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A continuous along-slope seismic proftle from the Upper Labrador Slope R. Hesse and I. Klaucke Department of Earth and Planetary Sciences. McGill 1Jnivers1ty. 3450 University Street. Montreal. Quebec. Canada. H3A 2A 7

lntroduction The Plio-Pleistocene Labrador Slope has been shaped by ice-marginal processes of the successive Laurentide ice-caps on the Canadian Shield. Drainage from the Laurentian ice-cap into the Labrador Sea occurred through fjords dissecting the bedrock of the Canadian Shield and through channels across the Labrador Shelf, where these channels are variously called troughs, saddles or channels. To the south of the main outlet in Hudson Strait, these are, in north-to-south order, Karlsefni Trough, unnamed trough, Hopedale Saddle, unnamed saddle, Cartwright Saddle, Hawke Saddle and Notre Dame Channel (Fig. 3.1).

There is a pronounced lithological difference between the detrital sediments supplied through the Hudson Strait, and the smaller outlets to the south. The ice age drainage area of the Hudson Strait comprised vast regions underlain by Paleozoic carbonates, i.e. Baffin Island, Foxe Basin and Hudson Bay and surroundings (Fig. 3.2). Turbidites in the Northwest Atlantic Mid-Ocean Channel (NAMOC) and its levees, which have their source in the Hudson Strait outlet and in the southern Baffin Bay fjords, are therefore rich in detrital carbonate (Chough et al. 1987). Detrital carbonate of northern Greenlandian and northern Canadian provenance, including the Canadian Arctic Archipelago, is also a characteristic component of Holocene ice-rafted debris in the Labrador Sea ( Gilbert and Barrie 1985) and western North Atlantic (Heinrich 1988). Significant proportians of the detrital carbonate have been ground to clay-sized particles and make up c. 40o/o of the < 2 pm fraction in the turbidite muds on the NAMOC levees (Chough et al. 1987). Their

Fig. 3.1. Location map showing the system of tributary channels and canyons (labelled counter­clockwise araund the basin periphery) to NAMOC and localities mentioned in text.

Fig. 3.2. Source areas of Lower Paleozoic carbonates supplied through the Hudson Strait outlet (from Josenhans et al. 1986).

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 RD.A. Smith. Published in 1995 by Chapman & Hall , London. ISBN 0 412 56110 7.

Fig. 3.4. Detailed branching pattern of the N-, D- and E­canyon systems on the northern Labrador Slope.

proportion in the silt fractions is probably even higher. This material was introduced as glacial flour into the Labrador Basin, but requircd special mechanisms for redistribution on the slope, rise and basin floor, discussed in the next section.

In cantrast to the Hudson Strait outlet, the fjord­connected sources on the shelf to the south supplied much greater proportians of locally derived igneous and metamorphic crystalline material of generally coarser grain size.

The continuous, 350 km long, seismic profile on the Upper Labrador Slope (Fig. 3.3) is the slope-parallel counterpart of the cross-slope-to-basin profile from the Labrador Sea published elsewhere in this atlas (Hesse, Ch. 2). It is the northern half of a 750 km long sleeve­gun profile (starting off Hamilton Bank at 54 °N) which

was acquired during CSS Hudson cruise 92-045 tagether with 3.5 and 12kHz seismic and bathymetric profiles (Hesse and Klaucke 1994). The portion shown in Fig. 3.3 starts c. 5JD45'N off the southern margin of Karlsefni Trough in c. 1600 m water depth and ends off Hudson Strait in 1500 m water depth at 60°40'N. Slope­parallel profiles from the middle and lower Labrador Slope and Rise have been published previously (Hesse 1992). Compared to these earlier profiles, the profile in Fig. 3.3 illustrates the dramatic increase in relief and canyon branching upslope. In addition to these variations in cross-slope morphology, Fig. 3.3 also reveals pronounced along-slope variations in relief and echo-character that can be interpreted in terms of sediment provenance and delivery mechanisms.

