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

WP3 - RPb MEGASEQUENCE

Distribution

Megasequence RPb is largely restricted to the basinal areas of the Rockall Trough and Porcupine Basin where its shelfward limit is marked by an erosional pinch-out on the slopes of these basins. Discrete outliers occur on the upper Hebrides Slope and in the Wyville-Thomson Ridge region, and a veneer of sediment has been locally proved on the axial seamounts of Rosemary Bank, Anton Dohrn and Hebrides Terrace (Stoker et al. 1993). 

External form and thickness

In the Rockall Trough, the basinal sediments commonly exceed 600ms TWTT in thickness in the western half of the basin, but are much thinner, and commonly eroded on the flanks of the basin. In general, RPb displays a ponded basin-floor-fill geometry but with locally significant upslope accretion onto the basin margin. This geometry reflects three main styles of sediment-drift accumulation: (1) broad, flat-lying to gently domed, sheeted drifts which occupy a large part of the axial region of the basin floor, (2) elongate mounded drifts onlapping the margins of the basin and, (3) the giant, elongate, Feni Drift in the southern Rockall Trough (Stoker et al. 1998; 2001). The latter is up to 80km wide, and separated from Rockall Bank by a wide erosional moat. It can be traced for about 400km parallel to the western margin of the southern Rockall Trough, but loses its classic morphological expression north of 55°N. This may, in part, be due to erosion associated with the C10 reflector. A smaller drift occurs adjacent to the Porcupine Bank. In the Porcupine Basin, the RPb megasequence shows a broad catenary profile that is up to 650ms TWTT thick in the centre of the basin. Superimposed on this broad depocentre is a N–S subsidiary depocentre lying to the west of the eastern basin margin. This corresponds to the Porcupine Drift, an elongate contourite drift that is coeval with the Feni Ridge but is more topographically subdued than the latter. 

Seismic facies

In the Rockall Trough and Porcupine Basin, RPb is characterised by parallel-bedded continuous reflections through the basins but is more variable towards the basin margins or adjacent to the seamounts, where gently domed to undulatory waveforms are present. The continuity of reflections is locally offset by small-scale normal faults (probably polygonal faults). Some of the undulations represent large-scale sediment waves, up to 40m high with a wavelength of 2–3km (Stoker et al. 2001), which have been locally eroded during the formation of the upper bounding surface, C10. In areas of upslope accretion, sediment drifts have migrated from the basin onto the slope, and display both onlapping and downlapping reflector terminations. Further complexity preserved as minor unconformities, truncations, channelised fills and slope failure, are attributed to contourite deposition and reworking.

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Upper boundary

In the central and northern Rockall Trough, the intra-early Pliocene unconformity, C10, marks the upper boundary. The nature of this boundary is characteristically erosional, varying from planar to irregular. Locally, where C10 is absent, the upper boundary is marked by the present-day sea bed. In the southern Rockall Trough and Porcupine Basin the unconformity is slight in the basin centres becoming more pronounced towards the basin margins.

Lower boundary

The reflective zone of C20 marks the lower boundary of RPb over most of the region, separating it from megasequence RPc that includes lower Miocene strata. In the centre of the Rockall Trough and the Porcupine Basin, it is a subtle unconformity marked by a change in reflector character but it becomes a pronounced unconformity towards the margins of both basins. Whilst C20 does not represent the true base of the Neogene succession, it does represent an easily recognised seismic boundary that can be utilised to subdivide the upper Cenozoic succession in this region. In the northern Rockall Trough, C20 onlaps onto Palaeogene strata, and north of Rosemary Bank, the lower boundary is marked by an erosional unconformity that truncates Oligocene and older strata. 

Lithology

The nature of the deep-water sediments has been tested at several sites. DSDP site 610 sampled the Feni Ridge and proved lower/middle Miocene to lower Pliocene nannofossil chalk and ooze (Hill 1987). Lower Pliocene nannofossil ooze was also recovered from basinal, sheeted drifts in the upper part of RPb at ODP site 981 (Jansen et al. 1996). Farther north, BGS borehole 94/1 proved upper Miocene–lower Pliocene biogenic sand and gravel from the moat of an elongate mounded drift on the slope of Rockall Bank. In the NE Rockall Trough, a sand-dominated succession was proved in well 164/25-2 that penetrated the feather-edge of a flat-lying basinal succession. At shallower depths, BGS borehole 90/15 proved middle Miocene, bioclastic sands and muds from the top of Anton Dohrn seamount, whereas 90/18 recovered middle to upper Miocene nannofossil ooze from the top of Rosemary Bank (Stoker et al. 1993). On the upper Hebrides Slope and Shelf, middle to upper Miocene glauconitic sandstones were recovered in BGS boreholes 88/7,7A, 90/12,12A and 90/13; a similar facies was sampled on the Malin Shelf in wells 13/3-1 and 19/5-1 (Stoker et al. 1993). On the western slope of Porcupine Bank, flanking the Rockall Trough, borehole 83/20-sb01 proved middle to upper Miocene silty clays (Stoker 1999). There is no information for this megasequence from the Porcupine Basin.

Subdivision

Although there are a number of indications of intra-RPb events it is not possible to subdivide the megasequence at this stage. 

