The Messinian Var canyon (Provence, southern France) — Paleogeographic implications

Abstract

From north to south, the lower course of the Var River cuts through three tectonic units: (1) the Nice arc (“arc de Nice”), (2) the oriental part of the Castellane arc (“arc de Castellane”), and (3) the autochthonous Provençal substratum. The Nice and Castellane arcs are sub-Alpine overthrusts, emplaced near the end of the Miocene and reactivated in Plio-Quaternary times. During the Pliocene, the downstream section of the river was invaded by the sea and transformed into a ria. The Pliocene sedimentary succession (Tabianian in its oldest levels) fossilizes a deeply incised erosional relief. In order to put together an orderly geographic reconstitution of this pre-Pliocene topography, one has to exclude from the study area the two sub-Alpine units, which were affected by neotectonics, and to limit the observations to the Provençal substratum. Some modest epeirogenic uplift of the substratum and its incision by erosion provide good outcrops of its sedimentary series. The Provençal substratum consists of Mesozoic and Cenozoic sediments, the youngest (Vence succession or “série de Vence”) ranging in age from Burdigalian to Tortonian. Chronostratigraphic constraints given by the underlying and overlying sediments respectively, indicate that the erosion surface is younger than Tortonian and older than Tabianian, and can, therefore, be dated as Messinian. Geological observations at the surface and geophysical investigations in the subsurface indicate that at some 40 km from the present abyssal plain of the western Mediterranean, the thalweg of the Messinian paleovalley cuts more than 500 m deep into the Provençal substratum. At 20 km from the abyssal plain, the same thalweg lies some 700 m beneath sea level, or some 1000 m below the former Miocene sea level. Cores and dredges obtained from the continental slope south of the Var Valley suggest a progressive connection of the Pliocene ria with the abyssal plain. This gigantic incision now filled with Pliocene sediments and located along the continental shelf and the emerged land, represents a remarkable example of a Messinian canyon partially exhumed by the Plio-Quaternary epeirogenic movements. This canyon has been eroded subaerially by the paleo-Var River in a setting fairly similar to the modern margin. The subaerial erosion is a product of the endoreic regression that followed the isolation of the Mediterranean during the latest Miocene. The base level coincided with the level of the abyssal plain where evaporites were being deposited. The presence of Messinian canyons, at present more or less filled by younger sediments, along the continental margins of the Mediterranean basin (the case of the Var River is just one example among others) leads to an independent confirmation of the deep-basin desiccation hypothesis for the genesis of Messinian evaporites.