Petrographic analyses of over one hundred Australian coal seams of Permian and Triassic age are used to determine within seam and between seam variations and the conditions prevailing during seam formation. Tests on groups of coals show that a change in coal type is dependent on the type of coal immediately preceding (Markovian properties). From this dependence most probable sequences of coal variations can be derived. For individual seams major changes in coal type are represented by characteristic plies, from which standard profiles of each seam are derived. Type seam sequences are derived from the standard profiles for each coal measure group, when more than half of the profiles are similar. Marked similarities exist between the type seam sequences. The distribution of the types of petrographic profiles of all seams is found unlikely to be due to chance. The sequences found from the Markovian properties, the type seam quences and the distribution of profiles are few and similar. All these results indicate that the seams are deposited under ordered conditions and that the major control is an internal one, most probably a response of plant growth and decay to the changing conditions associated with the increase of peat thickness. Comparison with Carboniferous coals from England and Canada shows that similar petrographic profiles are developed in them.



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