Age:

Silurian

Type designation:

Type section: 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, Illinois (Rexroad, 1986).

History of usage:

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 (Rexroad, 1986). 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; Rexroad, 1986).

Description:

The Schweizer Member generally consists of intermixed dolomitic shale and shaly to argillaceous dolostone (Rexroad, 1986). Silty intervals and patches of siliceous dolostone are present in some areas, but the unit has little or no chert (Rexroad, 1986). 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 (Rexroad, 1986). Otherwise, gray dominates in contrast with the more usual brown of the overlying part of the Sexton Creek (Rexroad, 1986). Maximum recognized thickness in Indiana is 39 ft (12 m) (Rexroad, 1986). The unit thins and disappears to the east and the south by depositional thinning and interfingering with the rest of the Sexton Creek (Rexroad, 1986).

Correlations:

The Schweizer Member is the basal unit of the Alexandrian Series in Illinois and in part of Indiana (Rexroad, 1986). 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; Rexroad, 1986). The Schweizer is about the same age as the Belfast Member of the Brassfield Formation (Dolomite) of southwestern Ohio and adjacent Kentucky (Rexroad, 1986).

Regional Indiana usage:

Kankakee Arch (COSUNA 14)
Supergroup: none
Group: none
Formation: Sexton Creek Limestone
Member: Schweizer Member
Michigan Basin (COSUNA 15)
Supergroup: none
Group: none
Formation: Sexton Creek Limestone
Member: Schweizer Member

Misc/Abandoned Names:

None

Geologic Map Unit Designation:

Sscs

Note: Hansen (1991, p. 52) in Suggestions to authors of the reports of the United States Geological Survey noted that letter symbols for map units are considered to be unique to each geologic map and that adjacent maps do not necessarily need to use the same symbols for the same map unit. Therefore, map unit abbreviations in the Indiana Geologic Names Information System should be regarded simply as recommendations.

COSUNA areas and regional terminology

Names for geologic units vary across Indiana. The Midwestern Basin and Arches Region COSUNA chart (Shaver, 1984) was developed to strategically document such variations in terminology. The geologic map (below left) is derived from this chart and provides an index to the five defined COSUNA regions in Indiana. The regions are generally based on regional bedrock outcrop patterns and major structural features in Indiana. (Click the maps below to view more detailed maps of COSUNA regions and major structural features in Indiana.)

COSUNA areas and numbers that approximate regional bedrock outcrop patterns and major structural features in Indiana.

Major tectonic features that affect bedrock geology in Indiana.

References:

Hansen, W. R., 1991, Suggestions to authors of the reports of the United States Geological Survey (7^th ed.): Washington, D.C., U.S. Geological Survey, 289 p.

Liebe, R. M., and Rexroad, C. B., 1977, Conodonts from Alexandrian and early Niagaran rocks in the Joliet, Illinois, area: Journal of Paleontology, v. 51, p. 844–857.

Rexroad, C. B., 1986, Schweizer Member, in Shaver, R. H., Ault, C. H., Burger, A. M., Carr, D. D., Droste, J. B., Eggert, D. L., Gray, H. H., Harper, Denver, Hasenmueller, N. R., Hasenmueller, W. A., Horowitz, A. S., Hutchison, H. C., Keith, B. D., Keller, S. J., Patton, J. B., Rexroad, C. B., and Wier, C. E., Compendium of Paleozoic rock-unit stratigraphy in Indiana–a revision: Indiana Geological Survey Bulletin 59, p. 138.

Rexroad, C. B., and Droste, J. B., 1982, Stratigraphy and conodont paleontology of the Sexton Creek Limestone and the Salamonie Dolomite (Silurian) in northwestern Indiana: Indiana Geological Survey Special Report 25, 29 p.

Shaver, R. H., coordinator, 1984, Midwestern basin and arches region–correlation of stratigraphic units in North America (COSUNA): American Association of Petroleum Geologists Correlation Chart Series.

Willman, H. B., 1973, Rock stratigraphy of the Silurian System in northeastern and northwestern Illinois: Illinois State Geological Survey Circular 479, 55 p.