~ THE MAcLEOD EVAPORITE BASIN ~ WESTERN

- Brian W. Logan -

t t t

SEEPAGE FACE SEEP AGE FACE

- ( Ha llie} FLOOR SEEPAGE INFLOW.._. SEEPAGE OUTFLOW

Downloaded from http://pubs.geoscienceworld.org/books/book/chapter-pdf/3837980/9781629811376_frontmatter.pdf by guest on 25 September 2021 THE MAcLEOD EVAPORITE BASIN

Holocene Environments, Sediments and Geological Evolution

by Brian W. Logan

Department of Geology The University of Western Australia Nedlands Western Australia

Published by The American Association of Petroleum Geologists Tulsa, Oklahoma, U.S.A. 74101

Downloaded from http://pubs.geoscienceworld.org/books/book/chapter-pdf/3837980/9781629811376_frontmatter.pdf by guest on 25 September 2021 Copyright © 1987 The American Association of Petroleum Geologists All Rights Reserved Published December, 1987

ISBN: 0-89181-321-7

Library of Congress Cataloging-in-Publication Data

Logan, Brian W. The Macleod evaporite basin, western Australia.

(AAPG memoir; 44) Bibliography: p. Includes index. 1. Evaporites-Australia-Macleod, Lake, Region (W.A.) 2. Geology, Stratigraphic-Recent. I. Title. II. Series. OE471.15.E8L64 1987 552'.5 87-33455 ISBN 0-89181-321-7

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Association editor: James Helwig Science director: Ronald L. Hart Special projects manager: Victor V. Van Beuren Project editor: Anne H. Thomas Special editor: Kathryne E. Pile Design and production: S. Wally Powell

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PREFACE

lllE MacLEOD BASIN PROGRAM ...... ix FUTURE RESEARCH PROGRAM ...... x ACKNOWLEDGMENTS ...... xi CHAPTER 1: MODELS FOR MARINE EVAPORITE BASINS AND EVAPORITE ENVIRONMENTS

INTRODUCTION ...... I WATER AND SAI.J" BALANCES ...... 2 WATER BALANCE ...... 2 Inflow Components and Mechanisms ...... 3 Outflow Components and Mechanisms ...... 4 Evaporative Outflow ...... 5 BRINE CONCENTRATION INDICES ...... 5 SALT BALANCE ...... 6 BASIN MORPHOLOGY AND HYDROLOGIC PARAMETERS ...... 8 LEVELS AND SURFACES ...... 8 HYDROLOGIC PARAMETERS ...... 9 FREE-FLOW EXCHANGE BASINS ...... 10 BASIN MODELS ...... 10 Model 1: Stratopycnal Basin ...... I 0 Model 2: lsopycnal Basin ...... 12 INTRABASINAL ENVIRONMENTS ...... 13 Metahaline Basin Environments ...... 13 Hypersaline Basin Environments ...... 13 Gypsum Basin Environments ...... 15 SEEPAGE BASINS ...... 15 Seepage Faces ...... 15 Brine Sinks ...... 15 SEEPAGE BASIN MODELS ...... 16 Model 1: Open Seepage Basin ...... 16 Models 2, 3, 4: Hydrosealed Seepage Basins ...... 17 SEEPAGE BASIN ENVIRONMENTS ...... 19 Brine Ponds ...... 20 Brine Sheets ...... 23 Majanna Environment ...... 24 Flood Sheets ...... 25 Interactions in the Prevailing Marine Brine System ...... 26 BRACKISH BASINS ...... 26 PETROLOGY OF GYPSITE AND HALITE ...... 27 GYPSITES ...... 27 Crystalline Gypsite ...... 27 Clastic Gypsite ...... 28 HALITE ROCKS AND SEDIMENTS ...... 29 Crystalline Halite ...... 29 Clastic Halite ...... 31 CHAPTER 2: GEOLOGICAL FRAMEWORK OF THE EVAPORITE BASIN

