[meteorite-list] North Sea Crater Shows Its Scars

From: Gerald Flaherty <grf2_at_meteoritecentral.com>
Date: Mon Mar 21 13:24:59 2005
Message-ID: <014401c52bfc$b8550620$6401a8c0_at_Dell>

Thanks Ron. Wonderful speculation and possible avenues to pursue suggested
by the counter argument.
Jerry Flaherty
----- Original Message -----
From: "Ron Baalke" <baalke_at_zagami.jpl.nasa.gov>
To: "Meteorite Mailing List" <meteorite-list_at_meteoritecentral.com>
Sent: Friday, March 18, 2005 2:35 PM
Subject: [meteorite-list] North Sea Crater Shows Its Scars


>
>
> http://news.bbc.co.uk/1/hi/sci/tech/4360815.stm
>
> North Sea crater shows its scars
> By Jonathan Amos
> BBC News
>
> What is thought to be the UK's only space impact crater has been mapped
> in detail in 3D for the first time.
>
> The so-called Silverpit structure lies several hundred metres under the
> floor of the North Sea, about 130km (80 miles) east of the Yorkshire
> coast.
>
> The new pictures show a spectacular set of rings sweeping out around a
> 3km-wide (1.8 miles) central hole.
>
> Researchers report their description and interpretation of the images in
> the Geological Society of America Bulletin.
>
> Dr Simon Stewart and Phil Allen detail how the crater's features would
> have developed from the cataclysmic fall of an asteroid or comet about
> 60-65 million years ago.
>
> "I'm 99% certain - as certain as you can be - that this is an impact
> structure," Phil Allen told the BBC News website.
>
> "The geomorphology of the crater is absolutely right. If you saw that on
> Mars or any of the other planetary bodies you wouldn't question it."
>
> But some have - and there is now a lively debate about the origin of
> Silverpit among those who study the geology of the North Sea.
>
> For their part, Allen and Stewart - who first reported Silverpit's
> features in 2002 - hope their latest assessment of seismic reflection
> maps will go a long way to silencing the doubts.
>
> Other worlds
>
> Today, Silverpit is covered by shales and sandstones almost one km deep.
>
> It is only with the seismic data collected by petroleum companies
> hunting for new oil and gas fields that we know anything about the
> remarkable features cut into the underlying chalk.
>
> The whole area has been folded over time - stretched on one side,
> compressed on the other.
>
> Allen and Stewart say the inner bowl contains a 300m-high central peak,
> or nipple, typical of impact craters.
>
> This bowl is then surrounded by closely spaced rings, produced by rocks
> that have collapsed along lines of weakness. The rings stretch out
> almost 10km from the central point.
>
> "As far as we're concerned, the structure is pretty near unique -
> certainly on Earth," said Mr Allen, a consultant geophysicist with
> Production Geoscience Ltd in Aberdeen.
>
> "We suggest the rings are post impact-deformation. We believe there were
> two phases. First, during impact, specific areas were weakened - the
> ring shape was defined during impact, if you like.
>
> "Then, much later - perhaps millions of years later - the rings were
> produced."
>
> Silverpit is 130km east of Yorkshire (BBC)
>
> Although nothing quite like Silverpit can be seen elsewhere on Earth or
> on the other inner planets, Stewart and Allen say the tight rings are a
> good match for those of impact craters on Jupiter's icy moons, such as
> Europa and Callisto.
>
> The two researchers think this may have something to do with the type of
> surfaces being bombarded.
>
> "It goes to what's under the ice in the Jovian examples, which is
> probably a briny ocean; and what's under the chalk at Silverpit, which
> are these shales that may transmit the energy. We are beginning to think
> the layering is important."
>
> Independent lines
>
> To the sceptics, though, there is a more mundane explanation for the
> Silverpit features which does not require an extraterrestrial impactor.
>
> It relates to a thick layer of salt of Upper Permian (248-256 million
> years ago) age that lies below the whole area. This layer is well known
> because it forms the sealing horizon for gas prospecting.
>
> The salt is highly mobile and, argues Professor John Underhill from the
> University of Edinburgh, moves in and out of rock regions, influencing
> the geomorphology above.
>
> He says the crater rings match exactly where the salt has withdrawn.
>
> "Features like this exist whenever you remove material at depth. It's
> true, for example, if you remove magma at depth; you have a collapse
> known as a caldera," explained Professor Underhill.
>
> "Likewise, if you have mine shafts collapsing around a central point -
> if the mass at depth is circular, the pattern of fractures is also
> circular."
>
> He added: "The best thing about this is that it has stimulated a debate
> and it is an interesting theory, but I just don't agree with their
> interpretation."
>
> Tyre crater on Europa (Lunar and Planetary Institute, Houston)
> Similarities exist with impact structures on Jovian ice moons
>
> There are several lines of inquiry that might help settle this argument
> once and for all.
>
> If, as Stewart and Allen believe, a seven-million-tonne, 120m-wide
> object struck the Earth at 20km/s, the local rocks should show evidence
> of melting and metamorphism.
>
> Drill samples pulled up during gas prospecting in the area may find
> this. They may also give a more tightly constrained age for the
> Silverpit structure.
>
> In addition, any impact would have thrown material out over a large
> area. These ejecta, which take very distinctive forms, may yet turn up
> at locations in the UK and Scandinavia.
>
> And then there are the tsunami deposits. Silverpit was covered by a
> shallow sea back at the start of the Tertiary. An impact would have sent
> giant waves surging across nearby land masses. There should be evidence
> of this disturbance in sediments.
>
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Received on Fri 18 Mar 2005 03:54:46 PM PST