THE KANAB WONDERSTONE: SIDERITE OXIDATION BY IRON-OXIDIZING MICROORGANISMS

Vertically jointed sandstone bodies in the Shinarump member of the Chinle Formation contain ∼5% Fe₂O₃. Much of this occurs as 1 to 5 mm thick, undulatory bands of iron oxide cement (IOC) that are generally sub-parallel to bedding and joint faces but which cross-cut and obscure sedimentary structures. The crests and troughs of the undulatory bands are rounded typically, but where sharp, point away from the interior of the body. The sandstone on the inner side of the IOC band is typically bleached. Bleaching is pronounced in areas where the IOC band is thick or concave toward the interior of the sandstone. Iron also occurs as iron oxide stain (IOS). Between each pair of IOC bands are alternating IOS and lightly stained or unstained bands of sand grains that also cross-cut and obscure sedimentary structures. Banding produced by the IOS is typically oriented at a high angle to the exterior-most IOC band and may appear to be truncated by it. The IOS bands become asymptotic to the interior-most IOC band. The interior-most portion of the sandstone bed may contain bleached sandstone enclosed by a band of IOC. Small accumulations of manganese oxide may occur on the inner contact between these circumferential bands of IOC and bleached cores. Siderite is abundant locally in the lower portion of the Chinle Formation on the Colorado Plateau. Although siderite occurs typically as concretions, locally; as at Temple Mountain, it cements large masses of sandstone. We interpret the IOC and IOS to be produced by oxidation of ferrous iron resulting in acid production and dissolution of siderite. Iron-oxidizing microbes colonized the interface between siderite-cemented sand and porous sandstone. Oxidation of iron along this interface produced acid that caused dissolution of siderite. Aqueous ferrous iron diffused back to the biofilm where it was oxidized. The convex-inward IOS bands are the remains of self-focused reaction fronts that advanced ahead of the biofilm. Periodic breakthroughs of oxygenated groundwater required microbes to colonize the new reaction front. Iron that had not diffused back to the biofilm prior to groundwater breakthrough was oxidized abiotically; staining the rock. Microbial oxidation of iron was complete in the center of the sandstone where cores of siderite were isolated by biofilms.