Stratigraphic Stacking Patterns through a Protracted Icehouse Interval Disclose Alternating Glacial and Non-Glacial Periods: The Permian of Eastern Australia

Christopher Fielding¹, Kerrie L. Bann², James A. MacEachern³, Tracy Frank¹, and Michael C. Rygel^1
1 University of Nebraska-Lincoln, Lincoln, NE
² University of Alberta, Edmonton, AB
³ Simon Fraser University, Burnaby, BC

Recent research indicates that rather than being a single, continuous interval, the late Paleozoic Gondwanan Ice Age was recorded in eastern Australia as a series of relatively short-lived (c. 1 m.y.) periods, separated by longer intervals of cool but non-glacial conditions. Following the identification of several such glacial intervals in the Permian record, the thick and laterally extensive marine Permian succession of the Bowen, Gunnedah and Sydney Basins in New South Wales and Queensland has been re-evaluated in terms of formative controls. Formations containing glacial indicators also show evidence of abrupt and high-amplitude (50-75 m) shifts in relative sea level, and typically comprise stacks of thin, condensed, incomplete and top-truncated sequences. This pattern is interpreted to record a low-accommodation regime in which frequent, large-scale drops in relative sea-level caused erosional truncation of sequences, and in which clastic sediment supply was relatively low, leading to an abundance of heterolithic facies. Contrastingly, the formations that enclose these glacial intervals, which are typically much thicker and more sand-prone, show facies stacking patterns that indicate only modest shifts in relative sea-level. In some of these intervals the stacking patterns are such as to preclude definition of sequences. This pattern is interpreted to record the greater levels of sediment flux into basins during non-glacial intervals and a regime in which sediment supply and accommodation were often delicately balanced. Synchroneity of major changes in sediment caliber and provenance throughout the region can be explained either by processes related to glacier formation and decay (such as isostatic unloading, or collapse of a proglacial forebulge) or by tectonic processes.