SEDIMENTARY BASINS OF THE W O R L D An Introduction to the Series
Etymology reveals much about the essence of a word. S...
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SEDIMENTARY BASINS OF THE W O R L D An Introduction to the Series
Etymology reveals much about the essence of a word. Science in German, Wissenschaft, is the art of observing whereas science in Chinese, koxue, is the study of classifying. Scientific observations, with the help of modem equipments, have made leaps and bounds in our century, but taxonomy seems irrelevant. Classification can be science. The rise of the natural sciences in Europe could be traced back to Carl Linnaeus in 1750 when he used criteria of mutual exclusiveness to establish the taxonomy of living organisms. Unfortunately, this prerequisite in dividing and subdividing is not always appreciated, and a common practice in geology has been to "classify" basins through reference to incidental attributes. So, we have coastal basins, back-arc basins, extensionally rifted basins, successor basins, deep-sea basins, flysch basins, etc. These qualifications describe the geography, tectonic setting, principal stresses, orogenic chronology, depositional environment or sedimentary association of a basin, but these all could be different aspects of one and the same. "A basin is a basin is a basin", paraphrasing Gertrude Stein. Even though no two basins are exactly alike, subsidence is the common denominator of all. A systematic classification of basins depends upon recognition of the mutually exclusive causes of subsidence. Forty years ago, when I first went to study in the United States, I was fascinated by the debate whether basin subsidence is isostatically induced by sedimentary load. Later, in the 1950's, I began to realize that depressions on Earth are underlain by thin crust and came to the conclusion that subsidence could be the surface manifestation of an endogenetic process of crustal thinning, in response to the Airy isostasy. Still later, in the early 1960's, after geophysical studies had revealed the heterogeneity of the Earth's mantle, subsidence could be related to mantle cooling and corresponding density change, in response to the Pratt isostasy. Meanwhile, we all concede that the weight of a sedimentary pile filling a subsiding depression will induce isostatic subsidence. The three different mechanisms of isostatic adjustment are, however, not mutually exclusive, and they may have operated concurrently. To classify basins on the basis of the various modes of isostasy can thus not be systematic. But, as indicated by gravity studies, not all sedimentary basins are isostatically adjusted, and not all subsidence is isostatic. For a first-order division we could thus recognize two classes of basins, those which have subsided isostatically and those which have not. Crustal thinning is a prerequisite to initiate Airy isostasy. Thinning can be induced by two different systems of principal stresses, with the least principal stress either horizontal or vertical. The former leads to the genesis of rifted basins under horizontal extension, and the latter results in pull-apart basins in transform or strike-slip fault zones. The orientation of principal stresses could thus be the criterion for second-order distinctions. Rift basins are commonly present in the continental interior. After the appearance of oceanic crust between separated continents, the rifts become narrow oceanic gulfs (like the Red Sea). Eventually the loci of subsidence are shifted to passive margins where coastal plains are underlain by thick basinal sediments. Rifted basins may also form on an active margin where a segment of continent is torn apart from the mainland to form islands arcs; those are back-arc basins. The position with respect to plate margins can thus be used as the criterion for third-order distinctions. Subsidence induced by horizontal compression, the third of the three possible configurations of principal stress, is not isostatic. Plate-tectonics theory relates the origin of trenches to the underthrusting of ocean lithosphere on an active plate margin and the origin of foreland basins to the underthrusting within the continental lithosphere. These two major basins of compressional origin are thus also distinguished by the third-order criterion concerning their position with respect to plate margins.
VI
SEDIMENTARY BASINS OF THE WORLD - - AN INTRODUCTION TO THE SERIES
When we first planned the series of the Sedimentary Basins of the World, we intended to adopt a genetic classification. Basins are subdivided into the three sets of criteria discussed above: 1. Isostastically adjusted basins 1.1. Extensional basins 1.1.1. Rift basins in the continental interior 1.1.2. Narrow oceanic gulf basins 1.1.3. Basins of deposition on passive margins 1.1.4. Rifted basins on active margins or back-arc basins 1.2. Transcurrent (transform or strike-slip) pull-apart basins 2. Isostastically not adjusted basins 2.1. Compressional basins 2.1.1. Foreland basins (in a continental interior) 2.1.2. Oceanic trenches (on a continental margin)
With this scheme in mind, the editors of Elsevier and I made up a list of the volumes for the projected series, and we started our search for volume editors. Our first priority, taking into account current demand, was to bring out a volume on China. We were concerned that the Chinese basins could not be fitted into our scheme, because we were told that they represent a special group of unclassifiable basins on "paraplatforms". However, as I became personally involved in researches on the geology of China, I came to the conclusion that the Chinese basins were not "unclassifiable". Rifted basins, back-arc basins, pull-apart basins, foreland basins, etc., exist in China, as they are present elsewhere in the world. The Chinese basins of different origins do share a common history in geologic evolution, and they are united by their geography. It would be illogical to discuss the various Chinese basins in separate volumes. Yet if we are to include all of them in one, we have to designate that volume by their unifying geography. When we started to work on our second volume on rifts, we were still trying to keep our genetic scheme, although we were resigned to make a single exception for the Chinese opus. We were thinking of the East African Rift Valleys, and the emphasis was on Africa. As chance would have it, I just happened to accept a consulting contract on Africa. After a year of working on the assignment, I realized that basins on that continent are as diverse in origin as those in China; there are foreland basins, pull-apart basins, as well as rift basins in Africa. About this time, our choice of the editor for a volume on rifted basins, Professor R.C. Selley, brought up another issue. He pointed out to me the impracticability of putting out volumes on the sole basis of their postulated origins. The purpose of the series is to provide information on the geology of sedimentary basins of the world in order to help a novice to start a project. Commonly, the one who seeks information knows the geographic extent of his interests, but not necessarily the genesis of his targets. Taking, for example, the case of a person who is to start an exploration venture in some region, how should he know if he is to study a monograph on pull-apart basins or one on foreland basins. He knows, of course, if the location is in China or in Africa, and could consult an opus on Sedimentary Basins of China, or that on Africa accordingly. The arguments by Professor Selley finally convinced me to change our scheme. The criterion of dividing the volumes will have to be geographical. In addition to those on China and Africa, volumes on sedimentary basins of Australia, South America, Central America, and the Soviet Union (Russian Platform and Siberia) are planned. Geographical groupings are satisfactory if the basins of various origins in a region share some common heritage, but to throw all heterogeneous entities into one big pot could be disconcerting. To produce, for example, a volume on the Sedimentary Basins of Europe to include all those in the Russian Platform, under the North Sea, and in the Prealps may make a good lexicon, but not an opus harmonized by a unifying theme. This consideration led us to the decision to place priority on certain natural boundaries, so that each volume would sustain a certain coherence in geology as well as in geography. The Cenozoic basins in the Tethyan orogenic belt, for example, are to be grouped under the title of Foreland Basins of the Alpine-Mediterranean Region. The pull-apart and back-arc basins on the shores of the Pacific will be included in two or more volumes of the
Sedimentary Basins of the Circum-Pacific Region. There are, of course, other sedimentary basins of the Earth, especially those of the Near East, North America and Antarctic, which should be included in the series if the coverage is to be complete. On the other hand, we shall also evaluate the demand of the profession for such volumes as the initial volumes of the series successively appear during the next decade.
SEDIMENTARY BASINS OF THE WORLD - - AN INTRODUCTION TO THE SERIES
VII
It is my hope that the series of volumes would not be compared to philately albums; there should be unity in style and in substance. Yet the accumulation of geological information has reached such immense proportions since Eduard Suess wrote his monumental work Das Antlitz der Erde that no single person could ever hope to master the geology of the world. The series of our volumes will, therefore, have to be collective efforts. Coordinations by volume editors are indispensable. My job, as the series editor, is to further enhance the unity and harmony of the whole. We have, however, to accept the fact that each article of a volume may "speak" a different dialect, and each volume of the series may "speak" a different language. Perfect consistency can only be achieved if a person has the time or the capacity to translate all those hundreds of articles in more than a dozen volumes into one universal script. This is not possible, and the practical alternative to the ideal is, therefore, to leave each author or group of co-authors a maximum freedom in their style of presentation and in their interpretations of geology. The articles are to be accepted as expressions of the present state of understanding of an area by leading geologists working in that area. They may or may not represent the understanding of the volume editor, or that of the series editor. Through my experiences in editing the first volume on Chinese basins, I appreciate the potential dangers of such freedom of expressions; a lack of precision in semantics could lead to grave misunderstandings. I felt impotent when I saw basic terms, like orogeny, platform, shelf sediments, intracratonic basins, etc., defined, in certain communities of our profession, on a basis distinctly different from that adopted by modem students of geology. The misconceptions in some instances are so deeply rooted, that nothing short of a rewrite could save the situation. Yet neither the volume nor the series editor could completely revise all the articles. To avoid complete chaos, I plan, therefore, to write a summary, at the end of each volume (or at least some), in my style, and to interpret the geology on the basis of my understanding. Such summaries may contain an overdose of personal opinions and may involve interpretative errors by a single geologist, but they should, at least, be consistent, and may eliminate misconceptions caused by the divergent meaning of the same words as they are used in various "dialects" and "languages". The preceding pages were written in January, 1988 for the first volume of the series on Chinese Basins. The reviewers of the Chinese volume gave me encouragement that I, as the series editor, should continue my role as an interpreter of different cultures. I have, therefore, taken the initiative to write the last chapter of the second volume m A Distant View of the South Pacific Geology. The volume edited by Peter Ballance and my summary are both written in English. There is little need for translation. Nevertheless, New Zealanders speak English with their local accent, and the same "words" are pronounced differently by one who speaks English with a strong Chinese and Swiss accent. It is not surprising that Peter, a dear old friend, found it difficult to "agree entirely" with me. On the other hand, he was tolerant enough not to protest too strongly, and I could have my Distant View for the reference of other distant readers. In reviewing the articles in the second volume of our series, I became more convinced than ever of the wisdom of Selley's advice that the basins should be grouped regionally. I have devoted most of my professional career with a process-oriented approach. When I edited a book on Mountain-Building Processes, the emphasis was on processes. Instead of regional syntheses, I adopted an analytical approach to look into the different processes involved in mountain-building, sedimentary, magmatic, deformational, and metamorphic. In editing this series on Sedimentary Basins, there is no better alternative than regional synthesis. Not only basins in China have diverse origins, those in the South Pacific are equally diverse. Yet they all seem to belong to the same set of diverse basins. If the geology of the South Pacific seems very different from that of China, the apparent distinction can be attributed to the fact that they have advanced to different stages of tectonic evolution. Four years have gone by since the publication of the second volume of the series of The Sedimentary Basins of the WorM. On the eve of the publication of the third volume, I am encouraged to find that the series will not be philately albums; they will not be collections of random observations. I saw the parallelism in the pattern of orogenic deformations in China and in South Pacific, and I could see the same pattern in Africa. There is a difference in the stage of the tectonic evolution. China, as a part of Eurasia has undergone a billion years of amalgamation. The South Pacific is still in an earlier phase of accretion. Africa has gone a long away since its separation of Pangaea, 7and it is being pushed toward Eurasia to its ultimate destiny of a place in a supercontinent. The figures and the colorations of the mosaic pieces are different, but they are all to be pieced together for a unifying theory of global tectonics. The next two volumes will be The
VIII
SEDIMENTARY BASINS OF THE WORLD m AN INTRODUCTION TO THE SERIES
Sedimentary Basins of the Former Soviet Union. I expect to find the same manifestations of the fundamental principles which give us The Face of the Earth. Golden, Colorado, April, 1997
KENNETH J. HSLr Series Editor Sedimentary Basins of the World
SEDIMENTARY BASINS OF AFRICA Introduction and Acknowledgements I speak of Africa and golden joys W. Shakespeare - - Henry IV, Part 2. V, iii, 100
I was very badly brought up as a boy, being allowed to read extensively from the currently politically incorrect works of Rudyard Kipling and Rider Haggard. Whatever else these authors may have done to me, they imbued in my boyish mind a deep fascination for Africa and all things African. One of several reasons that compelled me to become a geologist was the opportunity that it provided to travel the world in general, and Africa in particular. I first landed in Africa in 1963, have intermittently visited the continent many times, and lived there for several years. My particular interest has been in petroleum exploration, but I have also had assignments on gold and coal exploration. From the days of the Roman Empire onwards European explorers have constantly made new discoveries about Africa, as Pliny wrote in 78 AD, "ex Africa semper aliquid novi" (there is always something new out of Africa). Geologists, however, commonly remark on the uniformity of much of Africa's geology. Vast areas are occupied by fiat, though high, plains of Precambrian basement, composed of igneous and metamorphic rocks of immense and diverse ages. These basement rocks are locally overlain by a cover of shallow marine and continental sediments of Cambrian to Recent age. One of the points documented in this book is the regional uniformity of Lower Palaeozoic stratigraphy. This is best seen when tracing the outcrop from the Atlantic coast of Mauritania in the west, eastwards through Algeria and Libya (Chapter 1). Indeed this stratigraphy can be traced, with little variation, into Arabia. This similarity is not difficult to explain, because this terrain formed the southern shores of the Tethys, the ancient ocean that lay to the north of the old Gondwana continent. Geologists who have hammered the Lower Palaeozoic rocks of north Africa, however, will also feel at home in the Cape of Good Hope. The unconformity at Sea Point, described by Playfair and Hall in 1815, and again by Charles Darwin during the Beagle cruise of 1831-1836, invites direct comparison with the pan-Saharan Tassilian discordance whose regional significance was recognised by Kilian (1922). The overlying facies in the Table Mountain Group, with its braided alluvium, and Tiggillitiferous shallow marine sands capped by Silurian tillites, is directly comparable with the Lower Palaeozoic sediments of the Sahara (Tankard and Hobday, 1977). The sedimentary basins of Africa are largely of two types, sag basins, and failed rifts (Clifford 1986). Some of the sag basins, such as the Karoo, are clearly syn-depositional in origin, with clear evidence that subsidence was coeval with, and exerted a strong control on, sedimentation (Chapter 12). Many of the sag basins, however, notably those of north Africa, are clearly post-depositional in origin. The uniform stratigraphy of basins, such as those of Algeria, shows that their sediments were laid down on the gently sloping Saharan platform. Their present basinal shape developed during the late Carboniferous "Hercynian" tectonic phase. Similarly the Murzuk and Kufra basins of southern Libya, if they existed at all, were northerly plunging embayments, open to the Tethyan Ocean to the north, throughout the Palaeozoic, and much of the Mesozoic eras. They did not gain their present closed basinal shape until the middle of the Cretaceous Period (Chapter 2). The genesis of circular sag basins, such as those of Africa, have long attracted attention. Several modes of origin have been postulated. One of the most popular proposes that thermal doming over a mantle "hot spot" leads to the erosion of uplifted crustal rocks, followed by cooling and crustal collapse, initially into a rift, followed by gentle sag subsidence (Allen and Allen, 1990). More recently it has been suggested that crustal sags may result from "cold spots" due to mantle cooling, resulting in downwelling and a dignified gentle sagging of the crust (Hartley and Allen, 1994). This model implies that sag basins may lack a precursor rifting phase, and an early high heat
X
SEDIMENTARY BASINS OF AFRICA - - INTRODUCTION AND ACKNOWLEDGEMENTS
flux, important considerations when modelling basins for petroleum generation studies. Support for this mechanism is provided by the preservation of the uniform Lower Palaeozoic pan-Saharan stratigraphic sequence in a series of isolated basins separated by basement ridges (Chapter 1). Palaeocurrent data (presented in Chapter 2) clearly show that the basins post-date their sediment fill, and implies that the uniform northerly slope of the palaeo-Tethys was locally disrupted by crustal sag basins in the mid-Cretaceous. The Cretaceous Period was a very important time in the history of Africa. The break up of the Gondwana continent, and the concomitant opening up of the Atlantic Ocean, defined the present boundaries of the African continent. As the Cretaceous rifts extended across Gondwanaland some rifts opened to become the Atlantic Ocean, others failed, becoming infilled with thick sequences of sediments that often included organic-rich muds laid down in restricted marine or fresh-water environments. Failed rifts are characterised by high heat flow, due to crustal thinning. These failed rift basins are thus often important petroleum provinces. Epicratonic rifts, such as the Sirte embayment of Libya (Chapter 3), have largely carbonate reservoirs, while the intracratonic rifts,
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SEDIMENTARY BASINS OF AFRICA - - INTRODUCTION AND ACKNOWLEDGEMENTS
XI
such as those of the Sudan (Chapter 6) and east Africa (Chapter 9) are characterised by terrigenous sediments. Finally, the present Atlantic and Indian Oceanic coasts of Africa are defined by rifts that failed and extended by sea-floor spreading into oceans. The Atlantic coastal basins are significant petroleum provinces, because their sediments prograded out over organic rich muds that were deposited in the narrow anoxic Cretaceous seaway of the incipient Atlantic Ocean (Chapters 8 and 13). Where the failed rift system of the Benue trough cross-cuts the Atlantic coast, a vast influx of terrigenous detritus generated the major petroleum province of the Niger delta (Chapter 7). The analogous rift basins on the Indian Ocean coast seem to lack such rich source beds (Chapter 10). Thus the geological history of Africa, as revealed by its sedimentary basins, has gone from unity to diversity, a pleasing synthesis of the disparate views of Africa held by the modem and ancient world. Figure 1 shows the location and chapter numbers of the sedimentary basins of Africa that are described in this volume. Readers will note the uneven length and varied style of the contributions in the different chapters. There is good reason for this treatment. There are still many African sedimentary basins, such as the rifts of eastern Africa, that have barely been described in the literature to date. In such instances the value of this volume is that it publishes the first detailed accounts of their geology. Other basins, however, such as those of northwest Africa, have been described over many years in polyglot papers in many journals. It is perhaps useful, therefore, to produce concise review papers that give coherent accounts of these basins, and that direct the reader towards appropriate references for sources and further information. I thank the contributors to this volume, not only for their contributions, but either for their patience, while their co-contributors wrote their contributions, or thank the laggards, for finally contributing. It has been remarked that a million years means nothing to a geologist, and that this is why they should never be lent money. But this dictum also applies to some geological authors. Finally I must acknowledge the many people and organisations who have enabled me to study Africa and provided me with the geological perspective to assemble and contribute to this volume. These include Arabian Gulf Oil Company, B.H.P., Esso, Genmin, Island Oil Corporation, Mobil, Oasis Oil Company of Libya, Shell, and SOEKOR. I am also grateful to the many geologists who have either propelled me off into Africa on some geological venture or other, or who have travelled with me across the continent, sharing not only their geological knowledge, but even their last can of beer. I am in their debt. R.C. SELLEY (Editor)
REFERENCES
Allen, EA. and Allen, J.R., 1990. Basin Analysis Principles and Applications. Blackwell Scientific, Oxford, 451 pp. Clifford, A.C., 1986. African oil -- past, present and future. In: M.T. Halbouty (Editor), Future Petroleum Provinces of the World. Am. Assoc. Pet. Geol. Mem., 40: 339-372. Hartley, R.W. and Allen, P.A., 1994. Interior cratonic basins of Africa: relation to continental break-up and role of mantle convection. Basin Res., 6:95-113. Kilian, C., 1922. Aper~u grnrrale de la structure des Tassilis des Ajjers. C.R. Acad. Sci. Paris, 175: 825-827. Playfair, K.J. and Hall, B., 1815. Account of the structure of Table Mountain, and other parts of the peninsula of the Cape. Drawn up by Professor Playfair from observations made by Captain Basil Hall (then Lieutenant) R.N., ER.S. Edinb. (Read 31 May, 1813) Trans R. Soc. Edinburgh, 7: 269-278. Pliny (the Elder), 78. Natural History, V11, 17. Tankard, A.J. and Hobday, D.K., 1977. Tide-dominated back-barrier sedimentation, Early Ordovician Cape Basin, Cape Peninsula, South Africa. Sediment. Geol., 18: 135-159.
List of Contributors *
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M.A. ALA 8 Department of Geology, Royal School of Mines Imperial College of Science, Technology and Medicine London SW7 2BP, U.K. G.K. BRINK 13 Consultant P.O. Box 226 Franschoek 7690, South Africa D.S. BROAD 13
Exploration Department, SOEKOR P.O. Box 307 Parow 7500, South Africa A.D.M. CHRISTIE 12 Geological Survey of South Africa P.O. Box X112 Pretoria 0001, South Africa D.I. COLEI2 Geological Survey of South Africa P.O. Box X112 Pretoria 0001, South Africa
K.J. HSU Institute for Resource and Environmental Geosciences Colorado School of Mines Green Center 1500 Illinois Street Golden, CO 80401, USA J.J. MAIER~3 Exploration Department, SOEKOR P.O. Box 307 Parow 7500, South Africa E.I. MBEDE l~ Department of Geology University of Dar-es-Salaam P.O. Box 35052 Dar-es-Salaam, Tanzania I.K. McMILLAN 13 De Beers Marine (Pty) Ltd P.O. Box 87 Foreshore Cape Town 8000, South Africa
A. DUALEH l~ Department of Geology Mogadishu, Somalia
R. McG. MILLER II National Petroleum Corporation of Namibia Private Bag 13196 Windhoek, Namibia
A.S. EL HAWAT4 Department of Earth Sciences Garyounis University P.O. Box 543 Benghazi, Libya
R.T.J. MOODY 5 Department of Geology Kingston University Penrhyn Road Kingston-upon-Thames, Surrey KT1 2EE, U.K.
L.E. FROSTICK 9 Research Institute for Environmental Science and Management University of Hull Hull HU6 7RX, U.K.
C.S. NWAJIDE 7 Shell Petroleum Development Company of Nigeria Ltd. XGSW/3 Warri, Nigeria
M.R. JOHNSON 12 Geological Survey of South Africa P.O. Box X112 Pretoria 0001, South Africa
S.W. PETTERS 7 Department of Geology University of Calabar Calabar, Nigeria
* Superior ciphers refer to the chapter number.
XIV T.J.A. REIJERS 7 Shell Petroleum Development Company of Nigeria Ltd. XGSW/3 Warri, Nigeria D.L. ROBERTS 12 Geological Survey of South Africa EO. Box X112 Pretoria 0001, South Africa R.B. SALAMA 6 CSIRO, Division of Water Resources Private Bag EO. Wembley, WA6014, Australia R.C. SELLEY 1'2'3'8 Department of Geology, Royal School of Mines Imperial College of Science, Technology and Medicine London, U.K.
LIST OF CONTRIBUTORS C.J. VAN VUUREN 12 Geological Survey of South Africa EO. Box X112 Pretoria 0001, South Africa J.N.J. VISSER12 Geological Survey of South Africa EO. Box X112 Pretoria 0001, South Africa H. de V. WICKENS 12 Geological Survey of South Africa EO. Box X112 Pretoria 0001, South Africa
Chapter 1
The Sedimentary Basins of Northwest Africa" Stratigraphy and Sedimentation
R.C. SELLEY
preserved within a series of gentle sag basins, the similarity of stratigraphy implies deposition on a uniform shelf which underwent subsequent warping, thus leaving the strata preserved within the basins, and eroded from the intervening arches (Fig. 1). The sedimentary wedge of the Sahara Platform is separated from the third zone, the Atlas Fold Belt, by a major tectonic break, termed the Sahara Flexure. In the Atlas Mountains the sedimentary section attains its maximum thickness, with rocks of all geological periods represented. It is believed, however, that the Atlas Fold Belt accreted onto the African Shield during late lateral crustal movement, as the
INTRODUCTION
Northwest Africa consists essentially of three main structural units. From south to north these are: the Precambrian cratons of the central Sahara, the Sahara Platform and the Atlas Fold Belt. The Precambrian basement of the central Sahara dips gently northwards towards the present day Mediterranean beneath a cover of Phanerozoic sediments. These sediments were deposited on the southern shores of the ancient Ocean of Tethys. The strata of the Sahara platform possess a remarkable uniformity of stratigraphy and facies. Though these strata now lie
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are dealt with in chapters l and 2, respectively. The Sirte basin is described in Chapter 3.
African Basins. Sedimentary Basins of the World, 3 edited by R.C. Selley (Series Editor: K.J. Hsti), pp. 3-16. 9 1997 Elsevier Science B.V., Amsterdam. All rights reserved.
4
R.C. SELLEY north
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Tethyan Ocean closed into the Mediterranean sea (Clifford, 1986, Klitgord et al., 1988). Because the Atlas Mountains are newcomers, and thus not really part of mainland Africa, they are not described in the present volume. For over a century geologists have remarked on the remarkable uniformity of geology in Arabia and North Africa. As Doughty wrote, following his epic field work in Arabia: "The geology of the land of the Arabs is truly of the Arabian simplicity, a stack of plutonic rock, where upon lie sandstones, and upon the sandstones limestones. There are besides great landbreadths of lavas and spent volcanoes ... " (Doughty, 1888) This sentence expounds a fundamental truth about the geology of Arabia and North Africa, lands that together once formed the southern shores of the great Tethyan Ocean. A geotraverse southwestwards from the Gulf of Arabia, or southwards from the Mediterranean, reveals a broadly similar stratigraphy (Fig. 2). This commences with Pleistocene beach rock. Moving inland one traverses a flat sarir or dune field surface, every now and then descending a limestone escarpment. Intermittently one encounters Tertiary lava flows and volcanoes. Finally one descends the last limestone scarp, normally a Cretaceous chalk, at the base of which are some ubiquitous Cretaceous Greensands. The traverse continues across a dissected terrain of sandstone scarps, jebels and wadis, before finally reaching Precambrian metamorphic basement. Detailed examination of the stratigraphy reveals great similarities, particularly in the Palaeozoic strata. Because of the uniformity of stratigraphy it is sensible to deal with this aspect in a single chapter to encompass all the basins of north west Africa. The next chapter then describes the structural evolution of the main sedimentary basins, and notes local variations in stratigraphy. This organisation avoids the unnecessary repetition of stratigraphic detail. The only exception to this arrangement is the Sirte basin of Libya, whose stratigraphy and structural history is
markedly different from the rest. It is thus dealt with in a separate chapter.
