157 8. Hydro-stratigraphic study of the Hydrogeological Experimental Site of Poitiers, France The Trichites Limestone Formation is composed of pluricentimetric beds of packstone, each capped by a bioturbated horizon rich in Trichites sections. Based on the C1 core description, the formation begins with grain-supported lamination, producing a distinctive “snake-skin” facies on OPTV imagery (Gaillard et al., 2024). The initial stratum of the Trichites Formation is approximately 30 cm thick and is consistently bounded at the top by a clayey joint visible on all OPTV logs. The Trichites Formation directly overlies the Oolitic limestone Unit corresponding to the upper Laeviuscula Biozone. This oolitic interval is readily identifiable in core material. On OPTV logs, the Oolitic limestone appears as a white level composed of 3 grainstone strata that are about 1 m thick each. The base of this unit is marked by a discontinuous black clay observed at the following depths: M01 (84.35 m), M09 (84.45 m), M10 (85.37 m), M14 (84.97 m), and M17 (81.87 m). The upper sequence boundary corresponds to the contact with a 50 cm-thick bed identified on all OPTV logs. The contact between the 50 cm thick bed and the Trichites Formation is equivalent to the D7bis discontinuity. Figure 5 Clayey joint at the top of the “Assise à terriers” Formation (D8bis discontinuity). Figure 6 Transition between Trichites formation and the “Assise à terriers” formation.
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