Quaternary stratigraphic correlation and environmental changes in Hangjiahu plain

(Zhejiang Geological Survey Institute, Xiaoshan 3 1 1203)

According to the multiple stratigraphic division of lithostratigraphy, magnetic stratigraphy and biostratigraphy, the Quaternary strata in Hangjiahu Plain are divided into 9 sequences, 8 climatic cycles and 27 base-level cycles, and the strata in different boreholes and different regions are accurately compared. According to the paleogeographic characteristics of different climatic cycles, the quaternary environmental changes in Hangjiahu plain were studied. At the same time, the changes of Qiantang River and Hangzhou Bay after the land formation of Hangjiahu Plain are preliminarily analyzed.

Keywords: Hangjiahu Plain; Quaternary; Stratigraphic correlation; Base level cycle; Changes in circumstances

The first problem to be solved in Quaternary research is stratigraphic division and correlation, and "fossil sequence law" is almost the only practical global basis for stratigraphy. Traditional biostratigraphy uses "standard fossils" to determine the stratigraphic age and make regional or intercontinental correlation. Strata are divided according to rocks or rock combinations, and try to find widely distributed and continuous "marker beds" for regional comparison. However, the total time of Quaternary strata in Hangjiahu Plain is only 2.48Ma；; Fossils, such as foraminifera, ostracods, calcium microalgae, etc. The geological distribution time often increases by 1 ~ 2 orders of magnitude; Quaternary stratigraphic phase transition in this area is extremely frequent, and adjacent boreholes are hundreds or even tens of meters apart, so it is impossible to directly compare them. Before the sediments finally enter a stable geological state, they will undergo many times of erosion, transportation and transformation, and the lack of strata and frequent phase changes have become the important characteristics of Quaternary in this area. To a great extent, the marine plain is lateral accretion, which retrogrades and progressions to land or ocean with the rise and fall of sea level. Therefore, based on the theory of high-resolution sequence stratigraphy, the author uses the concept of "base-level cycle" and the research methods of lithostratigraphy, climate stratigraphy, chronostratigraphy, magnetic stratigraphy and sequence stratigraphy to establish the isochronous stratigraphic framework and correlation basis, solve the isochronous comparison problem between standard hole and ordinary borehole climate cycle, and further compare the strata in climate cycle with the isochronous theory of "base-level cycle" to make a quasi-instantaneous lithofacies paleogeographic map.

1 Quaternary stratigraphic division

Quaternary is limited to 2.48Ma represented by China geological year. According to the age calibrated by the paleomagnetic data of 1- 1 hole, the bottom boundary of Quaternary in Hangjiahu Plain is roughly estimated to be at 258.7 meters ... The thickness of Quaternary sediments in Hangjiahu Plain varies greatly, and the sedimentary types are complex and diverse, and the lithofacies changes sharply and the lithologic combination is complex. Now, from bottom to top, it is as follows.

1. 1 Jiaxing Group (North QPJ)

This layer is only found in boreholes, and its lithology is mainly gravel, gravel and medium-fine sand, with local gravelly loam or gravelly clayey silt, and hard plastic loam is common at the top. The bottom is not integrated on the bedrock, the buried depth of the bottom is 156 ~ 3 18m, and the thickness varies greatly laterally, 6 ~ 148m. 1- 1 hole length148m, which can be divided into three lithologic sections:

Lower segment1.1.1(n-qpj1)

It is mainly composed of gravel, gravel sand and medium coarse sand, with occasional thin layers of fine sand and clay, mainly yellow and brownish yellow. Regionally, it is mainly distributed in the structural depressions of Wang-Xitang line and Xinfeng-Pinghu line, and most other areas are missing, with a thickness of about 81m.

1. 1.2 middle part (n-qpj2)

Mainly distributed in the basement structural depression area, with a thickness of about 30m. The lithology of the lower part is mainly grayish green ~ grayish yellow silty fine sand, partially mixed with cohesive soil layer; The upper part is mainly composed of loam and clayey silt, containing more iron-manganese nodules.

