Crater linked to end-Permian extinction

In mid May news spread fast that a nearly circular feature that shows up in gravity data over the north-western continental margin of Australia could be a crater, about 220 km across, which formed at the end of the Permian (Becker, L. et al. 2004. Bedout: A possible end-Permian impact crater offshore of northwestern Australia.  Science Express 14 May 2004 – www.sciencexpress.org). Australian and US scientists have examined drill cuttings from exploratory oil wells that penetrate to the level of the hidden feature.  They describe breccias and associated melt rock. A plagioclase separate from the exploration well has an Ar/Ar age of 250.1 ± 4.5 Ma, that is within error of the age (251 Ma) of the largest Phanerozoic mass extinction.  Unfortunately, they have not discovered the easily recognised signs of shock damage to minerals – distinctive banded lamellae in quartz – nor any meteoritic chemical signature.  Nevertheless, the structure is huge and looks very like the gravitational expression of the Chixculub crater off the Yucatan Peninsula of Mexico, drill core from which shows all the signs of having formed by an impact at the end of the Cretaceous.  Evidence is accumulating from the Permian-Triassic boundary sequence that some event did produce all the signs usually attributed to a major impact in a global ejecta blanket (see Permian-Triassic boundary and an impact?, December 2003 EPN).  Despite glass being included in the breccias, many experts on impact processes and products are sceptical that the Bedout structure was produced by an impact.  But probably the only way in which such melts might have formed is by some kind of seismic shock, although that could have occurred during volcanism..  The structure is so huge that if it does have an origin by internal processes it ranks among the biggest to be found – could this ironically be a product of a Verneshot event (see Mass extinctions and internal catastrophes, above)?!

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