The Mesozoic-Cenozoic interior sag basins of Central Africa: The Late-Cretaceous-Cenozoic Kalahari and Okavango basins

摘要

The Kalahari basin formed as a response to down-warp of the interior of the southern Africa, probably in the Late Cretaceous. The down-warp, along with possible uplift along epeirogenic axes, back-tilted rivers into the newly formed Kalahari basin and deposition of the Kalahari Group sediments began. Initial deposition of basal gravels occurred in the channels of the Cretaceous rivers, with other unsorted gravel beds deposited at the base of scree slopes along the edges of valleys and fault-bounded structures. The accumulation of gravels continued as the down-warp of the basin progressed with interbedding of the gravel layers with sand and finer sediment carried by the rivers. Thick clay beds accumulated in the lakes that formed as a result of the back-tilting of 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. A period of relative tectonic stability during the mid-Miocene saw the silcretisation and calcretisation of older Kalahari Group lithol-ogies, and this was followed in the Late Miocene by relatively minor uplift of the eastern side of southern Africa and along certain epeirogenic axes in the interior. More significant uplift that followed in the Pliocene along epeirogenic axes may have elevated Karoo Supergroup and basal Kalahari Group sedimentary rocks above the basin floor where they were exposed to erosion. The eroded sand was washed into the basin where it was reworked and redeposited by aeolian processes during drier periods, resulting in the extensive dune fields that are preserved today. Propagation of the East African Rift System into southern Africa has led to the formation of local depressions and the disruption of drainages, notably of the Kwando and Okavango Rivers. Lakes periodically occupied these depressions in the past. Large alluvial fans have developed in the depressions as a result of loss of flow confinement, and they currently support extensive wetlands (Okavango and Linyanti swamps).