Grain-size Characteristics Of Aeolian Deposits In The Mu Us Desert

The Mu Us Desert as one of the four largest Desert in China,the region has a fragile ecological environment and numerous types of sand dunes,which is a hot area of arid and semiarid geomorphologic process research in China.Grain-size composition is an important sedimentary feature of aeolian sediments,which can reflect the transport-deposition process and sedimentary environment of sediments.Based on the field investigation,indoor sample testing,and the relevant literature,the spatial distribution of sand dune size,the comparison of surface(0-2cm)and shallow(15cm)grain size characteristics of five dune types(Barchan,parabolic dune,ridge-shaped dune,transverse dune and longitudinal dune),and the differences of grain size characteristics of three dune types(Barchan,parabolic dune and ridge-shaped dune)and different landforms of the three dune types(windward slope,crest and leeward slope)were analyzed,the cause of formation is also discussed.The main conclusions are as follows:(1)The surface sediment of the Mu Us Desert dune is generally thicker in the northern region than in the southern region,showing a gradual thinning of the mean grain size from the northwest to the southeast and from the northeast to the southwest,but in the downwind direction,that is,from the northwest to the southeast,the mean particle size does not become thinner all the time,but there is a local coarsening phenomenon under the background of the overall thinning.In the vertical direction,there was no significant difference in grain size composition and grain size parameters between the surface layer and the shallow layer of the dune.However,the surface layers of barchan and dune chains,parabolic dunes and longitudinal dunes are slightly thicker than those of shallow layers,while the surface layers of ridge-shaped dunes and transverse dunes are slightly thinner than those of shallow layers.(2)Compared with the Barchan and parabolic dunes,the average grain size of surface sediments of the ridge-shaped was the smallest of the three dune types,this is related to the downwind direction of the Barchan and parabolic dunes.The classification process of aeolian sand was significantly influenced by the vegetation coverage.The parabolic dune and the ridgeshaped dune were typical semi-mobile-semi-fixed dune.The vegetation coverage on the surface of the dune was much higher than that of the Barchan dune.Vegetation can intercept the movement of coarse particles in wind-blown sand streams,capture fine particles,and produce fine particles when sand dunes are fixed and weathered into soil,resulting in poorer sorting of parabolic dunes and Barchan dunes,the content of fine particle components(including clay,silty sand,very fine sand)is higher.(3)The comparison of the average grain sizes of different parts of the three types of dunes showed that 13 of the 22 barchans have the finest grain-size on the windward slope,accounting for 59.09%,while 14 of the 24 parabolic dunes have the finest grain-size on the leeward slope,accounting for 58.33%.The distinct types of barchan and parabolic dune are mainly associated with the opposite direction in which the horns extend(The barchan’s horns face downwind,while the parabolic dunes’ horns face upwind).There is no obvious predominance deviation in the 39 ridge-shaped dune,and the frequency of each type is close to each other.