PRECAMBRIAN BASEMENT AND INFRA-CAMBRIAN SEDIMENTS
It is beyond the scope of this volume to discuss the Precambrian basement on which the Phanerozoic sedimentary basins lie. Detailed accounts will be found elsewhere (Fullagar, 1980; Ghuma and Rogers, 1980; Bowen and Jux, 1987). Briefly the Precambrian basement is composed of a complex series of igneous and metamorphic rocks that reflect the gradual cratonisation of continental crust, culminating in a widespread tectono-thermal event dated at about 580 my BP (Fig. 3). This basement includes several sedimentary sequences that have undergone regional metamorphism in general proportion to their age. The identification of the Precambrian: Cambrian boundary thus presents something of a problem. Across much of the Sahara the Precambrian basement is unconformably overlain by a thick broadly conformable sequence of sediments whose upper part contains Lower Palaeozoic fossils. The unconformity at the base of this sequence is termed the Tassilian discordance (Killian, 1922). Where the base of the Palaeozoic sequence unconformably overlies igneous and metamorphic basement the boundary between the Precambrian and the Palaeozoic rocks is easily delineated. Locally, however, barren sedimentary rocks lie beneath the major sub-Palaeozoic unconformity and above igneous and/or metamorphic basement. Such sedimentary formations are given local lithostratigraphic names and have been referred to as "Infra-Cambrian". Subsequently, however, some of the "Infra-Cambrian" sedimentary formations have been found to be of Cambrian age, on the basis of radiometric dates from associated igneous rocks. Thus, though the Tassilian discordance extends across much of the Sahara, it ranges widely in age, and can no longer be assumed to mark the Proterozoic-Palaeozoic boundary (Legrand, 1985).
THE SEDIMENTARY BASINS OF NORTHWEST AFRICA: STRATIGRAPHY AND SEDIMENTATION
5
Fig. 3. Outcrop of Precambrian granite basement, Tibesti, southern Libya. Note fractures and characteristic pan-African elephant buttock weathering
Fig. 4. Generalised Palaeozoic stratigraphic column for the Sahara platform (from Selley, 1996). This sequence, with only minor local variations, can be found preserved in all the sedimentary basins of northwest Africa from the Tindouf basin in the west, to the Kufra basin in the east. It can also be recognised with little change, apart from different formation names, on the Arabian platform.
CAMBRO-ORDOVICIAN
As noted earlier, geologists have long remarked on the similarity of Lower Palaeozoic stratigraphy in Arabia and the Sahara. There is a remarkably uniform sequence of facies that may be traced from basin to basin, from the Atlantic Ocean to the Arabian Gulf (Fig. 4). The Tassilian discordance is normally overlain by coarse pebbly channelled sands. These pass up into better sorted finer sands,
commonly bioturbated. This facies passes up in to graptolitic shales, which in turn pass up, often via turbidite sands in to prograding deltaic, and finally fluvial sands (Bennacef et al., 1971; Clark-Lowes and Ward, 1991). This is a gross generalisation, but a very useful one to memorise. There are two complications. First, though this sequence is broadly correct, there are gradations between the sand facies. Furthermore the various facies are occasionally interbedded, notably
6
R.C. SELLEY
the sandstones, but sometimes there are tongues of shale within sandstone sequences. The second problem is that of age. The sandstones contain an interesting assemblage of trace fossils, but are largely devoid of body fossils that can be used biostratigraphically, The shales, however, contain graptolites and microfossils, such as acritarchs, that can be used to establish a biostratigraphy. Because the barren sandstones are commonest at the base of the sequence, and the shales become more abundant upwards, so the stratigraphy gradually becomes better refined up the sequence. As mentioned earlier, the base of the Cambrian can only be roughly related to the Tassilian discordance. Sediments can only be attributed to a Cambrian age by inference from radiometric dates from the Precambrian basement beneath the Tassilian discordance, and from the occurrence of Ordovician fossils in overlying shales. When traced from basin to basin the rock units have been given local formation names. When actually seen in the field, it at once becomes obvious that these are of the same facies, though their age and lateral relationships may be unclear. The three facies will now be described, and their environments deduced. The lowest facies is composed of coarse pebbly channel sands (Fig. 5). Formations of this facies have essentially sheet geometries with little regional thickness variation. The base of the sequence is commonly marked by the Tassilian discordance. The unconformity is a mature pediment surface with occasional residual inselbergs. The overlying sediments range from pebbles to silt, but are largely of
gra
silt
/
\ \
f f f
Fig. 5. Representative sedimentological log of Cambro-Ordovician cross-bedded pebbly channel sand facies. Interpreted as braided alluvial outwash, composed almost exclusively of active braided channel sands, with rare abandoned channel shales.
medium to very coarse sand grade. Intraformational conglomerates of reworked siltstone occur throughout the sequence. Petrographically these sands include arkoses, especially at the base of the sequence. But they often pass up in to more mature quartz arenites. They are normally red coloured, though many formations are bleached during subsequent meteoric flushing, but the early red colour is retained by impermeable claystones and siltstones. This facies is composed of a series of superimposed channel complexes. Three main types may be recognized. Two types are broad and shallow, either with heterogeneously infilled sandstones, or shale infilled. The third type of channel has steep sided walls, and contains a sequence that fines up from a basal intraformational conglomerate, via sand to shale. Channels of the first type comprise about 90% of the facies. They are typically some 300 m wide and 5 m deep and infilled by sandstones with no apparent regular vertical arrangement of grainsize and structure. The channels are floored by a thin conglomeration of pebbles, and are infilled by various types of cross-bedding. Cross-bedding dips are unimodal at any one locality. When plotted regionally they indicate palaeocurrents flowing northwards off the Sahara shield. Flat bedding is also present, sometimes with scattered pebbles. Quicksand deformation structures are common, including both recumbent foresets as well as convolute bedding. Channels of the second type are rare. They are similar in scale and profile to the sand-infilled channels, internally they are quite distinct. The channel floors are marked by an extraformational pebble lag, abruptly overlain by siltstone, whose laminae drape over the pebbles. The fill is almost entirely composed of laminated micaceous siltstone. These shale-infilled channels are overlain by the more common sand-infilled channels. Body fossils are normally absent from this facies. Obscure unidentifiable trails and tracks occur. Bilobate trails attributable to Cruziana are sometimes found in the shale-filled channels (Seilacher, 1991). From the preceding account it is clear that this facies was deposited in channels. The coarse texture and cross-bedding indicates sedimentation from the bed load of extremely powerful currents. The small variability of cross-bed orientations and channel trends shows that the currents were unidirectional and regionally persistent. The siltstones clearly originated from the infilling of abandoned channels. The predominant red colour suggests early diagenesis above the water table. There are no unequivocal marine fossils. This facies would thus appear have been deposited from nonmarine channel-confined unidirectional traction currents. These conditions are to be found in braided alluvial channels such as occur
THE SEDIMENTARY BASINS OF NORTHWEST AFRICA: STRATIGRAPHY AND SEDIMENTATION
7
Fig. 6. Large scale (>2 m) tabular planar cross-bedding in Tigillitiferous Ordovician shallow marine shoal sand. Jebel Eghei, southern Libya.
on alluvial fans, or on extensive braid plains formed from coalesced alluvial fan systems. Palaeomagnetic global reconstructions show that the western Sahara was close to the South Pole in the Early Palaeozoic, and there is abundant evidence of glaciation (to be presented later) in overlying late OrdovicianSilurian sediments. It can be argued, therefore, that these braided alluvial sands and gravels were deposited as glacial outwash on the flanks of polar ice caps The braided alluvial pebbly sand facies is overlain by, and sometimes interbedded with, the non-pebbly sheet sand facies. The non-pebbly sheet sand facies is overlain by, and sometimes interbedded with graptolitic shales. The non-pebbly sheet sand facies is composed largely of well-sorted medium and fine grained proto-quartzites. This facies is generally cross-bedded, with tabular planar cosets up to 3 m high with sets 5-15 cm high. Sometimes, however, individual sets up 2 m height occur (Fig. 6). Troughs are generally rare. Foresets are homogeneous and accretionary, reflecting the good sorting of this facies. Heterogeneous and avalanche foresets, such as occur in the poorly sorted pebbly sands beneath, are rare (Fig. 7). Cross-bed dip directions in the sandstones are generally unimodal, and indicate deposition from northerly currents flowing down the depositional slope from the Sahara shield towards the Tethyan Ocean. There are very few body fossils in these sandstones. There are, however, occasional trace fossils. The most characteristic type is the vertical burrow known throughout the Sahara as Tigillites, and elsewhere in the world as Sabellarifex, Scolithos, or
Fig. 7. Representative sedimentological log of Cambro-Ordovician Tigillitiferous nonpebbly sheet sand facies. Interpreted as shallow marine shelf environment, composed almost exclusively cross-bedded shoal sands, with shale sheets deposited on tidal flats.
Monocraterion. These burrows are sometimes so abundant that they destroy any original sedimentary structures that may have once existed. The sandstones are interbedded with rare grey argillaceous micaceous siltstones with thin very fine sandstone layers. These units are each between 1-3 m thick and have sheet geometries, in contrast to the abandoned channel silts of the facies beneath. The
8 siltstone sheets are generally laminated throughout with occasional thin beds of very fine sand and isolated sand ripples. These sandstones are rippled throughout. Micro-cross-laminated cosets are generally absent; the sands being composed of congeries of tippled lenses separated by argillaceous laminae and clay drapes. The bases of the siltstone sheets are generally transitional, their tops are abrupt, rarely erosional. These shale units contain a diverse trace fossil assemblage that includes Cruziana, Tigillites and Harlania. The abundance and orientation of cross-bedding in this facies points to deposition from unidirectional lower flow regime traction currents. The fine grainsize, however, shows these currents had significantly lower velocities than those which deposited the braided alluvial sands beneath. The predominance of tabular planar cross-beds, and the absence of channelling indicates that these were open-flow currents unconfined by channel banks. The vertical burrows characterize the Scolithos ichnofacies which is diagnostic of shallow marine conditions. The laminated shale units indicate sporadic lower energy conditions when suspended sediment settled out. The associated rippled very fine sands show that gentle traction currents sometimes occurred; while the absence of cross-laminated cosets and the presence of clay drapes on ripple crests suggests that these currents pulsated gently. Such conditions are more likely to be found in tidal realms rather than in the more regular flows of river channels. A shallow tidal environment is also suggested by the suite of trace fossils. Tigillites, Cruziana and Harlania are all characteristic of shallow marine deposits. The weight of the evidence suggests therefore that the Cambro Ordovician non-pebbly sheet sand facies of the Sahara and Arabia originated in a marine shelf environment. The cross-bedded sands were probably deposited from migrating megaripples and bars similar to those described from modern shelf sand waves. The thin shales and very fine sands suggest intermittent regressive phases when shallower tidal fiat deposits were laid down. Across Arabia and the Sahara the non-pebbly sheet sand facies normally overlies the pebbly channel sand facies. Locally, however, interbedded formations of the two facies types occur, and in some instances the two facies are interbedded. Sometimes, for example, braided channels are found capped by Tigillitiferous horizons. The later sequences indicate the deposits of the actual coastline, where braided channels reached the sea and were subjected to marine influences. Study of the boundaries between sequences of the 2 facies reveals the nature of transgressions and regressions acros the Arabian and Saharan shields. The sequence boundaries where the fluvial forma-
R.C. SELLEY tions overly the shallow marine sands are usually planar erosional surfaces, marked by a thin basal conglomerate, that are intermittantly dissected earlier by steep-sided channels. They are thus analogous to the sequence boundaries described earlier from within the pebbly sand facies. They thus imply a similar origin, namely a pediment retreating across a lithified substrate, succeeded by braided alluvial outwash deposits. Where sequences of pebbly channel sands are overlain by the non-pebbly sheet sand facies, by contrast, the sequence boundary though also planar, is unchannelled. This implies that the sea transgressed across a wave cut bench that was then overlain by the non-pebbly sheet sand facies. Figure 8 displays the relationship between sequence boundaries and fluctuating sea level envisaged for interbedded formations of the Cambro-Ordovician pebbly channel sand and non-pebbly sheet sand facies. Note that these are superimposed over a gradual marine transgression across the Saharan and Arabian shields.
SILURIAN The non-pebbly shallow marine sheet sands are overlain by and pass down northwards into black graptolitic shales. Over most of southern Algeria and Libya there is a major discordance between the shallow marine sands and the overlying black shales. This transgresses older formations to directly overly the Precambrian basement in the centre of the Sahara. The main tongue of this shale is termed
KEY
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FACIES Non-pebbly sheet sand Pebbly channel sand facies PreCambrian basement
ENVIRONMENT Shallow marine shelf Braided alluvial outwash
SEA LEVEL FALLING RISING
(a)
1. Falling 2. Slowly rising 3. Rising fast 4. Slowly rising 5. Slowly falling 6. Falling fast 7. Slowly falling
Fig. 8. Geophantasmograms to illustrate the relationship between Lower Palaeozoic facies, sequence boundaries and inferred sea level change in north Africa and Arabia. In general terms braided alluvial sands are overlain by and pass Tethys-ward into Tigillitiferous marine shoal sands, with occasional interfingering. Fluvial sands typically overly channelized (Type 1) sequence boundaries, Tigillitiferous shoal sands overly planar sequence boundaries.
THE SEDIMENTARY BASINS OF NORTHWEST AFRICA: STRATIGRAPHY AND SEDIMENTATION ment shield
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Fig. 9. The Silurian Tannezuft Shale-Acacus Sandstone transitional sequence, Jebel Eghei, southern Libya. Though diachronous when traced off the shield towards the palaeo-Tethys, this upward-coarsening progradationai sequence can be traced along the strike from Mauritania, across north Africa and into Arabia. the Tannezuft Formation, from Wadi Tannezuft in Algeria. It can be traced from basin to basin across much of the Sahara, being variously referred to as the Tannezuft or Gotlandian (French for Silurian) shale. It is, as this name suggests, largely of Silurian age. In many parts of the Sahara geomorphic features indicative of glacial erosion occur cut into exhumed surfaces of divers pre-Silurian rocks. Glacial features such as striated pavements and roche moutonn~es have been recorded from the Atlantic coast to the southeastern part of the Kufra basin (Beuf et al., 1969). In some localities these surfaces can be traced beneath the base of the Tannezuft shale. Furthermore, thin pebbly mudstones and debrites, attributable to glacial moraines, occur between the graptolitic shales and the underlying discordant the base of the Tannezuft Formation. Locally diamictites infill steep sided valleys cut into the sub-Tannezuft discordance. Elsewhere diamictites exhibit the sinuous form of exhumed eskers. Because the base of the Tannezuft shale contains Lower Llandoverian graptolites, these erosional and depositional features demonstrate a pre-Early Llandoverian glaciation for much of North Africa. Analogous broadly coeval glacial features have been extensively documented in Arabia (e.g., Abed et al., 1993). The Tannezuft shale is often rich in organic matter. It is an important petroleum source rock for petroleum in the Hassi Messaoud and other fields of Algeria (Tissot et al., 1975). The black shales pass up into a sequence of upward-coarsening shale to sand increments in which the sand percentage grad-
ually increases upward (Fig. 9). The upper sand is termed the Acacus Sandstone throughout the Sahara. The type section being Wadi Acacus on the western flank of the Murzuk basin. Regional biostratigraphic studies by Klitzsch (1965) and Bellini and Massa (1980) using graptolites show that the shale-sand transition is strongly diachronous down the depositional slope tYom the Sahara shield towards the Tethys (Fig. 10). The transition from the Tannezuft shales into the Acacus sands is often marked by interbedded sequences of shales and thin sands with erosional bases, graded bedding, and fragmentary Bouma sequences. These are the characteristic features of turbidites. The Tannezuft Shale grades up transitionally into the Acacus Sandstone Formation (Fig. 11). This is a medium to very fine grained clean well sorted sandstone. As sand content increases up the section, trace fossils become common. They include Cruziana, Harlania and sparse Tigillites. These sands are overlain by cross-bedded channel sands with shale pellet basal conglomerates. The Tannezuft-Acacus boundary is often composed of several upward-coarsening genetic increments with a general pattern strongly suggestive of deltaic sedimentation, viz.: Unit
Facies
Environment
I II
Cross-bedded channel sands Interlaminated,load-casted, bioturbated cross-laminated sands and shales Graptoliticlaminated black shales with occasional turbidites
Distributarychannels Mouth bar and delta slope
III
Pro-delta offshore mud zone
THE SEDIMENTARY BASINS OF NORTHWEST AFRICA: STRATIGRAPHY AND SEDIMENTATION "', Tunisia North
A
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~
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B
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Fig. 10. Cross-section to show the diachronous nature of the Tannezuft-Acacus boundary across the Saharan platform. This reflects the gradual progradation of the Acacus delta system over the deeper water muds of the Tannezuft Formation. (Based on Bellini and Massa, 1980.)
DEVONIAN
Acacus Formation
cross-bedded multi-storey channel sands
assorted trails & burrows
upward coarsening increment >lOOm
graded sands with fragmentary Bouma sequences
Tannezuft Formation
graptol ites
Fig. 11. Sedimentological log through the Tannezuft-Acacus boundary showing the transitional nature. Interpreted as offshore muds overlain by a prograding delta slope and alluvial flood plain.
The repetition of these increments may be attributed to global or tectonic causes, or to the autocyclic progradation and abandonment of successive delta lobes as the shoreline gradually regressed across the Sahara Platform towards the Tethyan Ocean.
The remarkable similarity of Saharan Lower Palaeozoic facies and stratigraphy begins to break down in the Upper Palaeozoic. In the north and west the Silurian Acacus delta is conformably overlain by Devonian sands, shales and limestones with a diverse marine fauna. This suggests a gradual deepening of the sea. To the south and east, however, the base of the Devonian is strongly discordant. Early Devonian sediments are absent, and Emsian and younger sediments unconformably overstep older formations down to the Cambro-Ordovician fluvial sands. In these areas the unconformity is overlain by the sandstones that are attributed to the Tadrart Formation. The Tadrart Formation is a coarse to medium grained well sorted sand with a sparse kaolin matrix. It contains rare fine sand and thin shale horizons. Cross-bedding occurs in a variety of forms which are often seen to infill channels. Much less frequently finer, flatbedded sands with Tigillites occur. Plant fragments (Lepidodendron sp.) and spores have also been found. These have been used to place the Tadrart Formation in the Siegenien and Emsian stages. The Tadrart Sandstone is often broadly comparable in facies to the Cambro-Ordovician rocks. A similar environment is proposed, of braided alluvial sedimentation with occasional marine incursions that reworked the sands on beaches, bars and shoals (Turner, 1980). The quartzose composition, finer grain size, lack of pebbles and good sorting of the sands of the Tadrart Formation indicate a polycyclic derivation from Lower Palaeozoic strata cropping out on the shield margin, rather than a direct derivation from the basement hinterland. The Tadrart Sandstone is overlain conformably by the Ouan Casa Formation. Klitzsch (1969) has published the type section from the Wadi Ouan Casa in southwest Libya. The Ouan Casa Formation consists essentially of laminated siltstone with, occasionally, thin laterally
12
R.C. SELLEY g,'aveJ
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Libya in the east, a distance of some 3,000 km. Two types of ironstone can be recognised according to their distribution. One type is thin, and laterally extensive, cropping out for distances of 200 to 300 km. The second type is stratigraphically restricted to the Lower and Upper Devonian. It occurs in localised lenses some 20-30 m thick. It is this second type that is economically important (Guerrak, 1987).
.
.
CARBONIFEROUS-PERMIAN
.
-....__
1-2 m
Fig. 12. Sedimentological log of typical Devonian upward-coarsening genetic increment. Interpreted as upward-shoaling offshore mud to coastal sand bank environment, capped by a significant bioturbated pause horizon (maximum flooding surface).
extensive laminae of very fine sand. It contains horizons of Tigillites and rare brachiopods. The latter are taken by Klitzsch (1969), Collomb (1962) and Burollet and Manderscheid (1967) to indicate an age from Emsian to Eifelian. The Uan Casa Formation is disconformably succeeded by the Aouinet Ouenine Formation over much of the Sahara. There is no marked angular contact, and no overstepping relationship. The type locality is at the well of Aouinet Ouenine on the western flank of the Gargaf arch. Collomb (1962) describes five laterally extensive upward-coarsening shale to sand cycles each about 30 metres thick. When studied in detail these are often composed of smaller upward-coarsening genetic increments (Fig. 12). Comparable sedimentology and faunas imply that the Aouinet Ouenine and Uan Casa formations share a similar depositional environment. The palaeontology shows that this was shallow marine to intertidal. The sedimentology points to deposition in a series of tidal flats and sand banks that prograded cyclically down slope over deeper water offshore muds. There is, however, one other distinctive facies that reaches its apogee in the Devonian. Ironstones occur intermittently throughout much of the northwest Sahara in rocks of Ordovician to Lower Carboniferous age. This facies is particularly well-developed in the Devonian. The ironstones are generally oolitic, and occur interbedded with the shallow marine sands and shales previously described. They crop out intermittently from Zemmour in the west to the Fezzan of
The type sections for Saharan Carboniferous rocks are within the Illizi-Gahadames basin where they attain a thickness of over 500 m. The contact with the underlying Devonian is disconformable. The Carboniferous sediments are broadly comparable to those of the Devonian in facies and distribution. Marine limestones and shales in the northwestern Sahara and Atlas, pass south and east into progressively more sandy and continental deposits. The limestones include oolitic, bioclastic and algal Collenia varieties. These are interbedded with, and pass southwards and eastwards into, upward-shoaling shale: sand sequences analogous to those of the Devonian. These in turn pass south and eastwards into coarse, cross-bedded and channelled sandstones, with Lepidodendron and other unidentifiable plant fragments. These are comparable in facies to the Tadrart and Lower Palaeozoic sands which were earlier identified as of braided alluvial outwash origin. Late Carboniferous evaporites occur in the Reggane and Bechar basins of Algeria, heralding the onset of the Triassic phase of aridity. Preserved Saharan Carboniferous rocks generally range in age from Tournasian-Westphalian, though the evaporites in the Bechar basin are dated as late as Stephanian. The generally barren nature of the continental sands and evaporites means that they may extend in age into the Permian. By the end of this phase of deposition, however, the sea retreated northwards, never ever to return to the Saharan Platform with the depth and extent that prevailed through much of the Palaeozoic Era. This retreat of the sea was followed by a major warping of the Palaeozoic strata, sometimes referred to as the Hercynian Orogeny. Where preserved, the Mesozoic sediments overlie Palaeozoic strata with a strong regional unconformity.
MESOZOIC
Two main facies were deposited during much of the Mesozoic Era. A thick section of marine Mesozoic sediments crops out in the complex folds of the Atlas Mountains. These can be traced southwards in
THE SEDIMENTARY BASINS OF NORTHWEST AFRICA: STRATIGRAPHY AND SEDIMENTATION the subsurface in the Algerian sedimentary basins. Over much of the Sahara, however, the Palaeozoic strata are unconformably overlain by nonmarine, largely, unfossiliferous, sandstones, variously termed "Continental Post-Tassilian" and "Continental Intercalaire" in Algeria, "Continental Mesozoic" in Libya, and "Nubian", in Sudan, Egypt and Arabia. The dateable marine Mesozoic of the northwest will first be described, followed by an attempt to produce a coherent account of the nonmarine Mesozoic sediments. In the Atlas Mountains, and in the subsurface of the Algerian basins, the Mesozoic sequence begins with a series of evaporites. The Stephanian evaporites of the Bechar basin have been already noted. Evaporites are notoriously difficult to date palaeontologically, so it is quite possible that evaporite deposition commenced in the Atlas area in the Permian Period or earlier. The evaporites include both anhydrite and halite, and are interbedded with dolomites, red shales and occasional sandstones. The evaporites provide a regional barrier to petroleum migration across the Algerian basins, acting as the seal to the Hassi Messaoud oil field. Basal sands (dated as Triassic) between the Hercynian unconformity and the evaporites serve as petroleum reservoirs in the Hassi er R'Mel and other Algerian fields. In the Atlas Mountains the Triassic sediments are overlain by Lower Jurassic dolomites and limestones, heralding a return to the open marine conditions that continued throughout much of the Mesozoic. The marine transgression advanced diachronously across the northwestern part of the Sahara shield in Algeria, Tunisia and northern Libya.
13
Arid conditions continued intermittently, with Early Jurassic and Senonian evaporites occurring in the Polignac and Timimoun basins of Algeria. For the most part, however, shallow marine limestones, shales and sands were deposited throughout the Jurassic in all these areas. Over much of the Sahara the dateable (to some extent) Palaeozoic strata are unconformably overlain by continental deposits that are largely barren of fossils. These are in turn locally overlain by fossiliferous Cretaceous limestones, commonly of Cenomanian age. As mentioned earlier, these barren sandstones were given a range of local names. They continue to be a stratigrapher's nightmare (e.g., Banerjee, 1980, and Klitzsch and Squyres, 1990). The sandstones are largely unfossiliferous, apart from wood fragments attributable to the genus Dadoxylon (Fig. 13), and leaf imprints attributable to the genus Cladophlebis. These plant remains are taken to indicate an Early Cretaceous (Neocomian) age. Sandstones of this type were first described from the Sudan by Russeger (1837), who gave them the appropriate name of the "Nubian Sandstone", and attributed them to an Early Cretaceous age, because they were overlain by fossiliferous, and hence dateable, Cretaceous limestones. Thereafter, for a century or more, the term "Nubian" was given to barren sandstones beneath dateable Cretaceous limestones and above basement, from Libya in the west, to Arabia in the east. Gradually, as research progressed, and fossils were discovered, it became apparent that the "Nubian", though largely continental in origin, and commonly a very distinctive
Fig. 13. Fossilized wood (Dadoxylonsp.) in the Continental Mesozoic sandstones of the Messak scarp, northern Murzuk basin.
1
4
R
.
C
.
