The Silverpit Crater: 3D seismic data reveals Britain's first impact structure

Phil Allen, Production Geoscience Ltd, Banchory

3rd December 2002, Jarvis City Hotel, Aberdeen

Introduction

Geologists have only recently begun to accept the notion that meteorite impact events have played a significant part in the Earth's history. It is now widely recognised, for example, that the Chicxulub impact in Mexico may have been responsible for the cataclysmic events that caused the mass extinctions at the end of the Cretaceous period.

Impact craters are found on virtually every body in the Solar System. However, the Earth only has about 170 known craters, and none have been found in the British Isles area - until this year!

A high-resolution 3D seismic survey over BP's Trent gas field was being appraised at Production Geoscience Ltd (PGL) in Banchory when some very strange geometries emerged from the data, noticeably at the top Chalk and base Chalk horizons. Analogy with other impact structures strongly suggests an impact origin for this feature, which has been called Silverpit after the local fishing ground.

Crater scientists have been amazed by the quality of the seismic data, which allows the fine detail to be mapped in three dimensions - probably for the first time at any crater anywhere. After a presentation to specialists at a recent impact meeting in Sweden, Silverpit was dubbed "the sensation of the conference".

Silverpit

Silverpit's discoverers are Phil Allen, a consultant geophysicist at PGL and Simon Stewart, a structural geologist who was working for BP in Aberdeen.

In a paper published in Nature on 1st August 2002, Stewart and Allen described the structure produced by an impact that occurred in a shallow sea 60 to 65 million years ago, 140 Km east of where Scarborough is now. Silverpit is buried by up to 1Km of Tertiary sediments which have preserved the fine detail of the structure. This is unusual for terrestrial craters, most of which are at the surface and subjected to erosion.

With a diameter of 3 Km, this crater is certainly not the largest in the world (Chicxulub has a diameter of about 150 Km). However, the projectile (asteroid or comet) which created it probably weighed about 2 million tonnes. Travelling at 20-50 Km/s, the energy generated by impact would have been nearly 100 megatons.

The most unusual, and puzzling, feature of Silverpit is a series of concentric fractures, which surround the crater up to a distance of 10 km from its centre. Stewart and Allen were astonished when they first saw this pattern emerge from the seismic data. They had never seen anything like it. In fact, they had to go quite a long way before they found an impact structure which could be compared to Silverpit. Two of the icy moons on Jupiter have a few large craters that bear an uncanny resemblance to the new discovery. Not only geologists, but also planetary scientists are keen to learn more about the North Sea crater.

The discovery was described by Jay Melosh, Professor of Planetary Science at the University of Arizona, as a "wonderful find". "Silverpit's multi-ringed morphology is unique among terrestrial craters, but it does resemble larger craters in Europa and Callisto," he notes. "Given the spectacular detail revealed by the 3D seismic reconstruction, Silverpit is likely to teach us a great deal about the mechanics of how such ring-systems arise."

Further Work

And Silverpit is about to yield more surprises. Since the "Nature" article was written, four more 3D seismic datasets have been located in the archives. These data are now being interpreted, and there is a puzzling variation in the morphology of the concentric rings.

There are drill cuttings from two exploration wells drilled through the Silverpit structure in the 1980's. Work on these samples includes a biostratigraphic study, to allow us to date of the impact more accurately, and a petrographic analysis to see if any of the rocks show signs of shock-metamorphism.

Proposals for further work include numerical modelling studies which will, hopefully, explain some of the peculiarities of Silverpit, and a search for evidence of the tsunami which would have been generated at impact.

Stewart and Allen are planning another Silverpit paper and lecture invitations have come from bodies such as the Royal Astronomical Society in London and the Lunar and Planetary Institute in Houston.

Silverpit may not rank among the planet's largest meteorite impacts, but it is certainly one of the most beautiful, and enigmatic…. And it's British.

We thank Production Geoscience Ltd, BP, Conoco-Phillips and Western-Geco for allowing the use of interpretation facilities and seismic data.

Further Reading:
French B.M. Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures. LPI Contribution No 954, Lunar and Planetary Institute, Houston. 120pp. (1998). (Viewable online)
Melosh H.J. Impact Cratering: a Geologic Process. Oxford University Press, New York (1989).
Stewart S.A. and Allen P.J. A 20-km-diameter multi-ringed impact structure in the North Sea. Nature, volume 418, pages 520-523 (2002)