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

摘要

Late Mississippian (Chesterian) to Early Pennsylvanian (Morrowan) conodonts in east-central Idahoand adjacent Montana establish biostratigraphic dating of continuously deposited Antler foreland basinsequences. Five conodont biozones are defined within a carbonate mud dominated succession in Idaho'sforeland basin and are traceable to thinner platform strata in Montana. These biozones temporally repre-sent and are recorded within a westward thickening shelf margin, intrashelf basin or embayment, shal-low shelf, and near-shore environment determined on the basis of associated conodont biofacies, faunalcorrelations, lithologies, and sequence stratigraphic analysis. Conodont faunas within the biostratigraphicinterval are dominated by species of Cavusgnathus, Adetognathus, Gnathodus, Vogelgnatbus, Hindeo-dus, and Rhachistognathus, which comprise nearly 95% of conodonts recovered. Chesterian conodontsrecovered include Cavusgnathus naviculus, C. altus, C. unicornis, Adetognathus unicornis, A. lautus, A.spathus, Gnathodus bilineatus, G. girtyi, G. bollandensis, Hindeodus cristula, H. minutus, Vogelgnathuscampbelli, and Lochriea commutatus commutatus. The five Chesterian and lower Morrowan biozonesare, 1) Hindeodus/Vogelgnatbus Biozone—lower Chesterian, 2) Cavusgnathus naviculus Biozone—mid-dle 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 acorrelation of marine strata across the Cordillera of Idaho and Montana.