INTRODUCTION ...... 33 GEOMORPHOLOGY ...... 33 BARRIER ...... 33 HINTERLAND ...... 33 Eastern Boundary Ranges ...... 36 iv

Downloaded from http://pubs.geoscienceworld.org/books/book/chapter-pdf/3837980/9781629811376_frontmatter.pdf by guest on 25 September 2021 Manberry and Hoari Dune Fields ...... 36 Floodway ...... 36 Boolathanna Basin ...... 36 ...... 36 Minilya River ...... 37 Lyndon River ...... 3 7 BASIN ...... 37 Pr esent Morphology ...... 37 Initial Pre-Holocene Morphology ...... 37 STRATIGRAPHIC UNITS, MacLEOD GRABEN ...... 38 FOUNDATION UNITS...... 38 Korojon Calcarenite, Toolonga Calcilutite...... 38 Tertiary Limestone Formations ...... 38 EOLIANITE DUNE FORMATIONS ...... 39 PLEISTOCENE MARINE UNITS ...... 40 Dampier Formation ...... 40 Bibra Formation ...... 40 Age of Marine Pleistocene Units ...... 40 HOLOCENE MARINE UNITS ...... 40 MacLeod Evaporite ...... 40 Boolathanna Formation ...... 43 MacLeod Evaporite/ Boolathanna Formation Transition ...... 43 FLUVIAL, LACUSTRINE AND SOIL UNITS ...... 44 Westphal Clay ...... 44 Depuch Formation ...... 44 Little Creek Formation ...... 44 MacLEOD GRABEN ...... 44 FOUNDATION STRUCTURE ...... 44 Barrier Horst ...... 44 Graben and Hinterland ...... 44 SEDIMENTATION PHASES ...... 45 Quobba Phase ...... •...... 45 Dampier Marine Phase ...... 45 Little Creek Phase ...... 45 Bibra Marine Phase ...... 46 Holocene Phase ...... 46 SUMMARY AND CONCLUSIONS ...... 48

CHAPTER 3: MODERN ENVIRONMENTAL SYSTEM

INTRODUCTION ...... 49 CLIMATE ...... 49 Evaporation ...... 49 Southerly Wind System ...... 49 Rainfall Events ...... 49 Cyclones ...... 50 BRINE SroRAGE ...... 51 EVAPORITE BASIN RESERVOIR ...... 51 Basinal Hydroseal ...... 5 1 Ibis Gypsite Reservoir ...... 52 Texada Halite Reservoir ...... 5 2 SUBBASINAL RESERVOlRS AND SEALS ...... 53 Pleistocene Sequence ...... 53 Tertiary Limestone Formations ...... 5 3 BASIN SYSTEM ...... 53 SEAWATER INFLOW ...... 53 Cygnet Marsh Seepage Face ...... 53 Quobba Seepage Face ...... 54 Bejaling Seepage Face ...... •...... 60 Gnaraloo Seepage Face ...... 60 ENVIRONMENTAL UNITS ...... 60 Permanent Brine Ponds ...... 60 v

