Based on high-resolution multibeam-sonar data and low-resolution (GEBCO) bathymetry data, we classify the geomorphology of Canada’s Pacific margin within the four bioregions designated by Fisheries and Oceans Canada for management of biological resources. We designate 14 units. Nine continental shelf units are fiords, bedrock terrain, offshore banks, Haida Gwaii platform, Haida Gwaii shelf, Vancouver Island shelf, incised shelf, glacial trough, and major delta. On the continental slope, we identify the canyon zone, the accretionary wedge (off Vancouver Island), and the transform (Queen Charlotte Fault) terrain. The abyssal zone is treated as a single unit with two components: seafloor-spreading terrain, and abyssal plain with fans, seamounts, and channels. Hexactinellid sponge reefs of various morphologies are found in three of the continental shelf geomorphic units and cover up to 10% of the seafloor in the glacial trough category. Examples based on multibeam sonar imagery are used to display the chief characteristics of the 14 units, as well as the geomorphic diversity within them. Compared with Canada's east-coast glaciated passive margin, geomorphic similarities include: (1) the panoply of glacial landforms and (2) shelf terrain dissected by sub-glacial meltwater. Major differences include: (1) the presence of unique “tectonic” terrains on the Pacific continental slopes; (2) hexactinellid sponge bioherm reefs that are unique to the Pacific margin; (3) the absence of glacio-tectonic terrains on the Pacific shelves; and (4) the absence of “classic” trough-mouth fans on the Pacific margin.

Holland (1964) showed that the British Columbia landscape could be subdivided into units that were topographically alike and that landforms could be described in such a way that their origins might be understood. Here we adopt an analogous approach to classify the submarine areas of British Columbia lying within the bioregions defined by the Department of Fisheries and Oceans Canada (DFO). The marine spatial planning framework for Canada’s national network of Marine Protected Areas (MPAs) comprises 13 ecologically defined “bioregions” that encompass Canada’s oceans and the Great Lakes (Department of Fisheries and Oceans 2009). Canada’s Pacific margin is divided into four bioregions (Fig. 1), namely: (1) the Strait of Georgia, (2) the southern Shelf, (3) Offshore Pacific, and (4) the Northern Shelf.

The shelf, slope, and deep water of the four bioregions have been foci of investigations over many decades and the surficial geology of the shelf is well understood (e.g., Barrie 1988; Barrie et al. 1991). It is commonly interpreted in terms of the surficial geology units typical of a glaciated shelf, i.e., ice-proximal, and ice-distal glacigenic sediments overlain by postglacial sediments. Over the past several decades, extensive mapping of the continental shelf using multibeam sonar has revealed seafloor morphologic complexity (e.g., Greene and Barrie 2011; Barrie et al. 2012) that was not evident on surficial geology maps based on sparse track lines.

The objective of this paper is to offer a perspective in which the seafloor physiography takes precedence over surficial geology. The specific goals are (1) to develop a geomorphic classification of the British Columbia seafloor within the bounds of the four DFO bioregions; (2) use a series of vignettes based on high-resolution multibeam sonar data to demonstrate the salient characteristics of the geomorphic units and the variability within them; and (3) compare the geomorphology of the British Columbia active margin with that of Canada’s Atlantic passive margin, in particular the Newfoundland–Labrador Shelves bioregion, recently classified geomorphically by Shaw et al. (2023a).

The study area (Fig. 1) is the continental shelf, the continental slope, and the abyssal plain, within the bounds of the four British Columbia bioregions. Shelf-crossing troughs shallow near the shelf break. Narrow shelves off Vancouver Island and Haida Gwaii are fringed by canyons, below which the terrain on the lower continental slope is highly irregular. Most of the abyssal plain has extremely low relief, but irregular terrain is present in the form of seafloor-spreading ridges, seamounts, and submarine channels.

The study area is located on an active margin encompassing boundaries between major plates, specifically the Pacific Plate, the North American Plate, Explorer Plate, and Juan de Fuca Plate (Riddihough et al. 1980; Rohr and Furlong 1995). The Queen Charlotte Transform Fault off Haida Gwaii separates the Pacific and North American plates and is associated with complex geomorphic terrains (Barrie et al. 2013; Greene et al. 2019). The Cascadia subduction zone off Vancouver Island results from convergence of the Explorer and Juan de Fuca plates with the North American Plate, and contains complex terrains that differ in style from those on the transform fault to the north. The Juan de Fuca Ridge is a mid-ocean spreading centre that is associated with irregular and complex relief features on the abyssal plain (Underwood et al. 2005).

During the local Last Glacial Maximum parts of the shelf were occupied by grounded ice (Herzer and Bornhold 1982; Howes 1982; Blaise et al. 1990; Clague and James 2002; Mathewes and Clague 2017; Shaw et al. 2019; Hebda et al. 2022; and others). Josenhans (1994) showed evidence of grounded ice in the three shelf-crossing troughs in Queen Charlotte Sound, an area not ice-covered in other reconstructions. The southern part of the independent ice cap on Haida Gwaii formed piedmont lobes (Shaw et al. 2019). In the far south, convergent flow into the Strait of Georgia terminated in the Puget Lobe and the Juan de Fuca Lobe (Clague and James 2002). Several glacial refugia have been proposed, notably the Hecate refugium off Haida Gwaii (Mathewes 1989; Mathewes et al. 2015; Mathewes and Clague 2017), and others in northwest Vancouver Island (British Columbia Ministry of Environment, Lands and Parks 1997; Hebda et al. 2022).

Relative sea-level histories vary from the fiord heads, which experienced relative sea levels up to +200 m upon deglaciation, to the mid- and outer-shelves where relative sea-level lowstands were as low as −150 m in places (Barrie and Conway 2002; Hetherington and Barrie 2004; Clague and James 2002; Fedge and Josenhans 2000; Hetherington et al. 2003, 2004; James et al. 2009; Josenhans 1994; Josenhans et al. 1993, 1995, 1997; McLaren et al. 2014; Shugar et al. 2014 and references therein).

Seabed mobility and sediment disturbance on the shelf portion of the study area due to tidal currents and waves on the shelf portion of the study area is described by Li et al. (2021). The highest seabed disturbance index (SDI) is found east of Haida Gwaii in Queen Charlotte Sound, on the three banks in Queen Charlotte Sound, and on the innermost shelf off Vancouver Island.

Major sources for this classification are several compilations of bathymetry, namely (1) an unpublished Canadian Hydrographic Service (CHS) digital elevation model based on single-beam soundings; (2) GEBCO bathymetry data (GEBCO Compilation Group 2020); and (3) the “Canada west coast topo-bathymetric digital elevation model” (Kung et al. 2023), a regional model gridded at 10 m spacing and based on multibeam bathymetry, single beam, and other data. Interpretation of geomorphic features was facilitated by on-line access to Geological Survey of Canada seismic data that have been scanned and converted into SEG-Y format.

