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Geo-Mar Lett (2011) 31:343–360
DOI 10.1007/s00367-011-0256-9

ORIGINAL

Contourites within a deep-water sequence

stratigraphic framework
Rachel Brackenridge & Dorrik A. V. Stow &
F. Javier Hernández-Molina

Received: 9 May 2011 / Accepted: 15 July 2011 / Published online: 9 August 2011
# Springer-Verlag 2011

Abstract Sequence stratigraphy has proven to be an first model reveals a CDS where bottom current activity is
extremely useful predictive tool in the search for hydro- markedly more vigorous during times of sea level high-
carbons along the continental margins. However, of the stand, whereas the second model indicates margin evolution
several models in use, none includes the effects of where bottom-water currents are most vigorous during
alongslope processes in deep-water. This paper, therefore, times of sea level lowstand. It is recognised that there are
is a first attempt to place contourite depositional systems additional controlling factors linked to sea level variation
(CDSs) firmly within a sequence stratigraphic framework, which can significantly modify the distribution and devel-
based on detailed examination of over 20 CDSs worldwide. opment of contourite elements.
It also presents a new view of how sea level variation
influences bottom current generation and intensity. Two key
controls on contourite drift formation are identified: Introduction
sediment influx and bottom current velocity. Sea level
directly influences the sediment influx to a basin and, One of the dominant paradigms for the description and
therefore, the contourite response fits nicely into the interpretation of continental margin sedimentary systems
downslope sequence stratigraphy model. Bottom current is sequence stratigraphy, as developed from the seminal
velocity variations in response to sea level are more work of Peter Vail and others in the late 1970s. Sequence
complex, and two key controls are identified: (1) oceanic stratigraphy provides a dynamic view of stratigraphy in
gateways can effectively constrict and accelerate water which direct linkages can be made between variations in
masses and are therefore closely associated with CDS sea level and sedimentation, and a hierarchy of cycles of
evolution; fluctuating sea level will affect the water eustatic sea level changes can be recognised at a
exchange through a gateway; (2) changing rates of worldwide scale (Mitchum et al. 1977; Vail et al. 1977;
bottom-water generation: some water masses appear more Haq et al. 1987). Much progress has been made since that
vigorous during periods of lowstand, whereas others appear time and several different schools have emerged which
more sluggish. In order to accommodate this variation, two promote rather different models. In their recent review of
new sequence stratigraphy models are herein presented, sequence stratigraphy, Catuneanu et al. (2009) state that
comprising both downslope and alongslope processes. The “each model is justifiable in the context in which it was
proposed and may provide the optimum approach under
Responsible guest editor: E. Llave the right circumstances.”
However, despite ever-growing interest in deep-water
R. Brackenridge (*) : D. A. V. Stow
IPE-ECOSSE, Heriot Watt University, sedimentation for hydrocarbon exploration, there is still
Edinburgh EH14 4AS, UK almost no mention in any of these models of contourites
e-mail: rachel.brackenridge@pet.hw.ac.uk and bottom currents. Even in Catuneanu et al. (2009), the
discussion of deep-water settings makes no reference to the
F. J. Hernández-Molina
Facultad de Ciencias del Mar, Universidad de Vigo, alongslope system. This is despite the recognition of
36200 Vigo, Spain contourites as a possible seismic mounded facies in early
344 Geo-Mar Lett (2011) 31:343–360

work on seismic stratigraphy (Mitchum et al. 1977). water model which places the contourite depositional
Furthermore, it is now clearly recognised that contourite system within a sequence stratigraphic framework.
deposits are a hugely important component of deep-water
depositional systems, everywhere from the upper continen-
tal slopes to the abyssal plains (e.g. Stow et al. 2002a; Sequence stratigraphy: existing models and problems
Viana and Rebesco 2007; Rebesco and Camerlenghi 2008).
These deep-water systems, especially along continental Sequence stratigraphy can be defined as “the analysis of
margins where many of the contourites are found, are genetically related depositional units within a chronostrati-
currently frontier areas for hydrocarbon exploration and graphic framework” (Reading and Levell 1996). It grew out
production (Stow and Mayall 2000; Haughton and Kendall of the subsurface analysis of continental margins during oil
2009; Nielsen et al. 2011, this volume). The integration of explorations based on seismic profiling and deep borehole
contourite depositional systems (CDSs) with downslope techniques, and has now become a fully fledged sub-
systems is of paramount importance in this context. discipline of geology (Vail et al. 1977, 1991; Haq et al.
Whereas individual drifts have been previously evaluat- 1987; Posamentier and Vail 1988; Posamentier et al. 1988;
ed in terms of sequence stratigraphy (e.g. Llave et al. 2001, Van Wagoner et al. 1988; Emery and Myers 1996;
2006, 2007), and sea level has been considered as one of Catuneanu 2006).
the important controls on contourite drift evolution (e.g. Individual units recognisable on seismic profiles are
Faugères et al. 1993, 1999), no generally applicable known as depositional sequences. These are bounded by
sequence stratigraphic model has yet been developed. The distinctive unconformity surfaces (type 1 or 2), and
problem is indeed challenging because the interaction of comprise an internal arrangement of sub-units (or systems
additional controls is complex, there is an apparent tracts) associated to particular variations of sea level. One
disconnection between Northern and Southern Hemisphere of the basic tenets of sequence stratigraphy is that sea level
systems, and there is no simple relationship between acts as a fundamental control on margin sedimentation by
alongslope and downslope systems. But, the time is well influencing the delicate balance between accommodation
overdue and the database does now exist for this important space and sediment influx (Myers and Milton 1996).
first attempt to place contourite depositional systems firmly Individual systems tracts result from the changing relation-
within a sequence stratigraphic framework by modifying ship between these two important controlling factors.
the conventional downslope model. This paper develops The most widely used sequence stratigraphic model is
concepts introduced by Faugères et al. (1993, 1999), Diez illustrated in Fig. 1. At the base, the sequence boundary
et al. (2008) and Hernández-Molina et al. (2008). It is an represents a surface along which significant erosion and/or
additional challenge to refine the model by detailed analysis a hiatus in sedimentation occurred. This can result from a
of industry-generated seismic data which are age-controlled strong drop in eustatic sea level, causing the continental
with well calibrations. shelf to be exposed to sub-aerial erosion (type 1 boundary)
The principal aims of this work, therefore, are (1) to or not (type 2 boundary). The subsequent lowstand systems
consider the range of controls which influence contourite tract (LST) is normally subdivided into three depositional
drift development and bottom current erosion in deep- units in deep-water: the basin-floor fan, slope fan, and
water; (2) to consider specifically the role of sea level lowstand wedge. The basin-floor fan signifies bypass of
variation in this regard; and (3) to develop a new deep- sediment from the shelf downslope into the deep basin.

Fig. 1 Basic sequence stratigraphic model for a continental margin sive surface, MFS maximum flooding surface. Important components of
sequence architecture where right is basinwards: in the threefold systems the LST are the basin-floor fan and slope fan. Most basin sedimentation
tracts model, LST lowstand, TST transgressive, HST highstand. occurs during the LST. Figure not to scale but approx. 10s–100s km
Important bounding surfaces are SB sequence boundary, TS transgres- across and 100 m–few km deep (modified from Haq et al. 1988)
Geo-Mar Lett (2011) 31:343–360 345

Following or simultaneous with basin floor-fan deposition, 10–100×103 years; Myers and Milton 1996). (7) Pliocene
muddier turbidite and debrite downslope processes result in and Quaternary sequence stratigraphy models are rather
the formation of a slope fan. Slow relative sea level rise and complex and present important differences to conceptual
restored slope stability is responsible for the generation of models. In contrast to idealised sequence stratigraphic
an overlying prograding lowstand wedge, which develops models, the existence of the regressive (forced or not)
into a transgressive wedge when the accommodation space systems tracts (RSTs) is likely to predominate in the Pliocene
exceeds sediment deposition. Collectively, the lowstand and Quaternary sedimentary record (e.g. Hunt and Gawthorpe
elements present a retrogradational architecture. The over- 2000; Hernández-Molina et al. 2000). (8) Finally, as stated
lying transgressive systems tract (TST) is bounded by the above, none of the existing models accounts for the very
transgressive surface at its base and the maximum flooding significant and sometimes dominant elements of alongslope
surface along its upper limit (Posamentier and Vail 1988; sedimentation—the contourite depositional systems. This is
Posamentier et al. 1988). This is then overlain by the the primary focus of the paper presented.
highstand systems tract (HST), which represents deposition
at a time of limited accommodation space resulting from
the slowing of relative sea level rise. If sediment supply is Controls on deep-water sedimentation
sufficient, then a progradational depositional architecture
will be observed. For the interpretation of any sedimentary system, including
While this model reflects a simplified response to deep-water systems, it is important to consider the full
eustatic sea level variation and presents a schematic range of factors which have influenced the accumulation of
sedimentary architecture for siliciclastic systems, the reality sediment and its preservation. These comprise both external
on many continental margins is more complex (Posamentier and internal controls. The principal external controls
and James 2009). Key issues include (1) carbonate- include (1) sediment supply—the nature, rate and source
dominated systems behave quite differently, as has been of supply, as well as the type of sediment; (2) sea level
clearly demonstrated by numerous authors (e.g. Emery and changes—eustatic and relative sea level fluctuations, as
Myers 1996; Schlager 2005; Catuneanu 2006). Mixed well as short-term tidal, seasonal and storm effects; and (3)
systems present a further challenge for interpretation. (2) climate—temperature, precipitation and wind regimes, as
Synsedimentary tectonic activity can significantly affect well as short- and long-term climatic changes.
accommodation space and/or sediment supply, in some Internal controls can be equally significant in affecting
cases completely masking sea level effects (Bridge and sedimentation. These include (1) tectonic activity—isostatic
Demico 2008). Therefore, ‘relative sea level’ is a more movements, subsidence and uplift, plate tectonic setting,
accurate term to be used when discussing continental seismicity and volcanicity; (2) local physical, biological
margin sedimentary architecture. (3) The presence of ice and chemical processes—the nature and intensity of bottom
along a margin can significantly alter the sedimentary currents, and the degree of interaction with other processes;
architecture (Powell and Cooper 2002). (4) In reality, the (3) post-depositional processes—compaction, deformation,
lowstand wedge and prograding highstand systems tract biogenic and chemogenic effects, amongst others; (4)
rarely downlap onto the deep-water elements, as demon- regional bathymetry—water depth, slope gradient and
strated by Haughton and Kendall (2009). Furthermore, the seafloor irregularities; and (5) accommodation space—
so-called slope fan is not everywhere distinct from the controlling progradation and/or aggradation.
basin-floor fan, but commonly represents its channel-levee The rates and length of time over which the controlling
feeder system. (5) Much of the continental margin system processes operate is also of vital importance. For example,
around the world has a slope-apron fringe of sediment and sediment supply and, hence, accumulation rates can be 2–3
no distinct slope or basin-floor fan, a point long emphasised orders of magnitude greater for deltas than for contourite
by various authors (e.g. Stow et al. 1996). In these cases, drifts. These various controls have been more extensively
the lowstand systems tract, in particular, will be very discussed by numerous authors (Walker and James 1992;
different from that of the standard model (Shanmugam Reading 1996; Allen 1997; Leeder 1999; Bridge and
2006). (6) In the standard model, depositional sequences Demico 2008, amongst others).
reflect mostly second- and third-order changes of sea In order to simplify our understanding of this complex
level (i.e. duration of 1–10×106 years), whereas systems interplay of controls, conventional sequence stratigraphic
tracts and parasequences reflect third- and fourth-order models assume two primary controls: sediment supply and
changes (100×103 to 1×106 years). The growth and sea level variation. Myers and Milton (1996) have more
development of large submarine fan systems may span recently expressed this as sediment supply and accommo-
two or more normal depositional sequences, while internal dation space (where the latter is a function of eustatic sea
variation or sequences can occur with higher frequency (e.g. level, tectonics and compaction), as shown in Fig. 2. The
346 Geo-Mar Lett (2011) 31:343–360

primarily by the existence of and variation in bottom

currents (see Stow et al. 2008, 2009). Bottom current
velocities control the facies and bedforms of a drift, in
addition to determining areas of deposition and erosion on
the seabed. Drift development requires bottom-water
currents to be stable and effective for a prolonged period
of time. Mean current velocities must be maintained at
>0.1 m/s to enable significant reworking, transportation and
deposition of contourites, whereas non-deposition and
seafloor erosion become more prevalent at velocities
>0.5 m/s (Stow et al. 2009). Long-term fluctuations in
bottom-water currents lead to an apparent cyclicity of
seismic facies from which palaeoceanographic information
may be extracted.
The cause of these fluctuations is still a major topic of
debate and ongoing research; however, general consen-
sus would identify the very important effects of (1)
oceanic gateways, and (2) prevailing conditions in the
source areas (or kitchens) for bottom-water generation.
There is a considerable body of research which seeks to
explain short-term (e.g. decadal) variation in ocean
circulation, especially following the findings of Bryden
et al. (2005) on the slowing of meridional overturning
circulation in the North Atlantic linked to climate change.
There is also an extensive palaeoceanographic literature
on climate change and circulation patterns over the recent
glacial–interglacial cycles (e.g. Rahmstorf 2002). Howev-
Fig. 2 Controls on downslope and alongslope depositional systems er, to the authors’ knowledge there has been no attempt to
directly link deep-water circulation with sea level change
(Bacon, personal communication 2010; Piola, personal
balance between accommodation space and sediment communication 2011). The theoretical considerations
supply forms the stratigraphic stacking patterns in the presented below are therefore proposed as a new expla-
original downslope model (Fig. 1). nation of just how variations of sea level act to influence
bottom current generation and intensity. This is an
important step in understanding the link between sea level
Controls on contourites and contourite development.

Previous attempts to understand the controls on contourite Gateway effects

development have clearly emphasised the numerous inter-
linked factors listed above (e.g. Faugères and Stow 1993, 2008; Oceanic gateways are critical to the exchange of both
Viana et al. 1998; Faugères et al. 1999; Shannon et al. 2005; surface- and deep-water masses between ocean basins
Diez et al. 2008). These can be resolved into two primary (Fig. 3). They are therefore pivotal in determining the
controls for contourite sedimentation: bottom current intensity presence or absence as well as the relative intensity of
and its variation, and sediment supply (Fig. 2). Unlike for bottom currents associated with intermediate- and deep-
near-shore depositional systems, accommodation space is a water masses. At times of high sea level, there is a normal
less significant factor (except in the case of shallow-water and full exchange of water masses through the gateway
contourites). Sea level affects the system mainly by influenc- and, hence, bottom currents will be strong and well
ing bottom current intensity, as discussed below. developed. At intermediate sea level (during either
transgressive or regressive phases), a gateway’s cross-
Bottom current variations sectional area will be reduced, thereby limiting bottom-
water exchange and intensifying the associated bottom
It has long been realized that the nature of contourite currents. It may be that upper, less dense strands of bottom
deposition, both in the long and short term, is controlled currents are intensified, whereas lower, denser strands are
Geo-Mar Lett (2011) 31:343–360 347

consequent active generation of cold dense water leads to

vigorous overturn, off-shelf spillover and downslope flow
of the dense bottom water (Fig. 4a). Where this reaches its
density equilibrium, the cascading water mass turns
alongslope under the influence of Coriolis force and
proceeds away from the source area, hugging the bottom
contours. Slightly warmer waters are then drawn over the
shelf to replace that lost to cold-water generation, and the
process continues.
At low relative sea level (Fig. 4b) the shelf becomes sub-
aerially exposed and, where an ice shelf had existed, it
becomes grounded. This results in a smaller, or non-
existent, shelf contribution to the cold-water kitchen area
and, hence, a reduction in the most effective bottom-water
generation. In other words, both the lack of shelf area and
the grounding of ice on shelves prevent the influx of
‘warmer’ surface water to drive the overturn. Heat

Fig. 3 Effects of changing sea level on the exchange of water masses

through oceanic gateways. BW Bottom water, MW middle water, SW
surface water

diminished. During sea level lowstands, the bottom-water

mass becomes severely limited or completely shut off,
such that bottom currents are also reduced in intensity or
are non-existent.

Bottom-water kitchen conditions

The global thermohaline circulation system is fed by the

continued and abundant generation of cold, saline bottom
waters in specific cold-water kitchens at high latitudes,
including the Norwegian-Greenland Sea, Labrador Sea,
Bering Sea, Weddell Sea and other locations around
Antarctica (Rahmstorf 2006). Empirical differences have
been noted in the timing of generation between Northern
and Southern hemispheres, and have been referred to as the
see-saw effect (Steig 2006).
For the Northern Hemisphere deep-water kitchens in
the Arctic regions, high rates of bottom-water generation
and accelerated overturning in the North Atlantic have
been noted during interglacials and associated highstands
(Rahmstorf 2002; Piotrowski et al. 2004; Lynch-Stieglitz
et al. 2007; Knutz 2008). The following mechanism is
here proposed.
Cooling of surface waters is most effective where it
occurs over broad shelf areas, both with and without Fig. 4 Effect of sea level on the generation of bottom water. a High-
latitude cold-water kitchens, sea level highstand. b High-latitude cold-
floating ice shelves, at periods of relative high sea level
water kitchens, sea level lowstand. c Low-latitude warm-water
when the heat-exchange dynamics between the atmosphere/ kitchens, sea level highstand. d Low-latitude warm-water kitchens,
ice shelf and ocean surface are particularly effective. The sea level lowstand
348 Geo-Mar Lett (2011) 31:343–360

exchange still occurs between the atmosphere and the open colder water and a greater level of freezing to form sea ice,
ocean surface but is less dynamic across the main body of there is potential for increased bottom-water generation
the open ocean. Although large areas of the open ocean during glacial periods. This would support the inferences of
may become covered in floating sea ice, the generation and higher bottom current velocities being linked to colder
sinking of denser water is relatively diffuse and the bottom climates for the Southwest Pacific gateway (Goosse et al.
water does not become focussed along a continental 2001; Carter et al. 2004), and along the Argentine margin
margin. The overall result is a slowing of cold dense water (Hernández-Molina et al. 2009, 2010).
generation, and less vigorous thermohaline overturn. This
leads to more sluggish bottom currents and circulation. Warm-water kitchen effects
In reality, the system is not necessarily as simple as
the model postulates. For example, the generation of The principal warm-water kitchen for intermediate/deep
Arctic Intermediate Water may show an increase during saline water today is found in the Mediterranean Sea
cold-climate lowstands. There are also changes and (Fig. 4c, d). This ultimately gives rise to the Mediterranean
interruptions to meridional overturning caused by short-term Outflow Water (MOW), which escapes through the Gibral-
climatic events. tar gateway into the North Atlantic Ocean. During previous
It is also very difficult to directly observe the formation greenhouse conditions on the planet, such warm-water
of cold water and its subsequent sinking (Rahmstorf 2006). kitchens of the geological past would have been still more
Thus, determining the relative contribution of the shelf and important than today and, at times (such as much of the
open ocean areas to the overall cold-water kitchen is Cretaceous), would have provided the dominant source of
problematic. Validation of the mechanism proposed above bottom water.
requires further observational data from high-latitude At high relative sea level stands, broad shallow shelf
kitchen areas. areas in arid and semi-arid climatic zones provide the
For the Southern Hemisphere there is even less unequiv- most effective regions for heat exchange, leading to a
ocal evidence for the link between climate/sea level and rapid evaporation of surface waters, increased salinity
bottom-water generation, and the whole process appears and, hence, density of the seawater left behind, and its
very complex. Whereas there is certainly evidence of an consequent sinking and downslope flow to form inter-
expanded influence of Antarctic Bottom Water into the mediate and bottom water masses (Fig. 4c). Coriolis force
Atlantic during glacial times and lowered sea level, there is deflects the newly generated water mass to flow along-
less direct evidence for increased bottom current velocity slope as a dense but warm-water bottom current. Cooler
(McCave et al. 1995; Orsi et al. 1999; Schmittner 2003; surface waters flow across the shelf region to replace that
EPICA Community Members 2006; Negre et al. 2010). It is lost to warm-water generation.
quite possible that this expanded influence is largely the At low relative sea level stands (Fig. 4d) the shelf may
result of the infilling of the ‘accommodation space’ left by become sub-aerially exposed and unable to provide such a
reduced generation of Northern Hemisphere deep waters. large warm-water kitchen area. Heat exchange still occurs
There is better evidence for an accelerated Antarctic between the atmosphere and the ocean surface but
Circumpolar Current at depth during glacial periods but, evaporation is less efficient across the main body of the
as this is driven largely by surface wind shear, it is not open ocean, warm saline and dense water is generated
necessarily correlated with increased bottom-water genera- more slowly and/or more diffusely, and there is less
tion in cold-water kitchen areas. vigorous overturn. This leads to less vigorous bottom
Furthermore, during times of extensive sea-ice develop- currents and circulation, in exactly the same way as for
ment, all water circulation is pushed basinwards, and cold-water kitchens.
therefore the core location for bottom-water generation is Whereas this model is true in general for warm-water
likewise shifted. This is particularly visible in the water kitchens and higher sea levels in the geological past,
masses surrounding Antarctica where the Antarctic Cir- there remains some controversy in the literature with
cumpolar Current moves northwards at times of sea-ice regard to Plio–Quaternary variations for the Mediterra-
development (Rebesco et al. 1997). This may push the nean and Red Sea warm-water kitchens. According to
bottom-water currents away from the continental slope Rohling and Zachariasse (1999), the Red Sea outflow was
where morphological forcing was likely to have caused severely reduced during the last glacial maximum (and
accelerated bottom-water velocities. On the other hand, at lowered sea level). For the Mediterranean, however, Cacho
least part of the extremely deep shelf areas around et al. (2000) see evidence for enhanced overturning rates
Antarctica still maintained floating ice shelves during during the last lowstand, while several authors working in
glacial lowstands and, hence, the same effective method the Gulf of Cadiz infer an increased bottom current flow of
of bottom-water generation as during highstands. With the lower strand of the MOW (Hernández-Molina et al.
Geo-Mar Lett (2011) 31:343–360 349

