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The series of twelve large sedimentary mounds, elongated orthog-
onal to the Antarctic Peninsula Pacific continental margin, are
examples of mixed drift systems produced by the interaction of
downslope and alongslope processes. The margin has been exten-
sively studied over the past decade by a series of Italian, German,
British and joint research programmes, and has also been the
focus of DSDP and ODP drilling. This paper focuses on Drift 7,
an elongate body some 200 km long, 70 km wide and up to 1 km
thick (Table 1, Fig. 1). It synthesises the principal results derived
from study of a variety of data types, including detailed bathy-
metric and swath bathymetric data, seismic reflection profiles and
sub-bottom (TOPAS) profiles, seabed photos, and sediment core
samples.
Geological and oceanographic setting
The Pacific Margin of the Antarctic Peninsula has been charac-
terised by rapid terrigenous sedimentation since the late Miocene
(Tucholke et al. 1976; Larter & Barker 1989). Glacially-derived
sediment has been redistributed downslope and alongslope by
turbidity currents and bottom currents. West of Drake Passage,
the axis of the eastward-flowing Antarctic Circumpolar Current
(ACC) lies about 60°S (Nowlin & Klinck 1986). A narrow counter
current flows south-westward close to the margin (Gordon 1966;
Nowlin & Zenk 1988; Camerlenghi et al. 1997a).
On the continental rise, in water depths of 2700–3700 m, there
are 12 large sediment mounds elongated orthogonally to the
margin (Fig. 1). They are thought to be formed of material
originally supplied from turbidity currents flowing in deep-sea
channels extending from the margin to the abyssal plain, and
redistributed by south-westerly-flowing bottom currents (Rebesco
et al. 1994, 1996; McGinnis & Hayes 1995).
Present day bottom current flow in the area of drift 7, one of the
largest drifts (Fig. 2) was reconstructed (Camerlenghi et al. 1997a;
A. Crise, pers. comm.) from data recorded in three deep moorings
deployed by the R/V OGS-Explora during two cruises of the
Progetto Nazionale Ricerche in Antartide (PNRA). Mooring ST-
01 was deployed at 3475 m depth on the south-west, steeper side
of drift 7, Mooring ST-02 at 3338 m depth on the gentler north-
eastern side, and Mooring ST-03 (equipped with two current
meters at 8 m and 60 m above the sea bed respectively) at 3580 m
on the distal north-western side.
The direction of the bottom water flow is controlled by drift
topography, as shown by the mean current direction. The general
SW flowing circulation follows the isobaths and appears to be
geostrophically adjusted at least for a large part of the year (Fig.
2). The mean current velocity 8 m above the seabed is between
4cm s–1 at ST-03 and 6.2 cm s–1 at ST-01 (± 2.8 cm s–1), and speed
never exceeded 20 cm s–1. This flow is capable of transporting fine
sediment particles, but not of eroding the sediment. The potential
temperature is remarkably stable between 0.11 ± 0.01ºC and 0.13
±0.02ºC. These values, even in the absence of salinity records,
suggest that the bottom layer consists of modified Circumpolar
Deep Water (CDW), which constitutes the largest volume of
water in the Southern Ocean (Carmack 1990).
The observed bottom water flow is consistent with deposition of
Holocene hemipelagic sediments during a ‘drift maintenance’
stage. Indicators of palaeoceanographic conditions during glacial
periods are at present too scarce to fully understand how the past
oceanographic conditions influenced the evolution of the drifts.
353
Sediment drifts and deep-sea channel systems, Antarctic Peninsula Pacific Margin
M. REBESCO1, C. J. PUDSEY2, M. CANALS3, A. CAMERLENGHI1, P. F. BARKER2, F. ESTRADA3&
A. GIORGETTI1
1Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Borgo Grotta Gigante 42/c, 34010 Sgonico (TS),
Italy (e-mail: mrebesco@ogs.trieste.it)
2British Antarctic Survey, Madingley Road, Cambridge CB3 0ET, UK
3Universitat de Barcelona, Campus de Pedralbes, Barcelona E-08028, Spain
Abstract: Twelve sedimentary mounds are identified on the upper continental rise of the Pacific Margin of the Antarctic
Peninsula. All these mounds are produced by a varying degree of interaction of along-slope bottom water flow with down-
slope turbidity currents. These mounds provide a complete range of intermediates between two end members: the sediment
drift and the channel levee. Surface sediments on drift 7 suggest that the mechanisms for the supply and transport of
sediment include entrainment of material from turbidity currents within ambient bottom currents, and pelagic settling from
the sea surface, including biogenic and glacially derived material. The long-lasting activity of these mechanisms is docu-
mented by the data provided by four DSDP and ODP drill sites. Bathymetric and seismic data, both at a large, comprehen-
sive scale and at a small, detailed scale, show the geometry of the sedimentary mounds and their relationships with the
adjacent turbidity current channel systems. These data allow the determination of some diagnostic criteria to identify the
sediment drifts.
From: STOW, D. A. V., PUDSEY, C. J., HOWE, J. A., FAUGÈRES, J.-C. & VIANA, A. R. (eds)
Deep-Water Contourite Systems: Modern Drifts and Ancient Series, Seismic and Sedimentary Characteristics.
Geological Society, London, Memoirs, 22, 353–371. 0435-4052/02/$15.00 © The Geological Society of London 2002.
Table 1. Principal characteristics
Location: Antarctic Peninsula Pacific Margin
Setting: Upper continental rise
Age: Mid Miocene-Present
Drift type: Detached drift (with influence of
turbidity currents)
Dimensions: Up to about 200 70 1 km
Seismic facies and attributes: Asymmetric, with a gentler side
underlain by conformable reflectors
and a steeper side underlain by more
discontinuous reflectors with frequent
unconformities and erosional
truncations.
