Lithofacies and depositional settings of the Coniacian–Campanian Chalks in the Mons Basin (Belgium)

Three drill holes in the Mons Basin yield together 350 m of Coniacian to Campanian chalk cores. Detailed geological logging was conducted on the well-known Trivières Chalk, Saint-Vaast Chalk, and Maisières Chalk formations. A thorough sedimentological characterization was performed, from macrofacies to SEM microtexture. Petrographic observations and geochemical analysis indicate eogenetic calcite cementation and phosphatization in two benchmark levels, already known for their high glaucony content, namely in the Trivières Chalk Formation and the Maisières Chalk Formation. Upwelling currents, prevalent during the Late Cretaceous, are likely the source of the phosphates, and the cause of reduced sedimentation rates. Strong anoxic currents resulted in the formation of hardgrounds, the development of glaucony pellets and pyrite formation within shallow subtidal sediments. Multiple conglomeratic chalk intervals were identified in the Trivières Chalk Formation. They likely result from local sediment destabilization due to extensional tectonic activity. Moreover, the identification of deformation bands attests to synsedimentary tectonic activity within the basin. Overall, this research unravels various phenomena, including marine currents, sea-level fluctuations, and tectonic activity, that influenced chalk deposition in the Mons Basin. This research enables enhanced comprehension and forecasting of the heterogeneities in chalk and its spatial variability.