Zhong-shuang Cheng, Chen Su*, Zhao-xian Zheng, Zhuang Li, Li-kang Wang, En-bao Wang

1 Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Science, Shijiazhuang 050061, China.

2 Key Laboratory of Groundwater Science and Engineering, Ministry of Natural Resource, Shijiazhuang 050061, China.

3 Shandong Geological Environment Monitoring Station, Jinan 250014, China.

4 Research of Regional Geological Survey of Heilongjiang, Harbin 150036, China.

Abstract: Thick loess is deposited on the platform in the piedmont zone of Muling-Xingkai Plain (Muxing Plain), but the genesis of the Muxing loess is still unclear. The aims of this study are to analyze the grain size characteristics of Muxing loess collected from the cores of a typical borehole (ZK1) in the piedmont zone of Muxing Plain, and to verify its genesis. The Muxing loess is mainly composed of the particles with diameter less than 50 μm, with an average content of 92.48%. The coarse silt particles with diameter of 10-50 μm are the basic composition of aeolian sediments, and their average content is 44.34% for the Muxing loess, which is the mode class among the particles with different diameters. The grain size parameters and frequency curves are similar to those of the typical aeolian sediments. The distribution characteristic of the Muxing loess in the C-M scatter diagram is consistent with that of the Xi Feng loess. In addition, the discriminant analysis shows the Muxing loess mostly consists of aeolian sediments. Therefore, it can be concluded that the Muxing loess mainly resulted from aeolian deposition based on the grain size characteristics. Muxing Plain is dominated by the monsoon climate, and the wind-blown dusts are gradually deposited after being transported over long distances.

Keywords: Muxing Plain; Loess; Grain size; Characteristics; Aeolian deposition

Introduction

The Muling-Xingkai Plain (Muxing Plain) is located in the east of Heilongjiang Province,China, and the Wandashan Mountain is along the northern side and western side of Muxing Plain.Previous studies suggested that the sediments on the platform of piedmont plain were mainly composed of residual slope deposits. However, a field survey conducted by the author shows the upper and middle parts of the sediments on the platform are fine-grained loess layers with a thickness of about 20 m. Coarse-grained slope deposits only occur in the deeper part of the sediments. This phenomenon is significantly different from the stratum law formed by the typical residual slope accumulation. Therefore, the grain size analysis is used to further understand the genesis of the Muxing loess on the platform of the piedmont plain.

Loess is widely distributed in China, and loess deposition is a special kind of dust-sediments forming in quaternary period. Loess is an important indicative factor in studying the paleoclimate in the Late Cenozoic, and the genesis of loess is the basis for the restoration of paleoenvironment (Liu, 1985;Ding et al. 2005; Cao et al. 2014; Peng et al. 2016;Chen et al. 2017). Generally, the grain size compositions of sediments vary with different transportation power and transportation manner.Therefore, the cause and source of loess can be identified based on the analysis of the particle size characteristics, especially the high-precision systematic analysis (Jiang et al. 2018; Li et al. 2010;Wei et al. 2015; Wang et al. 2018a). The grain sizes of sediments are dominated by many factors,such as the dynamic conditions, the material sources, and the climate factors and so on. Thus, it is possible to further understand the process of sedimentary formation by the analysis on the grain sizes of deposit sediments (Sun et al. 2018; Wu et al. 2019).

The phases and cycles of the East Asian monsoon changes are recorded by the grain size sequences of loess and paleosoil in China, and a series of achievements have been made currently(Lu and An, 1998; An and Liu, 2000; Liu et al.2005; Li et al. 2014; Wang et al. 2018b). The studies on loess distributed in China are mainly concentrated on the Loess Plateau and Xinjiang.Numerous studies were conducted in the Three Gorges Basin of the Yangtze River. But there are relatively few studies on the sedimentary characteristics and the loess genesis in the Northeast China (Huang et al. 2010; Zeng et al. 2016; Wang et al. 2018; Zhu et al. 2019). At present,Huangshan loess in Harbin has been identified as aeolian sediments, which is the basis for ascertaining cause of Muxing loess and palaeoclimatic environment in Northeast China (Li et al. 2010; Wu et al.2014; Li et al. 2014; Wei et al. 2015).

In this paper, a systematic analysis on the grain sizes of Muxing loess collected from the cores of a borehole at the platform of Muxing Plain is carried out. The aims are to compare the grain size feature of Muxing loess with that of other loess with verified causes, and eventually to identify the genesis of Muxing loess. This study will be beneficial for the research on the paleoclimatic evolution in Northeast China.

