The stratigraphy and sedimentology of the Dunscombe Mudstone Formation (late Triassic) of south-west England

The mid to late Triassic Mercia Mudstone Group exposed on the east Devon coast between Sidmouth and Seaton consists of c. 450 m of predominantly red mudstones that were deposited in low-relief sabkha environments in hot deserts. In marked contrast to this, the Dunscombe Mudstone Formation in the middle part of the group consists of 35-40 m of interbedded and interlaminated green, purple and grey mudstones, breccias, muddy limestones and lenticular siltstones/sandstones that were deposited in a wetter, possibly cooler climate. The formation is poorly exposed inland, but the striking colour difference from that of the adjacent formations enables its outcrop to be traced more or less continuously from the Devon to Somerset coasts. A detailed study of the sedimentology and ichnology of the formation, in particular that of laterally impersistent arenaceous members in its lower part, has shown that it was deposited in a succession of shallow, freshwater lakes in a low-relief topography that was at times crossed by broad shallow distributary channels. A combination of palynology and magnetostratigraphy is interpreted to represent that the Dunscombe Mudstone Formation is a condensed succession that occupied most of the late Triassic Carnian Stage, about 11.5 million years. It was deposited at a time of active tectonic subsidence, as a result of which it is laterally highly variable in thickness. In its extensive subcrop in the Wessex and Bristol Channel basins it locally reaches over 150 m in thickness in the more rapidly subsiding graben areas by the addition of halite. The northern boundary of the depositional area roughly followed the Variscan Front, along the line of the present-day Mendip Hills and South Wales coast.