DISTRIBUTION OF RECENT SEDIMENTS IN SALDANHA BAY AND LANGEBAAN LAGOON

Summary

Sediment distribution in Saldanha Bay is wave controlled and can be related to the energy levels created by the refraction pattern. Four energy zones are distinguished in the inner bay: a centrally exposed zone, two marginal semi-exposed zones, a sheltered zone in the north and a bay/lagoon transitional zone in the south. Sediment is supplied on the abrasion platform of the exposed zone and along the rocky shoreline of the North Channel. These areas display the coarsest sediments. In North Bay medium sands dominate, whereas the South Channel is characterized by very fine sands. Both semi-exposed zones as well as the sheltered zone consist of very fine sand. The bay/lagoon transitional zone is constructed of a tidal delta made of fine sand. Sediment distribution seems to be in equilibrium with the prevailing hydrodynamic regime. In contrast, Langebaan Lagoon is a current controlled regime. Four physiographic units can be separated: tidal channels, subtidal flats and sandbanks, intertidal flats, and salt marshes. These units reflect the major energy levels of the tidal system. Texturally the lagoon divides into two major zones separated roughly by the central channel. East of the channel fine sands dominate, whereas west of the channel medium sands are more frequent. Coarse sediments occur as lag deposits in the northern outflow channels. In contrast to the dominating presence of very fine sand in Saldanha Bay this size fraction is surprisingly deficient in Langebaan Lagoon. There is evidence for a Holocene high stand of the sea between 6 000 and 2 000 y B.P. The modern sediment distribution is the result of the subsequent regression and stabilization of the sea-level about 1 800 y B.P., and is therefore essentially a relict feature.