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Berthierine from the non-marine Wealden (Early Cretaceous) sediments of south-east England

Published online by Cambridge University Press:  09 July 2018

K. G. Taylor*
Affiliation:
Postgraduate Research Institute for Sedimentology, Reading University, Whiteknights, Reading, RG6 2AB, UK

Abstract

A thin (15–20 cm) Fe-rich intraformational conglomerate in the fresh- to brackish-water Early Cretaceous succession of south-east England contains pisoids and pseudo-ooids of a green clay mineral which has been identified as berthierine. The pisoids (up to 0·5 cm in size) are concentrically laminated, have complex structure, and are frequently nucleated around a sandstone fragment. They show evidence of weathering due to a change in their environment after formation. The ooids have well to moderately well developed concentric lamination, are very often multi-nucleated and do not always completely cover the nucleus. They suggest formation as a result of mineral precipitation as opposed to agitation and accretion. The climate and environment during the Wealden were suited to the formation of Fe-rich soils and it is likely that the pisoids and ooids were originally composed of iron oxyhydroxide and kaolinite, having formed in soil in the Weald Basin. They were then eroded and deposited in the base of a large scour structure as part of the conglomerate. On burial, and the attainment of low Eh conditions, berthierine formed by diagenetic transformation of the pre-existing minerals.

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1990

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