Abstract
Palaeoecology and gamma-ray spectrometry have been combined to study the controls on deposition of two well-known Jurassic organic-rich mudrocks. Portable gamma-ray spectrometry allows the rapid measurement of K, U and Th in the field. Th and K are contained mainly in the detrital clay fraction of the mudrocks and the Th/K ratio is found to decrease slightly with increasing water depth and distance from shoreline. Measured values of U have a detrital and an ‘authigenic’ component. Sediments deposited under anoxic conditions are consistently enriched in ‘authigenic’ U. Non-detrital ‘authigenic’ U has been estimated by the relative enrichment of U over Th, which is assumed to be immobile and present entirely in the detrital fraction.
The Kimmeridge Clay represents a several-million-year period of organic-rich mudrock deposition in which climate-induced stratification of the water column was the predominant control on the formation of the different lithologies. Sea level controlled the position of storm wave-base relative to the substrate and thus the frequency of oxygenation events during shale deposition. Coccolith limestones contain up to 10 ppm ‘authigenic’ U and represent the most anoxic sediments.
The lower Toarcian ‘anoxic event’ coincides with a period of rising sea level and transgression. The maximum ‘authigenic’ U value of 8.2 ppm was recorded 4.5 m above the base of the Jet Rock member and represents the most intense period of anoxia.
Organic carbon correlates well with ‘authigenic’ U in the lower Toarcian where the degree of bottom water anoxia appears to be the main control on organic carbon preservation. The organic carbon/‘authigenic’ U correlation is poor in the Kimmeridge Clay, where sedimentation rate and surface productivity are additional influences on organic carbon preservation. U/organic carbon ratios in the Jurassic case-study sediments are much lower than for Palaeozoic black shales.
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© 1987 J. k. Leggett & G. G. Zuffa (Graham and Trotman)
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Myers, K.J., Wignall, P.B. (1987). Understanding Jurassic Organic-rich Mudrocks—New Concepts using Gamma-ray Spectrometry and Palaeoecology: Examples from the Kimmeridge Clay of Dorset and the Jet Rock of Yorkshire. In: Leggett, J.K., Zuffa, G.G. (eds) Marine Clastic Sedimentology. Springer, Dordrecht. https://doi.org/10.1007/978-94-009-3241-8_9
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