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NGMDB

Geologic Unit: Decorah

Usage:

Decorah Formation (IL*,IA*,MN*,MO*,WI*)
Decorah Formation of Galena Group (IL,IA,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] of Galena Group (IL)

["Subgroup" not recognized as a formal stratigraphic rank term (CSN, 1933; ACSN, 1961, 1970; NACSN, 1983, 2005). Decorah subgroup considered informal and should not be capitalized.]


Subunits:

GROUP STATUS (alphabetical) --all in IL*,MO*: Guttenberg Limestone, Kings Lake Limestone, Spechts Ferry Formation.
/SUBGROUP STATUS (alphabetical) --all in IL: /Guttenberg Formation, /Kings Lake Formation, /Spechts Ferry Formation.
FORMATION STATUS (alphabetical): Deicke Bentonite Bed (IA,MN*), Guttenberg Limestone Member (IL*,IA*,MN*,WI*) or Guttenberg Member (MO), Ion Dolomite Member (IL*,IA*,MN*,WI*), Kings Lake Member (MO), Millbrig Bentonite Bed (IA,MO,MN*), Spechts Ferry Shale Member (IL*,IA*,MN*,MO*,WI*).


Geologic age:

Middle Ordovician (Mohawkian)*


Type section, locality, area and/or origin of name:

Decorah, Winnishiek Co., northeastern IA (Calvin, 1906).
Reference sections in MO (Thompson, 1991):
(1) roadcut on U.S. Interstate Highway 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 U.S. Interstate Highway 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 quadrangle;
(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.


AAPG geologic province:

Illinois basin*
Iowa shelf*
Ozark uplift*
Upper Mississippi embayment*
Wisconsin arch*


NGMDB

For more information, please contact Nancy Stamm, Geologic Names Committee Secretary.

Asterisk (*) indicates usage by the U.S. Geological Survey. Other usages by state geological surveys.

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

Slash (/) indicates name does not conform with nomenclatural guidelines (CSN, 1933; ACSN, 1961, 1970; NACSN, 1983, 2005). This may be explained within brackets ([ ]).

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