Lithostratigraphy: Maquoketa Shale Group >>Neda Formation
Chronostratigraphy: Paleozoic Erathem >>Ordovician System >>Cincinnatian Series >>Richmondian Stage
Allostratigraphy: Tippecanoe Sequence
H. B. Willman and T. C. Buschbach
The Neda Formation (Savage and Ross, 1916, p. 193, in which it was called Neda iron ore), the youngest formation in the Maquoketa Group in northern Illinois, is named for exposures near Neda, Dodge County, Wisconsin.
Extent and Thickness
The Neda Formation occurs in local areas in northern Illinois and is exposed along the Kankakee River southeast of Ritchey, Will County (26, 27, 35, 36, 32N-10E, particularly in NE NW SW 36). In that area it is 4-8 feet thick, overlies the green Brainard Shale, and is overlain by the Kankakee Formation or, locally, by 1-2 feet of the Wilhelmi Formation of Silurian age. The Neda is generally less than 10 feet thick in Illinois.
Although the Neda rests sharply on the underlying Brainard Shale, there is no evidence of an unconformity. The upper contact, however, is sharp and erosional. The basal Silurian rocks in places contain streaks of reddish shale and black oolites that were probably reworked from the Neda. The Neda occurs only in areas where the Brainard Shale is relatively thick and where the basal Silurian formations, the Wilhelmi and Elwood Formations, which generally occur in channels eroded in the top of the Brainard, are absent or very thin. The major unconformity, therefore, is on top of the Neda Formation, which favors its assignment to the Maquoketa Group (Workman, 1950; Buschbach, 1964) rather than to the basal Silurian Noix Oolite Member (Athy, 1928).
The Neda Formation is largely red shale interbedded with red-brown or black oolite consisting of flattened spheroids of geothite or hematite that average about 0.5 mm across. In places it contains a few beds of gray or green shale.
The Neda is not fossiliferous in Illinois, but Savage and Ross (1916) reported a Cincinnatian fauna in the Neda in its type region.
The Neda is believed to be a thin western tongue of the Queenston delta of the Appalachian region.
ATHY, L. F., 1928, Geology and mineral resources of the Herscher Quadrangle: Illinois State Geological Survey Bulletin 55, 120 p.
BUSCHBACH, T. C., 1964, Cambrian and Ordovician strata of northeastern Illinois: Illinois State Geological Survey Report of Investigations 218, 90 p.
SAVAGE, T. E., and C. S. Ross, 1916, Age of the iron ore in eastern Wisconsin: American Journal of Science, v. 41, p. 187-193.
WORKMAN, L. E., 1950, Neda Formation in northeastern Illinois: Illinois Academy of Science Transactions, v. 43, p. 176-182; Illinois State Geological Survey Circular 170, p. 176-182.
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