Pridoli (Silurian) to Lochkovian (Early Devonian) conodonts and the Silurian-Devonian boundary interval in the Decatur Limestone and Ross Formation in the Western Valley of Tennessee, USA

Abstract

The timing of the facies transition from the Decatur Limestone to the overlying Rockhouse Limestone Member of the Ross Formation in western Tennessee was evaluated using conodont faunas and the delta 13C carb curve. The facies transition corresponds to conodont extinctions associated with the Klonk Oceanic Event and with the Klonk isotopic excursion, which span the Silurian-Devonian boundary interval. The interpreted position of the Silurian-Devonian boundary is placed near the top of the Decatur, above the conodont extinction level and in the ascending limb of the isotopic excursion. The lowest occurrences of the Devonian species Caudicriodus murphyi and C. anitae are slightly higher, at the top of the Decatur, and both species characterize the Rockhouse Limestone Member. The conodont succession across the Silurian-Devonian boundary is like that reported from sections in West Texas and Oklahoma and represents a regional biological response to the Klonk Event in the southern part of the Appohimchi Subprovince of the Appalachian Province, one that is also represented by comparable changes in brachiopod and ostracode faunas. Correlation into the northern part of the Appohimchi Subprovince is best accomplished using the delta 13C carb curve. The western Tennessee conodont fauna includes two closely related species of Zieglerodina, Z. zellmeri and Z. planilingua, as well as distinctive forms of Pseudooneotodus.