Age:
Silurian
Type designation:
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).
Principal reference section: A section of the Pleasant Mills Formation in the Meshberger Bros. Stone Corp. quarry near Linn Grove, Adams County (SE¼SE¼ sec. 33, T. 26 N., R. 13 E.) was designated as a principal reference section by Droste and Shaver (1982, p. 21).
Subsurface reference section: A section of the Pleasant Mills Formation in Indiana Geological Survey drill hole 72 (Indiana Geological Survey Petroleum Database Management System No. 140455) in the abandoned Markland Avenue Quarry in Kokomo, Howard County (SW¼SW¼ LaFountain Res., T. 24 N., R. 3 E.; elevation 804 ft) was designated as a subsurface reference section by Droste and Shaver (1982, p. 21).
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 now consist of, in ascending order, three members: the Limberlost Dolomite Member, the Waldron Member, and the Louisville Member (Droste and Shaver, 1986). Further, the rocks recognized as the Louisville include the New Corydon Limestone type section of Cumings and Shrock (1928, p. 116) as shown by Droste and Shaver (1976, p. 19) (Droste and Shaver, 1986). (The New Corydon type section was incorrectly given by Shaver [1970, p. 151] as being within the Salamonie Dolomite [Droste and Shaver, 1986].)
Description:
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].)
The Pleasant Mills has a reef facies that grades from fully mature reef rock, satisfying most genetic requirements for reefs, to what may be called incipient reef rock (Droste and Shaver, 1986). The lower part of the barrierlike feature called the Fort Wayne Bank (Droste and Shaver, 1982, fig. 6) illustrates this range, as in far northwestern Indiana whitish granular vuggy pure dolostone dominates the Pleasant Mills interval, whereas southwestward in the Allen County area brown micritic to fine-grained dolostone, reminiscent of typical A-unit carbonate rocks of the basinal Salina Group, is fairly laden with laminar stromatoporoids to bring about a near doubling of thickness of the upper, Louisville-equivalent part of the Pleasant Mills (Droste and Shaver, 1986).
Very small to modest-sized discrete reefs are also known in the Pleasant Mills, but in particularly the eastern part of northern Indiana and generally south of the Fort Wayne Bank, both the lower and upper parts of the Pleasant Mills are intervals of reef abortions, whereas the middle part, the Waldron Member, is an interval of reef generation (Droste and Shaver, 1976; Shaver and others, 1978; Griest and Shaver, 1982; Shaver and Sunderman, 1983; Droste and Shaver, 1986).
North of the Fort Wayne Bank, where the formation becomes decidedly thicker, the reef facies is mostly absent, and two types of dolostone dominate, commonly in vertically alternating units a few feet to tens of feet thick: dark-brown micritic to fine-grained, partly laminated dolostones and lighter colored granular vuggy dolostones (Droste and Shaver, 1986). Chert is present in some places, particularly in the lighter colored rocks of a couple in a cyclic sequence (Droste and Shaver, 1986).
The Pleasant Mills is defined to extend throughout much of northern Indiana except southeastward of its eroded edge and except for parts or the whole of several far western counties (Droste and Shaver, 1986). Its southern (noneroded) and western limit is defined to coincide with the farthest south and southwestward extent of the wedge edge of the lower, Limberlost member (Droste and Shaver, 1982, figs. 5-7; Droste and Shaver, 1986). The middle and upper rock strata of the Pleasant Mills, however, extend beyond this limit and there are classified as the Waldron Shale (Formation) and the Louisville Limestone (Droste and Shaver, 1986). Droste and Shaver (1986) noted that the Pleasant Mills has a vertical cutoff relationship along this limit.
The formation is zero feet thick where eroded, less than 50 ft (15 m) thick in places along the vertical cutoff, and more than 300 ft (92 m) thick in the northeast corner of the state (Droste and Shaver, 1986). Both contacts, lower and upper, appear to be conformable nearly everywhere; wholly transitional to interbedded lithologies are general (Droste and Shaver, 1986). The upper transitional zone, with the Mississinewa Shale Member of the Wabash Formation, is particularly noteworthy and commonly spans 30 ft (9 m) of strata (Droste and Shaver, 1986). The upper contact, therefore, is often picked at different levels but is here considered to be at the base of the transitional interval in accord with the recommendation of Rexroad, Noland, and Pollock (1978, p. 2) (Droste and Shaver, 1986).
Correlations:
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).
The Pleasant Mills lithology can be traced physically and by means of interval stratigraphy widely in the Great Lakes area, as is explained for particularly lower Pleasant Mills rocks in the article on the Limberlost member (Droste and Shaver, 1986). In both the Michigan and Appalachian Basins, rocks of the A unit of the Salina Group correspond generally to the Pleasant Mills, which interval in the marginal parts of those basins within or nearest Indiana directly underlies the C shale unit of the Salina without an intervening B salt (Salina) (Droste and Shaver, 1986). In those classifications that recognize B carbonate rocks, such rocks may or may not correlate with upper Pleasant Mills rocks (Droste and Shaver, 1986). Definitive data are lacking. In these marginal-basin areas, units corresponding generally to the Pleasant Mills include: western Ohio, Greenfield and Tymochtee Dolomites and the upper part of the Lockport Group where this formation has been extended stratigraphically upward because of a westward facies change; southern Michigan, the A unit (as already noted) and possibly the B unit (both Salina) and including the Ruff Formation of Budros and Briggs (1977, p. 55-56); eastern Wisconsin, sections ranging from the upper part of the Cordell Member (Manistique Formation) into the lower part of the Racine Dolomite and including the so-called Lannon, or building-stone, beds; northeastern Illinois, the Sugar Run Formation and the lower part of the Racine Formation and Illinois Basin, the upper part of the St. Clair Limestone (Droste and Shaver, 1986).
In western New York and western Ontario, including the Bruce Peninsula, generally corresponding rocks range variably from within the Lockport Group (Eramosa Dolomite) through all or parts of the Guelph and Oak Orchard Dolomites (Droste and Shaver, 1986).
In these regional relations, one or both contacts of the correlative interval are likely time transgressive, as has been proposed especially for the lower contact (Droste and Shaver, 1986).
Economic Importance:
Industrial Minerals: Crushed stone products from the Pleasant Mills Formation (Silurian) include the following: aglime, chemical uses, crushed stone, and riprap from quarries in Grant and Wabash Counties (Shaffer, 2016).
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References:
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 Publication No. 75, 226 p.
Droste, J. B., and Shaver, R. H., 1976, The Limberlost Dolomite of Indiana, a key to the great Silurian facies in the southern Great Lakes area: Indiana Geological Survey Occasional Paper 15, 21 p.
Droste, J. B., and Shaver, R. H., 1982, The Salina Group (Middle and Upper Silurian) of Indiana: Indiana Geological Survey Special Report 24, 41 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.
Shaffer, K. R., compiler, 2016, Directory of industrial mineral producers in Indiana: Indiana Geological Survey Directory 11-2016, 287 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.
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