风成沉积物中不同粒径石英颗粒表面机械结构的变化特征

THE VARIATION CHARACTERISTICS OF SURFACE MICROTEXTURES OF QUARTZ GRAINS AMONG DIFFERENT SIZE GRADE IN EOLIAN SEDIMENTS

按搬运方式将风成石英颗粒分级，逐级测量并统计表面机械结构的类型、数量和大小，并计算其面积。发现悬移为主的颗粒表面结构的类型、数量最少，仅见少量不规则坑、V形坑，在颗粒表面分布面积最小；跃移搬运为主的颗粒表面结构类型最丰富，不规则坑、蝶形坑、V形坑、新月形坑、贝壳状断口大而深，在颗粒表面的分布面积最大；滚动搬运为主的颗粒表面结构数量最多，以小而浅的不规则坑为主，伴有V形坑、贝壳状断口等，在颗粒表面的分布面积大小适中。不同搬运方式在石英颗粒表面形成的结构不同，撞击频率、颗粒大小、搬运速度共同影响表面结构的发育，高撞击频率、大粒径可以形成并承载更多的表面结构，大动能撞击可形成多种类且大尺寸的表面结构。在借助石英颗粒表面结构进行成因判别时需根据粒径大小选择相应粒级的石英颗粒表面结构特征组合，以及结构的数量、大小特征等进行综合判别。

It is generally accepted that surface textures of quartz grain can be used to distinguish the original source,transportation process and some postdepositional process of sediments,which has been successfully applied in the study of eolian sediments,including loess,red clay and desert sand.But so far no unified standard on the characteristics of surface textures of eolian quartz has been established,and different people are using their own observed information in the eolian sediments identification.Some researchers have found that grain size influences surface textures of quartz grain potentially.In this way,a sequence of surface textures of different sized eolian quartz grain could be built and become the new and full-scale standard of identification of eolian sediments in reality.After sampling several typical eolian sediments in the field,we have pretreated the samples and analysed the grain size distribution under Mastersizer 2000,then picked out 9 samples whose grain size distributions are continuous in the range of 5~1000μm,including surface layer loess in Hongguang Mountain,dust fall of April to September in the north side of Kunlun Mountain,dust fall of whole year in the west margin of Cele Oasis,sand in the middle part of Takelamagan desert,sand of KumutageDesert in Lop Nur,sand in the northwest margin of Qaidam Basin Desert,and three eolian coarse sand in Lop Nur.These 9 selected samples are then purified by chemical method to gain quartz grains.Since different sized grains are transported by different ways,then wegraded all the purified eolian quartz grains into 5 grades by transportation forms,which arelong-distance suspension （5~20μm）,close-distance suspension （20~70μm）,long-distance saltation （70~100μm）,close-distance saltation （100~500μm） and rolling grains （500~1000μm）,and analyzed each grade one by one.By testing and counting the type,quantity and size of microtextures on the surface of quartz grains grade by grade through SEM,we have discovered that there is barely a small number of irregular pits and V-shaped pits developed on the surface of suspending transported grains,which also occupied the minimum microtexture distribution area.The type of microtextures on saltation load dominated by large and deep irregular pits,V-shaped pits,rounded pits,graded arcs,conchoidal fractures is the most diverse,which results in the maximum microtexture distribution area too.The rolling transported quartz grains have the greatest number of microtextures,including plenty of small and shallow irregular pits and few V-shaped pits,conchoidal fractures,but a little smaller microtexture distribution area than saltation load.It follows that different sized grains transported by different progress form in equable microtexture characteristics finally,three main factors produced from transporting progresssuch as impacting frequency,grain size and velocity of movement affect the development condition of surface microtextures in eolian quart grains.A large quantity of microtextures could be formed by high impacting frequency and supported by large surface area.Meanwhile,high velocity collision would lead to grow out diversified and large,deepmicrotextures.In summary,different sized eolian quartz grains develop different microtextural combination,we should choose corresponding characteristics of surface textures of quartz grains by grain size in use.Besides,a characteristic combination of surface textures of quartz grain is more effective than a single type of surface texture in the detection of sedimental environment of sediments,and we also do not think the V-shaped pit is only formed from underwater environment,and we believe it is just a representative texture of high energy impacting environment both in wind and water.