Soft Sediment Deformation Style and Distribution as a Method to Define Shelf-Edge Rollover in Low-Gradient Shelf-Edge Delta Systems

Shelf-edge delta systems are commonly defined by clinothem geometries; however, these are cryptic in low gradient systems. This study demonstrates how facies analysis can be used to help define shelf-edge rollover and evolution of the shelf-edge system in a very fine-grained sandstone dominated, low-gradient shelf-edge delta system in the northern Tanqua Karoo depocenter, South Africa. Lateral and vertical facies associated with slumping and other soft-sediment deformation features at sub-seismic to seismic scale (~1 km laterally, ~100 m vertically) on the Katjiesberg outcrop were examined. Correlation of these units using measured sections and ~3 linear km of digital outcrop models documents the stratigraphic position of two shelf-edge rollover points, which implies progradation of the shelf-edge over time. Progradation and aggradation of predominantly in situ deformation features and their transition into detached slump features are consistently representative of the evolution from shelf-to-slope through time; a view that contrasts with previous studies suggesting that style and extent of soft-sediment deformation are unpredictable in shelf-edge delta systems.

This study highlights intra-basinal process regime variability and facies expressions within a preserved shelf-edge delta system. It underscores the role of slump features in characterizing clinothems and signaling clinothem rollover, which is particularly useful in low-gradient systems with subtle clinothem geometry. The characterization 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.