More than chalk

In South Limburg, groundwater is mainly abstracted from rocks deposited between around 85 and 61 million years ago, during the Late Cretaceous and earliest Paleocene. These rocks form the main aquifer of the region and are traditionally grouped in the Chalk Group. Groundwater quality has deteriorated in recent decades, particularly because of high nitrate concentrations linked to agriculture and urbanisation, yet the geological and groundwater models used in the region do not always reproduce observations from pumping tests and groundwater monitoring. Chalk is a fine-grained limestone whose relatively homogeneous composition and texture and wide occurrence across northwest Europe have led to it being viewed as a uniform sedimentary system, and this view has shaped how chalk aquifers are understood: high matrix porosity but low permeability, with flow concentrated in fractures and dissolution pathways. South Limburg, however, occupied a proximal zone of the Chalk Sea, close to emergent land and to the Roer Valley Graben, which was uplifted and eroded during the Late Cretaceous. Sedimentation was therefore influenced by carbonate production, siliciclastic supply, changes in water depth, and tectonic movements, and the succession contains not only chalk but also mudstones, glauconitic sandstones, mixed carbonate–siliciclastic deposits, bioclastic packstones and grainstones, and coquinas. The region has historically been studied mainly from a palaeontological perspective, and the resulting stratigraphic subdivision does not always represent the lithological heterogeneity relevant for subsurface mapping and groundwater management. This thesis addresses this problem by characterising the facies heterogeneity of the Upper Cretaceous–Danian succession and investigating how it controls aquifer properties. I combined outcrop and borehole observations with thin-section petrography, mineralogical and chemical analyses, geophysical well logs, and laboratory measurements of porosity and permeability. On this basis, I propose a revised lithostratigraphic framework, recognisable in outcrops, borehole cores, and geophysical logs. The siliciclastic Aken and Vaals formations are placed outside the Chalk Group, which is instead subdivided into the Gulpen, Dorne, Kunrade, and Maastricht formations. Danian calcarenites previously assigned to the Houthem Formation are included in an expanded Maastricht Formation, whose six traditional members are better represented by a lower and an upper member. The distribution, thickness, and preservation of these units were strongly controlled by Late Cretaceous inversion tectonics, which divided South Limburg into structural domains with different histories of subsidence, uplift, and erosion. This heterogeneity has direct consequences for groundwater flow. Chalk of the Gulpen Formation has high matrix porosity, around 40%, but low permeability, so fractures, flint horizons, and dissolution pathways govern flow. Bioclastic calcarenites of the Maastricht Formation and grainstone beds of the Kunrade formation have larger, better-connected pores, with matrix permeability approaching 10 D, so groundwater can flow through the rock itself. In contrast, mixed carbonate–siliciclastic and cemented intervals may locally form baffles. The Upper Cretaceous–Danian aquifer of South Limburg should therefore not be represented as a single homogeneous chalk aquifer. Representing its units and their petrophysical properties separately provides a more realistic basis for geological and hydrogeological models of groundwater flow in the region.