Age:

Cambrian

Type designation:

Type locality: The Potsdam Sandstone was named by Emmons in 1838 (p. 214-217, 230) for Potsdam, St. Lawrence County, New York (Droste and Patton, 1986).

History of usage:

The Potsdam Sandstone was then considered to consist of sandstones lying above primary (Precambrian) rocks (Droste and Patton, 1986). 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 (Droste and Patton, 1986). 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 dolostone of the Knox Dolomite Megagroup (St. Croixan and Canadian Series) (Droste and Patton, 1986). Droste and Patton (1985) modified the name to "Potsdam Supergroup" and assigned the Mount Simon Sandstone and the five formations of the Munising Group to it (Droste and Patton, 1986).

Description:

The Potsdam Supergroup in Indiana contains formations whose components are dominantly siliciclastic, its base is unconformable with underlying rocks of Precambrian age, and its top is defined by the tops of the Franconia and Davis Formations of the Munising Group (Droste and Patton, 1986). 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; Droste and Patton, 1986). 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 (Droste and Patton, 1986).

The Munising Group of the Potsdam Supergroup overlies the Mount Simon everywhere in Indiana (Droste and Patton, 1986). Several formations of the Munising record the lateral gradation of the Potsdam rocks into the carbonate rocks of the Knox Supergroup (Droste and Patton, 1986).

The Mount Simon and Munising rocks are very arkosic in places, whereas feldspar is generally not an abundant component in younger Paleozoic rocks in Indiana (Droste and Patton, 1986). 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 (Droste and Patton, 1986). The Potsdam Supergroup contains about one-third of all Paleozoic rocks in Indiana and ranges in thickness from less than 1,000 ft (305 m) in eastern Indiana to more than 3,000 ft (915 m) in northwestern Indiana (Droste and Patton, 1986).

Correlations:

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 (Droste and Patton, 1986). As noted above, rocks of pre-Dresbachian age are a possibility (Droste and Patton, 1986). The Potsdam has broad regional equivalency with rocks that occupy a similar stratigraphic position and that are widely known by their formational names (Droste and Patton, 1986).

Regional Indiana usage:

Illinois Basin (COSUNA 11)
Supergroup: Potsdam Supergroup
Illinois Basin Margin (COSUNA 12)
Supergroup: Potsdam Supergroup
Cincinnati Arch (COSUNA 13)
Supergroup: Potsdam Supergroup
Kankakee Arch (COSUNA 14)
Supergroup: Potsdam Supergroup
Michigan Basin (COSUNA 15)
Supergroup: Potsdam Supergroup

Misc/Abandoned Names:

None

Geologic Map Unit Designation:

Cp

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 features in Indiana.

Major tectonic features that affect bedrock geology in Indiana.

References:

Droste, J. B., and Patton, J. B., 1985, Lithostratigraphy of the Sauk Sequence in Indiana: Indiana Geological Survey Occasional Paper 47, 24 p.

Droste, J. B., and Patton, J. B., 1986, Potsdam Supergroup, 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. 118–119.

Emmons, Ebenezer, 1838, Report of the second geological district of the State of New York: New York Geological Survey Annual Report 2, p. 186–252.

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

Schwalb, H. R., 1982, Paleozoic geology of the New Madrid area: U.S. Nuclear Regulatory Commission Report NUREG\CR-2909, 61 p.

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.

Swann, D. H., and Willman, H. B., 1961, Megagroups in Illinois: American Association of Petroleum Geologists Bulletin, v. 46, p. 471–483.