Syn-depositional, low temperature illite formation at the Jurassic-Cretaceous boundary (Purbeckian) in the Jura Mountains (Switzerland and France); K/Ar and delta 18 O evidence
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Abstract K/Ar ages and oxygen isotope compositions were determined on illitic clay fractions of Purbeckian marls from the Swiss and French Jura Mountains in order to examine their syn-depositional origin which had previously been suggested based on sedimentological and mineralogical data. Measured ages range between 122.3+ or -2.3 Ma and 136.5+ or -2.5 Ma, thus being either coeval or slightly younger than the accepted age of sedimentation (Jurassic/Cretaceous boundary). Because total burial depth was less than 1000 m, delta 18 O values around 20 per mil are compatible with a formation of illite by replacement of smectite in an evaporated sea-water. Therefore, in addition to the two common types of illite resulting either from the fragmentation of pre-existing micas or diagenetic formation in deeply-buried sediments, illitic clay minerals formed at low temperature do exist and may be useful for age determination.Keywords:
Illite
Marl
Radiometric dating
Authigenic
K–Ar dating
Abstract A 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 18 O-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.
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Abstract The use of field emission electron microscopy for the study of clay mineral petrography in mudrocks and sandstones is discussed. The methodology including sample preparation is outlined and three examples of the application of the technique are described: the formation of authigenic illite in mudrocks, the multiple generation of authigenic illites in sandstones and the effects of KCl drilling muds on shale fabrics. In the study of authigenic illite formation in Palaeocene mudrocks from the North Sea, the FESEM analyses have demonstrated the formation of illites with increasing burial depth that conventional SEM and XRD analyses had failed to show. The FESEM analyses of the authigenic illites in Carboniferous sandstones from the southern North Sea revealed at least three different habits representing different generation episodes rather than one illite formation event.This has important repercussions with regard to the interpretation of stable isotope and dating data for the illites. Significant petrographic changes in shales after treatment with KCl drilling muds have been observed from FESEM analyses, suggesting reactivity between the shales and the KCl muds.
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The 4th member of Xujiahe Formation in the western Sichuan Basin is composed of typical tight sandstones that contain abundant debris but very scarce feldspar in skeleton grains.Authigenic clay minerals are dominated by illite and kaolinite,wherein authigenic illite can be divided into three principal types: grain-coatings,netted aggregates filled in secondary intragranular dissolved pores and non-netted aggregates filled in intergranular space.The first type transforms from the early-formed smectite through I/S mixed layer,the second type is derived from illitization by K-feldspar during burial metamorphism,and the third type is a product of illitization by K-feldspar and the early-formed kaolinite during deep burial.The formation of authigenic illite has a dual effect on the development of relatively high-quality reservoirs in tight sandstones.On the one hand,it can reduce sandstone permeability and lower the quality of reservoirs,and on the other hand,the grain-coaling illite can preserve the primary porosity by preventing quartz from cementation,while the transformation of smectite to illite and the illitization of K-feldspar and kaolinite during burial diagenesis can accelerate the dissolution of K-feldspar to form secondary pores.Therefore,the formation of authigenic illite generally plays a positive role in the primary porosity preservation and the development of secondary pores of sandstones,and consequently favors the formation of relatively high-quality reservoirs.
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