Depositional Environments of Pinnacle Reefs, Niagara and Salina Groups, Northern Shelf, Michigan Basin1

Abstract

In the northern shelf zone of the Michigan basin, pinnacle reefs of the Niagara and Salina Groups underlie the A-2 evaporite of the Salina Group. They consist of four developmental stages--biohermal, organic-reef, supratidal-island, and tidal-flat--in order of evolution. Rocks of the first two stages are within the Guelph Formation, Niagara Group, whereas rocks of the last two stages make up the lower units of the Salina Group. Strata of the biohermal stage, composed predominantly of crinoidal and bryozoan allochems, initially developed in quiet water below wave base. The bioherm grew to wave base by the end of the biohermal stage. Wave-resistant tabulate corals and stromatoporoids of the organic-reef stage developed in the high-energy zone above wave base, in a normal marine environment. Supratidal and intertidal environments of the supratidal-island stage are characterized by dominance of algal stromatolites. At the bottom and top of the section deposited during the supratidal-island stage, vadose caliche, extensive solution leaching, iron oxide, and flat-pebble conglomerate are indisputable evidence of subaerial exposure and two episodes of erosion. Fragments of algal flat-pebble conglomerate are in reef talus below the Salina A-O carbonate and A-1 anhydrite, indicating that the supratidal-island stage is older than the lowermost evaporites of the Salina Group. The tidal-flat stage is recorded by carbonate rocks that cover tops of the pinnacle reefs; these beds were deposited contemporaneously with the upper A-1 carbonate (Ruff formation) of the off-reef section. The algal pelletal-wackestone lithofacies overlaps fine mudstone of the off-reef A-1 carbonate at the reef flanks, and unconformably overlies the finely laminated algal stromatolite of the supratidal-island stage.