1 Study area

The Muxing Plain is situated in the eastern part of Heilongjiang Province of China. The Wandashan Mountain is along the northern side and western side of the study area, and the eastern and southern boundaries are the Ussuri River and the Xinkai Lake, respectively. The Muxing Plain falls in the sub temperate zone with mid-temperate continental monsoon climate. The mean annual temperature is 1.9 ℃, and the mean annual natural precipitation is 540-680 mm (Cao and Zhu, 2014). It is windy and dry in the spring, and rainy and cool in autumn.The special topography and climate condition provide the material and energy sources for the dust weather, which is the basis of the formation of loess layer.

The Muxing Plain experienced at least two crustal movements before the Paleozoic. But the crust was in a relatively stable period with only slight movements from Cambrian to Devonian.The Wandashan Mountain began to form in the Carboniferous period, and lacustrine deposits with intermediate-acid volcanic rocks were deposited along the fault depression zone. In the early and middle Mesozoic, a deep fault depression occurred in the study area, which resulted in the geosynclinal formation. After the Upper Cretaceous period, large scale fissure eruptions along the edge of the fault depression occurred frequently. Small graben-type subsidence occurred in Miocene and Pliocene, making up the embryonic form of the modern plain. A series of minor graben-type faults occurred in the basin at the beginning of the Neogene Quaternary, accompanied by basalt eruption. And then, Muxing Plain entered the period of ongoing tectonic movement.

The platform mainly appears in the northern piedmont zones of the Wandashan Mountain. The sediments on the platform are deposited and exposed in strips, with thickness ranging from a few meters to thirty meters. The sediments on the platform mainly consist of loess, and the residual sediments are visible in the contact zone between the terrace and platform.

2 Materials and methods

2.1 Lithology

The location of borehole ZK1 is shown in Fig. 1,and the depth of the ZK1 is 55 m. The sediments within 23.2 m below the ground surface consist of loose clays and gravels, but gradually turn into hard rocks from 23.2 m to 55 m. The core samples from ZK1 were not tested for dating. But it can be inferred that the sediments within 23.2 m below the ground surface are composed of quaternary sediments based on the lithological change.

The lithology of the quaternary strata is mostly the gray-yellow clayey silt. The loess section on the platform of the Muxing Plain shows the development of vertical joints and iron infiltration along with them. The color of the core samples from ZK1 gradually darkens from top to bottom,and becomes nearly brick red at the bottom, where large pieces of crushed stone appear and the size can reach 10 cm. The sorting of sediments at the bottom is very poor, basically without rounding( Fig. 2 and Fig. 3). In addition, the sediments at the bottom are unconformable on the underlying layer.It was believed that the sources of the sediments on the platform were eluvium and glacial debris with the occurrence of a large amount of gravels.

2.2 Sampling and measurement

Fig. 1 Location of the study area

Fig. 2 Elurium and fissure at ZK1

Drilling and coring of ZK1 began in September 2017, and the diameter of core is 108 mm. The depth of borehole is 55 m, and the core recovery rate is above 70%. The loess samples were mainly collected from the cores from 1 m to 21.2 m below the ground surface. The samples were collected at 2 cm intervals, and were stored in transparent 100 g bags. A total of 102 loess samples were collected in this study.

The particle size of loess samples was analyzed at the Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences. Mastersizer 2000 laser particle size analyzer was used to test the particle size, and the test range is 0.02-2 000 μm. The error was close to but less than 2%.

The pretreatment steps on the samples are as follows. Firstly, 10 mL of 10% H2O2was added into the sample, and the samples were heated ultrasonically until no more bubbles were generated. Secondly, 10 mL of 10% HCl was added into samples, and the samples were heated continually. Then, 30 mL of distilled water was added into samples, and then the supernatants were poured out after one day, waiting for the pH of the sample to be neutral. Finally, 10 mL of 0.1 mol∙L-1sodium hexametaphosphate ((NaPO3)6) was added into the pretreated samples before the final test,and then, the samples were oscillated for 10 minutes in an ultrasonic cleaner. The tests of particle sizes were performed after the samples were thoroughly dispersed. Each sample should be tested repeatedly for three times.

Fig. 3 Lithologic column plot of ZK1

2.3 Particle size analysis

In this study, the particle size parameters involved are the average value, the standard deviation, the skewness, and the kurtosis. These parameters are mainly obtained by the graphical method. The cumulative distribution curve was plotted based on the particle size composition of every sample, and the particle sizes corresponding to different cumulative percentages can be read from the curve.Then, the parameter values is calculated based on the related formulas.

