Abstract

Qualitative and numerical simulation of regional groundwater flow in the Colorado Plateau during Late Jurassic and Early Cretaceous time has led to a model of uranium deposition in areas of upwelling solutions. Paleogeographic reconstruction shows that surface drainage and therefore groundwater flow was generally toward the northeast and east. Croundwater flowing in these directions, principally through Triassic-Jurassic and Permian sandstones, encountered sediments of variable thickness due to buried uplifted or downdropped Precambrian blocks. The buried uplifted blocks caused upward movement of groundwater around them. These inferred zones of upwelling are closely associated with concentrations of Jurassic- and Cretaceous-age uranium deposits. The results are consistent with hypotheses of an upwelling brine mixing and reacting with descending meteoric water and causing uranium precipitation at the fluid interface. Whether the uranium came from above or below the interface is an unsolved problem, but this model helps to identify better the sources of the waters and also suggests target areas for exploration.

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