The northern boundary of Sanjiang Plain is Heilongjiang, and it is surrounded by Xiaoxing 'anling, Nadanha Daling and Wanda Mountain in the west, south and east. The area is low and flat, and swamps and wetlands are developed. The terrain is 80m above sea level in Jiamusi area in the southwest, 50-60m in the middle of the plain, 34m at the lowest point of Heixiazi Island (Fuyuan Delta) in Fuyuan County in the northeast, and the ground slope is about11000. There are scattered residual hills and hills on the plain.

Generally speaking, the three steps on the ground of Sanjiang Plain are visible, and the first and second steps form a low plain with an elevation of 50 ~ 60m, and the platform elevation is about 100m. Heilongjiang and Wusuli rivers pass through the north and east of the plain, and Songhua River obliquely passes through the middle of the plain, forming a valley plain. In addition, we can see many residual hills and mountains standing on the plain, breaking the monotonous plain landscape.

Second, the landform formation factors

The geomorphic pattern of Sanjiang Plain is mainly controlled by geotectonics, especially the geological evolution since the late Jurassic, which is the main factor to form the geomorphology of Sanjiang Plain. External forces play a leading role in shaping the causes and morphological units at all levels. Internal force and external force are two contradictory aspects of geological action, but in different geological historical periods and under different geological conditions, the leading role of geomorphology is also transformed.

(A) internal dynamic geological process

1. geotectonics

This area consists of two traditional tectonic units, namely Jiamusi uplift belt of Jihei fold system and Nadanha Daling geosyncline fold belt. Before the late Jurassic, their temporal and spatial laws were different. About 1.9× 108a ago, the Pacific plate was formed and gradually moved to the northwest, and its role was obviously strengthened in the late Jurassic, and it became the master of the geological and geomorphological development in the eastern margin of the Asian continent. After the Late Cretaceous, the continental rift gradually formed and developed into a northeast continental rift system dominated by Songliao-Jieya Graben, including Yitong-Yilan Rift, Dunhua-Mishan Rift, Sanjiang-Amur Graben and its nearby fault basins, which together formed a complex geological structure pattern with mountains and plains alternating with corresponding uplift and depression, and laid the basic outline of the landform in this area.

2. Neotectonic movement

If the formation of the continental rift system in Northeast China has completed the basic outline of the landform in this area, then the joint action of new tectonic movement and external dynamic factors has shaped the present landform landscape. Obviously, neotectonic movement has played an important role in the development of landform and the formation of basic landform structure in this area. Its characteristics are as follows.

(1) forms a multi-step terrain.

Multi-level topography and valley terraces in mountainous areas, multi-level topography and sediments with different rhythms in plains are the direct products of neotectonic movement and external dynamics.

The mountainous area is equivalent to the planation plane in Xing 'an period, which was shaped in Paleogene and began to decompose in Neogene. It is equivalent to that the Brinellian surface was shaped from the end of Paleogene to the beginning of early Pleistocene in Quaternary, and decomposed before (300 ~ 120) × 104a at the end of early Pleistocene.

Terraces on the plain can generally be divided into three levels, the terrace is formed at (40 ~ 20) × 104a, the second terrace is formed at (20 ~ 7) × 104a and the first terrace is formed at (7 ~ 1) × 65438.

(2) Inheriting the old structural belt, with strong activity.

Mountain uplift and plain subsidence in this area generally occur at the structural boundary of rift system. The mountains around and inside the plain rise along the fault zone, and the plain sinks intermittently with the surrounding fault zone as the boundary.

(3) The landform is obviously different.

Because of the difference of rift intensity in time and space, there are obvious differences in landform.

The mountain uplift in this area is mostly arch-shaped, with the largest uplift in the central ridge line and smaller on both sides.

The subsidence amplitude and sedimentary thickness vary greatly in Sanjiang Plain, with the platform elevation 100m, the second terrace 60-80 m, the first terrace 40-80 m ... the lowest point is 34m, and the Quaternary sedimentary thickness100-300m.

(B) the control of external forces on the formation of landforms

The external forces that form the landform in this area are mainly the formation of running water and lakes.

1. Flow action

The water system in this area is relatively developed, with many rivers 100. The development of these rivers is of universal significance to the formation of landforms in this area.

Since the late Cenozoic, the low mountain and hilly areas have been in a period of differential uplift, and the flowing water is mainly characterized by erosion. Since the Quaternary, the vast plains have been intermittently deposited, so the flowing water is mainly accumulated, forming valley plains, fan plains and low plains.

