Chemostratigraphic trends and provenance of the Permian Tanqua and Laingsburg depocentres, southwestern Karoo Basin, South Africa

[摘要] ENGLISH ABSTRACT: Foreland basins commonly fill with sediment derived from the adjacent fold/thrust belt,providing a relatively simple source-to-basin configuration. However, that is not true for theearly southwestern Karoo Basin, since the composition of the Ecca Group sedimentary rocksdo not match the composition of the adjacent fold/thrust belt.The southwestern Karoo Basin is bordered to the west and south by the Cape Fold Belt(CFB) and provides the opportunity to study the linkage between its early structural evolutionand deposition in the two spatially and temporally distinct Tanqua and Laingsburgdepocentres. The CFB was formed when the early Palaeozoic passive continental margin,which formed a large section of the southern edge of Gondwana, evolved into an activeconvergent margin during the late Palaeozoic. Orogenesis resulted in a northwest-trendingCedarberg branch and an eastwest-trending Swartberg branch. The oroclinal bend betweenthe two branches includes large-scale northeast-trending syntaxis structures, such as the HexRiver and Baviaanshoek anticlinoria, which influenced the sedimentation path into the basin.Spectral gamma ray (SGR), mineralogical and geochemical studies of exposed rocksfrom the Tanqua and Laingsburg depocentres indicate a near uniform provenance for both,dominated by granitic and metamorphic material derived from a provenance seemingly farbeyond the CFB.SGR data, combined with lithology, show that regional stratigraphic correlation is possiblein the Skoorsteenberg, Kookfontein and Waterford Formations in the Tanqua depocentre. Thesame is true for the Laingsburg and Fort Brown Formations in the Laingsburg depocentre.There are no major changes in the SGR data set between the successive sandstone or shaleunits that could imply different origin, and no distinct signals in the SGR pattern of the shaleintervals that could potentially correspond to maximum flooding surfaces.The Tanqua and Laingsburg depocentre sandstones are very fine- to lower mediumgrained,tightly packed, poorly to well sorted, and have undergone mechanical compactionand pressure solution. The mineralogical composition and texture of these sandstonessuggest that they have undergone high-grade diagenesis to low-grade regional burialmetamorphism to the lower greenschist facies (250 ± 50ºC; ~2 kbars). They aremineralogically and geochemically classified as lithic arenites and greywackes, and theTanqua depocentre sandstones are slightly more mature than the Laingsburg depocentresandstones. REE patterns for the Tanqua and Laingsburg depocentre sandstones are similar,suggesting that both form part of the same evolutionary pattern and that the sediments haveone common origin, i.e. a provenance predominantly composed of granitic material.Homogenous εNd-values for all sandstone samples of around –5 at the time of depositionindicate that there is little or no variation in provenance between the Tanqua and Laingsburgdepocentre sediments. TCHUR model ages of 0.70 to 0.95 Ga, and TDM model ages of 1.19 to1.49 Ga, resulted from a mixture of Archaean and Proterozoic material in unknownproportions. The most likely source terrane is thought to be the North Patagonian Massif. Thelatter show Nd isotopic compositions corresponding to an average εNd-value of -5 at 265 Ma.

[发布机构] Stellenbosch University