Is Bedout an Impact Crater? Take 1

Abstract

L. Becker et al. 's recent proposal that the Bedout structure off northwestern Australia is a giant bolide impact crater of Permian-Triassic (P-Tr) boundary age (“Bedout: a possible end-Permian impact crater offshore of northwestern Australia,” Research Article, 4 June, p. 1469; published online 13 May; 10.1126/science.1093925) 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 ([1][1], [2][2]). 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 or a tsunamite. A trace metal assay also failed to find evidence for iridium enrichment. Core material is not available from sites nearer to Bedout, but mudlog data and wireline logs from boreholes as close as 400 to 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 northeastern 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. 1. 1.[↵][3]1. B. M. Thomas 2. et al. , Austr. J. Earth Sci. , in press. 2. 2.[↵][4]1. B. M. Thomas, 2. C. J. Barber , APPEA J. 44, 59 (2004). [OpenUrl][5] # Response {#article-title-2} We agree with Wignall et al. that the study of cores from Permian-Triassic (P-Tr) sections in western Australia potentially provides a valuable test of the hypothesis that Bedout is a large P-Tr boundary crater. What is at issue is whether any of the existing cores or well logs, including Hovea-3, contain a complete record across the boundary. In many of the onshore basins that are proximal to Bedout, much of the Permian and Early Triassic section is missing (up to 2 km in some basins). Wignall et al. also state that Hovea-3 is closer to Bedout than some K-T impact sites (e.g., northeastern Mexico) are to the Chicxulub crater, and, thus, Hovea-3 should have some preservation of an impact ejecta layer. The distance between Hovea-3 and Bedout (1000 km) is about the same (not closer) as that between Chicxulub and the northeastern Mexico sections such as El Mimbral. These Mexican sections, and even closer (to Chicxulub) ones in Haiti, contain less than 1 m of ejecta; thus, even minor erosion at the boundary could erase the record. Wignall et al. rely on biostratrigraphy to determine the P-Tr boundary in Hovea-3. As shown elsewhere for the P-Tr, biostratigraphy alone is not a reliable indicator of the completeness of the boundary layer ([1][6], [2][7]). However, if it can be further demonstrated (e.g., by isotope dating) that Hovea-3 does indeed represent a record of continuous sedimentation across the P-Tr boundary, and it contains no ejecta, then this would argue against Bedout being a large P-Tr boundary crater. Finally, the absence of iridium in Hovea-3 is consistent with other P-Tr sections worldwide. This may reflect, again, upon the completeness of the P-Tr boundary, the type of section (marine versus continental), or the actual impacting body (e.g., asteroid versus comet). Several impact tracers (see our Research Article) occur in P-Tr boundary sections worldwide, none of which contains elevated iridium. Thus, we await full documentation and future investigations of impact tracers in Hovea-3 and other onshore cores that may include the P-Tr boundary. 1. 1.[↵][8]1. S. A. Bowring 2. et al. , Science 280, 1039 (1998). [OpenUrl][9][Abstract/FREE Full Text][10] 2. 2.[↵][11]1. Y. G. Jin 2. et al. , Science 289, 432 (2000). 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