Silverpit crater is a buried sub-sea structure under the North Sea off the coast of the United Kingdom. The crater-like form, named after the Silver Pit — a nearby sea-floor valley recognized by generations of fishermen, was discovered during the routine analysis of seismic data collected during exploration for oil, and first reported in 2002 as a possible impact crater.^[1] If correct, it would be the first impact crater identified in or near the United Kingdom, however, the interpretation is controversial and alternative origins have been proposed.^[2] The age of the feature is constrained between 74 – 45 million years (Late Cretaceous – Eocene).^[3]

Discovery

The crater was discovered by petroleum geoscientists Simon Stewart of BP and Philip Allen of Production Geoscience Ltd, during routine analysis of seismic data while exploring for natural gas deposits in a region 130 km off the Humber estuary. Allen noticed an unusual set of concentric rings. Although they looked like they may have been caused by impact, he had no experience of impact structures. So he hung an image of them on the wall of his office, hoping someone else might be able to shed light on the mystery. Stewart, visiting Production Geoscience on an unrelated matter, saw the image and suggested it might be an impact feature. The discovery of the crater and the impact hypothesis were reported in the journal Nature in 2002. />

Silverpit crater is named after the Silver Pit fishing grounds in which it is located. The name is given by fishermen to a large elongated depressed area in the bed of the North Sea, which is thought to be an old river valley formed while the sea level was lower during the Ice Age.

Only three years before the announcement of the discovery of the Silverpit crater, it had been suggested that seismic data from the North Sea would have a good chance of containing evidence of an impact crater: given the rate of crater formation on the Earth and the size of the North Sea, the expected number of impact craters would be one.

The crater currently lies below a layer of sediment up to 1,500 m thick, which forms the bed of the North Sea at a depth of about 40 m. Studies suggest that at the time of the crater's formation, the area was under 50 to 300 m of water.<ref name="sa02" />

Additional images of the Silverpit crater: http://en.wikipedia.org/wiki/Image:Silverpit_8.jpg, http://en.wikipedia.org/wiki/Image:Silverpit.jpg

Origin

The origin of the crater is currently under debate within the Geoscience community with alternate theories of salt withdrawal and pull-apart basin^[5] proposed, raising doubts as to Silverpits categorization as an impact structure.^[6]

Evidence in favour of impact origin

Other mechanisms for producing a crater were considered and rejected by Allen and Stewart when they discovered the crater. Volcanism was excluded because there were no magnetic anomalies in the crater, which would be expected if eruptions had occurred there. Withdrawal of salt deposits below the crater, known to be a mechanism for the formation of some craters, was ruled out because the Triassic and Permian layers of rock beneath the crater appeared to be undisturbed. Another strong indication that an impact had created the crater was the presence of a central peak - something difficult to form except through a meteorite impact.

Evidence for alternative interpretations

Analysis of larger scale, but older seismic data by Professor John Underhill, a geologist at the University of Edinburgh, led to a suggestion that withdrawal of material at depth was in fact a better explanation.<ref name="u04" /> Underhill found that all layers of rock down to the Permian (with an age of about 250 million years) are synclinically folded, and that sediments of this era at the crater are thinned, suggesting that the crater was forming while Permian sediments were being laid down.<ref name="u04" />

The existence of the central peak which seemed to strongly support the impact hypothesis was considered by Underhill to be equivocal. He suggested it may have been an artifact of image processing,<ref name="u04" /> but subsequent seismic reflection mapping of the crater by Stewart and Allen seemed to confirm its existence.<ref name="sa05" />

In 2007, Underhill has continued to present evidence that he argues does not support the impact hypothesis. After analyzing seismic data over a wide region, he argues that Silverpit is just one of many similar features related to the withdrawal of the Permian-age Zechstein salt. This result is due to be presented at the April 2007 annual meeting of the American Association of Petroleum Geologists.^[7]

Structure

Silverpit crater is about 3 km wide at the top Cretaceous level.<ref name="sa05" /> Unusually for a terrestrial crater, it is surrounded by a set of concentric rings, which extend to about 10 km radius from the centre. These rings give the crater a somewhat similar appearance to Valhalla crater on Jupiter's moon Callisto, and other craters on Europa.^[8] Normally, multi-ringed craters tend to be much larger than Silverpit, and so, if the impact hypothesis is correct, the origin of Silverpit's rings is subject to debate. A complicating factor is that almost all known impact craters are on land, whereas two-thirds of impacting objects will land in oceans and seas, so the results of impacts on water are much less well established than those of impacts on land. Compare the Chesapeake Bay impact crater, probably the most thoroughly studied marine impact zone.

