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1 Introduction

Continental shelves represent the transitional zone between the subaerial landmass and the deep ocean, occupying an area of about 27 million km2, which is equivalent to almost 9% of the surface area of the oceans (Harris et al. 2014). The continental shelf extends from the shoreface to a marked change in slope known as the shelf break. In a global context, the shelf break varies from 20 m in tropical continental shelves to 550 m in polar regions, with a global average of 140 m water depth. The shelf width typically ranges between 30 and 60 km, although it can vary significantly worldwide. For instance, it is drastically reduced in active margins or in areas where submarine canyons deeply incise the continental shelf such as La Jolla and Scripps canyons in California, whereas in other settings such as the Siberian continental shelf into the Arctic Ocean, the width can be up to 1210 km.

Continental shelves occur in passive and active margins and in all climate environments, showing distinctive geomorphic characteristics (Shepard 1963). Several classifications have been proposed for modern continental shelves according to different criteria: tectonic setting (active and passive) (Emery 1980), shelf morphology (narrow, wide, shallow, and deep shelves) (Inman and Nordstrom 1971), vertical relief (low-, medium- and high-relief shelves) (Harris et al. 2014), prevailing energy regime (wave- and storm, tidal-current, and oceanic current dominated (Swift 1972), or climatic classification, based on latitudinal variations (high-latitude or glacial shelves, tropical or carbonate shelves and middle-latitude or siliciclastic shelves) (Thompson 1961). In spite of the simplifications associated with the climatic classification, we use it for a descriptive overview of the continental shelves types because of their associated characteristic morphological features (Fig. 1).

Fig. 1
figure 1

Schematic representation of the three shelf types based on the climatic classification: a glacial shelf, b mid-latitude (siliciclastic) shelf, and c tropical (carbonate) shelf. Note that horizontal scale differs among the three shelf types. Synthetised from Dowdeswell et al. (2002), Durán et al. (2014) and Lewis (2001)

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Glacial continental shelves range from a few tens to several hundred kilometres in width (Dowdeswell et al. 2002). Although shelf sediment is commonly deposited during ice sheet advance and retreat over the continental shelf, some shelves such as the East Siberian shelf are mainly supplied with sediment by major rivers rather than from glacial delivery of debris (Dowdeswell et al. 2002). Glacial continental shelves are characterised by a rugged seafloor produced by subglacial processes and/or grounded icebergs. The most common landforms described in these shelves include mega-scale lineations, drumlins, ribbed moraines and crag-and-tails (e.g. Clark 1993; Ottesen and Dowdeswell 2006).

Mid-latitude or siliciclastic continental shelves can be dominated by a variety of processes, such as tides, storms or currents (Swift 1972). Terrigenous sediment supply to the shelf occurs mainly at river mouths with a very uneven geographical distribution; a few rivers in Asia supply about 70% of the total annual amount of terrigenous sediment input to the ocean by rivers (Milliman and Meade 1983). Across the shelf, sediment distribution consists largely of sands on the inner shelf, muds occupying the middle shelf and sand and gravel on the outer shelf (Emery 1952). Overall, inner-shelf sands and middle-shelf muds are currently being supplied from the continent. Away from the river mouths, however, the shelf is a sediment-starved environment in which sedimentation is dominated by the reworking of older sediment. Main landforms in these continental shelves include tidal scours, palaeo-river channels, modern and relict prograding sediment bodies and a wide variety of bedforms, such as sand banks, sand ridges, ribbons, sediment waves, subaqueous dunes and sorted bedforms.

Tropical or carbonate continental shelves have a very low gradient depositional slope (commonly less than 0.1°) from the shoreline or lagoon to a basin floor (Burchette and Wrigth 1992). Shelf sediment is commonly characterised by biogenic material instead of terrigenous sediment supplied by rivers. Typical examples of tropical continental shelves are Bahamas, Florida shelf or the Great Barrier Reef in Australia. Continental shelf landforms in these environments include spurs and grooves, terraces, reef ridges, pinnacles, platforms, carbonate-sand shoals and lagoons (Harris and Davies 1989; Lewis 2001; Duce et al. 2016).

