Sedimentary facies and sequence stratigraphy of the Miocene Fujin Formation in Erlongshan coal-bearing basin

Erlongshan basin in eastern Heilongjiang Province is a Cenozoic continental coal basin and Miocene Fujin Formation is its coal-bearing strata. In this paper, we have studied its sedimentary facies, sequence stratigraphy and coal accumulation characteristics. Fujin Formation is composed of tufaceous siltstone and fine-grained sandstones, tufaceous conglomerates, and lignite, was deposited in lacustrine environments ranging from littoral and shallow lake, deep lake, medium deep lake to alluvial fan. Two depositional systems were classified; lacustrine and alluvial fan; the whole Fujin Formation composes a third-order sequence with the regional unconformities as its upper and lower bounds. The lowstand system tract corresponds to the bottom conglomerate member, the transgressive systems tract corresponds to the lower sandstone member and middle coal-bearing member, and the highstand systems tract corresponds to the upper sandstone member. The coal seams in Fujin Formation were formed at the end of the transgressive systems tract, and the thickest coal is distributed in the central zone of the basin, which thins out towards northwest and southwest. Coal accumulation has been significantly controlled by basin subsidence, and during the stable subsiding of the basin of the late transgressive systems tract, the increase rate of accommodation space was well balanced with the rate of peat accumulation, thus forming a regional thick coal seam.