The spatial distributions of faults and deep sea carbonate mounds in the Porcupine Basin, offshore Ireland

Marine and Petroleum Geology 20 (2003) 509–522 www.elsevier.com/locate/marpetgeo

The spatial distributions of faults and deep sea carbonate mounds in the Porcupine Basin, offshore Ireland Wayne Baileya,b,1, Patrick M. Shannonb,*, John J. Walsha, Vikram Unnithanb a Fault Analysis Group, Department of Geology, University College Dublin, Dublin 4, Ireland Marine and Petroleum Geology Group, Department of Geology, University College Dublin, Dublin 4, Ireland

b

Received 23 August 2002; received in revised form 12 June 2003; accepted 19 June 2003

Abstract Localised populations of deep water carbonate mounds occur throughout the NE Atlantic margin of Ireland, the UK and Norway, but the mechanisms responsible for their nucleation and growth are not well known. Based in part on the interpretation of seismic data from the Porcupine Basin, offshore Ireland, it has been proposed previously that deeply rooted faults are present immediately beneath mounds and act as conduits for the vertical migration of mound-feeding hydrocarbons. Several discrete carbonate mound populations or provinces are present in the Porcupine Basin above a number of distinct fault systems at different levels in the stratigraphy. However, detailed mapping of the distributions of both mounds and faults for two of these provinces in the northern part of the basin, using 3D and 2D seismic data, demonstrates that there is a poor spatial relationship between the two. Furthermore, virtually all the reflector offsets directly beneath the mounds, which have previously been interpreted as faults can be attributed to seismic artefacts such as velocity pull-ups and diffraction cones. Therefore, our findings strongly suggest that seismically mappable faults do not play a pivotal role, as conductive fractures, in the evolution of the mounds. However, mounds located towards the NW margin of the Porcupine Basin are underlain by a shallow, intensely faulted slide package, which provides one potential association between faults and mounds. q 2003 Elsevier Ltd. All rights reserved. Keywords: Seabed mounds; Faults; Porcupine Basin

1. Introduction Ever since the discovery of carbonate mounds in deep (500 – 1200 m), cold water environments along the NE Atlantic margin there has been considerable speculation concerning the mechanisms responsible for coral nucleation and growth (Hovland, Croker, & Martin, 1994). The primary framework organisms are the cold water corals Lophelia pertusa and Madrepora oculata, and the mounds have been shown to be stable over a wide range of water depths (150 – 2000 m) and temperatures (4 – 12 8C; Freiwald, Wilson, & Henrich, 1999; Henriet et al., 1998). Clusters of mounds within the Porcupine Basin, offshore Ireland, form discrete populations, fields or provinces that typically parallel isobaths within relatively constant depths (ca. 800 – 1000 m; see below). This observation has * Corresponding author. Tel.: þ 353-1-716-2331; fax: þ353-1-283-7733. E-mail address: [email protected] (P.M. Shannon). 1 Present address: CSIRO Petroleum, ARRC, 26 Dick Perry Avenue, Technology Park, Kensington, Perth, WA 6151, Australia. 0264-8172/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0264-8172(03)00079-5

fuelled debate concerning the controls on mound nucleation and growth. The coral forming organisms require a combination of factors in order to grow (DeMol et al., 2002; Kenyon, Ivanov, & Akmetzhanov, 1998 and references therein). Firstly, strong bottom currents are required in order to stop the mounds being buried by sediment and also to provide a nutrient source. Secondly, a firm substrate is required, which may be provided by hardgrounds or bedrock exposed by erosion or slumping (O’Reilly, Readman, Shannon, & Jacob, 2003). Thirdly, the corals require a nutrient source, which could be in the form of (a) organic matter sourced from the photic zone and/or rivers draining off adjacent land masses (Freiwald et al., 1999) or (b) a chemosynthetic food chain based on bacteria that feed on escaped hydrocarbons or gas hydrates (Hovland et al., 1994). Hydrocarbons and gas hydrates require transport mechanisms to get them to the seafloor and vertical migration along faults and liberation by slumps or seafloor slides have been proposed as possibilities (Henriet et al., 1998; Henriet, De Mol,

