Evolution Characteristics and Stratigraphic Division of Quaternary Sedimentary in the South Wing of Yangtze River Delta, East China

This paper discusses the sporo pollen assemblage, evolution of sedimentary environment, sedimentary facies and the lithotope characteristics revealed by boreholes since pliocene epoch by carrying out 14 C dating, sporo pollen and micro-paleontological analysis of sediments in borehole BK01 (depth 237.80 m) in the southern flank of the Yangtze River Delta Plain. According to the results of this study, there are 17 species of foraminifera that have been identified as 11 genera, including 16 benthic foraminifera and 1 planktonic foraminifera ； 8 species of Ostracoda that have been identified as 8 genera; sporo pollen analysis shows that there are 37 types, including 18 species of woody plant pollen, 11 species of herbaceous plant pollen, 8 species of fern spores, of which, in the sporo pollen assemblage, woody plants have the highest content (about 85.1%), ferns spores have about 8.7%, and herbaceous plants have only 6.3%. Sedimentary environment records can be divided into 18 sporo pollen assemblage zones according to borehole lithology, sporo pollen assemblage and micropaleontology analysis. The results show that the BK01 bore strata from bottom to top can be divided into carbonaceous mudstone (Ech), Jiaxing Formation (N-Qp 1 j), Qiangang Formation (Qp 2 q) Dongpu Formation (Qp 3 d), Ningbo Formation (Qp 3 n) and Zhenhai Formation (Qhzh).


Introduction
With the change of global environment increasingly becoming an important issue of concern to the international community, the study of the evolution history of global climate and environment since Pleistocene has attracted much attention of scholars (Huang et al., 2018;Edwards, 2003). The Yangtze River Delta is an alluvial plain before the Yangtze River enters the sea (Su et al., 2017;Chen et al., 2009;Zhao et al., 2015;Liu et al., 2010). Studying the sequence of sedimentary strata in the Yangtze River Delta since Pleistocene is not only of great significance to the study of the development history of sedimentation and the evolution of palaeogeographic environment in the Yangtze River Delta (Wang et al., 2008), but also plays an unique role in studying the formation of alluvial plain and the change of palaeogeographic environment in the Yangtze River Delta in this geographical position on the southern wing of the Yangtze River Delta as a research hotspot in recent years (Fan et al., 2011;Chen et al., 2008;Yu et al., 2016).
At present, it has made a lot of important achievements in the study of the changes of paleoclimate and sea level since the last glacial period in the Hangzhou Bay area (Wang et al., 2008;Li et al., 2011), which not only lays a foundation for the further study of sedimentary characteristics, stratigraphic sequence and paleoenvironment evolution since the Quaternary, especially the Holocene, but also provides a detailed basis for the study of the evolution of the Yangtze River delta plain in the Quaternary (Lin et al., 2019a;Zhang, 2005;Yu et al., 2016;Miao et al., 2016;Liu et al., 2010). However, due to the difficulty of drilling, the depth of boreholes and the limitation of the analysis of borehole sample collection and testing in the previous work, the existing research on the Paleoenvironment of Holocene period is more in-depth (Liu et al., 2007;Liu et al., 2019), while the research on the evolution of sedimentary environment since Pleistocene period in the southern wing of the Yangtze River Delta is relatively few, and there are some bottlenecks such as the lack of high-precision stratigraphic data since Pleistocene.
Through sampling and analysis of high resolution sporo pollen, foraminifera and ostracods in the sediment cores of borehole BK01, this paper studies the evolution characteristics of sedimentary facies and sedimentary environment in the plain area of the Yangtze River Delta, and carries out lithological stratification and stratigraphic division of boreholes by combining isotope dating data, which not only provides evidence for the division and comparison of Quaternary strata in the Yangtze River Delta, but also provides basic information for the further research.

