A potential global chronostratigraphic boundary for the base of the middle Visean

High-resolution foraminiferal biostratigraphy of carbonates immediately below and near the base of the Holkerian Substage in three sections of the South Cumbria Shelf allows the recognition of the Cf5 alpha and Cf5I3 subzones. The most complete section at Grubbins Wood is almost free of dolomitisation. At the White Scar Quarry and Barker Scar sections, minor faunal gaps are inferred at the base of one or both subzones, respectively. The Grubbins Wood section has the most limited exposure, whereas White Scar Quarry has extensive exposure of this interval, especially the basal horizons of the subzones, compared to Barker Scar. The Grubbins Wood section, because of its exceptional foraminiferal record is more suitable for the establishment of a coincident boundary stratotype for the mid Visean and Holkerian boundary at the base of the Cf5I3 subzone. However, the Barker Scar section should be retained as the Holkerian unit stratotype for Britain, with the base of the Holkerian repositioned lower in the section, at the base of the Cf5I3 subzone. White Scar is a suitable unit-parastratotype, filling data gaps due to possible faunal gaps and dolomitisation at Barker Scar. The rich foraminiferal assemblages allow good global correlation with other regional zonations, which if refined in other countries, would allow a more affirmative global chronostratigraphic basal boundary for the middle Visean.