河流对0.8MaB.P.环境突变事件的地貌响应研究

Geomorphic Response of the River to the Environmental Change Event at 0.8 Ma B.P.

以兰州盆地0.8 Ma B.P.阶地为例证,运用古地磁测年方法,通过收集相关文献,分析讨论了0.8 Ma B.P.阶地与0.8 Ma B.P.环境突变事件的联系。结果表明:①0.8 Ma B.P.环境突变事件主要表现在气候转型、构造运动等方面,具有群发性和全球性特点;②兰州盆地以及其他区域0.8 Ma B.P.阶地存在的证据,表明河流在0.8 Ma B.P.前后普遍发生过一次下切事件;③0.8 Ma B.P.阶地是河流对0.8 Ma B.P.环境突变事件的地貌响应,构造运动为提供了下切驱动力,而气候变化则控制了下切时间。

Based on a large number of research data on the environmental change events at 0.8 Ma B.P. and the 0.8 Ma B.P. terraces, this paper, which takes the Lanzhou Basin as an example, discusses the internal relationship between the 0.8 Ma B.P. terraces and the events of environmental change at 0.8 Ma B.P.. In the Lanzhou Basin, there are two river terraces, namely the Zaoshugou terrace and the Wuyishan terrace, which are selected respectively as the study terraces. For the Zaoshugou terrace, the altitude of gravel stratum is 80 m higher than the river level. The top of the gravel stratum is covered by at least 64 m eolian loess, and the paleosol S8 is at the bottom of the eolian loess. While for the Wuyishan terrace, the altitude of gravel stratum is 140 m higher than the river level. The top of the gravel stratum is overlain by about 100 m eolian loess, and the paleosol S8 is also at the bottom of the eolian loess. Through the paleomagnetic dating and loess-paleosol sequence matching, it is discovered that these two terraces have the same age, and both were developed at about 0.865 Ma B. P.. According to the analysis of sedimentary characteristics and the correlative literature of tectonic movement, these two terraces show significant attribute of the tectonic movement genesis. At the same time, they also have similar lithology that there is a paleosol developed on the top of fluvial silt. It shows that the Yellow River downcut during the transition from glacial to interglacial. Therefore, the 0.8 Ma B.P. terraces in the Lanzhou Basin were formed under the combination of tectonic movement and climate change. The main results show that： 1） With group-occurring and global characteristics, the environment change events at 0.8 Ma B.P. are mainly manifested in the climate transition, tectonic movement, loess expansion, biological evolution, astronomy meteorite and so on; 2） the Zaoshugou terrace and the Wuyishan terrace were formed at about 0.865 Ma B. P., which were developed under the combination of tectonic movement and climate change; 3） The universal existence of the 0.8 Ma B.P.P terraces in the Lanzhou Basin and other regions verifies that there was a large-scale river down cutting event which resulted in the formation of 0.8 Ma B.P. terraces; 4） The environment change events at 0.8 Ma B.P. are the direct cause of development of the 0.8 Ma B.P. terraces. The intensive tectonic movement provides the driving force for the development of river terraces, and the dramatic climate transition controls the time of river incision. Therefore, the 0.8 Ma B.P. terraces are the geomorphic response to the 0.8 Ma B.P. events of environment change.