Sedimentology and stratigraphic architecture of the Late Permian Betts Creek Beds, Queensland, Australia

Abstract

The Late Permian Betts Creek Beds form a succession of coal-bearing alluvial-coastal plain sediments in a basin marginal setting within the northeastern Galilee Basin, Queensland, Australia. The unit is ∼ 50–60 m in thickness at Porcupine Creek National Park where outcrop is laterally continuous for several kilometers. Eight facies have been identified within the formation and can be grouped into 2 facies associations: (A) channel deposits and (B) floodbasin deposits. The channel association consists of conglomerate (A1) and trough cross-bedded multistorey sandstone facies (A2), both interpreted as deposits of low-sinuosity river systems, tidally influenced fluvial channels (A3), interbedded sandstone and siltstone (A4) interpreted as the abandonment fill of the alluvial systems, and diamictite (A5) interpreted as debris flows. The floodbasin facies association is composed of sandy siltstone (B1; proximal–distal floodbasin), carbonaceous siltstone (B2; mire), and bioturbated siltstone (B3; estuarine) facies. The overall sediment body architecture can be resolved into 6 unconformity-bounded cycles interpreted as sequences in the genetic sense, which are sheet-like in geometry and, in general, consist of amalgamated multistorey, multilateral braided fluvial deposits at the base overlain by extensive sheet-like overbank mudstones, carbonaceous shales, and coals. Sea-level change is interpreted as the primary control on the sequence architecture of the formation, while the internal stratigraphic architecture, however, varies between sequences and is a function of a combination of sea level, tectonic, and autogenic controls.