The characteristics of rock, grain-size, sporo-pollen and depositional model in the oldest loess section in Lanzhou

The oldest loess deposits in Lanzhou area,formed 1.84 Ma.B P ago,was recognized for the first time in the Shanchengwanling Section,Digou village,Yongdeng county,North of Lanzhou.It has eighteen integral sequences of loessp aleosol.The average grain-sizes of loess are 23.57 μm,and of paleosol is 19.10 μm.The loess is composed of siltstone and 90% grass-flower flours in sporo-pollen a ssemblage.On the basis of field survey and indoor systermatic study,the authors summarized the genesis of loess and depositional model,determined the location of up and down siltstone layers through stratigraphic correlation.