H. B. Willman and Elwood Atherton
The Wilhelmi Formation is named for Wilhelmi Airport on the south side of Joliet, Will County.
The type section of the Wilhelmi Formation is a railroad cut about 5 miles southwest of Wilhelmi Airport (SE SW SE 35, 35N-9E).
Earlier the Wilhelmi Formation had been referred to as the lower shaly part of the Edgewood Formation.
The Wilhelmi is lithologically similar to the Mosalem Formation in northwestern Illinois. It probably correlates with the lower part of the Edgewood Formation in western Illinois but is more argillaceous than the latter. Strata of similar lithology occur in the basal Edgewood in Iowa and are locally present in the base of the Mayville in Wisconsin and the Brassfield in Indiana and Kentucky.
Extent and Thickness
The Wilhelmi Formation is as much as 100 feet thick where it fills deep channels in the Maquoketa Shale Group, but it is thin or absent in other areas. It is absent in southern Illinois.
The Wilhelmi Formation (Willman, 1973, p. 12) is the basal formation in the Alexandrian Series in northeastern Illinois (fig. S-3).
The lower part of the Wilhelmi Formation is medium to dark gray, very argillaceous dolomite and dolomitic shale. This unit is differentiated as the Schweizer Member and is present only where the formation is thick. The overlying moderately argillaceous dolomite, differentiated as the Birds Member, is characteristic of the relatively thin part of the formation. In some areas the basal dolomitic shale is difficult to differentiate from shale of the underlying Maquoketa Group, but the latter generally is greenish gray.
The Wilhelmi contains early Llandoverian graptolites (Ross, 1962a).
ROSS, C. A., 1962a, Early Llandoverian (Silurian) graptolites from the Edgewood Formation, northeastern Illinois: Journal of Paleontology, v. 36, no. 6, p. 1383-1386; Illinois State Geological Survey Reprint 1963-A.
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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