Mississippian System

Type and reference sections: The Sisson Member was named by Keller and Becker (1980) for a crossroads community in southern Knox County, Ind. The type section is in the T & H No. 1 Lane well, in sec. 21, T. 1 N., R. 10 W., about 3 miles (5 km) east of Sisson. The specified interval is from 2,105 to 2,226 feet (642 to 679 m) of depth. Reference sections in Gibson and Posey Counties were also designated (Keller and Becker, 1980, p. 11).

Description: The Sisson is recognized only in the subsurface and is the basal member of the St. Louis Limestone in an area that extends from Knox, Daviess, Dubois, and Perry Counties southwestward. It is distinguished by skeletal limestone much like that in the Salem Limestone but commonly a little more fine grained also present are fine-grained dolomite, crystalline limestone, thin green and black shale, and anhydrite. Over most of the area of its recognition the member is 110 to 160 feet (34 to 49 m) thick, but in Posey County thicknesses of more than 200 feet (60 m) are known (Keller and Becker, 1980).

Most previous workers have assigned rocks of the Sisson Member to the Salem Limestone, and in Illinois these rocks are still so classified. Excluding these rocks from the Salem, however, and regarding them as a distinctive member of the St. Louis avoid creating a step effect in the Salem-St. Louis contact; that contact, therefore, is now at a uniform horizon throughout the state.