成煤构造环境外文翻译.docx
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成煤构造环境外文翻译
Coal-ProducingTectonicEnvironments
Thisfinalchapterintheinvestigationofcoalsedimentationisconcernedwithdepositionalaspectsofthehighestorderofmagnitude,namely,theinfluenceofthecrustalsettingonpeataccumulation.Thisisabroadandcomplexfieldwhichdrawsoninformation,gatheredfrommanydifferentdisciplinesoftheearthsciences.Someofthesearecurrentlyevolvingquiterapidly,whileothersreina“moppingup”stage,insensuKuhn(1970)andWalker(1973),followingrecentscientificrevolutions.Anexampleofthelatteristhereplacementofthegeosynclinalhypothesisintheearly1970sbytheconceptofplatetectonics.Evenafteralifespanof20years,thisnewparadigmisstillintheprocessofbeingrefinedandfittedoutwithconceptualsubsets,asshownbythecurrentemphasisonterraneanalysis.Itisthereforenotpossibleatthisstagetomakeadefinitivestatementonthechosensubject,butmerelytooutlinetheprincipleonwhichamoderngeotectonicclassificationofcoalfieldscanbeestablished.Eventhismodestgoalisfraughtwithdifficulty,becausethechangefromthepredominantlystaticgeosynclinalviewofglobaltectonicstoitsmodern,largelymobilisticinterpretationhascomplicatedthetectonicclassificationofsomecoalfields.Whilethetectonicstatusofmanycoalfields,e.g.thoseinforedeepsorforelandbasinshaschangedrelativelylittle,thesettingofcoalsfoundininter-andintramontanetroughs,i.e.withinorogeniccordilleras,cannotbeproperlyassessedwithoutverycarefulstudy.Accordingtothegeosynclinalconcept,practicallyalloftheseintradeeps,togetherwithfore-andbackdeeps,theirextra-orogeniccounterparts,wereregardedaspartofagroupofmolassesbasins,thedevelopmentofwhichaccompaniesorfollows“terminalgeosynclinaltecto-orrgenic”(Aubouin1965).Thisfixistandstrictlysequentialinterpretation(highlightedbytheterm“epieugeosyncline”ofKay1951)hasnoplaceinmoderngeotectonicanalysis,whichviewsmostorogenicbeltsascollagesofautochthonousandallochthonousterranes,i.e.astectono-stratigraphicassemblageswithpossiblycoevalbutheterogeneousstratigraphicrecordsreflectingtheiroriginindifferentgeologicalandgeographicaldomains(MongerandPrice1979,Mongeretal.1982).Thetectonicsetting,whichinfluencedtheformationofanallochthonousterraneassemblagebeforeaccretion,mayhavebeenverydifferentinstyleandphysicallyfarremovedfromitsrestingplaceafterdocking.Itfollowsthatamulti-terraneorogenmaycontainavarietyofcoalsformedatdifferenttimesbeforeandafterterraneaccretion.Moreover,contemporaneouspre-accretionarycoaldepositsformedindifferentterranesarelikelytovaryincoaltypes,coalificationhistoriesandtectonicstyles,andallofthesewillinturndifferfromthepost-accretionarymolassescoals,whichalonereflecttheconditionaprevailingintheorogenitself.Indeed,thesituationmayevenbemorecomplex,aswillbediscussedinChap.9.3.2.2.
Platetectonicshascreateditsownnomenclature,ofwhichonlytheessentialtermswillbeusedhere.Theywillbesupplementedbytermswhichareeitherdescriptive,andthereforeindependentofgeotectonictheory,orwhichhavestoodthetestoftimebecausetheyareusefulinspiteoftheirgenericassociationwithnowobsoleteconcepts.Forexample,theexpressions“mio-”and“eugeosynclinalassemblage”havebeenkepthereasreferencetermforshallowwatermarine(mainlyshelf),andoceanfloorpelagite,turbiditeandophioliteassociationa,respectively.Moreover,reducedtoa“miogeocline”,themiogeosynclinehasintheNorth-Americanliteraturebecomeastandardtermforautochthonous,sedimentaryterracewedgesonlappingcontinentalmargins.Alsotectonicattributesofsediments,suchas“synorogenic”flyschand“latesyn-(folded)topostorogenic(non-folded)”molasses,respectively,canstillbeusedinaplatetectoniccontextwithoutundulycorruptingtheirrelativelyloosedefinitions.Particularlyinthediscussionofcoalfieldssituatednearconvergentplateedges,theconceptofmolassesastheproductofthedestructionoftheupliftedorogenisveryuseful.Asinthepreviousdiscussion,itisnotthepurposeofthischaptertogivedetaileddescriptionsofalargenumberofcasesbuttoselectafewtypicalexamplesofcoalfieldsandrelatetheessenceoftheirarchitecturetotheirrespectiveplatetectonicsettings.
