A sedimentation on the continental margin off the Orange river and the Namib desert

Abstract

Abstract by author:

On the Walvis shelf off Lüderitz residual glauconite was reworked from older Neogene sediments, particularly on Lüderitz Bank. North of latitude 26°S residual phosphorite pellets were probably formed in Neogene diatomaceous oozes and then concentrated during a Late-Tertiary regression. Erosion of semi-consolidated Neogene clays and ?Palaeogene quartzose limestones on the middle and outer Walvis shelf led to deposition of very fine residual quartz sand south of Lüderitz. Coarse quartz sand was reworked from littoral sandstones on the middle shelf south of the Orange River

The effect on sedimentation in the hinterland of poleward shifts of climatic belts during Quaternary interglacials and the reverse movement during glacials is assessed

Recent terrigenous sediments are derived by erosion of poorly consolidated Karoo sediments in the catchment of the Upper Orange. Erratic but powerful floods transport the sediments to the coast in suspension each summer. At the mouth intense wave action dominates the submarine delta of the Orange River and fractionates the sediment load. Sand is transported equatorwards by littoral drift and is blown off beaches towards the Namib Sand Sea during violent Trade-Wind gales each summer. In contrast, silt and clay are transported polewards by a counter-current, particularly during westerly winter storms

Recent biogenic sedimentation is controlled by upwelling in the Benguela Current System, which is most intense off the wind-dominated Sperrgebiet, south of Lüderitz. Weaker upwelling off the Orange River allows oceanic planktonic foraminifera to penetrate to the middle shelf, whereas on the Walvis shelf in the study area they only reach the outer shelf. Towards latitude 25°S the Benguela Current System decays and interacts with poleward-flowing warm oxygen-poor water. Periodic mass mortalities of oxygen-starved phytoplankton lead to some of the most organic-rich sediments in the world