This chapter focuses on mid-latitude shelves characterised by transport and deposition of siliciclastic sediment. Glacial and tropical continental shelf landforms are discussed in Chap 13 (Batchelor et al. 2018) and 23 (Lo Iacono et al. 2018).

2 Brief History of Research on Continental Shelf Landforms

The first studies on continental shelf morphology, sediment distribution patterns and material composition were carried out in the late 1950s, mostly focusing on the textural, mineralogical and biological aspects of sediment and diagenetic processes (e.g. Shepard and Moore 1955; Potter 1967; Emery 1968; Swift et al. 1971). In the late 1960s, the development of new instrumentation and techniques designed for marine use has advanced the knowledge of fluid and sedimentary processes on continental shelves, allowing scientists to make short-term observations of some important dynamic and sedimentary parameters such as currents, suspended sediment concentration, settling velocity, near-bed bottom shear stress and flocculation (Sternberg et al. 1986). Following this approach, several studies involving simultaneous records at the boundary layer have improved the understanding of the mechanisms, pathways and rates of sediment movement (Evans and Collins 1975; Flemming 1978). Particular attention was paid to examining the mutual interactions between landforms and the processes acting on them (Wright and Thom 1977).

In the following years, some comprehensive classifications have been proposed for continental shelf bedforms based on the relation between bedforms and sediment input, flow velocity/bed shear stress and sediment-size (Belderson et al. 1982; Ashley 1990; Southard and Boguchwal 1990). Belderson et al. (1982) proposed a qualitative model that related the formation of different sedimentary features with the current strength and sand availability. Subsequent experimental studies revealed that a number of bedform states exist that are stable only between certain values of flow velocity/bed shear stress and sediment-size (Southard and Boguchwal 1990). These authors proposed a sequence of equilibrium bedform states as the flow velocity increases for a given sediment-size: lower stage plane bed, ripples, dunes, plane bed, and finally antidunes. In the same year, Ashley (1990) presented a new classification for large-scale flow-transverse sandy bedforms in fluvial and shallow marine environments.

With the introduction of high-resolution mapping systems in the 1990s, such as the multibeam echosounder, bathymetric LiDAR, underwater photography and video, new information has become available on seafloor characteristics allowing the characterisation of the present shelf seafloor at a high level of detail. The detailed geomorphic mapping of submerged continental margins provided the basis for mapping physical habitats (Kostylev et al. 2001), palaeolandscape reconstruction (Micallef et al. 2013), assessment of the anthropogenic impact on the seafloor (Puig et al. 2012) and discovery of potentially significant archaeological sites (Bailey and Flemming 2008).

Coupled with improvements on direct observations, in situ measurements and advances in numerical modelling of hydrodynamics, knowledge about processes that govern the morphodynamics of bedforms has also been significantly enhanced in recent years. At present, investigations into the origin, morphology and evolution of bedforms on the continental shelf involve several distinctive approaches along three main lines of research: (a) the morphology and sediment characterisation of bedforms (e.g. Amos and King 1984; Li and King 2007; Durán et al. 2015), (b) their formation and dynamics (e.g. McBride and Moslow 1991; Calvete et al. 2011; Simarro et al. 2015), and (c) their internal structure (Dalrymple and Hoogendoorn 1997; Goff et al. 2005).

3 Processes

The present day geomorphology of most continental shelves results from combinations of relict and modern features, reflecting a complex interplay among different controlling factors acting at varying time scales. The most relevant factors include the geological structure, sea-level change, sediment delivery and dispersal systems. The geological structure, acting at scales of thousands and millions of years, plays two main roles. It directly controls the overall configuration of the margin (width and slope, and the deep structure of the subsurface), thus determining the gross morphology of the continental shelf, and it indirectly controls the supply of sedimentary material to the marine environment and their composition by influencing the morphometry and lithology of the watersheds.

