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

Jeans, C. V. and Fisher, M. J. and Merriman, R. J. (2005) Origin of the clay mineral assemblages in the Germanic facies of the English Trias: application of the spore colour index method. Clay Minerals, 40 (1). pp. 115-129. DOI https://doi.org/10.1180/0009855054010160

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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 one 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.

Item Type: Article
Additional Information: Nevis Associates Limited;cml
Uncontrolled Keywords: 2005 AREP IA46 IA47 2005 P
Subjects: 03 - Mineral Sciences
Divisions: 03 - Mineral Sciences
Journal or Publication Title: Clay Minerals
Volume: 40
Page Range: pp. 115-129
Identification Number: https://doi.org/10.1180/0009855054010160
Depositing User: Sarah Humbert
Date Deposited: 09 Mar 2011 10:34
Last Modified: 23 Jul 2013 09:59
URI: http://eprints.esc.cam.ac.uk/id/eprint/1700

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