Detrital zircon U Pb geochronology of Lower Permian sedimentary rocks of the southern Sydney Basin in southeastern Australia: Implications for provenance and basin evolution

We report a suite of detrital zircon U Pb age data from the southern Sydney Basin in southeastern Australia, including the lowermost Permian Wasp Head, Pebbley Beach, and Snapper Point formations as well as the Lower Ordovician basement that was proximal to the basin margin. The aim of this study was (a) to identify Permian volcanic zircons that provide maximum depositional age constraints for these units and (b) to obtain the age spectra of pre-Permian zircons to infer, along with sedimentological evidence, the source-to-sink history of the early southern Sydney Basin. In total, 466 detrital zircon grains from five samples were analysed, generating 471 spot ages. Three main age peaks (1100–900 Ma, 650–600 Ma and 480–440 Ma) were recognised and coincide with the age signatures of sedimentary and metamorphic rocks of the Lachlan Orogen situated to the west of the Sydney Basin; this suggests the Lachlan Orogen is the main provenance area for the southern Sydney Basin during Early Permian times. Additionally, a gradual increase of Carboniferous ages with a peak at ~337 Ma is evident through the Pebbley Beach and Snapper Point formations. This age peak coincides with the I-type Bathurst Batholith in the eastern Lachlan Orogen, suggesting that the exhumation of this body was a possible sediment source. A less likely alternative is derivation of these Carboniferous grains from the Tamworth Belt forearc basin within the southern New England Orogen. One Permian volcanic zircon was recovered from the Pebbley Beach Formation, dated at 293 ± 5 Ma (1σ), which is broadly consistent with the biostratigraphic age of this formation. Possible sources for this single Early Permian zircon grain are distal arc sources to the east or rift-related volcanism responsible for the initiation of the Sydney Basin as part of the Early Permian eastern Australia rift system. • First suite of detrital zircon U Pb ages provided for the three lowermost lithologic units of the southern Sydney Basin. • Lachlan Orogen confirmed as the primary source of sediments during the Early Permian. • A possible provenance shift detected by the occurrence of ~337 Ma age-peaked zircons in the Snapper Point Formation.