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Silicifications and associated clay assemblages in the cretaceous marine sediments of southern england

Published online by Cambridge University Press:  09 July 2018

C. V. Jeans
C. V. Jeans*
Affiliation:
Department of Applied Biology, University of Cambridge
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Abstract

Silicifications and smectite-dominated clay assemblages are associated in the marine Cretaceous sediments of England. Both are neoformed, and have developed at approximately the same time in the host sediment. Two silicification types, opal-CT and α-quartz, have a distribution pattern unrelated to the stratigraphy or lithology of the Lower Greensand, Upper Greensand and Lower Chalk. The petrography of five examples suggest that the two silicification types were both precipitated from the porewaters of their host sediment, and their silica mineralogy and their associated silicate minerals were controlled generally by the silica concentration of the porewaters and more locally by the pH gradients.

Résumé

Résumé

Des silicifications et des assemblages des mineraux argileux à prédominance smectite sont associées aux sédiments marins cretacés d'Angleterre. Les deuxsont néoformées et se sont développées approximativement en même temps que le sédiment hôte. Deux types de silicifications, opal-CT et α-quartz, ont un modèlè de distribution sans rapport à la stratigraphie ou lithologie du Lower Greensand, du Upper Greensand et du Lower Chalk. La pétrographie de cinq exemples fait penser que les deux types de silicification ont tous deux été précipités à partir des eaux interstitielles de leur sédiment hôte, et la minéralogie de leur silice et leurs minéraux silicates associés ont été généralement contrôlés par la concentration en silice des eaux interstitielles et plus localement par les gradients de pH.

Kurzreferat

Kurzreferat

In den marinen kretazischen Sedimenten von England sind silifizierte und mit Smektit angereicherte Schichten vergesellschaftet. Beide sind Neubildungen, die sich etwa gleichzeitig in den Ursprungssedimenten entwickelt haben. Zwei der Silifizierungsphasen, Opal-CT und α-Quarz haben ein Verteilungsmuster, das nicht mit der Stratigraphie oder Lithologie des Lower Greensand, Upper Greensand und Lower Chalk in Beziehung gebracht werden kann.

Petrographische Untersuchungen von fünf Proben lassen vermuten, dass die zwei Silifizierungsphasen über die Porenwässer des betreffenden Sedimentsgesteins ausgeschieden wurden, wobei der jeweilige Anteil an SiO2-Phasen und den mit ihnen vorkommenden Silikaten allgemein durch die SiO2-Konzentration der Porenwässer und mehr ortsbezogen durch den ph-Gradienten kontrolliert wird.

Resumen

Resumen

En los sedimentos cretáceos marinos de Inglaterra hay asociados conjuntos de silicificaciones y arcilla esmectética. Ambas son neoformaciones y se han desarrollado aproximadamente al mismo tiempo en el sedimento horspedante. Dos tipos de silicificación, ópalo-CT y α-cuarzo, tienen una forma de distribución que no guarda relación con la estratigrafía ni la litología de la arena Lower Greensand y Upper Greensand y la Lower Chalk. La petrografía de cinco ejemplos sugiere que los dos tipos de silicificación se precipitaron, ambos, de las aguas intersticiales de su sedimento hospedante, y su mineralogía del sílice y los silicatos con ellos relacionados estuvieron controlados en general por la concentración de sílice de las aguas intersticiales y, más localmente, por los gradientes del pH.

Type
Research Article
Information
Clay Minerals , Volume 13 , Issue 1 , March 1978 , pp. 101 - 126
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1978

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