176 A new concept of karst development based on hydrogeology and geophysics after the site name. In this model, fractures near the surface play a role in distributing groundwater recharge, while water in the saturated zone flows predominantly along horizontal karst horizons. For example, discontinuities D6 and D7 of Gabilly et al. (1985) correspond to two main karstic horizons. Each horizon acts as a drain for water percolating through the overlying limestone sequence. Some of these karst levels are not intersected by rivers (infra-fluvial karst horizons), and the relationship between such levels and modern river systems remains to be clarified. Karst dating The age of the Poitou Threshold karst has also been the subject of divergent theories. Gabilly et al. (1978, p. 19) correlated the three levels of caves around Poitiers with phases of valley downcutting of the Clain and Vienne rivers during the Quaternary, particularly during the Middle Pleistocene to the Riis glacial stage, the Mindel, and the Würm glacial stages. However, the infilling of some cavities by black clays and glauconitic sands dated from Cenomanian had long been recognized within the Middle Jurassic limestones (Cariou and Mathieu, 1959; Mathieu, 1960; Alvarez, 1980; Legendre, 1984). On the HES, palynological analyses of clays trapped within karstic conduits below the Parkinsoni biozone (Upper Bajocian) yielded ages ranging from the Cenomanian to the Santonian (Bassil, 2014). Recently, Valentin et al. (2021) discovered abundant and diverse fossil assemblages ranging from the Albian to the Cenomanian in cross-stratified deposits infilling a Bajocian paleokarst near Persac, on the east of the Poitou Threshold. However, Cenomanian formations do not outcrop on the Poitou Threshold itself. These examples demonstrate that karst horizons are older than the Cenomanian transgression (Tab. 1). These karst systems trapped marine sediments of Cretaceous age before their complete erosion on the Poitou Threshold (Fig. 3). Table 1 Location of Cretaceous sediment trapped in karst on the Poitou Threshold. Location X Longitude Y Latitude Reference Chardonchamp 0.3488° 46.6269° Alvarez, 1980 (p. 93) Persac 0.6985° 46.3577° Valentin, 2021 Vouneuil-sous-Biard 0.261° 46.6035° Mathieu, 1960 Poitiers SEH 0.4062° 46.5522° Bassil, 2014 Lavoux 0.5248° 46.5964° Cariou, Mathieu, 1959 Luzay -0.1965° 46.9205° Legendre, 1984 (p. 50) Moulins, Smarves 0.3482° 46.5314° Mathieu, 1960
RkJQdWJsaXNoZXIy NjA3NzQ=