安宁系的时代、成因类型和环境

本文对我国新生代地层中颇具争议的安宁系进行了综合对比分析,确认安宁系位于中新统咸水河底砾岩之上,早更新世初黄河阶地砾石层之下,为中新世中期至上新世沉积。沉积构造、粒度和石英砂表面特征等一致表明,安宁系是与干热气候相应的风砂活动的产物。安宁系及与之同时并存的中国东部风成红土说明,第三纪晚期中国大陆已存在沙漠-风成红土的分异格局。推测第三纪晚期,兰州附近地区处于副热带北侧,西风带南缘,半干旱草原至干旱荒漠草原环境。

THE AGE,GENETIC TYPE AND ENVIRONMENT OF ANNING SYSTEM

On the basis of analysis and correlation of strata in Lanzhou area, it is regarded in thisarticle that the youngest stratum under the Arming System naffed by C. C. Young andM.N.Bian in 1936 is a layer of gray-white sandy conglomerate formed in the early MioceneEpoch, basis on the evidences of vertebrate paleontology found at some places such as thenorth slope of Gaolan Mountain, Hanjinzi, Xianshuihe etc.，while the oldest stratumabove the Arming 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 (8.P.) dated by themethod of gypsum fission trace. Arming System, therefore, was formed in the period ofMiddle Miocene-Pliocene. Arming System is fossil eolian sand face of desert in light of many features such as largelaminae, sandstone with calcareous cement, sand grainsize compared with desert sand,micro-texture of quartz sand granule including dish-shaped concavity, curved grouve,curved edge and silica deposit, etc. Th,e type of physial environment for the formation. of Anning System may be fromsemiarid steppe to arid desert steppe, and it mainly formed by the arid climate of LateTertiary according to the parameters of environment e.g. redish sand colour, calcareouscemented sandstone, calcareous nodule, and relating to other conclusions ofpaleogeography and paleoboany. There must be a paleo- anticyclone centre in the west of Lanzhou controlling wholenorth China inLate Tertiary according to:(1) the occurence of foreset laminae in ArmingSxstem are mainly from west to east (2) the wide spreading of eolian red earth of LateTertiary in the east of Lanzhou such as Baode and Jinle formations.