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The Geological Society of America Geologic Time Scale 1888 2013
CELEBRATING ADVANCES IN GEOSCIENCE
1,† 2 3 4
J.D. Walker , J.W. Geissman , S.A. Bowring , and L.E. Babcock Invited Review
1
Department of Geology, University of Kansas, Lawrence, Kansas 66045, USA
2
Department of Geosciences, ROC 21, University of Texas at Dallas, Richardson, Texas 75080, USA, and
Department of Earth and Planetary Sciences, MSC 03 2040, 1 University of New Mexico, Albuquerque, New Mexico 87131, USA
3
Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge,
Massachusetts 02139, USA
4
Department of Geology, Lund University, SE-223 62 Lund, Sweden, and School of Earth Sciences, Ohio State University,
Columbus, Ohio 43210, USA
ABSTRACT INTRODUCTION over the past few decades in establishing both
numerical and relative ages, describe selected
The Geological Society of America has One of the most important aspects of research proxies for time in the rock record, and note and
sponsored versions of the geologic time scale in the geosciences is connecting what we examine comment on some future challenges. This paper
since 1983. Over the past 30 years, the Geo- in the rock record with ages of events and mea- is intended to provide a general overview of
logical Society of America Geologic Time sured rates and durations of geologic processes . geologic time scales. The most comprehensive
Scale has undergone substantial modifi ca- In doing so, geoscientists are able to place esti- treatment of the geologic time scale is contained
tions, commensurate with major advances mates on the rates of climate and evolutionary in the recent publication of Gradstein et al.
in our understanding of chronostratig- changes, use astronomically forced depositional (2012), the most current defi nitive work on the
raphy, geochronology, astrochronology, processes to tell time within sedimentary basins, geologic time scale from a global perspective.
chemostratigraphy, and the geomagnetic examine the ways in which tectonic processes This book is the most recent in the series of ma-
polarity time scale. Today, many parts of change crustal and mantle structure and infl uence jor publications by The Geological Society of
the time scale can be calibrated with preci- landscape evolution and global climate patterns, London (Harland et al., 1964) and subsequently
sions approaching less than 0.05%. Some and assess the temporal relations among magma- Cambridge University Press (Harland et al.,
notable time intervals for which collabora- tism, fl uid-rock interaction, and base/precious 1982, 1990; Gradstein et al., 2004; Ogg et al.,
tive, multifaceted efforts have led to dra- metal mineralization in many settings. A key re- 2008) and Elsevier (Gradstein et al., 2012). The
matic improvements in our understanding quirement is establishing accurate ages of rocks current Geological Society of America Geologic
of the character and temporal resolution that are directly associated with or bracket a geo- Time Scale (Fig. 1) incorporates information
of key evolutionary events include the Tri- logic event or process. With efforts to estimate presented in the International Commission on
assic-Jurassic, Permian-Triassic, and Neo- the chronology of geologic events beginning well Stratigraphy’s International Chronostratigraphic
proterozoic-Phanerozoic boundaries (or over two centuries ago, this was commonly ac- Chart (Cohen et al., 2012) and in Gradstein
transitions). In developing the current Geo- complished by establishing a relative geologic et al. (2012). Numerical dates used for bound-
logical Society of America Time Scale, we time scale using the ranges of fossils and strati- ary positions are from Gradstein et al. (2012).
have strived to maintain a consistency with graphic relationships. Beginning in the early The Geological Society of America (GSA) does
efforts by the International Commission on twentieth century, the relative geologic time scale not directly “maintain” an international geo-
Stratigraphy to develop an international was calibrated using numerical information. logic time scale. Rather, the society provides a
geologic time scale. A geologic time scale is the ordered com- geologic time scale in a concise, logically orga-
Although current geologic time scales are pilation of numerical ages and relative age de- nized and readable format that is largely based
vastly improved over the fi rst geologic time terminations based on stratigraphic and other on the work of the International Commission on
scale, published by Arthur Holmes in 1913, principles. Numerical ages, formerly called Stratigraphy (ICS) and related groups and pub-
we note that Holmes, using eight numeri- “absolute ages” (Holmes, 1962), form a chrono- lications. GSA follows the work and recommen-
cal ages to calibrate the Phanerozoic time metric time scale typically expressed in thou- dations of these groups in promoting a better
scale, estimated the beginning of the Cam- sands (ka) or millions (Ma) of years; relative understanding and use of geologic time through
brian Period to within a few percent of the ages form a chronostratigraphic time scale. A its time scale. Many of these organizations in-
currently accepted value. Over the past 100 geologic time scale is an invaluable tool for geo- clude geoscientists who are GSA members or
years, the confl uence of process-based geo- scientists investigating virtually any aspect of members of the associated societies of GSA.
