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

¹ Department of Geology and Geophysics, West Mains Road, University of Edinburgh, Edinburgh EH9 3JW, ² Isotope Geology Unit, Scottish Universities Research and Reactor Centre, East Kilbride G75 0QU, and ³ Department of Geology and Applied Geology, Glasgow University, Glasgow G12 8QQ, UK

^* E-mail: mwi{at}glg.ed.ac.uk

(Received 8 April 1998; revised 23 October 1998)

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 (88±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.

KEYWORDS: siderite, zonation, Brent Group, bacteria, microbial influence

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