168 A new concept of karst development based on hydrogeology and geophysics Conclusion Data acquired from various geophysical methods show a high degree of consistency and provide a coherent framework for the spatial distribution of karstic horizons identified at the Hydrogeological Experimental Site (HES) of Poitiers (France). Importantly, fracture density alone does not account for the localization of karst features. All authors concur in describing sub-horizontal karstic levels, highlighting the predominant control of stratigraphy over structural factors. The main sedimentological controls on karst development are stratigraphic discontinuities, as described by Gabilly et al. (1985), and porous formations composed of oolitic grainstone limestones. A synthesis of borehole optical logs (OPTV) enables the identification of distinct limestone units forming the Poitou Threshold. Through this correlation, the main karstic horizons within the supra-Toarcian aquifer are interpreted as follows: The karst horizon at a depth of ~50 m corresponds to a bioturbated interval of the Parkinsoni Biozone. These beds were previously described by Benvel (1978, p. 35). The karstic level between depths of 72 and 75 m, identified through crossflow logging, is associated with the D8 discontinuity. The most prominent karstic horizon occurs between depths of 82 and 85 m. It lies within the Oolitic Formation. Beneath the D7bis discontinuity, the upper dolomitized zone is highly permeable. The karstic zone observed between depths of 97 and 100 m is correlated with the D7 discontinuity. The D7 discontinuity is the upper limit of the middle-dolomitized zone contains open geodes. The 110-115 m depth horizon is marked by D6 discontinuity and the contact with the Marl limestone lamination formation of the Opalinum zone. The Lower dolomitized zone began just below the D6 discontinuity. The three dolomitized zones seem to appear under the discontinuities. The Upper one is probably related to the black clay layer, which supports pyrite and organic matter. The middle and lower zones contain centimetric microcavernes. These voids are probably the remains of fully dissolved calcite geodes. Additional isolated cavities have been observed outside of these main discontinuities; however, they appear discontinuous and spatially limited. In the unsaturated zone, some cavities may develop along fractures in the chalky Callovian limestone, but their hydraulic connection to the main karstic horizons remains unproven. The 35 m depth horizon seems to be correlated with a bioturbated interval in the Bathonian limestone.
RkJQdWJsaXNoZXIy NjA3NzQ=