Geology and geomorphology

Yinchuan plain is high in the west and low in the east. Helan Mountain in the west is a mid-high mountain heading for NE30, with an altitude of 1500 ~ 3200m and a peak of over 3500m, which has become a natural barrier against cold air and sandstorms in the northwest. The central part is plain, which consists of piedmont diluvial inclined plain, alluvial plain and lake plain rivers. The terrain is open and flat, sloping from southwest to northeast. The western part is a piedmont diluvial inclined plain with large terrain slope and developed gullies; Generally, it is bounded by the west main canal, with a plain area in the east, with a flat terrain, a slope of 0.2 ~ 1.8, a ground elevation of1106 ~130m, criss-crossing ditches and dotted with lakes and marshes.

According to the causes and forms of landforms, it can be divided into three types: accumulation denudation landform, accumulation landform and aeolian landform.

1. Accumulation and denudation landform

It includes two morphological types: piedmont diluvial oblique plain and alluvial plain, which are composed of diluvial and alluvial plain respectively.

Piedmont diluvial oblique plain: It is composed of diluvial deposits and distributed at the eastern foot of Helan Mountain. It is a banded oblique plain composed of diluvial fans. It is 8 ~ 15 km wide and consists of Quaternary proluvial. Altitude1140 ~1700m. It inclines from west to east with a slope of 5% ~ 10%, and gullies are developed.

Alluvial plain: it is adjacent to the piedmont alluvial plain in the west and the trailing edge of alluvial-lake terrace II in the east. Strike SN, distributed in a strip shape, with a width of 5 ~ 15 km from east to west. It is mainly composed of alluvium, with a slope of 1 ~ 3, flat terrain and fertile soil.

2. Accumulation landform

Mainly for the plains I and II terraces, low-order alkali beaches and fan-front depressions. Class I terraces are distributed along the banks of the Yellow River, and are deposited in modern rivers and lakes. The terrain is flat, the east-west slope is 0. 17 ~ 1, the platform width is 4 ~ 12 km, and the front edge of terraces is 0.5 ~ 1.0m higher than the floodplain. Lakes and salt marsh depressions along the back edge of terraces are well developed. The second terrace is composed of early and middle Holocene river and lake deposits, extending in the north-south direction, with a width of 10 ~ 20km and a maximum width of 26km. The average ratio of upstream and downstream is 0. 17, with low terrain, lakes and swamps everywhere, and serious salinization in some areas. Low-level saline-alkali beach, distributed in Pingluoxi Datan area, is a low-level saline-alkali beach with open and gentle terrain. The fan-front depression distributed in Mingshui Lake Farm is a closed-flow depression with shallow water level and serious salinization. The yellow river beach is composed of Holocene alluvium, which is irregularly distributed in strips along the banks of the yellow river. The terrain is relatively flat with an average slope of 0. 16. The floodplain is 0. 1 ~ 1.0m higher than the water level of the Yellow River and slightly inclined to the Yellow River.

3. Aeolian landform

Fixed and semi-fixed sand dunes: They are distributed in Wuduizi-Gao Ren Town, southwest of Pingjibao and northeast of Zhenbeibao, and are typical fixed grass sand dunes. Especially in the northeast of Zhenbeibao, the distribution range is small, the sand dunes are steamed bread-shaped, with a height of 1 ~ 3m, single and discontinuous, and the terrain is flat, mainly composed of desert plants such as Nitraria tangutorum.

Movable dunes: They are distributed in Wuduizi, east of Gao Ren, southeast of Yinchuan Forest Farm, both sides of Pingjibao Station, northeast of Nanliang Farm, etc. Most of them are movable crescent dunes and ridge dunes. The long axis of the crescent dunes is northwest, with a gentle windward slope of 7 ~ 20, a leeward slope of 30 ~ 40, and a height of 2 ~ 5m facing southeast, with individual heights of 65,438+00 ~ 65,438+05m. It is composed of aeolian fine sand with undulating terrain.

Second, the structure

Yinchuan fault basin is adjacent to the Yellow River fault in the east and connected with Ordos block. The western boundary is the fault zone at the eastern foot of Helan Mountain; The south boundary fault, striking northwest, is located at the northern foot of Niushou Mountain; The northern margin is controlled by a group of concealed faults in the south of Shizuishan. Faults are developed in the basin, the basement structure is complex and the Cenozoic sedimentary thickness is large.

