Sandy submarine braid plains: Potential deep ... - GeoScienceWorld

Sandy submarine braid plains: Potential deep-water reservoirs Reinhard Hesse, Ingo Klaucke, Saeed Khodabakhsh, David J. W. Piper, William B. F. Ryan, and NAMOC Study Group

ABSTRACT Sandy submarine braid plains, like their fluvial counterparts on land, are sand-rich depositional environments that may display excellent reservoir characteristics in terms of sediment volume, porosity, and permeability. The submarine examples may be laterally associated with potential source rocks such as the fine-grained levee deposits of deep-sea channels. A side-scan sonar study of the central Labrador Sea revealed the existence of a more than 700 km long and up to 120 km wide submarine sand and gravel plain that has been supplied with sediment by high-density turbidity currents, possibly resulting from subglacial lake outburst flooding in the Hudson Strait. The side-scan imagery of parts of the plain displays a conspicuous streaky pattern of alternating high and low backscatter intensity. High-resolution 3.5 kHz seismic profiles and 12 kHz bathymetric profiles show that the pattern represents a furrow-andridge (erosional) or channel-and-bar (depositional) topography, similar to a braided alluvial plain. The furrows or channels have low acoustic backscatter, are less than 10 m deep, and are separated by ridges or bars having high backscatter. Some channels terminate in depositional lobes. Individual channels and bars (or furrows and ridges) are less than 100 m wide and can be followed up to 40 km downcurrent. On sleeve-gun seismic profiles, the total sand thickness appears to be between 200 m (proximal) and 100 m (distal). Piston cores from the plain recovered massive sand layers up to 4 m thick, buried under 1 m of Holocene hemipelagic ooze. Texturally, the sands and gravelly sands display a trend of improving sorting with increasing mean grain size. Some very coarse grained samples are moderately well sorted and almost matrix free. The flooding events that deposited the sands might be the submarine counterpart of Heinrich events but need not be restricted to such events. Radiocarbon ages of about 10 k.y. from the base of the ooze overlying the youngest sand gave a minimum age for the sand that is similar to the age of Heinrich event 0. Estimates for the discharge volume of individual events are poorly constrained and range from 103 to 105 km3. Braided channel patterns in deep-water

Copyright 䉷2001. The American Association of Petroleum Geologists. All rights reserved. Manuscript received July 30, 1997; revised manuscript received July 7, 1999; final acceptance October 10, 2000.

AAPG Bulletin, v. 85, no. 8 (August 2001), pp. 1499–1521

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AUTHORS Reinhard Hesse ⬃ Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, Quebec H3A 2A7, Canada; [email protected]; current address: Institut fuer Geologie, RuhrUniversitaet Bochum, Universitaetsstr. 150, D-44 801, Bochum, Germany; [email protected] Reinhard Hesse obtained his Diplom (M.Sc. degree), Dr. rer. nat. (Ph.D.), and Habilitation (D.Sc. degree) from the Technical University of Munich, Germany. In 1969 he joined the Department of Geological Sciences of McGill University, Montreal, Canada, as assistant professor; he retired in 1996 and is now professor (postretirement). He is Honorar Professor at the Ludwig Maximilians University of Munich and currently visiting professor at the Ruhr-University, Bochum, Germany. His scientific interests include modern and ancient deep-sea sedimentation, clastic diagenesis, and plate tectonics. Ingo Klaucke ⬃ GEOMAR, Wischofstr. 1-3, D-24148 Kiel, Germany; [email protected] Ingo Klaucke is currently a research fellow at GEOMAR, Kiel, Germany. He studied geology at the universities of Freiburg (Germany), Lyon (France), and McGill (Canada), where he received his Ph.D. in 1995. His main research interests focus on the modes and pathways of sediment transfer from the continent to the deep sea and on the facies distribution of clastic deep-sea deposits. Saeed Khodabakhsh ⬃ Geology Department, Bu-Ali-Sina University, Hamedan, Iran, 65174; [email protected] Saeed Khodabakhsh received his B.Sc. degree from Shahid Beheshti University, Iran, in 1986 and his M.Sc. degree from Tehran University, Iran, in 1990. During 1987–1991 he worked for the Geological Survey of Iran and the National Iranian Oil Company. In 1992 he went to McGill University, Montreal, Canada, where he obtained his Ph.D. in 1997. He is currently assistant professor in the Geology Department, Bu-Ali Sina University, Hamedan, Iran. His research interest is clastic depositional environments.

David J. W. Piper ⬃ Geological Survey of Canada (Atlantic), Box 1006, Dartmouth, Nova Scotia B2Y 4A2, Canada; [email protected] David Piper is a research scientist with the Geological Survey of Canada at Bedford Institute of Oceanography. He gained his B.A. degree and Ph.D. from Cambridge University, England. His interests include modern and ancient turbidites, marine Quaternary of eastern Canada, neotectonics and sedimentation in Greece, and sediment instability constraints to hydrocarbon development offshore eastern Canada. William B. F. Ryan ⬃ Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York, 10964; [email protected] Bill Ryan is a Doherty Senior Scholar at the Lamont-Doherty Earth Observatory. His current research has taken him to the Gulf of Lyon in the Mediterranean to study the cutting and filling of submarine canyons related to sea level cycles. He has also been investigating a very rapid Holocene transgression on the Black Sea shelf. Bill received his Ph.D. in geological sciences from Columbia University in 1971 and the Shepard Medal from SEPM in 1993. NAMOC Study Group ACKNOWLEDGEMENTS Reviews of this article by Keith W. Shanley, L. G. Kessler II, and particularly by Carlos Pirmez, as well as the cooperation of Captain Lewis, officers and crew of CSS Hudson cruise 93-025, major funding from NSERC, Ottawa, Ontario, and NSF, Washington, D.C., and a major contribution of AAPG to the printing costs of the foldout, are gratefully acknowledged.

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Sandy Submarine Braid Plains

sandy depositional environments are not restricted to high latitudes but also have been identified in various submarine fan settings in the lower latitudes, for example, the Orinoco, Var, and Monterey deep-sea fans and in the Santa Monica Basin. The largest examples, however, are known from high latitudes, suggesting that melt-water discharge from continental ice sheets may favor the formation of this habitat of giant sands in the deep sea. The occurrence of sandy braided deep-water environments having favorable reservoir characteristics in a variety of tectonic settings makes this type of environment a potentially interesting deep-water target.

INTRODUCTION Sandy and gravelly braided alluvial plains and channels are characteristic elements of fluvial depositional systems that typically occur at the proximal, high-gradient and high-energy apex of such systems. They are generally associated with periodic high-discharge and coarse bed-load rivers (e.g., Schumm, 1981; Walker and Cant, 1984). That similar environments also occur on the modern deepsea floor is a recent discovery but comes as no surprise because many features of fluvial depositional systems have their deep-sea counterparts (e.g., Klaucke and Hesse, 1996). A submarine braided channel had already been postulated by Hein and Walker (1982) as a depositional environment of coarse clastic lower Paleozoic deep-sea sediments in the Quebec Appalachians. Braided channel segments from the deep sea also were described almost two decades ago (Belderson et al., 1984). Here we present results of the first detailed study directed at the extensive submarine sand and gravel plain that extends for more than 700 km across the basin plain of the northeastern Labrador Sea, using side-scan imagery, seismic profiles, and grain-size and petrographic analyses. This sand and gravel plain is up to 120 km wide, is braided in its distal parts, and occurs in the submarine extension of the drainage system of the former Laurentide ice sheet through the Hudson Strait. Based on their large dimensions and textural properties these sand bodies could become excellent reservoir rocks for hydrocarbons or mineral deposits, if preserved in suitable tectonic settings. The discovery of this depositional feature in a high-latitude deep-sea environment sheds new light on the conditions of ice-sheet drainage, including a possible relationship to Heinrich ice-rafting events that have risen to prominence in the debate about rapid late Pleistocene paleoclimatic change (e.g., Broecker et al., 1992; Bond et al., 1993). Few other examples of braided submarine sand plains or channels have been hitherto described from the modern sea floor (e.g., Belderson et al., 1984; Kenyon and Millington, 1995), but their significance for hydrocarbon exploration has not been appreciated previously. As the frontiers of hydrocarbon exploration move to increasingly more extreme settings, the occurrence of a potentially attractive new reservoir type such as very large sand bodies in a deep glacial-marine environment

would be welcome. The geologic setting, origin, and significance of this hitherto little-known sand-rich deep-sea depositional environment are discussed in this article.

