Stratigraphic and sedimentological evidence for late Wisconsinan sub-glacial outburst floods to Laurentian Fan

Palaeogeography, Palaeoclimatology, Palaeoecology 246 (2007) 101 – 119 www.elsevier.com/locate/palaeo

Stratigraphic and sedimentological evidence for late Wisconsinan sub-glacial outburst floods to Laurentian Fan David J.W. Piper a,⁎, John Shaw b , Kenneth I. Skene c a

Geological Survey of Canada (Atlantic), Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, N.S., Canada, B2Y 4A2 b Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada, T6G 2E3 c Department of Oceanography, Dalhousie University, Halifax, N.S., Canada, B3H 3J4 Received 8 April 2004; accepted 17 October 2006

Abstract Sub-glacial meltwater produces a distinctive stratigraphic and sedimentological response on the continental margin. Seismostratigraphy of Laurentian Channel reveals thick till deposits at its seaward end that pass laterally into stratified sediment in deeper basins, that may record periods of water build up beneath the ice. Two scales of meltwater discharge are recognised: large scale that caused catastrophic erosion and transported large volumes of coarse sediment to the abyssal plain and smaller scale, yielding principally muddy sediment. Sub-glacial outburst floods from the Laurentian Channel ice stream delivered distinctive red sediment derived from Permian–Carboniferous strata of the Gulf of St. Lawrence directly to Laurentian Fan between ca. 17 and 14 14C ka, separate from North Atlantic Heinrich events. On levees of Laurentian Fan, three major pulses of meltwater plume muds are separated by intervals dominated by hemipelagic sediments. These meltwater intervals are recognised distally as periods of plume sedimentation on the Scotian Slope and ice-rafting of hematite-stained quartz to the North Atlantic Ocean. In channels of Laurentian Fan, at least one major sediment transport event is recognised that eroded the upper slope and the major fan valleys, depositing a bed of gravel at least 3 m thick in the characteristically wide fan valleys and thick sand on the Sohm Abyssal Plain. The same event was probably responsible for giant flute-like scours. The age of the gravel bed is directly constrained only by the presence of local overlying Holocene sediment. Much of the surface of the gravel bed was re-worked by the 1929 “Grand Banks” turbidity current. An erosional event on the upper slope, likely correlative with the flood-generated gravel bed, has been dated at 16.5 14C ka. Such large scale erosional flood events can be recognised back through several glacial cycles and have played an important role in the architectural evolution of Laurentian Fan. Crown Copyright © 2006 Published by Elsevier B.V. All rights reserved. Keywords: Glaciation; Meltwater; Turbidite; Sohm Abyssal Plain; Laurentian Fan

1. Introduction and geological setting 1.1. Introduction and purpose

⁎ Corresponding author. Tel.: +1 902 426 6580; fax: +1 902 426 4104. E-mail address: [email protected] (D.J.W. Piper).

Laurentian Channel (Figs. 1 and 2) was the principal outlet for glacial ice from the Appalachian Ice Complex and the southeastern Laurentide Ice Sheet (Grant, 1989). Evidence for sub-glacial outburst floods has

0031-0182/$ - see front matter. Crown Copyright © 2006 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2006.10.029

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Fig. 1. Southeastern Canada and Sohm Abyssal Plain. Shows location of cores from Sohm Abyssal Plain in Fig. 8 and cores from south of the Grand Banks of Newfoundland referred to in text.

been presented for other margins of the Laurentide Ice Sheet (Shaw et al., 1996), including the Hudson Strait outlet to the ocean (Hesse et al., 1996). In this paper, we

reinterpret several enigmatic features of Quaternary sediment and morphology of the Laurentian Fan and Sohm Abyssal Plain on the southeastern Canadian

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Fig. 2. Map of Gulf of St. Lawrence and Scotian margin. 18 14C ka, 14.3 14C ka and 13.7 14C ka ice limits from Stea et al. (1998) and Josenhans and Lehman (1999). Boxes show Figs. 3 and 12. (A) to (H) are locations of cores in Fig. 9. Also shows location of Fig. 7D.

continental margin (Fig. 1). We use these features to evaluate the sub-glacial flood hypothesis. In particular, we ask the following questions: 1. Is there clear evidence for sub-glacial flood discharge on Laurentian Fan? 2. What can the Laurentian Fan tell us about what happens when sub-glacial flood discharge reaches the sea? 3. Can observations on Laurentian Fan be applied to other ice stream outlets? 4. Is there any systematic relationship with Heinrichtype events? 5. What is the stratigraphic record of major sub-glacial flood discharges on Laurentian Fan? Understanding the late Quaternary glacial record of Laurentian Fan is complicated by the effects of the 1929 Grand Banks earthquake. The magnitude 7.2 earthquake, with an epicentre on the upper slope seaward of Laurentian Channel, triggered widespread slope failure that evolved into a turbidity current travelling at speeds of at least 19 m/s (Piper et al., 1988) and transporting 200 km3 of sediment, principally sand (Piper and Aksu,

1987). In a series of papers, Hughes Clarke, Piper, Shor and colleagues interpreted extensive gravel megaripples on Laurentian Fan as a product of the 1929 turbidity current (Piper et al., 1985, 1988; Hughes Clarke et al., 1990; Shor et al., 1990). 1.2. Quaternary geological setting of the southeastern Canadian margin Quaternary glacial sediments on the southeastern Canadian margin are derived principally from the Paleozoic Appalachian orogen. The Gulf of St. Lawrence is developed over the Lower Paleozoic foreland to the orogen in the north and the Magdalen basin, a Devonian– Permian successor basin, in the south. The orogen, the foreland, and the successor basin each supply distinctive lithoclasts to glacial sediments. The continental shelves are underlain by a Mesozoic–Cenozoic sedimentary wedge that developed during opening of the North Atlantic ocean. The Laurentian Channel is a U-shaped glacially excavated trough extending from the St. Lawrence estuary across the Gulf of St. Lawrence to the shelf edge SW of Newfoundland. It was fed by a series of tributary troughs

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draining ice originating in the Appalachian Ice Complex (Dyke and Prest, 1987), which provided a distinctive red– brown sediment derived from erosion of Carboniferous lowlands. This red–brown sediment is of critical importance for understanding the glacial history of the Laurentian Channel. Erratics from the Canadian Shield are known only from the northern Gulf of St. Lawrence and it appears that most sediment glacially transported to the shelf edge was derived from the Appalachian orogen and its successor basins (Grant, 1989). The Laurentian Fan (Fig. 3) is a major Quaternary sediment accumulation seaward of Laurentian Channel (Stow, 1981; Skene and Piper, 2003). Two main fan valleys, with high levees, extend southward to the lower fan and Sohm Abyssal Plain. Pliocene–Quaternary sediment is up to 3 km thick on the fan. Study of detrital sediments on the continental margin suggests that the Laurentian Channel was principally excavated during isotopic stage 12 (ca.

