The Neogene stratigraphy of the glaciated European margin from Lofoten to Porcupine
WP3 - RPa MEGASEQUENCE
Megasequence RPa is widely distributed throughout the region. It is best developed along the eastern margin of the Rockall Trough, north of 55ºN, and in the area of the Feni Ridge in the SW Rockall Trough (Stoker et al. 2001). It is locally absent along the western flank of the Trough, on rock platforms on the adjacent shelves and banks, and on the top and flanks of the Rosemary Bank, Anton Dohrn and Hebrides Terrace seamounts, although these may be covered by a veneer of sediment. In the Porcupine Basin, RPa is relatively thin but can be mapped within the limits of the available database.
External form and thickness
In the Rockall Trough, megasequence RPa displays a marked east–west asymmetry, being thickest along the Hebrides and Malin margins associated with the development of the prograding wedges that form the Sula Sgeir and Barra/Donegal fans, which commonly exceed 600ms TWTT, and locally 800ms TWTT, in thickness. In the basin, beyond the limit of the fans, RPa is generally less than 200ms TWTT in thickness, except on the Feni Ridge where it is locally thicker. The western flank of the Trough, north of 56ºN, was largely a zone of erosion throughout the RPa interval. A discrete prograding wedge has been identified on the eastern flank of the Rockall Bank that exceeds 100ms TWTT. In the Porcupine Basin, RPa is a thin basinal succession about 100ms TWTT in thickness. However, local thickening occurs on the flanks of the basin, up to c. 200ms TWTT.
The Sula Sgeir and Barra/Donegal fans are dominated by hummocky, structureless to chaotic packages of debris-flow deposits (Stoker 1995). Slide events have been locally identified within these fan systems (Holmes et al. 1998). On the Hebrides and Malin shelves, glacigenic sediments preserved as structureless to chaotic sheet-like units dominate the upper part of the succession. Parallel-bedded, flat-lying to undulatory, sediment-drift deposits dominate the Rockall Trough. These vary from broad-domed, sheeted drifts in the basin, to single- and multi-crested elongate mounded drifts on the basin flanks (Stoker et al. 1998; 2001). In the southern Rockall Trough, the Feni Ridge displays high-amplitude, continuous reflections that display aggradation accompanied by large-scale sediment-wave development, with wavelengths of up to 2km. In the Porcupine Basin, reflectors are parallel continuous to discontinuous and hummocky in character. The latter are commonly associated with clusters of carbonate mounds that fringe the basin.
The upper boundary of the RPa megasequence in both the Rockall and Porcupine basins is the present-day seabed.
The lower boundary is the intra-early Pliocene unconformity, C10. Along the Hebridean and Malin margins, C10 is mostly downlapped by clinoforms of the prograding wedges. Farther landward, the base of RPa is a composite surface where the C10 reflector has been truncated by the Glacial Unconformity (see below: Subdivision). In the central and northern Rockall Trough, the boundary commonly truncates the underlying Miocene deposits. In the southern Rockall Trough and Porcupine Basin, the boundary rests with only a slight unconformity on the underlying succession in the basin centres, but becomes markedly unconformable towards the basin margins. In the SW Rockall Trough it is onlapped progressively by the Feni Ridge.
The Hebrides and Malin shelves are characterised by glacial diamictons, muds and sands (Stoker et al. 1993). On the shelf margin, the prograding deposits remain terrigenous in character. A sand-dominated lower Pliocene to middle Pleistocene section unconformably overlain by mud-dominated glacigenic deposits was proved in BGS borehole 88/7,7A (Stoker et al. 1994). The basal lag gravel shows a high gamma-ray response due to the incorporation of reworked glauconite from the underlying Miocene strata. On the lower slope of the Sula Sgeir Fan, well 164/25-1 proved a muddier sequence although sands increase towards the base of the section; muddy basinal deposits underlie the fan. On the slope west of Ireland, deep-marine contouritic sands and muds were proved in PIPCo RSG boreholes (Praeg & Shannon 2000). In deeper water, short cores from the distal edge of the Barra Fan recovered interbedded, thin-bedded turbidites and hemipelagites (Knutz et al. 2001). The basinal section was also penetrated at DSDP site 610 and ODP sites 980 and 981, which proved muddy calcareous biogenic contourites, with dropstones in the upper part of the section (Ruddiman et al. 1987; Jansen et al. 1996). Condensed, muddy sandy and gravel-lag contourites occur on the slopes of Rockall and George Bligh banks (Howe et al. 2001). Similar deposits cap the seamounts (Stoker et al. 1993).
On the Hebridean margin, the RPa megasequence incorporates a number of existing informal seismic-stratigraphic sequences that are detailed elsewhere (Stoker et al. 1993). 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 is dated as early mid-Pleistocene in age (about 0.44Ma) and has been informally termed the Glacial Unconformity (GU) (Stoker et al. 1994). This surface can be traced into the slope apron, and broadly correlates with the top of the NN19 zone in DSDP 610 and ODP 980/981, but cannot, at present, be confidently recognised seismically for any great distance within the basinal succession.
