Burial ages, ghost-rocks and karst network structure. Insights from the Vis canyon (Southern France)
Âges d’enfouissement, fantômes de roches et structuration karstique, cas de la vallée de la Vis (Sud de la France)
Résumé
New burial ages of cave deposits, based on the 26Al/10Be cosmogenic isotope ratio technique, from three caves in the Vis Valley, located in the south-east perimeter of the “Grands-Causses” in southern France are challenging the validity of classic karstic evolution models to explain the structure and evolution of the karstic network in the region. Generally speaking, the formation of horizontal galleries at different elevations above the canyon floor is commonly used as time-markers of former regional base level positions. The burial ages obtained for the Bergougnous cave (1.2 ± 0.3 Ma, +117 m above base level -a.b.l.-) and the Scorpions cave 1.2 ± 0.4 Ma, +105 m a.b.l.) are in agreement with the regional river-incision rate (83 ± 35 m.Ma-1) and conform to a classical per descensum evolution of the karst network. However, the emplacement of alluvium deposits within the Rocas cave at 0.94 ± 0.07 Ma is not consistent with a regular per descensum model. The Rocas cave is 317 m a.b.l. but its entrance is located on the plateau about 4 kilometres from the Vis canyon crest whereas, in contrast, the two caves listed above open directly onto the wall of the Vis canyon. To our knowledge, this is the first presentation of quantitative data which supports the observation that karstic horizontal levels are not always directly controlled by the regional base level position. We support our findings with absolute ages, which for the first time, demonstrates the primary importance of the ghost-rock process on the location of horizontal levels. Base level location is then a secondary parameter, mainly involved in a triggering event for ghost-rock erosion
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