68 A new concept of karst development based on hydrogeology and geophysics Typology of voids in Poitou threshold limestones For both study sites (HES and Cuchon Cave), the observed tectonic patterns do not coincide with the karst networks’ orientations. This prompted an investigation into the potential role of stratigraphy in controlling karst development, based on analyses of primary cliff outcrops described in several doctoral dissertations on the Poitou Threshold (Benvel, 1978; Beaulieu, 1978; Balusseau, 1980; Mourrier, 1983). This work was presented during a field excursion of the French Chapter of the International Association of Hydrogeologists (www.cfh-aih.fr/ colloques-et-visites/excursion-seuil-du-poitou-ag-12-au-15-03-2020.html and “Le karst du seuil du Poitou : Approche stratigraphique et rôle de la tectonique” on researchgate). The methodology consisted of identifying macroporosity features in stratigraphically well-constrained outcrop sections. An illustrative example is shown in Figure 7, taken from the former Puy-Hervé quarry in Latillé, approximately 27 km west of Poitiers (see Fig. 3). Stratigraphic correlation was established through the collection of two index ammonite specimens: Brasilia sp. (Aalenian) and Sonninia ovalis (Early Bajocian). A distinct discontinuity separates the oolitic Aalenian formation from the gravelly limestones of the Bajocian. This surface, infilled with red clay, corresponds to Discontinuity D7 as defined by Gabilly et al. (1985). Locally, this discontinuity has been subject to dissolution, resulting in the development of macroporous zones within the limestone massif. Using this method, two types of voids were identified, both in the examined outcrops and in the cores from the HES boreholes: • type 1: scattered vacuoles within massive limestone beds, variably interconnected. These vacuolar levels are predominantly concentrated in the Lower Bajocian, specifically within the Laeviuscula and Humphriesianum ammonite zones; • type 2: conduits aligned along stratigraphic discontinuities between two depositional sequences. These discontinuities are often filled with red clay, possibly corresponding to paleosol horizons. Their numbering follows the classification of Gabilly et al. (1985). These void types are illustrated in Figure 8. They are not associated with vertical fractures. The dolomitization observed in certain formations was not considered in this analysis, as stratigraphic correlation was not possible due to the absence of paleontological markers in those units. Type 1 voids are observed within the Bajocian limestones. In the area surrounding Poitiers (Boivre Valley, Clain Valley), these vacuoles are concentrated near the top of strata within the Humphriesianum zone and form distinct vuggy levels. Figure 9 shows an example from the Passelourdin cliff, located six kilometers south of Poitiers (Branger et al., 2026). In certain outcrops of Aalenian age (Gartempe Valley, east of Poitiers), calcite geodes of similar shape and size have been observed by the authors.
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