The stratigraphy of the Mercia Mudstone Group succession (mid to late Triassic) proved in the Wiscombe Park boreholes, Devon

The type section of the Mercia Mudstone Group is the almost complete exposure in the cliffs between Sidmouth and Axmouth on the south Devon coast. The group comprises four formations, in ascending order the Sidmouth Mudstone, Dunscombe Mudstone, Branscombe Mudstone and the Blue Anchor Formation. The type sections of the three oldest of these are at this locality. The partially cored Wiscombe Park No.1 and No.2 mineral-exploration boreholes, drilled by British Gypsum Ltd (now part of BPB UK Ltd) in 1982, were sited about 5.8 and 4.7km north of the cliff sections respectively. The deeper of these penetrated almost the whole of the Sidmouth Mudstone, the whole of the Dunscombe Mudstone and the lowest part of the Branscombe Mudstone. The lithological succession proved in the cored parts of the boreholes can be correlated in detail with that exposed in the cliffs. This has enabled geophysical logs made through the full length of the boreholes to be correlated with the cliff sections for the cored and uncored parts of the boreholes. The availability of a suite of geophysical logs that has been calibrated against the coastal exposures provides a key correlational link between the outcrop succession and a large number of uncored but geophysically logged hydrocarbon-exploration boreholes throughout the Wessex Basin. Eastwards from Wiscombe Park and the coastal exposures, beneath much of south Dorset, the Dunscombe Mudstone expands from 35m in thickness to over 180m by the addition of thick beds of halite (Gallois, 2003; Harvey and Stewart, 1998).