The of North American 1983 Geologic Scale

Compiled by Allison R. Palmer Centennial Program Coordinator Geological Society of America, P.O. Box 9140, , Colorado 80301

Preparation of the 27 synthesis volumes of The Geology of for the Decade of North American Geology Base of Companion to end of (DNAG) is now in progress. In order to encourage uniformity Berggren, W. A., Kent, D. V., and Flynn, J. J., 1984, Appendix, among DNAG authors in the citation of numerical ages for chron- in and the : Geological Society ostratigraphic units of the , an ad hoc Time of London (in press). Scale Advisory Committee was established by the DNAG Steering Base of to base of Committee in 1982. This advisory committee, consisting of Z. E. Harland, W. B„ Cox, A. V., Llewellyn, P. G„ Picton, C.A.G., Peterman (Chairman) and J. E. Harrison, U.S. ; Smith, A. G., and Walters, R., 1982, A geological time scale: R. L. Armstrong, University of British Columbia; and W. A. Cambridge, Cambridge University Press, 128 p. Berggren, Woods Hole Oceanographic Institution, was asked to Base of to base of Barremian (dating and chronostrati- evaluate numerical dating schemes that were either recently pub- graphic correlation of the "M" ) lished or in press and to provide recommendations for the best Kent, D. V., and Gradstein, F. M., 1984, A to Recent numbers to use in preparation of a DNAG time scale. The chart , in Tucholke, B. E., and Vogt, P. R., eds., The on the opposite side of this page was developed from the recom- Western , Volume M of The geology of North mendations of the Time Scale Advisory Committee. America: Boulder, Colorado, Geological Society of America Geochronometric ages (Ma, Ga) assigned to chronostrati- (in press). graphic boundaries are subject to several uncertainties in addi- Note: has been eliminated from the Late tion to those introduced by the numerical dating methods chronostratigraphic scale following Tozer, E. T., 1979, Latest Tri- themselves; boundary stratotypes for many units are not yet assic ammonoid and , western : Geo- chosen, so disagreement exists about exact biostratigraphic place- logical Survey of Canada Paper 79-1B, p. 127-135. ment and correlation of a boundary; and many materials that can Base of to base of be numerically dated are not known in good context with biostrat- Armstrong, R. L., 1982, - time scale igraphic data, so extrapolation to a chronostratigraphic boundary calibration in British Columbia, Canada, in Odin, G. S., ed., is commonly required. Furthermore, with respect to the late Numerical dating in : New York, John Wiley & Mesozoic and the , differing numerical calibrations Sons, p. 509-513. of the magnetic polarity-reversal scale based on differing choices Base of Scythian to base of of scattered isotopically dated tie points, differing interpretations Webb, J. A., 1982, Triassic radiometric dates from eastern Austra- of the positions of biostratigraphic boundaries with respect to the lia: in Odin, G. S., ed., Numerical dating in stratigraphy: New polarity-reversal scale, and uncertainties in the meaning of isotopic York, John Wiley & Sons, p. 515-521. ages derived from glauconies lead to disagreement about ages assigned to some chronostratigraphic boundaries. With these caveats, the numerical ages given in this chart All numerical ages except those for the upper and lower represent interpretations acceptable to the DNAG Time Scale boundaries of the Paleozoic are derived from Harland and others Advisory Committee. The uncertainty bars for Paleozoic and (see above, 1982, p. 52-55). Late numbers are for Mesozoic ages are from data in Harland and others (1982). continentally based ages (N = ""; W = ; Uncertainty bars for the Cenozoic are not available. S = ). The marine-based ages are from Harland and Sources for the numerical ages and for the chronostrati- others (1982, Fig. 5.6). The earlier estimate for the base of the graphic nomenclature are given below. at 570 Ma is retained.

CENOZOIC Berggren, W. A., Kent, D. V., and Van Couvering, J. A., 1984, Harrison, J. E., and Peterman, Z. E., 1982, North American geochronology and ; in Geo- Commission on Stratigraphic Nomenclature, Report 9, Adop- chronology and the geologic record: Geological Society of tion of geochronometric units for divisions of Precambrian London (in press). time: American Association of Bulletin, Berggren, W. A., Kent, D. V., and Flynn, J. J., 1984, v. 66, p. 801-802. geochronology and chronostratigraphy, in Geochronology and the geologic record: Geological Society of London (in press).

GEOLOGY, v. II, p. 503-504, September 1983 503 è DECADE OF NORTH AMERICAN GEOLOGY