North Sea Quaternary morphology from seismic and magnetic data: indications for gas hydrates during glaciation?

Buried circular depressions and channels in Quaternary strata have been investigated by 3D seismic data and, less commonly, by high-resolution aeromagnetic data. Sub-glacial melt-water drainage channels of various dimensions are the most distinct morphological features. In the same strata, numerous crater-shaped depressions were found, some coinciding with the initiation of channels. The diameter ranges from 500 m to 3000 m and the depth from 20 to 300 m - appreciably larger than common pock-marks. It is argued here that the craters were generated by gas expulsion from melted gas hydrates, combined with melt-water expulsion and erosion. This is supported by: (1) a similarity with published seafloor craters that originate from melted gas hydrates; (2) appropriate physical conditions for the formation and melting of gas hydrates during glacial and interglacial periods; (3) correlation with shallow gas occurrences and seismic gas indications. An increased number of craters occurs above a major fault and some Tertiary hydrocarbon discoveries, which may indicate thermogenic gas. At such locations, an increase in high-frequency magnetic anomalies can be explained by deposition of sediments with contrasting magnetic susceptibilities. This is an alternative explanation for the occasionally increased shallow magnetic anomalies above hydrocarbon fields, otherwise attributed to secondary changes in the magnetic mineralogy.