One possibility is that after the impact excavated a bowl-shaped depression, soft material surrounding it slumped towards the centre, leaving the concentric rings. It is thought that for this to happen, the soft material would have to be quite a thin layer, with more brittle material on top. A thin layer of mobile material beneath a solid crust is easy to understand in the context of icy moons, but is not a common occurrence on the rocky bodies of the solar system. One suggestion is that overpressured chalk below the surface may have acted as the soft mobile layer.^[9]

The impact

From the size of the crater and certain assumptions about the speed of an impacting object, the size of the impactor can be estimated. Impacting objects are generally moving at speeds of the order of 20–50 km/s, and at these speeds an object about 120 m across and with a mass of 2.0×10⁹ kg would be required to form a Silverpit-sized crater, if the object was rocky. If it had been a comet, the crater would have been larger.

For comparison, the object which struck the Earth at Chicxulub is estimated to have measured approximately 9.6 km across, while the object responsible for the Tunguska event in 1908 is thought to have been a comet or asteroid about 60 m across, with a mass of about 4×10⁸ kg.^[10]

An object 120 m across smashing into the sea at many kilometres per second would generate enormous tsunamis. Scientists are currently searching for any evidence of large tsunamis in the surrounding areas dating from around that time, but no such evidence has been uncovered yet.

Age

The position of the crater within the layers of rock and sediment on the sea floor can be used to constrain its age: sediments laid down before the crater's formation will be disturbed by the impact, while those laid down afterwards will not. In their discovery paper, Allen and Stewart stated that Silverpit was formed in Cretaceous chalk and Jurassic shale, but is covered by an undisturbed layer of Tertiary sediment.<ref name="sa02" /> The Cretaceous Period ended about 65 million years ago, but, on the evidence of nearby boreholes, the lowermost Tertiary sediments appear to be absent. Thus the age of the Silverpit event was initially stated to lie somewhere between 65 and 60 million years before present. However, after a more detailed apprasial of the seismic data, Allen and Stewart gave a more cautious estimate of the age as between 74 – 45 million years (Late Cretaceous – Eocene).<ref name="sa05" />

The stratigraphic method of estimating the age of a crater is somewhat crude, and the result is questioned by Underhill's non-impact hypothesis.<ref name="u04" /> Assuming an impact origin, other possible ways of dating the event include looking for evidence of ejecta material such as tektites, and deposits from the hypothesised tsunami, which might be found anywhere around the North Sea basin.<ref name="fildes" /> As well as allowing a more accurate age determination, finding such evidence would also strengthen the impact hypothesis. Two nearby oil exploration wells penetrate the ring system, and cutting samples from these are currently being analysed.

Analysis of samples taken directly from the central crater would also assist age determination as well as confirm one or other of the proposed theories; until this has occurred Silverpit cannot be confirmed as an impact structure.

Part of a multiple impact?

The early estimate of the age of the Silverpit event, stated as 65 – 60 million years before present, overlaps with the age of the Chicxulub impact, which occurred 65 million years ago and probably played a major role in the extinction of the dinosaurs. Several other large impact craters of around the same age have been discovered, all between latitudes 20°N and 70°N, leading to the speculative hypothesis that the Chicxulub impact may have been only one of several impacts that happened all at the same time.

The collision of Comet Shoemaker-Levy 9 with Jupiter in 1994 proved that gravitational interactions can fragment a comet, giving rise to many impacts over a period of a few days if the comet fragments should collide with a planet. Comets frequently undergo gravitational interactions with the gas giants, and similar disruptions and collisions are very likely to have occurred in the past.

While this scenario may have occurred on Earth 65 million years ago, evidence for this hypothesis is not yet strong. In particular, the ages of some of the possibly related craters are only known to an accuracy of a few million years. The increased uncertainity in the age estimate for Silverpit to 74 – 45 million years further weakens the hypothesis.

See also

• Impact event
• Bolide
• Chesapeake Bay impact crater
• Comet Shoemaker-Levy 9

References

External links

• North Sea crater shows its scars (BBC News, 18 March, 2005)
• UK's first impact crater discovered
• National Geographic news
• David Darling's space encyclopedia
• News item from PGL
• British Geological Society
• Further information from the British Geological Society

Cretaceous-Tertiary extinction event
Craters found at the K-T boundary
Boltysh crater	Chicxulub Crater
Shiva crater	Silverpit crater

Coordinates: