Geological Survey of Denmark and Greenland Bulletin
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Nr. 4, Review of Survey activities 2003, pp. 57-60 |
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57
Prior to the 1990s only few geological investigations of the
seabed and the shallow geology around the Faroe Islands had
been undertaken (Waagstein & Rasmussen 1975; Nielsen et
al. 1981). However, in the 1990s marine geological and in
particular seismic investigations were markedly intensified.
Since 1993 several studies on the structure of the Faroe
Islands margin and seafloor processes have been funded by
the European Union, namely the ENAM (European North
Atlantic Margin) project I and II (19931999) and the
STRATAGEM (Stratigraphy of the Glaciated European
Margin) project (20002003), and these have provided sig-
seabed and the shallow geology around the Faroe Islands had
been undertaken (Waagstein & Rasmussen 1975; Nielsen et
al. 1981). However, in the 1990s marine geological and in
particular seismic investigations were markedly intensified.
Since 1993 several studies on the structure of the Faroe
Islands margin and seafloor processes have been funded by
the European Union, namely the ENAM (European North
Atlantic Margin) project I and II (19931999) and the
STRATAGEM (Stratigraphy of the Glaciated European
Margin) project (20002003), and these have provided sig-
nificant new information on the mechanisms shaping the
Faroe Islands margin (e.g. Boldreel et al. 1998; Kuijpers et al.
1998a; Nielsen & van Weering 1998; van Weering et al.
1998). Due to the expertise and regional geological knowl-
edge obtained during these projects the Geological Survey of
Denmark and Greenland (GEUS) became involved in so-
called `geohazard' seabed studies of the FaroeShetland
Channel in 1997. These investigations were financed by the
petroleum industry that had begun to show significant inter-
est in exploration of the FaroeShetland Channel area. The
studies focused on possible natural risks that would affect
Faroe Islands margin (e.g. Boldreel et al. 1998; Kuijpers et al.
1998a; Nielsen & van Weering 1998; van Weering et al.
1998). Due to the expertise and regional geological knowl-
edge obtained during these projects the Geological Survey of
Denmark and Greenland (GEUS) became involved in so-
called `geohazard' seabed studies of the FaroeShetland
Channel in 1997. These investigations were financed by the
petroleum industry that had begun to show significant inter-
est in exploration of the FaroeShetland Channel area. The
studies focused on possible natural risks that would affect
Geohazard studies offshore the Faroe Islands: slope
instability, bottom currents and sub-seabed sediment
mobilisation
instability, bottom currents and sub-seabed sediment
mobilisation
Tove Nielsen and Antoon Kuijpers
Fig. 1. Overview of the Faroe Platform area with locations of major mass flow deposits, the pathway of high-energy Norwegian Sea overflow water
(NSOW), and location of the mud diapirs at the northern entrance of the FaroeShetland Channel. FS, FOIB slide; GR, GEM raft; SandF, Sandoy fan;
SandS, Sandoy slump; SudF, Su>
uroy fan; SudS, Su>
uroy slump.
Geological Survey of Denmark and Greenland Bulletin 4, 5760 (2004) © GEUS, 2004
submarine structures, such as slope instability and strong
bottom currents, and included both shallow seismic data
acquisition and sediment core analyses. Most of the work at
sea was undertaken with the Russian research vessel Prof.
Logachev, and carried out within the framework of the inter-
national, UNESCO-supported `Training-Through-Research'
(TTR) programme co-ordinated by Moscow State Univer-
sity, Russia. Since 1997, more than three million DKK have
been granted for various projects and this work has been doc-
umented in 14 classified reports. This paper presents some of
the main results from these `geohazard' studies, in particular
with respect to the sediment instability affecting the western
flank of the FaroeShetland Channel, the occurrence of very
strong bottom currents in the channel, and the newly discov-
ered mud diapirs at the northern entrance of the channel
(Fig. 1).
bottom currents, and included both shallow seismic data
acquisition and sediment core analyses. Most of the work at
sea was undertaken with the Russian research vessel Prof.
