Widely used sequence stratigraphic models predict that specific facies assemblages alternate in the stratigraphy of deep-sea fans, depending on the cyclic nature of sea-level variations. Our work tests this assumption through facies reconstruction of submarine fans that are growing in a small basin along the tectonically active Sicilian margin. Connected canyons have heads close to the coastline; they can be river connected or littoral cell–connected, the first receiving sediment from hyperpycnal flows, the latter intercepting shelf sediment dispersal pathways. Hyperpycnal flows directly discharge river-born sediment into the head of the river-connected canyon and originate a large turbidite fan. A drift formed by the longshore redistribution of sediment of a nearby delta introduces sediment to the head of the littoral cell–connected canyon, forming turbidity currents that flow within the canyon to reach the basin plain. However, since sediment failure and landslide processes are common in the slope part of the system, a mixed fan, consisting of both turbidites and mass-transport deposits, is formed. Disconnected canyons, with heads at the shelf edge far from the coastline, are fed by canyon head and levee-wedge failures, resulting in mass-transport or mixed fan deposition, the latter developing when the seafloor gradient or the lithology of the failed sediment allows turbidity current formation. Connected canyons form in areas with high uplift rates, where the shelf is narrow and steep and the shelf edge is at a relatively shallow depth. Disconnected canyons develop where there are lower uplift rates or subsidence, where the shelf is large and relatively gentle with a deeper shelf edge. It is deduced that the relative vertical movements of fault-bound blocks control whether canyons are connected to the coast at the present day. The role of tectonics in controlling the canyon feeding processes and the facies of submarine fan growth during highstand periods is therefore highlighted. A further view that arises from our paper is that in active margins, the slope portion of fan systems, through seafloor instability and variations in channel gradient, is a key factor in determining the final deep-sea fan facies, regardless of the distance between the coast and the canyon. The concomitant growth of turbidites, mass-transport deposits, and mixed fans demonstrates that models that predict changes in submarine fan facies on the basis of sea-level cycles do not necessarily apply to systems developed along tectonically active margins.
Early sequence stratigraphy models assumed that deep-sea submarine fans grow mainly during sea-level lowstands when a direct connection between rivers and submarine canyons and channels is established (Vail et al., 1977). These models were developed mainly from the analysis of passive margin stratigraphy and derived their main conceptual framework from the interpretation of conventional seismic and drill cores. Ideas coming from the functioning of modern depositional systems were not incorporated in the model. At that time, the available techniques of marine geology could not play an important part in the perception of the multiple aspects connected with the issue of sediment delivery processes to the oceans, one of the main assumption that underlies the sequence stratigraphic model (Posamentier and Kolla, 2003; Catuneanu et al., 2009, 2011). Since then, the techniques of marine geology have advanced; in particular, the advent of multibeam technology has led to the extensive mapping of continental margins worldwide, revealing submarine landscapes that were previously largely unexpected (Sager et al., 2004). The characteristics of the geomorphic elements that contribute to sediment delivery to the deep sea, particularly submarine canyons and channels, have been highlighted in increasing detail. Of particular interest for the understanding of sediment delivery to the deep sea is the discovery that in many cases, particularly along active margins, canyon heads can incise the shelf and reach the coastal areas (Harris and Whiteway, 2011). Canyon heads can be directly connected with rivers and consequently can sometimes be fed directly by hyperpycnal flows (Piper and Normark, 2009; Puig et al., 2014). In other cases, canyon heads are not in direct connection with rivers, but their activity during the present sea-level highstand is nonetheless made possible by longshore currents and littoral cells that supply sediment to them (Piper and Normark, 2009; Puig et al., 2014). Landslides are also a possible source of sediment for the deep sea and can act also in canyons that are far from the coastline and stranded at the shelf edge (Piper and Normark, 2009; Puig et al., 2014). Thus, taking into account the relationships between the canyon head and the sediment staging area, the role of hyperpycnal flows, storms, longshore currents, littoral cells, dense shelf-water cascading, and landslides as possible triggers for flows that can feed sediment to the deep sea during the present-day highstand has been reevaluated (Piper and Normark, 2009; Puig et al., 2014). Therefore, it has been recognized that landward shifts of shorelines do not always lead to the deactivation of channels and canyons, and the hypothesis that deep-sea sediment starvation always occurs during sea-level highstands is now negated (Covault et al., 2007; Covault and Graham, 2010).