Sediment facies and attributes: Very fine-grained, showing glacial-
interglacial cycles in composition,
texture and sedimentary structures
(interglacials are bioturbated, contain
ice-rafted debris and biogenic silica and
carbonate; glacials are laminated clays).
SF27 Rebesco (to/d) 12/18/02 12:23 PM Page 353
Bathymetry
General bathymetry
The bathymetric map of Figure 3,which covers the entire conti-
nental margin from shelf to abyssal plain, shows the main physio-
graphic characteristics of this glacial system. Like other parts of
the Antarctic continental margin (Cooper et al. 1991) the conti-
nental shelf is over-deepened with a seafloor that generally dips
landward. Shelf bathymetric relief is high (up to 1000 m) due to
the presence of overdeepened basins (Domack et al. 1994;
Rebesco et al. 1998a) and glacial troughs carved by ice streams
(Pope & Anderson 1992; Pudsey et al. 1994; Canals et al. 2000).
Four main lobes, defined by oceanward-convex trends of the
354 M. REBESCO ET AL.
Fig. 1. Location map of the study area. Sediment drifts and mounds of the continental rise are shaded in light grey and progressively numbered from north
to south according to the system used by Rebesco et al. (1996). Deep sea channels, indicated by dark grey arrows, are newly named in this paper.
Prograding lobes of the continental shelf break are shaded in dark grey and numbered from north to south according to Larter et al. (1997). Glacial troughs
are indicated by black arrows. Main fracture zones are indicated by dashed lines. Seismic profiles are shown as black lines. Location of DSDP and ODP
sites and cores are also shown.
SF27 Rebesco (to/d) 12/18/02 12:23 PM Page 354
continental shelf break, can be correlated with prograding wedges
consisting of coalescing banks of the outer shelf (Larter & Cun-
ningham 1993; Larter et al. 1997; Rebesco et al. 1998b). These four
lobes are separated by large symmetric, U-shaped troughs with
gentle sides (Vanney & Johnson 1976a, b; Pope & Anderson 1992;
Rebesco et al. 1998b). The continental slope (about 500 to 3000 m
water depth) is very steep. The gradient of the four lobes (>13º) is
generally steeper than that of the slope at the mouth of the
troughs. Despite such steepness, there is no evidence of present-
day major slope failure, nor of major canyons cutting into the
slope.
The upper limit of the continental rise south of 67ºS is deeper
(almost 3500 m) than in the northeast part (around 3000 m) as a
consequence of a major step (in correspondence of the Tula
Fracture Zone) within the stepwise younging of basement age
northeastward along the margin (Tucholke & Houtz 1976). The
Bellingshausen Abyssal Plain lies below 4800 m (Vanney &
Johnson 1976a, b). The irregular relief of the upper continental
rise results from the 12 sediment mounds separated by deep-sea
channel systems, originating at the base of the slope between the
lobes. The channels are named for the first time in this paper
(Fig. 1).
Since some of the channels run parallel to the margin for tens
of kilometres before turning seaward towards the outer rise, the
sediment mounds are commonly separated from the continental
slope by a broad 10–20 km wide erosional depression. All the 12
mounds have a distinct bathymetric expression. Nine of them
were numbered progressively from north to south and interpreted
as sediment drifts, with varying degree of interaction of bottom
currents with downslope turbidity current processes (Rebesco et
al. 1994, 1996, 1998b; McGinnis & Hayes 1995; McGinnis et al.
1997). Three additional mounds (mounds 3A, 4B, and 5A) are
newly mapped here, mainly on the basis of swath bathymetry and
seismic data. All the sediment mounds share many common
features, though each one is different from the others. They are
preferentially located between the shelf lobes (i.e. in front of the
shelf troughs). They are elongated in a direction approximately
orthogonal to the margin, with a wider, thicker central body and
two narrow ends. The largest mounds have their summits at an
average depth of 2700 m, are up to 200 km long by 70 km wide,
and attain an elevation of nearly 1 km above the adjacent
channels. The majority of the mounds are asymmetric with a steep
side (typically sloping 2º) and a gently-dipping side (typically 0.8º)
that meet to form a long and narrow crest. Most mounds (drift 1,
2, 3, 4A, 6 and 7) have their steep side oriented toward the
southwest.
The mounds merge into the lower continental rise, at a depth of
about 3500 m northeast of the Tula Fracture Zone, and about 4000
m farther southwest. The lower continental rise north of the Tula
Fracture Zone is a gently sloping region where the 3600 to 4000 m
isobaths show an outward-convex curvature (Palmer deep sea fan
of Vanney & Johnson 1976a, b). The 4100 to 4600 m isobaths are
roughly SW–NE striking and correspond to a more uneven area
where numerous seamounts protrude through a thin drape of sedi-
mentary cover (Tucholke & Houtz 1976; Vanney & Johnson
1976a, b; Larter & Barker 1991a). The 4700 m deep South
Shetland Trench, limited to the southwest by the Hero Fracture
Zone, belongs to the active part of the margin and is not discussed
here.
Swath bathymetry
The detailed swath bathymetric map of Figure 4, collected during
the GEBRAP’96 cruise onboard BIO Hesperides covers the
margin west of Palmer Archipelago from the outer shelf (325 m
water depth) to the upper continental rise (3800 m).
The outer continental shelf is a relatively flat, landward-dipping
area crossed by glacial troughs, marked by iceberg scours and
including some shallow banks that correspond to till deltas
(Canals et al. 1998). Water depth ranges from 325 m to 450 m, with
the north-eastern sector being particularly uneven. The shelf edge
is sinuous with two main seaward convex lobes. These lobes cor-
respond to the prograding lobes 1 and 2 of Larter et al. (1997).
