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2 Continental slope and basin sedimentation adjacent to an ice­margin: a continuous sleeve-gun profile across the Labrador Slope, Rise and Basin R. Hesse Department of Earth and Planetary Sciences, McGill University, 34;0 University Street, Montreal, Quehec, Canada, HA3 2A7

lntroduction PubBeation of continuous seismic profiles in atlas format permits direct comparison of differences in evolution of major architectural elements of modern continental margins. The accompanying > 700 km long sleeve-gun profile across the Labrador Margin (Fig. 2.1(a)) and basin floor (Fig. 2.1(b) and (c)) shows the Quaternary evolution of a continental margin adjacent to waxing and waning continental ice-caps. (Publication of separate portians of this profile in journal format resulted in fragmentation of the record and loss of essential information (cf. Figs 7, 21 and 22 of Hesse and Rakofsky 1992 and Fig. 4 of Hesse 1992). Startingon the middle Labrador Slope at c. 1700m water depth and extending across the lower slope, rise and basin centre to 3400 m water depth, the profile (Fig. 2.1(a)-(c)) covers major parts of the Labrador Margin, except the upper slope and shelf, including the prominent leveed channel of the basin floor, the Northwest Atlantic Mid-Ocean Channel (NAMOC). Figure 2.1(d), which parallels Fig. 2.1(a), and is located 60 km to the south, demonstrates along-slope variation in sedimentary processes and sediment preservation. The descriptions of architectural elements below are summaries of the main depositional and structural Features on and in the subsurface of the slope and adjacent basin. Detailed discussion of specific Features shown by the seismic profiles is found in the caption for Fig. 2.1.

Architectural elements of the Labrador Margin

Middle slope (between 1800 and 2300m water depth): Sediment­creep, sliding, slumping and debris­flow generation

Creep, sliding and slumping are the dominant processes shown in Fig. 2.1(a) and (d), representing a sequence of slow mass-movement processes with an increasing degree of internal deformation and loss of coherence of the sediment. Wherever the downslope component of the shear stress induced by the weight of the sediment exceeds sediment cohesion, failure will occur on discrete shear planes and cause slumping. If the failed sediment is accelerated sufficiently it will eventually lose its internal coherence entirely and be turned into cohesionless debris flows.

Lower Labrador Slope (between 2300 and 2800m water depth): Debris-flow slope

The debris-flow deposits of the lower slope are seismically largely transparent and separated by discontinuous, indistinct reflections. The latter are caused by the acoustic cantrast between some of the debris-flow units (DFD), but there are probably many more individual debris-flow deposits than there are units marked by reflections. A piston-core analysis of debris-flow deposits on the outer levee of the NAMOC indicated the presence of at least two or three stacked deposits (cores 87-02, -09 in Fig. 40.2, Hesse, Ch. 40), where a high-resolution seismic profile revealed only one, up to 20m thick, unit (Hesse et al. 1990, Fig. 6). The parallel surface echoes represent a drape of stratified sediment. Debris-flow bodies are generally elongated in the downslope direction (Aksu and Hiscott 1992). How extensive they are laterally on this sector of the Labrador Slope cannot be deduced from Fig. 2.1(a) and (d) because the distance between the represented sections is too great.

Atlas of Deep Water Environments: Architectural style in turbidite systems. Edited by K.T. Pickering, R.N. Hiscott, N.H. Kenyon, F. Ricci Lucchi and R.D.A. Smith. Published in 1995 by Chapman & Hall, London. ISBN 0 412 56110 7.

Mass wasting by debris-flow development occurs in sediment-oversupplied areas with very high sedimentation rates, such as the areas seaward of major shelf glaciers. On the Labrador Margin, the ice­front of the latest Laurentide ice-cap was probably grounded in 400-700m water depth (Fillon and Harmes 1982; Josenhans et al. 1986). Terminaland ice­push moraines are expected to have formed at these depths. Sediment-transfer from the subglacial and englacial melt-water drainage system to the sea probably involved both hyperpycnal and hypopycnal flow. Traction and bedload material was probably largely transferred to the heads of major submarine canyons forming channelized turbidity currents. Fine­grained suspended sediment would rise by fresh-water buoyancy and form turbulent surface plumes (Pfirman and Solheim 1989; Syvitski 1989), which would spread the sediment over !arge areas on the upper slope, supported by the effects of katabatic winds. The southward-flowing Labrador Current would entrain the surface plumes and cause deposition of fine-grained sediment south of the ice-tongues of shelf-glaciers. Some of the sediment-laden melt-water discharge might feed mid-depth nepheloid layers, which may also receive sediment from hemipelagic settling. These processes, augmented by the effects of ice rafting and turbidity currents spilling over the canyon walls, produce seismically well-stratified deposits in intercanyon areas on the upper slope and the upper half of the middle slope, as careful core analysis shows (Wang 1992). Slope angles are highest on the upper slope. Where the sediment input is most intense, oversteepening of the slope causes collapse.

