DEVONIAN TIME SCALE

All seven Devonian stages have been defined by GSSPs, but revisions of the base of the Emsian and of the Devonian-Carboniferous boundary are currently underway. Since some of the stages are much longer than those in other systems, and since there are important times of second-order global extinction and sedimentary perturbations, giving natural breaks within stages, the Subcommission on Devonian Stratigraphy is currently working on formal substage definitions. The detailed and high-resolution conodont–ammonoid zonation for the Devonian with over 60 zones is in stark contrast to the less than eighteen radiometric age dates employed in Devonian time scale building, some of which have ± 6 myr uncertainty.

The Devonian age model in GTS2012 is derived from a cubic spline that integrates (1) the stratigraphic positions with error bars for all age dates and (2) a subjective chronostratigraphic scale that incorporates estimates of successive relative spans of conodont zones within each stage/substage. The Givetian stage does not contain any radiometric dates; therefore, its span was estimated from a straight-line interpolation between the closest points along the scale and no confidence limits are calculated for this interval.

The Silurian-Devonian boundary is at 419.2 ± 3.2 Ma, and the Devonian-Carboniferous boundary is at 358.9 ± 0.4 Ma. The Middle Devonian begins at 393.3 ± 1.2 Ma, and Late Devonian at 382.7 ± 1.0 Ma. The Pragian is the shortest stage in the Devonian at 3.2 ± 1.2 myr, and the (unrevised) Emsian is the longest stage (14.3 ± 2.6 myr), followed by the Famennian (13.3 ± 0.8 myr).

Ages of all other biostratigraphic, magnetic polarity, stable isotope, sea-level changes, and other zones and events are derived from their calibrations to the conodont zonal scale. The next generation of Devonian time scales will incorporate astronomical cycles and additional high-resolution U-Pb dates.