STRATIGRAPHY OF THE CHEROKEE GROUP AND REVISION OF PENNSYLVANIAN STRATIGRAPHIC NOMENCLATURE IN IOWA

R.L. Ravn, J.W. Swade, M.R. Howes, J.L. Gregory, R.R. Anderson, and P.E. VanDorpe


Iowa Department of Natural Resources, Geological Survey Bureau,
Technical Information Series 12, 1984, 76 p.

ABSTRACT


Recent studies have revealed the need to revise and refine the stratigraphic nomenclature of the Pennsylvanian System in Iowa. Clear distribution is made between major chronostratigraphic and lithostratigraphic units in order to clarify confusion in the use of the term "Series" in previous literature. The term "Series" is restricted to its proper chronostratigraphic usage, and the term "Supergroup" is applied to strictly-defined lithostratigraphic units.

The major Pennsylvanian outlier in Scott and Muscatine counties is recognized as a structural extension of the Eastern Interior (Illinois) Basin. The Illinois formation names of Caseyville and "Spoon" are extended into Iowa for these strata. Strata of the Caseyville Formation are recognized as the oldest known Pennsylvanian (Morrowan Series) sediments in Iowa.

Systematic coring within the Des Moines Supergroup in southern Iowa has provided previously unavailable continuous sections through the Cherokee Group, and permits, for the first time, a consistent correlation through this interval. Based on major depositional episodes, four new formations are delineated within the Cherokee Group, in ascending order: Kilbourn, Kalo, Floris, and Swede Hollow. Coal bed nomenclature within the Cherokee also is formalized. The lower portion of the Cherokee Group is known to correlate with Illinois Basin strata considered to be of Atokan age. A practical chronostratigraphic boundary for the top of the Atokan Series is placed, through newly-named Blackoak and redefined Cliffland Coals. The new name Carruthers is proposed to replace the name Wiley for the uppermost coal of the Floris Formation because of previous miscorrelation between this coal and the properly designated Wiley Coal of Illinois.

The boundary between the Cherokee and Marmaton Groups in Iowa is redefined as the base of the Excello Shale, rather than its top. This change results from review of the depositional interpretation of lowermost Marmaton strata. With the inclusion of the Excello Shale, the lowermost Marmaton "Fort Scott" Formation is considered to comprise two marine cyclothems, each depositionally analogous to a single formation in overlying Marmaton strata. Three new formational units are defined to accommodate these two marine cyclothems and the intervening marginal marine to non-marine shale unit. In ascending stratigraphic order, these are: Mouse Creek Formation, Morgan School Shale, Stephens Forest Formation. Depositional interpretation of the upper Marmaton Formations in Iowa are also reviewed, and two new, but yet unnamed, formations are identified.

Group nomenclature in the Missouri Supergroup is revised. The basal unit, previously designated as the Pleasanton Group, is reduced to formation status, and placed within a resurrected and newly-defined Bronson Group, to include strata from the Pleasanton upward through the Dennis Formation. The overlying Kansas City Group is redefined to include strata from the Cherryvale formation upward through the Bonner Springs Shale. The definition of the Lansing Group is unchanged. The Westerville Limestone and Quivira Shale, previously considered formations in Iowa, are redesignated as members of the Cherryvale Formation, to bring Iowa nomenclature into conformity with that of Kansas. No changes are proposed for strata of the Virgil Supergroup.