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

摘要

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 similar to 40 km in diameter and a similar to 90 km-diameter collapse crater. An impact age ofca590 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, andca580 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(40)Ar/Ar-39 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 x 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 similar to 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.