Based on the study of Pleistocene ostracods in Henan Plain and the comparative analysis with Pleistocene ostracods in Wei Fen Basin, Xue Duo discussed the formation age of the eastern section of the Yellow River east of Sanmenxia.
Micro-paleoostracods are micro-arthropods. Except for a few molecules, most modern species live in rivers, lakes, ponds, swamps and oceans, and their geographical distribution is expanded or transferred through the communication of surface water bodies. Therefore, the similarities and differences of OSTRACODA combinations between the two places often reflect the communication degree of surface water between the two places. Henan plain strata, especially the upper strata of Zhoukoudian period, are rich in ostracods, which is the area with the most microfossils in Quaternary, providing sufficient evidence for the study of the formation age of the Yellow River east of Sanmenxia.
According to the data, the most common tertiary ostracod assemblages in the early Nihewan Wei Fen Basin are CYPRINIDAE, Leucaenidae, Illulidae, Candoni La and Candona. In addition, there are marine ostracods such as sanyunia, Callistochyhere, Sinocytheridea (in colorful flowers) and Tanella (Chen Shi) in the lower part, which coexist with foraminifera. It reflects the environment of brackish lakes (table 1 1. 1). During Nihewan period in Sanmenxia Basin, such as Lsh hole 28 1~390m ~ 390m and Huangdi River profile 2 ~ 16 (Yuan Fengdian, 1986), the main molecules of this assemblage, such as CYPRINIDAE, Tanella, Leucocythere and Sinocytheridea, were also found, indicating that. However, the common CYPRINIDAE, CYPRINIDAE, Callistocythere, Sinocytheridea and Tanella in Wei Fen Basin were not found in the Nihewan terrace of Henan Plain east of Sanmenxia. The OSTRACODA assemblages are mainly composed of Ilyocypris, Candona, Candoniella and other fresh water types, which are quite different from those in Wei Fen-Sanmenxia Basin, indicating that there was no large surface water exchange between the two places at that time, and they were in a relatively closed and isolated state (Xue Duo, 1996).
Table 1 1. 1 Wei Fen basin quaternary stratigraphic sequence and fossil assemblage comparison table.
(cited by Xue Duo, Lin Hemao and others, 1996)
In the late Nihewan period, the most common characteristics of the second OSTRACODA assemblage in Wei Fen Basin are Junwang and Condonilla, followed by Interleukin, Condona and Cyprinidae, which reflects that the salinity of the lake is lower than before, and it is a freshwater-brackish water environment. Although many genera and species of this combination are the same as the ostracod combination in Henan plain, such as Ilyocypris, Candoniella, Candona and other freshwater types, the difference is still obvious, which shows that there are no genera such as interleukin and CYPRINIDAE in Henan plain. So at this time, there is still no large surface water exchange between the two places.
However, by the time we arrived at Zhoukoudian, great changes had taken place. Ostracoda in Henan Plain is obviously similar to that in Wei Fen Basin. The five genera of the first Ostracoda assemblage in Wei Fen Basin are all found in Henan Plain. Li Shanian is the most common representative genus not only in Wei Fen Basin, but also in Henan Plain (table 1 1.2). At the same time, Li Shanian is a new genus and exotic species in Henan. All these are enough to show that in the early Zhoukoudian period, there was considerable surface water exchange between Wei Fen Basin and Henan Plain, that is, a large amount of lake water west of Sanmenxia had been discharged eastward through Henan Plain, and the eastern part of the Yellow River was gradually formed (Xue Duo, 1996).
Table 1 1.2 Brief Distribution Table of li mountain Virginia in Qp2-Qh Period of Henan Plain
(According to Xueduo 1996)
The above viewpoint can also be proved in the lithologic and sedimentological characteristics of Pleistocene in Wei Fen Basin and Henan Plain. In Wei Fen Basin, the underground Qinchuan Group and the underlying Sanmen Group are in unconformity contact (Chen Wanchuan et al., 1982), which is caused by a large amount of eastward drainage of lake water in the early Zhoukoudian period or the late Nihewan period, and the lake surface is greatly reduced. The name of the surface lagoon formation not only vividly shows that it was deposited after the lake was discharged eastward, but also consists of slope deposits and alluvial deposits, which are in unconformity or pseudo-conformity contact with the underlying strata (Lin Hemao et al., 1982). In the northern and eastern plains of Henan Province, the Nihewan terrace is mainly composed of brown, brown and grayish green loam, clayey silt and gravel layer with mixed grain structure, and its material composition obviously comes from nearby mountainous areas. In Zhoukoudian period, light yellow, grayish yellow, fine-grained, well-sorted sand layer and clayey silt are the main materials, and the material composition obviously comes from the western Loess Plateau, which is caused by the Yellow River transportation (Xueduo, 1996).
