Basinfill of The Permian Tanqua depocentre, SW Karoo basin, South Africa

[摘要] ENGLISH ABSTRACT: Basin subsidence analysis, employing the backstripping method, indicates thatfundamentally two different basin-generating mechanisms controlled Tanqua depocentredevelopment in SW Karoo Basin. The subsidence curves display initial dominantlydecelerating subsidence, suggesting an extensional and thermal control possibly in a strikeslipsetting during the depocentre formation; on the other hand, subsequent acceleratingsubsidence with time suggests that the dominant control on the depocentre formation in SWKaroo was flexure of the lithosphere. Based on these observations on the subsidencecurves, it is possible to infer that the first stage of positive inflexion (~ 290 Ma) is thereforerecognised as the first stage of Tanqua depocentre formation.Petrographic study show that most of the studied sandstones of the Tanqua depocentre atdepth of ~ 7.5 Km were subjected to high pressure due to the overlying sediments. They aretightly-packed as a result of grains adjustment made under such pressure which led also tothe development of sutured contacts. It is clear the high compaction i.e. grain deformationand pressure solution occurred on the sediments; leading to total intergranular porosityreduction of the quartz-rich sediments and dissolution of the mineral grains at intergranularcontacts under non-hydrostatic stress and subsequent re-precipitation in pore spaces.Furthermore, siliciclastic cover in the Tanqua depocentre expanded from minimal values inthe early Triassic (Early to Late Anisian) and to a maximum in the middle Permian (Wordian-Roadian); thereby accompanying a global falling trend in eustatic sea-level and favoured bya compressional phase involving a regional shortening due to orogenic thrusting and positiveinflexions (denoting foreland basin formation). The estimate of sediment volume obtained inthis study for the Permian Period to a maximum in the middle Permian is thereforeconsistent with published eustatic sea-level and stress regime data. In addition, this newdata are consistent with a diachronous cessation of marine incursion and closure of Tanqua depocentre, related to a compressional stress regime in Gondwana interior during the latePalaeozoic.

[发布机构] Stellenbosch University