Bedout
Encyclopedia
Bedout or more specifically the Bedout High, is a geological and geophysical feature centered about 250 km 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 30 km in diameter where older rocks have been uplifted as much as 4 km towards the surface. 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
.
John Gorter first suggested that the Bedout High might mark the centre of a very large buried impact crater
up to 250 km in diameter, based mainly on its internal geological structure as revealed on a single seismic
survey line, and suggested that it may be of near end Permian
age, based on the reported age of volcanic rocks in Lagrange-1. The impact hypothesis was developed further by Luanne Becker (University of California
) and coworkers who claimed to have confirmed that the supposed volcanic rocks in the petroleum wells are actually impact melt breccia
showing evidence of shock metamorphism
, and re-dated these rocks at 250.1 ± 4.5 Ma. The new age is essentially coincident with the Permian
-Triassic
boundary and associated extinction event
, and Becker speculates that there is a likely connection. Recently it has been reported, based on very limited preliminary data, that the impact hypothesis is further supported by chromium
isotope
evidence for the presence of extraterrestrial material.
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
in the Canning
Canning Basin
The Canning Basin is a geological basin located in Western Australia.The Basin covers approximately of which approximately is on land...
and overlying Roebuck basins. Although not obvious from sea floor topography, it is a roughly circular area about 30 km in diameter where older rocks have been uplifted as much as 4 km towards the surface. 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
Bedout Island
Bedout Island is a small Australian island in the eastern Indian Ocean. It lies 42 km offshore from Larrey Point and the mouth of the De Grey River, and 96 km north-east of Port Hedland, on the Pilbara coast of north-west Western Australia....
.
Impact hypothesis
In 1996 Australian geologistGeologist
A geologist is a scientist who studies the solid and liquid matter that constitutes the Earth as well as the processes and history that has shaped it. Geologists usually engage in studying geology. Geologists, studying more of an applied science than a theoretical one, must approach Geology using...
John Gorter first suggested that the Bedout High might mark the centre of a very large buried impact crater
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
up to 250 km in diameter, based mainly on its internal geological structure as revealed on a single seismic
Reflection seismology
Reflection seismology is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves. The method requires a controlled seismic source of energy, such as dynamite/Tovex, a specialized air gun or a...
survey line, and suggested that it may be of near end Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
age, based on the reported age of volcanic rocks in Lagrange-1. The impact hypothesis was developed further by Luanne Becker (University of California
University of California
The University of California is a public university system in the U.S. state of California. Under the California Master Plan for Higher Education, the University of California is a part of the state's three-tier public higher education system, which also includes the California State University...
) and coworkers who claimed to have confirmed that the supposed volcanic rocks in the petroleum wells are actually impact melt breccia
Breccia
Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, that can be either similar to or different from the composition of the fragments....
showing evidence of shock metamorphism
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 metamorphic effects unequivocally associated with explosions...
, and re-dated these rocks at 250.1 ± 4.5 Ma. The new age is essentially coincident with the Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
-Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
boundary and associated extinction event
Permian-Triassic extinction event
The Permian–Triassic extinction event, informally known as the Great Dying, was an extinction event that occurred 252.28 Ma ago, forming the boundary between the Permian and Triassic geologic periods, as well as the Paleozoic and Mesozoic eras...
, and Becker speculates that there is a likely connection. Recently it has been reported, based on very limited preliminary data, that the impact hypothesis is further supported by chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
evidence for the presence of extraterrestrial material.
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 basaltBasaltBasalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
, in part altered by contact with sea water and metamorphismMetamorphismMetamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
due to its deep burial, and the evidence for shock metamorphism is likely mistaken. - 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.
- The presented evidence for impact fails to meet established standards, and the dating methodology is flawed.
- 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.