The distribution of halite (rock-salt) in the Mercia Mudstone Group (mid to late Triassic) in south-west England

The subdivision of the Mercia Mudstone Group in south-west England into four formations (the Sidmouth, Dunscombe and Branscombe mudstones and the Blue Anchor Formation) and nine members, each with a distinctive suite of geophysicallog signatures, has made it possible to identify lateral lithological variations within the group. Prominent among these are halite-rich beds that are almost wholly confined to the Dunscombe Mudstone Formation. Their distribution is related to major faults in the underlying Variscan basement that gave rise to differential subsidence during the deposition of the Mercia Mudstone Group. Up to 30 m of halite is present within the Dunscombe Mudstone Formation in the Central Somerset Basin between the Quantock Hills and Mendip Hills structural highs, and up to 130 m of halite is present in the Wessex Basin on the south side of the Cranborne–Fordingbridge structural high. Thick beds of halite are absent from the Dunscombe Mudstone Formation over these structural highs and over a subsidiary structure that runs from the Quantock Hills to the east Devon coast between Sidmouth and the Axe Valley. Thin beds of collapse breccia in the Dunscombe Mudstone Formation in coastal exposures in east Devon may be residues derived from thin beds of halite.