stratigrraphyandgeologictime地层与地质时间课件

StratigraphyandGeologicTimeStratigraphyBasicprinciplesofrelativeagedatingUnconformities:MarkersofmissingtimeCorrelationofrockunitsAbsolutedatingGeologicTimeHowoldistheEarth?Whendidvariousgeologiceventsoccur?InterpretingEarthhistoryisaprimegoalofgeology.SomeknowledgeofEarthhistoryandgeologictimeisalsorequiredforengineersinordertounderstandrelationshipsbetweengeologicunitsandtheirimpactonengineeringconstruction.StratigraphyandGeologicTimeStratigraphy:Stratigraphyisthestudyofrocklayers(strata)andtheirrelationshipwitheachother.Stratigraphyprovidessimpleprinciplesusedtointerpretgeologicevents.Stratigraphy:TworockunitsatacliffinMissouri.(USGeologicalSurvey)TworockunitsatacliffinMBasicprinciplesofrelativeagedating

Relativedatingmeansthatrocksareplacedintheirpropersequenceofformation.Aformationisabasicunitofrocks.Belowaresomebasicprinciplesforestablishingrelativeagebetweenformations.PrincipleoforiginalhorizontalityPrincipleofsuperpositionPrincipleoffaunalsuccessionPrincipleofcross-cuttingrelationshipsBasicprinciplesofrelativeaPrincipleoforiginalhorizontality:Layersofsedimentaregenerallydepositedinahorizontalposition.Thusifweobservedrocklayersthatarefoldedorinclined,theymust,withexceptions,havebeenmovedintothatpositionbycrustaldisturbancessometimeaftertheirdeposition.

PrincipleoforiginalhorizontMostlayersofsedimentaredepositedinanearlyhorizontalposition.Thus,whenweseeinclinedrocklayersasshown,wecanassumethattheymusthavebeenmovedintothatpositionafterdeposition.HartlandQuay,Devon,EnglandbyTomBean/DRKPhoto.MostlayersofsedimentaredePrincipleofsuperposition:

Inanundeformedsequenceofsedimentaryrocks,eachbedisolderthantheoneaboveandyoungerthantheonebelow.Therulealsoappliestoothersurface-depositedmaterialssuchaslavaflowsandvolcanicashes.Principleofsuperposition:Principleofsuperposition.(W.W.Norton)Principleofsuperposition.(WApplyingthelawofsuperpositiontothelayersattheupperportionoftheGrandCanyon,theSupaiGroupistheoldestandtheKaibabLimestoneistheyoungest.(photobyTarbuck).ApplyingthelawofsuperpositPrincipleofcross-cuttingrelationships:

Whenafaultcutsthroughrocks,orwhenmagmaintrudesandcrystallizes,wecanassumethatthefaultorintrusionisyoungerthantherocksaffected.

Principleofcross-cuttingrelCross-cuttingrelationships:Anintrusiverockbodyisyoungerthantherocksitintrudes.Afaultisyoungerthantherocklayersitcuts.(TarbuckandLutgens)Cross-cuttingrelationships:AUnconformities:Markersofmissingtime

Whenlayersofrockformedwithoutinterruption,wecallthemconformable.Anunconformityrepresentsalongperiodduringwhichdepositionceasedanderosionremovedpreviouslyformedrocksbeforedepositionresumed.AngularunconformitiesDisconformityNonconformityUnconformities:MarkersofmisAngularunconformities:

Anangularunconformityconsistsoftiltedorfoldedsedimentaryrocksthatareoverlainbyyounger,moreflat-lyingstrata.Itindicatesalongperiodofrockdeformationanderosion.Angularunconformities:Formationofanangularunconformity.Anangularunconformityrepresentsanextendedperiodduringwhichdeformationanderosionoccurred.(TarbuckandLutgents)FormationofanangularunconfAngularunconformityatSiccarPoint,southernScotland,thatwasfirstdescribedbyJamesHuttonmorethan200yearsago.(HamblinandChristiansenandW.W.Norton)AngularunconformityatSiccarDisconformity:

