The Neogene stratigraphy of the glaciated European margin from Lofoten to Porcupine
WP2 - FSN1 MEGASEQUENCE
Megasequence FSN-1 is widespread throughout the region, extending from the North Sea Fan and north Faroes Slope to the Wyville-Thomson Ridge, and from the West Shetland and Faroes shelves into the Faroe-Shetland and Faroe Bank channels. It is mostly absent on the Munkagrunnur, Wyville-Thomson and Fugløy ridges, the Faroe Bank and the innermost shelves.
External form and thickness
The FSN-1 megasequence has three main depocentres. The largest depocentre is found in the north of the Faroe-Shetland Channel, where sediments over 1200ms TWTT thickness form part of the North Sea Fan. The two other large depocentres, both showing sediment thicknesses of up to 650ms TWTT, are found on the west and the east Faroese shelves, the West Faroe and East Faroe wedges. These sediment accumulations are associated with the construction of prograding wedges. Similar, albeit smaller, wedges — the Foula and Rona wedges — are found on the West Shetland margin, where sediment thicknesses range from about 200–400ms TWTT. These shelf-margin depocentres contrast with the basinal sections preserved in the southern part of the Faroe-Shetland Channel and the Faroe Bank Channel where FSN-1 is generally thinner and more patchily developed. Around the southern tip of the Munkagrunnur Ridge, on the Fugloy Ridge and on the inner shelves, FSN-1 rarely exceeds more than 100ms TWTT in thickness. On the north Faroes and West Shetland slopes, FSN-1 has been locally modified by mass-wasting processes (Nielsen & van Weering 1998; Bulat & Long 2002).
The prograding wedges are characterised by seismically chaotic and structureless packages of diamictons and debris flows (Stoker 1995; Andersen et al. 2000). The upper part of the wedges is commonly formed of sheet-like units of glacial diamicton that display an aggrading configuration on the West Shetland and Faroes shelves. However, the bulk of the wedges consist of parallel-oblique to sigmoid-oblique, prograding, clinoforms that separate thicker packages dominated by stacked debris-flow deposits. In the basins, the seismic character of FSN-1 shows a parallel to sub-parallel reflector pattern that mostly drapes the underlying structures. Generally, the internal reflectors of FSN-1 are continuous, except in the northernmost part of the Faroe-Shetland Channel, where they are broken and slightly disrupted due to de-watering of the underlying FSN-2 sequence (Andersen et al. 2000). Localised onlap and truncation of reflectors in the basins are attributed to the effects of bottom currents (Stoker et al. 1998; Davies & Cartwright In press). Such activity is responsible for the generation of contourite deposits found at the base of the slopes, which are identified by parallel internal reflectors that onlap up-slope, by the development of erosional moats that also show up-slope migration, and by the occurrence of sediment waves. On the northern West Shetland Slope, the slope apron consists of an interdigitation between the downslope-advancing prograding-wedge deposits and the upslope-accreting sediment drifts (Stoker 1999; 2002).
The upper boundary of the FSN-1 megasequence is the present-day sea bed.
The lower boundary of the FSN-1 megasequence is the Intra-Neogene Unconformity (INU), dated as early Pliocene in age. This is characterised by a medium- to high-amplitude reflector, which forms an erosional, angular unconformity on the shelves, slopes and in the basins, except in the northern Faroe-Shetland Channel where it is more conformable in style. Landward on the shelves, the base of FSN-1 is a composite surface where the INU has been truncated by the Glacial Unconformity (see below: Subdivision).
Lithological information is primarily derived from BGS boreholes and commercial wells in the UK sector of the Faroe–Shetland region. On the West Shetland Shelf, the lower part of the prograding wedge generally consists of interbedded sands and muds with dropstones below the glacial unconformity, whilst above, glacial ice-contact sediments comprise the upper part of the section (Stoker 1999). This assemblage is typified by BGS borehole 77/9, which sampled the proximal part of the Rona wedge, and proved sand-dominated lower Pliocene–Pleistocene sediments overlain by middle–upper Pleistocene muddy diamictons. A comparable succession has been tested on the West Shetland Slope (Stoker 1999), with abundant debris-flow deposits, especially in the glacial part of the prograding wedges. On the lower slope of the Rona wedge, glacially-influenced sediments directly overlie Palaeogene strata, and BGS borehole 99/3 penetrated several muddy glacigenic debris flows interbedded with thinner glacimarine and contouritic sandy muds and sands (Davison & Stoker In press). The basinal succession has been sampled in well 214/4-1, which proved mainly muds although the log response implies that sandy interbeds are also present (Davis & Cartwright In press).
