Lithologic and rock magnetic evidence for the Mid-Miocene Climatic Optimum recorded in the sedimentary archive of the Xining Basin, NE Tibetan Plateau

Thick Cenozoic deposits in the Xining Basin provide great potential for understanding the tectonic and climatic evolution of the NE Tibetan Plateau. However, the Middle Miocene Climatic Optimum (MMCO) in this basin has not been reported up until now, due to the poor exposure of the uppermost strata covering this interval. In this study, a 194 m-deep core of flood-plain to channel-flood deposits was obtained from the Tashan flats above the Xiejia Section in the Xining Basin. Magnetostratigraphic dating of the core determines its age at ca. 18.5-13.5 Ma. Lithological and rock magnetic analyses demonstrate that the most prominent changes recorded in the present study occurred between similar to 17 and similar to 14 Ma. During this period, the sedimentary sequence mostly consists of gray-white or bluish-gray claystone and muddy siltstone, intercalated with some thin red claystone. The rock magnetic parameters show a large variability, with higher values in thin red claystone beds and lower values in gray-white or bluish-gray layers. These features demonstrate that, due to a warm and wet climate, extensive rivers had developed in the Mid-Miocene and that a reducing sedimentary environment prevailed. These "favorable" climatic conditions coincide with the MMCO as based on global deep-sea deposit records, indicating that this warm event exerted a major impact on regional climate change in the NE Tibetan Plateau during the Mid-Miocene. (C) 2015 Elsevier B.V. All rights reserved.