Continental shelves are sensitive to sea-level change. During the Quaternary, sea-level cycles were characterised by slow, stepped sea-level falls and abrupt sea-level rises with short-lived lowstands and highstands (Siddall et al. 2007). As a consequence, most of the shelf was exposed several times to subaerial processes. The most recent sea-level fall coincided with the Last Glacial Maximum (LGM) about 18,000 years ago, when sea-level fell about 120 m below the present-day sea-level (Fairbanks 1989; Hanebuth et al. 2000) and the coastline was located near the shelf edge. Since the end of the LGM, sea-level rose rapidly, forcing the landward migration of the shoreline and coastal environments and drowning the previously exposed landscape. The successive shoreline migrations across the shelf led to the formation of distinctive morphological features such as drowned barrier islands, beachrocks, prograding sediment bodies and sand ridges located in the mid-outer shelf, which are used as indicators of sea-level positions. The geomorphology of continental shelf drowned landscapes is therefore a valuable tool for better understanding the formation and evolution of ancient landscapes in relation to the youngest climatic-eustatic cycles (Cohen and Lobo 2013).

Sediment supply on mid-latitude continental shelves mainly depends on the hydrology of contributing rivers, which in turn is controlled by climate and drainage basin characteristics (Syvitski and Morehead 1999). In addition to natural factors, human interventions on the river course may also cause changes in the sediment yield. Once the sediment reaches the continental shelf it is dispersed by processes that act on a wide range of temporal scales (from seconds to hundreds of years) such as buoyant plumes, waves, tides, wind-induced and thermohaline currents, internal waves and sediment gravity flows (Nittrouer and Wright 1994) (Fig. 2). The across- and along-shelf sediment fluxes caused by the interplay of hydrodynamic processes determine the sediment distribution, the development or evolution of bedforms and the presence of sedimentary structures on the seafloor.

Fig. 2
figure 2

Diagram showing main physical processes and sediment dispersal across the continental shelf. Redrawn from Geyer and Traykovski (2001)

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4 Continental Shelf Landforms

Landforms on the continental shelf reflect the combined effect of geological structure, glacio-eustatic changes in sea-level and dominant sedimentary processes acting at different time-scales. The variety of identified morphologies is huge and some of them are referred to using different terms. Therefore, the following sections are far from an exhaustive inventory of landforms on continental shelves, but they aim to highlight the main categories and processes. Landforms are grouped as: (a) associated to consolidated bottoms, (b) erosive morphologies, (c) prograding landforms, (d) bedforms, (e) gas-related features, and (f) anthropogenic features.

4.1 Consolidated Bottoms

Two main types of landforms are described in this section: (a) bedrock outcrops on the seafloor, and (b) coastal sedimentary bodies that were cemented by intertidal processes or during low-stand periods.

In areas of low sediment input or dominated by erosional processes, underlying structure can be exposed at the seafloor resulting in a rough bathymetry with variable relief (Fig. 3). Rocky outcrops are a common feature in the continental shelf that can occupy large areas (Galparsoro et al. 2010; Micallef et al. 2013; Durán et al. 2014). Locally, faults affecting the basement can outcrop at the seafloor, resulting in high escarpments that can be up to 50 m high and have been described in several continental shelves (Liquete et al. 2007; Micallef et al. 2013).

Fig. 3
figure 3

a Shaded colour multibeam bathymetry of the Kimmeridge Clay in the English Channel coast, Dorset, England. Tidal currents excavate the soft mudstones resulting in large scours. b Close view showing extensional faults affecting the outcrops. Data courtesy of the Channel Coastal Observatory (www.channelcoast.org)

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Marine terraces are shore platforms formed by subaerial weathering during lowstand exposure and wave erosion that drove the coastline landward during subsequent sea-level rise (Anderson 1999; Pirazzoli 2005; Micallef et al. 2013). They consist of a flat to gently inclined surface that can locally terminate in a step-like morphology. The position of the marine terraces is controlled by eustacy and/or tectonics; thus they are considered as reliable sea-level indicators, recording periods of stationary (Bradley and Griggs 1976) or slow rates of sea-level (Collina-Girard 2002; Martínez-Martos et al. 2016).

