156 A new concept of karst development based on hydrogeology and geophysics inclination (26°), the true vertical depth of the fossil is calculated at approximately 42 m. This finding supports the identification of the Bajocian–Bathonian boundary at 42 m, previously inferred from OPTV data based on the presence of two strongly bioturbated layers in boreholes M10, M14, and M15 (Fig. 3). The D9 discontinuity, which defines the base of the Bathonian, appears subtly on OPTV images at depths of 45.60 m in M14, 45.10 m in M15 (visible at a bedding joint), and 45.02 m in M17. A flinty limestone unit is consistently observed below the planar erosion surface identified as D9. This unit is characterized by intense bioturbation around a depth of 50 m and the presence of distinctive chert (flint) bands that can be correlated between boreholes using OPTV data. A wavy erosional surface, highlighted by a concentration of bivalve shells, marks the top of the first depositional sequence of the Parkinsoni Biozone. This surface is observed at the following depths: M01 (61.77 m), M10 (62.49 m), M14 (62.35 m), M15 (63.60 m), and M17 (59.12 m) (Fig. 4). A major planar erosional surface, underlain by a clay-rich horizon, defines the top of the bioturbated formation traditionally referred to as the “Assise à terriers” (Fig. 5). This key stratigraphic surface is clearly identified in several boreholes: M01 (64.81 m), M02 (65.48 m), M10 (65.48 m), M14 (65.31 m), M15 (63.60m) and M17 (62.25 m). It is interpreted as corresponding to the regional D8bis discontinuity. A bioclastic horizon, associated with the base of this formation, is recorded at 67.00 m in the C1 core and identified at 66.42 m in M01 and 67.05 m in M10. The base of this unit is marked by a wavy erosional surface overlying a 15 cm thick bed rich in Trichites sections (Mollusca). This surface is well expressed across several HES boreholes: M01 (72.15 m depth), M10 (71.99 m), M14 (71.13 m), M15 (72.00 m), and inferred at 70.63 m in M17. This erosional boundary is interpreted as the D8 discontinuity (Fig. 6). Figure 4 Top of the first “Parkinsoni” sequence.
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