Évolution paléogéographique et karstogenèse du bassin d'Issirac (Gard, France) du Crétacé supérieur au Miocène inférieur

Abstract : Since the emergence of the carbonated platform of the southeastern basin of France during the Albian-Cénomanian, a long process of complex polyphased karstification is being installed under joint influence of tectonics, eustatism and climate. The endorheic basin of Issirac includes terrains from Cretaceous until Eocene age. Connected to the Alès groove, it has been undergoing eustatic variations combined with flows coming at the same time from the Cevennes and, apparently, from the installing endokarst in the regional carbonated platform englobing the plateau of Méjannes-le-Clap, located in the north of the Gard department, and the Cèze-Ardèche interfluvium. These observations underline a karstogenesis going back to late Cretaceous and early Paleogene times, related to the installation of a basin in conjunction with a planation surface in and on which redesigned sediments have been deposed of which, at present time, only relics remain. Nevertheless its poor visibility at the level of Paleocene and Eocene terrains of the Issirac basin, the karstogenesis permits to observe in the underlying limestone levels of the late Barrémian and Aptian (Bédoulian), indications of excavation whose infillings show strong proportions of alterites containing minerals of volcanic or magmatic origin coming from the Central Massif. The development of karstic aquiphers is orienting hypogeous drains towards successive base levels installing into the endorheic basin during the same periods, before their accomodations during Oligocene and Miocene times. This dynamic is modeling new endokarstic shaping of various gradations. The most recent geological in situ observations are indicating a progressive inversion of hypogeous drains changing from sedimentary base level aggradations formed by the endorheic basin of Issirac towards those of successive low levels related to the incision of Cèze and Ardèche rivers during Cenozoïc eras.
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Joël Jolivet, Hervé Chapuis, Jordan Ré-Bahuaud, Didier Graillot, Bernard Guy, et al.. Évolution paléogéographique et karstogenèse du bassin d'Issirac (Gard, France) du Crétacé supérieur au Miocène inférieur. 2017. ⟨hal-01612184⟩

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