Geomorphology 241 (2015) 1–18

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Geomorphology

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Submarine canyons and channels in the Lower St. Lawrence Estuary (Eastern Canada): Morphology, classification and recent sediment dynamics

Alexandre Normandeau a,⁎,PatrickLajeunessea, Guillaume St-Onge b a Centre d'études nordiques, GEOTOP & Département de géographie, Université Laval, Québec, QC G1V 0A6, Canada b Canada Research Chair in Marine Geology, GEOTOP & Institut des sciences de la mer de Rimouski, Université du Québec à Rimouski, Rimouski, Québec, Canada article info abstract

Article history: Series of submarine canyons and channels observed in the Lower St. Lawrence Estuary (LSLE) provide an oppor- Received 1 December 2014 tunity to analyze in great detail the morphology, spatial distribution and modern activity of such systems in a rel- Received in revised form 27 March 2015 atively shallow (≤300 m) semi-enclosed basin. Based on their geomorphology and physical settings, the canyons Accepted 28 March 2015 and channels were classified into four categories according to their feeding sources (ancient or recent): glacially- Available online 8 April 2015 fed, river-fed, longshore drift-fed and sediment-starved systems. Their activity was interpreted based on geomor- fl Keywords: phological characteristics such as the presence of bedforms related to gravity ows, backscatter intensity, axial fi Submarine canyons incision and the presence of rapidly deposited layers in sur cial sediments. River-fed deltas were interpreted Gravity flows as inactive, mainly because suspended sediment concentrations at river mouths are low, preventing the genera- St. Lawrence Estuary tion of hyperpycnal currents or delta-lip failures related to high sediment supply. Longshore drift-fed canyons, Seabed morphology present where the coastal shelf narrows, were found to be episodically active probably due to earthquakes or ex- Sedimentary processes treme storm events. Unlike other longshore drift-fed canyons observed elsewhere in the world, they are active infrequently because of the modern low rates of sediment supply to their heads. The most active canyons are the sediment-starved type and were observed near Pointe-des-Monts. Their activity is probably due to slope fail- ures and to the presence of strong hydrodynamic processes. Therefore, sediment supply is not the main mecha- nism responsible for modern canyon and channel activity in the LSLE. However, sediment supply has been an important factor during the formation of the submarine channels and canyons. Their quasi-exclusive occurrence on the Québec North Shore is attributed to its larger watershed and important sedimentary delivery during deglaciation. The Québec North Shore watershed is 20 times greater than the Québec South Shore watershed, which favored the transport of greater volumes of sediment during the early-Holocene. Moreover, the slope proximity to the shore led to the formation of longshore-drift fed systems on the North Shore when sediment supplied to rivers were transferred on a narrow shelf. © 2015 Elsevier B.V. All rights reserved.

1. Introduction relative sea-level (RSL) positions (Boyd et al., 2008; Ducassou et al., 2009), as previously suggested by sequence stratigraphic models (Vail Submarine canyons and channels incise most of the world's et al., 1977). Tectonic and climatic factors, as well as the type and vol- continental shelves and act as preferential routes for the transport of ume of sediments, play a major role in canyon activity (Stow et al., continental sediments to the deep-sea. In some cases, canyons incise 1983; Bouma, 2004). For example, tectonic processes influence the the continental shelf up to a depth of b100 m and transfer coastal width and slope of the continental shelf and the rock susceptibility to sedimentstodepthsofmorethan1000m,mainlybygravityflows erosion and are known to have an important role in canyon activity in (e.g., Gaudin et al., 2006; Mulder et al., 2012). The activity of submarine California (Covault et al., 2007) and the Mediterranean Sea (Migeon canyons and channels depends largely on the rate of sediment supply at et al., 2006, 2012). The type and volume of sediment include the capac- their head and on oceanographic processes able to remobilize these ity of a material to be eroded and transported, while climate largely in- sediments (Puig et al., 2014). Many submarine canyons of the world fluences precipitations, which in turn influences river floods and are currently active in the present sea-level highstand (Boyd et al., sediment delivery to the coast. All these processes can have more im- 2008; Khripounoff et al., 2009; Xu et al., 2010; Biscara et al., 2011; pact than RSL positions on canyon activity (e.g., Ducassou et al., 2009). Mulder et al., 2012; Paull et al., 2013). However, recent studies have Deep-water submarine fans have been largely studied for their demonstrated that canyon activity does not necessarily depend on hydrocarbon potential (Mayall et al., 2006), whereas shallow submarine fans within continental shelves have been the focus of fewer studies ⁎ Corresponding author. Tel.: +1 418 656 2131x4508. (e.g., Conway et al., 2012; Normandeau et al., 2013, 2014; Warrick E-mail address: [email protected] (A. Normandeau). et al., 2013). Recent studies on shallow submarine canyons using

