Geomorphology of the NE Sicily continental shelf controlled by tidal currents, canyon head incision and river-derived sediments

Geomorphology 217 (2014) 106–121

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Geomorphology of the NE Sicily continental shelf controlled by tidal currents, canyon head incision and river-derived sediments Fabiano Gamberi ⁎, Marzia Rovere, Alessandra Mercorella, Elisa Leidi, Giacomo Dalla Valle Istituto di Scienze Marine, Consiglio Nazionale delle Ricerche, Via P. Gobetti 101, 40129 Bologna, Italy

a r t i c l e

i n f o

Article history: Received 27 June 2013 Received in revised form 24 March 2014 Accepted 25 March 2014 Available online 18 April 2014 Keywords: Relict continental shelf Sea-level variations Submarine canyon head Coastal barrier–lagoons system River delta Sandwaves

a b s t r a c t The NE Sicily continental shelf, imaged by multibeam bathymetry data and CHIRP/sparker seismic profiles, is less than 5 km-wide, and is located in a tectonically active margin characterized by strong regional uplift rates. In this paper, we show how variations of geomorphic elements in the study area are tied to spatial and temporal changes in the driving forces that control the seafloor processes. This study demonstrates that the geomorphology of continental shelves can vary over very short spatial scales depending on the uneven distribution of sediment supply from rivers and sediment transfer both across and along the shelf by oceanographic currents. In the northeastern part, three sandwave fields were mapped in the highstand sediment wedge that, due to the small size of rivers, is restricted to the inner shelf. The sandwave fields are found in proximity of the Messina Straits, a shallow water sill with strong tidal currents between the Tyrrhenian and the Ionian Seas. The bedform fields have sandwaves of variable shape, wavelength and orientation, reflecting along-shelf variations of tidal current strength and sediment grain size distribution. In the southwestern shelf, rivers are larger and form deltas that shape a considerable part of the shelf, often having their distal, still channelized delta front at the shelf edge. In some cases, deltas are built close to the heads of canyons and a large volume of the river-borne sediments is directly fed to the deep sea through delta front terminal distributary channels. Where rivers are small, the outer shelf lacks recent river borne sediment and presents a relict morphology consisting of submerged coastal systems formed during previous sea-level lowstands. The tectonics of the study area mainly consist of structures that have a NNE–SSW trend similar to the extensional faults responsible for the Siculo-Calabrian Rift Zone in the nearby emerged areas. Our study extends the area affected by the regional deformation belt into the NE Sicily offshore. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Continental shelves are the submarine areas closest to land and, as a consequence, fluvial sediment transport to the offshore, albeit in combination with oceanographic processes, is the main control for the development of geomorphic elements on the shelf. Waves, storms, currents and gravity-driven flows are the main oceanographic processes that are active in the shelf and can redistribute river-borne sediment across and along the shelf, creating sediment transport gradients and contributing to the creation of geomorphic elements on the shelf (Wright, 1995; Fagherazzi and Overeem, 2007). In addition, climate and tectonics operate at different spatial and temporal scales in controlling the way and the site of sediment accumulation (Wright, 1995; Fagherazzi and Overeem, 2007; Sommerfield et al., 2007). For example, temporal migration of different environments on the shelf occurs due to the advancement and retreat of the coastline in response to sea-level variations.

⁎ Corresponding author. Tel.: +39 051 6398889; fax: +39 051 6398940. E-mail address: [email protected] (F. Gamberi).

http://dx.doi.org/10.1016/j.geomorph.2014.03.038 0169-555X/© 2014 Elsevier B.V. All rights reserved.

The location of the rivers and the size of the discharge from their drainage basins remain the main controls on the site of sediment input to the coastal area and on shelf depocenters (Swift and Thorne, 1991; Wright, 1995; Olariu and Steel, 2009). Along continental margins, modern deltas started to develop, following the decelerated eustatic sealevel rise, in late Holocene (Stanley and Warne, 1994; Sommerfield et al., 2007). Depending on the amount and type of river-borne sediments, shelf morphology and oceanographic conditions, deltas of different sizes and nature were formed and their progradation can now be seen to have reached different locations across the shelf (Burgess and Hovius, 1998; Porebski and Steel, 2006). Although modern deltas are mainly restricted to the inner shelf, in some cases their distal portions can also reach the outer shelf and the shelf edge (Porebski and Steel, 2006). Continental shelves are also the areas where sea-level oscillations have the widest effects, resulting in phases of emergence and submergence that alternate over several thousands of years during cycles of sea-level variations. As a consequence, the geomorphology of the continental shelf not only records the present-day morphogenetic agents but it can also document past landscape shaping processes. The latter is the case where original landforms created during past sea-level stages have

