GSA Geologic Time Scale (v. 4.0)

Commentary

J.D. Walker1, J.W. Geissman2, S.A. Bowring3, and L.E. Babcock4, Compilers

  1. 1 Dept. of Geology, 1475 Jayhawk Boulevard, University of Kansas, Lawrence, Kansas 66045, USA; jdwalker@ku.edu
  2. 2 Dept. of Geosciences, ROC 21, University of Texas at Dallas, 800 West Campbell Road, Richardson, Texas 75080, USA; geissman@utdallas.edu; and Dept. of Earth and Planetary Sciences, MSC 03 2040, Northrop Hall, 1 University of New Mexico, Albuquerque, New Mexico 87131-0001, USA; jgeissatunm.edu
  3. 3 Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge Massachusetts 02139, USA; sbowring@mit.edu
  4. 4 Department of Geology, Lund University, SE-223 62 Lund, Sweden; and School of Earth Sciences, Ohio State University, Columbus, Ohio 43210, USA; loren.babcock@geol.lu.se

2013 marks the 30th anniversary of the first Geological Society of America Geologic Time Scale (Palmer, 1983), the 100th anniversary of the first geologic time scale based on radioisotopic dates (Holmes, 1913), and the 125th anniversary of the Geological Society of America. Here we briefly review the development of the GSA Geologic Time Scale. A more complete treatment can be found in Walker et al. (2013).

The effort to prepare the first Society time scale was concurrent 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 geologic time scale” (Palmer, 1983). The Time Scale Advisory Committee consisted of Z.E. Peterman (chairman), J.E. Harrison, R.L. Armstrong, and W.A. Berggren. Allison (Pete) Palmer, as Centennial Science Program Coordinator for GSA, was given the charge of compiling the Advisory Committee’s efforts. The goal of the then unique layout of the GSA/DNAG Geologic 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., including uncertainties in ages of chronostratigraphic boundaries) compilation of our current knowledge of geologic time.

Work on this GSA time scale started in 2012 in conjunction with preparation by the compilers of an article on the Geologic Time Scale for the GSA Bulletin (Walker et al., 2013). This effort is a revision of the 2009 GSA Geologic Time Scale (Walker and Geissman, 2009). Revisions focused on three aspects. The first was to update names and boundaries to capture changes presented in Gradstein et al. (2012) and Cohen et al. (2012) to reflect recent efforts of the many working groups of the International Commission on Stratigraphy (ICS). Second, we updated the boundary ages using these same sources. Lastly, the magnetic polarity time scale was modified. Significant changes from the previous GSA Geologic Time Scale principally reflect adjustments to the Cenozoic, including: (1) dropping the use of Tertiary, which previously was considered a period that was the same as the combined Paleogene and Neogene; and (2) dropping the informal early, middle, and late divisions for the Paleocene, Eocene, Oligocene, and Miocene.

The compilers plan to keep the GSA Geologic Time Scale as up to date as possible. For that reason, we are moving away from the previous practice of putting a calendar date on the time scale (i.e., 2009 Geological Time Scale) but rather we have adopted a “versioning” approach. The current GSA Geologic Time Scale is version 4.0 as it is the fourth one produced in this series. It is our opinion that the geological community no longer views the time scale as static, but one that should evolve as new research is done. Establishing new stratigraphic datums, determining new dates for boundaries, and advances in other aspects of geologic age determinations occur often and should be reflected in a more dynamic approach to time scale presentation with yearly updates. The time scale will be given a version number and posting date, and be available at no cost online. Previous versions will also remain available at www.geosociety.org.

GSA encourages the use of the time scale, boundary ages, and its terminology in all publications; strict enforcement, however, is not planned. Constructive comments are encouraged and should be addressed to editing@geosociety.org.

REFERENCES CITED

  1. Cohen, K.M., Finney, S., and Gibbard, P.L., 2012, International Chronostratigraphic Chart: International Commission on Stratigraphy, www.stratigraphy.org (last accessed May 2012). (Chart reproduced for the 34th International Geological Congress, Brisbane, Australia, 5–10 August 2012.)
  2. Gradstein, F.M., Ogg, J.G., Schmitz, M.D., and Ogg, G.M., editors, 2012, The Geologic Time Scale 2012, vol. 1: Boston, Elsevier, http://dx.doi.org/10.1016/B978-0-444-59425-9.01001-5.
  3. Holmes, A., 1913, The Age of the Earth: London and New York, Harper.
  4. Palmer, A.R., 1983, The Decade of North American Geology 1983 Geologic Time Scale: Geology, v. 11, p. 503–504, doi:10.1130/0091-7613(1983)11<503:TDONAG>2.0.CO;2.
  5. Walker, J.D., and Geissman, J.G., compilers, 2009, Geologic Time Scale: Geological Society of America, doi:10.1130/2009.CTS004R2C.
  6. Walker, J.D., Geissman, J.W., Bowring, S.A., and Babcock, L.E., 2013, The Geological Society of America Geologic Time Scale: GSA Bulletin, doi:10.1130/B30712.1.