Sedimentary Characteristics And Geological Significance Of Qingshuihe Basin Since Late Pleistocene

The northeastern margin of the Qinghai-Tibet Plateau arc tectonic belt is the tectonic transition zone between the developing Qinghai-Tibet Plateau and the collapsing North China Craton.It is also the newest and forming part of the Qinghai-Tibet Plateau.The uplift of the tectonic belt is closely related to the living environment of human beings since the Late Pleistocene.At present,the research mainly focuses on the geometry,kinematics and activity law of the active faults,while the research on the regional sedimentary characteristics is relatively weak.Qingshuihe basin is located in the northeastern margin of qinghai-tibet plateau the rear of the arc structure,adjacent to Liu Pan Shan orogenic belt,the late Pleistocene-holocene sedimentary sequence is complete,in fluvial silt,clay and alluvial fan gravel layer and eolian loess deposition is the main sedimentary characteristics,adjacent to complete records Liu Pan Shan structural belt in the middle uplift history of the latest.Guided by the basin-mountain coupling theory,this paper selects representative sections in the Qingshuihe Basin and systematically carries out the basic geological research on the combination of sedimentology,chronology and environment science.Based on several typical sections of Late Pleistocene and Holocene,combined with photoluminescence and ¹⁴C dating,the sedimentary sequence of Late Pleistocene and Holocene in Qingshuihe Basin supported by accurate dating data is established.Under the restriction of sedimentary cycle frame,the lithofacies palaeogeographic pattern of different evolutionary stages can be restored.On the basis of stratigraphic lithology analysis and paleoclimate index study of typical sections,the development and extinction process and extinction model of two paleo-lakes in the Qingshuihe Basin during the Late Pleistocene are established,and the coupling relationship between them and structure and climate is discussed.The study can provide reliable evidence of stratigraphy,chronology and environment for the sedimentary evolution history of the Late Pleistocene in Qingshuihe Basin.According to the study:(1)Qingshuihe basin is mainly composed of Salawusu Formation,Shuidonggou Formation and Holocene fluvio-lacustrine strata,and there are erosional unconformity contacts between Salawusu Formation and Shuidonggou Formation,and between Shuidonggou Formation and Holocene sediments;(2)The Salawusu Formation started at about 144?120 ka B.P.and ended at about50 ka B.P.,while the Shuidonggou Formation started at about 25?15 ka B.P.and the Qingshuihe Basin started at about 14?11 ka B.P.The depositional break of the two late Pleistocene unconformities occurred at?50-25 ka B.P.And?15-11 ka B.P.;(3)During the Salawusu period of the Late Pleistocene,the Qingshuihe Basin was dominated by alluvial fan,fan delta,fluvial facies and shore-shallow lacustrine facies.In the Shuidonggou stage of the Late Pleistocene,the sediments in the Qingshuihe basin are mainly composed of alluvial fan,fluvial facies,flood plain and shore-shallow lacustrine facies,and the provenance system is basically the same as that in the Salawusu stage.During the late Pleistocene to Holocene sedimentary period,the lacustrine basin completely withdrew from the Qingshuihe Basin after the early fluctuation,and the basin developed alluvial fan,flood plain,fluvial facies and swamp facies,and the provenance system was consistent with the Salawusu Formation and Shuidonggou Formation;(4)The paleoclimate evolution of the Qingshuihe Basin during the Late Pleistocene is basically the same as that of the global climate evolution.The paleoclimate evolution of the Qingshuihe Basin during 145?75 ka B.P.It was in a relatively warm and humid climate background,corresponding to the Mis5 phase of the deep-sea oxygen isotope curve,and in the 75?58 ka B.P.And 25?15 ka B.P.were in a relatively cold and arid climate background,corresponding to the deep-sea oxygen isotope curve Mis4 and Mis2 stages,respectively.From the end of the Late Pleistocene to the Holocene,the climate was relatively arid,which occurred in?14 ka B.P.And?7-6 ka B.P.in a relatively brief warm and humid climate background;(5)The frequent and significant tectonic uplift in the arc-structural belt on the northeastern margin of the Qinghai-Tibet Plateau since the Late Pleistocene resulted in a depositional gap(?50-25 ka B.P.)between Late Pleistocene Salawusu and Shuidonggou,and a depositional gap(?15-11 ka B.P.)between Late Pleistocene Shuidonggou and Holocene in the Qingshuihe Basin.The development and extinction of the Qingshuihe Basin were driven by the discontinuous sedimentation of the two periods.Late Pleistocene(?58-25 ka B.P.)tectonic uplift in the arc-shaped tectonic belt on the northeastern margin of the Qingshuihe Basin drove the disintegration of the paleo-lakes in the Late Pleistocene.At the end of the Late Pleistocene(?14-11 ka B.P.),the lacustrine sedimentary system in the Qingshuihe Basin disappeared completely and entered the stage of the evolution of the Holocene fluvial system.At the same time,the cold and arid climate background of the late Pleistocene(deep-sea oxygen isotope MIS2 stage)and the relatively arid environment during the Holocene are also important reasons for the lake basin contraction.There is a co-evolutionary relationship between structure and climate,which drives the development and extinction of ancient lakes in Qingshuihe Basin.