Adisconformityisaminorirregularsurfaceseparatingparallelstrataonoppositesidesofthesurface.Itindicatesahistoryofupliftingabovesea(water)level,undergoingerosion,andloweringbelowthesealevelagain.Disconformity:Formationofdisconformity.(W.W.Norton)Formationofdisconformity.(WDisconformitiesdonotshowangulardiscordance,butanerosionsurfaceseparatesthetworockbodies.Thechannelinthecentralpartofthisoutcroprevealsthatthelowershaleunitsweredepositedandthenerodedbeforetheupperunitsweredeposited.(HamblinandChristiansen)DisconformitiesdonotshowanNonconformityAnonconformityisabreaksurfacethatdevelopedwhenigneousormetamorphicrockswereexposedtoerosion,andyoungersedimentaryrocksweresubsequentlydepositedabovetheerosionsurface.(TarbuckandLutgens)NonconformityAnonconformityiAnonconformityattheGrandCanyon.Themetamorphicrocksandtheigneousdikesoftheinnergorgewereformedatgreatdepthsandsubsequentlyupliftedanderoded.Youngersedimentarylayerswerethendepositedontheerodedsurfaceoftheigneousandmetamorphicterrain.(HamblinandChristiansen)AnonconformityattheGrandCTypesofUnconformityThisanimationshowsthestagesinthedevelopmentofthreemaintypesofunconformityincross-section,andexplainshowanincompletesuccessionofstrataprovidesarecordofEarthhistory.View1showsadisconformity,View2showsanonconformityandView3showsanangularunconformity.[byStephenMarshak]PlayAnimationWindowsversion>>PlayAnimationMacintoshversion>>TypesofUnconformityThisanimDistinguishingnonconformityandintrusivecontact

Nonconformity:

Thesedimentaryrockisyounger.Theerosionsurfaceisgenerallysmooth.Dikesmaycutthroughtheigneousbodybutstopatthenonconformity.Intrusivecontact:

Intrusionisyoungerthanthesurroundingsedimentaryrocks.Thecontactsurfacemaybequiteirregular.Azoneofcontactmetamorphismmayformsurroundingtheigneousbody.Cross-cuttingdikesmaypenetrateboththeigneousbodyandthesedimentaryrocks.DistinguishingnonconformityaContrastingfieldconditionsfor(a)anonconformityand(b)anigneousintrusion.(West,Fig9.4)ContrastingfieldconditionsfThethreebasictypesofunconformitiesillustratedbythiscross-sectionoftheGrandCanyon.(TarbuckandLutgents)ThethreebasictypesofunconGeologicHistoryAcross-sectionthroughtheearthrevealsthevarietyofgeologicfeatures.View1ofthisanimationidentifiesavarietyofgeologicfeatures;View2animatesthesequenceofeventsthatproducedthesefeatures,anddemonstrateshowgeologistsapplyestablishedprinciplestodeducegeologichistory.[byStephenMarshak]PlayAnimationWindowsversion>>PlayAnimationMacintoshversion>>GeologicHistoryAcross-sectiostratigrraphyandgeologictime地层与地质时间Principleoffaunalsuccession:Groupsoffossilanimalsandplantsoccurthegeologichistoryinadefiniteanddeterminableorderandaperiodofgeologictimecanberecognizedbyitscharacteristicfossils.PrincipleoffaunalsuccessionFossilsaretheremainsofancientorganisms.Therearemanytypesoffossilization.(Top)naturalcastsofshelledinvertebrates.(Middle)Fishimpressions.(Bottom)Dinosaurfootprintinfine-grainedlimestonenearTuba,Az.Fossilsaretheremainsofancstratigrraphyandgeologictime地层与地质时间Theprincipleoffossilsuccession.Notethateachspecieshasonlyalimitedrangeinasuccessionofstrata.(W.W.Norton)TheprincipleoffossilsuccesCorrelationofrockunits

