NEW U-PB ZIRCON AGES AND THE DURATION AND DIVISION OF DEVONIAN TIME

Abstract

Newly determined U–Pb zircon ages of volcanic ashes closely tied to biostratigraphic zones are used to revise the Devonian time-scale. They are: (1) 417.6±1.0 Ma for an ash within the conodont zone of Icrioduswoschmidti/I. w. hesperius (early Lochkovian); (2) 408.3±1.9 Ma for an ash of early Emsian age correlated with the conodont zones of Po. dehiscens–Lower Po. inversus; (3) 391.4±1.8 Ma for an ash within the Po. c. costatus Zone and probably within the upper half of the zone (Eifelian); and (4) 381.1±1.3 Ma for an ash within the range of the Frasnian conodont Palmatolepispunctata (Pa. punctata Zone to Upper Pa. hassi Zone). U–Pb zircon ages for two rhyolites bracketing a palyniferous bed of the pusillites–lepidophyta spore zone, are dated at 363.8±2.2 Ma and 363.4±1.8 Ma, respectively, suggesting an age of ∼363 Ma for a level within the late Famennian Pa. g. expansa Zone. These data, together with other published zircon ages, suggest that the base and top of the Devonian lie close to 418 Ma and 362 Ma, respectively, thus lengthening the period by ∼20% over current estimates. We suggest that the duration of the Middle Devonian (Eifelian and Givetian) is rather brief, perhaps no longer than 11.5 Myr (394 Ma–382.5 Ma), and that the Emsian and Famennian are the longest stages in the period with estimated durations of ∼15.5 Myr and 14.5 Myr, respectively.