Downloaded from http://pubs.geoscienceworld.org/books/book/chapter-pdf/3837980/9781629811376_frontmatter.pdf by guest on 25 September 2021 Majanna ...... 61 Permanent Brine Sheets ...... 61 UNIT FLUX AND BRINE TRANSMISSION ...... 64 Unit Flux ...... 64 Brine Transmission ...... 65 BRACKISH INFLOW ...... 66 SEEPAGE OUTFLOW ...... 69 Ralph Sink ...... 70 Texada Sill Sink ...... 70 MacArthur Sink ...... 71 HYDROCHEMISTRY ...... 71 SYSTEM UNITS ...... 71 Seepage Faces ...... 71 Brine Sheets ...... 75 Permanent Ponds ...... 75 Phreatic Majanna (Ibis Gypsite Member} ...... 75 Brine Sinks (Vadose Majanna) ...... 75 Flood Sheets ...... 77 INTRAFORMATIONAL BRINES ...... 77 Texada Halite Brine ...... 77 Pleistocene Formations ...... 77 Tertiary Limestone Formations ...... 77 PRECIPITATES AND SEDIMENTATION ...... 78 HALITE FlELD ...... 78 GYPSUM FIELD ...... 80 Prismatic Crystalline Gypsite ...... 80 Coarse, Crystal-Clast Sand and Gravel ...... 80 Clastic Gypsite and Red Clay ...... 80 Mixed Clay, Hemipyramidal and Clastic Gypsite ...... 82 Medium to Coarse Gypsum Sand and Sandstone ...... 84 CARBONATE FIELD ...... 84 Pond and Brine Sheet Sediments ...... 84 Majanna Sediments ...... 85 SALT BUDGET ...... 85 SUMMARY AND CONCLUSIONS ...... 87 Basin System ...... 87 Holocene Basin Reservoir ...... 89 Intrabasinal Environment ...... 89 Precipitates and Sedimentation ...... 91

CHAPTER 4: GEOLOGIC EVOLUTION OF 1HE EVAPORITE BASIN

INTRODUCTION ...... 93 MacLEOD EVAPORITE ...... 93 COOLAN CARBONATE MEMBER ...... 94 Definition and Lithotypes ...... 94 Fossil Biota ...... 94 Geometry and Facies Distribution ...... 95 Modern Analogs ...... 95 CYGNET CARBONATE MEMBER ...... 95 Definition and Lithotypes ...... 95 Fossil Biota ...... 96 Geometry and Facies Distribution ...... 96 Modern Analogs ...... 97 TEXADA HALITE MEMBER ...... 97 Definition and Lithotypes ...... 97 Geometry and Facies Distribution ...... 97 Modern Analogs ...... 98

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Downloaded from http://pubs.geoscienceworld.org/books/book/chapter-pdf/3837980/9781629811376_frontmatter.pdf by guest on 25 September 2021 IBIS GYPSITE MEMBER ...... 99 Definition and Lithotypes ...... 99 Geometry and Facies Distribution ...... 100 EGRET CARBONATE MEMBER ...... 101 Definition and Lithotypes ...... 101 Geometry and Facies Relations ...... 101 STRATIGRAPHIC RELATIONS ...... 102 Coolan Carbonate Member ...... 102 Cygnet Carbonate Member ...... 103 Texada Halite and Coeval Units ...... 103 Ibis Gypsite and Egret Carbonate ...... 104 TRENDS IN BASIN EVOLUTION ...... 1 04 EXTERNAL FACTORS, SEA LEVEL AND SILL CLOSURE ...... 104 BASIN MORPHOLOGY AND SEDIMENTATION ...... 1 05 BASINAL BRINE LEVEL AND SURFACES ...... 1 05 SALT BUDGETS ...... 1 07 Salt Inflow ...... 107 Salt Retention and Outflow ...... 107 Salt Budget and Basin Evolution ...... 107 WATER BUDGETS ...... 109 Evolution of Basinal Brine Budget ...... 109 STAGES IN BASIN EVOLUTION ...... 110 HOLOCENE INTERVAL I ...... 112 General ...... 112 Hydrology ...... 112 Sedimentation ...... 113 HOLOCENE INTERVAL II ...... 113 General ...... 114 Hydrology ...... 114 Sedimentation ...... 114 HOLOCENE INTERVAL III ...... 114 General ...... 144 Hydrology ...... 114 Biota ...... 116 Sedimentation ...... 116 HOLOCENE INTERVAL IV ...... 116 General ...... 116 Hydrology ...... 117 Sedimentation ...... 118 HOLOCENE INTERVAL V ...... 118 General ...... 118 Hydrology ...... 118 Sedimentation ...... 121 Evolution of the Sedimentation System ...... 121 HOLOCENE INTERVAL VI ...... 121 General ...... 121 Hydrology ...... 122 Flood Impact ...... 122 Sedimentation, Ibis Gypsite (Ib 2) ...... 123 Sedimentation, Egret Carbonate (Eg 2, 3, 4) ...... 123 Evolution of the Sedimentation System ...... 123 HOLOCENE INTERVAL VII (PRESENT) ...... 124 General ...... 124 Sedimentation ...... 124 SUMMARY AND CONCLUSIONS ...... 125 CONTROL FACTORS AND EVENTS ...... 125 LONG-TERM TRENDS, KEY PARAMETERS ...... 125 Water Budget...... 125 Salt Budget ...... 126 Environmental Units ...... 126 Levels and Surfaces...... 12 6

