GEOGRAPHICAL CONTROLS ON SEDIMENT ACCRETION OF THE CENOZOIC ALGOA GROUP BETWEEN OYSTER BAY AND ST. FRANCIS, EASTERN CAPE COASTLINE, SOUTH AFRICA

Abstract

Borehole data provide an opportunity to explore the thickness distribution of Late Cenozoic Algoa Group sediments along the southern Eastern Cape coastline, between Oyster Bay and Cape St. Francis, South Africa. Bedrock lithology, bedrock elevation, surface relief and sediment supply are factors considered to influence thickness variations locally. Variations in accretion values allow recognition of four coast-parallel trending zones, referred to here as zones A to D. Zones A and B are characterised by a general trend of increasing thickness, with increasing distance from the coastal margin and occur in the first few hundred metres from shore. Zone A occurs closest to the shoreline and records the lowest sediment thicknesses. At Thyspunt zone A extends inland from the coastal margin for ~250 m and sediment thicknesses here range between 3 to 12 m. Zone B defines the inland area between ~250 to 750 m from the shoreline. A clear progression from zone A accretion values into thicker zone B sediment thicknesses is recognized at Thyspunt, where sediment thickness typically range between 12 to 26 m. Further inland, zone C, a zone of peak thickness values (61 m) occurs where the Algoa Group is underlain by the argillaceous Goudini and Cederberg Formations. Bedrock elevations below sea-level and the occurrence of fluvial deposits above these argillaceous bedrock lithologies indicate the presence of a possible palaeo-valley near Thyspunt and Cape St. Francis. Such a palaeo-valley allowed for greater accretion of sediments. Greatest accretion values occur in these areas especially where coupled with elevated surface relief as is the case towards the northwestern regions of the study area where elevated dune heights occur close to the area’s upwind sand supply. Furthermore here the occurrence of the Cape St. Francis anticline cored by quartzite of the Peninsula Formation form a bedrock ridge impeding sediment transfer to the east, concentration dune mass along the western limb of the anticline. The transition to zone D is marked by the dramatic decline from peak sediment thickness values to thinner accretion values (