| Description |
The effects of iron chemistry dominate the visual landscape of southwestern Utah, producing not only the well-known red rock scenery, but also a broad variety of other colorful features. Diagenetic coloration is perhaps best displayed in the magnificent cliffs of the Jurassic Navajo Sandstone, where selective chemical alteration by subsurface fluids has enriched the rock with iron in some areas (iron oxide cement precipitation), and locally depleted the rock of iron in others (chemical bleaching). This study examines the complex interrelationships between these coloration facies in a regional context in order to better understand the large-scale stratigraphic, hydrodynamic, and tectonic mechanisms that produced them. Primary goals of the study are to map diagenetic coloration facies, analyze geospatial relationships, establish the timing of major diagenetic events, and evaluate the impact of iron oxide cementation on reservoir architecture. Results indicate that the Navajo Sandstone has a prolonged and complex diagenetic history. At least four major events have occurred on a regional scale: (1) precipitation of primary grain coats to produce primary red rock; (2) bleaching of the upper Navajo Sandstone; (3) precipitation of dense concentrations of iron oxide (up to 30% by weight) in the lower formation; and (4) brightly colored secondary cementation along joints in the upper formation. Hierarchal taxonomies are presented for classifying and interpreting the major types of diagenetic features that are present. Sandstone bleaching patterns are influenced by multiple factors including variations in eolian bedform morphology, higher percent grainflow stratification in the upper formation, and the localized superimposition of horizontally contiguous bleached facies (loosely akin to sedimentary facies relationships of Walther's law). Dense concentrations of concretionary iron oxide cement occur primarily as discontinuous subhorizontal horizons in the lower portion of the formation. The wide occurrence of these horizons in southwestern Utah may indicate a relationship with regional tectonic processes, with cementation resulting from fluctuating geochemical conditions. Iron oxide precipitation locally follows previously high-permeability bedding features that, upon cementation, become barriers to subsequent fluid flow ("permeability inversion"). These relationships have application for both characterizing reservoir architecture and understanding the diagenetic history of the region. |
