Date of this Version
Loope, D.B., and Kettler, R.M., 2019, Rinded iron-oxide concretions in Navajo Sandstone along the trail to Upper Calf Creek Falls, Garfi eld County, in Milligan, M., Biek, R.F., Inkenbrandt, P., and Nielsen, P., editors, Utah Geosites: Utah Geological Association Publication 48, 7 p., https://doi. org/10.31711/geosites.v1i1.59.
Concretions are hard rock masses, usually spheroidal, but commonly oblate or discoidal, that are formed by strongly localized precipitation of minerals in the pores of an otherwise weaker sedimentary rock (see Bates and Jackson, 1980, for a more extensive definition). The iron-oxide-rich concretions in the Jurassic Navajo Sandstone in southern Utah are unusual in two fundamental ways. First, they are cemented by iron oxide (Fe2O3, or Fe(OH)3); most other concretions are cemented by silica (SiO2), calcite (CaCO3), dolomite (CaMg(CO3)2), or siderite (FeCO3). Second, unlike other concretions, they are not strongly cemented throughout, but instead, the iron oxide is concentrated in a very strongly cemented, sharply defined, exterior rind or shell. In the smaller concretions, the entire interior lacks iron-oxide cement, and is similar to the rock outside the concretion; in the larger concretions, there is a central zone that is strongly cemented by iron oxide, but the sandstone between the central core and the rind has no iron-oxide cement.
The processes involved in the formation of the Navajo concretions have been extensively studied by geologists ever since the Martian rover Opportunity imaged small, spheroidal, iron-rich concretions on the surface of Mars (Moore, 2004). The Navajo concretions were quickly recognized as possible terrestrial analogs to the Martian “blueberries” (Chan and others, 2004). At present there are three main explanations for how these concretions formed. The first interpretation of the Navajo concretions (Chan and others, 2004; Beitler and others, 2005) views the iron-oxide cement and the “rinded” aspect of today’s concretions as primary features—just a thin, three-dimensional band of sandstone (circular when viewed in two dimensions) that had been cemented by iron oxide (Chan and others, 2004). A complication of this interpretation is that, because iron is not transported by water containing oxygen, mixing of an iron-bearing water (devoid of oxygen) with a second, oxygenated water mass is required (Chan and others, 2004). Mixing is typically not required for growth of silica, calcite, dolomite, or siderite concretions.
The second interpretation (Loope and others, 2010; Weber and others, 2012) is that solid, spheroidal concretions were originally well-cemented throughout by siderite (FeCO3). The rinds formed secondarily, during much later alteration of the older, primary siderite. A weakness of this interpretation is that unaltered siderite has not been found in any Navajo concretions.
The third interpretation (Yoshida and others, 2018) holds that the original spheroidal concretions were solid spheres cemented by calcite (CaCO3). The rinds formed later when acidic, iron-bearing waters invaded the Navajo. The calcite of the concretions buffered the acidic waters, forcing iron oxide to precipitate. A weakness of this interpretation is that it does not explain the localization of iron into a distinct, strongly cemented rind nor the presence of abundant iron in a spatially isolated core zone.