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Palaeomagnetic study of Cenozoic sediments in North Sea boreholes: an example of a magnetostratigraphic conundrum in a hydrocarbonproducing area

Abstract Palaeomagnetic measurements from a southern North Sea borehole that penetrated 13 m of Holocene, 189 m of Plio-Pleistocene and 4 m of Oligocene sediments have yielded six polarity changes. One polarity boundary coincides with a regional mid-Tertiary unconformity and a second occurs at the base of the Holocene section. The other four boundaries are interpreted as the Kaena-Gauss and the Gauss-Matuyama polarity transitions, and the base and top of the Olduvai subchron. The borehole results provide a chronological framework for interpreting nearby seismic sections and for assessing the sedimentation rates as a major Plio-Pleistocene delta complex approached and then overwhelmed the borehold site. Rock magnetic and X-ray diffraction work show that the iron sulphide greigite is an important magnetic mineral in the prodelta and delta front marine clays and lagoonal delta top clays, whereas magnetite is the dominant magnetic mineral in the fluvio-deltaic sands and in the Holocene and Oligocene marine deposits. Thermal demagnetization studies demonstrate that the greigite carries a stable natural remanent magnetization. It may have formed through sulphide diagenesis induced by hydrocarbon seepage or in a reducing sedimentary environment. Greigite is surprisingly difficult to detect through the standard palaeomagnetic method of alternating field demagnetization and may be of much greater palaeomagnetic importance in hydrocarbon-producing areas than previously supposed.