EPISODES OF MICROBIAL SIDERITE OXIDATION PRODUCE DIVERSE IRON-OXIDE MORPHOLOGIES AND RECORD UPLIFT OF BEDROCK AQUIFERS (Invited Presentation)

On the Colorado Plateau of southern Utah, large, rinded oval to spheroidal concretions and scalloped, mm-to-cm-thick bands cemented by iron oxide are widespread in the Jurassic Navajo Sandstone and the Triassic Shinarump Member of the Chinle Formation. Both types of cementation are products of siderite oxidation by iron-oxidizing microbes. The shape and size of the rinded concretions were dictated by the shape and size of the precursory, siderite-cemented concretions. The scalloped bands were produced by microbes metabolizing euhedral crystals or tiny spheroids of siderite that were disseminated within the aquifer. When uplift and denudation of the Plateau caused oxygen to invade the sandstone, microbes colonized concretion perimeters. Diffusion of “entrapped” ferrous iron allowed rinds to thicken inward as the siderite in the concretion center dissolved. The diffusion rate of oxygen, rather than the diffusion rate of ferrous iron limited the rate of microbial growth. In contrast, bands of iron-oxide cement formed in open, advective portions of the aquifers. We interpret the bands as “oxygen dams” that microbes constructed parallel to flow and thus slowed the radial expansion of conduits of oxygen-rich groundwater. Preliminary U-Th/He ages and the control of some cement accumulations by shallow, slope-parallel joints indicate Quaternary ages for Plateau iron oxides. A third, bulbous morphology of iron-oxide accumulation that is convergent with the form of digitate stromatolites is present in the Cambrian Umm Ishrin Formation of southern Jordan. Scalloped, iron-oxide-cemented bands and spheroidal concretions are also widespread in this formation. In laboratory experiments, microbial biofilms take on digitate forms when nutrients are in low supply. We hypothesize that the digitate Jordanian oxides formed in portions of the sandstone aquifer where microbial growth was limited by the diffusion rate of ferrous iron. Preliminary U-Th/He ages and a published study of apatite fission tracks in the Umm Ishrin suggest that siderite was oxidized during an episode of exhumation (and cooling) during the Late Cretaceous.