450 ka) and prior to stage 14 sediment transport to the Laurentian Fan was by a proto-St. Lawrence River (Piper et al., 1994). There have been two schools of thought concerning the extent of Wisconsinan ice in the Gulf of St. Lawrence. The “minimalist” school, influenced by observations suggesting that the Magdalen islands were not recently glaciated, the lack of evidence for significant ice loading, and old radiocarbon dates now known to be contaminated with "dead" carbon, viewed the Gulf of St. Lawrence as largely ice free during the last glaciation (Dyke and Prest, 1987; Grant, 1989). In recent years, increasing evidence emerged that makes the “maximalist” view inescapable, that the Gulf of St. Lawrence was filled with ice at the late Wisconsinan maximum and an ice stream flowed out through the Laurentian Channel to the shelf edge (Boyd et al., 1988; Stea et al., 1998; Josenhans and Lehman, 1999; Piper and MacDonald, 2002).

Fig. 3. Map of Laurentian Fan to show erosional channels of Eastern and Central valleys, based on map by Hughes Clarke (1988), modified by more recent soundings. Dashed polygon shows limits of Seabeam data reported by Hughes Clarke et al. (1990). Also shown are sidescan images illustrated in Figs. 6 and 7 and cores illustrated in Figs. 4 and 11. B1, B2 are residual buttes of Pliocene sediment; S1, S2 are giant scours. Grey arrows show Central Valley and a marginal spillover channel. Locations of cores discussed in text with an erosion surface over red–brown clay turbidites are shown.

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On the upper slope off Laurentian Channel, coring in the base of landslide scars allows the sampling of a diamict that immediately overlies erosional gullies eroded into proglacial muds. Piper and MacDonald (2002) showed that this erosional event is bracketed by radiocarbon ages of 16.55 and 16.37 14C ka1 (± 0.1). Recessional till deposits in Cabot Strait date from about 13.5 14C ka (Josenhans and Lehman, 1999) and subsequent deglaciation of the Gulf is well constrained (Dyke and Prest, 1987). Numerous tunnel valleys, interpreted to result from erosion by sub-glacial water, cross the eastern Scotian Shelf, apparently originating from the Laurentian Channel south of Cabot Strait (Boyd et al., 1988; Loncarevic et al., 1992). Deeply buried channels (to N 1100 m below sea level) of postMiocene age seen in multichannel seismic profiles across the eastern part of outer Laurentian Channel (MacLean and Wade, 1992) may also be tunnel valleys. 2. Morphology of Laurentian Fan Laurentian Fan is a cone-shaped accumulation of sediment, crossed by two main valleys, Eastern and Western valleys (Figs. 2 and 3). The shelf break off Laurentian Channel is at about 400 m water depth. Several slope gullies feed the head of Western Valley, whereas Eastern Valley merges upslope with the erosional continental slope. The valleys have high western levees and lower eastern levees that have been partially removed by erosion in water depths of less than 3000 m. Western Valley is not unlike many turbidite fan valleys, with a slightly sinuous thalweg and a 2–4 km wide valley floor. In contrast, Eastern Valley floor has an average width of 25 km. Remnants of stratified muddy sediment (probably old levees) of late Pliocene to early Pleistocene age form buttes on the valley floor (B1, B2 in Fig. 3). Two flatfloored channels spill off the main channel at about 2800 m water depth (grey arrows in Fig. 3): one rejoins Eastern Valley and the other leads to Western Valley. In its lower reaches, Eastern Valley bifurcates. Eastern Valley is an order of magnitude wider than both ancient and modern turbidite channels summarized by Bouma et al. (1985) and Clark and Pickering (1996). It thus seems improbable that the valley is an equilibrium feature of normal turbidity current flows. Rather, in its morphological complexity, it resembles large glacial spillways on land, such as those described by Kehew (1982). 1

All ages presented in 14C ka in this paper are from radiocarbon dates to which a uniform −400 year reservoir correction applied. This reservoir correction may not be the most appropriate, but has been widely used in the previous literature.

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3. Late Wisconsinan sedimentation on Laurentian Fan and adjacent areas 3.1. Fan valley stratigraphy Sidescan imagery (Piper et al., 1985, 1988) shows that Eastern Valley is floored by extensive gravel megaripples overlain by sand ribbons. Cores from channel floors on Laurentian Fan generally recover sand or gravel, with a surface veneer of silty mud no more than 20 cm thick. In one core near the distal limit of the valley system, 8404002 (Figs. 3 and 4), the feather edge of the gravel on the valley floor is overlain by several turbidites and by a muddy debris flow deposit that appears to have flowed in from the eroded valley side. Farther up the valley, core 85001-07 (Fig. 4) has a small graded gravel to sand bed overlying a thick gravel deposit. Otherwise, cores appear to sample only a single depositional event on the valley floor. Submersible observations of two sections eroded through the gravel deposit on the valley floor show that the deposit consists of massive cobble gravel passing up into a graded sequence of pebble to granule gravel overlain by a veneer of sand and mud. Sections in the giant flute described by Shor et al. (1990) show a sharp contact between the 10–20 cm surface sand bed and the underlying gravel (Fig. 5). Otherwise, there is no evidence for multiple depositional events. Submersible observations also show isolated boulders on the megaripple surface, principally in troughs (Fig. 2.8 of Hughes Clarke, 1988). 3.2. Sidescan imagery in Eastern Valley The surficial depositional setting of Eastern Valley changes dramatically along its length, as revealed by SeaMARC 5-km-swath sidescan imagery (Piper et al., 1985, 1988). This imagery, together with seismic reflection profiles and observations of outcrops from submersibles, suggests that the upper part of Eastern Valley is an erosional slope scoured into well-consolidated older till, diamicts and proglacial sediment (Fig. 6). Where the slope is reduced from about 3.5° to less than 2.5°, near the 1600 m isobath, the valley contains the first downslope evidence of deposition of gravel deposits and megaripple bedforms. Fields of megaripples in gravel are well developed between the 2000 and 3700 m isobaths, but farther down-valley the gravel is progressively masked by ribbons and waves of coarse sand up to 10 m thick, until below the 5100 m isobath, all the observed bedforms are developed in sand (Hughes Clarke et al., 1990). Sandy bedforms within the gravel megaripple fields no longer have a pristine morphology, but show signs of degradation, including circular scours (Fig. 7A) similar

