A deep drillcore from the Morokweng impact structure, South Africa: petrography, geochemistry, and constraints on the crater size

Abstract

The Morokweng impact structure in South Africa was formed 145 Ma ago, at the time of the minor mass extinction that marks the Jurassic–Cretaceous boundary. Previous size estimates for the Morokweng impact structure ranged between 70 and 340 km, and those workers favoring a very large size speculated on the role that the Morokweng impact might have played in relation to this mass extinction. Consequently, the actual size of this impact structure has wide-ranging implications. Petrographic and geochemical analyses, combined with SHRIMP U–Pb single zircon chronology, of a >3400 m long drillcore from about 40 km west of the center of the Morokweng impact structure show that this borehole is most likely located outside the impact structure, limiting the maximum crater diameter to <80 km. While the formation of a <80 km impact structure in continental terrane probably did not engender global biological consequences, it is possible that a series of impact events at J/K boundary time could have caused environmental stress on a worldwide scale. The terrestrial impact crater record does list several smaller structures of similar ages to that of the Morokweng structure. The results of this study are also significant for South African Archean geology. Late Archean (2.6–2.8 Ga) granitoids in the crust of the Kaapvaal Craton are much more widespread than previously thought. The overprint of the Namaquan–Kibaran orogenesis at 1.2–0.9 Ga along the western and southern margin of the craton is significant even in the far North West Province of South Africa.