Sequence stratigraphic models also recognize that in areas with narrow shelves, sediment can be delivered to the deep sea during highstands, but the implication is that this process is brought about by coastal progradation (Catuneanu et al., 2009, 2011). As a consequence, the majority of river-born sediments are trapped in the coastal area and shelf and only low-density turbidites contribute to the growth of fine-grained deep-sea fans. Therefore, the models envisage the systematic development of submarine fan facies driven by the changing character of gravity flows as coastline positions shift during the different tracts of a sea-level cycle (Posamentier and Kolla, 2003; Catuneanu et al., 2009, 2011).
This simple model is, however, not in agreement with recent marine geology data indicating that the processes that feed sediment into canyons are highly varied during the present-day sea-level highstand (Piper and Normark, 2009; Puig et al., 2014). The nature of the processes that feed sediment into canyons has been shown to have a large impact on the volume of deep-sea highstand sedimentation and consequently to be largely responsible for the growth rate of deep-sea fans (Normark et al., 2009; Romans et al., 2009; Covault and Graham, 2010). Furthermore, the style of canyon feeding systems has been shown to have a large impact on the behavior and character of flows within canyons and channels and in the basin plain, controlling the efficiency of the system and consequently being largely responsible for the site of sediment deposition (Piper and Normark, 2009). The large variability of fan depositional style and facies as imposed by the nature of the flow-triggering mechanisms is further augmented by the possibility for flows to undergo modifications imposed by gradient variations along the slope (Piper and Normark, 2009).
It is well established that fans can be active during the present sea-level highstand (Covault and Graham, 2010); an important step forward in the study of deep-sea sedimentation is to investigate the potential of tying deep-sea fan evolution to the processes of sediment delivery to canyons and to the setting of the slope. Modern marine geology data must play a prominent role in the study of this issue. Process-oriented interpretation of multibeam data can be aimed at determining the sedimentary processes occurring in modern deep-sea fans, an essential step toward the understanding of facies development. The data deriving from old, but still amply used, techniques of marine geology can greatly enhance the strength of facies interpretation obtained from bathymetric information. Apart from few instruments with limited availability, sidescan sonars and high-resolution subbottom profilers have not undergone revolutionary development in recent years. However, they still furnish superior data, the importance of which is now better appreciated when they are used to complement interpretation of seafloor bathymetry (Sager et al., 2004). The routinely available techniques of seafloor sampling have remained largely unchanged in the last years, but nevertheless provide geophysical data ground truthing and facies evaluation.
In this paper we expand the analysis of present-day highstand fans to determine how their facies are controlled by the variation of canyon connectivity to the coastline along the margin, and by flow transformation imposed by uneven gradient along the slope part of the system. We integrate new multibeam bathymetric and reflectivity data with sidescan sonar and subbottom images and core data to characterize modern submarine fans along a tectonically active margin. We analyze fan dimensions, processes, and facies and correlate them with the location of the head of the feeding canyons and with processes occurring on the canyon course. We highlight that both turbidites and mass-transport deposits contribute to the recent makeup of the submarine fans, showing that in active margins, deep-sea fan facies development does not fit in models that rely exclusively on sea-level variations, but depends largely on tectonic deformation.
The Capo d’Orlando Basin is located along the central part of the northern Sicilian margin (Fig. 1). The continental shelf is in general narrow, attaining a maximum width of 10 km to the west and becoming progressively narrower toward the east (Fig. 1A). The shelf break is located at variable depths, at ∼140 m to the west of Cape Orlando, shallowing significantly to the east. The upper slope of the Capo d’Orlando Basin is characterized by canyons that have their heads at the shelf break or incise the shelf, having their heads very close to the coastline (Fig. 1A). In the lower slope, the canyons connect with leveed channels that have a general southeast-northwest trend. The basin plain is located at an average depth of ∼1500 m and is characterized by lobe deposits formed beyond the channel mouths. The Capo d’Orlando Basin plain is confined seaward by the submarine slope of the Aeolian Islands. An extensional fault bounds the basin to the west and is responsible for its westward dip (Gamberi and Dalla Valle, 2009). A submerged structural high, connecting the Sicilian margin with the Aeolian Island arc, bounds the basin toward the east. The area is affected by a high rate of regional uplift (Westaway, 1993) and ongoing fault activity as displayed by the seismicity of the area (Pondrelli et al., 2006).
