The Preliminary Study On Geomorphic Feature And Development Model Of Vegetated Linear Dunes In Southeastern Gurbantunggut Desert

The common types of vegetation linear dunes(VLDs)have round ridges,high vegetation coverage and high stability,they are distributed in inland deserts with vegetation under unidirection/acute angle bidirectional wind.The morphological characteristics,distribution pattern,material structure and evolution model of VLDs have differences,it record the regional desert landforms evolution and modern geographical environment.Gurbantunggut Desert is the largest fixed/semi-fixed desert in China,VLDs account for more than 80%,it is an important ecological screen for the Silk Road,and it is great for regional ecological economy sustainable development by study the VLDs evolution and monitor the long-term desertification trend.In this paper,four areas(F2,F3,F4,F5)along the national highway 216 and the other four areas(F7,F8,F9,F10)in Fukang-Shanan are selected in southeastern desert,based on field observation and sampling,the regional wind regime,wind dynamic and dune activity is discussed through sand drift potential,grain size parameters and quartz grains surface texture,supplemented by remote sensing region division to expound the dune distribution pattern and cause,based on ground penetrating radar(GPR)data,combined with other results,the relationship between morphology and internal structure of VLDs are discussed,furthermore conclude its development model.Results show that:(1)The annual mean wind velocity ranges in 1.46-3.78 m/s in Gurbantunggut desert,and it decreases gradually from northwest to southeast.The average sand-driving wind velocity ranges in 6.0-9.96 m/s,distributed in NW and W directions,it mainly occurs in spring,summer and early autumn,winter rarely.The annual drift potential(DP)and resultant sand drift potential(RDP)ranges in 0.58-1088.36 VU and 0.33-1021.26 VU,the resultant sand drift direction(RDD)ranges in 88.17°-149.81°,and the wind direction variability ranges in 0.58-0.96,The wind energy in the northern and western regions are high,the southern region is low.Under the influence of regional differentiation in wind regime,dune movement direction is from northwest to southeast,VLDs cross-section changes from very asymmetric to relatively symmetric.(2)From the remote sensing image analysis of dune types and spatial variation,there are simple,straight crest with dendritic,straight crest with equal spacing and sinuous crest VLDs in study area.From desert center to the edge,with the increase of dune density and profile asymmetry in study area,the morphological parameters such as height,spacing and termini vary regularly.From the change of dune termini,VLDs are interconnected by means of scrub plants or nebkhas as intermediaries,which is related to regional near-surface wind-sand flow characteristics,vegetation coverage,sand source and others.(3)The VLDs are dominated by fine and medium sand,and the grain size are mainly 0-6φ,the frequency distribution curves show the typical aeolian characteristics.The dunes are inclined to the coarse-crest grain size distribution pattern,the mean grain size at the top of dune is coarser,changes synchronously with the sorting.Due to the influence of local wind,wind-sand flow in different parts and biological soil crust,the variation of grain size parameters become complicated,the grain size become finer in dune interior.Scanning electron microscope results show that the roundness of quartz grains is moderate,the micromorphology is mainly dish-shaped,crescent-shaped pits and other aeolian characteristics,there are residual flow and glacier effects,which reflect the diversity of sand source,the complexity of aeolian sand activity and dune development.(4)Based on GPR datas results,combined with field section observation,it reveals the shallow(3-5m)structure information,which can identify high-angle dipping,convex and wedge cross-bedding in the middle-upper part of dunes and crest,and sub-horizontal to low-angle dipping bedding in interdunes and the middle-lower part of dunes,which reflects the strong wind erosion and deposition in the upper part of dune,and the accretion in the middle-lower parts of dune wings and interdunes.From desert edge to the center,the shallow sedimentary structure assemblage of dune slope changes from asymmetrical to symmetrical,the cross-section morphology showed the same variation,but no evidence of lateral migration have been found.According to different scale dune structure sequence,the evolution model could be divided into four stages: shrub dunes,shadow shrub dunes,longitudinal dune chains and vegetation linear dunes.