ABSTRACT:Upper Triassic sandy horizons in the Paris Basin were sampled at depths ranging from outcrop in the northeast to 2,700 m in the centre of the basin. The smallest clay sub-fractions (<0.2 Ixm) from the deepest central samples consist mainly of illite and chlorite whose K-Ar age is 190 Ma. These minerals formed at a relatively high temperature of 220-250~ as determined from the oxygen isotope fractionation between authigenic illite and associated quartz overgrowths, but at a burial depth of only 500 m. In a nearby drillhole that crosscuts a fault zone reactivated during the 190 Ma event, illite is less well-crystallized and has a higher 6~80 value suggesting different physical and chemical conditions of formation during the same hydrothermal episode. Two other generations of illite-smectite mixed-layer clays formed in the same Triassic horizon: one at 150 Ma and the other at ~ 80 Ma. These younger clays have higher 8180 values and thus may have formed at somewhat lower temperatures. The 8180 values of the fluids from which the different illite-smectite mixed-layer minerals form range from +9 to +13.5 per mil (SMOW).An Ar diffusion code was used to estimate, on the basis of the Ar loss of the clay-type material, the duration of these events. The results suggest that the duration of the hydrothermal events at 190 Ma and 150 Ma were rather short, <1 Ma, whereas the youngest event was protracted over a much longer period of ~37 Ma. Comparison between K-At ages of the different mixed-layer minerals and sedimentation rates of the sediments since the Palaeozoic shows significant accelerations of the rates at ~190-200 Ma and 150 Ma, and a less important one at ~80 Ma. These observations provide additional evidence that the first two events were promoted by basementrelated tectono-thermal activities. The third event is considered to be of diagenetic origin.The prime factors affecting burial and/or episodic thermal diagenesis of sedimentary rocks and formational fluids can be classified into those characterizing the sediments and fluids such as mineralogy, petrography, petrophysics and fluid chemistry, and those imposed on the sediments such as pressure, temperature and duration of the events. Most of these parameters can be measured and/or estimated, but making a precise determination of the timing (epoch and duration) and of the maximum temperature reached is very difficult.

Abstract--Progressively buried sandstones and shales of the Mahakam Delta Basin in Indonesia were studied. Mineralogical, morphological and K-Ar isotopic data were obtained for clay, mica and feldspar minerals. The data indicate that K necessary for the illitization of illite/smectite mixed-layer minerals was supplied mainly from K-feldspar alteration within the sandstones and from mica within the shales. Most of the K-feldspar alteration for both the shale and sandstone samples were observed outside the main zone of illitization, which is restricted to the upper 2000 m of sediment. The feldspar grains were altered below this depth for both lithologies. Therefore, illitization requires an open sedimentary system. This is in contrast to the illitization model for deeply buried shales of the Gulf Coast. That system is commonly assumed to be a closed K system.

Chemical and isotopic data on formation waters of oil‐fields from two parallel anticline axes of the Mahakam Delta Basin provide information about the present‐day mass transfers in the sedimentary sequence of this basin. Depletions in Ca, Sr and K, enrichment in Rb, and the Sr/Ca ratios in the waters are related to illitization of smectite and precipitation of carbonate minerals, as well as dissolution of K‐feldspar and precipitation of albite. These short‐lasting processes seem to have been more pronounced in the deeper segments of the proximal anticline. The Sr, oxygen and hydrogen isotopic data emphasize occurrence of lateral migrations of the formation waters within permeable units from Borneo Island to the proximal anticline under continental influence, and further to the distal anticline under marine influence. Description of a combined process including illitization of smectite, precipitation of carbonate minerals, dissolution of K‐feldspar and precipitation of albitic feldspar, during lateral migrations of the formation waters was only possible because of the young age and the restricted volume of the basin. In addition, the chemical signatures in the formation waters were not obscured by water‐rock interactions during long‐distance migrations that occur systematically in large sedimentary basins.

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

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