The Distribution,Age And Its Geomorphological Significance Of Weihe River Terrace

Fluvial Geomorphology is one of the main objects of study in Geomorphology. It records the information of climate and tectonic change, and also the process that the river response to the external changes. River terrace, as the palaeo-floodplain, is a basic river landform that formed by the river bed abandoned, not only can be used to restore the developmental of fluvial geomorphology, but also the history of climate and tectonic changes where the river drianaged.Through detailed fieldwork on the Weihe River upstream basin-Zhongtan Basin, we find that the river terrace in this area is extremely well preserved and is a perfect carrier of regional climate change and tectonic activity.Terraces in this region are distributed in three different tectonic units:Huangjiaping section, south of the river; Xiejiaping-Xiaowan section, north of the river; and the Zhongtan basin. There are6terraces in Huangjiaping section,9terraces in Xiejiaping-Xiaowan section, and4terraces in the basin.The study area is located in the Loess Plateau. Terraces are capped by loess of different thickness, which is convenient for the dating. As most of the sections are accompanied by agriculture terraces, we dated only some of these terraces. Palaeomagnetic and climate stratigraphy analysis for the high terraces, OSL dating for the lower ones, and age-height analysis for others. We find that, there are13terraces of staircase since the Pleistocene, the lowest paleosol of the loess stratigraphy are S14, S13, S11, S10, S9, S8, S7, S6, S5, S4, S1, Sm, S0, so we speculate the terrace formation ages are respectively1.24Ma,1.19Ma,1.08Ma,1.03Ma,982ka,866ka,790ka,712ka,621ka,424ka,130ka,57ka,10ka. It suggests that the terrace formation is controlled by the41ka and100ka climate cycle and the incision is occured in the glacial-interglacial transition. The staircase records13of16times glacial-interglacial cycle sine1.24Ma. Terrace sequence and climatic transition is not a one-to-one correspondence. It indicates that climate change is not the only main controlling factors to the development of the terraces.We obtained the incision history of Weihe river during1.24-0.62Ma, the average incision rate is0.46m/ka. The extreme incision may indicate the response to the surface uplift was forced by the Kunhuang movement. The river incised61m since0.13Ma in the Huangjiaping section, the rate is0.47m/ka, this may imply the response to the uplift of North Qinling orogen.The asymmetry among the Zhongtan basin, Huangjiaping and Xiejiaping-Xiaowan section indicate that the west extension of Fenwei graden affected the Zhongtan basin in621k a B.P., formed a series of little faults in the north of this region, and the Zhongtan basin is formed. After this, the basin is continued subsiding and depositing.The gravels above Yujiaxia T2and Liujiaping T4are bad roundness and sorting, It is a non-normal aggradation after the terrace formation. The Xinyang T2recorded four times of paleo-floods. We concluded that these gravels indicate many times of damming around50ka.