How Much Sediment Has Been Eroded off Southwestern South Africa Since the Cretaceous, and Where is it Now?

Abstract

Quantifying the flux and character of sediment generated from continents and transported to oceans remains a challenge, which requires an estimation of the volume and lithology of material removed and the timing of erosion to be constrained. The majority of exhumation onshore Africa occurred during the Late Cretaceous-Early Tertiary and is related to uplift associated with the African superplume. A better understanding of the long-term landscape evolution can be determined by constraining the volume removed from a c. 140,000 km² area in SW South Africa since the start of the Cretaceous. The study area covers three main drainage basins, the Olifants, the Breede, and the Gouritz. An advantage of investigating this area is that the drainage network is demonstrably antecedent, the onshore geology is well constrained, and that there has been little tectonic activity since the Cretaceous. Several assumptions need to be made in order to construct a grid of structural cross sections across the study area, with the estimated ‘maximum’ and ‘minimum’ thicknesses of lithostratigraphic groups extrapolated above present day topography. The extent of the lithostratigraphic units at the time of exhumation is constrained by provenance analysis of onshore Cretaceous deposits that exposed in the hanging-wall of major extension faults. The cross-sections have been georeferenced in 3DMove, with a surface fit between. The volumes calculated range from 0.72 × 106 km³ to 1.56 × 106 km³. The southern offshore basins are of interest to industry, and with a better understanding of the volume and composition of the eroded sediment; assessment of reservoir quality can be improved. The estimated volume of material is more than several of the largest submarine fans on the planet, and is an order of magnitude greater than the material found in offshore basins. Explanations for this missing sediment include breakdown and transport as fines, unidentified submarine fans, or that the majority of exhumation occurred during the early Cretaceous break-up of Pangaea and has been tectonically transported elsewhere.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015