Le karst de Casteljaloux (Lot-et-Garonne). Rôle du processus de fantômisation dans la genèse des grottes épikarstiques et du fonctionnement hydrologique

Abstract

The underground karst of Casteljaloux (Lot-et-Garonne, France) : role of ghost-rock processes on epikarstic cave evolution and hydrological functioning of the springs. The karst of Casteljaloux is located on the left bank of the Garonne river , 80 km southeast of Bordeaux (70-150 m a.s.l.). The epikarstic caves develop a few meters below the surface in Miocene lacustrine limestones of Lower and Upper Aquitanian. The southern part is covered by a quarternary eolian sandy formation (Sables des Landes) covering the Tertiary. The ghost-rock process in limestone was observed in sections of quarries, roads and in caves. In the quarry of Trouyre, there are false galleries showing an evolution with weathered rock, brown clay injection on top and into the altered rock, compaction of the sediments, inwash of quartz gravels (paleo-terrace of Garonne), and finally recent deposits (charcoal). In the subhorizontal cave of Saint-Julien, 3 m below the topographic surface, there is an alteration of the Aquitanian limestone and the marly limestone forming the ceiling, resulting in densely packed sodastraws. The petrographic analysis confirms thaqt porosity is due to alteration. The hydrochemical regime of springs is very specific. The concentration of calcium and bicarbonate increases with flow rate (concentrative effect), opposite to classical karst. This hydrochemical response indicates a very porous karst, directly related to the strong alteration of Aquitanian limestone and the sandy cover which enhances retarded recharge in water.