INTRODUCTION The effects of iron chemistry dominate the visual landscape of southwest Utah, producing not only the well- known red-rock scenery, but also a variety of distinctive cementation features such as iron-rich spheroidal concre- tions. These features are particularly abundant in the St. George area of Utah (igure 1), where iron oxide cements (including iron oxyhydroxides) concentrate along spatially discontinuous horizons in the lower part of the Jurassic Navajo Sandstone (igure 2). Willis and Higgins (1996) proposed that these dense iron oxides may indicate an an- cient water table, but the speciics of this potential rela- tionship remain to be explored. Iron-rich horizons have important implications for evaluating regional controls on iron oxide cementation as well as for interpreting modern luid low pathways in clastic aquifer and reservoir units. This study examines these iron-rich horizons to: 1) char- acterize the nature and extent of iron oxide cementation, 2) investigate the mechanisms that produced cementation, and 3) evaluate the impact of concentrated cementation on reservoir properties. Iron oxide cementation in the Navajo Sandstone is dominated by hematite (α-Fe 2 O 3 ) and goethite (α-FeOOH), both of which are thermodynamically stable reaction se- ries end-members under near-surface, oxidizing condi- tions (Cornell and Schwertmann, 2003). These sandstones display colors in the longer wavelengths of the visible spectrum (red, yellow, and brown) with variations in hue produced by multiple factors including iron oxide concen- tration, crystal size and distribution, and isomorphic substi- tution of other metals into the lattice structure (Schwertman and Cornell, 2000). Iron oxide cements may concentrate locally to produce visible features such as millimeter-scale spots, centimeter-scale concretions, and meter-scale pipes and sheets (for detailed discussions of iron oxide cementa- tion features in the Navajo Sandstone see Chan and others, 2000, 2004; Loope and others, 2010, 2011; Parry, 2011; Potter and Chan, 2011; Potter and others, 2011). Previous studies have described a diverse set of coloration facies IRON-RICH HORIZONS IN THE JURASSIC NAVAJO SANDSTONE, SOUTHWESTERN UTAH: PROGRESSIVE CEMENTATION AND PERMEABILITY INVERSION ABSTRACT Dense concentrations of iron oxide cement (up to 30% by weight) form discontinuous subhorizontal horizons that mark the top of a regionally extensive (100’s of km 2 ) iron enrichment zone in the Jurassic Navajo Sandstone of southwestern Utah. These horizons transect primary bedding fabric and their trend surfaces parallel the top of the formation. Cross-cutting relationships and apparent tilting of the horizons along the St. George syncline sug- gest that they formed during the Late Cretaceous and Paleocene. Horizons developed at substantial depths along laterally extensive oxidation-reduction fronts, with iron likely sourced from bleached sandstone in the upper part of the formation. Iron cementation was likely triggered by oxygenation related to a relative drop in regional paleo- groundwater levels. A hierarchal taxonomy is presented for classifying iron-enriched sandstones based upon the concentration and distribution of iron oxide cement. The permeability of these sandstones is highly variable due to heterogeneities in bedding fabric and differences in cement type and distribution. Average values of about 900 millidarcies are highest for red-colored sandstones with thin iron oxide grain coats and lowest, less than 10 millidarcies, for dark-colored ironstones with dense, pore-illing iron oxide cement. Remnant diagenetic features (cementation bands, spheroidal concretions, etc.) in ironstones are indicative of progressive cementation under favorable conditions. Iron oxide ini- tially concentrates within high-permeability strata (e.g., eolian grainlow deposits) that, upon cementation, become barriers to subsequent luid movement, thus creating zones of “permeability inversion.” Thus, characterization of iron oxide horizons has signiicant implications for reservoir evaluation in this and similar sandstone formations affected by mobilized and reprecipitated iron oxide minerals. by Gregory B. Nielsen 1 , Marjorie A. Chan 2 , and Brenda Beitler Bowen 3 1 Weber State University, Ogden, UT; gnielsen@weber.edu 2 Department of Geology and Geophysics, University of Utah, Salt Lake City, UT; marjorie.chan@utah.edu 3 Department of Geology and Geophysics, University of Utah, Salt Lake City, UT; brenda.bowen@utah.edu Nielson, G.B., Chan, M.A., and Beitler Bowen, B., 2014, Iron-rich horizons in the Jurassic Navajo Sandstone, southwestern Utah—progressive cementation and permeability inversion, in MacLean, J.S., Biek, R.F., and Huntoon, J.E., editors, Geology of Utah’s Far South: Utah Geological Association Publication 43, p. 215–236.