GEOLOGIC SETTING AND PREVIOUS WORK: DEEP-WATER DEPOSITIONAL SYSTEMS OF THE LABRADOR SEA The braided submarine sand and gravel plain of the northeastern Labrador Basin (Figure 1a) occurs amid other deep-water clastic depositional systems of the Labrador Sea. Its coarse grain size, its low relief, and the near-absence of major channels contrast markedly with the Northwest Atlantic Mid-Ocean Channel (NAMOC) depositional system in the southwestern half of the basin, which consists of interconnected tributary channels that converge into the NAMOC as a trunk channel. The NAMOC system is predominantly fine grained except for coarse channel-fill deposits. Deep-water glaciomarine deposition in the Labrador Sea during the last glaciation and back at least to the middle Pleistocene was dominated by these two juxtaposed systems (Hesse et al., 1997b), which are representatives of the basin-plain type and the channeldominated type, respectively. Huge debris-flow lobes on the slope and rise in front of the Hudson Strait and the smaller ice outlets of the fjords form a third depositional system supported by mass wasting in contrast to the two turbidity-current-dominated systems. Studies since the mid-1970s have revealed that the Hudson Strait outlet of the Pleistocene Laurentide ice sheets was the main source for the glacial detritus in the deep basin. This material was partitioned between the NAMOC system and the sand and gravel plain by the mechanism of ice-margin sifting discussed in the section “Origin of the Braid Plain.” The smaller outlets of the Laurentide ice sheets through the fjords on the Labrador Coast contributed material to the NAMOC system during glacial times, whereas Greenlandian sources have not yet been identified in the clastic sediments of the deep basin, probably because the Greenland ice sheet never melted completely (Chough et al., 1987). Previous studies concentrated on the NAMOC system and the meandering nature (Chough and Hesse, 1976; Hesse et al., 1987) and other fluvial aspects of its channels (Klaucke and Hesse, 1996) including submarine yazoo channels (Hesse, 1989). The dynamics of channelized turbidity currents in the

NAMOC were analyzed by Klaucke et al. (1997). Side-scan sonar imaging of the NAMOC provided the first comprehensive view of major parts of this giant deep-sea channel and some of its tributaries (Hesse et al., 1996). Myers and Piper (1988) established the seismic stratigraphy of the basin. In another seismic stratigraphy study, Klaucke et al. (1998a) elaborated on the evolution of the NAMOC channel-levee complex. Klaucke et al. (1998b) studied the middle to distal reaches of the NAMOC. Hesse et al. (1990) used the Labrador Sea as an example for the dispersal patterns of clastic detritus in a small, high-latitude ocean basin. The work of Hesse (1992), Wang and Hesse (1996), and Hesse et al. (1999) concentrated on the Labrador Slope. The importance of turbid surface plumes for supplying fine-grained sediments to the upper slope and through remobilization to the NAMOC system was emphasized by Hesse et al. (1997a). The braid-plain interpretation for the northeastern basin plain was first proposed by Hesse and Rakofsky (1992) based on (1) piston cores, (2) bathymetric profiles, and (3) 3.5 kHz and sleeve-gun seismic profiles but remained speculative without the bird’s-eye view offered by side-scan imagery. The new evidence from side-scan images (Hesse et al., 1996, 1997b) and sets of parallel seismic and 3.5 kHz profiles acquired during the side-scan survey, which is presented here, fully confirms the original interpretation and provides the data necessary to analyze the anatomy of this recently recognized deep-water depositional environment using remote-sensing tools. With the additional ground truth information from cores, insight is obtained into the facies organization of the environment. Some modifications of the previous interpretations of Hesse (1992) and Hesse and Rakofsky (1992) concerning the shelfto-basin supply routes of the sediment on the slope off the Hudson Strait are pointed out in their proper context.

METHODS We acquired 140,000 km2 of HAWAII MR-1 sea floor imagery in 1993 along a corridor up to six ship tracks wide, spaced 20 km apart, providing full coverage of the course of the NAMOC and adjacent sea floor from the upper slope off the Hudson Strait at 61⬚ to 44⬚30⬘N, including the western half of the braid plain. The HAWAII MR-1 system (Rognstad, 1992) operates at 11 and 12 kHz on the port and starboard sides, respectively, and is towed at a speed of 8–9 knots. Other Hesse et al.

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Figure 1. (a) Location map showing converging tributary system of the NAMOC of the Labrador Sea and position of the sandygravelly submarine braid plain. Tributaries are labeled counterclockwise around the basin in ascending branching order. Facies distribution of the Labrador Slope and the Labrador Basin is based on piston cores, seismic profiles, and side-scan sonar imagery (modified from Hesse et al., 1997b). See inset for location. Rectangular boxes: location of Figure 2a–d. (b) Location map for profiles of Figures 7–11. IMOC ⳱ Imarssuak Mid-Ocean Channel.

technical details of the system and the data acquisition procedures have been given in Hesse et al. (1996). The 40 in3 (655 cm3) sleeve-gun seismic reflection profiles and 3.5 kHz seismic profiles acquired simultaneously with the side-scan imagery provide a set of parallel profiles augmented by numerous cross-cutting lines collected during previous cruises (see index map in Figure 1b).

EVIDENCE FOR THE SUBMARINE SAND AND GRAVEL PLAIN AND ITS BRAIDED NATURE HAWAII MR-1 Side-Scan Sonar Imagery In the proximal region north of about 60⬚N (in the area northeast of the NAMOC where the channel has two branches) (Figure 2a, b), the sand and gravel plain appears as an area of relatively uniform low backscatter without a braided pattern. Between 59⬚50⬘ and 59⬚35⬘N, a huge slump has buried an older stage of the sand plain under it, displacing the present plain eastward beyond the limit of the image. The plain reappears at about 59⬚30⬘N where the braided pattern starts, showing alternating stripes of high and low backscatter (Figure 2b, d). The stripes are either curved or straight, parallel with each other, or sometimes overlapping (e.g., between 58⬚20⬘ and 57⬚30⬘N on the two eastern tracks) (Figure 2d). In the southern half of the braided plain, the orientation of the lineaments displays curvature toward the NAMOC. Individual streaks of the braid pattern, which are 100 m to several hundreds of meters wide, can be followed kilometers to tens of kilometers in the downcurrent direction; the longest have a length of up to 40 km. Whether the streaky pattern represents predominantly depositional (channels and depositional bars or lobes) or erosional features (furrows and residual ridges) is not immediately clear (see the section on bathymetric profiles). The braided pattern continues all the way south to the confluence of the Imarssuak Mid-Ocean Channel (IMOC) with the NAMOC (Figure 1a). At this point, the braid plain terminates, because sediment-laden currents, which have not yet run out of sediment, are trapped by the IMOC and directed into the NAMOC. The anastomosing IMOC is the only major tributary to the NAMOC from the Greenland side (Egloff and Johnson, 1975) because the tributaries coming from the West Greenland slope end where they enter the

plain and seem to be buried under the sands of the plain. The IMOC drains the West Reykjanes Basin, that is, the present outlets from the eastern Greenland ice sheet and continental margin. Piston Cores Piston cores from the braided part of the plain east of the NAMOC all contain sand occurring in massive layers several meters thick (Figures 3a, 4; cores from the proximal, nonbraided part of the plain are not available). Layers cannot be correlated between cores. The base of the stratigraphically deeper sand layers has not been penetrated in the cores. The thickest layer measures more than 4 m (core H84-030-9). The cores also contain minor graded sand and gravel and laminated sand and silt turbidites. In core H84-030-7, which is located about 10 km west of the B tributary (Figure 2c), 8 m of thin-bedded sand and mud turbidites alternate with pelagic layers containing ice-rafted debris (IRD)-rich Heinrich layers (Reiss, 2000). Cores from the nearby eastern levee of the NAMOC in contrast contain fine-grained spill-over turbidites from the NAMOC and pelagic layers including IRD deposited during and between Heinrich events (Hesse et al., 1987, 1996). All cores from the braid plain are capped by a Holocene pelagic ooze up to 1 m thick. Samples taken about 20 cm above the base of the pelagic ooze gave accelerator mass spectrometer (AMS) 14C ages (based on the left-coiling form of the foraminifera Neogloboquadrina pachyderma) of 9.45 Ⳳ 0.11 ka (core 84030-8) and 9.72 Ⳳ 0.14 ka (core 84-030-9), whereas samples taken immediately on top of the sands (⬍5 cm from the top) from the same cores gave ages between 16 Ⳳ 0.2 and 22.68 Ⳳ 0.24 ka, respectively (Figure 3c). The best explanation is that these greater ages reflect contamination by older reworked foraminifera, not recognized as such during picking, from the fine suspended-sediment cloud that followed the last sandy turbidity current. (If they were true ages of the sediment, sedimentation rates for the first 20 cm of sediment overlying the sands would be about 4 times less than what they are for the rest of the Holocene at these sites, i.e., 1.5–1.8 cm/103 yr instead of 6–7 cm/103 yr.) Texturally, the sands and gravelly sands display a trend of improving sorting with increasing mean grain size (Figure 5j); some of the very coarse grained samples (mode in the ⳮ1 to ⳮ2␾ fraction) are moderately well sorted, practically matrix free, and would provide Hesse et al.