The upper segment of 1. 1.3 (n-qpj3)

The lower part is brownish gray silty fine sand and medium coarse sand, with dual structure and rich in calcium; The upper part is composed of grayish blue, brownish yellow clay and loam, which is rich in calcareous nodules and carbonized plant debris. It is mainly developed in the northeast of Hangjiahu Plain, with a thickness of about 35m.

1.2 Qian 'gang Formation (Qpq)

It is mainly distributed in Hangjiahu area, with a small amount in Xiaoshan, Shaoxing and northern Cixi. Borehole exposure is mainly composed of fine sand and loam. The bottom of this formation is separated from Jiaxing formation by paleosol layer. The buried depth of the bottom plate is 9 1 ~ 232 m, and the thickness is 5 ~102 m. The bottom plate is thin in the southwest and thick in the northeast, which can be divided into upper and lower sections.

Lower segment1.2.1(qpq1)

Mainly gray; The lithology is mainly medium-fine sand and silty sand, containing more calcareous argillaceous nodules, with a thickness of about 43m. The top is generally gray-green, gray-yellow hard plastic loam, with iron-bearing manganese nodules.

Upper part of 1.2.2 (Qpq2)

There are two sedimentary rhythms, which are composed of loam, clayey silt and medium-fine sand alternately, and the thickness is about 20m. The upper part is gray-green hard plastic loam or clay, which is rich in iron-manganese nodules and calcium nodules; The lower part is gray fine sand, medium fine sand and calcareous argillaceous nodules. In the piedmont zone, the typical network structure is developed in clay with a thickness of about 20 m.

The marine foraminifera in this stratum indicates that there was a short-term transgression in this period, which was in the late Middle Pleistocene and belonged to the earliest small-scale transgression in this area.

1.3 Dongpu Formation (Qpd)

Widely distributed in plain areas, mainly continental deposits in the early stage and alternating land and sea in the later stage. It can be divided into two parts:

1.3.1(qpd1)

The lower part is composed of loam or clay, clayey silt or silt, and there are several sedimentary rhythms. The upper part is silty sand or clayey silty soil, with silty sand and loam locally. The single layer is several millimeters to tens of centimeters thick, mainly gray, and contains calcareous nodules.

The upper segment of 1.3.2 (Qpd2)

Upper clay or loam, grayish green, brownish yellow, containing iron manganese nodules and organic matter, mainly grayish green, plastic to hard plastic, containing iron manganese nodules and calcium nodules; The lower part is mainly composed of gravel and clayey silt, mainly gray and grayish yellow, and locally grayish brown.

1.4 Ningbo Formation (Qpn)

The upper part is dominated by river loam and clay, grayish yellow, grayish green, hard plastic and iron-bearing manganese nodules. The middle part is composed of fine sand, clayey silt and loam, containing calcium and organic matter, mainly gray; The lower part is loam, gray-dark gray, plastic. The buried depth of the bottom plate is 6 ~ 85m, and the thickness is 0 ~ 49m.

1.5 Hemudu Formation (Qhh)

It includes five cultural layers, namely Kuahuqiao, Hemudu, Majiabang, Songze and Liangzhu, and the Holocene sedimentary strata below them, with a time limit of 10000a. B.P (or 12000a. To 4300a a. Therefore, Hemudu Formation in different places has different cultural layers and lithologic combinations, but its cultural layer is a remarkable symbol.

1.6 Zhenhai Formation (Qhzh)

Widely developed in this area, it constitutes the present plain topsoil. The lower part is mainly muddy loam and clay, flowing plastic ~ soft plastic, gray, partially mixed with silt lens; The genetic type of the upper part is more complicated. In low-lying areas, loam and clay of lagoon facies and limnetic facies are mainly grayish brown and gray, forming a limnetic water network plain.

The estuarine area is dominated by clayey silt and silty sand, with gray and micro-bedding, forming the estuarine alluvial plain.

The coastal plain area is dominated by marine, alluvial silt and clayey silt, with thin layers developed and mainly gray. The buried depth of this layer is 5 ~ 76m.

2 Division of climate cycle and datum cycle

The Quaternary time span is short, and organisms have lost or partially lost their role in dividing chronostratigraphy. Based on the isochronous theory of climate cycle and datum level cycle, the author uses the methods and techniques of climate cycle, magnetic stratigraphy and sequence stratigraphy to make high-precision correlation of strata in the whole region, and makes paleogeographic maps of different periods with microfacies as the mapping unit to reveal the evolution of paleoenvironment.

