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The Sub-Kalahari Geology and Tectonic Evolution of the Kalahari Basin, Southern Africa

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The Sub-Kalahari Geology and Tectonic Evolution of the Kalahari Basin, Southern Africa THE SUB-KALAHARI GEOLOGY AND TECTONIC EVOLUTION OF THE KALAHARI BASIN, SOUTHERN AFRICA. by Ian Gerald Haddon A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, 2005 ACKNOWLEDGEMENTS The preparation of this thesis has involved the assistance of many people and my sincere thanks to all those who have helped with my research in any way. In particular, certain people need to be mentioned by name: Eddie Van Wyk and other staff at the Department of Water Affairs and Forestry as well as the staff at Sishen Iron Ore, Wessels Manganese and Black Rock Manganese Mines were of great help in providing much of the borehole data for South Africa. The staff at the Geological Survey Departments of Angola, Namibia, Zambia, Zimbabwe and Botswana are also thanked for their co-operation in providing data for the compilation of the isopach and geological maps. South African National Parks are thanked for allowing me access to Kalahari Group outcrops in the Kalahari Gemsbok Park. Mike de Wit and John Ward of De Beers provided valuable feedback on the palaeo-drainage reconstructions of the region as well as the stratigraphy of the northern parts of the Kalahari Basin. At the Council for Geoscience Greg Botha is thanked for his guidance during the early stages of this project. Barry Millsteed, Mike Johnson, Gerrit de Kock and Matt Du Toit provided excellent advice and scientific input and Nols Van Vuuren and Peter Zawada provided continuous support and encouragement for the project. Marcel Brits and Kryzia Guszek helped with the digitising of some of the base maps used for the compilations. Doreen Van der Walt spent endless patient hours teaching me how to use ArcInfo and helping me produce the GIS-generated maps. The director of the Council for Geoscience is thanked for the opportunity to do this research and for permitting the work conducted on the Kalahari Basin to be used for a thesis. My thesis supervisor Prof. Spike McCarthy provided invaluable criticism and insight and continually challenged me to re-examine preconceived and outdated ideas about the Kalahari Basin. Lastly, thanks to my parents and Julia for all of their support and encouragement. ABSTRACT Geophysical, borehole and mapped data from the Kalahari Basin were used to create maps of the sub-Kalahari geology, isopachs of the Kalahari Group and basal gravels and a sub-Kalahari topographical surface. These are the first basin-wide maps of this type to be produced. These new data were interpreted with the aid of an extensive literature review as well as data gathered at three localities in the southern part of the Kalahari Basin and enabled several conclusions to be made regarding the tectonic evolution of the area. The sub-Kalahari Geological Map shows that rocks dating from the Archaean to present are exposed on the edges of the basin as well as covered by the Kalahari Group sedimentary rocks. Many of the rocks shown on the sub-Kalahari geological map record a history of rifting and subsequent collision, with the NE and SW trending structures appearing to have been reactivated at various times in the geological past. The extent of Karoo Supergroup rocks is greater than previously thought and Karoo sedimentary and volcanic rocks cover a large percentage of the sub-Kalahari surface. The Karoo Supergroup lithologies have been intruded by dolerite dykes and sills and the massive Botswana Dyke Swarm is shown on the sub-Kalahari map extending in a northwest direction across Botswana. The subtraction of the thicknesses of Kalahari Group sediments from the current topographical digital elevation model (DEM) of Africa in order to prepare a DEM of the sub-Kalahari topographical surface and the preparation of an isopach map of the basal gravels gives some indication of the courses followed by Mid-Cretaceous rivers. Topographic profiles along the proposed courses of these rivers show that the floor of the Kalahari Basin has a particularly low elevation in certain areas suggesting that downwarp of the interior of the basin rather than adjacent uplift was the driving force behind Kalahari Group sedimentation. When down-warp of the Kalahari Basin began in the Late Cretaceous these rivers were back-tilted into the newly formed basin and deposition of the Kalahari Group sediments began. The basal unit of the Kalahari Group consists of gravels deposited by the Cretaceous rivers as well as on scree slopes. As down-warp of the basin continued, so more gravels were deposited as well as the sand and -i- finer sediment carried by the rivers. Thick clay beds accumulated in the lakes that formed by the back-tilted rivers, with sandstone being deposited in braided streams interfingering with the clays