AGE OF THE LOWER MISSISSIPPIAN SCIOTOVILLE BAR LOCALITY (CUYAHOGA FORMATION, PORTSMOUTH MEMBER) IN SOUTHERN OHIO BASED ON BIOSTRATIGRAPHY AND SEQUENCE STRATIGRAPHY

The Kinderhookian – Osagean stage boundary interval contains a well documented and widespread unconformity, which is a sequence boundary caused by a forced regression (Matchen & Kammer, 2006). In central Ohio, lithostratigraphic evidence of the unconformity is based on an incised valley filled with coarse fluvial sediments of the Black Hand Sandstone. In southern Ohio and northern Kentucky the sequence boundary is conformable, and is represented by marine red-beds of the Rarden Member of the Cuyahoga Formation. Conodont evidence from the Pike-Adams County line places the Kinderhookian – Osagean boundary within the Rarden equivalent portion of the Henley Member of the Borden Formation (Sandburg et al., 2002). Thus, those strata immediately above the sequence boundary contained within the Rarden are of early Osagean age.

Two ammonoid assemblages are considered herein: the Route 10 assemblage from the Nancy Member, Borden Formation in northernmost Kentucky, and the well-known Sciotoville bar assemblage from the Portsmouth Shale Member, Cuyahoga Formation in southernmost Ohio. The first assemblage from KY 10 is dominated by Muensteroceras oweni (Hall, 1860), undoubtedly of lower Osagean age (Lineback, 1963), but also contains specimens of the early Late Tournaisian Triimitoceras cf. epiwocklumeriforme (Korn et al., 2003). The age of the Sciotoville bar assemblage has long been debated (Manger, 1979; Gordon and Mason, 1985) in large part because the assemblage is dominated by the endemic form Imitoceras sciotoense (Miller & Faber, 1892), but also contains M. oweni and a single specimen of Gattendorfia andrewsi (Winchell, 1870), a form thought to be restricted to the Kinderhookian (Miller & Garner 1955). Both assemblages come from the Portsmouth Member, Cuyahoga Formation (equivalent of the Nancy Member, Borden Formation), which lies above Rarden Member. As the K – O chronostratigraphic boundary coincides with the sequence boundary contained within the Rarden, both assemblages must be of early Osagean age. Thus Imitoceras sciotoense is of early Osagean age, and the range of Gattendorfia andrewsi extends into the early Osagean. This study thus gives a clear example of how sequence stratigraphy may be used to resolve questionable ages of some fossil assemblages.