Conodonts of the Lower Ordovician Prairie du Chien Group of Wisconsin and Minnesota

摘要

The Oneota and Shakopee Formations of the Lower Ordovician Prairie du Chien Group of Wisconsin and Minnesota are readily differentiated on the basis of their distinctive conodont faunas. These faunas are part of the North American Midcontinent Province fauna (NAMP). The Oneota Formation contains elements of Ibexian fauna C, and represents the upper Skullrockian Stage of the Ibexian Series. The Shakopee Formation contains elements of Ibexian fauna D (NAMP), and represents at least part of the lower Stairsian Stage. The Skullrockian-Stairsian boundary and Ibexian C-D faunal boundary correspond to the Oneota-Shakopee disconformity. The Skullrockian and Stairsian Stages are the only stages of the Ibexian Series represented in the Prairie du Chien outcrop area studied for this report. Strata of the Shakopee in its type area at Shakopee, Minnesota, contain conodont elements of Ibexian fauna C. We interpret these strata to represent older strata of the Oneota Formation instead of younger, Stairsian-age strata of the Shakopee Formation, and accordingly, we reassign these strata to the Oneota.