CONODONT BIOSTRATIGRAPHY AND CORRELATION OFF THE MIDDLE DEVONIAN (GIVETIAN) CEDAR VALLEY GROUP OF NORTHERN IOWA AND SOUTHERN MINNESOTA

The biostratigraphy of the marine epeiric carbonate platform strata of the Middle Devonian Cedar Valley Group are poorly constrained in Northeast (NE) Iowa (IA) and southern Minnesota (MN). Where preserved, basal Cedar Valley Group strata of the Little Cedar Formation overlay Late Eifelian-Middle Givetian open marine and peritidal carbonates of the Wapsipinicon Group. To constrain the age range of those rock in northern IA and southern MN we samples the thickest known successions in Fayette County, IA, in and near the town of Fayette (23 m), and at the Varco Quarry section south of the City of Austin in Mower County, MN. The latter is the type locality of the Polygnathus varcus Stauffer. The P. varcus Zone, as originally proposed in the 1960s spans more than half of the Givetian Stage of the upper Middle Devonian Series (now divided into five zonal intervals). Polygnathus ansatus, P. linguiformis linguiformis Γ morphotype, P. xylus xylus, P. ovatinodosus, P. varcus, Icriodus brevis, I. difficilus, and Prioniodina sp. were reported or illustrated from the basal 3.5 m of Little Cedar in Howard County, IA, by Klapper and Barrick. The Little Cedar strata in and around the town of Fayette are 23-24 m thick are included in the “Lower and “upper” Solon Member. Our Fayette sections of the Lower Solon Member yield a similar fauna as reported by Klapper and Barrick, but includes P. timorensis, Tortodus sp. cf. T. bultynki, Latericriodus l. latericrescens, and I. lindensis, but no definitive P. varcus morphotypes. These new forms, and taxa reported by Klapper and Barrick’s, correlate with the upper ansatus Zone. Ozarkodina semialternans with P. linguiformis linguiformis ε morphotype first occur in the Upper Solon, correlated with the semialternans-latifossatus Zone.

Little Cedar Formation dolomitized mudstones in southern Minnesota are assigned to the Basset Member. Eleven samples at 100 and 50 cm spacing were taken through the lower 9.5 m of the Varco Quarry section. The fauna recovered from the Varco Quarry locality include: P. ansatus, P. l. linguiformis Γ morphotype, P. xylus xylus, P. ovatinodosus, Icriodus brevis, I. difficilus, and I. sp. The Varco Quarry succession is correlated with the upper ansatus Zone rocks of Iowa, thus fixing the stratotype of Polygnathus varcus within the upper ansatus Zone.