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

WP2 - FSN-2 MEGASEQUENCE

Distribution

Megasequence FSN-2 occurs widely throughout the region, but is best developed in the northern Faroe-Shetland Channel and east Faroes Slope, and on the north Faroes Slope and into the Norwegian Basin. A discrete depocentre is also found in the Faroe Bank Channel. The megasequence has everywhere been affected by early Pliocene erosion, but especially south of 61º30'N where it has been extensively eroded on the shelves and slopes, and is locally absent over structural highs, such as the Wyville-Thomson and Munkagrunnur ridges. 

External form and thickness

The thickest accumulations of FSN-2 occur to the north and east of the Faroes Shelf where they exceed 600ms, and locally 700ms, TWTT in thickness. On the north Faroes Slope, the thickest part of the mapped succession occurs on the middle to lower slope, thinning upslope and forming a wedge-shaped shelf-margin deposit (Nielsen & van Weering 1998). The basinward extension into the Norwegian Basin is poorly defined due, in part, to extensive base-of-slope mass failure. The accumulations in the northern Faroe-Shetland Channel and on the east Faroe Slope are predominantly basinal in character and appear to form an elongated, WSW–ENE trending set of depocentres, which may further continue to the NE on the lower slope, north of Shetland. The development of these basinal deposits appears to be structurally controlled (see Pre-Neogene section). The discrete FSN-2 deposit found on the SW side of the Faroe Bank Channel is up to 400ms TWTT thick and forms an eroded, mounded deposit resembling a fan or drift deposit. On the West Shetland margin, the FSN-2 sediments are commonly preserved as erosional remnants, mostly less than 100ms TWTT thick. These include formerly mounded sediment drifts and waves, small deltaic wedges and localised mass-flow deposits at the SW end of the West Shetland Slope, and an eroded sheet-like deposit on the adjacent shelf (Stoker 1999). 

Seismic facies

In the main depocentres in the northern part of the region, the shelf-margin and basinal sediments are mainly characterised by a seismically-layered configuration of parallel reflections. This seismic pattern is locally discontinuous with the internal reflectors disrupted by small faults due to de-watering of the sediments (Andersen et al. 2000). Commonly, the reflections display upslope accretion by onlap from the basin floor onto the slope, in association with complex waveforms, which are attributed to the effects of bottom currents (Nielsen & van Weering 1998; Stoker 1999; Davies et al. 2001; Davies & Cartwright In press). The eroded remnants of equivalent strata south of 61º30'N may be indicative of a previously more extensive development of FSN-2 contourite deposits. The small deltaic wedges on the southern West Shetland Slope display a prograding reflection configuration (Stoker 1999). These wedges become younger upslope. Localised fan bodies may also be preserved flanking the Munkagrunnur Ridge. Shallow-marine deposits occur on the West Shetland Shelf, where parallel-bedded strata display low-angle transgressive onlap onto the shelf. Erosion of the latter sediments, prior to the development of the Rona (FSN-1) prograding wedge, may have generated seismically-chaotic mass-flow deposits on the slope (Stoker 1999). 

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Upper boundary

The upper boundary of the FSN-2 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 northernmost Faroe-Shetland Channel where it is more conformable in style. Landward on the shelves, the top of FSN-2 is a composite surface where the INU has been truncated by the mid-Pleistocene Glacial Unconformity.

Lower boundary

The lower boundary of the FSN-2 megasequence is the Top Palaeogene Unconformity (TPU) that everywhere separates Palaeogene and Neogene strata. The TPU is an angular unconformity that truncates the FSP megasequence. It is best recognised in the SW Faroe-Shetland Channel, where a highly sculpted erosion surface still has sea-bed expression in the form of major erosional deeps (Stoker et al. In press). On the flanks of the basins, the TPU is commonly cut out by the INU. In the northern Faroe-Shetland Channel, a diagenetic horizon formed of porcellanite is observed on seismic profiles as a high-amplitude reflector (Davies & Cartwright In press). Locally, this reflector is coincident with the TPU, and hinders the definition of the unconformity in this part of the basin.

Lithology

Lithological information is primarily derived from BGS boreholes and commercial wells in the UK sector of the Faroe–Shetland region. On the West Shetland margin, muddy, glauconitic, bioclastic, slightly gravelly sandstones, as proved in BGS boreholes such as 77/7 and 90/3, characterise the shelf and upper slope succession (Stoker 1999). Subordinate interbedded lithologies include mudstones, siltstones, sandy limestones and lignites. The variable gamma-ray traces in boreholes 77/7 and 90/3 may reflect variation in glauconite content, clay mineralogy, and bulk lithology. For example, the change in gamma-ray value at 87m in 77/7 corresponds to a change from a lower glauconite-rich, kaolinite- and mica-dominated sandstone unit to an upper unit of sandstones and muddy sandstones, with reduced glaucony and high smectite and mica (Evans et al. 1997). The deep-water succession has only been tested in well 214/4-1, which proved a mudstone-dominated basinal section (Davies & Cartwright In press). 

