Is Bedout an Impact Crater? Take 2

Abstract

In their Research Article "Bedout: a possible end-Permian impact crater offshore of northwestern Australia," L. Becker et al. report having identified a buried impact structure, which they link to the Permian-Triassic mass extinction (4 June, p. 1469; published online 13 May; 10.1126/science.1093925). Becker et al. have scarcely extended the suggestion made by Australian petroleum workers (in industry trade journals). Our scrutiny of the alleged evidence indicates that there is no substantiation that this alleged structure is an impact crater. The gravity map (fig. 11) actually highlights the differences between Bedout and confirmed impact structures. There is actually no crater defined by the geophysical data, only a noncircular high in the seismic data, claimed to be a "central uplift." In comparison, the central uplift feature of a large impact structure, such as the 250- to 300-km-diameter Vredefort Structure, would reveal a significant central positive gravity anomaly due to the uplift of relatively denser mid- to lower crustal material. The highly altered rocks described by Becker et al. as impact products strongly resemble volcanic breccias and lack impact diagnostic textures. No true shock features are described from any of the samples. No mineralogical or geochemical evidence is provided that the purported "diaplectic glass" or "maskelynite" are indeed glasses, and mineral chemical information is missing. The "shock features" claimed to be presented in quartz grains from "ejecta horizons" (which remain of uncertain stratigraphic relation either to the alleged Bedout feature or to the end-Permian extinction) do not show any of the characteristics of unambiguous shocked minerals.

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