Geomorphology of the Acraman impact structure, Gawler Ranges, South Australia

Abstract

The late Neoproterozoic Acraman impact structure occurs mostly in felsic volcanic rocks (Mesoproterozoic Gawler Range Volcanics) in the Gawler Ranges, South Australia, and strongly influences the topography of the region. The structure is expressed topographically by three main features: a near-circular, 30 km diameter low-lying area (Acraman Depression) that includes the eccentrically placed Lake Acraman playa; a partly fault-controlled arcuate valley (Yardea Corridor) at 85–90 km diameter; and arcuate features at 150 km diameter that are visible on satellite images. Geological and geomorphological observations and apatite fission-track data indicate that Acraman is eroded several kilometres below the crater floor, with the structure originally comprising a transient cavity about 40 km in diameter and a final structural rim 85–90 km in diameter. Ejecta of shock-deformed fragments of felsic volcanic rock up to 20 cm across derived from the Acraman impact form an extensive horizon ≤40 cm thick in Ediacaran (about 580 Ma) shale in the Adelaide Geosyncline 240–370 km to the east of the impact site. A correlative band ≤7 mm thick of sand-sized ejecta occurs in mudstone in the Officer Basin up to 540 km to the northwest of Acraman. The dimensions of the impact structure and the geochemistry of the ejecta horizon imply that the bolide was a chondritic asteroid >4 km in diameter. Acraman ranks among the largest 4% of known terrestrial impact structures, and the impact would have severely perturbed the Ediacaran environment.