Geostrophic sand ridge, dune fields and associated bedforms from the Northern KwaZulu-Natal shelf, south-east Africa |
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| Authors: | PETER J RAMSAY ALAN M SMITH† THOMAS R MASON |
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| Institution: | Joint Geological Survey-University of Natal, Marine Geoscience Unit, University of Natal, PO Box 18092, Dalbridge, 4014, South Africa;Geo-Environmental Services, 59 Flemming Johnstone Rd, Umbilo, Durban, 4001, South Africa |
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| Abstract: | Subaqueous dunes are formed on the KwaZulu-Natal outer-shelf due to sediment transport by the Agulhas Current (geostrophic current). These dunes occur within two dune fields at depths of ? 35 to ? 70 m. The net sediment transport direction is south, but short-period reversals form northward-migrating bedforms. The dune fields are physically bounded by late Pleistocene beachrock and aeolianite ledges. A bedform hierarchy has been recognized in the dune fields comprising a system of three generations of climbing bedforms. The outer dunefield has given rise to a sand ridge (H=12 m; L=4 km; W=1.1 km; and an 8° lee slope) whereas the inner dune fields have achieved large-scale dune status. Bedload parting zones within the dune fields occur where the sediment transport direction switches from north to south due to reversals in the geostrophic flow; these zones occur at depths of ? 60, ? 47 and ? 45 m. An interpretative stratigraphic model is presented on what such geostrophite deposits would look like in the ancient sedimentary record. |
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