The exposed surface such as "hard soil layer" and iron-manganese nodule layer formed in cold period is the natural interface of climate cycle transformation. Taking the bare surface as the dividing line, the cycle from warm to cold is also a climate cycle.

The datum plane is the theoretical potential energy balance surface that separates deposition and erosion, and there is neither deposition nor erosion on this surface. The datum plane always moves to the maximum or minimum of its amplitude in one direction, forming a cycle of rising and falling, which is called datum plane cycle. The strata preserved in this time domain form a genetic sequence. Because its boundary is a time plane and a time stratigraphic unit, the subsequence is a geological body with relatively short base-level cycle time interval.

According to the division marks of climate cycles, superposition styles and interface characteristics of base-level cycles, the Quaternary system in Hangjiahu Plain is divided into 8 climate cycles and 9 subsequences. The obvious coupling relationship between subsequences and climate cycles indicates that subsequences are controlled by climate cycle changes [1]. Among them, the Pleistocene includes 7 climatic cycles, and the Holocene has 1 climatic cycle, and each climatic cycle can be divided into 3-4 base-level cycles (table 1).

Table 1 Division Table of Quaternary Climate Cycle and Base Level Cycle in Hangjiahu Plain

3 regional comparison

The regional correlation of Quaternary strata is limited to Shanghai and Jiangsu in the Yangtze River Delta Plain, and only the Quaternary strata in the marine plain are compared. Quaternary correlation in the plain area of the Yangtze River Delta is one of the important issues in Quaternary research. In the process of regional comparison, firstly, according to the signs of paleosol layer, weathering crust and river scouring surface, the author makes a more accurate comparison of the strata in climate cycle, and then according to the isochronous principle of base-level cycle, the comparison accuracy is greatly improved. At the same time, the age and climate stratigraphic signs determined by magnetic strata are taken as important signs. Regional comparison is made based on the G4 14 hole in Jiading, Shanghai, SK 10 hole in Nantong, Jiangsu and 1- 1 hole in this area [2].

3. 1 Late Pleistocene stratigraphic correlation

The lower age limit of China stratigraphic code (Quaternary) is 2.48Ma, which is located in Jiaxing Formation in Zhejiang, Anting Formation in Shanghai and Haimen Formation in Jiangsu respectively. The top boundary of the Lower Pleistocene is the interface of Burong positive polarity and Songshan negative polarity. Between these two chronological boundaries, there are Jiaxing Formation in Zhejiang, Anting Formation in Shanghai and Haimen Formation in Jiangsu, which can be compared with each other.

3.2 Middle Pleistocene stratigraphic correlation

The time limit of the Middle Pleistocene is 0.73 ~ 0. 13 Ma. The sediments in Zhejiang Province are called Qiangang Formation, and the top boundary is an ancient soil layer, which shows that it is a climate cycle from warm to cold. Shanghai is the Jiading Formation with a height of more than 0.73Ma, and the top interface is the river scouring surface, which is the product of climate cooling, sea level drop and seawater extension, and also indicates the temperature drop. Although the manifestations are different, they also reflect a climate cycle. The Middle Pleistocene in Jiangsu is called Qidong Formation, and its top interface is hard soil layer, which also reflects the cold climate and has the same climate change law as that in Zhejiang. Therefore, they can be compared.

3.3 stratigraphic correlation between late Pleistocene and Holocene

The late Pleistocene was 6.5438+0.3 million years to 6.5438+0 million years.

Zhejiang includes Dongpu Formation and Ningbo Formation. Dongpu Formation is located on the paleosol layer below the magnetic bottom boundary of Burong (1 14000), and the top boundary is the paleosol layer above the magnetic top boundary of Burong (108000). The top boundary of the Upper Ningbo Formation is marked by the end of the last glacier, and its sediments show "hard soil layer".

The Upper Pleistocene in Shanghai also includes Kunshan Formation and Gehu Formation. The bottom boundary of Kunshan Formation is also the scouring surface below the bottom boundary of the Burong magnetic event, the top boundary is the scouring surface above the magnetic event, and the top boundary of Gehu Formation above it is also the interface of the last ice age. Therefore, Dongpu Formation and Ningbo Formation in Zhejiang can be compared with Kunshan Formation and Gehu Formation in Shanghai respectively.