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to, the seabed. A demonstrable spatial relationship would help to support the thesis of Hovland et al. (1994) regarding a fault-controlled thermogenic food source for mound location and growth. The absence of a spatial relationship would negate the possibility of faults acting as the sole transport network for mound-feeding nutrients and would stimulate the search for alternative controls on mound location. The seismic reflection data used in this paper are from three 2D surveys and one 3D survey in the Porcupine Basin (Fig. 1). The 2D data comprise one extensive regional survey and two more localised surveys on the eastern and western basin margins. These data were recorded to 8 s TWT and have a line spacing varying from 1 to 7 km. The vertical resolution of the 2D data is ca. 5 ms at shallow depths decreasing to ca. 20 ms in the Jurassic. The 3D volume (ca. 1000 km2 area £ 6 s depth) has a comparable vertical resolution, but high lateral resolution of 12.5 m, which permits accurate mapping and correlation of geological structures.

2. Geological setting

Fig. 1. (a) Regional map showing the location of the Porcupine Basin, carbonate mounds (black circles) and the 2D (black lines) and 3D (grey polygon) seismic datasets. (b) Detail of the seismic coverage and extent of the mound provinces in the Porcupine Basin. Bathymetric contour interval is 100 m. The locations of mounds are those of Croker and O’Loughlin (1998).

Vanneste, Huvenne, Van Rooij, & the Porcupine-Belgica’97, 98 & 99 Shipboard Parties, 2001; Hovland et al., 1994). However, previous studies relied on either widespaced seismic grids or on high-resolution shallow seismics, which limited the ability to establish a 3D relationship, if any, between faults and mounds. In this study, we use high-quality 2D and 3D seismic databases from the northern part of the Porcupine Basin in order to carry out a detailed analysis of the spatial relationship between faults at different levels in the stratigraphy and clusters of carbonate mounds at, or close

The Porcupine Basin, offshore west of Ireland, is a deep water (200 – 3500 m water depths) underfilled sedimentary basin. The basin contains up to 13 km of Upper Palaeozoic to Cenozoic sediments (Shannon, 1991; Shannon & Naylor, 1998). The geological development of the basin reflects the sedimentary response to phases of rifting in the Permo-Triassic, Late Jurassic– Early Cretaceous and mid-Cretaceous with intervening periods of thermal subsidence. The rift phases saw the deposition of a range of clastic lithofacies ranging from deltaic sandstones to submarine fan sandstones with occasional lacustrine siltstones and mudstones (Sinclair, Shannon, Williams, Harker, & Moore, 1994; Williams, Shannon, & Sinclair, 1999). The thermal subsidence deposits are typified by fine-grained clastics and carbonates that onlap progressively onto the basin margins. Occasional non-rift marine regressions in the Early Cenozoic are interpreted as the response to a variety of ridge-push and mantle plume geodynamics (Jones, White, & Lovell, 2001; Shannon et al., 1999). Deep water sedimentation and the establishment of regional contour-hugging bottom currents is marked by a Late Eocene to Early Base Oligocene unconformity (herein referred to as C30 following the terminology of McDonnell and Shannon (2001), Stoker, van Weering, and Svaerdborg (2001)). A number of other significant unconformities are identified, the most pronounced being in the mid-Miocene (C20) and the early Pliocene (C10). These mark a combination of differential basin subsidence effects and of regional sea level and palaeoclimatic changes, but throughout the Oligocene and Neogene the sediment type was characterised by along-slope transport and redepositional

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processes yielding siltstone and mudstone contourites and pelagic sediments (McDonnell & Shannon, 2001).

