Magnetostratigraphy of the Oligocene and Miocene of the Linxia Basin, northwestern China

Abstract

The Cenozoic biochronology framework for northern China has been based on few vertebrate fossil localities with unverified age constraints. The Linxia Basin, located on the northeastern margin of the Qinghai-Tibetan Plateau, northwestern China, preserves thick, successive terrestrial sequences with an enormous quantity of vertebrate fossils of ages spanning the Late Oligocene to Early Pleistocene. This record has bearing on the definition of Cenozoic mammalian biochronology of North China, and its better dating. However, our understanding of detailed mammalian evolution and infilling history of the Linxia Basin has been hampered by poor age constraints and disagreement with new paleontological findings. To clarify the age range of the fluvial-lacustrine deposits and their faunas in the lower part of the succession, we present here a revised magnetostratigraphy and its chronology obtained from three well-correlated and overlapping sections in the eastern Linxia Basin. Magnetite and hematite were identified as the main ChRM (characteristic remanent magnetization) carriers in the sediments. Reliable specimens from 1142 sampled layers representing a total thickness of 484.4 m from the three sections yielded 28 normal and 23 reversed polarities. With the aid of updated biochronological constraints, the fluvial-lacustrine sequence of five stratigraphic units (Tala, Jiaozigou, Shangzhuang, Dongxiang, and Liushu/Hujialiang formations), was successfully correlated to chrons of the GPTS (geomagnetic polarity timescale). These represent the top of Chron C13r (ca. 33.8 Ma) to lower Chron C5n.2n (ca. 10.8 Ma). The recalibrated basin-infilling history shows response to episodic growth of topographic highs during these time intervals. Moreover, this new age model yields an estimated age of ca. 27 Ma (within Chron C9n of early Late Oligocene) for the paracerathere fauna from the eastern Linxia Basin, which is in accordance with other biochronological findings. The new age estimate for the paracerathere fauna supports a proposal for relocating the basal boundary of the Chinese Tabenbulukian Land Mammal Stage/Age to Chron C9r.