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GEOLOGICTIME basicprinciples AlessandroGrippo,Ph.D TheodoreRooseveltNa8onalPark,NorthDakota ©AlessandroGrippo Rela5veTime • Inordertoestablishasequenceofeventsin8me (rela8ve8me)weusefivedifferentprinciples • ThreeofthemareknownasSteno’sPrinciples (fromNicolausSteno):superposi8on,original horizontality,andlateralcon8nuity • Twomorewereaddedlater:theprincipleof cross-cuNngrela8onshipsandtheprincipleof inclusions Steno’sprinciples 1.Superposi5on • Inanundisturbed successionof sedimentaryrocks layers: – theoldestlayeris attheboPom – theyoungest layerisatthetop • Or,thesequence getsyoungerfrom theboPomtothe top ScoPsBluffNa8onalMonument,Nebraska ©AlessandroGrippo Steno’sprinciples 2.OriginalHorizontality • Becauseoftheinfluenceofgravity,sedimentis depositedinhorizontallayers(therearesome excep8ons) – So,whenyouseesequenceswherelayersarenothorizontal, thosesequenceshavebeentectonicallydeformed Horizontallayers Tectonicdeforma8onisminimalorabsent MoenkopiForma8on attheGrandFallsoftheLiPleColoradoRiver NavajoNa8onatLeupp,Arizona ©AlessandroGrippo Tiltedlayers Theselayerswereoriginallyhorizontal,andsubsequentlydeformed Valencia,LosAngelesCounty,California ©AlessandroGrippo Ver8callayers,asseenatthecoreofathrustfault. Theselayerswereoriginallyhorizontal. Firenzuola,Firenze,Italy ©AlessandroGrippo Howdohorizontal layersget8lted? Inthisexampleweseefivedifferentstagesof deforma8on: A–undisturbedhorizontallayers B–lateralcompressioncausesmildfolding (structurescalledsynclinesandan8clines) C–compressioncon8nuesandcausesafoldto developasteep8ltedside(limb)anda ver8calside D–aruptureoccurs(athrustfault*) E–con8nuedcompressionatthispointcauses almosthorizontalslidingofoneblockover theother 5ltedlayers ver5callayers *afaultisafracturealongwhichmovementoccurs whenthefaultsurfaceisnearlyhorizontalandallowsforveryold rockstooverlieveryyoungrocks,itiscalledathrustfault) Modifiedfrom“TheBigBendoftheRioGrande:AGuidetothe Rocks, Geologic History, and SePlers of the Area of Big Bend Na8onalPark”,TexasBureauofEconomicGeology,1987. Steno’sprinciples 3.LateralCon5nuity • Sedimentextendslaterally,inalldirec8onsun8litthinsandpinches out,orterminatesagainsttheedgeofthedeposi8onalbasin BryceCanyonNa8onalPark,Utah ©AlessandroGrippo otherprinciples 4.Cross-CuBngRela5onships 5.Inclusions • Whatcuts,isyoungerthanwhathasbeencut • Anigneousintrusion(aplutonoradike),ora fault(afracturealongwhichmo8onoccurs) mustbeyoungerthantherocksitintrudesor displaces • Aninclusion(orfragment)insideabodyof rockisalwaysolderthantherockitself Atthisloca8onlavaflowedoversedimentaryrocks,mel8ngandtrunca8ngtheoriginallayers.Thelavacooledintoa darkextrusiveigneousrock(basalt)interrup8ngtheoriginallateralcon8nuityofthelightcoloredsandstonelayersof theMoenkopiForma8on.Assuch,theblackrockisyoungerthanthereddishsandstones(itinterruptstheirpaPern) MoenkopiForma8on attheGrandFallsoftheLiPleColoradoRiver NavajoNa8onatLeupp,Arizona ©AlessandroGrippo Anormalfault:thelekblock(hangingwall)moveddownwardinrespecttotherightblock (footwall).Thelayerswerecon8nuous(correla8onisnotimmediatebutnotethekinked bededgesalongthefault)buthavebeencutbythefault. Hence,thefaultisyoungerthanthelayers(itoccurredakerthelayershadformed) HouseRockJunc8on,Arizona ©AlessandroGrippo Unconformi8es • Gapsintherockrecord • Gapsoccurfortwo reasons: – non-deposi8on – erosion(whichalso impliesnon-deposi8on) • Firstrecognizedby JamesHuPonatSiccar Point SiccarPoint,Berwickshire,Scotland Unconformi8es • Atunconformi8es,8meisnotrecorded,andthe geologicrecordisincomplete • Thegapcouldbeshort(lessthan1millionyears)or verylong(hundredsofmillionstobillionsofyears) • Unconformi8esaresurfaces,andnotlayers • Thegeologic8menotrepresentedatanunconformity