3. Mound provinces Several hundred mounds, mostly seabed features and sometimes buried or partly buried, have been identified using seismic and other geophysical data in the Rockall Trough and the Porcupine Seabight (Croker & O’Loughlin, 1998). An elongate cluster of mounds, sometimes more than 100 m high and lying in water depths of ca. 800 m occurs along the eastern margin of the Rockall Trough adjacent to the Porcupine Bank (Kenyon et al., 1998; Shannon, O’Reilly, Readman, & Jacob, 2001). These are sometimes associated with slope failure escarpments (O’Reilly et al., 2003). Within the Porcupine Seabight the mounds fringe the eastern and northern margins of the basin and typically lie in water depths ranging from 500 to 900 m (Fig. 1). The mounds along the eastern basin margin lie in a region of high seabed dips associated with mass wastage and margin parallel erosional channels. These mounds are characterised by barrier-like buildups 1 – 3.5 km wide, 250 – 1500 m long and 50 – 200 m high, which lie in water depths of 600 –900 m. These mounds are not discussed further in this paper and the reader is referred to DeMol et al., 2002 for further details. The mounds on the northern slope of the basin can be divided into two distinct populations based on their size and morphology. The more southwesterly population is composed typically of large (, 2 km diameter, , 250 m height) seabed mounds lying in water depths of 600 – 900 m. In contrast, the more northwesterly group is characterised by small (50 – 100 m height) mainly buried mounds at water depths of 500 –700 m (Henriet et al., 1998, 2001), with few seabed mounds. Both populations are located in an area of low seabed dips and the mounds are typically associated with pronounced N –S oriented erosional moats. Significantly, all these mounds root on the same regionally continuous sub-horizontal reflector (herein referred to as the ‘Base Mound reflector’; Fig. 2b) at approximately 200 – 250 ms below the seabed. This points to a geologically instantaneous mound growth event. Based on shallow site survey seismic reflection data Huvenne, Croker, and Henriet (2002) suggested that this horizon is ‘near base Quaternary’ in age. However, regional mapping of the reflector in the course of the present study, and correlation of the reflector with a similar major, dated, sequence boundary (C10) in the Rockall Trough (McDonnell & Shannon, 2001; Stoker et al., 2001), suggests an early Pliocene age. This horizon is locally an erosional unconformity within the Porcupine Basin. The two mound populations on the northern slope of the basin lie within the area covered by our seismic data sets and

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form the basis of the analysis in this study (Fig. 1). Twodimensional data cover both the buried and seabed mound populations, whilst 3D data are restricted to part of the field of largely buried mounds in the northwestern part of the region.

4. Faults in the Porcupine Basin Within the Mesozoic and Cenozoic sedimentary succession of the Porcupine Basin several stacked fault systems can be identified. These can be generalised to include: (1) Late Jurassic – Early Cretaceous tectonic growth faults; (2) intraformational faults contained within Palaeogene (Paleocene to Eocene) submarine fan sandstones and mudstones; (3) localised packages of polygonal faults contained within Palaeogene and Neogene deposits and (4) an intensely faulted slide interval situated within the Neogene and approximately 200 m below the seabed (Fig. 2). The lowermost fault system comprises large-scale (100 m displacement), predominantly Late Jurassic to Early Cretaceous growth faults, which formed in response to a pronounced rift episode (Shannon, 1991; Shannon & Naylor, 1998; Sinclair et al., 1994). The mean N – S strike of these faults (McCann, Shannon, & Moore, 1995) defines the Late Jurassic to present day basin architecture. Estimates of b (stretching) factors for the Late Jurassic to Early Cretaceous extension vary from 1.2 in the north of the Porcupine Basin to six in the south (Tate, White, & Conroy, 1993; White, Tate, & Conroy, 1992). Only the largest of these faults are reactivated and locally breach the Base Cretaceous and Tertiary unconformities. Reactivation has resulted in a localisation of strain in the overlying fault systems and in places their upper tip lines extend close to the seabed, displacing units as young as Pliocene in age (Fig. 2a). The Jurassic growth faults are not discussed any further with respect to the mounds because they are buried by approximately 3 s (ca. 4 km) of sediment and by overlying intraformational fault systems and have no direct spatial relationship with the surface and near-surface mounds. Intraformational faults, typically with a maximum displacement of up to 250 m, are commonly present along the basin margin within a ca. 1.2 s (1.5 km) thick package of Paleocene – Eocene submarine fan deposits (Shannon, 1992) that is bounded by the Base Tertiary and C30 reflectors (Figs. 2 – 5a). The faults show no evidence of synsedimentary displacement. They are mainly truncated by the regional C30 unconformity, which indicates that faulting took place predominantly in latest Eocene times. These faults, on both the eastern and western sides of the basin, are characteristically organised into N – S trending fault systems that directly overlie the abrupt transition between the uplifted Jurassic– Cretaceous margins and the downwarped basin centres. This spatial association suggests that faulting formed partly in response to differential subsidence and

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Fig. 2. (a) Section through the 3D seismic volume. Three stacked fault systems are shown and include, from base to top, the Jurassic growth faults, intraformational faults contained between the Base Tertiary and the C30 unconformity, and the overlying polygonal fault package in the NW half of the section. Note the two reactivated basement faults located at the NW end and in the centre of the section. Position of the Eocene horizon presented in Fig. 5 is shown (arrow). (b) Seismic profile showing the spatial relationships between mounds and the slide interval. SW extent of slide indicated by arrow. Approximate location of the profile is shown in (a) and also in Figs. 1 and 5.

