stratigrraphyandgeologictime地层与地质时间

StratigraphyandGeologicTimeStratigraphyBasicprinciplesofrelativeagedatingUnconformities:MarkersofmissingtimeCorrelationofrockunitsAbsolutedatingGeologicTimeHowoldistheEarth?Whendidvariousgeologiceventsoccur?InterpretingEarthhistoryisaprimegoalofgeology.SomeknowledgeofEarthhistoryandgeologictimeisalsorequiredforengineersinordertounderstandrelationshipsbetweengeologicunitsandtheirimpactonengineeringconstruction.Stratigraphy:Stratigraphyisthestudyofrocklayers(strata)andtheirrelationshipwitheachother.Stratigraphyprovidessimpleprinciplesusedtointerpretgeologicevents.TworockunitsatacliffinMissouri.(USGeologicalSurvey)BasicprinciplesofrelativeagedatingRelativedatingmeansthatrocksareplacedintheirpropersequenceofformation.Aformationisabasicunitofrocks.Belowaresomebasicprinciplesforestablishingrelativeagebetweenformations.PrincipleoforiginalhorizontalityPrincipleofsuperpositionPrincipleoffaunalsuccessionPrincipleofcross-cuttingrelationshipsPrincipleoforiginalhorizontality:Layersofsedimentaregenerallydepositedinahorizontalposition.Thusifweobservedrocklayersthatarefoldedorinclined,theymust,withexceptions,havebeenmovedintothatpositionbycrustaldisturbancessometimeaftertheirdeposition.Mostlayersofsedimentaredepositedinanearlyhorizontalposition.Thus,whenweseeinclinedrocklayersasshown,wecanassumethattheymusthavebeenmovedintothatpositionafterdeposition.HartlandQuay,Devon,EnglandbyTomBean/DRKPhoto.Principleofsuperposition:Inanundeformedsequenceofsedimentaryrocks,eachbedisolderthantheoneaboveandyoungerthantheonebelow.Therulealsoappliestoothersurface-depositedmaterialssuchaslavaflowsandvolcanicashes.Principleofsuperposition.(W.W.Norton)ApplyingthelawofsuperpositiontothelayersattheupperportionoftheGrandCanyon,theSupaiGroupistheoldestandtheKaibabLimestoneistheyoungest.(photobyTarbuck).Principleofcross-cuttingrelationships:Whenafaultcutsthroughrocks,orwhenmagmaintrudesandcrystallizes,wecanassumethatthefaultorintrusionisyoungerthantherocksaffected.Cross-cuttingrelationships:Anintrusiverockbodyisyoungerthantherocksitintrudes.Afaultisyoungerthantherocklayersitcuts.(TarbuckandLutgens)Unconformities:MarkersofmissingtimeWhenlayersofrockformedwithoutinterruption,wecallthemconformable.Anunconformityrepresentsalongperiodduringwhichdepositionceasedanderosionremovedpreviouslyformedrocksbeforedepositionresumed.AngularunconformitiesDisconformityNonconformityAngularunconformities:Anangularunconformityconsistsoftiltedorfoldedsedimentaryrocksthatareoverlainbyyounger,moreflat-lyingstrata.Itindicatesalongperiodofrockdeformationanderosion.Formationofanangularunconformity.Anangularunconformityrepresentsanextendedperiodduringwhichdeformationanderosionoccurred.(TarbuckandLutgents)AngularunconformityatSiccarPoint,southernScotland,thatwasfirstdescribedbyJamesHuttonmorethan200yearsago.(HamblinandChristiansenandW.W.Norton)Disconformity:Adisconformityisaminorirregularsurfaceseparatingparallelstrataonoppositesidesofthesurface.Itindicatesahistoryofupliftingabovesea(water)level,undergoingerosion,andloweringbelowthesealevelagain.Formationofdisconformity.(W.W.Norton)Disconformitiesdonotshowangulardiscordance,butanerosionsurfaceseparatesthetworockbodies.Thechannelinthecentralpartofthisoutcroprevealsthatthelowershaleunitsweredepositedandthenerodedbeforetheupperunitsweredeposited.(HamblinandChristiansen)NonconformityAnonconformityisabreaksurfacethatdevelopedwhenigneousormetamorphicrockswereexposedtoerosion,andyoungersedimentaryrocksweresubsequentlydepositedabovetheerosionsurface.(TarbuckandLutgens)AnonconformityattheGrandCanyon.Themetamorphicrocksandtheigneousdikesoftheinnergorgewereformedatgreatdepthsandsubsequentlyupliftedanderoded.Youngersedimentarylayerswerethendepositedontheerodedsurfaceoftheigneousandmetamorphicterrain.(HamblinandChristiansen)TypesofUnconformityThisanimationshowsthestagesinthedevelopmentofthreemaintypesofunconformityincross-section,andexplainshowanincompletesuccessionofstrataprovidesarecordofEarthhistory.View1showsadisconformity,View2showsanonconformityandView3showsanangularunconformity.[byStephenMarshak]PlayAnimationWindowsversionPlayAnimationMacintoshversionDistinguishingnonconformityandintrusivecontactNonconformity:Thesedimentaryrockisyounger.Theerosionsurfaceisgenerallysmooth.Dikesmaycutthroughtheigneousbodybutstopatthenonconformity.Intrusivecontact:Intrusionisyoungerthanthesurroundingsedimentaryrocks.Thecontactsurfacemaybequiteirregular.Azoneofcontactmetamorphismmayformsurroundingtheigneousbody.Cross-cuttingdikesmaypenetrateboththeigneousbodyandthesedimentaryrocks.Contrastingfieldconditionsfor(a)anonconformityand(b)anigneousintrusion.(West,Fig9.4)Thethreebasictypesofunconformitiesillustratedbythiscross-sectionoftheGrandCanyon.(TarbuckandLutgents)GeologicHistoryAcross-sectionthroughtheearthrevealsthevarietyofgeologicfeatures.View1ofthisanimationidentifiesavarietyofgeologicfeatures;View2animatesthesequenceofeventsthatproducedthesefeatures,anddemonstrateshowgeologistsapplyestablishedprinciplestodedu