The Kuparuk Formation (Neocomian) consists of glauconitic sandstones and mudrocks of marine origin. Authigenic siderite is common throughout the formation, either concentrated in distinct layers or as isolated rhombs. Examination of siderite-rich samples with back-scattered electron imaging reveals complex compositional zonation in the siderite. The samples are principally composed of mixtures of relatively early-formed Fe-rich siderite and relatively late-formed Mg-rich siderite, with mean compositions of (Fe (sub 77.5) Mg (sub 13.8) Ca (sub 7.9) Mn (sub 0.7 ) )CO 3 and (Fe (sub 55.8) Mg (sub 32.8) Ca (sub 10.8) Mn (sub 0.6 ) )CO 3 , respectively. Fe-rich siderites are most abundant in glauconitic sandstone intervals, whereas Mg-rich siderites are most abundant in non-glauconitic sandstones and mudrocks. The whole-rock isotopic composition of the siderite is highly variable, with delta 13 C values ranging from -20.3 to 7.9 per thousand PDB, and delta 18 O values from 21.4 to 31.1 per thousand SMOW. This variation in isotopic composition correlates with the relative proportions of the two siderite types in a given sample. The Fe-rich siderites have low delta 13 C and high delta 18 O values, whereas the Mg-rich siderites have high delta 13 C and low delta 18 O values. Both the elemental and carbon isotopic compositions of the siderite result from modification of the original marine pore waters during successive stages of microbial decomposition of organic matter. The oxygen isotopic compositions of the siderite indicate that early pore waters were depleted in delta 18 O, perhaps as a result of water/sediment interaction. Authigenic ankerite associated with the siderite-bearing samples is unzoned and has a mean composition of Ca (sub 1.11) Fe (sub 0.35) Mg (sub 0.53) (CO 3 ) 2 . The ankerites have higher delta 13 C values and lower delta 18 O values than the Fe-rich siderite, but have delta 13 C values less than and delta 18 O values greater than (with one exception) that of the Mg-rich siderite. The isotopic data suggest that ankerite precipitation began prior to precipitation of Mg-rich siderite and ended subsequent to or during Mg-rich siderite precipitation.