DATA AND METHODS
The available data set consists of multibeam bathymetry acquired in 1999 with a SIMRAD EM-12 multibeam at depths >500 m (Gamberi and Marani, 2004; Gamberi and Dalla Valle, 2009). Further multibeam data were successively acquired to complete the bathymetric coverage of the area during the cruise TORDE10 on board the R/V Urania and various cruises carried out by ISMAR (Istituto di Scienze Marine-Consiglio Nazionale delle Ricerche) in the frame of the MAGIC (Marine Geohazards along the Italian Coasts) project funded by the Italian National Agency for Civil Protection. The merged bathymetric data cover all the shelf, slope, and basin plain apart from a narrow zone, with variable extent, of the very shallow water areas close to the coastline (Fig. 1A). The 1999 multibeam bathymetric data were also processed to obtain a backscatter image of the lower slope and basin plain (Fig. 2). Besides the multibeam backscatter, a mosaic acquired during the TTR14 cruise on board the R/V Logatchev, with the MAK II (www.cggeinternational.com/MAK-1M.htm) deep-towed sidescan sonar is available in a portion of the study area covering the lower slope and the proximal basin plain (Fig. 3). A grid of two-dimensional single-channel seismic lines acquired in the late 1970s is also available (Fig. 1B). CHIRP (compressed high-intensity radar pulse) subbottom profiles spaced at an average interval of 2 km cover the entire study area (Fig. 1B). Seafloor samples, with both gravity and box corers, were collected during the TORDE10 cruise carried out in 2010 on board the R/V Urania (Fig. 1B); the 60 samples, with variable penetration (from no recovery to 6 m), provide control on the facies of the Holocene to modern succession and allow the ground truthing of the geophysical data.
We use the term mass-transport deposit as in Nardin et al. (1979) to include all kinds of gravity-driven deposits with the exception of turbidites. The term mixed refers to fans that show evidence of both turbidites and mass-transport deposits and not to the hybrid group of sediment gravity flows (sensu Haughton et al., 2009).
SUBMARINE FAN MORPHOLOGY, PROCESSES, AND FACIES
The available data set has been used to determine the sedimentary processes that prevail in the various parts of the basin. In particular, the character of the sediment gravity flows in the canyons, channels, and lobes, was determined through the combined interpretation of the genetic significance of the seafloor geomorphology and backscatter at different scales, and core interpretation. As a result, a map of the different discrete fans in the study area was produced (Fig. 4). In the study area, the Calavà and the Orlando canyons have their heads close to the coastline (<500 m; Fig. 5) and are thus defined as connected canyons. The Calavà canyon head is located in front of a river mouth (Fig. 4) and is therefore defined as a river-connected canyon. The Orlando canyon head does not face a major river, but is located downcurrent from a major river delta and is called a littoral cell–connected canyon (Fig. 4). The Naso and the Zappulla canyons have their heads stranded at the shelf edge, far from the coastline (>2.5 km; Figs. 4 and 5); they are referred to here as disconnected canyons.
Connected Canyon Fans
Calavà River Connected Turbidite Fan
The head of the Calavà connected canyon is very close to the coastline adjacent to Cape Calavà and a few tens of meters from the coastline the canyon floor is >100 m deep (Figs. 1, 3, and 5). Further downslope, the Calavà canyon has a relatively flat floor where the subbottom profile lacks penetration and the sidescan sonar shows a high backscatter, indicating the presence of coarse-grained deposits (Fig. 6A). At the base of slope, the canyon passes to the Calavà channel that heads toward the northwest (Figs. 2 and 4). At the canyon to channel transition and further downchannel, sediment waves at the seafloor provide evidence of turbidity current activity (Fig. 6B). The sediment wave area has high backscatter in the sidescan sonar and lacks penetration in the subbottom profile (Fig. 6B), suggesting that it is composed of coarse-grained bedforms. The sediment waves have a wavelength of ∼100 m, a suitable indicator of coarse-grained bedforms (Wynn and Stow, 2002; Gamberi and Marani, 2011). Coarse-grained, graded, and laminated sands sampled at the seafloor in the sediment wave field (Fig. 6C) provide evidence of Holocene turbidites, confirming the geophysical interpretation. The Calavà channel feeds a large lobe (50 km long and 10 km wide) with a series of diverging channels that head toward the western part of the basin (Figs. 2 and 4). The channels have low relief, and are best imaged in the multibeam backscatter image (Fig. 2). In the middle part of the lobe, the core of Figure 6E shows that 8 turbidites make up most of the last 2.80 m of the sedimentary succession. Turbidites are mainly composed of a thin, medium sand basal part grading upward into a thicker mud cap. However, an ∼1-m-thick finer grained parallel-laminated turbidite and a mudclast-rich coarse to medium sand turbidite are present. Further westward, the channels die out distally into a featureless fan fringe (Figs. 2 and 4), where subbottom profiles show that a succession of relatively thin bedded reflections account for the upper 50 m of the fan stratigraphy (Fig. 6D). This seismic facies is taken as an indication that turbidites predominate in the entire most recent sediment package of the Calavà fan.
