关键词: 洛川, 原生碳酸钙, 次生碳酸钙, 碳酸钙的沉积速率, 地质年代

Abstract: The difference of deposition rates of CaCO₃ between in loess and paleosol with different ages is found out through the analysis of 276 samples collected from the Hei-mugou standard loess section in Luochuan County. Shaanxi Province, located in dry or semidry areas of Northwest China. The deposition rates of CaCO₃ tend to be greater from bottom to ,top as loess gets younger. For example, the deposition rates of CaCO₃ in Wucheng loess is 0.09g/cm²·10³a, 0.29 in the lower part of Lishi loess, 0.36 in the upper part; 0.21 in Malan loess; 0.07 in paleosol of Wucheng loess, 0.21 in the lower part of Lishi loess, 0.19 in the upper part; 0.18 in dark loessal soil of Holocene.The deposition rates of CaCO₃ in loess with various ages are all greater than that: in their corresponding paleosol. For example, the deposition rate of CaCO₃ in Wucheng, loess is 1.3 times as big as in its paleosol; 1.4 times in the lower part of Lishi loess, and 1.9 times in the upper part.The deposition rates of CaCO₃ in loess-paleosol series increase synchronously from bottom to top, and the deposition rates in loess are greater than in their corresponding, paleosol, which coincides with the fact that the paleoclimate during the period of Quaternary turned from warm- humid to dry-cold, and the climate changed alternatively between warm-humid and dry-cold. From above we conclude that the deposition rate of CaCO₃ and its variation can be used as a mark to divide Quaternary strata.In order to date unknown strata and to determine the activity period of fault in a certain area, it is of great importance for us to evaluate deposition rate of CaCO₃ in dry or semidry areas, to find out empirical parameters and to establish certain models. The aforesaid method of dating with calcic soil is a simple and feasible way of geologic age determination.

Key words: Luochuan, Original CaCO₃, Secondary CaCO₃, Deposition rate of CaCO₃, Geologic age