37 © EDP Sciences, 2026 DOI: https://doi.org/10.1051/978-2-7598-3934-6.c002 QUAL I TÉ GÉOPHYSIQUEAPPLIQUÉE 2 The stratigraphy of the Middle Jurassic P. Branger, T. Gaillard and H. Geairon Paleontological division of the Middle Jurassic Since the end of the Toarcian period, the Poitou threshold and its Parisian and Aquitaine slopes have corresponded to a carbonate platform that persisted throughout the Middle Jurassic (Aalenian, Bajocian, and Bathonian). At that time, Poitou was located at the edge of the Tethys, a vast ocean between Gondwana and Laurasia (Fig. 1). The first sediments deposited on this platform date back to the Aalenian (174 Ma± in ICS, 2024). Two ammonite lineages are commonly used to date the sediments of this stage: Leioceratinae and Graphoceratinae. The Toarcian-Aalenian boundary is difficult to define paleontologically because the last Pleydellia have affinities with the first Leioceras (Cariou and Hantzpergue, 1997). In platform regions such as Poitou, the Toarcian-Aalenian boundary is linked to an ecological change with a drop in sea level. The first ammonite zone corresponds to the appearance of L. opalinum. This is followed by two species of ammonites that are widespread in Western Europe: Ludwigia murchisonae and Brasilia bradfordensis, whose horizons determine the Middle Aalenian and are separated by a discontinuity. In Poitou, the Middle Aalenian ends with the Brasilia gigantea horizon. The Upper Aalenian corresponds to the appearance of Graphoceras concavum.
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