59 © EDP Sciences, 2026 DOI: https://doi.org/10.1051/978-2-7598-3934-6.c003 QUAL I TÉ GÉOPHYSIQUEAPPLIQUÉE Hydrogeology of the Poitou Threshold T. Gaillard and M. Moreau Aquifers of the Poitou threshold Two main aquifers The sedimentary cover of the Poitou Threshold (Branger et al., 2026) consists of Lower and Middle Jurassic deposits. Consequently, only two aquifers were historically defined by the first geologists who studied the region’s hydrogeology (Welsch, 1912). These are referred to as the Supra-Toarcian Aquifer and the InfraToarcian Aquifer, named after the intervening Toarcian marls that separate them (Welsch, 1912). Continuous coring from boreholes drilled at the Hydrogeological Experimental Site of Poitiers University (HES) has provided a detailed lithostratigraphic and hydrogeological description of the Jurassic sequence (Fig. 1). Beneath the site, the granitic basement occurs at a depth of 158 m. The InfraToarcian Aquifer overlies this basement and is approximately 21 m thick. Its basal section comprises clays and dolomites of Hettangian–Sinemurian age (150–158 m), overlain by bioclastic marly limestones of Pliensbachian age (135–150 m). This aquifer is overlain by the Toarcian marls, which are about 15 m thick (120–135 m). 3
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