“RATTLESTONES” FROM THE DAKOTA FORMATION OF NEBRASKA AND THEIR RELEVANCE TO IRON-OXIDE-CEMENTED CONCRETIONS IN UTAH'S NAVAJO SANDSTONE

Crossbedded, fluvial sandstones of the Cretaceous Dakota Formation contain abundant intraformational clasts composed of heavy rinds of iron oxide that surround mud-rich, iron-poor cores. The cores contain enough void space (45%-89%) that collisions can be heard when the clasts are shaken. In thin-sections of rinds, silt particles surrounded by iron-oxide cement are commonly arranged in arcs and circles, indicating disruption of the original sediment fabric by mineral growth. In muddy floodplain facies of the Dakota Formation, mm-scale spherosiderites are abundant in paleosols. We attribute the high porosity of rattlestone cores and the distinctive distribution of silt grains in the rattlestone rinds 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 very similar to Pleistocene Dutch rattlestones (van der Burg, 1969), and are closely analogous to the iron-oxide-rich, marble-like concretions in the Navajo Sandstone of south-central Utah. All three examples of rinded concretions were generated by the oxidation of siderite precursors. During formation of the iron-oxide rinds, concretion interiors remained anaerobic until all siderite was dissolved and ferrous iron had migrated to the perimeter of the structure where it combined with oxygen.