Stratigraphy and Conodont Paleontology of the Brassfield (Silurian) in the Cincinnati Arch Area

Abstract Conodonts were collected from 49 sections of the Brassfield Formation (Brassfield Limestone) in its outcrop belt around the Cincinnati Arch in Indiana, Kentucky, and Ohio. Thirty-four indigenous species represent the genera Acodus, Ambalodus, Distomodus, Drepanodus?, Euprioniodina, Icriodina, Ligonodina, Lonchodina, Neoprioniodus, Ozarkodina, Paltodus, Panderodus, Plectospathodus, Spathogna thodus, Synprioniodina, Trichonodella, and a generic group that includes species variously referred to Keislognathus?, Rhynchognathodus?, Roundya, and Trichonodella. Seven new species named herein are Icriodina stenolophata, Ligonodina? extrorsa, Paltodus costulatus, P. debolti, P. dyscritus, P. migratus, and Spathognathodus oldhamensis. The new subspecies Panderodus unicostatus serratus is also named. Conodonts show that the Brassfield Limestone as it is now recognized belongs in the upper part of Bereich I of the European conodont zonation. Younger strata, which overlie the Brassfield from Jefferson County, Ind., into Bullitt County, Ky., possibly should be placed within the Brassfield, and if they were included, the formation in this area would include beds belonging in the celloni-Zone. The Brassfield correlates with the Kankakee Dolomite and the Sexton Creek Limestone of the Alexandrian Series in Illinois. Conodonts support the evidence based on brachiopods that the upper part of the Brassfield is younger west and north of the type area than in the type area of central Kentucky. From about 5 miles southeast of Stanford, Ky., northeastward to northern Adams County, Ohio, the Brassfield Formation is overlain conformably or with slight unconformity by the Noland Formation, which also contains Bereich I conodonts. From Adams County north to Piqua, Ohio, the formation is unconformably overlain by the Dayton Limestone. In Indiana and Kentucky on the west side of the Cincinnati Arch, the Osgood Member of the Salamonie Dolomite or its equivalents overlie the Brassfield with major unconformity except where strata intermediate in age are present from Jefferson County, Ind., into Bullitt County, Ky. The Brassfield unconformably overlies Ordovician strata throughout the Cincinnati Arch area. Tie Ripley Island positive area of southeastern Indiana was the only active structure influencing Brassfield sedimentation in the area studied.