Cosmogenic Nuclides Isochronal Burial Dating And Geomorphological Significance Of The Haiziping Xigeda Formation In Luding Section Of Dadu River

The Xigeda Formation is a famous Late Cenozoic stratum in the southwest of China,mainly composed of gray-green,gray-yellow river-lake facies clay,silt,and silty clay,with inter-layer structure as the prominent feature.It is widely distributed in the river valleys of the trunk stream and tributaries in the southeast of Tibet Plateau and roughly intermittently distributed in a north-south direction.This set of strata was first discovered by Chang Longqing during the geological survey in the area of Xichang,and was named "Hundan strata".Later,because the stratum was more typical in the Xigeda Basin,its name was changed to the Xigeda Formation.The study of the Xigeda Formation is the key to the development of the water system pattern in the southeastern margin of the Tibet Plateau,and it also has an important indication for the uplift process in this area,thus,the chronology of the Xigeda Formation and its formation mechanism have attracted wide attention for many years.However,due to the lack of paleontological fossil,strata exposure and the constraint of dating methods,the age of the Xigeda Formation at different locations can span from 4.2 Ma to 0.2 Ma.Different dating methods for the Xigeda Formation at the same location also yield different dating data.There are many viewpoints on the formation mechanism of the Xigeda Formation,including the genesis of glacial lake,rift basins and dammed lakes.Due to the uncertainty of the sedimentary age and formation mechanism of the Xigeda Formation,the connotation of the Xigeda Formation is becoming increasingly unclear,and different scholars have different interpretations of its geological and geomorphic significance.In this paper,the Haizhiping Xigeda Formation in the Luding section of Dadu River is taken as the research object.Based on field investigation,sampling and laboratory experiments,the sedimentary age of the Haizhiping Xigeda Formation is constrained by the cosmogenic nuclide burial ages of the top gravel layer and the bottom sand-gravel layer.Based on the analysis of sedimentary facies,provenance and geological conditions,the genetic mechanism of the Xigeda Formation in Haizhiping is identified.Based on the study of the age and genetic mechanism of the Haiziping Xigeda Formation in Luding,the geomorphological significance of the Xigeda Formation there is analyzed.The main conclusions are as follows:(1)According to the burial dating of cosmogenic isochronous radionuclides of the Xiaohaizi sand-gravel layer in the top of the Xigeda Formation and the Ganhaizi gravel layer at the bottom of the Xigea formation,the burial age of the Xiaohaizi sand-gravel layer is 1.04±0.15 Ma,and the burial age of the Ganhaizi gravel layer is 0.53±0.07 Ma.Therefore,the sedimentary age of the Xigeda Formation in Dadu River is limited to the range of 1.04-0.53 Ma.The measured polar column of the geomagnetic field in the Haiziping Xigeda Formation is reinterpreted and compared with the standard polar column.The bottom of the measured polar column can correspond to the galamillo normal polarity subchron at the top of the Matsuyama reversed polarity chron,and the top of the measured polar column can correspond to the lower part of the Brunhes normal polarity chron.(2)The Xiaohaizi sand-gravel layer at the bottom of the Xigeda Formation is fluvial accumulation,which indicates that the ancient Dadu River existed before the deposition of the Xigeda Formation.The Wanwantou breccia layer at the bottom of the Xigeda Formation has poor roundness and sorting.The rock composition is single and the source is nearby bedrock,indicating that the breccia layer is a nearby landslide.The lacustrine sedimentation of the Haiziping section changes abruptly from coarse-grained material to fine-grained,which is not consistent with the sedimentary characteristics of faulted basins,but conformed to the sedimentary characteristics of dammed lakes.Combined with the distribution of faults and the earthquakes and landslides in this region,the paper believes that the Haiziping Xigeda Formation was caused by the dammed lake.(3)Utilizing the burial time of the Xiaohaizi sand-gravel layer and the Ganhaizi gravel layer,the long-term incision rate of the Dadu River Luding section since 1 Ma is about 480 m/Ma and the incision rate since 0.53 Ma is constrained to be less than 940 m/Ma.Compared with the rivers in the southeast of the Tibet Plateau,it shows that the river incision rate in the region is basically consistent,and the structural uplift in the southeast of the plateau is characterized by integrity.Based on the incision rate obtained in this study since 1 Ma,it is estimated that the formation time of the main surface in the Dadu River Luding section was no later than 6.7 Ma and the formation time of the ablated surface was no later than 4.6 Ma.(4)Based on the Haiziping Xigeda Formation and the seven terraces in Luding section,the sedimentary and geomorphic evolution process of the Luding section of Dadu River is reconstructed.It can be roughly divided into five stages: river incising,landsliding,deposition of the Xigeda Formation,the development of terraces based on the Xigeda Formation,and the development of terraces based on the bedrock.The formation of the Xigeda Formation in the Luding Haiziping of the Dadu River indicates that there was a high long-term dam downstream,which hindered the trace erosion of the river and became the local erosion reference level of the upstream lake,slowing down the incision of the rivers upstream.