Geodynamic evolution of the peri-Mediterranean karst during the Messinian and the Pliocene: evidence from the Ardèche and Rhône Valley systems canyons, Southern France

During the Messinian–Pliocene eustatic cycle, the Mediterranean Sea was characterized by a short lived (5.95–5.32 Ma) sea-level fall, which attained − 1500 m in some areas. The study of benchmark levels permits the chronology and dynamics of this event to be established. In the Rhône's middle valley, our investigations allow a new interpretation for the genesis of the Ardèche endokarst. A fall in base-level was responsible for both the incision of the so-called Messinian canyons as well as a deep karst development. Karst systems were formed in association with the Messinian canyons of the Ardèche and Rhône Rivers. During the flooding of the Mediterranean Basin (5.32 Ma), these karst systems were filled by water and plugged by sedimentary infilling of the rias. This mechanism pushed groundwater backward through the karst system, which in turn formed diagnostic “chimney-shafts”. These pathways were geometrically connected to the position of the Pliocene benchmark levels. Consequently, the Messinian Salinity Crisis was responsible for two karst responses.The first was concomitant with the crisis itself and corresponds to the formation of a karst system. The second followed the Messinian Salinity Crisis and corresponds to the adaptation of this karst system in Vauclusian karsts by the formation of “chimney-shafts”.