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doi:10.2204/iodp.proc.307.101.2006

Background

Geographic setting

A geographic overview of Porcupine Seabight and the mound provinces is shown on Figure F1. Three distinct mound provinces have been identified: the Hovland, Magellan, and Belgica mound provinces.

Hovland mounds

The Hovland mounds are the first mound occurrences reported from industrial data on the northern slope of Porcupine Basin (Hovland et al., 1994) that led to the unveiling of a complex setting with large multiphased contourite deposits and high-energy sediment fills, topped by a set of outcropping mounds or elongated mound clusters as high as 250 m (Henriet et al., 1998; De Mol et al., 2002).

Magellan mounds

The Hovland mounds are flanked to the north and west by the crescent-shaped, well-delineated Magellan mound province, which is characterized by a very high density of buried medium-sized mounds (1 mound/km²; average height = 60–80 m) (Huvenne et al., 2003). High-resolution seismic data combined with 3-D industrial seismic data (Huvenne et al., 2002) has shed some light on the presence of a past slope failure that partly underlies the mound cluster.

Belgica mounds

On the eastern margin of Porcupine Basin, a 45 km long range of large mounds towers from a strongly eroded surface (Fig. F2). The mounds partly root on an enigmatic, deeply incised, very faintly stratified seismic facies (Unit P2) (Fig. F3) (De Mol et al., 2002; Van Rooij et al., 2003) that De Mol (2002) interpreted as a nannofossil ooze of Pliocene age analogous to the similar seismic facies of ODP Site 980 in the southwestern Rockall Trough (Unit P1) (Jansen, Raymo, Blum, et al., 1996) and partly on a layered sequence capped by a set of short-wavelength, sigmoidal depositional units (De Mol et al., 2002, 2005b; Van Rooij et al., 2003).

The Belgica mound province consists of 66 conical mounds (single or in elongated clusters) in water depths ranging from 550 to 1025 m. The mounds are partly enclosed in contourite deposits (Van Rooij et al., 2003). Mounds typically trap sediment on their upslope flank, which is consequently buried, whereas their seaward side is well exposed and forms a steep step in bathymetry. Average slope angles range from 10° to 33°. The largest mounds have a height of ~170 m. In the deeper part (>900 m water depth) of the Belgica mound province (Beyer et al., 2003), an extremely “lively” mound was discovered in 1998 on the basis of a very diffuse surface acoustic response. This mound, known as Thérèse Mound, was selected as a special target site to study processes involved in mound development for European Union (EU) Fifth Framework research projects. Video imaging revealed that Thérèse Mound, jointly with its closest neighbor, Galway Mound, might be one of the richest cold-water coral environments in Porcupine Seabight, remarkably in the middle of otherwise barren mounds. Challenger Mound, to the southwest, also shares the acoustic properties of Thérèse Mound but is only covered by dead coral rubble (Foubert et al., 2005; De Mol et al., 2005a; Huvenne et al., 2005; Wheeler et al., 2005).

Geologic setting

Porcupine Seabight forms an inverted triangle opening to the Porcupine Abyssal Plain through a narrow gap of 50 km at a water depth of 2000 m at its southwest apex between the southern and western tips of the Porcupine Bank and terraced Goban Spur, respectively (Fig. F1). It gradually widens and shoals to depths of 500 m to the east on the Irish continental shelf and north to Slyne Ridge. Porcupine Seabight is the surface expression of the underlying deep sedimentary Porcupine Basin (Fig. F4), which is a failed rift of the proto-North Atlantic Ocean and is filled with a 10 km thick series of Mesozoic and Cenozoic sediments (Shannon, 1991). Basin evolution can be summarized in three major steps: a Paleozoic synrift phase, a predominantly Jurassic rifting episode, and a Late Cretaceous–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 ~30 km thick (Johnston et al., 2001). The prerift succession probably commences with Devonian clastic sediments overlain by 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 (Shannon, 1991; Moore and Shannon, 1995). Permian and lowermost Mesozoic deposits are early rift valley continental sediments which can be >2 km thick. During Permian times, predominantly fluvial and lacustrine sedimentation took place with nonmarine mixed clastic deposits and evaporites. Triassic sediments contain nonmarine to marine facies (Ziegler, 1982; Shannon, 1991). Lower Jurassic deposits are not found over the entire basin but, where present, could comprise limestones and rare organic-rich shales with sandstones.

Jurassic rifting phase

The middle Kimmerian rifting phase marked an increase in tectonic events in the Arctic, Atlantic, and Tethys 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 Jurassic deposits (Ziegler, 1982). Middle Jurassic fluvial claystones and minor sandstones might lie unconformably above 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.

