Abstract:

There is a continuous Cenozoic sedimentary sequence in the Lantian-Weinan area in the southeastern Weihe Basin, which
contains a large number of mammal fossils. The Bahe Formation is especially famous for the three-toed equid “Hipparion ” and the
other mammal fossils, and it has been accordingly established as a standard of Bahe stage. However, there are still debates about the age of the Bahe Formation. In order to investigate the importance of the Bahe Formation and its sedimentary environmental changes and paleontological characteristics in the Late Neogene period in Northern China, we carried out a detailed magnetostratigraphic study of the type Bahe Formation called the Duanjiapo section (246 m thick), which is located on the southern bank of the Ba River, Lantian County, Shaanxi Province. The magnetic polarity sequence can be clearly calibrated to the Geomagnetic Polarity Time Scale (GPTS). The result shows that the age of the Bahe Formation is 11~7 Ma, corresponding to the Late Miocene. This study believes that the Bahe Age should be a separate stratigraphic unit that is different from the Baode Age. It could provide a more precise age for the regional mammal fossils comparison. Meanwhile, the Bahe Age is also different from the Lantian Age, according to the comparison of sedimentary environment and paleoclimate with other basins, i.e. the Linxia Basin and the Tianshui Basin. The Bahe Age is characterized by a much drier semiarid steppe climate whereas the Lantian Age is humid forest climate.

Key words: Weihe Basin, Bahe Formation, magnetostratigraphy, Miocene