Logachev, and carried out within the framework of the inter-
national, UNESCO-supported `Training-Through-Research'
(TTR) programme co-ordinated by Moscow State Univer-
sity, Russia. Since 1997, more than three million DKK have
been granted for various projects and this work has been doc-
umented in 14 classified reports. This paper presents some of
the main results from these `geohazard' studies, in particular
with respect to the sediment instability affecting the western
flank of the FaroeShetland Channel, the occurrence of very
strong bottom currents in the channel, and the newly discov-
ered mud diapirs at the northern entrance of the channel
(Fig. 1).
Material and methods
Seismic data acquisition was carried out with a 100 kHz air-
gun and a 6-channel streamer to map subbottom structures.
Seismic profiling was carried out in combination with a 10
kHz long-range (2
gun and a 6-channel streamer to map subbottom structures.
Seismic profiling was carried out in combination with a 10
kHz long-range (2
× 6 km) side-scan sonar for obtaining
information on seabed surface sediment and topography. In
selected areas a higher resolution of the seabed features was
necessary, and a deep-towed side-scan sonar was deployed,
operating at 30 or 100 kHz with ranges of 2
selected areas a higher resolution of the seabed features was
necessary, and a deep-towed side-scan sonar was deployed,
operating at 30 or 100 kHz with ranges of 2
× 1000 m and
2
× 350 m, respectively. The latter device was also equipped
with a 5 kHz subbottom profiler, whereas during all survey
activities another, hull-mounted subbottom profiler was rou-
tinely operated. Bottom samples were retrieved with a 6-m
activities another, hull-mounted subbottom profiler was rou-
tinely operated. Bottom samples were retrieved with a 6-m
gravity corer, a box corer, and a large video-controlled grab.
In addition, underwater video was deployed. After retrieval of
the sediment cores, the cores were described and magnetic
susceptibility measurements were carried out on board.
Selected samples were investigated using a microscope to
determine mineral and microfossil content. After the cruise
more extensive core studies were made, and sediments were
dated using the AMS
In addition, underwater video was deployed. After retrieval of
the sediment cores, the cores were described and magnetic
susceptibility measurements were carried out on board.
Selected samples were investigated using a microscope to
determine mineral and microfossil content. After the cruise
more extensive core studies were made, and sediments were
dated using the AMS
14
C method.
Slope instability
Prior to of the `geohazard' studies, mass flow deposits had not
been reported from the western flank of the FaroeShetland
Channel. In contrast, a major slide complex was known to
extend over most of the north-eastern Faroe Islands margin
(Fig. 1). Seismic studies carried out in the latter area during
the ENAM project demonstrated that large-scale slumping
and sliding had affected the middle and lower slopes below
1500 m water depth since the Miocene (Nielsen & van
Weering 1998; van Weering et al. 1998). High-resolution
side-scan sonar surveying in the area downslope of the main,
c. 300 m high headwall, where water depth is about 2300 m,
demonstrated the presence of a large number of downslope-
trending tracks on a low slope gradient, locally displaying
cross patterns, and occasionally a markedly irregular pattern
(Fig. 2). At the termination of the tracks, outrunner blocks of
sediment were observed, up to 18 m high, and with a maxi-
mum length of 70 m. Some of the blocks were found at a dis-
tance of up to 25 km from the initial mass flow terminus at
the main headwall. The sub-bottom profiles in the trail-mark
area indicate that most of the tracks have been filled with
transparent sediment acoustically comparable to the Holo-
cene hemipelagic surface unit, and thus may have an age
older than Holocene.
been reported from the western flank of the FaroeShetland
Channel. In contrast, a major slide complex was known to
extend over most of the north-eastern Faroe Islands margin
(Fig. 1). Seismic studies carried out in the latter area during
the ENAM project demonstrated that large-scale slumping
and sliding had affected the middle and lower slopes below
1500 m water depth since the Miocene (Nielsen & van
Weering 1998; van Weering et al. 1998). High-resolution
side-scan sonar surveying in the area downslope of the main,
c. 300 m high headwall, where water depth is about 2300 m,
demonstrated the presence of a large number of downslope-
trending tracks on a low slope gradient, locally displaying
cross patterns, and occasionally a markedly irregular pattern
(Fig. 2). At the termination of the tracks, outrunner blocks of
sediment were observed, up to 18 m high, and with a maxi-
mum length of 70 m. Some of the blocks were found at a dis-
tance of up to 25 km from the initial mass flow terminus at
the main headwall. The sub-bottom profiles in the trail-mark
area indicate that most of the tracks have been filled with
transparent sediment acoustically comparable to the Holo-
cene hemipelagic surface unit, and thus may have an age
older than Holocene.
