness of surface water or groundwater drawn to the surface by evaporative pumping (Hsii & Siegenthaler, 1 969). Dissolution, on the other hand, is fractional, with the most soluble salts being removed first. Most of the alkaline earth carbonates contained in the crusts are not redissolved and thus are permanently lost from the evolving brines. Such losses, as well as that of silica, have been discussed by Eugster ( 1 970), Jones, Eugster & Rettig ( 1 977) and Eugster & Jones ( 1 978). Other important losses were observed for potassium, removed through ion exchange reactions, and sulphate, involved in bacterial reduction. The most extensive gains in efflorescent crust dissolution are achieved by Na, Cl and some of the less abundant elements, such as Br, B, F. By now the solute load of the brines may have increased more than a thousand-fold over that of the inflow waters and the stage is set for final mineral precipitation. This can occur in one of two modes, (a) at the surface from the open lake brine, or (b) within the sediment from occluded brine. The nature of the minerals precipitated from such brines is of course dictated by the evolutionary track the brines have followed previously. Common products are mirabilite (Na2 S04. 10H20), thenardite (Na2 S04), halite (NaCl), trona (Na2 C03.NaHC03.2H2 0), burkeite (Na2C03.2Na2 S04), bloedite (Na2 S04.MgS04.4H2 0), epsomite (MgS04.7H2 0). Common salts formed within the sediment by reaction of the earlier-formed precipitates with occluded brine are gaylussite (Na2 C03.CaC03.5H20), pirssonite (Na2 C03.CaC03.2H20), glauberite (Ca­ S04.Na2 S04), nahcolite (NaHC03). Interaction of occluded brine with detrital sediment may also produce less soluble minerals such as authigenic silicates. The best-known examples are the zeolites formed by volcanic glass reacting with alkaline brines (for a recent summary see Eugster & Hardie, 1 978). Many ofthese deposits exhibit mineral zonation, presumably recording concentration gradients in the subsurface brine. Common successions, from dilute perimeter to the saline centre, are: montmorillonite-zeolite (phillipsite, *For kinetic reasons aragonite may form instead of calcite, particularly in dilute surface lakewaters with initially high Mg/Ca ratios (Fiichtbauer & Hardie, in preparation).

Saline lakes and their deposits

13

clinoptilolite, erionite, mordenite)-analcime-K feldspar. A wide variety o f other minerals formed in a similar manner has been reported, the most spectacular list being assembled for the Eocene Green River Formation (Milton, 1 97 1 ).

B I O L O G I C A L P R O C E S S E S I N S AL I N E L A K E S

Tolerance of some organisms to high salinity (over 50o/00) and/or high alkalinity is extraordinary, for example, the unusual Tilapia fish, the brine shrimp A rtemia, several species of rotifers, copepods, nematodes, insect larvae, worms, blue-green algae, bacteria and halophyte higher plants (see Beadle, 1 974, pp. 259-282 for a summary of saline lake biota). These salHolerant organisms may have an important input to the sedimentary record of saline lakes, particularly in connection with carbonate precipitation, bioturbation of bottom sediment, and deposition and decomposition of organic matter. In non-saline temperate zone perennial lakes, summer blooms of planktonic algae are thought to be responsible for precipitation of calcite. Photosynthesis removes C02 from the water, increasing pH and hence CO/- activity, which may lead to supersaturation with respect to CaC03• This is the model used by Nipkow ( 1 920) to explain the annual CaC03 laminae in Lake Zurich. Whether a similar model applies to saline lakes is not clear. In the Dead Sea aragonite precipitation is not seasonal but continuous throughout the year and appears to be inorganically precipitated (Neev & Emery, 1 967). So, too, in Deep Springs Lake, California, the alkaline earth carbonates apparently are inorganic precipitates (Jones, 1 965; Peterson, Von der Borch & Bien, 1 966; Clayton, Jones & Berner, 1 968). Blue-green algae certainly are associated with precipitated tufas and travertines in inflow stream channels and around spring orifices in saline lake basins (e.g. Dunn, 1 953; Scholl, 1 960; Slack, 1 967). Again, however, it is not clear whether the metabolic processes of the algae have induced carbonate precipitation or whether the algae are simply entombed by inorganically precipitated carbonate. The tufas and travertines are discussed in more detail under 'spring subenvironment.' Bioturbation of the sediments of saline lake basins by organisms is particularly obvious in deposits surrounding the lake. For example, in alluvial fans, sandflats, dunes and mudflats the depositional layering may be disrupted or destroyed by halophyte plant roots (cf. dikaka of Glennie & Evamy, 1 968; Glennie, 1 970, pp. 1 1 3- 1 1 7), by insect burrows, by oligochaete worm burrows, by bird feeding pits and by terrestrial animal burrows. In perennial saline lakes the bottom sediments may be burrowed and pelleted by worms and brine shrimp. In lake brines that contained dissolved SO/ - , sulphate-reducing bacteria (Baas­ Becking & Kaplan, 1 956; Goldhaber et a!. , 1 977) in bottom sediments can play an important diagenetic role: they induce anaerobic conditions that favour dissolution of gypsum and precipitation of iron sulphides and phosphate (as discussed below under 'perennial lake subenvironment'). Blue-green algal mats and coccoid ooze may accumulate on the bottoms of saline lakes. These algal deposits will be the prime candidates for diagenetic conversion to 'kerogen', the hydrocarbon complex that makes the organic components of the 'oil shale' of the Eocene Green River Formation (Bradley, 1 973). Before the saline lakes

14

L. A . Hardie, J. P. Smoot and H.

P.

Eugster

become too saline, diatoms may thrive and diatom ooze may collect on the bottom, effectively removing Si02 from the lake waters and storing it as a sediment deposit (see, for instance, the Dead Sea, Neev & Emery, 1967, p. 8 1). Finally, phreatophyte plants (Robinson, 1 958) that typically occur as widely spaced but abundant growths on fan toes, sandfiats and mudflats where the water table is shallow, act to increase the dissolved solute concentration of groundwater by evapotranspiration. Hunt ( 1 966) has excellent descriptions of such phreatophytes (as well as xerophytes) in Death Valley, California.

S U B E N V IR O N M E N T S O F S A L I N E L A K E D E P O S I T I O N A L COMPLEXES

Saline lakes are surrounded by a complex of genetically interrelated depositional subenvironments* that result mainly from the characteristics of the inflow to the lake. So, from a sedimentological point of view, it is necessary to consider the closed saline lake basin as an integrated whole with the saline lake as simply one part of a complex system of subenvironments. It is our experience that no two modern saline lake basins are quite the same, and so it is difficult to present a typical model for saline lake deposition. Our approach instead is to isolate the individual depositional subenviron­ ments that are associated with modern saline lakes, and to describe the diagnostic features of their sediments (sedimentary structures, textures, geometry, etc.) and the processes responsible for these features. With this approach the individual subenvironments, like building blocks, can be assembled as necessary to provide an overall sedimentological description of any particular saline lake depositional complex. Which subenvironments are actually present and how they are arranged in both space and time represent sensitive responses to the particular processes that are operating, or have operated, in a particular saline lake basin. This kind of approach has been outlined for carbonate tidal-fiat deposits by Hardie ( 1977, pp. 1 88- 1 8 9) and has great utility in reconstructing ancient depositional environments, provided the appropriate criteria are available from modern deposits. With this approach in mind we have outlined below the significant features of the different major sedimentary subenvironments that we have recognized in modern saline lake basins. The subenvironments we have considered are, (I) alluvial fan, (2) sandfiat, (3) mudflat, (4) ephemeral saline lake (saline mudflat and salt pan), (5) perennial saline lake, (6) dune field, (7) perennial stream floodplain, (8) ephemeral stream floodplain, ( 9) springs, ( 1 0) shoreline features of saline lakes. We will first discuss the characteristic features of each subenvironment in turn and then we will very briefly present a few examples of typical combinations of subenvironments found in modern saline lake basins, such as those of Death Valley (Fig. 4), Saline Valley (Figs Sa and b) and Deep Springs Valley (Fig. 6) in California; Lake Eyre basin (Fig. 7) in Australia; the Dead Sea basin in the Middle East; and the Great Salt Lake basin (Fig. 8) in Utah. By 'subenvironment' we mean any part of the surface of the basin that has a distinctive physiography and on which a distinctive set of physical, chemical and biological processes operate (see Hardie, 1 977, p. 3 ) The rock record of a subenvironment is a 'subfacies' (Reinhardt & Hardie, 1 976, p. 1 6). •

.

Saline lakes and their deposits

15

Alluvial fan and sandflat subenvironments

A large number of saline lake basins are formed as a result of block-faulting and rifting. The resulting high relief in such horst-and graben systems (Fig. 3c) leads to the development of coarse gravelly alluvial wedges that surround the saline lake of the fiat valley floor (see, for example, figs 3, 4, 5 , 28, 57, 58, 6 1 and 75 in Hunt & Mabey, 1 966). These gravelly wedges are coalescing alluvia/fans, which in turn are cone-shaped piles of very coarse sediment built out radially onto a fiat valley floor from a stream point source in the mountains (see Bull, 1 972; Cooke & Warren, 1973, p. 1 74). We have found that alluvial fans in saline lake basins commonly grade downslope into a fringing sandy apron that has features different from the alluvial fan as well as from stream floodplains. This distinctive sandy apron to alluvial fans Hardie ( 1973) has been called the sandjlat subenvironment. Alluvial fans are concave in radial profiles and convex in cross-fan profiles (Bull, 1 972, p. 63) with average slopes generally less than 1 0° (Cooke & Warren, 1 973, fig. 3.5); in the rugged Basin-and-Range province of the western U.S. fan slopes are commonly greater than 4°. Relief from apex to toe of the fan may be as much as 500 m over a distance of 20 km as in Baja California at the northwest corner of the Gulf of California. If faulting is long-lived then very thick (but rather narrow) fan deposits measured in thousands of metres can accumulate (see e.g. Bull, 1 972, p. 80 and fig. 16). Alluvial fan surfaces are dissected by a characteristic radial pattern of braided ephemeral stream channels that become shallower and less distinct toward the toe of the fan. This pattern reflects the essential depositional events that build alluvial fans: catastrophic storm-flooding in the mountains produces a surge of water which gushes down the canyon feeder streams and spreads out onto the fan (McGee, 1897, pp. 99- 1 05). These flood events produce four maj or kinds of deposits on fans (cf. Bull, 1972, pp. 66-7 1 ; Blissenbach, 1 954, pp. 1 78- 1 79): ( 1) shallow braid channel deposits; (2) fills of deeper, incised channels; (3) sieve deposits; and (4) debris flow deposits. Braid channel deposits (sheetflood sediments of Bull, 1 972, pp. 66-68) are seen on the fan surface as coarse gravel bars isolated by a braid system of very shallow channels ( < 1 m deep), or 'washes', floored by coarse sand, grit and fine gravel (Fig. 2a, see also figs 77 and 78 in Hunt & Mabey, 1 966). In vertical sections these deposits, like typical braid stream sediments (see Doeglas, 1 962, Williams & Rust, 1 969) are a series of cross-cutting lenses (Spearing, 1 975, fig. 1 ). Thickness is typically less than 1 m. The gravel bars consist of lenses of framework-supported boulders and pebbles (Fig. 2b), moderately sorted, and, in some cases, imbricated. Sand and grit may fill the framework voids by infiltration. The interbar 'wash' sediments are recognized by the planar-parallel horizontal lamination and low angle inclined laminated gritty sand. Maximum grain size in the gravel fraction decreases dramatically down-fan (Krumbein, 1 942). It appears that at maximum flood velocity the gravel lenses are produced as large lenticular longitudinal bars. As the flood wanes, the water shallows and the stream competence decreases so that fine gravel, grit and sand are deposited in the interbar channels as upper flow regime bedforms (plane beds, antidunes and 'washed out' dunes, Williams, 197 1 , p. 8; Simons, Richardson & Nordin, 1 965, p. 37). Incised channelfills are narrow gravelly lenses elongated downstream and generally much thicker than the braid channel deposits, up to several metres. They represent fills of sinuous channels cut earlier by the same flood at maximum discharge or by previous .floods. These fills commonly show a fining-upwards in grain size (see Glennie, 1 970,

16

L . A . Hardie, J. P. Smoot and

H.

P. Eugster

Fig. 2. (a) View of the surface of an alluvial fan, B aja California, showing a gravel bar (right) and a gritty,

sandy, braid channel or wash (left). Trenching tool for scale. This is a typical view of the mid-fan zone. (b) Cross-section through an alluvial fan deposit, Death Valley, California, showing typical flat lense-shaped bedding of gravels and pebbly grits and sands. (c) Cross-section through sandflat deposit, Baja California, showing wavy and inclined bedding (antidune bedding) and scour-and-fill structures. The surface of the sandflat was covered by a thin efflorescent halite crust (chips at the surface are crust fragments broken during trenching). Ballpoint pen for scale. (d) Polygonally cracked surface of layered halite of salt pan, Saline Valley, California. Pick handle for scale (approx. I m long). A fluffy efflorescent halite crust is forming at the surface of the pressure-cracks as brine is drawn up the cracks by evaporation. This salt pan is surrounded by the efflorescent crust covered saline mudflat shown in Fig. 3c.

fig. 18; Picard & High, 1973, pp. 1 98- 1 99), and overall sorting is not as good as that of the gravel bars of the braid system (Bull, 1964). Mudflow deposits are also found as channel fills. Sieve deposits on fans were first recognized and described by Hooke ( 1 967, pp. 453-456). These unusual deposits consist of longitudinal tongues of relatively well sorted open-framework gravel (see fig. 5 in Bull, 1 972) that sieve out coarse gravel by allowing floodwaters to sink down into the sediment, carrying with them the finer sediment and leaving only boulders on the surface. Debris flows are uniformly thick (up to 3 m) lobes of very poorly sorted 'pebbly mudstones' or 'sandy mudstones' that have resulted from downslope flow of a dense, viscous, wet, muddy sediment in which coarser grains are suspended. Bull ( 1 964, table 1 7) showed that debris flows are 40-90% mud. Debris flows that are made up of sandy mud, rather than gravelly mud, are usually identified as 'mudflo ws'. Profiles across old mudflows in Death Valley (Hunt & Mabey, 1 966, fig. 48) show well defined

Saline lakes and their deposits

17

channel-levee systems. Bull ( 1972, p. 70) noted that if the debris flow is very fluid then graded bedding and imbricate structure can be seen in the gravel component. For very viscous flows, the internal texture is uniform, although vertical orientation of platy boulders may be seen. On the basis of the surface features of fans and the distribution of the above four kinds of fan deposits we subdivide the fan subenvironment into three zones, ( I ) fan apex, (2) mid-fan, and (3) fan toe (see also Spearing, 1975). At the fan apex the radiating channels are well defined, few in number and commonly incised (e.g. fig. 1 in Bull, 1 972). The sediments are typically channel fills and sieve deposits. In the mid-fan zone the shallow braided channel-gravel bar system dominates so that the sedimentary record is mainly boulder-gravel lenses interbedded with planar horizontal to inclined laminated sands and grits (Fig. 2b). At the fan toe the braid channels are still the main dispersal systems but the channels are extremely shallow and the gravel bars are mainly in the pebble size range. Sand and grit dominate overall in these fan toe sediments. At the distal end of the fan toe where it passes into the sandfiat subenvironment the sand in the channels may be rippled (mainly linguoid current ripples), and, in some cases, the waning currents may produce a 'powering down' vertical sequence of poorly laminated gritty sand overlain by horizontally laminated medium sand and capped by a ripple cross-laminated fine sand (Hardie, 1 973), a sequence much like the Bouma sequence usually attributed to waning turbidity currents in deep water (Bouma, 1 964). The distal end of the fan toe will grade into the sandjlat subenvironment where the braid channels lose their identity and the floodwaters disperse as unchannelled, unconfined sheetfioods across a narrow fiat ( < < 1 slope) sand plain (Hardie, 1 973). Sand is the main component of the sediment and characteristically occurs as planar­ parallel horizontal laminae and wavy laminated beds (Fig. 2c). These structures are produced by plane bed and antidune flow regime bedform migration: even though the flow competence is only in the sand range the flow is upper regime because the water depths of the flood sheets are very small (perhaps only a few centimetres). Shallow ponding of water on the sand fiat as the adjoining saline lake expands with the flood may result in wind wave reworking of the surface of the sandfiat, producing a variety of shallow-water wave-ripple bedforms (e.g. interference ripples, fiat-top ripples) on top of the sheet-flood laminated layer. Deposition by each flood event may range from a package of laminated sand tens of centimetres thick to a thin sheet one or two centimetres thick (Fig. 3d). Both the sandfiat and fan toe surfaces are normally subaerially exposed and so are subject to significant reworking and redeposition of sand by the wind, a point emphasized by Glennie ( 1970, pp. 29-56) for 'wadi' sediments. Indeed, the sandfiat surface and sediment may take on features very similar to the backshore of a beach, with cut-and-fill structures, low-angle inclined bedding and heavy mineral lag laminae. Small wind-blown dunes, with avalanche cross-bedding, slump structures, etc., may march across the sandfiat and up the fan toe where they may coalesce into a large dune field. Significant post-depositional diagenetic processes operate on alluvial fans and sandfiats. Perennial springs that surface along fault-zones at the fan apex will give rise to short travertine-lined channel bottoms (Slack, 1967) along which the spring waters flow before sinking into the porous fan sediment. Travertine and tufa may also form from springs that emerge along the fan toe edge (Jones, 1 965, pp. 34-35; and Hunt & •

18

L. A . Hardie, J. P. Smoot and H. P. Eugster

Mabey, 1 966) (see 'spring subenvironment'). Evaporative pumping (Hsii & Siegenthaler, 1 969) from the groundwater body below the fan surface, particularly in the braid channel 'wash' sediments, rnay result in the formation of calcite pore cements and caliche crusts, coatings, root moulds and nodules within the fan sediment (Bull, 1 972, p. 65; Lattman, 1973; Glennie, 1 970, pp. 33-36; Hardie, 1 968, p. 1 288; Blissenbach, 1 954, p. 185; Hunt et a!. , 1 966, p. 1 4). At the distal end of the fan toe and beneath the sandflat in the vadose zone the pore and vug filling cements are more likely to be gypsum and/or high-Mg calcite (perhaps even 'protodolomite'). If gypsum does form in these zones then dehydration to anhydrite and/or bassanite may occur (Hardie, 1 967). The groundwater from which these chemical precipitates formed could be recharged by the aperiodic surface flooding (ephemeral recharge) and from perennial springs and mountain streams (perennial recharge, as for example, in Saline Valley, California, see Hardie, 1 968). Phreatophyte and xerophyte bushes and trees with large root systems, like creosote bush (Larrea tridentata), paloverde (Cercidium sp.) and mesquite (Prosopis sp.) of the western U.S. desert basins (see Hunt, 1 966), will not only disrupt the layering in alluvial fan and sandflat sediments, but also act by evapotranspiration to concentrate both ephemeral and perennial groundwater (phreatophytes are particularly effective on fan toes where perennial groundwater is shallow but still relatively dilute; see Hunt, 1 966, fig. 5). On death and decay of the roots, the network of root holes provide important conduits for groundwater movement and hence can also become plugged with calcareous caliche or gypsum. Bushes may also act as baffles for wind blown sand and build small pyramidal mounds of sand on the fan toe and sandflat. Finally, pebbles and boulders exposed for long periods on fan surfaces can become coated with 'desert varnish' (Hunt et a!., 1 966, p. 6; Lustig, 1 965, p. 1 34; Hunt & Mabey, 1 966, pp. 90-92; Glennie, 1 970, pp. 1 9-20), a film of iron-manganese oxides thought to be produced by in situ weathering. Some authors, such as Bull ( 1 972, p. 80), believe that desert varnish can be used as a criterion for recognizing ancient alluvial fans.

Dry mudflat subenvironment

Saline lakes, particularly ephemeral ones, are fringed by a subaerially exposed plain of fine-grained sediment, a supralittoral mudflat. This mudflat subenvironment would fall within the definitions of 'playas' and 'inland sabkhas' of other workers (e.g. Cooke & Warren, 1 973, pp. 2 1 5 and 2 17; Glennie, 1 970, p. 60; Neal, 1975, p. l). We have distinguished between a 'saline mudflat', saturated with brine from which a mass of salt crystals have grown to completely destroy the depositional sedimentary structures, and a 'dry mudflat' in which the depositional sedimentary structures are well preserved. The 'saline mudflat' has strong affinities with ephemeral salt pan deposition and so is dealt with under the 'ephemeral saline lake' subenvironment. Here we will discuss only the dry mudflat subenvironment. The surface of the dry mudflat subenvironment is characterized by polygonal mudcracks and thin saline crusts. The mudcracks are generally narrow (a millimetre or two wide) and irregular to wavy, isolating polygons typically 5-25 em across (Fig. 3a; see also Cooke & Warren, 1 973, figs 2 . 1 4 and 2. 1 6). The mudcracks may extend to depths of a few millimetres to tens of centimetres. The surface crusts are of two kinds: ( I ) very thin (a millimetre or two thick), hard, dense crusts made of micritic alkaline

Saline lakes and their deposits

19

3. (a) Close-up view of mudcracked surface of dry mudflat, B aj a California. Pencil for scale. Note the secondary mudcracks within the primary polygons. Gypsum is crystallizing within the mudcracks. (b) Vertical slab of plastic-impregnated core through dry mudflat sediments, Baja California, showing characteristic millimetre-lamination. Pencil point for scale. Gypsum (white euhedral shapes) has crystallized within sheetcracks, mudcracks and fenestral pores. (c)View ofblocky efflorescent crust covering saline mudflat of Saline Valley, California. This porous crust, about 30 em thick, is so hard it must be broken with a pick in order to get through to the underlying gypsum- and glauberite-studded, brine-soaked soft mud. Note the very steep faulted valley walls that rise 3000 m above the mudflat. At the base of these precipitous mountains can be seen coalescing alluvial fans which are some 300 m high. (d)Trench dug into a saline mudflat bordering a sandflat, B aj a California. B allpoint pen and buried sample core-can for scale. The sandy-silt sediments are crowded with anhydrite nodules and lenses of nodules, and partly dehydrated gypsum crystals, the growth of which has destroyed most of the layering. The surface of the shallow brine groundwater body is exposed at the bottom of the trench. The efflorescent crust that once covered this saline mudflat was dissolved during a massive storm which washed a thin layer of wavy-laminated sand over the mudflat (top of trench), prograding the distal end of the sandflat over the mudflat. Since this photo was taken an efflorescent halite crust has once again covered the surface.

Fig.

earth carbonate (commonly high-Mg calcite or 'protodolomite' as in Salt Flat Graben, Texas); these brittle crusts may crack into small (millimetre or centimetre scale) platy fragments and be reworked by wind or flood water into pockets of fiat-chip gravel, grit or sand intraclasts; and (2) puffy, porous crystalline crusts up to several centimetres thick of soluble saline minerals such as halite, thermonatrite, trona or thenardite; these

20

L. A. Hardie,

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Smoot and H. P. Eugster

salt crusts are dissolved with each succeeding stoim flooding event and so are not preserved in the underlying sediment, but their effects may be seen as disruption and destruction of the upper part of each depositional unit. The mudcracks are desiccation cracks while the crusts are surface efflorescences chemically precipitated from subsurface brines drawn up through the vadose zone to the surface, probably by evaporative pumping (Hsii & Siegenthaler, 1 96 9). Where the subsurface brines are relatively dilute, as typically occurs near the outer edge of the mudflat, the crusts are likely to be the carbonate micrite type, but where the subsurface brines are quite concentrated and hence have evolved beyond the alkaline earth carbonate stage (Hardie & Eugster, 1 970; Eugster & Hardie, 1 978), the crusts are of the soluble salt type. Other surface features of the mudflat are patches of sand (both clastic grains and intraclastic peloids) that are starved wind ripples or longitudinal trains blown across the mudflat from the adjoining sandfl.at and alluvial fan or derived from erosion of the mudcrack polygons on the mudflat itself. In some places at the outer edge of the mudflat, small sandy rills may protrude from the sandfl.at onto the mud surface. Also at the outer edge of the mudflat where it meets the sandfl.at, springs may surface and deposit mounds of travertine and spring tufa (see under 'spring subenvironment'). The sedimentary features within the sediments of the mudflat subenvironment of saline lakes are not well known, and this represents a major gap in our documentation of saline lake sedimentation. We present here some of our cursory findings as well as apply data from analogous settings, e.g. marginal marine supratidal saline mudflats such as those of the extensive playa-like sabkha at the northwest corner of the Gulf of California (Thompson, 1 96 8 ; Hardie, 1 973). The predominant sedimentary structures that should be expected in the sediments of the mudflat subenvironment are: ( I ) millimetre-scale lamination which consists o f either (a) lenticular laminae o fvery fine sand and coarse silt alternating with more continuous laminae of fine silt and clay (Fig. 3 b) or (b) graded laminae and very thin beds; (2) disruption of the laminae by shallow and/or deep mudcracks, commonly filled or partly filled with fine sand and silt (including peloids); deep cracks may be filled with several different generations of sand and silt; (3) disruption of the laminae by horizontal sheet-cracks that propagate from mudcracks along bedding planes (Fig. 3b); (4) crystals of gypsum, glauberite, mirabilite, etc. may fill or partly fill mudcracks and sheet-cracks which have acted as conduits for subsurface brine movement beneath the mudflat (Fig. 3b); (5) disruption of the laminae by displacive growth of saline minerals like gypsum in the vadose zone; (6) destruction of layering by efflorescent crust growth of soluble saline minerals like halite at the mudflat surface; (7) thin dense alkaline earth carbonate micrite crusts interlayered with the unlithified sediment laminae; and (8) thin lenses of mudchip or micrite crust intraclast sands and grits. We speculate that there might be three basic types of mudflat, each produced by a different kind of depositional process and resulting in a different kind oflayering, ( 1) sheetwashed mudflats, (2) ponded water mudflats, and (3) exposed old perennial lake­ bottom mudflats. Deposition on sheetwashed mudflats would occur when thin sheets of sediment-charged stormwaters stream off the sandfl.ats and across the mudflats en route to the central saline lake. The essential process here would be traction deposition from moderate velocity, but very shallow, unchannelled flow (hence fine-grained sediment but upper flow regime bedforms). The resulting layering probably would be graded, with traction load, fiat lenticular sandy or silty laminae capped by a fine mud

Saline lakes and their deposits

21

(clay-sized sediment) drape deposited when the flow waned. These mud drapes, as well as the finer-grained lenticular lamination, would become mudcracked (centimetre scale) on drying out; such small shallow mudcracks should be beautifully preserved when the next flood washes sandy sediment over the bottom and into the old surface cracks. Deposition in ponded (standing) water on mudflats occurs during storm flooding when the saline lake temporarily expands outward over the fringing mudflat subenvironment. Under these conditions the sediment-charged sheetwash off the sandfiats should rapidly decelerate as it enters the expanded lake and quickly deposit its load. We envisage that deposition would take the form of a graded thin bed or thick lamina deposited (a) from the sheetwash that entered the shallow standing lake water as a waning turbid underflow (a 'turbidite'), or (b) from the mixed inflow-lake water body as a simple 'settle-out'. After deposition these graded layers could be reworked by wind waves to produce either coarse silt-fine sand lenses and muddy drapes or, if the layer is thick enough, simple surface rippling. Finally, as the lake waters recede by evaporation, the mudflats are exposed once more and mudcracking will disrupt the layering. The last mudflat type, the old lake-bottom, is not an active depositional mudflat but is a response to a major long-term climatic change that caused a perennial lake to dry up. Reeves ( 1 968, p. 1 20) identified many ofthe playas ofthe Great Basin of the western U.S. as being of this type. The mudflats to the east of the Bonneville Salt Flats in Utah may be an example of the exposed perennial lake-bottom type of mudflat. At present the surface is covered with polygonal mudcracks (tens of centimetres across) and a thin efflorescent halite salt crust, but the features in the sediments below suggest perennial lake deposition rather than mudflat deposition. The evidence is as follows: the sediments are finely laminated alkaline earth carbonate micrites with no internal mudcracks to signal exposure between depositional events; only the uppermost 20-30 em, which is a highly churned soil-like zone, shows evidence of recent exposure to mudcracking, salt-crust deformation, oxidation, insect burrowing and, in places, rooting by halophytes. Ephemeral saline lake subenvironment

By ephemeral saline lake we mean a shallow body of water, normally a concentrated brine, that at least once every few years dries up, leaving in the low central area an exposed layer of salt(s) that precipitated out as the brine evaporated. This subenvironment has been described as a salina, alkali lake or playa lake when wet and as a playa, dry lake, alkali fiat, salt fiat, salt pan or inland sabkha when dry (see Reeves, 1968, p. 87; Cooke & Warren, 1 973, p. 2 1 5 ; Glennie, 1 970, p. 60; Neal, 1 975, p. 1). Modern ephemeral saline lakes may cover at maxiumum stand an area as large as 8000 km2 (Lake Eyre, Australia) or as small as 0·008 km2 (Basque Lakes, British Columbia). Maximum depth of water may be several metres (e.g. 4 m in Lake Eyre during the 1 949- 1 952 flood, Johns & Ludbrook, 1 963, p. 23). The important aspect of the ephemeral saline lake subenvironment is that recharge is by surface runoff during infrequent catastrophic storms and by springs. In the long periods between fioodings the standing storm waters of the newly expanded lake will slowly shrink and become saline brines by evaporative concentration, ultimately reaching saturation with respect to salts such as halite or trona before drying up completely or almost completely. This cycle of catastrophic expansion (with freshening) and gradual contraction (with increasing salinity) of the lake (see

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L. A. Hardie,

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P. Smoot and H. P. Eugster

Langbein, 1 96 1 ) leads to a sub-division of the subenvironment into two parts based on the features of the bottom sediment: ( 1) a salt pan, underlain by layered salts (Fig. 2d), in the lowest part of the lake area; and (2) a saline mudflat, underlain by muddy clastic sediment crowded with crystals of salt minerals, surrounding the salt pan. Deposition of clastic sediment in the saline lake subenvironment appears to take place mainly as 'settle-out' of silt-clay sized grains from suspension when the turbulence of the flood waters subsides. This leaves a lamina or thin bed of mud as a storm layer over the expanded lake bottom. The thickness of this settle-out layer will depend on the concentration of suspended sediment and the depth of water, but would rarely exceed a few to tens of millimetres because mud-sized grains are not a major product of weathering in arid zones and because the floodwaters are seldom very deep. After settle-out of the storm-layer, wind induced waves may cause rippling of the surface of the layer or even reworking of the entire layer into silt-clay lenticular lamination. Blue-green algal spores washed in by the storm may bloom into a pervasive lake-bottom mat, but because the mat develops after deposition there will be no algal influence on the sedimentation. During the flood, the freshwater runoff streaming over the efflorescent crust covered mudflats surrounding the lake will preferentially dissolve the most soluble saline minerals from the crusts, producing a chemically 'simple' brine dominated by one or two major solute species, e.g. NaCl, Na2 S04 or Na2C03, a brine different from one produced by evaporative concentration of dilute inflow waters and sequential precipitation of carbonates and sulphates (as outlined by Hardie & Eugster, 1970) . This ponded brine will, over the ensuing weeks and months, slowly evaporate until halite or trona (or other soluble salt appropriate to the brine chemistry) precipitates. This stage of precipitation of salt out of the ponded surface brine may not occur until the lake area has been reduced considerably, for example, Lake Eyre expanded to 8000 km2 in 1950 but after evaporation to dryness by 1 952, a layer ofhalite a few centimetres thick had been deposited over only 1 / 1 0 of this area (from data in Bonython, 1956). A similar value for lake/salt pan ratio was found for Saline Valley, California (Hardie, 1968), which is only one hundredth the size of Lake Eyre. At Lake Magadi, on the other hand, the lake/salt pan ratio is much larger; the shorelines are steep in this graben lake and so annual flooding does not drastically expand the lake area (Eugster, 1 970). The thickness of the salt layer deposited by complete evaporation of the ponded brine will generally be in the order of centimetres, e.g. a sheet of brine l m deep and saturated with respect to halite will yield a halite crystal layer about 1 5 -20 em thick. The textural features of the salt layer reflect nucleation of seed crystals at the brine-air interface and subsequent epitaxial overgrowth on the foundered seeds. Shearman ( 1 970) and Eugster & Hardie ( 1 978) have described the major features of halite layers in salt pans while Eugster ( 1 970) has discussed trona crystallization. Interesting post­ depositional features include polygonal cracking (Fig. 2d) and overthrusting of the dry salt layer (see descriptions in Eugster & Hardie, 1 978), growth of salt crystals in the underlying mud layer, and bacterial reduction of S04 to produce H 2 S and iron sulphides in the mud layer making it black and anaerobic (Reeves, 1968, p. 78; Baas­ Reeking & Kaplan, 1 956). Overall, then, in the salt pan a single storm will result in the deposition of a couplet of a thin mud layer (millimetre scale, black, iron sulphide-rich, crowded with salt crystals) overlain by a thicker crystalline salt layer (centimetre scale) (see Bonython, 1 956, plate VIII, fig. b). Repeated storms will superimpose one couplet upon another

Saline lakes and their deposits

23

making a salt pan facies that could reach tens or even hundreds of metres in thickness. For example, more than 300 m of salt-mud interbeds underlie the Death Valley salt pan (Hunt & Mabey, 1 966, table 1 9), while more than 40 m of trona underlie Lake Magadi (Baker, 1 958). Immediately surrounding the salt pan the newly deposited mud is left exposed as the saline lake shrinks, and may become extensively mudcracked. Also, as the lake shrinks the brine will soak into the mud and persist as a subsurface brine body or join an existing perennial groundwater brine, the upper level of which will be controlled by the salt pan brine level. Evaporative pumping (Hsu & Siegenthaler, 1 969) will cause continued evaporation of the subsurface brine in the upper vadose zone and lead to widespread intrasediment precipitation of salts as displacive and/or poikilotic crystal growths. At the very surface an efflorescent salt crust will quickly form (Fig. 3c; see also Eugster & Hardie, 1 978). Where the brine body is close to the surface, massive intrasediment growth of salts will destroy any layering so that only a structureless mud full of salt crystals will result (Fig. 3d). This zone of efflorescent salt-encrusted, brine­ soaked, massive saline mud fringing the salt pan we have called the 'saline mudflat'.

