Section III
Submarine Mass Movements in Margin
Construction and Economic Significance
Investigating the Timing, Processes
and Deposits of One of the World’s Largest
Submarine Gravity Flows: The ‘Bed 5 Event’
Off Northwest Africa
R.B. Wynn, P.J. Talling, D.G. Masson, C.J. Stevenson,
B.T. Cronin, and T.P. Le Bas
Abstract An extensive dataset of shallow sediment cores is used here to describe
one of the World’s most voluminous and extensive submarine gravity flows. The
Bed 5 event, dated at ∼60 ka, originated on the upper slope offshore Atlantic
Morocco, in the vicinity of Agadir Canyon. The volume of initial failure was
∼130 km3 of sediment, and the failure appeared to rapidly disintegrate into a highly
mobile turbidity current. Widespread substrate erosion beneath the flow occurred
up to 550 km from the interpreted source, and is estimated to have added a further
30 km3 of sediment. The flow spread upon exiting Agadir Canyon, with deposition
occurring across both the Agadir Basin and Seine Abyssal Plain. Evidence for flow
transformations and linked turbidite-debrite development can be found in both
basins, and there are also indications for sediment bypass and fluid mud behaviour.
A portion of the flow subsequently spilled out of the western Agadir Basin, and
passed through the Madeira Channels prior to deposition on the enclosed Madeira
Abyssal Plain at 5,400 m water depth. The total run-out distance along the flow
pathway is about 2,000 km, with only about half of the pathway confined to canyon
or channel environments. Our results show that large-volume submarine landslides
can rapidly disintegrate into far-traveling fluid turbidity currents, and that depositional processes within such flows may be complex and spatially variable.
Keywords Landslide • turbidity
• Morocco
current • sandwich
bed • sediment
cores
R.B. Wynn (), P.J. Talling, D.G. Masson, C.J. Stevenson, T.P. Le Bas
National Oceanography Centre, Southampton, European Way, Southampton, SO14 3ZH, UK
e-mail: rbw1@noc.soton.ac.uk
B.T. Cronin
Deep Marine, 9 North Square, Footdee, Aberdeen, AB11 5DX, UK
D.C. Mosher et al. (eds.), Submarine Mass Movements and Their Consequences,
Advances in Natural and Technological Hazards Research, Vol 28,
© Springer Science + Business Media B.V. 2010
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Introduction and Data
The northwest African continental margin has hosted some of the World’s largest
submarine landslides during the last 200,000 years. For example, the Sahara Slide
off Western Sahara, dated at ∼50–60 ka, involved 600 km3 of sediment and has a
run-out distance of 700 km (Gee et al. 1999). In this example the failed sediment
block partially disintegrated into a plastic debris flow, but there is no evidence for
transformation into a fluid turbidity current. In contrast, the Moroccan Turbidite
System has hosted several large-volume landslides that have transformed into
highly mobile flows soon after failure (Wynn et al. 2002b). Deposits of these flows
include some of the most voluminous turbidites in the World, with deposit volumes
exceeding 100 km3 and run-out distances of up to 2000 km. Most of the large-volume
flows are siliciclastic and derived from the Moroccan margin. Flow deposits are
best developed in three inter-connected depositional basins: the Agadir Basin, the
Seine Abyssal Plain and the Madeira Abyssal Plain, all at water depths > 4,000 m
(Fig. 1).
Here, we present an overview of one of the largest flows to have occurred in the
Moroccan Turbidite System during the late Quaternary. The deposit of this flow is
called Bed 5 (Wynn et al. 2002b; Frenz et al. 2009), and it is particularly interesting
as it underwent multiple transformations during its passage downslope (Talling et al.
2007). Our study builds upon previous work by incorporating new sediment (piston)
cores that penetrated the Bed 5 deposit in the eastern Seine Abyssal Plain. These
cores were collected during research cruise JC027 in autumn 2008, and provide further evidence for flow transformations in Bed 5. In addition, the new cores allow
comprehensive analysis of the source area, timing, flow pathways, deposits and processes of the Bed 5 event across the entire Moroccan Turbidite System (Fig. 1).