Materials and Methods
Borehole BK01 (30˚44'45''N, 120˚53'59''E), located in Daqiao Town, Nanhu District, Jiaxing City (Figure 1), is 237.80 m deep, of which the part below 236.50 m is bedrock. This study cuts, flattens and describes the lithology of drilled cores from the center along the longitudinal direction, collects 104 samples of micropaleontology (foraminifera, ostracoda) and sporo pollen analysis respectively, 184 samples of grain size analysis, 175 samples of paleomagnetism and 8 samples of photoluminescence dating, with a sampling interval of 1 m.
Micro paleontological analysis includes identification and quantitative statistics of foraminifera and ostracoda species. The specific analysis method is to dry sediment samples below 60˚C, weigh about 50 g dry samples and add a little 15% H 2 O 2 solution, wash the samples with 0.052 mm pore sieve after dispersal, carry out identification and quantitative statistics of genera and species under Nikon E 200 bio-microscopy after flotation and filtration (calculated with 100 g dry samples). The process of sporo pollen analysis is to take 50 g of each sample, extract the sporo pollen flotation material to identify, count and calculate its percentage content under the microscope after acid-alkali treatment and gravity liquid flotation (Zhang, 2005;Tao et al., 2005;Li & Zhao, 2018;Jia & Zhang, 2006).
Eight samples of sediment containing carbon chips are packaged in sealed packages for photoluminescence dating (OSL), which is carried out by the conventional methods (Nian et al., 2018;Gao et al., 2019), that is, to take out about 100 g of unexposed samples in a 1000 ml beaker and soak them in distilled water; first remove organic matter with 30% hydrogen peroxide, then remove carbonate minerals with 30% hydrochloric acid, and then wash the suspension to neutrality with distilled water, separate 4 -11 µm fine-grained mixed minerals according to Stokes theorem, then immerse them in fluosilicic acid for 3 days, remove feldspar and other minerals, purify fine-grained quartz, and evenly precipitate the purified fine-grained quartz sample on stainless steel sheet with 9.7 mm diameter by ethanol, complete it using Risø TL/OSL-DA-20 thermoluminescence/photoluminescence instrument produced by Risø Laboratory in Denmark.
The Department of Geosciences, Sun Yat-sen University complete sporo pollen analysis and micropaleontology analysis. The Guangzhou Institute of Geo-chemistry, Chinese Academy of Sciences is commissioned to complete sediment dating analysis.

Sporo Pollen Assemblage
The 104 sporo pollen samples from borehole BK01 are identified as 37 sporo pollen types, including 18 woody pollens, 11 herbaceous pollens and 8 fern spores. Among the sporo pollen assemblages, woody plants have the highest content (about 85.1%), ferns spores have about 8.7%, and herbaceous plants have only 6.3%. The pollen of coniferous plants is mainly Pinus and Taxodiaceae, and contain a small amount of pollen such as Tsuga. The pollen of broad-leaved plants is mainly Liquidambar, and contains a certain amount of Ulmus, Fagus, Juglans, Quercus and Carpinus. The pollen of herbaceous plants is Cyperaceae and Polygonum, followed by Poaceae and Caryophyllaceae, and a small amount of Liliaceae and Plantago. The pollen of pteridophytes is mainly Polypodiaceae, Trilete spore and Polypodium. It can be divided into 18 sporo pollen assemblage zones according to the changes of sporo pollen types and assemblage characteristics ( Figure

Micropaleontology
There are 104 foraminifera samples in borehole BK01, of which 11 genera and 17 species are identified, including 16 benthic foraminifera and one planktonic fo-

Chronological Sequence
The data of relevant ages measured by OSL method are shown in Table 2. The OSL ages at 24.0 m, 29.0 m and 59.0 m are 60.5 ± 3.3 ka, 74.8 ± 3.8 ka and 120.6 ± 8.9 ka, respectively. And the strata at that time belong to the Middle Pleistocene. In addition, OSL ages at hole depths of 108.5 m, 129.0 m and 138.2 m are all over 150 ka.

