PETROLOGY AND REGIONAL SIGNIFICANCE OF A DEVONIAN CARBONATE/EVAPORITE COMPLEX, EASTERN MICHIGAN BASIN ~

ABSTRACt: Reef limestone and interreef dolomite of the Detroit River Group occur in remarkably close association ear Formosa, Bruce County, Ontario. The stromatoporoid- and coral-built reefs are nearly pure, gray calcite; the nongrain volume is about 50 percent microspar, 40 percent pseudospar, and 10 percent cement. The interreef matrix is fine-grained, orangeish dolomite-containing fossil molds or calcite-dolomite-celestite pseudomorphs of precursor sulfate evaporites. The transition zone between the reef and interreef rocks is thin (4-20 cm) and consists of a complex mixture of carbonate types including: a) fine-grained dolomite similar to the interreef matrix, b) mosaics of calcite or mixed calcite and dolomite mi-crospar and pseudospar, c) three types of calcite cement, d) two types of dolomite cement, and e) two types of mixed calcite and dolomite cement. Micrite, dolomicrite, terrigenous clastics, and other insolubles are nearly absent. Diagenesis of the interreef sediments involved a succession of pore-water chemical changes that ranged from hypersaline to fresh, whereas in the reefs the (synchronous?) changes were from marine to fresh. Very early transition-zone neomorphism, replacement, and cementation by marine, brackish, and hyper-saline waters caused rapid decrease in permeability and prevented migration of waters between the reef and interreef sediments/"rocks. " These complex diagenetic events were probably due to mid-Detroit River regression that produced sabkha-related nvironments over most of the Michigan Basin. Shoreline fluc-tuations and general basinward progradation produced an array of interfingefing basin-centered halite and basin-margin (sabkhal sulfate vaporites and dolomite. The continuous sequence of Detroit River limestone near Ingersoll, Ontario, suggests that this may have been the location of the inlet to the hypersaline Mich-igan Basin from the normal-marine Appalachian Basin to the east