DISTINCTIVE REMNANTS OF SIDERITE CONCRETIONS OXIDIZED IN ADVECTING GROUNDWATER (NAVAJO SANDSTONE, SOUTH-CENTRAL UTAH)

In south-central Utah, in the headwaters of Harris Wash, spheroidal concretions composed of an iron-oxide-cemented rind and an iron-poor core are abundant in the Navajo SS. The outermost surfaces of many of the rinds are stepped. Steps are up to 5 mm high and descend across the concretion surface as a series of concentric rings. On each in situ concretion, steps drop progressively downward toward the SE, and a “comet tail” of iron-oxide-stained sandstone extends SE from the top of each step. Cross-sectional views reveal the rinds to be nearly isopachous (they do not progressively thin to the SE). Oblate, unstepped concretions up to 8 m in diameter with 25-mm-thick, iron-oxide rinds are abundant in the Navajo SS about 15 km N of the stepped concretions. The innermost portions of these larger concretions contain rhomb-shaped patches of poikilotopic iron oxide cement. We interpret the iron oxide rinds on both types of concretion as precipitates that thickened inward as precursor siderite concretions dissolved and Fe(II) diffused outward to meet O₂ at the outer margin of the structures. The iron oxide remaining in the core zones of the larger concretions is pseudomorphous after siderite that was oxidized in situ (i.e. iron that did not diffuse outward to contribute to the rind). Siderite originally formed under reducing conditions within a confined aquifer, down-gradient from an anticline charged with CO₂ and CH₄. With Plateau uplift, the concretions were later subjected to oxidizing groundwater flowing SE toward the Colorado River. As the aquifer slowly shifted from reducing to oxidizing, the siderite concretions started to dissolve, releasing acid. An iron-oxide rind initially formed on the up-gradient (O₂-rich), outer margin of the spheroidal concretions and thickened as Fe(II) was released and diffused NW. The Fe(II) released on the down-gradient side, however, did not immediately meet O₂ and was carried down-gradient to form a comet tail. As O₂ increased, a rind eventually fully enclosed the structure. The steps mark growth stages of the rind during its down-gradient extension over the surface of the dissolving siderite precursor. Once the rind covered the down-gradient surface of the concretion, it proceeded to grow inward, eventually reaching a thickness similar to that of the rind in the up-gradient position.