δ18O values of authigenic minerals in shallow (< 1,500 m) Cretaceous rocks from Alberta suggest that ground water exerts an important control upon the diagenesis of sandstones. In Alberta, ground waters are significantly depleted in 18O relative to seawater.
Minerals precipitated in equilibrium with ground water should have δ18O values predictably lower than similar phases formed from more 18O-rich fluids. The δ18O values of 150 clay samples from the Milk River, Belly River, and Viking formations range from 6 to 20‰ (SMOW). An equally large variation in δ18O (+11 to +28‰, SMOW) and δ13C (− 10 to +2‰, PDB) is shown by over 125 carbonate samples. Detrital clays from the Milk River have δ18O values of +16 to +20‰ (SMOW). The δ18O (+24 to +28‰, SMOW) and δ13C (− 3 to +1‰, PDB) values of dolomite clasts are typical of platform carbonates. The much lower δ18O values of the authigenic kaolinite, smectite (+10 to +15‰, SMOW), and calcite (+15 to +19‰, SMOW) in the sandstone aquifer reflect neoformation from low-18O ground waters at temperatures as low as 5°C. Involvement of organically derived CO2 during calcite formation is indicated by low δ13C values (− 10 to − 3‰, PDB). In sandstone of the Belly River Formation, early porelining chlorite and later porefilling kaolinite and calcite have quite low average δ18O values of 6.3, 12.0, and 13.1‰ (SMOW) respectively. The kaolinite and calcite approach oxygen isotope equilibrium at a temperature of 55 ± 10°C with ground waters of about − 8 to − 10‰(SMOW). The chlorite is out of isotopic equilibrium and formed either at lower temperatures and(or) from more 18O-rich fluids. The δ18O values of < 2μm clays from the Viking Formation (18-20‰ SMOW) may reflea a detrital origin. Illite-smeaite, however, which is concentrated in the < 0.2μm fraaion, has lower δ18O values (+12 to 16‰), perhaps suggesting ground-water involvement in its genesis.