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Downloaded from http://pubs.geoscienceworld.org/books/book/chapter-pdf/3837980/9781629811376_frontmatter.pdf by guest on 25 September 2021 CHAPTER 5: SUMMARY AND CONCLUSIONS

INTRODUCTION ...... 127 PREDICTIONS: GEOLOGICAL FUTURE ...... 127 SEA LEVEL OSCILLATION ...... 127 SEA LEVEL RISE AND SYSTEM REVITALIZATION ...... 127 TRANSGRESSION AND BREACHING OF THE BARRIER ...... 128 SEA LEVEL REGRESSION ...... 128 BRACKISH INFLOW ...... 128 GENERAL ...... 128 ACCRETION RATES ...... • ...... 12 8 INTRABASINAL REGRESSION: CIRCA 5100 YBP ...... 1 28 BRINE SINKS ...... 129 IMPORTANCE OF BASINAL HYDROSEAL ...... 129 EVAPORITES: WHY ONLY HOLOCENE? ...... 130 A BARRIER OF PERMEABLE LIM ESTONE? ...... 130 DEFICIENClES ...... 130 REFERENCES CITED ...... 1 32 INDEX ...... 1 35

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Downloaded from http://pubs.geoscienceworld.org/books/book/chapter-pdf/3837980/9781629811376_frontmatter.pdf by guest on 25 September 2021 Preface