logical thought with observed and approxi- Earth’s development, anywhere on the planet,
mated geologic rates has led to coherent and and at almost any time in Earth’s history. History of Chronometric-
quantitatively robust estimates of geologic This paper describes the history of the devel- Chronostratigraphic Geologic Time Scales
time scales, reducing many uncertainties to opment of the Geological Society of America
the 0.1% level. Geologic Time Scale and provides a brief his- “For it was evident to me that the space between
tory of geologic time scales and their compo- the mountain ranges, which lie above the City of
†E-mail: jdwalker@ku.edu nents. We also discuss important advances made Memphis, once was a gulf of the sea, like the regions
GSA Bulletin; March/April 2013; v. 125; no. 3/4; p. 259–272; doi:10.1130/B30712.1; 1 fi gure; 1 table.
For permission to copy, contact editing@geosociety.org
259
© 2013 Geological Society of America
Walker et al.
.
BDY (Ma)541635 850 1000 1200 1400 1600 1800 2050 2300 2500 2800 3200 3600 4000
AGES
MMIAN CIAN
NIAN ASIAN Y HERIAN A
PERIOD O T
YOGENIANT STENIAN ECT A RHY SIDERIAN
EDIACARANCR CAL ST OROSIRIAN e shown
RA
E ALEO- the Cenomanian
NEOPRO- ALEOPRO- MESO-ARCHEAN P ARCHEAN
TEROZOIC MESOPRO-TEROZOIC P TEROZOIC EOARCHEAN
NEOARCHEAN ozoic Eon. Names of
PROTEROZOIC ARCHEAN ka) for om 0.13 to 0.01 Ma.
ositions follow Gradstein but only two arr
PRECAMBRIANEON HADEAN
AGE(Ma) 750 1000 1250 1500 1750 2000 2250 2500 2750 3000 3250 3500 3750 4000 ages,
1
PICKS(Ma)252254260265269272279290296299304307315323331347 359 372 383388393 40841 419423426427430433439441444445453458467470478485490494497501505509 514521529 541
NIAN 0
GE A TIAN IBIAN UNIAN
A TINSKIAN YCHIAN AGE 1A AGE 5AGE 4AGE 3AGE 2T e the Eras of the Phaner
WORDIANROADIANASSELIANGZHELIAN VISEAN MENNIAN EMSIAN LUDFORDIANGORSTIANHOMERIANTELAERONIANHIRNANTIANKAFLOIANPDRUMIAN
CAPIT KUNGURIANARSAKMARIANKASIMOVIANMOSCOVIANOURNAISIANAFRASNIANGIVETIANEIFELIANPRAGIANSHEINWOODIANRHUDDANIANSANDBIANDAPINGIANGUZHANGIANFOR
CHANGHSINGIANWUCHIAPINGIAN BASHKIRIAN T F LOCHKOVIAN DARRIWILIANTREMADOCIANJIANGSHANIAN
SERPUKHOVIAN
Y Y Y Y Y Age estimates of boundary pom 0.78 to 0.13 Ma, and Late f
gian lian TE TE TE TE
Lopin-Guada-lupianCisura-LAMIDDLEEARLLA MIDDLEEARL LA MIDDLE EARL PRIDOLILUDLOWLLANDO-VERLAMIDDLEEARLFURON-GIANEpoch 3Epoch 2TERRE- ounded to one decimal place (100
EPOCH WENLOCK NEUVIAN
ANIANV SIPPIAN
ALEOZOIC PERMIAN PENNSYL- MISSIS- DEVONIAN SILURIAN ORDOVICIAN CAMBRIAN
P PERIOD CARBONIFEROUS The Pleistocene is divided into four
AGE(Ma) 260 280 300 320 340 360 380 400 420 440 460 480 500 520 540
3 6 1 4 9 5 2 7 6 8 0 4 3 1 9 1 1 2
PICKS(Ma)66.072.1 83.686.389.893.9100 11 12 13 13 13 14 15 15 16416161717 18 19 1920 209 228 23724 24725025 ovisional.