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Fig. 4. Sediment sections in cores 84040-2 and 85001-7 from Eastern Valley of Laurentian Fan.

to the “kolk” (vertical vortex) erosion inferred by Baker (1978) from the Channelled Scabland of Washington State, USA. Their acoustic response is not sharp: this wispy appearance (Fig. 7B) also suggests that the sand was once more continuous and has been eroded. Long wavelength sand waves also appear to have suffered later erosion (Fig. 7C). In contrast, rare trains of “fresh” looking sand waves occur on the lower fan (Fig. 7D) and are probably the result of deposition in the 1929 event. Petrography of gravels from the floor of Eastern Valley suggests that their source differs from that of more widespread ice-rafted detritus. Sorted gravels in lower

Eastern Valley have relatively high proportions of indurated grey sandstone, crystalline limestone and plutonic igneous rocks and only rare red sandstone clasts (Piper and deWolfe, 2003). The limestones are unlike those in carbonate-rich Heinrich beds from Hudson Strait and are probably derived from the Lower Paleozoic around Anticosti Island. We interpret the petrographic assemblage as of a local northern Gulf of St. Lawrence provenance. In contrast, the lithologies of ice-rafted gravel from cores on Laurentian fan and nearby continental slopes (Stow, 1977, his Fig. 40), including ice-rafted brick-red gravelly sandy mud beds (Piper and Skene,

Fig. 5. Photographs from DSV Alvin showing sharp contact of 5 cm sand bed over gravel deposit that rests with erosive contact on early Pleistocene consolidated mudstone. From giant scour S2 in Fig. 3. Another cross-section of a 3 m thick deposit was illustrated by Hughes Clarke et al. (1990), their Fig. 5.

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Fig. 6. Composite SeaMARC sidescan sonar image from head of Eastern Valley showing rapid transition from erosional slope to deposition of gravel waves. Further details in Piper et al. (1985).

1998), consist of friable red, green and grey sandstones and lesser limestone (Piper and deWolfe, 2003). These appear to be derived principally from Carboniferous

sedimentary rocks, with lesser amounts of igneous and metamorphic lithologies from the Applachian orogen, suggesting an origin in the Appalachian Ice Complex.

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Fig. 7. Selected SeaMARC images from Eastern Valley showing interpretation of features. Light backscatter corresponds to sand, darker backscatter to gravel. Locations shown in Fig. 3. (A) gravel waves with overlying sand ribbons, both showing subcircular patches of “kolk” erosion (K); (B) eroded (wispy) sand ribbons overlying gravel waves. (C) gravel waves overlain by longer wavelength sand waves with a very “ratty” eroded appearance; (D) fresh sand waves in axis of South Branch (location in Fig. 2).

3.3. Abyssal plain stratigraphy The distal equivalent of the gravel bed in Eastern Valley is likely a thick sand bed on northern Sohm Abyssal Plain. Deposition on the Sohm Abyssal Plain records sediment derived principally from transport across Laurentian Fan (Piper and Hundert, 2002). Most

available cores are old Lamont cores that have dried out and have substantial coring disturbance, particularly in sandy intervals. Core stratigraphy is summarised by Fruth (1965) and Horn et al. (1971): key cores from the northern Sohm Abyssal Plain are illustrated in Fig. 8. These cores have a surface silt or sand bed ~ 1 m thick resulting from the 1929 event, whereas correlative sediment on the central and southern abyssal plain is generally a silty mud b 20 cm thick (Piper and Aksu, 1987). The 1929 deposit overlies bioturbated muds with variable concentrations of dissolution-resistant foraminifera and in places thin (b10 cm) silt and fine sand beds. This principally pelagic sediment is generally 0.5 to 1 m thick and overlies a thick silt to sand bed, 0.6 to N 1.6 m thick. In core A180-2 a coarse sand that stopped the corer is probably correlative (Fig. 8). This regional thick sand bed overlies more bioturbated foram-bearing muds with thin interbedded sands and silts. Regional rates of pelagic sediment accumulation (e.g. Keigwin and Jones, 1995), a bulk C-14 age in core A180-1, and a change downsection from warmer to cooler-water foraminifera in core A180-2 all suggest that the thick sand bed is probably of latest Pleistocene age. We argue below that it correlates with the gravel deposit in Eastern Valley. On the distal Sohm Abyssal Plain (Fig. 1), core HU80016-50 (Piper and Hundert, 2002) contains in downward order a mud turbidite corresponding to the 1929 event, some pelagic sediment, and then a thick succession of mud turbidites of latest Pleistocene age, overlying a 12 m deep hemipelagic interval with a mean age of about 11.8 14C ka (Benetti et al., 2004). The corer was stopped by a N1 m thick medium sand bed which was sampled in nearby cores. Surface textures of sand grains (Wang et al., 1982) include numerous collisional features interpreted as forming in an energetic highdensity flow. We interpret this sand bed as correlative with the thick late Pleistocene sand bed on the northern Sohm Abyssal Plain, as shown by the grey unit in Fig. 8. 3.4. Levee stratigraphy and age control The most abundant lithology in cores from Laurentian Fan levees is a thick succession of red–brown clay turbidites with thin silt laminae (the “silt-laminated mud” of Skene and Piper, 2003; see also Stow and Bowen, 1980; Curran et al., 2004) of late Pleistocene age. Core MD95-2029 completely penetrates this red– brown clay turbidite unit, which is about 12 m thick (core F in Fig. 9), and overlies 12 m of grey muds. Three main units of red–brown clay turbidites are present in this and other cores (Skene and Piper, 2003, their Fig. 9), separated by two 2–10 cm thick intervals of bioturbated grey

