The stratigraphy of the Penarth Group (Late Triassic) of the East Devon coast

The cliff and foreshore exposures in the Devon part of the Dorset and East Devon Coast World Heritage Site expose an unbroken late Triassic to early Jurassic succession. The change from the terrestrial, red-bed facies of the Triassic Mercia Mudstone Group to the fully marine conditions of the Jurassic Lias Group takes place via the Penarth Group, a succession of mudstones, siltstones and limestones deposited in lagoonal and sheltered shallow-marine environments of varying salinities. The Penarth Group as currently defined is divided into the Westbury Formation overlain by the Lilstock Formation, based on type sections in the Severn Estuary area. The lithology and sedimentology of the Westbury Formation strata exposed on the east Devon coast are closely comparable with those of the type area, but those of the Lilstock Formation are not. It is therefore proposed on lithological and historical grounds that this formation should be replaced by a Cotham Formation overlain by a White Lias Formation. This would reinstate, without any change in their original definitions, two of the oldest formally defined stratigraphical names in the British Phanerozoic. All three formations are lithologically distinctive, and are separated from their neighbours by erosion surfaces that represent non-sequences. Those at the bases of the Westbury and Cotham formations are overlain by pebble beds rich in vertebrate remains (‘bone beds’). The Cotham Formation is a highly condensed succession comprised of thinly interbedded mudstones and limestones with ripple trains, stromatolites, desiccated surfaces and slumped beds, the last of which have been attributed to earthquakes or a bolide impact. The limestones of the White Lias Formation exposed on the east Devon coast are sedimentologically complex with channels, slumps and desiccated surfaces. The position of the Triassic-Jurassic boundary is currently under review. Possible positions include the base of the Cotham Formation, a horizon within the formation, the base of the White Lias, the base of the overlying Blue Lias Formation or a horizon within the Blue Lias Formation.