Why is Sichuan Basin also called "Red Basin"?

Sichuan Basin is surrounded by connected mountains, covering an area of about 6.5438+0.6 million square kilometers. Sichuan Basin is the home of most people in Sichuan and Chongqing. It is one of the most densely populated areas in China and even the world, and it is also the cradle of Bashu culture. Known as the "land of abundance". Zhuge Liang, the prime minister of Shu, once praised it as "fertile soil for thousands of miles".

China is a famous red bed basin with the most typical shape, the southernmost latitude and the lowest elevation in the great basin of China. Located in the east of Sichuan Province, on the upper reaches of the Yangtze River, it covers an area of more than 260,000 square kilometers, accounting for 46% of the area of Sichuan Province. Sichuan Basin borders Qinghai-Tibet Plateau and Hengduan Mountains in the west, Qinling Mountains in the north, Loess Plateau in the east and Yunnan-Guizhou Plateau in the south.

Geological landform Sichuan basin belongs to Sichuan depression of Yangtze paraplatform. Paleozoic was relatively uplifted, lacking Devonian and Carboniferous. Indosinian movement transformed into a large depression, and fold uplift occurred in the late Yanshan period, especially after Himalayan movement. The basin pattern is mainly controlled by NE-SW and NW structural lines, forming a typical rhombic basin. Guangyuan, Ya 'an, Xuyong and Yunyang are the four vertices of the rhombus, with the east and west sides slightly longer, 380-430 km, and the north and south sides slightly shorter, 365,438+00-330 km. The connecting line of the four vertices of the diamond is roughly equivalent to the contour line of 650 ~ 750 meters in the basin, and the bottom of the basin and the marginal mountains are also divided by this line. Sichuan Basin was an inland lake basin10.40 billion years ago. Until 66 million years ago, the mountainous areas on the edge of the basin rose rapidly, and the water systems in the middle and upper reaches of the Yangtze River began to communicate. The eastward flow of lake water in the basin laid the present landform.

There are many low mountains and Zhongshan mountains on the edge of the basin, and the mountains are steep. Most of the rivers in the mountainous areas on the edge of the basin are V-shaped valleys, and the height difference of ravines is above 500 ~ 1 000 meters, and the surface is rugged, so there is a saying in history that it is difficult to get through the Shu Road and get into the sky. Ridges are mostly at an altitude of 2,000-3,000 meters, and can exceed 3,000-4,000 meters in the northwest and west, such as 4,984 meters in Longmen Mountain, 3,099 meters in Emei Mountain and 479 1 meter in Xiaoxiangling Mountain. Limestone before Paleozoic is widely exposed on the surface, followed by slate, schist, crystalline limestone, quartzite, sandstone mudstone and conglomerate, and granite and basalt in some places. Karst landforms such as stone forest, karst cave, underground river and valley can be seen in limestone distribution area. Xingwen County in the southern margin of the basin is called "Shilin Cave Township" (see Xingwen Stone Forest). Famous mountains such as Twelve Peaks in Wushan and Jinfo Mountain are also mainly developed from limestone. Emei Mountain, which is composed of limestone, basalt and granite, and Qingcheng Mountain, which is composed of sandstone, mudstone and conglomerate, is known as "Emei is beautiful on earth" and "Qingcheng is quiet on earth", and is a famous tourist attraction in China.

Most of the bottom of the basin is 250 ~ 700 meters above sea level, and the terrain is inclined to the southeast. All rivers in the basin flow into the main stream of the Yangtze River from the marginal mountains at the bottom of the basin, forming a centripetal water system. The surface is covered by a large area of Mesozoic purplish red sandstone and mudstone, so it is called "red bed basin", which is the most concentrated area of Mesozoic continental red beds in China. Sichuan basin is a hilly basin with hills at the bottom, followed by low mountains and plains.

