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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 (Ab99, Or98), 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/m3 moving at 25 km/s, with kinetic energy of 6 × 1022 J.

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