It is considered that foreign molecules such as li mountain Nya do not all appear on the boundary between Zhoukoudian Stage and Nihewan Stage in Henan Plain, and a few appear near the boundary. Therefore, the formation time of the eastern section of the Yellow River does not rule out the possibility of the late Nihewan. At the same time, the appearance of a small number of foreign molecules also shows that before the large-scale eastward discharge of lakes, the Yellow River was once an intermittent river formed by occasional overflow. To sum up, Xue Duo thinks that the formation time of the eastern part of the Yellow River is about 1.00 ~ 0.78 Ma in the early Zhoukoudian period or the late Nihewan period.
11.1.1.2 The eastern Yellow River was formed in the early Sarawusu period, about 0. 130Ma ago.
Jiang Fuchu, Wu Xihao and Guohua Xiao discussed the cutting age of Sanmenxia from the sedimentary characteristics of Mangshan loess and the disappearance of Sanmenxia ancient lake, and then studied the formation of the eastern section of the Yellow River.
Mangshan Plateau is located on the south bank of the Yellow River in the west of Henan Plain, and consists of aeolian loess with a thickness of 170m above S8. The existing platform is about 18km long from east to west, about 5km wide from north to south, and the highest elevation is 262m, which is the southeast edge of the transition zone between the Loess Plateau and the Henan Plain.
Sedimentary characteristics and material sources of (1) Mangshan loess
1) Characteristics of sediment magnetic susceptibility measurement and horizon correlation. According to the characteristics of lithology and magnetic susceptibility, the strata above S2 in Taohuayu section of Mangshan Mountain are divided into 17 layers from top to bottom (Figure 1 1. 1). For the strata above S 1, Zhiming Sun studied the paleomagnetism at 50cm, and the results showed that all of them were positive magnetization, which should be when they were distributed in positive polarity.
Figure 1 1. 1 loess-paleosol sequence, magnetic susceptibility and grain size curve of Taohuayu section in Mangshan.
(According to Fu Jiang Primary School, 1998)
1-loess; 2- Shallow paleosol; 3- paleosol; 4-paleomagnetic positive polarity time
According to the sequence of strata, combined with TL, 14C dating data and the characteristics of magnetic strata, Jiang Fuchu and others think that the two layers of deep-developed paleosols in Taohuayu section can be compared with S 1 and S2 in the Loess Plateau, respectively, and more than 7 1.4m are Malan loess and Holocene loess. The section depth of 0 ~ 0.6m is Holocene paleosol and loess; 0.6 ~ 7 1.4m is Malan loess in Salawusu period, of which 0.6~30.4m is L 1LL 1 loess, 30.4~49.9m is L 1SS 1 weakly developed paleosol, and 49.9 ~. 7 1.4 ~ 8 1.3~93.7m is S 1 paleosol, 8 1.3~93.7m is Zhoukoudian L2 loess, and 93.7~94.7m is S2 paleosol.
2) Characteristics of sediment granularity and deposition rate. The thickness of Malan loess in Sarawusu period in Taohuayu loess section of Mangshan Mountain, Henan Province is huge, reaching 70.8 meters, far exceeding the thickness of contemporary strata in the Loess Plateau. Jiang Fuchu and others investigated the grain size and deposition rate of loess. The results of grain size analysis show that the grain size of loess in Salawusu period is coarse, and the content of grain components > 20μ m and < 2μ m varies between 45% ~ 85% and 4% ~ 16% respectively (Figure 1 1. 1), so it should be mainly silty sand and fine sand. Compared with Luochuan, Xifeng and Lantian Liujiapo loess in the Loess Plateau, the grain size is obviously larger (table 1 1.3, table 1 1.4).
The calculation results of the average settlement rate of Mangshan loess show that there are great differences in different periods. The average sedimentation rate of the last glacial period was the highest, reaching 247cm/ka (table 1 1.5). The average sedimentation rate of paleosol S 1 in the last interglacial period was obviously lower than that of loess, but it still reached 18cm/ka.
