Evaluation of the Kinderhookian-Osagean (Lower Mississippian) Boundary in the Cordillera, Western United States

Abstract Paleontologic definition of the Kinderhookian-Osagean (K-O) series boundary is hindered by a regional unconformity in the type area in the Midcontinent. In the western U.S. Cordillera sedimentologic and biostratigraphic data indicate that a number of sections represent continuous sedimentation across the K-O boundary. Previously defined conodont and foraminiferal zonations can be recognized with variable degrees of confidence along the western U. S. carbonate platform and adjoining basin. The conodont faunas allow recognition of a depositional sequence between the top of the Siphonodella isosticha-Upper S. crenulata Zone and the first occurrence of Polygnathus communis carina, which includes the K-O boundary. We refer to this sequence as the K-O boundary interval. The low diversity and abundance of conodonts across the boundary interval suggest that most previously proposed boundary markers are not practical, and also imply that conodonts underwent a selective extinction and recovery in the late Kinderhookian and early Osagean. The conodont abundance and diversity did not significantly recover from the extinction event until about the first occurrence of Polygnathus communis carina. Therefore we suggest that the best candidates for a K-O boundary marker in the Cordillera are the first occurrence of Polygnathus communis carina and Pseudopolygnathus multistriatus Morphotype 2, which may be worldwide recognizable chronostratigraphic horizons. In shallow water biofacies the extinction of the conodont Patrognathus can be used to approximate the base of the K-O boundary interval. The low abundance of the tuberculate Foraminifera in certain sections reduces their usefulness as a boundary marker, but their occurrence supports recognition of the boundary interval throughout the western U. S.