Perennial saline lake subenvironment

By perennial saline lake we mean a surface body of brine that persists for many years (tens, hundreds or even thousands) without drying up. It may be shallow or deep (metres to hundreds of metres) but if more than several metres deep it is usually stratified (meromictic). Examples are Great Salt Lake, Utah (southern basin, about 1 2 m deep) and the Dead Sea on the Israel-Jordan border (northern basin 400 m deep). Perennial saline lakes require a substantial perennial inflow, normally a large river or rivers (e.g. Jordan River flows into the Dead Sea; the Weber, Bear and Jordan rivers empty into Great Salt Lake). The perennial stream inflow not only keeps the lake constantly supplied with water so that it does not dry up by evaporation, but also supplies both dissolved solutes and clastic sediment. Considerable amounts of dissolved solutes can be provided by perennial springs around the perimeter ofthe lake (e.g. the many fault-line brine springs around the Dead Sea; Bentor, 1 96 1 ) while aperiodic flash floods can bring in clastic sediment from all parts of the drainage basin as ephemeral sheetflood inflow. The primary process in perennial saline lakes is evaporation from the lake surface. In the arid climate necessary for a saline lake to persist, evaporation is essentially continuous and leads to ( 1 ) concentration of the surface brine, and (2) nucleation and growth of saline minerals in this surface brine. Both the newly concentrated brine and the saline minerals precipitated from it will sink down toward the bottom of the lake, and the less dense, less concentrated inflow will 'float' in over the brine to evaporate and sink down in turn. This continuous repetition of inflow � evaporation � saline mineral precipitation � sinking of brine and settling of chemical sediment, is the essential feature of stratified perennial lakes in which evaporation exceeds inflow. Which saline minerals will actually precipitate from the surface waters will depend, apart from kinetic factors, upon the evaporation/inflow ratio, the chemical composition of the inflow, and the stage in the history of the lake (see Hardie & Eugster, 1970; Eugster & Hardie, 1978). For example, for a non-alkaline brine, the saline minerals could be alkaline earth carbonates, or alkaline earth carbonates +

24

L. A. Hardie, J. P. Smoot and H. P. Eugster

gypsum, or alkaline earth carbonates + gypsum + halite. If evaporation only slightly exceeds inflow then it is probable that only alkaline earth carbonate saturation will be reached. On the other hand in order to precipitate a halite-bearing assemblage, not only must the evaporation/inflow ratio be very high but the bottom brine must be dense enough to support the concentrated surface brine at the high density a halite­ precipitating brine achieves, otherwise the surface brine will sink before halite saturation is reached. Hence, the mineralogy of the chemical sediments of a perennial saline lake clearly is a sensitive record of the relative evaporation/inflow ratio as well as the 'maturity' of the lake. This leads to a simple geochemical evolution model for perennial saline lakes deep enough to be stratified. Let us consider the changes that might take place in a non-alkaline perennial lake in an arid climate as inflow decreases because of changes in rainfall in the enclosing highlands. If inflow initially was greater than evaporation then the lake would be deep and quite dilute. At this stage the lake would support a diverse freshwater fauna and flora in spite of the aridity, an aridity that would be evidenced only in the fringing mudflats and isolated 'lagoons' where saline mineral precipitates would be abundant. A good example of this stage is Lake Chad in Africa (Maglione, 1 974). Now, if inflow rate was decreased so that it became less than the evaporation rate, lake expansion and deepening would stop, reverse, and evaporative concentration of the surface waters would lead to chemical precipitation of alkaline earth carbonates (aragonite, low-Mg calcite or high-Mg calcite depending on the Mg/Ca ratio and ionic strength, see Fiichtbauer & Hardie, 1 976). Initially, the evaporating surface waters, because of their increased density, would sink before saturation with respect to saline minerals could be reached. The sinking surface brine may either mix with the bottom waters increasing the overall brine density, or simply displace the bottom waters forcing them toward the surface where they could evaporate. Eventually the whole brine body would be dense enough that the residence time of the surface waters was long enough to reach supersaturation with respect to an appropriate alkaline earth carbonate mineral. * As the tiny crystals of precipitated carbonate (see, for example, Neev & Emery, 1 967, pp. 88-90 for size data on micron­ scale aragonite needles precipitated from the Dead Sea surface waters) slowly settled through the mixed or displaced bottom waters, most would dissolve, further increasing the salinity and density of the lake brine body. An extended period might ensue before the entire brine was saturated throughout with respect to the carbonate precipitate and net accumulation of carbonate sediment on the lake bottom would occur. Walker Lake in Nevada is a good example of this stage of development, but the carbonate precipitate is the unusual monohydrocalcite (CaC03.H2 0) (R. Spencer, personal communication, 1 976). Continued evaporative concentration of the surface waters will slowly increase the overall concentration of the brine body because the residence time of the surface waters becomes progressively longer as the brine density progressively increases. Ultimately, in a manner analogous to the carbonate saturation stage, the lake brine becomes saturated with respect to gypsum and a stage of gypsum + carbonate co­ precipitation and accumulation sets in. The Dead Sea is at this stage today (Neev & Emery, 1 967), although it appears to have reached this point from the opposite *S upersaturation of the surface waters may be reached before sinking of the brine because the surface waters are likely to be warmer and hence fractionally lighter than the undersaturated but cooler lower waters.

Saline lakes and their deposits

25

direction, that is, increased inflow rate since early Holocene times has replaced a halite stage with a gypsum + aragonite stage. As long as the inflow rate of our hypothetical perennial saline lake remains much smaller than the evaporation rate, the lake will eventually reach saturation with respect to halite in the same manner as described for the carbonate and gypsum accumulation stages. The solubility of halite, however, is an order of magnitude greater than that of gypsum (in relative mass terms) and so a long period of time will be required to reach the halite stage. And, of course, the level of the lake will have fallen considerably, perhaps it would have no more than 1 / 1 00 the volume of the lake at carbonate saturation stage. If the lake basin sides are not too steep, this drop in lake level could expose a large part of the lake-bottom underlain by carbonate or carbonate + gypsum. Great Salt Lake, Utah, appears to be at this stage today, with wide fringing mudflats that represent old perennial lake-bottom. At this stage, upon further evaporation, the fate of the perennial lake will follow one of two possible paths depending upon the depth of the brine saturated with respect to halite (or some other salt, say trona) and the inflow-evaporation balance. If the brine body was shallow, say no more than a few or a few tens of metres deep, the lake may dry up completely and deposit an appropriate layer of salts. This happened at Owens Lake, California, in response to inflow cutoff by man and it nearly happened at Great Salt Lake during the drought of 1 930- 1 935 when a thick halite layer was deposited over the lake bottom. If the drought had continued, Great Salt Lake would quickly have joined the many examples of ephemeral salt lakes. Accumulation of salt could have continued as long as springs brought in the necessary solutes, as they do now at Lake Magadi. Instead, inflow increased again and Great Salt Lake remained a perennial lake, but now considerably diluted. In fact, much of the solute load of the dense bottom brine may have been derived by dissolution of the bottom salt crust. The transition from perennial lake to ephemeral salt lake is documented many times in the geologic record, such as in the transition from the High Magadi beds to present Lake Magadi (Baker, 1958; Eugster, 1 970), the transition from Bottom Mud to Lower Salt and from Parting Mud to Upper Salt at Searles Lake (Smith, 197 8), from oil shales to trona beds in the Wilkins Peak Member of the Green River Formation (Bradley & Eugster, 1 969) and the transition back is equally common. In contrast, if the saline brine body is very deep, say hundreds ofmetres, the lake will not readily turn into an ephemeral salt lake. Instead, a massive evaporite deposit must eventually accumulate from such a perennial salt lake. This is what would happen to the Dead Sea, if the Jordan River were cut off. We do not know of a contemporary example where this is taking place and we therefore can only suggest a reasonable model. This model of halite or trona precipitation from a deep lake is simply an extension of the model of carbonate and gypsum precipitation from a perennial lake discussed earlier. Evaporation takes place at the surface, producing a concentrated surface brine, that when its density becomes great enough, will flow to the bottom of the lake, displacing lighter bottom water. This process continues until the whole brine body is saturated with, say, halite, when massive halite precipitation would set in. The lake could remain such a perennial halite-depositing lake for thousands of years or more, provided the proper and delicate evaporation/inflow balance is maintained. The model we have outlined here is similar in principle to that suggested by Schmalz ( 1 969) for deep water marine evaporites. It is not restricted to monomineralic deposition and applies equally well to complex brines.

26

L. A . Hardie, J. P. Smoot and H. P. Eugster

The vertical sequence of chemical sediments that would record the stages in the evolution of the hypothetical non-alkaline lake outlined above would simply be a halite + minor gypsum + trace of carbonate package overlying a gypsum + minor carbonate package which in turn overlies a carbonate package. The thickness of each package will depend very much on the chemical composition of the inflow, the evaporation/inflow ratio, the original lake volume, and the length of time involved, but probably would be in the order of metres, tens of metres or even hundreds of metres. Of course if a climatic reversal occurs and inflow increases at any point, then the sequence would record it faithfully by a reversal of stage, as for example in the Dead Sead where cores through the bottom show a thin late Holocene carbonate package (80 em) overlying an early Holocene-late Pleistocene halite package (Neev & Emery, 1 967). The sedimentary structures typical of chemical sediments of perennial saline lakes deep enough to be stratified are relatively uncomplicated. The major feature is areally widespread (laterally continuous) saline mineral laminae and thin beds with clastic sediment (mud) partings. Many authors identify each saline mineral layer-clastic layer 'couplet' as a 'varve' (e.g. Reeves, 1 968, fig. 50). In both Searles Lake (Smith, 1 978) and the Dead Sea (Neev & Emery, 1 967, pp. 84-85), however, the carbonate laminae are not annual but are rather irregular, only one lamina deposited per several years (the Dead Sea, 1 5-20 laminae in 70 years). The varve interpretation arises because saline lake deposits have been compared to temperate zone non-saline lake deposits (e.g. Bradley, 1929). In saline lakes, however, evaporation is normally continuous so that chemical precipitation is continuous throughout each year, as Neev & Emery ( 1 967, pp. 82-85, p. 93) showed for co-precipitation of aragonite and gypsum in the Dead Sea. Perennial saline lake layering, then, is not due to seasonal pulses of chemical precipitation but rather seems to be due to storm-flood influxes of clastic sediment (mainly mud) that punctuate the continuous rain of chemical sediment (compare with Calvert's, 1 966, explanation for the clastic mud-diatom ooze couplet lamination in the sediment of the Gulf of California). The clastic influxes could be seasonal but are more likely to be quite irregularly spaced, as for example is typical of the Great Basin of the western U.S. (e.g. Hardie, 1 968) or the Lake Eyre, Australia, basin (e.g. Bonython, 1956). The typical thicknesses of the chemical layers (i.e. thickness between clastic partings) depend upon the solubility of the saline mineral, the net annual evaporation (typically 1 -2 m) and the average interval between storm­ floodings (perhaps 1-5 years). Roughly, we would expect carbonate laminae to be fractions of a millimetre, gypsum laminae to be several millimetres, and halite to occur in thin beds of several centimetres. The most probable mechanism of deposition is simple 'settle-out' to produce uniformly thick layering. The clastic sediment is also likely to be deposited by 'settle-out' as fine mud laminae, but an occasional massive storm over the whole basin could introduce unchannelled sheet-floods (carrying coarse sediment from the surrounding mudflats) as a dense underflow from which a graded clastic layer could be deposited. Other maj or sedimentary features besides the layering are diagenetic in origin. The fine-grained chemical sediments soaking in the bottom brine might recrystallize to a more stable coarse crystalline mosaic, destroying in the process the 'settle-out' fabric. Where organic matter has accumulated in the bottom sediment, the interstitial brines may undergo bacterial reduction of sulphate ions (Bass-Becking & Kaplan, 1 956). If gypsum, or other sulphate mineral, is present this bacterial reduction can result in

Saline lakes and their deposits

27

dissolution of the mineral, such as Neev & Emery ( 1 967, p. 94) believe is responsible for the loss of gypsum in the bottom sediments of the Dead Sea. The sulphide-rich anaerobic conditions produced in the sediment by the bacterial reduction of sulphate will also allow ferric oxides and oxyhydroxides brought in with or on the clastic grains to dissolve, raising the ferrous ion concentration. This can lead to precipitation of black iron sulphides like mackinawite and greigite which ultimately will convert to pyrite or marcasite (Berner, 1 970) and/or precipitation of iron phosphate (Bray, B ricker & Troup, 1 973). At the same time bubbles of H 2 S or CH4 gas generated by the bacterial reduction process may disrupt or deform the layering in the sediment, leaving a fenestral fabric (cf. Forstner, Muller & Reineck, 1 968). Very shallow perennial saline lakes that are always thoroughly mixed by wind action would probably give a sedimentary record like ephemeral saline lakes and perhaps should be included with such ephemeral lakes.

Windblown dune field subenvironment

The sandy alluvial fan toes, the sandflats and the inflow stream floodplains in saline lake basins are dry and exposed most of the time and without a pervasive vegetation cover. So, whenever protecting efflorescent salt crusts are absent, the surface sediments of these subenvironments are highly vulnerable to wind erosion and deposition (see, for example, Glennie, 1 970, pp. 45-47). Dune fields and deflation flats and hollows are the major products of this wind action. In deep, narrow elongate closed basins, such as the graben valleys ofthe Great Basin of the western U.S., small sand dune fields (a few square kilometres in area, with dunes perhaps no more than 1 0 m high) become established (essentially non-migratory) at the upsloping ends of the basin, fringing the playa flats. This takes place because the winds are forced by the topography to blow up or down the long axis of the valley. In Death Valley (Hunt & Mabey, 1 966, plate 1 ) and Saline Valley (Lombardi, 1 963, plates 1 and 2) in California, the dune fields cover less than 5% and 1 0%, respectively, of the depositional basin area, and rest on sandflat and fan toe substrates. In wide shallow closed basins like those of Lake Eyre, Australia, or the Etosha Pan, Southwest Africa, the dune fields cover most of the basin interior. These massive dunefields mainly take the form of extensive longitudinal dune ridges with bare 'claypans', deflation pavements, caliche- and silcrete-covered pavements, and isolated small ephemeral saline lakes in the interdunal troughs (see King, 1 956, fig. 1 and plates 3, 4, 5; Twidale, 1 972, fig. 1 and pp. 6 1-65; Goudie, 1 973, p. 1 1 7; Glennie, 1 970, enclosure 1 ). Dune sand is not always derived by wind erosion of alluvial sand fringing a playa or salt lake, but may be derived by erosion of the playa or dried saline lake surface. In these cases the dunes may be composed of saline minerals like gypsum originally precipitated within the playa or lake mud (McKee, 1966; Glennie, 1 970, p. 1 36) or perhaps even mud intraclasts (Huffman & Price, 1 949). The diagnostic sedimentary features of aeolian sediments have been well described by many authors, particularly McKee & Tibbitts ( 1 964), McKee ( 1 966), Sharp ( 1 966), Glennie ( 1970), Bigarella ( 1 972) and Hunter ( 1 977) and we refer the reader to these works.

28

L. A. Hardie, J. P. Smoot and H. P. Eugster

Perennial stream floodplain subenvironment

Inflow to perennial saline lakes is invariably dominated by one or more perennial streams. For example, almost 80% of the water reaching the Dead Sea annually is brought in by the Jordan River which rises from spring-fed Lake Tiberias and flows 105 km down the narrow Jordan Valley before reaching the Dead Sea (see Neev & Emery, 1 967, pp. 72-73). To maintain Great Salt Lake, Utah, 82% of the present annual inflow is supplied by the perennial Weber, Jordan and Bear Rivers (Hahl & Langford, 1 964) that rise in the towering Wasatch Range which makes the eastern buttress of the basin. The depositional elements of perennial stream floodplains, both meander and braid streams, are among the best known of all modern sedimentary environments, and so will not be repeated here. We refer the reader to works such as those of Doeglas ( 1 962), Harms, Mackenzie & McCubbin ( 1 963), Allen ( 1 965), Harms & Fahnestock ( 1 965), Coleman ( 1 969), Williams & Rust ( 1 969), Allen ( 1 970, pp. 1 1 8- 1 48), McGowan & Garner ( 1 970) and Reineck & Singh ( 1 975, pp. 225-263), where point-bar, scroll-bar, levee, crevasse-splay, flood basin, ox-bow lake, braid-bar, braid-channel, etc. deposits are described and diagnostic vertical sequences outlined. We would add only that in saline lake basins, perennial stream floodplains would be subject to arid conditions. So the normally subaerially exposed subenvironments like levees and upper point-bars may have significant caliche and/or silcrete deposits and intrasediment growth of saline minerals such as gypsum, while the floodbasins and ox-bow lakes may become ephemeral saline lakes and/or desiccated mudflats.

Ephemeral stream floodplain subenvironment

Ephemeral streams are sporadically-flooding river systems that are usually dry. Such systems are distinguished from alluvial fan-sandflat complexes by their longitudinal extent away from the source highlands. They are a major subenviron­ ment component of wide shallow closed basins like Lake Eyre, Australia (Bonython & Mason, 1 953) and the Etosha Pan, South-west Africa (Gevers, 1 930), or of sandy deserts like those of Libya or the Arabian Peninsula where the ephemeral streams are known as 'wadis' (Glennie, 1 970, pp. 29-56, p. 198). Ephemeral stream floodplains are simply an expanse of braid channels and braid bars (Bonython & Mason, 1 953, plate 1 ; Twidale, 1 972, fig. 17; Glennie, 1 970, pp. 30-32; Williams, 1 97 1). Grain size of the sediment decreases away from the highlands (Williams, 197 1 , p. 2; Picard & High, 1 973) so that gravel bars and gritty channels may predominate near the source areas and fine downstream to sandy material. The floodplains are commonly partly covered with windblown dunes that have derived their sand from the dry stream beds. The surface sediment may be coated with caliche crusts, silcretes or even gypsum (Bonython & Mason, 1 953; Bonython, 1 956; Gevers, 1 930; Goudie, 1 973), or may be cemented by alkaline earth carbonate vadose cements, particularly the braid channel bottoms (Glennie, 1 970, pp. 33-36). The diagnostic sedimentary structures of ephemeral stream floodplain deposits have been described by Williams ( 1 97 1 ), Karcz ( 1 972) and Frostick & Reid ( 1 977), and have been discussed and illustrated at some length by Glennie ( 1 970, pp. 1 1 - 1 4, pp. 29-56) and Picard & High ( 1 973). Because ephemeral streams are such important features of saline lake basins, both in floodplains and in alluvial fans, we will briefly summarize

Saline lakes and their deposits

29

their sedimentary record. In typical braid stream fashion ephemeral stream deposits consist of cross-cutting lenses of coarse braid bar sediments and finer channel sediments (Doeglas, 1 962; Williams & Rust, 1 969) that show inclined planar lamination and thin bedding, antidune wavy bedding, megaripple cross bedding, horizontal planar lamination, ripple cross lamination, mud-cracked clay drapes, mud­ chip pockets, and steep-edged scour-and-fill. These lenses occur as fining-upward sequences that reflect a waning flow-regime. Near the source area highlands the deposits are mainly channel-fill conglomerates that show inclined bedding and grits with horizontal to low inclination lamination. Trough cross-bedded and planar to wavy laminated sands and grits and ripple cross-laminated sands dominate the middle of the floodplain. At the distal ends near the saline lake finer-grained sand with horizontal planar to wavy lamination and ripple cross lamination is the norm. The crucial evidence for an ephemeral, as opposed to a perennial, braid stream is the presence of ( 1 ) mud-cracked mud drapes (Glennie, 1 970, pp. 49-55) at the top of thin (tens of centimetres) waning flow sequences that are dominated by relatively fine­ grained upper flow regime structures, and (2) mud-chips (peloids) scattered through the sands or as grit, breccia or conglomerate pockets and lenses. Glennie ( 1 970, p. 1 2) stressed the presence of rippled and horizontally-laminated aeolian sand beds separating the water-laid sequences. Picard & High ( 1 973) suggested terraced channel sides formed by receding flood waters are characteristic of ephemeral streams. Finally it is the presence of caliche, silcrete and gypsum crusts, alkaline earth carbonate vadose cements and saline mineral intrasediment growths (e.g. gypsum crystals, anhydrite nodules) that record the essentially arid climate ofthese ephemeral stream deposits in saline lake basins.

Springs and spring-fed ponds

Springs, although small features, are important in saline lake basins because they not only are perennial suppliers of water and particularly of dissolved solutes to many lakes (see, for example, Bentor, 196 1 ; Jones, 1 965; Hardie, 1968; Eugster, 1 970) but they are settings for primary production of chemical and biochemical sediment such as travertine and tufa. Springs, which are surface outlets of groundwater, usually discharge in one of two situations: ( 1 ) along faults, commonly at the apices of alluvial fans, at saline lake margins and even beneath lakes (Scholl, 1 960; Jones, 1965; Hunt et a!., 1 966, p. 29; Hardie, 1 968; Neev & Emery, 1967); and (2) at the intersection of a porous sediment (aquifer) and impermeable layer (aquiclude) such as where alluvial fan debris has built over mud flat or lake bottom sediments (Jones, 1965, p. 1 5 ; Hunt et a!., 1 966, p. 29; Hahl, 1968). There are many other spring settings, of course, but these two modes are apparently the most common in saline lake basins. Once the springs discharge, the outflow may quickly seep into a porous substrate after flowing a few metres (a situation commonly found in alluvial fans, e.g. Jones, 1 965, p. 14; Hardie, 1 968), they may feed perennial streams (Slack, 1 967; Dunham, 1972, p. 1-5 8), they may form ponds, marshes or 'lagoons' (Baker, 1 958; Jones, 1 965, p. 1 5 ; Hunt et a!., 1966, pp. 32-36, fig. I I ; Eugster & Jones, 1 968), or they may mix directly with saline lake waters as they exude into them (Scholl, 1960; Scholl & Taft, 1 964). The compositions of spring waters vary considerably reflecting the history of the

30

L. A. Hardie, J. P. Smoot and H. P. Eugster

groundwater. Some examples of this variation are: ( l ) springs are dilute where groundwater is near its source such as the apices of alluvial fans or the proximal portions of ephemeral streams (Jones, 1 965, table 7; Hardie, 1 968, table 2). These springs can reach supersaturation with alkaline earth carbonates, particularly low-Mg calcite, by 'degassing' col from the spring water on encountering the atmosphere, or by evaporative concentration of the waters as they flow on the surface; (2) groundwaters may be concentrated by evaporation in the vadose zone of porous sediments (such as the alluvial fan) resulting in spring waters which are supersaturated with respect to high-Mg calcite, protodolomite or even gypsum (Hunt et al. , 1 966, figs 39, 44, 45 and 47); (3) solutes may be added to ground waters by seepage of storm runoff that has dissolved surface saline crusts. This groundwater can emerge near the lake as brine springs, such as described by Jones ( 1 965, table 7, p. 30), Hunt et al. ( 1 966, tables 50 and 52), Hardie ( 1 968, table 2) and Eugster ( 1 970); (4) springs may be fed from deep circulating groundwater that dissolves old evaporites (see Bentor, 196 1 on the Dead Sea brine springs); (5) hot groundwaters may react with bedrock to produce brines with unusual compositions (see Muffler & White, 1 969 on the Salton Sea, California). The most obvious and probably most diagnostic depositional products of springs are travertines and tufas. These structures are composed of alkaline earth carbonates, usually low-Mg calcite (Slack, 1967; Irion & Muller, 1 968) and occasionally high-Mg calcite (Barnes & O'Neil, 197 1 ) . Travertine and tufa form low mounds, sheets, coated grains, and pore-filling cements at spring orifices, along outflow channels and margins of ponds and marshes (see for instance Hunt et al., 1 966, p. 1 6; Slack, 1 967; Dunham, 1972, p. I-58 and fig. I-5 8) and apparently form 'pinnacles ' where springwaters exude into lakes (Scholl, 1 960; Scholl & Taft, 1964). Travertines are laminated, commonly with alternating layers of colour-banded micrite and fenestral, palisade-structured micrite (Smoot & Hardie, in preparation; Irion & MUller, 1 968, figs 3, 4 and 5; Dunham, 1 972, figs I-59-I-63; Monty, 1976, fig. 7). Tufas are generally less well laminated, showing predominately a fenestral fabric, like the porous layers of travertines (Scholl, 1960; Scholl & Taft, 1964; Golubic, 1969; Monty, 1 976, figs 28 and 29) . The laminar layers probably form by precipitation of carbonate minerals from films of water while the porous layers and tufas are produced by precipitation around plants such as algae or moss, so that the structures produced are 'chemical stromatolites'. The role of the plants is probably as a static substrate, but they may aid precipitation by providing a preferred nucleation surface or increasing supersatura­ tion by photosynthetic uptake of C01 (see Weed, 1 889, or discussion in Hardie, 1 977, pp. 1 70- 175). The porous tufas and fenestral travertines are very friable and can break down into carbonate sediments (peloids from granule to mud size) which Smoot ( 1976, 1978) emphasizes as a major source of non-skeletal carbonate sediment production in arid basins. McGannon ( 1 975) presented a similar model in describing a Pleistocene, cross-bedded, fining-upward fluviatile deposit composed of tufa fragments, ooids and pisoids that could be traced back to a spring source. Other chemical deposits reportedly made by springs include: ( l ) 'chemical deltas' which result from springs mixing with saline lake water. Deposits that have been interpreted in this way include the oolitic sands of northern Pyramid Lake, Nevada (Surdam & Wolfbauer, 1 975, p. 343) and the protodolomite muds(?) ofMound Playa, Texas (Reeves, 1 968, pp. 64-65, figs 5 1 , 53 and 54); (2) Hunt et al., ( 1 966, pp. 56-59) describe spring-fed marshes floored with precipitated masses of crystalline gypsum as well as crusts containing glauberite, thenardite, halite and trona; (3) hot springs

Saline lakes and their deposits

31

surfacing along faults in Lake Magadi are precipitating alumino-silicate gels (Eugster

& Jones, 1 968); and (4) silica sinters are forming around hot springs in Yellowstone National Park, Wyoming (Weed, 1 889; Walter, 1 976; Walter, Bauld & Brock, 1 976)

and similar sinters have been reported in Pleistocene Lake Lahontan marginal sediments (Morrison, 1 964, fig. 1 2). Finally, spring ponds and marshes may support, in addition to a rich flora (algae, fungi, saltgrasses, rushes, pickleweed, cattail, mesquite, tamarask, palm, etc., see Hunt, 1 966), brineshrimp (Jones, 1 965, p. 17) and desert fish (Hunt et a!., 1966, p. 35; Beadle, 1 974). The remains of the vegetation can accumulate at the bottom of these spring ponds, marshes and 'lagoons' to produce organic-rich layers (Jones, 1965, p. 1 5) which could become buried by storm runoff sediment layers or chemically precipitated saline mineral layers. Shoreline features

Shoreline features such as deltas, beaches, beach ridges, spits, bars and platform and mound build-ups have not been widely reported from saline lakes. This may well be due to the fact that most of these features are products of strong currents and/or energetic wave action found only in large relatively deep perennial lakes, a setting that is uncommon in arid closed basins. During the flooding of Lake Eyre, Australia, in 1 949-1 950 the dune sands and ephemeral stream sediments were worked into spits, longshore bars, beaches and beach ridges along the temporary lake shore (see Bonython & Mason, 1 953, plate 3 and King, 1 956, fig. 4). Presumably these features would look like their non-saline temperate perennial lake (Gilbert, 1890, pp. 23-89) and marine counterparts (see Reineck & Singh, 1 973, pp. 280-349). Deltas in freshwater perennial lakes have been examined (e.g. Houbolt & Jonker, 1 968; Forstner et a!., 1 968) but to our knowledge similar detailed studies on saline lake deltas have not been published. Two unusual kinds of 'delta' in saline basins have been reported: ( 1) Krinsley ( 1970, see reprint in Neal, 1 975) has mentioned a 'fan delta' from the Sabzevar Basin, Iran, which seems to be a lobate deposit (characteristics not described) that shed directly from an alluvial fan into a temporary lake; and (2) a 'chemical delta' appears to be forming at the mouth of the narrow channel between the Gulf of Karaboghaz and the Caspian Sea (Teodorovich, 1 96 1 ; Dickey, 1 968) due to precipitation of alkaline earth carbonates as the inflowing Caspian Sea water concentrates by evaporation on entering Karaboghaz. Broad submerged shoreline platforms and isolated mounds built up of marly sediment have been reported from freshwater 'marl lakes' (Hooper, 1956; Wilson, 1936, 1 938; Moxham & Eckhart, 1956; Wetzel, 1 970) but not as far as we know, from arid climate saline lakes. This may simply be an oversight. Subaqueous algal mounds and ooid sand shoals have been described from Great Salt Lake (Eardley, 1 938; Carozzi, 1 962) but it is not clear whether these features are being actively produced today or under what conditions they formed (see Halley, 1976). S O M E E X AM P L E S O F S AL I N E LA K E D E P O S I T I O N AL COMPLEXES

No two saline lake basins are exactly the same (cf. Twidale, 1972, p. 2 1 2) because mode of origin, tectonic setting, bedrock types and patterns, regional and local climate,

32

L. A . Hardie,

J.

