Environmental and diagenetic records from a new reference section for the Boreal realm: The Campanian chalk of the Mons basin (Belgium)

In the Mons basin (Belgium), the long-term trend of both δ13C and Sr/Ca reflects stable high and then falling relative sea levels in the Boreal realm through the Campanian. The transition between the stable high and the falling sea levels coincides with the Early Campanian-Late Campanian boundary. Two short-term sea-level changes were also recognised: a major transgressive period at the Early Campanian-Late Campanian boundary associated with an important hydrothermal activity of sub-marine ridges and a regressive period at the mid-Late Campanian characterised by a negative shift in the δ13C profile. The transgressive period at the Early Campanian-Late Campanian boundary is marked by a positive Mn anomaly, a negative anomaly in the δ13C and Sr/Ca curves and lithological evidence (a simple hardground with mineralized intraclasts above and below its surface, bioclastic concentrations with coarse debris). The δ18O curve characterised by a depth-related depletion could support a Campanian long-term cooling trend. These chemostratigraphic data were interpreted in the light of the diagenetic history of the investigated section. The SEM and macroscopic observations do not show any evidence of burial diagenesis. On the other hand, a simple hardground allowed to clarify the impact of the early diagenesis on chalk. The geochemical signature of the hardground is characterised by a depletion in Sr (≈ 350 ppm) and δ13C (≈ 0.5‰) values and an increase in Mn (≈ 210 ppm), Mg (≈ 500 ppm) and δ18O (≈ 0.5‰) values. The lithification is associated with important modifications of the pore-space network: a total porosity depletion from 31.9-38.5% to 18.9-27.6% and a decrease in pore aperture from 0.25-0.38 to 0.14-0.15 μm. The diagenetic evolution of the pore-space is mostly controlled by an overgrowth of calcitic particles. The lack of oriented particles (SEM observations) suggests that the lithification requires a sediment stability.