La tectonique du bassin de Marseille à l'Oligocène

Abstract

The basin of Marseilles was formed on the site of a tectonic cicatrice resulting from late Eocene tangential movements. During the tangential tectonics, the jurassic and cretaceous layers has been strain torn at the back boundary of the over-thrusting mass of the Etoile mountain. Thus bared, the Triassic was in numerous points directly underlaid by oligocene deposits. The filling of the basin consists of about 1.000 m thick continental sequence, with a detrital and lacustrine unit in the lower part, and coarse clastics in the upper part. The tectogenesis of Marseilles' basin went on all over the oligocene period. Deformations of the bottom and of the edges, underlined by lacunas and ravinements, have alterned with subsident sedimentation episods. The different stages of the tectonic units of the edges are assigned to several periods of the Oligocene. The whole evolution has been synthetised by retrotectonic profiles and paleogeographic sketches. These reconstitutions show that the Marseilles' basin area has been progressively reduced consequently to the up-rising of some of its edges during the middle Oligocene. We also debate over the standing idea according to which the Marseilles' basin is a downdropped structure. It would be necessary to assume a 4.000 m subsidence which isuncompa-tible with its smalless and presence of Triassic and Permian lying shallow directly at the center of the basin and on its edges. The sinking of the bottom under the pressure of very shallow water deposits cannot account for the indisputable subsidence. That one has been initiated by the void corresponding to a missing mesozoic formation. We find again these layers in Etoile overthrust, wich is a tectonic unit displaced on about 15 km to the North in comparison with Arc basin. The tectonic cicatrice has evolved into synclinal depression under the influence of successive stresses. Important faults which occur all along the subsidence don't thus result from a passive sinking of the basin. Marseilles' basin area has been progressively reduced conse¬ quently to the up-rising of some of its edges during the middle Oligocene. We also debate over the standing idea according to which the Marseilles' basin is a downdropped structure. It would be necessary to assume a 4.000 m subsidence which isuncompa-tible with its smalless and presence of Triassic and Permian lying shallow directly at the center of the basin and on its edges. The sinking of the bottom under the pressure of very shallow water deposits cannot account for the indisputable subsidence. That one has been initiated by the void corres¬ ponding to a missing mesozoic formation. We find again these layers in Etoile overthrust, wich is a tectonic unit dis¬ placed on about 15 km to the North in comparison with Arc basin. The tectonic cicatrice has evolved into synclinal depression under the influence of successive stresses. Important faults which occur all along the subsidence don't thus result from a passive sinking of the basin.