65 3. Hydrogeology of the Poitou Threshold As early as the 19th century, Longuemar (1856) highlighted the role of vertical fractures in controlling spring emergence at the contact with Toarcian marls. This model implicitly assumes that karst conduits develop preferentially along such fractures, supplied by concentrated recharge via sinkholes. Similarly, Welsch (1912) proposed a conceptual model in which dry valleys act as preferential drainage pathways toward springs. In this interpretation, decompression of the limestone massif within the dry valleys facilitates the karst conduit development. Official reports by hydrogeologists accredited by public health authorities consistently describe groundwater flow in these Jurassic limestones as occurring through fissure and conduit porosity, which locally generates preferential flow zones with high transmissivities (Mourrier et al., 1986). Comparison between lineaments and speleological networks The dominant structural orientations in the study area include faults trending N115–125°, representing the main tectonic axis, and faults oriented N40–50°, corresponding to conjugate extension directions (Burbaud-Vergneaud, 1987). Additional orientations are also present, including N20–30°, N90°, N150°, and N180°. To evaluate the influence of tectonics on the spatial organization of karst networks (restitution karst), a comparative analysis was carried out between lineament orientations and Speleological geometries in two study areas. The first area is centred on the largest known karst system of the Poitou Threshold (the Cuchon Cave system). The second encompasses the Hydrogeological Experimental Site (HES) of Poitiers University (Fig. 4). Lineaments were mapped by extracting elevation data from a 5 m resolution digital terrain model. Their orientations and lengths were used to construct stereo-net plots depicting the local tectonic framework. Karst void orientations for the Cuchon area were derived from a detailed speleological survey of the cave system (approximately 4 km of mapped passages). At the HES, fracture networks were characterized using the OPtical TeleViewer (OPTV) method, which produces high-resolution, magnetically oriented colour images of the borehole wall. The results are shown in Figures 5 and 6. In the Cuchon area, the dominant lineament orientation is N45–55°E, whereas the surveyed karst conduits predominantly trend N130–140°E. This suggests that local fracturing does not exert primary control on the orientation of the Cuchon network galleries. At the HES, two regional fracture sets were identified, trending N50–65°E and N120–130°E. However, discontinuities observed in boreholes are mainly oriented between N80–100°E. These two examples indicate that restitution-type karst systems are only weakly influenced by regional fracturing patterns.
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