Mega-pockmarks and linear pockmark trains on the West African continental margin

Marine Geology 244 (2007) 15 – 32 www.elsevier.com/locate/margeo

Mega-pockmarks and linear pockmark trains on the West African continental margin Robin Pilcher ⁎, John Argent 1 Hess Corporation, 500 Dallas Street, Houston, Texas, 77006, USA Received 28 January 2007; received in revised form 11 May 2007; accepted 14 May 2007

Abstract Seabed pockmarks, the manifestation of the natural process of fluid escape at the seabed, are a widespread feature of the equatorial West African continental margin. Pockmarks occur singly, in small groups, in large random fields and in organized arrays or ‘pockmark trains’. Pockmark trains are associated with areas of steeper seabed gradient and evolve though time to form deep gullies. Pockmark gullies may exceed 1 km in width and extend for 10–20 km down slope, and form through the interaction of slope failure and fluid escape processes. Gullies maintain a rugose internal geometry throughout their development and do not represent sediment transport pathways to the deep basin. The geological processes that form seabed pockmarks and pockmark gullies are active today and these features may represent a hazard to subsea infrastructure. © 2007 Elsevier B.V. All rights reserved. Keywords: seabed morphology; pockmarks; West Africa; fluid escape; slump; seabed hazard

1. Introduction In this paper we illustrate a suite of fluid escape features, discovered during the course of petroleum exploration on the West African continental margin (Fig. 1). Two separate, but genetically related, features that differ significantly from those already reported in the literature are described: firstly, mega-pockmarks, which are the largest contemporary pockmarks known, and secondly, linear pockmark trains associated with areas of slope instability. Additional related features, some of which are similar to examples previously ⁎ Corresponding author. E-mail address: [email protected] (R. Pilcher). 1 Green House, Pearson Road, Sonning-on-Thames, Berkshire, RG4 6UL, UK. 0025-3227/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2007.05.002

described elsewhere, are illustrated where they help to explain the morphology or evolution of these newly described features. In this section we provide a summary and analysis of published examples of pockmarks and related fluid escape features. The regional setting of our study area is described in Section 2 and our methodology is laid out in Section 3. These newly recognized features and their mode of origin are described in detail in Section 4. In Section 5 we compare and discuss the results of this study to previously described features. 1.1. Pockmark morphology and size The term “pockmark” was introduced by King and MacLean (1970) to describe small “blips” on echosounder records. Since then these crater-like depressions have been recognized in many areas around the world.

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Fig. 1. Location of study areas on the West African continental margin. Regions 1 and 2 lie offshore Gabon and Region 3 lies offshore Equatorial Guinea.

Circular to elongate pockmarks have been described in a variety of marine settings, from estuarine through continental shelf and slope to deep marine. These features were first recognized and described in Nova Scotia (King

and MacLean, 1970) and the subsequent observation of similar structures in other areas was promoted by the advent and increased use of high resolution echo-sounder, side-scan sonar and 3-D seismic surveys employed in the exploration for hydrocarbons. Pockmarks are usually described as circular or nearcircular depressions, generally 10–200 m in diameter and up to 35 m deep. Several special cases of non-circular pockmarks and regular arrangements of pockmarks are discussed in Section 1.2. Globally, contemporary pockmarks have been reported with diameters of less than 1 m to hundreds of meters (Fader, 1991; Haskell et al., 1999; Hovland and Judd, 1988). Pockmarks greater than 250 m in diameter are coined “giant” by Foland et al. (1999), who describe a pockmark field on the Californian margin. A compilation of published data on the size (diameter and depth) of pockmarks described in the literature is presented graphically in Fig. 2. It is clear that a wide range of sizes of pockmark exist, covering more than 4 orders of magnitude, however the majority of those observed fall within the 10–250 m diameter and 1–25 m depth range. In the sub-surface, exceptionally large irregular craters, up to 4 km in diameter and developed in Palaeogene age strata, have been interpreted to have formed through large-scale fluid eruption; however these

Fig. 2. Graph illustrating the size of 57 published occurrences of contemporary pockmarks from around the world. The large diamond represents megapockmarks described in this study. The X and Y scales are logarithmic. Various ways of describing pockmark size have been used in the source literature — we attempt to honour these different formats in this graph. Single points represent either measurements of single pockmarks or average measurements where no range is given. Error bars represent the range of sizes in a pockmark field or region, with the associated point representing either the average or maximum values as quoted in the source. The mean diameter of pockmarks is 128 m and the mean depth is 9.6 m. Source data from Carvalho and Kuilman (2003), Dimitrov and Woodside (2003), Fader (1991), Games (2001), Hasiotis et al. (2002), Haskell et al. (1997, 1999), Hovland (1982, 1991, 1992, 2003), Hovland and Judd (1988), Hovland et al. (1987), Kelley et al. (1994), Maestro et al. (2002), Mosher et al. (2004), Pickrill (1993), Rise et al. (1999), Schroot and Schüttenhelm (2003), Söderberg and Flodén (1992) and Soter (1999).

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are interpreted to be composite craters made up of several smaller (100–200 m) pockmarks (Cole et al., 2000). The internal shape of pockmarks ranges from cone shaped to flat-bottomed. The internal slopes of pockmarks average 9° with a range of 6°–18° (Fader, 1991). It seems likely that fresh pockmarks are conical depressions which become rounded or flat-bottomed through time due to slumping or coverage by hemipelagic sedimentation. Close up investigations of the structure of a pockmark using a remotely operated vehicle (ROV) revealed no evidence of an ejecta rim around the depression (Hovland et al., 1987). Many larger pockmarks are long-lived features and, where seismic reflection data are available, can be seen to have grown at the seabed over an extended period of time (Gay et al., 2006a,b), while others have been buried by recent sediment, indicating they have ceased to be active (Cole et al., 2000; Games, 2001; Gay et al., 2003, 2006a,b; Hovland, 1982). 1.2. Pockmark arrangement and density Pockmarks are found to occur in both random and non-random distributions and in a wide range of spatial densities. We use the term “random” to describe those pockmarks which either occur singly or appear to have an irregular distribution on the seabed, with no discernable spatial relationship to each other or to a resolvable surface or sub-surface feature. They typically occur on low-gradient shelf and deep-marine environments underlain by simple layered stratigraphy. Random pockmarks are described in Section 1.2.1. Non-random pockmarks show one of several types of organized spatial arrangement, related either to more complex underlying geology or to local disturbance of the seabed. Non-random pockmarks occur in a variety of settings and their formation has been attributed to a variety of mechanisms. In this paper we propose a new mechanism for non-random pockmarks so these types are described in more detail in Sections 1.2.2–1.2.10. 1.2.1. Random pockmarks Random pockmarks are the more commonly described type and occur singly, in small groups or in extensive fields with spatial densities of up to 1000/km2 (Fader, 1991; Hovland, 1982; Hovland and Judd, 1988; Rise et al., 1999). Random pockmarks may be produced by a single episode of fluid expulsion or be the result of prolonged constant or episodic expulsion. Random pockmarks are usually associated with flat or gently dipping homogenous mud-prone sediments in structurally simple geological substrates.

