60 A new concept of karst development based on hydrogeology and geophysics Above the Toarcian marls lies the Supra-Toarcian Aquifer. Its base consists of marly limestones passing upward into more carbonate-rich and oolitic facies of Aalenian age (101–120 m). The overlying Bajocian limestones (46–101 m) display a range of facies, from bioclastic to gravelly textures, and include a distinct interval of white oolites between 80 and 83 m. This unit is followed by fine-gravelly limestones interbedded with flint of Bathonian age (21–46 m) and capped by “chalky” white limestones of Callovian age (6–21 m). Near the surface, these limestones are weathered to form a red clay horizon (0–6 m). The Infra-Toarcian Aquifer is confined, whereas the Supra-Toarcian Aquifer is unconfined. The substantial thickness of the Toarcian marls ensures effective hydraulic separation between the two, except in faulted zones where vertical displacements exceed 20 m. Piezometric map At the HES, groundwater levels in the Infra-Toarcian Aquifer range from 95 to 100 m NGF (meters above sea level, French elevation datum), while those in the Supra-Toarcian Aquifer range from 100 to 105 m NGF. The piezometry of the Supra-Toarcian Aquifer is relatively well documented as a result of several monitoring campaigns carried out between the Vienne River (east of Poitiers) and the Clain River (Coirier et al., 1968). In 2004, two additional piezometric surveys were conducted on the Supra-Toarcian Aquifer. Each survey covered approximately 800 measurement points (Marchais and Bichot, 2005). The piezometric map derived from the October 2004 survey is considered representative of low-water conditions (Fig. 2). The main drainage axis in the Clain River basin trends south–north from Vivonne to Poitiers. The downcutting by the Clain began during interglacial stages of the Mindel and Riss glaciations. Secondary drainage axes, oriented perpendicular to the main one, correspond to younger valleys incised during the Weichselian: the Clouère River (Gencay–Vivonne axis), the Vonne River west of Vivonne, the Auxance River north of Poitiers, and the Boivre River. In these valleys, Supra-Toarcian limestones are deeply eroded, and riverbeds may locally cut down to the Infra-Toarcian Aquifer. Piezometric gradients in the area range between 0.5% and 1%. The piezometric watershed of the Clain differs markedly from its topographic watershed (Fig. 3). To the east, a north–south-oriented groundwater divide separates the Vienne and Clain basins. This divide is near the axis of the Miosson River, which is perched and hydraulically disconnected from the Supra-Toarcian Aquifer in its upstream reaches. The piezometric divide defines the hydrogeological catchments of the Vienne and Clain, which do not coincide with their surface watershed boundaries. The piezometric catchment of the Clain is smaller than its topographic watershed due to a westward offset of the divide. As early as 1968, Coirier et al. attributed this offset to the lower base level of the Vienne
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