Table 1 1.3 clay content in the upper part of Luochuan and Xifeng loess profiles (%)
(According to Fu Jiang Primary School, 1998)
Table 1 1.4 Table of grain size distribution content (%) in the upper part of Liujiapo loess section in Lantian
(According to Fu Jiang Primary School, 1998)
Table 1 1.5 Table of Average Deposition Rate of Mangshan Loess in Sarawusu Period
(According to Fu Jiang Primary School, 1998)
3) The source of Mangshan sediments. Jiang Fuchu and others believe that the sandy loess with a sedimentation rate as high as 247cm/ka appears in the mountainous area far away from the desert, which should be caused by specific local events and climatic conditions. The aeolian dust in Mangshan area is obviously coarser than that in the Loess Plateau, so the composition from the desert crossing the Loess Plateau is extremely limited, and most of them are near-source materials. They think that the alluvial fan of the Yellow River east of Jin Meng is the main source of Mangshan loess. Sediment produced by erosion in the Loess Plateau is transported by tributaries and main streams of the Yellow River and enters the Henan Plain through Sanmenxia. As the riverbed widens, the water flow disperses, the sediment transport capacity drops sharply, and the sediment carried by it quickly deposits, forming a huge alluvial fan of the Yellow River to the east of Jin Meng. After the northerly winter wind from high latitude passes through Taihang Mountain, it produces topographic downhill wind, which blows up the sediment transported by the Yellow River at the top of the fan into dust, and quickly accumulates at the downwind diffusion place due to the sudden drop of wind speed, forming the near-source aeolian loess of Mangshan Mountain with coarse grain size, high deposition rate and large stratum thickness.
Malan loess stratum in Salawusu period of Mangshan Mountain has a high sedimentation rate and needs rich material source supply. The thickness of loess-paleosol sequence in Mangshan Loess L2 and its following areas is roughly equivalent to the thickness of contemporary strata in the Loess Plateau. The thickness of S 1 composite paleosol is obviously thicker than that of S 1 paleosol on the Loess Plateau, and the thickness of aeolian Malan loess above S 1 is extremely large. According to the study of loess-paleosol sequence in the Loess Plateau, the soil-forming period of S 1 is128 ~ 75ka. They believe that this fact shows that the sediment, the erosion product of the Loess Plateau, started at about 130ka and was brought to the Henan Plain by the Yellow River through Sanmenxia, and then began to accumulate, forming a thick aeolian loess near the source area. At the same time, it also shows that since about 74ka, especially since 24ka, it is the most intense period of erosion on the Loess Plateau.
(2) The end of Sanmen Ancient Lake.
Sanmen ancient lake is distributed from Sanmenxia in the east to Baoji in the west, with a set of strata dominated by river and lake deposits. 1959, Pei Wenzhong divided the sediment profile of Dongpogou River and Lake in Sanmenxia (exposed thickness of 273m) into three rock profiles from bottom to top: 1 rock profile, with brownish red, brownish yellow and grayish white gravel layers in the lower part and brownish red gravel sub-sand and loam layers in the upper part, with the thickness of108./kloc-0. The second rock section, the lower layer of brownish red, brownish yellow, grayish yellow sand, gravel and sandy clay, with a thickness of 43m, is in unconformity contact with the underlying 1 rock section; The upper light yellow clayey silt (loess) is sandwiched with five layers of light brown-red paleosol, with vertical joints developed and 80.5m thick, and the upper and lower parts are in parallel unconformity contact. The third rock section, the lower part of which is brownish red, brownish yellow and apricot yellow clayey silt and loam, is 6.7m thick. The upper loess clayey silt is sandwiched with a thin layer of brownish red loam and gravel lens with a thickness of 34.7 meters. There is an erosion surface between the upper and lower parts. The research results of magnetic strata show that the loess layer above the second rock segment to the third rock segment is positive polarity, which belongs to the positive polarity of distribution.
