38 A new concept of karst development based on hydrogeology and geophysics A major renewal of marine fauna marks the first Bajocian transgression at the boundary between the Concavum and Discites zones. The carbonate platform is well dated by the Tethyan ammonite fauna in Western Europe, as the boreal fauna did not penetrate the epicontinental seas of the western Tethys. The ammonite zones of the Lower Bajocian are defined by Hyperlioceras discites, Witchellia laeviuscula, Sonninia propinquans, and Stephanoceras humphresianum. The Upper Bajocian transgression lies between the Humphriesianum zone and the Niortense zone. The epicontinental seas underwent a reactivation of the basement faults and an oolitic barrier extending along the Massif Central to the Pyrenees, marking the sedimentation in the Aquitaine Basin (Cariou and Hantzpergue, 1997). The zonation of the Upper Bajocian is based on the ammonites Strenoceras niortense, Garantiana garantiana, and Parkinsonia parkinsoni. The appearance of Zigzagiceras zigzag marks the base of the Bathonian stage. In Poitou, boreal ammonites (Gonolkites convergens) coexist with Mediterranean ammonites (Morphoceras parvum). This mixture of fauna is a distinctive feature of this stage. The Lower Bathonian ends with the Procerites aurigerus zone. The Middle Bathonian includes the Procerites progracilis, Tulites subcontractus, Morrisiceras morrisi, and Cadomites bremeri zones. The Upper Bathonian includes the Prohecticoceras retrocostatum zone and the Clydoniceras discus zone. Deposit sequences The dating of carbonate deposits using ammonite fauna has enabled a detailed reconstruction of the sequence of deposits in the Paris Basin (Rioult et al., 1991). On the Poitou threshold, dating using ammonites is hampered by the rarity of pelagic fauna. Furthermore, the study of Middle Jurassic terrains on the edge of the Figure 1 175 My paleogeographic map of the western Tethys with the Poitou area (GPlates 2.5).
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