Willman, H. B., Elwood Atherton, T. C. Buschbach, Charles Collinson, John C. Frye, M. E. Hopkins, Jerry A. Lineback, and Jack A. Simon, 1975, Handbook of Illinois Stratigraphy: Illinois State Geological Survey Bulletin 95, 261 p.
H. B. Willman and T. C. Buschbach
Dubuque Formation (Sardeson, 1907, p. 193).
The Dubuque Formation is named for Dubuque, Dubuque County, Iowa.
The Dubuque Formation type section is a quarry in Dubuque, east of Columbia College at the top of the West 14th Street hill, where 33 feet of Dubuque, its top eroded, is exposed.
The Dubuque is overlain unconformably by the Maquoketa Group. Although the contact is sharp, there is slight, if any, truncation of the Dubuque in the outcrop area in Illinois.
Extent and thickness
In Illinois the Dubuque Formation occurs only in the northwestern part of the state, where it is 40-45 feet thick (Templeton and Willman, 1963, p. 128). It is truncated by Maquoketa Group strata south of Galesburg in Knox County. It is also absent in subsurface in northeastern Illinois, but equivalent beds may be included in the Wise Lake Formation.
The Dubuque Formation is the shaly dolomite at the top of the Galena Group. The lower 20 feet of the Dubuque Formation grades uniformly from the pure, thick-bedded dolomite of the underlying Wise Lake Formation to shaly dolomite. A 4-inch bed of dolomite set off by relatively strong shale partings occurs 8 feet above the base of the formation and is a widely traceable marker bed. In the upper part of the formation, beds of dolomite 1-6 inches thick that are dense, fine-grained, and argillaceous are interlayered with beds of dolomitic shale 1-8 inches thick. A thin bed of red-brown shale occurs 20 feet below the top. Calcite-filled vugs are common in the upper 2 feet.
Well log characteristics
Paucicrura [Dalmanella], Sowerbyella, Pseudolingula iowensis, and crinoid debris are common in the shaly beds in the upper 15 feet. Some beds contain numerous, minute, brown-red, spore-like forms that are probably algae. The Dubuque is equivalent to the Oxoplecia Zone of early reports.
Age and correlation
The Dubuque Formation is correlated with the Hillier Member at the top of the Cobourg Formation in New York, the Cynthiana Limestone in Kentucky, and the Catheys Formation in Tennessee.
Environments of deposition
SARDESON, F. W., 1907, Galena Series: Geological Society of America Bulletin, v. 18, p. 179- 194.
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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