Sedimentary And Pedogenic Characteristics Of Chengdu Clays And Its Underlying Clays:Implication For Paleoenvironment Changes

To further understand regional environmental evolution in the Chengdu plain,it is necessary to identify the sedimentary environment characteristics of the Chengdu clays and its underlying strata which are brown clays and reticulated red clays.Particle size distribution,particle size frequency curve,particle size parameter,fractal dimension,magentic susceptibility,organic matter,calcium carbonate and series of geochemical parameters were analyzed.The comparative analysis of soth with other regions is of great significance for further understanding of regional environmental evolution.The research results are as follows:1.Particle sizes of silt(5?50 ?m)dominated the Chengdu clays and its underlying clays,while clays(<5 ?m)concentrations and Kd values were higher andil formation strenglower than that of northern loess,respectively;coarse particles of>100 ?m were presented in the curves of particle size frequency for all samples.(2)The three-layer clays were similar to the northern loess and different from the fluvial and lacustrine sediments from the characteristics of particle size parameters.(3)In comparison,the brown clays showed higher values of skewness and kurtosis as well as coarse silt(10?50 ?m)concentrations,and lower values of mean particle size(?)and fractal dimension.While higher values of standard deviation and lower kurtosis values were presented in the Chengdu clays;and the values of mean particle size(?)and fractal dimension from the reticulated red clays were the largest.(4)The fractal dimension was significantly correlated with particle size composition,average particle size and kurtosis from the three clay layers,while only the brown clays showed a significant correlation between the values of fractal dimension,standard deviation and skewness.(5)The Chengdu clays and reticulated red clays rather than the brown clays once experienced a strong weathering and pedogenesis process,showing a climatic change trend of warmth-humidity to cold-drought and to warmth-humidity.2.By comparing and analyzing the physical and chemical properties and element geochemical characteristics of Chengdu clays and underlying clays,the differences in the strength characteristics of weathering and soil formation of three-layer clays are discussed.It shows that the content of reticulated red clays,carbonate,and organic matter is higher than that of Chengdu clays and brown clays,and the magnetic susceptibility is lower than that of Chengdu clays and brown clays.Chengdu clays has the lowest carbonate content and the highest magnetic susceptibility.All the elements of the three-layer clays are mainly composed of four elements:SiO2,Al2O3,Fe2O3,and K2O.The three layers have the lowest CaO content in the reticulated red clays,followed by the higher content of Chengdu clays and brown clays;the Al2O3 and Fe2O3 content in the reticulated laterite.Higher than Chengdu clays,brown clays content is the lowest.Chemical parameters indicate that the average value of Ba/Sr,Rb/Sr,Ba/Nb,CIA,residual coefficient,and potassium-sodium ratio of reticulated red clays is the largest in the three-layer clays;the leaching coefficient,alkali rejection coefficient,and silicon-aluminum ratio mean content of the three-layer clays The smallest.It shows that it has been subjected to strong chemical weathering,and the soil formation intensity is the strongest.The chemical parameters of brown clay indicate that the CIA value is the smallest,and the other chemical parameter values are between the average values of the three-layer clay,and are similar to the values of Chengdu clays;the average content of Chengdu clays;The largest in clay.The mean values of residual coefficient and potassium-sodium ratio were the smallest.According to the geochemical parameters and the A-CN-K triangle diagram,the reticulated laterite is more affected by weathering and soil formation than Chengdu clays and brown clays.The geochemical characteristics of Chengdu clays and underlying clays elements indicate that reticulated laterite is subjected to more severe weathering and soil formation than Chengdu clays and brown clays.3.Based on a comparative analysis of the geochemical characteristics of the three-layer clays elements of Chengdu clays,its underlying brown clays,and reticulated laterite in the Chengdu Plain area,and the soil genesis characteristics of the loess in the upper Minjiang River and the Malan loess in the Loess Plateau,There are differences in soil formation strength.The performance of the Chengdu clays and its underlying clay three-layer clays in the Chengdu Plain area is stronger than the weathering and soil formation of the Malan loess in the upper reaches of the Minjiang River and the Loess Plateau.The regional climatic characteristics indicate that the Chengdu Plain is warmer and wetter than the upper Minjiang River and the Loess Plateau,while the upper Minjiang River and the Loess have a high-principle dry and cold climate.