Type locality: The name "Colchester Coal" was introduced by Worthen (1868, p. 11) for exposures in McDonough County, Illinois (Burger and Wier, 1970; Ault and Harper, 1986). Wanless (1956, p. 10) designated as the type section exposures near Colchester (secs. 12 and 13, T. 5 N., R. 4 W.), McDonough County (Ault and Harper, 1986).
History of usage:
Extended: The name "Colchester Coal Member" (of the Linton Formation) was adopted in Indiana (Wier and Gray, 1961) for the coal designated Coal IIIa by Ashley (1909, p. 55-57) (Burger and Wier, 1970; Ault and Harper, 1986). In Pike and Warrick Counties this unit was previously known as the Velpen Coal (Fuller and Ashley, 1902, p. 2), but the name "Velpen" is now used only for the overlying limestone (Burger and Wier, 1970; Ault and Harper, 1986).
The Colchester Coal Member is a thin (0.1 to 3 ft [< 0.1 to 0.9 m]) but widespread coal that is bright banded and commonly displays a thin medial shale parting (Ault and Harper, 1986). The Colchester may lie from a few inches to 65 ft (20 m) above the Seelyville Coal Member but generally lies 20 to 40 ft (6 to 12 m) above the Seelyville (Ault and Harper, 1986). The Colchester coal lies 5 to 50 ft (1.5 to 15 m) below the Survant Coal Member (Ault and Harper, 1986).
The Colchester Coal Member overlies a persistent underclay, 1 to 4 ft (0.3 to 1.2 m) thick (Burger and Wier, 1970; Ault and Harper, 1986). It is overlain by the Mecca Quarry Shale Member, a black fissile shale that is 1 to 7 ft (0.3 to 2.1 m) thick (Burger and Wier, 1970; Ault and Harper, 1986).
The name "Colchester" is now used for this coal in Illinois and in the subsurface in Kentucky, but correlation with surface exposures in Kentucky is controversial and not definitely established (Jacobson and others, 1985).
Regional Indiana usage:
Illinois Basin (COSUNA 11)
Coal IIIa, Colchester Coal Member (IIIa), Velpen Coal
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.
Ashley, G. H., 1909, Supplementary report to the report of 1898 on the coal deposits of Indiana: Indiana Department of Geology and Natural Resources Annual Report 33, p. 13–150.
Ault, C. H., and Harper, Denver, 1986, Colchester Coal 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. 31.
Burger, A. M., and Wier, C. E., 1970, Colchester Coal Member (IIIa), 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. 38–39.
Fuller, M. L., and Ashley, G. H., 1902, Description of the Ditney quadrangle: U.S. Geological Survey Geologic Atlas, Folio 84, 8 p.
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.
Hasenmueller, W. A., and Ault, C. H., 1991, Reference core and correlation of key beds in the Petersburg and Linton Formations (Pennsylvanian) in Indiana: Indiana Geological Survey Occasional Paper 57, 8 p.
Jacobson, R. J., Trask, C. B., Ault, C. H., Carr, D. D., Gray, H. H., Hasenmueller, W. A., Williams, D., and Williamson, A. D., 1985, Unifying nomenclature in the Pennsylvanian System of the Illinois Basin: Illinois State Academy of Science Transactions, v. 78, p. 1–11.
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.
Wanless, H. R., 1956, Classification of the Pennsylvanian rocks of Illinois as of 1956: Illinois State Geological Survey Circular 217, 14 p.
Wier, C. E., and Gray, H. H., 1961, Geologic map of the Indianapolis 1° x 2° quadrangle, Indiana and Illinois, showing bedrock and unconsolidated deposits: Indiana Geological Survey Regional Geologic Map, Indianapolis Sheet, scale 1:250,000.
Worthen, A. H., 1868, Coal Measures and Lower Carboniferous limestones, in Geology and paleontology: Illinois State Geological Survey, v. 3, p. 1–19.
For additional information, contact:
Nancy Hasenmueller (email@example.com)Date last revised: April 13, 2017