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1.2.2. Pockmark clusters and coalescing pockmarks In several areas, including the Gulf of Maine and Passamaquoddy Bay in the Northwestern Atlantic (Fader, 1991), the Ionian Sea, Greece (Hasiotis et al., 2002), the North Sea (Hovland, 1982; Hovland et al., 1987) and the Mediterranean (Dimitrov and Woodside, 2003) pockmarks are observed to form groups or clusters. High resolution side-scan sonar surveys in the Emerald and LaHave Basins are interpreted to show large pockmarks composed of hundreds of very small internal pockmarks (Fader, 1991). These are similar to the observations of “unit” pockmarks made by Hovland et al. (1984) in the North Sea and Scotian Shelf. Hovland (1992) also describes clusters of unit pockmarks increasing in density to a central depression — the result of many episodic fluid escape events. 1.2.3. Fault-strike pockmarks We use the term “fault-strike” pockmarks to describe the linear arrangement of pockmarks aligned along the strike of a sub-surface fault. Care is required in interpreting these pockmarks as other mechanisms, especially those acting from above the seabed, can form linear arrangements of pockmarks (see Sections 1.2.9 and 1.2.10). Fault-strike pockmarks have been described in the northern Gulf of Lawrence, Canada (Syvitski and Praeg, 1989), the Baie des Chaleurs, offshore New Brunswick, (Fader, 1991), the Gulf of Corinth (Soter, 1999) and the South China Sea (Hovland and Judd, 1988). Although several examples exist where linear strings of pockmarks have been genetically linked to underlying faults, detailed descriptions of the position of the individual pockmarks in relation to the fault are scarce; this issue is discussed in more detail in Sections 1.2.4 and 5.1. 1.2.4. Fault hanging-wall pockmarks Maestro et al. (2002) describe large (350 m diameter) pockmarks in the Ebro Delta, associated with biogenic gas generated in the organic-rich clays of the pro-delta. These pockmarks are found to occur in the hanging-walls of listric faults and appear to lie above the footwall cut-offs of the organic-rich unit (Figs. 6 and 8 of Maestro et al., 2002). Abrams (1996) describes fault-related seepage occurring up to several hundred meters from seabed fault scarps in the Gulf of Mexico and schematically illustrates seepage to be located on the hanging-wall of a fault (Fig. 2 of Abrams, 1996). An important distinction can be made between faultstrike pockmarks (Section 1.2.3) where the fault plane acts as a fluid conduit to the surface and pockmarks form along its strike, and fault hanging-wall pockmarks

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where the pockmark develop offset from the fault trace on the hanging-wall side. The fault creates a change in structural elevation of a fluid-charged layer in the subsurface, locally tilting the layer and bringing the footwall side closer to the seabed, while dropping the hangingwall side down. We discuss this mode of pockmark formation in more detail in Section 5.1. 1.2.5. Buried channel pockmarks In many areas of passive margins, straight or sinuous submarine channels have been identified crossing the continental shelf and slope. Where courser-grained channels occur in mud-dominated settings these channels form a preferred fluid migration pathway in the subsurface. Linear pockmark arrays have been observed above the length of buried channels, indicating the escape of fluid from the channel sediment, vertically upward to the seabed (Haskell et al., 1997; Gay et al., 2006a,b). In many areas these shallow channels can be identified and mapped in the sub-surface by their seismic facies, minor incision or compactional faulting, or their acoustic amplitude contrast, and a close relationship between channel and overbank facies and seabed pockmark occurrence can be established (Gay et al., 2003). Minor (sub-seismic) compactional faulting caused by the lithology contrast between channel sands and surrounding muds may provide the fluid migration pathways to the seabed (Haskell et al., 1999). 1.2.6. Mud diapir pockmarks Pockmarks have been described in several areas where mud diapirs are interpreted to exist in the shallow subsurface (Dimitrov and Woodside, 2003; Hovland, 1991; Hovland, 1992; Hovland and Judd, 1988). In the Skagerrak region between Norway and Denmark, Hovland (1991) describes large pockmarks that appear to be spatially related to the underlying steep sides of clay diapirs. The same pattern is observed in the Adriatic where again gas seepage appears to emanate from the steepest parts of the underlying structures. Hovland (1992) proposes that the source of the gas is the mud diapir itself, charged by percolation of gas into the clay pore space from below, while the flaky clay fabrics formed along the “skin” of the diapir provide a preferential fluid migration pathway. We suggest that the steep flanks of diapirs, whether clay or salt, provide a lateral barrier to fluid flow in tilted beds, and are deflecting and directing fluids upward to the seafloor, where the pockmark forms. 1.2.7. Slump pockmarks A slump is defined as the movement of a coherent sedimentary mass along a discrete shear plane, where the

