Interval includes Little Cedar, Coralville, Lithograph City, and Shell Rock formations; Shell Rock Fm in northern Iowa only. Primary Lithologies: limestone, fossiliferous, variably argillaceous, part biostromal; dolomite, part fossil-moldic to vuggy, variably argillaceous. Secondary Lithologies: limestone, dense, “sublithographic”; dolomite/limestone breccia (evaporite collapse); limestone, sparse to unfossiliferous, argillaceous to shaly; shale, gray to green-gray, dolomitic. Minor: anhydrite/gypsum (outcrop belt only in Grundy, Tama, Poweshiek counties; extensive in subsurface of central and southern Iowa); chert; glauconite; sandy limestone/sandstone. Maximum thicknesses in outcrop belt: southeast Iowa, 80-135 ft (24-41 m); east-central Iowa 130-210 ft (40-64 m); northern Iowa 250-350 ft (76-107 m). Cedar Valley Group thins and is overstepped by Dl westward in Winnebago County.
Interval includes Pinicon Ridge, Otis, Spillville, and Bertram formations. Otis and Bertram formations restricted to east-central Iowa only; Spillville Formation in northeast Iowa only. Pinicon Ridge Formation oversteps Spillville and Otis edges in Fayette, Bremer, Black Hawk, Buchanan, southern Chickasaw counties to directly overlie Sh, Sw, or Om. Primary Lithologies: dolomite, part laminated, variably argillaceous, part fetid; limestone, dense, “sublithographic”, part laminated to intraclastic; dolomite, fossil-moldic to vuggy (Otis-Spillville fms). Secondary Lithologies: limestone/dolomite breccia (evaporite collapse); dolomitic shale and shaly dolomite, gray to green-gray, part silty-sandy; limestone, part peloidal to fossiliferous (Otis Fm). Minor: chert and chalcedony nodules (Pinicon Ridge Fm); sandstone; oolitic limestone (Otis Fm). Extensive anhydrite/gypsum in subsurface outside of outcrop belt. Maximum thicknesses in outcrop belt: east-central Iowa, 60-160 ft (18-49 m); northern Iowa, 10-130 ft (3-40 m), locally absent in Bremer County beneath Dc.
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).
Dolostone, limestone, and shale; Includes the Lithograph City Formation, Coralville Formation, and Hinkle and Eagle Center Members of the Little Cedar Formation.
Dolostone, sandy dolostone, limestone, and shale; includes the Chickasaw Shale and Bassett Member of the Little Cedar Formation, and Pinicon Ridge and Spillville Formations.