compaction (McCann et al., 1995). This is supported by regional mapping and subsidence analysis that point to a phase of rapid basin subsidence during the late Eocene in the Porcupine (Tate et al., 1993) and Rockall (Stoker, 1997) basins. Furthermore, evidence of sediment mobilisation, such as small-scale diapiric structures (, 50 ms high), observed using seismic data at the transition from mud- to overlying sand-dominated deposits, points to a possible role of fluid overpressuring and the generation of gravitational instability associated with faulting (Watterson, Walsh, Nicol, Nell, & Bretan, 2000). Locally, packages of polygonal faults are present in the basin and are observed in both Palaeogene and Neogene units (Figs. 2 and 5b). Polygonal fault systems are

characteristically layer-bound and are therefore another form of intraformational fault system (Cartwright, 1994). They are best identified using 3D seismic data, which reveals the 3D, multi-directional pattern of faulting. In the absence of 3D data, a layer-bound fault system may be interpreted as being polygonal if geometrically similar patterns of conjugate faults are observed on sets of perpendicular 2D lines. Two main polygonal fault systems have been identified and are located in the NW (3D data; Figs. 2 and 5b) and SE (2D data; not shown) of the basin. In the NW, they are observed in 2D and 3D datasets and are contained within a , 500 ms (625 – 750 m) thick package of mudstone-dominated Eocene deposits and truncated by the C30 unconformity to the SE (Fig. 2a). This fault system

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Fig. 3. Positions of faults and mounds interpreted on the 2D seismic surveys in the northern parts of the Porcupine Basin (2D lines are shown Fig. 1). Positions of intraformational faults are shown where they offset an Eocene reflector (Fig. 2a). The locations of mounds are those of Croker and O’Loughlin (1998).

directly overlies the intraformational faults described above, which locally extend upwards complicating the characteristic polygonal fault pattern. The polygonal faults in the SE of the basin are localised within a Miocene mounded sedimentary body (, 500 ms thick, ca. 40 km N – S and 20 km E – W), which may be a submarine fan deposit. The lowermost bounding surface displays evidence of sediment mobilisation in the form of small ‘diapiric’ structures at the intersections of conjugate faults. This is consistent with a model of faulting resulting from density inversion induced by high fluid pressures (Henriet, De Batist, & Verschuren, 1991; Watterson et al., 2000). These faults are located over 30 km south of the mounds and buried by 200 ms of sediment. Because of the lack of any obvious spatial association with mounds they are not discussed further. A thin (ca. 100 m), shallow (ca. 50 – 200 m below the seabed), intensely faulted ‘slide’ interval is identified in

the NW part of the basin (Figs. 2b and 6). This feature, which has previously been described by Huvenne et al. (2002), comprises a network of relatively undeformed, polygonal blocks bound by small (# 5 m displacement) faults defining conjugate arrays (Fig. 2b; between Base Slide and Top Slide reflectors). Both extensional and reverse offsets are observed along the faults, but reverse offsets dominate towards the SE compressional toe (inset Fig. 6a). The distinctive 3D map view pattern is reminiscent of polygonal fault systems, which can form due to the generation of a gravitational instability caused by density inversion and fluid overpressuring (Henriet et al., 1991; Watterson et al., 2000). The polygonal fault pattern is particularly well developed along the Base Slide reflector along which there is evidence of sediment mobilisation at the base of the slide that support the theory that high fluid pressures were important in the deformation of this interval (Figs. 2b and 6b). As a slight variation to the model of

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Fig. 4. (a) Detailed interpretation of 2D data on the NE Porcupine Basin margin showing the positions of mounds (grey), intraformational faults and seismic lines superimposed on bathymetric contours in ms (all derived from 2D data). All fault locations are where they offset an Eocene reflector (shown in b and c). The grey fault is the main basement reactivated structure that offsets the C30 unconformity. (b, c) Two-dimensional seismic profiles showing the distributions of faults and mounds. Note that the upper tips of the basement reactivated fault that offsets the C30 unconformity are still deeply buried beneath the mounds.

