B De Mol

Large deep-water coral banks in the Porcupine Basin, southwest of Ireland.

The Porcupine Basin, southwest of Ireland, was one of the earliest sites from where the deep-water corals Lophelia sp. and Madrepora sp. were recovered. These deep-water corals have since been found all along the Atlantic margins of Europe, in water depths ranging from 50 to more than 2000 m. Recent geophysical studies have demonstrated the mound-building potential of deep-water corals. Available data indicate that three major provinces of coral bank occurrences can be identified in the Porcupine Basin: (1) high-relief surface mounds which have a dimension of 1 by 5 km and a height up to 200 m (‘Hovland’ mounds), flanked to the north by (2) a swarm of buried mounds, somewhat smaller (up to 90 m), and with more irregular shapes than those recognised in area 1 (‘Magellan’ mounds), and (3) outcropping or buried, conical mounds (single or in elongated clusters, up to 150 m high) occurring on the southeastern slope of the basin (‘Belgica’ mounds). As far as can be inferred from shallow cores, the surface lithology predominantly consists of an upper layer rich in foraminiferal sand and terrigenous silty clay with intercalations of biogenic rubble. The banks host a remarkable number of colonies of living and dead Lophelia pertusa and Madrepora oculata. The living and dead assemblages are underlain by a significant layer of coral debris in a muddy matrix. Deep-water coral debris together with a living association of the same species covers the surface of the ‘Belgica’ and ‘Hovland’ mounds, which may suggest that these corals have played a significant role in the development of the mound structures. The capacity for mound formation by scleractinian corals in the aphotic zone has been known for some time. Examples are found at different locations along the shelves and the continental margins of the North Atlantic. The role of the corals in these deep-water build-ups is still a point of debate. Though the genesis and initial control of mound settings in this basin might be related to hydrocarbon seeps, it appears that the major development of the Porcupine coral banks in recent geological times has most likely been controlled by oceanic circulation and dynamics in water masses and nutrient supply.

Gas hydrate crystals may help build reefs

During a recent cruise in the Porcupine Basin, off southwest Ireland, we discovered two extensive and hitherto largely unsuspected deep-water reef provinces, including a giant cluster of hundreds of buried mounds. The ring shapes of many reefs suggest that they are caused by an axial fluid expulsion at the sea bed, a transient flow well confined in space and time. We are exploring various hypotheses, but a stimulating avenue for research is opened by a glacially controlled growth pulse and subsequent decay of a shallow layer of gas hydrates as a methane buffer and probably indirectly as a ground for overlying biological communities.

Seismic evidence of current-controlled sedimentation in the Belgica mound province, upper Porcupine slope, southwest of Ireland

The Porcupine Seabight is an embayment that takes a particular position in the NE Atlantic slope. Sonographs, a few current measurements and hydrodynamic modelling suggest the presence of a strong northward-flowing bottom current, locally enhanced by internal tides, affecting the eastern slope of the Seabight. At this location a province of coral banks is described, expressed as mounds lined up in along-slope-trending ridges. In this paper, very high-resolution single-channel seismic profiles are used to evaluate to what extent the bottom currents influenced the deposition of the sediments surrounding the mounds throughout the Late Cenozoic. Three seismostratigraphic units (P1, P2 and P3) can be identified in the Belgica mound area, separated by two margin-wide discontinuities (RD2 and RD1). Within Unit P1 (probably Early to Middle Miocene) upslope-migrating sediment waves are observed, suggesting strong bottom currents were already active in the Miocene. After an early Middle Miocene erosion event, represented by reflector RD2, an acoustically transparent layer (Unit P2) of as yet unknown lithology was deposited in the studied area. A second margin-wide erosional event, marked by the Late Pliocene RD1 reflector, removed a large part of Unit P2 and has cut deeply into Unit P1. Subsequently, the Belgica mounds were constructed spectacularly fast on topographic irregularities on the RD1 paleobathymetry. The onlap within the Quaternary Unit P3, which surrounds these mounds, suggests that the mounds were already present before the deposition of P3 and were big enough to affect the intensity of the currents around them. Furthermore, the channels and the mounds are, together with the complex oceanographic regime, the key morphological elements responsible for the shaping of a contourite system in the Belgica mound area during the Quaternary. One drift body is formed by an inferred south–north-directed current, with a drift levee and associated channel located on its western side. Between this channel and the mounds, large-scale sediment waves suggest an intensified bottom current running along the foot of the steep flanks of the mounds. The Belgica mounds are embedded in another drift body. Here, an interaction of bottom and turbidity currents is suggested, creating short turbidite channels at the southern and northern flanks of the mounds. Locally, small confined drifts can be observed where Unit P3 is deposited in a narrow passage made by the paleobathymetry of RD1 and the mounds.

