Sections of graptolitic shale within the Cordilleran foldbelt are very condensed. An average accumulation rate of 4 ∓ 1 m/m.y. calculated for these shales falls within the range of 2 to 5 m/m.y. for modern deep-sea clay and siliceous ooze. The Deep Sea Drilling Project has shown that once an age is determined for a stratigraphic horizon within a pelagic section, it is possible to estimate the ages of other horizons by assuming constant rates of sediment accumulation. This method, used on graptolitic shale, suggests that the time span represented by a graptolite zone is proportional to its thickness. The thickest zone analyzed is that of Nemagraptus gracilis, which is calculated to have spanned about 8 m.y., whereas the thinnest zone, Glyptograptus persculptus, took less than 1 m.y. to accumulate. The readily recognized zones of Nemagraptus gracilis and Climacograptus bicomis, which together consistently form thick biostratigraphic intervals, are the zones most commonly found, whereas the G. persculptus zone is very rare. A new subdivision of the Ordovician and Early Silurian time scale has been made on the basis of the calculated time duration of graptolite zones.

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