The influence of topographically complex slopes on deepwater processes and stratigraphic architecture

Topographically complex slope to basin floor profiles are increasingly recognised in modern seafloor, seismic reflection and outcrop datasets, and range from simple slope profiles, through stepped slopes with high gradient ramps linking low gradient steps, to slopes with 3D enclosed minibasins linked by tortuous corridors. This study investigates a range of slope to basin floor topographic configurations with multi-scale (mm to 100 km) variability, using regionally extensive exposures from the Permian slope to basin floor deposits in the Laingsburg depocentre, Karoo Basin, South Africa. Over 400 outcrop logs, totalling 14 km in thickness, combined with a large database from earlier Stratigraphy Group studies, are used to assess the influence of dynamic seabed relief on turbidity current processes and depositional patterns across a range of scales, and their transfer into the stratigraphic record. Degradation plays a large role in shaping submarine slopes. The formation and evolution of a submarine slide complex is investigated, including time transgressive lateral margins of basal shear surfaces/zones and varyingly confined of remobilized and turbidite infill. The base of slope is a key area of gradient change. Here, the spatial and temporal variations of a channel-lobe transition zone (CLTZ) are documented, including how topographic influence on turbidity currents varies and evolves, causing CLTZ expansion/contraction and migration. In addition, how these topographically complex areas are transferred into the stratigraphic record, as single surfaces and volumes of rock, is discussed. Areas of dynamic and fixed topography have been recognised as having long-term effects, leading to the evolution of a stepped slope profile. In the Laingsburg depocentre stepped slope topography initiated before major clastic input and increased temporally, gradually outpacing sediment supply. The effects of slope orientation and gradient change on flow processes and stratigraphic architecture are presented as a range of intraslope lobe deposits and bypass dominated zones.