| The core XYC was taken from the XingYi village in Zhengjiang Province, ShangYu City, which is in the NingShao Plain and the lower reaches of Cao-E river. The drilling is in the South Bank of the Qian Tang River paralic area, where the sedimentation is extensive developed, especially, it is a extensive development of marine sediments, lacustrine deposits and fluvial deposits since Holocene in Zhejiang. The sedimentary characteristics of the core has a great significance for the reconstruction of Holocene environmental in Zhejiang. In this paper, based on the particle size measurement of 1717 samples of the core XYC, Attempt to explore the grain size characteristics of the sedimentary core. Combined with magnetic susceptibility, lithologic characteristics, historical data, I4C dating data, make a further analysis of the characteristics of sedimentary environment changes recorded by the core XYC. We have some conclusions:(1) The sediment of XYC Core is mainly composed of silt (4-63μm). In the 1717 samples, the silt content of the siltly sample which is higher than 50% account for 88.35%.8.68% of the samples contain silt between 30-50%, Only 2.97% of the samples were less than 30%. All of the sample, the content of silt is 69.65%, the content of clay is 18.69%, the content of sand (>63 m) is least which is 11.66%. In the size of silt, small silt (4-10μm), compared to the coarse silt (10-63 m), is more in advantage. The average content percent of former is 21.49%, the average content of the latter is 48.16%. The content of clay and sand is different in different depth. The component of clay, the content is lower than 10% in some layer, some layers are higher than 20%, The maximum content of clay is 42.96%, and the minimum is only 4.02%. The component of sand, the maximum is up to 74.86%, the minimum is 0, the variation coefficient is up to 143.34%. So, grain size composition changes with depth, especially clay, sand content changes with the depth of constructive reflect the borehole records the changes of sedimentary environment and sedimentary phase change. So, grain size component changes with the depth, especially clay, sand content changes for different depth, reflect the drilling records the changes of sedimentary environment and sedimentary phase change.(2)According to the composition of grain size and its parameters, the XYC Core sediments can be classified into four types of sedimentary, That is silty sand (I), sandy silt (II), clay silt (III), silty sand (IV).Silty sand (I). Sand is the dominant particle, Content between 4O%-75%. Silty sand is second, content between 15%-40%. The frequency curve shows a multiple peaks, and the samples are mainly distributed in the depth of 23.10m-25.30m and 32.10m-34.34m. Sandy silt (II). Silty sand is dominant particle, Content between 40%-70%.Sand is second, content between 20%-40%. The content of clay is less than 20%, the frequency curve showed multiple peaks or single peak. This kind of sample is distributed in the depth of 22.62m-23.08m and 25.32m-27.72m. Clay silt (III). Silty sand is dominant particle, content between 60%-80%. The content of clay is more than 20%, which is in the second position, and its maximum content can be 40.69%. The content of sand is lower than 20%. The frequency curve is single peak, mode peak is at 6Φ-6.5Φ, and this kind of sample is distributed in the depth of 4.24m-13.32m and 19.06m-22.60m. Silty sand(IV). Silty sand is the dominant particle, content more than 70%, and its maximum content can be 91.29%. The content of clay is lower than 20%, is also the sand. The frequency curve is single peak, the mode peak is at 5.5Φ-6Φ, and this kind of sample is distributed in the depth of 2.22m-4.22m,13.34m-19.04m, and 27.74m-32.08m.(3)The main components of XYC Core grain-size and the change of grain-size indexes with depth are also obvious. Along the XYC Core from bottom to top, the silt content showed a trend of fluctuations increase.In the longitudinal direction silt has three high-value and three low-value.The depth of three high values respectively 27.74m-32.08m,22.62-9.22m and 4.24-2.22m.Three mean values of silt content are all over 70%, the highest section is over 85%.The depth of three low values respectively 34.34～32.10m,25.32～23.10m and 9.22～4.24m.The mean values of silt content are under 40% and 70%. Along the XYC Core from bottom to top, slightly clay content showed a trend of increased volatility.There are three high-value and three low In the longitudinal direction, the depth of three high values are 32.10～27.74m,19.06～13.34m and 9.22～4.24m. The depth of three low values are in the middle.The mean value of clay content is close to or more than 20%, the clay content is mainly less than 20%. Along the XYC Core from bottom to top,and sand content showed a trend of fluctuations decrease.Sand content has three peaks three valleys, Peak depths are 34.34～32.10m,27.74～23.10m,19.06～13.34m.The mean sand content in more than 20%, even more than 45%.The three valleys are 32.10～27.74m,22.62～19.06m and 13.34～4.24m,respectively, less than 10%.The average particle diameter, presenting the variation of "two high and three low",two high value respectively corresponding to the lithology column 32.10～27.74m (average mean particle size of about 6.40 Φ),22.62～4.24m (average mean particle size of 6.68Φ), the average mean particle size reached 6.40Φ above;Three low segment depths are 34.34～32.10m,25.32～22.62m and 4.24～2.22m. The mean average particle size of these three sections corresponding respectively 4.56Φ,5.13Φ and 6.09Φ.The average particle diameter of the low segment were lower than 6.10Φ. From the view of its longitudinal trend, the average values showing a slight increase and then decrease volatility trend along the XYC Core from bottom to top.For the part of the longitudinal variation of sorting coefficient, presenting the variation of "two high and two low".Two high value respectively corresponding to the depths of 34.34～32.10m and 27.74～23.10m.The average values of sorting coefficients are 2.68 and 1.98.Two low value depths are 32.10～27.74m and 22.62～2.22m, the average of the two low is lower than 1.80.From the view of longitudinal trend,the sorting coefficient fluctuated decreasing trend from bottom to top along the XYC Core.(4)According to the characteristics of grain sizes, combined with depositional facies discriminant function, sedimentary marker, characteristics of magnetic susceptibility, and compared with the typical characteristics of lithology and grain sizes in nearby areas sediments, we can draw a preliminary judge of XYC core from bottom to top that the study region had experienced the changes of four phases in depositional environment including fluvial facies, tidal-flat facies, neritic facies and limnetic facies. Meanwhile, there are four AMS14C age dates(5440 cal a B.P,7505 cal a B.P,9085 cal a B.P, and 9525 cal a B.P) of XYC core in the depth of 9.78 m, 2.76 m,26.72 m and 31.74 m respectively. The depositional environment of the study area of XYC core can be divided four stages from the early Holocene to the late Holocene. (1)The warmer and wave period during the early Holocene (9525 cal a B.P) corresponded to the fluvial facies. (2)Rapid transgression (9086 cal a B.P) corresponded to the tidal-flat facies. (3)The high sea level period of Holocene Megathermal (7505 cal a B.P) corresponded to neritic facies. (4)The cooling and fluctuant period of climate in the late Holocene(5440 cal a B.P) corresponded to the limnetic facies. The changes of depositional environment in XYC core in the study area during the Holocene indicate abundant formation between the sedimentary facies and the changes of climate and the sea level. Therefore, the evolution of XYC core depositional environment to a certain extent reflects the environmental changes in the region during Holocene. |
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