Orlando Littoral Cell–Connected Mixed Fan
The Orlando canyon has its head very close to the coastline adjacent to the Cape Orlando (Figs. 1 and 5). It connects with the Orlando channel that crosses the headwall of a landslide involving a large portion of a preexisting levee wedge (Fig. 4). Further, smaller scale collapse events also occurred in the area, as shown by an abundance of landslide scars and blocky mass-transport deposits in the levee and at the channel mouth (Figs. 7A, 8A). At the channel mouth, a network of distributary channels forms a relatively large lobe (20 km long and 8 km large) that in its distal part is in contact with the Calavà fan lobe (Fig. 4). In the proximal part of the fan lobe, sediment waves and scours are indicative of turbidity current activity (Figs. 7B, 7C). However, a blocky seafloor is also relatively common, indicative of mass-transport deposits (Fig. 7C). Comet marks are formed in the lee of some blocks, indicating that turbidity currents reworked the mass-transport deposits (Fig. 7C). The core in Figure 7D, located at the channel to lobe transition, shows that two parallel and cross-laminated medium sand turbidite layers overly a mass-transport deposit, consisting of folded and disrupted thin-bedded turbidites and mudclast, within a silty matrix. A similar subbottom sedimentation pattern is found in a core further downsystem (Fig. 7E), confirming that both turbidites and mass-transport deposits are present in the Orlando fan.
Disconnected Canyon Fans
Naso Mixed Fan
The Naso canyon head is far from the coastline, at a distance of ∼2 km, corresponding with an embayment in the shelf break (Fig. 5). Upslope from the Naso fan, the sediments of the last transgressive and highstand sea-level stages are restricted to the shelf (Pepe et al., 2003). The Brolo abandoned canyon, 4 km to the east, has a head physiography similar to that of the Naso canyon. It has a low backscatter and shows a thin-bedded drape in the subbottom profile (Fig. 6A), suggesting that is it not a coarse-grained setting. It is therefore reasonable to assume that a similar sedimentary setting currently characterizes the Naso canyon. The Naso disconnected canyon links with the Naso channel (Figs. 1 and 4). The straight western flank of the channel is interpreted to be fault controlled, including gullies and small landslide headwall scarps that are positioned on the channel flank (Figs. 8A, 8C). Downchannel, significantly larger landslide headwall scarps continue to modify the western channel margin. Mass-transport deposits are abundant on the channel floor, as shown by the widespread blocky sidescan sonar facies (Fig. 8C). However, turbidity current activity is also exhibited in the channel floor through the excavation of two inner thalwegs, discontinuous fields of sediment waves, and longitudinal furrows (Fig. 8C). Furthermore, on the eastern side of the channel, turbidity current overspill forms a levee with sediment waves and scours (Figs. 2 and 4). Seafloor sampling confirms that both turbidites and mass-transport deposits are present in the channel floor (Fig. 8B).
Zappulla Mass-Transport Deposit Fan
In the shelf adjacent to the head of the Zappulla canyon, filled incised valleys show that the lowstand feeding system is currently shut off (Fig. 9C). The Zappulla channel is fed by disconnected canyon heads within a large shelf edge embayment affected by sediment failure (Figs. 1, 5, and 9C). The channel parallels the base of slope and has a very high backscatter and blocky seafloor indicative of mass-transport deposits (Figs. 2 and 3). Distally, the Zappulla channel connects to small lobes (8 km long and 5 km wide) with a similar backscatter pattern (Figs. 3 and 9A), again suggesting mass-transport deposits (Fig. 4). The mass-transport nature of the high backscatter lobes is confirmed by seafloor samples (Figs. 9B, 9E). Mass-transport deposits compose the major part of the subbottom succession of the Zappulla fan lobe, as shown by a stack of 4 transparent layers in the last 50 m of the lobe sequence (Fig. 9D).
