The oxygen-isotope compositions of authigenic clay and carbonate minerals from the Pembina I-Pool (basal Belly River sandstone) provide useful constraints on the diagenesis of this unit. Early diagenetic chlorite has delta 18 O values (+ 4.2 to + 6.3, SMOW) compatible with low-temperature crystallization from brackish water. The delta 18 O values of diagenetic calcite (+ 11.8 to + 14.7, SMOW) and kaolinite (+ 10.4 to + 12.8, SMOW) are best explained by crystallization at temperatures of at least 45 degrees to 70 degrees C from pore fluids similar in delta 18 O to the existing formation water (-9, SMOW). The implication of such temperatures is that the sandstone was much more deeply buried in the past than it is at present. At least 1,300-1,400 m of overlying material may have been eroded since maximum burial.