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Fig. 8. Stratigraphic sections of selected cores from Sohm Abyssal Plain. A- and V-cores are Lamont cores described by Fruth (1965). Small cores are corresponding trigger-weight cores. For location, see Fig. 1.

hemipelagic sediment, which have an estimated duration of 500–1000 years. On the basis of several radiocarbon dates, Skene and Piper (2003) showed that these hemipelagic intervals date from ca. 14.1–14.9 14C ka and ca. 15.6 14C ka. On the basis of a detrital carbonate unit, identified as Heinrich layer 2 (H2), near the base of MD95-2029, Skene and Piper (2003) extrapolated an age of about 18 14C ka for the base of the red–brown clay turbidite unit. A short distance above the top of the red– brown clay turbidite unit is a brick-red gravelly sandy mud bed, overlain by a thin detrital carbonate bed, identified as

Heinrich layer 1 (H1) derived from Hudson Strait (Piper and Skene, 1998) and dated at about 14.0 14C ka. The brick-red gravelly sandy mud bed, and a shallower similar bed dated at about 12.8 14C ka, resulted from a short period of intensified ice-rafting from the Gulf of St. Lawrence, analogous to Heinrich events (Piper and Skene, 1998), corresponding to rapid ice-retreat phases recognised by Josenhans and Lehman (1999). It is not possible to directly correlate the gravel deposits in the fan valleys with the levee stratigraphy. A prominent erosion event cuts the top of the red–brown clay turbidite

110 D.J.W. Piper et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 246 (2007) 101–119 Fig. 9. Summary stratigraphic sections showing distribution of red–brown muds on the Scotian Margin. Details of cores in Piper (2001) and Piper and deWolfe (2003). Fogo Seamounts modified from Mudie (1992). Ages are radiocarbon years, with a −0.4 ka marine reservoir correction. Core locations shown in Fig. 2. At locality C, dates in parentheses are based on dated adjacent cores correlated by Gauley (2001).

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unit in several cores on the lower fan (Fig. 3), but the recovery of overlying sediment was poor and all that can be said is that recovery of several decimetres of overlying sediment cannot reasonably date from 1929. Alternatively, this erosion may have been caused by the flow that transported the gravel down the fan valleys. A thin red– brown clay overlies the gravel in core 84040-02 and is overlain by a green–grey sand, similar to the late Pleistocene turbidite sands above H1 on the eastern part of the fan derived from Grand Banks valley (Skene and Piper, 2003). From this correlation, the gravel bed that floors the valley is interpreted to be older than the youngest sediment in the red–brown clay turbidite unit (about 14 14C ka), but not as old as the base of the unit (about 18 14C ka). 3.5. Regional distribution of red–brown clays The latest Pleistocene–Holocene lithostratigraphy of the Scotian Margin is similar to that of the Laurentian Fan (Hill, 1984; Mosher et al., 1994; Piper and deWolfe, 2003). Cores on the upper Scotian Rise contain the detrital carbonate bed of Heinrich layer 1 (H1) and the immediately underlying brick-red gravelly sandy mud bed. Beneath this level, some cores contain only turbidites and/or debris-flow deposits, generally with a brownish or greyish colour. Cores from topographic elevations, however, commonly contain a red–brown mud unit that is texturally and petrographically similar to the red–brown clay turbidites of the Laurentian Fan. These red–brown muds overlie grey or red sand and mud turbidites, from which they can clearly be distinguished by their colour and finer grain size. Radiocarbon ages (Fig. 9) and correlation of the brickred gravelly sandy mud markers with Laurentian Fan suggest the red–brown clays date from about 14 to 16 14C ka (perhaps older in the east), corresponding to the top of the red–brown clay turbidite sequence on Laurentian Fan. The thickness of the red–brown clay unit on the Scotian margin decreases westward and downslope, from over 13 m on the eastern Scotian Slope (core E in Fig. 9, located in Fig. 2) to about 3 m on the central Scotian Slope (core B) and is thin or absent on the west Scotian Slope (core A) (see also Hundert, 2003). To the east of the Laurentian Channel, red– brown muds are rare on St Pierre Slope (present for example in core G) and absent on the SW Grand Banks slope, but thin red–brown mud units are present on the SW Grand Banks rise. On the Fogo Seamounts (Alam et al., 1983; Mudie, 1992), a 25 cm thick brick-red mud is interbedded with pelagic ooze (core H). The age of the red–brown mud and the thickness variations away from Laurentian Channel suggest that it had an origin related

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to that of the red–brown clay turbidites on Laurentian Fan. 3.6. The upper slope at the head of Eastern Valley On a large scale, SeaMARC sidescan imagery (Fig. 6) shows linear erosional features on the continental slope at the head of Eastern Valley and submersible dives (Hughes Clarke et al., 1989) showed that these result from erosion of bedded, well-consolidated older diamicts and proglacial mud. There is evidence in seismic reflection profiles for thick acoustically incoherent diamict units at least to 1500 mbsl and pebbly diamict outcrops at 1910 mbsl (Hughes Clarke et al., 1989). These deposits are interpreted to resemble trough–mouth fans described from other ice stream outlets (e.g. Taylor et al., 2002). On the upper slope, thick proglacial muds extend downslope to 550–750 m and were eroded by retrogressive failure in the 1929 earthquake (Piper et al., 1999), down to the level of a prominent diamict that overlies a highly gullied erosional surface (Piper and MacDonald, 2002) (Fig. 10). This erosional surface cuts well-stratified proglacial muds that yielded a radiocarbon age from an unidentified bivalve of 16.55 ± 0.15 14C ka and proglacial sediment immediately above the diamict was dated (again using a bivalve) at 16.37 ± 0.05 14C ka (Piper and MacDonald, 2002). These ages constrain the erosional event to about 16.5 14C ka.