e-Cenomanian, and re pr
ANIANONIAN om 1.8 to 0.78 Ma, Middle fr
AGE ANGIAN the pr
ALBIAN APTIAN LANGINIAN THONIAN OARCIAN NORIAN ANISIANINDUAN
CAMP SANTCONIACIANTURONIAN BARREMIANA BERRIASIANTITHONIANOXFORDIANCALLOVIANBABAJOCIANAALENIANTRHAETIAN CARNIANLADINIANOLENEKIAN
CENOMANIAN HAUTERIVIANV KIMMERIDGIAN PLIENSBACHIANSINEMURIANHETT The Cenozoic, Mesozoic, and Paleozoic ar
MAASTRICHTIAN .
Y Y Y
TE TE TE (1 Ma) for
EPOCH LA EARL LA MIDDLE EARL LA MIDDLEEARL
ACEOUS CRET JURASSIC TRIASSIC ime Scale
PERIOD ee ages: Calabrian fr
MESOZOIC CHRON.C30C31C32C33 C34 CHANGES CHANGESRAPID POLARITYRAPID POLARITY
ANOM.30313233 34 M0rM1M3M5M10M12M14M16M18M20M22M25M29
MAGNETICPOLARITYHIST
70 80 90 0 est whole number
GEOLOGIC TIME SCALEAGE(Ma) 100 11 120 130 140 150 160 170 180 190 200 210 220 230 240 250
0.011.82.63.65.37.2 1.6
PICKS(Ma) 1 13.8 16.0 20.4 23.0 28.1 33.9 37.8 41.2 47.8 56.0 59.2 61.6 66.0 America Geologic Ted series and stages of the Cambrian ar
ALLIAN NIAN
ONIAN V A TTIAN ONIAN
AGE T T ounded to the near
CALABRIANGELASIANR DANIAN
PIACENZIANZANCLEANMESSINIANOLANGHIAN CHA RUPELIAN BAR LUTETIAN YPRESIAN THANETIANSELANDIAN
T SERRA BURDIGALIANAQUIT PRIABONIAN e r
*
OCENE MIOCENE OLIGOCENE EOCENE ALEOCENEP Geological Society of
EPOCHHOLOCENEPLIOCENE stratigraphic units follow the usage of the Gradstein et al. (2012) and Cohen et al. (2012).
PLEIST e 1. What is shown as Calabrian is actually thr
TER-Y NEOGENE ALEOGENEP ono e.
CENOZOICPERIODQUANAR Figurchret al. (2012) but arto Pleistocene interval. Numberher
. CHRONC1C2 C3 C4 C5 C6 C7 C8 C9 1 C21 C23 C26
C2A C3A C4A C5A C5BC5CC5DC5EC6AC6BC6CC7A C10C1 C12 C13C15C16C17C18C19 C20 C22 C24 C25 C27C28C29C30
ANOM.122A 3 3A 4 4A 5 5A 5B5C 5D5E6 6A6B 6C77A8 9 1011 12 13151617 18 19 20 21 22 23 24 25 26 27 28 29 30
MAGNETICPOLARITY.HIST
(Ma) 5 10 15 20 25 30 35 40 45 50 55 60 65
AGE
260 Geological Society of America Bulletin, March/April 2013
The Geological Society of America Geologic Time Scale
about…Ephesos and the Plain of the Maiander, if it The discovery of radioactivity, and, more In his fi rst edition of On the Origin of the Spe-
be permitted to compare small things with great. And specifi cally, the relation between radioactive cies by Means of Natural Selection, Charles
small these are in comparison, for of the rivers, which parent elements and their intermediate and ulti- Darwin provided a crude estimate of the age
heaped up the soil in those regions none is worthy to mate daughter products through a fundamental of Earth of several hundred million years
be compared in volume with a single one of the mouths
of the Nile, which has fi ve mouths.” half-life of radioactive decay, was the seminal based on both geology and his assumption of
—Herodotus, likely the world’s fi rst geologist, event that led to establishing the numerical phyletic gradualism. Interestingly, estimates
fi fth century B.C., in his Histories, 2.10.0-2. ages of geologic materials and ultimately de- of the duration of the Phanerozoic incorporat-
veloping the fi rst chronometric time scale. The ing radiometric ages in the work of Holmes
The quest to understand geologic time has fi rst attempt was by Arthur Holmes (1913) in (1913) at ~550 Ma (inferred from Holmes’ fi g.