The swath bathymetry map reveals that, in detail, the northeast-
ernmost lobe (Lobe 1) is actually made up of two minor lobes.
Farther to the north of Lobe 1 there is another minor shelf lobe,
indicated by a change in the steepness of the continental slope.
The northeast trending continental slope of Lobe 1 is narrow (10
km) and very steep (up to 22°), following the sinuous outline of
the shelf edge. The slope is incised by small straight gullies that do
not extend up to the shelf edge. The depth of the base of the con-
tinental slope ranges from approximately 2500 m in the south,
down to 3000 m in the north.
The upper continental rise is characterised by five elongate
sediment mounds whose NW-trending crests are roughly orthogo-
nal to the shelf edge. Four of these mounds correspond to drifts 1,
2, 3, and 4 of Rebesco et al. (1996). A new mound (mound 3A) is
here identified between drifts 3 and 4. The largest imaged mound
is drift 3, some 125 km wide between the Palmer and South
Anvers channels to either side (Fig. 1). The gentler sides of the
mounds are mostly undisturbed, as exemplified by the level
surface north of the crest of drift 3 (Fig. 5). Conversely, the crests
and steep sides of the mounds are commonly affected by small
curved scarps. Drifts 1, 2, and 3 have a steeper SW side than NE
side. Drift 4, which apparently displays a steep NE side, is only
SEDIMENT DRIFTS AND DEEP-SEA CHANNEL SYSTEMS, ANTARCTIC PENINSULA PACIFIC MARGIN 355
Fig. 2. Location map of current-moorings on drift 7. See location in
Figure 1. The mean direction of the current measured 8 m above the
seabed is indicated with arrows at the location of the three moorings. The
inferred bottom water flow path along the isobath between the moorings
is shown by a dashed line.
SF27 Rebesco (to/d) 12/18/02 12:23 PM Page 355
partially imaged by the swath bathymetric survey. Mound 3A is a
relatively more symmetrical and subdued feature. These mounds
are separated by depressions, mostly parallel to the mounds’
crests, which contain erosional deep-sea channels with poorly
developed levees (Tomlinson et al. 1992). Slope gradients can
reach up to 10° locally in the channel margins. Three seamounts
of probable volcanic origin have been identified in the deepest
region of the northernmost mound.
Channel systems vary greatly both in shape and morphological
complexity. The southern systems (Renaud Channel between drift
4 and mound 3A, and Palmer Channel between mound 3A and
drift 3) consist of two slightly sinuous channels that originate at
the base of the continental slope. The Palmer Channel is wide and
only slightly incised, in contrast to the straight and strongly incised
Renaud Channel between drift 4 and mound 3A. The northern
channel system has a complex morphology, displaying a dendritic
pattern with many tributaries that extend back to the base of the
slope. Three main catchments, collecting many gullies and smaller
channels, feed the system, and evolve downslope into two main
channels: North Anvers Channel that runs between drifts 1 and 2,
and South Anvers Channel between drifts 2 and 3. These channels
finally converge to form the biggest channel in the area.
356 M. REBESCO ET AL.
Fig. 3. General bathymetric map of the western margin of the Antarctic Peninsula between 62–70°S and 60–80°W, from Rebesco et al. (1998b). A coloured
version of this figure, including both contours and data locations, is available at the web site of Terra Antartica (http://www.mna.unisi.it/TAP/mapcol.eps).
Contours in corrected metres. Contour interval: 50 m on the continental shelf (shallower than 500 m); 500 m on the continental slope (between 500 and
2500 m); 100 m on the continental rise (deeper than 2500 m). SM, seamount.
SF27 Rebesco (to/d) 12/18/02 12:23 PM Page 356
SEDIMENT DRIFTS AND DEEP-SEA CHANNEL SYSTEMS, ANTARCTIC PENINSULA PACIFIC MARGIN 357
Fig. 4. Multibeam coloured bathymetric map of the five northern mounds (contour interval 25 m). See location in Figure 1. The upper inset is a location
map superposed on the GEBCO bathymetry. The lower inset is an interpretation of the main morphosedimentary features.
SF27 Rebesco (to/d) 12/18/02 12:23 PM Page 357
Stratigraphic context
In piston and gravity cores, late Quaternary lithological cyclicity
has been related to glacial-interglacial cycles using diatom and
radiolarian biostratigraphy and barium stratigraphy (Pudsey &
Camerlenghi 1998; Barker et al. 1999; Pudsey 2000). Intervals of
diatom-bearing foraminifer-bearing mud alternate with intervals
of almost barren mud with poorly-preserved diatoms. Radiolari-
ans are common in core-top samples and are of omega zone age
(0–0.43 Ma; Hays 1965). The diatoms are also of late Quaternary
age (Thalassiosira lentiginosa zone, 0–0.6 Ma; Gersonde &
Burckle 1990), with Hemidiscus karstenii identified in isotope
stage 7. Foraminifera are also consistent with a Late Quaternary
age (Silvia Spezzaferri, pers. comm). Barium data show 900–1600
ppm biogenic Ba in the interglacial facies (see Section 6b) and
0–150 ppm in the glacial facies (Pudsey & Camerlenghi 1998;
Pudsey 2000).
The continental rise of the Pacific margin of the Antarctic
Peninsula was drilled at four sites during one DSDP leg and one
ODP leg (see Fig. 1), providing a Neogene stratigraphy.
Site 325 of DSDP Leg 35 (Hollister et al. 1976) was located in
3748 m water depth near the transition between the upper and
lower continental rise according to the recent definition of
Rebesco et al. (1998b). The main objective of drilling was to
establish the age of the oceanic basement, with palaeoceanogra-
phy as a side objective. Site 325 is located in the distal part of the
wide zone of channels that separate drift 5 and mound 4B. This
site penetrated 718 m of a sedimentary succession composed of
terrigenous turbidites and ice-rafted debris (Fig. 6a). The hole was
spot cored and recovered only 35 m of sediment from ten cores.
