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

Abstract

Sediment-filled caves, conduits and voids are common in many karst regions. These voids and the sediment they contain are important palaeoclimatic and palaeoenvironmental archives, but often have an adverse impact on engineering projects, mineral extraction and hydrogeology. Most studies into fluvial sedimentation in karst aquifers have focussed on more traditional karst areas. However, the nature and extent of fluvial sedimentation within caves and conduits in the important Upper Cretaceous Chalk Group aquifer (NW and Central Europe), and their impacts are less well known. This is principally due to a lack of accessible Chalk caves with exposed 3D sediment archives for study. Fortunately, the discovery of the World's longest Chalk cave system by underground quarrying at Caumont in the Seine valley near Rouen, northern France, has exposed numerous sediment sections along 2.4 km of passage. Detailed analysis of the stratigraphy, mineralogy, sedimentology, provenance and the chronology of the exposed sediments including the novel use of Gamma-ray spectrometry, reveals complex stratigraphy and lateral facies distribution along a karst conduit. The depositional model comprises five allostratigraphical units since the mid-Chibanian, separated by periods of erosion. The units are derived from hyper-concentrated and sediment-laden flows, and include thalweg, channel, slackwater, backswamp speleothem facies and debris flow deposits that are interbedded. Speleothems precipitated during MIS 7, 6, 5e and 1. During MIS 7–6, detrital sediments filled almost all Chalk conduits, similar to other caves in the European Atlantic Margin, coevally with the Penultima (Saalian) Glacial Cycle and a maximum of the Earth eccentricity. Detrital sediments are derived from the erosion of local Chalk bedrocks as well as metamorphic and igneous rocks of remote areas, such as Morvan massif and Massif Central. The depositional model is consistent with the conception of the Chalk as a karst aquifer. Significant sediment aggradation caused upwards dissolution (paragenesis), conduit occlusion and subsequent genesis of new conduits by flow diversion, potentially altering the functioning of the chalk aquifer and the interpretation of Chalk hydrogeology (e.g., dye-tracing tests).