| A variety of external morphology and mysterious formation causes that showed by yardang landforms always stimulate the interest of geographers and explorers in this landform. Compared with wind accumulation landforms, investigations to yardang landform, which is a wind erosion landform, are very little. In China yardang landforms mainly distribute in middle and lower reaches of Shule River. Lop Nor region, Xinjiang Wu’erhe region and Qaidam Basin. Among these regions Qaidam Basin is the largest distribution area of yardang landforms. This paper selected the yardang landforms in the Qarhan Salt Lake regions, which located in the central south of Qaidam Basin, as study object. Through thoroughly field investigations, indoor experimentation and analysis, remote sensing image processing, this paper mainly studied yardangs’external morphology, regional wind dynamic conditions and material composition characteristics of yardang landforms. Based on the above mentioned results, this paper legitimately infer the forming processes of yardang landforms in the study area, hoping that this study can fill the gap in the study of yardang landforms in the Qarhan Salt Lake regions. The results indicated:(1) The external morphological characteristics of yardang landforms. The range of I, w, R is31.88～574.63m,11.15～169.83m,1.19:1～5.53:1. and the mean value is141.07m,48.31m,2.94:1,respectively.The yardangs in study area are all belong to meso-yardang (length varies from10m to1000m).The directions of yardang bodies’ long axis are NW-ES,WNW-ESE and NNE-SSE. The number of NW-ES directional yardangs is the most, accounting for91%of the total. While the other two directions account for4%,5%, respectively. The space between yardang bodies varies much more widely, ranges from3.8m to247.5m, and the mean value is48.94m.(2) Wind regime and sand drift potential characteristics in the study area. Sand-driving wind accounts for22.93%of the annual wind records, and the monthly mean speed of sand-driving wind varies6.2～7.4m s-1. The main sand-driving wind direction are WNW and W, accounting for41.05%,20.02%of the annual sand-driving wind frequencies, respectively. The annual DP of study area is326VU, and RDP is235VU, so the study area belongs to intermediate wind energy environment. The annual net sand transport direction is ESE110.14°.The value of wind direction variability (RDP/DP) is0.72, belongs to intermediate ratio and wide unimodal wind regimes.(3) The grain-size characteristics of yardang landforms’sediments. The main components of the deposits are silt sand, very fine sand, clay and fine sand. There are mainly four types of distribution curves, namely asymmetric bimodal, near-symmetric bimodal, unimodal, etc. The main peak and the secondary peak of the bimodal distribution curves are located at the silt sand fraction and very fine sand fraction respectively, the only difference is the contents of the two fractions. The mean size (Mz) of the deposits ranges from3.95to7.18Φ,and the mean value is5.97Φ; standard deviation (σ1) varies from1.37to2.47Φ,and the mean value is1.99Φ;the skewness (SK1) ranges from-0.27to0.58,and the mean value is0.10;the kurtosis (Kg) varies from0.71to1.28,and the mean value is0.93.According to Sahu’s discrimination formula and the characters of both the grain size composition and distribution curves, found that the main sedimentary environments are lacustrine deposits, lakeshore deposits and fluvial deposits, delta deposits, as well as turbidity deposits and a few aeolian deposits. The sedimentary environments of the deposits are very complex.(4) Heavy mineral characteristics. The percentage mass content of heavy minerals in yardang landform sediments ranges0.008%～1.028%, and the mean value is0.281%.And it contains up to24types of heavy minerals. The content of unstable heavy mineral is the most, namely57.64%:then is relatively stable heavy mineral, that is25.08%; and the content of stable and extremely stable heavy mineral is the least,namely17.27%.Among the characteristics index of heavy mineral, the ATi index is83.43,the GZi index is81.19,the ZTR index is1.63,and the stability coefficient is0.29.(5) The formation of yardang landforms. Qarhan Salt Lake area become the subsidence and accumulation center of Qaidam Basin since the Quaternary, and the thickness of sedimentary layer up to2300-2800m.Then with the integral and fast uplift of Tibetan Plateau and warming and drying tendency of climate change, the water area of Qarhan Salt Lake is gradually shrinking. The salt lake is gradually drying and the salts extract. The wind in the Qarhan Salt Lake area is very powerful and the wind eroded the lake bottom along the cracks, which makes the cracks become more and more wide. While the sediments between cracks, shaping by the abrasion and deflation of wind, gradually forming the yardangs. And the cracks in the bottom of salt lake are becoming more and more wide, which forming the corridor between yardang bodies. The seasonal water flow and gravitational collapse also play some important roles in the processes of yardang landform forming. During the processes of yardang landform forming, the extracted salts form hard salt crust, which reduces erosion and damaging effects of external forces and has a protective effect on yardang landforms. However, yardang landform has its own processes of coming into being, developing and passing away. By the erosion of external forces, the yardang bodies will gradually flatten and disappear. |
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