Geomorphology and Stratigraphy of the south Kenyan continental shelf: Coral reef growth, backstepping and carbonate-siliciclastic interaction

The southern Kenyan shelf is amongst the narrowest in the world, averaging ca. 1 km. It has an irregular topography with alternating areas of topographically high hardground and mobile sediment in topographic lows. The shelf edge is characterised by multiple slumps. Seismic surveys reveal the stratigraphy to comprise a basal erosional surface, overlain by a coral reef unit (average interpreted thickness ca. 20 m) that spans the entire shelf width and contains two discrete ridges where interpreted reef thickness exceeds 50 m. The sequence is topped by a highly irregular surface that reflects the contemporary reef morphology. The surface bathymetry, backscatter and reef stratigraphy indicate periodic growth interrupted by backstepping that is attributed to variable rates of sea-level rise. Surface sediment comprises transgressive siliciclastic sand derived from lowstand river deltas. Carbonate sediment input from the reef is limited by low energy conditions, although coarse debris is present at the base of the contemporary reef slope. Reef evolution was accompanied by deposition of transgressive sands and the two are intimately associated, with sand being trapped on the seaward edge of growing reef crests and deposited in inter-reef depressions.