The Mesozoic–Cenozoic interior sag basins of Central Africa: The Late-Cretaceous–Cenozoic Kalahari and Okavango basins
Introduction
Research on the geology of the Kalahari basin has been conducted over the last hundred years, commencing with Passarge’s (1904) detailed account of the geology of Botswana, Die Kalahari. Much of the previous geological research in the region has been well summarized in various syntheses of data (e.g. Dingle et al., 1983, Thomas and Shaw, 1990, Thomas and Shaw, 1991, Thomas and Shaw, 2002, Haddon, 2000) and this chapter attempts to build on this previous work by including more recent interpretations.
Kalahari Group sedimentary rocks were deposited in a large basin stretching some 2200 km from South Africa in the south northwards through Botswana and Angola into the Democratic Republic of the Congo. The sediments were predominantly deposited by rivers in the Late Cretaceous and early Tertiary with later aeolian reworking of the uppermost unconsolidated sands occurring during the Pliocene and Quaternary. These unconsolidated sands are believed to form the largest continuous sand body on earth (Baillieul, 1975). The thickness of the Kalahari Group sediments varies from a few cm to around 450 m, with the thickest deposits along the border between Namibia and Angola, in central Angola, in southeastern Namibia, along the border between South Africa and Botswana and to the northeast of Okavango Delta in northern Botswana (Fig. 1). Although Fig. 1 shows the extent of Kalahari Group sediments as only just extending into the Democratic Republic of the Congo, it is possible that prior to uplift in the late Tertiary and Quaternary and erosion by the Congo River and its tributaries, Kalahari Group sediments covered much of the Congo Basin.
Section snippets
Pre-Kalahari geological setting
Directly underlying the Kalahari Group sediments are rocks representing every time period from the Archaean to the Cretaceous, the result of a long history of uplift and erosional events in the region prior to Kalahari Group deposition. A large proportion of the sub-Kalahari floor consists of rocks belonging to the Karoo Supergroup as well as post-Karoo dolerite sills and dykes (Haddon, 2001). Although pre-Kalahari Cretaceous rocks have been recorded beneath the Kalahari sediments over much of
Kalahari group stratigraphy
The litho-stratigraphy of the Kalahari Group has been determined mainly by analysis of borehole cores from around the basin as well as from the infrequent outcrops of the rocks in river valleys and in mine exposures. Local variations in the stratigraphic succession occur throughout the basin (Fig. 2) making correlations between different areas difficult, but generally, a similar stratigraphy has been recognised in most parts of the basin. Basal conglomerates and gravels are commonly overlain by
Conclusions
Following uplift around the margins of southern Africa in the mid to late-Cretaceous, drainage patterns in southern Africa were dominated by several rivers flowing southeastwards from Angola before flowing into the Indian Ocean via the Limpopo as well as rivers draining westwards via the Kalahari and Karoo Rivers. Subsidence of the interior in the late Cretaceous back-tilted the drainage and resulted in sediment deposition in the newly formed Kalahari Basin. Later uplift and subsidence and
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