Clay minerals as records of temperature conditions and duration of thermal anomalies in the Paris Basin, France: a reply to the discussion by Spötl et al.

Abstract

Sp6tl et al. (1996) have re-examined studies published by us on clay materials of sandstones in the Paris Basin. They refute the XRD characterization, K-Ar dating and formation temperature of one of the interstratified illite-smectite phases identified and have provided an alternative explanation of the published results. However, the authors ignored some basic data provided in the publications under discussion, and they have not been able to explain readily why some of the data were discarded. Furthermore, part of their disagreement is unsubstantiated. We maintain that, on the basis of coherency and consistency of all the data obtained, as well as on independent published results, the most satisfactory interpretation for the thermal evolution of the Paris Basin is the one that we suggested in our publications. Sp6tl et al. (1996) have re-examined the studies of Mossmann et al. (1992) and Clauer et al. (1995a) on clay materials of Rhaetian sandstones in the Paris Basin. To them, the interpretations provided in the two studies are "at odds with data on the diagenetic, thermal and hydrological evolution of the basin". They further claim that the "data show no evidence of hydrothermal alteration" and conclude that "the chain of argument is flawed and based on several questionable assumptions". They provide an "alternative way" which they think is compatible with "normal burial diagenesis".Sp6tl et al.'s arguments hinge essentially on two points: (1) the characterization and dating of the I-S (illite-smectite) phase 1, and (2) the reliability of the palaeotemperature of 220-250~ that was estimated on the basis of oxygen isotope geochemistry of the illite-quartz assemblage. In this reply, we refute their argument which we consider to be unsubstantiated and untenable. Our purpose is not to respond to every issue that they raised, but to provide, whenever appropriate, answers to their concerns. In fact, Sp6tl et al.'s discussion raises two other aspects which we will take the opportunity to address in this comment, namely: (1) the reliability of physical methods applied to geological problems, and (2) the way investigators interpret analytical data in the light of the best information available to them. THE CHARACTERIZATION AND DATING OF I-S PHASE 1Sp6tl et al. (1996) clearly disagree that illite formed 190 Ma ago in the Rhaetian sandstones of the Paris Basin during a hydrothermal event at a temperature of possibly 220-250~ To them, the 190-Ma illite is an artefact that corresponds to a resetting of Hercynian detrital micaceous material that was transported to the basin and then progressively buried. We totally reject this view. We have based our interpretation on X-ray diffraction (XRD) determinations of smear slides treated in three different ways to determine the clay mineral compositions and the amounts of smectite layers in the mixed-layer phases based on computer XRD decomposition, and scanning electron microscope (SEM) and transmission electron microscope 9 1996 The Mineralogical Society

Clay mineral diagenesis in sedimentary basins — a key to the prediction of rock properties. Examples from the North Sea Basin

“…The ages obtained from Jurassic reservoir sandstones range from 100-30 Ma, often with a concentration of ages between 40-60 Ma (Thomas, 1986;Liewig et al, 1987;Jourdan et al, 1987;Glasmann et al, 1989a,b). Analyses of Jurassic sandstones from Haltenbanken gave ages from 55-31 Ma, but Ehrenberg & Nadeau (1989) interpreted these ages to be much too old due to contamination of old feldspar and illite and interpreted the illite to have formed in the last few Ma at temperatures close to 140~ Also from other basins, like the Paris Basin, the possible detrital contamination of illite and the validity of K/Ar datings have been debated (SpiStl et aL, 1996;Clauer et al, 1996). If these illite dates represent the time of the main phase of illite growth, this poses several problems.…”