The Permian-Triassic boundary in Australia: where is it and how is it expressed?

Placement of the Permian-Triassic boundary in Australia, and particularly in Queensland, relied initially (1950s) on lithological criteria. The boundary was taken as coincident with the change from coalmeasures (Permian) to non coaly sediments, principally redbeds. This was, and remains, a convenient field and subsurface mapping point. Subsequently, palaeontological evidence (1960s) focussed on the demise of the Glossopteris Flora as a marker for the upper boundary of the Permian System. Palynological studies (1970-90s), linking at least broadly to the macrofloras, and, most importantly, to palynofloras from the independently dated Salt Range section, indicated that a major floral change which began during the close of coal measure sedimentation, was of early Changhsingian age. The interpretation is necessarily dependent on stratigraphic range data from the Pakistan section, and on establishing migration pathways for the parent floras. The almost global appearance of monolete, sculptured, cavate lycopod spores of Aratrisporites, which first appear in the non-coaly sediments, above the last Permian coal seams in eastern Australia, and in faunally dated Early Triassic (upper Greisbachian) sediments in western Australia, has been suggested as a marker for the basal Triassic. But acceptance of his criterion is not agreed by all workers. SHRIMP (Sensitive High Resolution Ion Microprobe) dating of single zircons from the type Meishan section, China, has provided a numeric age for the P-T boundary (based on the appearance of the conodont Hindeodus parvus) of about 252 Ma. SHRIMP dates of tuffs from eastern Australia, occurring several hundreds of metres below the base of the coalmeasures have been dated at 251 Ma, which if correct would infer that the coals were of Triassic age! However, these ages have been established using different zircon standards, and the standard used for eastern Australia seems to give younger ages. This problem is under intensive investigation, and so, at present, SHRIMP does not provide a boundary solution. Changes in carbon isotopic composition of organic matter have been suggested as a proxy for the boundary in Australia. But these criteria too, are imprecise, as the origin of the organic matter effects the value of the isotopic composition; such that woody derived kerogen is isotopically heavy (-24‰) and nonwoody kerogen is consistently lighter (to -32‰). The resultant signature reflects constraints imposed by admixtures of organic matter, biofacies, and depositional history. A lack of carbonates at the P-T boundary in Australia precludes the use of carbon isotopes from this source. The problem of recognition of the P-T boundary in Australia, and in Gondwana in general, is exacerbated by rarity of marine index fossils, in particular conodonts, so that correlation with the marine standard sections of the warmer northern hemisphere deposits is not possible. After 25 years, the P-T boundary in Australia remains elusive and between two anchor points of Late Permian (but not latest) and late Early Triassic age. Correlation with the Late Permian (early Changhsingian) uses palynological criteria in eastern Australia, and the late Early Triassic is defined by marine faunas in western Australia.