Multiple generations of dolomitization in the catoche formation of Port au Choix, Newfoundland

Greene, Michael G. (2008) Multiple generations of dolomitization in the catoche formation of Port au Choix, Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Catoche Formation of the St. George Group in the Port au Choix area consists of about 123 m of limestone overlain by 38 m of dolostone. Deposited on a shallow carbonate platform in subtidal to peritidal settings, the upper 38 m of the Catoche Formation has undergone multiple phases of dolomitization. A fabric-retentive micritic dolomite (D1) replaces limemuds. The preservation of mud laminae and near-micritic grain size implies that dolomitization began early after deposition. Dolomite 2 (D2) replaces peloidal packstone/grainstone and wackstone and exhibits two ranges of crystal sizes, subhedral to euhedral crystals that range from 50 to 120 μm and anhedral to euhedral crystals that range between 150 to 250 μm. Crystals belonging to D2 commonly have cloudy Fe-rich cores with clear Fe-free rims. Locally crystals have well developed faces and are associated with abundant intercrystalline porosity suggesting a limited supply of Mg⁺² at the time of dolomitization. Overprinting early dolomite generations is a locally developed stylolite-related dolomite (D3). Crystals round along stylolites are generally subhedral to euhedral and range in size from 70 to 150 μm. A replacement and pore-filling euhedral to sometimes anhedral dolomite (D4) commonly occludes vugs and typically may reach several millimeters in size. Rare saddle-dolomites (D5), 300 to 600 μm in size, exhibit sweeping extinction and tend to occlude biomolds, veins but rarely lines vugs. -- Trace element distribution shows an increase of Sr with depth for D1 and D2 suggesting a possible downward movement of the early dolomitizing fluids for the respective dolomite generations. Dolomite 1 has a Sr/Ca molar ratio that ranges from 0.0037 and 0.00094 suggesting an origin from a mixed meteoric/seawater source. Stable isotopes calculations yield a range of -10.7 to -6.4‰ SMOW and agree with a mixed meteoric/seawater source for the earliest dolomitizing fluid. Microthermometric measurements recorded from D4 indicate a temperature of formation ranging from 89.2 to 99.5°C suggesting deposition at significant depth. Oxygen isotope signatures for the dolomitizing fluid associated with D4 yield a range from -1.2 to 2.4‰ SMOW and suggest an origin derived from evolved diagenetic waters.

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