Abandoned Canyon Systems
Both the eastern and western slope sectors bounding the described active fan systems display intermediate backscatter in the canyons (Fig. 2), providing evidence that coarse-grained sediment is not currently delivered to the systems. In the 3-km-wide eastern and 6-km-wide western shelves, canyons do not indent the shelf break and thus are currently shut off from a sediment supply (Figs. 1 and 4).
Connected Canyons Fan Facies
The Calavà and the Orlando connected canyons form, respectively, a turbidite and a mixed fan (Figs. 4 and 10A). The fans show abundant evidence of sediment waves, scours, comet marks, and sandy turbidites at the seafloor, suggestive of highly turbulent flows, such as those initiated due to oceanographic processes and hyperpycnal flows (Piper and Normark, 2009; Romans et al., 2009). Northern Sicily is a mountainous, high-relief region and the rivers have an intermittent regime (called fiumara) with river floods normally occurring twice a year in correspondence with rainy seasons (Regione Siciliana, 2010). The solid discharge of the rivers facing the study area can reach values of 80,000 m3/yr (Brambati et al., 1995). The area is also prone to flash floods, exemplified by three events in northern Sicily between 2007 and 2009 (Aronica et al., 2012). During flash floods, exceptionally large volumes of sediment can be transported to the river mouths, as shown by the mobilization of 780,000 m3 of sediment in the 2009 event in a catchment of only 10 km2 close to the study area (Aronica et al., 2012). During this episode, the occurrence of hyperpycnal flows was observed at fiumara mouths (Casalbore et al., 2011).
The Calavà canyon head is located in front of the mouth of a relatively large river (Fig. 4). In the coastal area between the canyon head and the river mouth, Holocene deposits have a thickness of <10 ms twt (two-way traveltime; Fig. 11A), equivalent to about 10 m. The deposits thicken away from the very narrow shallow-water area between the river mouth and the canyon head and form two separated sediment bulges (Fig. 11A). This setting is here interpreted as the evidence that the area that connects the river, and the canyon head is characterized mostly by sediment bypass. We therefore conclude that much of the river-born sediments bypass the coastal area and are fed directly to the Calavà canyon head through hyperpycnal flow processes during river floods. A field of sand waves is located west of the canyon at a depth of ∼50 m (Fig. 11A). To the west of the Calavà canyon, two large rivers form deltas upslope from the disconnected Naso and Brolo canyons (Figs. 4, 5, and 10A). Longshore currents trend east (Brambati et al., 1995), and thus the north-south–trending axis of the sediment waves is indicative of an eastward drift of sediment that, intercepted by the canyon head, forms turbidity currents. Thus the proximal part of the Calavà canyon, on the west side of a northward-projecting cape, is fed both by hyperpycnal flows and by an eastward-flowing littoral cell of deltaic sediment (Figs. 4 and 10).
The Orlando canyon head is not connected with a major river, but there is a large river mouth only ∼5 km to the west (Fig. 4). To the east of the major river mouth, two large west-east elongate depocenters of Holocene sediment are separated in coincidence with the canyon head (Fig. 11B). They are the evidence that the eastward-flowing littoral cell redistributes the river-born sediment to form an asymmetric sediment distribution in the delta developed at the river mouth. We therefore interpret this fan as being fed by sediment redistributed by a littoral cell that is intercepted by the canyon. The Orlando fan shows that even a coast-connected system can develop a mixed facies (Figs. 4 and 10A). In this case, the substantial mass-transport component of the fan facies is the result of the abundant landslides generated in its slope portion, where extensive failure of the channel levee wedge occurs (Figs. 4 and 10A).
The thickness of the Holocene sediment is much higher in the coastal and shelf areas surrounding the Orlando canyon than in those surrounding the Calavà canyon (Figs. 11A, 11B). This setting can be the evidence that in the case of the river-connected canyon much of the river-born sediment is bypassed to the deep sea directly from hyperpycnal flows, and only a little amount is stored in the shelf surrounding the canyon head. On the contrary, in the case of the littoral cell–connected canyon, a large part of river-born sediment is stored in the shelf, and only that involved in the littoral cell escapes the shelf and is fed to the canyon head. The different capability of sediment storage in the shelf is in turn reflected in the size of the two fans, the river-connected one being much larger than the littoral cell–connected one.
