USGS Home
Contact USGS
Search USGS

Mineral Resources > Online Spatial Data > Geology > by state > Iowa Geology

Geologic units in Washington county, Iowa

Kinderhook Series (Phanerozoic | Paleozoic | Carboniferous Mississippian-Early) at surface, covers 19 % of this area

Gilmore City Limestone- light gray fossiliferous limestone, commonly oolitic. Approx thickness 155 ft. Hampton Formation- limestone and dolomite; fossiliferous gray chert in lower portion. Approx. thickness 150 ft. Starrs Cave Formation- bio-fragmental limestone; oolitic in part. Approx thickness 15 ft. Prospect Hill Formation- greenish-gray siltstone. Approx thickness 90 ft. McCraney Limestone- very pale orange to pale yellowish-brown sublithographic limestone and brown dolomite. Approx thickness 65 ft.

Lithology: limestone; dolostone (dolomite); siltstone; chert

Yellow Spring Group (Phanerozoic | Paleozoic | Devonian-Late) at surface, covers 22 % of this area

English River Formation- gray siltstone; southeastern Iowa; locally in north-central Iowa. Approx thickness 23 ft. Maple Mill Shale- greenish-gray shale, silty in upper part; contains spore carps; discoidal, concentrically laminated limonite pellets at top in the subsurface of central and southwestern Iowa. Approx thickness 300 ft. Aplington Formation- argillaceous, silty dolomite and minor chert; quartz geodes. Approx thickness 40 ft. Sheffield Formation- greenish-gray shale in central Iowa and in the subsurface of southwestern Iowa; in decending order, dusky yellowish-brown shale followed by greenish-gray shale, very light olive-gray shale, and dark olive-gray shale in the subsurface of southeastern Iowa. Approx thickness 95 ft.

Lithology: shale; dolostone (dolomite); siltstone

Cherokee Group (Phanerozoic | Paleozoic | Carboniferous Pennsylvanian-Middle [Des Moines]) at surface, covers 4 % of this area

Cyclic deposits with carbonaceous shale, clay, siltstone, with lesser sandstone, and thick coal beds; minor but persistent limestone beds; may include parts of Atoka or Morrow Series.

Lithology: shale; claystone; siltstone; sandstone; coal; limestone

Meramec Series (Phanerozoic | Paleozoic | Carboniferous Mississippian-Late) at surface, covers 12 % of this area

Ste. Genevieve Limestone- fossiliferous limestone and red and green shale. Approx. thickness 50 ft. St. Louis Limestone- limestone and dolomite, sandstone locally perdominant; locally contains chert. Spergen Formation- sandy micaceous dolomite.

Lithology: limestone; dolostone (dolomite); shale; sandstone; chert

Osage Series (Phanerozoic | Paleozoic | Carboniferous Mississippian-Early) at surface, covers 43 % of this area

Warsaw Formation- gray, dolomitic shale and argillaceous dolomite; chalcedonic chert. Locally contains many geodes.Approx thickness 85 ft. Keokuk Limestone- fossiliferous, gray or brown limestone and dolomite; gray and brown chert with white spicules, locally predominant in lower portion; minor brown or gray shale. Approx thickness 90 ft. Burlington Limestone- gray, fossiliferous limestone and darker gray dolomite; white and gray mottled fossiliferous chert, locally contains dolomite crystals; two widespread glauconite zones; basal sandstone locally in southeastern Iowa. Approx thickness 80 ft.

Lithology: dolostone (dolomite); limestone; shale; chert; sandstone

Lime Creek Formation (Phanerozoic | Paleozoic | Devonian-Late) at surface, covers 0.8 % of this area

Fossiliferous limestone or dolomite in upper part with local stromatoporoid biostromes in north-central Iowa; extremely fossiliferous gray shale and argillaceous limestone in middle; gray shale in lower part with spore carps toward base.

Lithology: limestone; dolostone (dolomite); shale