Linking source and sink: Evaluating the balance between onshore erosion and offshore sediment accumulation since Gondwana break-up, South Africa

The geomorphic origin and evolution of the tectonically unique interior highland of southern Africa, the Kalahari Plateau, and its flanking low-lying coastal planes, remain largely unresolved because of a lack of regional quantitative analyses of its uplift and erosion history. Here we focus on the southern Cape, South Africa and link onshore denudation, based on new apatite fission track thermochronology results, to offshore sediment accumulation, using abundant well data and a seismic reflection profile. We attempt to relate source and sink in order to resolve some first order issues concerning timing of the exhumation and development of the topographic features of southern Africa. The volume of sediment accumulated off South Affica's south coast is calculated using 173 wells and a seismic reflection profile. A total, uncompacted, sediment volume of 268,500 km(3) accumulated off South Africa's south coasts since similar to 136 Ma, in the Outeniqua and Southern Outeniqua Basins. Accumulation volumes and rates were highest in the early Cretaceous (48,800 x 10(4) km(3) at similar to 8150 km(3)/Ma from similar to 136 to 130 Ma, and 57,500 x 10(4) km(3) at 5750 km(3)/Ma from similar to 130 to 120 Ma) and midlate Cretaceous (83,700 x 10(4) km(3) at 3200 km(3)/Ma from similar to 93 to 67 Ma). Volumes and accumulation rates were lowest for the early-mid-Cretaceous (47,400 x 10(4) km(3) at 1750 km(3)/Ma from similar to 120 to 93 Ma) and the Cenozoic (31,200 x 10(4) km(3) at 450 km(3)/Ma from similar to 67 to 0 Ma). Although our analysis shows that the accumulated volume of offshore sediments does not match the calculated volume of onshore erosion, as quantified through apatite fission track thermochronology (e.g. Tinker, J.H., de Wit M.J., Brown, R., 2008. Mesozoic exhumation of the 439 southern Cape, South Africa, quantified using apatite fission track thermochronology. Tectonophysics, doi: 10.1016/j.tecto.2007.10.009), the timing of increased sediment accumulation closely matches the timing of increased onshore denudation. This suggests that the greatest volumes of material were transported from source to sink during two distinct Cretaceous episodes, and that the processes driving onshore denudation decreased by an order of magnitude during the Cenozoic. (C) 2008 Elsevier B.V. All rights reserved.