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Abstract

Niagara pinnacle reefs in northern Michigan are located in a band between a massive shelf-edge reef complex and deeper water facies in the Michigan basin. Pinnacles at the shelf ward edge of this band are dolomite, pinnacles at the basinward edge are predominantly limestone. Two distinct types of dolomite have been observed in the pinnacles.

The first type of dolomite occurs as clear rhombs about 30 to 100 microns in diameter. Its origin is attributed to mixing of meteoric and seawater at the base of a freshwater lens which was lowered down through the pinnacles during their emergence at the end of the Niagaran period. The source of the required magnesium was the stabilization of the original high magnesium calcite in the reefs.

The second type of dolomite is associated with tidal fiat environments which were present over the reefs during A-l Carbonate deposition. This dolomite occurs as brownish, anhedral crystals with a widely varying size range which have usually completely replaced the preexisting limestone. The degree of dolomite replacement decreases downward from the overlying tidal flat strata. Reflux of hypersaline brines is presumed to have been the magnesium source for this dolomite.

Reef material unaffected by this brine reflux has δ13C values of about + 1.5‰ (vs. PDB) and Sr2+ concentrations of about 200 ppm. Rock through which this brine passed has δ13C values of about 4.0‰ and strontium values as low as 75 ppm. The large reduction in Sr2+ concentrations from values encountered in recent carbonate sediments has been attributed to fresh water diagenesis and later dolomitization. The more positive 13C values related to brine reflux are believed due to isotopically heavy carbonate species related to methane production in a reducing tidal flat environment.

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