Conodont Biostratigraphy of Chesterian Strata (Late Mississippian–Early Pennsylvanian), East-Central Idaho and Southwestern Montana

Abstract Late Mississippian (Chesterian) to Early Pennsylvanian (Morrowan) conodonts in east-central Idaho and adjacent Montana establish biostratigraphic dating of continuously deposited Antler foreland basin sequences. Five conodont biozones are defined within a carbonate mud dominated succession in Idaho’s foreland basin and are traceable to thinner platform strata in Montana. These biozones temporally represent and are recorded within a westward thickening shelf margin, intrashelf basin or embayment, shallow shelf, and near-shore environment determined on the basis of associated conodont biofacies, faunal correlations, lithologies, and sequence stratigraphic analysis. Conodont faunas within the biostratigraphic interval are dominated by species of Cavusgnathus, Adetognathus, Gnathodus, Vogelgnathus, Hindeodus, and Rhachistognathus, which comprise nearly 95% of conodonts recovered. Chesterian conodonts recovered include Cavusgnathus naviculus, C. altus, C. unicornis, Adetognathus unicornis, A. lautus, A. spathus, Gnathodus bilineatus, G. girtyi, G. bollandensis, Hindeodus cristula, H. minutus, Vogelgnathus campbelli, and Lochriea commutatus commutatus. The five Chesterian and lower Morrowan biozones are, 1) Hindeodus/Vogelgnathus Biozone—lower Chesterian, 2) Cavusgnathus naviculus Biozone—middle Chesterian, 3) Adetognathus unicornis Biozone—upper Chesterian, 4) Adetognathus Biozone— uppermost Chesterian, and 5) Rhachistognathus primus Biozone—lowest Morrowan. The Mississippian-Pennsylvanian boundary is marked by the appearances of Rhachistognathus primus and R. websteri (Morrowan), with the concurrent disappearance of Cavusgnathus. These biostratigraphic zones provide a correlation of marine strata across the Cordillera of Idaho and Montana.