Ancient impact structure identified beneath Hay Plains

This ‘total magnetic intensity’ image of the Deniliquin impact structure shows its 510km diameter, multi-ring pattern, central core, radial faults and location of shallow drill holes. Geoscience Australia data published in Gliksin and Yeates 2022.

By Tertia Butcher

The Hay Plains have become the focus of geoscientists and researchers who believe the largest known asteroid impact structure on Earth lies buried beneath.

Geologist Tony Yeates, originated the view of the Deniliquin multi-ring structure as an impact structure. He visited the area last week as guest of Edward River Council and U3A (University of the Third Age) in Deniliquin.

Mr Yeates told The Riverine Grazier the structure is yet to be further tested by drilling, but is believed to span up to 520 kilometres in diameter.

This is larger than the Vredefort impact structure in South Africa which to date has been considered the world’s largest, and three times bigger than the Chicxulub crater in Mexico which represents the impact that caused the extinction of the dinosaurs.

The ancient impact occurred 25 kilometres north-west of Deniliquin, roughly west of Pretty Pine towards Stevens Weir.

Mr Yeates said the discovery came as a surprise.

“It was by accident,” he said. “I was reviewing the Darling Basin for prospecting purposes some 25 years ago when I came across magnetic patterns beneath the surface which likely represented a massive buried impact structure.

“Fortunately, I had come across meteorite structures before and thought this may be one.

“I had hoped the powers to be would do a bit of drilling, but the structure is too deep and this would be an expensive activity.”

“Discoveries from NASA's lunar and deep space programs provided a great deal of understanding about the processes involved in meteorite impact cratering.

“The damage done to rocks by large impacts leaves signatures in geophysical data, which are crucial to identifying buried impact structures such as this one.

“A lot of knowledge came from those.

“Magnetic imaging shows there must be something sitting down there.”

Imaging also shows de-magnetised rocks around the Booroorban area and according to Mr Yeates this quiet magnetic zone was critical in identifying the structure.

He said it was difficult to know its age without drilling for dateable samples.

“Based on tentative regional rock relationships, it could be about 514 million years-old (when the Flinders Ranges formed and there is a gap in the early fossil record) or about 444 million years old when an event occurred that preceded granite formation within the structure and may have coincided in time with a known extinction of 85 per cent of species.

“Formation of the structure affected rocks down to about 30 km deep.

“You would need a huge impact to do that.

“We need to do a new deep seismic survey across its entire diameter before we can confidently determine what the structure looks like at depth.

“However, all the geophysical characteristics are known features of a large impact structure and I think we have nailed it already.

“It is the biggest in the world known at this stage and I wouldn’t be surprised if this is in fact found to be much larger.”

In recent research published by Mr Yeates and geologist and climate scientist, Dr Andrew Glikson the region’s updated geophysical data between 2015 and 2020 confirmed the existence of a 520-kilometre diameter structure with a seismically defined dome at its centre.

Magnetic readings of the area have revealed a symmetrical rippling pattern in the crust around the core of the structure.

This was likely produced during the impact as extremely high temperatures created intense magnetic forces.

Their research also showed the top of this dome is about 10 kilometres shallower than the top of the regional mantle.

The Deniliquin structure was named by Mr Yeates and is likely located on the eastern part of the Gondwana continent, prior to it splitting off into several continents (including the Australian continent) much later.

The Australian continent and its predecessor continent, Gondwana, have been the target of numerous asteroid impacts.

These have resulted in at least 38 confirmed and 43 potential impact structures, ranging from relatively small craters to large and completely buried structures.