60 April /mAy 2009, GS A ToDAy
J.D. Walker, Department of Geology, 1475 Jayhawk
Boulevard, University of Kansas, Lawrence, Kansas 66045,
USA
J.W. Geissman*, Department of Earth and Planetary Sciences,
MSC 03 2040, Northrop Hall, 1 University of New Mexico,
Albuquerque, New Mexico 87131-0001, USA
Last year marked the 25th anniversary of the first Geologi-
cal Society of A merica Geologic Time Scale (Palmer, 1983).
The effort to prepare the first Society time scale was concur-
rent with the preparation of the 27 volumes of The Geology
of North America to celebrate the Decade of North American
Geology (DNAG). In 1982, an ad hoc time scale advisory
committee was formed by the DNAG steering committee to
encourage “uniformity among DNAG authors in the citation
of numerical ages for chronostratigraphic units of the geo-
logic time scale” (Palmer, 1983). The Time Scale Advisory
Committee consisted of Z.E. Peterman (chairman), J.E. Har-
rison, R.L. Armstrong, and W.A. Berggren. Allison (Pete)
Palmer, as centennial science program coordinator for GSA,
was given the charge of compiling the committee’s efforts.
The goal of the then-unique layout of the GSA-DNAG Geo-
logic Time Scale, with each Phanerozoic era given identical
column length, along with the Precambrian, was to provide a
compact, succinctly organized yet suitably detailed (e.g., in-
cluding uncertainties in ages of chronostratigraphic boundar-
ies) compilation of our current knowledge of geologic time.
In a recent communication to the second author, Pete wrote,
I do remember that I hand-drafted it at 2 or 3 times
publication scale and pasted up the words. I also had to
figure out how to format it so that it would all fit on one
sheet (thus changing the scale for the numerical ages as
things got older). I think the small plastic cards came
later. I also think I consulted with various geochronolo-
gists to see which of several published numerical time
scales was considered the most reliable, so none of the
compilation was original—I just cribbed existing infor-
mation. Fortunately, the stage-level nomenclature was a
bit less messy than it is now…
Just think of how much has changed in 25 years (!), as re-
cent and future modifications to the GSA Geologic Time Scale
have and will continue to be done in a moment, in front of a
com puter.
Work on the current time scale started in 2007, and revision
efforts focused on three aspects. The first was to update
names and boundaries to capture changes documented in
Gradstein et al. (2004) and to reflect more recent results of
the many working groups of the International Commission
on Stratigraphy. Other relevant work was also incorporated as
needed (e.g., new publications on the Triassic time scale).
Second, we updated the boundary ages using whatever
sources were available. Last, the magnetic polarity time scale
was extensively modified, especially for the early Mesozoic,
in particular from the earliest Jurassic to the Permian-Triassic
boundary. Some aspects of the GSA Geologic Time Scale do
not conform to the recommendations of the International
Commission on Stratigraphy. The names “Tertiary” and “Pre-
cambrian” were not dropped on the new time scale. The
Quaternary, the status and boundaries of which are still be-
ing debated, was modified to reflect some of the pending
recommendations. These differences were retained to best
reflect the needs of GSA members and Divisions.
The compilers plan to keep the GSA Geologic Time Scale
more up-to-date by having yearly reviews of important
changes adopted by the International Commission on Stratig-
raphy, new publications, and information made available
through other efforts, such as the Earth-Time Initiative. Loren
Babcock (Ohio State University) will join us as a Geologic
Time Scale compiler for the next revision. In the past, the
time scale has been static for 10 years at a time, but we now
anticipate more frequent updates. The time scale will be giv-
en a posting date and be available online, in both color and
black-and-white versions.
GSA encourages the use of the time scale, boundar y ages,
and its terminology in all publications; strict enforcement,
however, is not planned. The time scale will be kept as cur-
rent as possible. Constructive comments are encouraged and
should be addressed to [email protected].
REFERENCES CITED
Gradstein, F., Ogg , J., Smith, A., et al., 2004, A Geologic Time Scale
2004: Cambridge University Press, 589 p.
Palmer, A.R., 1983, The Decade of Nor th American Geology 1983 Geo-
logic Time Scale: Geology, v. 11, p. 503–504, doi: 10.1130/0091
-7613(1983)11<503:TDONAG>2.0 .CO;2.
Commentary
2009 GSA Geologic Time Scale
