Augusta Group

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).
State Iowa
Name Augusta Group
Geologic age Middle Mississippian, Osagean
Lithologic constituents
Major
Sedimentary > Carbonate > Limestone (Bed)dolomitic, fossiliferous; fossiliferous, (especially crinoidal packstone-grainstone); minor glauconitic limestone/dolomite
Sedimentary > Carbonate > Dolostone (Bed)part argillaceous to shaly; incidental phosphatic dolomite/limestone (“bone bed”)
Minor
Sedimentary > Clastic > Mudstone > Shale (Bed)gray to green-gray, part dolomitic, part silty
Sedimentary > Chemical > Chert (Bed)nodular to bedded; in northern Iowa - dolomite/chert breccia
Incidental
Sedimentary > Clastic > Siltstone (Bed)dolomitic
References

Witzke, Brian J., Anderson, Raymond R., and Pope, John P., 2010, Iowa Geological and Water Survey, DNR, Bedrock Geologic Map of Iowa, OFM-2010-1. Scale 1:500,000.

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Counties Boone - Calhoun - Davis - Des Moines - Grundy - Hamilton - Hardin - Henry - Humboldt - Ida - Iowa - Jasper - Jefferson - Johnson - Keokuk - Lee - Louisa - Mahaska - Marion - Marshall - Monona - Polk - Poweshiek - Sac - Story - Tama - Van Buren - Washington - Webster - Woodbury - Wright