Stratigraphic distribution and palaeo-oceanographic significance of European early Palaeozoic organic-rich sediments

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Abstract
Summary: Marine, organic-rich sediments of early Palaeozoic age are thicker, and were deposited over longer time-periods, than those of other Phanerozoic intervals. They occur widely throughout North and West Europe. Their palaeogeographic setting is less certain than those of their Mesozoic and Caenozoic counterparts, making genetic models difficult to construct. The stratigraphic distribution of organic-rich sediments in the area from the Ural Mountains to Portugal is briefly reviewed for the Cambrian, Ordovician and Silurian Systems. The occurrence of organic-rich sediments in different palaeoenvironments (shelf, slope, basin) is compared for the three palaeocontinents Gondwanaland, Baltica and Laurentia, using the reconstructions of Cocks and Fortey (1982). Organic-rich sediments are shown to be more widely distributed at certain times: the middle and late Cambrian, the Caradoc and the Llandovery-Wenlock intervals. Two areas are selected for more detailed discussion: the Middle and Upper Cambrian Alum Shales of Scandinavia, illustrative of shelf organic-rich sedimentation and Caradoc-Llandovery sediments of the British Isles, which include common black shales deposited in varying abyssal to shelf environments. Possible controls on black shale deposition include variations in sea level stand, degree of geographical restriction, climatic variation, oxygen levels in the atmosphere and hydrosphere, variations in organic productivity and sedimentation rates. Our ability to recognize the relative effects of these controls in the past is variable. Where information is sufficient, it suggests that high sea-level stands associated with warm climatic conditions, and low hydrospheric oxygen contents are important in the development of early Palaeozoic organic-rich sediments.