Boggs S. Principles of Sedimentology and Stratigraphy

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15.2 Geologic Time Units

517

series, because series are rock its, not units of time. Most names for eons and eras

are the same as the names of the corresponding eonoems and erath ems.

Geochronometric units are pure time units. They are not based on the time

spans of designated chronostratigraphic stratotypes but are simply time divisions

of an appropriate magnitude or scale, with arbitrarily chosen boundaries. At this

time, a geochronometric time scale is used to express the ages of Precambrian

rocks (see Fig. 15.2) because no globally recognized and accepted chronostrati

graphic scale has been developed for these rocks. Precambrian rocks have not yet

proven generally susceptible to analysis and subdivision by superposition or by

application of other lithologic or biologic principles that we commonly use in sub

dividing the Phanerozoic rocks; however, a chronostratigraphic scale for Precam

brian rocks may be developed in the future. Subdivision of Precambrian rocks is

further discussed  the next section.

Eonothem

Erath em

System and Subsystem

Series

Numerical

Age (Ma)

Quaternary

Holocene

0.1

Pleistocene

1.8

Pliocene

Cenozoic

Neogene

Miocene

23.8

Te rtiary

Oligocene

Paleogene

Eocene

Paleocene

65.0

Cretaceous

Upper

Lower

144.2

Upper

Mesozoic

Jurassic

Middle

Lower

206

Upper

Triassic

Middle

0 Lower

248 

Upper



Permian



Lower

290

Carbon- I

Pennsylvanian

.

Upper

323

iferous

Mississippian

Lower

354

Upper

Devonian

Middle

Lower

Paleozoic

417

Silurian

Upper

Lower

443

Upper

Ordovician

Middle

Lower

490

Cambrian

Upper

Lower

543

Not

formally



subdivided















2500



Not





formally

.

subdivided





Source of ages: Geological Society of America 1999 Geologic Time Scale.

Figure 15.2

Nomenclature of Phanerozoic chronostratigraphic

units commonly used throughout the world. Pre

cambrian rocks are divided into the Archean and

Proterozoic; however, no scheme for further sub

division of the Precambrian is globally accepted.

518

Chapter 15 I Chronostratigraphy and Geologic Time

15.3 THE GEOLOGIC TIME SCALE

Purpose and Scope

Classifying rocks on the basis of time involves systematic organizaon of strata

into named units, each corresponding to specific intervals of geologic time. These

units provide a basis for time correlation and a reference system for recording and

systematizing specific events in the geologic history of Earth. Thus, the ultimate

aim of creating a standardized geologic time scale is to establish a hierarchy of

chronostratigraphic units of international scope that can serve as a standard refer

ence to which the ages of rocks everywhere  the world can be related. Establish

g the relative ordering of events in Earth's history is the main contribution that

geology makes to our understanding of time.

A standard geologic time scale should express any age in any place, and it

should be understandable, clear, and unambiguous. It should also be independent

of opinion and therefore have some objective reference that is accessible. Finally, it

should be stable, that is, not subject to equent change, and it should be agreed to

and used inteationally in all languages (e.g., Harland, 1978).

Development of the Geologic Time Scale

Chronostratigraphic Scale

Geologists have been working for more than 200 years to develop a systematic

scheme for a global time-stratigraphic classification of rock units. This slow

process has evolved through two fundamental stages of development:

1. Determining time-stratigraphic relationships from local stratigraphic sections

by applying the principle of superposition, supplemented by fossil control

and, more recently, radiometric ages.

2. Using these local stratigraphic sections as a basis for establishing a composite

international chronostratigraphic scale, which serves as the material reference

for constructing a standardized international geologic time scale.

The international chronographic scale has evolved gradually over the past two

centuries into its present form (Table 15.1, Fig. 15.2). Figure 15.2 shows the hierar

