Reassessing the chronostratigraphy and tempo of climate change in the Lower-Middle Permian of the southern Sydney Basin, Australia: Integrating evidence from U–Pb zircon geochronology and biostratigraphy

Abstract

The Permian of the southern Sydney Basin (Australia) comprises both marine and non-marine sedimentary sequences with evidence of glacial deposition and syndepositional magmatism represented by tuffs, lava flows and hypabyssal intrusions. This succession has been well-studied in lithostratigraphy and sedimentology, but the Lower-Middle Permian chronostratigraphy remains poorly constrained due to a lack of radiometric isotope ages and strong faunal endemism. In this study, we report the first suite of reconnaissance U–Pb zircon ages from this succession, comprising two Wandrawandian Siltstone (WS) tuffs (Green Point tuff 272.6 ± 6.5 Ma population, Callala Bay tuff ≥290 Ma Permian populations) and magmatic zircon ages from three discrete syndepositional intrusions: the Termeil Complex intruding the Snapper Point Formation dated 278.9 ± 3.7 Ma (95% confidence), the Milton Complex intruding the Snapper Point Formation and likely the WS dated 275.5 ± 4.3 Ma, and a dyke/sill at Kinghorn Point intruding the WS dated 275.6 ± 4.3 Ma. In light of the new preliminary U–Pb ages, combined with stratigraphic and biostratigraphic evidence, the age of the Wasp Head Formation is reassessed to be late Asselian-Sakmarian, Pebbley Beach Formation Artinskian, Snapper Point Formation early Kungurian, Wandrawandian Siltstone late Kungurian-Roadian, Nowra Sandstone and Berry Siltstone both confined within early Wordian, and Broughton Formation from mid-Wordian to early Capitanian. The ages and duration of the Permian glacial events are also revised, with the first episode (P1) lasting from basal Permian to early Sakmarian (~299–293 Ma), P2 spanning Artinskian (~290–283 Ma), P3, possibly the longest of all Permian glacial intervals, extending from mid-Kungurian to early Capitanian (~278–263 Ma), and P4 from mid-Capitanian to late Wuchiapingian (~260–254.5 Ma). The ≥290 Ma Permian zircon populations in the Callala Bay tuff reflect either an early Permian volcanism or older zircons recycled from elsewhere.