internally undeformed mass undergoes backward-tilting rotation (Schwab and Lee, 1988). Like other mechanisms that locally reduce the lithostatic pressure on a fluidcharged layer in the shallow stratigraphy, sediment slumping may promote preferential pockmark formation in the area of the slump (Foland et al., 1999; McNeill et al., 1998; Sultan et al., 2004). Slumps are common on unstable (steep) slopes and may be triggered by seismic activity (earthquakes), storm-wave action, or local subsurface movement such as diapirism or faulting. Sediment slumps may be associated with active faults or salt/shale diapirism in the sub-surface, so the sub-surface geology, rather than the slumping itself, may be localizing pockmark formation. 1.2.8. Current-modified pockmarks Although pockmarks originate as circular or nearcircular features in plan view, they may be subsequently elongated through scouring and current action (Bøe et al., 1998; Josenahns et al., 1978). Pockmarks in Placentia Bay, Newfoundland, occur in association with an area of mega-flutes at the seabed; the pockmarks are interpreted to have provided an initial seabed roughness for subsequent generation of mega-flutes by tsunami-generated turbidity currents (Fader, 1991). Fader and Miller (1988) suggest that the 4 km by 100 km area of mega-flutes is the result of a tsunami generated during a large earthquake in 1929. On the southern slope of the Norwegian Trench, elongate depressions with widths of up to 400 m and lengths of up to 2 km are interpreted as current-modified pockmarks by Bøe et al. (1998). 1.2.9. Iceberg scour pockmarks In arctic regions the seabed may be disturbed by iceberg scouring. Linear pockmarks on the Labrador shelf and Laurentian Channel, Canada (Fader, 1991), the Barents Sea (Hovland and Judd, 1988) and offshore mid Norway (McNeill et al., 1998) have been attributed to this mechanism of formation. The local scouring by grounded moving icebergs causes sufficient reduction in lithostatic pressure to allow fluid in the shallow sediments to escape to the seabed, forming a pockmark (Fader, 1991; Piper et al., 1985). 1.2.10. Anthropogenic pockmarks Like iceberg scours, anthropogenic mechanisms can disturb the seabed from above, locally reducing lithostatic pressure and causing fluid to be released to the seabed. These triggers can include trawling during bottom-fishing, dredging, dumping of dredged material, anchoring and propeller wash (Fader, 1991; Hovland and Judd, 1988). In heavily bottom-fished areas, such as

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the Scotian Shelf of Canada, pockmarks are commonly associated with the linear and curvilinear trawl marks that cover the seafloor. These pockmarks are aligned along the length of the trawl marks (Fader, 1991). The observation of pockmarks associated with anthropogenic features confirms that pockmark formation is an ongoing process as bottom-fishing operations only commenced in the 1950's (Fader, 1991). 1.3. Geographical distribution of pockmarks Pockmarks are known to occur in a variety of marine environments from estuarine to marine shelf, slope and abyssal plain settings and have been recorded at all water depths from b 10 m to ∼5000 m (Hovland and Judd, 1988). They are also reported in lacustrine environments (Hovland and Judd, 1988; Pickrill, 1993). Besides a sub-aqueous environment, certain other factors seem to be important in pockmark formation. First, a source of fluid to produce the pockmark: gas of thermogenic or biogenic origin or pore fluid from rapidly buried and compacting sediments. Second, a finegrained clay-rich and soft substrate; thus true pockmarks cannot form in a crystalline or lithified substrate. Examples of crater-like structures that cut lithified strata are interpreted to form through catastrophic eruption of gas through a competent seal layer such as volcanoclastic tuffs (Cole et al., 2000) or gas hydrates (Solheim and Elverhøi, 1993). While fluid escape will occur through course-grained sandy substrates no pockmark will be formed; however, other indications of fluid escape may be observed, for example bioherms or bacterial mats (Fader, 1991). Despite the above criteria, pockmarks have been observed in some surprising settings; for example in the Swedish Baltic Sea pockmarks occur in an area of crystalline rocks with only a very thin sedimentary cover. Here they are interpreted to be sourced by thermogenic gas that has migrated through basement fractures from source beds to the west (Hovland, 1992). Along the Atlantic coast of North America the only bays and estuaries that are known to contain pockmarks are those along the Fundy/Gulf of Maine coasts. One factor in the formation of pockmarks may be the macrotidal setting of these areas. Changes in the delicate balance between the pressure of confined shallow gas and the pressure imparted by the overlying water, brought about by the extreme tidal range, may be enough to trigger periodic release of gas (Fader, 1991). Pockmarks have previously been noted to occur in a variety of environments, including: in association with mud diapirs and mud volcanoes (Dimitrov and Wood-

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side, 2003; Hovland, 1992; Hovland and Judd, 1988), on active faults (Dimitrov and Woodside, 2003; Fader, 1991; Soter, 1999; Syvitski and Praeg, 1989), associated with submarine slumps (Dimitrov and Woodside, 2003; Foland et al., 1999; Schwab and Lee, 1988). Basins with these geological or structural phenomena may be more likely to develop pockmarks. A genetic link between pockmark occurrence and petroleum basins has been implied both in the literature (Hovland and Judd, 1988) and within the petroleum industry, particularly with respect to new venture exploration. In a basin with an active petroleum system there is certainly an additional source of migrating fluids (thermogenic hydrocarbons), that may form pockmarks and in some cases gas escaping from pockmarks has been demonstrated to be thermogenic in origin (Hovland and Sommerville, 1985). While pockmarks may be more common in petroleum basins, many of the described occurrences are in non-petroleum bearing basins as pockmarks will form equally from biogenic gas or sedimentary pore fluids. While several observations regarding the geographical distribution of pockmarks have been made or implied in the past, we feel that most of the apparent clusters of pockmark occurrence are largely due to sampling bias; thus many examples are known from shallow coastal waters, often in heavily fished northern latitudes and many examples come from basins where active petroleum exploration and production is occurring. The seafloor of these areas tends to be heavily investigated with echo-sounders, sonar and seismic surveys. 2. Regional setting In this paper we describe a variety of features from the equatorial West African continental slope, including mega-pockmarks, random, and non-random pockmarks and various linear pockmark trains. We focus on the continental slopes of Gabon and Equatorial Guinea (Fig. 1), part of the passive margin formed during the Mesozoic rifting of the South Atlantic. The offshore areas of both Gabon and Equatorial Guinea, like much of the West African margin, have been extensively surveyed in the exploration for petroleum reserves. We utilize seismic data acquired for petroleum exploration in this study. 3. Methods The dataset used in this study comprises three 3-D seismic reflection surveys from the West African

