The c. 600 Ma Acraman impact and ejecta horizon of South Australia: An update

The Acraman impact site, located in the 1.59 Ga Gawler Range Volcanics on the Gawler Craton, South Australia, is a complex impact structure that is now eroded >2.5 km below the original crater floor. Palaeomagnetic data for meltrock from the Acraman impact site indicate a stable magnetization and a virtual geomagnetic pole that is in close agreement with the palaeomagnetic pole for the c. 600 Ma Bunyeroo Formation 200-300 km E in the Adelaide fold belt. The palaeomagnetic data are therefore consistent with the correlation of the Acraman impact event and deposition of the widespread impact ejecta horizon. Recently several new ejecta locations have been identified in the Officer Basin up to 540 km NW of Acraman. Petrological and geochemical correlations are in accord with the palaeomagnetic evidence. Restored (unfolded) distances from Acraman versus decompacted thickness for the ejecta horizon are consistent with a transient cavity diameter of c. 40 km. The estimated impact energy for Acraman exceeds the threshold of a million Mt set by earlier workers for global catastrophe. The impact occurred at a low palaeolatitude (c. 15°) and probably dramatically affected the atmosphere in both the northern and southern hemispheres. Independent data from the Vendian palynology of Australia show that the Acraman impact induced a biotic crisis indicating that the Acraman impact caused a severe perturbation in the late Vendian.