The φ standard are used to represent the particle size D, and the conversion formula is as follows:

The classifications of the grain size represented by φ are shown in Table 1.

Mean grain size (Mz)

The mean grain size is the basic data in the grainsize analysis and can indicate the concentrated state of the grain size distribution. The grain sizes corresponding to the cumulative percentages at 16%, 50% and 84% are used in this study for their good representativeness.

Table 1 Classifications of the particle size expressed by φ

Where: φ84, φ50, and φ16 represent the grain sizes corresponding to the cumulative percentage at 84%, 50%, and 16%, respectively.

Standard deviation (σ)

Standard deviation is used to indicate the sorting of grain sizes, the formula is as follows:

Where: φ95, φ84, φ16, and φ5 represents the grain sizes of cumulative percentage at 95%, 84%,50%, and 16%, respectively.

It is defined that the sorting is excellent when σ＜0.35, good when σ=0.35-0.5, at a medium level when σ=0.5-1, relatively poor when σ=1-2, poor when σ=2-4, and extremely bad when σ＞4.

Skewness (Sk)

Skewness can reflect the degree of asymmetry of the cumulative frequency curve. This parameter is often coupled with the standard deviation to evaluate the sedimentary environment.

When the curve is close to a normal distribution,the SK value ranges from -0.1 to 0.1. When the SK value is negative, the curve is skewed to the left.When SK value is positive, the curve is skewed to the right, indicating the loess consisting of high contents of fine-grained sediments (Table 2).

Table 2 Classifications of grain size skewness

Kurtosis (Kg)

Kurtosis mainly reflects the width of the cumulative frequency curve of grain size of samples, and it is closely related to the sources of the material and the depositional environment. The kurtosis value can be expressed as follows:

When the value is positive, the cumulative frequency curve appears in narrow peak shape.When it is negative, the curve shows a wide peak shape (Table 3).

3 Result analysis and cause discussion

3.1 The particle size composition of loess

Previous studies have proved the coarse silt sediments were the basic constituents of typical aeolian loess, and the clay sediments were the submode components of typical aeolian loess (Liu,1985; Li and Yang, 2001). It represents the background value of atmospheric dust in the aeolian loess area, and indicates the accumulation of wind-blown dusts is the basic way to form the loess sediments.

In this paper, the grain size borderline between clay, fine silt, coarse silt and fine sand is set as 5 μm,10 μm and 50 μm, respectively. As shown in Fig. 4,the content of fine sand (＞50 μm) in the Muxing loess ranges from 0.02% to 32.08%, with the average of 7.52%. The content of coarse silt (10-50 μm) ranges from 11.61% to 71.93%, with the average of 44.34%, which is the highest in the core samples. The content of fine silt-grade (5-10 μm)is 10.14%-31.09%, with the average of 21.31%.And the content of clay particles (＜5 μm) is 13.33%-57.10%, with the average of 26.8%, which is the sub-mode group in the core samples.Therefore, the Muxing loess has high content of the coarse silt and the clay, which is consistent with the composition of the aeolian loess.

According to the comparison on the composition of Muxing loess with other typical loess from different regions (Fig. 4), it can be seen that particle composition of the Mixing loess is similar to that of the aeolian Xiashu loess from the Loess Plateau and the modern aeolian Harbin loess (Liu,1985; He et al. 2009; Li et al. 2010). All of them contain high level of coarse silt and clay, distinctly different from the composition of fluvial sediments(mainly fine sand). The Muxing loess is further identified as the aeolian deposition based on its components. However, some differences in thecontents of particles between Muxing loess and other loess have been shown, which may be due to the difference in geographical location, climate environment, and sources of material. Both the Muxing Plain and Harbin are located in the northeast of China, and the similar climatic conditions and sources of sediments lead to the similar grain sizes of loess. Therefore, the particle composition of the Mixing loess is closer to that of Harbin loess.

Table 3 Kurtosis classifications of sediment

Fig. 4 Comparison of grain size compositions between Muxing loess and other loess

3.2 The frequency curve

The grain size frequency distribution curves of sediments with different sources are different from each other. Therefore, the frequency curve is an important method to assess the genesis of the sediments.