SELLEY
Fig. 14. Coset of tabular planar cross-bedding in braided alluvial Continental Mesozoic sandstones of the Messak scarp, northern Murzuk basin.
facies of braided alluvial origin, actually ranged in age from Early Cretaceous back to Cambrian or earlier (Van Houten, 1980; Klitzsch and Squyres, 1990). The term "Nubian" became abandoned, and indeed, in the late nineteen-sixties more papers published about Nubian semantics, than about the rocks themselves. Nowadays if researchers are in doubt about the stratigraphical affinity of a barren Saharan sandstone they .provide it with a new formation name. Sedimentologically the "Continental Mesozoic" (for want of a better term) consists of a range of facies indicating deposition in a corresponding range of different depositional environments. The facies include conglomerates that formed as fans around the basin margins. The fanglomerates are overlain by, and pass basinwards into, thick sequences of cross-bedded pebbly channelled sands (Fig. 14), comparable to the braided alluvial deposits described in earlier formations (Fig. 15A). There is also a facies of upward-fining increments of conglomerate-sandstone-red shale (Fig. 15B). Superficially this suggests deposition in a meandering alluvial environment. With the sands being deposited as channel point bars, and the shales as flood plain
deposits. Sometimes, however, shale laminae on the channel floors exhibit desiccation cracks, indicating that discharge was ephemeral (Fig. 16). The shales are commonly massive, with a conchoidal fracture, and they contain rare scattered grains of well-rounded quartz and feldspar. This distinctive texture is characteristic of playa lakes, across which isolated sand grains blow, as in parts of the present day Sahara. Thus it is more probable that these upward-fining increments are more probably indicative of alternations of braid plain and playa lake, than of meandering alluvial flood plain conditions. There are also occasional thick units of red and variegated mudstones, with rare scattered well-rounded sand grains that are diagnostic of a playa lake environment. In the Kufra basin typical continental Mesozoic sandstones formations overly and underly a curious silicified limestone, termed the Chieun Formation. This locally contains abundant Hydrobia, indicating a lacustrine environment, and a post-Triassic age. A description of the dateable marine limestones will be given in the chapter on the Sirte basin, since this is where they are best developed. It should be noted, however, that in some parts of the Sahara
T H E S E D I M E N T A R Y BASINS OF N O R T H W E S T AFRICA: S T R A T I G R A P H Y A N D S E D I M E N T A T I O N
15
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Fig. 15. Representative sedimentological logs of two of the most common facies of the Continental Mesozoic. (A) Cross-bedded pebbly channel sand facies. Interpreted as braided alluvial outwash, composed almost exclusively of active braided channel sands, with rare abandoned channel shales. (B) Upward-fining genetic increments of coarse cross-bedded channel sands overlain by shales9 Superficially these deposits are comparable to the point bars and flood plains of humid meandering fluvial channel systems. Note, however, the desiccation cracks on channel floors, indicative of ephemeral flow, and the scattered rounded (wind blown?) sand grains in the massive siltstones. Interpretable as ephemeral braided channels passing out into playa lakes.
there are isolated jebels of chalk, with a thin basal greensand, unconformably overlying a broad range of stratigraphic units, right down to Precambrian basement. These outliers are crucial markers of the break up of Gondwanaland, serving to delineate the extent of narrow Cretaceous seaways that extended right across the Sahara shield and into what is now N i g e r i a ( K o g b e , 1980).
Fig. 16. Desiccation cracks in abandoned braided channel shale within the Continental Mesozoic sandstones of the Messak scarp, northern Murzuk basin9
16
SELECTED BIBLIOGRAPHY Naturally an area as large as north-west Africa, with its great mineral wealth, has attracted a large literature. Important source books include: Holland, C.H., 1985. Lower Palaeozoic of North-Western and West-Central Africa. Wiley, Chichester, 512 pp. Salem, M.J. and Busrewil, M.T. (Editors), 1980. Geology of Libya, Vols. I-III. Academic Press, London, pp. 1-1712. Salem, M.J. and Belaid, M.N. (Editors), 1991. Geology of Libya, Vols. IV-V. Elsevier, Amsterdam, pp. 1713-2095. Salem, M.J., Sbeta, A.M. and Bakbak, M.R. (Editors), 1991. Geology of Libya, Vol. VI. Elsevier, Amsterdam, pp. 20992491. SONATRACH, 1979. Geology of Algeria. In: J.L. Chardac (Editor), Well Evaluation Conference 1979. Schlumberger, Paris, pp. 1-26.
REFERENCES Abed, A.M., Makhlouf, I.M., Amireh, B.S. and Khalil, B., 1993. Upper Ordovician deposits in southern Jordan. Episodes, 16: 316-328. Banerjee, S., 1980. Stratigraphic Lexicon of Libya. Socialist People's Libyan Arab Jamahiriyah Industrial Research Centre, Tripoli, 300 pp. Bellini, E. and Massa, D., 1980. A Stratigraphic contribution to the Palaeozoic of the Southern Basins of Libya. In: M.J. Salem and M.T. Busrewil (Editors), Geology of Libya, Vol. I. Academic Press, London, pp. 3-56. Bennacef, A., Beuf, S., Biju-Duval, B., De Charpal, O., Gariel, O. and P. Rognon., 1971. Example of cratonic sedimentation: Lower Palaeozoic of Algerian Sahara. Bull. Amer. Assoc. Petrol. Geol., 55: 2225-2245. Beuf, S., Biju-Duval, B., Stevaux, J. and Kulbicki, G., 1969. Extent of Silurian Glaciation in the Sahara: its influences and consequences on sedimentation. In: W.H. Kanes (Editor), Geology, Archaeology and Prehistory of Southwestern Fezzan, Libya. Petrol. Explor. Soc. Libya, Tripoli, pp. 103-116. Bowen, R. and Jux, U., 1987. Afro-Arabian Geology. Chapman and Hall, London, 295 pp. Burollet, P.F. and Manderscheid, G., 1967. Le Devonian en Libye et en Tunisie. Int. Symp. Devonian System, Calgary, Vol. 1, pp. 205-213. Clark-Lowes, D.D. and Ward, J., 1991. Palaeoenvironmental evidence from the Palaeozoic "Nubian Sandstones" of the Sahara. In: M.J. Salem, A.M. Sbeta and M.R. Bakbak (Editors), Geology of Libya, Vol. VI. Elsevier, Amsterdam, pp. 2099-2154. Clifford, A.C., 1986. African oil m past, present, and future. In: M.T. Halbouty (Editor), Future Petroleum Provinces of the World. Mem. Amer. Assoc. Petrol. Geol., 40: 339-372. Collomb, G.R., 1962. Etude g6ologique du Jebel Fezzanet de sa bordure Palaeozoique. Notes et Mem. Comp. Fr. Petrol, No. 1, 35 pp. Fullagar, P.D., 1980. Pan-African age granites of northeastern Africa: New or reworked sialic material? In: M.J. Salem and M.T. Busrewil (Editors), Geology of Libya, Vol. III. Academic Press, London, pp. 1051-1058. Ghuma, M.A. and Rogers, J.J.W., 1980. Pan-African evolution
R.C. S E L L E Y in Jamahiriya and north Africa. In: M.J. Salem and M.T. Busrewil (Editors), Geology of Libya, Vol. III. Academic Press, London, pp. 1059-1064. Guerrak, S., 1987. Palaeozoic oolitic ironstones of the Algerian Sahara: a review. J. African Earth Sci., 6: 1-8. Kilian, C., 1922. Aper~u g6nerale de la structure des Tassilis des Ajjers. C.R. Acad. Sci. Paris, 175: 825-827. Klitgord, K.D., Hutchinson, D.R. and Schoton, H., 1988. U.S. Atlantic continental margin; structural and tectonic framework. In: R.E. Sheridan and J.H. Grow (Editors), The Geology of North America, the Atlantic Continental Margin, U.S. Decade of North American Geology Series. Geological Society of America, Washington, DC, Vol. 1-2, pp. 19-55. Klitzsch, E., 1965. Die Gotlandien-Transgression in der Zentrul Sahara. Z. Deusch. Geol. Ges. Hannover, 117: 492-501. Klitzsch, E., 1969. Stratigraphic section from the type areas of the Silurian and Devonian strata at western Murzuk basin, Libya. In: W.H. Kanes (Editor), Geology, Archaeology, and Prehistory of Southwestern Fezzan. Petrol Explor. Soc. Libya., pp. 83-90. Klitzsch, E. and Semtner, E., 1993. Silurian palaeogeography of NE Africa and Arabia m an updated interpretation. In: U. Thornweihe and H. Schandelmeier (Editors), Geoscientific Research in Northeast Africa. Springer-Verlag, Berlin, pp. 341-344. Klitzsch, E. and Squyres, C.H., 1990. Paleozoic and Mesozoic geological history of northeastern Africa based upon new interpretation of Nubian Strata. Bull. Amer. Assoc. Petrol. Geol., 74:1203-1211. Kogbe, C.A., 1980. The Trans-Saharan Seaway during the Cretaceous. In: M.J. Salem and M.T. Busrewil (Editors), The Geology of Libya, Vol. I. Academic Press, London, pp. 91-96. Legrand, P.H., 1985. Lower Palaeozoic rocks of Algeria. In: C.H. Holland (Editor), Lower Palaeozoic of North-Western and West Central Africa. J. Wiley, Chichester, pp. 5-90. Russeger, J. 1837. Kreide und Sandstein: Einfluss von Grait auf letzteren. N. Jb. Min, pp. 665-669. Seilacher, A., 1991. An updated Cruziana stratigraphy of Gondwanan Palaeozoic sandstones. In: M.J. Salem, M.T. Busrewil and A.M. Ben Ashour (Editors), The Geology of Libya, Vol. IV. Elsevier, Amsterdam, pp. 1565-1582. Seilacher, A., 1993. Problems of correlation in the Nubian Sandstone Facies. In: U. Thornweihe and H. Schandelmeier (Editors), Geoscientific Research in Northeast Africa. SpringerVerlag, Berlin, pp. 329-340. Selley, R.C., 1996. Ancient Sedimentary Environments and Their Subsurface Diagnosis. Chapman and Hall, London, 4th ed., 300 pp. Tissot, B., Deroo, G. and Espitali6, J., 1975. Etude compar6e de l'6poque de formation et d'expulsion du petrole dans diverses provinces g6ologique. Proc. 9th World Petrol. Cong. Tokyo, Applied Sci. Pubs, London, 2: 159-169. Turner, B.R., 1980. Palaeozoic sedimentology in the Southeastern part of the AI Kufrah Basin, Libya: A model for oil exploration. In: M.J. Salem and M.T. Busrewil (Editors), Geology of Libya, Vol. I. Elsevier, Amsterdam, pp. 351-374. Van Houten, F.B., 1980. Latest Jurassic-Early Cretaceous regressive facies, Northeast Africa Craton. Bull. Am. Assoc. Petrol. Geol., 64: 857-867. Whiteman, A.J., 1971. Cambro-Ordovician rocks of A1-Jazair (Algeria). A Review. Bull. Am. Assoc. Petrol. Geol., 55: 1295-1335.
Chapter 2
The Basins of Northwest Africa" Structural Evolution
R.C. SELLEY
INTRODUCTION
As described in the previous chapter, the sedimentary basins of Northwest Africa share a remarkably uniform Early Palaeozoic stratigraphy, that gradually became more varied regionally until the great Late Carboniferous marine regression. It was because of this uniformity that their stratigraphy was described, in general terms, in a single tidy chapter, to avoid unnecessary repetition. In this chapter the structure of the various basins will be described one by one, in a general west to east direction. Only the Sirte basin will be omitted from this chapter and dealt with in one of its own. This is because it is younger than the other basins, formed in a different way, and has a different stratigraphy and structural history. Considered at is simplest north west Africa consists of the Sahara shield. This is a vast area of Precambrian continental crust that has undergone little structural deformation since the end of the Proterozoic Era. When traced northwards towards the present day Mediterranean the Sahara shield dips gently beneath the cover of Phanerozoic sediments that was described in the previous chapter. These sediments were deposited on the southern shores of Tethys. In the Atlas Mountains the sedimentary section attains its maximum thickness, with rocks of all geological periods represented. It is believed, however, that the Atlas fold belt accreted onto the African Shield during late lateral crustal movement as the Tethyan Ocean closed in to the Mediterranean sea (Clifford, 1986). Because the Atlas Mountains are newcomers, and thus not really part of mainland Africa, they are not described in the present volume. It is not easy to clearly define the sedimentary basins of the northwest Sahara. Some are easily recognised as quadripetally closed basins, but many are northerly plunging embayments that originally opened out into the great Tethyan ocean, to become subsequently closed. Furthermore some
basins change name where they cross an international boundary, or according to different authors. Bearing these qualifications in mind the main northwestern Saharan sedimentary basins from west to east are the Tindouf, Reggane, Ahnet, Mouydir and Illizi/Ghadames, Murzuk and Kufra (Fig. 1). The structure and tectonic evolution of these basins will now be defined.
TINDOUF BASIN
The Tindouf basin is the most westerly of the Saharan basins. It is elongated east to west. Its western edge is truncated by the Cretaceous Atlantic coastal sag basin in Western Sahara. It is closed off some 700 km to the east in western Algeria. The basin is asymmetric in cross-section, with a steep northerly limb, and a gentle southerly limb. It is infilled by some 8km of sediments of Cambrian to Carboniferous (Namurian) age, that broadly conform to the pan-Saharan stratigraphy described in the previous chapter. Unusually for much of the Sahara, the presence of archaeocyathid limestones at the base of the section demonstrates a (Lower) Cambrian age. There are several horizons of oolitic glauconite and phosphate in the Lower Ordovician section. A major glaciated erosion surface of pre-Ashgillian age transgresses across earlier sediments on to Precambrian basement (Deynoux et al., 1985). There are some Late Devonian dolerite and andesite lavas. The Tindouf basin has a history comparable to that of many of the Saharan basins. It was a northerly plunging embayment into which the Tethyan seas advanced intermittently from the Ordovician until the Carboniferous. The Tindouf embayment became a closed basin in the Late Carboniferous. This was a widespread phase of tectonic movement that occurred across much of the Sahara. This episode is commonly, but rather unfortunately, referred to
African Basins. Sedimentary Basins of the World, 3 edited by R.C. Selley (Series Editor: K.J. Hsti), pp. 17-26. 9 1997 Elsevier Science B.V., Amsterdam. All rights reserved.
18
R.C. S E L L E Y
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F i g . 1. Structural map to shown the distribution of the various sedimentary basins of Algeria. Simplified from S O N A T R A C H ( 1 9 7 9 ) . Note that there is a considerable variation of terminology for the different basins and subbasins. In particular the Illizi basin metamorphoses into the Ghadames basin when traced eastwards into Libya.
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Fig. 2. Pre-Mesozoic subcrop map of Algeria, simplified from Legrand (1985). This demonstrates that many basins had formed during the Hercynian tectonic event at the end of the Carboniferous Period, and before the deposition of Mesozoic strata. Lines A - B , C - D and E - F refer to cross-sections illustrated in Fig. 3.
THE BASINS OF NORTHWEST AFRICA: STRUCTURAL EVOLUTION
19
Fig. 3. Cross-sections to illustrate the architecture of Algerian sedimentary basins. Locations shown in Fig. 2. A-B, and C-D based on SONATRACH (1979), E - F based on Chiarelli (1978).
as the Hercynian Orogeny, though it involved little more than the uplift and northerly closure of many of the Saharan basins. There are no post-Carboniferous sediments in the Tindouf basin, apart for some 100 m of limestones, sandstones and shales of Miocene to Pliocene age.
REGGANE BASIN
The Reggane basin lies between the Tindouf and Ahnet basins. It is elongated northwest to southeast, with a length of some 350 km and a width of some 150 km. Like the Tindouf basin, it is also asymmetric, with a gentle southwestern limb that dips gently off the Reguibat massif, and a steeper northeastern limb that is bounded by the Ougarta range. The Reggane basin is infilled by some 5 km of sediments of Cambrian to Carboniferous (Namurian) age, that
broadly conform to the pan-Saharan stratigraphy described in the previous chapter. Dolerite and andesite lavas near the base of the section give radiometric dates that indicate a Cambrian age. There are also Late Devonian and Late Carboniferous (Namurian) lavas intercalated with the sedimentary section. Thin evaporites occur interbedded with dolomites and limestones in the Upper Visean section. The Reggane basin has a history comparable to that of the Tindouf and many of the Saharan basins. It was a northeasterly plunging embayment into which the Tethyan seas advanced intermittently from the Ordovician until the Carboniferous. The Reggane embayment became a closed basin by the uplift of the Ougarta range in the Late Carboniferous during the Hercynian orogenic event. Unlike the Tindouf basin, however, the Reggane basin contains over 200 m of "Continental Intercalaire", the local name for the largely barren
20
R.C. SELLEY
continental Mesozoic clastics. There are also intermittent outcrops of Plio-Pleistocene alluvial and eolian sands, and lacustrine marls and limestones that locally attain thicknesses of 100 m or so.
AHNET, MOUYDIR AND ILLIZI/GHADAMES BASINS
The Tindouf and Reggane basins are clearly defined centripetally dipping features. Further east, however, the structure becomes more complex. As noted earlier, during the Palaeozoic Era the Sahara platform consisted of northerly plunging embayments that opened out into the great Tethyan ocean. These embayments become closed basins during the Hercynian tectonic phase. East from the Reggane basin the southern ends of the Ahnet, Mouydir and Illizi/Ghadames basins are clearly defined by their unconformable contact with Precambrian basement. When traced northwards into the subsurface their relationships with one another, and adjacent basins and subbasins become unclear. The Illizi basin of Algeria metamorphoses into the Ghadames basin where it extends northeastwards into Libya. These basins contain the classic pan-Saharan Palaeozoic stratigraphy described in the previous chapter. Indeed, many of the type sections occur where the formations crop out around the flank of the Hoggar Massif. Similarly, it was in this region that the evidence for the late Ordovician glaciation was first recognised. When traced northwards the Palaeozoic sediments of the Ahnet, Mouydir and Illizi basins are overlain by Mesozoic sediments. A sub-Mesozoic map is a useful way of demonstrating how the Hercynian tectonic event defined the architecture of the various basins and subbasins (Fig. 2). Beginning with Triassic evaporites, and following with shallow marine clastics and carbonates, the Mesozoic sediments form a cover that thickens northwards towards the Atlas Mountains. The Algerian basins host major oil and gas fields, such as Hassi Messaoud (Balducci and Pommier, 1970; Bachellen and Peterson, 1992) and Hassi er R'Mel (Magliore, 1970) respectively. These fields occur on the north-south aligned ridges that separate the various basins and subbasins. The negligible variation in Palaeozoic stratigraphy shows that these ridges did not exist at that time. As shown by the pre-Mesozoic subcrop map and cross-sections (Figs. 2 and 3) they developed during the Hercynian phase. This event defined the present boundaries of the various basins and subbasins. There is also evidence of rejuvenation of these positive features during the Cretaceous Period (Sonatrach, 1979). This last phase of movement was crucial, because it predated the maturation of petroleum in the Silurian Tannezuft shale and its migration into adjacent
Fig. 4. Cross-sections to illustrate the evolution of the Triassic salt basin of Algeria (after SONATRACH. 1979). Note that the Cambro-Ordovician reservoir sands of the Hassi Messaoud oil field are capped by Triassic evaporites. Thus the Tannezuft Shale (Silurian) source rock could not have generated petroleum until post-Triassic time.
petroleum reservoirs that range in age from CambroOrdovician to Triassic (Fig. 4). Thus the structures were formed in time to trap the migrated petroleum (Tissot et al., 1975, 1984; McGregor, 1996) When the Ahnet, Mouydir and Illizi/Ghadames basins are traced northwards it becomes hard to resolve basin architecture beneath the ever-thickening sequence of monoclinally dipping Mesozoic strata. This region is best described under the broad regional term of "the Triassic Salt basin". This includes within its bounds the Bechar basin, the Oued Mya basin, the Western Great Erg basin, the Mac Mahon basin, and the Polignac basin. The northern limit of the Triassic Salt basin is defined by a major tectonic feature at the foothills of the Atlas Mountains. This is termed the Atlas flexure, or more dramatically and appropriately, by French geologists "Le Accident sud-Atlasian". This line marks an abrupt increase in sediment facies and thickness from the Sahara platform in the south to the Atlas trough in the north. Palaeogeographic maps commonly show that this line coincides with major
THE BASINS OF NORTHWEST AFRICA: STRUCTURAL EVOLUTION
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At the end of the Eocene and during the Oligocene these global tectonic activities were culminated by regression, siliciclastic sedimentation and volcanism throughout North Africa. In fact, the extent of the late Palaeogene and early Neogene regression and siliciclastic influx is not to be under estimated. During the Late Oligocene and Early Miocene time the southern Mediterranean area of Egypt, Libya and Tunisia, as well as, the offshore areas of Tripoli-Gabes and Sirt basins were transformed from carbonate-dominated to siliciclastic-dominated basins as the African shield began to rise (Benomran et al., 1987). This was accompanied by sinking of the Mediterranean and down faulting all along the North African continental margin in response to anticlockwise rotation of Arabia and the opening of the Red Sea and the Gulf of Suez (Sestini, 1984).
Palaeogene Palaeocene The Palaeocene sequence in the pericratonic basins of Egypt rests conformably on the Senonian carbonates but forms an unconformable relationship with the Late Cretaceous on the surrounding structural highs (El Zarka and Radwan, 1986). At the centre of Gindy basin, the Palaeocene sequence reaches a maximum of 283 m in thickness. It consists of light coloured, fine grained lime mudstone with chert nodules alternating with chalk and shale. Further to the south, the Late Cretaceous-Early Tertiary boundary in the intracratonic basins of
Egypt was not accompanied with intense tectonics. The Dakhla Shale Formation found in these basins appears to be lithologically continuous from the Late Cretaceous into the Palaeocene. However, the Cretaceous-Tertiary boundary is marked within the sequence by a thin unit of intraformational conglomerate rich in reworked Cretaceous fauna (Hendriks et al., 1987; Said, 1990). Away from the basin centre the Cretaceous-Tertiary hiatus is followed by glauconitic, conglomeratic material of Middle Danian age (Barthel and Hermann-Degen, 1981). The Palaeocene sequence in the intracratonic basins forms a tripartite subdivision consisting of Danian shale unit (upper part of Dakhla Shale Fm.), a middle limestone unit (Tarawan, Garra Fm. etc.) followed by an upper shale sequence of Landanian age (Esna Shale Fm.). Like the Dakhla shale, the Esna shales consist of varicoloured, marine, often euxinic marl and shale sequence with carbonate interbeds. The middle carbonate unit, on the other hand, consists of chalk, marly limestone. On the basinal margin, south and southeast of the Upper Nile basin these middle and outer shelf facies change into claystone, siltstone and sandstone alternating with coquinoid limestone and overlain by a sequence of massive, nodular fossiliferous limestone, claystone and marl. These facies successively represent inner shelf, lagoonal and middle shelf sedimentation (Hendriks et al., 1984). The boundary of the middle limestone unit with the underlying Dakhla shale forms a significant regional disconformity surface recognized through out
SEDIMENTARY BASINS OF EGYPT: AN OVERVIEW OF DYNAMIC STRATIGRAPHY southern and central Egypt and the Gulf of Suez and the Red Sea area. It corresponds with boundary between the planktonic foraminiferal zones P3 and P5, and is referred to as the "Velascoensis Event" (Strougo, 1986). The limestone above the boundary is extensively bioturbated, develop nodular texture and contains phosphate nodules, vertebrate remains, reworked coral heads and dwarfed fauna (Strougo, 1986; Hermina, 1990). These features may suggest slow rate of sedimentation, early lithification and hardground development. In other places, the basal part of the middle limestone unit consists of peritidal facies rich in sandy marls and vermetid debris (Barthel and Herrmann-Degen, 1981). In the Gulf of Suez-Red Sea area the middle unit consists of olistostromes of boulder size carbonate clasts floating in marly matrix and associated with coarse grained cross-bedded sandstone filled channels. Taken together these features points towards the occurrence of debris flow and mass movements. These were developed in response to syndepositional tectonism associated with faulting and increased of deformation that preceded Red Sea rifting (Strougo, 1986). Eocene
The depositional sequence of the Eocene epoch in Egypt exhibits a general shallowing upward succession of depositional units reflecting continuous and progressive uplift of the African craton as it was responding to the compressive tectonics between Africa and Eurasia. The Eocene megacycle consists of three progressively shallowing-up depositional cycles each of which reflect phases of transgression and sea level falls of global magnitude. In response to tectonics A1 Faiyum basin was subdivided by structural highs into northern and southern depocentres during the Eocene time (El Zarka, 1983). The southern depocentre, Gindy basin, attained a north-south orientation (Fig. 20), that forms a transition between the NE-SW basinal trends of the Western Desert such as Abu Gharadig and the NNW-SSE trend of the Gulf of Suez and the Red Sea high. West of the Western Desert, Siwa and Matruh basins were also trending in NNW-SSE directions in accordance to the structural trend of the Sirt basin of Libya. In the subsurface of the pericratonic basins the Early Eocene-Palaeocene attains a gradational boundary with no clear break in sedimentation. In Gindy basin the Early Eocene sequence constitutes up to 930 metres of alternating lime mudstone, chalk and thin shale and marl interbeds. Locally, the limestone is glauconitic and dolomitic, and the shales are pyritic and calcareous (El Zarka and Radwan, 1986). In the intracratonic basins to the south, the Early Eocene-Palaeocene boundary shows no physical stratigraphic manifestation as the palaeontological
71
boundary is located within Esna Shale Formation. These shales are overlain by the Thebes limestone which offers a recognizable physical stratigraphic Early Eocene boundary in the sequence. In the Thebes Formation in Upper Nile basin forms 300 metres thick shoaling-up cycle of rhythmically bedded lime mudstone and chalk with occasional chert nodules, grading upwards into oyster reefs and alviolinid rich sand shoals west of the basin. To the east, in the proximity of the Red Sea high, the sequence consists of limestone turbidites, intraformational conglomerates and rhythmically bedded cherty limestones, suggesting unstable depositional conditions related to reactivation of the structures along the high. At the basin's centre, the top of the Early Eocene sequence is marked by the development of nodular chalk hardgrounds. In the region of the Red Sea high this horizon is marked by subaerially exposed caliche and micro-karst surfaces (Snavely, 1979). In the Gulf of Suez area, the Early Eocene facies are similar to those found in the Upper Nile basin. However, the top of the sequence consists of reworked phosphatic deposits, which is equivalent to the hard ground and karstified horizon of the Upper Nile basin. The Early Eocene facies were deposited in deeply submerged stable basins throughout Egypt except in the vicinity of the structural highs. The extensive shallowing features at the top of the sequence, on the other hand, are consistent with the global regression which resulted from lowering of sea level at the end of the Early Eocene (Snavely, 1979; Abul Nasr and Thunell, 1987; Strougo et al., 1990). The terminal Ypresian regression have also resulted in the development of a pronounce disconformity surface within a deep marine sequence in AI Jabal al Akhdar (El Hawat, 1985); and it was also associated with restriction of marine water circulation and extensive evaporite precipitation in Sirt basin in Libya. This disconformity extends further to the west in the offshore area of Tripoli-Gabes basin and the onshore of Tunisia. It marks the top of the shoaling-up Ypresian nummulitic sequence and contribute to the development of Metlaoui Group oil reservoir (Bishop, 1988; Bernasconi et al., 1991) The sea regression at the end of Early Eocene from latitude 22 ~ 30'N to latitude 27~ in Upper Egypt, just south of the town of Asyut, led to the emergence of the cratonic areas of Dakhla and Upper Nile basins. During the Middle Eocene, the Lower Nile or Gindy basin was developed as the main depocentre between Asyut and Cairo. In Gindy Basin, the Middle Eocene forms a 717 m thick shoaling-up depositional cycle of bioclastic-algal limestone rich in large forms such as Nummulites gezahensis, alviolina, echinoids and oysters. This facies association was deposited in tidal fiat, bay, back reef shoals and shelf edge environments (Philobbos
72
A.S. EL HAWAT
and Keheila, 1979; Wahab and Khalifa, 1984). At the basin margin in Cairo area, the sequence is 200 m thick and consists of yellowish white, hard, poorly fossiliferous chalky limestone and dolomitic limestone, overlain by white, hard chalky limestone representing deposition in lagoonal, and shallow neritic conditions (Strougo et al., 1992). Further to the west and north of the Western Desert, the Middle Eocene consists of a thin clastic-dominated sequence of sand, silt and shale which was derived from the erosion of the Cretaceous structural highs (El Zarka and Radwan, 1986). Similar facies changes are found in the Gulf of Suez basin, where the Middle Eocene also, constitutes a shoaling-up nummulitic carbonate sequence. The sequence changes north of the basin into an increasingly clastic-dominated succession of shales, silts and interbedded limestones. The introduction of these clastics were also, attributed to the occurrence of Late Cretaceous structural highs source north of Sinai. The Middle Eocene sequence in the area is capped by a second reworked phosphatic 28 ~ I
bed (Abul Nasr and Thunell, 1987). The shallowing event of the Middle Eocene sequence which is recognized in different areas of Egypt is consistent with the global eustatic lowering of the sea level which marks the end of the Middle Eocene (Abul Nasr and Thunell, 1987; Strougo et al., 1992). A progradational, shoaling-up nummulitic Lutetian sequence overlain by peritidal facies and followed by a disconformity is exposed in Cyrenaica (El Hawat, 1986; E1 Hawat and Shelmani, 1993). The Lutetian sequence in the subsurface in Sirt basin consists of large-scale, progradational clinoform structures recognized in seismic lines. Continued lowering of the sea level during the Late Eocene resulted in the progressive emergence and erosion of the structural highs that provided clastic sediments to the basins of northern Egypt (Fig. 21). The Late Eocene in the Western Desert consists of a 220 m thick sequence of sandy limestone sandstone and shales associated locally with oyster banks. These represent deposition in a shallow, neritic, lagoonal and deltaic conditions. In the
30 o
I
I
I
32 ~ 32 o
Alexondrio /
7/I
/~//
,,z\
30 ~ -
/
'/// .30 ~
:r
.~.
iJj
:>8~ -
.28 o
I
!
