Siderite zonation within the Brent Group: microbial influence or aquifer flow?

A three-fold zonation can he imaged within authigenic siderite from sandsto nes of the Brent Group using back-scatter SEM techniques. We interpret this zonation in terms of the biogeochemical zonation of shallow but icd sedime nt. The innermost siderite crystal zone is very Fe rich (95.0+/-0.5 mol.% F eCO3), with high Mn Levels relative to Ca and Mg. This is interpreted as fo rming 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), a nd this corresponds to the sulphate reduction zone. The outer crystal zone is intermediate in composition, and is equated with the zone of methanogene sis (88 +/- 1 mol.% FeCO3). Isotopic values cannot be assigned to individua l crystal zones. Bulk delta(18)O values (-2.7 to -13.0 parts per thousand V -PDB) are not consistent with precipitation from seawater at low temperatur es, but suggest meteoric pole-waters. delta(13)C data(-4.3 to -15.7 parts p er thousand 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 re duction zone, and hence the formation of pyrite, do not favour the formatio n 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.