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Fig. 5. Coherence maps showing the distribution of intraformational and polygonal faults in the 3D area. (a) Intraformational faults at the level of an Eocene reflector (shown in Fig. 2a). Note that the positions of mounds (shown in white) at the Base Mound reflector are shown (from Huvenne, De Mol, and Henriet (2003)) and reactivated basement fault zones are annotated (arrows). The irregular SW boundary to the map is caused by erosion by the C30 unconformity. Location shown as the grey area in Fig. 1. (b) Detail of the polygonal fault pattern that overlies the intraformational faults. Note the N–S trending basement reactivated fault to the far west (arrow in a). The southwesterly extent of the polygonal fault system is shown in (a) and is caused by erosion corresponding to C30 and Miocene erosional events.

Huvenne et al. (2002) we interpret the slide as a highly evolved and overpressured polygonal-type fault system that was subjected to minor downslope gravitational sliding.

5. Distribution of faults and mounds 5.1. 2D seismic data Faults were proposed by Hovland et al. (1994) as conduits for thermogenic hydrocarbon seepage to feed

the mounds. On the basis of 2D seismic data, they interpreted offsets of high-amplitude seismic reflectors directly beneath mounds as faulted sediments charged with fluid/gas. It is now suggested that most of the features identified as faults by Hovland et al. (1994) are seismic artefacts related to the velocity contrast between carbonate buildups and the background sediments. We have verified this conclusion by analysis of the same seismic lines figured by Hovland et al. (1994). Faults directly underlying the mounds could be distinguished from seismic artefacts such as diffraction cones, velocity pull-ups and vertical breaks

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Fig. 6. Details of the deformation within the slide immediately beneath the mound population (Fig. 2). (a) Coherence map extracted from a sub-horizontal slice extracted 25 ms below the top slide reflector. Inset shows the detail of the pronounced compressional toe features. The positions of mounds (white) at the Base Mound reflector are shown (from Huvenne et al., 2003). Location shown as the grey area in Fig. 1. (b) Coherence map of the Base Slide reflector illustrating a distinct polygonal pattern.

because (i) they would extend away from the mounds and away from the underlying zones of poor seismic data on adjacent seismic lines and (ii) they would show horizon offsets beyond the lateral dimensions of the overlying mounds. In the case of the Porcupine examples, however, the lateral impersistence of apparent offsets and the lateral continuity of reflectors on either side of mounds, suggests that the immediately underlying offsets are seismic artefacts rather than faults (Fig. 4). In the case of the figures illustrated and interpreted from the Porcupine Basin by Hovland et al. (1994), dipping faults are interpreted along

the edges of diffraction cones which extend downwards away from the mound edges, and vertical faults through velocity pull-ups and through diffuse seismic data caused by the degraded stack response. These features are clearly seismic artefacts, which were also identified by Hovland et al. (1994). We were unable to identify any clear offset in the uppermost 1 s of section (latest Oligocene and Neogene) of the profiles in the vicinity of the mounds. This is also consistent with our regional mapping of faults in the mound region, (Figs. 2a and 4b,c) where the upper part of the sedimentary succession beneath the mounds is devoid of

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faults. Therefore, it is our conclusion that the features previously interpreted in the Neogene succession beneath the mounds as faults and fault offsets are seismic artefacts. The main problem with using widely spaced (ca. 3 –5 km) 2D seismic lines to infer spatial relationships between faults and mounds is that there is a high probability of either miscorrelating faults between lines (e.g. linking separate fault segments as one fault) or missing features between the lines. Nevertheless, it is possible to generate a map which accurately reflects the distribution of mounds, on the one hand, and faults (. 5 ms maximum throw) at different stratigraphic levels, on the other. Fig. 3 shows the distributions of mounds and of intraformational faults offsetting a mapped Eocene horizon. There is no clear spatial relationship between the two. The intraformational faults were chosen because they are the most widespread of the three fault system types and they contain occasional structural linkage with the underlying Jurassic reactivated faults. Therefore, if any spatial relationships exist between faults and mounds they should be observed on this map. Only a handful of these faults offset the C30 unconformity and are therefore the best candidates for fault-related migration pathways. Again, there is no spatial correlation between these faults and the mounds (Fig. 4). Fig. 4 shows the best example from the 2D data in which the positions of mounds and faults roughly coincide. The main fault (indicated in grey on Fig. 4) is a 130 ms maximum displacement (at Base Tertiary) basement reactivated structure that locally offsets the C30 unconformity. The upper tip-line of the fault in map view is located ca. 3.5 km upslope from the mounds and more significantly it is still approximately 800 ms (ca. 1 km) below the Base Mound reflector. Therefore, for any mound-feeding fluids/ gases to reach the mound they must migrate vertically through a considerable thickness of mud-dominated sediments. No other faults (. 5 ms displacement) are imaged at shallower levels so such migration would have to be through the sediment. If faults were always located beneath mounds then a case could be made for a genetic relationship. The fact that they are rarely located in such a setting suggests that faults are not a critical control on mound development. 5.2. 3D seismic data To constrain better whether or not there is a spatial relationship between the mounds and faults we have mapped their distributions using the 3D seismic volume. Fig. 7a shows the bathymetry for the survey area, which highlights the position of the mound population in the SE and the simple NE – SW striking contours to the NW. The strong N – S trending bathymetric contours associated with the mound population reflect the presence of moats on either side of mounds, which suggests the activity of N –S reversing currents. Fig. 8a –c show coherence maps for reflectors mapped at different levels through the mound

