PENNSYLVANIAN CYCLOTHEMS AND PRELIMINARY CORRELATIONS OF THE SO-CALLED “CASEYVILLE” FORMATION OF EASTERN IOWA-NORTHERN ILLINOIS

Outliers of Lower Pennsylvanian strata are known from eastern Iowa and northwestern Illinois, separated by over 400 km from correlative strata in southern Illinois. These outliers have been given the label of “Caseyville Formation” in Iowa and northern Illinois, a name imported from the southern Illinois Basin. This correlation and usage was further encouraged by the discovery of well-defined Morrowan spore assemblages in lower “Caseyville” strata of Iowa (especially the studies of Robert Ravn). The “Caseyville” Formation in both Iowa and northern Illinois ostensibly has been used as a stratigraphic label to include all Pennsylvanian strata below the Abbott/Tradewater formations, and the interval is known to include a succession of seven cyclothems (as first outlined by Searight and Smith, 1969) up to 70 m thick. It has been assumed that these strata correlate with the Caseyville Fm. of southern Illinois, and, hence, are entirely of Morrowan age. Each of the northern “Caseyville” cyclothems display a similar vertical succession: 1) basal coal bed; 2) marine-influenced estuarine mudstone-siltstone facies, commonly pyritic; and 3) an upper paleosol, with local erosional channeling. Sandstone channels are common but laterally discontinuous. Maximum transgression is marked by estuarine facies with brackish faunas and burrows, and tidally-modulated sedimentary features are common. The lowest two cyclothems include diverse palynofloras that corroborate a Morrowan correlation. However, preliminary palyostratigraphic sampling of upper “Caseyville” strata in Iowa suggests that the upper three (and possibly five) cyclothems are likely Atokan units (Endosporites globiformis and others). As such, much of the interval that has been termed “Caseyville” in Iowa-Illinois does not correlate with the Caseyville Fm. of southern Illlinois-Kentucky. Instead, upper “Caseyville” strata likely correlate with the Kilbourn Formation (Cherokee Group) of south-central Iowa, and the lower Abbott/Tradewater of Illinois. The Caseyville Formation probably should be abandoned as a stratigraphic label in eastern Iowa, and new terminology introduced.