Subdivision

On the West Shetland margin, a distinctive glauconitic sandstone unit (e.g. boreholes 77/7 and 90/3) was named the Muckle Ossa Sandstone by Stoker (1999). On the NE Faroe margin, and in the northern Faroe-Shetland Channel, an intra-Miocene Unconformity (IMU) of mid-Miocene age has been identified that forms the basis for a potential subdivision of the FSN-2 megasequence (Andersen et al. 2000; Davies & Cartwright In press). Details of this preliminary subdivision are presented elsewhere (Nielsen et al. 2002), with a summary in figure 9 of this atlas. 

Regional correlation

The FSN-2 megasequence can be correlated with the Kai Formation offshore Norway, and with the RPb and upper part of RPc megasequences in the Rockall–Porcupine region.

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Genetic interpretation

Strong bottom currents have been active in the area throughout the deposition of FSN-2, as is evident from the predominance of contourite deposits in the basins, and on adjacent flanks. Deposition of sediment drifts and waves is most prevalent in the northern part of the region, where they have contributed to the construction of the shelf margins flanking the Faroe-Shetland Channel and the north Faroe margin. In contrast the southern half of the region has been an area of net erosion throughout the Neogene, and only remnants of sediment drifts are preserved. There is evidence in the south of subaerial exposure of the West Shetland Shelf followed by marine transgression during the Miocene. Deltaic wedges on the southern West Shetland Slope imply a lowstand of relative sea level at or just below the contemporary shelfbreak (Stoker 1999). The progressive upslope-backstepping development of these wedges further implies a rising sea level that may have resulted in the deposition of the transgressive Muckle Ossa Sandstone on the West Shetland Shelf. The erosional planation of the latter and its possible re-deposition by mass-flow processes on the slope suggest episodic changes in relative sea level, which may be linked to tectonic activity. This is supported by possible fan developments on the innermost eastern Faroe Shelf and Faroe Bank Slope.

Biostratigraphic characterisation

BGS boreholes from the West Shetland Shelf reveal a rich microfossil content (Stoker 1999). Foraminiferal and algal cyst assemblages record the earliest Miocene Bolboforma spinosa / Bolboforma rotunda bioevent, and the earliest mid-Miocene Heteropleura dutemplei peelensis bioevent. The dinoflagellate cyst assemblages reveal a late early Miocene to late Miocene age range on the basis of several biomarkers, including Labyrinthodinium truncatum, Unipontidinium aquaductum, Palaeocystodinium golzowense, Systematophora placantha and Apteodinium spiridoides. In the Faroe-Shetland Channel, Davies & Cartwright (In press) record Miocene calcareous microfauna such as Bolboforma clodiusi, Globorotalia praescitula, Martinottiella bradyana and Neoglobigerina continuosa from well 214/4-1. They also cite the common occurrence of Orbulina universa as being indicative of lower Pliocene sediments immediately below the INU.

Strontium isotopes

Strontium isotope (87Sr/86Sr) analyses have been undertaken on bivalve fragments and foraminifera for a number of BGS boreholes on the West Shetland Shelf, including 77/7 and 90/3 (Stoker 1999) (Fig. 12). For 77/7, samples from 100m and 110m gave ages between 18.35–20.73Ma; for 90/3, a sample from 85.09 gave an age of 16.34–16.78Ma. 

Age

Early Miocene to early Pliocene.

References

Andersen, M.S., Nielsen, T., Sørensen, A.B., Boldreel, O.L. & Kuijpers, A. 2000. Cenozoic sediment distribution and tectonic movements in the Faroe region. Global and Planetary Changes, 24, 239-259.

Davies, R. & Cartwright, J. In press. Character and origin of a fossilised bottom simulating reflector originating from opal A to C/T transformation: an example from the Neogene of the NE Atlantic margin. Basin Research.

Davies, R., Cartwright, J., Pike, J. & Line, C. 2001. Early Oligocene initiation of North Atlantic Deep Water formation. Nature, 410, 917-920.

Evans, D., Morton, A.C., Wilson, S., Jolley, D. & Barreiro, B.A. 1997. Palaeoenvironmental significance of marine and terrestrial Tertiary sediments on the NW Scottish Shelf in BGS borehole 77/7. Scottish Journal of Geology, 33, 31-42.

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

Nielsen, T., Mathiesen, A., Stevenson, A.G., Hoult, R., Sørensen, A.B., Kuijpers, A & Gillespie, E.G. 2002. Regional Stratigraphy WP2. Stratagem Deliverable 19.

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., Long, D. & Bulat, J. In press. A record of mid-Cenozoic strong deep-water erosion in the Faroe-Shetland Channel. In: Mienert, J. & Weaver, P. (eds), European Continental Margin Sedimentary Processes: An Atlas of Side-Scan Sonar and Seismic Images, Springer Verlag.

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