Download GEOLOGIC TIME basic principles

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