chy of major chronostratigraphic units in general use throughout most of the

world. A more detailed chronostratigraphic scale that also shows subseries and

stages

is given in Appendix D; this chronostratigraphic and geochrohometric scale

was compiled by Salvador (1985) as part of the COSUNA (Correlation of Strati

graphic Units of North America) project. [Note: The ages of the boundaries be

tween chronostragraphic units shown in Appendix D may not agree with the

more recent age determinations given in the new geologic time scale shown in

Figure 15.3.] Some of the provincial stage names commonly used in North Ameri

ca are also shown in Appendix D; however, there now appears to be a general

movement among North American stratigraphers to abandon these provincial

stage names and adopt the European (global) stages as standards for North Amer

ica. Stratigraphers in Europe and many other parts of the world have for many

years subdivided the Tertiary into two subsystems, the Paleogene and the

Neogene, with the top of the Oligocene Series as the dividing boundary between

the two (Fig. 15.2). Geologists in North America have now also adopted this prac

tice. They have likewise adopted the European usage of the Carboniferous as a

system name, but with subdivision in North America into the Mississippian and

Pennsylvanian subsystems. Other versions of the chronostratigraphic scale exist

(e.g., Cowie and Basse

tt, 1989; Harland et al., 1990) that differ somewhat from

15.3 The Geologic Time Scale

519

1999 GEOLOGIC TIME SCALE

PA LEOZOIC

LTE

MIDD

•Jrternat1onal ages have not beer estab1ishecL These are regional (laurel"tian) only.

Sources for nomenclature ana ages: Primarily ftom Gradstel n, , and Ogg, j,, 1996, Ep isodes, v. 19, nos. i & 2: Gradstein, F. ,  al., 1995,

GEOLOGICAL SOCIE1Y

OF AMERICA

Figure 15.3

SEPM Special Pub. 95-128; Bergg ren, W. A., et at, 1995, SEPM Special Pub. 54, p, 129·212; Cambnan and basal OrdoViCian ages

adaed from Landing.

998, Canadian Joumai of Earth Scie nces, v. 35, p. 329-338; and Dav1dek, K, et al., 1998, Geological Magazme,

v. 135, p. 305-30R Cambrian age names from Pa!mer, A. R., 1998, Canadian Journal of Earth Sciences, v. 35, p. 323-328.

1999 geologic time scale published by Geological Society of America.

this scale, particularly in naming of series and stages and partial subdivision of

the Precambrian into erathems and systems. The geologic community has not yet

achieved the ideal of a truly international chronostratigraphic scale that is accept

ed and used by all geologists worldwide.

Geochronologic (Time) Scale

Figure 15.2 is a chronostratigraphic scale with units and boundaries based on

physical divisions of the rock record, but it is not in itself a time scale. To function

as a geologic time scale for expressing the age of a rock unit or a geologic event,

the chronostratigraphic scale must be converted to a geochronologic scale consist

ing of units that represent intervals of time rather than bodies of rock that formed

during a specified time interval. The geologic time scale is derived from the

chronostratigraphic scale by substituting for chronostratigraphic units the corre

sponding geochronologic uts (Table 15.1). Thus, the geologic time scale is ex

pressed in eras, periods, epochs, ages, and chrons rather than erathems, systems,

series, stages, and chronozones. The subdivision boundaries of the geologic time

scale

are calibrated in absolute ages; however, the geologic time scale differs from

900

520

Chapter 15 I Chronostratigraphy and Geologic Time

a true geochronometric scale, which is based purely on time without regard to the

rock record. By contrast, the subdivisions of the Phanerozoic time scale are of un

equal length, because they are based on chronostratigraphic units that were de

posited during unequal intervals of time.

The geologic time scale has been in existence for several decades, and during

that time it has continued to evolve, with refinements being made particularly in

subdivision of the epochs and ages and absolute-age calibration of the boundaries

between periods, epochs, and ages. Figure 15.3 shows the most recent version of

the geologic time scale published by the Geological Society of America in 1999.

This time scale is subdivided into ages based on the European stages, and bound

aries between ages are calibrated in absolute time. Absolute ages are given in mil

lions of years before the present (Ma), where the present refers to 1950. Methods

for absolute age calibration of the geologic time scale are discussed below. Note

that the magnetic polarity scale for the most recent approximately 160 million

years is also included in the time scale. Note also the use of a geochronometric

scale for the Precambrian, with the dividing boundary between the Archean and

the Proterozoic set arbitrarily at 2500 million years.

Box 15.1 Calibrating the Geologic Time Scale

As mentioned, the geologic time scale has evolved slowly over a long period

of time. To develop the scale to its present level of usefulness for fixing the po

sition in time of a particular rock unit or geologic event, two types of informa

tion had to be available to stratigraphers: (1) some method of arranging rocks

in an orderly succession on the basis of their relative position in time, or rela

tive ages, and (2) a method of determining the ages of the boundaries between

rock units on the basis of their absolute position in time with respect to some

fixed time horizon, for example, the present.

