Late Mississippian (Chesterian)-Early Pennsylvanian (Morrowan) Conodont Biostratigraphy of East-Central Idaho and Southwest Montana

Jason M. Abplanalp¹, Peter E. Isaacson², and Anna S. Gilmore². (1) University of Idaho, University of Idaho, Department of Geological Sciences, Moscow, ID 83843, phone: (814) 574 4121, [email protected], (2) Department of Geological Sciences, University of Idaho, Moscow, ID 83844-3022

Biostratigraphic investigation and biofacies analysis of Late Mississippian (Chesterian) to Early Pennsylvanian (Morrowan) conodonts in east-central Idaho and adjacent Montana were required to delineate sequences on a platform to foreland basin transect. Three conodont biozones were encountered and time lines through thin and thick successions are evident. Formations containing these zones are interpreted to represent westward thickening shelf margin, cratonic trough or embayment, shallow shelf, and near-shore deposition on the basis of associated conodont biofacies, faunal correlatives, and lithology. Conodont faunas present within the boundary interval are dominated by species of Cavusgnathus, Adetognathus, Gnathodus, and Rhachistognathus, which compose nearly 95% of conodonts recovered. Late Mississippian (Chesterian) conodonts recovered include Cavusgnathus naviculus, C. altus, C. regularis, C. windsorensis, C. convexus Adetognathus unicornis, Gnathodus bilineatus, and G. commutatus commutatus. Several broadly occurring and correlatable biostratigraphic zones are delineated for the Chesterian and include: 1) the mid-Chesterian Cavusgnathus sp. zone; 2) the upper Chesterian Adetognathus unicornis/Cavusgnathus naviculus zone; 3) the uppermost Chesterian Rhachistognathus muricatus/Gnathodus bilineatus-commutatus commutatus zone. Early Pennsylvanian (Morrowan) conodonts recovered include Adetognathus lautus, A. spathus, A. gigantus, Rachistognathus muricatus, and R. primus. The Mississippian-Pennsylvanian boundary is marked by the overlapping occurrence of Rachistognathus primus, Adetognathus lautus, and A. spathus with the concurrent decimation of Cavusgnathus sp. and Gnathodus bilineatus–commutatus commutatus. These defined biostratigraphic zones allow for the accurate correlation of shallow marine strata across the study area.