and covering them in some areas as the shallow lakes filled up with sediment. During the Mid-Miocene there was a period of tectonic stability that saw the silcretisation and calcretisation of older Kalahari Group lithologies. At the end of the Miocene there was some uplift along the eastern side of southern Africa as well as along certain epeirogenic axes in the interior. In general this uplift was fairly gentle. Later more significant uplift in the Pliocene possibly elevated Kalahari Group and Karoo Supergroup sedimentary rocks above the basin floor and exposed many of them to erosion. The eroded sand was washed into the basin and reworked into dunes during drier periods. This uplift occurred along epeirogenic axes and was greater than the Miocene uplift. The development of the East African Rift System (EARS) in the Late Eocene or Oligocene has had a significant influence on the Kalahari Basin. Reactivation of older NE-SW trends by SW- propagating rifts extending from the main EARS is evident by recent movement along faults along the Damara Belt and those that were associated with Karoo sedimentation and post-Karoo graben formation. The propagating rifts have resulted in uplifting, faulting and in some cases, graben formation. In some cases lakes have formed in the grabens or half-grabens themselves and in other cases they have been formed between the uplifted arches related to parallel rifts. The propagating rifts have had a strong influence on the drainage patterns and shape of the Kalahari Basin, in particular in the middle parts of the basin where they have controlled the formation of the Okavango Delta and the Makgadikgadi pans. -ii- LIST OF FIGURES Chapter 1: Introduction Fig. 1.1 Locality map of the Kalahari Basin Fig. 1.2 Main roads, railway lines and towns in the Kalahari Basin. Chapter 2: Methodology Chapter 3: Sub-Kalahari Geology Fig. 3.1 Aeromagnetic coverage of southern Africa (data from Council for Geoscience). Fig. 3.2 Gravity coverage of southern Africa (data from Council for Geoscience). Fig. 3.3 Estimated depth to magnetic basement in Botswana (after Pretorius, 1984). Fig. 3.4 Summary of main Precambrian structures referred to in the text (mainly after Reeves, 1979; Carney et al. , 1994). Fig. 3.5 Locality map, showing the outcrop distribution of the Transvaal Supergroup in South Africa and southern Botswana (after Moore et al. , 2001). Fig. 3.6 The distribution of the late middle Proterozoic basins (after Borg, 1988). Fig. 3.7 The evolution of the KSG Rift (after Borg, 1988). Fig. 3.8 Distribution of the Damara, West Congolian and Katanga Supergroup rocks underlying the Kalahari Group sedimentary rocks. Fig. 3.9 A correlation of the Damara Belt tectonic zones between Namibia and Botswana (modified from Carney et al. , 1994). Fig. 3.10 Distribution of Karoo Supergroup rocks underlying the Kalahari Group sedimentary rocks. Fig. 3.11 Tectono-geographic map of the Late-Carboniferous to Early Permian transition (300-280 Ma) of the African segment of Gondwana (after Visser and Praekelt, 1996). Fig. 3.12 Tectono-geographic map of the Early to Late Permian transition (260- 255Ma) of the African segment of Gondwana (after Visser and Praekelt, 1996). Fig. 3.13 Tectono-geographic map of the Permian to Triassic transition (250-245 Ma) of the African segment of Gondwana (after Visser and Praekelt, 1996). Fig. 3.14 Distribution of rifts in southern Africa (mainly after Vail,1967; Lambiase,1989; Shoko and Gwavava,1999). Fig. 3.15 The distribution of the ~180 Ma dykes in southern Africa (modified from Reeves, 2000). Fig. 3.16 (a) Aeromagnetic and (b) gravity coverages of the Morokweng Impact Structure (data from the Council for Geoscience). Chapter 4: Distribution and lithostratigraphy of the Kalahari Group Fig. 4.1 Isopach map of the Kalahari Group showing the main depocentres or sub-basins. Fig. 4.2 Distribution of pedogenic duricrusts in southern Africa (after Botha, 2000). Fig. 4.3 The stages of the formation of calcretes (after Netterberg, 1980). Fig. 4.4 The vegetated sand dunes of the southern Kalahari. (a) The road between the Auob and Nossob Rivers, Kalahari Gemsbok Park, South Africa. (b) The broad interdune areas to the south of the Kalahari Gemsbok Park. Fig. 4.5 Surface sand types in Botswana (after Baillieul, 1975). Fig. 4.6 The three major dune fields of the Kalahari (after Thomas and Shaw, 1991a). Fig. 4.7 Summary of implied wind directions from Kalahari sand dunes (after Mallick et al. , 1981). Fig. 4.8 Summary of the main characteristics of the five dune classes in the southwestern Kalahari dunefield (after Bullard et al. , 1995). Fig. 4.9 Linear dune class distributions throughout the southwestern Kalahari dunefield (after Bullard et al. , 1995). Fig. 4.10 Histograms of dune and aeolian sediment luminescence ages derived from: (a) the southern Kalahari and (b) the middle and northern Kalahari (after Thomas and Shaw, 2002).
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