Megasequence RPa can be correlated with megasequence FSN-1 from the Faroe–Shetland region and the Naust Formation offshore Norway.
The Sula Sgeir and Barra/Donegal fans form the main RPa depocentres in this region. The progradation of the shelf margin off NW Britain and Ireland resulted in the seaward migration of the shelfbreak by up to 50km during the Plio-Pleistocene interval (Stoker et al. 1993). Deltaic and shallow-marine progradation across the outer shelf was followed by extensive shelf glaciation in the early mid-Pleistocene, as ice sheets expanded out to the shelfbreak. In deep water, bottom-current activity prevailed throughout this interval in the Rockall Trough and Porcupine Basin. However, contourite development in the RPa megasequence was less aggressive in terms of aggradation and build-up than during deposition of the underlying RPb megasequence.
In DSDP 610 (and 610A) and ODP 980 and 981 (Ruddiman et al. 1987; Jansen et al. 1996), calcareous nannofossils record the late early Pliocene to Holocene NN15–21 zones of Martini (1971) above the C10 reflector. This zonation has also been recognised in BGS borehole 88/7,7A on the upper Hebrides Slope, although the oldest sediments above C10 may extend back into the NN13 zone (Stoker et al. 1994). However, the shelf-margin record contrasts with that of the basin in that the NN13/15–19 sequence is greatly condensed with respect to the deep-water sites, whereas the NN20–21 sequence, which corresponds with the glacial succession on the Hebrides Shelf, is expanded. The boundary between NN19 and NN20 marks the LAD of Pseudoemiliania lacunosa; this is recorded in the deep-water sites. On the upper Hebrides Slope, this boundary coincides with the glacial unconformity. The degree of erosion is uncertain at 88/7,7A, but this extinction event provides a useful marker for the onset of expansive glaciation in this region.
Planktonic foraminifera are well represented at site 980/981, where the Pleistocene Neogloboquadrina pachyderma (sinistral) acme zone, the latest Pliocene Globorotalia inflata zone, the late Pliocene Globogerina bulloides zone, the mid-Pliocene Neogloboquadrina atlantica zone and the early Pliocene Globorotalia puncticulata zone have all been recognised (Flower 1996). The latter zone straddles the C10 reflector. A similar assemblage has been recognised at site 610 (610A) (Ruddiman et al. 1987). On the upper Hebrides Slope, both the Gr. inflata and Gr. puncticulata zones were recognised despite the condensed nature of the section (Stoker et al. 1994). The Gr. puncticulata zone plus the early Pliocene benthonic foraminifera, Uvigerina venusta saxonica, immediately overlie the C10 reflector.
Latest early Pliocene to Holocene.
Flower, B.P. 1999. 2. Data report: Planktonic foraminifers from the subpolar North Atlantic and Nordic seas: sites 980-987 and 907. In: Raymo, M.E., Jansen, E., Blum, P. & Herbert, T.D. (eds), Proceedings of the Ocean Drilling Program, Scientific Results, 162, 19-34.
Holmes, R., Long, D. & Dodd, L.R. 1998. Large-scale debrites and submarine landslides on the Barra Fan, west of Britain. In: Stoker, M.S., Evans, D. & Cramp, A. (eds), Geological Processes on Continental Margins: Sedimentation, Mass-Wasting and Stability. Geological Society, London, Special Publication, 129, 67-79.
Howe, J.A., Stoker, M. & Woolfe, K.J. 2001. Deep-marine seabed erosion and gravel-lags in the northwestern Rockall Trough, North Atlantic Ocean. Journal of the Geological Society, London, 158, 427-438.
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.
Knutz, P.C., Austin, W.E.N. & Jones, E.J.W. 2001. Millennial-scale depositional cycles related to British Ice Sheet variability and North Atlantic paleocirculation since 45 kyr B.P., Barra Fan, U.K. margin. Paleoceanography, 16, 53-64.
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.
Praeg, D. & Shannon, P.M. 2000. Shallow Seismic Site Surveys from Blocks 83/24, 83/20, 75/10, 16/28, 11/20, 78/28: An Interpretation and Stratigraphic Integration with Boreholes. UCD Contribution to RSG Project 98/23.
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. 1995. The influence of glacigenic sedimentation on slope-apron development on the continental margin off Northwest Britain. In: Scrutton, R.A., Stoker, M.S., Shimmield, G.B. & Tudhope, A.W. (eds), The Tectonics, Sedimentation and Palaeoceanography of the North Atlantic Region, Geological Society, London, Special Publication, 90, 159-177.
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.
This page was Last updated 17 September 2002