2006; Voelker et al. 2006; Toucanne et al. 2007; Schmiedl 150 scientific drill sites (DSDP, ODP, IODP) which have
et al. 2010). The upper strand, by contrast, appears to have penetrated contourite/bottom current systems; (2) published
higher velocities during warm-water highstands (Stow et seismic profiles from over 30 separate drifts and CDSs,
al. 2002b). some with good seismic grids but poor age control
(Faugères and Stow 2008). Published industry data with
Sediment supply variations good well control through contourite systems are very rare,
partly because the alongslope component has either not
The second fundamental control on contourite development been recognised or is too shallow in the section to be of
is sediment supply. For contourite accumulation to occur, economic interest. From a survey of this database,
sediment supply must be greater than background pelagic information for 20 well-documented drift systems has been
and hemipelagic fallout. Stow et al. (2008) identified the compiled in Table 1. They cover a wide range of environ-
potential sediment influx as sourced from turbidity currents, ments including those from high and low latitudes,
pro-delta plumes, slope spillover, and hemipelagic and Northern and Southern hemispheres and different tectonic
pelagic settling from both upstream and directly at the drift settings. Of these, four examples have been selected as case
site. Additionally, erosive bottom currents are capable of studies; the CDS off the Antarctic Peninsula in the SE
reworking previously deposited sediments. Pacific Ocean, the Argentine Basin CDS in the SW Atlantic,
The influence of sea level on these different sediment the Eirik Drift in the North Atlantic, and the Gulf of Cadiz
sources is not easy to ascertain and is not everywhere the CDS. They have been chosen to enable a direct comparison
same. As recognised in conventional sequence strati- of Northern and Southern Hemisphere systems in addition
graphic models for siliciclastic systems, deep-water to cold-water vs. warm-water systems.
clastic sedimentary systems are enhanced during sea
level fall and lowstand periods (RST and LST) on Antarctic Peninsula drifts
account of an increase in clastic sediment influx to the
deeper basins. This is due to enhanced erosion of the Location and setting Twelve drifts have been identified on
shelf, direct sediment supply to the shelf edge/upper the continental rise off the Antarctic Peninsula, in the SE
slope, increased activity of downslope processes and Pacific Ocean. They have been deposited by a boundary
increased sediment bypass of the slope. Although more current which flows south-westwards from the Weddell Sea
material is therefore likely to be available for redistribu- region, via the Drake Passage (Fig. 5). This current flows
tion by bottom currents, contourite depositional systems counter to the eastward-flowing Antarctic Circumpolar
may become masked by the dominance of downslope Current. As a result, the evolution of these drifts is closely
systems (Faugères et al. 1999; Fulthorpe et al. 2010). related to the palaeoceanography of the Antarctic region
On the other hand, during times of high relative sea and the opening of the Drake Passage. The most thoroughly
level, clastic sediment is more likely to become trapped on researched of these, Drift 7 (Rebesco et al. 1997, 2002; van
the shelf until the accommodation space has reduced Weering et al. 2008), is documented here.
sufficiently to enable progradation to the shelf edge.
However, large rivers will still feed pro-delta plumes across Evolution and controls Pre-drift sediments are mainly
the shelf, especially in the case of narrow shelves, and turbiditic, aged between 36 and 15 Ma, and cover the
contribute to some degree of hemipelagic sedimentation. oceanic basement. Drift growth began at ca. 15 Ma (Fig. 6)
Off-shelf sediment spillover processes will become more when bottom current velocities increased sufficiently to
prevalent as more mobile sediment is fed to and reworked dominate over downslope processes. This has been linked
across the shelf (Viana et al. 1998; Stow et al. 2002c). It to continued deepening of the Drake Passage, and the drop
should be noted that sediment supply to the deep ocean in sea level following the mid-Miocene climatic optimum
basins is by no means driven solely by eustatic sea level and subsequent expansion of the East Antarctic Ice Sheet.
fluctuation. Climate and local tectonics are additional Climatic changes continued to affect the drift throughout its
concerns, although such factors are outside the scope of evolution, as the Antarctic ice sheets extended seawards
this study. and glacio-eustatic levels fluctuated. ‘Drift growth’ and
‘drift maintenance’ stages have been identified (Fig. 6).
Drift growth is accounted to combined high bottom-water
Sequence stratigraphy of drifts: case studies velocities and high sediment influx during glacial con-
ditions and associated eustatic lowstands. There is some
This paper investigates 20 contourite drifts for which good change in drift accumulation rates during the Plio–Pleisto-
data exist, either in the literature and/or from first-hand cene and this succession is named the ‘drift maintenance
studies. An extensive published database includes (1) over stage’ (Fig. 6). This was probably related to the onset of
350 Geo-Mar Lett (2011) 31:343–360

Table 1 Summary of drifts used to create a revised sequence stratigraphy model which incorporates alongslope deposits

Drift Influencing water Time of most Time of highest Time of greatest contourite Model
mass(es)a vigorous BW velocity sediment influx sediment accumulation

Antarctic Peninsula drifts AABW; ACBW Glacial lowstand Glacial lowstand Late Miocene lowstand 2
Argentine Basin, elongate AABW Glacial lowstand Glacial lowstand Late Oligocene–Early Miocene lowstand 2
mounded
Balke Bahama outer ridge NADW Interglacial highstand Glacial lowstand Pliocene transgression and highstand 1
Barra Fan Drift NADW; LDW Interglacial highstand Glacial lowstand Holocene transgression and highstand 1
Canterbury Basin drifts SC Glacial lowstand Glacial lowstand* Middle–Late Miocene lowstand 2
Ceuta Drift MU Glacial lowstand Glacial lowstand Quaternary lowstand 2
Chatham Terrace Drift AABW Glacial lowstand Glacial lowstand Masked by local tectonic activity 2
Cosmonaut Sea margin AABW Glacial lowstand Glacial lowstand Middle–Late Miocene lowstand 2
drifts
Eirik Drift NADW Interglacial highstand Glacial lowstand Pliocene transgression and highstand 1
Faro-Albuferia Drift MOW Glacial lowstand Glacial lowstand Plio–Pleistocene lowstands 2
Feni Drift NSOW; LDW Glacial lowstand Glacial lowstand Mid–late Pleistocene lowstand 2
Hatton Drift NADW Interglacial highstand Glacial lowstand Plio–Quaternary transgression and regressions 1
Lofoten Drift AIW Interglacial highstand Glacial lowstand Quaternary lowstands n/a
NE Rockall Trough Drift NADW; NSOW; Interglacial highstand Glacial lowstand Unknown 1
LDW
Nyk Drift AIW Interglacial highstand Glacial lowstand Quaternary highstands 1
Upper slope Campos BC Interglacial highstand Glacial lowstand Present highstand 1
Basin drifts
Vema contourite fan AABW Glacial lowstand Glacial lowstand Transgressive and highstand** n/a
Vesterålen Drift AIW Interglacial highstand Glacial lowstand Quaternary lowstands n/a
West Shetland drifts ISOW Interglacial highstand Glacial lowstand Neo-Quaternary transgression and highstands 1
Wilkes Land drifts AABW Glacial lowstand Glacial lowstand Neo-Quaternary lowstands 2
a
AABW Antarctic bottom water, ACBW Antarctic circumpolar bottom water, AIW Antarctic intermediate water, BC Brazil current, ISOW Iceland-
Shetland overflow water, LDW lower deep water, MOW Mediterranean outflow water, MU Mediterranean undercurrent, NADW North Atlantic
deep water, NSOW Norwegian sea overflow water, SC Southland current
*
AND low relative sea level resulting from uplift; local uplift has now resulted in the downslope sediment ‘drowning’ (covering) this contourite system
**
Does not fit with model due to unusually high erosion of contourite sediments occurring during lowstand (information compiled from Stow and
Holbrook 1984; Robinson and McCave 1994; Laberg et al. 2001; Ercilla et al. 2002; Escutia et al. 2002; Faugères et al. 2002; Howe et al. 2002;
Rebesco et al. 2002; Stow et al. 2002c; Viana et al. 2002; Laberg and Vorren 2004; Carter et al. 2004; Hohbein and Cartwright 2006; Hernández-
Molina et al. 2006; Llave et al. 2007; Hunter 2008; Diez et al. 2008; Fulthorpe et al. 2010; Hernández-Molina et al. 2010)

permanent ice in the Arctic and the increased impor- bottom-water currents and the drifts. The bottom-waters
tance of bipolar deep-water generation mechanisms. At along this margin rarely reach velocities capable of eroding
present there is little bottom-water activity over the sediment. When observations on seismic records and
area. evidence on shorter timescales using core data are consid-
ered with respect to drift evolution off the Antarctic
Sequence stratigraphy This high-latitude CDS is under a Peninsula, it is seen that glacial periods are preferential to
strong glacial control which has three main effects on the drift growth due to higher-velocity bottom-water currents
drift. First, numerous authors have noted an increased rate and increased sediment supply which can be pirated by this
in Antarctic Bottom Water (AABW) formation during times enhanced flow.
of glacial advance (see EPICA Community Members 2006;
Rahmstorf 2006). Second, glacial advance across the
continental shelf alters the sediment input to the margin Argentine Basin drifts
significantly, with enhanced contourite sedimentation rates
being observed at times of sea level lowstand. Third, it has Location and setting A complex CDS has recently been
been hypothesized that the Antarctic circumpolar currents described from the continental margin offshore Argentina
are pushed northwards during glacial times (Rebesco et al. (Hernández-Molina et al. 2009, 2010; Violante et al. 2010).
1997) and, therefore, there is less interaction between the Alongslope activity in the basin initiated close to the
Geo-Mar Lett (2011) 31:343–360 351

a mounded drift formed under the influence of complex

basin circulation pathways.

Evolution and controls This CDS is made up of distinct

erosional and depositional features developed in response
to tectonic gateway opening and bottom-water variations
caused by climatic and eustatic changes. Three major
depositional units (Fig. 7) have been identified on seismic
records. Contourite activity in the basin commences with an
erosional surface relating to the onset of AABW formation
at approx. 33 Ma (Hinz et al. 1999). This occurred due to a
deepening of the Drake Passage, and the extension of the
East Antarctic ice sheet and associated cooling which
triggered the thermohaline circulation system in the
Southern Hemisphere (Goosse et al. 2001; Carter et al.
2004). The subsequent sediment accumulation forms the
lower seismic unit (LU) and signifies a phase of major
progradational drift growth. Bottom-water velocities
throughout LU formation are thought to have fluctuated in
response to the changing cross-sectional area of the Drake
Passage. The main phase of vertical drift growth occurred
during a major global transgressive event. A second major
erosional discontinuity is thought to be related to the
Middle Miocene lowering of eustatic sea level. This is
followed by low accumulation rates and aggradational
stacking patterns in the intermediate seismic unit (IU)
associated with third-order highstand (Ogg and Ogg 2008;
Hernández-Molina et al. 2009). The final major seismic
unit, the upper unit (UU), has developed under the present-
day oceanographic configuration of the basin. Three sub-
units have been recognised in the LU and three in the UU
Fig. 5 Principal bottom-water circulation trends (arrows) around the which may have formed in response to third-order T–R
Atlantic continental margins. Green Bottom-water source kitchens: cycles (Fig. 7).
low-latitude cold-water kitchens in the Arctic and Weddell seas, warm-
water kitchen in the Mediterranean. Main water masses: NADW North
Atlantic Deep Water, AABW Antarctic Bottom Water, MOW Mediter- Sequence stratigraphy Although seismic analysis is the
ranean Outflow Water. Numbered contourite drifts are those referred to main source of data in this region, some conclusions may
in the text: 1 Antarctic Peninsula drifts; 2 Argentine Basin drifts; 3 be drawn on the sequence stratigraphy of the Argentine
Eirik Drift; 4 Gulf of Cadiz drifts (modified from McCave and
Basin CDS. Bottom-water velocities tend to be more rapid
Tucholke 1986; Faugères et al. 1993)
at times of glacio-eustatic lows and this may lead to
erosion, although sediment influx will be high. The
Eocene–Oligocene boundary (Fig. 7) and has been attrib- Argentine margin therefore provides an example of a CDS
uted to the opening of the Drake Passage. The Argentine under the influence of a bottom-water current which is
Basin has since been affected by the deep Antarctic water accelerated during glacial times and global eustatic low-
masses. From the Middle Miocene onwards, the develop- stands. The evidence presented above shows that erosion is
ment of thermohaline circulation in the Northern Hemi- likely to occur during lowstands, whereas sediment accu-
sphere has facilitated the additional influence of the North mulation preferentially occurs during the lowstand to
Atlantic Deep Water on this contourite depositional system. transgressive phases of systems tract development.
Water masses enter the Argentine Basin via deep narrow
passageways in the topographic highs which enclose it. The
water masses are then pushed against the continental The Eirik Drift
margin due to the Coriolis force, to form an intensified
boundary current. Plastered drifts developed along the Location and setting The Eirik Drift is a large Cenozoic
upper continental slope and, on the lower continental slope, elongated drift located off the southern tip of the Greenland
352 Geo-Mar Lett (2011) 31:343–360

Fig. 6 Palaeoceanographic
events and seismic characteris-
tics associated with the evolu-
tion of Drift 7, Antarctic
Peninsula. The main phase of
drift building coincides with
rising relative sea level. It
should be noted that the local
tectonic evolution of the margin
is the dominant control over
relative sea level, and glacial
activity is an additional impor-
tant control (adapted from Haq
et al. 1987; Rebesco et al. 2002;
Hunter 2008; Ogg and Ogg
2008)

continental margin (Hunter et al. 2007; Hunter 2008). It is SW orientation in response to the oceanographic setup of
one of a number of contourite drifts formed in the northern the Greenland margin. Here, several deep-water masses
Atlantic Ocean which are closely linked to gateways and overspill from the Arctic Basin through gaps in the
overflow waters from the Nordic and Arctic seas (Fig. 5). Greenland-Iceland Ridge and combine to form the Deep
Located at depths up to 3,400 m, it is elongated in a NE– Water Boundary Current (Hunter 2008).

Fig. 7 Palaeoceanographic
events and seismic characteris-
tics associated with the evolu-
tion of the Southern Drift,
Argentine Basin. After contour
current initiation in the Early
Miocene, drift growth was rapid.
Despite a global lowstand in the
Middle–Late Miocene, localised
deepening of the Drake Passage
enabled drift growth to continue,
illustrating the importance of
gateways on CDS evolution
(adapted from Haq et al. 1987;
Hunter 2008; Ogg and Ogg
2008; Hernández-Molina et al.
2010)
Geo-Mar Lett (2011) 31:343–360 353

Evolution and controls The palaeoceanographic events are during the warm interglacials (sea level highstands and
summarized in Fig. 8. There is evidence for bottom-water transgressions) strong bottom-water currents prevailed,
currents in the region from the Middle Miocene, when the leading to erosional surfaces, active bedform growth and
relative sea level over the Greenland-Iceland Ridge reached CDS development. During lowstands, high downslope
sufficiently high levels to enable the exchange of water sediment influx can dominate and current velocities are
masses between the Atlantic and Arctic oceans. At that seen to wane.
time, bottom-water formation in the Northern Hemisphere
was vigorous and contourite sediment accumulation in-
creased as more deep-water was able to escape into the The Gulf of Cadiz CDS
North Atlantic. Through the Late Miocene and Early
Pliocene the bottom-water circulation increased in intensity, Location and setting The Gulf of Cadiz CDS has devel-
as is evident from the formation of migrating sediment oped over the past 4–5 Ma in response to Mediterranean
waves, drift progradation and localised erosion (Diez et al. Outflow Water (MOW) exiting through the Gibraltar
2008). Late Pliocene global sea level fall and ice sheet gateway (Hernández-Molina et al. 2003, 2006; Llave et al.
advance in response to cooling is coincident with a 2007). The MOW generates an intermediate mid-slope
weakening of the bottom-water current in the Eirik Drift bottom current comprising relatively warm but saline water
region between 3 and 0.9 Ma (Knutz 2008), a marked produced in the warm-water kitchen of the eastern
reduction in drift progradation and slowing in the rate of Mediterranean Sea (Fig. 5). In this respect it is thought to
accumulation. The Holocene has seen evidence of renewed resemble palaeoceanographic conditions during the ex-
intensification of the Deep Water Boundary Current, tremely high sea levels and warm greenhouse conditions
coincident with increased rates of bottom-water formation of the middle and Late Cretaceous. Diapiric ridges
during times of global eustatic highs. orientated perpendicular to MOW flow (Tasianas 2010)
form morphologic obstructions and create distinct channels
Sequence stratigraphy The sequence stratigraphic re- through which the water mass splits into two pathways: the
sponse of the Eirik Drift is in concurrence with Mediterranean upper and lower waters. As a consequence, a
observations in many other Northern Hemisphere con- complex drift system has developed along the northern
tourite depositional systems influenced by deep Arctic margin of the Gulf of Cadiz, including both erosional and
water masses (e.g. Howe 1995; Stoker et al. 1998; Weaver depositional domains and numerous different drift types
et al. 2000; Gröger et al. 2003; Øvrebø et al. 2006). Thus, (Hernández-Molina et al. 2006).

Fig. 8 Palaeoceanographic
events and seismic characteris-
tics associated with the evolu-
tion of the Eirik Drift, North
Atlantic Ocean. The main phase
of drift building coincides with
highstand conditions. The Late
Pliocene lowstand leads to
slower deep-water circulation in
the region (adapted from Haq et
al. 1987; Hunter 2008; Ogg and
Ogg 2008)
354 Geo-Mar Lett (2011) 31:343–360

Evolution and controls Drift initiation occurred when the Sequence stratigraphy Various regions of the Gulf of Cadiz
Gibraltar gateway opened and deepened sufficiently to CDS appear to respond differently to eustatic changes. This
enable significant MOW escape into the Atlantic. This is most likely due to the complexity of the region and the
occurred during the Early Pliocene, ca. 5.3–4 Ma (Fig. 9). additional morphological and tectonic control along the
Contourite drifts such as the Faro-Albufeira Drift are seen margin. The available evidence indicates that greatest
to be highly cyclical in nature, comprising numerous contourite accumulation occurs during times of glacio-
seismic sequences and sub-sequences (Llave et al. 2001, eustatic lowstand.
2006; Stow et al. 2002a). These are interpreted as being due
to major long-term fluctuations in bottom current intensity
(and location), although it is not yet clear to what extent
these have been caused by climatic-eustatic fluctuations or New sequence stratigraphic model
tectonic adjustments at the Gibraltar gateway. Several
authors have demonstrated higher velocities of the lower This section attempts to combine the more theoretical
branch of the MOW during glacial times (Cacho et al. considerations of controls on deep-water sedimentation,
2000; Hernández-Molina et al. 2006; Llave et al. 2006; including the influence of sea level variation on bottom
Toucanne et al. 2007). Other work, however, points to an currents and sediment supply, with the observational data
increased velocity of the upper branch of the MOW during gained from a detailed study of contourite depositional
the recent highstand period (Stow et al. 2002b). There systems, especially those documented in Table 1 and in the
appears to be a basinward shift in the Mediterranean four case studies presented above. These data show that
Outflow Water during times of lowstand due to increased eustatic sea level changes affect bottom current generation
MOW density and lowered sea level. Other key observa- and intensity differently, which is especially evident
tions which relate to sequence stratigraphy are increased between the two hemispheres. It has therefore been
sedimentation rates on drifts in the upper core of the MOW, necessary to propose two new sequence stratigraphic
and a change from progradational to aggradational stacking models of shelf–slope–basin sedimentation which focus
patterns in response to rising sea level. Nelson et al. (1993) on alongslope (contourite) elements developed on
proposed a direct link between drift accumulation and alongslope-dominated margins (Fig. 10).
relative sea level due to the changing cross-sectional area of
the Strait of Gibraltar, although other factors such as Model 1: enhanced bottom-water currents during HST
fluctuating density of the MOW and tectonic activity have
also played an important role in the development of the Sequence boundary and LST As with the conventional
CDS during each evolutionary stage (Llave et al. 2007). downslope ‘slug’ diagram (Fig. 1), the new model containing

Fig. 9 Palaeoceanographic
events and seismic characteris-
tics associated with the evolu-
tion of the Faro-Albuferia Drift
region in the Gulf of Cadiz
(adapted from Haq et al. 1987;
Nelson et al. 1993; Stow et al.
2002a; Llave et al. 2007; Hunter
2008; Ogg and Ogg 2008). M
Messinian, LPR lower Pliocene
revolution, BQD base Quaterna-
ry discontinuity, MPR mid-
Pleistocene revolution
Geo-Mar Lett (2011) 31:343–360 355