been integral to the geosciences for over 200 his book The Age of the Earth. Holmes (1913, 17) and Barrell (1917, p. 892) at ~552 Ma are
years. James Hutton fi rst formally presented the Chapter X) extensively reviewed the early well within a few percent of the currently ac-
scientifi c hypothesis that Earth is ancient in a methods of geochronology using U as the cepted value.
reading at the Royal Society of Edinburgh on parent element. Work at that time showed that The time scale was greatly refined by
4 April 1785. He concluded his revolutionary the decay of U produced He as a by-product Holmes in the second edition of his book
text, Theory of the Earth, which was based on and probably had as its ultimate daughter the (Holmes, 1937). At that time he used almost
his lectures to the Royal Society of Edinburgh element Pb. The half-life of U and thus pro- 30 U-He, U-Pb, and Th-Pb age determina-
and published in 1788, with the now-famous duction rates of these elements were roughly tions. Because of the recognition of multiple
and often-quoted sentence concerning the natu- established by the time of Holmes’ (1913) isotopes, geochronology relied not just on de-
ral history of the Earth: “The result, therefore, publication. At the time of Holmes’ work, termining the parent-daughter ratio, but in the
of our present enquiry is, that we fi nd no ves- it was simply the ratio of U to He or Pb that cases of the U-Pb and Th-Pb decay systems,
tige of a beginning,—no prospect of an end” was measured—the discovery that elements the actual atomic mass and, to some extent,
(p. 304). His work in part inspired the great had multiple isotopes was reported separately the isotope ratios of the daughter products
advances in the geosciences over the following in the same year (Soddy, 1913; Thomson, (Holmes, 1937, Chapter V). Holmes continued
century, over many parts of the world, and, after 1913). Holmes (1913) reviewed all available to refi ne the time scale over the next 25 years.
the discovery of radioactivity, prompted the ini- age determinations for U-Pb and U-He that Concurrently, analytical methods increased in
tial attempts to quantify the age of the planet had bearing on tying numerical age estimates their precision while the number of elements
Earth and geologic time. to meaningful geologic ages. In this effort, and thus minerals used for isotopic age deter-
The fi rst attempts by geologists to quantify he noted that U-He age estimates were typi- minations increased. For example, Kulp (1961)
the chronostratigraphic time scale and to es- cally too young, represented minimum ages, primarily incorporated K-Ar dates as numerical
tablish some bounds on the age of Earth fall and were most useful for younger (Cenozoic) estimates for his compilation of the time scale.
under the category of “hourglass” methods. The rocks. Holmes established the pre-Cenozoic A subsequent symposium in honor of Holmes
two most important of these involved consid- time scale using a total of fi ve U-Pb determina- resulted in a major effort toward developing a
erations of the thickness of sedimentary strata tions. Of these, only three were from rocks of geologic time scale by a broader community
and the salinity of the oceans. Both of these ap- Phanerozoic age: an age of 340 Ma for the end of geoscientists (Harland et al., 1964). In the
proaches relied on using estimated rates of geo- of the Carboniferous, 370 Ma for the end of the symposium volume, the authors reviewed all
logic processes to establish a more quantitative Devonian, and 430 Ma for the end of the Ordo- of the signifi cant aspects of and developments
time scale. For the fi rst hourglass, the rates of vician. Ultimately, Holmes used fi ve U-He and with the geologic time scale from numerical
sediment accumulation were compared to thick- fi ve U-Pb dates to calibrate his geologic time dates, to hourglass methods, and stratigraphic
nesses of sedimentary strata of known chrono- scale. The ages of other parts of the chrono- constraints. In all, over 300 dates were used to
strati graphic age to compute the duration of strati graphic time scale not covered by avail- construct a Phanero zoic time scale.
