Characteristics Of Natural Sedimentary Deposits Of Loess Plateau And Its Environmental Significance

The soil erosion problem in the Loess Plateau has been always paid much attention, and the study on the law of sediment deposition in the small watershed of the Loess Plateau is of great significance to the control of soil erosion. As the natural poly marsh, Huangtuwa exists longest, and it has a very typical representative. Studying on the characteristics of grain size and element composition can reveal the law of soil erosion in the small watershed on the Loess Plateau in Northern Shaanxi, and reflect historical sedimentary environment. And, it will provide soil erosion control of basin with scientific basis by deepening the understanding of the law of soil erosion on the Loess Plateau.This study analysis the deposition profile of Huangtuwa natural poly marsh andsediment grain size parameters and the characteristics of geochemical elements todiscusse the distribution of sediments, reveal the relevance between the grain size and element and its control factors sedimentary physical and chemical characteristics,, and finally expound the indication meaning of the grain size and element geochemical characteristics to the sedimentary environment. Conclusions as the follows:(1)In Huangtuwa, the grain size of the sediment in the natural poly marsh profiles is more finer on the whole, mainly silt sand; the content of clay sediment and fine sand is second than others; and the least distribution is medium sand and coarse sand which only appear in the individual layers. The sorting of the sand is the worst and the sand is belongs to lapse components in the process of sediment transport and deposition. The sand components containing medium sand and coarse sand are the sign with strong water power, which indicate the effect of sediment particles on the sedimentary environment or the rainstorm. The sediment skewness concentrating in between 0.1 and0.47,which belonging to positive-skewed, indicates that particle size in profile sediment is almost coarse sand. From the deposition profile kurtosis curve, the kurtosis is generally medium to narrow.(2) The sediment triangle has the indicative function to the distribution law of sedimentary cycle andit is well corresponded to the distribution law of "upper fine and lower rough". The type of grain size frequency curve is mostly single peak, which is symmetry, positive-skewed or negative-skewed. The main particle size distributes in the silt sand (fine powder sand, coarse silt) component, reflecting the fairly stable hydrodynamic conditions of sediment deposition.(3) The major elements in sediment including seven elements in Huangtuwa natural poly marsh, whichare Si, Al, Fe, Ca, Na, Mg and K and are in the form of oxide. The abundance value of Si elements is highest, and the content of Al, Fe elements comes the next. It shows that the sediments are mainly silicate and alum inosilicate. As for trace elements, this paper focuses on the study of ten elements, Cu, Mn, Rb, V, Sr, Cr, Zr, Co, Ce and Y. The vertical distribution of all the elements content in the profile is the alternation of peak and valley values, and the distribution of the seven major elements and most of trace elements have obvious sedimentary cycle.(4) The distribution of chemical elements in the sediments of the study area has a correlation, which indicates that the elements have specific characteristics of the combination. Si element has the most abundant content, so it has a certain relationship with the growth and decline of most elements. As the composition of clay minerals, the element of Al、Fe、Mg、K and other elements has a high correlation with clay minerals. Most of the other elements also have the similar correlation, which shows that the chemical behaviors between the elements have the "crony" relationship.(5) Through the analysis of the relationship between element content and grain size parameters in deposition profiles, the sedimentary section elements in Huangtuwa natural poly marsh are divided into three categories. The first category is an element with a distinct "particle size effect", which includes all the major elements and trace elements and has a high correlation with grain size parameters, and thehe sedimentary cycle is obvious in the section. The second type of elements includes Sr, Cr, Zr, Co and other trace elements, whichhave the poor or general correlation with grain size parameters, and the cycle distribution law is not obvious. In addition, there still exists the third kind of elements which have a poor correlation with the grain size parameters and basically have no sedimentary cycles in the profile.(6) The sediment deposition has "dual structure" bedding in Huangtuwa natural poly marsh, showing that the secondary loess is formed by flowing water in the study area; but the similar structure was not found in the original loess layer in the basin, proving that the cause of the formation was non-flood deposits from the reverse. From the view of the vertical direction in Huangtuwa natural poly marsh, the interphase layer distribution of coarse and fine particles showed obviously sedimentary cycle; From the view of horizontal direction, the particle size distribution isthat fine particles from upstream to downstream gradually become fine.(7) Analysis of sedimentary environment of the study area by Sahu formula shows that Huangtuwa natural poly marsh sedimentary environment is relatively simple and mainly the rivers phase sedimentary environment, sometimes sand deposition and turbidities sedimentary environment.(8) By comparing the average contents of major elements in each section of the main channel and the branch ditch in the horizontal direction and analyzing the deposition pattern of "ancient poly marsh " in the small beach ditch at the early stage, it reveals the evolution of the sedimentary environment of local erosion. Analyzing the C profile of the main channel, which is in the lowest in dam, and the element distribution of three profiles of Ad, Ab and P, which is in the branch gully, the C profile is defined as "standard poly marsh" deposition and the three sections of Ad, Ab, and P are attributed to the causeof the sedimentary environment of modern "non-standard poly marsh". At the same time, this paper defines preliminarily the range of the modern "(non-standard poly marsh)" deposition by using the DEM (digital elevation model).