Geochemical and mineralogical variations in the upper Mercia Mudstone Group (late Triassic), southwest Britain: correlation of outcrop sequences with borehole geophysical logs

The Mercia Mudstone Group comprises up to 1200 m of predominantly red mudstones and siltstones laid down in rift-related basins during a period of regional subsidence. Up to 150 m of undifferentiated red mudstones of Norian and Rhaetian age (Late Triassic) were examined in coastal outcrops in Devon and Somerset. Gamma-ray measurements were taken at outcrop to provide correlation with published borehole logs. One of the borehole subdivisions was identifed at outcrop within a sequence of otherwise undifferentiated red mudstones and was related to a transition in clay mineral assemblage and stable isotopic composition. The red mudstones below this transition have a higher proportion of magnesian clay minerals and enriched carbonate oxygen isotope compositions, indicating deposition from Mg³⁺-rich marine-derived waters. Clay mineral assemblages in the succeeding red mudstones are dominated by illite, and oxygen isotopic compositions indicate greater influence of K⁺-rich continental-derived waters. -from Authors