U-Pb detrital zircon age determination and provenance of the lower Karoo succession from the Karoo Research Initiative (KARIN) borehole KWV-1, Eastern Cape Province, South Africa

Abstract

Abstract In recent years, the maximum age of sedimentation and possible sources of detritus to the sedimentary successions of the Karoo Supergroup has received a lot of interest, with various authors presenting age data from across the main Karoo Basin (MKB). Our paper aims to contribute to this ever-growing geochronological database as we report weighted mean ages for the youngest zircons as well as detrital zircon age distributions for the lower Karoo Supergroup succession sampled from the Centre of Excellence for Integrated Mineral and Energy Resource Analysis (CIMERA)-Karoo Research Initiative (KARIN) drill core KWV-1. A total of 880 near concordant zircon ages were determined using Laser Ablation Quadrupole Inductively Coupled Plasma Mass Spectrometry (LA-Q-ICP-MS) from fifteen samples representing the Dwyka Group, the Collingham-, Ripon-, Fort Brown- and Waterford formations of the Ecca Group and the Koonap Formation of the Beaufort Group as intersected by this ~2 352 m deep research borehole drilled in the southeastern sector of the basin. The weighted mean ages for the youngest zircons calculated for the formations of the Ecca- and Beaufort groups compares with findings reported by other authors and with the ages reported for tuffaceous layers associated with these units. The weighted mean ages for the youngest zircons of the Dwyka Group reflect the age of the youngest source area and do not aid in constraining the maximum age of deposition. A significant change in detrital zircon age distribution was noted between the Dwyka Group and the overlying formations of the Ecca and Beaufort groups, which we interpret as a change in provenance. The major Meso-to Neoproterozoic detrital zircon age fractions present in the Dwyka Group are attributed to source areas located towards the north and northeast of the MKB, in line with regional ice flow directions. A significant late Carboniferous to Permian detrital zircon age fraction is however observed throughout the formations of the Ecca- and Beaufort groups, with a lesser fraction of older zircon ages. The prevailing youngest age fraction reflects ages associated with magmatic source areas in the Gondwanide Orogeny located towards the south of the MKB at the time of deposition, while the recycling of deformed sedimentary strata of the Cape Supergroup contained in the Cape Fold Belt is considered the source of older detritus. The source of detritus to the MKB remained unchanged during the deposition of the respective formations of the Ecca- and Beaufort groups, despite changes in the depositional environment brough on by regional tectonics.