Structural and magmatic controls on the turbidites of the Karoo Basin, South Africa

We investigated the relationship between tectonism and sedimentation in the Karoo Basin by integrating U-Pb single-grain detrital zircon analyses from seven sandstones with U-Pb zircon analyses from 30 volcanic tuffs. U-Pb detrital zircon data from the Karoo Supergroup strata indicate that the source of the turbiditic, deltaic, and fluvial sediments included an active volcanic province, with increasing contribution from the nearby Cape fold belt through time. The depositional ages obtained from the turbiditic strata of the Karoo Basin, based on U-Pb zircon tuff ages, and the published ages for Cape fold belt deformation suggest that the influx of coarse clastic sediment was synchronous with active deformation of the fold belt during the Gondwanan orogeny. Our tuff ages indicate that peak magmatism began prior to a major deformation event and predated turbidite deposition; initial sedimentation in Karoo turbidite systems coincided with a major deformational phase in the Cape fold belt. U-Pb detrital zircon ages reveal that mid-Permian Karoo turbidites are largely composed of Permian volcaniclastic sediment, whereas the Late Permian and Early Triassic sediment was increasingly sourced from the Cape Supergroup, now exposed in the Cape fold belt. While structural development of the Cape fold belt likely controlled the entry points of sediment into the basin, orogenic uplift may have partitioned the sediment routing systems, severing the connectivity between the active magmatic arc and the basin. We present a model in which a combination of volcanic ejecta, transported via atmospheric suspension, and the formation of entrenched drainages in the catchment areas allowed partial bypassing of continental drainage divides and deposition onto the leeward side of the Cape fold belt. © 2018 The Geological Society of America. All rights reserved.