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AAPG Bulletin, V.
Dolomites of the Boat Harbour Formation in the Northern Peninsula, western Newfoundland, Canada: Implications for dolomitization history and porosity control
1Department of Earth Sciences, Memorial University of Newfoundland, St. John's, Newfoundland, Canada, A1B 3X5; [email protected]
2Department of Earth Sciences, Memorial University of Newfoundland, St. John's, Newfoundland, Canada, A1B 3X5; [email protected]
3Department of Earth Sciences, Brock University, Ontario, Canada, L2S 3A1; [email protected]
The Boat Harbour Formation constitutes the upper part of the Lower Ordovician St. George Group on the Northern Peninsula, western Newfoundland. It ranges in thickness from 140 m (459 ft) at Main Brook to 96 m (315 ft) at Daniel's Harbour. Dolomitization of the carbonate sequence is more pervasive in the lower 30 to 40 m (98 to 131 ft) at Main Brook, whereas at Daniel's Harbour, the section is entirely dolomitized.
Petrography suggests that the Boat Harbour Formation has been affected by three phases of dolomitization. The earliest (near surface or synsedimentary) phase is D1 dolomicrite (4–55 m), which exhibits dull to no luminescence. It commonly occurs as laminae-capping cycles and as breccias in the younger dolomite phases. It has low Sr (228 30 ppm) and an average 18O value of 6.0 0.8 (Vienna Peedee belemnite [VPDB]) in the Main Brook section but more depleted signatures for 18O of 8.8 1 (VPDB) and lower Sr contents (45 8 ppm) in the Daniel's Harbour section. The geochemical composition suggests that D1 was developed from fluids of a mixture of meteoric and marine waters.
The midburial phase D2 dolomite consists of coarse planar subeuhedral crystals (30–400 m) that show concentric cathodoluminescence zoning and are also crosscut by microstylolites. Its 18O values range between 6.6 1.3 (VPDB) at Main Brook and 9.0 0.5 (VPDB) at Daniel's Harbour. This dolomite likely precipitated from fluids that circulated through crustal rocks with progressive burial (Th value of 114C 11C and salinity value of 23 1.8 eq. wt. % NaCl).
The late-stage D3 dolomite has large and coarse nonplanar crystals (125 m–7 mm) that exhibit sweeping extinction under crossed polars, which is characteristic of saddle dolomite and also sometimes shows thin brightly luminescent rims. It was likely precipitated during deeper burial in pulses and from hot fluids (Th values of 148C 19C and 115C 19.6C and mean salinities of 23 2 and 22 2 eq. wt. % NaCl at Main Brook and Daniel's Harbour, respectively). This is also supported by their relatively more depleted 18O (11.1 1.2 and 12.3 1.4 VPDB, respectively) and low Sr contents (88 36 and 38 5.9 ppm, respectively).
Porosity in the Boat Harbour Formation is mainly associated with the midburial D2 dolomite. Intercrystalline porosity is the dominant type, and it ranges in the formation from less than 1% to 8% at Main Brook and from 7% to 12% at Daniel's Harbour. Vugs are less common but are associated with D3 dolomite. The porous zone in the formation at Main Brook starts approximately 10 to 15 m (33 to 49 ft) below the lower Boat Harbour disconformity and extends down to the lower formational boundary, whereas porous zones in the formation at Daniel's Harbour are indiscriminately distributed throughout the section.
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