关键词: 黄土, 风化剖面, 深层淋滤, 降水事件

Abstract: On the basis of the illuvial theory raised by the author,weathered and leached section identification in the field,CaCO₃ content,susceptibility and grain size analyses in laboratory,it was found that a deep weathered section under the fourth and the fifth paleosos had developed best since loess deposition.The weathered layer is 0.3~0.6 m thickness and it can be divided into two sublayers:the upper sublayer is weathered loess with ferruginous clay film and the lower sublayer is weathered loess without forruginous clay film.Macrocharacteristic is that leached vertical cracks developed in weathered loess and loess exists in the form of lumps of 20~40 cm in diameter from weathering.The low CaCO₃ content about 1% shows that the loess layer was affected by leaching on which overlying paleosol developed.The susceptibility value of weathered loess is between 40 and 60(10^-6SI) and is less than that of paleosol,but bigger slightly than that of unweathered loess layer,which indicates weathered loess layer was affected by the temperature under which overlying paleosol developed.Grain size,microtexture and susceptibility of weathered loess are similar to that of unweathered loess layer,which indicates it doesn't belong to the component of paleosol.Low CaCO₃ content and a lot of leached cracks in weathered loess layer demonstrate it doesn't belong to normal loess layer and is deep weathered residual layer formed when weathering exceeds the range of soil formation zone.There is not illuvial layer of CaCO₃ under the paleosol above weathered loess layer,but illuvial layer of CaCO₃ can be seen at the bottom of weathered loess layer,indicating that the illuvial layer of CaCO₃ has removed away from paleosol.This kind of illuvial layer of CaCO₃ doesn't belong to the component of paleosol,which shows richer rainfall and strong leaching.That the fourth and the fifth paleosols are middle or acid leached forest soils can be determined in the light of deep weathered loess layer and illuvial layer of CaCO₃ removed away from paleosol as well as ferruginous clay film in deep loess layer in Changwu area.When the two paleosols developed the depth of leaching and weathering not only was bigger than that of modern soils in same area,but also bigger than that in the southeast part of Loess Plateau.The development of deep weathered section is possibly because subtropical climate migrates to the middle part of Loess Plateau,and it represents climatic event with rich rainfall when the average annual rainfall was about 900 mm.

Key words: loess, weathered section, leaching in deep loess, rainevent