Summary

Southwestern Ontario is located at the southeastern margin of the Michigan Basin, which is a circular intracratonic basin. In the subsurface of southwestern Ontario, the Guelph Formation is a carbonate ramp sequence developed along the basin margin during the Middle Silurian, which is dominated by thick upper slope patch reef and lower slope pinnacle reef facies and thin inter-reef facies. The Guelph Formation is mainly composed of tan to dark brown, microcrystalline to fine crystalline, pervasive and massive dolomites, and it is overlain by the Upper Silurian Salina Formation, which consists of interceded carbonates and evaporites.

Several intervals of partly to almost completely altered rocks, previously reported as marine limestones or paleosols by other workers, are firstly recognized as dedolomitized carbonates in the cores from five pinnacle reefs within a narrow transitional zone between platform dolomites and basinal limestones. There is no evidence or indication to show that the dedolomitization is fault-controlled or unconformity-related. Some dedolomitized intervals are important oil and gas reservoir rocks. The dedolomitized rock in the Guelph Formation appears to be very different from its dolomite precursor and primary limestone since its light color, loose structure, poor lithification, and containing numerous dolomite lenses or patches.

Integrated stratigraphic, petrographic, geochemical, and fluid inclusion evidence suggests that localized dedolomitization of Guelph Formation encountered in the transition zone may occur in a subsurface environment shortly after the early dolomitization and dolomite recrystallization. The calcium-rich (high Ca/Mg ratio) dedolomitizing agents are most likely the fluids evolved from the hypersaline waters responsible for the dolomitization and dolomite recrystallization in the same reflux flow system.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah