Silurian System

Type section: The Stroh Member of the Cataract Formation was named by Rexroad (1980) for Stroh, Steuben County, in extreme northeastern Indiana. 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 feet; 288 m).

Description and use of name: The Stroh Member is generally composed of impure argillaceous dolomite and thin shale interbeds and has a maximum thickness of about 14 feet (4.3 m). Generally at the base of the Stroh is a few inches to about 2 feet (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. Otherwise the carbonate rock is grayish or tannish gray, but it also has a green east that is even more apparent in the shaly parts.

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. Therefore, the member is limited to northeastern Indiana north of northern Randolph County and east of LaPorte County. It is conformable with the overlying Salamonie Dolomite, and the boundary between the two is between the highly argillaceous Stroh and the relatively pure dolomite of the Salamonie. This lithologic change is marked by a key signature pattern on geophysical logs. The Stroh unconformably overlies either the Cabot Head Member of the Cataract Formation or Cataract rocks that are undifferentiated by member. 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)

Correlation: As indicated above, the Stroh is continuous with the Clinton Group (undifferentiated) of Michigan and with the basal beds of the Salamonie Dolomite in Indiana. 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. The Stroh represents the upper part of the Pterospathodus amorphognathoides-Kockelella ranuliformis Assemblage Zone (conodonts) (Rexroad, 1980) and belongs in the Niagaran Senes.