http://dx.doi.org/10.1016/j.geomorph.2015.03.023 0169-555X/© 2015 Elsevier B.V. All rights reserved. 2 A. Normandeau et al. / Geomorphology 241 (2015) 1–18 high-resolution multibeam echosounders have allowed imaging in 3. Methodology great detail the recent and frequent sedimentary processes acting with- in them (Hill et al., 2008; Conway et al., 2012; Hughes Clarke et al., 3.1. Data and methods 2014). Such shallow submarine canyons or channels are often observed at the head of fjords up to a depth of 400 m, at the mouth of rivers sup- Multiple multibeam echosounder datasets were merged in order plying considerable amounts of sediments (e.g., Conway et al., 2012). to produce the high-resolution bathymetric maps of the LSLE. Depths More recently, submarine fans were also discovered in very shallow sec- greater than 30 m were mapped by the Canadian Hydrographic tors of continental shelves at depths of less than 60 m (Normandeau Service (CHS) on board the CCGS Frederick G. Creed using a et al., 2013; Warrick et al., 2013). More attention is now being drawn Kongsberg Simrad EM-1000 (95 kHz; ~10-m grid resolution; before to these small-scale systems in shallow environments because they 2005) and an EM-1002 (95 kHz; ~10-m grid resolution; since can be mapped at a higher resolution using higher frequency multibeam 2005) and on board the Guillemot using a Simrad EM-3000 (before echosounders and can provide high-resolution visualization of gravity- 2005) and an EM-3002 (since 2005). New surveys were undertaken driven processes. Similar shallow-water systems have been reported over submarine canyons in 2012 on board R/V Coriolis II, using a in the Lower St. Lawrence Estuary (LSLE) (Duchesne et al., 2003; Kongsberg EM-2040 (300 kHz; 1-m grid resolution) to improve the Gagné et al., 2009; Pinet et al., 2011; Normandeau et al., 2014), one of horizontal resolution of the images. This system also allowed the largest and deepest estuary in the world with depths reaching extracting backscatter intensities to produce reflectivity maps of ~350m. the seafloor. Shallower areas of the LSLE were mapped by the Centre Submarine canyons are also known to transfer eroded coastal sedi- interdisciplinaire de développement en cartographie des océans ments to deeper marine basins (e.g., Romans et al., 2009; Budillon (CIDCO) on board the R/V Bec-Scie and R/V Gabrielle C using a SEA et al., 2011). Yoshikawa and Nemoto (2010) demonstrated that subma- SwathPlus-M (234 kHz; 2-m grid resolution) in 2009–2011 and our rine canyons can profoundly affect coastal sediment dynamics because group on board the R/V Louis-Edmond-Hamelin in 2012 using a they often constitute the end of a littoral cell. In the LSLE, coastal erosion Reson Seabat 8101 (240 kHz; 3-m grid resolution). Seismic data affects a large part of the coastline (Bernatchez and Dubois, 2004). It were acquired using an Edgetech X-Star 2.1 Chirp (2–12 kHz; cm- was first hypothesized that the numerous submarine canyons and scale vertical resolution) subbottom profiler and an Applied channels present along the margin of the LSLE were still active and Acoustics Squid 2000 sparker (2.4 kJ; m-scale vertical resolution). could contribute significantly to coastal erosion by transferring coastal In total, more than 500 km of seismic lines were acquired during sediments to the Laurentian Channel (Gagné et al., 2009; Pinet et al., the 2012 expedition on board R/V Coriolis II. 2011; St-Onge et al., 2011). However, evidence for this hypothesis at Box cores were collected during the 2012 Coriolis II cruise in differ- the LSLE scale is still lacking. Based on a high-resolution geomorpholog- ent sectors of the LSLE and in 2006 in the Les Escoumins area (Gagné ical mapping and surface sediment analysis (box cores) approach, this et al., 2009). The box cores were first opened, visually described and study aims to: 1) provide a classification of submarine canyons and photographed. The cores were then analyzed through a Siemens channels present in the LSLE, where they were formed in a similar somatom volume sensation CT-Scan that allowed a non-destructive environment and in similar climatic and tectonic conditions, but with visualization of longitudinal and transverse sections of cores using X- distinct morphologies; 2) define the factors controlling the distribution ray attenuation. Gray levels vary mainly according to density (Fortin of canyons and channels in a semi-enclosed basin; and 3) assess the et al., 2013) and allow identifying sedimentary structures and establish- sedimentary processes currently shaping these systems. This paper ing a high resolution stratigraphy (e.g., St-Onge and Long, 2009). Grain- highlights the heterogeneity of types of incisions in an otherwise size analysis was performed using a Horiba laser sizer. Sediments were relatively similar bathymetric environment. diluted into a calgon solution for ≥3 h, shaken and then submitted to an ultra-sound bath in order to disaggregate the particles. At least 2. Physical setting three runs were averaged. Statistical parameters were obtained using the Gradistat software (Blott and Pye, 2001). The approximate age of The LSLE is one of the largest estuaries in the world, reaching 200 km the surficial sediments was determined by counting the activity of the in length, 50 km in width and ≤350 m in depth (Fig. 1). It is located radiogenic isotope 210Pb (half-life = 22.3 yrs) via gamma emission at between the Grenville (north) and Appalachian (south) geological the Centre d'études nordiques. Dating encompasses approximately a provinces, in eastern Canada. The Grenville geological province is main- century. ly composed of hard igneous and metamorphic rocks (Franconi et al., 1975). The littoral is composed of many deltas, which are at the origin of the sand-size sediment found throughout the LSLE shoreline 3.2. Terminology (Lessard and Dubois, 1984; Sala and Long, 1989; Dionne and Occhietti, 1996; Bernatchez, 2003; Jaegle, 2014). The base-level of the canyons Because the submarine canyons and channels studied in the LSLE and channels, which is the lowest point a particle can attain in the are located in an inner-shelf shallow-water environment, the marine realm, is the Laurentian Channel, at ≤350 m (Loring and Nota, terminology used here differs slightly from the one used in deep- 1973). The Appalachian region bordering the LSLE is formed by the water settings. For example, Talling (2014) used the term canyon sedimentary rocks of the Gaspé Peninsula and is Paleozoic in age. for deep incisions (N100 m) formed primarily by erosion and gully The LSLE basin contains in some sectors ~400 m thick of sediments was used for smaller incisions (b100 m). If we used this terminology, that recorded glaciation, deglaciation and the establishment of postgla- all the incisions in the LSLE would be considered as gullies. We thus cial conditions (Cauchon-Voyer et al., 2008; St-Onge et al., 2008; use a different terminology that is adapted to the bathymetric setting Duchesne et al., 2010). Eight seismic units were identified in the LSLE, of the LSLE: (a) Canyon is used here for incisions (generally 10s of ranging from ice-contact sediments, to deglacial (ice-proximal and meters) formed primarily by erosion that reach the base-level, ice-distal) and recent sediment deposition. More local units are related which is the bottom of the Laurentian Channel. Canyons are general- to submarine mass movements and Holocene fans, located punctually ly, but not exclusively, V-shaped. (b) Gullies are small-scale incisions on the Québec North and South shores. A surficial deposit map of the (b10 m deep), that generally do not reach base-level. (c) Channels LSLE was realized by Pinet et al. (2011) and illustrates the silt-sand are defined as conduits that are formed primarily by depositional nature of the margins of the Laurentian Channel and the finer silts of flows (Talling, 2014) and that reach the shelf edge or the base- the trough, where most sediments originate from the North Shore level. They are generally U-shaped and have wide and relatively (Jeagle, 2014). flat floors. A. Normandeau et al. / Geomorphology 241 (2015) 1–18 3

A

B

C

Fig. 1. Location of the study area. A) Multibeam bathymetry of the Lower St. Lawrence Estuary (LSLE) with location of core COR1203-27BC shown in Fig. 6F. The location of the other cores are indicated on following figures specific to each turbidite system. B) Slope map of the LSLE with location of the different types of canyons and channels (see Discussion). C) Main direction of longshore drift and sediment composition of the shoreline along the North Shore of the LSLE. Sediment composition is derived from Dubois et al. (2005) where a) high volume of erod- ible sediments consists of clay, clay with blocks, gravel, organic matter, sand, sand with blocks, silt or silty sand; b) mid volume of erodible sediments consists of the latter sediments on bedrock; c) low volume of erodible sediment consists of till or till on bedrock, and d) bedrock.

4. Results Manicouagan, it is ~50 km wide (Fig. 2). The slopes along the Laurentian Channel also vary from Tadoussac to Pointe-des-Monts. Near the head of 4.1. LSLE geomorphology the Laurentian Channel (Tadoussac), the margins are generally steep (≥10°). The slopes become gentler near Manicouagan and Godbout, at The Laurentian Channel, a deep (N300 m) trough, extends over the less than 10°. The northern slope generally has stronger gradients, espe- entire LSLE region. On the Québec North Shore, the extent of the coastal cially in Portneuf and Pointe-des-Monts, where the slope is much steeper, shelf (from shoreline to slope break) varies between 0 and 15 km while reaching ≥20°, compared to the South Shore, where it is b5°. on the South Shore, it varies between 5 and 20 km (Fig. 1). The width of The LSLE receives sediments from many rivers along its North and the Laurentian Channel varies from Tadoussac to Pointe-des-Monts. South shores. The combined area of these watersheds totals Near the head of the Laurentian Channel (near Tadoussac), it is ≤10 km ~183,000 km2 on the Québec North Shore (excluding the St. Law- wide and gently becomes wider near Les Escoumins (N15 km). Near rence River and Saguenay Fjord watersheds) and ~9000 km2 on the 4 A. Normandeau et al. / Geomorphology 241 (2015) 1–18

0 aa’ Gradient (°) 20 Tadoussac -200 a’

a Depth (m) Les EscouminsN -400 0 b’ b 0 bb’ 20 Gradient (°) c’Portneuf c -200 Depth (m) Rimouski Betsiamites -400 0 0 cc’Gradient (°) Mitis 30 -200 Outardes Depth (m) -400 0 Manicouagan 0 d d’ Gradient (°) d’ d 10 -200 Depth (m) -400 0 0 e

e’ Gradient (°) 15 e Godbout -200 e’ Depth (m) -400 0 0 f

f’ Gradient (°) f f’Pointe-des-Monts 25 -200 Depth (m) -400 0 0 1020304050 Distance (km)

Fig. 2. Bathymetric and gradient profiles of the Lower St. Lawrence Estuary in sectors incised by submarine canyons and channels.

