Micromilieu-controlled glauconitization in fecal pellets at Oker (Central Germany)

Although numerous models for the formation of glauconite have been presented, the precise process and micro-environment of glauconitization are still poorly constrained. We characterize the special micromilieu of glauconitization developed during early diagenesis and present a model for glauconite formation in fecal pellets. Glauconitization at Oker (Central Germany) occurred predominantly in fecal pellets deposited in a shallow marine-lagoonal environment during the Kimmeridgian. Within the fecal pellets, rapid oxidation of organic matter provides the post-depositional, physicochemical conditions favourable for glauconitization. Replacements of matrix calcite, dissolution of detrital quartz, K-feldspar, and clay minerals, and Fe redox reactions were observed within the early micro-environment, followed by the precipitation of euhedral pyrite, matrix-replacive dolomite, and megaquartz accompanied by IS formation as thin section analyses and SEM observations show. Carbonate geochemical compositions based on ICP-OES and stable oxygen and carbon isotope signatures demonstrate that glauconite formation started in a suboxic environment at a pH of 7-8 and a temperature of 22 +/- 3 degrees C to 37 +/- 2 degrees C at maximum. TEM-EDX-SAED and XRD analyses on separated glauconite fecal pellets and on the <2 mu m clay mineral fraction reveal the predominance of authigenic 1M(d)-glauconite, 1M(d)-glauconite-smectite, and 1M(d) cis-vacant I-S, besides accessory detrital 2M(1)-illite and montmorillonite. Kinetic modelling of the glauconite (93-94% Fe-illite layers and 6-7% Fe-smectite layers, R3) and of I-S (66-68% Al-illite layers and 32-34% Al-smectite layers, RI) leads us to conclude that the I-S formed solely by slow burial diagenesis, whereas the glauconite formed close to the seafloor, suggesting significantly faster kinetics of the glauconitization reaction compared with smectite-illitization related to burial diagenesis. Thermodynamically, the substitution of octahedral Al3+ for Fe3+ and Mg2+ during the Fe-Mg-smectite to glauconite reaction via the formation of glauconite-smectite mixed-layered clay minerals may have resulted in a higher reaction rate for this low-temperature glauconitization process.