From May 65438 to May 0997, Jiang Fuchu and Wang Sumin, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, started the field geological survey of lacustrine-fluvial strata in Sanmen system. It is found that Dongpogou section is incomplete, and a large section of strata is missing in the middle due to faults and other reasons. Judging from the topography of Dongpogou section, the loess layer above the second rock section may overlap with the third rock section. The relationship between the second rock segment and the third rock segment is erosion unconformity. When the loess in the upper part of the second rock section is piled up in the high place, the fluvial facies and lacustrine facies in the lower part of the third rock section may be deposited in the low place at the same time, and they are deposited in different stages at the same time. From 65438 to 0957, Jia Fuhai and others measured the most complete Sanmen section in Huangdi River, Pinglu, Shanxi Province, which was covered with loess early or late. The deposition of white sand layer at the top of Sanmen system marks the end of Sanmen ancient lake, and the third-grade loess covered terrace system developed later was formed by the Yellow River. When lacustrine facies and fluvial-lacustrine facies were deposited in the lower part of Sanmenxia Basin, red clay-loess-paleosol stratigraphic sequence was accumulated in the upper part, both of which belonged to the simultaneous heterogeneous correlation system, that is, the classic "Sanmen system" included bottom gravel layer, lower Sanmen Formation and upper Sanmen Formation, all of which were covered by loess. Jiang Fuchu and others re-investigated the section of Huangdi River in Pinglu and collected relevant samples. The results of paleomagnetic test and analysis show that the accumulation of Sanmen system began in Gilbert's reversal of polarity, and experienced many river and lake environment successions, generally ending in the late positive polarity of Boujon.
(3) The Yellow River runs through Sanmenxia.
To sum up, Jiang Fuchu and others believe that the Yellow River began to flow eastward through Sanmenxia with loess sediments about 0. 130Ma ago from the thick paleosol of Mangshan loess and the subsequent development of the huge Malan loess. The disappearance of Sanmen ancient lake was also about the same time. Due to the infiltration of Sanmenxia, a large amount of lake water flowed out and drained quickly, ending the deposition of the "white sand layer" of the Upper Sanmen Formation, which was covered with Malan loess. Sanmenxia was connected in the early Salawusu period, forming the main stream of Sanmenxia section of the Yellow River. At this time, the Yellow River can really flow eastward through Sanmenxia and come to Henan Plain with a large amount of sediment eroded from the Loess Plateau. After leaving the gorge, the huge ancient alluvial fan of the Yellow River and the vast North China Plain were formed. Moreover, the strong northerly monsoon blows the fine powder at the top of the alluvial fan of the Yellow River into dust, which quickly accumulates in the downwind diffusion part, forming a very thick Malan loess in Salawusu period of Mangshan Mountain.
11.1.1.3 The East Yellow River was formed in the early Zhoukoudian or the late Nihewan, about 0.78Ma ago.
The research results of Quaternary geological evolution in Henan Plain show that the eastern part of the Yellow River was formed in the early Zhoukoudian period, that is, about 0.78Ma ago. Its main geological basis is as follows:
1) The sediment source is different from the lithofacies paleogeographic environment. Before and after the formation of the Yellow River, the material source of the Quaternary sedimentary circle in Henan Plain was obviously different from the lithofacies paleogeographic environment. According to a large number of borehole data in Henan plain in Quaternary and the changes of lithofacies paleogeographic environment in various periods of Quaternary, the sediments in Nihewan period before the formation of the eastern section of the Yellow River mainly came from Luo Yi River and surrounding mountainous areas; However, the sediments in Zhoukoudian period and later period after the formation of the eastern part of the Yellow River mainly came from distant places, which are similar to the material characteristics of the Loess Plateau, and the lithologic characteristics of the upper and lower sets of strata are obviously different. The upper part is carried by the Yellow River, and the alluvium mainly comes from the western Loess Plateau, which is light yellow, grayish yellow, yellowish gray thick sandy soil and clayey silt (there is clayey accumulation only in the edge zone and downstream area of the ancient river belt, mostly lenticular). The particles are coarse, silty and loose, and contain scattered calcium and calcium cores. The lower limit of this set of alluvium is the lower limit of Zhoukoudian stage. However, the underlying Nihewan terrace stratum is mainly brown, gray-green, brown or reddish brown, with loam, clay mixed with clayey silt and sand layer or gravel layer with mixed grain structure, which is deposited in ice water and glacial lake facies. In this regard, it can also be seen from the Zhengzhou-Xinxiang Yellow River cross-section (Figure 1 1.2) that the materials in the lower section of Nihewan River come from the mountainous areas in the south, north and west, with coarse gravel and sand distributed, which is poorly sorted; However, the upper sand layer of Zhoukoudian has fine particles and good sorting, and its material characteristics are obviously different from those in the northern and southern mountainous areas, which is undoubtedly the product of long-distance transportation of the Yellow River. The provenance, lithologic characteristics and paleogeographic environment of the upper and lower strata are obviously different, and the boundary age is about 0.78Ma, which is the early Zhoukoudian or the late Nihewan.
