Siderite zonation within the Brent Group: microbial influence or aquifer flow?
A three-fold zonation can be imaged within authigenic siderite from sandstones of the Brent Group using back-scatter SEM techniques. We interpret this zonation in terms of the biogeochemical zonation of shallow buried sediment. The innermost siderite crystal zone is very Fe rich (95.0 ± 0.5 mol.% FeCO3), with high Mn levels relative to Ca and Mg. This is interpreted as forming within the Fe reduction zone, with Mn from the closely associated Mn reduction zone. The second siderite crystal zone is frequently represented either by an episode of dissolution, or is impure (80 ± 1 mol.% FeCO3), and this corresponds to the sulphate reduction zone. The outer crystal zone is intermediate in composition, and is equated with the zone of methanogenesis (887 plusmn; 1 mol.% FeCO3). Isotopic values cannot be assigned to individual crystal zones. Bulk 18O values (-2.7 to -13.0‰ V-PDB) are not consistent with precipitation from seawater at low temperatures, but suggest meteoric pore-waters. 13C data (-4.3 to -15.7‰ V-PDB) are consistent with microbially-mediated precipitation. Pyrite and siderite are usually mutually exclusive within a single sample. Sedimentary conditions which favour the development of a strong sulphate reduction zone, and hence the formation of pyrite, do not favour the formation of a strong sub-oxic zone, where siderite is preferentially precipitated, and vice versa. There is a strong facies control upon siderite formation, with ripple cross-laminated sands being most strongly siderite cemented.
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