Abstract:

The orangered unconsolidated sandstone which consists almost entirely of quartz is widely distributed from the east of Hongcheng to the west of Gaolan on the north side of the Yellow River in Lanzhou, Gansu Province. The research on unconsolidated sandstone not only has high economic value but also an extremely significance for the drought condition study in northwest China. For a long time, the genesis of this sandstone has attracted wide attention and dispute at home and abroad. By field observation of the sedimentary structure, indoor particle size analysis test and SEM scanning, the genesis of this sandstone were discussed in aspects of the sedimentary characteristics，the grainsize characteristics and the surface characteristics of the quartz in this  sandstone. The unconsolidated sandstone widely distributed from Shuifu to Caojiawan has the following characteristics of underwater sedimentary: the horizontal bedding is evident in the sedimentary structures; the change of cumulative grainsize distribution curves consists with the neptunian theory; the conchoidal fracture and scaly peeling caused by erosion on the quartz surface are developed. The unconsolidated sandstone distributed from Zhujiajing to Hongcheng has the following aeolian features: the tabular oblique bedding is evident in the sedimentary structures; the change of cumulative grainsize distribution curves meets the aeolian theory; the surface of the quartz in the unconsolidated sandstone possesses pockmarked face, butterfly pit, and so forth. These results indicate that the genesis of this sandstone is very complicated and is regionally different; it cannot be simply analyzed by aeolian theory or neptunian theory.

Key words:  unconsolidated sandstone；genesis；aeolian theory；neptunian theory；sedimentary environment, Lanzhou, Gansu