The grain size frequency curves of each loess samples collected from the cores at ZK1 (Muxing loess) are characteristic of similar bimodal shape,and the main peak of each curves locating at the coarse silt particles (Fig. 5). The mode of sample sizes nearly lies between 5φ and 7φ on the x-axis,and individual samples have a secondary peak between 0φ and 1φ. Previous studies have shown that the particles with size of 5-7φ (10-50 μm)were usually the principal particles transported by wind. With the increase of grain size, the floating capacity of particles in the air became worse. The particles with size less than 4φ (greater than 63 μm)could not suspend in the air, and could only move in the form of jumping (Pye and Tsoar, 1987;Zhang et al. 1998). The grain size frequency curves of Muxing loess samples show that the Muxing loess is mainly composed of the silt-grade particles, which further reflects its aeolian cause.The multi-modal curves may be related to the participation of water in some stages or shortdistance transportation by wind (Sun et al. 2000).

The frequency curves of the Muxing loess are similar to that of the eolian Wushan loess (Fig. 5).It can be speculated that the Muxing loess on the platform of the Muxing Plain are also aeolian sediments. However, the shapes of the Muxing loess curves are thinner and their slopes are steeper compared with those of Wushan loess, indicating better sorting of Muxing loess.

3.3 The grain size parameters

The grain size parameters have a good correlation with the formation environment of the sediments,therefore sediments with different sources have different parameters. The mean grain sizes (Mz) of the core samples from ZK1 (Muing loess) range from 5.48φ to 7.91φ, with the medium of 6.59φ.The median diameters (Md) of samples are 5.05-7.83φ, with the medium of 6.50φ. In addition,standard deviations of samples range from 1.16φ to 2.20φ, with the average of 1.59φ. The skewness values (Sk) of samples range from -0.08φ to 0.36φ, and the values of kurtosis (Kg) are 0.81-1.31φ.

Fig. 5 Grain-size frequency curves of Muxing loess (a) and Wushan loess (b)

The grain size parameters of the Muxing loess are very similar to those of Xiashu loess and Harbin loess (Fig. 6), further indicating that the Muxing loess results from the eolian deposition.

Fig. 6 Comparison of grain size measured between loess in ZK1 and different sediments

3.4 C-M diagram

The C-M diagram has been widely used in identifying the sedimentary environment and source of loess (Lu and An, 1998). In the C-M diagram, C is the grain size corresponding to 1%of the cumulative curve, and M is the median size of samples. The sedimentary environment and source of loess can be further determined by comparing the position of loess with unknown genesis and the loess with a known genesis in the diagram. As shown in Fig.7, the Muxing loess samples are distributed closely around the Xifeng Loess samples, suggesting their similar depositional environment and genesis. Hence, the Muxing loess can be regarded as the eolian deposition.

3.5 Discriminant function

The discriminant function is another important way to identify the sedimentary environment and genesis of loess. The parameter values or discriminants with different sedimentary types can be inferred based on the multivariate statistical analysis of sediment grain size parameters. The formula to identify the sedimentary environment is as follows:

Fig. 7 Position of the Muxing loess samples(Triangular points) and the Xifeng loess samples(Cross points) in the C-M diagram

Mz, σ, Sk and Kg are the mean grain size,standard deviation, Skewness value, and Kurtosis value, respectively.

Generally, when the sediments result from the eoposition, the value of the discriminant should be less than -2.741 1 (Sahu, 1964). Nearly all the results of the Muxing loess samples are less than-2.741 1, except one sample No. 51(Fig. 8). The results based on the discriminant function support the aeolian sedimentary environment of Muxing loess on the platform of the Muxing Plain.

Fig. 8 Results of each core loess samples based on the discriminant function

3.6 The genesis of Muxing loess

Loess with thickness of more than 20 m is deposited on the platform of Muxing Plain, and the gravel sediments only appear under the loess sediments. Based on the grain size analysis of the loess, it can be concluded that the Muxing loess is mainly aeolian sediments and resulted from wind transportation. The Muxing Plain is dominated by the monsoon climate. The sediments are gradually deposited after being transported over long distance.This study has a better understanding on the loess genesis and sedimentary environment in the northeast of China.

4 Conclusions

The genesis of loess on the platform in the piedmont zone of Muxing Plain was studied in this paper. The results show that the Muxing loess is the eolian deposits based on the grain size composition, C-M diagram, and discriminant function. The loess sediments are gradually deposited over many years after being transported from distant sources under the impact of the monsoon climate. This study is beneficial for the research on the paleoclimatic evolution in northeast China.

Acknowledgements

This study was financially supported by the China Geological Survey’s project (No. DD20160311)and National Natural Science Foundation of China(No. 41602268).