Factors controlling shelf sediment dispersal along the southeast African continental margin

Abstract

Four major factors control sediment dispersal along the southeast African continental margin: (1) morphology of the upper continental margin; (2) wave regime and wind-driven circulation; (3) influence of the Agulhas Current; and (4) sediment supply. These factors interact on various scales to produce a unique regional sediment dispersal pattern. The continental shelf is very narrow and the adjacent slope extremely steep. The shelf platform is interrupted by a number of structural offsets, and locally the shelf break is dissected by numerous submarine canyons. The nearshore zone down to about 60 m waterdepth is dominated by a high-energy swell regime, and in most places is underlain by a thin wedge of sandy sediment. This wedge appears to have achieved dynamic equilibrium with the prevailing energy regime, and additional sediment is rapidly dispersed and fed into a sandstream, situated on the central shelf, where the Agulhas Current dominates sediment transport. Along some parts of the coast the sediment wedge is dammed behind a drowned coastal dune ridge. Within the sandstream, sediment is transported alongshelf until a structural offset is reached, where sediment spills over the shelf break. Four major localities for off-shelf transport are recognized between Maputo and Port Elizabeth. Most of the suspended load appears to be expelled from the system without being deposited on the shelf. In the lee of each offset a clockwise gyre is developed, which produces return-flow currents capable of moving sediment northward, thereby creating bedload partings. In the vicinity of these partings, depositional features are observed that are similar to those normally associated with tidal regimes, e.g., bedforms migrating in opposite directions. Most of the bedload sediment entering the system is eventually deposited in well-defined sinks situated in the lee of structural offsets. This model applies only to inter-glacial high sea-level situations accompanied by strong western boundary flow of the Agulhas Current.