I. PORCUPINE SEABIGHT (Leg I) 1.1. Objectives and geologic@ setting
N. Kenyon and M. Ivanov
Objectives
The objectives of the first leg of the 7th TTR were:
1. Deep-water carbonate mounds (north Porcupine Seabight and Celtic Margin of the Porcupine Seabight)
2. Deep-water turbidite channels, the Gollum channel system (Celtic margin of the Porcupine Seabight)
3. Slope stability (Biscay margin of the Celtic Sea)
These TTR objectives were planned to be met by the following surveys:
1. A reconnaissance of the eastern and northern margins of the Porcupine Sea Bight, based on OKEAN-long-range sidescan sonar records and seismics. For this purpose two adjacent tracks were generated. The aim was to collect information on the upper reaches of the Gollum channel system, on the presence of “barrier-type” mounds and on the seismo-stratigraphic expression of the upper sedimentary sequences in this area;
2. detailed investigation of mound structures described by Hovland et al. (1994) in the northern part of the Porcupine Sea Bight and the small mound structures discovered by the R/V Belgicu cruise to north-west of the large mound structures. Furthermore attention was paid to the recently discovered “barrier-type” mounds in the eastern part of the Porcupine Seabight. This was achieved by surveys with the OREtech medium to high resolution 30/100 kHz deep-towed sidescan sonar over several of the mound sites. Furthermore, gravity coring of the periphery, the moat, the flank and the crest of these carbonate mounds should provide information on their setting and uppermost structure. Underwater TV/video survey was expected to gain additional and detailed information on the seafloor processes taking place in the areas of mound occurrence.
Important information on the mound composition would be contained in bottom samples retrieved by the Preussag TV-controlled grab sampler. Particular attention was paid to biological investigation of these bottom samples.
3. detailed investigation of the central part of the Gollum channel system by the OREtech medium to high resolution 30/100 kHz deep-towed sidescan sonar, profiling and gravity coring of different sedimentary sub-environments (levees, terraces, channel, etc.);
4. detailed investigation of some possible instability features on the Biscay margin were planned but not carried out, as permission to work arrived too late.
Surveys 1, 2 and 4 were of particular interest to the CORSAlRES partner, while representatives from SOC, UC Cork and U Aberdeen were additionally interested in survey 3.
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Physiography of the Porcupine Seabight
N. Kenyon and P. Hunter
The Porcupine Seabight sits on the map rather like a pear that has had a bite taken out of the wide part and then been placed upright on a table such that the bite, the exit to the deep Atlantic, is on the left hand side. Bathymetric charts of the Porcupine Seabight were lacking in detail prior to the
map made by the Institute of Oceanographic Sciences, UK (Hunter and Kenyon, 1984). This improved greatly on the previous charts that had been prepared by Brenot and Berthois (1962) from much more limited data.
The technique for making the map was a novel one. It has been shown that it is better to make some assumptions about geological processes when contouring from scattered soundings (Laughton, 1986). There are usually benefits over maps that are contoured by computers. However it is still possible to make errors because ones assumptions may be wrong. An example of this is seen on the maps of the canyoned margin of the bay of Biscay (e.g. Roberts et al., 1979), which assume that unless there is evidence to the contrary, the canyons run down the line of greatest gradient. However the more recently available swath mapping techniques have shown that the canyons can be fault guided and run oblique to the greatest slope (Kenyon et al., 1978; Sibuet et al., 1984). The basic sounding sheets were compiled from all the data that had been supplied to 10s as a part of the GEBCO deep-sea mapping programme. In order to connect up the contours in the best way to depict the true shape of the ground, sidescan sonar data were analysed for relief features and the interpretation overlaid on the sounding sheets. Contours can then be drawn that are a great improvement on what would otherwise be drawn on the basis of soundings and assumption of process alone. Features on the sidescan sonar that are due to those small scale roughness contrasts that are not associated with relief, were not used. The resulting map has proven to be generally correct within the area where sidescan sonar was available. The sidescan sonar data were obtained with both the long- range GLORIA system from cruises in 1977 and 1981 and from medium range, 36 kI-Iz hull mounted, sidescan sonar used on the same cruises. The range of the GLORIA data is about 20 km and the data obtained cover the eastern slope and the Gollum Channel. The range of the hull mounted sidescan sonar system was 2.5 km to each side but in deeper water much of the record is taken up by the water column. However useful narrow beam profiles and narrow strips of sidescan sonar could be obtained with this equipment and it could be operated with little loss in data quality at ships cruising speeds of
12 knots.
