Sam Boggs Jr. Principles of Sedimentology and Stratigraphy 4th Edition 2005.docx

8/11/2019 Sam Boggs Jr. Principles of Sedimentology and Stratigraphy 4th Edition 2005.docx

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15Chronostratigraphyand Geologic Time15.1 INTRODUCTION

tinguished by lithology, magnetic characteristics, seismic reflection characteristics, or fossil content. s such, they are obser!able or measurable

material reference units that de"ict the descri"ti!e stratigra"hic features of a region. Definition of these units allo#s the !ertical and lateral relationshi"s be t#eenroc$ units to be recogni%ed and "ro!ides a means of correlating the units from onearea to another. s &rumbein and 'loss (1)*+ "oint out, ho#e!er, descri"ti!estratigra"hic units do not lend themsel!es to inter"retation of the local stratigra"hic column in terms of -arth history To inter"ret -arth history re uires thatstratigra"hic units be related to geologic time/ that is, the ages of roc$ units must be $no#n. -stablishing the time relationshi" among roc$ units is calledchronostratigraphy, and stratigra"hic units defined and delineated on the basis of time are geologic time units. The relationshi" be t#een chronostratigra"hy andother branches of stratigra"hy is illustrated in 0igure 15.1.

In this cha"ter, #e e amine the conce"t of geologic time units and e "lorethe relationshi" of time units to other ty"es of stratigra"hic units. 2e #ill also seeho# geologic time units are used to create the 3eologic Time 'cale and #e #illdiscuss methods of

calibrating the time scale.

0inally, #e #ill e amine methods

for chronocorrelation4correlation of roc$ units on the basis of their ages.

15. 3-O6O3IC TI7- UNIT'3eologic time units are conce"tual units rather than actual roc$ units, althoughmost geologic time units are based on roc$ units. In fact, #e recogni%e t#o distinctty"es of formal stratigra"hic units that can be distinguished by geologic age8

The stratigraphic units described in the preceding chapters are rock units


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units, called stratotypes, based on actual roc$ sections, and units inde"endent of reference roc$ sections (see ""endi C . Ideally, the reference roc$ bodies for geologic time units are isochronous units. That is, they are roc$ units formed duringthe same s"an of time and e!ery#here boun de d by synchronous surfaces, #hichare surfaces on #hich e!ery "oint has the same age.

51+514Chapter 15 / Chronostratigraph y and Geologic Time

0igure 15.1Diagram illustrating the "rocedures and " ro cesses in!ol!ed inc h r o n o s t r a t i g r a " h y a nd th e relationshi" of geolog ic t ime

Geochronometry

Dates In years

units to other $inds of strati9gra"h ic units. 3olden s"i$erefers to internat ional ly agreedu"on "oint s or b ou nd a ri e s ins t ra to ty"e strat igra"hic sect ionsselected to ser!e as referencesect ions for chronos t ra t ig ra"h icunits. : fter ;olland, C. ;.,

Biostratigraphy


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3olden Spil

tliostratigraphy

u"ergrou"

ou"

ormation

mber

Tlie North merican tratigra"hic Code and The nternational 'tratigra"hic

3uide ('al!ador, 1))A recogni%e t#o fundamental ty"es of isochronous geologicime units8 chronostratigra"hi

c units and geochronologic units. Chronostratigraphic

units (Table 15.1 are tangible bodies of roc$ that are selected by geologists toser!e as reference sections, or material referents, for all roc$s formed during thesame inter!al of time. In other #ords, a "art icular section of sedimentary roc$ ha!ing a $no#n age s"an is selected to re"resent that "art icular inter!al of geologic time. 0or e am"le, the inter!al of time from about 59 5E million years agois called the >ermian >eriod and is re"resented by roc$s of the >ermian 'ystem located in the >ro!ince of >erm, Russia (see Table 15. . Geochronologic units, bycontrast, are di!isions of time distinguished on the basis of the roc$ record as e "ressed by chronostratigra"hic units. They are not in themsel!es stratigra"hicunits. If the distinction bet#een these t#o ty"es of units seems some#hat confusing, the follo#ing illustration ma y hel" to clarify the difference. Chronostratigra"hic units ha!e been li$ened to the sand that flo#s through an hourglassduring a certain "eriod of time. recambrian.

Erathemssub!i"isions of an eonothem/ none in the >recambrian/ the >hanero%oic erathems, names originallychosen to reflect maFor changes in the de!elo"ment of life on -arth, are the >aleo%oic (Gold lifeG , 7eso%oic(Gintermediate lifeG , and Ceno%oic (Grecent lifeG

#ystemthe "r imary chronostratigra"hic unit of #orld#ide maFor ran$ (e.g., >ermian 'ystem, ?urassic'ystem / can be subdi!ided into subsystems or grou"ed into su"ersystems bu t most commonly are di!idedcom"letely into units of the ne t lo#er ran$ (series

#eriesa subdi!ision of a system/ systems are di!ided into t#o to si series (commonly three / generallyta$e their name from the system by adding the a""ro"riate adFecti!e G6o#er,G G7iddle,G or GU""erG to thesystem name (e.g.. 6o#er ?urassic 'eries, 7iddle ?urassic 'eries, U""er ?urassic 'eries / useful for chronostratigra"hic correlation #ithin "ro!inces/ many can be recogni%ed #orld#ide

#tagesmaller sco"e and ran$ than series/ !ery useful for intraregional and intracontinental classification and correlation/ many stages also recogni%ed #orld#ide/ may be subdi!ided into substages

