64 A new concept of karst development based on hydrogeology and geophysics valley. Indeed, downcutting of the Vienne valley during the Mindel and Riss periods extended well beyond the paleovalleys of the Clain, which do not extend northward beyond Vivonne (46.42°N). The deeper incision of the Vienne valley into Supra-Toarcian formations south of Chauvigny enhances groundwater drainage toward the Vienne at the expense of the Clain basin. More recent, localized piezometric investigations have shown that this groundwater divide fluctuates spatially and temporally over several kilometers (Gaillard and Moreau, 2024). The piezometric map in Figure 3 depicts the easternmost position of the divide recorded to date. In the upstream part of the Clain watershed, another piezometric divide occurs south of the Availles-Limouzine Fault (ChampagnéSaint-Hilaire Horst; see Gaillard, 2026). This divide separates the Clain from the Charente Basin, which drains westward to the Atlantic Ocean. Finally, to the southwestern part of the Clain Basin, a portion of groundwater is captured by the springs of the Sèvre Niortaise river via several karstic sinkholes located in the Dive-du-Sud valley. This karstic capture has been demonstrated by dye tracing experiments (Coirier, 1964). The supra-Toarcian aquifer Groundwater flow patterns in the supra-Toarcian aquifer Groundwater flow in the Supra-Toarcian Aquifer exhibits characteristics of both a porous aquifer system—evidenced by piezometric mapping—and a karstic system, as demonstrated by numerous dye tracing tests and the exploration of karst networks by speleologists (Déribéré, 1937; SCP, 1990; Sibert et al., 2008). The speleological networks explored within the Vienne département can be classified into two main types: introduction karsts and restitution karsts. Introduction karsts (also referred to as sinkholes or “gouffres”) are circular depressions, sometimes reaching diameters of up to 100 m (e.g., the Gouffre du Grand Soubis in the Moulière Forest; Gouffre de la Troussaye at Marnay). Their depths generally do not exceed 50 m (e.g., 48 m at the Gouffre du Charreau de Boussec, Chauvigny). These features are predominantly vertical shafts, occasionally interrupted by ledges corresponding to bedding plane discontinuities. Restitution karsts are most commonly accessed through caves, and less frequently by cave diving. These systems include laminoirs (passages developed along bedding planes) and trémies (collapse zones connected to the surface). The galleries are typically sub-horizontal (Bigot, 2004, p. 105) and often display parallel orientations (SCP, 1990, p. 58).
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