Geologic Unit: Decorah

Usage:

Decorah Formation (IL*,IA*,MN*,MO*,WI*)
Decorah Formation of Galena Group (IL*,IA*,MN*,MO*,WI*)
Decorah Shale (IA*,MN*)
Decorah Shale of Galena Group (IA*,MN*)
Decorah Formation of Sinnipee Group (WI)
Decorah Shale of Sinnipee Group (WI)
Decorah Limestone (IL*)
Decorah Group (IL*,MO*-notably in St. Louis region)

Subunits:

GROUP STATUS (alphabetical) --notably in St. Louis region IL*,MO*: Guttenberg Limestone, Kings Lake Limestone, Spechts Ferry Formation.
FORMATION STATUS (alphabetical): Carimona Member (IA*,MN*), Castlewood Limestone Member (IL*), Deicke K-bentonite Bed (IA*,MN*), Guttenberg Limestone Member (IL*,IA*,MN*,WI*) or Guttenberg Dolomite Member (IL) or Guttenberg Member (MO,WI), Ion Dolomite Member (IL*,IA*,MN*,WI*), Millbrig K-bentonite Bed (IA*,MN*), Spechts Ferry Shale Member (IL*,IA*,MN*,MO*,WI*) or Spechts Ferry Member (WI).

Geologic age:

Late Ordovician (Mohawkian)*

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

[Named from exposures at 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*