The approach to classification broadly follows the hierarchical approach of Emery and Uchapi (1972) whose system included first-order elements (land and ocean) and second-order elements (continental shelves, continental rises, abyssal hills, etc.). The basic geomorphic units in our classification are obtained by subdividing the second-order elements into their third-order components. This results in nine units on the continental shelf, three on the continental slope, and two on the abyssal plain. In addition to an overview of each of the units, we describe representative examples based on multibeam sonar mapping. For the Offshore Pacific bioregion (abyssal plain), for which high-resolution multibeam bathymetry data are not available, we are forced to adopt a slightly differing approach to describing geomorphology.

The boundaries between geomorphic units were drawn manually, unlike the semi-automated classification approach that has been used in some marine areas (e.g., Nanson et al. 2022). Additionally, the goal of this paper was not to produce a map of the entire four bioregions, but rather to illustrate the geomorphic diversity with illustrative examples. A future mapping approach might well be more systematic and make use of international classifications such as that of Dove et al. (2020) and Nanson et al. (2023). 

4.1. Geomorphic units on the shelf, slope, and abyssal plain

Nine geomorphic classes are distinguished on the continental shelves, three on the continental slope, and two in deep water (Table 1).

4.2. Unit 1: fiords

The fiords of British Columbia are deep, elongated troughs that extend far inland. During the Quaternary Period (2.6 myr to present), they were conduits for grounded ice leaving the interior of the Cordilleran Ice Sheet (Margold et al. 2013). Retreat was punctuated by standstills, with ice margins initially halting at fiord mouths and subsequently at sidewall constrictions. The final glacial phase involved infilling by thick glaciomarine sediments. Upon deglaciation, relative sea levels were high (Clague 1985), and then dropped steeply. Fiord troughs were then modified by postglacial processes such as bottom currents, mass transport, and fluvial deposition.

The fiords encompass archetypal fiord morphologic characteristics (Syvitski et al. 1987; Dowdeswell et al. 2016), including channel networks, great length/width ratio, steep sidewalls, thick fills of glacial and postglacial sediments, mass-transport landforms, submarine fan deltas (Prior and Bornhold 1989; Shaw et al. 2017), submarine channels (e.g., Conway et al. 2012), transverse submarine moraines, and fiord-head deltas (Stacey and Hill 2016). The wide range of submarine slope failures (e.g., Shaw and Lintern 2016; Shaw et al. 2017) include failures on fiord-head deltas (Prior et al. 1982), and development of large bedrock slumps (sackung) (Conway and Barrie 2015). Notably, the fiords of British Columbia have been test beds for understanding turbidity currents, with the research at Squamish delta, 40 km distant from Vancouver providing insights into the origin of bedforms on the delta front (Hughes Clarke 2016).

The example (Fig. 2) is part of the mainland Kitimat fiord network (Bornhold 1983; Shaw and Lintern 2016; Shaw et al. 2017), which extends ∼140 km inland from the −205 m sill. A series of sub-basins are separated by transverse submarine moraines. With transverse submarine moraines (A) being characterized by high backscatter gravelly substrates and intervening basins by low backscatter (mud), there are alternating textural variations throughout the fiord system (see Fig. 2B, Shaw et al. 2017). While the basins have little to no relief (B), there are exceptions. Thus, the channel at C at a depth of 530 m is current scoured, resulting in exposure of glaciomarine sediment, with high backscatter muddy gravel lag at the seafloor (see Fig. 6, Shaw et al. 2017). Only a short distance away, 80 m thick sediment drifts of gas-charged postglacial mud are accumulating (e.g., D). The steep sidewalls are imprinted by gullies in many areas (e.g., E). A singular feature on this figure is the large “sackung” block at F, which results from collapse of the fiord sidewalls (Conway and Barrie 2015).

As will be seen with most of the geomorphic units, because of the geomorphic diversity of fiords, this imagery is a “snapshot” rather than a characterization of the class. A classification of the Kitimat fiord network (Shaw and Lintern 2016) includes 13 classes: two types of sidewalls, two types of fiord floor, moraines, deltas, fan deltas, four types of mass transport landforms, bioherms, and anthropogenic terrain.

4.3. Unit 2: Bedrock terrain

Bedrock terrain has irregular topography and high relief and is found in water depths up to several hundreds of metres in a fringe along mainland coasts. Glacial landforms are superimposed, including those formed parallel to former ice flow (flutes, megaflutes, drumlins, and crag and tails). Moraines that formed transverse to former ice-flow directions are most substantial near modern coasts, where a major ice margin was established following retreat from the shelf (e.g., see Figs. 6 and 7 in Shaw et al. 2019). Glaciomarine sediment deposited from meltwater plumes is draped over the bedrock in places. Glacial landforms and sediment drapes have high backscatter (gravel and muddy gravel). The seafloor in bedrock terrain is thus a mosaic of morphologies and textures (e.g., see Greene and Barrie 2011), and outcrops are rare. Postglacial reworking and redistribution of glacigenic sediments result in areas of low-backscatter sand with bedforms (Barrie et al. 2005) and deposits of finer-grained sediment in depressions.

The example (Fig. 3a) is in Queen Charlotte Sound, just seaward of the Kitimat fiord network (Shaw and Lintern 2016; Shaw et al. 2017). Bedrock outcrops (A) with relief of 30 m are found on wide platforms averaging 60 m depth. Between the platforms are troughs (B) extending up to 280 m below sea level, with smooth seafloors underlain by postglacial sediment. Landforms created by the southwest-ward flow of Cordilleran ice into Hecate Strait are small crag and tails (C), and an array (D) of mega-scale glacial lineations (MSGL). The latter landforms disappear beneath the blanket of postglacial mud (E) in the adjacent glacial trough. Morainal ridges (F) up to 40 m high and oriented in a northwest direction overlie bedrock. The curved moraine at G is in an area of iceberg-furrowed glacial sediment. Figure 3b, in which coloured backscatter is draped over the gray-scale elevation model, demonstrates the complexity and variability of bedrock terrain. Areas of high backscatter (dark gray/blue) represent bedrock and glacial landforms, and areas of low backscatter (pale gray) are muddy fine sand.

4.4. Unit 3: Shelf banks

The offshore banks in Queen Charlotte Sound were ice-free in the last glacial cycle (Josenhans 1994; Shaw et al. 2019), in contrast to adjacent troughs, which hosted grounded ice that extended to the shelf break. The two examples in this geomorphic class, namely Moresby Bank and Goose Island Bank, are shallow (minimum depths 110 and 30 m, respectively), low-relief platforms that slope gently seawards to a shelf break at a depth of ∼250 m. In contrast to the adjacent troughs, which contain an average of 50 m of Quaternary sediment (Josenhans 1994), they host only a veneer of Quaternary sediments. Bedrock that is exposed or near the surface is part of the late-Miocene to late-Pliocene Skonum Formation, poorly consolidated sand and siltstone (Higgs 1989).

