Hydrocarbon-related chemical remagnetization in the upper Ordovician Zhaolaoyu Formation, southern Ordos Basin, North China: Paleomagnetic and geochemical insights

Remagnetization is widespread in sedimentary basins worldwide, complicating the reconstruction of paleogeographic positions. Understanding its timing and mechanisms is therefore essential not only for assessing the preservation of primary remanence, but also for deciphering stratigraphic burial histories, diagenetic processes, and hydrocarbon-related fluid activities. In the Ordos Basin, North China, extensive remagnetization of the upper Ordovician strata provides a valuable opportunity to investigate remagnetization associated with organic-matter activity. To investigate the timing and mechanisms of remagnetization, we conducted systematic paleomagnetic and geochemical studies on limestone and tuff strata from the upper Ordovician Zhaolaoyu Formation in the Fuping area, along the southern margin of the Ordos Basin. Rock magnetic and demagnetization results indicate that magnetite is the dominant magnetic carrier, with a characteristic remanent magnetization direction of Ds = 330.5°, Is = 28.8°, k = 11.3, α95 = 4.3°. The corresponding paleomagnetic pole aligns with the Early−Middle Triassic segment of the North China Block apparent polar wander path. Anisotropy of magnetic susceptibility results reveal a primary sedimentary fabric, indicating that the strata were largely unaffected by subsequent tectonic deformation. Scanning electron microscopy and energy-dispersive spectroscopy results show that most magnetite grains occur as spherical or framboidal aggregates. Fluorescence microscopy observations reveal the presence of organic matter within microfractures. Additionally, strong correlations are observed among total organic carbon, pyrolysis gas yield, and natural remanent magnetization. Therefore, these results indicate that the Zhaolaoyu Formation recorded a chemical remagnetization event likely associated with organic-matter maturation during the Early−Middle Triassic. This study provides critical constraints on the evolution of hydrocarbon generation from Ordovician source rocks along the southern margin of the Ordos Basin.