Sequence stratigraphy of the Prairie du Chien Group, Lower Ordovician, Midcontinent, U.S.A.

The Oneota and Shakopee Formations of the Lower Ordovician Prairie du Chien Group compose the Sauk C 1 and C 2 second-order sequences, or supersequences, which are recognized in both outcrop and the subsurface. These second-order sequences are bounded by type 1 sequence boundaries, the regional Cambrian-Ordovician, mid-Lower Ordovician, and Lower to Middle Ordovician (Sauk-Tippecanoe) unconformities. Karst development and patchy silicification beneath all three sequence boundaries indicate regional subaerial exposure of the outcrop area during unconformity development. In outcrop the Lower Ordovician Prairie du Chien Group consists of four depositional sequences corresponding to the Stockton Hill and Hager City Members of the Oneota Formation and New Richmond and Willow River Members of the Shakopee Formation. These four depositional sequences correspond to the Sauk C 1.1, C 1.2, C 2.2, and C 2.3 third-order sequences, each pair of which is separated by a type 2 sequence boundary. Sequences within the Prairie du Chien Group can be subdivided into systems tracts typically composed of one or two laterally continuous minor lithofacies. Individual systems tracts are composed of meter-scale, shoaling-upward parasequences, but parasequence sets have not yet been identified within Prairie du Chien strata.