CONODONT FAUNAS ACROSS THE SILURIAN-DEVONIAN BOUNDARY AND THE KLONK ISOTOPE EVENT IN WESTERN TENNESSEE, AND THE APPEARANCE OF ENDEMIC EARLIEST DEVONIAN ICRIODUS SPECIES

The Silurian-Devonian boundary in the Western Valley of Tennessee corresponds with the shift in carbonate facies from the Decatur Limestone to the overlying Ross Formation. The transition from the light-colored medium- to coarse-grained echinoderm packstone and grainstone of the upper part of the Decatur to the more thinly bedded, often glauconitic, dark-colored echinoderm grainstone and fossiliferous shale of the lower part of the Ross Formation coincides with the major shift in conodont faunas associated with the Klonk Carbon Isotope Event (KCIE). The transition is best seen in a roadcut along US 64 near Flat Gap Creek, east of Savannah, TN. In the upper beds of the Decatur, the late Pridoli Oulodus detorta-Belodella coarctata fauna occurs. As δ¹³C values rise to above 2.0‰, thin shale partings appear between the carbonate beds and conodont abundance falls, Pridoli species begin to disappear, and rare Icriodus elements appear. Through the next one meter, δ¹³C values increase to 3.0‰, the grainstone darkens in color and shale beds increase, and the coniform taxa Pseudoonetodus and Decoriconus strongly dominate. In the overlying two meters of coarser-grained glauconitic grainstone, with two layers of scyphocrinoid bulbs, δ¹³C values reach maximum values, >4.0‰, and then decline to just less than 3.0‰. Icriodus and Zieglerodina elements become moderately common.

The clearly defined KCIE in the Flat Gap Creek section, in association with the extinction of late Pridoli taxa, as already documented in southern Oklahoma and West Texas, provides a definitive basis for placing the position of the Silurian-Devonian boundary just below the peak value of δ¹³C, within the Decatur-Ross transitional interval. The two Icriodus morphotypes present in the lowermost Devonian beds in Tennessee, as well as those in Oklahoma, West Texas, and possibly in the Appalachian Basin, are not the same as the morphotypes assigned to Icriodus woschmidti in Europe nor I. hesperius in western North America. We suggest that separate clades of Icriodus species radiated in the different geographic regions from a common Pridoli ancestor. The existence of well-defined brachiopod and trilobite provinces during the latest Silurian and earliest Devonian is well established, and it is likely that conodont faunas were isolated in a similar manner.