Placing strata in stratigraphic order in terms of their relative ages has

been the guiding principle used by stratigraphers in constructing the geologic

time scale. Relative ordering was determined by applying the principle of su

perposition, aided by use of fossils. The principle of superposition means sim

ply that in a normal succession of strata which have not been tectonically

overturned since deposition, the youngest strata are on top and the ages of the

strata increase with depth. Most of the divisions in the current global chronos

tratigraphic scale are based on fossils, and early efforts to create an inteational

chronostratigraphic scale before methods of absolute-age determinations were

developed would have been impossible without the use of fossils.

Fortunately, methods are now available not only for determining the rel

ative ages of strata but also for fixing within reasonable limits of uncertainty

the absolute ages of some strata. Development of these methods of absolute

age estimation have made it possible to place approximate absolute ages on

boundaries of the chronostratigraphic scale initially established by relative

age determination methods. Absolute age data can also be used for determin

ing ages of poorly fossiliferous Precambrian rocks that cannot be placed in

stratigraphic order by relative-age determination methods. The principal

method for determining the absolute ages of rocks is based on decay of ra

dioactive isotopes of elements in minerals. Other methods of determining the

absolute passage of geologic time include counting; lake-sediment varves,

which are presumed to represent annual sediment accumulations; growth in

crements in the shells of some invertebrate organisms; growth rings in trees;

and Milankovitch climate cycles in sediments. These alternative methods are

15.3 The Geologic Time Scale

521

useful only for marking the passage of short periods of time in local and re

gional areas and are not of importance in calibrating the geologic time scale,

except possibly some parts of the Pleistocene and Pliocene.

Thus, e major tools for finding ages of sedimen to calibrate the geolog

ic time scale are relative-age determinations by use of fossils-biochronology

and absolute age estimates based on isotopic decay-radiochronology. These

tools may be used both for calibrating the chronostratigraphic scale directly

and for calibrating the succession of reversals of Earth's magnetic field; this

succession constitutes e magnetostratigraphic time scale discussed in

Chapter 13. We shall now discuss each of these dating methods, beginning

with biochronology.

CALIBRATING THE GEOLOGIC TIME SCALE BY

USE OF FOSSILS: BIOCHRONOLOGY

Biochronology is the organization of geologic time according to the irre

versible

process of evolution in the organic continuum (Chapter 14). Useful

fossil horizons are more widespread and abundant in Phanerozoic rocks than

are horizons whose ages can be estimated by radiochronology. Furthermore,

biologic events can commonly be correlated in time more precisely than can

radiometric data in all but Cenozoic rocks. Because of these factors, fossils

have convenonally provided the most readily available tool for dating and

long-distance correlations of Phanerozoic rocks. It is necessary, however, to

make a clear distinction between biochronology and biostratigraphy. Bios

atigraphy (Chapter 14) aims simply at recognizing the disnctive fossils that

characterize a known stratigraphic level  a sedimentary section without re

gard to the inherent time significance of the fossils. For example, William

Smith was able to use fossils very effectively for identifying and correlating

strata, even though he had little or no idea of the time relationships or time sig

nificance of the fossils. Biochronology, on the other hand, is concerned with

the recognition of fossils as having ages that fall at known points in the span of

evolutionary time, as measured by fossils of a reference biostratigraphic sec

tion. Therefore, by establishing identifiable horizons in reference sections

based on fossils, biochronology provides a tool both for international correla

tion and for worldwide age determination.

The aim of biochronology is to make possible correlation and dating of

the geologic record beyond the limits of local stratigraphic sections. To do this

most effectively, stratigraphers use features or events in the paleontologic

record that are widespread and easily identifiable and that occurred during

short periods of geologic time. These events are considered to be biochrono

logic datum events because they mark a particular short period of time in the

geologic past. The datum events most commonly used are the immigrations

(first appearances) and extinctions (last appearances) of a fossil species or

taxon. The first appearance of a species as a result of immigration from anoth

er area commonly occurs very rapidly after its initial appearance, owing to

evolution from its ancestral morphotype. The first appearance is so rapid, in

fact,

that geologically speaking we consider speciation and immigraon as es

sentially synchronous events. Extinction of a taxon may also occur very rapid

ly, although commonly not as rapidly as speciation.

