About This Item
Share This Item
The Middle Silurian pinnacle reef system of northwest Michigan consists of a narrow, northeast-southwest-trending band of isolated reefs encased in thick Upper Silurian evaporites. All reefs display a similar pattern of diagenetic evolution: neomorphism of metastable carbonate components followed by precipitation of a closure cement; modification of porosity by a combination of cementation, solution and dolomitization; and emplacement of hydrocarbons and stylolitization. The exact sequence of diagenetic events and the resulting texture of the carbonate rock vary considerably from reef to reef, but several regional trends can be identified.
There is an obvious change in reef mineralogy across the reef trend, from predominantly calcitic reefs basinward to predominantly dolomitic reefs near the shelf. There appears to be a corresponding increase in ^dgr13C and ^dgr18O isotopic ratios shelfward, both in matrix limestone, according to Sears and Lucia in 1979, and in the neomorphic replacement of former marine cements. A change in the mineralogy of closure cements is observed along the reef trend, from calcite-pyrite in the southwest to pyrite or dolomite-quartz in the northeast. Oil emplacement also varies along the reef trend. Sparry dolomite and pyrite mineralization are associated with bitumen in the southwest parts of the reef trend, whereas leaching appears to accompany or just predate oil entry in the northeast. Finally, the dominant pore-filling phases in each reef vary in an irregular fashion throughout the trend from calcite and halite in some reefs to anhydrite and laminated dolomite in others.
These variations on a diagenetic theme appear to be related only to the presence or absence of diagenetic fluids in the reef's history, not to the lithology involved. Differential migration of fluids, caused by different hydrostatic heads on each fluid or migration through different pathways, would account for the diversity of diagenetic sequences in the pinnacle reef system. Careful petrographic and chemical analysis of each individual reef is needed to identify the local diagenetic history, and these local histories must be correlated in order to infer the larger picture of basin development.
End_of_Article - Last_Page 438------------