41 2. The stratigraphy of the Middle Jurassic shows stratigraphic growth of fine clayey limestone beds. This is where the Gryphaea beaumonti lumachels are found, which constitute a regional stratigraphic marker. D6 From the Middle Aalenian onwards (units 3-4-5-6), the paleogeography changes and two areas can be distinguished: • a distal platform domain extending along the northern Aquitaine margin, characterized by highly condensed and lacunar sedimentation of calcareous biomicrites with ferruginous oolites, where nektonic fauna (ammonites, belemnites) and benthic fauna (mollusks) are equally abundant; • a proximal carbonate platform edge domain, with much thicker sedimentation, consisting of fine dolomitic limestones with flints and predominantly benthic fauna, from a shallower environment with moderate energy. In the basin area, from Niort to Saint-Maixent, the Middle Aalenian (Murchisonae zone) consists of thin marl beds and a more massive bank of clayey limestone. Above a marked discontinuity (D5bis?) is a level of reddish clayey limestone with ferruginous oolites and numerous fossils, gastropods, bivalves, belemnites, and ammonites characterizing the upper part of the zone. The whole is less than 1 m thick. The Upper Aalenian, Concavum zone, is only represented from the Saint-Maixent graben and also at Vitré, north of Celles-sur-Belle, in the form of reddish, sometimes purplish, highly fossiliferous clayey limestones with ferruginous oolites and not exceeding 10 cm in thickness (unit 7). In the Couhé-Vérac-Lusignan region, the Middle Aalenian thickens considerably (approximately 20 m) and occurs as dolomitic limestones with flints attributed to the Murchisonae subzone. The next 6 meters, Bradfordensis subzone, consist of often dolomitic limestones with flints alignments. The Upper Aalenian and the extreme base of the Bajocian (Concavum and Discites zones), nearly 5 m thick, correspond to pseudo-oncoides limestones. From Poitiers to Chauvigny, the Aalenian has the same facies with even greater thicknesses, from 19 to 27 m, for the whole. Islands dotted the platform with mangrove flora, giving rise to polyp limestones and driftwood. Continuing eastward into the Gartempe Valley, the Aalenian thins again as it approaches the northern edge of Limousin, with the return of ferruginous oolites in the Bradfordensis subzone and abundant pelagic fauna. D7 – Lower Bajocian Everywhere, the base of the Lower Bajocian is indicated by levels rich in ammonites, numerous Sonniniidae, and evidence of a major transgressive event (units 8 and 9). Along the entire northern Aquitaine border, there is a layer of gray limestone with ferruginous oolites, generally about 20 centimeters thick, rich in fossils, including
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