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Figure 2. (a, b) Side-scan sonar mosaic, four tracks wide, between 60⬚30⬘ and 58⬚30⬘N with interpretations. (c, d) Mosaic up to six tracks wide between 58⬚30⬘ and 56⬚N. The broken line shows the western limit of the braided sand and gravel plain that borders the eastern levee of the NAMOC. The channel marked B may or may not connect at the surface with tributary canyons on the West Greenland slope (cf. Figure 1) but most likely did so in the past. Display polarity is normal, that is, areas of low backscatter strength appear dark and areas of high backscatter strength are bright. For location see Figure 1.

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Figure 2. Continued.

1506 Sandy Submarine Braid Plains Figure 3. Piston cores from the braid plain. (a) Lithology and facies. (b) 18O stratigraphy (with AMS 14C ages), d13C values (with marine oxygen isotope stages 1, 2, 3, and 4) for core 90-013-013 (modified from Hillaire-Marcel et al., 1994). (c) d18O and d13C values and AMS 14C ages based on N. pachyderma (sinistral) for cores 84-030-8, -9, and -16. Oxygen isotope curves for the three cores H84-30-8, H84-30-9, and H84-30-16 permit correlation with core 90-013-013P from the Greenland margin whose oxygen isotope stratigraphy is constrained by radiocarbon dates, the presence of the Vedde ash, and a detrital record of Heinrich events 1 and 2. Core 84-30-16 is close to 013P and has similar isotopic values for d18O (⬃2.5‰) and d13C (⬃0.6‰) from N. pachyderma (sinistral) in the middle to late Holocene, suggesting that the isotopic determinations from the two cores can be directly compared. (The ⬃0.5‰ lighter d18O values for cores 84-8 and -9 may reflect a stronger meltwater effect at the more western core sites.) The dip in d18O to 3.5‰ immediately above the sand deposits (which is replicated in the magnitude of the deviation from middle Holocene values in our other cores) is identical with the Younger Dryas signal in 013P, as is the dip in d13C immediately overlain by a small peak (Figure 3c). The isotopic values for O and C at 125–150 cm in core 84-8 closely match those between H1 and the Younger Dryas. (Possible matches late in isotopic stage 3 do exist, but this correlation seems improbable on the basis of sedimentation rates.)

good reservoir rocks, if subjected to favorable cementation/dissolution conditions during burial. Provenance indicators point to a Canadian source, supporting the hypothesis of a Hudson Strait provenance of the braid-plain sands. These indicators include the abundance of detrital carbonate derived from Paleozoic limestones in Hudson Bay, Hudson Strait, Foxe Basin, and the Canadian Arctic Archipelago, and the feldspar composition (Chough et al., 1987; Khodabakhsh, 1997). Plagioclase having a high anorthite content (80–90%) (Figure 6) typical of anorthosites in Greenland (Windley, 1969) was found only in core 88-025-7 from the Greenland slope but is absent in the braid-plain cores. The absence of source-specific heavy minerals such as chrome spinel, eudialyte, arfvedsonite, aegirine, or twinned hypersthene derived from the alkali basalt province of southwest Greenland (Cro¨mmelin, 1937) points in the same direction, excluding a Greenlandian source, at least for the youngest braid-plain sands penetrated by the piston cores. Bathymetric, 3.5 kHz, and Sleeve-Gun Seismic Profiles The echo character on the 3.5 kHz profiles from the braid plain (Figures 7, 8) is compatible with the sandy substrate from the cores. The profiles show generally low penetration, a prolonged bottom echo, and a varying number of diffuse or indistinctly parallel subbottom echoes. They reveal a tendency toward better stratification in the more distal regions of the plain in the east and south but contrast markedly with the deep-penetration profiles from the muddy levees of the NAMOC (up to 70–80 m penetration, e.g., profiles XY, BA, and WXX in Figure 7 and along-channel profiles of Figure 9) with their sharp, highly continuous parallel reflections and overall transparency, seen both in the 3.5 kHz and sleeve-gun profiles (Figures 9, 10, 11). In the east, the braid plain extends for about another 60 km east of the cutoff of the B tributary in Figure 2c, d, based on bathymetric and seismic profiles and a single piston core (88-024-6). Profile XY, of which only a part is shown in Figure 7, traverses the full width of the plain of about 120 km. Bathymetric 12 kHz profiles show shallow, generally less than 10 m deep channels on the braid plain east of the NAMOC (profile KL in Figure 8), which cannot be traced from profile to profile with the exception of the up to 40 m deep B channel (Figures 1, 2d). The 12 kHz profiles record the surface morphology of the braid plain better than the 3.5 kHz profiles (compare profile KL of Figure 8 with nearby profile

LM). Careful correlation of the side-scan imagery with the 12 kHz profiles suggests that low-backscatter areas are associated with depressions in the sand surface, whereas high-backscatter areas are positive topographic features. The poor surface expression of the streaks on the 3.5 kHz profiles appears to arise from blanketing of the entire region by the Holocene pelagic-ooze cover. The backscattered sound from the side-scan instrument apparently penetrated this meter-thick cover, and possibly deeper, thus potentially superposing morphological features of the sand bodies in the braid plain that are vertically overlapping. Depressions in the upper sand surface may not always appear on the sea floor, because they may be covered by thicker mud as a result of weak bottom-current winnowing, but they give a weaker backscatter response. Over elevated sand bottom, the mud may be thinner and the backscatter energy higher. (See, however, an interpretation by Gardner et al. [1996], who relate low backscatter to thick massive sands and high backscatter to thinner sand/mud intercalations.) If our interpretation of the high- and lowbackscatter streaks is correct, then the feathered shape of the upstream ends of some of the dark (low-energy) streaks (e.g., the three streaks immediately south of profile AB in Figure 2b, north of 59⬚N, marked S1, S2, and S3 on Figures 2a, 7, 12) suggests that they are erosional features carved by sand-carrying currents. Erosion is imperceptible, however, on the 3.5 kHz profile crossing the features (profile WXX on Figure 7), except by the interruption of the second subbottom reflector (see blowup in Figure 12). Light patches on the side-scan images (e.g., north of 58⬚N), however, suggest the presence of depositional bars that may terminate in depositional lobes, judging from their outlines on a trace map overlay on the side-scan images (Figure 2a, c). These interpretations partly answer the question raised previously of whether the streaky pattern on the side-scan images represents predominantly depositional (channels and depositional bars or lobes) or erosional (furrows and erosional residual ridges) features; apparently both are present. On the side-scan mosaic a streaky pattern also occurs west of the NAMOC (near the western edge of the mosaic in Figure 2c, d) where a separate small sand plain exists, as interpreted from seismic profiles. Erosion and sand deposition by turbidity currents that originated from spill-over from the E/F tributary at its sharp eastward bend shortly upstream of its junction with the NAMOC (Figure 2c, d) generated this Hesse et al.

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Figure 4. Massive sandy gravel layer (core H84-030-16; see Figure 3a for stratigraphy). (a) Radiograph (87–111 cm subbottom) showing top of the sand layer, which displays grading, followed by ripple-cross laminated division of fine-sand-to-silt turbidite (Tc division), which may or may not be related to the same event that deposited the massive sand. The coarse sand layer at 96–97 cm subbottom appears to be an artifact (sand injection during core retrieval) rather than being the Ta division of a subsequent turbidite followed by another Tc division (sand flowage as a coring disturbance is also indicated between 100 and 106 cm). The granules at 88–89 cm subbottom are ice-rafted debris occurring below a fine-grained sand turbidite. (b) Core photograph 59–162 cm subbottom showing top of massive sand layer followed by Tc division and subsequent fine-grained sand turbidite. (c) Core photograph 163–267 cm subbottom showing the middle part of the massive gravelly sand layer.

Figure 5. (a–h) Histograms of grain-size distributions of braid-plain sands and gravel in ␾ units; (i) Cumulative curves. (j) Sorting vs. mean grain size. (k) Main grain-size parameters: Md ⳱ median, Mz ⳱ mean, Mo ⳱ mode.

Figure 6. Feldspar composition in braid-plain massive sands and Greenland slope sediments. See text for explanation.

Greenland greenland 88-7

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Greenlandian slope sediments, core 88-7 Braid plain sands: cores 84-8, 9, 16

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Canadian provenance

Ab pattern. At this locality the E/F tributary is locally filled by debris-flow deposits that blocked the channel and forced the spillover (dfd in profile 1 of Figure 9; the channel is partially filled in profile 2 and empty near the junction with the NAMOC in cross-channel profile KL in Figure 8). The contact relationship on the 3.5 kHz profiles between the youngest braid-plain sands and the eastern NAMOC levee varies, ranging from onlap at the surface (OL) with basal truncation of the muddy levee turbidites (profiles BA in Figure 7; DE in Figure 8), evidence for interfingering (IF, western part of profile XY, profile WXX in Figure 7), a combination of onlap and interfingering (profile CD in Figure 7) to complete levee erosion (in Figure 8, all profiles except DE; see next section), showing that the last episode of turbidity-current activity on the braid plain outlasted levee building by the NAMOC. The side by side occurrence of the levee facies and the sand-plain facies, which show interfingering in the subsurface, however, attests to the coexistence of the braid plain and the NAMOC in the late Pleistocene. Both depositional systems appear largely independent from one another as far as individual depositional events and the supply routes of the sediment are concerned (Hesse et al., 1997b; see also the Interpretations section). 1510


Greenlandian pr.

