Mineral origin and siderite cementation processes in the Missourian Clastic rocks of the Anadarko Basin, OK

Abstract

The processes and physicochemical conditions leading to siderite precipitation in Missourian sediments of the Anadarko Basin, Oklahoma, have been identified and related to groundwater migration, microbial Fe-reduction, and relative abundances of phyllosilicates. Using optical microscopy, X-ray diffraction, electron microscopy with microanalysis, as well as carbon and oxygen stable isotope analyses, four siderite types were revealed and related to parental eogenetic processes. Concretion and band type siderite are the most common and have higher phyllosilicate content (illite/muscovite, illite-smectite, and Fe-rich chlorite) compared to the adjacent poorly siderite cemented sediment. The occurrence of spherical to subspherical siderite nanoglobules is connected to siderite mineralizing on bacteria cell walls. They are present in every siderite type and indicate that microbial organisms played a significant role in overcoming the geochemical barriers which inhibit siderite precipitation. The amount and type of cation substitution within a single siderite nanoglobule changes from center (FeO: 86, CaO: 10, MgO: 5, MnO2: 1) to rim (FeO: 78, MgO: 16, CaO: 4, MnO2: 2). The gradual Mg enrichment is accompanied by the emergence of Mg-rich rhombohedral crystals (FeO: 76, MgO: 17, CaO: 6, MnO2: 1) indicate a second, abiotic phase of siderite mineralization. Development of siderite bands and concretions is linked to periods of reduced sediment supply with the formation of each type being dependent upon host sediment porosity. Areas with high porosity correlate with concretion formation while areas with low porosity are linked to the formation of a band. δ13CVPDB values range between -12.01 to -1.65 ‰ indicate all siderite mineralized in a post-oxic environment. The δ18OVPDB values range from -10.79 to 0.08 ‰ and are inferred to be associated with the second, Mg-rich phase of siderite cementation which has likely amended the original δ18OVPDB.

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