deposition of the rock unit. The second method able age estimates were approximated by using Over the next 20 years, the standardization
relied on understanding the input from streams compilations of sediment thicknesses. Holmes of isotopic decay constants by Steiger and Jäger
to the oceans to compute an overall age for the concluded this chapter by stating (p. 165): (1977) and major improvements to the chrono-
oceans. In both cases, these were very rough strati graphic time scale, fostered by the Inter-
approximations of the duration of processes, “Most of the available evidence drawn from radio- national Union of Geological Sciences (IUGS)
because the estimates of rates were inexact. active minerals has now been passed in review. As yet through the International Commission on Stra-
Importantly, these estimates indicated that the it is a meager record, but, nevertheless, a record brim- tigraphy (ICS) and its subcommissions, greatly
ful of promise. Radioactive minerals, for the geologist,
Earth was much older than was accepted at are clocks wound up at the time of their origin. After advanced the geologic time scale. In particular,
the time. This was also true of estimates by a few years’ preliminary work, we are now confi dent the efforts of the ICS established worldwide
Thomson (see below). Remarkably, during the that the means of reading these time-keepers is in our standard defi nitions of the relative geologic
second half of the nineteenth century, prior to possession. Not only can we read them, but if they time scale. ICS has and continues to establish
the discovery of radioactivity, a number of have been tampered with and are recording time in- global chronostratigraphic standards known as
correctly, we can, in most cases, detect the error and
workers recognized astronomically forced sedi- so safeguard ourselves against false conclusions.” global boundary stratotype section and points
mentary deposits and used them as a means to (GSSPs) (see following discussion). The work
calibrate geologic time (reviewed in Hilgen, Besides establishing a rough time scale, this of IUGS and ICS has culminated in more re-
2010). The fi rst attempts were built on the astro- work was radical in that it expanded the age of fi ned geologic time scales for the Phanero-
nomical theories for ice ages, and they used Earth beyond any previous estimate. Up until zoic published by Harland et al. (1982), Odin
eccen tricity maximums to tune deposits of the that time, the particularly infl uential work of (1982), and numerous other authors. The initial
last glaciation but were also tuning Miocene and Lord Kelvin had placed an upper limit of ca. GSA time scale (Palmer, 1983) relied heavily
Cretaceous sedimentary rocks (Hilgen, 2010). 40 Ma on the age of Earth (Thomson, 1865). on the contributions by these authors. A major
Geological Society of America Bulletin, March/April 2013 261
Walker et al.
treatment, including the statistical assessment scale that was North American–centered but RECENT ADVANCES AND THEIR
of uncertainties of the estimated boundary ages, with more far-reaching consequences. With IMPACT ON THE GEOLOGIC
was presented by Harland et al. (1989). Con- progress of the DNAG project and considerable TIME SCALE
siderable progress was made over the next 15 changes in chronostratigraphic nomenclature
years, with the next major and comprehensive taking place internationally, refi nements of the Here, we review some of the key advances
revision to the geologic time scale published time scale were inevitable, and updated versions that have led to changes and refi nements of the
by Gradstein et al. (2004). This publication were published as GSA’s 1999 Geologic Time geologic time scale. These include, of course,
fully integrated the advances made in global Scale (Palmer and Geissman, 1999) and 2009 changes in stratigraphic and geochronologic
stratigraphy through the work of the ICS, in- Geologic Time Scale (Walker and Geissman, approaches that are at the heart of a combined
cluding astrochronology, and took advantage 2009). What set the original DNAG time scale chronometric/chronostratigraphic time scale,
of considerably more precise radioisotope apart from earlier versions of time scales pub- but also astrochronology, which is revolution-
40 39 lished in textbooks and summary papers was izing the time scale effort, chemostratigraphy
geo chronol ogy including Ar/ Ar step heat-
ing and single-crystal laser methods as well as the overt application of integrated stratigraphic and related rock magnetic stratigraphy, and the
U-Pb zircon dating methods. and magnetostratigraphic data sources (includ- geomagnetic polarity time scale (see Table 1 for
ing both relative and numerical age dating tech- brief descriptions of these methods).
History of the Geological Society of America niques used to assemble the time scale) and the
Geologic Time Scale style of presentation. Advances in Stratigraphy—The
The rationale behind and history of the work International Commission on Stratigraphy
Besides being the 125th anniversary of the to develop the fi rst Geological Society of Amer-
Geological Society of America, 2013 marks ica Geologic Time Scale itself were described Leading the way in the advancement of
the 30th anniversary of the Geological Soci- by Palmer (1983) and Walker and Geissman chronostratigraphic information is the Inter-
ety of America Geologic Time Scale, the 50th (2009). Allison (“Pete”) Palmer was the Cen- national Commission on Stratigraphy, including
anniversary of the publication of the Vine- tennial Science Program Coordinator for GSA’s the many subcommissions formed by the ICS.
