Summary

Silicification fabrics within the Cenomanian and basal Turonian of Devon may be classified by their morphology and petrological relationships. The replacement of bioclasts by silica was often a two stage process; the early stage has δ¹⁸O values ranging from 30.0 to 32.4‰ SMOW. The palaeotemperatures calculated from these are close to values for Cenomanian sea water and thus the early silicification took place in marine porewaters and before significant burial. The consistently lighter values for the later replacement (ranging from 27.2 to 29.4‰) cannot be explained by an increase in burial depth. Partial recrystallization in meteoric waters is believed to be the mechanism responsible. Isotopic data suggest that the formation of siliceous nodules (chalcedony cemented sands) within the arenaceous lower members of the Cenomanian was an early process, although petrographic evidence points to possible (later?) recrystallization. The formation of late-stage drusy quartz occurred entirely in meteoric porewaters.