DEPOSITIONAL RATE OF THE MALAN LOESS AND ITS GEOLOGICAL SIGNIFICANCE

Malan loess section in Heimugou location of Luochuan County, Shaanxi Province, has been studied. From the top of the section at 2.56m to the end at 10.36m, we collected samples by intervals of 20cm, total 40 samples were collected. The samples were dated by thermoluminescent(TL) method. Depositional rate of the Malan loess and the regularity of its variation as well as its geological significance are discussed in the paper. Average depositional rate of the Malan loess for whole section is 0.078mm/ a. According to the variation in the depositional rate, the section can be divided into three horizons. Horizon 1 is in interval 2.56-3.56m, with its average dipositional rate of 0.091mm/ a. Horizon 2 is in interval 3.56-7.76m with average depositional rate of 0.35mm / a. Horizon 3 is in interval 7.76-10.36m, with average depositional rate 0.058mm / a. The variation of dcpositional rate has following geological significance. (1) Depositional rate of the Malan loess is closely related to the grain size. The grain of the lower part is coarse, well graded the depositional rate is least. The grain of middle part is fine, not well graded, the dcpositional rate is higher.The upper part is similar to the lower part. (2) The depositional rate of the Malan loess is somewhat correlated to the distrbution and character of¹⁰Be concentration along the cross section. The average ¹⁰Be precipitation rate in the middle part is highest. The dcpositional rate or loess in this layer is also highest. But the average depositional rate of loess in the upper and lower parts is lowest, the average¹⁰Be precipitaion rate is also lowest. ¹⁰Be camefrom the atmosphere and absorbed by aerosol, and then reached the earth's surface by rainfall and dustfalls. Finegrained aerosol probable more absorbed ¹⁰Be and its precipitation rate is higher than the coarse grains, so the ¹⁰Be concentration in fine grains is higher than in coares. (3) The variation of the depositional rate of the loess depends on the climatic evolution. from Zhang Deer, the more dry and arid the climate, the more frequent the precipitation of the loess. The loess has also deposited during cold climate. On the contrary, the warm and humid climate did not favorable to loess dcposite. The locss-palcosoil sequence reflects climatic cycle in that time. Cold-warm and dry -humid climates alternated frequently. The climatic change reflected by loess-palcosoil sequence was in large scale and long inter time. Authors call it the first-ordcd indicator of paleoclimatic change. The paleoclimatic change reflected by depositional rate is called second-order indicator of the paleoclimatic change, because it is a change in the loess of sub-order stratigraphic unit. The high depositional rate was during the dry-cold climate and deositional rate is lower during the warm-humid climate.