Chronozonethe smallest

chronostratigra"hic unit/

its

boundaries

may be

inde"endent of

those

of

ran$ed stratigra"hic unitsGeochronologic Unita di!ision of time distinguished on the basis of the roc$ record as e "ressed by chronostratigra"hic units/ not an actual roc$ imit, but corres"onds to the inter!al of time during #hich an established chronostratigra"hic unit #as de"osited or formed/ thus, the beginning of a geochronologic unit corres"onds to the time of de"osition of the bottom of the chronostratigra"hic unit u"on #hich it is based and the ending corres"onds to the time of de"osition of the to" of the reference unit/ the hierarchy of geochronologic units and their corres"onding geochronos9

y


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tratigra"hic units are8Geochronologic Unit Correspon!ing Geochronostratigraphic Unit

51$

Chapter 15 % Chronostratigraphy an! Geologic Timelocalities

for "eriods

and lo#er ran$ed geochronologic

units are identical #ith those for their corres"onding chronostratigra"hic units. 0or e am"le, the ?urassic >eriod isthe time during #hich the ?urassic 'ystem of roc$s #as de"osited. >eriods are di!ided into e"ochs. -"ochs re"resent the time during #hich a series #as de"osited.They ta$e their name from the "eriod by adding the adFecti!e -arly 7iddle, and6ate (e.g.. -arly ?urassic -"och, 7iddle ?urassic -"och, 6ate ?urassic -"och . Notefrom Table 15.1 the different usage of 6o#er, 7iddle, and U""er for subdi!ision of

'ystem name Ty"e locality Name "ro" os ed by Date "ro" osed Remar$s

Huaternary 0rance ?ules Desnoyers 1= ) Defined by lithology,including some unconsolidated sediment

Tertiary Italy 3io!anni rduino 1 *E Originally defined bylithology/ redefined #ithty"e section in 0rance onthe basis of distincti!efossils

Cretaceous >aris recambrian has not yet been di!ided into internationally acce"ted systems.


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15.2 Geologic Time Unitsseries, because series are roc$ units, not units of time. 7ost names for eons and erasare the same as the names of the corres"onding eonothems and erathems.

Geochronometric units are "u re time units. They are not based on the times"ans of designated chronostratigra"hic stratoty"es bu t are sim"ly time di!isionsof an a""ro"riate magnitude or scale, #ith arbitrarily chosen boundari es. t thistime, a geochronometric time scale is used to e "ress the ages of >recambrianroc$s (see 0ig. 15. because no globally recogni%ed and acce"ted chronostratigra"hic scale has been de!elo"ed for these roc$s. >recambrian roc$s ha!e not yet

"ro!en generally susce"tible to analysis and subdi!ision by su"er"osition or bya""lication of other lithologic or biologic "rinci"les that #e commonly use in subdi!iding the >hanero%oic roc$s/ ho#e!er, a chronostratigra"hic scale for >recam br ian roc$s ma y be de!elo"ed in the future. 'ubdi!ision of >recambrian roc$s isfurther discussed in the ne t section.51 &

Proterozoic; however, no scheme or !rther s!"#division o the Precam"rian is glo"ally accepted.

onothem $rathem %ystem and %!"system %eries &!merical 'ge ()a*

+!aternary olocenePleistocene

-.1

1.

Cenozoic&eogene

Pliocene)iocene 2 .

05.-

1 .2

2-0

TertiaryPaleogene

ligocene$ocenePaleocene

Cretaceo!s Upper 3ower

)esozoic 4!rassicUpper )iddle3ower

'cc(Uz5

TriassicUpper )iddle3ower

Permian Upper 3ower

)C( Car"on# Pennsylvanian Upper

i ero!s )ississippian 3ower

Paleozoic

6evonianUpper )iddle3ower

%il!rian Upper 3ower

rdovicianUpper )iddle3ower

Cam"rian Upper

3ower

z5

ccC*

**+occ(U1 occC(

&ot

ormallys!"divided

5

o(UCCC(

z5(U

occ5

&ot

ormallys!"divided


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cam"rian roc7s are divided into the 'rchean and51- Chapter 15 % Chronostratigraphy an ! Geologic Time

15. T/E GE*(*G)C T)+E #C (E

Purpose an!

#copeClassifying roc$s on the basis of time in!ol!es systematic organi%ation of stratainto named units, each corres"onding to s"ecific inter!als of geologic time. Theseunits "ro!ide a basis for time correlation and a reference system for recording andsystemati%ing s"ecific e!ents in the geologic history of -arth. Thus, the ultimateaim of creating a standardi%ed geologic time scale is to establish a hierarchy of chronostratigra"hic units of international sco"e that can ser!e as a standard reference to #hich the ages of roc$s e!ery#here in the #orld can be related. -stablishing the relati!e ordering of e!ents in -arth s history is the main contribution thatgeology ma$es to our understanding of time.

standard geologic

time scale should e "ress

any

age

in any "lace,

and itshould be understandable, clear, and unambiguous. It should also be inde"endentof o"inion and therefore ha!e some obFecti!e reference that is accessible. 0inally, itshould be stable, that is, not subFect to fre uent change, and it should be agreed toand used internationally in all languages (e.g., ;arland, 1) = .

0e"elopment o the Geologic Time #cale

Chronostratigraphic Scale

3eologists ha!e been #or$ing for more than EE years to de!elo" a systematicscheme for a global time9stratigra"hic classification of roc$ units. This slo#

"rocess has e!ol!ed through t#o fundamental stages of de!elo"ment8

1. Determining time9stratigra"hic relationshi"s from local stratigra"hic sections by a""lying the "rinci"le of su"er"osition, su""lemented by fossil controland, more recently, radiometric ages.

. Using these local stratigra"hic sections as a basis for establishing a com"ositeinternational chronostratigra"hic scale, #hich ser!es as the material referencefor constructing a standardi%ed international geologic time scale.

