Mesozoic marine organic-rich shales

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Abstract
Summary: A survey of Mesozoic organic-rich shales in space and time indicates that they are widely distributed through much of the era but are concentrated especially at two levels: Upper Jurassic (Kimmeridgian-Volgian/Tithonian) and Middle Cretaceous (Aptian-Turonian). Stable carbon isotope studies prove to be a particularly useful tool in both palaeoenvironmental and diagenetic analysis. Anoxic events in the ocean may be characterized by high concentrations of δ 13 carbonate carbon. The carbon isotope values obtained may, however, reflect diagenesis in the sulphate reduction zone rather than conditions in the original sea water. Calcite concretions formed at successive stages in diagenesis may record a change from strongly negative to strongly positive values of δ 13 C, indicating a switch from the sulphate-reduction to the fermentation zone. Analysis of benthic palaeoecology leads to a classification of marine mudrocks into barren laminite, shelly laminite, shelly shale and shelly mudstone, of which the last lacks any significant quantity of kerogen. By analogy with Recent sediments an attempt is made at a tentative estimate of the oxygen content at the sediment/water interface during the original deposition. Environments of deposition are considered in general terms and the respective merits of several facies models for the Mesozoic deposits are briefly discussed.