On the West Shetland margin, the FSN-1 megasequence incorporates a number of existing informal seismic-stratigraphic sequences that are detailed elsewhere (Stoker et al. 1993; Stoker 1999). Of particular note is a distinct reflector that essentially subdivides the shelf-margin succession into pre-expansive (below) and expansive (above) ice-sheet-dominated strata. This reflector has been informally termed the Glacial Unconformity (GU), which is probably of early mid-Pleistocene age (Stoker 1999). A correlative surface has also been identified on the Faroes Shelf (Stoker et al. 2002).
Megasequence FSN-1 can be correlated with the Naust Formation offshore Norway, and the RPa megasequence in the Rockall–Porcupine region.
Megasequence FSN-1 represents a major phase of progradation of both the Faroes and West Shetland shelf-margins. Deltaic and shallow-marine sediments rapidly built out the prograding wedges, such as the West Faroe, East Faroe, Rona and Foula wedges, which were further constructed during widespread mid- to late Pleistocene shelf glaciation (Stoker 1999; Andersen et al. 2000; Stoker et al. 2002). Sediment pathways and ice streams draining from the Faroes, Shetland and NW Scotland fed these depocentres. The vast amount of sediment that accumulated on the North Sea Fan is interpreted to have been supplied by a major glacial outlet from the Norwegian Channel (King et al. 1996). In deep water, bottom-current activity prevailed throughout this interval on the north Faroes Slope and in the Faroe-Shetland and Faroe Bank channels. Deposition of sediment drifts and waves on the slopes and basin floor is most prevalent north of 61ºN, whereas erosional activity predominates in the narrower part of the channel system, to the south. The interplay of downslope and alongslope processes is revealed by sea-bed images (Bulat & Long 2002; Kuijpers et al. 2001; 2002).
BGS borehole 77/9 and commercial well 214/4-1 demonstrate that the sediments immediately overlying the INU are of early Pliocene age. On the West Shetland Shelf, borehole 77/9 proved the presence of the dinoflagellate cysts Amiculosphaera umbracula and Spiniferites splendidus and the planktonic foraminifer Globorotalia crassaformis, at the base of the proximal part of the Rona wedge (Stoker 2002). This assemblage correlates with the early Pliocene NN13/14 biozones of Martini (1971). In the Faroe-Shetland Channel, well 214/4-1 recorded the early Pliocene foraminifers Sigmoilopsis schlumbergeri and Cibicidoides pachyderma above the INU (Davies & Cartwright In press). Significantly, perhaps, the bulk of the sediments that comprise the prograding wedges are of late Pliocene–Pleistocene age, as proved by the common occurrence of the dinoflagellate cysts A. umbracula and Habibacysta tectata, and the benthic foraminifer Cibicides grossa, e.g. BGS boreholes 84/1 and 84/2. Above the glacial unconformity, the sediments are generally characterised by an impoverished, commonly barren, foraminiferal and dinoflagellate cyst assemblage, although a mid-Pleistocene–Holocene age is envisaged (Stoker 1999). In the Faroe-Shetland Channel, late Pliocene–Pleistocene foraminifera, such as C. grossa and Neogloboquadrina atlantica, are present in well 214/4-1, but the lack of resolution at the top of the well precludes the determination of the glacial unconformity within the basin.
In well 6506/12-4, the oldest part of the Naust Formation corresponds to the Cibicides grossus–Elphidiella hannai–Globigerina bulloides–N. atlantica (sinistral) zone of Eidvin et al. (1998; 2001), and the base of the zone is marked by highest consistent occurrence of N. atlantica (dextral). In well 34/8-3A, the base of the Naust Formation is defined by the highest occurrence of Ehrenbergina variabilis (Eidvin et al. 2001).
Latest early Pliocene to Holocene.
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This page was Last updated 17 September 2002