The top boundary of late Pleistocene Kunshan Formation in Jiangsu Province is an ancient soil layer that can be compared with Dongpu Formation, and the top boundary of Gehu Formation is also the last glacial period, so it can be compared.

The Holocene in Zhejiang, Shanghai and Jiangsu is marked by the last glacial period or the Gothenburg magnetic event, so Hemudu Formation and Zhenhai Formation in this area can be compared with Shanghai Formation and Rudong Formation (Table 2).

4 Environmental changes in Hangjiahu Plain

4. Quaternary development and evolution of1Hangjiahu plain

The development of Quaternary in Hangjiahu Plain can be divided into two stages, marked by the first flood and local conditions. In the early stage, it was a continental lakeside river, which was called the "river-lake" development stage. The later stage is dominated by tidal flat deposits, which is called "tidal flat" development stage.

4. 1. 1 development stage of rivers and lakes

The early development stage of rivers and lakes includes the first to fourth climatic cycles, which is equivalent to the lower part of Jiaxing Formation to Qiangang Formation. The existing landscape pattern of Hangjiahu Plain is high in the southwest, low in the northeast and hilly in the southwest, which was formed in Pliocene in the new century and has been controlling the Quaternary sedimentation.

At first, lakes were mainly developed in the north, and only a few small lakes were seen in Cixi in the south. They are all surrounded by ancient land or bare areas and are strictly controlled by the terrain. As time goes on, lakes generally push south and west, approaching the hilly areas in the south and west. Small lakes gradually merge into larger lakes, and depressions in hilly areas also form lakes of different sizes. This phenomenon reflects the changing law of global warming.

Table 2 Quaternary Stratigraphic Division and Regional Correlation Table

Rivers will appear in almost every climate cycle, and become more and more developed, while the source area gradually moves south and the scale gradually becomes larger. Alluvial fans can be seen in all climatic periods, and they vary in size, either living between hills or on plains. The development degree of rivers is closely related to rainfall, and its gradual development characteristics also confirm the climate change characteristics of climate warming and increasing rainfall. The movement of the source area to the hilly area seems to indicate the strengthening of leveling. Alluvial fans can be seen not only in hilly areas, but also in plain areas, indicating that not only the relative height difference in hilly areas is higher than today, but also there are many hills in plain areas.

Although it generally reflects climate warming, there are four secondary climate fluctuations at this stage, showing a positive correlation that the lake area increases with the increase of climate temperature and decreases with the decrease of climate temperature. In the early stage of each climate cycle, the distribution of lakes is limited to the northeast, and it expands to the south in the middle stage. Lakes of different sizes can also be seen in the hilly areas of the south and west. In the late period of climate cycle, due to the decrease of lake level, the lake shrank to the northeast, and the original great lake was divided into two lakes, east and west.

The distribution of denuded area is contrary to the evolution of lakes, and each climate cycle shows the changing law of increasing in the early stage, decreasing in the middle stage and increasing in the later stage. In the early stage, it appears in fragments, in the middle stage, it only appears sporadically in the western and southern hills and the original lake area, and in the later stage, it appears in fragments, sometimes reaching into the lake to split the lake in two or form a peninsula.

From the analysis of the fluctuation law of lakes and denuded areas, they are coupled with climate cycle, that is, climate cycle controls the development and evolution of lakes and denuded areas. The temperature rises, the rainfall increases, the rivers develop, and the lake surface rises, which causes the denudation area to shrink and the lake area to expand. In hilly areas, due to the increase of rainfall and the elevation of groundwater level, lakes of different sizes are formed by collecting water in low-lying areas. When the climate becomes cold and dry, the situation is just the opposite.

It is also obvious that tectonic activity controls the deposition at this stage. At the end of the first climate cycle, the denudation area distributed in the northeast, which may be caused by the differential settlement caused by the reactivation of the northeast fault. At the end of the second climate cycle, Jiaxing and Zhapu Peninsula extended to the lake area, but the lake surface declined and the peninsula shrank, reflecting that the crust may have declined. In the early and late stages of the third climate cycle, the bottom topography of shallow lakes in the northeast and deep lakes in the southeast appeared, which can only be caused by the tilt of "blocks" caused by structural strengthening. At this stage, tectonic activity and climate change control sedimentation and sediment distribution.