Themethodofrelatingrockunitsfromonelocalitytoanotheriscalledcorrelation.Onewayofcorrelationistorecognizetherocktypeorrocksequenceattwolocations.Anotherwayofcorrelationistousefossils.Abasicunderstandingoffossilsisthatfossilorganismssucceededoneanotherinadefiniteanddeterminableorder,andthereforeatimeperiodcanberecognizedbyitsfossilcontent.CorrelationofrockunitsTheprincipleofcorrelationofrockunits.Therockcolumnscanbecorrelatedbymatchingrocktypes.(W.W.Norton)Theprincipleofcorrelationostratigrraphyandgeologictime地层与地质时间WilliamSmith,acivilengineerandsurveyor,couldpiecetogetherthesequenceoflayersofdifferentagescontainingdifferentfossilsbycorrelatingoutcropsfoundinsouthernEnglandabout200yearsago.Inthisexample,FormationIIwasexposedatbothoutcropsAandB,thusFormationIandIIwereyoungerthanFormationIII.(PressandSiever).WilliamSmith,acivilengineeCorrelationofstrataatthreelocationsontheColoradoPlateaurevealsthetotalextentofsedimentaryrocksintheregion.CorrelationofstrataatthreeThegeologiccolumnwasconstructedbydeterminingtherelativeagesofrockunitsfromaroundtheworld.(Next)Bycorrelation,thesecolumnswerestackedoneontopoftheothertogiverelativeagesofrockunits(W.W.Norton)Thegeologiccolumnwasconstrstratigrraphyandgeologictime地层与地质时间Absolutedating

Thegeologictimebasedonstratigraphyandfossilsisarelativeone:wecanonlysaywhetheroneformationisolderthantheotherone.Absolutedatingwasmadepossibleonlyafterthediscoveryofradioactivity.AbsolutedatingRadioactivityAttheturnofthe20thcentury,nuclearphysicistsdiscoveredthatatomsofuranium,radium,andseveralotherelementsareunstable.Thenucleioftheseatomsspontaneouslybreakapartintootherelementsandemitradiationintheprocessknownasradioactivity.Wecalltheoriginalatomtheparentanditsdecayproductthedaughter.Forexample,aradioactive92U238atomdecaysintoastablenonradioactive82Pb206atom.RadioactivityexampletypesofradioactivedecayAlphadecay:anaparticle(composedof2protonsand2neutrons)isemittedfromanucleus.Theatomicnumberofthenucleusdecreasesby2andthemassnumberdecreasesby4.Betadecay:abparticle(electron)isemittedfromanucleus.Theatomicnumberofthenucleusincreasesby1butthemassnumberisunchanged.exampletypesofradioactivedIllustrationofalphaandbetadecays.(adaptedfromTarbuckandLutgens)stratigrraphyandgeologictime地层与地质时间ThedecayofU238.Afteraseriesofradioactivedecays,thestableendproductPb206isreached.(TarbuckandLutgents)ThedecayofU238.AfteraserDecayconstant

Therateofdecayofanunstableparentnuclideisproportionaltothenumberofatoms(N)remainingatthetimet.dN/dt=-l*NThereasonthatradioactivedecayoffersareliablemeansofkeepingtimeisthatthedecayconstantlofaparticularelementdoesnotvarywithtemperature,pressure,orchemistryofageologicenvironment.

DecayconstantHalf-life

Thehalf-lifeofanradioactiveelementisthetimerequiredforone-halfoftheoriginalnumberofradioactiveatomstodecay:T1/2=0.693/l.Thehalf-livesofgeologicallyusefulradioactiveelementsrangefromthousandstobillionsofyears.TheageoftheEarth(4.6billionyears)wasfirstobtainedusingU/Th/Pbradiometricdating.Thehalf-lifeofU238is4.5billionyears.Half-lifeTheradioactivedecayisexponential.Halfoftheradioactiveparentremainsafteronehalf-life,andone-quarteroftheparentremainsafterthesecondhalf-life.(TarbuckandLutgens)TheradioactivedecayisexponTheconceptofahalf-life.Theratioofparent-to-daughterchangeswiththepassageofeachsuccessivehalf-life.(W.W.Norton)Theconceptofahalf-life.ThGeologicTime

