Abstract

The Pliocene period in the Montpellier region, considered as being emblematic since the middle of the 19th century because of its diverse mammal fauna, has not received much innovative treatment over the last 30 years. In particular, there has been no reinterpretation of geological data in the light of the Messinian-Zanclean double eustasy and related mechanisms. And yet, the increase in the number of urban projects affecting the Pliocene terrains and substratum has allowed for a renewed analysis of these areas in conformity with the results obtained in the nearby valley of the Rhône river. Highlighting the discordance, the regressive Messinian erosion has allowed for the Pliocene fluviatile network of the Lez to emerge. This in turn has made possible the reconstruction of a digitate network that recorded all of the phases of the filling in of these Pliocene rias. This is particularly clear thanks to the study of the Messinian discordance in the hard rocks of the Montpellier fold on the one hand, and on the other, within the Neogene ditches. Since the Messinian, the course of the Mosson and Lez rivers, just as that of the Vidourle river, tracked heavily to the east. This is how the Pliocene Mosson river crossed the Montpellier fold. In fact these modifications of the river courses, made possible by the reuse, until the end of the Pliocene, of the erosion surfaces of the coastal links (the Montpellier fold, the Gardiole), correspond to the accentuation of the deformations of these surfaces. The amplitude and variability of these movements is a result of the altitude of the marine-continental Pliocene transition. This altitude is doubtless often rather approximate, but it differs considerably with respect to the coastal Jurassic ranges (the Montpellier fold to the north, the Gardiole to the south), which frame the Pliocene “Gulf” of Montpellier. The two coastal ranges, calcareous and sub-parallel to the sea surface, have succeeded in channeling the transgression at the point of the saddleback axes, conferring Dalmatian coastal characteristics to the Pliocene ria.

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