Diagenetic Coloration Patterns in the Jurassic Navajo Sandstone of Zion National Park, Utah

Nielsen, Gregory B. and Marjorie Chan
University of Utah, Salt Lake City, UT

     Coloration patterns in the Jurassic Navajo Sandstone of Zion National Park are examined using geospatial, petrographic and geochemical analysis. In the northern Kolob Plateau, the Navajo Sandstone is dominantly red colored with a uniform pigmentation resulting from thin, iron oxide grain coatings. These grain coatings formed during early diagenesis to produce the “primary” sandstone color.
     In contrast, Navajo Sandstone of the main Zion Canyon was previously divided into three informal subunits: white (upper), pink (middle), and brown (lower). Analysis of these subunits indicates a more complex diagenetic history than previous interpretations suggested, with multiple episodes of iron oxide depletion (bleaching) and enrichment (cement precipitation). The white and pink subunits are characterized by a combination of prevalent bleaching, areas of remnant primary sandstone, small concretionary lenses and brightly colored, secondary mineralization. The brown subunit represents a separate episode of iron oxide enrichment and is characterized by widespread dark spotty cement concentrations. The upper contact of this brown subunit is regionally extensive as a subhorizontal surface. In Zion Canyon, a narrow zone (~10 m) of concentrated bleaching occurs immediately above the brown subunit.
     Widespread bleaching in the upper Navajo Sandstone ends abruptly in the Kolob Plateau, but narrow, well-defined bleaching bands locally follow high-permeability units in the lower Navajo Sandstone throughout the central park and into the Kolob Plateau. These narrow bands indicate horizontal movement of fluids. Multiple episodes of subsurface fluid movement on both regional and local scales contributed to the distinctive coloration variations of Zion National Park.

AAPG Search and Discovery Article #90071 © 2007 AAPG Rocky Mountain Meeting, Snowbird, Utah