D. Long

The Storegga Slides: Evidence from eastern Scotland for a possible tsunami

Abstract The Second Storegga Slide on the continental slope off western Norway has been dated at between 8000 and 5000 yrs B.P. A prominent sand layer in Flandrian (Holocene) deposits along the coast of eastern Scotland, and dated at approximately 7000 yrs B.P. may have been deposited by a tsunami generated by the slide. The altitude and stratigraphy of the layer allow estimates to be made of the magnitude of the earthquake which initiated the slide.

Late Quaternary palaeontology, sedimentology and geochemistry of a vibrocore from the Witch Ground Basin, central North Sea

Abstract Results are detailed from analytical procedures carried out on a single closely sampled 5.7 m long vibrocore from the Witch Ground Basin, central North Sea. Information about the environmental conditions during the deposition of the Witch Ground Formation, of late Weichselian to Holocene age, was obtained by combining the various palaeontological, sedimentological and geochemical results. This has suggested that a complete glaciomarine to marine sequence from before 13,000 to after 10,000 B.P. is present in the central North Sea. An ash layer equated with the Vedde Ash of western Norway has been identified for the first time in the UK sector.

A relict ice-scoured erosion surface in the central North-Sea

Abstract The interpretation of shallow seismic records from the central North Sea has revealed the existence of an irregular erosion surface within late Pleistocene sediments. A morphological study of this surface has identified two main types of relief: (1) an irregular, rough topography with depressions varying in depth from 1 to 6 m, and in width from 25 to 300 m; and (2) a much smoother topography with relatively few depressions. On a palaeobathymetric map the rough topography extends from ca. 110 to 160 m below sea level (OD), while the smoother topography extends beyond the 160 m below OD contour. This surface is interpreted as an ice-scoured erosional feature formed by the grounding of sea ice in a shallow shelf environment. The stratigraphic position of the ice-scoured surface shows it to be a relict late Weichselian feature formed at ca. 17,000–18,000 B.P.

Slope Failures in the Faroe — Shetland Channel

Submarine slides varying from 0.002km3 to more than 360km3 have been identified in the Faroe — Shetland Channel using a wide range of surveys. Although not a major constraint to seabed use slides need to be considered as a potential geohazard. Dynamic loading of contouritic horizons is considered the triggering mechanism of these thin layer failures.

Tsunamis in the Norwegian Sea and North Sea Caused by the Storegga Submarine Landslides

Giant submarine landslides in the Storegga area on the continental slope west of Norway took place on at least three occassions during the Late Quaternary. This paper provides a summary of present knowledge regarding tsunamis generated as a result of the Storegga Slides. Most attention, however, is given to the tsunami generated by the Second Storegga Slide that took place circa 7,000 years ago. The tsunami generated by this landslide is believed to have struck most coastlines bordering the eastern North Atlantic. The paper summarises the geological evidence for the former occurrence of this tsunami. These results are compared with the results of recent mathematical modelling of the landslide and tsunami. Remarkably, there is relatively good agreement between estimates of tsunami run-up derived from the sedimentary evidence and run-up values obtained from the modelling experiments.

A Record of Mid-Cenozoic Strong Deep-Water Erosion in the Faroe-Shetland Channel

The Faroe-Shetland Channel is a narrow, elongate, deep-water basin located on the continental margin between northern Britain and the Faroe Islands (Fig. 1). To the NE, the channel widens and slopes down into the Norwegian Basin; to the SW, it turns northwestwards and continues as the Faroe Bank Channel, which joins the Iceland basin.Water depths in the channel increase, generally, from 1,000 m in the SW to 1,700 m in the NE, with a corresponding increase in its width (at the 1,000 m isobath) from 15 to 100 km. Whilst the floor of the channel is largely smooth and gently inclined to the NE, an irregular, relict basin-floor topography is preserved at its narrower SW end. The latter consists of several erosional scarps and hollows forming localised open deeps, the Judd Deeps, that are cut into Eocene bedrock (Stoker 1999) to depths in excess of 1,200 m. Open image in new window Fig. 1. Location and bathymetric setting (in metres) of Faroe-Shetland Channel. Boxed area in main map shows location of detailed study area depicted in Figs. 2 and 3

Sea bed morphology of the Faroe-Shetland Channel derived from 3D seismic datasets

