Stratigraphic Prediction of a Range of Soft-Sediment Deformation Phenomena in a Submarine Slope Setting: An Example from the Tanqua Karoo, South Africa

Richard Wild, David Hodgson, Stephen Flint, and Carlos Oliveira
University of Liverpool, Liverpool, United Kingdom

The Permian Karoo Basin margin evolved from being dominated by mixed bypass and depositional processes (Unit 5, Skoosteenberg Formation), to a largely accretionary system defined by shelf edge deltas and slope lobes (Kookfontein Formation). The changes in slope morphology (an overall reduction in slope angle and increase in slope length) and usable accommodation through time is reflected by a change in the dominant deformation assemblages. The spatial and temporal distribution of the deformation facies suggests that different deformation assemblages occur in distinct palaeogeographical settings governed by a combination of profile position, depositional gradient, sediment flux and position within the relative sea level cycle.

Integration of the sediment deformation data set with the sequence stratigraphic framework developed for the submarine slope succession demonstrates a predictable stratigraphic distribution. During the deposition of Unit 5, deformation occurred predominantly within the falling stage and lowstand systems tracts of 5th order sequences and was characterised by the development of slope ‘creep' bodies. Within Cycles 2-5 of the Kookfontein Formation, falling stage systems tracts of 5th order sequences are characterised by the occurrence of slumps/MTCs whereas, foundered mouth bars are more typical of the lowstand systems tract. It is proposed that falls in relative sea level triggered the generation of slumps/MTCs whereas rapid progradation and deposition onto a gradient led to gravitational instability, creep and failure of delta front mouth bars. This contrasts with studies that emphasise seismicity as the trigger mechanism for sediment deformation.