155 8. Hydro-stratigraphic study of the Hydrogeological Experimental Site of Poitiers, France The Middle Bathonian is characterized by bioturbated limestone with basal chert layers. The formation thickness exceeds 2.20 m in M01 and 3.30 m in M10. At the Poitiers cliff outcrop, a comparable unit was described as gravelly and oolitic limestone containing Procerites sp. (Gabilly, 1978, p. 150). The Bathonian flinty limestone corresponds to a micrograinstone facies, previously described as sub-oolitic limestone by Gabilly (1978). Two distinct chert bands are observed at the top of this formation, clearly identifiable in boreholes M15 and M10. This double chert band is characteristic of the “Assise à silextite” (flint band) described by Mourier (1986) and Gabilly (1978). A significant change in sedimentation occurs above the 42 m depth, where very fine-grained limestones overlie a bioturbated interval between 42 and 45 m depth (noted in M10, M14, and M15). This lithological transition is interpreted as marking the Bajocian–Bathonian boundary (Fig. 3). A Ctenostreon fossil was recovered from the inclined borehole C4 at a depth of 66.25 m. Considering the borehole’s Figure 2 Bathonian / Callovian transition with D11 discontinuity. Figure 3 Bajocian – Bathonian transition (D9 discontinuity).
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