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The AAPG/Datapages Combined Publications Database

Utah Geological Association

Abstract


Geology and Geologic Resources and Issues of Western Utah, 2009
Pages 67-96

Diagenetic Coloration Facies and Alteration History of the Jurassic Navajo Sandstone, Zion National Park and Vicinity, Southwestern Utah

Gregory B. Nielsen, Marjorie A. Chan, Erich U. Petersen

Abstract

Coloration patterns in the Jurassic Navajo Sandstone of Zion National Park and vicinity are examined using a broad variety of geochemical, geospatial, petrographic, and bedform analysis techniques. Six diagenetic coloration facies (including 12 subfacies) are defined and characterized. Results indicate a prolonged and complex diagenetic history with variations in color resulting largely from changes in the concentration and distribution of iron oxides. In the northern Kolob Plateau, the Navajo Sandstone has a uniform red pigmentation (red primary facies) that formed during early diagenesis to produce the “primary” sandstone color. In contrast, Navajo Sandstone of main Zion Canyon displays “secondary” alteration features occurring in three distinct vertical coloration facies: brown (lower), pink (middle), and white (upper).

The white and pink facies in Zion Canyon are characterized by a combination of prevalent bleaching, areas of remnant “primary” sandstone, and small concretionary iron-enriched lenses. Bleaching is concentrated in the upper Navajo Sandstone where alteration occurred during middle diagenesis (deep burial). Widespread bleaching and alteration in Zion Canyon terminates abruptly in the central park but narrow, well-defined, white bleached bands locally follow high-permeability beds northward for several kilometers into the red-colored Kolob Plateau (red/white facies). The brown facies is characterized by widespread dark iron oxide cement concentrations precipitated beneath a well-defined subhorizontal boundary. Isolated lenses of dense ironstone in overlying facies represent possible ancient conduits for transporting mobilized iron. Later episodes of limited iron oxide precipitation produced brightly colored cementation (multicolored facies) along some joints in the upper part of the formation.

The spatial distribution of alteration features at Zion National Park is related to permeability variations corresponding to stratigraphic shifts in eolian bedform morphology. Moving stratigraphically upward through the Navajo Sandstone, bedform changes here include increased foreset dip (from 18 to 22 degrees), a clockwise rotation of mean transport direction (from S to SW) and higher percent grainflow strata (from 17 to 25%). Increased prevalence of grainflow strata in the upper part of the formation likely facilitated bleaching fluids in this zone. These examples show the importance of primary textures in controlling fluid flow within an eolian reservoir system.


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