Neogene Sediments And Paleoenvironmental Evolution Of The Middle Reaches Of The Yellow River

Based on a great quantity of detailed reviews in field,sample's analyses and tests in laboratory ,this thesis makes a detailed research on Neogene red clay characteristics of Laogaochuan profile which is typical in the middle reaches of the Yellow River, especially on characteristics of facial pattern,grain –size and susceptibility. This thesis gain a series of number indexes which indicate the variation of paleoclimate and paleoenvironment．This work can be summarized as the followings : Firstly,Laogaochuan profile consists of 18 red clay layers intercalated with 17 layers of calcareous concretions．This imply the cyclic change of paleoclimate.Through the comparative study of magnetostratigraphy and biostratigraphy in two banks of Yellow River, Laogaochuan and Jijiagou country,this thesis gains that the Hipparion faunas in the two regions had the uniform dwelt environment and experienced the same evolution since Baode Stage.Secondly , the component particles of the red clay are predominantly less than 250μm,with the content of silt(5-50μm) average 52.17%．And the content of 10-50μm is higher than that of 5-10μm．Few particles over 250μm can be observed.. We choose four groups of grain size data:the mean, >63μm,>30μm and <2μm．We could see that the mean, >3μm,>0μm grain-size curves change distinctly,while <2μm grain-size curve is relatively stable．The features indicate that the Neogene climate was mainly dry and cold and the strength of winter monsoon had a great change,and also popular．The summer monsoon is relatively weaker with little change。With this background climate there were alterations of warm-wet and cool-dry climate. Three partical-size modes exist in the red clay,with ranging from 20 to 100,2 to 20,and 0.3 to 1μm .This shows the red clay with different components is likely formed by multiform origin。Thirdly ,the susceptibility curve shows strong change since 18th layer,which probably means the climate of the early stage of Yushe Stage was warmer and wetter than that of the middle, late stage of Baode Stage. Finally , the stratigraphic structure,chemical composition,spatial comparisons of grain size substitute indexes and the types of fossil shows that the red clay deposit in the middle reaches of the Yellow River is aeolian origin.