178 A new concept of karst development based on hydrogeology and geophysics the Eocene (Gabilly et al., 1978, p. 19). From the Eocene onward, the plateau was once again overlain by continental deposits coming from the Massif Central mountains. A fluvial network draining westward gave rise to the so-called Ypresis River, named after its Ypresian age, determined at its mouth on the Atlantic coast (Godard et al., 1994). The overlying lacustrine limestones, though poorly dated — except at Fonliasmes (Mazerolles commune), where a Bartonian age was determined (Brunet and Gabilly, 1981), also contributed to covering the Jurassic karst. The Poitou Threshold was then entirely covered by the argillaceous-sandy complex of the Bornais Formation during the Plio-Pleistocene, lying between the ThouarsMirebeau and Champagné-Saint-Hilaire fault zones. Assuming that the downcutting of the valleys eroded the Bornais formation, the onset of plateau erosion likely corresponds to the lowering of sea level during the Günz glacial period (1.2 to 0.7 My BP). The corresponding fluvial terraces (Fv on the French geologic map) are clearly visible only north of the Threshold, around Châtellerault city, and may correspond to the Günz/Mindel interglacial (Bourgueil et al., 1976). The high terraces of the Vienne and Clain valleys (Fw) are mainly Middle Pleistocene in age. Their formation occurred in the context of a rising base level associated with the Mindel-Riss interglacial (421–395 ky). The karst systems of the Passelourdin and Lussac-les-Châteaux sites are each located at elevations below those of the Fw terraces (Gaillard et al., 2024). In both cases, these cavities are altitudinally above the Fx terraces, which are dated to the Saalian (>110 ky BP) through the presence of Acheulean-Levallois and Mousterian industries, as well as faunal remains of Elephas primigenius and Rhinoceros tricorhinus (Facon, 1955; Voinchet et al., 2020). The reactivation of karst is therefore constrained between these two alluvial deposition events, i.e., between 421 and 110 ky BP. This karstic morphology extends beneath the entire Poitou Threshold plateau. A significant issue remains, however: at the HES site, the karstified levels are clearly controlled by the Middle Jurassic stratigraphy, with no connection to the Clain valley downcutting or the terrace altimetry. The elevation of lower karst levels is below the current riverbed elevation of the Clain (Poitiers HES) and the Vienne (Civaux nuclear power plant area in Gaillard et al., 2024). These rivers could not have exported carbonates from below these levels. Syngenetic karstification hypothesis Karst systems located beneath river valleys cannot be explained by Quaternary valley downcutting. Furthermore, late-stage karstification of a carbonate massif requires the export of calcium and bicarbonate ions. Such export via drainage pathways is difficult for deep groundwater located below the current riverbeds. Syngenetic karstification therefore, remains a plausible hypothesis (Jennings, 1968; Grimes, 2006). Such karstification processes, associated with discontinuities, have been described in the
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