Paleonet: International Geological Timescale [SEC=UNOFFICIAL]
Erik
orbulina at gmail.com
Wed Oct 3 18:14:42 UTC 2012
Concerning the discrepancy in absolute ages mentioned earlier....
The age models for most intervals of geologic time rely on international
agreement on stage boundaries (GSSPs) and interpolation of combined
radio-isotopic data and cycle-stratigraphy durations to those boundaries.
The GTS2012 book provides extensive peer-reviewed documentation for each
radio-isotopic date (and its revision to currently accepted monitor
standards, error analysis, biostratigraphic uncertainty, etc.) and
explanation of cycle-stratigraphy and other constraints. In addition, for
stage boundaries that lack GSSPs, there is a discussion of contending
options and why a particular working version and calibration was selected.
We encourage people to read the details behind each age model, estimates of
uncertainty on interpolated ages assigned to stage boundaries (GSSPs or
working definitions) and caveats on such age models. As more stage
boundaries are officially defined, additional quality radio-isotopic dates
are acquired, and cycle-stratigraphy scalings are extended and verified,
then the geologic time scale will have progressively more precise ages for
international divisions.
When the ICS chart was compiled simultaneously with the final
editing of GTS2012 in April, each subcommission was asked to provide their
preferred set of numerical ages. Nearly all subcommissions decided to use
the GTS2012 (Chairs and some other members of all subcommissions had been
sent advance copies of the GTS2012 chapters at the end of summer 2011 for
review).
Unfortunately, the Early Cretaceous part of the ICS chart as
contributed by the subcommission officers did not use the GTS2012 set for
these yet-to-be-defined stages. Instead, there seemed to be an oversight
of using the GTS2008 age set that had been based on an older Ar-Ar monitor
standard. In contrast, the GTS2012 radio-isotopic age controls (and all
other Ar-Ar ages in the GTS2012 databases) have been revised to the Ar-Ar
monitor standards of Kuiper et al. (2008). This factor was one reason that
the former “GTS2008” ages shifted approximately 1 myr older in GTS2012 for
that interval. Cycle stratigraphy of the Aptian interval from Italian
drill cores and additional U-Pb ages imply that the working definition for
the base-Aptian (beginning of Chron M0r) is close to 126.5 Ma. Thanks to
cycle-stratigraphy work, including those cited by Bruno Granier, this
change from the previous 125 Ma as used in GTS2004 to the new 126.6 Ma
cascaded through the age estimates for the younger to-be-defined stage
boundaries. These new estimates of “working definitions” also agree with
the limited recalibrated radio-isotopic ages. Therefore, the ICS version
in this interval should be adjusted. Details are in the geomagnetic and in
the Cretaceous chapters of GTS2012, plus the appendix of radio-isotopic
ages.
The Triassic is beset by a combination of (1) undefined stage
boundaries, (2) lack of unambiguous biostratigraphic-constrained
radio-isotopic ages for the Late Triassic and portions of Early Triassic,
and (3) disputed correlations of the cycle-magnetostratigraphy of the
Newark Basin to magnetostratigraphy from ammonite- or conodont-bearing
marine sections. Therefore, in GTS2012, two contrasting “fits” for Late
Triassic are shown. The selected age model in GTS2012 was influenced by
in-press U-Pb ages from lower Norian provided by Mike Orchard (former chair
of Triassic subcommission of ICS) and by enhanced Rhaetian
magnetostratigraphy and suggested Norian cycle-stratigraphy from marine
sections by Hüsing et al. (*EPSL*, 2011); although Lucas et al. (Earth
Science Reviews, 2012) favor the alternative fit. There is an insightful
set of papers on Triassic magnetostratigraphy (by Hounslow and Muttoni) and
radio-isotopic dating (by Mundil et al.) in “The Triassic Time Scale”
(Lucas et al., Geol. Soc. Spec. Publ., 2010). The ICS version of ages
seems to be a compromise, although that set would not lend itself to either
of those magnetostratigraphic-fit options. The beginning of the Olenekian
of Early Triassic differs between GTS2012 and ICS by about 1 myr, but
cycle-magnetostratigraphy in the Germanic Basin (Szurlies, Menning,
Backmann, Kozur, etc.) and U-Pb ages from ammonite-bearing beds in Nevada
(Schmitz et al., in prep.) favor the GTS2012 age relative to
base-Triassic. However, the age will also depend on the future decision
for the placement and inter-regional correlation of the Induan-Olenekian
stage boundary!
The continued work of ICS to establish GSSPs for the
approximate 30 remaining undefined stage boundaries is the important first
step to establishing an international scale and its age model. The
accomplishments of many active groups in radio-isotopic dating, in cycle
stratigraphy and in statistical interpolation methodology will enable a
much more precise age model with realistic uncertainties for those stage
boundaries. Until that time, it is important that the assumptions, working
definitions, and data constraints for any age model be properly documented
and open to alternative interpretations.
