Sediment supply to the Orange sedimentary system over the last 150 My: an evaluation from erosion/sedimentation balance
Résumé
The South African plateau is bordered by passive margin basins preserving the terrigeneous sediment produced during onshore erosion. As such, these basins potentially provide a record of the variation in onshore elevation and relief over time. Here we bring new constraints on the uplift and erosion of the Southern African plateau over the last 150 Ma from the perspective of the stratigraphic architecture of these basins. We review published data to quantify the terrigeneous supply eroded in the drainage area and preserved in the basins. The novel aspect of our approach is the integration of the evolution of the whole domain in sedimentation (i.e. not only the platform) as well as the onshore eroding region. Along the South African and Namibian Atlantic margins, we determined the long-term signal of sedimentary supply (at the 10 Ma scale) from 3D mass balance calculations comparing sedimentary volumes deduced from offshore isopach maps (with a particular attention to associated uncertainties) on one hand and denudation volumes deduced from thermochronology data on the other. We show that, in this case, the volumes of denudation and of sediments preserved in the basin are consistent both in total and incremental amount. The latter is therefore a reliable estimation of the denudation in the associated continental domain, subject to certain assumptions such as the drainage area over time. The long-term signal of sedimentary supply of the South African and Namibian Atlantic margins shows that the major part of the denudation occurred during the post-rift Lower Cretaceous and the Upper Cretaceous and subsequently decreased. This is not the expected behavior for the simple relaxation of topography acquired during rifting that would show a progressive decrease of the sediment supply over time. This suggests a significant rejuvenation of relief during the Upper Cretaceous, 50 Ma after the rifting event. If erosion is directly linked to elevation or relief variation, this implies 2 generations, the first before break-up (i.e. before 130 Ma; presumably during rifting) and the second during the Upper Cretaceous.