Stratigraphy and facies architecture of the uppermost fan system in the Tanqua sub-basin, Permian Ecca Group, South Africa

[摘要] ENGLISH ABSTRACT:Fan System 5 forms the uppermost submarine fan system of the Permian-age Tanqua FanComplex (Ecca Group) of the southwestern Karoo Basin. It is the most widespread system andrepresents the final phase of fan deposition in the Tanqua sub-basin. Depositional characteristicsdiffer markedly from the rest of the fan systems, mainly because it lacks sedimentary featuresindicative of a single point source basin floor fan. The entire system consists of six differentstages of fan growth and development in the lower slope settings.A hypothetical model was composed for Fan System 5 to understand the spatial/temporaldistribution of reservoir and seal facies in slope turbidite settings. The facies vary from massiveamalgamated sandstone beds to thin-bedded, ripple cross-laminated sand and siltstone beds. Athick shale unit identified as a regional marker layer overlies Fan System 5. lts base is defined bythe presence of a regionally developed 20 cm thick hemipelagic shale unit.Six sand-rich units with channel-complexes are present in the Klein Hangklip, Groot Hangklip,Kalkgat, Tongberg, Skoorsteenberg and Blauwkop localities. The facies characteristics in thesouthernmost outcrops of Fan System 5 (Groot Hangklip, Tongberg and Kalkgat) reflectdeposition in a lower slope setting where local structural control seems to have played a majorrole in the distribution and regional development of channel-fill and overbank depositionalelements.The channel-fills are arranged in vertical to off-set stacking patterns and are comprised ofmassive, amalgamated [me to very fine-grained sandstone units up to 30 m in thickness. They areseparated by thinner sandstone/siltstone units of varying thickness. The channelization displayedby the more proximal outcrops are interpreted to represent an upper fan, deposited in a lower- tomid-slope setting. In contrast to the channel-fill deposits at Skoorsteenberg, Klein Hangklip andGroot Hangklip, ripple cross-laminated overbank deposits, associated with smaller channel-fillunits, predominate in the northeastern and eastern parts of the outcrop area. Massive- and thinbeddedfrontal sheet sandstones constitute the down-dip extensions to the most northern outcropsof Fan System 5. Highly erosive, stacked base-of-slope channel complexes, seemingly controlled by subtle earlystructural features, were able to construct significant thicknesses of regionally well-developedoverbank deposits, marginal to the channel complexes. These facies changes occur over relativelyshort distances, which hold significant implications for the prediction of and the heterogeneity ofreservoir facies in slope settings.Gradients are much steeper in the lower slope to mid-slope area than on the proximal basin floor.The occurrence of soft-sediment deformation in the overbank and upper parts of the channel-filldeposits supports a slope origin. Weakly developed wave-ripple marks in the uppermost layers ofFan System 5 further indicate that water depths approached wave base prior to deposition of theupper markerbed shales.Paleotransport for Fan System 5 was towards the north, northeast and east. The palaeocurrentdirections of the channel-fill complexes in Klein- and Groot Hangklip seem to roughlycorrespond to the structural trend of synclinal depressions in this area. However, the effect andinfluences of basin floor topography and structural features on deposition were determined to beminimal on the regional development and local facies control of the fan.

[发布机构] Stellenbosch University