Sandstone petrofacies expressions of multiphase basinal tectonics and arc magmatism: Permian–Triassic north Bowen Basin, Australia

Abstract

Abstract Modal analyses of 209 sandstone samples from the Permian–Triassic succession of the northern Bowen Basin, eastern Australia, identify two stratigraphically and compositionally distinct petrofacies. The Lower to Mid Upper Permian Back Creek Group is characterised by Petrofacies A which is quartz-rich (Q82F8L10). It was sourced primarily from cratonic basement terranes to the west, where relief was subdued, and the quartz content of the sandstones may also reflect some reworking in the marine realm with a consequent loss of labile grains. Petrofacies B is volcanolithic and characterises alluvial sediments of the Upper Permian Blackwater and Lower Triassic Rewan Groups. It was sourced from an undissected to transitional magmatic arc provenance located in the contemporary New England Orogen to the east, which supplied abundant pyroclastic debris to the depositional complex. Petrofacies B of the northern Bowen Basin was derived primarily from felsic volcanics, compared to an intermediate association recorded for the southern basinal sector, indicating significant along-arc variation in volcanic style. As a consequence, its framework grain population is enriched in quartz and is not readily accommodated in the schemes of provenance interpretation currently in use. The consistency of framework detrital modes for sandstones distributed throughout the Blackwater Group (Q24F10L66) allows the recognition of subpetrofacies B1 and shows that the volcanism associated with the magmatic arc system was remarkably uniform in character and activity during deposition of the entire group. The magmatic arc delivered almost identical sedimentary debris over a period of some 9 My in the Late Permian. Relative enrichment of quartz within the Early Triassic Rewan Group (Q49F6L45) relative to the Blackwater Group discriminates subpetrofacies B2 and is attributed to a climatic change and shift in palaeotemperature at the Permian–Triassic boundary. The contact between the marine Back Creek Group (Petrofacies A) and the coal-bearing Blackwater Group (Petrofacies B) represents an abrupt switch in provenance, with little or no evidence of source mixing. The boundary marks the onset of volcanism and associated tectonic uplift in the northern part of the New England Orogen to the east. Sediment from the new source prograded rapidly westwards across the basin, coincident with regression. Direct evidence of contemporaneous volcanism is widespread in the Blackwater Group as shown by tuffs and tonsteins which are distributed throughout the entire late Permian succession.