Cosmogenic Radionuclide Dating Of River Terrace In Hechuan Member Of Jialing River And Its Geomorphic Significance

The Jialing River is the second largest tributary of the Yangtze River after the Minjiang River.The development characteristics and evolution era of the river terraces of the Jialing River will help to further enhance the scientific understanding of the evolution process,tectonic movement,and climate change of the Yangtze River system.Due to the limitations of terrace field profiles and dating materials and methods,previous studies on the geomorphological evolution of the Jialing River basin mainly focused on the unique"free meander"development characteristics of the middle reaches.The chronology of related river terraces is relatively weak.In recent decades,cosmogenic nuclide dating methods have developed rapidly and have been widely used in the dating of river terraces.Taking the Hechuan area,the dividing point between the middle and lower reaches of the Jialing River basin,as the research area,this paper uses the cosmogenic nuclide dating method to carry out scientific exploration on the dating of the T3 and T4 terraces.Based on the field profile characteristics of the terraces,the cosmogenic nuclide 10Be exposure dating and²⁶Al/¹⁰Be burial dating of the T3 and T4 terraces in the Hechuan section of the Jialing River have been completed.Finally,based on the results of terrace dating and previous research,the development era of river terraces is analyzed,and the evolution process of watershed geomorphology is discussed.The main conclusions are as follows:（1）The gravel layer of the northern T4 terrace of the Nanping Bridge in the Hechuan section of the Jialing River was exposed using a cosmic origin nuclide ¹⁰Be depth profile for dating.The fitting exposure age period is 0.037-0.215 Ma,and the²⁶Al/¹⁰Be simple burial age is 0.2-0.5 Ma.The burial age of the isochron is 1.17±0.13 Ma.Comparing the basin terrace sequence with previous research results in adjacent areas,it is found that the isochronal burial age of T4 terrace measured this time is relatively old.It is speculated that the dating samples collected this time may have multiple burial histories,and the dating results are greatly affected by the sediments of higher terraces.Based on previous research results,it is believed that the formation age of the T4 terrace in the Hechuan section of the Jialing River is between0.3-0.49 Ma.（2）A simple burial age survey was conducted for Terrace T3,and the burial age of the gravel layer in the terrace was measured to be 0.22 Ma.Compared with previous OSL and ESR research results,it was found that the measured age was relatively old.Due to the fact that the samples collected in this cosmic genetic age survey were at the lower part of the gravel layer,the samples collected by previous generations were usually floodplain facies sandy sediments above the gravel layer,and the sample collection horizon was significantly lower,resulting in older measured age data,Therefore,the age value of T3 terrace measured in this paper is reliable.（3）From the perspective of geomorphological characteristics of the basin,the development level of terraces in the Jialing River basin may be affected by the connection of the Three Gorges.However,by comparing the uplift process of the Qinghai-Tibet Plateau and combining with the chronological analysis of the terraces,it is speculated that the formation dynamics of the terraces in the Jialing River basin may have come from the Kunming-Huangshan Movement and the Gonghe Movement.The river terraces in the Hechuan section of the Jialing River generally show a stable sedimentary environment,and the relationship between the development of the terraces and climate change is not obvious.