Sequence-paleogeography and coal -accumulation of Xujiahe Formation in Jingang mining area of eastern Sichuan basin

Sequence-paleogeography and coal-accumulating characteristics of the Late Triassic Xujiahe Formation in Jingang mining area of eastern Sichuan basin were studied based on the data from outcrops, borehole cores, and geophysical logging. According to the color discontinuity surface of strata, basal erosional surfaces of the incised valley filled with sandstone and the vertical facies reversal surface, four third-order sequences were subdivided in Xujiahe Formation, in which sequence III and sequence IV representing the coal-bearing members in the study area, were corresponding respectively to the intervals from member 4 to member 5 and from member 6 to member 7. Taking the third order sequences as the mapping unit, we have reconstructed the lithofacies-paleogeography pattern of sequence III and sequence IV. During deposition of sequence III and sequence IV, the main paleogeographical units were the meandering river and fluvial delta, with Jiangnan Oldland to the east as the major provenance. The mineable coal seams were mainly developed in the transgressive systems tract (TST), with relatively thick coals in sequence IV. The coal in sequence III was deposited in the interdistributary bay of the lower delta plain environment and the coal in sequence IV was formed in the back swamp of the meandering river environment.