MAJOR DIAGENETIC FEATURES ASSOCIATED WITH IRON OXIDE ENRICHMENT AND DEPLETION, JURASSIC NAVAJO SANDSTONE, SOUTHWESTERN UTAH

The diverse range of colors and precipitation or alteration features in the Jurassic Navajo Sandstone of Snow Canyon, Red Cliffs Desert Reserve, and related areas of southwestern Utah are largely associated with two major diagenetic processes: iron oxide enrichment and depletion. A taxonomy presented here classifies major diagenetic features within each of these two process categories.

Iron enrichment features are classified based upon their scale, geometry, and degree of mineralization. Initial mineralization is commonly diffuse with discoloration and small-scale (mm-cm+) oxidation spots, mineralization rings and Liesegang-type bands. Continued or subsequent mineralization with favorable nucleation conditions results in denser cemented features such as iron oxide concretions and mineralization bands. With sustained precipitation, amalgamation of small-scale features forms medium-scale (m+) sheets that act as barriers to subsequent fluid flow. In southwestern Utah, many of these individual small to medium-scale features occur in association with a regionally extensive iron precipitation front.

Iron depletion (or “bleaching”) features are classified based upon their scale, geometry, and spatial orientation. The migration paths of reducing fluids within eolian sandstone reflect subtle permeability variations within and between bedsets. A variety of alteration features result based on the chemical nature of the fluid, the direction of fluid migration (relative to depositional surfaces), and spatial position within a reservoir system. Where bleaching occurs on a regional scale, the greatest diversity in alteration features typically occurs along the margins of the main bleached zone where depletion produces bleached rock with highly varied geometries ranging from small-scale pinstriping, mottling, and reduction spots, to medium-scale patches, lobes, and fingers. Further out from the reducing infiltration zone, depletion is typically localized and elongate fingers of bleaching may follow high-permeability bedsets for several kilometers beyond the main bleached zone.

Analysis of these iron oxide enrichment and depletion features provides a framework for reconstructing ancient fluid migration pathways as well as a foundation for characterizing modern reservoirs.