Dolomitization of Northern Michigan Niagara Reefs by Brine Refluxion and Freshwater/Seawater Mixing
S. O. Sears, F. J. Lucia, 1980. "Dolomitization of Northern Michigan Niagara Reefs by Brine Refluxion and Freshwater/Seawater Mixing", Concepts and Models of Dolomitization, Donald H. Zenger, John B. Dunham, Raymond L. Ethington
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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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Special Publication 28 has its roots in the 22nd Annual Research Symposium of SEPM entitled Concepts and Models of Dolomitization – Their Intricacies and Significance held on April 3,1979 in Houston, Texas as part of the joint annual meetings of AAPG and SEPM. The purpose of that symposium was to express the state-of-the-art of the study of the elusive process(es) of dolomitization. Most of the contributions in this volume are concerned with apparent early, nearsurface dolomitization, either by hypersaline brines, by the marine-meteoric mixing model or some variant thereof, or by both mechanisms where more than one phase or kind of dolomite exists, or where the origin of a particular dolomite is uncertain. Other models and aspects of dolomitization are treated here as well.