Lithologies were mainly clay/claystone to sand/sandstones with ice
rafted debris, with poorly preserved diatoms in places, rare nan-
nofossils in the lower cores and conglomerates at the base of the
hole. The upper lithologic unit (0–528 mbsf) is generally finer
grained than the lower one. The oldest sediment recovered was
claystone of Oligocene to Early Miocene age range. A major
hiatus was identified between the two lithologic units, from
approximately 15 to 8 Ma. This mid-Miocene hiatus has been
attributed to the reduction in terrigenous supply as a result of
margin uplift following ridge-crest subduction (Larter & Barker
1991b), and is therefore diachronous along the margin, but it can
be used to place limits on the timing of transition from ‘pre-drift’
deposition to ‘drift growth’ on the continental rise (Rebesco et al.
1997).
Sites 1095, 1096, and 1101 of ODP Leg 178 (Barker et al. 1999)
were located on the upper continental rise. The objective of
drilling was to recover a continuous high resolution record of
glacial processes occurring on the continental margin over the last
10 million years. Sites were located on drifts 7 and 4 because the
elevation of these sites above the channels was believed to shield
these sediments from high-energy turbidity current flows.
Sites 1095 and 1096 (Fig. 6b, c) were drilled on drift 7. The
entire sequence from the Mid-Miocene to Present was obtained
from two sites. Site 1096 recovered the more expanded upper part
of the section, down to the Early Pliocene (4.7 Ma) by penetrat-
ing 608 m in 3152 m water depth. The more distal Site 1095 was
located in 3842 m water depth at the transition between sediment
drift and turbiditic lobe. It sampled 570 m of a more condensed
succession at least down to the Late Miocene (10 Ma and possibly
older). Continuous coring, multiple-hole drilling, and excellent
magnetostratigraphic control provided a composite record of sedi-
mentation over the last ten million years.
Site 1101 (Fig. 6d) was drilled in 3280 m water depth on drift 4
in order to obtain a comparative lithostratigraphic record from a
more northerly position than the other two sites. 218 m of contin-
uously deposited fine grained sediment extending to the mid-
Pliocene (3.1 Ma) was sampled.
The uppermost few tens of metres of sediment (down to about
50 m) at the three sites, are composed of alternating grey ter-
rigenous laminated and brown massive, bioturbated, foraminifer-
and diatom-bearing silty clays. These lithologies are very similar
to those recovered in piston and gravity cores from all other
sediment drifts of this margin (Pudsey 2000), and indicate the
glacial–interglacial alternation of sedimentation. No evidence of
358 M. REBESCO ET AL.
Fig. 5. 3D digital terrain model of the continental slope and rise area of the five northern mounds, same area as Figure 4. View from west. Note the
variations in size of the various mounds, the distinct morphologies of the channel systems, the gullies that cut the continental slope, and the three volcanic
seamounts near the northeastern corner.
N
Seamounts
Drift 1
Drift 2
Drift 3
Drift 3A
Drift 4
50 km
SF27 Rebesco (to/d) 12/18/02 12:23 PM Page 358
turbiditic deposition can be found in these sediments, and depo-
sition appears to be dominated by redeposition of suspended fines
from terrigenous turbidity currents in deep sea channels during
glacials, and by hemipelagic/pelagic deposition during inter-
glacials.
Deeper in the sections colour changes are less obvious, and
parallel-laminated silt and mud turbidites become common. At
Site 1096, a mixed contourite-turbidite succession occurs from
32.8 to 173.0 mbsf, with generally low biogenic content. Turbidite
silts are thin, and subordinate to muds of likely contouritic origin.
From 143–608 mbsf, very thinly laminated and generally non-
bioturbated clays deposited from dilute turbidity currents
alternate with intensely bioturbated homogenous silty clays. At
Site 1095, thick and repetitive sequences of green laminated silt
and mud were recovered, becoming dark greenish grey laminated
claystones towards the base of the recovered section. Sharp-based,
graded, variably laminated fine sands and silts and laminated silty
clays, interbedded with more massive facies, represent a largely
turbiditic succession. At Site 1101, foraminifer-bearing layers
alternate with barren laminated or massive intervals between 53.3
and 142.7 mbsf. Deeper in the section, the biogenic component is
low, and massive clayey silt and diamict occur. Ice-rafted debris is
scattered throughout the facies drilled at the three sites, and
appears concentrated within bioturbated intervals.
SEDIMENT DRIFTS AND DEEP-SEA CHANNEL SYSTEMS, ANTARCTIC PENINSULA PACIFIC MARGIN 359
Fig. 6. Location of DSDP and ODP sites on
multichannel seismic profiles, with estimated
penetration on a two-way travel time scale.
See location in Figure 1. (a) DSDP Site 325
re-positioned on a modern seismic profile
(modified after Larter & Barker 1991b); (b),
(c), and (d) ODP sites located on Leg 178 site
survey profiles (Barker, Camerlenghi, Acton
et al. 1999).
SF27 Rebesco (to/d) 12/18/02 12:23 PM Page 359
In summary, deposition at these three sites was different,
ranging from dominantly hemipelagic on the drift crest and centre
to dominantly turbiditic at the distal site. All sites revealed a more
or less pronounced cyclicity in turbidite abundance, bioturbation
and ice-rafted debris, reflected in cyclicity in colour, magnetic sus-
ceptibility, and bulk density. This is considered to reflect the cyclic
provision of glacial sediments to the uppermost continental slope.