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not been overprinted by subsequent sedimentation or erased by successive erosional processes and are found outcropping on the seafloor of relict shelf portions (Shepard, 1932; Emery, 1968; Swift et al., 1971; Mallett et al., 2013). Particularly in active margins, tectonic structures, both active and quiescent, play an important direct role in the resulting morphology of the continental shelf and can have a significant impact on the development of other processes, such as current pathways and intensities, sediment entry points, the creation and destruction of accommodation space. Offshore tectonic structures are sometimes more revealing than their terrestrial counterparts, which are often largely obscured by the weathering of the subaerial environment. Furthermore, the structural style of the shelf allows for the extension of the interpretation of regional tectonic structures into the offshore and to refine the pattern of potential seismogenic sources in seismically active areas. On continental shelves, large-scale tidal bedforms, such as megaripples and sandwaves are important geomorphological elements for their impact on human activities (energy, communication and shipping industries) and their relevance in marine spatial planning (sand extraction). Sandwaves are large flow-transverse bedforms formed by a reversing flow such as a tidal current (Allen, 1980). The use of the descriptive term “sub-aqueous dunes” was recommended by Ashley (1990) for all large-scale flow-transverse bedforms. These bedforms show height classes of 0.25–0.4 m (small), 0.44–2.8 (large) and N 2.8 m (very large) (heights calculated after Berné et al., 1993) and corresponding spacings of 5, 10 and 100 m. Sometimes the term dune is used when the sediment grain size is unknown (e.g. Gómez et al., 2010), but the term “sandwave” is also used irrespective of the grain size (Barnard et al., 2006). Van Landeghem et al. (2009) used the term “sediment wave” rather than “sub-aqueous dune” in the Irish Sea to indicate those bedforms where the sediment composition could not be identified, leaving the term “sandwave” for those where the grain size was available. On the other hand, in the literature, sediment waves are often regarded as those bedforms created by either downslope-flowing turbidity currents or along-slope-flowing bottom currents in deep-water settings (Wynn and Stow, 2002). These sediment waves display wave heights of 1–70 m and wavelengths of 0.1–6 km (Wynn et al., 2000). In this paper, we use the term “sandwave”: i. To avoid confusion with the sediment waves that are present in the slope channels of the study area (Gamberi and Rovere, 2011; Gamberi et al., 2013); ii. To be consistent with the literature, where the term “sandwave” is more frequently used instead of dunes, when dealing with modeling and coastal engineering studies (e.g. Besio et al., 2004). Compound sandwaves are defined as flow-transverse marine subaqueous dunes with superimposed megaripples (10 m wavelength and 1 m height) and have a typical length of 100 to 800 m and heights of several meters (Rubin and McCulloch, 1980; Ashley, 1990; Besio et al., 2004; van Dijk and Kleinhans, 2005). In this paper, we describe the geomorphology of the NE Sicily shelf through the interpretation of multibeam bathymetric data and CHIRP seismic profiles. We show how its current morphodynamics are largely controlled by the varying size of the river drainage basins along the margin and how the tectonic structures, mapped in the shelf, mimic the complex tectonic framework that characterizes the deformation belt (Siculo-Calabrian Rift Zone) present across northeast Sicily and southern Calabria. 2. Geological setting The study area is located along the NE Sicily margin between Capo Milazzo and the area offshore Capo Rasocolmo (Fig. 1a). On land, the Peloritani Mountains are the southern portion of the Calabrian Arc where the tectonic basement nappes of the Kabilo-Calabride units, that tectonically overlie the Apenninic–Maghrebian Chain, outcrop (Lentini et al., 1996, Fig. 1a). The orogenic wedge was affected by the

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extensional tectonics that led to the opening of the Tyrrhenian backarc basins at the rear of the Maghrebian thrust belt (Patacca et al., 1987). As a result, NNE–SSW-trending normal faults and NW–SEtrending strike–slip faults give rise to the horst and graben structural setting of NE Sicily (Di Stefano and Lentini, 1995; Lentini et al., 2000). Basement nappes of the Kabilo–Calabride units are exposed on the NE–SW elongated horsts (i.e. Castanea Ridge), whereas Miocene to Quaternary sediments fill the grabens (i.e. Barcellona Trough; Fig. 1a). Within this complex tectonic setting, the largest tectonic feature of the Calabrian Arc (Fig. 1b) is the Siculo-Calabrian Rift Zone (SCRZ, Fig. 1c), a 370 km long belt that runs continuously along the inner side of the Calabrian Arc, through the Messina Straits and along the Ionian coast of Sicily and extends also westward into the Aeolian Islands Arc (Monaco and Tortorici, 2000). High regional uplift rates affect the NE Sicily area (Westaway, 1993) and are amplified by local uplift connected with the vertical movements of single fault blocks of probably active structures (Catalano et al., 2003; Scicchitano et al., 2011). Although uplift rates are high and co-seismic slip has been suggested, in the study area no significant seismicity, focal mechanisms (Gasperini et al., 2012) or stress indicators are mapped (Montone et al., 2012). 3. Data set The Digital Terrain Model (DTM) used for the interpretation of the geomorphology of the study area (Fig. 1a) was compiled from data acquired with different multibeam systems on board several oceanographic vessels. High resolution bathymetric data were acquired during two cruises in the upper slope and shelf area, carried out on board R/V Mariagrazia in 2009 and 2010 using a hull-mounted Kongsberg EM3002D (300 kHz) and a pole-mounted Reson 7111 (100 kHz) multibeam system respectively. In 2011 some sectors of the outer shelf were re-mapped with a hull-mounted Kongsberg EM710 (70–100 kHz) multibeam system on board R/V Urania. All the multibeam data have been merged and post-processed using CARIS HIPS and SIPS software; a 5-m-resolution DTM was produced for the shelf area, while a 20-m-resolution DTM was attained for the slope area. Together with multibeam data acquisition, high resolution CHIRP seismic profiles were acquired on board R/V Mariagrazia and Urania with a hull-mounted Teledyne BENTHOS III CHIRP system having a frequency modulation between 2 and 20 kHz. The CHIRP profiles shown in this paper were acquired in 2009. They are spaced at about 2 km (Fig. 1a) and have 0.5-m-vertical resolution. Two seismic profiles were acquired in August 2103 with a 1 kJ sparker source and recorded by an EdgeTech 265 hydrophone cable towed behind the vessel (Fig. 1a). 4. Data description and interpretation 4.1. Shelf sector n. 1 The shelf sector n. 1 is comprised between the northeastern limit of our data and the Sindaro canyon head (Figs. 1a, 2). Here, the inner continental shelf down to a depth between 80 and 70 m has an average dip of 1.85°, with slope ranges of 0.8° and 2.7°. Three fields of sandwaves with different characteristics are present (Fig. 2). The southern sandwave field has bedforms with NW–SE directed crests (Fig. 2). They consist of relatively smaller and more discontinuous bedforms with wavelengths of 250 m and heights of 1 m superimposed on larger bedforms with a spacing of 500 m and heights of 2 m (Fig. 3a). The central sandwave field has bedforms with crests oriented in an E–W direction and variable sizes (Fig. 2). They consist of very large (sensu Berné et al., 1993) bedforms with wavelengths of 500 m and heights of 5 m and superimposed large sandwaves with spacing of 250 m and heights of 2.5 m (Figs. 3b, 4a, b). Due to the close range of sizes, these cannot be described as compound sandwaves. Simple sandwaves are

