A fossil landslide preserved offshore at Lyme Regis, Dorset, UK

Multidisciplinary studies carried out in advance of site investigations of the areas at Lyme Regis, Dorset most threatened by a combination of landslides and marine erosion included sidescan-sonar, bathymetric and seismic-reflection surveys in the adjacent offshore area. These revealed a large area (over 1500 m long x 700 m wide) 500 to 700 m offshore from the present-day coastline in which an irregular sea bed is strewn with rock debris. This area is underlain by a layer of heterogeneous material up to 30 m thick with traces of disturbed bedding and shear planes. Comparison of the stratigraphy and structure of the underlying in situ beds, as determined from seismic-reflection surveys, suggests that the disturbed material is the residue from a single large (> 10 million tonnes) landslide that resulted from a shear failure in seaward-dipping mudstones in the lower part of the Jurassic Charmouth Mudstone Formation. A reconstruction of the geology immediately prior to the landslide indicates that the failure occurred at a time when the cliff line was c. 350 m south of its present position, possibly in the mid Holocene 5000 to 6000 year ago when sea level was sufficiently high to re-erode a Pleistocene cliff line.