Submerged barrier islands and beachrocks are examples of coastal sedimentary deposits that can be cemented through the precipitation of carbonate cements, mostly of high-Mg calcite and aragonite (Bricker 1971). Lithification usually takes place in the intertidal zone and may involve all sorts of sediment, such as sands and gravels of both clastic and biogenic origin (e.g. Russell 1963; Kelletat 2006). Submerged barrier islands are elongate morphologies that can reach tens of kilometres in length and several metres in height (Gardner et al. 2007). Beachrocks are sharp, convex structures that range from tens to centimetres to several metres in height, and 150 m to 10 km in length (Liquete et al. 2007). The location of these morphologies on the continental shelf, caused by subsidence or sea-level rise, is also considered a proxy for Quaternary sea-level and neotectonic displacements (Ramsay and Cooper 2002; Kelletat 2006).

4.2 Erosive Morphologies

Erosive morphologies on continental shelves are generated by strong bottom currents and exhibit a wide variety of shapes and dimensions. They include channels, scours (irregular shaped depressions), obstacle marks, large-scale lineations and furrows. The most common erosive features are the palaeo-river channels, which originate during periods of low sea-level when fluvial systems extended their limits basinward, widening the exposed shelf and eroding the underlying strata (Harris et al. 2013). These channels show V-shaped cross-sections and can be linear or sinuous in plan view (Micallef et al. 2013).

Scours are erosional features that show a variety of shapes, sizes and configurations; from small (around 0.5 m deep) to large scale features up to 20–30 m deep (Shaw et al. 2012; Durán et al. 2014). They are frequently associated with the interaction of intense bottom currents with geomorphological restrictions such as straits, narrow openings between islands, coastal headlands or rocky outcrops. Where constricted, the flow is enhanced and turbulence from the strong currents scours the seafloor. An excellent example of large scours generated by strong bottom currents is the Minas Passage in the Bay of Fundy, Canada, where a large scour trough occupies an area of 240 km2, incising 20–30 m into the seabed (Shaw et al. 2012) (Fig. 4).

Fig. 4
figure 4

Shaded colour bathymetric map of the Minas Passage in the Bay of Fundy, Canada. Inset map of the western part of the scour trough system shows different seafloor features such as: (A) the sharply-defined western edge of the principal trough; (B) a pair of banner banks; (C) a small scour trough; (E) transversely-aligned symmetrical dunes. (F) and (G) are shadow sand banks. (Reprinted from Shaw et al. 2012, with permission from Elsevier)

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At a smaller spatial scale, when a bottom current encounters an individual obstacle, such as scattered rocky outcrops, the flow is constricted and accelerated, which may result in erosion in front of the obstacles. Downstream from the obstacle, a shadow zone is generated where turbulence intensity slowly diminishes (Allen 1982). These features are named obstacle marks. Depending on the bottom currents intensity, obstacles may cause sediment deposition but also non-deposition or even sediment erosion at the downstream side of the obstacle generating crescent scour and comet marks (Werner and Newton 1975).

Finally, large-scale lineations and furrows represent flow-parallel bottom structures that develop under episodic, energetic and directionally stable bottom currents that generate helical motions within the benthic boundary layer (Dyer 1970; Flood 1983). Lineations can be up to several kilometres long with a negative relief of several metres (Lo Iacono et al. 2010; Durán et al. 2014). Furrows are also longitudinal bedforms that develop in fine-grained, cohesive sediment (Flood 1983). They consist of a series of parallel, regularly spaced grooves in the seafloor that can be 20 m wide and 2 m deep (Flood 1983).