Fig. 10 Revised deep-water

sequence stratigraphic models,
applicable along margins where
bottom-water circulation is more
vigorous during times of a
highstand or b lowstand. Darker
colours Downslope deposits,
lighter colours alongslope
deposits, LST lowstand, TST
transgressive, HST highstand,
SB sequence boundary, TS
transgressive surface, MFS
maximum flooding surface.
Figure not to scale but approx.
10s–100s km in the horizontal
and 100 m–few km in sediment
thickness (modified from an
original model by Haq et al.
1988)

contourite deposits begins at the base with a sequence greatly favouring drift development. Erosional unconform-
boundary which is overlain by the lowstand systems tract ities are also common throughout the succession because
(LST). During low relative sea level, the continental shelf minor fluctuations in bottom current velocity lead to
may be subjected to sub-aerial erosion and the slope repeated cycles of erosion and deposition.
generally experiences sediment bypass. As a result, there is This last point reveals an important departure from the
enhanced sediment deposition in the deep ocean basins, and existing models. Using conventional sequence stratigra-
basin-floor and slope-apron fans typically develop. This phy laws (Van Wagoner et al. 1988), any erosional
increase in downslope sediment volume will affect contour- unconformity is tied to sub-aerial erosion and therefore
ite drift development by masking alongslope processes. requires a sea level fall. By contrast, the revised model
Furthermore, the marked reduction in bottom current activity shows that regionally extensive unconformities can be
and velocity at times of low relative sea level provides associated with increased bottom current velocities and sea
conditions unfavourable for contourite development level rise. This is amply supported by observational as
(Fig. 10a). However, at least on some margins there is well as theoretical data (e.g. Shannon et al. 2005, for the
evidence for muddy contourite deposition leading to the NW UK continental margin).
accumulation of thin, fine-grained sheeted drifts. In other
cases these are commonly interbedded with more HST The gradation between transgressive and highstand
dominant turbidite and debrite sediments, leading to the systems tracts is indistinct with respect to contourite
formation of mixed contourite drifts as seen off the development and depends on the oceanographic setting of
eastern USA and the Argentine continental margins the margin in question. However, a trend of increasing
(Faugères and Stow 2008; Huppertz, personal communi- bottom-water generation in response to sea level rise will
cation 2010). see more active bottom-waters throughout the TST, climax-
ing in the HST. Downslope processes become muted, so
TST During the transgressive systems tract (TST), the that bottom current activity is uninterrupted and better
influence of downslope processes wanes and there is preserved. Elongate, mounded contourite drifts continue to
renewed activation of bottom currents and contourite accumulate, showing both alongslope progradational and
deposition. This is typically expressed in the reworking of aggradational patterns. Assuming that the maximum
downslope sediment and the formation of elongate mounds bottom-water velocities reached are sufficiently high,
along the slope apron, showing active alongslope progra- contourite facies will have larger mean grain sizes, so that
dation and/or aggradation (Fig. 10a). Sediment supply over sandy contourites will be more widely dispersed, common-
the shelf edge is moderate and much of it fine-grained, ly as sandy sheeted drifts (Fig. 10a). Under still stronger
356 Geo-Mar Lett (2011) 31:343–360

currents, non-deposition surfaces and erosional features As with the model proposed for enhanced bottom-water
become significant, including channels, gullies, furrows, currents during HSTs, erosional unconformities are com-
moats and other less regular scour features. These too may mon throughout the succession. These are formed in
be the focus of sandy contourite deposition. response to fluctuating bottom current velocities over the
Sediment supply to the bottom currents which construct duration of deglaciation.
contourite drifts will be variable during the HST, depending
partly on geographic location of the margin and partly on HST Where high eustatic sea level is associated with
other additional controls—especially tectonics and climate. relatively low rates of bottom-water formation, contourite
During development of the prograding highstand wedge in development is severely limited in the HST (Fig. 10b). At
the conventional model, sediment supply to the slope will such times, downslope sediment supply to ocean basins is
increase and there is thus potential for enhanced sediment low as sediment is trapped on the continental shelves. The
supply to any drift system active at this stage. combination of low bottom-water velocities and limited
sediment supply results in little to no contourite drift
development, and hemipelagic and pelagic sedimentation
Model 2: enhanced bottom-water currents during LST may become the dominant process along the continental
slopes. Some degree of contourite drift deposition may
Sequence boundary and LST As with the previous model, occur where current speed is adequate for reworking of
low sea level triggers enhanced erosion of the shelf and sediments supplied by slope spillover or normal pelagic
terrigenous sediment bypass into the oceanic basins. settling (Stow et al. 2008).
Wherever bottom-water current velocities peak during As with model 1 HST, certain circumstances such as
times of sea level lowstand, the pirating of downslope tectonic or climatic changes enable downslope sediment
sediments by bottom-water currents can result in the progradation across the continental shelf and into the
domination of contourite systems over the normally pathway of contour currents. If water velocities are
expected downslope LST sequence (Fig. 10b). The result insufficient to redistribute sediment entering the basin, then
is high drift growth rates along the slope apron; some downslope processes may prevail.
margins show accumulation rates which are an order of
magnitude higher during LSTs where enhanced bottom
waters redistribute large sediment influxes associated with
glacial margins (Laberg et al. 2001; Rebesco et al. 2002). Discussion and conclusions
Erosional processes driven by contour currents may also
be prevalent along continental margins at the times when The stratigraphy of contourite drifts has been used
bottom-waters reach sufficient velocities. This is particular- extensively to identify major global palaeoceanographic
ly common in regions where gateways may constrict and, events. This paper is a first attempt to link these
therefore, further amplify bottom-water velocities (e.g. observations, together with theoretical considerations of
Hernández-Molina et al. 2009) or where specific morpho- sea level and other controls on bottom current variation,
logical forcing accelerates the flow (e.g. Viana et al. 2002). to sequence stratigraphic concepts by revising the
Expected sediment facies include sandy contourites where original downslope model. This has been met with a
the maximum bottom-water velocities are sufficiently high to number of challenges.
transport larger grain sizes. Where erosional features become The original sequence stratigraphy model for downslope
significant, channels, moats and terraces are likely to develop. processes provided the definitions of systems tract bound-
These too may be the focus of sandy and, in exceptional cases, aries—the changing balance between sediment supply and
also gravel-lag contourite deposition. accommodation space determines the systems tracts. For
example, the maximum flooding surface (separating the
TST During transgressive systems tract (TST) develop- transgressive and highstand systems tracts) is the point over
ment, downslope processes become more limited to the which there is a shift from sediment supply being unable to
continental shelf and contour current cores begin to shoal. fill the accommodation space to sediment supply exceeding
This is a transitional time for the system, during which the available accommodation space. Since accommodation
accumulation rates slow down. Aggradational units are space is not a primary control on deep-water sediments, this
highly likely and sheeted drifts thus prevail (Fig. 10b). distinction between systems tracts cannot be strictly valid in
Bottom-water currents begin to slow down and therefore that case. As a result, it can be expected that there is a more
smaller average grain sizes are expected, although highly gradual change between systems tracts in the deep-water.
cyclical deposits will accumulate and ice-rafted debris may Hence, the contourite systems tracts identified in the
be found in drifts deposited at higher latitudes. revised model may not exactly match the downslope
Geo-Mar Lett (2011) 31:343–360 357

systems tracts of the conventional model. More work is in duration of 1–10×106 years), while systems tracts and
progress to elucidate any possible disconnection. parasequences reflect third- and fourth-order changes
A further consideration in placing alongslope processes (100×103 to 1×106 years). The same will be true for
into the existing sequence stratigraphic model is related to alongslope systems; however, as with deep-water
the orientation of system evolution. Downslope processes turbidite fans, drift systems are capable of spanning
develop from the continent basinwards. Conversely, con- two or more normal depositional sequences. This is
tourite depositional systems evolve parallel to the conti- clearly evident along the Canterbury Basin drifts,
nental margin and, therefore, along a different axis from which continue to grow through numerous downslope
that of downslope systems. This should be carefully systems sequence boundaries (Fulthorpe et al. 2010).
considered when applying the models to an existing
Where applied with care and a good understanding of a
continental margin, since a depositional sequence found at
margin system, the models proposed here can greatly facilitate
a location where downslope processes dominate (i.e. at a
the interpretation of margins which include or are dominated
turbiditic fan) will differ from that of a continental margin
by contourites. External controls, in addition to sea level,
adjacent to a turbidite fan. This is particularly important
should always be carefully considered with respect to how they
when examining contourites downcurrent of turbiditic
might affect a given contourite depositional system—models
processes where pirating of sediment plays a key role.
will never be applicable everywhere. Sequence stratigraphy
It should be noted that the new models (Fig. 10) are, in fact,
has proven to be a highly valuable tool in hydrocarbon
idealised conceptual end-member models and, in reality, there
exploration of downslope sedimentary systems and it is
will be complications when applying the models to any
therefore highly likely that a sequence stratigraphic model for
existing geological system. This is exactly the same as for
contourite systems could prove to be a strong predictive tool.
any existing sequence stratigraphic model and, as a conse-
A next step in this line of research is to assess how robust the
quence, many of the same problems arise when considering
models are in other settings and drift types—for example, in
the sequence stratigraphy of downslope and alongslope
shallow-water and abyssal plain contourites or sheeted con-
systems. These problems have been discussed above in this
tourite systems. Work by Viana et al. (2002) has begun to
paper. The considerations which are most significant to
address this topic in the Campos Basin shallow-water
alongslope sequence stratigraphy are elaborated upon below.
contourites. Here, sea level lowstand events are associated
with distinctive current waning and increased downslope
1. Synsedimentary tectonic activity can significantly affect
facies deposition. Subsequent sea level rises lead to the
sedimentary architecture along continental margins, in
deposition of sandy facies with bedforms and increasing
some cases completely masking sea level effects (Bridge
erosion. This suggests that the sequence stratigraphy model
and Demico 2008). Local tectonic adjustments will affect
put forward in this paper can also be applied to shallow-water
both downslope and alongslope systems similarly.
contourites in addition to the deep sea contourite systems for
Added complications to contourite systems arise from
which it was developed. Future work should specifically
the extremely important role played by tectonics in CDS
focus on the collection of datable material in order to provide
placement and development, since oceanic circulation is
better time constraints for the assessment of the sequence
controlled by gateways. All the case study examples of
stratigraphic evolution of a given drift at numerous timescales.
contourite depositional systems presented in this paper
illustrate the intimate relationship between contourite Acknowledgements The Spanish Comisión Interministerial de
drift development and gateway opening. Perhaps the Ciencia y Tecnología supports this research through the Project CTM
creation of gateways is the primary control on oceanic 2008-06399-C04/MAR (CONTOURIBER). Funding towards travel
costs to attend and present this work at the International Congress
circulation on a first- or second-order timescale.
‘Deep Water Circulation: Processes and Products’ was provided by the
2. The presence of ice along a margin can significantly International Association of Sedimentologists. We extend thanks to E.
alter the sedimentary architecture, sediment influx to a Llave (guest editor), J.S. Laberg, two anonymous reviewers and the
basin and/or rate of bottom-water generation (Goosse et journal editors for their interest, corrections and suggestions which
have helped us to improve the final version of our manuscript.
al. 2001; Powell and Cooper 2002; van Weering et al.
2008). High-latitude systems can therefore be difficult
to predict and may not always fit into the end-member
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 Marine and Petroleum Geology 46 (2013) 36e50

Contents lists available at SciVerse ScienceDirect

Marine and Petroleum Geology

journal homepage: www.elsevier.com/locate/marpetgeo

Review article

A Pliocene mixed contouriteeturbidite system offshore the Algarve

Margin, Gulf of Cadiz: Seismic response, margin evolution and
reservoir implications
Rachel E. Brackenridge a, *, F.J. Hernández-Molina b, d, D.A.V. Stow a, E. Llave c
a
IPE-ECOSSE, Heriot Watt University, Edinburgh EH14 4AS, UK
b
Facultad de Ciencias del Mar, Universidad de Vigo, 36200 Vigo, Spain
c
Instituto Geológico y Minero de España (IGME), c/Ríos Rosas 23, 28003 Madrid, Spain
d
Department of Earth Sciences, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK

a r t i c l e i n f o a b s t r a c t

Article history: A buried mixed contouriteeturbidite system has been identified in the Pliocene succession of the Algarve
Received 7 January 2013 basin in the northern Gulf of Cadiz. This margin is currently dominated by the Mediterranean Outflow
Received in revised form Water and associated contourite deposition. Analysis of seismic data along with careful geographical and
20 May 2013
oceanographic reconstructions of the margin show the gradual move from a turbidite-dominated to-
Accepted 21 May 2013
Available online 11 June 2013
wards a contourite-dominated margin, and the subsequent ‘birth’ of an impressive elongate mounded
contourite drift system- the Faro-Albufeira drifts. The contourite drift can be distinguished from down-
slope (turbiditic and mass transport deposits) based on the acoustic character, distribution analysis and
Keywords:
Contourite drift
through careful margin reconstruction. In the earliest Pliocene, Seismic Unit P1 has been interpreted as a
Mixed system dominant down-slope (most likely turbidite) system sourced mainly from the northeast. There is clear
Algarve Margin evidence of contourite reworking at Seismic Unit P2, where upslope progradation and a sheeted
Mediterranean Outflow Water morphology are observed. High amplitude reflections are thought to be a result of more vigorous bottom
Gulf of Cadiz currents in the early Upper Pliocene that were capable to redistributing coarser sediments. However, in
Contourite hydrocarbon reservoir potential the northeast of the study area a thick sequence of chaotic seismic facies has been interpreted as mass
transport deposits sourced from the north indicate that the bottom currents were unable to dominate
over the entire margin due to high down-slope clastic influx. Semi-transparent Seismic Unit P3 indicates
that the Upper Pliocene initially experienced a reduction in bottom current intensity; however upslope
progradation shows that a mixed system was maintained. Above the Base Quaternary Discontinuity (ca.
2.6 Ma), highly erosive discontinuities and high amplitude seismic reflections are evidence of pro-
nounced intensification of the Mediterranean Outflow Water and a move to a fully contourite-dominated
slope. Mixed turbiditeecontourite systems such as the one identified in the Algarve Basin could provide
impressive petroleum potential where downslope clastics are winnowed and reworked by bottom water
currents to leave good reservoir properties. Here, we present a conceptual model for sheeted drifts as
hydrocarbon reservoirs in the subsurface.
! 2013 Published by Elsevier Ltd.

1. Introduction contourite deposits are referred to as “drifts”. Where currents are

strong enough, a variety of erosional and depositional features are
The term ‘contourite’ is accepted for those sediments deposited developed (Evans et al., 1998; García et al., 2009; Hernández-
or substantially reworked by bottom currents (Heezen et al., 1966; Molina et al., 2008; Masson, 2001; Nelson et al., 1993; Preu
Rebesco and Camerlenghi, 2008). Major accumulations of et al., 2013; Stow et al., 2009; Stow and Mayall, 2000). An asso-
ciation of various drifts and related erosional features is commonly
termed a contourite depositional system (CDS) (Hernández-
Molina et al., 2003, 2008). Mixed contouriteeturbidite systems
* Corresponding author. Tel.: þ44 (0)131 451 3699.
are developed where bottom currents have interacted with down-
E-mail addresses: rachel.brackenridge@pet.hw.ac.uk (R.E. Brackenridge), fjhernan@
uvigo.es (F.J. Hernández-Molina), dorrik.Stow@pet.hw.ac.uk (D.A.V. Stow), e.llave@ slope processes (Faugères et al., 1999; Rebesco and Camerlenghi,
igme.es (E. Llave). 2008).

0264-8172/$ e see front matter ! 2013 Published by Elsevier Ltd.

http://dx.doi.org/10.1016/j.marpetgeo.2013.05.015
 R.E. Brackenridge et al. / Marine and Petroleum Geology 46 (2013) 36e50 37

The use of seismic data is vital to contourite drift identification external morphology. Much less is known about the identification of
and interpretation. Drifts are primarily identified by external ge- sheeted drifts both on the seabed and in the subsurface. In recent
ometry and internal architecture. This is useful for drifts of mounded years, the hydrocarbon potential of contourite deposits has been
morphology, but much more challenging with aggradational sheeted explored (Viana et al., 2007), and it is seen that clastic reservoir
drifts. For these, the seismic facies could be an additional important (sand-rich) facies are most commonly found as sheeted drifts, or as
diagnostic tool. It is, therefore, of great importance to fully under- localized channel accumulations. It is therefore of great importance
stand the acoustic response of sedimentary drifts. Work by Faugères that these drifts are characterized and made identifiable in the sub-
et al. (1999), Howe (2008), Nielsen et al. (2008) and Shanmugam surface and distinguishable from down-slope deposits.
(2006) have provided an initial overview to the seismic response of This work examines a buried contourite sheeted drift within a
contourites and outline the processes by which to analyse and mixed system in the Gulf of Cadiz using seismic data. The key aims
describe drifts using geophysical data. There are several types of are therefore; 1) Characterize the along- and down-slope compo-
contourite drifts, defined mainly on their morphological, sedimen- nents of the system on all scales (drift-, depositional unit- and
tological and seismic characteristics (Faugères and Mulder, 2011; seismofacies-scale); 2) Assess the interaction between along- and
Faugères et al., 1999; McCave and Tucholke, 1986; Rebesco, 2005; down-slope processes; 3) Propose a depositional model for the
Rebesco and Camerlenghi, 2008; Rebesco and Stow, 2001; Stow et al., Algarve Margin Pliocene section; 4) Identify any key criteria to aid
2002b). The generation of each drift type is controlled by a complex the identification of sheeted contourite drift systems elsewhere; and
set of factors, most importantly the local morphology of the seabed, 5) Use the above information to make conclusions on the identifi-
the bottom water current conditions and the sediment supply cation of contourites in the subsurface for hydrocarbon exploration.
(Faugères and Stow, 1993; Faugères et al., 1999; Shannon et al., 2005;
Viana et al.,1998). Often, a complex CDS is formed, consisting of many 2. Geological and oceanographic setting
different drift types and erosional elements. A given contourite
accumulation may also evolve over time between drift types. Since 2.1. Geological setting and margin evolution
the primary identification of contourites is usually by overall drift
geometry (Nielsen et al., 2008), many known drifts are those with The study area is located along the Algarve Margin in the western
some topographic relief from the seabed, i.e. those of mounded Gulf of Cadiz (Fig. 1A). This region has had a complex geodynamic

Figure 1. A) Location map for the study area (boxed). AABW ¼ Atlantic Bottom Water; AALW ¼ Atlantic Intermediate Water; MOW ¼ Mediterranean Outflow Water; NADW ¼ North
Atlantic deep Water. B) Tectonic and Location Map for the study Area. Modified from Maldonado and Nelson (1999). C) The study area is located along the northern margin of the
Gulf of Cadiz and is influenced by the Mediterranean Outflow Water. The seismic surveys used for this study are indicated. Study area highlighted in box. Red dashed line indicated
position of the seismic lines on Fig. 3. MOW ¼ Mediterranean Outflow Water. Atlantic SW ¼ Atlantic Surface Water.
38 R.E. Brackenridge et al. / Marine and Petroleum Geology 46 (2013) 36e50

history as a result of its situation between the African and Eurasian (Baringer and Price, 1997) and eventually the MOW leaves the
continental plates (Flinch et al.,1996; Medialdea et al., 2009; Zitellini seabed and reaches a neutral buoyancy at a depth of 1400 m
et al., 2009). Geophysical studies have aided the interpretation of the offshore the Cape of St Vincent, where it begins to raft above the
margin evolution, which has been varied and complex (Gutscher North Atlantic Deep Water.
et al., 2002; Maldonado et al., 1999; Medialdea et al., 2004; Nelson The MOW is restricted to a core approximately 10 km wide as it
and Maldonado, 1999). The plate boundary between the African accelerates through the Strait of Gibraltar. Along the mid-slope of
and Eurasian plates has ‘jumped’ (Srivastava et al.,1990) and at times the Gulf of Cadiz however, the core can exceed 80 km in width
the Iberian plate acted independently (Roest and Srivastava, 1991). (Baringer and Price, 1997) and forms a broad, sluggish flow. The
Three key events have been identified as crucial for the modern set- velocity is locally enhanced where late Cenozoic neotectonics have
up of the margin (Maldonado et al., 1999): 1) Late Mesozoic passive created diapiric ridges oblique to the MOW flow direction (García
margin development that occurred post-rift and sea-floor spreading et al., 2009). These ridges, predominantly marly in composition
as the Tethys and Atlantic Oceans opened; 2) the Cenozoic conver- (Maldonado et al., 1999), have undoubtedly been responsible for
gence of the African and Eurasian plates in an NeS direction; and 3) the splitting of the water mass into numerous distinctive cores
the extensional collapse of mountains to the east and resulting (Fig. 1C), although vertical layering within the water core has also
olistostrome emplacement, named the Cadiz Allochthonous Unit been proposed as an additional control. There are two principal
(Medialdea et al., 2004) over the Gulf of Cadiz. A change in water cores- one main upper water core and a lower core that
compression orientation in the Late MioceneePliocene may also further divides to form the Intermediate Branch, the Principal
have played an important part in affecting the sedimentation pattern Branch and the Southern Branch (Llave et al., 2007). Each branch
of the margin (Zitellini et al., 2009). demonstrates unique physical oceanographic properties, such as
Today, a compressional tectonic regime is ongoing in the Gulf of salinity, temperature and average velocity (Ambar and Howe, 1979;
Cadiz, although it is slowing and the margin is moving towards Borenas et al., 2002; Zenk, 1970). Overall, the Upper Core (which
more stable conditions (Rosenbaum et al., 2002). The Eurasian and influences the study area) is a smaller, warm, less saline water mass
African plates are currently converging at a rate of approximately when compared to the lower core (Llave et al., 2007).
4 mm/yr in an NEeSW orientation (Maldonado et al., 1999). To the The movement of water masses through the Strait of Gibraltar
west, the Azores-Gibraltar Fracture Zone (AGFZ) results in strike- was first recorded in salinity data acquired in the early 1900s
slip tectonics in the west of the Gulf (Fig. 1B). To the East, (Nielsen, 1912) and since then, numerous studies on the water
mounting evidence points towards a region of forced lithospheric shows that physical properties have remained fairly constant. There
subduction of oceanic crust eastwards under the Alboran and is more controversy surrounding the palaeo-evolution of the MOW.
Mediterranean Seas (Gutscher et al., 2002; Royden, 1993). This re- Data from the Mediterranean Basin shows that the Messinian
sults in slab rollback and subsequent uplift and extensional tec- Salinity Crisis ended in the latest Miocene (Hsü et al., 1973). This
tonics in the eastern Gulf of Cadiz and Gibraltar regions. Elsewhere signifies that the Strait of Gibraltar opened at this time so that the
neotectonics play an important role along the margin. The depo- exchange of water was possible between the Atlantic Ocean and the
sition of an unstable allocthonous unit in the middle to late Mediterranean Sea. The precise date of the initiation of the MOW is
Miocene has led to intense halokinesis along the margin, which in dispute, but it is generally accepted that bottom water formation
affects the modern day sea floor. Triassic and Mid-Miocene marls in the Mediterranean initiated sometime after the influx of the
and salts have pierced overlying sediments forming diapiric ridges Atlantic water into the basin, probably in the beginning of the Early
and adjacent depocentres (Hernández-Molina et al., 2006; Llave Pliocene (Hernández-Molina et al., 2009b; Llave et al., 2010;
et al., 2007; Somoza et al., 2003). In all, the margin is a tectoni- Raddatz et al., 2011; Rogerson et al., 2012). Since that time, varia-
cally complex and active one. This has important implications for tions in the MOW are clear from proxy data and the high cyclicity
both modern and past sedimentation trends. observed in acoustic data. These variations point towards a more
vigorous and denser lower branch of the MOW during glacial times
2.2. Oceanographic setting (Cacho et al., 2000; Hernández-Molina et al., 2006; Llave et al.,
2006; Toucanne et al., 2007; Voelker et al., 2006) and a more
The hydrodynamic framework of the Gulf of Cadiz is dominated vigorous les denser upper branch of the MOW during the recent
by the water exchange between the Atlantic Ocean and the Medi- highstand period (Rogerson et al., 2005; Stow et al., 2002a). A clear
terranean Sea. The Strait of Gibraltar acts as the gateway for ex- separation between responses to climatic/eustatic-induced
change between the warm saline Mediterranean Outflow Water changes and tectonic movements remains challenging, and will
(MOW) and the overlying Atlantic Inflow Water (Baringer and Price, be explored further in this study.
1999; Llave et al., 2007). Much physical oceanographic research has
been carried out on the MOW in recent decades and its current 2.3. Contourite depositional system (CDS)
properties and route are now well constrained (Baringer and Price,
1999; García Lafuente and Ruiz, 2007; Serra et al., 2010). The MOW directly affects the sediments deposited in the study
The MOW is composed of a mixture of water masses sourced area, and the rest of the Gulf of Cadiz. Sufficient velocities have been
from the Mediterranean Basin; the Levantine Intermediate Water maintained to form large contourite drifts along the entire length of
forming approximately two thirds, and the remainder sourced from the margin from the Strait of Gibraltar to the Cape of St Vincent and
the Western Mediterranean Deep Water. The constricted basin of beyond along the western and northern Iberian continental mar-
the Mediterranean with its arid climate provides favourable con- gins (Hernández-Molina et al., 2011; Van Rooij et al., 2010). As both
ditions for the formation of warm, highly saline, dense water of depositional and erosional products can be present along a margin
13 ! C, 36.5& (Ambar and Howe, 1979). The water mass accelerates influenced by bottom currents, it is practical to refer to contourites
through the narrow gateway of the Strait of Gibraltar, where they as depositional systems. A contourite depositional system (CDS)
are thought to locally reaching velocities in excess of 2.5 m s"1 will form along any continental margin where along-slope pro-
(Ambar and Howe, 1979; Mulder et al., 2003) and moves north- cesses dominate over mass wasting, turbidity currents and pelagic/
westwards along the mid-slope of the Gulf of Cadiz (Fig. 1C). hemipelagic settling processes (Hernández-Molina et al., 2008).
Density-driven descent and mixing with overlying Atlantic Waters Each depositional system will begin in some region that can pro-
results in decreasing salinity along the margin from SE to NW vide a mechanism for enhanced bottom water velocity. It will
 R.E. Brackenridge et al. / Marine and Petroleum Geology 46 (2013) 36e50 39