3~o
28 ~ 0 L
50 I
I00 I
150 I
-
!
2 0 0 KM. I
LAND
~ I ~t
STRUCTuRAL HIGHS, ISLAND
DELTAIC SEDIMENTS
////! ///I
BASINAL
MUD
BASINAL
MARL
Fig. 21. Palaeogeography of Late Eocene time in northern Egypt (after Salem, 1976).
SEDIMENTARY BASINS OF EGYPT: AN OVERVIEW OF DYNAMIC STRATIGRAPHY subsurface, the Late Eocene facies are often undistinguished from the overlying Oligocene deposits. The Late Eocene siliciclastic deposits generally thicken northwards, but may also, thicken locally toward basin centres as they prograde away from the surrounding structural highs (El Shazly, 1977; Salem, 1976).
Oligocene The Tertiary marine regression and tectonic upheaval continued in Egypt during the Oligocene as the Arabian plate started to move in an anti-clockwise direction around a pivotal point in the Jordan (Fig. 22). This movement was initiated in response to resistance to differential drifting of the northwestern foreland of Africa in comparison to the northeast. Causing the movement of Sinai to the southwest, and the subsequent opening of the Gulf of Suez (Klitzsch 1986; Schandelmeier et al., 1987). These movements also led to the development of the NW-SE, ENEWSW fault systems which were associated with extensive volcanicity throughout Egypt. These were combined to contribute to the development and subsidence of the Gulf of Suez and the Nile Delta basins (El Shazly, 1977; Rizzini et al., 1978; Said, 1981). The Oligocene in Egypt was a time of uplift, regression, volcanicity and continental sedimentation
73
(Fig. 24). Outcrops southwest of Cairo consist of 250 m of sandstone and gravel with local limestone and shale interbeds of fluviomarine origin (Said, 1962). These facies change westwards into fluvial sands and gravels, and grade northwards into deltaic siltstone, shale and glauconitic sandstone. Locally, continental Oligocene facies are overlain by basaltic lava flows up to 250 m thick (El Zarka and Radwan, 1986). North of the Gulf of Suez basin, the Oligocene consists of fossiliferous foraminiferal marls. These change southwards into red beds, consisting of reddish, clayey and pebbly calcareous sandstones, associated locally with post rifting basaltic flows (Salem, 1976; Chowdhary and Taha, 1987).
Neogene Miocene The Early Miocene transgression was associated with deposition of siliciclastics (Moghra Formation) in the Western Desert (Said, 1962). In the subsurface the Miocene consists of 615 m thick coarsening-up deltaic sequence of sandstone, shale and limestone intercalations. This sequence pinches out eastwards towards the Nile Delta in the region of Tiba basin, and changes northwards into pro-delta silts and marine shale (Fig. 25). Following the initial Miocene
/ t
//
ARABIAN ) PLATE
,,,;/ T-EARLY
TERTIARY
I) Arabian Plate moves faster than Nubian Plate. 2) Development of NW-SE Riedel shear. 3) Development of NE-SW Syrian Arc folds.
"n"- OLIGOCENE- MIOCENE
"rrr- LATE
4) Anti- clockwise movement of Arabian Plate. 5) Opening of Gulf of Suez.
6) N.
MIOCENE- PLIOCENE 8= QUATERNARY
movement of Arabian Plate along Aqaba fault. 7) Opening of Red Sea graben.
Fig. 22. Sketch diagram of the structural developmentof Gulf of Suez-Red Sea-Gulf of Aqaba graben system (after Klitzsch, 1986).
.,,j
EOCENE AFTER FAULTING
Q L. EOCENE
~...~...........~. LATE CRETA. . . . ~ ... --_...--_--~! I I ~ valeocene . . . . I~ I I ~
~
I
/
Reworked lower Eocene breccia / a n d Conglomerate ln Igoonol deposits
f
~
Q .
(/., \ r~~~
../
END OF OLIGOCENE
Pre Miocene
j
ormlty
Fan Conglomerate ~ Lower- Middle Mlocene
/
Q
I
MIDDLE MIOCENE LOWER MIOCENE REER
/ / I
w/
/
/ ~ J
d
~
.
.
.
.
m
J
> f./3
Fig. 23. Rift margin block faulting development on the western side of Gulf of Suez (after Klitzsch, 1986).
t-n t--' :i: >
SEDIMENTARY BASINS OF EGYPT: AN OVERVIEW OF DYNAMIC STRATIGRAPHY 26 ~
?_8 =
I
I
,
,I
5 0o I
I
75
:52 ~ I
:54 ~
I
:52 ~
52 ~ M e d i t e r r a n e a n Sea
\
\
Shelf \
Carbonates ?
\ \ 30 ~
N
30 ~
28 ~
:)8~ 0
~
SLOPEFAN
I00
150
2C
Red
T 26 ~
50
28 ~
:50 ~
I!l /I II I]
PRODELTA
32 ~
[~'~'i'ii]
Sea
5 4~
sANos
LAND
DELTA
Fig. 24. Palaeogeography of the Oligocene, northern Egypt (after Salem, 1976).
transgression in the Delta basin, the Aquitanian was a time of low sea level and uplift throughout the delta as indicated by the low sedimentation rate of 60 to 70 m/Ma. East of the Delta a higher sedimentation rate was estimated (400-500 m/Ma) during the Burdigalian (Wray, 1985). In the Gulf of Suez basin the Oligocene rifting was followed by Early Miocene marine transgression over an irregular basin floor. The northern part of the basin centre was filled by 2200 m thick Globigerina marls that thins southwards to 200 m at the Gulf's entrance (Salem, 1976). These marls change laterally and upwards towards the rift's margin into calcareous sandstone, limestone and interbeds of marls and gypsiferous shales. The outer margin of the rift consists of crystalline basement half horsts and graben structures forming a series of platforms and basins. These are covered by Early Miocene sediment apron of mixed alluvial gravels conglomerate and sabkha evaporites that grade basinwards into platform carbonates and basin rim reefs (El Haddad et al., 1984; Coniglio et al., 1988; James et al., 1988). It was noted that the Early Miocene sequence was interrupted by tectonic movements, causing reactivation of faults and tilting of fault blocks within the basin (Fig. 23). These tectonics were associated with the development of an unconformity in the basinal sedimentary sequence, and the development of karstification, subaerial diagenesis of platform carbonates and synsedimentary collapse of basin rim reefs (James et al., 1988).
Salem (1976) suggested that the Middle Miocene transgression was associated with the eastwards shift of the Palaeonile from the Western Desert to its present day position (Figs. 25 and 26). The resulting cessation of siliciclastic influx into the Western Desert gave way to an increase in carbonate productivity and the deposition of Marmarica Formation that extended into Cyrenaica. During the Middle Miocene (Langhian), the Nile Delta was affected by tectonic instability and gravity faulting. This was followed during late Middle Miocene (Mid and Late Serravalian) by a pronounced sea level fall causing a depositional hiatus that ranges from 6 to 2 Ma in duration, and may extend to Early Tortonian (Wray, 1985; Harms and Wray, 1990). The Middle-Late Miocene delta cycle is a coarsening-up depositional sequence that thins gradually southwards in the direction of Cairo (Fig. 28). The early part of the cycle consists of more than 700 metres of green, gray claystone interbedded with fossiliferous marls and rare quartzose sandstone interbeds. Following an unconformity, the second part is of Late Tortonian-Early Messinian age. It consists of 1-3 km thick, northwest prograding clinoform sequence the deposition of which was taking place in association with rapid subsidence of the eastern part of the delta, which was influenced by an easterly longshore drift. During this phase the delta maintained a high sedimentation rate that reached 680 m/1 Ma (Wray, 1985). Sediments constituting this depositional phase consist of poorly
76
A.S. EL HAWAT 28 ~
26 ~ !
32 ~
30 ~
!
32 ~ i
3 4~ !
i
32 ~
\ \ \ \ \ 30 ~
\ 32 ~
\ \ \
e@
9
28 ~ 28 ~
I
I
26 ~
I
I
28 ~
I
30 ~ T
50
I00
I
I
150
I
32 ~
i% m ~
y&ll
V
34 ~
200KM.
I
LEGEND LAND
CARBONATES [VvV vV ~ E V A P O R I T E S
DELTA
ALLUVIAL
FAN
Fig. 25. Palaeogeography of Early Miocene time, northern Egypt (after Salem, 1976).
sorted conglomeratic sands, clays and calcareous sandstone representing a prograding fluvio-deltaic and coastal deltaic sedimentation and associated swamps and lagoons. Wray (1985) and Harms and Wray (1990) pointed out that during this phase of the delta history, a major integration of the Nile drainage system has taken place for the first time. North of the delta region, the Late Miocene cycle is capped by 40 m thick unit of Late Messinian anhydrite interbedded with thin clay layers (Fig. 27). In places, the top part of the delta sequence exhibits deeply incised channels and slump structures that produce an angular unconformable relationship with the overlaying Pliocene delta cycle (Rizzini et al., 1978). In the Western Desert, the Tortonian-Messinian sequence consists of 30-40 metres thick cross-bedded, well sorted, medium grained sands, clays and limestone, that grade up into gypsiferous clays and coarsely crystalline selenite (Omara and Sanad, 1975). The sequence exhibits a disconformable relationship with the Early and Middle Miocene below, and the overlaying Pliocene carbonates. In the Gulf of Suez basin the Middle Miocene consists of calcareous shale and marl interbedded with anhydrite, and grading upwards into massive anhydrite and rock salt and associated with gray shale interbeds. The sequence is overlain by Late
Miocene anhydrite with minor interbeds of sandstone and shale (Chowdhary and Taha, 1987). These evaporites attain a maximum thickness of 3540 metres at the southern end of the basin and decrease northwards to zero (Salem, 1976). Taken together with thickness variation of the Early Miocene open marine Globigerina marls, it was concluded that these evaporites were deposited in a barred basin which was opened to the Tethys from the north (Said, 1962; Salem, 1976, fig. 16). On the basinal margin, the Middle-Late Miocene sequence is similar to that of the Early Miocene. It consists of peritidal carbonates associated with basin-rim reefs and subtidal stromatolites on the gulf side. Away from the basin centre these sediments change into alluvial clastics near basement highs. This depositional suite of carbonates and clastics were covered by Late Miocene evaporites and was followed by evaporite solution collapse breccia that resulted from subaerial exposures and diagenesis (El Haddad et al., 1984; Coniglio et al., 1988; James et al., 1988). Pliocene Following the Messinian lowering of sea level during the Mediterranean salinity crisis (Hsti et al., 1973), and due to lowering of the base level of erosion, the River Nile drainage became deeply
SEDIMENTARY BASINS OF EGYPT: AN OVERVIEW OF DYNAMIC STRATIGRAPHY 26
28
v
v
30
!
!
MEDITERRANEAN
!
32 !
A
77 54 !
i
:52
SEA
Ale
\
\ \ 30
>, gg
SINAI
i i i
i
0 I
50 I
____J~_
I00 150 200 Km I
I
EASTERN DESERT
~(~./,~O,f
28
I
__._m,..--._...L
Delta fan
V~.~_~I
Shale
Alluvial fan
~
Calcareouss.s.
Slope fan
~
Carbonates
Land
~
Evaporites.
Fig. 26. Palaeogeography of Middle Miocene time, northern Egypt (after Salem, 1976).
entrenched as far south as the city of Aswan 1200 km inland (Ryan, 1978). The occurrence of the Pliocene (Plaisancian) marine and estuarine clays and sands in the vicinity of Aswan (Chumakov, 1968) was attributed to the opening of the Atlantic floodgate and refilling of the Mediterranean basin during the Early Pliocene transgression (Hsti et al., 1973). The Early Pliocene estuarine deposits were followed by Late Plio-Pleistocene fluviatile sequence. In the Nile Delta basin, an Early Pliocene transgressive sand unit (Abu Madi Formation) forms the base of the Plio-Pleistocene Nile Delta cycle (Fig. 27). It consists of thick- bedded, rippled, crossand plane-bedded, bioturbated sandstone, interbedded with clay and occasional conglomerate at the base. The following delta cycle is 1500 m thick coarsening-up sequence that consists of deep shelf and slope clays with few quartzose sand interbeds. These sand beds increase in thickness and frequency upwards, as they grade into 300 m thick, large scale prograding foresets of coarse- to medium-grained deltaic sand units. The following 700 m of the sequence is thick-bedded, conglomeratic, coarse- to medium-grained quartzose sandstone with reworked chert, quartzite and dolomite pebbles. The top of the sequence also contains coquina and peat deposits representing coastal and lagoonal sedimentation that
extended up to the Quaternary (Rizzini et al., 1978). West of the delta in the Faiyum area, the Pliocene consists of limestone, sandstone and bioclastic shoreline facies. These may change laterally in Wadi Natrun into an association of clay, limestone and sandstone with vertebrate remains indicating fluvio-estuarine and marginal marine conditions. Further to the west in the Western Desert and away from the influence of the Nile, the Pliocene sequence consists of 60 metres of pink oolitic limestone which is unconformably underlain by the Miocene (El Shazly, 1977). In the Gulf of Suez basin, the Late Miocene regression and erosion was followed by subsidence associated with the opening of the Red Sea to the Indian Ocean. The post-Miocene sequence in the subsurface is up to 950 m thick, coarse to mediumgrained sandstone interbedded with red brown claystone and siltstone. These contain minor intercalations of limestone and anhydrite, associated with fauna of Indo-Pacific affiliation (Chowdhary and Taha, 1987). On the basin margin, active extensional tectonics and the associated depositional suites that began in Early Miocene time continued until the present (Purser et al., 1987). It exhibits rapid lateral variation of carbonates (reefs), evaporites and siliciclastic sediments. This is a typical facies association
78
A.S. EL HAWAT
Rock Units AGE
Environments u
AVG.
Formation
Lithology
Thickness Metars
Holocene
Bilqas
50
a _u ~ a'6o .~._
"6 ~
E.~
to.
Neritic
~. J.
"-- 9149 o
o
o
o
o
o
o
Pleistocene
Mit Ghamr
700
9 ~
.
9
~176
.
.
.
~
~
~
.
Upper El Wastani
300
....
9
,,
9
,,
~
~
,
9
Middle Kafr al Sheik
U.I Z uJ c.) 0 _J 13.
1500
Lower Abu Madi
300 "
"'"'~
~
Rosetta
50
~
~
v Vv Vv
'OoOoO ~
"~
9 ~176
r ~
Messinian Qawasim
UJ Z UJ
9
9
~
....
0
Tortonian
Serravalian
Sidi _ .. Salim
>700
~
!
Langhian Fig. 27. Stratigraphic column of the Nile Delta (after Rizzini et al., 1978).
related to rift margin depositional climatic conditions (Fig. 29). Under sporadic wet periods causing flash coarse clastics from the hinterland.
set up in arid these conditions floods provided These were fol-
lowed by a rapid return to a siliciclastic-free, clear marine water leading to carbonate sedimentation and evaporite mineral precipitation in areas of restricted circulation (Purser et al., 1987).
SEDIMENTARY BASINS OF EGYPT: AN OVERVIEW OF DYNAMIC STRATIGRAPHY
79
N
S It
Pleistocene
~
~
~
t-z-o~
o".
o
.~- .-'~-, -"-~." ". ".'.'-'.
Holocene
~=
9
~
"'"
"-~----- :-~
."5"- . .
.
~
_ L ~ - - ~
.J'-~." ! . .---C
e /
1.
\
\\
(
\
I
o ~ -
"-,d
.,
"
--
50
~oo
'
I
150 I
-
36"E
I
Fig. 18. Central Sudan groundwater troughs. Map showing the Atabara trough in the Atbara rift system, Soba and Gezira trough in the Blue Nile rift, Nuba trough in the White Nile rift and the Sudd trough in Bahr El Arab rift. (After Salama, 1987.)
132 percentage of fines and clays, the aquifer is not homogenous. The Tertiary aquifer can be considered as consisting of small different water-bearing bodies (aquifers), with highly variable hydrogeological properties, separated by other water bodies (aquitards), which play an important role in the hydrogeological properties of the aquifer (Salama and Salama, 1974). Due to the presence of thick clay deposits in the northern part, south of Bahr E1 Arab some wells drilled by spudding to depths of 300 m, failed to reach a water-bearing stratum. Some hand dug wells in the vicinity of Bahr E1 Arab also proved to be dry. This rules out Bahr E1 Arab as a potential recharge source, at least in the southern part of Abu Gabra trough. Transmissivity was calculated on the basis of the lithology of sediments from the bore-logs, as compared with other similar areas in the Baggara basin. It was found to range from 25-50 m 2 day -l. Correcting for partial penetration and well losses T values ranging from 100-500 m 2 day -l, for 50 m of saturated aquifer thickness. These values are low, compared with the values of Transmissivity in the Baggara basin, but are acceptable taking into consideration the high percentage of fines and clays in the Sudd basin. Blue Nile rift basin This basin covers an area of 76,000 km 2. The basin has been divided into three subbasins, roughly coinciding with the structural divisions of the Blue Nile rift, these subbasins are: Khartoum, Gezira and Singa (Salama, 1976) (Fig. 18). Khartoum subbasin. Three main aquifer are identified in Khartoum basin (Andrew, 1948; Abdel Salam, 1966; E1 Boushi and Whiteman, 1968; E1 Boushi, 1972; Kheiralla, 1966; Saeed, 1974, Maimberg and Abdel Shafie, 1975; S.G.E.P., 1979). (1) Upper semi-confined Gezira aquifer; this is found mainly along the White Nile, where it is capped by a thin clayey layer, the aquifer is extensively utilised for irrigation purposes from hand dug wells. (2) Lower semi-confined Gezira aquifer; this covers the area between the White and Blue Nile, the northern part of Khartoum north to Sabaloka (Six Cataract), on the left side of the White Nile and the right side of the Blue Nile. This aquifer receives direct recharge from the Nile and is considered as the most highly utilised aquifer in Sudan. More than 500 wells tap this aquifer. The upper 10-15 m are fine sands and clays, while in the central part, south of the green belt the thickness increases. Water is usually found in a layer of sand and gravel ranging in thickness from 3 to 10 meters.
R.B. SALAMA (3) Khartoum Mesozoic semi-confined aquifer; the Mesozoic deposits forming the aquifer, crop out in the area west of the White Nile, and east of Khartoum North. It is also found below the surface in all the other areas of the basin. Ground-water maps prepared for the basin shows the following characteristics (Abdel Salam, 1966; Kheiralla, 1966; Salama, 1976; Saeed, 1974; S.G.E.P., 1979): (a) Ground water moves away from the two rivers. (b) There are two ground-water troughs, one in the northeast of the Blue Nile and the other in the central area between the Blue Nile and White Nile. (c) The hydraulic gradient varies considerably. In the area adjacent to the River Nile, it ranges from 0.002-0.005 close to the Nile and 0.001-0.0009 at a distance of more than 5 km away from the Nile (S.G.E.P., 1979). (d) The influence of the Nile water levels on the ground-water levels fluctuations has been related to distance away from the Nile (Saeed, 1974, 1978). This was found to be 5 m at a distance of less than 200 m; below 1 m at a distance of less than 2 km; and no effect in observation wells more than 2 km away from the river. But this is not true over the whole area, in some places the effect does not occur beyond one kilometre, especially near the Nile at Kosti and Rebek where the effect is minimal. The fluctuations of the ground-water levels resuiting from high rates of abstraction are noticed in the irrigation areas, where levels are known to have dropped more than 10 m in the upper semi-confined aquifer, at the northern part of Khartoum Province. Transmissivity ranged from 38.4 m 2 day-~ to 5950 m 2 day -l (Saeed, 1974). Salama (1976) from the analysis of more than 500 pumping test data in the area, showed that the transmissivity ranged from 500 m 2 day -1 to 2000 m 2 day -~. With transmissivity increasing with depth S.G.E.P. (1979). Storativity values range from 10-2 to 10 -3 for the upper semi confined aquifer, 10 -3 to 10 -4 for the lower semi confined aquifer, and 10 -3 to 10-5 for the confined aquifer. Gezira subbasin. The Gezira subbasin covers the Wad Medani graben of the Blue Nile rift (Abdel Salam, 1966; E1 Boushi and Whiteman, 1968; E1 Boushi, 1972; E1 Boushi and Abdel Salam, 1982). The water-bearing strata in the Gezira are similar to those of Khartoum basin. The ground water occurs under semi artesian and leaky conditions, as a result of impervious layers of clays in the Gezira formation (El Boushi, 1972). The upper Tertiary semi-confined aquifer have transmissivity values ranging from 100-500 m 2 day -l, and storativity of 10 -2 to 10 -3. The lower Tertiary semiconfined aquifer transmissivity values of 500-1500 m 2 day -I
RIFT BASINS OF THE SUDAN and storativity of 10 -2 to 10 -3. The deep confined aquifer T of 300-2000 m 2 day -~ and storativity 10 -3 to 10 -5 . Ground-water level fluctuations ranged from three meters near the Blue Nile to nearly steady conditions near the central part. Singa subbasin. This subbasin covers the Singa graben of the Blue Nile rift. Detailed hydrogeological study carried out in the area show that these aquifers are similar to the types found in Khartoum basin. The ground water occurs under semiconfined and leaky conditions. The general direction of the ground water is away from the rivers towards the central part of the aquifer, with a general ground-water component from south to northwest. Aquifer characteristics showed marked similarity to the Gezira basin. White Nile rift basin This ground-water basin extends over the White Nile rift, covering an area of 100,800 km 2. It includes Umm Ruwaba basin and the northern part of Sudd basin (Salama, 1976). The basin is formed of three subbasins; Bara, Umm Ruwaba and the White Nile. Bara basin. Bara basin includes Bara trough of the White Nile rift. It is fault bounded on the north, east and west, and it is separated from the Umm Ruwaba subbasin in the southeast by Umm Dam ridge (Hunting Technical Services, 1970); extending in a NW-SE direction (Rhodis et al., 1963; Hunting Technical Services, 1970; Mabrook, 1972; Maimberg and Abdel Shafie, 1975; E1 Boushi et al., 1975; All, 1978). All the strata below the unconfined water table are water saturated. Those include the sediments of the Tertiary and Mesozoic deposits. The types of aquifer recognised in Bara basin are: (a) Unconfined aquifer: 20-30 years ago, most of the villages in the Bara basin have a system of open wells varying in depth from a few meters to about 20 meters, some of them would go dry during summer, or the yield would decrease significantly. These wells were abandoned after the drilling of the deep wells and the construction of a permanent water point. All these wells tap the unconfined aquifer, which seems to spread over the whole of the Bara basin. This unconfined aquifer is utilised for irrigation in the eastern part of the basin, in Bara and in the Kheiran district. The aquifer is formed of a few metres of gravels and sands, in some places the gravel is intercalated with clayey sand and sandy clay. (b) Tertiary semi-confined aquifer: The semi-confined aquifer lies below the unconfined aquifer and is separated from it by a layer of clays and fine
133 sandy clays and clayey sand, of variable thickness, but it reaches its maximum thickness in the southern part of the basin, where a thickness of more than 500 m is known, i.e Bara. Usually the water is under semi-confined to confined conditions. (c) Mesozoic confined aquifer: In the northern part of the basin, where the Mesozoic faulted blocks are overlain by the thick Tertiary deposits, the water is found under artesian conditions (Umm Balgei flowing well; E1 Boushi et al., 1975). Transmissivity values ranged from 100-500 m 2 day -~ and storage coefficient of 10-3-10 -4 with delayed yield effect. Umm Ruwaba basin. Umm Ruwaba basin covers the Umm Ruwaba graben of the White Nile rift (R.E.G.W.A., 1979; T.N.O., 1979). The basin is fault bounded and extends in a SE direction. The water-bearing formations in this subbasin are similar to those of the Bara basin, except for the fact that the Mesozoic aquifer is semi confined and not confined as in the Bara basin. The Mesozoic aquifer is restricted to Kosti area only, and unexpectedly, it is dry even near the White Nile and with thicknesses exceeding 200 m. Transmissivity values for the Mesozoic sediments are almost twice that for the Tertiary values. T ranges from a low of 12.5 m 2 day -l to 120 m 2 day-i for the Tertiary deposits. Water level fluctuations ranged from two meters near the Nile to about 50 cm two kilometres away (R.E.G.W.A., 1979). White Nile basin. This basin extends over the White Nile graben, it was previously considered as part of the Sudd basin (Salama and Salama, 1974; Salama, 1976; Geophysics and Strojoexport, 1977). The water-bearing formations are similar to those of the Umm Ruwaba basin, with one major difference; the Mesozoic sediments although encountered in few wells (Salama and Salama, 1974) does not seem to have the same aquifer properties previously known in the other subbasins. River Atbara rift basin This rift basin is divided into two subbasins; the River Atbara and the River Gash subbasins. The River Atbara subbasin extends north from the Abu Haraf water divide to the Atbara River, and covers an area of 23,896 square kilometres. Two types of aquifer are recognised: (1) Semi-confined river alluvium deposits extending along the River Atbara from Atbara town, south to Qoz Regeb. The top layer is usually clayey sand and sandy clay, followed downwards by gravels and sand. The water is usually under semiconfined conditions. (2) Semiconfined Mesozoic deposits; occurring in almost all
134 the other areas of the subbasin. It attains its maximum thickness at W. El Makabrab. Several observation wells along the river Atbara, show seasonal fluctuations of about 10 m. The semi confined aquifer has a T value of 100-1000 m 2 day-1 and storativity 10 -2-10 -4. The River Gash subbasin extends over the alluvial deposits of the river Gash in Kassala town, and extends downstream to cover the Gash delta to the north of Wagara. Previously the alluvial river deposits were considered as one aquifer only, which is separated sometimes by aquitard layers (Saeed, 1969; E1 Amin, 1979). Further detailed work (T.N.O., 1982), showed that the deposits form two aquifers; Upper and Lower aquifer, with a top clay layer and a continuous aquitard layer which separates the two aquifer layers. The depth to the aquitard layer varies from 6 m to about 30 m below ground surface, with an average depth of 12.5 m. Where the upper aquifer is not developed, the aquitard and the top layer may form one unit. The general direction of the ground-water flow is following the Gash river downstream towards the delta. The aquifer exhibit marked seasonal fluctuations (El Amin, 1979), the magnitude of which depends on the distance of the observation well from the Gash river and or whether the observation well is in direct hydraulic connection with the recharging river. Ground-water recovery at the banks of the Gash river amounts to about 10-12 m, this decreases rapidly to about 3 m one kilometre away, and to only 1 m two kilometres away (T.N.O., 1982). Transmissivity values ranged from 100-690 m 2 day -! (El Amin, 1979), which is low in comparison to other alluvial aquifers (Salama, 1971). The storativity ranged from 10 -2 to 10 -3.
