GEOLOGICTIME
partIV:Stra1graphicUnitsandTerminology
AlessandroGrippo,Ph.D
CalcarenitesoftheBismantovaRock,MiddleMiocene
ReggioEmilia,Italy
©AlessandroGrippo
Stra1graphicTerminology
•  OurstraDgraphicrecordismadeofrocks
•  Thefirstthingweseewhenoutinthefieldisrocks
•  RocksneedtobedisDnguishedonefromtheother
•  TheconceptofstraDgraphicunitisborn:
–  astraDgraphicunitisanysubdivisionoftherecordbased
oncertainrockcharacterisDcs
Geochronologicand
Chronostra1graphicUnits
•  StarDngintheXIXcentury,geologistsbegan
subdividingtherockrecordinintervals
•  TheseintervalsindicateddifferentrelaDve
Dmesbutwererock-based
•  Thesewerecalledsystemsofrocks
•  SystemsofrocksfromduringDmeperiods
•  Systemsweregivennamesbasedonplaces(e.g.
Cambrian,Permian)orothercharacterisDcs(e.g.
Silurian,Carboniferous,etc.)
•  So,theCambrianSystemofrockswasdeposited
duringtheCambrianPeriodofDme.
•  Systemscanbetouched,seen,described
•  PeriodsareabstractunitsofDme
•  ASystemrepresentsa
Chronostra1graphicUnit
•  APeriodrepresentsa
GeochronologicUnit
•  SystemsandPeriodhave
thesamenames,but
representdifferent
concepts
•  BothSystemandPeriods
arepartofhierarchyof
Units
Intheend:
•  AChronostraDgraphicUnitsisabodyofrockthat
wasdepositedduringacertainDmeinterval(a
GeochronologicUnit)
–  ChronostraDgraphicunitsareboundedby
synchronoushorizons(boundaries)
–  Achronostra1graphichorizonisastraDgraphic
surfaceorinterfacethatissynchronous
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CenozoiccomesfromtheGreekkainos(new)andzoikos(animal),
zoion(livingbeing).Itthusmeans"(periodof)newlife".
MesozoiccomesfromtheGreekmeso(inbetween,middle)and
zoikos(animal),zoion(livingbeing).Itthusmeans"(periodof)middle
life".
PaleozoiccomesfromtheGreekpalaios(old)andzoikos(animal),
zoion(livingbeing).Itthusmeans"(periodof)oldlife”
Cretaceous(d'Halloy,1822)comesfromtheLaDncreta,orchalk.It
meansthus"theDmeofthechalk",anditoriginatedbecauseofthe
abundanceofchalkofthisageintheParisBasin,inFrance
Jurassic(VonHumboldt,1795)originatesfromtheJuraMountains,a
mountainchainlocatedbetweenSwitzerlandandFrance
Triassic(VonAlberD,1834)comesfromtheLaDnTrias,meaninga
Systemorganizedinthreepartsnamed,fromthebobomtothetop,
Buntsandstein(GermanforNewRedSandstone),Muschelkalk
(Germanformussellimestone,orshelllimestone)andKeuper(anold
Germanminernamefor"iridiscentmarls",amarlbeingamixture
betweenshaleandlimestone).
ThePermian(Murchison,1841)wasnamedaferrocksfromthePerm
Basin,inRussia.SomesourcesmaintainthatMurchisonwantedto
makeareferencetotheancientkingdomofPermia,ratherthanthe
cityofPermitself;inanycase,thatlostkingdomistodaypartofa
RussianfederalrepublicwhosecapitalisthecityofPerm
ThePennsylvanian(Williams,1891)isnamedaferthestateof
Pennsylvania,intheUSA*TheMississippian(Williams,1891)owesits
nametothefactthatrocksofthisagecropoutintheMississippi
valleyintheUSA*
*pleasenotethatthenamesPennsylvanianandtheMississippianare
onlyusedintheUnitedStatesinplaceoftheCarboniferous
TheCarboniferous(Phillips,1835)wassonamedbecauseofthe
abundanceofcoal(carbon)idenDfiedinrocksofthisageinwestern
Europe
ThenamefortheDevonian(MurchisonandSedgwick,1840)comes
fromtheregionofDevon,inEngland,UK
ThetermSilurian(Murchison,1835)comesfromtheLaDnSilures(an
ancientCeltpopulaDonofWales,intheUK)
ThetermOrdovician(Lapworth,1879)comesfromtheLaDn
Ordovices(anancientCeltpopulaDonofWales,intheUK)
ThetermCambrian(Sedgwick,1835)comesfromtheLaDnword
Cambria(theancientnameofWales,intheUK)
AgesandStages
• 
Astage(anditsDmeequivalent,theage)isasmall-rankchronostraDgraphicunit
(geochronologicunit).Inthepicturebelowisanexampleofstages(andages)
relaDvetotheCretaceousSystem(Period):
• 
ThestagehasbeencalledthebasicworkingunitofchronostraDgraphy
• 
Astageisdefinedbyitsboundarystratotypes
Lithostra1graphicUnits
•  SDll,whenwegointhefield,wejustsee
rocks,notDme
•  So,straDgraphicunitsbasedonrocksarethe
mostcommonunits
•  ThesearedefinedasLithostratographicUnits
•  ThebasiclithostraDgraphic
unitistheForma1on
–  AFormaDonisaphysical
bodyofrocksthatisdisDnct
fromsurroundingones,and
thickenoughtobe
mappable
–  FormaDonsarenamedafer
alocalnameora
characterisDcoftherock,
accompaniedbyeitherthe
nameoftherockitselfor,if
morethanonelithology,
thename“FormaDon”
•  Examples
–  theSantaMonicaShale
–  theGreenRiverFormaDon
Thisimageshowsthreeunits,frombobomtotop:aquartzite(slightlymetamorphosedsandstone),ashale,alimestone.