The MacLeod Basin Program the MacLeod case, brine retention has been mostly through development of an impermeable, basin-lining hydroseal during The MacLeod evaporite basin (Lake MacLeod) at 24 o S on the early stages of evaporite accretion and partly as a consequence semi-arid northwest coast of Australia (Figure 1) contains a 12 of the regional hydrologic system. The long-term preservation m-thick formation of Holocene marine evaporites. A large-scale of highly soluble evaporites in juxtaposition with the ocean also evaporite system, sustained by seawater inflow, continues to requires efficient mechanisms for hydrologic isolation. operate in the basin today. Thus, MacLeod is one of the few Presently, MacLeod basin is an example of a "desiccation Quaternary basins that resembles the evaporite basins of earlier basin," wherein brine surfaces mostly are suppressed to subsur• epochs, and it therefore offers unique research opportunities in face locations and much of the sedimentation surface is only evaporite sedimentation, systems, and evaporite-basin evolu• periodically covered with brine; permanent brine ponds and tion. The MacLeod basin research program was initiated about brine sheets are relatively minor components of the environ• 1975 and has grown to be a main engagement for the Sedimen• mental system. Stratigraphic analysis indicates that most of the tology Research Group, Department of Geology, University of basin-filling evaporite sequence accreted under these condi• Western Australia, with ongoing staff research and a range of tions, although the first stages of accretion were in a basin con• postgraduate projects on various aspects of basinal geology. nected to the ocean across a shallow sill. Concepts and themes central to evaporitology are the barred The "desiccation basin" model has long been a basic concept basin, barrier, or sill (Ochsenius, 1877), salt precipitation in evaporite studies. However, it has been somewhat theoreti• sequence (Usiglio, 1849); seepage outflow or reflux (many cal because it was principally formulated from geologic evi• authors); hydrologic structure and dynamics (e.g., Scruton, dence for low brine levels in ancient basins and from 1953); and deep-water vs. desiccated basin models (many experimental data concerning relations between evaporative authors). Some of these concepts and themes were developed volume reduction and precipitation fields. Corollaries of the during interpretive reconstruction of ancient evaporite basins. desiccation basin concept also have emerged in the literature, Because of the rarity of modern evaporite basins, there has mostly by imaginative reconstruction-insights that reach their been only limited opportunity to examine them in real terms. height with the desiccated Mediterranean basin of Messinian The MacLeod basin has provided opportunities for assessing times (Hsii, 1972). MacLeod basin now provides an actualistic most of the basic concepts. The program has confirmed (1) the basis for the desiccation-basin concept. It also is possible to barrier and seepage as important factors in controlling evapo• embellish the general model, for some features at MacLeod rite systems and (2) the validity of Usiglio's sequence in a large• have not been defined hitherto. scale basinal setting. A number of models for evaporite basins My approach is to analyze contemporary environments also have been developed, in terms of hydrologic structure and (Chapter 3) and reconstruct paleoenvironments (Chapter 4) as hydrodynamics. dynamic, open systems defined in terms of interrelated water The most important new insights from the MacLeod research and salt budgets. This approach combines aspects of systems regard the definitive role of seepage in sustaining a large basin analysis, hydrology, and oceanography with the salt precipita• system and in controlling system status. In this topographically tion sequence from seawater feedstock (Figures 2, 3). Lack of closed basin, seepage inflow of seawater presently sustains a prior work of this type on a modern seepage basin presented 2000 km 2 system. The inflow is driven by an ocean/basin difficulties in terms of concepts and terminology. As a result it hydrostatic head of -3 m to - 4 m that is maintained as a func• has been necessary, for purposes of documentation, to develop tion of the large differential between inflow and evaporative new conceptual models and to coin new names for features in outflow. Earlier in the Holocene, basin-filling formations of the MacLeod system. Thus in Chapter 1, majanna, brine sheet, halite and gypsum were laid down in brine ponds lying at - 11 brine pond, and flood sheet are defined as basic environmental m to - 5 m, and were fed by seepage alone. Seepage outflow of units of a seepage basin system. Seepage faces, brine sinks, dense brine also is (and has been) a large-scale, definitive factor basinal hydroseals, closure thresholds, and critical hydrologic in the system, setting status in terms of Usiglio's precipitation levels are discussed in relation to the general case. sequence. Earlier programs of the Sedimentology Group were located The emphasis on the closed or restricted nature of evaporite immediately south of MacLeod, in Shark Bay, where a central basins, arising from requirements of deficit water budgets, has interest was the development of hypersaline, barred basins as a tended to obscure the reality that evaporite systems must be result of barrier-bank growth (Hagan and Logan, 1974a). The hydrologically open and dynamic. In this context, the problem Shark Bay basins extend across a concentration range from oce• of achieving the deficit budget required for significant evaporite anic to hypersaline (Hamelin Pool), and evaporites are laid accretion by limiting inflow is only equal to the problem of down only in restricted marginal tidal flats. The shift to retaining dense brines in a basin that is open to access by sea• MacLeod basin was thus a move along a spectrum of barred water. There is thus a requirement for mechanisms or factors marine basins to one with high-order evaporites, that is, from that inhibit brine outflow if basin systems are to attain high• carbonate-field through gypsum, halite to bittern-field systems. order status and accrete significant thicknesses of evaporites. In The Shark Bay experience was valuable in interpretive aspects

