REVISED CONODONT-, GRAPTOLITE-, AND CHITINOZOA-BASED SILURIAN COMPOSITE DEVELOPED USING GRAPHIC CORRELATION AIDS NEW CALIBRATION OF CURRENT SILURIAN CHRONOSTRATIGRAPHY

Little agreement exists about the durations of epochs and ages in most Silurian time scales developed since 1989. Reasons for the lack of consensus include the use of newly determined isotopic dates for the various time scales, differences in results of the two radiometric dating methods most commonly used since 1990, Mass Spectromic Isotope Dilution (MSID) and Sensitive High Resolution Ion Micro-Probe (SHRIMP), the absence of any stratigraphic units with isotopic dates at/near the Global Boundary Stratotype Section and Point in any Silurian boundary stratotype section, and thus, the necessity of interpolation to calibrate Silurian chronostratigraphy. In order to develop a revised calibration of Silurian chronostratigraphy that addresses the reasons for those differences, since the limited choice of isotopic dates remains the same, we recommend that the method of interpolation be changed. Standard Time Units (STUs), units that represent equal rock thickness and also equal time (at least conceptually), are used as chrons for interpolation instead of graptolite zones. The STUs are based on the composite standard units (CSUs) which comprise a recently revised Silurian composite standard (CS) developed using graphic correlation on range-data of more than 450 species of conodonts, graptolites, and chitinozoa in over 60 stratigraphic sections in North America and Europe. Since the Silurian CS currently includes only Telychian through lower Devonian stratigraphic sections, durations for the Rhuddanian and Aeronian can only be approximated. A new Silurian time scale is developed by calibrating Silurian chronostratigraphy based on STUs and isotopic dates for rocks obtained mainly by using the MSID-dating method (the method used to determine the isotope dates used for the Silurian by the International Commission on Stratigraphy), rather than isotope dates obtained using the SHRIMP-dating method. The new Silurian time scale recognizes the following durations of Silurian epochs: Pridoli, 3.3 Ma; Ludlow, 3.2 Ma; Wenlock, 4.85 Ma; and Llandovery, 13.85 Ma. Durations of Silurian ages are: Ludfordian, 2.4 Ma; Gorstian 0.8 Ma; Homerian, 2.7 Ma; Sheinwoodian, 2.15 Ma; Telychian, 8.45 Ma; Aeronian, 3.5 Ma; and Rhuddanian, 1.9 Ma.