COLORFUL DIAGENETIC FACIES AND FLUID-RELATED ALTERATION FEATURES OF THE JURASSIC NAVAJO SANDSTONE, SNOW CANYON STATE PARK, UTAH

Six diagenetic facies representing four major alteration events were defined, mapped, and characterized in the Jurassic Navajo Sandstone of Snow Canyon State Park, Utah using geochemical, geospatial and petrographic techniques. The first event was formation of iron oxide grain coatings during early compaction producing an even, pervasive pigmentation termed the primary red facies. The second event was large-scale bleaching by paleofluids that reduced and removed up to 65% of the iron oxide to produce both white and marginal yellow/white facies. An intermixed zone of partially bleached, and locally iron-enriched sandstone named the red/white transition facies occurs in the southern part of the field area, and suggests that fluids infiltrated for at least several kilometers past the main reaction front. The third event involved precipitation of dense secondary iron oxides within a discrete reaction zone to produce the brown/black ironstone facies. Associated local features include dense iron-cemented sandstone sheets, caps, bands and concretions. Iron oxide mineralization commonly followed previously high-permeability bedding to subsequently become a low-permeability barrier upon cementation. The upper boundary of this facies locally overprints bleaching, indicating that it is the more recent event. Cross-cutting relationships with N-NW trending joints suggest iron precipitation predates regional Basin and Range extension. The fourth event involved fluids permeating through joints to locally produce large patches of bright red and yellow mineralization in the upper Navajo Sandstone called the tan upper facies.

Geospatial relationships in Snow Canyon indicate a complex diagenetic history with distinct episodes of bleaching and iron precipitation controlled by local bedding and tectonic features. Well exposed diagenetic features at Snow Canyon serve as valuable analogs for characterizing the Navajo Sandstone and other subsurface reservoirs worldwide.