31 1. The Poitou Threshold the proposed schematic fault map, the various areas of sedimentation are superposed onto tectonic blocks. High-energy facies (oolitic limestones) were placed between the faults that delimit the Pictave block. Gravelly limestones that extended to the west were pinched out against the Vasles-Availles fault. Bioclastic limestone with sponge beds was confined to the area between the Vasles-Availles fault and the Parthenay fault. To the west of the Parthenay fault, sedimentation was marly. Poitou threshold definition Gabilly et al. (1978) mapped the facies distribution for the Toarcian, lower Bajocian, and Callovian. Mourier and Gabilly (1985) mapped it in the Vienne and Charente valleys. The pattern of the fault system is similar to Figure 9. The northern axis, which better limits the extension of high-energy facies, corresponds to the gravity anomaly parallel to the Thouars-Mirebeau fault. Sedimentation north of the Vasles-Availles-Limouzine axis is a carbonate platform environment, while to the south it becomes openly marine; this is the Vendean domain (Gabilly and Cariou, 1974; Mourier, 1983; Branger, 1989). The VaslesAvailles axis (roughly equivalent to the Pouzauges-Oradour axis of Gabilly, 1962; Mourier 1983; Branger, 1989) marks the end of the Poitevin Strait and the beginning of the Aquitaine Basin. Defined in this way, the Poitou threshold appears to be a mid-Jurassic shoal, with the northern and southern limits of the platform aligned with the structuring axes inherited from the basement. The shorelines are controlled by these granitic horst sets from the Toarcian to the Bathonian. The platform continues into the Bajocian and Bathonian towards Berry. In the Callovian, however, the inherited higher elevations no longer seem to constrain sedimentation, and the gap in some Callovian ammonite zones extends over a vast plateau (Cariou, 1980). Although some authors have sometimes compared the Poitou threshold to a vast NW-SE anticline with a large radius of curvature (Gabilly, 1978), the geometry of the strata is sub-horizontal and is offset by normal faults. In fact, the geometry of the limestone strata on the Poitou platform conforms to cycles of sea-level variation and the creation of available space for proximal and distal depositional facies (Mourier, 1983; Branger, 1989; Gonnin et al., 1992). Based on structural maps, it is possible to give a purely geological definition to the Poitou Threshold as the region between the Thouars-Mirebeau fault to the north and the Vasles-Champagné-Saint-HilaireAvailles-Limouzine axis to the south (Fig. 10). The advantage of this proposal is that it also represents the platform of the Pictave domain during the Jurassic (Gabilly and Cariou, 1974; Mourier and Gabilly, 1985).
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