Differential early diagenetic low-Mg calcite cementation and rhythmic hardground development in Campanian-Maastrichtian chalk

The Campanian-Maastrichtian limestones in the south of the Netherlands are well-sorted fine-grained mudstones and silt- to fine sand-sized bioclastic grainstones. These limestones show a distinct lithological cyclicity manifested by fining-upward grain-size cycles with calcite-cemented layers or hardgrounds capping each cycle. In the lower part of the succession, the cyclicity is further enhanced by nodular chert layers, These fining-upward cycles are caused by fair-weather- or storm-wave-related energy fluctuations and consequently changing rates of deposition, differences in composition and texture and differences in the degree of early diagenesis. Cemented layers and hardgrounds are the result of differential early marine calcite cementation. In these limestones early calcite cementation cannot be explained by the supply of cementing materials from saturated seawater, An alternative model for early marine calcite cementation is proposed, in which early calcite cementation occurred within the sediment at some distance below the seafloor as a result of organic matter degradation and internal redistribution of bioclastic carbonate. Bacterial organic matter degradation caused dissolution of relatively unstable high-Mg calcite (and/or aragonite) in the oxic zone followed by precipitation of calcite cement as a consequence of bacterial iron reduction. During periods with a lower sedimentation rate, any significant early cementation and replacement of high-Mg calcite occurred when younger oxic burrows dissected older burrowed sediment with a more advanced stage of organic matter degradation, in particular iron reduction. Cementation also occurred when redox boundaries repeatedly moved through the same layers during periods with low nett sedimentation and increased storm activity and erosion. The differences in the degree of early diagenetic calcite cementation were further enhanced during Paleocene karstification and development of secondary porosity by dissolution of remaining unstable carbonate grains and by associated meteoric water calcite cementation.