IMPROVING THE GEOLOGIC TIME SCALE (Invited Presentation)

Arthur Holmes, the Father of the Geologic Time Scale once wrote: “To place all the scattered pages of earth history in their proper chronological order is by no means an easy task”. Ordering these scattered and torn pages requires a detailed and accurate time scale.

Geologic Time Scale 2012 (GTS2012) is more precise and more accurate than GTS2004. Precambrian now has a detailed proposal for chronostratigraphic subdivision instead of an outdated and abstract chronometric one. Of the 100 chronostratigraphic units in the Phanerozoic 63 now have formal definitions.

GTS2012 builds on over 265 carefully calibrated radiogenic age dates. Detailed age calibrations now exist between radiometric methods and orbital tuning, making 40Ar-39Ar dates 0.64% older and more accurate; U-Pb dating is much refined. Although radiometric ages can be more precise than zonal or fossil event assignments, the uneven spacing and fluctuating accuracy and precision of both radiometric ages and zonal composite scales demands intricate stratigraphic reasoning, quantitative biostratigraphic methods and clever statistical and mathematical techniques to calculate the geologic time scale.

Bases of Paleozoic, Mesozoic and Cenozoic are bracketed by analytically precise ages, respectively 541 ± 0.63, 252.16 ± 0.5 and 66.03 ± 0.05 Ma. High-resolution, direct age-dates now also exist for base-Carboniferous, base-Permian, base-Jurassic, base-Cenomanian and base-Eocene. Relative to GTS2004, 26 of 100 time scale boundaries have changed age, of which 14 have changed more than 4 Ma, and 3 (in Middle to Late Triassic) between 6 and 12 Ma. There is much higher stratigraphic resolution in Late Carboniferous, Jurassic, Cretaceous and Paleogene, and improved integration with stable isotopes stratigraphy. Cenozoic and Cretaceous have a refined magneto-biochronology.

An astronomical tuning solution now exists for the whole of the Cenozoic. Ages and durations of Neogene stages derived from the orbital tuning are considered to be accurate to within a precession cycle (~20 kyr) assuming that all cycles are correctly identified. Paleogene dating combines orbital tuning, radiometric and C-sequence splining, hence stage ages uncertainty is larger; it varies between 0.2 and 0.5 myr.