Acraman, located in the Gawler Range Volcanics (1592 ± 2 Ma), South Australia, is Australia’s largest known impact structure and the evident source of an ejecta horizon containing shocked volcanic fragments within the Neoproterozoic (about 590 Ma) Bunyeroo Formation 220 to 350 km east of Acraman and in coeval shales 470 km northwest of Acraman. The deeply eroded structure comprises a central uplift area at least 10 km across marked by sparse outcrops of intensely shattered Yardea Dacite, within an inner topographic depression about 30 km in diameter containing the Lake Acraman salina. Rocks of the central uplift exhibit shatter cones and multiple sets of shock lamellae in quartz grains that indicate shock pressures of up to 15 GPa. An apparent ring structure occurs at 85 to 90 km diameter and arcuate surface features are evident at about 150 km diameter. A dike of melt rock in the central uplift area consists mainly of laths of albite and scattered grains of titanomagnetite in a matrix of K-feldspar and finely inter-grown quartz. Both the Na- and K-feldspar phases have virtually pure end-member compositions (Ab 99 , Or 98 ), and evidently are low-temperature authigenic phases that formed by secondary alteration of the melt rock. Matrix K-feldspar and quartz may be devitrification products of glassy material. The melt rock is enriched in potassium but is not anomalous in cosmogenic siderophile elements. A negative gravity anomaly of about 6 mGal amplitude and 30 to 35 km in diameter is centered on the inner topographic depression. This depression is marked also by a subdued aeromagnetic signature, and a dipolar aeromagnetic anomaly indicates that a shallow (approximately 300 m depth) magnetic source occurs in the central uplift area. Paleomagnetic study of the melt rock indicates a stable remanent magnetization and a virtual geomagnetic pole that agrees closely with the paleomagnetic pole determined previously for the Bunyeroo Formation. Apatite fission track analyses and estimated rates of erosion together suggest that as much as 2 km thickness or more of overlying rocks has been eroded from the Acraman region since the impact at about 590 Ma. The transient cavity and excavated area thus may have been as much as 40 km in diameter, 30% greater than the diameter of strongly disrupted bedrock at the present level of erosion. The limits of the final structural rim after gravitational slumping of the crater walls may be marked by the apparent ring feature at 85 to 90 km diameter. Diameters of the uneroded structural features—central uplift, excavated area, and possible final structural rim—therefore may have been ≥10 km, about 40 km, and 85 to 90 km, respectively. Arcurate features at about 150 km diameter may be faults or fractures marking the outer limit of disturbance. Acraman could have been formed by impact with an Earth-crossing chondritic asteroid estimated to be 4.7 km in diameter and of density 3500 kg/m 3 moving at 25 km/s, with kinetic energy of 6 × 10 22 J.

Abstract Sedimentary features characteristic of ice-rafting are present in the Bunyeroo Formation (Wilpena Group) of the Adelaide Geosyncline and in the coeval Dey Dey Mudstone (Ungoolya Group) of the eastern Officer Basin (Figs 66.1 & 66.2), providing evidence of a mid-Ediacaran glacial climate in South Australia. The Acraman asteroid impact, a negative shift in marine δ 13 C, and a major acritarch turnover coincided with this frigid epoch.

ABSTRACT Airborne radiometric (gamma-ray) data provide estimates of the concentrations of potassium (K), thorium (Th), and uranium (U) in soil, regolith, and bedrock. Radiometric data constitute an important source of geochemical information, commonly used in mineral exploration and for geological mapping of Earth and other planets. Airborne radiometric data have rarely been applied to the exploration and analyses of impact structures, in contrast with other conventional geophysical tools (e.g., gravimetry, magnetism, and seismic reflection/refraction). This work represents the first systematic survey of the K, Th, and U radiometric signatures of Australian impact structures, based on the continent-wide airborne radiometric coverage of Australia. We first formulated several hypotheses regarding the possible causes of formation of circular radiometric patterns associated with impact structures. Then, the radiometric signatures of 17 exposed impact structures in Australia were documented. Our observations confirmed the supposition that impact structures are commonly associated with circular radiometric patterns. We then selected the five structures with the most prominent circular radiometric patterns (Gosses Bluff, Lawn Hill, Acraman, Spider, and Shoemaker), and we discuss the possible origin of these anomalies. Based on these five case studies, we argue that such patterns result from either crustal deformation induced by the impact event and/or from postimpact superficial processes controlled by the crater topography. This work also suggests that airborne radiometric data may be useful, in combination with other geophysical tools, in the search for new possible impact structures.

ABSTRACT There are currently 31 confirmed structures of impact origin in Australia. More than 49 additional structures have been proposed to have formed due to asteroid impact but await confirmation. Many discoveries have been made in Australia in the time since the last comprehensive review of the Australian impact cratering record was published in a peer-reviewed journal in 2005. These include further expanding the record of confirmed craters, and providing new insights into a variety of impact-related processes, such as shock deformation, phase transitions in accessory minerals, new impact age determinations, studies of oblique impacts, and more. This update is a review that focuses principally on summarizing discoveries made since 2005. Highlights since then include confirmation of five new Australian impact structures, identification of Earth’s oldest recognized impact structure, recognition of shock deformation in accessory minerals, discovery of the high-pressure phase reidite in Australia, determination of the links between impact craters and some ore deposits, and publication of the first generation of numerical hydrocode models for some Australian craters.