Type locality: Formal use dates from 1914, when Walcott (p. 354), crediting an unpublished manuscript by E. O. Ulrich, applied the name "Eau Claire Sandstone" to about 100 ft (30 m) of thin-bedded and shaly sandstones overlying the Mount Simon Sandstone in exposures along the Eau Claire River, Eau Claire County, Wisconsin (Droste and Patton, 1986).
History of usage:
Summary of early history: The terms "Eau Claire Grit" and "Eau Claire trilobite beds" were first used in a casual manner (Wooster, 1878) for clastic rocks exposed near the Dalles of the Chippewa River in Wisconsin (Trowbridge and Atwater, 1934; Droste and Patton, 1986).
The Eau Claire consists of the following rock types: (1) dolostone, feldspathic, and partly glauconitic siltstone; (2) very fine grained to fine-grained, generally well sorted sandstone; (3) maroon and dark-brown micaceous shale; (4) silty dolostone; and (5) oolitic limestone. The oolitic limestone zones in the Eau Claire occur near or at its top and are distributed only in southern Indiana (Droste and Patton, 1986). The Eau Claire Formation ranges in thickness from about 400 ft (122 m) in northeastern Indiana to more than 1,000 ft (305 m) in southwestern Indiana (Droste and Patton, 1986).
The Eau Claire Formation everywhere overlies the Mount Simon Sandstone, the contact being conformable (Droste and Patton, 1986). In northwestern Indiana the Eau Claire rocks are predominantly sandstones that grade upward into the Galesville Sandstone (Droste and Patton, 1986). Elsewhere the Eau Claire is generally overlain conformably by the Davis Formation (Droste and Patton, 1986). In southwestern Indiana the Eau Claire grades upward into the Potosi Dolomite of the Knox Supergroup (Droste and Patton, 1986).
The Eau Claire Formation is known by the one name in Indiana, Illinois, Wisconsin, Michigan, western Ohio, and western Kentucky; its equivalent in eastern Missouri is the Bonneterre Dolomite (Droste and Patton, 1986). Cores of the Eau Claire taken from Vermillion County in western Indiana have yielded a trilobite assemblage consisting of representatives of the Cedaria Zone (early Dresbachian age) and the Crepicephalus Zone (middle Dresbachian) (Palmer, 1982).
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 featuers in Indiana.
Major tectonic features that affect bedrock geology in Indiana.
Droste, J. B., and Patton, J. B., 1986, Eau Claire Formation, 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. 40-41.
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.
Medina, C. R., and Rupp, J. A., 2012, Reservoir characterization and lithostratigraphic division of the Mount Simon Sandstone (Cambrian): Implications for estimations of geologic sequestration storage capacity: Environmental Geosciences, v. 19, p. 1-15.
Medina, C. R., Rupp, J. A., and Barnes, D. A., 2011, Effects of reduction in porosity and permeability with depth on storage capacity and injectivity in deep saline aquifers: A case study from the Mount Simon Sandstone aquifer: International Journal of Greenhouse Gas Control, v. 5, p. 146-156.
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.
Trowbridge, A. C., and Atwater, G. I., 1934, Stratigraphic problems in the upper Mississippi Valley: Geological Society of America Bulletin, v. 45, p. 21-80.
Walcott, C. D., 1914, Cambrian geology and paleontology: Smithsonian Miscellaneous Collections, v. 57, p. 345-412.
Wooster, L. C., 1878, Work in St. Croix, Dunn, and adjacent counties: Wisconsin Geological Survey Annual Report for 1877, p. 36-41.
For additional information, contact:
Nancy Hasenmueller (firstname.lastname@example.org)Date last revised: February 16, 2016