Abstract:

The Qaidam Basin is the largest basin on the northeastern Tibetan Plateau, where the Cenozoic sediments attain thicknesses up to over 10 km. These sediments record the growth process of the plateau and the paleoclimatic changes, and thus attract wide attention. Previous studies focused on describing these strata and qualitatively determining their sedimentary facies, with few quantitative studies. In this study, to quantitatively determine the sedimentary facies, grain-size analysis has been conducted on typical sandstone samples from the Cenozoic Xiaganchaigou to Shizigou formations in the Dahonggou section of the Qaidam Basin. Various methods, such as Sahu discriminant, grain size frequency distribution, end-member component analysis, and cumulative frequency distribution, have been employed to reveal the transport medium, transport mode, and sedimentary environment of the Cenozoic sediments in this section. The results show that the sediments of the Xiaganchaigou to Shangyoushashan formations were deposited mainly in an off-shore, shallow lake setting, with the hydrodynamic conditions successively weakening from the Xiaganchaigou, Shangyoushashan, Shangganchaigou, to Xiayoushashan formations. In these formations, the transported sediments include the suspended lake background component, the suspended turbidity current component, the jumping fine sand component, and rolling coarse sand component, with the last three coming from subaqueous distributary channels. In contrast, the sediments of the Shizigou Formation were mainly deposited in an on-shore, fluvial setting, containing transported sediments of suspended river background, and alluvial suspended turbidity current, jumping fine sand and rolling coarse sand components. These results provide quantitative constraints for determining the sedimentary facies of the Cenozoic sediments of the Dahonggou section of the Qaidam Basin. In a broader perspective, this study provides an analytical procedure for quantitatively determining the sedimentary facies using the grain-size results. 

Key words: Qaidam Basin, Cenozoic sediments, grain-size analysis, sedimentary facies