Objective  Understanding the Permian-Early Triassic sedimentary environments along the southern margin of the Ordos Basin (also referred to as the SMOB) and their tectono-sedimentary responses to the Paleotethys Ocean plays a key role in determining the paleogeographic evolution and energy resource potential of the southern margin of the North China Craton.

Methods This study investigated the Qinling Orogenic Belt (QOB) and the SMOB. Through systematic sedimentological examination based on drilling data and outcrop sections, along with a systematic review of existing paleontological findings, this study re-determined the Permian-Early Triassic tectono-paleogeographic frameworks along the SMOB. Furthermore, this study explored the paleogeography of the SMOB and the North Qinling Orogenic Belt (NQOB), as well as the connectivity between the Ordos Basin and the Mianlue Ocean (a northern branch of the Paleotethys Ocean), during the Permian to Early Triassic.

Results  The results of sedimentological examination indicate that from the Permian to the Early Triassic, distal-source delta-front - coastal sedimentary systems were primarily deposited along the SMOB and in the Yaoshi area, NQOB. The widespread quartz sandstones corroborate the supply of terrigenous clasts by distal source areas. The sedimentary facies are dominated by distal-source marine-continental transitional facies, with no pronounced near-source deposits like alluvial fans identified. The review of paleontological findings confirms the extensive development of marine fossils, including brachiopods, gastropods, bivalves, ammonites, foraminifera, crinoids, and trace fossils, from the upper member of the Permian Taiyuan Formation to the Triassic Heshanggou Formation along the SMOB. This result confirms the presence of extensive marine transgression or marine-continental transitional environments. The reconstructed depositional framework reveals that coastal and deltaic deposits predominate along the SMOB and in the NQOB and then transition southward to a marine carbonate platform environment in the South Qinling Orogenic Belt (SQOB). This finding demonstrates the connection between the Ordos Basin and the Paleotethys Ocean and corroborates that the NQOB represents the southward extension of the deltaic system along the SMOB. The organic matter deposited in prolonged marine-continental environments exhibits significantly greater hydrocarbon generation potential under saline conditions than under freshwater environments. The Permian shales show higher hydrocarbon generation potential than the Triassic and Jurassic equivalents, and the alternating development of the Late Paleozoic coal measures and marine shales leads to the formation of multiple coal seams. Compared to those in the north-central Ordos Basin, the Permian-Lower Triassic deposits in the NQOB exhibit finer grains and are dominated by fine-grained shales. These deposits exhibit both high-quality source rocks and reservoir-cap rock assemblages, establishing them as a new target for coal and hydrocarbon exploration.

Conclusions During the Permian-Early Triassic, the SMOB was a distal offshore depositional region on the northeast side of the Paleotethys Ocean. Its water bodies were characterized by marine-influenced saline environments rather than traditionally considered freshwater environments, posing challenges to the conventional continental-dominated sedimentary pattern. This insight provides a critical basis for the reconstruction of the paleogeography, the understanding of the basin-mountain coupling mechanisms, and the selection of optimal energy exploration targets along the southern margin of the North China Craton.