Cretaceous subsidence and Paleogene–Neogene sedimentation

Porcupine Basin began at the start of the Cretaceous as a failed rift structure with a typical steer’s head profile (Moore and Shannon, 1991). A major rifting pulse during the Early Cretaceous, associated with the Late Kimmerian orogeny, was accompanied by a significant eustatic sea level fall and gave rise to a regional unconformity that is largely of a 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 onset of the Late Cretaceous 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 pelagic 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 Late Cretaceous to early Paleocene sedimentation is characterized by a high-amplitude seismic reflector marking the change from carbonate to clastic deposition (Shannon, 1991). Most of the Paleogene postrift sediments are dominantly sandstones and shales, influenced by frequent sea level fluctuations. 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 characterized by southerly prograding complex deltaic events overlain by marine transgressive deposits (Naylor and Shannon, 1982; Moore and Shannon, 1995). The controls on the relative rises and falls in sea level are dominantly due to the North Atlantic plate tectonic regime. During the late Paleogene and Neogene, 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, deposition, and an increased probability of sedimentary slides and slumps. Therefore, overall Oligocene and Miocene sedimentation is characterized by along-slope transport and redepositional processes yielding contourite siltstones and mudstones and hemipelagic–pelagic deep-marine sediments, caused by a combination of differential basin subsidence and regional sea level and paleoclimate changes. The youngest unconformity mapped in Porcupine Basin is correlated with an early Pliocene erosion event in Rockall Basin and is considered to be a nucleation site for present-day cold-water coral mounds (e.g., McDonnell and Shannon, 2001; De Mol et al., 2002; Van Rooij et al., 2003).

Pleistocene and Holocene sedimentation

Recent sedimentation is mainly pelagic to hemipelagic, although foraminiferal sands (probably reworked) 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 northern Porcupine Seabight bears predominantly north-south-trending plough marks on several levels within the Quaternary sedimentary succession. Smaller plough marks 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 in this area (Games, 2001).

Seismic studies/site survey data

Studies carried out during the past seven years under various EU Fourth and Fifth Framework programs, European Science Foundation programs, UNESCO Training Through Research Program, and various European national programs 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 (surface and deep-towed) 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 mosaicking (using ROV). High-resolution seismic data (penetration = ~350 m; resolution = 1–3 m) have been acquired over the Belgica mound province (1125 km of seismic lines over a 1666 km² area). All drill sites are located on high-quality cross lines. 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 acoustic imaging data, and towed ocean bottom instrument side-scan sonar (30 kHz). A multibeam survey was completed in June 2000 (Polarstern), and the area was covered again by the Irish Seabed Program. The ROV VICTOR (Institut Francais de Recherche pour l’Exploration de la Mer) was employed twice (Atalante and Polarstern) to video survey different mounds in the Begica mound province (Thérèse Mound and a transect from Challenger Mound to Galway Mound). Previous subbottom sampling includes more than 40 gravity and piston cores in the Belgica mound province (penetration = 1.5–29 m), numerous box cores, and ~1.5 tons of television-controlled grab samples.

Three main seismostratigraphic units can be identified in the Belgica mound area separated by two regional discontinuities (Van Rooij et al., 2003) (Fig. F3). The lowermost Unit P1 is characterized by gentle basinward-dipping, continuous parallel strata with moderate to locally high amplitude reflectors. A clinoform pattern formed by a number of superposed sigmoid reflectors is encountered in the upper strata of Unit P1 below and adjacent to Challenger Mound (Fig. F5). These clinoforms are frequently characterized by a high-amplitude top sigmoid reflector. They appear to reflect high-energy slope deposits and have the possibility, based on reversals of signal polarity, to contain traces of gas. An alternative explanation for the phenomenon is a contrast in lithology between the top of the clinoforms and the overlying sediments in combination with the geometry of the unit, which enhances the amplitude of the reflection (De Mol et al., 2005b). This seismic facies is interpreted as migrating drift bodies of Miocene age (Van Rooij et al., 2003; De Mol et al., 2005b). The upper boundary of Unit P1 is an erosional unconformity which strongly incises the underlying strata. Unit P2 is characterized by a nearly transparent acoustic facies on top of the erosional unconformity bounding Units P1 and P2. Only a few sets of continuous, relatively high amplitude reflectors are observed within Unit P2. De Mol (2002) interpreted this seismic facies as a nannofossil ooze of Pliocene age analogous to the similar seismic facies of ODP Site 980 in the southwestern Rockall Trough (Jansen, Raymo, Blum, et al., 1996). The uppermost seismic Unit P3, characterized by slightly upslope migrating wavy parallel reflectors, represents Quaternary drift deposits partly enclosing the mounds. The reflectors of Unit P3 onlap the mound, suggesting that the mounds were already present before deposition of the most recent drift. Scouring and moat features around the mounds suggest that they affect the intensity of the currents and the deposition of the enclosing sediments (De Mol et al., 2002; Van Rooij et al., 2003). Challenger Mound roots on the regional erosional unconformity separating Units P1 and P3 (Figs. F3, F5). The mound appears on seismic profiles as an almost acoustically transparent dome-shaped structure. The mound is bounded by diffraction hyperbolae originating at the summit of the mound. Inside the mound, no internal reflectors have been recognized, indicating a uniform facies without any large acoustic impedance differences. The mound acoustic facies might also be interpreted as a loss of seismic energy due to scattering or absorption by the rough seabed and internal structure of the mound. However, an important observation is that the reflectors underneath the mounds show reduced amplitudes, although the reflectors have not completely disappeared. This argues for the fact that not all the seismic energy is absorbed or dispersed inside the mound facies (De Mol et al., 2005b). The internal structure of the mounds is derived from the observation of shallow cores (Foubert et al., in press) and seismic velocity analyses (De Mol, 2002). The seismic facies of the coral banks is homogeneous and transparent with an estimated internal velocity of 1850 ± 50 m/s based on velocity pull-ups of single-channel seismic. This velocity suggests carbonate-rich sediment (velocity = 2300 m/s) intermixed with terrigeneous material (velocity = 1700 m/s) as groundtruthed by the surficial sediment samples.

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