Precambrian rocks are exposed in cores of uplifts, and possibly early Precambrian granites were the source of uranium in Tertiary host rocks. Paleozoic rocks occur along flanks of Laramide uplifts and in low-angle thrusts. The only significant uranium deposits are low-grade, but extensive, in phosphatic rocks. Mesozoic rocks are thicker, and locally Cretaceous sandstones are major hosts for uranium. Deformation began in Cretaceous, and in early Eocene relief reached a maximum and thick beds of coarse conglomerate were deposited. Lakes formed in the basins and uraniferous phosphates occur in the lake deposits. Ash beds are abundant in the Oligocene and later Tertiary. Alternatives for formation of Eocene uranium deposits are considered; it is suggested that they may be an end product of a long complex sequence of events, with redistribution near the end of the Pliocene.

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