Cambrian System

Type locality and use of name: The Potosi Dolomite was named for cherty carbonate rocks exposed at Potosi, Washington County, Mo. (Winslow, 1894, p. 331, 351, 355). This unit was traced from the type locality, through the subsurface, and to its exposures in northern Illinois, where it has been called the Trempealeau Formation (Workman and Bell, 1948) and the Potosi Dolomite (Buschbach, 1964) and where it overlies the Franconia Formation and underlies the Eminence Formation. From this Illinois understanding, use of the name was extended to Indiana by Droste and Patton (1985), but in modified concept. Rocks equivalent to the Eminence of Illinois are included in the Potosi of Indiana because the Eminence of Illinois becomes much less sandy eastward and gives way to predominantly dolomitic rocks that are badly distinguishable in Indiana from the main body of Potosi rocks.

Description: The Potosi Dolomite consists of fine- to medium-grained dolomite and a few thin interbeds of shale or siltstone and sporadic quartz sand grains that are most common in northern Indiana. In color it grades downward from light shades of gray and brown to medium and dark shades of gray and brown. Glauconite is sporadic in many places and is typical in the upper and lower beds of the Potosi in northern Indiana. Chert is not nearly so abundant in the Potosi as it is in rocks of the Prairie du Chien Group. Small cavities lined with drusy quartz are characteristic of the Potosi, but similar cavities are less common in some zones in Prairie du Chien rocks above the Potosi.

In Indiana the Potosi Dolomite is present throughout the subsurface. It ranges in the subsurface from less than 20 feet (6 m) in northwestern Indiana, where it overlies the Franconia Formation, to more than 2,000 feet (610 m) in southwestern Indiana, as judged from data from deep wells in Kentucky and Illinois. The southward increase in thickness results in part from downstepping of the Potosi through the whole of the Franconia, Ironton, and Galesville stratigraphic interval to where the Potosi lies on the Eau Claire Formation in southwestern Indiana. (See also the discussion under "Munising Group.")

In northwestern Indiana the Potosi lies conformably on the Franconia Formation. Elsewhere the Potosi lies conformably on the Davis Formation or on the Eau Claire Formation. The top of the Potosi is generally conformable with the overlying Oneota Dolomite except in northwesternmost Indiana where the St. Peter Sandstone lies unconformable above the Potosi.

Correlation: Traditionally, the Potosi Dolomite has been considered as the youngest rocks in the Cambrian System (Trempealeauan Stage). The Potosi of Indiana correlates with the Trempealeau Group (Formation) of Wisconsin and Michigan; a lower part of the undifferentiated Knox Dolomite and an upper part of the Eau Claire Formation as recognized by Janssens (1973) in Ohio the Elvins Formation and the Copper Ridge, Potosi, and Eminence Dolomites of Kentucky; and the Derby-Doe Run Member of the Franconia Formation, the Potosi Dolomite, and the Eminence Formation of Illinois. (See Droste and Shaver, 1983, and Shaver and others, 1985.)