Date of this Version
The Journal of Geology, 2011, volume 119, p. 505–520; DOI: 10.1086/661110
Concretions cemented by iron oxide are abundant and diverse in the Jurassic Navajo Sandstone of southern Utah. Some of these structures are considered terrestrial analogs for concretions imaged on Mars. The Navajo concretions can be spheroidal, pipelike, or tabular with multicompartmented boxworks. These iron oxide concretions typically display a rinded structure: dense sandstone rinds cemented by iron oxide surround pale, iron-poor sandstone cores. Within a single structure, the iron-rich rinds may be single or multiple. Pseudomorphs of siderite are present in local residual, iron-rich cores of boxworks. Workers in the late nineteenth through mid-twentieth centuries, many of whom found evidence for siderite precusors, concluded that many spherical, rinded, iron oxide-cemented concretions were formed by centripetal precipitation of iron oxide inward from the perimeter of the concretion; in contrast, the walls of pipelike concretions of iron oxide grew centrifugally outward. We interpret the Navajo spheroids and boxworks as centripetal products of the oxidation of siderite-cemented (precursor) concretions that were very similar in both size and shape to the current concretions: rinds grew (thickened) inward toward the internal source of Fe(II). Siderite pseudomorphs appear to be absent from spheroids and many boxworks because all siderite was dissolved. In the cores of the larger boxworks some siderite was oxidized in situ; the Fe(II) did not migrate away from the original siderite crystals. The oxidation process was mediated by iron-oxidizing microbes and began at concretion perimeters when oxidizing groundwater started to displace the CO2- and methane-bearing water that had precipitated the siderite. In contrast, pipelike concretions are centrifugal—rinds formed around a cylindrical reaction front and thickened outward toward Fe(II) and away from the oxygenated water flowing within the cylinders. The cylindrical shape of the reaction front was produced by self-organizing feedbacks between dissolution of dispersed siderite cement and focused flow through a heterogeneous sandstone matrix.