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    Low temperature, authigenic illite and carbonates in a mixed dolomite-clastic lagoonal and pedogenic setting, Spanish Central System, Spain

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    Authors
    Huggett, Jennifer cc
    Cuadros, Javier
    Gale, Andrew S.
    Wray, David cc
    Adetunji, Jacob cc
    Affiliation
    Natural History Museum
    University of Portsmouth
    University of Greenwich
    University of Derby
    Issue Date
    2016-06-30
    
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    Abstract
    The aim of this study was to further our understanding of the pedogenic and lacustrine modification of clay minerals. Some of these modifications are of special interest because they constitute reverse weathering reactions, rare in surface environments, and because there is not yet an accurate assessment of their global relevance in mineralogical and geochemical cycles. For this study, two sections from the Central System in Spain were selected. Both are sections through the Uppper Cenomanian-Turonian mixed clastic and carbonate succession, containing both calcite and dolomite, in the Sierra de Guadarrama. Mid-Turonian sea level fall resulted in the formation of a coastal plain environment in which extensive pedogenesis occurred around saline lagoons. The mineralogical changes that have occurred as a result of sedimentation in saline lagoons and as a consequence of pedogenesis are described. Textural relationships indicate that the dolomite cement pre-dates the calcite. Silicate minerals are represented by quartz, kaolinite, illite-smectite, illite, minor plagioclase and alkali feldspar, and trace chlorite and palygorskite. There is a positive correlation between the intensity of pedogenesis and the proportion of illite in the clay assemblage in one of the sections, indicating pedogenic illitisation. In this section, the intensity of the illitisation process increases up, reaching a maximum where pedogenesis is most intense in the middle part, and then decreases as marine influence increases towards the top of the Alcorlo Formation and the overlying marine Tranquera Formation. The clay assemblages are consistent with a slow transformation process from kaolinite to illite by way of illite-smectite, taking place under surface conditions. The illitisation process has resulted in a less Fe-rich, more Mg-, and Al-rich illite than the majority of previously documented cases in the near surface. Formation of Al-rich illite is not therefore restricted to the deep subsurface. The mechanism for low temperature illitisation involves enhanced layer charge resulting from Mg2 + substitution for Al3 + (or Fe3 +) and Fe3 + to Fe2 + reduction. Mg2 + enrichment may have occurred principally in saline lagoons or lakes, while Fe3 + to Fe2 + reduction occurred as a result of wetting and drying in a pedogenic environment. So far as it has been possible to establish, this dual mechanism has not previously been documented. This study indicates clearly that the dolomite and calcite are authigenic cements that precipitated in a clastic sediment, probably soon after deposition. Dolomitisation and Mg enrichment of the clay may have occurred at the same time. Seawater is the most probable source of Mg.
    Citation
    Huggett, J. et al (2016) 'Low temperature, authigenic illite and carbonates in a mixed dolomite-clastic lagoonal and pedogenic setting, Spanish Central System, Spain', Applied Clay Science, 132-133:296
    Publisher
    Elsevier
    Journal
    Applied Clay Science
    URI
    http://hdl.handle.net/10545/621566
    DOI
    10.1016/j.clay.2016.06.016
    Additional Links
    http://linkinghub.elsevier.com/retrieve/pii/S0169131716302642
    https://gala.gre.ac.uk/id/eprint/15611/
    Type
    Article
    Language
    en
    ISSN
    1691317
    ae974a485f413a2113503eed53cd6c53
    10.1016/j.clay.2016.06.016
    Scopus Count
    Collections
    Environmental Sustainability Research Centre

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