Diagenesis of English Lower Jurassic limestones as inferred from oxygen and carbon isotope analysis

Fifty-seven samples of mostly Liassic limestones and calcite spar were subjected to mass spectrometer analysis to determine δ¹³C and δ¹⁸O. The results are used to compare and interpret the changing isotopic composition through time of calcite-precipitating pore fluids and to assess the influence of rate and type of sedimentation, diagenetic reactions and meteoric water influx. In general microsparite beds and concretions formed sooner after deposition in Dorset than in Yorkshire, where sedimentation rates were usually higher. Calcite horizons in the Bridport Sands Formation of Dorset are entirely diagenetic in origin. Carbon isotope data from concretions in the Jet Rock Formation of Yorkshire support a model of changing organic reactions with increasing depth of burial. The influence of meteoric waters is substantially restricted to the formation of post-Liassic veins.