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 Elwood Atherton
Elwood Formation (Willman, 1973, p. 14).
Named for the town of Elwood, Will County.
The type section, which is 5 miles north of the town of Elwood, lies in a ravine 3 miles southwest of Joliet, on the southeast side of the Des Plaines River Valley (NW NW SE 36, 35N-9E).
The Elwood Formation overlies the Wilhelmi Formation in northeastern and north-central Illinois (fig. S-3). The Elwood Formation appears to be conformable to the Wilhelmi below and the Kankakee above.
Extent and thickness
The Elwood Formation is 27 feet thick in the type section. Although widely present in the Joliet area, it is erratic in occurrence elsewhere. It is exposed locally along the Fox River Valley north of Aurora (SE SW 3, 38N-8E) and has been traced westward in subsurface as far as Peru, La Salle County.
The Elwood, which previously was referred to as the upper cherty zone of the Edgewood Formation, is a slightly argillaceous, light brownish gray, thin- to medium-bedded, fine-grained dolomite that contains layers of dense white chert as much as 4 inches thick. Chert forms 40-50 percent of the upper part of the formation but is less abundant downward. The abundance of chert decreases laterally, and in places equivalent beds may not be separable from the Wilhelmi Formation.
Well log characteristics
Age and correlation
In some areas, particularly in subsurface, very cherty strata previously correlated with the Kankakee Dolomite probably belong to the Elwood Formation. It is lithologically similar to the Blanding Formation in northwestern Illinois and to strata included in the Mayville Formation in Wisconsin.
Environments of deposition
WILLMAN, H. B., 1973, Rock stratigraphy of the Silurian System in northeastern and northwestern Illinois: Illinois State Geological Survey Circular 479, 55 p.
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