THE ROLE OF IRON IN MUDSTONE DIAGENESIS - COMPARISON OF KIMMERIDGE CLAY FORMATION MUDSTONES FROM ONSHORE AND OFFSHORE (UKCS) LOCALITIES

Siliciclastic muds and mudstones commonly contain 5% iron, At depositi on, most of this iron is in the oxidized form, Fe-III, whereas in anci ent mudstones it is predominantly in the reduced form, Fe-II. In most fine-grained siliciclastic sediments iron reduction is an important pr ocess during burial diagenesis. A combination of geochemical and petro logic techniques has been applied to organic-rich mudstones of the Kim meridge Clay Formation to investigate this valence change, These sedim ents were collected from a variety of depositional (shallow shelf to d eep 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 (toget her with Al) in dioctahedral micaceous clays (notably smectite), In su ch sites, it is apparent that Fe-III is very difficult to reduce, Our data suggest that diagenetic redox reactions are unlikely to be signif icant in the burial interval 0.5-4.0 km. It is also apparent that subs tantial iron reduction took place early during burial diagenesis with precipitation of pyrite, siderite, and ferroan carbonates in the sulfa te-reduction, methanogenic, and decarboxylation zones, respectively, A ll these minerals are found in the studied mudstones from the shelf lo calities, In contrast, within the graben mudstones, pyrite is the domi nant 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) bei ng converted to the pyrite prior to sediment remobilization, Resedimen taton then allowed a second phase of sulfate reduction with replenishe d pore-water sulfate from the anoxic bottom graben waters, The remaini ng reactive iron was converted to pyrite, thereby preventing precipita tion of Fe-rich carbonates in the deeper diagenetic zones, Hence, the diagenetic iron-mineral assemblages in the different facies are quite different, Diagenetic assemblages present in ancient siliciclastic sed iments thus offer valuable insights into both sedimentary and diagenet ic processes, but the links are complex and must be interpreted with c are.