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

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doi: 10.1180/000985500546512
Authors:Wilkinson, M.; Haszeldine, R. S.; Fallick, A. E.; Osborne, M. J.
Author Affiliations:Primary:
University of Edinburgh, Department of Geology and Geophysics, Edinburgh, United Kingdom
Scottish Universities Research and Reactor Centre, United Kingdom
Glasgow University, United Kingdom
Volume Title:Mineral diagenesis and reservoir quality, the way forward
Volume Authors:Bain, D. C., editor; Hall, P. L.; Shaw, H. F.; Spears, D. A.
Source:Clay Minerals, 35(1), p.107-117; Sixth Cambridge conference on Mineral diagenesis and reservoir quality, the way forward, Cambridge, United Kingdom, March 26-27, 1998, edited by D. C. Bain, P. L. Hall, H. F. Shaw and D. A. Spears. Publisher: Mineralogical Society, London, United Kingdom. ISSN: 0009-8558
Publication Date:2000
Note:In English. 23 refs.; illus., incl. 1 table, sketch map
Summary: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 δ18O values (-2.7 to -13.0 per mi; V-PDB) are not consistent with precipitation from seawater at low temperatures, but suggest meteoric pore-waters. δ13C data (-4.3 to -15.7 per mil 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.
Sections:Clay minerals
Subsections:Petrology; weathering; soils
Subjects:Alteration; Aquifers; Authigenic minerals; Bajocian; Brent Group; C-13/C-12; Carbon; Carbonates; Clastic rocks; Diagenesis; Geochemical indicators; Ground water; Isotope ratios; Isotopes; Jurassic; Mesozoic; Middle Jurassic; O-18/O-16; Oxygen; Petroleum; Petroleum exploration; Reservoir rocks; Sandstone; Sedimentary rocks; Siderite; Stable isotopes; Atlantic Ocean; North Atlantic; North Sea; Bacteria
Coordinates:N600000 N610000 E0020000 E0010000
Abstract Numbers:01M/118
Record ID:2000057287
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute.
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