PALEOZOIC-PRECAMBRIAN TIME SCALE 2004

The Geologic Time Scale 2004 program under the guidance of Felix Gradstein, Chair of the International Commission on Stratigraphy, reviews international definitions of divisions of geologic time at their "Global boundary Stratotype Section and Point" or GSSPs; integrates stratigraphy (bio-, chemo-, magneto- and sequence-stratigraphy scales); and gives numerical ages for boundaries and major events within each stage. Except for the Carboniferous and Cambrian, which have only a single intra-system GSSP, most Paleozoic stages are defined by GSSPs. The new Ediacaran period of the uppermost Proterozoic was assigned a basal GSSP in Australia in 2004. The boundaries of all other international Precambrian eras and periods are fixed at assigned numerical ages. Most of the Paleozoic time scale is derived by combining selected radiometric ages (mainly U-Pb) with different mathematical interpolation methods (constrained optimization or CONOP, graphical correlation, and/or spline-fitting of integrated biozonations for each period). Utilization of astronomical (Milankovitch) cycles for scaling is still in its infancy within the Paleozoic.

Computed ages (rounded to nearest 1 myr) for the BASES of geologic periods are:

251 Ma=Triassic (base of Mesozoic Era) 444 Ma=Silurian 299 Ma=Permian 488 Ma=Ordovician 359 Ma=Carboniferous 542 Ma=Cambrian (base of Paleozoic Era) 416 Ma=Devonian 630 Ma=Ediacaran (youngest period of Precambrian)

The compilation of GTS2004 is summarized in A Geologic Time Scale 2004 (~600 pages with summary poster, Cambridge University Press, www.cambridge.org). Summary diagrams and tables of GSSPs are on the ICS website (www.stratigraphy.org), and the source data will be accessible through the CHRONOS database system. Coordinators for the Paleozoic-Precambrian time scale include Felix Gradstein, Frits Agterberg Bruce Wardlaw, Vladimir Davydov, Michael House, Mike Melchin, Roger Cooper, Pete Sadler, John Shergold, Andrew Knoll, Wouter Bleeker and Laurence Robb.