Québec South Shore. A basic surficial geological map produced by the Geological Survey of Canada (Turner et al., 2003) was used to extract general information concerning the northern and southern water- Watershed fi sheds (Fig. 3). In both watersheds, sur cial geology consists mainly Mud of rock, till, sand and gravel, and mud. On the North Shore, ~62% of Peat the total area (~59,600 km2) consists of rock, ~33% (~31,200 km2) of till, and ~5% (~4750 km2) of sand and gravel. On the Québec Sand and Gravel South Shore, ~72% of the total area (~6400 km2) consists of rock, Till ~15% (~1350 km2) of till, ~10% (~903 km2) of sand and gravel and Rock ~3% (~250 km2) of mud. In these two watersheds, sand and gravel are mainly located on the shores of the LSLE while rock and till are mainly located inland. On the Québec North Shore, where most of the submarine canyons and channels are observed, the coastline totals ~400 km in length. Dubois et al. (2005) identified the type of sediments composing the shoreline (the data are unfortunately not available for the South Shore). These results were extracted in order to give an accurate representation of the northern shoreline. From Tadoussac to Pointe- des-Monts, 44% of the shoreline consist of rock, 5% consist of low volume of erodible sediment (mainly till), 5% consist of mid volume of erodible sediment (mainly sand on till/bedrock), and 46% is composed of a high volume of erodible material (mainly sand and clay) (Fig. 1C). Therefore, ~50% of the coastline is susceptible to erosion and can be a source of sediment for submarine canyons and channels. The mid- and Depth (m) high-volume of erodible sediments are mainly located where land 0 protrudes into the water, i.e., in deltaic settings. 360 4.2. Tadoussac

A series of submarine canyons are present in the Tadoussac sector. A fi rst canyon, named here the Saguenay canyon, is located near the 0 45 90 180 center of the LSLE and incises the head of the Laurentian Channel with km an 85° azimuth (Fig. 4 and Table 1). The head of the canyon incises a b chaotic seabed, at a depth of ~30 m, 3 km from the coast. The canyon Fig. 3. Map of surficial geology and watersheds on the North and South Shores of the LSLE. reaches ~4.5 km in length at a depth of 140 m but its fan progresses into Source of surficial geology map: Turner et al. (2003). A. Normandeau et al. / Geomorphology 241 (2015) 1–18 5

Fig. 4. Geomorphology of the Saguenay canyon: A) Multibeam bathymetry of the sector; B) Slope of the sector; C) Backscatter imagery of a dune field on the lower fan with presence of a wreck; D) Photograph of the seabed shown in A; and E) Matrix grain-size distribution of the samples collected over the submarine canyon and fan shown in A. water depth of 200 m. Its width varies between 250 and 450 m and the on the fan and at the head of the canyon (Fig. 4B,C). The dunes near the mean slope is ~1–1.5°. Two mass movements disturb the surface mor- head of the canyon are located at a depth of ≤40 m and are 10–50 m phology of the fan (Fig. 4A). Series of subaqueous dunes are also present long and less than ~ 3 m high. They correspond to large dunes

Table 1 Physical properties of submarine canyons and channels from the Lower St. Lawrence Estuary.

Name Azimuth Watershed Distance from the Average thalweg Average Average upper Average lower Lobe? Head water Feeding (°) (m2) coast (m) length (km) width (m) slope (°) slope (°) depth (m) source

Saguenay 85 – N3000 4.5 250–450 1–1.5 1–1.5 Yes 30 Ancient glacier Tadoussac 120 – 230–900 1–1.5 300–400 6–11 5 No 6–20 Longshore drift Les Escoumins River 60 793 500 1 200–300 18 7 Yes 5 River Les Escoumins 150 – 750–1250 1.5 150–200 15 5 Yes 5–8 Longshore drift Portneuf 90 3085 b2700 Buried Buried – 1 Yes Buried River Betsiamites 115 18700 – Buried Buried 3–3.5 1 Yes Buried River Manicouagan 100 45908 4000–6000 5–20 500–1500 1–5 0.5–1.5 Yes 10–20 River Godbout 170 1577 500–1500 8–9.5 400–900 2–4 1 Yes 20–30 River Pointe-des-Monts 180 – 130–500 2–3 100–300 15–20 2–3 Yes 10–25 Starved Rimouski 350 1600 9000 5 300–400 – b1 Yes 75 River Mitis 325 1800 7000 4 200–300 – 1 Yes 80 River 6 A. Normandeau et al. / Geomorphology 241 (2015) 1–18

(Ashley, 1990). The dunes located on the fan (125 m deep) have wave- A series of canyons are located closer to the shoreline and are named lengths of ~30–40 m and heights of ~1 m. Where subaqueous dunes are here the Tadoussac canyons (Fig. 5 and Table 1). They are observed near absent, bottom photographs and sediment cores on the fan reveal the shore surrounding the Petite-Bergeronne River (watershed: rounded clasts and a sediment matrix with primary grain-size modes ~570km2) in a 120° azimuth. The shores are composed of erodible sed- of ~200–1000 μm(Fig. 4D, E). iments, more specifically of sand and glacio-marine clays with boulders

A

360

B

C

D

Fig. 5. Geomorphology of the Tadoussac canyons: A) Multibeam bathymetry of the sector; B) Backscatter imagery draped on multibeam bathymetry revealing the homogeneous intensity over the submarine canyons; C) Slopes and sediment composition of the shore (see Fig. 1C for legend) of the sector with photograph of an onland mass movement; and D) Seismic profile of the base of the canyons revealing mass movement deposits (MMDs). A. Normandeau et al. / Geomorphology 241 (2015) 1–18 7 on till (Fig. 5C). The head of these canyons are all located close to the canyons, a seismic profile reveals the presence of numerous mass shoreline (between 230 and 900 m). They incise the margin at a movement deposits (MMDs) within the seismic sequence (Fig. 5D). depth of 6–20 m down to a depth of 160 m. The Tadoussac canyons However, there is no apparent accumulation or fan-like deposits are short, having length less than 1.5 km. The upper and lower slopes observed on the bathymetry data (Fig. 5A,C). The base of the canyons are 5–11°. They are characterized by numerous headscarps and is rather characterized by an escarpment (Fig. 5), triangular facets and amphitheater-shaped erosional scars (Fig. 5A,C). At the base of the arelativelyflat base level, leaving some of the canyons “hanging”

Fig. 6. Grain-size properties of cores A) 06BC (Tadoussac); B) 77BC (Les Escoumins); C) 76BC (Les Escoumins); D) 19BC (Godbout); E) 26BC (Pointe-des-Monts); and F) 27BC (Pointe-des- Monts). The shaded areas illustrate fining-upward sequences, interpreted as gravity flow deposits. 8 A. Normandeau et al. / Geomorphology 241 (2015) 1–18

(e.g., Harris et al., 2014). Furthermore, there is little variation of while the small axial incision within the canyon is 50 m wide. The backscatter intensity over the canyons. The only two visible features lobe and axial channel are characterized by high backscatter values on the backscatter map are two mass movements near the base of the (Fig. 7B) and the absence of acoustic penetration in sub-bottom profiles canyons (Fig. 5B). A core collected at the mouth of one of the canyon (Fig. 7F), particularly where the recent channel with crescentic (COR1203-06BC) reveals a fining-upward sequence with mud clasts bedforms is observed. and D50 (median grain size) decreasing from 600 to 100 μm(Fig. 6A). Three other canyons are located east and were first described by The base of the core is thus interpreted as a gravity flow deposit. 210Pb Gagné et al. (2009) using lower resolution multibeam bathymetry analysis reveals that it reaches its supported values around 13 cm (10 m grid). These canyons are not associated with a terrestrial drainage (Fig. 6A), indicating that the gravity flow deposit is slightly older than system. They are located where the coastal shelf narrows to b1km,ata 100 yrs. Mass movement processes can also be observed onshore, depth of 5–8 m and have a 150° azimuth (Fig. 8 and Table 1). The shores near the head of the canyons. A mass movement observed onshore east of the canyons are mainly composed of high volumes of erodible was triggered during the winter of 2009–2010 (Fig. 5C). sediment, such as sand and glaciomarine clay (Figs. 1C and 8C). New high-resolution multibeam bathymetry (1 m grid) allowed us to obtain 4.3. Les Escoumins a more detailed representation of the morphological character of these canyons. The three canyons are ~1.5 km long, ~200 m large and incise The first canyon observed in the Les Escoumins sector is located at the margins of the Laurentian Channel down to a depth of ≥200 m. the mouth of the Les Escoumins River (watershed: 793 km2; Fig. 7 and The mean slopes of the three canyons are ~15° decreasing downslope Table 1) in a 60° azimuth. The sandy shore surrounding the canyon to ~5°. The westernmost canyon is the closest to the shoreline head is 3 km long and is isolated between rocky shores (Fig. 7C). The (~550 m) whereas the two others located east are farther from it head of the canyon is located b 1 km from the river mouth, at a depth (~1 km). However, all three canyons are located at a similar distance of 5 m. Based on the rugosity of the multibeam imagery and the from the shore at low tide. The easternmost canyon is more deeply in- presence of rocky sediments and boulders in the intertidal zone, the cised than the two others, as illustrated by higher slopes on its sidewalls coastal shelf in this sector appears to have low rates of modern sediment (Fig. 8C). The upper section of the eastern canyon is characterized by a accumulation near the head of the canyon (Fig. 7A). The canyon is 1 km small axial channel and a terrace (Fig. 8E). It also contains crescentic long and its fan reaches 275 m deep. The upper slope is ~18–20° and bedforms in its thalweg, whereas the two other canyons are draped decreases to ≤10° downslope. The upper part of the canyon is by fine silts. Crescentic bedforms are similar in size to those observed composed of a terrace and a small axial channel (Fig. 7D). Further at the mouth of the Les Escoumins River. The backscatter intensity is downslope, the main canyon axis is composed of small crescentic high over the easternmost canyon and fan, and is relatively low over bedforms and the channel bifurcates right when arriving on the lobe the center and westernmost fan and canyon (Fig. 8B). Sub-bottom (Fig. 7E). The bedforms are asymmetric downslope, have a wavelength profiles collected over the sector reveals an absence of acoustic penetra- of ~25–50 m and are ~1–4 m high. The canyon width is 200–300 m tion below the fans. A core collected on the eastern fan (COR0602-77BC)

