177 9. The Deffend hydrogeological model Thus, between the Cenomanian transgression and the reactivation of karst associated with Quaternary valley downcutting, several million years of karst evolution remain to be reconstructed. The absence of Albian and even Purbeckian sediments on the Poitou Threshold led some authors to conclude that the platform remained fully emergent during the Lower Cretaceous (Gabilly et al., 1978, p. 17; Alvarez, 1980, p. 86; Mourier et al., 1986). An initial phase of karstification is thus theoretically conceivable. This long-lasting subaerial exposure would therefore correspond to an ancient karstification episode, at least partially predating the Cenomanian. The absence of Lower Cretaceous sediments implies that the Poitou threshold uplifted by at least 100 m, simply assuming that the sea has risen in accordance with the Hardenbol sea level chart (1998). This tectonic phase could correspond to the beginning of the Atlantic north opening during the Tithonian stage (Dercourt et al., 1993). The Cenomanian transgression subsequently deposited black clays, followed by glauconitic green sands over part of the Poitou Threshold. These formations, now largely eroded, extended southwards as far as the area around Poitiers (Mathieu, 1960) within a context of high stand sea level (Hardenbol et al., 1998). The discovery by Valentin et al. (2021) confirms that this marine transgression covered the entire Poitou Threshold and filled the karst from the surface (Fig. 3). The red clays overlying the Bajocian and Bathonian limestones were interpreted as the result of weathering of the limestones from the late upper Cretaceous to Figure 3 Location of evidence of ante-Cenomanian karsts.
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