Types Of Coal Forming Vegetation And Succession Of Paleoenvironment Of Middle The Huge Thick Jurassic Coal Seams In Eastern Dzungaria Basin,China

The eastern coal field of Dzungaria Basin,not only containing abundant coal resources but also possessing large coal seam thickness,is an important coal energy base in China.At present,its predicted reserves in eastern coal field are 390 billion tons,and the proved reserves are 213.6 billion tons.Based on the geological investigation,data collection,experimental testing,data processing and theoretical analysis,with the huge thick coal seam of Xishanyao Formation(Middle Jurassic)in Wucaiwan coal field from eastern Dzungaria Basin as the research object.In this paper,sampling,sporopollen separation and extraction and microscopic observation and the identification of sporopollen species were adopted in the study area,the study on paleovegetation types and paleoenvironment succession of it was carried out.The results show that the coal forming swamp of Xishanyao Formation in Wucaiwan mining area experienced shallow water swamp,deep water swamp,wet swamp and dry swamp successively from bottom to top.The coal forming vegetation is dominated by woody vegetation,appearing a mixture of grass and wood locally,which is possible to accumulate in situ.According to the coniss clustering method,the vertical vegetation succession of the thick coal seam of Xishanyao Formation in the study area is divided into three areas.Zone A includes 26 pterdophyte genera,9 gymnosperm genera and 1 bryophyte genera.Zone B includes 29 pterdophyte genera,20 ymnosperm genera and 2 bryophytes genera.The content of pteridophytes decreased,and gymnosperms began to increase.Zone C includes 27 pterdophyte genera,18 gymnosperms genera and 1 bryophyte genera.The types of the paleovegetations in the eastern coalfields of Dzungaria Basin are mainly the herb of Lycopodiaceae-Selagioellaceae plants,shrub of Osmundaceae and Polypodiaceae plants and the coniferous broad-leaved of Cyatheaceae,Cycadaceae and Pinaceae plants.From the bottom to the top,the quantity of pteridophyte decreases gradually,while the Pinaceae plants show continuous increase.Combining the ancient vegetation with coal facies,we can see that the characteristics of coal forming vegetation in huge thick coal seam are as follows: at the bottom of the coal seam,Osmundaceae is dominant In the middle part of the coal seam,the coal forming plants are mainly those of Osmundaceae,Alsophila and cladosporaceae in the early stage,and those of Lycopodiaceae and Selaginellaceae in the middle and late stage.In the upper part of huge thick coal seam,the coal forming plants are mainly cycads and conifers,and some of them are mainly Lycopodiaceae.The paleoclimate changed from warm-damp to humidification-semi-humidification,and finally to semi-humidification-semi-dry,which belonged to the Tropical-subtropical zone.And the study area is divided into four paleovegetation community: Nearshore wet land paleovegetation community,Lowland Cycads and Filicinae plants community,Slope broad-leaved and coniferous plants mixed community,Slope broad-leaved and coniferous plants mixed community,High land coniferous forest community,while the Nearshore wet land paleovegetation community and Lowland Cycads and Filicinae plants community are the main coal-forming plant communities in the study area.The research results show that the study area experienced a large scale of lake transgression and lake retreat in the basin,which led to the changes of Paleoenvironment in three stages: warm-damp to humidification-semi-humidification,and finally to semi-humidification-semi-dry.