Abstract:

A number of paleokarst fillings have been investigated. The most important of them represents an ancient underground river. It appears as a sequence of filled passage segments, which have been de-roofed by karst denudation. These segments are developed at 360 to 380 m above sea level and have been followed for 5.2 km. Three distinct cave fillings were put into evidence: 1) beige micaceous silts and sands which represent exogenic immature alluvials and were dated with rodent bones as Uppermost Miocene; 2) mature red clay and sandstone of local origin, whose age might vary from Uppermost Miocene to Recent; 3) monogenic breccia generated by wall gelifraction during the Pleistocene. The petrographic composition of the immature alluvials is similar to the one of the Ardèche River which flows in the vicinity, but deeply entrenched in a canyon, at the altitude of 60-70 m ASL. Therefore, the paleo-underground river was fed by ponors located on the bank of the Ardèche River, when it was flowing more than 300 m higher than its present bed. The weak variations in elevation of the fossil channel suggest a development within the immediate vicinity of the water table. The biostratigraphic age of the immature alluvials as well as the paleokarstic context suggest that the cave was still active ~5.6 to 5.45 Ma ago. In this timespan falls the drastic dryout of the Mediterranean Sea and the beginning of the incision of the Messinian canyons in this area. In general, this fossil water-table cave is also informative on the morphological evolution of the Ardèche Karst and underline the usefulness of palaeontology in dating speleogenesis.