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population (Fig. 2b). Coherence seismic attribute volumes are used to highlight lateral changes in amplitude between adjacent seismic waveforms and are therefore ideal for identifying features such as faults, unconformities, channels and mounds across which there is an abrupt change in seismic character (Marfurt, Kirlin, Farmer, & Bahorich, 1998). Dark shading in Fig. 8a –c indicates the positions of mounds and also their N – S elongate erosional moats. The mound population defines a belt of . 10 km width, which approximately parallels bathymetric contours. Significantly, the largest mounds are located to the SW and also on the NW margin of the belt. This location coincides with the SE and NW margins of large wavelength (200 ms thick, 15 km long), mounded (contourite drift) deposits within the top slide-base mound and base mound-seabed intervals, respectively (Fig. 7c and d). Fig. 2 is a typical cross-section through the seismic volume showing the three main fault systems that characterise Jurassic, Palaeogene and (Palaeogene-) Neogene intervals. The spatial distributions of faults and mounds are easily evaluated using a coherence map from within the intraformational fault system (Fig. 5a). The map is of an Eocene reflector within the upper part of the intraformational fault system and shows that the relatively simple N –S trending fault pattern is only locally disrupted by reactivation of two main NNE- to NE-striking Jurassic basement fault zones (arrows in Fig. 5a; see also Fig. 2a). The overlying polygonal fault system (Fig. 5b), whose extent is marked on Fig. 5a, is truncated by the C30 unconformity and possibly also by a mid-Miocene unconformity. What is clear from Fig. 5a is that there is no clear spatial relationship between the mounds and faults. The intraformational faults are locally coincident with the mound population, but they strike N – S in contrast to the NE-trending mound belt and they are at least 500 ms (ca. 650 m) below the Base Mound reflector. The polygonal faults are located 1 km (minimum) upslope of the mounds and 300– 500 ms below the Base Mound reflector. Finally, the main reactivated basement faults, which locally reach the Base Mound reflector, are also located upslope (. 7.6 km) of the mounds. Small faults, close to the limit of resolution (ca. 5 m displacement), are observed at shallow stratigraphic levels. Fig. 9 shows a coherence map for a reflector located 50 ms (ca. 65 –75 m) below the Base Mound reflector (Fig. 2b). The upper tips of the reactivated basement faults are present at this level. The other small linear discontinuities (dark) on the map are small faults with dip lengths , 200 ms and trace lengths on average ca. 2.5 km. Their fault traces are irregular, occasionally concave in the down dip direction and they parallel the present day bathymetry, which all suggests that they are compaction related. Again, there is no clear relationship between the distribution of these faults and the mound population except that is for a single, relatively long (7.6 km) fault (marked £ on Fig. 9) that is coincident with the NW limit of some of the mounds.

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Fig. 7. Depth and isochore maps from the 3D seismic area. (a) Bathymetry. Note the positions of the mounds in the south and SE of the area defined by disruption of NE-trending contours by N –S oriented moats and by localised elliptical features which correspond to the larger mounds (also see (d) and Fig. 8). S ¼ seabed mound. (b) Time depth map of the Base Mound reflector. (c and d) Isochore maps for the Base Mound-seabed (d) intervals. Note that the positions of large mounds along the NW margin of the mound field, which is also evident by the N– S pattern of erosional moats, are clearly discernible in (d).