P. Smoot and H. P. Eugster

v v v vC' v v v v v v v 0.>-.>- v v v v v v
=

=

(m)

a/. ( 1 97 1 . 1 973 ) and Hecky & De gens

KiYU

Tanganyika

Catchment

7. 1 40

Lake

2.060

23 1 .000 (exclusive of Kivu) 32.600

240 485 5 83 0·33-0-4 3·2 * 1 10 190

Z Mean depth zm, maximum depth Volume (km3) v Volume storage-area relationship t>. VI t>.A L Outflow (km3 /year) Residence time water (years) Refill time (runoff alone) (years)

Depth

et

=

=

570 1 .470 1 8.880 0·66 2·7 430 1 .000

*Ten year average discharge supplied by Amenagement Hydro-Electrique des Chutes de Mururu for its dam on the upper Ruzizi River.

Table 2. Chemical composition of Lakes Kivu and Tanganyika and major tributaries to Lake Tanganyika.

Source of data: a, b and c from Degens et a/. ( ! 97 3 ), d from Beauchamp ( 1 939). n.d., not determined Na (mg/1)

K (mg/1)

Ca (mg/1)

Mg (mg/1)

Cl (mg/1)

so, (mg/1)

co, (mmol/kg)

a Kivu 1 2 1 ·6 b R uzizi (near B ukavu) 1 1 7·0 c Tanganyika 66·3 d Malagarisi 1 6-4

97-4 1 00·2 34·2 2·4

4·8 4·6 8·2 12·9

87 88 4 1 ·5 9· 1

55 n.d. 21 1 5 ·5

23·8 n.d. 3-4 2· 1

1 2·5 1 3· 1 5·64 1 · 55 (mEq/ 1 )

The density increase produced b y higher salinities just offsets the density decrease inherent with the higher temperatures which results in a stable thermohaline density structure. This meromixis greatly reduces the exchange of water between the upper 70 m of water which circulates at least seasonally and the anoxic water of greater depth. Lake Tanganyika is thermally stratified with a perennial thermocline at approximately 1 00 m. Vertical temperatures and major ion distributions can be found

(a )

100

20

l...

.

�

40

60

Concentration (mg /1) 80

0

I. . "

-·

-

100 I

..

. .. . .. . .

{.

100 200

150 I

.. .

200 I

0

250 I

Mg

100 I

\

300 I

� .... ...... . ..: ·-...

150 '

� i 250

400 I

•

350 --l

..

24

25

�

26

'

� .... ..,

'I>

(_____

V:!

� � �

!::> ;:s !::>..

� �

I

I

. ... .. ...

. '" ·

.

• • .

.

..

K

·: :.

..

··

300

.

.

200 I

.. .

Temperature (•c)

-

Ca

Q) c

23

50

-·

Q.

(b)

400

•

-

.s:.

400

300 I

-

. •. .

300

e

200 I

!

.

200

400

100 I

.

.. ,..

Na

�.'1

).. .

450

Fig. 2. (a) D istribution of major cations (Ca, Mg, K, Na) in Lake Kivu waters from several stations, after Degens et al. ( 1 973). (b) Temperature profile versus depth for Lake

Kivu (deep northern basin) after Degens et al. ( 1 973).

� (")

�

Kivu- Tanganyika Basin

47

in Degens et a!. ( 1 9 7 1 ). The Ruzizi River presently flows from Lake Kivu to Lake Tanganyika and is the major contributor to the water and salt budget of Lake Tanganyika (cf. Table 1 ). The impact of the Ruzizi on Lake Tanganyika is clearly reflected in the chemistry of the surface waters (Table 2) especially the proportions of potassium and magnesium.

S EDIMENTS Lake Tanganyika

Eleven cores were studied. B ased on sediment colour, diatoms and mineralogical composition, four stratigraphic units can be recognized (Fig. 3). Sedimentation rates are in the order of 30 - 50 em per 1 000 years in the deep basins and approximately ten times less on the shallow sill which separates the two deep basins (Degens et a!., 1 9 7 1 ) . The longest core studied was 1 0 · 74 m ; i t was taken b y the Department o f Zoology of Duke University, Durham, N.C., in the southern part of the lake. This core (core TL 2) is dark grey and stratigraphically uniform; the average sedimentation rate is 49 em per 1 000 years (Livingstone, 1 965). 0 Sill Lake Tongonyi ko Sediment stratigraphy

5 .c::

a.

500

"' "0

Northern Basin

[:::J I Corbonotes/5Y2 / I - N 6 EJ 2 Koo linite/N4 [:::J 3 Smectite/595/1 I!]]J 4 Smectite/N6

Nontronite layer

Ar- Aragonite Ioyer MQC - - Mg - Colcite Ioyer Carbonate content>IO 0/o

Strati graphic units

0

oooo

1 00 Di s t a n c e

( kml

200

250

Fig. 3. Gross stratigraphy of selected cores from Lake Tanganyika based on characteristic minerals and

sediment colour.

Unit 1 (the youngest) is characterized by alternating light (diatoms) and dark layers (clay) forming a varve-type pattern. High-magnesium calcite (9- 1 1 mol per cent MgC03 in the northern basin, 7-9 mol per cent in the southern basin) is present as well as three thin aragonite layers which only occur in the cores from the northern basin. Kaolinite is by far the dominant clay mineral. The abundant diatoms all belong to the

48

Peter Stoffers and R. E. Hecky

genus Nitzschia. Organic carbon content is high ( > 4%) and pyrite in the form of framboids is found frequently. Unit 2 is medium grey in colour. Lamination is not as pronounced as in the first unit. The dominant components are kaolinitic clays and diatoms (Stephanodiscus, Melosira). Organic carbon content is high (6- 1 2%) and pyrite layers are frequent. No carbonates were detected. Concerning their texture, these two upper units are mainly clayey silts although sand layers are present occasionally which are mostly of turbiditic origin. Units 3 and 4 are only found in the cores fmm the sill area. Unit 3 is bluish-grey in colour whereas Unit 4 is light grey. Both are composed of carbonate-free fine-grained mud. The clays are predominantly smectite . Organic carbon content is low ( 1-2%) reaching its lowest values in Unit 4. No pyrite w as found in these units. Microfossils are rare.

Lake Kivu

Ten con :s from the various basins of Lake Kivu were studied. Details on the mineralogy and chemistry of the sediments can be found in Reeky & Degens ( 1 973), Degens & K ulbicki ( 1 973) and Stoffers ( 1 975). In Fig. 4 a stratigraphic correlation of some repres �ntative cores based on the occurrence of characteristic minerals is attempted. T he 14C dates are from Reeky & Degens ( 1 973). It seems that during the coring operat: on a substantial part of the upper sediment section was lost in some cores (A. Driscoll, 1 Jersonal communication). Especially at Station 4, superpenetration of 1 he corer was I toticed which is confirmed by stratigraphic considerations, as well as by ; tge dating. A c 'ate of 1 1 , 200 years B .P. was obtained from the top of this core. The base 1 ;ave an age of 1 3,560 years B.P. which is the oldest sediment recovered from the lake. ! :edimentation rates for the upper parts of the cores can only be given as an ; tpproximation They are greater than 30 em per 1 000 years based on core 9 and 1 0, .vhere not too Tiuch sediment seems to be missing. In the lower parts of the cores, sedimentation t ates are much higher, ranging between 1 and 5 m per 1 000 years. Core 1 4, take1 t in 3 1 0 m water depth in the western par� of the lake, is particularly noteworthy. In , he lower part of the core, well-rounded poorly sorted pebbles­ prin ;ipally of ml tam orphic rocks-and shell fragments, are the dominant constitu­ ents No sign of ! :rading was observed. F ·om Fig. 4 it c an be seen that the oldest sediments deposited in Core 1, 1 2 and 1 3 are s iderite-rich se iiments In Core 1 3 and 1 2 these sediments are coarse grained with a . 'and content of me re than 25%. Especially in Core 1 3, these sediments have a crumbly tt. xtu -e similar to a soil. During the deposition of the siderite-rich sediments, finely­ lan.'in 1ted organic-rich muds irregularly interrupted by maj or diatomite intercalations ( > 0· 5 em) were dc.oosited at Station 4 which is situated in the deepest area of Lake Kivu. , \lso at Statio.1. 4 are found infrequent layers of manganosiderite, one of which wa s ap Jroximately 1 em thick and nearly pure. The siderite-rich sediments are topped by a se< liment sequence alternating between carbonate-free pyrite-rich sediments and ara. �onitic muds. It t the lower parts of the cores, the pyrite-rich muds are dark grey, texturally stiff and struc tureless. At Stations 1 and 9, these sediments are mainly clays (kaolinite) whereas at St� tions 1 0, 1 2 and 1 3 silts dominate. Towards the top of the cores, the pyrite-rich muds .ue dark brown in colour. These muds often have a gel-like consistency with a high m·ganic carbon content ( > 1 0%). Water content is very high (90-95%). Only in ·

49

Kivu- Tanganyika Basin K 15

K 13

K1

K 12 - -? - -

LOST

LOST

LOST

LOST

I

-�

K 4 ? LOST

K 10

'

?

440m

86m

?

STRATIGRAPHY

OF

LAKE KIVU SEDIMENTS

�o�o;o:o�o�o�o�o;� Aragonite � :��ydro­ (:·:·:·:·:·:·:·:·:·:·:·:·1: ��t;da�f��i�e c=J �����n�ich h C=:J �!�;':,!::�

! 1t0

90m

( W.ter Depth ) 600

700

13 1\0 ! llO

800 l

_____J 402m

Fig. 4. Gross stratigraphy of selected cores from Lake Kivu based on characteristic minerals.

Cores 9 and 10 the carbonate intervals start with high-magnesium calcite ( - 1 2 mol per cent MgC03) and pass up into aragonite. Aragonite muds are found in the lower part of Station 15 which is located in the Bukavu basin. This basin is separated from the main lake by a shallow sill (-30 m). Core 15 is characterized by the presence ofmonohydrocalcite, high-magnesium calcite and protodolomite. The monohydrocalcite intervals are laminated with brownish­ yellow 0·5-3 mm thick layers of monohydrocalcite alternating with brownish-green layers of diatoms. Below this sequence are layers of black sapropelic sediments containing hardly any carbonates. o1RO values for the aragonites present in Station 1 5 fall within the narrow range of + 2 · 2-3· 1 per mil relative to PDB. Also the o13C data are very constant ranging from + 6 · 3 to 7 ·0. The isotopic composition of the monohydrocalcite/protodolomite shows lighter o1RQ and o13C values falling into the range of + 1 · 1 - 1 · 5 and 2 · 5-5 · 7, respectively.

50

Peter Stoffers and R. E. Hecky

Diatoms

Characterization of the biostratigraphy of Lake Kivu is based on siliceous microfossils present in the northern basin cores, 1 0 and 4. These two cores are considered, on the basis of their stratigraphy and their radiocarbon chronology, to give a nearly complete record of sedimentary events through the last 1 4,000 years. The methods of sample preparation and counting are described in Reeky & Kilham ( 1 973). Sampling frequency was at least once every lO em. Ecological interpretations are based on Richardson ( 1 968, 1 969). From the bottom of the superpenetrated Core 4 to 1 30 em in Core 10 only three species comprise well over 90% ofthe flora at any sample level. These three are Stephanodiscus astraea v. minutula, Nitzschia fonticola, and Nitzschia spiculum. Among the other diatoms only Nitzschia acicularis achieves abundances over 5% for short periods of time. The relative abundance of Stephanodiscus astraea declines almost linearly from about 90% at the base of Core 4 to zero at 1 30 em in Core 10 (Fig. 5). Nitzschia fonticola also disappears at this depth interval which forms a sharp contact between laminated aragonitic mud below and dark brown highly organic pyrite-rich mud above. At this interval, a thin layer of volcanic ash is also present. In a space of 5 mm the diatoms change from Nitzschia spiculum to purely Stephanodiscus astraea to relatively short and wide species of Nitzschia such as N. Palea and N. acomodata which are present in the dark brown material. Even more striking are the changes in absolute abundances, as diatoms are scarce in the brown material, and minute siliceous scales of the chrysophyte Paraphysomonas vestita became the most abundant microfossil. In the upper sediment section of Core 1 0, Nitzschia spiculum is joined by N. bacata and N. mediocris as new elements in the diatom plankton. Another new genus is a Chaetoceros sp. This genus is rarely recorded from inland waters, as it is typically marine.

[ B.P.J yrs. X

Frequency % 0

50

100

10 3

2

4

.. "' 9 mol per cent MgC03) is only found in the cores from the northern basin whereas only high­ magnesium calcite ( < 9 mol per cent MgC03) is present in the southern basin. No carbonates could be detected in Core TL2 taken at the extreme southern end of the lake. Fluctuations in the carbonate content reflect the changing amount of water supplied by the Ruzizi to Lake Tanganyika. The stratigraphic record for the two lakes deduced from mineralogical and diatom studies has provided an insight into the interaction of climate and geology in determining the hydrochemistry and sedimentation during the Late Pleistocene­ Holocene period. The climatic interpretation obtained here is more or less correlative with other East African data over the available time period. (cf. Livingstone, 1 975; Stoffers & Holdship, 1 975; Richardson & Richardson, 1972).

REFERENC ES BEAUCHAMP, R.S.A. ( 1 939) Hydrology of Lake Tanganyika. Int. Rev. Hydrobio/. 39, 3 1 6-353. CAPART, A. ( 1 952) Le milieu geographique et geophysique. Exploration hydrobiologique du Lac

Tanganyika ( 1 946- 1 947). Inst. Roy. Sci. Nat. Be/g. 1, 3-27. CouLTER, G.W. ( 1 963) Hydrogeological changes in relation to biological production in southern Lake

Tanganyika. Limno/. Oceanogr. 8, 463-477. DAMAS, H. ( 1 937) La stratification thermique et chimique des lacs Kivu, Edouard, et Ndalaga (Congo

Beige). Verh. Int. Limno/. 8, 5 1 -68. DEGENS, E.T. & KULBICKI, G. ( 1 973) Data file on metal distribution in East African rift sediments. Tech.

Rep. Woods Hole Oceanogr. Ins/. 73- 15, 1-280. DEGENS, E.T. & Ross, D.A. (Ed.) ( 1 969) Hot Brines and Recent Heavy Metal Deposits in the Red Sea.

Springer, New York. DEGENS, E.T. & Ross, D.A. ( 1 976) Strata-bound metalliferous deposits found in or near active spreading

centers. In: Ores in sediments, sedimentary and volcanic rocks (Ed. by K. H. Wolf), pp. 1 65-2 1 1 . Elsevier, Amsterdam. DEGENS, E.T., VoN HERZEN, R.P. & WONG, H.K. ( 1 9 7 1 ) Lake Tanganyika: water chemistry, sediments, geological structure. Naturwissenschaften, 58, 229-24 1 . DEGENS, E.T., VON HERZEN, R.P., WONG, H.K. & J A N NASCH, H.W. ( 1 973) Lake Kivu: structure, chemistry, and biology of an East African rift lake. Geo/. Rdsch. 61, 245-277. HECKY, R.E. & DEGENS, E.T. ( 1 973) Late Pleistocene-Holocene chemical stratigraphy and paleolimnology of the Rift Valley Lakes of Central Africa. Tech. Rep. Woods Hole Oceanogr. Ins/. WHOI 73--28. HECKY, R.E. & K t L HAM , P. ( 1 973) Diatoms in alkaline, saline lakes: ecology and geochemical implications. Limno/. Oceanogr. 18, 53-72. HuTCHINSON, G . E. ( 1 957) A Treatise on Limnology, V ol. I. Wiley and Sons, New York. K UFFERATH, J. ( 1 952) Le milieu biochimique. Exploration hydrobiologique du Lac Tanganyika ( 1 946-47). Inst. Roy. Sci. Nat. Be/g. 1, 3 1 -47 LiVINGSTONE, D .A. ( 1 965) Sedimentation and the history of water level change in Lake Tanganyika. Limnol. Oceanogr. 10, 607-6 tO. LIVINGSTONE, D.A. ( 1 975) Late Quaternary climatic change in Africa. Ann. Rev. eco/. Systematics, 6, 249-280.

Kivu- Tanganyika Basin

55

MOLLER, G . & FORSTNER, U. ( 1 973) Recent iron ore formation i n Lake: Malawi, Africa. Mineral. Deposita, 8, 278-290. RICHARDSON, J . L. ( 1 968) Diatoms and lake typology in East and Central Africa. Int. Revue Ges. Hydrobiol. 53, 299-338. RICHARDSON, J . L. ( 1 969) Characteristic planktonic diatoms of the lakes of tropical Africa. Int. Revue Ges. Hydrobiol. 54, 1 75- 1 76. RICHARDSON, J . L. & RICHARDSON, A. E. ( 1 972) History of an African rift lake and its climatic implication. Ecol. Monogr. 42,499-534. SCHMITZ, D.M. & KuFFERATH, J. ( 1 955) Problemes poses Ia presence de gaz dissous dans les eaux profondes du Lac Kivu. Bull. Seances Acad. roy. Sci. Coloniales, N.S. 1, 326-356. STOFFERS, P. & FISCH BECK, R. ( 1 974) Monohydrocalcite in the sediments of Lake Kivu. Sedimentology, 21, 1 63- 1 70. STOFFERS, P. & HOLDSHIP, ST ( 1 975) Diagenesis of sediments in an alkaline lake: Lake Manyara, Tanzania. IXth International Congress of Sedimentology, Nice, theme, 7, 2 1 1 -2 1 7. STOFFERS, P. ( 1 975) Sedimentologische, geochemische und paliioklimatische Untersuchungen an ostafrikani schen Riftseen. Habilitationsschrift Universitiit Heidelberg, 1 1 7 pp. VERBEKE, J . ( 1 957) Recherches ecologiques sur Ia faune des grands lacs de !'est du Congo Beige. Exploration Hydrobiologique des Lacs Kivu, Edouard et Albert, Vol. 3, Fasc. I , 1 77 pp.

Spec. Pubis int. Ass. Sediment. ( 1 97 8) 2, 57-8 1

Holocene carbonate evolution in Lake Balaton (Hungary): a response to climate and impact of man*

G ER M A N M U L L E R and F R A N K W A G N E R

Institut fur Sedimentforschung, Universitiit Heidelberg, P. O. Box 1 0 30 20 D-6900 Heidelberg, West Germany

AB STRACT

Mineralogy, geochemistry and oxygen isotope composition of Lake Balaton carbonate sediments reflect fluctuations in the composition of the lake water, which were strongly influenced by both climate and man during the past 8000 years. During the 'Pre-Roman Stage' (about 7500-2000 y. B.P.) when the lake had no outflow (closed basin), calcite with low Mg, Sr and Na concentrations was precipitated at high water levels during periods with a relatively low rate of evaporation. High magnesian calcite with up to 20 mol per cent MgC03 and elevated Sr and N a concentrations and protodolomite formed at low water levels during periods of high evaporation from solutions with higher Mg/Ca and Mg + Ca/Sr ratios and elevated Na concentration. These conclusions are also strongly supported by oxygen isotope data of the autochthonous carbonates. Lithium, associated with clay minerals correlates positively with Mg, Sr and Na. In addition to the vertical fluctuations in carbonate composition within each core, pronounced lateral changes are found between the different cores: From core A (closest to the main inflow, the Zala River) to core F (farthest distance from the Zala River) the concentrations of Mg, Sr and Na incorporated in carbonates increase more or less steadily. The interstitial waters in the cores show a similar development: The Mg/Ca ratios and the Na concentrations increase generally from core A to core F. Within each core the highest Mg/Ca ratios were found to occur in the lower half of the core where they are still close to the zone where the highest Mg concentrations of carbonate minerals are found. Two periods with evaporation maxima can be traced along the long axis of the lake: one towards the end of the Atlanticum (about 5000 y . B.P.), another towards the end of the Subboreal (about 3000 y. B . P.). After an artificial outflow was built by the Romans about 2000 y. B.P., the lake changed from a closed to an open basin with only minor fluctuations in water level and hydrochemistry

Since then, high magnesian calcite with a more or less

constant rate of MgCO,-, Sr and Na incorporation has been precipitating during periods of algal blooms. *Dedicated to Professor Dr Martin Schwarzbach, Ki:iln on the occasion of his 70th birthday.

Modern and Ancient Lake Sediments Edited by Albert Matter and Maurice E. Tucker © 1978 The International Association of Sedimentologists. ISBN: 978-0-632-00234-4

58

German MUller and Frank Wagner FOREWORD

Already in his first semester, a student of geology will learn about the importance of some sediments as climatic indicators. Tillites derived from glaciers are witness of cool climate, whereas coral reefs are believed to have been grown in warm seas. He will also learn of the paradox that raindrop imprints are more an indicator of a warm and arid climate than of a climate of abundant rainfall. During the past decades, methods used in the reconstruction of palaeoclimates have become more sophisticated. The application of geochemical and isotope-geochemical methods and the addition of clay minerals, zeolites and other sedimentary materials have enlarged our knowledge of how climatic changes can be recognized in sediments. The present study is an attempt to apply such methods to homogeneous lacustrine carbonate muds with the aim of reconstructing the historical development of a lake and its sediments and to develop a stratigraphy based mainly on the chemistry and mineralogy of the autochthonous carbonates. Since carbonate chemistry is strongly related to the chemistry of the water from which precipitation took place, the carbonates strongly reflect the chemistry ofthe lake water at the time of their deposition. As the water level and the hydrochemistry of a lake (especially of a closed basin), is a response to the ratio evaporation : inflow + precipitation, climatic changes might be expressed in the carbonates precipitated in the lake. This is not the first attempt to reconstruct palaeoclimatic conditions from carbonate composition. In a study of numerous lakes from different climatic zones, Muller, Irion & Forstner ( 1 972) established the general relationship between carbonate mineralogy and Mg/Ca ratio of the lake or interstitial water from which precipitation or transformation took place. Stoffers ( 1 975) applied changes in carbonate mineralogy of sediments from Lake Victoria (Africa) to reconstruct palaeo­ Mg/Ca ratios. Stoffers & Reeky ( 1 97 8) used mineralogical and oxygen isotope changes in sediments of the Kivu-Tanganyika Basin to establish palaeo-salinities. Rothe, Hoefs & Sonne ( 1 974) observed an enrichment in 180 in Tertiary carbonate sediments from the Mainz Basin (Germany) by repeated evaporation of a closed basin which was paralleled by increasing concentration of Sr. Preliminary results of a combined oxygen isotope and trace element study of Precambrian carbonate sediments from the Bambui Group, B razil (in preparation), clearly indicate that even in very old and diagenetically altered carbonates, these methods might be useful to show changes in water composition which could then be related to general climatic conditions during the time of carbonate deposition. INTRODUCTION

Lake B alaton is one of the best studied lakes in the world. Towards the end of the last century, Louis Loczy ( 1 849- 1 920) organized a research team for the investigation of the lake and its surroundings. Between 1 897 and 1 920 a Balaton Monograph was published in Hungarian (A Balaton Tudomanyos Tanulmanozasanak Eredmenyey ) and German (Resultate der wissenschaftlichen Erforschung des Balatonsees) by the Hungarian Geographical Society with the contributions of sixty scientists 'dealing with the problems of geology, geomorphology, archaeology, ethnology, anthropology and history of the region' (Ronai, 1 969) - an excellent example of inter-disciplinary cooperation.

Holocene carbonate evolution in Lake Balaton

59

The investigation of the lake bottom by underwater coring (thirteen drillings with max. depths of 23· l m) was part of the programme and first results were published in the Appendix ( 19 1 1) to Vol. I, 1 (which appeared in 1 9 16) of the Balaton Mongraph: Uber die Sande des Balatonbodens (G. Melczer), Der Grund des Balatonsees, seine mechanische und chemische Zusammensetzung (P. Treitz) and Die chemische Zusammensetzung des Schlammes und des Untergrundes vom Balaton-Baden (K. Emszt). Treitz assumed two sources for the fine-grained sediment sequence ofthe lake: wind-transported clastic material and chemically precipitated lime-a conclusion which is confirmed by our studies. An investigation of the surface sediments of Lake Balaton (MUller, 1 969, 1 970) revealed that sedimentation in this lake is mainly governed by the precipitation of high-magnesian calcite. According to our knowledge, this was the first report of Recent high-magnesian calcite formation in a fresh water environment. Since then, this mineral has been found in many other lakes and other non-marine environments (Muller et al., 1 972). Analyses of a 1 · 1 5 m sediment core taken in the Lake Balaton surface sediment study showed that the rate of Mg incorporation in the high­ magnesian calcite and other chemical parameters varied with depth. As a consequence, a series of sediment cores comprising the full Holocene sedimentary sequence was taken along the long axis of the lake. The results of the investigations of these cores are presented in this report.

S ETTIN G

A compilation of data concerning the limnology of Lake Balaton was presented by Entz & Sebestyen ( 1946); the period 1 946-1960 is covered in a compilation by Sebestyen ( 1 962). Both reports are printed in German and Hungarian. An Excursion guidebook Study Tours (in English) prepared for the International Symposium on Palaeolimnology at the B iological Research Institute of the Hungarian Academy of Sciences at Tihany, 28-3 1 August 1 967, sums up the most important data on the lake. Geology

Lake Balaton, situated in the centre of the western region of Hungary called Transdanubia (Fig. 1 ), is the largest and shallowest lake in Central Europe. It stretches in a WSW-ENE direction along the border of the foothills of the Hungarian Central Mountains, which are built up of Palaeozoic and Mesozoic sedimentary rocks, partly covered by Tertiary and Quaternary sedimentary rocks (including basalts of Late Tertiary age). Clastic sedimentary rocks of Upper Pliocene age (Pannonian), mostly covered by Quaternary loess and soils, border the southern and eastern side of the lake. The lake itself is in an area of Pannonian sediments which are overlain by a thin cover of Late Pleistocene/Holocene sediments (Ronai, 1 969). Pollen analyses carried out on the lake sediments by Z6lyomi ( 1 953) clearly revealed 'that the present lake as a whole was born at the beginning of the Holocene' (Ronai, 1 969). Climate

The Lake Balaton area has a particular meso-climate within the general climatic conditions of Hungary. It belongs to the climatic belt with prevailing Westerlies and

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German Muller and Frank Wagner

68

behaviour clearly speaks for a mineral with the properties of protodolomite rather than high-magnesian calcite or 'pseudo-dolomite' (Berner, 197 1) . Gaines's ( 1 977) proposal, t o apply the term protodolomite strictly only t o minerals with order reflections in diffraction patterns and to apply the term 'pseudodolomite' to minerals in which cation-order cannot be unequivocally demonstrated, does not take into account other important physical and chemical properties. Until a better definition for these types of sedimentary dolomite-like Ca-Mg carbonates, which are quite common in Recent non-marine sediments, has been found, the designation 'cation-disordered protodolomite' is chosen to describe a mineral with properties, some of which are typical for either protodolomite or pseudodolomite. Morphologically the cation-disordered protodolomite appears to be identical to high-magnesian calcite and cannot be differentiated by purely optical means, including stereoscan observations (Fig. 2). Fig. 5 shows typical X-ray diffractograms of different grain size classes within a sediment sample from the 'Rosa Zone', core E. The sand fraction ( > 63 ttm) contains mainly detrital calcite; in the medium and coarse silt fractions (6· 3-63�-tm) detrital dolomite is abundant. The clay fraction ( < 2 ttm) consists nearly exclusively of primary high-magnesian calcite and cation-disordered protodolomite. This holds true also in the fine silt fraction (2-6·3 ttm) in which smaller amounts of detrital calcite are also present. Since more than 90% of the carbonates in the muds encountered in this study are made up of fine silt and clay fractions, allochthonous carbonate minerals play only a minor role in the carbonate mineralogy.

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comparative purposes, the various thicknesses of the carbonate mud sections were converted to common (arbitrary) thickness.

