外文翻译中国西北地区新疆的煤炭火灾年代.docx

外文翻译中国西北地区新疆的煤炭火灾年代.docx

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外文翻译中国西北地区新疆的煤炭火灾年代

附录

DatingofcoalfiresinXinjiang,north-westChina[23]

XiangminZhang,SalomonB.KroonenbergandCorB.deBoer

DepartmentofEarthResourcesSurveys,ITC,POBox6,Enschede,theNetherlands;FacultyofAppliedEarthSciences,DelftUniversityofTechnology,POBox5028,2600GADelft,theNetherlands;PalaeomagneticLaboratory,FacultyofEarthSciences,UniversityofUtrecht,3584CDUtrecht,theNetherlands

ABSTRACT

CoalfiresinChinaconsumevastamountsoffuelandcauseseriousenvironmentalproblems.Mostofthesecoalfiresarerelatedtominingactivity.However,naturallyproducedpalaeocoalfiresinXinjiang,north-westChina,havebeenrecognizedviaburntrocks.Theburntrocksinthestudyareaarefoundatdifferentriverterracesunderlyingunburntalluvialandriverterracedeposits.Severalagegroupsofcoalfireshavebeenidentifiedbasedonthepositionsofburntrocksatriverterracesandtherelationshipbetweentheburntrocksandtheterracedeposits.Thesepalaeocoalfiresare:

（1）Pliocene–EarlyQuaternaryinageat200mabovepresentriverterracedeposits;

（2）MiddlePleistoceneinage,at>90m;（3）LatePleistocene,at90–70m;（4）Holocene;（5）burntrocksrelatingtoactivecoalfires.Palaeomagneticdataoftheburntrocksfromdifferentterracesgivenormalremanentmagnetizationandhelpfurthertoconstraintheagesofthecoalfires.

TerraNova,16,68–74,2004

Introduction

CoalfiresareoneofthemostseriousproblemsfortheChinesecoalindustry.TheestimatedannuallossofcoalbyfiresinChinarangesfromabout10–20milliontones（Guanetal,1998）to100–200milliontones（Schalkeetal,1993;CassellsandVanGenderen,1995）.Besidesthehugelossofcoalresourcesandminingsafety,coalfirescauseconsiderableenvironmentalproblems,suchasairpollutionandlanddegradation.Coalfireshaveaglobalimpactaswell;theemissionofCO2mightcontributeconsiderablytotheincreaseingreen-housegasesintheatmosphere.Ifthelatterestimateforannuallossofcoaliscorrect,CO2emissionbycoalfiresinChinawouldaccountfor2–3%oftheworldCO2outputfromtheburn-ingoffossilfuelsfortheyear1992（CassellsandVanGenderen,1995）.ActivecoalfiresinChinaareusuallyrelatedtominingactivity;however,thedirectcauseofthecoalfiresisessentiallyspontaneouscombustion.Spontaneouscombustionisaprocessofoxidationofcoalinwhichthetemperatureofthecoalincreasesuntilafirestarts.Firescanalsostartatsurfaceoutcropsasaresultofnaturalprocessessuchasforestfires,lightningorevensolarheating.Exposureofthecoalseamisessentialforthesetypesofcoalfire.Severalgeologicalprocesses,suchasfaulting,foldinganderosionbyriveraction,canbringthecoaltothesurfacethusleadtocoalfires.Inourstudyarea,coalfireshaverepeatedlyoccurredbecauseofriverdowncuttingandexposureofcoalseams.Thecaprocksandsedimentsenclosingtheseamshavebeengreatlyalteredthermallybythecoalfires.Thesethermallyalteredrocks（burntrocks）indicatethatthepalaeocoalfiresoccurredduringgeologicalhistory.Theagesofthepalaeocoalfirescanbeconstrainedfromtheagesoftheriverdenudationthatexposedthecoalseamandfromtheterracedepositsoverlyingtheburntrocks.ThetemperatureoftheburntrocksisbelievedtohaveexceededtheCuriepoint（Guan,1963）andhencetheyrecordthegeomagneticdirectionofthatpointintime.Tenorientatedspecimensofburntrockfromdifferentagegroupswerecollectedandexaminedinthelaboratory.Thepalaeomagneticdataprovidefurtherconstraintsontheabsoluteagesofthecoalfires.OurstudyshowsthatcoalfireshaverepeatedlyoccurredsincethePlioceneandthatmostoftheburntrocksareofPleistoceneage.

Geologicalsetting

TheToutunhestudyareaissituated30kmsouth-westofUrumqi,thecapitalcityofXinjiang,China,inthetransitionzonebetweentheTianshanmountainrangeandtheJunggarbasin,atanaltitudeof1000–1400m.ItiscrossedbytheToutunheriver,whichisfedbytheglaciersoftheTianshanMountainsandbyitstributaryriverstheQianshuihe,GangouandHaojiagou（Fig.1）.ThecoreoftheE–W-trendingTianshanMountainrangeconsistsofpreMesozoicbasementrocks（PengandZhang,1989;BGXJ,1993;Carrolletal,1995）.OnthenorthernsideoftheTianshan,MesozoicandCenozoicsedimentaryrockshavebeendetachedfromtheunderlingpre-MesozoicrocksandfoldedintothreelinesofE–W-trendinganticlinesandsynclines.TheKelazhaanticlineinthestudyareaissituatedinthefirstline,withJurassicstrataformingthecoreofthefolds.UnfoldedPliocenesedimentsunconformablycovertheJurassicandCretaceousrocks.Themaincoal-bearingstratathathaveburntoutbelongtotheMiddleJurassicXishanyaoGroup,whichconsistsoffreshwaterdeltaicsediments（BGXJ,1993;Schneider,1996）.ThecoallayersareconcentratedinthelowerpartsoftheXishanyaoGroup,withthicknessvaryingfrom1to27m.ThelowestmineablelayeristheDacaocoallayer,withaconstantthicknessof15–17m.TheToutunheriverdissectsthefoldedJurassicrocksandhasaflightofatleastsixriverterracesinthevalley（Fig.2;HuangandZhao,1981;Qiao,1981;Molnaretal,1994）.Theuppermostterracesaresituatedabout90mabovetheriver,thelowermostoneabout15mabovethepresentriverlevel.Theotherterracesaresituatedbetweenthese,witharegularheightintervalofabout10–15m.Mostoftheterracesarecutintobedrockandcappedby1–3mofcoarsegravel.