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Fig. 1. a) Shaded relief map of the NE Sicily shelf resulting from the compilation of different multibeam surveys. Boxes correspond with the detail images of Figs. 2, 6, 8 and 11, showing the four shelf sectors described separately. b) Regional location of the study area; the black arrows show the extensional regime responsible for the Siculo-Calabrian Rift Zone above the Calabrian Arc subduction zone. c) Schematic reconstruction of the faults described in this paper (offshore) as compared to those of the Siculo-Calabrian Rift Zone (SCRZ) in NE Sicily and in Calabria. Land geology is from Lentini et al. (2000).

also present in this field (Fig. 3b). The northern sandwave field consists of only two sandwaves with ENE–WSW trending crests; they are simple features and are the largest bedforms of the study area with an amplitude of 10 m and a wavelength of about 1000 m (Figs. 3c, 4a, c). Between the central and the northern sand waves field, an isolated bedform is also present (Fig. 2). In the sand waves fields, the CHIRP profiles show very low penetration and no reflections; since penetration is largely inversely proportional to sediment grain size, a relatively coarse grained seafloor is interpreted.

Further offshore, the outer shelf is relatively flat with an average seaward dip of 0.43°, with angles ranging between 0.2° and 0.7°. Here a morphologically complex seafloor is present (Fig. 2). Two narrow ridges with arcuate planforms (interpreted as coastal barriers in Fig. 5a) are on average 3 m higher than circular or elongate shaped depressions developed landward from the ridges. The arcuate ridges are interpreted as coastal barriers that were developed seaward from lagoons corresponding with the depressed areas. Ridges with a more linear planform are also present (linear ridges in Fig. 5a). In some areas, v-shaped incisions

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Fig. 2. Shaded relief map of the shelf sector n. 1 (location in Fig. 1). The box corresponds with the area of Fig. 5.

about 150-m-wide and few meters deep can also be observed crossing the shelf for a length up to 200 m (Fig. 5a). The incisions are interpreted to be drowned incised valleys corresponding with a past river course eroded during the last sea-level lowstand when the shelf was subaerially exposed (Fig. 5a). As a whole, the geomorphology of the area can be interpreted as representing submerged coastal barrier–lagoon systems and incised valleys corresponding with past river courses, spanning the bathymetric range from about 70 to 100 m (Fig. 2). The outer part of the northeastern shelf sector represents therefore a relict shelf, where coastal and subaerial morphologies are present at the seafloor because they have not been eroded or buried by the later deposition of the highstand wedge deposits that correspond with the inner shelf sediment wave fields (Fig. 2). A bathymetry step present between 80 m and about 110 m water depth limits the submerged coastal barrier–lagoon systems seaward and is interpreted as corresponding to a 10 km tectonic escarpment composed of discrete NE–SW and NNE–SSW oriented fault segments (Fig. 2). The tectonic lineament is less marked westward, although north from the Sindaro canyon head it still corresponds with a relatively steeper area in the outer shelf (Fig. 2). Incisions interrupt the continuity of the tectonic escarpment landward from the heads of Rasocolmo 1 and 2 canyon heads (Figs. 2, 5a). The incisions represent previous riverincised valleys indicating that during a lower stand of sea level, rivers breached the tectonic escarpment to directly connect with the canyon heads. Two steep linear escarpments with a length of 3.5 km and a relief of about 7 m, which can also be interpreted as faults, are present in the southwestern sector of this shelf sector (Figs. 2, 5b). Relict coastal morphologic elements, consisting of linear elongated ridges, are aligned along some segments of the faults (Figs. 2, 5a). The faults disappear westward approaching the Sindaro canyon head, being presumably buried and perhaps healed westward by the southern field of sandwaves (Figs. 2, 5c). Seaward from the tectonic escarpment, the distal shelf is on average 200-m-wide and steep, with an average gradient of 3.95°. Its width is

