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Dating the Acraman asteroid impact, South Australia: the case for deep drilling the ‘hot shock’ zone of the central uplift

Abstract The deeply eroded Acraman structure in the Mesoproterozoic Gawler Range Volcanics, Gawler Craton, South Australia, ranks among the world’s largest known impact structures, with a transient crater ∼40 km in diameter and a ∼90 km-diameter collapse crater. An impact age of ca 590 Ma is suggested by Rb–Sr shale dating of Ediacaran strata hosting a related ejecta band of volcanic fragments in the Adelaide Rift Complex and Officer Basin, and ca 580 Ma by chemostratigraphy. Ascertaining an accurate and precise impact age would greatly benefit Ediacaran biostratigraphic, biogeochemical and magnetostratigraphic global correlations. However, numerous attempts to date the impact, using material from exposures at Acraman and the ejecta band, have been unsuccessful. U–Pb dating of zircons from shattered dacite basement, melt rock and ejecta by three laboratories gave ages related to the 1592 Ma Yardea Dacite. K/Ar and 40Ar/39Ar dating of end-member K-feldspar in melt rock by two laboratories provided early Paleozoic ages, indicating later fluid movement. Although 80% of terrestrial impact structures ≥40 km in diameter have been drilled, Acraman has not. Deep drilling of the ‘hot shock’ zone of the unexposed central uplift is required to obtain zircons with U–Pb ratios that were likely reset by the impact. High-resolution aeromagnetic data for Acraman reveal an elliptical magnetic low measuring 19.5 × 18.3 km, with NNE–SSW-trending major axis and a high-amplitude dipolar anomaly of the central uplift that is offset to the SSW. The magnetic signature suggests oblique impact from the SSW. The source of the dipolar magnetic anomaly, centred ∼1 km below the Lake Acraman salina and with a remanence direction concordant with that of the melt rock, marks the ‘hot shock’ zone and a potential target for drilling. The scientific rewards of such drilling could be substantial. KEY POINTS Acraman in the Mesoproterozoic Gawler Range Volcanics, South Australia, is one of the world’s largest known impact structures, with a related ejecta band of volcanic fragments hosted by Ediacaran (ca 590–580 Ma) strata in the Adelaide Rift Complex and Officer Basin at unfolded distances from 250 km up to 550 km from the impact site. Numerous attempts at U–Pb dating of zircons from shattered volcanic basement and melt rock at Acraman, and zircons from the ejecta, gave un-reset basement ages, and K/Ar and 40Ar/39Ar dating of melt rock gave early Paleozoic ages related to later fluid movement. An accurate and precise age for the impact would provide a reliable age for the ejecta-hosting Ediacaran strata and greatly benefit Ediacaran biostratigraphic, biogeochemical and magnetostratigraphic global correlations. Deep drilling of the ‘hot shock’ zone of Acraman’s central uplift, inferred to be the source of a high-amplitude dipolar aeromagnetic anomaly centred ∼1 km below the Lake Acraman salina, is required to obtain zircons with U–Pb ratios most likely to have been reset by the impact.