Sedimentation rates (see Fig. 7) were highest on the drift crest
(18 cm ka–1 in Unit III, Site 1096) and lowest on the distal flank
(5 cm ka–1 in the time equivalent units of Site 1095). At all three
sites, the rates decreased through the Pliocene and into the
Pleistocene (as low as 2.5 cm ka–1 in the Late Pleistocene of Site
1095). The gradual decrease in the rate of sedimentation observed
at all rise sites is believed to reflect an overall trend of decreasing
input of glacial sediment from the continental shelf, rather than
changing palaeoceanographic conditions (i.e. bottom current
direction and intensity) in the deep sea (cf. Barker 1995).
Seismic characteristics
Reflection profiles
The 900 km long composite multichannel seismic profile of Figure
8, striking parallel to the margin between 62°45S and 68°15S,
crosses the 12 sedimentary mounds, with the largest (drifts 6 and
7 in the southwest) attaining a relief of about 1 km. In addition to
the data in Figure 8, a considerable multichannel seismic
360 M. REBESCO ET AL.
Fig. 8. Composite multichannel seismic profile striking parallel to the margin. See location in Figure 1. This 900 km long profile is composed of six different
profiles acquired during four cruises of R/V OGS-Explora.
Fig. 7. Simplified lithostratigraphic logs of
DSDP and ODP sites on the continental rise
west of the Antarctic Peninsula, with main
chronostratigraphic ties. Note that stratigraphy
at site 325 is poorly constrained. Age of the base
of each site is also indicated.
SF27 Rebesco (to/d) 12/18/02 12:24 PM Page 360
SEDIMENT DRIFTS AND DEEP-SEA CHANNEL SYSTEMS, ANTARCTIC PENINSULA PACIFIC MARGIN 361
Fig. 9. Seismic profile I95-130a showing the crest of drift 6. The internal structure of the drift is different on either side. The steeper side (SW) is
characterized by high reflectivity, abundant terminations and undulations of reflectors. A prominent change in reflectivity parallel to the sea bottom is
particularly evident on this side of the drift. In contrast, the gentler side (NE) is relatively smooth, and underlain by continuous, highly reflective units, with
linear, parallel or sub-parallel internal reflectors conformable with the sea floor.
SF27 Rebesco (to/d) 12/18/02 12:24 PM Page 361
reflection coverage exists for this margin, acquired mainly on
board R/V OGS Explora in 1990, 1992, 1995, and 1997 but with
minor contributions also from UK, USA and Spanish cruises. The
northeastern half is covered also by GLORIA sidescan and single-
channel (watergun) seismic survey reported by Tomlinson et al.
(1992) and Rebesco et al. (1996).
The majority of the mounds (drifts 1, 2, 3, 4A, and 7) have an
asymmetric external shape with a steeper, rougher SW side and a
gentler, smoother NE side. Two mounds (drifts 5 and 8) have the
opposite geometry (NE sides are steeper). Drift 6 and possibly
drift 4 both consist of a concave-up plateau formed by the gentler
sides of two coalesced drifts. The remaining three mounds
(mounds 3A, 4B and 5A) are smaller and more symmetrical. Dip
seismic profiles show that the mounds generally have gently-
dipping NW sides merging with the lower continental rise, and
steeply-dipping SE sides facing the continental slope.
The internal structure of these mounds is different beneath the
gentle and the steeper sides (Fig. 9). The steeper sides are charac-
terized by high reflectivity, abundant lateral terminations and
undulations, and sea-floor terminations of reflectors, by either
erosion or non-deposition. Prominent changes in reflectivity
parallel to the seafloor, probably produced by a diagenetic change
362 M. REBESCO ET AL.
Fig. 10. Seismic profile I95-130a showing Alexander Channel that separates drift 6 and 7. The acoustic facies of the channel levee systems is very different
from that of the drift. The channel system, composed by two branches, is evidenced by high amplitude, discontinuous, reflectors at its floor, surrounded by
transparent facies. A levee, evident on the NE of the channel, is characterized by relatively well stratified and rapidly wedging-out deposits. Older, buried
channels with dimensions comparable to the modern one are possibly detectable within the inter-drift area.
SF27 Rebesco (to/d) 12/18/02 12:24 PM Page 362
deep in the sediments, are particularly evident on the steep sides
of the mounds. In contrast, the gentler sides are relatively smooth,
and are underlain by continuous, highly reflective units, with
linear, parallel or sub-parallel internal reflectors conformable with
the sea floor.
The mounds are separated by deep-sea channels traversing the
deep areas between mounds. Channels are only evident between
the mounds and not on the mounds themselves. The acoustic
facies of these channel levee systems is very different from that of
the mounds. Channel floors are characterised by high amplitude,
discontinuous, reflectors, surrounded by transparent facies (Fig.
10). Levees, where present, are shown by relatively well stratified
deposits abruptly wedging out at the side of the channel. Buried
channels with dimensions comparable to the modern ones are
detectable in seismic profiles within the three upper sequences.
They show a limited lateral shift and are confined to the inter-
mound areas. In the three deeper seismic sequences, the channels
are replaced by enigmatic northeast dipping seismic reflectors that
appear to cut across other horizons. These are interpreted to
represent the traces of local, diachronous hiatuses of limited
temporal extent enclosed within continuous depositional areas
(Rebesco et al. 1997).
The history of sedimentation on the continental rise was recon-
structed by Rebesco et al. (1997), who identified six major seismic
units. Units were dated (very approximately in some cases) by
correlation with DSDP Site 325 and known tectonic events, and
by regional climatic events. This history is summarized in a three-
stage scheme, as follows: (1) a ‘Drift-maintenance Stage’ (5 Ma to
the present) characterized by preservation and enhancement of
the elevation of the drifts; (2) a ‘Drift-growth Stage’ (15–5 Ma)
showing substantial variations in thickness as a consequence of
increasing bottom current activity and larger glacial sediment
supply from the margin; (3) a ‘Pre-drift Stage’ (36–15 Ma) charac-
terized mainly by subparallel reflectors representing a dominantly
turbiditic sequence.