even reduced where the Rasocolmo 1 and 2 canyon heads incise landward from the otherwise relatively continuous shelf edge (Fig. 2) that corresponds with a sudden gradient increase to 20°. 4.2. Shelf sector n. 2 The shelf sector n. 2 is comprised between the Tarantona and the Villafranca canyon heads (Figs. 2, 6). The landward part of the continental shelf here is relatively featureless with a gradient of 1.52°. Only in the area facing the Gallo river, very low relief linear channels are present (Fig. 6, inset), possibly resulting from river-derived delta flows. In the middle shelf, a steeper NE–SW trending subtle escarpment most likely corresponds with a mostly healed fault trace that connects the heads of the Tarantona and the Villafranca canyons (Fig. 6). Seaward from the fault, the shelf remains steep and straight channels develop, and in general widen and deepen downslope; the largest one reaches a depth of 3 m and a width of 100 m. The channelized area is located seaward from the Gallo river delta, and the channels are interpreted as the result of erosion created by delta-derived sediment gravity flows. Offshore from the Santa Caterina and the Saponara rivers, the Villafranca canyon head has a 2000-m-wide, continuous semi-circular morphology at a depth of about 70 m that corresponds with a sudden gradient increase with values as high as 10° (Fig. 6). Landward from the canyon head, the shelf is less steep with a gradient of 4.5° and displays channels (Figs. 6, 7a). The main, deepest channel directly faces the present day Saponara river mouth and is about 15 m deep and 150 wide (Fig. 7a). Further channels are developed eastward, facing the Santa Caterina river mouth and showing a progressively shallower depth of incision away from the main channel (Fig. 7a). To the west of the main channel, only one channel is present with a depth of 10 m and a meandering planform (Figs. 6, 7a). The Saponara and Santa Caterina river mouths are located at a distance of only 500 m from the coastline, thus the channelized area is interpreted as an offshore, subaqueous delta front with channels formed by sediment gravity flows

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Fig. 3. Bathymetric profiles over the sandwave fields extracted from the 5-m-resolution DTMs displayed alongside (location in Fig. 2). a) Southern sandwave field. b) Central sandwave field consisting of both superimposed and simple bedforms. c) Northern sandwave field consisting of simple bedforms.

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Fig. 4. a) CHIRP profiles over the sandwave fields. b) CHIRP profile crossing the central sandwave field. c) CHIRP profile crossing the northern sandwave field. CHIRP profiles are not perpendicular to the sandwave direction and horizontal scales are different (location of profiles in Fig. 2).

during river floods, similarly to river-dominated delta systems described in Olariu and Bhattacharya (2006). Further offshore, at 1500 m from the coast, the Villafranca Canyon consists of four main erosional trunks separated by interfluves where deposition is, on the contrary, evident (Fig. 7b). The canyon represents a significant erosional feature that is incised about 100 m with respect to the surrounding shelf (Fig. 7b).

4.3. Shelf sector n. 3 The shelf sector n. 3 is located between the Villafranca and Cocuzzaro canyon heads (Figs. 2, 8). In the inner shelf, to the west of the Villafranca canyon head, an area with seaward convex bathymetric contours is present. It is located offshore from the Rometta river delta and, for this reason, is interpreted as representing its delta front offshore continuation (Fig. 8). It has a gradient that from 2.38° at a depth of 50 m, the upper limit of our data, decreases to 1.06° distally. The delta is 1200-m wide and 6-m high at a depth of about 47 m, being about 2-km wide and only 4-m high at a depth of about 55 m. Sand waves are widespread over the whole surface of the Rometta Delta offshore continuation (Fig. 8). The sandwaves in front of the delta extend into deeper water eastward, showing that the delta is asymmetric (Fig. 8). The delta asymmetry is confirmed by the CHIRP profiles (Fig. 9a), showing that its upward convex lens of sediment with strong and discontinuous reflection thin eastward, approaching the head of the Villafranca Canyon. Seaward from the Rometta Delta, a complex morphology with linear escarpments, that are interpreted as faults, characterizes the outer shelf, at a depth greater than 70 m (Fig. 8). Between the distal part of the Rometta Delta and the Rometta canyon head, the fault pattern results in a horst elongated in a NE–SW direction (Fig. 8). Faults are also present in the area between the Rometta and the Cocuzzaro canyon heads (Fig. 8). Here they result in a staircase seafloor morphology that progressively lowers the single fault blocks going offshore; the faults have a variable trend between NE–SW and E–W, but N–S oriented faults cause further block fragmentation. In this area, one of the faults causes an offset in the sediment on the seafloor suggesting its possible recent activity (Fig. 9b).