Abstract First returns from 3D exploration surveys have been utilized to display seafloor morphology of the Faroe-Shetland Channel between the UK and the Faroes. The image combines 32 datasets creating a regional perspective of Quaternary sedimentary processes. Geomorphic information is of significance for sea bed geohazard evaluation, environmental studies and as an analogy for former sedimentary environments. The image covers more than 25000 km2 extending from the shelf (water depth ∼ 120 m) to the basin floor (water depth up to ∼ 1600 m). On any margin knowledge of the sea bed morphology is essential for understanding the environmental setting and for safe operations in deepwater. Under favourable circumstances, the sea bed can be picked from 3D exploration seismic surveys in a similar manner to any other horizon to provide detailed images of the seafloor, thereby negating the need for dedicated sea bed surveys. Combining first returns from several surveys creates a regional perspective, essential when considering importance of features e.g. the rarity of a certain seafloor environment or the presence of a potential landslide upslope from an operations area. The Faroe-Shetland Channel displays a wide range of sea bed features including, sediment waves, contourite deposits, polygonal cracking, landslides, debris flows, turbidity current channels, glacial moraines and iceberg ploughmarks. Resolving the spatial aspects of these features greatly assists the interpretation of shallow profile data for geohazard and environmental studies and provides a backdrop onto which biologists, oceanographers, sedimentologists and engineers can overlay their data sets and thus their interpretations.

Morphology of an Ice-Sheet Limit and Constructional Glacially-Fed Slope Front, Faroe-Shetland Channel

The submarine Rona Apron has built out into the Faroe-Shetland Channel as one of several Pliocene-Quaternary depocentres on the deep-water Atlantic margin north-west of Britain (Fig. 1). Fed from an ice sheet which was presumed to have extended to the shelfbreak during the late Pleistocene (Stoker 1995), the Rona Apron has line-to-arcuate sourced debris flows which are typical of a constructional slope front or apron (terminology after Galloway 1998). Open image in new window Fig. 1. Location and regional Pliocene-Holocene depositional setting of the Rona Apron. Pliocene-Pleistocene isopachytes modified after Evans (1997)

The Afen Slide — A Multistaged Slope Failure in the Faroeshetland Channel

The Afen slide submarine slope failure has comprehensive 2D and 3D seismic coverage as well as geotechnical data from sediment cores. This allows a detailed assessment of the phases, modes and characteristics of failure. These findings can be applied to the recognition and understanding of other submarine slope failures.

Methane hydrates: problems in unlocking their potential

Methane hydrates have been recovered or postulated for virtually all continental margins around the world and a few areas onshore. Volumes of about 2 × 10 14 m 3 have been estimated for this potential resource. However, only a few sites have been suggested offshore northwest Europe, despite extensive hydrocarbon exploration and academic studies of the margin. Reasons for this anomaly are unclear. To aid the search a new hydrate stability zone map for the UK is presented. As well as identifying a resource, hydrate studies are also important in assessing geohazards to deep-water exploration and development. Stability, processes and distribution information contribute to the wider climate change debate as methane hydrates are estimated to hold a significant part of the global organic carbon budget. To quantify reserve potential and to identify suitable methods of methane extraction, a full understanding of how hydrates are held within sediments is required. Although modelling (physical and theoretical) can contribute to an understanding, it is important to evaluate in situ conditions to ‘ground truth’ acoustic data and imagery. How hydrate is held and its control of dynamic geotechnical behaviour within the sedimentary system is still very poorly understood. Parameters such as pore size, fluid saturations, sediment mineralogy and cementation will affect hydrate morphology, distribution, behaviour (during dissociation) and potential recovery from porous media. Assessing physical parameters and processes under in situ conditions provides the next step along the route to exploiting methane hydrates as a resource. The requirement to recover samples under in situ pressures and temperature conditions provides a significant technological challenge that has been attempted over the last few years with some success. Currently, the European HYACINTH project is developing systems to recover, analyse and manipulate hydrate-bearing sediments under in situ pressures and temperatures. On Leg 204 of ODP this equipment was used for the first time to recover hydrate cores at in situ pressure, transfer them without loss of pressure into laboratory chambers and to log them geophysically. As the database of in situ properties grows, integrated laboratory studies of synthetic sediment-hosted hydrates can be developed to provide important benchmarking, which is crucial for the study of rare natural core samples.