中更新世晚期以来高阶地发育与中游黄河贯通

FU Jianli , ZHANG Ke , MA Zhanwu , et al.The terrace ( T5and T4 ) formation since the late Middle Pleistocene and its implication in the through cutting of the middle reach of Yellow River

黄河的形成演变,是综合反映东亚构造地貌和自然环境变迁的重大地质事件,中游是黄河演化的关键河段,也是研究意见分歧集中的河段。发育于晋陕峡谷北段的寺沟T5阶地代表了晋陕峡谷未沟通前的雏形黄河发育阶段,T4阶地代表了黄河的贯通形成。文中选择了自三门峡峡谷至河曲寺沟间不同地貌部位的5个T5阶地和1个T4阶地典型剖面进行了系统研究,通过对阶地上覆黄土野外地层划分、OSL和ESR年代测定及磁化率测试结果与黄土高原段家坡典型黄土地层的对比,确定T5阶地和T4阶地上覆黄土最底部地层分别为S2和S1古土壤层,由黄土地层年代序列确定两级阶地分别形成于250～200ka BP和130～80ka BP。根据阶地的分布和沉积特征,认为250kaBP前后的构造活动是T5阶地形成的主因,此时雏形黄河自三门峡溯源侵蚀到了河津地区,但未沟通河套盆地。形成于130～80ka BP期间的T4阶地发育,代表晋陕黄河北段与河套盆地得以沟通,现今黄河最终形成。

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.