4.3 Prograding Landforms

Along the inner shelf, modern deposits are organised in prograding sediment bodies that comprise prodeltas and infralittoral prograding wedges. When clastic sediment is delivered to the coast and inner shelf, it is reworked by marine processes forming a delta (Elliot 1986). Large deltas build huge submarine prograding sediment bodies that can extend tens of kilometres seaward. The main delta physiographic zones include the subaerial and the subaqueous delta plain, which comprises the delta front and the prodelta (Wright 1985) (Fig. 5). The morphology of a delta depends on the discharge regime, but also on the relative magnitudes of tides, waves and currents (Colleman 1975). Based on the dominant process, three main types of deltas are distinguished: river-dominated deltas such as the Mississippi River delta, wave-dominated deltas (e.g. the Nile River delta), and tide-dominated deltas such as the Amazon River delta (Galloway 1975).

Fig. 5
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Shaded colour multibeam bathymetry of the Llobregat prodelta in the Catalan continental shelf, NW Mediterranean Sea, showing undulated sedimentary features (from Urgeles et al. 2011 with permission of Springer)

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The infralittoral prograding wedge (IPW) develops in the infralittoral environment but is not directly linked to a significant fluvial supply. It is generated by downwelling currents and associated seaward transport of sediment. The IPW extends from the supralittoral to the storm wave-base level, showing lateral continuities of tens of kilometres and thickness of several metres (Hernández-Molina et al. 2000). Across-shelf, it forms a low-angle slope (0.6° on average) that is bounded seaward by an abrupt increase in the slope gradient (about 2° on average) that extends to water depths of 40–50 m (Pickrill 1983). The main progradational development of modern IPWs has taken place during the Holocene sea-level highstand stabilisation (Hernández-Molina et al. 2000; Ercilla et al. 2010).

Large prograding sediment bodies have been also documented on the mid-outer shelf, and particularly at the shelf edge (Gardner et al. 2007; Liquete et al. 2007; Ercilla et al. 2010; Durán et al. 2013). Their location detached from the shoreline is related to the last sea-level rise. At times of slow rise, or stillstand, the shore position remained nearly constant allowing the development of prograding coastal deposits, which were drowned during subsequent episodes of rapid sea-level rise.

4.4 Bedforms

Bedforms are morphological features formed by the interaction between a flow and a sedimentary bed. They can be composed of gravel, sand or mud and of siliciclastic or carbonate sediment, displaying a wide range of spatial scales. This section focuses on different types of bedforms that are ubiquitous in continental shelves, with emphasis on their morphology, sediment characteristics, present-day activity and the processes that govern their formation and evolution.

Sand Banks, Sand Ridges and Sand Ribbons

Sand banks, sand ridges and sand ribbons are common features on many continental shelves. Sand banks are found on continental shelves where large amounts of sand are available from the local seabed or coastal erosion, and where strong currents are capable of moving the sediment (Dyer and Huntley 1989). Most sand banks appear to have been created during the post-glacial rise in sea-level, but they have been subsequently modified by changing currents and waves. A descriptive classification of sand banks was provided by Dyer and Huntley (1989) based on the morphology and main sediment transport pathways associated to each sand bank type; these include estuary mouth ridges, headland banner banks and open shelf ridges. Banner banks are located near headlands, islands or large rocks. They are separated from them by a channel and can be generated by unidirectional and bidirectional currents (Belderson et al. 1982; Kenyon and Cooper 2005). These bedforms are few kilometres in size and have an elongated pear-shaped form (Dyer and Huntley 1989). Sand banks are asymmetrical in cross-section, with the steeper leeward slope oblique to the direction of the flow and net sand transport.

Sand ridges are a specific type of sand bank (Dyer and Huntley 1989), also referred to as large and very large subaqueous dunes when they are active bedforms (Ashley 1990). They are pervasive bedforms on many continental shelves worldwide (Figs. 6 and 7). In tidal-dominated settings, sand ridges show elevations up to 40 m with orientations that are primarily determined by the peak tidal direction (Dyer and Huntley 1989; Liu et al. 2007). Sand ridges in non-tidal continental shelves, however, are smaller (up to 12 m high) and show an orientation oblique to the shoreline (Snedden et al. 2011). They show a linear, elongated shape and a predominantly asymmetric transverse profile, with steeper down-current flanks (Bassetti et al. 2006; Li and King 2007).

