Magnetostratigraphy of a drilling core from the Baiyanghe alluvial fan at the western margin of the Junggar Basin, NW China and its paleoenvironmental significance

Although alluvial fans are widely distributed along the western margin of the Junggar basin, NW China, uncertainties in their formation age and associated environment need to be resolved. In a well-developed alluvial fan in the upper reaches of the Baiyang River, a 137 m-deep hydrogeological drilling core (ZK1) has been extracted. Paleomagnetic analysis shows that this section has been deposited since 8 Ma, during three distinct phases: (1) a lower segment of muddy facies formed during 8–5.6 Ma; (2) a middle segment of alternating muddy and sandy and gravel layers formed during 5.6–3.6 Ma, and (3) the upper layer of gray conglomerate formed from 3.6 Ma to present. The fan began to develop at ~5.6 Ma, perhaps driven by the Mediterranean Messinian salinity crisis originally. At 3.6 Ma, the deposition rate increased significantly and the climate further dried and cooled, likely due to the uplift of the Qinghai-Tibet Plateau and its far-field structural and environmental impacts. However, the direct causes of the two events recorded in this section are related to the rapid uplift of Wuerkashier Mt., and to regional and global climate cooling.