An The sleeve-gun and air-gun profiles of Figures 10 and 11 with their greater seismic penetration illustrate the differences between the well-stratified, highpenetration NAMOC levee facies (penetration down to 400 ms, profile 3 and parts of 4 in Figure 10) and the highly reflective facies typical of coarse-grained sediments (channel-fill facies of the NAMOC, profile 3; western sand plain, profile 1; braid plain, profiles 4– 6 in Figure 10), which have lesser penetration down to 200–300 ms. Notwithstanding their much lower penetration depth, the 3.5 kHz profiles show the differences equally well at shallow subsurface levels: deeper sound penetration and excellent surface-parallel stratification under the western NAMOC levee (profile 3, northern part of profile 2 near the E/F tributary and north of it, middle part of profile 4 in Figure 9) and low penetration on the sand plains both west and east of the NAMOC (profiles 1 and 6, southern part of profile 2, northern parts of profiles 4 and 5 in Figure 9). Based on echo character, the sand-plain and braidplain facies are difficult to distinguish from the coarsegrained channel-fill facies of the NAMOC (e.g., BP and DCF, respectively, profile WXX in Figure 11; ground truth for the channel sands and gravels is provided by the cores of Hesse et al. [1987], their figures 8A, B and 15). The along-channel sleeve-gun profiles of

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Figure 7. Profiles (3.5 kHz) showing variation in contact relationship between the eastern NAMOC levee (EL) and the submarine sand and gravel plain. AEAF: Absence of eastern levee in the proximal part of the plain. Subbottom reflectors under the nonbraided part of the plain are relatively continuous, and surface erosion is restricted to the vicinity of the eastern branch of the NAMOC, where the side-scan image (Figure 2b) shows a streaky pattern. XY: Multiple contact relationship. Onlap (OL) of the western branch of the plain (note decrease in penetration under surficial sand facies) and interfingering (IF) at the contact with the main plain farther east shown by continuation of some of the levee reflectors under and between more sand-rich layers. AB: Onlap (OL) associated with marginal truncation of the muddy levee. WXX: Interfingering (IF) and erosion of a subsurface reflector (between arrows at S1, S2, and S3) corresponding to dark streaks in the side-scan sonar image in Figure 12. CD: Combination between onlap and interfingering (OL/IF). The variable width of the NAMOC levee (40 km in CD compared with 25 km in WXX and 15 km in AB) in nearby profiles is a function of crossing angle and downcurrent braid-plain development (note different scales for lower three profiles).

1512 Sandy Submarine Braid Plains Figure 8. Bathymetric profiles (3.5 kHz and 12 kHz) across the NAMOC and the southern part of the submarine braid plain south of 59⬚N. Note the presence of the eastern mud-rich levee (EL) of the NAMOC in the northernmost profile (DE, 3.5 kHz) and onlap (OL) by braid-plain sands. In profiles KL, LM, NO, ST, and UV farther south, the levee has been eroded by sand-rich flows. Shallow channels (⬍10 m deep) appear only in the 12 kHz bathymetric profile (KL, marked by arrows), where a seamount (SMT) forms the eastern channel wall of the NAMOC. The channel floor of the E/F tributary in KL is deeper than the main channel. At the junction with the NAMOC, however, the E/F tributary forms a hanging valley because of a steep channel gradient of the NAMOC in this region. Approximate position of cores 84-030-7 to -10 have been projected into the 3.5 kHz profile LM. This profile shows some stratification in the braid plain and erosion in small channels at the surface. NO shows the up to 40 m deep B tributary. The sandy nature of the western levee in profiles KL, ST, and UV results from flows that have spilled over from the E/F, G, and H tributaries. EEL ⳱ eastern (partly) eroded levee.

Figure 9 Seismic profiles (3.5 kHz) parallel with the NAMOC. Crossover points with profiles on Figures 7 and 8 are marked by the letter labels of (cross section) profiles. The six parallel profiles between 56⬚ and 58⬚N cover the sand plain west of the NAMOC (profiles 1 and 2), the western NAMOC levee (profile 3), and the braid plain east of

the NAMOC (profiles 4–6). Although the NAMOC appears several times on profile 3, the channel does not cross from one side to the other of the ship track but stays east of the track except near the southern end. The irregular levee surface at points near the channel indicates gullying of the channel walls. Note the shallow channels, the

deeper B tributary, possible sand bars and lobes, and evidence of erosion in profiles 4–6. The low-amplitude wavy surface of the western levee (profile 2 between about 58⬚ and 59⬚30⬘N) reflects meandering of the NAMOC with about a 20 km meander wavelength, the “troughs” correspond to eastward convex meanders, and the crests

correspond to westward convex meanders. Profile 5 shows the relatively well-stratified proximal sand and gravel plain north of the giant slump (closeup inserted below profile 5) discussed in the text. dfd ⳱ debris-flow deposit. NAMOC W and NAMOC E ⳱ western and eastern branches of the NAMOC, respectively.

Figure 10 Along-channel sleeve-gun profiles (corresponding to the 3.5 kHz profiles of Figure 9). Profile 1: Sand plain west of the NAMOC. Profile 3: NAMOC levees and channel. Note contrast between high-penetration seismic facies of muddy NAMOC levees (penetration down to 400 ms or 350 m below sea floor assuming a sound velocity

for two-way traveltime of 1800 m/s) and the highly reflective facies of the coarse channel-fill deposits of the NAMOC as well as the braid-plain sands. For distinction of the seismic facies of the buried braid-plain sands and the channel-fill sands of the NAMOC, see the text. The sands immediately underlying the western branch of the

NAMOC (NAMOC W) and the NC tributary are channel sands according to their position. Profiles 4 and 5: Braid plain east of the NAMOC. Transparent NAMOC levee facies appears visible only adjacent to the NAMOC in profile 4 in the northern part of the profile, whereas in profile 5 its presence is inferred from the 3.5 kHz profile (see Figure 9).

Figure 11. Sleeve-gun (655 cm3) and air-gun seismic profiles across the NAMOC and adjacent sand and gravel plain to the east. AEAF: Crossing of the proximal NAMOC displaying two branches and absence of an eastern NAMOC levee. Note relatively wellstratified nature of the proximal sand plain, which is not braided except in the immediate vicinity of the NAMOC. The former channel wall (western levee) in the subsurface approximately 10 km west (CM, arrow) of the present wall overlies coarse-grained channel-fill facies, indicating eastward channel migration. M9: Stepwise eastward shift of the NAMOC paleochannel (by 35 and 15 km, respectively) indicated by former western levee position (CM ⳱ former channel margin, shown by arrows). WXX: Profile showing marked contrast in echo character between the relatively transparent and well-stratified NAMOC levees and the braid plain (BP). In this cross section, the deep-channel fill (DCF) under the NAMOC floor is seismically indistinguishable from the subbottom echoes under the braid plain. OL: Onlap of braid-plain sands on the truncated eastern NAMOC levee (for detail see Hesse and Rakofsky [1992], figure 12). SCF: Secondary channel fill. FFF: Mud-filled tributary. GH: Reduced width of the eastern NAMOC levee due to westward expansion of the braid plain through levee erosion by sandy currents. Hesse et al.

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Figure 12. Blowup of seismic profile crossing the three lowbackscatter-intensity (dark) streaks (S1, S2, and S3) in the braid plain near 59⬚15⬘N. The profile is superposed on sidescan imagery in an attempt to show suspected erosion of a subsurface sand layer (second subbottom reflector from top) at the position of the streaks.

