Magnetochronology of the Oligocene mammalian faunas in the Lanzhou Basin, Northwest China

The fluvio-lacustrine sequence in the Lanzhou Basin, located at the northeastern margin of the Tibetan Plateau, is a rich source of Oligocene–Miocene mammalian fossils, critical to our understanding of the terrestrial Asian mammal and environmental evolution. While the Miocene mammalian faunas have been dated with magnetostratigraphy, the numerical age of the Oligocene faunas remains controversial. Here, we present new high-resolution magnetostratigraphic records from two fluvio-lacustrine sections in the central Lanzhou Basin, which provide age constraints for two Oligocene mammal assemblages in the Lower Xianshuihe Formation, the Nanpoping and Xiagou Faunas. The Lower Xianshuihe Formation is suggested to span from polarity chrons C12r to C7n, ranging from ca 31 Ma to 24 Ma. The Early Oligocene Nanpoping Fauna correlates to the C12r–C11n.2n polarity chron intervals, yielding an age of ∼31–30 Ma, while the Late Oligocene Xiagou Fauna correlates to chrons C7An–C8n.2n, with an age of ∼26–25 Ma. This magnetostratigraphy provides a more accurate age than biochronology for the Oligocene giant mammal Indricotheriinae contained in the Nanpoping Fauna and sheds new light on the chronology and distribution of Indricotheriinae in Eurasia. The faunas, in particular the Nanpoping Fauna, suggest a mixed setting of woodlands and grasslands associated with a humid environment in the Lanzhou Basin during Early Oligocene, in contrast to its present arid environment with few woodlands.