Revised Magnetostratigraphy of the Linxia Basin (ne Tibetan Plateau) Constrained by Mammalian Fossils

The Linxia Basin, situated on the northeastern margin of the Tibetan Plateau, preserves exceptionally well-developed Cenozoic sedimentary sequences and abundant mammalian fossils, making it a key region for investigating regional tectonic evolution, paleoenvironmental changes, and bio-evolution in Central Asia. However, the geochronology of the sedimentary sequences is in great dispute owing to ambiguous lithostratigraphic correlation and poorly calibrated magnetostratigraphy in previous studies, especially inaccurate fossil horizon assignments. From 2017 to 2021, we conducted lithostratigraphic correlation of the strata from the southern part of the basin, which is rich in fossils, with the Maogou section in the central basin, establishing a solid intra-basinal lithostratigraphic framework with accurate fossil locations. Then, high-resolution magnetostratigraphic investigations were performed on the Maogou section. This continuous sedimentary sequence comprises, from oldest to youngest, the Jiaozigou, Shangzhuang, Dongxiang, Liushu/Hujialiang, and “Liushu” formations. A total of 718 paleomagnetic samples were collected at 20–50 cm intervals, and rock magnetic results indicate that hematite and single-domain magnetite are the dominant magnetic carriers, ensuring the reliability of paleomagnetic signals. Magnetostratigraphic results show that the Maogou sedimentary sequence (from Jiaozigou to Liushu/Hujialiang formations) spans from chron C10r to chron C5r.2n, yielding an age of 29‒11.6 Ma, with constraints from four mammalian assemblage levels. The revised magnetostratigraphy indicates that the Jiaozigou Formation corresponds to chrons C6Cn.3n‒C10r (~23-29 Ma), with Paraceratherium fauna from the lower Jiaozigou Formation being determined as ~29 Ma, and that the Hujialiang Formation, containing Platybelodon fauna, corresponds to chrons C5r.2n‒C5Ar.2r (~11.6-12.8 Ma). Integrated sedimentary and paleontological evidence indicates a semi-arid woodland environment in the Late Oligocene, increasing aridity in the Early Miocene, a widespread lacustrine system during the Middle Miocene Climatic Optimum, and the lake disappearance at ~12.8 Ma driven by regional tectonic uplift of the Jishi Shan and West Qinling. The revised magnetostratigraphy significantly refines the chronology of the Linxia Basin, providing a robust foundation for understanding the middle Cenozoic tectono-sedimentary history, paleoclimate changes, and associated mammalian evolution in Central Asia.