Reef limestone and interreef dolomite of the Detroit River Group occur in close association near Formosa, Ontario. The reefs are gray calcite. The interreef matrix is dolomite with fossil molds or calcite-dolomite-celestite pseudomorphs of evaporites. The transition zone between reef and interreef rocks is a complex mixture of carbonates including: fine-grained dolomite, mosaics of calcite or calcite and dolomite microspar and pseudospar, calcite cement, dolomite cement, and mixed calcite and dolomite cement. Diagenesis of interreef sediments involved pore-water changes from hypersaline to fresh, whereas in the reefs changes were from marine to fresh. Early transition-zone neomorphism, replacement, and cementation caused decrease in permeability and prevented migration of waters between reef and interreef. Complex diagenetic events were due to mid-Detroit River regression that produced sabkha-related environments. The continuous sequence of Detroit River limestone near Ingersoll, Ontario, suggests that this was the location of the inlet to the hypersaline Michigan Basin from the normal-marine Appalachian Basin.--Modified journal abstract.