44 A new concept of karst development based on hydrogeology and geophysics In the Chauvigny area, at a height of 21 meters, the Middle Bathonian consists of finely grained limestone with thin oolitic and oncolitic intercalations and a few beds rich in Entolium towards the top. Clayey intercalations of brackish origin persist at this level in the Creuse valley. The Upper Bathonian (25 to 30 meters) corresponds to the Chauvigny oolite. The lower part (10 to 12 meters) is formed of coarse limestone with colonies of polyps (Lochmaeosmilia), topped by fine oolite in massive banks. Here again, we find discontinuity D11 in the form of a flattened and threaded surface. The various deposit sequences (systems tracts) are shown in Figure 2. Carbonate deposits are significantly thicker (gravelly and oolitic facies) on the threshold than on the Aquitaine slope (bioclastic facies). The discontinuities separating the deposit sequences extend from the basin to the platform. Correlations between the basin and the Poitevin platform with sedimentary discontinuities make it possible to define the geometry of the sedimentary bodies. An overall diagram of these deposit sequences is shown in Figure 2. The ammonites indicating the biozones are shown in Plate 1. Stratigraphy of the Bajocian south of Poitiers: the cliffs of Passelourdin In Saint-Benoît, south of Poitiers, the Passelourdin cliffs in the Clain Valley provide an opportunity to observe the stratigraphy of the Lower and Upper Bajocian. The area has been briefly described by various authors (Gabilly, 1978; Benvel, 1978; Mourier et al., 1986). The cliff provides a detailed description of the Bajocian deposit sequences, thus providing a precise stratigraphic framework for this stage for the study of the results obtained at the Hydrogeological Experimental Site (HES) of the University of Poitiers. The structural context is linked to the Ligugé horst (Fig. 3). The site is in fact at the center of a horst bounded to the south by the Mauroc fault and to the north by the Saint Benoit fault (Benvel, 1978). The Mauroc fault is parallel to the Ligugé horst faults and has a southward offset of approximately 10 to 12 m. It is not sufficient to bring the Bajocian and Pliensbachian into contact, as the Toarcian marls are approximately 22 m thick (see section of borehole 05894X0053/1030 on the horst). Together with the Saint Benoît fault, it thus delimits a small horst within which the equivalent of the Bajocian III of Benvel, 1978, in Figure 3 (Gaillard and Branger, 2024) outcrops. A new survey was conducted in February 2025 with the help of a rope access technician. Due to very dense vegetation, the description is based on a synthetic survey of the entire height of the cliff (“survey with climbing”), illustrated by photographs (Plate 2).
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