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Anatomy and Stratigraphic Development of a Lower Slope to Base-of-Slope Siliciclastic Wedge from the Tanqua Depocentre, SW Karoo Basin, South Africa

Van der Merwe, Willem 1; Wickens, H. De Ville 2; Hodgson, David 1; Flint, Stephen 1; Wild, Richard 3
1 Earth and Ocean Sciences, University of Liverpool, Liverpool, United Kingdom.
2 Department of Geology, University of Stellenbosch, Stellenbosch, South Africa.
3 Reservoir Characterization Unit, Chevron Energy Technology Company, Houston, TX.

Permian deepwater deposits of the Tanqua Depocentre, SW Karoo Basin, South Africa, include Fan System 5 (Unit 5), a ~100m thick base-of-slope sand-rich succession, which overlies a 0.2m thick mudstone above basin-floor Fan 4, and is capped by a 12m thick regionally-developed mudstone. Unit 5 lies above point-sourced sand-rich basin-floor fans, and is overlain by progradational shelf-edge deltaics. As such, Unit 5 represents the transition from a bypass- to accretion-dominated slope system. Unit 5 is exposed for 70km down depositional dip and >20km across strike, which has allowed an extensive outcrop-based dataset to be collected. This is augmented by four research boreholes sited away from outcrop.

Unit 5 consists of three discrete stages of stratigraphic growth. Several points of channelisation occur across the outcrop belt and indicate a change from point- to mulitple-source sediment supply. Channel-fills are dominated by structureless amalgamated sandstone, with basal mudstone clast conglomerates. Deposits away from channel-fills are thin-bedded, ripple-laminated layered sandstones, and are interpreted to represent a combination of frontal splay deposits from earlier up-dip slope feeder systems, and lateral spill deposits formed during channel incision and aggradation. The ratio of frontal: lateral splay deposits at any section depend upon the stratigraphic position and spatial location with respect to the genetically related channel-complex. Sedimentological criteria for establishing the depositional setting of thin-bedded facies include stratigraphic packaging and bed thickness (thickening upward trends and greater average bed thickness in frontal deposits), abrupt changes in palaeoflow, and beds with wedge geometries and isolated channel forms (lateral deposits).

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009