Post-Gondwana drainage and the development of diamond placers in western South Africa

The post-Gondwana history of the major livers in the western part of South Africa is important because these rivers were instrumental in the developme nt of diamond placers along the west coast of southern Africa. The evolutio n of the drainage systems that developed after breakup of west Gondwana can be viewed in three time-slots: the middle to Late Cretaceous, the early to middle Cenozoic, and the late Cenozoic periods. During the middle to Late Cretaceous there were two main river systems drai ning the interior. The one in the south, also referred to as the Karoo Rive r, had its source in the present upper Orange/Vaal drainage basin and its o utlet was at the present Olifants River mouth. The second and more northerl y system, also known as the Kalahari River, drained southern Botswana and N amibia and entered the Atlantic Ocean via the lower Orange River. Erosion d ominated the period immediately after breakup of west Gondwana and most of the diamonds released during erosion of Cretaceous kimberlites in central S outh Africa were transported by the Karoo River to the coast. By early Cenozoic times, the lower Kalahari River had captured the upper pa rt of the Karoo River and the broad configuration of the present Orange Riv er network was established. This capture and northerly shift of the Orange River, on the newly exhumed pre-Karoo surface, was the result of an acceler ated uplift of the southern and eastern subcontinental margins ca. 100 to 8 0 Ma. During the early and middle Cenozoic, the climate was arid to semiari d. This resulted in a substantial reduction in erosion rates and hence few diamonds were re leased from the primary bodies during that time. Late Cenozoic fluvial gravels, however, dated as either middle Miocene or P lio-Pleistocene, contain diamonds that were reworked out of older Tertiary fluvial deposits. Sediments at the base of the Koa Valley and in the upper terraces in the Sak Valley formed tine Koa River, a major tributary of the Orange River during the Miocene, and drained most of the area previously oc cupied by the lower Karoo River. The Koa River thus reworked diamonds trapp ed in the Cretaceous Karoo River deposits or terraces. Younger sediments of the Carnarvon Leegte were never part of the Koa system . In fact, the Sak River captured the upper goa River by late Pliocene time s and the Plio-Pleistocene lower terraces in the Sak Valley and the paleo-C arnarvon Leegte joined as the paleo-Hartbees River-another major tributary of the Orange in the Plio-Pleistocene. Although climatic changes were the major controls that initiated the alluvi al pulses during the Cenozoic, asymmetric uplift of the subcontinent was ul timately responsible for the northwesterly shift of the Orange River.