Discrete clay diagenesis in a very low-permeable sequence constrained by an isotopic (K–Ar and Rb–Sr) study

Abstract

Multimethod analyses of several size fractions of clays were used to reconstruct the diagenetic history of the shallow buried claystones within the Paris Basin. A systematic decrease occurred in K–Ar dates relative to the decrease in size of the clay fractions, signifying higher amounts of newly formed clay material in the finer fractions. We suggest that the authigenic clay minerals occurring in the fine fractions had an Al-montmorillonite composition. By assuming that the isotopic K–Ar and Rb–Sr dates obtained on a bentonite layer in the sedimentary sequence stand for pure, authigenic clay minerals, one may interpret all K–Ar dates as mixtures of one authigenic and two detrital end-members. The results imply that a period of low sea level favoured diagenetic smectite-type clay formation about 10–15 million years after deposition of the sediments. Signatures of limited-scale chemical and isotopic homogenisation mean that the rock volumes affected by the diagenetic modifications had to be quite limited. The study of clay minerals extracted from some stylolites further suggests that any overpressure related to the origin of stylolites had no effect on clay authigenesis.