Disconnected Canyons and Mass-Transport Deposit Fan Facies
The Zappulla and Naso disconnected canyons feed a mass-transport deposit and a mixed fan, respectively.
In the study area, active block faulting creates surfaces tilted as much as 0.8° (Sulli et al., 2013) that can represent a precondition for sediment failure. The Capo d’Orlando Basin is characterized by local widespread seismicity (Fig. 1A). Earthquake-triggered sediment transport within a canyon 10 km east of the study area was revealed by the rupture of a submarine cable in coincidence with two regional seismic events (Ryan and Heezen, 1965). It can therefore be concluded that earthquake-related landslides favored by seafloor steepening are responsible for submarine fan growth in the area where the canyons are not connected.
In these fans, the facies of the failed sediment is variable, consisting of both muddy slope deposits of the Zappulla canyon areas and thin-bedded sandy turbidites formed on the levees of the Naso submarine channels, as shown by cores. Sandy mass-transport deposits are transformed into turbidity currents more easily than muddy ones (Tripsanas et al., 2008). The capacity of mass-transport deposits to transform into turbidity flows is also enhanced by higher seafloor gradients (Piper et al., 1999b; Piper and Normark, 2009). The gradient of the Naso channel is 2.5°, whereas that of the Zappulla is only 1.0°. Thus, in the Naso channel, the seafloor gradient and the facies of the collapsed material cause landslides to readily transform into turbidity currents in a way similar to the Grand Banks (Piper et al., 1999a). As a result, the Naso fan develops a mixed nature that contrasts with that of the Zappulla fan, where mass-transport deposits are prevalent (Figs. 4 and 10A).
Tectonics as Ultimate Control on Fan Facies
All the canyons of the study area were linked with river mouths during the last sea-level minimum, when incised valleys formed through river excavation of the exposed shelf, as shown in the nearby northeastern Sicilian continental shelf (see Gamberi et al., 2014). During sea-level fall and lowstand, rivers incise deeper where the shelf is narrower and steeper and where the shelf edge is at shallower depth (Tornqvist et al., 2006; Mattheus and Rodriguez, 2011). In the study area, a narrower, steeper shelf and a lower depth of the shelf edge characterizes the areas where the two connected canyons are located (Figs. 4 and 10). On the contrary, a relatively wide, gently sloping shelf is present in the areas where the Zappulla and the Naso disconnected canyons are located (Figs. 4 and 10). The geomorphology of the shelf therefore substantiates that the connected canyons are now present where the deeper incised valleys formed during river incision of the emerged continental shelf.
Active tectonics in the studied margin are responsible for fault-block tilting and differential rates of uplift (Fig. 10A). The Orlando and Calavà Capes are part of a structural high with high uplift rates (1 mm/yr; Di Stefano et al., 2012) (Fig. 10A). To the west of the Cape Orlando, a northeast-southwest–trending extensional fault parallels the coastline and causes the lowering of the coastal area and of the shelf (Figs. 4 and 10A). It is apparent that in the study area, the connected canyons are located in the areas with the highest uplift rate. We therefore conclude that it is the rate of vertical movement of blocks that determines the character of canyons, by controlling shelf gradient and width and thus the degree of incision of rivers during sea-level lowstand. Connected canyons feeding turbidite fans are developed only in connection with blocks characterized by the highest rate of relative uplift.
