253-3 Hirnantian (Upper Ordovician) through Aeronian (Llandovery, Silurian) conodonts from Indiana and Ohio

Session: Conodonts from North America and Beyond - Honoring the Career of Dr. James E. Barrick

Presenting Author:

Authors:

Abstract:

The position of the Ordovician-Silurian boundary is difficult to determine in the Appalachian Basin and surrounding arches. In Ohio and Indiana, the Ordovician-Silurian boundary was placed above the Cherokee Unconformity, which is typically identified at the base of the Belfast Member, or, at the base of the Centerville Member of the Whippoorwill formation (new) where the Centerville is recognized. However, the Whirlpool Sandstone and Manitoulin Dolomite—strata overlying the Cherokee Unconformity in New York and Ontario—have been recently assigned a Hirnantian (Late Ordovician) age, indicating that strata overlying the unconformity farther west in Ohio and Indiana might also be Hirnantian. This possibility spurred much recent research on the Whippoorwill formation in Ohio and Indiana, where detailed litho/sequence stratigraphy, subsurface correlation, macropaleontological data, and carbon isotope chemostratigraphic data indicate a Hirnantian through earliest Rhuddanian (Llandovery, Silurian) age for the unit.

To add to this research, we collected 9 conodont samples from the Whippoorwill though “golden Brassfield” formations in Sand Creek, Decatur County, Indiana, and reviewed an existing conodont collection from a 1975 study on the Whippoorwill and Brassfield formations from Adams County, Ohio. From the Indiana section we recovered Late Ordovician taxa such as Drepanoistodus, Plectodina, and Phragmodus from the Centerville through Brassfield formations. The occurrence of Rexroadus kentuckyensis? and Ozarkodina oldhammensis in the upper part of the Centerville Member and lowermost Belfast indicates a position within the Hirnantian Ozarkodina hassi Zone. Combination of conodonts typical of the Late Ordovician with Ozarkodina hassi Zone conodonts may be due to reworking, but this faunal overlap has been found in numerous Hirnantian sections around the world and may provide additional evidence that the Centerville is Hirnantian. Review of the collection from Adams County, Ohio indicates that the lower part of the Belfast (Belfast A) is within the Ozarkodina hassi Zone. The presence of Distomodus kentuckyensis in the middle of the overlying Belfast B submember indicates that the Ordovician-Silurian boundary occurs within the Belfast B. Conodonts tentatively identified as Pseudolonchodina expansa as well as Oulodus aff. jeannae indicate an upper Rhuddanian or Aeronian position for the upper part of the Brassfield Formation.  

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8702

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