At the start of the studies of the western flank of the
FaroeShetland Channel, it soon became evident that slope
instability and associated mass flow had also occurred in this
area. Seismic evidence (Fig. 3) clearly shows that these
processes have taken place repeatedly since late Pliocene time.
Within this context, it should be noted that no evidence has
been found for any major mass-wasting activity having
occurred subsequent to the early Holocene sea level rise
(Kuijpers et al. 2001); i.e. during the past c. 7000 years the
Faroe Islands margin appears to have been generally stable.
instability and associated mass flow had also occurred in this
area. Seismic evidence (Fig. 3) clearly shows that these
processes have taken place repeatedly since late Pliocene time.
Within this context, it should be noted that no evidence has
been found for any major mass-wasting activity having
occurred subsequent to the early Holocene sea level rise
(Kuijpers et al. 2001); i.e. during the past c. 7000 years the
Faroe Islands margin appears to have been generally stable.
High-energy bottom current environments
Export of deep waters formed in the North Atlantic occurs
via two major gateways: one between Greenland and Iceland
and one between Iceland and Scotland (e.g. Dickson et al.
1990). Sediment core studies have demonstrated that these
via two major gateways: one between Greenland and Iceland
and one between Iceland and Scotland (e.g. Dickson et al.
1990). Sediment core studies have demonstrated that these
58
Fig. 2. Deep-tow side-scan sonar record and sub-bottom profile from the
trail mark area downslope of the main slump scar in the mass flow area
north-east of the Faroe Islands (see Fig. 1). The sonograph shows a large
number of outrunner block tracks of varying width, with several blocks
(circles) at the ends of their respective tracks. Crossing slide paths are
also observed. Arrow indicates markedly irregular pattern of some
tracks. From Kuijpers et al. (2001).
overflow currents were generally reduced or ceased during
cold (stadial) climate periods, relative to interstadial and par-
ticularly interglacial conditions (e.g. Kuijpers et al. 1998b).
Noteworthy in this context is the recent observation in the
FaroeShetland Channel of a decreasing overflow since 1950
(Hansen et al. 2001). For the purpose of providing informa-
tion on the high-energy overflow current environments along
the FaroeShetland gateway, an inventory of current-induced
bedforms detected by side-scan sonar was undertaken, which
revealed the flow path where near-bottom current speed
reaches around 1.0 m/s (Kuijpers et al. 2002). For compari-
son, supplementary information from actual current meter
measurements has been added in order to determine whether
the bedforms recorded (Fig. 4) could be relict features, or can
be considered to be in equilibrium with the recent current
regime. Our knowledge of overflow processes, which were
cold (stadial) climate periods, relative to interstadial and par-
ticularly interglacial conditions (e.g. Kuijpers et al. 1998b).
Noteworthy in this context is the recent observation in the
FaroeShetland Channel of a decreasing overflow since 1950
(Hansen et al. 2001). For the purpose of providing informa-
tion on the high-energy overflow current environments along
the FaroeShetland gateway, an inventory of current-induced
bedforms detected by side-scan sonar was undertaken, which
revealed the flow path where near-bottom current speed
reaches around 1.0 m/s (Kuijpers et al. 2002). For compari-
son, supplementary information from actual current meter
measurements has been added in order to determine whether
the bedforms recorded (Fig. 4) could be relict features, or can
be considered to be in equilibrium with the recent current
regime. Our knowledge of overflow processes, which were
previously based only on information from current meter sta-
tions and ship-borne hydrographic sections, has thus been
extended and a regional overview of the areas most inten-
sively influenced by the overflow currents has been obtained.
tions and ship-borne hydrographic sections, has thus been
extended and a regional overview of the areas most inten-
sively influenced by the overflow currents has been obtained.
Sub-seabed sediment mobilisation
Submarine mud volcanoes, or diapirs, can range in size
between 0.5 and 800 m high. Two main mechanisms are con-
sidered to lead to the formation of mud diapirism, i.e. high
sedimentation rates and/or lateral tectonic compression.