1EarlyExamplesofaTectonicClassificationofCoalfields
Large-scalecoalformationcantakeplaceonlyinactivelysubsidingregions,forexampleinsedimentarybasins.Itispossiblethereforetocharacterisethegeotectonicenvironmentoccupiedbyacoalmeasuresequenceinamannersimilartothatwhichisappliedtoothersedimentaryenvironments.Stutzer(1920)andStille(1926)wereamongthefirsttorecognisethegeneticlinksbetweentectonismandtheformationofcoal.Stille,inparticular,referredtothestrikingdifferenceintermsofbasinfill,numberofcoalseamspresent,theiraveragethicknessandproportioninrelationtototalcoalmeasurethickness,whichexistbetweentheCarboniferousandTertiarycoalmeasuresofEurope.HeattributedsuchdissimilaritiestocontrastingdegreesofcrustalmobilityintheareasaffectedbythetwomainEuropeancoal-formingperiods.HisresultsaresummarizedinTable9.1.EvenifdifferencesincompactionratiosbetweentheTertiarybrownandCarboniferousbituminouscoalsaretakenintoaccount(toalesserdegreethecompactionappliestointer-seamsediments)thecontrastisquiteremarkable.
LateritwasshownbyvonBubnoff(1937)thatthedistributionoftheworldreservesofcoalisalsorelatedtothegeotectonicsettingofcoalfields.HisconclusionsaresummarizedinTable9.2,whichindicatesthatofallcoaldepositsknownupto1937,some71%developedinformertectonicallyveryactiveenvironments,particularlyinthemolassesforedeepswhichdevelopadjacenttoorogenicbeltsandreceivemuchoftheweathereddebriswasheddownfromtheuplands.
CarboniferouscoalmeasuresinmobileVaricanbasins
TertiarycoalmeasuresincratonicbasinsofCentralEurope
Averagecoalmeasurethickness
3000m
150m
Averagenumberofcoalseams
200
2
Averageseamthickness
1m
15m
Cumulativethicknessofcoal
180m
25m
Proportionoftotalcoal
6%
16.7%
Proportionofeconomicinsitucoal
3.6%
12%
Table9.1.Stille’s(1926)comparison(slightlymodified)betweensomecharacteristicsofcoalmeasuresformedintectonicallymobileandcratonisedpartsofEurope,respectively
Table9.2.Thedistributionofworldreservesofcoalinreferencetothegeotectonicsettingofcoalfields.(AftervonBubnoff1937)
Foredeepsmarginaltoorogenicbelts70%
Intradeepswithinorogenicbelts1%
Shelfbasinsoncratonicmargins21%
Cratonicinterior8%
Theconcentrationofcoalintheregionsassociatedwithorogenicbeltsisevenmorehighlightedwhenthelateralextentofthedepositsisconsidered.Coalfieldssituatedwithinorontheshelfmarginsofcratonscoverawiderareathanthecomparativelynarrowforedeeps,butitsarealrestrictioniscompensatedbythefrequencyofcoalseamsoccurringinathickstackofcoalmeasures.Aswillbediscussedlater,thisisrelatedtothesubstantialandprolongedsubsidencethatthecontinentalmarginissubjectedtonearasubductioncomplex,asanorogenicbeltisaccretedontotheplateedge.Itisnotsurprising,therefore,thatvonBubnoff(1937)foundalsoaclosetemporalrelationshipbetweenorogeniesandcoalformationinNorthAmerica,Europe,AsiaandSouthernContinents.Ofcourse,therearemajororogeniesknownwhicharenotassociatedwithcoaldeposits.However,invariably,theirabsenceisrelatedtofactorsaffectingthevegetablesource.Forexample,allpre-Devonianorogeniesoccurredatatimewhentheplantkingdomwasstillinsufficientlyequippedbyevolutiontofulfilitsroleasaneffectiveproducerofpeat.
Thecontinentalshelfenvironment,beinglessmobile,hasproducedfewercoaldepositsthantheorogenicdomain.Inthiscontextitisimportanttodefinethetermshelf.Tothegeographer,theshelfregionisusuallythatpartoftheseawhichextendsbetweenthestrand-lineandthecontinentalslope.However,asvonBubnoff(1948a)noted,thepositionofthestrandlineisquiteincidentaldependingoncrustalmovementsandrelativesealevelpositions.Fromthegeologicalviewpoint,itappearsthereforeusefultoextendthedefinitionoftheshelfsothatthetimefactorcanbeaccommodated.Shelfregionsmaythenberegardedasthosemarginalbutfuullyintegratedzonesofcontinentswhichareoccasionallyaffectedbyshallowmarinetransgressions.Typicalareasarethetrailingedgesofcontinentalplatesandthecratonicmarginsofforedeeps.Commonlytwotypesofshelfenvironmentsaredistinguished,calledstableandunstable,respectively(vonBubnoff1948a;KrumbeinandSloss1963).Themajorityfortheirassociatedcoalfieldsisparalicincharacter,whichishighlightedbytheintercalationofcoalmeasureswithmarinestrata,afeaturethatisalsocommontothemolassesforedeeps.However,marinestratamaynotalwaysberecognizedbecauseoflackoffossils,whichisrelatedtothedilutionofseawaterbyanexcessiveinfluxoffreshwaterfromthenearbycoastalswamps(DuffandWalton1962).
Intracratoniccoalfieldsandthoseformedinintramontancbasinsarefrequentlylimnicincharacter,i.e.theyhavenohydrologicalconnectiontothesea,becausetheyhavebeenformedinland-lockedbasinsabovethethenprevailingsealevel.AspectacularmodernexampleofintramontanepeatformationoccursinthereedmarshesontheshoresofLake