Bedout , or more specifically the Bedout High, is a

geological Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other E ...

and

geophysical Geophysics () is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. The term ''geophysics'' some ...

feature centered about off the northwestern coast of Australia in the Canning and overlying Roebuck basins. Although not obvious from sea floor topography, it is a roughly circular area about in diameter where older rocks have been uplifted as much as towards the surface and may mark the centre of a very large buried

impact crater An impact crater is a circular depression in the surface of a solid astronomical object formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal collapse, impact crater ...

up to in diameter.Gorter J.D. 1996. Speculation on the origin of the Bedout high - a large, circular structure of pre-Mesozoic age in the offshore Canning Basin, Western Australia. PESA News, October/November 1996, p. 32-34. The Bedout High was penetrated by two petroleum exploration wells (Bedout-1, Lagrange-1) in the 1970s and 1980s. It is named after nearby Bedout Island.

Impact hypothesis

In 1996 Australian

geologist A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes Earth and other terrestrial planets, as well as the processes that shape them. Geologists usually study geology, earth science, or geophysics, althoug ...

John Gorter first suggested that the Bedout High might mark the centre of a very large buried impact crater up to in diameter, based mainly on its internal geological structure as revealed on a single

seismic Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other ...

survey line, and suggested that it may be of near end

Permian The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last ...

age, based on the reported age of volcanic rocks in Lagrange-1.Gorter J.D. 1998. The petroleum potential of Australian Phanerozoic impact structures. The APPEA Journal 38, 159-187. The impact hypothesis was developed further by Luanne Becker (

University of California The University of California (UC) is a public land-grant research university system in the U.S. state of California. The system is composed of the campuses at Berkeley, Davis, Irvine, Los Angeles, Merced, Riverside, San Diego, San Franci ...

) and coworkers who claimed to have confirmed that the supposed volcanic rocks in the petroleum wells are actually impact melt

breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of ...

showing evidence of

shock metamorphism Shock metamorphism or impact metamorphism describes the effects of shock-wave related deformation and heating during impact events. The formation of similar features during explosive volcanism is generally discounted due to the lack of metamorp ...

, and re-dated these rocks at 250.1 ± 4.5 Ma.Becker L., Poreda R.J., Basu A.R., Pope K.O., Harrison T.M., Nicholson C. & Iasky R. 2004. Bedout: A possible end-Permian impact crater offshore of northwestern Australia. Science 304, 1469-1476
Abstract
/ref> The new age is essentially coincident with the

–

Triassic The Triassic ( ) is a geologic period and system (stratigraphy), system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago (Year#Abbreviations yr and ya, Mya), to the beginning of the Jurassic Period 201.36 ...

boundary and associated extinction event, and Becker speculates that there is a likely connection. It was subsequently reported, based on very limited preliminary data, that the impact hypothesis is further supported by chromium

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numb ...

evidence for the presence of extraterrestrial material.Becker L., Shukolyukov A., Macassic C., Lugmair G. & Poreda R. 2006. Extraterrestrial Chromium at the Graphite Peak P/Tr boundary and in the Bedout Impact Melt Breccia. Lunar and Planetary Science XXXVII (2006), abstract # 232
PDF
/ref>

Rebuttals to the impact hypothesis

The Bedout impact hypothesis has not been widely supported by impact cratering specialists. Specific counter claims include: # The alleged impact melt breccia actually has the characteristics of a volcanic

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

, in part altered by contact with sea water and

metamorphism Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of ch ...

due to its deep burial, and the evidence for shock metamorphism is likely mistaken.Glikson A. 2004. Comment on ‘‘Bedout: A possible end-Permian impact crater offshore of northwestern Australia’’. Science 306, 613
PDF
/ref>Glikson A. 2004. Bedout basement rise, offshore northwestern Australia: evidence of an unshocked mafic volcanic hyaloclastite volcanic breccia. Eos Trans. AGU, 85(47), Fall Meet. Suppl., Abstract B33C-0269
Abstract
# A crater of the purported size would have produced a widespread layer of ejecta, but no such eject is evident at the nearest Permian-Triassic boundary sites in Australia.Wignall P., Thomas B., Willink R., Watling J., Becker L., Poreda R.J. & Pope K.O. 2004. Is Bedout an impact crater? Take 1. Science 306, 609-610. # The presented evidence for impact fails to meet established standards, and the dating methodology is flawed.Renne P.R., Melosh H.J., Farley K.A., Reimold W.U., Koeberl C., Rampino M.R., Kelley S.P., Ivanov B.A., Becker L., Poreda R.J., Basu A.R., Pope K.O., Harrison T.M., Nicholson C. & Iasky R. 2004. Is Bedout an impact crater? Take 2. Science 306, 610-612. # A detailed re-evaluation of the geophysical data indicates that the evidence is not consistent with the expected properties of a giant impact crater, but rather the Bedout High was likely the result of episodes of continental rifting.Müller R.D., Goncharov A. & Kristi A. 2005. Geophysical evaluation of the enigmatic Bedout basement high, offshore northwest Australia. Earth and Planetary Science Letters 237, 265-284.

References

{{reflist

External links

Stephen Earle, "Double jeopardy at the end of the Permian - new evidence for a large impact"
Possible impact craters on Earth Impact craters of Western Australia Permian impact craters Triassic impact craters Permian Australia Triassic Australia Geography of the Indian Ocean Permian–Triassic extinction event

Impact hypothesis

Rebuttals to the impact hypothesis

See also

References

External links