Slumping and Sediment Storage at the Shelf-Edge: A Case Study From the Kookfontein and Waterford Formations, Tanqua Karoo Depocenter, South Africa

Abstract

A low-gradient shelf-edge delta system preserved at the Katjiesberg outcrop in the northern Tanqua Karoo depocenter (South Africa) records the evolution from shelf- to slope-depositional processes in a fine-grained sandstone dominated system. Lateral and vertical facies associated with slumping and other soft-sediment deformation features at near seismic scale (~1 km laterally, ~100 m vertically) are documented. Stratigraphic correlations of these units using measured sections and ~3 km of digital outcrop models document the stratigraphic position of two shelf-edge inflection points, which implies progradation of the shelf-edge. Progradation and aggradation of detached slump deposits and their transition into dominantly in situ deformation features are consistently representative of the evolution from slope-to-shelf through time. The exposed Permian–Triassic Kookfontein Formation prodelta/slope succession at Katjiesberg is likely a time-correlative deposit to clinothem cycles previously described 10s of km up depositional-dip to the southwest. Katjiesberg represents the more distal expression of these cyclothems in the lower Kookfontein Formation, whereas the upper cycles are interpreted as collapse-dominated clinothems that are overlain by basinward prograding mouthbar deposits of the Waterford Formation. This study highlights intra-basinal process regime variability and facies expressions within a preserved shelf-edge delta system. Results underscore the role of slump features in characterising clinothems and signalling clinothem rollover, which is particularly useful in low-gradient systems with subtle clinothem geometry. The characterisation of low-gradient clinothem geometries and variability in this shelf-edge delta system improves our understanding of shelf-margin accretion and deepwater sediment delivery across the shelf and slope.