Beside faults, features that can be interpreted as the morphologic evidence of past coastal systems further complicate the setting of this area (Fig. 10). As an example, in the bathymetric map of Fig. 10 an arcuate ridge and small circular or elongated elevated areas bound a low bathymetry area landward from a fault. This morphology is interpreted as representing a drowned coastal barrier–lagoon system which represents deposits of a lower stand of sea level now submerged in the outer relict shelf. In the same area, v-shaped, relatively continuous incisions can be interpreted as incised valleys formed in coincidence with past rivers. Some of the ridges corresponding with the inferred drowned barriers are shown in the CHIRP profiles (Fig. 9b); they appear as reliefs cored by highly reflective facies, indicative of coarse-grained sediments, that are in general outcropping on the seafloor. Offshore from the Cocuzzaro River, the offshore part of its delta has a relief of only 2.5 m and in its upslope part, at a depth of about 50 m, it has a width of 1 km (Figs. 8, 9a). In the shallower part of the offshore delta, small-scale bedforms (25–50 m in wavelength and 0.5–1 m in height) are present (inset in Fig. 8). Further downslope, a narrow, 20-m-wide and 0.5-m-deep channel develops and connects with the head of the Cocuzzaro Canyon (Fig. 8). To the west of the delta, a steeper shelf sector might mark the southward continuation of the NE–SW trending faults that are present further east (Fig. 8). 4.4. Shelf sector n. 4 The shelf sector n. 4 is located in the southwestern margin of the study area (Fig. 11). The area is dominated by the Niceto and Corriolo canyons developed in front of the homonymous rivers (Fig. 11). The Niceto canyon head has a semi-circular 3000-m-wide area characterized by a sudden increase of the seafloor dip to 15°, occurring at about 80 m water depth (Fig. 11). Landward, the shelf is less steep with a gradient of about 4° and consists of a channelized area that connects with the eastern portion of the Niceto canyon head (Fig. 11). A CHIRP profile acquired at a distance of about 1 km from the coastline (Fig. 12a), corresponding with the upper limit of our data coverage at 30 m water depth, shows that the largest channels are located eastward and directly face the Niceto river mouth. A network of smaller tributary channels join the main channel (Fig. 11). To the west, further channels

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Fig. 5. a) Bathymetric contours of the relict outer shelf in the shelf sector n. 1 (location in Fig. 2). The relict morphology consists of coastal barrier–lagoon systems, traces of incised valleys and linear ridges. Contours are every 1 m, bold contours every 10 m. b) Sparker profile across the outer shelf in shelf sector n. 1 (location in Fig. 1). One of the faults corresponds with a seafloor step. The highstand sediments correspond with the wedge that in the inner shelf thins seaward and closes in correspondence of the fault. c) Sparker profile over the western portion of the shelf sector n. 1 (location in Fig. 1). Here, the highstand wedge reaches the shelf edge and covers one of the faults.

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Fig. 6. Shaded relief map of the shelf sector n. 2 (location in Fig. 1). The inset shows a higher resolution image of the more proximal part of the channelized portion of the Gallo river delta. No relict outer shelf is present here and the highstand wedge extends to the shelf edge.

face the Muto river mouth with a relief that decreases progressively westward (Figs. 11, 12a, b). A CHIRP seismic profile located further offshore shows that the channels could be erosional or depositional, whereas the inter-channel areas are always depositional (Fig. 12b). The area between the Cocuzzaro and the Niceto canyon heads is characterized by downslope convex contours, straight central channels and bending lateral channels (Fig. 11). The straight central channels die out downslope, whereas the lateral channels converge seaward into the Cocuzzaro and the Niceto canyon heads (Fig. 11). This area is interpreted as the offshore channelized eastern continuation of the Niceto river delta, comparable in width to the Niceto river alluvial valley (Fig. 11). This part is also characterized by the presence of bedforms, which are 50–100 m in wavelength and 0.5–1.5 m in height (Fig. 11, inset). The Corriolo canyon head has a semi-circular area with a width of about 2 km and is characterized by a sudden gradient increase to 12°. Upslope from the canyon head, a channelized area is present on the shelf, with a gradient of 5.44°. Two main channels, which are 15 m deep and 150 m wide, are located in front of the Corriolo river mouth (Figs. 11, 12c). Further offshore, these channels converge into a single erosional trunk (Fig. 12d). Smaller channels are present eastward and westward from the two main ones, showing decreasing relief (Figs. 11, 12c) and the seafloor slopes gently westward from the main channels (Fig. 12c). This area is interpreted as the channelized offshore part of the Corriolo river delta, whose mouth is only 800 m landward. A steep lineament, with a NW–SE trend and possibly corresponding with a fault is present between Capo Milazzo and the western side of the Corriolo canyon head (Fig. 11). 5. Discussion 5.1. Tectonics The study area is part of the Calabrian Arc, the segment of the Mediterranean orogen connecting the Southern Apennines and the

Maghrebian thrust belts (Fig. 1b). The Calabrian Arc orogenic thrust stack was formed mainly during Oligocene and Miocene due to the collision between the African and Eurasian plates. Successively, the Calabrian Arc was affected by extensional and strike–slip tectonics due to the opening of the Tyrrhenian Sea back-arc system and orogenic wedge extension in the rear of the chain. Within this complex tectonic setting, the largest Quaternary regional feature of the Calabrian Arc is the Siculo-Calabrian Rift Zone (SCRZ), a 370 km-long extensional belt that runs continuously along the inner side of the Calabrian Arc, through the Messina Straits and along the Ionian coast of Sicily, also extending westward into the Aeolian Islands Arc (Monaco and Tortorici, 2000) (Fig. 1c). This rift belt is the result of a regional extension domain with an ESE–WNW direction and has an overall NNE–SSW direction (Fig. 1c), being characterized by separate branches and 10- to 50-kmlong distinct fault segments (Catalano et al., 2008). While the structures that make up the SCRZ are well known on land, less knowledge is available on their distribution in the offshore. The faults imaged by our data are in agreement with an ESE–WNW extension direction and thus document that structures pertaining to the SCRZ are present in the northeastern Sicilian shelf. In the hinterland of the northeastern part of the study area, the extensional structures originate a staircase of faults that progressively lower the Tyrrhenian coastal side. The age of fault activity decreases from the hinterland toward the coastal area. The faults observed in the shelf sector n. 1 (Figs. 2, 13) can represent the most recent structures responsible for the foundering of the northeastern Sicilian offshore with respect to the Peloritani Mountains culmination along the Tyrrhenian–Ionian divide. Furthermore, the movement along this fault can also be responsible for the large masstransport deposits that are located in the continental slope (Gamberi and Marani, 2006; Gamberi et al., 2011; Rovere et al., 2013). A more complex tectonic style is evident in shelf sector n. 3 (Figs. 8, 13). Although, NE–SW trending faults are still predominant, and arranged in a series that progressively lowers the bathymetry on the outer shelf portions, NW–SE faults are also present. In the SCRZ, faults

