Chronostratigraphic units Geology By Misson Choudhury

Hey I'm misson Choudhury pursuing my m.sc in Applied geology at Bangalore university

1. Chronostratigraphic Units
Purpose of choronostratigraphic classification
Thegeneralpurposeofchronostratigraphicclassificationisto
organizesystematicallytheEarth’ssequenceofrockstratainto
namedunits(Chronostrtigraphicunits),correspondingto
intervalsofgeologictime(geochronologicunits),toserveasa
basisfortime-correlationandareferencesystemforrecording
eventsofgeologichistory.
Specificobjectivesare
1.Todeterminelocaltimerelation;localtime-correlationof
strata;determinationoftherelativeageofstratainlocal
section;organizationofstrataintonamedchronostratigraphic
unitsofworldwideapplication
2. To establish a standard Global Chronostratigraphic Scale

2. Definitions
Chronostratigraphy
Theelementofstratigraphythatdealswiththeageofstrata
andtheirtimerelations.
ChronostratigraphicClassification
Theorganizationofrockstrataintounitsonthebasisoftheir
ageortimeoforigin.
ChronostratigraphicUnit
Abodyofrockstratathatisunifiedbybeingtherocksformed
duringaspecificintervalofgeologictime.
Chronozone
Azonalunitofallrocksformedanywhereduringthetime
rangeofsomegeologicfeatureorsomespecifiedintervalof
rockstrata.

3. Thechronozonebasedontherangeofacertaintaxonshould
beclearlydistinguishedfromthebiozonebasedontherange
ofthatsametaxon(taxon-range-zone).
Thetermchronozonemaybeusedformallyasa
chonostratigraphicunitofminorrank.
ChronostratigraphicHorizon(Chronohorizon)
Astratigraphicsurfaceorinterfacethatisisochronous-
everywhereofthesameage.
Thetermchronohorizoniscommonlyappliedalsotoverythin
anddistinctiveintervalsthatareessentiallyisochronousover
theirwholegeographicextend.

4. Kinds of chornostratigraphic Units
Hierarchy of chronostratigraphic and Geochronologic
Unit-Terms
Followingtermsandgeochronologicequivalentsto
expressunitsofdifferentrankortimescope.
Chronostratigraphic Geochronologic
Eonothem Eon
Erathem Era
System Period
Series Epoch
Stage Age
Chronozone Chron

5. Chronozone (and Chron)
Chronozoneisaformaltermforthelowestranking
divisioninthehierarchyofchronostratigraphicterms.
Chronisthecorrespondinggeochronologicterm.
Thechronozonemaybedefinedineitheroftwoways.
First,itmaybemadetocorrespondtothetimespanof
thestratotypeoftheunit;inthiscasethetimespanof
thechronozonewouldbepermanentlyfixed.
Second,thetimespanofthechronozonemaybemade
tothetotaltimespanoftheunit(whichmaybelarger
thanthatofthestratotype).

6. Thechronozonetakesitsnamefromthestratigraphic
unitonwhichitisbased,eg.ExusalbusChronozone
(derivedfromtheExusalbusRange–zone),Barrett
Chronozone(derivedfromtheBarrettFormation).
Achrontakesthesamenameasitschronozone.
Thetimespanofachronozoneisusuallydefinedin
termsofthetimespanofapreviouslydesignated
stratigraphicunitsuchasaformationoramemberora
biozone.

7. Stage (and Age)
Thestageisachronostratigraphicunitofrelativelyminor
rankintheconventionalhierarchyofformal
chronostratigraphicterms,representingarelativelyminor
intervalofgeologictime.
Itsgeochronologicequivalentsisknownasanage.
Thestagehasbeencalledthebasicworkingunitof
chronostratigraphybecauseitissuitedinscopeandrank
tothepracticalneedsandpurposesofintraregional
chronostratigraphicclassification.
Itisoneofthesmallestunitsinthestandard
chronostratigraphichierarchythatisprospectmaybe
recognizedworldwide.
Stagesmaybesubdividedintosubstagesand
superstages.

