Abstract

A comprehensive seismic reflection survey supported by side-scan sonography, box-coring, and grab-sampling has revealed the characteristics of the continental shelf. Holocene sediments form an inner shelf sediment wedge, between 2 and 40 km wide, extending quasi-continuously over 1500 km. It lies unconformably on an incised erosion surface truncating folded and faulted Palaeozoic and Mesozoic strata. The south and east coast shelves have important environmental differences, yet similar sediments have accumulated on both shelves. The east coast shelf is very narrow (minimum 3 km), is dominated by the Agulhas Current and has few coastal bays. The south coast shelf is wide (maximum 270 km), is dominated by southwesterly swells and features several headlands and associated bays. On the east coast, simple linear Pleistocene aeolianite cordons dam Holocene sediments, whereas on the south coast, multiple, subparallel aeolianite ridges interfere with sedimentation. The Agulhas Current, and inshore counter-currents in lee eddies, transport bed load sediments vigorously, generating large transverse bed forms. Where these, and littoral drift transport paths converge, sediment sinks occur. Prevailing swell energy is considered to limit upward accumulation, promoting progradation and construction of “submerged spit-bars” at water depths of between 20 and 90 m. Similar features accumulate on the south coast where littoral sediments driven eastward by swells cannot bypass prominent capes. Sediments build out beyond the headland forming a sediment sink and the eastern margin of a sedimentary compartment. In these situations, the sediment wedge reaches a maximum thickness of 67 msec (54 m) forming sediment bodies up to 8 km long and 5 km wide, which comprise over 5 km³ of sediment. These bodies are characterized by large cross-beds, seen in seismic profiles, and small-scale cross-bedding, observed in box-cores. Such features incorporating porous, medium-grained sand prograding over organic-rich muds may provide important hydrocarbon reservoirs. The linear, narrow South African shelf wedge, which incorporates both geostrophic, current-generated, transverse bed forms and cross-bedded, submerged spitbars, represents a novel facies model for the inner shelf. Such sediment bodies in the rock record may be mistaken for beach/barrier/subaerial spitbar complexes accumulating in shallow tidal seas.