DIFFERENTIAL EARLY DIAGENETIC LOW-MG CALCITE CEMENTATION AND RHYTHMICHARDGROUND 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 b ioclastic grainstones. These limestones show a distinct lithological c yclicity manifested by fining-upward grain-size cycles with calcite-ce mented layers or hardgrounds capping each cycle. In the lower part of the succession, the cyclicity is further enhanced by nodular chert lay ers, These fining-upward cycles are caused by fair-weather- or storm-w ave-related energy fluctuations and consequently changing rates of dep osition, differences in composition and texture and differences in the degree of early diagenesis. Cemented layers and hardgrounds are the r esult of differential early marine calcite cementation. In these limes tones early calcite cementation cannot be explained by the supply of c ementing materials from saturated seawater, An alternative model for e arly marine calcite cementation is proposed, in which early calcite ce mentation occurred within the sediment at some distance below the seaf loor as a result of organic matter degradation and internal redistribu tion of bioclastic carbonate. Bacterial organic matter degradation cau sed dissolution of relatively unstable high-Mg calcite (and/or aragoni te) in the oxic zone followed by precipitation of calcite cement as a consequence of bacterial iron reduction. During periods with a lower s edimentation rate, any significant early cementation and replacement o f high-Mg calcite occurred when younger oxic burrows dissected older b urrowed sediment with a more advanced stage of organic matter degradat ion, in particular iron reduction. Cementation also occurred when redo x boundaries repeatedly moved through the same layers during periods w ith low nett sedimentation and increased storm activity and erosion. T he differences in the degree of early diagenetic calcite cementation w ere further enhanced during Paleocene karstification and development o f secondary porosity by dissolution of remaining unstable carbonate gr ains and by associated meteoric water calcite cementation.