ORGANIC FACIES OF SURFACE SEDIMENTS IN THE NORTH-SEA

Surface sediments of the North Sea were investigated by organic geochemical and sedimentological methods including organic carbon and nitrogen determination, Rock-Eval pyrolysis, protokerogen microscopy, gas chromatography of aliphatic hydrocarbons, grain-size fractionation and clay mineral analysis. The total organic matter was found to be a mixture of mostly reworked or oxidized, marine and terrestrial constituents with a generally strong influence of the latter throughout the platform. Locally, improved hydrodynamic conditions yielded an incorporation of more freshly produced marine organic matter. Differences in water current velocities and turbulence also appeared to determine whether this supplement is incorporated into the fine or coarse fractions of the sediments. A general constancy of the organic carbon contents, within the individual size fractions, was recognized independently of sediment type and age, i.e. in recent and pre-recent deposits. This suggested that the latter have been effectively overprinted by the shelf processes operating at present, or that these processes have been of comparable intensity since about the beginning of the Holocene transgression. The major factors controlling the spatial variations in the organic facies are the sediment grain-size and the present-day hydraulic regime. © 1990.