The example (Fig. 4) is on southeastern Goose Island Bank, in water depths of 180–230 m. While the banks are featureless on GEBCO bathymetry, at higher resolution great geomorphic complexity is evident. Escarpment “A” is up to 8 m high, has high backscatter, and is interpreted as bedrock. To its northeast, the seafloor has micro-relief of several metres, and backscatter imagery reveals a mosaic of coarse sediment and bedrock (high backscatter) and fine sediment (low backscatter). On escarpment “B” (up to 8 m high) and to its southwest, backscatter is low everywhere. South of escarpment “B” the smooth seafloor is interpreted as muddy fine sand overlying bedrock. The sediment drift at “C” is at a depth of 220 m and is 8 m thick at its maximum.

Multibeam sonar imagery reveals a plethora of enigmatic landforms on southwestern Goose Island Bank. Scattered chevron-style ridges (Fig. 4b) with high backscatter, relief up to 5 m and length up to 4 km, occur in water depths of 96–260 m. At one location, a ridge is partly overlain by mobile sand bedforms, indicating that it is not an active bedform type. The ridges are provisionally interpreted as residual landforms, resulting from erosion of the Skonum Formation rocks. A second landform type (Fig. 4c) consists of a field of mounds in depths of 120–140 m with a combined area of 15 km2. They are 20 m in diameter, slightly elongated, several metres high, with high backscatter. It is also speculated that these are excavated structures in the Skonum Formation, but alternatively they may be bioherms of an unknown type. An irregular ridge 4 m high and 14 km long (Fig. 4d) on the southeast flank of the bank in a water depth of 160 m has mainly high backscatter, and is provisionally interpreted as a moraine, formed by a lobe of grounded ice on the flanks of an ice stream in adjacent Goose Island Trough.

4.5. Unit 4: Haida Gwaii platform

Unit 4 is defined as a shallow (mostly <30 m), low-relief platform located between Hecate Strait to the east and Haida Gwaii to the west. Part of this platform was glaciated in the last glacial cycle by ice emanating from Haida Gwaii (Clague et al. 1982), and the unglaciated part constituted a glacial refugium (Mathewes et al. 2015; Mathewes and Clague 2017; Shaw et al. 2019). Quaternary sediment cover is relatively thick, as can be seen in coastal exposures (Clague et al. 1982). Seismic surveys show that the platform is underlain by networks of channels, with no surface expression. Glacial and proglacial landforms that may have existed on the platform have been effaced by the combined effects of the postglacial relative sea-level transgression and modern waves and currents, although Barrie and Conway (2016) identified a glacial outwash plain in water depths of 37–48 m.

The southeast margin of the platform, in water depths of 90–175 m, comprises progradational sand bodies (D, Fig. 5a) interpreted as drowned spits (Barrie et al. 2009). At the extreme north, the boundary of the platform is an escarpment with a break of slope at −25 m. Wave–current shear velocity (Li et al. 2021) increases from southeast (deepest parts of the platform) to northwest (shallowest), and the platform has very high levels of disturbance, so much of the seabed is covered by arrays of mobile sandy bedforms with average relief of 5 m.

The example (Fig. 5b) is derived from a compilation of LiDAR and multibeam sonar data. It depicts an area at the north end of Graham Island, offshore from the Argonaut Plain, an area of thick outwash sediments exposed in coastal bluffs (Clague and Bornhold 1980; Clague et al. 1982). This is in a region in which elevated levels of seabed disturbance by waves and currents are expected (Li et al. 2021). The seafloor displays an array of sandy bedforms at varying scales, superimposed on a low-relief, gently sloping platform with surface gravel. Relief on the bedforms ranges up to 10 m at E. The extreme western part of the unit is a 5 m thick littoral sediment wedge organized into two main nearshore bar systems (F) and a strip of 20 m high coastal dunes (G).

4.6. Unit 5: Haida Gwaii shelf

Reconstructions of the glacial history indicate that Haida Gwaii supported its own ice cap, separated from mainland ice (Clague et al. 1982). However, the exact location of the margin of grounded ice on the Haida Gwaii shelf remains uncertain. The fiords of western Haida Gwaii terminate on a shelf that averages only 3–4 km in width, with a maximum width of about 20 km in the vicinity of Rennell Sound, in the north of the area. The shelf platform has an average slope of <2° and is bounded in the west by a shelf break highly indented by canyons.

North of Rennell Sound (Fig. 6, insert), bedrock covers the entire shelf, with relief averaging 5–10 m, but up to 100 m in places (To retain a simple classification, this bedrock zone, and lesser areas of bedrock on the outer shelves of Haida Gwaii and Vancouver Island, has not been designated within the Unit 2 (bedrock) category.). South of Skidegate Channel, the shelf is subdivided into two zones. A smooth, gently sloping outer-shelf platform is strewn with bedrock ridges and knolls with relief up to 30 m. An inner-shelf sediment prism forms a landward-sloping platform 1–2 km wide, with a break of slope at ∼−60 m. This platform is also strewn with bedrock outcrops. Several detached shelf blocks are separated from the shelf so that, off Rennell Sound, the shelf break is at 20 km (200 m depth) beyond which is a 7 km gap, and then an elongate, detached “sliver” of continental shelf, 20 km long and 3 km wide at a depth of 210 m.

The example (Fig. 6) is from the vicinity of Barry Inlet. This part of the shelf is subdivided into the two zones noted above. A gently shelving inner-shelf sediment prism (A) is composed of sand and gravel, and extends 1 km seaward to a break of slope at −70 m, below which a relatively steep escarpment descends to −150 m. Beyond the inner-shelf sediment prism, the outer shelf (B) is 4 km wide with a slope of 1°, and terminates in a shelf break at −240 m. This outer shelf platform is a mosaic of sand and gravel, interrupted by numerous bedrock pinnacles with height averaging 5 m. There are several larger areas of sandy seafloor with no bedrock outcrops (C). The outermost part of the shelf platform (D) has few bedrock outcrops and has mostly low backscatter (i.e., sand).

4.7. Unit 6: Vancouver Island shelf

Like Haida Gwaii in the north, Vancouver Island was a nucleus for glaciation independent of the mainland Cordilleran ice, and several lines of evidence indicate that grounded ice extended onto the continental shelf (see below). However, other evidence suggests that some coastal areas were not glaciated, but constituted glacial refugia (Hebda et al. 2022). Vancouver Island shelf (insert, Fig. 7a) is a gently sloping platform with width increasing from 20 km in the north to 50 km in the south. The shelf edge is an indented break of slope at ∼−200 m. The shelf surface is smooth, except for bedrock outcrops more common on the innermost shelf. A distinctive feature is that the fiord troughs that may extend some distance onto the shelf have been filled from their seaward end by sediments. Thus, Kyuquot Sound (arrowed, Fig. 7a), with depths up to 175 m near its seaward end, has been partly infilled by spillover of sediment from the adjacent 50 m deep shelf. Irregular terrain in several areas is likely indicative of the development of lobate ice margins on the shelf.

