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Ordered illite-smectite and kaolinite-smectite: pedogenic minerals in a Lower Carboniferous paleosol sequence, South Wales?

Published online by Cambridge University Press:  09 July 2018

D. Robinson
Affiliation:
Department of Geology, Wills Memorial Building, University of Bristol, Queen's Road, Bristol BS8 1RJ
V. P. Wright
Affiliation:
Department of Geology, Wills Memorial Building, University of Bristol, Queen's Road, Bristol BS8 1RJ

Abstract

Mixed-layer clay minerals of illite-smectite, having smectite contents ∼25% and IS ordering, kaolinite-smectite, and a three-component variety with kaolinite-smectite-chlorite-like material are described from a Carboniferous paleosol in South Wales. The clays, which are restricted to particular horizons, are unusual occurrences in a pedogenic sediment and pose particular problems as to their genesis. Positive geological evidence, principally in the form of mineralogical and organic thermal maturation data, demonstrates that the rocks of the region cannot have reached diagenetic temperatures > 100°C. This would appear to suggest a pedogenic rather than a diagenetic origin for these clays. In these sediments the only plausible mechanism of illitization to produce the illite-smectite would be by repeated wetting and drying cycles causing irreversible K fixation. Such cycles would be entirely realistic for a vertisol-like paleosol formed on overbank deposits in a floodplain environment subject to strong seasonal moisture contrasts. If this interpretation is correct, this illite-smectite represents the most illitic yet recorded from a non-burial or non-hydrothermal environment.

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1987

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