Both mechanisms can result in over-pressure of a mobile sed-
iment layer at sub-bottom depth. In the mid-1990s mound
features were observed immediately east of the Fugloy Ridge
(see Fig. 1) by the British Geological Survey (BGS), and were
reported as possible cold-water coral mounds. Further high-
between 0.5 and 800 m high. Two main mechanisms are con-
sidered to lead to the formation of mud diapirism, i.e. high
sedimentation rates and/or lateral tectonic compression.
Both mechanisms can result in over-pressure of a mobile sed-
iment layer at sub-bottom depth. In the mid-1990s mound
features were observed immediately east of the Fugloy Ridge
(see Fig. 1) by the British Geological Survey (BGS), and were
reported as possible cold-water coral mounds. Further high-
59
Fig. 3. Single-channel airgun
profile from the western flank
and basin of the FaroeShetland
Channel and a schematic
interpretation (bottom) showing
the presence of a large mass flow
unit of presumably late Pliocene
early Pleistocene age.
Fig. 4. Deep-tow side-scan sonar record of
Norwegian Sea overflow water (NSOW)-induced
sandwaves at the southern end of the
FaroeShetland Channel. Water depth is
11001200 m. From Kuijpers et al. (2002).
resolution seismic work by BGS and the Royal Netherlands
Institute for Sea Research (NIOZ) revealed, however, that the
mounds were most likely mud-diapirs, an interpretation later
supported by a TOBI side-scan sonar survey carried out by
the Southampton Oceanography Centre (SOC). During the
2002 TTR-cruise with R/V Prof. Logachev GEUS made a
detailed study of the mounds at the northern entrance of the
FaroeShetland Channel (Fig. 5).
Institute for Sea Research (NIOZ) revealed, however, that the
mounds were most likely mud-diapirs, an interpretation later
supported by a TOBI side-scan sonar survey carried out by
the Southampton Oceanography Centre (SOC). During the
2002 TTR-cruise with R/V Prof. Logachev GEUS made a
detailed study of the mounds at the northern entrance of the
FaroeShetland Channel (Fig. 5).
The results of this work confirm that the mound struc-
tures can be classified as mud diapirs originating from sub-
surface sediment mobilisation. This sediment mobilisation is
probably due to the excessive load of dense, glacigenic sedi-
ments of the North Sea Fan deposited on top of low-density
(Miocene) diatomaceous ooze. Several stages of maturity
have been observed: (1) an initial stage where the diapirs do
not pierce the seabed, (2) a young (up to 50 m high) stage dis-
playing a marked relief, and (3) an up to 100 m high, mature
stage where the diapirs have a smoother appearance.
Preliminary results from AMS
surface sediment mobilisation. This sediment mobilisation is
probably due to the excessive load of dense, glacigenic sedi-
ments of the North Sea Fan deposited on top of low-density
(Miocene) diatomaceous ooze. Several stages of maturity
have been observed: (1) an initial stage where the diapirs do
not pierce the seabed, (2) a young (up to 50 m high) stage dis-
playing a marked relief, and (3) an up to 100 m high, mature
stage where the diapirs have a smoother appearance.
Preliminary results from AMS
14
C dating of sediment cores
collected from the diapirs suggest an episode of major activa-
tion of the diapirs around the time of the Last Glacial
Maximum (LGM).
tion of the diapirs around the time of the Last Glacial
Maximum (LGM).
Acknowledgements
The studies were supported by the Faroese offshore consortium GEM
(now FOIB) and the European ENAM-II and STRATAGEM projects. The
contributions from colleagues at GEUS, NIOZ, BGS and the TTR-pro-
gramme are gratefully acknowledged.
(now FOIB) and the European ENAM-II and STRATAGEM projects. The
contributions from colleagues at GEUS, NIOZ, BGS and the TTR-pro-
gramme are gratefully acknowledged.
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60
Fig. 5. Mosaic image of deep-tow side-scan sonar records of the mud diapir area at the northern entrance of the FaroeShetland Channel. Water depth
is 16001700 m. From Nielsen et al. (2002).
Authors' address
Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: tni@geus.dk
Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: tni@geus.dk
