The Mercia Mudstone Group of the Severn Basin

This report is an output of the UK Stratigraphy thematic research area within BGS National Geoscience. The rationale for the work was to support the development of an improved stratigraphic framework for part of the Mercia Mudstone Group in the Severn Basin region (Newell, 2024) in light of recently published research on the topic (Milroy et al., 2019), and building on the framework established by Howard et al. (2008). We find that, in the Severn Basin, the Formation-level identification scheme proposed by Milroy et al. (2019) holds true for newly studied drillcore from the Newport area. Furthermore, we suggest that the ‘Dunscombe Mudstone’, recently adopted by Newell (2024) in a national-scale correlation of borehole wireline data, may be a more appropriate term to encapsulate the markedly heterogeneous interval observed at outcrop in this study between what is currently (sensu Howard et al., 2008) the uppermost Sidmouth Mudstone and lowermost Branscombe Mudstone formations. Herein we present new logs of several sections visited by Milroy et al. (2019) which were used to devise a stratigraphic framework for the region based on physical characteristics of the units such as fracture habit and sedimentology, as well as the logs of two previously unrecorded boreholes from the Newport area. Core from a borehole at Twyning was also studied.