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Silverpit "not crater"


A Petroleum Group debate voted overwhelmingly against the hypothesis that the Silverpit Structure
(North Sea) was caused by meteorite impact. Dawne Riddle reports.

Science, as we all know, is not a democracy and being popular is not the same thing as being right. However if the majority view counts for anything, then a recent debate at the Geological Society has indicated that the less glamorous “withdrawal” hypothesis for the origin of the Silverpit Structure (S. North Sea) has now overtaken the impact crater idea, which (in a sign of the times) was portrayed throughout as the “traditional” explanation.

Map showing Silverpit locationThe crater-like structure was discovered by petroleum geoscientists Simon Stewart of BP and Philip Allen, then of Production Geoscience Ltd, during routine analysis of seismic data. Allen noticed an unusual set of concentric rings and 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 the company on an unrelated matter, saw the image and suggested it might be an impact feature. The discovery coupled with the impact hypothesis were reported in Nature in 2002. Since that time considerable debate has surrounded the idea, with other geologists – notably Prof. John Underhill of Edinburgh, advocating some form of collapse due to the withdrawal of material – salt - from below.

Stewart Rush Underhill

The debate, which took place on 6 October, formed the first of a series of Petroleum Group debates sponsored by BP and attracted an audience of about 100. Channel 4 News science correspondent Julian Rush opened the proceedings and chaired the debate. Speaking first, Simon Stewart presented a simple model that in his view was diagnostic for any withdrawal structure, namely that the profile of collapse should be traceable throughout the section below the structure and the zone of withdrawn material. He contended that this condition was not fulfilled in the seismic sections of Silverpit, and adduced other lines of evidence – such as the (disputed) central “rebound” spike – which he said could not be explained by withdrawal.

Underhill Speaking for the opposing view, John Underhill presented a detailed analysis of the structural trends in this part of the North Sea, including new data on the presence of NW-SE trending dykes (related to the igneous centres on the Isle of Mull, invisible on older, low-resolution scans). These, he believes, are what mobilised the salt and then initiated its withdrawal in the Paleogene. Underhill maintains that his theory is consistent with aeromagnetic results, the known extent of the distribution of devolatissed coal and unusual Chalk diagenesis, and velocity effects. Underhill further suggested that the correct interpretation of these structures would have commercial implications for prospectivity through depth conversion and the understanding of Bunter gas occurrence – and, crucially, of the gas’s chemical and isotopic composition.

The audience, made up dominantly of hydrocarbon industry seismic interpreters with a scattering of researchers and academics, preferred the Underhill model - for which, after a 20-minute period of debate, they voted overwhelmingly, 80:20.

Vote - 80:20 against the impact theory (red)