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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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

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GEOLOGIC UNIT CONTACTS

BEDROCK WELL DATA POINTS

COUNTY OR TOWN BOUNDARY

MAJOR HIGHWAYS

Interstate Route

U.S. Route

State Route

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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