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STRATIGRAPHY AND BASIN MODELLING OF THE GEMSBOK SUB-BASIN (KAROO SUPERGROUP) OF BOTSWANA AND NAMIBIA Valerie Nxumalo A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of requirements for the degree of Master of Science Johannesburg, 2011 DECLARATION I declare that this dissertation is my own, unaided work. It is being submitted for the Degree of Master of Science at the University of the Witwatersrand, Johannesburg. It has not been submitted before for any degree or examination in any other University. The information presented in this dissertation was obtained by me while employed by the Council for Geoscience, Pretoria. ___________________________________ (V. Nxumalo) ______________day of _____________201____ i ABSTRACT The Gemsbok Sub-basin is situated in the south-western corner of the Kalahari Karoo Basin and extends south from the Kgalagadi District of Botswana into the Northern Cape (South Africa); and west into the Aranos Basin (southeast Namibia). The Sub-basin preserves a heterogeneous succession of Upper Palaeozoic to Lower Mesozoic sedimentary and volcanic rocks of the Karoo Supergroup. Because the succession is largely covered by the Cenozoic Kalahari Group, the stratigraphy of the succession is not as well understood as the Main Karoo Basin in South Africa. Most research in the Gemsbok Sub-basin is based on borehole data. This study focuses on the intrabasinal correlation, depositional environments and provenance of the Karoo Supergroup in the Gemsbok Sub-basin in Botswana and Namibia. Based on detailed sedimentological analyses of 11 borehole cores of the Karoo Supergroup in the Gemsbok Sub-basin of Botswana and Namibia, 8 facies associations (FAs) comprising 14 lithofacies and 2 trace fossil assemblages ( Cruziana and Skolithos ichnofacies) were identified. The facies associations (FA1 to FA8) correspond to the lithostratigraphic subdivisions (the Dwyka Group, Ecca Group, Beaufort equivalent Group, Lebung Group [Mosolotsane and Ntane formations] and Neu Loore Formation) of the Karoo Supergroup. Sedimentological characteristics of the identified facies associations indicate the following depositional environments: glaciomarine or glaciolacustrine (FA1, Dwyka Group), deep-water (lake or sea) (FA2, Ecca Group), prodelta (FA3, Ecca Group), delta front (FA4, Ecca Group), delta plain (FA5, Ecca Group), floodplain (probably shallow lakes) (FA6, Beaufort Group equivalent), fluvial (FA7, Mosolotsane and Neu Loore formations) and aeolian (FA8, Ntane Sandstone Formation). The Dwyka Group (FA1) forms the base of the Karoo Supergroup in the Gemsbok Sub- basin and overlain by the Ecca Group deposits. Three types of deltas exist within the Ecca Group: fluvial-dominated; fluvial-wave interaction and wave-dominated deltas. The Gemsbok Sub-basin was characterised by rapid uplift and subsidence and high sediment ii influx during the deposition of the Ecca Group. Petrographic and geochemical analyses of the Ecca Group sandstones revealed immature arkose and subarkose type sandstones dominated by angular to subangular detrital grains, sourced from transitional continental and basement uplifted source areas. The sandstones of Ntane Sandstone Formation are classified as subarkoses and sourced from the craton interior provenances. Key words : Gemsbok Sub-basin, Kalahari Karoo Basin, Karoo Supergroup, facies associations, provenance. iii ACKNOWLEDGEMENTS This project forms part of a larger project titled ‘Tri-Nations Karoo Basin Correlation Project’ (Project No. CO-2007-562) that was initiated by the Council for Geoscience. Financial support was provided by the Department of Science and Technology (DST), South Africa. I would like to thank the geological surveys of Botswana and Namibia for giving access to borehole cores for this study. Dr. J. Neveling, Prof. B.S. Rubidge and Prof. G. Drennan are thanked for their wonderful supervision, patience and understanding. I would also like to thank my colleagues at the Council for Geoscience for their support on the project. Particular gratitude is extended to Prof. J.A. MacEarchern (Simon Fraser University, Canada) and Associate Prof. L. Buatois (University of Saskatchewan, Canada) for assisting me in identifying trace fossils in the borehole cores in this study. iv TABLE OF CONTENTS DECLARATION ................................................................................................................. i ABSTRACT ........................................................................................................................ ii ACKNOWLEDGEMENTS ............................................................................................... iv TABLE OF CONTENTS .................................................................................................... v LIST OF FIGURES .......................................................................................................... vii LIST OF TABLES ........................................................................................................... xiv LIST OF APPENDIX FIGURES...................................................................................... xv CHAPTER ONE: INTRODUCTION ................................................................................. 1 1.1 General Introduction ............................................................................................... 1 1.2 Tectonic framework of the Karoo-age basins across southern Africa .................... 2 1.3 Stratigraphic review of southern African Karoo-aged basins ................................. 5 1.4 Provenance of the rocks in the Kalahari Karoo Basin .......................................... 22 1.5 Structural framework of the Kalahari Karoo Basin .............................................. 25 1.6 Correlation of the Karoo Supergroup in the Kalahari Karoo Basin of Namibia and Botswana ............................................................................................................... 26 CHAPTER TWO: AIMS AND METHODS .................................................................... 28 2.1 Introduction ........................................................................................................... 28 2.2 Location of the study area ..................................................................................... 28 2.3 Methodology ......................................................................................................... 31 CHAPTER THREE: SEDIMENTOLOGY ...................................................................... 35 3.1 Introduction ........................................................................................................... 35 3.2 Facies analysis ...................................................................................................... 35 3.3 Lithofacies description .......................................................................................... 37 3.4 Invertebrate Palaeontology ................................................................................... 45 3.5 Facies associations ................................................................................................ 52 v CHAPTER FOUR: SANDSTONE PETROLOGY, GEOCHEMISTRY AND PROVENANCE .................................................................................................... 72 4.1 Introduction ........................................................................................................... 72 4.2 Sandstone petrography .......................................................................................... 72 4.3 Detrital framework grains ..................................................................................... 77 4.4 Detrital modes ....................................................................................................... 82 4.5 Geochemistry of sandstones ................................................................................. 86 4.6 Sandstone classification using geochemistry ........................................................ 92 4.7 Sandstone provenance and tectonic settings ......................................................... 92 CHAPTER FIVE: DISCUSSION ..................................................................................... 97 5.1 Stratigraphy ........................................................................................................... 97 5.2 Depositional Environments ................................................................................. 108 5.3 Geographic distribution of facies associations ................................................... 126 5.4 Geological history of the Gemsbok Sub-basin ................................................... 129 CHAPTER SIX: CONCLUSIONS ................................................................................. 135 CHAPTER SEVEN: REFERENCES ............................................................................. 138 7.1 List of References ............................................................................................... 138 APPENDICES ................................................................................................................ 151 vi LIST OF FIGURES Figure 1.1 The distribution of Karoo-aged basins in southern Africa ............................... 1 Figure 1.2 Pan-Gondwanian foreland system generated through compression, collision and terrain accretion along the southern margin of Gondwana and the associated foreland basins..................................................................................................................................
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