The lithostratigraphy of the Mercia Mudstone Group (mid-late Triassic) of the south Devon coast

An almost complete section through the Mercia Mudstone Group is exposed in the cliffs between Sidmouth and Axmouth on the south Devon coast. This is the only section in North West Europe that exposes such a complete sequence of the mid to late Triassic 'Keuper Marl' facies. The c. 450.m-thick succession dips gently eastwards in a series of long, continuous sections that are separated from one another by minor faults which, with one exception, do not cut out any of the succession. At Seaton about 50 m of strata occur within a complexly faulted zone in which fault-bounded fragments of the succession cannot be readily correlated with the main outcrop. The availability of a continuous core from a nearby borehole has enabled the sequence in the faulted section to be pieced together for the first time, and to provide a complete stratigraphy for the group. The previously un-named lower part of the Mercia Mudstone Group (the beds below the Blue Anchor Formation) exposed in the Devon coastal sections can be divided into three formations and nine members based on gross lithology. The lowest and highest of these formations consists of relatively monotonous red mudstones for which the names Sidmouth Mudstone and Branscombe Mudstone are proposed. The middle formation, for which the name Dunscombe Mudstone is proposed, consists of a 35 m-thick sequence of laminated green, purple and grey mudstones which expands in some inland successions proved in boreholes to over 100 m by the addition of thick beds of salt. The lithologies exposed in the coastal sections can be correlated with the geophysical-log signatures of the Mercia Mudstone Group successions proved in inland boreholes throughout the Wessex Basin.