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margin, two from offshore Gabon and one from offshore Equatorial Guinea. We refer to these three areas as Region 1, Region 2 and Region 3. Region 1 covers an area of 1600 km2 on the Gabonese continental margin, centered approximately 110 km southwest of the coast at Libreville, in water depths of between 540 m and 1860 m. This survey was acquired in a north–south orientation, with a line spacing of 12.5 m. Region 2 covers an area of 290 km2 on the Gabonese continental margin, centered approximately 140 km south-southwest of the coast at Libreville, in water depths of between 75 m and 800 m. This survey was acquired in a northwest–southeast orientation, with a line spacing of 12.5 m. Region 3 covers an area of 2800 km2 on the Equatorial Guinea continental margin, approximately 40 km from Mbini, in water depths of between 60 m and 1700 m. This survey was acquired in a northeast– southwest orientation, with a line spacing of 12.5 m. All three seismic surveys are time-migrated and of high resolution and good interpretation quality in the shallow section of interest. Where depths are quoted these have been calculated directly from the seismic data using a seawater velocity of 1450 m/s. Distances, areas and the dimensions of individual features were measured from the maps using the planimeter tool in the seismic interpretation software. Interpretation was carried out on a fine grid basis with the resulting maps being auto-tracked then hand edited to remove spikes and areas of poor tracking. Dip attribute maps were then computed on the edited seabed maps. These maps remove the effect of a large depth

range across the dataset and allow the small-scale features, i.e. pockmarks, faults etc., to be interpreted and illustrated in detail. The structural models are illustrated by taking vertical profiles through type structures at various stages of the proposed evolutionary sequence. As we are looking at a geological present-day snapshot these do not represent a true time sequence, but an interpreted evolution based on the data available to us. Seismic profiles are illustrated with a modest vertical exaggeration to facilitate the interpretation and display of the structures described. The large-scale 3-D visualizations are illustrated with a 3× vertical exaggeration. 4. Results In the examples illustrated here from the West African margin we have observed both random and non-random pockmarks, some similar to those already described (see Section 1) and several features which differ significantly from features previously described. We restrict the detailed discussion to the unusual and new features. 4.1. Description of pockmarks in Region 1 (Gabon) Region 1 lies in offshore Gabon in water depths ranging from 540 m to 1860 m (Figs. 1 and 3). A variety of pockmark features are observed including random pockmarks in the west and southeast of the area, and non-random pockmarks and pockmark trains in the central and northern areas (Fig. 4).

Fig. 3. 3-D visualization of the seabed structure map in Region 1. Viewing direction is from the West. Cool colours indicate deep water, warm colours indicate shallower water. Vertical exaggeration is 3x. For scale see Fig. 4.

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shallow horizon (c. 200 ms below seabed) is interpreted, a complex polygonal pattern of faults is revealed (Fig. 5). Similar fault patterns have been described in the North Sea (Cartwright, 1996) and the Lower Congo Basin (Gay et al., 2004), where polygonal fault networks are interpreted to form in response to threedimensional compaction and dewatering of fine-grained sediments during early burial. We agree with Gay et al. (2004) that the two phenomena, polygonal faults and random pockmarks, may be genetically linked, and that the minor faults associated with the dewatering of shallow overpressured shales may provide the fluid migration conduits that are necessary for the formation of pockmarks. Polygonal fault patterns are present in stratigraphically bounded layers and several distinct networks of polygonal faults can be stacked in a given area, suggesting that the faults are restricted to thin intervals rather than being through-going (Cartwright, 1996; Gay et al., 2004). We suggest this observation helps explain why the locations of the seafloor pockmarks do not form a corresponding polygonal pattern. The upper slope random pockmarks are somewhat different. They range in size from large (∼ 100 m) to in

Fig. 4. Seabed dip of Region 1. The location of the sub-areas illustrated in Figs. 5, 6, 7 and 8 are highlighted.

4.1.1. Random pockmarks Randomly distributed pockmarks exist in two areas in the landward (upper slope) and basinward (lower slope) parts of Region 1. The lower slope pockmark field, which is located in water depths of 1400–1800 m, is characterized by circular features ranging in diameter from approximately 50 m (similar to those described in other basins) to extremely large depressions which reach up to 750 m and are commonly over 500 m in diameter. The depths of the pockmarks range from approximately 20 m to over 95 m. The pockmark density in the area covered by our data is approximately 1.4/km2. This is lower than concentrations described in other areas (e.g. Fader, 1991; Foland et al., 1999; Hovland and Judd, 1988), probably because of the significantly greater size of these structures. We do not see any clear evidence for a relationship between water depth and pockmark size. In the lower slope setting where sediments are generally fine-grained, stratigraphic dips are low (1.3– 1.8°) and tectonic structures are rare, a random distribution of pockmarks can be expected. When a

Fig. 5. (a) Dip-map interpretation of a horizon ∼ 200 ms below the seabed (see Fig. 4 for location). Although the interval appears flat and continuous the dip-map reveals a fine network of polygonal faults. (b) Seabed dip-map of the same area showing pockmark distribution.

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4.1.2. Non-random pockmarks In Region 1, non-random associations of pockmarks occur in a variety of settings including: in fault hangingwalls, at fault terminations, and in dip-parallel strings associated with slump faulting in an area of steeper seafloor dips. A few examples of pockmarks along fault strike were observed; these have a similar mode of formation to the seemingly random pockmarks described above in that the fault provides the conduit for fluid expulsion at the seafloor. Here the faults reach the seabed, unlike the polygonal dewatering fault array described in Section 4.1.1. However, in many of the pockmarks observed in our study area we note that the pockmarks are not at the fault intersection with the seabed, but are offset from the fault on the hanging-wall side (Fig. 7). Here we interpret the initiating factor to be the release of overburden pressure caused by extensional thinning, allowing the fluid to escape through the soft unconsolidated

Fig. 6. (a) Seabed dip-map in an area of mega-pockmarks in the central part of Region 1 (see Fig. 4 for location). The pockmarks are circular, up to 1500 m in diameter and up to 150 m deep. (b) Seismic reflection profile across three mega-pockmarks.

excess of 1500 m, with diameters of over 1000 m being common. This scale of contemporary fluid escape structure has not previously been described (Fig. 2), and here we coin the phrase mega-pockmark to differentiate those features that are greater than 1000 m in diameter. The depth of some of these mega-pockmarks has been calculated (using seawater acoustic velocity of 1450 m/s to convert from time to depth) to be over 150 m (Fig. 6). The concentration of pockmarks on the upper slope is much lower (approximately 0.15/km2), supporting a relationship between size and density of pockmarks. On the upper slope where the pockmarks are observed to be significantly larger, but much less common, polygonal fault patterns are not observed in the shallow sediments. Thus without the dewatering mechanism to initiate a pervasive network of fluid conduits in the soft sediments, the dominant control is tectonic; pockmarks are restricted to areas that have abrupt changes in the underlying geology and are therefore less common, but larger. Some of these upper slope mega-pockmarks also show a relationship to active faults in the shallow subsurface.