Fig. 6
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Shaded colour multibeam bathymetry of Sable Island Bank, showing shoreface-connected and offshore sand ridges. (Reprinted from Li and King 2007, with permission from Elsevier)

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Fig. 7
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Shaded colour multibeam bathymetry of the Gulf of Valencia (Western Mediterranean Sea) outer shelf and continental slope showing sand ridges in the middle shelf and sediment waves in the outer shelf and continental slope. Courtesy of M. Ribó

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Sand ridges have been observed in the inner shelf but also in the middle and outer shelf at water depths greater than 40 m (Bassetti et al. 2006; Simarro et al. 2015). Sand ridges located in the middle and outer shelf are interpreted as formed in the shoreface environment through similar mechanisms to those proposed for shoreface-connected ridges, and subsequently detached from the shoreline during sea-level rise (Goff et al. 2005; Calvete et al. 2011; Nnafie et al. 2014).

Sand ribbons are flow-parallel bedforms that are much smaller than sand ridges; they are less than 200 m wide and 0.5 m high (Kenyon 1970; Amos and King 1984). They are characterised by thin, elongate bands of sand overlying a coarser lag sediment that is exposed between them (McLean 1981). They are interpreted as generated by longitudinal roll vortices or helical flow cells (McLean 1981; Amos and King 1984).

Sediment Waves

Sediment waves are large-scale (generally tens of metres to a few kilometres wavelength and several metres high) undulating, depositional bedforms that are generated beneath a current flowing at, or close to, the seafloor (Wynn et al. 2000). They consist of fine sediment (mud and silt dominated) and have been described in prodeltas (Mulder and Syvitski 1995; Trincardi and Normark 1988) (Fig. 5) as well as in the outer continental shelf (Ribó et al. 2016) (Fig. 7).

Prodeltaic sediment waves show variable sizes, with wavelengths of 20–400 m and maximum amplitudes of 4 m (Urgelés et al. 2011) (Fig. 5). They commonly have sinuous to rectilinear crests that are parallel to the bathymetric contours. Undulated sediment features on modern prodeltas have been initially interpreted as a result of sediment deformation and slope failure phenomena (Corregiari et al. 2001). However, subsequent studies suggest that the most likely mechanisms for the formation of these morphologies are related to processes in the bottom boundary layer, including sediment resuspension by internal waves (Puig et al. 2007), bottom currents (Urgeles et al. 2011) and hyperpycnal flows (Wheatcroft et al. 2006; Urgeles et al. 2011). Sediment waves have also been described in the outer shelf and upper slope showing wavelengths of several hundred metres and wave heights of up to several metres (Ribó et al. 2016). The main processes responsible for their formation include bottom currents (Lonsdale and Hollister 1979) and internal waves (Ribó et al. 2016); although in many cases multiple processes are involved (Faugères et al. 2002).

Subaqueous Dunes

Subaqueous dunes is the general term suggested by Ashley (1990) for sandy, dynamic, flow-transverse bedforms on shallow waters. The term sand waves is also used to describe the same type of bedforms, although they are mostly related to tidal currents (Amos and King 1984). These bedforms are ubiquitous on the continental shelf and exhibit a wide range of dimensions with heights ranging from centimetres to 30 m, and spacing of up to 100 m (e.g. Bassetti et al. 2006; Franzetti et al. 2013) (Fig. 8). Dunes are classified based on primary descriptors of shape (2-D or 3-D) and size. Based on the wavelength (λ), Ashley (1990) classifies dunes as small (0.6–5 m), medium (5–10 m), large (10–100 m) and very large dunes (λ > 100 m). Another important morphology parameter is the dune height. Flemming (2000) provides evidence for the existence of an equilibrium geometrical relationship between wavelength and height, placing an upper limit in dune height.

