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Abstract

In the Middle and Late Jurassic, the structural configuration of the area occupied by the San Juan basin was different from the present-day configuration, which developed in response to Laramide deformation in the latest Cretaceous to early Tertiary. An isopach map indicates a thickening of Middle to Upper Jurassic strata near the southern margin of the area. The thickening is attributed to downwarping, mainly in the Jurassic, which may have controlled depositional patterns and influenced the distribution of primary uranium ore deposits in the host rocks of the Jurassic Morrison Formation. Paleotectonic reconstruction indicates the existence of a magmatic arc immediately west of the Colorado Plateau in the Jurassic. Jurassic folding in the basin and magmatic activity in the arc have important implications bearing on the uranium geology of the area. The folding probably exerted some control on depositional trends in the host rocks which, in turn, influenced the distribution of uranium deposits in them. Volcanism in the magmatic arc supplied silicic ash that may be the source of some, if not all, of the uranium in the host rocks.

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