Climate The terrain of Sichuan Basin is closed, and the temperature is higher than other areas at the same latitude. The average temperature in Leng Yue is 5 ~ 8℃, which is 2 ~ 4℃ higher than that in Shanghai, Wuhan in the same latitude and Guiyang in the south latitude. Extreme minimum temperature -6 ~-2℃. Frost and snow are scarce, and the frost-free period is 280 ~ 350 days throughout the year. Changning, located in the Yangtze River valley, is frost-free all year round.

Summer begins at the end of May and lasts for more than 4 ~ 5 months. The temperature in the hottest month is as high as 26 ~ 29℃, the Yangtze River basin is nearly 30℃, and the extreme maximum temperature in the southeast of the basin often exceeds 40℃. Chongqing and Pengshui used to have 44℃, so Chongqing is also one of the three "stoves" in the Yangtze River Basin. The sunny, hot and high temperature weather in midsummer caused a severe summer drought in the southeast of the basin.

The annual average temperature is 16 ~ 18℃. The active accumulated temperature above 10℃ is 4500 ~ 6000℃ and lasts for 8 ~ 9 months, belonging to the middle subtropical zone. The Yangtze River basin in the southeast is over 6000℃, such as Qijiang River is over 6 100℃, which is equivalent to the tropical climate of South Asia south of Nanling in China. The temperature in the basin is high in the east and low in the west, high in the south and low in the north, high in the bottom and low in the edge, and the isotherm distribution is concentric. The temperature in the mountainous area on the edge of the basin has vertical distribution characteristics. The elevation of Emei Mountain and Jinfo Mountain rises by 100 m, and the temperature drops by 0.55℃ and 0.6 1℃ respectively. The average annual temperature at the top of Mount Emei is only 3℃, and the active accumulated temperature above 10℃ is 586℃, which is equivalent to cold temperate zone and sub-cold zone in climate.

The annual precipitation in Sichuan Basin is 65,438+0,000 ~ 65,438+0,300 mm, and the precipitation in the mountainous areas around the basin is very rich. For example, the annual precipitation in the western mountainous area between Leshan and Ya 'an is 1.500 ~ 1.800mm, which is a prominent rainy area in China and is called "West China Rain Curtain". However, winter drought, spring drought, summer waterlogging and autumn Mao Mao rain are unevenly distributed in the year, and 70 ~ 75% of the rainfall is concentrated in 6 ~ 10. The maximum daily precipitation can reach 300 ~ 500mm. "Night rain" has been famous since ancient times, and it accounts for more than 60 ~ 70% of the total rainfall. The fog in the basin is big and wet, and the clouds are low and cloudy. Emei Mountain and Jinfo Mountain are the foggy areas in China, and the annual relative humidity is also the highest in China. The annual sunshine in the basin is only 900 ~ 1300 hours, and the annual solar radiation is 370 ~ 420 KJ/cm2, both of which are the lowest in the country, so it is called "the tree spring barks at the sun".

There are nearly 10,000 species of plants in the flora and fauna basin, and there are many ancient endemic species that are far behind in other parts of China. Metasequoia glyptostroboides, Liriodendron chinense, Zelkova schneideriana, Davidia involucrata, Celastrus mandshurica, Ailanthus altissima, Chimonanthus praecox, Eucommia ulmoides, Taxus chinensis, Cinnamomum camphora, Fokienia hodginsii, Taxus chinensis, Platycladus orientalis and so on. It can still be seen in the mountains on the edge of the basin and the valleys in the east of the basin. In the hot and humid valley, you can see ancient tropical relict plant, such as spinulosa spinulosa, Dryopteris pubescens, Osmunda japonica and Li Bai. Jinfo Mountain and Jinyun Mountain have established nature reserves respectively. Youyang, on the southeast edge of the basin, also has the highest white paulownia in the world, up to 44 meters.

