| In this study,the petrological characteristics,sedimentary characteristics,and sequence stratigraphic,coal accumulation and thick coal seam genesis of the Early Cretaceous in the northeast China are studied by collecting boreholes,cores,logs,outcrops,and paleontology data with reference to previous research results.(1)In the Lower Cretaceous of northeastern China,five lithologic types and corresponding 14 types of lithofacies were identified:conglomerate,sandstone,siltstone,mudstone and organic rock.Seven depositional systems were identified,including alluvial fan,fan delta,braided fluvial,braided fluvial delta,meandering fluvial,meandering fluvial delta,and lacustrine.The main coal-forming environments in the northeastern China are fan-braided deltas and littoral-shallow lakes.(2)A total of six sequence surfaces were identified including regional unconformity,incised valley erosional surface,paleosol,and sedimentary facies shift surface.The Lower Cretaceous in northeast China can be divided into five third-order sequences,including sequences ?-?.Sequences ?-? correspond to the initial subsidence,rapid subsidence,stable subsidence,early shrinkage,and late shrinkage stages,respectively.The thick coal seams in the western zone are mainly located in the transgressive system sequences ?-?.The thick coal seams in the central zone are mainly located in the transgressive system of sequence ? and sequence ?,while that in the eastern zone are mainly located in the transgressive system of sequences ?-?.(3)The sedimentary filling sequences of faulted basins in the northeastern China are very similar.They could be subdivided into five members from bottom to top:the bottom coarse clastic sediment member,coal-bearing clastic sediment and lacustrine mudstone members,lacustrine mudstone member,coal-bearing clastic sediment member,and coarse clastic sediment member.Similar sedimentary filling sequences illustrate the synchronicity of the tectonic evolution in the northeastern China,which provides the possibility for the comparison of sequence stratigraphic in the northeastern China.A large number of thick coal seams were developed in the western zone and the metamorphism of coal seams is relatively low,with lignite as the dominant component.The thick coal seams are mainly distributed in the eastern part(Yimin sag,Huhehu sag and Jiuqiao sag)and the northern part(Zhalainuoer sag)of the Hailar Basin,in the northeastern part of the Erlian Basin(Giergalangtu sag,Baiyinhua sag,Huolinhe sag,and Anan sag).The coal accumulation in syn-rifting stage is worse than that in the shrinking stage.Compared with the western zone,the coal seam thickness thins in the middle zone and coal seams are widely and continuously developed.The thick coal seams are mainly distributed in the eastern and northern part of the Songliao Basin and the northern part of the Fuxin Basin.The coal accumulation in syn-rifting stage is better than that in the shrinking stage.The thickness of the coal seam in the eastern zone is the thinnest,and the thick coal seams are mainly distributed in the Jixi Basin,Shuangyashan Basin and Hegang Basin of the Pan Sanjiang Basin,mainly formed in the syn-rift stage(Chengzihe Formation).The coal accumulation in the shrinking stage(Muleng Formation)is poor.(4)Based on the depositional model of faulted basin proposed by Olsen(1990)and lake basin type proposed by Carroll and Bohacs(1999),and combined with depositional environment and coal accumulation characteristics of Lower Cretaceous in faulted basin of western zones,three coal accumulation models were proposed based on different tectonic evolution stage of Lower Cretaceous in faulted basin of western zones.Huolinhe type coal accumulation model corresponds to the steady subsidence stage of basin evolution,and the lake type 1s similar to the Newark type and balanced-fill lake basin.Shengli type coal accumulation model corresponds to the early shrinking stage of basin evolution,and the lake type is similar to the Newark to Richmond type and overfilled to balanced-fill lake basin.Yimin type coal accumulation model corresponds to the late shrinking stage of basin evolution,and the lake type is siuilar to the Richmond type and overfilled lake basin.The central zone sets the Xujiaweizi Sag of Songhao Basin as an example,and the coal accumulation model of different position of Xujiaweizi Sag was summarized.Coal accumulation model 1:In the north of Xujlaweizi Sag,the dip of Xuxi fault is small,and the fault subsidence rate is equal to the basement subsidence,the eoals are developed both in the steep slope zone and the gentle slope zone.Coal accumulation model 2:In the central of Xujiaweizi Sag,the central uplift is developed.Compared with the northern area,the dip of fault is larger,and the eoals are better developed in the gentle slope zone than the steep slope zone.Coal accumulation model 3:In the south of Xujiaweizi Sag,the dip of fault is larger than the central area,the fault subsidence rate is larger than the basement subsidence,and the coals are mainly developed in the gentle slope zone.The eastern zone sets the Sanjiang Basin Group as an example,and coal accumulation models during two major coal-forming periods were summarized.The major characteristics of coal accumulation during the rapid subsidence stage(sequence?)corresponds to the balanced-fill lake basin stage.The thick coals are mainly developed in the delta plain and littoral-shallow lake depositional environments during the peacefiul stage of volcanic activity.The thick coals are developed in the transgressive systems tract.The major characteristics of coal accumulation during early shrinking stage(sequence IV)corresponds to the overfilled lake basin stage.Compared with the sequence ?,the depositional environments are steady during the sequence IV.Influenced by strong tectonic activities,the coal accumulation became worse during the sequence ?.(5)Due to different tectonics,paleogeography,paleoclimate,volcanic activity and depositional environment,each area shows distinctive characteristics of coal-bearing basins.The northeast China is located in the N 40°?N 60°,and is situated in the warm and humid dimate zone.The humid elimate provides the favorable eonditions for plant development and peat accumulation.Coal accumulation in the Songhao Basin of central zones and eastern zones are not developed during the shrinking stage.This may be related to the increased volcanic activities caused by the subduction of the paleo-Pacific plate to the Asian plate during the shrinking stage.The presence of volcanic rocks and pyroclastic rocks prove this,while the volcanic rocks and pyroclastic rocks are not presented in the western zones.Frequent occurrence of volcanic activities leads to the intense alteration of paleoclimate,and the arid paleoclimate destroys the development of peat swamps and is not favorable for large-scale coal accumulation.Only thin coal seams are developed in part of areas.The tectonics are relative steady in the western zones and influence of volcanic activity is little during the shrinking stage,and these conditions are favorable for large-scale coal accumulation.(6)Five sequence stratigraphic surfaces were identified in the thick coal seams of the northeast China:paludification surface(PaS),terrestrialization surface(TeS),accommodation-reversal surface(ARS),nonmarine flooding surface(NFS),and give-up transgressive surface(GUTS).The peat swamp of No.6 thick coal seam in Erlian Basin is m an environment of oxygen and deep wate.The 6#thick coal seam is vertically composed of 7 upward-deepening cycles,7 upward-shallowing cycles,and sequence stratigraphic surfaces.They are controlled by high frequency changes between the accommodation rate and peat production rate controlled by the subsidence and paleoclimate.The peat swamps in the 16#thick seam of the Hailaer Basin are in an oxygen-rich and shallow environment.The 16#thick coal seam vertically consists of 12 upward-deepening cycles,5 upward-shallow cycles,and sequence stratigraphic surfaces. |
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