The Posidonia Shale in the Dutch Offshore: An Optical, Electron Optical and Geochemical Study
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The Posidonia Shale Fm in the Dutch Central Graben is well known for its high concentration of orgnic matter. The preservation of organic matter previously have been explained by permanent anoxic conditions and restricted water conditions. Recent studies have shown, however, that low energy settings or quiet water conditions and the subsequent suspension settling are not a prerequisite for deposition of muds and shales, but that depositional energy condition are commonly sufficiently high to rework and advect the sediment. In order to effectively determine the lithofacies variability within a succession it is important to make observations of grain-size, determine the origin of individual components, and make microtextural observations of the rock. Therefore, this study uses a combination of optical, electron optical and geochemical analyses of unusually thin (< 20 µm) polished thin sections to show that the Posidonia Shale Fm is deposited by multiple depositional mechanisms in a dynamic fine-grained siliclastic environment. The thin section analyses shows that the Posidonia Shale Fm is characterised by a stacked succession of mud-sized dominated thin beds often characterised by three-fold internal stratigraphy. These thin beds have an erosional base with on top a silt-dominated unit which is either gradually fining upwards or arranged in low angle laminae. The second unit consists of lamina sets of clay and silt dominated layers which are either horizontal or curved and vary from continuous to discontinuous. The uppermost unit, which is predominantly recognised in the upper part of the Posidonia Shale Fm, is homogeneous and clay dominated. Furthermore, the upper part of the Posidonia Shale Fm is characterised by carbonate micro concretions and a high abundance of pyrite. The geochemical analyses show a concomitant negative carbon isotope excursion, increase in TOC concentration and an increasingly heavier sulphur isotope trend in the upper part of the Posidonia Shale Fm. Furthermore, redox and productivity proxies are enriched in this part of the succession. This typical vertical trend in each thin beds suggests that the sediment-water interface was located below the fair-weather wave base, but in relatively shallow water above the storm wave base. Anoxic conditions below the sediment-water interface prevailed in the lower part of the Posidonia Shale Fm and extended to anoxic to sulphidic bottom water conditions in the upper part of the Posidonia Shale Fm. The three-fold internal stratigraphy, however, shows that these conditions were interrupted by short events, possibly by storm induced wave activity, and the subsequent inflow of oxygen-rich surface waters. The Posidonia Shale Fm is, thus, deposited in a dynamic fine-grained siliclastic environment with an alternation of anoxic and short-term oxic conditions.