Three groups of fault structures are mainly developed in the basin, among which NE-trending and NNE-trending faults are the most developed, which control the east-west boundary of the basin. North-west faults and nearly east-west faults control the north-south boundary of the basin. See figure 3-3- 1 and figure 3-3-2.

1. NE-trending fault.

This group of faults controls the boundary of the basin, and some of them are hidden in the plain, which is the most developed group of faults in the basin. The fault is steep towards the center of the basin, which makes the basin in a stepped fault depression, belonging to extensional normal fault and dextral shear strike-slip fault. From west to east, the large control faults are as follows:

(1) Helan eastern foot fault zone

This fault zone is the structural boundary on the west side of Yinchuan Basin in Quaternary, starting from Shizuishan in the north and ending at Touguan in the south, with a length of 130km. The overall strike is NNE, and the dip angle is greater than 60. It is a fault zone composed of a series of right-lateral strike-slip normal faults. The eastern side of the fault is the piedmont diluvial inclined plain, and the western side is the towering Helan Mountain, with a relative height difference of 2,200 ~ 2,400, which is a fault triangle or diluvial fan.

(2) Luhuatai fracture

Located about 20 kilometers west of Yinchuan, it starts from Honggou in the south and ends at Pingluo Railway Station in the north, with an extension length of 73 kilometers. It is a normal fault, with NE strike, east dip angle of 565,438+0 ~ 78, and maximum fault distance of 5965 meters. The fault distance in the middle is large and the north and south ends become smaller. Paleogene is the western boundary of the lake basin and the west side of the fault, and Neogene directly covers Paleozoic or older strata. At the end of Neogene, the fault activity was not obvious.

(3) Yinchuan fault

The fault starts from Huangqu Bridge in the north and reaches Yongning in the south, with a total length of 85 kilometers. It is a normal fault with a dip angle of 45 ~ 77 and a maximum fault distance of 3220m, which decreases from south to north. In Paleogene, the fault controlled the eastern boundary of the sedimentary basin and continued to be active until modern times, so it was an active fault.

Figure 3-3- 1 Structural Outline of Yinchuan Plain (according to Wu et al., 2008)

Figure 3-3-2 Geological Profile of Yinchuan Plain (according to Wu et al., 2008)

(4) Yellow River fault

The fault controls the eastern boundary of Yinchuan Plain, starting from Shizuishan in the north and passing through Tao Le to Lingwu South, with a total length of 130km, striking NNE, with a steep profile and a dip angle of 49 ~ 66. It is a normal fault inclined to NWW, cutting Paleogene and Neogene. The east fault distance of Yinchuan City is 2800m, and the north fault distance gradually decreases, and the northeast fault distance of Helan County is 65438+. Only some areas were active in Quaternary.

2. East-west fracture

This group of faults mainly control the south and north boundaries of Yinchuan Plain, and some of them are concealed in the Plain. Different distribution positions have great differences in fracture properties.

This group of faults is mainly distributed at the northern end of Yinchuan Plain. The largest is the Zhengyiguan fault to the north of Shizuishan, which crosses Helan Mountain and extends eastward across the Yellow River to Laseng Temple, showing a concealed state. The Paleogene and Neogene scattered along the line are staggered, and the modern activities are mainly compression and sinistral strike-slip, which is weaker than the NNE fault. In addition, there are three south-dipping east-west buried faults between Huangqu Bridge and Shizuishan, which directly control the plain basement and make the basement fall 5km to the south within the range of 15km.

3. Northwest Fault

It is mainly distributed in the southern half of Yinchuan Plain, including the northern foot fault zone of Niushou Mountain, the Sanguankou-Jinjizhen fault zone and the NW-trending fault hidden in the plain.

(1) Niushou fault zone at the northern foot of Shandong Province

This fault zone is an integral part of Longshoushan-Liupanshan fault, and it is the boundary between China-Korea paraplatform and Qilian geosyncline. The fault zone consists of mountain side faults and platform front faults. The front-end fault of the platform is a hypothetical fault. Niushou Mountain is a cliff in front of it, with a height of 10m and a strike of NW 3 10. The platform is based on Paleogene, and the Quaternary deposition is very thin. The northeast of the platform is the alluvial plain of the Yellow River, and the thickness of Quaternary is more than 100m. Two earthquakes of magnitude 4 occurred in the southern part of the steep slope at 1970.

(2) Sanguankou-Jinjizhen Fault

The fault is large in scale, starting from Qingshikou, Zuo Qi, Alashan, Inner Mongolia in the north and extending to the south of Baitashui, Lingwu in the southeast, with a total length of 150 kilometers and an overall strike of about 330. It has compression and torsion properties and is concealed underground in Yinchuan Plain.