terminate where the transport capacity of the contour current is

diminished so that it is no longer capable of dominating over other
depositional processes. A CDS will evolve over time and space,
controlled by climatic and oceanographic changes in addition to
local tectonic activity that can affect the margin and water masses
such as oceanic gateways and halokinesis.
The Gulf of Cadiz CDS initiates where the MOW overspills into
the Atlantic Ocean as a highly saline dense water mass and is
accelerated through the narrow conduit which connects the two
oceans: the Strait of Gibraltar (Howe, 1982). Coriolis Forces push the
water mass northwards, moving it around the mid-slope in the Gulf
of Cadiz and extending round the west Iberian Margin. Close to the
Strait of Gibraltar, current-meter moorings have recorded bottom
water velocities in excess of 1 m s!1 (Baringer and Price, 1999;
García et al., 2009; Hernández-Molina et al., 2011) and as a result,
erosion and sand-deposition are the dominant ongoing processes.
Features are closely tied to the erosive capacity of the bottom water
mass (i.e. its velocity) and the local sea bed morphology
(Hernández-Molina et al., 2008) with laterally extensive erosional
surfaces in the east moving to discrete contourite channels in the Figure 2. A number of past studies have been carried out. General consensus is
west where outcropping diapiric structures manipulate the bottom nearing in the Quaternary section; however the Pliocene is refusing to yield a robust
stratigraphy to date. This study identifies three units in the Pliocene and a highly
water core. Contourite moats occur more distal still (from the Strait eroded section in the Lowermost Quaternary. From Llave et al. (2007, 2011), Lopes et al.
of Gibraltar) and are associated with depositional features. Many (2006), Maldonado and Nelson (1999), Marches et al. (2010) and Roque et al. (2012).
contourite drifts are observed along the margin, evolving between
drift types over space and time. There is also a broad trend of frequency, deep-penetrating data that gives much lower resolution.
decreasing mean grain size along the margin as the transport ca- Most recent work points towards agreement on Quaternary dis-
pacity of the MOW decreases with distance from the Strait of continuities, however the Pliocene section has remained poorly
Gibraltar (Nelson et al., 1999). Detailed investigations of the systems studied and unwilling to provide a robust stratigraphy.
has exposed five distinct ‘morphosedimentary sectors’ (Hernández-
Molina et al., 2003, 2006; Llave et al., 2007) each showing different
characteristics and depositional/erosional features. (1) Proximal 4. Data and methods
scour and sand ribbons sector; (2) Outflow-sedimentary lobe
sector; (3) Channels and ridges sector; (4) Active contourite drift An extensive 2D seismic survey database was used (Fig. 1C),
sector; (5) Submarine canyons sector. The study area under inves- comprising two surveys. Survey PD00 was acquired by TGS-NOPEC
tigation here is located mainly in the active contourite drift sector in 2000 using a tuned bolt array with a shot point interval of 12.5 m
(sector 4) and extends some way into the submarine canyons sector (TGS, 2005). Survey PR00 was acquired at an unknown earlier date,
(see Fig. 5 in Hernández-Molina et al., 2006). but reprocessed in 2000 with survey PD00 (George, 2011). A total of
On a larger scale, this CDS can be compared and contrasted to 95 lines along the Portuguese margin of the Gulf of Cadiz make up
others found on continental margins around the oceans. This sys- the data set, between 160 and 320 km WNW of the Gibraltar
tem is classed as a mixed contouriteeturbidite system, but unlike Gateway. These seismic lines cover the outer shelf and extend
many other margins (Locker and Laine, 1992; Rebesco et al., 2002; down-slope to water depths of up to 3000 m. The 2D seismic grid is
Viana, 2001; Viana et al., 2002) along-slope processes now domi- relatively dense, with offsets of 5e10 km between lines that range
nate along the mid-continental slope and down-slope processes from 140 to 300 km in length. Additional data includes a detailed
dominate at greater depths on the lower continental slope and bathymetric map and marine gravity (TGS, 2005; Zitellini et al.,
abyssal plain (Hernández-Molina et al., 2011). Other features of this 2009). All available data was compiled and uploaded to the inter-
margin that makes it markedly different from others, is the lack of a pretation software, in this case IHS Kingdom 8.5.
continental rise and relatively fewer down-slope submarine can- In general, the survey shows moderate to high quality,
yons than would be expected (attributed to the large nature of the decreasing with depth. Poor resolution is observed at depths
margin fluvial system). The first feature could be accounted for by greater than 2.5e3 s TWT. Additional zones of poor resolution can
the unstable substratum in the Gulf of Cadiz: the ‘Cadiz Alloctho- be accounted for by the presence of salt or gas, which are known to
nous Unit’, and the second has been linked to the vigorous water adversely affect seismic resolution (Loseth et al., 2009). The data
masses circulation, which inhibit the down-slope processes on the was also checked for polarity and has been processed to European
upper and middle slope (Hernández-Molina et al., 2006). standard (negative events represent increased impedance con-
trasts). No velocity model is available for reliable time-to-depth
3. Past work in the region conversion at this stage.
Work in the region has been carried out to identify key dis-
Early studies on the regional tectonostratigraphic evolution of continuities in the subsurface, which relate to changes in the
the Gulf of Cadiz identified major discontinuities bounding at least depositional style associate with oceanographic events along the
two seismic units within the Pliocene (Maldonado et al., 1999). slope. The data has been tied to industry borehole Algarve-2
More localized studies on the Algarve Margin have resulted in (Fig. 1C), which was drilled by Esso in 1982 (Algarve-2, 1982).
dispute over 1) the names, 2) the age and 3) the number of major This has been used to confirm the depths and ages of the major Late
discontinuities in the study area (Llave et al., 2011; Lopes et al., Miocene and Pliocene discontinuities with some confidence at a
2006; Marches et al., 2010; Roque et al., 2012) (Fig. 2) Much of regional scale. Although there will be increased risk in reflection
this confusion and conflict derives from interpretations based on mapping away from the boreholes, care was taken to map discon-
the analysis of very old seismic data from the 1970s, or low tinuities by observing strong reflections in seismic (Sheriff and
40 R.E. Brackenridge et al. / Marine and Petroleum Geology 46 (2013) 36e50

Figure 3. Location of seismic lines indicated on Fig. 1C. Seismic sequences are labelled and the key discontinuities named. MPR ¼ Mid Pleistocene Reflection; MQD ¼ Mid Qua-
ternary Reflection; BQD ¼ Base Quaternary Discontinuity; UPD ¼ Upper Pliocene Discontinuity; LPR ¼ Lower Pliocene Revolution; M ¼ Top Miocene Discontinuity. Depositional
sequences P1-QII refer to those outlined in Fig. 2.
 R.E. Brackenridge et al. / Marine and Petroleum Geology 46 (2013) 36e50 41

Geldart, 1995) and, more importantly, observing reflection termi- along with isochron maps for different sections in the Pliocene
nations (onlapping and downlapping as well as erosional trunca- succession (Fig. 4). This work builds on that carried out by
tions of reflections). In addition, the regional Quaternary ages given Hernández-Molina et al. (2006), Llave et al. (2001), (2007) and Stow
by Llave et al. (2001, 2007, 2011); Lopes et al. (2006); Hernández- et al. (2002a).
Molina et al. (2002, 2006); Marches et al. (2010) and Roque et al.
(2012) have been considered for the final chronostatigraphic 5. Results: the seismic response of the Pliocene section
framework.
Once picked, discontinuities were mapped. Key horizons are: The Late Miocene to Quaternary sediment cover along the Gulf
the seabed, six discontinuities within the Pliocene and lowermost of Cadiz margin is characterized by major discontinuities which
Quaternary section, and the acoustic basement (Fig. 3). Contour have been used to aid interpretation (Fig. 3). Each seismic sequence
maps for these horizons were produced using standard methods, has different acoustic characteristics, and here they will be

Figure 4. Isochron (time) maps for depositional sequences P1-QII. Reds are depocentres. A clear contrast in the focus of deposition between the Quaternary and the Pliocene
sections. A major region or absent sedimentation (erosion) is seen in PQ.
42 R.E. Brackenridge et al. / Marine and Petroleum Geology 46 (2013) 36e50

outlined, including the first detailed description of the features of a and it should be noted that this differs in age from the Roque et al.
contourite sheeted drift in the Lower Pliocene (Unit P2 in Fig. 3). (2012) Discontinuity M, which instead marks the base of the
The Quaternary succession has previously been carefully analysed Miocene. There are some local evidences across the study area that
(Llave et al., 2011) and care will be taken to note any comparisons this discontinuity represents a smooth erosional surface. The
and contrasts between the Quaternary and Pliocene successions. overlying seismic unit is here named P1 to correlate to the (Llave
et al., 2010, 2011) classification scheme and associates, in part, to
5.1. Upper Miocene the base of U1 in the Roque et al. (2012) sequence classification
(Fig. 2). The unit is dated as the Early Zanclean.
Although outside the scope of this paper, it should be noted that
a considerable thickness of Miocene and older sediments underlay 5.2.2. Distribution
the Gulf of Cadiz contourite system. It is most likely composed of Figure 4 shows an isochron map of Seismic Unit P1 and clearly
sediments of hemipelagic and turbiditic origin. Maximum sedi- shows a considerable depocentre in the NE of the study area. Here,
ment thickness between the base Pliocene and top acoustic base- it reaches thicknesses of 0.25 ms but pinches out basinwards to the
ment is up to 0.75 ms TWT, although the formation pinches out south and southwest. To the north, it is onlapping onto the palae-
along many of the structural highs. The Miocene succession is oslope relief and in places onto tectonic highs.
heavily deformed, now observed as palaeorelief in the seismic data.
Deposition of the olistostrome mass: the Cadiz Allochthonous Unit 5.2.3. Seismic character and evolution across the study area
(Maldonado et al., 1999; Medialdea et al., 2004) throughout the The Seismic Unit P1 is made up of stacked high amplitude,
Miocene resulted in a complex array of depositional basins and laterally continuous reflection events. Locally, moderate reflection
tectonic highs. Neotectonism has since punctured the sediments in amplitudes appear at the base of this unit. In general, a weak
some regions, most notably in the east of the study area. response is observed at the base, increasing up the sequence,
evolving towards high amplitude, laterally extensive reflections
5.2. Seismic Unit P1 that top the sequence. This is in stark contrast to the underlying
weak to semi-transparent and more chaotic seismofacies of the
5.2.1. Bounding discontinuities Late Miocene. The package has been deformed by late Tertiary
The basal discontinuity of Seismic Unit P1 is defined by a marked tectonics. Deformation includes faulting, displacement due to the
change in seismofacies and forms a major onlap surface (Fig. 5). It uplift of the Guadalquivir High (Fig. 3) and localized deformation by
has been dated as the latest Miocene at w5.3 Ma (Llave et al., 2011) halokinesis.
and approximately marks the MioceneePliocene boundary. It has
been previously named Discontinuity M (Llave et al., 2010, 2011) 5.3. Seismic Unit P2

5.3.1. Bounding discontinuities

The discontinuity separating Seismic Units P1 and P2 is the most
subtle of the discontinuities in studied sedimentary record across
the study area. Although largely conformable with the underlying
P1 seismic package, it locally marked as an onlap surface (Fig. 5). In
compliance with Llave et al. (2011), this discontinuity is named the
Lower Pliocene Discontinuity (LPR). Onlap is seen primarily along
the continental slope, and to some extent in minor depocentres
formed between the salt and mud diapirs. There appears to be
localized upslope progradation along the continental slope in
contrast to the onlap observed in the underlying P1 sequence.

5.3.2. Distribution
P2 reaches its maximum thickness in the northeast of the study
area, where it is 0.15 ms (Fig. 4). Maximum thicknesses coincide
with an area of chaotic seismic facies (Fig. 3A). There is also a
depocentre in the southeast (Fig. 4). Towards the west, the package
thins considerably and in places it is completely absent where it has
been cut by later erosional surfaces.

5.3.3. Seismic character and evolution across the study area

In the east of the study area, there is a marked change in seis-
mofacies across the LPR discontinuity. There is a change from
laterally continuous reflections in Seismic Unit P1 to highly chaotic
seismic facies in P2. The package itself shows varied character across
the region. To the northeast, a thick succession of chaotic seismic
facies dominates. However, basinwards and westwards along-slope,
this evolves towards high amplitude, parallel reflections.

5.4. Seismic Unit P3

Figure 5. Detailed seismic section from line PD00-830 (Fig. 3B.) Onlap and truncation
trends can be seen in the reflections. Different seismofacies can clearly be identified,
5.4.1. Bounding discontinuities
most notably, transparent and chaotic in the Pliocene section close to the slope moving The high amplitude reflections of Seismic Unit P2 make way for
to more laterally continuous reflections in the Quaternary section. more transparent facies over large swaths of the study area (Fig. 3A).
 R.E. Brackenridge et al. / Marine and Petroleum Geology 46 (2013) 36e50 43

The thin Upper Pliocene P3 unit is bounded by the Upper Pliocene seen. The stacked sequences are highly complex, and outside the
Discontinuity (UPD) at its base and the Base Quaternary Disconti- scope of the presented paper. They have been examined in detail in
nuity (BQD) on top. This unit is previously identified by Roque et al. Llave et al. (2001, 2007).
(2012). The basal discontinuity is marked by a change in seismofa-
cies in the south east of the study area and localized onlap and 6. Discussion
downlap. There is some localized evidence that this is an erosional
discontinuity (Fig. 5), and (Llave et al., 2001) have noted its erosive 6.1. Pliocene depositional model for the Algarve Margin
character. Data from industry borehole Algarve-2 indicate the
overlying sequence P3 is Late Pliocene (Early Piacenzian) in age. Using seismic stratigraphic analysis, this study identifies four
key successions within the Pliocene to Lowermost Quaternary
5.4.2. Distribution section along the Algarve Margin. These have been attributed to the
Seismic Unit P3 has been heavily eroded by later events along changing dominance of along- and down-slope processes leading
the continental margin and is completely absent across much of the to the eventual evolution of a contourite depositional system (CDS).
north study area. Basinwards, it has been preserved, although sig- Seismic unit development is determined by the interplay of the
nificant thinning is seen over the diapiric ridges in the east. Else- Mediterranean Outflow Water (MOW), tectonic activity and glacio-
where a consistent thickness of approximately 0.1 ms is maintained eustatic sea level changes. Llave et al. (2011) have expressed these
with some thinning to the west (Fig. 4). sequences as contourite evolutionary stages and interpret the
Lower Pliocene as a pre-contourite phase, the Upper Pliocene as an
5.4.3. Seismic character and evolution across the study area early contourite phase and the Quaternary as a late and contourite-
Seismic Unit P3 marks a change in acoustic response, with a dominant phase. The evidence from this study supports this
move to a weak semi-transparent seismic response throughout the regional interpretation.
study area. As with the underlying Seismic Unit P2, chaotic seis- In addition, a Pliocene mixed system has been described in
mofacies characterize the northeast of the study area whereas semi- detail for first time with interacting down-slope and along-slope
continuous, parallel reflections are seen in the remaining region. processes, confirming some of the findings by Roque et al. (2012).
The region therefore provides a good opportunity to analyse the
5.5. Seismic Unit PQ differing acoustic responses of the sediments deposited by these
processes and uses the conclusions to better identify sheeted
5.5.1. Bounding discontinuities contourite drifts elsewhere in the subsurface.
The BQD, which is easily distinguished by its high amplitude,
marks the base of Seismic Unit PQ and the onset of contourite- 6.1.1. Seismic Unit P1; Lowermost Pliocene
dominated deposition along the margin (Fig. 3). Previous studies Discontinuity M is defined by a change in acoustic response, and
have dated this discontinuity as w2.6 Ma (Llave et al., 2011; Roque acts as a clear onlap surface, especially where the overlying Seismic
et al., 2012). The upper boundary is as yet unnamed and is identi- Unit P1 appears to ‘infill’ the Miocene palaeotopography. Locally,
fiable due to its locally highly erosive nature in the northeast of the there is some evidence of erosion that could indicate some short
study area where erosional truncations are seen (Fig. 3A). In this hiatus associate to Discontinuity M. Overall, the discontinuity has
study, the erosional discontinuity has been named the Lower- been interpreted as an important change in the deposition process.
Quaternary Discontinuity (LQD). The overlying P1 sequence is highly reflective and made up of
laterally extensive parallel seismic reflections up to 0.15 ms TWT in
5.5.2. Distribution thickness and the distribution map of this sequence shows the
The seismic unit has been heavily eroded in the northeast of the main depocentre in the NE of the study area (Fig. 4).
study area and in places it is completely absent along the slope. The isochron map for the Seismic Unit P1 shows sedimentation
Basinwards, the seismic unit is preserved and is seen to thicken focused broadly in an along-slope orientation with lobes fanning
considerably to the west (Fig. 4), which is in stark contrast to the out to the Southwest (Fig. 4). Careful reconstruction of the regional
underlying Pliocene Seismic Unit P3, which thinned to the west. palaeogeography in the Gulf of Cadiz gives clues to the likely origin
Significant thinning over diapirs in the east of the study area is seen of this seismic unit. Riaza and Marínez Del Olmo (1996) and
throughout the entire section (e.g. Fig. 3A). Martínez del Olmo (2004) proposed that throughout the latest
Miocene and Early Pliocene a down-slope system was supplying
5.5.3. Seismic character and evolution across the study area the region with sediment from the northeast. They suggest that
The intense erosion along the slope has left a partially ‘missing there was an influx of clastic sedimentation across the northeast
sequence’ that can only be seen in the south of the study area. Very Gulf of Cadiz at this time which resulted in the deposition of the
little can be said about the original external geometry of the de- Guadiana Sands via down-slope (most likely turbiditic) processes.
posits, but up to four regional, highly erosional discontinuities have These sands have been identified in industry boreholes to the east
been identified within the section. High amplitude, laterally of the study area (Riaza and Marínez Del Olmo, 1996). Based on this,
extensive aggradational seismic reflections with many erosional and the distribution trends observed in the data, we can expect that
truncations are seen within the sequence. this could be a distal turbiditic fan system, moving across the
continental slope and being deflected basinwards. Systems with
5.6. Mid-upper Quaternary similar distribution patterns have been observed in the Peira Cava
turbidite system in France (Amy et al., 2004; McCaffrey and Kneller,
The Quaternary to recent section is marked by numerous dis- 2001). Within the study area, the high amplitude, laterally exten-
continuities, many highly erosional in nature. Seismic sequences sive ‘tram line’ acoustic response at the base of Seismic Unit P1 is
are highly progradational and aggradational in nature and show a indicative of sandier facies (although it cannot be quantitatively
clear progradation in an upslope direction. Two main depocentres stated). The seismic facies of Unit P1 make it difficult to distinguish
are identified: one in the northeast, and one in the southwest of the along- and down-slope deposits, and the additional distribution
study area (Fig. 4). The Quaternary section is the first sequence analysis is required to make conclusions on the depositional
where erosional features have been preserved, and channels can be process.
44 R.E. Brackenridge et al. / Marine and Petroleum Geology 46 (2013) 36e50

Locally in regions where the thickness is significant (close to the 6.1.2. Seismic Unit P2; Late-Lower Pliocene
modern upper slope), complex relationships can be seen in the The Lower Pliocene discontinuity (LPR), located at the base of
form of localized onlap relationships (Fig. 5). The proximity to the Seismic Unit P2, signifies the clear onset (at seismic scale) of the
upper slope suggests that two discrete down-slope sedimentary MOW’s influence on the study area. This occurred sometime in the
systems were feeding this area e one minor down-slope system Late-Early Pliocene (Late Zanclean) (Llave et al., 2011) when the
sourcing from the north and a large-scale down- and across-slope Gibraltar Gateway allowed for the exchange of water masses be-
system sourcing form the northeast, which quickly dominated tween the Atlantic Ocean and the Mediterranean Sea and exchange
margin deposition (Fig. 6). initiated. Such an oceanographic reorganization would have had a
The MioceneePliocene boundary is generally accepted to be the substantial effect on the depositional style of the margin, causing a
time when the Strait of Gibraltar was opened, and the subsequent change in dominant depositional processes.
flooding of the Mediterranean ended the Messinian Salinity Crisis The overlying package, P2, is highly variable in character across
(Hsü et al., 1973). However, there is much more ambiguity over the region and this has been interpreted to be as a result of multiple
when bottom water generation in the Mediterranean Sea reini- depositional processes ongoing thorough the Late Zanclean.
tiated and when the MOW was established in the Gulf of Cadiz. Figure 6 maps the extent of down-slope vs. along-slope sediments,
Based on the present study, the seismic data shows little evidence as based on their seismic facies and distribution. A thick succession
of along-slope processes in the lowermost Pliocene. This is in (0.2 ms TWT) of chaotic seismic facies in the easternmost study
agreement with interpretations from Hernández-Molina (2009), area is interpreted as down-slope mass transport deposit accu-
Hernández-Molina et al. (2009a), Llave et al. (2011) and Roque et al. mulation. Their close proximity to the upper slope suggests debris
(2012). Therefore, if exchange of water masses was occurring flow deposits (debrites). Basinwards and westwards, these chaotic
through the Strait at this time, they were insufficient to dominate facies become increasingly interbedded with more laterally
margin deposition at a seismic scale. continuous seismic reflections, interpreted to be a Lower Pliocene
sheeted contourite drift based on its overall regional geometry.
Evidence for a contouritic origin for P2 include; (1) progradation of
seismic units upslope, a characteristic commonly used as a identi-
fication criteria in many contourite depositional systems (Faugères
et al., 1999); (2) an along-slope orientation; and (3) evidence for a
highly cyclic nature of the sequence.

6.1.3. Seismic Unit P3; Upper Pliocene

One of the most subtle of the discontinuities along the Algarve
Margin, the Upper Pliocene discontinuity (UPD) marks a change in
sediment acoustic response of the unit with a move from high
amplitude to semi-transparent seismic facies. The overlying Upper
Pliocene sequence shows localized areas of high amplitude re-
flections in some areas, but laterally these give way to semi-
transparent regions showing poor continuity. In the NE of the
study area, chaotic seismic facies still dominate indicating that in
this region mass transport processes were still active at this time.
The distribution of the seismic unit shows a largely sheeted
morphology, orientated, but thinning along-slope. This observa-
tion, combined with the semi-transparent seismic facies, leads to
the conclusion that a homogeneous muddy-sheeted contourite
drift was deposited at this time, influenced by a relatively sluggish
MOW. In addition, there is evidence that there was significant
tectonic movement at this time, with P3 thinning considerably over
the diapiric ridges and onlapping of seismic reflections onto the
underlying UPD discontinuity. The local occurrence of high ampli-
tude reflections can be linked to where diapir growth has affected
bottom water flow and locally intensified it, resulting in localized
patch drifts of coarser material (Fig. 3C). In the northwest of the
study area there is still strong interbedding of down-slope (chaotic)
and along-slope (parallel) seismic facies (Fig. 6) indicating that at
this time, contourite deposition was not completely dominant.

