Geologic units in Harrison county, Iowa

Dakata and Windrow Formations (“Mid” Cretaceous; upper Albian-upper Cenomanian) at surface, covers 37 % of this area

Dakota Formation widespread in western Iowa, lower sandstone-dominated Nishnabotna Member, upper mudstone/shale-dominated Woodbury Member. Correlative Windrow Formation found as erosional outliers in northeastern and north-central Iowa. Primary lithologies: sandstone, quartzose, very fine to medium grained; mudstone/shale, light to dark gray, variably silty-sandy, noncalcareous (Woodbury Mbr.). Secondary lithologies: sandstone, medium to very coarse grained, part pebbly to gravelly, locally cemented by iron oxides (Nishnabotna Mbr., Windrow Fm.), gravel, quartz and chert clasts; siltstone; mudstone, red, pink, yellow-brown, black (carbonaceous). Minor: lignite; siderite (concretions, pedogenic sphaerosiderite pellets, cemented siltstone); massive iron ore, silty to sandy (Windrow Fm.). Maximum thickness Dakota Fm. 500 ft (150 m), commonly 100-300 ft (30-90 m); Windrow Fm. 40 ft (12 m).

Marmaton Group (Middle Pennsylvanian, upper Desmoinesian) at surface, covers 32 % 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).

Upper Cherokee Group (Middle Pennsylvanian, middle Desmoinesian) at surface, covers 15 % 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).

Lower Cherokee Group and Raccoon Creek Group (Lower-Middle Pennsylvanian; Morrowan-lower Desmoinesian) at surface, covers 9 % 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).

Bronson Group (Middle to Upper Pennsylvanian, uppermost Desmoinesian-lower Missourian) at surface, covers 7 % of this area

Interval includes Pleasanton, Hertha, Elm Branch, Swope, Galesburg, Dennis formations. Primary Lithologies: limestone, fossiliferous; shale/mudstone, light to dark gray. Secondary lithologies: mudstone to siltstone, gray to green-gray; limestone, coated grain. Minor: black shale, phosphatic; red/maroon mudstone; coal; chert. Maximum thicknesses 100-130 ft (30-40 m).

Marmaton Group (Middle Pennsylvanian - Desmoinesian) at surface, covers 0.1 % of this area

Shale and limestone. Shale is dark gray to light gray, greenish gray, black, red, silty, sandy carbonaceous, fissile, calcareous; contains scattered limestone nodules. Limestone is medium gray to light gray, red, mottled thin bedded to thick bedded, sandy, shaly, vuggy, fossiliferous. Approx. max thickness 125 ft

Kansas City Group (Late Pennsylvanian - Missouria) at surface, covers < 0.1 % of this area

Limestone and shale. Limestone is dark gray to light gray, brownish gray, very thin bedded to massive, argillaceous, fossiliferous; contains very thin layers of chert, pyrite crystals, and small flakes of mica near base. Shale is dark gray to light gray, greenish gray, red, and black; locally slighty sandy, calcareous, carbonaeous, fissile, and fossiliferous. Approx. max thickness 200 ft