Holocene carbonate evolution in Lake Balaton

69

Chemistry of interstitial solutions

From cores A, B, C, E and F, a total of thirty-six sediment sections with a length of 1 0 em each were collected and centrifuged not later than 2 h after retrieving the cores. From the filtered solution the cations (Ca, Mg, Sr, Na and K) were determined by atomic absorption spectroscopy, the anions (C 1, S04, HC03) by standard methods of water chemistry. The results are listed in Table 2. In Fig. 6 some of the data are presented graphically. For comparative purposes, the various thicknesses of the Holocene carbonate mud sections in the different cores were converted to a common (arbitrary) thickness. Results (a) Calcium. Ca concentrations range between 1 0·8 and 25·5 ppm. As a rule higher values normally occur in the top and basal layers of a core, minimum values in the middle or upper third. With one exception (core F), the maximum concentration is always to be found in the top layer. Ca generally tends to decrease from core A to core F. (b) Magnesium. M g concentrations range between 34·8 and 63 ·0 ppm. With one exception (core B), the minimum value always occurs in the top layer. Maximum values are to be found in the middle or in the lower part of the cores. There are no major lateral concentration differences amongst the different cores. (c) Strontium. Sr concentrations range between 0·05 and 0 · 1 7 ppm. Within a core, Sr has some similarity (but less pronounced) with Ca: higher values are found in the top and basal layers, a minimum occurring in the upper third or middle of a core. No pronounced differences between the different cores can be recognized. (d) Sodium. Na concentrations range between 1 8 · 8 and 50 · 8 ppm. With the exception of core B, the N a concentration increases continuously with depth, thus the minimum concentration is to be found in the top layer, the maximum concentration in the deepest layer. The increase is most pronounced in core E ( 1 9 · 8-50·8 ppm). With depth, a general increase of Na occurs within the upper seven sections then lower values were measured in the two lowest sections. From core A to core E the Na concentrations increase continuously. Core F has somewhat lower concentrations than core E, especially in the lower third of the core. (e) Potassium. K concentrations range between 5 ·9 and 1 5·0 ppm. As with Na, there is a general trend towards an increase in K concentrations with depth; the minimum values are always to be observed in the top layer. The lateral variations are less pronounced as with N a, however, cores E and F show distinctly higher concentrations than the other cores. (j) Chloride. Cl concentrations range from 1 0·6 to 23 · 1 ppm. Chloride has a clear tendency to increase from the top to the bottom layer of a core, the steps of increase are, however, smaller than with Na. Lateral differences are also present: Cores E and F have higher concentrations than cores A through D. Core A exhibits the lowest concentration of all. (g) Sulphate. S04 concentrations are the most variable. They vary between 6· 1 and 56 · 4 ppm, i.e. nearly within an order of magnitude. No general trend of distribution can be seen from the data, neither in a vertical nor in a lateral direction. (h) Bicarbonate. H C03 concentrations range between 297 · 8 and 5 85 · 8 ppm. Within a core low values are to be found both in the top and in the bottom layers with a

70

German Muller and Frank Wagner

maximum occurring near the middle of a core. No clear lateral trends in distribution can be traced. (i) Mg/ Ca (atomic) ratio. The Mg/Ca ratio shows the typical development plotted in Fig. 6. Within each core two minima occur: one in (or close to) the bottom and one in the top layer. Between these minima, the Mg/Ca curve bends towards higher Mg/Ca ratios and reaches its maximum at about the top ofthe lower third or the lower half of a core. Only in core E is the development of the curve more irregular. The broad maximum occurring in the other curves is split into two less extensive maxima by a minimum. From core A to core F, the Mg/Ca curves shift continuously from lower to higher Mg/Ca ratios. Again core E behaves differently: its curve crosses the curves of both core C and F irregularly. UJ Sr/Ca 1 000 (atomic) ratio. The Sr/CalOOO ratio varies between 1 ·60 and 3·97. Within a core no general distribution trend is to be observed. Laterally, a general increase from core A to core F may be recognized. (k) NajK (atomic) ratio. The Na/K ratio varies between 3·66 and 7·25. Within a core no common trend of variation can be observed, but generally, the lower parts of a core exhibit somewhat lower ratios than the upper half (exception: core A). From core A to core C, the ratio shows a slight increase. In cores E and F, the ratios are again in the range of core B . (l) Na/Cl (atomic) ratio. This ratio, which permits the calculation o f the portion of Na connected with Cl to form NaCl, varies between 2·02 and 3·69. Within a core, the ratio generally increases with increasing depth. No general trend is to be observed laterally. The results obtained may be summarized as follows. ( 1 ) Vertical distribution (from top to bottom); (a) Na, K, and Cl and the Na/Cl ratio decreases; (b) Mg increases; (c) Ca, Sr, HC03 and the Mg/Ca ratio have minima in the top and bottom layers with a broad maximum occurring towards the middle of the core; (d) Sr and the Sr/CalOOO ratio do not seem to be dependent upon the location in the core. (2) Lateral distribution (from core A to core F): (a) Mg, Sr, HC03 and the Na/Cl ratio do not change significantly; (b) Ca decreases; (c) the Mg/Ca ratio, the Sr/Cal OOO ratio, and Na, K, and Cl increase.

D EV E LO P M E NT O F C A RB O N A T E M I N E R A L O G Y , C HEMI STRY, G EO C H E M I S TRY AND I SOTOPE G EOCHEMISTRY

Core E

The results of mineralogical, chemical and geochemical investigations of core E, including isotope data from Linz ( 1 976) are presented in Tables 3 and 4 and depicted in Fig. 7. It should be mentioned, that the chemical data were obtained from the total sediment, whereas the isotope measurements refer to the < 2 /lm fraction of the sediment only, in which non-detrital carbonates are about the only carbonate minerals. Detritic carbonates occur only in trace amounts (Fig. 5).The original chemical MgC03- and Sr determinations were recalculated on the basis of 1 00% carbonates. Calcite precipitation commences above the gastropod layer with a 4 mol per cent MgC03-calcite which is poor in Sr ( < 500 ppm) and low in 8180 (-3°/00). Within the

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Holocene carbonate evolution in Lake Balaton

77

Two absolute datings carried out on carbonate muds ( < 2 Jlm fraction, practically free of detrital calcite and dolomite Fig. 5) of the 'Rosa Zone', Mg 1 ( 1 26 - 133 em) and the Mg 2 maximum (82-88 em) revealed ages of6640 and 4500 years, respectively. The 'true' ages may be considerably younger, since a 'hard water effect', in freshwater carbonates is capable of falsifying the ages by + 1300-3000 years. If 1 500 years are assumed for such a 'hard water effect', the radiocarbon ages would fit perfectly into the time scale depicted in Fig. 7 . There is no doubt, however, that the Mg 1 maximum developed during the Atlanticum and the Mg 2 maximum during the Sub-Boreal.

DISCUS SION OF RESULTS AND CONCLUSIONS

If the major events of carbonate evolution illustrated in Fig. 9 are redrawn on a time scale independent of the thickness of the different cores, the model of Fig. 10 results. In addition to the vertical chemical and mineralogical differentiation of the sequence, a pronounced lateral differentiation exists as well. From core A (closest to the major inflow of the lake, the Zala River) to core F (in the easternmost part of the lake) a gradient of increasing Mg, Na, Sr, and Li concentration in the sediments is observed. The results of the examination of the interstitial water chemistry lead to a similar conclusion. With the exception of sulphate, whose present irregular distribution is the result of the activity of sulphate-reducing bacteria, all other ions examined in this study (and some ratios derived therefrom) show a pronounced vertical and lateral distribution pattern which can generally be related to the vertical and lateral chemical differentiations observed within the sedimentary carbonates. It should be kept in mind, however, that the present pore solutions do not represent the original composition of the lake water at the time when it was included into the newly­ formed sediment as interstitial water. Mineral-water reactions, diffusion and, to a much greater extent, an upward movement of the pore solution due to sediment compaction must have led to changes in the original chemistry of the solutions. Thus one cannot expect that short-term fluctuations in lake water chemistry - which are well reflected in the sediment's geochemistry - will be preserved in the pore water chemistry. The same lateral gradient as observed in the Holocene carbonate sediments and their interstitial solutions, although less pronounced, exists in the MgC03 contents of the contemporaneous primary carbonate deposits of the lake (Fig. 3), and it seems only logical to assume the same mechanisms for the Holocene carbonate sediments that are in effect today: (a) an increase of the concentration of all ions (with the exception of CaH and HC03 - ) along the long axis of the lake as a result of evaporative dominance over inflow, and (b) the precipitation of calcite (with some MgC03 incorporation) depletes the water in Ca and HC03- and at the same time increases the Mg/Ca ratio. If such changes occur in an open basin (such as the lake represents today), a magnifying effect may be expected at times when the lake had no outlet (closed basin) as was the case before the Romans built an artificial outflow. At least three old terraces ( 140- 1 50 em, 1 80-250 em, and 300-400 em) above the present water level (Keresmaros, 1 939) were witness to this period which hereafter is designated as the 'pre-roman stage' in contrast to the 'post-roman stage' of the lake development during which the lake can be considered to be an open system.

78

German Muller and Frank Wagner 8

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Origin of carbonate sediments

1 17

almost every lamina (see Fig. 1 0), although other mudstones contain no in situ crusts. Dense, white dolomite clasts of sand size are very common in dolomitic sandstones and sandy dolomite mudstones. These are probably derived from erosion of the crusts.

Figs. 8-10. (8) Dolomite crust (light layer in centre of photograph) overlying a heavily desiccated ephemeral

lake mudstone (light wisps are very compacted mudcracks) (Section 7, Fig. l ). Overlying silt scours the mudstone just to the right of the crust. Scale bar is l em long. (9) Thin-section of dolomite crust shown in Fig. 8 . The crust (dark area) grades into and unconformably overlies the laminated peloidal silt underneath. The crust in turn is sharply overlain by a quartz-rich silt. Scale bar is 0·5 mm long. ( 1 0) Dry mudflat mudstone with numerous dolomite crusts (thin white layers) (Section 4, Fig. l ). Note flat clasts of crusts in sediments. Mudcracks (dark vertical disruptions) stop abruptly and form horizontal sheetcracks at the crust just right of centre, while cracks extend across the interface where the crust has been eroded on the left. Mudcracks stop abruptly at the very thin crust at top of the basal layer in this photograph. Scale bar is l em long.

Caliche crusts and cements

Caliches are found in the alluvial fan deposits (Sections 1 0 and 1 1 , Fig. 1 ) as crusts, cements and rinds around the Palaeozoic limestone and dolomite clasts. The caliches occur as both calcite and dolomite, the former as cements in the deposits close to the Uinta Mountain front (unmeasured sections south of Section 1 0, Fig. 1 ), the latter as crusts, rinds and cements in the fan toe deposits. The crusts and rinds are dense micrite with colour banding that has diffuse boundaries while the cements are usually simple micrite intergranular pore fillings. In thin section, the Wilkins Peak caliches have a clotted texture with irregular spar-lined voids, spar-coated quartz grains and disseminated quartz silt grains (Fig. 1 1 ), as are seen in Recent caliche crusts.

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Joseph P. Smoot

Fig. 1 1 . Comparison of thin-sections of a caliche crust from an alluvial fan deposit in Wilkins Peak (a)

(Section 10, Fig. l) and a modern caliche crust from Texas (b). Note the clotted texture, the irregular sparlined voids (v), spar-coated quartz grains (q), and finely disseminated quartz silt (white specks). Wilkins Peak sample is 2 mm wide and the Texas caliche is 3 mm wide.

In modern alluvial fans caliches also commonly occur as crusts, cements and rinds on limestone boulders (Bull, 1 972, p. 65; Lattman, 1 973). The composition of modern caliches ranges from low-Mg calcite to protodolomite (Gevers, 1 930; Friedman, 1 965, p. 266; Goudie, 1 973, p. 21 ). The Wilkins Peak caliches are commonly found as recognizable cobble to granule-size fragments of crusts and cemented lumps of quartz sand in the alluvial fan toe deposits. The more delicate rinds and cements almost certainly provided some fine-grained micrite but such grains would be too small to preserve fabrics diagnostic of their source.

Fig. 12. Mound-shaped, stromatolitic travertine (Section 5, Fig. l) (centre of photo to left of knife) coating eroded surface of siliciclastic sand and in turn covered with siliciclastic silt (sediment in outer fringe of photograph). Note porous internally laminated character of the travertine crust. Knife is 35 em long.

Origin of carbonate sediments

1 19

·Travertine tufas

The travertine tufas occur as stromatolitic mounds on the eroded surfaces of siliciclastic and dolomitic mudstones and sandstones (Fig. 1 2) and as bumpy crusts on desiccated dry mudflat mudstones. The evidence for these carbonate features being travertines is based on their composition and fabric. The composition ofthe stromatolitic structures is independent of the surrounding sediments, for example, a pure dolomite mound will drape a pure quartz sandstone and in turn be covered by siliciclastic silt (Fig. 1 2). This indicates that the mounds are not sediment trapping algal stromatolites, but rather precipitated structures. The internal fabric of the mounds consists of alternating porous micrite layers, commonly with a crude palisade structure, and dense micrite layers with internal lamination (Fig. 13). In thin section the dense laminar layers are either sparry radial 'sinter' fabric (Irion & Muller, 1 968, figs 1 2 and 1 3 ) or very dense with vague vertical structures (the densely calcified zones of Monty, 1 976, fig. 7).

Fig. 13. Internal textures of travertine crusts. A travertine crust from the Wilkins Peak (a) is compared to a

modern travertine from a stream deposit near San Antonio, Texas. (b) The key characteristics are the dense laminar layers with vague internal colour banding alternating with fenestral layers with a tubular, clotted fabric. Wilkins Peak travertine with a vertical palisade fabric (c) is similar to modern stream tufas shown by Irion & Muller ( 1 968, fig. 5) and Monty ( 1 967, fig. 7). Scale bar on each photo is 0·5 em long.

The porous layers are made of micrite clots separated by vertical irregular voids (Fig. 14) which are like the tufa layers of modern travertines where carbonate precipitates around algal filaments or moss thalli (Irion & Muller, 1968; Monty, 1976, p. 207).

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Joseph P. Smoot

Modern travertines of this variety are precipitating at spring orifices and along spring­ fed streams (Slack, 1 967; Irion & Mi.iller, 1 968) where ground waters degas C02 to equilibrate with pC02 in the air and the stream waters are evaporated while flowing along their course. Spring outlets are common in modern playa systems, particularly at the toes of fans, where they feed small streams or shallow ponds on the mud flats (Jones, 1 965; Hardie et a!. , 1 978). The scoured surfaces upon which the Wilkins Peak stromatolitic structures are precipitated are interpreted as small stream cuts and the crust-covered mudstones are believed to result from precipitation of travertine from shallow sheets of water flowing on the mud flat surface. The dolomitic composition of the structures is not inconsistent with the travertine origin as high Mg calcite and protodolomite travertines have been reported in the Recent (Barnes & O'Neil, 197 1 ; Stalder, 1 975), their composition apparently dependent on the MgH /Ca2 + ratio of the waters.

Figs. 14 and 15. ( 1 4) Thin section of porous tufa layers in travertine crust (Section 4, Fig. 1). Note clotted texture of the porous layers and the crude vertical orientation of voids. Dense layers are either dark calcified zones or light-coloured sinter layers. Photograph shows area 1 ·5 em high. ( 1 5) Thin-section showing close­ up of porous tufa layer in Fig. 14. Note the delicate micrite fabric and fine-sand to silt size clots. Compare this to the textures of the peloids in Fig. 7. Photograph shows area 2·5 m m high.

Coarse sand- and gravel-sized fragments of the dense laminar stromatolitic layers are commonly found in the Wilkins Peak sands, but fragments of the porous tufa layers are rare. An analogous situation exists in the modern deposits at Deep Springs Lake, California, where travertines precipitated along spring-fed stream channels on the Birch Creek alluvial fan are almost completely eroded with each major storm (every 3-4 years) (Slack, 1 967) but only a few dense fragments have been found near the fan toe (Jones, 1 963). Specimens of the B irch Creek travertine provided by Slack easily broke down with gentle rubbing to fine sand- and silt-sized micrite peloids. The porous layers of the Wilkins Peak travertines are believed to be an excellent source for the fine-grained dolomite micrite peloids that make up the bulk of the carbonate sediments (Fig. 1 5). The abundance of these structures is difficult to ascertain as they

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121

have limited lateral extent (usually less than 5 0 m) and are frequently covered in weathered outcrops. For instance, in Section 4 (Fig. 1) seventeen travertine layers were found while in Section 5, twenty horizons were found. However, only eight layers were in equivalent horizons between the measured sections, which means there are at least twenty-nine horizons of travertine layers that were not completely removed by erosion there and many more horizons were found in unmeasured portions of the outcrops. Other evidences of travertine layers where no in situ mounds were found are sand layers with gravel fragments that are travertine. These layers are usually traceable into typical sand flat sandstones. Some of the gravelly sands are oolitic, with the ooids being poorly sorted, irregular shaped and often with only one or two layer thick cortices (Fig. 16). These features are very similar to those ofthe Pleistocene fluviatile ooids described by McGannon (1975) which he interpreted as having precipitated from a spring-fed river.

Fig. 16. Poorly sorted oolitic sand from sand flat sandstone (Section 4, Fig. I ). Note the irregular shapes of

the ooids and the grains with only one or two coatings (for instance the two large quartz grains in the upper left hand corner). Compare this with fig. I I in McGannon ( 1 975). Width of photograph represents about 4 mm.

M A S S B A L ANCE OF C A RB ON ATE P R O DU CTI ON

The dolomite crusts, caliches and travertines apparently provided some carbonate sediments in the Wilkins Peak, but did they provide enough to account for all of the carbonate? To test this out the following mass balance simulates the precipitation of travertine from spring waters using a computer programme devised by Hardie & Eugster ( 1 970). A ground water with low average values of solute concentrations from a limestone terrain in an arid or semi-arid region (White, Hem & Waring, 1963) was allowed to 'degas' to equilibrium with atmospheric pC0 2 then was evaporated to four times the initial concentration as would occur in modern springs as they surface and

1 22

Joseph P. Smoot

flow along open stream channels (Slack, 1 967). In this simulation the initial calcium in solution (0·01 g/1) was 90% removed as calcite while the water was still fresh enough for drinking (ionic strength < 0· 1 )! The simulated evaporative concentration and precipitation of calcite led to significant increase in alkali and CO/ - concentration, which is consistent with the sodium carbonate salts (mostly trona) which precipitated from the final brines in the basin centre during Wilkins Peak time. To produce 8-44 X 10'4 kg of carbonate sediment (Bradley & Eugster, 1 969) at the basin edges just by travertine precipitation in 1 X 1 06 years (estimated by Bradley, 1 964, but could be low by a factor of three, Eugster & Hardie, 1 975, p. 327), the yearly total spring water input would need to be 2·8 X 1 0'2 1/year. Table 2 shows a comparison of this value with measured spring inputs into two modern saline basins. Note that the differences in drainage area vary at the same scale as the spring water input, suggesting that the required Wilkins Peak spring inflow is not at all unreasonable. Table 2. Comparison of yearly spring inputs measured for Recent Deep Springs Lake and Great Salt Lake with the calculated value for the Wilkins Peak's Lake Gosiute.

Spring input (!/year) Area (km') Deep Springs Lake Great Salt Lake Lake Gosiute

·

5-4 X 1 09 ! ·O x 1 0 ' ' 2·8 X 1 0 1 2

< 10 350 9500

The computer program in this mass balance could not simulate the precipitation of Mg calcite, which is what is needed for the Wilkins Peak model. The difference in spring input, however, is not considered important enough to invalidate the mass balance for the following reasons: ( 1 ) Fuchtbauer & Hardie ( 1 976) demonstrated experimentally that the MgC0 3 content of a Mg calcite is directly related to the Mg/Ca ratio of the solution and that high Mg calcite (up to 60 mol per cent MgC03 !) will precipitate easily under the appropriate kinetic conditions (see discussion in Hardie, 1 977, p. 1 74), (2) modern high Mg calcite and protodolomite travertines have been reported, so the kinetic conditions in those circumstances must be favourable, (3) the Mg2+ /Ca2 + ratio of most ground waters will rise as calcite precipitates from them (Hardie & Eugster, 1 970) a phenomenon observed in modern alluvial fans as calcite cements precipitate from subsurface waters (Eugster & Hardie, 1 978). That this also occurred in the Wilkins Peak alluvial fans is indicated by the distribution of caliche cements, and (4) the mass balance is based only on travertine precipitation and the initial precipitation of calcite, while the actual conditions included the caliches and dolomite crusts and probably occurred until all of the Ca2+ and Mg2+ ions were removed.

C AR B ON ATE SE DIMENT P R O D UCTION : THE MO DEL

B ased on the environmental reconstruction of the basin, the observed carbonate textures and the hydrologic and chemical constraints imposed by the mass balance, the

Origin of carbonate sediments

1 23

following model is proposed for carbonate sediment production in the Wilkins Peak deposit. Ground waters draining the Palaeozoic limestone-dolomite-orthoquartzite terrain of the Uinta Mountains precipitated low Mg calcite cements within the sediments of the alluvial fan apices, thus increasing the Mg2 +/Ca2 + ratio ofthe waters. At the toes of the fans, high Mg calcite precipitated as caliche crusts, cements and rinds near and at the sediment-air interface. Where the ground waters surfaced as springs at the toes of the alluvial fans, high Mg calcite and protodolomite travertines (chemical stromatolites) precipitated as the waters degassed. More travertine precipitated along shallow cut stream channels as the spring outflow coalesced and drained onto the inactive mudflats. Some travertine also precipitated on the mudflat surface as dense laminar crusts, where the waters were reduced to thin flowing films on the surface. On the dry mudflat surface, very thin high Mg calcite (protodolomite?) crusts precipitated from vadose waters. All of the precipitation occurred during each of the many long periods between sedimentation events. Small storms initiating in the mountains caused flash floods on the fans which eroded caliche crusts and travertines at the toes and as the waters spread out into sheetfloods on the playa surface they continued to erode travertines and dolomite crusts on the surface. The hydrology of the basin ensured that the coarse material was left at the basin edge (quartz and Palaeozoic carbonate sand) while the finer sediment was washed into the basin centre. The fine sediments were almost exclusively fine-sand and silt fragments of travertine, caliche and dolomite crust since those grain sizes were not produced from the source rocks. It should be stressed that this absence of siliciclastic mud is the main reason why the Wilkins Peak Member is such a pure carbonate deposit. Each flood not only carried freshly eroded precipitates, but also intraclasts of desiccated, previously deposited carbonate sediment. Therefore, the accumulation in the basin centre is made up of a large fraction of carbonate sediments that have been through several stages of transport followed by subaerial exposure and desiccation then re-erosion. Floods capable of moving sand flat sands into the central portions of the basin eroded the crusts and carbonate muds (particularly the desiccation mudcrack polygons), thus the initially almost pure quartz sand was gradually diluted by carbonate intraclasts during transport into the centre. The coarser sand grains were left at the basin edge (which includes most of the quartz fraction) so the quartz component of the sands was also decreased by size sorting (the sorting may have been aided by the lower density of the micrite grains due to their porosity). The perennial lake laminites (including oil shales) are made of the small finer-than-silt fraction of the eroded crusts plus a minute amount of clay derived from the source area or brought in by wind. These settled out in the standing water and were reworked by bottom currents. Some calcite-rich oil shales may have been precipitated out of the water column. This model is favoured as the best explanation for carbonate sediment production in the Wilkins Peak Member for the following reasons: ( 1 ) the carbonate sediments are predominantly detrital intraclasts deposited in a subaerially exposed, evaporative environment; (2) the three crustose precipitates are the only obvious source of material similar to the intraclasts in texture and composition; (3) modern dolomite crusts, caliches and travertines all form in conditions similar to those postulated for the Wilkins Peak; (4) the various crusts all can easily break down into the necessary grain sizes, especially the tufa layers of the travertines; and (5) the mass balance suggests that adequate amounts of carbonate minerals can be produced by precipitating crusts at the basin edge to provide all of the carbonate sediments in the Wilkins Peak within a

1 24

Joseph P. Smoot

reasonable depositional time period. The possibility of direct precipitation from standing lake water, intrasediment growth or erosion of Palaeozoic rocks as sources for carbonate sediments have not been disproven. However, the evidence presented does not favour any of these as a major source in the Wilkins Peak, although they may be important in other conditions.

IMPLIC ATION S OF THI S STU D Y

This study has demonstrated the importance of knowing the physical conditions of a sedimentary basin before developing models for 'chemical problems' such as carbonate sediment production. As a mechanism of non-skeletal carbonate production the following implications can be made: ( 1 ) a major carbonate deposit can be made without a major body of standing water; (2) hard, precipitated crusts can provide fine-grained carbonate sediments; and (3) large 'primary' dolomite deposits can be made by the erosion of syndepositional high Mg calcite and protodolomite crusts and their deposition in a place where no other sediment is brought in. This last implication could include the Recent marine crusts, which only form a small fraction of the surface sediments, but could be deposited as the major sediment particles under the proper conditions. The implications for Precambrian carbonate deposits are centred on the aspect of precipitated stromatolites as sources of sediment. The Precambrian contains many examples of stromatolites which seem to be precipitated structures as opposed to sediment-trapping algal structures (see Bertrand-Sarfati, 1 976; Donaldson, 1 976; Hoffman, 1 976). These stromatolites are interpreted as having formed under marine conditions (although some are not obviously so) but this does not mean they cannot be providing sediment. The modern fresh-water algal marshes of the basically marine deposits on Andros Island, Bahamas and the Everglades, Florida are accumulating carbonate muds derived from in situ precipitated algal tufas (Monty, 1 967; Monty & Hardie, 1 976; Gleason & Spackman, 1 973) . This suggests that at least some of the Precambrian carbonate deposits could be entirely derived from precipitated stromatolites. Also some of the large Precambrian carbonate deposits could conceivably be non-marine! The presence of precipitated stromatolites should alert the worker to these possibilities.

A CKNOWLE DGMENT S

This paper is part of the author's doctoral thesis under the guidance of L. A. Hardie and H. P. Eugster. I would like to thank them for introducing me to the Green River Formation and teaching me the tools I needed to work in it. I am especially grateful to Dr Hardie for being a constant source of encouragement and insight. Dave Yanko provided invaluable assistance in measuring sections and collecting specimens for two field seasons and also helped in the preparation of the samples. I am very thankful to Mr James Montgomery and his family for letting Dave and me stay on their ranch

Origin of carbonate sediments

1 25

while working in Wyoming and for providing assistance in finding outcrops. Keith Slack of the U.S.G.S. kindly provided specimens of the Birch Creek travertines. I would like to thank the reviewers, Dr Hans Fuchtbauer and Dr C. C. Reeves, and the editors for their comments and criticisms. Field work was supported by NSF grant No. GA 3 1 076, GSA student grant No. 2 1 22-76 and the Shell Oil Research Fund at Johns Hopkins University. The I.A.S. graciously provided funds to allow me to attend this symposium.

RE FERENCE S ATWOOD, O.K. & BUBB, J.N. ( 1 970) Distribution of dolomite in a tidal flat environment, Sugarloaf Key, Florida. J. Geol. 78, 499-505. BARNES, I. & O'NEIL, J.R. ( 1 97 1 ) Calcium-magnesium carbonate solid-solutions from Holocene con­ glomerate cements and travertines in the Coast Range of California. Geochim. cosmochim. A cta, 35, 699-7 1 8 . BATHURST, R.G.C. ( 197 1 ) Carbonate Sediments and Their Diagenesis. Developments in Sedimentology, 12. Elsevier Publishing Co., Amsterdam. BERTRAND-SARFATI, J. ( 1 976) An attempt to classify Late Precambrian stromatolite microstructures. In: Stromatolites (Ed. by M. R. Walter), Developments in Sedimentology, 20, pp. 25 1-260. Elsevier, Amsterdam. B RADLEY, W.H. ( 1964) Geology of the Green River Formation and associated Eocene rocks in southwestern Wyoming and adjacent parts of Colorado and Utah. Prof Pap. U.S. geol. Surv. 496-A. BRADLEY, W.H. & EuGSTER, H.P. ( 1 969) Geochemistry and paleolimnology of the trona deposits and cssociated authigenic minerals of the Green River Formation of Wyoming. Prof Pap. U.S. geol. Surv. 496-B. BULL, W.B. ( 1972) Recognition of alluvial-fan deposits in the stratigraphic record. In: Recognition of A ncient Sedimentary Environments (Ed. by 1. K . Rigby and W. K. Hamblin), Spec. Pubis Soc. econ. Paleont. Miner., Tulsa, 16, 63-83. CHILINGAR, G . V. ( 1956) Relationship between Ca/Mg ratio and geologic age. Bull. Am. Ass Petrol. Geol 40, 2256-2266. CULBERTSON, W.C. ( 196 1 ) S tratigraphy of the Wilkins Peak Member of the Green River Formation, Firehole Basin quadrangle, Wyoming. Prof Pap. U.S. geol. Surv. 424-D, 170- 1 73 . DoNALDSON, J.A. ( 1976) Paleoecology of Conophyton and associated stromatolites i n the Precambrian Dismal Lakes and Rae Groups, Canada. In: Stromatolites (Ed. by M. R. Walter), Developments in Sedimentology, 20, pp. 523-534. Elsevier Publishing Co., Amsterdam. EuGST:2R, H.P. & HARDIE. L.A. ( 1 975) Sedimentation in an ancient playa-lake complex: the Wilkins Peak Member of the Green River Formation of Wyoming. Bull. geol. Soc. Am. 86, 3 1 9-334. EUGSTER, H.P. & HARDIE, L.A. ( 1 978) Saline Lakes. In: Physics and Chemistry ofLakes (Ed. by A. Lerman). Springer Verlag, New York. FRIEDMAN, G . M. ( !965) Occurrence and origin of Quaternary dolomite of Salt Flat, West Texas. J. sedim. Petrol. 36, 263-267. FOCHTBAUER, H . & HARDIE, L.A. ( 1 976) Experimentally-determined homogeneous distribution coeffici­ ents for precipitated magnesium-calcites: application to marine carbonate cements. A bs. Prog. geo/. Soc. A m. Meetings, 8, 877. GEVERS, T.W. ( 1 930) Terrester Dolomit in der Etoscha-Pfanne, Sudwest-Afrika. Z. Mineral. 6, 224-230. G INSBU RG , R.N., REZAK, R. & WRAY, J.L. ( 197 1 ) Geology of Calcareous A lgae (Notesfor a Short Course), Sedimenta I. Comparative Sedimentology Laboratory, University of Miami. GLEASON, P.J. & SPACKMAN, W. ( 1 973) The algal origin of a freshwater lime mud associated with peats in the southern Everglades. Abs. Prog. geol. Soc. Am. Meetings, 5, 398-399. GOUDIE, A.S. ( 1 973) Duricrusts in Tropical and Subtropical Landscapes. Oxford University Press, London. HAGAN, G.M. & LOGAN, B.W. ( 1 974) Development of carbonate banks and hypersaline basins, Shark Bay, Western Australia. Am. A ss. Petrol. Geol. Mem. 22, 61- 1 39.

