Graphic Rejuvenation of Ordovician Chronostratigraphic Scale

Walter C. Sweet

For much of the past century, the North American Middle and Late Ordovician has been divided into a succession of 9 or 10 stadial units, whose stratotypes are demonstrably diachronous lithic units of variable temporal significance in Nevada, New York, and the Cincinnati arch region of Kentucky, Indiana, and Ohio. Distribution of most faunal elements within these stratotypes has never been well documented. Hence, nonstratotypic equivalents are difficult to recognize with precision, and resolution below the stadial level is impossible. In the last decade, however, data on the ranges of more than 100 conodont species in closely sampled sections at more than 80 localities in the United States and Canada have been used graphically to build a composite scale that includes the s ratotypes of all traditional units. Currently, this scale can be used to resolve about 80 (rather than 9 or 10) divisions in the Mohawkian and Cincinnatian Series, and it may ultimately provide the basis for resolving that many more divisions in the Whiterockian. Elaboration of the framework now controlled solely by conodonts, through addition of scaled range data from other fossil groups, should eventually enable even finer resolution in a segment of the Phanerozoic record that is of substantial stratigraphic, tectonic, and paleobiologic interest.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.