Silurian System
Type section and use of name: 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, Ind. (center sec. 4, T. 26 N., R. 15 E.). Two reference sections were designated, one in the nearby Linn Grove Quarry and the other consisting of subsurface rocks penetrated by Indiana Geological Survey drill hole 72 in the abandoned Markland Avenue Quarry in Kokomo, Howard County, Ind. (See details in Droste and Shaver, 1982, p. 17 and 21.)
When this new classificatory unit was created, the terms Waldron Formation and Louisville Limestone, in northern Indiana use for 20 years, were dropped, which has proved to be an unsatisfactory arrangement. As stated in the Louisville and Waldron articles, however, these terms are here reintroduced, with member status in the Pleasant Mills Formation, for northern Indiana use. At the same time (1982, Droste and Shaver), the preexisting Limberlost Dolomite was reduced to member status and assigned as the lowest part of the Pleasant Mills. Also, this sequence of rocks was assigned for the first time to the then-redefined (for Indiana use) Salina Group. The Pleasant Mills now consists in ascending order of three members: the Limberlost Dolomite Member, the Waldron Member, and the Louisville Member. Further, the rocks recognized as the Louisville include the New Corydon Limestone type section of Cumings and Shrock (1928a, p. 116) as shown by Droste and Shaver (1976, p. 19). (The New Corydon type section was incorrectly given by Shaver, 1970, p. 151, as being within the Salamonie Dolomite.)
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 dolomite that on outcrop appears thin to rather massively bedded but that is also color banded and faintly laminated. 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. Approximately in the middle part of the formation is found 1 foot to as much as 30 feet (0.3 to 9.2 m) of gray to dark-gray fine-grained argillaceous to shaly thin-bedded dolomite, which between 1961 (Shaver and others) and 1982 had been called the Waldron Shale (Formation). (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.)
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. 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 dolomite dominates the Pleasant Mills interval, whereas southwestward in the Allen County area brown micritic to fine-grained dolomite, reminiscent of typical A-unit carbonate rocks of the basinal Salina Group, are fairly laden with laminar stromatoporoids to bring about a near doubling of thickness of the upper, Louisville-equivalent part of the Pleasant Mills.
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).
North of the Fort Wayne Bank, where the formation becomes decidedly thicker, the reef facies is mostly absent, and two types of dolomite dominate, commonly in vertically alternating units a few feet to tens of feet thick: dark-brown micritic to fine-grained, partly laminated dolomites and lighter colored granular vuggy dolomites. Chert is present in some places, particularly in the lighter colored rocks of a couple in a cyclic sequence.
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. 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, 198: ], figs. 5-7). 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. The Pleasant Mills, therefore, has a vertical cutoff relationship along this limit.
The formation is zero thick where eroded, less than 50 feet (15 m) thick in places along the vertical cutoff, and more than 300 feet (9: 3 m) thick in the northeast corner of the state. Both contacts, lower and upper, appear to be conformable nearly everywhere wholly transitional to interbedded lithologies are general. The upper transitional zone, with the Mississinewa Shale Member of the Wabash Formation, is particularly noteworthy and commonly spans 30 feet (9 m) of strata. 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).
Correlation: 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. Limited conodont studies of Pleasant Mills and partly equivalent Louisville rocks in Indiana have not yet provided wholly definitive ages. Occurrences include, however, Kockelella variabilis in several places and Spathognathodus snajdn from either high in the Pleasant Mills or just above in subsurface rocks north of the Fort Wayne Bank. (See Rexroad, Noland, and Pollock, 1978, p. 3, and Shaver and others, 1971, p. 64.) All these indicators are compatible with a Pleasant Mills age ranging from middle Wenlockian into early Ludlovian (middle and upper Niagaran).
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. 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). In those classifications that recognize B carbonate rocks, such rocks may or may not correlate with upper Pleasant Mills rocks. 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.
In western New York and western Ontario, including the Bruce Peninsula, generally corresponding rocks range variably from within the Lockport Group (Eramosa Dolomite) through ail or parts of the Guelph and Oak Orchard Dolomites.
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. (See Limberlost, Waldron, and Louisville articles; see also Droste and Shaver, 1977, and Shaver and others,1985.)