2
Landslide Source Area and Timing
The frequency of large-scale turbidity currents passing through the Moroccan
Turbidite System is low, with roughly one event every 10,000 years (Wynn et al.
2002b). Correlation of individual flow deposits between basins is based upon (1)
sand fraction mineralogy, (2) mud geochemistry and colour, (3) coccolith ratios in
the mud cap, (4) microfossil-based dating of bounding hemipelagites, and (5) relative stratigraphic position compared to seafloor and volcaniclastic marker turbidites
(Wynn et al. 2002b). The Bed 5 deposit has been extensively sampled throughout
the Moroccan Turbidite System, and mineralogical and geochemical analyses indicate a source area on the outer shelf and/or upper slope of northwest Morocco,
adjacent to upper Agadir Canyon (Fig. 1; Weaver et al. 1992; Davies et al. 1997;
Wynn et al. 2002b). However, a lack of geophysical data from the source area
means that the exact shape and size of the initial failure are unknown.
The Bed 5 event has been dated at ∼60 ka in each of the three depositional basins
of the Moroccan Turbidite System, using both coccoliths and planktonic foraminifera
Fig. 1 Overview map of the Moroccan Turbidite System. Bathymetry is based upon GEBCO data, with contour intervals of 500 m. Core locations indicated
by white circles; those illustrated in Figs. 3, 5 and 7 are highlighted in yellow. Red arrows highlight the main flow pathways for the Bed 5 event. Dashed red
circle shows location of interpreted source area for the Bed 5 landslide. White rectangles indicate locations of Figs. 2, 4 and 6. Abbreviations as follows:
AC = Agadir Canyon; SAP = Seine Abyssal Plain; AB = Agadir Basin; MC = Madeira Channels; MAP = Madeira Abyssal Plain: CDF = Canary Debris
Flow
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extracted from bounding hemipelagic sediments (Weaver and Kuijpers 1983;
Davies et al. 1997; Wynn et al. 2002b). The source landslide occurred at the transition between oxygen isotope stages 4 and 3, at a time when sea-level and global
temperatures were rising. It therefore seems likely that landslide initiation was
linked to rapid sea-level rise of ∼40 m, following a 10 kyr period of exceptionally
low sea-level (Siddall et al. 2003). Just prior to the Bed 5 landslide, eustatic sealevel was actually at its lowest point for 70,000 years. However, a lack of available
data from both the submarine source area and adjacent landmass, mean that the
pre-conditioning factors and the specific trigger mechanism for the initial landslide
cannot be determined.
3
Flow Pathways, Run-Out Distance and Deposit Volume
Analysis of grain size and bed thickness trends has allowed flow pathways for the
Bed 5 event to be reconstructed in detail (Wynn et al. 2002b; Frenz et al. 2009). The
coarsest basal grain sizes (gravel of 5–20 mm), and largest erosional hiatuses below
the base of the bed (up to 1.0 m), are found in the eastern Agadir Basin adjacent to
the mouth of Agadir Canyon (Figs. 2 and 3). This suggests a flow pathway through
Fig. 2 Slope map of Agadir Basin, based upon GEBCO data. Slope angles across most of Agadir
Basin are < 0.1°. For location see Fig. 1. Bathymetric contours spaced at 500 m intervals. Core
locations shown by white circles, and those illustrated in Fig. 3 are numbered and highlighted in
yellow. Yellow dashed lines indicate transect locations in Fig. 3. SAP = Seine Abyssal Plain
Investigating the Timing, Processes and Deposits of One of the World’s
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Fig. 3 Core correlation transects, showing the geometry and sediment facies of the Bed 5 deposit
across Agadir Basin: (a) dip transect, (b) eastern (proximal) strike transect, (c) western (distal)
strike transect. See Fig. 2 for locations. Transects are hung off base of Bed 5. Note the development of a thick (≥2 m) linked turbidite-debrite bed in the central basin, downstream of a marked
slope break from 0.05° to < 0.01°. Cores 48 and 51, located upstream of the slope break, display
highly erosive bases (marked with an asterisk) that are incised into underlying sediments for several tens of cm. Grain-size scale as follows: (A) clay and silt (<63 μm), (B) fine sand (63–250 μm),
(C) medium sand (250–500 μm), (D) coarse sand (500–2,000 μm), (E) gravel (>2,000 μm)
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R.B. Wynn et al.
