How is the loess on the Loess Plateau formed?

The primary loess on the Loess Plateau is an aeolian deposit in the dry and cold climate of Quaternary Glacial Period, and the secondary loess is transformed from the primary loess by flood and alluvial.

During the Quaternary loess accumulation period, with the climate rotation of glacial and interglacial periods, the loess strata showed the alternation of loess and paleosol. According to the ancient soil in loess, loess stratum can be divided into Wucheng loess, Lishi Huang Shang loess, Malan loess and Holocene loess from bottom to top.

According to the Heimugou loess profile in Luochuan, the early Pleistocene Wucheng loess is located under the lower interface of the 15 layer loess. The middle Pleistocene Lishi loess is located between the upper interface of the first paleosol and the lower interface of the fifteenth loess. The first layer of loess is Malan loess formed in late Pleistocene. Black Shanghai soil above Malan loess is an ancient soil layer formed in Holocene.

The distribution thickness of loess strata in the Loess Plateau is generally between Liupanshan and Luliang Mountain 150-250m, and generally within 100m to the west of Liupanshan. The thickness of loess in different strata is also different: the thickness of loess in Wucheng is not large, which is 58 meters in Heimugou, Luochuan, and 0/7.5 meters in Wucheng/Kloc, Shanxi.

Lishi loess is the main part of loess stratum in the Loess Plateau, with a general thickness of 100- 150m, and the maximum thickness is distributed in the middle reaches of Jinghe River and Luohe River. Malan loess is widely distributed, with a general thickness of 10-30m, less than 10m near Tianshui, 9.6m in Dong Zhiyuan and 10m at the source of Luochuan. The thickness of Holocene loess is generally 2-3 meters, of which the thickness of black Hu soil layer is 1-2 meters.

Extended data:

Structure of loess;

Loess generally has well-developed tubular pores, the pore size is 0.5 ~ 1 cm, most of the pores are filled with different amounts of carbonate, and some pores are almost completely filled with carbonate. The porosity of loess is relatively high, generally 33% ~ 64%. Experience has proved that the porosity of loess is the root cause of loess collapse.

On the one hand, vertical joints in loess are caused by vertical tensile stress under the action of gravity, on the other hand, there are many vertical pores in loess, which weaken the resultant force in the horizontal direction, so cracks are easy to occur vertically. On the slopes in the loess area, it often collapses along the joint surface, forming cliffs and sometimes causing large-scale landslides.

There is no bedding in loess. Although the grain size of loess changes in vertical section, this change does not cause obvious bedding alternation, which is different from general bedding. The porosity of loess sample soil is less than that of loess, but the bedding is very clear.

Baidu Encyclopedia-Loess Plateau