Fig. 10. Summary of stratigraphic evidence on the upper slope at the head of Eastern Valley for the age of the late Wisconsinan gullying event (modified from Piper and MacDonald, 2002).

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3.7. Glacial stratigraphy of Laurentian channel Glacial till in outer Laurentian Channel is overlain by ice-proximal glaciomarine sediments, passing up into more distal glaciomarine sediments, a few to several tens of metres thick, that have been scoured by icebergs (King, 1976). Farther east on St Pierre Slope, Bonifay and Piper (1988) determined sedimentation rates of 1 m per 40 years in proximal glaciomarine muds. The age of the youngest till is thus not directly known, but is argued to date from about 14.2 14C ka by Piper and MacDonald (2002). Farther north in Laurentian Channel, the thick till deposits pass laterally into stratified sediment in deeper basins (King and Fader, 1986, 1990) that may record periods of water build up beneath the ice. 4. Synthesis: the 16.5 1929 event

14

C ka outburst flood and the

The gravel megaripples on the floor of Eastern Valley were interpreted in the late 1980s as the product of the 1929 turbidity current, on the grounds that the deposits appeared morphologically fresh and, except for a few late debris flow deposits, were not overlain by other sediment (Piper et al., 1985, 1988; Hughes Clarke et al., 1990; Shor et al., 1990). Fresh sediment failures in muddy sediments on the continental slope could account for only a small proportion of the 1929 turbidite deposits (Piper and Aksu, 1987) and Hughes Clarke et al. (1990) proposed that most sand was eroded from the floor of Eastern Valley, with the gravel deposits concentrated by deflation of the valley floor. This interpretation of the megaripple fields as a 1929 deposit raises some troubling questions. Initiation of a turbidity current through sediment failure, even if prolonged as a result of retrogressive failure (Piper et al., 1999) or dissociation of gas hydrates (M. Morrison, pers. comm., 1996), is unlikely to produce a turbidity current that flowed for more than a few tens of hours. Yet the megaripple fields require substantial deflation of the valley floor, formation of megaripples with wavelengths of hundreds of metres and heights of 2–10 m, deposition of sand ribbons, and local erosion of both sand and gravel, including formation of a giant flute scour and valleymargin thalwegs (Shor et al., 1990). We argue below that the gravel bed with megaripples flooring Eastern Valley is an older feature generated by a catastrophic late Pleistocene event and was only modified by the 1929 turbidity current. Several lines of evidence suggest that the widespread gravel megaripples on the floor of Eastern Valley were deposited by an event of considerable duration and complexity long before the 1929 “Grand Banks” turbidity current. The sequence of events during this earlier flow

included erosion of older glacial deposits on the upper slope, deposition of several metres of gravel on the floor of Eastern Valley, moulding of the gravel into well-developed megaripples, and deposition of up to 10 m thickness of sand in sand ribbons. A thick latest Pleistocene sand bed on the northern and central Sohm Abyssal Plain is evidence for a depositional event that transported coarser sediment farther than the 1929 turbidity current (Fig. 8). The gravel appears derived from the northern Gulf of St. Lawrence, in contrast to the widespread ice-rafted gravel from the Appalachian Ice Complex. Locally, the gravel wave surface was eroded to form a giant flute and an incised thalweg at the base of the channel wall. Other evidence of erosion is the degraded appearance of some of the sandy bedforms in the lower valley and the presence of a marginal thalweg. Following formation of the gravel megaripples, isolated boulders, probably ice-rafted, were deposited on the valley floor. The feather edge of the gravel is overlain by several turbidites in core 84040-02. These observations suggest that the gravel megaripples are of late glacial age. The pattern of deposition followed by erosion is characteristic of flood hydrographs, but not of slump-generated turbidity currents (Mulder et al., 1998). The 1929 turbidity current was initiated by slumping of muddy and silty sediments on the continental slope, with turbidity current ignition taking place where debris flows passed through a hydraulic jump on steep slopes (Piper et al., 1999). The current reworked some of the sand on the floor of the valley and may have moved icerafted boulders short distances as tractional bedload into the troughs of the megaripples. The great abundance of sand on the abyssal plain resulted from the abundance of late Pleistocene sand available for erosion overlying the megaripples on the floor of Eastern Valley. Several lines of evidence indicate that the red–brown clay turbidites are not normal overbank turbidite muds. From 3.5 kHz profiles, they show no change in thickness across western levee of Eastern Valley, and in cores are thickest on the western levee of Western Valley (Skene and Piper, 2003). Petrographically similar facies are found on the Fogo Seamounts, 1 to 1.5 km above the seafloor of the Laurentian Fan, and all along the Scotian margin to about longitude 63°, even though it is clear that by the time they were deposited (14–16 14C ka) ice had retreated from the shelf break into shelf basins such as Emerald and La Have basins (Fig. 2). In the northern Labrador Sea, Hesse and his colleagues have used X-radiographs of thin slabs of core to show that several distinctive muddy facies are temporally associated with carbonate-rich Heinrich ice-rafting events (Wang and Hesse, 1996; Hesse and Khodabakhsh, 1998; Hesse et al., 1999). On the upper slope immediately south of major ice

