181 9. The Deffend hydrogeological model On the Poitou Threshold, the catch-up/keep-up model recurred several times during the Middle Jurassic. The main discontinuities identified by Gabilly et al. (1985) correspond to phases of low sea level. Figure 4 shows a synthetic diagram of the systems tract during a transgression-regression cycle. A table shows the correspondence with ammonite zones, and porous bodies are represented by dolomitized strata with vuggy levels and discontinuities. At the outcrop scale, the genetic units corresponding to lowstand and highstand deposits repeat this pattern. This model also makes it possible to suggest a sequence of diagenetic events affecting the limestones during deposition. During sea-level falls, dolomitization found favourable conditions due to the evaporation of marine waters trapped within the sediments around isolated shoals, combined with meteoric water input across the platform, promoting the precipitation of calcium and sulphates (Morda et al., 2012). The advance of the dolomitization front within the limestones would then be controlled by the freshwater/saltwater interface. This process could repeat with each cycle and would be particularly intense in the eastern part of the Poitou threshold, where the Montmorillon dolomite is located. Within this framework, the depositional sequences that influenced the porosity of the supra-Toarcian limestone aquifer are as follows: (i) at the end of highstand, when sedimentation has filled the available accommodation space, the platform is subjected to intense currents that scour the limestones and formed a more or less cemented sub-horizontal discontinuities, expressed as a bioturbated hardgrounds (Gabilly, 1962). These discontinuities, sometimes underlined by remnants of palaeosols, are conducive to the formation of highly porous hollow bedding planes; (ii) during the lowstand, for example, the discites horizon (corresponding to the lowstand of sequence Bj1 of Gonnin et al., 1992), per descensum dolomitization occurred (Mourier, 1983). The limestone becomes more porous and forms vacuoles and vuggy levels, probably due to the dissolution of calcite geodes into the laeviuscula limestone; (iii) the transgressive system tract is formed by thin strata, rich in pelagic fauna, but with relatively low porosity; (iv) the highstand system tract is made up of metric strata, often oolitic and rich in crinoid debris, which can form a porous aquifer (grainstone facies). In this schema, the absence of the niortense/subfurcatum zone could be related to a relative uplift of the Poitou threshold. Karstification associated with the D8 (and D8bis) discontinuity could be explained by this depositional gap, accompanied by intense bioturbation that enhanced subsequent dissolution (Figure 4). The emersion event at the end of the Parkinsoni zone (Bomfordi subzone), identified by ostracod-bearing clays in the Vienne valley (Mourier and Almeras, 1986), is associated with the deposition of black clays in submerged areas and karstification marked by red clays observed in boreholes from the HES site (Gaillard, 2026).
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