ContinentalShelfResearch,Vol. 11, Nos 8-10, pp. 897-937,1991.
Printedin GreatBritain.
0278-4343/91$3.00+ 0.00
¢~)1991PergamonPresspie
T o w a r d s an u n d e r s t a n d i n g o f s e d i m e n t d e p o s i t i o n o n glaciated
continental shelves
JAMES P. M. Svvrrsgt*
(Received 22 January 1990; in revised form 2 October 1990; accepted 25 February 1991)
Abstraet--A simple vertical stratigraphic sequence of Quaternary deposits is recognized on 20
glaciated shelves from around the world reflecting the transition from the most recent glacial to
interglacial period. This sequence includes some or all of the following: (1) ice-contact (ice:
deposited and/or ice-loaded) sediments; (2) ice-proximal sediments; (3) ice-distal sediments; (4)
paraglaeial coastal sediments; arid (5) post-glacial sediments. Typically, the sequence unconformably overlies sedimentary rocks on the outer shelf and a mixed bedrock basement on the inner
shelf. The relative volume of these deglacial units provide important clues for the reconstruction of
ice sheet dynamics. Deposition of unit (1) occurred initially during the period of globally (eustatic)
lowered sea-level, and later with units (2) and (3) during a period of high sea levels related to
isostatic loading. Deposition of unit (4) occurred during the initial fall in sea level (isostatic
recovery) when ice had retreated and was rapidly ablating. Deposition of unit (5) occurred during
more complex and local sea level fluctuations, that include such effects as collapse of the crustal
forebulge. Minor variations to this association of sea level and sedimentation pattern occur,
particularly if the glacial cycle is out of phase with the global eustatic cycle. Outstanding seismostratigraphic problems include: distinguishing ice-loaded glacial-marine sediments from subglacially deposited till; distinguishing till from other ice-contact deposits such as grounding-line
fans; and distinguishing till from non-glacial debris flow deposits. The incorrect identification of till
provides poor data control on ice sheet reconstruction, ice sheet properties and dynamics,
especially as related to sediment transport.
INTRODUCTION
QUATERNARYice sheets have directly affected sediment deposition on 55% of the Earth's
continental Shelves. The volume of Quaternary sediment found on these shelves is roughly
1.2 × 1 0 6 km 3 (average thickness 30 m, range 0-1.8 km, over an area of 40 x 106 km2).
Although the composite age of this sediment is variable, most of these shelf deposits are
considered to be associated with the advance and retreat of the Late Pleistocene ice sheets.
In contrast, glacially-derived sediment located along continental slopes reflects the much
longer period of multiple glaciation throughout the Pleistocene (BOULrON, 1990), and
since the Oligocene in the case of Antarctic slopes (Burrt~aWOaTIa, 1990). Together these
volumes of sediments far exceed the extrapolated modem delivery rate of fluvial sediment
to all of the world's continental shelves at 10 km 3 a -1. Yet only 9% of this fluvial load is
presently delivered to these glaciated shelves (data from MILLIMANand MEADE, 1983).
This sedimentation imbalance is accounted for by Quaternary deposition associated with
the advance and retreat of ice sheets (POWELLand ELVERrIOI,1989).
*Geological Survey of Canada, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada.
897
898
J.P.M. SYvrrsto
To understand this radical change in sedimentation regimes, we must address the
environmental conditions associated with the growth and decay of ice sheets. This
information will also help in the management of offshore resources, including mineral
prospecting (aggregates, placer deposits), the interaction of seabed lithology and fisheries,
and the development of offshore installations (communication networks and pipelines).
The petroleum industry has long had problems drilling on glaciated shelves, due in part to
subsea horizons of boulders and gas hydrates, but also because of icebergs furrowing the
seabed and impacting seafloor installations. Artificial islands constructed as ice-protected
drilling platforms, in the Beaufort Sea for instance, can thaw the underlying permafrost
layers and liquefy the supporting medium. Exploration companies must also understand
the glacial history of high latitude margins so as to ascertain the impact of isostasy on
petroleum reservoirs. In the Barents Sea, Late Cenozoic glaciations have caused cyclic
loading and unloading and tilting of oil and gas reservoirs (A. ELVERHOI, personal
communication, 1990).
Although research on glacial marine environments has had a long history (e.g. PHILLIPI,
1910), most of the literature and research post-dates 1975. At present there is a plethora of
conceptual stratigraphic models used to describe the sedimentation processes on continental shelves typically based on modern analogues of ice shelf or tidewater environments.
This paper sets out to review and comment on: (1) the role of using modern glacial settings
as analogues to describe glacial-marine deposits; (2) our understanding of the physics of
water-ice-sediment interactions through the development of numerical models; (3) the
methods of sequence stratigraphy; (4) the terminololgy used in the study of stratigraphy of
glaciated continental shelves; (5) the stratigraphy recognized on glaciated continental
shelves; and (6) some of the outstanding problems hampering the development of a unified
stratigraphy on these glaciated shelves. The paper places emphasis on northern continental shelves in general, and Canadian shelves in particular, as other reviews on glacialmarine sedimentation have been more concerned with Alaskan and Antarctic continental
shelves (MoLNIA,1983a; ANDERSONand MOLNIA,1989). A special focus is provided on
problems in the interpretation of marine diamicton.
THE ROLE OF THE ANALOGUE
A principal method employed by sedimentologists is to relate older sedimentary
deposits to modern analogue environments, so as to understand the types and magnitude
of earth processes that once operated within the palaeo-environment. Contemporary
glacial-marine studies provide information on the size and movement of glaciers, conditions at the grounding line, the thermal and hydrologic regime of ice sheets, the
terrestrial and marine climate, and the dynamics of iceberg calving (e.g. Antarctica:
JACOBS,1989; Svalbard: ELVERHOIet al., 1989; Baffin Island: ANDREWSand SYvrrsrd, in
press; Alaska: POWELL and MOLNIA, 1989). However, modern glacial environments
largely reflect the waning stages of ice sheet retreat, and palaeo-ice sheets may have
operated under conditions different from contemporary glaciated shelves (HUGHES,
1987).
The ice shelf analogue
One early model of glacial-marine sedimentation (CAMPYand AH~,D, 1961) suggested
Antarctic-style ice shelves as an appropriate modern analogue environment for the
Sedimentdepositionon glaciatedcontinentalshelves
899
interpretation of older glacial-marine sedimentary sequences. The model considered two
scenarios: (1) dry-base ice shelves that deposited marine pebbly mudstones subaqueously
(waterlain till); and (2) wet-based ice shelves associated with thick sequences of bulldozed
flow-till, partially redistributed by meltwater plumes and turbidity currents. Recently,
MOLNIA (1983) and EYt.ES et al. (1985) have argued that this early model has lead to an
overemphasis on ice shelves in explaining glacial-marine sequences. Ice shelves are limited
to deep water due to buoyancy considerations. They also depend on properties of the ice
such as high velocities and low calving rates, geometric constraints including valley walls in
the case of floating ice tongues, and pinning points in the case of more open ocean ice
shelves.
Ice shelves have been used to explain the conformable nature of some glacial-marine
deposits, a result of the spread of buoyant plumes under the ice (a low energy environment). However, as argued by Sx,vrrs~a and Pr.AEG(1989), a number of factors combine to
provide a similar sedimentary environment near tidewater glaciers. Well-stratified waters
and low bottom currents form as a result of winter sea ice cover and the summer input of
buoyant meltwater which together limit the action of tidal and wave-induced currents near
the seafloor. By definition, the presence of waterlain till suggests the presence of an ice
shelf, where basal glacial debris is released from the sole of the glacier seaward of the
grounding line (I-IAMB~v et al., 1989). The deposition of waterlain till was considered by
DREWRVand COOPER(1981) to occur over tens Of kilometres from the grounding line. New
evidence from iced cores collected through ice shelves (P. BAm~rr, 1990, personal
communication) suggests that basal debris is deposited even closer to the grounding line.
Thus, the sedimentation regimes of tidewater glaciers and ice shelves may be more
similar than we once imagined. One difference is that the height of the debris pile fronting
a tidewater glacier is not constrained by a floating ice ceiling: such a pile could grow in size
to form a moraine. In the case of the ice shelf environment, the deposition of sediment is
confined to seaward of the grounding line, thus forming a diamict apron ( ~ l ~ y
et al.,
1990). Another difference between the two regimes is that the seafloor fronting a
tidewater glacier can be affected by iceberg calving processes and the release of supraglacial material. Dv.EwRv and CooPER (1981) also note that tidewater glaciers have a higher
sediment content at their ice termini and in their calved icebergs, than do ice shelves and
their calved icebergs. This suggests that subglacial basal debris may be released, due to
iceberg rafting, over greater distances in a tidewater environment than in an ice shelf
environment.
Arctic ice shelves, such as the Milne and Ward Hunt, are a Holocene phenomenon
formed initially through expansion of cold-based ice through fiords and into a coastal
environment protected by landfast ice (JEFrPaES, 1987). Their mode of formation has
increased speculation on the possibilities of ice caps formed within the marine environment on continental shelves (A. ELVEmt0I, 1989, personal communication; A. J~Nr~ir~os,
1988, personal communication). In these scenarios, an ice shelf would grow in thickness,
due to both snow accumulation on their surface and freezing of seawater to their base, until
the base of the ice shelf reached the seafloor to create a marine-based ice sheet. There is
also some speculation that the Antarctic ice shelves may be a Holocene phenomena, a
response of increased ice sheet bouyancy as a consequence of Late Quaternary global sea
level rise (see Ar~DERSONet al., in press).
In certain circumstances, our observations of modern analogues are not able to directly
help in the reconstruction of palaeo-ice sheet dynamics, for theory and data appear at
900
J . P . M . SYvrrsKi
sea
level
Fig. 1. Stable margins of an ice sheet advancing into the marine environment (after HUGHES,
1987): (a) terrestrial melting margin; (b) intertidal margin; (c) tidewater margin; (d) grounding line
of a confined ice shelf; (e) calving front of a confined ice shelf; (f) grounding line of an unconfined
ice shelf; and (g) calving front of an unconfined ice shelf.
odds. In a recent example, ANDREWS (1990) employs a novel hypothesis to explain the
great thickness of sediment located in Arctic fiords. He suggested that during glacier readvance into the fiords, rapid iceberg calving in association with pervasive seasonal fast ice
and iceberg jamming within the sill regions of fiords resulted in the formation of an ice shelf
and possible preservation of older sediments within the fiord basin. This is supported by
the seismo-stratigraphic interpretation of GILBERT(1985), although SYvrrsKI and HEIN
(1991) and STRAVERSand SVVITSKI(1991) provide sediment mass balance arguments to
suggest that these fiord basinal sediments reflect tidewater sedimentation from the last
glaciation.
Hu6rms (1987) suggests that ice sheets advance into the marine environment typically
with an ice shelf at their leading edge (Fig. 1). However this cannot be a general rule for
many tidewater termini are advancing today (POWELL, in press). We therefore are
beginning to recognize that ice shelves are neither necessary nor always appropriate in
explaining the deposition of sediment on glaciated continental shelves. Even the role of ice
shelves in depositing much of the sediment on the Antarctic continental shelves is hotly
debated. For instance, LINDSTROM(1988) provides a model of glacial-marine sedimentation that does not give prominent play to the role of ice shelves in Antarctic sedimentation. This may be contrasted with a conceptual model of HAMBREYet al. (in press) that
invokes ice shelves in order to explain the presence of what is identified as thick sequences
of waterlain till.