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W.R. ( 1 965) Geology of the Flaming Gorge Area Utah-Colorado-Wyoming. Prof Pap. U.S. geol. Surv. 490. HARDIE, L.A. ( 1 977) Algal structures in cemented crusts and their environmental significance. In: Sedimentation on the Modern Tidal Flats of Northwest Andros Island, Bahamas (Ed. by L. A. Hardie), pp. 1 59-177. Johns Hopkins University Press, Baltimore. HARDIE, L.A. & EUGSTER, H.P. ( 1 970) The evolution of closed-basin brines. Spec. Pap. Miner. Soc. A m. 3, 273-290. HARDIE, L.A. & GINSBURG, R.N. ( 1 977) Layering, the origin and environmental significance of lamination and thin bedding. In: Sedimentation on the Modern Tidal Flats of Northwest Andros Island, Bahamas (Ed. by L. A. Hardie), pp. 50- 1 23. Johns Hopkins University Press, Baltimore. H ARDIE, L.A., SMOOT, J.P. & EuGSTER, H.P. ( 1 978) Saline lakes and their deposits: a sedimentological approach. In: Modern and A ncient Lake Sediments (Ed. by A. Matter and M. E. Tucker). Spec. Pubis int. Ass. Sediment. 2, 7-4 1 . HOFFMAN, P . ( 1 976) Environmental diversity o f Middle Precambrian stromatolites. In: Stromatolites (Ed. by M. R. Walter), Developments in Sedimentology, 20, pp. 599-6 12. Elsevier, Amsterdam. IRION, G. & M OL LER , G. ( 1968) Mineralogy, petrology and chemical composition of some calcareous tufa from the Schwabische Alb, Germany. In: Recent Developments in Carbonate Sedimentology in Central Europe (Ed. by G. Muller & G. M. Friedman), pp. 157- 1 7 1 . Springer-Verlag, New York. JONES, B.F. ( 1 963) The hydrology and mineralogy of Deep Springs Lake Inyo County, California. Ph.D. Dissertation. Johns Hopkins University, B altimore. JoNES, B. F. ( 1 965) The hydrology and mineralogy of Deep Springs Lake, In yo County, California. Prof Pap. U.S. geol. Surv. 502-A, l -56. LATTMAN, L.H. ( 1 973) Calcium carbonate cementation of alluvial fans in southern Nevada. Bull. geol. Soc. A m. 84, 301 3-3028. LOGAN, B.W. ( 1 974) Inventory of diagenesis in Holocene-Recent carbonate sediments, Shark Bay, Western Australia. Am Ass. Petrol. Geol. Mem. 22, 1 95-249. MCGANNON, D.E. ( 1 975) Primary fluvial oolites. J. sedim. Petrol. 45, 7 1 9-727. MILLIMAN, J.D. ( 1974) Marine Carbonates. Springer Verlag, Berlin. MONTY, C.L.V. ( 1967) Distribution and structure of Recent stromatolitic algal mats, eastern Andros Island, Bahamas. Ann. Soc. geol. Be/g., Bull. 90, 55-100. MONTY, C.L.V. ( 1 976) The origin and development of cryptalgal fabrics. In: Stromatolites (Ed. by M. R. Walter), Developments in Sedimentology, 20, pp. 1 93-250. Elsevier, Amsterdam. MONTY, C.L.V. & HARDIE, L.A. ( 1976) The geologic significance of the freshwater blue-green algal calcareous marsh. In: Stromatolites (Ed. by M. R. Walter), Developments in Sedimentology, 20, pp. 447-478. Elsevier, Amsterdam. NEUMANN, A.C. & LAND, L.S. ( 1975) Lime mud deposition and calcareous algae in the Bight of Abaco, Bahamas: a budget. J. sedim. Petrol. 45, 763-786. PICARD, M . D . & HIGH, L.R. ( 1 972) Criteria for recognizing lacustrine rocks. In: Recognition of A ncient Sedimentary Environments (Ed. by J. K. Rigby & W. K. Hamblin). Spec. Pubis Soc. econ. Paleont. Miner., Tulsa, 16, 1 08-145. RAUP, D.M. & STANLEY, S.M. ( 1 97 1 ) Principles of Paleontology. W. H . Freeman & Co., San Francisco. SHINN, E.A. ( 1 969) Submarine lithification of Holocene carbonate sediments in the Persian Gulf. Sedimentology, 12, 109-144. SHINN, E. A. ( 1 973) Carbonate coastal accretion in an area of longshore transport, NE Qatar, Persian Gulf. In: The Persian Gulf ( Ed. by B. H. Purser), pp. 1 79- 1 92. Springer Verlag, New York. SHINN, E.A., GINSBURG, R.N. & LLOYD, R.M. ( 1 965) Recent dolomite from Andros Island, Bahamas, In: Dolomitization and Limestone Diagenesis, A Symposium (Ed. by L. C. Pray & R. C. Murray). Spec. Pubis Soc. econ. Paleont. Miner., Tulsa, 13, 1 1 2- 1 23 . SIMONS, D . B . , RICHARDSON, E.V. & NORDIN, C . F . ( 1965) Sedimentary structures generated b y flow in alluvial channels. In: Primary Sedimentary Structures and their Hydrodynamic Interpretation (Ed. by G . V. Middleton). Spec. Pubis Soc. econ. Pa/eon. Miner., Tulsa, 1 2 . 34-52. SLACK, K.V. ( 1 967) Physical and chemical description of Birch Creek a travertine depositing stream, Inyo County, California. Prof Pap. U.S. geol. Surv. 549-A. SMOOT, J.P. ( 1977) Sedimentology of a saline closed basin: the Wilkins Peak Member, Green River Formation (Eocene), Wyoming. Unpublished Ph.D. Thesis, Johns Hopkins University, B altimore. SMOOT, J.P. ( 1 978) Sedimentary subenvironments in the Wilkins Peak Member of the Green River Formation (Eocene), Wyoming. (In preparation.) HANSEN,

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P.J. ( 1975) Cementation of Pliocene-Quaternary fluviatile clastic deposits in and along the Oman Mountains. Geologie Mijnb. 54. 148- 1 56. STOC KMAN, K . W . , G IN S B U RG, R.N. & S H I N N . E.A. ( 1 96 7 ) The production of lime mud by algae in South Florida. J. sedim. Petrol. 37, 633-648. WHITE, D. E., HEM, J . D . & WARING, G . A . ( 1963) Chemical composition of subsurface waters. Prof Pap. U.S. geo/. Surv. 440- L.

STALDER,

Spec. Publs int. Ass. Sediment. (1978) 2, 129-145

Late Neogene chemical sedimentation in the Black Sea*

K E N N E T H J . H S 0 and K E R R Y K E L T S Geological Institute, Swiss Federal Institute of Technology, Zurich, Switzerland

AB STR ACT

Deep-sea drilling penetrated a largely lacustrine sequence more than 1 km thick in the Black Sea. The oldest sediments are Late Miocene black shales, deposited in a brackish water­ body, which was a part of the Paratethys lac-mer. The middle (and the bulk) of the penetrated section, ranging from Late Miocene to early Pleistocene in age, was laid down during a time of periodic chemical sedimentation, when calcite, magnesian calcite, aragonite, dolomite, and siderite were precipitated. The youngest sediments are middle to late Quaternary and are largely terrigenous. The Black Sea was a lake during much of the late Neogene; only rarely was the Black Sea, as it is now, marine brackish, when sea water from the Mediterranean could enter. Calcite chalks were deposited in a deep freshwater lake environmentally similar to the Holocene Lake Zurich. Aragonite and magnesian calcite were laid down in the Black Sea at times when it was brackish-marine or hypersaline. Dolomite muds were precipitated during a period when the Black Sea changed from a brackish lac-mer into a shallow salt lake. Stromatolitic dolomite was formed diagenetically in a sabkha-like environment. Siderite occurred when deep-weathering of low-lying coastal plains resulted in a high influx of dissolved iron to the Black Sea.

*Contribution No. 1 06 of the Laboratory of Experimental Geology, Swiss Federal Institute of Technology, Zurich, Switzerland.

Modern and Ancient Lake Sediments Edited by Albert Matter and Maurice E. Tucker © 1978 The International Association of Sedimentologists. ISBN: 978-0-632-00234-4

130

Kenneth J. Hsii and Kerry Kelts INTR O DUCTION

The Black Sea is now a giant brackish lagoon of the Mediterranean Sea. However, the Black Sea basin hosted a lake for much of the time during the last 6 or 8 million years. The lower Quaternary and the Plio-Miocene sequences there include considerable chemical and biogenic sediments. Calcitic lacustrine chalk is the main chemical sediment; sideritic, dolomitic, and aragonitic deposits are also present. Diatoms comprise the most common biogenic component. Ostracodes, molluscs, and other calcitic fossils are present as minor constituents. This paper is an attempt to interpret the chemical sediments of the Black Sea and their changing depositional environments, with reference to Holocene analogues, in particular Lake Zurich. We would like to mention at the outset that this work resulted from a post-cruise study of cores obtained by the DSDP Leg 42B cruise to the Black Sea. Our interpretations rely heavily on geological data to be published in a forthcoming cruise report (Ross, Neprochnov et a!., 1 978); we refer particularly often to the work by Peter Stoffers, Alfred Traverse, and Hans Schrader. We are grateful to the JOIDES organization, and to the Deep Sea Drilling Project for having given us the opportunity to study the Black Sea cores. We would also like to thank the many scientists who served as the shipboard staff and who engaged in post-cruise studies; discussions with them have contributed greatly to our understanding of the Neogene geology of the Black Sea. We are indebted to the help of our colleagues, especially Judy McKenzie who furnished data on stable isotopes and Helmut Franz, who made the excellent SEM photographs. Peter Stoffers, John Milliman, and the Editors of this Volume read the first draft of the manuscript; their comments led to a substantial improvement.

Geology of the Black Sea

The Black Sea basin probably owed its origin to a marginal basin, south of the Eurasian continent, and behind a Cretaceous island arc, which extended from Bulgaria, through Anatolia, to the Caucasus (Hsii, Nachev & Vuchev, 1 977). The sedimentary sequence of the Black Sea is probably more than 10 km thick (Neprochnov, Neprochnova & Mirlin, 1 974). A deep-sea drilling cruise to the Black Sea bored six holes at three sites, with a maximum penetration of 1 073·5 m at Site 380 (Ross, Neprochnov et a!., 1 975). A generalized stratigraphy of the section penetrated at DSDP Site 380 is shown by F ig. l . The sediments are believed by us to range from Late Miocene to Quaternary in age, on the basis of interpreting palaeomagnetic stratigraphy, climatic variation, sedimentation rate, and other data (Hsii, 1 978). It should be noted that this interpretation is tentative and has not been accepted by a consensus of the shipboard staff (see Ross, 1 978). The Black Sea was a part of the ancient Tethys and a site of marine sedimentation until the Middle Miocene when its connections to the Mediterranean were severed (Fig. 2a). The restricted sea of eastern Europe extended from Vienna to regions beyond the Aral Sea and was called the Paratethys. It became brackish with time and its salinity at the start of Late Miocene was reduced to 1 5-20%0 (Koj umdgieva, 1 976) when black shales were deposited. With the disintegration of the Paratethys the Black Sea became a giant lake (Fig. 2b). Periodic chemical sedimentation took place under changing climate. The youngest sediments of the Black Sea are upper Quaternary and mainly terrigenous, laid down in a freshwater or brackish environment.

Late Neogene chemical sedimentation in the Black Sea

Salinity

Freshwater

Climate

Time

13 1

Lithologic Units

Fig. I. Stratigraphy, climate and water salinity of the deep Black Sea. (Modified from Figures in Hsii, 1978; Traverse, 1978; Schrader, 1978.)

Chemical sediments of the Black Sea

The late Neogene chemical sediments of the Black Sea are intercalated in a dominantly marly sequence, which is more than 600 m thick at Site 3 80 on the western side of the abyssal plain (Ross, Neprochnov et a!., 1 978). The sequence has been divided into the following units on the basis of the different carbonate interbeds (Fig. 1); they are in descending order: (7) Upper Siderite Unit; (6) Upper Chalk; (5) Lower Siderite Unit; (4) Lower Chalk; (3) Aragonite Unit; (2) Gravels and Dolomites; ( 1 ) Laminated Carbonates, including dolomite.

132

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These sediments were deposited in a large lake of considerable water depth. The only exceptions are the Late Miocene gravels and dolomites, which are deposits of shallow subaqueous or subaerial origin (Ross, Neprochnov et a/. , 1 978). The intercalation of these sediments in a deep-water sequence is believed to signify an episode of desiccation when there was a hydrographic deficit for the Black Sea, and much of the Paratethys water was drained into the desiccated Mediterranean (Hsu, 1 978; see also Fig. 2b). The late Neogene was a time of gradual climatic deterioration. The cooling trend led to the extinction of thermophyllic floras during the Late Miocene. With the onset of glaciation in northern Europe and in the Alps and Caucasus, forest vegetations were replaced by steppe floras. Traverse ( 1 978) portrayed the climatic fluctuations with the help of steppe index, which is the percentage of steppe pollen in a total pollen assemblage (see Fig. 1). Chemical sedimentation started in Late Miocene when the climate was still warm, continued throughout the Pliocene, reached a zenith of chalk

Late Neogene chemical sedimentation in the Black Sea

133

deposition during the Alpha Glacial Stage in the early Pleistocene, and was replaced by terrigenous sedimentation at the beginning of the Beta Glacial Stage* . The correlation of lithology with palaeoclimate suggests that calcite was on the whole deposited during colder and siderite during warmer tim.::s . Diatom floras are common in the sediments of Black Sea. They indicate that the lake water was mostly fresh or brackish; only rarely did the salinity approach that of seawater (Schrader, 1978; see also Fig. 1). C H ALK S

Calcite is present as a chemical sediment in practically all units, but the precipitation of calcite exclusive of all other carbonate minerals took place only during the times when the Upper and Lower Chalks were laid down. Nearly pure calcitic sediments occur either as paper-thin laminations in varves (Fig. 3), or as homogeneous layers (Fig. 4).

Figs 3 and 4. (3) Cycles of Laminated Chalks alternating with grey marl. Lower Chalk Unit, Black Sea.

(Sample 380A-5 5 - l / 1 08- 1 25 em.) (4) Cycles of homogeneous chalk alternating with grey marl. Upper Chalk Unit, Black Sea, showing the sharp basal contact and a burrowed transition zone (Samples 379A-58-4/35-50 em corresponds to cycles b and c referred to in Table 1). Scales in centimetres. *Traverse proposed a terminology specifically for the Black Sea because of the difficulty of correlating D SD P data with established European glacial stages.

1 34

Kenneth J. Hsii and Kerry Kelts

Laminated chalk

Laminated chalks are characterized by dark laminae rich in organic matter (up to 2%) and light laminae rich in calcite. Several samples were examined under the scanning electron microscope (Fig. 5). The calcitic grains are nearly uniform in size, ranging from 5 to 1 5 Jlm. The blocky subhedral crystals show well-defined crystal edges. Twinning is present, but uncommon. Surfaces of crystals show signs of selective etching or fluting through dissolution; skeletal surfaces with etch pits resulted from advanced dissolution. Some small, relic, rod-shaped crystals 0·2-0· 5 Jlm in size are present (Fig. 6). They are calcareous, and may have been derived from the breakdown of larger crystals. Diatoms are present, and many tests show advanced degrees of dissolution.

Fig. 5. Lacustrine Chalk, Black Sea. (a) Dissolution features on euhedral to subhedral calcite polyhedra from a cyclically deposited Seekreide of the Upper Chalk. Note the very selective surface etching of the larger grains. Dissolution proceeds along preferred lattice directions and some faces are more resistant than others. X-ray diffraction indicates that some calcite in this almost pure chalk may contain up to 4% Mg in solid solution. (b) same, detail of grain surface. (c) same, some crystals are sculptured, other hollowed by dissolution. (Sample 380A- 1 2-4/7 1 -7 2 em.)

Laminated chalks are varve-like, similar to the varved chalks of Lake Zurich (Fig. 7). White laminae are essentially pure or very low Mg calcite, whereas the dark laminae consist of detrital clay-sized particles, diatom frustules and organic material. Calcite grain-size, form and dissolution features also match those from Lake Zurich (Kelts & Hsu, 1 978). The similarity suggests that individual laminae-couplets represent an annual cycle. Calcite precipitated during the summer and detrital laminae were deposited during the winter. Cyclic deposition of chalk

Laminated chalks are most common in the Lower Chalk Unit. They are interbedded with grey marls (Fig. 3). The alternation indicates periodic environmental changes. Not all chalks are varves. In fact many of the chalks from the Upper Chalk Unit form thin homogeneous layers a few centimetres thick and these layers are interbedded with marls (Fig. 4). Examined under SEM, the chalk is found to consist of the same blocky polyhedra of subhedral calcite crystals as the laminated chalks. A qualitative estimate, however, indicates more advanced dissolution effects in the non-laminated beds.

Late Neogene chemical sedimentation in the Black Sea

135

Fig. 6 . Lacustrine Chalk, Black Sea. A n example o fdissolution of fine-grained calcite grains i n a greyish-tan homogeneous chalk, Upper Chalk Unit. (a) a rhombic surface with knobby dissolution textures. (b) selective in situ dissolution leaves a mere skeletal relic of a calcite grain. (Sample 3 80A- 1 7-6/49-5 1 em.)

Fig. 7. Recent non-glacial varves, Lake Zurich. Dark layers comprise diatom frustules, organic sludge and detrital minerals; light layers mainly non-magnesian calcite crystals, 2-5 lim large in diverse stages of dissolution. Thick layer at the base is a lutite bed deposited from a low-density turbidity current in 1 9 1 8. Scale in centimetres.

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Hsii and Kerry Kelts

The cyclic sediment commonly ranges from 2 to 8 em and is typically 4-5 em thick. The base of the cycle is invariably a clastic sediment, which may be a silt or mud. This sediment lies with a sharp contact on the chalk of the previous cycle. The silt, if present, grades upward into a dark grey mud or marl, which contains some calcite. A zone of burrows is commonly present between the mud (or marl) and the chalk, the burrows having a light-coloured infilling from the overlying chalk. (Fig. 4). The burrows belong to the ichnogenus Chondrites, which is known mainly from marine sediments (Ekdale, personal communication). However, the annelids that produced the Black Sea burrows probably belonged to the genus Nereis, which is present in the Caspian Sea today (Zenkevich, 1 957). Nereis can compete successfully where there is insufficient circulation, as it has the remarkable ability to withstand short periods of absence of oxygen. The highest lithology of a cycle is the light-grey chalk. The chalk was probably also laminated before it was reworked by burrowing. The burrowing processes were apparently interrupted by the turbidity-current deposition of the overlying terrigenous clastics; the upper contact of the chalk is sharp and is not affected by bioturbation. So the cycle repeats itself.

Table I . Isotopic composition of calcite in cyclically deposited mud and chalk, Black Sea. Analyses by J. McKenzie, ETH, Zurich

Cores

"0

PDB

"C PDB

Cycle a 379A/5 8-4-27-28, chalk 379A/5 8-4-29-30, burrowed chalk 379A/5 8-4-30-3 l , mud 379A/5 8-4-32·5-33·5, silt

- 5· 1 2 - 5 ·57 - 5 ·86 - 6·22

+ 2·22 + 1 · 64 + 0·64 - 0·58

Cycle b 379A/5 8-4-3 8-39, chalk 379A/5 8-4-39-5-40·5, burrowed chalk 379A/5 8-4-4 l-42, mud

- 4·94 - 5 ·34 - 5 ·73

+ 1 ·60 + 1 ·37 + 0·00

Cycle c 379A/5 8-4-48-49, chalk 379A/58-4-50-5 l , mud

- 5 ·20 - 5 ·53

+2·19 + 0·57

Cycle d 3 80A/l 4-3-4-5, chalk 380A/ l 4-3-5·5-6·5, mud

- 5 -46 - 5 ·75

+ 1 ·39 + 1 ·39

Cycle e 380A/ l 4-3-7 ·5-8·5, chalk 380A/ 14-3-9·5- 1 0·5, chalk 380A/14-3- 1 2·5- 1 3 ·5, mud

- 5 ·52 - 5 ·44 -5·19

+ 1 · 72 +2·18 + 1 -49

Cycle f 380A/ 1 4-3- 1 4·5- 1 5 ·5 , mud

- 5 ·35

+ 1 · 96

The isotopic composition of calcite in the cyclical deposits has been determined and the results are shown in Table 1. The changes of the o180 and oUC values are systematic; the calcite in the chalk almost invariably has a less negative o180 and more positive oUC value than the underlying mud: the change in o180 is more than 2 per

Late Neogene chemical sedimentation in the Black Sea

137

mille and was apparently caused by fractionation a s a consequence o f increasing evaporation. The increase in o 1 3C is up to 3°,00 and the trend is similar to that found in cyclically deposited Cretaceous pelagic sediments of the southern Alps, where the change was related to increasing stagnation (H. Weissert, personal communication). Exclusion of clastics

A prerequisite for chemical sedimentation is the exclusion of terrigenous clastics. For example, Lake Zurich has been a site of chalk sedimentation since the turn of the century, because the waters emptying into the lake contain hardly any suspended particles. The detritus from the main tributary, the Linth, has been deposited in the lakes upstream (Walensee, Obersee) and the sediments from the minor side-tributaries have been trapped behind small-reservoir dams which were built for the purpose of flood-control prior to the turn of the century. Considering the material balance of the Black Sea, we note that the inflowing rivers are now supplying annually 1 50 X 1 06 tons of detritus (Shimkus & Trimonis, 1 97 4). If the influx is evenly spread out over the 0-42 x 106 km of the area, the rate of terrigenous sedimentation should be 1 5 em per 1 000 years. However, the rate of terrigenous accumulation on the abyssal plain is only 6 em per 1 000 years (t.y.). The discrepancy between the calculated and actual rates indicates that more than two thirds of the detritus has been trapped on the shelf in shallow waters (Ross & Degens, 1 974). The present slow rate of terrigenous influx permits the deposition of biogenic calcitic sediments (Muller & Stoffers, 1 974; Stoffers & Muller, 1 978). The rate of terrigenous sedimentation during the last glacial stage was as high as 90 cm/t.y.; the detrital supply then must have been at least six times greater than the present and the bulk of that was dumped onto the abyssal plain. Any pelagic chemical or biogenic components produced were obscured by the detrital influx. One was thus inclined at first to attribute a critical role to climatic factors; chemical sediments should only be forming during tern perate to warm preglacial or interglacial times, when erosion was moderate and when much of the terrigenous influx was deposited on the shelf. It was, therefore, a great surprise when the palynological results (Traverse, 1 978) proved that the main period of the chalk deposition took place during a glacial stage! A regional synthesis has revealed that the palaeogeography of eastern Europe during the early Pleistocene was significantly different from that of the present (Hsu, 1 978). The Bosphorus was then not yet open, and the Danube was emptying into a lake in Romania east of the Carpathian Range. Although the Danube water may have been emptied into the Black Sea, the Danube detritus, which constitutes more than half of the terrigenous influx to the Black Sea, was then trapped in this peri-Carpathian lake. The Dnestre detritus might also have been directed there, and that of the Dneper and Kuban may have been trapped by the Sea of Azov. Deprived of those sources, the Black Sea received annually only about a quarter as much terrigenous detritus as it does now, so that the rate of terrigenous sedimentation during an early Quaternary glacial stage should have been about 22 cm/t.y. The annual load of dissolved Ca2 + of rivers draining into the Black Sea is 14 X 1 06 tons (Shimkus & Trimonis, 1 954) and may have been similar in quantity then. If this load was combined with dissolved carbonate and deposited in the Black Sea, the rate of chalk deposition should have been about 9 cm/t.y. Thus, the total rate for clastic and chalk sedimentation should

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Hsii and Kerry Kelts

have been 3 1 cm/t.y. This rate is about the same as that calculated on the basis of the thickness ofthe early Pleistocene sediments (Hsii, 1 97 8). Also the ratio (9:22) of calcitic to terrigenous sedimentation coincides more or less with our visual estimates of the proportion (about 1 :2) of carbonate to clastic sediments. This analysis suggests that the deposition of the Upper Chalk Unit during the Alpha stage of glaciation took place because much of the detritus carried down by rivers was trapped in peripheral basins. The Lower Chalk was deposited during a warm preglacial epoch at 5·4 cm/t.y. This low rate indicates not only a trapping of clastics in peripheral basins, but also that the rivers carried much smaller terrigenous and dissolved loads than they do now. Depositional environments of chalks

The Lower and the Upper Chalks were deposited in considerably different environments. The Lower Chalk was deposited during relatively warm Pliocene time; the Upper Chalk was deposited during a glacial stage. The Lower Chalk has a rich planktonic diatom-flora and the Melosira-Stephanodiscus assemblage is typical of an oligohaline environment (3-5o/00)(Schrader, 1 97 8). Diatoms are very abundant in some layers, constituting more than half of the bulk volume. The Upper Chalk contains an unusual planktonic flora of freshwater dinoflagellates (Traverse, 1 97 8). Diatoms are commonly absent; they are either not produced, or they are not preserved from dissolution (see Fig. 8). The one exception is an almost monospecific assemblage ofthe freshwater diatom Melosira undulata at one level, indicative of a water salinity less than 3o/00 (Schrader, 1 97 8). The Lower Chalk contains no benthonic fossils. The Upper Chalk contains a rich benthonic ostracod fauna; the Candona-Loxoconcha assemblage indicates a deep freshwater lake environment like that of the Great Lakes of North America (Olteanu, 1 97 8). The chalks of the lower unit are commonly laminated, and the structure indicates completely stagnant bottom conditions, devoid of life and

Fig. 8. Dark lamina in a varve, Lower Chalk, Black Sea. (a) Dark greenish-grey part of a lamina couplet in

the Lower Chalk shows mostly layered silicates and some scattered diatom frustule remnants with some quartz and low Mg calcite. (b) as above. In some rare instances there is evidence of overgrowth in diatom pores rather than dissolution. (Sample 380A-49-5/ 1 1 9- 1 20 em.)

Late Neogene chemical sedimentation in the Black Sea

139

current activity. The chalks of the upper unit contain both laminated and structureless varieties; the latter has been burrowed by annelids in bottom environments not completely devoid of oxygen. The Upper Chalk also contains abundant silts and mud layers, which were probably deposited by low-density turbidity currents. The Black Sea lacustrine environments of chalk deposition had much in common with the environments of Lake Zurich. The lake has been eutrophic since the turn of the century, and no benthonic life exists on the deep lake floor. Despite the general stagnation of the deepest bottom, partial annual overturn of the lake water is still taking place. The water movement at the beginning of the spring brings cold and bicarbonate-rich deep water to the surface. C alcite precipitation takes place in the late spring and throughout the summer in response to seasonal temperature and pH changes (Kelts & Hsu, 1 978). A maj or diatom bloom occurs early in the spring; diatom tests sink to the bottom, where they form the base of an annual varve. Coarser calcite crystals are also formed during the early to late spring. The summer harvest of calcite tends to be fine-grained. This chalk is overlain by a dark lamina of organic-rich clay deposited during the late autumn and winter. The lack of bioturbation permits the preservation of annual deposits as carbonate varves (Fig. 7). The present-day Lake Zurich environment is thus an excellent analogue of Lower Chalk deposition, except the Lake Zurich water is probably even less saline. The stagnation of the deepest bottom of Lake Zurich is caused by lack of bottom circulation. Studies of the water movement in Swiss lakes indicated that bottom­ current activities are commonly related to the emplacement of density underflows (Lambert & Luthi, 1 977). Cores from Lake Zurich show no major turbidite­ sedimentation after 1 900. Flood-control constructions since that date have apparently prevented the continuation of stream-floods on land as turbidity underflows in the lake. The absence of turbidity-current activity may in fact be responsible for the oxygen-deficiency of the lake. Stagnation of the Black Sea at the time of the Lower Chalk deposition might be likewise explained, because this unit contains practically no turbidites; a density-stratification of the slightly brackish water may have contributed further to the stagnation. The present chalk-deposition of Lake Zurich is not a unique event during the Holocene. The previous period was the Preboreal to Boreal time, 1 0,300-7,500 years ago. This older unit consists mainly of light grey chalk (65-85% calcite) with intercalations of silt laminae and was deposited at a time when there was little detrital input to Lake Zurich (Kelts, 1 97 8). Those chalk beds are mostly bioturbated and homogeneous. Like the Boreal chalk, the Upper Chalk of the Black Sea is also characterized by the predominance of a non-laminated variety; its lack oflamination can also be attributed to bioturbation. The Boreal chalk of Lake Zurich and the Upper Chalk of the Black Sea are furthermore similar because diatoms are rarely preserved or altogether absent in these sediments. One maj or difference is the prevailing climate. The Upper Chalk of the Black Sea contains mainly steppe pollen, indicative of deposition during a glacial stage. The Boreal sediments contain abundant tree pollen, and were deposited during a post-glacial climatic optimum. The data confirm our interpretation that the exclusion of clastics, not climate, is the critical factor for chalk deposition. This condition was achieved for the Upper Chalk deposition in the Black Sea, when the climate was cold and arid, and for the Boreal chalk of Lake Zurich when the climate was warm and when erosion was minimized by forest-growth. Another factor is the

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Kenneth J. Hsii and Kerry Kelts

physical limnology. A freshwater lake in the Black Sea basin may have had a thermal structure similar to that of Lake Zurich during the Boreal time. Glaciers were far away, but the lake water was cold in the winter and warm in the summer. Surfacing of cold waters rich in dissolved carbonate during the annual spring-overturn and the subsequent warming provided the condition for supersaturation and precipitation of calcium carbonate. Cause of cyclic deposition

Cyclical deposits of the Upper Chalk in the range of centimetres in thickness cannot be annual varves. The cycles signify climatic oscillations of a few hundred years in period. However, the influence of climate is complicated. The Black Sea cycles were initiated when chalk sedimentation was interrupted by the deposition of terrigenous silts, carried to the abyssal plain by turbidity currents. The intrusion of the underflows, like their counterpart in Swiss lakes, has apparently promoted bottom circulation. With the exclusion of turbidites the bottom became more stagnant, while chalk deposition began. The periodical increase of the terrigenous influx was probably related to periodic retreat ofglaciers during the Alpha stage of glaciation. Pollen studies of cyclically deposited sediments by Traverse ( 1 978) indicated that the steppe pollen are dominant in the chalk whereas the forest pollen are dominant in the terrigenous layers of the cyclic units. The steppe index (S.I.) reached as high as 83 in a chalk (sample 3 80A/ l 2/4/4 1 -42 em), but went down to as low as 1 in the terrigenous sediment 2 em below. Other S.l. values include variations from 73 to 19, 17 to 22, 48 to 36, 5 5 to 9, 83 to 35, 93 to 1 8, 67 to 67, 85 to 1 0, 63 to 60 in the chalk and mud layers, respectively; all except one show that the chalk was deposited during colder and mud during warmer times of short-termed climatic cycles. Such drastic changes of climate took place in cycles of several hundred years; the periodicity is comparable to the Holocene climatic variations as evidenced by the advances and retreat of mountain glaciers in the Swiss Alps, where four 'miniglacial' and 'mini­ interglacial' stages during the last 2,000 years have been recorded (Schneebeli & Rothlisberger, 1976).

