Submarine Slope Progradation and Shelf Construction in the Early Permian Karoo Basin, South Africa

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

The early Permian Kookfontein Formation is an extremely well-exposed 250 m-thick lower slope to shelf succession that overlies the well known Tanqua basin floor fans. Detailed sedimentological analysis coupled with the identification of key surfaces and stacking patterns in continuous outcrop and research borehole data has allowed development of a model for the detailed stratigraphic evolution of the succession. Thirteen coarsening-upward cycles have been mapped for over 15 km down depositional dip and for 20 km along strike. The mechanism of shelf construction was via repeated episodes of progradation of mixed influence shelf-edge deltas onto the upper slope. This process resulted in punctuated failure events characterised by foundering at the shelf edge, which initiated down dip mass transport complexes (MTCs) on the mid-to-upper slope. The failure events added extra material/volume to the upper slope, assisting subsequent basinward progradation of the system. From integration with the underlying deepwater succession, the Kookfontein Formation is interpreted to represent a 3rd order highstand sequence tract (HST) composed of several 4th order sequences. These 4th order sequences are comprised of parasequences. The lower, slope-dominated parasequences are thicker than overlying, accommodation-limited shelf parasequences. Individual shelf parasequences can be traced over the shelf edge, thicken down dip and exhibit a proximal to distal variation in lithofacies, reflecting deposition in deeper water. Fourth order sequence boundaries are marked by sharp based shoreface deposits that overlie slumps and MTCs while a 3rd order sequence boundary truncates the Formation and is marked by the first fluvial deposits with extrabasinal clasts.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005