Comparison of Grain-size Distribution of Red Clay and Loess -paleosol Deposits in Chinese Loess Plateau

Key words: 
• red /
• clay /
• grain /
• size

Abstract: Red clay, a predominant deposit in Chinese loess plateau during the late Tertiary,is regarded as a good record of paleoclimate changes during the Late Mi ocen e and Pliocene.Some fossils have been found in this deposit and a number of samp les have been measured for magnetic susceptibility,grain size etc.In order to ob tain climatic information in the past several million years,however,it's deposit ion process and origin are still discrepant.In this paper,a detail grain size d istribution of the Late Tertiary red clay deposit in North China is presented,c omparisons of grain size distribution of red clay with that of typical Quaternar y aeolian loess-paleosol deposit in this region demonstrate both sediments are s imilar origin.And a specific difference between their grain size distributions a lso exist.It implies that some different transportation media and depositional e nvironments to form these sediments.Our detailed grain size distribution compari sons demonstrate that the dust to form the red clay deposit was transported by w eaker wind system than that in Quaternary.A unification of the grain size distri bution of red clay may infer that red clay has strongly been weathered. Climate was progressively cold and dry in the Late Tertiary in the North Chian and t errestrial sediments were dominated in this region.The red clay is not lacustrin e and weathering materials origin because:(1) it was impossible there existed a l ake as big as several hundred thousand kilometers in this region since the clima te was quite dry at that time,(2) evidence from chemical element analysis are bi g different from that underlying bedrock,demonstrating that its heterogeneous or igin was from bedrock.In addition,a very good unification of grain size distribu tion from top to bottom of the red clay sequence has also verified that it was n ot weathering origin. Noticeable differences of grain size distributions of red clay and fluvial sediments have corroborated that it was not fluvial origin, als o this conclusion can be proved by that there are no laminations and multi inter calation of calcium carbonate in the red clay sequence by field observations. A very good similarity of grain s ize distribution of red clay and its overlying loess-paleosol sediments should b e strong evidences showing their similar origin. Detailed comparisons of granulo metry characteristic of C-M, L-M,L-M and A-M plots Q1-Md-Q3 fluctua tions empirical judging equation and vector analysis supported this conclusion.M ore specific analyis shows that grain size characterisitcs of red clay are more close to that of paleosol than that of loess.Finer particle size distribution,st rong we athering and pedogenesis and more reddish color of red clay may indicate that th e wind system was weaker, climate was much drier and warmer,and stronger oxidati on in the Late Tertiary than that in Quaternary. However,some red clay removed b y water was involved. From above evidences we conclude that the red clay deposit in North China are ae olian origin.The dust to form the red clay deposits was transported by the wind system which was weaker than that of loess-paleosol formed during Quaternary.The dust may has been strongly weathered by weathering and pedogensis p rocess.