MOROKWENG, SOUTH-AFRICA - A LARGE IMPACT STRUCTURE OF JURASSIC-CRETACEOUS BOUNDARY AGE

Geophysical, petrographical, and geochemical data indicate the presenc e 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 ro cks with high abundances of Cr, Ni, Co, and the platinum-group element s, consistent with the presence of up to 5% of a chondritic component. Ion probe dating of zircons extracted from the impact melt yielded a Pb-206/U-238 age of 146.2 +/- 1.5 Ma and a Pb-208/Th-232 age of 144.7 +/- 1.9 Ma; these are indistinguishable from the age of the Jurassic-C retaceous boundary. Following the identification of the Chicxulub impa ct structure of Cretaceous-Tertiary boundary age, the discovery of a s econd large impact structure at a previously established major chronos tratigraphic boundary strengthens suggestions that large impact events have been major factors in the evolution of the Earth.