Cold-climate, fluvial to paralic coal-forming environments in the Permian Collinsville Coal Measures, Bowen Basin, Australia

The Middle Permian Collinsville Coal Measures of the northern Bowen Basin illustrate a range of cold to cold-temperate, coal-forming environments. Cold climate is indicated by Glossopteris flora in the coal measures, and by restricted marine fauna dominated by brachiopods and bryozoa in correlative marine sequences of the Back Creek Group which contains also abundant lonestones (dropstones). Sedimentation was characterised by an overall transgression, interrupted by local fluvial and coastal progradation in a shallow, epicontinental sea during a relatively quiescent tectonic period.Six sedimentary environments are represented: fluvial, fluvio-paralic, barrier-strandplain, back-barrier, tidal flat and open marine. The basal coal formed from peat of swamps of abandoned areas of gravelly braided streams, and is massive, dull, and with high ash (20%), low sulphur (1%) contents. Overlying coals developed from peats formed in fluvio-paralic and paralic environments, and thicker seams are generally brighter, with low to moderate ash (8–17%) and moderate to high total sulphur (1–6%) contents. Seams associated with fluvial influence show splits and high ash yield, while seams associated with coastal deposits show high sulphur levels (up to 21%).In contrast to reported models of coal-forming environments, no clearly defined deltaic or inter-distributary bay-fill sequences were identified in the area studied. Rather, vast freshwater wetlands backed low-gradient, progradational coasts locally having bars and barriers. The barriers were not prerequisites for substantial peat accumulation, although may have locally assisted peatland development by raising the profile of coastal equilibrium.