40 A new concept of karst development based on hydrogeology and geophysics With this sequential approach, the deposit sequences of the Poitou threshold can be precisely defined (Table 1). Discontinuities are associated with low sea levels on the eustatic curve. The sedimentary bodies correlate well with the discontinuities for the Aalenian and Bajocian. In the Bathonian, the deposit sequences follow a regressive trend. On the Poitou threshold, the determination of sequences with discontinuities is less precise. Stratigraphy of the Middle Jurassic on the Poitou Threshold Starting in the Early Jurassic period, the Poitou Threshold and its surroundings were gradually invaded by an epicontinental sea connected to the western Tethys Sea to the southeast. Throughout the Middle Jurassic, the region was a depositional area whose general physiography, inherited from the Permian-Triassic differential erosion of its Hercynian basement, allows several distinct areas to be identified. In simplified terms, an axis running from Pouzauges to Oradour-sur-Glane (Gabilly et al., 1978) separates, to the northeast, a shallow domain with abundant carbonate sedimentation and benthic fauna from a deeper domain to the southwest, where sedimentation was less abundant but often accompanied by a rich pelagic fauna of ammonites. This Pouzauges-Oradour-sur-Glane axis corresponds fairly closely to the Vasles fault line, which runs along the Nantes-Parthenay granite axis towards the Champagné SaintHilaire horst and the Availles-Limouzine fault (see southern limit in Gaillard and Branger, 2026). In detail, the shallowest deposits were formed on the northwestern edge of the Massif Central, which at that time formed a carbonate aureole bordering the central platform. The eastern edge of the Vendée Massif, also shallow, appears as a shoal backing onto the Armorican Landmass. These two areas are connected by the Pictavian domain. To the south of the paleogeographic axis lies the Melusine trough (see Gaillard and Branger, 2026), a depressed area linking the northern edge of the Aquitaine Basin, which can be traced as far as the Vendée coastline. The remarkable permanence of these sedimentary domains can be explained by low accumulation of deposits and the stability of the geological bedrock. Indeed, during this interval, the remobilization of the main Hercynian tectonic axes according to a tilted block model, which was paleogeographically decisive during the Callovian and Upper Jurassic periods, appears to have been very limited. The deposit sequences are labeled D in reference to the discontinuities described by Gabilly et al. (1985). Aalenian Throughout the region, the sedimentary evolution that began in the Upper Toarcian with a reduction in clay inputs continued during the Aalenian (bodies labeled 1 and 2 in Fig. 2). The Lower Aalenian (Opalinum zone), which is very thin (2 to 3 m),
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