Elwood Formation
Lithostratigraphy: Hunton Limestone Megagroup >>Elwood Formation
Chronostratigraphy: Paleozoic Erathem >>Silurian System >>Alexandrian Series
Allostratigraphy: Tippecanoe Sequence
Primary source
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.
Contributing author(s)
H. B. Willman and Elwood Atherton
Name
Original description
Elwood Formation (Willman, 1973, p. 14).
Derivation
Named for the town of Elwood, Will County.
Other names
History/background
Type section
Type location
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).
Type author(s)
Type status
Reference section
Reference location
Reference author(s)
Reference status
Stratigraphic relationships
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.
Lithology
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.
Core(s)
Photograph(s)
Contacts
Well log characteristics
Fossils
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
Economic importance
Remarks
References
WILLMAN, H. B., 1973, Rock stratigraphy of the Silurian System in northeastern and northwestern Illinois: Illinois State Geological Survey Circular 479, 55 p.
ISGS Codes
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