Upper Devonian (Frasnian–Famennian) conodont biostratigraphy in the subsurface of north-central Iowa and southeastern Nebraska

Abstract

The Upper Devonian–Lower Mississippian sequence in the subsurface of north-central Iowa and southeastern Nebraska consists of the Lime Creek, undifferentiated Sheffield–Maple Mill, English River, Prospect Hill, and Chapin Formations. The Lime Creek is restricted entirely to the Polygnathus biofacies where conodonts are present and correlates with an interval from Zone 4b to Faunal Interval 8 of the Canadian (Alberta) Rockies zonation based on conodonts and foraminifers. The oldest identifiable Famennian units range in age from the crepida–rhomboidea Zones at locality 1 to the Upper marginifera Zone or younger at localities 2 and 3. The youngest Devonian strata may occur in the Upper expansa Zone at locality 1 and Middle expansa Zone at localities 2 and 3. Famennian faunas are present in the Polygnathus biofacies at localities 1 and 3 and in the Icriodus biofacies at locality 2. A total of 50 species and subspecies are identified, four of which have not been previously described (Icriodus sp. A, Hemilistrona sp. A, Polygnathus depressus n. sp., and Polygnathus sp. B).