Depositional Facies and Paleoenvironmental Record of the Late Tertiary Stratum of Lanzhou Area, Gansu

Key words: 
• Lanzhou area

Abstract: Lanzhou Area is located on the northwestern margin of the Loess Plateau and the northeastern margin of the Tibetan Plateau, which Tertiary stratum are specially significant for the research of the Tibetan Plateau uplifts and the development of the eolian sediment. With a thickness of 210 m, the Gaolan section belongs to the late Tertiary according to the magnetostratigraphy. 901 samples were obtained from the field in a sampling pace of 0.25 m and subsequently the grainsize, magnetic susceptibility and chroma experiments were accomplished in laboratory. The measured particle size data were separated by a mathematical method to gain the components of different origins as well as the ratio of each component. The analytical results of environmental proxies show that Gaolanshan section is eolian deposition with 15 layers of sand accumulated by rivers. The reconstruction results of paleoenvironment imply that the sedimentary stratum of the section had changed from fluvial deposits to eolian deposits at 7 Ma. That means the initial drying of the internal Asia continent begun at least 7 Ma ago, the conclusion of which is consistent with the main Loess Plateau. Lanzhou area was a relatively open plain and structurally stable with a wet and warm climatic condition in 6～5.2 Ma； in 5.2～3.5 Ma, the tectonic activities were frequent thus the septial river sand formed and the climate was dry cold; the lithosphere of Lanzhou area began to become relatively depressed at 3.5 Ma and Wuquan conglomerate is a result of the phase A of QinghaiXizang movement.