Identifying fluvio–lacustrine intervals in thick playa-lake successions: An integrated sedimentology and ichnology of arenaceous members in the mid–late Triassic Mercia Mudstone Group of south-west England, UK

Abstract

A complete Triassic succession is exposed in the cliffs of the Devon coast in south-west England, UK where it forms part of the Dorset & East Devon World Heritage Site. The upper part of the succession, the Middle to Late Triassic Mercia Mudstone Group, consists of c. 450 m of predominantly red mudstones and associated evaporites that were deposited in playa lakes in hot deserts. The Group has an extensive onshore and offshore outcrop and subcrop in the U.K. where it locally comprises over 3000 m of red beds with thick halites. The Devon coast section is the type for the Group and is the most complete exposure of the Keuper Marl facies in north-west Europe. In its middle part, the Dunscombe Mudstone Formation comprises a 25 to 40 m-thick laterally variable succession of green, purple and grey laminated mudstones with thin interbeds of calcareous siltstone/sandstone, and thicker beds of autobreccia related to gypsum/anhydrite and halite dissolution. It represents a fluvio–lacustrine and probably cooler climatic interval that is thought to have lasted, on the basis of palaeontological and magnetostratigraphic evidence, for most of the Carnian Stage (c. 12 Ma). Elsewhere in Europe the Carnian Stage is represented by fluvial deposits including the Esterienschichten and Schilfsandstein Formations in Germany. At the type section on the Devon coast the Dunscombe Mudstone includes a lenticular unit of bioturbated, predominantly fine-grained sandstone up to 4 m thick, the Lincombe Member, that has not been recorded in inland sections in the county. When traced northwards into Somerset, thin lenticular sandstones, the North Curry and Sutton Mallet Sandstone Members, are locally present at similar stratigraphical levels in the lower part of the Dunscombe Mudstone. A detailed analysis of the sedimentology of the three principal occurrences of these arenaceous units revealed 13 lithofacies and 6 ichnofabrics that suggest deposition in similar fluvio–lacustrine settings. Their palaeoenvironmental interpretation and correlation is more complex than previously thought. The Lincombe Member is interpreted as a geographically isolated sedimentary body that was deposited in a shallow, oxygenated freshwater lake. In Devon it passes upwards and laterally into lake-margin and lake delta-plain deposits. The North Curry Sandstone was deposited in a series of broad, shallow channels that crossed a low-relief topography. Laterally, along strike, the channel sands either pass into or terminate against more argillaceous lacustrine deposits. In contrast, the depositional environment of the Sutton Mallet Sandstone is interpreted as relatively small-scale channels with low flows and possible channel switching. Fluvio–lacustrine intervals are relatively common within playa-lake successions, but there are few published accounts of ancient examples that are sufficiently detailed to allow the complex interactions of these ephemeral shallow-water environments to be understood. The sedimentological and ichnological descriptions and analyses of the Triassic example presented here will be applicable to other fluvio–lacustrine sediments deposited in dryland environments.