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Fig. 7. a) Bathymetric profile crossing the Saponara and Santa Caterina rivers delta (location in Fig. 6). b) CHIRP profile crossing the Villafranca canyon head (location in Fig. 6).

with similar direction act as transfers between different branches and have a strike–slip character (Monaco and Tortorici, 2000). A similar interpretation is advanced for the NE Sicily continental shelf. The SCRZ is the major seismogenic source for southern Italy (Catalano et al., 2008), therefore our data show that the deformation belt structures are also present in the offshore area, and pose new constraints on hazard evaluation for the Sicilian area. At the southwestern limit of our study area, Scicchitano et al. (2011) have shown that Capo Milazzo was affected by two paleo-earthquakes with coastal displacement in the Late Antiquity, with the source likely corresponding with an offshore fault. The fault along the western margin of the Niceto canyon (Figs. 11, 13) can be identified as one possible source for the observed coastal deformation. 5.2. Rivers and large scale shelf morphology The tectonic pattern of northeast Sicily also controls the on-land drainage basin morphologies, with rivers that are small on the eastern side and become progressively larger to the west (Fig. 1; Table 1). In NE Sicily, rivers are mainly represented by 1st order streams (Strahler, 1952). Fiumara (Table 1) is the local name for seasonal streams characterized by high gradient and short length, an ephemeral regime and catchment areas developed almost entirely in high relief mountains

(Sabato and Tropeano, 2004). Torrente is the Italian name for an intermittent stream, characterized by a torrential regime, consisting of one or more seasons of dramatically reduced flow during the year (Table 1). On the Sicilian shelf, to the west of the study area, the sediments deposited during the present highstand of sea level can reach a thickness of about 50 m and are arranged in a prograding wedge of sediment that tapers seaward, in places covering the whole shelf (Pepe et al., 2003; Caruso et al., 2011; Sulli et al., 2013). A similar wedge of highstand sediment is present in the study area (Fig. 5b, c). In the northeastern sector of the study area (sector n. 1) (Fig. 5c), and in sector n. 3, the highstand wedge is limited to the inner shelf not reaching depths higher than 70 m, leaving uncovered the past coastal system outcropping at the seafloor of the outer relict shelf (Figs. 2, 13b). Since the relict shelf areas are located in correspondence of the smaller rivers, it is evident that it is the amount of sediment input from rivers that controls the seaward extent of the inner shelf highstand wedge and the presence of a relict outer shelf. Eastward from the Tarantona canyon head, no deltas have been mapped in 50 m water depth at a distance of only 1 km from the coastline (Fig. 13b). The offshore parts of deltas might be present in the area, but they do not reach the depth of 50 m. On the contrary, deltas are well developed at depths higher than 50 m in the southwestern sector of the

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Fig. 8. Shaded relief map of the shelf sector n. 3 (location in Fig. 1). The inset shows the small-scale bedforms associated with the channelized part of the Cocuzzaro Delta.

study area, where rivers are larger (Fig. 13b, Table 1). As a whole, therefore the distal parts of the deltas reach larger areas of the outer shelf portions going westward, mirroring the increase in the river drainage basin size (Fig. 1, Table 1, Fig. 13b). 5.3. The relict outer shelf The outer shelf of sectors n. 1 and 3, are characterized by relict geomorphic elements that represent past landscapes. The relict outer shelf morphology includes coastal barrier–lagoon systems and incised valleys (Figs. 5a, 10) that formed at different locations on the shelf, thus tracking the variations in coastline position during sea-level variations. Past coastal systems are mainly developed in the 70–100 m bathymetric range. If the coastal systems were formed during the last falling stage of sea level, taking into account the composite sea level curve of Waelbroeck et al. (2002), they should have been successively exposed for about 15,000 years to subaerial erosional processes and to reworking processes during the last transgression of sea level. However, the majority of the coastal systems are very well preserved and do not show any evidence of erosion. Thus it is reasonable to conclude that they are the expression of coastal systems developed during the last rise of sea level. The last sea-level rise has not occurred continuously but has proceeded in steps, with tracts of rapid rising punctuated by periods of relatively slower rates of rise (Fairbanks, 1989; Lambeck et al., 2011) (Fig. 11b). One of the tracts with a relatively lower rate of rise of seafloor

occurred when the sea level rose from about 80 to 60 m below its present position. This corresponds with the depth where, in the Sicilian relict outer shelf, the coastal systems are best developed. We therefore conclude that the submerged geomorphic elements in the study area could have formed during an interval of relatively reduced rate of sealevel rise allowing sediment supply to balance the relatively reduced pace of accommodation creation. The widespread presence of coastal barrier–lagoon systems in the relict shelf also shows that in the past, coastal areas were characterized by an environment that was much different from the coastal setting developed at the present-day highstand of sea level along the Sicilian margin, where lagoons are not present. This observation shows that different types of coastal systems develop across the shelf depending on the shift of sea-level position during cycles of sea-level variations, as proposed by Porebski and Steel (2006). However, our study also points out that the topography of the underlying basement can play a significant role in conditioning coastal system development. In addition, this finding poses interesting suggestions on the environmental conditions for early human settlements in the northeastern Sicilian area. 5.4. Controls on sandwave characteristics In the northeastern sector of the study area, where rivers are small and deltas do not reach the 50 m water depth, oceanographic currents are the prevailing geomorphic agents on the inner shelf, as shown by

