Tectonic Modification to Timing, Distribution and Architecture of Deepwater Basin Floor and Slope Deposits Within a Third Order Composite Lowstand Systems Tract, Karoo Basin, South Africa

David M. Hodgson and Stephen Flint
University of Liverpool, Liverpool, United Kingdom

At tectonically active basin margins, modification of the sediment supply system can result in apparently anomalous stacking patterns, analysis of which may lead to incorrect sequence stratigraphic models and therefore inaccurate prediction of reservoir. This scenario is illustrated in the 3rd order deepwater siliciclastic sequence of the SW Karoo Basin, South Africa.

Early long term progradation of the basin margin in the tectonically-active Laingsburg area is marked by an 800 m-thick succession of 4th order basin-floor fan complexes (each comprising two or more 5th order fans) overlain by 4th order lower- to upper-slope channel complex sets (that comprise several 5th order slope channel complexes). A general increase in the efficiency of the system is marked by the upward decrease in the proportion of mud. Overall, a predictable thinning- and fining-upward succession is identified as sand supply waned through the 3rd order late lowstand (LST) to transgressive systems tract (TST).

In contrast, the coeval Tanqua depocenter records an overall coarsening-upward 400m thick succession of four 5th order basin floor fans and a 4th order lower slope channel/overbank complex set. Regional analysis, however, shows this succession belongs to the same late LST to TST. The progradational stacking pattern, which in isolation could be interpreted as a highstand systems tract, is due to the gradual increase in sediment supply resulting from diversion of sediment from the Laingsburg depocenter, driven by northward propagation of the subaqueous proto Cape Fold Belt.