In cantrast with Fig. 2.1(a), nearby slope section Fig. 2.1(d) shows stratified sediments throughout, which are also involved in creep and sliding, but show a lesser extent of slumping and debris-flow development. Figure 2.1(d) is located south of the major shelf glacier of Karlsefni Trough (cf. Fig. 3.1, Hesse and Klaucke, Ch. 3) in an area shielded from the direct sediment fall-out from the ice-tongue and protected from the effects of the most intense injection of sediment into the sea, thus having been less subjected to oversteepening and mass wasting.

Labrador Rise (between 2800 and 3300 m water depth): Spillover turbidites

The lower slope to rise transition is characterized by the change from predominant mass-wasting and denudational to predominantly depositional processes associated with a distinctly lower slope gradient. The sediment-wave field on the lower part of Fig. 2.1(a) occurs on the right-hand levee of canyon EAB. Extensive sediment -wave fields on the rise and slope and in the subsurface have previously been interpreted as contourites (e.g. Myers and Piper 1988). The re­interpretation of the mud waves on the Labrador Rise as spillover turbidites is supported by core analyses (Wang and Hesse, unpublished data), sidescan sonar imagery (Praeg and Schafer 1990), and is in line with current interpretations of similar wave fields elsewhere (Damuth 1979; Normark et al. 1980; EEZ-Scan 84 Scientific Staff 1988). Development of these Features preferentially on the right-hand (southern) canyon levees is a result of the Coriolis force which enhances asymmetric spillover (to the right). Turbiditic mud waves probably represent a draping phenomenon in which a stack of younger turbidites drapes a single mud-wave event at the base.

Basin floor: Yazoo-type tributaries, debris-flow tongues and main trunk channel deposits

The basin floor of the Labrador Sea is covered by a network of converging tributary channels that, in a yazoo-type fashion, flow subparallel to the main trunk channel (the NAMOC) for long distances (Hesse 1989). These tributary channels are fed by the slope canyons with turbidity currents and debris flows and form an interconnected, basin-wide 'submarine drainage system' (Hesse and Rakofsky 1992). Some of the debris flows have overtopped the channel banks and formed huge debris-flow tongues on the basin floor (Hesse et al. 1990), as seen in Fig. 2.1(b) between channels EA and D and between D and the NAMOC. The NAMOC

(Fig. 2.l(c)) displays the strongest size fractionation between channel-fill and adjacent levee deposits as a result of its great depth Cup to 200m). Its levees are muddy, consisting predominantly of mud turbidites; the channel-fill facies consists of gravel and sand (Hesse et al. 1987). Tributary (or submarine yazoo­type) levees are sandier and thus much more similar to their associated channel deposits than the NAMOC levees. East of the NAMOC, the prevailing sand-rich facies are interpreted as deposits of a submarine braid­plain (BP). One of the tributary channels (channel B) is continuous and can be traced from profile to profile, whereas the numerous very shallow channels cannot be correlated. Recent sidescan sonar imagery suggests that this braid plain may have formed by cataclysmic subglacial Iake outburst flooding through Hudson Strait (Hesse et al. 1993). The basin floor is not flat but displays considerable relief generared by channels of varying depths and their associated levees that may rise markedly above the base Ievel of the basin, 80 m in the case of the NAMOC. The deposits of the channels and levees of the basin floor are distinct and will have recognizable ancient analogues in the geologic record that may have been incorporated into accretionary wedges or collision zones.

Acknowledgements Editorial suggestions by R.N. Hiscott are acknowledged with thanks. The profiles of Fig. 2.1 were obtained during Cruise 90-13 with the excellent cooperation of Master, officers and crew of CSS Hudson and logistical and financial support from the Atlantic Geoscience Centre, Bedford Institute of Oceanography, Dartmouth, NS. Funding came from NSERC and DEMR, Ottawa, Ontario, Canada.

Heferences Aksu, A.E. and Hiscott, R.N. 1992. Shingled Quaternary

debris flow Jenses on the northeast Newfoundland slope. Sedimentology, 39, 193-206.

Damuth, JE. 1979. Migrating sediment waves created by turbidity currents in the northern South China Basin. Geology, 7, 520-523.

EEZ-Scan 84 Scientific Staff 1988. Physiography of the western United States Exclusive Economic Zone. Geology, 16, 131-134.

Fillon, R.H. and Harms, R.A. 1982. Northern Labrador Shelf glaciation: Chronology and Iimits. Canadianjoumal of Earth Sciences, 19, 162-192.

Hesse, R. 1989. 'Drainage systems' associated with mid­ocean channels and submarine yazoos: Alternative to submarine fan depositional systems. Geology, 17,

1148-1151. Hesse, R. 1992. Continental slope Sedimentation adjacent to

an ice-margin. I Seismic facies of Labrador Slope. Geomarine Letters, 12, 189-199.

Hesse R. and Rakofsky, A. 1992. Deep-sea channel/submarine yazoo system of the Labrador Sea: A new deep-water facies model. American Association of Petroleum Geologists Bulletin, 76, 680-707.

