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

• Published in: Geological magazine 2020-07, Vol.157 (7), p.1181-1198
• Main Authors: Mullineux, S, Sparks, RSJ, Murphy, MD, MacNiocaill, C, Barfod, D, Njorka, J, Schumacher, JC
• Format: Article
• Language: English
• Subjects: absolute age; Anatase; Apatite; Ar/Ar; Bentonite; Carboniferous; chemical composition; clastic rocks; Clay; Clay minerals; columnar joints; contact metamorphism; dates; Derbyshire England; diabase; Dimensions; electron microscopy data; electron probe data; England; Europe; Ferric oxide; fractures; Geology; Great Britain; Haematite; heating; Hematite; High level radioactive wastes; high-level waste; igneous and metamorphic rocks; igneous rocks; Illite; Illites; intrusions; Joints (timber); Laboratories; Lower Carboniferous; Mesozoic; Metamorphism; Metamorphism (geology); mineral composition; Minerals; Montmorillonite; Montmorillonites; Original Article; Oxidation; Palaeomagnetism; Paleomagnetism; Paleozoic; Petrology; plutonic rocks; prograde metamorphism; protoliths; pyroclastics; Quarries; radioactive waste; Radioactive waste disposal; Radioactive wastes; Rocks; sedimentary rocks; SEM data; sills; Smectites; spectra; Tideswell Dale; Triassic; tuff; United Kingdom; Volcanic activity; Volcanic ash; volcanic rocks; Volcanism; waste disposal; Western Europe; X-ray diffraction data; X-ray fluorescence spectra
• ISSN: 0016-7568; 1469-5081
• DOI: 10.1017/S0016756819001535

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.