Seismic stratigraphy of the Bill Bailey and Lousy Bank area: implications for subsidence history

The present study discusses some results of a reconnaissance seismic survey carried out in 1988 in the area around the prominent Bill Bailey and Lousy Banks, located SW of the Faeroe Islands. The area is thought to be part of the Rockall-Faeroe microcontinent, which was flooded by Palaeocene plateau basalts. The seafloor topography is largely due to the present-day organization of the basalt surface. Important structural features exhibited by this basement are sub-surface diverging reflectors and a major fault. Sediment accumulation is confined to the basinal area where four unconformities were identified. The lower boundary corresponds to the well-known reflector R4 of the North Atlantic. The other unconformities are proposed to correlate with hiatuses at the beginning and end of the middle Miocene, and at the end of the Pliocene, respectively. The sediment units are briefly described by their upper and lower boundary, seismic facies and thickness variations. About the lithology, however, little is so far known. The sediments are deformed by intraformational faults and a diapir-like structure. Both deformational styles are probably related to temporary overpressurisation in fine-grained sediments, but resulting from different causes. Stratal geometric patterns indicate that initially the subsidence of the basalt surface was rather uniform, but became non-uniform after the Eocene/Oligocene boundary, differentiating the two banks from the surrounding basin. In May-June 1988 a reflection seismic survey was set up jointly by the Geophysical Institute of Kiel (Germany) and the Renard Centre of Marine Geology in Gent (Belgium). The aim was to investigate the sedimentary patterns on the flanks of the Iceland-Faeroe Ridge. The area southwest of the Faeroe Islands, marked by prominent seamounts, was involved in the investigations: six reconnaissance profiles were shot across the Bill Bailey and Lousy (or Outer Bailey) Banks. An airgun array totalling between 2.61 and 3.81 was used as acoustic source. The reflected signals were detected by a 24channel streamer with an active length of 600 m. Only the analogue monitor recordings and the common-offset plots are presented here. The Bill Bailey and Lousy Banks delimit the northern edge of the Rockall Trough. Together with the Faeroe Bank these banks link the Rockall Plateau in the SW with the Faeroe insular block in the NE. Their location is shown in Fig. 1, along with the available seismic lines. Seismic (Bott et al. 1971, 1974; Casten 1973) and isotopic (Hald & Waagstein 1983) evidence shows that the Faeroe Island block may be underlain by continental crust which probably extends beneath the Faeroe, Bill Bailey and Lousy Banks (e.g. Roberts et al. 1983), thus adjoining the Rockall microcont inent in a southwestward direction. This so-called Rockall-Faeroe microcontinent became isolated by two distinct extensional phases: the area was split from the main European continent by an early to mid Cretaceous rifting episode (Roberts 1975; Roberts et al. 1983) which created the Rockall Trough and the Faeroe-Shefland Channel. And in late Palaeocene times the RockallFaeroe microcontinent broke away from Greenland, giving way to the opening of the northern North Atlantic. This second extensional phase was accompanied by the massive subaerial extrusion of plateau basalts which flooded large parts of the microcontinent within a few million years. The presence of a thick pile of Palaeocene plateau basalts is well established on the Faeroe Islands, on the nor thern Rockal l P la teau (Laughton et al. 1972; Roberts 1975) and in the area bounding the northern Rockall Trough From Scrutton, R. A., Stoker, M. S., Shimmield, G. B. & Tudhope, A. W. (eds), 1995, The Tectonics, Sedimentation and Palaeoceanography of the North Atlantic Region, Geological Society Special Publication No. 90, pp. 125-139 125 126 K. VANNESTE E T AL. Fig. 1. Generalized bathymetric map of the area, compiling available seismic grid, basaltic outcrop (from Waagstein 1988), plutonic centres (after Roberts et al. 1983), zone of dipping reflectors (Andersen 1988), oceancontinent boundary (from Smythe 1983), the southern lava front (from Wood et al. 1988), together with relevant DSDP sites. Mercator projection at 60 ~ N. (e.g. Roberts et al. 1983; Wood et al. 1987). The edges of the microcontinent were identified on the northern margin of the Faeroe shelf by the recognition of oceanward dipping reflectors (Smythe 1983; Andersen 1988), but have not been detected along the northern flanks of the Bill Bailey and Lousy Banks up to now.