Disconformities in Grants Mineral Belt and Their Relation to Uranium Occurrence: ABSTRACT

Abstract

At least two major, regionally extensive, intraformational disconformities and numerous local ones are present in sedimentary rocks of Triassic, Jurassic, and Cretaceous ages in the Grants mineral belt. These disconformities, some of which have been known for many years, have proven useful in stratigraphic correlation and in differentiating genetically related rock sequences in the mineral belt. In addition, two of these disconformities, one local and the other regional, are significantly associated with the distribution of sandstone-type uranium deposits in the Jurassic Morrison Formation and the overlying Cretaceous Dakota Sandstone. The Westwater Canyon Member and laterally equivalent beds of the Recapture and Brushy Basin Members of the Morrison Formation are separated from underlying Jurassic rocks by an intraformational disconformity which marks a major change in depositional environment from predominantly sabkha-eolian dune to fluvial-lacustrine within the Jurassic sequence. This disconformity is economically significant in that all of the large uranium deposits in rocks of Jurassic age in the mineral belt occur within fluvial sandstone facies of the Morrison above the disconformity. Recognition of this disconformity in outcrop and in the subsurface is important in delineation of exploration target areas and resource assessment in the San Juan basin. Uranium deposits in the basal part of the Dakota Sandstone are associated with the well-known regional, southward-beveling disconformity present at the lower boundary of the Dakota throughout the San Juan basin and adjacent region. In the western part of the mineral belt, impermeable Brushy Basin shales have been truncated at this disconformity and uranium-bearing groundwaters from the Morrison Formation have migrated into basal organic-rich sandstone of the Dakota to produce several small to medium-sized uranium deposits. End_of_Article - Last_Page 689------------