Geological survey of Heihe river basin

(1) general situation of physical geography

Heihe River is a large inland river in northwest China, which is located in the middle of Hexi Corridor and Qilian Mountain and flows through Qinghai, Gansu and Inner Mongolia provinces (autonomous regions). Geographical location is 97 37' ~102 06' east longitude and 37 44' ~ 42 40' north latitude. The basin is bounded by Qilian Mountain in the south, Juyanhai in the north, Mongolians and Shiyang River in the east, and Heishan and Shule River in the west. It is more than 400 kilometers long from east to west and 800 kilometers long from north to south, with a drainage area of about10.3 million square kilometers.

Heihe river basin has a superior geographical position and is the only place where the ancient Silk Road and the Eurasian Continental Bridge pass. There is a satellite launch base in China and a long border with Mongolia. It has flourished and changed a lot in history. The disappearance of ancient countries such as Heishui and Loulan and the Gobi Desert today are all related to the water system. Therefore, it is of great significance to establish a three-dimensional geological structure model of groundwater in Heihe River basin and find out its geological structure for water exploration, ecological environment construction and basin economic and social development in arid areas.

1. Meteorological

Heihe river basin is located in the hinterland of Eurasia, far from the ocean and has a typical continental climate. In summer, the warm and humid air flow in the southeast Pacific can pass through Chinese mainland, Qinling and Loess Plateau, affecting this area. Due to the influence of the Qinghai-Tibet Plateau, the southwest airflow can bring water vapor from the Indian Ocean and the Bay of Bengal to the east of the region. The air currents in the western Atlantic Ocean and the northern Arctic Ocean travel long distances, cross Eurasia, pass through Central Asia and the Black Sea, cross the Zhungeer boundary mountain and Tianshan Mountain, and reach the western part of this area as tail wings, which makes it lack of water vapor and dry air and has a weak impact. In winter, this area is controlled by Mongolian and Siberian high pressure, so it is extremely cold and dry. Generally speaking, the precipitation in the southern mountainous areas is the largest, the precipitation in the central and northern plains is the least, and the precipitation in the east is greater than that in the west. On the contrary, the evaporation in the north and west is large, and the evaporation in the east and south is small.

According to 1957- 1983 meteorological data, the average annual precipitation in this area is 42mm, and 60% ~ 70% of the annual precipitation is concentrated in July ~ September, and few people have precipitation above 10mm; The average annual evaporation is 3755 mm, which is about 89 times of precipitation. The strongest evaporation season occurs from May to August, accounting for about 50% of the annual evaporation. Precipitation and evaporation vary with the ups and downs of the terrain. Generally, the higher the terrain, the greater the precipitation and the smaller the evaporation. For every elevation of 1000 m, the precipitation increases by 3.5 ~ 4.8 mm, and the evaporation decreases by 25 ~ 32 mm; The annual average temperature is 8.04℃, the highest is 465438 0.8℃, and the lowest is -36.4℃. The daily temperature difference is generally 14 ~ 17℃, and the highest is 34.2℃. Annual sunshine hours are 3325.6 ~ 3432.4t；; The annual average absolute humidity is 4.2mb, the relative humidity is 32% ~ 35%, and the humidity coefficient is lower than 0.009% ~ 0.0 12%. Northwest wind prevails, with an annual average wind speed of 4.2m/s and a maximum wind speed of 15.0m/s, with an average of 54 days of gale of magnitude 8 and above, 29 days of sandstorm and gale.

2. Hydrology

The main river in the basin is Heihe River, also known as weak water (or Ejina River), which originates from the northern foot of Qilian Mountain and is divided into two tributaries. The eastern branch is called Heihe River, which is the main stream. The west branch is called Beida River, which flows into Ejina Banner through Shuangchengzi, and divides into East and West Rivers to Bayan Bo Gu at the western foot of Langxin Mountain. The East River is divided into eight branches to the north, which flows into the East Juyan Sea (Suoguo Naoer), and the West River is divided into four branches into the West Juyan Sea (Gashun Naoer). The total length of the river is 82 1km, and the drainage area is 1.28 million km2. Above Yingluo Gorge and upstream of the mountain pass, the river is 303km long and the drainage area is 1 0,000km2. Charlotte to Justice Gorge is the middle reaches; Just below the canyon is the downstream.

