Keywords: 
• middle /
• reaches /
• of

Abstract: The reasonably interpretation of the genesis of the Late Neogene red clay has attracted increasingly attention due to its importance for better understan ding of problems such as the evolution of the Quaternary paleoclimate and the Pa leomonsoon.Several opinions have been documented.But a more systematic petrologi c study on red clay has not been reported. Recently we have made a careful field observation and conducted a systematic stu dy on petrology,microtexture and morphology,geochemistry on several red clay sec tions in different parts in the middle reaches of the Yellow River,using thin se ction,SEM,X-ray diffraction,particle size and geochemical analysis.Field and lab oratory studies show that the pedostratigraphic structure in different red clay sections can be fairly well correlated although they are located over far distan ces and the texture,mineral components exhibit an obvious homogeneity in differe nt places.Our studies also found that preferentially oriented optical clay films are common in red clay.They mainly occur along the walls of pores and cracks a s flowing colloid or as rim films around framents. They were formed during soil -forming process and fheir development is controlled by paleoclimate.The develo ped optical caly films in red clay indicate a warm,wet forest climate accompanie d by an intensely leaching process,while undeveloped ones imply an arid or semia rid prairie environment coupled with slight leaching.Our studies also show that calcareous concretion layer beneath the red clay horizon is an illuvial horizon which was resulted from eluviation of calc form the upper clay and by its migr ation and reperecipitation during pedogenic process of the red clay,impluing a w arm and wet climate with intensely leaching. The Hipparion red clay was formed domintly under an arid-semiarid pr airie environment with subhumid-humid forest climate alternative.This climatic p attern was probably related with the paleogeographic pattern that Qing-Zang Plat eau had elevated to a height that could blocked the warm and wet paleomonsoon bl owing from the Indian Ocean to northe China.The variation of the climate and di fferentiation of temperature among red clay layers became greater after about 6. 0Ma,this also attributed to the deteriorated climate resulted in accelerating el evation rate of Qing-Zang Plateau.Taking the Hipparion red clay in R idge Wangdaifu,Fugu,norther Shaanxi province as an example,its formation experi enced four main climatic cycles,according to the petrology,texture and micromor phological features as well as geochemical characteristics of the red clay.Each cycle initiated with arid or semiarid climate and ended with humid or subhumid c limate.