ABSTRACT

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_77.5Mg_13.8Ca_7.9Mn_0.7 )CO₃ and (Fe_55.8Mg_32.8Ca_10.8Mn_0.6 )CO₃, respectively. Fe-rich siderites are most abundant in glauconitic sandstone intervals, whereas Mg-rich siderit s are most abundant in non-glauconitic sandstones and mudrocks.

The whole-rock isotopic composition of the siderite is highly variable, with ¹³C values ranging from -20.3 to 7.9 PDB, and ¹⁸O values from 21.4 to 31.1 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 ¹³C and high ¹⁸O values, whereas the Mg-rich siderites have high ¹³C and low ¹⁸O 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 ¹⁸O, 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_1.11 Fe_0.35Mg_0.53(CO₃)₂. The ankerites have higher ¹³C values and lower ¹⁸O values than the Fe-rich siderite, but have ¹³C values less than and ¹⁸O 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 side ite precipitation.