The presence of the intensely faulted slide package constitutes the best spatial correlation with the mounds (Figs. 2 and 6). However, we note that this package has been identified using 2D seismic lines away from the mound field (Huvenne et al., 2002) and similar structures have not been identified beneath the other mound provinces. Nevertheless, the spatial association between mounds and this interval is striking and the southeastern limit of the ‘slide’ lies within the relatively narrow transition from the buried to the seabed mound fields (Figs. 1b, 6 and 7). As stated previously, we interpret the intense faulting within this interval to be akin to polygonal faulting and the result of high fluid pressures (Watterson et al., 2000), which also resulted in minor (ca. 500 m) downslope translation of the whole interval. Therefore, a potential link exists between high fluid pressures and faulting at shallow depths directly beneath the mounds.

6. Discussion Using 2D and 3D seismic data we have demonstrated that there is no clear spatial relationship between faults and either buried or seabed mound provinces in the northern part of the Porcupine Basin. In support of the hypothesis that hydrocarbons act as a food source for the mounds, Hovland et al. (1994) noted that of the 25 exploration wells drilled in the basin prior to 1994 most reported hydrocarbons shows and four yielded significant discoveries. The best evidence of hydrocarbon migration to the seafloor in the Porcupine Basin is from the Connemara oil field (Games, 2001) and from site survey results on block 35/19 which, according to Hovland et al. (1994) show numerous pockmarks. However, these areas are located approximately 90 km north of the mound provinces and no mounds have been identified in

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Fig. 8. Coherence maps of reflectors mapped at various levels through the mounds in the 3D area. Dark areas are mounds (rounded central areas) or their elongate N –S erosional moats (see inset in (c)). Note that mound populations shown on the Mid Mound (c) and Upper Mound (b) reflectors include both buried and emergent mounds. Location of coherence maps shown in Fig. 7a and stratigraphic positions of reflectors shown in Fig. 2b.

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these areas. Significantly, we have not observed any pronounced or convincing association of pockmarks with mound clusters, nor has any been documented in the literature for the region (Kenyon et al., 1998). Additionally, we note that preliminary surveying of the seabed in the area of the 3D survey area did not find any evidence of gas seepage (Huvenne, Personal Communication). Furthermore, analyses of pore fluids and carbon isotopes from shallow cores on the mounds have failed to find evidence for hydrocarbons or methane (DeMol, Keppens, Swennen, & Henriet 1998; Ivanov, Kenyon, Henriet, Swennen, Limonov, & TTR-7 Shipboard Party, 1998). We cannot completely discount the possibility of a shortlived hydrocarbon seepage ‘event’, as suggested by Henriet et al. (2001) to trigger initial mound nucleation. However, the most obvious candidate faults that could facilitate hydrocarbon migration are located upslope of the mounds and/or at least 500 ms (ca. 650 m) beneath the mounds, which would require vertical migration through a thick, mud-dominated succession. In summary, models that link faults, hydrocarbon migration and mound evolution in these areas are highly tenuous. If there is not a direct spatial association between mounds and faults, what are the likely controls on mound development? The answer(s) to this question is beyond the scope of this paper, which focuses on an analysis of faults and mounds. The question is addressed in separate publications, but a number of salient points are worth highlighting here. The mounds show a spatial association with a number of underlying sedimentary and tectono-sedimentary features which may offer the potential for fluid sourcing for mound instigation. For instance, the mounds on the northern slope of the Porcupine Basin generally overlie a succession of coalbearing deltaic strata at Early Tertiary level (Moore & Shannon, 1992; McDonnell, 2001). Mounds also often occur above the margins of Miocene (presumed silty and permeable) contourites. A route for the migration of thermogenic gas, over a long period of time from the coals to the surface could be postulated through focussed diffusion along the edges of the contourites or through small-scale fracture networks, rather than along metre-scale displacement faults. Alternatively, slope failures may facilitate the release of fluids or gas during a ‘trigger event’ and such a (slide) feature has been identified beneath part of the buried mounds (Henriet et al., 2001; Huvenne et al., 2002). The close spatial relationship between the mound population and the slide may suggest a genetic association. However, such ‘trigger events’ are likely to be relatively rapid and are expected to provide only a very short-term mound-feeding source. While this could assist with the provision of an initial food source, it is unlikely to provide the required long-term food supply necessary for continued mound development. One possible link may be that the slide package represents a relatively high permeable layer along which basinal fluids may be transported to the surface over time to provide either nutrient support or the chemistry for hardgrounds on which

