Large-scale periglacial creep folds in Jurassic mudstones on the Dorset Coast, UK

Abstract

The unglaciated part of southern England was subject to periglacial climates that lasted for more than 20 ka on at least eight occasions during the last 750 ka. There are large outcrops of Jurassic mudstones in south-west England, principally the Lias Group and the Oxford Clay and Kimmeridge Clay formations, and extensive exposures of these beds in the cliffs on the Dorset coast. Notwithstanding the susceptibility of this type of mudstone to permafrost damage and deformation, there is no published record of large-scale folding in the region that has been attributed to periglacial disturbance. Three examples of folding are described here, in the Lias Group at Charmouth and Seatown in west Dorset, and in the Kimmeridge Clay on the Isle of Portland that are attributed to intermittent downhill creep of surface layers up to 20 m thick when in a partially frozen condition. The style of folding in the mudstones and the geometry of the disturbed deposits indicates that they are not tectonic in origin, nor were they formed by valley bulging or landsliding. These are the first large-scale structures of their kind to be recorded in southern England: similar folds elsewhere have been interpreted as valley bulges or tectonic in origin. At the Seatown and the Isle of Portland localities, the deformed mudstones have been preserved beneath younger landslides. The absence of similar structures elsewhere on the Dorset coast is attributed to the rapid removal of similarly weakened materials by marine erosion at times of high sea level during the last c. 6000 years