Late Wenlock sequence and bentonite stratigraphy in the Malvern, Suckley and Abberley Hills, England

Abstract

The Late Wenlock Series (Homerian Stage) of the central Midland Platform occupies an area stretching from Ledbury to the Malvern, Suckley and Abberley Hills. Based upon the establishment of a sequence stratigraphic framework for the Much Wenlock Limestone Formation, and the immediately under- and over-lying Coalbrookdale and Lower Elton Formations, comparisons can now be made with key sections across the northern Midland Platform and beyond. These correlations have been strengthened by the determination of apatite rare earth element (REE) geochemical signatures obtained from four volcanic ash layers (bentonites) at Whitman's Hill Quarry (Herefordshire), which allow for comparisons with published coeval sections at Wren's Nest Hill (West Midlands) and Wenlock Edge (Shropshire), as well as with bentonites described from the Island of Gotland (Sweden).Across the study area fifteen parasequences associated with two pronounced regressive episodes, separated by a marked transgression, can be identified. The lithological responses to these relative sea-level changes are the same as those reported from the West Midlands, including the threefold division of the Much Wenlock Limestone Formation into Lower Quarried Limestone, Nodular Beds and Upper Quarried Limestone Members. Apatite REE geochemical signatures from Whitman's Hill Quarry identify three bentonites which probably originated from a granodiorite magmatic source, while a fourth bentonite has a distinctively mafic composition, more akin to that of a gabbro or syenite. This distinctively mafic bentonite is preserved on a marked flooding surface within the Nodular Beds Member and appears compositionally and stratigraphically equivalent to a bentonite at Wren's Nest Hill (West Midlands). Furthermore in both sections this bentonite is of notable thickness (120–200mm) allowing for its identification in other sections across the region. Comparisons with Gotland identify three closely spaced bentonites of a similar mafic composition to the bentonite described from the Midland Platform. While similarities in stratigraphic position and composition do not, at present, allow for the identification of a single ash fall event covering both the Midland Platform and Gotland, they are indicative of a shared source region, which may offer the possibility for future bentonite correlation between these regions.