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

A Armandine Les Landes,V Vergnaud-Ayraud,P Davy,L Aquilina,C Le Carlier

doi:10.5194/hess-19-1413-2015

Abstract

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 Armorican 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.

Highlights

In recent decades, saline fluids have been sampled at great depths (0.5–5 km) in continental basements (Bucher and Stober, 2010; Frape et al, 2003)
Salinities ranging from 60 to 1400 mg L−1 were recovered at depths ranging from 41 to 200 m, except for the Cinergy drilling project, where water was collected at a depth below 450 m in a fractured schist, with a chloride concentration of 1240 mg L−1
Gests that the saline fluids in the Armorican basement are of marine origin and correspond to paleoseawater diluted by meteoric waters (Bottomley et al, 1994; Casanova et al, 2001; Frape et al, 1984; Fritz, 1997; Gascoyne and Kamineni, 1994; Nordstrom et al, 1989)

Summary

Introduction

Saline fluids have been sampled at great depths (0.5–5 km) in continental basements (Bucher and Stober, 2010; Frape et al, 2003). Two alternative origins related either to glacial recharge of brines due to cryogenic mechanisms (Starinsky and Katz, 2003) or to anthropogenic sources (Kelly et al, 2008; Mullaney et al, 2009; Panno et al, 2006; Perera et al, 2013) have been proposed. These three hypotheses have led to extremely different estimates of the saline fluid residence time (Devonian or Pleistocene for the Canadian brines, for example) and of the paleohydrogeology of the continental aquifers.

Results

Discussion

Conclusion