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

Two Neogene megasequence boundaries, C10 and C20, bound the RPa and RPb megasequences (McDonnell & Shannon 2001; Stoker et al. 2001). These boundaries are illustrated in the geoseismic section and seismic panels opposite (Fig. 10). It should be noted that C20 represents an intra-Miocene event and not the base of the Neogene, for which there are problems of seismic definition in the Rockall Trough. However, in order to adopt a uniform system of nomenclature across the Rockall–Porcupine region it was felt most appropriate to use C20 to define the boundary between the Neogene RPb megasequence and the late Palaeogene–early Neogene RPc megasequence. This is further discussed below. The mid-Pleistocene, shelf-margin, glacial unconformity, and the regional late Eocene, C30 Unconformity, which defines the base of RPc, are also shown. 

The Glacial Unconformity

The Glacial Unconformity (GU) marks the onset of widespread glaciation of the Hebrides–Malin shelf, at about 0.44Ma (Stoker et al. 1994; Stoker 1995). On the shelf, the GU is a distinct, planar to irregular erosion surface that commonly truncates prograding clinoforms in the underlying Plio-Pleistocene strata (Fig. 10b). In deeper water, the GU is a predominantly planar surface that is downlapped by packages of debris flows on the lower slope, and which becomes a more-conformable, seismically-indistinct, draped surface in the Rockall Trough. 

The C10 Unconformity 

This C10 Unconformity is a high-amplitude, continuous reflection that characteristically forms an erosional, angular unconformity truncating strata on the flanks and within the Rockall Trough and Porcupine Basin (Fig. 10a, Fig. 10c, Fig. 10d, Fig. 10e and Fig. 10f). The boundary is also characterised by a change in seismic reflection character, with the overlying strata invariably characterised by a succession of high-amplitude reflections in contrast to the lower amplitude reflections of the underlying sediments. Adjacent to the Hebrides and Malin slopes, reflector C10 forms the base of the RPa prograding shelf-margin succession that includes the Barra-Donegal and Sula Sgeir fan systems and associated interdigitating/overlapping sediment-drift deposits (Fig. 10b and Fig. 10c). Beyond the limits of these fans, the removal of a significant section of the RPb succession implies submarine erosion associated with deep-water bottom currents (Fig. 10a, Fig. 10c, Fig. 10d, and Fig. 10e). In such areas, the overlying strata typically display variable downlap and onlap onto the flanks of the basins. On the western flank of the Rockall Trough, reflector C10 is, itself, commonly eroded and truncated by the present-day sea bed (see Geoseismic Panel 8a). In the basins, the continuity of C10 is locally offset by small-scale faults (Fig. 10c). In the NW Rockall Trough, C10 has been locally affected by neotectonic doming. (see Geoseismic Panel 7a).

The C20 Boundary

The C20 boundary is a basinal reflector that has no physical expression on the margin of the basins, nor on intrabasinal highs, such as seamounts. In the Rockall Trough, it is restricted to the central and southern part of the basin, south and west of the Rosemary Bank Seamount. In this area, it is characterised by a subhorizontal, high-amplitude acoustic signature that varies from a sharp, single reflection to a reflective zone several tens of milliseconds thick (Fig. 10c, Fig. 10d, and Fig. 10e). At DSDP site 610, the reflective zone has been attributed to the diagenetic alteration of clay minerals and the development of massive smectite, originally derived from volcanic ash (Dolan 1986). This is consistent with the observation that the highest amplitude reflectors within the reflective zone commonly vary with space and time, and concurs with the preposition of Dolan (1986) that the spread of smectite across this zone may have occurred with sporadic hiatuses and periods of deposition. Around the margin of the basin and adjacent to Rosemary Bank, C20 generally onlaps Oligocene and older strata (Fig. 10a). It commonly marks the base of an intense phase of sediment-drift growth in the basin (Fig. 10c and Fig. 10e). In the Porcupine Basin, C20 is more sharply defined and is one of several unconformities identified in the bottom of the RPb and the top of the RPc megasequences. Towards the margins of the basin the basal reflectors of the RPb megasequence rest with a marked unconformity (onlap) on the underlying deposits (Fig. 10f) while in the basin centre it becomes conformable to subtly unconformable with low-angle onlap or downlap onto the underlying megasequence. A deep-water origin has been invoked for C20 (Stoker et al. 2001).

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The Problem of defining Base Neogene

Whilst C20 provides a means of correlation between the Rockall Trough and the Porcupine Basin, older Miocene strata have been proved below C20 in both of these basins (see Fig. 13). An early Miocene hiatus has been identified in the Porcupine Basin (Fig. 10f); however, an equivalent boundary cannot be identified with any confidence in the Rockall Trough (e.g. Figs 10a & fig. 10e). North and east of Rosemary Bank and on the upper Hebrides slope well and borehole data have proved that the lower Miocene succession is largely absent, and middle–upper Miocene strata rest with angular unconformity on Oligocene strata of the RPc megasequence (Fig. 10a and Fig. 10b; see Geoseimic Panel 8c).

The C30 Unconformity

This is a regional unconformity that forms the base of the RPc (upper Eocene–lower Miocene) megasequence (McDonnell & Shannon 2001; Stoker et al. 2001). It is a high-amplitude reflection that is characteristically onlapped by the overlying RPc–RPa succession around the margin of the basins (Fig. 10b and Fig. 10e). 

References

Dolan, J.F. 1986. The relationship between the 'R2' seismic reflector and a zone of abundant detrital and authigenic smectite, Deep Sea Drilling Project Hole 610, Rockall Plateau region, North Atlantic. In: Ruddiman, W.F., Kidd, R.B., Thomas, E. et al. (eds), Initial Reports of the Deep Sea Drilling Project, 94. Washington (U.S. Government Printing Office), 1109-1115.

McDonnell, A. & Shannon, P.M. 2001. Comparative Tertiary stratigraphic evolution of the Porcupine and Rockall basins. . 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, 323-344.

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. 2002. Late Neogene development of the UK Atlantic margin. In: Doré, A.G., Cartwright, J., Stoker, M.S., Turner, J.P. & White, N. (eds), Exhumation of the North Atlantic Margin: Timing, Mechanisms and Implications for Petroleum Exploration. Geological Society, London, Special Publications, 196, in press.

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. (eds) The Petroleum Exploration of Ireland's Offshore Basins. Geological Society, London, Special Publication, 188, 411-438.

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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