Rinded iron‐oxide concretions: hallmarks of altered siderite masses of both early and late diagenetic origin

Abstract

Iron-bearing concretions are valuable records of oxidation states of subsurface waters, but the first concretions to form can be altered drastically during later diagenetic events. Distinctive concretions composed of heavy rinds of iron oxide that surround iron-poor, mud-rich cores are common along bases of fluvial cross-bed sets of the Cretaceous Dakota Formation, Nebraska, USA. Concretion rinds thicken inward and cores contain 46 to 89% void space. Millimetre-scale spherosiderites are abundant in palaeosols that developed in floodplain facies. Evolution of rinded concretions began when intraformational clasts were eroded from sideritic soils, transported, abraded and deposited in river channels. Alteration of siderite and formation of rinds occurred much later, perhaps in the Quaternary when sandstone pore waters became oxic. Dakota concretions are analogous to 'rattlestones' in Pleistocene fluvial channels of The Netherlands, and their rinded structure is analogous to that of iron-rich concretions in the aeolian Navajo Sandstone of Utah. In all three deposits, rinded concretions formed when pre-existing, siderite-cemented concretions were oxidized within a sand matrix. Unlike fluvial examples, siderite in the Navajo Sandstone was autochthonous and of late diagenetic origin, having precipitated from carbon dioxide and methane-enriched waters moving through folded and jointed strata. Iron-rich rinds formed in all these strata because concretion interiors remained anaerobic, even as oxygen accumulated in the pore waters of their surrounding, permeable matrix. Iron oxide first precipitated at redox boundaries at concretion perimeters and formed an inward-thickening rind. Acid generated by the oxidation reaction drove siderite dissolution to completion, creating the iron-poor core. Iron-oxide rinds are indicators of the former presence of siderite, a mineral that forms only under reducing conditions, during either early or late diagenesis. Siderite is vulnerable to complete oxidation upon exposure, so the distinctive rinded concretions are valuable clues that aid in deciphering diagenetic histories and for recognizing methanic floodplain palaeoenvironments and wet palaeoclimate.

Diagenetic origin of nodules in the Sheepbed member, Yellowknife Bay formation, Gale crater, Mars

“…The second possibility is that hollow nodule voids represent secondary porosity caused by the dissolution or selective leaching of a more soluble mineral phase that once existed in the cores of the hollow nodules (Figure a). Iron oxide‐cemented rinds in concretions of the Cretaceous Dakota Formation [ Loope et al , ], the Navajo sandstone [ Kettler et al , ; Loope et al , ], and Quaternary sediments in the Netherlands [ Van der Burg , , ] are interpreted as forming during the dissolution of early diagenetic siderite spherules caused by changing redox conditions [ Loope et al , ]. In cohesive muddy sediments, siderite nodules grow displacively, and subsequent oxidation of these nodules during diagenesis leaves behind an iron oxide rind surrounding a central cavity [ Loope et al , ].…”

The footprints of ancient CO2-driven flow systems: Ferrous carbonate concretions below bleached sandstone

“…Following van der Burg (1969), Loope et al (2012) showed that spindle-shaped intraclasts that form lag gravels in fluvial channels of the Cretaceous Dakota Formation (eastern Nebraska) were originally composed of sideritic floodplain silt and developed rinds only after they were deposited in the channels. The abraded shapes of the transported Dakota intraclasts dictated the shapes of the rinds that formed after transport and now surround them.…”

“…This hypothesis guided field-based work , bench experiments (Chan et al, 2007;Barge et al, 2011), and iron isotope studies (Chan et al, 2006;Busigny and Dauphas, 2007). Loope et al (2010Loope et al ( , 2011Loope et al ( , 2012 and Kettler et al (2011) reinterpreted the concretions as the altered (oxidized) remains of precursor concretions cemented by ferrous carbonate minerals. Weber et al (2012) presented evidence that iron-oxidizing microbes mediated the oxidation of the carbonates.…”

The origin of native selenium microparticles during the oxidation of sideritic mudstones in the Veřovice Formation (Outer Western Carpathians)

“…The appearance of strongly weathered clasts of sideritic mudstones is a reminder of the initial stage of the formation of the so-called rattle stones (Van Loef 2000;Loope et al 2012). This weathering structure that is relatively rare, especially in less weathered clasts of sideritic mudstone, is intersected by tiny secondary fissures of which walls are covered by thin coating of oxyhydroxides of Fe and Mn.…”