Conodont biostratigraphy of the Little Cedar and Lower Coralville Formations of the Cedar Valley Group (Middle Devonian) of Iowa and significance of a new species of Polygnathus

Abstract

The Little Cedar and lower Coralville formations of the Cedar Valley Group (Middle Devonian) of Iowa were deposited on a cratonic, shallow-water, carbonate and evaporite shelf during the Taghanic onlap. Four conodont subzones, zones, or cratonic biofacies faunas can be recognized in this sequence of strata. They are, in ascending order, the Middle varcus Subzone (recognized in the lower Little Cedar Formation), the hermanni zone (recognized in the middle Little Cedar Formation), the Lower subterminus Fauna (recognized in the upper Little Cedar Formation), and the Upper subterminus Fauna (recognized in the lower Coralville Formation). The Lower subterminus Fauna and the Upper subterminus Fauna represent a possible zonal span from the Lower hermanni Zone to the Upper disparilis Zone of the standard conodont zonation.In the Polygnathus- and Icriodus-dominated conodont faunas studied is a biostratigraphically useful new species. Polygnathus klugi new species is a Polygnathus dubius-like form that occurs sporadically and in small numbers in the Middle varcus Subzone, the hermanni Zone, and the Upper subterminus Fauna. However, it occurs consistently and in large numbers in the Lower subterminus Fauna, where it is the characterizing species. It has also been recovered from core samples both in central Alberta, Canada, and the Russian Platform. In both areas, it is associated with conodonts of the subterminus faunal interval.