Uppermost parasequences of the Grand Rapids Formation and Upper Mannville Group record progradation of open-coast wave/storm-dominated deltas in the west, passing eastward into brackish-water bay-head deltas. Distributary channel deposits occur in close proximity to both settings. Stratigraphic cross-sections indicate that autocyclic delta lobe switching occurred in both delta types.
Open-coast deltas contain low-diversity trace fossil suites consisting of slightly more robust ichnogenera, most of whom reflect opportunistic colonization of storm beds. Ichnogenera typical of more marine conditions are sporadically distributed, indicating that environments were not as persistently salinity stressed as in the east.
Bay-head deltas in brackish-water settings display the lowest trace fossil diversities and pronounced size reductions. Facies lack ichnogenera associated with more marine conditions. Brackish-bay deposits are the most intensely burrowed, attesting to low deposition rates. Ichnological diversities are slightly greater than in bay-head delta deposits, reflecting less marked salinity fluctuations in persistently brackish-water conditions.