An asteroid impact origin of the Hirnantian (end-Ordovician) glaciation and mass extinction

Abstract

The buried ∼520 km-diameter Deniliquin multiple-ring impact structure (DMS), southeastern Australia, is regarded as likely to have triggered the ∼ 1.4 million years-long Late Ordovician (Hirnantian) glaciation and mass extinction event (445.2 and 443.8 Ma), which eliminated about 85% of species. The Hirnantian, second in severity relative to the Permian-Triassic boundary extinction (251 Ma) and almost twice as severe as the K–T impact and extinction event (66 Ma), is considered likely to represent the consequence of the Deniliquin mega-impact event. The geophysical evidence for a deep-seated impact origin of the DMS includes its distinct symmetric multiple-ring pattern (Fig. 1), a central magnetically quiet TMI core, radial faults and an underlying mantle dome about 10 km shallower than the regional MOHO. The magnitude of the DMS is consistent with the scale of the Hirnantian glacial and extinction events and much larger than Cambrian extinction events, which occurred on a smaller scale, suggesting the DMS is likely to represent the trigger for Hirnantian glacial and extinction events. A search for impact-deformation effects in proximal and distal Ordovician strata around the Deniliquin mega-impact structure is required to further test this suggestion.