Matthews-Morley-Larochelle hypothesis that DNAG project, and was charged with compil- These groups actively promote the acquisi-
marine magnetic anomaly patterns adjacent to ing the results of the efforts of the Time Scale tion and dissemination of information vital to
mid-ocean ridges were a record of the polarity Advisory Committee, consisting of Z.E. Peter- making informed decisions about stratigraphic
reversal history of the geomagnetic fi eld and man, J.E. Harrison, R.L. Armstrong, and W.A. boundary positions and numerical calibrations
thus that the ridges were sites of ocean-fl oor Berggren. Pete Palmer had a clear passion for of those positions. A major goal of the ICS is
spreading (Vine and Matthews, 1963), the 100th the work and devoted considerable energy to the to develop an unambiguous, globally applica-
anniversary of the fi rst geologic time scale pre- project. An innovative contribution was the at- ble nomenclature for geologic time units and
sented by Holmes (1913), and the 225th anni- tempt to organize the time scale onto a single their chronostratigraphic equivalents, a com-
versary of Hutton’s Theory of the Earth (1788). 8.5 by 11 inch sheet of paper using the “tools mon language in which concepts are identical
The GSA time scale grew out of the Decade of the trade” in those days—Mylar, zipatone for all localities across the globe. This effort is
of North America Geology (DNAG) project letters, a Leroy lettering machine, and a lot of part of a larger project to develop unambiguous
(Palmer, 1983), which had as its goal a syn- patience. His efforts resulted in a unique lay- chronostratigraphic units for the entire geologic
thesis of the geology then known of the North out for the time scale, in which each era of the time scale (Gradstein et al., 2004, 2012; Ogg
American continent. Before that time, geologic Phanerozoic, and all of the Precambrian, was et al., 2008; Cohen et al., 2012). The main driv-
information on North America had been scat- given identical column length. Scaling of the ing forces for the refi nements and restructuring
tered through the literature. With a few excep- magnetic polarity time scale to fi t this format to the chronostratigraphic side of the geologic
tions (e.g., Eardley, 1951; King, 1959), little required numerous trials. When Jim Clark, the time scale are changes in philosophy about
in the way of comprehensive summaries of the director of publications for GSA, suggested that how stratigraphic units are defi ned, as well as
geology of North America existed. A necessary the geologic time scale should also be published high-resolution studies utilizing chronostrati-
prerequisite for discussing the geology of the as a pocket wallet card, the effort became even graphic proxies, including biostratigraphy,
continent was a common chronostratigraphic more challenging. A key concern by both the ad- chemostratigraphy, magnetostratigraphy, rock
vocabulary and internally consistent sense of visory committee and Palmer was whether the magnetic stratigraphy, and astrochronology/
both the relative and numerical ages of geo- time scale should be North American centric or orbital tuning.
logic and evolutionary events. This provided global. The advisory committee and Palmer rec- Over the past several decades, different strati-
ages of specifi c stratigraphic units addressed in ognized the likely pitfalls and hurdles associated graphic philosophies have been applied to defi -
the multivolume compilation involving scores with trying to develop a global time scale and nitions of chronostratigraphic units. Defi nitions
of authors that resulted from the DNAG proj- set as a goal trying to develop a “common vo- in many places around the world were com-
ect. The DNAG project was the fi rst detailed cabulary” for North America, still a suffi ciently monly based on the unit-stratotype concept, in
synthesis of North American geology follow- great challenge. In discussions with Palmer which a type section serves as the standard of
ing widespread acceptance of plate-tectonic (2012, personal commun.), he emphasized the reference for the defi nition and characterization
theory and provided a sense of the evolution diffi culties with defi ning the base of the Ordovi- of a unit (Salvador, 1994). The lower and upper
of the continent in the context of global events. cian (which has more recently been solved with boundaries of a unit are normally specifi ed by
Before the DNAG compilation could proceed the defi nition of an Ordovician GSSP). The fi - reference to a type section. Beginning in the
very far, a uniform geologic time scale had to nal product (Palmer, 1983) served the intended 1970s, the concept of a boundary-stratotype was
be adopted, and it was the fi rst major product purpose well. “There were no naysayers, and no promoted. Under this concept, only the base of a
of the DNAG initiative. This inevitably led to major disagreements,” stated Palmer. “The basic chronostratigraphic unit is formally defi ned, and
advancement of a version of the geologic time subdivisions were all accepted.” it is marked by a point in strata (Salvador, 1994),
262 Geological Society of America Bulletin, March/April 2013
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