| including clastic, carbonate, coal and pyroclastic. According to regional tectonicunconformities, sedimentary transformation surface, incise valley erosion surface andthe boundary between upper Permian and lower Triassic，a total of three third-ordersequences are subdivided. Three sequences reflect a vertical and lateral pattern ofpalaeogeography，which shows that，from the bottom to the top and from southwest tothe northeast，there is a variation in facies units from fluvial plain，through tidal-deltacombination，to shallow marine. In the three sequences，depositional environments areall deeper in the east，and shallower in the west，with the transgressive pulse ofadvancing from east to west. The favorable coal-forming unit is mainly the deltaplain，follwed by the bay-tidal flat facies, the worst coal-forming units are river andeluvial plain. Based on third-order sequence, the lithofacies paleographic frameworkof each unit is reconstructed. The coal accumulation is mainly controlled bypalaeotectonics, sea level changes. First-order tectonic units and surroundingmountains determin the positon and derection of coal development, while second-oderand third-order tetonic units determine the characteristics of depositional systems andwhere and how the coal accumulates. |
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