The zonal vegetation in Sichuan Basin is subtropical evergreen broad-leaved forest, and the representative tree species are Castanopsis fargesii, Castanopsis emeiensis, Castanopsis hystrix, Cyclobalanopsis glauca, Cyclobalanopsis glauca, Quercus Baoshan, Schima superba, Schima superba, Phoebe bournei, Machilus Runnan and so on. The altitude is generally 1600 ~ 65438+. Followed by subtropical coniferous forest and bamboo forest composed of Pinus massoniana, Cunninghamia lanceolata and cypress. The marginal mountain areas are evergreen broad-leaved forest, evergreen broad-leaved and deciduous broad-leaved mixed forest, cold temperate mountain coniferous forest, and some areas are subalpine shrub meadow.

Sichuan Basin is one of the most complete areas in China. According to statistics, except fish, there are 4 17 species of animals at the bottom of the basin, 487 species, 3 17 species and 288 species in the western, northern and southern mountainous areas of the basin, among which economic animals account for more than half. The mountainous area on the western edge of the basin is the best preserved and most concentrated area of endemic and ancient animals in China, including giant panda, golden monkey, pronghorn, grey golden monkey and white-lipped deer. There are also rare and unique animals such as red pandas, snow leopards, gazelles, macaques, crested deer, otters and mandarin ducks, blood pheasants, golden pheasants with red bellies, green-tailed pheasants, golden pheasants with white bellies and golden pheasants with red bellies.

Pingwu, Qingchuan, Beichuan, Baoxing, Tianquan, Hongya and Mabian in the western margin of the basin are the main distribution areas of giant pandas in China. Seven nature reserves, including giant pandas and golden monkeys, have been established, including Tangjiahe, Wang Lang, Wolong (see Wolong Nature Reserve), Fengtongzhai and Horn River.

The giant salamander in the gully on the edge of Youyang, Mabian and Pingwu basins, the Chinese sturgeon and the white sturgeon in the Yangtze River and Jinsha River are also endemic to Sichuan, and all belong to the national protected animals.

Natural resources and economic overview There are coal, iron, natural gas, petroleum, salt, mirabilite, gypsum, phosphorus, aluminum, sulfur, copper, manganese, gold, graphite, mercury and other minerals in the basin, among which natural gas and mirabilite rank first in China, and there are also important strontium minerals in China. The reserves of salt rocks in central Sichuan reach 200-300 million tons. The reserve of hydraulic resources in the basin is nearly 50 million kilowatts. Industrial concentration, the main industrial cities are Chengdu, Chongqing, Zigong, Yibin, Neijiang, Nanchong, Luzhou and other cities.

This basin is a famous agricultural area in China. The purple soil distributed in the river basin covers an area of 6.5438+0.4 million square kilometers, which is neutral or neutral and alkaline, rich in mineral nutrients such as phosphorus and potassium, moderate in texture and good in permeability. It was cultivated and planted as early as the Western Han Dynasty.

The land utilization rate in the basin area is as high as 30 ~ 40%, and it is the largest rice and rape producing area in China. The output of sericulture, citrus, tung tree, Fraxinus mandshurica, gallnut, tremella fuciformis and coptis chinensis all ranks first in China.

Physical geographical division Sichuan Basin can be clearly divided into marginal mountain and basin bottom, with an area of 654.38+10,000 square kilometers and1.6,000 square kilometers respectively. There are many middle and low mountains in the marginal mountainous areas. The elements of the landscape are obviously transitional, such as the composition of animals and plants permeating the components of Central China, Southwest China, Qinghai-Tibet Plateau and North China respectively. Generally, there are 2 ~ 5 vertical natural zones from bottom to top in marginal mountainous areas. The marginal mountainous areas are the bases of various economic trees and timber forests in Sichuan. There is serious soil erosion in agriculture.

