Diagenesis of Niagaran (Middle Silurian) Pinnacle Reefs, Northwest Michigan (Carbonate, Stable Isotopes, Thermal Modelling).

Abstract

The presence of geothermal gradients 10(DEGREES) to 20(DEGREES)C higher than the current average gradient (25(DEGREES)C/km), and of one kilometer of now eroded overburden in the Michigan Basin during the Paleozoic can be inferred from the high organic maturity of basin strata using the Lopatin method. These data are used to reconstruct the burial history of a single pinnacle reef from northwest Michigan, allowing absolute time constraints to be placed on reef diagenesis. Partial dolomitization occurred during subaerial exposure just after reef growth; regional dolomitization occurred between the late Silurian and the Devonian; and late mineralization by calcite, dolomite and pyrite occurred after Mississippian hydrocarbon emplacement and before the end of the Jurassic. There is no evidence that a well-developed fresh-water lens was ever present in this reef. Regional controls on carbonate diagenesis in the northwest reef trend include: size and hydraulic conductivity of reefs, thickness and lithology of adjacent and overlying evaporites, migration paths of late gypsum-derived and basinal brines, and the timing of anaerobic fermentation in organic-rich carbonates. The interaction of these factors can cause genetically-related fluids to precipitate diagenetic phases of differing geochemistry and /or mineralogy. A non-diagenetic trend toward more ('13)C enriched marine components observed throughout the middle Silurian in northwest Michigan appears to reflect the growing restriction of Michigan Basin water prior to evaporite deposition.