Origin of dolomites in the Boat Harbour Formation, St. George Group, in western Newfoundland, Canada: implications for porosity development

Azmy, Karem and Knight, I. (Ian) and Lavoie, Denis and Chi, G. (2009) Origin of dolomites in the Boat Harbour Formation, St. George Group, in western Newfoundland, Canada: implications for porosity development. Bulletin of Canadian Petroleum Geology, 57 (1). pp. 81-104. ISSN 0007-4802

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Abstract

The lower part of the St. George Group of western Newfoundland consists of Tremadocian shallow marine platform carbonates of the Boat Harbour (about 180 m thick) and the underlying Watts Bight (about 60 m thick) formations. In the Boat Harbour Formation, dolomitization is pervasive at the top of most shallowing-upward, metre-scale, peritidal hemicycles. Petrographic examination of the Boat Harbour Formation carbonates suggests that the succession has been affected by at least three phases of dolomitization, which influenced the final rock porosity. These phases have crystal-size ranges of about 4 to 40 μm (earliest dolomite D1), 50 to 150 μm (D2), and 300 μm to 20mm (saddle dolomite D3), respectively. They occur as both replacements and cements and exhibit dull (D1 and D3) to zoned (D2) luminescence under the cold cathodoluminoscope. The occurrence of near-micritic size dolomites (about 4–40 μm) may suggest that dolomitization started at low temperatures during early stages of diagenesis. The lack of evaporite interbeds in the formation and the depleted δ18O values (–6.2±0.8‰ VPDB) as well as the low Sr contents (168±45ppm) of the earliest dolomites likely exclude a brine origin from evaporated seawater. The Sr/Ca molar ratios (0.0069 to 0.0017), calculated for the earliest dolomitizing fluid, suggest a mixture of marine and meteoric waters possibly in a mixing zone environment. The petrographic features and geochemical attributes of D2 and D3 phases, such as their depleted δ18O values (–6.9±1.5‰ and –8.3±0.9‰ VPDB, respectively) and Sr contents (177±76 and 117±33ppm, respectively), suggest that they were formed under relatively deeper burial conditions and possibly from hydrothermal fluids which is supported by homogenization temperatures (up to 135º C) and estimates of salinities (up to 23 wt. % NaCl) in the latest dolomites (D3). Based on visual estimates from thin sections, the porosity varies from <1% in most of the formation to about 10% in a dolomitized algal lime mudstone bed in the upper part of the formation, a few metres below the Boat Harbour Disconformity. Except for some vugs, the majority of pores are intercrystalline and associated with D2. The dolomitization events recorded by the Boat Harbour Formation carbonates resemble, in their succession and geochemical signatures, those previously documented in the Aguathuna Formation. However, significant differences are noted with the dolomitization events recorded in the Lower Ordovician Romaine Formation on Anticosti Island; these differences are tentatively associated with the very distinct tectonic settings of each case.

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