--> Depositional Architecture of a Slope Fan Fed by Multiple Channel Systems

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Depositional Architecture of a Slope Fan Fed by Multiple Channel Systems

Abstract

Prediction of reservoir and seal distribution in submarine slope fans fed by multiple channel systems is challenging as seismic data rarely has sufficient resolution to constrain the coeval shelf interval, the relative ages of the channel-fills, and their connection to down-dip lobe complexes. Unit 5 (Permian Skoorsteenberg Formation, Tanqua depocentre, Karoo Basin, South Africa) is a shelf-edge through slope to base-of-slope and basin-floor system fed by multiple channel systems exposed over ~1700 km2 and intersected by 8 cored research boreholes. Unit 5 overlies four well constrained basin-floor lobe systems and provides a rare opportunity to investigate the sub-seismic 3D architecture and down-dip variation of a slope fan.

Unit 5 thickens then thins over ~45km basinward, demonstrating a clinothem geometry. Four evolutionary stages have been identified in proximal outcrops. Stage 1 deposits (~60m thick) are characterised by multiple channel complexes (up to 40m deep) bounded by external levees and proximal lobes, indicating an organised deep-marine system. Stage 2 (<35m thick) is marked by soft sediment deformation, with evidence of downslope movement, associated with hummocky cross-stratification and symmetric ripples on bed surfaces, indicating a wave-dominated shallow-marine system on an unstable slope. Stage 3 (<4m thick) comprises bioturbated fining upward sandstones interpreted as a transgressive deposit. Stage 4 (<5-35m thick) is a basinward thinning thin-bedded turbidite unit capped by a regional mudstone. Some 30 km down-dip, Unit 5 consists of ripple laminated silt- and sandstone thin-bedded turbidites (~85m thick) overlain by laterally extensive ripple laminated and structureless sandstones (~25m thick) interpreted as basin-floor lobe complexes.

That shallow-marine deposits with hummocky cross-stratification, Stage 2, overlie deep-marine deposits, Stage 1, without a significant stratigraphic break within Unit 5, suggests a relatively shallow water depth at the time of deposition (~100 m) but with a slope steep enough to form channel-levee complexes with basin-floor lobe complexes down-dip. In contrast to the underlying point-sourced fan systems, the transition from channel-dominated to lobe-dominated strata is more extensive and is characterised by thin-bedded turbidites. These differences point to a distinctive architecture for fans fed by multiple channels and provides an important alternative analogue to the classic point-sourced model.