Print this pageTesting of High Resolution Sequence Stratigraphic Concepts in Turbidite Successions of the Karoo Basin, South Africa
JOHNSON, STEPHEN. D. and STEPHEN S. FLINT
The Permian Ecca Group deep marine deposits of the Karoo foreland basin, South Africa provide continuous exposures over 40 km and provide an opportunity to test the utility of high resolution sequence stratigraphic concepts in turbidite reservoir geology. Three consistently recognizable facies associations have been related to different degrees of sediment starvation. Type-1 horizons are marked by nodular concretionary horizons in the fine grained successions between each fan. The nodules are 7-50 cm diameter and/or in extensive "fans" or more localized lozenges at specific horizons. Type-1 horizons are interpreted as times of maximum sediment starvation, time-correlative to low-order inter-fan maximum flooding surfaces on the coeval shelf. Type-2 horizons comprise 20cm-3m thick shales with thin laminae of silt. They form intra-fan permeability barriers that isolate vertically individual lobes and are traceable for over 20km. Type-3 horizons are less than 30 cm thick, occur within individual amalgamated sheets (lobes) or within channel complexes and are traceable for up to 5km. Type-2 horizons are interpreted to represent periods when the whole fan system effectively "shuts down" thus representing high order maximum starvation ("flooding") surfaces. We cannot reliably determine if the origin of the laterally less extensive Type-3 horizons is autocyclic switching related to compensation cycles or high frequency shelfal flooding. Thus, within a turbidite "fan" setting, several types of starvation surface with different geometries and implications for permeability distribution can be defined, based on their sedimentological expression and stratigraphic context.