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outlets such as Hudson Strait, irregularly stratified muds with some ice-rafted detritus are identified as suspension fall-out from surface plumes (Hesse et al., 1997). More distally, poorly graded mud beds with scattered ice-rafted detritus are interpreted as deposits of lofted mid-water turbidity currents (Hesse et al., 2004). Muds with distinct silt laminae and lacking ice-rafted detritus, except between layers, are interpreted as turbidites. We interpret the red–brown clays in shallower parts of the Laurentian Fan and Scotian margin as an outwash plume facies similar to that interpreted by Hesse et al. (1997, 1999) from the Labrador margin south of Hudson Strait. In deeper water, turbidite facies are present (Skene and Piper, 2003). Stow and Bowen (1980) demonstrated that if the clay turbidites were deposited by slow thick flows, deposition over periods of many days was required. Such long duration flows are presumably sourced either by settling of surface plume sediments or direct hyperpycnal flow. The distribution of red–brown clay turbidites suggests that these currents covered a vast area of the Laurentian Fan and east Scotian Rise seaward of the Laurentian Channel outlet. Three major periods of such red– brown clay turbidite deposition on the Laurentian Fan between 14 and 18 14 C ka (Fig. 9) were interrupted by periods of hemipelagic sedimentation on levees. Based

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on regional rates of hemipelagic sedimentation, the two correlatable hemipelagic layers represent periods of 102 –103 years of cessation of turbid plume delivery. In the north-central Atlantic Ocean, Bond and Lotti (1995) recognised a series of peaks in hematite stained quartz sand, including peaks at H1 that correspond to the brick-red gravelly sandy mud iceberg-rafted horizons on Laurentian Fan (Fig. 11). Between H1 and H2, Bond and Lotti recognised three short-lived peaks (a, b and c) that have also been found by magnetic susceptibility records on Orphan Knoll (Stoner et al., 1996). Given uncertainties in precise dating of these peaks (Fig. 11), they may correspond to the three thick intervals of brown–red clay turbidites on Laurentian Fan, separated by hemipelagic intervals. At this time, cold water may have extended far enough south across the fan that there was minimal melting of icebergs. 5. Evidence for earlier flood events 5.1. The red mud record on the Fogo seamounts and Sohm Abyssal Plain Older red–brown muds, petrographically similar to the red–brown clay turbidites, are found in cores from the Fogo Seamounts (Fig. 1) and are particularly thick in

Fig. 11. Oxygen isotope record from (A) MD95-2029, also showing red–brown clay turbidite intervals (Skene and Piper, 2003); (c) HU73021-7 (Keigwin and Jones, 1995) and (B) comparison with % hematite coated-grains at VM23-81 in the central Atlantic Ocean (Bond and Lotti, 1995). Ages in radiocarbon years with a − 0.4 ka marine reservoir correction.

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marine isotope stage (MIS) 6, but are also present in MIS 4, 10 and 12 (Alam et al., 1983). Farther to the southeast, Frank Hall (pers. comm., 1997) has identified several horizons dominated by hematite-type magnetic properties in core HU87008-03 from the J-anomaly ridge (Piper et al., 1994). The youngest was deposited at about MIS 4; other intervals occur in MIS 8, 10 and 12. Some hematiterich sediments occur in intervals that are not likely to be strongly glaciated: for example, late MIS 11, MIS 19 and MIS 25. There is no hematite record in this core during the last-glacial maximum (MIS 2), when thick hematite rich muds were discharged to Laurentian Fan and hematite stained quartz is common in the central Atlantic Ocean. The record on the J-anomaly ridge may therefore be influenced as much by oceanic circulation as by major discharge through Laurentian Channel. 5.2. Erosional events in Eastern Valley of Laurentian fan In the lower part of Eastern Valley, a few major erosion events alternate with long periods of levee growth (Fig. 12). Chronologic control is provided by a late Pliocene biostratigraphic pick (F.M. Gradstein, in Piper and Normark, 1989) and the H2 horizon in core

MD95-2029 (Piper et al., 1999). Skene and Piper (2006) have developed a regional correlation of seismic reflectors and argue that the major erosional events illustrated in Fig. 13 date from MIS 2, 4, and 12. The event in MIS 2 is correlated with the 16.5 14C ka event that deposited the gravel bed in Eastern Valley. 5.3. Evidence of sub-glacial meltwater erosion elsewhere on the southeastern Canadian margin We argue below that many of the features described from Eastern Valley of the Laurentian Fan are a consequence of sub-glacial meltwater discharge. Here we briefly review other evidence on the southeastern Canadian margin for such discharge. Tunnel valleys, interpreted to be cut by sub-glacial meltwater, are widespread on the east Scotian Shelf. They dominate the morphology of the inner part of the shelf (Loncarevic et al., 1992) but are largely filled with sediment on the outer banks (Boyd et al., 1988). Seismic reflection profiles of the fill of tunnel valleys suggest that they are of various ages and King (2001) has suggested that the youngest dates from a few thousand years after the last-glacial maximum (LGM). The general trend of valleys on the eastern Scotian Shelf (Loncarevic

Fig. 12. Seismic section from Laurentian Fan showing interpretation of earlier erosional valley-widening events. Chronology based on Skene and Piper (2006).

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Fig. 13. Tunnel valleys on east Scotian Shelf mapped from industry multichannel seismic profiles by Flynn (2000).

et al., 1992) suggests that many may originate from meltwater discharging southeastward from Laurentian Channel near Cabot Strait (Fig. 2). Large tunnel valleys on Banquereau and Sable Island Bank have been mapped from industry multichannel seismic profiles by Flynn (2000) (Fig. 13) and confirmed by higher resolution seismic profiles where available (e.g. Boyd et al., 1988; Amos and Miller, 1991; King, 2001). The pattern shows a coalescence of the valleys towards the heads of the major canyons that indent the shelf break. Within these canyons, erosional widening is interpreted from seismic reflection profiles. The age of the last major widening event is uncertain, but appears older than 14 14C ka (Jenner et al., in press). Elsewhere on the southeastern Canadian margin, Piper and Gould (2004) demonstrated that canyon widening events south of Whale Bank (Fig. 2) were synchronous in multiple canyons and correlated approximately with the last-glacial maximum (LGM) ice advance on the Grand Banks of Newfoundland. 6. Discussion We have argued above that the Eastern Valley has a very unusual morphology that resembles glacial spillways on land. Features of the thick gravel bed on the valley floor show it was deposited from an exceptional discharge originating from the Laurentian Channel. Our limited stratigraphic control suggests that this gravel bed was deposited from a single major flow. It most reasonably correlates with the major gullying event on the

