BLEACHED ROCK AND FERROUS CARBONATE CEMENTS ARE THE FOOTPRINTS OF ANCIENT CO2 RESERVOIRS: NAVAJO SANDSTONE OF SOUTHERN UTAH

In the “Grand Staircase” that lies north of Grand Canyon, the upper part of the Navajo Sandstone crops out as the 75 km-long White Cliffs. The lower Navajo comprises the Vermillion Cliffs. University of Utah geologists have argued that the upper Navajo: 1) was originally red and was bleached by buoyant hydrocarbons; and 2) that the iron in the abundant iron-oxide concretions of the lower Navajo came from iron that was liberated during the bleaching of the overlying rocks. These researchers interpret the concretions as primary precipitates that formed in zones where iron-bearing, reducing fluids mixed with oxidizing water. Our work indicates that the iron oxides are secondary, and formed via microbial oxidation of (primary) concretions that were cemented by siderite and ferroan calcite. Rather than calling on two waters that mix, we argue that the carbonates precipitated in reducing water and that the iron oxides formed later when, as the Colorado Plateau was uplifted, oxidizing waters invaded.

When carbon dioxide dissolves in water, it increases that water’s density. Geologists, motivated to understand the fate of carbon dioxide injected into vertically confined aquifers, have shown that dissolution of the gas into formation water causes gravity-driven, convective transport systems to develop. Carbon dioxide (likely sourced by Oligocene intrusions) and methane migrated up-dip into the Navajo along the crests of the Kaibab and Escalante Anticlines. Bleaching of the upper Navajo Sandstone required two processes: 1) Methane dissolved the iron-oxide coatings on the sand grains, putting Fe²⁺ into solution; and 2) Dissolution of the CO₂ added CO₃^-- and HCO₃^- to the reducing formation water and increased its density, causing the down-dip and down-section transport of the dissolved ferrous iron. Growth of ferrous carbonate cements took place in this reducing environment (beneath the bleached zone, far from oxidizing waters). As the Colorado Plateau was uplifted, the Navajo on structural highs was breached by erosion, and oxidizing, meteoric invaded the formation. Iron-oxidizing microbes facilitated the alteration of siderite, forming dense rinds cemented by iron oxide on the perimeters of the precursor concretions. Microbes could not metabolize ferroan calcite; those concretions remain unaltered.