Lower Cretaceous coal-forming environment in Huhehu depression in Hailar basin

Coal-forming environment of Lower Cretaceous in Huhehu depression was analyzed from structure and filling sequence. Lower Cretaceous of Huhehu depression went through two tectonic evolution stages, i.e. extensional faulted depression and post-fault thermal subsiding fault sag. Extensional faulted depression stage can be divided into initial tension crack stage, strong tension crack stage, rapid subsidence stage of fault sag; post fault thermal subsidence stage can be divided into fault-sag transition subsiding stage and fault-sag subsidence stage. According to the association characteristics of filling sequence of faulted basin, the strata of Lower Cretaceous in Huhehu depression were identified from bottom to top as coarse clastic alluvium member(Fm) at the base, lacustrine fine-grained deposit member(Lm), coal-bearing clastic rock member and lacustrine member(Cm-Lm), lacustrine fine-grained detrital deposit member(Lm), coal-bearing clastic rock member and lacustrine fine deposit member(Cm-Lm) and coal-bearing clastic rock member(Cm). Combining the structure evolution and filling sequence, this paper discussed the coal-forming environment of the second member of Nantun Formation, the second member of Damoguaihe Formation and Yimin Formation. During sedimentation of the second member of Nantun Formation, coal-bearing strata were mainly developed in the lake shore(where swamp was developed in long term and stably in faulted basin) and the abandoned braided river delta. During sedimentation of the second member of Damoguaihe Formation, fault-depressed shallow water basin, the northern coal accumulation was mainly developed in fan delta sand body, and the southern coal accumulation was mainly developed in the lake shore where swamp was developed in long term. During sedimentation of Yimin Formation, depression was filled, and the shallow marshes became major coal-bearing places.