Figure 10, however, suggest that differentiation between the channel-fill facies and braid-plain facies may be possible based on seismic continuity. The discontinuous highly reflective facies under the western levee in profile 3 (southern part in Figure 10) is interpreted as coarse-grained channel-fill facies, that is, the same facies that underlies the present NAMOC floor and ad1514


jacent parts of the levee. Where this facies is buried in the subsurface, it may contain small-scale low-angle dipping reflectors (middle part of profile 3 in Figure 10) that may be lateral accretion surfaces on submarine point bars. The more continuous, highly reflective facies under the eastern levee of Figure 10 (profile 3), but also under the western levee that follows imme-

diately to the north (northern part of profile 3), may be buried braid-plain facies, because it is very similar to the seismic facies under the braid plain itself (profiles 5, 6, and southern, middle, and northernmost parts of profile 4). These profiles suggest that predecessors of the present braid plain may have extended farther west, because seismically the braid-plain facies can be identified at least as far west as profile 3 under the eastern levee and locally even under the western levee (in the same profile). In the region north of 59⬚30⬘N, there is evidence that in the past the NAMOC, with its growing muddy levees shifted eastward to its present position (as shown by the positions of older, buried levees in the west on profiles AFAE and M9 in Figure 11; locally, also short westward shifts occurred in the course of the meandering of the channel, as e.g., on profile B, figure 7 of Klaucke et al. [1998a]). When this shift occurred, the western braid-plain margin was also displaced eastward. Myers and Piper (1988) established a seismic stratigraphy for the Labrador Sea by tying prominent seismic reflectors to Deep Sea Drilling Project Site 113 and Ocean Drilling Program Site 646. Their reflectors A, B, C, and D have been tentatively dated middle Pleistocene, earliest Pleistocene/latest Pliocene, early late Pliocene, and middle Pliocene, respectively. Reflector A corresponds to the seismic boundary between the muddy spill-over turbidite facies of the NAMOC levees (transparent seismic facies having sharp continuous reflectors) and the coarse-grained facies equivalent to the braid-plain sands (low penetration seismic facies having a prolonged bottom echo and a varying number of diffuse or indistinctly parallel subbottom echoes). This suggests that the formation of the present NAMOC levees may have started in middle Pleistocene (around 1 Ma), but the establishment of the braid plain predated them. Reflectors B, C, and D are too deep to appear on our profiles. In most parts of profiles 4–6 (Figure 10) there is no sharp lower acoustic boundary of the seismic braidplain facies. At the southern end of profile 4, however, the braid-plain facies is underlain by a transparent layer below about 100 ms, which is followed at depth by a more reflective unit having wavy reflections suggesting that the transparent layer may be a pelagic unit (or alternatively, a debris-flow deposit) that, together with the underlying sediments, underwent differential compaction. In the distal braid plain the total sand thickness would therefore not exceed 100 m. In northern, more proximal parts (e.g., profile 4), the highly reflec-

tive facies extends to about 200 ms sub–sea floor, giving a minimum thickness of slightly less than 200 m. Levee Erosion by Flows from the Sand Plain Entering the NAMOC In the region south of 57⬚30⬘N, where the streaky features curve markedly toward the NAMOC (Figure 2c, d), the left (eastern) levee of the NAMOC, which farther north is up to 100 m higher than the adjacent braid plain, is absent (on all profiles south of profile ED in Figure 8). The levee has been eroded over a distance of more than 100 km along the channel. Curvature of the lineaments indicates that the currents, which breached the levee by approaching it slightly oblique to strike, flowed into the channel, not out of it (Figure 2c). Currents within the NAMOC, which overtop the channel banks with their mud-carrying flow tops, are not powerful enough to erode the levee. Instead they deposit the muddy spill-over turbidite facies on the levees. The significance of the breached levee is that it attests to the erosive power of sandy flows after they have traversed more than 600 km of the plain. The flow path of the currents from the plain into the NAMOC was approximately parallel with the B tributary, which joins the main channel at 56⬚30⬘N (Figures 1, 2c; profiles 4 and 5 in Figures 9, 10). The B channel is a major NAMOC tributary from the braid plain; in fact it is the only channel on the plain that can be traced for some distance (in Figure 2c up to 58⬚N, where it leaves the eastern margin of the side-scan mosaic). Contrary to previous interpretations (Hesse and Rakofsky, 1992), this channel does not result from diversion of the NAMOC by major debris flows and the giant slump at latitude 59⬚30⬘N (see the following discussion) that filled the main channel to flood-plain level; instead it develops within the proximal part of the braided sand plain north of 59⬚N, where the channel head is shallow, very broad, and indistinct (profiles AB, WXX in Figure 7). Now it is also clear that the flows that generated the sand and gravel plain did not originate from the NAMOC by spillover at the point where the main channel is filled by the said debris-flow deposits (region of the NAMOC WEST and EAST in Figure 2a), contrary to the assumption of Hesse and Rakofsky (1992, their figure 11, profile A) and Hesse (1992, his figure 3, lower profile), which was made before the side-scan imagery became available. On the side-scan images the plain can be traced northward beyond (i.e., Hesse et al.

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upstream of) the choked part of the channel. In this region of the proximal, nonbraided part of the plain, the flows, carrying sand and gravel, probably formed wide sheet flows, coming from an area of the slope in front of the strait where no major tributary canyons to the NAMOC exist (Figure 1a; see also Hesse et al. [1999], their figure 2A). The flows therefore bypassed the proximal parts of the NAMOC system, coming directly from the slope in front of the outer strait.

During large meltwater discharges, the coarse fraction, however, can form hyperpycnal flows and directly generate turbidity currents carrying sand and gravel across the slope in front of the strait to the deep plain. Part of this bed load is temporarily deposited in submarine ice-margin fans and later also remobilized. We have called this grain-size separation process “icemargin sifting” (Hesse et al., 1997a, b).

INTERPRETATIONS

The true volume of individual sand-carrying flows that supplied the braid plain with sediment is most difficult to assess because we have neither the evidence for their duration, nor the data to gauge their thickness or width reliably. In the future, video observations or side-scan imaging of seamounts on the braid plain using a deeptowed instrument might provide information on flow depth from flood markers associated with the thickest flows, and from these it may be possible to extrapolate flow widths. We have no firm evidence that every flow that entered the plain covered it in its entire width. The flat proximal part of the plain upstream of the braid plain proper (see discussion in the section “Braided vs. Nonbraided Part of the Sandy-Gravelly Flood Plain”), however, suggests that at least in this region the sandy-gravelly turbidity currents may have been wide sheet-flow events possibly covering the entire width of the plain. For subglacial meltwater floods, which are assumed to have carved “bed-rock drumlins” under the Laurentide ice sheet, the parallelism of the drumlins has been used to infer flows that were as wide as the drumlin fields themselves: up to 60–150 km; the height of the drumlins was used to estimate minimum flow depths (i.e., 20 m) (Shaw, 1989). Converging, diverging, or criss-crossing patterns on our side-scan imagery suggest braided turbidity currents. It cannot be excluded, however, that these patterns may represent overlapping linear features penetrated by the sound, in which case the plain might not be truly braided. Discharge estimates for the largest of the subglacial events on land are in the 106 m3 /s range, and conservative volume estimates give figures on the order of 105 km3 such that individual floods could have caused a global sea level rise of up to 0.3 m (Shaw, 1989). Where measurements exist, it has been found that such floods may carry twice as much bed-load material than suspended-load material (Beecroft, 1983). As high-

Origin of the Braid Plain The coexistence of two large depositional systems in the deep basin of the Labrador Sea (one predominantly fine grained, the other predominantly coarse grained), both of which received their sediment from the same source in the Hudson Strait, requires a large-scale grain-size separation process at the entrance point of the sediment-laden glacial meltwater to the sea (Hesse et al., 1997a). This process, comparable to wind sifting, operates by entraining much of the fine suspended sediment in surface plumes while discharging the coarse bed-load fraction directly as gravity flows across the shelf edge to the deep sea. At the glacier front, fresh water rises from ice tunnels to the surface of the sea or stays at the surface if discharged from supraglacial rivers, carrying with it the fine suspended sediment and forming turbid hypopycnal surface plumes. These plumes are engulfed by the south-flowing Labrador Current and rain out their suspended load on the continental slope south of the Hudson Strait, forming a thick mud blanket (Hesse et al., 1997a). Unless suspended loads exceed about 40 g/L, underflows do not develop in cold seawater; such high concentrations are rarely reached during normal meltwater discharges (Mulder and Syvitski, 1995). Through this grain-size separation process a large fraction of the fine-grained sediment is separated from the coarse bed load (medium to coarse sand and gravel) and transferred to the slope south of the strait. Remobilization of the dominantly fine-grained slope sediment by sliding and slumping generates a high-relief slope sector of canyons and intercanyon ridges south of the strait (Hesse et al., 1999). The resulting mass transport, mostly by channelized debris flows and turbidity currents, supplies the mud-dominated NAMOC system with fine-grained sediment. 1516