The basin is about 350 km long and is open to the deep ocean in the south west through a constricted gap. The Irish shelf and the Celtic Sea are to the east, the Goban Spur to the south, the Porcupine Abyssal Plain (about 4700 m deep) and the Porcupine Bank to the west and the less than 300 m deep Slyne Ridge to the north. The basin margins are at their steepest (> 2.7”) where the Gollum Channel system is located, west of the Celtic Sea shelf edge. Overall gradients are greater than 1” here but are generally less than 2 degrees, These lower gradients are less than those for the slopes of the Bay of Biscay and the Rockall Trough. The basin floor lies between about 2000 and 3000 m.
The Gollum Channel system is one of the very few lengthy, leveed channel systems known from the NW European margin. It is named after a particularly unpleasant creature in J.R.R. Tolkien’s book “Lord of the Rings”, the more heroic and positive places, tribes and characters having been used for other features on the margin here. It is a striking looking tributary system with low sinuosity and narrow, steep sided channels. The heads of the channels are in depths of about 300m. They do not form steep walled amphitheatre shaped canyon heads like those in the Bay of Biscay (Kenyon et al., 1978). The widths remain fairly constant from top to bottom of the system. The deepest and widest slope channels are in the north. The gradient is greatest on the slope, decreasing to a minimum between 2300 m and 2500 m where a few meanders are found. Below 2500 m the gradient and the height of the walls increase. This implies that the channel has cut down through the steeper slope at the mouth of the Porcupine Seabight and has not yet reached an equilibrium grade. The levees are low, but can be seen on the bathymetric chart and appear to be higher on the right hand side. All profiles across the lower reaches of the channels show terraces. There is a sharp bend to the left at the
.I_ T--- __ .--__~
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mouth of the channel which is probably guided by older, more resistant rocks. There are rugged areas of probable outcrop near the mouth of the Seabight.
North of the Gollum Channel system there are two broad deeps cut into the slope. They are about 20 km and 25 km long, 10 km wide and up to 400 m deep. There appear to be accumulations of sediment at their foot. As there are no open channels at the base it is assumed that they are old slide scars.
The narrow linear deeps to the north of the two broad deeps include the closed deeps and the down slope trending deeps, described in the section on the OKEAN line. These are either partly filled channels or the site of erosion by gas seepage.
On the high resolution sidescan sonar data belonging to SOC there are iceberg ploughmarks.
This is their southernmost occurrence recorded in the NE Atlantic. In water deeper than the ploughmarks there are small sand waves, recorded down to a depth of 550 m. The occurrence of sand waves at these depths has been a mystery that is starting to unravel as a result of this cruise.
The presence of carbonate mounds on the eastern and northern slopes of the Seabight was not suspected until the paper by Hovland et al. (1994) and the R/V Be&a cruise of 1997. The 198 1 RRS Discovery cruise had mapped two mounds, one is on the bathymetry map and the other was too low, where crossed, to be on the map.
The western margin of the Seabight increases in steepness to the south. There is a pronounced outward bulge in the contours that lie in the north, the reasons for which are not yet known.
