ADDITIONAL EVIDENCE FOR THE SIDERITE OXIDATION HYPOTHESIS FOR NAVAJO SANDSTONE CONCRETIONS

The recent interest in spheroidal rinded iron oxide concretions of the Navajo Sandstone has resulted in the development of several hypotheses for the formation of these concretions. We favor the siderite oxidation hypothesis (Loope et al., 2010 Geology 38: 999–1002) for the formation of the Navajo concretions. It is useful to recall that spheroidal rinded concretions are only one manifestation of the iron oxide mineralization in the Navajo and that the forms of iron oxide mineralization observed in the Navajo occur in other formations. The Navajo contains not only the spheroidal rinded concretions but also pipelike and boxwork concretions. Pipelike concretions are cylindrical, rinded iron oxide masses that are typically orthogonal to a fracture. Iron oxide boxworks are prismatic masses of iron oxide-cemented sandstone wherein the iron oxide mineralization accumulates in fracture selvages. These forms of iron oxide mineralization are not unique to the Navajo, however. All three types of mineralization occur in the Caseyville Formation (Carboniferous, Mid-Continent, North America), rinded spheroidal concretions are known from the Dakota Formation (Western Interior, Cretaceous, North America), whereas rinded spheroidal concretions and boxwork have been observed in the Mesa Verde Formation (Western Interior, Cretaceous, North America). The similar morphologies and mineralogy of iron oxide mineralization in these formations is evidence that the mineralization had a similar origin in each case. We interpret the Navajo mineralization as the product of successive dissolution of siderite, mm to cm-scale transport of aqueous Fe(II), microbially-mediated oxidation of aqueous Fe(II) to Fe(III), and finally, precipitation of ferric oxyhydroxide. The Caseyville, Dakota, and Mesa Verde are all known to contain siderite. The occurrence of "Navajo-style" iron oxide mineralization in rocks that are known to have contained siderite is supporting evidence for the siderite oxidation hypothesis.