(3) NW-trending faults concealed in the plain.

There are many NW-trending faults hidden in the south of Yinchuan Plain. According to artificial seismic detection, the NW-trending fault extends to the vicinity of Suyukou in the northwest.

Third, class

Yinchuan basin is a Cenozoic faulted basin, and its center is located in the west of Pingluo-Fu Yao-Yinchuan line, between Yongning and Lingwu. It is a graben-type step basin with gentle east and steep west. In Pingluo-Fu Yao, Yinchuan and Yongning-Wuzhong areas, the thickness of Quaternary is greater than 1600 m, and the sedimentary thickness decreases rapidly towards the edge of the plain.

Quaternary sediments in the basin are mainly diluvial, alluvial and lacustrine deposits, with lacustrine deposits and aeolian deposits. The diluvial fan skirt at the eastern foot of Helan Mountain is well developed and gradually turns into alluvial fine soil belt to the east. Near the mouth of Qingtongxia gorge, there is an alluvial fan of the Yellow River, which is composed of gravel, and gradually transits to the north (downstream) into medium-fine sand and silty mixed cohesive soil, and the cohesive soil layer gradually increases.

Pleistocene under 1.

Yinchuan Formation is mainly deposited by rivers and lakes, distributed in the lower part of Yinchuan Plain, with the buried depth below 190 ~ 200 m, and it is grayish brown, grayish black fine sand mixed with gray, grayish brown, brownish brown clayey soil, grayish brown gravel and gravel. From south to north, the color changes from red to black, and the particle size changes from coarse to fine. The alluvial fan at Qingtongxia mouth of the South Yellow River is pebble or gravel, which is round. To the north, gravel layer, gravelly medium sand, medium sand and fine sand layer are in turn. In Yinchuan and Yin Bei areas, silty sand is dominant, and in some areas, the cohesive soil layer increases, containing humus or mud gravel.

2. Middle Pleistocene series

(1) River and lake sediments

It is distributed in the lower part of Yinchuan Plain, with a buried depth of 80 ~ 120 m, belonging to a set of fluvial deposits, and it is gray, grayish black and brownish gray fine sand mixed with grayish white, brownish gray and gray cohesive soil, and some fine sand contains mud and gravel. From south to north, the particle size gradually becomes smaller. Alluvial fans in Qingtongxia estuary of the Yellow River are all gravels, which gradually turn into gravels and medium-coarse sand to the north, and medium-fine sand with gravels to the south of Yinchuan. Horizontal bedding is developed, some of which are oblique bedding, containing more plant residues, humus and snail shells. The buried depth of the roof is 80 ~ 120m, the buried depth of the floor is 190 ~ 200m, and the thickness is 70 ~ 100m.

(2) Alluvial and diluvial sediments

It is distributed in the lower part of the alluvial plain of Wenquan-Yuquanying Farm, with a buried depth of 130 ~ 150m and a thickness of about 60m. It is mainly composed of dark gray, grayish yellow fine sand and blue gray silty fine sand mixed with cohesive soil, and the fine sand in the lower part of the alluvial plain on the west side contains gravel. From west to east, the color turns from yellow to black, which is alluvial deposit.

3. Upper Pleistocene series

(1) Pliocene diluvium

Distributed in the slope plain in front of Lushan Mountain in Helan Mountain. There are sporadic outcrops in the nearby mountains. The middle and front parts of the alluvial plain are 60-80m deep and 70-90m thick, and are composed of stones, boulders, gravels, sand, clayey sand and sandy clay. From west to east, the particles gradually become thinner until the front edge of alluvial plain becomes a fine-grained zone.

(2) alluvial deposits in the lower part of Upper Pleistocene

It is distributed in the lower part of the alluvial plain of Wenquan-Yuquanying Farm, with a buried depth of 60-80m and a thickness of 50-90m. The lithology is gray fine sand mixed with sandy clay, and the slope plain near the piedmont is gravel fine sand, mainly alluvial. Compared with the lacustrine plain strata, it is determined that this stratum is the lower member of Upper Pleistocene.

(3) Pliocene Upper proluvial.

Distributed in the eastern foot of Helan Mountain, it constitutes the main body of the piedmont inclined plain. From the top of the fan to the front edge, the particles change from coarse to fine, and the lithology of the top of the fan is block stone, boulder and gravel, which is sub-angular-semi-circular, and the sorting is poor; In the middle of alluvial fan, the lithology is gravel with gravel sand; The front soil area is characterized by clay sand or sandy clay interlayer. This layer is 60 ~ 80m thick.

