Palaeoenvirmental Evolution Recorded From The Palynological And Organic Geochemical Indexes Of The Middle Jurassic Yaojie Formation,gansu Province

The Jurassic is an important coal-forming period in China.In contrast,thick red clastic deposits were widely deposited in North American continent,which was influenced by various factors such as the global sea-land pattern and atmospheric circulation.During this period,the supercontinent(Pangaea)has not been completely disintegrated,and the accompanying Megamonsoon climate obviously impacted terrestrial environment of China,where located at the eastern edge of the Pangaea.In order to explore this process,this study mainly focused on the Middle Jurassic continuous coal-bearing sedimentary sequences from the Yaojie Formation of Minhe Basin,Gansu Province,by using the palynological and organic geochemistrical methods,the palynoflora and palaeo-environment changes during the Middle Jurassic in Minhe Basin were studied.The sporo-pollen results showed that the palynoflora mainly consists of bisaccates(such as Pseudopinus,Pinuspollenites,Pseudopicea,Protopicea and Podocarpidites)while petridophytes took second place,and mainly represented by Cyatheaceae/Dicksoniaceae.There are always a small number of Osmundaceae(Osmundacidites),Taxodiaceae(Perinopollenites and Taxodiaceaepollenites)and Cheirolepidiaceae(Classopollis).In this area,the flora was dominated by conifer and ferns,probably indicating a relatively warm and humid climate environment.The palynoflora composition showed significant changes upwardly,and can be divided into four stages,1)Cyathidites/Deltoidospora-based fern flora,2)Conifers-Ginkgophytes flora with bisaccates and Chasmatosporites,3)Cyathidites/Deltoidospora-based fern flora,4)Conifers flora dominated by bisaccates.The palynoflora changes of the studied ares during the Middle Jurassic reflected a significant fluctuation of palaeo-climate,with an evolution of hot and humid to warm and humid,than to hot and humid,and ended with cool and semihumid.The results of organic geochemistry results showed that the palynofacies are dominated by amorphous organic matter(AOM)and Phytoclast,and the content of Palynomorph is less.The total organic carbon(TOC)and hydrogen index(HI)is mostly concentrated in the oil shale and mudstone sections.Lithologic characteristics,palynofacies,TOC and HI indicated that the sedimentary environment changed significantly upwardly,and can be also roughly divided into four stages,swamp facies environment with terrestrial higher plant input,half deep lacustrine environment with higher plants and algae input,fluvial facies environment with higher plants input and the deep lacustrine environment with algae input.In conclusion,vegetation flourished in the Minghe Basin during the Middle Jurassic,and the palaeo-climate was warm and humid,suitable for organic matter deposition such as coal and oil shale.The vegetation evolution alternated by the pteridophytic dominated flora and conifer dominated flora,and lake sediments the transformaed from higher vegetation input to lacustrine aquatic algae input.Therefore,the significant changes of palaeo-environment of Minghe Basin during the Middle Jurassic may related to the special geographical location and global climatic pattern.Firstly,the studied area was situated at the boundary between the northern and southern climatic zones of China in the Jurassic,and the movement of climate zone may have a significant impact on the vegetation.Secondly,influencing by the global "Megamonsoon" during the Middle Jurrasic,northern China was characterised by heavy rainfall and lush vegetation,therefore,a large number of coal-bearing sediments developed here.Southern China was characterised relatively dry climate.However,the climate affected by the "Megamonsoon" is not stable,which is indicated by the intermittent coal,red layer,limestone deposition in the coal-bearing sediments of northern China.