6.1.4. Seismic Unit PQ: uppermost PlioceneeLowermost Quaternary

The BQD marks a major change in the depositional style of the
margin. Throughout the Pliocene, deposition has been focused to
the east of the study area. This shifts westwards above the BQD.
This seismic unit is completely absent from the north of the study
Figure 6. Gross depositional environment reconstructions across the study area for area due to intense erosion along the middle slope. Only a ‘window’
depositional sequences P1-QII. It is seen that there is a move from downslope- into the sequence is preserved basinwards (Figs. 3A and 4) and
dominated to contourite-dominated sedimentation. The highly erosive nature of the therefore it is difficult to be certain on its complete interpretation.
bottom water currents is clear to see in along the upper slope and contourite channels.
AD ¼ Albufeira Drift; FEMD ¼ Faro Elongate Mounded Drift; BDSD ¼ Bartolomeu-Dias
Clues to the origin of this sequence can however be gathered. A
Sheeted Drift; FSD ¼ Faro Sheeted Drift; PH ¼ Portimao High; GH ¼ Guadalquivir High; major change in the system is evident e with an increase in seismic
PSD ¼ Pliocene Sheeted Drift; MOW ¼ Mediterranean Outflow Water. amplitudes and a move to discontinuities that are highly erosive in
 R.E. Brackenridge et al. / Marine and Petroleum Geology 46 (2013) 36e50 45

nature and the drift begins to develop a more pronounced moun- 2009a) and around the Antarctic margin (Maldonado et al.,
ded morphology. These are characteristics of a high-energy con- 2003). The majority of these have been recognized, as with the
tourite system. Thinning over the diapiric ridges in the east imply Algarve Margin buried sheeted drift, due to their close relationship
ongoing halokinesis, further intensifying the MOW at this time. As with turbidite and mounded contourite deposits respectively.
erosion dominated in the east of the study area (where erosional Here we examine in detail, the seismic characteristics of the
truncations are common in the unit), the west was a region of sheeted drift found in the Lower Pliocene succession of the Algarve
contourite deposition, where transport capacity of the bottom basin (Gulf of Cadiz) and discuss the close relationship between
water fell as it exited the diapiric ridge province. Here, upslope down-slope and along-slope processes at the beginning of drift
progradation is evident, a feature common to contourite drift formation. This is carried out at the scale of the drift to that of the
growth (Fig. 5) (Faugères et al., 1999). seismofacies.
Due to the absence of the seismic unit in the north of the study
area, conclusions cannot be made of the regional drift type as the 6.2.1. Seismofacies-scale characteristics
external and internal morphologies cannot be clearly analysed. Seismofacies are extremely variable in any sediment body and,
What is clear is that this unit signifies a major period of repeated in general, cannot solely be used to determine contouritic origin of
MOW intensifications and that the ongoing tectonic and diapiric a deposit. In the case of the Upper Pliocene drift within the Algarve
movements were likely to be instrumental in the change to basin however, seismofacies do give a strong indication of the
contourite-dominated system. presence of a contourite sheeted drift as it can clearly be distin-
guished from other deposits based on the acoustic response. High
6.1.5. Mid-upper Quaternary amplitude and laterally continuous reflections can be identified and
After a period of intense erosion in the lowermost Quaternary, are in stark contrast to the chaotic down-slope sequence seen close
the modern set-up of the margin truly began to establish itself to the continental slope (Fig. 3A). These seismic facies are seen to be
(Fig. 6). It is at this time that the contourite system in the study area interbedded in Seismic Units P2 and P3. A similar seismic facies
moves from a sheeted (aggradational) drift to a remarkable relationship is observed in the Storegga slide complex and associ-
mounded elongated and separated drift (progradational and ag- ated contourites offshore NW Europe, albeit on a larger scale
gradational). This sedimentary record has been studied in great (Solheim et al., 2005).
detail by Llave et al. (2007) and Roque et al. (2012) among others Contrasting Seismic Unit P1 with P2 illustrates the problem of
and is outside the scope of this paper. However, it forms a good distinguishing depositional processes from seismic facies alone.
contrast to the underlying Pliocene/Lower-Quaternary sheeted drift Locally, both sequences show very similar acoustic characteristics,
and contouriteeturbidite mixed system. It is separated from the and so they have been interpreted based on other observations on a
sheeted drift system by major discontinuities that are interpreted larger scale.
to be in response to tectonic readjustment of the margin. The
mounded drift shows the development of a well-formed contourite 6.2.2. Depositional unit-scale characteristics
moat along the middle slope, and all the features outlined by Typical characteristics of sheeted drifts on the sea-floor have
Faugères et al. (1999) as being characteristic of contourite drifts been outlined in the literature. Faugères et al. (1999) note that
such as upslope progradation and regional unconformities or sheeted drifts, both accumulating on the abyssal plain and plas-
erosional surfaces. tered along continental slopes (as in the case of the Gulf of Cadiz)
show little to no progradation of seismic units. They observe that
6.2. Sheeted drift identification and characteristics “gently downlapping reflections show only slight downcurrent
progradation with either a basinward or landward oblique
Seismic data almost always possesses uncertainty in the inter- component”. In the Pliocene sheeted drift, this is certainly the case.
pretation stage, with many possible outcomes (Sheriff and Geldart, Sequences identified across the drift are aggradational in nature.
1995). Knowledge of the regional tectonic setting, palaeoceano- There is some evidence of minor progradation, including upslope
graphic set up of the margin and additional data such as bathy- progradation which is often an indication of contourite influenced
metric, petrophysical and sedimentological aid the clarification of sedimentation (Fig. 5). The intense interaction with down-slope
the seismic data into a realistic and robust geological interpreta- processes to the east of this system (Fig. 3A) may disguise further
tion. The recognition of a contourite drifts on seismic is however prograding packages. Individual units are seen to downlap and are
still problematic, particularly where downslope processes are also interpreted as prograding along-slope to the west. Sequences also
ongoing since the seismic response is often similar (Shanmugam, thin towards many of the diapirs in the east of the study area,
2012). Additional problems are encountered where the resolution indicating active halokinesis at the time of deposition.
of the seismic data is poor due to deep-penetrating low frequency
acoustic data (as is commonly used in the petroleum industry) or 6.2.3. Discontinuities and seismic unit identification
by the use of older seismic data. Three key criteria for identification Regionally extensive erosional discontinuities are often impor-
have been proposed; 1) the presence of major discontinuities across tant criteria for contourite identification. Previous work in this re-
the entire drift region that represent erosional or hiatus surfaces gion has identified key discontinuities based on laterally
formed by major hydrological events; 2) the presence of deposi- continuous high amplitude reflections (Lopes et al., 2006;
tional units exhibiting convex-upwards geometry; and 3) pro- Maldonado et al., 1999; Marches et al., 2010). However, disconti-
gradational or aggradational stacking patterns (Faugères and nuities in the study area are not always clearly erosive at a seismic
Mulder, 2011; Faugères et al., 1999; Rebesco and Camerlenghi, scale or across a wide area, but can be due simply to a reorgani-
2008). Using these criteria, many mounded drifts have been iden- zation of the hydrodynamic set up of the margin. In such cases they
tified both on the modern sea floor (Faugères et al., 1993) and in the can be identified by a distinct regional change in the acoustic
subsurface (Hüneke and Stow, 2008). Identification of sheeted response of the package. Where the nature of the discontinuity is
drifts has proven more problematic, particularly in the sub-surface. erosional, the acoustic impedance (AI) between the under- and
Examples do exist in the Campos Basin (Moraes et al., 2007), Danish over-lying seismic packages may not necessarily be significant. The
Basin (Surlyk and Lykke-Andersen, 2007), the North Rockall Trough nature of high-energy contourite deposition (as is the case in the
(Stoker et al., 1998), Argentine slope (Hernández-Molina et al., Gulf of Cadiz) is one of gradual waxing and waning flow. Erosional
46 R.E. Brackenridge et al. / Marine and Petroleum Geology 46 (2013) 36e50

discontinuities correlate to times of enhance bottom water velocity, since the Algarve Margin is influenced by the upper core of the
such as is seen in the uppermost Pliocene to Lowermost Quaternary MOW, it would be expected that the most vigorous bottom waters
in the study area. During such times the current has erosional ca- are associated with highstand conditions. However, problems
pabilities. The gradual waning of flow results in deposition of arise with this theory when the depositional sequences are
sediment particles from the moment that the transport capacity compared to the global eustatic sea level curve (Miller et al.,
drops below the erosional threshold. Therefore there is not 2005). Figure 7 summarizes the relationship between sea level
necessarily a hiatus in the sediment record where digenesis, or hard and the drift evolution. Neotectonics will further complicate this,
ground formation may form a remarkable seismic reflection, but and care must be taken to distinguish the two controls on bottom
the AI depends solely on the difference acoustic response of the water velocity by looking at the margin-wide versus the localized
over- and underlying sediment packages. As a result, many of the response.
discontinuities identified in this detailed study are highly variable Within the Algarve Basin contourite succession, high cyclicity is
in amplitude and can demonstrate low acoustic impedance be- observed in the acoustic data, indicating that there have been times
tween under- and over-lying seismic packages (low-amplitude re- of increased velocity and times of waning bottom water currents.
flections). Consequently, discontinuities in contourites should be Sea level fluctuations during the Pliocene and Quaternary are var-
identified through the observation of truncations, onlaps and ied. Broad trends seen include a lowering of sea level during the
downlap morphologies of the seismic reflections and not using Quaternary and a move to more extreme sea level fluctuations as a
solely the identification of laterally continuous high amplitude result of the change to dominance of the 100 ka Milankovitch cy-
reflections. cles. Here we examine the paleaoceanographic implications of the
Pliocene sequence.
6.2.4. Drift-scale characteristics Discontinuity M separates the Miocene from the lower Pliocene
Since contourite drift types are named on account of their and signifies the end of the Messinian Salinity Crisis at 5.33 Ma
external morphology (Faugères and Mulder, 2011; Faugères et al., (CIESM, 2008). Subsequent discontinuities represent times when
1999; McCave and Tucholke, 1986; Rebesco and Stow, 2001; Stow bottom water velocities were enhanced sufficiently to cause
et al., 2002b), the drift identified in the Pliocene offshore the widespread or localized erosion of the sea floor. Seismic Unit P2
Algarve Margin can be classified as a sheeted drift within a mixed represents the full onset of the Mediterranean Outflow Water and
system. The overall morphology is that of consistent thickness and the resulting discontinuity, the LPR, has been dated at approxi-
is mapped extending over an area of over 2000 km2 (Fig. 6). mately 4.0e4.2 Ma (Llave et al., 2011). This occurred at a time of
Thinning occurs only close to the basin margins (along the upper low then rising global eustatic sea level (Miller et al., 2005). The
slope and Guadalquivir Bank and against some (but not all) diapirs high amplitude acoustic response of the contourite section of
in the east of the study area). Since its deposition, the drift has been Seismic Unit P2 topped by an erosive discontinuity indicates
deformed by ongoing halokinesis and faulting along the margin, accelerating bottom water velocities at this time and the deposi-
but the original broad, low-mounded morphology is still evident tion of a sandy-sheeted drift. Global sea-level trends (Fig. 7) show
and is strikingly different to the overlying mounded elongated and a relative high at this time (Miller et al., 2005). In addition, there is
separated drift (Faro-Albufeira drift). a strong influx of debrite deposits. Downslope processes are most
Overall distribution of the drift is elongated in an NEeSW commonly associated with sea level lowstand (Catuneanu, 2006),
orientation, following the contours of the continental slope. This indicating that there may have been some additional control on
highlights an important consideration when identifying sheeted the system at this time, for example major tectonic events. Seismic
drifts in the sub-surface; prior knowledge of the oceanographic set Unit P3 has been interpreted as a time when relatively weak
up of a margin can aid contourite recognition. Along the Algarve bottom water currents were in action over the drift. During this
basin, the Pliocene oceanographic set-up is moderately well con-
strained (Hernández-Molina et al., 2011; Rogerson et al., 2012). It is
well known that the opening of the Gibraltar Gateway occurred
around the MioceneePliocene boundary (Nelson et al., 1999;
Maldonado et al., 1999). Estimates for the full onset of the MOW are
around the time of discontinuity Lower Pliocene Discontinuity
(LPR) when upslope progradation begins to be seen in the seismic
data. This provides good evidence for oceanographic conditions
favourable to drift formation. Elongation of a sediment body along-
slope can, when combined with knowledge of the palaeoceano-
graphic set-up along a margin, aid the recognition of contourite
sheeted drifts in the subsurface.

7. Implications

7.1. Palaeoceanographic implications

This mixed turbiditeecontourite system and its acoustic

response can tell us a great deal about the oceanographic evolu-
tion of the Algarve Margin. Authors have analysed the effects of
relative sea level and global climatic changes on the MOW (Llave
et al., 2006; Nelson et al., 1993; Stow et al., 2002a). Although it is
challenging to completely isolate the climatic effects to the bot- Figure 7. The palaeoceanographic implications of each depositional sequence.
Although some potential ties are seen between times of enhances bottom water cur-
tom water current, it appears that there is a basinwards shift of rents and eustatic sea level, there must addition allogeneic controls on the system.
the dominant MOW core during times of lowstand due to These are ties to alternative forcing such as tectonics and halokinesis. Eustasy curve
increased density of the water mass (Llave et al., 2006). Therefore, from Miller et al. (2005).
 R.E. Brackenridge et al. / Marine and Petroleum Geology 46 (2013) 36e50 47

time, there was a dramatic fall in sea level followed by a relative 7.2. Economic importance
highstand (Fig. 7), although fluctuations in eustasy continued
throughout the latest Pliocene. Proxy data shows that in the Late Deep-water continental margins can provide all aspects of a
Pliocene (3.5e3.3 Ma) the density of the MOW significantly petroleum system; mature source rocks, ample seal facies and
increased (Khelifi et al., 2009) and it is likely that the semi- trapping structures. These systems have therefore had growing
transparent Seismic Unit P3 is representing a weaker Upper Core economic interest in recent decades. The search for reservoir facies
of the MOW, while the Lower Core was intensified (outwith the has focused on turbidite systems, which are capable of supplying
study area). Seismic Unit PQ returns to high amplitude reflections sand-facies to the continental slope. Turbidite sand sheets are often
with multiple erosional discontinuities. The section is partially regarded to be among the most economically viable reservoirs on
absent due to erosion along much of the slope. This indicates a account of their excellent lateral continuity, simple reservoir ge-
time of enhanced MOW circulation in the upper core. The global ometry, and well-sorted sediments (Weimer and Slatt, 2004). These
sea level curve shows repeated high amplitude fluctuations at this lead to reservoirs that have high porosity and permeability values
time, interpreted at 4th order glacio-eustatic cycles. In addition, and high-rate, high-recovery reserves.
the seismic indicates that this was a time of accelerated diapir Mixed systems such as the Pliocene system offshore the Algarve
growth to the east of the study area, and therefore the excep- Margin could provide a further reservoir facies for hydrocarbon
tionally vigorous bottom water currents can be attributed to the exploration (Fig. 8). The interplay of down-slope clastic influx with
combination of high amplitude eustatic fluctuations enhanced by strong along-slope current reworking can produce well-sorted
neotectonics (Fig. 6). sandy accumulations with excellent reservoir potential (Antich
Three important conclusions from details palaeoceanographic et al., 2005). Seismic Units P2 and PQ show promising signs of
studies can be made based on this work. Firstly, there does appear sand-rich contourite deposition. They are made up of high ampli-
to be some link between eustasy and bottom water velocity, how- tude reflections when compared to other units and show evidence
ever it does not seem always be directly tied to the 3rd or 4th order of a high-energy environment of deposition (erosional truncation
cycles, but is more complex. This is in agreement with the findings of reflections is seen in the uppermost Pliocene). Where con-
of Rogerson et al. (2012). This in turn will affect the evolution of the touriteeturbidite mixed systems can be identified in the subsur-
margin on a seismic scale. However, caution must be executed to face, bottom water reworking of clastic down-slope sediment holds
consider additional controls such as tectonics. Secondly, the the potential for excellent reservoir quality facies deposition
amplitude and duration of climatic and eustatic fluctuations cause (Mulder et al., 2008).
differing responses to the contourite system. It is a highly complex This has been demonstrated in the Santos Basin offshore Brazil
relationship. Finally, the MOW itself has an important influence on (Bulhões et al., 2012; Viana et al., 1998; Viana and Rebesco, 2007).
the type of system that will develop and onset of processes that Here, sediment overflow from the shelf has been directly feeding
trigger turbulence in a bottom water mass can significantly alter the into the bottom currents since the Oligocene. Additionally, there
development of a contourite depositional system. The above ob- are downslope systems that are greatly modified where they are
servations show that seismic facies, acoustic amplitudes and deposited in the path of vigorous bottom currents. The result is
seismic stratigraphic techniques can be used to analyse palae- regions of high amplitude acoustic response and large-scale bed-
oceanography on an along-slope and downslope mixed system, forms where contourite sands are deposited. These deposits show
although all must be used to come to a robust conclusion. some similarities to the Algarve Margin contourites of the Pliocene,

Figure 8. The Gulf of Cadiz contourite depositional system and mixed system provides a good example of many contourite sands. Upon further burial these could make excellent
petroleum reservoirs. Sands accumulate within contourite channels or as broad sheet-like drifts.
48 R.E. Brackenridge et al. / Marine and Petroleum Geology 46 (2013) 36e50

albeit under a less aggressive current regime compared to that of the earliest Pliocene, Seismic Unit P1 has been interpreted as a
the Santos Basin (which is largely transported away and leave only dominant down-slope (most likely turbidite or mass transport)
discrete patch drifts within a dominantly erosional regime). Along system sourced mainly from the northeast. There is clear evidence
the Algarve Margin, Seismic Unit P2 shows seismic evidence of this of contourite reworking at Seismic Unit P2, where upslope pro-
process, with major sediment influx up-current (debrites repre- gradation and a sheeted morphology are observed. Nevertheless, in
sented by chaotic seismic facies) resulting in the deposition of a the northeast of the study area a thick sequence of chaotic seismic
sand-rich sheeted contourite drift downcurrent and basinwards facies has been interpreted as mass wasting deposits sourced from
(Fig. 5). the north. Evidence for more vigorous activation of the MOW in
An alternative reservoir facies deposited in mixed systems could sequence P2 is seen basinwards in the form of extremely high
be winnowed turbidite deposits. Where down-slope processes, amplitude reflections. However, the bottom currents were unable
generally occurring on a timescale of minutes to hours, are influ- to dominate over the entire margin due to high down-slope clastic
enced by longer-lasting (operating of a geological timescale) influx. Semi-transparent Seismic Unit P3 indicates that the upper
moderate bottom currents, the finer grain sizes may be removed by Pliocene initially experienced a reduction in bottom water in-
winnowing. The result is a sand-rich deposit exhibiting good tensity; however upslope progradation shows that a mixed system
sorting, porosity and permeability, and therefore good potential was maintained. Above the Base Quaternary Discontinuity (ca.
reservoir properties on burial. Examples of such deposits have been 2.6 Ma), highly erosive discontinuities and high amplitude seismic
identified both on the sea-floor, for example in the Gulf of Mexico reflections are evidence of pronounced intensification of the MOW
(Shanmugam and Moiola, 1982; Shanmugam, 2012), and in the and of fully contourite-dominated slope.
subsurface as in the Campos basin (Mutti et al., 1980). If there is any As deepwater exploration becomes more and more important to
minor onset of the MOW in the Lowermost Pliocene in the Algarve meeting energy needs, it is important to further our understanding
Basin (Seismic Unit P1), this mixed deposit could consist of clean of these regions. Water masses circulation is particularly important
sands and hold some reservoir potential. in these deeper settings and as a result contourite deposits are very
A final mechanism for sand influx into a contourite system is common and can hold the potential for good reservoirs, particularly
outlined by Viana et al. (2007) as “downwelling of dense saline where they are closely associated with a downslope system or
waters developing sand-rich channels”. This is a probably source of other source of coarse clastic sediment. Where contouriteeturbi-
additional sediment influx to the modern Gulf of Cadiz Sand Sheet dite mixed systems can be identified in the subsurface, bottom
in the easternmost Gulf of Cadiz. Shelf spillover, downwelling of the water reworking holds the potential for excellent reservoir quality
MOW and bottom water current erosion contribute to the accu- facies deposition. Misinterpretation of sheeted contourite deposits
mulation of sand across an area estimated to be 4000 km2. The as turbidites has important implications for reservoir modelling
contourite sand sheet is located at the exit of the Strait of Gibraltar and it is therefore of great importance that the interpreter has a
across an area where the MOW speeds out and decelerates on good understanding of how to distinguish deepwater depositional
exiting the confined Gibraltar Oceanic Gateway (Buitrago et al., processes using acoustic data. Key steps are to analyse seismic
2001). Regions of bottom water acceleration through narrow con- facies, sequence distribution and palaeo-margin set up prior to
duits evidently provide a mechanism for impressive contourite defining any depositional process.
sand accumulation and it may be this mechanism that resulted in
the deposition of Seismic Unit PQ in the Algarve Basin on exiting a
very active region of diapirism to the east of the study area. Acknowledgements
The above examples highlight that where there is 1) sufficient
transport capacity in a given bottom current for the transport and The study was funded through Institute of Petroleum Engi-
deposition of grain sizes >0.063 mm and 2) a sediment influx from neering Ali Danesh Scholarship. The study is related to the CON-
the shelf to a bottom current system, then there is the possibility of TOURIBER (CTM 2008-06399-C04/MAR) and MOWER (CTM2012-
accumulating contouritic sediments on the slope that have very 39599-C03-01) projects. Many thanks to TGS for allowing seismic
attractive properties for the hydrocarbon industry. Seismic data is a sections to be published and to two reviewers who suggested
dominant source of information for the sub-surface and it is widely modifications to improve the original manuscript.
used to great effect in the hydrocarbon industry. Since misinter-
pretation of sheeted contourite deposits as turbidites has important
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OTC Number 23697-PP

Geohazards and Ocean Hazards in Deepwater:

Overview and Methods of Assessment
Dorrik Stow, Rachel Brackenridge, Urval Patel, Suzannah Toulmin
Institute of Petroleum Engineering, Heriot-Watt University, Edinburgh EH14 4AS, UK

Copyright 2012, Offshore Technology Conference

This paper was prepared for presentation at the Offshore Technology Conference held in Houston, Texas, USA, 30 April–3 May 2012.

This paper was selected for presentation by an OTC program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been
reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material does not necessarily reflect any position of the Offshore Technology Conference, its
officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Offshore Technology Conference is prohibited. Permission to
reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of OTC copyright.

