This lecture reviews Geologic Time Scale 2004 (Gradstein, Ogg et al., 2004; Cambridge University Press), constructed and detailed by 40 geoscience specialists, and indicates how it will be further refined. Since Geologic Time Scale 1989 by Harland et al., many developments have taken place: (1) Stratigraphic standardization through the work of the International Commission on Stratigraphy (ICS) has greatly refined the international chronostratigraphic scale. In some cases, traditional European-based stages have been replaced with new subdivisions that allow global correlation. (2) New or enhanced methods of extracting high-precision age assignments with realistic uncertainties from the rock record. These have led to improved age assignments of key geologic stage boundaries and other global correlation horizons. (3) Orbital tuning has greatly refined the Neogene, and improved parts of Paleogene and Mesozoic. (4) Statistical techniques of compiling integrated global stratigraphic scales within geologic periods.
Anticipated advances to the Geologic Time Scale during the next 8 years include: a geologically realistic Precambrian scale, formal definition of all Phanerozoic stage boundaries, orbital tuning of polarity chrons and biostratigraphic events for entire Cenozoic and Cretaceous, a detailed database of high-resolution radiometric ages that includes “best practice” procedures, full error analysis, monitor ages and conversions, resolving age dating controversies (e.g., zircon statistics and possible reworking) across Devonian/Carboniferous, Permian/Triassic, and Anisian/Ladinian boundaries, improved and standardized dating of several ‘neglected’ intervals (e.g., Upper Jurassic – Lower Cretaceous, and Carboniferous through Triassic, and detailed integrated stratigraphy for Upper Paleozoic through Lower Mesozoic.
The geochronological science community and ICS are focusing on these issues. A modified version of the time scale to accompany the standardization (boundary definitions and stratotypes) of all stages is planned for 2008 (to be presented at the 33th International Geologic Congress in Oslo), with a totally revised version of GTS available in 2012.