There are many hills, low mountains and plains at the bottom of the basin. The surface composition is new and single, sandy mudstone and Quaternary sediments. The climate belongs to the middle subtropical zone, and the heat is much higher than that in the marginal mountainous areas, but the precipitation is not as good as that in the marginal mountainous areas. Vegetation is replaced by artificial secondary forest trees, with low coverage and contiguous cultivated land. Drought and flood in summer are the biggest disasters in this area. The bottom of the basin is bounded by Longquan Mountain and Huaying Mountain, which can be divided into three parts: Chengdu Plain, hills in the middle of Sichuan and parallel ridges and valleys in the east of Sichuan.

Sichuan Basin is a part of Yangtze Platform, which is called Sichuan Platform. It is a relatively stable area, but it has experienced two large-scale sea immersion. The first time started from the Cambrian more than 500 million years ago, and continued to the Silurian more than 370 million years ago, and it continued to sink into an ocean basin. The Silurian Caledonian movement took place, and the rest areas rose to land except the Longmenshan geosyncline in the west. At the end of Carboniferous period, 270 million years ago, due to the second seawater immersion, the basin was once again occupied by the ocean. The Permian sea and land alternated, forming Nankong, Songzao, Tianfu and other coal mines near Chongqing. At the end of Permian, magma erupted in the west of the basin, and basalts formed in Xiaojinding and Qingyinge areas of Emei Mountain at this time.

Triassic, 654.38+0.9 billion years ago, the Indosinian movement gradually raised the basin edge into a mountain, and the area submerged by seawater gradually rose to land, from a sea basin to a lake basin. At that time, the lake occupied almost all the territory of the present Sichuan Basin and was called "Bashu Lake", ending the history of seawater immersion. During the long period of 6,543.8 billion years in Mesozoic, the climate in the basin was warm and humid, and ferns, cycads and gymnosperms grew everywhere, which was another coal-forming period. Yongrong Coal Mine was formed in Triassic and Jurassic. From Changshou and Dianjiang in the east, Jiangyou and Qionglai in the west, at the foot of Daba Mountain in the north and Chishui in Guizhou in the south. This is also a region rich in natural gas. During this period, reptiles and dinosaurs ruled for a period of time. Mamenxilong in Hezhou discovered in 1957 is 22 meters long and 3.5 meters high. It is the largest and most complete dinosaur fossil in Asia.

At the end of Cretaceous 70 million years ago, another strong crustal movement "Yanshan Movement" took place. The mountains around the basin are constantly uplifting, and at the same time, many large faults are produced, such as Longmenshan fault in the west and Huayingshan fault in the east, which divides the basin into three parts. Bashu Lake has shrunk to only 20,000 square kilometers. The closed basin topography and sharply reduced water surface make the climate gradually dry and hot, and the sediments change from marine phase and land-sea alternating phase to continental phase. A large number of weathered, eroded and denuded materials have accumulated thousands of meters thick in the basin, forming purple sand, mud and shale. Gymnosperms decreased and dinosaurs became extinct. Under drought conditions, the inland lake was strongly evaporated, the concentration increased, and the salt accumulated continuously, forming a salt lake. Later, the sediments were buried and preserved in the stratum, and after a long geological process, the strata were formed. Zigong area is a famous well salt producing area.

The Neogene was influenced by Himalayan orogeny more than 20 million years ago. Two or three million years ago, in the Quaternary, the earth's crust underwent another tectonic movement. The water system on both sides of Wushan was eroded from the source, and * * * crossed Wushan, forming the world-famous Three Gorges of the Yangtze River, and the water from the basin was incorporated into the Yangtze River water system. As a result, Sichuan basin has experienced the changes of sea basin, lake basin and land basin from internal flow basin to external flow basin, from closed internal flow area to external flow area, and from accumulation to erosion.

Quaternary is an era when glaciers are widely distributed, and a large number of glaciers are developed in the northwest mountainous area of the basin. After the melting of glaciers, a large number of sediments were carried by Minjiang River and Tuojiang River and accumulated in Qianshu Lake in the west, eventually forming Chengdu Plain.

Sichuan Basin is rich in plains 7%, hills 52% and low mountains 4 1%.