Bedform evolution and dynamics of a geostrophic current-swept shelf, northern KwaZulu-Natal, South Africa

Green A. N.; Flemming B. W.; Cooper J. A. G.Wanda T. F.

摘要

When compared to tide-dominated shelves, shelves swept by geostrophic flows are relatively understudied, yet geostrophic currents have the potential to construct substantial current-generated shelf bedforms. This paper examines the evolution of a series of bedforms encountered along the narrow Agulhas Current-swept northern KwaZulu-Natal shelf. Their evolution is placed in the context of progressive current impingement and variation in flows associated with the postglacial transgression and culminating in the present-day highstand. Ultra-high resolution seismic reflection, multibeam, side-scan sonar and single beam data sets reveal several bedform scales and morphologies; wave ripples and shoreface-connected ridges are associated only with the inner shelf, and 2D and 3D very large dunes are more cosmopolitan and span the entire shelf sector. The inner shelf is marked by rock outcrop (aeolianite), surrounded by sandy sediment, grading seaward into sediment starved bedforms associated with bioclastic gravels. Where sufficient sediment exists in local depocenters, very large dunes form discontinuous fields along the outer shelf. The seismic stratigraphy of the outer shelf reflects the effects of postglacial flooding by rising sea levels and increasing current impingement by the Agulhas Current. The Holocene wave ravinement surface is overlain by flat-lying strata (early dune development and dune amalgamation with first current exposure), in turn covered by hummocky, sub-horizontal aggrading beds (amalgamation), overlain by inclined cross-bedded packages (lee faces of the bedforms formed during migration and full current interaction). Morphometric analyses show that for both the inner and outer shelf, no relationships exist between water depth, wavelength and spacing. Height to spacing (H/L) relationships are weak but nevertheless show a broadly positive trend. Bedform heights are lower on the inner shelf compared to the outer shelf, but bedform spacing is greater on the inner shelf, with a 40% overlap in H/L indices observed between the two areas. The departures in overlap can be linked to the competing offshore Agulhas Current and the inshore wave-dominated processes. Bedforms of the inner shelf plot below the global H/L mean and can be related to the more infrequent incursions of the Agulhas Current core. At the time of survey, the Agulhas Current was likely situated well offshore, resulting in reduced current activity in the survey area resulting in rounding of the dune crests, degradation of the dune crests and trough infilling. On the outer shelf, the H/L values plot above the mean global trend, suggesting vertical accretion due to the faster currents. Crest rounding and downlapping of the upstream lee faces onto the downstream stoss faces indicate dune degradation, which is related to a seaward location of the Agulhas Current at the time of survey.

Bedform evolution and dynamics of a geostrophic current-swept shelf, northern KwaZulu-Natal, South Africa

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