Estuaries are semi-enclosed coastal bodies of water that have a free connection with the open sea and, within which, seawater is diluted with fresh water derived from land drainage. The location of estuaries at the transition zone between the fluvial and continental shelf environments implies an inherent geological complexity because of the interaction of terrestrial and marine sedimentary processes (Dalrymple and Choi 2007). Several parameters exhibit major changes both along and across the strike of estuaries, including depth, slope, intensity of tidal waves, salinity, and amount of suspended solid, among others. The variability of these parameters and their interaction may lead to a complex sedimentary architecture and influence sediment characteristics such as grain size, sedimentation rate, type of sedimentary structures and geomorphological signature of sedimentary accumulations.

The St. Lawrence River system is a major hydrographic feature of North America and is characterized by a large drainage area including and originating at the Great Lakes. In the St. Lawrence River Estuary, Quaternary sediments vary from less than 30 m to over 400 m in thickness (Duchesne et al. 2010). Previous studies of these sediments mainly investigated their overall architecture and the impact of the last glacial episode on the development of the Quaternary succession (e.g. D’Anglejan and Brisebois 1978; Syvitsky et al. 1983; Syvitski and Praeg 1989; Massé 2001; St-Onge et al. 2003, 2008; Duchesne et al. 2010). In this contribution, we focus on the geology of the seafloor and shallow subsurface. This approach became conceivable through the analysis of recently acquired geophysical (multibeam bathymetry and very high-resolution seismic reflection data; Pinet et al. 2011a) and geological (samples and photographs; Campbell 2007; Bolduc 2008) datasets for the St. Lawrence River Estuary. Through a basin-scale approach, the current study attempts to provide a snapshot of present-day sedimentation patterns and accumulations in this large estuarine sedimentary system. Such a snapshot may serve as an analogue for other sedimentary successions for which the physiographic, hydrographic and geologic conditions are uncertain.

The St. Lawrence River Estuary is among the largest in the world, extending for a distance of over 400 km, a width up to 50 km, and a depth of up to 350 m (Fig. 1). On the south shore, the Appalachian Mountains (Fig. 1A) exhibit relatively smooth relief, and elevation is generally less than a few hundred metres. The north shore, under-lain by Grenvillian metamorphic rocks of the Canadian Shield (Fig. 1A), rises steeply and reaches elevations of more than 1000 m south of the Saguenay River.

The St. Lawrence River Estuary is traditionally divided into two segments on the basis of bathymetric signature (Fig. 1B); the Lower Estuary, which extends from the mouth of the Saguenay River northeast to Pointedes-Monts, and the Middle Estuary, which encompasses salt water southwest of the Saguenay River. A distinctive feature of the Lower Estuary is the Laurentian Channel, which extends more than 900 km from the confluence with Saguenay River to the edge of the Scotian Shelf in the Atlantic Ocean (Fig. 1A). In the southwest part of the Lower Estuary, the Laurentian Channel exhibits the morphological signature of an incised-valley bounded by steep escarpments, whereas to the north, it is characterized by more shallowly dipping margins. Differential erosion has traditionally been considered as the main geological process responsible for the parallelism of the Laurentian Channel with bedrock boundaries, as it is hosted by softer rocks of the St. Lawrence Platform that are less resistant to erosion than the adjacent Grenvillian and Appalachian rocks (Pinet et al. 2008). The Middle Estuary is shallower and characterized by a ‘rougher’ bathymetry, including a network of interconnecting channels and basins between northeast-striking, kilometre-wide banks and islands.

The mean tidal range of the study area increases toward the southwest, from 3.0 m near Pointe-des-Monts to 4.4 m at Tadoussac (extreme high water of 4.9 and 6.4 m respectively; Fisheries and Oceans Canada 2010). The estuary is characterized by a salinity-induced density stratification leading to the development of an estuarine circulation in which the denser, more saline bottom water flows slowly (~ 0.5 km/day) landward at the bottom of the estuary, whereas fresher water moves rapidly (~ 10–20 km/day) seaward at the surface (Fig. 1C; Syvitski et al. 1983). Intensive mixing between water layers occurs at the head of the Laurentian Channel, which is characterized by a sudden rise of the seabed near Tadoussac (Fig. 1C).

