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

Abstract

<jats:p>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 ≥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<jats:italic>c</jats:italic>. 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 (<jats:italic>c</jats:italic>. 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<jats:italic>c</jats:italic>. 40 km. The estimated impact energy for Acraman exceeds the threshold of 10<jats:sup>6</jats:sup>Mt set by earlier workers for global catastrophe. The impact occurred at a low palaeolatitude (<jats:italic>c</jats:italic>. 12.5°) 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.</jats:p>