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Downloaded from http://pubs.geoscienceworld.org/books/book/chapter-pdf/3837980/9781629811376_frontmatter.pdf by guest on 25 September 2021 • these insights are detailed in conclusions to the chapters of this (") book, but the basic requirements have been topographic clo• sure, a highly permeable barrier, and mechanisms for retaining INDIAN dense brine. In the MacLeod case, final closure has been due not to growth of carbonate barriers, but to sediment accretion 2 2 ° 2 2 ° associated with the growing Gascoyne delta lobe to the south. EXMOUTH The transition from a freely connected marine basin with OCEAN GULF/ metahaline/carbonate facies to an evaporite basin was very rapid; thus hypersaline/carbonate facies, typical of Hamelin Pool, mostly are absent from the basinal sequence. w This memoir is a synthesis that marks the end of stage 1 of an ongoing research program. Stage I has been lengthy but not unduly so, given the dimensions of the basin, the complexity of the system, and interrelated technologic problems involved in 23° access, core drilling, and precise levelling. Techniques for cor• ing evaporite rocks were perfected in 1978, concurrent with partial solutions to access problems. This, along with the inci• dence of major support from Dampier Salt (Operations) Pty Ltd in early 1979, is the threshold of the MacLeod program as reported here. Techniques for precise levelling also were devel• oped in 1978, and many tens of kilometers of lines were laid out on the basin floor from bench marks that fortuitously had been installed by theW. A. Department of Lands and Surveys in con• 2 4 ° nection with road planning. However, a large-scale flood event RIDGE in mid-1980 facilitated the essential delineation of basin-floor topography, since there was enough water for small craft and rapid bathymetric surveys. Flood sheets, developed as a result of cyclones and winter rainfall events, have also been a serious impediment to the overall program, since they frequently ter• minated drilling operations on the lake bed and disrupted the course of systematic hydrologic sampling. 25° Future Research Program, Stage 2

~SHAAK Stage I of the research program established a framework of knowledge concerning modern environmental systems, evolu• tion of the evaporite basin, sediments and sedimentation, and biology. Stage 2, which builds on this foundation, commenced in 1985 with initiation of a number of new projects. A central focus of stage 2 is defining the larger-scale hydro• 2 6 ° logic system surrounding the basin. Knowledge about the regional system is a major deficiency of the present memoir, but a start has been made to delineate this with exploration wells that probe formations in deeper parts of the MacLeod depres• 100 K m sion and marginal platforms. Stage I investigations demonstrate that MacLeod is a leaky 0 Ll) basin which has retained only a few percent of salts inflowing .-. over the past 8,000 years, and that brine outflow, mostly by seepage, has been into formations thai' underlie the basin and Figure 1. Map showing the location of the MacLeod basin crop out in the surrounds. Also, most brine outflow is through (Lake MacLeod) on the Western Australian coast and in rela• sinks located in marginal platform areas, The fate of the out• tion to regional features and drainage systems. During Quater• flowing brine is of considerable interest in terms of fundamental nary man·ne phases the basin was a marine gulf connected research and estimation of commercial brine reserves. Thus a southward to Shark Bay. stage 2 investigation currently is underway to (I) further investi• gate mechanisms of brine outflow in the sinks, (2) determine brine types in aquifer formations, and (3) investigate the impact of the MacLeod program because of commonality in climatic of these brines on receptor formations, particularly in relation and oceanographic factors and to some extent in biota; diver• to dolomitization. gence to evaporite-basin conditions at MacLeod being an event Stage 2 also includes a number of investigations of evaporitic of the late Holocene, more than 5000 years ago. The Shark Bay sediments and processes. Facies associated with permanent to MacLeod shift also has been productive in terms of insights brine sheets that run down basin slopes from the base of the regarding specifications for transitions of marine basins to evap• barrier are of special interest because of similarities to certain orite systems and accretion of basin-filling evaporites. Most of tidal-supratidal facies. There also are current projects on gener-

