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.”

Because of the early paucity of Silurian conodont studies, it is still necessary to establish a comprehensive zonation and provide the sound systematics requisite for zonation of the Silurian System in North America. Current research indicates that the broad outlines of North American zones will closely parallel those defined in the Carnic Alps by Otto Walliser in 1964. In detail, however, modification is required, and the problems of provincialism and migration indicate need for some geographically restricted zones. The oldest Silurian conodont zone recognized in the Midwest, northern Michigan, and Ontario is yet to be named. Above it the limits of the Icriodina irregularis Assemblage Zone have been established in the same area, but provincial differences between the northern and southern areas are recognized. The next overlying zones, the Neospathognathodus celloni and the Pterospathodus amorphog-nathoides-Spathognathodus ranuliformis Assemblage Zones, have been documented in the Midwest and in the Niagara Gorge area. Younger zones have not been established, but a number of guide fossils of Walliser's zones have been recognized. Kockelella has been found in the upper part of the Louisville Limestone and in the Mississinewa Shale Member (Wabash Formation) in Indiana; Spathognathodus sagitta is present in the Lockport Dolomite in the Niagara Gorge and the St. Clair Limestone in Arkansas; Polygnathoides siluricus and Kockelella variabilis are included in the diversified fauna of the Bain-bridge Formation in Missouri and in the Lafferty Limestone in Arkansas; P. siluricus is also present in the Brownsport Formation and Decatur Limestone in Tennessee; Spathognathodus snajdri is found in the Kokomo Limestone Member (Salina Formation) in Indiana and the top of the Lockport Dolomite in New York; Spathog-nathodus eosteinhornensis is present in the Stonehouse Formation in Nova Scotia; and Spathognathodus remscheidensis is reported from the Syracuse Formation and younger Silurian strata in New York and from the upper part of the Decatur Limestone in Tennessee.