Upper Cambrian–Middle Ordovician carbonates and mixed siliciclastic rocks in the Michigan Basin form a shallow-water marine sedimentary package that thickens greatly into the basin center to more than 2000 ft (600 m). These sedimentary rocks, representing the upper part of the Sauk megasequence, are truncated or completely absent at the edges of the basin, whereas sedimentation may have been continuous across the major interregional Sauk-Tippecanoe unconformity in the basin center. The Michigan Basin strata have been correlated with the Prairie du Chien Group and to the underlying Trempealeau Formation, both of which crop out farther west in the upper Mississippi Valley, especially in southwestern Wisconsin. Current stratigraphic nomenclature assigns the upper interval in the central part of the Michigan Basin to the Foster Formation, whereas the middle interval is unassigned and the base of the package is the Trempealeau Formation.

More than 250 wells penetrate most of this entire sedimentary package that reaches a thickness exceeding 2300 ft (700 m) in the center of the Michigan Basin. Using this well data, along with the few available cores and drill cuttings, mostly from the Foster Formation, a regional stratigraphic and lithologic framework can be established. The package around the basin margins is pervasively dolomitized but is generally pure carbonate except for some cherty horizons. In the basin center, strata are also mostly dolomite, but fabrics and facies characteristics are better preserved. Local, thin, discontinuous layers of quartz sandstone and shale are interbedded with the carbonates. Some of the carbonate strata contain abundant siliciclastic silt, and diagenetic nodular anhydrite is found sporadically throughout the Foster Formation. A single continuous core of more than 1200 ft (360 m) in length through the Foster Formation in the north-central basin shows multiple meter-scale shallowing-upward packages dominated by shallow subtidal to peritidal carbonate facies. It appears that shallow-marine depositional environments existed throughout the Michigan Basin during the latest Cambrian and Early Ordovician.

Although the stratigraphic sequence is affected by a major unconformity at the basin margins, sedimentation may have kept up with significant subsidence in the basin center. These sediments, preserved in the center of the Michigan Basin, represent some of the youngest deposits of these rocks anywhere in the midwest.