岷江上游叠溪古滑坡坝-堰塞湖研究进展

Recent research on the Diexi paleo-landslide:dam and lacustrine deposits upstream of the Minjiang River,Sichuan,China

四川岷江上游叠溪发育有一套厚度超过200 m、保存较为完整的湖相沉积,被定名为叠溪古堰塞湖相沉积,其形成于距今30 ka前,存活了约15 ka,因此记录了青藏高原东缘晚更新世-全新世(包括末次冰期)的重大地质与环境事件。现有研究初步揭示了古堰塞的沉积特征,但对叠溪古滑坡及古堰塞湖形成与演化的系统研究还十分不足。本文通过详细的野外调查,结合现代遥感测绘技术(无人机载LiDAR),构建叠溪古滑坡的三维地质模型,研究了其地质与地貌特征。同时,采用高密度电阻率法ERT,在滑坡体上布设2条长870 m和990 m的测线,探明了滑坡体内部结构特征。通过古堰塞湖相沉积露头和钻孔的调查,结合激光粒度测试,重建了古堰塞湖的范围、规模与沉积特征。在此基础上,通过对古湖相沉积坡面上多级阶地的分析,初步探讨了古堰塞湖的消亡及其对下游史前古聚落变迁的影响。研究结果表明,叠溪古滑坡不仅完全堵塞岷江而且还堵塞了对岸支沟,堆积体方量达到(1400~2000)×10^(6) m^(3)。古堰塞湖在滑坡坝后向上游延伸26 km,所形成的最大湖面覆盖面积约21.4 km 2,库容蓄水量约1670×10^(6) m^(3)。叠溪古滑坡-堰塞湖在岷江上游形成了陡峭的河道裂点(Knickpoint),对山区河道与地貌演化具有长期影响。

A paleo-lacustrine deposit,more than 200 m thick,named the“Diexi paleo-landslide-dammed lake deposit”was found upstream of the Minjiang River in Sichuan Province.The paleo-lake was formed around 30 ka BP and the dam started to fail around 15 ka BP.Therefore,the paleo-lacustrine deposit could have recorded important geological and geoenvironmental events(earthquakes,paleo-climate changes,etc.)during the period from the late Pleistocene to Holocene.The formation and evolution of the Diexi paleo-landslide and the landslide-dammed lake are poorly understood.Through detailed field investigation and LiDAR measurements,we constructed a 3D geological model and found the geomorphological and geological evidence to determine the boundary and volume of the paleo-landslide that formed the Diexi paleo-lake deposit.Using geophysical measurements(ERT)and laser grain size analysis,we determined the internal structure of the landslide deposit.We then reconstructed and analyzed the extent,volume,and sedimentological features of the paleo-lake based on a survey of outcrops and boreholes,and discussed the possible failure processes of the paleo-dam and its implications for the migration of prehistoric settlements.The results indicate that the Diexi paleo-landslide not only completely blocked the Minjiang River but also blocked its upstream branches,with the deposit volume reaching 1400×10^(6)-2000×10^(6) m^(3).The paleo-lake extended upstream to about 26 km behind the dam;the maximum surface area and volume of the lake was approximately 21.4 km^(2) and 1670×10^(6) m^(3),respectively.The paleo-landslide caused a knickpoint that may have a long-term impact on the mountain landscape evolution in the region.