Sub-bottom profiles
The forty parametric source (TOPAS) profiles acquired during
the GEBRAP’96 cruise onboard BIO Hesperides show that the
outer continental shelf and slope are characterized by opaque and
hyperbolic acoustic facies, while the continental rise displays
mainly parallel stratified and chaotic acoustic facies, supple-
mented by opaque and hyperbolic facies. The mounds are mostly
represented by parallel stratified facies. The generally high lateral
continuity of this facies is locally interrupted by sheet and lens
shaped bodies characterized by chaotic to transparent facies (Fig.
11). The crests of the mounds commonly have small scarps on
both sides. Channel heads are mainly occupied by opaque and
hyperbolic acoustic facies (Fig. 12a). Channels become highly
erosive downslope (Fig. 12b), although they can also be filled and
smoothed by massive deposits showing chaotic and transparent
facies (Fig. 12c). Opaque acoustic facies prevail in the deepest,
widest channel reaches (Fig. 12d).
Sediments
Sediments: seabed photos
Studies of bottom photographs from the area by Hollister &
Heezen (1967), Sullivan et al. (1973) and Dangeard et al. (1977)
showed evidence for strong bottom currents (in the form of aligned
motile organisms, partly obliterated bioturbation structures, and
scours round small obstacles) on the continental slope near
Adelaide Island and in the vicinity of the Polar Front. Turbid
bottom water was observed on the lower continental rise west of
90°W. Elsewhere on the continental rise, bottom photographs
showed abundant tracks, trails and faecal structures, with scattered
ice-rafted pebbles, suggesting weak bottom currents (Fig. 13).
Sediments: core description and facies
A total of 17 gravity cores have been collected from drift 7 and 15
from the other drifts (Camerlenghi et al. 1997b; Pudsey & Camer-
lenghi 1998; Pudsey 2000; Lucchi et al. in press). The sediments
cored on drift 7 are predominantly terrigenous in composition and
very fine-grained; sediment facies confirm the extent of the drift
inferred from seismic data. Cores show a cyclicity between brown,
bioturbated, diatom-bearing mud with foraminifera (core tops and
additional thin units downcore) and grey, laminated, barren mud
(thicker units between the brown layers). Contacts between
brown and grey units are gradational and bioturbated. Ice-rafted
debris is present but generally sparse. Cores on the steep sides of
the drift recovered a condensed section with thinner cycles and
probable hiatuses.
SEDIMENT DRIFTS AND DEEP-SEA CHANNEL SYSTEMS, ANTARCTIC PENINSULA PACIFIC MARGIN 363
Fig. 11. Parametric source (TOPAS) profile
across the crest of drift 3. Note the dominant
parallel stratified facies, and the chaotic and
transparent facies near the surface on the NE
side, related to local lateral instability. Vertical
exaggeration x 33. See location in Figure 1 (see
also Figures 4 and 12).
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364 M. REBESCO ET AL.
Fig. 12. TOPAS profiles illustrating the inter-drift channelled drainage systems on the continental rise, from upper to lower channel reaches. (a) Hyperbolic facies typical of the uppermost reaches of small
erosive channels. (b) Erosive channel showing chaotic and hyperbolic facies in the thalweg and southwest wall; the east wall is slumped. (c) Mud-flow filled channel reach; post-flow erosion in the thalweg is also
apparent. (d) Distal channel 12 km wide, characterized by chaotic, opaque and hyperbolic facies; inner small channels eroding the main channel floor can be also observed. Vertical exaggeration x 33. See
location in Figure 1, see also Figure 4.
SF27 Rebesco (to/d) 12/18/02 12:24 PM Page 364
The top of each core (Unit A of Pudsey & Camerlenghi 1998)
consists of 0.1–0.2 m of structureless olive brown diatom-bearing
mud with common dispersed IRD, overlying 0.2–0.4 m of olive
brown to grey or dark grey bioturbated, faintly laminated mud.
Carbonate content (foraminifer fragments) is 0.4–0.8% and
organic carbon up to 0.4%, both decreasing downwards in the
unit. The burrows are mainly of Planolites type and parallel to
bedding.
Below Unit A, grey and dark grey silty clay forms a thick unit
(2.2–5.5 m thick) in each core, designated Unit B. Parallel to
lenticular lamination and thin bedding are outlined by slight
contrasts in colour and in X-ray absorption. The lamination is
rather indistinct and irregular; there are neither sharp-based
graded units nor any signs of erosion. Bioturbation is rare and the
burrows are very small (1–2 mm). Some distinctive reddish
marker layers 1–5 cm thick can be correlated from core to core; in
X-radiographs, these layers are seen to be laminated on a sub-
millimetre scale. Wavy lamination is common in the upper part of
Unit B in most cores, while the lower part tends to be more homo-
geneous. IRD is less common than in Unit A and occurs in thin
layers (3–15 mm) rather than being dispersed. Unit B is thickest
near the centre and north-east side of the drift, and thins markedly
to the southeast, southwest and northwest.
The next brown unit, Unit C, attains a thickness of 1.0 m in
cores 6 and 7; in core 9 its apparent base may be a hiatus (Fig. 14).
