Smectite, zeolite, biotite and apatite in the Corallian (Oxfordian) sediments of the Baulking area in Berkshire, England

Abstract

Significant quantities of clinoptilolite-heulandite, lesser amounts of low tridymite-cristobalite, sparse euhedral biotite, apatite and fresh plagioclase accompany the abundant smectite in the mudrocks from the Corallian sequence in the Baulking and Uffington boreholes. The association is thought to be genetically related, of volcanic origin and formed from the in situ decomposition of volcanic ash. Smectite concentrations fluctuate, showing five maxima, but are unrelated to lithology and hence to environment. This suggests repeated ash-falls during the Corallian sedimentation in the Berkshire area. The initial volcanic episode appears to have been contemporaneous with the shoaling which initiated Corallian sedimentation in southern England, suggesting a common tectonic control.