Formation and evolution of fan-shaped land and its oasis

Fan-shaped land is the main area of oasis formation and evolution in arid area, and it is also an important area of human activities. Xinjiang is the largest oasis distribution area in arid areas of China, mainly distributed in fan-shaped, alluvial plain, alluvial plain and lake plain. Because of its excellent soil and water conditions, fan-shaped land is suitable for the formation and development of oasis, and it is the main area of oasis distribution in Xinjiang. The scale and characteristics of fan-shaped land are related to the hydrological characteristics of rivers, and the suitability of oasis formation and development is related to the genetic types of fan-shaped land.

I. Distribution and types of fan-shaped land

The concept of fan-shaped land mainly includes: ① refers to the semi-cone or fan-shaped accumulation landform formed at the outlet of valley or river, including alluvial cone, alluvial fan, alluvial fan and debris flow fan (Nanjing University, etc.,1983); (2) When rivers or seasonal floods flow from valleys into open desert areas ... they carry a large amount of debris and accumulate at the exit of mountain passes, forming fan-shaped alluvial fans or alluvial fans on the plane (Zhang et al.,1995); (3) The so-called fan-shaped land (alluvial fan) refers to the accumulation of materials eroded by storm flow at the outlet of the ravine, and its plane shape is like a fan, hence the name (Zheng Wu, 1999). This book refers to rivers, seasonal mountain torrents or mountain torrents and mudslides originating from Altai Mountain, Tianshan Mountain, Kunlun Mountain and Altun Mountain. After flowing out of the mountain pass and entering the plain, due to the decrease of riverbed slope, the river speed slows down, the flowing water is dispersed, and the carrying materials accumulate to form fan-shaped land, including alluvial fan, alluvial fan, debris flow fan and delta at the estuary. Fan-shaped land is the main area of oasis formation and development. Among 87 oasis towns in Xinjiang, there are 47 fan-shaped towns.

(A) the distribution of fan-shaped land

Fan-shaped land is mostly distributed along the edge of intermountain basin, and its scale is proportional to river flow. For example, the water network in the west of Urumqi on the northern slope of Tianshan Mountain is dense, and the catchment area and water volume of Toutun River, Hutubi River and Kuitun River are large, and the scale of alluvial fans is also large; There are fewer and fewer water systems in the east, and the scale of fan-shaped land is getting smaller and smaller. In the east of Yiwu, only the diluvial fan formed by seasonal flood accumulation is seen, and the scale is very small. The composition of fan-shaped land is related to mountain lithology. For example, the bedrock on the northern slope of Tianshan Mountain is covered with loess, and the vegetation coverage of forest grassland is high, while the bedrock on the southern slope of Tianshan Mountain is mostly exposed, and the composition of fan-shaped land is finer than the latter. Fan-shaped land has the basic characteristics of distribution along the edge of foothill basin or valley.

(2) Types of fan-shaped land

According to the formation scale, composition materials and water source conditions, fan-shaped land can be divided into five categories: alluvial fan, alluvial fan, alluvial fan, delta fan and debris flow fan, each with its own characteristics (Table 8- 1).

Table 8- 1 Types and characteristics of fan-shaped land in Xinjiang

Second, the suitability analysis of the formation and development of fan-shaped oasis

Due to the different water and soil conditions of the above-mentioned fan-shaped land, the formation and development of oasis are directly or indirectly affected, mainly as follows.

(1) alluvial fan

It is formed by alluvial rivers with flowing water all the year round. Alluvial fan is flat, and the slope of the fan rarely exceeds 10, with an average of 3 ~ 6. Such as Urumqi River, Kuitun River, Tekes River, Aksu River, Ganwei River and other alluvial fans, with flat terrain and a slope of < 3. Weathered bedrock in the basin is the main source of alluvial fan formation, and the surface components are interbedded with sandy pebbles and sandy soil on the upper part of alluvial fan. The fine soil substance in the middle part increases and the soil layer thickens; The lower part is fine soil, the clay content at the fan edge increases, and the groundwater level is high, forming swamp or saline-alkali land. For example, the alluvial fan of Kuitun River is covered with pebbles with a diameter of 2 ~ 15 cm, which is suitable for the development of industry, mining and urban oasis. The middle and lower soil layers are 30 ~ 50 cm thick, exceeding 100cm in some areas and up to 30m in some areas. The soil is cohesive soil, with a silt content of 28% ~ 40%, which is suitable for the development of farmland oasis. The soil at the fan edge is thick and salinized strongly. In addition, for example, the alluvial fan of Ganwei River, because the coarse-grained materials carried by the river have been deposited in Baicheng Basin, the upper part of the alluvial fan is also fine-grained soil.

