Abstract

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.

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