39 2. The stratigraphy of the Middle Jurassic Massif Central, particularly in Haut-Poitou, is difficult due to lateral variations in facies (Glangeaud, 1895). Under the impetus of Professor Gabilly, the stratigraphy of the Poitou threshold was revised in the 1970s with the definition of 107 ammonite biozones from the Sinemurian to the Oxfordian (Gabilly and Rioult, 1971; Gabilly and Cariou, 1974). This zoning was clarified using a lithological and sedimentological approach. Even before the advent of sequential stratigraphy, Gabilly understood the importance of taking into account major sedimentary discontinuities and deposit facies (Gabilly, 1962), which were often ignored by earlier authors (Welsch, 1895). The Poitou threshold series was thus revised and divided into 13 sequences separated by discontinuities and presented in 1974 during the excursion organized by Gabilly and Cariou on the Poitou threshold. The comparison of pelagic ammonite zones (Deux-Sèvres and Charente) with the discontinuities separating each sequence made it possible to define a sequential stratigraphy of the Jurassic platform on the threshold where ammonites are rare or even absent in the outcrops studied (Gonnin et al., 1993; Branger and Gonnin, 1994). Table 1 shows the initial stratigraphic division by Gabilly and Cariou (1974), which was subsequently used in the work of the Faculty of Geology at Poitiers and in the notes on the 1:50,000 geological maps. The numbers of the discontinuities (D6 to D11) are borrowed from the scale used in the work of the University of Poitiers (Mourier, 1983; Gabilly et al., 1985) and follow the logic of the 1974 sequences. Table 1 Sequence stratigraphy of the Poitou Threshold.
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