St. Croixan Series

Cambrian System

Type locality and use of name: The Potsdam Sandstone was named by Emmons in 1838 (p. 214-217, 230) for Potsdam, St. Lawrence County, N.Y. It was then considered to consist of sandstones lying above primary (Precambrian) rocks. The name Potsdam has since been applied widely, at least in the correlative sense, to many bodies of sandstones occupying a somewhat similar stratigraphic position in Canada, Pennsylvania, Virginia, Iowa, Wisconsin, Minnesota, Michigan, and Indiana. As the Potsdam was redefined by Swann and Willman (1961) and called the Potsdam Sandstone Megagroup, it included the Cambrian rocks composed of predominantly siliciclastic components that show extensive lateral gradation into dolomite of the Knox Dolomite Megagroup (St. Croixan and Canadian Series). In accord with latest standard nomenclatural recommendations, Swann and Willman's name was modified to "Potsdam Supergroup" by Droste and Patton (1985), who assigned the Mount Simon Sandstone and the five formations of the Munising Group to it.

Description: The Potsdam Supergroup in Indiana contains formations whose components are dominantly siliciclastic, its base is unconformable with underlying crystalline rocks of Precambrian age, and its top is defined by the tops of the Franconia and Davis Formations of the Munising Group. On the basis of data now available the Mount Simon Sandstone is the basal formation of the Potsdam, but deep drilling in southwestern Indiana may penetrate Paleozoic rocks, older than Late Cambrian, below the Mount Simon that are comparable to the Mermen Sandstone in Illinois (Schwalb, 1982). Such pre-Mount Simon sandstones of Cambrian age would be included in the Potsdam Supergroup; such sandstones may be difficult to distinguish from the Mount Simon Sandstone.

The Munising Group of the Potsdam Supergroup overlies the Mount Simon everywhere in Indiana. Several formations of the Munising record the lateral gradation of the Potsdam rocks into the carbonate rocks of the Knox Supergroup.

The Mount Simon and Munising rocks as very arkosic in some places, whereas feldspar is generally not an abundant component in younger Paleozoic rocks in Indiana. The Potsdam rocks have not been exposed since their deposition in Indiana, and in this respect they are the only sedimentary rocks in the state that have not been subjected to erosion one or more times after their initial burial. The Potsdam Supergroup contains about one-third of all Paleozoic rocks in Indiana and ranges in thickness from less than 1,000 feet (305 m) in eastern Indiana to more than 3,000 feet (915 m) in northwestern Indiana.

Correlation: Sparse biostratigraphic data from Potsdam rocks in Indiana preclude firm age assignment, but on the basis of regional information taken mainly from areas of outcrop, the Potsdam is placed in the St. Croixan Series and includes rocks of Dresbachian and Franconian age. As noted above, rocks of pre-Dresbachian age are a possibility. The Potsdam has broad regional equivalency with rocks that occupy a similar stratigraphic position and that are widely known by their formational names.