Interaction of Density Currents, Depositional Topography and Submarine Slope Progradation: Unit C, Laingsburg Depocentre, Karoo Basin, South Africa

Rufus L. Brunt, Claudio N. Di Celma, David Hodgson, and Steve Flint
Department of Earth and Ocean Sciences, Liverpool University, Liverpool, United Kingdom

Regional submarine slope angles of around 1° are documented in modern systems, however the effects of slope progradation, deposition, differential compaction or compressional stresses may generate local variations in gradient. These subtle variations in slope angle and orientation impact the locus and style of deposition and erosion by density currents. Accurate measurement of submarine slope angle in ancient outcrop successions is not possible, but the response to gradient change in flow behaviour can be identified by analysis of facies and unit thickness. These variations may be below seismic resolution but are important when predicting reservoir potential.

Unit C of the Laingsburg depocentre, Karoo Basin, South Africa provides an ideal depositional template to test whether systematic variations in facies and depositional thickness can be used to identify subtle variations in slope topography. Gentle post-depositional folding of the Laingsburg succession enables a quasi-3D image of regional facies variations to be built. As absolute age control is not available for Unit C two internal mudstone markers were used to define three depositional sequences (C1, C2 and C3) that can be mapped for over 250 km². Unit C ranges in thickness from 110 m to 0 m over distances as little as 14 km. C1, C2 and C3 differ in terms of aerial extent, thickness, and distribution of depositional environments in space and time. These changes are attributed to compensatory stacking of depositional elements and compactional effects across a range of hierarchical scales, and were influenced by progradation of the submarine slope.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery