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

The Strangways impact structure occurs within the McArthur sedimentary basi n, Northern Territory, Australia. The target rocks are primarily quartzites and subordinate shale-siltstone sequences of the Mesoproterozoic Roper Gro up. These are primarily underlain by granitoid gneiss, which is exposed in a 9-11 km diameter central uplift. Exposure is relatively limited and the u pper parts of the impact structure have been removed by erosion. Based on t he extent of the disturbance of the regional geology, the diameter of Stran gways 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 a s post-Roper Group (<1370 Ma) and pre-Cretaceous (>145 Ma). In order to bet ter define the time of impact, infrared laser spot fusion Ar-40/Ar-39 data have been obtained from two impact melt lithologies and a highly shocked br eccia clast. These yield a combined age of 646 +/- 42 Ma. Biotite dated fro m 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 ne w age of 646 +/- 42 Ma for Strangways places it firmly in the Neoproterozoi c, rather than being Ordovician or younger, as previously proposed. (C) 199 9 Elsevier Science B.V. All rights reserved.