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Morphometric Comparison of Modern Deepwater Systems With the Permian Laingsburg-Karoo: Is the Present the Key to the Past?


Ancient siliciclastic basin margin systems are rarely exposed at outcrop with enough spatial continuity to fully explore both down-dip and lateral relationships. The Permian Fort Brown Formation is exposed in the Laingsburg-Karoo depocentre, South Africa, over an area of some 2500 km2 providing down dip control of >120 km and across strike control of up to 25 km. The 200–300 m thick mudstone-dominated slope to basin floor succession includes four sand-prone 30–70 m thick deep-water units, Units C to F, each of which is interpreted as a lowstand sequence set comprising three lowstand systems tracts. The sand-prone units are intercalated with regionally mapped 20–30 m thick mudstone drapes interpreted as combined transgressive to highstand sequence sets, together which build four composite sequences. A detailed correlation framework permits each system to be traced from slope valley-fill, through channel-levee dominated slope deposits to the distal fringe of basin-floor fans. Observation of the system margins provides sub-seismic detail on the extent and characteristics of gradually thinning and fining depositional architecture. Edge-to-edge reconstruction of each deep-water system has allowed the dynamic distribution and volumetric partitioning of sedimentary environments to be mapped, providing insight into autogenic and allogenic controls on the development of the Karoo basin margin. A systematic disparity in volumes is evident when the Fort Brown deep-water lowstand sequence sets are compared to published datasets of modern systems, where Fort Brown Fans occupy a similar area, but are an order of magnitude thinner. The issues may arise through differences in estimation technique from outcrop and seismic data or a difference in the architectural hierarchy being compared; alternatively the volume and rate of sediment supply to the late Permian Karoo Basin may have been affected by the climatic setting of the Pangean catchment. Either way, caution is therefore required in reconstructing ancient source-to-sink relationships and deepwater sand volumes using morphometric parameters derived from modern systems.