Sequence-palaeogeography And Coal-seams Change Law Of Benxi-shanxi Formation In North China Platform

This research has collected all kinds of early research reports,and boreholes,cores,well logging,paleontological data in geological exploration units.The method of field observations and office analyses has been used,petrology and deposition characteristics have been analyzed,sequence stratigraphy has been divided,coal-accumulating laws of Benxi-Shanxi formations in North China Basin have been finally studied.The main conclusions are as follows:(1)According to syntagmatic relation of 25 lithofacies in vertical direction and on the plane,a total of 6 sedimentary systems and 24 sedimentary facies associations are identified in this formation.Lithofacies include imbricate conglomerates,evenly bedding sandstones,bioturbated siltstones,dark mudstones,bauxite,carbonate rock,coals and so on.Sedimentary systems include fluvial,alluvial fan,fan delta,delata,barrier-lagoon-tidal flat and carbonate platform.Fluvial sedimentary system is mainly developed in the vicinity of northern Yinshan ancient land and southern Qinling ancient land.Alluvial fan sedimentary system is mainly developed in the northern margin of the North China Basin.North China Basin fan delta depositional system distribution is limited in Xinglong area,Hebei province.As most important coal forming environment,delta sedimentary system occupies a very important position in the North China Basin in the Late Carboniferous and Early Permian period.Barrier-lagoon-tidal flat system and carbonate platform system compose epicontinental sea sedimentary system,which develops in Benxi and Taiyuan formation,and terminates peat accumulation repeatedly.(2)According to the identification of sequence stratigraphic interface in North China platform,combined with the characteristics of sequence interface,late Carboniferous and early Permian strata are divided into 3 third order sequences(SII1-SII3)and the corresponding 7 fourth level sequence(SIII1-SIII7).Typical third order sequence interfaces are weathering crust of the upper part of the Ordovician System,9# coalseam bottom surface in Xishan in Taiyuan,Beichagou sandstone bottom surface,and Luotuobozi sandstone bottom surface.Typical fourth order sequence interfaces are weathering crust of the upper part of the Ordovician System,interface of Benxi formation and Taiyuan formation,9# coalseam bottom surface and Qiligou sandstone bottom surface of Xishan profile in Taiyuan,Beichagou sandstone bottom surface,Dazhan sandstone bottom surface and Xiangtan sandstone bottom surface in Henan province,Luotuobozi sandstone bottom surface.Based on the analysis of a large number of boreholes in the study area,thick coals mainly develope in TST and HST of SIII3(respectively 8-9 coalseams and 7 coalseams),TST and HST of SIII4(respectively 6 coalseams and 6 upper coalseams),TST of SIII5 and SIII6(respectively 5 coalseams and 2-3 coalseams),of which coal accumulation of SIII3 and SIII4 present a stronger regularity in connecting boleholes section map of North China platform in NS direction.This shows difference between coal accumulation of epicontinental sea and transition delta sedimentary system.Coal development of SIII3 and SIII4 has a strong positive correlation with transgression,and some positive correlation with regression.Coal accumulation in TST is beeter than that in HST.6 coalseams and 7 coalseams have a poor development in eastern North China platform.(3)Based on the sequence stratigraphy research,strata between Benxi formation and Shanxi formation could be divided into 7 mapping units(equivalent to fourth order sequence)from bottom to top.On the basis of statisticcal data of single factors of boreholes in research area,the isopleths maps of each single factor have been drawn.On the basis of analyzing single factor isopleths and boreholes composite columnar comprehensively,this research has reconstructed lithofacies paleographic maps quantitatively.Overall,North China platform has experienced the process of sedimentary environment from transitional facies to continental facies in late C and early P,and transgressive direction change and other major events.Provenances of SIII1 and SIII2 are Yinshan ancient land in the north and Qinling-Funiu ancient land in the south.Northern region is dominated by continental fluvial flood sendimentary system,represented as flood plain facies.Sedimentary environment of southern region is transited to lagoon,tidal flat and barrier island.Western region(e.g.Shizuishan and Yinchuan)is dominated by carbonate platform sendimentary system.Transgressive directions are west to east in the west,east to west in the east(e.g.Liaodong Peninsula,Xuzhou,Linyi).Due to the influence of central uplift of Ordos basin and transgression scale,eastern and western carbonate platforms have not communicated.During