The Bedout Crater – no sign of impact
Becker et al.’s recent proposal that the Bedout structure off NW Australia is a giant bolide impact crater of Permian-Triassic (P- Tr) boundary age (1) provides many readily testable hypotheses, not least of which is that there should be some evidence of the impact in the sedimentary record of the surrounding area. The Bedout structure is located in the Roebuck Basin which is part of a 2000 km extent of Late Palaeozoic-Mesozoic rift basins developed from Perth to Darwin. Marine conditions during the latest Permian and Early Triassic are recorded by the Kockatea Shale in the Perth Basin and equivalents in other basins. We have examined core material from the Kockatea Shale in the Hovea-3 borehole, located around 1000 km south of the Bedout structure (2,3). This reveals a P- Tr transition in which bioturbated mudstones with a diverse latest Permian fauna, dominated by brachiopods, are replaced by laminated, anoxic shales with earliest Triassic bivalves. At no level in the core, which spans a Wuchiapingian to Dienerian interval, is there evidence for a layer of impact ejecta nor a tsunamite. Trace metal assay also failed to find evidence for Ir enrichment. Core material is not available from sites nearer to Bedout, but mudlog data and wireline logs from boreholes as close as 400-500 km from Bedout also indicate a shale-on-shale transition across the P- Tr boundary. It is significant that the Hovea-3 core is substantially closer to the proposed impact site than the celebrated K-T impact sites in NE Mexico are to the Chicxulub Crater. For example, the famous Mimbral site, with its spherule layers and thick tsunamite record, is over 1500 km distant from Chicxulub.
We suggest that either the impact did not occur in the late Permian to early Triassic interval (and therefore it has no relevance to the P- Tr mass extinction event) or it is not an impact crater, and is more likely to be a volcanic structure.
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