The evolution of the ground-water flow systems Based on hydrogeological and hydrochemical data from the Sudd basin the general direction of ground-water movement is from the basin boundaries towards the central part (Salama and Salama, 1974). The water levels of the Sudd basin forms a closed trough, isolevel 300 m is the lowest level in the trough and the lowest reduced water level in all the rift basins. In Bara basin, detailed water level maps (Rhodis et al., 1964; Hunting, 1970; Salama, 1976; Ali, 1978) shows that ground-water movement is from the west to the southeast with a hydraulic gradient of 0.00053 (Salama, 1976). In Umm Ruwaba basin ground-water movement is from the east to the west in the northern area, and from the west to the SE in the southern part. It also shows two closed ground-water troughs at the
R.B. SALAMA northeastern and southem ends, the northern one at 350 m a.m.s.1, and the southern one at 330 m a.m.s.1 (Salama, 1976). In the White Nile basin the ground-water flow is completely reversed, in contrast to the direction of ftow in the northern area, the general direction of ground-water flow is from the west to the east with a major trough south of Kosti and another trough in W. Adar. The general ground-water movement in River Atabara basin is from the SW to the NE. There is a big trough at the northeastern part of the basin (Salama, 1976). A large ground-water trough exists in the central part of the Gezira basin (Abdel Salam, 1966; Salama, 1976). Ground-water flow in large sedimentary basins is controlled by four main processes (Verweij, 1993): (1) sedimentation in a subsiding sedimentary basin; (2) introduction of heat into a basin; (3) tectonic processes acting on a basin, and (4) infiltration of meteoric water in a sub-aerial basin. These four processes were at one stage or another working separately or together in the rift basins of Sudan. The rifting phases which formed the Sudanese Rift System caused the formation of deep basins. Due to the continuous subsidence in the basins, together with continuous recharge of meteoric water which was taking place at the aquifer boundaries (rift basins boundaries), caused the formation of a deep ground-water trough in each one of these basins. The general direction of ground-water flow in all these basins is from the basin boundaries toward the troughs which in all cases were occupying a distal end of a river system. Although no ground-water discharge is taking place at these troughs in the present time, it is logical to assume that in wet pluvial periods (which are well recorded) that these troughs would act as ground-water discharge areas. E1 Boushi and Abdel Salam (1982), in their discussion of the troughs of the Khartoum and the Gezira basins, mentioned that: " Those two troughs represent windows of replenishment to the Nubian aquifer. If water did not leak to augment the Nubian water, those troughs would have been filled long ago". According to the results of this work it is evident that this trough represent a replica of a palaeohydrologic system, and there is no connection between the Gezira aquifer and the Mesozoic sandstone aquifer. The ground water in the lower Mesozoic aquifer is under pressure, and it can seep upward, but the water from Tertiary sediments cannot move downward against the pressure. This is also reflected in the marked contrast between the water qualities of each aquifer. The existing hydrogeological pattern is a very old phenomenon, which has been established since
RIFT BASINS OF THE SUDAN the inception of the rift basins. It has been slightly modified after the filling of the sedimentary troughs, and subsequent recharge during the wet periods take place at a slow rate, separated by periods of no recharge, during the dry periods. The continuous subsidence of the basins, and the fact that the areas where the troughs are existing are the deepest parts of the basins, which coincide with the ground-water trough, proves that the ground water is a replica of the surface water pattern that was existing at the time of formation. Ground-water resources Several research workers studied the alluvial basins, as the collection of data from the existing hand-dug wells was less expensive, and there were problems of over-withdrawal due to the development around those basins (Iskander, 1967; Salama, 1971; Saeed, 1969; Hussein, 1975). All the other studies carried out by consulting firms, were concentrated in the problem areas, i.e. hard basement rocks. Since 1966, several detailed investigations have been carried out by research workers in some of the groundwater basins (either in part or in whole): Mabrook (1972), Saeed (1974), Mohamed (1975), E1 Tohami (1977). Most of these studies delineated basement boundaries, aquifer characteristics and used analytical methods to estimate safe yield of the aquifer systems. Three basins were studied in detail using groundwater models (Salama, 1985a), they are Baggara basin of Bahr El Arab rift, the Umm Ruwaba basin of the White Nile rift and the three basins of the Blue Nile rift. In all three cases two simulations were carried out. The first one with the present ground-water recharge and discharge rates (including abstraction for domestic and irrigation) and the second one with increased rates of abstraction. Separate sets of simulations were carried out to test certain aquifers for heavy abstraction rates for irrigation purposes and for the new urban sites which depends mainly on ground water (i.e. Daein, Babanusa, Muglad (Baggara basin), Bara, Umm Ruwaba, Tendelti (Bara and Umm Ruwaba basins)) and the heavy abstraction for irrigation in Khartoum Province. The important conclusions from this study are: (1) The ground-water resources of these basins are quite adequate to sustain the water requirements of the expanding rural and urban centres. In nearly all cases it was found that better well design, better well completion and deeper wells to tap high yielding aquifer layers are required. (2) That abstraction rates can be safely increased ten folds and in some cases 50-fold without causing extensive drawdown in the area. (3) On the other hand it was found that using
135 these aquifers for irrigation purposes, will cause heavy drawdowns. In Bara basin where recent recharge is decreasing due to the long trends decrease in rainfall, it is highly recommended that irrigation developments be phased out. In Khartoum province, all the recent studies which excludes the Blue Nile as a substantial source of recharge as previously claimed, show that the heavy pumping will adversely affect the water levels. (4) In the alluvial aquifers (W. Nyala, K. E1 Gash, Arbaat, Tokar) which supply domestic and irrigation water for these urban centres, the studies show that the ground-water resource is limited and the expansion of these centres has to be controlled. Petroleum resources of the Sudanese Rift System and the role of ground water in its migration and accumulation Petroleum discovery and resources In 1975, Chevron Overseas Petroleum Inc. started a major petroleum exploration operation in the west and southern part of Sudan. During their 12 year operation in Sudan they acquired vast amount of geological and geophysical data. These included extensive aeromagnetic and gravity survey, 58,000 km of seismic data and drilled 86 wells (Schull, 1988). These extensive detailed investigations shed more light on the history and development of the rift basins of Sudan. The productive and prospective structures resulting from extensional movement and the compressional forces created within resulted in a complex structures created by rotated fault blocks, drape folds, and reverse drag folds. These structures created producing oil traps. The reservoir rock range from quartz arenites and wackestones to arkosic arenites and wackestones. These include sandstones deposited in fluvial channel, lacustrine delta-plain-distributary channel, and delta front environment. Schull (1988) summed up the characteristics of the reservoirs from data compiled from 30 cored wells; reservoir quality decreases with increasing depth (due to compaction, quartz overgrowth), with decreasing grain size and with increasing amounts of feldspars and lithic grains. Petroleum was discovered in the three explored rift systems; Bahr El Arab rift, in the White Nile rift and in the Blue Nile rift. The first oil was recovered from a well in the Muglad basin, the first significant oil flow occurred in Abu Gabra basin and the first important discovery was made in the Unity basin (Anon, 1981a, b, 1982; Schull, 1988). Oil was discovered in Abu Gabra Formation Cretaceous sands in the Muglad basin, in the Bentiu Formation in Babanusa basin, in the sand deposits of Darfur Group in the Unity basin. In the Tertiary oil was discovered in the sands of the Areal Formation.
136 The lacustrine claystones deposited in suboxic environment provide good oil-prone source rock. The depositional environment of these claystones and shales are within large lakes distal from the primary elastic influx. The organic material deposited in the lake was preserved in the suboxic conditions. The primary sources are degraded algal and plant material. The lithological description of the sedimentary sequence of the Sudanese rift basins indicate that it has been deposited in shallow lacustrine environment, occupied by intermittent swamps, lakes, surrounded by continental edge delivering terrestrial organic material which is periodically exposed to subaerial degradation and water flooding. These conditions usually produce high wax crude oil, which is the case for Sudan oil. In the shallow parts of the recharge area, infiltrating meteoric water flush hydrostatic trapping positions, on the other hand the deeper parts of a recharge area are comparatively favourable for the entrapment of hydrocarbons. Lateral hydrocarbon migration towards discharge areas enhances the volumes of hydrocarbons available for entrapment in these areas. Biodegradation and water washing through continuous discharge resulted in an increase in the density of the residual hydrocarbons. Total organic carbon content of the source rock of Sudan averages 1.3%. The generated oils are paraffinic, low sulfur, high pour point. The recoverable reserve in Unity and Heglig areas has been estimated to be 250,300 million bbl (Schull, 1988). Several other wells have recovered significant amounts of oil during stem tests. Flow rates as high as 4000 BOPD on a 5 cm choke have been measured. All oils have low gas/oil ratios and high pour points (Schull, 1988).
Hydrocarbon migration and accumulation through ground-water flow in the rift basins Three major mechanisms control the primary hydrocarbon migration (Verweij, 1993): (!) primary migration of continuous separate phase hydrocarbons driven by hydrocarbon potential gradients; (2) ground-water driven primary migration of hydrocarbons in aqueous solution, and (3) diffusion-driven primary migration of hydrocarbon in aqueous solution, and diffusion-driven primary migration of hydrocarbons through organic matter network. On the other hand secondary phase hydrocarbon migration is driven by hydrocarbon potential gradients which in turn are controlled by: magnitude and direction of the force of gravity, densities of the hydrocarbon and the ground water, magnitude and direction of the net driving force for ground-water flow and magnitude and direction of the capillary pressure gradient. The actual rate of secondary hy-
R.B. SALAMA drocarbon migration is controlled by the hydrocarbon potential gradient, the density and viscosity of the hydrocarbons and the effective permeability of rocks to hydrocarbons. On a basin wide scale, the pattern of secondary migration is determined by (Verweij, 1993): The hydrogeological frame work of the basin; the hydrodynamic condition of the basin and the associated ground-water flow systems in the basin; the density differences between the hydrocarbons and water. Under hydrostatic conditions, the hydrocarbons will become trapped in the reservoir rock when buoyancy-induced lateral upward hydrocarbon migration in the carrier-reservoir rock is stopped by a capillary pressure boundary. Hydrostatic trapping positions include structural traps, stratigraphic traps and combination traps. Hydrodynamic conditions affect the sealing capacity of a rock or a fault and consequently influence the holding capacity of hydrostatic traps. Vertically downward ground-water flow may increase the resistant force to hydrocarbon movement of certain layers and make them impermeable to hydrocarbons, creating hydrodynamic trapping possibilities. This does not seem to be the case in Abu Gabra trough where hydrocarbon signatures have been noticed in the shallow top 1000 m. Which coincide with a relatively high salinity zone in the aquifer and from the ground-water flow direction a possible discharge area. The general pattern of ground-water flow systems which controlled hydrocarbon migration and accumulation in the rift basins can be summarised in: (a) Several studies have been published showing the relation between gravity induced ground-water flow and hydrocarbon accumulation (Toth, 1980; Toth and Corbet, 1986; Toth and Otto, 1989). The results of the study suggest that long distance lateral migration of hydrocarbons can be explained by a basin-wide gravity-induced ground-water flow focusing ground-water and hydrocarbons into laterally continuous hydrogeological units and provide an additional driving force to transport the hydrocarbons laterally across the basin. The relationship between the location of known hydrocarbon accumulations and the regional hydrodynamic condition has been identified by several authors (Toth, 1980; Toth and Otto, 1990). Favourable regions for accumulation and entrapment of hydrocarbons are created by the combined influence of buoyancy forces, net driving forces for ground-water flow and capillary forces. Effective recharge in the Sudanese Rift basins is taking place mainly along the basin boundaries, while discharge is most probably taking place in the trough areas of the closed basin, although there is no contemporary evidence of discharge. The lithologi-
RIFT BASINS OF THE SUDAN cal and chemical evidence indicate that ground-water discharge was taking place at these areas at different time during the pluvial periods. (b) Tectonic processes may also influence both the ground-water pressure condition and the hydrogeological framework in a basin. These forces might lead to the escape of ground-water and hydrocarbons from such tectonically geopressured zones vertically upward along fractures and active faults. Oil contaminated ground-water has been noticed in several wells in the Abu Gabra basin, with some oil showing during shallow drilling for ground-water in Abu Gabra trough. This area is also associated with a fault system which generated high temperature ground water along the fault area. (c) Generally, the frequency of hydrocarbon accumulations was observed to increase and be maximum in areas of ground-water discharge and associated stagnant zones (Toth, 1980). In the stagnant zone, ground-water flow is negligible and the separate phase hydrocarbons introduced into these zones, will be moved by vertically upward directed buoyancy forces alone. In discharge areas the ground-water flow is vertically upwards and consequently the net driving force for separate phase hydrocarbons is directed vertically upwards. This will eventually lead to the entrapment of hydrocarbons in available hydrostatic traps of sufficient sealing capacity. Or even in the absence of these traps hydrocarbons may be trapped by the opposite lateral hydrodynamic forces (Verweij, 1993). The gravity induced ground-water flow conditions enhances lateral migration of hydrocarbons towards ground-water discharge areas. In case of Sudan basins where each rift system is characterised by the presence of ground-water troughs which indicate a closed hydrogeological basin and possible ground-water discharge areas. These sites happens to be the best oil producing areas in the rift basins. Buried saline lakes of the Sudanese Rift System Salama (1985a, b, 1987, 1990, 1994) presented evidence for the presence of highly saline groundwater bodies that occupy the flowing end of each of the rift systems. These have been interpreted as buried saline lakes, sabkhas or playas (Fig. 19). The widespread presence of calcrete, kanker and other carbonate deposits, over, and in, most of the Tertiary deposits, showed that conditions were favourable for the deposition of carbonates. It is postulated that the shallow standing waters evaporated forming salt crusts. In the next flood, (fresh water) dissolved the most soluble salts; (NaC1 and NazSO4), and transported these towards the deepest part of the basin, leaving carbonates behind in the form of kanker nodules. This explains the wide
137 distribution of kanker nodules over the upper Tertiary horizons, and the concentration of the sodium, chloride and the sulphates in the saline zones. At the same time, in the deepest part of the graben which was always a lake, playa or sabkha, the lake water evaporated, and became increasingly saline. During dry arid periods the lakes were completely or partly evaporated thus creating layers of salts, which were later dissolved by ground water to form saline ground-water bodies. Lake Sudd Salama (1987) showed that a series of fresh water lakes exists at the edges of Bahr E1 Arab; Lake Keilak, Lake Abyad and Lake Kundi, together with the main Sudd lake which covers most of the central part of this river system. Based on hydrological data he concluded that if the Victoria Nile was not connected to the White Nile the Sudd would be a closed lake system. Independent evidence showing that the White Nile was not connected to the main Nile prior to 12,500 yr BP was presented by Shukri (1949) and Hassan (1975), based on mineralogical analysis of the Nile deposits and by Kendall (1969), Livingstone (1980) and Adamson and Williams (1980). All this evidence indicates that the Sudd depression of southern Sudan was a closed lake system (Fig. 20). Willcocks (1904) postulated that the ancient lake was 250 miles in length from north to south and that the Blue Nile flowed southwards to join this lake. Lawson (1927) elaborated the Lake Sudd hypothesis and was first to call it by this name. Ball (1939) developed the Lake Sudd hypothesis further, assigned a length of over 655 miles to the lake. Ball (1939) made calculations similar to those of Lawson (1927) and concluded that for a lake of these dimensions, an average evaporation rate of 3 mm day -! over the lake would be sufficient to dispose of all rain and river water entering the lake, since the average annual rate of evaporation from open water surfaces at Mongalla, Malakal and Khartoum are 3.0, 4.5 and 7.5 mm respectively (Hurst and Phillips, 1931). Berry and Whiteman (1968) and Whiteman (1971) went to great lengths to prove that Lawson and Ball were wrong. Salama (1987) agreed that the lake may never have achieved the size assumed by the pioneering workers, yet there is enough evidence to show that there was always a water body in the Sudd region (Berry, 1962; Salama, 1987). However, the lake was not formed by a dammed up Nile. It was formed in a closed basin caused by the block subsidence in Bahr E1 Arab rift (Salama, 1985a, b). The size fluctuated according to the palaeoclimatological events prevailing at the time. Salama (1987), using saturation indices of minerals and salinity parameters, showed that a saline
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FORE-ARC/SUBDUCTION AREAS Late Palaeozoic fore-arc basin (Gust eta/., 1985)
!" 9" "] (?)Carboniferous-Triassic forearc basin (Smellie, 1981)
ill Ill Carb~176 (-Permian?) subduction zone (Visser, 1987) ~,\ ~ j (?)Carboniferous-Triassic subduction complex (Smellie, 1981)
Late Palaeozoic_&_. Mesozoic subduction site (De Wit et al., 1988) + + + Microplate boundary Gondwanide structural trend
EMFB EIIsworth Mountains Fold Belt PMFB Pensacola Mountains Fold Belt Fig. 32. Gondwana reconstruction (after De Wit et al., 1988), showing the probable location of the magmatic arc, Gondwanide Orogen and other tectonic elements. Modified after Johnson (1991).
o -.4
308 in sandstones of the Ecca and Beaufort Groups in the southern Karoo Basin (Elliot and Watts, 1974; Martini, 1974; Johnson, 1976, 1991) may have been derived from this arc. However, the distance separating the postulated arc from the Karoo Trough (~1700 km) represents a problem, and it may be necessary to consider the possibility of an originally closer arc source located along the northern edge of the Falkland/Malvinas Plateau (Ramos, 1986; Johnson, 1991; Collinson et al., 1992; Fig. 32). The progressive northeastward decrease across the Karoo Basin in thickness and frequency of tufts in the Ecca Group led Viljoen (1987) to suggest a source in Patagonia, where Permian volcanic centres presently crop out. Mitchell and Reading (1986) point out that as the basin deepens many successions pass from a shallow water (pre-orogenic) phase through a starved basin (pre-flysch) phase to a deep marine clastic (flysch) phase and finally to a continental clastic (molasse) phase. The Cape-Karoo succession fits this classical geosynclinal cycle rather well, although a major glacial episode is here interposed between the preorogenic stage (Cape Supergroup) and pre-flysch stage (Prince Albert-Collingham Formations). Dwyka glaciers and ice sheets The Dwyka basin was located within the polar circle since the end of the Devonian (Smith et al., 1981). Decreases in temperature during the mid-Carboniferous caused the global Namurian regression (Veevers and Powell, 1987) as well as the build-up of an extensive ice cover over the southern mountain chain (fringing the palaeo-Pacific margin) and the cratonic highlands to the north and east. The onset of the major glacial depositional phase at about 300 Ma reflects the spreading of a large marine ice sheet across the basin. The areal extent of the basin was at least double its present size, and probably included the present Falkland Islands, situated at the southeastern comer of the Karoo Basin (Mitchell et al., 1986; Fig. 32). At first deposition took place largely from a grounded ice sheet, but the onset of slightly warmer conditions resulted in rain-out debris accumulating from a predominantly floating ice shelf. Further warming led to dissolution of the ice sheet, with glacial deposition confined largely to valleys bordering the cratonic highlands, and inundation of the basin by a shallow body of water into which argillaceous material was transported. Ecca seas and deltas The change-over from primarily glacial diamicton to offshore mud sedimentation (Prince Albert
M.R. JOHNSON et al. Formation) in the Permian is attributed to a climatic amelioration in the region and a major regional transgression. These resulted in a marine basin with a drowned northern margin. The basin, which was located between palaeolatitudes 50 ~ and 70 ~ (Smith et al., 1981), was greatly influenced by the influx of sediment-laden cold meltwaters from the glaciers on the highlands. Outwash fans formed where streams debouched into the basin, underflows carried mud and silt basinwards to settle as a blanket deposit on the bottom while possible mud fans built up along bottom depressions in the western and eastern sectors of the basin. Upwelling of cold water in certain areas enhanced a rich marine life. The abrupt change in depositional conditions between the Prince Albert and Whitehill Formations can be attributed to shallowing of the basin and termination of oceanic circulation which led to stratification of the water and reducing conditions during Whitehill time. This organic-rich mud facies together with fossils of the reptile Mesosaurus tenuidens are also present in the Warmbad, western Kalahari and Parana Basins which suggests that the areal extent of the interconnected basins may have been as large as 4.5 x 106 km 2. By the end of the Artinskian (~270 Ma) all the ice on the highlands had melted and a lush vegetation developed in the region which would have lowered the effluent as well as sediment supply to the basin. In the south the accumulation of organic-rich muds under low-energy conditions (Whitehill Formation) was followed by deposition of numerous thin air-fall tufts which periodically interrupted the suspension settling of mud (Collingham Formation). The sudden influx of coarser detritus from a provenance situated south of the present basin which followed this distal volcanic episode marked the transition from "pre-flysch" to flysch stages. Rapid downwarping of the Karoo Trough accompanied the build-up of sandy and silty submarine fans and basin plain turbidites of the Ripon, Laingsburg, Vischkuil and Skoorsteenberg Formations, which were deposited in a water body which according to Visser and Loock (1978) was up to 500 m deep. The turbidity processes in turn gave way to suspension settling of rhythmically bedded pro-delta mud (Fort Brown Formation) as the delta slope prograded across the turbidite fans at its foot. Turbidite deposition did not, however, extend very far into the basin and to the north of the Karoo Trough suspension settling of mud (Tierberg Formation) took place in relatively shallow water. In the northeastern part of the basin the Ecca sea was initially starved of sediment due to glacial cover in the source areas. As the climate warmed, detritus was carried mainly from a granitic highland situated towards the northeast and to a lesser extent
THE FORELAND KAROO BASIN, SOUTH AFRICA from a relatively low-lying quartzitic provenance to the north (Witwatersrand a r c h Fig. 1) and deposited as a prominent fluvio-deltaic wedge (Vryheid Formation). The presence of prominent coal seams in the Vryheid Formation points to luxuriant plant growth and also indicates that the organic-rich muds of the Whitehill Formation are probably a distal equivalent of this formation. Mud carried in suspension from the deltas was distributed further basinward and deposited as a thick blanket during the advances of the coastline (Pietermaritzburg Formation) and also during its retreat (Volksrust Formation). During this period several major transgressions caused by epeirogenic subsidence and/or sea level rises interrupted delta progradation, while shoreline sands formed in places. The climate was cold throughout deposition of the sequence. In the south the transition from flysch to molasse is denoted by the progradation of delta front sands (Waterford Formation) across pro-delta Fort Brown Formation muds. In the west and northwest deltaic sediments advanced more or less simultaneously into the Tierberg sea. Parts of these units may, however, represent coastline rather than deltaic deposits.
Beaufort alluvial plains Progradation of sandstone-rich delta front and lower delta plain sediments into the Ecca sea was followed by the subaerial deposition of upper delta plain and fluvial mud and sand of the Adelaide Subgroup during the Late Permian. Gradual denudation of the provenance caused a sourceward shift of the mixed-load fluvial deposits that were replaced by flood basin and lacustrine muds in the western part of the basin (Turner, 1985). In the northeast the Estcourt Formation provides evidence that deltaic conditions persisted locally for some time after they had ceased elsewhere. Palaeocurrent studies indicate that while the main source areas were located south and southeast of the basin, provenances to the west, north and east (Theron, 1975; cf. Fig. 24) also supplied sediments during the Late Permian. An intrabasinal provenance, the Clocolan Dome, may have been the source of localised coarse fluvial sands and granulestones prior to its burial by younger sediments (Theron, 1970). Strong uplift associated with the major Cape Fold Belt orogeny along the southern margin of the basin at the beginning of the Triassic led to the influx of medium-grained, pebbly, bed-load fluvial sandstones of the Katberg Formation. These sediments extended right across the basin, but with denudation of the provenance a sourceward shift of facies occurred, resulting in the overstep of bed-load fluvial deposits by mixed-load and flood basin deposits of
309 the Burgersdorp Formation (Hiller and Stavrakis, 1984). The outline of the basin may have almost coincided with its present limits apart from an extension towards the northwest as shown by Beaufort Group xenoliths in a kimberlite pipe from the Finsch Mine, 140 km west-northwest of Kimberley (Visser, 1972).