Description of morphology and echo character Morphologically, the portion of the sleeve-gun profile shown in Fig. 3.3 consists of four sectors which are associated with the northernmost mapped three major canyon systems on the Labrador Slope (labelled N, D and Ein counter-clockwise order; progressively higher­arder branches are designated by a hierarchical lettering scheme; Fig. 3.4; see also Hesse and Rakofsky 1992). From north (top right-hand corner of Fig. 3.3) to south (lower left-hand corner) these four sectors are:

1. A northernmost low-relief portion c. 30 km lang (canyons NBABBB to NBABBD) which is part of the low-relief debris-flow slope off Hudson Strait.

2. A maximum-relief zone with deeply incised, mostly narrow, canyons of the N- and D-systems that are between 200 and 700 m deep and show progressively decreasing relief southward (NCABA to DBBABA).

3. A low-relief sector to the south comprising the EA canyons and the northernmost canyons of the EB group (including EBABB). Relief between the canyons and flanking sediment ridges is generally less than 100m.

4. A moderate-relief segment associated with the remaining northern canyons of the EB group (canyons EBBAAAA to EBBABC) with reliefnot exceeding 300m.

In detail, the four slope sectors have the following morphological and echo characteristics, respectively:

1. Relief differences between canyons and levees on the northernmost segment of the profile are less than 120m. The canyon walls display hyperbolic echos indicative of wall-gullying and slumping. The canyon floors are underlain by more highly reflective sediment with prolonged echos, whereas the levees consist of !arge pockets of transparent sediment with a few distinct reflections. The more highly reflective sediment extends southward under the adjacent (right­hand) levee at greater sub-bottarn depth indicating northward canyon migration. Sound penetration is up to 900 ms two-way travel time (or up to 450m sub-bottom).

2. The maximum-relief zone (NCABA to DBBABA) is morphologically and structurally most remarkable, particularly in comparison with the adjacent low-relief slope sectors immediately to the north and south. This high-relief sector is

c. 200 km lang and shows a distinct southward decrease in relief. The highest relief is found on the southern flank of the more than 5 km wide plateau at the northern end of the sector where the flank of the plateau drops 700 m into the NCABA canyon. Ridge-crest to canyon-floor depth differences of 400-500 m are not uncommon along the next five to ten ridges to the south, whereas further south relief decreases to 300m and finally 200m at the southernmost of the D-canyons (DBBABA).

The canyons are either narrow and more or less empty, or broad and sediment-filled. Same of the narrow, seemingly empty canyons (e.g. canyons DBAABACA, and DBAABACB and DBAABCB) are nevertheless underlain by higher reflectivity-sediment with prolonged echos suggesting the presence of some coarse-grained canyon-fill material. The floor deposits of the sediment -containing canyons are either: (a) nearly flat-topped (e.g. NCBAAB); (b) show a thalweg­like depression on one side (e.g. NCBBABBA); (c) show a strongly north-to-south sloping surface with a small sediment apron on the south-canyon wall (e.g. NCBBAAB and, with a small central ridge, DBAAAABC), underlain by higher­reflectivity sediment with conformable reflections; (d) vice versa of (c) showing a south-to-north sloping surface (e.g. canyons between NCABA and NCBAAB) with the same characteristics as (c); (e) display a nearly symmetrical profile (e.g. DBABA) or CD an irregular surface (e.g. NCBBBBD, canyons south of DABB). In almost all canyons, the reflectivity of the canyon-fill sediment is higher than that of the adjacent ridges, which is partly an artifact due to overlapping side-echos and the angle of dip of the deposits.

The ridges are either symmetrical or have a shoulder on the south side. Same are double ridges, but all (except the plateau-like northernmost one) appear very steep at the vertical exaggeration of the profiles, c. 25 times. Their internal reflectors repeat the morphological outline with dip angles often decreasing at greater sub-bottarn depth. As a consequence of this, some of the narrow, acute ridges display a diapir-like appearance in cross-section (e.g. ridge between canyons DBAAAABA and DBAAAABB). Sediment-reflectivity under the ridges is distinctly lower than under the canyons and is at a minimum under the ridge crest where a white-out may occur. These ridges reveal a maximum

sound penetration of up to 1 s two-way travel time (or up to 500m sub-bottom, e.g. northern plateau with its parallel reflections in a largely transparent sediment column; ridge south of canyon NCBAAB; ridge north of canyon NCBBAAB).

