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

Modal analyses of 209 sandstone samples from the Permian-Triassic successio n of the northern Bowen Basin, eastern Australia, identify two stratigraphi cally and compositionally distinct petrofacies. The Lower to Mid Upper Perm ian Back Creek Group is characterised by Petrofacies A which is quartz-rich (Q(82)F(8)L(10)). It was sourced primarily from cratonic basement terranes to the west, where relief was subdued, and the quartz content of the sands tones may also reflect some reworking in the marine realm with a consequent loss of labile grains. Petrofacies B is volcanolithic and characterises al luvial sediments of the Upper Permian Blackwater and Lower Triassic Rewan G roups. It was sourced from an undissected to transitional magmatic are prov enance located in the contemporary New England Orogen to the east, which su pplied 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-are variation in volcanic style. As a consequence, its framework grain population is enriched in quartz and is no t readily accommodated in the schemes of provenance interpretation currentl y in use. The consistency of framework detrital modes for sandstones distri buted throughout the Blackwater Group (Q(24)F(10)L(66)) allows the recognit ion of subpetrofacies B1 and shows that the volcanism associated with the m agmatic are system was remarkably uniform in character and activity during deposition of the entire group. The magmatic are delivered almost identical sedimentary debris over a period of some 9 My in the Late Permian. Relativ e enrichment of quartz within the Early Triassic Rewan Group (Q(24)F(10)L(6 6)) relative to the Blackwater Group discriminates subpetrofacies B2 and is attributed to a climatic change and shift in palaeotemperature at the Perm ian-Triassic boundary. The contact between the marine Back Creek Group (Pet rofacies A) and the coal-bearing Blackwater Group (Petrofacies B) represent s an abrupt switch in provenance, with little or no evidence of source mixi ng. The boundary marks the onset of volcanism and associated tectonic uplif t in the northern part of the New England Orogen to the east. Sediment from the new source prograded rapidly westwards across the basin, coincident wi th regression. Direct evidence of contemporaneous volcanism is widespread i n the Blackwater Group as shown by tuffs and tonsteins which are distribute d throughout the entire late Permian succession. (C) 2000 Elsevier Science B.V. All rights reserved.