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Septarian concretions from the Oxford Clay (Jurassic, England, UK): involvement of original marine and multiple external pore fluids

Authors

  • J. D. Hudson,

    1. 1 Department of Geology, University of Leicester, Leicester LE1 7RH, UK (E-mail: jdh4@leicester.ac.uk) 2Postgraduate Research Institute for Sedimentology, University of Reading, Whiteknights, Reading RG6 6AB, UK3NERC Isotope Geosciences Laboratory, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK4Natural Environment Research Council, Polaris House, North Star Avenue, Swindon SN2 1EU, UK
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  • 1 M. L. Coleman,

    1. 1 Department of Geology, University of Leicester, Leicester LE1 7RH, UK (E-mail: jdh4@leicester.ac.uk) 2Postgraduate Research Institute for Sedimentology, University of Reading, Whiteknights, Reading RG6 6AB, UK3NERC Isotope Geosciences Laboratory, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK4Natural Environment Research Council, Polaris House, North Star Avenue, Swindon SN2 1EU, UK
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  • 2 B. A. Barreiro,

    1. 1 Department of Geology, University of Leicester, Leicester LE1 7RH, UK (E-mail: jdh4@leicester.ac.uk) 2Postgraduate Research Institute for Sedimentology, University of Reading, Whiteknights, Reading RG6 6AB, UK3NERC Isotope Geosciences Laboratory, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK4Natural Environment Research Council, Polaris House, North Star Avenue, Swindon SN2 1EU, UK
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  • and 3 N. T. J. Hollingworth 4

    1. 1 Department of Geology, University of Leicester, Leicester LE1 7RH, UK (E-mail: jdh4@leicester.ac.uk) 2Postgraduate Research Institute for Sedimentology, University of Reading, Whiteknights, Reading RG6 6AB, UK3NERC Isotope Geosciences Laboratory, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK4Natural Environment Research Council, Polaris House, North Star Avenue, Swindon SN2 1EU, UK
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Abstract

Septarian concretions occur at several horizons within the Oxford Clay Formation, a marine mudstone containing pristine aragonite and immature biomarker molecules. They record the passage of at least four generations of pore fluids, the first of marine origin and the last still present in cavities. Concretion bodies formed, cracked, and calcite and pyrite precipitated in and around the cracks within the sulphate reduction zone, as demonstrated by C, O, S and Sr isotopic composition (Pore fluid 1). Before major compaction, sandstone dykes were intruded locally, and baryte precipitated, followed by coarse calcite cements with isotopically light oxygen and radiogenic strontium, indicating the introduction of meteoric-derived water (Pore fluid 2). Later, coarse celestine within concretions has distinct sulphur-isotopic composition and requires a further, geographically restricted, water source (Pore fluid 3). Celestine-bearing concretions contain water in tight cavities whose isotopic composition is close to that of modern precipitation. Its chemistry shows that it is equilibrating with pre-existing minerals implying a relatively recent origin (Pore fluid 4). The mineralogy of the Oxford Clay concretions shows that complex results can follow from a simple burial and uplift history, and that multiple generations of pore fluids can pass through a low-permeability clay.

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