DOWNWARD TRANSPORT OF IRON IN THE GRAND STAIRCASE OF SOUTHERN UTAH: THE ROLE OF CARBON DIOXIDE

In the “Grand Staircase” north of Grand Canyon, the upper Navajo Sandstone crops out as the 75 km-long White Cliffs. The lower Navajo comprises the Vermillion Cliffs. Some geologists argue the upper Navajo: 1) was originally red and was bleached by buoyant hydrocarbons; and 2) that iron in abundant iron-oxide concretions of the lower Navajo was liberated during bleaching of overlying rocks. They interpret the concretions as primary precipitates that formed where iron-bearing, reducing fluids mixed with oxidizing water. Our work indicates iron oxide is secondary, and formed via microbial oxidation of (primary) concretions cemented by siderite (FeCO₃). Rather than calling on two waters that mix, we argue that siderite precipitated in reducing water, and that later, as the Colorado Plateau was uplifted and oxidizing waters invaded the Navajo, siderite altered to iron oxide.

Our reinterpretation of original mineralogy of concretions avoids the problem of how oxidizing water could have pervasively penetrated the lower Navajo while reducing fluids were bleaching the upper Navajo. We also answer the question of why iron was transported downward. When CO₂ dissolves in water, it increases that water’s density. Researchers interested in the fate of CO₂ injected into vertically confined aquifers have shown that dissolution of CO₂ into formation water causes gravity-driven, convective transport systems to develop. In the study area, CO₂ (likely sourced by Oligocene intrusions) and methane migrated updip into the Navajo along the crests of the Kaibab and Escalante Anticlines. Bleaching of the upper Navajo and downward transport of iron required two processes: 1) Methane stripped the iron-oxide coatings from the sand grains, putting Fe²⁺ into solution; and 2) Dissolution of CO₂ added CO₃^-- and HCO₃^- to the reducing water and increased its density, causing down-dip and down-section transport of Fe²⁺. Growth of ferrous carbonate cements took place in reducing waters in and beneath the bleached zone, far from sources of oxidizing water. As the Colorado Plateau was uplifted, the Navajo on structural highs was breached, and oxidizing, meteoric water invaded the formation. Iron-oxidizing microbes facilitated alteration of siderite, forming dense rinds cemented by iron oxide on the perimeters of precursor concretions.