Age:

Cambrian

Type designation:

Type locality: The Potosi Dolomite was named for cherty carbonate rocks exposed at Potosi, Washington County, Missouri (Winslow, 1894, p. 331, 351, 355; Droste and Patton, 1986).

History of usage:

The Potosi Dolomite 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 (Droste and Patton, 1986). From this Illinois understanding, use of the name was extended to Indiana by Droste and Patton (1985), but in modified concept (Droste and Patton, 1986). 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 (Droste and Patton, 1986).

Description:

The Potosi Dolomite consists of fine- to medium-grained dolostone and a few thin interbeds of shale or siltstone and sporadic quartz sand grains that are most common in northern Indiana (Droste and Patton, 1986). In color it grades downward from light shades of gray and brown to medium and dark shades of gray and brown (Droste and Patton, 1986). Glauconite is sporadic in many places and is typical in the upper and lower beds of the Potosi in northern Indiana (Droste and Patton, 1986). Chert is not nearly so abundant in the Potosi as it is in rocks of the Prairie du Chien Group (Droste and Patton, 1986). 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 (Droste and Patton, 1986).

In Indiana the Potosi Dolomite is present throughout the subsurface (Droste and Patton, 1986). It ranges in the subsurface from less than 20 ft (6 m) in northwestern Indiana, where it overlies the Franconia Formation, to more than 2,000 ft (610 m) in southwestern Indiana, as judged from data from deep wells in Kentucky and Illinois (Droste and Patton, 1986). 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 (Droste and Patton, 1986). (See also the discussion under "Munising Group" [Droste and Patton, 1986].)

Boundaries:

In northwestern Indiana the Potosi lies conformably on the Franconia Formation (Droste and Patton, 1986). Elsewhere the Potosi lies conformably on the Davis Formation or on the Eau Claire Formation (Droste and Patton, 1986). 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 (Droste and Patton, 1986).

Correlations:

Traditionally, the Potosi Dolomite has been considered as the youngest rocks in the Cambrian System (Trempealeauan Stage) (Droste and Patton, 1986). 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 (Droste and Patton, 1986). (See Droste and Shaver, 1983, and Shaver, 1984 [Droste and Patton, 1986].)

Regional Indiana usage:

Illinois Basin (COSUNA 11)
Supergroup: Knox Supergroup
Group: none
Formation: Potosi Dolomite
Illinois Basin Margin (COSUNA 12)
Supergroup: Knox Supergroup
Group: none
Formation: Potosi Dolomite
Cincinnati Arch (COSUNA 13)
Supergroup: Knox Supergroup
Group: none
Formation: Potosi Dolomite
Kankakee Arch (COSUNA 14)
Supergroup: Knox Supergroup
Group: none
Formation: Potosi Dolomite
Michigan Basin (COSUNA 15)
Supergroup: Knox Supergroup
Group: none
Formation: Potosi Dolomite

Misc/Abandoned Names:

None

Geologic Map Unit Designation:

Cpt

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.

See also:

Davis Formation

References:

Buschbach, T. C., 1964, Cambrian and Ordovician strata of northeastern Illinois: Illinois State Geological Survey Report of Investigations 218, 90 p.

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, Potosi Dolomite, 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. 117–118.

Droste, J. B., and Shaver, R. H., 1983, Atlas of early and middle Paleozoic paleogeography of the southern Great Lakes area: Indiana Geological Survey Special Report 32, 32 p.

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.

Janssens, Adriaan, 1973, Stratigraphy of the Cambrian and Lower Ordovician rocks in Ohio: Ohio Geological Survey Bulletin 64, 197 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.

Winslow, Arthur, 1894, Lead and zinc deposits: Missouri Geological Survey, v. 6, 387 p.

Workman, L. E., and Bell, A. H., 1948, Deep drilling and deeper oil possibilities in Illinois: American Association of Petroleum Geologists Bulletin, v. 32, p. 2,041–2,062; Illinois Geological Survey Report of Investigation 139, 22 p., 1949.