Post-Beaufort floodplains and deserts A Middle Triassic hiatus which increases northwards (Turner, 1983) was followed by a Late Triassic cycle consisting of a northward-thinning bedload-dominated fluvial wedge (Molteno Formation). This cycle is linked by H~ilbich (1983) with intensification of Cape Fold Belt tectonism dated at 229 4- 5 Ma, but a closer provenance than for the previous cycle is implied by the presence of Cape Supergroup quartzite pebbles and boulders. Turner (1983) attributed northward-tapering wedges in the Molteno Formation to phases of fault-controlled uplift of provenances along the southeastern basin margin. Denudation of the provenance again led to a sourceward shift of the facies with overstepping of the bedload fluvial deposits by mixed-load fluvial and flood basin/lacustrine deposits of the Elliot Formation. These deposits are overlain by the Late Triassic-Early Jurassic Clarens Formation consisting largely of aeolian fine-grained sands derived from a western source (Beukes, 1970). A progressive increase in aridity is evident in the MoltenoElliot-Clarens depositional sequence (Visser, 1991 ).
Igneous events and Gondwana break-up The main Karoo Basin was completely filled during the Jurassic with the outpouring of at least 1400 m of basaltic lavas (Drakensberg Group). The initial break-up of the southern African component of Gondwana commenced during the Middle Jurassic with the formation of rift-associated sedimentary basins around the continental margins of southern Africa and within the Cape Fold Belt (Dingle et al., 1983). The Falkland Islands also appear to have rotated 120 degrees as they moved from their position off the southeast coast of South Africa (Fig. 32) to a site approximately 500 km southeast of Cape Town (Mitchell et al., 1986). The main breakup probably commenced during the Early Cretaceous (Larson and Ladd, 1973) with the opening up of the Atlantic Ocean, lateral movement along the Agulhas/ Falkland Fracture Zone including a further rotation of 60 degrees of the Falkland Islands microplate (Mitchell et al., 1986) and separation of East Antarctica from southern Africa (Dingle et al., 1983).
310 ECONOMIC RESOURCES
Coal The main Karoo Basin is host to the major coal resources of South Africa, though significant deposits are also present in the minor, contemporaneous basins to the north. Coal is developed in both the Permian Vryheid Formation and the mid-Triassic Molteno Formation. The extent of the economic coal deposits is illustrated in Fig. 33 together with the arbitrarily defined boundaries of the various coalfields. Coal seams throughout the basin are virtually horizontal. The only significant disturbances are those associated with dolerite sills and dykes which not only displace and replace the strata but also devolatilise the coal. Distribution and thickness of coal seams in those coalfields peripheral to the basin (for example, the Springs-Witbank coalfield) are controlled by pre-Karoo and Dwyka glaciation topographic features and, to a lesser extent, by sedimentological factors. Coal seams more distally situated were influenced to some degree by basin-floor topography and tectonic events but sedimentological criteria, such as the type of peat environment, local rates of subsidence, and timing of marine transgressions and fluvial clastic influxes exerted much stronger controls (Fig. 34). The wide range of depositional settings within which peats accumulated, combined with variations in climate and plant communities as well as Jurassic dolerite intrusions, impart to the coals significant differences in grade, type and rank. These differences have important practical implications with respect to beneficiation methods and utilisation in the metallurgical, synthetic fuels and power (steam) generation processes. In general, coals of the Karoo Basin are more variable in type and contain a much higher inertic and transitional-reactive inertic component than those of Europe and the USA (Falcon, 1986). With total recoverable Karoo Supergroup coal reserves of 55 333 Mt (in situ resources of 121 218 Mt), of which 37 625 Mt are present in the main Karoo Basin (Bredell, 1987), South Africa ranks fifth in the world. Although total saleable (beneficiated) reserves within the basin are in the order of 29,000 Mt, a large component (77%) of this is lowgrade (< 25.5 Mj/kg) bituminous coal. High-grade (noncoking) bituminous reserves (12%), which contributed the bulk of the 50 Mt of coal exported in 1992 (Tinney, 1993) are mainly the product of coal seams from which prime and middlings products are prepared. Of the approximately 174 Mt of coal produced during 1992 (Tinney, 1993), noncoking bituminous coal constituted about 94% of the total. Coking coal and anthracite together account for approximately 4% and 1.5% of saleable reserves re-
M.R. JOHNSON et al. spectively but, with the introduction of direct reduction processes and declining demand for anthracite (especially in the export market), these reserves appear to be adequate in the short to medium term. The Molteno Formation, although providing most of South Africa's coal between 1900 and 1904, is no longer productive. An extensive investigation into the feasibility of exploiting a torbanite deposit (developed within the No. 5 seam in the Highveld coalfield) with a view to establishing a retorting plant to produce shale oil was undertaken recently. Oil and gas
Numerous oil shows are known in the northern part of the Karoo Basin but only two small uneconomic accumulations have been found. It was established early in the exploration for oil during the 1960s and early 1970s that only the rocks north of latitude 28~ were still in a diagenetic stage consistent with the generation and preservation of oil, except where metamorphosed by dolerite intrusions (Rowsell and De Swardt, 1976). The source rock potential of the Pietermaritzburg Formation was rated as fairly good near its top, but otherwise poor, that of the Vryheid Formation as fair to good and that of the Volksrust as mostly poor and lignitic. Because the best oil shows were encountered in the upper part of the Vryheid, this formation was probably the source. The volume of shale in the Vryheid is, however, too insignificant to be an important source. The primary porosity and permeability of the Vryheid are in general poor, although leaching has improved the quality considerably in places towards the north. Widespread intrusion by dolerites probably led to large-scale conversion of oil into gas and some escape along fractures. The Ecca shales qualify as fairly good gas source rocks but only two small uneconomic gas fields have been discovered during exploration for other mineral deposits in the northern part of the Karoo Basin. Uranium and molybdenum
Uranium occurrences, which are presently subeconomic, are located in the western and central parts of the Karoo Basin within the Adelaide Subgroup and Molteno and Elliot Formations (Cole and Labuschagne, 1985; Le Roux and Toens, 1986). The occurrences are epigenetic, tabular and sandstone-hosted, forming discrete pods and lenses less than 10,000 m 3 in volume. The sandstones represent fluvial channel deposits and are interbedded with mudrock of flood basin and/or lacustrine origin. The thickest sandstone bodies (up to 60 m thick) contain the highest proportion of mineralisation. In the Adelaide Subgroup the sandstone bodies cluster
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into packages (members), with the Moordenaars and Poortjie Members, in the middle of the subgroup, containing approximately 50% of the uranium occurrences.
The dominant uranium is commonly associated senopyrite, chalcopyrite, uraninite may also occur.
mineral is coffinite, which with calcite, pyrite, arbornite and chalcocite; In the Adelaide Subgroup
312
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in the southwestern part of the basin molybdenum, in the form of molybdenite and jordisite, is present (Cole and Wipplinger, 1991). Here, a few occurrences exceed one million tons of ore but the possibility of exploiting the uranium and recovering molybdenum as a by-product is mitigated against by the marginal grades, which average less than
1500 ppm U and 800 ppm Mo. Further negative factors are the presence of calcite and clay minerals, which would cause the reagent consumption to be high in an acid leach, and the presence of sulphides, which would cause a high reagent consumption in an alkaline leach (Le Roux and Toens, 1986).
T H E F O R E L A N D K A R O O BASIN, S O U T H A F R I C A REFERENCES
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M.R. J O H N S O N et al. Van Vuuren, C.J., 1972. Geological well completion report of the Swartberg (SW1/67) borehole. Rep. Southern Oil Explor. Corp. (unpublished). Van Vuuren, C.J., 1981. Depositional models for the Vryheid Formation in the northeastern part of the Karoo Basin - - a review. Ann. Geol. Surv. S. Afr., 15: 1-11. Van Vuuren, C.J., 1983. A basin analysis of the northern facies of the Ecca Group. Ph.D. Thesis, Univ. Orange Free State, Bloemfontein, 249 pp. (unpublished). Van Vuuren, C.J. and Cole, D.I., 1979. The stratigraphy and depositional environments of the Ecca Group in the northern part of the Karoo basin. In: A.M. Anderson and W.J. van Biljon (Editors), Some Sedimentary Basins and Associated Ore Deposits of South Africa. Spec. Publ. Geol. Soc. S. Afr., 6:103-111. Veevers, J.J. and Powell, C.M., 1987. Late Paleozoic glacial episodes in Gondwanaland reflected in transgressive-regressive depositional sequences in Euramerica. Bull. Geol. Soc. Am., 98: 475-487. Viljoen, J.H.A., 1987. Subaqueous fallout tuffs of the Ecca Group in the southern Cape Province. In: G. Brown and V.A. Preston (Compilers), Workshop on Pyroclastic Volcanism and Associated Deposits. Dept. Geol. Miner., Univ. Natal, Pietermaritzburg, pp. 45-48. Viljoen, J.H.A., 1992a. Lithostratigraphy of the Collingham Formation (Ecca Group), including the Zoute Kloof, Buffels River and Wilgehout River Members and the Matjiesfontein Chert Bed. Lithostrat. Ser. S. Afr. Comm. Strat., 22. Viljoen, J.H.A., 1992b. Lithostratigraphy of the Laingsburg Formation (Ecca Group). Lithostrat. Ser. S. Afr. Comm. Strat., 20. Viljoen, J.H.A. and Wickens, H. de V., 1992. Lithostratigraphy of the Vischkuil Formation (Ecca Group). Lithostrat. Ser. S. Afr. Comm. Strat., 19. Visser, J.N.J., 1972. Sediment6re insluitsels van Karoo-ouderdom in kimberliet van die Finsch-diamantmyn. Tydskr. Natuurwet., 12: 32-36. Visser, J.N.J., 1982. Upper Carboniferous glacial sedimentation in the Karoo Basin near Prieska, South Africa. Palaeogeogr., Palaeoclim., Palaeoecol., 38: 63-92. Visser, J.N.J., 1983. Glacial-marine sedimentation in the Late Paleozoic Karoo Basin, Southern Africa. In: B.F. Molnia (Editor), Glacial-Marine Sedimentation. Plenum Publishing Corporation, New York, pp. 667-701. Visser, J.N.J., 1986. Lateral lithofacies relationships in the glacigene Dwyka Formation in the western and central parts of the Karoo Basin. Trans. Geol. Soc. S. Afr., 89: 373-383. Visser, J.N.J., 1987. The palaeogeography of part of southwestern Gondwana during the Permo-Carboniferous glaciation: Palaeogeogr., Palaeoclimatol., Palaeoecol., 61:205-219. Visser, J.N.J., 1989. The Permo-Carboniferous Dwyka Formation of Southern Africa: deposition by a predominantly subpolar marine ice sheet. Palaeogeogr., Palaeoclimatol., Palaeoecol., 70: 377-391. Visser, J.N.J., 1991. Geography and climatology of the Late Carboniferous to Jurassic Karoo Basin in south-western Gondwana. Ann. S. Afr. Mus., 99(12): 415-431. Visser, J.N.J., 1992. Deposition of the Early to Late Permian Whitehill Formation during a sea-level highstand in a juvenile foreland basin. S. Afr. J. Geol., 95:181-193. Visser, J.N.J. and Botha, B.J.V., 1980. Meander channel, pointbar, crevasse splay and aeolian deposits from the Elliot Formation in Barkly Pass, northeastern Cape. Trans. Geol. Soc. S. Afr., 83: 55-62. Visser, J.N.J. and Loock, J.C., 1978. Water depth in the main Karoo basin, South Africa, during Ecca (Permian) sedimentation. Trans. Geol. Soc. S. Afr., 81: 185-191. Visser, J.N.J., Loock, J.C. and Jordaan, M.J., 1980. Permian
T H E F O R E L A N D K A R O O BASIN, S O U T H A F R I C A deltaic sedimentation in the western half of the Karoo Basin. Trans. Geol. Soc. S. Afr., 83: 415-424. Visser, J.N.J., Loock, J.C. and Colliston, W.P., 1987. Subaqueous outwash fan and esker sandstones in the Permo-Carboniferous Dwyka Formation of South Africa. J. Sed. Petrol., 57: 467478. Von Brunn, V., 1981. Sedimentary facies related to Late Paleozoic (Dwyka) deglaciation in the eastern Karroo Basin, South Africa. In: M.M. Creswell and P. Vella (Editors), Gondwana Five. A.A. Balkema, Rotterdam, pp. 117-123. Vos, R.E. and Hobday, D.K., 1977. Storm beach deposits in the late Palaeozoic Ecca Group of South Africa: Sed. Geol., 19: 217-232. Walker, R.G., 1979. Turbidites and associated coarse clastic deposits. In: R.G. Walker (Editor), Facies Models. Geol. Ass. Canada, Waterloo, pp. 91-107. Wickens, H. de V., 1984. Die stratigrafie en sedimentologie van die Groep Ecca wes van Sutherland. M.Sc. Thesis, Univ. Port Elizabeth, 86 pp. (unpublished). Wickens, H. de V., 1985. Sedimentologiese ondersoek van die
317 Formasies Vischkuil en Laingsburg, Groep Ecca, Laingsburgomgewing. Rep. Geol. Surv. S. Afr. (unpublished). Winter, H. de la R. and Venter, J.J., 1970. Lithostratigraphic correlation of recent deep boreholesin the Karoo-Cape sequence. In: Second Gondwana Symposium: Proceedings and Papers. Counc. Sci. Ind. Res., Pretoria, pp. 395-408. Winter, M.E, 1985. Lower Permian palaeoenvironments of the northern Highveld Coalfield and their relationship to the characteristics of coal seams. Ph.D. Thesis, Univ. Witwatersrand, Johannesburg, 261 pp. (unpublished). Wright, L.D., 1978. River deltas. In: R.A. Davis (Editor), Coastal Sedimentary Environments. Springer-Verlag, New York, pp. 5-68. Zawada, P.K., 1987. The stratigraphy and sedimentology of the Ecca and Beaufort Groups in the Fauresmith area. M.Sc. Thesis, Univ. Witwatersrand, Johannesburg, 192 pp. (unpublished). Zawada, P.K., 1988. The stratigraphy and sedimentology of the Ecca and Beaufort Groups in the Fauresmith area, south-western Orange Free State. Bull. Geol. Soc. S. Afr., 90:48 pp.
C h a p t e r 13
Late Mesozoic Sedimentary Basins Off the South Coast of South Africa
I.K. McMILLAN, G.J. BRINK, D.S. BROAD and J.J. M A I E R
Late Mesozoic sedimentation off the south coast of South Africa records a history of initial continental rifting (?Middle-Late Jurassic to latest Valanginian), followed by a transitional episode (latest Valanginian to Early Aptian) and a drifting episode (Early Aptian to present day), as Africa separated from South America. Rifting appears to have been initiated by separation of East and West Gondwana during the Middle to Late Jurassic. Sediments associated with rifting are now confined to four major basins Bredasdorp, Pletmos, Gamtoos and Algoa, which are underlain and bounded by rocks of the Ordovician-Devonian Cape Supergroup that form prominent arches between the basins. During the rifting phase when half-graben basin styles were typical, sediments accumulated in a wide range of environments (non-marine to slope). In the Bredasdorp Basin, where major bounding faults are less well developed, sediments were laid down in non-marine and marginal marine environments resulting in widespread development of red and green claystones overlain by clean, porous glauconitic littoral sandstones. In contrast, transitional early drift sedimentation, which began after a major regional unconformity (seismic horizon 1Atl) in the latest Valanginian, is characterised by deep-marine, poorly oxygenated conditions. The pre-lA, 13A and 14A sequences are of considerable economic significance for hydrocarbons, particularly in the Bredasdorp Basin where commercial gas and condensate production began in 1992. Late drift sedimentation since Late Aptian has occurred in generally well-oxygenated environments, and has led to the steady southwards development of the continental shelf and the formation of an elongate basin parallel to the relict shelf break. This basin, the Outeniqua Basin, is composed of essentially mid-Aptian to Maastrichtian deposits, and overlies the pre-existing rift basins with a transverse structural grain.
INTRODUCTION
Regional setting The locations of the offshore basins, and boreholes drilled to date (January 1992), on the southern South African continental margin are shown in Fig. 1. The basins lie at the southernmost end of the African continent, where the plate margin was sheared by fight-lateral movement along the Agulhas-Falkland Fracture Zone: in contrast elsewhere in Southern Africa the margins are extensional pull-apart in style. Four major depocentres, the Bredasdorp, Pletmos, Gamtoos and Algoa basins, with the smaller Infanta Embayment, formed at the c o m m e n c e m e n t of rifting along the southern margin of the African plate. Cross-sections from west to east across the southern South African margin illustrate the variations in structural style (Fig. 2). All five basins have now been extensively explored via acquisition and interpretation of multi-channel seismic,
deep borehole drilling, and a wide variety of service geology disciplines. The latter include petrography, core analysis, palaeontology (foraminifera, ostracods, dinoflagellates, acritarchs, pollen and spores, as well as some work on nannofossils), geochemistry and vitrinite reflectance and seismic attribute studies. A summary of the number of boreholes drilled, kilometres of seismic line shot and the area of each basin is shown in Table 1. As can be seen, greatest interest has focused on the Bredasdorp Basin. Drilling for hydrocarbons was initiated in 1967 in the late Mesozoic rocks of the onshore part of the Algoa Basin, though one hole in this area dates back to 1908. Offshore drilling c o m m e n c e d in 1968 with the exploration of the Superior High and surrounding region in central Pletmos Basin. In the early 1980s interest concentrated on the gas fairway along the northem flank of the Bredasdorp Basin, where significant gas discoveries led to the Mossgas development project which began gas and
African Basins. Sedimentary Basins of the World, 3 edited by R.C. Selley (Series Editor: K.J. Hsti), pp. 319-376. 9 1997 Elsevier Science B.V., Amsterdam. All rights reserved.
tao to
Fig. 1. Location map of South African southern offshore sedimentary basins. Numbered boreholes are referred to in the text. In part after Broad (1990) and reproduced with permission of the Geological Society of South Africa. 1,,=.I
tt" >. Z
t"
9 N 9 t") t'rl
7~ -] >.
>. o~
9 ,-4 9
9 9
>.
>.
Fig. 2. Schematic profiles across the Bredasdorp, Pletmos, Gamtoos and Algoa basins, South African offshore. to i--,t
322
I.K. McMILLAN et al.
Table 1 Areas, kilometres of seismic coverage and number of wells drilled in the southern offshore basins and their onshore extensions (as of January 1992) Basin
Area, onshore and offshore (km 2)
Seismic line, offshore (kin)
Boreholes
Bredasdorp Pletmos-lnfanta Gamtoos
18,150 21,350 5,038
46,617 23,700 4,272
8,193
5,000
132 37 2 onshore 10 offshore 22 onshore 9 offshore
Algoa
condensate production from the F-A platform in early 1992. Little exploration interest has been centred to date on the distal parts of the Outeniqua Basin (Fig. 1), primarily because of the generally excessive water depths and strong Agulhas Current. Consequently, comparatively little will be found herein on the Southern Outeniqua Basin sensu stricto: a more comprehensive review of its salient features can be found in Dingle et al. (1983). It has long been recognised that the southern offshore basins exhibit features characteristic of rift basins, and more specifically of divergent margin basins (Atlantic-type passive margin basins of Bally and Snelson, 1980) which have been modified by transform movements. Their geological history is discussed in this paper in terms of the following stages of development: rift, transitional-early drift and late drift, which follows the nomenclature used by Edwards and Santogrossi (1990). A similar terminology had been used previously for the adjacent west coast of southern Africa (Gerrard and Smith, 1982). Previous work The vast majority of work undertaken by Soekor on the southern offshore basins remains in internal reports, and what has been published is essentially a synthesis. Published compilations dealing with aspects of the southern offshore geology, stratigraphy, palaeontology and geochemistry, together with relevant onshore data from the southern Cape coast include those of Du Toit (1976, 1979), Winter (1973, 1979), Leith and Rowsell (1979), McLachlan et al. (1976), McLachlan (1977), McLachlan and McMillan (1979), De Swardt and McLachlan (1982), Light et al. (1982) and Marot and McLachlan (1982). A summary of much of this work has been made by Dingle et al. (1983), and additional comments can be found in Tankard et al. (1982). More recently, intensive seismic-stratigraphic analyses of the transitional to late drift interval
(1Atl to horizon K: latest Valanginian to about Santonian) have been undertaken for the Bredasdorp and Pletmos basins (Beamish et al., 1988 and Brink et al., 1994, respectively), and resulted in detailed subdivision of the sedimentary sequences of these two basins. Between the previously identified major unconformities 1Atl, 5Atl/6Atl, 13Atl and 15Atl (see definition of a type 1 unconformity in Van Wagoner et al., 1987), there were found to occur other smaller (higher order) seismic events, each associated with a lowstand sedimentary episode on the upper slope, and separated from each other by a highstand episode. These detailed sequence-stratigraphic studies have led to the compilation of an atlas of seismic stratigraphy which draws examples from the Bredasdorp and Pletmos basins (Brown et al., 1995). Attempts have been made therein to correlate sequences and sequence boundaries with eustatic relative sea-level curves. Seismic stratigraphic terminology used in the present article follows the definitions given by Van Wagoner et al. (1987). Although the original seismic-stratigraphic studies recognised the synchroneity of the major unconformities 1Atl, 5Atl/6Atl, 13Atl and 15Atl between the Pletmos and Bredasdorp basins based on the palaeontological age-datings above and below the breaks, the smaller unconformities were identified independently in each basin, and no correlation is implied between the two basins. Consequently, although 8At l, for example, has chronostratigraphic value in the Bredasdorp Basin, its time relationship with 8Atl of the Pletmos Basin remains to be established. Although seismic and sequence-stratigraphic identification of highstand and lowstand sedimentary packages has resulted in ever-finer stratigraphic subdivisions of the sedimentary infill in the South African offshore basins, confirmatory evidence of sea-level change has often proved ambiguous or inconclusive. Repeated changes in sea-level would be expected to result in sedimentary packages containing a wide variety of deep, medial or shallow water benthonic microfaunas at any one locality. However, from foraminiferal evidence, borehole sections tend to show either little change in depositional environment or a gently shallowing upward trend. Though the relationships of lowstand and highstand sedimentary packages and associated deposits are not in dispute, it is considered here that they reflect changes in rates of continental margin subsidence and rates of sediment input amended by seismic, tectonic and sea-floor erosion processes. The reader is referred to Brown et al. (1995) and Brink et al. (1994) where the eustatic influence on sedimentary packages is emphasised. Details of holes drilled in the southern offshore basins can be found in the American Association
LATE MESOZOIC SEDIMENTARY BASINS OFF THE SOUTH COAST OF SOUTH AFRICA of Petroleum Geologists' annual reviews for central and southern Africa. General structure and history of the southern offshore basins The Bredasdorp, Pletmos, Gamtoos and Algoa Basins have comparable histories, although responses to specific events affecting the four basins are often distinctly different in each case. The oldest datable sediments drilled to date are of Kimmeridgian age (Late Jurassic) in the Gamtoos and Algoa basins, and it is considered that rocks of equivalent age also occur in the depocentres of the other basins. Even older Mesozoic sediments may occur, but lie too deeply buffed to be of economic significance in oil exploration. These earliest deposits commenced accumulation at the initiation of rifting. In southern Africa rifting is regarded as having commenced with the preliminary fracturing of Gondwana into east (Antarctica-Australia-India) and west (South America-Africa) portions. Kimmeridgian or Portlandian sedimentary rocks are known from northern Mozambique (Silva, 1966), Bajocian-Bathonian rocks in the Majunga Basin of northwest Madagascar (Espitali6 and Sigal, 1963) and coastal Tanzania (Quennell et al., 1956), and Toarcian deposits in Somalia (Kent, 1974). These ages give an impression of a seaway opening down the length of the east coast of Africa from Early Jurassic in the north to Late Jurassic in the south (Norton and Sclater, 1979). Generation of the earliest oceanic crust between East and West Gondwana is regarded as Kimmeridgian for the Mozambique Basin (in the present day southern Mozambique Channel), as described by Srgoufin (1978), Norton and Sclater (1979) and Powell et al. (1980). More recently Martin and Hartnady (1986) dated initial separation of Antarctica from the eastern margin of the Falkland Plateau as 145-122 my (M21-M 10). A summary of the plate tectonic setting of the Mesozoic southern offshore basins of South Africa is presented by Fouch6 et al. (1992). Deep Sea Drilling Project results from holes drilled on the eastern flank of the Falkland Plateau (sites 330 and 511) revealed the oldest sediments, unconformably overlying Precambrian granitic and gneissic rocks, to be of Kimmeridgian-Portlandian age, perhaps extending as far back as Oxfordian (Jones and Plafker, 1977; Jeletzky, 1983). Considerable similarities exist between the stratigraphic sequences and lithologies intersected at sites 330 and 511 and boreholes in the deeper parts of the southern Gamtoos Basin. Martin et al. (1981) have also previously commented on the similarity in structure and stratigraphy of the Falkland Plateau and Outeniqua (sensu lato) basins, and they considered
323
that the two constituted a single feature during the Late Jurassic to Early Cretaceous rift-onset to driftonset period. Thus, for the rift period, the Bredasdorp, Pletmos-Infanta, Gamtoos and Algoa basins can be regarded as proximal tongues of the large Falkland Plateau Basin that commenced subsiding during the Kimmeridgian or slightly earlier. Dingle et al. (1983) have regarded the Middle Jurassic date of 162 -+- 7 my derived from a whole-rock K/AR determination of basalt from the Suurberg Group in the northernmost onshore Algoa Basin as indicative of the commencement of basin formation. However, although pre-Kimmeridgian rocks are to be expected at several localities in the offshore Gamtoos and Algoa basins, nowhere onshore in the southern Cape basins can marine rocks be dated older than Portlandian (Colchester Member and its equivalents), and closely associated nonmarine rocks (Enon Conglomerate) are likely to be only slightly older. The relationship of the volcanic rocks of the Suurberg Group with basin formation remains uncertain. Some difficulty exists in interpretation of the drift-onset or break-up unconformity across the southern continental margin of South Africa. The term was first used by Falvey (1974) and has recently been reviewed by Braun and Beaumont (1989) in terms of its relationship to flank uplift. Two major unconformities have been proposed in the past, though they have been somewhat confused in interpretation. McLachlan and McMillan (1979), Du Toit (1979) and De Swardt and McLachlan (1982) all comment on the substantial change in sedimentary and tectonic style seen at the unconformity associated with seismic horizon C (later renamed 1At 1), which was previously dated microfaunally as near the Hauterivian-Barremian boundary. Subsequently it has become clear that the I Atl unconformity is of latest Valanginian age and although representing only a short time break, profound changes in depositional environment occurred. In contrast, in the onshore Algoa Basin although the stratigraphic position of the 1Atl unconformity can be identified microfaunally within the Sundays River Formation it cannot be recognised seismically or with electric logs. In the Pletmos, Gamtoos and Algoa basins, and to a lesser extent in the Bredasdorp Basin, a major unconformity, associated with seismic horizon 6Atl (5Atl in the Bredasdorp), and regarded as latest Hauterivian to earliest Barremian in age, marks the end of half-graben infilling and the commencement of prograding shelf sedimentation. The northern, onshore portions of the Algoa and Gamtoos basins ceased active subsidence and sedimentation at this time. The 6Atl hiatus reflects a phase of profound erosion of the underlying Hauterivian rocks, so much so that in the southern Pletmos, Gamtoos and Algoa,
324 horizons 1Atl and 6Atl combine as a compound unconformity (6Atl = 1At l) often as a result of canyon-cutting. Even in the Bredasdorp Basin where erosion was less severe, there is evidence of canyons at this level. Aspects of this are dealt with more fully later. For the present work, the Late Valanginian 1Atl unconformity is regarded as the drift-onset or break-up unconformity and marks the change from rift to transitional-early drift sedimentation, although this is probably an oversimplification. It may be that the intense erosion and canyon cutting noted at the level of 6Atl in the southern parts of the Pletmos, Gamtoos and Algoa basins reflects their proximity to the uplifting marginal fracture ridge; less severe erosion at this time in the Bredasdorp Basin perhaps accords with the greater distance of this basin from the marginal fracture ridge (Fig. 1). As noted above, latest Valanginian and later sediments accumulated as drifting between South America and Africa was activated. In terms of the southern continental margin of South Africa, this involved the shearing of the greater Falkland PlateauOuteniqua basins along the Agulhas Fracture Zone over a distance of approximately 1300 kilometres. The subsequent history of transverse movement of the Falkland Plateau past the Agulhas Bank has been described in detail by Martin et al. (1981, 1982) and Martin and Hartnady (1986). The latter authors, relying on a revised sequence of plate tectonic reconstructions, suggest that the eastern end of the Falkland Plateau cleared the tip of the South African continental margin in the late Albian. It is not unexpected that these major transform movements had a profound effect on the structural development of southern Africa. The proximity of the Agulhas Fracture Zone to the basins discussed in this paper implies that many structural aspects of these basins may be explained in terms of strike-slip or wrench faulting, particularly in those basins closest to the fracture zone. This subject was first addressed by Du Toit (1976) who observed the progressive clockwise rotation of faults from a roughly easterly orientation in the Bredasdorp Basin to a southerly orientation in the Gamtoos and eastern Pletmos basins near the fracture zone, thus implying that movement along the shear zone was responsible for the southerly curvature of the faults. However, a recent structural study of the Gamtoos and Algoa basins by Cartwright (1989) proposes that the clockwise bending of these faults, basins and arches is more likely to be due to either "tension gash" sinusoidal pullapart movements dating from the early propagating transform, or inheritance of a trend in the underlying Cape Fold Belt, rather than late bending during continental separation (Malan et al., 1990). The Agulhas marginal fracture ridge (Scrutton and Du Plessis, 1973; Ben-Avraham et al, 1993)
I.K. McMILLAN et al. and the contiguous Agulhas Arch (De Swardt and McLachlan, 1982) are regarded as having been uplifted during the drifting episode, due to thermal expansion and phase boundary migration within the lithosphere. In turn this led to crustal thinning by erosion (Falvey, 1974), although this may have been a more complex procedure for the sheared AgulhasFalkland margin than for the pull-apart Atlantic type margins described by Falvey (1974) and Braun and Beaumont (1989). Further discussion of the marginal fracture ridge and its history is provided by Ben-Avraham et al. (1993). However, neither the Agulhas marginal fracture ridge, nor the departing Falkland Plateau appear to have substantially hindered marine incursions into the basins of the South African southern offshore for any length of time, either during the rift, transitional-early drift or late drift phases. The rifted basins were smothered by deep-marine prograding and aggrading sediments during the latest Valanginian to mid-Aptian. Although subsidence remained substantial in the Bredasdorp Basin until Cenomanian times, deposition thereafter resulted in the development and subsequent outbuilding of progressively more linear continental shelf and slope systems. Figure 3 represents a generalised foraminiferal correlation of the southern offshore basins from latest rift times (immediately pre-lAtl) to late drift times (15Atl), in order to provide a biostratigraphic framework of episodes of sedimentation within this critical period (considered both in a tectonic and economic sense) for the three following sections of this article: Bredasdorp, Infanta and Pletmos, and Gamtoos and Algoa basins.