Brief Geological History of the Porcupine Basin
A. McDonnell
The Porcupine Basin is one of the largest offshore Irish basins and lies approximately 150 km to the southwest of Ireland. It is a Mesozoic to Cenozoic basin, which is elongate in a north-south direction and displays a roughly symmetrical cross section. Water depths in the present day basin vary from 300 m in the north to over 2000 m in the south. As a consequence of this southerly deepening most exploration wells to date have been concentrated in the northern shallower part of the basin. The Basin is bounded to the north and west by Precambrian to Lower Palaeozoic basement highs and is bounded to the east by the Irish Continental shelf. The history of the Porcupine Basin is described in terms of pre-rift (Devonian-Permian), syn-rift (Triassic-Jurassic) and post-rift (Cretaceous-Tertiary) phases of basin evolution (Croker and Shannon, 1987).
Pre-Rift
The Pre-rift succession in the basin contains Devonian, Carboniferous and Permian deposits.
The Devonian appears to be sparsely preserved, being encountered in only a few wells where it records the interfingering of continental fluvial deposits and nearshore marine strata. It is interpreted to represent a series of transgressions and regressions. These fluctuations continued through to the Carboniferous where a thick deltaic succession is preserved. Thick coarsening upward sandstone units are capped by coals deposited in a marshy delta top environment. The coals act as a potential gas source rock in the basin while the deltaics hold limited reservoir potential. The overlying Permian succession is dominantly continental, dominated by shales but also containing evaporites and is interpreted to represent continental playa lake deposition. The Permian and Carboniferous are only locally preserved and are thought to have been deposited in a series of north-east south-west oriented sub-basins which exploited an underlying Caledonian trend (Shannon, 1991).
Svn-Rift
The syn-rift phase of basin evolution contains strata of Triassic and Jurassic age. The Triassic is preserved locally in sub-basins and an oscillating nearshore environment of deposition is interpreted. It was deposited during an initial rift phase marking the break up of Pangea. The main phase of rifting occurred during the Mid to Late Jurassic during which time the basin developed its present day north-south orientation, oblique to both Caledonian and Variscan trends. The north-south trend is thought to be a response to extensional stresses in the nearby evolving North Atlantic Ocean.
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A marine transgression marks the Lower Jurassic while in the Middle Jurassic an ancient shoreline is interpreted to have existed in the centre of the basin, separating marine deposition to the south from continental deposition to the north. During this time, in the north of the basin braided stream deposits were laid down, grading upwards into finer sediments of meandering river channels.
Active rifting took place in the Late Jurassic accompanied by a regional sea level rise. Upstanding areas were submerged as the sea transgressed northwards and a range of lithologies and facies were developed throughout the basin reflecting syn-rift deposition. Depositional environments range from estuarine, tidal channel and beach deposits to fluvial and lacustrine deposits in the west. In the basin centre a submarine fan complex was developed. Rifting waned during the early part of the Cretaceous and a major unconformity marks the Jurassic-Cretaceous boundary.
The Jurassic contains the greatest potential in the basin for oil rich source rocks and also contains considerable reservoir quality sandstones, with a range of potential plays both structural and stratigraphic, present.
Post-Rift
A period of thermal subsidence followed rifting, with sediments of Cretaceous age onlapping and infilling the topography. The Cretaceous consists of a deepening marine environment although deposition is interrupted by a minor rift episode of Aptian-Albian .age. A deltaic complex was developed in the basin at this time. In the Upper Cretaceous a major rise in sea level occurred which resulted in chalk deposition. At the base of the Tertiary however, a relative fall in sea level marked the end of carbonate deposition.
Palaeocene and Eocene times record the shedding of elastics into the basin in the form of submarine fans from the western and eastern basin margins while a delta complex prograded from the north. Relative sea level rose throughout the Upper Tertiary and quieter water marine strata were deposited interrupted occasionally by channeling. There has been relatively little study carried out dealing with the Pleistocene and Holocene history of the basin however during this time sediments were likely to have been sourced from the Irish continental shelf to the east. Present deposition in the basin dominantly comprises quiet water deep marine mud although a deep sea channel system (Gollum System) is developed in the south of the basin, sourced from the eastern margin. A number of mounded features are also evident which have been interpreted by Hovland et al. (1994) as carbonate mounds possibly initiated by gas seeps to the surface.
