Impact of alteration corridors on karst reservoir organisation and evolution of groundwater flow path: An example from the southern border of the Larzac Causse, southern France

Abstract

All the features of karstic reservoirs result from the chemical and/or mechanical erosion of an initial rock volume, which modifies the initial petrophysical properties (i.e., porosity and permeability). The spatial distribution and organisation of the karst system in a carbonate massif are often overlooked in studies on karst hydrological functioning. However, these parameters are key to understand and accurately model dynamic flow. This contribution aims at characterising the nature and impact of alteration corridors on the organisation and evolution of the karstic reservoir and its present-day functioning. We focus on characterising the specific impact of the late onset of pocket valley regressive erosion on the organisation of the present-day drainage system. We used a 3D approach to correlate field observations on the surface and in caves, with remote sensing. The expression of alteration corridors is analysed in a 40 km2 area on the southern border of the Larzac Causse. This Jurassic carbonate massif is affected over its entire thickness by a network of vertically elongated alteration corridors containing dissolution-collapse breccia, mainly oriented in a N-S direction. Ghost-rock karstification played a significant part in the karstic reservoir structure and evolution. Alteration corridors result from the in-situ dissolution of the bedrock along an initial jointing pattern. The dissolution-collapse breccia corridors correspond to ghost-rock corridors selectively emptied of their alterite, under the effect of a hydraulic gradient. The vertically elongated structure of alteration corridors that cross-cut the Jurassic sequence enables fluid circulation from an upper to a lower aquifer, which were initially separated by a Toarcian marly seal unit. The subsequent initiation of pocket valleys led to the rapid evacuation of the residual alterite contained in ghost-rock corridors, under the influence of gravity. These alteration corridors determine the position of present-day pocket valley springs and enhance the regressive erosion dynamics, that progressively capture a north-east flowing watershed (Vis River). Finally, residual alterites are evacuated below the overflow spring altitude during high-flow events by flooding/dewatering of galleries. Such mechanisms of alterite evacuation provide insight into i) the part of the karstic reservoir that is connected to the spring, and ii) the dynamics of the associated flows. This study suggests that corridor networks constitute a substantial volume of porous and permeable materials that plays a major role in the present-day groundwater flow path. Such features should be considered valuable drilling targets for water exploration when located below the piezometric level.