3. The low-relief slope sector to the south off the southern end of Saglek Bank is c. 55 km lang with rather uniform morphology and echo characteristics. Relief between canyons and adjacent ridges does not exceed 100m. This sector is located just north of a major outlet from the ice-cap that occurs in front of a major slope canyon, (the EB canyon).

Sound penetration in this sector of the profile is low, not exceeding 200-300 ms two-way travel time (or 100-150m sub-bottom). Stratification is poor, consisting of indistinct, discontinuous reflections and a prolonged surface reflection. Off Saglek Bank, the upper sediment layer is transparent. The high reflectivity compared to immediately adjacent sectors of the profile is in part an artifact caused by modified recorder settings imposed by changing weather conditions.

4. The moderate-relief sector off the northern part of Nain Bank is c. llOkm lang and shows relief differences between V-shaped canyons and adjacent ridges of up to 300m. Reflectivity of the upper 100-200 m of the ridge sediment is high with closely spaced sharp reflections that more or less conform with the topography. Returns from below that depth are mostly noise. Penetration in the ridge sediment decreases southward. Under the canyons, penetration is significantly less than under adjacent ridges; below a strong, prolonged bottom-reflection there are generally no sub­bottarn reflections. The higher reflectivity of the canyon-floor sediment is in part an artifact caused by side echos.

Provenance as a major factor affecting transparency of seismic facies The main difference observed along the slope profile of Fig. 3.3 is between the northern seismically highly transparent segments and the southern highly reflective segments. The difference in proportion of detrital carbonate (high in the north and low in the south) is thought to be the main reason for the generally much poorer seismic penetration and higher

reflectivity of the near-surface layers in slope sectors 3 and 4 compared to the transparent sectors 1 and 2, which contain high proportians of fine-grained, carbonate-rieb sediments.

Effects of sediment input and transport mechanisms on upper-slope morphology Differences in sediment input and transport mechanisms explain the morphologic differences between adjacent slope sectors 1 and 2 in the highly transparent seismic facies and between sectors 3 and 4 in the low-transparency facies. The transfer of glacially derived sediment to the marine environment involved a number of distinct mechanisms that occurred in two major steps.

1. Delivery mechanisms to ice-margin First the sediment is transported to an ice-marginal position by the glacier as englacial material or by subglacial or supraglacial streams. Since a major ice-tongue extended most likely as a grounded shelf glacier to the shelf/slope transition off Hudson Strait, which in this region occurs at c. 1000 m water depth, coarse sediment was delivered directly to the upper slope by ice transport. Same of this material was rafted southwards by icebergs and distributed along the entire Labrador Slope and even beyond the confines of the Labrador Sea. Shelf-glaciers issuing from the smaller fjord outlets to the south probably also extended to the shelf edge, which here occurred at 500 m water depth, much shallower than off Hudson Strait. Glaciers may even have extended further seaward on to the upper slope, at least temporarily (Josenhans et al. 1986; Josenhans and Zevenhuisen 1987).

In addition, fine-grained material, released either directly from melting at the ice-front or from meltwater streams entering the sea subglacially or supraglacially, rase to the sea surface or stayed at the surface to form turbid plumes that were incorporated by the Labrador Current and drifted southward. Surface plumes result from the buoyancy of fresh water entering the sea (Pfirman and Solheim 1989). Meltwater discharge from glaciers, even if heavily sediment­laden, does not normally Iead to the development of turbidity currents in the marine environment (Syvitski et al. 1987), in cantrast to glaciallakes. The presence of nepheloid-layer deposits on the

