H. B. Willman and Elwood Atherton
The Racine Formation (Hall, 1861), the uppermost Niagaran Formation in northern and western Illinois (figs. S-3, S-4, S-5), is named for exposures in quarries at Racine, Wisconsin. The quarries at Racine are in reefs, and the term "Racine" was introduced in northeastern Illinois for similar rocks (Savage, 1926, p. 524).
Savage gave the name "Waukesha" to the impure cherty dolomite thought to underlie the reefs but in fact equivalent to them. He also included in the Waukesha the strata under the reefs that were used for building stone. Later, Waukesha was restricted in northeastern Illinois to the latter strata, and the overlying reef and interreef strata were combined in the Racine (Willman, 1943). At the same time the Racine was redefined to include the uppermost Niagaran strata that had been differentiated by Savage on the basis of their fossils as being equivalent to the Port Byron Formation of northwestern Illinois, although similar in lithology to the Racine. For the same reason, the term "Port Byron" was discontinued in northwestern Illinois, and Racine was extended to include the entire sequence of reef-bearing rocks.
The Racine is equivalent to the upper part of the St. Clair and to all except the youngest part of the Moccasin Springs in southern Illinois. It is correlated with and is similar in lithology to the Gower in Iowa, the Engadine in northern Wisconsin, and the Wabash in Indiana.
Extent and Thickness
The Racine is well exposed in the Chicago area (fig. S-2A, B), especially in the large quarries at Thornton, McCook, La Grange, Hillside, and Elmhurst (Bretz, 1939; Willman, 1943, 1962, 1973; Willman et al., 1950; Lowenstam et al., 1956). In northwestern Illinois it is well exposed at the top of the Mississippi bluffs in Palisades State Park north of Savanna, in the bluffs south of Fulton to Port Byron, and at Morrison, Whiteside County (Sutton, 1935; Willman, 1943, 1973). In western Illinois it is exposed only in a small area at Grafton, Jersey County. The Racine is as much as 300 feet thick.
In some areas it is almost entirely pure reef rock (fig. S-2E); in others it is largely silty or argillaceous, cherty interreef rock (fig. S-2F). In places the two types of rocks intertongue. Dark gray to black shaly beds are present locally. At Blue Island, a thickness of several feet of such beds was encountered during the digging of the Calumet Sag Channel and they have been informally called the "Blue Island beds" or "Lecthaylus shale" (Lowenstam, 1948a). Reefs start at various levels in the Racine. Many are well defined reefs with flank beds dipping steeply away from a central massive core. In other areas the reef bodies overlap. The Racine reef rock is exceptionally pure dolomite, largely vesicular to coarsely vuggy, medium grained and light gray to white. It commonly is mottled with various shades of gray. In some reefs, many of the vugs are filled with asphaltum. The steeply dipping flank beds are generally pure dolomite, but some beds grade into argillaceous dolomite as the slopes diminish. About 45 feet of Racine is exposed at Grafton, but immediately west of there the formation is truncated by Middle Devonian strata. The interreef rocks in northeastern Illinois are largely impure, varying from moderately silty to very silty or very argillaceous, and lenticular bodies of shale are locally present on the flanks of some reefs. The interreef rocks are commonly very cherty, the chert generally in irregularly scattered nodules. The interreef rocks in many places contain beds of relatively pure dolomite, probably wash from the reefs. Near the reefs some beds have a strongly nodular character, suggesting deposition in water that was subject to frequent agitation.
The Racine reefs are highly fossiliferous. In northwestern Illinois much of the Racine consists of flat-lying, pure, reef type of dolomite, and reef structures do not appear to be as common as they are in northeastern and central Illinois. No reefs are present in the small area where the Racine is exposed at Grafton in Jersey County, western Illinois.
BRETZ, J. H., 1939, Geology of the Chicago region, Part 1-General: Illinois State Geological Survey Bulletin 65, 118 p.
HALL, JAMES, 1861, Descriptions of new species of Crinoidea; investigations of the Iowa Geological Survey, preliminary notice: Albany, New York, 19 p.
LOWENSTAM, H. A., 1948a, Biostratigraphic studies of the Niagaran inter-reef formations in northeastern Illinois: Illinois State Museum Science Papers, v. 4, 146 p.
LOWENSTAM, H. A., H. B. WILLMAN, and D. H. SWANN, 1956, Niagaran reef at Thornton, Illinois: Illinois State Geological Survey Guidebook Series 4, 19 p.
SAVAGE, T. E., 1926, Silurian rocks of Illinois: Geological Society of America Bulletin, v. 37, p. 513-533.
SUTTON, A. H., 1935, Stratigraphy of the Silurian System of the upper Mississippi Valley: Kansas Geological Society Guidebook, 9th Annual Field Conference, p. 268-280.
WILLMAN, H. B., 1943, High-purity dolomite in Illinois: Illinois State Geological Survey Report of Investigations 90, 89 p.
WILLMAN, H. B., 1962, Silurian strata of northeastern Illinois, and Descriptions of stops second day, in Silurian rocks of the southern Lake Michigan area: Michigan Basin Geological Society Annual Field Conference, p. 61-67, 81-96; Illinois State Geological Survey Reprint 1962-M.
WILLMAN, H. B., 1973, Rock stratigraphy of the Silurian System in northeastern and northwestern Illinois: Illinois State Geological Survey Circular 479, 55 p.
WILLMAN, H. B., H. A. LOWENSTAM, and L. E. WORKMAN, 1950, Field conference on Niagaran reefs in the Chicago region: Illinois State Geological Survey Guidebook Series 1, 23 p.
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