Next Section | Table of Contents
doi:10.2204/iodp.sp.307.2005
The Porcupine Seabight is an amphitheater-shaped embayment in the Atlantic Irish shelf, off the southwestern coast of Ireland. It is enclosed by four shallow platforms: Porcupine Bank and Porcupine Ridge on the western side, Slyne Ridge in the northeast, the Irish mainland shelf in the east, and the terraced Goban Spur in the south (Fig. F1). The only opening toward the Porcupine Abyssal Plain lies between Porcupine Bank and Goban Spur. Porcupine Seabight, which is the surface expression of the underlying deep sedimentary Porcupine Basin, is a failed rift of the proto-North Atlantic Ocean, which is filled with a 10 km thick series of Mesozoic and Tertiary sediments (Shannon, 1991). The basin evolution can be summarized in three major steps: a Paleozoic synrift phase, a predominantly Jurassic rifting episode, and a Late Cretaceous to Holocene thermal subsidence period.
Basin Development and Synrift Sedimentation
The basement of Porcupine Basin is composed of Precambrian and lower Paleozoic metamorphic rocks, forming continental crust of ~30 km thickness (Johnston et al., 2001). Because the greater part of the basin is located north of the main Variscan deformation front, it has known a lesser degree of metamorphism than time-equivalent rocks of southern basins. The prerift succession commences with probably Devonian clastic sediments, overlain with lower Carboniferous carbonates and clastics. The upper Carboniferous rocks feature deltaic to shallow-marine deposits with Westphalian coal-bearing sandstones and shales and possibly Stephanian redbed sandstones.
During a Permo–Triassic early rifting phase period, a first minor east–west regional extension regime is inferred by the development of a series of small rift basins (Shannon, 1991; Moore and Shannon, 1995). The lowest Mesozoic deposits are early rift-valley continental sediments, which can have a thickness of more than 2 km. During Permian times, predominantly fluvial and lacustrine sedimentation took place with nonmarine mixed clastic deposits and evaporites (Fig. F4). Triassic sediments contain nonmarine to marine facies (Ziegler, 1982; Shannon, 1991). The Early Jurassic (Lias) is characterized by a tectonic tranquil period and a relative sea level rise, resulting in a marine sedimentary environment (Shannon, 1991). Lower Jurassic deposits are not found over the entire basin but could comprise limestones and rare organic-rich shales with sandstones (Fig. F4).
At the end of the Early Jurassic period, the middle Kimmerian rifting phase marked an increase of tectonic events in the Arctic, Atlantic, and Thetys rift systems. This major tectonic event was apparently accompanied by a renewed eustatic lowering of sea level and is likely responsible for erosion of a large part of the Triassic and Liassic deposits (Ziegler, 1982). Middle Jurassic fluvial claystones and minor sandstones might lie unconformably over earlier deposited strata and can be considered to be products of this major rifting episode. During the Late Jurassic, differential subsidence was responsible for the transition from a continental to a shallow-marine sedimentary environment in Porcupine Basin. Shallow-marine fans and scarp deposits were developed close to the basin margins, while in the center of the basin deepwater sand-rich mass flow deposits interdigitate with background hemipelagic sedimentation. These continuous series were topped by an unconformity of latest Jurassic age, a result of the late Kimmerian tectonic phase (Shannon, 1991).
At the start of the Cretaceous, the general structure of Porcupine Basin could be compared with a rift structure prior to breakup. Its specific failed rift structure has a typical steer's head profile (Moore and Shannon, 1991). The increased climatologic and plate tectonic reorganizations since the Paleogene also affected deepwater circulation and sedimentation within Porcupine Basin. Most of the postrift sediments are dominantly sandstones and shales, influenced by frequent sea level fluctuations. In contrast to the Celtic Sea basins, lower Paleocene deposits of Porcupine Basin underwent minimal inversion influence caused by the Alpine Orogeny (Shannon, 1991).
A major rifting pulse during the Early Cretaceous, referred to as late Kimmerian tectonism, was accompanied by a significant eustatic sea level fall and gave rise to a regional unconformity that is largely of submarine nature (Ziegler, 1982; Moore and Shannon, 1995). This undulatory unconformity marks the base of the Cretaceous, where marine strata onlap Jurassic sequences (Shannon, 1991). The contemporaneous Transition Sequence, however, is only found in some confined subbasins filled with clastic fans (Moore and Shannon, 1995). The Lower Cretaceous deposits (PK1) are northward-onlapping deep-sea sandstones and are still deposited on the wedge-shaped subbasins. The Albian PK2 sequence is affected by a minor rifting pulse of early actual North Atlantic seafloor spreading that started in the late Aptian (Shannon, 1991; Moore and Shannon, 1995). A local uplift of the basin margins produced a regressive period with an outer shelf deltaic sedimentation environment with locally sand rich deltas and associated beach complexes (McDonnell and Shannon, 2001). The onset of the Late Cretaceous (PK3) was characterized by a further relative sea level rise, featuring offshore sandstone bars, followed by a northward thinning and onlapping outer shelf to slope sequence of carbonates (chalk). Along the southwestern and southeastern margins of the basin, Moore and Shannon (1995) recognized the presence of biohermal reef buildups.