The international chronogra"hic scale has e!ol!ed gradually o!er the "ast t#ocenturies into its "resent form (Table 15.1, 0ig. 15. . 0igure 15. sho#s the hierarchy of maFor chronostratigra"hic units in general use throughout most of the#orld. more detailed chronostratigra"hic scale that also sho#s subseries andstages is gi!en in ""endi D/ this chronostratigra"hic and geochronometric scale#as com"iled by 'al!ador (1)=5 as "art of the CO'UN (Correlation of 'tratigra"hic Units of North merica "roFect. :Note8 The ages of the boundaries bet#een chronostratigra"hic units sho#n in ""endi D may not agree #ith themore recent age determinations gi!en in the ne# geologic time scale sho#n in0igure 15.+.B 'ome of the "ro!incial stage names commonly used in No rt h merica are also sho#n in ""endi D/ ho#e!er, there no# a""ears to be a generalmo!ement among Nort h merican stratigra"hers to abandon these "ro!incialstage names and ado"t the -uro"ean (global stages as standards for North merica. 'tratigra"hers in -uro"e and many other "art s of the #orld ha!e for manyyears subdi!ided the Tertiary into t#o subsystems, the Paleogene and the'eogene, #ith the to" of the Oligocene 'eries as the di!iding bound ar y bet#eenthe t#o (0ig. 15. . 3eologists in Nort h merica ha!e no# also ado"ted this "ractice. They ha!e li$e#ise ado"ted the -uro"ean usage of the Carboni erous as a%o!rce o ages8 Geological %ociety o 'merica 1999 Geologic Time %cale.

igure 15.2&omenclat!re o Phanerozoic chronostratigraphic!nits commonly !sed thro!gho!t the world. Pre#


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system name, but #ith subdi!ision in No rth merica into the +ississippian andPennsyl"anian subsystems. Other !ersions of the chronostratigra"hic scale e ist(e.g., Co#ie and O6 RITK

' sI.?9

P$< 6$P C P C?%

()a*

6U

O 6o

3-O6O3IC 6 'OCI-TKO0 7-RIC

)''%T< C T '&

C$& )'& '&%$rimarily from 3radstein, R, an d Ogg, F., 1))*, Episodes, !. 1), nos. 1 @ / 3radstein, R, et al., 1))5,'->7 '"ecial >ub. 5A, ". )591 =/


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"ressed in eras, "eriods, e"ochs, ages, and chrons rather than erathems, systems,series, stages, and chrono%ones. The subdi!ision boundari es of the geologic timescale are calibrated in absolute ages/ ho#e!er, the geologic time scale differs from

52 Chapter 15 % Chronostratigraphy an ! Geologic Timea true geochronometric scale, #hich is based "urely on time #ithout regard to theroc$ record. hanero%oic time scale are of une ual length, because they are based on chronostratigra"hic units that #ere de "osi ted during une ual inter!als of time.

The geologic time scale has been in e istence for se!eral decades, and duringthat time it has continued to e!ol!e, #ith refinements being made "art icularly insubdi!ision of the e"ochs and ages and absolute9age calibration of the boundaries bet#een "eriods, e"ochs, and ages. 0igure 15.+ sho#s the most recent !ersion of the geologic time scale "ubl ished by the 3eological 'ociety of merica in 1))).This time scale is subdi!ided into ages based on the -uro"ean stages, and boundaries bet#een ages are calibrated in absolute time. bsolute ages are gi!en in millions of years before the "resent (7a , #here the "resent refers to 1)5E. 7ethodsfor absolute age calibration of the geologic time scale are discussed belo#. Notethat the magnetic "olari ty scale for the most recent a""ro imately 1*E millionyears is also included in the time scale. Note also the use of a geochronometri

cscale for the >recambrian, #ith the di!iding bou nd ar y bet#een the rchean andthe >rotero%oic set arbitrarily at 5EE million years.

lacing strata in stratigra"hic order in terms of their relati!e ages has been the guiding "rinci"le used by stratigra"hers in constructing the geologictime scale. Relati!e ordering #as determined by a""lying the "ri nci"le of su "er"osi tion, aided by use of fossils. The "rinci"le of su"er"osition means sim "ly that in a normal succession of strata #hich ha!e not been tectonicallyo!erturned since de"osition, the youngest strata are on to" and the ages of thestrata increase #ith de"th. 7ost of the di!isions in the current global chronostratigra"hic scale are based on fossils, and early efforts to create an internationalchronostratigra"hic scale before methods of absolute9age determinations #erede!elo"ed #ould ha!e been im"ossible #ithout the use of fossils.

0ortunately, methods are no# a!ailable not only for determining the relati!e ages of strata but also for fi ing #ithin reasonable limits of uncertaintythe absolute ages of some strata. De!elo"ment of these methods of absolute9age estimation ha!e made it "ossible to "lace a""ro imate absolute ages on boundari es of the chronostratigra"hic scale initially established by relati!e9age determination methods. bsolute age data can also be used for determining ages of "oorly fossiliferous >recambrian roc$s that cannot be "laced instratigra"hic order by relati!e9age determination methods. The "rinci"almethod for determining the absolute ages of roc$s is based on decay of radioacti!e isoto"es of elements in minerals. Other methods of determining the


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bsolute "assage of geologic time include counting/ a$e9sediment ar!es,

#hich are "r esumed to re"resent annual sediment accumulations/ gro#th increments in the shells of some in!ertebrate organisms/ gro#th rings in trees/and 7ilan$o!itch climate cycles in sediments. These alternati!e methods are

15. The Geologic Time #caleuseful only for mar$ing the "assage of short "eriods of time in local and regional areas and are not of im"ortance in calibrating the geologic time scale,e ce"t "ossibly some "art s of the >leistocene and >liocene.

