Abstract

The integration of outcrop and recently acquired cores and wireline logs from research boreholes, extensive paleocurrent data, and accurately mapped key surfaces has produced a revised model for the stratigraphic evolution of Fan 4, the youngest fine-grained submarine fan in the Skoorsteenberg Formation, Tanqua depocenter, southwest Karoo basin, South Africa. Mapping of alternating sand- and silt-rich intrafan units reveals a progradational–aggradational–retrogradational stacking pattern in Fan 4. This pattern occurs in the older submarine fans as well and allows the prediction of geographic and stratigraphic distribution of lithofacies and architectural elements. Subsurface data have revealed that Fan 4 has a distinct lower division that is poorly exposed at outcrop. This lower division represents the progradational phase, which typically forms a coarsening- and thickening-upward succession that locally healed the basin-floor topography. The progradational and aggradational phases contain sand-filled channels across the southern, more fan-proximal outcrop belt. A distinct change in paleocurrent in the northern part of the study area suggests the impact of subtle seabed topography and/or an entry point on the western basin margin. Basinward of the main feeder system in the south, the fan axis is dominated by tabular turbidites with discrete zones of high amalgamation in the progradational and aggradational phases. The retrogradational phase is not well preserved at outcrop but is clearly recognized in cores.

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 sequences so that each fan is a composite sequence. The overall basinward-to-landward stepping of Fan 4 means that reservoir sandstones and section are lost from the base (through progradation) and top (through retrogradation) at progressively more basinward locations. Therefore, movement downdip in any basin-floor fan system will mean the lowermost sandstone that is preserved is progressively younger. This stacking pattern imparts an important predictable control on reservoir and seal geometries. This study therefore 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.