Marine Quaternary deposits located on Canadian continental shelves have had a wide
range in interpretations: from sedimentation under extensive ice shelves surrounding the
outer continental margin, to that associated with ice margins restricted close to the
shoreline (FULTON, 1984). Ice sheet reconstruction models of LINOSTROMand MACAYEAL
(1986) and DENTON and HU6HES (1981) suggest the formation of extensive ice shelves
around the North Atlantic and Arctic oceans. Yet field Observations of calving rate vs
water depth relationships (PowELL, 1988) argues strongly against the unpinned ice shelves
suggested by the models. The North Atlantic experiences swells as high as 10 m and
breaking storm waves in excess of 30 m: from purely mechanical considerations unpinned
ice shelves could not withstand such forces. The ice shelf proponents have offered no
Sediment depositionon glaciatedcontinental shelves
901
supporting stratigraphic data (BOULTON, 1979) and fossils suggest warm water conditions
existing under the location of some proposed ice shelves (AKsu, 1985).
The tidewater ice terminus analogue
At present, most glaciers with marine termini are tidewater in nature. To understand the
rate of sedimentation and the distribution of sediment at or near the margin of tidewater
glaciers, POWELL (1983, 1984, in press) identified the following parameters: iceberg
production (a function of ice regime, crevassing, water depth and flow velocity), retreat
rate of the glacier terminus (a function of ice flow, melting, evaporation/sublimation and
calving), debris distribution within the terminus (a function of basin relief, glacier thermal
regime, and ice flow dynamics: EYLES et al., 1985), meltwater stream hydrology, and
oceanographic conditions both near and distal to the ice terminus (Svvrrsra, 1989a). Water
depth near a tidewater glacier will depend on regional tectonism, isostasy, sedimentation
and eustasy. Sedimentation near the grounding line will depend on both sediment yield,
grounding line geometry and sediment dispersal processes. POWELL(in press) considers six
mechanisms for sediment to accumulate at a grounding-line: subglacial meltout and
lodgement, subglacial squeeze and push, conveyor-belt recycling, meltout of englacial
debris, dumping of supraglacial debris, and discharge of subglacial streams. The role of
dynamic sea level fluctuations was given more prominence in the later models of glacialmarine sedimentation (ANDERSOr~et al., in press; Svvrrsra and PP,~mG, 1989).
SYvrrsm (1989a) described the hydrodynamic principles operating at and near a
tidewater glacier. It was demonstrated that: (1) melt along the face of an ice terminus has
only a minor affect on terminus retreat and sediment deposition; (2) water discharged at an
ice front occurs principally through vertically rising buoyant jets; and (3) ice-contact
processes deposit most sediment very near the ice terminus. Field evidence collected near
the terminus of tidewater glaciers also suggests that most sediment is deposited within the
first 5 km (GORLICHet al., 1987; COWANand POWELL,in press; BOULTON,1990).
When choosing a modern analogue environment to describe the 6 million year old
Yakataga Formation in Alaska, ARMENTROUT(1983) used information from modern
Alaskan studies on tidewater glacial environments. Similarly ANDERSONet al. (in press)
could employ knowledge of modem tidewater glacial environments found in Antarctic to
interpret older Antarctic sequences dating from the Oligocene. Choosing an appropriate
modern analogue is more difficult, however, where continental shelves have been subject
to glaciation but no nearby modern glacial setting exists. For instance KII~G and FADER
(1986) chose the modem Antarctic ice regime to help interpret the glacial stratigraphic
sequences located on the shelves of eastern Canada. MCCLENNEN (1989) disagreed and
suggested that the Barents Sea models (e.g. ELVEm-IOIet al., 1989; VORRENet al., 1989)
provide appropriate modern analogues to describing northwestern Atlantic shelves,
rather than those developed for Alaska or Antarctica. PFmMANand SOLHEIM(1989), who
studied the Nordaustlandet ice sheet, Svalbard, also suggested it as an analogue environment for many northern continental shelves. The Nordaustlandet ice sheet terminus
experiences high rates of sedimentation in the summer associated with numerous meltwater outflow, some forming beaded eskers. The seafloor sediments were found extensively reworked by contact with the glacier, following a recent surge. Subaqueous push
moraines 2-5 m high and greater than 25 m wide, were identified at the modem ice
902
J.P.M. S~rrrssa
terminus; observations that complement those of the formation of push moraines on
Baffin Island and Iceland (BouLTON, 1986).
THE ROLE OF NUMERICAL MODELS
Glacial-marine sedimentation may be explored through the construction of physicallybased numerical models that attempt to simulate the growth of large ice sheets (e.g.
FISCHERet al., 1985; LINDSTROM,1988). Models are designed to balance mass and energy
while describing ice sheet dynamics (Hul-reR and ENGELHARDT,1988). Numerical models
are a useful learning tool where parameters can be prioritized in order of importance
(SYVrrSKL 1990) and may even lead to the collection of new field measurements. If model
simulations do not agree with field data, the physical basis of the model may be lacking, or
better boundary conditions and input data must be sought, or we may need to reinterpret
our field data (SYvrrsia, 1989b). Unfortunately we lack comprehensive four-dimensional
(spatial and temporal) models of ice-sediment-water interactions.
LtNDSTROM(1988) simulated the growth of an ice sheet in the Antarctic and determined
where an ice shelf would ground with a lowered sea level. Model results were used as a
basis for an understanding of basal till deposition, ice loading on glacial-marine sediment,
and sedimentation processes in general. FISCHER et al. (1985) developed a threedimensional, steady state, plastic ice sheet model that incorporates low yield stress for
areas with deforming beds (similar to the models of BOULTONand JONES, 1979; BOULTONet
al., 1984). The model, which incorporates abrupt ice flow changes across major substrate
contacts, suggests a low-angle surface of relatively thin ice, for ice flow over deforming
areas. Large ice streams are generated at the boundaries between normal yield stress (nondeformable) and low yield stress (deformable) beds. Of increasing importance in modelling marine ice sheet behaviour is the lag time between ice load and mantle response
(BouLTOH, 1990). ThUS, ice bed elevations are time variable and out of phase with ice
thickness.
Although debate still continues on the inception of ice sheets in the northern hemisphere in the Tertiary (cf. WRIGHT,1989; SOCO, 1990), consensus is beginning to centre on
basic changes in the ocean and atmospheric circulation arising from the formation of the
Panama Isthmus and other tectonic events. Model predictions, however, have failed to
converge on an accepted ice sheet reconstruction for the Late Quaternary, and vary in
their size, timing, isostatic loadings and even the position of the forebulge. The thin ice
sheet model of BOULTONet al. (1985) can be contrasted with the thick ice load model of
BUDD and SMITH (1985). Similarly, the aerially limited ice sheet model of FISCHERet al.
(1985) can be contrasted with the maximal ice sheet model of HUGHES(1987). Most models
still ignore isostatic adjustments beneath ice streams and few include the effects of
enclosure of ice streams by sidewalls that impart drag, especially at the junction of fiord/
shelf transitions (HUGH,.S, 1987).
Of more concern to glacial-marine geologists is the disappearance of ice sheets, for
sedimentation on glaciated continental shelves volumetrically reflects the deglacial processes. An ice sheet collapse is an extensive, irreversible loss in elevation of an ice sheet
(HuGx-ms, 1987). The areal extent of an ice sheet might increase temporarily during
collapse, but ultimately it must decrease (HUGHES, 1987). This distinguishes collapse from
shorter, recoverable deglaciation episodes observed as interstadials. HUGHES (1987)
considered five mechanisms that initiate a retreat of the grounding line (Fig. 2): ice stream
Sediment deposition on glaciated continental shelves
A
ice stream surge
and/or lowered sea
level
___.1___
B
lowered sill
C
sea level rise
-\
D
~
n
g
E
e
l ratedcavl ,ng
Fig. 2. Mechanisms for destabilizing the marine margin of an ice sheet (after HUGHES, 1987).
Grounding-line retreat is facilitated by: (a) an ice-stream surge that lowers the ice-sheet surface;
(b) delayed isostatic sinking that lowers the bed; (c) rising sea level that raises the ice shelf; (d)
surface and basal melting that thin the ice shelf; or (e) circulation of warm shelf water and basal
meltwater discharged at the grounding-line to together ferry icebergs out to sea as the grounding
line retreats.
903
904
J.P.M. SYviTsrd
surge; a lowered basement; sea level rise; melting; and accelerated calving. DENTON and
HUGHES(1981) proposed downdraw as the major mechanism of ice sheet collapse, because
ice stream drainage basins must expand in area if the pulling power of the ice streams
lowers the ice surface faster than ice precipitation raises it. The pulling power of marine ice
streams is great because they move fast, and the reduced basal shear stress along the ice
stream allows the pulling force to reach farther into a marine ice sheet (HUGHES,1987).
Taken to extreme then, this concept sees ice sheet collapse as a result of a mechanical
feedback loop, rather than of climatic improvement (cf. BUDD and SMITH,1985).
One of the more complex and variable (spatial and temporal) processes of ice sheet
stability, and a difficult process to model, is basal traction with bed freezing. It is
recognized that basal traction is determined primarily by bed roughness, permeability and
softness of the underlying basement, and by the hydrology of basal meltwater (PATERSON,
1981). In the initial stage of glacier flow, there is a lack of communicating drainage
channels, while in the advance stage these would be well-developed (ROTHLISBERGERand
IKEN, 1981). Very high water pressures are generally considered to increase the rates of ice
flow (BROWN et al., 1987). The subglacial thermal regime can also fluctuate across the
freezing point, both spatially and temporally, and over wide areas of the glacial bed
(MENZlES, 1981). The freezing front propagates by progressive pore water freezing, and
this depends on pore water availability, debris porosity and permeability (MENZIES, 1981).
ORHEIMand ELVEmtOI (1981) note that an ice stream grounded in the offshore could allow
the development of tens of metres of permafrost.
METHODS OF SEQUENCE STRATIGRAPHY
Sequence stratigraphy is the recognition of major packages of sediment on continental
shelves that reflect periods of significant sediment input, bounded by periods of widespread erosion: typically sea level induced on nonglaciated shelves and both glacier and
sea level induced on glaciated shelves. Once recognized, the glacial sequence stratigraphy
can be interpreted both in the sense of global events and more regional events to establish
the glacial/deglacial/post-glacial history. Seismo-stratigraphy provides the regional geometry of the various depositional units, while strategically collected cores provide details of
the lithologic properties, the biologic environment, and time frame of these units (VORREN
et al., 1989). Isostatically-uplifted marine sections along coastlines also provide stratigraphic information on near-shore glacial-marine processes (SHORT and MODE, 1985).
Stratigraphic information is used in conjunction with observations from modern analogue
environments, to infer the palaeo-environmental conditions operating during the Late
Quaternary. Ice sheet reconstructions are constrained by this offshore glacial-marine
information (e.g. BOULTON,1979; DYKE and PI~EST, 1987).
Investigations into the nature of glacial sequence stratigraphy attempt to provide
answers for: (1) the marine extent of ice sheets; (2) the ice load on the seafloor; (3) the
speed of ice advance and retreat; (4) the portions of the marine ice sheet terminus
grounded and tidewater, or floating as an ice shelf; (5) the oceanography conditions on the
shelf following the advance and retreat of the ice sheet; (6) the thermal regime in the
marine environment under the ice sheet; (7) the basal shear stress of the ice sheet over
deformable and non-deformable substrate; (8) the proportions of subglacial transport,
glaciofluvial transport, and ice-rafted transport; and (9) the history of seafloor modification following deglaciation. As an example, high or continuous retreat rates of a
Sediment deposition on glaciatedcontinental shelves
905
submarine ice margin might be indicated if: (1) subglacial features such as eskers and
drumlins, and sole markings from the base of the ice sheet, are preserved; (2) ice-contact
deposits such as push moraines are absent; (3) and the volume of ice-proximal glacialmarine sediment is small compared to ice-distal sediment. Thick ice-proximal deposits,
and the presence of push moraines, provide evidence for a more slowly retreating ice
terminus. Ice-contact terminal fans and regional moraines indicate a quasi-stable ice
terminus position.