A DE

B

F

C

Fig. 7. Geomorphology of the Les Escoumins River canyon: A) Multibeam bathymetry of the sector; B) Backscatter imagery draped on multibeam bathymetry revealing the higher intensity over the submarine fan and canyon; C) Slopes and sediment composition of the shore (see Fig. 1C for legend) of the sector; D–E) Multibeam imagery of the canyon and fan illustrating a terrace, a small axial channel and crescentic bedforms (vertical exaggeration = ×6); and F) 3D fence diagram of Chirp sub-bottom profiles illustrating the canyon and fan system. A. Normandeau et al. / Geomorphology 241 (2015) 1–18 9

A BC

D EF

Fig. 8. Geomorphology of the Les Escoumins canyons: A) Multibeam bathymetry of the sector; B) Backscatter imagery draped on multibeam bathymetry revealing higher intensity over the eastern submarine canyon; C) Slopes and sediment composition of the shore (see Fig. 1C for legend) of the sector; D) 3D fence diagram of Chirp sub-bottom profiles illustrating the three submarine canyons; and E–F) Multibeam imagery of the canyon and fan illustrating a terrace, a small axial channel and crescentic bedforms (vertical exaggeration = ×6). contains a fining-upward sequence that was interpreted as a turbidite suggest that the turbidite was deposited during the 1860 AD Rivière-

(Fig. 6B) (Gagné et al., 2009). Over the turbidite, D50 values range Ouelle earthquake (M ~6) or the 1870 AD (M ~6–6.5) Baie-Saint-Paul between 200 and 300 μm with a coarsening-upward trend. 210Pb data earthquake (Gagné et al., 2009). Conversely, a core collected over the

A C

B D

Fig. 9. Geomorphology of the Les Escoumins–Manicouagan shelf composed of submarine channels: A) Multibeam bathymetry of the sector; B) Slopes and sediment composition of the shore (see Fig. 1C for legend) of the sector; C) Seismic profiles collected over the Betsiamites channel illustrating its buried nature; and D) Multibeam imagery illustrating undulations and gullies at the mouth of the Betsiamites River. 10 A. Normandeau et al. / Geomorphology 241 (2015) 1–18

western fan (COR0602-76BC) reveals silty to sandy sediments, with D50 500 and 1500 m in width. They all show varying levels of sinuosity. values ranging between 10 and 40 μm at the base and coarsening up The southernmost has the highest incision gradients at a depth of around 50–100 μmstartingat15cm(Fig. 6C). 175 m (Fig. 10B). The two channels located at the river mouth show lower gradients on their margins and are characterized by sediment 4.4. Les Escoumins–Manicouagan infilling (Fig. 10C, D). Sedimentary processes on the shelf mainly include longshore-drift related bedforms as indicated by the presence of The coastal shelf gently becomes wider between Les Escoumins and transverse bars (Fig. 10D). Manicouagan, reaching 15 km off Forestville (Fig. 9). The deltas of three major rivers (Portneuf, Betsiamites and Outardes) are present in this 4.6. South Shore sector and their submarine components are composed of channels (Table 1). These channels formed on the coastal shelf above the slope On the South Shore, two channels are observed offshore the Rimou- break of the Laurentian Channel (Fig. 9B). The upper slope of the shelf ski (watershed: ~1600 km2) and Mitis Rivers (watershed: ~1800 km2) directly near the mouth of the Betsiamites River is 3.5° and decreases (Fig. 11 and Table 1). They are both ~200–400 m large and have a relief to less than 1.5° offshore. A seismic profile collected off the Betsiamites of ≤10 m. The head of the channels are 70–80 m deep and are 7–10 km River reveals the buried nature of these channels (Fig. 9C). Gullies are away from the shoreline. Their mean slope is 1°. The channels are observed near the mouth of the river (Fig. 9D). ~4–5 km long and their fans extend in the Laurentian Channel but are hardly visible on the multibeam imagery. Both of them show a smooth 4.5. Manicouagan morphology attributed to a sediment drape.

The channels observed at the mouth of the Manicouagan River are 4.7. Godbout located offshore the city of Baie-Comeau (Fig. 10 and Table 1). This river has the largest watershed of the North Shore (~45 900 km2). The The Godbout canyons are located at the mouth of the Godbout River, head of the channels are located directly at the mouth of the main which has a watershed of 1577 km2. The river mouth consists of sandy river channel but are still relatively far from the coast (~5 km). The sediments that extend on 6 km between rocky shores (Fig. 12). Two head of the channels are observed at a depth of ~10–20 m. The upper other canyons located west of the river mouth probably relate to its slope is short and varies alongshore at ≤10°. The gradient then de- ancient positions, 1500 m from the shoreline. The easternmost canyon creases to 1.5° downslope. These channels are the longest observed in incises the shelf to 500 m from the shoreline and its head is located the LSLE, with the southernmost reaching 20.5 km. The other channels 2 km east of the river mouth (Fig. 12 and Table 1), at a depth of vary between 5 and 10 km in length. All the channels vary between 20–30 m. The three canyons vary in length between 8 and 10 km. The

A C

B D

Fig. 10. Geomorphology offshore the Manicouagan River mouth. A) Multibeam bathymetry of the sector; B) Slopes and sediment composition of the shore (see Fig. 1C for legend) of the sector; C) Seismic profile collected on a channel located at the mouth of the current Manicouagan River discharge; and D) Multibeam imagery of the channels at the mouth of the current river position illustrating longshore-related bedforms and the infilling of the channels. A. Normandeau et al. / Geomorphology 241 (2015) 1–18 11