Thus, the maFor tools for finding ages of sediments to calibrate the geologic time scale are relati!e9age determinations by use of fossils4 biochronology4 and absolute age estimates based on isoto"ic decay4radiochronology. Thesetools may be used both for calibrating the chronostratigra"hic scale directlyand for calibrating the succession of re!ersals of -arth s magnetic field/ thissuccession constitutes the magnetostratigra"hic time scale discussed inCha"ter 1+. 2e shall no# discuss each of these dating methods, beginning#ith biochronology.

C () 6 T)'G T/E GE*(*G)C T)+E #C (E 7U#E * *##)(#8 )*C/6*'*(*G7

iochronology is the organi%ation of geologic time according to the irre!ersible "rocess of e!olution in the organic continuum (Cha"ter 1A . Usefulfossil hori%ons are more #ides"read and abundant in >hanero%oic roc$s thanare hori%ons #hose ages can be estimated by radiochronology 0urthermore,biologic e!ents can commonly be correlated in time more "recisely than canradiometric data in all bu t Ceno%oic roc$s.


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0 Ds and 6 Ds are not totally synchronous o#ing to the fact that e!en thoughimmigrations and e tinctions can ta$e "lace uite ra"idly, as mentioned, they

521

522 Chapter 15 % Chronostratigraphy an! Geologic Time

are not actually instantaneous e!ents. 'ome "lan$tonic s"ecies ha!e been re "ort ed to s"read #orld#ide in 1EE to 1EEE years/ ho#e!er, bi oturba tion of sediment

after

de"osition can

mi

fossils

through a %one se!eral

centimeters

thic$, and accidents in "reser !ation as #ell as bias in collection and analyticalmethods can combine to create uncertainties in the age of the 0 Ds and 6 Dsthat can amount to thousands of years. Ne!ertheless, the duration of the0 Ds of many "l an$tonic s"ecies ma y be as little as 1E,EEE years/ that is, theages of the first a""earance datum of a s"ecies #ill not !ary by more than1E,EEE years in different "a rt s of the #orld (een 0 ; < an! ( 0 ;0een 0 ; < an! 0 ; < ; m%+a ? 15 m @ 5 +a< an! be:


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t>een ( 0 ;0< an! ( 0 ;C< ; m%+a ? 1 m @ +a


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igure 15.1.2n e?ample o biochronoiogical !ating by use o nanno ossil !atum

e"ents correlate! >ith magnetic polarity e"ents. A ter Gartner, #., 13&&,Calcareous nanno ossil biostratigraphy an! re"ise! zonation o th e Pleis:tocene8 +arine +icropaleontology, ". 2, ig. 5, reprinte! by permissiono Else"ier #cience Publishers.B

age determination are sho#n in Table 15.1.1. The carbon914 method is a""liedto direct dating of !ery young sediments. The protactinium92 1 and thorium92 methods are also a""lied to direct dating of sediments ranging in age toabout 5E,EEE years. The usefulness and limitations of these methods for directdating of sediments are discussed further in succeeding sections.

7ost radiometric dating methods cannot be a""lied to direct dating of sedimentary roc$s. They are used to determine the ages of igneous and meta9mor"hic roc$s, #hich indirectly "ro! ide ages for associated sedimentary roc$s(to be discussed . The potassium%argon metho! is #idely used because it can

be a""lied to a number of minerals that are common in igneous and metamor9 "hic roc$s, and it gi!es generally reliable results. It can be used for dating "lu9tonic, igneous, !olcanic, and metamor"hic roc$s (metamor"hism resets theradioacti!e cloc$ , and e!en some sedimentary minerals (e.g,, glauconite . The "r inci"al "roblem #ith the "ot assium argo n methods is that the decay "roduct, argon9AE, is a gas that can lea$ out of a crystal.

The argon94 %argon9 3 method is a related techni ue in #hich "otassi9um9+) is con!erted to argon9+) by irradiation #ith fast neutrons in a nuclear


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15. The Geologic Time #caleTable 15.1.1 #i+u9mi s or princip!l nii lhi"ls ot i+ilioini li ir

525

PNot used in calculating radiometric ages.PPCan be used for dating older roc$s under fa!orable circumstances.

reactor. The ratio of "otassium9+) to "otassium9AE is $no#n, so argon9+) canser!e as a "ro y for "otassium9AE. This relationshi" "ermits the "o ta ss iu m determination for a "otassium9argon age to be made as "art of the argon isoto"eanalysis. In other #ords, measurement of the amo mt of argon9+) (#hich "ro ies for "otassium9AE renders it unnecessary to se"arate "o tassium from a mineral and measure the amount of "otassium9AE. arent Daughter ;alf9life dating range used for

nuclide nuclide (years (years .P.< dating

arbon91A PNitrogen91A 5 +E GGL9 AE,EEE 2ood, charcoal,CaCO shells

rotactinium ctinium9 + ,A=E


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measured during the argon analysis. n age can be determined from theargon9AE argon9+) ratio once the con!ersion rate of argon9AE to argon9+) hasbeen determined by irradiating a standard of $no#n age along #ith the

52$ Chapter 15 % Chronostratigraphy an! Geologic Time

sam"le (e.g.,


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on #ithout significantly inter ru"ting the sedmentation "ro

cess. olcanic mate

rials may be eru"ted onto GsoftG unconsolidated sediment and then bur iedduring subse uent, continued sedimentation, leading to a succession of interbedded sedimentary roc$s and !olcanic roc$s that are essentially contem"oraneous

15. The Geologic Time #cale

Table 15.1.2 Categories o rocDs most use ul or geochronologiccalibration o the geologic time table