Fig. 8
figure 8

Shaded colour multibeam bathymetry of the Banc du Four (Western Brittany, France). The morphology of the Banc du Four field is composed of large dunes exhibiting a great diversity of shapes and sizes developed in a deep (70–105 m depth) tidal-dominated environment. (Reprinted from Franzetti et al. 2013, with permission from Elsevier)

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Dunes may appear in extensive fields over broad sandy shelf areas or as secondary features superimposed over large-scale features, such as large and very large dunes or sand ridges (Lobo et al. 2000; Goff et al. 2005; Li and King 2007; Durán et al. 2017c). Forcing mechanisms able to induce dune formation on the continental shelf are related to tidal currents (Berné et al. 2002; Franzetti et al. 2013), wind-induced bottom currents (Cacchione et al. 1987; Guerrero et al. 2017) and water masses flowing under geostrophic conditions (Bassetti et al. 2006; Lo Iacono et al. 2010). Dunes can be stationary bedforms (Whitmeyer and FitzGerald 2008) or migrate at different rates (Ernstsen et al. 2006; Franzetti et al. 2013), providing valuable information on the local and regional current patterns and sediment transport. In non-tidal shelves, recent observations indicated that dunes migrate at very low rates (Durán et al. 2017c), but they migrate at rates of 1–17 m year−1 in tidal settings (Le Bot et al. 2000; Schwab et al. 2014).

Sorted Bedforms

Sorted bedforms are spatially extensive (m to km) features with subtle bathymetric relief (cm-m) present on many continental shelves. They are characterised by a sharply edged sequence of coarse-grained and fine-grained domains with little topographic relief (about 1 m) relative to the bedform spacing, which varies from metres to kilometres (Murray and Thieler 2004) (Fig. 9). They have been observed on a variety of continental shelves at water depths ranging from 10 to 90 m (e.g. Hume et al. 2003; Durán et al. 2017a, 2017b). Their formation has been related to a feedback between bed composition and sediment flux in a self-organised pattern (Murray and Thieler 2004; Coco et al. 2007). Several studies have shown that sorted bedforms tend to be ephemeral in shallow waters (down to 15–20 m) (Hume et al. 2003), while in deeper waters they are more persistent (Lo Iacono and Guillén 2008; De Falco et al. 2015).

Fig. 9
figure 9

a Shaded multibeam bathymetry and b backscatter imagery of the Catalan continental shelf (NW Mediterranean Sea) showing the morphology of sorted bedforms developed in the inner shelf (from Durán et al. 2017b, with permission of Springer)

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4.5 Gas-Related Features

The presence of fluid escape features, such as seafloor pockmarks, was first introduced by King and MacLean (1970) over the Nova Scotian shelf. Pockmarks are seafloor depressions with steep sides and flat floors. In plan-view, they are usually circular, elliptical or elongated, and may be composite in shape (Fig. 10). Pockmarks are constrained to fine-grained substrates and show variable dimensions that can be up to 150 m wide, 200 m long, and 10 m deep (Andresen et al. 2008). Their shape, dimensions and spatial distribution depend on the capacity and composition of fluid reservoirs sourcing the venting fluids, type of the fluid escape, sediment characteristics and configuration of underlying structures, water depth and morphology of the surrounding seafloor (Hovland and Judd 1988; Hovland et al. 2010).

Fig. 10
figure 10

a Perspective view of a shaded relief digital terrain model from a series of pockmarks in the Norwegian Trough. Black arrow points at a trawl-scar associated unit pockmark and white arrow points at a possible ‘incipient’ pockmark. White line indicates the location of the Sub-bottom profiler (SBP) record crossing the pockmark (b). (Reprinted from Hovland et al. 2010, with permission from Elsevier)

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4.6 Anthropogenic Features

The increasing anthropogenic activity on the continental shelf has a direct impact on the seabed. Most of the impacts are derived from bottom trawling, mineral exploitation, waste disposal and the increasing production of renewable energy. The direct physical effects of bottom trawling include scraping and ploughing of the seabed and sediment resuspension (Jones 1992; Palanques et al. 2001, 2014). Scars left by the trawl doors have a particularly strong signature on the seafloor, as evidenced by large depressions of 5–20 cm depth in sandy sediment (Krost et al. 1990). Furthermore, in areas of intense fishing, the reworking of the seafloor by trawling is so severe that it gradually modifies the shape of the submarine landscape over large spatial scales, smoothening the seabed (Fonteyne 2000; Puig et al. 2012)

