在对四川岷江叠溪古堰塞湖已有研究的基础上,进一步讨论古堰塞湖形成与消亡过程对岷江下游和成都平原演化及古文化发展的影响。以笔者1999年在调查叠溪地震滑坡时发现的形成于30 ka B.P.(即晚更新世末次盛冰期)的巨厚湖相沉积物为基础,并将其命名为“叠溪古堰塞湖”,再联系到1933年8月25日发生的71/2级叠溪强烈地震,其诱发的众多崩滑灾害在岷江流域范围内形成了系列堰塞湖,溃决后洪水一直冲到成都平原,造成严重的灾害这一事件。通过取样测试分析和对比相关考古资料,结果表明叠溪古堰塞湖溃决始于15 ka B.P.;根据岷江河谷上发现的6级河流阶地,说明堰塞湖是经过多次溃决逐步消亡的,可划分为溃决瀑发阶段、溃决中期阶段和消亡阶段,叠溪下游还发现有同时期形成的不同规模的古堰塞湖。岷江上游的古堰塞湖,是古人从西北进入四川盆地的重要水陆通道和文化遗址的聚集地,其溃决消亡的演化过程直接影响了岷江的堵塞与改道,成为这一时期古文化地迁徙的重要因素。
Valuable geological and environmental information can be obtained from the 200 m thick lacustrine sediments in the Diexi lake(an ancient landslide-dammed lake) of the Minjiang River. The shaking table test method was employed to study the disturbance phenomena which occurred in the Diexi lake sediments. The results show that the disturbance phenomena were caused by liquefaction-induced flows in the unconsolidated lacustrine sediments, due to triggering by earthquakes. The deformations only occurred in unconsolidated sediment layers and not in consolidated layers. This means that a consolidated layer cannot be liquefied and disturbed again by an earthquake for a second time. The disturbance on one layer corresponds to only one earthquake. The temporal occurrence of earthquakes could be determined by disturbance layers generated at different ages. In total, 10 disturbed layers were found in the lacustrine sediments of the Diexi lake. The experiments showed that there were more than 10 earthquakes between 30 ka B.P. and 15 ka B.P. in the Diexi lake area based on the dating of the disturbed sediment layers.
In 1999, Diexi paleo-dammed lake(2349 m a.s.l.) was discovered around Diexi town along the Minjiang River in Sichuan province. Diexi is located where the eastern edge of the Tibetan Plateau and the Sichuan Basin meet. The dammed lake was formed during the Last Glacial Maximum of the Late Pleistocene(~30,000 years ago) and began to empty about 15,000 years ago. The lacustrine sediments(up to 240 m thick) preserve abundant paleoenvironment information. In this paper, a mass of oxygen isotopes and 14 C dating from drilled cores are analyzed and discussed. The δ18 O curve on the paleo climate from this section is comparable with the coeval paleo climatic curves of ice cores and karsts in China and others. Furthermore, the physical model testing has confirmed that the disturbed zones in the core are caused by strong earthquakes occurred at least 10 times, which implies strong crustal deformation, as an important driving force, affecting climate change. This study provides a new window to observe East Asian monsoon formation, paleoenvironmental evolution and the global climate change.
WANG Lan-shengWANG Xiao-qunSHEN Jun-huiYIN GuanCUI JieXU Xiang-ningZHANG Zhi-longWAN TaoWEN Li-wang