4. 1.2 tidal flat development stage

In the fifth climate cycle, tidal water invaded this area, and the corresponding tidal flat deposits began to appear, which opened the development stage of Hangjiahu Plain dominated by marine deposits. At the initial stage of transgression, the coastline was located in Huzhou-Yuhang-Haiyan-Cixi line. At the peak of transgression in the sixth climate cycle, the coastline has reached Huzhou South-Sandun-Xiaoshan-Shaoxing-Cixi. The coastline of the seventh climate cycle pushed southward to Hangzhou-Xiaoshan area at the peak of transgression. Therefore, the fifth climate cycle to the seventh climate cycle is the peak of transgression, that is, from the late middle Pleistocene to the late Pleistocene. The sea area in this area continued to expand and the coastline moved southward, indicating that the temperature was generally warming during this period. Every climate cycle has a law of cold-warm-cold, which shows three development stages: initial transgression-peak transgression-seawater recession. The response of sediments is that the sediments in intertidal zone or subtidal zone move southwest, the ancient land and denudation area shrink, and the sand bars or dikes decrease or advance southwest. When the sea recedes, the coastline shifts to the sea, the ancient land increases, and the denuded area increases rapidly, often only the sediments in the supratidal zone are seen. However, at the end of the seventh climate cycle, that is, at the end of the late Pleistocene, there was a large-scale regression phenomenon, which was characterized by a large area of denudation area, the coastline moving northeast and a large number of sand dikes, but only tidal deposits were seen in this area, which may be a reflection of the arrival of the ice age. According to the analysis of paleogeographic map, the tide mainly invaded from northeast to southwest at this stage, and Cixi and Yuyao had two tidal inlets, which were east-west and southeast-northwest respectively.

The early Holocene was a post-glacial transgression period, which was mainly manifested in that the denudation area and the ancient land divided the beach into several unequal sedimentary areas, all of which were deposited in the supratidal zone, and the ancient Taihu Lake was much larger than it is now. By the second base-level cycle, the sea area expanded sharply, and almost all of it was submerged by seawater except the present hilly area; Shallow sea deposits appear in Hangzhou Bay, with subtidal, intertidal and supratidal deposits around it, which represents the characteristics of high sea level. The biggest feature of the third base-level cycle is the sharp expansion of denudation area, and the coastline moves to Huzhou, Luoshe, Wangdian and Zhapu. 1 ~ 3 base-level cycles constitute the secondary climate cycle. By the fourth base-level cycle, large-scale transgression appeared again, and the coastline was similar to the second base-level cycle, reaching the existing hilly area directly, marking the beginning of a new secondary climate cycle.

The ancient Qiantang River was formed in the fifth climate cycle. Although it appeared and disappeared in the following period, from the analysis of tidal inlets, especially in Holocene, the land was basically flooded by rivers. So it has existed since its formation, at least in Holocene. It's just that sometimes they are forced to leave this area because of the sea invasion.

Like the development stage of rivers and lakes, tectonic activity also plays a role in controlling sedimentation, but in this stage, especially after Holocene, tectonic action is much weaker than that in the development stage of rivers and lakes.

4.2 Changes of Qiantang River and Hangzhou Bay since 2500

According to the data analysis, after the Hangjiahu Plain was fully landed, the coastline of Qiantang River and Hangzhou Bay began to move northward from 5000a a. Blood pressure rose to 2600a a. About 4000 years ago, the prototype of Qiantang River and Hangzhou Bay basically took shape. From 2 100 to 2600, land has appeared in the south of Chengxiang Town-Cixi-Zhenhai Line in Xiaoshan, Hangzhou, forming Xiaoshao and Ningfeng Plain. At that time, Hangzhou Bay was called "Big Horn", and its mouth width was over 100 km. The top of the bay is located in today's Hangzhou, and the southern boundary is in today's Xiaoshan-Cixi-Zhenhai line.

