Objective The Permian - Early Triassic sedimentary environment along the southern margin of the Ordos Basin and its tectonic-sedimentary response to the Paleo-Tethys Ocean are key to understanding the paleogeographic evolution and energy resource potential of the southern margin of the North China Craton.
Methods Taking the Qinling and the Southern Ordos Basin as the research subjects, a systematic sedimentological study was conducted through drilling and field profile investigations. Existing paleontological findings were systematically reviewed, leading to a re-evaluation of the Permian-Early Triassic tectonic and paleogeographic patterns along the southern Ordos Basin. The study further explored the paleogeography of the southern Ordos Basin and North Qinling during the Permian - Early Triassic, as well as the connectivity between the basin and the Mianlue Ocean.
Results The results show that: (1) Sedimentological studies indicate that the southern Ordos Basin and North Qinling primarily developed a distal-source delta-front to coastal depositional system during the Permian - Early Triassic. The widespread occurrence of quartz sandstones demonstrates the influence of distal terrigenous clastic supply. Sedimentary facies are dominated by distal-source marine-terrestrial transitional environments, with no significant near-source deposits like alluvial fans. (2) A synthesis of paleontological data confirms the widespread development of marine-origin fossils (including brachiopods, gastropods, bivalves, ammonites, foraminifera, crinoids, and trace fossils) in the upper Taiyuan Formation to Heshanggou Formation along the southern Ordos Basin, evidencing extensive marine transgression or persistent marine-terrestrial transitional environments. (3) The reconstructed depositional framework reveals that coastal and deltaic deposits dominated the southern Ordos Basin and North Qinling, transitioning southward to the marine carbonate platform environment of the South Qinling. This demonstrates connectivity between the Ordos Basin and the Paleo-Tethys Ocean, with the North Qinling representing the southward extension of the deltaic system from the southern Ordos Basin. (4) Organic matter deposited in long-term marine-terrestrial environments under saline conditions exhibits significantly superior hydrocarbon generation potential compared to freshwater environments. Permian mudstones/shales possess better hydrocarbon generation potential than Triassic and Jurassic equivalents. Furthermore, coal measures interbedded with marine shales form multi-seam coal structures. (5) The Permian - Lower Triassic sediments in the northern Qinling are finer-grained than those in the central-northern Ordos Basin, dominated by fine-grained mudstones/shales, which provide both high-quality source rocks and reservoir-seal combinations, offering new targets for coal and hydrocarbon exploration.
Conclusion This study proposes a new understanding: during the Permian - Early Triassic, the southern Ordos Basin was a distal offshore depositional area on the northeastern Paleo-Tethys Ocean. Its water body was not a traditionally assumed freshwater environment, but a marine-influenced saline environment, challenging the conventional "terrestrial-dominated" depositional model. These results provide critical evidence for reconstructing the paleogeography of the southern North China Craton , understanding basin-mountain coupling mechanisms, and selecting optimal energy exploration targets.
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