Permian coals of eastern Australia: geological control of petrographic variation

Abstract

Coal type and its geological controls in the Permian basins of eastern Autralia are described and discussed. The Permian basins comprise the large foreland Sydney and Bowen Basins in the east, the large cratonic Galilee and Cooper Basins in the west, and the small cratonic Blair Athol, Wolfgang and Oaklands Basins. Coal type is described by the following petrographic parameters: vitrinite content, inertinite composition, and microlithotype composition. Vitrinite content measures the degree of preservation of organic matter in the peat-forming stage; inertinite, which comprises organic material oxidized in the peat-forming stage, consists of larger structured macerals and smaller detrital macerals; and the microlithotypes, as indicated here, measure the degree of microscopic mixing of vitrinite and inertinite. Variation of these three local petrographic parameters, particularly inertinite and microlithotype composition, closely follows temporal and spatial patterns of subsidence in the Permian basins. Vitrinite content is, in places, independent of subsidence rate, which may reflect climatic control. Petrographic composition of the Australian Permian coals is attributed primarily to tectonic control. The influence of climate on coal type is postulated in some areas, but is difficult to quantify. On the basis of limited data, petrographic composition of the Permian coals appears to be largely independent of variation in the Permian flora.