CONODONT FAUNA AND BIOSTRATIGRAPHY OF THE MIDDLE DEVONIAN (GIVETIAN) LOWER CEDAR VALLEY GROUP IN THE TYPE AREA OF POLYGNATHUS VARCUS, MOWER COUNTY, SOUTHEASTERN MINNESOTA

The Polygnathus varcus Zone as originally proposed by Bischoff and Ziegler in 1957 as the lowest in a series of subzones, encompassed the lower part of the Givetian Stage of the Middle Devonian. Ziegler, Klapper and Johnson outlined three subdivisions of the varcus Zone in their 1976 revision/redefinition after it was elevated to zonal status by Ziegler in 1962. The same interval is now divided into five zonal intervals (timorensis, rhenanus-varcus, ansatus, semialternans and hermanni) in the current conodont zonation of the Givetian Stage. First Appearances (FAs) of species of the P. varcus group define the bases of the P. timorensis and P. varcus-rhenanus zones. The type specimens of P. varcus and P. xylus were recovered from outcrop and core samples at the Varco Quarry and Homestead Well core in Mower County, Minnesota. At both locations Devonian and Ordovician conodonts are reworked in shales with Cretaceous plant fossils overlying dolostones of the Little Formation. To constrain age of P. varcus in its’ type area the 9.5 m of dolomitized mudstones of Basset Member of the Little Cedar Fm. at the Varco Quarry section was sampled for conodonts, trace element and δC¹³_carb geochemistry. The fauna recovered includes: P. varcus, P. xylus, P. l. linguiformis ɣ, Icriodus brevis, Prioniodina sp., I. difficilus, Tortodus sp., T. sp. cf. T. variabilis? of Uyeno, P. timorensis, Polygnathus ansatus, Ozarkodina semialternans and P. aff. ovatinodosus. The interval spanning samples 1-9 below the FA of Ozarkodina semialternans is assigned to the upper part of the P. ansatus Zone. The FA of the O. semialternans in our sample 10 denotes the base of the Schmidtognathus latifossatus/O. semialternans Zone.

δC¹³_carb values of the Taghanic δC¹³_carb excursion in the Appalachian Basin record an shift of over +3 0/00 in the coeval Tully Limestone. Correlations indicate that the Varco section should record peak values as seen in New York. Our data from Iowa show little variation with δC¹³_carb values ranging between +0.82 0/00 and + 0.65 o/oo. The Varco δC¹³_carb. δO¹⁸_carb isotope data cross-plot does not indicate a diagenetic resetting of δC¹³_carb values. This might suggest that the shallow Little Cedar shelf water mass was a distinct hydrofacies than more open marine waters of the outer shelf and the Illinois-Appalachian basins to the south and east.