M AGNE SI AN C A LCI TE AN D AR AGONI TE

Magnesian calcite is present only in appreciable quantity in the Aragonite Unit. Aragonite is present in this unit, in the Laminated Carbonates unit, and near the top of the Upper Siderite unit. Aragonite Unit

The Aragonite Unit was deposited during the warm Early Pliocene time. Aragonite and magnesian calcite occur as thin layers of laminated sediments in a mudstone sequence. The laminae are less than 1 mm in thickness and have a varved structure. The Aragonite Unit contains brackish-marine floral assemblages. The nannofossil assemblage is monospecific, and consists of a Braarudosphaera species, which is tolerant of a less than normal marine salinity down to 22°/00 (Bukry, 1 974). The diatom assemblage A ctinocyclus ehrenbergli - Hermesinum adriaticum is mixo-euhaline, 300 40 /00 salinity (Schrader, 1 97 8). The absence of a foraminiferal and a diversified

Late Neogene chemical sedimentation in the Black Sea

141

Figs 9- 1 1 . (9) Aragonite, Laminated Carbonate Unit, Black Sea. Aragonite prisms i n a marl. X-ray diffraction indicates quartz and aragonite are the dominant minerals in this sample with minor amounts of low Mg calcite and ankeritic dolomite. Aragonite is particularly concentrated in clusters or radial sprays of subhedral prisms in the SEM. Note a tapering off of some prisms at one end. (Samples 3 80A-64-4/93-95 em.)

( 10) Dolomite in Gravel Unit, Black Sea. The euhedral dolomite rhombs occur together with low Mg calcite in thin white laminae from green marl stones in a mud-pebble horizon. The knobby grains are dissolution surfaces of a Mg calcite (5-7% Mg). Non-stoichiometric dolomite constitutes over 65% of the sample, with calcite (5%) and minor quartz (7%). Dolomite crystals appear to have grown diagenetically as replacement and interstitial filling. Several grains interpenetrate and many are twinned. (Sample 380A-58-l /6 1-63 em.) ( l l a) Siderite from Upper Siderite Unit, Black Sea. The sample is from a reddish brown hard layer, and shows smooth, rounded, anhedral siderite crystals growing on a substratum. X-ray diffraction indicates that the sample contains only 1 0% manganosiderite, in a calcitic sediment, with quartz and some dolomite. (Sample 380A- l -3/28-3 l em.) ( l i b) Siderite from Lower Siderite Unit, Black Sea. The sample is from a laminated reddish brown hard layer in pale grey sediments. Manganosiderite, with up to 1 5 % Mn in solid solution, constitutes over 80% of the bulk. It occurs as irregular 'wheat grains' suggestive of a primary rather than replacement origin. Fine­ grained matrix contains mainly quartz and iron oxides. (Sample 380A-37-3/57-58 em.)

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nannofossil assemblage indicates that the Black Sea then, like now, was not exactly marine. There was probably a density layer then, as now, with a stagnant bottom where varve-like laminations could be preserved. The bulk of the carbonates in the Aragonite Unit are aragonite and magnesian calcite. The aragonite occurs as stubby, imperfect prisms, commonly clustered into sprays with a tapered shank (Fig. 9). The magnesian calcite contains commonly 6% Mg, but more than 1 2% Mg in some instances. That aragonite and Mg calcite should occur in place of calcite indicates an environment where abundant Mg2 + ions inhibit the crystallization of the stable phase calcite (e.g. Berner, 1 975). Mudstone was deposited in an environment different from that of laminated carbonates. Diatoms are common in the mudstone, but rare or absent in the carbonate. Apparently the water was alternating from CaC03 saturation and Si02 undersatura­ tion to CaC03 undersaturation and Si02 saturation. The cause ofthe changes in water chemistry was ultimately related to climatic variations; an excess ofCa2 + was imported during temperate times, but an excess of dissolved Si02 was imported because of deep weathering under tropical or subtropical conditions. Laminated Carbonate Unit

Aragonite, but not magnesian calcite, is present in the Laminated Carbonate Unit, which was deposited at times of fluctuating lake-levels and changing salinity within the Black Sea basin. The alternation of calcite, dolomite, and aragonite sedimentation also indicates changes in water-chemistry. Calcite was precipitated when the lake was freshwater or oligohaline, and aragonite precipitated when the Mg2 + ion concentration was high. DOLOMITE

Dolomite in the Gravel Unit is stromatolitic, similar to that formed in modern sabkha environments. SEM investigations (e.g. Fig. 1 0) show euhedral, smooth, rhombs 4- 10 p,m in size, some twinned, which appear to have been formed in pore spaces of intertidal sediments. Dolomite in the Laminated Carbonate Unit was formed in a lake of fluctuating water-levels. lts origin may be similar to the Holocene dolomite of Lake Balaton (described by Muller, Irion & Forstner, 1 972; Muller & Wagner, 1 978). SI DERITE

Siderite occurs as hard layers or as soft muds. Some hard layers are apparently concretionary, with encrusting anhedral crystals growing on a substratum. Others are apparently cemented mud, consisting of wheat-grain-like crystals in a finer-grained mat:ix. Depositional environments of siderite

The steppe index indicates warm climate at the time of siderite deposition (Fig. 1 ). With a warming trend after a cold episode during the Pliocene, Lower Chalk deposition was replaced by Lower Siderite. Similarly the change from the Alpha-

Late Neogene chemical sedimentation in the Black Sea

143

Glacial to the A(Anna)-Interglacial Stage coincides with the change from the Upper Chalk to the Upper Siderite sedimentation. Diatoms are common in both units of sideritic deposits. The Lower Siderite has an oligohaline assemblage, and the Upper Siderite has an assemblage indicative of oligohaline to mesohaline conditions. The diatomaceous sediments are commonly grey and laminated. The light grey diatom-rich laminae contain up to 80% diatoms whereas the clay-rich contain only 10 or 1 5 % diatoms. The existence of a rich diatomaceous flora suggests a eutrophic water-mass, and the laminated structure indicates anoxic bottom waters. Origin of siderites

Siderite occurrences are rare in Holocene environments. A small amount of siderite is formed diagenetically in the brackish-water sediments of Chesapeake Bay (Bricker & Troup, 1 97 5), and layered siderite occurs in Late Quaternary deposits of Lake Kivu in equatorial Africa (Stoffers, 1 975). The conditions for the Black Sea siderite deposition were comparable to that of Lake Kivu. The climate of a middle latitude region like the Black Sea during an interglacial stage was similar to the climate of the tropical Lake Kivu during a glacial stage; it was temperate rather than tropical. The eutrophic watermass of the Black Sea then was evidently low in Eh, as Lake Kivu was 1 2,000 years ago. That an iron carbonate should form instead of a calcium carbonate requires that the Ca/Fe ratio of the water be less than 20/ I (Berner, 1 97 1 ). Sluggish streams of the southeastern United States where relief is low and rainfall is high, have a high content of organic matter, iron and silica, and a low content of other dissolved ions such as Ca2+ and MgH (Beck, 1 972). The present-day stream from the coastal plain of Georgia, for example, has a Ca/Fe of about I (Garrels, Perry & McKenzie, 1 973). The rivers draining into the Black Sea today have a Ca/Fe ratio far higher than 20 (Shimkus & Trimonis, 1 974). However, their water-chemistry may have been different during the times of siderite deposition. We can assume that the low-lying coastal plains surrounding the Black Sea were then heavily forested, the prevailing climate being similar to that of the southeastern United States. A deep chemical weathering led to a breakdown of silicates so as to yield abundant iron and silica to river waters. The formation of iron carbonate in place of iron sulphide in a reducing environment suggests that the sulphate ion concentration of the Black Sea was low. Since sea water is rich in dissolved sulphate, the salinization of the Black Sea during the time of siderite deposition was probably not caused by an invasion from the Mediterranean, but resulted from evaporitic excesses. A marine invasion terminated siderite formation, and led to the deposition of pyritic and aragonitic muds on top of the Upper Siderite. Sideritic concretions are a diagenetic phenomenon, formed at or near the sediment-water interface like the siderite of Chesapeake Bay. On the other hand, wheat-like siderite grains in the soft sideritic muds are probably subaqueous precipitates.

SUMM ARY

Chemical sedimentation in the Black Sea started in Late Miocene when the water­ body was changing from a brackish sea into a lake. During the first stage of isolation,

1 44

Kenneth J. Hsii and Kerry Kelts

the lake was being progressively desiccated. Oscillations of water level and variations of water chemistry resulted in the deposition of a wide assortment of sediments. Calcite was precipitated when the water was fresh; dolomite, magnesian calcite and aragonite were deposited when the water was brackish to saline. The sediments are commonly laminated and indicate stagnant bottom conditions. Eventually the edge of the Black Sea abyssal plain was exposed and the Laminated Carbonates were overlain by gravels, sands and stromatolitic dolomites. At the beginning of the Pliocene, the Black Sea was again submerged; aragonite and magnesian calcite were deposited. The marine connection was, however, soon severed, and the lake was rapidly desalinified. The Lower Chalk Unit includes both calcitic and diatomaceous sediments. The water mass was probably eutrophic and slightly brackish, and the bottom stagnant. Chalks are commonly laminated like the carbonate varves of Lake Zurich. During a warming trend of the late Pliocene, the lake became brackish. The influx of iron-rich waters led to the formation of siderite. With the onset of continental glaciation in northern and central Europe and a changed climate, the Black Sea was again desalinified. Cyclically deposited chalks were laid down then, when the bottom was not completely anoxic. After a change from a glacial stage to an interglacial, chalk sedimentation was replaced for a second time by deposition of siderite. The latter was terminated by a marine invasion during the middle Quaternary; aragonite was then precipitated. The interval of periodic chemical sedimentation was ended soon afterward, when the D anube brought in enough clastics to herald an epoch of terrigenous sedimentation. However, the Black Sea remained a great lake until the Bosphorus Strait came into existence and marine waters spilled over. During the last glaciation, when the sea level dropped below the sill of Bosphorus, the Black Sea basin again became a freshwater lake. Finally, as the Holocene sea level rose, Mediterranean waters re-entered via the Bosphorus. The Black Sea became a brackish lagoon; the density-layering of the watermass led to the present bottom stagnation and anoxic condition.

REFERENCE S BECK, K.D. ( 1 972) Sediment-water interactions of some Georgia rivers and estuaries. Office of Water Resources Project B-033GA, Rept. Georgia Inst. Tech. BERNER, R.A. ( 1971) Principles of Chemical Sedimentology. McGraw Hill, New York. BERNER, R.A. ( 1 975) The role of magnesium in the crystal growth of calcite and aragonite from sea water. Geochim. cosmochim. A cta, 39, 489-504. BRICKER, O.P. & TROUP, B.N. ( 1 975) Sediment-water exchange in Chesapeake Bay. In: Estuarine Research (Ed. by L. E. Cronin), pp. 3-27. Academic Press, New York. BUKRY, D. ( 1 974) Coccoliths as paleosalinity indicators. In: The Black Sea, Geology, Chemistry and Biology (Ed. by E. T. Degens and D. A. Ross). Am. Ass. Petrol. Geo/. Mem. 20, 353-363. GARRELS, R.M., PERRY, JR, E.A. & McKENZIE, F.T. ( 1973) Genesis of Precambrian iron formations and the development of atmospheric oxygen. Econ. Geol. 68, 1 1 73- 1 179. HsO, K.J. ( 1 978) Stratigraphy of the lacustrine sedimentation in the Black Sea. In: Initial Reports of the Deep Sea D rilling Project, Vol. XLIIB (D. A. Ross and Y. Neprochnov et a/.). U.S. Government Printing Office, Washington D.C. (In press.) HsO, K.J., NACHEV, I.K. & VucHEV, V.T. ( 1 977) Geologic evolution of Bulgaria in light of plate-tectonics, Tectonophysics, 40, 245-256. KELTS, K . ( 1 978) Geological and sedimentary evolution of Lakes Ziirich and Zug, Switzerland. Dissertation, ETH, Ziirich, NR 6 1 46. KELTS, K. & HsO, K.J. ( 1 978) Calcium carbonate sedimentation in freshwater lakes and the formation on non-glacial varves in Lake Ziirich. In: Lakes, Physics, Chemistry, Geology (Ed. by A. Lerman). Springer-Verlag, New York. (In press.)

Late Neogene chemical sedimentation in the Black Sea

1 45

E. ( 1 976) Paleoecologie des communautes des mollusques du Miocene en Bulgarie du Nord­ Ouest. IV Communautes des Mollusques du Bessarabien et du Chersonien (Sarmatien moyen et superieur). Geologica Balk. 6, 37-56. LAMBERT, A. & L OTH !, S. ( 1977) Lake circulation induced by density currents: an experimental approach. Sedimentology, 24, 735-7 4 1 . MOLLER, G . & STOF FERS , P. ( 1 974) Mineralogy and Petrology o f Black Sea Basin sediments. I n : The Black Sea, Geology, Chemistry and Biology (Ed. by E. T. Degens and D. A. Ross). Am. A ss. Petrol. Ceo/. Mem. 20, 200-248. MOLLER, G., IRION, G. & FORSTNER, U. ( 1972) Formation and diagenesis of inorganic Ca-Mg carbonates in the lacustrine environment. Natunvissenschaften, 59, 1 5 8 - 1 64. MOLLER, G . & WAGNER, F. ( 1 978) Holocene carbonate evolution in Lake Balaton (Hungary): a response to climate and impact of man. In: Modern and A ncient Lake Sediments (Ed. by A. Matter and M . E. Tucker). Spec. Pubis int. Ass. Sediment. 2, 57-8 1 . NEPROCHNOV, Yu. P., NEPROCHNOVA, A.F. & MtRLIN, YE. G . ( 1 974) Deep structure o f Black Sea Basin. In: Black Sea, Geology, Chemistry and Biology (Ed. by E. T. De gens and D . A. Ross). Am. Ass. Petrol. Ceo/. Mem. 20, 35-49. OLTEANU, R. ( 1978) Ostracoda from DSDP Leg 42B . In: Initial Reports ofthe Deep Sea Drilling Project, Vol. XLIIB (D. A. Ross and Y . Neprochnov et al.). U.S. Government Printing Office, Washington D.C. (In press.) Ross, D . A. & DEGENS, E.T. ( 1 974) Recent sediments of the Black Sea. In: The Black Sea, Geology, Chemistry and Biology (Ed. by E. T. Degens and D. A. Ross), Am Ass. Petrol. Ceo/. Mem. 20, 1 8 3 - 1 99. Ross, D.A. ( 1 978) Black Sea Stratigraphy. In: Initial Reports of the Deep Sea Drilling Project, Vol. XLIIB (D. A. Ross, Yu Neprochnov et a/.). U.S. Government Printing Office, Washington D.C. (In press.) Ross, D .A., NEPROCHNOV, Yu. et a/. ( 1 975) G LOMAR CHALLENGER drills the Black Sea. Geotimes, 20/10, 1 8-2 1 . Ross, D . A., NEPROCHNOV, Y u . et a/. ( 1978) Initial Reports ofthe Deep Sea Drilling Project, Vol. XLIIB. U.S. Government Printing Office, Washington D.C. (In press.) SCHNEEBELL, W. & ROTHLISBERGER, F. ( 1976) 8000 Jahre Walliser Gletschergeschichte. Die A /pen, Zeitschrift Schweiz. A lpenclub, 52, 3/4, 5- 1 52. ScHRADER, H.J. ( 1978) The Diatom Units and the Paleogeography of the Black Sea in the Late Cenozoic (DSDP Leg 42B). In: Initial Reports of the Deep Sea Drilling Project, Vol. XLIIB (D. A. Ross, Yu Neprochnov et a!.). U.S. Government Printing Office, Washington D.C. (In press.) SHIMKUS, K.M. & TRIMONI S, E.S. ( 1 974) Modern sedimentation in Black Sea. In: The Black Sea, Geology, Chemistry and Biology (Ed. by E. T. Degens and D. A. Ross). Am. Ass. Petrol. Ceo/. Mem. 20, 249-278. STOFFERS, P . ( 1 975) Sedimentpetrographische, geochemische und paldoklimatische Untersuchungen an ostafrikanischen Riftseen. Habilitationsschrift UniversiUit Heidelberg. STOFFERS, P. & M O L LER, G. ( 1978) Mineralogy and Lithofacies of Black Sea Sediments, Leg 42B, Deep Sea Drilling P roj ect. In: Initial Reports of the Deep Sea Drilling Project Vol. XLIIB (D. A. Ross. Yu Neprochnov et a!. ). U.S. Government Printing Office, Washington D.C. (In press.) TRAVERSE, A. ( 1 978) Palynological Analysis of DSDP Leg 42B ( 1975) Cores from the Black Sea. In: Initial Reports of the Deep Sea Drilling Project, Vol. XLIIB (D. A. Ross, Yu Neprochnov et a/. ). U.S. Government Printing Office, Washington D.C. (In press.) ZENKEVICH, L.A. ( 1 957) Caspian and Aral Seas. In: Treatise on Marine Ecology and Paleoecology (Ed. by J. W. Hedgpeth). Mem. Ceo!. Soc. A m. 67, 89 1-9 1 6. KOJUMDGIEVA,

Spec. Pubis int. Ass. Sediment. ( 1 978)

2,

147- 168

Turbidites and varves in Lake Brienz (Switzerland): deposition of clastic detritus by density currents

M I C H A E L S T U R M * and A L B E R T M A T T E R Geologisches Institur der Universitdt Bern, Sahlistrasse 6, CH-3012 Bern, Switzerland

AB STR ACT Lake Brienz is a 14 km long and 26 1 m deep oligotrophic valley lake which lies in the front ranges of the Swiss Alps. Sedimentation is entirely clastic and is dominated by two rivers which enter the lake at opposite ends. The sediment load is transported and deposited in the lake by overflows, interflows and underflows (low- and high-density turbidity currents) depending on the density difference between river and lake water. Whereas high-density turbidity currents, which deposit up to 1 50 em-thick graded sand layers. occur only once or twice per century after catastrophic flooding. low-density turbidity currents occur annu ally during periods of high discharge and deposit centimetre-thick faintly graded sand layers. Fine-grained sediment supplied by overflows and interflows rains down continuously during summer thermal stratification to form the dark-grey summer half-couplet of a varve; at turnover in the autumn the remaining sediment trapped at the thermocline settles out and forms the light-grey winter layer. Turbidites grade distally into thin dark-grey layers indistinguishable from the dark-grey summer half-couplet. Turbidites on the basin plain can be correlated with varves on the slopes. Therefore, in Lake Brienz the formation of varves and turbidites is genetically related and depends on the existence of over- and interflows. turbidity currents and seasonal thermal stratification.

INTRO DU CTION

While working on turbidite sequences of ancient Flysch basins in the Alps we became interested in turbidity currents and their deposits that occur today in lakes. First observed in lakes by Forel ( 1 8 85) and Heim, Moser & B iirkli-Ziegler ( 1 8 88), turbidity currents are now recognized as an important depositional process. In * Present address: Swiss Federal Institute of Technology, EA WAG, CH-8600 Diibendorf/Zii rich, Switzerland

Modern and Ancient Lake Sediments Edited by Albert Matter and Maurice E. Tucker © 1978 The International Association of Sedimentologists. ISBN: 978-0-632-00234-4

1 48

Michael Sturm and A lbert Matter

comparison with ocean basins, lakes can be regarded as closed, natural sedimentation laboratories whose parameters are comparatively easy to measure and monitor, and hence to a certain extent lakes can be used for modelling processes in the oceans. We believed that by applying modern coring and seismic techniques and sedimentological methods to lake sediments much information could be obtained about sedimentary regimes in lakes, especially on the origin, nature, abundance and distribution of turbidity currents, which would also be applicable to marine sedimentation. Furthermore, we wanted to study the origin of clastic varves in lakes and find out whether they are related to turbidity currents as postulated by Kuenen ( 1 95 1 ) and B anerj ee ( 1 968) or to overflows as suggested by Antevs ( 1 9 5 1 ).

SWITZ[RLANO SAMPLE STATIONS •

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Fig. I. Index map of Lake Brienz showing bottom topography, sample stations and position of cross sections. The two main tributaries, the Aare and Liitschine Rivers, are at either end of the longitudinal basin. Note steep slopes and narrow shore terraces of the basin. Solid contour lines indicate 20 m intervals; dashed lines indicate 10 m intervals.

In order to investigate these problems Lake Brienz was chosen for this study, firstly because it is a deep longitudinal trough with a sediment source at each end and therefore has much in common with ancient flysch basins of the Alps (Hesse, 1 974), and secondly because it has a long hydrologic record, including a known input of sediment load through time and a well studied current pattern and physical limnology. Lake B rienz is one of several longitudinal valley lakes within the front ranges of the Swiss Alps. It is situated about 70 km southeast of Berne, 5 64 m above sea level (Fig. 1 ) The background data summarized in Table 1 show that 18% of the drainage basin is covered by glaciers and lies at a mean altitude of 1950 m. .

Turbidites and varves, Lake Brienz

1 49

Table 1. Physical parameters of Lake Brienz. Data from Nydegger ( 1 967) and from the Swiss Fed. Bureau of Water Management.

Maximum length Maximum width Maximum depth Mean depth Surface area Volume Altitude Drainage area Mean altitude of drainage area G laciated part of drainage area Ratio drainage area : lake area Aare water discharge Liitschine water discharge Total water discharge Mean water retention time Aare suspended load Liitschine suspended load Total suspended load

14 km 2·5 km 26 1 m 174 m 30 km' 5·2 km3 566 m above sea level 1 1 40 km' 1 950 m above sea level 1 8·2% 38 : l 33 m3 sec- ' (mean 1 954-1975) 1 9 m3 sec- ' (mean 1924- 1975) 61 m3 sec - ' (mean 1 935- 1975) 2·8 years 1 23·7 x 103 tons year - ' (mean 1965- 1 975) 1 24· 1 X 103 tons year - ' (mean 1 965-1975) approx. 285 x 1 03 tons year - ' (mean 1 965- 1 975)

Most of the catchment is drained by two major rivers, the Aare and the Uitschine, which carry enormous amounts of glacial and snow-melt waters and sediment into the lake, especially in spring and early summer (Table 1 ). Based on our experience in the neighbouring Lake Thun (Sturm & Matter, 1 972c) we expected that because the Aare drains largely crystalline terrane whereas the Liitschine drains sedimentary and crystalline terrane the rivers would carry detritus of distinctly different mineralogic composition, and also that the steep-gradient streams debouching from lateral slopes into the lake would influence sedimentation in only a minor way and could be neglected. This was confirmed during a study of the surficial sediments that revealed that each sample containing coarse silt to sand sizes could be attributed to either the Aare or Liitschine source and that sediment load of lateral tbrrents had very little influence on the sedimentation within the basin (Sturm, 1 976). Lake Brienz is a holomictic, oligotrophic lake (Nydegger, 1957, EAWAG, 1 967), and the lake sediments are therefore predominantly allochthonous in origin (Sturm, 1976). Thermal stratification develops from March to November with an average thickness of the thermocline of about 25 m. Current measurements made at several depths within the uppermost 60 m reveal a counter-clockwise rotation of the water mass (Nydegger, 1967, 1 976). This circulation pattern is attributed to the continuous driving force of the inflowing rivers which are deflected to the right as a result of the earth's rotation. Flow velocities measured at 1 9 and 25 m depth reach 5 ·6 em sec - 1 and 5·0 em sec - 1 (Nydegger, 1 967, 1 976). Nydegger also reported the existence of a turbid layer within the thermocline. He thought that it was built-up through accumulation of suspended matter carried into the lake by the rivers. The basin morphology of Lake Brienz is rather simple; a flat central basin plain is flanked by steep (30-40.) lateral slopes with a delta at each end of the lake. The shore terrace is generally very narrow or missing entirely where the shores are formed by cliffs. A slightly broader shore terrace occurs only near the village oflseltwald (Fig. 1 ).

1 50

Michael Sturm and A lbert Matter

The lateral slopes as well as the central basin plain show a smooth bottom morphology whereas in the delta areas distinct morphological features such as channels and levees are present. The channels on the Aare delta foreslope for example are up to 200 m wide and up to 35 m deep (Sturm, 1 975; Sturm & Matter, 1 972b). The lake trough has been eroded deeply into Mesozoic bedrock by fluvial and glacial action. The maximum depth of the basement is about 860 m below lake level i.e. approx. 300 m below sea level. During Pleistocene and Holocene times about 600 m of clastic sediments accumulated (Matter et al., 1973). We expected that turbid meltwater that enters the lake each spring would provide material for turbidity currents in Lake Brienz. By collecting a large number of cores from the entire basin we wanted to correlate individual turbidites across the basin, use the mineralogy to identify which of the two rivers was the source and then determine the path of each turbidity current and the amount of erosion, if any, at the base of each turbidite. If successful, we would thus be able to reconstruct the pattern of basin filling and arrive at a model for clastic sedimentation in such lakes.

METH O D S

In order to determine the sedimentary pattern and the sediment dispersal in the lake 80 cores and 140 surface samples were collected during several field campaigns between 1973 and 1 976. The cores were taken with a 3-6 m Reineck piston corer with l 0 em diameter (Reineck, 1967) and a modified 8- 1 0 m Kullenberg piston corer with a diameter of 6 em. The surface sediment cores were taken with a Reineck box corer (Reineck, 1 963) and a 1 m gravity corer. The locations of the coring sites were determined from sextant readings on distinct shore features. Water depths were measured by echo sounder. The cores were taken to the laboratory, cut lengthwise with an electro-osmotic guillotine (Sturm & Matter, 1 972a), photographed and sampled. Samples and cores were sealed in plastic bags or polyethylene foils and stored in a core library at about Ire. The analytical programme included grain-size analyses by standard sieve and sedimentation balance procedures, heavy mineral counts, gasometric analysis of the carbonate content, and clay mineral analysis of the clay size fraction ( < 4,um) by X-ray diffraction. The sequence of lithologies in each core and the core-to-core correlations were worked out using core photographs. During 1 973 and 1974 transmissometer and temperature profiles were measured at fixed stations in the Aare delta to identify turbid layers in the water column. At the same time 1 litre water samples were taken at various depths at these stations to determine the amount of suspended matter.

RE S U LT S Description of sediments

Inspection of a large number of cores revealed that the sediment lithology could be described in terms of four different sediment types, (a) delta sand and mud, (b) laminated mud, (c) homogeneous mud and (d) laminated mud with interbedded graded sand/silt.

Turbidites and varves, Lake Brienz

151

Fig. 2. Box cores from delta area of the Aare River: (a) core B K-42, 240 m water depth, from bottom of main channel, showing massive, poorly graded sand; (b) core BK-42, 240 m water depth, from levee of main channel showing alternations of thin sand and silt beds. Note lenticular shape of some of the layers. Scale in centimetres.

Delta sand and mud is found on the foreslopes of the Aare and Liitschine deltas. Coarse-grained well sorted massive sand beds occur in channels as 1 5-30 em thick layers with sharp lower and upper contacts (Fig. 2a). Occasionally pebbles up to 2 em in size were found at the base of these layers, and very rarely a faint lamination was seen. The delta muds consist of lenticular bedded sand/silt alternations with subordinate clay layers. The sandy and silty layers are commonly less than 1 ·5 em thick and frequently swell and pinch out over a distance of a few centimetres. Ripple cross­ bedding, usually characteristic of lenticular bedding (Reineck & Singh, 1975, p. 1 00) has not yet been recognized, but grading was observed in silt layers (Fig. 2b). Lenticular bedded sand/silt is found in the levee and interchannel areas as well as in the lowest foreslopes of the deltas. This kind of bedding was also reported from lake sediments in front of small developing deltas by Coleman ( 1 966, cit. in Reineck & Singh, 1975, p. 1 02). Because of the irregular topography in the channelled portion of the delta, there are abrupt lateral lithologic changes from a channel to a levee or interchannel area. The irregular nature of the lenticular bedding and the abrupt changes from channel to levee deposits preclude core-to-core correlation in the delta areas. Laminated muds consist ofregularly alternating dark and light grey laminae 1 -3 mm thick which form varve-like couplets (Fig. 3 a). The maximum thickness of a couplet never exceeded 10 mm. Microlaminae of about 1 mm in thickness were sometimes observed. Contacts between individual lamina includingthe contact between dark and light coloured laminae of a couplet are sharp and nongradational. From visual inspection the dark lamina always appeared to be slightly coarser grained and less

152

Michael Sturm and A lbert Matter

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Regular varves are devloped on top and bottom of core divided by a thick homogeneous mud section; (b) core BL-8, 26 1 m water depth, from centre of the basin plain. Irregular varve bedding is occasionally interrupted by graded beds of Aare (TA) and Liitschine (TL) turbidites. Youngest sediment on top of turbidite TA -1 is more regularly bedded than older sediments. Note frequent gas expansion holes. Scales in centimetres.