It consists of light olive brown mud (8–12% diatoms, 4–6%
foraminiferal carbonate) overlying olive grey to greyish brown
mud (2–5% diatoms, 1% carbonate) then olive brown to light
SEDIMENT DRIFTS AND DEEP-SEA CHANNEL SYSTEMS, ANTARCTIC PENINSULA PACIFIC MARGIN 365
Fig. 13. Seabed photos taken in 1963–1964
from USNS Eltanin (Goodell 1964, 1965). (a)
Eltanin 5–16 (63° 58S, 67° 56W, 2950 m water
depth) Pale brown clayey silt (4.3% sand,
71.1% silt, 24.6% clay). Many echinoid trails
and worm tracks can be seen. Sparse pebbles,
no evidence for current activity; (b) Eltanin
5–22 (65° 06S, 70° 41W, 3109 m) Dark
yellowish brown clayey silt (7.0% sand and
gravel, 60.7% silt, 32.3% clay). Echinoid trails
and worm tracks similar to above; note also
patchy distribution of coarse angular ice-rafted
debris.
SF27 Rebesco (to/d) 12/18/02 12:24 PM Page 365
366 M. REBESCO ET AL.
Fig. 14. Detailed core logs from visual descriptions, dip transect on drift 7 (Fig. 1). Only the larger ice-rafted pebbles are shown. Units A–D were defined
using sediment colour changes combined with magnetic susceptibility data. ‘Pre-C’ denotes older sediments below a sharp contact at the base of unit C
Also: X-radiographs of (a) homogeneous and (b) layered IRD occurrence in core 9. The photographs illustrate the central 6 cm of the 9 cm core diameter.
The small black dots in (a) are manganese micronodules; the white lines are coring-induced fractures. From Pudsey & Camerlenghi (1998).
SF27 Rebesco (to/d) 12/18/02 12:24 PM Page 366
SEDIMENT DRIFTS AND DEEP-SEA CHANNEL SYSTEMS, ANTARCTIC PENINSULA PACIFIC MARGIN 367
Fig. 15. Grain-size data, measured by sieving (sand %) and Sedigraph (silt and clay). (a) Cores 4 and 7, spliced together at top of Unit C (arrow). Size frequency histograms of the fine fraction (100% = total
sediment) at 0.25class interval at core top (0.01 m), 3.99 m and 5.58 m correspond to the star symbols on the downcore plot. Note that there is 30–45% of material finer than the Sedigraph measurement limit
of 11. There is commonly a mode near the silt-clay boundary. (b) Core 6, including size frequency histograms for units C and D. The sample at 5.28 m contains 38% sand. (c) Core 9, most of Unit C shows
negative skewness. The very firm sediments underlying Unit C are all very poorly sorted, but have variable sand:silt:clay ratios and median diameters. They have more medium and coarse silt (10–15% in the
range 4–6) than the other cores, and most samples have a mode in the range 5.5–6. From Pudsey & Camerlenghi (1998).
SF27 Rebesco (to/d) 12/18/02 12:24 PM Page 367
olive brown mud (20–27% diatoms, 4–16% carbonate). The
planktonic foraminifer Neogloboquadrina pachyderma (sinistral)
is the most common species. Colour transitions are gradational,
and any original sedimentary structures have been obliterated by
bioturbation. Dispersed IRD is common (Fig. 14c).
Core 6 (lowest 1.6 m) recovered another unit of grey laminated
mud, Unit D, also seen in the lowest 0.1 m of core 7. Unit D is
faintly laminated and bioturbated and the contact with Unit C is
gradational and burrowed. IRD is common and occurs mainly
dispersed in the upper half of the unit, and in thin layers in the
lower half. Additional cores from the distal part of drift 7 and in
the adjacent channels contain coarse-grained turbidite beds.
Sediments: summary analytical results
Particle size analyses on selected cores from drift 7 show the
sediments are fine-grained and very poorly sorted. We present the
data as downcore plots of sand-silt-clay, median diameter, sorting
and skewness, with representative size frequency histograms of
the fine fraction, in Figure 15.
Unit A was measured in all core tops except core 9, and
downcore in core 4 (Fig. 15a). It contains 2–4% sand, 21–29% silt
and 67–77% clay. There is no evident relationship between grain
size and position of the core on the drift. Median grain size is
about 10 , there is a mode in the range 8–9 and sorting is very
poor (Gabout 3).
Unit B generally has less than 1% sand, 21–28% silt, and >70%
clay (Fig. 15a, b). Median grain size is 10–10.5 and the mode is
in the range 7.3–8.4 with very poor sorting (Gabout 3.5). Some
samples have up to 32% silt with a weak secondary mode at
6–6.5 . Throughout Units A and B, the proportion of medium
and coarse silt (4–6 ) is very low, generally <8% of total
sediment. Unit C contains 2–6% sand (locally up to 30–40%),
30–37% silt and 60–68% clay. Up to half of the sand is coarse
(>0.5 mm), angular and interpreted as ice-rafted. Core 6 has
somewhat less silt and more clay. Median grain size is 9–9.5 and
the mode is at 7.5–8 , commonly with a secondary mode at
5.5–6 . Sorting is very poor with Gof 2.5–3.5.
Unit D in core 6 has generally less than 1% sand except in two
thin layers at 4.58 and 5.28 m, where both the sand and the fine
fraction are extremely poorly sorted (Fig. 15b). Elsewhere silt
forms 18–28% and clay 72–82%. Median diameter is 10–11.2 ,
mode 8–8.5 and sorting is very poor (G= 3–3.5).
Discussion
Supply and transport of sediment in the late Quaternary
As shown by core data, drift 7 sediments are predominantly ter-
rigenous, very fine-grained, very to extremely poorly sorted, and
generally lack a mode in the silt size range. These features point
to deposition from suspension with negligible current winnowing.
In particular, the positive (fine) skewness throughout most cores
attests to the absence of any process that removed fine material.
