WATER TABLE CONTROLS ON THE MORPHOLOGY OF IRON-OXIDE CONCRETIONS IN THE NAVAJO SANDSTONE OF SOUTHERN UTAH

Iron-oxide concretions in the Navajo Sandstone are the altered remains of iron carbonate precursors that precipitated from late diagenetic, reducing, CO₂-charged pore waters. Near Escalante, UT, spheroidal concretions 0.2 -10 cm in diameter are composed of iron-oxide-cemented rinds that surround central, light-colored, iron-poor cores. These structures were oxidized within the phreatic zone. Iron-oxidizing microbes colonized the outer surfaces of spheroidal siderite concretions and exploited them as sources of energy and carbon. As siderite dissolved, ferrous iron diffused to the structure’s perimeter, thickening the rinds. Rinds continued to thicken inward until all siderite was dissolved and all iron had been transported from the interior to the rind. At Russell Gulch, 150 km westward and only 20 km E of the Hurricane Fault (Colorado Plateau’s western margin), rinded, spheroidal concretions contain central, iron-rich cores with rhombic, iron-oxide pseudomorphs after siderite. A light, iron-poor zone separates the rind from the dark core. Iron-oxide stains extend SW from concretions, indicating the flow direction of oxidizing water. Why, in these structures, did thickening of the rinds stop before all interior iron was transported to the perimeter? At this site, the concretions lie as much as 200 m below the base of Pleistocene paleo-valleys that were cut into the Navajo Sandstone and subsequently filled by the Lava Point basalt (1.02 Ma). The lava occupied the lowest points on the landscape, and therefore was likely emplaced near the level of the water table. Basalt is more resistant to erosion than the Navajo Sandstone. As rapid uplift along the Hurricane Fault and fluvial incision continued, the basalt became an elongate interfluve. Russell Gulch (now 400 meters deep) is the SW-draining canyon that formed on the SE side of the lava flow. As stream flow deepened Russell Gulch, the water table dropped 400 m to its present position. The iron-rich rinds and pseudomorph-bearing cores formed during rapid Quaternary uplift and record the passage of concretions from the phreatic zone to the vadose zone. Below the water table, rinds formed and thickened as iron diffused to the perimeter. Above the water table, cm-scale diffusion ceased and siderite was oxidized in situ, allowing preservation of the pseudomorphs.