Abstract
The Petoskey Formation type section is exposed near the shore of Little Traverse Bay, in the city of Petoskey, Michigan. The rocks are Middle Devonian and are the uppermost unit of the Traverse Group in western Michigan. The outcrop consists of limestone and dolomite, containing abundant stromatoporoids. At the top of the outcrop is a biostrome, consisting of a massive bed of stromatoporoids. Thin-section staining with Alizarin Red-S and potassium ferricyanide indicates that dolomite selectively replaced the calcite matrix, but has not altered the stromatoporoids greatly. Other faunas show varying degrees of dolomitization. Gypsum molds are found within some of the stromatoporoids, indicating hypersalinity nearby. The depositional environment for the Petoskey Formation type section was nearshore shallow marine. Carbonate buildups were present in the form of biostromes and bioherms. Evaporites were formed in nearby areas, which were restricted from influx of normal marine waters. Dolomitization occurred in the Petoskey Formation rocks soon after deposition, as hypersaline waters moved through the sediments or poorly cemented rocks. Well-cemented beds were apparently unaffected by the dolomitizing waters.
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