The evolution of the Molopo drainage

[摘要] The appearance of the dry Molopo River with its generally wide and shallow valley cut into theKalahari Group sediments, but more especially its few impressive rock-cut gorges, has intriguedmany people over the ages, and led to many theories concerning its history. The rock-cutgorges, in particular, have traditionally been attributed to ancient, previously much moreextended drainage lines, more or less in the same geographical position as the present MolopoValley. An analysis of the sediment body of the Kalahari Group, and both, alluvial gravels andgeomorphic features on the rim of the Cainozoic Kalahari Basin, have indicated that the Molopodrainage has gradually shifted westward over time, in response to a tilting of the drainage area,which lies across the south-eastern rim of the Kalahari Basin, The present geographical positionof the Molopo River is thus a relatively recent one in its evolution. The earliest traces ofdrainage lines in the area predate the Permo-Carboniferous glaciation of Gondwana. A preglaciationvalley system with only some similarities to the present-one flowed in a northwesterlydirection. There is a long hiatus in the evidence from the end of the glaciation to thebeginning of the formation of the Calnozoic Kalahari Basin. The most significant feature of thatintermediate period, is a large meteorite impact which occurred near Morokweng at the J-Kboundary.The Cainozoic evolution of the Molopo drainage has been strongly influenced by both tectonicsand climatic change. Tectonics, which include both the initial formation of the Kalahari Basinand Neogene warpings of the intra-continental axes of uplift, caused the interruption of a pre-Kalahari southward flowing drainage system, an extended upper Molopo, the existence of theMolopo as an endoreic system for an extended period of time, and a westward shift of the entireMolopo drainage system. Progressively more arid conditions interrupted by humid climaticpulses of decreasing intensity have occurred since the Cretaceous. The aridifying conditionscaused the existence of playa-like conditions over long periods oftime in the back-tilted sectionof the proto-Molopo. This was followed by a rapid sediment infilling of the sub-basin and amajor rejuvenation phase, which caused the incision of the Molopo River into the duricrustedsediments of the Kalahari Group, and the re-establishment of the Molopo River as an exoreicdrainage system in its present position. Rock-cut terrace remnants in the upper Molopo giveevidence of much smaller climatic changes during the Quaternary. There has been no integratedflow in living memory.

[发布机构] University of the Witwatersrand