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Geologic Unit: Decorah

Decorah Formation (IA*,IL*,MN*,MO*,WI*)
Decorah Formation of Galena Group (IA,IL,MN,MO,WI)
Decorah Formation of Sinnipee Group (WI)
Decorah Shale (IA*,MN*)
Decorah Shale of Sinnipee Group (WI)
Decorah Limestone (IL*)
Decorah Group (IL*,MO*)
/Decorah subgroup [informal] (IL)
["Subgroup" not recognized as a formal stratigraphic rank term (CSN, 1933; ACSN, 1961, 1970; NACSN, 1983). Considered informal and should not be capitalized.]

Ordovician, Middle*

Geologic Province:
Iowa shelf*
Illinois basin*
Wisconsin arch*
Ozark uplift*
Upper Mississippi embayment*
Areal Extent:

Type Locality:
Decorah, Winnishiek Co., northeastern IA (Calvin, 1906).
Reference sections in MO: (1) roadcut on I-55, mileage 182+, in NW/4 sec. 6, T. 41 N., R. 6 E., Herculaneum 7.5-min quadrangle, Jefferson Co.; (2) roadcut on north frontage road of I-44, west of Allenton-Six Flags exit, in sec. 33, T. 44 N., R. 3 E., western St. Louis Co.; (3) exposure on St. Louis County Road W., 1 mi south of Eureka, in NW/4 NW/4 NE/4 sec. 12, T. 43 N., R. 3 E., Pacific 7.5-min quad; (4) roadcut on U.S. Highway 61, north of bridge over Spencer Creek, in SE/4 SE/4 sec. 21, T. 55 N., R. 4 W., Ralls Co.; and (5) roadcut on west frontage road to U.S. Highway 61, 1.25 mi north of New London, in NW/4 SW/4 NE/4 sec. 36, T. 56 N., R. 5 W., New London 7.5-min quadrangle, Ralls Co. (Thompson, 1991).

Unit Name History:
Named (Calvin, 1906).
GNC remarks ca. 1923 (IN Wilmarth, 1938).
Revised (Kay, 1928, 1935; Kay and Atwater, 1935).
Used as formation of Galena Group (Trowbridge and others, 1935).
Used as member of Galena Formation (Stauffer and Thiel, 1941).
Used as Decorah Formation; Spechts Ferry Member extended into MO (Larson, 1951).
Overview (Weiss, 1955).
Assigned Carimona Limestone as member of Decorah (Agnew, 1956).
Described (undiff.) in Fillmore Co., MN (Weiss, 1957).
Described in upper Mississippi Valley zinc-lead district (Heyl and others, 1959).
Used as /Decorah Subgroup (Willman and others, 1975).
Used as Decorah Group; Reference sections (Thompson, 1991).
Correlation information (Frey, 1995).
Revised; Overview (Nelson, 1996; Nelson and others, 1996).
Overview (Harrison, 1997; Nelson, 1998).
Areal limits (Harrison, 1999).

Note: If you need more information, please contact Nancy Stamm (
Asterisks (*) indicate usage by the U.S. Geological Survey. Other usages by state geological surveys.

"No current usage" implies that a name has been abandoned or that it has fallen into disuse. Former usage and, if known, replacement name given in parentheses ().

Slashes (/) indicate unit name usage violates the 1983 North American Stratigraphic Code. This violation may be explained within brackets ([ ]).

U.S. Department of the Interior, U.S. Geological Survey, Reston, VA, USA
Database project personnel
Last modified: 15-Jan-2013