Late Oligocene-Early Miocene magnetochronology of the mammalian faunas in the Lanzhou Basin-environmental changes in the NE margin of the Tibetan Plateau.

Hong Ao,Zhisheng An,Peng Zhang,Mark J. Dekkers,Yongxiang Li

doi:10.1038/srep38023

Abstract

A large number of terrestrial mammalian fossils were reported in the well-exposed Paleogene and Neogene fluvio-lacustrine strata in Western China. Their accurate ages are crucial to understand the mammalian and environmental evolution associated with the step-wise uplift of the Tibetan Plateau. At present their ages are surprisingly poorly constrained. Here, we present a high-resolution magnetostratigraphic dating of the Late Oligocene–Early Miocene mammal assemblages from a 233-m thick fluvio-lacustrine section in the Lanzhou Basin located at the northeastern margin of the Tibetan Plateau, China. The results suggest that the section spans from the polarity subchron C6Cn.2r to C5En, i.e. ranging from ca 23 to 18 Ma. This magnetochronology provides considerably more robust ages for three associated land mammalian faunal assemblages. Updated ages end the debate on existing ambiguous and distinctly different magnetostratigraphic correlations for those Late Oligocene–Early Miocene assemblages. The new ages now enable precise correlation of these faunas to the European Land Mammal and North American Land Mammal Ages. The faunal assemblages further suggest a mixed setting of woodlands and grasslands associated with a humid environment in the Lanzhou Basin during the Late Oligocene–Early Miocene, in contrast to its modern poor vegetation cover and arid environment.

Highlights

The Cenozoic continental environmental and mammal evolution of North America, Europe, and Africa have a much higher dating resolution and are better known than that of Asia
Many sites rich in mammalian faunas of Oligocene–Miocene age were collected from the fluvio-lacustrine sequences in the Lanzhou Basin located at the northeastern margin of the Tibetan Plateau in Western China (Fig. 1); importantly a basic biochronology has been established[2,21,22,23]
A total of 81 samples did not yield reliable characteristic remanent magnetization (ChRM) directions because they show unstable demagnetization trajectories at higher temperatures, or maximum angular deviation (MAD) values ≥10° and 33 samples are rejected for the declinations trending roughly north but with upward inclinations inconsistent with the expected geomagnetic field

Summary

Introduction

The Cenozoic continental environmental and mammal evolution of North America, Europe, and Africa have a much higher dating resolution and are better known than that of Asia. Many sites rich in mammalian faunas of Oligocene–Miocene age were collected from the fluvio-lacustrine sequences in the Lanzhou Basin located at the northeastern margin of the Tibetan Plateau in Western China (Fig. 1); importantly a basic biochronology has been established[2,21,22,23] Because these Cenozoic fluvio-lacustrine sediments are devoid of suitable material for radiometric dating, magnetostratigraphy has been used to numerically date these faunas[2,17,24]. The recent magnetostratigraphic dating of the Late Miocene Xingjiawan Fauna in the northwest Lanzhou Basin has provided an age model for the upper part of the fossiliferous Xianshuihe Formation[17] With these new results in mind, here we report on a high-resolution magnetostratigraphic study of the fossiliferous DTG section, aiming to provide precise ages for the associated mammalian faunas and to close the current debate on their ambiguous and unclear ages.

Methods

Results

Conclusion