During the spring and summer, the water masses have a well-developed thermocline characterized by three layers (Syvitski et al. 1983; Fig. 1D). The minimum temperature (2°C) is found in the intermediate water layer (50–250 m). Salinity of the surface layer (0–50 m) ranges from 26–31 psu (practical salinity unit) and steadily increases to 34.5 psu on the river bottom (Syvitski et al. 1983; Fig. 1D). Since the 1930s, a steady decrease of oxygen concentration in bottom waters has been documented (Gilbert et al. 2005).

The main sources of fresh water are the St. Lawrence River (12 000 m³/s near Tadoussac), the Saguenay River (2100 m³/s) and three large rivers on the north shore ─ the Betsiamites, Aux Outardes and Manicouagan rivers, which have a combined discharge of 1600 m³/s (Fisheries and Oceans Canada 2010). The St. Lawrence River alone transports more than 6.5 million tonnes of suspended solid into the Lower Estuary annually (Fisheries and Oceans Canada 2010). North of Saguenay River, most of these sediments are deposited on the floor of the Laurentian Channel, which previous studies have indicated to consist mainly of dark grey, bioturbated, fine silts (Nota and Loring 1964; St-Onge et al. 2003). Estimates of recent sedimentation rates range from 0.70 cm/yr off Rimouski to 0.22 cm/yr in the northern part of the study area (Smith and Schafer 1999).

Complete multibeam bathymetry coverage for water depths greater than 30 m is available between Îles aux Coudres to the southwest and Pointedes-Monts to the northeast (Figs. 1B, 2A). The bathymetric coverage includes several surveys conducted between 1997 and 2007 by the Canadian Hydrographic Service using the Canadian Coast Guard Ship Frederick G. Creed and the bathymetric launch Guillemot. Multibeam systems onboard the F.G. Creed consisted of a Kongsberg EM-1000 (before 2005) and a Kongsberg EM-1002 (since 2005). On the Guillemot, multibeam systems consisted of a Kongsberg EM-3000 (before 2005) and a Kongsberg EM-3002 (since 2005). In addition to multi-beam bathymetry, very high-resolution seismic reflection data (not shown) have been employed to image the shallow subsurface of the St. Lawrence River Estuary. These seismic data, which were collected in 2007 using a Huntec DTS source with a peak frequency of ~900 Hz, have a vertical resolution of ~0.8 m (Campbell 2007; Bolduc 2008).

One hundred and forty-five seafloor sediment samples (86 grab samples and 59 short gravity cores less than 2 m long) and 79 seafloor photographic transects (consisting of 8 to 21 images) were collected at strategic sites to characterize surficial units (Campbell 2007; Bolduc 2008; Fig. 1A). Seafloor samples and photographs provide the critical link between the geomorphological units identified on multibeam bathymetry and the lithological characteristics of sediments.

During this study, surficial geology units (Fig. 2B) were primarily defined on the basis of their geomorphological signature (Pinet et al. 2011a). Surficial units consist either of homogeneous and well-sorted sediments (such as the Holocene Laurentian Channel fine silt unit; see below) or sediments with variable grain-size characteristics (such as the Holocene Laurentian Channel-margin sand to silt unit; see below). In most cases, surficial geology units have relatively sharp boundaries on the multibeam bathymetry map. Where not obvious on bathymetric images, unit boundaries were defined on very high-resolution seismic lines and extrapolated from one line to the other. This was the case for the lower and upper boundaries of the latest Pleistocene to early Holocene sediment unit; these boundaries are easily identifiable on several seismic lines yet exhibit no clear geomorphological signature. Very high-resolution seismic lines were also critical for the interpretation of linear features associated with buried structures such as bedrock ridges (Pinet et al. 2011a).

