Silurian System

Type section and use of name: The Schweizer Member of the Wilhelmi Formation was named by Willman (1973) for Schweizer School 1 mile (1.6 km) west of the type section, which is part of the Wilhelmi type section, in the SE¼ sec. 35, T.35 N., R.9 E., Will County, Ill. The lithologic distinctions between the Birds Member of the Wilhelmi and the overlying Elwood Formation of Illinois are lost in northwestern Indiana, and the lithologic equivalents of each are included in the Sexton Creek Limestone. Because the lower member of the Wilhelmi, the Schweizer, is both laterally and vertically gradational with the Sexton Creek in Indiana, it is included as a member of the formation in Indiana (Rexroad and Droste, 1982).

Description: The Schweizer Member generally consists of intermixed dolomitic shale and shaly to argillaceous dolomite. Silty intervals and patches of siliceous dolomite are present in some areas, but the unit has little or no chert. Reworked Ordovician material, including fossils, is present throughout the member, but particularly in the lower part, and commonly imparts a greenish color to the Schweizer. Otherwise, gray dominates in contrast with the more usual brown of the overlying part of the Sexton Creek. Maximum recognized thickness in Indiana is 39 feet (12 m). The unit thins and disappears to the east and the south by depositional thinning and interfingering with the rest of the Sexton Creek.

Correlation: The Schweizer Member is the basal unit of the Alexandrian Series in Illinois and in part of Indiana. The few indigenous conodonts are those of the Panderodus simplex Assemblage Zone, but this interval is recognized primarily on negative evidence as the zone underlying the lowest level of occurrence of Distomodus kentuckyensis in the Midwest (Liebe and Rexroad, 1977). The Schweizer is about the same age as the Belfast Member of the Brassfield Formation (Dolomite) of southwestern Ohio and adjacent Kentucky .