Analysis of the Ragged Basin, Western Australia: Insights into syn-orogenic basin evolution within the Albany-Fraser Orogen

Australian Albany-Fraser Orogen evolution insights from Ragged Basin analysis. Sedimentary basins occur within a variety of tectonic settings, both within plates and near plate boundaries. We explore the complex history of the Mesoproterozoic Ragged Basin, located in the eastern Nornalup Zone of the Albany-Fraser Orogen, which is part of the West Australian Craton. Sediments of the Ragged Basin were deposited within a shallow basin by a large fluvial system dominated by shifting, sandy braided channels, forming a quartz-rich succession defined as the Mount Ragged Formation. The gradual coarsening upwards sequence indicates a distal fluvial environment characterised by channel migration and abandonment, changing to a proximal fluvial environment characterised by rapid periods of sedimentation and coarser deposits. Ion microprobe (SHRIMP) U-Pb analysis of detrital zircons constrain a maximum depositional age of 1314 ± 19. Ma for the Mount Ragged Formation, so it is feasible that deposition started during the latter part of Stage I (c. 1330-1260. Ma) of the Albany-Fraser Orogeny. The detrital zircon U-Pb data demonstrates that the Mount Ragged Formation contains c. 1810-1320. Ma detritus, most of which appears to be derived locally from the reworked craton margin that forms the Albany-Fraser Orogen basement. However, the smaller c. 1560 and c. 2490. Ma zircon age components have no known source within the West Australian Craton, and were potentially originally sourced from the Gawler Craton or other unknown sources beneath the Eucla and Bight Basins. These exotic ages support the interpretation that outboard accretion occurred prior to Stage II of the Albany-Fraser Orogeny. New structural data, field observations and aeromagnetic image interpretation indicate that the Mount Ragged Formation was deformed by a northwest-vergent fold and thrust system. A minimum age for deposition, and structural emplacement, is provided by a crystallisation age of 1175 ± 12. Ma for a cross-cutting monzogranite exposed at Scott Rock, part of the Esperance Supersuite. Upper-crustal thrusting in the Mount Ragged Formation can be linked to deeper, large-scale regional structures such as the Tagon and Rodona Shear Zones, the latter of which represents the eastern edge of the Albany-Fraser Orogen.