FROM INCISION TO OVERSTEPPING: THE EVOLUTION OF A RELICT SEGMENTED LAGOON ON THE HIGH ENERGY SE AFRICAN CONTINENTAL SHELF

Newly acquired bathymetric and seismic data from the mid shelf of the Durban Bight, KwaZulu-Natal reveal a series of previously undocumented seafloor and sub-bottom features that closely resemble features of contemporary segmented lagoon and coastal lake systems. These comprise semi-circular seafloor depressions, arcuate ridges, cuspate spits and prograding submerged barrier and overlie a well defined incised valley complex with up to five distinct fill facies. The fills document the evolution of the valley system from initial fluvial to low energy lagoonal conditions. The lagoonal fill in this instance has overtopped the valley and spread laterally away from the original incised valley. The submerged shoreline complex resembles contemporary segmented lagoon systems on the sandy northern KwaZulu-Natal and southern Mozambican coastal plain. Formation of the submerged system began with valley incision during the Last Glacial Maximum ~ 18 000 BP. Thereafter, continued transgressive infilling occurred to a point where an extensive lagoon and back-barrier system was established. In contrast to contemporary systems, transgressive infilling rather than shoreline modification dominated these valley fill processes. Once the valley was overtopped, sea levels remained static, causing the net segmentation of the system and slow closure of the tidal basins/circular depressions. The preservation of such seafloor morphology is remarkable and rarely observed in such detail. This is the product of fortuitous cementation (as beachrock and aeolianite) after deposition, rapid overstepping of the shoreline during transgression and sustained periods of non-deposition.