DTI Strategic Environmental Assessment Area 8 : superficial seabed processes and hydrocarbon prospectivity

The SEA8 area lies along the southern margin of the UK continental shelf, extending from the inner English Channel to the shelf break at ~200 m water depth. The main physiographic areas within SEA8 are the Bristol and English channels, the Western Approaches and the Celtic Sea. The Bristol Channel forms a major re-entrant in the north of the area with water depths up to 50 m. The English Channel extends eastward to the southern margin of the North Sea. The Western Approaches and Celtic Sea are mainly planar and slope gently southwestward, although on their southwest margins below 120 m lie a series of Tidal Sand Ridges (TSR’s) up to 50 m high, oriented orthogonal to the shelf edge. Sedimentary Processes The present understanding of the sedimentary processes operating within SEA8 has been developed over decades of research. The area has been relatively stable since the midlate Tertiary when a period of erosion resulted in a major unconformity surface upon which the later Quaternary sediments were deposited. During the Quaternary, the area was located on the southern margin of the continental ice sheets that dominated the environment of northwest Europe for the past 1.5 million years. During periods of maximum ice advance, sea level was lowered by up to 120 m, thereby exposing the present seabed to terrestrial conditions. Although the Bristol Channel was ice covered, to the south lay outwash plains on which glaciofluvial sediment was laid down. Major river channels were eroded by the Meuse and Seine as they flowed to the sea. At the end of the glacial periods, as the ice retreated and sealevel rose, the glacial sediments were reworked. On the shelf edge this resulted in the formation of the large tidal sand ridges, 200 km long and up 50 m metres high. Where the sand ridges are absent, winnowing of the (mainly) periglacial sediment resulted in the formation of a coarse-grained residual sediment deposit, termed Layer B. Once the post glacial sea level had stabilised, the sedimentary regime across most of the area became dominated by tidal induced seabed stress and the present mobile sediment layer, termed Layer A was formed by the winnowing of glacial deposits. On the shelf edge although tidal stresses dominate there is an influence on sedimentation from wind driven waves. The SEA8 area is at present sediment starved, there is little fluvial input. In the most highly stressed seabed environments, exposed bedrock and the unsorted gravelly, sandy and muddy sediments of Layer B are swept clean of their finer grained components. Parts of the seabed in these swept areas may consist of cobbles and boulders. Environments of least seabed stress are characterised by fine-grained muddy sediments. Where there is active sediment movement, mobile sand bodies have formed. The specific form of the bedforms is due to the tidal velocity in association with the grain size and volume of sediment available for transport. Generally speaking, these areas are located between the areas of extremely high seabed stress and the very low seabed stress represented by the mud belts in the northwest. The sense of regional seabed sediment transfer is from and across areas of high seabed stress to areas of lower seabed stress. Thus we find regions where sediment is swept clear in the inner English and Bristol channels, from where seabed sediment fines westward towards the shelf edge. The only muddy areas are located either in semi-enclosed bays or in the northwest margin of SEA8. The overall sediment starved nature of the area has resulted in a complex interdigitation between the sediments laid down during the post-glacial sealevel rise (TSR’s and Layer B) and those resulting from the present day sedimentation regime (Layer A). Based on the results from the Irish Sea we have attempted to subdivide the SEA8 area into marine habitats based on sediment grain size, seabed stress (as interpreted from maximum spring tide velocity) and bedform type. This subdivision applies to the largest areas identified but, in the absence of biological data, together with more detailed sedimentary data, does not allow the identification of smaller, and possibly more vulnerable habitats, that may require particular management to ensure survival when development takes place (e.g., as in the Irish Sea). In the coastal regions, the detailed data for habitat classification may be more commonly available, but this needs to be considered in more focussed habitat studies that are beyond the scope of this report. Coastal Coastal processes may be considered on the regional scale, but it is the local scale that is important in the context of environmental assessment. General subdivisions maybe made on the regional coastal differences in the SEA8 area, such as those based on the geological control on coastal morphology. In addition to the geological control on the coastal morphology, the coastal morphology influences the type of waves impacting the coast. Sediment movement too reflects broader-scale regionality with, for example, the four sediment cells identified along the south coast. Although our understanding of the processes acting on the coastal zone is good in general it is poor in detail. This reflects the application and scale for which knowledge and understanding are required, i.e, in the human context over shorter timescales. The main requirement is for knowledge of coastal processes that are required to underpin the design of coastal protection.