THE AGE, GENETIC TYPE AND ENVIRONMENT OF ANNING SYSTEM
Li Xiaoze Yan Mancun Zhou Liping Wang Guiyong (Lanzhou institute of Desert Research, Chinese Academy of Sciences)
On the basis of analysis and correlation of strata in Lanzhou area, it is regarded in this article that the youngest stratum under the Aiming System named by C. C. Young and M.N.Bian in 1936 is a layer of gray-white sandy conglomerate formed in the early Miocene Epoch, basis on the evidences of vertebrate paleontology found at some places such as the north slope of Gaolan Mountain, Hanjinzi, Xianshuihe etc., while the oldest stratum above the Anning System is the sixth terrace gravel layer of Huanghe River (Yellow River) under the Wucheng loess layer at Jiuzhoutai, formed 1.488 ±0.11 Ma (B.P.) dated by the method of gypsum fission trace. Anning System, therefore, was formed in the period of Middle Miocene-Pliocene. Anning System is fossil eolian sand face of desert in light of many features such as large laminae, sandstone with calcareous cement, sand grainsize compared with desert sand, micro-texture of quartz sand granule including dish-shaped concavity, curved groove, curved edge and silica deposit, etc. The type of physial environment for the formation, of Anning System may be from semiarid steppe to arid desert steppe, and it mainly formed by the arid climate of Late Tertiary, according to the parameters of environment e.g. redish sand colour, calcareous cemented sandstone, calcareous nodule, and relating to other conclusions of paleogeography and paleoboany. There must be a paleo-anticyclone centre in the west of Lanzhou controlling whole north China in Late Tertiary according to : (1) the occurence of foreset laminae in Anning System are mainly from west to east; (2) the wide spreading of eolian red earth of Late Tertiary in the east of Lanzhou such as Baode and Jinle formations.