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Age, origin and climatic signal of English Mesozoic clays based on K/Ar signatures

¹ Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, ² 29 Greystoke Park, Gosforth, Newcastle upon Tyne NE3 2DZ, ³ Nevis Associates Ltd., Helensburgh, Argyll and Bute, G84 8DD, and ⁴ School of Earth Sciences, University of Greenwich, Pembroke, Chatham Maritime, Kent ME4 4AW, UK

^* E-mail: SJL11{at}esc.cam.ac.uk

(Received 3 January 1999; revised 20 March 2001)

The K/Ar characteristics of 53 clay assemblages (Triassic–Cretaceous), representing the detrital, volcanogenic and arid-facies clay mineral associations, are interpreted in relation to their mineralogy, chronostratic age and geological origins. The K-bearing mineral components of the 1–2 µm, 0.2–1 µm and <0.2 µm fractions of each clay assemblage together display one of two characteristic patterns of K₂O and ⁴⁰Ar values (the K/Ar signature of the assemblage) on a ⁴⁰Ar/K₂O correlation diagram. Interpretation of the K/Ar signatures indicates that: (1) all of these clay assemblages are apparently unaffected by burial diagenetic illitization; (2) the Jurassic and Cretaceous detrital clay assemblages are derived from the reworking of weathered Caledonian metasediments (420–500 Ma) and weathered kaolin-bearing sediments of Upper Devonian/Carboniferous age; and (3) the role played by palaeoclimate in developing the pattern of clay minerals in the Mesozoic sediments of England is much less significant than previously believed.

KEYWORDS: K/Ar signature, climatic signal, clays, England, Triassic, Jurassic, Cretaceous

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