The impacts of dredging activities on the seafloor, either for aggregate mining or dredging of navigation channels and harbours, entail modification of the seabed morphology and sediment dispersal during dredging and the subsequent disposal of the dredged material. As a consequence of these activities, large depressions form on the seafloor and massive sedimentation of fine sediment occurs in the dredged and disposal areas (Du Four and Van Lanker 2008; Durán et al. 2017a). Hard structures, such as pipelines on the seafloor or wind farms, can also modify sediment dynamics nearby the structures, resulting in large scours on the seafloor (Whitehouse 1998; Harris et al. 2010).

5 Key Research Questions and Future Directions

Continental shelves represent a vulnerable environment due to their extensive use for navigation, recreation, fishing and aquaculture, mineral exploration, waste disposal, and the production of renewable energy from waves, tidal currents and wind (Barrie and Conway 2014; Chiocci and Chivas 2016). Furthermore, continental shelves constitute archives of long-term environmental changes in the complex interaction among climate, sea level and sediment input (Gao and Collins 2014; Lobo and Ridente 2014). They, therefore, can be used to better understand how coastal regions have evolved in response to past sea-level changes in order to predict how they will respond to future climate change.

Over the past decades, great efforts have been made to better understand the geomorphology, seafloor characteristics and sediment transport processes on the continental shelf. However, despite the huge progress made in recent years, only a small fraction of the continental shelf is covered with high-resolution bathymetric data and several key research questions still remain to be addressed, which constitute the core of ongoing and future research. These include: (a) the evolution of continental shelves during the youngest climatic-eustatic cycles; (b) the sediment transport processes associated with the formation and dynamics of bedforms and the transfer of sediment from the continent to the deep sea; (c) the impact of exploitations activities on the continental shelf, such as dredging and sediment disposal, fishing activities, deployment of wind farms, wave power and tidal/current energy plants on the continental shelf landscape; (d) the accurate characterisation of seabed processes and landforms using permanent monitoring observatories and their integration in the marine spatial planning, particularly those associated to new uses of the marine environment such as wind farms, marine protected areas or aquaculture; and (e) the use of new accurately and fully geomorphic data mapping of continental shelves landscapes for understanding the evolutionary history of ancient landscapes and identifying potentially significant archaeological sites on the seabed.

In the last years, the characterisation of the geomorphic variability of continental shelves has seen significant evolution with innovations in seafloor mapping, especially through the advances in multibeam echosounding, laser-light detection and ranging (LiDAR), side scan sonar, ship-deployed remotely operated vehicles (ROVs), autonomous underwater vehicles (AUVs) and seismic sub-bottom profiling techniques. In association with these enhancements, improved and new methods of analysis and dating of sediment cores have provided better knowledge of the continental shelf landscape and its recent evolution during the last glacial-interglacial cycles. With the development in seafloor mapping, methods of bedform analysis are also progressing substantially to analyse high-resolution spatial data that allow quantifying bedform geometry and evolution (changes in shape and migration) and seafloor morphological changes. This, together with advances in the predictive ability of morphodynamic models and the integration of morphological, sedimentological and hydrodynamic data, contribute to advancing the understanding of the formation, evolution and present-day dynamics of bedforms and to assess the impact of human interventions on continental shelf morphology.

In addition to innovation in the approaches and methods, progress in the geomorphological research of continental shelves evolves thanks to knowledge transfer between different scientific disciplines and research areas. One of the most representative examples is the integration of seabed geomorphology and benthic habitats to support government spatial marine planning, management and decision making, and the designation of marine protected areas. Recently, several investigations have also paid attention to the potential of studying continental shelf morphology to update the submerged archaeological record and understand patterns of human settlement and dispersal (Bailey and Flemming 2008; Evans et al. 2014; Harff et al. 2016). This represents a new and multidisciplinary field for research where geomorphology will play a major role.