The Mercia Mudstone Group comprises up to 1200m of predominantly red mudstones and siltstones laid down in rift-related basins during a period of regional subsidence. The mudstones are commonly dolomitic and contain horizons of sulphate and halide salts. Up to 150m of undifferentiated red mudstones of Norian and Rhaetian age (Late Triassic) were examined in coastal outcrops in Devon and Somerset, southwest Britain, and mineralogical and stable isotopic studies were carried out in order to identify any subtle changes in the succession. Gamma-ray measurements were taken at outcrop to provide correlation with published borehole logs in which the Mercia Mudstone Group has been subdivided on the basis of gamma-ray and sonic response. One of the borehole subdivisions was identified at outcrop within a sequence of otherwise undifferentiated red mudstones and was related to a transition in clay mineral assemblage and stable isotopic composition over 20 to 30 m of section. The red mudstones below this transition have a higher proportion of magnesian clay minerals and enriched carbonate oxygen isotope compositions, indicating deposition from Mg2+-rich marine-derived waters. Clay mineral assemblages in the succeeding red mudstones are dominated by illite, and oxygen isotopic compositions are relatively depleted, indicating a greater influence of K+-rich continental-derived waters.