Stragraphers speak of the first and last appearances of a taxon as the

rst appearance datum (FAD) and the last appearce datum (LAD). These

FADs and LADs are not totally schronous owing to e fact that even though

immigrations and extinctions can take place quite rapidly, as mentioned, they

522

Chapter 15 I Chronostratigraphy and Geologic Time

are not actually instantaneous events. Some planktonic species have been re

ported to spread worldwide in 100 to 1000 years; however, bioturbation of

sediment after deposition can mix fossils through a zone several centimeters

thick, and accidents in preservation as well as bias in collection and analytical

meods can combine to create uncertainties in the age of the FADs and LADs

that can amount to thousands of years. Nevertheless, the duration of the

FADs of many planktonic species may be as little as 10,000 years; that is, the

ages of the first appearance datum of a species will not vary by more than

10,000 years in different parts of the world (Berggren and Van Couvering,

1978). The error caused by an age discpancy of is magnitude becomes in

significant when applied to estimation of e ages of cks that are millions to

hundreds of millions of years old. Thus, the FADs and LADs of many fossil

species can be considered essentially synchronous for e utilitarian purposes

of biochronology.

FADs and LADs are the most easily utilized and communicated types of

fossil information upon which to base biochronology, and they can be used

across great distances within the range of the defining taxa. Therefore, ey

have come to domate global biochronological subdivision. The procedure

for establishing the biochronology of any fossil group based on FADs and

LADs involves the following steps (described by Haq and Worsley, 1982) and

is illustrated graphically in Figure 15.1.1:

1. Identify and locate in local biostratigraphic units e FADs and LADs of

distinctive fossil taxa at have wide geographic distribution.









.

Figure 15.1.1

FAD (B) = 75 Ma calculat from sedimentatio

rata

D (D)

85 Ma from radiometric data

Schematic illustration of the application of biochronology to age cal

ibration of a local stratigraphic section. The ages of the FAD for

Species A and the D for Species D are established by radiometric

dating of some closely associated physical feature (e.g., an ash bed).

The FAD for Species B and the D for Species C cannot be dated

radiometrically; however, the ages can be calculated from the sedi

mentation rate determined beeen FAD (A) and LAD (D). This rate

(3 m/Ma) can then be used to determine the age difference be

tween FAD (A) and FAD (B) ( 3 m/Ma x 15 m 5 Ma) and be

tween LAD (D) and D (C) (3 m/Ma x 10 m= 3 Ma).

15.3 e Geologic Time Scale

523

2. If possible, assi ages to these events by direct or indirect calibration

through radiochronology or magnetostratigraphy. If ages can be assigned

to any two events, the sedimentation rates for strata between these events

can be calculated by dividing the age difference between the o by the

thickness of sediment separating em. The sedimentation rates can then

be used to calculate e approximate age of each event enclosed within the

dated succession (Fig. 15.1.1).

3. If radiometric or magnetostratigraphic calibration of FADs or LADs in the

local section cannot be accomplished, then the ages of e datum levels

must be found in a different way. Under these conditions, ages of the

FADs and LADs are estimated on the basis of their stratigraphic position

with respect to calibrated datum levels of other fossil groups that also

occur in the sedimentary succession and whose ages have been found by

study of one or more successions elsewhere.

An example of biochronologic calibration is illustrated in Figure 15.1.2,

which shows the use of calcareous nannoplankton to establish a biochronol

ogy for the Pleistocene by direct correlation with magnetostratigraphic units.

CALIBRATING BY ABSOLUTE AGES: RADIOCHRONOLOGY

General Principles

Radiochronology is based on the principle that radiogenic minerals such as

uranium-235 and potassium-40 decay spontaneously at a fixed rate to a "daugh

ter" product. Thus, the age of a radiogenic mineral can be calculated from the

measured ratio of parent radionuclide to daughter product in the mineral, by

use of the known decay rate of the parent material. The decay rate is common

ly expssed as the half-life of the radioactive isotope (i.e., the time required

for one-half of the parent material to decay to the daughter product). The

number of atoms of the parent radioactive material and the inert daughter

product are measured by a mass spectrometer, which is an instrument that

separates and counts atoms of diffent masses or charges in a radiogenic min

eral such as zircon (Table 15.1.1).

