High-Resolution Magnetostratigraphy of the Continental Middle–upper Triassic Ermaying and Yanchang Formations: A Reference for the Ordos Basin, North China

The Middle–Late Triassic constitutes a pivotal interval for the assembly and consolidation of early modern-type ecosystems, and reliable marine–terrestrial stratigraphic correlation is essential for deciphering this transformative evolutionary phase. Magnetostratigraphy ranks among the most powerful tools for achieving such correlations, and China, characterized by a distinctive “marine south, terrestrial north” Triassic paleogeographic configuration, provides an unparalleled natural archive for this endeavor. The Ordos Basin within the North China Block preserves a thick and stratigraphically continuous succession of continental Triassic strata, supported by a well-established biostratigraphic framework. Nevertheless, the inherent difficulty of obtaining paleomagnetic samples from the predominant mudstone lithologies has hindered the development of a high-resolution magnetostratigraphic timescale, thereby impeding precise regional stratigraphic correlation, integration of marine and terrestrial records, and the accurate temporal calibration of major climatic and biotic events. This study focuses on stratigraphically continuous sections at Xuefengchuan in Hancheng and Zhangjiayan in Wubao, and undertakes an integrated investigation combining high-resolution magnetostratigraphy, rock magnetism, and sedimentary facies analysis of the fine-grained sedimentary rocks of the Ermaying and Yanchang formations. The primary objectives are to establish a magnetic polarity sequence and chronostratigraphic framework for the continental Middle–Late Triassic of North China and to elucidate the sedimentological controls on magnetostratigraphic recording. Preliminary results indicate that hematite and magnetite are the dominant remanence carriers in samples from the Ermaying and Yanchang formations. Characteristic remanent magnetization directions were effectively isolated through a combination of systematic thermal and alternating-field demagnetization and have been validated by polarity reversal tests. A total of 18 primary magnetozones are recognized within the studied sections. Isotopic ages obtained from volcanic ash beds at the bases of normal-polarity magnetozones HC4n and HC7n constrain the entire upper Ermaying Formation to the late Anisian (Middle Triassic), thereby placing the Anisian–Ladinian boundary at the base of the Yanchang Formation. The overlying interval, dominated by reverse-polarity magnetozone HC8 and normal-polarity magnetozones HC9–HC10, corresponds to the Ladinian, whereas the uppermost magnetozones HC14–HC18 are correlated with the reverse-polarity chrons of the early Carnian. Coupled analysis of polarity sequences and sedimentary facies evolution further reveals that organic-rich mudstones and lacustrine turbidites within the Yanchang Formation significantly interfere with paleomagnetic signals, thereby providing essential lithofacies constraints for the interpretation of continental magnetostratigraphic data. This study establishes a systematic Middle–Late Triassic magnetostratigraphic sequence for the continental basins of North China, refines key chronostratigraphic boundaries including the Anisian–Ladinian and Ladinian–Carnian transitions, and provides critical constraints for understanding the temporal relationship between terrestrial environmental changes and biotic evolution during the Triassic.