Seismo-stratigraphic units are successive acoustic intervals that can be recognized and
traced on the basis of distinctive acoustic attributes, bedding styles, and/or unit geometry.
Acoustic attributes are the presence and relative strength of both coherent reflectors
(stratification) and incoherent backscatter (tone), coherency being limited to the resolution of the system. Both types of reflection impart information about changes in sediment
texture or geotechnical properties; the absence of reflections usually implies a homogeneous sediment. Bedding styles refer to the internal acoustic stratification in relation to the
bounding surfaces of the unit. Unit geometry is the nature of the upper reflector and its
relation to the basal reflector, that is the unit boundaries, and the relation of the interval as
a whole to basal topography. Units are established at one or more areas of well-defined or
typical sections, and then traced laterally to establish a regional stratigraphy within which
units may display facies variability in their acoustic attributes, bedding style, and geometry
(Svvrrsrd and PRAEG, 1989).
Core analysis includes obtaining basic textural characteristics of the units. A poorly
sorted sediment, for example, may indicate fluctuating energy conditions in the case of
layered sediment, or a conveyor-belt transport mechanism such as subglacial transport, or
imprinting of one process onto another, such as ice-rafted gravel dropped onto mud
deposited through hemipelagic sedimentation. Mineral data can help determine the
provenance of sediment, including the mode of deposition. Magnetic susceptibility data
may also distinguish different sediment sources, for instance particles derived from
granitic terrane from those derived from a sedimentary terrane (ANoREWS and J~.r~Nir~s,
1987) (e.g. Fig. 3). Clay mineralogy has been used to characterize shelf sediments that
have been subaerially exposed and weathered during the ice sheet-induced global lowering
of sea level (SEGALLet al., 1987). A marked upward reduction in the number of ice-rafted
carbonate clasts found in Labrador Shelf sediment cores documents a source change in the
production of icebergs--from a Laurentide or Innuitian ice sheet source to a modern
Greenland ice sheet source (JoSENHANSet al., 1986). Geotechnical properties of stratigraphic units provide information on the structural stability of the deposit, the mode of
sediment deposition and post-depositional affects. Key parameters include shear strength,
water content, critical void ratio, and consolidation ratio. These parameters can help
determine whether sediments has been loaded by a glacier for instance (Fig. 3; MACLEAN
et al., 1989).
Palaeoecology and biostratigraphy help describe the paleoenvironmental conditions
occurring at the time of sediment deposition, providing an indication of water temperature, salinity, oxygen levels, sedimentation rates, and resuspension events (OsTEP,MAN,
1985 ; VILKS and MUDIE, 1983; VORRENet al., 1978). Marker assemblages can help locate
palaeo-grounding-lines, palaeo-productivity levels (including areas of upwelling), and
palaeo-ice cover (Arsu, 1985). Both microfossil (Fig. 3; VILKSet al., 1987) and macrofossil
(SIMENSTADand POWELL, in press; SvvrrsKl et al., 1989) assemblages may define conditions
unique to bottom waters and/or surface waters (WILLIAMS, 1985; MUDIE et al., 1984).
906
J.P.M. SYVITSKI
BARROW STRAIT CORE 86-027-154
SEDIMENT
UNITS
SHEAR
STRENGTH
kPa)
0
10
i
i
=
i
L
POSTGLACIAL
I
2-
"II"- 3 n
LU
E3
4-
GLACIALMARINE
BULK
DENSITY
(g/cm3)
1.2 1.6 2.0 2.4
,
,
/
/
,
=
=
i
BENTHIC
FORAMINIFERA
(No./o'n 3)
0
3000 6000
WATER
CONTENT
(%)
50 100
I
i
I
I
MAGNETIC
SUSCEPTIBILITY
(x 10.5 SI units)
0 100 200
t
--
C.
reniforme
E, excavatum
-- -
others
5DRIFT
6-'
BARROW STRAIT CORE 86-027-144
SEDIMENT
UNITS
0
SHEAR
STRENGTH
(kPa)
5 10
BULK
DENSITY
(g/cm3)
1.2 1.6 2.0 2.4
WATER
CONTENT
(%)
0
50 100
J
BENTHIC
FORAMINIFERA
(No./cm3)
0
3000 6000
MAGNETIC
SUSCEPTIBILITY
(x 10-5 SI units)
0 100 200
,
I
I
1 ~
Q
3
POSTGLACIAL
GLACIAL-
i
MARINE
DRIFT
Fig. 3. Stratigraphic parameters measured for two cores collected in Barrow Strait, Arctic
Canada (after MACLEANet al., 1989). Both cores penetrate throughpost-glacial,glaciomarine,and
glacial drift sediment. Note that the shear strength does not always increase in the drift unit.
Similarly, marine palynology can define aeolian pathways, vegetation zones, sea ice cover
and climatic conditions (DE VERNAL and HILLAIRE-MARCEL,1985; AKsu and MUDIE,
1985).
Dating the various stratigraphic units is important in ascertaining the rates of processes.
Radiometric techniques, such as the new 14C tandem accelerator mass spectrometers
(TAMS), allow for the analysis of small sample weights and provide an absolute
chronology (+300 years) over the last 60,000 years. Other dating techniques include
thermoluminescence, uranium-series disequilibrium, atom counting, amino acid racemization, tephrochronololgy and fission-track methods, and electron spin resonance (RUTTER, 1985). Techniques such as paleomagnetism (ANDREWS et al., 1986) and marine
oxygen isotope chronology (AKsu, 1985) identify the basic Quaternary stratigraphic
stages. None of the dating methods can be used indiscriminately (see KELLOGG and
Sediment depositionon glaciated continental shelves
907
SCHAFER,1986), and this has led to argument of the timing of ice sheet advances and
retreats, dynamics of sea level fluctuations and rates of sediment deposition (e.g.
SALVaGSEN, 1977).
Sediment from glaciated shelves often lacks sizeable carbon-based material (e.g. bone,
wood, fossils), and therefore bulk samples containing organic carbon are sometimes
dated. Such material almost always contains "old" carbon, such as coal or peat with the
expected contemporaneous carbon (ANDREWS et al., 1985). Interestingly, differences
between a 10,000 year old TAMS shell date and a conventional 14C bulk organic carbon
date appears to increase towards mid-latitudes, being + 10,000 years on the Baffin Island
shelf (ANDREWSet al., 1985), + 15,000 years on the Labrador shelf (JOSENHANSet al., 1986,
and +20,000 years on the Scotian shelf (GIPP and PIPER, 1990). In the later cases TAMS
dating techniques led to revision of the timing of Laurentide ice retreat from the Mid
Wisconsinan (30,000 years BP) to the Late Wisconsinan (10,000 years BP). In the Baffin
Island example, sedimentation rates were revised upward by a factor of two. In the
Labrador shelf study, spurious age determinations had ice contact deposits range in age
from 10,000 to 23,000 years, yet the overlying glacial-marine sediments had an even wider
range in age (10,000-31,000 years), suggesting that both units were probably deposited
circa 10,000 years ago and contain considerable reworked material.
STRATIGRAPHIC TERMINOLOGY FOR GLACIATED CONTINENTAL SHELVES
Typically, arguments over the meaning of scientific terms reflects the rather imprecise
nature of our understanding of a particular topic. The study of glacial-marine sedimentation is such an example. Below some of the basic stratigraphic terms used in the study of
glaciated continental shelves are defined with occasional comments on disagreements in
the literature.
Ice-contact sediments
Ice-contact sediments record the presence of grounded glacial ice and are commonly
(but not always!) characterized on acoustic records by moderate to strong tone, poor to
absent stratification, and by upper and lower bounding surfaces that may be complex and
variable (Fig. 4). The sediments may .form a depositional facies based on a constructional
geometry. They could include deposits of sheet till, eskers, drumlins, submarine push
moraines, grounding-line fans, submarine frontal-dump and lateral moraines. An iceloaded facies may occur as a basin-fill geometry, consisting of glacigenic sediments, nonglacigenic sediments, and proglacial sediments, that together have been sculpted and
loaded by an advancing ice sheet or ice stream. The ice-loaded facies may have a distinct
microfossil facies when compared to the ice-deposited facies where normally microfossils
are absent.
Glacial drift
Glacial drift includes sediment transported and deposited by or from a glacier (glacial
till), or by water emanating from a glacier (glacial-fluvial). Drift may consist of unstratified
sediment such as moraines, and stratified deposits such as outwash fans, eskers, kames,
varves, glaciofluvial sediments (AGI definition). Drift differs from ice contact sediments,
for it includes stratified sediments. Glacial drift has been used to describe offshore glacial
deposits that are not acoustically stratified (VILKS et al., 1989; MACLEANet al., 1989).
908
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Sediment depositionon glaciatedcontinentalshelves
909
Till
Till is an aggregate whose components are brought together by the direct agency of
glacier ice and does not undergo subsequent disaggregation and redeposition. BOLrUNOV
(1970) described basal till as having no indigenous fauna/flora, no sorting, a preferred clast
long axis orientation, and no stratification. As till is not transported by water, the clasts are
angular and may show tool (striation) markings. Basal till includes both lodgement
(plastering on) and meltout [ice is inactive during basal debris release (HAMBREYet al.,
1989)]. Deformation till can contain internal stratification although layering is not aerially
extensive. "Till" is often used interchangeably with the more encompassing terms drift,
diamicton and ice-contact sediments (DREIMANISand LUNDQVaST, 1984).
Glacial-marine sediments
Glacial-marine sediments are a mixture of glacial detritus and marine sediment
deposited more or less contemporaneously (ANDREWS and MArSCn, 1983), with the
proportion of marine sediment varying from significant in the most distal zone, to
insignificant near an ice terminus. The glacial component may be released directly from
glaciers and ice shelves, or delivered to the seafloor from those sources by gravity, moving
fluids, or iceberg rafting. The marine component comprises mainly terrigenous and
biogeneous sediment. MOLNIA (1983) evaluated the many definitions of glacial-marine
sediments and concluded most of the early "Antarctic" definitions stipulate ice-rafting as
the principal mechanism, whereas in "subarctic" environments glacial-marine deposits are
dominated by fluvial-transported debris. BOLXUNOV (1970) described glacial-marine
mixtons (s/c) as sediment having macrofauna in living position, marine plant fragments,
definite sorting with a high clay content, random clast axis, and stratification. It has since
become common practice to differentiate between glacial-marine deposits formed proximal to the grounding line of an ice shelf or tidewater glacier and composed of coarsergrained sediment, from principally muddy deposits formed distally to the ice terminus
(ELVERt-IOI et al., 1989; MOLNIA, 1983). Unfortunately, few authors specifically differentiate the distance meaning in the terms "proximal" and "distal": proximal glacial-marine to
one author may be equivalent to distal glacial-marine to another, and "post-glacial" to one
author may mean "distal glacial-marine" to others.
