The genesis and evolution of karstic conduit systems in the Chalk

Abstract

The Upper Cretaceous Chalk Group is renowned as a major aquifer, but the development of secondary porosity due to karstic conduits is poorly understood. Hydrogeological data and evidence from boreholes, sections, and tracer tests indicate that dissolutional conduits occur throughout the Chalk aquifer. Here, we assess the evidence for Chalk karst, and combine it with theoretical models of dissolution and cave formation to produce a conceptual model for the development of karstic conduits. Dissolution due to the mixing of saturated waters of contrasting chemistry along key lithostratigraphical inception horizons form extensive but isolated conduit networks. These form a significant proportion of the secondary porosity and enhance permeability. They prime the aquifer for the development of more integrated conduit networks formed by focussed recharge of unsaturated surface derived water. However, the porous, well-fractured nature of the Chalk means that the time needed to form large integrated cave systems is often longer than timescales of landscape change. Continued landscape evolution and water table lowering halts conduit development before they can enlarge into cave systems except where geological and geomorphological settings are favourable. Groundwater models need to consider the formation of secondary karst permeability as this has a major influence on groundwater flow.

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

“…Fluvial and debris-flow sediments observed in the Caumont cave system are comparable to those seen in many classical karst caves (e.g., Campaña et al, 2017;Martini et al, 2018). This reinforces the idea, based on previous geomorphological and hydrogeological studies (see Farrant et al, 2023;Maurice et al, 2023), that the Chalk often acts as a karstic aquifer. In detail, the lithofacies and facies association established by Bosch and White (2004) and White (2007) for classical karst caves was successfully applied to the study of a Chalk aquifer conduit system for the first time.…”

“…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.…”

Comparing the Fate and Transport of MS2 Bacteriophage and Sodium Fluorescein in a Karstic Chalk Aquifer

“…They are very common in the field area and are generally concentrated on or near to the geological contact between the Late Cretaceous Chalk and the overlying Paleogene sand and clay deposits [ 43 , 44 ]. Sediment eroded from these deposits is transported into the aquifer via stream sink features, impacting conduit development and the hydrogeochemical functioning of the aquifer [ 45 ]. Catchments where arable land use is predominant can experience enhanced sediment influxes due to poor soil health and high sediment loads in run-off events.…”