Stromatoporoids of the Detroit River group and adjacent rocks (devonian) in the vicinity of the Michigan Basin

The Appalachian Fauna! Province stromatoporoid acme zone occurs in a succession of Lower (Emsian) and Middle (Eifelian) Devonian carbonate rocks exposed along the margin of the Michigan Basin in southwestern Ontario (including the Niagara Peninsula) and adjacent areas of Michigan, Ohio and Indiana. These rocks include (from older to younger) the upper Bois Blanc Limestone, the entire Detroit River Group, the upper Columbus Limestone and the lower Dundee/Delaware Limestone. Stromatoporoids are typically much more abundant and diverse in limestones, especially reefs and biostromes, but locally the Detroit River dolomites also contain notable assemblages. Systematic studies of Michigan Basin Emsian and Eifelian stromatoporoids began in 1873 and specimens from these rocks were important in the pioneering research of H.A. Nicholson and W.A. Parks. The species described herein are based on examination of thin-sections from over 500 coenostea collected from about 35 outcrops in an area exceeding 12 000 square km. Cellular/microlaminar forms belonging to the genera Syringostroma and Habrostroma (new genus) dominate most of the fossil assemblages in both abundance and diversity. The entire stromatoporoid fauna includes 26 (possibly 31) species distributed among 7 (possibly 9) genera; three species (Habrostroma formosensis, H. beachvillensis, Pseudoactinodictyon stearni) are new. Results of the taxonomic aspects of this study include the clarification of the levels of morphological variation of most species based on large samples and the recognition of several synonymous previously described species. The faunas of the Bois Blanc, Columbus and Dundee/Delaware Limestone consist chiefly of long-ranging species also found in adjacent rock units whereas about half of the Detroit River Group species are absent from the adjacent rocks. Greatest fauna! similarities exist among outcrops within the Detroit River but even here the levels of similarity are very low, even among samples from rocks of the same age and environmental setting. Comparison of the Detroit River stromatoporoid species with other Appalachian Province faunas outside the Michigan Basin indicates greatest similarity to the Kwataboahegan Formation in the Hudson Bay Lowlands and lesser similarities to the Columbus Limestone in central Ohio and the Jeffersonville Limestone in southeastern Indiana and north-central Kentucky. Thus, the value of individual stromatoporoid species in local and regional chronostratigraphy and as indicators of particular paleoenvironments appears to be severely limited.