Abstract

Glaciotectonic deformation of the Late Devensian sediments exposed at St Bees, Cumbria is represented by minor extensional faults resulting from ice-sheet loading of the unconsolidated sediments accompanied by thrusting and folding. The highly deformed northern part of the section has numerous thrust faults associated with fold structures that verge predominantly to the SE, but with some NW-vergent (backthrust) structures. A high-resolution seismic reflection survey confirms that the thrust structures form a linked fault system that detaches at, or slightly above bed rock, which is here composed of Triassic, St Bees sandstones. In the less deformed southern part of the section, the seismic survey has imaged bedding with a southerly component of dip in the St Bees sandstone cut by steep north-dipping extensional faults which, to some extent, control rockhead topography. An integrated approach combining structural geology and high-resolution seismic reflection surveying has enabled the construction of a balanced cross-section which estimates a minimum of 115 m (22%) cumulative shortening due to glaciotectonic processes. Minor structures observed in the St Bees cliffs and larger structures interpreted from the seismic profile are compatible with a critical wedge model for deformation caused by an overlying thick ice wedge with a SE-dipping surface slope.

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