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The north flank of the Datil Mountains is underlain by gently south-southwest dipping strata of the Late Cretaceous Crevasse Canyon Formation, which is unconformably overlain by the Eocene Baca Formation. This erosional unconformity is well exposed where the upper 30 to 60 m of the Crevasse Canyon forms a 15-km-long belt of mesas, which probably represents an east-trending, fine-grained, meander-belt complex.
Numerous uranium anomalies and several small orebodies (Red Basin area) generally form a coaxial uranium belt within (and below) a 20 to 30-m-thick, intensely bleached zone at the top of the Crevasse Canyon. In drill cuttings and canyon walls, the light-gray sandstones and light-purplish-gray shales of the bleached zone grade downward into the dark-gray sandstones and carbonaceous shales typical of the Crevasse Canyon Formation. Tabular uranium deposits commonly occur in bleached channel sands where they are in contact with carbonaceous shales. Thin laminae of black hematite are common along the mineralized contacts. Carbonized wood and pyrite concretions, common in the lower Crevasse Canyon, occur as silicified logs and limonite concretions in the bleached zone.
The basal 0 to 10 m of the Baca Formation is usually a light-brownish-gray conglomeratic sandstone containing abundant hematite flakes, broken limonite concretions, and bits of silicified wood, suggesting that bleaching (and mineralization?) may have predated Baca deposition.
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