Deformation history and basin-controlling faults in the Mesozoic sedimentary rocks of the Somerset coast

Abstract

Structures in the Triassic and Liassic sedimentary rocks of the Somerset coast indicate spatial and temporal variations in the deformation history. From Kilve to Watchet, there was Mesozoic north-south extension and Tertiary north-south contraction. At Lilstock, about 3 km to the east, there was a more complex history, including: (1) the development of joints, normal faults and veins striking about 060°; (2) approximately north-south extension on 095° striking normal faults, with sinistral transtension; (3) east-west contraction, with sinistral shear on some 095° striking normal faults; (4) dextral reactivation of some 095° striking normal faults; (5) north-south contraction, and thrusts and strike-slip faults, with the reverse-reactivation of the largest 095° striking normal faults. The joints in the district mostly postdate the faults. Two large approximately east-west striking faults are postulated to form the northern edges of the Quantock and Exmoor hills, here called the North Quantocks Fault (NQF) and the North Exmoor Fault (NEF). These were basin-bounding normal faults during the Mesozoic, with maybe more than 1000 m of throw, suggesting that the Bristol Channel Basin is not a half-graben developed above a south-dipping Variscan thrust which underwent Mesozoic extensional reactivation. The NQF and NEF may have been reverse-reactivated during the Alpine contraction. The NQF probably caused variations in the stress history of the Mesozoic sedimentary rocks of the Somerset coast over only a few kilometres along strike. The possible relationships of the NQF and NEF with the Cothelstone Fault are discussed. Several broad orders of relay ramp scale occur, within which are developed smaller antithetic normal faults. Between the north-dipping NQF and NEF, the relay ramp contains south-dipping (antithetic) normal faults with tens to hundreds of metres displacement. In relay ramps between the south-dipping faults are north-dipping normal faults with metre-scale displacements, which in turn have relay ramps with south-dipping normal faults with millimetre-scale displacements.