Siliciclastic muds and mudstones commonly contain 5% iron. At deposition, most of this iron is in the oxidized form, Fe III , whereas in ancient mudstones it is predominantly in the reduced form, Fe II . In most fine-grained siliciclastic sediments iron reduction is an important process during burial diagenesis. A combination of geochemical and petrologic techniques has been applied to organic-rich mudstones of the Kimmeridge Clay Formation to investigate this valence change. These sediments were collected from a variety of depositional (shallow shelf to deep graben) environments and diagenetic (0.5-4.5 km burial) settings, and our analyses have shown that significant Fe III survives burial to depths of the order of 4 km. At these depths Fe III is located (together with Al) in dioctahedral micaceous clays (notably smectite). In such sites, it is apparent that Fe III is very difficult to reduce. Our data suggest that diagenetic redox reactions are unlikely to be significant in the burial interval 0.5-4.0 km. It is also apparent that substantial iron reduction took place early during burial diagenesis with precipitation of pyrite, siderite, and ferroan carbonates in the sulfate-reduction, methanogenic, and decarboxylation zones, respectively. All these minerals are found in the studied mudstones from the shelf localities. In contrast, within the graben mudstones, pyrite is the dominant Fe II -rich mineral species present. We propose that the different style of diagenesis in the two settings was produced by a fraction of the reactive iron (i.e., that contained within soil sesquioxides) being converted to the pyrite prior to sediment remobilization. Resedimentaton then allowed a second phase of sulfate reduction with replenished pore-water sulfate from the anoxic bottom graben waters. The remaining reactive iron was converted to pyrite, thereby preventing precipitation of Fe-rich carbonates in the deeper diagenetic zones. Hence, the diagenetic iron-mineral assemblages in the different facies are quite distinct. Diagenetic assemblages present in ancient siliciclastic sediments thus offer valuable insights into both sedimentary and diagenetic processes, but the links are complex and must be interpreted with care.

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