Geologic units in Polk county, Iowa

Lower Cherokee Group and Raccoon Creek Group (Lower-Middle Pennsylvanian; Morrowan-lower Desmoinesian) at surface, covers 73 % of this area

Lower Cherokee Group in southern, central, and western Iowa includes Kilbourn, Kalo, Floris formations (primarily Atokan-lower Desmoinesian, locally Morrowan at base); Raccoon Creek Group in eastern Iowa includes “Caseyville” and Tradewater formations (primarily Morrowan at most localities; locally includes Atokan-lower Desmoinesian Tradewater Formation in upper part). Primary Lithologies: shale/mudstone, light to dark gray, part silty to sandy; sandstone, very fine to medium grained; siltstone, gray. Secondary Lithologies: carbonaceous shale/mudstone, gray to black; phosphatic black shale; limestone, dense, part fossiliferous, part sandy; coal (beds locally > 2 ft). Minor: sandstone, coarse-grained to granular, part conglomeratic; mudstone, red to pink; limestone concretions (may be septarian); cone-in-cone limestone; siderite/ironstone concretions and pellets; pyrite. Maximum thicknesses Raccoon Creek Group in Muscatine-Scott counties: 230-250 ft (70-75 m). Maximum thicknesses Lower Cherokee Group across outcrop belt: 200-370 ft (60-113 m). Maximum thickness in southwest Iowa subsurface: 650 ft (200 m).

Upper Cherokee Group (Middle Pennsylvanian, middle Desmoinesian) at surface, covers 22 % of this area

Interval includes Verdigris and Swede Hollow formations; base of map unit drawn at the widespread Whitebreast Coal. Primary Lithologies: shale/mudstone, light to dark gray, part silty to sandy; siltstone, gray. Secondary Lithologies: limestone, part fossiliferous, part sandy; sandstone, very fine to fine-grained. Minor: mudstone, red/maroon to brown; shale, black, phosphatic; carbonaceous shale; coal (beds locally > 2 ft); ironstone/siderite concretions; limestone concretions (may be septarian); pyrite. Average thickness 100 ft (30 m); maximum thicknesses to 130 ft (40 m).

St. Louis and Pella Formations (Middle-Upper Mississippian, Meramecian-lower Chesterian) at surface, covers 3 % of this area

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).

Augusta Group (Middle Mississippian, Osagean) at surface, covers 2 % of this area

Interval includes Burlington, Keokuk, and Warsaw formations. Upper Warsaw strata locally preserved at top of interval in extreme southeast Iowa are lower Meramecian age . Primary Lithologies: dolomite, part argillaceous to shaly; dolomitic limestone, fossiliferous; fossiliferous limestone (especially crinoidal packstone-grainstone). Secondary Lithologies: glauconitic limestone/dolomite; shale, gray to green-gray, part dolomitic, part silty; chert, nodular to bedded. Minor: quartz crystals, quartz geodes, chalcedony; phosphatic dolomite/limestone (“bone bed”); siltstone, dolomitic. Lithologies noted only in northern Iowa: oolitic limestone; “sublithographic” limestone; dolomite/chert breccia. Interval is erosionally beveled beneath sub-“St. Louis” and sub-Pennsylvanian unconformities. Maximum thicknesses in northern Iowa: 85-105 ft (26-32 m). Maximum thicknesses in southern Iowa: 155-240 ft (47-73 m).

Marmaton Group (Middle Pennsylvanian, upper Desmoinesian) at surface, covers 0.5 % of this area

Interval includes Morgan School, Mouse Creek, Labette, Pawnee, Bandera, Altamont, Nowata, Lenapah, Memorial, Lost Branch formations. Primary Lithologies: shale/mudstone, light to dark gray; limestone, fossiliferous. Secondary Lithologies: mudstone, green-gray to red/maroon; shale, black, phosphatic; sandstone, very fine to medium grained. Minor: coal; siltstone, conglomerate. Maximum thicknesses: 125-175 ft (38-52 m).