Offshore from Checleset Bay (Fig. 7a), a large semi-circular lobe (A) that extends almost to the shelf edge (30 km from the coast) attains 80 m elevation above the surrounding featureless shelf (B). A second arcuate ridge (C) lies inland from the main ridge. Figure 7b shows part of this area as mapped with multibeam sonar. The rough terrain at D has high backscatter and relief up to 4 m and is provisionally interpreted as a glacial diamict (till). These observations are consistent with a lobe of grounded ice that extended onto the continental shelf. Similar but smaller features farther south are also tentatively identified as moraines, showing that ice from Vancouver Island likely extended onto the shelf in places, in addition to the large lobe off Barkley Sound/Juan de Fuca Strait. The area at E on Fig. 7b is interpreted as bedrock, morphologically similar to the inner shelf bedrock zone along the entire shelf as seen on multibeam sonar imagery. The area at F shows the infilling of a fiord entrance (Esperanza Inlet) by spillover of shelf sediment; the sill here is at −25 m.

4.8. Unit 7: Incised shelf

Networks of channels on glaciated continental shelves result from meltwater activity under grounded glacial ice (Kirkham et al. 2022). On eastern Canadian shelves, they occur either as open channels, as on the Scotian Shelf (e.g., Sankarelli 1998) and the Grand Banks of Newfoundland (King 2014; Shaw et al. 2023a), or as buried channels (tunnel valleys) on the Scotian Shelf (Macrae and Christians 2013). Here we focus on the open channels, noting that buried channels with no surface geomorphic expression appear to exist on the Haida Gwaii Platform.

The process of incision by meltwater also operated on Canada’s Pacific margin, in four areas: (1) to the east of Haida Gwaii Platform; (2) off southeast Haida Gwaii; (3) on Cook Bank (off the northeast tip of Vancouver Island); and (4) off Juan de Fuca Strait. While sharing the attribute of shelf incision, the four areas of dissected terrain differ sufficiently in terms of extent, morphology, and inferred genesis that it is necessary to describe them separately. To facilitate comparison, they are shown in Fig. 8, in 100 km square areas at the same scale and with the same colour bar; the relatively shallow area east of Haida Gwaii platform (Fig. 8a) required a separate colour bar.

The first example is located east of Haida Gwaii. On Fig. 8a, area “A” is classified as the Haida Gwaii Platform, area “B” is assigned to the glacial trough category, and “C” indicates the location of the submerged (−90 m) early Holocene delta in Hecate Strait (Barrie and Conway 2002). The incised shelf (D) consists of plateaus with accordant depths averaging −40 m, dissected by channels with maximum depths of 70 m; total area is 1400 km2. Seismic reflection data show that the terrain has been “smoothed” as a result of sediment infill, and the channels are deeper than on the digital elevation model: some channels incised into bedrock reach a maximum depth of −90 m. Several of the large channels appear to drain east, but their depth is variable, and there is no eastward trend of thalweg depth. Based on the limited quality of available data, it is tentatively proposed that the irregular terrain here is partly due to meltwater activity under grounded ice. The adjacent submerged delta (C) at −90 m confirms that the area was subaerial in the late glacial and early Holocene, so drainage eastwards from Haida Gwaii and subsequent inundation and sedimentation also contributed to the complex terrain.

Example 2, southeast of Haida Gwaii, occupies an area of 2500 km2 (Fig. 8b), most of which is a basin (A) that is 50–100 m below the Haida Gwaii Platform to the east. The east margin of the basin is a steep, irregular escarpment notched by 100 m deep channels (B). In the west, the basin connects with the fiords of Haida Gwaii (C). In the southeast, much of the basin floor (D) at 140 m depth is underlain by acoustically stratified (glaciomarine) and unstratified sediments (moraine). Channels incised into bedrock reach a depth of 250 m (see Fig. 9, Shaw et al. 2019). Beyond a single deep channel (E), channels 50–75 m deep splay across a large arcuate lobe (bounded by the yellow line on Fig. 8b).

Shaw et al. (2019) interpret this dissected area as having formed under a piedmont glacier lobe emanating from southern Haida Gwaii (see their Fig. 13). The systems of basins and channels are consistent with erosion by grounded ice and subglacial meltwater. Large pro-glacial lakes developed in basin A (Josenhans 1994; Josenhans et al. 1995), and at least one glacial lake outburst has left a geomorphic imprint in the form of channels and scoured seafloor at “F” (Shaw et al. 2019). The great geomorphic diversity of this area is further shown by the presence of a large arcuate submarine moraine (G) at the entrance to Juan Perez Sound, and a well-preserved sandur plain and fluvial channels (see Fig. 10, Shaw et al. 2019) that formed during the −150 m postglacial relative sea-level low stand.

The third area of dissected terrain is Cook Bank (Fig. 8c), a bedrock platform with an area of 1200 km2. Plateaus with tops around 50 m depth are separated by 25 m deep channels. The channels do not connect in a dendritic network. The largest channel meanders in places and tends to slope northwards; along its length, it has relief variations of almost 20 m. Seismic cross-sections of Cook Bank (Josenhans 1994) show thick deposits of postglacial sand (A) that infill tunnel valleys incised 50 m into bedrock. Overlapping till tongues in adjacent Cook Trough are indicative of retreating grounded ice. The seafloor is highly mobile, and the classification by C. Stacey (personal communication, 2019) shows large areas of bedforms including sand and gravel ribbons, and sand waves. The proposed mechanism for the dissected terrain is that ice sourced in northern Vancouver Island was deflected by the strong flow of Cordilleran Ice in Cook Trough, flowed across the Nahwitti Lowland, as demonstrated by Howes (1982), and reached the bank.

The southern dissected zone (Fig. 8d) occupies 4500 km2 of continental shelf off southern Vancouver Island. As described by Herzer and Bornhold (1982), it consists of a system of deep troughs (up to >400 m below sea level) cut into the Tertiary bedrock, together with morainal ridges and an outwash plain on the unglaciated continental shelf to the northwest. Parts of the irregular topography have been levelled by the postglacial transgression, although the ridge at A is interpreted as a moraine. The troughs are bounded by escarpments 50–100 m in elevation. Illustrations in Herzer and Bornhold (1982) show that the troughs formerly extended farther across the shelf than now.

Herzer and Bornhold (1982) argued that the area was occupied by a large piedmont glacier fed by ice originating in southern Vancouver Island and the Juan de Fuca Strait. The ice extended across the shelf and incised a series of troughs as well as morainal ridges. Topography, including moraines, has been levelled by the postglacial sea-level transgression. Troughs buried up to 400 m below sea level extend completely across the shelf.