Fig. 7. (a) Seabed dip-map in an area of fault hanging-wall pockmarks (see Fig. 4 for location). The pockmarks are circular and are offset by 200–600 m from the fault trace in the hanging-wall side. (b) Seismic profile showing high-angle extensional faults with pockmarks in their hanging-walls.

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sediments. The fault itself does not act as the migration pathway. The pockmarks are offset from the fault trace in the hanging-wall side by a few hundred meters (Fig. 7); the faults dip at between 50–60° suggesting that, if fluid migration is vertical, the source of the fluids is at a depth of approximately 300–800 m below the seafloor. We believe this process of fluid escape and pockmark formation, triggered by reduction in lithostatic stress in fault hanging-walls is also the mechanism which, when occurring in areas of steepening dip and instability on the slope, is responsible for the growth of pockmark chains that ultimately coalesce into gullies. These unusual features are the subject of Section 4.1.3. 4.1.3. Pockmark trains and gullies In the central part of Region 1, the gradient of the slope increases; seismic reflection data show this area of steeper dips to be related to the presence of a high ridge of salt, trending north–south in the sub-surface. In map view the seabed in this area is cut by an array of gullies that trend down slope and die out up and down slope in the gently dipping areas to the east and west (Fig. 3). The gullies are 1 km or more in width and approximately 10 km in length. Examination of these gullies, particularly using dip-maps which reveal the structures in fine detail, shows them to be made up of linear strings of circular pockmarks (Fig. 8). Within each pockmark train or gully, the pockmarks are fairly uniform in size and spacing. In mapping the pockmark gullies in Region 1, we can identify a spectrum of morphologic styles ranging from incipient or immature strings of small discrete pockmarks to mature and coalesced trains or gullies.

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We propose that this spectrum of styles also represents an evolutionary sequence in the formation of these previously undescribed features. This evolutionary sequence is illustrated by the six seismic profiles in Fig. 9. In Stage 1 (Fig. 9a) an area of steeper seabed slope can be seen underlain by a large elevated diapiric salt mass. We interpret the presence of this north–south trending salt wall and the uplifted strata flanking it on either side to be responsible for the local increase in structural dips at the seabed. A series of ridges and troughs is present at the seafloor in this stage, however there are no pockmarks. The ridges and troughs are linear and run perpendicular to the slope. At this early stage of deformation it is clear on the seismic profile that the ridges are related to listric faults. The listric faults are shallowly dipping (∼ 15°) and detach into multiple bedparallel detachment surfaces. In Stage 2 (Fig. 9b) pockmarks develop locally in the hanging-wall of the listric faults. As described above in Section 4.1.2, the pockmarks do not lie on the fault strike at the seabed, but rather are offset from the fault in the hanging-wall. In Stage 3 (Fig. 9c) the number of pockmarks increases and they grow in size. As in Stage 2 the pockmarks are clearly offset down slope from the faults, however each fault now has a pockmark associated with it. As the number and depth of pockmarks increases the underlying listric faults become harder to interpret on the seismic data. As the process continues the faults become less and less obvious and it is only the spectrum of stages preserved in this area that allows our interpretation of a genetic link between listric slump faults and pockmarks to be made.

Fig. 8. Dip-map of the central portion of Region 1 showing the linear pockmark trains and gullies (see Fig. 4 for location). Immature pockmark trains are made up of smaller distinct pockmarks. More mature trains grow in length and width, until the pockmarks start to coalesce into a gully. Several of the pockmark trains bifurcate down slope.

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Fig. 9. Spectrum of styles of pockmark trains and gullies, interpreted to be an evolutionary sequence in the formation of these features. (a) Stage 1: Listric slump faulting occurs on areas of oversteepened slope. Faults sole into multiple detachment zones. (b) Stage 2: Pockmarks develop locally in the hanging-wall of listric faults; insert shows detail of pockmark in hanging-wall of slump fault. (c) Stage 3: Pockmarks grow in size and number. Some pockmarks start to coalesce. (d) Stage 4: Linear pockmark train coalesced. (e) Stage 5: Pockmark gully growing in depth and width. (f) Stage 6: Pockmark gully starts to become smooth-bottomed as pockmarks fully merge.

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Fig. 10. Strike section (north–south) through a series of pockmark trains illustrating various stages in the proposed evolution of a pockmark gully.

In Stage 4 (Fig. 9d) the train of discrete pockmarks starts to coalesce and the size of the individual pockmarks has grown to the point that the faults are no longer distinct. In the dip sense (down the slope) the pockmark train appears as a regular alternation of “v”shaped depressions and rounded “n”-shaped ridges. At this stage the depressions are local to the pockmark train, that is they do not extend as valleys across the slope. Another important interpretation supported by the early stages of the evolutionary sequence is that the gully cannot be a sediment transport pathway into the deep basin. There is no through-going channel that would allow current or sediment flow, rather the axial part of the gully comprises a string of alternating highs and lows. In Stage 5 (Fig. 9e) the pockmarks continue to grow in diameter and depth resulting in the pockmarks having a “u”-shaped profile and the intervening highs becoming more sharply angular. As the width of individual pockmarks increases so does the width of the evolving gully.

In Stage 6, a mature pockmark gully (Fig. 9f), the circular form of individual pockmarks is largely lost as adjacent pockmarks merge. In the dip direction the axis of the gully has a spoon-shaped profile. As stated above this morphology is restricted to the axis of the pockmark train and the adjacent seabed is unaffected. The bottom of the gully starts to smooth out in the late stages of evolution; however the presence of remnant highs and lows and the absence of any down-dip sediment dumps support our interpretation that these features do not act as the locus of basinward sediment or current flow. The seismic profiles in Fig. 9 illustrate the growth of pockmark trains and gullies in the dip direction, down the slope; in Fig. 10 we illustrate the evolutionary sequence in the orthogonal direction, along the strike of the slope. Not all the stages can be illustrated on one line, however Fig. 10 shows; incipient pockmarks (Stage 1), small shallow “v”-shaped pockmarks (Stage 2), large “v”-shaped pockmarks (Stage 4), wide and deep “u”-shaped

Fig. 11. 3-D visualization of the seabed structure in Region 2 which lies offshore Gabon in water depths of 75 m to 800 m. Viewing direction is from the West. Cool colours indicate deep water, warm colours indicate shallower water. Vertical exaggeration is 3x. For scale see Fig. 12.

