Abstract
Summary. The late Triassic (Rhaetian) Cotham Member in southwest Britain was deposited in a shallow, dominantly freshwater lagoon and contains a bed of synsedimentary deformation which can be traced for at least 25 km. The bed includes microfaults and plastically deformed slump-folds. In south Wales and north Somerset the top of the bed is slightly eroded and desiccation cracks up to I m deep are present. The deformation was probably caused by earthquake activity and associated uplift in the north Somerset-south Wales area. Earthquake activity is inferred to have been part of the early Cimmerian tectonic phase in northern Europe. The bed of synsedimentary deformation is a stratigraphic 'event' which can be used for correlation in this area.