52&

Ty"e of roc$ 'tratigra"hic relationshi" Reliability of age data

olcanic roc$ (la!a flo#s andash falls

>lutonic igneousroc$s

7etamor"hosedsedimentaryroc$s

'edimentary roc$scontainingcontem"oraryorganic remains(fossils, #ood

'edimentary roc$scontaining

Interbedded #ithGcontem"oraneousGsedimentary roc$s

Intrude (cut acrosssedimentary roc$s

6ie unconformably beneathsedimentary roc$s

Constitute the roc$s #hoseages are being determined

6ie unconformably beneathnon9metamor"hosedsedimentary roc$s

3i!e actual ages of sedimentary roc$s in closestratigra"hic "ro imity abo!eand belo# !olcanic layers

3i!e minimum ages for the roc$sthey intrude

3i!e ma imum ages for o!erlying sedimentary roc$s

3i!e minimum ages for metamor"hosed sedimentaryroc$s

3i!e ma imum ages for theo!erlying non9metamor"hosedsedimentary roc$s

3i!e actual ages of sedimentaryroc$s

3i!e minimum ages for sedimentary roc$s

authigenicminerals suchas glauconite

in age. Thus, estimates of the ages of such associated !olcanic roc$s also establish the ages of contem"oraneous sedimentary roc$s.

ges of #hole !olcanic roc$ can be estimated relati!ely easily by the "ota ssium9argon method, and ages of minerals in these roc$s can be determined by the "otassium9argon or other methods. olcanic roc$s that occur inassociation #ith nearly contem"oraneous sedimentary roc$s #hose ages canalso be determined by fossils "ro!ide e tremely useful reference "oints for calibration. In fact, establishing the absolute ages of fossiliferous sedimentaryroc$s by association #ith contem"oraneous !olcanic flo#s #hose ages can be

radiometrically estimated has "robably been the single most im"ortantmethod of calibrating the geologic time scale.0or this method to #or$, the contem"oraneity of the interbedded !ol

canic and sedimentary roc$s must first be established. If a "yroclastic flo#such as an ash fall or a la!a flo# eru"ts o!er an older, e "osed sedimentaryroc$ surface #here erosion is ta$ing "lace or sedimentation is inacti!e, the


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flo# is not conte m"oraneous #ith the underlying sedimentary r oc$. The agecalculated for suc h a flo# indicates only that the roc$ belo# the flo# is older and the roc$ abo! e younger than the flo#. geologist can establi sh contem"oraneity by determi ning if fossils in sedimentary layers abo!e a nd belo# theflo# belong to th e same biostratigra"hic %one or by loo$ing, a long the basalcontact of the flo# unit, for "hysical e!idence that ma y sho# that the underlying sediment #as still soft at the time of the !olcanic eru"tion. 0or e am"le,ash fall material m ay be mi ed by bi oturba tion into underlying s ediment, soft

52-

igure 15.10iagram illustrating ho > the contemporaneity o se!imentary rocDsto an associate!, !atable "olcanic layer can be establishe!. Theshale be!s belo> an! abo"e the "olcanic ash be! belong to th esame oramini eral biozone an! the base o the ash be! has beenbioturbate!, in!icating that th e un!erlying se!iment >as still so tat th e time o th e ash all. There ore, the shale be!s are appro?i:mately the same age as the ash be! ;- +a


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it rests unconformably (0ig. 15.1.A . 0or e am"le, a sedimentary successionde"osited on the eroded, #eathered surface of a granite batholit h ma y subse

uently be intruded by a di$e or a sill. The sedimentary unit is ob!iouslyyounger than the batholi th but older than the di$e or the sill. Unfortunately,there is no #a y to determine ho# much younger or older unless other e!i

dence is a!ailable.


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The calibration methods discussed abo!e allo# the estimation of ages of sedimentary roc$s only through their association in some manner #ith igneous or metamor"hic roc$s #hose ages can be determined by radiometric methods.Clearly the uncertainties in!ol!ed in finding ages of sedimentary roc$s bythese indirect methods could be a!oided if ages could be estimated directly. smentioned, terrigenous minerals in sedimentary roc$s are not useful for radiochronology because they yield ages for the "arent roc$s, not the time of de"osition of the sediment. The only materials in sedimentary roc$s that can beused for direct radiochronology are organic remains (#ood, calcium carbonatefossils, an d other such remains that #ere de"osited #ith the sediment and au9thigenic minerals that formed in the sediment #hile still on the seafloor or shortly after burial . The "rinci"al methods that ha!e been used for direct radiochronology of sedimentary roc$s are (1 the carbon91A techni ue for organicmaterials, ( the "otassium9argon and rubidium9strontium techni ues for glauconites, (+ the thorium9 +E techni ue for ocean floor sediments, and (Athe thorium9 +E "rotactinium9 +1 techni ue for fossils and sediment.

short discussion of the ad!antages and disad!antages of each of thesemethods follo#s. 0or a descri"tion of other "ossible direct dating methods, suchas amino9acid racemi%ation and other methods based on radioacti!e dise uilibrium of uranium, thorium, and "rotactinium, see 3eyh and 'chleicher (1))E .


Carbon914 +etho!. The carbon91A method can be a""lied to the radiochronol9ogy of materials such as #ood, "eat, charcoal, bone, lea!es, and the CaCOshells of marine organisms. The method has been used e tensi!ely for estimating ages of archaeological materials, but its a""lication in geology is limited toHuaternary geology because of the !ery short useful age range of the method.Carbon91A

is

"r oduced in

the

atmos"here o#ing to

the

im"act of

cosmic9rayneutrons on ordinary nitrogen91A atoms. The nitrogen atoms lose a "roton andare thus con!erted to carbon91A, #hich, in turn, decays bac$s to nitrogen91A#ith a half9life of 5 +E years. Carbon91A is incor"orated into carbon dio ide(CO , #hich is assimilated by "lants and animals during their life cycles.2hen organisms die, their tissue no longer assimilates ne# radioacti!e car bon/ thus, the amount of radiocarbon in the organisms diminishes #ith time.The age of a sam"le is determined by measuring the amount of radiocarbon "e r gram of total carbon in a sam"le and com"aring this amount #ith the initial amount at the time the organism died. The age e uation is

