Niagaran Series

Silurian System

Type and reference sections: The Salamonie Dolomite was named for exposures of dolomite in the headwaters area of the Salamonie River near Portland, Jay County, Ind. (Pinsak and Shaver, 1964, p. 24). The type section is the exposure near Portland in the Meshberger Bros. Stone Corp. quarry (formerly the Rockledge Products, Inc., quarry) and the rocks penetrated by Indiana Geological Survey drill hole 44, cored from the floor of that quarry in the NW¼NW¼ sec. 30, T 23 N., R. 14 E. Two principal reference sections in Indiana are the exposures in the Meshberger Bros. Stone Corp. quarry (formerly the H and R Stone Co. quarry) near Ridgeville, Randolph County (E2SE¼ sec. 12, T. 21 N., R. 13 E.), and the rocks cored in the Northern Indiana Public Service Co. Carl Wyneken No. 1 well near Wallen, Allen County (SE¼SW¼SE¼ sec. 11, T. 31 N., R. 12 E.). The two cores mentioned are in the core library of the Indiana Geological Survey.

History of name: The name was originally applied only in northern Indiana to dolomitic rocks that were then considered to lie above the Brassfield Limestone and below the Waldron Formation except in the northern two to three tiers of counties, where it underlay the Salina Formation, and in far western counties, where it underlay (and still does) the Louisville Limestone. In 1967 (French, p. 18) recognition of the unit was extended to the southeastern Indiana outcrop area, where it was assigned two preexisting formations reduced to member status, the Osgood Member below and the Laurel Member above. In 1974 (Becker, p. 17-18) nearly statewide recognition was completed with description of the unit in the subsurface of southwestern Indiana. In 1976 (Droste and Shaver, p. 4) the formation name Limberlost Dolomite was assigned to what had been the unnamed upper brown fine-grained dolomite member of the Salamonie. This unit was later reduced to member rank and assigned to the Pleasant Mills Formation of the Salina Group (Droste and Shaver, 1982, p. 11). In 1980 (Rexroad, p. 12) the lower, approximately Osgood equivalent of the Salamonie in the parts or whole of 17 northeastern Indiana counties was assigned to the Stroh Member of the Cataract Formation. With assignment of what was originally the upper Salamonie to the Limberlost Dolomite (later becoming part of the Pleasant Mills), the Portland quarry section (see above) became very nearly a single, complete type section for the Salamonie Dolomite, exhibiting about 140 feet (48 m) of this formation, from which possibly only a very few feet of rocks have been eroded at the top.

Description: The Salamonie has these boundaries with other units: In northern Indiana it variably overlies the Brassfield Limestone, the Sexton Creek Limestone, and the Cataract Formation; further, in northern Indiana, a lower part of the Salamonie has a vertical cutoff boundary with the upper part of the Cataract, so that the oldest Salamonie rocks in northern Indiana are in western counties, there lying on the Sexton Creek. In southern Indiana the Salamonie overlies the Brassfield and Sexton Creek Limestones and, in small areas, Ordovician rocks. In southwestern Indiana the Salamonie has a vertical cutoff boundary with the approximate lower half of the St. Clair Limestone. In most of northern Indiana the Salamonie underlies the Pleasant Mills Formation, and in the rest of state, including far western counties in northern Indiana, it underlies variably the Waldron Shale (Formation) and the Louisville Limestone. The upper contact is conformable; the lower one is unconformable except in far northwestern Indiana and in northeastern Indiana where the Salamonie overlies the Stroh Member of the Cataract. Present criteria for recognition of the lower boundary (for example, those of Rexroad, 1980, and Rexroad and Droste, 1982) differ in some detail from those originally proposed by Pinsak and Shaver (1964).

The Salamonie has two principal lithologies. Lower Salamonie rocks are generally impure, especially in southern Indiana, and include finer grained argillaceous limestone and dolomitic limestone (as well as dolomite) and shale (Osgood Member in the south). The upper rocks are much purer and consist of whitish coarser grained bioclastic vuggy dolomite in much of the state. Such dolomite of great purity dominates the upper Salamonie especially in northern Indiana. Chert is present sporadically within the Salamonie but mostly in the upper, Laurel rocks in part of southeastern Indiana. The upper Salamonie has a pure carbonate reef facies in many places in Indiana, from the Illinois Basin to northeastern Indiana outcrop (Becker and Droste, 1978; Shaver and others, 1978; Droste and Shaver, 1980).

The Salamonie ranges in thickness from zero to about 60 feet (18 m) along eroded edges in southeastern Indiana outcrop, to about 140 feet (43 m) near its type section, to more than 250 feet (76 m) in far northeastern Indiana, to about 100 feet (30 m) in northwestern Indiana, and to 50" feet 15 m) in central and southwestern Indiana.

Correlation: As noted above, lower Salamonie rocks in northwestern Indiana are equivalent to the upper Cataract rocks of northeastern Indiana, and in southwestern Indiana the entire Salamonie equates with lower St. Clair rocks of the Illinois Basin. The Salamonie also correlates closely to approximately with these rocks in adjacent states: central Kentucky, the Osgood Formation and the Laurel Limestone; western Ohio, the Dayton Formation through the middle part of the Lockport Group or through the Cedarville Dolomite; in southern Ohio, the Estill Shale through the Peebles Dolomite; Michigan, the Clinton and Niagara Groups; and northern Illinois and southeastern Wisconsin, the Joliet Dolomite except for the lowermost few feet.

The brachiopod Pentamerus oblongus is a common Salamonie guide fossil that reaches its known upper range in Indiana few feet above the upper Salamonie contact, that is, in Limberlost rocks.

Conodont studies by Rexroad (1980) and Rexroad and Droste (1982) record possibly three standard Silurian conodont zones within the Salamonie. The lowest of these, the Ozarkodina celloni Assemblage Zone, has been noted at one northwestern Indiana locality lowermost in the Salamonie. The next lowest zone, the Pterospathodus amorphognathoides-Kockelella ranuliformis Assemblage Zone, has been found elsewhere lowermost in the unit, and also elements of the Kockelella amsdeni Assemblage Zone have been found (rarely) in lower Salamonie rocks.

In northwestern lndiana a zone of abundant ammodiscid foraminifers, the Ammodiscus-Thurammina Assemblage Zone, has been noted near the base of the formation (Mound, 1968 Rexroad and Droste, 1982), there in association with elements of the P. amorphognathoides-K. ranuliformis conodont zone. These fossil occurrences suggest that the Salamonie belongs generally in the lower part of the Niagaran Series and, in British terms, probably in parts of both the Llandoverian and Wenlockian Series. In accord with most common midwestern and midcontinent practices, the ammodiscid zone places the lowest Salamonie rocks in the upper part of the Alexandrian Series (North American standard). (See also Shaver and Sunderman, 1983.)