黄河中游的地貌与地文期问题

The Landforms in the Middle Reaches of The Yellow River and Problem of Physiographic Stage

上起积石峡下至三门峡的黄河中游地区在晚新生代期间发育了一级夷平面、一级剥蚀面和七级河流阶地。夷平面形成于２．４Ｍａ以前的晚第三纪，剥蚀面形成于２．４～１．８ＭａＢ．Ｐ的第四纪初期；黄河的七级阶地分别是开始于１３００、１１００、７８０、５９０、１４０、５０、１０ｋａＢ．Ｐ的河流下切形成的，其原因在于构造的脉动式上升。基于上述研究成果，我们将每一地貌面形成的时期定义为一个地文期，它与其后因构造运动造成的地形切割或河流下切构成一个地文期旋回。在晚新生代黄河中游至少存在九个地文期旋回。

There are a planation surface, a denudation surface andseven terraces in the middle reaches of the Yellow River from the Jishi Gorge to the Shanmen Gorge. The planation surface deveioped during the Neogene before 2. 4 Ma B. P. according to the mammal fossils buried in the relative deposit and dating of overlying loess. The denudation surface was formed during the initial Quaternary from 2. 4 Ma B. P. to 1. 8 Ma B. P. on the basis of the ages of downlying stratum annd overlain loess. Based on the studies of loess magnetic stratigraphy and paleosol sequence on terraces, seven terraces of the Yellow River were formed because of stream downcuttings which respectively began at 1300, 1100, 780, 590,140, 50 and 10 ka B. P. from older to younger. The reason for stream downcuttings is pulsating tectonic uplifts. Based on the above researches, a physiographic stage is defined as a period in which a geomorphic surface was formed. This stage and its following short period in which stream downcutting or landforms destruction happened because of tectonic uplift compose a physiographic cycle. The end time of each physiographic stage has been given out according to age of geomorphic surface. There are at least nine physiographic cycles in the middle reaches of the Yellow River during the Late Cenozoic Era.