Sedimentology of False Bay (Western Cape, South Africa): geological background information for the integrated environmental management of a physically confined coastal compartment

Abstract

Abstract False Bay is a large, physically confined embayment located along the southwest coast of South Africa. It is a classic example of a coastal compartment or littoral cell that acts as a receptor and sink of both terrestrial (siliciclastic) and marine (bioclastic) material. It is almost square in shape, measuring ∼35 km N-S and ∼39 km W-E and covering ∼1130 km2. Only a few small rivers discharge into the bay, as a consequence of which only small amounts of sediment are episodically supplied to the bay. The bathymetry reveals two well-defined terraces, one between 30 and 45 m, the other between 50 and 55 m water depth, which are indicative of extended Pleistocene sea-level stillstands. Sediment is dispersed by long-period ocean swells approaching from the southwestern quadrant, as well as nearshore swell- and wind-driven currents. The presence of a prominent rock pinnacle at the entrance to the bay (Rocky Bank) causes wave orthogonals to converge on its leeward side, and which results in substantial wave amplification along the eastern shore of the bay. Coarse-grained sediments (gravel, very coarse sand, coarse sand and medium sand) line the rocky shores in the west and east, and are dispersed around and away from submarine rock outcrops, whereas fine-grained sediments (fine sand, very fine sand and mud) are largely confined to an arcuate belt extending along the centre of the bay from shallow water in the northwest to the deepest parts in the south. This is documented by the distribution of individual size fractions and the mean grain size, and applies to both the siliciclastic and the bioclastic sediment components which appear to be in hydraulic equilibrium. There is a distinct N-S gradient from lower to higher bioclastic content with increasing water depth. The textural parameters (mean grain size, sorting and skewness) reveal the existence of two hydraulic populations that are mixed in various proportions in the course of their dispersal in the form of either bed load or suspended load transport. The mud fraction is closely linked to the dispersal pattern of the fine and very fine sand fractions, indicating that it is transported in the form of aggregates and/or faecal pellets, the bulk of which is evidently in hydraulic equilibrium with the finer-grained sands. It can be anticipated that, during lower Pleistocene sea levels, the palaeo-False Bay valley was occupied, or at least regularly visited, by large and small African mammals and early humans.