^dgr¹⁸O 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 ^dgr¹⁸O values predictably lower than similar phases formed from more ¹⁸O-rich fluids. The ^dgr¹⁸O values of 150 clay samples from the Milk River, Belly River, and Viking formations range from 6 to 20^pmil (SMOW). An equally large variation in ^dgr¹⁸O (+ 11 to +28^pmil, SMOW) and ^dgr¹³C (- 10 to +2^pmil, PDB) is shown by over 125 carbonate samples. Detrital clays from the Milk River have ^dgr¹⁸O values of +16 to +20^pmil (SMOW). The ^dgr¹⁸O (+ 24 to +28^pmil, SMOW) and ^dgr¹³C (- 3 to +1^pmil, PDB) values of dolomite clasts are typical of platform carbonates. The much lower ^dgr¹⁸O values of the authigenic kaolinite, sme tite (+ 10 to +15^pmil, SMOW), and calcite (+ 15 to +19^pmil, 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 ^dgr¹³C values (- 10 to -3^pmil, PDB). In sandstone of the Belly River Formation, early pore-lining chlorite and later pore-filling kaolinite and calcite have quite low average ^dgr¹⁸O values of 6.3, 12.0, and 13.1^pmil (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^pmil(SMOW). The chlorite is out of isotopic equilibrium and formed either at lower temperatures and(or) from more SUP>18O-rich fluids. The ^dgr¹⁸O values of <2µm clays from the Viking Formation (18-20^pmil SMOW) may reflect a detrital origin. Illite-smectite, however, which is concentrated in the < 0.2µm fraction, has lower ^dgr¹⁸O values (+ 12 to 16^pmil), perhaps suggesting ground-water involvement in its genesis.

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