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