STRUCTURAL AND MAGMATIC CONTROLS ON THE TURBIDITES OF THE KAROO BASIN, SOUTH AFRICA

We investigate the relationship between tectonism and sedimentation in the Karoo basin by integrating 571 new U-Pb single-grain detrital zircon analyses from 7 sandstones with 729 U-Pb zircon analyses from 30 volcanic tuffs (including 107 new U-Pb zircon analyses from 4 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 occurred between deformation events and predates 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, while 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 might have partitioned the sediment routing systems severing the connectivity between the active magmatic arc and the basin. We present a model where 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.