EachoneisdisDnctformtheotherbasedontheirlithologicalcharacterisDcs.Asaconsequence,eachunitisaForma1on.
NoDcethatthelimestoneincludeswithinitsboundariesafewlevelsofshale.Thetrailatthebaseofthecliffworksasscale.
MojaveDesert,SanBernardinoCounty,California
©AlessandroGrippo
FormaDons(lithostraDgraphicunits)
•  FormaDonscanbepartofGroups,orcanbe
subdividedintoMembers
•  Asyourememberfrombefore,bodiesofrock
donotrepresent1me
•  So,aFormaDonisnotthesameofa
ChronostraDgraphicUnit
•  ForinstancetheBright
AngelShaleofthe
AmericanSouthwestis
LowerCambrianinage
inCaliforniaand
Nevada,butisMiddle
Cambrianinagein
UtahandArizona
•  FormaDonsareavery
pracDcaltoolinthe
fieldbutbearnoDme
significance
Biostra1graphy
•  TheuseoffossilsforstraDgraphiccorrelaDoniscalled
biostra1graphy.
•  BiostraDgraphyisbasedonWilliamSmith'sPrincipleof
FaunalandFloralSuccession.
•  BiostraDgraphicunitsarebasedonthestraDgraphicrange
offossiltaxa.
•  Thestra1graphicrangeofataxonisthetotalverDcal
intervalthroughwhichthattaxonoccursintherockrecord,
fromitslowermosttoitsuppermostoccurrence.
Biozones(biostraDgraphicunits)
•  ThemostfundamentalbiostraDgraphicunitisthezone
or,moreformally,thebiozone.Azoneisaphysical
bodyofrockwhoseboundaries,lowerandupper,are
basedontherangesofoneormoretaxa.
ChemicalStra1graphy
•  ChemicalstraDgraphy,orchemostra1graphy,
referstothestudyofthefrac1ona1onofstable
isotopesinnature
•  FracDonaDonissimplytheseparaDonofisotopes
accordingtotheirdifferentatomicweights
•  FracDonaDoncanoccur,forinstance,asa
consequenceofoceanographicandclimaDc
changes
•  ThemostimportantfracDonaDonprocesses
wearegoingtodiscussrefertotheisotopesof
–  oxygen(18O/16O)
•  studiesaboutclimatechange
–  carbon(13C/12C)
•  studiesaboutoxygenaDonoftheoceanbobom
–  stronDum(87Sr/86Sr)
•  studiesabouttheageofcertainfossils
StronDumisotopesstudies(87Sr/86Sr)
• 
• 
86Srisabundantandconstantinnature
87SrinsteadisincreasingoverDme,becauseit
derivesfromradioacDvedecayof87Rb
•  Asaconsequence,theraDo87Sr/86Sris
increasingoverDme
•  Thesmallestthisvalue,theoldertherock
Wheredowefind87Srand86Sr?
• 
87Srand86SrcanreplaceCaionsincalciteand
aragonite
•  Marineanimalsusecalciteand/oraragoniteto
buildshellsorotherpartsoftheirstructure
•  Anycalciteoraragoniteskeletonwillcontain
sometracesofSrinitsCaCO3mineral
structure
•  ByknowingtherelaDon
betweentheraDovalue
andDme,wecan
establishwhenthe
organismlived
•  Srisotopesworkas
straDgraphicclocks.
•  Thismethodisactually
notreliableforsamples
olderthantheEocene
Stra1graphy
ENDofpartIV