Abstract

Exceptional exposures of diagenetic alteration at Snow Canyon State Park, Utah are valuable analogs for characterizing the Navajo Sandstone and other subsurface reservoirs worldwide. Mapping of six diagenetic coloration facies/subfacies was completed at a 1:10,000 scale and covers a total area of ~60 km². Measured stratigraphic sections further define the spatial relationships between facies. A compiled taxonomy relates bleaching type to the duration of alteration and relative distance from the main bleached zone.

The various diagenetic coloration facies at Snow Canyon were produced along or near a spatially extensive chemical reaction front boundary (or “redox transition zone”). These facies document the complex lateral transition between red-colored “primary” rock in the southern park (enriched in iron oxide) and yellow to white colored “bleached” rock in the northern park (depleted in iron oxide). Subsequent precipitation of dense iron oxide produced brown to black coloration that “overprints” other facies in the lower Navajo Sandstone. Geospatial relationships indicate a complex diagenetic history with distinct episodes of bleaching and iron oxide precipitation influenced by local eolian stratification features. Paleo-fluid migration associated with bleaching was likely toward the south or southwest. Structural relationships tentatively suggest that bleaching preceded the development of Sevier-age (Late Cretaceous to early Tertiary) deformation features.

Spatial correlation of diagenetic facies involves both horizontal and vertical components. Horizontal migration of bleaching fluids within higher permeability eolian strata produces well-defined bands of white, bleached sandstone that extend for kilometers into unbleached red sandstone. A variety of bleaching geometries may be roughly indicative of relative distance from the main bleached zone. The order of succession for bleaching features in vertical section often mirrors lateral transitions observed in the field. For example, the upper bleached zone at Snow Canyon typically overlies an intermixed red/white zone with bleaching becoming less prevalent moving stratigraphically downward. Factors that may contribute to this pattern include changes in primary bedding fabric and the vertical superimposition of horizontally contiguous coloration facies (loosely akin to sedimentary facies relationships of Walther’s law). Repetition and complexity in the architecture of diagenetic facies along the reaction front boundary result from multifaceted interaction between sedimentary, structural, and diagenetic processes.