-- Jim Ogg
(Purdue University; co-coordinator for GTS2012)
[For GTS2012 documentation, GSSP and data tables, radiometric
recalibrations and other chronostratigraphic information: *Geologic
TimeScale 2012* (2 volumes, with about 250 full-color figures, 1100 pages,
50-contributors coordinated by Felix Gradstein, James Ogg, Mark Schmitz and
Gabi Ogg; published August 2012; and can be ordered from either Elsevier
(for $115):
http://store.elsevier.com/The-Geologic-Time-Scale-2012-2-Volume-Set/isbn-9780444594259/
or from Amazon.com (for $100):
http://www.amazon.com/Geologic-Time-Scale-2012-2-Volume/dp/0444594256 .
Direct contributors include: Gradstein, F.M, Ogg, J.G., Schmitz, M.D., Ogg,
G.M., Agterberg, F.P., Anthonissen, D.E., Becker, T.R., Catt, J.A., Cooper,
R.A., Davydov, V.I., Gradstein, S.R., Henderson, C.M., Hilgen, F.J.,
Hinnov, L.A., McArthur, J.M., Melchin, M.J., Narbonne, G.M., Paytan, A.,
Peng, S., Peucker-Ehrenbrink, B., Pillans, B., Saltzman, M.R., Simmons,
M.D., Shields, G.A., Tanaka, K.L., Vandenberghe, N., Van Kranendonk, M.J.,
Zalasiewicz, J., Altermann, W., Babcock, L.E., Beard, B.L., Beu, A.G.,
Boyes, A.F., Cramer, B.D., Crutzen, P.J., van Dam, J.A., Gehling, J.G.,
Gibbard, P.L., Gray, E.T., Hammer, O., Hartmann, W.K., Hill, A.C., Paul F.
Hoffman, P.F., Hollis, C.J., Hooker, J.J., Howarth, R.J., Huang, C.,
Johnson, C.M., Kasting, J.F., Kerp, H., Korn, D., Krijgsman, W., Lourens,
L.J., MacGabhann, B.A., Maslin, M.A., Melezhik, V.A., Nutman, A.P.,
Papineau, D., Piller, W.E., Pirajno, F., Ravizza, G.E., Sadler, P.M.,
Speijer, R.P., Steffen, W., Thomas, E., Wardlaw, B.R., Wilson, D.S., and
Xiao, S., plus many other advisers, consultations, and reviewers.
The forthcoming GSA will have a series of posters and talks for each
time interval by the different groups of contributors.]
On Wed, Oct 3, 2012 at 7:40 AM, Martin Farley <mbfarley at hal-pc.org> wrote:
> First, given the uncertainties in the age estimates for these stage
> boundaries, the differences are not huge (or even important).
>
> Second, there will always be a conflict between having a standard that is
> usable within an organization (or more generally) and having information
> that is completely up to date. My advice is to pick a standard (e.g., that
> used by TS Creator) and not worry about improved information until the
> standard is updated. The alternative (updating the timescale on an ad hoc
> basis) is chaos.
>
>
> On Wed, October 3, 2012 2:27 am, John.Laurie at ga.gov.au wrote:
> >
> > The Geological Timescale in the International Chronostratigraphic Chart
> on
> > the ICS website
> > (http://www.stratigraphy.org/ics%20chart/ChronostratChart2012.pdf)
> differs
> > from that in the two volume "A Geological Timescale" released at the IGC
> > in August. While most of the numerical ages are the same, some differ
> > considerably. Some of these are listed below:
> >
> > Stage ICS Book
> > Hauterivian 132.9 133.9
> > Barremian 129.4 130.8
> > Aptian 125.0 126.3
> > Carnian 235.0 237.0
> > Rhaetian 208.5 209.5
> > Olenekian 251.2 250.0
> >
> > While these differences are not huge, they immediately cause problems
> when
> > the object across an organisation like mine is to have a standard
> > timescale to which everyone adheres, as the ICS timescale is the one
> > everyone has on their wall. However, to generate many of our
> stratigraphic
> > diagrams/charts we use Timescale Creator (which follows the book). Can we
> > please have a consensus? If not, we will be back where we were in the
> > 1990s when everyone had their own version.
> >
> >
> > -----------------------------------------------------------------
> > Dr John R. Laurie
> > Energy Division
> > GEOSCIENCE AUSTRALIA
> > GPO Box 378
> > Canberra ACT 2601
> > Australia
> >
> > Tel: (02) 6249 9412; Fax: (02) 6249 9980
> > E-mail: John.Laurie at ga.gov.au
> > Street Address:
> > Cnr Jerrabomberra Avenue & Hindmarsh Drive
> > Symonston ACT 2609
> > ABN 80 091 799 039
> > ----------------------------------------------------------------
> >
> >
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>
>
> Truly yours,
>
> Martin Farley
> mbfarley at sigmaxi.net
>
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