BREDASDORP BASIN
Introduction
The Bredasdorp Basin is defined by basement arches aligned parallel to the structural grain of the orogenic Cape Fold Belt. The bounding Infanta Arch to the northeast and the Agulhas Arch to the southwest define a southeasterly elongate basin approximately 200 kilometres long by 80 kilometres wide and about 18 000 square kilometres in area (Table 1, Fig. 1). Minor onshore extensions occur to the west of Cape Infanta (Dingle et al., 1983, Malan and Viljoen, 1990). Economic basement (horizon D, Fig. 4) attains a maximum depth of about 7 kilometres (4 seconds of two-way time), and has been intersected only along the northern margins and on basement highs flanking both the northeastern and southwestern margins of the basin. With the exception of three wells on the north eastern margin of the basin, intersected
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LATE MESOZOIC SEDIMENTARY BASINS OFF THE SOUTH COAST OF SOUTH AFRICA basement consists of black slates of the Devonian Bokkeveld Group: only close to the Infanta Embayment have quartzites of the Ordovician-Silurian Table Mountain Group been encountered. Initial rifting was characterised by the development of horst and graben tectonics in an extensional stress regime. Active boundary faults largely controlled rift clastic deposition. Thick successions laid down in continental, paralic and shallow-marine environments are interpreted to have developed in response to major marine transgressive and regressive cycles principally induced by syndepositional normal faulting events. Termination of active rift sedimentation is marked by the 1Atl unconformity, which records significant uplift and truncation of underlying deposits along the basin margins during the late Valanginian (Figs. 3 and 5). Features interpreted as caused by inversion tectonics occur at several localities in the Bredasdorp Basin, and at several horizons, particularly in the D to 1Atl interval, the 1Atl to 6Atl interval, and at 14Atl (van der Merwe and Fouch6, 1992). Post-lAtl to 13Atl onlap-fill sequences are associated with both rapid thermal subsidence along the basin axis and episodes of re-activated faulting. They represent transitional tectonic processes subsequent to the onset of drifting. Permanent marine conditions in the central Bredasdorp Basin were established from 1Atl times onward, with the early sediments accumulating under deep marine, poorly oxygenated conditions. The regional 13Atl unconformity in the Early Aptian (Fig. 6) marks the onset of renewed and gradually accelerating subsidence, especially in the central and southern Bredasdorp Basin. Relatively thick progradational sequences are widespread, and reflect the epeirogenic nature of Late Cretaceous sedimentation from 15Atl to horizon L (end of the Cretaceous) in the more distal parts of the basin. Subsidence of the Bredasdorp Basin ended at horizon L, at the end of the Cretaceous, with sediments accumulating very slowly during the Tertiary (McLachlan and McMillan, 1979) under stable shelf conditions. Sporadic seaward tilting caused thickness differences in an otherwise very uniform Tertiary sedimentary veneer. From the Tertiary the basin was a component of the Agulhas Bank continental margin and greater Outeniqua Basin. The Agulhas Arch was no longer a positive feature from about Late Oligocene times. Consequently, Tertiary deposits accumulating over the Bredasdorp Basin region generally reflect well-oxygenated, open ocean regimes, and are composed mainly of biogenic and limey muds, often rich in glauconite. Igneous activity is evident at several stratigraphic levels as indicated by changes in seismic character, and confirmed by drilling and petrographic evidence. Minor tuff layers within the rift succession have
329
been recognised, particularly along the northeastern flank of the Bredasdorp Basin, and Early Tertiary lamprophyres have been encountered in boreholes in the southwestern and southeastern parts of the basin. Seafloor outcropping intrusions of a similar age in the northwestern Bredasdorp Basin are trachytic and have been described by Dingle and Gentle (1972). Basin evolution Rift tectonics and sedimentation (D to 1Atl) Widespread normal faults and complex horsts and grabens are seen on seismic sections across the basin (Figs. 7, 8 and 9), indicating that the basin was subjected to an extensional stress regime during the rifting phase. Maximum known throw on the Arniston Fault which defines a major half-graben in the extreme northwest of the basin, is approximately 3850 metres, which is comparable to the throw on the Superior and Pletmos faults in the Pletmos Basin. Other large half grabens are present elsewhere within the Bredasdorp Basin but are less clearly delineated except locally in the Mossel Bay gasfields area. Detailed lithostratigraphic study of rift sedimentation along the northeastern flank of the basin indicates that differential subsidence of the basement floor strongly influenced sedimentation rates from horizon D to 1Atl (?Kimmeridgian to Late Valanginian): all lithogenetic units thicken within graben and condense over horsts. For economic reasons, rift sedimentation history has been studied in greatest detail for the gas fields area (Light et al., 1982; Strauss et al., 1990; Fatti et al., 1995) but similar lithostratigraphic successions have been described elsewhere on the flanks of the Bredasdorp Basin (Broad and Turner, 1982). Throughout the rift episode, clastic supply into the basin was mainly from the north and northeast and was derived from erosion of orthoquartzites and slates of the Cape Supergroup and sandstones and shales of the Karoo Supergroup. During Late Jurassic and Early Cretaceous times the Cape mountains are presumed to have been much higher than their present maximum of about 2000 metres due to the generally high elevation of the southern African part of Gondwana (De Swardt and Bennet, 1974; Partridge and Maud, 1987). De Swardt and Rowsell (1974) considered the Ordovician-Devonian rocks of the Cape Supergroup to have been metamorphosed by burial to a depth greater than 7000 metres prior to being subjected to folding. This implies that the Dwyka, Ecca and Beaufort Groups of the Carboniferous to Triassic Karoo sequence formerly overlay the Cape Supergroup in the Cape Mountains, and that they have subsequently been removed by erosion.
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344 Valanginian. A youngest possible age would suggest that this upper glauconitic sandstone interval accumulated in the Late Valanginian, and thus equates to the basal Sundays River Formation of the onshore Algoa Basin and its equivalent in the northern Pletmos Basin (Fig. 3). The marine components of progradational cycles in the western gas fields are characterised by a greater abundance of coarse sandstones and conglomerates, and carbonaceous detritus, corresponding with a disappearance of the meagre shell debris, compared with the eastern sandstones. This is interpreted to indicate a greater proximity to source, less wave and current reworking, and probably slightly lowered salinity levels. Gradation from typical foreshore sandstones, exhibiting low-angle planar cross-lamination and heavy mineral laminar concentrates, to fluvio-deltaic distributary bar-sands and interdistributary bay muds within individual cycles (Wickens, 1989) reflect a progradation of the palaeoshoreline towards the southeast. These progradational cycles are from 100 to 200 metres thick and are considered to have been caused by fault activated increases in depositional slope. This is indicated by widespread and abundant penecontemporaneous faulting, and by soft sediment deformation observed in cores. The upper shallow marine sandstones are the best reservoirs of the gas fields area and possess significant porosities and permeabilities (Light et al., 1982; Marot and McLachlan, 1982, Questiaux et al., 1985, Strauss et al., 1990, Pferdek~imper et al., 1992). Termination of active rift tectonics is indicated by the basinwide angular unconformity 1Atl which is nearly everywhere a compound unconformity, and by the termination of faults at 1Atl in the Bredasdorp Basin (Figs. 7, 8 and 9). This substantial break in sedimentation appears to have been predominantly tectonically controlled. Rift sediments were variably eroded, especially over structural highs and along the margins of the Bredasdorp Basin.
Transitional-early drift tectonics and sedimentation (1Atl to 13Atl) General remarks. Tectonic development and depositional history of only the 6Atl to 13Atl part of this Late Valanginian to Early Aptian interval has been intensively studied using seismic sequence techniques (Beamish et al., 1988, Brown et al., 1995). Major subsidence of the basin occurred in latest Valanginian (1At l) times, following which relatively uniform, slow, thermally-driven regional subsidence prevailed. 1Atl to 5Atl sedimentation is confined to several concurrent depocentres, notably in the northwest against the bounding Arniston Fault, and in the central and southern areas along
I.K. McMILLAN et al. the basin axis. Deep-marine sedimentation predominated from 1At 1 to 13At 1; poor circulation in the overlying water column led to profoundly lowered oxygen levels at the sea-floor, and benthonic faunas (ostracods and foraminifera) are correspondingly relatively rare, and confined to proximal boreholes close to the Arniston Fault and on the flanks of the Agulhas and Infanta arches (Fig. 3). In the central basin area, benthonic faunas are lacking, and faunal composition frequently consists only of Radiolaria with shell debris (particularly Inoceramus prisms and echinoderm debris) transported from the shelf. Argillaceous marine sequences exhibiting onlapfill geometries were repeatedly eroded in proximal areas due to slower rates of subsidence, and the material carried by turbidity flows into deep water.
1Atl to 5Atl (Late Valanginian to Hauterivian). During this period the northern flank of the Bredasdorp Basin, including the gas fields area, either lacked active sedimentation or suffered erosion. Southerly trending submarine valleys and canyons were cut into pre-1Atl sediments and provided conduits for sediment passing into deeper parts of the basin. Distal deposits are mainly argillaceous, accumulated in poorly oxygenated conditions and locally exhibit source rock potential (Davies, 1990). Rare turbidite sandstones occur, as detailed by Hodges and Winters (1990). The channels and canyons effectively subdivide the pre-lAtl upper shallow marine sandstones into discrete areas and provide part of the trapping mechanism for the gas reservoirs. 5Atl to 13Atl (Barremian to Early Aptian). At 5Atl time several changes in the area of active sedimentation occurred. The bounding Arniston Fault in the north ceased effective movement, the depocentre to the south of the fault merged with the depocentre in the central and southern Bredasdorp Basin, and, most significantly, a major shoreward advance of sedimentation occurred along the entire northeastern flank of the basin, so that deposition consequently occurred over the entire region of the gas fields. Concurrently, marine sedimentation commenced in the Infanta Embayment (see later). Channelised and mounded structures are recognisable in the central Bredasdorp Basin at 5Atl times, and some are of potential economic significance. Sedimentation during 5Atl to 13Atl times was again dominated by turbidity flows into a poorly circulating and poorly oxygenated, deep marine basin. Progradation from the northern margin and from the Infanta Arch is reflected by distinctly shallowing-upward sequences. In proximal areas to the north and cast (including the gas field area) these sequences culminate in shallow marine and shelf
LATE MESOZOIC SEDIMENTARY BASINS OFF THE SOUTH COAST OF SOUTH AFRICA sands that are often clean and highly porous, and constitute the shallowest parts of the highstand systems tracts. Locally matrix-supported conglomerates occur (Hodges and Winters, 1990: Facies I). An episode of channelling occurred during the Early Barremian (6At l) and three major channel systems were cut, originating on the northwest, west and southwestern flanks of the basin, and extending towards the basin deep, which at that time was located about 25 kilometres due south of the gas fields area. The main channel, which is easterly-trending and originates from the Agulhas Arch near borehole 5, can be traced for 60 kilometres (Brink and Wickens, 1990). It lies slightly to the north of the later and much larger 13A canyon system illustrated in Fig. 12, thus demonstrating the southerly shift of the basin axis during the Early Cretaceous. Channels on the 6Atl surface are also recognised throughout the gas fields area. These southerly-trending submarine valleys and canyons were cut deeply into pre-lAtl rocks and provided conduits for sediment passing into deeper parts of the basin. These canyons cross-cut the pre-lAtl west-east structural grain and disrupt the continuity of the upper shallow marine reservoir sandstones. The canyons are clay-plugged and provide part of the trapping mechanism for the gas fields. Towards 13At 1 times (Early Aptian) subsidence rates and faulting show marked declines, presaging a more stable Bredasdorp Basin. A major change to a northerly sediment input direction can be seen at this time: sandstones became widespread over almost the entire basin in highstand tracts proximally, and in lowstand turbidites distally. Lithologies are described by Hodges and Winters (1990: their Facies III), and petrography details are provided by Hill (1990). Turbidite systems intersected in boreholes comprise slightly carbonaceous, fine-grained sandstones in lobe, depositional channel and abandonment environments, much as described by Mutti and Normark (1987). Organic enrichment of the 5Atl to 13Atl interval is intermittent. Over the northeastern flank of the Bredasdorp Basin, some enrichment is seen in the 5A sequence and locally in the 6A sequence whereas in the central Bredasdorp Basin the 9Atl to 12Atl interval also shows some organic enrichment (Davies, 1990). The pattern of distribution is not yet fully understood.
Late drift tectonics and sedimentation (13Atl to present day) The 13Atl unconformity in the Early Aptian ushered in a very different sedimentation regime. Although there is seismic evidence for major erosion around the basin margin, erosion deeper in the basin is confined primarily to cutting of the 13Atl canyon which is a submarine channel about 5 km wide and
345
approximately 50 km long, that trends in an easterly direction across the central part of the basin (Fig. 12). Updip tributaries of the system are clayplugged. The channel system served as a conduit supplying mass-flow deposits, predominantly thinly bedded turbidites, to the deeper parts of the basin. The 13A channel is the site of oil accumulations which are located in seismically-defined mounded sequences comprising submarine fan-channel complexes. In the overlying 14A sequence sandy basinfloor fans also occur in the central Bredasdorp basin, and again have oil-bearing reservoirs. Argillaceous slope-front fans have also been identified, located immediately beyond the relict shelf-edges of successive lowstand systems tracts. The interval overlying horizon 13Atl is distinguished by high-gamma organic rich claystones in the central and southern Bredasdorp Basin that only proximally contain benthonic fauna but almost everywhere are rich in planktonic foraminifera and Radiolaria (Fig. 3: 13Atl to Top Anoxic). Though distinctly thinner in the proximal, shelf areas, these claystones accumulated under severely lowered oxygen conditions and they extend northwards almost to the basin margin, and northeastwards across the gas fields area. Along the steeper southern flank against the Agulhas Arch, the 13A transgression did not advance as far as in the north and northeast. The anoxic interval, though tending to be sand-rich in the west, is elsewhere one of the most organically enriched and best source rock intervals yet intersected in southern offshore drilling (Davies, 1990). A maximum thickness of 200 metres is developed along the Bredasdorp Basin axis, which at that time was sited close to the southern flank of the basin. Source rocks are mature and viable and have supplied hydrocarbons (mainly oil) to adjacent 13A and 14A deep-water sandstone reservoirs. The Top Anoxic surface, as defined by foraminiferal data (Fig. 3), marks an abrupt return to well-oxygenated sea-floor conditions, reflecting a great improvement in the water mass circulation; benthonic faunas are thereafter mainly abundant and often diverse. Sedimentation rates and basin subsidence rates generally show signs of decline, especially in the gasfield areas of the northeastern shelf of the Bredasdorp Basin. To the northwest however, clastic input remained high, and high-stand shelf sandstones are common throughout the interval, attaining a maximum advance into the central basin area between horizons 14Ctl and 15Atl. These sands are thickest towards the basin axis and thin towards the northeastern and southwestern flanks. Distinctive lowstand-tract sandstones developed as deepwater fan-channel complexes at 14Atl times, and extend considerably into the distal, southern parts of the basin from the western and southern
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LATE MESOZOIC SEDIMENTARY BASINS OFF THE SOUTH COAST OF SOUTH AFRICA flanks. Their maximum thickness appears to be controlled by the position of the basin axis at 14Atl times. Some detail of their character is presented by Gilbert (1990), Hill (1990) and Hodges and Winters (1990, Facies II). These sandstones have substantial economic significance as oil reservoirs, and their full potential is still being evaluated. Maximum sedimentation within the 13A sequence is confined to a small region in the vicinity of borehole 3 (Figs. 7 and 8), that lay close to the shelf-break (outer shelf to upper slope) whereas maximum thickness of the 14A sequence lies about 37 kilometres east-southeast. The 15Atl unconformity shows clear microfaunal evidence of erosion, suggesting minor warping and uplift during the Late Cenomanian. Maximum erosion appears to have occurred distally, particularly in the easternmost part of the basin (McMillan, 1990a). An episode of sediment-starved, oxygen-starved, organically-enriched, plankton-enriched claystone is found immediately overlying 15Atl and shows some value as a source rock in the southern half of the Bredasdorp Basin, but is absent further north. However, the interval is generally too immature for it to be regarded as a viable source rock except in the distal, southernmost parts of the basin. This interval is dated as Early Turonian on the basis of planktonic and benthonic foraminifera. Overlying sediments show pronounced progradation, especially in the eastern parts of the Bredasdorp Basin. This prograding episode, of Turonian to mid-Coniacian age (15Atl to horizon K) can be recognised eastwards as far as the Pletmos Basin. Above horizon K sediments are predominantly biogenic, and clastic input to the basin was greatly reduced, though progradation and truncation can be recognised on several surfaces, especially at horizon X (late Santonian-early Campanian) and at horizon L (Maastrichtian-Palaeocene boundary). In the southeastern Bredasdorp Basin, a major domal structure (the F - F structure), as yet little explored and lying seaward of the end of seismic line B-B', appears to have formed during the latest Cretaceous, as indicated by the thin interval between horizon X and horizon L. It is one of the few major late structures in the basin, and it may have developed in association with, or immediately before, the many volcanic intrusions mapped in that part of the basin and which were intersected in the crestal well. Considerable thicknesses of horizon 15Atl to horizon L sediments occur in the vicinity of the F-X prospect described by Fouch6 (1990) in the southeastern part of the basin. Post-L sedimentation (Tertiary to present day) is exclusively composed of highstand shelf deposition of glauconitic clays, occasional sands and widespread biogenic clays. Relative uplift of the Agulhas Arch since shortly before horizon L led
347
to erosion of Late Cretaceous sediments from the flanks of the arch and redeposition of the fossiliferous debris into Palaeocene shallow-marine clayey and glauconitic sands. To the east, away from the Agulhas Arch, middle to outer shelf deposits accumulated. A major Late Oligocene unconformity can be recognised, in keeping with many other localities worldwide (Vail et al., 1977). Early Miocene biogenic clays are widely developed over the entire southern half of the basin, and their depositional environments imply that by this time the Agulhas Arch had essentially foundered. Later deposition has been minimal, and where sampled, thin veneers of one metre or less, of Holocene and latest Pleistocene sediments unconformably overly Early Miocene rocks.
PLETMOS BASIN AND INFANTA EMBAYMENT Introduction
The Pletmos Basin together with the Infanta Embayment, is the largest of the southern offshore basins, being some 270 km long and 110 km wide (Fig. 1). As with the Bredasdorp Basin, these areas are defined by arches of Palaeozoic rocks (the lnfanta and St. Francis arches), and are aligned parallel to the east-southeasterly-trending structural grain of the Cape Fold Belt. The pre-Mesozoic basement has been penetrated on basement highs only and consists mainly of Ordovician-Silurian Table Mountain Group quartzites belonging to the Palaeozoic Cape Supergroup. The Pletmos Basin is a structurally intricate basin, much more so than the Bredasdorp Basin. It is divided into two major areas by the complex Superior Fault (Fig. 1), and during the rift phase distinctly different sub-basins existed to the north and south of the fault. The major bounding faults, notably the Plettenberg Fault in the northeast, and the Pletmos Fault in the southwest, virtually define the limits of rift sedimentation (Figs. 1, 2). The northwestern parts of the basin have been little explored even though some major local depocentres have been recognised there. Drilling results indicate that depositional environments in that region are mainly proximal, with red and green non-marine beds and shallow marine claystones and sandstones predominating: source rocks seem to be lacking and gas levels while drilling have not been encouraging. A number of onshore extensions of the Pletmos Basin, notably in the vicinity of Mossel Bay, at Knysna, and at Plettenberg Bay, are present. Where sediments are exposed, lithologies comprise varicoloured claystones of non-marine origin or shallow marine claystones and sandstones, and have been
348 described by Rigassi (1970), Rigassi and Dixon (1972), McLachlan and McMillan (1976), McLachlan et al. (1976), McLachlan and McMillan (1979) and Rust (1983). Additional comments and a summary are given by Dingle et al. (1983) and Malan and Viljoen (1990). The tectonostratigraphic evolution of the Pletmos Basin has been discussed by Bate and Malan (1992). Because of its poor hydrocarbon potential the Infanta Embayment has been little drilled. The embayment, approximately 80 km by 40 km, is aligned parallel to the Cape Fold Belt structural grain and lies seawards of, and is partly enclosed by, basement highs of the Infanta Arch. The embayment extends southeastwards over a low basement ridge into the Bredasdorp Basin, and similarly northward into the Pletmos Basin. A major graben is developed eastwards down the axis of the distal Infanta Arch (Fig.
