Type area: The term “Osgood Beds” was applied by Foerste (1896, p. 191) to the fossiliferous lower part of what was then called the "Laurel Formation" near Osgood in Ripley County, Indiana (Rexroad 1970, 1986). A type section was not designated, and now the member is not well exposed in the Osgood area (Rexroad, 1986).
History of usage:
The Osgood beds attained formational status when Foerste (1897, p. 217, 230) restricted the Laurel to the beds above the Osgood (Rexroad, 1986). The Osgood Formation was later reduced to member rank by French (1967), who assigned it to the lower part of the Salamonie Dolomite (Rexroad, 1986).
As recognized by Foerste (1897), the Osgood consisted of lower and upper shaly units, an intervening middle carbonate unit, and in many places a basal dolomitic limestone (Rexroad, 1986). These lithologies are gradational, and the units have limited lateral extent in southeastern Indiana (Rexroad, 1986). The carbonate content of the Osgood increases to the north and the west, so that it cannot be separated from the overlying Laurel Member of the Salamonie Dolomite, and the undifferentiated equivalents of the Osgood and the Laurel are referred to the Salamonie Dolomite (Rexroad, 1986). Osgood thickness ranges from 10 to 30 ft (3 to 9 m) and averages about 15 ft (4.6 m) (Rexroad, 1970, 1986). The Osgood unconformably overlies the Brassfield Limestone or rocks of Ordovician age where the Brassfield is absent; it is overlain conformably by the Laurel Member (1970, 1986).
Conodonts of the Pterospathodus amorphognathoides-Kockelella ranuliformis Assemblage Zone are present in the lower part of the Osgood (Nicoll and Rexroad, 1968), which suggests that most of the member is of early Wenlockian age (early Niagaran in the North American standard) (Rexroad, 1986). The Osgood is the thinning edge of a tongue of the Estill Shale of east-central Kentucky, a unit that contains the same zonal conodonts (Rexroad and Nicoll, 1972; Rexroad and Kleffner, 1984; Rexroad, 1986). In west-central Kentucky where the upper shaly unit thins and disappears, the boundary between the Osgood Formation and the Laurel Dolomite is dropped to the base of the limestone that in Indiana is the middle carbonate unit of the Osgood (Rexroad, 1986). The upper Osgood of Indiana, therefore, has a lower Laurel equivalent in west-central Kentucky (Rexroad, 1986). The Osgood of west-central Ohio is limited to a thin shaly interval between the Dayton Formation and the Laurel Limestone, so that the Osgood of Indiana equates with the Osgood of Ohio and with rocks above and possibly below (Rexroad, 1986). The Osgood correlates approximately with the Stroh Member of the Cataract Formation of northeastern Indiana (Rexroad, 1980) and approximately with the upper part of the Brandon Bridge Member of the Joliet Formation of northeastern Illinois (Liebe and Rexroad, 1977; Rexroad, 1986).
Regional Indiana usage:
Illinois Basin Margin (COSUNA 12)
Geologic Map Unit Designation:
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.
Foerste, A. F., 1896, An account of Middle Silurian rocks of Ohio and Indiana: Cincinnati Society of Natural History Journal, v. 18, p. 161-200.
Foerste, A. F., 1897, A report on the geology of the Middle and Upper Silurian rocks of Clark, Jefferson, Ripley, Jennings, and southern Decatur Counties, Indiana: Indiana Department of Geology and Natural Resources Annual Report 21, p. 213-288.
Hansen, W. R., 1991, Suggestions to authors of the reports of the United States Geological Survey (7th ed.): Washington, D.C., U.S. Geological Survey, 289 p.
Liebe, R. M., and Rexroad, C. B., 1977, Conodonts from Alexandrian and early Niagaran rocks in the Joliet, Illinois, area: Journal of Paleontology, v. 51, p. 844-857.
Nicoll, R. S., and Rexroad, C. B., 1968, Stratigraphy and conodont paleontology of the Salamonie Dolomite and Lee Creek Member of the Brassfield Limestone (Silurian) in southeastern Indiana and adjacent Kentucky: Indiana Geological Survey Bulletin 40, 73 p.
Rexroad, C. B., 1970, Osgood Member, in Shaver, R. H., Burger, A. M., Gates, G. R., Gray, H. H., Hutchison, H. C., Keller, S. J., Patton, J. B., Rexroad, C. B., Smith, N. M., Wayne, W. J., and Wier, C. E., Compendium of rock-unit stratigraphy in Indiana: Indiana Geological Survey Bulletin 43, p. 122-124.
Rexroad, C. B., 1986, Osgood Member, 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. 106-107.
Rexroad, C. B., and Kleffner, M. A., 1984, The Silurian stratigraphy of east-central Kentucky and adjacent Ohio, in Rast, Nicholas, and Hay, Helen, eds., Field trip guides for Geological Society of America annual meeting, Southeastern and North-Central Sections: Lexington, University of Kentucky and Kentucky Geological Survey, p. 44-65.
Rexroad, C. B., and Nicoll, R. S., 1972, Conodonts from the Estill Shale (Silurian, Kentucky and Ohio) and their bearing on multielement taxonomy: Geologica et Palaeontologica, Special Volume 1, p. 57-74.
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.
For additional information, contact:
Nancy Hasenmueller (email@example.com)Date last revised: February 12, 2015