this period,coal gathering is weak.Sedimentary facies types of SIII3 and SIII4 are same as that of SIII1 and SIII2,but transgression scale is bigger,especially in SIII3 period.At this time,sea water of east and west has been communicated,marine biological types such as mammalian fossils can be traced and contrasted.Carbonate platform is widely distributed in Anhui,Henan,Southwestern Shanxi,Southern Shannxi in the east,and Shizuishan,Yinchuan in the west.Northeastern region is dominated by carbonate platform sendimentary system,and changes into continental deposit in the further north.Eastern and western carbonate platforms have communicated.Provenances are Yinshan ancient land in the north and Qinling-Zhongtiao ancient land in the south.Transgressive directions have changed into west to east in the west,southeast to northwest in the south(e.g.Huainan,Huaibei).SIII3 and SIII4 are important coal accumulation periods,eapecially in SIII3 period,coverage area of coalseams is large,single and accumulative thickness are large.Due to extrusion from north and south side,North China platform gradually upliftes,sea water slowly retreates,continental river and transitional delta gradually occupes the dominant position.Sea water slowly retreates in the west,Zhidan-Daning-Jixian in Ordos Basin developes a lot of lakes(lagoons).There is still a large area of sea water in the east,the area to the south of Zibo-Jinan-Hebi-Jincheng developed lagoon and tide flat sediment.Provenances are Yinshan ancient land in the north and Qinling-Funiu ancient land in the south,Weibei coalfield and Sanmenxia develope fluvial delta deposits.Transgressive direction is mainly southeast tonorthwest in Huainan and Huaibei.SIII5 is an important coal accumulation period.Since then,coal accumulation period almostly finishes,except Anhui province.(4)In late Carboniferous and early Permian,North China platform mainly has two large-scale coal accumulation periods,lower coalseams of SIII3 and upper coalseams of SIII5.Main transgressive direction of SIII3 is south to north,sedimentary facies belt is obvious in NS direction.From the south to the north,carbonate platform,barrier lagoon,transitional delta facies,continental fluvial facies belts are successively distributed,peat accumulation rate increased gradually.According to statistics,the southern part of the study area,which is between Huaibei and Taian,developed highest number of coalseams,the number of coalseams decreases gradually northwards.The northern part of the study area,which is between Cangzhou and Zhangjiakou,developes maximum thickness of coalseams,which decreases gradually southwards.Change tendency of coalseam number and coalseam accumulative thickness are in opposite order in the NS direction.The rule “Sediment transport law in coastal plain transition zone under third-order relative sea level changes” is reflected in the fourth order sequence SIII3.Areas close to land develope tidal flat and continental river sediments,and the accumulation rate of peat is large,and the center of coal accumulation appears near the mfs,when banlance realizes between accommodation space creation rate and peat/coal accumulation rate.Areas close to sea develope carbonate platform sediments,and the accumulation rate of peat is small,and the center of coal accumulation appeare near the initial sea transgressive surface or equivalent sea regressive surface relative to mfs,when banlance realizes between accommodation space creation rate and peat/coal accumulation rate.Upper coalseams are formed in the period of SIII5,there are mainly 6 coal accumulating centers,which are distributed uniformly in all directions of the study area.Coal accumulating centers can be divided into 2 types,(1)-(3)are influenced by paleogeography,(4)-(6)are influenced by basement subsidence rate.(1)-(3)are located in areas where clastic activities are not strong,such as regions among barrier islands,rears of barrier islands,regions among delta lobes and so on.The water environment is relatively closed,the clastic sediments are not active,and the peat source is abundant.(4)-(6)are located mainly in the region with 30-40 m stratigraphic thickness.This shows that 30-40 m of basement subsidence rate is suitable for coal accumulation,when banlance realizes between basement subsidence rate and peat/coal accumulation rate,between base-level change rate and peat/coal accumulation rate,and the remaining space is relatively stable.(4)-(6)are dominated by delta plain,delta front and tidal flat deposit,where water depth is suitable,can realize that the main "coal source" that is peat deposits survive and grow,and that dead plant debris will not be oxidized.Balance can be struck between peat accumulation rate and accommodation increasing rate,the thickness and the extension of of the coalseam can be met,thick coalseams forms at last.