Stratigraphical influence on chalk cave development in Upper Normandy, France: implications for chalk hydrogeology

Abstract

Classically, the Upper Cretaceous Chalk Group aquifer of northwest Europe is conceptualized as a homogenous dual-porosity aquifer, with high porosity related to its fine-grained porous matrix, and intermediate hydraulic conductivity associated with fractures. However, an increasing number of hydrological studies visualize the Chalk as a heterogeneous karst aquifer due to the localised presence of dissolutionally enlarged conduits. Field investigation suggests that cave development is guided by distinct stratigraphical and tectonic discontinuities within the rock mass. Identifying which potential inception horizons within the Chalk aquifer are favoured, and why, is important for developing future robust conceptual models of groundwater behaviour. This study focusses on the Chalk of the Upper Normandy region in France where karstic conduits are common and are linked to major sources of groundwater for public water supply. We analyse the geometry and geomorphology of six chalk caves exposed in the Seine Valley with an aggregated length of over 5.7 km, along with other caves in southern England, and identify the key inception horizons associated with their development. The data shows that prominent Turonian, Coniacian and Santonian hardgrounds have influenced the development of 68% of the studied caves length, with sheet-flints and marl seams also playing a prominent role. Caves developed on or between hardgrounds typically display a complex interlinked anastomotic passage network, whereas passages subjected to paragenetic conditions caused by a high sediment flux tend to be concentrated into fewer, larger conduits. The new evidence from Normandy and Southern England demonstrates the role of lithostratigraphy, and in particular stratigraphical discontinuities on conduit development. The data reinforces the idea that the Chalk aquifer should be viewed as a heterogeneous triple porosity karstic aquifer, in which conduit development is influenced by key stratigraphical discontinuities. This improved conceptual model can be used to develop better groundwater flow models and improved catchment delineation.

The genesis and evolution of karstic conduit systems in the Chalk

“…They are especially important in low-dip settings. Evidence from cliff and quarry sections, boreholes, accessible chalk caves, adits and springs suggest these horizons play an important role in the development of conduit networks (Ballesteros et al, 2020).…”

“…Marl seams can influence recharge processes significantly, creating horizons with high matric potential (Gallagher et al, 2012). In parts of the succession, hardgrounds are important inception horizons, notably the Chalk Rock in the Chiltern Hills and in Normandy where they host conduits and cave systems (Ballesteros et al, 2020) and springs (Figure 7C). The majority of the small solutionally enlarged conduits and dissolution tubules developed along these inception horizons are likely to be formed by mixing dissolution, locally enlarged by later epigenic dissolution.…”

Going with the flow: Sedimentary processes along karst conduits within Chalk aquifers, northern France

“…1c) (Genuite et al, 2021) and fluvial terraces (Lautridou et al, 1999;Antoine et al, 2000Antoine et al, , 2007. Coevally, the karstification has created branchwork and maze caves (Rodet, 2013), typically associated with inception horizons on low permeability beds (hardgrounds, marls and sheet flints) within the Chalk (Ballesteros et al, 2020;Farrant et al, 2023). At present >8 km of cave passage has been explored.…”

Historical underground quarrying: A multidisciplinary research in the Caumont quarry (c. 13th–19th centuries), France

“…A huge volume of chalkstone was exported from this area to the buildings of the Seine Valley during the Middle Ages (Ballesteros et al, 2021) and following centuries, competing with other construction workshops in Caen, Vernon and Paris, among others (Dujardin, 2004(Dujardin, , 2006. The quarried bedrock in Caumont corresponds to dedolomitized chalk, a singular coccolithic limestone of the Upper Cretaceous in age (Ballesteros et al, 2020(Ballesteros et al, , 2021. This quarrying area includes more than 10 exploitations (e.g., Caumont, Maquisards, Pylone, Jacqueline); the major one is specifically named Caumont quarry (Sibout, 2011;Tomat, 2009), the openings of which are located at 200 m from the River Seine (Figure 1b).…”

“…Indeed, the Gros Lien bed shows abundant rhombohedral moulds filled by calcite related to the development of hardgrounds during chalk diagenesis. Consequently, the occurrence of hardgrounds (detailed in Ballesteros et al, 2020) explains the quality of the Caumont chalkstone as a historical construction material.…”