MOSSBAUERAND CHEMICAL INVEST IGAT IONS OF MUDROCKS

sequences have been studied by M6ssbauer spectroscopy and chemical methods. Kimmeridge Clay Formation sediments proved to be richer in total Fe than Gulf Coast shales. In such sediments, Fe may be present in clay minerals (Fe(II) in chlorite, Fe(III) in illite and illite/smectite), pyrite, and ferroan carbonates (siderite, dolomite and ankerite). Pyrite accounts for a much greater proportion of the total Fe in the Kimmeridge Clay samples in which it is difficult o reconcile chemical data with M6ssbauer data. There is major doublet overlap of Fe(III) in silicates with Fe(II) in pyrite and spectra cannot be satisfactorily deconvoluted. This would appear to be a fundamental limitation for simple applications. In the pyrite-poor Gulf Coast material, however, chemical and spectroscopic evaluations of overall valence state are much more consistent. Confidence in both determinative techniques i generated, and useful information documenting progressive r duction and redistribution f Fe with burial is obtained. It is important o know how the oxidation state of Fe in fine-grained clastic sediments changes as they are buried, principally because Fe in the ferric state is the only significant solid oxidant present. As sediments are buried, Fe tends to be reduced progressively to the ferrous state by organic matter with the result that new (authigenic) minerals form, organi