(15.1.+

#here is the measured acti!ity of the sam"le at the "r esen t moment in disintegrations "e r minute "e r gram of carbon (d"m g and is the initial acti!ity (e.g.,


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he usable ages in some cases to as much as *E,EEE9=E,EEE years. These s"ecial

ethods are !ery e "ensi!e and ha!e not been #idely used in he "ast . lso,

hey are e ce"tionally subFect to systematic err or because of contamination

of

sam"les #ith young carbon.The carbon91A method has been used successfully for such a""lications

as estimating ages of !ery young sediment in cores of dee"9sea sediment andunra!eling recent glacial history by analysis of #ood in glacial de"osits. Its e tremely short range renders the method of little !alue in calibrating the geologic time scale e ce"t for !ery recent Huaternary e!ents.

6a!iochronology o Glauconites by Use o Potassium94 % rgon94 an!6ubi!ium9-&%#trontium9-&. Radioacti!e "otassium9AE !!"# is incor"oratedinto glauconite grains (green clay minerals com"osed of com"le "otassium9aluminum9iron silicates as they e!ol!e by alteration "rocesses on the seafloor.2hen the glauconite grains are fully formed, they theoretically become closedsystems #ith res"ect to gain or loss of "ota ssium or argon/ that is, no additional radioacti!e "o tass ium is ta$en into the grains and the !$r that forms bygradual decay of "o tass ium remains tra""ed #ithin the glauconite grains(e.g., Odin and Dodson, 1)= . 7easurement of the "%$r ratio in the glauconite grains thus allo#s the age of the grains to be estimated. The half9life of

"otassium9AE is 1 5E million

years/ therefore, it is

theoretically

"ossible to

15. The Geologic Time #cale

a""ly the &9 r method to radiochronology of roc$s ranging in age from aboutone million years (less in some cases to the age of -arth.

'e!eral #or$ers ha!e re"orted that glauconite ages tend to be 1E9 E"ercent too young o#ing to some argon loss. On the other hand, calculatedglauconite ages ma y be too old in some cases o#ing to the "resence of inherited radiogenic argon that #as already in sediment at the time the glauconitegrains formed. lso, the formation of glauconite grains and their closure toloss of argon do not occur simultaneously #ith de"osition of the enclosingsediment. 3lauconite grains, therefore, must yield a slightly younger age thanthe sediment in #hich they occur, e!en if uncertainties about inherited or lostargon are not a "roblem. Odin and Dodson (1)= suggest that the time re

uired for glauconites to e!ol!e and become closed systems ma y range u" to5,EEE years or more. Thus, in relation to biostratigra"hic %onation, the glau

conite &9 r ages are closer to those of fossils in the hori%on immediately abo!ethe glauconites than to the fossils de"osited #ith the glauconites.

The ages of glauconites can also be estimated by the rubidium9strontiummethod (Table 15.1.1 . Radioacti!e rubidium &b# is incor"orated into glauconites as they form, along #ith "otassium9AE. The long half9life of rubidium9= limits the use of the rubidium9strontium method to

radiochronology of

roc$s older than about 1E million years. Details of the Rb9'r method as a""liedto the radiochronology of sedimentary roc$s are gi!en by Clauer (1)= .

Estimating Piges o #e!imentary 6ocDs by Use o *ther uthigenic +inerals.In addition to glauconite, se!eral other authigenic minerals ha!e been used indirect radiochronology of sedimentary roc$s by the &9 r and Rb9'r methods.These minerals include clay minerals such as illite, montmorillonite, and chlorite/ %eolites/ carbonate minerals/ and siliceous minerals such as chert and o"al.


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ds. The choice of method de"ends u"on the age of he roc$s an

d the ty"es of

aterials "resent in them. n general, c

alibration of he time scal

e by estimat

ng ages of !olcanic roc$ s associated #ith essentialy contem"

oraneous sed

entary roc$s hat can be e

asily correlated by marine fossils is the most useful

nd reliable a""roach. Radiochronology of sedimenary glaucontes or brac$

ing the ages of sedimentary roc$s from associaed "lutonic ntrusi!e roc

$s

ay also yield usable ages

4the only ages a!ailable in some cases. Therefor e,

different methods ma y ha!e to be a""lied to estimating ages of roc$s in eachgeologic system. Details of the methods used for estimating ages of boundaries be t#een and #ithin the different systems are gi!en in Odin (1)= ,'nelling (1)=5a , and ;arland et al. (1))E .

0igure 15.+ sho#s the calibration of the 3eological 'ociety of merica1))) 3eologic Time 'cale on the basis of absolute ages obtained from a num

ber of different sources. Readers should be a#are, ho#e!er, that other "ublished geologic time scales ha!e slightly different !alues for some of these


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15.4 Chronocorrelation boundaries (e.g., Odin, 1)= / Curry et a6, 1)= / 'nelling, 1)=5b/ ;arland et a6,1))E , indicating differences in o"inion about the ages of the boundaries. Cali

brat ion of the geologic time table has changed steadily through the years as ra9diochronologic methods ha!e im"ro!ed and more absolute ages ha!e becomea!ailable.

lthough the

ages no#

used

to

calibrate

the

maFor boundari es

of

the

geologic time scale are unli$ely to undergo maFor re!ision in the future, it issafe to assume that refinements in these ages #ill continue for some time.