4.9. Unit 8: Glacial trough

Canada's Pacific margin glacial troughs are long (100–250 km) wide (∼30 km) depressions with relatively steep sidewalls extending from near modern coasts and shallowing towards the shelf break. They are Dixon Entrance, three troughs in Queen Charlotte Sound (Moresby Trough, Mitchell's Trough, and Goose Island Trough), and the Strait of Georgia/Juan de Fuca Strait. The glacial troughs carried grounded ice from the interior of ice sheets to the shelf break during glacial maxima (Fig. 12, Shaw et al. 2019). They contain thick glacial deposits (Barrie and Bornhold 1989; Barrie and Conway 1999), including recessional till tongues (lenses of glacial diamict) indicative of incremental ice retreat (Josenhans 1994).

The glacial trough category encompasses a great amount of geomorphic variability including: (1) upstream trough areas with streamlined glacial landforms organized into convergent arrays (e.g., Strait of Georgia, Fig. 2, Shaw et al. 2019); (2) long ridges oriented parallel to the former ice flow direction (MSGL); (3) residual ridges and banks resulting from the partial erosion of earlier Quaternary sediments (Barrie et al. 2012; Mosher and Hamilton 1998); (4) morainal ridges that formed on top of streamlined landforms during ice retreat (e.g., Shaw et al. 2019); and (5) glass sponge reefs that favour the glacial substrates found in troughs (). Seafloor texture is thus highly variable, ranging from mud and sandy mud in postglacial sediments to muddy gravel and gravel where glacial materials are exposed at the sea floor.

Two examples illustrate the geomorphic variability of glacial trough terrain. In Hecate Strait, a wide trough at depths of ∼180 m contains a curving array of ridges (Fig. 9a) with relief of 10–20 m, interpreted as mega scale glacial lineations (MSGL), indicative of ice flowing down the trough (Shaw et al. 2017). Superimposed on the MSGL are arrays of small retreat moraines several metres high. They are not discerned using the illumination direction in Fig. 9a but are very evident with illumination direction from the north—see Fig. 4, Shaw et al. (2019). The MSGL and retreat moraines are composed of glacial diamict and are overlain by a drape of glaciomarine mud. Such substrates are ideal for the establishment of glass sponge reefs (), and indeed large areas within the image are occupied by such reefs (A).

The second example (Fig. 9b) is from Goose Island Trough, in Queen Charlotte Sound, at a mean depth of 210 m. Here the glacial diamict in the trough (see cross-section in Josenhans 1994) is imprinted by relict iceberg furrows (A). Glass sponge reefs favour this type of seabed, and in this example (see also ) they cover a large area with mounds up to 15 m above the surrounding sea floor (B). Areas of smooth seafloor are postglacial fine-grained sediment (C). The northwest quadrant of the image is occupied by irregular terrain (D) shallowing from −250 m to −180 m on the flank of Goose Island Bank. The irregular terrain has a terraced appearance, is dissected by leveed channels, and its margin (dashed line, Fig. 9b) has a lobate appearance, with lobes resting directly on the glacial surface. This terrain is not well understood at present, and perhaps indicates mass transport of sediment down the flanks of the glacial trough some time after deglaciation.

4.10. Unit 9: Major deltas

Mapping has uncovered examples of deltas that formed during low stands of relative sea level and are now submerged. The best example is the large delta at the extreme north end of Hecate Strait (within Unit 8, glacial trough) that is graded to −90 m water depth and formed at the end of deglaciation (Barrie and Conway 2002). The modern deltas in the study region (Luternauer 1984), developed in the postglacial period, against a background of geographically varying changing relative sea levels (e.g., see Fig. 8, Clague 1998). The Skeena River delta of northern British Columbia is rather unusual. It has largely infilled a fiord trough and debouches into deeper water at several locations separated by islands (Conway et al. 1996). Multibeam sonar imagery of the several submarine delta fronts shows stacked debris flows dissected by meandering channel systems. The Fraser Delta, much larger and more complex than all the other modern deltas (Clague et al. 1998), has evolved over the past 10 000 years (Clague 1998; Figs. 1 and 7; Mosher and Hamilton 1998) against a background of initial relative sea level fall and a subsequent Holocene rise.

The Fraser River Delta (Fig. 10) has prograded into the Georgia Strait glacial trough. Evidence of the strong ice flow (northerst to southeast) within the trough is evident on McCall Bank (A), where glacigenic sediment deposited prior to the last ice advance has been partially eroded (see Fig. 2, Mosher and Hamilton 1998). The delta front is a concave slope strongly indented by debris flow channels offshore from modern and older tributaries (B). The so-called Foreslope Hills at C are of problematic origin, and were perhaps current formed (Mosher et al. 1995). The irregular terrain at D is the Roberts Bank failure complex, formed by collapse of the delta front. The large lobe of sediment at E is up to 300 m thick.

At the north end of the Vancouver Peninsula, Burrard Inlet was never an outlet for the Fraser River. The entrance to the inlet hosts thick drifts of pock-marked sediment (F) that originated on the delta and has bypassed the headland at (G). To the east, the channel (G) narrows in several places, marked by tidal scour troughs that alternate with relatively thick ebb and flood sediment drifts.

4.11. Unit 10: Canyon zone

The upper continental slope over a distance just under 1000 km is etched by canyons and valleys. Like those on Canada’s Atlantic passive margin (Gao et al. 2022), during glacial phases the canyons hosted turbidity currents that carried glacigenic sediments from the shelf into deep water. Except, perhaps, for canyons in Haida Gwaii, they are less active today. For example, Goldfinger et al. (2017) argue that Barkley Canyon (the most southerly on the margin) is largely relict with little Holocene recharge. Canyons off Vancouver Island extend onto the terrain of the accretionary wedge (Unit 11), whereas those off Haida Gwaii are intersected by the Queen Charlotte fault (Barrie et al. 2013; Harris et al. 2014a). In both instances, sediment transport via canyon systems has been interrupted by complex terrain between them and the abyssal plain. Off Haida Gwaii, in addition to west-facing canyons developed on the margin of the narrow shelf, canyons are developed on the Queen Charlotte Terrace (9 on Fig. 1a), and may be east- or west-facing, depending on location. Because they exist on a tectonically active margins, the Pacific canyons show evidence of submarine slope failure, and exhibit seafloor geomorphic features (mounds, vents) caused by gas venting and gas hydrates (Pauli et al. 2015).

Examples from four areas reveal the geomorphic diversity of canyons. Barkley Channel (Fig. 11a) is located adjacent to the Juan de Fuca-dissected terrain on the shelf. The headwater channels converge into a broad channel (A) that widens downstream to 3 km at “B”. The channel cuts through the irregular terrain of the Cascadian Subduction zone to reach the abyssal plain at a depth of 2400 m. The thalweg is irregular in places, notably at the end of the profile, where a ridge (C) is encountered at a depth of 2000 m.