The transition from Upper Cretaceous to Lower Paleocene sedimentation is characterized by the high-amplitude reflector C40, marking the change from carbonate to clastic deposition (Shannon, 1992). In general, the Paleocene succession is more mud dominated, whereas the main coarse clastic input occurred in the middle Eocene to earliest late Eocene (McDonnell and Shannon, 2001). The Paleocene–Eocene is subdivided into five sequences (PT1–PT5) (Fig. F4). Within PT1 to PT4, every sequence is characterized by a southerly prograding complex deltaic event overlain by marine transgressive deposits (Moore and Shannon, 1992). The controls on the relative rises and falls in sea level are dominantly due to the North Atlantic plate tectonic regime. The upper Eocene is marked by a sea level rise with development of offshore bar sandstones (Naylor and Shannon, 1982). The upper Eocene sequence PT5 differs from previous sequences in a deep-marine mudstone facies (Moore and Shannon, 1992). A major basinwide unconformity developed in the latest Eocene to early Oligocene. This unconformity C30 marks the onset of deepwater sedimentation and establishment of regional contour-hugging bottom currents (McDonnell and Shannon, 2001).
During the late Paleogene and Neogene, a passive uplift of the Norwegian, British, and Irish landmasses was very important in shaping the present-day Atlantic margin. Although the origin of this uplift remains unclear, it probably resulted in enhancement of contour currents, causing local erosion and deposition and an increased probability of sedimentary slides and slumps. Therefore, overall Oligocene and Neogene sedimentation is characterized by along-slope transport and redepositional processes yielding contourite siltstones and mudstones and (hemi)pelagic deep marine sediments, caused by a combination of differential basin subsidence and regional sea level and paleoclimate changes. Eocene–Oligocene sediments are subdivided into three sequences (PT6–PT8) (Fig. F4). The base of the PT6 sequence is characterized by the high-amplitude continuous C30 reflector, while the top reflector is the erosive base Miocene C21 reflector. Within this Oligocene succession, a reworked Eocene microfauna has been described, suggesting shelfal incision or reworking by bottom currents. These Oligocene deposits are interpreted to be a sediment drift (Shannon, 1992). In sequence PT8, a slope-parallel sediment drift is described in the southeast part of the basin. The C20 reflector, which forms the lower boundary of PT8, is related to a relative sea level fall and is noticeably erosive in the south part of the basin (McDonnell and Shannon, 2001). This intensified scouring may be related to intensified bottom current activity elsewhere in the North Atlantic. The youngest unconformity mapped in Porcupine Basin is correlated with the Early Pliocene C10 event in the Rockall Basin. This event is considered to be a nucleation site for present-day coral banks (McDonnell and Shannon, 2001; De Mol et al., 2002).
Prior to the start of the EC-FP5 GEOMOUND and ECOMOUND projects studying cold and deepwater coral banks, little was known about the Quaternary evolution of the PSB. Recent sedimentation is mainly pelagic to hemipelagic, although (probably reworked) foraminiferal sands can be found on the upper slope of the eastern continental margin. The main sediment supply zone is probably located on the Irish and Celtic shelves, whereas input from Porcupine Bank seems to be rather limited (Rice et al., 1991). In contrast to the slopes of the Celtic and Armorican margins, which are characterized by a multitude of canyons and deep-sea fans, the east-west–oriented Gollum channels are the only major downslope sediment transfer system located on the southeastern margin of the seabight (Kenyon et al., 1978; Tudhope and Scoffin, 1995), which discharges directly onto the Porcupine Abyssal Plain. Rice et al. (1991) suggest that the present-day channels are inactive. According to Games (2001), the upper slope of the northern PSB bears predominantly north-south–trending ploughmarks on several levels within the Quaternary sedimentary succession. Smaller ploughmarks are also observed and interpreted as Quaternary abrasion of the continental shelf caused by floating ice grounding on the seabed. An abundance of pockmarks is also apparent on the seabed of this area. Within some of these Connemara pockmarks, an associated fauna of the coldwater coral Lophelia sp. has been observed (Games, 2001). Together with Madrepora sp., Lophelia sp. is found along the entire northwest European margin, manifested as coral patches to giant coral banks.
Seismic Studies/Site Survey Data
Studies carried out during the past 7 y under various EU Fourth and Fifth Framework programs (CORSAIRES [Coring Stable and Instable Realms in European Seas], ECOMOUND, GEOMOUND, and ACES), European Social Fund (ESF) programs (Euromargins project MoundForce) and national programs (GeNesis, etc.) have gathered substantial information from the area of interest, including box cores, long gravity cores, piston cores, high-resolution seismics (surface and deep towed), side-scan sonar at various frequencies and elevations over the seabed, surface multibeam coverage, and ultra-high-resolution swath bathymetry (using a remotely operated vehicle [ROV]) and video mosaicing (using ROV).
High-resolution seismic data (penetration = ~350 m; resolution = 1–2 m) have been acquired over the Belgica mound province (over a 1000 km2 area). A very high resolution pseudo–3-D seismic grid over an area of 10 km2 with a nominal line spacing of 180 m was shot over Thérèse mound. All proposed sites have been prepared by a minimum of a set of high-quality cross lines (Figs. F5, F6, F7, F8, F9, F10, F11, F12, F13, F14).
Side-scan sonar data have been acquired at various resolutions and elevations: deep-tow 100 kHz side-scan sonar and 3.5 kHz profiler, resolution = 0.4 m (95 km2 in the Belgica mound province), high-resolution Makanchi seismic array (MAK) data (Training Through Research [TTR] program), and full coverage of towed ocean bottom instrument (TOBI) side-scan sonar (30 kHz) (Figs. F15, F16).
A multibeam survey was completed in June 2000 (Polarstern) and the area was covered again by the Irish Seabed program (Fig. F2).
ROV VICTOR (Institut Francais de Recherche pour l'Exploration de la Mer [IFREMER]) was dedicated during two cruises (Atalante and Polarstern) to video surveying both Thérèse and Challenger mounds.
Previous subbottom sampling includes more than 40 gravity cores and piston cores in the Belgica mound province (penetration = 1.5–29 m), numerous box cores, and some television-controlled grab samples of ~1.5 ton.