Cyclic Deposition of Silurian Carbonates and Evaporites in Michigan Basin

Abstract

The existence of hydrocarbon-bearing Silurian pinnacle reefs along the southeastern margin of the Michigan basin has been known for many years, with reef discoveries reaching a peak during the 1950s and early 1960s in that area. Renewed exploration activity around the basin margin resulted in the discovery, beginning in 1969, of an extensive pinnacle-reef fairway extending for 160 mi across northern Michigan. This discovery, in conjunction with new reef discoveries in southern Michigan, suggests that a hydrocarbon-bearing pinnacle-reef fairway of Silurian age extends around the entire Michigan basin. Coring of Middle and Upper Silurian strata associated with the northern Michigan pinnacle-reef fairway has revealed three basic vertical lithologic successions: (1) nonreef, (2) barrier-reef complex, and (3) pinnacle-reef complex. The nonreef vertical lithologic succession consists of dense micritic carbonates cyclically interbedded with evaporites (halite and anhydrite). Cores through the barrier-reef trend show a vertical succession of lithologies indicative of basinward progradation of the barrier-reef complex. The reef-core facies of the barrier-reef complex is dominated by stromatoporoids. Cores through the pinnacle reefs show an ascending vertical zonation consisting of a crinoidal zone, a coral-algal zone, an algal zone, a laminar stromatoporoidal zone, and a stromatolitic zone Paleontologic data indicate at least two separate phases of reefing associated with the northern Michigan Silurian reef fairway. The Niagara barrier-reef trend, the crinoidal and coral-algal zones of the pinnacle reefs, and the micritic interreef Niagara strata are Wenlock or older in age. Fossils indicate a Ludlow-Pridoli(?) age for the upper parts of the pinnacle reefs (algal zone, laminar stromatoporoidal zone, stromatolitic zone) and for algal fringing reefs along the front of the Wenlock barrier-reef trend. A reef-interreef depositional model which invokes sea-level fluctuations is most consistent with data obtained from the Silurian of the Michigan basin and may be applicable to the reef-evaporite controversy outlined by Sloss (1969). High-sea-level phases are equated to carbonate deposition (Niagara, A-1 carbonate, A-2 carbonate) and low-sea-level phases are equated to evaporite deposition (A-1 evaporite, A-2 evaporite, and B evaporite). In effect, this model proposes the existence of two distinct, quasicontemporaneous depositional settings (one carbonate and the other evaporite) which were very close in geologic time and yet were not synchronous. The distribution and thickness patterns of the Niagara and lower Salina carbonate and evaporite rocks show two major trends: (1) a reversal in carbonate thickness trends from basinward thinning of the Niagara and A-1 carbonate to basinward thickening of the A-2 carbonate, and (2) a progressive marginward spreading of successive evaporite units even though each has the same thickness in the basin interior. These two major trends probably reflect the progressive deterioration of basin-margin reefing from late Niagaran into early Cayugan time. The Niagara Group carbonates are thick, very fossiliferous, and reefy along the basin margins. They thin basinward into nonreef carbonates deposited in a somewhat starved-basin interior. The A-1 carbonate is similar to the Niagara Group in that i is characterized by basin-margin reefing, but its biotic abundance is reduced considerably from that of the Niagara and, as a result, the A-1 carbonate produced less basin-margin depositional topography. The A-2 carbonate, which is generally devoid of basin-margin reefing, produced little or no basin-margin depositional topography. Basinward thickening of the A-2 carbonate suggests that physical, rather than biologic processes became the dominant factor during deposition of this unit. Thus, with each successive carbonate phase producing less basin-margin depositional topography, the evaporite phase which followed each carbonate phase spread increasingly farther from the center of the basin on to the basin margins.