RINDED CONCRETIONS FORMED BY OXIDATION OF SIDERITE: RATTLESTONES AND MARBLES FROM NON-MARINE SANDS AND SANDSTONES OF QUATERNARY, CRETACEOUS, AND JURASSIC AGE

Siderite (FeCO₃) forms at subsurface sites where the rate of iron reduction exceeds the rate of sulfate reduction—a situation that is common during the diagenesis of non-marine clastic sediments. When exposed to oxidizing groundwater or to O₂ in the vadose zone, siderite is altered to iron oxide. Crossbedded, fluvial sandstones of the Cretaceous Dakota Formation in eastern Nebraska contain abundant intraformational clasts composed of heavy rinds of iron oxide that surround mud-rich cores. The cores contain sufficient void space (46-89%) that, when the clasts are shaken, the internal collisions produce an audible “rattle”. In thin-sections of rinds, silt particles within the iron oxide are commonly arranged in arcs and circles, indicating disruption of the sediment fabric by mineral growth. Mm-scale spherosiderites are abundant in the paleosols that developed in muddy floodplain facies of the Dakota Formation. We attribute the distinctive distribution of silt grains in the rattlestone rinds and the high porosity of the cores to spherosiderite growth in floodplain soils prior to the transportation, deposition, and oxidation of the intraclasts. The iron oxide rinds and iron-poor cores of the Dakota rattlestones are direct analogs of Pleistocene Dutch rattlestones (van der Burg, 1969, Palaeo3, v. 6, 105-124). Modern and ancient rattlestones are, in turn, analogous to the iron-oxide-rich concretions in the Navajo Sandstone of south-central Utah. All of these rinded concretions were generated by the oxidation of siderite precursors. During their formation, concretion interiors remained reducing until all siderite was dissolved and ferrous iron had migrated to the perimeter of the structure where it combined with oxygen.