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1.2. Seismic profiling data
A. McDonnell, 0. Krylov, A. Limonov
General description of collected seismic lines and preliminary seismic stratigraphy of the area
Seismic profiling was carried out in the Porcupine Basin (Porcupine Seabight) from the 9th until the 1 lth July 1997. Twelve profiles (PSAT-l to PSAT-12) were taken with a ship speed of approximately 6 knots and recording time of 3.0 sec. The profiles have a total length of 567 km. In terms of location (Fig. 5) profiles PSAT-l to PSAT-3 define a NW-SE line along the continental slope on the southeastern margin of the Porcupine Basin. Profiles PSAT-5, 6 and 8 were run parallel and westwards of the northwestern section of profile PSAT-3. These lines were spaced about 9 km apart to provide overlapping of the OKEAN sonographs. Profiles PSAT-4, 7 and 9 are short connecting sections. PSAT-IO, 11 and 12 were shot parallel and approximately 9 km westwards of PSAT-3, 2 and 1 but don’t extend as far south, stopping short of the Gollum Channel zone.
The profiled seismic section can be divided into 3 main seismo-stratigraphic sequences resting on a highly irregular, buried topography representing acoustic basement which may be Cretaceous to Lower Tertiary in age. A number of sub-sequences can also be identified. The upper sequence is tentatively correlated with Base Miocene to Holocene deposits, while the middle sequence is inferred to be Upper Oligocene in age. The lower sequence is suggested to comprise Oligocene strata. Information from Hovland et al. (1994) was used to aid in dating these deposits. The slope of the margin becomes progressively steeper from north to south, with line PSAT-l intersecting the steepest section of the slope, which is incised by canyons and gullies. This slope change may be of some importance, as there appears to be a relationship between slope gradient and style of deposition.
Seismic line PSAT-01
Line PSAT-01 (117 km long) is located at the southeastern corner of the study area and is characterised by very variable and irregular sea floor relief. The distinctive topography is primarily created by a large channel system, which is transected by the profile. Bathymetry maps display the channels extending from the eastern margin of the Seabight out into deeper water areas in the west.
Six main channels are crossed. The channels have flat bottoms and some are flanked by terraces and levees.
Inspection of the section allows three main sequences (Sl-3) to be identified (Fig. 6). Sl is the oldest sequence and displays very low amplitude reflectors which were affected by signal attenuation. S2 shows moderate amplitude events while S3, the youngest package, displays a moderate to high amplitude character. The sequences rest on an acoustic basement, which defines a very irregular, locally faulted, topography highlighted by a very high amplitude upper boundary.
Internally the basement displays a much weaker, dominantly transparent character, but in the upper part locally continuous stronger events are seen, suggesting sediment stratification. The basement is quite shallow in the most southern and the most northern parts of PSAT-01 where it is at a depth of approx. 0.5 s (TWIT). In the intervening section, a distance of approx. 85 km, the basement is much deeper reaching 1.5 s (TWIT) at its deepest. It is in this basal low that the thickest accumulation of sediment occurs, infilling and levelling off topography. The Gollum Channel system exploits this region suggesting an underlying basement control on location of the system.
Sl onlaps and infills this buried topography. It comprises low amplitude, laterally continuous events. The upper boundary of the sequence is undulatory in nature and is defined by the lowermost continuous reflector of a package of moderate to high amplitude events. S 1 makes up very large-scale (12 km diameter approx.) mound features (Fig. 7), which appear to be composed of at least three depositional sub-sequences. A dominantly concordant contact exists between these sub-sequences, but locally marginal truncation of underlying reflectors is evident. These packages appear to build up
directly on top of each other, reaching a crestal height of 500 ms and thinning laterally. All three sub- sequences display the same low amplitude character. The palaeolows between these mounds are onlapped and infilled by sediments with very low amplitude continuous reflectors similar to the
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mounds themselves. The infilling strata can also be subdivided into different depositional units, although they generally demonstrate the same internal pattern suggesting there was little variation in
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