Are Diamictites Impact Ejecta?—No Supporting Evidence from South African Dwyka Group Diamictite1

Diamictites, especially those deposited before the break-up of Gondwana in the Late Carboniferous and Permian, have recently been suggested to represent ejecta deposits from large comet or meteorite impact events. This is in contrast to the commonly held interpretation that these rocks represent glaciomarine sedimentary deposits. To test this controversial hypothesis, we carried out a detailed petrographical study of over 75,000 mineral and rock clasts from a large number of Dwyka Group diamictite samples from localities covering an extensive part of the Karoo basin in southern Africa. No definitive evidence of impact-diagnostic shock metamorphic deformation in mineral or lithic clasts from any of these samples was detected. We conclude, therefore, that to date no unequivocal evidence for an impact origin of these diamictites in the South African stratigraphic record has been documented. What is more, the general hypothesis that some diamictites in the stratigraphic record could represent impact ejecta is not supported by first-order observations of bona fide shock (impact) related phenomena in such rocks.