ABSTRACT

This study attempts to trace the mineralogical and textural changes observed in a pelitic unit comprised of shale, slate and phyllite with respect to diagenesis/low-grade metamorphism, and to determine if there is any correlation between the field terms shale, slate and phyllite and their thermal maturity determined by illite crystallinity and vitrinite reflectance. The unit of study is the Stanley Shale Formation (Mississippian), which crops out in the Ouachita Mountains of west-central Arkansas. Surface grab-bag samples were analysed using thin-sections, sodium bisulfate fusion, and petrographic grain-mount analysis to determine mineralogic and textrual patterns; x-ray diffraction of ethylene-glycolated oriented clay-mounts of the less than two micron fraction to determine the degree of illitization (Weaver's sharpness ratio and Kubler's peak-width index) and monochromatic reflected white light on kerogen concentrates to evaluate thermal maturation.

Results of the study indicate that with increasing diagenesis/low-grade metamorphism (based upon rock type) the percent and grain size of chert increases, the percent of monocrystalline quartz and feldspar decreases and their respective grain sizes increase. Differences between the shale/slate and slate/phyllite groups are subtle and reflect gradational change between "diagenesis" and "low-grade metamorphism". The increase in percent and grain size of chert is most significant between the shale and phyllite groups. Illite crystallinity and vitrinite reflectance data were inconclusive. The Stanley contains too little organic material (especially the slate and phylllite samples) to make reflectance measurements meaningful. Illite crystallinity data, when compared to standard Weaver and Kubler maturation scales, were found to be ambiguous, but when analysed internally showed the expected general increase in maturation from shale to slate to phyllite. Weaver's sharpness ratios did not always correlate with Kubler's peak-width index values and no definite correlation was found between illite crystallinity, thermal maturation data and rock field terms.