The effect of river dynamics induced by the Messinian Salinity Crisis on karst landscape and caves: Example of the Lower Ardèche river (mid Rhône valley)

Abstract

The karstic canyon of Lower Ardèche is located in the Middle Rhône valley, which is directly tributary to the Mediterranean Sea. The Rhône River is emblematic of the Messinian Salinity Crisis (MSC) impact on landscape morphology. Along the edge of the Saint-Remèze Plateau, the Rhône valley displays four benchmark levels generated by the MSC: the Pre-evaporitic abandonment surface (1), the Messinian erosional surface (2), the Marine/non-marine surface of the Pliocene ria (3) and the Pliocene abandonment surface (4). The study of these benchmark levels allows us to reconstruct the evolution of the regional base level over the last 6 Ma. We obtain a curve for base-level evolution that provides a geodynamic reference, which is used to investigate the morphogenesis of the Saint-Remèze karstic plateau. The Ardèche River downcuts the Saint-Remèze Plateau in a deep canyon, from Vallon-Pont-d'Arc to the West, to its confluence with the Rhône to the East. Several abandoned valleys are present along the western edge of the Saint-Remèze Plateau at the inlet of the Ardèche canyon. In these abandoned valleys, the fluvial deposits are related to several periods, from the Pliocene onwards. They provide important insights into the fluvial dynamics: a 160 m-thick aggradation sequence infilled the Ardèche canyon during the Pliocene. This aggrading river caused the first lateral shifting, as an aggradation epigenesis. This first infilling shows that the Ardèche canyon already existed before the Pliocene. Secondly, it has been demonstrated that the Ardèche Canyon is downcut into the Pre-evaporitic surface of the Saint-Remèze Plateau, dated to 5.45 Ma [Martini, J., 2005. Etude des paléokarsts des environs de Saint-Remèze (Ardèche, France): mise en évidence d'une rivière souterraine fossilisée durant la crise de salinité messinienne. Karstologia 45–46, 1–18]. Consequently, the canyon downcutting is entirely due to the MSC, and occurred during a time span of only 100 000 years. Based on these observations, it is possible to elucidate the curve of the regional base-level evolution. Hence, we are able to propose a new interpretation of the geomorphological evolution of the Saint-Remèze karstic plateau and its cave levels for the last 6 Ma. The cave levels consist in underground short-cuts of the surface meanders. They mainly developed during the Pliocene aggradation cycle. The Chauvet Cave, famous for its Palaeolithic paintings, corresponds to one of these underground short-cuts. The aggradation period ends at the end of the Pliocene with long high-level riverbed stability. It favours the development of large low gradient surfaces as pediments. The complete Messinian–Pliocene eustatic cycle is responsible for the downcutting of the Ardèche canyon and its infilling during the Pliocene. Consequently, karst developed according to the base-level oscillations, as low gradient surfaces and as cave levels. For the study of the peri-Mediterranean caves and karst areas, we propose to apply the Lower Ardèche valley evolution model, based on the base-level oscillations during and after the MSC.