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Abstract:

The oxygen isotope compositions of diagenetic minerals from sandstones and conglomerates have been determined for a Permian to Upper Cretaceous section through the Alberta deep basin. These data have been used to reconstruct variations in the δ18O values of pore water during diagenesis of intercalated sandstones and shales. The results confirm earlier observations on the oxygen isotope evolution of pore water in the western Canada sedimentary basin. First, meteoric water was abundant during early diagenesis, even in sediments deposited in shallow marine environments. Second, the maximum δl8O values attained by pore waters during burial diagenesis were generally < + 3‰, much lower than in other shale-dominated basins (e.g., Gulf Coast). However, pore waters in sandstones located adjacent to, or intercalated with, carbonates and shales of the underlying Paleozoic section had substantially higher δ18O values (+ 7 to +9‰) at or near maximum burial. Third, late diagenesis was dominated by low-18O meteoric waters.

Saturation of the sedimentary section by meteoric water during early diagenetic processes probably resulted from infiltration during subaerial exposure associated with sea level fluctuations of the inland sea. Influx of meteoric water probably continued episodically throughout burial diagenesis. Contribution of meteoric water during early diagenesis is reflected in the low maximum porewater δ18O values that characterize the peak of burial diagenesis. These results also indicate that the smectite-to-illite reaction in Mesozoic shales did not dominate oxygen isotope evolution of sandstone pore waters in the western Canada sedimentary basin. Any increase in the δ18O value of pore water during burial probably resulted from variable mixing between early diagenetic, meteoric-dominated pore waters and 18O-rich pore waters that had equilibrated with the underlying Paleozoic carbonate platform. The δ18O values of late diagenetic minerals and present pore waters reflect both this process and the subsequent, substantial influx of surface-derived meteoric water during post-Laramide uplift and erosion.

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