The Neogene stratigraphy of the glaciated European margin from Lofoten to Porcupine

No unified high-resolution stratigraphy exists for the Faroe–Shetland region at the present time. A number of Pleistocene seismic-stratigraphic units have been identified on the West Shetland margin (Stoker et al. 1993, but their regional extent remains unknown. As this stratigraphy has recently been summarised elsewhere (Stoker 1999) the emphasis here is on a general summary of processes and a new compilation of short-core data, located on the map opposite, from the Faroes margin (Fig. 25) that represent the last 200ka. It is hoped that the latter may provide an aid to future integrated high-resolution stratigraphic interpretation.

Both alongslope and downslope sedimentary processes have been active throughout the Faroe–Shetland region during the last 200ka, with downslope processes largely dominant during glacial intervals. The pattern does vary across the region, with alongslope deposits important to the north of 61°N outside of the North Sea Fan, where thick sediment drift accumulations occur along the lower slope. Downslope sedimentation is more dominant between 59.5o– 61°N, and also within the area of the North Sea Fan, which is fed from the Norwegian Channel. Due to the effects of the Norwegian Sea Deep Water (NSDW) currents, the basinal Faroe-Shetland Channel is an area of sediment export (Stoker et al. 2002), and this effect is exacerbated in the SW where the channel narrows and erosion by bottom currents is intensified.

Generally, the Faroe and West Shetland Margins are constructional in nature with thick debris-flow deposits forming the slope aprons. On the shelf an aggradational geometry reflects the waxing and waning of the ice sheets during the mid- to late Pleistocene, whereas the slope apron has a prograding geometry. Prograding wedges of debris flow deposits with interglacials or interstadials represented as intervening condensed sections represent periods of maximum ice advance. These features are manifest as the Rona, Foula, West Faroe and East Faroe wedges, and the North Sea Fan (see map on figure 25). The last glacial maximum (marine oxygen isotope stage (MIS) 2) marked the maximum advance of the ice sheets, which represents the last major build out of the prograding wedges. Additionally, smaller discrete fans, such as the Sandoy Fan, accumulated on the lower eastern slope of the Faroe margin.

Ice-rafted debris (IRD) and till tongues have been seen on recently acquired seismic in present day water depths of 500–700m. In the basinal area drifts and contourites predominate, and to the north of 60.5°N these overlap and interfinger with the downslope debris-flow wedges. Seismic records from the N Faroe margin also show features of widespread erosion of subsurface strata (Nielsen & Van Weering 1998; Van Weering et al. 1998; Kuijpers et al. 2001). These erosional features can be attributed to ice scouring during extreme glaciation, tentatively dated to MIS 6. Iceberg scouring has been found to be widespread down to about 700m water depth. 

Over large areas of the Faroe-Shetland and Faroe Bank channel systems and adjacent to the Faroe, Bill Bailey's and Lousy banks, the actual sedimentation conditions are characterised by reworking, transportation and erosion of at least the finer sediment fraction (Kuijpers et al. 2002). In areas where erosion occurs a lag sediment of coarse glacial IRD is formed, whereas silty and fine-grained sandy sediments prevail elsewhere, e.g cores ENAM 33 & 94-13 (Fig. 25). These conditions are typical of a full-interglacial climate regime (MIS 1, 5e and 7), but enhanced NSDW bottom-current activity also prevailed during the other warm substages (a, c) of MIS 5. In addition, comparable conditions prevailed at the beginning and also in the later part of MIS 3. It is also notable that the interstadial stages of the full-glacials MIS 2 and 6 are characterised by enhanced thermohaline circulation, but generally at a current strength below that required for reworking and erosion of the fine-grained sediment fraction, (Rasmussen et al. 1996; Kuijpers et al. 1998; Bäckström 2000).

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Studies of slope instability of the Faroe and West Shetland margins, and dating of the ensuing mass-flow deposits indicate that slope instability appears to be predominantly linked to maximum glaciation and sea-level changes. However, aseismic tectonic activity is also considered to have been a major influence, as illustrated by the Afen Slide (c. 5ka) (see map on figure 25), which is located across the Victory Transfer Zone. This and other NW–SE trending transfer zones within the Faroe–Shetland region (Fig. 16) have been periodically reactivated throughout the Neogene and are still 'active' at the present day (Ritchie, J.D., pers. comm. 2002).