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pockmarks (Stage 5), and deep mature coalescing pockmarks (Stage 6).

portion of Region 1, we did not observe an association with polygonal fault networks in this area.

4.2. Description of pockmarks in Region 2 (Gabon)

4.2.2. Linear pockmark trains The most obvious feature of Region 2 is a series of east–west trending channels crossing the area and apparently merging down dip with a larger northwest– southeast trending feature. Superficially these features appear similar to the linear pockmark trains and gullies described in Region 1 (Section 4.1.3), however interpretation of the seismic data in this area revealed no evidence of slumps or listric faults underlying the pockmark trains. A maximum amplitude extraction from the 3-D seismic volume, over a depth interval of 150 ms below the seabed, reveals a series of slope channels underlying the linear pockmark trains. The central part of each channel system is sinuous and relatively high amplitude (white–red) and interpreted to be coarser grained — probably sandy facies. The areas immediately on either side of the sinuous channel are low amplitude (grey–

Region 2 lies offshore Gabon, approximately 140 km south-southwest of the coast at Libreville and 50 km south of Region 1, in water depths of between 75 m and 800 m (Fig. 11). As in Region 1, the seabed structure map reveals only the large-scale structures; again we use dip-maps to illustrate the detailed structure (Fig. 12a). Here we also use acoustic amplitude to illustrate sedimentary features just below the seabed (Fig. 12b). 4.2.1. Random pockmarks Random pockmarks are not as common in this area as they are in Region 1, however apparently random pockmarks are observed in the north of the area. These pockmarks are between 300 m and 600 m wide and 20 m and 30 m in depth. Although they have a similar morphology to the pockmarks described in the western

Fig. 12. a) Seabed dip-map. (b) Maximum amplitude map, 0–150 ms below the seabed of Region 2. Grey and black colours are low amplitude, white to red colours are high amplitude. The morphology of slope channel systems and their associated splays and overbank deposits can be clearly interpreted in this area. The seabed pockmarks align perfectly with the buried channels.

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Fig. 13. 3-D visualization of Region 3 which lies offshore Equatorial Guinea in water depths of 60 m to 1700 m. Viewing direction is from the West. Cool colours indicate deep water, warm colours indicate shallower water. Sediment transport channels can be clearly differentiated from pockmark gullies. Vertical exaggeration is 3x. For scale see Fig. 14.

black) and are interpreted to be levee mud facies. Beyond the levees lies an irregular area of bright amplitudes which we interpret to be minor splays and sandy overbank deposits. Comparison of the seabed dip and maximum amplitude extraction (Fig. 12a and b) shows the pockmarks to be very well organized and closely follow the path of the sinuous main channel. A similar sinuous trend of pockmarks mimicking underlying channel sinuosity has been reported from the Congo basin by Gay et al. (2003). Pockmarks are also associated with some areas of overbank material and with the large NW–SE trending channel in the north of the area. The mode of formation of these pockmark trains appears to be related solely to the presence of the shallow channels, and not to slump faulting as interpreted in Region 1. It is important to emphasize that while the presence of a channel has caused the string of pockmarks, the channel itself is buried and thus inactive; the pockmark string is not a suitable sediment transport pathway due to its highly rugose internal morphology (see Section 4.1.3). 4.3. Description of pockmarks in Region 3 (Equatorial Guinea) Region 3 lies on the Equatorial Guinea continental margin, approximately 40 km from Mbini, in water depths of between 60 m and 1700 m. Region 3 is approximately 200 km to the north of Region 1 in Gabon. 4.3.1. Random pockmarks Within Region 3 there are there are a very few random pockmarks. Most of the pockmarks occur in the central steeper dipping portion of the dataset, and although several groups of pockmarks occur somewhat

in isolation we interpret these to be precursors to future pockmark trains and gullies (see Section 4.3.2). One of the characteristics of the extensive random pockmark fields in Region 1 was the presence of polygonal fault networks in the shallow sub-surface underlying these areas. In Region 3 we have observed pervasive polygonal faulting at shallow depths below the seabed, yet do not see the same development of pockmarks. 4.3.2. Pockmark trains and gullies Region 3 is characterized by a gently sloping continental shelf, a very abrupt shelf break, approximately 20 km from shore in 100 m water depth, and a steeply dipping slope (3.4°). The most obvious feature of Region 3 is the many deep gullies that trend WNW–ESE across the continental slope (Fig. 13).Two distinct types of features can be seen to cut the slope area; submarine sediment transport channels, and pockmark gullies similar to those described in Region 1 (Section 4.1.3). The submarine channels are sinuous, incising features that reach far out onto the basin floor, and are actively incising back onto the shelf to provide conduits for sediment transport. The pockmark gullies are present only on the slope, die out down dip and do not incise landward into the shelf. Again, a dip-map is used to better illustrate the detailed structure of this area (Fig. 14). The pockmark trains and gullies are similar in appearance to those described 215 km to the south in Region 1. We do not illustrate individual stages in the evolutionary sequence here as we propose that the model presented for Region 1 (Fig. 9) is generic and may be applied here also. The pockmark trains are of similar length, in places somewhat longer, reaching up to 25 km. In the early stages the pockmark trains are made up of pockmarks with diameters of between 200 m and 400 m and depths of between 30 m and 65 m. In the mature

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Fig. 14. Dip-map of Region 3. The details of the pockmark trains and gullies can be seen. As in Region 1 a spectrum of styles is seen as well as many branching forms. P = pockmark gully, S = sediment channel.

