Submarine canyons associated with alternating sediment starvation and shelf-edge wedge development: Northern KwaZulu-Natal continental margin, South Africa

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Multibeam bathymetry and high resolution boomer data reveal a shelf-edge wedge embayed by numerous canyons on the currently sediment starved northern KwaZulu-Natal continental margin. Submarine canyons in the southern portions of the study area may be differentiated from those in the north by the presence of an underlying erosive surface marked by palaeo-depressions and a thinner shelf edge-wedge. Canyons exhibit square interfluves in the mid-slope, slump and landslide failure surfaces in their walls and high amplitude chaotic reflectors, interpreted as slump deposits, infilling older thalwegs. The shelf-edge wedge architecture where it is embayed by canyons is progradational into the canyon axes and marked, particularly in the north, by block like features. These are interpreted as syn-depositional growth faults. Lenses comprising chaotic, high amplitude reflector packages are common within the wedge and are interpreted as laterally discontinuous slump masses. Instability of the shelf-edge wedge is considered the driving force for canyon inception. Initial failures generated downslope eroding sediment flows and submarine rills that evolved into their modern forms by retrogressive slumping when the rill walls became oversteepened. Continued delivery of sediment to the shelf-edge wedge was contemporaneous with canyon widening and headward erosion. Mid-slope canyon topography formed as the result of sandy turbidity currents that ignited downslope of these slump sources, coupled with non-aggradation of the adjoining interfluves. Differences in the thickness of the shelf-edge wedge are the result of varying fluvial influences. Where larger rivers crossed the shelf during sea-level lowstands the shelf-edge wedge has been cannibalised. This is more pronounced in areas with subsurface palaeo-depressions that may have constrained the downslope eroding phase, especially during lowstands. Where there were insignificant fluvial influences, no subsurface depressions and a better preserved shelf-edge wedge are apparent. Post dating the shelf-edge wedge, a protracted period of sediment starvation occurred, halting canyon development and stunting more elaborate erosion patterns such as those of other margins.