8. Thelowerandupperboundary-stratotypesofastage
representspecificmomentsingeologictime,andthe
timeintervalbetweenthemisthetimespanofthestage.
Ontheaveragetheyrangefrom3to10millionyearsas
indicatedbyisotopicagedeterminations.
Thenameofastagepreferablyshouldbederivedfroma
geographicfeatureinthevicinityofitstypesectionor
typearea.
Moststageshavebeengivengeographicname,but
manycurrentlyusedstageshavethenameofthe
lithostratigraphicunitseg.Cenomanianstage

9. Substages and Superstages
Thesubstagesisasubdivisionofastage.
Somestageshavebeendividedcompletelyintoformally
namedsubstages;othershavehadonlycertainparts
designatedassubstage.
Thegeochronologicequivalentsofasubstagemightbe
termedasubage.
Preferableyitiscalledsimplyasage.
Asubstageisdefinedbyboundary-stratotypes.
Severaladjacentstagesmaybegroupedintoa
superstage.
Itispreferablysimplytosubdividetheoriginalstageinto
twoormorenewstages.

10. Series (and epoch)
Theseriesisaunitintheconventional
chronostratigraphichierarchy,rankingaboveastageand
belowasystem.
Thegeochronologicequivalentofaseriesisanepoch.
Aseriesisalwaysasubdivisionofasystem
Mostsystemshavebeendividedintothreeseries,but
thenumbervariesfromtwotosix.
Seriescommonlyincludefromtwotosixstages.
Thetermssuperseriesandsubserieshavebeenused
onlyinfrequently
Mostseriescanberecognizedworldwide.

11. Currentlyacceptedseriesvaryintimespan,butaverage
about15millionyears.
Thetimespanofaseries,ifdividedfullyintostages,is
thesumofthetimespanofitscomponentstages.
Anewseriesnameshouldpreferablybederivedfroma
geographicfeaturesinthevicinityofitstypesectionor
typearea.

12. System (and Period)
Thesystemisaunitofmajorrankintheconventional
chronostratigraphichierarryaboveaseriesandbelowan
erathem.
Thegeochronologicequivalentofasystemisaperiod.
Occasionalneeditisdividedintosubsystemand
supersystem.
Timespanofasystemcanbedefinedasthetimespan
ofthesumofitscomponentseriesorcomponentstages.

13. ThetimespansofthePhanerzoicsystems
rangefrom35to70millionyearsandaverage
about50to60millionyears.
Thenamesofcurrentlyrecognizedsystemsare
ofdiverseorigin.Someareindicativeofposition
egTetiaryandQuaternary);othershavea
lithologiccannotation(eg.Carboniferous,
Cretaceous);andothersaretribal(eg.
Ordovician,Silurian)andstillothersare
geographic(eg.Permian,Devoanian)

14. Erathem (and Era)
Anerathem(fromtheGreekrootseraandthem“the
depositsofanera”).
Itisthelargestformalunitcommonlyrecognizedinthe
chronostratigraphichierarchyandusuallyconsistsof
severaladjacentsystems.
Theintervalofgeologictimecorresponsingtoan
erathemisanera.Itcarriesthesamenameasits
equivalenterathem.
TheErathemshavetraditionallybeennamedtoreflect
majorchangesinthedevelopmentoflifeontheEarth:
Paleozoic(oldlife),Mesozoic(IntermediateLife),and
Cenozoic(recentLife),Archeozoic(mostancientlife)

15. Eonothem (and Eon)
Thetermeonhasbeenusedforageochronologicunit
greaterthananera.
Thechronostratigraphicequivalentswouldbean
eonothen.
Twoeonsaregenerallyrecognized.
OneisthePhenerozoiceon(timeofevidentlife),which
encompassesthePaleozoic,MesozoicandCenozoic
eras.
Theothereoncoverspre-phanerozoic(pre-Paleozoic)
timeandhasbeenknownastheCryptozoicEon(timeof
hiddenlife),andArcheozoiceon(timeofmostancient
life),orsimplythePrecambrianEon.