isdefinedasahiatus Unconformi8es • Unconformi8esareoffourdifferenttypes: – Paraconformi8es • Thesequencesofstrataaboveandbelowtheunconformity areparallel,andtherewasnoerosion,justnon-deposi8on – Disconformi8es • Thesequencesofstrataaboveandbelowtheunconformity areparallel,andtherewaserosion – AngularUnconformi8es • Thesequencesofstrataaboveandbelowtheunconformity areatanangle – Nonconformi8es • Sedimentaryrockscoveringerodedplutonicand/or metamorphicrocks Unconformi8es 1.Paraconformity • Paraconformi8esand disconformi8esareoken difficulttotellonefromthe other • Timeismissingbecause rockswereneverdeposited duringthat8me,butthere isnoerosion • Cluescanbefoundin fossils,hardenedsurfaces, bioturba8on,etc. ParaconformityorDisconformity? Whatseemslikeacon8nuoussec8onisinreality depositedasa“stopandgo”sequence.Thisisa submarinehighthatshowsa“condensed”sec8on PesaroeUrbino,Italy ©AlessandroGrippo Unconformi8es 2.Disconformity Unconformi8es 3.AngularUnconformity Inthisimagefromaquarry, weseethick,8ltedbanks ofgypsumoverlainby horizontal,thinlayersof bluishshales Theircontactisanangular unconformity Bologna,Italy ©AlessandroGrippo Unconformi8es 4.Nonconformity AnexamplefromtheGrandCanyon,Arizona Theimagetothelek,takenfromDoyleetal.(2001)shows: A-aver8calprofileoftheGrandCanyonsec8on B - a stra8graphic sketch of the Units and Forma8ons present in the area, including their name, which also shows the three different kinds of Unconformi8es,labeledasA,D,andN C - part of the Geologic Time Scale covering the Paleozoic Era, showing the ageoftherockunits,orForma8ons Correla5on • Tocorrelatemeanstoestablishequivalencyin8me • Correla8oncanalsobepursuedbetweenrocks,butitwilllikelylose itstemporalmeaning • Therearethreemainwaystocorrelatebetweenrocksequences: – Byphysicalcon8nuity:youcanseeortraceyourlayersdirectly.Itis usuallylimitedtosmallareas – Bysimilarityofrocktypes:thisistrickyandyoushouldbeabsolutely certainthatyourarelookingatthesamerock – Byuseoffossils • PrincipleofFossilSuccession • ConceptofIndexFossil • ConceptofFossilAssemblage(rememberAlbertOppel’szones) Correla8on 1.PhysicalCon5nuity Correla8on 2.SimilarityofRockType Correla8on 3.Fossils • Fossilsareremnantsofancientformsoflife, oroftheirac8vity • Sincefossilschange(evolve)in8meaccording toaknownandpredictableorder,theycanbe usedasacorrela8ontool SignificanceofFossils • Fossilgiveustwolinesofinforma8on: • theEnvironmentofDeposi8on – thephysicalplacewhereorganismslived • Rela8veTime(becauseofWilliamSmith’sPrincipleof FossilSuccession:faunasandflorafolloweachotherin 8meaccordingtoaknownandpredictableorder) – the8meintervalwhensaidorganismslived BasicuseofFossilsforCorrela8on • Findingonesinglefossilperseisnotenough – ifanorganismlivedforaverylong8me,itmight notbeveryusefulforcorrela8on Nau8lus is considered to be a "living fossil," as the species has undergone liPlechangeinthelast400millionyears. The nau8lus first appeared about 265 millionyearsbeforethefirstdinosaurs. becauseofthis,aNau8lusfossilwillnot be very indica8ve of any 8me period in EarthHistory IndexFossils • AnIndexFossilisthefossil ofaspecieswhich: – livedforaveryshort amountof8me – waswidespread – livedindifferent environments – itiseasytofindinthe fossilrecord • IndexFossils, unfortunately,arerare Mucrospirifermucronatus,aMiddleDevonianbrachiopodindex fossilfromLebanon,NewYork from: hPp://www.thefossilforum.com/index.php?/gallery/image/38004-mucrospirifer-mucronatus-frommadison-co-ny/ FossilAssemblages • Oken,wehavetoresorttotheuseofFossil Assemblages,associa8onoffossilswith differentranges • Arangeisthever8caldistribu8onofafossil speciesinrocks,fromthemomentofitsfirst occurrence(FO)tothemomentofitslast occurrence(LO) STRATIGRAPHY endofpart2