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The Impact of Slope Topography on Sediment Partitioning and Depositional Architecture in a Deepwater Lower Slope Setting

Abstract

Seismic reflection datasets of slope and base-of-slope systems are widely employed to resolve stratigraphic and architectural features and improve characterization and prediction of sediment delivery to the deep basin. Here, detailed outcrop studies integrated with core and well log data from Unit 5 of the Permian Skoorsteenberg Formation in the Karoo Basin, South Africa reveal important stratigraphic and architectural features (seismic- and sub-seismic scale) of channel and fan systems on a variable slope profile. The interpreted stepped-slope morphology is characterized by spatial and temporal changes in depositional facies and architecture, channel aspect ratio and stacking pattern. Key interpreted depositional environments include channels and overbank deposits, channel-lobe transition zones (CLTZ), frontal splays and lobes, and hemipelagic drape complexes. These architectural features reflect a combination of active and relatively passive sediment supply systems. Channel incision results from knickpoint migration headward from a relative increase of slope gradient while deposition of frontal splays and lobes is associated with a reduction in gradient and confinement. The CLTZs represent areas where flow confinement decreases abruptly, and are characterized by an assemblage of erosional and depositional features. Preferential accumulation of sediment and ‘healing’ of the slope is believed to reflect the deceleration of sediment gravity flows as they encountered lower-gradient steps and partial (flow stripping) or full confinement. We speculate that changes in the equilibrium profile and slope morphology also caused variations in the amount and rate of erosion with increased incision where knickpoints cut through bathymetric highs in an attempt to establish a graded profile. This exhumed slope succession provides an opportunity to constrain the stratigraphic and spatial distribution of this critical part of a deep-water system and emphasizes the role topography plays in partitioning sediment on siliciclastic continental slopes. Importantly, studies such as this facilitate the development of predictive models for application to sub surface datasets and are of particular relevance when predicting reservoir presence and quality in areas poorly constrained by wells and seismic.