DTI Strategic Environmental Assessment Area 4 (SEA4) : geological evolution Pilot Whale Diapirs and stability of the seabed habitat

The DTI 2002 programme of new deep-water seabed multibeam and sample data acquisition in SEA4 included surveys of a field of seabed mud mounds, collectively named the Pilot Whale Diapirs. The largest of these occur over a buried anticline and they are set in sediment debris flows that originated from grounded ice and submarine landslides. Other diapirs and mud mounds are sited on and adjacent to the north-east plunging Fugloy Ridge and buried transfer fault zones within a region subject to modern earthquakes. The focus of this study is on the SW group of the five main groups of large-scale mud diapirs with seabed elevation of 30m or more above the surrounding seabed and with very complex seabed geometries. Diapiric sediment has been transferred to seabed from deep sources, in places from more than 500m below modern seabed and from strata more than 24 million years old. Interpretations of the fossil biota, sediment properties and seismic reflection profiles indicate that there is submetre scale heterogeneity in the composition and age of sediments cropping at or near seabed on the large-scale mud diapirs. Interpretations from the seismic reflection profiles and the fossil data from one site on the large-scale diapirs indicate that the large-scale mud diapirism postdates approximately 5 million years ago and might have been initiated as late as 1.1 million years ago. The evidence suggests that rapid large-scale mud diapirism is not occurring at the present day. In contrast, interpretations of the regional geological setting and the sub-seabed data indicate that there are large areas with potential for modern, active and small-scale diapirism. Reconnaissance sample surveys indicate that some of the steepest slopes on the large-scale diapirs are composed of rock and overlain by thin soft sediments with gravel at seabed. A numerical static stability model is presented that predicts the general conditions under which the modern seabed will become unstable on the large-scale diapirs. The model predicts that thin-skin seabed failures prevent thick accumulations of normally consolidated sediment on the steep flanks of the large-scale mud diapirs. These failures will contribute to the variability of the seabed substrates

Sedimentary and oceanographic responses to early Neogene compression on the NW European margin

The lower Neogene stratigraphy of the NW European Atlantic margin, from the Vøring to the Porcupine basins, is interpreted to record a discrete phase of compressional tectonism that spanned at least 8 Ma from the earliest to the early mid-Miocene. This compressional tectonism may be coeval with a local reorganisation of the NE Atlantic plate system with the transfer of the Jan Mayen micro-plate from Greenland to Europe. The compressional tectonics has resulted in a number of stratigraphic sequences of complex character bounded by regional base Neogene and intra-Miocene unconformities. These are traceable across a range of depths and record distortion of the basin margins and changes in deep-water circulation patterns. This episode of compressional tectonics has also resulted in the creation of a number of anticlinal domes along the Norwegian, Faroese and UK Atlantic margins. The stratigraphic and structural evidence are interpreted to record two stages in the development of the margin: the first being characterised by a prolonged period of regional flexure in response to the build-up of compressive stresses; the second stage is the development of anticlinal structures that led to a rapid release of stress. In the Wyville–Thomson–Faroes region, compressional deformation influenced the creation of the present-day deep-water conduit of the Faroe Bank Channel, which is interpreted to be an early Neogene syncline. Together, the Faroe Bank and Faroe–Shetland channels represent the deepest water passageway across the Greenland–Scotland Ridge. The early Neogene development of this conduit is considered to mark the onset of deep-water exchange across this oceanic gateway

Palaeoslides and other mass failures of the Pliocene to Pleistocene age along the Atlantic continental margin of NW Europe

A number of pre-Holocene submarine mass-failure events of various sizes have been recognised within the Plio-Pleistocene succession along the European margin from Lofoten to Porcupine. These may occur as canyon or debris-flow deposits, but especially as slumps and slides that may be either buried within the sedimentary column or have seabed expression. The present paper deals mostly with the larger slides and palaeoslides within the region, some of which show evidence of reactivation during the Holocene. The known geographical distribution of the palaeoslides is presented, and the best estimates for their ages proposed. The best-known zone of palaeosliding is the Storegga Slide Complex in which the most significant period of sliding was during the mid-Pleistocene and earlier. Sliding here may have been initiated during the late Pliocene, as may have been the case for the North Faroes Slide Complex. Other palaeoslides, with a variety of ages, have been identified along the European Atlantic margin from the Nyk Slide in the north to the Foreland Slide Complex and the Rockall Bank Mass Flow in the south. Several of the palaeoslides and Holocene slides are associated with glacial depocentres, for instance the North Sea, Sula Sgeir, Barra and Donegal fans. While it is speculated that there may be several causal mechanisms, most striking is the common association of the paleoslides with tectonic features, suggesting that seismic activity or other processes associated with such lineaments have had a major influence on palaeoslide distribution