Type locality: The Scales Shale was initially named the "Scales Formation" by Templeton and Willman (1963, p. 135) for the village of Scales Mound, Jo Daviess County, Illinois, near which the formation is exposed in railroad cuts (Gray, 1986).
History of usage:
A three-part division of the Maquoketa Group (then a formation) in northwestern Indiana was recognized by Gutstadt (1958, p. 76), but he did not name the divisions, and it remained for Gray (1972, p. 20-21) to extend the name Scales Shale to the lowest of these (Gray, 1986). The formation is identifiable in most of northern, central, and southwestern Indiana (John B. Droste, oral communication, 1983; Gray, 1986).
The Scales Shale is commonly about 100 ft (30 m) thick in northwestern Indiana, but it thickens eastward and southeastward (Gray, 1986). In much of eastern Indiana, where it makes up by far the largest part of the Maquoketa Group, it is 500 to 700 ft (150 to 200 m) thick (Gray, 1986). The upper part is gray shale containing thin beds of limestone that become more abundant southeastward (Gray, 1986). This part grades downward into the lower part, which is dominantly dark brown shale and is 100 to 300 ft (30 to 100 m) thick (Gray, 1986).
Throughout its extent the Scales Shale is underlain by the Trenton Limestone (Ordovician) and overlain by the Fort Atkinson Limestone (Ordovician) (Gray, 1986). The upper contact is generally considered to be conformable, but the lower contact is thought by some authors to be disconformable (Rooney 1966; Willman and Buschbach, 1975, p. 84-85), whereas others (for example, Gray, 1972, p. 6-7 and 14-15) question this interpretation and consider this contact to be essentially conformable (Gray, 1986). On the basis of a regional study of the Trenton Limestone, Brian D. Keith (oral communication, 1984) suggested that the contact is a regionally time-transgressive discontinuity, but not a disconformity (Gray, 1986). Age relationships discussed below strongly support the latter view (Gray, 1986).
The Scales Shale of the northwestern Illinois outcrop area was long considered to be Richmondian in age (Twenhofel and others, 1954) but later was thought to be "equivalent to all the Cincinnatian strata older than the Richmondian" (Templeton and Willman, 1963, p. 136; Gray, 1986). Shaver (1984) assigned the Scales of Illinois to the Maysvillian Stage and implied that the Scales-Galena (in Indiana, Scales-Trenton) contact descends in time eastward, so that much of the Scales in Indiana belongs to the Edenian Stage and is contemporaneous with higher parts of the Galena Group in Illinois (Gray, 1986).
Regional Indiana usage:
Illinois Basin (COSUNA 11)
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.
Gray, H. H., 1986, Scales Shale, 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. 137-138.
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.
Rooney, L. F., 1966, Evidence of unconformity at top of Trenton Limestone in Indiana and adjacent states: American Association of Petroleum Geologists Bulletin, v. 50, p. 533-546.
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.
Templeton, J. S., and Willman, H. B., 1963, Champlainian Series (Middle Ordovician) in Illinois: Illinois State Geological Survey Bulletin 89, 260 p.
Twenhofel, W. H., Bridge, Josiah, Cloud, P. E., Jr., Cooper, B. N., Cooper, G. A., Cumings, E. R., Cullison, J. S., Dunbar, C. O., Kay, Marshall, Liberty, B. A., McFarlan, A. C., Rodgers, John, Whittington, H. B., Wilson, A. E., and Wilson, C. W., Jr., 1954, Correlation of the Ordovician formations of North America: Geological Society of America Bulletin, v. 65, p. 247-298.
Willman, H. B., and Buschbach, T. C., 1975, Ordovician System, in Willman, H. B., Atherton, Elwood, Buschbach, T. C., Collinson, Charles, Frye, J. C., Hopkins, M. E., Lineback, J. A., and Simon, J. A., Handbook of Illinois stratigraphy: Illinois State Geological Survey Bulletin 95, p. 47-87.
For additional information, contact:
Nancy Hasenmueller (email@example.com)Date last revised: October 30, 2014