The equation for calculating radiometric age is

t = -

+ 1

[D-Do

(15.1.1)

where N is the number of parent atoms of an element (e.g., uranium) present

in any given amount of the element,  is log base e, D is the total number of

daughter atoms (e.g., lead), Do is the number of original daughter atoms, and

A is the decay constant, which is calculated from the relationship

(15.1.2)

where T1 ;2 is the half-life of the radioactive element (Faure, 1986, Chapter 4). N

and D are measurable; D0 is a constant whose value is either assumed or cal

culated from data for cognetic samples of the same age.

Radiometric Methods

incipal Methods. Some of the most useful radionuclides for estimating ab

solute ages and the minerals, rocks, and organic materials most suitable for

524

Chapter 15 I Chronostratigraphy and Geologic Time

MAGNETIC

STRATIGRAPHY

AGE

0.5

1.0

1.5











JARA �

Figure 15.1.2





J

Ds

NANNOFOSSIL

ZONES

Emiliania

huxleyi

Emiliania

huxleyi

Acme

Zone

Gephyrocapsa oceanica

Zone

Pseudoemiliania lacunosa Zone

small Gephyrocapsa Zone

Helicopontosphaera sellii Zone

Cyclococcolithina

macintyrei

Zone

FADs

Discoaster brouweri

Zone

An example of biochronological dating by use of nannofossil datum

events correlated with magnetic polarity events. [After Gartner, S., 1977,

Calcareous nannofossil biostratigraphy and revised zonation of the Pleis

tocene: Marine Micropaleontology, v. 2, Fig. 5, reprinted by permission

of Elsevier Science Publishers.]

age determination are shown in Table 15.1.1. The carbon-14 method is applied

to direct dating of very young sediments. The protactinium-231 and thorium-

230 methods are also applied to direct dating of sediments ranging in age to

about 250,000 years. The usefulness and limitations of these methods for direct

dating of sediments are discussed further in succeeding sections.

Most radiometric dating methods cannot be applied to direct dating of

sedimentary rocks. They are used to determine the ages of igneous and meta

morphic rocks, which indirectly provide ages for associated sedimentary rocks

(to be discussed). The potassium/argon method is widely used because it can

be applied to a number of minerals that are common in igneous and metamor

phic rocks, and it gives generally reliable results. It can be used for dating plu

tonic, igneous, volcanic, and metamorphic rocks (metamorphism resets the

radioactive clock), and even some sedimentary minerals (e.g., glauconite). The

principal problem with the potassium/ argon methods is that the decay prod

uct, argon-40, is a gas that can leak out of a crystal.

The argon-40/argon-39 method is a related technique in which potassi

um-39 is converted to argon-39 by irradiation with fast neutrons in a nuclear

Approximate

useful

Parent

Daughter

Half-life

dating range

nuclide

(years) (years B.P.)

Carbon-14

*Nitrogen-14

5730

<�40,000

Protactinium

*Actinium-227

32,480

<�150,000

-231 (daughter

nuclide of

uranium-235)

Thorium-230

*Radium-226

75,200

<�250,000

(daughter nuclide

of uranium 238/234)

Uranium-238 Lead-206

4500

10->4500

million

Uranium-238

Spontaneous

<�65

fission tracks

million

Uranium-235

Lead-207

710

10->4500

million

Potassium-40

Argon-40

1250

1->4500

million million

Rubidium-87

Strontium-87

48 billion

10->4500

million

Samarium-147

Neodymium

106

> 200 million

-143

billion

Lutetium-176

Hafnium-176

35 billion

> 200 million

Half-life data om Bowen (1998).

*Not used  calculang radiomeic ages.

*an be used for dating older rocks under favorable circumstances.

