The Ordovician, Silurian and Devonian periods

Publisher Website
Google Scholar
Abstract
Summary: A reassessment of published age determinations, including some recent dates on Middle Devonian, Late Silurian, basal Caradoc and middle Arenig rocks, enables the construction of a more precise time-scale. The most accurate points on our scale are: basal Frasnian, 368 ± 8 Ma; early Ludlow, 421 ± 2 Ma; end of early Llandovery, 431 ± 6 Ma; early Caradoc, 450 ± 7; middle Arenig, 482 ± 5 Ma. Interpolations from these data indicate that the base of the Devonian is 412 ± 5 Ma, and the base of the Silurian is 435 ± 7 Ma. The end of the Devonian is less precise, we estimated it to be at 354 + 10, -5 Ma (depending on stratigraphic and sedimentological estimates for the duration of the Frasnian and Famennian stages). The base of the Tremadoc is perhaps 513 ± 10 Ma. Dating by K/Ar, 40 Ar/ 39 Ar, U/Pb and Sm/Nd all show good agreement, but some Rb/Sr isochrons on acid volcanic rocks lie off our time-scale line. Most fission track dates on zircons from bentonites have very large errors; a few with the smallest errors are shown on our graph, but none of them have been precise enough to use in the construction of the time-scale.