SHELF MORPHOLOGY AND SUBMERGED SHORELINE PRESERVATION: EXAMPLES FROM KWAZULU-NATAL, SOUTH AFRICA

Shelf stratigraphy and shoreline preservation on the high wave energy shelf of KwaZulu-Natal South Africa were examined using high resolution seismic and multibeam bathymetric methods. This paper examines variations in shoreline preservation in relation to shoreline slop and width.

The northern shelf sector is 2-4 km wide with a gradient ca. 2.7°. Three seismic units are preserved and represent calcareous sedimentary barriers (shorelines), back barrier sediments and a highstand sediment wedge. Barriers are best preserved at -100m and -60m with an intervening erosional surface between each. Scarps in the surface represent now buried headland-embayment cells separated by rocky headlands on the high gradient coast.

The central shelf sector is 8-15 km wide with a gradient of ca. 1°. Five seismic units are identified that record a large-scale cemented barrier complex with back-barrier systems comprising lake/lagoon depression and extensive, well-preserved parabolic dune fields.

Differences in shelf setting cause variations in the style of barrier-preservation and associated transgressive stratigraphy. A much thicker post-transgressive sediment drape, higher degrees of ravinement and only partial barrier preservation exist on the steeper and narrower shelf section. A simpler transgressive arrangement of shelf elements is produced. In contrast, the more gently sloping shelf shoreline complex contains large-scale equilibrium forms, dune fields and significantly higher degree of preservation of the shoreline features.