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Quarterly Journal of Engineering Geology and Hydrogeology
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Quarterly Journal of Engineering Geology and Hydrogeology; 1991; v. 24; issue.4; p. 363-374;
DOI: 10.1144/GSL.QJEG.1991.024.04.04
© 1991 Geological Society of London

Article

The use of dating techniques to constrain the age of fault activity: a case history from north Somerset, United Kingdom

J. C. Gutmanis1, E. A Hailwood2, R. H. Maddock1 & C. Vita-Finzi3

1 GeoScience Limited, Silwood Park, Buckhurst Road, Ascot SL5 7QW, UK
2 Department of Oceanography, Southampton University, Southampton SO9 5NH, UK
3 Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, UK

Attempts were made to obtain absolute ages constraining the most recent episode of fault activity in the North Somerset Coastal Fault Belt as part of the seismic hazard assessment for the proposed Hinkley Point ‘C’ power station. Radiometric (uranium series, 14C) and palaeomagnetic dating techniques were applied to calcite from vein systems which cut the Hinkley Point Fault and 14C dating was applied to bone material from undeformed Quaternary gravel deposits overlying faults in Liassic bedrock at Helwell Bay. Detailed field mapping and microscopy indicated that the samples chosen for dating had not undergone any significant fault-related deformation. Dates obtained from the radiometric techniques ranged from c. 33 ka to greater than 300 ka and reflect the influences of isotopic exchange and contamination as well as the inherent age limitations of the techniques. Palaeomagnetic dating techniques identified reverse polarizaton in a calcite sample from the Hinkley Point Fault, indicating an age greater than 730 ka, the time of the last geomagnetic field reversal. Palaeomagnetic vectors suggest a pre-Pliocene (and conceivably an early Jurassic) age for the calcite vein emplacement.