Research On The Extinction Process And Impact Of Diexi Ancient Barrier Lake

The Diexi area is located on the southeastern edge of the Qinghai-Tibet Plateau.It is severely affected by strong geological tectonic movements,and the terrain conditions of the mountain valleys lead to frequent occurrence of geohazards such as landslides,collapses,and debris flows.Furthermore,after the happening of landslides,it is also easy to dam the river and form the landslide dammed lakes,and the dam break may cause floods,which seriously threatens the lives and property safety of downstream people.A large number of ancient landslide deposits are distributed in the Diexi area,and the lake deposits with the most well-preservation and the largest thickness are currently developed,indicating that the ancient disasters in this area were extremely developed.In 1999,Mr.Wang Lansheng first discovered the ancient lake sediments in this area,and named it as the Diexi ancient dammed lake sediment,and the formation time was determined to be about 30,000 years ago by preliminary dating.However,the scale of the ancient barrier lake and its extinction,and how did the extinction affect the migration of the ancient Shu civilization in the downstream,these questions are still worthy of intensive study.Therefore,in this paper,by a large number of field investigations,according to the distribution of lacustrine sediment outcrops,grain size change,sedimentary characteristics and the location of landslide dam,the range of Diexi ancient dammed lake was determined,and the multi-level terraces existing in lacustrine sediment were investigated in detail to reconstruct the extinction process of the Diexi ancient dammed lake.In addition,through flood inversion and historical data review,combing with the geological and environmental data in the downstream of the Minjiang River,the possible impact of dam-break flood disasters on the migration of ancient Shu civilization in the lower reaches were discussed thoroughly in this paper.The main results are as follows:?1?Field survey results show that there are four types of lacustrine sediments exposed in the Diexi area:lacustrine sediments with pebbles,disturbed and deformed lacustrine sediments,the highest elevation lacustrine sediments and common lake sediments.The lacustrine sediments with pebbles are mainly distributed in the form of terraces in Tuanjie Village and Taiping Town.The height of the lacustrine sediments with the highest elevation is 2330m,which corresponds to the height of the largest lake surface of the Diexi ancient dammed lake.From the distribution point of view,the grain size of the lacustrine sediments from the reservoir head to the reservoir tail is gradually reduced and the sortability is improved.The sedimentary characteristics transition from many materials and disorder to a single material with rhythm,showing a typical riverside beach sediment feature.According to the topography and landform features and the spatial distribution of downstream dam-break deposits,the location of the Diexi ancient landslide dam was determined,and the maximum depth of the Diexi ancient dammed lake was 240m,and the maximum storage capacity was 1.67 x 10⁹ m³,the area of the largest lake is 2.14 x10⁷m²,and its scale ranks the sixth in the world-known largest landslide dammed lakes.?2?There are seven levels of terraces in Tuanjie Village.By analyzing the gravel size,gravel shape and gravel direction of gravels on the top of each terrace,it can be known that the gravel layer of this terrace is the near-middle source alluviation by the Minjiang River.There are three terraces in Taiping Town.According to the theory of sedimentary facies,it is known that the profile conforms to the typical debris flow deposition characteristics.Therefore,it can be determined that the pebbles on the top of the Taiping terrace belong to the debris flow accumulation body.Based on the identification of the sources of the gravel layers of two terraces and the ¹⁴C dating method to limit the time,it was determined that the ancient dammed lake in Diexi was extinct in the form of phase-breach about 15,000 years ago.Through the seven-level terraces developed in Tuanjie Village,it can be determined that the Diexi ancient dammed lake experienced at least seven stages of decay.During the entire extinction,the climate showed a general trend of alternating cold and hot-hot-cold.?3?By studying the hydraulic conditions of the gravel layers on the top of Tuanjie terraces,it is found that the outburst flood caused by the extinction of the Diexi ancient dammed lake is a mega-every flood.The Diexi ancient dammed lake was extinct at 3.4ka B.P,causing the outburst flood,which carried a large amount of lacustrine sediments.the grain size of the sediments was consistent with the grain size of the soil formed by3.4 ka B.P in the Jinsha site of the Twelve Bridges civilization.The organic matter content is slightly higher than that of the Jinsha site,which is consistent with the phenomenon that the outburst flood scours the vegetation on both sides of the river causing the accumulation of organic matter in the downstream,indicating that it is very likely that the outburst floods caused by the extinction of the Diexi ancient dammed lake inundated the Jinsha site.In addition,the fourth stage of the extinction of the Diexi ancient dammed lake coincides with the rise of Sanxingdui civilization,which also shows that the outburst flood may lead to the extinction of Baodun civilization,causing the migration of the ancient Shu people to develop the Sanxingdui civilization.The phase-breach of the Diexi ancient dammed lake and the rise of the Yingpanshan civilization,Sanxingdui civilization,and the Twelve Bridges civilization are all in a period of climate change,which indicates that the inflow of the Diexi ancient dammed lake may be sharply increased due to abnormal climate changes.The breach and the outburst flood further affected the migration of the ancient Shu civilization in the downstream area.By studying the geological information such as the provenance,microstructure and sedimentary structure of the clay in the Chengdu Plain,it can also be confirmed that the lake sediments carried by the outburst flood have a corresponding relationship with the Chengdu Plain where the ancient Shu civilization is located.The results of previous archaeological operations also illustrate the relationship between the extinction of ancient Shu civilizations and flood disasters.Based on the fore-mentioned information,it can be inferred that the outburst flood caused by the extinction of the Diexi ancient dammed lake in the upper reaches of the Minjiang River had a profound impact on the migration of the ancient Shu civilization.