HYDROGEN-ISOTOPE GEOCHEMISTRY OF DIAGENETIC CLAY-MINERALS FROM CRETACEOUS SANDSTONES, ALBERTA, CANADA - EVIDENCE FOR EXCHANGE

The δD and δ18O values of diagenetic kaolinite and 10 Å clay minerals (illite, illite/smectite) have been determined for Lower Cretaceous Viking Formation marine sandstones and conglomerates, Upper Cretaceous basal Belly River Group shoreline and deltaic sandstones, and overlying Belly River/Brazeau Groups continental sandstones from the western Canada sedimentary basin. Early and late diagenetic kaolinites have a remarkably small range of δD values (-132 to - 112%, Viking Formation; -137 to-128%, Belly River/Brazeau Groups) relative to their δD18O compositions (+13.8 to +26.9%, Viking Formation; +5.9 to +12.7%, Belly River/Brazeau Groups). The δD values are also much lower than commonly reported for kaolinite. Calculations indicate that the diagenetic kaolinite from all units is in, or close to, hydrogen-isotopic equilibrium with present formation waters at present subsurface temperatures. Within the limits of available fractionation data, similar observations can be made for diagenetic 10 Å clay minerals from these units (δD = -132 to -107%, Viking Formation; -138 to -123%, Belly River/Brazeau Groups). By comparison, a general state of oxygen-isotope equilibrium does not exist between diagenetic kaolinite and present formation waters. Kaolinite has retained oxygen-isotope compositions characteristic of neoformation during earlier stages of diagenesis. Some diagenetic 10 Å clay minerals (e.g. Belly River/Brazeau Groups) may be in oxygen-isotope equilibrium with present formation waters because of neoformation from these waters (in sandstones), or oxygen-isotopic exchange during the smectite to illite transformation (in shales). The results of this study suggest that hydrogen-isotope re-equilibration between diagenetic kaolinite and formation water in sandstones can occur independent of oxygen-isotope exchange at temperatures as low as ≈40°C. Interpretation of hydrogen- and oxygen-isotope compositions for clay minerals from such systems (i.e. high water/rock ratio, low temperatures) should therefore be decoupled. © 1990.