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Fig. 9. a) CHIRP profile along the inner shelf showing the offshore parts of the Rometta and Cocuzzaro Deltas (location in Fig. 8). b) CHIRP profile along the outer shelf showing the horst bounded by extensional faults and the relict coastal structures (location in Fig. 8).

fields of sandwaves in the highstand wedge (Figs. 2, 13). The sandwave fields are close to the Messina Straits (Fig. 13), a 500-m-deep sill between the Tyrrhenian and the Ionian Seas, characterized by large gradients of tidal displacement that channel strong tidal currents with velocity up to 3 m/s in the sill region (Vercelli, 1925; Bignami and Sallusti, 1990; Brandt et al., 1999). Large sandwave fields, connected with the Messina Straits tidal currents, are abundant in the areas adjacent to the Messina Straits (Poluzzi et al., 1997; Santoro et al., 2002). Similarly, we therefore conclude that the sandwave fields in the study area are formed by tidal currents amplified in the nearby Messina Straits. The sandwaves pertain to three distinct fields with specific dimension attributes (Figs. 2–4, 13). Sandwave characteristics are mainly determined by sediment availability and grain size, current and wave energy and water depth (Allen, 1968; Rubin and McCulloch, 1980; Allen, 1982; Ashley, 1990; Flemming, 2000; Bartholdy et al., 2002; Francken et al., 2004; Van Landeghem et al., 2009). The southern sandwave field has the most limited cross-shelf areal extent (about 1 km) and bathymetric range (40–67 m), whereas the central and the northern sandwave fields span a larger cross-shelf extent (2–2.5 km) and bathymetric range

(40–83 m) (Fig. 2). The height of the sandwaves are not controlled by the water depth because different heights are present at the same water depth along the shelf. The sediment grain size shows a trend of coarsening from silt to sand from the NW Sicily margin toward the NE Sicily margin and the Messina Straits (Amore et al., 1995). Therefore, the size of the sandwaves in the northern field can be a result of sediment distribution. The coarsening of sediment toward the Messina Straits can also reflect the increasing velocity of currents going toward the Straits. The areas with no sandwaves in between the different fields also suggest that an uneven sediment distribution is present in the study area, with local grain size distribution that at places does not allow sand wave formation. Superimposed sandwaves, such as those formed in the central field (Fig. 3b), are formed as a result of local flow separation associated with larger bedforms morphology, unsteady flows or secondary flows (Allen and Collinson, 1974; Allen, 1978; Barnard et al., 2011). The northern sediment wave field, closer to the Messina Straits, does not develop superimposed bedforms, possibly indicating that more uniform flow conditions are encountered approaching the Straits.

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Fig. 10. Bathymetric contours of the relict outer shelf in the shelf sector n. 3 (location in Fig. 8). Contours are every 1 m, bold contours every 10 m.

5.5. Deltas and canyons From the Tarantona canyon head to Capo Milazzo, relatively larger rivers develop deltas, whose offshore portions are visible on the shelf and make up most of the prograding highstand sediment wedge of

the study area (Fig. 13b, Table 1). The Saponara, the Muto and the Corriolo river deltas develop upslope, from the head of the Villafranca (Figs. 6, 7, 13b), the Niceto and the Corriolo canyon heads (Figs. 11, 12, 13b) respectively. Our data image the deltas from a distance of 600 m from the coastline at an average depth of 40 m, showing that they

Fig. 11. Shaded relief map of the shelf sector n. 4 (location in Fig. 1). The inset shows the small-scale bedforms associated with the channelized part of the Niceto Delta.

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Fig. 12. a) Bathymetric profile of the Niceto and Muto river channelized deltas; b) CHIRP profile crossing the Niceto and Muto river channelized deltas; c) bathymetric profile of the Corriolo river channelized delta; d) CHIRP profile crossing the Corriolo river channelized delta. Location of profiles in Fig. 11.

are characterized by a channelized delta front with channels connected to the canyon mouth. The channelized sector has a width of about 1 km. At the present-day, the confinement and the construction of by-pass

structures force the rivers to discharge into the sea as single channels, without developing a subaerial pattern of distributary channels. Therefore, it is possible that the onset of the distributary channelized delta

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Fig. 13. a) Sea-level curve in the last 20,000 years after Lambeck et al. (2011), based on data collected in the northern Sicily margin. b) Schematic representation of the geomorphic elements in the study area.

part occurs in the proximal part of the subaqueous delta front, which is not imaged by our data, as terminal distributary channels (sensu Olariu and Bhattacharya, 2006) and that the submarine channels are not formed at the junction with subaerial distributary river channels. Another possibility is that, before anthropogenic impact on the terminal parts of rivers, the emergent delta plains consisted of a distributary channelized area, with subaerial channels directly connected with the channels in the subaqueous delta portion. This would imply that the channels away from the present-day confined river mouths are now inactive. Similar channels developed in connection with abandoned