Ice-proximal glacial-marine sediments
These are beds of alternating sand/gravel and mud, acoustically characterized by strong,
closely spaced reflectors (Fig. 4; Svvrrsra and PRAE~, 1989). Proximal may include
Fig. 4. Airgunseismicreflectionprofilesof Late Quaternary sedimentswithinthe St Lawrenceestuary (after
S~na's~a and PRAE~, 1989). Profiles show a complete glacial-deglacial stratigraphic sequence of: (1) an
acousticallyunstratifiedlowerunit of ice-contactsediment,interpreted as loadedglacial-marinesedimentsrather
than till; (2) a well-stratifiedice-proximalglacial-marineunit; (3) a more transparent ice-distal glacial-marine
unit; (4) a well-stratifiedparaglacial unit; and (5) a largelytransparent estuarine unit of post-glacialmuds. (A)
Lateral transition of unit (1) from acousticallyunstratified to stratified sediments. (B) Ice-contactunit (1) is
largelyunaffectedby basal bedrock topography;its surface maeroreliefis interpreted as sole marks from an ice
advance. (C) Paraglacialunit (4), composedlargelyof gravityflowsand and silt, is ponded over unit (3). Also
note gas horizonsthat typifythe post-glacialunit (5).
910
J.P.M. SYVITSKI
distances between 0 and 100 km out from the ice terminus, depending on the size and
dynamics of the ice system. Ice-proximal sediments may be deposited either as a thin
comformable layer during the rapid retreat of an ice terminus, or as a wedge-shaped fan
marking the position of an ice terminus still stand.
Ice-distal glacial-marine sediments
These tend to be characterized on acoustic records by distinctive low to medium tone,
weak to moderate stratification, and a conformable bedding style in which basal topography underlying the units is translated through internal reflectors to the surface of the unit
(Fig. 4). Distal can include being at the ice terminus if the ice is relatively sediment free
and with no nearby glacial-fluvial discharge, or involves distances of several hundred
kilometres from the margin of an active and sediment-rich ice terminus. Ice-distal deposits
are muddy with a variable amount of ice-rafted debris, deposited principally from
meltwater plumes emanating from rapidly ablating ice margins at times of high sea levels.
In ice shelf environments, ice-distal sediments may be composed of coarser-grained icerafted sediment mixed with biogenic oozes.
Paraglacial sediments
These are coarse-grained fluvial-marine deposits related to the period of abnormally
high rates of sediment transport from a terrestrial ice sheet that is experiencing rapid
ablation (CHURCH and RYDER, 1972; SvvrrsKI et al., 1987). Vast quantities of glacial,
proglacial and exposed marine terrace sediments may be available for fluvial erosion and
transport, and account for abnormally high sediment yields. Acoustically and lithologically, these deposits resemble deltaic wedges that have prograded rapidly from a coastline
(Fig. 4). That is they contain a mixture of sediment gravity flow and hemipelagic deposits
with syn-depositional mass flow deposits, buried channels, and shear planes related to
submarine slides. These sediments have been recognized on the Alaskan shelf (PowELL
and MOLNIA, 1989; CARLSON, 1989) and on the eastern Canadian shelf (SYVITSKI and
PRAE~, 1989).
Post-glacial sediments
Post-glacial sediments are the marine deposits (as used here) that post date the ablation
of marine or terrestrial ice sheets, or are beyond the influence of a distal ice sheet.
Sediments are presently accumulating principally from non-glacial processes ( E L w ~ o I et
al., 1989) and reflect the establishment of modern sedimentation patterns concomitant
upon the establishment of contemporary currents and circulation, and sea level. Postglacial sediments are no longer influenced by turbid glacial meltwater plumes. These
sediments can occur either as a basinal mud facies or as sand and gravel lags in the
shallower water surrounding basinal mud (S~v-rrsrd and P~E~, 1989; ELvEPa~¢I and
SOT.HEIM, 1983; KINC and FAÜER, 1986; PIPEI~, 1988; ELV~I~I and KlUSTO~RSEN, 1977;
VORRENet al., 1989). The basinal mud facies is characterized acoustically by low tone and
weak reflectors. They are typically organic-rich, reflecting the development of terrestrial
carbon sources and may be charged with methane (Fig. 4). The lag deposits are composed
acoustically of strong tone with discontinuous reflector surfaces (very dependent on
Sediment depositionon glaciated continental shelves
911
seismic system). Post-glacial sediments may remain influenced by ice, in the form of sea ice
or iceberg rafted sediment deposition.
SEQUENCE STRATIGRAPHY ON GLACIATED CONTINENTAL SHELVES
The vertical statigraphic sequence of depositions from twenty glaciated shelves from
around the world are compared in Table 1. The identified sequence stratigraphies are
mostly those of the original author(s), from seismo-stratigraphic data, lithologic data, or
some combination thereof. A complete deglacial, vertical stratigraphic sequence at any
location consists of some or all of the following: (1) till, glacial drift, or ice-contact
sediments; overlain by (2) glacial-marine sediment--which is sometimes divided into a
lower ice-proximal unit and an upper (3) ice-distal unit; (4) paraglacial deltaic sands and
prodelta muds, and other nearshore and coastal sediments; and (5) post-glacial lags, basin
fill, and estuarine muds and pelagic oozes. Typically, the sequence unconformably overlies
sedimentary rocks on the other shelf and a mixed bedrock basement on the inner shelf.
Deposition of unit (1) occurs initially during the period of globally (eustatic) lowered sea
level, and later with units (2) and (3) during a period of high sea levels related to isostatic
loading of the inner shelf. Deposition of unit (4) occurs during the initial fall in sea level
(isostatic recovery) when ice has retreated on land and is rapidly ablating. Deposition of
unit (5) occurs during more complex and local sea level fluctuations, that include such
affects as the collapse of the crustal forebulge. Minor variations to this association of sea
level and sedimentation pattern occur, particularly if the glacial cycle is out of phase with
the global eustatic cycle (for details see BOULTON,1990). To support the regional sequence
stratigraphy identified in Table 1, the next section briefly reviews, the stratigraphic facies
models developed to describe four of the world's glaciated shelves: Antarctic shelves,
northeast Pacific shelves, northeast Atlantic shelves and northwest Atlantic shelves.
Antarctic continental shelves
Research on glacial-marine sedimentation on Antarctic continental shelves is well
advanced, although seismic coverage has been hampered by heavy pack ice. Sediment
deposits are largely coarse-grained, except for deposits of biogenic ooze, and in the lower
latitudes of the Antarctic peninsula where meltwater production is higher. Most stratigraphic facies models employ processes associated with ice shelves. Glacial conditions
have lasted nearly an order of magnitude longer on Atlantic shelves compared with Arctic
shelves.
ANDERSON(1972) considered four ice shelf sedimentation regimes: advancing dry base;
advanced dry base; receding wet base; and receded wet base. The model describes the
lateral transition in lithologies and biofacies, and emphasizes a fluctuating carbonate
compensation lysocline. The early model includes some unrealistic ideas: turbidity
currents generated from the undermelt of ice shelves, hypersaline stagnant basins on an
exposed continental shelf, and vertical facies boundaries--ideas that have been superceded (ORHEIMand ELVERHOI,1981; ELVERHOI,1984). More recent papers suggest low to
negligible rates of sedimentation beneath large portions of an ice shelf (e.g. KELLOGGand
KELLOGG, 1988; HAMBREYet al., in press), although ANDERSON and MOLNIA (1989) still
support the notion of underflows occurring beneath floating glacier-tongues. Are these
turbid layers advected under the ice shelves, do they relate to the settling of seasonal
Barents Sea Shelf
(ELVERH~Iand SOLHEIM,1983)
(ELWVatOIet al., 1989)
(LAUTERNAUERet al., 1989)
British Columbia
Antartic peninsula
(ANDERSONet al., in press)
Weddell sea shelf
(ELVERHOi,1984)
George V shelf
(HAMPTONet al., 1987)
McMurdo Sound
(HAMBREYet al., 1989)
Prydz Bay Shelf
(HAMBREYet al., 1990)
NE Gulf of Alaska
(MOLNIAand CARLSON, 1980)
Yakataga Pm. Alaska
(A~ENTROUT, 1989)
Shelf environment
(reference)
Till?--moraines, eskers
and outwash
Stiff pebbly mud--till
and ice-loaded glacialmarine
Subglacial stream and
outwash
Till: lodgement basal,
waterlain
Till: lodgement basal,
waterlain
Moraine facies
Not identified (seen on
seismics)
Proximal G.-M.--pebbly
mud
Coastal and
nearshore sands
Ribbon sands
proximal
rhythmites deltaic
sands
Sandy mud and
mud
Not present
G.-M. mud facies
Glacial-fluvial outwash and
ice-rafted facies
Diamictite frontal
dump--onlap bedded
Glacial-marine with
dropstones
Distal G.-M.--mud/
pebbly mud
Lags out of section
organic-rich muds
Holocenellagmuds
Rhythmites
Lag facies
Not present
Distal G.-M.
Proximal G.-M.
Ice-distal G.-M.
diamictite
Marine
Shore facies
Post-glacial--lags--pelagic oozes
Soft pebbly mud
with bioclastics
Holocene diatom,
mud
Marine
Distal G.-M.
Not present
Proximal G.-M.
Glacial-marine diamicton
Soft pebbly mud
Glacial-marine (= G.-M.)
Vertical stratigraphic sequence
Comparison of stratigraphic deposits found on glaciated shelves
Subglacial deposit and
glacial-marine (loaded)
Till--stiff pebbly mud
Table 1.
~o
tO
Norwegian Shelf
(HOLTEDAHL,1986; 1988)
Inverness Shelf (U.K.)
(BotJLTON et al., 1981)
Canadian Arctic archipelago
(MAcLE^I~ et al., 1989)
SE Baffin Shelf
(PRAEGet aL, 1986)
Hudson Strait
(VILKS el al., 1989)
Baffin Fiords
(GILBERT, 1985)
Labrador Shelf
(JosENHANSet al., 1986)
Lake Melville
(VmKs et al., 1987)
Gulf of St Lawrence
(SYvrrsrd and PRAEG,1989)
Scotian Shelf
(ICdN~and FADE~, 1986)
Maine Inlet
(BELKNAP e / a l . , 1986)
Early holocene-wedge-shaped fill
Paraglacial deltaic
Sands and gravels
Ice-distal
Ice-distalG.-M.
Silt facies B
Stratified G.-M.
Ice-proximal
Ice-proximal--retreat
phase G.-M.--still stand
Silt facies A
Glacial-marine mud
Glacial drift--ice-contact
till, waterlain till
Ice-contact till and
loaded G.-M.
Till
Till
Ice-contact---glacial
drift--ice-loaded
Drift--till
Till
Estuarine-fluvial
mud
Glacial-marine
Unstratified G.-M.
Glacial-marine
Distal G.-M.
Proximal G.-M.
Not present
Early holocene
Basin fill
Stratified G.-M.
Not present
Glacial drift
Glacial drift
Basal till? discontinuous
Well stratified and
ponded
Lag deposits
Proximal G.-M.-Distal G.-M.-acoustically stratified
transparent
Glacial-marine---on seismics may be divided into
two units (proximal and distal?)
Glacial-marine
Ice-keel turbate and
post-glacial muds
Late holocene-organic-rich muds
Post-glacial--lags--basinal muds
Post-glacial--lags--basinal muds
Estuarine mud
Holocene organicrich muds
Post-glacial
Sediments
Ice-keel turbate and
post-glacial muds
Late holocene-muds and lags
Surface muds
Post-glacial mud
L~
~D
a
.el
E
0
8
o
O
0
t~
o
o
914
J.P.M. SYvrrsKl
layers, or are they true highly turbid dense flows? Similarly, DOMACK(1988) suggests the
generation of turbidity currents, some landward flowing, within an outershelf environment known for having extremely low sedimentation rates (<<0.1 mm a-l). Are these
deposits of turbidites or tempestites?
