North Sea crater shows its scars

By Jonathan Amos
BBC News science reporter

The Silverpit chalk structure is deep under the sea bed Enlarge Image

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.

It is an interesting theory, but I just don't agree with their interpretation Prof John Underhill, Edinburgh University

"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.

As far as we're concerned, the structure is pretty near unique Phil Allen, consultant geophysicist

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."

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 seismic maps peel off overlying rocks to reveal the chalk features

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."

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.