Fig. 11. Geomorphology of the Rimouski and Mitis channels: A) Multibeam bathymetry of the sector; B) Slopes of the sector; C) Multibeam bathymetry illustrating the Rimouski channel and its draping morphology; and D) Multibeam imagery illustrating the Mitis channel and its partly buried nature. slope of the upper part of the canyon is ~4° while the lower part 300 m. The canyons relief at their head varies between 10–15 m. Further decreases to ≤ 1°. Scarps are observed along the eastern canyon downslope, it increases to 30 m and gently decreases down to 10 m (Fig. 12A,B). A box core (COR1203-19BC) collected in the thalweg of before becoming unconfined. Numerous crescentic bedforms (Fig. 13E, the eastern canyon reveals silty-sand to sandy-silt sediment (Fig. 6D). F) are observed within the thalweg of the canyons. They are asymmetric No distinct facies are observed in the CT-Scan imagery. Sedimentary downslope, have ≤ 60 m wavelengths and are ~1–3 m high. bedforms at the Godbout River mouth are characterized by undulations Sub-bottom profiles collected in the Pointe-des-Monts canyons that are ~2 m high and 20–50 m long (Fig. 12C). These bedforms are show an absence of acoustic penetration. A hemipelagic drape is mainly present on the shelf break but do not extend to the eastern observed on each side of the canyons (Fig. 13D). Two cores were collect- canyon thalweg. They are localized in the first kilometer of the river ed in the Pointe-des-Monts sector: a first in the thalweg of the eastern mouth. A sub-bottom profile collected in the eastern thalweg reveals canyon (COR1203-26BC; Fig. 6E) and a second just outside the subma- the absence of acoustic penetration over the canyon, whereas its base rine fans (COR1203-27BC; Fig. 6F). The core collected in the thalweg consists of ≤4 m of hemipelagic sediment (Fig. 12E). Furthermore, no of the eastern canyon is 10 cm long and consists of homogenous fine 210 bedforms are observed within the thalweg on the sub-bottom profiles, sand (D50 =110–120 μm). Pb analysis reveals that this core did not nor on the multibeam imagery (Fig. 12D). attain its supported values, indicating recent sediment deposition (b100 yrs). The core collected just outside the submarine fans is

4.8. Pointe-des-Monts 47 cm long and consists of fine to medium silt (D50 =12–23 μm) but is composed of two modes (~10 and ~50 μm). Three distinct submarine canyons are observed in the Pointe-des- Monts sector and were first described by Normandeau et al. (2014) 5. Discussion (Fig. 13 and Table 1). The two canyons located to the east have similar physical settings. The shores surrounding the area and the heads of 5.1. Classification and modern sediment dynamics the canyons almost entirely consist of bedrock, except for small pocket beaches (Fig. 13C). These canyons have length of ≤3.5 km, down to Four genetic types of submarine canyons and channels are observed ≤300 m deep. Their width varies greatly and ranges between 100 and in the LSLE. These systems were identified and classified based on their 300 m. The upper slope is ~15–20° until ~125 m deep. This section is sediment source: 1) glacially-fed canyon; 2) longshore drift-fed composed of numerous small gullies that feed the tributaries of the canyons; 3) river-fed canyon and channels; and 4) sediment-starved canyons (Fig. 13A). The canyon floor slope then decreases to 2–3°, canyons. A qualitative summary of their characteristics is provided in 390 m from the head until the end of the fans located at a depth of Table 2. A combined geomorphological and sedimentological approach 12 A. Normandeau et al. / Geomorphology 241 (2015) 1–18

ABC

D

E

Fig. 12. Geomorphology of the Godbout canyons: A) Multibeam bathymetry of the sector; B) Slopes and sediment composition of the shore (see Fig. 1C for legend) of the sector; C) Multibeam bathymetry illustrating undulations at the river mouth; D) Multibeam imagery illustrating the absence of bedforms along the thalweg of the canyon; and E) Seismic profile revealing the absence of acoustic penetration in the thalweg of the canyon and the draping post-glacial sediments on the fan.

(e.g., Lastras et al., 2007; García-García et al., 2012; Hill, 2012; 5.1.1. Glacially-fed canyon Babonneau et al., 2013; Obelcz et al., 2014)wasusedtodescribe The chaotic morphology of the seabed at the head of the Saguenay the typical morphological characteristics of these systems and to as- canyon (Fig. 4) is interpreted as the remains of a temporary stillstand sess their recent activity. Activity of submarine canyons is defined as of the Laurentide Ice Sheet margin, which is in accordance with the in- amodification of canyon morphology by erosion and deposition in terpretation of marine charts by Dionne and Occhietti (1996). There- recenttimesasopposedtotheoccurrence of general background fore, the canyon that incises this morainic system is interpreted as a sedimentation (Mountjoy et al., 2014). It can also be defined as the relic of a proglacial canyon formed during ice stabilization phases at frequency of particle-laden currents that are intrinsic to submarine the mouth of the Saguenay Fjord (Dionne and Occhietti, 1996; Locat canyons (Mulder et al., 2012). Here, activity is defined as a modifica- and Sanfaçon, 2000). Submarine canyons and channels related to glacial tion of submarine canyon and channel morphology by particle-laden outflow are often located off cross-shelf trough (e.g., Rise et al., 2013; currents directly related to these systems. If currents unrelated to Roger et al., 2013). Trough-mouth fans often comprise numerous gullies thecanyonserodetheseafloor in a given region, the canyons are that are related to meltwater outflows (Dowdeswell et al., 2006; considered inactive. In order to assess their activity, the canyons Pedrosa et al., 2011). Unlike other glacially-fed systems, the Saguenay located within the same geological and geomorphological context fan is incised by only one canyon. This absence of widespread incisions were grouped together. Therefore, the most active canyon or channel on the fan could be due to the presence of the Laurentian Channel and is used when defining the activity of each group. This way the strong tidal currents that confined the flows on a 5 km width. It can inactivity of ancient channels that were abandoned to the profitof also be related to the stagnation of the glacier during its retreat which another are not accounted for. lasted less than 1000 yrs around 11 ka BP (Dionne and Occhietti, 1996). A. Normandeau et al. / Geomorphology 241 (2015) 1–18 13

ABC

D EF

Fig. 13. Geomorphology of the Pointe-des-Monts canyons: A) Multibeam bathymetry of the sector; B) Backscatter imagery draped on multibeam bathymetry revealing higher intensities over the submarine canyons; C) Slopes and sediment composition of the shore (see Fig. 1C for legend) of the sector illustrating the low volume of sediment; D) 3D fence diagram of Chirp sub-bottom profiles illustrating the three submarine canyons; and E–F) Multibeam imagery illustrating the high number of crescentic bedforms along the thalweg of the canyons (vertical exaggeration = ×6).

The incision of the Saguenay canyon into gravelly sediments indi- known for its strong tidal currents. Numerous dune fields are present cates that it was formed by energetic gravity flows, which is most likely in areas of finer-grained sediments. These dunes are observed on the the result of hyperpycnal flows or mass movements related to high sed- chaotic seafloor at the head of the Laurentian Channel, on the large iment discharge at the mouth of the glacier (e.g., Ó Cofaigh et al., 2004; mass movement located south-west of the canyon and on the distal Piper et al., 2007; Armitage et al., 2010; Roger et al., 2013). The coarse part of the fan. They are not present within the canyon (Fig. 4A). nature of the sediments composing the seafloor (Fig. 4D, E) supports These dunes are most likely associated with tidal currents or internal this hypothesis, because such coarse material needs energetic flows to tides, as interpreted for similar dunes located upstream in the St. be transported. Today, strong currents due to upwelling at the head of Lawrence Estuary (Bolduc and Duchesne, 2009), and are not related to the Laurentian Channel and internal tides limit the deposition of fine recent sedimentary processes within the canyon. Therefore, there is sediments in the sector (Ingram, 1983) and lead to a lag deposit on no evidence for the recent passage of gravity flows in the canyon. The the seafloor. Strong currents are evidenced on the seafloor by the pres- St. Lawrence River or the Saguenay Fjord cannot produce hyperpycnal ence of a tidal scour (Fig. 1) similar to the one observed by Shaw et al. flows that could currently be remolding the seafloor of the Saguenay (2014; their Fig. 11) on the seabed of the Bay of Fundy, a region canyon because sediment concentrations are too low (Mulder and

Table 2 Qualitative properties of submarine canyons and channels in the Lower St. Lawrence Estuary.

