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Last Updated: Friday, 30 March 2007, 11:31 GMT 12:31 UK
UK impact crater debate heats up
By Jonathan Fildes
Science and technology reporter, BBC News

Silverpit Structure (Image credit:Phil Allen (PGL) and Simon Stewart (BP))
Seismic surveys show a trough surrounded by concentric fractures
A deep scar under the North Sea thought to be the UK's only impact crater is no such thing, claims a leading geologist. Professor John Underhill, from the University of Edinburgh, says the Silverpit structure, as it is known, has a far more mundane explanation.

Detailed surveys reveal nine similar vast chasms in the area, he says.

This suggests it was part of a more widespread process, probably the movement of salt rocks at depth, not a one-off meteorite impact, he believes.

"I feel like I'm spoiling a party," said Professor Underhill. "It's a less glamorous explanation, but that's what the scientific data is saying."

Professor Underhill first put forward his theory in 2004. New evidence, he says, now backs it up.

I just felt that there was a bit more to the story than met the eye
John Underhill
However, the group that discovered the structure in 2002 stands by its original theory of a cataclysmic asteroid or comet impact about 60-65 million years ago.

"I can't understand why John keeps banging away at an alternative model," said team member Dr Simon Stewart, a geologist with BP.

"The crater interpretation of Silverpit still stands, in my opinion."

Regional view

The 3km-wide (1.8 miles) wide bowl was discovered in 2002 by Dr Stewart and his colleague Phil Allen, of geoscience firm PGL, about 130km (80 miles) east of the Yorkshire coast.

The structure, which comprises concentric, closely-spaced rings, is punched through a band of chalk. Today, it is covered by shales and sandstones almost one kilometre deep.

It can only be seen on seismic data, collected by petroleum companies hunting for new oil and gas fields.

Silverpit is 130km east of Yorkshire (BBC)
Two studies by Dr Stewart and Mr Allen, the latest of which mapped the structure in 3D, concluded that it was the result of a space impact. But Professor Underhill has never been convinced.

"I just felt that there was a bit more to the story than met the eye," he told BBC News.

To establish whether the feature was unique, he examined a 3,750-sq-km-area around the structure.

"I decided to throw a more regional view at it, and ended up finding a whole load of these features with very similar cross sections," he said.

He says he has identified at least nine major bowl-shaped depressions, known as synclines, and over 15 subsidiary structures including Silverpit itself. More have also been identified elsewhere, he claims.

Salt push

He says that the swarm of structures is the result of movement of a thick layer of salt of Upper Permian (248-256 million years ago) age that lies below the whole area.

The salt is highly mobile and flows between areas of high and low pressure.

The alternative theory for the formation of Silverpit

In some regions, huge blisters of salt force the overlying rocks up into domes, known as anticlines; elsewhere the salt flows entirely away and the overlying layers buckle and subside.

This is what caused the crater-like Silverpit structure, argues Professor Underhill.

"The key observation is that every single syncline is exactly coincident with where the salt has thinned or withdrawn," he said.

"There is an absolute one-to-one correlation between these two levels."

Professor Underhill also points out that there have been no tsunami deposits of the right age found flanking the North Sea.

If a space object did crash into the shallow basin, the argument goes, it would have caused great waves to dash the coastlines of surrounding countries.

"There is a lack of any independent evidence for a meteorite impact for the time that they say in the place that they advocate," said Professor Underhill.

Missing links

Dr Stewart is un-moved. He points to a 300m-high central peak, or nipple, in the centre of the inner bowl, typical of impact craters.

In addition, he argues the seismic surveys show areas of undeformed rock underlying the crater.

The Silverpit structure has been mapped in 3D

He explained it was like finding a hole in the roof of your house at the same time as you were digging in the basement.

"With only this information, one might conclude your roof collapsed because of subsidence into the hole you made in the basement," he says.

"But if you then point out that the first floor is intact, undeformed, we would conclude the roof hole was unrelated to the basement hole and indeed was most likely to be caused by something dropping through it."

Professor Underhill is unconcerned by this argument. He says that different rocks are mechanically stronger than others and will react in different ways when the salt withdraws.

Conclusive proof

The debate has drawn in other researchers from the geological community.

Impact expert Dr Gareth Collins from Imperial College London has also examined the evidence and says the circular structure is geometrically similar to other craters, particularly those found on other planets.

Tyre crater on Europa (Lunar and Planetary Institute, Houston)
Similarities exist with impact structures on Jovian ice moons

"On balance an impact origin is the simplest and most likely explanation," he says. "But to qualify that - it has absolutely not been proven to have an impact origin."

To unequivocally show Silverpit is a crater, he says, geologists would have to drill through its centre and look for evidence of minerals, such as shocked quartz, catastrophically altered by the crushing forces of the impact.

"The rocks and minerals affected by the impact would have been changed in a way which is absolutely diagnostic of high pressures that happen over a very short period of time," he said.

Other geologists with experience of the North Sea say that the large number of similar structures found by Professor Underhill strongly favours salt withdrawal.

"Given the abundance of these features and their distribution, it looks more like a salt withdrawal phenomenon than an impact, unfortunately," said Professor John Gluyas, of Durham University and co-founder of North Sea oil firm Fairfield Energy.

"On balance, I think John has it at the moment; but I think I'd like to see more evidence before I side with one camp."

The new work will be presented at the annual American Association of Petroleum Geologists meeting in Long Beach, California, in early April.

A REGIONAL VIEW OF SILVERPIT
Seismic map of Silverpit (Courtesy of  PGS, WesternGeco, British gas and Shell)
Peeling away the complex geology around Silverpit reveals a series of basins thought to have been created by salt withdrawal
(Seismic data courtesy of PGS, WesternGeco, British Gas and Shell)




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