Magnetostratigraphy And Paleoenvironmental Evolution Of The Middle Eocene-early Miocene Deposits In The Lanzhou Basin, Northwest China

The continental aridification and environmental evolution of the Central Asiasince Eocene are generally attributed to the land-sea redistributions and the uplift ofthe Tibetan Plateau, but some studies argue that the environment of the Asia isprimarily dominated by global cooling. Precise age control of the geological andclimatic records is key to distinguish the effect of the regional tectonism and/or theglobal climate on the environmental evolution in Central Asia. Here we present arelatively high-resolution magnetostratigraphic dating of the Eocene–Early Miocenefluvio-lacustrine sediments from a 757-m thick Duitinggou section in the LanzhouBasin located at the northeastern margin of the Tibetan Plateau, to establish a morepresice magnetochronology than previous biochronology and magnetochronology.Based on the sedimentary environment, environmental magnetism and speciesanalysis of fossil mammals, we rebuild the environmental evolution in the LanzhouBasin from Eocene to Early Miocene. Along with other climatic records from the NETibetan Plateau, we further discuss the effect of regional tectonism and global coolingon Asian climate. The preliminary conclusions are listed below:1. Magnetostratigraphic results show that 21 pairs of normal and reversed polarityzones can be recognized from the DTG section. With constraints from biostratigraphy,palynostratigraphy and lithostratigraphy, the most plausible correlation of themagnetostratigraphic polarity sequence in the DTG section to the GPTS, spans fromthe C19 r chron to the C5 En chron, ranging from ca 42 to 18 Ma, i.e., MiddleEocene–Early Miocene. The age of the boundary between the Xiliugou Formation andYehucheng Formation is 42 Ma, Yehucheng Formation and Lower XianshuiheFormation 31 Ma, and Lower Xianshuihe Formation and Middle XianshuiheFormation 24 Ma. The magnetostratigraphic age model also provides precise ageconstrains for the associated five faunal assemblages: 19 Ma for the Duitinggou Fauna,21–20 Ma for the Zhangjiaping Fauna, 24–23 Ma for the Miaozuizi Fauna, 26 Ma forthe Xiagou Fauna and 31–29 Ma for the Nanpoping Fauna.2. The initial appearance of gypsum layers in the section at 42 Ma is interpreted asthe result of the shift from open freshwater lakes to the playa lakes linked to theearliest aridification in the Asian interior. The central and south uplift of the TibetPlateau and the retreat of the Paratethys Sea would played an important role in thearidification of Central Asian in the Middle Eocene. The disappearance of gypsumlayers at 34 Ma may result from decreasing evaporation linked to cooling. The ratio ofhematite against goethite and sodium against aluminum also decreases significantly at34 Ma, which indicates that the climate in the Lanzhou baisn should turn cold. This isconsistent with global cooling at Eocene–Oligocene climate transition(EOT),implying the effect of global climate on the Asian climate. The significant changes ofthe magnetic susceptibility anisotropy of the sediments at ca 29 Ma may implyincreasing tectonic strain related to the uplift of the Northeastern Tibetan Plateau.3. The faunal compositions suggest that the Oligocene–Early Miocenepaleoenvironment in the Lanzhou Basin was dominated by grasslands intervened by acomparatively brief period of woodlands dominance between ca 24 and 23 Ma aroundthe Oligocene–Miocene boundary.In short, the Lanzhou Baisn was warm and moist before 42 Ma. The aridificationat 42–40 Ma in the NE Tibetan Plateau is attributed to the uplift of the south andcentral Tibet Plateau and the retreat of the Paratethys Sea. Later, the environmentalevolution of the Lanzhou Basin is consistent with global cooling between 40 and 29 Ma, especially during the EOT. After the tectonic activity in the NE Tibetan Plateau at29 Ma, the climate in the Lanzhou Basin is inconsistent with global climate. This mayindicate that the NE Tibetan Plateau has reached certain altitude which can influencethe regional environment significantly.