K–Ar and Rb–Sr dating of authigenic illite–smectite in Late Permian coal measures, Queensland, Australia: implication for thermal history

Abstract

K–Ar and Rb–Sr isotopic studies were carried out on authigenic illitic clay minerals in Late Permian coal measures from the Bowen Basin (Australia), in order to determine the timing of maximum paleotemperatures, which were responsible for coal maturation and coal seam gas generation. The results indicate two major thermal events affected the Bowen Basin region, at 205–215 and 140–155 Ma. The narrow range of age data from different size fractions, lithologies and stratigraphic depths indicates episodic, short-lived thermal events, rather than gradual temperature increase due to progressive burial as previously believed. The earlier, thermal event in the latest Triassic postdates maximum burial of the Bowen Basin strata, which occurred during the Middle to Late Triassic. The ages of 205–215 Ma correspond with the published evidence for regional Late Triassic extension in eastern Australia. The younger (140–155 Ma), thermal phase during the Late Jurassic–Early Cretaceous is related to the initial rifting and associated widespread igneous activities prior to break-up of Gondwana. The K–Ar dates in conjunction with vitrinite reflectance data as paleotemperature indicators indicate that the younger thermal event occurred at lower temperatures than the earlier one, except in the shallow part of the southern Bowen Basin. This event is recorded exclusively in less illitic, R=0 illite–smectite (I–S) mixed-layer samples mainly in the shallow part of the southern Bowen Basin (Baralaba Coal Measures).