15.4 C/6*'*C*66E( T)*'Chronostratigra"hic units are e tremely im"ortant in stratigra"hy because theyform the basis for "ro!incial to global correlation of strata on the basis of agee ui!alence. 2e ha!e already established that chronostratigra"hic correlation iscorrelation that e "resses corres"ondence in age and chronostratigra"hic "ositionof stratigra"hic units. To many geologists, correlation on the basis of age e ui!alence is by far the most im"ortant ty"e of correlation and, in fact, it is commonlythe only ty"e of correlation "ossible on a truly global basis. 7ethods of establishing the age e ui!alence of strata by magnetostratigra"hic and biologic techni uesha!e already been discussed (Cha"ters 1+, 1A . 'e!eral other methods of time9stratigra"hic correlation are also in common use, including correlation by short9term de"ositional e!ents, correlation based on transgressi!e9regressi!e e!ents,correlation by stable isoto"e e!ents, and correlation by absolute ages. These methods are discussed belo#.

E"ent Correlation an! E"ent #tratigraphy-!ent correlation constitutes "a rt of #hat has come to be $no#n as e"ent stratig:raphy. -!ent stratigra"hy focuses on the s"ecific e!ents that generate a strati9gra"hic unit or succession rather than on the "hysical or biological characteristicsof the unit. 0or e am"le, a eustatic rise in sea le!el can affect sedimentation "atterns #orld#ide. s a result of this e!ent, sedimentary facies are generated in a!ariety of en!ironments in !arious "arts of the #orld. These facies may not bee ui!alent in terms of their "hysical characteristics/ ho#e!er, they are e ui!alentin the sense that they #ere "roduced as a result of the same e!ent. Thus, they arechronological e ui!alents.

-!ents can be considered to ha!e different scales de"ending u"on their duration (0ig. 15.A , intensity, and geologic effect. 'ome con!ulsi!e e!ents are e traordinarily energetic, occur uic$ly, and ha!e regional influence (e.g., e "losi!e!olcanic eru"tions, im"act of large e traterrestrial bodies (bolides , great earth

ua$es, catastro"hic floods, large !iolent storms, large tsunamis . These e!entsmay "roduce #ides"read effects, including mass e tinctions.


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Chapter 15 % Chronostratigraphy an ! Geologic Time

'ge "e ore present (years* 'pproDimate d!ration o

,

olocene ,, . ,E $Dperimental>,istorical Pleistocene Tert F)esoz certain classes o eventso signi icance to the1- 10 10 1- 1-H 1-H 10 1-H geological record

1 ho!r; Ts!nami, gravity low1 > 1- days8 'sh all, lava low

1 year8 Aormation o ann!al deposits

I- years8 !man li espan1-- > 1--- years8 Continent>wideeDpansion o s!ccess !l immigrant;deposition 1 >cm pelagic ooze-.1 my8 'verage paleomagnetic event-.5 my8 'verage glaciation cycle1 my; Cyclothem, paleomagnetic epoch

1 E1- my; %pecies li espan5 E 5- my8 rogenic cycle

igure 15.46esol"ing po>er o geochronologic systems in th e Cenozoic on th e basis o an! 9 rabsolute ag e !iscrimination an! biochronological !iscrimination. The "ertical a?is sho>sthe !uration o e"ents ranging rom hours to hun!re!s o millions o years, an! the hori:zontal a?is sho>s ag e be ore the present ranging rom hours to hun!re!s o millions o years. 'ote that !ating can resol"e e"ents that range in ag e rom tens o years to lessthan 1 , years an! that are years to tens o thousan!s o years apart. 9 r !ating canresol"e e"ents that are ol!er than 1 , years an! that are separate! by at least 1 ,years. iochronology is most e ecti"e in resol"ing e"ents that are ol!er than about on emillion years an! that are space! at least one million years apart. A ter erggren, H. .,an! hysical e!ents that meet this criterion include tsunamis,storms, floods, sediment gra!ity flo#s, !olcanic eru"tions, meteorite and cometim"acts, ra"id sea9le!el changes, and abru"t re!ersals of -arth s magnetic field(e.g., -insele, 1))=/ 'hi$i, Chough, and -insele, 1))* . Chemical e!ents, #hich may

be related to "hysical e!ents, include sudden changes in stable isoto"e (e.g., o ygen, carbon content of the ocean and de!elo"ment of ano ic (lo#9o ygen conditions in the ocean.


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"hysical, chemical, and biological e!ents generate corres"onding e!ent de"osits(e.g., a !olcanic eru"tion "roduces an ash bed . Combining se!eral $inds of e!ent

%edimentary %!ccession

=lac7 shales&od!lar lime#stone o specialpattern

=lac7 shales

)onomorphossil layer

Tephra

T!r"idites

%torm layer

=lac7 shales

, such as ano?:ic e"ents that cause mass e?tinctions or mass mortality, ma y be o global scope. Combin:ing "arious Din!s o e"ents lea!s to i!enti ication o high9resolution stratigraphic units;holostratigraphic units< an ! biostratigraphic units that ha"e chronostratigraphic signi i:cance. Column sea le"el8 2 all, 6i @ rise. Column biostratigraphic units8 Et @ earliest,E @ early, + @ mi!!le, ( @ late. Column e"olution8 thicD lines o species ranges @ in!e?species. A ter arnes et al., 1335 , Global e"ent stratigraphy, in Halliser, *. /. ;e!.


25/185

falls are called ash layers, te"hra layers, bentoni te beds (if the ash alters to ben9tonite clays , or tuff layers. The ash fall from a single eru"tion may "roduce ashlayers se!eral centimeters thic$ that can co!er thousands to hundreds of thousands of s uare $ilometers. 0or e am"le, ash from the eru"tion of 7t. 7a%ama in

5 $ Chapter 15 % Chronostratigraphy an! Geologic Timesoutheastern Oregon about *5EE9 EEE years ago, an eru"tion that subse uentlyled to the formation of the Crater 6a$e caldera, #as carried northeast#ard by#inds and de"osited as far a#ay as 'as$atche#an and 7anitoba, Canada. s

hfrom the 7ay 1)=E eru"tion of 7t. 't. ;elens also s"read o!er thousands

of

s uare $ilometers east an d north of 7t. 't. ;elens in 2ashington and Idaho.

