Middle Devonian Stratigraphy and Conodont Biostratigraphy, North-Central Ohio

Abstract

Devonian sedimentation in north-central Ohio began in warm, somewhat restricted seas in which the Bois Blanc and Amherstburg formations (latest Early to earliest Middle Devonian) accumulated. Increased restriction, reflected by the Lucas Dolomite facies, prevailed until middle costatus-2.one time when marine transgression initiated deposition of the Columbus Limestone. A wave-erosional episode left a disconformity atop the Columbus that truncates the lower australis Zone near Sandusky and progressively lower levels southward; the uppermost costatus Zone is thus missing in central Ohio. The Delaware Limestone represents a transgressive-regressive cycle, apparently during kockelianus-'Z.one time, which was followed by an extensive erosional period in which three endemic species of Icriodus died out. The Plum Brook Shale is in the upper ensensis Zone (lowest upper Middle Devonian) and reflects the If T-R cycle. The Prout Dolomite contains a Middle varcus Subzone fauna and represents the Ila T-R cycle (Taghanic onlap). Regional correlatives of the Prout currently assigned to the Lower varcus Subzone are probably of Middle-varcus age. Considerable disagreement exists between the fairly extensive Middle Devonian conodont-biostratigraphic data in the Lake Erie region and recent correlations based on volcanic ash beds and/or geophysical logs.