Thegeologictimescalesubdividesthe4.6-billion-yearhistoryoftheEarthintomanydifferentunits,whicharelinkedwiththeeventsofthegeologicpast.Thetimescaleisdividedintoeons:PrecambrianandPhanerozoicanderas:Precambrian,Paleozoic("ancientlife"),Mesozoic("middlelife"),andCenozoic("recentlife").Theerasareboundedbyprofoundworldwidechangesinlife-forms.Theerasaredividedintoperiods.Theperiodsaredividedintoepochs.GeologicTimeThestandardgeologictimescalewasdevelopedusingrelativedatingtechniques.Radiometricdatinglaterprovidedabsolutetimesforthestandardgeologicperiods.(W.W.Norton)ThestandardgeologictimescaTheawesomespanofgeologictimeThegeologictimerepresentseventsofawesomespansoftime.Ifthe4.6-billion-yearEarthhistoryisrepresentedbya24-hourdaywiththebeginningat12midnight,thefirstindicationoflifewouldoccurat8:35am.Dinosaurswouldappearat10:48pmandbecomeextinctat11:40pm.Therecordedhistoryofmankindwouldrepresentonly0.2secbeforemidnight.

Theawesomespanofgeologicstratigrraphyandgeologictime地层与地质时间TheKTextinctionAttheboundarybetweenCretaceous(thelastperiodofMesozoic)andTertiary(thefirstperiodOfCenozoic)about66millionyearsago,knownasKTboundary,morethanhalfofallplantandanimalspeciesdiedinamassextinction.Theboundarymarkstheendoftheerainwhichdinosaursandotherreptilesdominatedandthebeginningoftheerawhenmammalsbecameimportant.Thewidelyheldviewoftheextinctionistheimpacthypothesis.AlargeobjectcollidedwiththeEarth,producingadustcloudthatblockedthesunlightfrommuchoftheEarth’ssurface.Withoutsunlightforphotosynthesis,thefoodchainscollapsed,whichaffectedlargeanimalsmostseverely.TheKTextinctionStratigraphyandGeologicTimeStratigraphyBasicprinciplesofrelativeagedatingUnconformities:MarkersofmissingtimeCorrelationofrockunitsAbsolutedatingGeologicTimeHowoldistheEarth?Whendidvariousgeologiceventsoccur?InterpretingEarthhistoryisaprimegoalofgeology.SomeknowledgeofEarthhistoryandgeologictimeisalsorequiredforengineersinordertounderstandrelationshipsbetweengeologicunitsandtheirimpactonengineeringconstruction.StratigraphyandGeologicTimeStratigraphy:Stratigraphyisthestudyofrocklayers(strata)andtheirrelationshipwitheachother.Stratigraphyprovidessimpleprinciplesusedtointerpretgeologicevents.Stratigraphy:TworockunitsatacliffinMissouri.(USGeologicalSurvey)TworockunitsatacliffinMBasicprinciplesofrelativeagedating

Relativedatingmeansthatrocksareplacedintheirpropersequenceofformation.Aformationisabasicunitofrocks.Belowaresomebasicprinciplesforestablishingrelativeagebetweenformations.PrincipleoforiginalhorizontalityPrincipleofsuperpositionPrincipleoffaunalsuccessionPrincipleofcross-cuttingrelationshipsBasicprinciplesofrelativeaPrincipleoforiginalhorizontality:Layersofsedimentaregenerallydepositedinahorizontalposition.Thusifweobservedrocklayersthatarefoldedorinclined,theymust,withexceptions,havebeenmovedintothatpositionbycrustaldisturbancessometimeaftertheirdeposition.

PrincipleoforiginalhorizontMostlayersofsedimentaredepositedinanearlyhorizontalposition.Thus,whenweseeinclinedrocklayersasshown,wecanassumethattheymusthavebeenmovedintothatpositionafterdeposition.HartlandQuay,Devon,EnglandbyTomBean/DRKPhoto.MostlayersofsedimentaredePrincipleofsuperposition:

Inanundeformedsequenceofsedimentaryrocks,eachbedisolderthantheoneaboveandyoungerthantheonebelow.Therulealsoappliestoothersurface-depositedmaterialssuchaslavaflowsandvolcanicashes.Principleofsuperposition:Principleofsuperposition.(W.W.Norton)Principleofsuperposition.(WApplyingthelawofsuperpositiontothelayersattheupperportionoftheGrandCanyon,theSupaiGroupistheoldestandtheKaibabLimestoneistheyoungest.(photobyTarbuck).ApplyingthelawofsuperpositPrincipleofcross-cuttingrelationships:

Whenafaultcutsthroughrocks,orwhenmagmaintrudesandcrystallizes,wecanassumethatthefaultorintrusionisyoungerthantherocksaffected.

