The terrace (T5 and T4) formation since the late Middle Pleistocene and its implication in the through cutting of the middle reach of Yellow River

The formation of the Yellow River was one of the important geological events, which comprehensively reflects the East Asian neotectonic movement and natural environmental changes. The middle reach of the Yellow River is a key to the study of the Yellow River Evolution. In the Xiaosigou located in the northern section of the Shanxi-Shaanxi Gorge, the T5 terrace represents the prototype of the Yellow River terrace before the communication of the gorge and the T4 terrace represents the run-through of the Yellow River. In this research, we choose the typical profiles of five T5 terraces and one T4 terrance from the Sanmenxia Gorge to Xiaosigou for a systematic study. Comparing the analysis results of the overlying loess on the terraces with those of Duanjiapo typical loess on the Loess Plateau by field stratigraphy, OSL and ESR dating, and magnetic susceptibility, we make sure that the most old loess layer on the T5 and T4 terraces is S2 and S1 paleosol, respectively. From the results of loess stratigraphic sequences, it is confirmed that the two terraces were formed at 250-200 ka and 130-80 ka, respectively. From the distribution and deposition characteristics of the terraces, it is found that the tectonic activities at 250-200 ka were the main causes of the terraces of T5, when the Miniature Yellow River eroded headward from to Sanmenxia to Xiaosigou in Hejin region; while the north of Yellow River in Jin-Shaan region was formed at 130-80 ka when the Miniature Yellow cut through the Hetao Basin, resulting in the eventual formation of the current Yellow River.