DTI Strategic Environmental Assessment Area 4 (SEA4) : continental shelf seabed geology and processes

This review presents a summary of the published data and their interpretations derived from the continental shelf seabed in the mature oil and gas areas of the West Shetland Shelf occurring to the north and west of the British Isles. The basis for this review is the premise that variations in the seabed geology will influence variations in the modern seabed habitat. The purpose is to review the seabed geomorphology, near-bottom currents, types of rock outcrop, variations in unconsolidated sediment texture, the variety and distributions of seabed bedforms and selected aspects of sediment inorganic geochemistry. It is intended that the review will provide a basis for a better understanding of conditions in the modern seabed environment and for possible future scenarios of strategic environmental interest. The large-scale physiography of the inner-to-middle continental shelf has originated from varying degrees of rock and unconsolidated sediment resistance to the effects of erosion by icesheets during several glaciations since approximately 1.1Ma. Although ice-sheet erosion has also locally dissected the outer continental shelf, other variations in shelf physiography are identified with bedforms that were generated when sediments were deposited as the former ice sheets advanced and retreated across the continental shelf. There is little modern sediment input to the continental shelf. Thus the modern seabed environment now largely reflects the effects of reworking by near-bottom currents on the topography and the sediments that originated during the glaciations. The reworked sediments typically consist of large areas of relict seabed gravel on elevated topography, mobile sheet sand deposits on relatively flat seafloor and muddy sands and sandy muds in the sheltered coastal and nearshore areas and in basins on the open continental shelf. Rock crops at seabed and just below seabed in the areas of the inner and middle continental shelf that had previously undergone the strongest sub-ice erosion. Tidal sand banks, tidal sand ridges and fields of migrating sandy bedforms typically form in water depths ranging from 20-100m or more and in the areas that are prone to the strongest wave and tide generated near-bottom currents. These bedforms and surrounding seabed areas locally consist of more than 60% shell fragments derived from the prolific post-glacial benthic biota in the nearshore environments. Sedimentary sinks are typified by high metal values associated with fine-grained sediments and clay minerals, metal oxides and hydroxides and organic compounds. The anomalous values of zinc, for example, occur in sheltered basins adjacent to land but also in discrete areas on the outer continental shelf.