Silurian System

Type and reference sections, synonyms, and use of name: The Kokomo Limestone Member was considered by Cumings and Shrock (1927, p. 76) as a formation and was named for about 50 feet (15 m) of color-banded, thinly laminated eurypterid-bearing Waterlime Beds of Foerste (1904b, p. 33) that were exposed in the Markland Avenue Quarry in Kokomo, Howard County, Ind. (SW¼ sec. 36, T. 24 N., R. 3 E.). This use restricted the more or less informal use by Foerste and others of the term Kokomo, which also applied to overlying rocks now assigned to the Kenneth Limestone Member. (See also Cumings, 1922, p. 459-461, for early assignments of the Kokomo to the Waterlime, Lower Waterlime, Lower Helderberg, and Eurypterid Beds, Divisions, etc.)

The type quarry was filled in during the 1960's, which led Shaver (1970, p. 84), following the suggestion of Shaver and others (1961, p. 23), to designate the Kokomo rocks cored at the edge of the quarry in Indiana Geological Survey drill hole 72 as the principal reference section. The core is in the Survey core library and was described in the last-mentioned source (p. 13).

The rank of the Kokomo was reduced to that of member in the Salina Formation (Pinsak and Shaver, 1964, p. 50). The Kokomo Limestone Member was later transferred to the Wabash Formation when the latter unit was assigned to the elevated-in-rank Salina Group (Droste and Shaver, 1982, p. 21).

Description: The Kokomo Limestone Member characteristically consists of strikingly banded tan and gray micritic to very fine grained, thinly laminated and dolomitic limestone. Besides the thin lamination, which may be considered as made up of couples, each consisting of a dark lamina and a light lamina, a more gross alternation of light and dark beds characterizes the Kokomo in some places. At such places the thin lamination is pervasive through the gross couples whose separate light and dark parts are 2 to 10 feet (0.6 to 3.0 m) thick. In addition, as many as four still larger units have been traced in the Cass County area (Tollefson, 1979).

The basis for such recognition has been, besides the gross lithology noted above, the different combinations of lesser Kokomo lithologies that consist of intraformational breccias and contorted beds (considered to represent soft-sediment deformation), algal stromatolites, cut-and-fill structures, molds of evaporate crystals, and other features generally ascribed to very shallow subtidal to intratidal environments, including environments of desiccation. In some places macro-fossiliferous rocks are present in parts of the Kokomo and include the tabulate coral Halysites, so-called "dwarf" cephalopods of the Protokionoceras type, eurypterids, and leperditiid ostracods in coquinas. (See especially Cumings and Shrock, 1928a; Pinsak and Shaver, 1964; Tollefson, 1979; and Shaver and Sunderman, 1983, p. 177-180.)

As recorded especially by Droste and Shaver (1983), the Kokomo variably overlies parts of the cherty Liston Creek Limestone Member (Wabash Formation), very likely conformably or with only very minor localized unconformity, and in some places the Kokomo completely replaces the Liston Creek and rests directly on the Mississinewa Shale Member (Wabash Formation). In the Silurian outcrop area of central northern Indiana and in the subsurface just to the north, the Kokomo underlies either the cherry Kenneth Limestone Member (Wabash Formation) conformably or the Muscatatuck Group (Middle Devonian) unconformable, The Kokomo-Kenneth contact appears sharp in places but not in others, and evidence of unconformity of regional scope is lacking In fact, the Kokomo-Liston Creek relations (noted above) and the existence of a probable Kokomo reef facies (for example, see Indiana University Paleontology Seminar, 1980) appear to deny strongly the classic idea of the Kokomo having grossly unconformable relations with the reef-bearing rock units that include without doubt all the other members of the Wabash Formation.

Distribution of the Kokomo beyond its type area in Howard County and the adjacent Cass County area is poorly understood, although various notations of Kokomo rocks in the Jasper County and Allen County areas have been made from about the turn of the century. Pinsak and Shaver in their 1964 Salina thickness map (pl. 2) defined an approximate Kokomo distribution in the Cass County and Howard County area, but beyond that area they applied the term Kokomo only in Allen County (p. 79). More recently, the term Kokomo has been assigned to rocks in Marshall and Pulaski Counties in areas that are variably north and south of the Fort Wayne Bank (Droste and Shaver, 1983), but Okla (1976, p. 38) recognized the Kokomo only in the counties around the type area. Physical continuity of the Kokomo in typical lithology among ail these occurrences seems most unlikely.

In the cored type section, the Kokomo is about 50 feet (15 m) thick in the type area, including Cass County, it ranges from zero to about 125 feet (38 m) in thickness, the zero being a deposition Al zero for a facies relationship and in the above-mentioned areas beyond the type area, reported thicknesses range from 30 to 125 feet (9 to 38 m).

Correlation: Within the Wabash Formation, the Kokomo correlates, variably, with part of or nearly the whole of the Liston Creek member. In recent years it has been correlated with other rocks of the Salina Group in stratigraphically high positions, as high as the E unit of the Michigan and Appalachian Basins and higher. Much confusion has attended past correlations, however, beginning with correlation with the Lower Devonian Waterlime Beds (so-called) of New York during the period on either side of 1900. The problem was one of commonality of the thinly laminated eurypterid-bearing beds and one of lack of normal-marine index fossils. This problem continued as late as the time of publication of the 1956 bedrock-geology map of Indiana (Patton, 1956), and Pinsak and Shaver's (1964) and Okla's (1976) correlations possibly involve rocks that Droste and Shaver (1983) and Tollefson (1979) assigned to a pre-Mississinewa position as well as to a post-Mississinewa position. Further, the Kokomo correlates, as do Liston Creek rocks, with much of the Bailey Limestone of the Illinois Basin.

The type Kokomo as well as some other parts of the Kokomo are known to underlie the Kenneth Limestone Member Wabash Formation), whose brachiopods, conodonts, and ostracods collectively require a rather Late Silurian age assignment. Moreover, conodonts identified as Spathognathodus eosteinhornensis and S. snajdri have been found in the Kokomo itself (Pollock and Rexroad, 1973). For all these reasons, the Kokomo was assigned a latest Ludlovian to early Pridolian (middle Cayugan) age by Shaver and others (1985).