The Osage Series in southeastern Iowa is composed primarily of cherty carbonate rocks. Thin shale beds appear in the middle of the series and increase upward in number and thickness. The lithologic character and thickness of the series are relatively uniform throughout the area. This gives no suggestion of major changes in environment of deposition or tectonics either laterally or vertically.
It is convenient to consider the area in two parts, the south-eastern district and the western district. Subsurface sections in the southeastern district are readily matched to the classic surface exposures of the Burlington Limestone, Keokuk Limestone, and Warsaw Formation near the Mississippi River. The lithologies of the three formations are so similar that small changes, and especially dolomitization toward the west, make tracing of boundaries difficult.
The Burlington Limestone is divided in the southeast into three members that are named and described in this report. The Dolbee Creek Member at the base and the Cedar Fork Member at the top are mainly recrystallized crinoidal bioclastic limestone. The Haight Creek Member in the middle is very cherty and contains much dolomite even in the east. Glauconite at the base of the Haight Creek and disseminated in the Cedar Fork is a persistent horizon marker. Haight Creek and Cedar Fork are found throughout the area; Dolbee Creek is restricted to the southeastern district.
The Keokuk Limestone is characterized by mottled gray bioclastics and chert in the southeast, and brownish-black cherts with white spicules and argillaceous dolomite in the west. The carbonate is argillaceous and the formation contains shale beds throughout the area. No sharp boundary marks the contact with the overlying Warsaw Formation. The latter consists of grayer dolomite beds and much more shale than the Keokuk; chalcedonic chert and crystalline quartz are generally abundant.
The Warsaw Formation is included in the Osage Series be cause of its gradational relationship to the underlying Keokuk Limestone and because of the unconformity above it.
Graphic well sections and photomicrographs illustrate the lithologic nature of each unit. Cross sections, thickness maps, and a lithofacies map of the Keokuk-Warsaw Formations depict the changes and regional variations. Structural features are shown by a map using as a datum the base of the Haight Creek Member of the Burlington Limestone.
The original sediments were marine limestone and shale deposited under stable shelf conditions. The limestones consisted mainly of disarticulated crinoid skeletons which were spread evenly over the sea floor but appear not to have been carried far by waves or currents. There is little evidence of bar accumulation. The occurrence of thicker shale sections peripheral to anticlines suggests that these structures may have been active at the time of deposition. Both dolomite and chert are probably diagenetic.
Iowa was beyond the reach of terrigenous deposits from the Appalachian tectonic belt. Any land in the direction of the Canadian Shield was certainly very low, and the seas extended widely to northwest and southwest.