ABSTRACT

Reef limestone and interreef dolomite of the Detroit River Group occur in remarkably close association near 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 microspar and pseudospar, c) three types of calcite cement, d) two types of dol mite 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 hypersaline 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 environments over most of the Michigan Basin. Shoreline fluctuations and general basinward progradation produced an array of interfingering basin-centered halite and basin-margin (sabkha) sulfate evaporites 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 Michigan Basin from the normal-marine Appalachian Basin to the east.