Figure 1 1.2 Cross-section of Yellow River alluvial fan in Zhengzhou-Xinxiang, Henan Plain
2) The upper sand layer is developed, and the characteristics of alluvial fans are remarkable. Alluvial sand layers are developed in the upper reaches of the Yellow River, and the spatial distribution of sand layers has typical characteristics of river alluvial facies, mostly in thick layers and strips, and the thickness of a single layer is generally 10~20m ~ 20m. The mainstream facies of the ancient river belt are mostly medium sand, fine medium sand and medium fine sand, and the marginal facies are fine sand and silt. The sand content of river zone strata is generally above 40%, up to 80%, and the sand layer is horizontally distributed and mostly fan-shaped. Sandy soil mainly comes from the Loess Plateau in the west, forming a fan handle from wen county to Mengzhou. After entering the plain, it spread to the northeast, east and southeast, and the early edge has reached the areas of placanticline, Kaifeng and Fugou (Figure 1 1.3). Only the Yellow River can form such a large typical alluvial fan, and other rivers can't finish it in a short time. The sedimentary particles of alluvial fan have the characteristics of thickening from the top of fan to the front of fan and from the axis of fan to the side of fan. In the vertical direction, it has many characteristics of "dual structure" and horizontal bedding, and the development of sedimentary sand layer and alluvial fan characteristics is extremely remarkable. This is obviously different from the lower ice water, alluvial fan-shaped land and river-lake facies accumulation before the development and formation of the Yellow River. Before, the sand layer was not very developed, mostly a small amount of cohesive soil and sand, and the fan-shaped characteristics and scale were not prominent.
Figure 1 1.3 Isograms of alluvial fan distribution and sedimentary thickness of the Yellow River in Henan Plain
3) The characteristics of heavy minerals in sediments are obviously different. Taking amphibole as an example, the alluvial deposits of the Yellow River in the upper part of Henan Plain are characterized by continuous high values, which can reach more than 40%. From west to east, the relative content changes from more to less, as shown in Chapter 7, Section 4. However, the content of sediments in the lower part of Nihewan terrace suddenly decreased, and there was no regularity from west to east. The boundary between the characteristics of heavy minerals in the upper and lower sediments is precisely the boundary between the positive and negative distribution of magnetic strata, that is, the bottom boundary of alluvial deposits in the Yellow River, and the time is also the early Zhoukoudian period or the end of Nihewan period.
4) The species and genera of OSTRACODA fossils are obviously different. Through the comparative analysis of the ostracod assemblages in the Quaternary strata of Sanmen Ancient Lake in Henan Plain and Wei Fen Basin, it is found that the species and assemblages of ostracods in the two areas are obviously different before the early Zhoukoudian or the end of Nihewan. Since then, the ostracods of Henan Plain are very similar to those of Sanmen Ancient Lake in Wei Fen Basin. See Chapter 7, Section 3 for details. Just like Xue Duo's research results, many genera of OSTRACODA assemblage in Sanmen ancient lake in Wei Fen basin can be seen in Henan plain, among which Lishania has become a new genus and exotic molecule in Henan, and it is the common representative genus of Wei Fen basin and Henan plain. These indicate that in the early Zhoukoudian period, there was communication between Wei Fen Basin and Henan Plain, that is, Sanmen ancient lake water began to flow eastward into Henan Plain, and the eastern section of the Yellow River was gradually formed.
Based on the above facts, the author thinks that the eastern section of the Yellow River began to form in the early Zhoukoudian or the late Nihewan, that is, about 0.78Ma, and its basis is sufficient. At this time, under the tracer erosion of the Yellow River in the east, the bedrock horst of Sanmenxia was cut through, and the huge Sanmen ancient lake began to flow eastward in the early Quaternary, and Sanmen ancient lake disappeared. At present, the rock island standing under the dam of Sanmenxia Reservoir is the bedrock residual mound left by the river tracing erosion and cutting the horst during this period.