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Journal of the Geological Society; 2000; v. 157; issue.1; p. 187-200
© 2000 Geological Society of London

Paper

Carbonate concretions from the London Clay (Ypresian, Eocene) of southern England and the exceptional preservation of wood-boring communities

JENNIFER M. HUGGETT1, ANDREW S. GALE1,2 & SIAN EVANS2

1 School of Earth and Environmental Sciences, University of Greenwich, Chatham Maritime, Kent ME4 4AW, UK
2 Department of Palaeontology, Natural History Museum, Cromwell Road, London SW7 5BD, UK

Scientific editing by John Cope.

Marine driftwood bored by teredinid bivalves (‘shipworms’) is common in the London Clay Formation (Ypresian, Eocene) of southern England. The London Clay of Sheppey, Kent has yielded six genera of teredinids and is the most diverse fossil assemblage of the family known. The presence of several genera that are known exclusively from mangroves at the present day demonstrates the potential value of teredinids in the recognition of ancient mangroves. The rapid sea-floor burial of logs containing numerous individuals resulted in the borings becoming completely or partially sealed by the plug-like pallets. In this exceptional mode of preservation, valves, pseudofaeces and pallets are found in life position, and early cementation prevented compaction of the wood and borings. The bored wood acted as centres of carbonate concretion formation. Cement compositions, mineral associations and {delta}13C and {delta}18O stable isotope data indicate that most precipitation occurred in the iron reduction zone, with sulphate reduction also being an important source of carbonate for the Sheppey concretions. Fractures and their infilling cements are demonstrably contemporaneous with the cement zones in which they terminate. It is likely that rapid changes in cement chemistry are a consequence of sudden release of fluid by dewatering events, whilst more gradual shifts in chemistry were controlled be evolution through the oxic, Fe, Mn and S reduction zones. Teredinid boring linings and the cements which infill them may have recrystallized by a dissolution:reprecipitation process which has preserved the chemistry of successive cements and has probably not significantly changed {delta}18O values.


Keywords: Eocene, UK, bivalves, borings, concretions.




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