Timescales of regional circulation of saline fluids in continental crystalline rock aquifers (Armorican Massif, western France)

Abstract : In recent decades, saline fluids have been sampled worldwide at great depths in continental basements. Although some of them have been attributed to marine transgressions , the mechanisms allowing their circulation are not understood. In this paper, we describe the horizontal and vertical distributions of moderately saline fluids (60 to 1400 mg L −1) sampled at depths ranging from 41 to 200 m in crystalline rock aquifers on the regional scale of the Ar-morican Massif (northwestern France). The horizontal and vertical distributions of high chloride concentrations are in good agreement with both the altitudinal and vertical limits and the succession of the three major transgressions between the Mio-Pliocene and Pleistocene ages. The mean chloride concentration for each transgression area is exponentially related to the time spanned until the present. It defines the potential laws of leaching (displacement) of marine waters by fresh meteoric waters. The results of the Armorican aquifers provide the first observed constraints for the timescales of seawater circulation in the continental crystalline basement and the subsequent leaching by fresh meteoric waters. The general trend of increasing chloride concentration with depth and the time frame for the flushing process provide useful information to develop conceptual models of the paleo-functioning of Armorican aquifers.