Regional correlation

Megasequence RPb can be correlated with megasequence FSN-2 from the Faroe–Shetland region and the Kai Formation offshore Norway.

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Genetic interpretation

The RPb interval witnessed the most significant development and accumulation of sediment drifts and waves in the Rockall Trough and Porcupine Basin. It was characterised by a vigorous, deep-water bottom-current circulation pattern and extensive lateral migration of sediment by upslope accretion onto the flanks of the basins. Thick biogenic sections proved in the southern Rockall Trough imply high biological productivity and pelagic sedimentation at this time. Indications of downslope input into the Rockall Trough, in terms of either submarine fans or mass failure, remain ambiguous. However, Austin (2000) suggests that significant mass wasting of the slope off NW Ireland occurred during this interval.

Biostratigraphic characterisation

In DSDP 610, the late early Miocene to latest Miocene/early Pliocene NN4–12 zones of Martini (1971) have been recognised (Ruddiman et al. 1987) between the C20 and C10 reflectors. The youngest (NN12) part of the section, between 233m and 243m, is based on an assemblage consisting of Coccolithus pelagicus, Calcidiscus leptoporus, Calcidiscus macintyrei, Reticulofenestra pseudoumbilica, Sphenolithus abies, Discoaster surculus, Discoaster intercalaris and Discoaster variabilis. It should be noted that between 348m and 463m, no age-diagnostic species were found; thus the NN7–10 assignment for this interval remains undifferentiated. In the lower part of the section, reflector C20 overlaps with the Sphenolithus heteromorphus zone (NN5) and the Helicosphaera ampliaperta zone (NN4).

On the upper Hebrides Slope, the 2.85m-thick condensed section in BGS borehole 88/7,7A, containing R. pseudoumbilica, Helicosphaera sellii, Discoaster exilis and, Discoaster antarcticus, was assigned a broad NN4–10 age (Stoker et al. 1994). This age assignment is supported by algal cysts, such as Bolboforma metzmacheri, B. reticulata, the FAD of the foraminifer Orbulina suturalis and the dinoflagellate cysts Pentadinium laticinctum, Labyrinthodinium truncatum and Reticulasphaera actinocoronata.

Foraminifera provide the first indication of definite Miocene strata in well 164/25-2, with mid- to late Miocene species, such as Hoeglundina elegans, Globorotalia praegulloides and O. suturalis between 576m and 603m. Significantly, perhaps, late Oligocene and early Miocene species, such as the foraminifer Globorotalia praescitula and the palynomorphs Hystrichokolpoma granulata, Thalassiphora pelagica and Emslandia spiridioides are mixed with the younger assemblage. This suggests that the late Oligocene–early Miocene component is reworked. 

Age

Late early / early mid-Miocene to early Pliocene.

References

Austin, B.J. 2000. RSG Project No: 98/23 Rockall Trough Shallow Stratigraphy and Geohazards Study 2000. Hydrosearch Report No: 99239-PIP.

Hill, P.R. 1987. Characteristics of sediments from Feni and Gardar drifts, sites 610 and 611, Deep Sea Drilling Project Leg 96. In: Ruddiman, W.F., Kidd, R.B., Thomas, E., et al. (eds.) Initial Reports of the Deep Sea Drilling Project, 94, U.S. Government Printing Office, Washington D.C., 1075-1082.

Jansen, E., Raymo, M.E., Blum, P. et al. 1996. 3. Sites 980/981. In: Jansen, E., Raymo, M.E., Blum, P. et al. (eds.), Proceedings ODP, Initial reports, 162: College Station, TX (Ocean Drilling Program), 49-90.

Martini, E. 1971. Standard Tertiary and Quaternary calcareous nannoplankton zonation. In: Farinacci, A (ed.), Proceedings of the II Planktonic Conference, Rome, 1969. Rome: Edizioni Tecnoscienza, 739-785.

Ruddiman, W.F., Kidd, R.B., Thomas, E. et al. 1987. 6. Site 610. In: Ruddiman, W.F., Kidd, R.B., Thomas, E. et al., Initial reports of the Deep Sea Drilling Project, 94: Washington (U.S. Government Printing Office), 351-470.

Stoker, M.S. 1999. Irish Rockall Shallow Drilling 1999: Stratigraphic Summary. British Geological Survey Technical Report, WB/99/22C.

Stoker, M.S., Hitchen, K. & Graham, C.G. 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., van Weering, T.C.E. & Svaerdborg, T. 2001. A Mid-Late Cenozoic tectonostratigraphic framework for the Rockall Trough. In: Shannon, P.M., Haughton, P.D.W. & Corcoran, D.V. (eds) The Petroleum Exploration of Ireland's Offshore Basins. Geological Society, London, Special Publication, 188, 411-438.

Stoker, M.S., Akhurst, M.C., Howe, J.A. & Stow, D.A.V. 1998. Sediment drifts and contourites on the continental margin off northwest Britain. Sedimentary Geology, 115, 33-51.

Stoker, M.S., Leslie, A.B., Scott, W.D., Briden, J.C., Hine, N.M., Harland, R., Wilkinson, I.P., Evans, D. & Ardus, D.A. 1994. A record of Late Cenozoic stratigraphy, sedimentation and climate change from the Hebrides slope, NE Atlantic Ocean. Journal of the Geological Society, London, 151, 235-249.

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