Preservation of Highstand Coals in a Basin Marginal Context: the Late Permian Betts Creek Beds, Queensland, Australia

Jonathan P. Allen and Christopher Fielding
University of Nebraska-Lincoln, Lincoln, NE

The placement of coal within a sequence stratigraphic framework has received increasing attention over the last 10 years. However, current models for coal in a sequence stratigraphic framework, while generally accepted, do not fully explain the nature (lateral extent and thickness) and occurrence of all coal deposits. The preservation of peat requires high rates of accommodation. Therefore, during transgression at the timing of maximum flooding, when rates of accommodation are at their maximum, extensive and thick coals are envisaged to accumulate. In the Late Permian Betts Creek Beds, laterally extensive, thick coals occur within late highstand deposits. This timing and characteristic of coal development differs somewhat with the traditional models of coal accumulation.

The Late Permian Betts Creek Beds form a succession of alluvial-coastal plain sediments deposited in a basin marginal setting within the northeastern Galilee Basin, Queensland, Australia. The formation consists of 6 highly condensed sequences 10-14 m in thickness with clearly differentiated sequence tracts. Each sequence is composed of laterally extensive, amalgamated braided fluvial sandstones (LST), heavily bioturbated mudstones containing a restricted expression of a mixed Skolithos/Cruziana ichnofacies (TST), and carbonaceous mudrocks and coals (HST). Carbonaceous mudstones and coals are laterally extensive over the study area (~5 km) and up to 2 m in thickness. Within coals several discrete horizons of bioturbation are also observed. These deposits correlate to coals over 10 m in thickness elsewhere in the basin. It is proposed that slow rates of base level fall and the low accommodation basin margin setting play major roles in the preservation of highstand coal deposits.