Geologic units in Johnson county, Iowa

Cedar Valley Group (Middle-Upper Devonian, middle Givetian-middle Frasnian) at surface, covers 34 % of this area

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.

Famennian formations (Upper Devonian, lower to upper Famennian) at surface, covers 30 % of this area

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).

Lime Creek Formation (Upper Devonian, upper Frasnian) at surface, covers 18 % of this area

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).

Wapsipinicon Group (Middle Devonian, upper Eifelian-middle Givetian) at surface, covers 8 % of this area

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.

Scotch Grove Formation (Lower Silurian, upper Llandovery-lower Wenlock) at surface, covers 5 % of this area

Formation includes Welton, Buck Creek Quarry, Waubeek, Palisades-Kepler, Johns Creek Quarry members; Palisades-Kepler and Johns Creek Quarry members contain carbonate mound facies. Erosionally beveled and truncated beneath Dw. Primarily Lithologies: dolomite, porous, fossil-moldic to vuggy, part very crinoidal (includes Welton Mbr); dolomite, cherty to very cherty, dense (Buck Creek Quarry Mbr). Secondary Lithologies: dolomite, sparsely fossil-moldic, dense, part vuggy (includes Waubeek Mbr); dolomite, coarsely crystalline, part very crinoidal (within Johns Creek Quarry, Palisades-Kepler mbrs); dolomite mudstone, dense. Minor: dolomite, slightly argillaceous; quartz druse, chalcedony, silicified fossils. Maximum thicknesses: 94-240 ft (29-73 m); may reach thicknesses to 300 ft (90 m); beveled and truncated beneath Dw.

Lower Cherokee Group and Raccoon Creek Group (Lower-Middle Pennsylvanian; Morrowan-lower Desmoinesian) at surface, covers 3 % 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).

Gower Formation (Lower-?Upper Silurian, Wenlock-?Ludlow) at surface, covers 2 % of this area

Formation includes Anamosa, Brady, LeClaire (Scott-Muscatine counties) members; Brady and LeClaire members are carbonate mound facies. Erosionally beveled and truncated beneath Dw. Primary Lithologies: laminated dolomite (Anamosa Mbr), part fetid/organic to east (Scott-Clinton counties); dolomite mudstone, dense, featureless. Secondary Lithologies: dolomite, fossiliferous to vuggy, moldic, part brachiopod-rich (Brady Member); dolomite, part coarsely crystalline, vuggy, fossiliferous to sparsely fossiliferous, part crinoidal (LeClaire Member). Minor: chert; intraclastic dolomite. Maximum thickness: 180 ft (55 m); beveled and truncated beneath Dw.

Kinderhookian formations (Lower Mississippian, Kinderhookian) at surface, covers < 0.1 % of this area

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.

Augusta Group (Middle Mississippian, Osagean) at surface, covers < 0.1 % 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).