Résumé
The Lodève-Graissessac Basin is a ca. 150-km2 Stephano-Permian basin, located 60 kmnorthwest from Montpellier, in southern France. This basin was particularly studied as it hostsabout 20 000 tons of U, which were exploited from 1956 to 1997. In the framework of theNEEDS-URALOD project, a multidisciplinary study has been initiated in 2022, in order to revisitthe uraniferous mineral system. In this context, the geometry of the basin as well as itsevolution with time are key factors on the mineralization. The Basin is classically consideredas a south dipping half graben, covered unconformably by a thick horizontal Mesozoic cover.Unfortunately, if the shallow NW part of the basin is relatively well constrained by boreholedata, the southern part is not so well known in terms of depth and relationships with boundingfaults. Therefore, we have performed a new gravimetric survey using Scintrex CG5 and CG6portable relative gravimeters, complementing previous data from the Bureau GravimétriqueInternational (BGI; 435 datapoints). The 229 new datapoints have been acquired along threeNS profiles and one EW profile, in order to have a continuous set of data (ca. 3 datapoints/km).All available data (gravimetric, structural, topographic) have been integrated in a GIS project.Bouguer anomaly and residual anomaly maps reveal that the Basin has a relatively lownegative gravimetric signature, bound to the NW and the SE by two strong negative signaturesof the Mendic granite and Clermont l’Hérault Oligo-Miocene basin respectively. From thesedata, 2D forward gravimetric modelling of the Basin and surrounding rocks have beenperformed using GM-SYS modelling software along the three NS and the WE cross-sections.These models were constrained by the outcrop and borehole data (several of them reachingthe base of the basin) and also by newly acquired density measurements performed on themain lithologies of the basin. The three NS sections suggest that the depth profile of the basinis more symmetric from N to S and less triangular than proposed in the literature. Furthermore,a variation in the maximum depth of the basin is documented along the WE section with adeepening of the basin from about 1000 m to the W to about 2000 m to the E. Globally, theprofiles also display significant, kilometric-scale variations of the gravity that have still to beexplained (basalt, topography, faults). The results of these models, as well as the othergeological data will be integrated in the gocad 3D modelling software, in order to model moreaccurately the 3D geometry of the basin. In fine, the obtained geometries will be used to modelcoupled thermoconvective circulations, to investigate which type of circulation at whichtemperatures may be expected in the basin at the time of deposition and during the Mesozoic.