IMPLICATIONS OF IRON OXIDE CEMENTATION HORIZONS IN THE JURASSIC NAVAJO SANDSTONE, SOUTHWESTERN UTAH

Dense iron oxide cements are concentrated along discontinuous horizons in the lower part of the Navajo Sandstone in southwestern Utah. Spatial analysis indicates that these horizons roughly parallel the top of the formation and laterally correlate over large distances (up to tens of kilometers). Multiple generations of iron oxide cement provide evidence of a geochemical reaction front. Precipitation occurred on a broad scale and may be related to regional tectonic controls.

Dense iron oxide cement concentration (up to 30%) results in a dark sandstone color. Primary ferruginous sandstones have a reddish color produced by thin hematite grain coatings. In contrast, darker secondary ferruginous sandstones have variable mixtures of hematite and goethite cement. Geochemical modeling indicates that these secondary cements may have precipitated from the interaction of deep formation fluids with relatively shallow meteoric water. Spatial relationships suggest that original red sandstone grain coatings removed from the now white-colored (“bleached”) areas in the upper part of the Navajo Sandstone were the most likely iron source.

Liesegang-type bands are commonly associated with iron oxide horizons. These bands are generally oriented parallel to the main reaction front, suggesting that reactants were locally transported by vertical diffusion. Estimated diffusion rates are very low when applied over large distances. This indicates that advective transport processes were also necessary to replenish the large quantities of iron and oxygen consumed along the reaction front. Stratigraphic and structural relationships suggest that diagenetic precipitation of iron oxide cements might have occurred at substantial depths (300+ m). Mechanisms for introducing oxygen into the deep subsurface may include the development of large fracture zones in the Navajo Sandstone and/or uplift/unroofing of the formation.

Measured permeability values indicate that cemented horizons act as high impedance barriers to vertical fluid transmission. Interaction between these iron oxide horizons and other diagenetic, depositional, and structural barriers can locally segment fluid flow. Thus, characterization of Navajo Sandstone iron oxide horizons has significant implications for reservoir evaluation and for diagenesis in similar porous formations.