Volume of Meltwater Discharge Events in the Hudson Strait and Resulting Turbidity Currents

magnitude, low-frequency events, these subglacial outburst floods are capable of considerable geomorphic work, having generated large-scale depositional bed forms and erosional features such as the bed-rock drumlins already mentioned and the tunnel valleys of Boyd et al. (1988). For the submarine case, flow rates and volumes may be smaller than the preceding estimates by an order of magnitude because only the bedload–carrying parts of the flows would find their way to the sea floor; most of the meltwater would still either rise to or simply stay at the surface of the ocean. Comparison with large turbidity currents, for which volume estimates have been published, suggests even smaller volumes, by another order of magnitude. The 1929 Grand Banks turbidity current (Piper et al., 1988) distributed sand over an area of the lower Laurentian Fan of similar extent and having relief elements of comparable elevation to the Labrador Basin braid plain. Mean sand bed thickness on the lower fan was 1–2 m, and total sediment delivery was 100–200 km3 (Piper and Aksu, 1987). From the flow character in the channeled part of the flow, Hughes Clarke et al. (1990) inferred a mean volume concentration of 6%, implying a water discharge of 2–5 ⳯ 103 km3, much of which would have been the result of entrainment. Although we do not know that individual flows on the Labrador Basin braid plain deposited across the entire plain, sand bed thickness is a little greater than that of the 1929 deposit, and the proportion of the flow that continued down the distal reaches of the NAMOC is unknown. If the 1929 event was a suitable analog, individual flows may have had initial water discharges, prior to entrainment, on the order of 103 km3. As the side-scan sonar study (Hesse et al., 1996) has shown, the sandy flows that traveled for about 1000 km from the shelf break off Hudson Strait mostly without the guidance of a major channel like the NAMOC, finally joined the NAMOC north of the junction with the B channel in the region of the eroded eastern NAMOC levee and through the B channel itself, or through the IMOC, which forms the southern limit of the braid plain. Some of those flows, which had not yet run out of sediment at this point, were guided into the NAMOC, where apparently they caused spillover on the levees of the distal channel south of 56⬚N. Some may have traveled all the way to the NAMOC terminus on the Sohm Abyssal Plain at 37⬚N, or another 2500 km. The occurrence of a more highly reflective, probably sandier, seismic levee facies of the NAMOC south of the junction with the IMOC than north of it, which extends to our southernmost

crossing of the channel at 44⬚30⬘N (Klaucke et al., 1998b, figure 3), attests to the significance of sand input in this part of the system, which probably stems largely from the sandy flows on the braid plain, but possibly also from the IMOC and from the E/F and H tributaries on the Labrador margin (Hesse et al., 1999). At present, petrographic evidence to pinpoint the different sources for the flows in the distal channel south of the confluence with the IMOC is not yet available. Potential Water Reservoirs and Delivery Mechanisms at the Source: Subglacial Lakes, Equivalents of Jo¨kulhlaups, and Possible Relationship to Heinrich Events If the flows were indeed very large, then reservoirs of appropriate volume must have existed to release the amount of sediment and water needed to sustain the flows. Potential reservoirs would have been large subglacial lakes, perhaps augmented by supraglacial and englacial water bodies, in the Hudson Strait and connected glacial drainage areas of the Laurentide ice sheet. The possible former existence of such lakes is indicated by large depressions, for example, in the floor of the strait (MacLean et al., 1992). Next a mechanism of catastrophic reservoir emptying would have been needed, for example, lifting an ice barrier or breaching a submarine end-moraine dam and initiating a subglacial outburst flood. Smaller events of this kind are well known under their Icelandic name, “jo¨kulhlaups,” from present and past glacial environments (Waitt, 1985), particularly during times of glacier advance (Beecroft, 1983), and have been proposed as models for postulated Hudson Strait meltwater discharge events (Johnson and Lauritzen, 1995). The proposed mechanism for braid-plain formation might be related to the same ice dynamics that produced Heinrich ice-rafting events (Broecker et al., 1992), namely, surges of the ice stream in the Hudson Strait every 4–16 k.y. (Bond et al., 1993; MacAyeal, 1993). The isotopic and radiocarbon dating results suggest that the topmost sands may correspond in age to Heinrich event H0 in the Younger Dryas climatic oscillation. More deeply buried sands may show a similar genetic relationship to earlier Heinrich events, in particular, if the lower sand in core 84-030-8 could be related to H1. The process of large subglacial water discharges involving subglacial lakes was proposed by Johnson and Lauritzen (1995) on the basis of gorges exposed on the northern shore of Hudson Strait. If the lake-outburst hypothesis is viable, rising lake levels may have lifted the ice dams and triggered ice surging Hesse et al.

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and Heinrich events. In this case the flooding events need not be restricted to Heinrich events; they might have occurred in phase with the more frequent Dansgaard-Oeschger cycles (Dansgaard et al., 1993) on a 1000–2000 yr scale rather than a 10,000 yr scale (Johnson and Lauritzen, 1995). Longer cores are needed to determine the periodicity of turbidite sands on the braid plain.

DISCUSSION AND CONCLUSIONS Braided vs. Nonbraided Part of the Sandy-Gravelly Flood Plain North of 59⬚50⬘N (Figures 1; 2a, b) the streaky sidescan signature of the braid plain is absent except immediately adjacent to the NAMOC (see profile AEAF in Figure 7). Instead a uniformly low backscatterenergy region occurs where the seismic profiles still indicate sand and gravel as far north as the base of the slope (Figure 1a); our data coverage of the plain ends north of 60⬚45⬘N and west of 57⬚30⬘W. The boundary between the two regions, that is, the braid plain in the south and the monotonous sand and gravel plain in the north, is marked by a highly gullied terrain (Figure 2a, b) interpreted as a giant slump in the seismic profiles (e.g., profile 5 in Figure 9). This slump originated east of the now inactive western branch of the NAMOC in the region around 60⬚N. The more recent eastern branch (Figure 2a, b; profile 5 in Figure 9) cuts the slump and reflects a displacement of the active channel 20 km to the northeast (Klaucke, 1995). The slump probably deflected major parts of the turbidity currents from the proximal part of the plain to the east, but gullying of its surface also suggests that some of these flows entering from the north were split by the obstacle of the slump into western and eastern spills forming elongate vortices. The western spills overflowed the slump and caused the streaky (?erosional) feature on the eastern NAMOC levee around 59⬚30⬘N (S1–S3 on Figures 2a, 7, 12, profile WXX). In the region south of the slump, it seems that the eastern levee is protected from erosion by currents where the slump is highest. The fact that the eastern levee extends farther south on the more easterly profile 5 in this region than on profile 4 (Figures 9, 10) may reflect the breakup of individual flows, where the western spills cause levee erosion farther north in profile 4 than in profile 5. This is the only evidence so far to suggest that the width of individual flows was consid1518


erable, greater than a minimum of 20 km, if the preceding interpretation is correct. The occurrence of the braid plain downslope from the nonbraided part of the plain may be an artifact of side-scan sound penetration through the Holocene mud cover, which is about a meter thick in the distal regions of the plain, where the plain appears braided, but may be considerably thicker in the proximal regions nearer the Hudson Strait source, where we do not have ground truth control from piston cores. Alternatively, the smooth surface of the sand layers in the proximal part of the plain may be real and correspond to a smooth surface of sheet sand deposits because they are well stratified on seismic profiles and do not show evidence of erosion except in the immediate vicinity of the NAMOC (Figures 2a, b; 9). Comparison with Other Examples Braided sand sheets in the deep sea are not restricted to ice-marginal environments in high latitudes. With the increasing number of recent side-scan and backscatter imaging surveys in deep water, braided sand sheets have been recognized from the deeper parts of several deep-sea fans (Table 1). In the high latitudes, the southern Bering Sea braid plain (Kenyon and Millington, 1995) is a feature unrelated to fans and of dimensions comparable to the Labrador Sea braid plain. In low latitudes, lower fan sands deposited on lobes beyond the limits of leveed channels in some cases show braided patterns. These were first described by Belderson et al. (1984) from the distal Orinoco Fan off the Barbados accretionary prism (recently confirmed by Ercilla et al. [1998]). In addition, braided segments of submarine channels have been reported by several authors (Gardner et al., 1991, 1996; Cronin et al., 1995). Low-latitude braided lobes are generally smaller than the high-latitude examples, and commonly the characteristic anastomosing pattern is less clearly developed. Their gradients range from 1:350 to less than 1:700, corresponding to the range of gradients over which leveed channels typically show meandering planform (Clark et al., 1992). Because the levees of fan valleys appear to prograde over sandy depositional lobes (Normark et al., 1983; Pirmez and Flood, 1995), the occurrence of a laterally unconstrained flow may promote the formation of a braided channel pattern. Levee breaching and avulsion of a meandering fan channel, as observed on other mud-rich deep sea fans (e.g., Amazon Fan, Hiscott et al., 1997; Pirmez et al., 1997) can also

Table 1. Submarine Braid Patterns in Modern Deep-Sea Fan and Basin Plain Settings Location

Area (km2)

Gradient

Labrador Sea

700 ⳯ 100

1:550

Bering Sea

250 ⳯ 70

1:600

Old passive margin/ Cretaceous ocean crust Back-arc basin

⬎80 ⳯ 30

1:350

Foot of accretionary prism Tropical river

GLORIA

1:600 1:350

Young passive margin Strike-slip basin

EM12 GLORIA

Kenyon and Millington, 1995 Belderson et al., 1984 Unterseh et al., 1998 Edwards et al., 1996

GLORIA

Gardner et al., 1996

Orinoco Fan Var Fan Santa Monica Basin Monterey Fan

5⳯3 45 ⳯ 30

Tectonic Setting

Active continental margin

lead to sheet-sand deposition in interchannel lows, seen as high-amplitude reflection packets on seismic profiles, which may or may not possess braided patterns. Not all lower fan sand plains have a braided pattern, however: for example, the Indus (Kenyon et al., 1995) and Mississippi fans (Twitchell et al., 1995) show a distributary channel pattern. Most large passive-margin fans that have meandering fan valleys leading to distributary lower fan channels are built by turbidity currents of relatively uniform size, probably triggered by shallow prodeltaic sediment failure (Normark and Piper, 1991). In contrast, several of the described low-latitude examples of braid plains probably have a much greater range of discharges, as a result of hyperpycnal flow occurring at times (Piper and Savoy [1993] for Var; Piper et al. [1999a] for Santa Monica). Subglacial outburst floods, as postulated for the Labrador Sea, represent an extreme end member of such hyperpycnal flow. Hyperpycnal flow events (e.g., Mulder et al., 1998) may have a longer duration (days) than failure-generated turbidity currents (10–20 hr) (e.g., Piper et al., 1999b), and this longer duration may be necessary to develop the apparent braided pattern of channels and bars (furrows and ridges). The occurrence of large-scale braided sand and gravel plains in highlatitude subpolar regions may be related to the effective separation of sand and mud by meltwater discharge associated with major glacier outlets from continental ice sheets. According to Mulder and Syvitski (1995), the separation is up to 20% more effective in cold than in warm seawater. We thus draw an analogy with fluvial systems, in which braiding appears to be promoted by “flashy” discharge and large amounts of bed-load sediment (Schumm, 1977).