Alluvial fans are generally good in soil and abundant in water, which are suitable for the formation and development of oases. According to statistics, there are 39 oasis towns with alluvial fans in Xinjiang, accounting for 44.83% of oasis towns in Xinjiang and 83% of fan towns.

(2) diluvial fan

In mountainous areas dominated by dry erosion, such as Altun Mountain, Kuruktag Mountain, Beita Mountain and baruch Mountain, it is formed by accumulation of seasonal flood carriers. The scale of alluvial fans is small, and most slopes are greater than 3. The composition is mainly gravel, with angular gravel and mixed accumulation. The fan edge is clayey silt and clay, and saline-alkali land or cracked land can be seen. Lacustrine fan is a desert with poor soil quality, water shortage and few plants, which is only suitable for developing industrial and mining oases. For example, Karamay, located at the eastern foot of Ma Yili, is an industrial city oasis developed with the development of petroleum industry.

(3) alluvial-diluvial fan

Rivers originating from the southern slopes of Tianshan Mountain and Kunlun Mountain, due to the lack of forest or water conservation in mountainous areas, mountain torrents are formed when there is heavy rain in mountainous areas, and the river water volume changes greatly in seasons. The rivers carry sand and gravel and accumulate in the foothills to form alluvial fans. For example, the southern slope of Bogda Mountain originated from the alluvial and diluvial fans of Santanggou, Heigou, Talanggou and the rivers along the river, with relatively flat terrain, and the top of the fan is generally composed of sand, gravel and pebbles, with little sand and drought, and few or no plants; There are more sandy soil, deep soil layer and shallow groundwater level in the middle, which is convenient for water irrigation from mountain pass and suitable for the formation and development of artificial oasis. The lower loam is increased, which is the overflow zone of spring water, and the soil salinization is obvious.

delta

In the area where the river enters the lake, it is formed by the dispersed flow of the river and the accumulation of substances carried by the lake. For example, the deltas of Kaidu River and Wulungu River are flat, with many tributaries, mainly fine sand, more clay, high groundwater level, dense plants and many swamps, which are good areas for forming natural oases. The development of artificial oasis is easy to produce swamp and salinization.

(5) Debris flow fan

In the arid and denuded mountain areas with poor vegetation and more gravel on the slope, once heavy rain occurs, a mixture of sand and water will be formed, and debris flows will accumulate in the foothills to form debris flow fans. Mainly distributed in the southern foot of Alagou and Huola Mountain in Tianshan Mountain. Debris flow fan is small in scale, slightly undulating, mainly composed of rock debris, disorderly accumulation, no bedding and less vegetation. Oasis development is difficult, such as the relocation of Alagou Dongfeng Machinery Factory due to frequent mudslides.

Thirdly, the reason and process of the evolution of fan-shaped oasis.

(A) the reasons for the evolution of fan-shaped oasis

The water source of fan-shaped land mainly comes from rivers originating in mountainous areas, and the precipitation in mountainous areas is related to regional climate conditions. The formation of oasis is limited by soil and water conditions. When the geomorphic process causes fan-shaped deformation and the soil and water conditions change, the oasis also changes in time and space. The geomorphic processes that cause the evolution of fan-shaped oases are mainly as follows.

1. tectonic movement

At the same time, the mountain is uplifted by tectonic movement, and the fan-shaped land at the foot of the mountain is also uplifted, leading to river cutting or diversion. For example, the alluvial fan oasis of Peacock River at the southern foot of Tianshan Mountain is affected by the uplift of advanced structures, which makes Peacock River turn from north to south to west and then flow eastward. Alluvial fans moved from east to west, forming new and old alluvial fans. The old alluvial fan in the east was uplifted by the structure, which led to the exhaustion of the water source of the old river, the decline of the groundwater level, the death of plants and the evolution from oasis to desert.