Fig.1Geologicalmapofthestudyareawithdistributionofburntrocksandactivecoalfires.ThestraightlinefromBeigoutoQianshuiheindicatesthelocationofthesectionshowninFig.2[20].

Fieldcharacteristicsofburntrocks

Burntrock（pyrometamorphosedrock）isageneraltermforthermallymetamorphosedrocks（Tyra′cek,1994）originatingfromheatingbyburningcoalseams.Burntrockscanbeclassifiedaccordingtocharacteristicssuchascolour,texture,structureandmetamorphictemperatures（Guan,1963）4Inascarp-slopeexposureofslightlydippingburntrocks,acharacteristicthree-layerprofileisdeveloped.

（1）Atthebottomaretheunburntrocksunderlyingthecoalseams.Nearingthecontactwiththeburntcoalseam,therocksbecomegraduallybakedinappearance;inmudstonesthisisobviousfromthebrick-redcolour,ceramiccharacterandsound,andsometimeswithacharacteristicmillimetre-sizedsix-sidedcolumnarstructure（chopstickrock）causedbyshrinkagejointsarrangedinasheaf-likepattern,likejointinginbasalticlavaflows.Locally,thepresenceofacharacteristichardenedkaolinlayer（bakedunderclay）seemstohaveactedasaninsulator,thusprotectingtheunderlyingrocksfrombaking.Sandstonesappearlessaffectedbybakingalone.

（2）Thecoalseamitselfhasbeenreducedtoathinashlayerofonlyafewcentimetersthickness,oftenrichingypsum.（3）Abovetheashlayertheroofofthecoalseamhasusuallycollapsed,resultinginabreccia-likestructure.Asthetemperatureofcoalfiresusuallyincreasesupwardsasaresultofincreasedoxygensupply,theserocksarealsopartiallymolten:

betweenthepartlymoltenfragmentsofcollapsedroofrocks,adarkvesicularglassymatrixisoftenfound,sometimeswithmicroflowstructuresasinropylavas.Loessoverlyingburntrockmayhavebeenbakedtobrick-likesubstances.Themaximumthicknessoftheburntrocksinthestudyareaisabout100–150m.Thereddish–yellowishcolourenablesunequivocalidentificationofburntrocksfromremotesensingimagery.Henceburntrocksareusuallyusedasanindicatorofundergroundcoalfires（Kang,1991;Zhangetal,1999,2003）.

Fig.2Cross-section（fromBeigoutoQianshuihe）ofToutunheriverterracesandburntrockoccurrences.

Riverterracesanddatingofpalaeocoalfires

Theoutcropconditionsofthepalaeocoalfiresinourstudyareaareintimatelyrelatedtothedevelopmentofdifferentriverterraces（Fig.2）.Therelativeageofthepalaeocoalfirescanbedistinguishedaccordingtotheirpositionsatdifferentterracesandtherelationbetweentheburntrocksandterracedeposits.AgeneralageclassificationoftheterracedepositsspanningthewholeQuaternaryhasbeenobtainedbycorrelationwithmorainicstagesintheTianShanMountainsintheupperreachesoftheToutunherivers（Bo,1981;Qiao,1981;BGXJ,1993）.Fiveagegroupsofburntrockshavebeenrecognizedinthisarea:

（1）TheBeigouburntrocksoccuratanunconformitybetweensteeplydippingJurassiccoalsandblackshalesandunbakeddepositsofadissectedalluvialfan,200mabovetheleveloftheBeigoutributaryoftheToutunheriver.TheallivialfancouldbeLatePlioceneorEarlyPleistoceneage（Q1highterrace,inChineseterminology,0.7–2Ma）.ThesedepositsmightcorrespondtotheXiyuFormation,exposuresofwhichelsewherehaveproducedabundantLatePliocenefaunahavebeenfound,andwhichisgenerallyassociatedwiththeinceptionofglaciation（Qiao,1981;Molnaretal,1994）.Becausethealluvialfan

depositsarenotbaked,thecoalfiresoccurredbeforedepositionofthesesediments.

（2）IntheQianshuihe–Louzhuangziareaaclassicprofileofburntrocksshows20mofmoltenscoriaceousbrecciasontopofaburnt-outDacaocoallayer,inturnunderlainbyabakedkaolin（‘porcellanite’）layer,possiblyofvolcanicashorigin.Theirpositioninthefieldsuggeststhemtobeolderthanthe90-mterracenearby,asthedepositsareincontactwiththeburntrockandshownosignsofbaking.Asimilarsituationisfoundinthesouth-westernpartoftheareanearLouzhuangzialongthewesternsideoftheToutunheriver.TheQianshuiheandLouzhuangzi90-mterracesarefluvi