Rock And Mineral Characteristics Of Sand Particles In The Kumtagh Desert And Their Genesis

Deserts are widely distributed in the northwest of China.Exploring the formation and evolution trends of deserts is an important tool for developing and transforming deserts,and the shaping of desert environment by desert sediments under the control of dynamical system,material supply and substratum is also the focus of desert research.As the fifth largest desert in China,the basic research on the geology,geomorphology and vegetation of the Kumtagh desert has been relatively well developed by previous authors,but the research on the desert sediments themselves is still relatively weak.On this basis,this paper carries out a systematic study on the rock and mineral characteristics of the sediments of the Kumtagh desert.Because of the vast area of the Kumtagh desert and the differences between the sediments on different geomorphic types in the south and the north,the sediments in the north and south regions of the desert are compared and analyzed in this paper.Based on field exploration and sampling in the Kumtagh desert,indoor experiments and data analysis,the particle size,chromaticity,magnetization rate,rock and mineral characteristics of the Kumtagh desert sediments are summarized,and the main conclusions are as follows:(1)The grain composition of surface sand in Kumtagh desert is mainly fine sand,followed by coarse sand,medium sand and extremely fine sand.The sediment in the northern part of the desert shows a clear trend of thinning from west to east.The sand particles in the central part of the desert gradually become finer and the average particle size gradually increases from north to south.The overall sorting of sand particles is poor;The deviation in the north of the desert is positive,while the deviation in the south is negative;There is a negative correlation between average particle size and skewness,and there are significant differences in correlation between different regions but northern desert is higher than that in the southern desert.(2)The chromaticity of sediment shows a gradual increase as the particle size becomes thicker.There is a significant difference in particle size between the surface sand particles and the underlying sand particles in the northern desert,resulting in a significant difference in color but the sand particles in the southern dunes have good uniformity in both upper and lower layers,mostly of medium to fine sand,Therefore the sand particles have a low chromaticity and therefore appear yellow.The variation in shade of feather shaped sand dunes is caused by wind power,resulting in the accumulation and dispersion of surface coarse particles due to wind accumulation or erosion.The adjacent areas have extremely coarse sand(-1.0～0.0Ф)on the surface or very coarse sand-coarse sand(-1.0～0.5Ф)a relative change of 5-10% in the content can lead to the appearance of light and dark spots on the surface of sand dunes.(3)The magnetic susceptibility of sediments in the north of Kumtagh desert is high in the east and low in the west,and the magnetic susceptibility of sand dunes with similar longitude in the middle of the desert is high in the south and low in the north.Sediments in the southern desert while exceed 3Ф and less than 1Ф magnetic particles are mainly controlled by particle size.The higher the particle size frequency is,the higher the magnetic susceptibility is.The magnetic susceptibility of coarse grain sediment in the northern desert is affected by the chromaticity of sand particles under the control of grain size and the larger the chromaticity value,the higher the magnetic susceptibility.While >3Ф Fine sand particles are mainly controlled by particle size,and magnetic susceptibility increases with the increase of particle size.Primary rock debris and desert paint exist in coarse-grained sediments,and ferrous metal minerals(magnetite and limonite)are rich in fine-grained sediments,resulting in high magnetic susceptibility of coarse and fine grain sizes of sediments.(4)The medium fine gravel and coarse sand rocks in Kumtagh desert are mainly intermediate acid volcanic rock and regional metamorphic rock and the most common lithology is rhyolite,felsite,andesite,trachyte and quartzite.The main lithology of dark particles is intermediate acid volcanic rock,and the main lithology of light particles is metamorphic rock and felsic minerals.Coarse sand particles are relatively common in rock debris with larger mineral particles,while fine sand particles in rock debris and smaller parent rock mineral particles in rock debris will increase in content along with the content of monominerals in sand particles will increase exponentially.Cryptocrystalline and fine-grained rocks that are resistant to weathering become the main role of desert sand debris,and coarse grained(crystalline)granitic rocks and medium deep metamorphic rock are easier to decompose into monominerals while is the main material source of desert fine sand.(5)The content of heavy minerals in Kumtagh desert ranges from 0.3% to 30% and the heavy minerals are rich in variety but low in monominerals.There are 29 kinds of heavy minerals,the main heavy minerals are ordinary hornblende,epidote and limonite and 9 types of light minerals,mainly quartz,plagioclase,and potassium feldspar.Both light and heavy mineral components contain a large amount of rock debris,with a content of over 70% for heavy minerals and up to 40% for light minerals.The maturity of light minerals shows that the transport distance of sand particles in the north of Kumtagh desert is higher than that in the south;The analysis of heavy mineral characteristic index shows that the weathering degree of clastic sediments in the northern and southern parts of the desert is similar,and the source distance is similar.The surrounding mountains such as the Altun Mountains and the Beishan Mountains are the main sources of their clastic materials,which is consistent with the inference of the desert’s "in-situ sanding".