Shifting the Paradigm: Redefining the Chronostratigraphy of the Triassic Rewan Group, Bowen Basin, Australia

Matthew Scipione, Romain Vaucher , Eric Roberts, Alex J McCoy-West, Joan Esterle, Ashten Turner, Espen Knutsen

The Triassic continental Rewan Group in the northern Bowen Basin, Queensland, Australia, consisting of the Sagittarius Sandstone and the Arcadia Formation, preserves a key record of terrestrial environments and faunas that have been assumed to document recovery following the end‑Permian mass extinction (EPME). The Rewan Group accumulated in a retroarc foreland basin during the Hunter–Bowen Orogeny, but its chronostratigraphy has remained poorly constrained because previous age models relied mainly on limited, geographically restricted biostratigraphy (esp. palynostratigraphy). Here, we couple detailed lithostratigraphic analysis with high‑density U–Pb detrital‑zircon (DZ) geochronology by LA‑ICP‑MS (ca. 300 analyses per sandstone), calibrated against latest Permian (ca. 252–251 Ma) tuffs and a detrital apatite U–Pb dataset, to establish a robust chronostratigraphic framework for the Sagittarius Sandstone and Arcadia Formation. We compare several maximum depositional age (MDA) metrics and show that maximum‑likelihood ages (MLA) at 10% discordance provide a stratigraphically coherent MDA estimate, with younger single‑grain and cluster‑based estimators used as internal checks and minimum bounds. Preferential zircon picking further show that targeted grain selection enriches the youngest Triassic populations, strengthening the robustness of the resulting MDA constraints. The resulting MDAs demonstrate that the Rewan Group spans ca. 250–233 Ma (Olenekian to Carnian) and that the base-Rewan contact is strongly time-transgressive. In the Taroom Trough (foredeep), fluvial successions young consistently up section from the latest Permian (based on tuff ages) through Olenekian–Anisian Sagittarius Sandstone into the late Ladinian–earliest Carnian Arcadia Formation (based on sandstone MDAs). In contrast, in the Denison Trough (back-bulge), the latest Permian coal measures are directly overlain by Middle–early Late Triassic Rewan Group deposits, implying a hiatus or condensed interval of at least ca. 12–15 Myr based on MDAs. We show that known Arcadia Formation vertebrate fossil‑bearing horizons are late Ladinian (239 Ma) to early Carnian (236 Ma) rather than earliest Triassic, with the younger date also corroborated by a detrital apatite lower‑intercept age of ca. 239 Ma. These revised ages show that the Arcadia Formation vertebrate assemblages do not come from the immediate post‑EPME interval but from the late Ladinian to early Carnian, across the onset of the Carnian Pluvial Episode. They provide a dated framework for testing Triassic continental ecosystem evolution in the Bowen Basin and in comparable basins globally.