A CONODONT-, GRAPTOLITE, AND CHITINOZOA-BASED SILURIAN (UPPER LLANDOVERY-PRIDOLI) CHRONOSTRATIGRAPHY

The conodont- and graptolite-based Silurian composite developed by Kleffner (1989, 1995), using the graphic correlation method, has required significant revision as a result of recent taxonomic study of conodont species that were important in initial development of the composite, addition of 11 stratigraphic sections from North America and Europe, and recorrelation of several sections already in the composite. Recorrelation of the older sections was facilitated by the acquisition of more comprehensive and refined ranges for taxa represented in those sections. The newly revised Silurian composite includes range-data on more than 370 conodont, graptolite, and chitinozoa species and forms the basis for a revised Silurian chronostratigraphy that is now comparable with the one proposed by the Subcommission on Silurian Stratigraphy in 1995 (SSS).

All of the original Silurian conodont zones proposed by Walliser (1964), as well as all upper Llandovery-Pridoli conodont zones recognized by Aldridge and Schönlaub (1989), by the SSS, and by Corradini and Serpagli (1999) can be recognized as chronozones or subchronozones in the revised Silurian chronostratigraphy, but not always at precisely the same stratigraphic positions as in those Silurian conodont zonations. All of the graptolite biozones utilized by the SSS from the upper Llandovery (lower, but not lowest, Telychian) through the Pridoli can be recognized as chronozones, and in the same ascending order. It is also possible to recognize as chronozones seven of the 11 chitinozoa biozones utilized by the SSS, and eight of the 12 chitinozoa biozones recognized by Verniers and others (1995) for the upper Llandovery (Telychian)-Pridoli in the revised Silurian chronostratigraphy.

Isotopic data from near the Llandovery/Wenlock, Wenlock/Ludlow, and Silurian/Devonian boundaries and the revised Silurian chronostratigraphy are used to develop a calibrated time scale for the Wenlock-Pridoli. The time scale makes possible not only high-resolution correlation among faunal zones, but also determination of durations of all conodont, graptolite, and chitinozoa zones recognized for the Wenlock, Ludlow, and Pridoli.