The Acraman asteroid impact, South Australia; magnitude and implications for the late Vendian environment

Acraman, located in the 1.59 Ga Gawler Range Volcanics on the Gawler Craton, South Australia, is a complex impact structure that is now eroded > or =2.5 km below the original crater floor. The geology, geomorphology, apatite fission-track geochronology, and geophysical signature of Acraman suggest that the original crater comprised highly disturbed rocks of a central uplift, a transient cavity up to c. 40 km in diameter, and a possible final structural rim at 85-90 km diameter. Radial unfolded distance from the centre of Acraman, versus decompacted thickness for the Acraman ejecta horizon identified in late Vendian (c. 580 Ma) mudstone in the Adelaide fold belt, Torrens Hinge Zone and Officer Basin up to 540 km from the impact site, accords with a transient cavity diameter of c. 40 km. The estimated impact energy for Acraman exceeds the threshold of 10 (super 6) Mt set by earlier workers for global catastrophe. The impact occurred at a low palaeolatitude (c. 12.5 degrees ) and probably perturbed the atmosphere in both the northern and southern hemispheres. The Acraman impact probably caused a severe perturbation of the late Vendian environment, a finding consistent with independent data from the Vendian palynology of Australia that the Acraman impact induced a biotic crisis.