Upper Devonian and Lower Mississippian Conodont faunas, north-central Iowa

Conodonts are rare in the Shell Rock Formation, but they support the previous assignment of this formation to the lower part of the Upper Devonian. Conodont species diagnostic of the Lower Palmatolepis gigas Zone (Upper Devonian, toIγ) occur in the upper part of the Juniper Hill and in the lower Cerro Gordo members of the Lime Creek Formation. The ranges of conodont species occurring in the Sheffield Formation are indicative of zones toIδ -tollα. The conodont faunas from the Aplington Formation is meager but may indicate zone toIV or toV. Conodonts from the English River Siltstone and the Maynes Creek Member of the Hampton Formation are distinctive of zones upper cuI through cuII. The Devonian-Mississippian boundary in north-central Iowa occurs between the Aplington Formation and either the English River Siltstone , which is not everywhere present, or the Hampton Formation. At one locality the Maple Mill Shale overlies the Aplington Formation and here the boundary is between the Maple Mill and the overlying English River Siltstone