Abstract: The Linxia Basin, a depression bounded by the Leijishan Fault in the west, Xiqinling Fault in the south and Maxianshan Mountain in the northeast in the northeastern edge of the Tibetan Plateau, is very sensitive to the climatic and environmental changes.Many effective research programs have been carried out in this region. Substantial amounts of mammalian fossils have been discovered in the thick Late Cenozoic sediments of the Basin. Through detailed exhumation in this region there have found 5 most typical mammalian faunas:The Late Oligocene Dzungariotherium fauns, the Middle Miocene Platybelodon fauns, the Late Miocene Hipparion fauns, and the Early Pleistocene Equus fauns.
The Heilinding section lies on the south margin of the Linxia basin with a total of 227 m Miocene deposits from the top to the bottom. The fossils Hipparion sp. and Platybelodon sp. were excavated in the red clay layer of the upper Liushu Formation and sandstone of Dongxiang Formation respectively. Analysis of thermal demagnetization of natural remanent magnetization (NRM) indicates that hematite, maghemite,and magnetite are primary magnetic minerals in the sediments. According to the high-resolution magnetostraitigraphic research on this section, 14 normal polarities (N1~N14) and 14 reversal polarities (R1~R14) are interpreted, compared with the GPTS of Cande and Kent, from the top 15 m to the bottom 227 m systematically. Based on this result, sedimentation rates and geological age of the fossils-bearing strata can be defined. The Hipparion fauna is constrained at 6.27 MaBP and the Platybelodon fauna is dated 11.12 Ma.