| In the past500years, soil erosion influences the treatment of the Loess Plateau seriously, which should be studied in depth. And it is one of the important issues to study the rules of sediment yield of a long time series in the typical small watershed. Some scholars, taking modern alluvial loess as the information sources, carry out research on the mechanism of soil erosion and sediment yield rules. However, the deposition of modern loess dam can not achieve the ideal effect, because of its short time series and the accuracy limitations of time division. The loess depression of Pangjiagou basin in the south of Zizhou county, is known as the oldest natural depression formed by alluviation, which is formed440years ago. And it is a perfect information source. It can reveal the evolution rules of soil erosion of the Loess Plateau in the north of Shanxi province and provide basic data and scientific evidence for the treatment of soil erosion of the Loess Plateau to analyze its sediment.Supported by the national natural science fundation, environmental information of sediment in the natural loess depression is collected and analyzed by the method of pit mining and experimental analysis. And the physical and chemical characteristics of sediment in loess depression, such as physical stratification, granularity, constant elements and trace elements, are analyzed. The conclusions are:(1) By analyzing the partical size and elements and observing the physical form of four meters profile, it founds that the sediment of warping dam in whole profile shows the changes of thick-thin-thick-thin, the rules of sedimentary cycle is apperent. Deposition layer which is below2.2meters of sedimetary profile is more thicker and the stratification is very clear. In the deep of1.8meters to2.2meters, it can be denfined as the duration of lacking the flood.(2) By comparing the average percentage and changes of grain size fraction which distributes on the profile of sediments of warping dam, it founds that the sedimen of warping dam is mainly made of coarse silt, there is a little medium sand and coarse sand which are only appeared in a certain depth. (3) By analyzing the Percentage changes of each grain size fraction distributed on the profile, it can be inffered the process of sedimentation in warping dam. At the preliminary stage of forming the erosion of rainfall and runoff, it mainly is made of coardse silt and fine sand. With increasing of runoff, the components of fine sand, medium sand, coarse become depositing and increasing; With the reducing of runoff, the sediments in warping dam are changing into the components which include colloidal partical, clay and the silt.(4) By analyzing the sepecial sedimentary layers of each grain size fraction and the data of the partical size in sedimentary profile, it founds that the deposited profile in0.2to0.36meters,0.38to0.6meters,2.26to2.42meters,2.44to3.05meters are correspoinding to each other. As for flood deposition events, it can be inffered that in0.38to0.6meters, there is the maximum flood level which corresponds with the profile according to the rules of inundation to suna and flood scale. Flood intensity in3.14to3.46is weaker than that in0.2to0.36meters,0.38to0.6meters,2.26to2.42meters,2.44to3.05meters.(5) By analyzing the data of Dam silt sediment grain size, it known that the whole Poor sorting of Dam silt sediments and Sorting coefficient are among1.25to1.88Φ. Dam silt sediments.From the skewness curve of deposited profile, Sediments in profile are mostly positive-skewed, particles are comparatively coarse. Kurtosis of the sedimentary sect are among0.77to1.92, the kurtosis is sharp.(6) By analyzing the size frequency curve of deposited profile sample in warping dam and the probability curves of grain size, it shows that types of each sample of grain-size curves deposited profile are relatively simple which are mostly unimodal, Minor are belong to doule peak. In the profile of0.70to0.74m,1.38-1.40m,3.66-3.68m,4.04to4.06m, the bimodal form in the size-frequency curves indicates that sequence rainfall is smaller or shorter. Detrital material carried by erodent rainfall mixed with debris at sampling point in a simple way, which failed to make full sorting. Grain size frequency curves in loess depression natural sediment silting dam, with a uniform form, are mainly divided into three saltatory steps. Two-steps characteristics of the three are the typical grain size frequency curves.(7) By analyzing the relative content and distribution rules of elements in the section, it is found that the relative content of Si is ranked first and Al and Ca’s are ranked second. In the trace elements, Ti has the most relative content. The elements are divided into three categories according to their distribution rules in the section:The elements of I, which are distributed regularly in the profile, have the evident sediment deposit. They are similar to the constant elements. Ga, Y, V, Mn, Ti, Rb, Sr, Ba, Cu, Pb, Zn, Cs and Ni belong to this type. The relative content of the elements of Ⅱ has a small variation in the deposition profile, which only appears the obvious peak at specific depth, such as Zr, Cr and Hf. The relative content of the elements of Ⅲ changes frequently in the section and have no evident peak, including Co, Cs, Ce, Th, P and Nb. What’s more, the distribution characteristics of the constant elements in the profile show that the profile in the depth from1.8m to2.2m maybe forms a relatively stable sediment environment. And the relative content of N elements is influenced by artificial nitrogen in the depth above1.4m.(8)Through the correlation analysis between particle size and elements, and among elements, it is found that the profile distribution of elements complies with the "elements granularity control law". The elements, Si and Na, are mainly distributed in the sediments with coarse particle size. The elements, Al, Fe, Ca, K and Mg, are distributed in the sediments with fine particle size mainly. Si has the diluting effect on the distribution of other elements. The correlation among elements is high, except Zr, Hf, Cs, U and P. The distribution of elements in the profile is influenced by material sources and chemical property, for example the parasitotropy of P. |
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