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Shelf-Edge Recognition Criteria and the Impact of Process Regime Variation on Deepwater Sediment Delivery

Abstract

The shelf-edge is a zone of significant gradient change separating a flat-lying shelf from the steeper slope. The geometry of this gradient change is observable in continuous, dip-oriented outcrops, but requires definition when continuous exposure is obscured. This definition is essential as the shelf-edge separates areas dominated by shelf currents from those dominated by gravity-driven processes. This study uses outcrop data from shelf-to-slope clinoforms of the Permian Kookfontein Formation (Karoo Basin, South Africa) to derive qualitative- and quantitative-recognition criteria for the shelf-edge where the physical gradient change is not visible due to the current geomorphology of the area. We quantify facies proportions and distribution throughout the shelf-margin profile and map 3D relationships between: 1) clinoform thickness; 2) soft-sediment deformation; 3) facies presence; 4) facies transitions; and 5) channel-fill type and thickness to infer the location of the shelf-edge for three individual shelf-to-slope clinoforms. A zone of maximum correlation was calculated for a single clinoform and tested against a 3D reconstruction using a mapped horizontal datum, underlying bounding surface, and a digital elevation model. Using this reconstruction, the interpreted shelf-edge was found to be correlative to the zone of maximum gradient shift between the flay-lying shelf and steeper slope. The variability of process regimes (e.g. fluvial, tidal, or wave processes) at-and-above the shelf-edge was mapped and quantitatively assessed to determine the impact on sediment delivery to the slope. Where the shelf-edge is dominated by strong fluvial drive, the upper slope is characterized by packages of sand-rich, delta-front turbidites downlapping onto slope deposits. These packages are overlain by large collapse aprons incised by laterally accreting channels, which become increasingly wider and thicker further down the slope. In contrast, where the shelf-edge is characterized by strong tidal influence, packages of delta-front deposits that prograde onto the slope are significantly thinner and sand-poor, and only one small channel was observed. These results demonstrate that differential progradation and the potential for deep-water sediment delivery are dependent on process-regimes at the shelf-edge, and in the absence of seismic data, facies transitions and stratigraphic parameters can be used to constrain the location of the shelf-edge.