Sequence architecture within a low-accommodation setting: An example from the Permian of the Galilee and Bowen basins, Queensland, Australia

Abstract

The coal-bearing, Upper Permian Betts Creek Beds (BCB) of the northeastern intracratonic Galilee Basin in north-central Queensland, Australia, record sedimentation within an alluvial-coastal plain in a low-accommodation basin-margin setting. Six third-order depositional sequences are preserved within the BCB and are characterized by a sequence architecture that is overall relatively thin (15 m; 49 ft), condensed, and top truncated. Sequences have a lowstand systems tract composed of extensive braided fluvial sheet sandstones, a transgressive systems tract characterized by tidally influenced fluvial sheet sandstones and bioturbated estuarine mudrocks, and a highstand systems tract composed of coastal-plain mudrocks and coals. Sequences can be correlated throughout the entire Galilee Basin and related to the lithostratigraphy established in the adjacent Bowen Basin, a foreland basin to the east. The overall Late Permian fill is considered a second-order sequence. The stacking patterns show an overall retrogradational to progradational architecture suggestive of relatively low accommodation space. Sequences show an extremely laterally continuous geometry (1000 km; 600 mi) and are relatively uniform in thickness. Sequences do not show evidence of onlap, toplap, or erosional truncation, but instead are thin in areas of low accommodation and expand into areas of higher accommodation. That these third-order sequences can be traced across basins with differing tectonic regimes, together with the persistent, sheetlike nature of the sequences, suggests that tectonic effects on the sequence architecture were minimal. The basin fill covers a time interval during which two glacial events occurred in the final stages of the late Paleozoic Ice Age, suggesting that changes in glacioeustasy may have been the dominant controls on the sequence architecture of the Late Permian succession in the Galilee Basin of northeastern Australia.