Principleofcross-cuttingrelCross-cuttingrelationships:Anintrusiverockbodyisyoungerthantherocksitintrudes.Afaultisyoungerthantherocklayersitcuts.(TarbuckandLutgens)Cross-cuttingrelationships:AUnconformities:Markersofmissingtime

Whenlayersofrockformedwithoutinterruption,wecallthemconformable.Anunconformityrepresentsalongperiodduringwhichdepositionceasedanderosionremovedpreviouslyformedrocksbeforedepositionresumed.AngularunconformitiesDisconformityNonconformityUnconformities:MarkersofmisAngularunconformities:

Anangularunconformityconsistsoftiltedorfoldedsedimentaryrocksthatareoverlainbyyounger,moreflat-lyingstrata.Itindicatesalongperiodofrockdeformationanderosion.Angularunconformities:Formationofanangularunconformity.Anangularunconformityrepresentsanextendedperiodduringwhichdeformationanderosionoccurred.(TarbuckandLutgents)FormationofanangularunconfAngularunconformityatSiccarPoint,southernScotland,thatwasfirstdescribedbyJamesHuttonmorethan200yearsago.(HamblinandChristiansenandW.W.Norton)AngularunconformityatSiccarDisconformity:

Adisconformityisaminorirregularsurfaceseparatingparallelstrataonoppositesidesofthesurface.Itindicatesahistoryofupliftingabovesea(water)level,undergoingerosion,andloweringbelowthesealevelagain.Disconformity:Formationofdisconformity.(W.W.Norton)Formationofdisconformity.(WDisconformitiesdonotshowangulardiscordance,butanerosionsurfaceseparatesthetworockbodies.Thechannelinthecentralpartofthisoutcroprevealsthatthelowershaleunitsweredepositedandthenerodedbeforetheupperunitsweredeposited.(HamblinandChristiansen)DisconformitiesdonotshowanNonconformityAnonconformityisabreaksurfacethatdevelopedwhenigneousormetamorphicrockswereexposedtoerosion,andyoungersedimentaryrocksweresubsequentlydepositedabovetheerosionsurface.(TarbuckandLutgens)NonconformityAnonconformityiAnonconformityattheGrandCanyon.Themetamorphicrocksandtheigneousdikesoftheinnergorgewereformedatgreatdepthsandsubsequentlyupliftedanderoded.Youngersedimentarylayerswerethendepositedontheerodedsurfaceoftheigneousandmetamorphicterrain.(HamblinandChristiansen)AnonconformityattheGrandCTypesofUnconformityThisanimationshowsthestagesinthedevelopmentofthreemaintypesofunconformityincross-section,andexplainshowanincompletesuccessionofstrataprovidesarecordofEarthhistory.View1showsadisconformity,View2showsanonconformityandView3showsanangularunconformity.[byStephenMarshak]PlayAnimationWindowsversion>>PlayAnimationMacintoshversion>>TypesofUnconformityThisanimDistinguishingnonconformityandintrusivecontact

Nonconformity:

Thesedimentaryrockisyounger.Theerosionsurfaceisgenerallysmooth.Dikesmaycutthroughtheigneousbodybutstopatthenonconformity.Intrusivecontact:

Intrusionisyoungerthanthesurroundingsedimentaryrocks.Thecontactsurfacemaybequiteirregular.Azoneofcontactmetamorphismmayformsurroundingtheigneousbody.Cross-cuttingdikesmaypenetrateboththeigneousbodyandthesedimentaryrocks.DistinguishingnonconformityaContrastingfieldconditionsfor(a)anonconformityand(b)anigneousintrusion.(West,Fig9.4)ContrastingfieldconditionsfThethreebasictypesofunconformitiesillustratedbythiscross-sectionoftheGrandCanyon.(TarbuckandLutgents)ThethreebasictypesofunconGeologicHistoryAcross-sectionthroughtheearthrevealsthevarietyofgeologicfeatures.View1ofthisanimationidentifiesavarietyofgeologicfeatures;View2animatesthesequenceofeventsthatproducedthesefeatures,anddemonstrateshowgeologistsapplyestablishedprinciplestodeducegeologichistory.[byStephenMarshak]PlayAnimationWindowsversion>>PlayAnimationMacintoshversion>>GeologicHistoryAcross-sectiostratigrraphyandgeologictime地层与地质时间Principleoffaunalsuccession:Groupsoffossilanimalsandplantsoccurthegeologichistoryinadefiniteanddeterminableorderandaperiodofgeologictimecanberecognizedbyitscharacteristicfossils.PrincipleoffaunalsuccessionFossilsaretheremainsofancientorganisms.Therearemanytypesoffossilization.(Top)naturalcastsofshelledinvertebrates.(Middle)Fishimpressions.(Bottom)Dinosaurfootprintinfine-grainedlimestonenearTuba,Az.Fossilsaretheremainsofancstratigrraphyandgeologictime地层与地质时间Theprincipleoffossilsuccession.Notethateachspecieshasonlyalimitedrangeinasuccessionofstrata.(W.W.Norton)TheprincipleoffossilsuccesCorrelationofrockunits