Abstract

Drilling for hydrocarbons in the deep marine environment provides a unique set of challenges for industry.
Amongst these are the distinct hazards caused by natural geological and oceanic processes such as: (a) semi-
permanent bottom currents, (b) episodic turbidity currents, slope instability and mass transport events (slides,
slumps, debris flows), and (c) gas hydrate escape. We present data on the nature, effects and assessment of
these deepwater hazards, including current velocities, transport/erosion capacities, recurrence intervals, and
hydrate distribution. It is of upmost importance that the oceanographic conditions are carefully considered prior to
deepwater operations to ensure the work can be carried out safely. Thorough risk assessment requires
knowledge of existing bottom currents, an assessment of potential mass transport events and turbidity currents,
and understanding of conditions likely to induce the formation and destabilisation of gas hydrates. Pipelines,
cables, subsea installations, key connections such as the riser and any other seabed infrastructure are all
susceptible to damage. Correct assessment of hazard will allow for the right equipment to be used for the
operations including blowout preventers, riser size, vibration suppressors, and for the safest siting of cables and
pipelines. It will also aid the subsea architecture design and planning operations with minimal downtime. All these
considerations lead to a safer exploration and production while eliminating unnecessary cost.

Introduction

Deepwater is defined by different communities in slightly different ways. For the marine geologist and
oceanographer, it is often taken as that area of the ocean beyond the shelf break, i.e. deeper than about 100-200
m water depth. For the sedimentologist, it is below storm wave base, which is also around 100-200 m. For the
industry, deepwater exploration is considered to be that in water depths in excess of 500 m. For the purposes of
this paper, we consider the shelf-slope transition (i.e. shelf break) as the key boundary between shallow and deep
water.

This deepwater environment is far from tranquil. It is subject to a range of processes that will significantly affect
drilling operations for hydrocarbon exploration and recovery. Bottom currents are everywhere present and in
some areas they are especially active and with considerably elevated velocities. Here we first focus on deepwater
bottom currents and the hazards they present. Their occurrence and distribution, nature and variability, and
damage potential are discussed, followed by consideration of the methods of hazard assessment. We present a
new method using the bedform-velocity matrix.

The second topic, mass transport events and turbidity currents, is discussed in a similar way. These processes
occur on all types of continental margins and slopes, and are of great importance, both scientific and industrial.
They are the dominant processes through which large amounts of sediments are transferred across the
continental slope to the deep ocean. Enhanced knowledge of these processes and their depositional features is
2 OTC Number

not only important for continued success of oil exploration in deep-water settings, but also for the potential impact
on human life and settlements (i.e. tsunamis), and protection of offshore infrastructure (e.g. platforms, pipeline,
telecommunication cables etc).

The third topic covered is that of gas hydrates in ocean sediments. These are nearly ubiquitous in near-surface
sediments on continental margins, as a result of high pressures generated by the overlying water column. They
too pose a serious hazard to deepwater petroleum operations.

Bottom Current Hazards

Occurrence and distribution

Currents are present throughout the world’s oceans and seas. There has, to date, been no recording of any

oceanic water mass that is completely calm with no movement (Zhenzhong and Eriksson, 1998). They occur at
the surface (usually wind-driven) and throughout the entire water column (Gross and Gross 1996). Where wind is
the driving force, intensity decreases significantly with depth. It is therefore unlikely that wind-driven currents will
penetrate past the top few 100 metres of water except in some exceptional circumstances, for example, major
storm events (Brooks, 1984). Little was known about deep ocean currents until just a few decades ago. Despite
being first identified over seventy years ago (Wust, 1936), it was not until the 1960s that technological advances
allowed for focused work on bottom and deep ocean currents to begin (e.g. Heezen and Hollister, 1963; Heezen
et al., 1966). Direct current measurements, seabed photography, seabed and subsurface sampling and acoustic
methods have been used to quickly advance our understanding of the hydrodynamics of the deep ocean
(Shanmugam et al., 1995; Zenk, 2008).

It is now widely accepted that bottom currents are formed by various means: (1) wind-driven; (2) tidal-driven; (3)
thermohaline-driven; (4) internal wave-driven (Stow et al., 2002; Shanmugam, 2006). Due to this range of current
types, operations in all water depths can be affected. In deepwater, most bottom currents are generated as part of
the thermohaline conveyor belt (Broecker, 1991; Rahmstorf, 2006), and form in the cold-water kitchens of Polar
regions (Brackenridge et al., 2011). The global thermohaline circulation system is driven by intense cooling of
highly saline surface waters in the Arctic and Antarctic regions. This leads to sinking of dense water masses and
ventilation of the deep ocean. Subsequent mixing with overlying water masses and heating allows upwelling, and
so the process is maintained. Two key ingredients are required to create sufficiently high water density to allow
sinking: salinity and temperature. As a result, there are a limited number of source areas for bottom water
generation; namely the Norwegian-Greenland Sea, Labrador Sea, Bering Sea, Weddell Sea, Ross Sea and other
locations around Antarctica (Rahmstorf, 2006).

Warm-water kitchens, in which warm saline bottom water is generated by intense evaporation in low-latitude
regions are relatively rare in today’s oceans. The principal source is the Eastern Mediterranean Sea, with escape 
to the global ocean via the Gibraltar Gateway. Locally generation also occurs in the Red Sea and Persian Gulf,
but escape to the ocean is prevented by shallow sills.

At depth it is also possible to encounter internal tides and waves. These commonly occur at the interfaces
between water masses of different density and have the capacity to mix and transport large volumes of water and
sediment (Zhenzhong and Eriksson, 1998). They are broadly comparable to surface waves in that they have
highly variable amplitudes and wavelengths depending on the density gradient and other conditions.

Accelerated bottom currents are generated experienced along many of the continental margins under
investigation by the hydrocarbon industry (Fig 1). These water masses form ‘rivers’ of highly vigorous water in the 
deep ocean along continental boundaries were Coriolis Forces and seabed morphology act to further increase
velocity. In the Atlantic Ocean, deep oceanic currents affect many petroleum basins. The continental slope
offshore Brazil is influenced by the Brazil Current, the South Atlantic Central Water, the Antarctic Intermediate
Water, the North Atlantic Deep Water and the Antarctic Bottom Water (Viana et al., 2002). These water masses
all contact with the continental slope at different depths and have various velocities ranging from sluggish to
40cm/s (Farrant and Javed, 2001). Some are northward flowing boundary currents, whereas others flow in a
southward direction. The Gulf of Mexico is subject to highs of 70cm/s along the course of the Loop Current, and is
generally seen to be around 50cm/s (Brooks, 1984). The current speeds of the North Atlantic Deep Water will
pose additional challenges in the frontier regions offshore Greenland where large bottom-water sediment deposits
have been identified (Hunter et al., 2008; Nielsen et al., 2011) as well as offshore the NW European margin, West
of Shetland (Fig 1).
OTC Number 3

Where these currents reach sufficient velocities, they can erode, pick up, transport and deposit sediments. Great
accumulations of sediments are named contourite drifts, and these can be covered in bedforms and erosional
features (Stow et al. 2002; Rebesco and Camerlenghi 2008). Most commonly, any depositional or erosional
feature influenced by bottom water currents will be orientated alongslope and can extend for many hundreds of
kilometres. Where higher velocities are observed, regionally-extensive erosional surfaces or terraces cut into the
continental slope are found (Hernández-Molina et al., 2008). Sediments range from fine muds to sands, and in
some regions, gravelly accumulations can be found. Most contourite drifts are composed of mixed
siliciclastic/biogenic material and may be closely associated with other deposits such as turbidite or glaciogenic
material.

Nature and variability

Unlike episodic turbidity currents or continuous pelagic settling processes, contourites are deposited by a
continuous to semi-continuous processes. They are semi-continuous since flow will wax and wane on numerous
timescales and at numerous magnitudes, but the flow will be maintained over a geological timescale.

The key features of bottom currents have recently been compiled by Stow et al. (2008). Bottom currents (also
known as contour currents) generally move in an alongslope direction, following the contours of the continental
margin. They are capable of moving upslope and downslope, for example to move around morphological
obstructions or sea floor features. The dominant direction will however, be parallel to the continental margin.
Where they are present over low gradient slopes and abyssal plains, the bottom water will form a broad sluggish
-1
core moving at <10 cm s . The water mass is be distinguished from other masses on account of temperature and
salinity data, as well as its relative velocity. It is the sea floor morphology itself that is the chief control on the
route of the bottom current core, its velocity and complexity (Zenk, 2008). Higher gradient slopes, such as the
continental rise and slope, or some morphological obstruction such as a seamount, or submarine canyon can
elevate the velocity of a bottom current. Where any water mass is constricted, its velocity will increase. Coriolis
forces can further elevate velocity by pushing the water mass against the feature and further constricting it.
Features such as subsea channels or gateways provide conditions for greatest increase in water velocity.

The velocity and nature of a current is highly variable over space and time. On examination of contourite deposits,
changes in sedimentary structures, grain size and other properties represent different current intensities across
the drift (e.g. Stow et al., 1986). Over space, currents decelerate over distance down current, until some
morphological feature allows for further acceleration. Direct measurements of water velocities show highs of a few
metres per second along the path of some water masses where they are constricted and accelerated (Gonthier et
al., 1984).

The current velocity will also fluctuate over time, on various scales. Sedimentological work shows large scale
(million year) changes, glacial-interglacial cycles on 100kyr scale and a smaller scale fluctuation on a 1000 yr
timescale (Stow et al., 1986; Knutz, 2008). Direct measurements of bottom currents show further fluctuations of
intensity on a seasonal and even daily cycle (Zhenzhong and Eriksson, 1998; Stow at al 2012). As a result, these
currents are highly complex, difficult to study and challenging to predict.

Damage potential

Ocean currents provide additional risks in regions of petroleum exploration and production; Surface currents
cause additional strains on automatic positioning equipment on drilling vessels and oil tankers. Intermediate
currents cause huge amounts of additional strain on equipment connecting the rig with the seabed, and deep
currents can affect sub-sea infrastructure. Here we examine the risks associated with vigorous bottom currents
specifically. These pose a potentially great risk to deepwater operations including drilling, production and
development (Chow et al., 2006). Key risks identified are: (1) triggering of downslope mass transport processes;
(2) pipe-line walking and fatigue; (3) additional stress to risers and tethers.

Bottom water currents and mass-wasting. Both bottom currents and their resulting deposits (contourites) are
capable of promoting large-scale slope instability (see also below). There are numerous examples where
contourite and mass transport deposits co-exist in the geological record (e.g. Bryn et al., 2005), and it is now clear
that contourite deposits can promote large-scale slope instability through numerous factors (Laberg and
Camerlenghi 2008). Firstly contourites show characteristically high sedimentation rates when compared to many
other deepwater processes (Stow et al., 2002a). This can lead to high fluid content and therefore high pore
pressures and low slope stability. Secondly, the nature of contourite deposition results in moderately to well-
sorted sediments being deposited. These are significantly weaker than their poorer sorted counterparts of higher
4 OTC Number

internal shear strength. Finally, contourite accumulation can result is the formation of mounded drifts. These
directly modify the shape of the continental margin, and locally increase the slope gradient which can lead to
failure. Additional slope stability issues arise when vigorous currents come in contact with the seabed. Internal
waves can promote slope instability. Where failure does occur along a contourite-dominated margin, the resulting
deposit is impressively widespread. The majority of contourite-associated mass wasting events observed in the
ancient record are considerably larger than the events experienced in recordable history.

Pipeline walking and fatigue. In shallow water operations, pipelines are laid in trenches to protect them from
movement, however in deep water, infrastructure lies directly on the seabed in poorly consolidated sediments with
extremely high clay and water content. This, combined with bottom water currents can result in movement of the
pipeline and addition stress on connectors with tie-ins and risers (Carr et al., 2008). When combined with other
hazards exerted on the equipment such as thermocycling of the pipeline (Rong et al., 2009; Whooley, A., pers.
comm., 2011) and corrosion (McKinnel, 2011), premature failure is possible. Careful modeling and planning of
pipeline routes is a highly complex procedure, requiring many variables to be considered, and the avoidance of
localized regions of accelerated bottom water is important to avoid unnecessary strain on equipment. .

Stress to risers and tethers. Fatigue of risers, tethers and drill-strings is common in regions of constant ocean
currents and can drastically increase production costs. Risers act as obstructions to flow and cause vortexes to
form in the water column. This amplifies the strain on equipment – causing vibrations in the connections and
increasing wear of the mechanism. Analysis on vortex-induced vibrations (VIV) has identified the cause to be due
to water currents and wave actions. The main point of wear is close to the top of the riser or close to the touch
down point, where bottom water currents are in operation (IntecSea, n.d.). Additional strain on connectors is
common in deepwater regions swept by vigorous water currents due to increased riser curvature (Howells, 2000)
and corrosion (McKinnel, 2011).

These are the main issues that arise from vigorous bottom currents and the ones that pose the greatest safety,
environmental and economic cost. An additional problem is also re-entry procedures in deep water (Bullock et al.,
1979). Such risks can result in premature fatigue of subsea infrastructure and increase the time and costs of
operations. A detailed understanding of currents can allow engineers to design subsea structures to withstand
necessary current conditions, without added costs. Current velocity data is essential for design of structures such
as pipelines, manifolds and risers and can be particularly challenging in frontier areas swept by bottom water
currents. This will be a future challenge in the West Shetland Region where bottom water velocities may exceed
1m/s (Kuijpers et al., 2002) and where 500 tonnes of subsea structures will be awarded for the Clair field
development in 2012 alone (Sheehan, 2011).

Hazard Assessment

Currents are challenging to study and analyse. They can be directly identified on the basis of temperature, salinity
and velocity anomalies in the water column. Subsea floats and tracers are widely used to examine the dominant
routes of bottom waters (Richardson et al., 2000; Zenk et al., 2008) and velocities (Smith and Jacobs, 2005).
From such measurements, the nature and characteristics of deepwater currents have been assessed. They are
generally contained within an elevated velocity water ‘core’ that creates a boundary current, from which eddies 
and loop currents may peel off and may rejoin the main water core.

Current velocity measurements may be taken directly through the water column, using moored current metres
and shipboard acoustic Doppler current profilers, but nature and variability of the current route and intensity make
direct measurements unreliable. A study by Smith and Jacobs (2005) considered over 50,000 current
measurements from various devices to produce a more reliable and realistic current circulation pattern in the Gulf
of Mexico. Despite added boundary constraints and data smoothing, large errors were observed close to
significant topographic features such as canyons and the shelf-break in addition to water mass boundaries. For
hydrocarbon infrastructure to be safe and reliable for the duration of operations, the mean and maximum possible
current intensity at the site of drilling is required. Discrete current velocity measurements should be recorded from
moored tools in place for a number of months (e.g. Hamilton, 1990). These long timescale recordings are required
to identify trends such as seasonal variations. Direct measurements on this magnitude would be very expensive
and time-consuming to survey prior to operations beginning. Alternative methods of assessing current intensity
over an area are possible, based on the knowledge of contourite depositional and erosional features.

Geotechnical companies can provide extensive acoustic datasets of the seabed in order to assess the regional
geohazards prior to drilling operations commencing. Multibeam, side scan and sub-bottom profiler methods are
used from ship-board or AUV (Autonomous Underwater Vehicle) instruments (Chow, 2006). When combined with
OTC Number 5

seismic acquired at an exploratory stage, many potential hazards can be mapped such as irregular topography,
faulting, gas hydrates and sand facies mapping. However, these methods have yet to be used to their full
potential as they do not fully assess bottom current hazards. Here we outline the main ways in which the industry
can utilise this data to further assess the geohazards relating to oceanic currents.

Damuth (1980) uses high frequency echograms of the sea floor to assess the seabed facies and distinguish
between the different near-bottom processes. Contourite depositional and erosional features typically respond
differently from downslope sediments such as turbidites and debrites. Migrating sediment waves and erosional
furrows cause disrupted and hyperbolic acoustic responses at high frequency (Damuth, 1975). Although features
such as sediment waves are present in some turbidite-dominated regions, acoustic character can be combined
with existing knowledge of bottom water currents to positively identify regions most affected by currents. Another
way in which acoustic data can be used to positively identify regions influenced by vigorous currents is to identify
contourite features based regional trends of orientation and geometry. Seismic data can be used to identify
contourite drifts on the basis on external and internal geometries, and the presence of laterally-extensive
erosional discontinuities (Faugeres et al., 1999; Nielsen et al., 2008).

Bedform-Velocity matrix

Although the above methods are extremely useful in identifying bottom currents and acquiring a regional
understanding of the hydrodynamic regime of the margin, they do not yield quantitative data on mean current
velocity. Here we present a novel way of extracting this information using existing geophysical methods applied to
the latest contourite research.

Heezen and Hollister (1971) first realised the succession of bedforms developing under ever-increasing bottom
water velocities in deep water. Now, detailed sedimentological analyses of regions influenced by bottom water
currents have led to the compilation of a bedform-velocity matrix (Stow et al 2009). From this, it is possible to
estimate mean bottom water velocity from the sediment grain size and/or from bedforms on the sea floor (Fig 2).
High-energy environments have the transport capacity for larger grain sizes and therefore will result in coarser
grained deposits (Tucker, 1991; Reading and Levell 1996). Other considerations should be taken into account,
such as grain density and distance from sediment source or sources, but on a whole, as the velocity of a bottom
water mass decreases, so too does its transport capacity. The matrix is, in essence, very similar to other bed
form-velocity matrix constructed for other depositional processes. It examines likely dominant grain sizes in
addition to the expected bed forms on the sea bed formed by currents.

As noted above, bottom currents tend to show variability in flow speed over many different timescales. The
bedform present on the seabed is, therefore, a record of the mean or dominant flow characteristics over the
timescale of deposition. The grain size velocity matrix could therefore provide a robust means of identifying
bottom water core routes and average velocities. The matrix can be used with high frequency, high resolution
acoustic surveying methods which allow the seabed to be mapped in great detail. Bedforms can be identified
using methods such as side-scan sonar (Kenyon and Belderson 1973), and images from Autonomous
Underwater Vehicles (Chow, 2006). Bottom photographs can be used to gain yet more knowledge of the
bedforms and features actively forming on the sea floor. If utilised to its full potential, the bedform-velocity matrix
could provide a cost-effective method gaining detailed regional information on the risks posed by bottom water
currents.

Mass Transport Events and Turbidity Currents

Nature and Variability

Slides and Slumps. Submarine slides and slumps involve the movement of coherent masses of sediments
bounded on all sides by distinct failure planes (Mulder and Cochonat, 1996). The internal structure of the moving
mass is largely undisturbed as most of the shear is localised along a basal failure surface. However, if the moving
mass is unconsolidated, and depending on the strength and heterogeneity of the material, it may undergo
3
complex internal deformation as it moves downslope. They range in size and volume from a few m to several
3
hundred to thousands of km and are known to have run out distances in excess of 200km. Differentiation of
slides and slumps is based on the values of the Skempton ratio h/l, where h is the depth of the slip surface and I
is the length of failure: slides are translational with Skempton ratios of <0.15, whereas slumps are rotational and
deep rooted with a h/l ratio between 0.15 and 0.33 (Skempton and Hutchinson, 1969).

Most submarine slides appear to be translational and are characterised by a fairly flat, slope-parallel basal failure
OTC Number 5

seismic acquired at an exploratory stage, many potential hazards can be mapped such as irregular topography,
faulting, gas hydrates and sand facies mapping. However, these methods have yet to be used to their full
potential as they do not fully assess bottom current hazards. Here we outline the main ways in which the industry
can utilise this data to further assess the geohazards relating to oceanic currents.

Damuth (1980) uses high frequency echograms of the sea floor to assess the seabed facies and distinguish
between the different near-bottom processes. Contourite depositional and erosional features typically respond
differently from downslope sediments such as turbidites and debrites. Migrating sediment waves and erosional
furrows cause disrupted and hyperbolic acoustic responses at high frequency (Damuth, 1975). Although features
such as sediment waves are present in some turbidite-dominated regions, acoustic character can be combined
with existing knowledge of bottom water currents to positively identify regions most affected by currents. Another
way in which acoustic data can be used to positively identify regions influenced by vigorous currents is to identify
contourite features based regional trends of orientation and geometry. Seismic data can be used to identify
contourite drifts on the basis on external and internal geometries, and the presence of laterally-extensive
erosional discontinuities (Faugeres et al., 1999; Nielsen et al., 2008).

Bedform-Velocity matrix

Although the above methods are extremely useful in identifying bottom currents and acquiring a regional
understanding of the hydrodynamic regime of the margin, they do not yield quantitative data on mean current
velocity. Here we present a novel way of extracting this information using existing geophysical methods applied to
the latest contourite research.

Heezen and Hollister (1971) first realised the succession of bedforms developing under ever-increasing bottom
water velocities in deep water. Now, detailed sedimentological analyses of regions influenced by bottom water
currents have led to the compilation of a bedform-velocity matrix (Stow et al 2009). From this, it is possible to
estimate mean bottom water velocity from the sediment grain size and/or from bedforms on the sea floor (Fig 2).
High-energy environments have the transport capacity for larger grain sizes and therefore will result in coarser
grained deposits (Tucker, 1991; Reading and Levell 1996). Other considerations should be taken into account,
such as grain density and distance from sediment source or sources, but on a whole, as the velocity of a bottom
water mass decreases, so too does its transport capacity. The matrix is, in essence, very similar to other bed
form-velocity matrix constructed for other depositional processes. It examines likely dominant grain sizes in
addition to the expected bed forms on the sea bed formed by currents.

As noted above, bottom currents tend to show variability in flow speed over many different timescales. The
bedform present on the seabed is, therefore, a record of the mean or dominant flow characteristics over the
timescale of deposition. The grain size velocity matrix could therefore provide a robust means of identifying
bottom water core routes and average velocities. The matrix can be used with high frequency, high resolution
acoustic surveying methods which allow the seabed to be mapped in great detail. Bedforms can be identified
using methods such as side-scan sonar (Kenyon and Belderson 1973), and images from Autonomous
Underwater Vehicles (Chow, 2006). Bottom photographs can be used to gain yet more knowledge of the
bedforms and features actively forming on the sea floor. If utilised to its full potential, the bedform-velocity matrix
could provide a cost-effective method gaining detailed regional information on the risks posed by bottom water
currents.

Mass Transport Events and Turbidity Currents

Nature and Variability

Slides and Slumps. Submarine slides and slumps involve the movement of coherent masses of sediments
bounded on all sides by distinct failure planes (Mulder and Cochonat, 1996). The internal structure of the moving
mass is largely undisturbed as most of the shear is localised along a basal failure surface. However, if the moving
mass is unconsolidated, and depending on the strength and heterogeneity of the material, it may undergo
3
complex internal deformation as it moves downslope. They range in size and volume from a few m to several
3
hundred to thousands of km and are known to have run out distances in excess of 200km. Differentiation of
slides and slumps is based on the values of the Skempton ratio h/l, where h is the depth of the slip surface and I
is the length of failure: slides are translational with Skempton ratios of <0.15, whereas slumps are rotational and
deep rooted with a h/l ratio between 0.15 and 0.33 (Skempton and Hutchinson, 1969).