Other historic e am"les of #ides"read ashfalls include the 1)+ eru"tion of

Hui%a"u in

Chile,

an

eru"tion that

distributed

!olcanic

ash

east#ard

for

15EE

$m

across 'outh merica and into the tlantic Ocean, and the eru"tion of >erbuatanolcano at &ra$atoa Island, Indonesia, in 1==+, an eru"tion that s"read !olcanic

dust around the #orld.Te"hra layers ma$e e tremely useful reference "oints in stratigra"hic sec

tions. They "ro! ide a means for reliable time9stratigra"hic correlation if they areof sufficient lateral and !ertical e tent and if they can be identified as the "roductof a "articular !olcanic eru"tion. Identification of indi!idual ash layers or ben

9tonite beds can often be made on the basis of "etr ogra"hic characteristics4ty"esof mineral grains, roc$ fragments, glass shards, or other com"onents4or trace9el

ement com"osition. ges of these layers ma y be determined also by radiometricmethods, allo#ing the layers to be identified and correlated by contem"oraneousage. Te"hra layers are "articularly useful in correlating across marine basins, andit ma y e!en be "ossible to correlate ash layers in marine basins to #ell9dated la!aflo#s or ash layers on land, thereby e tending marine correlations onto land.

Turbidity currents constitute another ty"e of Ginstantaneous geologic e!entthat can "roduce thin, #ides"read de"osits (e.g., -insele, 1))= . Turbidites may ha!echronostratigra"hic significance if a "articular turbidite bed, or succession of beds,can be differentiated from other turbidite units and traced laterally Unfortunately,most turbidites commonly consist of rhythmic or cyclic successions of units that ha!e!ery similar a""earance and are !ery difficult to differentiate. Thus, in "ractice, theusefulness of turbidites in time9stratigra"hic correlation is rather limited.

Other ty"es of Gcatastro"hicG short9term geologic e!ents include dust

storms that "roduce fine9grained loess de"osits on land or silt9sand layers in marine basins. 'torms at sea can stir u" and trans"ort sediment on the continent shelf to "r oduce thin Gstorm layersG of sand or silt, as discussed in a "receding cha"ter.

'lo#er, noncatastro"hic de"ositional conditions also may generate thin, distincti!e, #ides"read stratigra"hic mar$er beds under some de"ositional conditions. De"osition of these beds does not necessarily ta$e "lace GinstantaneouslyG Ne!ertheless, they can be used for time9stratigra"hic correlation if they formed asa result

of

de"osition that

too$

"lace o!er

a large "art

of a basin

during a relati!elyshort "eri od of time under essentially uniform de"ositional conditions. 0or e

am"le, changes in ocean circulation "a tt erns ma y br ing about ano ic conditions(0ig. 15.5 , leading to #ides"read de"osition of organic9rich blac$ shales. thin,#ides"read limestone bed #ithin a dominantly shale or silt succession im"lie

s


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de"osition of the imestone under

conditions that #ere in effect essentially simul

taneously throughout a geologic "ro!ince. 'uch a thin limestone bed #ithin a suc

cession of nonmarine clastic units may re"resent a brief incursion of marine

conditions into a nonmarine en!ironment or the tem"orary "onding of fresh

#ater to form a large, shallo# la$e. Thin limestone units in a thic$ succession of

marine clastic de"osits ma y indicate shelf carbonate de"osition during brief "eriods #hen clastic detritus #a s tem"orarily tra""ed in estuaries or deltaic en!ironments and thus "r e!en ted from esca"ing onto the shelf.


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3andward %eaward

' =3ocalities

igure 15.$Time correlation by position in a transgressi"e9regressi"e cycle. Th e line connect ing points o !eepest9>ater con!ition is a time line. A ter)sraelsDi, +. C., 1 3 4 3 , *scillation chart8 m. ssoc.Petroleum Geologists ull., ". , ig. , p. 3-.B

5.- Chapter 15 % Chronostratigraphy an! Geologic Time>

igure 15.&Transgressi"e9regressi"e cycie se!imentation an! e"ent correiation in theEocene o the isie o Hight in southern Englan!. A rom ger, 0. F., 133 , Thenature o the stratigraphical recor!, r! e!.. ig. &.2, p. 1 . 6eprinte! bypermission o Iohn Hiley J #ons (t!.

m CRO' ' 9


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throughout the #orld. ariations in isoto"ic com"osition of sediments or fossilsallo# geochemists to construct isotopic composition cur"es that can be used asstratigra"hic mar$ers for correlation "ur"oses. To be useful for correlation, fluctuations in isoto"ic com"osition must be recogni%able on a global scale and must beof sufficiently short duration to sho# u" as a shift on isoto"ic com"osition cur!es.

lso, stratigra"hers must be able to fi the relati!e stratigra"hic "osition of thesefluctuations in relation to biost rat igra"hic, "aleomagnetic, or radiometric scales.Of the !arious "otentially useful isoto"es, o ygen isoto"es seem most nearly tomeet these re uirements and ha!e "ro!en to be "art icularly useful for chronostratigra"hic correlation of Huaternary and late Tertiary sediments. Carbon, sulfur,and strontium isoto"es are also useful for correlating roc$s of certain ages.

!ygen "sotopesThe natural isoto"es of o ygen are listed in Table 15.+. 7ost of the o ygen in theoceans occurs as o ygen91*. O ygen91= is much rarer (about E. "ercent of total


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Sam Boggs Jr. Principles of Sedimentology and Stratigraphy 4th Edition 2005.docx