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The Long-Term Evolution of an Exhumed Deepwater Stepped-Slope Profile

Abstract

The impact of changes in slope angle and orientation on turbidity current behaviour has been investigated in many physical and numerical experiments and interpreted from outcrop, subsurface and modern systems. However, the long-term impact of subtle and dynamic seabed topography on the architecture of deep-water systems is more challenging to constrain. Phases of deposition and erosion associated with a long-lived stepped slope profile have been mapped in the Karoo Basin, South Africa, providing the opportunity to investigate the stratigraphic record of turbidity current behaviour and sediment dispersal patterns over topographically complex slope profiles. This study focuses on the Permian Laingsburg and Fort Brown Formations, where multiple large sand-rich systems (Units C, D, E and F), have been mapped from entrenched slope valleys, through channel-levee systems to basin-floor lobe complexes over 2500 km2. Here, we investigate the Unit E slope fan system, and several thinner (typically <5 m in thickness) and less extensive Units, A/B, B/C, D/E and E/F, which developed as precursors to the larger scale systems. Thickness and facies distributions of these broadly lobate thinner units indicate more subtle responses to seabed topography, suggesting deposition on a dynamic slope profile. Their similar thickness distributions indicate persistent areas of increased slope accommodation, inferring an underlying structural control. Larger-scale systems were also affected by the development of this slope accommodation, but this is less clearly recorded as they significantly modified the slope profile as they evolved. A complex slope during Unit E times is indicated by preservation of both perched lobes and lateral variations in thickness and facies across the base-of-slope, locally forming a bypass dominated channel-lobe transition zone (CLTZ). The maximum extent of this dynamic CLTZ is estimated at 14 km in width and 6 km in length, formed by at least 4 stages of CLTZ expansion/contraction or shifting of the zone. This study highlights the importance of smaller systems in understanding subtle changes in dynamic seabed topography in evolving slopes as well as demonstrating for the first time an entire exhumed CLTZ from channel/levee to lobes and spatial and temporal changes within. This exhumed system can supplement seismic datasets in understanding sedimentary facies, stratigraphic evolution and sand distribution over topographically complex slope profiles.