the canyon, which is consistent with the location of the interpreted source area (Fig. 1;
Frenz et al. 2009). In addition, initial analysis of several cores recovered from lower
Agadir Canyon reveals the presence of significant erosional hiatuses, at a stratigraphic level consistent with Bed 5 (Talling et al. 2007).
Upon exiting Agadir Canyon, a portion of the Bed 5 flow passed westwards
across Agadir Basin, before spilling into the Madeira Channels and ultimately
spreading across the Madeira Abyssal Plain at a water depth of ∼5400 m (Figs. 1, 4
and 5; Wynn et al. 2002b). The straight line distance from the interpreted source
area and the southern Madeira Abyssal Plain is ∼1600 km (Fig. 1), however, direct
tracing of likely flow pathways indicates that the total flow run-out distance is probably in excess of 2,000 km. The scale of this flow, which is non-channelised for
about half of its passage, is exceptional; other deep-sea flows with comparable runout distances are largely confined within channels, e.g. Bengal Fan (Curray et al.
2003) and the Northwest Atlantic Mid-Ocean Channel (Klaucke et al. 1998).
Another portion of the Bed 5 flow moved north (and downslope) upon exiting Agadir
Canyon, and entered the Seine Abyssal Plain from the southwest (Figs. 1, 6 and 7).
Fig. 4 Bathymetry map showing the Madeira Abyssal Plain and distal Madeira Channels, based
upon GEBCO data. Note that data in this region are of insufficient quality to allow a slope map to
be constructed. For location see Fig. 1. Bathymetric contours spaced at 100 m intervals; the basin
floor is at ∼5,400 m. Core locations shown by white circles, and those illustrated in Fig. 5 are
numbered and highlighted in yellow. Yellow dashed line indicates transect location in Fig. 5. CDF
= Canary Debris Flow
D11810
ABCDE
1m
D11812
ABC
A
A
A
A
ABCDE
D10699
D11322
D10698
D11319
D10698-4
?
Thick mud cap
in central basin
D10688
ABCDE
150 km
D11817-TC
ABCDE
Flow direction
D11816
ABCDE
?
NE
Fig. 5 Core correlation transect (∼700 km long), showing the geometry and sediment facies of the Bed 5 deposit across the Madeira Abyssal Plain. See Fig. 4
for location. Transect hung off base of Bed 5. Note the development of a thick (∼1 m) mud cap in the deep central basin. Lithofacies, sedimentary structures
and grain size schemes same as Fig. 3
D11809
Bed 5
not present
(pinch out)
SW
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Fig. 6 Slope map of the Seine Abyssal Plain, based upon GEBCO data. Slope angles across most
of the plain are < 0.1°. For location see Fig. 1. Core locations shown by white circles, and those
illustrated in Fig. 7 are numbered and highlighted in yellow. Yellow dashed line indicates transect
location in Fig. 7. AC = Agadir Canyon. Note that cores 40 and 73 are located on the crest and
flank of a sedimentary ridge separating Seine Abyssal Plain and Agadir Basin
The flow then moved east across the plain for at least 500 km, as shown by its presence
in core JC027-19 (Fig. 7). It is possible that a portion of the Bed 5 flow also passed
through the series of canyons located ∼150 km north of Agadir Canyon (Fig. 1).
