Sequence-Palaeogeography and Coal Aaccumulation of Late Permian in Southwestern China

The Upper Permian in southwestern China was developed with abundant coal resources,and is divided into three formations including Emeishan Basalt Formation,Longtan Formation( lower and middle memberss of Xuanwei Formation or Wujiaping Formation) and Changxing Formation( upper member of Xuanwei Formation or Wangjiazhai Formation) from bottom to top. The sequence stratigraphy,paleogeography,and coal accumulation of the Late Permian in this region have been studied based on outcrop and borehole sections. According to regional unconformities,sedimentary facies reversal surface,base of incised valley fill sandstones,and distribution of marine limestone beds which reflects variation of the transgressions,we have subdivided the Later Permian into 3 third-order sequences and related lowstand,transgressive,highstand systems tracts. Sequence 1 corresponds to the intervals of early Wujiaping stage,and is distributed mainly in the northeastern Sichuan,eastern Yunnan and western and eastern Guizhou. Sequence 2 corresponds to the middle Wujiaping stage,and Sequence 3 corresponds to late Wujiaping stage though Changxing stage. Both Sequence 2 and Sequence 3 are all distributed in the whole region of southwestern China. The vertical and lateral variation of facies in three sequences reveals a overall transgression pattern of palaeogeography.From bottom( Sequence 1) to top( Sequence 3) and from west to east,there was a variation in facies units from fluvial plain,through tidal-delta combination,to shallow marine. In these three sequences,depositional environments are all deeper in the east,and shallower in the west,with the transgressive pulse of advancing from east to west. The coal-forming environments are swamps developed from fluvial overbank,interdistibutary bay,lagoon-tidal flat environments,while the favorable coal-forming environment is the interdistributary bay of the delta plain. In the three sequences,with the overall westward transgression,beneficial coal-forming environments and coal-accumulating centers migrated to the west,towards the Khangdian Oldland.