Genesis of siderite nodules from the lower carboniferous terrigenous sequence in the Subpolar Urals

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The paper presents the lithological, mineralogical, and geochemical characteristics of the composition, structure, and organic matter of siderite nodules and host mudstones in the Lower Carboniferous (Tournaisian–Visean) siderite-bearing sequence exposed along the Kozhym River on the western slope of the Subpolar Urals. The obtained results revealed that organic matter in the mudstones is dominated by C₁₆ and C₁₈n-alkanes, suggesting a significant microbial activity in the sedimentation environment. The formation of nodules was promoted by the activity of diverse bacterial communities, the abundance of which was related to processes of methanogenesis in bottom sediments owing to gaseous fluid seepages that promote the saturation of sediments with methane and the flourishment of bacterial colonies. Such processes in a marine basin led to the local freshening or some salinization of water and the development of euxinic setting and specific bacteria. Consequently, siderite nodules therein are marked by bacteriomorphic textures and specific authigenic mineralization (framboidal pyrite, sphalerite, galena, sulfoselenides, and tellurides). Bacteria utilized the substrate of the redeposited weathered crust, resulting in a large-scale formation of the nodular siderite.