Stratigraphic Evolution of Fine-Grained Submarine Fan Systems, Tanqua Depocenter, Karoo Basin, South Africa

Abstract

Abstract The integration of correlated outcrop and newly acquired core and wireline logs, extensive paleocurrent data, and accurately mapped surfaces has enabled a common model of the stratigraphic evolution to be developed of four Permian fine-grained submarine fan systems (Fans 1–4) from the Tanqua depocenter, SW Karoo Basin, South Africa. Additionally, this data has revealed the influence of subtle seabed topography on the fan systems' boundaries, internal facies architecture, and paleocurrent directions. Furthermore, the internal stratigraphy of individual fan systems are now known to be more complex than first believed. Mapping high-frequency intrafan units reveals a progradational–aggradational–retrogradational stacking pattern common to each submarine fan, which allows the geographic and stratigraphic distribution of lithofacies and architectural elements to be predicted. Basinward of the main feeder system, fan axes are dominated architecturally by sheet turbidites with discrete zones of high amalgamation during progradational and aggradational phases, whereas isolated channel forms that cut thin-bedded turbidites are more common in the retrogradational phase. This change is interpreted to be due to local increase in the lower slope gradient through sediment aggradation increasing the potential for channel avulsion and the development of splay lobes during decreasing sediment supply. The progradational, aggradational, and retrogradational phases are assigned to the early, middle, and late lowstand systems tract of a fifth-order sequence respectively. Each phase is built of higher-frequency (sixth-order) sequences so that each fan is a composite sequence. The overall basinward-to-landward stepping of the individual fans means that moving down dip in any basin-floor fan system, the lowermost sandstone preserved is progressively younger, whilst the uppermost sandstone is progressively older. This stacking pattern imparts an important predictable control on reservoir and seal geometries. This study aids the development of predictive models for fine-grained submarine fan initiation, growth, and abandonment, and lithofacies and architectural element distributions in space and time.