Source Subglacial discharge

Data

HAWAII MR1 This article

Subglacial volcanicl. (?) GLORIA

High-gradient river High bed-load river

Reference

Significance for Exploration The Labrador Sea submarine braided plain is a largescale example of a depositional pattern found seaward of mixed mud and sand channel-levee systems in many lower latitude fans. An understanding of the processes and geometry of this system can thus be applied to other deep-water braided sand plains. Such sand plains occur in a wide range of tectonic settings (Table 1). They commonly overlie basinal shales deposited either before turbidite progradation or at marine highstands. In the Labrador Sea, juxtaposition of the braided plain to the NAMOC with its muddy levees containing between 0.8 and 1.0% organic carbon (Chough et al., 1987; Hesse and Rakofsky, 1992; Khodabakhsh, 1997) places it next to sediments having marginal source rock character. Small braid systems are found in basins developed on continental crust that has a high preservation potential. Braided sand (and gravel) plains are most likely to develop where sand is efficiently separated from mud in the source area for turbidity currents and in which the size (discharge volume) of turbidity currents is highly variable. They may also be favored by prolonged discharge associated with hyperpycnal flow, rather than short-lived discharge resulting from sediment failure. The Labrador Sea example may be unique in terms of the size of the discovered sand body (700 ⳯ 100 ⳯ 0.150 km or about 104 km3, corresponding to a gross pore volume of 10–20 billion bbl), which is enormous and, if oil filled, would make it one of the largest fields on earth. As the discussion has shown, however, its characteristics may serve as guidance to numerous smaller scale examples in various continental margin settings. Hesse et al.

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REFERENCES CITED Beecroft, I., 1983, Sediment transport during an outburst from glacier de Tsidjiore Nouve, Switzerland, 16–19 June, 1981: Journal of Glaciology, v. 29, p. 185–190. Belderson, R. H., N. H. Kenyon, A. H. Stride, and C. D. Pelton, 1984, A ‘braided’ distributary system on the Orinoco deep-sea fan: Marine Geology, v. 56, p. 195–206. Bond, G., W. Broecker, S. Johnsen, J. McManus, L. Labeyrie, J. Jouzel, and G. Bonani, 1993, Correlations between climate records from North Atlantic sediments and Greenland ice: Nature, v. 365, p. 143–147. Boyd, R., D. B. Scott, and M. Douma, 1988, Glacial tunnel valleys and Quaternary history of the outer Scotian shelf: Nature, v. 333, p. 61–64. Broecker, W. S., G. Bond, M. Klas, E. Clark, and J. McManus, 1992, Origin of the North Atlantic’s Heinrich events: Climate Dynamics, v. 6, p. 265–273. Chough, S. K., and R. Hesse, 1976, Submarine meandering talweg and turbidity currents flowing for 4,000 km in the Northwest Atlantic Mid-Ocean Channel, Labrador Sea: Geology, v. 4, p. 529–533. Chough, S. K., R. Hesse, and J. Mu¨ller, 1987, The Northwest Atlantic Mid-Ocean Channel of the Labrador Sea: IV. petrology and provenance of the sediments: Canadian Journal of Earth Sciences, v. 24, p. 731–740. Clark, J. D., N. H. Kenyon, and K. T. Pickering, 1992. Quantitative analysis of the geometry of submarine canyons: implications for the classification of submarine fans: Geology, v. 20, p. 633–636. Cro¨mmelin, R. D., 1937, A sedimentary petrological investigation of a number of sand samples from the south coast of Greenland between Ivigtut and Frederiksdal: Meddedelser om Grønland, v. 113, p. 1–32. Cronin, B. T., N. H. Kenyon, J. Woodside, T. den Bezemer, A. van der Wal, J. Millington, M. K. Ivanov, and A. Limonov, 1995, The Almeria Canyon: a meandering channel system on an active margin, Alboran Sea, Western Mediterranean, in K. T. Pickering, R. N. Hiscott, N. H. Kenyon, F. Ricci Lucchi, and R. D. A. Smith, eds., Atlas of deep-water environments: architectural style in turbidite systems: London, Chapman and Hall, p. 276–281. Dansgaard, W., et al., 1993, Evidence for general instability of past climate from a 250-kyr ice-core record: Nature, v. 364, p. 218– 220. Edwards, B. D., M. E. Field, and N. H. Kenyon, 1996, Morphology of small submarine fans, inner California Continental Borderland, in J. V. Gardner, M. E. Field, and D. G. Twitchell, eds., Geology of the United States’ seafloor: the view from GLORIA: Cambridge, Cambridge University Press, p. 235–249. Egloff, J., and G. L. Johnson, 1975, Morphology and structure of the southern Labrador Sea: Canadian Journal of Earth Science, v. 12, p. 2111–2133. Ercilla, G., et al., 1998, New high-resolution acoustic data from the ‘braided system’ of the Orinoco deep-sea fan: Marine Geology, v. 146, p. 243–250. Gardner, J. V., M. E. Field, H. Lee, B. E. Edwards, D. G. Masson, N. Kenyon, and R. B. Kidd, 1991, Ground truthing 6.5-kHz side scan sonographs: what are we really imaging?: Journal of Geophysical Research, v. 96, p. 5955–5974. Gardner, J. V., R. G. Bohannon, R. M. E. Field, and D. G. Masson, 1996, The morphology, processes and evolution of Monterey Fan: a revisit, in J. V. Gardner, M. E. Field, and D. G. Twitchell, eds., Geology of the United States’ seafloor: the view from GLORIA: Cambridge, Cambridge University Press, p. 193– 220. Hein, F. J., and R. G. Walker, 1982, The Cambro-Ordovician Cap

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Enrage´ Formation, Que´bec, Canada: conglomeratic deposits of a braided submarine channel with terraces: Sedimentology, v. 29, p. 309–329. Hesse, R., 1989, Drainage system associated with mid-ocean channels and submarine yazoos: alternative to submarine fan depositional system: Geology, v. 17, p. 1148–1151. Hesse, R., 1992, Continental-slope sedimentation adjacent to an icemargin: I. seismic facies of Labrador Slope: Geo-Marine Letters, v. 12, p. 189–199. Hesse, R., and A. Rakofsky, 1992, Deep-sea channel/submarine Yazoo system of the Labrador Sea: new deep-water facies model: AAPG Bulletin, v. 76, p. 680–707. Hesse, R., S. K. Chough, and A. Rakofsky, 1987, The Northwest Atlantic Mid-Ocean Channel of the Labrador Sea: V. sedimentology of a giant deep-sea channel: Canadian Journal of Earth Sciences, v. 24, p. 1595–1624. Hesse, R., A. Rakofsky, and S. K. Chough, 1990, The central Labrador Sea: facies and dispersal patterns of clastic sediment in a small ocean basin: Marine and Petroleum Geology v. 7, p. 13– 28. Hesse, R., I. Klaucke, W. B. F. Ryan, M. E. Edwards, and D. Piper, 1996, Imaging Laurentide ice sheet drainage into the deep sea: impact on sedimentation and bottom water: GSA Today, v. 6, no. 9, p. 3–9. Hesse, R., S. Khodabakhsh, I. Klaucke, and W. B. F. Ryan, 1997a, Asymmetrical surface-plume deposition near ice-outlets of the Pleistocene Laurentide ice sheet in the Labrador Sea: GeoMarine Letters, v. 17, p. 179–187. Hesse, R., I. Klaucke, W. B. F. Ryan, and D. Piper, 1997b, Ice-sheet sourced juxtaposed turbidite systems in Labrador Sea: Geoscience Canada, v. 24, no. 1, p. 3–12. Hesse, R., I. Klaucke, S. Khodabakhsh, and D. Piper, 1999, Continental slope sedimentation adjacent to an ice margin: III. the upper Labrador Slope: Marine Geology, v. 155, p. 249–276. Hillaire-Marcel, C., A. de Vernal, G. Bilodeau, and G. Wu, 1994, Isotope stratigraphy, sedimentation rates, deep circulation, and carbonate events in the Labrador Sea during the last ⬃200 ka: Canadian Journal of Earth Science, v. 31, p. 63–89. Hiscott, R. N., C. Pirmez, and R. D. Flood, 1997, Amazon submarine fan drilling: a big step forward for deep-sea fan models: Geoscience Canada, v. 24, p. 13–24. Hughes Clarke, J. E., A. N. Shor, D. J. W. Piper, and L. A. Mayer, 1990, Large-scale current-induced erosion and deposition in the path of the 1929 Grand Banks turbidity current: Sedimentology, v. 37, p. 613–629. Johnson, R. G., and S. E. Lauritzen, 1995, Hudson Bay–Hudson Strait Jo¨kulhlaups and Heinrich events: a hypothesis: Paleogeography, Paleoclimatology, Paleocology, v. 117, p. 123–137. Kenyon, N. H., and J. Millington, 1995, Contrasting deep-sea depositional systems in the Bering Sea, in K. T. Pickering, R. N. Hiscott, N. H. Kenyon, F. Ricci Lucchi, and R. D. A. Smith, eds., Atlas of deep-water environments: architectural style in turbidite systems: London, Chapman and Hall, p. 196–203. Kenyon, N. H., A. Amir, and A. Cramp, 1995, Geometry of the younger sediment bodies of the Indus Fan, in K. T. Pickering, R. N. Hiscott, N. H. Kenyon, F. Ricci Lucchi, and R. D. A. Smith, eds., Atlas of deep-water environments: architectural style in turbidite systems: London, Chapman and Hall, p. 89– 93. Khodabakhsh, S., 1997, Pleistocene Laurentide ice sheet drainage into the Labrador Sea: sedimentary facies, depositional mechanisms, stratigraphy and significance of Heinrich events: Ph.D. thesis, McGill University, Montreal, Quebec, Canada, 263 p. Klaucke, I., 1995, The submarine drainage system of the Labrador Sea: result of glacial input from the Laurentide icesheet: Ph.D. thesis, McGill University, Montreal, Quebec, Canada, 216 p.

