Abstract

“Rattlestones” are rinded, iron-oxide-rich concretions that form when siderite-cemented masses of floodplain muds are reworked into fluvial channels. Similar iron-oxide concretions in Utah’s Navajo Sandstone (Jurassic) have been suggested as Earth analogs for Martian concretions imaged by the rover Opportunity at Gusev Crater.  Initial work concluded that the Navajo concretions are primary iron-oxide precipitates formed by mixing of reduced and oxidized waters.  Our work shows that these concretions (like Pleistocene rattlestones from The Netherlands and Cretaceous rattlestones from Nebraska) formed via oxidation of a siderite-cemented precursor. The Navajo concretions record changing aquifer chemistry during uplift of the Colorado Plateau. Breaching of anticlines charged with methane and CO2 caused widespread precipitation of siderite concretions under reducing conditions.  Continued uplift led to alteration of the siderite by iron-oxidizing microbes.