ANDERSON et al. (1983) described stacked facies assemblages that reflect the advance
and retreat of a glacier: a basal ice shelf glacial-marine facies overlain by a lodgement till
facies, together interdigitating with an iceberg glacial-marine facies. Recently, ANDERSON
et al. (in press) proposed a dynamic (time-progressing) stratigraphic model based largely
on seismic data. The model is complex, accounting for sea level fluctuation and ice
dynamics covering the period Late Oligocene to the present, and includes both shelf and
slope deposits. Major unit boundaries and unconformities are shown, and sediment facies
include subglacial and glacial-marine deposits, canyon and fan deposits, contourites,
pelagic muds and oozes, outer shelf beach deposits, carbonate bioclastic deposits, and
lags. The five stages of the model are: (1) an initial temperate maritime glacial setting with
glacial outwash streams and a strand plain; (2) waxing and waning of a temperate ice cap
when submarine canyons and fans developed; (3) glacial retreat and development of a
subpolar ice cap with sedimentation confined to the inner shelf; (4) re-advance of
Antarctic (polar) ice sheet when submarine canyons were filled with glacigenic sediment;
and (5) the modern ice front that has retreated to the nearshore, a consequence of the Late
Quaternary global sea level rise.
ALLEYet al. (1989) describe the modern Ross Sea regional unconformity that is overlain
by a diamicton several tens of metres thick. They hypothesize that this diamicton is a
deformation till and that the Ross Sea sediments record one or more expanses of the tilllubricated west Antarctic ice sheet to the edge of the continental shelf. Ice shelf
development is a result of rapid flow of cold ice from outlet glaciers or ice streams into
protected embayments with localized high spots. The rapid ice velocity arises from
deformation of a several metre thick water saturated (highly unconsolidated) basal till that
is eroding an unconformity of sediment beneath. The result is a "till-delta" tens of metres
thick and tens of kilometers long at the gounding line (Fig. 5).
Similarly, but based on a number of CIROS and ODP drill holes, HAMBREYet al. (1989;
in press) suggest most sediment is transported from the Antarctic landmass to the sea
within the basal debris layer, a layer that is only a few meters thick. The transport ends at
the grounding line where basal debris is released. The entire surface of the continental
shelf is considered as a mega "till-delta" or diamict apron. Debris deposited from icebergs
is almost entirely supraglacial and englacial sediment. They note that the volume of glacial
meltwater is insufficient to generate visible turbid sediment plumes in surface waters
around east Antarctic and the Ross Sea. The HAMBREYet al. (1989) facies model for
sedimentation around a floating glacier tongue in McMurdo Sound includes: (1)
lodgement/basal till; (2) waterlain till; (3) proximal glacial-marine sediments; (4) distal
glacial-marine sediments; and (5) marine sediments (mainly diatomaceous oozes).
ANDERSON (1985), DREWRY (1986) and ANDERSON and MOLNIA (1989) describe the
sedimentation regime presently found in Antarctic bays and fiords. More recently,
GIUFFITn and ANDERSON(1989) observed that climatic and orographic gradients appear to
control ice terminus dynamics, particularly the level of meltwater and sediment production. Moister areas experience sediment gravity flows and coastal basins accumulate
ponded sediment. In drier areas, terrigenous sediment production is low and biogenic
production high--ocean currents disperse and deposit these sediments as a conformable
TILL
A
DELTA
COUPLING
LINE
GROUNDING
LINE
ICE
FRONT
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~
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•
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/
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~"~'*"
r~ ~ 180 I-
~
0
1
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,.1~"OJO140 k- - "
•
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""
~
~
~
~
,
2
3
4
$
6
7
km
i
I
"6*
I
i
ir~o
Fig. 5. (A) Conceptual model of the likely configuration of a "'till-delta'" from a deforming bed
back from a grounding line of an ice shelf (after ALLEY et al., 1989). (B) A possible candidate
(author's interpretation) for a "'till-delta" from Wellington Channel, N.W.T., Arctic Canada, an
environment that may have been covered in by ice shelf (MAc:LEANeta/., 1989). High-resolution
HUNTEC ® DTS reflection seismic profile was provided by B. MACLEAN(1989).
916
J.P.M. Svvrrsrd
drape over a rough basement. In more mountainous regions that experience higher levels
of precipitation, sediment production is high and in the marine setting sediment is
dispersed by meltwater plumes.
Northeast Pacific continental shelves
If Antarctic continental shelves are an end-member with a large number of ice shelves of
largely cold-based ice, where large tabular icebergs are relatively sediment free, then the
Alaskan and British Columbian shelves are exactly the opposite. There, Quaternary
glaciers are considered warm-based with tidewater fronts through which large volumes of
turbid meltwater discharge and calved icebergs are small (bergybits). These Pacific
continental shelves are tectonically active, in response to plate convergence, although ice
sheet-related sea level fluctuations remained a dominant control during the Pleistocene
and early Holocene (Ltrr~AU1ZR et al., 1989). POWELLand MOLNIA (1989) document a
setting where up to 8 m a-1 in precipitation in a very rugged setting (peaks rising to >5 km)
combine to cause very rapid glacial flow (3 km a-l), which with high basal loads (up to 1.5
m of debris) produce large volumes of siliclastic glaciomarine sediment. Some of the
coastal mountains are themselves composed of glaciomarine sediment: the Yakataga
Formation provides a complete (>3 km thick) section having accumulated over a 6 million
year period (Fig. 6A; ARMENTROUT,1983).
Glacial advances into the Pacific are slow due to great coastal water depths, and high
rates of calving (PoWELL and MOLNIA, 1989). As a result of slow ice advance, unconsolidated sediment is scoured from fiords and embayments during each glaciation (unless
tectonically uplifted like the Yakataga Formation). The only confirmed morainal banks
are at the mouths of the fiords and along offshore ridges, probably terrestrial deposited at
times of lower sea level. POWELL(1988) provides a stratigraphic model of a morainal bank
association, with a back wedge zone where lodgement till is emplaced, a core zone
receiving frontal dump sediments and and a forewedge zone that experiences ice-proximal
glacial-marine deposition.
MOLr~IA (1983) developed a glacial-marine sedimentation model that considers three
phases in the postion of the glacier with respect to the shoreline. In the advance phase, the
ice stream extends beyond the shelf-slope break as an ice shelf: the continental shelf is
limited to subglacial ice-contact deposits. In the middle phase, the ice stream is grounded
on the shelf and due to the dominance of glacial-fluvial discharge ice-contact fans are
formed. In the retreat phase, the glaciers are onshore and a nearshore bedload-dominated
facies is interdigitated with an offshore prodelta facies.MoL~qIA (1983) notes that fossils
need not be present in offshore glacial-marine sedimentary environments. SYVITSKIet al.
(1987, Ch. 6) document how benthic communities are largely absent from environments
with high sedimentation rates (>0.5 m a-l), a value that is commonly exceeded within the
proximal prodelta environments in modern Pacific fiords.
POWELL (1981) proposed three facies associations that depend on the conditions of the
retreating tidewater glacier: (1) rapid retreat with icebergs calved into deep water; (2) slow
retreat or a still stand with active calving into medium depth water; and (3) slow ice
terminus retreat dominated by subaerial ablation of the glacier while in very shallow
water. A typical vertical sequence might include a thin basal till unit, overlain by bergstone
(ice-rafted) mud, marine-outwash (prodelta) mud, deltaic sands and fluvial gravels (Fig.
6B; POWELL, 1983; 1984).
917
Sediment deposition on glaciated continental shelves
During ice advance onto the western Canadian shelf, pre-Late Wisconsinan sediments
were truncated, above which ice-contact and proglacial sediments were deposited (LtrrER~AVeR and MURRAY, 1983): terminal and recessional submarine moraines, stagnation
deposits (kame and kettle), eskers and outwash sand and mud. Many of these features are
B
PALAEO-ICE MARGINAL STILL STAND
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Fig. 6. (A) Conceptual model for the Yakataga Formation, Robinson Mountain, Alaska--a
fiord-fill megasequence (modified after ARMESTROUT,1983). (B) Hypothetical section of Nacialmanne sediment showing general sedimentary facies and facies association (modified a~er
POWELL,1984).
918
J.P.M. Svw~sra
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Fig. 7. Correlation between (A) grain size fractions and (B) seismic reflectors. (C) provides the
regional setting for this glacial-marine sequence from the western Canadian continental shelf
(from LUTERNAUERet al., 1989).
preserved due to rapid deglaciation of the shelf through calving of grounded ice (LUTERNAUER et al., 1989). Shelf lags were produced during a lower stand of sea level, that is
before isostatic recovery began, Acoustic-lithologic correlations through their glacialmarine sequence indicate that seismic reflectors are a result of impedance changes that
develop across thin sandy intervals (Fig. 7).
Northeastern Atlantic continental shelves
Although the eastern Atlantic continental shelves are well studied, there exist divergent
views on the timing and extent of glaciations (BouLTON, 1979). In general, these shelves
Sedimentdepositionon glaciatedcontinentalshelves
919
have experienced low sea levels, prior to 15,000 years BP, followed by crustal depression
(circa 10,000-15,000 years BP) and uplift and regression during the Holocene. More
regionally, the deglaciation of the European ice sheet was synchronous with global
eustasy, the Spitsbergen ice sheet lagged behind and the British ice sheet was ahead
(BouLTON, 1990). Sea ice has always had an important control of sediment pathways on
these shelves (ELVERHCqet al., 1989), except recently on the more southern shelves (North
Sea and Irish Sea seabeds). Glaciers still affect sediment deposition on the Kara Sea,
Barents Sea, Icelandic and Greenlandic shelves. Sediment accumulates at rates much
higher than found on Antarctic shelves, and considerably lower than observed on the
eastern Pacific shelves. The maximum accumulation rates are 0.2 m ka -1 for the
continental slope, 0.4 m ka -1 for the shelf break, 0.1 m ka -1 for stratiform diamictons, I m
ka -1 for shelf trough fills, and 60 m ka -1 for coastal fiords (VORREN et al., 1989). Large
parts of these passive margin shelves are presently experiencing sediment erosion coupled
with biogenic carbonate deposition (ELvERI-I~Iet al., 1989).
ELVERHOIet al. (1989) observed that the sediment distribution in the northern Barents
Shelf reflects both modern processes (including those associated with tidewater glaciers)
and those associated with former glaciations of the shelf. Both post-glacial (sediment
source is not directly from an ice sheet) and pro-glacial sedimentation occurs synchronously. Sedimentary facies were divided into: (1) a glacier-proximal zone, influenced by
grounding-line processes and high input of sediment; and (2) a glacier-distal, sea-ice and
current controlled zone. The distal zone is associated with the deposition of fine-grained
mud from plume drift, coarser particle ice-rafting, primary production, and basin fill from
the erosion of shallow banks.
On the northern Barents continental shelf, ice sheet advance and retreat has been
recorded by sediment deposition and ELVERHOI and SOLHEIM (1983) identified the
following sequence stratigraphy: (1) ice-loaded glacial-marine sediment; overlain by (2)
basal till; (3) ice-proximal glacial-marine sediment; (4) ice-distal marine sediment; and (5)
post-glacial lags on banks and muddy fill in basins. Research around Spitsbergen, both
within the fiords (e.g. ELVERHI21Iet al., 1983) and on the continental shelf (e.g. PFIRMANand
SOLrlEIM, 1989), provided important information on the dynamics of glacier surges and
their interaction with the seafloor. Sole marks preserved from Weichselian glacial
advances have been identified from sidescan sonograms collected on the continental shelf
(A. ELVERHOIand A. SOLHEIM,personal communication 1989). They differ from normal
pseudo-random orientation of iceberg scour marks, and show a consistent set of parallel
groves.
