Influence of local fluid flow on properties of low permeability Cretaceous siltstones (South-Eastern France): Implications for a nuclear waste deep repository

A difference of Pb-isotope composition between diagenetic (formed in the matrix during early and burial diagenesis) and epigenetic (precipitated in the gouge during late-stage fluid flow) pyrite is thought to reflect the Pb-isotope evolution over time in the siltstones until the formation of pyrite in the gouge. Assuming that the diagenetic pyrite formed during very early diagenesis in the sequence (c. 97-98 Ma based on glauconite dating [Rousset, D., Leclerc, S., Clauer, N., Lancelot, J., Cathelineau, M., Aranyossy, J.F., 2004. Age and origin of Albian glauconites and associated clay minerals inferred from a detailed geochemical analysis. Journal of Sedimentary Research 74, 631-642.]), a modelled-evolution of the Pb-isotope composition leads to an estimate of the time elapsed between diagenesis and fluid flow of some 60 Ma. Consequently, fluid circulation which precipitated pyrite could have occurred 39 ± 7 Ma ago, during the Pyrenean thrusting.