The Genesis Of The Upper Weihe River Terraces And Geomorphology Evolution

Fluvial geomorphology is an important hotspot in geoscience. As a major aspect of fluvial geomorphological evolution, river terrace is used to recover the history of fluvial geomorphological evolution and get related information about surface uplifting and climate change. In the last century, by studying river terraces, reseachers were in agreement with the genesis and evolution of Yellow River. However, there is no consistent opinion about the process of evolution of Weihe River, which is the largest tributary of Yellow River. Most studies about Weihe River focused on its middle and lower sections. The upper Weihe River is located at the southern Longxi basin and the northern flank of the Qinling Mountains. Influenced by the active tectonic movements during Tertiary and frequent climate changes during Quaternary, the forms of landscape in this area are various and surface was eroded strongly. Niaoshushan Mountain, from which Weihe River originates, divides Yaohe River, another tributary of Yellow River. This area drew attention of geomorphologists and was considered as a key region to uncover the genesis and evolution of Yellow River and paleo- Weihe River.We selected Zhongtan and Longxi as the study area in our work, and proposed the history of Weihe River and Yellow river and genesis of the terraces in study area. Based on palaeomagnetic data, optical stimulated luminescenece (OSL) detection, ¹⁴C dates and loess-paleosol sequence, the ages of seven terraces along upper Weihe River around Longxi and Zhongtan have been confirmed. The seven river terraces preserved along the upper Weihe River in Longxi basin were formed about 870 ka BP, 790 ka BP,620 ka BP,420 ka BP,130 ka BP,60 ka BP and 10ka BP; The seven river terraces preserved along the upper Weihe River in Zhongtan basin were formed about 1.5 Ma BP,1.1 Ma BP,870 ka BP,620 ka BP,410 ka BP,128 ka BP and 11 ka BP.By comparing the data about ages and elevations of those terraces in other geological and geomorphological researches, we got incision rates about different sections and tributaries of Weihe River. The incision rates of Weihe River and its tributary are as following (river name, incision rate): Weihe River around Longxi, 0.2 m/ka; Weihe River around Baoji, 0.99 m/ka; Luohe River, 0.11 m/ka; Jinghe River, 0.16 m/ka; Bahe River, 0.17 m/ka. Those data indicate that the incision rate of upper Weihe River is higher than its middle-lower, but lower than Mingjiang in the interior of Qinling Mountains. It was maybe caused by different tectonic movement, which was stronger in inner areas than at northern flanks of Qinling Mountains and at upper area than at lower area of Weihe River.The paleosol overlying all terraces around Longxi along Weihe River(for example, from T7 to T1, the paleosol was S8, S7, S5, S4, S1, Sm and S0, conrrespondingly.) indicate that climate change played an important role in terrace formation. So, tectonic uplifting and climate change may each have played variable roles, and from our study their effects cannot be clearly separated. In Longxi zone there was at least ten times of climate transition, but only seven terraces have been founded. One reason is these terraces destroyed because of river erosion and tectonic movement, another is relative to the degree of warm-moist of climate and climate instability.Generally the highest terrace has been considered as an important sign as river appearance. Based on the highest terrace along Weihe River, the results indicated that: at 1.5 Ma BP, Sanmen palaeolake began to contract, when Weihe River appeared in Longzhong basin, joining into paleolake around Tianshui. Then Weihe River cut through Baoji gorge at 1.4-1.2 Ma BP, and appeared in Longxi basin. At the same time, the water system of Yellow River also had great changes. Yellow River cut through Sanmen gorge and flowed toward to the east into China Sea, and Weihe River joined into Yellow River. Sanmen palaeolake disappeared. At 1.2 Ma BP, Luohe River,Bahe River and Jinghe River formed, which flowed to Weihe River, whichindicated that entire Weihe River had been built, and modern river system formed.