TRACING THE ORIGIN OF KARSTIC WATER AT THE MARGIN OF CRYSTALLINE BASEMENT BY STRONTIUM ISOTOPES - AN EXAMPLE FROM THE CAUSSES-LOZERIENS (FRANCE)

The ''Grands Causses'' Mesozoic sedimentary cover unconformably overly ing the Variscan basement is a favored zone for tracing the origin of surface and groundwaters. A strong contrast in the Rb-Sr system differ entiates the silicate basement from the dominantly carbonate cover. Th is contrast, best expressed in the Sr-87/Sr-86 ratio, also characteriz es the waters flowing along and inside these formations and allows one to trace their origin and to identify and quantify mixing phenomena a s well as evaluate the influence of lithologic, geometric and hydrodyn amic parameters of the aquifers.