However, there is no evidence in cores from Seine Abyssal Plain for multiple
graded intervals (Fig. 7), and it seems unlikely that multiple flows traveling along
different pathways would result in a single fining-upwards deposit.
Investigation of bed thickness in the three depositional basins allows estimates
of flow volume to be calculated for Bed 5. Volume calculations assume that bed
surfaces between core locations are linear, and that no significant Bed 5 deposits
occur outside of depositional basins. The bed volume is estimated at 40 km3 on the
Madeira Abyssal Plain (Jones et al. 1992), 22 km3 in the Agadir Basin (Frenz et al.
2009), and 100 km3 on the Seine Abyssal Plain (in this volume). This leads to a total
volume estimate of ∼162 km3, although this is likely to be an under-estimate as
additional Bed 5 deposits are patchily distributed outside of depositional basins,
e.g. in the Madeira Channels. The Bed 5 deposits on Seine and Madeira Abyssal
Plains are mud-dominated (Figs. 5 and 7), whereas in Agadir Basin there is a higher
proportion of sand (both clean turbidite sand and muddy debrite sand; Fig. 3).
Analysis of benthic foraminifera assemblages from the base of the Bed 5 deposit
in several cores from Agadir Basin, has revealed that ∼20% of the assemblage is
composed of species that live below 2,500 m water depth (Talling et al. 2007).
Fig. 7 Core correlation transect (∼800 km long), showing the geometry and sediment facies of the Bed 5 deposit across the Seine Abyssal Plain. See Fig. 6
for location. Transect hung off base of Bed 5. Note the thick (≥2 m) mud cap in the deep central basin and development of a linked turbidite-debrite bed in
the eastern basin. Lithofacies, sedimentary structures and grain size schemes same as Fig. 3
Investigating the Timing, Processes and Deposits of One of the World’s
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R.B. Wynn et al.
If we assume that the source landslide occurred on the upper slope/outer shelf (i.e.
shallower than 2,500 m), then this material was likely incorporated into the flow
through seafloor erosion in the lower Agadir Canyon and eastern Agadir Basin.
This is supported by evidence for significant erosion in the lower canyon, in the
form of km-scale scours and major erosional hiatuses (Wynn et al. 2002a, b, Talling
et al. 2007). If we also assume that 20% deep-water benthic foraminifera equates
to 20% of the deposit volume, then we estimate that ∼32 km3 of sediment was
eroded from water depths > 2,500 m; this leads to a revised estimate of 130 km3 for
the initial source landslide volume.
4
Flow Processes and Deposits
Bed 5 is the most recent large-volume flow to have passed through Agadir Canyon
(Wynn et al. 2002b), and widespread erosion on the canyon floor appears to be
attributable to a strongly turbulent flow (Wynn et al. 2002a; Talling et al. 2007).
In addition, deposits in the easternmost Agadir Basin are composed of thin gravel
lags or cut-and-fill scours; these scours are up to 1.0 m deep and contain upwardfining turbidites (Figs. 3 and 7; Talling et al. 2007), again indicating well-mixed
and turbulent flow conditions.
The Bed 5 flow spread rapidly upon exiting Agadir Canyon, with separate portions moving west into Agadir Basin and northeast into Seine Abyssal Plain. In the
central Agadir Basin, the flow decreased in velocity upon reaching a marked
decrease in slope (Fig. 3a). Although the head of the flow remained fully turbulent,
and deposited upward-fining clean sands across the basin floor, the central portion
of the flow collapsed and deposited ungraded muddy sand in an elongate tongue
along the basin axis (Fig. 3). This flow collapse was probably a response to excessively high near-bed mud concentrations, resulting from updip erosion of finegrained sediments in Agadir Canyon and eastern Agadir Basin. The tail of the flow
formed a dilute suspension cloud that deposited a weakly graded or ungraded mud
cap (see Talling et al. 2007 for a detailed discussion of depositional processes of
this ‘linked turbidite-debrite’ deposit).