13). In both the Pletmos Basin and Infanta Embayment, rift, transitional-early drift and late drift phases of sedimentation are recognised and the basin-wide unconformities D, 1Atl and 13Atl delineate the onset of these episodes. From the top of basement (horizon D) to horizon 1Atl (Kimmeridgian to Late Valanginian), an extensional stress regime led to horst and graben tectonics and, locally, extremely thick accumulations of sediment, most notably in the graben just south of the Plettenberg Fault (Figs. 4, 5). Thick D to 1Atl intervals also occur just to the north of the Superior Fault, west of section line E-E' (Fig. 14), as well as in the southernmost Pletmos Basin, north of the Pletmos Fault. D to 1Atl sediments are composed of inner to outer shelf sandstones and claystones with localised non-marine red and green beds. Since oxygen levels at the time of deposition appear to have been near normal in both the Pletmos Basin and Infanta Embayment, organic enrichment in these rocks is very rare. Like the Bredasdorp Basin during this time period, the local relative sea level variations probably reflect an interplay of fluctuations in rates of subsidence caused by syn-depositional faulting and sediment supply. A marked change in sedimentation pattern occurs above 1Atl. Compared with the underlying sediments the Late Valanginian to Hauterivian (1Atl to 6At l) deposits accumulated in a substantially deeper marine environment, and in profoundly lowered oxygen conditions with local organic enrichment occurring over the 1At 1 surface. The 6Atl unconformity marks a phase of uplift and erosion prior to deposition of a second, mainly deep marine, poorly oxygenated sequence (6Atl to 13Atl). As with 5Atl marking the termination of the Arniston bounding fault in the Bredasdorp Basin, 6Atl marks the end of substantial normal movement of the Pletmos,
I.K. McMILLAN et al. Plettenberg and Superior faults, though subsequent strike-slip reactivation can be seen along the Superior Fault. Concurrent with the 6Atl phase of uplift, sedimentation rates in the Pletmos Basin declined substantially north of the Superior Fault. Only the southernmost Pletmos Basin shows thicknesses comparable to the Bredasdorp Basin from 6Atl through to horizon L at the top of the Cretaceous. The Tertiary sediments accumulated exclusively on the continental shelf where they form a relatively thin veneer thickening gradually in a southerly, distal direction. Basin evolution Rift tectonics and sedimentation (D to l A t l ) Pletmos Basin. Initial rifting was developed by complex horsts and grabens, with the rift sediments of widely variable lithologies being deposited mainly in the graben and reflecting rapid changes in depositional environment. The horizon D surface (economic basement) is shown in Fig. 4. The rapid changes in facies in the earliest sediments, together with a lack of complete sequences drilled in the basin deeps has hindered a full understanding of the early history of the Pletmos Basin and the Infanta Embayment. The interval is too deeply buried in much of the Pletmos Basin for it to be of economic significance. The D to I Atl interval thicknesses intersected in boreholes range from 2500 metres on the southern margin of the Plettenberg Graben to as little as 500 metres on bevelled highs. Sedimentation probably commenced in Kimmeridgian times, especially in the deep Plettenberg Graben in the northeast of the basin (Fig. 15). Here the basement surface probably had a gentle northeasterly dip into the Plettenberg Fault. However, over the Superior High (line E-E', Fig. 14a), on the eastern flanks of the southern parts of the Infanta Arch, and on the northern margins of the basin off Plettenberg Bay, the oldest sediments are Portlandian. On the basement highs such as the Superior High, the earliest sediments are often stained red and comprise a veneer of pebbly and sandy beds. These are overlain by shallow marine claystones and sandstones which contain benthonic microfaunas that suggest a slightly restricted environment. Foraminiferal correlation with other basins is not easy, but the assemblages found in the lower glauconitic sandstone at horizon V in the Bredasdorp Basin (Fig. 11) show some elements in common with those seen in the Portlandian "Colchester" equivalent on the northern margin of the Pletmos Basin (McLachlan et al., 1976). The deep marine, poorly oxygenated environments present in the Gamtoos and Algoa Basins during the Kimmeridgian and Portlandian have not been recognised in the Pletmos and Bredasdorp
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Fig. 14. Seismic profile and geological interpretation across the Pletmos Basin. Profile E-E' in Fig. 1. (a) Southern part. (b) Northern part.
354
I.K. M c M I L L A N et al.
Fig. 15. Seismic profile and geological interpretation of the Plettenberg Graben located in the northeastern portion of the Pletmos Basin. Profile F-F' in Fig. 1.
LATE MESOZOIC SEDIMENTARY BASINS OFF THE SOUTH COAST OF SOUTH AFRICA basins. The deepest marine environments during this time are restricted to that part of the basin north of the Superior Fault, where they contain diverse microfaunas that appear no deeper than outer shelf. To the south of the Superior Fault environments were shallower, ranging from inner shelf to transitional, and the resulting variably glauconitic and shelly sandstones are well-developed. However, microfaunas are rare. Slight deepening occurred in the later Portlandian when marine sedimentation became more argillaceous. Similar depositional conditions appear to have been maintained during the Berriasian, though sandstones are less well developed probably due to a shift in the point source. On the northern rim of the basin during this period, minor red beds with shallow-marine sandstones (often very shelly with crinoid skeletal debris) indicate a localised regression. The red beds occupy the same time-span as the Kirkwood Formation red beds of the northern Algoa Basin. However, although conditions are marginally marine throughout the interval south of the Superior Fault, no equivalent regression can be identified there. In the Early Valanginian, depositional environments almost everywhere show signs of shallowing upward, with shelly and glauconitic sandstones being widespread. Most of these sandstones are marginal marine and tidally-influenced in character. Only in the Plettenberg Graben to the northeast did middle to outer shelf claystone and sandstone sedimentation continue. In the Pletmos Basin sandstones were best developed during late Early to Late Valanginian. Permeabilities, porosities and thicknesses of sandstone units attain a maximum in this interval, and constitute economically significant reservoirs in the Superior High area (Fig. 16) (Maier, 1990). These sandstones are generally fine grained and glauconitic with varying amounts of argillaceous matrix, and are interbedded with glauconitic and locally calcareous claystones and siltstones. Sandstone distribution is indicated on the log correlation of the Pletmos boreholes (Fig. 16). Tectonically the basin is relatively complex. Large normal faults developed during the early rift period, and movement on the hanging wall contemporaneous with sedimentation is reflected in section E-E' and F-F' (Figs. 14 and 15). Maximum known throw of basement on the Pletmos, Superior and Plettenberg faults is approximately 2600 metres, 5000 metres and 5600 metres, respectively. The irregularity of the main fault lineaments reflects stresses originating from later strike slip movement along the Agulhas Fracture Zone, to be discussed later. The resulting fault patterns created potential structural traps for hydrocarbons in the D to 1Atl interval.
355
Infanta Embayment. Pre- 1At 1 sedimentation in the Infanta Embayment is comparable to that seen in the Pletmos Basin southeast of the Superior Fault, and in the Bredasdorp Basin. Initial sediments in the embayment are non-marine, with red beds dominant, and these have been interpreted to represent fluvial meandering-channel sandstones with overbank siltstones and claystones. Sedimentation probably began during the Kimmeridgian but no datable microfossils have yet been found. Deposition was initiated in two depocentres, one being the Infanta Embayment proper, the other being the graben along the length of the distal Infanta Arch. A summary of the evolution of the Infanta Embayment during the Late Jurassic to Early Cretaceous period (D to 1At 1) is presented by Turner (1990). Glauconitic sandstones and interbedded claystones were laid down during a Portlandian marine episode and overlie an unconformity mapped as seismic horizon IV (Fig. 13). The sandstones contain comminuted crinoid and oyster debris, ooliths and ostracods, and rare foraminifera, and are interpreted to have accumulated in a shoreline system. Though palaeontological data is not entirely confirmatory, it seems likely that horizon IV of the Infanta Embayment and the graben is equivalent to horizon V of the northeastern Bredasdorp Basin. In the vicinity of borehole 7 (section D-D', Fig. 13) in the graben, and west of the distal Infanta Arch, the basal deposits overlying the horizon IV unconformity consist of oolitic and shelly grey limestones and claystones, although the majority of the marine sequence is as described above. In general terms the thickness of marine beds is much greater here than in the Bredasdorp Basin, and is more in keeping with the Pletmos Basin. Following a regressive episode, during which thin but widespread non-marine red and minor green beds were deposited, a further marine transgression occurred, and variably shelly glauconitic sandstones and interbedded claystones were again deposited. Reliable micropalaeontological data from these upper marine beds is lacking: their age may be as young as Late Valanginian, and thus they could be equivalent to the lower beds of the Sundays River Formation in the Algoa Basin, and to the upper shallow marine gas-bearing sandstones of the northern flank of the Bredasdorp Basin. Transitional-early drift tectonics and sedimentation (1Atl to 13Atl) 1Atl to 6Atl interval. The rifling episode was terminated at 1Atl times by major tectonism in the Late Valanginian. In common with the Bredasdorp Basin, 1Atl to 6Atl sediments, of latest Valanginian to Hauterivian age, were deposited in markedly deeper water than those immediately underlying
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LATE MESOZOIC SEDIMENTARY BASINS OFF THE SOUTH COAST OF SOUTH AFRICA horizon 1At1, implying abrupt subsidence of the Pletmos Basin. In addition, oxygen levels at the sea-floor were much reduced, leading to widespread accumulation of organic-rich claystones above horizon IAtl. These claystones have some potential as source rocks locally in the Plettenberg Graben and at isolated sites southwest of the Superior Fault. They contain a distinctive foraminifera fauna composed mainly of a variety of benthonic agglutinated foraminifera, notably Trochamminoides sp. A, with floods of dictyomitroid and spherical Radiolaria. The overlying beds consisting of cyclic deep marine claystones with local channel sandstones, are poorly fossiliferous with much land-derived plant debris and few Radiolaria. The overall 1Atl to 6Atl succession is regressive and becomes sandier up section. The 1Atl to 6Atl sediments are bounded in the northeast by the Plettenberg Fault, and in the southwest by the Pletmos Fault. Along the northern margin of the Pletmos Basin are several boreholes which indicate that the 1A to 5A sequences were deposited in an environment shallower than much of the basin, probably on the shelf. Indications are that sea-floor oxygen levels were nearer to normal, with the benthonic foraminifera faunas correspondingly more diverse and with calcareous forms dominant. Here, sequences consist of grey claystones with thin argillaceous sandstones. Locally, thicker channel and bar facies occur, composed of fine- to medium-grained, glauconitic and porous sandstones, that were sourced from the north and northeast of the Pletmos Basin. The cyclicity of the 1A to 6A sequences is regarded as having developed in response to third order eustatic sea-level changes (Brink et al., 1994); however, movement on the major bounding fault planes may well have influenced cyclical sedimentation. Adjacent to the Plettenberg Fault sedimentation rates were high greater than 200 metres per million years ~ and subsidence rates were also correspondingly high. Borehole thicknesses of the 1Atl to 6Atl interval attain 2800 metres in the Plettenberg Graben, but only 50 to 100 metres distally in the southernmost Pletmos Basin. Substantial uplift at 6Atl times led to severe erosion of the 1A to 5A sequences as well as the upper pre-lAtl section in the central and southern Pletmos. The most complete pre-6Atl sequences are thus found along the northern margin of the Pletmos Basin, such as that intersected by borehole PB-A1 (McLachlan et al., 1976), and elsewhere close to the Plettenberg Fault. Southward, sequences are increasingly abbreviated, as in boreholes 12 and 13 (Fig. 16), and south of the Superior Fault only the basal portion (1Atl to 2Atl or 3Atl?) is preserved in basin deeps. The 1A to 5A sequences have not been recognised in the Infanta Embayment, and it is
357
unclear whether they were removed by 6At 1 erosion, or whether they were never deposited. 6Atl to 13Atl interval. Following major uplift and erosion at 6Atl time, sedimentation reflects marked changes in clastic supply and basin shape. Horizon 6Atl marks the end of substantial movement on the bounding faults (especially the Pletmos and Plettenberg faults). Again, sedimentation during this interval was cyclic, and shows substantial aggradation and progradation. High frequency sequences and sequence sets have been interpreted as responses to the interplay between lower order eustatic fall and rise cycles, high sediment input rates and a period of relatively stable subsidence rates (Brink et al., 1994). Sedimentation in the Pletmos Basin initially occurred simultaneously in two major sub-basins, one north of the Superior High, and one to the south. During 6A deposition, sediment sources migrated from the north to the northwest margin of the basin, and progradation advanced down the axes of the two sub-basins. In time the Superior High was inundated: in general the northern sub-basin was infilled by proximal, sandier sequences, while the southern sub-basin was infilled by distal clays. Gouws (1990) has described a lowstand prograding wedge in the 8E sequence which was explored for hydrocarbons. Distal sedimentation occurred in poorly oxygenated, deep marine conditions on the 6Atl surface and led to the accumulation of organic-rich claystones having source potential. The interval is characterised by distinctive agglutinated benthonic foraminifera (see Fig. 3) with occasional floods of planktonic foraminifera, and more widespread floods of Radiolaria. The overlying prograding beds in the 6Atl to 13Atl interval are poorly fossiliferous, often containing restricted foraminifera faunas of low diversity, with scattered Radiolaria. These sediments display progressive shallowing upwards features and show a tendency to become sandier, with lignite and localised coal beds indicative of marginal marine conditions near 13Atl. The Superior High tended to cause sediment starvation in the southern Pletmos Basin, so that the Barremian to mid-Aptian sequences become very thin distally, as in the vicinity of borehole 9 (Figs. 14 and 16). Although high porosities sometimes occur in the proximal sandstones, source rocks are too distant for these sandstones to be of economic significance. The 6A to 12A sequences in the Infanta Embayment show close similarities to those on the northeastern flank of the Bredasdorp Basin, but are distinctly more proximal in character. In a borehole located close to the depocentre, uppermost slope grey claystones with some organic enrichment, and variable Radiolaria, grade upward to thick, shallow marine, bar sandstones which are glauconitic, fine
358 to medium grained and well sorted. The maximum basinward advance of shelf sandstones in all three basins, Bredasdorp, Pletmos and Infanta, appears to have been roughly coeval during 10Atl to 13Atl times (Early Aptian) (Fig. 3). Thicknesses of the 6At 1 to 13At 1 interval penetrated in boreholes attain a maximum of 2000 metres, with the interval thinning northward to the basin margin and southward into deep water, where they are of the order of 100 metres.
Late drift tectonics and sedimentation (13Atl to present day) After the mid-Aptian, sedimentation rates show a marked decline over the entire Pletmos Basin and Infanta Embayment, and sandstones become comparatively rare. Thereafter, the basin subsided slowly within a widening continental shelf setting. The Superior High and the distal Infanta Arch have little or no influence on sedimentation patterns from 13Atl times onward. Faunal associations and sediment thicknesses become very uniform from the eastern Bredasdorp Basin across to the Pletmos Basin. Only in the southernmost Pletmos Basin have higher sedimentation and subsidence rates been encountered. The Early Aptian to mid-Cenomanian interval, 13Atl to 15Atl, is characterised by an organicenriched episode at its base (13Atl to Top Anoxic, see Fig. 3) that is distinguished by floods of planktonic foraminifera and Radiolaria and a lack of benthonic species. This claystone has some potential as a source rock. Though widespread on the upper slope in the southern Pletmos Basin, it is everywhere considerably thinner than its equivalent in the Bredasdorp Basin (where it is a major source rock), and it cannot be clearly recognised on the shelf in the northern Pletmos Basin. The Top Anoxic to 15Atl interval consists of well-oxygenated shelf and slope claystones with minor sandstones: only on the inner shelf overlying the old Plettenberg Graben are sandier sequences found, with shelly and glauconitic sandstones common. Diverse foraminiferal faunas are widespread, and are correlatable with Bredasdorp Basin sequences. These correlations suggest that over much of the Pletmos Basin the majority of the Cenomanian section has been eroded away following mild tectonic uplift at 15Atl times (Late Cenomanian), as described by McMillan (1990a). Thicknesses of the 13Atl to 15Atl interval intersected in boreholes are never greater than 300 metres on the shelf but thicken to 700 metres or so on the upper slope. Overlying the 15Atl surface is an Early Turonian interval of oxygen-starved, organically enriched sediments that may prove to be of some value as a source interval in the south, although it is generally regarded as thermally immature. Thereafter follows
I.K. McMILLAN et al. a phase of substantial progradation which culminated on the shelf with a latest Turonian fall of sea level and accumulation of fine to medium grained, clean, glauconitic, shelly sheet sands (horizon 11 sand). In deeper water, progradation continued until horizon K (top middle Coniacian). Sediments consist of clays with local channel-derived argillaceous, fine-grained sandstones. From mid-Coniacian to mid-Campanian (horizon K to horizon X) sedimentation rates in both the Pletmos Basin and the Infanta Embayment show a marked decline, corresponding to an increase in the biogenic component and a predominance of orientated lime rich claystones. Thereafter, later Cretaceous sediments are glauconitic claystones with occasional sandy stringers that indicate a reactivation of southerly-orientated progradation. A mid-Campanian transgression at horizon X is inferred along the northern margin of the Pletmos Basin. This phase of increased subsidence and faster sedimentation was brought to an abrupt termination by tectonic activity at horizon L, at the end of the Maastrichtian. Sedimentation thereafter was extremely condensed. Deposition commenced early in the Palaeocene, and consists of glauconitic claystones. Biogenic limey clays predominate in the Middle and Late Eocene, with greenish siltier glauconitic clays in the Early Oligocene. Local channel and minor bar sandstones can be recognised. Unconformably overlying the Early Oligocene are Early Miocene white biogenic limey clays topped by a substantial hardground, in turn overlain by an extremely thin veneer of latest Pleistocene and Holocene. All over the Pletmos Basin and Infanta Embayment explored to date, Cainozoic rocks accumulated in shelf environments and they show little variation in foraminiferal faunas or depositional environments. Maximum borehole thicknesses of the 15Atl to horizon L interval are of the order of 1500 metres, and of the horizon L to sea-floor interval about 700 metres in the southernmost Pletmos Basin.
GAMTOOS AND ALGOA BASINS
Introduction The Gamtoos and Algoa Basins (Fig. 1) are half grabens bounded by major faults to the northeast. Although each basin is substantially smaller than the Bredasdorp and Pletmos Basins (see Table 1) they nevertheless contain comparable thicknesses of sediment (Fig. 2). The Gamtoos Basin is a relatively simple half graben (Figs. 17 and 18), but the Algoa Basin is subdivided into three fault troughs, the Port Elizabeth Trough, the Uitenhage Trough, and the Sundays River Trough in a west to east direction
LATE MESOZOIC SEDIMENTARY BASINS OFF THE SOUTH COAST OF SOUTH AFRICA (Figs. 1, 19 and 20). The last-named trough occurs almost entirely onshore and contains over 4200 metres of Portlandian to Hauterivian sediments. The Gamtoos Basin and the three Algoa Basin troughs are separated from each other by highs of Palaeozoic sediments in the form of arches on their western margins and faulted upthrown blocks on their eastern sides (Figs. 17, 19 and 20). These arches are composed mostly of the Palaeozoic Cape Supergroup (mainly Ordovician-Silurian Table Mountain quartzites and Devonian Bokkeveld slates) which are aligned along the grain of the Permo-Triassic Cape Fold Belt. Deep drilling offshore has intersected basement rocks (Table Mountain quartzites) only on the flanks of the basement arches and on basement highs. Onshore drilling in the Uitenhage Trough has everywhere terminated in Table Mountain quartzites, while in contrast, every hole drilled to basement in the Sundays River Trough bottomed in Bokkeveld black slates. The major bounding faults (Fig. 1), particularly the Gamtoos Fault and the St. Croix Fault, extend deep into the crust and probably have complex histories (Friedinger, 1986; Malan et al., 1990). Onshore the Gamtoos Fault has a throw of about three kilometres; offshore the fault plane can be traced to a depth of approximately 12 kilometres (5.5 seconds two-way time). Seismic profiles across the Gamtoos Fault illustrate its listric nature (Figs. 17, 18). Additional major faults in the Gamtoos Basin occur on the eastern flanks of the shallow St. Francis Arch. In the Algoa Basin the Uitenhage Fault, which divides the southernmost Uitenhage Trough into two half grabens, appears to be a late feature and was possibly developed during 1Atl to 6Atl times. The Port Elizabeth Trough was probably continuous at its northern end with the Uitenhage Trough prior to fault movement during the latest Valanginian to Hauterivian (1 At 1 to 6At 1). In both basins, subsidence and sedimentation were rapid during the pre-Kimmeridgian to Hauterivian (D to 6At 1), but later sediments (excepting the 13Atl to 15Atl canyon fills) are much thinner compared with the equivalent intervals previously described for the Pletmos and Bredasdorp Basins (Fig. 3). A simplified chronostratigraphic table is provided in Fig. 21. No detailed sequence-stratigraphic study has been undertaken for these basins because of the lack of economic success, limited thicknesses in the 1Atl to 15Atl interval and the prevalence of severe faulting in the pre-1At 1 section which makes seismic correlation difficult. Structural development of these basins has been summarised recently by Malan et al. (1990), and their hydrocarbon potential discussed by Broad (1989, 1990). Additional comments on the Algoa, Gamtoos and Pletmos basins and their
359
tectonostratigraphic evolution are given by Bate and Malan (1992). Basin evolution Rift tectonics and sedimentation (D to 1Atl) Development of the Gamtoos and Algoa Basins occurred during the Late Jurassic and earliest Cretaceous, but may well have been initiated in the Middle Jurassic. The oldest dated sediments encountered in drilling are Kimmeridgian, but substantial thicknesses near depocentres remain unexplored, particularly in the Gamtoos Basin where the depocentre contains approximately 7000 metres of undrilled Mesozoic section. Here basement horizon D exceeds 5.5 seconds two-way time (approximately 12 km) and it is possible that the undrilled rift sediments are of Middle to Late Jurassic age (see Fig. 17). Gamtoos Basin. Early sedimentation in the Gamtoos Basin shows substantial lateral variation in depositional environments. Kimmeridgian sedimentation on the flanks of the St. Francis Arch (borehole 16, Fig. 17) is characterised by a basal non-marine conglomeratic and red bed interval, overlain by shallow-marine interbedded sandstones and siltstones. In contrast, to the east, close to the Gamtoos Fault and near the Gamtoos Basin depocentre, upper slope black claystones with minor turbiditic sandstones accumulated at this time in lowered oxygen conditions. Microfaunas here are sparse, mostly agglutinated foraminifera, with Radiolaria always present and often occurring in floods. Several intervals show organic enrichment and have good potential as source rocks, even though they are buried to depths around 4000 metres. Minor gas shows occurred while drilling thin sandstones in the sequence (Broad, 1989, 1990). Portlandian sedimentation in the depocentre is a continuation of that of the Kimmeridgian interval, but shows a gentle shallowing and increase in oxygen levels upwards, corresponding to a slow increase up-section of benthonic microfaunas. On the flanks of the St. Francis Arch, Portlandian sedimentation (borehole 16) commenced with an abrupt deepening, and outer-shelf well-oxygenated claystones with diverse benthonic microfaunas were laid down over the Kimmeridgian shallow marine sandstones. During Portlandian to Berriasian times the area of sedimentation in the Gamtoos Basin appears to have enlarged considerably, both towards the St. Francis Arch (borehole 15, Fig. 17) and towards the Recife Arch (boreholes 20, 21, Fig. 18). At these localities environments of deposition deepen upward towards the top of the Berriasian. Poorly fossiliferous basal pebble beds and sandstones give way up-section to outer shelf, well oxygenated claystones
360 with a diverse microfauna. In contrast, boreholes in the central part of the basin show a steady shallowing-upward to outer shelf conditions by the top Berriasian, coupled with increasing sea-floor oxygen levels and increasingly diverse and abundant microfauna that for the first time shows an appreciable calcareous component in the benthonic foraminifera. Sediments remain predominantly grey claystones with minor sandstones. In the offshore Gamtoos Basin Valanginian sedimentation reflects trends established in the Portlandian and Berriasian. The Valanginian shows much more uniformity of depositional environment across the southern half of the basin than seen lower in the section. Diverse and abundant foraminiferal faunas indicate well-oxygenated conditions at the sea-floor, and sediments everywhere are middle to outer shelf claystones with rare sandstones. Only in the northern half of the offshore basin are more proximal depositional environments seen. Boreholes drilled in the north close to the Gamtoos Fault intersected shallow-marine (mainly innermost shelf) interbedded sandstones and claystones (borehole 20, Fig. 18), or, further northwest, transitional and estuarine sandstones with fewer claystones and occasional red bed intervals. An entirely different sedimentation regime prevailed during this period in the onshore part of the Gamtoos Basin, where over 3000 metres of redstained conglomerates grading up-section to sandstones and rare red beds with very rare shallowmarine claystones, were intersected (McLachlan and McMillan 1976). Marine foraminifera in one interval high in the sequence suggest a pre-lAtl late Valanginian age, but the majority of the interval is undated, though it may well range back to at least the Portlandian.
Port Elizabeth Trough. In the Algoa Basin the three troughs display different rift sedimentation histories (Fig. 21) and consequently they are described separately. In the Port Elizabeth Trough basement horizon D attains about 3.5 seconds two way time (approximately 6500 metres) (Figs. 4, 19). Boreholes 23 and 24 (Fig. 19) intersected rift sequences, similar to those in borehole 16 in the Gamtoos Basin. In borehole 24 a basal shallow-marine interval, regarded as ?Kimmeridgian age consists of shallow-marine to transitional claystones and sandstones, locally rich in lignite, and with very poor, marginal marine foraminifera faunas. This sequence shallows westwards to coarse, pebbly sandstones and claystones containing lignite and siderite spheres, with some red beds in borehole 23. A marked deepening of the depositional environment can be seen at the base of the Portlandian, with outer shelf conditions predominating thereafter. The Portlandian
I.K. McMILLAN et al. sediments are mainly claystones which display intervals of organic enrichment, with significant source rock potential in boreholes 23 and 24. Significant oil shows occurred in adjacent porous and permeable sandstones (Broad, 1990). Microfaunas are abundant, though the foraminifera faunas are not diverse. Through the Berriasian interval in the two boreholes a general shallowing-upward trend can be recognised from the foraminifera faunas, and lithologies consist of interbedded sandstones and claystones deposited on a well-oxygenated shelf. Sedimentation in the two more northerly Port Elizabeth Trough boreholes commenced in the mid-Berriasian: a characteristic that seems very similar to that seen on the flanks of the Gamtoos Basin, as discussed previously. The Berriasian to Early Valanginian intervals of these northerly wells consist of interbedded sandstones and claystones with foraminiferal faunas that indicate well-oxygenated inner shelf environments. Early Valanginian sediments are preserved only locally due to erosion and indicate that the sequence becomes progressively more sandstone rich, and transitional in depositional environment, towards 1At 1. Over the entire Port Elizabeth Trough the 1At 1 surface is intensively planed, so that the most complete sequences (?Late Valanginian) lie close to the Port Elizabeth Fault.
Uitenhage Trough. The Uitenhage Trough (Fig. 20) shows considerable lateral variation in lithologies through the D to 1Atl sequence. Drilling in the onshore portion of the trough, where horizon D attains a maximum depth of about 1800 metres (Fig. 4) has revealed basal non-marine red sandstones and conglomerates (Enon Formation, Fig. 21) overlain by fluvial sandstones (Swartkops Member). These are overlain by a marine and fluvial-influenced interval of greenish and grey claystones with sandstones, that contain Portlandian foraminifera (Colchester Member equivalent). This Colchester Member equivalent is overlain by non-marine red and minor green beds with sandstones (main body of the Kirkwood Formation) which is in turn overlain by shallow marine grey claystones and sandstones with a foraminifera fauna indicative of a Late Valanginian age (lower Sundays River Formation). Some localised organic enrichment occurs in the Colchester Member equivalent, but it has not been buried deeply enough for it to be a viable source rock. The sequence outcrops near Port Elizabeth and has been described by McLachlan and McMillan (1976, 1979). In 1908, one of the first boreholes drilled for oil, at Swartkops, just north of Port Elizabeth, failed to discover hydrocarbons, but located a thermal mineral water supply that supported a health spa for several years. Offshore in the Uitenhage Trough drilling has occurred only in the southern half, well
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