Table 1 Dimensions of the drainage basins of the streams discussed in the text. Name

Type

Drainage basin (km2)

Perimeter (km)

Regime

dei Corsari Tarantona Gallo Saponara Cocuzzaro Niceto Muto Corriolo Mela

Fiumara Fiumara Fiumara Torrente Rio River Torrente Torrente Torrente

16.40 n.d. 27.70 31.90 n.d. 81.48 40.23 22.28 65.40

16.20 n.d. 14.40 29.00 n.d. 48.52 36.31 24.71 51.47

Recurring Recurring Recurring Recurring Recurring Recurring Recurring Recurring Recurring

subaerial distributary channels due to human activity are present in the Fraser Delta in British Columbia (Hart et al., 1998) and in the Gulf of Corinth river delta (Piper et al., 1990). The Niceto Delta exhibits two sectors with different morphology. It has a western part with characteristics similar to those of the Corriolo, Muto and Saponara deltas, consisting of a 2500-m-wide channelized area developed landward from the head of the Niceto canyon located at a depth of about 70 m, at a distance of 1500 m from the coastline (Figs. 11, 12, 13b). In its eastern part, a channelized area reaches the shelf edge at a distance of 4 km from the coastline (Figs. 11, 13b). This area is not linked to the present-day river mouth but it is developed seaward from the alluvial valley that in the coastal region is up to 3 -km -wide (Fig. 11). Therefore it is possible that this part of the delta is inactive, but was previously fed by delta distributaries, before the anthropogenic confinement of the rivers. The Gallo, the Rometta and the Cocuzzaro Deltas develop in shelf areas where canyon heads are far from the coastline (Fig. 13b). The offshore part of the Rometta Delta and, to a lesser degree, of the Cocuzzaro Delta are characterized by the extensive development of sandwaves (Figs. 8, 13b). Widespread sandwave fields, such as the one developed in the Rometta Delta, have been documented in the Noieck river delta in the South of Alaska (Bornhold and Prior, 1990) and in the Guadalfeo river delta in SW Spain (Lobo et al., 2006). These sandwaves were interpreted as being the result of the action of hyperpycnal flows,

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connected with periodic high-energy seasonal large river floods. We suggest a similar origin for the sandwave fields in the deltas of the study area which are also characterized by episodic large river floods due to heavy rains. The sandwave fields are the result of tractive bed load transported by sustained turbulent flows exiting the river mouth, as hyperpycnal flows, as suggested by Mutti et al. (2003) and Zavala et al. (2011) for similar bedforms. In the Rometta Delta, the sandwaves show an asymmetric distribution reaching higher depths in the eastern delta side (Fig. 8). We interpret this asymmetry as resulting from the tectonic structures present on the shelf, with a fault that lowers the eastern side of the delta that therefore has more space to develop basinward on that side (Fig. 8). On the contrary, the Cocuzzaro and the Niceto Deltas show abundant channelization in their offshore parts (Figs. 8, 11). The channels are associated with small-scale bedforms (Figs. 8, 11, insets). We interpret these features as resulting from the local action of highly turbulent flows connected with hyperpycnal flow evolution along the delta slope. 6. Conclusions The present study highlights how the geomorphology of continental shelves can vary over very short spatial scales depending on the various agents that feed sediment to the shelf and rework it across and along the shelf. We show that in areas where rivers have limited sediment discharge, the recent highstand wedge is mainly shaped by oceanographic processes and does not reach the outer shelf where a relict geomorphology reflecting processes that occurred during previous lower sea-level stands is recognized. On the contrary, where rivers have higher sediment discharge, deltas are developed and much or the whole shelf is shaped by processes connected with river-fed depositional bodies outbuilding and the highstand wedge reaches the shelf break. In the study area, the amount of sediment supply from rivers, largely controlled by the size of the river drainage basins, in turn determined by on-land tectonic structures, is the first control on the environmental setting of the shelf. Deltas are present on the shelf at depths greater than 50 m (the limit of our data) only in front of the mouths of the largest rivers. Our study shows that the amount of river-borne sediment that is trapped on the shelf is largely determined by the available accommodation space. Where canyon heads are close to the coastline, the riverborne sediment yield forms deltas that have their front within the canyon heads and can feed sediment to the deep sea. Where the shelf edge is far from the coastline, deltas can fully develop and much of the river input is trapped on the shelf. Along-shelf sediment redistribution is mainly active where rivers are small in terms of sediment supply and where the regional physical oceanography is conducive to sediment reworking forming sandwave fields. Tidal currents channeled in the nearby Messina Straits are capable of shaping a large part of the inner shelf of the study area. The discontinuous and highly diverse nature of the resulting bedform fields show, however, that sediment distribution is an important factor in determining the shape and dimension of sandwaves. Our data also show that sediment flux and sediment reworking can be negligible processes for long periods in specific areas of the outer shelf, thus resulting in the preservation of past landscapes. The coastal barrier–lagoon systems formed during the last sea-level rise mark coastal system building during a long-term transgression trend. These submerged landscapes show that past coastal settings were different from the present-day ones, posing interesting suggestions on the environmental conditions for early human settlements in the northeastern Sicilian area. Acknowledgments We thank all the participants to the various cruises of data acquisition in the study area. We are grateful to Katrien Van Landeghem and Cornel Olariu, for their thorough, critical evaluations of the original

manuscript, as well as for their valuable suggestions. This is contribution 1812 of ISMAR Bologna.

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