Over 300 mn yrs old asteroid impact zone found in Australia

Scientists in Australia have found a major ancient asteroid impact zone on Earth in the country's outback, dating back to over 300 million years.

The impact zone, which centres on the East Warburton Basin in north-eastern South
Australia, was caused by an asteroid up to 20 kilometres-wide that slammed into the planet between 298 and 360 million years ago, researchers at Australian National University (ANU) and University of Queensland have claimed.

Terrain around the impact site shows evidence of changes caused by shock-wave related deformation and heating of the ground by an impact event, Andrew Glikson of ANU was quoted as saying by Australian Broadcasting Corporation.

"This shock metamorphic terrain covers an area of over 30,000 square kilometres making it the third-largest site of its kind ever discovered on Earth," Glikson said.
To confirm the area was an impact zone, Glikson and colleagues studied quartz grains retrieved from drill holes.

Optical and electron microscopic examination revealed tiny fractures, which indicate the quartz grains had been shocked by an asteroid or meteor impact.
"This is the only way these features are formed," he said. Follow up observations detected deep seismic anomalies below the terrain where the samples were taken.

"This allowed us to determine the scale of the impact site which is buried under four kilometres of younger sediments," he said.
He said that there is a link between this impact site and three or four other large impact sites of the same age scattered around Australia.

"Asteroid impacts commonly occur in clusters of two or more projectiles," he said, adding
"Where impacts are near- contemporaneous they're usually fragments of a larger body broken apart by the gravitational effect of the Earth-Moon system.

"This new discovery is a twin for one we reported on last year in the Eromanga Basin in south-western Queensland called the Tookoonooka Crater.

"It looks like both impacted at the same time."

He also said that there is a link between this site and a nearby potential impact site on the South Australian/Northern Territory border known as the West Warburton geophysical anomaly, and another site at Woodleigh in Western Australia.