【作者】 刘小丰；

【导师】 潘保田；

【作者基本信息】 兰州大学 ， 自然地理学， 2007， 博士

【摘要】 河流地貌历来是地学界研究的重点和热点问题之一。阶地作为流域地貌的基本形态之一，通过对其研究不但可以恢复河流地貌的演化历史，而且可以提取与构造运动和气候变化相关的信息。近一个世纪以来，中外学者通过对河流阶地的研究，已经对黄河的形成演化取得了比较一致的认识。但是，作为黄河最大的一级支流—渭河，其形成演化的过程目前分歧较大，且多数研究主要集于中下游地区。渭河上游位于陇中盆地的南缘，西秦岭的北麓地带，新生代期间构造活动剧烈，气候变化频繁，从而造成了该区流域地貌类型多样，地表侵蚀切割强烈。其源头鸟鼠山以其低矮的分水岭与黄河的另一支流洮河相隔，该区倍受地貌学家的关注，被认为是解决古黄河与古渭河形成发育的关键地带。本文选择渭河上游陇西段和中滩段的河流阶地进行研究。通过古地磁、光释光、¹⁴C及黄土-古土壤序列等定年方法，研究发现渭河陇西段7级阶地的形成年代分别为860ka BP、790ka BP、620ka BP、410ka BP、130ka BP、60ka BP与11ka BP。渭河中滩段7级阶地的形成年代分别为1.5Ma BP、1.1Ma BP、870ka BP、620ka BP、410ka BP、128ka BP与11ka BP。同时，综合该流域其它地质地貌资料，根据阶地的形成年代和拔河高度等参数，得到了渭河不同河段和不同支流的下切速率。陇西地区渭河自860ka BP以来下切速率为0.2m／ka，宝鸡段为0.99m／ka；渭河的支流中，洛河下切速率为0.11m／ka，泾河为0.16m／ka，灞河为0.17m／ka。可以看出渭河上游地区河流的下切速率明显高于中下游，而与秦岭内部的岷江下切速率相比，则要低的多，可能是由于构造活动的不同导致了河流下切速率的差异，秦岭内部抬升要强于北麓地区，而渭河上游地区则要高于下游。渭河陇西段各级河流阶地河漫滩顶部都发育了一层古土壤，如T7-T1分别对应于古土壤S8、S7、S5、S4、S1、Sm与S0，表明气候变化在阶地形成过程中发挥着重要作用，河流下切可能发生在冰期向间冰期的转化阶段。因此，在陇中盆地，渭河阶地是在构造运动和气候变化共同作用下形成的。更新世以来，陇中盆地至少经历了10次气候转型事件，而渭河却只发育了7级阶地，可能由于阶地被后期的河流侧向侵蚀和构造运动所破坏，或与气候温暖湿润程度不够及气候变化的不稳定性有关。最高级河流阶地是河流开始出现的重要标志。通过渭河不同河段的最高级阶地的研究表明，距今1.5Ma BP左右，渭河在陇中盆地开始发育，注入天水古湖，此时渭河下游三门古湖面积急剧缩小。1.4-1.2Ma BP，渭河在陇西地区出现，渭河切穿宝鸡峡谷，渭河上下游贯通；这一时期黄河流域水系也发生了重大的调整，黄河切穿三门峡东流入海，三门古湖消失，渭河注入黄河共同入海。1.2Ma BP左右，洛河、灞河和泾河形成，丰富了渭河水系，现代渭河水系基本形成。更多还原

【Abstract】 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.更多还原