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Large, Heterolithic Channel Fills of the Upper Permian Rangal Coal Measures, Queensland, Australia: Well-Exposed Analogues for the McMurray Formation


The Upper Permian Rangal Coal Measures and equivalents formed during a phase of declining continental arc volcanism and active thrust loading in the complex retroarc foreland Bowen Basin of eastern Queensland, Australia. The unit, around 100 m thick in total, comprises sandstone bodies, thick heterolithic sandstone-siltstone intervals with Inclined Heterolithic Stratification (IHS), mudrocks with thin-bedded sandstones, and coal bodies up to 8 m thick. A number of large, opencut mines have provided extensive though temporary exposures of the Rangal Coal Measures, allowing an evaluation of their sedimentology and stratigraphy. In an earlier study, Fielding (1993, Sedimentary Geology, 85, 475–497) carried out a facies analysis and concluded that the depositional environment was an extensive, low-lying alluvial plain crossed by rivers that alternated between two distinct fluvial styles; 1. Moderately sinuous (< 1.5) streams that formed highly complex, heterolithic channel fills, and 2. More mobile, perhaps braided rivers that formed sheet-like sandstone bodies. In this study, a re-evaluation of the dataset is presented, and the conclusion is drawn that significant portions of the Rangal Coal Measures were likely formed in tidally-influenced rivers (estuaries) that drained the north-south Bowen Basin axially during the latest Permian. The principal lines of evidence in support of this reinterpretation are 1. abundance and diversity of IHS-filled channel forms, and abrupt lateral facies variations that indicate significant areal partitioning of sand in the formative rivers, 2. presence of abundant small-scale sedimentary structures commonly associated with tidal activity (flaser bedding, mud drapes, etc.), 3. presence of a low-diversity, sporadically distributed trace fossil suite, 4. presence of fish fossils of types that have been previously regarded as marine, and 5. bimodal to bipolar palaeocurrent distributions. Given this reinterpretation, the Rangal Coal Measures can serve as a well-exposed analogue for the internal stratal architecture and reservoir heterogeneity that is known to characterize the Cretaceous McMurray formation in the Athabasca oil sands province.