Keywords: 
• eluvial zone /
• CaCO₃ movement /
• illuvial types /
• illuvial depth /
• new geological theory

Abstract: In the research of many geological problems about weathered eluvial zone, the conten theory is widely adpoted, which is based on content of chemical composition. Although the content is very important to resolve lots of geological problems, this theory is not perfect because the content of chemical composition is easily effected by the weathering process. In order to remedy the defect of traditional content theory and reveal the law of nature, it is necessary to establish anew theory. During the investigation of paleosol and weathered eluvial zone in the Loess Plateau, the author found unsuccessive illuviation, thick layer or multi-layer illuviation of CaCO₃ and other chemical composition-a special geological phenomenon. In the paper, the author established a new theory on depth of CaCO₃ illuvial horizon of weathered zone combining with CaCO₃ content analysis and penetrating experiment, this theory is very effetive to study paleosol, weathering crust, paleoenvironment and many geological problems of weachered eluvial zone. The theory reveals that the more precipitation is, the bigger the movement depth of CaCO₃ illuvial horizon is, though the eluvial process is very short; the less the precipitation is, the shallower the movement depth of CaCO₃ illurial horizon is, though eluvial process is very long. Because the process in which CaCO₃ moves to the illuvial horizon is very short, the influence of time could be neglected. So the moved depth of CaCO₃ illuvial horizon is reliable basis for studying many geological problems about weathered eluvial zone. Since CaCO₃ belongs to dissoluble salt, the illuviation process of the salts such as NaCl, CaSO₄, which are more dissoluble than CaCO₃, is much shorter, and the illuvial theory on the depth of CaCO₃ can be applied to them. The movement of colloid material such as Fe₂O₃ and Al2O₃ is influenced clearly by time factor, however, as long as their illuvial horizons form, that is to say, the process needed in which they move to the illuvial horizon have been met. So their illuvial depth can also be regarded as areliable foundation for studying the geological problem of weathered eluvial zone. There are three reasons why the movement time of CaCO₃ to illuvial depth is very short. The first is that CaCO₃ can move under the condition of basicity. Movement and illuviation can occure in the early period of soil-forming processes. The second is that a part of CaCO₃ comes from precipitation and CaCO₃ can illuviate at the bottom of soil as long as eluvial occurs. The third is that CaCO₃ exists in the form of Ca²⁺and HCO^-₃ in the water. The two ions will follow the penetrating water whereever it goes. According to the quantitative correlation (y=₃05x+168.5) between mean annual precipitation (y) and depth of CaCO₃ illuvial horizon(x), the mean annual precipitation in the developmental period of paleosol and weathering crust can be calculated quantitatively and provides a basis for determining the formula on precipitation of rainy season. The author can draw three deductions in accordance with the illuvial theory on depth of CaCO₃ and the types of CaCO₃ illuvial horizon. Deduction 1：when movement depth of CaCO₃ illuvial horizon exceeds the thickness of clay-grouting layer of soil, it can be seen that the soil is the neutral or acid soil of leaching type. Deduetion 2： When moved depth of CaCO₃ illuvial horizon exceeds the thickness of developing zone of soil (about 2 m) , it can be determined that the soil is changing to weathering crust. Deduction ₃： when two, three layers or unusually thick CaCO₃ illuvial horizon exists under the same paleosol or weathering profile, it indicates that there are two or more soil-forming periods and corresponding climate change at that time.