Morokweng, South Africa: A large impact structure of Jurassic-Cretaceous boundary age

Abstract

Geophysical, petrographical, and geochemical data indicate the presence of a large impact structure in the area around Morokweng, Northwest Province, South Africa, possibly up to 340 km in diameter. Drill cores from the center of the structure show a thick layer of impact melt rocks with high abundances of Cr, Ni, Co, and the platinum-group elements, consistent with the presence of up to 5% of a chondritic component. Ion probe dating of zircons extracted from the impact melt yielded a 206 Pb/ 238 U age of 146.2 ± 1.5 Ma and a 208 Pb/ 232 Th age of 144.7 ± 1.9 Ma; these are indistinguishable from the age of the Jurassic-Cretaceous boundary. Following the identification of the Chicxulub impact structure of Cretaceous-Tertiary boundary age, the discovery of a second large impact structure at a previously established major chronostratigraphic boundary strengthens suggestions that large impact events have been major factors in the evolution of the Earth.