VIRGILIAN (UPPER PENNSYLVANIAN) CYCLOTHEMS IN SOUTHWEST IOWA

Virgilian stratigraphy in southwest Iowa has received little attention since the report of Hershey et al. (1960). Available core and outcrop sections were re-examined as part of a STATEMAP-sponsored bedrock mapping project for southwest Iowa. Several long continuous cores provide master reference sections with which to compare shorter core and outcrop intervals. The longest reference core (340 m) was drilled near Riverton, Fremont Co., and penetrates strata from the Stotler Fm (Wabaunsee Gp) into the Marmaton Gp. The cyclothemic framework for the Midcontinent Virgilian proposed in various papers by Boardman and Heckel is clearly discernible across southwest Iowa, and all major and intermediate cyclothems and most minor cyclothems are recognized. However, two minor cyclothems (rather than one) are identified in the Lawrence Fm: a lower Amazonia (with paleosol cap) and an upper unnamed (fossiliferous shale above the “upper Williamsburg” coal). Strata of the Kereford, Clay Creek, and Spring Branch cyclothems are thinned and difficult to differentiate in Iowa; the Clay Creek overlies a local coal and is separated from the Spring Branch by a very thin to absent Stull Shale. Likewise, the Curzon-Sheldon cylothems are closely spaced and thin (<2m), but “birdseye” fenestrae mark subaerial exposure at the top of the interval. Three cyclothems are recognized in the Rulo through Soldier Creek interval in Kansas, but four cyclothems seem to be present in the same stratigraphic interval in Iowa. The highest Pennsylvanian strata recognized in Iowa belong to the Jim Creek cyclothem.