金沙江中游奉科湖相层ESR年代学研究及成因探讨

ESR chronology of lacustrine sediment in the Fengke reaches of the middle Jinsha River:implications for landslide dammed lake formation

不同流域大量发育的沉积物不仅记录了晚新生代的气候信息,也为地貌演化等研究提供了重要依据。我国著名的第四纪地质单元——昔格达组,在青藏高原东南缘地区的大河流域中广泛发育,如攀枝花地区金沙江湖相地层。因此,对该地区湖相地层的沉积特征、形成时代及其成因机制的研究具有重要意义,但其形成时代至今仍存在较大争议。昔格达组颜色上以黄色、灰白色为主,厚度介于几十米到几百米,主要由黏土、粉沙、细沙等细粒组分组成,发育水平层理多数呈半成岩状态;该套地层的分布受现今河流的严格控制,呈孤立状态分布于金沙江及其支流的河谷中。本文在金沙江中游奉科地区发现出露典型湖相地层,厚约20 m,在奉科剖面采集5个样品,开展ESR年代学研究,结果表明:该套湖相地层年龄为440—556 ka,属于中更新世沉积。野外地质地貌考察结果显示:该套湖相地层直接超覆于基岩之上,且在上覆湖相地层中未发现砾石层和沙层等典型的河流相二元结构,结合ESR测年结果,初步判断奉科湖相地层为大型滑坡堵塞金沙江形成的堰塞湖沉积。

Background,aim,and scope Sediments developed by different basins not only record the climate information in the Late Cenozoic but also provide an important basis for researches including geomorphic evolution.As a famous Quaternary geological unit in China,Xigeda Formation,like lacustrine strata of Jinsha River in Panzhihua area,develops extensively in the large river basins situated in southeastern Tibetan Plateau.So it is of great significance to do research on sedimentary characteristics,formation age and formation mechanism of lacustrine strata there.This article takes the Fengke lacustrine facies strata in the middle reaches of the Jinsha River as the research object.Materials and methods In the middle Jinsha River,lacustrine strata of Fengke are about 20 m thick,from which the author collected 5 samples to do chronological research by ESR.First,the samples were processed chemically to remove carbonate and organic matter,then 11-50 μm quartz components were selected from the sample of purification,for the quartz Ti-Li center ESR dating experiment.Results Take the average of the ESR age of the 5 samples as the age of the lacustrine formation,the result is 440-556 ka.Discussion Previous studies have used different dating methods to measure different sections of Xigeda Formation,and the dating results are quite different.In this paper,the dating results of Fengke lake facies strata are also different from 4.2-1.78 Ma of paleomagnetic dating results.In the southeast edge of Tibetan Plateau in China,the phenomenon of bank slope instability and landslide siltation is very common.The area is one of the world’s most common landslides and blockages caused by the formation of dammed lakes in the river as well as loose sediment formed by heavy rainfall and frequent earthquakes.Most quake lakes collapse and disappear within a year.However,under the influence of special factors,some ancient barrier lakes can exist for thousands or even tens of thousands of years.The lacustrine sedimentary strata in the Fengke area are composed of grayish-yellow silty clay and clay,which generally develop horizontal bedding and have obvious sedimentary characteristics of lacustrine strata.In the field,lacustrine sediments can be seen overlapping with bedrock,and the sediments are continuous and uninterrupted,without sand and pebbles and other typical fluvial facies binary structures.Meanwhile,there is no tributary development of the Jinsha River in the Fengke area.On the plane,the lacustrine sediments are distributed along the mainstream valley of Jinsha River and distributed on both banks of Jinsha River in a long strip.The distribution characteristics are consistent with the characteristics of barrier lake sediments formed by the landslide blocking of non-tributary rivers.Landslides occur frequently in the Sichuan-Yunnan region.The different ages of each section may be due to the formation of lacustrine sediments in each section caused by different landslide events in different periods.Conclusions The age of the lacustrine layer in Fengke area is 440-556 ka.Preliminarily considers the lacustrine strata of Fengke as the sediments of dammed lake which are formed as a result from large landslides blocking the Jinsha River.Recommendations and perspectives It is suggested that other dating methods be used to study the Fengke lake facies strata for chronological correlation.