Conodonts from Middle Devonian Strata from the Michigan Basin

Abstract

Study of the stratigraphic distribution of 20 species of the platform genera Icriodus, Polygnathus, Palmatolepis?, and Schmidtognathus at 11 subsurface localities in northern Indiana and at 10 supplementary outcrop localities in northern Indiana, Michigan, and western Ontario makes possible the recognition of five conodont zones in middle Devonian strata of the Michigan Basin. The lower part of the Detroit River Formation of northern Indiana contains a fauna characterized by Polygnathus angusticostatus, P. robusticostatus, and P. “webbi.” This faunal association is similar to that of the Spathognathodus bidentatus Zone (Eifelian) of Europe. The upper Eifelian fauna of the Dundee Limestone of Michigan, Ohio, and western Ontario, which is characterized by Icriodus angustus, is not present in northern Indiana, where the Detroit River is succeeded unconformably by the Traverse Formation. The lower part of Traverse strata of Indiana and the Bell Shale, Rockport Quarry Limestone, Ferron Point Formation, Genshaw Formation, Newton Creek Limestone, and lowermost part of the Alpena Limestone (all the Traverse Group) of Michigan contain a lower Givetian fauna distinguished by I. latericrescens latericrescens below the lowest position of P. varcus. The upper part of the Traverse Formation of Indiana and the upper part of the Alpena Limestone and the Four Mile Dam Limestone, Norway Point Formation, Potter Farm Formation, and Thunder Bay Limestone (all the Traverse Group) of Michigan lie within the P. varcus Zone (upper Givetian). The lowermost part of the Antrim Shale of northern Indiana contains the fauna of the Schmidtognathus hermanni-P. cristatus Zone of probable late middle Devonian (Givetian) age. Forty -four species of conodonts that are referable to 18 genera are present in the collection. The new taxon Icriodus latericrescens robustus is proposed herein.