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Abstract

Graphic correlation of 12 previously uncompiled stratigraphic sections wiih the Silurian composite standard of KJeffner (1989) results in a revised Silurian composite standard (CS) that has Worldwide applicability as a high-resolution chronostraligraphy. The additional range-data on 52 graptolite species, 39 conodont species, 10 events, and one boundary stratotype make it possible to graphically correlate virtually any stratigraphic section (which meets the data requirements of the graphic correlation method) containing representatives of diagnostic conodont and/or graptolite species with the newly revised Silurian CS. The nonannular absolute chronology based on the Silurian CS divides with confidence into 92 standard time units (STUs), a resolution that is a minimum of twice that of any previously proposed Silurian chronostraligraphy. Most sections graphically correlate with the Silurian CS by fitting a straight line of correlation, indicating that the standard reference section (Cellon, Austria) consists of rock which accumulated at a relatively constant rate. The absolute chronology based on the Silurian CS is thereby consonant (or nearly so) with an annular scale, and the 92 STUs it divides into are of equal annular length. Conodont and graptolite chronozones are defined in the Silurian CS according to international rules of stratigraphy and, if they contain at least one STU, can be recognized with confidence in any section that is a part of the Silurian CS or that can be added to it by the graphic correlation method. The conodont and graptolite chronozones defined in the Silurian CS are based on zones proposed by Walliser (1964), Barrick and Klapper (1976), Jeppsson (1988), Aldridge and Schönlaub (1989), Kleffner (1989), and Cocks and Nowlan (1993, for a proposed standard left-hand column for international Silurian correlation Charts).

All post-Aeronian Silurian series and stage boundaries can be recognized with confidence in any section that is already or can become a part of the Silurian CS. Three of the series boundaries, the Llandovery/Wenlock, Ludlow/Pridoli, and Pridoli/Lochkovian (Silurian/Devonian), are recognized in the Silurian CS based on the position of the “golden spike” in their boundary stratotypes. Lower boundaries of the graptolite zones that are at the same or approximate leveis as the other Silurian boundaries are used to recognize the positions of those boundaries in the Silurian CS. The Silurian chronostratic scale, based on the range-data on conodont species, graptolite species, events, and boundary stratotypes represented in the Silurian CS, is calibrated by using the Wenlock/Ludlow and Silurian/Devonian tie-points of Harland and others (1989) and STUs as chrons of equal duration to interpolate between and below those tie-points. The Silurian time scale developed by this method provides the best means at present for estimating the durations of the Wenlock, Ludlow, and Pridoli Epochs, and all of the ages that comprise them (except perhaps for the Sheinwoodian and Homerian). The Pridoli was the longest epoch with a duration of 8.4 Ma, compared to 7.1 Ma for the Ludlow and 2.6 Ma for the Wenlock.

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