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 mu 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 delta O-18 value of -6.0 parts per thousand +/- 0.8 parts per thousand (Vienna Peedee belemnite [VPDB]) in the Main Brook section but more depleted signatures for delta O-18 of -8.8 parts per thousand +/- 1 parts per thousand (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 (30-100 mu m) that show concentric cathodoluminescence zoning and are also crosscut by microstylolites. Its delta O-18 values range between -6.6 parts per thousand +/- 1.3 parts per thousand (VPDB) at Main Brook and -9.0 parts per thousand +/- 0.5 parts per thousand (VPDB) at Daniel's Harbour. This dolomite likely precipitated from fluids that circulated through crustal rocks with progressive burial (T-h value of 114 degrees C +/- 11 degrees C and salinity value of 23 +/- 1.8 eq. wt. % NaCl). The late-stage D3 dolomite has large and coarse nonplanar crystals (125 mu 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 (T-h values of 148 degrees C +/- 19 degrees C and 115 degrees C +/- 19.6 degrees C 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 delta O-18 (-11.1 parts per thousand +/- 1.2 parts per thousand and -12.3 parts per thousand +/- 1.4 parts per thousand 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.