Sedimentary Characteristics Of Aeolian Sediments In The Mu Us Sandy Land

As one of the four largest sandy lands in China,the Mu Us Sandy Land is known as the "youngest desert".Located in the ecotone of agriculture and animal husbandry,the irrational use of large-scale land in modern times has resulted in the activation of loose sandy sediments on the surface and intensified the desertification process,which becomes a hot spot of desertification research in China.Sediment is the material basis for the formation and development of aeolian Geomorphology.Its characteristics record significant information in the formation and development of the desert landscape.It does not only help to determine the sedimentary environment and material sources of the desert but also of great significance for desertification control and ecological construction.Based on field investigation and relevant theory of aeolian sedimentology,the grain size,geochemical elements,minerals and micro-morphological characteristics of surface sediments in different regions and different dune types in the Mu Us Sandy Land are studied in detail,which provides a vital scientific basis for discussing the source and sedimentary environment of sediments in the Mu Us Sandy Land.The conclusions drawn in this paper mainly include:(1)The roundness of quartz sand particles in surface sediments is generally poor in the Mu Us Sandy Land,and the majority of quartz particles are sub-angular,while there are few round quartz particles.The typical symbolic features of the aeolian environment such as dish-shaped impact scars,crescent-shaped impact scars,and pits appear in large quantities,while V-shaped pits,parallel striations,and cracks appear less under the action of flowing water and chemistry.It indicates that the sediments in the Mu Us Sandy Land have experienced strong physical weathering,but little chemical weathering.(2)The grain size composition of surface sediments is mainly composed of medium and fine sand in the Mu Us Sandy Land.In space,the contents of fine and very fine sand increase gradually from north to south,while the contents of medium,coarse and very coarse sand decrease gradually.The dominant grain size of different dune types is also medium sand and fine sand.The difference in grain size distribution is significant.The mean grain size of surface sediments in the Mu Us Sandy Land is 2.42?.In space,the mean grain size is getting fine gradually from northwest to southeast and from northeast to southwest in the Mu Us Sandy Land.The average value of sorting is 0.66?,which is better in sorting,and sorting shows no obvious trend from northwest to southeast.The average value of skewness is 0.02,which shows a nearly symmetric distribution.The average value of kurtosis is 0.99,which demonstrates a medium peak.The grain size parameters of different dune types are significantly different.The mean grain size of surface sediments of dune networks,parabolic dunes,and nabkha dunes is dominated by fine sand,while the crescent dunes are mainly composed of medium sand.In addition,the sorting of interdunes is medium,the others are moderately well sorted or well sorted.The skewness is basically a nearly symmetric distribution,and the kurtosis is medium.(3)The composition of the major elements of surface sediments in different regions and dune types in the Mu Us Sandy Land is similar,which are dominated by SiO2 and Al2O3.Compared with the average chemical composition of the upper continental crust,all of the major elements are relatively depleted except SiO2.In space,the contents of SiO2 gradually increased from northeast to southwest,while the elements Al2O3,Na2O,and K2O are gradually decreased.The contents of the major elements of MgO,Fe2O3,and P2O5 are gradually reduced from the northwest to the southeast,while the contents of CaO,MnO and TiO2 show unobvious spatial changes in the Mu Us Sandy Land.The migration of trace elements in different regions and dune types in the Mu Us Sandy Land is roughly similar,which are dominated by Ba and Sr.Compared with the average chemical composition of the upper continental crust,Co and Mo show extremely obvious enrichment,and Sr,Zr,and Pb are close to the upper continental crust,while the other elements suffered loss.In space,the contents of Ba,Sr,Rb,and Pb gradually decreased from northeast to southwest,while the elements Cr,Zn,Ni,Cu,Nb,and Th are gradually increased.The contents of trace elements Co and V are gradually increased from the northwest to the southeast in the Mu Us Sandy Land,while the contents of La reduced.Trace elements Zr and Y show extremely unobvious spatial changes·(4)The light minerals composition of surface sediments in the Mu Us Sandy Land are dominated by quartz and feldspar,with an average content of 95.4%.The heavy minerals are mainly amphibole and garnet,with an average content of 1.86%and various types.The Mu Us Sandy Land is mainly composed of stable-extremely stable and unstable heavy minerals.(5)The sediments in the Mu Us Sandy Land were deposited under extremely arid climatic conditions,and the chemical weathering of the sandy land is still in the primary stage of continental weathering and the Mu Us Sandy Land suffered relatively low chemical weathering.(6)There exist significant differences in sedimentary sources between the western and eastern regions in the Mu Us Sandy Land,and the sediments in the southwestern region are mainly derived from the sediments in the Ningxia-Inner Mongolia reach of the nearby Yellow River.