Testing the limits of chronostratigraphic resolution in the Appalachian Basin, Late Devonian (middle Frasnian), eastern North America: New U-Pb zircon dates for the Belpre Tephra suite

摘要

The Belpre Tephra suite from the Chattanooga Shale in eastern Tennessee and the lower Rhinestreet Shale in western New York has yielded high-precision chemical abrasion- thermal ionization mass spectrometry (CA-TIMS) U-Pb zircon dates of 375.55 +/- 0.10 Ma from "tephra 01"and 375.25 +/- 0.13 Ma from "tephra 06"at Little War Gap, Hancock County, Tennessee, and 375.14 +/- 0.12 Ma from "tephra 7.67"at Eighteenmile Creek, Erie County, New York. While the latter two ages provide an apparently isochronous marker horizon for stratigraphic correlation, the conodont zonation for the two localities is disjunct: The tephra beds from the Chattanooga Shale at Little War Gap are, at least in part, Frasnian zonation (FZ) 8, based on the co-occurrence of Ancyrognathus barba and Palmatolepis housei, while the tephra-bearing interval from the Rhinestreet Shale in Erie County, New York, is apparently older, in FZ 7, based on the occurrence of Ancyrognathus sp. L? of Klapper and the goniatite Naplesites inyx. We discuss various hypotheses to explain this chronostratigraphic conflict, including the proposition that we have reached the current limits of resolution of both radioisotopic and biostratigraphic methodologies in this epoch.