1 53

Turbidites and varves, Lake Brienz

clayey than the light lamina. This was confirmed by grain-size analysis of thirteen couplets from core B L-6 (Fig. 3a and Fig. 1 2). Although the mean grain sizes of both kinds of laminae lie in the silt range, the dark laminae are generally about 0·5 coarser grained. Moreover the light laminae contain very little carbonate ( < I 0%) whereas the dark laminae contain 8-36% carbonate. Therefore, if these couplets are varves, the dark basal lamina represents the summer and the light lamina the winter layer, which is j ust the opposite of classical glacial varves. Laminated muds accumulate on the lateral slope and on the basin plain (see below). Homogeneous light grey muds were present in several cores, for example in the middle section of core B L-6 (Fig. 3 a). Homogeneous muds occasionally show a very faint lamination. No macroscopic evidence of bioturbation was observed, however. They are virtually carbonate free ( < I %) and contain no sand-sized admixtures. Their mean grain size lies within the fine silt grade. The observed thickness ranges from 70- 1 80 em. Homogeneous muds were never observed in cores from water depths greater than 40 m, nor in the uppermost sediments in shallower water. It is thought that they are not forming in the lake today. Their distribution in the sediments is restricted to a small area on the upper slope and shore terrace near Iseltwald (Fig. 4). Laminated muds interbedded with graded sand or silt beds were found in cores from the basin plain. The graded beds are either of dark grey or light grey colour. They vary not only in colour but also in thickness, carbonate content and heavy mineral composition. The light grey graded beds are generally much thicker and contain less carbonate ( < 1 0%) than the dark grey graded beds ( > 1 5 % carbonate). The heavy mineral suite of the dark grey beds is dominated by garnet, apatite and hornblende whereas the light beds contain a hornblende-epidote suite. These graded beds are the deposits of turbidity currents generated by the Aare River (light grey) and the Li.itschine River (dark grey).

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surface but have been mapped from occurrence in cores. Contour lines indicate 50 m intervals.

The distribution of these four sediment types was plotted on a bathymetric map (Fig. 4). It is obvious that each sediment type or facies is closely related to a morphologic unit. Delta sand and mud cover the delta areas at each end of the lake. Laminated mud is found on the lateral slopes. Homogeneous mud is restricted to the shelf area near Iseltwald. Layers of graded sand/silt intercalated with laminated muds are found on the fiat basin plain at depths between 250 and 26 1 m.

1 54

Michael Sturm and A lbert Matter

Characteristics and correlation of turbidites

Turbidites found in cores from the basin plain are of particular interest. One of the more prominent Aare turbidites (TA-3), nearly 70 em thick in core 1 7 from mid-basin, was analysed in detail in order to show the structural and textural variations within an individual turbidite (Fig. 5). The base of this turbidite is a scoured surface, overlain by coarse sand, grading upwards into fine sand, then silt and finally silty clay at the top. Wood fragments and plant leaves occur at the base. Simple graded bedding is the only sedimentary structure present in this and in the rest of the large number of turbidites that we have studied. Other structures such as parallel lamination or ripple bedding are not developed. A distinct very light coloured 2 em thick clay layer overlies with sharp contact the graded sequence (Fig. 5). Within this single turbidite the carbonate content increases from 2·5 to 8 · 5% at the top. Changes in carbonate content and grain size are more pronounced in the upper half to two-thirds of the turbidite. Clay is virtually absent in the lower half of the bed. The overlying clay layer contains very little carbonate. Sorting is poor throughout the bed, but slightly less so in the clay­ deficient lower part.

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after Folk & Ward ( 1 957).

155

Turbidites and varves, Lake Brienz

Liitschine and Aare turbidites can b e identified i n cores o n the basis o f different grey hues, carbonate content and heavy mineral composition. We succeeded in correlating eleven Liitschine turbidites (TL- 1 to 1 1 ) and six turbidites that had been deposited by Aare turbidity currents (TA- 1 to 6) (Figs 6-7). The correlation is based on the sequential pattern of dark and light turbidites, their thicknesses and on varve counts and correlations in the laminated muds between the turbidites. Varve correlation also provided estimates of the amount of erosion caused by a turbidity current (see erosional contacts in Figs 6 and 7). Two particular turbidites (TA- l and T A-3) were closely sampled and grain-size analyses were carried out to check the textural variations and the core-to-core correlation.

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Individual turbidites vary in thickness from 2 to 1 5 0 em (Figs 6 and 7). Thick turbidites are much less common than thin ones. Lateral variation in thickness is

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Turbidites and varves, Lake Brienz

157

greatest i n thick turbidites such a s TA- 1 whereas the thin turbidites are more uniform in thickness. Because the basin plain sediments overlying TA- 1 contain no thick turbidites approximately even amounts of sediments (50-60 em) have accumulated over the entire basin plain subsequent to the deposition ofT A- 1 (which probably took place in 1 896, see below). Aare turbidites attain maximum thickness and frequently have eroded underlying sediments in the eastern part of the basin plain (Fig. 6a). Liitschine turbidites are thickest at the other end of the basin but no erosion was observed. Whereas Aare turbidites can be traced up to the foot of the Liitschine delta, Liitschine turbidites are often absent in cores from the Aare delta region due to erosion by subsequent Aare turbidites (Fig. 7). For example in core 1 7 TA- l eroded at least 40 em of previously deposited silt and clay including Liitschine turbidites TL- 1 , 2 and 3 (Fig. 6a). Even at a distance of 9 km from the mouth of the Aare River this turbidity current still was able to erode at least 25 em of mud including the 3-4 em silt layer of TL- 1 (Fig. 7, core 3 2). As would be expected mean grain size of the sediment deposited by an individual turbidity current decreases with increasing distance from the source, for example TA- 1 (Fig. 7). Sorting, however, remains fairly constant. Correlation of varve-like couplets

The correlation of turbidites was only possible within the central basin plain where cores of sufficient length could be obtained. On the steep lateral slopes box corer and gravity corer were used and yielded cores 1 0-30 em long. These cores were used in an attempt to correlate individual varve couplets from slope to slope across the profundal basin plain (Fig. 8). Key couplets showing distinct features such as microlamination, sandy admixtures, and a more pronounced dark colour (Fig. 8, 0- 1 5 , Q- 1 0, T- 1 ) aided in correlation of the other more uniform varves. The regular couplets on the slopes never exceed 1 0 mm in thickness and the dark basal silt lamina of each couplet is generally thinner than the light-coloured clayey lamina above it. Towards the basin plain couplet thickness becomes more irregular and varies between 5 and 20 mm. This is also true for the couplets of the laminated intervals ofthe basin plain facies. On the basin plain the dark lamina is thicker than the light lamina, and would thus correspond to proximal deposition according to Ashley ( 1 972). Our observations indicate that the darker sand/silt layer is up to twenty times thinner on the slopes than in a proximal core of the basin plain whereas the light clayey layer remains nearly constant. Core BP-56 from the basin plain near the Aare delta shows an Aare turbidite that consists of a 57 mm thick graded sand/silt and a 4 mm thick clay layer (Fig. 8, BP-56, 01 5) . In cores from the basin plain (Fig. 8, BK-69, 70, 84, 85) this sand/silt layer decreases to an ungraded 5-7 mm silt lamina with increasing distance from the Aare mouth, whereas the clayey lamina above is of constant thickness (about 3 mm) in all cores. In cores from the northern and southern slope (Fig. 8, B K -48, 68, 77, 83) the silt layer is even thinner, 1 -3 mm, but the light layer is constantly 3 mm thick. Apparently a distinct turbidite may grade laterally into a thin lamina indistinguish­ able from a dark lamina of a varve. A similar observation was reported by Ludlam ( 1 969, 1 974) in Fayetteville Green Lake (New York). The most striking point is that a turbidite on the basin plain can be correlated with a dark lamina ofwhat we believe are true varves on the upper lateral slope, an area certainly beyond the reach of turbidity currents.

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do varves on the basin plain. Note change of key-bed 0- 1 5 from a graded bed in the eastern basin plain to an ungraded basal lamina of varve on the slopes. Gas expansion holes are responsible for cracks within some of the cores. Index map gives location of correlated cores. Scale in centimetres.

Turbidites and varves, Lake Brienz

159

DI S C U S SION Turbiditic sedimentation

Sedimentation of the uppermost Aare turbidite TA- 1 is typical of the larger turbidites in Lake Brienz. Fig. 9 shows the areal distribution of TA- 1 and a longitudinal cross-section. Unfortunately no cores were obtained in the proximal part of the turbidite because of the difficulty in coring the deltaic sands. By analogy with modern turbidity currents observed in Lake Brienz TA- 1 probably originated as sediment-laden river water flowed downslope following a delta channel to the basin plain. Where the turbidity current overflowed the delta channel, lenticular sands and silt layers were deposited. The channel becomes less deep and loses its identity at about 240 m water depth. From this point the main body of the turbidity current spread out and eroded sediments across a large part of the fiat lake floor (Figs 6a and 7). The head of the current lost its erosional capacity about 9 · 5 km from the Aare inflow. This limit is indicated by a dashed line in the western part of the basin on Fig. 9a.

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Near its source the current deposited a coarse-grained and poorly sorted thin sand layer of about 22 em along the axis of the flow (BP-25, Fig. 9a), which thins out laterally to a 10 em thick layer in core BP-23 (Fig. 9a). The much deeper erosion (higher velocity) and greater deposition in the centre of the basin as shown on Fig. 6a indicates that at cross-section A-A' the main mass of the flow was still more or less confined and thinned and slowed laterally towards the base of the slopes. The 1 40 em thick graded bed which had rapidly accumulated in the basin centre only 1 km downcurrent of B P-25 suggests that the turbidity current may have undergone an hydraulic j ump (Komar, 197 1 ) from an erosional supercritical flow to a subcritical depositional flow. By about 6 km from the Aare inflow the turbidity current had spread a uniform sediment layer over the entire basin plain as evidenced by the constant thickness observed in cores 7- 1 2 and B P-40 (Fig. 9a). Beyond core 32 the thickness of TA- 1 decreases rapidly to 4 em in core 25, and it finally remains as a dark thin lamina at the foot of the Liitschine delta.

160

Michael Sturm and A lbert Matter

Assuming a bulk sediment density of 1 ·7 g. cm - 3 (Jackli, 1958) this particular turbidity current deposited a total mass of about 8 x 1 06 tons (dry weight). This corresponds to about thirty times the mean amount of detritus delivered annually to the lake (Table 1). According to Forstner, Muller & Reineck ( 1 968) an exceptional flood of the Rhine River carried three times the average annual suspended load into Lake Constance within a few hours. During the past 30 years the yearly load carried by the Aare River was up to ten times the 30 year-average. It is concluded therefore that in Lake Brienz TA- 1 was deposited by a turbidity current of much larger size than any other that had occurred during the past decades and may have been the result of a catastrophic flood in the drainage basin. The last such event was a catastrophic mudflow which moved out from a lateral valley onto the Aare delta and into the lake in 1 896 (von Steiger, 1 896). According to varve counts TA- l occurred about 70 to 90 years ago which would fit the date of the mudflow. TA- l , however, has a typical Aare River heavy mineral suite that is different from that of the river draining the mudflow source area. Two alternative explanations are then: ( 1 ) the turbidity current depositing TA- l originated when part of the upper delta slope consisting of Aare-derived material became unstable due to overloading by the mudflow from the lateral valley and slid off, merging eventually with the much smaller turbidity current generated directly by the mudflow; or (2) a similar catastrophic event occurred in the main valley of the Aare causing the exceptionally large turbidity current in the lake which picked up additional material by eroding the delta foreslope. Approximate varve counts of the laminated intervals separating large turbidites indicate that large turbidity currents occurred on the average once or twice per century. This is compatible with historical records reporting catastrophic events such as large floods or mudflows. Such reports, however, are often too vague and incomplete to be precisely correlated with turbidites in the lake. Whereas this first type or 'catastrophic' turbidity current .is able to carry large amounts of sand -sized detritus far out onto the basin plain and to erode the lake floor, a second kind of turbidity current is caused by river floods (annual spring floods etc.). This type deposits a much thinner graded sand/silt layer that decreases much faster in thickness with increasing distance from its source and becomes soon indistinguishable from the basal silty layer of varves. If several floods produce turbidity currents of this kind during a single year, a microlaminated silt layer results. Although no direct current measurements are available for Lake Brienz, we can crudely estimate the velocities of turbidity currents from the mean grain size at the base of a graded bed. Using the sediment threshold curve of Miller & Komar ( 1 977) for quartz grains in pure water at 2o·c, a sloping lake floor of 1 ·, and a maximum grain size of 1-2 r-

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from core 8 taken in the centre of the basin (see Fig. 7) ; water samples were collected on 28 May 1974--2 km from the mouth of the A are River and filtered through 0-45 fLm Millipore filters (see Table 2). In the turbidite mean grain size grades upwards from medium sand to finest silt. Mean grain sizes in the top of the turbidite, varves and suspended matter are approximately the same. All samples are poorly to very poorly sorted, • and ... indicate mean grain- size (M,) and sorting ( a1 ) of samples of turbidite and dark varve laminae; oand 6 indicate M, and a1 of samples of light laminae and of suspended material.

Turbidites and varves, Lake Brienz

1 65

explains the greater thickness of dark laminae in these areas compared to those from the slopes. This may be checked by a comparison ofgrain sizes of a large turbidite (TA3), varves and suspended material of water samples (Fig. 1 2). In all cases sorting is poor. Mean grain sizes of TA-3 are distinctly coarser than mean grain sizes of varves and suspended material, but similar grain sizes can be observed within the top layer of the turbidite, in light-coloured laminae of varves and constituting the suspended material. Regardless of their origin, dark layers in the basin and on the slope generally form between spring and autumn, and both are capped by a light-coloured 'winter' layer. Thus, although deposited during the course of only a few hours, turbidites on the basin plain correspond with half-couplets on the slope. If pelagic sedimentation is continuous over the year, an unlaminated sediment would accumulate such as the homogeneous mud found only in cores on the upper slope and shore terrace off Iseltwald. This happens when the thermocline does not develop or if it is at water depths greater than the depositional surface. This was also reported by Ashley ( 1 975) who noticed that no varves were formed on morphologic highs in glacial Lake Hitchcock. No such homogeneous muds are accumulating today in Lake Brienz. The occurrence of homogeneous muds, however, in cores from the upper slope indicates that according to varve counts about 600 years ago the level of Lake Brienz and hence its thermocline was about 10- 1 5 m lower than today. S UMM A R Y

Sedimentation in Lake Brienz is dominated by two rivers, the Aare and the Liitschine, which enter at opposite ends of the long, narrow and deep basin. The sediments are clastic. The rivers drain areas of distinctly different lithologies, and thus the source of the sediments can be clearly delimited. Four major types of sediment can be distinguished, massive and lenticular sands and silts near deltas, laminated muds and homogeneous muds on slopes and laminated muds interbedded with graded sands and silts on the basin plain. The sediments are related to two different depositional processes, (a) turbidity currents (underflows) along the lake bottom and (b) interflows and surface currents (overflows) in and on the lake water. Massive turbidites up to 150 em thick apparently are the product of high-density turbidity currents that occur only once or twice per century as a result of catastrophic flooding or landslides. These turbidites represent only the graded A-interval of the Bouma sequence, suggesting rapid deceleration of turbidity currents. The currents cut channels into deltas and erode basin plain sediments up to 9 km from the river mouth. Such turbidity currents cover most of the basin plain with coarse sand fining distally into graded silt layers and finally into thin ungraded silt/clay laminae. One such turbidite can be clearly mapped over 1 1 km2 and contains a volume of about 5 X 106 m3 sediment. Turbidites from both sediment sources interfinger. A second type of turbidite is represented by thin dark grey layers apparently deposited by low-density turbidity currents which occur one or more times each year as a result of spring meltwaters and heavy rainfalls. These turbidites consist of slightly graded coarse-to-fine sands in the delta region but thin rapidly into ungraded silt laminae on the basin plain. Neither type of turbidite was found on the lateral basin slopes or morphological highs.

166

Michael Sturm and A lbert Matter

Laminated muds on the lateral slopes and on the flat basin plain (here interbedded with the thick turbidites) apparently result from an annual sedimentation cycle related to thermal stratification in the lake water and seasonal inputs of sediment-laden river water. If the suspended load and water temperature are such that river water is less dense than cold bottom lake water, instead of flowing down along the bottom as an underflow, the river water will form a turbid layer at or above the thermocline (interflow or overflow). Counter-clockwise currents will spread the entrapped suspended sediment over the entire basin. Coarser particles fall out continuously during the summer months and form the dark grey basal laminae of varve couplets. Light-coloured layers represent fine-grained particles that become trapped by the density gradient at the thermocline . during the summer and settle out after the thermocline is destroyed in the autumn. On the basin plain, the occurrence of one or more low-density turbidity currents during the spring and summer results in additional thin, dark grey layers (microlaminations) which are indistinguishable from the basin-wide 'summer' layer. Dark layers are therefore thicker on the basin plain than on the lateral slopes which are beyond the effects of turbidity currents. The light-coloured 'winter' layer is of uniform thickness over the entire basin. Lake Brienz sediments show that the mechanisms ofturbidite deposition and clastic varve formation are integrally related. A half-couplet on the lateral slope can grade into a thick graded turbidite on the basin floor and both have a common sediment source but are the products of an overflow or interflow on the one hand and an underflow, i.e. a turbidity current, on the other hand. Thus, neither the models of de Geer ( 1 9 1 0), Kuenen ( 1 95 1 ) and Laj tai ( 1 967), and others, which invoke only turbidity currents as the varve-forming mechanism, nor the model of Antevs ( 1 95 1 ) which requires only overflows, provides a complete explanation for deposition of laminated clastic sediments in lakes. Our study suggests that both over- and interflows as well as turbidity currents are required, and that the existence of seasonal thermal stratification is of critical importance.

A CKN OWLE DGMENT S

We wish to thank especially R. F. Wright for criticizing earlier drafts of the manuscript and suggesting important changes as well as for correcting the English of the final draft. Further thanks are due to K. Kelts and P. Shannon for useful comments in reviewing this paper and to A. Lerman for stimulating discussions. This study was supported by the Swiss National Science Foundation, grant No. 2.427.75.

REFERENCE S ANTEVS, E.( 195 1 ) Glacial clays in Steep Rock Lake, Ontario, Canada. Bull. geo/. Soc. A m. 62, 1 223- 1 262. ASHLEY, G . M . ( 1 972) Rhythmic sedimentation i n glacial Lake Hitchcock, Massachusetts-Connecticut. Contr. Geo/. Dept Univ. Mass. 10. ASHLEY, G.M. ( 1 975) Rhythmic sedimentation i n glacial Lake Hitchcock, Massachusetts-Connecticut. In: Glaciofluvial and Glaciolacustrine Sedimentation (Ed. by A. V. Jopling and B . C . McDonald). Spec. Pubis Soc. econ. Paleont. Miner., Tulsa, 23, 304-320.

Turbidites and varves, Lake Brienz

1 67

BANERJEE, I. ( 1 968) A study of glacial varves as turbidites. A bstr. Meet. A m. geol. Soc. Wash. D. C. Feb. 1 968, 335-336. BANERJEE, I. ( 1 977) Experimental study on the effect of deceleration on the vertical sequence of sedimentary structures in silty sediments. J. sedim. Petrol. 47, 77 1-783. BATES, CH. C. ( 1 953) Rational theory of delta formation. Bull. Am. A ss. Petrol. Geol. 37, 2 1 1 9-2 1 62. BELL, H.S. ( 1 942) Density currents as agents for transporting sediments. J. Geol. 50, 5 12-547. EAW (Eidg. Amt. f. Wasserwirtschaft) ( 1 939) Deltaaufnahmen des Eidgenossischen Amtes fiir Wasserwirtschaft. Mitt. 34. EA WAG ( 1 967) Einfluss der veriinderten Zuflussverhaltnisse auf den Thuner- und Brienzersee. Report Nr. 3435, EAW AG-ETH, Ziirich. FoREL, F.A. ( 1 885) Les ravins sous-lacustre des fleuves glaciaires. C. r. hebd. Seanc. A cad. Sci., Paris, 101, 725-728. FOLK, R.L. & WARD, W.C. ( 1957) B razos River bar: a study in the significance of grain size parameters. J. sedim. Petrol. 27, 3-26. FORSTNER, U., MOLLER, G. & REINECK, H.-E. ( 1 968) Sedimente und Sedimentgefiige des Rheindeltas im Bodensee. Neues. lb. Miner. A bh. 109, 33-62. DE GEER, G. ( 1 9 10) A geochronology of the last 1 2,000 years. Int. Geol. Congr. XI. Sess. Stockholm, 241-253. GoU L D , H.R. ( 1 960) Turbidity Currents. In: Comprehensive Survey of Sedimentation in Lake Mead (Ed. by W. 0. Smith et al.). Prof Pap. U.S. geol. Surv. 295, 201-207. GusTAVSON, T.C. ( 1975) Bathymetry and sediment distribution in proglacial Malaspina Lake, Alaska. J. sedim. Petrol. 45, 738-744. HElM, ALB., MOSER, R. & B ORKLI-ZIEGLER, A. ( 1 888) Die Catastrophe von Zug. 5. Juli 1887. Veri. Hofer & Burger, Zurich. HESSE, R. ( 1 974) Long-distance continuity of turbidites: possible evidence for an early Cretaceous trench­ abyssal plain in the East Alps. Bull. geol. Soc. A m. 85, 859-870. Ji\cKLI, H. ( 1 958) Der rezente Abtrag der A!pen im Spiegel der Vorlandsedimentation. Eclog. geol. Helv. 51, 354-365. KOMAR, P.O. ( 1 97 1 ) Hydraulic jumps in turbidity currents. Bull. geol. Soc. A m. 82, 1477-1 488. K u EN EN, P.H. ( 195 1) Mechanics of varve formation and the action of turbidity currents. Geol. Fiiren. Fiirhandl. 73, 69-84. LAJTAI, E.Z. ( 1 967) The origin of some varves in Toronto, Canada. Can. J. Earth Sci. 4, 633-639. L AM B ERT, A.M. ( 1 978) Eintrag, Transport und Ablagerung von Feststoffen im Walensee. Eclog. geo/. Helv. 71. (In press). LAMBERT, A.M., K E LTS, K . R. & MARSHALL, N.F. ( 1 976) Measurements of density underflows from Walensee, Switzerland. Sedimentology, 23, 87- 1 05. LUDLAM, S.D. ( 1 969) Fayetteville Green Lake, New York. III. The laminated sediments. Limn of. Oceanogr. 14, 848-857. LUDLAM, S.D. ( 1 974) Fayetteville Green Lake, New York. VI. The role of turbidity currents in lake sedimentation. Limnol. Oceanogr. 19, 656-664. MATTHEWS, W.H. ( 1 956) Physical limnology and sedimentation in a glacial lake. Bull. geo/. Soc. A m. 67, 537-552. MATTER, A., 0ESSOLIN, D., STURM, M. & S O SST RU N K , A. E. ( 1973) Reflexionsseismische Untersuchung des Brienzersees. Eclog. geol. Helv. 66, 7 1-82. MILLER, M.C. & KOMAR, P.O. ( 1 977) The development of sediment threshold curves for unusual environments (Mars) and for inadequately studied materials (foram sands). Sedimentology, 24, 709-72 1 . NORMARK, W.R. & DICKSON, F . H . ( 1 976) Man-made turbidity currents i n Lake Superior. Sedimentology, 23, 8 1 5-832. NYDEGGER, P. ( 1 957) Vergleichende limnologische Untersuchungen an sieben Schweizerseen. Beitr. Geol. Schweiz-Hydrol. 9, l-80. NYDEGGER, P. ( 1 967) Untersuchungen iiber Feinstofftransport in Fliissen und Seen, iiber Entstehung von Triibungshorizonten und zuflussbedingten Stromungen im Brienzersee und in einigen Vergleichsseen. Beitr. Geol. Schweiz-Hydrol. 16, l-92. NYDEGGER, P. ( 1976) Stromungen in Seen. Untersuchungen in situ und an nachgebildeten Modellseen. Beitr. Geol. Schweiz, kl. Mitt. 66, 1 4 1 - 1 77.

1 68

Michael Sturm and A lbert Matter

0STREM, G. ( 1 975) Sediment transport in glacial meltwater streams. In: Glacioflu vial and Glaciolacustrine Sedimentation (Ed. by A. V. Jopling & B. C. McDonald). Spec. Pubis Soc. econ. Paleont. Miner., Tulsa, 23, 1 0 l- 1 22. REINECK, H.-E. ( 1 963) Der Kastengreifer. Natur Mus. Frankfurt, 93, 1 02- 108. REINECK, H.-E. ( 1 967) Ein Kolbenlot mit Plastik-Rohren. Senckenberg. leth. 48, 285-289. REINECK, H.-E. & SINGH, I. B. ( 1 957) Depositional Sedimentary Environments. Springer-Verlag, New York. voN STEIGER, H. ( 1 896) Der Ausbruch des Lammbaches vom 3 1 Mai 1 896. Mitt. Naturf Ges. Bern, 265-275. STURM, M. ( 1 975) Depositional and erosional sedimentary features in a turbidity current controlled basin (Lake Brienz). !Xth Int. Congr. Sedim., Nice, 5, 385-390. STURM, M. ( 1 976) Die OberfHichensedimente des Brienzersees. Eclog. geol. Helv. 69, l l l - 1 23 . STURM, M. & MATTER, A. ( l972a) The electro-osmotic guillotine, a new device for core cutting. J. sedim. Petrol. 42, 987-989. STURM, M. & MATTER, A. ( l972b) Geologisch-sedimentologische Untersuchungen im Thuner- und Brienzersee. Jb. Thuner- und Brienzersee, 52-72. STURM, M. & MATTER, A. ( l 972c) Sedimente und Sedimentationsvorgiinge im Thunersee. £clog. geo/. Helv. 65, 563-590.

Spec. Pubis int. Ass. Sediment. ( 1 97 8) 2, 169- 1 87

Lacustrine facies in the Pliocene Ridge B asin Group: Ridge B asin, California

M A R T I N H . L I N K and R 0 B E R T H . 0 S B 0 R N E Department of Geology, Los Angeles Harbor College, Wilmington, California 90744, and Department of Geological Sciences, University of Southern California, Los Angeles, California 90007, U.S.A.

A B STR ACT

The Ridge Basin is a wedge-shaped trough 1 5 by 40 km which contains over 9000 m of lacustrine sedimentary rocks. Lacustrine sedimentation in the Pliocene Ridge Basin occurred in this elongated trough formed during active strike-slip displacement along the San Gabriel fault. The lacustrine and other terrestrial deposits reflect syntectonic deposition related to steep faults with components of slip along the eastern and northeast and strike-slip to oblique­ slip components along the western margin of the Ridge Basin. Asymmetrical development of facies resulted from this differential tectonism along the margins of this basin. The Violin Breccia, which crops out along the western margin, consists of breccia, conglomerate and sandstone deposits that chiefly represent narrow talus and small alluvial fans that accumulated along the San Gabriel fault scarp. The Peace Valley 'beds' which occur in the central part of the basin, contain nearshore to offshore dark analcimic and ferroan dolomitic mudrock, organic-rich shale, and localized turbidite sandstone. This unit has a lacustrine origin as indicated by molluscs, ostracods, stromatolites, plants, and insect and vertebrate remains. The Ridge Route 'formation' crops out along the eastern margin and consists of sandstone and conglomerate which accounts for the greatest volume of sedimentary rock exposed in the Ridge Basin. It interfingers with both the Peace Valley 'beds' and Violin Breccia to the west. The sediments comprising this large alluvial fan-fluvial, marginal lacustrine and offshore turbidite complex were derived from the north and east. The lower part of the Ridge Basin Group is transitional from underlying marine strata. Ridge Basin lake evolved from an externally-drained, relatively deep lacustrine and/or marine system with thick sequences of turbidite and slump-folded strata to an internally­ drained, rather shallow, closed-lake system with thick sections of dolomitic mudrock. Lacustrine sedimentation in the Ridge Basin ended with the termination of movement along the San Gabriel fault. At that time the basin was filled with fluvial and fanglomerate deposits assigned to the Hungry Valley Formation.

Modern and Ancient Lake Sediments Edited by Albert Matter and Maurice E. Tucker © 1978 The International Association of Sedimentologists. ISBN: 978-0-632-00234-4

1 70

Martin H. Link and Robert H. Osborne INTRO DUCTION

The Ridge Basin, California (Fig. 1) is a relatively small wedge-shaped basin (Eaton, 1 939) or trough ( 1 5 X 40 km) which contains an extremely thick ( > 1 2,000 m) composite marine, lacustrine and fluvial stratigraphic section (Fig. 2). The Ridge Basin was tectonically active from Mohnian (late Miocene) to Blancan (early Pleistocene), which is an interval of about 10 million years (Crowell, 1 976, personal communication). The siliciclastic deposits have been deformed, uplifted and eroded forming spectacular and nearly continuous outcrops across the width of the basin (Fig. 3); therefore, it provides an excellent three-dimensional view of the structure and sedimentology of a recently dissected lacustrine basin (Fig. 4). Inasmuch as the Ridge Basin occurs at the j unction of the San Gabriel and San Andreas fault systems, it has been the subject of intensive tectonic investigations (Crowell, 1 974a,b) and is of economic interest because of oil shows, potential source beds, and its proximity to oil provinces. The authors are engaged in a comprehensive study of the sedimentology of the Ridge Basin principally to integrate its sedimentary and tectonic evolution. Data from stratigraphic sections, electric logs from boreholes, petrography, and mapping are integrated in this paper. The purpose of this paper is to concisely describe and discuss the major lacustrine facies in the Ridge B asin.