Figure 11b shows the uppermost part of Barkley Channel, and reveals numerous escarpments, only a few of which are indicated (A). These are evidence of widespread mass wasting. Riedel et al. (2022) reported on some of these, as well as methane escape structures on the seabed of the principal channel.

Figure 11c is from the northern tip of the Vancouver Island shelf, about 20 km distant from the modern coast, and due south of Cook Bank. The shelf break here is −230 m. Within this image the canyons reach depths of 1700 m, and extend beyond the image to depths of ∼1850 m. The “V” shaped canyons have dendritic patterns, with numerous shelf-edge tributaries converging. The thalweg of the principal channel (A) in this example slopes at ∼7°, while slopes of 40° and more are found on the ridges.

The final example (Fig. 11d) is from offshore the south end of Haida Gwaii and illustrates how canyon development is constrained by the Queen Charlotte Transform fault (A). A sliver of continental shelf (B) is separated from the shelf by a canyon system (C) that runs parallel to the coast, and links with a series of small canyons that drain the shelf break (D). The combined channel (E) reaches the abyssal plain at a depth of 2400 m. The truncation of canyons seen at “A” is explored in detail by Barrie et al. (2013) and Harris et al. (2014a).

4.12. Unit 11: Accretionary wedge

Whereas the previous geomorphic units owe much of their character to Quaternary cycles of glaciation, the geomorphology of this unit is the result of tectonism, modified by bypassing of sediment from the canyon zone. The unit is located off Vancouver Island, occupying the continental slope in the depth interval 1200–2500 m. It comprises irregular, coast-parallel ridges generated by subduction of the Juan de Fuca Plate below the North American Plate. The sediments scraped from the subducting plate form an accretionary wedge and frontal anticlinal ridges (Davis and Hyndman 1989). This is part of the Cascadia subduction zone, stretching from northern Vancouver Island to northern California (Scholtz et al. 2016).

In the example (Fig. 12), the accretionary wedge is located on the continental slope between the canyon zone at A (depth 1000 m) and the low-relief abyssal plain (B, depth 2600 m). Numerous ridges aligned parallel to the shelf break range in elevation up to 750 m. At the western boundary of the zone are the two ridges described by Scholtz et al. (2016). Orca Ridge (C) is 800 m above the surrounding seafloor, and Slipstream Ridge (D) is 500 m.

Numerous arcuate escarpments (arrows, Fig. 12) etched into the sides of ridges testify to slope failure and mass transport processes. Both Orca Ridge and Slipstream Ridge exhibit submarine slope failures (Scholtz et al. 2016), likely facilitated by the presence of gas hydrates (Yelisetti et al. 2014). The 3 km wide failure at D on Slipstream Ridge occurred at c. 11 ka (Hamilton et al. 2015); it exhibits a rectilinear depression with a depositional apron spread out on the abyssal plain. The depositional apron is overlain by turbidite deposits triggered by earthquakes (Hamilton et al. 2015).

4.13. Unit 12: Transform terrain

Like Unit 11, this terrain owes its distinctive morphology to tectonic forces. The Queen Charlotte Fault (Barrie et al. 2013; Barrie et al. 2018; Harris et al. 2014a; Hyndman 2015) is a large strike–slip fault that forms the boundary between the Pacific and North American Plates off Haida Gwaii (9, Fig. 1). A 40 km wide submarine terrace, the Queen Charlotte Terrace, formed along the west side of the fault (Tréhu et al. 2015), is a “subduction zone accretionary sedimentary prism” (Hyndman 2015). The terrace consists of a melange of deformed marine sediments and basalt. It is elongated, and slopes seaward, with shallowest areas at −160 m and deeper parts >2000 m depth.

This zone is so highly variable in geomorphology that a representative example cannot be shown. Barrie et al. (2013) and Harris et al. (2014a) show an interpretation of an 80 km long stretch that has been surveyed with multibeam and illustrate some portions in high-resolution, including a series of “hanging Canyons.” The example (Fig. 13) is an area depicted on their figure, at the south end of a relatively shallow (−200 m) platform, part of the Queen Charlotte Terrace (see also Fig. 12, Hyndman 2015). The area at B is on the Haida Gwaii continental shelf, and shows typical terrain: low-relief shelf with irregular bedrock outcrops. The area east of the Queen Charlotte Fault in this figure is designated as part of the Canyon geomorphic unit (Unit 10). Unlike canyons in the Accretionary Wedge zone (Unit 11), the canyons (C) do not descend to the abyssal plain, but are interrupted by the shallow terrace, and only reach a depth of 800 m. They are closely spaced, and linear, and strikingly different in appearance from the four examples in Fig. 11.

In the canyon zone (C), the closely spaced (1 km apart) upper-slope gullies and canyons merge downslope to form fewer, wider channels (D) up to 1 km wide, with flat bottoms, steep sides, and transverse ridges that are up to 5 m in height and average 75 m apart. The channels lead into basins E and F. The active Queen Charlotte fault runs along the E–F line. Barrie et al. (2013) show that basin F has an outlet to deeper water (G). There is another breach 5 km to the southeast, and yet another 25 km distant, all contributing to the complexity of this terrain. The escarpment at H is a failure headwall, with a displaced block at J. Dunes up to 5 m high in the 1100 m deep basin at K are evidence of sediment transport towards the south, into deeper water.

4.14. Unit 13 Abyssal plain-seafloor-spreading terrain

The abyssal “Offshore Pacific” bioregion (Figs. 1 and 14) encompasses a vast area of seabed in the 2300–3800 m depth interval. It has been strongly conditioned by seafloor spreading and tectonics (e.g., Riddihough et al. 1980; Rohr and Furlong 1995; Rohr and Tryon 2010), vulcanism (Chase et al. 1975), sedimentation (Underwood et al. 2005), deep-sea fan development (e.g., Dobson et al. 1998; Knudson and Hendy 2009), and deep-sea channelization (Underwood et al. 2005). With some exceptions (e.g., Chaytor et al. 2007), the abyssal areas have not been surveyed by multibeam sonar, so the present analysis is constrained to the use of low-resolution GEBCO and CHS data compilations. In consequence, compared with the other three bioregions, a different approach has been taken to describing geomorphology. We divide the bioregion into two geomorphic units.

The Abyssal plain seafloor-spreading terrain (within dashed black line, Fig. 14) is a mosaic of closely spaced seafloor-spreading ridges oriented up to 250 m high, and chains of seamounts oriented normally to the spreading ridges and up to 1700 m high. Underwood et al. (2005) describe how this basement seafloor-spreading terrain of the Juan de Fuca Ridge emerges from the turbidite deposits that extend farther eastwards towards Nitimat Fan.

