182 A new concept of karst development based on hydrogeology and geophysics Conclusion Geophysical (Mari and Porel, 2008, 2009; Delay et al, 2022; Mari, 2026), hydrogeological investigations (Audouin et al., 2008; Bodin et al, 2022; Boulais et al., 2026), and borehole logging conducted at the Poitiers HES site have completely renewed the understanding of the supra-Toarcian aquifer. The first scientific contribution is the demonstration that the productive horizons within the Middle Jurassic limestones (from Aalenian to Bathonian) are sub-horizontal. Fracturing, which was previously invoked as the main factor controlling the spatial distribution of transmissivity and porosity within the limestones, is in fact restricted to the upper part of the aquifer, particularly within the unsaturated zone. Below a certain depth, within the Bajocian limestones at the HES, horizontal structures form distinct superimposed karstic levels. The second contribution is the demonstration that these levels are not randomly distributed. Stratigraphic analysis of borehole sections has shown that discontinuities associated with marine lowstands are well correlated with ravinement of the depositional surface, combined with dolomitization (Gaillard, 2026). The correlation between stratigraphy and the depths of karstic levels is a major scientific contribution. The resulting hydrogeological model explains the formation of syngenetic karstic horizons within the context of a non-rimmed carbonate shelf. These levels were subsequently reactivated throughout geological time. The karst is thus highly polyphasic, evolving from an initial pre-Cenomanian stage to the Quaternary, with the downcutting of fluvial valleys that actually drain the Poitou Threshold. This model explains the presence of Cretaceous sediments within the Bajocian or Callovian limestones. Similarly, the presence of karstic levels beneath present-day riverbeds’ elevation is consistent with the theory of syngenetic karstification at the top of the highstand systems tract. References Alvarez P. (1980). L’Oxfordien, le Cénomanien et le Turonien au NW de Poitiers (Vienne): stratigraphie, sédimentologie, cartographie et hydrogéologie [The Oxfordian, Cenomanian and Turonian to the NW of Poitiers (Vienne): stratigraphy, sedimentology, mapping and hydrogeology]. Ph.D. thesis, University of Poitiers, France. Audouin O., Bodin J., Porel G., Bourbiaux, B. (2008). Flowpath structure in a limestone aquifer: multi-borehole logging investigations at the hydrogeological experimental site of Poitiers, France. Hydrogeology Journal, 16: 939–950. https://doi.org/10.1007/s10040-008-0275-4
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