Type designation:

Type section: The Stroh Member of the Cataract Formation was named by Rexroad (1980) for Stroh, Steuben County, in extreme northeastern Indiana (Rexroad, 1986). The type section consists of the rocks penetrated by the Northern Indiana Public Service Co. Arden Tubbs No. 1 well in the SW¼SE¼SW¼ sec. 27, T. 36 N., R. 12 E. (Altitude, 944 ft; 288 m) (Rexroad, 1986).


The Stroh Member is generally composed of impure argillaceous dolostone and thin shale interbeds and has a maximum thickness of about 14 ft (4.3 m) (Rexroad, 1986). Generally at the base of the Stroh is a few inches to about 2 ft (0.2 to 0.6 m) of very fine grained to sublithographic argillaceous carbonate that is tannish except for widely spaced laminations of gray and commonly another color, such as pink or green (Rexroad, 1986). Otherwise the carbonate rock is grayish or tannish gray, but it also has a green cast that is even more apparent in the shaly parts (Rexroad, 1986).

The Stroh represents a southwestward extension of the Clinton Group of the Michigan subsurface, the southern and western limits of which are arbitrarily prescribed because of facies changes between it and the Salamonie Dolomite (Rexroad, 1986). Therefore, the member is limited to northeastern Indiana north of northern Randolph County and east of LaPorte County (Rexroad, 1986).


The Stroh is conformable with the overlying Salamonie Dolomite, and the boundary between the two is between the highly argillaceous Stroh and the relatively pure dolostone of the Salamonie (Rexroad, 1986). This lithologic change is marked by a key signature pattern on geophysical logs (Rexroad, 1986). The Stroh unconformably overlies either the Cabot Head Member of the Cataract Formation or Cataract rocks that are undifferentiated by member (Rexroad, 1986). Because the underlying rocks are lithologically similar to the Stroh, the lower Stroh boundary may be difficult to pick on a physical basis even though the basal bed of the Stroh is generally distinctive (Rexroad, 1980; Rexroad, 1986).


The Stroh is continuous with the Clinton Group (undifferentiated) of Michigan and with the basal beds of the Salamonie Dolomite in Indiana (Rexroad, 1986). It correlates with the upper part of the Brandon Bridge Member of the Joliet Formation of northeastern Illinois, with the Osgood Member of the Salamonie in southeastern Indiana, with part of the Estill Shale of east-central Kentucky, and approximately with the interval of the Rockway Dolomite and the Willowvale Shale of New York (Rexroad, 1986). The Stroh represents the upper part of the Pterospathodus amorphognathoides-Kockelella ranuliformis Assemblage Zone (conodonts) (Rexroad, 1980) and belongs in the Niagaran Series (Rexroad, 1986).

Regional Indiana usage:

Kankakee Arch (COSUNA 14)
Supergroup: none
Group: none
Formation: Cataract Formation
Member: Stroh Member
Michigan Basin (COSUNA 15)
Supergroup: none
Group: none
Formation: Cataract Formation
Member: Stroh Member

Misc/Abandoned Names:


Geologic Map Unit Designation:


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 featuers in Indiana.

Major tectonic features that affect bedrock geology in Indiana.

See also:

Osgood Member


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

Rexroad, C. B., 1980, Stratigraphy and conodont paleontology of the Cataract Formation and the Salamonie Dolomite (Silurian) in northeastern Indiana: Indiana Geological Survey Bulletin 58, 83 p.

Rexroad, C. B., 1986, Stroh 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. 151.

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.

For additional information contact:

Nancy Hasenmueller (
Date last revised: February 12, 2015