Chapter 9 - A reappraisal of large, heterolithic channel fills in the upper Permian Rangal Coal Measures of the Bowen Basin, Queensland, Australia: The case for tidal influence

Abstract

Abstract This chapter documents a reappraisal of an Upper Permian coal-bearing succession (Rangal Coal Measures (RCM) and equivalents) that forms part of the fill of the retroarc foreland Bowen Basin of northeast Australia. Previous research concluded that the unit was the product of entirely continental, alluvial environments, but in this chapter, the case is made for a tidal influence in the middle part of the formation. The RCM have been divided into seven lithofacies, interpreted to record channel, floodbasin, and mire environments of deposition. The middle part of the formation is dominated by heterolithic channel fills with macroform inclined bedding (Inclined Heterolithic Stratification, IHS) that are interpreted to be the product of deposition in moderately sinuous rivers in which construction of point bars was a widespread process. Several lines of evidence collectively argue for a tidal influence on sediment deposition: (1) the stratigraphic context of the central part of the RCM, which appears to represent the transgressive (TST) and early Highstand Systems Tract (HST) of a depositional sequence; (2) the character of IHS sets, in which fine-grained partings extend from the brinkpoint of cross-sets down to their toesets, and in many exposures displays a rhythmicity to the bedding style; (3) the abundance of small-scale sedimentary structures associated with tidal activity, such as flaser and wavy bedding, rhythmic interlamination of sandstone and mudstone, synaeresis cracks, and fluid-mud deposits, as well as a distinctive, low diversity and sporadically distributed trace fossil suite; (4) bipolarity to palaeocurrent distributions; and (5) preservation within the channel fills of fossil fish that elsewhere are associated with marine or coastal deposits. Accordingly, the middle part of the RCM is reinterpreted as the deposits of a southward-draining, tidally influenced fluvial system. Palaeogeographic considerations suggest that this system likely occupied a north–south elongate, southward-opening tidal embayment, or gulf, that extended for 1000 km from central Queensland to central New South Wales before exiting to the Panthalassan Sea. Several of the more enigmatic elements of the stratal architecture evident from mine exposures are difficult to reconcile purely in terms of extant models for point bar formation and evolution. Some of these features may owe their origin to modifications of estuarine geomorphology evident from modern systems that are oversupplied with sediment, but which are not extensively documented in the literature. The details of IHS bedding structure summarized herein can assist in the description of heterolithic channel fill reservoirs in hydrocarbon provinces such as the Athabasca oil sands of western Canada where correlation between wells over short distances (100–300 m) is problematic.