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Highlights

  • The role of stratigraphic discontinuities on chalk speleogenesis has been analyzed
  • The analyses combine cave geometry, karst geomorphology and stratigraphical sections
  • Chalk caves and karst springs are related to hardgrounds, sheet-flints and marl beds
  • Hardgrounds, sheet-flints and marl beds concentrated groundwater flow forming caves
  • The results reinforce the visualization of the Chalk as a karst aquifer

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.

DOI

https://doi.org/10.5038/1827-806X.49.3.2319

Creative Commons License

Creative Commons License
This work is licensed under a Creative Commons Attribution-Noncommercial 4.0 License

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