(JBD & RHS)
Pleasant Mills Formation,
Type and reference sections and use of name: The Limberlost Dolomite Member was originally named as a formation (Droste and Shaver, 1976, p. 4) for exposures of dolomite in the Limberlost area (for example, including Limberlost Church and Limberlost Creek) near Geneva in Adams and Jay Counties, Ind. The type section consists of the exposures in the John W. Karch Stone Co. quarry 42 miles (7.2 km) east-southeast of Geneva, Adams County (SW¼SW¼ sec. 31, T. 25 N., R. 15 E.). The principal reference section is in the Meshberger Bros. Stone Corp. quarry 1¼ miles (2 km) southeast of Fairview, Randolph County (SW¼NW¼ sec. 11, T. 21 N., R. 12 E.). Two other reference sections were originaily designated, including a subsurface section consisting of rocks penetrated by Indiana Geological Survey drill hole 72 in the abandoned Markland Avenue Quarry in Kokomo, Howard County (SW¼SW¼ sec. 36, T. 24 N., R. 3 E.)
The Limberlost Dolomite (original name) was later reduced to member status and assigned as the bottommost unit to the then-new Pleasant Mills Formation (Droste and Shaver, 1982, p. 11).
Description: The Limberlost is made up of several subtly different carbonate facies, the dominant one consisting of light-brown micritic to fine-grained rather pure dolomite. Variations include fine-grained, faintly to strongly laminated and color-banded dolomite, oolitic dolomite, coarser grained bioclastic vuggy dolomite, and bluish-gray mudstone dolomite (the latter two lithologies in association with reef-flank and reef-core rocks).
In general, granularity decreases and laminated appearance increases northward and eastward toward the Michigan and Appalachian Basins, in that the Limberlost takes on typical characters of the basin carbonate facies of the Salina Group in those directions. Oolites are conspicuous to dominant in many places in the central and eastern parts of northern Indiana, and dolomite containing molds of euhedral gypsum crystals is present in some places. The Limberlost is a relatively unfossiliferous (usual sense) unit, and it represents a restricted environment in which many reefs, growing upward from within the underlying Salamonie sediments, became aborted. Generally westward from eastern Indiana and western Ohio, reefs were less adversely affected, so that in places the Limberlost is made up entirely of its reef facies that is variably flanked by other facies, including the oolitic and coarser bioclastic facies. The range in such relationships is well illustrated in quarries near Celina, Ohio, and New Corydon (the type quarry), Pleasant Mills, and Montpelier, Ind. (See Droste and Shaver, 1976, and Shaver and Sunderman, 1983.)
The Limberlost has the same defined southern and southwestern boundaries in Indiana as do the Pleasant Mills Formation and the Salina Group, including an eroded limit in central eastern Indiana and a practically recognizable depositional wedge edge extending northwestward from Shelby County and thence northward to southern Lake County. (See figs. 5-7 in Droste and Shaver, 1982.) A northern practical limit extends approximately across the second tier of northern Indiana counties. In part of this area and northward, the Limberlost becomes indistinguishable because it is involved with the lower part of hundreds of feet of reef facies (the Fort Wayne Bank) and because the Waldron lithology, normally seen above the Limberlost, is hardly recognizable anywhere north of the Fort Wayne Bank.
In this distribution the Limberlost ranges in thickness from zero along both eroded and depositional edges to more than 70 feet (21 m) in a few northern locales (Droste and Shaver, 1976, fig. 1, and 1982, appendix, secs O-Z).
The Limberlost is underlain by the Salamonie Dolomite and is overlain by the Waldron Member of the Pleasant Mills Formation. Both the lower and the upper Limberlost contacts appear to be conformable, as thoroughly transitional to interbedded lithologies apply, but there probably is minor unconformity in some places along the lower contact, for example, possibly in the reference-section quarry in Randolph County.
Correlation: A common late Llandoverian-middle Wenlockian (late Alexandrian-middle Niagaran) guide fossil, the brachiopod Pentamerus oblongus, is found at its highest stratigraphic range in the Limberlost. It is followed Closely above in middle Pleasant Mills and Louisville rocks by species of the pentamerid genus Rhipidium (late Wenlockian-early Ludlovian).
The lower Limberlost boundary and at least lower Limberlost-equivalent rocks can be Correlated by physical tracing and by sequential position throughout much of the Great Lakes region, possibly including even western New York and western Ontario. The Limberlost is a key unit, representing onset of restricted, near-evaporitic conditions that followed deposition of a great, widespread blanket of light-colored coarse-grained bioclastic carbonate rocks of the Niagara and Lockport Groups, the Salamonie Dolomite, and the Joliet Formation. In those areas (for example, western Ohio) where Lockport rocks extend stratigraphically higher than they do in most places, the Limberlost position is within an upper part of the Lockport Group, that is, just below or within what is called the Guelf Dolomite in some places. Closely equivalent units include the Greenfield Dolomite (Salina Group) of western Ohio, brown carbonate rocks of the Michigan Basin that have been variably assigned to the upper part of the Niagara Group and to the lower part of the A unit of the Salina Group, and the Sugar Run Formation of northeastern Illinois. In each of these examples, however, the upper Limberlost boundary can hardly be coordinated precisely because the Waldron, present in Indiana, is mostly lacking beyond the borders of the state.
The Limberlost is likely a time-transgressive unit, particularly between what were basin-and shelf-depositional areas. For example, the depositional wedge in northern Indiana is thought to be the same age as upper Salamonie (Laurel) rocks of southern Indiana, but the relationship is expressed as a wedge and not as a vertical cutoff. (See also Droste and Shaver, 1977.)