The oxygen isotope compositions and paragenetic sequence of diagenetic minerals from the Upper Cretaceous basal Belly River sandstone, southwestern Alberta, have been used to identify changes in pore-water composition during diagenesis. These changes can be related to major geologic events within the western Canada sedimentary basin. Low- 18 O meteoric water has played an important role in the diagenesis of these Upper Cretaceous rocks on a basinwide scale during deposition and early diagenesis and following uplift of the basin in early Eocene time. Diagenetic phases include early chlorite (delta 18 O = +6.1 to +8.3 per thousand ) and calcite (delta 18 O SMOW = +21.0 per thousand ; delta 13 C PDB = -1.6 per thousand ), followed by quartz overgrowths (delta 18 O = + 13. 1 to + 18.0 per thousand ), calcite (delta 18 O SMOW = + 10.4 to + 14.4 per thousand ; delta 13 C PDB = - 10.6 to - 1.2 per thousand ), and then kaolinite (delta 18 O = +7.8 to + 11.9 per thousand ), illite/smectite (delta 18 O = +10.9 to +13.1 per thousand ) and smectite (delta 18 O = +12.6 to +13.7 per thousand ). The interpretation of these results is that shallow diagenesis, early in the burial history (chlorite, calcite) occurred in the presence of brackish or fresh water. With burial, the delta 18 O value of the pore water increased due to water/rock interaction. Burial diagenesis was terminated in the early Eocene by uplift (Laramide Orogeny). Recharge of the basin by low- 18 O meteoric water occurred at this time. The meteoric water then became involved in the formation of diagenetic quartz and calcite at or near maximum burial temperatures, and in the crystallization at lower temperatures of kaolinite, illite/smectite, and smectite as post-Eocene cooling and erosion progressed.