The insert on Fig. 14 is based on GEBCO data in vicinity of Juan de Fuca Ridge. The largest seamount is Explorer Seamount (D). Elsewhere seamounts are aligned in chains (E) that rise 1000 m above the surrounding terrain. In addition to the larger geomorphic features, large areas are occupied by long, low-relief (50–100 m) ridges (F) oriented parallel to the spreading axes of the Juan de Fuca Ridge. East of the spreading ridges, the basement of the abyssal plain lies under a thick blanket of sediments; Fig. 2 in Underwood et al. (2005) clearly illustrates the transition from spreading ridges to sedimented abyssal plain.

4.15. Unit 14: Abyssal plain with fans, seamounts, and channels

In Unit 14, turbidite deposits result in relatively low relief in comparison with the terrain of Unit 13. Nevertheless, there is significant geomorphic variability. On the abyssal plain sensu stricto thick sediments bury the underlying terrain resulting in virtually no relief (Fig. 14). Seismic profiles in the Cascadia Basin (Fig. 1; e.g., see Fig. 2, Underwood et al. 2005) show acoustically stratified sediments up to 400 m thick completely burying the basement, but thinning westwards until the irregular topography of the open basement (Juan de Fuca Ridge) emerges.

Submarine fan systems are adjacent to former sources of high sediment input from the continental shelf. Nitinat Fan (A, Fig. 14) (Knudson and Hendry 2009; Goldfinger et al. 2017) rises 500 m above the abyssal plain, slopes at ∼0.5°, and hosts leveed channels (Underwood et al. 2005) not clearly evident on the GEBCO data. The Queen Charlotte Sound fan (B) has developed in tectonically complex terrain, notably recently active volcanic seamounts such as the Dellwood Knolls (Riddihough et al. 1980; Rohr and Furlong 1995), and the southeastern extension of the Queen Charlotte Transform Fault. The fan reduces 1000 m in elevation over a distance of 150 km and is dissected by leveed channels up to 300 m deep, including the Moresby deep sea channel (Fig. 14; Chase et al. 1975), which has been deflected 40 km to the northward by dextral movement along the Queen Charlotte Fault. Fan development off Dixon Entrance (C) has been modified by relative motion (4.4 cm a−1) of the Pacific Plate, resulting in northwards migration of channel systems, notably the Horizon and Mukluk channels (Fig. 14; Dobson et al. 1998; Walton et al. 2014). Channels connecting with Dixon Entrance thread their way through the ridged terrain of the Queen Charlotte Terrace (Fig. 7, Dobson et al. 1998) to reach the Mukluk Fan at about −2000 m depth.

Submarine channels associated with Nitimat Fan, the Queen Charlotte Sound fan, and Dixon Entrance (Walton et al. 2014) extend far out onto the abyssal plain, beyond the bioregion boundary. The Mukluk and Horizon channels originate north of the study area and traverse the abyssal plain over a distance of ∼350 km. They are 3–8 km wide, with leveed banks. Volcanic seamounts and/or abyssal hills are scattered across the abyssal plain, either solitary or in chains. Chaytor et al. (2007) show their morphologic diversity.

The first of two examples from Unit 14 (Fig. 15a) shows the abyssal plain offshore from Dixon Entrance (and includes areas just outside the bioregion boundary). Two channels are associated with the Baranof Fan (Walton et al. 2014), namely Horizon Channel (A) and Mukluk Channel (B). The latter traverses the tectonic terrain off Dixon Entrance (C). The channels have levees and are about 5 km wide within the bioregion, but wider to the north (D). Superimposed on the low-relief abyssal plain are several seamounts, including Bowie Seamount (E) and Denson Seamount (F). The north flank of Denson Seamount is undercut by scour in Mukluk Channel. Chaytor et al. (2007) provide a comprehensive analysis of these and other seamounts in the region. In the multibeam sonar image, Bowie Seamount (Fig. 15b) rises 3000 m above the abyssal plain to reach within 30 m of the sea surface and has slopes of up to 40°. It has a narrow plateau at 235 m depth (G), with several peaks superimposed (e.g., H). Denson Seamount to the north has a similar summit plateau. Such plateaux are not associated with relative sea-level change but result from vulcanism. The second example (Fig. 16) is the upper part of the Queen Charlotte Sound fan, in an area than includes the abyssal unit, the continental slope with canyons, and the continental shelf. The three shelf-crossing troughs in Queen Charlotte Sound (A, B, and C on Fig. 16) have a combined width of 60 km. The canyons on the upper continental slope extend up to 35 km inshore from the projected former shelf break (white dashed line) and have eroded both the trough floors and the banks (see insert). The submarine fan begins at about -2000 m, and submarine channels with levees extend westward across the abyssal plain, traversing tectonically complex terrain, including recently active volcanic seamounts (Riddihough et al. 1980; Rohr and Furlong 1995). Illustrations in Chase et al. (1975) show how the Tuzo Wilson Seamounts (active in the Quaternary) project above the surrounding thick turbidite sediments associated with the fan.

4.16. Unique fourth-order geomorphic elements: Hexactinellid sponge reefs

Reefs formed by Hexactinellid (glass) sponges are found in parts of the bioregions (Barrie and Conway 2007; Conway et al. 1989, 1991, 2001; 2005a, 2005b, 2007, 2019; Shaw et al. 2018). They developed in the postglacial period upon glacigenic sediments with a gravelly texture at the seafloor. Within the four bioregions they are found in geomorphic units 1, 2, and 8. In Unit 1 (fiords), reefs occur primarily on moraines (Conway et al. 2007; Marliave et al. 2009; Stone et al. 2014), Shaw et al. (2017). In Unit 2 (bedrock), they form a large, morphologically complex area in Chatham Sound (Shaw et al. 2018; Shaw et al. 2023b), where they have developed on glaciomarine sediment draped over bedrock ridges. In Unit 8 (glacial troughs), they are established on top of glaciomarine surface lag gravel (see Fig. 9), so that large reefs are found in Hecate Strait and Goose Island Channel (Krautter et al. 2001), and elsewhere. Their apparent absence in the other geomorphic units is due to geological and environmental controls. For example, the reefs are sensitive to excess sedimentation, and reefs in the Chatham Sound area have been buried due to fine-grained sedimentation originating from the nearby Skeena River (Shaw et al. 2018).

Conway et al. (2005a) report that the northern Queen Charlotte Basin sponge reef complex is spread across 700 km2 of Hecate Strait, and the southern Queen Charlotte Basin complex covers 120 km2 of Queen Charlotte Sound. These areas are in the glacial trough category in Queen Charlotte Basin (area 15 900 km2) and thus are 5% of the seafloor.

