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A gravity interpretation of the Orcadian Basin area

Kimbell, G.S.; Williamson, J.P.. 2016 A gravity interpretation of the Orcadian Basin area. British Geological Survey, 41pp. (CR/16/034N) (Unpublished)

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Abstract/Summary

A gravity investigation of the Orcadian Basin area has been conducted which involved the following stages: - compilation, imaging and qualitative interpretation of BGS gravity and magnetic data from the region; - compilation of rock densities from geophysical well logs and modelling of density variations within the sedimentary sequence; - construction of a structural model of the cover sequence down to the top of the Permian, based on depth-converted seismic interpretation; - calculation of the gravity effect of the sequence to top Permian using the structural and density models; - removal of this calculated gravity effect and a regional background field from the observations to leave a residual stripped gravity anomaly; - analysis of the signatures within the residual stripped gravity anomaly map, integrated with seismic evidence of Upper Palaeozoic structure and magnetic imaging. The residual stripped gravity anomaly map reveals features that can be correlated with the West Bank Basin and the eastern end of the Caithness Graben of Arsenikos et al. (2016), and with the thickened Upper Palaeozoic sequence in the East Orkney Basin inferred by those authors. Gravity signatures indicative of a thickening of the Upper Palaeozoic sedimentary rocks are also identified in the Dutch Bank Basin and the South Buchan Basin, areas in which seismic interpretation of Palaeozoic structure was difficult because of problems with data quality and line spacing. The influence of granitic intrusions is seen in a belt that extends in a north-north-east direction from Quadrant 19 into Quadrant 13, although the magnetic characteristics of the bodies might indicate separate post-tectonic and late-tectonic suites of Caledonian intrusions. Further granites are inferred in the Inner Moray Firth, in Quadrants 12 and 17. A Caledonian age is possible for these but an alternative interpretation invokes Palaeoproterozoic calc-alkaline basement, at least for the more magnetic component. Gravity signatures in the Inner Moray Firth are also influenced by low density Dalradian (Grampian Group) basement and Devonian sedimentary rocks, making it difficult to partition the response accurately between the different sources. Positive gravity signatures are associated with the Buchan Block and its offshore extension, and with Jurassic intrusives beneath the Forties Volcanic Province. Dense/shallow basement extends in a west-north-west direction from the Forties area in a broad axis, and this is an important component of the long-lived structural configuration of the region that may be linked to an early transform offset in the Laurentian margin. Recommendations for further work include more detailed and extensive gravity modelling, quantitative magnetic modelling and a geochemical/isotopic study of the samples available from offshore granite well penetrations.

Item Type: Publication - Report (UNSPECIFIED)
Funders/Sponsors: British Geological Survey
Additional Information. Not used in RCUK Gateway to Research.: This report is a published product of the 21st Century Exploration Road Map (21CXRM) Palaeozoic project. This joint industry-Government-BGS project comprised a regional petroleum systems analysis of the offshore Devonian and Carboniferous in the North Sea and Irish Sea. This report made open April 2017
Date made live: 04 Apr 2017 10:34 +0 (UTC)
URI: http://nora.nerc.ac.uk/id/eprint/516772

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