A 3D seismic study of the morphology and spatial distribution of buried coral banks in the Porcupine Basin, SW of Ireland

An industrial 3D seismic data volume, supplemented by high-resolution 2D seismics, was used to study part of a province of buried mound structures in the Porcupine Basin, southwest of Ireland. These ‘Magellan’ mounds and their associated moat structures, interpreted as scour marks, were mapped semi-automatically from time-structure and isopach maps. Image analysis techniques such as a tophat transformation (mathematical morphology) were applied for feature extraction. Size measures of both mounds and moats were derived from the resulting maps and summarised by means of some descriptive statistics. Spatial variability in mound occurrence and characteristics was investigated. Comparison with other mound structures in the area allowed the Magellan mounds to be identified as ‘coral banks’, associated with the growth of cold-water deep-sea coral species such as Lophelia pertusa (L.) and Madrepora oculata (L.). Mound growth clearly started in a single ‘event’, confined in time and space. Bottom currents and oceanographic characteristics of the surrounding water masses influenced this sudden process and the further mound development. However, the analysis of the 3D seismic data set did not allow us to identify unambiguously the actual cause for the sudden mound start-up. The mounds appear to have formed a dense cluster of structures of moderate size, which are significantly elongated in a N–S direction. They are associated with even more elongated moats, implying a periodically reversing N–S-directed current influence. A spatial density of one mound per km2 was measured, which remains more or less constant over the area investigated. Mound width and cross-sectional area and moat shape gradually change across the mound province, due to spatially changing environmental conditions at the initial growth stages of the mounds and during their further development (interplay between current regime and sedimentation).

Carbonate mounds and slope failures in the Porcupine Basin: a development model involving fluid venting

Abstract High-resolution reflection seismic investigations carried out in the Porcupine Basin, SW of Ireland, have shed light on the presence of several provinces of giant carbonate mounds. An intriguing setting is found on the northern slope of the basin. A cluster of surface mounds appears to be flanked by a large upslope, crescent-shaped province of buried mounds. Below the transitional zone, large imbricated slide scars suggest repeated failures. The buried mounds rise from an undisturbed basal horizon and seem to represent a single event, confined in time and space. Both high-resolution and industrial seismic data reveal a close vertical match of the mound cluster with a lower, buried sea-bed failure, where hydrate build-up may have played a role. The latter association may not be entirely fortuitous. It is suggested that gas venting may have triggered the formation of the mound clusters, and that the underlying sea-bed failure forms a previous but different expression of gas venting, on a common, episodic fluid migration pathway but under strongly contrasting bottom water temperature conditions.

Mounds and Sediment Drift in the Porcupine Basin, West of Ireland

The Rockall and Porcupine Basins, west of Ireland (Fig. 1), feature probably some of the world’s most spectacular carbonate mound provinces (Croker and 0’Loughlin 1998). In addition to the cluster of large surface mounds in the Porcupine Basin described by (1994) and consequently referred to as “Hovland” mounds, two additional provinces have been unveiled in this basin by 1997 cruises of the RV Belgica and Professor Logachev (Henriet et al 1998; Ivanov et al. 1998): a large crescent of buried mounds — the “Magellan mounds” — north of the Hovland province, and an elongated cluster of partly buried mounds on the eastern slope of the Porcupine Basin: the “Belgicamounds” (Fig. 1). Open image in new window Fig. 1. Location of the mound provinces in the Porcupine Basin (shaded areas)