ABSTRACT

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-¹⁸O 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 (¹⁸O = +6.1 to +8.3) and calcite (¹⁸O_SMOW = +21.0; ¹³C_PDB = -1.6), followed by quartz overgrowths (¹⁸O = + 13. 1 to + 18.0), calcite (¹⁸O_SMOW = + 10.4 to + 14.4; ¹³C_PDB = - 10.6 to - 1.2), and then kaolinite (¹⁸O = +7.8 to + 11.9), illite/smectite (¹⁸O = +10.9 to +13.1) and smectite (¹⁸O = +12.6 to +13.7). 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 ¹⁸O 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-¹⁸O 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.