Hesse, R., Chough, S.K. and Rakofsky, A. 1987. The Northwest Atlantic Mid-Ocean Channel of the Labrador Sea. V. Sedimentology of a giant deep-sea channel. Canadianjoumal of Earth Sciences, 24, 1595-1624.

Hesse, R., Klaucke, I., Konyukhov, B., Ryan, W.B.F., Keeley, C., Piper, D.].W., Sparks, R., Mayer, LA., Edwards, M.H., Johnson, D.J., Shor, A.N. and Won, J.S. 0993). Hawaii MR-1 sidescan sonar imagery of the Northwest Atlantic Mid-Ocean Channel of the Labrador Sea. Geological Society of America, Ahs. with Programs, 25(6), A-444.

Hesse, R., Rakofsky, A. and Chough, S.K. 1990. The central Labrador Sea: Facies and dispersal patterns of clastic sediment in small ocean basin. Marine and Petroleum Geology, 7, 13-28.

Josenhans, H.W., Zevenhuissen, J. and Klassen, R.A. 1986. The Quaternary geology of the Labrador Shelf. Canadian Journal of Earth Sciences, 23, 1190-1213.

Myers, R.A. and Piper, D.J.W. 1988. Late Cenozoic Sedimentation in the northern Labrador Sea: a history of bottarn circulation and glaciation. Canadian Journal of Earth Sciences, 25, 2059-2074.

Normark, W.R., Hess, G.R., Stow, D.A.V. and Bowen, A.J. 1980. Sediment waves on the Monterey fan levee: A preliminary physical interpretation. Marine Geology, 37, 1-18.

Pfirman, S. and Solheim, A. 1989. Subglacial meltwater discharge in the open marine tidewater glacier environment. Observations from Nordaustlandet, Svalbard Archipelago. Marine Geology, 86, 265-281.

Praeg, D.B. and Schafer, C.T. 1990. Sea-bed features of the Labrador slope and rise near 55°N revealed by SEAMARK1 side scan sonar imagery. Geological Survey of Canada, Open File, 29 pp.

Syvitski, J.M.P. 1989. On the deposition of sediment within glacier-influenced fjords: Oceanographic controls. In: Powell, R.D. and Elverhoi, A. (eds) Modem Glaciomarine Environments: Glacial and marine controls of modern lithofacies and hiofacies. Marine Geology, 85, 301-329.

Wang, D. 1992. Continental-slope Sedimentation, Labrador Sea: Depositional facies and glacial cycles. MSc thesis, McGill University, 92 pp.

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Fig. 2.1. The 'crumpled-rug geometry' of the upper half of the middle slope (1700-2300 m, upper quarter of (a) and most of (d)) is typical of sediment creep and sliding. The 'pseudocylindrical fold-structures' (SL in (a) and (d)) are neither true tectonic structures nor sediment waves but represent deformation structures due to creep and, in part, sliding. Slide blocks can be identified by their asymmetry. Horizontal or slightly landward dipping reflectors that terminate upwards abruptly against the slope surface form the toe of slide blocks that are disguised by hyperbolic side-echoes on their steep downslope side (near 'SL' in (a) and (d)). (a) Profile shows two slump scars (SC at about 1700 and 2250 m water depth) where the failed sedimenr has moved out and formed debris flows.

Below 2250 m down to canyon EAB at c. 2900 m, the entire Lower Slope in (a) is underlain by debris-flow deposits. (Note that the Ietter designation of the canyons denotes the order of branching: first-order tributaries to the NAMOC, one Ietter; second-order tributaries, two letters, etc., with the alphabetical order being counter­clockwise araund the basin.) The debris-flow slope in the centre of (a) displays a narrow sequence of parallel surface echoes and thick, relatively transparent, sediment -bodies separated by discontinuous, indistinct reflectors, which represent the bounding surfaces of some, but not all, individual debris-flow deposits (DFD). Some DFD are seen in (d) in the subsurface and a slump scar appears tobe present at the right margin of this section where subsurface reflectors surface, but compared to (a) to the north, debris-flow development and slumping are less intense.

The Labrador Rise (2900-3300 m) between canyons EAB and EA in (a) is dominared by spillover-turbidite sequences forming a sediment-wave field. The section transects the right-hand levee of canyon EAB.

The basin floor (below 3300 m water depth in (b) and (c)) is dominared by the central trunk channel of the NAMOC with its natural levees rising up to 80 m above the adjacent basin plains. The echo character of the well­bedded, seismically semi-transparent levees with continuous reflectors is distinct from the other geomorphic elements of the basin floor, i.e. the trunk channel with its Jens of underlying highly reflective material (channel-fill facies) and the more reflective tributary channels (SCF, secondary channel-fill facies) and tributary levees (YL, yazoo levee). The western levee of tributary B onlaps (OL) and partially eroded the eastern NAMOC levee. DFD form seismically distinct sediment bodies that occur as channel-fill in some of the tributary channels (e.g. EA) or as huge tongues on the levees between the channels (e.g. between EA and D and between D and the NAMOC).

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