H. B. Willman and T. C. Buschbach
The Oneota Dolomite (McGee, 1891, p. 331-333) is named for exposures along the Oneota River (now Upper Iowa River), Allamakee County, northeastern Iowa.
The Oneota is equivalent to the Gasconade Formation of Missouri and to the upper non-sandy part of the Chepultepec Formation in the Kentucky subsurface.
Extent and Thickness
In Illinois it is exposed in quarries and ravines along the Ashton Arch near the Fox and Rock Rivers (Willman and Templeton, 1951). The Oneota underlies all of Illinois except the northernmost part of the state (fig. O-9). Where overlain by New Richmond or Shakopee, the 0neota ranges in thickness from about 100 feet in the north to slightly over 300 feet in central Illinois. It is 200-500 feet thick to the southwest and probably considerably thicker to the southeast, where its boundaries are not distinct in the subsurface.
In northern Illinois the formation is subdivided into the Arsenal Member, a very cherty lower unit, and the Blodgett Member, a slightly cherty and slightly sandy upper unit (Buschbach, 1964). The Oneota overlies the Gunter Sandstone, or, where the sandstone is absent, rests unconformably on the Cambrian Eminence Dolomite. The contact between the Oneota and the overlying New Richmond is sharp.
The Oneota consists of fine- to coarse-grained, light gray to brownish gray, cherty dolomite that contains minor amounts of sand and, at its base, thin shaly beds. The chert is generally white, light or pinkish gray, or banded, and is in part sandy and oolitic. The chert occurs in layers, lenses, isolated nodules, and irregularly shaped bodies that have a distinctive branching habit. The coarse grain size characterizes the Oneota.
Macrofossils are rare in the Oneota and are mostly gastropods and algal masses (fig. O-2E).
BUSCHBACH, T. C., 1964, Cambrian and Ordovician strata of northeastern Illinois: Illinois State Geological Survey Report of Investigations 218, 90 p.
MCGEE, W. J., 1891, Pleistocene history of northeastern Iowa: USGS Annual Report 11, part 1, p. 189-577.
WILLMAN, H. B., and J. S. TEMPLETON, 1951, Cambrian and Lower Ordovician exposures in northern Illinois: Illinois Academy of Science Transactions, v. 44, p. 109-125; Illinois State Geological Survey Circular 179, 1952.
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