Deep-Sea Fan Facies: Active versus Passive Margins
Sequence stratigraphy models, developed on passive margins with long-term subsidence, show that the facies of deep-sea fans are mainly dependent on shifts of the coastline position (Catuneanu et al., 2009, 2011) (Fig. 10B). Models relate deep-sea fan facies to specific points in time along a cycle of sea-level variation, each with its own character of sediment delivery to the deep sea (Catuneanu et al., 2009, 2011). A long-term 1 mm/yr regional uplift affects the northeast Sicilian margin (Westaway, 1993) and block faulting results in local uplift rates as high as 5 mm/yr (Di Stefano et al., 2012). Our work therefore offers the possibility to highlight the divergence between deep-sea fan facies development in tectonically active margins and in passive margin models. (Fig. 10B). Our work shows that it is the landward migration of the canyon heads that drives the possibility of sediment delivery to the deep sea during the present highstand. Where a more efficient connection with river discharge exists, most of the river-born sediment is directly fed to the canyon heads, allowing the building of sandy turbidite fans (Fig. 10A). In addition, our study suggests that the largest facies variability in deep-sea fans is to be expected during highstands. During highstands the disconnected canyons are cut off from coastal sediment supply, and, depending exclusively on sediment supply from seafloor instability, build mass-transport fans. Our work shows that landslides and slope channel gradient can initiate or modify sediment delivery to the deep sea, thus controlling the final facies of deep-sea fans and the formation of mixed fan deposition. Attributes intrinsic to the deep-water slope portions of the systems are therefore vital in modifying deep-sea fan deposition in active margins. The essential role of the slope is thus a further aspect that sets apart deep-sea fan facies in active and passive margins, the latter being controlled merely by the position of the coastline and the resultant variation of sediment delivery processes to the deep sea.
The integrated interpretation of geophysical data and seafloor samples provides the opportunity to study the sedimentary processes and the facies of present-day deep-sea fans along an active margin as a function of canyon connectivity to sediment sources, the initiation mechanisms of sediment transfer to canyons, and flow response to gradient changes. Most of the deep-sea fans of the study area continue to receive sediment during the current highstand, as shown by other examples worldwide. Thus our findings consolidate the idea that, particularly in active margins, present-day sediment delivery to the deep sea is relatively common.
The processes that feed sediment within the canyons, in turn controlled by the type of connectivity of the canyons to sediment sources, are a major control on the facies of the deep-sea fans along the studied active margin.
Coast-disconnected canyons are fed by landslides and form mass-transport deposit fans; however, depending on the lithology of the failed masses and on the slope gradient, landslides can transform into turbidity currents and as a final result a mixed fan is formed.
Coast-connected canyons, with heads close to the coastline, can be river connected and are fed by hyperpycnal flows, or littoral cell–connected canyons fed by longshore currents that rework coastal and shelf sediment.
Single coast-connected canyons can also be characterized by multiple sediment supply mechanisms when hyperpycnal flows and littoral cells combine as sediment sources.
Coast-connected canyons have the ability to build large turbidite fans; eventually sediment failure in the slope part of the systems can cause landslides that result in the building of mixed fans.
Both coast-connected and disconnected canyons can form mixed fans, in the first case as a result of sediment failure in the slope part of the system, and in the latter as a result of flow transformation of mass-transport processes into turbidity currents.
Coast-connected canyons form in areas with high uplift rates, with a narrow and steep shelf; disconnected canyons develop where lower uplift rates or subsidence is occurring and the shelf is large and relatively gentle.
Overall, we show that, in active margins, sediment transfer to the deep sea is not merely a function of sediment supply to the margin and shelf accommodation space driven by subsidence, as in passive progradational margins. Coastal progradation is in fact not a prerequisite for sediment delivery to the deep sea, as demonstrated by the Sicilian margin source to sink sediment transport system. Regardless of their absolute volume, sediments can bypass the shelf where river-connected canyons are fed by hyperpycnal flows from rivers that, not having space for the construction of prograding deltas in the shelf, directly feed their load to the canyon heads. In this case, the lack of accommodation for river-born sediment in the shelf is brought about by canyon landward migration rather than by coastal progradation as in passive progradational margins. Furthermore, even when rivers, being not directly connected with canyon heads, have enough accommodation space to build deltas in the coastal areas, littoral cell–connected canyons can be active where there is a favorable interplay among canyon head, river delta location, and the direction of sediment transport by littoral cells.
We thank the technical crews and colleagues who made possible the acquisition of the data set during oceanographic cruises over a long time span. Alessandra Mercorella and Elisa Leidi reprocessed the compilation of swath bathymetry data; Andrea Gallerani assisted with sample collection. We thank the participants and the Chief Scientists Michael Ivanov and Neil Kenyon of the TTR-15 cruise of R/V Professor Logachev. We also thank Brian Romans and Lorna Strachan for their reviews on an early version of this manuscript, and David Piper for insightful comments and suggestions through the various stages of development of this paper. This work was supported by the “MAGIC project” (Marine Geohazards along the Italian Coasts) of the Dipartimento della Protezione Civile and by the “Ritmare project” of the Programme Nazionale della Ricerca funded by the Ministero dell’Università e della Ricerca.