The Geologic Context of Wonderstone: A Complex, Outcrop-Scaled Pattern of Iron-Oxide Cement

Abstract

Although siderite is a widespread early diagenetic mineral in fluvial systems, it is unstable in oxidizing environments and destroyed in permeable rocks that experience uplift and exhumation. The products of siderite oxidation, however, (mm-to cm-scale rhombs, concretions, and complex bands of iron-oxide cement) are widespread in the rock record of fluvial systems. The fluvial channels of the Shinarump Member of the Chinle Formation in southern Utah and northern Arizona, U.S.A., provide an excellent suite of examples of diagenetic features produced by Triassic and Neogene oxidation of early diagenetic siderite. These diagenetic features also provide direct evidence of the level of the water table during deposition of the Shinarump member. Large, in situ, discoidal concretions containing preserved siderite are present in Shinarump floodplain siltstones. Rip-up clasts derived from the siltstones developed iron-oxide rinds during late-stage, near-surface oxidation. These two structures show that floodplain silts contained abundant organic matter and methanic pore water. Groundwater recharging through these silts carried reducing water through underlying sand bodies and discharged into active channels. Degassing of CO 2 and methanogenesis caused rhombic crystals of siderite to precipitate in channel sands during these wet intervals. Some of this siderite may have been oxidized during dry intervals when groundwater circulation reversed, but most siderite in the channel sands was preserved until the Shinarump was exhumed during the Neogene. As oxygenated near-surface water entered joints in the lithified Shinarump, colonies of iron-oxidizing microbes living in the phreatic zone occupied redox boundaries and used the rhombic crystals of siderite in the sandstone and the spherulitic siderite in transported siltstone intraclasts as their sources of energy and carbon. The ferrous iron released from dissolving siderite within the intraclasts was oxidized at the siltstone-sandstone contact, generating rinded concretions similar to those in the Cretaceous Dakota Formation. Complex banding known as wonderstone was produced in the channel sandstones from oxidation of the rhombic siderite; the pattern is a combination of Liesegang bands and microbially mediated cements. The preserved rhombs are pseudomorphs after siderite crystals that were either oxidized during Triassic dry intervals, or escaped Neogene microbial oxidation in the phreatic zone, only to be oxidized abiotically in the vadose zone. Microbes are likely oxidizing Shinarump siderite a few kilometers down dip of outcrops with exposed wonderstone. At such locations, the Shinarump is in contact with overlying watersaturated Quaternary alluvium.

Interactions of A Paleocene River, A Rising Fold, and Early-Diagenetic Concretions

“…The oxidized remains of siderite concretions are distinctive, visually prominent, and highly informative. In meandering fluvial systems, early siderite concretions are commonly reworked by autocyclic processes (Loope et al 2012;Burgess et al 2016). This study demonstrates that reworked, iron-rich concretions can also reveal the interplay of deposition, early diagenesis, and erosion in tectonically active areas.…”

“…Reworked, sideritic concretions are common in the deposits of post-Silurian meandering fluvial systems, where migrating channels erode marshy or forested floodplain deposits. Rinded concretions (the oxidized remains of early-diagenetic, sideritic mudballs) are common in both the Triassic Shinarump Member of the Chinle Formation (northern Arizona; Burgess et al 2016) and the Dakota Formation (eastern Nebraska; Loope et al 2012). Reworked, very young, iron-rich concretions can also be found on marine sand flats and tidal creek beds (Pye et al 1990).…”

“…Those siderite concretions that grow in permeable sandstones and unconsolidated alluvium are likely to be altered to iron oxide when, due to tectonic uplift, they approach the land surface. In oxidizing, shallow groundwater, iron-oxidizing microbes can transform siderite concretions into dense iron-oxide rinds surrounding friable, iron-poor interiors (van der Burg 1969;Loope et al 2012), or into a combination of dense, iron-oxide-cemented bands and iron-oxide stain (true Liesegang; Kettler et al 2015;Burgess et al 2016). Large siderite concretions in sandstones are commonly fractured into small, threedimensional compartments; oxidation along the fractures generates boxworks delineated by dense accumulations of iron oxide (Taylor 1949;Loope et al 2011;Loope et al 2012;Burgess et al 2016).…”

The influence of climate on early and burial diagenesis of Triassic and Jurassic sandstones from the Norwegian–Danish Basin

“…Consequently, the reddening process seems to be associated with oxidizing conditions in this case. However, red coloration in other sandstones may origin from in situ soil formation (Mora et al ., ), eroded lateritic soils (Van Houten, ), ferruginization associated with fluctuating groundwater level, and hence iron mobilization under reducing conditions and precipitation under oxidizing conditions (Besly et al ., ; Mücke, ) or from iron‐oxides formed due to microbial activity in siderite‐cemented sandstones (Burgess et al ., ). Reddish oxidation rims in recycled glauconite grains and iron‐oxide/hydroxide replacement of Fe‐rich mica and heavy minerals confirm the overall oxidizing conditions.…”

Paleoenvironmental Implications of Selected Siderite Zones in the Upper Atoka Formation, Arkoma Basin, Oklahoma-Arkansas: A Look Back and Current Views on Siderite Genesis in a Sedimentary Environment

“…The end-product, of course, is iron oxide, from mm-scale rhombs to concretions and complex bands of iron-oxide cement. The outcropping rocks of the Shinarump member of the Chinle Formation in Utah and Arizona have been examined in some detail [84].…”