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.
Includes Elgin, Clermont, Fort Atkinson, Brainard, and Neda members. Overlain by Silurian strata in east-central Iowa, incised paleovalleys beneath Mosalem Fm.; truncated beneath Devonian strata in northern-most counties. Primary lithologies: shale, green-gray, variably dolomitic, mostly unfossiliferous; dolomite, argillaceous, part shaly, variably fossiliferous (part with common trilobites); dolomite, part argillaceous, part cherty to very cherty (northern Iowa). Secondary lithologies: shale, brown to brown-gray, organic, part graptolitic, part finely laminated (lower strata); dolomitic limestone to limestone, argillaceous, part cherty to very cherty, variably fossiliferous, part crinoidal (northern Iowa); interbedded dolomite and shale, part nodular, part fossiliferous. Minor: phosphorite, granular to massive (basal unit); dolomite, phosphatic to very phosphatic, argillaceous (Elgin Mbr.); ooidal ironstone and red shale (Neda Mbr.); pyrite, finely disseminated to nodular, pyrite cements. Maximum thicknesses: generally 200-275 ft (60-85 m) beneath Silurian, locally 100-200 ft (30-60 m) beneath Mosalem Fm.; 75-180 ft (23-55 m) where capped by Devonian strata in northern Iowa.
Mosalem Formation infills erosional paleovalleys incised into Maquoketa Shale; absent outside of paleovalleys. Lower Hopkinton Formation locally contains carbonate mound facies in Bremer County. Blanding Formation oversteps Tete des Morts-Mosalem edge in southern outcrop area. Primary Lithologies: dolomite, fossil-moldic to vuggy, fine to coarse crystalline; dolomite, dense to porous, cherty to very cherty, nodular to bedded chert (especially Blanding Fm). Secondary Lithologies: dolomite, dense, sparsely fossiliferous; dolomite, argillaceous, part laminated (Mosalem Fm). Minor: chalcedony, silicified fossils. Maximum thicknesses total interval: 160-330 ft (49-100 m), thickest in Dubuque, Jackson, Jones counties where Mosalem present; southern outcrop total thickness 65-130 ft (20-40 m). Formation thicknesses: Hopkinton, 65-160 ft (20-49 m); Blanding, 25-65 ft (8-20 m); Tete des Morts, 0-35 ft (11 m); Mosalem, 0-100 ft (30 m).
Interval is a northwestern limestone facies that correlates with dolomite strata (Tete des Morts, Blanding, lower Hopkinton formations) to the south and east. Sw forms the highest Silurian unit throughout most of its extent, generally overlain by Dw; locally interfingers with Sh dolomite strata to south. Primary lithology: limestone, dense, fossiliferous. Secondary lithologies: limestone, dense, cherty to very cherty; dolomitic limestone, dense; dolomite (locally at base). Minor: dolomite, silty to sandy (locally at base above Om). Maximum thickness, 80 ft (25 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).