ORIGINS OF THICK ACCUMULATIONS OF IRON-OXIDE CEMENT ALONG JOINTS IN THE NAVAJO SANDSTONE, SOUTHERN UTAH

Spheroidal, iron-oxide concretions (“Moki marbles”; 1 mm to 15 cm in diameter) are widespread in the Navajo Sandstone of southern Utah, and have garnered attention due to their similarity to structures on the surface of Mars. Much larger oblong to tabular concretions (several meters long and about one meter-wide) are abundant in the middle Navajo at several localities between Escalante and Fruita, Utah and along the East Kaibab Monocline north of Utah 89. Rind-like, iron-oxide-cemented sandstone defines the perimeters of the large concretions; these rinds are up to 25 mm thick. Friable sand occupies the interiors of some of the smaller concretions, but in most of the large ones, core stones are present. Cores contain abundant, mm-scale, iron-oxide pseudomorphs after siderite (FeCO₃) and are usually “partitioned” by vertical, iron-oxide-coated joints, giving the fully oxidized concretion a cuboid or "boxwork" aspect.

We interpret the large, iron-oxide-rich concretions to be the altered remains of siderite concretions of similar size and shape. The oxide-lined joints are unique records of Plateau uplift. The in situ, maximum horizontal compressive stress in this region is WNW (approximately orthogonal to that in surrounding regions), and the stress regime is extensional. Many of the joints that cut the siderite concretions likely formed via thermal contraction during uplift. Some of these joints terminate against (and post-date) horizontal joints that also likely formed in the shallow subsurface. Vertical joints acted as conduits for meteoric water that oxidized the siderite. Iron-oxidizing microbes colonized redox boundaries and precipitated the oxides along the joints and concretion perimeters. Oxidation took place below the water table because the ferrous iron liberated from the dissolving siderite had to diffuse lateral distances of several decimeters to form the joint linings. Oxidation of the cores (and formation of the pseudomorphs), however, probably took place above the water table. An age obtained from iron oxide mineralization along these joints would identify that period when the rocks were within the zone of neotectonic fracture formation (<500m), but still below the water table.