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Geological Society, London, Special Publications; 1991; v. 58; p. 381-395;
DOI: 10.1144/GSL.SP.1991.058.01.24
© 1991 Geological Society of London

Ancient Shelf Anoxia

Distribution, dynamics and palaeoecology of Kimmeridgian (Upper Jurassic) shelf anoxia in western Europe

Wolfgang Oschmann

Institut für Paläontologie Universität Würzburg, Pleicherwall 1, D-8700 Würzburg, Germany

Facies and related macrobenthic associations in the Kimmeridgian epeiric basin in Western Europe are arranged along an onshore-offshore gradient. It ranges from an intermediate-to-high-energy nearshore zone to a low-energy oxygen-controlled offshore environment.

Anoxia in the Kimmeridgian epeiric basin was not a permanent feature and cannot be described by the classical models which define aerobic, dysaerobic and anaerobic facies zones. These models have been proposed for deep (slope and basin) environments, in which the oxygen values at a certain depth remain in general constant. In contrast, anoxia in the Kimmeridgian is more dynamic and resembles modern anoxia occurring in eutrophic shelf and epeiric basins (e.g. the coastal areas of Mid-Atlantic Bight, North Sea and Baltic Sea) and exhibit remarkable annual oxygen fluctuations.

Therefore, in contrast to the classical models, a modified oxygen zonation for shelf environments is introduced, in order to explain the palaeoenvironment and dynamic alternation of Kimmeridgian anoxia. Both end members, the aerobic and the anaerobic environment, do occur as well. However, the short-term fluctuations prevent establishment of the dysaerobic zone. Instead, there is a seasonal anaerobic to anoxic environment, here defined as poikiloaerobic zone, which is colonized only by a few opportunists.





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