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).