Decoupling the Marine-Terrestrial Transition: A Revised High-Resolution Carboniferous-Permian Chronostratigraphy for the Junggar Basin and Northern Xinjiang

The Carboniferous-Permian transition in the Junggar Basin and its periphery (Northwest China) preserves a critical sedimentary archive of the final closure of the Paleo-Asian Ocean (PAO). Despite its importance for reconstructing Pangea assembly and guiding hydrocarbon exploration, the regional chronostratigraphic framework has long been controversial. The widespread regression and scarcity of marine index fossils in these marine-terrestrial transitional successions have forced reliance on low-resolution biostratigraphic methods, leading to conflicting tectonic models and persistent miscorrelations of key hydrocarbon-bearing strata. To resolve these uncertainties, this study establishes a comprehensive high-resolution chronostratigraphic framework anchored by 53 new zircon U-Pb ages. We systematically sampled volcanic ash layers from key outcrop sections and deep drill cores across the Junggar, Turpan-Hami, Santanghu, and Yili basins, employing a hierarchical analytical strategy that integrates LA-ICPMS and SIMS screening with high-precision CA-ID-TIMS dating. Our geochronological results necessitate a fundamental revision of the traditional stratigraphy and reveal three distinct developmental stages for regional source rocks. First, the Fengcheng Formation (Mahu Sag), representing the basin’s oldest major source rock, is precisely constrained to the Late Carboniferous (Kasimovian) to earliest Permian (Asselian), significantly older than its previous Early Permian assignment. Second, the widespread Lucaogou Formation and the correlative Tarlong Formation are definitively dated to the Sakmarian–Artinskian (Cisuralian), overturning the long-held Guadalupian view. Crucially, we identify that the “Lucaogou Formation” previously identified in deep sags (e.g., Fukang Sag) is diachronous, with some intervals actually representing older Late Carboniferous deposits. Third, the Pingdiquan Formation in eastern Junggar is re-dated to the Lopingian (Wuchiapingian). This reveals a ~25-million-year age difference compared to the Lucaogou Formation, proving they are not coeval and indicating a significant depositional hiatus in the eastern basin during the Cisuralian. This refined temporal control unveils a clear spatiotemporal pattern in the region’s tectonic evolution. We document a pronounced diachroneity in the marine-terrestrial transition, which youngs progressively from the northwest (Kasimovian, ~307 Ma) to the east (Gzhelian, ~302 Ma). This trend provides robust sedimentological evidence supporting a “scissor-like” closure model for the western PAO, driven by oblique collision. Furthermore, we propose a new “three-stage” model for source rock development: (1) a Kasimovian–Asselian stage (alkaline lacustrine Fengcheng Fm); (2) a widespread Sakmarian–Artinskian stage (fresh/brackish lacustrine Lucaogou/Tarlong Fms); and (3) a localized Wuchiapingian stage (swamp-lacustrine Pingdiquan Fm). This framework resolves decades of stratigraphic ambiguity and offers a precise temporal guide for exploring Carboniferous-Permian petroleum systems in the Central Asian Orogenic Belt.