Petrographic characterization of Late Jurassic to early Cretaceous sandstones in the offshore Bredasdorp sub-basin, South Africa: An example of a dynamic recycled provenance interpretation

Abstract

The Mesozoic syn-rift sediments of the Bredasdorp Sub-basin were deposited during the structural inversion of the orogenic Cape Fold Belt (CFB), a crucial time during the break-up of Gondwana. The Late Jurassic to Early Cretaceous non-marine to marine sediments of the Bredasdorp Sub-basin were studied to infer their composition, parent-rock lithologies, tectonic settings and their dynamic response to tectonic perturbations. The knowledge on the provenance terrains in the Bredasdorp Sub-basin is poorly understood and therefore this research shed light on these provenance terrains. Petrographic quantitative analysis revealed that the mineralogical composition of the sediments comprises mainly of litho-quartzose and feldspatho-litho-quartzose sediments. Modal petrographic data indicates a recycled orogen provenance on QFL plots suggesting the occurrence of a collisional provenance terrain. This setting can be interpreted as the reactivation of the CFB structures eroding the Bokkeveld Group (and younger Karoo Supergroup) and the Table Mountain Group metasediments (and Pre-Cape sediments). The shedding of these sediments in the Bredasdorp Sub-basin were coeval with the deposition of the D-V, V-BUSM and BUSM-1AT1 ‘informal’ sequences. Furthermore, an evolving recycled provenance from sedimentaclastic (un-dissected) to basementaclastic (dissected) sediments are proposed and could be a defining characteristic to the enigmatic offshore Jurassic-Cretaceous stratigraphic nomenclature.During the deposition of the D-V Sequence, a spatial variation in provenance due to intrabasinal uplifted fault blocks (possibly reactivated shear zones) occurred between the erosion of the Bokkeveld Group (and Karoo Supergroup) and the Table Mountain Group (TMG) metasediments. The deposition of the V-BUSM Sequence showed a similar compositional trend within their provenance terrain but showed a stronger correlation to an increased quartz and feldspar content, possibly because of the erosion of the dominant TMG, which was further supported by the occurrence of distinct type 3 polycrystalline quartz. The deposition of the BUSM-1AT1 sequence saw an increase in plutonic lithic fragments and feldspar content, indicative of an igneous source, which was attributed to the Cape Granite Suite. This distinction is crucial in creating a unifying stratigraphic nomenclature for the Jurassic-Cretaceous sequences of South Africa. A dynamic recycled orogen is envisaged for the Bredasdorp Sub-basin and therefore the use of these type of dynamic models should be incorporated in provenance models especially if there is a well-documented geological setting.