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Abstract

In the three-corner area of Indiana, Michigan, and Ohio, much of the 500-ft (150 m) Silurian section younger than the Salamonie Dolomite is a facies of the reef-bearing rocks of the Wabash platform areas of Illinois, Indiana, and Ohio. Rocks of the Salina Group and stratigraphical-ly equivalent rocks of the platform area do not contain salts and anhydrites, but they reflect the Michigan-basin cyclic sedimentation as far south as central Indiana in the form of transgressive-regressive facies within named rock units. No major unconformity is in the section, and the up-dip carbonate rocks probably are lateral equivalents of salts in the basin.

Approximate correlation of Wabash-platform rocks in Indiana with units of the Salina in the Michigan basin is: (1) Limberlost Dolomite—lowest part of A unit; (2) Waldron Formation through Louisville Limestone—much of remainder of A unit, especially A-1 carbonate; (3) Wabash Formation—from upper part of A unit (B unit in some areas) through uppermost Salina; (4) Kokomo Limestone Member (Salina Formation)—D unit and possibly younger; and (5) Kenneth Limestone Member (Salina)—probably younger than D. Three reef-start episodes on the platform were coordinated with periods of more normal salinity during late deposition of the Salamonie, late deposition of the Louisville, early in deposition of the Mississinewa, and during deposition of the Kenneth. Some of the earliest reefs aborted during A-unit periods of above-normal salinity, including periods represented by part of the Limberlost and middle Louisville rocks, but many reefs grew during all the time of Salinan cyclic deposition.

Even in the northern platform area, where the upper part of Silurian rocks has been eroded, a complete platform buildup that included reefs could have continued to accrete during deposition of the uppermost parts of the Salina Group. These interpretations do not readily favor some current ideas on thick sabkha evaporites, hundreds of feet of drawdown, and near-desiccation in the proto-Michigan basin—nor do they favor regional development of a so-called “Niagaran-Cayugan unconformity.”

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