Quaternary continental margin sedimentation off the southeast coast of South Africa

Abstract

A sedimentological study of Quarternary deposits in a deep-sea channel off the South African continental margin reveals that terrigenous and shelf (T and S) components from the sand fraction are concentrated on the upper continental slope and the channel floor. In comparison, sediments from the lower flanks of the channel contain relatively low amounts of these components. Based on these and seismic studies, four major controls on this distribution pattern have been established: (1) a marginal fracture ridge; (2) fluctuations in sea level; (3) dilution of the T and S components by biogenic carbonate; and (4) redistribution of the T and S components and dissolution of the carbonate by turbulent, corrosive bottom currents. Variability in the T and S component distribution patterns is related to changes in the influence of the last three controls due to climatic fluctuations. During interglacial periods, higher sea levels lead to the trapping of coarse-grained T and S components on the shelf; the shallow carbonate compensation depth (CCD) coupled with turbulent bottom currents leads to the dissolution of the (mainly planktonic) carbonate component on the floor of the Agulhas Passage; and redistribution of the T and S components by strong bottom currents is effected. Glacial periods are characterized by lower sea levels enabling T and S components to be transported directly offshelf into the passage; the deep CCD and weak bottom water turbulence leads to the dilution of the T and S component on the passage floor by carbonate; bottom currents are too weak to effectively redistribute this component.