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G EOLOGI C AL S ETTING

Sedimentation in the Ridge Basin was the result of infilling of an elongated trough formed during active strike-slip (transcurrent) movement along the San Andreas transform fault margin (Crowell, 1 973, 1 975). The Ridge B asin was adjacent to the San Gabriel fault system which was the major Pliocene strand of the San Andreas fault. The basin formed at a bend along this fault where one side was stretched, depressed, and filled with a thick sequence of sediments by gradual overlapping of

171

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older strata by younger as the depocentre moved northward through time (Fig. 4). Although restricted source areas to the west shed sediment along the San Gabriel fault (Violin Breccia), the maj or sediment input came from the east and northeast (Ridge Route and Hungry Valley Formations). An idealized cross-section of the Ridge Basin (Fig. 2) shows a half-graben bounded by Mesozoic and older basement rock on both sides and separated on the west by the San Gabriel fault and on the east by the Clearwater, Liebre, and associated fault systems. The San Gabriel fault was active from Miocene into Pleistocene time and shows over 60 km of right slip displacement (Crowell, 1 975). The Clearwater and Liebre fault systems are dominantly high-angle reverse faults that were active during sedimentation in the Ridge Basin (Fig. 4). In Pleistocene time the modern trace of the San Andreas fault originated in this area as strike-slip displacement splayed eastward from the San Gabriel to the modern San Andreas fault. This composite zone may be considered as the San Andreas transform fault margin. A total of 240 km of right slip on the San Andreas fault system since early Pliocene for the Ridge Basin is suggested by Ehlert & Ehlig ( 1 977). The lowest deposits in the basin are the Miocene nonmarine Mint Canyon Formation (Fig. 2). It is overlain by the late Miocene marine Castaic Formation which is about 2200 m thick and is transitional upward into the Ridge Basin Group. The Violin Breccia is laterally and vertically transitional into the Castaic Formation and the Ridge Basin Group. It is 1 1 ,000 m thick but extends along strike for a maximum distance of only 1 500 m (Fig. 3). The Ridge Basin Group is about 9000 m thick and consists of four units, which are the Violin Breccia, Peace Valley 'beds', Ridge Route 'formation', and the Hungry Valley Formation. The Peace Valley 'beds' and Ridge Route 'formation' do not yet have formational status. Based on faunal and

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stratification, and is devoid of fossils (Fig. 5B, C and D). Well rounded, friable basement clasts (Fig. 6A) are a common constituent. Finer-grained (sand- and gravel­ sized clasts predominate), well-stratified (Fig. 6B and C) and moderately to well-sorted deposits suggest deposition near the lacustrine shoreline. These deposits include tabular beds of cross-stratified and horizontally-laminated sandstone (Fig. 6D), erosive channels filled with conglomerate (Fig. 6E) and cross-stratified sandstone, and contain load and flame structures (Fig. 6F), convolute laminae, ripple marks, mudcracks and a few vertebrate remains. At times, perhaps during seasonal flooding, coarse-grained sediment was transported by braided streams on the alluvial fan complexes into the lake. These streams greatly modified the marginal lacustrine environments. Such sediment influxes are indicated by channelling and by basal conglomerate or sandstone containing intraclasts of stromatolites, mudstone, siltstone and sandstone. The eastern alluvial fan-fluvial facies consists of light-coloured granitic debris and minor amounts of volcanic and sandstone clasts derived from adj acent basement terrain and from Palaeocene to Eocene strata (San Francisquito Formation) along the eastern side of the basin (Crowell, 1 954). These clasts may have been derived even farther east from the Mojave Desert region (Ehlert & Ehlig, 1 977). Local fanglomerate

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Mudrock-carbonate facies

A mudrock-carbonate facies is characterized by massive to horizontally-laminated mudstone (Figs 1 2A, B and 1 3A), shale and siltstone, which contain ferroan dolomite, other iron carbonates, analcime, pyrite, j arosite, and gypsum nodules (Irvine, 1 977). This facies was deposited lakeward of the coarser-grained marginal facies. Sedimentary structures include parallel, varve-like laminations (Fig. 1 3B), concre­ tions (Fig. 1 3C), soft sediment deformation (Fig. 1 3 D), small-scale internal faulting

Lacustrine facies, Pliocene Ridge Basin, California

1 83

Fig. I I . Photographs of the offshore turbidite facies: (A) Large-scale slump-folded strata showing growth

faults with laterally continuous overlying and underlying strata; (B) brecciated bed bounded by laterally continuous strata; (C) small-scale slump-folded and disrupted strata; (D) dish structures with pillar columns (water escape features); (E) graded sandstones (Bouma a intervals) which contain rip-up clasts separated by darker mudstone; (F) thick-bedded laterally continuous sandstone separated by darker mudstone; and (G) cross-bedded and amalgamated conglomeratic sandstone interbedded with thinner mudstone.

(Fig. l 3 E), desiccation features and locally graded and brecciated beds. Biological constituents include plant remains, fish, insects, peloids, shell debris, and minor burrowing (Fig. l 3 E). Total carbon (as C%) ranges from 1 ·5 to 3 · 8 and bulk ferrous iron content ranges from 1 ·5 to 5·0% FeO for a few selected samples (Irvine, 1977). The analcime and ferroan dolomite both appear to be early diagenetic with the analcime formed from the reaction of montmorillonite with saline, alkaline waters and the ferroan dolomite formed by diagenesis of a high Mg calcite precursor phase in a reducing environment (Irvine, 1977). The association of black, highly-organic mudstone and shale, various carbonates, analcime, pyrite, and the paucity of in situ fossils and burrowing may suggest reducing conditions at least within the sediment. Reducing conditions may have been enhanced by chemical and/or thermal stratification within the water column. Minor burrowing also may indicate anaerobic conditions along the sediment-water interface. Periodic subaerial exposure of these sediments or subaqueous syneresis cracks are suggested by desiccation features and brecciated beds.

1 84

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CONCLU SION S

Ridge Basin lake (Fig. 14) formed in a narrow, small, tectonically active basin in which a great quantity of coarse-grained terrigenous sediment accumulated in alluvial fan-fluvial complexes which flanked the lake. Stromatolitic algae, fresh-water molluscs, ostracods and fish comprised the maj or biota, which lived in marginal lacustrine environments that were subj ect to periods of major terrigenous influx, subaerial exposure and wave agitation. In the offshore lacustrine environments, pro­ deltaic turbidites and mud-rich carbonate rocks accumulated. These offshore, non­ turbidite sequences contain dark, organic-rich dolomitic and analcimic mudstone and shale with pyrite. The physical and chemical nature of the Ridge B asin lake changed considerably through its 10 million year history. The lacustrine phase started as an infilling of a lake or semi-restricted marine embayment as represented by the thick turbidite-deltaic sequence of the lowermost Ridge Route 'formation'. At that time the lake or embayment was relatively deep (Fig. 1 4) with some external drainage into the adj acent

Lacustrine facies, Pliocene Ridge Basin, California

1 85

Fig. 13. Photographs of the offshore lacustrine mudrock-carbonate facies;(A) Carbonate mudrock

composed of ferroan dolomite and analcime at the base of Pyramid Dam; (B) light-coloured dolomite and analcime interbedded with dark-coloured, organic-rich shale; (C) nodular concretions of gypsum (?); (D) small-scale deformed beds; and (E) mottled (burrowed) and internally faulted carbonate mudrocks.

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Basin, California.

1 86

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Ventura marine basin. These conditions are suggested by palaeocurrent directions to the southeast, the extremely thick turbidite section (== 1000 m) and the large-scale (�30 m thick) slump folded intervals within the turbidite section. With continued strike-slip displacement along the San Gabriel fault, external drainage from the Ridge Basin lake was blocked to the south and thus it became a closed lacustrine system. A relatively shallow, internally-drained lake formed as suggested by thick carbonate-mudrock sequences, organic-rich shale, evidence of desiccation and the dominance of 'shallow­ water' lacustrine environments that interfinger with marginal alluvial fan-fluvial deposits (Fig. 1 5). The final major event in the history of the Ridge Basin lake was the termination of strike-slip displacement on the San Gabriel fault coupled with infilling of the lake with fluvial and fanglomerate deposits of the Hungry Valley Formation.

A CKNOWLE DGMENT S

We wish to thank J. C. Crowell for his interest, advice, encouragement and use of unpublished information concerning the geology of the Ridge Basin. Mark Newton assisted with part of the field work on which this paper is based. J. C. Crowell, J. D. Cooper, and C. E. Turner-Peterson kindly reviewed this manuscript. The illustrations were drafted by Janet Dodds and the manuscript was typed by Anne Snell. A Research and Publication Grant from the University of Southern California partially offset the cost of illustrations. Contribution No. 369, Department of Geological Sciences, University of Southern California.

Lacustrine facies, Pliocene Ridge Basin, California

1 87

REFERENCE S

D.l. ( 1950) The Piru Gorge flora of Southern California. Carnegie Inst. Washington, Pub/. 590, 1 59-2 14. CROWELL, J.C. ( 1 950) Geology of Hungry Valley area, Southern California, Bull. A m. Ass. Petrol. Geol. 34, 1 623- 1 646. CROWELL, J.C. ( 1 952) Probable large lateral displacement on the San Gabriel fault, Southern California. Bull. A m. Ass. Petrol. Geo/. 36, 2026-2035. CROWELL, J .C. ( 1 954) Geologic Map of the Ridge Basin area, California. California Div. Mines Bull. 170, Mapsheet 7. CROWELL, J . C . ( 1 973) Ridge Basin, Southern California. Soc. econ. Paleont. Miner., Pacific Sec. , Guidebook, Trip 3, l-7. CROWELL, J.C. ( l974a) Sedimentation along the San Andreas fault, California. In: Modern and Ancient Geosynclinal Sedimentation (Ed. by R. H. Dott, Jr and R. H. Shaver). Spec. Pubis Soc. econ. Pa/eont. Miner., Tulsa, 19, 292-303. CROWELL, J . C . ( l 974b) Origin of late Cenozoic basins in Southern California. In: Tectonics and Sedimentation (Ed. by W. R. Dickinson). Spec. Pubis Soc. econ. Paleonl. Miner., Tulsa, 22, 1 90-204. CROWELL, J . C . ( 1 975) The San Gabriel fault and Ridge Basin, Southern California. In: San A ndreas Fault in Southern California (Ed. by J. C. Crowell). Spec. Rep. California Div. Mines Geol. 1 18, 208-233. DAVID, H.B. ( 1 945) A Neogene stickleback from the Ridge formation ofCaalifornia. J. Paleont. 19, 3 1 5-3 1 8. EATON, J.E. ( 1 939) Ridge Basin, California. Bull. Am. A ss. Petrol. Geo/. 23, 5 1 7-558. E H LERT, K.W. & EHLIG, P.L. ( 1 977) The 'Polka-dot' granite and the rate of displacement of the San Andreas fault in Southern California. A bs. Prog. geol. Soc. Am. Meetings, 9, 4 1 5-4 1 6 . IRVINE, P.H. ( ! 977) The Posey Canyon Shale - A Pliocene lacustrine deposit of the Ridge Basin, Southern California. M.S. Thesis, University of California, Berkeley. JENNINGS, C.W. ( 1 953) Geology of the southern part of the Quail Quadrangle, California, Spec. Rep. California Div. Mines Geol. 30, l - 1 7 . LINK, M . H . , OsBORNE, R . H . & AWRAMIK, S . M . ( 1 978) Lacustrine stromatolites and associated sediments of the Pliocene Ridge Route Formation, Ridge Basin, California. J. sedim. Petrol. 48, 143- 1 5 8 . MILLER, W.E. & D ow Ns, T. ( 1 974) A Hemphillian local fauna containing a new genus ofantilocapvid from Southern California. Contrib. in Science, Los A ngeles County Nat. Hist. Museum. No. 258. M U TTI, E. ( 1 974) Examples of ancient deep-sea fan deposits from Circum-Mediterranean Geosynclines. In: Tectonics and Sedimentation (Ed. by W. R. Dickinson). Spec. Pubis Soc. econ. Paleont. Miner. , Tulsa, 22, 92- 1 05. NILSEN, T.H., BARTOW, J.A., STUMP, E. & L IN K , M.H. ( 1 977) New occurrences of dish structure in the stratigraphic record. J. sedim. Petrol. 47, 1 299- 1 304. SELLEY, R.C. ( 1 970) Ancient Sedimentary Environments. Cornell University Press, Ithaca. S HE PARD, J . B . , J R ( 1 962) San Gabriel fault zone. Bull. Am. Ass. Petrol. Geol. 46, 1 938- 1 94 ! . SQUI RES, R.L. ( 1 978) Medial Pliocene dragonfly nymphs Ridge Basin, Transverse Range, California. Abs. Prog. geol. Soc. Am. Meetings, Cordileran Sec. WI LLIAMSON, C.R. & PICARD, M.D. ( 1 974) Petrology of carbonate rocks of the Green River Formation (Eocene). J. sedim. Petrol. 44, 738-759.

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Spec. Pubis int. Ass. Sediment. ( 1 978) 2, 1 89-203

Lacustrine sedimentation in an evaporitic environment: the Ludian (Palaeogene) of the Mormoiron basin, southeastern France

GEORGES TRUC Departement des Sciences de Ia Terre, 15-43 Bd du 1 I Novembre 1 918, Universite Lyon 1, 69621 Villeurbanne, France et Lab. associe au C. N. R. S. No. 1 I 'Paleontologie stratigraphique et Paleoecologie'

A B STR A CT

The Palaeogene basin of Mormoiron is one of many sedimentary basins and grabens developed along the Rh6ne-Sa6ne axis of southeastern France. During the Palaeogene the Rhodanian trough, between the Massif Central and the Alps, consisted of several subsiding grabens separated by stable platforms. From west to east small grabens were located at Ales, Nimes and at Manosque, bordering the Durance fault, and basins at Mormoiron and Apt­ Forcalquier. The graben structures in the Ludian were formed as a result of an east-west extensional tectonic phase following the Lutetian and Bartonian compression of the 'pyreneo-provenale' phase. The structural regime produced a palaeogeography of isolated lake basins in which evaporites were formed. In the basin of Mormoiron, a thin sequence of evaporites developed on the platform areas, and a much thicker sequence was deposited in the more rapidly subsiding basin centre. In the Lower Ludian, terrigenous clastics were deposited around the margins of the basin, with finer material reaching the basin centre where organic­ rich clay accumulated. In the eastern part of the basin, near the Monts-de- Vaucluse, limestones with brackish-water molluscs (Potamides, Tympanotonus and Me/anopsis), ostracoda and foraminifera were deposited. Elements of a marine fauna indicate adaptation of certain species derived from a nearby marine area. Evaporites also formed in the basin centre during the Lower Ludian so that lenticular gypsum crystals are scattered in the organic-rich clay. During the Upper Ludian, a decrease in supply of terrigenous material led to desiccation and the Mormoiron lake basin became part of a much larger evaporitic area. Dolomite, sepiolite and magnesian smectites formed in the basin centre together with gypsum. At the basin margins limestones became more dolomitic. A fauna persisted in this evaporitic situation through ground-water springs from the Monts-de-Vaucluse. Periodically,

Modern and Ancient Lake Sediments Edited by Albert Matter and Maurice E. Tucker © 1978 The International Association of Sedimentologists. ISBN: 978-0-632-00234-4

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desiccation of the basin produced mud-cracked horizons. locally with bird and mammal footprints. At the end of this arid period, an extensive development of algal mats covered the whole area of the basin. This was followed by a renewed phase of terrigenous clastic sedimentation indicating a return to a more humid climate. Diagenetic phenomena in the sulphate deposits include anhydrite after gypsum and secondary gypsum (alabastrine-type) after anhydrite.

Re sume

L e bassin Paleogene de Mormoiron fait partie d'un ensemble de bassins et de grabens repartis le long de !'axe Rh6ne-Sa6ne, dans le sud-est de Ia France. Une tectonique en extension est-ouest ayant pris naissance au Ludien apres les phases compressives nord-sud du Lutetien et du Bartonien, conduit il ia formation de grabens separes les uns des autres par des plates-formes plus stables ou se developpent quelques bassins it subsidence moderee. Au Ludien, ce schema morphotectonique favorise !'existence d'un endoreisme important et l'isolement des bassins ou s'accumulent des depots evaporitiques. Au Ludien inferieur, le bassin de Mormoiron est envahi par une formation detritique d'origine fluviatile. Le materiel grassier est localise sur les marges du bassin tandis que seules les particules les plus fines atteignent le centre du bassin, milieu euxinique ou se forment les premiers cristaux de gypse. Sur les marges orientales du bassin, au pied des Monts-de- Vaucluse. apparaissent des calcaires it faune saumatre avec mollusques (Potamides, Tlmpanotonus et Melanopsis) ostracodes et foraminiferes adaptes it un rriilieu oligo- it mesohalin et provenant d'un reservoir marin peu eloigne. Pendant le Ludien superieur. Ia decroissance des apports detritiques, correlative d'une aridification de plus en plus affirmee du milieu. conduit it une sedimentation ou alternent dolomite, sepiolite, smectites magnesiennes et gypses tan dis que sur les marges Ia sedimentation carbonatee devient de plus en plus dolomitique. La me me faune persiste dans le secteur oriental grace aux emergences provenant de Ia nappe aquifere des Monts-de­ Vaucluse. A Ia fin de cette periode chaude et sou vent a ride, Ia plus grande partie du bassin est recouverte par un vaste manteau stromatolitique immediatement surmonte par des apports detritiques qui marquent le retour it des conditions climatiques plus humides. Les phenomenes diagenetiques les plus caracteristiques se manifestent au niveau du gypse et plus particulierement sur les marges du cirque evaporatoire (transformation/remplacement gypse-anhydrite at gypse secondaire de facies albatre).

INTRO D UCTION

The Palaeogene basin of Mormoiron is part of a complex of sedimentary continental basins in southeastern France, developed in the region of the Sa6ne and Rhone valleys. From the Valence graben situated between the Massif Central and the western foothills of the Alps, the Rhodanian trough enlarges to the south and is replaced by several subsidiary grabens (Ales, Nimes, Manosque) separated by stable platforms with very localized zones of subsidence (Fig. I ) such as the Mormoiron and Apt basins. In this region, all the grabens and basins contain Ludian evaporitic deposits and/or bituminous limestones. Structural framework

Before the Ludian, the region had been submitted to north-south compressive stresses from the Middle Cretaceous until the Bartonian. Some anticlines formed during the Cretaceous are still recognizable, such as the Comtadin anticline (Masse & Philip, 1 976). During the Lutetian and Bartonian the north-south 'pyn!neo-proven­ �ale' orogenic phase produced a strong anticlinal/synclinal fold pattern over the whole

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Lacustrine sedimentation

1 93

Valence and Mt Luberon suffered intense weathering that resulted in karstification of the Barremian (Urgonian) limestones (Fig. 2). Calcretes and silcretes also developed at this time. Terrigenous material derived from the Valence-Mt Luberon area was transported into a lacustrine basin extending from Mt Luberon to Marseille. The beginning of the Ludian period was marked by an episode of east-west tension following the Lutetian and Bartonian compression. This rifting affected the 'pyreneo-proven�ale' folds and resulted in northeast-southwest oriented structures, such as at Ales, Ni'mes, Manosque and Valence, in which great amounts of halite, anhydrite and bituminous limestones were deposited. More stable platforms in the neighbourhood of Apt and Mormoiron separated these grabens and were areas oflittle subsidence. Shallow-water carbonate sedimentation took place on these platform areas and sulphates were deposited in local areas of enhanced subsidence (Fig. 1). This structural framework produced a palaeogeography of grabens and basins of interior drainage which were isolated from each other and separated from contemporaneous marine areas by an important highland region (Gubler et al. , 1 975). Evaporites and their structural context

The Palaeogene of the Rhone valley (Ludian to Chattian) is comprised of several thick sequences of persistent clastic horizons which pass up into evaporites (Triat & True, 1 974). The alternation of clastics and evaporites, and the gradual passage of the former into the latter, are taken to indicate a climatic control on sedimentation of more humid followed by more arid phases. Within this tectonic framework, however, deposits of marls and limestones occur containing no trace of evaporites but with some freshwater faunas which existed during more humid periods. Penecontemporaneous tectonism, chiefly in the form of fault movements (Fig. I ), controlled the fluviatile clastic contribution to the basins during some periods of the Palaeogene. These sediments, commonly of stream-flood type, were concentrated at basin margins with only silts and clays spreading out towards the basin centres. Such a pattern is similar to other evaporitic basins and grabens (such as the Dead Sea) where marginal areas are dominated by a narrow strip of coarse detrital material which passes basinward into laminated mudstones (Gubler et a!., 1 975). Evaporite mineralogy is also controlled by the morphology and structure of the sedimentary basin. Halite is always localized in rapidly subsiding grabens while on the adj acent platforms there is no halite at all but many occurrences of gypsum. The preferential concentration of halite in basin centres is attributed to solution by undersaturated waters of any halite precipitated on platform areas and the progressive migration of these waters to basin centres. Evaporative concentration during arid periods then produces substantial halite deposits. Busson ( 1 968, 1 974) and others have involved a similar mechanism for the preferential concentration of halite in localized areas of subsidence.

T HE EV A PORITE B A SIN OF MORMOIRON

During the Ludian, the basin of Mormoiron behaved as a platform, with only moderate and even subsidence so that the formation of evaporites was not as substantial as in graben structures.

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Evaporites and their diagenesis

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To date lacustrine limestone and dolomite have no economic significance. Trona and other carbonates of soda (soda ash, natron) are basic industrial chemicals, but do not commonly occur in commercially-sized deposits in many playa lake basins (Smith et at. , 1 973). Most of the world's demand for trona is supplied from lacustrine deposits at Searles Lake, California, in the Green River Basin (Green River beds), and from Lake Magadi, Kenya, Africa. Sulphates

The principal sulphates found in playa lake deposits are bloedite, epsomite, gypsum, glauberite and thenardite, with gypsum being the most widespread. Lacustrine gypsums range from the very fine grained 'gypsum mush' so often found at playa surfaces to gigantic, exaggerated blades (Reeves, 1 968) produced by oxidation of sulphides. Playa surfaces commonly contain appreciable quantities of salt-resistant vegetation or vegetative debris which blows onto the surface, much of which is incorporated into the playa sediments. Sulphides, such as pyrite and hydrogen sulphide, are then produced by reduction during bacterial decay. In turn the oxidation of the sulphides provides the sulphate which combines with available carbonate to form the gypsum. How much gypsum in any one zone may be produced by bacterial action is not easily determinable, however, sulphur oxidizing bacteria T denitrificans, novellus, coproliticus, thioparus, and thiooxidans are common in gypsum brines. Thenardite (sodium sulphate), epsomite (magnesium sulphate), glau berite (calcium sodium sulphate) and bloedite (magnesium sodium sulphate) frequently occur with trona salts or mixed with halite and gypsum. Deposits of these sulphates are widespread, forming in both large playa environments such as Great Salt Lake, Utah (Cohenour, 1 966) as well as in small basins. Economic significance

Bloedite, epsomite, glauberite and thenardite are basic industrial salts, being used by many companies for production of everyday essentials (explosives, clothing, fertilizers, soaps/ detergents, glass, pharmaceuticals), thus even relatively small deposits, if strategically located, may be profitably utilized. Deposits of these sulphates, of considerable magnitude, are known in remote areas (for example, in northwestern Chihuahua, Mexico) which are not yet exploited. Thus no apparent shortages exist. Gypsum, used by the construction industry (cement use, plaster, sheetrock) and agriculture, is supplied principally by marine deposits.

Economic significance ofplaya lake deposits

285

Halides

Lacustrine deposits frequently contain significant deposits of halite (e.g. Lake Eyre, Australia), whereas most of the potassic chlorides, like sylvite, stay in solution or the miniscule amounts precipitated are rapidly removed by deflation. However, under certain environmental/depositional conditions which are largely unknown, sylvite lenses may occur with associated halite beds (Smith et a!., 1 973).

Economic significance

Playa lacustrine sediments contain little potassic halides and attempts to recover potassic salts from such deposits, except as by-products, have not been widely successful. However, sodium chloride is mined, as at Saltair, Utah (Cohenour, 1 966), for commercial, industrial and domestic use. Playa-lake basin brines, from California, Utah, and the Dead Sea, Israel, do yield potassium compounds (Smith et al. , 1 973).

Borates and nitrates

The borates (borax, kernite, searleasite, colemanite) and nitrates (soda nitre and nitre are usually found in most playa deposits formed in basins surrounded by volcanic terrains, yet commercially-sized deposits of the borates and nitrates are not common. It is suspected that this is mainly the result of the past reluctance to drill playas, combined with the ready availability of such minerals from such classic areas as Death Valley and Searles Lake, California. Borax and kernite are mined from lacustrine deposits at Boron, California and lacustrine deposits yield calcium borates near Death Valley, California (Smith et al. , 1 973). However, borate production from Searles Lake originates from the highly alkaline lacustrine brines.

Economic significance

Borax is a basic industrial chemical, being used in a multiplicity of products (Bateman, 1 956), from soaps and detergents in the chemical industry to use as a chemical in gasoline additives and fire-retardants. Nitrates are not mined deliberately from lacustrine deposits.

Silicates

Playa lake basins in semi-arid to arid areas commonly contain silicates in their sediments from the devitrification of volcanic ash in the highly alkaline environments. However, the original silicate may have actually been a precipitate like the hydrous sodium silicate magadiite (Eugster, 1 967) or perhaps a siliceous gel. Chert and opal lenses are also often found in lacustrine beds (as at Virgin Valley, Nevada), Eugster ( 1 967) showing that bedded chert could originate from kenyaite which originated from magadiite.

Economic significance

No economic significance is presently known for lacustrine silicates.

286

C. C. Reeves Jr

Lithium

Lithium in the lacustrine environment is so soluble that it concentrates during the late evaporative stages, with the potassium, boron, chloride or sulphate brines (Cannon, Harms & Hamilton, 1975; Vine, 1 975) and with the montmorillonite clays. Lithium-bearing clays (smectites), which represent the only lithium-rich playa lake basin sediment known, are commonly associated with calcite, dolomite, sepiolite and attapulgite. Several small occurrences oflithium-bearing clays from West Texas playa sediments are similar to hectorite, a lithium-rich clay from hydrothermal deposits near Hector, California. The West Texas lithium-rich clays, however, contain somewhat more Mg2 + and Fe2 + and about 0·8% less lithium (Goolsby, Reeves & Lee, 1 977). Lithium (as a phosphate) is produced as a by-product of the salt production at Searles Lake, California, and lithium (as a carbonate) is produced from playa lake brines in Clayton Valley, Nevada (Vine, 1 975). A major lithium occurrence in the Salar de Atacama, Chile, was also recently reported (Vine, 1 975) Economic significance

Lithium is used (Vine, 1 975) by several commercial industries (glass-making, ceramics, industrial chemicals, dry-cell batteries), perhaps the most notable being its use as lithium carbonate in the medical field to stabilize manic-depressive behaviour. However, the development of electric vehicles requires the lithium battery which could, by the early 1 980s, double lithium demand. A second technological development which may soon spur lithium demand would be the development of thermonuclear fusion plants which require a lithium blanket around the reactor unit (Bogart, 197 5). Uranium

Radioactive anomalies are common to lacustrine sediments and particularly to those deposited in closed basins. Such occurrences are known from Poland, Czechoslovakia, Turkey (Fakili), Japan (Mingyo-toge), France (Loveve), the Sila Plateau, Italy, western Australia and throughout the western United States. This propensity results from the extreme mobility of the uranyl ion under alkaline conditions, the availability of suitable reductants in the lacustrine environment, and the natural trapping mechanism of a closed basin. Frequently the uranium minerals will be closely associated with opaline beds (Virgin Valley, Nevada, or the Southern High Plains, Texas), with calcareous zones (Yeelirrie, western Australia or western United States), with zeolites (Tono Mine and Oochi deposits, Japan), or with volcanic ash (western United States and southern Texas). Although some authigenic uranium may occur in playa lake sediments, most appears to have been transported by meteoric water until encountering a reducing environment to cause precipitation of the uranyl carbonate complex. Uranium minerals encountered below the water table in playa lake sediments will usually occur as uraninite or coffinite whereas that above the water table will usually occur as carnotite, tyuyamunite or autunite. Uranium in lacustrine sediments is always in close proximity to igneous bedrock and adjacent deposits of arkosic sands and volcanic ash, but which acted as the original source of the uranium has long been debated (Grutt, 1 972; Rosholt & B artel, 1 969).

Economic significance of playa lake deposits

287

Although we have now identified additional causative factors (such as structure, fault control, spring locations), the role of the playa lake environment (alkaline pore water, plant material, humic acids) can hardly be overestimated.

Economic significance

By the year 2000 about 60% of the nation's electric energy is expected to be produced by nuclear power plants, requiring at least ten times the present annual domestic uranium requirement. This realization has resulted in an approximate ten-fold increase in the price of U308 in the last few years, creating an exploration 'boom' of classic proportions. The discovery of a commercially-sized uranium deposit in playa lake sediments (Jones, 1 970) therefore focused exploratory attention on playa lake environments throughout the world. It is expected that there will be several significant discoveries of massive uranium deposits from both ancient and modern playa lake basins during the next decade.

A UT H I G E N I C S

Authigenic lacustrine sediments, by definition, cannot form a primary deposit, yet the relative amounts in select zones may be uncommonly high. Certainly the recognition of over seventy authigenic minerals in the Green River Formation, Wyoming (Milton, 197 1 ) illustrates that fantastic mineralogic variations may occur in the playa depositional environment.

Zeolites

Zeolites, which are common in alkaline playa lake sediments, occur as clinoptilolite, erionite and phillipsite with some chabozite, mordenite and analcime. All apparently form from volcanic ash and tuff which accumulate in the basins. Although the progressive sequence of volcanic glass to alkalic zeolite to analcime to potassium feldspar is well documented, the particular alkalic zeolite which may form in a local lacustrine environment is not predictable (Surdam & Eugster, 1 976). What has been shown is that volcanic glass to alkalic zeolites, with the exception of analcime, form in the peripheral (freshest) lake areas. Analcime then occurs further basinward, and finally potassium feldspars form in the most alkaline/saline environments (Sheppard & Gude, 1 973). Erionite and phillipsite form mainly in saline lacustrine deposits while clinoptilolite and mordenite may form in either fresh or saline environments (Hay, 1 964, 1 966). The presence of zeolites in playa lake sediments then supplies important depositional environmental clues. However, perhaps the most important aspect is the possible relationship between zeolite formation and release of uranium from the volcanic glass. Although Rosholt, Prij ana & Noble ( 1 97 1 ) do not believe volcanic ash can supply the amounts of urarnum required, the massive amounts of volcanic ash in many basins and the close mineralogic association of uranium minerals with remnant ash shards suggests we may not know all the answers here.

C. C. Reeves Jr

288 Economic significance

Zeolites have become an important industrial resource in the last few years due to their unusual ion-exchange, dehydration, absorptive and catalytic properties. Of particular interest are the uses of zeolites for municipal sewage control, metal recovery and in pollution control. It therefore seems that the increasing importance of zeolites will lead to additional exploration of playa lake environments.

CONCLUSIONS

Even from this brief review it is apparent that the playa lacustrine environment is, and throughout geologic history has been, responsible for a magnificent variation of common and even uncommon sediments and minerals. What was once considered as 'worthless basin fill' is now the subject of an unusual concentration of geologic investigation. The recent need for several of the uncommon minerals produced from the lacustrine sediments will inevitably divest data which cannot, under ordinary investigative budgets, be secured. Thus, not only will geologists trained in working with the lacustrine facies be able to adequately contribute to exploration/exploitation ventures, but will be rewarded with those 'dry' academic morsels so necessary to future research.

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