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

The type sections of the Sidmouth Mudstone, Dunscombe Mudstone and Branscombe Mudstone formations of the Mercia Mudstone Group are the almost complete sections exposed in the cliffs between Sidmouth and Axmouth on the Devon coast. The partially cored Wiscombe Park No. 1 and No. 2 mineral-exploration boreholes, drilled by British Gypsum Ltd in 1972, were sited about 5.8 and 4.7 km north of the cliff sections respectively. The first of these penetrated the whole of the Sidmouth Mudstone and Dunscombe Mudstone formations and the lower part of the Branscombe Mudstone Formation. The lithological succession proved in the cored parts of the boreholes can be correlated with that exposed in the cliffs. Geophysical logs made through the full length of the boreholes enable the complete succession proved there to be correlated with that exposed in the cliffs. The calibrated geophysical logs have been used to correlate the succession at outcrop with those proved in uncored but geophysically logged hydrocarbon-exploration boreholes throughout the Wessex Basin. The Sidmouth Mudstone and Branscombe Mudstone successions proved in the Wiscombe Park boreholes are similar in thickness and lithology to those elsewhere in the Wessex Basin. In contrast, the Dunscombe Mudstone succession in the boreholes expands from 35 m in thickness to over 500 m by the addition of thick beds of halite in parts of the basin.