Downslope processes affecting the slopes of the Norwegian Sea and Faroe-Shetland Channel have been proved at several sample sites. Cores TTR9-AT-146G and 150G from the Sandoy Fan, and TTR8-AT-106G from the N Faroes margin illustrate that major stages of mass-flow deposition occurred at the Pleistocene–Holocene boundary (event A), at the Last Glacial Maximum (event B1) and at some time during the later part of MIS 3 (event B2). A fourth episode (event C of Kuijpers et al. 2001) is inferred to have an age that is also younger than the last interglacial (MIS 5). Several notable slides have also been correlated with this interval, including the Afen Slide (MIS 1) and recently imaged post-Tampen-age slides, which could be from MIS 2 or 1. The Storegga Slide is the largest known of the shelf-margin failures to have occurred within the last 200ka and lies just to the NE of the study area (Fig. 24 and map opposite). It is dated as c. 7200ka (early Holocene) (Bondevik et al. 1997). Due to the limited sediment coring depth, no cores have so far been retrieved containing mass flow deposits of an older age, (e.g. MIS 6). However, from seismic-stratigraphic analyses it is known that mass-flow episodes have also occurred during earlier glacial stages, consistent with the West Shetland margin (Stoker 1999). For example, it is thought that it was during MIS 6 (late mid-Pleistocene) that the Tampen Slide event occurred in the North Sea Fan (Fig. 24d and distribution map), and this appears to be the oldest recognised failure event within the last 200ka. These and several others slides, including the pre-Tampen, Møre and Miller slides, and the area of mass wasting on the N Faroes margin, are shown on the map opposite.

References

Bäckström, D.L. 2000. Late Quaternary paleoceanography and paleoclimate of the North Atlantic Ocean. PhD thesis, Department of Marine Geology, Göteborg University, Earth Science Centre Series A60, 2000.

Bondevik, S., Svendsen, J.I. & Mangerud, J. 1997. Tsunami sedimentary facies deposited by the Storegga tsunami, in shallow marine basins and coastal lakes, western Norway. Sedimentology, 44, 1115-1131.

Kuijpers, A., Andersen, M.S., Kenyon, N.H., Kunzendorf, H. & Van Weering, T.C.E., 1998. Quaternary sedimentation around Bill Bailey Bank, north-eastern Atlantic. Marine Geology, 152, 101-127.

Kuijpers, A., Nielsen, T., Akhmetzhanov, A., de Haas, H., Kenyon, N.H. & Van Weering, T.C.E. 2001. Late quaternary slope instability on the Faroe margin: mass flow features and timing of events. Geo-Marine Letters, 20, 149-159.

Kuijpers, A., Hansen, B., Hühnerbach, V., Larsen, B., Nielsen & Werner, F. 2002. Norwegian Sea Overflow through the Faroe-Shetland gateway as documented by its bedforms. Marine Geology, ENAM-II Special Issue (in press).

Nielsen, T. & van Weering, T.C.E. 1998. Seismic stratigraphy and sedimentary processes at the Norwegian Sea Margin northeast of the Faroe Islands. Marine Geology, 152, 141-157.

Rasmussen, T.L., Thomsen, E., Van Weering, T.C.E. & Labeyrie, L. 1996. Rapid changes in surface and deep water conditions at the Faroe Margin during the last 58,000 years. Paleoceanography, 11, 757-771.

Stoker, M.S., Hitchen, K. & Graham, C.C. 1993. United Kingdom offshore regional report: the geology of the Hebrides and West Shetland shelves and adjacent deep-water areas. London: HMSO for the British Geological Survey.

Stoker, M.S. 1999. Stratigraphic nomenclature of the UK North West Margin. 3. Mid- to late Cenozoic Stratigraphy. British Geological Survey, Edinburgh.

Stoker, M.S., Nielsen, T., van Weering, T.C.E. & Kuijpers, A. 2002. Towards an understanding of the Neogene tectonostratigraphic framework of the NE Atlantic margin between Ireland and the Faroe Islands. Marine Geology, in press.

Van Weering, T.C.E., Nielsen, T., Kenyon, N.H., Akentieva, K. & Kuijpers, A. 1998. Sediments and sedimentation at the NE Faroe continental margin: contourites and large-scale sliding. Marine Geology, 152, 159-176.

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