Depositional Environments and Sequence Stratigraphy of an Exhumed Permian Mudstone-Dominated Submarine Slope Succession, Karoo Basin, South Africa

Abstract

Abstract The physical stratigraphy of a 470-m-thick, claystone-dominated exhumed middle to upper submarine slope succession was constrained within an area of 400 km2 in which five sand-prone units were characterized (Units D/E, E, F, G, and H). Units D/E to Unit F show an overall pattern of thickening upward and basinward stepping. This stacking pattern is reversed from the top of Unit F to the base of Unit H, above which basinward stepping is again observed. Different architectural styles of sand-prone deposits occupy predictable stratigraphic positions within the basinward-stepping section, starting with intraslope lobes through channel–levee complexes to entrenched slope valleys. Sandstone percentage is highest in the intraslope lobes and lowest in the slope valley fills, reflecting a change from depositional to bypass processes. The landward-stepping stratigraphy is dominated by claystone units with thin distal fringes of distributive deposits. The upper basinward-stepping succession (Unit H) is a distributive system possibly linked to a shelf edge delta. Across-strike complexity in the distribution of sand-prone units was controlled by cross-slope topography driven by differential compaction processes. Hemipelagic claystones separating the sand-prone units represent shutdown of the sand delivery to the whole slope and are interpreted as relative sea-level highstand deposits. Eleven depositional sequences are identified, nine of which are arranged into three composite sequences (Units E, F, and G) that together form a composite sequence set. The highly organized physical stratigraphic stacking suggests that glacioeustasy, during the Late Permian icehouse period, was the main driving process for the analyzed succession.