Most submarine slides appear to be translational and are characterised by a fairly flat, slope-parallel basal failure
6 OTC Number

surfaces (slope gradients <2°). Such low angle failure surfaces have been imaged on 2D seismic studies of the
Storegga and Traenadjupet slides on the mid-Norwegian margin and the Afen slide from the Faeroe-Shetland
channel (Canals et al., 2004). For these failures, the sliding surface is predetermined and normally corresponds to
a discrete layer with low shear resistance, such as permeable sand layers or clay and sand interbeds
(hemipelagic, pelagic and contouritic deposits) (Mulder, 2010).

However, with recent developments in high-resolution three-dimensional seismic surveying, significant

topographic variations have been shown to occur along the basal slip surface (Gee et al., 2005). Such subsurface
studies agree with well known outcrop examples, and suggest stepped, striated, concave or a combination of
these slide surface topography can be present (Gee et al., 2005; Lee and Stow, 2007). As with translational
slides, the slide surface is predetermined and relates to an interval of bedding-parallel weak layer. The
topography is created as a result of the slide surface propagating upwards due to decreasing downslope driving
stresses or resistive forces, such as sediment shear strength (Lee and Stow, 2007; Martinsen, 1994). These
topographic features may initiate the development of internal deformation and dispersive pressure in the sliding
mass, which could lead to the morphological transformation of the slide into a debris flow or turbidity current, as in
the Gaviota Slide in the Santa Barba Channel (Greene et al., 2006), the Gondola Slide on the Southwestern
Adriatic Margin (Minisini et al., 2006) and the 1929 Grand Banks event (Piper et al., 1999).

Slides are commonly associated with a variety of extensional and compressional features (e.g. normal, reverse
and strike-slip faults etc). Extensional features (i.e. normal and listric faults) are most common in the upslope
parts of the slide, especially in the headwall area (Martinsen and Bakken, 1990), where they are predominantly
orientated perpendicular to the transport direction. In complex slides, where the motion of the initial mass
transport event lead to instability of neighbouring area, such extensional features can generate large volume,
linear troughs that control local sediment pathways until the relief is filled. Such features are common in shallow
deltaic environments (e.g. Clark Fjord, Baffin Island) (Mulder and Cochonat, 1996; Syvitski and Farrow, 1989).
These normal faults at the headwall scarp of slide gradually sole out at the level of the basal décollement (Stow et
al., 1996). Downslope, at the front (often referred to as the ‘toe’) of the slide deposit, compression is dominant due 
to frictional freezing of the mass transport deposit, and is often characterised by imbricate thrust slices of
chaotically deformed or coherently folded strata. The toe region is also known to have elevated topography
created by the resistance to downslope movement. In addition to normal and reverse faults, strike-slip faulting
may develop perpendicular to the maximum stress direction to accommodate the differential movement within the
mass of sediments. Such features are difficult to recognise in outcrops and subsurface data, but may help explain
longitudinal shear ridges observed on modern slide surfaces using side-scan sonar images (Hampton et al., 1996;
Lee and Stow, 2007).

Submarine slumps exhibit many features of slides and are gradational with them (Stow et al., 1996). However,
unlike slides, the basal failure plane of slumps is commonly concave upwards and the motion of the displaced
material is rotational (Hampton et al., 1996). The exact reasons for the differing movement styles is still poorly
understand, and as such, various authors have suggested that the poorly consolidated nature of the sediments is
the controlling factor, while others have emphasised the importance of mechanical mixing due to the interaction of
the moving mass with slide surface topography (Dykstra, 2005; Hampton et al., 1996; Mulder and Cochonat,
1996). This latter reasoning is borne out of the simple observation of more disturbed and chaotic features present
in distal parts of a mass transport deposit. The internal structure of slumps is often chaotic and highly deformed,
and the degree and style of deformation varies with position of the moving layer and the strength and
heterogeneity of the material (Stow et al., 1996). Since slumps commonly involve plastic deformation, they will
cease lateral motion once the applied shear falls below a critical value.

Slides and slumps are not isolated processes and often form complex structures with multiple phases of failures.
The most common are multiple phases of retrogressive failures that form because of upslope propagation of the
failure (Mulder, 2010; Mulder and Cochonat, 1996). Other less frequent complex slope failures are overlapping,
additive and successive slumps and slides, where the term ‘overlapped’ is applied if the failure surface of the
main body is merged with successive events, or ‘additive’ if the failure surfaces of induced events are not merged. 
Successive slides and slumps are generated when the initial failure of a mass of sediments triggers mobility in an
underlying second material mass. Such slides have been inferred in the area around the Titanic wreck (Mulder,
2010).

As mentioned briefly above, slides and slumps evolve downslope into debris flows and turbidity currents through
gradually increasing ambient water mixing and entrainment, and disintegration of coherent blocks. However, the
exact nature and cause of this transformation is still poorly understood as landslides can travel hundreds
kilometres without transformation into plastic flows or turbidity currents, while others transform close to the
OTC Number 7

source. In reality slides and slumps are complex events, and elements of slides, slumps, debris flows and turbidity
currents may all be present after a mass transport event (Masson et al., 2006).

Plastic Flows. Plastic flows are a type of mass transport events in which sediment and water are fully mixed such
that no internal stratification is preserved. They are common in continental slope settings and are represented by
debris flows, of which there are two types: cohesive flows (cohesive debris flows) and frictional flows (non-
cohesive or cohesionless debris flows) (Dasgupta, 2003). Both cohesive and non-cohesive debris flows are
characterised by finite yield strength due to either the cohesive strength of the material provided by high clay
content (cohesive debris flows) or frictional strength due to interlocking of grains. The cohesive strength of the
material imparts a pseudoplastic behaviour and an overall laminar state to the moving mass (Mulder and
Alexander, 2001; Shanmugum, 1996). Debris flows keep on moving until the shear forces exerted by the
downslope component of gravity falls below a critical value and the flow freezes en-masse. However,
experimental and field observations suggest that deposition from debris flows can also occur from incremental
aggradation of flow surges due to internal shearing throughout the moving mass. Consolidation of individual flow
surges is not possible before the arrival of the next surge and thus the resulting deposit will be amalgamated
(Dasgupta, 2003). Debris flows are known to move on low slope gradients, travel rapidly (speeds of several
meters per second observed from subaerial debris flows) over long distances and are only very slightly erosional;
characteristics which could be explained by the presence of a water layer beneath the moving mass
(hydroplaning) that reduces the resistance due to drag on the seafloor (Mulder and Alexander, 2001; Pickering et
al., 1989; Piper et al., 1999). Transformation of a debris flow to a more turbulent and dilute flow (i.e. turbidity
currents) takes places if dispersive pressure and buoyant lift comes into existence as clast-support mechanisms.

Cohesive debris flows usually consists of fine-grained matrix with a significant amount of clay content, although
movement as debris flows has been shown to occur in finest sand sized material where bulk clay content is as
low as 2% (Mulder and Alexander, 2001; Stow et al., 1996). In addition, the high clay content is also responsible
for the low rate of dilution of the flow by either deposition or entrainment of ambient fluid, thus the flow remain
coherent for longer. The clay-water mixture constitutes the fluid phase in cohesive debris flows and provides the
main clast-support mechanism for coarse grains and clasts during flow conditions. In contrast, in cohesionless
debris flows the rheological pseudoplastic behaviour is due to the very high grain/water ratio. Truly cohesionless
debris flows are not normally expected in nature as small amounts of clay impurities remain in the system. These
types of debris flows develop primarily in well-sorted sand and gravel and occur on very steep slopes. The
internal structure of both cohesive and non-cohesive flows is typically chaotic and disorganised, although reverse
grading and coarse tail grading of larger clasts is known to occur.

Turbidity Currents. Turbidity currents are one of the most important ways by which fine, medium and coarse-
grained material is transferred from shallow to deep water. They are turbulent suspensions of mud and sand in
water, which are propelled by the downslope component of gravity acting on the excess density. They may occur
as short-lived surge events that travel for only a matter of kilometres downslope, or go through a process of flow
ignition such that an autosuspension process is generated in the flow. This permits very long distance transport
over several thousands of km, both downslope and across flat abyssal plains. They can even travel a certain
distance in an upslope direction before they come to a halt by a combination of frictional resistance, loss of
sediment from the base of the flow and reverse gravitation pull.
0 5
Individual turbidity currents are discrete events with very variable recurrence intervals (10 - 10 y) and of very
different sizes. The largest flows are known to overtop channel margins of 850 m in height. These are likely to be
several kilometres in width and probably more in length. Much smaller turbidity currents also occur. Such currents
can be channel confined or flow across open slopes with little apparent confinement. They can deposit beds from
< 1 cm to > 10 m thick. Mean accumulation rates, therefore, are also very variable, typically from 10 cm to > 1
m/ky. The frequency of occurrence of turbidity currents ranges from 1/1000 years (approximately) for the distal
Bengal fan, to one every few years for parts of the Amazon and Congo fan systems.

Occurrence and Distribution

Submarine mass transport events and turbidity currents are extremely widespread in both ancient and modern
sedimentary environments, particular where fine-grained sediments predominate (Stow et al., 1996; Masson et al
2006). Hampton et al. (1996) used the term ‘landslide territory’ to designate the environments where they are 
most common, these include; open continental slopes, submarine canyon-fan systems, fjords, active river deltas,
flanks of volcanic islands and along convergent margins. The largest known submarine mass movements have
been known to occur on open continental slopes and on the flanks of volcanic islands. This could be in response
to the unique morphological and geological conditions of these areas (Masson et al., 2006). Open continental
8 OTC Number

slopes are often characterised by low slope gradients (<2°) and parallel-bedded, homogeneous, fine-grained
sediments deposited over large areas (Bryn et al., 2005; Hampton et al., 1996). If mechanical inhomogeneities
(i.e. parallel-bedding) control slope failures on open slope settings, it could potentially affect a significant area and
produce a large translational slide (e.g. Storegga Slide on the mid-Norwegian Margin). Oceanic islands on the
other hand are some of the steepest topographic features on the planet and volcanic processes tend to build,
steepen, and overload these slopes with time, inducing slope failure (e.g. Canary and Hawaiian Islands) (Masson
et al., 2002). The exact cause for failure on low angle (<2°) submarine slopes is still poorly understood, although
excess pore water pressure is considered to be a major factor.

The factors that control the long term stability of subaqueous slope and therefore the occurrence of mass
transport events and turbidity currents within these environments range from the obvious and short-lived
processes, such as earthquakes, to those that are less obvious and operate on timescales of tens to thousands of
years, such as climate change. In many cases the cause of the mass transport event is obvious, while in others
the triggering mechanism of these and of turbidity currents can be cryptic. By considering the ‘landslide territory’ 
of Hampton et al. (1996), a number of temporally varying factors that control slope stability have been identified.
These include: (1) the quantity, type and rate of sediment delivered to the continental margin, (2) sediment
thickness of the depocentre, (3) changes in seafloor pressure and temperature, which can influence hydrate
stability and the generation of free gas, (4) variations in seismicity, and (5) changes in groundwater flow
conditions within the slope and shelf (Lee, 2009; Tappin, 2010). However, both Lee (2009) and Twichell et al.
(2009) have argued that these controls may be secondary to the main driving force, namely climate change.

We suggest that it is exactly these factors that also influence the occurrence of turbidity currents, as many are
derived directly from mass transport events. Other factors more specific to turbidity current generation include: (1)
sudden excess sediment supply by rivers in flood (i.e. their generation from hyperpycnal flows); (2) rapid glacial
discharge events; (3) re-suspension of shelf edge to upper slope sediment as a result of storm stirring and the
incidence of internal tides/waves; and (4) storm build up of water across a continental shelf and its rapid
discharge down submarine canyons. Taken together, these two sets of factors are considered as ‘turbidity current 
territory’. 

Damage potential

The hazards associated with submarine mass transport events are numerous and will depend on the scale, type
and location of the movement. One of the more obvious consequences of submarine mass movements is the
generation of tsunamis that may have devastating consequences for coastal areas. The Holocene Storegga Slide,
one of the largest and well studied mass movements off the Norwegian continental margin, removed up to 2500-
3 2
3500km of sediments, affected an area of 90,000km and had a run-out distance of about 800km (Bryn et al.,
2005). Although the exact cause of the slide is still debated, both Kvalstad et al. (2005) and Bryn et al. (2005)
have suggested that rapid sediment loading during interglacial periods resulted in the development of excess pore
pressure and reduction of the shear strength in contourite marine clays and oozes. The slide was most likely
triggered by a large earthquake with the initial failure occurring within the over-pressured marine clay and oozes.

The tsunami associated with this event caused widespread inundation in Norway, Scotland, Faroe Islands and
NW Iceland (Smith et al., 2004). The run-up of the tsunami in the Shetlands exceeded 25m, while on the
mainland it exceeded 5m locally. This compares with run-up heights between 2m and 30m for the Indian Ocean
th
tsunami on December 26 2004 that killed over 280,000 people, and destroyed coastal settlements and
infrastructure across the Indian Ocean Basin (Synolakis and Kong, 2006). Another example tsunami hazards
st
associated with mass failure occurred on April 1 , 1946 when a M 7.3 earthquake struck offshore the Aleutian
Islands causing major slumping in the Alaskan Trench. The resultant tsunami had a run up of 30m along the
Scotch gap area, killed 69 people and caused $25 million in infrastructure damage.

Tsunamis generated by landslides on this magnitude are not restricted to open continental shelves as evidenced
by giant landslide scars on the flanks of volcanic islands such as Hawaiian-Emperor ridge, the Canary Islands,
Reunion, the submerged flanks of Mt Etna and Stromboli and the Marquesas Island (Hampton et al., 1996;
Masson et al., 2006; Masson et al., 2002). Along the Hawaiian-Emperor Ridge alone there are 68 major slumps
3
and debris avalanches over 20km in length, with some over 200km long and volume in excess of 5000km (Moore
et al., 1989). On the Canary Island, 14 major slumps and debris avalanches, and genetically-related turbidites and
debris flows, have been associated with the islands of El Hierro, La Palma, and Tenerife over the last one million
years. On average, the typical submarine mass transport event on the Canary Island involves a volume of 50-
3 2
200km , covers an area of a few thousand km , and has a run-out distance of 50-100km (Masson et al., 2006).
Although research into landslide generated tsunamis is still in its infancy, many of the slides mentioned above
OTC Number 9

may have had the potential to generate devastating tsunamis that could have affected distant coastal areas.

In the context of offshore oil and gas infrastructure, submarine hazards due to mass transport events and turbidity
currents have the potential to cause significant loss of life or damage to the environment and field installations
(Kvalstad, 2007). The triggering mechanisms for such events are primarily controlled by geological and climatic
processes (see above) or by human activity. Offshore infrastructure at risk of damage by submarine mass
transport events may include, but not limited to, exploration drilling rigs, piles, conductors, caissons, pipelines,
flow lines, cables, manifolds, well heads and gravity base structures. The potential impacts of mass transport
events on these structures are dependent on the scale, type, location and orientation of the movement. The
structures can be subjected to loading, burial or erosion by gravity flows (i.e. debris flows and turbidity currents) or
down-drag, uplift, rotation and translation by mass slides and slumps (Thomas et al., 2010b). For instance,
seafloor erosion and sediment scouring due to the passage of a turbidity current or debris flow may result in the
loss of support around foundations or beneath pipelines, while a rotational or translational failure will result in
down-drag near the headwall, or uplift near the toe for the same structure.

Examples of damage to offshore infrastructure as a result of mass movement processes are numerous. Perhaps
the most famous is the 1929 Grand Banks event on the continental slope south of the island of St Pierre. A series
of small, thin skinned, regressive slumps where initiated as a result of a M 7.2 earthquake located 250km south of
Newfoundland, in about 2000m water depth (Piper et al., 1999). These slumps underwent a series of surface and
-1
body transformations into debris flows and turbidity currents, and travelled at speeds up to 30ms and deposited
3
>150km of sediments on to the Sohm abyssal plain (Mulder, 2010; Piper et al., 1999). The mass movement
generated a tsunami that struck the southern end of Newfoundland’s Burin Peninsula, killing 29 people and 
causing millions of dollars of damage. Perhaps more interestingly, the resultant turbidity current broke twelve
transatlantic cables downslope, which allowed the speed of the current to be measured for the first time.

The 1979 Nice slide also generate a debris flow and turbidity current downslope that broke several
telecommunication cables. The initial slide involved mud from the prodelta slope and occurred in response to the
landfilling operations for the Nice international airport extension, but regressed onto land, removing part of the
runway along with construction equipment and killing several people working on site (Mulder et al., 1997). In
1998, cable damaging submarine slides and turbidity currents were generated when a powerful earthquake struck
offshore Papua New Guinea, killing over 2000 people (Masson et al., 2006). The resultant submarine slides and
turbidity current travelled at least 280km and in water depths greater than 6000m. Other, more recent examples
of cable breaks associated with submarine mass movements include the 2003 M 6.8 ‘Boumerdes’ earthquake
onshore Algeria that generated submarine landslides and turbidity currents, which subsequently damaged 6
cables and disrupted all submarine network in the Mediterranean region. And between 2003 and 2010, multiple
cables breaks were reported as a result of earthquake triggered submarine mass movements offshore Taiwan.

One of the few published examples of submarine geohazards damaging oil and gas installations comes from
wave and wind induced submarine slides and slumps during Hurricane Camille, which stuck the U.S Gulf of
Mexico in August 1969 (Hampton et al., 1996). As a result of these mass movements, three oilfield platforms on
the Mississippi River delta were displaced and damaged. Post storm surveys demonstrated that hurricane
generated 20m high waves induced stresses on the seabed that were strong enough to cause foundation-
disrupting (translation and uplift) slides in water depths greater than 100m (Bea et al., 1983).

In recent years, the passage of hurricanes Andrew, Katrina and Rita along the Gulf of Mexico has also damaged
hundreds of pipelines when strong waves and currents trigger landslides and slumps in the weak under-
consolidated clays issued from the Mississippi river. Presently, many offshore oil and gas prospects are located
where slope instability can be a major geohazard. Examples of such prospects include the Atlantis and Mad Dog
fields along the Sigsbee Escarpment in the Gulf of Mexico, numerous fields in the West Nile Delta offshore Egypt,
and the Ormen Lange gas field on the Norwegian continental slope (Storegga headscar).

The prediction, assessment and/or mitigation of geohazards in offshore settings are therefore key to the
continued successes of exploration and exploitation of hydrocarbons, particularly in deepwater areas.

Hazard Assessment

From the offshore oil and gas perspective, the Society for Underwater Technology estimates that the cost of
damage to submarine structures caused by mass failure is more than $400 million annually. The preferred
industry method of dealing with submarine geohazards is to identify and avoid them. However, for many locations
their large extent may make them difficult to avoid, as is the case with the Storegga slide offshore Norway which
10 OTC Number

overlies the Ormen Lange gas field, or the Gulf of Mexico where wave and current induced slides and slumps are
frequent (Clayton and Power, 2002). In terms of risk assessment, it is essential to answer the following questions:
(1) where did past and where will future mass movements and turbidity currents occur; (2) how frequently do they
occur; (3) what are the triggering mechanisms; (4) what is the area of influence; (5) what is the nature and
magnitude of the impact; and (6) can previous failures be re-activated (Locat and Lee, 2002; Thomas et al.,
2010b). While not a detailed or comprehensive study, this section will outline some of the key tools used in the
assessment of submarine geohazards.

For the determination and assessment of submarine geohazards a multidisciplinary approach is required that
involves the acquisition of geophysical, geological and geotechnical data. However, one of the first steps is to
undertake a comprehensive desk-based study incorporating all existing data (including site specific seismic data)
and to compile a regional geohazard register or inventory to establish a 3D regional ground model (Bryn et al.,
2007; Clayton and Power, 2002; Thomas et al., 2010b). The regional ground model allows attention to be placed
on relevant geohazards, detailing their geomorphological, bathymetrical and geological character, while
eliminating unrealistic geohazards, thus reducing the risk to an offshore development. The ground model
progressively evolves from a desk-based conceptual model, to a geological/geotechnical model, and finally into a
development specific engineering model as new data from the project specific site investigation becomes
available. Key data considered for input into the 3D regional ground model are itemised below.

Seafloor Mapping and Seismic Data. To establish seabed topography and geomorphology, bathymetric data need
to be acquired. The preferred tools used for bathymetric surveys are “multibeam” or “swath” echo sounder 
systems. Multibeam echo sounder systems are mounted to the ship’s hull and collect depth data by transmitting 
acoustic signals in sixteen or more beams arranged in a fan pattern. They can be operated in water depths
greater than 1500m, however the accuracy and resolution will decrease with increasing water depth due to beam
spreading and increased foot print (Kvalstad, 2007). The use of long tethers in deeper water surveys is less
productive and costly; therefore a recent development has been in the use of Autonomous Underwater Vehicles
(AUVs) to provide high resolution bathymetric. AUV can now operate in water depths up to 4500m and provide
invaluable information in the evaluation of geohazard risk assessment and site investigation, as shown by recent
studies in the West Nile Delta and the Ormen Lange gas field within the Storegga Slide area (Bryn et al., 2007;
Thomas et al., 2010a). Complimentary to the multibeam echo sounder survey is the use of side-scan sonar
(SSS). Side-scan sonar, either tethered or mounted on an AUV for deepwater investigation, can provide
information about seafloor morphology and reflectivity (backscatter) (Kvalstad, 2007). In this case, the variations
in the reflectivity can be used to map out targets such as pock marks, seabed hard grounds, gas seeps and other
geohazards, as well as sediment types.

In conjunction with seafloor surveys, it is also necessary to acquire 2D and 3D high and ultra high-resolution
seismic data during development specific investigations. These techniques allow the identification of previous
slide activity in the area, including mapping of features such as head scarps, side walls, topographic features on
basal surfaces, extensional and compressional ridges and translated blocks. In addition, good quality seismic
data provide an improved understanding of the frequency, magnitude and slope processes of previous mass
movement events in the development area. Conventional 3D streamer seismic surveys give good penetration
depth of about 5km, with vertical resolutions of 12m in the upper hundred metres of the sediments, and cover
large areas effectively (Kvalstad, 2007). However, to assess the lateral and vertical extent of smaller mass
movements and to allow seismostratigraphic mapping, ultra high-resolution (UHR), shallow penetration (50-75m),
deep towed or AUV mounted CHIRP seismic survey profiles need to be acquired (Thomas et al., 2010b). CHIRP
seismic data have vertical resolutions of less than 0.5m and allow integration with long piston and box cores for
deriving frequency, magnitude and potential impact of geohazard on the development. Furthermore, due to the
suitability of seismic facies to map-based interpretation, subsurface seascape evolution, sediment thickness
distribution and deepwater geomorphology can now be viewed by means of horizon slices, strata slices, and
interval attributes, further aiding the assessment of mass failure events (McConnell, 2004).