After 2 100a. B.p. Due to the sea level drop, the ancient Yangtze estuary moved southward, the delta continued to extend to the northeast sea area of the investigation area, and the tidal current of Zhapu-Jinshan deep trough washed away. And gradually formed the present triangle bay coast. Due to the complex hydrodynamic conditions in the estuary, the wide and shallow river channel and bank slope are composed of loose light loam and silt, which makes the river channel extremely unstable, leading to large-scale erosion and deposition changes on both banks, and the riverbed swings back and forth from south to north.

1600a later. B.p. The Qiantang River estuary and Hangzhou Bay have strong uplift and the coastline changes frequently. The estuary section recorded the great changes of Qiantang River from the north gate, the small and medium gate and the south gate into the Haihe River trough (Figure 1), and the coastline fluctuated by 20km.

A large area of land north of Xiaoshan-Cixi-Zhenhai in the south bank of Hangzhou Bay was formed in the process of the evolution of the mouth landform of Hangzhou Bay from "big corner" to "middle corner" in the last 600 years, because the top of Hangzhou Bay moved eastward and the southern beach gradually silted northward. From14th century (Ming dynasty) to now, the top of Hangzhou Bay has pushed eastward for nearly 50 kilometers.

Figure 1 Schematic Diagram of the Evolution of Qiantang River and Hangzhou Bay

Since 1950s, the north shore of Hangzhou Bay has been eroded, and systematic regulation and revetment works have been carried out, gradually making the north shore a relatively stable shore, but the erosion and siltation of some river sections still overlap. The south bank of Hangzhou Bay has been in a state of siltation and the local coast is relatively stable, which belongs to the continuous accumulation type.

From the above, it can be seen that the muddy coastline of the coastal plain of Hangzhou Bay has been in the process of erosion and siltation in the last 50 years, and the stability is only relative, and it has eased in the late stage of the previous upheaval, but the general trend of collapse in the north and siltation in the south still exists.

5 Conclusion and discussion

(1) Based on the theory of climate cycle and datum cycle, combined with the method of multiple stratigraphic division of rocks, the correlation problem between different boreholes is well solved, and the accuracy of quaternary stratigraphic division and correlation in Hangjiahu Plain is further improved.

(2) Based on the study of climate cycle and base level cycle, the Quaternary strata in Hangjiahu Plain are divided into 9 subsequences, 8 climate cycles and 27 base level cycles, and the environmental development and evolution of the Quaternary Hangjiahu Plain are systematically and comprehensively analyzed, and further divided into two stages: river and lake and tidal flat.

(3) Although the erosion coast on the north bank of Hangzhou Bay is mainly erosion, it has become a relatively stable coast after systematic regulation and revetment, but some areas still have erosion and siltation, and the south bank has been in a state of siltation, and some areas are relatively stable and continuous accumulation.

refer to

Chen, Qin,, etc. High-precision comparative study method of Quaternary strata in Hangjiahu Plain. Geology of China, 2002,29 (3): 275 ~ 280.

Chen, Gu Mingguang, Peng Zhenyu, et al. Quaternary geological study on Hangjiahu plain. 2004, internal edition, 4 ~ 138.

Environmental changes and correlation of Quaternary strata in Hangjiahu Plain

Chen, Gu Mingguang, Peng Zhenyu,

(Zhejiang Geological Survey Institute, Xiaoshan 3 1 1203)

Abstract: Based on the division of lithostratigraphy, magnetic stratigraphy and biostratigraphy, according to the isochronous theory of base-level cycle, the Hangjiahu Plain is divided into 9 subsequences, 8 climatic cycles and 27 base-level cycles, marked by ancient topsoil and river scouring surface. High-resolution stratigraphic correlation is carried out in different formation holes and different areas. In addition, combined with the paleogeographic characteristics of different climate cycles, the changes of quaternary stratigraphic conditions in Hangjiahu plain are studied, and the plain is divided into two development and evolution sections: river and lake and tidal flat. At the same time, the changes of Qiantang River and Huangzhou Bay after the land formation of Hangjiahu Plain were studied for the first time.

Keywords: Hangjiahu Plain; Quaternary (q); Stratigraphic correlation; Base level cycle; environmental change