NEWLY REVISED AND CALIBRATED CONODONT-, GRAPTOLITE-, AND CHITINOZOA-BASED SILURIAN CHRONOSTRATIGRAPHY DEVELOPED USING THE GRAPHIC CORRELATION METHOD

Ranges of over 400 conodont, graptolite, and chitinozoa species in 63 stratigraphic sections in North America, Europe, and Australia comprise the framework for a Silurian (upper Llandovery-Pridoli)-lowest Devonian (Lochkovian) chronostratigraphy developed using the graphic correlation method and recently revised with the inclusion of additional stratigraphic sections containing much new range data. The revised Silurian chronostratigraphy compares favorably with the one proposed by the Subcommission on Silurian Stratigraphy in 1995 (SSS).

The upper Llandovery-lowest Lochkovian chronostratigraphy recognizes 20 conodont chronozones, 24 graptolite chronozones, and seven chitinozoa chronozones, as well as seven conodont subchronozones and four chitinozoa subchronozones. All of the original Silurian conodont zones proposed by Walliser (1964), and all of the upper Llandovery-Pridoli conodont zones recognized by Aldridge and Schönlaub (1989), by 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.

Isotopic data from near the Llandovery/Wenlock, Wenlock/Ludlow, and Silurian/Devonian boundaries, the Ludlow-Pridoli succession of the Simpson Park Range 1, Nevada, United States section, and the upper Llandovery-Wenlock succession of the Cellon, Austria section facilitate the development of a calibrated time scale for the Silurian (late Llandovery-Pridoli). The Silurian time scale makes possible determinations of durations of the Wenlock (4.6 my), Ludlow (3.7 my), Pridoli (2.8 my), and all conodont, graptolite, and chitinozoa chronozones recognized in the revised Silurian chronostratigraphy.