SHRIMP zircon dating of the Permian System of eastern Australia∗

Abstract

SHRIMP zircon dates from Permian ignimbrites and tuffs associated with fossiliferous strata within the Sydney‐Bowen Basin and New England Orogen are used to establish a time scale for the Permian System in eastern Australia. For the first time this enables direct correlation of the eastern Australian Lower Permian succession with similarly measured ages in the Permian type sections in the Russian Urals; new SHRIMP ages from tuffs within the Russian Lower Permian succession are being published elsewhere. An additional tie point with the Russian Kazanian Stage is achieved by means of a SHRIMP date in Australia constrained by foraminifers of the Pseudonodosaria borealis Zone. Almost all the other Permian biota in eastern Australia are Gondwanan; they provide the basis for regional biostratigraphic correlation, but cannot be used to correlate with the type Russian Permian. Integration of the numerical time and biostratigraphic scales indicates diachronous relationships between formations within both the Sydney and Bowen Basins. In the former, the Greta Coal Measures and overlying Branxton Formation are diachronous, the boundary between the two units being younger on the Muswellbrook than on the Lochinvar Anticline. In the Denison Trough of the Bowen Basin, some of the Upper Permian marine to continental formations may be diachronous, with sediments at the top of and immediately above the Ingelara Formation in the northern part of the trough having the same age as the Black Alley Shale in central and southern parts of that structure. SHRIMP ages indicate that deposition probably began earlier in the Sydney than the Bowen Basin, and that Late Permian volcanism in the Warwick region of the southern New England Orogen took place at about the same time as intrusion of the first plutons of the New England Batholith. The top of the Carboniferous‐Permian Reversed Superchron in Australia is approximately 263 Ma in age and older than Kazanian. The age of the the Permian‐Triassic boundary presently measured in the Chinese stratotype section requires further investigation.