(2) Aeolian foundation of loess

In the past, the origin of loess has been controversial, which is caused by water and wind. Hydrologists see the existence of gravel layers transported by rivers in loess, while aeolian theorists see that the loess in the upper part of Huangtuling is uniform, but they can't see the stratification of hydrological deposits, that is, there is no gravel layer distribution. In fact, one of the differences between the two understandings is the different phenomena they observed in different parts. Secondary loess is found in basin area, piedmont slope and river alluvial area, and has obvious characteristics of water-borne deposition. In the upper part of the loess beam, it is all blown by the wind, of course, there is no foundation for water deposition.

Academician Liu Dongsheng, an expert on loess in China, made a systematic and comprehensive investigation in Xinjiang desert area, from Hexi Corridor to the hinterland of Shaanxi-Gansu-Ningxia Loess Plateau, and then to Shanxi Loess Plateau after decades of research, and demonstrated the aeolian nature of loess in many aspects. The aeolian theory of loess was finally unanimously recognized by geologists, and won the Taylor Prize, the highest geological award of the United Nations, becoming the only geologist in China who won this honor.

The relationship between wind and sand

One of the evidences of loess aeolian theory is that the mineral composition of loess is uniform, which will not be different due to different regions and different properties of exposed rocks in different regions. If loess is water-bearing, its mineral composition must be obviously different because of the different properties of upstream rocks. Because water-based sediments can only come from weathering and denudation of upstream bedrock, and then they are carried by water flow and deposited downstream. For example, the upper reaches are granite areas, and the feldspar content of granite in sediments is definitely high; The upper reaches are limestone areas, so there can be no feldspar in the downstream sediments, only calcite and dolomite; The upstream is metamorphic rock, so there will be a lot of metamorphic minerals such as chlorite and amphibole in the downstream sediments.

At present, the mineral composition and chemical composition of loess in different places are basically the same, reflecting the same source, so the bedrock lithology in loess areas is very different, but the accumulated loess composition is basically the same. Among loess minerals, feldspar accounts for 40% ~ 70%, timely accounts for 20% ~ 30%, clay minerals account for 10% ~ 30%, and heavy minerals account for 0. 1% ~ 0.5%. In the chemical composition of loess, alumina accounts for 12% ~ 14%, iron oxide accounts for 4% ~ 4.5%, ferrous oxide accounts for 1% ~ 1.2%, magnesium oxide accounts for 2% ~ 2.3%, and calcium oxide accounts for 4.5% ~ 7.

Loess is composed of minerals with particle size less than 0.25mm. When the wind speed is 2 ~ 3 m/s, they will be blown away, and when the wind speed is 4 ~ 5 m/s, the particles will feel when they hit the face. When the wind speed exceeds 10 m/s, sand will hurt when it hits your face. When the sandstorm in the weather forecast comes, the wind speed generally exceeds 10 m/s, and the sky is yellow. Without a mask, the sandstorm will get into the entrance nose.

The second basis of loess aeolian: after sampling and analyzing the loess on the Loess Plateau, the grain size of loess becomes smaller from northwest to southeast. The grain size of loess in Jingbian, Yulin and northwest Baode of Shaanxi Province is greater than 0.045 mm; The particle size of loess in the northwest of Shaanxi Huanxian, Zhangzi and Wutai lines is 0.035 ~ 0.045 mm; The particle size of northwest loess from Luochuan, Shaanxi to Lingshi and Taiyuan, Shanxi is 0.025 ~ 0.035 mm; The grain size of loess in Tongchuan, Jixian and Anze in Shanxi is between 0.015 and 0.025 mm, and the grain size of loess in the south and east of the line is less than 0.015 mm. The gradual change of loess grain size system clearly reflects the movement of loess from northwest to southeast. Because the transport characteristics of wind are always coarse particles falling from the wind source area, the lower the wind speed, the finer the particles will be.

The third basis for loess weathering is that it accumulates on different landforms of valleys, plains and hills, that is, it is like a coat, covering all parts of the body except the head. "Tou" refers to the Zhongshan area above 2000 meters above sea level, which may be a place where the wind can't blow. The upper limit of loess in Shanxi is generally 1400 ~ 1600 meters above sea level. In fact, loess is distributed in the mountains from1600 to 2000m, but its thickness is thin and there are many outcrops of bedrock, so geologists often use the lithology of bedrock to express it and ignore it (which is allowed to be ignored).