Possible Variscan thrusting beneath the Somerton Anticline, Somerset

J. A. DONATO

doi:10.1144/gsjgs.145.3.0431

Abstract

Vibroseis seismic data were acquired by Goal Petroleum plc during 1982 in the vicinity of the Somerton Anticline, Somerset. These data show the simple form of the anticline but in addition show a southward-dipping, discontinuous seismic event beneath. Gravity modelling demonstrates that this event may be correlated with an E–W-trending gravity gradient which can be followed for approximately 150 km. The dipping seismic event may represent a Variscan thrust plane which dips to the SSW in contrast to similar planes seen to the east which dip to the SSE. The variation in dip seen between these postulated thrust planes may explain the curvature of structural trends seen within, and to the east of, the Mendip Hills. The Polden Hills Fault runs sub-parallel to the proposed thrust plane beneath the Somerton Anticline and may be linked to a Mesozoic reactivation of the postulated thrust.

References

1. Bott M. H. P.,
2. Day A. A.,
3. Masson-Smith D.
(1958) The geological interpretation of gravity and magnetic surveys in Devon and Cornwall. Philosophical Transactions of the Royal Society (London), Series A 251:161–91.
OpenUrl CrossRef
British Geological Survey 1986. 1:250,000 Series, Bouguer Gravity map (Provisional Edition) Bristol Channel and Portland sheets. British Geological .Survey, Keyworth.
British Geological Survey 1986. Bouger anomaly map of the British Isles (southern sheet) 1:625,000. British Geological Survey, Keyworth.
1. British Institutions Reflection Profiling Syndicate (BIRPS) & Etude de la Croûte Continentale et Oceanique par Reflexion et Refraction Sismique (ECORS)
(1986) Deep seismic reflection profiling between England, France and Ireland. Journal of the Geological Society, London 143:45–52.
OpenUrl Abstract/FREE Full Text
1. Brooks M.,
2. Thompson M. S.
(1973) The geological interpretation of a gravity survey of the Bristol Channel. Journal of the Geological Society, London 129:245–74.
OpenUrl Abstract/FREE Full Text
1. Brooks M.,
2. Bayerley M.,
3. Llewellyn D. J.
(1977) A new geological model to explain the gravity gradient across Exmoor, N Devon. Journal of the Geological Society, London 133:385–93.
OpenUrl Abstract/FREE Full Text
1. Chadwick R. A.,
2. Kenolty N.,
3. Whittaker A.
(1983) Crustal structure beneath southern England from deep seismic reflection profiles. Journal of the Geological Society, London 140:893–912.
OpenUrl Abstract/FREE Full Text
1. Cook A. H.,
2. Thirlaway H. I. S.
(1952) A gravimeter survey in the Bristol and Somerset coalfields. Quarterly Journal of the Geological Society of London 107 255–86, (for 1951).
OpenUrl
1. Cornwell J. D.
(1986) The geological significance of some geophysical anomalies in western Somerset. Proceedings of the Ussher Society 6:383–8.
OpenUrl
1. Day G. A.,
2. Edwards J. W. F.
(1983) Variscan Thrusting in the Basement of the English Channel and SW Approaches. Proceedings of the Ussher Society 5:432–6.
OpenUrl
1. Edmonds E. A.,
2. Williams B. J.
(1985) Memoir of the British Geological Survey, Geology of the country around Taunton and the Quantock Hills. (HMSO, London) sheets 277 and 293.
1. Kenolty N.,
2. Chadwick R. A.,
3. Blundell D. J.,
4. Bacon M.
(1981) Deep seismic reflection survey across the Variscan Front of southern England. Nature 293:451–3.
OpenUrl CrossRef
1. Mechie J.,
2. Brooks M.
(1984) A seismic study of deep geological structure in the Bristol Channel area, SW Britain. Geophysical Journal of the Royal Astronomical Society 78:661–89.
OpenUrl Web of Science
1. Meissner R.,
2. Bartelsen H.,
3. Murawski H.
(1981) Thin-skinned tectonics in the northern Rhenish Massif, Germany. Nature 290:399–401.
OpenUrl CrossRef
1. Ussher W. A. E.
(1891) On the probable nature and distribution of the Palaeozoic strata beneath the Secondary, etc., rocks of the Southern Counties, with special reference to the prospects of obtaining coal by boring S of the Mendips Proceedings of the Somersetshire Archaeological and Natural History Society 36 88–136, (for 1890).
OpenUrl
1. Whittaker A.
(1972) The Watchet Fault—a post-Liassic transcurrent reverse fault. Bulletin of the Geological Survey, No. 41.
1. Whittaker A.
(1975) Namurian strata near Cannington Park, Somerset. Geological Magazine 112:325–6.
OpenUrl Abstract
1. Whittaker A.
(1978) Discussion of the gravity gradient across Exmoor, N Devon. Journal of the Geological Society, London 135:353–4.
OpenUrl FREE Full Text