Interval includes Prospect Hill, Chapin, Maynes Creek, and Wassonville formations; locally includes basal Crapo (“McCraney”) Formation in southeastern Iowa. Primary Lithologies: dolomite, part cherty to very cherty, part fossiliferous (Maynes Creek-Wassonville formations); limestone, skeletal to oolitic (Chapin Fm; lower Wassonville Fm/Starrs Cave Member; middle Maynes Creek/“Eagle City” member; basal Crapo Fm); siltstone (Prospect Hill Fm). Secondary Lithologies: shale, gray, silty (Prospect Hill Fm of southeastern Iowa); limestone, dense, “sublithographic” to stromatolitic (upper Maynes Creek Fm); limestone and dolomite, dense, nodular to banded, part silty (Crapo Fm). Maximum thicknesses for total interval: 100-175 ft (30-53 m). Thickness variations: Crapo (“McCraney”) Fm, 0-65 ft (20 m); Prospect Hill Fm, 0-90 ft (27 m), generally < 25 ft (8 m); Chapin Fm, 0-30 ft (9 m), generally < 10 ft (3 m); Maynes Creek Fm, 35-155 ft (11-47 m), thickest in central to northern Iowa; Wassonville Fm (southeast Iowa only), 0-70 ft (21 m), replaced to northwest by Maynes Creek and Chapin formations, beveled beneath sub-Burlington unconformity to southeast.
Interval includes Grassy Creek Shale, Saverton Shale, “Maple Mill” Shale, English River Formation, Louisiana Limestone (Lee County only), Aplington Formation (northern and western Iowa only), Sheffield Shale (central and northern Iowa). Famennian strata onlap Lime Creek Fm to the northwest. Primary Lithologies: shale, gray to green-gray, part silty; siltstone (especially English River Fm). Secondary Lithologies: shale, olive-brown and medium to dark brown, part laminated (Grassy Creek Fm, southeast Iowa); dolomite, part fossiliferous, part cherty, part argillaceous (Aplington Fm). Minor: interstratified dolomite and limestone, part fossiliferous (Louisiana Limestone); ooidal ironstone and phosphorite; phosphatic siltstone (“bone bed”); shale, red-brown. Maximum thicknesses in outcrop belt: southeast Iowa, 135-310 ft (41-95 m), thickest in Washington, Louisa, Des Moines counties; northern and central Iowa, 25-135 ft (8-41 m).
Interval includes Sweetland Creek Shale in southeast Iowa, and “Amana beds” of Iowa County. Lime Creek Fm onlaps eroded Ordovician surface in northwestern Iowa. Primary Lithologies: shale, gray to green-gray, dolomitic to calcareous; limestone, variably argillaceous, fossiliferous, part biostromal; dolomite, variably argillaceous, part fossiliferous. Secondary Lithologies: limestone, dense, “sublithographic” (upper part of carbonate-dominated facies in central and northwestern Iowa); shale, green-gray to brown, silty (Sweetland Creek Shale). Minor: siltstone; chert; oolitic limestone, carbonate breccia (central to northwestern Iowa). Thickness variations in outcrop belt: Sweetland Creek Shale of southeastern Iowa, 3-30 ft (1-9 m); Lime Creek Fm of southeastern to north-central Iowa, 40-200 ft (12-60 m); Lime Creek Fm of northwestern Iowa, 200-350 ft (60-105 m).
Interval includes lower Marble Valley Member, upper Humboldt Member; “Iowa Falls Dolomite” (dolomite facies); formation shares partial regional facies relationships with Burlington Formation; not present in southeastern Iowa. Primary Lithologies: limestone, primarily coated grain/oolitic to peloidal, variably fossiliferous to intraclastic. Secondary Lithologies: dolomite; dolomitic limestone; limestone, dense, “sublithographic.” Minor: shale, gray, calcareous; oncolitic limestone; glauconite; chert; breccia. Maximum thicknesses in northern Iowa 150-170 ft (45-52 m); interval thins to south and southeast 20-70 ft (6-21 m).
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).
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).