coalesced trains and gullies the gullies reach 1200 m in width and 220 m in depth. Examination of the seismic reflection data in this area reveals an extremely complex sub-surface geology characterized by multiple intervals showing polygonal faulting patterns, and highly slumped regions. The same model for formation of these pockmark gullies through listric extensional faulting on unstable slopes is supported in this area. Another observation from the seismic data is that the area has been cut by many generations of sediment transport channels, two of which are active at the seabed at the present day. Although sediment is clearly entering the basin in the region, as evidenced by these two channel systems, the pockmark gullies are not acting as sediment pathways and like the examples in Gabon have a very rugose internal morphology. 5. Discussion 5.1. Formation of pockmarks Pockmarks are widely accepted to form by fluid escape or expulsion through the seabed, a process coined gasturbation first proposed by Josenahns et al. (1978), and as a result their distribution is interpreted to be controlled by the distribution of fluid migration pathways in the shallow sediments. Hence, observed linear strings of pockmarks, i.e. non-random pockmarks, may be associated with fault zones within the shallow sediments or shallow buried channels (Haskell et al., 1999), or both. Two modes of formation have been proposed: catastrophic eruption of gas from overpressured shallow gas pockets and continuous fluid discharge, hindering sediment deposition around the seep (Hovland and Judd 1988). The process may be episodic or continuous. Both processes disperse fine-grained sediment into the water

column, from where it later settles out or is moved by bottom current activity. Pickrill (1993) describes a spectrum of stages in pockmark development from shallow depressions in the surface relief, through pockmarks that cut through shallow sub-surface reflectors, to decaying and relict pockmarks that have been partially or completely infilled. In some cases pockmarks appear to have been persistent through a long time, particularly in areas underlain by structural highs that may form a source and/or focus for fluid migration. In coarse, non-cohesive sediments, sites of fluid seepage at the seabed may be marked by bioherms, smaller indistinct pockmarks or bacterial mats (Fader, 1991; Hovland, 1992). Mechanisms that locally reduce the lithostatic pressure can initiate and localize the formation of pockmarks; these triggering mechanisms may include glacial scouring, tectonic faulting, sediment slumping and anthropogenic activities such as seabed trawling (see Section 1.2). Often both random and non-random patterns of pockmark distribution are observed in close proximity. Where pockmarks are associated with faults that reach the seabed, additional indicators of hydrocarbon migration are commonly indicated on vertical seismic profiles by shallow gas pockets or “flags”, along the fault plane. In addition to the common association of pockmarks along faults that extend from the seabed and cut fluidcharged shallow sediments, pockmarks have also been described occurring at the headscarp of a submarine slide (Foland et al., 1999). These authors suggest that this represents a separate fluid expulsion mechanism. Where pockmarks have been investigated using geochemical “sniffers”, and remotely operated vehicles (ROV's) compelling evidence for a hydrocarbon origin has

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been found. They are often found to contain carbonate crusts and carbonate-cemented rocks, interpreted to form from percolating methane, as described from the Norwegian sector of the North Sea (Hovland and Judd, 1988). Direct evidence for hydrocarbon seepage at pockmark sites has been observed in the UK North Sea in the form of actively seeping methane (Hovland and Sommerville, 1985). In addition carbonate rock retrieved from a pockmark off Baffin Island contained brown oil trapped in internal cavities (Hovland et al., 1987). A close association of pockmarks and shallow gas accumulations has also been observed. Side-scan sonar has been used to detect trapped gas less than 30 m below a group of pockmarks in the Norwegian North Sea (Hovland et al., 1987). The examples of pockmarks presented in this study come from three regions spanning c. 300 km of the equatorial South Atlantic margin of Africa. The abundance and variety of pockmark structures presented here support the notion that pockmarks are a global phenomenon and that apparent clusters of published data are heavily influenced by sampling bias. While the mechanisms of pockmark formation are similar in different areas,

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in this study we have described several unusual and new types of pockmarks. Firstly, we describe and illustrate mega-pockmarks on the Gabon continental margin that are significantly larger and deeper than any pockmarks previously described. These features exist in water depths ranging from 540 m to 1860 m and reach diameters of 1500 m and depths of 150 m, making these individual fluid escape structures an order of magnitude larger than those more commonly described elsewhere in the world's oceans (Fig. 2). Soft and fine-grained seabed sediments and a persistent supply of migrating fluids must be required to create such large pockmarks, however we do not believe their mode of formation differs significantly from that described for smaller features. Secondly, we described a new type of pockmark arrangement, which we have termed pockmark trains and gullies, in which gravity slumping on areas of slope instability causes pockmarks to be localized into linear arrays. These arrays evolve from short trains of small pockmarks, which grow over time to long trains of large

Fig. 15. Comparison of the internal morphology of a pockmark gully and a submarine sediment transport channel. (a) Pockmark gully — the axial part of the gully is highly rugose as it retains the steep geometries of multiple fault scarps and pockmark margins. (b) Sediment channel — the axial part of the channel (thalweg) is smooth and dips consistently basinward along its length.

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pockmarks and ultimately coalesce to form pockmark gullies where the individual pockmarks become hard to distinguish. These pockmark gullies trend down slope (in the direction of slump and listric fault movement) and, in map view, appear superficially similar to erosional submarine channels cutting the slope. While sediment from within the gully has clearly been removed and redistributed by the processes of pockmark eruption and gasturbation, the gullies do not act as conduits for externally derived material, as implied by Peel et al. (2001). Detailed mapping and computed dip-maps show that the axes of pockmark gullies retain a highly rugose morphology throughout their evolution; hence we see no evidence for these features as sediment transport pathways. In addition we have not observed any evidence for sediment dumps at the down-dip culminations of the trains. Fig. 15 shows a comparison between a seismic profile down the axis of a sediment channel and a pockmark gully. The sediment channel has a meandering sinuous shape in plan view and a seismic profile picked down the axis of the channel shows a constant basinward dip, with no internal barriers to flow. Conversely, the axial structure of a pockmark gully is highly rugose due to the numerous fault scarps and pockmark margins that make up the structure. Although pockmark trains and gullies are a newly recognized phenomenon, the mechanism of formation of the individual pockmarks that make up the train is similar to that described in Sections 1.2.4 and 4.1.2 for fault hanging-wall pockmarks. The observation that active faults, i.e. those forming present-day seafloor scarps, are not generally the preferred fluid migration pathway for

shallow fluids is an important one. We suggest that local uplift of the faulted cut-offs of an overpressured fluidcharged layer results in vertical migration from the footwall cut-off to form a pockmark in the hanging-wall of the fault. We illustrate this proposed mechanism of formation in Fig. 16. This process seems to operate equally in both planar extensional faults and highly listric slump faults. While the pockmarks can be seen to be offset from the main fault plane we acknowledge that sub-seismic antithetic extensional faults or fractures may be facilitating the fluid migration in the hanging-wall to the main fault. The implication is that even in shallow unconsolidated sediments fault zones are rarely migration conduits. If this is the case at the surface in unconsolidated recent sediments it may also be the case for faults deeper in the sub-surface, and therefore the popular model of faults as significant migration pathways for hydrocarbons in the sub-surface may not hold true. 5.2. Significance of pockmarks Pockmarks have received considerable interest since their discovery for several reasons, including their relevance to hydrocarbon exploration and production, their effect on local biodiversity, and as a source of the greenhouse gas methane into the environment. There has been much discussion of pockmarks as an indicator of sub-surface hydrocarbons (Carvalho and Kuilman, 2003; Hasiotis et al., 2002; Hood et al., 2002; Hovland et al., 1987; Hovland and Judd, 1988), however we believe that pockmarks may not be a reliable tool in this respect. Although many pockmark fields have been

