Sedimentology of the southern Karoo ripon formation and the southwestern Karoo Vischkuil/Laingsburg formations of the Ecca Group: implications for basin and palaeoenvironmental reconstruction

[摘要] ENGLISH ABSTRACT: Between the towns of Prince Albert (Western Cape) and Willowmore (Eastern Cape) the well-known Laingsburg depocenter and less-known Ripon depocenter of the southern Main Karoo Basin (South Africa) meet and exhibit an interfingering relationship involving the Laingsburg and Vischkuil formations (Laingsburg depocenter) on the one side and the Ripon Formation (Ripon depocenter) on the other. Studies regarding the link between these two submarine fan-dominated depocenters have been avoided to date due to the complex sedimentology and stratigraphic relationships involved. New sedimentologic and stratigraphic analysis presented in this thesis suggests that the Ripon depocenter is much more widespread than initially suggested. Channelized sandstones in the Pluto's Vale Member, as well as in the upper Wonderfontein Member, also suggest that up-dip feeders for the Ripon depocenter are more widespread than previously thought. Through detailed facies and architectural analysis, this study provides a basin reconstruction model, suggesting that the lower Vischkuil Formation is the distal equivalent of the lower Pluto's Vale Member submarine fan complex. Sedimentary petrology, including sandstone petrology and U-Pb detrital zircon geochronology, indicates that the two depocenters have slightly different source regions. Specifically, sandstones of the Laingsburg depocenter are characterized by feldspathic litharenites, whereas sandstones from the Ripon depocenter are characterized as ultralithofeldspathic. Zircon ages indicate that submarine fan development in the Laingsburg depocenter only initiated after deposition of the Pluto's Vale Member in the Ripon depocenter. The zircon populations and U/Th ratios further indicate that zircon grains were derived from South American as well as a source in the southwestern part of Antarctica, known as the Dyer arc.

[发布机构] Stellenbosch University