Themethodofrelatingrockunitsfromonelocalitytoanotheriscalledcorrelation.Onewayofcorrelationistorecognizetherocktypeorrocksequenceattwolocations.Anotherwayofcorrelationistousefossils.Abasicunderstandingoffossilsisthatfossilorganismssucceededoneanotherinadefiniteanddeterminableorder,andthereforeatimeperiodcanberecognizedbyitsfossilcontent.CorrelationofrockunitsTheprincipleofcorrelationofrockunits.Therockcolumnscanbecorrelatedbymatchingrocktypes.(W.W.Norton)Theprincipleofcorrelationostratigrraphyandgeologictime地层与地质时间WilliamSmith,acivilengineerandsurveyor,couldpiecetogetherthesequenceoflayersofdifferentagescontainingdifferentfossilsbycorrelatingoutcropsfoundinsouthernEnglandabout200yearsago.Inthisexample,FormationIIwasexposedatbothoutcropsAandB,thusFormationIandIIwereyoungerthanFormationIII.(PressandSiever).WilliamSmith,acivilengineeCorrelationofstrataatthreelocationsontheColoradoPlateaurevealsthetotalextentofsedimentaryrocksintheregion.CorrelationofstrataatthreeThegeologiccolumnwasconstructedbydeterminingtherelativeagesofrockunitsfromaroundtheworld.(Next)Bycorrelation,thesecolumnswerestackedoneontopoftheothertogiverelativeagesofrockunits(W.W.Norton)Thegeologiccolumnwasconstrstratigrraphyandgeologictime地层与地质时间Absolutedating

Thegeologictimebasedonstratigraphyandfossilsisarelativeone:wecanonlysaywhetheroneformationisolderthantheotherone.Absolutedatingwasmadepossibleonlyafterthediscoveryofradioactivity.AbsolutedatingRadioactivityAttheturnofthe20thcentury,nuclearphysicistsdiscoveredthatatomsofuranium,radium,andseveralotherelementsareunstable.Thenucleioftheseatomsspontaneouslybreakapartintootherelementsandemitradiationintheprocessknownasradioactivity.Wecalltheoriginalatomtheparentanditsdecayproductthedaughter.Forexample,aradioactive92U238atomdecaysintoastablenonradioactive82Pb206atom.RadioactivityexampletypesofradioactivedecayAlphadecay:anaparticle(composedof2protonsand2neutrons)isemittedfromanucleus.Theatomicnumberofthenucleusdecreasesby2andthemassnumberdecreasesby4.Betadecay:abparticle(electron)isemittedfromanucleus.Theatomicnumberofthenucleusincreasesby1butthemassnumberisunchanged.exampletypesofradioactivedIllustrationofalphaandbetadecays.(adaptedfromTarbuckandLutgens)stratigrraphyandgeologictime地层与地质时间ThedecayofU238.Afteraseriesofradioactivedecays,thestableendproductPb206isreached.(TarbuckandLutgents)ThedecayofU238.AfteraserDecayconstant

Therateofdecayofanunstableparentnuclideisproportionaltothenumberofatoms(N)remainingatthetimet.dN/dt=-l*NThereasonthatradioactivedecayoffersareliablemeansofkeepingtimeisthatthedecayconstantlofaparticularelementdoesnotvarywithtemperature,pressure,orchemistryofageologicenvironment.

DecayconstantHalf-life

Theh