Late Burial Diagenesis of Niagaran (Middle SUurian) Pinnacle Reefs in Michigan Basin^

Core samples from Middle Silurian pinnacle reefs in northern Michigan exhibit a regionally consistent assemblage of late diagenetic phases including geopetal diagenetic sediment, disseminated pyrite, pyrobitumen, rhombic dolomite cement, and equant calcite spar. High (>80°C) fluid inclusion homogenization temperatures and inclusions of Silurian-sourced oil in these diagenetic phases indicate that they formed between the Mississippian and Jurassic, when pinnacle reefs resided in the deep subsurface. Fluid inclusion salinities and stable carbon/oxygen isotopic ratios suggest that late diagenetic carbonates did not precipitate from connate fluids but from basinal brines that were conducted to pinnacle reefs by two regional carbonate aquifers. These data confirm that late burial diagenesis can affect carbonate rocks residing in high-salinity, low-permeabiUty fluid environments.