ABSTRACT

¹⁸O_PDB and ¹³C_PDB values have been measured on 107 calcite cement samples from a laterally extensive (> 3 km) and continuous calcite-cemented layer 0.5 m thick in the coastal exposures of the Lower Jurassic shallow-marine Bridport Sands in Dorset, southern England. The samples were taken from a two-dimensional grid with 10-cm horizontal and vertical spacing between samples and along individual vertical lines across the calcite-cemented layer. ¹⁸O_PDB values vary between -4.8 and -9.2 and decrease radially outwards from points with lateral spacings on the order of 0.5-1 m in the middle of the calcite-cemented layer. The ¹⁸O_PDB values therefore indicate that the calcite-cemented layer was formed by merging of concretions. All ¹³C_PDB values measured are in the narrow range -2.2 to -0.5, which suggests that the dominant source of calcite cement in the layer was biogenic carbonate.