IRON-RICH CONCRETIONS DELINEATE PALEOFLOW WITHIN A CO2-CHARGED AQUIFER: NAVAJO SANDSTONE OF SOUTH-CENTRAL UTAH

Many Colorado Plateau anticlines contain trapped carbon dioxide. Subsurface rocks within the Escalante anticline in south-central Utah sequester over 1 trillion cubic feet of CO₂. Spheroidal concretions with thick, iron-oxide-cemented rinds and lightly cemented, iron-poor, sandstone cores are abundant within bleached outcrops of Jurassic Navajo Sandstone on the southeast flank of the anticline. These concretions have been proposed as Earth analogs for Martian "blueberries". Many concretions display iron-oxide “tails” that extend southeastward, indicating precipitation during SE flow. Additionally, NW-SE oriented, pipy concretions with iron-oxide cemented rinds are abundant on the SE side of NE-SW oriented joints, defining a 38-km-long portion of an ancient flow path that lies parallel to the modern Escalante River. Further southeastward within a 53-km-long band stretching to the canyon of the Colorado River, irregular masses cemented by ferroan calcite are also present. Spheroids and pipes are the oxidized remains of siderite (FeCO₃) concretions that formed under reducing conditions in a shallow flow system initiated by Quaternary dissection of the region by the Colorado River. With recharge from the 3-km-high Aquarius Plateau, groundwater flowed southeastward through an undissected, CO₂ - and methane-bearing Navajo section within the Escalante anticline. Reducing waters dissolved iron oxide grain coatings from red sandstone, bleaching the rock and generating aqueous Fe²⁺. At elevations of 1-2 km, the CO₂ degassed through fractures and other high-permeability pathways. The abrupt rise in carbonate ion activity caused siderite to precipitate on the downflow side of joints; ferroan calcite precipitated further downflow as Fe²⁺ activity decreased. Further dissection exposed siderite to oxidizing waters, leading to oxidative dissolution and replacement by iron oxide, giving rise to the concretions we see today. We hypothesize that the rinds on concretions were generated by micoaerophilic, iron-oxidizing microbes that colonized the surfaces of concretions, and (through their metabolic activity) maintained themselves at oxic-anoxic boundaries. Reduced iron within the cores continued to migrate to the biofilms until the siderite was entirely dissolved and the rinds were fully formed.