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Niagaran pinnacle reefs in northern Michigan are located in a band between a shelf-edge reef complex and deeper water facies in the Michigan basin. Pinnacles at the shelfward edge of this band are dolomite; pinnacles at the basinward edge are predominantly limestone.
Two types of dolomite have been observed in the pinnacles. The first type is composed of clear rhombs about 30µ in diameter which only partly replaced the preexisting limestone. Its origin is attributed to a freshwater/seawater mixing zone at the base of a freshwater lens which moved down through the pinnacles during their emergence at the end of Niagaran deposition. The source of the required magnesium was the stabilization of the original high-magnesian calcite in the reefs.
The second type of dolomite is associated with tidal-flat environments which existed over and adjacent to the reefs during deposition of upper Niagaran and/or upper Salina units. This dolomite occurs as brownish, irregular crystals which have usually completely replaced the preexisting limestone. The intensity of this dolomitization decreases away from the associated tidal-flat environments. Reflux of hypersaline brines is presumed to have been the magnesium source for this dolomite. Reef material unaffected by this brine reflux has ^dgr13C values of about +1.5 parts per thousand (versus PDB) and Sr2+ concentrations of about 200 ppm. Rock through which this brine passed has ^dgr13C values of about +4.5 parts per thousand and Sr2+ concentrati ns of about 100 ppm. Both of these geochemical differences have been attributed to diagenesis.
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