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Fig. 3.3. The slope profile consists of the following four sectors in north to south order: 1. The northernmost low-relief slope with relief differences between canyons and levees of less than 120m is part of the !arge debris-flow slope off Hudson Strait; however, with some modifications in that the levees consist of !arge pockets of transparent, probably hemipelagic sediment with a few distinct reflections. 2. The maximum-relief zone south of Hudson Strait consists of seismically transparent sediment, probably of hemipelagic origin, with continuous reflections interpreted as ice-rafted coarser-grained layers. The highest relief of 700 m is found on the southern flank of the more than 5 km wide plateau at the northern end of the sector; southward the relief decreases distinctly to 400-500 m along the next five to ten canyon-ridge systems, then to 300m and finally 200m at the southern end of the D-canyons (DBBABA), reflecting the decreasing effect of turbid-plume fall-out with increasing distance from Hudson Strait. 3. In the low-relief debris-flow slope sector south off the southern end of Saglek Bank, relief between canyons and adjacent ridges does not exceed 100m. 4. In the moderate-relief sector off the northern part of Nain Bank with relief differences between V-shaped canyons and adjacent ridges of up to 300m, older debris-flow deposits appear to be capped by younger hemipelagic or terrigenaus sediment.

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Labrador Slope (Wang and Hesse, in press) shows, however, that some flows became dense enough to spread out as mid-depth suspension layers (nepheloid layers) and others, particularly those carrying a bedload of sandy and gravelly sediment, were able to form turbidity currents. The normal mode of generating turbidity currents in an ice-marginal environment, however, is not by meltwater-flooding events, but by remobilization of glacial-front upper-slope or glacial-lake sediment. Turbidity currents are therefore among the redepositional or secondary transport mechanisms discussed below, although they may form part of the primary transport system that includes surface plumes and nepheloid layers.

Turbid surface plumes and nepheloid layers are thought to be responsible for deposition of a significant portion of the fine-grained sediments (25-75% in retrieved piston cores; Wang and Hesse, in press) on the Labrador Slope. They are interpreted as two principal primary transport mechanisms of the fine-grained sediments of the seismically transparent and well-stratified slope sector 2. Transport by ice-rafting as a primary transport process includes coarser grained material, but is volumetrically probably an order of magnitude less important than input from turbid plumes and turbidity currents. A significant proportion of the fine-grained slope sediments is of turbidite origin.

2. Slope erosion and redeposition mechanisms The principal question concerning the origin of the canyon-and-ridge topography on the upper slope is whether it represents (a) canyon-levee complexes, or at least includes a strong (re-) depositional element, or (b) an erosional topography of an originally much smoother depositional surface. Our previous interpretations (e.g. Hesse 1992) were biased by experience from the rise and lower slope where the relief is largely depositional in origin. In particular, the ridges between canyons are depositional levees formed by spillover of turbidity currents and other mass-flow processes from the canyons.

The new data from the high-relief sector of the upper-slope profile soggest, however, that this is not a tenable interpretation for the upper slope. The upper slope south of the Hudson Strait outlet received its sediment to a !arge extent as a mud blanket by hemipelagic settling from glacial-flour­rich turbid plumes that were carried to the south parallel to the slope by the Labrador Current.

Nepheloid layers and ice-rafting also contributed to sediment accumulation (Wang and Hesse, in prep.). Retrograde syn- or post-depositional erosion by headward gullying in the canyons then generated the present-day dendritic, high­relief canyon pattern. Erosion in the canyons was caused by slumping from the steep canyon walls, generating debris-flows and turbidity currents. Ridge accumulation was most rapid during glacial times when the supply of terrigenaus detritus by turbid plumes and nepheloid layers substantially increased the normal pelagic settling rates. lce­rafting occurred both during glacial and interglacial times, but during glacial times required that the sea was not frozen over. Deposition rates of ice-rafted material were much lower during interglacial times when the iceberg drift was mostly restricted to sources from Greenlandian fjords. Erosion by headward gullying continued during interglacial times, as at present. The present boundary between the smooth uppermost slope at 500-700m water depth and the gullied, steep high-relief canyon­slope below probably represents the upper Iimit to which headward erosion has occurred to date. Higher up on the slope near the shelf break, the sediment probably also became more erosion­resistant because of an increase in grain size due to an increase in ice-rafted material and/or the presence of glacial till.

