28 A new concept of karst development based on hydrogeology and geophysics Rolin (2001). Granites intruded the Paleozoic sediments during the Cambrian, Ordovician, Devonian-Visean, and Namurian-Westphalian (Carboniferous). The primary granitic intrusion activity occurred between the Devonian and the Westphalian (Moscovian). Calc-alkaline magmas may be the result of the fusion of a mantle layer intruded into the tectonics of the threshold between the Parthenay fault and the Vasles-Availles fault (Rolin et al., 1999). Sedimentary coverage Granites and Paleozoic metamorphic formations make up the bedrock that is covered by Jurassic formations. Triassic sediments are located only at the threshold border (Infra Lias clay in the Aquitanian basin and arkose in the Paris basin). During the Early Jurassic (Sinemurian and Hettangian), shallow marine environments dominated the Poitou Threshold. The sediments consist primarily of shallow marine carbonates. During this period, three sedimentation areas were active: (i) the Atlantic area west of Fontenay-le-Comte, (ii) a Vendean area between Fontenayle-Comte and Thouars, and (iii) the “Pictave” area around Poitiers (Gabilly and Cariou, 1974). The Toarcian is marked by the deposition of dark, organic-rich marls and shales, indicative of deeper, more anoxic conditions associated with the global Toarcian Oceanic Anoxic Event (TOAE). Between the Aalenian and the Callovian, sedimentation transitioned to shallow marine carbonate platforms. These include micritic limestones, bioclastic limestones, oolitic limestones, and occasional reefal buildups. The Oxfordian is represented by limestones in the Pictave area. The sedimentary facies are marly from the Vendean to the Atlantic area. During the middle Jurassic, the sedimentation area was limited by the fault(s) inherited from Paleozoic tectonics (Mourier and Gabilly, 1985). Fossils, particularly ammonites and brachiopods, are critical for establishing relative ages due to their stratigraphic distribution and global correlation. While ammonite dating is accurate towards the basins, the scarcity of pelagic fauna in the middle of the Poitou threshold makes dating a delicate matter. For this reason, Gabilly studied signs of exposure and erosion registered in limestones and the biozone gap (Gabilly, 1962). This evidence is used to make correlations between the Poitou threshold and the Aquitanian basin margin (Gabilly et al., 1985). Gabilly et al. (1978) proposed a cross-section of the Poitou threshold, which is still valid (Fig. 8). The threshold affects sedimentation in several ways. The first is a thinning of the geological layers. The second is a facies variation, with carbonate facies on the threshold and marly facies towards the basins. The anticlines and synclines of the cross sections are associated with the fault system shown in Figure 8. No faults were identified as being related to the Paris Basin. As a result of the new fault pattern shown in Figure 7, the facies distribution of the middle Jurassic deposits could be revised. For example, Figure 9 shows the facies distribution during the upper Bathonian, as mapped by Gabilly et al. (1978). With
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