15.3 e Geologic Time Scale

525

Materials

commonly

used for

dating

Wo od, charcoal,

CaC0

shells

Deep-sea sediment,

aragonite corals

Deep-sea sediment,

aragonite corals

Zircon

monazite, sphene,

uranium/thorium

minerals

Volcanic glass,

zircon, apatite,

sphene, gaet

Zircon, monazite,

sphene, uranium/

thorium minerals

Muscovite, biotite,

feldspars,

glauconite, whole

volcanic rock

Micas, K-feldspar,

whole metamorphic

rock

glauconite

Pyroxene,

plagioclase, garnet,

apatite, sphene

Pyroxene,

plagioclase, garnet,

apatite, sphene

reactor. The ratio of potassium-39 to potassium-40 is known, so argon-39 can

serve as a proxy for potassium-40. This relationship permits the potassium de

termination for a potassium-argon age to be made as

part of the argon isotope

analysis. In other words, measurement of the amount of argon-39 (which prox

ies for potassium-40) renders it unnecessary to separate potassium from a min

eral and measure e amount of potassium-40. Both argon-39 and argon-40 are

measured during the argon analysis.  age can be determined from the

argon-40/ argon-39 ratio once the conversion rate of argon-40 to argon-39 has

been determined by irradiating a standard of known age along with the

526

Chapter 15 I Chronostratigraphy and Geologic Time

sample (e.g., Bowen, 1998). The method is so sensitive that very small sam

ples can be used, and it has the further advantage that it allows correction

for loss of argon by leakage. Because of these advantages, it is being in

creasingly used.

Like the potassium/ argon method, the rubidium/strontium method can

also be applied to a number of common minerals; however, it is less common

ly used. Rubidium is so rare that a long decay period is required to generate a

measurable amount of strontium. The uranium/lead methods make use of

minerals such as zircon, sphene, and monazite as well as some less common

uranium/thorium minerals. These methods give generally reliable ages for

older rocks and can be used for dating some rocks as young as about 10 million

years.

Fission-track dating is a technique that relies on counting fission tracks

in minerals such as zircon (e.g., Waer and Van den Haute, 1992). Emission of

charged particles from decaying nuclei causes disruption of crystal lattices, cre

ating the tracks, which can be seen and counted under a microscope. The older

the mineral the more tracks are present. The samarium/neodymium and

lutetium/hafnium methods are less commonly used dating techques that

may be applied to some rocks that are less amenable to dating by convention

al methods. Samarium and lutetium are rare earth elements with long half

lives, making them useful for dating very old (Precambrian) rocks.

Additional, specialized dating methods (e.g., amino-acid racemization

method, obsidian hydration method) are available also (Faure, 1986; Geyh and

Schleicher, 1990). Details of radiochronologic methods and discussions of er

rors and uncertainties in radiometric age determinations are available in sev

eral published volumes (e.g., Bowen, 1988, 1998; Dickin, 1995; Easterbrook,

1988; Faure, 1986; Geyh and Schleicher, 1990; Mahoney, 1984; McDougall and

Harrison, 1988; Odin, 1982; Parrish and Roddick, 1985; Williams, Lerche, and

Full, 1988).

Application to Dating Sedimentary Rocks. Although radiochronologic

methods can be applied to a variety of rock materials and organic substances

(Table 15.1.1), they have limited application to the direct estimation of ages of

sedimentary rocks. Most of the potentially usable minerals in sedimentary

rocks are terrigenous minerals that when analyzed yield the age of the parent

source rock (see Appendix B), not the time of deposition of the sedimentary

rock, although a few mare minerals such as glauconite can be used for direct

dating of sedimentary rocks. Therefore, much of the geologic time scale has

been calibrated by indirect methods of estimating ages of sedimentary rocks

on the basis of their relationship to igneous or metamorphic rocks whose ages

can be determined by radiochronology. The types of rocks that are most useful

for isotopic calibration of the geologic time scale are described in Table 15.1.2.

We will now examine in greater detail the most common methods used to find

ages of the sedimentary rocks of the international chronostratigraphic scale.

These methods are not, of course, restricted to determining the ages of sedi

mentary rocks that make up the inteational chronostratigraphic scale. They

can be applied to determining the ages of sedimentary rocks in generaL

Finding Ages of Sedimentary Roc by alysis of Interbedded "Contem

poraneous" Volcanic Roc. Lava flows and pyroclastic deposits such as ash

falls can be incorporated very quickly into an accumulating sedimentary succes

sion without significantly interrupting the sedimentation process. Volcac mate

rials may be erupted onto "soft" unconsolidated sediment and then buried

during subsequent, continued sedimentaon, leading to a succession of interbed

ded sedimentary rocks and volcanic rocks that are essentially contemporaneous