Posted on 03/30/2007 2:44:14 PM PDT by blam
UK impact crater debate heats up
By Jonathan Fildes
Science and technology reporter, BBC News
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 and has spent the last three years collecting evidence to back it up.
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.
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.
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."
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.
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.
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 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.
A REGIONAL VIEW OF SILVERPIT
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)
Would not some cores of the crater add more information? The presence of minerals known to be created in cosmic impacts would seem to confirm the impact theory, but the lack of minerals couldn't discount it, either, current erosion and all that.
Maybe they'll demand that his credentials be revoked and that his funding be rescinded?
-PJ
And what about a strike into the ice-pack during the last major freeze when the sea would have been frozen to the bottom?
It was found by those evil oil companies. Nothing good can come from this.
well then I guess the evidence would be limited to a Tommy Lee Jones movie produced by Al Gore.
"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 don't know what the climate was like in that period. I think the dino's were just clobbered by the Chixlub impact down in Mexico.
"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. " Their note does place it into one of the major ice sheet coverings, I think.
I've read that cosmic impacts on land causes it to get colder while an impact in water causes things to warm up. I think impacting into ice would be similar to results of a water impact, warming.
BTW, some pretty respectable scientists think a comet or comet fragment impacts caused the last Ice Age to end and we're still enjoying the benefits from that.
Note: this topic was posted on March 30, 2007.Thanks blam.
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Note: this topic was posted on March 30, 2007.Thanks blam.
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Maine Crater Related to Dino-Killer Asteroid?
Discovery News | April 3, 2003 | Larry O’Hanlon
Posted on April 6, 2003 12:39:18 AM EST by SteveH
http://www.freerepublic.com/focus/news/886240/posts
This chart is an indication of temperature changes over the last one million years. The article is about an event 60-65 million years ago. This chart has nothing to do with that time period. Also, any evidence of tsunami wash would have long ago been destroyed by many, many ice sheets.
Those evil oil companies supplied the data that enabled the Alvarez team to discover the Yucatan meteor crater of 65 million years ago (mya). Apparently oil deposits and the extreme pressure caused by these strikes can go hand in hand. There is an apparent crater off the Brazilian coast which seems to be an oil source. Given the oil in the North Sea, this goes along with that idea. Also, multiple craters around the same time period are not unheard of. The Chesapeake (Bay) Meteor strike of 34 mya, also had companions of roughly the same age, including a 10 mile wide crater off Toms River, NJ, and a 60 mile wide crater (Popogai (sp?) in Siberia. Another comment here mentions a possible crater off Maine of the right age (65 mya).
The Chesapeake Meteor was verified by deep coring which showed breccia and shocked quartz. They also hope to find oil around there.
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