Geologic units in Pocahontas county, Iowa

Dakata and Windrow Formations (“Mid” Cretaceous; upper Albian-upper Cenomanian) at surface, covers 52 % 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).

Manson Group, Terrace Terrane (Megablock Zone); Manson Impact Structure (Upper Cretaceous; upper Campanian) at surface, covers 16 % of this area

Upper Interval (bedrock surface): sedimentary-clast breccia with shaly matrix, clasts and blocks of Cretaceous shale and varied Paleozoic lithologies entrained in silty-sandy clay matrix; minor clasts/grains of Proterozoic “red clastics “ (mudstone/siltstone), crystalline basement rock, devitrified melt rock. Lower Interval (variably present, locally forms bedrock surface): overturned ejecta flap, breccias of Paleozoic lithologies and Proterozoic “red clastics” preserved in general inverted stratigraphic order. Breccias overlie down-dropped blocks (ring graben) of Cretaceous and Paleozoic strata in normal stratigraphic succession. Maximum thicknesses: upper shaly breccia up to 700 ft (210 m); lower interval approx. 1000 ft (300 m).

Manson Group, Crater Moat area; Manson Impact Structure (Upper Cretaceous; upper Campanian) at surface, covers 13 % of this area

Primary Lithology: sedimentary clast breccia with shaly matrix; clasts and blocks of Cretaceous shale and varied Paleozoic lithologies in silty-sandy clay matrix. Minor Lithologies: grains or clasts of Proterozoic “red clastics” (mudstone/siltstone), Proterozoic crystalline basement, devitrified melt grains. Maximum thickness unknown, may reach thicknesses to 10,000 ft (3000 m).

Kinderhookian formations (Lower Mississippian, Kinderhookian) at surface, covers 7 % 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.

Manson Group, Central Peak; Manson Impact Structure (Upper Cretaceous; upper Campanian) at surface, covers 6 % of this area

Primary Lithology (at bedrock surface): sedimentary clast breccia with shaly matrix; clasts and blocks of Cretaceous shale and varied Paleozoic lithologies in silty-sandy clay matrix (see above; thickness 0-200 ft; 60 m). Secondary Lithologies: crystalline rock megabreccia (locally forms bedrock surface), fractured and brecciated Proterozoic basement lithologies, dominantly gneiss but including granite and other igneous-metamorphic rocks (maximum thickness unknown). Suevite and impact-melt breccias (locally forms bedrock surface), clasts of Proterozoic crystalline basement rocks, melt-rock, and minor Proterozoic “red clastics” in a matrix of melt-rich material (devitrified) and/or sandy-silty debris (thicknesses to 380 ft; 115 m). Keweenawan shale-clast breccia (locally forms bedrock surface), dominated by gray to black shale clasts (small to large) derived the lower “red clastics” sequence, common melt clasts, minor basement and Phanerozoic clasts, in a silty-sandy shaly matrix or melt-rich matrix (thicknesses to 250 ft, 75 m).

Gilmore City Formation (Lower Mississippian, upper Kinderhookian to Middle Mississippian, lower Osagean) at surface, covers 4 % of this area

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).

Lime Creek Formation (Upper Devonian, upper Frasnian) at surface, covers 1 % 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).

Famennian formations (Upper Devonian, lower to upper Famennian) at surface, covers 0.8 % 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).