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The Kanab Wonderstone is sandstone (Shinarump Member, Chinle Formation) that is cemented and stained with iron oxide. The iron-oxide cementation and staining in these rocks have been considered examples of the Liesegang phenomenon, but we will show that they comprise a microbially induced structure. The spacing of bands of iron-oxide stain follow the Jablczynski spacing law (wherein the spacing between bands of iron-oxide stain increases as one traverses a series of bands) characteristic of Liesegang. Bands of iron-oxide cement exhibit more variable spacing and exhibit a weak but significant correlation between band thickness and distance between bands of cement. The pore-filling cement contains morphotypes that are similar in size and habit to those exhibited by microaerophilic iron-oxidizing bacteria. Other disseminated iron-oxide mineralization occurs as rhombohedra interpreted to be pseudomorphs after siderite. We interpret the cement to be produced by microbially mediated oxidation of siderite (a typical early diagenetic mineral in fluvial sandstones). Iron-oxidizing bacteria colonized the redox interface between siderite-cemented sand and porous sandstone. Microbes oxidized aqueous Fe(II), generating acid that caused siderite dissolution. The iron-oxide cement is the microbial product of a geochemical drive for organization; whereas the iron-oxide stain is true Liesegang. Together, they comprise a distinctive microbially induced structure with high preservation potential. Key Words: Biosignatures-Iron oxides-Diagenesis-Iron-oxidizing bacteria-Shinarump. Astrobiology 15, 616-636. [ABSTRACT FROM AUTHOR]

Concretions with thick, iron oxide-cemented rinds and lightly cemented, iron-poor sandstone cores are abundant within the Navajo Sandstone near the southeast flank of the Escalante anticline (Utah, United States). Previous workers suggested the spheroidal concretions as analogs for Martian "blueberries" (hematite concretions), and linked the origin of concretions and bleaching of the Navajo Sandstone to the buoyant rise of hydrocarbons toward anticlinal crests. We measured azimuths of 163 pipe-like (pipy), joint-associated concretions and those of 58 "comet tails" that extend from some spherical concretions. Both data sets have vector magnitudes >90%, and subparallel, southeast-directed resultant vectors. Our data indicate that groundwater transported and deposited iron over a flow path that spanned more than 100 km from the anticlinal crest to the Colorado River. Buoyant fluids could not have transported iron from bleached rock to sites of concretion growth, however, because iron moved downdip. We propose that iron was transported southeastward by reducing groundwater that flowed through a CO2- and methane-charged reservoir within the Escalante anticline. Iron-rich carbonates precipitated during CO2 degassing. After methane was flushed from the aquifer, microaerophiles oxidized Fe2+ in concretions, dissolving the carbonates and forming the iron oxide-rich rinds and iron-poor cores. These concretions define the flow direction and geochemical evolution of a paleoaquifer. This approach could be used to follow the water on other planets. [ABSTRACT FROM AUTHOR]

The discovery of spherical accumulations of iron oxide mineralization on Mars has prompted renewed interest in widespread but enigmatic accumulations of iron oxide on Earth. The rinded iron oxide concretions found in the Navajo Sandstone (Utah, USA) have been among the most extensively studied of these occurrences. At least four different models have been proposed for the formation of iron oxide concretions in the Navajo Sandstone. These include 1) mixing of reduced Fe(II)-bearing and oxygenated groundwaters, 2) neutralization of oxidized, acidic waters by calcite concretions, 3) neutralization of reduced, acidic waters by calcite concretions, and 4) oxidation of siderite by microbes during invasion of the Navajo aquifer by oxygenated groundwaters. We measured δ56Fe values of iron-bearing minerals from whole-rock Navajo Sandstone, concretionary iron oxide cements, iron oxide stains that are oriented in the direction of paleo-groundwater flow, iron oxide stains on the interior of rinded concretions, and ferroan carbonate cements. We have then combined these data with published measurements to evaluate each of the four published models. Whole rock Navajo Sandstone from the Kolob Plateau has been interpreted to be unaltered by bleaching fluids and has a δ56Fe value of 0.21‰. Iron in the Kolob Plateau sample comprises a mixture of hematite grain coatings (δ56Fe = 0.12‰) and a magnetic mineral fraction (interpreted to comprise largely detrital magnetite) that has a higher δ56Fe value (0.45‰). Iron oxide cements have δ56Fe values that are typically negative and as low as −0.74‰. In contrast, iron oxide-stained Navajo Sandstone in close proximity to a rinded iron oxide concretion has δ56Fe values that are 0.9 to 1‰ greater than the δ56Fe value of the associated iron oxide concretion. Finally, ferroan carbonates from the Navajo Sandstone yield δ56Fe values as low as −0.86‰. These values are in good agreement with measurements published previously for lithologies for which comparable measurements have been made. The presence of ferroan carbonates with negative δ56Fe values is evidence that aqueous Fe(II) was transported through the Navajo Sandstone aquifer and produced carbonate mineralization with negative δ56Fe values. The difference in δ56Fe values between the iron oxide concretions and associated iron oxide staining is more consistent with equilibrium fractionation between aqueous Fe(II) and Fe(II) adsorbed on quartz grains than it is with equilibrium fractionation between aqueous Fe(II) and any reduced or oxidized iron oxide mineral species. The negative δ56Fe values of the iron oxide concretions are best explained by oxidation of a reduced iron phase in a closed system from which no aqueous Fe(II) could escape. We interpret the iron isotope data as evidence that iron was stripped from Navajo sand grains by reducing fluids, reprecipitated in the Navajo aquifer as siderite, and oxidized by groundwaters under closed conditions during incision of the Colorado Plateau. • Iron oxide concretions with negative δ56Fe values occur in Navajo Sandstone • Ferroan carbonate in the Navajo Sandstone also has low δ56Fe values • The Navajo Sandstone aquifer contained reduced iron and carbonate-bearing waters • Low δ56Fe values of Navajo concretions unlikely if primary precipitate was iron oxide • Low δ56Fe values are evidence that iron oxide concretions formed from siderite [ABSTRACT FROM AUTHOR]