Eustatic Sea-Level Control of Silurian (Niagaran) Reefs, Michigan Basin: ABSTRACT

Eustatic sea-level changes controlled Niagaran reef and off-reef facies and eogenesis both in the Michigan basin and on the adjacent platform, as shown by surface (Thornton, northeast Illinois; Pipe Creek Jr., central Indiana) and subsurface reef studies (Onandaga, south Michigan). We recognize four stages of development defined by alternating highstands and lowstands of sea level. (1) During Llandoverian-Wenlockian time, a highstand resulted in growth of reefs with 10s to 100 m depositional relief with a basal stromatactis mudstone facies capped by volumetrically dominant crinoidal wackestone to grainstone-coral boundstone facies. Reef growth was below wave base and was characterized by extensive submarine cementation. (2) A relative fall of sea level in the late Wenlockian caused a saline brine to develop in the restricted Michigan basin, halting pinnacle reef growth and resulting in A-1 Evaporite deposition and anhydrite replacement of reef fossils and sediment. This fall of sea level did not expose the shelf or bring reef tops above wave base. It may be expressed in the surface reefs as distal megabreccias containing normal marine stromatoporoid-coral-Renalcis fauna and in the subsurface reefs (basin) by a hiatal break. (3) A Ludlovian-Pridolian highstand resulted in basinal reef rejuvenation (stromatoporoid-algal boundstone facies and followed by the stromatolite facies) and dissolution of replacement anhydrite. The deep-water basinal A-l Carbonate was deposited at this time. (4) A subsequent low-stand (Pridolian?) resulted in basinal hypersalinity, cessation of pinnacle reef growth, and A-2 Evaporite deposition.