Keywords: 
• ultra-thick coalseam /
• depositional dynamics /
• deeper water coal accumulation /
• Fushtm coalfield

Abstract: Numerous Meszoic Cenozoic small scale fault basins in China contain extermely thick coal seam with low ash content,but it is difficult to explain the genesis of such kind of coal seam only by in situ coal forming theory.Based on the research of the depositional dynamics of Fushun coal bearing basin,and analysis on coal bodies in main coal seam of Guchengzi Formation,the depositional environment of this kind of coal seam has been interpreted,together with the analysis on sedimentary features of root and trunk fossils within the coal seam. It shows that the basins depositional dynamic process was controlled by the lake level,tectonic subsidence and provenance.The paleoclimate influence the lake level changes,wave storm,function,source type and rate of sediment supply to a certain extent,even the plant type Fushun basins water depth is about 10 m at the basin edge where developed a large amount of water plants.The relative ideal water depth of depocenter may be 30～100 m. Because of syn sedimentary falt network activity,variation of water depth is in the form of sudden change in horizontal water body.Its depositional dynamic system is composed of perturbation lagging dynamic system,hypautochthonous dynamic system and allochthonous dynamic system.Allochthony depositional dynamics acts as the leading factor in lakeside(10 m) slope (10～30 m) basin(30 m) environment.Because Fushun basin is in high speed subsidence with low speed sediment supply during Guchengzi Period,the intraclastic deposition is active,therefore,the extremely thick coal seam can be used as an example of deeper water coal accumulation mode.