THE COMPREHENSIVE STUDY ON DEPOSITIONAL AGE OF JIOUZHOUTAI LOESS, LANZHOU
Chen Fahu Zhang Yutian Zhang Weixin Cab Jixiou (Geological Department, Lanzhou University)
The thickness of Jiouzhoutai loess section of Lanzhou is 318.2m which is the thickest loess section in China and in the world.The upsand loss and low-sand loess marker beds are respectively at 103.5 to 127.0 meters and at 45.0 to 52.5 meters high, which shows that the loess section was formed at 1.35 Ma ago and aeolian-loess began to deposit at about 140 Ma ago.The B / M boundary and Jaramiuo normal event are located at 135.0m and at 75.5-92.0m high, which means that the loess section should be formed at 1.30 Ma. Also there are thirty-four palaeosols among which the first eight layers of palaeosols are so called the first Wucheng palaeosol group formed at 1.48 Ma So the profile was formed at 1.40-150 Ma.The fission tract age of gypsum samples collected at profile bottom and at 5.0m high are respectively at 1.48Ma and 1.45 Ma. Therefore, the eaolianloess of Jiouzhoutai loess profile began to deposit at about 1.30 Ma and Alluvial-loess began to deposit at about 1.40-1.50 Ma.