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Geological Society, London, Special Publications; 1991; v. 58; p. 363-379;
DOI: 10.1144/GSL.SP.1991.058.01.23
© 1991 Geological Society of London

Ancient Shelf Anoxia

The Lower Oxford Clay: production and preservation of organic matter in the Callovian (Jurassic) of central England

J. D. Hudson1 & David M. Martill2

1 Department of Geology, University of Leicester, Leicester LE1 7RH, UK
2 Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, UK

The Lower Oxford Clay of the English Midlands is a thermally immature, organic-rich mudrock with a rich and well preserved invertebrate and vertebrate fauna. Biological productivity in the water column was high, as inferred from abundant cephalopods and the presence of large predatory marine reptiles and giant plankton-feeding fish. The sea floor was never anoxic but remained mostly dysoxic, resulting in a dysaerobic benthic biofacies. The ‘soupy’ substrate was inhabited by deposit-feeding bivalves, as well as gastropods and arthropods. The skeletons of large vertebrates at times provided ‘islands’ for the attachment of sessile benthos. Aragonite is well preserved, allowing isotopic palaeotemperature estimates of around 15°C for the bottom waters. Abundant pyrite formed within and beneath a shallow zone of bioturbation. At certain horizons early diagenetic concretions formed, preserving the original, pelletal, sediment texture, and three-dimensional fossils. Net sediment accumulation rates were extremely slow, around 1 to 15 mm of compacted sediment per 1000 years; sedimentation must have been episodic. Some diastems are represented by shell-beds, but others are cryptic. Nutrient supply is thought to be mainly from a well vegetated, swampy hinterland. Variations in bottom water oxygenation, sedimentation rate and substrate consistency can be inferred from the subtle facies variations within the Lower Oxford Clay, and controlled the accumulation of organic matter.





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