Short-term marine hydrodynamics is an important factor controlling sedimentation in estuaries (Dalrymple and Choi 2007). Because of the narrowing of the St. Lawrence River upstream of Saguenay River and the shallowing of the river floor – two characteristics largely inherited from bedrock geology (Pinet et al. 2008) – tidal height is amplified in the Middle Estuary compared to the Lower Estuary. In the Middle Estuary high-energy environment, which is characterized by a complex interplay of waves and tides, the Holocene sediment fill is reduced or absent (Fig. 7A).

Upstream of Saguenay River, bottom currents are strong enough (generally 50–100 cm/sec, locally up to 150 cm/sec; D’Anglejan and Brisebois 1978) to locally generate large-scale migrating submarine bedforms in sandy material (Syvitski et al. 1983; Bolduc and Duchesne 2009; Fig. 6). The sand may have been remobilized either from the bathymetric high close to the Saguenay River mouth, which is more exposed to currents, or from Quaternary deposits on the shore that have been reworked by tides, fluvial erosion and ice rafting. However, these bottom currents are unlikely to have mobilized larger cobbles, which are major constituents of the seafloor in some areas (Fig. 4E). Alternative hypotheses for the formation of coarse-grained deposits include: 1) erosion of the fine-grained fraction by tidal re-suspension and formation of a residual seafloor comprising only the coarse fraction; 2) remobilization of coarse-grained sediments from the shore, followed by relatively short transport and sedimentation during the yearly ice break-out period, as documented by Dionne (1993); and 3) remobilization by storm waves of coarse-grained pre-Holocene sediments forming the shallow (10–100 m) part of the St. Lawrence seafloor close to the mouth of Saguenay River.

North of Saguenay River, in the Laurentian Channel, sediments are interpreted to have been deposited from the suspended load present in the Lower Estuary. At the depths of the Laurentian Channel seafloor (>250 m), turbulence from tides and storm surges is not intense enough to cause re-suspension of fine silts, and erosion by post-sedimentation processes is minor.

In contrast, and as proposed by Loring and Nota (1973), the grain-size distribution of sediments deposited on some relatively shallow parts of the Laurentian Channel margins can be explained by turbulence from tides and storm surges, causing re-suspension of fine silts. The grain-size distribution of sediments on the margins is also a function of local sedimentary sources (i.e. St. Lawrence River tributaries) that may carry significant amounts of sediment. Fluvial input generally supplies fine-grained sands and muds whereas the coarser sands are supplied by bed and shore erosion.

Compared with other estuaries, in which facies characteristics and distribution record a complex but often progressive transition of physical processes from river-dominated (river current) to marine-dominated (waves and tides), the St. Lawrence River Estuary is characterized by a sharp variation in sedimentary processes in the Middle versus the Lower Estuary. This variation is a consequence of contrasting hydrodynamic conditions related to major changes in the shape of the St. Lawrence River Estuary. In the relatively shallow (generally less than 100 m) Middle Estuary, bedload reworking by bottom currents is the main process and precludes the deposition of Holocene fine-grained sediments. In some areas, erosional processes predominate over sedimentation and pre-Holocene sediments form the seafloor. In the Lower Estuary (northeast of Saguenay River), sedimentation from suspended load is the predominant mechanism. Surficial sediments are Holocene in age and correspond to fine silt except close to local sedimentary inputs and in shallow areas where turbulence from tides and storm surges may cause re-suspension of fine-grained sediments.

A 1:250 000 surficial geology map as well as twelve 1:50 000 shaded seafloor relief maps are available for free download at: http://geoscan.ess.nrcan.gc.ca/star-web/geoscan/servlet.starweb?path=geoscan/geoscan_f.web