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Morphologic Clues to the Origins of Iron Oxideâ•Ficemented Spheroids

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Morphologic Clues to the Origins of Iron Oxideâ•Ficemented Spheroids University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Faculty Publications in the Biological Sciences Papers in the Biological Sciences 2011 Morphologic Clues to the Origins of Iron Oxide–Cemented Spheroids, Boxworks, and Pipelike Concretions, Navajo Sandstone of South-Central Utah, U.S.A. David B. Loope University of Nebraska - Lincoln, [email protected] Richard M. Kettler University of Nebraska - Lincoln, [email protected] Karrie A. Weber University of Nebraska-Lincoln, [email protected] Follow this and additional works at: https://digitalcommons.unl.edu/bioscifacpub Part of the Life Sciences Commons Loope, David B.; Kettler, Richard M.; and Weber, Karrie A., "Morphologic Clues to the Origins of Iron Oxide–Cemented Spheroids, Boxworks, and Pipelike Concretions, Navajo Sandstone of South-Central Utah, U.S.A." (2011). Faculty Publications in the Biological Sciences. 198. https://digitalcommons.unl.edu/bioscifacpub/198 This Article is brought to you for free and open access by the Papers in the Biological Sciences at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Faculty Publications in the Biological Sciences by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Morphologic Clues to the Origins of Iron Oxide–Cemented Spheroids, Boxworks, and Pipelike Concretions, Navajo Sandstone of South-Central Utah, U.S.A. David B. Loope,1,* Richard M. Kettler,1 and Karrie A. Weber1,2 1. Department of Earth and Atmospheric Sciences, University of Nebraska, Lincoln, Nebraska 68588, U.S.A.; 2. School of Biological Sciences, University of Nebraska, Lincoln, Nebraska 68588, U.S.A. ABSTRACT 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. Introduction Concretions cemented by iron oxide are prominent iron poor (fig. 1). A full understanding of the Navajo in many outcrops of the porous and permeable Na- concretions requires a clear explanation of the or- vajo Sandstone of south-central Utah (fig. 1). The igin of this distinctive, rinded structure. Although concretions range in diameter from !1mmto8m Beitler et al. (2005) and Chan et al. (2007) argued and can be spheroidal, cylindrical, or tabular. Nava- that the concretions are primary precipitates jo spheroids have been proposed as terrestrial an- formed during the mixing of two dilute fluids, we alogs for the hematitic spherules imaged by Mar- interpret the rinds as byproducts of siderite tian rover Opportunity (Chan et al. 2004). Although oxidation. diverse in size and shape, nearly all the Navajo con- Our interpretation of the origins of the spheroidal cretions that we studied share a common structural and tabular rinds has been guided by observations feature: a dense rind or crust cemented by iron ox- made by geologists working as long as a century ide surrounds a sandstone core that is relatively ago. Dana (1896, p. 98) described concretions with rinded structure and interpreted them as having Manuscript received October 29, 2010; accepted May 13, 2011. formed via “consolidation that progressed inward * Author for correspondence: e-mail: [email protected]. from the exterior—a centripetal process.” Todd [The Journal of Geology, 2011, volume 119, p. 505–520] ᭧ 2011 by The University of Chicago. All rights reserved. 0022-1376/2011/11905-0005$15.00. DOI: 10.1086/661110 505 506 D. B. LOOPE ET AL. Figure 1. Spheroids, pipes, and tabular boxworks. A, Pea-sized spheroids in structureless sandstone. Note that co- alesced spheroids are defined by a single rind of iron oxide cement. B, Spheroid with manganese oxide dendrites growing inward from the inner surface of the iron oxide rind. C, Pipelike concretions with iron oxide rinds, Capitol Wash site. Note abundant spheroidal concretions (white arrow) and ridges on pipe surfaces (black arrow). Pipe splits just below the white S. D, Tabular boxwork with multiple compartments. Thick, iron oxide–cemented rinds are developed on perimeter and along vertical joints (white arrows), and interiors are occupied by iron-poor sandstone. (1903) showed that such rinds can form during the from open-ended cylindrical concretions showing progressive oxidation of siderite concretions, with that (in contrast to the fully enclosing centripetal the rinds reflecting the shapes of their siderite- rinds on spheroids and boxworks) the cylindrical cemented precursor concretions. Van der Burg rinds thickened outward. We argue that these pipe- (1969, 1970) provided another example (from Pleis- like concretions record the shapes of self-organized tocene fluvial sands of the Netherlands) of rinded, reaction fronts that formed when oxygenated water iron oxide–cemented concretions that formed via started to move through a permeable sandstone the oxidation of a siderite-cemented precursor. that contained dissolving, reduced-iron cements. This article builds on our earlier work (Loope et al. 2010) by providing more detailed explanations Setting for the morphologies of the Navajo concretions and the processes that formed them. Here we report Previous Studies. Chan et al. (2004) pointed out pseudomorphs of siderite crystals within the cen- the similarities of spheroidal, iron oxide–cemented tral, iron oxide–rich portions of large, tabular con- concretions in the Navajo Sandstone to the “blue- cretions. These provide support for our claim that berries” imaged on Mars by the rover Opportunity. the rinded structures are oxidation products of a Beitler et al. (2005, their fig. 12) and Chan et al. reduced iron phase. We also present new evidence (2005, 2007) interpreted the spheroids within the JournalofGeology CLUES TO THE ORIGINS OF NAVAJO CONCRETIONS 507 Navajo as well as pipelike concretions and box- through the Navajo Sandstone at Glen Canyon ini- works found with them as primary precipitates— tiated vigorous southeastward flow through the products of the mixing of two waters: upward-mov- Navajo aquifer. Based on downcutting rates, sid- ing, reduced fluids delivered iron, and coexisting erite cementation may have occurred about 2 mil- shallow groundwater provided oxygen. Beitler et al. lion years ago; the iron oxide concretions are even (2005, their fig. 12) envisioned the two fluids in- younger (Loope et al. 2010). teracting along a redox reaction front. Potter et al. Study Areas. We have studied iron oxide– (2008, 2011) showed that the dense, dark rinds that cemented concretions in two areas within south- surround and define most Navajo Sandstone con- central Utah (fig. 2): (1) the drainage of the Escalante cretions are cemented by hydrous ferric oxide. River (within Grand Staircase–Escalante National Chan et al. (2006) and Busigny and Dauphas (2007) Monument and the Glen Canyon National Recre- studied the variation of iron isotope ratios within ation Area) and (2) the eastern limb of the Water- the Navajo Sandstone and its concretions. The first pocket Fold (within Capitol Reef National Park). study concluded that the strongly negative d56Fe Most of our work has been carried out within the values in the rinds of concretions were generated Escalante drainage, and only a single site from the by dissimilatory dissolution of iron oxide coatings Capitol Reef (Capitol Wash, a tributary of the Fre- on sand grains and that variations in the d56Fe val- mont River) will be discussed here. ues of the concretions supported an open-system Geologic Setting. In south-central Utah, the Early model of concretion formation. The latter study Jurassic Navajo Sandstone is about 300 m thick
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