Episodic Sedimentary Evolution And Factors Of An Alluvial Fan(Huangshui Catchment,NE Tibetan Plateau)

Alluvial-fan successions record changes in hydrological processes and environments that may reflect tectonic activity,climate conditions and changes,intrinsic geomorphic changes,or combinations of these factors.This paper focuses on the sedimentary characteristics,evolution and controlling factors of a stream-dominated fan in the Ping'an depression,a tectonic depression of the Xining basin of China,laid down under a semi-arid climate in the northeastern Tibetan Plateau(NETP).In this study,a detailed description of the sedimentary characteristics of the section,including sediment composition,sedimentary structures and spatial distributions,was carried out in the field.In addition,147 powder samples and 15 samples were collected for grain-size analysis and optically stimulated luminescence(OSL),respectively.The main results are as follows:(1)The fan succession is about 22 m thick,which is composed of three facies associations from bottom to top,including six sedimentary units:(a)matrix to clast-supported,sub-rounded-rounded,poorly sorted,planar cross-stratified to crudely stratified sheets of coarse-grained sediments;(b)horizontal laminated sand extending for more than one hundred and fifty meters,covered by laminated layers of reddish fine silt and light yellow coarse silt or silty sand with stacked mounds of sand;and(c)clay-rich deposits with incipient paleosols and massive finer silt.The succession shows rapid sediment aggradation from high-energy to low-energy alluvial fans and finally to a floodplain.(2)The dating results using optically stimulated luminescence(OSL)method(K-feldspar pIRIR290)show the fan succession developed?168 ka-124 ka.Based on the dating results,a gravelly,high-energy fan was deposited during MIS 6,after which a low-energy fan,mainly composed of sand and silt,was deposited and finally covered by flood loam during the MIS 6-5 transition and the warmer last interglacial.(3)Stacked sand mounds with arching,symmetrical shape with dome beddings,as a special sedimentary structure developed in the lower-energy fan deposits.The aspect ratio(width/height)of a single mound or stacked mounds is positive correlation with the average of 7.5,which indicates that the development of the sand mounds was induced by standing vegetation.In addition,the grain-size distributions of all samples from different mounds and from sediments at different locations in a single mound are almost similar,and the sediments contain significant amounts of well-sorted very fine sand and coarse silt with modal values of 56-79 ?m.The low modal values,narrow grain-size range and the planar base of the mounds support accumulation by aeolian processes.Therefore,the wind reworked the exposed alluvial material transported onto the fan,forming the stacked mounds under the effect of shrub-coppice,which reveals the interaction of alluvial and aeolian processes in a semi-arid environment.(4)The stream-dominated alluvial fan in the Ping'an depression may be a response to a neotectonics uplift along faults at the boundary of Xiao gorge area.When Xiaoxia uplifted,the river cut down rapidly and then formed the gorge.In addition,accumulation both occurs in the upstream and downstream of Xiao gorge,simultaneously forming the accumulation terrace in the upstream and the alluvial fan in the downstream.However,the sedimentary characteristics and evolution of the alluvial-fan succession is inferred to represent intrinsic fan evolution and also related to the semi-arid climate.