Revising the Cambro-Ordovician depositional margin of Gondwana in eastern Australia: geochronology constraints on the Girilambone Group, central NSW

Cambrian deposition along eastern Gondwana extends from the western Koonenberry and Loch Lilly–Kars belts to the eastern Saint Arnaud and Warrata groups (and other covered Thomson Orogen units). Farther east, the Lachlan Orogen’s Ordovician depositional system comprises turbidite packages of the Adaminaby, Castlemaine and Girilambone groups. Though assigned to the Ordovician, the lower Girilambone Group has long been considered distinct in character and composition but has defied subdivision. We present new and reinterpreted zircon provenance data alongside minimum depositional ages for autochthonous mafic rocks to redefine Girilambone Group stratigraphy. The lower Narrama Formation consists of monotonous, texturally mature turbidite containing zircon derived from orogenic belts associated with the assembly of Rodinia (1250–900 Ma) and Gondwana (660–540 Ma), with minor inputs from the 520–500 Ma Delamerian Orogeny. This contrasts with the middle Narrama Formation, which features channelised, compositionally immature marker beds dominated by Delamerian I- and S-type granite derived zircons deposited synchronously with the orogeny. Mid-ocean ridge basalt composition gabbro/dolerite intruded ca 506 Ma tracking active back-arc processes and constraining minimum Cambrian aged deposition. We suggest zircon provenance alongside texture and composition in Narrama Formation sandstones require a proximal source consistent with derivation from the Moralana Supergroup (Adelaide Superbasin). Post-Delamerian Girilambone Group units—the mature Budgery Sandstone Member and quartzose turbidites with thick cherts of the upper Ballast Formation—display zircon signatures marking a return to continental interior sediment sources following Delamerian foreland denudation. This post-Delamerian sequence contains predominantly 660–540 Ma and 1250–900 Ma zircons and features intraplate basaltic magmatism (Mount Dijou, 488 ± 7 Ma), indicating deeper mantle melting from Delamerian crustal thickening. Crucially, our new interpretation of the Girilambone Group correlates the Narrama Formation and Koonenberry Belt stratigraphy and extends the Delamerian depositional system significantly farther east.