黄河晋陕峡谷1.1 Ma阶地的形成及其构造背景*

对黄河晋陕峡谷河曲、黑峪口、延水关和壶口等4个地区进行了详细的野外考察。河曲地区共发现3级河流阶地,更高的则为唐县期宽谷;唐县期宽谷的海拔高度约1000m(拔河高度150m),而3级河流阶地的拔河高度分别约为110m、80m和12m。黑峪口地区也存在唐县期宽谷,宽谷之下发育5级河流阶地,唐县期宽谷西高东低,海拔高度位于970m和940m之间,5级河流阶地的拔河高度分别约为130m,80m、50m、12m和4m。延水关地区共发现6级河流阶地,全部为第四纪期间形成,6级河流阶地的拔河高度分别为180m、130m、95m、50m、20m和4m。壶口地区共存在8级阶地,也全部为第四纪期间的阶地,阶地的拔河高度分别约为260m、210m、180m、120m、80m、60m、35m和15m。对壶口最高阶地进行了地层学研究,发现这一阶地上覆厚度约110m的黄土地层,黄土层的最底部为L13,古地磁研究结果和古土壤断代都指示了这一阶地的形成时间在距今1.1Ma左右。综合晋陕峡谷地区现有的研究结果认为,3.3Ma之前,鄂尔多斯地块内部构造极为稳定,发育了唐县期夷平面,古黄河在此夷平面上主要以侧蚀拓宽为主,下蚀极其微弱;3.3-1.1Ma,鄂尔多斯地块的构造稳定可能被打破,黄河小幅度下切;而1.1Ma以来,受鄂尔多斯地块快速抬升的影响,黄河发生剧烈下切,1.1 Ma阶地和晋陕峡谷的主体在这一时期形成。

Formation of the Yellow River terrace about 1.1Ma along the Shanxi-Shaanxi Gorge and its tectonic background

Detailed field investigation on river terraces was carried out in Hequ,Heiykou,Yanshuiguan,and Hukou along the Shanxi-Shaanxi Gorge of the Yellow River. Three terrace staircases and one strath were found in the Hequ area. The strath was formed during the Neogene(Tangxian Age)and hence is named Tangxian Strath,whereas the terraces were formed during the Quaternary based on the overlapped aeolian deposits. The altitude of the Tangxian Strath is about 1000m(150m above local modern Yellow River). Heights of three terraces above local modern Yellow River are 110m,80m and 12m. Development of five terrace staircases below the Tangxian Strath was also found in the Heiyukou area. The Tangxian Strath is higher on the western bank and lower on the eastern bank and has an altitude ranging from 940m to 970m. The heights of these five terraces above modern Yellow River are 130m,80m,50m,12m,and 4m. Six terrace staircases are present in the Yanshuiguan area. The heights of these terraces above local modern Yellow River are 180m,130m,95m,50m,20m,and 4m. Eight terrace staircases were counted in the Hukou area. The heights of river terraces in this area are 260m,210mm,180m,120m,80m,60m,35m,and 15m. In both Yanshuiguan and Hukou areas,all the terraces were formed during the Quaternary. Stratigraphic studies at the Hukou area indicate that the topmost terrace is overlain by a layer of loess which is about 110m in thickness. Oriented loess samples were also collected from the highest terrace in the Hukou area. Twelve paleosols were counted,and L13 is recognized in the lowest part of the section. Magnetostratigraphic analysis indicates that the terrace was formed around 1.1Ma B.P. Three stages of the Yellow River can be divided during the development of the river terraces and Tangxian Strath along the Shanxi-Shaanxi Gorge. Firstly,Ordos Block experienced a long stable phase before 3.3Ma B.P.,when the Tangxian Strath formed. Dominant processes of the Yellow River during this stage was lateral cutting with minimal downcutting. The second stage was during 3.3Ma B.P. to 1.1Ma B.P. The stable Ordos Block was probably disrupted,and the Yellow River experienced a slight downcutting. After 1.1Ma,the Yellow River suffered significant downcutting. The Yellow River terrace about 1.1 Ma and the Shanxi-Shaanxi Gorge were formed due to a quick uplift of the Ordos Block during this period.