[meteorite-list] Impact Origin of Silverpit Structure Disputed

From: matt <metlist_at_meteoritecentral.com>
Date: Fri, 20 Apr 2007 06:29:47 +0100
Message-ID: <46284FCB.4050904_at_plu.to>

This story was also covered in the second half of the BBC Radio 4 show
'Material World' that aired yesterday, and includes discussion from Phil
Allen who discovered the structure. You can here the show by clicking
'Listen to the latest program' at:

http://www.bbc.co.uk/radio4/science/thematerialworld.shtml

Ron Baalke wrote:
>> 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.
>
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Received on Fri 20 Apr 2007 01:29:47 AM PDT