BEDROCK GEOLOGY OF EAST-CENTRAL IOWADIGITAL GEOLOGIC MAP OF IOWAPHASE 6: EAST-CENTRAL IOWA
prepared by
Brian J. Witzke, Raymond R. Anderson,Bill J. Bunker, and Greg A. Ludvigson
Iowa Geological SurveySeptember 2003
Index Showing Area of Geologic Map
Prepared by the Iowa Department of Natural ResourcesIowa Geological Survey
Open File Map 03-2
Prepared under the STATEMAP program, in cooperation with theU.S. Geological Survey Department of Interior, supported by
Cooperative Agreement 02HQAG0034
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Vinton
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Prairie
South
Minerva
Keigley
Squaw Creek
NORTH
SKUNK R
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Branch
South
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Wolf
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Rock
Creek
Creek
Pratt
HinkleCreek
Cedar
River
Cedar
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Bear
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92 30'
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93 00'93 30'
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R09WR10WR11WR12WR13WR14WR15WR16W
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R24W R23W R22W R21W R20W R19W R18W R17W R16W R15W R14W R13W R12W R11W R10W R09W
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Mg
Mg
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Mg
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Mg
Mg
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Mg
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Mg
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Mg
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Ma
Ma
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Ma
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Ma
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Ma
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Ma
Ma
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Ma
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Ma
Ma
Ma
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Ma
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DcDc
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Dc
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Dc
Dc
Dc
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Dc
Dc
Dc
Dc
Dc
Dc
Dc
Dc
Dc
Dc
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Dc
Dc
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Dc
Dc
Dc
Dc
Dc
Dc
Dc
Dc
Dc
DcDc
DcDc
Dc
Dc
Dc
DcDc
Dc
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Dc
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STORY MARSHALL
TAMA BENTON
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TAMA BENTON
STORY MARSHALL
Rock Exposures in the Study Area
Areas of bedrock exposure or bedrock surface within the soil horizon##
0 5 10 15 20 Miles
scale
0 10 20 30 Kilometers
GEOLOGIC UNIT CONTACTS
BEDROCK WELL DATA POINTS
COUNTY OR TOWN BOUNDARY
MAJOR HIGHWAYS
Interstate Route
U.S. Route
State Route
TOWNSHIP BOUNDARY
RIVER OR STREAM
FAULT ZONE(dashed where inferred)
- identifies up-thrown block
- identifies down-thrown block
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LEGENDDescription of Rock Units
PENNSYLVANIAN
Cherokee Group, undifferentiated; Middle Pennsylvanian (Atokan-Desmoinesian).Maximum thickness 300 ft (91 m) in Story Co., 270 ft (82 m) in Benton Co. Primarylithologies: shale and mudstone, gray to black, part silt y to sandy. Secondarylithologies: sandstone, siltstone. Minor: coal, limestone, pyrite, siderite.
MISSISSIPPIAN
“St. Louis” and P ella formations, Middle Mississippian (Meramecian). Maximumthickness 80 ft (24 m) in Story Co.; beveled and truncated beneath Pc; Pella Fm mostlyabsent from map area. Primarily lithologies: dolomite, part sandy; sandstone; limestone,part sandy to sublithographic. Secondary lithologies: limestone/dolomite breccia;siltstone; shale, green-gray, calcareous. Minor: chert, chalcedony.
Augusta Group, includes Burlington, Keokuk, Warsaw formations; MiddleMississippian (Osagean) . Maximum thickness 155 ft (47 m) in Story Co. Primarylithologies: dolomite, part argillaceous to shaly; dolomitic limestone, fossiliferous.Secondary lithologies: shale, gray, dolomitic; glauconitic dolomite; chert, nodular tobedded. Minor: chalcedony, quartz geodes.Minor: quartz druse; dolomite/chert breccia.
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Gilmo re City Fm and “Eagle City” beds; Mississippian (upper Kinderhookian, lowerOsagean); informal stratigraphic grouping, regional stratigraphic relationships uncertain.Maximum thickness 70 ft (21 m) in northern Story Co. Primary lithologies: limestone,fossiliferous, oolitic, coated grain. Secondary lithologies: dolomite; dolomiticlimestone; limestone, dense to peloidal. Minor: glauconite, chert.
Maynes C reek, Chapin , Prospect Hill fo rmations; Mississippian (Kinderhookian).Maximum thickness 100 ft (30 m); Maynes Creek Fm 35 to 70 ft (11-21 m); Chapin Fm5 to 20 ft (1-6 m); Prospect Hill Fm 0 to 20 ft (6 m). Primary lithologies: dolomite,cherty to very cherty (Maynes Creek); limestone, oolitic (Chapin) ; siltstone (ProspectHill). Secondary lithologies: dolomite; fossiliferous limestone. Minor: calcareous shale,green-gray.
DEVONIAN
Famennian inte rval; primarily undifferentiated shale unit across map area (“SavertonShale”); subdivisible into “Maple Mill,” Aplington, and Sheffield formations innorthwest Tama Co.; Upper Devonian (Famennian) . Maximum thickness 150 ft (46 m).Primary lithologies: shale, green-gray to gray, part dolomitic to calcareous. Secondary:argillaceous dolomite; siltstone, part argillaceous. Minor: brown shale; red shale; chert;oolitic ironstone (hematite/limonite).
Lime C reek Formation; includes Juniper Hill, Cerro Gordo, Owen members; UpperDevonian (upper Frasnian) . Maximum thickness 200 ft (61 m) in western Tama Co.Primary lithologies: gray to green-gray shale, dolomitic to calcareous; dolomite, partargillaceous. Secondary lithologies: fossiliferous limestone; dolomitic limestone, partargillaceous.
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Cedar Valley Group; includes Little Cedar, Coralville, Lithograph City formations;Middle and Upper Devonian (upper Givetian, lowermost Frasnian). Maximumthickness 310 ft (95 m) in northwest Tama Co; thins southeastward to full thickness of110 ft (34 m) in southeast Benton Co. Primary lithologies: limestone, fossiliferous, partargillaceous; dolomite to dolomitic limestone. Secondary lithologies: unfossiliferouslimestone, argillaceous to sublithographic; chert. Minor to secondary:gypsum/anhydrite (Tama Co.).
Wapsipinicon Group; includes Pinicon Ridge Fm (Kenwood, Spring Grove,Davenport members), Otis Fm, Bertram Fm; Middle Devonian (upper Eifelian-middleGivetian). Full thickness 65 to 130 ft (20-40 m) in Benton Co. Primary lithologies:dolomite; sublithographic limestone; limestone breccia. Secondary lithologies: silty tosandy dolomite; shale, gray to green-gray, part sandy. Minor: chert, chalcedony,sandstone. Note: includes gypsum/anhydrite in Tama Co.
SILURIAN
LaPorte City F ormation; Lower Silurian (upper Llandovery-lower Wenlock).Maximum thickness 140 ft (43 m). Primary lithology: limestone, dense to fossiliferous.Secondary lithologies: chert (includes chert breccias at top); dolomite. Minor: shale,green-gray calcareous shale; clay residuum (at top).
Scotch G rove Formation; Lower Silurian (upper Llandovery-middle Wenlock).Maximum thickness 220 ft (67 m). Primary lithology: dolomite, dense to porous.Secondary: chert (absent to abundant). Minor: quartz druse; dolomite/chert breccia.
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Description of Map Symbols