The Late Triassic Callide Coal Measures, Queensland, Australia: Coal petrology and depositional environment

The Late Triassic (Carnian-Rhaetian) Callide Coal Measures are preserved in a partly fault-bounded basin remnant in east-central Queensland, Australia. The sequence comprises up to 150 m of interbedded clastic sedimentary rocks and at least four major coal seams, including one coal body up to 23 m thick. The sequence was deposited initially in high-gradient alluvial fan systems which gave way through time to sandy, low-sinuosity rivers. The restricted, intermontane and entirely alluvial nature of sediment accumulation is here considered to have influence the petrographic characteristics of Callide coals, and their external geometry.The main coal seam from the Callide Measures shows variation in the predominance of some macerals, indicating successions of environmental changes. The application of transmission electron microscopy (TEM) to the study of the Callide coal has enabled a better understanding of the nature and origin of some of the less understood macerals such as micrinite and vitrinite B. Two forms of vitrinite have been observed, each with a distinct reflectance range. The wood-sourced vitrinite A displays an average reflectance of 0.56%, implying a higher rank than the 0.49% Ro total vitrinite reflectance recorded in previous publications. Vitrinite B and A together represent the most commonly occurring macerals in the Callide coal samples of the present study. The lower-reflecting vitrinite B which forms bands, often several hundred μm in thickness, in TEM shows sub-micron electron transparent laminae of lipid-rich material alternating with a more conventional vitrinite material. The vitrinite B is interpreted to represent accumulations of leaves.The Callide coal has entered the oil window, and oil has been generated from some exinite, cutinite, and resinite, as evident from change in fluorescence and the presence of exsudatinite in cell cavities and cleats. Vitrinite B is seen under the microscope to also be generating oil. Micrinite, the origin of which has been much debated, occurs in selected horizons only, as lenticular bodies suggesting cell filling or filling of spaces between laminations in vitrinite B. TEM shows micrinite to consist predominantly of sub-micron pyrite and possibly other mineral particles adsorbed on humic acids.