2. Mountain torrents and mudslides

Sudden rainstorm and high temperature in mountainous areas in summer promote the melting of snow and ice, and snow in low mountainous and hilly areas in spring melts and accumulates due to high temperature, which can form a fan-shaped oasis endangered by mountain torrents and mudslides. For example, on July 26th 1953, heavy rain suddenly fell in the mountainous area of Urumqi River Basin, resulting in mountain torrents and mudslides, which destroyed four bridges and interrupted the east-west traffic of Urumqi River. The urban flood width is more than 120m, and the water depth is1.0 ~1.2m. A large number of houses collapsed and 60,000 mu of farmland in the downstream Wujiaqu was flooded. On August 1958 and 13, heavy rain suddenly fell in the northern mountainous area of Kuqa, resulting in dam failure of reservoirs in mountainous areas and destruction of Kuqa county; On June 24th, 1988, a sudden rainstorm in Alagou area triggered a mudslide, which caused the Dongfeng Machinery Factory located on the debris flow fan to suffer, and the economic loss reached 6 million yuan. On July 12 of the same year, a mudslide occurred in Toutun River in Urumqi, which caused some factories of Bayi Iron and Steel Plant located on the alluvial fan of Toutun River to be flooded and houses collapsed.

Step 3: landslides

In summer, rainfall in mountainous areas causes valley tops or fan-shaped landslides, sometimes causing oasis destruction. For example, in July 1987, continuous rainfall in the middle reaches of Kuitun River triggered a valley landslide, and the flood peak reached 22m after the landslide blocked the river. The flood destroyed the old and new headwaters and the main canal of Kuitun River, and the alluvial fan oasis was seriously affected, resulting in a direct economic loss of 20 million yuan.

4. Human action

Mainly include: ① quarrying, digging holes, taking sand and gravel materials at the upper edge of fan-shaped land. Due to the need of urban building materials, rocks and soil at the foot of the hillside are mined to form a free surface, causing landslides or collapses of rocks or soil and endangering oases. For example, two consecutive landslides occurred in the quarry of Building Materials Bureau located on the west side of alluvial fan of Urumqi River, and the landslide in Heijiashan quarry led to the relocation of 10 households; (2) Fan-shaped water pollution spread, such as 249.2× 104t industrial wastewater produced by Dushanzi Refinery at the top of alluvial fan of Kuitun River every year, which flows downward into Quangou Reservoir through Heiyougou, causing plant degradation and ecological deterioration. (3) Man-made development of water and soil in the middle and upper part of the fan-shaped land, drilling wells and pumping water, led to the decline of groundwater level in the lower part and the edge of the fan-shaped land, the disappearance of spring water, and the salinization of marshes, such as the alluvial fans of Manas River and Toutunhe River.

(2) Evolution process of fan-shaped oasis.

As mentioned above, most sectors have the characteristics of gradually slowing down the slope from the top of the sector to the edge of the sector, thinning the composition and increasing the groundwater level. The terrain of the fan edge is particularly flat, the composition is fine, it is a phreatic overflow zone, rich in aquatic plants, and it is the earliest place to form a natural oasis. The middle and lower part of fan-shaped land has flat terrain, deep soil layer, excellent soil quality, shallow groundwater level and convenient irrigation and drainage, which is suitable for developing farmland oasis. The upper part of the terrain is inclined and slightly undulating, mainly composed of sand and pebbles, and the groundwater level is low, which is conducive to urban development.

The formation of fan-shaped oasis is related to its soil and water conditions, and its evolution stages can be roughly divided into: ① early natural oasis, located in the middle and lower part of fan-shaped land and on the edge of fan-shaped, mainly swamp, meadow, grassland and shrub oasis, which human beings use to develop animal husbandry; (2) In the middle stage of artificial oasis development, the middle and lower parts of fan-shaped land were reclaimed to develop agriculture, resulting in a small amount of meadows and grasslands becoming farmland; (3) In the later stage of large-scale artificial reclamation, with the development of land reclamation, towns and mines, the Gobi desert on the upper part of fan-shaped land became an oasis in towns and mines, and the marshes and meadows on the edge of fan-shaped land disappeared and were replaced by saline-alkali deserts. Part of the cultivated land in the middle and lower part of the fan-shaped land is abandoned as desert due to salinization.

Under the action of human activities, the evolution process of fan-shaped land oasis mainly includes: ① With the expansion of artificial oases such as farmland and towns in the middle and upper part of fan-shaped land, the groundwater level at the edge of fan-shaped land drops, reed marshes disappear, replaced by meadows, or evolved from natural oases into saline-alkali land; (2) Natural oases, such as grasslands or shrubs, have evolved into farmland artificial oases due to the development of water and soil in the middle and lower parts of fan-shaped land; Part of farmland evolved from farmland to saline-alkali land or grassland desert due to secondary salinization; (3) The development of the towns above the fan-shaped land has turned the Gobi Desert into an artificial oasis; If the river is deep, and the town is difficult to use water because of tectonic uplift, the town will move and evolve from oasis to desert.