Paleoclimate change and sedimentary environment evolution,coal accumulation: A Middle Jurassic terrestrial

In order to reveal the relationship between paleoclimate change and sedimentary environment,as well as coal accumulation in the terrestrial basin,the principle of isotope geochemistry and petrology has been used to study the maceral,the isotope composition of organic carbon ( δ13 CO) and the paleoclimatic significance of main mining coal seam of the Middle Jurassic Upper Dameigou Formation (Bajocian stage) in northern Qaidam Basin. The average val- ue of δ13 CO of No. 7 coal and coal F is -23. 9‰,it varies from -26. 7‰ to -21. 6‰ and from -25. 6‰ to -22. 9‰respectively. The Upper Dameigou Formation is divided into eight stages corresponding to a long period cycle ( C1) and three short period cycles (C2) according to the characteristics of alternating evolution of swamp and terrigenous system and the variation of the organic carbon δ13 CO value. The δ13 CO value of low metamorphic coal is considered to not only reflect the isotope composition and content changes of palaeoatmospheric CO2 ,but also record the changes of paleoclimate (temperature and humidity) during the coal accumulating period ( plant growth to the end of peatifica- tion):Wet-warm paleoclimate is conducive to the carbon isotope fractionation of the coal-forming plants and the forma- tion of poor13 C humus (predecessor of vitrinite),and the δ13 CO value of coal decreased;dry-hot paleoclimate is not conducive to the carbon isotope fractionation of plants,but it is conducive to the formation of inertinite ( fusinite) in enrich13 C,and then cause the δ13 CO value of coal increases. From the point of the relationship between the climate change reflected by the δ13 CO values of the coal seam and the abandonment and revival of terrigenous system and the commencing and termination of coal accumulation,a model of the relationship between paleoclimate,sedimentary envi- ronment and coal accumulation in the terrestrial basin is proposed. It points out in the process of coal measures deposi- tion when the tectonic is active and basement subsidence is stable and uniform,the change of absolute lake level and the rate of sediment production and transportation ( supply) are controlled by the climate change through the runoff depth of drainage basin (annual average rainfall minus annual average evaporation) and the development and evolu- tion of plants,and then cause the alternating evolution of swamp and terrigenous system in the basin.