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Isopleth mapping of the Westwater Canyon and Brushy Basin Members of the Upper Jurassic Morrison Formation in the southern San Juan basin, New Mexico, constructed from approximately 1,800 geophysical logs and 100 measured sections, show that structural elements controlled depositional patterns in these two members. These compilations, which include isopach, sandstone-mudstone ratio, percent sandstone, net sandstone, and average number of shale interbeds per 100 ft (30.5 m) of section, illustrate the geometry of depositional units, the distribution of sandstone depocenters, and large-scale facies variations within the units. Paleotopographic mapping of the base of the Westwater Canyon compiled during this study shows a series of highs and lows trending east-southeast. The estwater Canyon is thin and sand-poor over the paleotopographic highs and thick and sandy along the paleotopographic lows, suggesting active structural control of facies distribution during deposition of this unit. Sedimentation of the Brushy Basin also was affected by some of the same active structural elements. Basement faults, reactivated through time and defined by detailed reflection seismic studies conducted by other workers, apparently exerted a significant influence on depositional patterns in the Morrison Formation.
Depositional patterns appear to control the location of uranium deposits. Primary uranium ore in the Westwater Canyon Member is restricted to sandstone depocenters associated with east-southeast-trending isopach thicks and large sandstone-mudstone ratios. Redistributed ore deposits also are concentrated in the vicinity of isopach thicks but in rocks with relatively low sandstone-mudstone ratios. Their location, however, is much more closely related to the position of a regional oxidation-reduction interface whose three-dimensional configuration was influenced regionally and locally by Laramide structures. Remnant ore deposits are relict primary deposits that lie updip of the redox interface in oxidized ground. Sedimentologic controls are similar to those of primary ore. In general, th se remnant deposits have been preserved from oxidation by a unique stratigraphic or structural setting.
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