Geohazard Core Logging. Often, significant sections of the rock record are removed for geotechnical analysis,
with the result that one or more events will be missed, creating uncertainties when deriving quantitative data (i.e.
dates and sediment accumulation rates) for input into deterministic and probabilistic slope stability analysis
(Thomas et al., 2010b). Therefore, an independent geohazard core logging strategy is required to determine
event frequency and magnitude. By placing the mass movement events within a temporal context, estimations for
frequency and magnitude can be derived. The most important geochronological dating methods for dating mass
14
movements include: (1) biostratigraphic dating, (2) radiometric dating ( C), (3) optically stimulated luminescence
dating, (4) amino acid ratios in fossils, and (5) correlation to well dated standards such as the oxygen isotope
record (Kvalstad, 2007). Furthermore, a well constrained chronostratigraphic framework allows you to correlate
OTC Number 11

mass movement events to the climostratigraphic curve with the aim of reducing the perceived risk profile of the
offshore development. Dating of various Atlantic margin slides indicates that the primary control on submarine
slope failure is global climate change (Lee, 2009; Piper and McCall, 2003; Twichell et al., 2009). By correlating to
the climostratigraphic curve, particular mass transport events dependent on certain conditioning factors (e.g.
glacial periods) can be eliminated as they may no longer be present during the lifetime of the offshore
development, thus reducing the risk profile of the development.

In addition, an independent geohazard core logging strategy allows integration with ultra-high resolution seismic
data (e.g. AUV CHIRP seismic profile) and facies based frequency and magnitude estimation. For example, a
recent study by Thomas et al. (2010a) in the West Nile delta demonstrates that, based on subsurface
geophysical data alone, event frequency is often underestimated and magnitude overestimated. Multiple stacked
mass movement deposits in the West Nile Delta appeared as a single seismic layer on CHIRP seismic records
because individual layers where below the limits of resolution. However, integration with detailed facies based
core logging highlighted the individual layers, thus increasing the frequency of events, but decreasing the
magnitude (Thomas et al., 2010a). This last example further highlights the use of an independent facies based
geohazard core logging in modern geohazard risk assessments.

Samples taken from turbidite beds within the cores studied can be anlaysed for grain size properties. We have
established a simple link between maximum grain size (using the one percentile value) and inferred flow velocity
of the turbidity current. This is shown in Figure 3 and can be used as a ready reckoning guide to assess the likely
velocity (or relative energy) of future turbidity currents in the area. This is presented as a new compilation.

GIS (Geographical Information System). The use of GIS for data organisation, manipulation, graphic
representation, regional analyses and slope stability assessments has become routine practice when
investigating geohazards both onshore and offshore. In the simplest case, a GIS could be used as a database to
compile, manage and present a mass movement inventory, and because associated slide parameters (e.g.
location, area etc) are stored in a GIS attributes table, spatial interrelationships can be investigated. For example,
Hitchcock et al. (2006) produced simple mudflow susceptibility maps in the Gulf of Mexico using GIS by ranking
and assigning point scores for each contributing factor in map layer (i.e. maps of sediment accumulation rate,
slope inclination, geology etc). The point scores are then summed up and depicted in a single mudflow
susceptibility map. More recently however, the GIS platform has been implemented in studies dealing with
predictive methods of mass movement occurrence. A recent study by Mackenzie et al. (2010) describe the use of
deterministic slope stability assessment using GIS. A deterministic approach is used to relate the safety factor
(FOS) as being dependent on parameters, such as shear strength, slope geometry, external loading or plane
orientation, which are modelled probabilistically, enabling fully quantified risk assessment. In this case, GIS
spatial analysis techniques could directly be used to derive safety factors against slope failure, with the additional
advantage of providing quantitative outputs for subsequent geotechnical risk assessment (Mackenzie et al., 2010;
Power et al., 2011). These as well as other examples emphasise the importance of GIS in modern slope
instability assessments as it enables sophisticated, numerical-based mapping of slope failure susceptibility.

Gas Hydrates

Occurrence and distribution.

Gas hydrate is a near-surface, sediment cementing, temperature and pressure sensitive substance, with nearly
ubiquitous presence on deepwater continental margins. It is a serious potential geohazard that must be assessed
prior to deepwater petroleum industry operations.

Gas hydrate, or clathrate, is a solid ice-like substance made up of frozen water molecules that form a hollow cage
and enclose a molecule of gas, most commonly methane (Kvenvolden, 1988). Gas hydrate formation requires low
temperature, high pressure, water and a volume of gas in excess of solubility (Sloan, 2008). Where these
conditions exist, hydrate can spontaneously form.

Gas hydrate was discovered in a laboratory 1810, detected in petroleum pipelines in the 1930s (Hammerschmidt,
1934; Sloan, 2008), but not proven to form natural, geological in situ deposits until the 1960s (Sloan, 2008). Ideal
temperature and pressure conditions for hydrate formation occur in onshore permafrost regions, shallow water
permafrost tongues, near-seafloor oceanic sediments, and in deep fresh water lake sediments; numerous
geologically formed hydrate samples have been recovered from each of these environments. Recent estimates of
15 17
the global volume of carbon sequestered in gas hydrate ranges from 0.83 x10 to 1.2 x10 (4.18 – 74,000 Gt of
methane carbon), (Burwicz et al, 2011; Buffett & Archer, 2004; Klauda & Sandler, 2005; Davie & Buffett, 2001;
12 OTC Number

15
Milkov, 2004) with the consensus value on the order of 1 – 5 x 10 (Milkov, 2004). With significant reserves of
carbon stored in gas hydrate, it is potentially a viable economic future energy resource, but presents a concern for
climate change and slope stability, in addition to presenting a hazard to offshore oil industry operations.

Gas hydrate in ocean sediments. While temperatures on the seafloor are usually above the freezing point of ice,
high pressures generated by the overlying column of cool water are sufficient to create conditions suitable for gas
hydrate formation. Figure 4 illustrates a phase boundary curve for [free gas + water]—[gas hydrate] system, and
pressure-temperature conditions for an oceanic environment. Hydrate is stable at a given depth if temperature is
lower than the hydrate/free gas equilibrium at that same depth. Gas hydrate can be stable within ocean water,
and will often coat buoyant upwelling gas bubbles that dissociate to free gas when they pass the phase boundary
on their upward journey. If conditions at or below the seafloor are conducive to hydrate formation, hydrate may
form in place. The maximum depth beneath the seafloor at which hydrate is stable is determined predominantly
by the geothermal gradient. The sub-seafloor interval that hydrate is stable over is termed the gas hydrate stability
zone (GHSZ); the GHSZ extends on average to depths of 300 – 500 m beneath the seafloor (Milkov, 2004),
although factors such high pore-water salinity and heat flow can retard the formation of hydrate.

Near surface sub-sea sediments are typically highly water saturated. If there is a volume of gas in excess of
solubility and the seafloor is at appropriate pressure/temperature conditions, there is a good probability that gas
hydrate will be present. Gas hydrate accumulations are nearly ubiquitous on continental margins (Fig. 5) and
samples have been recovered from drill holes on most margins, including the Gulf of Mexico (Brooks et al, 1986),
Prudhoe Bay offshore Alaska, MacKenzie Delta offshore Canada, the Mid-American Trench off Guatemala, both
eastern and western US continental margins, Peru, New Zealand (Pecher et al., 2011; Bialas, 2011), offshore
India (Winters et al., 2008), and in the Nankai Trough (Tsuji et al., 2009). Most new gas hydrate accumulations are
initially identified through the manifestation of a unique signal in seismic data: the bottom simulating reflection
(BSR). This is a reflection that parallels the seafloor and has a reverse polarity compared to the seafloor
response. The BSR correlates with the base of gas hydrate stability and is thought to occur when gas hydrate at
the base of the gas hydrate stability zone directly overlies free gas.

In addition to conducive temperature and pressure conditions, a volume of gas in excess of solubility is required
to form gas hydrate. Gas can be of thermogenic origin (migrated upwards from deeper gas reservoirs); in situ
biogenic origin (product of biomethanogenic processes within sediments within the GHSZ); or deeper biogenic
origin (product of biomethanogenesis occurring below the GHSZ and carried up by fluid flow).

Nature and variability.

The structural relationship between porous, permeable sediment and gas hydrate at the grain scale takes one of
four forms (Fig 6). Gas hydrates can be (a) disseminated freely within pore space; (b) coating sediment grains; (c)
cementing and strengthening the bulk sediment at grain contact points; (d) cementing and strengthening the bulk
sediment by forming part of the load-bearing matrix (Kleinberg and Dai, 2005; Jones et al., 2007; Worthington,
2010). For load bearing and cementing modes (b – d) hydrate contributes significantly to the strength of semi- and
un-consolidated sediments (Dvorkin et al., 1999; Durham et al., 2003).

Natural gas hydrate accumulations can form as massive layers, thin laminae, nodules, veins and fracture fill,
discrete grains disseminated through pore space, and as massive seafloor mounds. Vast quantities of hydrate
form at low saturation, and are widely dispersed in the pore space of unconsolidated muds. The highest
saturations of oceanic gas hydrate are found in highly permeable, high porosity shallow sands and sandstones,
commonly interbedded with non-hydrate bearing finer-grained layers; marine sands with high saturations of gas
hydrate recently drilled in the Gulf of Mexico (Boswell et al., 2010), Nankai Trough, Japan (Tsujii, 2009), India (Lee
and Collett, 2009) and Cascadian Margin (Riedel et al., 2006). The recent Korean Ulleung Basin drilling program
(UBGH2) revealed high saturations of pore-filling hydrate in the sandy layers of turbidites, with nodules,
disseminated grains and vein and fracture filling hydrate in the interbedded finer grained muds (Bahk et al., 2011;
Lee et al., 2011).

Methane is the most common hydrate forming gas that forms in situ (Kvenvolden, 1988); however, more than 130
different compounds have been known to form clathrate hydrates (Sloan, 2008) with ethane, propane, butane,
isobutane and carbon dioxide all occasionally observed in nature. Whether a particular gas can form a hydrate is
determined by the size of the cavity within the water-ice crystal lattice. The thermodynamic phase stability of
hydrate will be different for different gases, with longer chained hydrocarbons stable at higher temperatures (lower
pressures).
OTC Number 13

Damage potential.

Gas hydrates represent a genuine hazard to deepwater drilling, but documentation of specific incidents is scarce,
partly because of the limited number of deepwater wells and partly because hydrates are not immediately
recognised as the cause of an issue (Nimblett et al., 2005). However, hydrate related incidents in the Arctic
permafrost include gas kicks, blow outs, gas leaks outside the casing, fires, stuck pipes and well subsidence
(Yakushev and Collett, 1992). Traditionally, the oil industry has avoided the risk presented by gas hydrates by
choosing to drill away from known hydrate deposits. With deepwater oil exploration expanding to ever greater
depths where the seafloor is at stable hydrate forming conditions, it is becoming harder to avoid drilling through
hydrate.

Hydrates represent a direct hazard to deepwater drilling in three primary modes: (1) Melting of in situ gas hydrate;
(2) gas hydrate formation on subsea structures; (3) uncontrolled release of gas.

Melting of in-situ gas hydrate. Melting of gas hydrate occurs when the local temperature and pressure conditions
are perturbed, or the gas hydrate is mechanically disturbed. Drilling itself will mechanically disturb hydrate, but
can also generate heat through friction of drilling equipment against casing. Heat from drilling muds and hot fluids
produced from below the GHSZ (e.g. water, hydrocarbons) is transferred into sediments surrounding the wellbore
(or buried pipelines transporting hydrocarbons). Models of heat transfer show that the destabilization of hydrate
can occur within a 100 m radius of seafloor and sub-seafloor structures, though the melting of hydrate will be
concentrated and most rapid nearest to the structure (Peters et al., 2008). The rate of response of the hydrate
reservoir to heating in a subsea component, and the radius of vulnerability will depend on the thermal conductivity
of the component, as well as the permeability, porosity, gas hydrate saturation, water saturation, in situ stress
state, shear strength, pore pressures and thermal conductivity of the gas hydrate and host sediments (Peters et
al., 2008).

Melting of gas hydrate reduces the solid hydrate volume, releases water and methane into sediment pore-space,
and decreases water salinity. These changes cause an increase in both the permeability and porosity of
sediments. Additionally, if: (a) hydrate-bearing layers are sealed between low-permeability layers, the pore fluid
pressure and effective stress will increase; or, (b) hydrate-bearing layers are not sealed and fluid flow carries
excess water away, then sediment volume will expand (Dillon et al, 2002; Kayen and Lee, 1991; Waite et al.,
2009). These changes combined with the development of interstitial gas bubbles, and the loss of cement and/or
the load bearing framework have the potential to cause sediments to lose strength and become unconsolidated
(Prior and Coleman, 1984; Paull et al., 2003). The loss of strength may cause localised fracturing or large-scale
failure of sediments around subsea components triggering the failure of casing, wellbore, pipelines, rig supports
and other ocean-bottom supported equipment (Dickens et al., 1997; Nimblett, 2005; Peters et al., 2008).

Gas hydrate formation on subsea structures. Methane, either dissociated from gas hydrate or escaping to the
seafloor through vents or seeps, will form into gas hydrate if conditions are right; this presents a significant risk for
seafloor components. When hydrate forms on the exterior of movable parts, including blow out preventers, valves
and switches, it can hinder its performance with potentially serious consequences.

Uncontrolled release of gas. Uncontrolled release of gas may occur when in-situ gas hydrate melts and the
release gas flows into the well, or when over-pressured gas immediately beneath the GHSZ sealed by an
impermeable layer of gas hydrate is encountered. The rapid influx of gas into the well will cause drilling muds to
become highly gasified, potentially triggering gas kicks, blow outs and, in some cases, fire (Collett and Dallimore,
2002).

Other modes of damage potential from hydrate. Hydrates can also be a drilling hazard where melting not-related
to drilling has already occurred, or is occurring. Hydrate dissociation can be caused by an increase in seafloor
and seawater temperature, resulting from a warming or directional change in bottom water currents (Westbrook et
al., 2009), or to climatic affects. Dissociation can also be triggered by a decrease in seafloor pressure resulting
from eustatic or tectonic sealevel changes. These changes have the potential to cause the dissociation of gas
hydrate over extensive regions, potentially causing mass sliding or slumping events and damaging nearby, or
distal subsea components. Gas hydrate dissociation has been suggested as an aggravating factor contributing to
the triggering of the Storegga landslide on the Norwegian margin (Berndt et al., 2002; Bunz, et al., 2003; Berndt et
al., 2004; Mienert et al., 2005) and in many slides on the United States Atlantic margin (Booth et al., 1994).

Hydrate can form spontaneously within the drill hole or production pipelines even when no hydrate zone exists.
This will occur when methane mixes with water (drilling fluids) under favourable temperature and pressure
14 OTC Number

conditions. If hydrate formation leads to a blockage of the pipe, it may lead to pressure build-ups behind the
blockage leading to an explosion, or, forcing the movement of the plug through the pipeline, causing damage
along the way. If hydrates decompose within a limited volume, sealed drill hole very high pressures can be
generated, leading to potential blow out/rupture.

Examples Nimblett et al., (2005) report that deepwater wells in Southeast Asia suffered seafloor cracks adjacent
to well sites and attributed this, at least in part, to hydrate melting in the zone around the wellsite. Nimblett et al.
(2005) also noted operational issues attributable to gas hydrate dissociation in deepwater wells in Southeast Asia
and North Africa, including gas flow out of wellhead ports between the surface and the casing. Casing failures
occurred in the permafrost Messoyakha drill site in Russia (Goodman, 1979).

Rapid upwelling of gas bubbles (from dissociation or a blow out) over a wider area will a) drive water upwards,
and outwards as it reaches the surfaces, leading to strong near-surface currents affecting the stability of rigs,
ships and other vessels and, b) cause seawater to become aerated reducing vessel buoyancy (Garcia et al.,
2008). Though recent studies indicate rigs and vessels should be able to withstand the loss of buoyancy and that
high currents from mass hydrate dissociation or blowouts will have little effect in deepwater (Hammet, 1985; de
Andrade Jr, 1997; Adams et al., 2003). Examples of vessels surviving massive gas releases from blowouts
include the West Vanguard at Haltenbanken in the Norwegian Sea and Actinia, offshore Vietnam (Garcia et al.,
2008).

Highly gasified muds and gas escaping to the surface, particularly between the casings, presents a large fire risk.
Gas bubbling and fires attributed to gas hydrate have been documented, for example, at permafrost drill sites in
Canada (Goodman, 1979).

Hazard assessment

In order to reduce the risk provided to oil industry infrastructure by gas hydrates, it is necessary to have a
thorough knowledge of gas hydrate distribution, volume and saturation, reservoir temperatures, formation pore
pressure, rock porosities and permeabilities (Collett & Dallimore, 2002) and, ideally, hydrate formation mode and
bulk sediment strength before and after hydrate dissociation.

Petroleum industry pipelines, drill holes and infrastructure will only be at risk of suffering damage or downtime due
to hydrate related problems if the structures themselves are located within water or sediments conducive to the
formation of gas hydrate. This can be evaluated to a first order, by establishing the pressure, temperature and
salinity conditions at a site of interest, using a combination of multibeam bathymetry, CTD casts (conductivity-
temperature-depth), expendable bathythermograph (XBT), moored thermistor measurements and water chemistry
measurements (particularly chlorinity, salinity and methane saturation).

The base of the gas hydrate stability is frequently correlated with the depth-converted position of a BSR observed
in seismic data. A BSR is not present everywhere gas hydrate occurs, so if hydrate is expected to be present, the
geothermal gradient is needed to predict the thickness of hydrate stability. Geothermal gradients can be
measured by heatflow probes, or logging-while-drilling (LWD) tools, or by reconstruction from BSRs observed in
adjacent locations (e.g. Yamano, et al., 1982; Minshull and White, 1989; Townend, 1997; Henrys et al., 2003).

Actual gas hydrate occurrence is more difficult to determine. As mentioned previously, accumulations are
frequently identified through the appearance of a reverse-polarity BSR on seismic data, but where no BSR is
visible, other methods can be used to identify gas hydrate accumulations. Seafloor observation methods,
including deep camera tows, side-scan sonar, sub-bottom and water column profiles can be utilised to identify
outcropping hydrate mounds. These techniques can also identify any chemosynthetic communities or bubble
plumes associated with cold methane seeps, methane from hydrate dissociation or methane-rich fluids advecting
through seafloor sediments. If the seafloor is at favourable hydrate forming conditions, then high concentrations of
methane at the seafloor present a risk to ocean bottom structures even when there is no gas hydrate in the
underlying sediments. Hydrate samples recovered from piston and gravity coring, seawater chemistry and pore
water geochemistry can be analysed to determine the composition of hydrate forming gases. Recovered samples
can also be inspected to ascertain the primary hydrate forming modes (see Fig. 6) and affect of hydrate on
sediment strength.

The extent of the gas hydrate reservoir is delineated by combining seafloor observations and the observed BSRs
with other geophysical observations. Seismic data can also be analysed in more detail to determine both the
presence of gas hydrate and to calculate saturations, through high-resolution velocity of hydrate and non-hydrate
OTC Number 15

bearing intervals (Wood et al., 1994, Mishra, 2004; Ryu et al., 2009), from full waveform inversion, e.g. (Dai et al.,
2008; Crutchley et al., 2011), and by interpretation of seismic attributes from seismic cubes (Hato et al., 2006, Lee
et al., 2009). Controlled source electromagnetic methods can be used to identify electrically resistive gas hydrate
(e.g. Mehta et al., 2005; Schwalenberg et al., 2005; Weitemeyer et al., 2006a, b; Evans, 2007; Weitemeyer and
Constable, 2011), and estimate gas hydrate saturations through application of Archie’s Law (Archie, 1942). 

If drilling has already commenced, wireline logs and LWD tools are useful for estimating hydrate saturations and
measuring the petrophysical properties of hydrate bearing intervals. The most commonly used are acoustic
velocity (e.g. Lee and Waite, 2008) and electrical resistivity tools, but recently developed integrated nuclear
magnetic resonance logging and formation testing have proved invaluable for determining how hydrate is
distributed through sediment pore space (Collet and Lee, 2011).

With a comprehensive suite of geophysical and borehole data it is possible to produce a good model of wellbore
stability (e.g. Birchwood et al., 2006; Freij-Ayoub, 2007; Salehabadi, 2009; Khabibullin, 2011) and to plan for
possible formation over-pressures, and gas release from the hydrate zone during drilling; additives can be added
to the drilling fluids to inhibit hydrate formation in the pipelines, or chemically stabilize hydrates near the well bore.

Gas hydrates present a complex and dynamic risk to subsea structures, rigs and drill holes. With the expansion of
petroleum exploration into ever deeper waters it is becoming increasingly difficult to avoid drilling through gas
hydrate intervals to exploit deeper hydrocarbons. However, with an integrated analysis of commonly-acquired
oceanographic, geophysical, geological and petrophysical data, with the addition of targeted LWD tools, a
thorough understanding of the local gas hydrate system can be acquired. Such detailed knowledge of the local
gas hydrate system will allow drilling engineers to anticipate and therefore mitigate the hazards presented by
uncontrolled gas release from gas hydrate intervals, sediment destabilization, and hydrate formation within and on
ocean bottom utilities.

Conclusion

In conclusion, it is worth re-emphasising that it is of upmost importance that the oceanographic conditions are
carefully considered prior to deepwater operations to ensure the work can be carried out safely. Thorough risk
assessment requires detailed knowledge of existing bottom currents, a thorough assessment of potential mass
transport events and turbidity currents, and a clear understanding of conditions likely to induce the formation and
destabilisation of gas hydrates. Pipelines, cables, subsea installations, key connections such as the riser and any
other seabed infrastructure are all susceptible to damage. Correct assessment of hazard will allow for the right
equipment to be used for the operations including blowout preventers, riser size, vibration suppressors, and for
the safest siting of cables and pipelines. It will also aid the subsea architecture design and planning operations
with minimal downtime. All these considerations lead to a safer exploration and production while eliminating
unnecessary cost.

References

Adams, N. and Economides, M. 2003. Characterization of Blowout Behavior in Deepwater Environments. Paper SPE 79879
presented at the SPE/IADC Drilling Conference, Amsterdam, The Netherlands, 19-21 February.
http://dx.doi.org/10.2523/79879-MS
Archie, G. E. 1942. The electrical resistivity log as an aid in determining some reservoir characteristics. Petroleum
Transactions of AIME , 146 (4): 54-62. The Society of Petroleum Engineers.
Bahk, J.J., Kim, D.-H., Chun, J.-H., Son, B. K., Kim, J.-H., Ryu, B.-J., Torres, M., et al. 2011. Gas hydrate occurrences and
their relation to hosting sediment properties: results from UBGH2 , East Sea. Proceedings of the 7th International
Conference on Gas Hydrates (IC G H 2011). Edinburgh, Scotland, United Kingdom.
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Figure 1. Principal bottom currents active in the North Atlantic. NADW = North Atlantic Deep Water, AABW = Antarctic Bottom Water, AAIW =
Antarctic Intermediate Water. Shaded areas represent areas of maximum contourite deposition.
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Figure 2. Bedform-velocity matrix for deepwater bottom current systems, showing mean grain size of sediment versus flow velocity at or near
the seafloor, with schematic representation of the bedforms present under specific velocity-grain size conditions. Modified from Stow et al.,
2009.