A ferruginous oolite bed, intercalated among marine shales of Late Cretaceous age, underlies a large area in the Peace River district of northwestern Alberta. Samples of the oolite bed are composed of chlorite-like and limonitic oolites, silty rock fragments, and cryptocrystalline mudstone aggregates, cemented by a green, isotropic substance and authigenic carbonates. X-ray diffraction powder patterns show that the rocks contain quartz, goethite, and siderite, and suggest that the constituents of the mudstone fragments are poorly crystallized clay minerals of uncertain composition. The green, isotropic cement, present both in the oolites and as an intergranular filling, appears to be a ferruginous, gel-like substance amorphous to X-rays. The composition of the oolites and rock fragments, which become progressively more limonitic towards the top of the Fe-rich bed, shows that oolites were derived from outside of the depositional area, the primary ferrous constituents of the grains having been oxidized prior to deposition. Portions of this partially oxidized clastic framework not impregnated by mudstone matrix were then cemented by a ferruginous gel, chalcedony, and siderite in that sequence of deposition.