Analysis of new JC027 cores in this study has revealed that a similar linked
turbidite-debrite deposit is present in the eastern Seine Abyssal Plain (Fig. 7), over
500 km from the mouth of Agadir Canyon. This deposit is similar to that seen in
Agadir Basin, and comprises (1) an upward-fining basal sand turbidite with planar
laminae, overlain by (2) an ungraded structureless muddy sand debrite with mud
clasts, overlain by (3) a weakly-graded laminated silt turbidite with an ungraded
mud cap. The presence of this apparently isolated linked turbidite-debrite, near the
southeast margin of Seine Abyssal Plain, may also be related to slope gradient, with
the central portion of the flow ‘collapsing’ and depositing en masse due to a velocity decrease as it crossed a particularly flat section of basin floor (slope < 0.01°)
and/or approached the basin margin (Fig. 6).
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Inter-basin areas are dominated by sediment bypass, for example, the channels
separating Agadir Basin from Madeira Abyssal Plain (see core 22 in Fig. 3a). In
these areas the Bed 5 deposit is either absent or comprises medium sands with
climbing ripples and thin mud caps, indicating dilute flow conditions and/or sediment bypass in the head and centre of the flow. The lateral margins of Agadir Basin
are characterised by abrupt thinning of the Bed 5 deposit, and it is noticeable that
mud caps thin more abruptly onto basin margins than the basal clean sand layer
(Figs. 3b and c). Conversely, mud caps thicken dramatically in the deepest parts of
the enclosed Seine and Madeira Abyssal Plains (Figs. 5 and 7). If the mud was
depositing from a dilute suspension it might be expected to drape evenly across the
low slope angles encountered here (see Fig. 2), instead the marked lateral variations
in mud cap thickness may point towards post-depositional remobilisation and fluid
mud behaviour (cf. McCave and Jones 1988). Detailed grain size analysis of Bed 5
mud caps is currently in preparation, and will contribute to a future paper investigating depositional processes of thick ‘turbidite’ muds in the Moroccan Turbidite
System.
5
Conclusions
This overview of the Bed 5 event has revealed some surprising phenomena, with
evidence for large-scale erosion, flow transformations, fluid mud behaviour and
sediment bypass occurring within a single flow. Our results reveal that some largevolume slope failures, involving > 100 km3 of sediment, can rapidly disintegrate
into fluid gravity flows capable of traveling up to 2,000 km across very gentle
slopes (generally < 0.1°). The ability to track deposits of a single gravity flow
across distances > 1000 km is unique, and is made possible here by a robust chronostratigraphic framework (Weaver et al. 1992; Davies et al. 1997; Wynn et al.
2002b). Future work on the Moroccan Turbidite System will focus on the processes
and deposits of other large-scale flows during the late Quaternary, while more
detailed work on Bed 5 will involve investigation of erosional processes within
Agadir Canyon, and depositional processes of thick mud caps in the Madeira and
Seine Abyssal Plains.
Acknowledgments We would like to thank the large number of scientists and ship staff who have
assisted with data collection on research cruises to the Moroccan Turbidite System. In particular,
RBW (as Chief Scientist) would like to acknowledge the contribution of the deep-water coring
team on the two most recent cruises, CD166 and JC027. Research cruise CD166 was funded by
NERC and several companies involved in the UK-TAPS (Turbidite Architecture and Process
Studies) Agadir Project consortium, including BHP Billiton, ConocoPhillips, ExxonMobil, Norsk
Hydro and Shell. Research cruise JC027 was funded through the NERC Oceans 2025 strategic
research programme. We are also grateful to Michael Frenz, Esther Sumner, Sünje DallmeierTiessen, Gayle Hough and Andrey Akhmetzhanov for their significant contributions towards sedimentological and geophysical data collection and processing. Reviewers Mark Deptuck and David
Hodgson are thanked for their constructive comments that helped improve the paper.
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