ESR And Cosmogenic Nuclides ²⁶Al/¹⁰Be Burial Dating Of Terrace Sequence In The Sanyangchuan Basin Of The Weihe River

The geomorphological evolution of the drainage basin and the development of the river valley have always been the focus of traditional geomorphology.However,different subjects have different theoretical hypotheses and research objectives in view of the evolution history of even the same section.In view of the river terraces as ancient abandoned river channels,it is the direct evidence of the evolution of river systems and the development of river valleys.At the same time,the formation and development of terraces are closely related to regional structure and climate change.Detailed topography,sedimentary analysis and chronological study on them can restore the evolution history of fluvial geomorphology and the development process of river valley.There are 13 terraces are developed and preserved in the Sanyangchuan Basin,the Weihe River upstream.There are 10 terraces on Beishan,6 terraces on Nanshan and 3 fill terraces in the confluence area of the Hulu and Weihe rivers.The previous research is mainly based on partial terraces covering loess-paleosol stratigraphies?T3,T4 terraces on Beishan?and OSL?T2 on Beishan,T3 terrace on Nanshan,T2 terrace in the basin?,ESR of the gravel layer?T5 on Beishan?,T6 terrace,the T2 terrace on Nanshan)and the ¹⁴C of the organic matter in the floodplain?T3 and T1 terraces in the basin?.At the same time,relying on the upper and lower contact relationship of the geomorphic stratigraphy to infer and discuss the formation and development process of the terrace sequence in this area,it has great ambiguity.In this paper,the ESR and cosmogenic nuclides ²⁶Al/¹⁰Be burial dating methods are used to test and analyze the accumulation age of the gravel layers in the main terraces of the area,and compare with the previous estimation age to explore the development pattern of river terraces and reconstruct the processes of valley development.The main conclusion are as follows:?1?Dating the accumulation age of the gravel layer using ESR and cosmogenic nuclides ²⁶Al/¹⁰Be burial dating methods.The ESR age of the gravel layer in the T2terrace on Beishan is 683±102⁶42±30 ka;The ESR age of the T4 terrace gravel layer is 830±124⁸13±122 ka,and the cosmogenic nuclides ²⁶Al/¹⁰Be burial age is900±390 ka;The ESR age of T5 terrace is 876±131⁸52±128 ka;The ESR age of T6terrace is 1.01±0.15 Ma;The ESR age of T7 terrace is 0.98±0.15 Ma,and the cosmogenic nuclides ²⁶Al/¹⁰Be burial age is 0.97±0.32 Ma;The ESR age of T8terrace is 1.09±0.14¹.08±0.16 Ma,and the cosmogenic nuclides ²⁶Al/¹⁰Be burial age is 1.10±0.28¹.10±0.25 Ma;The ESR age of T9 terrace is 1.13±0.19¹.11±0.17 Ma;The ESR age of T10 terrace is 1.33±0.19¹.26±0.15 Ma,and the cosmogenic nuclides ²⁶Al/¹⁰Be burial age is 1.45±0.69¹.04±0.43 Ma,and the average age is 1.16±0.37 Ma.The ESR age of the gravel layer in the T2 terrace on Nanshan is 46±6.9⁴5±6.75 ka,the T4 terrace is 760±114⁷22±111 ka,for T5terrace is 791±119⁷54±127 ka,the age of T6 terrace is 901±135 ka.The accumulation age of the layer in this area roughly corresponds to the loess accumulation period,including L14?Beishan T10?,L13?Beishan T9?,L12?Beishan T8?,L11?Beishan T7?,L10?Beishan T6?,L9?Beishan T5 and Nanshan T6?,L8?Beishan T4 and Nanshan T5?,L7?Beishan T3 and Nanshan T4?,L6?Beishan T2?,L2?Nanshan T3?,L1?Nanshan T2?.Combined with the past estimates of the paleosol strata at the bottom of the loess on these terraces,corresponding to S13,S12,S11,S10,S9,S8,S7,S6,S5,S4,S1,Sm,respectively,consistent with previous inference,indicating the universality of geomorphology,there is no sedimentary discontinuity between the alluvium and the aeolian loess.?2?The chronological data of the gravel strata in the river terraces of the Sanyangchuan Basin also indicate that the sedimentary processes of the dual-phase structure mainly occur during the glacial period,that is,the rivers are dominated by lateral erosion during the glacial period,and the bottom of the loess above the floodplain are paleosols.A layer of paleosol indicates that the major episodes of river incision occurred during the transition from the glacial to the interglacial period,while in the interglacial period,combined with increasing rainfall,the appearance of dense vegetation cover would have reduced sediment supply,resulting in aggradation and valley widening within rivers.Since 1.2 Ma,13 terraces developed in this area,which correspond to almost 18 climatic cycles on the deep sea oxygen isotope curve and 15glacial-interglacial cycles recorded in the loess-paleosol layer,indicating that river geomorphology is very sensitive to the climate change response at orbital scale.However,not every climate cycle has a terrace record,such as the corresponding two-stage terraces of L4-S3 and L3-S2,which may be caused by the terrace formation but not preserved due to later erosion,or was not found on the loess overlay in the field.?3?Based on the research results of the step-like geomorphic surfaces on the eastern margin of the Qinghai-Tibet Plateau?planation surface,erosion surface,fluvial terrace?,it is found that there are a few valleys before 1.4¹.2 Ma,but the incision rate is only between 0.04⁰.22 m/ka,in the meantime,the regional topography is dominated by erosion and flattening;Since 1.2 Ma,the average incision rate has increased to 0.1⁰.32 m/ka,indicating the tectonic uplift was crucial for river incision and the formation and development of the modern valley in the early late Pleistocene.The asymmetry of the sequence of the Nanshan and Beishan terraces in the Sanyangchuan Basin indicates the westward extension of the tectonic depression,and the tectonic inversion occurs in the Sanyangchuan basin around 0.6 Ma.Based on the elevations and chronological data of the terraces in the Sanyangchuan Basin,the overall pattern of incision rate is?high-low-high?,and the period of fast incision rate roughly corresponds to the Kunhuang and Gonghe Movements.