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Time-stratigraphic relations within Silurian strata of the northern Michigan basin provide a model for reciprocal deposition of carbonates and evaporites. The Niagaran of the basin interior consists of crinoidal hematitic biomicrites. Toward the basin margin the biomicrites thicken and lose their hematitic character. A belt of pinnacle reefs marks the approach to the basin margin. The Niagaran pinnacle reefs have a lower crinoidal zone and an upper coral-algal zone. At the basin margin the Niagaran thickens abruptly into a dolomitized barrier reef complex. The barrier reefs were constructed mainly by corals and massive stromatoporoids which prograded basinward over thick skeletal forereef calcarenites.
Niagaran barrier and pinnacle reef construction was halted in the early Cayugan by an episode of evaporite deposition. Karst features within Niagaran carbonates suggest subaerial exposure of the barrier and pinnacle reefs at this time during a period of lowered sea level. Return of high sea level caused cessation of evaporite deposition and rejuvenation of the pinnacle reefs; also, fringing reefs developed along the margin of the former barrier reef trend. However, the renewed reef development was of a considerably different biotic character. These early Cayugan reefs possess a lower massive encrusting algal zone and an upper laminar stromatoporoid-stromatolite zone. Corals are notably absent. Reef development again was halted by evaporite deposition followed by another episode of car onate deposition generally devoid of reef rejuvenation.
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