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Columnar-jointed bentonite below a Doleritic Sill, Tideswell Dale, Derbyshire, UK: formation during prograde contact metamorphism

Published online by Cambridge University Press:  12 February 2020

S Mullineux
Affiliation:
School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, BristolBS8 1RJ, UK
RSJ Sparks*
Affiliation:
School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, BristolBS8 1RJ, UK
MD Murphy
Affiliation:
Natural England, Northminster House, PeterboroughPE1 1UA, UK
C MacNiocaill
Affiliation:
Department of Earth Sciences, University of Oxford, South Parks Road, OxfordOX1 3AN, UK
D Barfod
Affiliation:
Scottish Universities Environmental Research Centre, East Kilbride, GlasgowG75 0QF, UK
J Njorka
Affiliation:
Core Research Laboratories, Natural History Museum, Cromwell Road, London, UK
JC Schumacher
Affiliation:
Department of Geology, Portland State University, Portland, OR, USA
*
Author for correspondence: RSJ Sparks, Email: Steve.Sparks@bristol.ac.uk

Abstract

Columnar-jointed tuffs (bentonites) are located below the Lower Carboniferous Tideswell Dale Sill, Derbyshire, in an abandoned quarry. There are three zones of prismatic joints, columnar joints (∼60 cm thick and mean column width of 4.1 cm) and massive, conchoidally fractured rock sequentially from the dolerite contact downwards. The rocks are very fine-grained (1–10 µm) and consist mainly of sanidine, interlayered illite/smectite, and hematite, with minor quartz, apatite, montmorillonite, anatase and detrital minerals. High K2O and Fe2O3, low Na2O and low MgO are interpreted as due to alteration of felsic volcanic ash. The bentonite was contact-metamorphosed by the sill to temperatures of c. 300 °C. The progression of fracture geometries, thermal considerations and application of theories of column formation indicate that columns formed downwards away from the sill, due to prograde contact metamorphism of the originally clay-rich bentonite. The formation of columnar fracture networks by shrinkage due to igneous heating may have implications for the effectiveness of bentonite as a barrier for disposal of high-level radioactive waste. Regional dimensions include documenting widespread felsic explosive volcanism and inferring a Triassic oxidation event from palaeomagnetic data.

Information

Type
Original Article
Copyright
© Cambridge University Press 2020

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