William M. Aubrey, 1986. "The Nature of the Dakota-Morrison Boundary, Southeastern San Juan Basin", A Basin Analysis Case Study: The Morrison Formation, Grants Uranium Region, New Mexico, Christine E. Turner-Peterson, Elmer S. Santos, Neil S. Fishman
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A thin, discontinuous, fluvial, locally conglomeratic sandstone at the base of the Dakota Sandstone in the vicinity of the southeastern San Juan basin, New Mexico has been named the Encinal Canyon Member of the Dakota Sandstone. In the past, the sandstone beds, placed here in the Encinal Canyon, have been included in the Jackpile sandstone, (an economic unit in the Morrison Formation), in the Burro Canyon Formation, or in the Oak Canyon Member of the Dakota Sandstone. Distinction between the Encinal Canyon Member and the Jackpile sandstone, which are separated by an unconformity that probably spans most of the Early Cretaceous, is economically important. The Jackpile is a primary uranium exploration target, whereas the Dakota contains little known uranium.
In the past, the sub-Dakota erosional surface in the southeastern San Juan basin generally was thought to be at the base of the marine and paralic Oak Canyon Member of the Dakota Sandstone, which overlies the Encinal Canyon Member. The unconformity is shown here to be at the base of the fluvial rocks of the Encinal Canyon Member.
Local relief at the base of the Encinal Canyon indicates that the sub-Dakota erosional surface formed during a time of regional degradation. Easterly flowing streams scoured underlying units and in some places cut completely through the Jackpile sandstone and the Burro Canyon Formation. The Encinal Canyon was deposited in response to the initial transgression of the Dakota sea. As the sea inundated the area, a transgressive erosional surface formed, and the overlying paralic and marine sediments of the Oak Canyon Member were deposited.
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A Basin Analysis Case Study: The Morrison Formation, Grants Uranium Region, New Mexico
This volume summarizes results of a U.S. Geological Survey multidisciplinary basin analysis research effort that encompasses all aspects of the geology of the Morrison Formation in the Grants uranium region, located in the San Juan basin of northwestern New Mexico, U.S.A. Tectonic, stratigraphic, sedimentologic, structural, petrographic, mineralogic, geochemical, and resource studies are drawn together to provide a geologic synthesis of the Jurassic Morrison Formation, the main uranium host rock in the region, and to provide background data for the formulation of genetic models for ore genesis. The result is a compendium of 21 papers that incorporates many recent and significant advances in our understanding of factors that favored uranium mineralization in the Morrison, and several new genetic models that incorporate these recent advances. The basin analysis approach used here has proved fruitful in that ore genesis can now be viewed in the context of the evolution of a sedimentary basin rather than as an isolated event. This approach to ore genesis has application not only to uranium deposits but also to all sediment-hosted ore deposits.