Robert M. McKay
The St. Louis through Pella formations comprise the upper 20 to 40 meters of Mississippian strata over much of southeast to north-central Iowa. Several unconformity-bounded shelf sequences are known or suspected within the overall package, and these sequences display a wide spectrum of facies from evaporites to normal marine carbonates to terrestrial sandstones.
The basal sequence overlies a widespread unconformity developed on the Keokuk, Warsaw, and locally, Salem/Sonora formations. This sequence is best preserved in south-central Iowa, and is dominated by gypsum and anhydrite representative of widespread evaporative shelf conditions. Elsewhere, breccias suggest the existence of former evaporite deposits. It is uncertain whether an unconformity truncates this sequence.
The second sequence, commonly referred to as the lower St. Louis or Croton Member, is dominated by normal marine skeletal dolostones and limestones that shallow in some sections to channeled peritidal facies. A second breccia is suggestive of former evaporites of a coastal salina/sabkha. Truncation is suggested by the local presence of overlying sandy conglomerates.
The regressive Yenrougis Sandstone Member comprises the lower part of the third sequence in western counties. Stratification and trace fossils suggestive of shoreface and near shoreface conditions dominate the upper Yenrougis. This sandstone is conformably (?) overlain by a marine carbonate, the lower Verdi Member, that represents normal marine through supratidal conditions culminating with paleosol development and local paleokarst.
A fourth sequence of restricted-marine sandstone, shale and limestone of the upper Verdi Member is locally and conformably (?) overlain by brackish to fresh-water ostracode/fish bearing limestones that contain paleokarst partially filled with tetrapod-bearing conglomerates. These grade upward through nonmarine ostracode/fish-bearing shale and limestone and are capped by an extensively rooted sandy paleosol with thin and poorly developed coal.
The normal-marine shale and limestone succession of the overlying Pella Formation forms the uppermost sequence that is truncated by the Pennsylvanian/Mississippian unconformity.