Fig. 16. Model for the formation of pockmarks in areas of slope instability. (a) Regional dip of the seabed and shallow stratigraphy is basinward. Listric slump faults, detaching on multiple bedding-parallel detachments develop in areas of unstable slope and also dip basinward. Local stratigraphy in areas of listric faulting is rotated and dips landward. Overpressured (OP) intervals present in the shallow section are offset and rotated by the slump faults. In the area of the fault heave the confining overburden above the overpressured interval is reduced from normal (OBN) to a thinner amount related to the throw on the fault (OBF). (b) If the magnitude of the pressure in the overpressured zone is constant, the local reduction of overburden pressure at the fault results in an effective stress minimum at the point of the footwall cut-off (FWC) of the overpressured layer. If pore-pressure increases or overburden pressure decreases, the effective stress may reduce to zero, resulting in vertical leakage of fluid to the seabed and formation of a pockmark.

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recorded in hydrocarbon bearing basins such as the North Sea and Gulf of Mexico and gas escaping from pockmarks in several areas has been demonstrated to be of a thermogenic source (Hovland and Sommerville, 1985; Hovland and Judd, 1988), there are many instances where pockmarks are recorded in non-hydrocarbon provinces and many pockmarks are associated with biogenic gas. Even in mature hydrocarbon provinces such as the Gulf of Mexico much of the shallow gas observed on seismic data, penetrated during drilling operations, and even produced in some fields, is of biogenic origin. In addition, with a highly complex sub-surface geology including steeply dipping strata and widespread salt diapirs and canopies, the migration of thermogenic hydrocarbons would be expected to take a circuitous route to the seabed. Thus in an exploration sense the pockmarks themselves cannot reveal the presence of a mature thermogenic hydrocarbon kitchen, nor can their location reveal the presence of a sub-surface hydrocarbon accumulation. In an operational sense, however, pockmarks are an important feature, both as an indicator of shallow gas which may be a drilling hazard, and as a seafloor hazard to the installation of infrastructure such as rigs and pipelines. With pockmarks and pockmark gullies so widespread in many areas of petroleum exploration such as West Africa, they must be considered as a seafloor hazard to exploration and production infrastructure. Pockmarks are clearly active over extended periods of geological time and vent fluid, often gas, periodically. Pockmarks are usually avoided during drilling operations for this reason. While we do not believe that pockmark gullies are sediment transport pathways themselves they do indicate regions of slope instability. We suggest that observation of pockmark gullies on a slope should be considered as an indicator of increased risk for slumping or faulting that could disrupt subsea infrastructure. 6. Conclusions A variety of pockmark features are observed on the equatorial West African margin, including mega-pockmarks in excess of 1500 m in diameter and up to 150 m deep. While pockmark features have been recognized in many basins, with these examples we extend the known range of sizes of these features. Linear pockmark trains are observed on the continental slope in areas of slope instability and are associated with listric slump faults that sole into multiple bedding-parallel detachment surfaces. Pockmark trains initiate on the steepest dipping portions of a slope, and extend in length down slope with time. As the feature evolves, trains of

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discrete pockmarks coalesce to form pockmark gullies, which reach 1.5 km in diameter and 20 km in length. Pockmarks form in the hanging-wall of faults, not at the surface scarp. This is true in isolated fault-associated pockmarks in areas of stable slope, and for slumprelated pockmark trains. Shallow faults therefore are not the preferred migration pathway to the seabed for fluids escaping from an offset or deformed sub-surface source bed. In petroleum geology terms these faults can be considered to “seal”. Pockmark gullies, although they cut the slope and look superficially like sediment channels, do not provide preferential sediment transport routes into the deep water. However they do indicate areas of slope instability and slumping. Isolated pockmarks, linear pockmark trains and pockmark gullies are seabed hazards which need to be considered when installing subsea infrastructure for petroleum production or communications. Acknowledgements We thank Hess Corporation for permission to show the seismic data and publish our findings and our colleagues for their constructive criticism of the manuscript. We are very grateful to Joe Cartwright and an anonymous reviewer for their constructive reviews and comments on the manuscript. References Abrams, M.A., 1996. Distribution of subsurface hydrocarbon seepage in near-surface marine sediments. In: Schumacher, D., Abrams, M.A. (Eds.), Hydrocarbon Migration and Its Near-Surface Expression. AAPG Memoir, vol. 66, pp. 1–14. Bøe, R., Rise, L., Ottesen, D., 1998. Elongate depressions on the southern slope of the Norwegian Trench (Skagerrak): morphology and evolution. Mar. Geol. 146, 191–203. Cartwright, J.A., 1996. Polygonal fault systems: a new type of fault structure revealed by 3D seismic data from the North Sea Basin. In: Weimer, P., Davis, T.L. (Eds.), Applications of 3-D Seismic Data in Exploration and Production. AAPG Studies in Geology, no. 42 and SEG Geophysical developments Series, vol. 5, pp. 225–230. Carvalho, J., Kuilman, L.W., 2003. Deepwater Angola; seafloor pockmarks as hydrocarbon indicators? AAPG International Conference Abstracts, Barcelona, Spain. Cole, D., Stewart, S.A., Cartwright, J.A., 2000. Giant irregular pockmark craters in the Palaeogene of the Outer Moray Firth Basin, UK North Sea. Mar. Pet. Geol. 17, 563–577. Dimitrov, L., Woodside, J., 2003. Deep sea pockmark environments in the eastern Mediterranean. Mar. Geol. 195, 263–276. Fader, G.B.J., 1991. Gas-related sedimentary features from the eastern Canadian continental shelf. Cont. Shelf Res. 11 (8–10), 1123–1153. Fader, G.B.J., Miller, R.O., 1988. Megaflutes in Placentia Bay, Grand Banks of Newfoundland. Abstract, Geol. Assoc. Can. Annual Meeting, St John's, Newfoundland, vol. 13, pp. A38–A39.

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