Clay minerals as records of temperature conditions and duration of thermal anomalies in the Paris Basin, France: discussion

Abstract

AbstractA highly illitic phase containing <5–10% interstratified smectite was described by Mossmann et al. (1992) and, more recently, by Clauer et al. (1995) from Upper Triassic sandstones of the Paris Basin (France). This phase is present in all size-fractions of all samples and was interpreted as authigenic. The K/Ar model ages of this illite range from 189–200 Ma and, combined with oxygen isotope data, were cited as evidence of high-temperature (220–250°C hydrothermal precipitation from highly 18O-enriched fluids at burial depths of only 500 m.We suggest that this type of illite is more likely to be diagenetically altered detrital illitic material and unlike authigenic illite-smectite which is also present in many of their samples. This reassessment of their petrographic observations leads to a much more realistic diagenetic interpretation consistent with previous studies of the basin and avoids the need to invoke basin-wide hydrothermal activity during shallow burial.