Understanding siderite mineralization in phyllosilicate-associated cementations in the mid-Carboniferous Anadarko Basin clastic series, U.S.A.

The present study provides insights into the origin of siderite cementation in closely interbedded bipartite mudstone to sandstone Pennsylvanian strata from the Anadarko Basin. Mineralogical, geochemical, and stable-isotope data were collected from 80 siderite samples and their immediate non-siderite-bearing regions. Geometrically, siderite mineralization occurs in the form of concretions or bands, with the latter being the most common textural type and occurring solely in mudstone, whereas the former is found in both sandstone and mudstone. This microtextural and geochemical investigation posits siderite as a derivate of biological processes at the sediment–water interface. Bacteria cell walls denoted by an omnipresent nanoglobule structure dominate the areas of mineralization. Mineral quantifications indicate higher phyllosilicate content within the mineralization compared to the non-mineralized sediment reflecting the role the clay minerals provide as a source of bio-essential cations, labile FeOx, and organic matter needed for microbial colonies to flourish. Following the formation of biological siderite, the energetically favorable mineralization surfaces served as nuclei for further precipitation of mesogenetic inorganic siderite enriched in 16O. The second mesogenetic cementation features rhombohedral siderite overgrowths with increasing Mg-concentration on the outer rims of nanoglobules. The identified bands and concretions were formed during periods of relative sea-level highs, whereas the siderite-cemented intraclasts were eroded and deposited downstream during times of relative sea-level lows. This is corroborated by relatively low (Ca-Mg)/Fe substitution in eogenetic siderite, typical of mineralization in meteoric-water-dominated realms. Finally, based on enrichment in 12C and textural observations, which suggest suboxic geochemical conditions, we conclude that the ability of siderite to form early on allowed it to maintain net rock porosity by encasing quartz and inhibiting its overgrowth process.