2. Lake formation

Since the Quaternary, Sanjiang Plain has experienced multi-stage lake ups and downs. In the late Early Pleistocene, large lakes appeared in Sanjiang Plain. At the peak of lake development in the Middle Pleistocene, large lakes were formed in the south and west of Sanjiang Plain. The lake reached its peak about (40 ~ 20) × 104a ago, and the foothills were submerged by the lake, accumulating lacustrine clay layers. The late Pleistocene was the extinction period of lakes, about (20 ~ 15) × 104a ago. Due to the deterioration of paleoclimate, the lakes in Sanjiang Plain tend to die out. Around (15 ~ 7) × 104a, the paleoclimate warmed and the lake expanded again. The broad low plain east of Tongjiang-Jixian-Fujin is often submerged by the river, forming a vast river-flooded lake and depositing a silty clay layer with a thickness of 3 ~ 17m. Before (7 ~1.1) ×104a, the paleoclimate deteriorated again and the lakes in Sanjiang Plain disappeared. Therefore, some areas of Sanjiang Plain have formed lake facies low plains.

Three. Geomorphological classification and morphological description

Classification of landforms

According to the classification principle of geomorphic types in China Geomorphological Mapping Code, combined with the characteristics of this area, it is divided into 3 genetic types, 6 morphological subtypes and 7 lithologic-morphological units (Table 2- 1 and Figure 2- 1).

Table 2- 1 landform classification table

(2) Morphological description

ⅰ structure-flowing water landform

ⅰ 1 granite metamorphic hills

Scattered in Sanjiang Plain, the terrain elevation is 200 ~ 400 m, and the specific height is 60 ~100 m. It is mainly composed of granite and metamorphic rocks. The terrain is hilly, with a long and gentle slope, generally at 10 ~ 15, covered with residual slope deposits and black soil, and the Buxi planation plane remains at the height of 300 ~ 350 m at the top of the hill.

Figure 2- 1 Sanjiang Plain Landform Map

Its geomorphic formation time is n-qh.

Ⅰ 2 sandy gravel fan-shaped plain

Located in front of the eastern slope of Xiaoxing 'anling Mountain, it is adjacent to Songhua River in the east and Heilongjiang Old Road in the west. It consists of Ya Dan River, Dulu River, Wutong River and alluvial fan. Ice water fans and alluvial fans began to accumulate in the late Pleistocene, and continued to accumulate in the Holocene. Obviously, this fan-shaped plain is a multi-layered fan. It is composed of argillaceous gravel layer, and the particles become thinner at the front edge of the fan. Generally, the top of the sector is silty sand mixed with gravel, with a thickness of 1 ~ 3m, a deposition thickness of 15 ~ 25m, an elevation of 50 ~ 70m, and a specific height of less than 5m. The top surface of the sector is slightly inclined downstream.

Its landform was formed in QP3-QH.

Ⅰ 3-1Gravel Valley Plain

It is distributed in Songhua River, Heilongjiang River, Wusuli River and its main tributaries and valleys, including Class I terraces and high and low floodplains. The terrain elevation is 40 ~ 80m, and the specific height is 5 ~ 10m. The width of the valley plain varies with the size of the river. Generally, a large river valley is 5 ~ 15 km wide and a small river valley is 2 ~ 5 km wide, which is composed of silty clay, gravel and gravel layers. In the middle and lower reaches of large rivers, there are common side beaches, central beaches, cross flows, meanders, natural dikes, oxbow lakes, swamps behind banks, large and small lakes and so on.

Its landform was formed in QP3-QH.

Ⅰ 3-2 Mud-gravel Low Plain

It is distributed in the Songhua River Valley Plain, and the terrain is 60 ~ 80m above sea level. It is composed of loess silt, silty clay fine sand and gravel. The terrain is extremely flat, and the slope of the ground is generally less than 1‰, so swamp wetlands can be seen. Formed a landform of (7 ~ 1) × 104a.

Ⅱ Volcano-flowing water landform

Basalt mound: It is located in the area of 853 Farm, with a small distribution area, with a topographic elevation of 300-400 m and a specific height of 50-100 m. It is composed of Neogene basalt, with a flat top. Due to the action of running water in the later period, steep walls and deep valleys were formed.

Its landform was formed in Quaternary.

Ⅲ Lake landform

ⅲ 1 sandy clay platform

The platform distributed in front of the foothills around the plain has an elevation of 80 ~ 100 m, which is 10 ~ 20m higher than the local datum level, and is generally about 20m higher than the local datum level. It is connected with the low plain or fan-shaped plain and has obvious scarps. The surface lithology is the silty clay layer of Nongjiang Formation, and the gravel layer is below it. Due to the later erosion and denudation, the terrain fluctuated slightly.

Its landform was formed in Qp2.

