Upper Ordovician Stratigraphy in Eastern Interior Region

ABSTRACT The name Cincinnatian Series, a time-stratigraphic term, is restricted to rocks of Late Ordovician age, wherever they occur. Most of the “formations” assigned to the Cincinnatian Series are actually biostratigraphic zones. The name Cincinnati group is revived for the body of rocks continuous with the type Cincinnatian Series in the Cincinnati arch region, without time connotation. The Cincinnati group in subsurface is composed, in descending order, of the Maysville-Richmond formation and the Eden shale. Upper Ordovician rocks in western Indiana and eastern and southern Illinois are assigned to the Maquoketa group, composed, in descending order, of the Orchard Creek shale, Cape limestone (Templeton and Willman, in press), and Eden shale. Upper Ordovician rocks in northwestern Illinois and eastern Iowa are assigned to the Maquoketa shale. Contrary to existing correlations, rocks representing Edenian and Maysvillian, as well as Richmondian deposition, are present in Illinois and Iowa.