Surficial sediments of the wave-dominated Orange River Delta and the adjacent continental margin off south-western Africa

Rogers, J; Rau, A J Rua

Surficial sediments of the wave-dominated Orange River Delta and the adjacent continental margin off south-western Africa

Journal Article

2006

Journal Title

African Journal of Marine Science

Link to Journal

http://www.tandfonline.com/toc/tams20/current

Publisher

University of Cape Town

Department

Department of Geological Sciences

Faculty

Faculty of Science

Abstract

The textural and compositional characteristics of the surficial shelf sediments north and south of the Orange River Delta are reviewed and compared. Sediments are fractionated and dispersed both north- and southwards of the Orange River mouth by wave action, longshore drift and subsurface currents. The mean grain size decreases both offshore and southwards in response to decreased wave influence at the seabed and the competence of the weak poleward undercurrent respectively. The increasing dominance of marine biogenic components in sediments south of the prodelta indicates a greater marine influence, modifying previous inferences that the Namaqualand mudbelt is primarily derived from the southward transport of Orange River sediments. A sharp distinction can be drawn between sediments of the Orange Shelf to the south and the Walvis Shelf to the north. Foraminiferarich deposits that dominate the Orange middle shelf and slope indicate that upwelling is an inner-shelf phenomenon. On the Walvis Shelf, foraminiferal sediments are confined to the slope and outer shelf. Fish debris is more common in Walvis Shelf sediments. Although phosphorite and glauconite sands often occur together in the same deposits on the Orange Shelf, the two minerals are concentrated in separate deposits on the Walvis Shelf.