upper slope dated by Piper and MacDonald (2002) at 16.5 ± 0.15 14C ka, suggesting a correlation with the lowest thick red–brown clay turbidite unit on the levees. Although three major and one lesser red–brown clay turbidite units are recognised on Laurentian Fan (Fig. 9), each corresponding to a meltwater discharge event, there is no evidence that each was accompanied by a major bedload gravel discharge. We do not have suitable seismic-reflection profiles, however, to rule out the possibility that each was accompanied by a small erosional bedload discharge. Nor is there evidence on the levees that the lowest thick unit of red–brown clay turbidites (likely correlative with the gravel bed) differs significantly from the overlying units. A lower limit to the size of the discharge can be estimated from sedimentological data. The 1929 Grand Banks event distributed a sand bed on the Sohm Abyssal Plain of somewhat lesser thickness and extent to the 16.5 14C ka event. Total sediment delivery in 1929 was 100–200 km3 (expressed as bulk sediment volume deposited) (Piper and Aksu, 1987). From the flow character in the channeled portion of the flow, Hughes Clarke et al. (1990) inferred mean volume concentration of 6%, implying a water discharge of 2–5 × 103 km3, much of which would have been the result of entrainment, with a duration of 101–102 h. During the 16.5 14C ka event, 15– 25 km3 of gravel was also transported and deposited, in addition to perhaps 200 km3 of sand. The 16.5 14C ka turbidity current was clearly more powerful than the 1929 event, but the total volume of sediment transported

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suggests it was unlikely to have been as much as an order of magnitude larger. The volume of red–brown clay deposited on the Laurentian Fan and Scotian margin from one of the major plume events is of the order of 102 km3, equivalent to a fluid volume of the order of 104 km3 with a suspended sediment concentration of 40 gm/l. This concentration is that necessary to form a hyperpycnal flow (Mulder and Syvitski, 1995): the fact that a surface plume formed demonstrates a lower mean suspended sediment concentration and thus a higher discharge. In contrast, the volume of the brick-red gravelly sandy mud bed immediately below H1 has an estimated volume of b10 km3 (Piper and Skene, 1998). We interpret the gravel bed on the floor of Eastern Valley as originating from a catastrophic bedload flow from a glacial ice margin at or near the shelf edge. Flow into deep water would have been inertial, similar to the mechanism proposed by Prior and Bornhold (1989) for high-discharge events on an alluvial fan, but on a larger scale. The flow would have been at least 25 km wide, eroding a broad swath of the continental slope and filling Eastern Valley. It may have been focussed into Eastern Valley by re-excavation of sandy fill of earlier canyon heads on the east side of Laurentian Channel (MacLean and Wade, 1992). Where sections through the deposit are seen, a gravel bed up to 3 m thick appears to have been deposited and older sediments have been eroded or reworked. At the same time as 1–2 × 102 km3 of coarse bedload was being discharged into the deep sea, associated meltwater with suspended load may either have flowed down fan as hyperpycnal flows, or may have lofted to the surface to form a turbid surface plume in the manner proposed by Hesse et al. (2004). We cannot distinguish whether the thick red–brown clay turbidites are a result of direct hyperpycnal flow, or whether they result from nearbottom concentration of fall-out from a surface turbid plume. The surface plume, and associated nepheloid layers at density interfaces in the ocean, were advected westward along the Scotian Slope and deposited a 1–3 m thick red–brown clay on the central Scotian Slope. We estimate the volume of water in the sub-glacial flood to be of the order of 104 to 105 km3, equivalent to a sea-level rise of 0.1 to 1 m. Our petrographic data indicate that most gravel deposited could have had a source in the Appalachians or northern Gulf of St. Lawrence. Although Shaw (1996) has argued that flood discharge beneath the Laurentide ice sheet may have been both widespread and essentially synchronous over large areas, we lack unequivocal evidence for derivation of clasts from the

Canadian Shield. On the other hand, the paucity of red sandstone and siltstone clasts is surprising, considering the abundance of these lithologies beneath ice streams draining the Appalachian Ice Complex. The impact of plume discharge locally is to yield lighter isotopic values in planktonic foraminifera (Fig. 11). The very sparse foraminifera may be derived from rare, possibly seasonal, incursions of clear Labrador Current water into the area. At more distant sites it may be possible to recognise a distal meltwater signal. The inferred timing of the first major discharge event at 16.5 14C ka (19.5 ka cal) is synchronous with the first meltwater spike after the LGM in both the planktonic and benthonic foraminifer record on the Bermuda Rise (Hagen and Keigwin, 2002). Rapid calving of ice in Laurentian Channel and the Gulf of St. Lawrence between 14 and 13 14C ka led to the regional deposition of brick-red gravelly sandy mud beds on the Scotian margin, by plume and ice-rafting sedimentation (Piper and Skene, 1998), analogous to the mechanisms by which Heinrich layers were deposited seaward of the Hudson Strait outlet. The described subglacial outburst flood plumes and bedload deposits are older. They may have been accompanied by ice retreat, but there is no evidence that this retreat was substantial (e.g. Piper and MacDonald, 2002). On Laurentian Fan, there is evidence from seismic reflection profiles for large flood discharge events at the end of MIS 2, 4, and 12 and smaller events at other times (Skene and Piper, 2006). The morphology of Eastern Valley shows the effects of several large discharge events. Thick suspended load deposits similar to those west of the Laurentian Fan are known from other major ice outlets (Vilks et al., 1977; Hesse et al., 1999, 2004), but may result from continuous sub-glacial discharge rather than short-lived floods. Although no wide gravel-floored channels similar to Eastern Valley are present seaward of Hudson Strait (unpublished data of D.J.W. Piper and R. Hesse, Hudson cruise 97-048), supporting the interpretation of Shaw (1996) that there may not have been large volume floods there, the braided sand and gravel plain in the Labrador Sea suggests that smaller outburst flood events may have taken place (Hesse et al., 1996, 2001). The Mackenzie Trough, in the Beaufort Sea, is of similar dimensions to Eastern Valley of Laurentian Fan and thus unlike normal prodelta turbidity-current channel. It is buried beneath Holocene prodeltaic deposits. Putative outlets more distal to the ice margin (e.g. Mississippi, Susquehanna, Hudson) would have discharged onto the outer shelf rather than the shelf break except at glacial maximum lowstands, and probably would not have had sufficient bedload density and inertia for a hyperpycnal flow to continue into deep water. The preservation of such

