51 2. The stratigraphy of the Middle Jurassic By comparing the different observation points on the Passelourdin cliffs, it is possible to see the very clear discontinuities along which the cavities are aligned. The dolomitized benches and vacuole zones are contained between the discontinuities previously identified by the authors: • the Lower Bajocian (Laeviuscula zone) forms a locally dolomitized sequence constituting the karst at the base of the Bajocian, located between elevation 78 m (D7, top of the Aalenian) and below elevation 91 m (D7bis); • above the Laeviuscula zone and D7bis, hollowed-out conduits line this surface, while overhanging benches feature aligned subhorizontal voids (vacuolar levels, Fig. 5). This complex ends with the pebble surface of the ‘Passelourdin cave’ (D8). This karst is therefore located between 91 and 99 to 101 m; • above D8 and an altitude of 101 m, karst is initiated within the tubular limestones (burrowed zone). This so-called ‘upper’ karst has deepened to the pebble bench (D8), where the harground has limited its development. Conclusion The depositional sequences of the Poitou threshold end with discontinuities that appear to have planned the lower strata. These discontinuities allow correlations between, on the one hand, the distal facies in the Aquitaine basin and, on the other hand, the proximal facies in the center of the threshold defined by tectonics (Gaillard and Branger, 2026). This precise stratigraphic framework allows to characterize the cliffs around Poitiers and highlight karst levels consisting either of cavernous levels in the Laeviuscula and Humphriesianum horizons or along discontinuities. These levels are developed and may have been filled with sediments (red clay or alluvial facies). With this stratigraphic diagram, the position of the karstified levels can be more easily understood. The levels are not arranged randomly but are located at the edge of deposits or are concentrated within certain biozones in the form of vuggy levels. References Branger P. (1989). La marge nord-aquitaine et le Seuil du Poitou au Bajocien : stratigraphie séquentielle, évolution biosédimentaire et paléogéographique. Ph.D. thesis, University of Poitiers, France. Branger P., Gonnin C. (1994). Distribution des ammonites et dynamique sédimentaire sur le seuil du Poitou de l’Aalénien au Bajocien. Proceedings 3rd International Meeting on Aalenian and Bajocian stratigraphy, Marrakech, 5: 293-295.
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