Ⅲ 2 clay low plain

Located in the vast low plain east of Tongjiang, Fujin and Jixian. The terrain is low and flat, with an altitude of 50 ~ 60m. It is composed of loess silty clay and muddy silty clay. Ancient rivers, swamps and lakes are widely distributed, with dense river networks and lush aquatic plants. However, due to the large-scale land reclamation in recent decades, many areas have been reclaimed as farmland, and the original ecosystem has been destroyed.

Landform formation (20 ~ 7 years ago) × 104a.

Fourth, micro-geomorphology

There are residual hills, swamps and wetlands, ice mounds and lakes, ancient rivers and natural dams in this area.

(1) residual mound

It is distributed in parts of Fujin, Li Qinde, Luobei, Baoqing, Raohe and other places, 80 ~ 200m above the plain ground. The area of a single residual hill is 1 ~ 3km2, and the top of the hill is slightly flat and the slope is steep, which is mostly connected with the plain ground by scarps. Residual hills are mostly composed of granite and volcanic rocks. The largest remnant hill in this area belongs to the rich Ulhuri Mountain, which rises abruptly on the muddy and sandy plains, with a relative height of more than 400 meters, and is made of granite. According to the data, Huamashan extends below the plain ground, still low and steep, with the western slope as a cliff, the thickness of Quaternary is over 200m, and the eastern slope is slightly gentle, but the Quaternary is also above1000 m. The formation of surface residual hills is closely related to geological structures.

(2) Swamp wetland

Sanjiang Plain is the largest area of swamp vegetation distribution in China, which is widely distributed in all kinds of low-lying land and low flood plain. However, due to the drought year by year and the comprehensive development of Sanjiang Plain in recent years, the area of swamp wetland is shrinking day by day, and a considerable part of it has been reclaimed as farmland. According to the interpretation results in 2000 and the remote sensing interpretation results in the field, the current swamp wetland area is 4 489km2, mainly distributed in national wetland nature reserves such as Honghe, Sanjiang and Qixinghe, and provincial and county wetland nature reserves.

(3) Qiu Binghu

It is mainly distributed in the northeast of the survey area and the ancient river channel in Heilongjiang, and often appears symmetrically, which is called "Duihu" locally. The largest is Shuichengzi Ice Mound Lake, with an area of 2.5km2. There are more than 300 ice mounds and lakes of different sizes in the ancient river channel in the northeast of the survey area, and the smallest is 1 000m2. There are also ice mounds and lakes in the swamp around Naoli River, but the area is small and the lake is deep, which is close to groundwater recharge.

Most of these mounds were formed at the peak of the last ice age, about (2.2 ~ 1.5) × 104a ago. In Holocene, with the climate warming, ice mounds melted into lakes. Because most ice mounds are round, ice mounds and ice lakes are also round or fisheye-shaped.

(4) Ancient rivers

The ancient rivers mentioned here refer to those abandoned rivers with obvious negative topography and typical characteristics of ancient hydrological network. See Luobei Ancient River in Heilongjiang Province, the ancient river north of Fujin in Songhua River, the northeast of the survey area, Yalu River and Nujiang River.

The ancient river course in Luobei, along the famous mountains such as Luobei, Tuanjie and Lianhua, runs northeast-southwest and flows into Songhua River in the west of Puyang Farm, with a total length of 50km and a width of 6 ~ 12 km. The ancient river channel is about 3 ~ 5m lower than the first terrace on both banks, and the steep bank is obvious. The river is full of swamps, with ice mounds and lakes, as well as peat and permafrost under the peat layer.

The Songhua River ancient channel is located in Lianhua River and qinglong river, north of Fujin and east of Tongjiang, with a width of 5 ~ 15 km and a length of about 65km. The ancient river channel has negative topography and is covered by swamps and wetlands, which is about 2 ~ 3m lower than the first terrace on both sides of the river.

The abandoned time of these ancient rivers is generally at the beginning of the last glacial maximum, about 2.2× 104a ago.

(5) Natural dikes

In this area, most of the I terraces of Heilongjiang, Songhua River and their main tributaries have natural dikes. On the east side of Tongjiang-Fujinjiang section of Songhua River, there are several natural dikes along the river. The length of Dayushu Shagang natural dike exceeds 10km. Tongfu Expressway is built on it. The ancient natural dikes in Heilongjiang are distributed along the banks of Luobei ancient river, which is tens of kilometers long. In addition, there are five natural dams along the Dulu River.

The ancient natural dike is 5 ~ 10m higher than the surrounding ground and located on the gravel layer of the first terrace. The roof is flat, the cross section is trapezoidal, and the width is about 100 ~ 300 m. It is composed of medium-coarse sand and silty sand, and the surface layer is loess silty sand about 0.5m below the soil layer.