Record of sea-level changes, oxygen depletion and the δ 13C anomaly across the Cenomanian-Turonian boundary, Bohemian Cretaceous Basin

Abstract

The sedimentary record across the Cenomanian-Turonian boundary in Bohemia confirms the crucial role of eustasy as the driving force of the Cenomanian-Turonian anoxic event. The late Cenomanian siltstone facies of the Peruc-Korycany Formation was deposited after major sea-level rise at the beginning of the Metoicoceras geslinianum Zone (above the UZA 2.4/2.5 3rd-order sequence boundary of Haq et al., 1988). Increasing depletion in oxygen during the late Cenomanian is reflected in dark, pyrite-rich siltstones characterized by low C/S ratios and progressive deterioration of benthic foraminiferal assemblages. Small-scale steps in sea-level rise followed by oxygen depletion during the M. geslinianum Zone can be correlated between the Bohemian Cretaceous and the Plenus Marls of SE England. This indicates that the impingement of oxygen-poor waters on the European shelf, which reached inland as far as the Bohemian Massif, was driven by eustasy even at the level of 4th-order cyclicity. The late Cenomanian part of the global δ 13C anomaly is in part overprinted by diagenesis, and in part falls within a stratigraphic hiatus between the upper M. geslinianum and the Neocardioceras juddii Zones. A record of a waning δ 13C anomaly helped to assign the basal Bı́lá Hora Formation to the early Turonian W. coloradoense Zone. The basal Bı́lá Hora carbonates reflect a major early Turonian sea level rise and waning of dysaerobic conditions.