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Downloaded from http://pubs.geoscienceworld.org/books/book/chapter-pdf/3837980/9781629811376_frontmatter.pdf by guest on 25 September 2021 ation of sediments composed of magnesium carbonate miner• Texada Mines and Dampier Salt personnel carried out brine als, on gypsite lithotypes in brine ponds, and on organic analyses and provided meteorologic information over the study compounds in evaporitic environments. period. I would like to thank Ralph Ninham, Gerry Theunissen, Ken Grinter and others at the mine site laboratory for their ACKNOWLEDGMENTS ready assistance in this phase of the study. Dampier Salt per• sonnel have always been interested and helpful. I am unable to The MacLeod basin research mainly has been supported by cite all of the company people who have helped, but I would Dampier Salt (Operations) Pty Ltd with grants, services, and use especially like to thank Jack Sutton, Jock and Trudie Dargie, of facilities at the MacLeod mine site. I am deeply indebted to Ray and Phyllis Gansberg, John Williams and Bill Van Dinter at company management for this assistance. In particular I would the store, and Rosalind Humphries for many kindnesses. like to thank John MacArthur, Neil Swan, Bob Long, Gerry Landsat imagery has proved to be a valuable aid in delineat• Theunissen and Ralph Ninham for their sustained support and ing the hydrologic environment by permitting an overview of encouragement through all facets of this investigation. brine migration and flood-sheet parameters. The enthusiastic The Australian Research Grants Committee and Mobil assistance of J. H. Houghton and W. F. Holman, W.A. Depart• Research Corporation provided funding for the program, and ment of Lands and Surveys, and F. R. Honey and I. J. Tapley, there has been long-term basic support from the University of CSIRO Land Resources Management, is gratefully acknowl• Western Australia. Richard Koepnick of Mobil is thanked for his edged. Thanks also are due to A. Scott, Commonwealth Bureau interest and support. Texada Mines Pty Ltd also supported pro• of Meteorology, and R. Sheridan, W.A. Public Works Depart• jects in the early years of the program, thanks to E. T. Miller. ment, for environmental data. Core drilling, a major part of the field work, has been carried Members of the Sedimentology Group have made significant out by teams from the Department of Geology, University of contributions to the MacLeod basin program. My colleague, Ray Western Australia. Peter Radford, Senior Technician, has been Brown, has provided much support by assisting with adminis• responsible for the success of most of our coring operations and trative aspects of the overall sedimentology program in the is a key personality in other aspects of the project. I also thank Department of Geology. Contributions to knowledge of the Peter for his support and companionship through so many MacLeod basin are contained in unpublished honors and PhD events, hazardous to hilarious. theses: R. H. Caldwell and C. C. Ford on gypsite sedimentation; B. R. Bolton, K. Gayski, B. Pink, T. Scott, N. V. Logan, P. D. Kuek and M. J. Shepherd on evaporitic carbonates; M. Whib• Masierowski, B. O'Brien, M. J. Shepherd, J. E. Tang, and H. S. ley and S. E. Dowling on halite; M. T. Lewis, N. B. Thompson, Kehal took part in the drilling at various times and their efforts and H. S. Kehal on the stratigraphy of the barrier; and M. T. through bog, flood sheet, cyclone, and heat wave rate both Lewis on the stratigraphy of the Quaternary sequence in the admiration and grateful acknowledgment. Levelling, bathyme• MacLeod Graben. I have drawn on some of these works for try, and hydrologic sampling also were important aspects of the information, and they are acknowledged in the text when field studies, and I would like to thank M. T. Lewis, S. E. Dowl• appropriate. However, the synthesis given iR this memoir is ing, and N. V. Logan, who took part in this work under some• based on my own assessment of project data and material. It times trying circumstances. Technical staff of the Department of also represents an assessment at the end of MacLeod research Geology who have contributed importantly to the program are program stage 1; as a result, it diverges importantly from some J. Williamson, K. C. Hughes, S. Thompson, and D. Westblade. of the interim interpretations presented in the unpublished Renee Stienstra is especially thanked for her meticulous work works and reports. in finalizing the manuscript and color figures.

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