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clear evidence for a sub-glacial flood on Laurentian Fan is thus a consequence of its position close to a shelf-break ice margin and the lack of subsequent fluvio-deltaic sedimentation. 7. Conclusions 1. There is clear evidence for sub-glacial flood discharge on Laurentian Fan. Laurentian Channel was a major outlet for sub-glacial meltwater floods at three intervals between ca. 16.5 14C ka and 14.2 14C ka. The earliest late Wisconsinan event resulted in a hyperpycnal flow that transported large amounts of gravel and sand through an unusually wide fan valley to the Sohm Abyssal Plain. The two younger events provided principally mud, some of which accumulated on Laurentian Fan levees and some of which was transported in a plume farther westward along the Scotian Slope than during the hyperpycnal event. 2. On Laurentian Fan, two distinct styles of deposition occur when sub-glacial flood discharge reaches the sea. Rarely, there is a major hyperpycnal flow of gravel and sand, together with widespread mud deposition. More frequently, coarse-grained hyperpycnal flow is minor or lacking and principally mud is discharged either in a hypopycnal or a lofting hyperpycnal plume. Water discharge in each late Wisconsinan event was at least 104 km3. 3. These interpretations from Laurentian Fan suggest that at the Hudson Strait ice stream outlet, which lacks a single broad erosional fan valley with a gravel floor, not all sub-glacial meltwater events resulted in major hyperpycnal flows. The Mackenzie Trough may be analogous to the Eastern Valley of Laurentian Fan, but is deeply buried by Holocene sediment. 4. The major discharge events from Laurentian Channel do not correlate with Heinrich events. The ice-rafted horizon of brick-red sandy gravelly mud that immediately underlies Heinrich layer 1 on the Laurentian Fan and Scotian margin was sedimentologically quite distinct from the deposits of major sub-glacial discharge events and involved much smaller sediment discharge. 5. The stratigraphic record of major sub-glacial flood discharges on Laurentian Fan can be interpreted from seismic reflection profiles. The youngest hyperpycnal flow event is well constrained to 16.5 ± 0.15 14C ka from a dated erosion surface on the upper slope. Several similar major hyperpycnal discharge events are recognised in the past 0.5 Ma from their erosional effects on Laurentian Fan.

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Acknowledgements Core acquisition was funded by the Geological Survey of Canada. Core MD95-2029 was collected from Marion Dufresne using funding from the Natural Sciences and Engineering Research Council (NSERC). Skene was supported by an NSERC graduate student fellowship and NSERC operating grants to Piper and Paul Hill (Dalhousie). Laboratory work was supported by the Geological Survey of Canada. This is a contribution to the NSERCfunded CSHD project. Reviews by D. Hutchinson, R. Hesse and C.F.M. Lewis substantially improved this manuscript. References Alam, M., Piper, D.J.W., Cooke, H.B.S., 1983. Late Quaternary stratigraphy and paleo-oceanography of the Grand Banks continental margin, eastern Canada. Boreas 12, 253–261. Amos, C.L., Miller, A.A.L., 1991. The Quaternary stratigraphy of southwest Sable Island Bank, eastern Canada. Geol. Soc. Amer. Bull. 102, 915–934. Baker, V.R., 1978. Large scale erosional and depositional features of the Channeled Scabland. In: Baker, V.R., Nummedal, D. (Eds.), The Channeled Scabland. National Aeronautical and Space Administration, Washington, D.C., pp. 81–115. Benetti, S., Piper, D.J.W., Weaver, P.P.E., 2004. Styles, rates and timing of turbidite deposition along the western North Atlantic margin. Norsk Geologisk Forening Abstracts and Proceedings, Deep Water Sedimentary Systems of Arctic and North Atlantic Margins, p. 13. Bond, G.C., Lotti, R., 1995. Iceberg discharges into the North Atlantic on millenial time scales during the last glaciation. Science 267, 1005–1010. Bonifay, D., Piper, D.J.W., 1988. Probable Late Wisconsinan ice margin on the upper continental slope off St. Pierre Bank, eastern Canada. Can. J. Earth Sci. 25, 853–865. Bouma, A.H., Normark, W.R., Barnes, N.E., 1985. Submarine Fans and Related Turbidite Systems. Springer, New York. Boyd, R., Scott, D.B., Douma, M., 1988. Glacial tunnel valleys and the Quaternary history of the Scotian Shelf. Nature 333, 61–64. Clark, J.D., Pickering, K.T., 1996. Architectural elements and growth patterns of submarine channels: applications to hydrocarbon exploration. Am. Assoc. Pet. Geol. Bull. 80, 194–221. Curran, K., Hill, P.S., Schell, T.M., Milligan, T.G., Piper, D.J.W., 2004. Inferring the mass fraction of floc deposited mud: application to fine-grained turbidites. Sedimentology 51, 927–944. Dyke, A.S., Prest, V.K., 1987. Late Wisconsinan and Holocene history of the Laurentide ice sheet. Géogr. Phys. Quat. 41, 237–263. Flynn, R.F.J., 2000. Tunnel valleys under the southeastern Scotian Shelf. B.Sc. (Hons.) thesis, Saint Mary's University, Halifax, N.S., 48 pp. Fruth, L.S., 1965. The 1929 Grand Banks turbidite and the sediments of the Sohm Abyssal Plain. M.A. thesis, Columbia University, 257 pp. Gauley, B.-J.L., 2001. Lithostratigraphy and sediment failure on the central Scotian Slope. Unpublished M.Sc. thesis, Dalhousie University, 214 pp. Grant, D.R., 1989. Quaternary Geology of the Atlantic Appalachian region of Canada, Chapter 5. In: Fulton, R.J. (Ed.), Quaternary Geology of Canada and Greenland. Geology of Canada, vol. 1, pp. 393–440.

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