5.1. Comparison with the global ocean classification

One of the goals of this paper was to classify the seafloor within the four bioregions. Comparing the results of the classification with those of the global ocean classification (Harris et al. 2014a), similarities and differences emerge. Within the study area, Harris et al. (2014b) mapped elements of their shelf, slope, and abyssal base layers, with superimposed classification layers based on relief and feature layers. Overall the Harris classification compares well, but differences emerge when, for example, we compare treatment of the shelf zone. They identified a glacial trough category (similar to ours except they included fiords), but the troughs and remaining shelf areas are divided into areas of low, medium, and high relief rather than the nine shelf units of the present study (glacial trough, banks, dissected shelf, etc.). These and other differences show that whereas the global classification is strongly relief-based (with features as defined by the International Hydrographic Organization), ours is hybrid in the sense that units have been mapped according to both morphology and genesis. Thus we identify the dissected shelf category (arguably shaped by sub-glacial meltwater) as well as the accretionary wedge and Queen Charlotte Transform categories (shaped by differing tectonic processes).

5.2. Seafloor complexity

Turning to the second goal of the paper, by comparison with the low-resolution DEM of Fig. 1, the illustrative examples reveal a seemingly endless array of component geomorphic units within the 14 classes. The concept that seafloor complexity increases with resolution was explored by Emery and Uchupi (1972, p. 56) who opined that “…contours on charts are more irregular and complex in direct proportion to the density of soundings.” Thus, when the 14 units of the classification are examined through the lens of the higher-resolution illustrative examples (Figs. 215), they are seen to be comprised of mosaics of fourth-order geomorphic components. The examples on Goose Island Bank–featureless on GEBCO data but very complex on multibeam bathymetry data shown on Fig. 4–well illustrate this point.

Some further indication of the geomorphic complexity of the Pacific bioregions is gained by considering those relatively small areas on Canada’s Pacific margin that have been surveyed with multibeam sonar and geomorphically mapped at high resolution. The Kitimat fiord system (area of 4600 km2) contains 20 geomorphic classes (Shaw and Lintern 2016), and the Chatham Sound area (3300 km2) has 13 (Shaw et al. 2023b). Sheet 1 of the Gulf Islands map set (Greene and Barrie 2011) has 61 classes (in that instance classification was habitat-based, and included textural, morphologic, and genetic classes). A recent classification of Queen Charlotte Strait (1500 km2), an area designated as bedrock terrain in our scheme, has 11 classes (Li et al. 2023).

5.3. Geomorphic similarities with Canada’s Atlantic passive margin

Glacial landforms such as those in the study area also exist on Canada’s Atlantic passive margin, attesting to the parallel development of submarine glacial landsystems, although with some scope for variability. For example, the small retreat moraines in Hecate Strait (Fig. 4, Shaw et al. 2019) differ in size and extent from the vast fields of genetically similar (De Geer) moraines on the Scotian Shelf (Todd et al. 2007). Shelf zones dissected by sub-glacial meltwater such as those on the Pacific margin (Unit 7) also have their counterparts on the Atlantic passive margin (Sankeralli 1998; King 2014), pointing to comparable processes in grounded ice on the periphery of the former ice sheets.

5.4. Distinctive geomorphic elements of Canada’s Pacific active margin

Tectonic zones such as those of Units 11 and 12 on the Pacific margin are absent on the Atlantic passive margin, where continental slopes have profiles that only depart slightly from best-fit exponential decay curves (Mosher et al. 2017). Hexactinellid sponge reefs such as those on the British Columbia margin (Shaw et al. 2017; Conway et al. 2020) are absent on the Atlantic passive margin, where the silicic acid content is relatively low (Cermeño et al. 2015).

There is no glaciotectonic terrain on the British Columbia margin akin to that on the northeast Newfoundland Shelf (Shaw et al. 2012; Shaw and Longva 2017), where deep glacial troughs lie within the gas hydrate stability zone (Majorowicz and Osadetz 2003), and gas hydrate dissolution may have facilitated dislocation and transport of Quaternary sediments.

A final, and very significant, distinction is that the British Columbia active margin lacks the “classic” glacial trough-mouth fans (TMFs) (Vorren and Laberg 1997; Dowdeswell et al. 2016) that form convex bulges on continental slopes of passive margins, including on Canada’s Atlantic passive margin (Hiscott and Aksu 1996; Deptuk et al. 2007; Shaw et al. 2017; Mosher and Piper 2007a, 2007b). In the example of Queen Charlotte Fan (Fig. 16), headward canyon erosion of both banks and troughs, and the 35 km retreat of the shelf break, suggests that the amount of glacial sediment moving down the three troughs in glacial stages was very much less than, for example, the Laurentian Channel on Canada’s east coast passive margin: the mouth of Laurentian Channel projects 12 km seaward of the shelf break on adjacent banks, due to accretion of glacial diamict (Todd 2016). It can be further inferred that much of the sediment transported onto the fan system is not glacigenic in origin.

The Pacific continental shelf and adjacent deep water off British Columbia, within the four bioregions defined by DFO, is classified into a series of 14 third-order geomorphic units: nine on the continental shelves, three on the slopes, and two in deep water. The continental shelf units are: (1) fiords; (2) bedrock terrain; (3) offshore banks; (4) a shallow platform east of Haida Gwaii; (5) continental shelf west of Haida Gwaii; (6) the shelf west of Vancouver Island; (7) areas of deeply dissected continental shelf; (8) glacial troughs extending from the fiord zone to the shelf break; and (9) large deltas. Unit 10 is the canyon zone on the upper continental slope. Units 11 and 12 (Cascadia subduction zone and Queen Charlotte Transform Fault zone) are conditioned by tectonic processes. Deep-water terrain is classified as Unit 13 (seafloor-spreading terrain) and Unit 14 (abyssal plain with seamounts, fans, and channels).

A series of examples shows how the seafloor is composed of a mosaic of fourth-order geomorphic elements. Such is the spatial variability evident at higher resolution that these examples cannot be viewed as representative of the larger units, but rather as samples. Comparison with the Newfoundland and Labrador Shelf bioregion on Canada’s Atlantic passive margin, reveals some similarities and significant differences. Similarities include the presence of glacial landforms and meltwater incised areas. Differences include: (1) tectonic continental slope terrains are unique to the Pacific margin; (2) Hexactinellid sponge reefs are limited to the Pacific shelf; (3) glacio-tectonic terrains are unique to the Atlantic passive margin; and (4) depocentres offshore from major glacial outlets on the Pacific margin are channelized submarine fans at the base of the tectonic zones, rather than classic TMFs of the passive margin.

This work was undertaken as part of the Pacific coast bioregions activity (Cooper Stacey project leader) under the aegis of the Geological Survey of Canada’s Marine Geoscience for Marine Spatial Planning Program. Robert Kung facilitated access to multibeam sonar data. We acknowledge the efforts of the Canadian Hydrographic Service who collected most of the multibeam bathymetry data described in this paper. NRCan contribution number/Numéro de contribution de RNCan: 20230173.

Bathymetric data used are available via Kung et al. (2023).

Conceptualization: JS

Formal analysis: JS, KB, ZL, JE

Funding acquisition: CDS

Investigation: KB, ZL, CDS

Methodology: JS

Project administration: CDS

Resources: CDS

Validation: JE

Writing – original draft: JS

Writing – review & editing: JS

This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.