Facies diversity of the Campanian to Danian proximal Chalk Group in the Mons Basin, Belgium: first facies characterization and comparison to South Limburg, the Netherlands

Abstract

During the Late Cretaceous to Danian in Northwest Europe, the mainly pelagic sediments of the Chalk Group were deposited in the Chalk Sea. Proximal zones such as the Mons Basin, Belgium and the South Limburg area of the Dutch and Belgian Campine Basin were characterized not only by the typical coccolithophorid pelagic chalk but also by coarser sediments in a carbonate, siliciclastic, and mixed carbonate-siliciclastic depositional environment. However, the facies diversity of proximal chalk and its processproduct relationships are not yet well-understood. The Mons Basin and South Limburg were both situated in the proximal Chalk Sea just northwest of the Ardennes-Rhenish Massif and east of the Anglo-Brabant Massif, but have different tectonic histories. Comparing and contrasting the Chalk Group facies of these two basins will therefore gain valuable insights on their similar depositional environments and what the influence is of tectonics on sedimentary processes. We studied one borehole and three outcrops across the Mons Basin and conducted litho- and microfacies analysis of their lower Campanian to upper Danian rocks. The facies successions of the Mons Basin and South Limburg both show a fining upwards, followed by a coarsening upwards, and another fining upwards in mostly similar facies. However, key differences include the presence of phosphatic calcarenite in the Mons Basin, glauconitic sandstone in South Limburg, and the fact that facies shifts before the late Maastrichtian occur later in South Limburg. Our results suggest that these differences can be attributed to the inversion of the Roer Valley Graben that acted as a sediment source in South Limburg and influenced the basins sedimentary processes, resulting in the deposition of sediments with higher siliciclastic content and a later response to eustatic sea level induced facies shifts. The absence of inversion tectonics near the Mons Basin resulted in a lower siliciclastic content and the deposition of phosphatic sediments. We suggest that the similarities in the recognized facies reflect the more widespread effect that eustatic sea level changes had on the whole Chalk Sea of Northwest Europe. It is unlikely that the facies similarities were due to a potential sedimentary connection between the two basins during the flooding of the Anglo-Brabant Massif. Thus, facies diversity and facies successions in proximal chalk basins can reflect eustatic sea level changes, but these sedimentary systems can also be dominated by the influence of regional tectonics instead. The improved understanding on the facies diversity, their genetic processes, and the depositional architecture of proximal chalk are essential to improve subsurface models, so that the Chalk Group deposits in the subsurface can be used in a safe, efficient, and sustainable way.