Processus de fantômisation dans les calcaires du Jurassique supérieur du Jura tabulaire

Phantomisation processes in the upper Jurassic limestones of the tabular Jura mountains. The works of Yves Quinif and his collaborators, following observations in the quarries of Hainaut (Belgium), yielded the description of a special process of karstification, where, under some circumstances, an in-situ alteration of the carbonates occurs. This leads to underground caves with a peculiar morphology. These phantomisation phenomena were so far described in Wallonie, in Italy, and in the french regions of the Causses and the Charente. The diversity of geological contexts and the widespread reach of these observations open new perspectives for the knowledge of some karsts and their evolution. During the construction of the national highway AI 6, linking Biel to Delle across the folds of Jura mountains, geotechnical investigations prior to the construction of a tunnel in Bure (Canton Jura) in the tabular part of the mountain, permitted to recognize a system of small caves which present phantomisation phenomena and associated passage morphologies. Different factors contributed to this morphogenesis on the Bure site : (i) the geological context of the area, with the sedimentation of an Upper Jurassic unit that contains an alternating sequence of marls and limestones, deposited in a lagoon environment, and (ii) a very long emersion period from the end of the JJurassic perion, and a regional geomorphological context who was only lately affected by tectonic processes. These factors have given the time to start the process, then the géomorphologie evolution helped these karstic forms and underground networks to take place. These observations allow to see karstification processes in the Jura mountains under a new angle, in order to understand the setting of cave networks and their structuration in the tabular zones.