The indistinct, parallel to lenticular lamination in Units B and D
suggests small and irregular fluctuations in the supply and
transport of suspended mud. The style of lamination is very
similar to ‘plumites’ on the Labrador continental margin (Hesse et
al. 1997). Such lamination may also have been present in Units A
and C, prior to thorough bioturbation. These sediments are tran-
sitional between hemipelagites and muddy contourites. The
absence of graded laminated units (see Stow & Bowen 1980), or
indeed of any sharp-based silt-sand beds except in condensed
sections from the steep sides of the drift, indicates a turbidite
origin is unlikely.
The similarity of fine-fraction size distributions downcore
suggests there was little variation in bottom current strength
between glacial and interglacial parts of cycles. In each core, unit
C has the coarsest median grain size and least poor sorting, which
is evidence for marginally stronger currents in Unit C time. Grain-
size variations that are more clearly attributable to bottom-
current activity occur only in cores on the steep sides of the drift,
where sandy, negatively skewed samples may have had fine
material winnowed out by currents. Their texture contrasts with
the unsorted sandy muds in Unit D in core 6, which have near-
zero skewness and are thought to result from ice-rafting without
current sorting.
The benthic nepheloid layer can be supplied with suspended
sediment by a number of mechanisms, including: (1) current
erosion farther upstream; (2) entrainment of material from
turbidity currents; and (3) pelagic settling from the sea surface,
including biogenic and glacially derived material (McCave 1986).
On the Antarctic Peninsula Pacific margin, bottom currents are
weak and rather steady, so mechanism (1) is probably insignifi-
cant. Mechanisms (2) and (3) are both important, and the amount
and type of sediment supply are likely to have varied over glacial-
interglacial cycles (Pudsey & Camerlenghi 1998).
Glacial Unit B is thickest near the centre and northeast side of
drift 7, and thins to the southeast, southwest and northwest. This
suggests that most fine sediment was supplied to the contour-
following current and nepheloid layer near the slope, or from the
channel on the north-east side of the drift (consistent with the
entrainment of the fine fraction of turbidity currents; Fig. 16). The
relative importance of supply to the nepheloid layer by turbidity
currents and by meltwater plumes is, as yet, unknown.
Large scale geometry and depositional model
As pointed out by Rebesco et al. (1996), the largest mounds are
most plausibly interpreted as sediment drifts. They cannot be
explained as large levees on the northeastern side of the channels,
because the Coriolis effect would cause overbank deposition
southwest of the channels, with a short, steep northeast slope,
which is the opposite of what we observe.
A generic model (Fig. 17), applicable to the (Plio-Pleistocene)
drift maintenance stage, and most probably also to the preceding
stage of rapid drift growth (late Miocene), was proposed by
Rebesco et al. (1997). It takes into account the entrainment of
material from turbidity currents as the major source of supply to
the benthic nepheloid layer. The model shows a section through
the axis of a progradational lobe and the adjacent drift during
glacial maximum when a grounded ice stream transported
unsorted basal till to the continental shelf edge. Small-scale
slumps on the uppermost slope undergo downslope transition into
debris flows, then turbidity currents feeding the main channel via
tributaries on the uppermost rise. Suspended fines are entrained
in SW-flowing bottom currents and deposited down-current to
develop and maintain the drifts. However, subsequent turbidity
current flow in the channels and slope instability on the steeper
drift slopes tend to remove sediment from those areas, leaving a
permanent sediment increment only on the gentle sides of the
drifts.
The 12 mounds are hence generally interpreted as sediment
drifts controlled by along slope bottom-water flow with varying
degree of interaction with downslope turbidity current processes.
These mounds provide a complete range of intermediary steps
between two end members: the sediment drift (best represented
by drift 7) and the channel levee (best represented by mound 5A,
the SW levee of the north Tula channel).
The criteria that we consider as diagnostic to distinguish
sediment drifts from channel-levees are the following:
(a) asymmetry: drifts have one side distinctly steeper than the
other;
(b) orientation of the steep side: in the drifts, the steep side is the
368 M. REBESCO ET AL.
SF27 Rebesco (to/d) 12/18/02 12:24 PM Page 368
SEDIMENT DRIFTS AND DEEP-SEA CHANNEL SYSTEMS, ANTARCTIC PENINSULA PACIFIC MARGIN 369
Fig. 16. Inferred sediment transport
processes during glacial periods in the area
of drift 7. See location in Figure 1.
Downslope flow shown as open arrows,
alongslope flow shown as black arrows
where measured, dotted where inferred
(after Pudsey & Camerlenghi 1998).
Fig. 17. Synthesis cartoon model of the
depositional and oceanographic processes
inferred to occur along the Antarctic
Peninsula Pacific margin during a glacial
maximum (after Rebesco et al. 1997).
SF27 Rebesco (to/d) 12/18/02 12:24 PM Page 369
370 M. REBESCO ET AL.
SW one, hence not facing the NE upstream (feeding)
channel;
(c) large dimensions: drifts are generally wider than 40 km;
(d) large elevation above the adjacent channels: the crest of the
drifts rises at least 400 m above the adjacent thalweg;
(e) internal geometry: drifts have continuous, subparallel, con-
formable reflectors beneath the gentler side, and truncated,
chaotic reflectors beneath the steeper side;
(f) relationship with the adjacent continental slope: drifts are
mostly separated from the slope by erosive depressions;
(g) location: drifts are preferentially located in correspondence
to the present-day shelf troughs, in between the shelf lobes.
This work has been funded by the Progetto Nazionale Ricerche in
Antartide (PNRA) through the SEDANO (Sediment Drifts of the
Antarctic Offshore) Project, the British Natural Environment Research
Council (NERC), and the Spanish cooperative project 99120 from
‘Comisión de Intercambio Cultural, Educativo y Científico entre España y
los Estados Unidos de América’. Kristeen Roessig at the Antarctic
Research Facility, Florida State University, kindly supplied the photo-
graphs in Figure 13.
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