岷江叠溪古堰塞湖的发现

Discovery of huge ancient dammed lake on upstream of Minjiang River in Sichuan, China

1933年8月25日,四川岷江上游叠溪发生了一次7.5级地震。在叠溪地震引发的较场大滑坡上覆盖有较古老的厚土层,国内学者对其成因有不同解释。作者在开展叠溪地震滑坡调研中,发现并确认这是一套古堰塞湖相沉积物,当时的堰塞坝处在原叠溪古镇至下游马老顶乡一带长达数千米的河段,因此定名为叠溪古堰塞湖。古堰塞湖相沉积物厚度最大可达200余米,为一套层理清晰的粉土-黏土层。由叠溪向上游约30km河段,沿江断续可见这套沉积物,且有明显的相变。14C测年,湖相沉积物底部形成时代距今22ka,顶部距今约10ka,据此判断叠溪古堰塞湖维持了12ka。此外作者在较场一带还发现了以湖相沉积物为"基座"的五级"阶地",这些"阶地"上保存完好的二元结构河流相沉积物。该文较系统地介绍了叠溪古堰塞湖的基本特征,初步论述了其中包含的地质环境信息。

On August 25th, 1933, an earthquake of Magnitude 7.5 occurred near Diexi County seated in the upstream of Minjiang River of Sichuan Province and induced a series of avalanches and landslides. Some of them blocked the river and formed many dammed lakes ( the local people called them Haizi). In this calamity, more than 4000 people losed there lives by the direct damage of avalanches, landslides and floods due to the bursting of natural dams. 70 years past, a lot of rare geological phenomena resulted from earthquake can be identified. Among those dammed lakes, the large Haizi and the small Haizi are the largest in the major drainage area of Minjiang River and well preserved. They have been the local famous beauty spots. During the investigation of landslides, a suite of dammed lacustrine sediments more than 200 m thick were discovered, which formed 20 000 years ago in the area of Diexi. The natural dam of the ancient dammed lake just started near Diexi, so the lake was named Diexi ancient dammed lake. The lacustrine deposits are composed of veined silty soil and silty clay with yallow-grey color. Five river terraces were descovered on the lacustrine deposits, which formed during the vanishing process of the dammed lake. So that the ancient lacustrine deposits and terraces acumulation have put down the historical records of the evolution of geological envirorment as well as the variations of ancient climaie during the past 20 000 years. This paper introduces the basic characteristics of the ancient dammed lake, and the evolution of geological enviroments and the variation of the ancient climate are discussed in brief.