St. Louis and Pella Formations

Interval includes lower “St. Louis” (Meramecian) Croton and Yenruogis members; upper “St. Louis” (lower Genevievian) Verdi and Waugh members (interval does not correlate with type St. Louis of Missouri); Pella Fm (upper Genevievian); base includes Sonora Fm (Meramecian) in parts of southeast Iowa (Lee, Van Buren, Henry, Des Moines, Jefferson counties). Primary Lithologies: dolomite, part silty/sandy, part fossiliferous; limestone, part sandy to fossiliferous; part “sublithographic,” part argillaceous. Secondary Lithologies: dolomite/limestone breccia; shale/marl, green-gray, calcareous; sandstone, fine to medium-grained; siltstone; gypsum/anhydrite (south-central Iowa). Minor: limestone, oolitic; shale, gray; shale, red to pink; chert/chalcedony. Croton Member contains gypsum-anhydrite in subsurface outside of outcrop belt. Interval is erosionally beveled to locally absent beneath Pcl; Pella Formation absent over broader areas. Maximum thicknesses for total interval 60-150 ft (18-45 m); maximum thickness Pella Formation 75 ft (23 m); maximum thickness “St. Louis” Formation 75 ft (23 m); maximum thickness Sonora Formation 35 ft (11 m).
State Iowa
Name St. Louis and Pella Formations
Geologic age Middle-Upper Mississippian, Meramecian-lower Chesterian
Lithologic constituents
Major
Sedimentary > Carbonate > Dolostone (Bed)part silty/sandy, part fossiliferous
Sedimentary > Carbonate > Limestone (Bed)part sandy to fossiliferous; part “sublithographic,” part argillaceous; incidental oolitic
Minor
Sedimentary > Clastic > Sandstone (Bed)fine to medium-grained
Sedimentary > Clastic > Siltstone (Bed)
Sedimentary > Clastic > Mudstone > Shale (Bed, Calcareous)shale/marl, green-gray, calcareous; incidental units, one gray, one red to pink
Sedimentary > Clastic > Sedimentary-breccia (Bed, Carbonaceous)dolomite/limestone breccia
Sedimentary > Chemical > Evaporite > Gypsum (Bed)gypsum/anhydrite (south-central Iowa)
Incidental
Sedimentary > Chemical > Chert (Bed)chert/chalcedony
References

Witzke, Brian J., Anderson, Raymond R., and Pope, John P., 2010, Iowa Geological and Water Survey, DNR, Bedrock Geologic Map of Iowa, OFM-2010-1. Scale 1:500,000.

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