H. B. Willman and T. C. Buschbach
The Decorah Subgroup is named for exposures at Decorah, Winneshiek County, Iowa.
Decorah strata in Illinois correlate with the Rockland Formation in New York, the Curdsville and Logana Members of the Lexington Limestone in Kentucky, and the lower part of the Hermitage Limestone in Tennessee.
Extent and Thickness
The Decorah is extensive in Illinois and is exposed in central northern and extreme northwestern Illinois in a small area near West Point Landing in Calhoun County, and at Valmeyer in Monroe County. It is 15-30 feet thick in western Illinois, but it thins eastward and is generally only 1-5 feet thick in the central and eastern parts of the state.
The Decorah Subgroup (Calvin, 1906, p. 84; Kay, 1935b, p. 289; Herbert, 1949) consists of the shaly limestone and dolomite formations at the base of the Galena Group. At Decorah it is called the Decorah Formation and includes the Spechts Ferry, Guttenberg, and Ion Members. Kay took the Spechts Ferry out of the Decorah and put it in the Platteville, but Herbert showed that the Spechts Ferry belonged in the Decorah. Templeton and Willman (1963) removed the strata equivalent to the Ion from the Decorah in Illinois because they are not shaly.
The Decorah consists of three formations- the Spechts Ferry (at the base) is green shale and limestone; the overlying Kings Lake is largely limestone, partly silty and sandy; and the Guttenberg (at the top) is limestone with beds of brown-red shale. The Kings Lake thins out northward and is not present in northern Illinois. Both the Kings Lake and the Spechts Ferry thin out eastward, and in central and eastern Illinois the Decorah consists only of the Guttenberg Formation.
The Decorah strata in Illinois are very fossiliferous, containing abundant bryozoans and brachiopods. Refinesquina, Sowerbyella, and Pionodema (fig. O-5) are especially abundant.
CALVIN, SAMUEL, 1906, Geology of Winneshiek County: Iowa Geological Survey, v. 16, p. 37-146.
HERBERT, PAUL, JR., 1949, Stratigraphy of the Decorah Formation in western Illinois: University of Chicago Ph.D. thesis; Illinois State Geological Survey manuscript PH-1.
KAY, MARSHALL, 1935b, Ordovician System in the Upper Mississippi Valley: Kansas Geological Society Guidebook, 9th Annual Field Conference, p. 281-295.
TEMPLETON, J. S., and H. B. WILLMAN, 1963, Champlainian Series (Middle Ordovician) in Illinois: Illinois State Geological Survey Bulletin 89, 260 p.
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