Characteristics/classification Glacially-fed Longshore drift-fed River-feda Sediment-starved

Source of sediment Glacially derived Remobilized fluvial and coastal Fluvial sediments Sediments on the margins of the sediments erosion Laurentian Channel Proximity to a source of 3km b1km 0–5 km No sediment supply sediment supply Coastal shelf 3 km b1km 0–5km b1km Main inferred processes Hyperpycnal flows and Slope failures and Hyperpycnal flows and slope failures Slope failures and internal tides (?) slope failures storm-induced gravity flows Modern activity No evidence Evidence of episodic activity No evidence Evidence of frequent activity Slope 1–2° N5° 0–5° N10° Names Saguenay Les Escoumins, Tadoussac Betsiamites, Rimouski, Mitis, Portneuf, Pointe-des-Monts Godbout, Manicouagan

a Applies only to river-fed delta systems. 14 A. Normandeau et al. / Geomorphology 241 (2015) 1–18

Syvitski, 1995). Moreover, the Saguenay Fjord is composed of three to the AD 1860 (M ~6) or 1870 (M ~6–6.5) earthquakes in the deep basins and a sill at its mouth, favoring sediment capture in the Charlevoix-Kamouraska seismic zone (CKSZ). The amphitheater- fjord rather than in the LSLE (Locat and Lévesque, 2009; St-Onge et al., shaped scars at the head of the canyons suggest that the canyons 2012). This sector of the LSLE is rather today host of upwelling and in- evolved by retrogressive slope failures (e.g., Twitchell and Roberts, ternal tides that induce important bottom currents (Ingram, 1983). 1982; Farre et al., 1983). These slope failures are probably related to earthquakes because these canyons are located in the CKSZ 5.1.2. Longshore drift-fed canyons (Lamontagne, 1987). The CKSZ is the most active seismic zone in East- Both the eastern canyons of Les Escoumins (Fig. 8) and the ern Canada where five earthquakes of M ≥ 6 or more occurred during Tadoussac canyons (Fig. 5) are classified as longshore-drift fed subma- the last 350 years (Lamontagne, 2009). More than 100 mass move- rine canyons. The morphology of these systems is characterized by a ments were identified in the LSLE and the Saguenay Fjord (Duchesne narrow shelf, a disconnection to a terrestrial drainage system and a et al., 2003; Locat et al., 2003; St-Onge et al., 2004, 2012; steep slope. The sediment supplied to their heads is transported Cauchon-Voyer et al., 2008, 2011; Pinet et al., 2015) and most of them through longshore-drift. The narrow shelf allows sediments to be were interpreted to be the result of earthquakes from the CKSZ or the trapped between the Laurentian Channel margin and the shoreline. Saguenay area (Locat, 2011). St-Onge et al. (2004) proposed a recur- Sediments transported through longshore drift do not bypass the rence of large earthquakes (M ≥ 6.75) of 300 years before 4 ka BP and canyons. They are rather trapped at their head before being of 2000 years after 4 ka BP for the Saguenay Fjord. However, smaller redistributed along the canyon floor and fan. earthquakes could have caused smaller slope failures, such as proposed The three eastern canyons of Les Escoumins have distinct morpho- for slope failures observed in these longshore-drift fed submarine can- logical signatures. The western and central canyons are draped by fine yons. The absence of sediment accumulation at the base of these can- hemipelagic sediment. The multibeam imagery does not reveal any yons suggests that slope failures were relatively infrequent during the bedforms along the axis of these two canyons. The presence of fine late-Holocene which is in agreement with the earthquake recurrence silts in core 76BC (Fig. 6C) indicates that sediment gravity flows did rate of St-Onge et al. (2004). The absence of fan-like deposits could not occur in the recent past. Unlike the central and western canyons, also be explained by the presence of an escarpment at the base of the the eastern canyon presents numerous bedforms along its thalweg canyons (Fig. 5C) that has all the morphological characteristics of a and fan (Fig. 8E,F). Small crescentic bedforms and axial incision are ob- fault-scarp. The Tadoussac canyons are similar morphologically to hang- served along its axis. Its margins also show the highest slope gradients ing canyons described in Haida Gwaii, British Columbia (Harris et al., of the sector due to axial incision (Fig. 8C). According to Baztan et al. 2014). In both systems, an escarpment separates the slope from the (2005), axial incision can be interpreted as the downcutting of the can- base-level and there is absence of fan-like morphologies. Hanging can- yon thalweg during an erosion phase. These morphological properties yons form where the rate of canyon erosion is lower than the vertical suggest that gravity flows recently eroded the canyon bottom. Crescen- displacement of a fault, leaving the canyon mouth “hanging”. We spec- tic bedforms, possibly associated with supercritical currents (i.e., cyclic ulate that the similar geomorphology observed in the Tadoussac can- steps; Cartigny et al., 2011), are known to be present where gravity yons is attributed to vertical displacement along a fault due to late- flows are frequent (Smith et al., 2007; Babonneau et al., 2013; Quaternary tectonic activity, forming a fault-scarp. Moreover, the pres- Mazières et al., 2014). The higher values of backscatter intensity ence of strong bottom currents in this sector can also mobilize sedi- (Fig. 8B) and the absence of acoustic penetration (Fig. 8D) over the east- ments transported through the canyons and reduce the relief of their ern canyon further support the presence of coarse particles related to fans. the recent deposition of gravity flow deposits. Gagné et al. (2009) re- In both longshore-drift fed submarine canyons of Tadoussac and Les ported the presence of a turbidite at 30 cm depth on the fan (Fig. 6B), Escoumins, sediments and bedforms indicate that recent sedimentary dated to ~1850. This date is in the range with two earthquakes that processes occurred along them during the last few centuries. The could have remobilized shelf sediments in 1860 (M ~6) and 1870 (M triggering mechanisms of these recent gravity flows are most likely ~6–6.5) that occurred in the Charlevoix-Kamouraska seismic zone related to mass movements along the head of the canyons, as observed (CKSZ; Smith, 1962; Gouin, 2001). A mass movement triggered turbi- in many other longshore-drift fed settings (Budillon et al., 2011; Biscara dite is thus the most likely explanation for this deposit. Core 77BC also et al., 2013; Normandeau et al., 2013). Other processes such as shows no sign of fine hemiplegic sediments such as those observed in advection of shelf sediments during major storms can play a role in sed- the neiboughring fan (Fig. 6B), but rather contains sand-size particles iment transfer, although there is currently no evidence for these pro- (200–300 μm) increasing towards the surface. This increase of sand cesses in these sectors. These canyons most likely formed by a suggests recent hydrodynamic processes related to the eastern canyon combination of upslope retrogressive failures and downslope eroding rather than hemipelagic sedimentation. However, there is no distinct gravity flows. The Tadoussac canyons are a good example where up- sedimentary facies within this increase in grain-size related to a typical slope retrogressive failures are still ongoing. Numerous headscarps are gravity flow deposit. slope-confined rather than shelf-indenting. These slope-confined The Tadoussac canyons were also classified as longshore drift-fed headscarps suggest that the shelf-indenting canyons were formed by canyons even if their morphologies are not typical of other documented similar processes (e.g., Farre et al., 1983; Green et al., 2007). longshore drift-fed canyons. Unlike the Les Escoumins canyons, they do not incise the slopes where the shelf narrows but are nonetheless locat- 5.1.3. River-fed channels and canyons ed on a narrow shelf. Further, their potential source of sediment could The majority of the canyons and channels observed in the LSLE are be related to longshore drift, because they are not directly located at located at the mouth of major rivers. These river-fed systems were the mouth of the Petite-Bergeronne River, but rather incise the shelf subdivided into two categories: river-fed delta systems and river-fed close to the shoreline. Sediment inputs from the Petite-Bergeronne canyon systems. The former were formed during the progradation of River are redistributed along the coast and are likely trapped near the deltas in the LSLE. These systems are observed at the mouth of major canyon heads. This redistribution is possibly limited because there has rivers of the Québec North and South Shore (Portneuf, Betsiamites, not been any erosion and accumulation along the coast during the Manicouagan, Godbout, Rimouski and Mitis), suggesting a formation 20th century (Dubois et al., 2005). Erosion and deposition along the by either hyperpycnal currents or slope failures related to high accumu- coast are generally good indicators of active coastal processes lation rates. During deglaciation, sedimentation rates and suspended (e.g., Yoshikawa and Nemoto, 