VORREN et al. (1989) identified an upper unconformity with a glacially eroded morphology which extends over most of the southern Barents Sea shelf. Up to 300 m of
stratiform glacigenic sediments overlie this unconformity. Three dominant seismic signatures were discerned: semi-transparent; stratified; and chaotic reflection patterns. These
sediments are partly glacifluvial in origin, but on the whole they represent more-or-less
tectonized glaciomarine deposits. Only the deep troughs on the continental shelf contain
an undeformed glaciomarine to post-glacial marine basin fill. The greatest thickness (up to
I km) of glacigenic sediment occurs at the shelf break and below the upper slope as troughmouth fans (Fig. 8A). VORRENet al. (1989) provide the following stratigraphic model: (1)
lodgement of basal till; (2) deposition of laminated clay under an extensive sea ice cover;
(3) pebbly mud deposition in the troughs while banks were iceberg ploughed; and (4)
920
J.P.M. Sw]~sr~
p r e s e n t - d a y current winnowing o f b a n k s c o n c o m i t a n t with lag f o r m a t i o n and fine-grained
sediment deposition in the troughs. Similar to the A l a s k a n continental shelf e n v i r o n m e n t
(cf. CArLSOn, 1989), tills are generally limited to the threshold (sill) areas in fiords. Ice
contact deltas are located o n the inner shelf. T w o forms o f unconformities are recognized:
A
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Fig. 8. (A) A conceptual model of the main sedimentary processes on the continental shelf
break-upper slope of the Barents Sea during glacial and interglacial periods (modified from
VO~EN et al., 1989). (B) A conceptual model of massive "till" deposition on the Norwegian shelf
including the formation of lift-off moraines under an ice shelf (modified after stages 4 and 5 of the
K]No et al., 1987 conceptual model).
Sedimentdepositionon glaciatedcontinentalshelves
921
smooth unconformities having a glacial origin and associated with high shear strengths
>250 kPa, and irregular unconformities having a fluvial or subglacial fluvial erosive
origin.
KING et al. (1987) identified much of the Quaternary sediments on the mid-Norwegian
shelf as till deposits, by assuming all acoustically unstratified units represent till (Fig. 8B).
As argued below, that is a very tenuous assumption, for although some of this material is
undoubtedly till, a significant percentage could be ice-loaded glacial-marine or ice-front
outwash material. KiNG et al. (1987) also suggested that some of the internal reflectors
within thick "till" deposits represent erosional surfaces from ice advances---an interpretation that may be correct while still not supporting the till label for the deposits (they
could represent surges of the ice front onto glacial-marine sediment). HOLTEDAHLand
BJERKLI (1986), following the work of VO~REN et al. (1978), proposed the following
scenario for the deposition of glacial-marine sediment on the Norwegian shelf. Following
the retreat of the ice on the continental shelf during the Middle Weichselian, the seas were
inundated with sea ice and icebergs, contributing ice-rafted debris and the formation of
glacial diamictons. A change from arctic to boreal conditions took place at the onset of the
Holocene, concomitant with drastic change in the hydraulic regime with currents and
waves winnowing the shallower banks. HOLTEOAHLand BJERKLI (1982) suggest that this
hydraulic change reflected the induration of the Atlantic current onto the shelf. Holocene
winnowed lags presently overlie the glaciomarine mud.
On the southern Norwegian shelf, HOLTEDAHL(1986) recognized four stratigraphic
units: (1) a lower ice-proximal, acoustically well-stratified unit of glacial-marine sediment;
overlain by (2) an acoustically transparent, ice-distal glacial-marine sediment with copious
dropstones; (3) a well-stratified and ponded unit that was deposited during the period of
glacial withdrawal from the sea (similar to paraglacial deposits identified on other shelves;
and (4) a surficial transparent unit of post-glacial Holocene mud. Although the seismic
data were less clear, this pattern of Quaternary seismic units was found inshelf surveys to
the east (HoLrEDAHL, 1988).
The glacial-marine stratigraphy of the shelves surrounding the United Kingdom are
similar to their counterparts observed on the Norwegian shelves. BOULTONet al. (1981)
describe the following late Quaternary scenario for the western Inverness shelf: (1)
deposition of a discontinuous basal till unit, very much bedrock controlled; overlain by (2)
a glaciomarine deposit consisting of a conformable, acoustically stratified, sediment (it
appears possible to separate this unit into a well-stratified ice-proximal unit and a lessstratified ice-distal unit); (3) winnowing of these glaciomarine sediments during isostatic
recovery; and associated with (4) deposition of Holocene organic-rich sediments, containing gas in the basins.
PANTIN (1977) provides similar seismic profiles from Late Quaternary deposits on the
northern Irish Sea shelf. Unit descriptors include: (1) boulder clay (till); (2) proglacial
water-laid sediment (would be typically interpreted as proximal glacial-marine on other
shelves: Table 1); (3) marine, including glacial-marine (ice-distal unit on other shelves);
and (4) variegated marine sediments (typically described as Holocene or post-glacial
muds; Table 1). The southern Irish Sea shelves contain thicker glacial-marine sediments
(GERRARD, 1977). The identified meltwater channels may be an artifact of shelf winnowing
following the sea level fall, whereby the channel-like basins were filled with glacial-marine
sediments and subsequently preserved with truncated surfaces.
922
J.P.M. Svvrisra
Arctic and eastern Canadian continental shelves
The arctic and eastern Canadian continental shelves are subject to strong environmental
gradients over their geographic spread between 43°N to 85°N. For example, annual
precipitation ranges from 0.2 to 2 m a -1 from north to south. Likewise, Late Quaternary
deposits generally increase in thickness from <30 m in the north (MACLEANet al., 1989) to
>1300 m in the south (Svvrrsra and PRAEG,1989). JOSE~qnANSand FADER(1989) consider
that this trend may reflect colder basal conditions for the more northern glaciers, with
concomitant reduction in subglacial and englacial debris transport and a more rapid
deglaciation of these shelves.
MACLEAN et al. (1989) described the following marine stratigraphic units for deposits
found on the arctic archipelago shelves:
(1) glacial drift, an unstratified, unsorted diamict, of variable thickness and surface relief
which forms both positive constructional features and till of bedrock depressions. These
deposits are typically less than 10 m thick, but may exceed 50 m at submarine moraines or
morainal banks. They are unstructured, faunally barren, glacigenic deposits with relatively high shear strength and bulk density, and low water content. Figure 5 may represent
a "till-delta" (ALLEYe t al., 1989; also called a diamict apron by HAMBREYet al., in press)
which would support the notion of an ice shelf. The upper portion (1 m) of the drift can
have very low shear strengths (<5 kPa) although the water content is also low (<30% by
volume);
(2) glaciomarine sediments consist of stratified muds and sandy muds, <4 m thick,
having a restricted faunal assemblage (occasionally barren of foraminifera), moderate
shear strength and bulk density, and low water content;
(3) post-glacial sediments are acoustically transparent, non-laminated muds of local
extent and <7 m in thickness. They are characterized by high water content, low bulk
density and shear strength and a relatively diverse foraminifera population. TAYLOR(1988)
noted that permafrost does not exist in these offshore shelves, although it may exceed 500
m in thickness onshore. He suggested that this may indicate a long period of permafrost
degradation (>25,000 years) with these archipelago channels, or that the channels were
water filled and possible covered by ice shelves, or the glacier was warm-based ice during
the last glaciation.
The centre of the Laurentide ice sheet resided over Hudson Bay and James Bay.
Covering an area of 1,300,000 km 2, the Quaternary deposits within this intracratonic sea
are very thin, averaging <5 m (JoSENHANS et al., 1990). Three stratigraphic units are
resolved: (1) tills; (2) glaciomarine stratified sediments; and (3) postglacial mud. The latter
are largely confined to shore basins near river mouths. To the east, Hudson Strait was
occupied by a major Laurentide ice stream. Quaternary deposits there can exceed 130 m in
thickness (VILKSet al., 1989), and in addition to the Hudson Bay units, a paraglacial timeequivalent unit is recognized (between the glaciomarine deposits and the post-glacial
sediments). The paraglacial unit is acoustically stratified and ponded within basins as a
consequence of strong currents. Except for the paraglacial unit, a similar stratigraphic
sequence is recognized on the southeast Baffin Island continental shelf (PRAEG et al.,
1986). GILBERT(1985) interpreted the sedimentation in Baffin Island fiords as reflecting
a tidewater glacier undergoing a general retreat: (1) ice-contact and ice-proximal sediments, including till and deformed glaciomarine sediments; (2) stratified ice-proximal
Sediment depositionon glaciatedcontinental shelves
923
glaciomarine sediments; (3) unstratified ice-distal glaciomarine sediments; (4) stratified
glaciomarine deposits, reflecting glacial readvance; and (5) a surface veneer of acoustically transparent post-glacial mud. SvwxsIo and HEIN (1991) used sediment mass
balance calculations to suggest that Gilbert's unit (4) may represent paraglacial deltaic
deposition.
JOSENItANSet al. (1986) and JOSENHANSand FADER(1989) interpreted basal unstratified
seismic units found on the Labrador Shelf as till and thus recognize a thick multiple till
sequence. In their stratigraphic model, tills are derived from subglacial meltout of buoyant
ice sheets, yet an X-radiograph of their "till" (Fig. 6, section J of JOSENHANSand FADER,
1989) appears well-stratified. Glaciomarine sediments are described as having medium to
high intensity, parallel continuous coherent seismic reflectors that are conformable to
their basal topography (a discrete well-stratified unit, circa 5 m thick, overlying their "till"
unit could represent ice-proximal glaciomarine sediments). The glaciomarine sediments
may interfinger, conformably overlie, and abruptly terminate against the till surface. They
are in turn overlain by weakly stratified, ponded, post-glacial muds that were eroded from
the shallow bank surfaces, principally by iceberg scouring. The shelf banks are covered in a
<10 m thick iceberg keel turbate.
The Quaternary sequence in the northwest Gulf of St Lawrence and adjoining seas can
be locally missing or can excced 1.3 km in thickness (Svvrrsm and PRAEG, 1989). Five major
stratigraphic units that relate to the advance and retreat of the Laurentide Ice Sheet are
recognized:
(1) ice-contact deposits that record the presence of a grounded glacier including
glacially-deposited or ice-loaded sediments;
(2) ice-proximal glacial-marine sediment that reflects high energy deposition associated
with submarine discharge form the glacier terminus;
(3) ice-distal finer-grained sediment deposited during a period of elevated sea level and
under highly turbid and buoyant glacial plumes;
(4) paraglacial deltaic sediment that records the melting of terrestrially-based ice caps,
and the concomitant growth into a rapidly falling sea;
and (5) post-glacial sediment that reflects the winnowing of shallow areas and the
deposition of organic-rich mud in deep basins, principally under modern oceanographic
conditions.
SYVITSKI and PRAEG (1989) examine two forms of ice sheet advance into the marine
environment--an ice stream scenario, and a grounded ice sheet scenario. Their model
accounts for the speed of ice advance and retreat, fluctuating water depth, collapse of the
mantle's forebulge, and complete collapse of the ice sheet. The model assumes that steady
state conditions are seldom achieved.
KING and FADER (1986), KING et al. (1987) and JOSENHANSand FADER (1989) describe
Quaternary sediments on the Grand Banks of Newfoundland, the Scotian Shelf and the
Bay of Fundy. They invoke stable ice shelves as suggested by CAREYand AHMAD(1961) to
account for much of their identified "till" unit. However, extensive subglacial meltwater
channels have been identified on the outer Scotian Shelf (BoYD et al., 1988). I also question
the KING and FADER(1986) hypothesis that an ice shelf may form in the middle of a fully
grounded ice sheet residing on a geothermally and tectonically stable shelf (cf. Fig. 8B).
