A GEOLOGIC TIME SCALE 2004

The geologic time scale is the framework for deciphering the history of the Earth. The Geologic Time Scale 2004 program under the guidance of Felix Gradstein, Chair of the International Commission on Stratigraphy (ICS, under IUGS), had three components:

1. Divisions of Earth History: A concise summary was assembled of international definitions (“Global boundary Stratotype Section and Point” or GSSP) of divisions of geologic time (e.g., the Miocene Epoch, the Paibian Stage of Cambrian, etc.).

2. High-Resolution Stratigraphy: An integrated stratigraphy for each component period was compiled of biostratigraphic zonations, chemostratigraphy variations, magnetic polarity zones, major sea-level excursions and other events or trends that are important for global and regional correlation.

3. Absolute Age Scale: This detailed chronostratigraphic framework was scaled to absolute time. Depending upon the time interval, the derivation of the absolute time scale merged astronomical (Milankovitch) cycles, selected high-resolution radiometric ages, seafloor spreading models, strontium-isotope segments, and different geomathematical interpolation methods (spline-fitting, CONOP optimization, graphical correlation, etc.). The emphasis was on quantitative scaling of events within each stage, not just determining the age of the stage boundary. An important aspect is realistic uncertainties on all ages.

Computed ages (rounded to nearest 1 myr) for the BASES of geologic periods are: 23 Ma=Neogene; 65 Ma=Paleogene

145 Ma=Cretaceous; 200 Ma=Jurassic; 251 Ma=Triassic

299 Ma=Permian; 359 Ma=Carboniferous; 416 Ma=Devonian

444 Ma=Silurian; 488 Ma=Ordovician; 542 Ma=Cambrian

~600 Ma=Ediacaran (youngest period of Precambrian)

The compilation of GTS2004 involved a large number of specialists, and the book A Geologic Time Scale 2004 (~500 pages with summary poster, Cambridge University Press) will be published in summer, 2004. A summary diagram and tables of GSSPs can be downloaded from www.stratigraphy.org (ICS website), and the source data will eventually be accessed through the CHRONOS database system.