154 A new concept of karst development based on hydrogeology and geophysics grainstone) and provided a preliminary sequence stratigraphy. However, erosional surfaces were poorly sampled and often absent in the cores. To address this shortcoming, borehole optical imagery was analysed for the first time and presented at the “Journées Scientifiques de l’AGAP-Qualité” at the University of Poitiers (Gaillard et al., 2024). OPTV logs offer high-resolution imaging of bedding surfaces and discontinuities, enhancing the identification of sequence boundaries described on the Poitou Threshold (Gabilly, 1978; Gabilly et al., 1985; Gaillard et al., 2024; Branger et al., 2026). Initial OPTV analyses were focused on boreholes with minimal karstification and long open-hole sections (M01, M09, M10, M14, M15, M17). Subsequent analyses targeted boreholes extensively studied through geophysics and hydrogeological testing. Additional data include resistivity and natural gamma-ray logs. Table 1 List of open holes and drilling cores studied. Borehole X_RGF93 Y_RGF93 Z NGF Depth of top OH in m Depth of bottom OH OH length in m C1 501299.97 6609513.918 125.36 23.8 129.7 129.70 C2 501449.316 6609299.466 123.12 / / C3 501359.815 6609493.411 125.65 / / M01 501395.377 6609566.963 124.95 20.4 125.7 105.3 M09 501357.926 6609516.374 125.48 33.0 121.0 88.0 M10 501362.346 6609464.802 125.98 20.0 117.0 97.0 M14 501308.442 6609562.542 124.71 23.4 120.0 96.6 M15(b) 501357.803 6609564.507 124.89 23.5 124.5 101.0 M17 501460.577 6609517.356 124.05 23.0 128.0 105.0 RGF93: French geodesic system in meters; NGF: meter above sea level (masl) in the French elevation system; OH: open hole section. A summary of the results is proposed to properly constrain the HES stratigraphy with reference to the regional geological context. Figure 1 shows the locations of boreholes and core drilling. Lithostratigraphy of the HES The Callovian/Bathonian boundary can be observed in core logs. The Callovian limestone is a chalky, porous micropackstone to packstone containing peloids. On the OPTV log, discontinuity D11 is clearly visible in borehole M01 at a depth of 23.15 m and 23.02 m in M10, appearing as a sharp planar erosion surface (Fig. 2). A comparable surface is identified at 23.17 m depth in the PZ6 located between MP7 and M21.
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