Silurian System

Type area and history of name: The Mississinewa Shale Member, originally having formation rank, was named by Cumings and Shrock (1927, p. 72) for shaly weathering argillaceous silty dolomite and dolomitic siltstone, more than 50 feet (15 m) thick in single exposure, along the Mississinewa River between Marion, Grant County, and the southwest corner of Wabash County, north-central Indiana. Its status was reduced to that of member in the Wabash Formation by Pinsak and Shaver (1964, p. 35).

Mostly used until the 1970's only in central northern Indiana, the term was established in southern Indiana use especially by Becker (1974) and Becker and Droste (1978), who defined a vertical cutoff boundary against rocks of the Bainbridge Group, so that the Mississinewa, as defined, is absent from an area of eight-county size in the southwest corner of the state. For the southern Indiana outcrop and near-outcrop area, French (1967) and Rexroad, Noland, and Pollock (1978) advocated nonusage of the term Mississinewa, but the latter persons recommended that the lower boundary of the Mississinewa-equivalent interval with the Louisville be placed at the base of transitional lithology between the two intervals. For northern Indiana, the term Mississinewa had been in use only for the area of the Fort Wayne Bank and southward (Pinsak and Shaver, 1964), but by 1982 (Droste and Shaver, fig. 7 and p. 22) it had been extended to some far northern counties where argillaceous or shaly rocks lie above typical Pleasant Mills rocks and below rather pure upper Wabash rocks.

Description: In its type area the Mississinewa member consists of argillaceous dolomitic siltstone and silty dolomite, fairly calcareous in places, that is in various shades of gray and is dense to fine grained and massive appearing in unweathered exposures. Although clay and quartz silt make up more than 50 percent of some samples (Cumings and Shrock, 1928a; Erdtmann and Prezbindowski, 1973; Mathews and Sunderman, 1975; Owens, 1981), the designation of the Mississinewa as a shale is incorrect fissility is notably lacking from fresh exposures and cored sections. The bottom several to 30 feet (9 m) of the member consists of interbedded and otherwise gradational lithologies of the Mississinewa and Louisville types, the latter consisting of brownish fine-grained dolomite that is purer than the Mississinewa type. The member also includes subordinate amounts of light-colored granular cherty dolomite and dolomitic limestone of the Liston Creek type. Bioturbation and other primary sedimentary textures and structures have been described (Erdtmann and Prezbindowski, 1973) as those that occur only in very fine sediments deposited on a generally quiet sea floor.

The Mississinewa also has a reef facies, commonly called the Huntington Lithofacies (term applied to Silurian reef rocks at any stratigraphic level in Indiana) that ranges from impure-carbonate, immature-reef rock to nearly 100-percent-pure-carbonate, mature-reef rock, depending on both stratigraphic level and distal-to-proximal position in the reef. (See Textoris and Carozzi, 1964, and the discussion of the Huntington Lithofacies in the Wabash article.)

The lower Mississinewa contact with the Louisville in southern Indiana and Louisville-equivalent rocks in northern Indiana is conformable and placed at the stratigraphic level (noted above) below which typical Louisville lithology continues downward without intercalation of the Mississinewa type of impurities. The upper Mississinewa contact with the Liston Creek Limestone Member, although conformable, generally involves fewer transitional rocks than does the lower boundary. In the type areas of these two Wabash members, for example, the basalmost Liston Creek consists contrastingly of the thin glauconitic Red Bridge Limestone Member. In some other areas, however, the Kokomo Limestone Member (a generally micritic, thinly laminated carbonate rock) or rocks not given member status but having the Kokomo type of lithology overlie the Mississinewa member, probably conformably. (See, for example, Droste and Shaver, 1982, p. 31, and Shaver and Sunderman, 1983, p. 178.)

In still other areas, moreover, the Mississinewa and Liston Creek types of lithologies, as well as the reef facies, replace one another in seemingly any pattern from top to bottom of the Wabash Formation. For this reason, the Mississinewa and Liston Creek distributions as mappable rock units are somewhat limited in Indiana, so that the term Wabash Formation is the only term applicable to some sections.

In southeastern Indiana the Mississinewa and Mississinewa-equivalent rocks are absent from the Silurian outcrop area because of erosional truncation below the Silurian-Devonian unconformity and down onto the Louisville and older Silurian rocks, but the erosional wedge edge of these rocks, below the Muscatatuck Group (Middle Devonian), is present in the shallow subsurface. (See Rexroad, Noland, and Pollock, 1978, p. 2.)

In its somewhat spotty distribution in typical and noneroded and nonreef lithology, the Mississinewa ranges from an average of about 60 feet (18 m) in southern sections, to about 110 feet (34 m) in its type area, and to about 200 feet (61 m) in some northern parts of distribution.

Correlation: In its type-area and in its normal thickness, the Mississinewa correlates with upper Moccasin Springs rocks of the Illinois Basin, with a middle part of the Racine Formation of northern Illinois and eastern Wisconsin, C shale unit (Salina Group) and possibly some subjacent and superjacent Salina rocks in the Michigan and Appalachian Basins, and with a part of the Vernon Shale (Salina Group) of New York. Where the Mississinewa is at its thickest, however, and occupies nearly all the Wabash interval, these correlations extend appreciably higher stratigraphically. This means, then, that some upper Mississinewa rocks correlate with some Liston Creek and Kokomo rocks within the Wabash Formation.

Stratigraphically useful index fossils in the Mississinewa include conodonts that represent zones ranging from that of Kockelella variabilis upward into that of Spathognathodus snajdri. (See Shaver and others, 1971, p. 54; Rexroad, Noland, and Pollock, 1978, p. 3; and Indiana University Paleontology Seminar, 1980, p. 120.) In the pentamerid brachiopod lineage, the Mississinewa is represented by Kirkidium cf. K. knighti, which in Indiana is found consistently between zones represented by Rhipidium spp. (below, in Louisviile rocks) and K. cf. K. laqueatum (above, in upper Wabash rocks). (See Droste and Shaver, 1977, fig. 1 and p. 99.) Other Mississinewa fossils having international correlative significance include the graptolites Monograptus fatciformis (= M. bohemicus) (Cumings and Shrock, 1928a) and Pristiograptus jaegeri (Erdtmann and Prezbindowski, 1973, p. 347-349) and the acritarch Deunffia eisenacki (Wood, 1975). These indices, together with the subjacent and superjacent faunal zones, indicate an age for the Mississinewa within the Ludlovian Epoch, one that spans time across the Niagaran-Cayugan Series boundary in the New York standard.