Type section: The Pleasant Mills Formation was named by Droste and Shaver (1982, p. 11 and 17) for exposures of dolomitic rocks in the Meshberger Bros. Stone Corp. quarry 3 miles (4.8 km) south of Pleasant Mills, Adams County, Indiana (center sec. 4, T. 26 N., R. 15 E.; elevation at the bedrock surface is about 785 ft [239.3 m]) (Droste and Shaver, 1986).
History of usage:
When Droste and Shaver (1982) created this new classificatory unit, the terms “Waldron Formation” and “Louisville Limestone,” in northern Indiana use for 20 years, were dropped. Because Droste and Shaver later felt that this was an unsatisfactory arrangement, they reintroduced these terms with member status in the Pleasant Mills Formation, for northern Indiana use, in 1986. In 1982, Droste and Shaver reduced the Limberlost Dolomite to member status and designated it as the lowest member of the Pleasant Mills. Also, in 1982 they assigned this sequence of rocks to the then-redefined (for Indiana use) Salina Group.
The Pleasant Mills Formation consists of several subtly different but mostly rather pure carbonate facies including, dominantly in its type area, tan to brown micritic to fine-grained and sugary dolostone that on outcrop appears thin to rather massively bedded but that is also color banded and faintly laminated (Droste and Shaver, 1986). Oolites are common to very abundant (in the form of oolite shoals?) in the lower, Limberlost part of the formation throughout much of the greater type (outcrop) area (Droste and Shaver, 1986). Approximately in the middle part of the formation is found 1 foot to as much as 30 ft (0.3 to 9.2 m) of gray to dark-gray fine-grained argillaceous to shaly thin-bedded dolostone, which between 1961 (Shaver and others) and 1982 had been called the "Waldron Shale (Formation)" (Droste and Shaver, 1986). (See the Waldron article for further details of this lithology, which is restricted to the area from the Fort Wayne Bank as shown by Droste and Shaver [1982, fig. 5] southward [Droste and Shaver, 1986].)
In the pentamerid brachiopod lineage, species of the genus Rhipidium represent the upper, Louisville-equivalent Pleasant Mills, whereas the uppermost part of the range of Pentamerus oblongus represents the lower, Limberlost part of the formation (Droste and Shaver, 1986). Limited conodont studies of Pleasant Mills and partly equivalent Louisville rocks in Indiana have not provided wholly definitive ages (Droste and Shaver, 1986). Occurrences include, however, Kockelella variabilis in several places and Spathognathodus snajdri from either high in the Pleasant Mills or just above in subsurface rocks north of the Fort Wayne Bank (Droste and Shaver, 1986). (See Rexroad, Noland, and Pollock, 1978, p. 3, and Shaver and others, 1971, p. 64 [Droste and Shaver, 1986].) All these indicators are compatible with a Pleasant Mills age ranging from middle Wenlockian into early Ludlovian (middle and upper Niagaran) (Droste and Shaver, 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.
Budros, Ron, and Briggs, L. I., 1977, Depositional environment of Ruff Formation (Upper Silurian) in southwestern Michigan, in Fisher, J. H., ed., Reefs and evaporites–concepts and depositional models: American Association of Petroleum Geologists Studies in Geology 5, p. 53-71.
Cumings, E. R., and Shrock, R. R., 1928, The geology of the Silurian rocks of northern Indiana: Indiana Department of Conservation Publications 75, 226 p.
Droste, J. B., and Shaver, R. H., 1986, Pleasant Mills 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. 114-116.
Griest, S. D., and Shaver, R. H., 1982, Geometric and paleoecologic analysis of Silurian reefs near Celina, Ohio: Indiana Academy of Science Proceedings, v. 91, p. 373-390.
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.
Rexroad, C. B., Noland, A. V., and Pollock, C. A., 1978, Conodonts from the Louisville Limestone and the Wabash Formation (Silurian) in Clark County, Indiana, and Jefferson County, Kentucky: Indiana Geological Survey Special Report 16, 15 p.
Shaver, R. H., 1970, Salamonie Dolomite, 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. 150-152.
Shaver, R. H., and Sunderman, J. A., 1983, Silurian reef and interreef strata as responses to a cyclical succession of environments, southern Great Lakes area (Field Trip 12), in Shaver, R. H., and Sunderman, J. A., eds., Field trips in midwestern geology: Bloomington, Indiana, Geological Society of America, Indiana Geological Survey, and Indiana University Department of Geology, v. 1, p. 141-196.
Shaver, R. H., Ault, C. H., Ausich, W. I., Droste, J. B., Horowitz, A. S., James, W. C., Okla, S. M., Rexroad, C. B., Suchomel, D. M., and Welch, J. R., 1978, The search for a Silurian reef model–Great Lakes area: Indiana Geological Survey Special Report 15, 36 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.
Shaver, R. H., Doheny, E. J., Droste, J. B., Lazor, J. D., Orr, R. W., Pollock, C. A., and Rexroad, C. B., 1971, Silurian and Middle Devonian stratigraphy of the Michigan Basin–a view from the southwest flank, in Forsyth, J. L., Geology of the Lake Erie islands and adjacent shores: Michigan Basin Geological Society Guidebook, p. 37-59.
Shaver, R. H., with contributions by Gray, H. H., Pinsak, A. P., Sunderman, J. A., Thornbury, W. D., and Wayne, W. J., 1961, Stratigraphy of the Silurian rocks of northern Indiana: Indiana Geological Survey Field Conference Guidebook 10, 62 p.
For additional information, contact:
Nancy Hasenmueller (email@example.com)Date last revised: June 13, 2017