Calcite cements and the stratigraphical significance of the marine δ13C carbonate reference curve for the Upper Cretaceous Chalk of England

Abstract

The hypothesis of Jarvis et al. (2006) that a δ¹³C (calcite) reference curve based upon bulk samples from the Upper Cretaceous Chalk of England can be used as a primary criterion for trans-continental correlation is reviewed in the light of new stable isotope data from the Upper albian and Cenomanian chalks of eastern England and from the Cenomanian to Campanian chalks of southern England. Evidence demonstrates that in the coloured chalks of eastern England the cements invariably have positive δ¹³C values (up to 3.5‰) except where they have been affected by hardground development when the cements have negative δ¹³C values down to –6.5‰. in contrast, the White Chalk of southern England may have cements with δ¹³C values as negative as –8‰. Modelling indicates that the coloured chalks may preserve a truer record of the primary palaeo-oceanographic δ¹³C signal than the white and grey chalks of southern England. it is suggested that (1) many of the 72 isotope events described from the δ¹³C (calcite) reference curve and proposed for correlation may reflect the effects of variations in the type and extent of calcite cementation; and (2) until much more is known about the patterns of calcite cementation in the Upper Cretaceous Chalk the use of minor isotope events for trans-continental stratigraphic correlation can only be applied with the utmost caution.