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Overview of Porcupine Seabight Mounds

A geographic overview of Porcupine Seabight and the mound provinces is shown on Figure F1. Three different types of mound provinces were identified: the Hovland, Magellan, and Belgica mounds.

Hovland Mounds

The first mound occurrences reported from industrial data on the northern slope of Porcupine Basin (Hovland et al., 1994) 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 with a very high density of buried medium-sized mounds (1 mound/km2; average height = 60–80 m). High-resolution seismic data (Henriet et al., 2001) combined with 3-D industrial seismic data (Huvenne et al., 2003) shed light on the presence of a past slope failure that underlies most of the mound cluster.

Belgica Mounds

On the eastern margin of Porcupine Basin, a 45 km long range of large mounds towers from a strongly erosional surface (Fig. F1). The mounds partly root on an enigmatic, deeply incised, very faintly stratified seismic facies (Unit P2) (De Mol et al., 2002; Van Rooij et al., 2003). De Mol et al. (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) and partly on a layered sequence capped by a surprising set of short-wavelength, sigmoidal depositional units (De Mol et al., 2002; 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 an impressive set of contourites (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 slopes are 10°–15°. The largest mounds have a height of ~170 m. In the deeper part 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 deepwater coral environments in Porcupine Seabight, remarkably in the middle of otherwise barren mounds (Galanes-Alvarez, 2001; Foubert et al., 2005, De Mol et al., in press).

Geological 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. Porcupine Seabight gradually widens and shoals to depths of 500 m to the east on the Irish continental shelf and north to Slyne Ridge. Porcupine Seabight, which is the surface expression of the underlying deep sedimentary Porcupine Basin (Fig. F2), 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 commences with probably 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). The 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 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 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 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.

Postrift Thermal Subsidence

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). A major rifting pulse during the Early Cretaceous, referred to as late Kimmerian tectonics, 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 onset of the Upper 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 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 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 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 coral mounds (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. Within some of these Connemara pockmarks, an associated fauna of the cold-water coral Lophelia pertusa has been suggested (Games, 2001). Together with Madrepora oculata, L. pertusa is found along the entire northwest European margin, manifest 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, European Science Foundation programs, 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 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 = 1–3 m) have been acquired over the Belgica mound province (1125 km of seismic lines over a 1666 km2 area). All drill sites were prepared by a minimum of a set of 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 (Training Through Research Program), 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 both Thérèse and Challenger Mounds. 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 some television-controlled grab samples of ~1.5 ton.

Recent very high resolution seismic investigations (Belgica) identified three seismic units (P1–P3) and provide an intriguing picture of the main possible sedimentary facies forming and underlying the mound (Fig. F3). Seismic imaging suggests that Challenger Mound roots on the sharp slope break off of seismic Unit P1 with an erosional upper boundary (Fig. F4). One interpretation of this seismic data, tested during Expedition 307, is the need to introduce a slightly higher velocity (2000 m/s) in the mound core seismic model. This suggests the beginning of lithification (Henriet et al., 2002). The uppermost seismic unit (P3) at Site U1318 is thought to represent late Neogene drift deposits (Van Rooij et al., 2003). Unit P3 partly overlies a seismic facies with low-amplitude reflectors and an unknown lithology (Unit P2) that underlies the southern part of the Belgica mound province but disappears below Challenger Mound (Sites U1316 and U1317). The uppermost surface of Unit P2 also appears to be an erosional surface (De Mol et al., 2002). Below this erosional surface, seismic Unit P1 contains a parallel series of high-amplitude reflectors that dip toward Porcupine Basin. Unit P1 has characteristic wave-shaped reflectors within its upper strata. These sigmoid-shaped waveforms enclose small lenticular bodies, frequently characterized by a high-amplitude roof, which also appear at the root of Challenger Mound and are thought to be of Miocene age (Van Rooij et al., 2003). 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 sigmoidal bodies and the overlying sediments in combination with the geometry of the unit, which enhanced the amplitude of the reflection. The top reflectors of this unit might consist of more compact sediments or a diagenetic cap creating the observed higher amplitudes (De Mol et al., in press).

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