Origin of the Gravel Layer and Evolution of the Qingyi River in Front of the Southern Longmen Shan Foreland Basin:Insights from the Burial Age of Cosmogenic Nuclides 26Al/10Be
Since the Miocene,strong thrust uplift has occurred in the Longmen Shan nappe tectonic belt,resul-ting in the formation of a steep topographic gradient zone.In the foreland region,ancient alluvial fans composed of gravels have been widely developed,primarily including the Mingshan-Qionglai(MQ)and Danling-Simeng(DS)gravel layers.These layers,deposited by the ancient Qingyi River,are ideal for exploring the river's evo-lution and the paleoenvironmental changes in this region.We conducted field investigation in the Longmen Shan Foreland region and performed cosmogenic nuclide 26Al/10Be dating analysis on the MQ and DS gravel layers.The analysis indicates that the burial age of the MQ gravel layer is 0.608,9±0.018,7 Ma,and the burial age of the DS gravel layer is 0.218,0±0.094,1 Ma.Therefore,we conclude that the MQ gravel layer formed in the late Early Pleistocene to early Middle Pleistocene(Q31-Q12),and the DS gravel layer formed in the Middle Pleistocene(Q22).Combing previous research results,the two gravel layers correspond to the δ180 curve of deep-sea ice core and the sequence curve of Chinese loess,indicating they formed during a warm interglacial period.Based on statistical analysis of gravel composition,size,orientation,and paleoflow directions in the re-gion,three major diversion events have been identified in the Qingyi River system.In the early stage,the Qingyi River flowed from Ya'an in a north-northeast direction towards Jianshan.With the uplift of the Xiongpo anticline and the continued uplift of the Mingshan Qionglai terraces,the Qingyi River,which initially flowed from the southwest-north to the northeast,was forced to cut through the Xiongpo anticline and divert southeast at Caoba.
cosmogenic nuclideMingshan Qionglai gravel bedDanling-Simeng gravel bedthe Qingyi river
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