EARLY DIAGENETIC FERRIC OXIDE ACCUMULATIONS FORMED ALONG REDOX GRADIENTS: EXAMPLES FROM MODERN AND ANCIENT FLUVIAL SEDIMENTARY UNITS

Accumulations of authigenic ferric oxides are found in both modern sediments and ancient lithified rocks. These accumulations are the product of redox gradients developed where oxidizing and reducing fluids meet. Iron-enriched zones that formed soon after sediment deposition provide insight into early diagenetic conditions, particularly organic matter reactivity and trace metal mobility. Sediments deposited by the Meghna River in eastern Bangladesh contain up to ten weight percent ferric oxyhydroxide at the top of the capillary fringe. Dissolved ferrous iron is transported upward from reduced gray sediments that typically contain only 0.2 weight percent organic carbon. Precipitating ferric hydroxides accumulate arsenic to concentrations as high as 300 ppm and are an important part of the arsenic cycle in Bangladesh. Development of the ferric hydroxide bands is sensitive to grain size and distance between the top of the capillary fringe and the ground surface. Shorter distances result in steeper gradients and precipitation of ferric hydroxides in thin, defined bands. Within and below the capillary fringe ferric oxides are reduced resulting in abundant exchangable ferrous iron, and minor amounts of siderite. Zones of hematite-cemented fluvial sandstone in the Jurassic Morrison Formation in western Colorado and eastern Utah are tens of centimeters thick and extend a few kilometers parallel to the channel axes. These sandstones contain up to 10 weight percent iron in complex Liesegang bands that cross bedding. These hematite accumulations formed during shallow burial where oxic water from flood plains moved laterally to recharge the generally reducing main fluvial channels. The main channels contain detrital wood and plant fragments that locally became accumulation sites of V, U, Se and Mo. These and other examples share several characteristics necessary to produce the early diagenetic iron accumulations. Each sedimentary unit: 1) was deposited along a low hydraulic gradient, 2) tend to have low average content of organic matter with local accumulations of peat or woody material, and 3) had sediment source rocks characterized by reduced iron. The reduced iron is oxidized during weathering to form reactive ferric oxides that are redistributed by successive reduction and oxidation across the redox interface.