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Fig. 9. Coherence map for an arbitrary reflector located 125 ms below the Base Mound reflector in the 3D area showing the distribution of small (.5 ms throw), shallow-level faults. The positions of mounds (white) at the Base Mound reflector are shown (Huvenne et al., 2003).

many of the mounds appear to nucleate (T.C.E. van Weering, Personal Communciation). However, no direct linkage to seismically imaged faults (metre-scale displacements) can be demonstrated between the faulted package and the mounds, and no other shallow fault systems are seen associated with mounds in the basin. Previous alternative and diverse models for mound nucleation and growth are linked in that they are based on the spatial coincidence of mounds and certain geological and oceanographic processes. However, such processes are typically focussed along basin margins and this is compounded in the Porcupine Basin by the fact that the bathymetric shape of the basin has remained essentially fixed since Palaeogene times. The location of the basin margins is defined by syn-rift Jurassic faults, which have been continually reactivated in the Cretaceous and Tertiary. The tectonically uplifted topography at the basin margin resulted in the enhancement of contour currents in the Late Paleogene and in the Neogene, causing local erosion and deposition and the increased probability of sedimentary slides and slumps. The superimposition of these tectonic and sedimentary processes with the position of the mound provinces could conveniently be attributed to a genetic relationship. In particular, the stacking of fault systems, contourite deposits, submarine slides, erosional events and drift deposits could individually or collectively be associated with the carbonate mounds. The mounds typically define a series of bathymetrically constrained elongate clusters, with individual mounds showing the effects of clear current activity (O’Reilly et al., 2003). This raises the possibility that there is no underlying geological control on the location of the mounds. Instead the critical ingredients for mound development may be topographic and oceanographic. The only way to determine whether or not

a particular geological feature along the margin may be involved in mound evolution is to isolate and analyse the spatial relationships between the two. This is the approach we have used here for the faults. Our conclusion is that within the limits of the data area there is no spatial relationship between mounds and faults with metre-scale displacements. The structural controls on mound evolution may be indirect in controlling the bathymetric evolution of the basin and fluid or gas migration along conductive fault systems is not a prerequisite to mound growth.

7. Conclusions 1. Localised elongate clusters or ‘provinces’ of deep water mounds occur throughout the NE Atlantic Margin and are well developed in the Porcupine Basin, offshore Ireland. Several models have been presented in the literature in an attempt to explain the nucleation and location of the mounds. The most commonly cited generic model proposes that the mounds developed above faults that leaked thermogenic gas to the surface; a hypothesis, which is based in part on faults interpreted beneath mounds in the Porcupine Basin (Hovland et al., 1994). However, re-evaluation of these structures indicates that vertical offsets of seismic reflectors beneath the mounds are not due to faulting, but instead to seismic artefacts relating to their size and shape and their velocity contrast with the surrounding sediments. Furthermore, there is no documented evidence of gas seepage at the surface in the mound regions. 2. A detailed analysis of fault systems in the Late Mesozoic and Cenozoic succession in the region of the main mound clusters was carried out using 2D and 3D seismic data.

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Four stacked fault systems occur within the Porcupine Basin. These are (a) Late Jurassic– Early Cretaceous growth faults, (b) intraformational faults within Palaeogene submarine fan clastics, (c) localised packages of polygonal faults contained within Late Palaeogene and Neogene strata and (d) a thin, shallow, intensely faulted slide interval. 3. There is no spatial relationship between mounds and faults except for the intensely faulted slide interval. Any coincidence with deeply rooted faults appears to be entirely fortuitous. This suggests that other models are required to satisfactorily explain the nucleation and development of the mounds. These could include both geological and oceanographic models.

Acknowledgements The authors would like to thank Statoil Exploration (Ireland) Limited and their partners (Chevron, Conoco, Enterprise, Dana) for permission to publish this work. Thanks are also expressed to Peter Croker (Petroleum Affairs Division, Department of Communications, Marine and Natural Resources) for access to the MS81 digital data. WB, PMS and VU acknowledge the support provided by the EU FP5 project Geomound, Contract No. EVK3-CT-199900016. We would like to thank Veerle Huvenne for discussion and providing the positions of mounds shown in Figs. 5, 6 and 9 and to Jean-Pierre Henriet for introducing us to the slide. Daniel Praeg is acknowledged for discussions concerning the Rockall and Porcupine basins.

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