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

Abstract

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 diagesis. Guelph dolomite mainly consists of three types of replacive dolomites, including microcrystalline (20-50 um) anhedral dolomite (Type 1), finely crystalline (50-150 um) euhedral to subhedral dolomite (Type 2), and medium to coarsely crystalline (150-400 um) 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 general basinward dolomite-decreasing trend in the Guelph Formation indicates that dolomitizing fluids were derived from shelfward sources. Combined stratigraphic, petrographic and geochemical data suggest that this initial dolomitization probably resulted from regional subsurface (<300 m) reflux of normal to near-normal seawater that was induced by evaporative drawdown during the Late Silurian. Depleted o^18O values in Type 1 dolomites may have resulted from early recrystallization in a fluid similar to that for initial dolomitization but at deeper burial (320-1200 m).

The coexistence of three dolomite fabrics with relict textures and their crosscutting relationships indicate that Type 1 dolomite was altered to Type 2 and Type 3 dolomite. The systematic covariance between increasing crystal size with increasing ^87Sr/^86Sr ratios and increasing Fe and Mn contents reflects an advanced alteration of early-formed dolomite in pore fluids with increasing ^87Sr, Fe and Mn input from associated siliciclastics during burial.

Several altered intervals containing variable amounts of Type 2 dolomite and finer crystalline (20-50 um) 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. Guelph dedolomite shows depleted Sr, enriched Fe, Mn and ^87Sr/^86Sr values relative to limestone, but similar elemental and isotopic values to Type 2 dolomite. Dedolomitization is interpreted to have resulted from subsurface circulation of modified seawaters in the same conduit system as that for Type 2 dolomite formation, but extra Ca was acquired from the dissolution of remaining limestone under similar burial depth and temperature conditions to those for earlier Type 2 dolomite formation.

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 diageniesis, especially halite cementation, also played important roles in controlling the final porosity in Guelph carbonate.