The correlation of Lower Wenlock Series (Silurian) bentonites from the Lower Hill Farm and Eastnor Park boreholes, Midland Platform, England

Abstract

Twenty bentonites sampled from the Wenlock Series (Silurian) strata of the Lower Hill Farm and Eastnor Park boreholes indicate widespread volcanic ash fall across the Midland Platform. This paper describes the clay and phenocryst mineralogy of these bentonites. Furthermore, based upon the rare earth element and yttrium concentrations of apatite grains contained within five of the bentonites, it considers the chemical similarity between ash fall events and the nature of the source magma. Based on this analysis it seems likely that the volcanism originated from a subduction-related calc-alkaline, granodiorite magmatic source, and that such a source was probably well within 250 km of the sample sites. Furthermore, close geochemical similarities between bentonites argues strongly for the occurrence of one and the same bentonite within the uppermost metre of the Buildwas and Woolhope Limestone formations, as developed within the Lower Hill Farm and Eastnor Park boreholes, respectively. Such a correlation provides an important lower Wenlock Series time-line between the stratotype area and the southern Midland Platform.