PIPE-LIKE CONCRETIONS FORMED BY DISSOLUTION OF A CARBONATE MINERAL, NAVAJO SANDSTONE OF SOUTH-CENTRAL UTAH, USA

Pipe-like concretions up to several m long and several dm in diameter are widespread in the Navajo Ss of our study area. The pipes are composed of a core of light-colored sandstone surrounded by a dark, cylindrical rind cemented by iron oxide. Numerous, parallel pipes form dense clusters where adjacent pipes have coalesced. Near Escalante, clusters of pipes extend SE from prominent, NE-SW oriented joints. At Capitol Gorge, pipes are clustered east of a vertical fault within the east limb of the Waterpocket Fold. Using trends of pipe-like concretions exposed near Escalante, we delineated an ancient, 100 km-long, groundwater flow system that reached from the Aquarius Plateau to the ancestral Colorado River. Our work has shown that, like many other iron-oxide cements in concretions, the oxides in the Navajo structures are alteration products of pre-existing concretions that were first cemented by a reduced-iron mineral. Because pipes lie directly down gradient from a CO₂ reservoir, and because they are spatially associated with joints, we conclude that their precursors were siderite-cemented and formed during aquifer degassing. Although we originally assumed that the cylindrical shape of the modern concretions was inherited from siderite precursors, we here argue that (for a large majority of the structures) this shape first appeared later, during the oxidation process. This conclusion is based on: 1) differences between pipe-like structures and boxworks in the Navajo Sandstone that we confidently interpret as oxidation products of large, fusiform, siderite-cemented masses, and 2) similarities of the pipes to the morphologies of nonplanar reaction fronts (“fingers”) that develop in sandstones via positive feedback mechanisms when moving fluids dissolve cement. The cylindrical, iron-oxide rinds that originate at joint surfaces formed when colonies of iron-oxidizing microorganisms established themselves along the perimeters of dissolution fingers and metabolized Fe⁺⁺ (derived from siderite cement), using O₂ as an electron acceptor. Pipes formed in structureless rock cemented by siderite that had precipitated earlier on the down-gradient side of fractures. Like other pipe-like concretions, these formed parallel to groundwater flow, but they record dissolution, not precipitation of a carbonate mineral.