2010). A gravity flow deposit is observed sediment concentration were higher (Duchesne et al., 2010) and led in a core collected at the mouth of one of the canyons (Fig. 6A). Based on to the formation of these deltas. Between Les Escoumins and 210Pb dating, this deposit is older than 100 yrs and could also be related Manicouagan, the channels formed mainly on the coastal shelf, while A. Normandeau et al. / Geomorphology 241 (2015) 1–18 15 their fan formed in the Laurentian Channel (Fig. 9). On the counterpart, only ones that show such well-defined crescentic bedforms along the the channels and canyons of the Manicouagan and Godbout deltas were canyons and that have evidence of very recent activity (≤5 years, see mainly formed on the margins of the Laurentian Channel (Figs. 10 and also Normandeau et al., 2014). Although sediment is not supplied to 12). These two delta systems are characterized by relatively low mean the canyons, crescentic bedform movements reveal that gravity flows slopes (less than 2°) and are the largest features in the LSLE. are more frequent and recent within these canyons than in any other Bedforms or axial incisions were not observed within the channels submarine canyon or channel in the LSLE. and fans on any of the delta systems. In addition, box cores collected in the Godbout canyon did not reveal any sign of rapidly deposited 5.2. Canyon and channel emplacement layers (Fig. 6D). The majority of the channels in the LSLE are buried or draped by postglacial mud (Figs. 9, 10 and 11), most likely due to the The majority of canyons and channels observed on the LSLE occur on lower modern rates of sediment supply by the rivers. According to the the Québec North Shore while only two are observed on the Québec Inland Waters Directorate (1978), these rivers contain less than South Shore. Their predominant occurrence on the Québec North 50 mg L−1 of suspended sediment. This value is considerably less than Shore and their rare occurrence on the Québec South Shore can be ex- the 35–45 kg m−3 needed to generate a hyperpycnal flow (Mulder plained by: 1) the proximity of the slope to the shoreline on the Québec et al., 1998). Sediment outflowing from the rivers are mainly deposited North Shore; 2) the steepness of the slopes bordering the Laurentian near the river mouths and remobilized along the coast by longshore- Channel on the Québec North Shore; and 3) the high volumes of sedi- drift (Fig. 10D). The absence of recent delta-slope failures such as ob- ment that were transported towards the LSLE during deglaciation on served in deltas and fjord-head deltas from British Columbia (Conway the Québec North Shore (Bernatchez, 2003; Dietrich et al., 2014). et al., 2012; Hill, 2012; Hughes Clarke et al., 2014) also indicates low ac- The longshore drift-fed submarine canyons have steep slopes cumulation rates at the mouth of the rivers while the longshore drift (~10–15°) and are located less than 1 km from the shoreline. These sys- bedforms (transverse bars) indicate remobilization of sediment along- tems can thus intercept sediment transported alongshore and remobi- shore. Transport rates by longshore drift have thus outpaced the rate lize them downslope. On the Québec South Shore, the slopes of the of sediment supply that could form large mass movements due to Laurentian Channel can also be steep (~10°) and could act as routes to high rates of sedimentation. Where sediment supply is slightly higher, remobilize sediments downward. However, canyons in this sector can- such as at the mouth of the Betsiamites and Godbout Rivers, undulations not form by intercepting longshore drift sediments because the shore- and gullies are observed on the slope. These undulations and gullies are line is located more than 5 km away from the slope break. Therefore, restricted to the nearshore environment. The undulations are most like- longshore sediment transport occurs several kilometers away from ly due to local small instabilities since hyperpycnal currents cannot form the shelf break. The Québec South Shore is rather incised by mass move- due to low sediment suspended concentration in the rivers. The slightly ments at different sites (Pinet et al., 2015). Unlike the Tadoussac can- higher rates of sediment supply at the mouth of these rivers can also be yons, mass movements on the South Shore did not retrogress upslope due to important coastal erosion at the mouth of the deltas (Bernatchez to form canyon systems. and Dubois, 2004) and more accumulation on the coastal shelf (Fig. 9D), On the Québec North Shore, between Les Escoumins and rather than to higher supply from rivers. Manicouagan, the width of the coastal shelf is similar to the South The river-fed canyon observed at the mouth of the Les Escoumins Shore (≥5 km wide) and numerous submarine channels formed on River is unrelated to the construction of a delta (Fig. 7). This canyon oc- the seafloor. In this sector, sediment supply during deglaciation most curs on a steep slope on the shoulder of the Laurentian Channel. Its axial likely played a major role in forming the channels by hyperpycnal channel contains crescentic bedforms similar to those observed on the flows. The Québec South Shore has a watershed that reaches longshore drift-fed canyons of Les Escoumins (6 km apart) (Fig. 8). 9000 km2 while the Québec North Shore's watershed reaches High backscatter values on the fan and thalweg also reveal the coarse 183,000 km2. The Manicouagan River by itself has a larger watershed nature of its surface sediments (Fig. 7B). Therefore, we suggest that it (~45,000 km2) than the entire Québec South Shore. Two of the largest was also active during the 1860 or 1870 earthquake that likely led to rivers on the Québec South Shore, the Mitis and Rimouski rivers, have the turbidite layer observed in the longshore-drift fed canyons. Al- 1600 and 1800 km2 watersheds respectively. Sediments from these riv- though there is no sedimentological evidence for its episodic activity, ers were capable of forming channels on the slope but they did not en- its geomorphology shows many similarities with the longshore drift- large them as much as those of the Québec North Shore. The Québec fed canyons of Les Escoumins. Their recent activity appears to be closely South Shore rivers rather deposited their sediments on the gentle slopes related. of the coastal shelf and likely led to rarer gravity flowsthanontheQué- bec North Shore due to lower sediment concentrations in rivers during 5.1.4. Sediment-starved canyons deglaciation. A similar watershed to the Mitis and Rimouski rivers is ob- Having no sediment input at their heads, the Pointe-des-Monts sub- served on the northern shore for the Godbout River. However, unlike marine canyons are the only sediment-starved canyons of the LSLE. the Rimouski and Mitis rivers, the Godbout River directly delivers sedi- However, as they are located on a steep slope, slope instabilities and hy- ment to the slope of the Laurentian Channel due to the absence of a drodynamic processes can remobilize their enclosing sediments, lead- coastal shelf, leading to a direct sediment transfer from land to the Lau- ing to active gravity flows. Normandeau et al. (2014) provided rentian Channel (e.g., Babonneau et al., 2013). evidence that the Pointe-des-Monts canyons are currently active even without sediment supply at their heads through repeated multibeam 5.3. Controls on recent sedimentary processes bathymetry. The presence of crescentic bedforms that were displaced upslope between 2007 and 2012 suggest they are related to supercriti- Stow et al. (1983) and Bouma (2004) suggested that the activity of cal currents (i.e., cyclic steps; Cartigny et al., 2011; Hughes Clarke et al., submarine canyons is influenced by tectonics, the type and volume of 2014). These results, combined with higher backscatter values over the sediments, RSL changes and climate. In the LSLE, the proximity to the canyons indicate that sediments were transferred downslope recently. coast (tectonic setting) is important for the presence of a canyon or A core collected over the submarine canyons contains fine sands while channel. However, there is no clear relationship between the proximity a core collected just outside the fan contains the typical silty sediment to the coast and the activity of gravity flows in the last centuries. The of the Laurentian Channel (Fig. 6E,F). The two cores were collected at longshore drift-fed canyons (Tadoussac and Les Escoumins) show evi- approximately the same depth (~300 m). The sandy sediments in the dence of episodic activity while the sediment-starved canyons canyon thus reflect the presence of active gravity flows, possibly related (Pointe-des-Monts) show evidence of frequent activity (less than to the migration of the bedforms. The Pointe-des-Monts canyons are the 5 years). The head of these canyons are all located within 1 km from 16 A. Normandeau et al. / Geomorphology 241 (2015) 1–18 AB 0 Escoumins (river) 0 Escoumins (longshore drift) Pointe-des-Monts No evidence of activity -50 Tadoussac -50 Godbout Evidence of episodic activity -100 Betsiamites Saguenay -100 Frequently active Portneuf -150 Manicouagan -150

