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Ancient Shelf Anoxia |
Institute of Petroleum and Organic Geochemistry (ICH-5), Research Centre (KFA) Jülich, Postbox 19 13, D-5170 Jülich, Germany
1 Department of Geology and Geophysics, Rice University, Houston, Texas 77251, USA
The Posidonia Shale from northwest Germany displays a threefold stratigraphic subdivision: lower marlstone, middle calcareous shale with bivalve shells, and upper calcareous shale. Deposition commenced after transgression over a disconformity on the Pliensbachian. Absence of current features indicates depositional conditions below wave base. Sediment infilling to water depths coincident with wave base probably caused cessation of Posidonia Shale deposition.
Calcitic carbonate, organic matter and detrital clay are principal primary components. TOC values are 2–15%. Macerals are mostly from marine phytoplankton; terrigenous macerals are minor. HI values are high (500–800). Detailed sampling reveals a thin (4.5 m) low carbon zone within the middle shale; water column oxidation and bacterial alteration may account for this facies. Clay and organic matter relationships suggest that land-derived nutrients controlled phytoplankton productivity; carbonate constituents were a diluent to organic matter accumulation.
Absence of burrowing, abundance of organic matter, and pervasive diagenetic pyrite demonstrate continual anoxia below the sediment-water interface. However, C/S relationships suggest that a euxinic water layer did not extend far above the depositional surface. The occurrence of benthic bivalves in the middle shale indicates that bottomwater tolerable to epibenthos sometimes prevailed. Sulphate-reducing bacteria modified the primary organic matter causing a 20–30% loss.
