The Strangways impact structure, Northern Territory, Australia: geological setting and laser probe 40Ar/39Ar geochronology

The Strangways impact structure occurs within the McArthur sedimentary basin, Northern Territory, Australia. The target rocks are primarily quartzites and subordinate shale–siltstone sequences of the Mesoproterozoic Roper Group. These are primarily underlain by granitoid gneiss, which is exposed in a 9–11 km diameter central uplift. Exposure is relatively limited and the upper parts of the impact structure have been removed by erosion. Based on the extent of the disturbance of the regional geology, the diameter of Strangways is estimated to be 24–26 km. It is thus a complex crater and probably a central peak basin. Stratigraphic constraints place the time of impact as post-Roper Group (<1370 Ma) and pre-Cretaceous (>145 Ma). In order to better define the time of impact, infrared laser spot fusion 40Ar/39Ar data have been obtained from two impact melt lithologies and a highly shocked breccia clast. These yield a combined age of 646±42 Ma. Biotite dated from the granitoid gneiss central uplift yields 1686–1237 Ma, with the younger age reflecting the effects of shock-induced resetting and/or hydrothermal alteration due to hot melt sheet–water interaction following impact. The new age of 646±42 Ma for Strangways places it firmly in the Neoproterozoic, rather than being Ordovician or younger, as previously proposed.