The Division of Quaternary Strata
The lithological stratification and stratigraphic division of borehole BK01 are shown in Figure 3. Based on the lithological stratification characteristics and the division of stratigraphic units in the study area (Qi et al., 2017;Lin et al., 2019b), the borehole are Ech (Ech,, and N-Qp 1 j, Qp 2 q, Qp 3 d, Qp 3 n, and Qhzh since early Pleistocene from bottom to top.
N-Qp 1 j: its hole depth is exposed from 147.0 to 236.5 m. Among them, 147.0 -148.3 m is the al-lN-Qp 1 j 3 , with a thickness of 1. Qp 2 q: whose hole depth is exposed from 84.0 m to 147.0 m, is marked by hard soil, which is divided into two sections with a thickness of 63.0 M. 84.0 -106.3 m is the Qp 2 q 2 of alluvial lake, marsh and alluvial origin, with a thickness of 22.3 m, mainly clayey soil and sub-sandy soil. 106.3 -147.0 m is the Qp 2 q 1 , which is formed by alluvial lake and marsh deposits, with a thickness of 40.7 m; 106.3 -132.4 m is the clayey soil layer with more iron and manganese oxides and more fine silt particles in part; 132.4 -137.8 m is the clayey soil and sub-sandy soil with developed bedding and partly contains semi-carbonated plant residues; Journal of Geoscience and Environment Protection  137.8 -147.0 m is a sub-clay and medium sand layer with slightly bedding and contains more ferromanganese oxides.
Qp 3 d: whose hole depth is exposed from 58.5 to 84.0 m, is marked by hard soil layer with a thickness of 25.5 m. 58.5 -62.5 m is Qp 3 d of alluvial-lacustrine origin, with a thickness of 4.0 m, mainly the clayey soil. The grains in the lower part are coarser and contain more ferromanganese oxides and calcareous argillaceous nodules. 62.5 -84.0 m is the Qp 3 d of alluvial-marine origin, with a thickness of 21.5 m, which is mainly composed of silt layer with thin sub-sandy soil.
Qp 3 n: whose hole depth is exposed from 19.6 to 58.5 m, is marked by hard soil. The main sediments are clayey soil, clayey soil with sub-sandy soil and sub-sandy soil of alluvial lake, marine and alluvial sub-clay, with a thickness of 38.9 m. 19.6 -43.1 m is the Qp 3 n 2 of alluvial-lacustrine, alluvial-marine and ma- of alluvial and marine origin, with a thickness of 15.4 m, mainly composed of clayey soil, clayey soil with sub-sandy soil. 43.1 -51.4 m is the alluvial lake origin, which contains more ferromanganese oxides and shows slightly stratification locally. 51.4 -58.5 m is mainly the sub-sandy soil and the clayey soil with alluvial and thin layers of marine sediments, partially silt, wormholes and more shell fragments can be seen in the marine layer. Qhzh: whose hole depth is exposed from 0 to 19.6 m. 0 -3.6 m is the Qhzh 2 , which is mainly composed of clayey soil layer of miscellaneous fill and alluvial Lake origin, with a thickness of 3.6 m. 3.6 -19.6 m is the Qhzh 1 , its lower part is silty clay. 3.6 -8.5 m is the Qhzh 1 of alluvial-lacustrine origin, with a thickness of 4.9 m and mainly clayey soil. 8.5 -14.0 m is alluvial-marine origin, with a thickness of 5.5 m, mainly composed of clayey soil with silty clay. 14.0 -19.6 m is marine origin, with a thickness of 5.6 m and mainly silty loam.

Conclusion
1) In this borehole, the ages of the photoluminescence strata at depths of 24.0 m, 29.0 m and 59.0 m are 60.5 ± 3.3 ka, 74.8 ± 3.8 ka and 120.6 ± 8.9 ka, respectively, with the strata age of the Late Pleistocene; The age of photoluminescence strata at depths of 90.2 m is 142.7 ± 9.6 ka, while that in the depths of 108.5 m, 129.0 m and 138.2 m is more than 150 ka.
2) The record of sedimentary environment of borehole BK01 can be divided into 18 sporo pollen zones. The early Pleistocene corresponds to the sporo pollen P1 -P3 zones and the early P4 zone. The middle Pleistocene corresponds to the late stage of the sporo pollen P4 zone and the P5 -P9 zones. The late Pleistocene corresponds to the sporo pollen P10 -P16 zones. The Holocene is the tidal flat facies deposited in the Qhzh 1 in the early stage and the fluvial and lacustrine facies deposited in the Qhzh 1 in the late stage.