Somewhere on the mid-slope the transition must occur between hemipelagic-dominated inter-canyon ridges above and turbidite­dominated ridges below. This transition will be gradual as the activity and volume of turbidity currents and related redeposition processes increase gradually downslope and the canyon depth decreases, allowing more spillover to occur. Turbidite deposition will not be absent from the upper slope, as our cores show, and vice versa, hemipelagic interlayers are still present on the lower slope, as documented in piston cores (Hesse et al. 1987; Wang and Hesse, in prep.).

Sediment mass-transport via sediment sliding, slumping and debris flows (seismic fades 1 and 2, respectively, of Hesse 1992) is prevalent, however, on those sectors of the upper slope which have low relief, i.e. on sector 3, and, to a lesser extent relative to other processes, also on sector 1 and probably sector 4. Sector 3 is a typical debris-flow slope, as documented in downslope profiles (see cross-slope profile from

the Labrador Sea; Hesse Ch. 2), which show a characteristic downslope succession from slide­block fields to slump scars and debris-flow deposits. Rapid slope progradation in these sectors has led to slope oversteepening and partial self-disintegration. Debris-flow deposition Ieads to smoothing of the relief. lt also seems that debris-flow slopes are less amenable to retrograde erosion than are slope areas originally dominated by hemipelagic deposits.

Slope sector 4 is probably underlain by older debris-flow deposits, as also indicated by the cross-slope profile (section a, Hesse, Ch. 40), and capped by younger hemipelagic or terrigenaus deposits - therefore the moderate relief.

Slope sector 1, which shows only a small southernmost portion of the !arge low-relief slope in front of Hudson Strait, is of composite origin. lt contains !arge, partly buried channels with highly reflecting sediment which have migrated northward with time. Further downslope (not shown in Fig. 3.3), it contains extensive debris­flow deposits. In addition, it contains !arge ridges of seismically transparent and well-stratified sediment similar to the plateau at the northern end of sector 2, and like sector 2 is probably also predominantly of hemipelagic origin. Dumping of !arge quantities of sediment from the huge shelf­glacier tongue in Hudson Strait led to debris-flow development which smoothed the relief, rather than creating relief. Because the debris-flow deposits and the presumed coarse-grained channel-fill deposits are less susceptible to erosion than the hemipelagic and nepheloid-layer deposits, the smooth relief was preserved.

Acknowledgements R.N. Hiscott is thanked for editorial review and suggestions to improve the manuscript. Captain Strum, officers and crew of CSS Hudson provided their customary excellent cooperation at sea; D.].W. Piper helped to defray costs; the Atlantic Geoscience Centre, Dartmouth, Nova Scotia, affered logistical support; the Department of Fisheries and Oceans, Ottawa provided shiptime. The project was funded by NSERC and DEMR, Ottawa, Ontario, Canada.

Heferences Chough, S.K., Hesse, R. and Müller,]. 1987. The Northwest

Atlantic Mid-Ocean Channel of the Labrador Sea. IV. Petrography and provenance of the sediments. Canadian Journal of Earth Sciences, 24, 731-740.

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Heinrich, H. 1988. Origin and consequences of cyclic ice­rafting in the Northeast Atlantic Ocean during the past 130000 years. Quaternary Research, 29, 143-152.

Hesse, R. 1992. Contineotal slope sedimentation adjacent to an ice-margin. I Seismic fades of Labrador Slope. Geomarine Letters, 12, 189-199.

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. CanadianJournal ofEarth Sciences, 24, 1595-1624.

Hesse, R. and Klaucke, I. 1994. Continuous seismic profiling along an ice-margin affected continental slope: Upper Labrador Slope. In: Hesse, R. and Klaucke, I. (eds) Cruise Report, CSS Hudson Cruise 92-045.

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

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

Syvitski, J.P.M., Burrell, D.C. and Skej, J.M. 1987. Fjords: processes and products. Springer-Verlag, New York, 375 pp.

Wang, D. and Hesse, R. (in press) Contineotal slope sedimentation adjacent to an ice-margin. li. Gladomarine depositional fades on Labrador Slope and glacial cycles. Marine Geology.