MCLENNEN (1989) used simple buoyancy considerations to argue that ice shelves could not
exist on these eastern Canadian shelves.
924
J.P.M. SYvrrsra
S O M E U N R E S O L V E D P R O B L E M S IN GLACIAL-MARINE SEQUENCE
STRATIGRAPHY
There are a number of hypotheses that have been proposed that are presently outside
the generally accepted theories on how an ice sheet behaves in the marine environment.
They are typically invoked to explain peculiar seafloor features or deposits located on
glaciated shelves. The acceptance or rejection of these novet explanations would influence
our present interpretation on the dynamics of an ice sheet: for instance whether an ice
sheet was cold based with ice shelves on parts of its terminus, or warm based and
grounded.
Buoyancy-line moraines
KING and FAI)ER (1986), KIN6 et al. (1987), JOSENnANSet al. (1986), raN~ et al. (1991)
suggest a novel hypothesis to describe acoustic diapir-like structures found within their
glaciomarine sequences: "lift-off" or "buoyancy-line" moraines, formed on the periphery
of basins and/or isolated topographic highs where ice shelves made contact with the seabed
(cf. Fig. 8B). The structures are considered to be associated with ice fractures formed by
basal melting processes, and lift off of the landward portion of an ice sheet--the ice
fractures accumulated sediment from englacial discharge and under the influence of tides
deposited these lift-off moraines. I question how a subglacial body of water gets to be
tidal? Push-morainal banks may provide a simpler explanation of these features (i.e.
EYLES and EYLES, 1984; BOULa'ON, 1986). Other possible explanations include such
features as either Roggen moraines or de Geer moraines. I have observed identical
features in a prodelta non-glacial sequence in Goose Bay, Labrador, where data suggest
that sand was injected into low permeability mud within a high sedimentation rate
environment. Could these features be related to the push/squeeze deposits under investigation by POWELL(in press).
Acoustically unstratified sediment
An outstanding problem is the genesis of thick acoustically-unstratified glacigenic
sediments that are stratigraphically overlain by glacial-marine sediments. KING and FADER
(1986), KIN6 et al. (1987) among many others, have interpreted basal seismic layers
located on a glaciated shelf, that are not acoustically stratified (uniform dense pattern of
incoherent reflections) and that have a positive surface relief, as being till. KINO et al.
(1987) further proposed that transgressive till could be distinguished from regressive till
based on their acoustic stratigraphic position and morphology. There are three parts to this
acoustic problem: (1) can we distinguish ice-loaded glacial-marine sediments from
subglacially deposited till; (2) can we distinguish till from other ice-contact deposits such as
grounding-line fans; and (3) can we distinguish till from non-glacial debris flow deposits.
The incorrect identification of till can provide poor data control on ice sheet reconstruction, ice sheet properties and dynamics, especially as related to sediment transport. Many
till units identified in the literature are solely based on their stratigraphic position and
acoustic attributes.
PIPER (1988) argues: "high resolution seismic methods cannot adequately distinguish
between different types of till and coarse ice-margin deposits". A. DYKE, 1989 (personal
Sediment depositionon glaciated continental shelves
925
communication), questions whether acoustic methods could distinguish between a stoney
layer of lodgement till found wedged between intervals of stoney glacial-marine sediment
containing copious shells in life position. Seismic systems would not distinguish these
layers because their mass physical properties are too similar. The acoustic backscatter
would be high for all three layers and even the unit boundaries may not be differentiated.
In this light, CARLSON(1989) interprets similar acoustically unstratified basal units on the
Alaskan shelf as being undifferentiated till, outwash and proximal glacial-marine sediment.
There may be little difference in the acoustic properties of glacial drift and those of a
layer loaded by an advancing ice sheet, especially if the surfical portion of the loaded unit
provides appropriate sound scattering. The upper portion of an ice loaded glacial-marine
sediment may be covered in a till layer, the lower portion of the sequence would be simply
overconsolidated. Good acoustic wave-scattering objects include boulders, and rough
(psuedo-random) surface features such as from iceberg ploughing, and sole marks from
the base of a glacier (SYVITSKIand PRAEG, 1989). If the scattering surface of a seismic unit is
significant, then the entire unit may not show acoustic stratification even if the unit is
lithologically stratified: only a very large impedance contrast, say at the boundary with
bedrock, would be observed below the scattering surface.
Non-glacial deposits such as debris flow deposits are also acoustically unstratified (cf.
SvvrrsKi and FARROW, 1989) and furthermore, mass failure off the face of a prograding
submerged ice-contact fan is considered a relatively common process (PoWELL, 1988). TO
avoid this last problem, VmKS et al. (1989) identified one acoustic unit from Hudson Strait
as representing either till or a debris flow deposit. The problem of interpreting such
diamictons is not limited to marine data (cf. recent discussion and reply with respect to the
interpretation of diamictons as till or debris flows: MANDRYKand ROTTER, 1990; EYLESet
al., 1990), and marine deposits are typically sensed only remotely.
I have found no unambiguous stratigraphic data to support the concept that acoustically
unstratified glacial units are till; portions of these deposits could be till, ice-loaded
glaciomarine sediments or sand and gravel from a submerged grounding-line fan.
JOSENHANSet al. (1986) use the term till to describe Labrador shelf deposits that have a
strong surface reflection, undulating surface, uniform non-stratified acoustic character,
poorly sorted sediment with relatively low shear strength (+25 kPa) and low to normal
consolidation ratios (0.8-2.8). They suggest that the deposits were formed under low basal
loading of a hydrostatic-supported ice sheet in a marine setting. Their "till" units contain
foraminifera, although the numbers are low, fragmented or abraded, but not any lower
than sections of the overlying glacial-marine sediments (long cores through most offshore
"till" sections are almost non-existent: iceberg turbation of the surface of the "till" unit
may have mixed skeletal fragments into the surface of the till). The poor sediment sorting
of the "till" (6% gravel, 25% sand, 25% silt and 44% clay) differs little from the overlying
glacial-marine sediment (5-15% gravel, 15-28% sand, 22-23% silt, and 34-56% clay); nor
does the shear strength, which can be as high in portions of the overlying glacial-marine
sediment. In Hudson Strait, north of the Labrador Shelf, shear strength values up to 15
kPa were observed in glacial-marine sediments with variability related to the degree of
bioturbation, pyritization, and sediment structure (VILKS et al., 1989).
KING and FADER'S (1986) acoustically unstratified "till" units on the eastern Canadian
shelf had mean textural ranges 1-38% gravel, 7-53% sand, 11-43% silt, and 5-49% clay,
although most samples contained < 15 % mud. The overlying glacial-marine sediments are
926
J . P . M . SYVlTSKI
Table 2. Comparison of sequence stratigraphy between Antarctic continental shelves (after HAMm~EYet al., in
press) and eastern Canadian Shelves ( after Srvffsr~ and PP.AE6, 1989) using relative sediment volumes of
individual stratigraphic units
Antarctic shelves
Stratigraphic units
1. Lodgement/basal till
2. Waterlain till
ice-contact (units 1 + 2)
3. Proximal glacial-marine sediments
4. Distal glacial-marine sediments
5. Paraglacial coastal sediments
6. Marine sediments
McMurdo*
Prydzt
19
21
40
5
28
23
6
81
10
91
4
2
0.2
2
E. Canadian shelves
Laurentian,
(Volume, %)
16
39
28
6
12
Northwest Gulf§
33
13
28
4
21
* McMurdo Sound.
t Prydz Bay.
~Upper Laurentian Trough (St Lawrence Estuary).
§Northwest Gulf of St Lawrence.
generally finer-grained, although some units are texturally indistinguishable from the unit
identified as till. These deposits may relate to subglacial deposition, however, the lack of a
significant fine fraction suggests that some of these "tills" may be grounding-line fan
deposits composed primarily of sand and gravel and similar to those identified on the
Alaskan shelf (cf. CARLSON, 1989). ELVERHOIand SOLHEIM(1983) identified a stiff pebbly
mud on the Barents shelf, having shear strength values (40-60 kPa) one order of
magnitude greater than the overlying glacial-marine mud (3-5 kPa) and suggested that the
stiffness could result from either till deposition and/or ice loading onto glaciomarine
deposits.
The implications of incorrectly labelling a sediment unit as being till include poor data
control for ice sheet reconstructions, ice sheet properties, and ice sheet dynamics,
especially as they relate to sediment transport. Dates obtained from a misidentified till
could suggest, for instance, an ice sheet was on the continental shelf when it may have
actually been ablating on land. Confusing ice loaded glacial-marine sediment with till,
would not affect the dimensions of ice sheet reconstruction, but rather hamper our
understanding of the sediment pathways through an ice sheet with implications on ice
stream dynamics and thermal properties.
If these acoustically unstratified layers were deposited subglacially, they would account
for >60% (by volume) of the Quaternary sediment located on the eastern Canadian shelf
(data after JOSENHANSand FADER, 1989), >80% on the Barents shelf (data after VORRENet
al., 1989), >90% on large parts of the Norwegian shelf (data after KING et al., 1987), and
>75% in the inter-island channels in the arctic archipelago (data after MACLEAN et al.,
1989). Table 2 compares mass balance calculations based on Antarctic core data (the
CIROS and ODP data of HAMBREYet al., in press) and seismo-stratigraphic and core data
from the Gulf of St Lawrence (after SvvrrsKt and PRAEG, 1989). The wide range in the
proportions of the stratigraphic units should be noted.
In all cases, the volume of till deposits is high when compared to sediment mass balance
studies from modern glacier and ice shelf regimes (ELVERHOIet al., 1989; GRIFFrrHs and
Sediment deposition on glaciated continental shelves
927
ANDERSON, 1989; POWELL,1988). For instance, SvvlxsKI (1989a) calculated the flux of
sediment through an arctic tidewater glacier as being 86% by glaciofluvial discharge, 10%
by supraglacial dumping and ice-rafting, and 4% by subglacial (till) deposition, values
supported by other researchers (for example, DOWDESWELL, 1987; MACKIEWICZ,1984).
This enormous discrepancy between modern sediment-mass balance studies and volumes
indentified for Pleistocene subglacially transported sediment must be addressed. Where
are the appropriate outwash deposits located on these continental shelves? Is a portion of
the identified "till" units composed of loaded glacial-marine sediments? In summary, I
suggest that application of genetic names such as till and till tongues should be considered
carefully or avoided; these units could be referred to solely as ice-contact sediments.
Marine diamictons
VORRENet al. (1978) considered the origin of diamicton on the Norwegian continental shelf
in terms of the following hypotheses: (1) till deposited at the base of a grounded glacier; (2)
deposition from sea ice (pack ice); (3) deposition from the undermelt of an ice shelf; and
(4) deposition or turbation from icebergs. They eliminated hypothesis (1) as their unit had
too low a shear strength, and contained both foraminifera and laminations; hypothesis (2)
on the basis that the clasts were too angular for a beach origin; and hypothesis (3) on the
basis of too high a foraminiferal content. They accepted hypothesis (4) because paleDiceberg furrows were observed, and the pebble lithology and foraminiferal content
supported an iceberg rafting origin. However, if an unstratified acoustic unit is located
below a glacial-marine unit, an argument can be made for either an ice loaded or ice
deposited scenario, and stratigraphic arguments based on unit geometry and geographic
location must be used to choose between these hypotheses in the absence of samples.
