SMECTITE-TO-ILLITE CONVERSION

-The <0.5-tzm fraction of 120 samples from a lithologically complex Permian sedimentary sequence, underlying dolerite intrusive sheets, has been characterized by means of X-ray diffraction to establish I/S compositions as a function of temperature, lithology and time duration. Illitization has been active over the entire 210 m depth range and the clay data reflect both the local pattern of contact metamorphism and the more regional trend of heat flow during burial. A continuum exists in the illite proportions of the illite/smectite interstratifications with increasing distance from the intrusive sheet ranging from R = 3 with less than 5% smectite via R = 2 and R = 1 to R = 0 with up to 70% smectite. In the mixed-lithology section, individual component layers in the I/S within similar distance levels, but between contrasting lithologies, appear to vary only within a very restricted compositional range. In the massive mudstone strata, however, more silty parts contain I/S of a higher degree of ordering and lower expandability. Calcite contents are reflected in a higher rate of chlorite formation, but not in the I/S composition. A satisfactory inverse correlation was found between percent smectite in I/S and vitrinite reflectance in the lithologically complex section. R = 1 interstratifications are associated with a maximum vitrinite reflectance of 1.07-1.29 and R > 1 phases with 1.93-2.7, indicating that time duration is not a controlling factor in the illitization process in this facies. R = 0 interstratifications are present in a massive mudstone/siltstone sequence situated furthest from the igneous intrusives, and display vitrinite reflectance values of 1.42-1.52. No satisfactory explanations for this finding can be offered. Key Words-Burial diagenesis, Contact metamorphism, Lithology, Paleotemperature, Vitrinite reflectance. I N T R O D U C T I O N The conversion of smectite to illite is a major clay mineral reaction that occurs in a range of low-temperature geologic environments and is governed by several chemical and physical parameters (Ramseyer and Boles, 1986; Whitney, 1990). Temperature is considered the most important control (Hower et aL, 1976), and compositional trends in illite/smectite interstratifications (henceforth referred to as I/S) have been used as a geothermometer in several sedimentary basins (Heroux et al., 1979; Hoffman and Hower, 1979; McDowell and Elders, 1980; Inoue and Utada, 1983; among many others). Calculated thermal gradients were constructed by Hoffman and Hower (1979) using isograds based on temperatures of the disappearance of smectite (70~ the occurrence of R = 1 I/S (100~ and the first occurrence o f R = 3 I/S (180~ '~ While reports on I/S compositions in burial environments are common, information on clay mineralogical trends in the vicinity of intrusive bodies are scarce (Nadeau and Reynolds, 1981; Smart and Clayton, 1985; Aaron and Lee, 1986). The major difference between burial diagenetic and contact metamorphic processes is the t ime duration and contrasting information is available as to its influence on the illitization reaction. According to Smart and Clayton (1985), for example, the progressive illitization of I/S occurs too slowly to reflect the thermal effects of a minor, 50 m-thick igneous intrusive, except close to the contact. Copyright 9 1992, The Clay Minerals Society Srodofl (1979) reported another situation in which I/S clays failed to reflect a major synsedimentary overthrust. Bruce (1984), in contrast, concluded that geologic age is not important to the reaction, and Bouchet et al. (1988) reported an increase in the illite proportions in the vicinity of even very minor veins and suggested only a different transformation mechanism. Rock type, with its related porosity/permeability, also plays a role in the composit ion of I/S. Howard (1981), Smart and Clayton (1985), Boles and Franks (1979) and Eberly and Crossey (1989) all observed that the I/S clays tended to be much more expansible in sandstones than in adjacent shales. These data contrast with those o f Ramseyer and Boles (1986) who found that I/S in shales tended to show a slightly higher content ofsmect i te layers than I/S in closely-spaced sandstones. For paleotemperature evaluations only homogeneous mudstones are therefore assumed to give consistent results (Smart and Clayton, 1985). Smectite diagenesis occurs simultaneously in t ime and space with the maturat ion of organic matter. Vitrinite reflectance is now considered one o f the best parameters for defining the thermal maturity of organic matter, and certain critical I/S composit ional ranges can be correlated with levels of organic maturity (Heroux et al., 1979). The opinion is generally expressed, however, that clay minerals react much less readily to short-lived temperature increases than does organic matter (Srodofi, 1979; Aoyagi and Asakawa, 1984).