Stratigraphic Sequence And Evolution Of Sedimentary Environment Of Upper Paleozoic In Xingcheng, Western Liaoning

Base on the analysis of the Upper Paleozoic of the Yangjiazhangzi syncline in Xingcheng area,Western Liaoning,and data of the core(JK-1),the sequence of Upper Paleozoic in this area has been determined in this thesis.With previous studies available for consultation, the collection of regional strata was made which region include Liaoning province and North China Platform. The sedimentary environment and sedimentary facies are discerned by the grain-size analysis and sedimentary facies division. And then the evolution of paleoenviroment during Later Paleozoic is reconstructed.The Upper Paleozoic in Xingcheng area is continental clastic deposition. On the bottom, Benxi, Taiyuan, Shanxi Formation are composed of mudstone and shale mostly,mixed limestone and coal. Shihezi, Hamashan Formation are upper the part of the Upper Paleozoic, their lithological composition include mainly conglomerate and middle tu coarse grain sandstone whose coarse component increased Obviously. The sediment of Shihezi and Hamashan Formation had no coal, and organic content decreased drastically. The bauxite can be identified in 12 layers.By using the method of grain-size analysis,there are four kinds of sedimentary environments in the Later Paleozoic could be discerned in Xingcheng, Western Liaoning.1. Meandering river environment. The average grain-sizes of sediments are mainly 3Φ. The frequency curves are slight skewed positively distribution and the grain-size probability cumulative curves are typical bi-segment pattern and “low suspension” bi-segment pattern. 2. Delta plain environment.The sediments’ average grain-size is about 3.2Φ. The frequency curves of sediments of delta plain reveal both positively and negatively skewness. Ediments of this environment have a good gradation and the average grain-sizes of sediments are mainly 3Φ. Their frequency curves are slight skewed positively distribution. The grain-size probability cumulative curves have two types: “low-suspension” bi-segment pattern(distributary channel) and one-segment pattern(swamp). 3. Braided river environment. Its average grain-size is near 2.5Φ. It has middle to better gradation and the frequency curve of it is slight skewed positively distribution. The type of grain-size probability cumulative curves is the transitional type between the typical bi-segment pattern and the “high suspension” bi-segment pattern. 4. Alluvial fan environment. It has middle gradation and its average grain-size is about 2Φ. The frequency curve of it shows common and extreme skewed positively distribution. “High suspension” bi-segment pattern is the type of its probability cumulative curves.There are four sedimentary facies could be classified under the method of sedimentary facies marks identification which summarize the grain-size of sediment, maturity of rock structural and content, organic content, grain supported type, and so on.The four sedimentary facies are meandering river facies, delta plain facies, braided river facies and alluvial fan facies. And they could be divided in 7 subfacies and 16 microfacies futherly. The result of sedimentary facies classification shows that the evolution processes of Upper Paleozoic in this area is meandering river facies(or delta plain facies)-delta plain faciesmeandering river facies- braided river facies-alluvial fan facies.Considering the conclusions of grain-size analysis and sedimentary facies classification, this area could be deemed as a basin in close-shore environment which was warm and moist during Late Carboniferous to the middle of Middle Permian. Whereafter, the environment changed in direction of inland environment progressively from late Middle Permian. Up to the end of Late Permian, the environment had turned into inland which was dry and torrid.