[meteorite-list] Impact Origin of Silverpit Structure Disputed

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 30 Mar 2007 10:55:13 -0700 (PDT)
Message-ID: <200703301755.l2UHtDY24019_at_zagami.jpl.nasa.gov>

>
> UK impact crater debate heats up by Jonathan Fildes
> BBC News, March 30, 2007
>
> http://news.bbc.co.uk/2/hi/science/nature/6503543.stm
>

UK impact crater debate heats up
By Jonathan Fildes
BBC News
March 30, 2007

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 and has spent
the last three years collecting evidence to back 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 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.

Along with a colleague, Dr Zana Conway, he has identified at least nine
major bowl-shaped depressions, known as synclines, and over 15
subsidiary structures including Silverpit itself. He says that more have
also been identified elsewhere.

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.

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."

In addition, Dr Conway has examined the coastlines of Denmark and the
east of England for evidence of tsunami deposits of the right age.

If a space object did crash into the shallow North Sea, the argument
goes, it would have caused great waves to dash the coastlines of
surrounding countries. In addition, it would have left a layer with high
levels of an element known as iridium in the rocks.

"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.

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.

"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 the University of Durham 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."

Professor Underhill's and Dr Conway's work will be presented at the
annual American Association of Petroleum Geologists meeting in Long
Beach, California, in early April.
Received on Fri 30 Mar 2007 01:55:13 PM PDT