Folding and faulting in the Chalk at Dykes End, Bridlington Bay, East Yorkshire, resulting from reactivations of the Flamborough Head Fault Zone

Abstract

Summary: At Dykes End, Bridlington Bay, [National Grid Reference TA 216 692], a 200 m section of Chalk cliffs exposes folds with NW–SE-trending axes and north-eastward thrusting, accompanied by dome and basin structures in the foreshore, structures that are not exposed elsewhere in this region. The folding and thrusting resulted from transpression emanating from sinistral strike-slip reactivations of a right-hand bend, giving a local ESE–WNW trend, in the major E–W Langtoft Fault within the pre-Chalk basement just to the south. The northern side of the fault displaced westwards, underthrusting the Chalk cover which developed bedding-parallel shears, detachments, NW–SE axis folding, and thrusting north-eastwards. The underthrusting effects were localized and transient, ceasing as the strike-slip fault incised upwards through the Chalk. Subsequent flexuring, on ENE–WSW axes, curved foreshore bedding and enhanced closures of the earlier non-cylindroidal folding, producing domes and basins, sporadically exposed in the foreshore. Here, they have been downfaulted in a graben that is tilted, with downthrow increasing southward (seaward) towards the Langtoft Fault. Folding and faulting at Selwicks Bay, 4 km ENE, has been related to four deformation phases (‘D1–D4’). D1 produced gentle NNW–SSE folds and north-eastward shearing on bedding planes, comparable with the Dykes End cliff structures, and sinistral strike-slip movement on ESE–WNW faults, parallel to the Langtoft Fault. D2 was a tensional period. D3 produced E–W folds, comparable with the Dykes End cross-flexures, and D4 comprised extensional phases, comparable with the Dykes End faulting and graben development.