There are few natural lakes in this basin. Only at the tail of Heihe River, there are Dongju Salt Sea (Suoguo Naoer) and Xiju Salt Sea (Gashun Naoer), which are freshwater lakes replenished by surface water and groundwater. Historically, the water area of the West Juyan Sea was 350km2；; 196 1 year dries up, and Dongju Salt Sea 1958 Lake covers an area of 35.5km2, with an existing water surface of more than 20km2, an average water depth of 1m and a storage capacity of about 20m3.

There are 428 large and small glaciers in the basin, which are distributed at the source of the river, with an area of 129.79km2. It is estimated that the ice reserve is about 3.3km3, and the annual glacier melt water supplies about 365 million m3 to the river, accounting for 9.8% of the surface runoff in the basin.

(2) Regional landform

Heihe river basin is high in the south and low in the north, high in the west and low in the east. Qilian Mountain in the south has a high altitude, with the peak altitude of 3000-5000m and the highest peak of 5564 m; The elevation of the central part is mostly 1500~2500m ~ 2500 m, and the highest elevation of Qinglong Mountain is 3600m；. The altitude in the north is mostly around 1000 ~ 1500m, and the altitude changes relatively gently.

The southern part of Heihe River Basin is Qilian Mountain, with steep mountains, severe valley cutting and cliffs, mainly water erosion and glacier erosion. The middle part of the basin is mostly low mountain terrain, with mountains such as Heli Mountain and Longshou Mountain. Although the modern landscape is generally similar to the alpine region, due to the dry climate, the weathered terrain is more developed, and the semi-desert and desertification landscape in some areas is more obvious. The northern plain is located in the alluvial-diluvial plain in the lower reaches of Heihe River, with low terrain and relatively small altitude. Due to the strong winds and erosion in modern times, the plains have formed vast desert, wind-eroded depressions, accumulation topography, hills and wavy quasi-plain landforms. At the same time, most of the mountainous areas in the north are wide and low, with gentle peaks and undeveloped valleys.

This area is rich in landforms. According to the basic causes and forms of landforms, the landforms in this area can be divided into several types: mountains, plains, Gobi and desert, and each type can be divided into several subtypes. Such as fold fault-block mountain, fold fault-block mountain, structural denudation low hills, denudation accumulation ridges and hills, etc. There are steep alluvial fan plains, alluvial plains, alluvial plains and alluvial plains in the plain. To sum up, there are mainly the following types of geomorphic units: structural denudation geomorphology, mainly some folded high, middle and low hilly areas; Denudation and accumulation topography, mainly some mountain plains and valley plains; Accumulation, alluvial and diluvial deposits, lake plains and aeolian landforms; Gobi; Desert.

(3) Regional geological structure

1. Structure overview

The structure of this area is complex, including many geological units. The edge of each structural unit is basically controlled by large faults. For example, at the foot of Qilian Mountain, a series of thrust faults form imbricate faults, which control the hydrogeological conditions of the basin. In addition, there are many uplift zones and depression zones, which together with faults cut the basin plain into several great basin.

The tectonic movement is characterized by the rise of the southern mountainous area and the relative decline of the corridor basin, which provides a good place for Quaternary sedimentation. Such as Jiuquan East Basin and Zhangye Basin. The mountainous areas in central and northern China are relatively stable, with a small increase. The Quaternary loose layer sediments in the basin mainly come from the southern basin, with thin thickness and fine sedimentary particles.

Quaternary strata in the basin are relatively complete, and the basement of each basin can be divided into metamorphic rocks and igneous rocks before Lower Paleozoic, clastic rocks from Jurassic to Upper Paleozoic, and fine-grained rocks dominated by mudstone in Tertiary and Cretaceous. There are also some Q 1 rocks.

2. Tectonic movement

The tectonic movement in this area is mainly manifested in the following aspects:

Up and down movement: the mountain is always rising, while the basin is sinking. For example, the north-south mountains are rising and the plains are sinking. Qilian Mountain and Dahuangshan Mountain were upgraded to mountain dramas, and Damaying and Yongchang basins had the largest subsidence.

Fold changes: Fold structures were widely formed in the Early and Middle Pleistocene of Eogene and Quaternary, which confirmed that the neotectonic movement was very strong in the early Quaternary.