(4) Upgraded and unified proluvial.

It is distributed in the alluvial plain of Wenquan-Yuquanying Farm and the platform on the eastern and southern edges of the working area. Hot spring-Yuquanying farm area is 60 ~ 80m thick, mainly composed of alluvial gray and yellowish brown fine sand mixed with gray and yellowish brown sandy clay. Alluvial plain is fine sand with gravel.

(5) Upper Pleistocene alluvial-lacustrine deposits

This group is a set of medium-thick silty sand interbedded with thin sand and clay, and there are multi-level bedding and oblique bedding in the sand layer, often mixed with clayey gravel. There are five small sedimentary cycles and three large sedimentary cycles, which are mainly distributed in the lower part of lake plain in Yinchuan plain, with a buried depth of 2 ~ 30m and a thickness of 60 ~ 120m, and the thickness in the north is greater than that in the south. From Qingtongxia of the Yellow River to the north, the grain size gradually becomes smaller, from pebbles and gravels to gravel layers and medium-fine sand layers.

4. Brand-new system

(1) diluvium

It is mainly distributed in the west of Yinchuan new city, the bus station and the southeast edge of the plain. The lithology is gravel, gravel and gravelly coarse sand mixed with cohesive soil, and the thickness is less than 5m.

(2) Alluvial and diluvial sediments

Distributed in the eastern edge of Yinchuan Plain, the lithology is fine sand, cohesive soil and gravel, and the thickness is less than 20m.

(3) Lake and swamp sediments

It is distributed in the first and second terraces of rivers in lakes and plains, with more in the north than in the south. The lithology is mainly silty sand and cohesive soil, and the thickness is less than 3m. In addition to modern lake and swamp deposits, irrigation silt deposits have been found in most areas of the surface of the first and second terraces. The lithology is mainly silty sand, clayey sand and sandy clay, and the thickness is generally less than 2m.

(4) aeolian deposits in the late Holocene

It is mainly distributed in the east of Tao Le County, Yinchuan Forest Farm and the south of Husha. The lithology is medium-fine sand and silty sand, and the maximum thickness is 20 ~ 30m.

Fourthly, the history of river basin development.

Yinchuan basin is a Cenozoic extensional fault basin. At the end of Jurassic, Yanshan movement caused the present Helan Mountain and Yinchuan area to uplift together, forming Yinchuan ancient fault uplift, and Yinchuan area thrust towards Helan Mountain area, with the highest uplift, which led to local stratigraphic inversion. In Cenozoic Eocene, Yinchuan ancient fault uplift began to disintegrate, and the compressive fault zone along the uplift axis appeared tension and rift, resulting in a large height difference and gradually expanding westward. At the end of Neogene, the Yinchuan basin continued to fracture, and the western boundary of the basin extended to the eastern foot of Helan Mountain. During the Quaternary, the uplift of the Qinghai-Tibet Plateau led to NE compression, which made the southern boundary faults of Yinchuan Basin active and led to the increase of vertical subsidence of NE faults, and Yinchuan Basin received extremely thick Quaternary sediments.

In the early Pleistocene, Yinchuan basin was a closed internal flow basin, and the ancient Yellow River flowed into the basin from Qingtongxia in the south, with a warm climate. Huge thick river and lake deposits are accumulated in the basin, and the subsidence center is located in Yinchuan central depression and Lingwu shallow depression zone. From south to north, the grain size of the rock changes from coarse to fine. The lithology of alluvial fan in Qingtongxia of the Yellow River is gravel or gravel in the south, gravel layer, gravel medium sand, medium sand and fine sand in the north, and silty sand in Yinchuan and Yin Bei.

In the early Middle Pleistocene, the Yellow River crossed Shizuishan and became an outflow river, and Yinchuan Basin also became an outflow basin. The ancient Yellow River road may also develop along two sedimentary depressions, and the river surface is relatively wide.

In the late Pleistocene, the Yellow River flowed through Ninghuazhai, Xincheng District, Wenquan Farm and Gaomiao Lake. South of Wenquan Farm is the late Pleistocene alluvial plain, also known as the third terrace of the Yellow River, on which there are many sand dunes or wind erosion lands.

In Holocene, the Yellow River turned to the east, leaving the second terrace and flood plain. It has recently formed on the river bed and has a tendency to move eastward.