DISTINGUISHING BETWEEN MICROBIALLY-INDUCED SEDIMENTARY STRUCTURES AND GEOCHEMICAL SELF-ORGANIZATION RELATED TO IRON OXIDE MINERALIZATION IN FLUVIAL SANDSTONES

The Kanab Wonderstone is sandstone from the Shinarump member of the Chinle Formation that is variably cemented and stained with iron oxide. Although these features are unusual, they are not unique to the Shinarump member: similar features have been reported from other fluvial rocks worldwide. These features have been typically referred to as Liesegang bands, a type of geochemical self-organization. Closer examination reveals that the rocks in each locality comprise sandstones that host one to 5 mm thick, undulatory bands of iron oxide cement (IOC) that crosscut and obscure sedimentary structures. Between each pair of IOC bands are alternating bands of rock with iron oxide stain (IOS) and lightly stained or unstained bands of sand grains that also crosscut and obscure sedimentary structures. The interior-most portion of the sandstone bed may contain a bleached sandstone core enclosed by a band of IOC. The IOC and IOS are related spatially to vertical joints that cut the sandstone at regular intervals. The IOC in the Shinarump comprises a mixture of acicular grains (goethite) and hexagonal plates (hematite).

The joint control and presence of iron-poor cores surrounded by IOC in the Shinarump are analogous to box work IOC observed in the eolian Navajo Sandstone. We argue that the Navajo mineralization is the product of siderite that has been oxidized by iron-oxidizing microorganisms and that the Shinarump IOC formed similarly. Although the Shinarump Wonderstone IOC is not an example of Liesegang banding, the IOS is an example of the Liesegang phenomenon. Bands of IOS largely retain the morphology of reaction front fingers, presumably as a result of differential adsorption of aqueous Fe(II) during siderite dissolution. Variations in the width of the bands and in the spacing between these bands follow relationships observed in true Liesegang systems.