Fault movement: Fault structures are common in this area, some of which are exposed on the surface and some are hidden underground. These structures are mostly found in foothills, hilly areas and plains, and the length of fault lines varies from several kilometers to dozens of kilometers. In nature, most of them belong to compressive or compressive-torsional reverse faults and flat inference layers. The faults exposed to the surface are obvious, and there are generally fault structural fracture zones. Buried faults are mostly distributed in foothills and plains, such as the big fault at the northern foot of Qilian Mountain. These faults control the formation and migration of groundwater in the basin to varying degrees and can be used as the dividing line of the basin.

Earthquake action: According to historical records, there are frequent earthquakes in this area, some of which have high magnitude and serious destructiveness, which is also a reflection of the active modern crustal movement and another evidence of neotectonic movement.

(4) Regional level

The exposed strata in the basin are relatively complete, both old and new strata appear, and there are many Paleozoic and Mesozoic strata in mountainous areas, such as Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic and Cretaceous, which are all displayed in the modeling section. This kind of modeling mainly involves basins in plain areas, and it is not considered here because of the lack of exploration data in mountainous areas.

There are many intrusive rocks in the strata, such as granite, quartz diorite, granodiorite, ultrabasic rocks, etc., which invade the strata of different times in the form of branches or dikes. It also appears from time to time in the Quaternary of the basin.

Quaternary is widely developed in various basins in the region and is also the main object of our modeling. The following is a detailed description of the Quaternary system with different genetic types in each era.

Holocene (Qh): This layer is widely distributed in the surface of the region, but its sedimentary thickness is thin and its genetic types are complex, including flood, alluvial, siltation, lakes and aeolian types. The representative lithology of diluvium is gravel layer with high argillaceous content and poor gravel sorting. Alluvium is distributed in the ancient river channel of alluvial plain valley, mainly composed of sand and gravel, which is well sorted and circular. The sand is mainly fine sand and medium fine sand, with well-developed horizontal bedding, loose and locally mixed with thin clay. Siltation is distributed in denuded depressions, where the materials carried by modern valley floods are finally deposited, mainly clay loam mixed with fine sand. Lacustrine sediments are mainly distributed in the lake basin, and the lithology is clay mixed with thin sand layer. Aeolian deposits mainly appear in the form of sand dunes, and the lithology is mainly silty sand. Alluvial, limnetic and limnetic deposits are distributed on both sides of the riverbed in the middle and lower reaches of Heihe River Basin and in the ancient rivers, with a relatively narrow distribution range.

Upper Pleistocene (Q3): distributed in the vast plain. Generally, the upper renewal is mainly gravel sand and sand. The diluvial strata are distributed in the piedmont inclined plain, mainly composed of gravel, and its composition varies from place to place. The lithology of alluvial fan is mainly pebbles, and the fine soil plain is composed of clayey silt, sand and loam, with a thickness of tens to hundreds of meters.

Middle Pleistocene (Q2): The lithology is mainly pebbles, loam, clayey silt, interbedded clay and sand, and the upper layer is mostly covered by the gravel layer of Upper Pleistocene. Due to strong denudation, outcrops can be seen around the depression, mainly grayish yellow clay mixed with thin silt. The sediments of the Middle Pleistocene were deposited in the sedimentary environment inherited by the Lower Pleistocene, so the vertical and horizontal changes of lithology and thickness are basically consistent with those of the Lower Pleistocene.

Lower Pleistocene (Q 1): widely distributed in the basin plain, with clay sand as the main lithology, with great changes in sedimentary thickness and lithology. Thick layered conglomerate, glutenite mixed with sandstone, and calcareous cementation, and the Quaternary loose bed basement dominated by mudstone is formed in the northern and northern basins of the southern basin.

(5) Regional hydrogeological conditions

Due to the differences of Quaternary sedimentary lithofacies, groundwater recharge and discharge conditions, and the influence of basement fluctuation, the hydrogeological conditions in this area are obviously different.

The Quaternary loose layer in the front basin of Qilian Mountain is thick, loose and porous, which is a favorable place for storing groundwater. The buried depth of groundwater is relatively deep, generally 100 ~ 300 m. From south to north, to the central area, the basement rises, and the groundwater level becomes shallow. In many places, groundwater even overflows the surface in the form of springs. The northern basin is mainly a fine soil plain belt, and the groundwater type is a multi-layer phreatic-confined water structure.

Faults have a great influence on groundwater level, which is often accompanied by the decline and overflow of groundwater. The buried depth of water level on both sides of the fault is very different.