3 Calibration of the
Ordovician deep-water shales contain the prerequisites for a high-resolution timescale: rich successions of graptolite faunas, datable ash-fall K-bentonites, and minimally interrupted accumulation. Traditionally, the first appearances of selected graptolite taxa define provincial sets of zones, into...
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| Format: | Text |
| Language: | English |
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| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.423.6857 http://earthscience.ucr.edu/docs/Sadler%26Cooper_2004ColumbiaUP.pdf |
| Summary: | Ordovician deep-water shales contain the prerequisites for a high-resolution timescale: rich successions of graptolite faunas, datable ash-fall K-bentonites, and minimally interrupted accumulation. Traditionally, the first appearances of selected graptolite taxa define provincial sets of zones, into which radiometrically dated bentonites are subsequently arrayed to achieve a numerical timescale. Provincial differences and the modest numbers of zones impose the primary limits on resolution. We have taken a different approach that avoids the constraint of zones. ■ The New Approach To achieve a unifying timescale for this book, we applied computer-assisted optimization to combine graptolite range charts from all provinces directly, without using zones. The optimization process searches for a model sequence of all range-end events that best fits all the locally observed taxon ranges. Dated ashfall events are included with the range-end events from the outset of the search; thus, they too receive optimal placements in the model sequence and permit numerical ages for biostratigraphic events to be estimated by interpolation. By this method, we interpolated ages for the Australasian graptolite zone boundaries (VandenBerg and Cooper 1992). Ages for the boundaries in other subdivision schemes were derived, using all available criteria to correlate into the Australasian graptolite zonation (chapter 2). ■ The Raw Data The timescale is based on an ordered and scaled sequence of 2,306 events: 22 dated bentonites that are associated with graptolites or other fossils for which contemporary graptolites are known (table 3.1); 12 undated bentonite beds that help tie together short sections from the Mohawk Valley (C. E. Mitchell pers. comm.); and the first and last appearances of 1,136 taxa (1,119 graptolites, plus 17 trilobites and conodonts) as reported from almost 200 stratigraphic range charts worldwide, from the basal Ordovician to early Devonian. The range charts represent Arctic |
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