-200 -200 Depth (m) -250 -250

-300 -300

-350 -350 0 5 10 15 20 25 0 5 10 15 20 25 Distance (km)

Fig. 14. Plots of bathymetric profiles grouped by A) region and B) evidence of recent activity. the shoreline but show different rates of activity and different volumes characteristic of the LSLE is the activity of the sediment-starved subma- of sediments at their heads. Furthermore, the head of the channels in rine canyons. Normandeau et al. (2014) suggested that slope failures the river-fed delta systems are also located near the shoreline (less and internal tide-related currents could be responsible for the observed than 2 km in some cases) and do not show recent gravity flow deposits activity. However, no slope failures were observed during the 5-year re- and bedforms. Therefore, a proximity to the sediment source cannot peated bathymetric surveys, and internal tide-related currents are not solely explain recent submarine gravity flow activity in the LSLE. known for generating crescentic bedforms in other environments. The Plotting the slopes of each channel and canyon system (Fig. 14)re- comparison of canyons and channels located in the LSLE also indicates veals that all the inactive channels are located on the lower slopes, that the Pointe-des-Monts canyons are the only frequently active sys- while the canyons showing evidence of recent activity are located on tems despite the absence of sediment supply at their head, outlining the steepest ones. Slope is an important factor for the activity of subma- the fact that unique and poorly understood processes are ongoing in rine canyons (Piper and Normark, 2009). Therefore, the steeper the this sector. There is a need to investigate the triggering mechanism for slope, the stronger are the probabilities that gravity flows will remobi- generating turbidity currents in the Pointe-des-Monts sector, when lize sediments. The importance of the slope compared to the proximity other sectors of the LSLE having similar physical settings and more sed- of a sediment supply in modern times is probably due to the fact that iment supply (Les Escoumins for example) reveal little or no sign of re- sediment supply is generally low in the LSLE. Compared to other regions cent gravity flow activity. of the world, the Laurentian Highlands (Québec North Shore) consist of very hard igneous rocks in a ~72% proportion, while sand and gravel 6. Conclusions only account for ~5% of the total Québec North Shore watershed. With- out rivers delivering large volumes of suspended sediment to the coast, The LSLE is a unique Quaternary sedimentary system that allowed coastal erosion would be the main source of sediment supply today. Re- the initiation and development of morphologically contrasting subma- cent studies emphasize that the LSLE coastline is currently undergoing rine canyons and channels within a semi-enclosed basin. The diversity intense coastal erosion (Bernatchez and Dubois, 2004). However, coast- of types of canyons and channels located in a similar tectonic, eustatic, al erosion does not appear to be sufficient to maintain frequent canyon climatic and morphological setting allowed the observation of the dif- activity as observed on the Californian coasts (Smith et al., 2005; Paull ferent factors controlling their distribution and their modern sedimen- et al., 2013) or the Bay of Biscay coasts (Mulder et al., 2012; Mazières tary processes. The combined geomorphological and surface sediment et al., 2014) where annual events of sediment transfer were recorded. analysis of the canyons and channels of the LSLE lead to the following Furthermore, eroded coastal sediments are probably redistributed conclusions: along the deltas because there is no evidence for strong sediment trans- port near the canyons of Tadoussac, Les Escoumins and Pointe-des- 1) Four genetic types of submarine canyons and channels are observed Monts. These low rates of sediment transport limit the activity of the in the LSLE based on their feeding sources: glacially-fed, longshore Les Escoumins and Tadoussac canyons because sediment is not drift-fed, river-fed and sediment-starved systems. The river-fed sys- transported to their head as frequently as in other longshore drift-fed tems are further divided into two sub-categories, based on their re- submarine canyons offshore Gabon (Biscara et al., 2011), California lationship to a delta or not. (Smith et al., 2005, 2007)orFrance(Mazières et al., 2014). Furthermore, 2) The larger river watersheds and the proximity of the slope break to the low abundance of sediment at the mouth of rivers flowing into the the shore were shown to be responsible for the higher number of LSLE can explain the inactivity of the river-fed delta systems (Inland submarine canyons and channels on the Québec North Shore rather Waters Directorate, 1978). All the rivers of the Québec North Shore ex- than on the Québec South Shore of the LSLE. cept the Godbout River have less than 50 mg L−1 of suspended sedi- 3) The genetic type of canyons and channels defines their modern sed- ment. Gravity flows cannot form if the concentration of suspended iment dynamics. While glacially-fed and delta related river-fed can- sediment is insufficient to create dense water that can flow by yons and channels are currently inactive, longshore drift-fed hyperpycnal currents. The channels are thus being buried by normal canyons and sediment-starved canyons show evidence of episodic settling of suspended sediment or are remobilized by bottom currents. or frequent activity. In summary, lower slopes need higher sediment supply to generate 4) Unlike other marine deltas along the glaciated coastline of Canada, hyperpycnal currents or slumping whereas steeper slopes do not need the deltas of the Québec North Shore cannot generate gravity flows as much sediment supply in order to transfer continental sediments to today because sediment concentration from rivers is too low to pro- the Laurentian Channel. Slopes appear to be the main factor controlling duce hyperpycnal flows and accumulation rates are too low to pro- the episodic activity of turbidite systems in the LSLE today. duce frequent delta-slope failures. Sediment supply strongly limits the very recent activity of some sub- 5) Longshore drift-fed systems are episodically active today (N100 yrs marine canyons and channels in the LSLE. However, an intriguing recurrence?), probably due to the occurrence of large earthquakes A. Normandeau et al. / Geomorphology 241 (2015) 1–18 17

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The presence of bedforms, the analysis of surficial sediments, sub- 19th International Sedimentology Congress, Geneva, Switzerland. bottom profiles and backscatter intensity values all allow us to Dionne, J.-C., Occhietti, S., 1996. Aperçu du Quaternaire à l'embouchure du Saguenay, Québec. Géog. Phys. Quatern. 50, 5–34. define whether submarine canyons and channels are active or Dowdeswell, J.A., Evans, J., O Cofaigh, C., Anderson, J.B., 2006. Morphology and sedimen- inactive. tary processes on the continental slope off Pine Island Bay, Amundsen Sea, West Antarctica. Geol. Soc. Am. Bull. 118, 606–619. Dubois, J.-M.M., Bernatchez, P., Bouchard, J.-D., Daigneault, B., Cayer, D., Dugas, S., 2005. Acknowledgments Évaluation du risque d'érosion du littoral de la Côte-Nord du Saint-Laurent pour la période de 1996–2003. Conférence régionale des élus de la Côte-Nord (291 pp.). fi Ducassou, E., Migeon, S.B., Mulder, T., Murat, A., Capotondi, L., Bernasconi, S.M., Mascle, J., We sincerely thank the captain, crew, and scienti c participants of 2009. Evolution of the Nile deep-sea turbidite system during the Late Quaternary: in- the COR1203 cruise on board the R/V Coriolis II. This study was support- fluence of climate change on fan sedimentation. Sedimentology 56, 2061–2090. ed by the Fonds québécois de la recherche sur la nature et les technologies Duchesne, M.J., Long, B.F., Urgeles, R., Locat, J., 2003. New evidence of slope instability in the Outardes Bay delta area, Quebec, Canada. Geo-Mar. Lett. 22, 233–242. and Geotop through a scholarship to A.N., by the Natural Sciences and Duchesne, M.J., Pinet, N., Bédard, K., St-Onge, G., Lajeunesse, P., Campbell, D.C., Bolduc, A., Engineering Research Council of Canada through Discovery and Ship- 2010. 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