Sequence stratigraphy and fill history of the Bowen Basin, Queensland

A regional seismic synthesis of the 10 km-thick continental and shallow-marine succession of the Bowen Basin has revealed the basin-filling episodes and nine depositional supersequences (A-I). An Early Permian extensional phase, characterised by thick volcanics and half-graben development in a number of separated troughs, allowed fluviolacustrine sediments, including coal, to accumulate (Supersequence A). In the subsequent thermal subsidence phase, four marine supersequences (B-E) were generated. The onsets of supersequences B and C appear to be primarily tectonically controlled, but there is no evidence of tectonism initiating D, E and F. Supersequences B, D and E start with marine-flooding events, and in the Denison Trough end with either non-marine progradation (B), or high-energy nearshore sands (D and E). The first evidence of the compressional deformation associated with the development of a foreland basin occurs at the base of deltaic to marine Supersequence C. The thrust-induced loading of the foreland caused rapid subsidence in the adjacent Taroom Trough. It took about 10 Ma for the effects of thrust loading to spread across the basin from east to west. The foreland loading first affected the entire basin at the time of Late Permian Supersequence F, and lasted until the end of Triassic deposition. Four supersequences (F-I) have been identified in this phase of basin evolution. The later part of Supersequence F is almost wholly non-marine coal measures. The tectonism and base-level fall that generated the sequence boundary between Supersequences F and G, and ended coal deposition, represents a significant hiatus, and in places is characterised by low-angle unconformity and erosion. The pattern of the remaining units of the foreland basin phase indicates episodic basin filling as accommodation space became available, related to pulses of thrust loading. The regional geometry of the units, and the overstepping of Supersequence G by H, and then by I, indicates continuing westward progradation of foreland loading and corresponding changes in accommodation. It is striking that although the late foreland-loading phase displays the greatest rate of subsidence affecting the basin since the initial rift phase, there is little evidence of widespread flooding by the sea. The reasons were probably that the Late Permian-Early Triassic was a time of global first-order lowstand, and that foreland loading was able to provide simultaneously high rates of subsidence in the basin and greatly increased sediment supply from erosion in the mountains thrust up to the east.