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Origin of the clay mineral assemblages in the Germanic facies of the English Trias: application of the spore colour index method

Published online by Cambridge University Press:  09 July 2018

C. V. Jeans ,
M. J. Fisher  and
R. J. Merriman
C. V. Jeans*
Affiliation:
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
M. J. Fisher
Affiliation:
Nevis Associates Limited, 34 West Argyle Street, Helensburgh, Dumbartonshire G84 8DD, UK
R. J. Merriman
Affiliation:
British Geological Survey, Keyworth, Nottinghamshire NG12 5GG, UK
*
*E-mail: [email protected]
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Abstract

The origin of the regional and stratigraphical variation in the Triassic authigenic clay assemblages of England is discussed in relation to new estimates of the palaeotemperatures experienced by their host sediments and a preliminary study by transmission electron microscopy of their microtextural features. Spore colour index measurements, based on the spore type Deltoidospora s.l. occurring in the sediments (Penarth Group) at the very top of the Triassic sequence, give estimated palaeotemperatures ranging from 60–74°C (south Devon) to 89–97°C (northeast Yorkshire). Calculated palaeotemperatures, based on a gradient of 25°C/km, for the main zone of authigenic clay minerals range from 63–77°C to 89–97°C for the top to 71–85°C to 94–104°C for the base. Irregular mixed-layer smectite-chlorite, corrensite and Mg-rich chlorite are associated with calculated palaeotemperatures of 66–86°C, 66–104°C and 75–104°C respectively. The suggestion that elsewhere in the UK corrensite and Mg-rich chlorite were formed at temperatures in excess of 100°C finds no support. Geothermal gradients would have to have been of the order of at least 100–300°C/km to obtain these temperatures within the Triassic sediments; such values are associated typically with high-level magmatic intrusions or geothermal systems of which there is no geological evidence. The balance of evidence suggests that the Triassic authigenic clay assemblages formed by neoformation during the early stages of sediment diagenesis under the influence of variation in the alkalinity of the depositional environments.

Keywords

Triasauthigenicclay assemblageEnglandspore colour indexGermanic facies
Type
Research Article
Information
Clay Minerals , Volume 40 , Issue 1 , March 2005 , pp. 115 - 129
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2005

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