42 A new concept of karst development based on hydrogeology and geophysics numerous benthic forms, bivalves, and gastropods, as well as cephalopods, ammonites, and belemnites. Above this, are thicker banks of very hard micritic limestone (paved limestone from the Niort region) dating from the Laeviuscula and Propinquans zones (units 10-11-12-13). In the Mellois region, these levels are very poorly developed and may still contain ferruginous oolites and phosphate nodules. The base of the Humphriesianum zone remains very fossiliferous (unit 14), particularly the Umbilicum horizon (Humphriesianum subzone). The rock contains large quantities of glauconite. Throughout the area, spongy bioherms developed during the Subblagdeni subzone. The top of the Humphriesianum zone is truncated by a major discontinuity, D8, as described by Gabilly et al. (1985). In the Melusine gully and the Poitiers region, the Lower Bajocian reaches a thickness of around 20 meters, consisting of limestone with numerous pseudo-oncoides with nubeculars and flints levels. An oolitic bed marks the top of the zone at Laeviuscula, topped by calcarenites with crinoids containing very rare ammonites from the Propinquans and Humphriesanum zones (Teloceras cf. subblagdeni in the Vonne valley, Branger 1989). In the Boivre valley, polyp levels crown this series and probably constitute the lateral equivalent of the spongian bioherms developed in the distal domain. Towards the eastern part of the threshold, in the Gartempe valley, the Lower Bajocian ends with a flint-rich bank (silexite of Mourier, 1980, p. 35). Further east, in the Civaux-Chauvigny region, it consists of granular limestone with tube worms, approximately 16 m thick, containing numerous flints and frequent dolomitic passages. D8 – Upper Bajocian A new transgressive episode marks the base of the Upper Bajocian (units 19-20-21). In Niort and Vendée, the Niortense and Garantiana zones, as well as the extreme base of the Parkinsoni zone, are recognized by a decimeter-thick layer of phosphate stromatolite nodules with reworked fossils (Branger, 1989). This bed becomes significantly more carbonated in the Mellois with multi-decimetric limestone banks rich in fossils. Moving towards the proximal platform, the transition from the Lower Bajocian to the Upper Bajocian is less distinct, but the fossiliferous beds of the Garantiana zone and the Acris subzone persist. Above, and throughout the sector, there are about ten meters more of massive beds of gravelly limestone, with abundant sponges and tuberoids (ferruginized intraclasts). These are the spotted limestones of the Saint-Maixent area (Cariou et al. 2006). Flints beds appear from the Saint-Maixent graben onwards. On the Poitou platform, ammonites have also been collected at the base of the sub-stage. This is highlighted in the Poitiers cliffs by one or two burrow beds (units 20-21). Above, over about 20 meters, there are flint-dotted limestones (units 22 to 27). In the Vienne and Gartempe valleys, the facies indicate shallower environments. At a height of 25 to 40 meters, there are granular packstone limestones with isolated colonies of polyps (Lochmaeosmilia) interspersed with crinoid
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