Carbonate diagenesis and porosity evolution in the Guelph Formation, southwestern Ontario

The Middle Silurian Guelph Formation in southwestern Ontario is composed of shelfward patch reefs, basinward pinnacle reefs, and interreef facies developed on a gently-sloping carbonate ramp. Core and petrographic studies indicate that the Guelph Formation has undergone a complicated diagenetic history including pre-dolomitization diagenesis, pervasive dolomitization and post-dolomitization diagenesis. Guelph dolomite mainly consists of three types of replacive dolomites, including microcrystalline (20–50 μm) anhedral dolomite (Type 1), finely crystalline (50–150 μm) euhedral to subhedral dolomite (Type 2), and medium to coarsely crystalline (150–400 μm) euhedral to anhedral dolomite (Type 3).; Type 1 dolomite has the best preserved limestone textures, identical 87Sr/86Sr ratios to both limestone and Middle Silurian seawater, and similar low Fe and Mn contents to limestone. This dolomite is interpreted to represent the ‘least-altered’ dolomite phase that is geochemically the closest to the initial dolomite.; The coexistence of three dolomite fabrics with relict textures and their crosscutting relationships indicate that Type I dolomite was altered to Type 2 and Type 3 dolomite.; Several altered intervals containing variable amounts of Type 2 dolomite and finer crystalline (20–50 μm) dedolomite are observed in eight cores from five pinnacle reefs on the lower ramp. Common replacement fabrics such as numerous corroded dolomite relicts, poikilotopic fabrics, rhombic calcite pseudomorphs, and micrometer-sized dolomite inclusions indicate that Guelph dedolomite resulted from the replacement of preexisting Type 2 dolomite.; Low-porosity primary limestone resulted from early calcite cementation and porous limestone intervals resulted from extensive dissolution. Type 1 dolomite commonly shows similar low porosity to associated low-porosity limestone due to fabric-preserving dolomitization and early recrystallization. Most Type 2 and Type 3 dolomite intervals are porous due to fracturing, dissolution, and dolomite alteration. Dedolomite exhibits low porosity relative to Type 2 dolomite, suggesting that dedolomitization is a porosity-reducing process. Other post-dolomitization diagenesis, especially halite cementation, also played important roles in controlling the final porosity in Guelph carbonate. (Abstract shortened by UMI.)...