Cenozoic magnetostratigraphy of the Xining Basin, NE Tibetan Plateau, and its constraints on paleontological, sedimentological and tectonomorphological evolution

Abstract

The Xining Basin is well known for its thick and continuous sequence of fine-grained Cenozoic sediments. In addition, it contains important Xiejia fauna which define the Xiejian Stage of the early Miocene in the standard Chinese land mammal zonation, and it also provides detailed histories of eolian dust deposition which document the aridification of Asia, monsoon evolution and the initiation of the Yellow River in relation to the uplift of the Tibetan Plateau. However, the results of magnetostratigraphic dating of the fossil-bearing sequence has yielded conflicting ages for the fauna and stratigraphy, hindering the use of the sequence for addressing major paleoenvironmental and paleoclimatic issues. Here, we review the paleontological, lithological and magnetostratigraphic record of the Xining Basin and provide a new, long and continuous high-resolution magnetostratigraphy from the basin center. The results show that the observed magnetic polarity zones from the various sections at different sites in the basin exhibit similar magnetic polarity patterns which can be readily correlated, in terms of both magnetic polarity zonation and lithofacies; in addition, they show that the sedimentary sequence of the basin was sub-continuously deposited from ~54 Ma to 4.8 Ma. The magnetostratigraphic correlations confirm previous constraint of the Xiejia fauna in the late Oligocene at ~25 Ma, challenging the current Chinese mammal land zonation for the early Miocene, and we suggest that it provides a superior record of eolian dust deposition and river incision in the late Pliocene than previous studies. There is a close match of the climatic proxy records of the Xiejia section, dated by magnetostratigraphy, with the global climatic record which corroborates the paleomagnetic correlations. The evolution of the sedimentary environment provided by the lithofacies demonstrates a complete cycle of basin formation and termination, which records the eastern Qilian Shan experienced three main phases of uplifts: slow episodic uplifts at ~54 Ma and 22.5 Ma and a late rapid episodic uplifts at 8–7 Ma and since 4.8–3.6 Ma. This tectonic uplift in the NE Tibetan Plateau was mostly nearly synchronous responses to the initial and continuing collision of India and Asia since ~50 ± 5 Ma.