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Upper Wenlock bentonites from Wren's Nest Hill, Dudley: comparisons with prominent bentonites along Wenlock Edge, Shropshire, England

Published online by Cambridge University Press:  19 April 2011

DAVID C. RAY ,
ADRIAN V. J. COLLINGS ,
GRAHAM J. WORTON  and
GAVIN JONES
DAVID C. RAY*
Affiliation:
School of Earth & Environmental Sciences, University of Portsmouth, Burnaby Building, Burnaby Road, Portsmouth, PO1 3QL, UK Neftex Petroleum Consultants Ltd, Abingdon, OX14 4RY, UK
ADRIAN V. J. COLLINGS
Affiliation:
Arup Geotechnics, The Arup Campus, Blythe Valley Business Park, Solihull, B90 8AE, UK
GRAHAM J. WORTON
Affiliation:
Dudley Museum & Art Gallery, St James Road, Dudley, West Midlands, DY1 1HU, UK
GAVIN JONES
Affiliation:
248 The Broadway, Dudley, West Midlands, DY1 3DN, UK
*
†Author for correspondence: [email protected]
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Abstract

The upper Wenlock Series (Homerian Stage) of the northern Midland Platform, England, contains numerous volcanic bentonite clay layers. At Wren's Nest Hill, Dudley, 15 bentonites have been investigated and comparisons with the type-Wenlock have been made by means of two key sections along Wenlock Edge, Shropshire. In total 22 bentonites have been investigated and their clay and sand-grade mineralogies determined. Rare earth element (REE) and yttrium concentrations of apatite grains contained within ten of the bentonites have been established allowing geochemical fingerprinting as an indication of provenance of source magmas and identification of geochemical marker beds. Based on the analysis of REE and yttrium concentrations it seems likely that the majority of these bentonites originated from a granodiorite magmatic source. Comparisons with published Llandovery and lower Wenlock age bentonites indicate generally more enrichment in light REEs relative to heavy REEs. In addition, close geochemical similarities between bentonites along Wenlock Edge and at Wren's Nest Hill strongly argue for their presence as precise stratigraphic equivalents within the upper Much Wenlock Limestone Formation. These correlations are further supported by geophysical data from borehole wire-line logs across the West Midlands. Finally, a chemically distinct mid-Homerian episode of volcanism is identified and represents a potentially important marker interval between the study area and other similarly aged bentonites reported from the Island of Gotland, Sweden.

Keywords

apatitebentonitescorrelationrare earth elementsSilurianWenlock
Type
Original Article
Information
Geological Magazine , Volume 148 , Issue 4 , July 2011 , pp. 670 - 681
Copyright
Copyright © Cambridge University Press 2011

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