EDWARDS et al. (1987) contrast two ways that an overconsolidated diamicton can be
achieved: (1) glacial-marine sediment is over-ridden and loaded by an advancing ice
stream; and (2) lodgement till is produced through subglacial deposition beneath a
grounded ice stream (see Fig. 9). They suggest that the overconsolidated diamicton that
covers much of the Antarctic shelf was formed as a lodgement till (Fig. 9). ELVEV,n~I and
SOLHEIM (1983) note other alternate mechanisms in the development of stiff pebbly
sediment: (3) surficial sediment removal, i.e submarine sliding or traction currents; and (4)
dewatering associated with the progression of a freezing front as with the development of
permafrost. A fifth mechanism is from icebergs furrowing the seafloor where stiff pebbly
mud with strengths up to 100 kPa can be produced (H. CHRISTIAN, 1989, personal
communication). Shear strengths in excess of 200 kPa are considered to represent true
subglacial deposits (A. ELVERrI~I, 1990, personal communication).
Till tongues
Wedge-shaped deposits of an acoustically uniform dense pattern of incoherent seismic
reflections interlayered with stratified galcial-marine sediment have been termed "till
tongues" (KING and FADER, 1986; KING et al., 1987; JOSENnANSand FADER, 1989; IrdN~ et
al., 1991) (Fig. 10). They are considered to represent the migration paths of the gounding
(buoyancy) line of glaciers, where till is deposited from the meltout of debris immediately
proximal to the lift-off point of an ice sheet (KINc et al., 1987). The formation
928
J . P . M . SYVlTSm
BASAL TILL MODEL
LOADED GLACIAL-MARINE MODEL
A
Sea level
lowered = 120 m
B
Present sea level
D
Sea level
lowered - 120 m
[ Loaded glacial-marine sequence
Fig. 9. A schematic profile through an ice sheet/ice shelf showing contrasting models that may
lead to the development of heavily overconsolidated diamicton on the Antarctic shelf (after
EDWARDSet al., 1987).
of till tongues would therefore require the presence of an ice shelf (Fig. 10). ANDERSONand
MOLNIA(1989) argue against the proposed interpretation of these "till tongues". The KING
and FADER (1986) "till tongues" always occur near basin margins and have the geometry
and acoustic properties of slump deposits and have been so interpretated by GIPP (1990).
VORREN et al. (1989) describes similar features as grounding-line fans composed of ice
contact or proximal glaciomarine sediment, not from an ice shelf, but from a tidewater
glacier. The ALLEYet al. (1989) "till-delta" was also supposed to reflect the deposition of
glacigenic sediment at the grounding line of an ice shelf (Fig. 5), but has quite a different
geometry compared to the "till tongue" (Fig. 10).
In almost all cases where till tongues are reported, these features are intimately
associated with moraines being found seaward and downslope. In consideration of the
above discussion, I propose that these features relate to quasi-stable ice terminus positions
wherein a morainal bank is developed. Because moraines are tidewater glacial features,
TILL TONGUE MODEL
1
3
Lowered Sea Level
M~rm~nseaward
2:::22
'
s
s
s
s
.
.
s
.
~
.
.
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.
.
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.
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.
.
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.
.
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.
.
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s
.
.
2
s
s
s
s
s
I
s
,
l
~
l
l
l
l
/
I
/
/
l
l
l
l
l
/
l
l
S
l
t
l
S
l
l
l
l
/
' s ~ s s ~ s ~ l s s s s s s s s s ~ l s ~ s s ~ s ~
. . . . . .
4
sea Level Ri~e
Glaciomarine
l
l
........
Retreat of
Buoyancy Line
/-Buoyancy Line
S..*.,'SS,.',rSSS.,'.,'*'..'*'.,',.','*'SllJSSSSJ/
GULF
=,o t
•~
OF
MAINE
!
i¸
!
• i
!
I
I;
E
ff
uJ
a
280
W
~300
a.
I1,
320
340
0
1
2
km
Fig. 10. (A) A conceptual model (1 through 4) for the development of till tongues through the
migration of the buoyancy (grounding) line during sea level rise (after KINGand FADER,1986). (B)
The lower seismic profile is an example of a "till tongue" (G. FADER,personal communication,
1990) based on high-resolution HUNTEC ~ DTS reflection seismic data.
I
930
J . P . M . SYVXTSFO
DEVELOPMENT OF A FRONTAL
DUMP MORAINE OR FAN
I I
INCREASED SEDIMENTATION
AND FAN PROGRADATION
ICE SHEET ADVANCE
Fan
r l
|
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
RETREAT OFICE SHEET
c
Burial of
, //~/¢///////~
DECREASED SEDIMENTATION
AND BURIAL OF FAN
E
Buried Fan
ii!i!i!ii!iiiii!iiiii!iii!i!!iiiiiiiiiiiiiiii-:):i
Fig. 11. Two alternate hypotheses to the formation of what have been identified as till tongues
(ef. Fig. 10; IONG and FADER, 1986; KINO et al., 1991) starting with the aggradation and
progradation of a frontal moraine by a tidewater ice terminus (A). The first hypothesis involves the
advance of the ice sheet terminus over the moraine and the generation of a debris flow onto
proximal glacial-marine sediments (B). Subsequent retreat of the ice terminus just landward of the
moraine buries the debris flow by glacial-marine sediment (C). The second hypothesis involves a
period of rapid growth of a grounding line fan (D), followed by a reduction in sediment discharge
and burial of the fan terminus (E).
the ice sheet had a grounded and relatively vertical terminus. During the aggradation
processes, slumping may occur in the form of a debris flow, particularly if the glacier
advanced into the moraine deposit (Fig. 11). A modified version of this hypothesis would
have these interfingering tongues and glacial-marine mud relate to fluctuating discharge of
Sediment depositionon glaciatedcontinental shelves
931
water and sediment during moraine development and therefore relate to fingers of
outwash fans (Fig. 11, cf. Fig. 6).
SUMMARY
The study of sediment deposition on glaciated continental shelves is a new science,
driven in part by offshore development and environmental concerns. Process-oriented
studies of end-member glaciated shelf environments provide important models that
together constrain our interpretation of ancient glacial-marine sequences. We must
continue to examine the causal relationships between ice dynamics, climate, and
oceanography--particularly with a view to problems related to global change. We need
more field data on subglacial dynamics within marine-based ice sheets, including the
ground water hydrology beneath an ice sheet. Numerical models must address the issue of
time-variable deformable beds over the entire history of ice sheet development and
collapse. Such models should include lag effects, for instance between ice load and mantle
response. What is the nature of buoyancy drawdown as a causal factor in ice sheet
collapse.'? We need finer grid three-dimensional finite-element models for moving ice
sheets into the marine environment and predicting sediment deposition through a highly
complex and interrelated set of oceanographic and sedimentation processes.
Of the stratigraphic tools examined, all provide important data for paleoenvironmental
interpretation. However, none are without problems. Outstanding seismo-stratigraphic
problems include: recognizing ice-loaded glacial-marine sediments from till, and distinguishing till from other ice-contact deposits, that is grounding-line fans, and from nonglacial debris flow deposits. Interestingly, seismo-stratigraphic information in combination with a full suite of lithologic, biologic, and chronostratigraphic data still cannot
unambiguously distinguish a "till" unit from an ice-loaded or an ice-proximal glacialmarine deposit. Arguments based on unit geometry and location, and stratigraphic
position appear to be more capable in resolving such issues. There are six ways for glacial
sediment to become overconsohdated, and five ways that a marine diamicton may form.
The implications of incorrect genetic labelling, for example of till, include providing poor
data control on ice sheet timing, ice sheet properties and dynamics, especially as they
relate to sediment transport. Till tongues and lift-off moraines are examples of reconstructing ice sheet dynamics from the sedimentary record in the absence of modern
analogue environments. Until more conclusive information or theory is published, I
suggest that these seafloor features be given nongenetic terms, for mapping purposes.
Antarctica provides an end-member in the spectrum of glacial-marine environments,
having a preponderance for ice shelves, ice flow by internal creep, and limited meltwater.
Research should provide better evidence for turbidity current and underflow generation
considering the lack of meltwater production and the extremely low rates of open shelf
sedimentation. Northeast Pacific shelves provide the other end of the spectrum, dominated by turbid meltwater processes from tidewater glaciers that are highly erosive during
their advance phase. Research should focus on combining high-resolution and sequential
geophysical surveys and coring operations fronting glaciers in active retreat, considering
the enormous rates of sedimentation available. The Atlantic shelves lie somewhere
between these two end-member glaciated shelves. They appear dominated by tidewater
glaciers, except possibly in the more northern polar margins. There appears to be a
growing concensus that ice shelves were not common at the marine margin of temperate
932
J . P . M . Svvrrsra
ice sheets. E v e n t h e ice shelves o f A n t a r c t i c a m a y r e p r e s e n t o n l y a H o l o c e n e d y n a m i c
r e s p o n s e to g l o b a l d e g l a c i a t i o n o f t h e arctic ice sheets. I c e shelves m a y n o t h a v e b e e n
i m p o r t a n t d u r i n g p a s t A n t a r c t i c g l a c i a t i o n s (P. J. BARRET, 1990, p e r s o n a l c o m m u n i cation).
B a s e d o n t h e v e r t i c a l s t r a t i g r a p h i c s e q u e n c e o f d e p o s i t s f r o m 20 o f t h e w o r l d ' s g l a c i a t e d
s h e l v e s , a c o m p l e t e d e g l a c i a l s e q u e n c e consists o f s o m e o r all o f t h e following: (1) icec o n t a c t ( i c e - d e p o s i t e d a n d / o r i c e - l o a d e d ) s e d i m e n t s ; (2) i c e - p r o x i m a l s e d i m e n t s ; (3) icedistal s e d i m e n t s ; (4) p a r a g l a c i a l c o a s t a l s e d i m e n t s ; a n d (5) p o s t - g l a c i a l s e d i m e n t s . T y p i cally, t h e s e q u e n c e u n c o n f o r m a b l y o v e r l i e s s e d i m e n t a r y r o c k s o n t h e o u t e r shelf a n d a
m i x e d b e d r o c k b a s e m e n t o n t h e i n n e r shelf. T h e r e l a t i v e v o l u m e o f t h e s e d e g l a c i a l units
p r o v i d e i m p o r t a n t clues for t h e r e c o n s t r u c t i o n o f ice s h e e t d y n a m i c s . D e p o s i t i o n o f unit
(1) o c c u r s initially d u r i n g t h e p e r i o d o f g l o b a l l y ( e u s t a t i c ) l o w e r e d s e a level, a n d l a t e r with
units (2) a n d (3) d u r i n g a p e r i o d o f high s e a levels r e l a t e d to i s o s t a t i c l o a d i n g . D e p o s i t i o n
o f unit (4) o c c u r s d u r i n g t h e initial fall in sea level (isostatic r e c o v e r y ) w h e n ice has
r e t r e a t e d o n l a n d a n d is r a p i d l y a b l a t i n g . D e p o s i t i o n o f unit (5) occurs d u r i n g m o r e
c o m p l e x a n d local s e a level fluctuations, t h a t i n c l u d e such affects as t h e c o l l a p s e o f t h e
m a n t l e f o r e b u l g e . M i n o r v a r i a t i o n s to this a s s o c i a t i o n o f s e a level a n d s e d i m e n t a t i o n
p a t t e r n o c c u r , p a r t i c u l a r l y if t h e glacial cycle is o u t o f p h a s e with t h e g l o b a l e u s t a t i c cycle.
Acknowledgements--I would like to thank Drs John Anderson (RU), John Andrews (INSTAAR), Geoffrey
Boulton (UE), Anders Elverh¢i (UO) John Luternauer (GSC), Brian MacLean (GSC), David Piper (GSC), and
Ross Powell (UNE), who have shared their ideas and data, and for encouraging a critical analysis of their work. I
subsequently thank them for providing critical comments on an earlier version of this manuscript. This forms
Geological Survey of Canada Contribution No. 39590.
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