Klaucke, I., and R. Hesse, 1996, Fluvial features in the deep-sea: new insights from the glacigenic submarine drainage system of the Northwest Atlantic Mid-Ocean Channel in the Labrador Sea: Sedimentary Geology, v. 106, p. 223–234. Klauke, I., R. Hesse, and W. B. F. Ryan, 1997, Flow parameters of turbidity currents in a low-sinuousity giant deep-sea channel: Sedimentology, v. 44, p. 1093–1102. Klaucke, I., R. Hesse, and W. B. F. Ryan, 1998a, Seismic stratigraphy of the Northwest Atlantic Mid-Ocean Channel: growth pattern of a mid-ocean channel-levee complex: Marine and Petroleum Geology, v. 15, p. 575–585. Klaucke, I., R. Hesse, and W. B. F. Ryan, 1998b, Morphology and structure of a distal submarine trunk-channel: the Northwest Atlantic Mid-Ocean Channel between 53⬚ and 44⬚30⬘N: Geological Society of America Bulletin, v. 110, no. 1, p. 22–34. MacAyeal, D. R., 1993, Binge/purge oscillations of the Laurentide ice sheet as a cause of the North Atlantic Heinrich events: Paleoceanography, v. 8, p. 775–784. MacLean, B., G. Vilks, and B. Deonarine, 1992, Depositional environments and history of late Quaternary sediments in Hudson Strait and Ungava Bay: further evidence from seismic and biostratigraphic data: Geógraphie physique et Quaternaire, v. 46, p. 311–329. Mulder, T., and J. P. M. Syvitski, 1995, Turbidity currents generated at river mouths during exceptional discharges to the world oceans: Journal of Geology, v. 103, p. 285–299. Mulder, T., B. Savoye, D. J. W. Piper, and J. P. M. Syvitski, 1998. The Var submarine sedimentary system: understanding Holocene sediment delivery processes and their importance to the geological record, in M. S. Stoker, D. Evans and A. Cramp, eds., Geological processes on continental margins: sedimentation, mass wasting and stability: Geological Society Special Publication, v. 129, p. 145–166. Myers, R. A., and D. J. W. Piper, 1988, Seismic stratigraphy of late Cenozoic sediments in the northern Labrador Sea: a history of bottom circulation and glaciation: Canadian Journal of Earth Sciences, v. 25, p. 2059–2074. Normark, W. R., and D. J. W. Piper, 1991, Initiation processes and flow evolution of turbidity currents: implications for the depositional record: SEPM Special Publication 46, p. 207–230. Normark, W. R., D. J. W. Piper, and D. A. V. Stow, 1983, Quaternary development of channels, levees and lobes on middle Laurentian Fan: AAPG Bulletin, v. 67, p. 1400–1409. Piper, D. J. W., and A. E. Aksu, 1987, The source and origin of the 1929 Grand Banks turbidity current inferred from sediment budgets: Geo-Marine Letters, v. 7, p. 177–182. Piper, D. J. W., and B. Savoye, 1993, Processes of late Quaternary turbidity current flow and deposition on the Var deep-sea fan, north-west Mediterranean Sea: Sedimentology, v. 40, p. 557– 582. Piper, D. J. W., A. N. Shor, and J. E. Hughes Clarke, 1988, The 1929 Grand Banks earthquake, slump and turbidity current: Geological Society of America Special Paper 229, p. 77–92. Piper, D. J. W., R. N. Hiscott, and W. R. Normark, 1999a, Outcropscale acoustic facies analysis and latest Quaternary development

of Hueneme and Dume submarine fans, offshore California: Sedimentology, v. 46, p. 47–78. Piper, D. J. W., P. Cochonat, and M. L. Morrison, 1999b, Side-scan sonar evidence for progressive evolution of submarine failure into a turbidity current: the 1929 Grand Banks event: Sedimentology, v. 46, p. 79–97. Pirmez, C., and R. D. Flood, 1995, Morphology and structure of Amazon Channel, in R. D. Flood et al., Proceedings of the Ocean Drilling Program, initial reports, v. 155: College Station, Texas, Ocean Drilling Program, p. 23–45. Pirmez, C., R. N. Hiscott, and J. D. Kronen Jr., 1997, Sandy turbidite successions at the base of channel-levee systems of the Amazon Fan revealed by FMS logs and cores: unraveling the facies architecture of large submarine fans, in R. D. Flood, D. J. W. Piper, A. Klaus, and L.C. Peterson, eds., Proceedings of the Ocean Drilling Program, scientific results, v. 155: College Station, Texas, Ocean Drilling Program, p. 7–33. Reiss, N., 2000, Stable isotopic composition (oxygen- and carbon isotopes) of planktonic and benthonic foraminifera in late Pleistocene sediments of the Labrador Sea: M.Sc. thesis, Ruhr University, Bochum, Germany, 52 p. Rognstad, M., 1992, HAWAII MR1: a new underwater mapping tool: International Conference on Signal Processing Technology, v. 92, p. 900–905. Schumm, S. A., 1977, The fluvial system: New York, Wiley, 338 p. Schumm, S. A., 1981, Evolution and response of the fluvial system, sedimentologic implications, in F. G. Ethridge and R. M. Flores, eds., Recent and ancient nonmarine depositional environments: models for exploration: SEPM Special Publication 31, p. 19– 29. Shaw, J., 1989, Drumlins, subglacial meltwater floods, and ocean responses: Geology v. 17, p. 853–856. Twitchell, D. C., W. C. Schwab, and N. H. Kenyon, 1995, Geometry of sandy deposits at the distal edge of the Mississippi Fan, Gulf of Mexico, in K. T. Pickering, R. N. Hiscott, N. H. Kenyon, F. Ricci Lucchi, and R. D. A. Smith, eds., Atlas of deep water environments: architectural style in turbidite systems: London, Chapman and Hall, p. 282–286. Unterseh, S., P. Cochonat, M. Voisset, G. Ollier, and F. Harmegnies, 1998, Acoustic data ground truthing through in-situ measurements of physical properties: Proceedings of the 8th International Offshore and Polar Engineering Conference, v. 3, p. 686– 692. Waitt, R. W., 1985, Case for periodic, colossal jo¨kulhlaups from Pleistocene glacial Lake Missoula: Geological Society of America Bulletin, v. 96, p. 1271–1286. Walker, R. G., and D. J. Cant, 1984, Sandy fluvial systems, in R. G. Walker, ed., Facies models: Geoscience Canada Reprint Series 1 (2d ed.), p. 71–89. Wang, D., and R. Hesse, 1996, Continental slope sedimentation adjacent to an ice-margin: II. glaciomarine depositional facies on Labrador Slope and glacial cycles: Marine Geology, v. 135, p. 65–96. Windley, B. F, 1969, Anorthosites of southern West Greenland, in M. Kay, ed., North Atlantic—geology and continental drift: AAPG Memoir 12, p. 899–915.

Hesse et al.

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